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Immunologic messenger molecules: Cytokines, interferons, and chemokines

Published:November 25, 2009DOI:https://doi.org/10.1016/j.jaci.2009.07.008
      Cytokines and chemokines are secreted proteins involved in numerous aspects of cell growth, differentiation, and activation. A prominent feature of these molecules is their effect on the immune system with regard to cell trafficking and development of immune tissue and organs. The nature of an immune response determines which cytokines are produced and ultimately whether the response is cytotoxic, humoral, cell mediated, or allergic. For this chapter, cytokines are grouped according to those that are predominantly antigen-presenting cell or T lymphocyte derived; that mediate cytotoxic, humoral, cell mediated, and allergic immunity; or that are immunosuppressive. A discussion of chemokine function and their role in cell trafficking and disease follows.

      Key words

      Abbreviations used:

      ABPA (Allergic bronchopulmonary aspergillosis), AHR (Airway hyperreactivity), APC (Antigen-presenting cell), APRIL (A proliferation-inducing signal), BAFF (B-cell activation factor from the TNF family), DC (Dendritic cell), Foxp3 (Forkhead box protein 3), gp130 (Glycoprotein 130), ICAM (Intercellular adhesion molecule), IFNGR (IFN-γ receptor), IL-1ra (IL-1 receptor antagonist), IL-2R (IL-2 receptor), IL-4R (IL-4 receptor), IL-5R (IL-5 receptor), IL-6R (IL-6 receptor), IL-10R (IL-10 receptor), IL-12R (IL-12 receptor), IL-13R (IL-13 receptor), IL-17R (IL-17 receptor), IL-20R (IL-20 receptor), IL-22R (IL-22 receptor), IRS (Insulin response element), iTreg (Induced regulatory T), JAK (Janus kinase), MAPK (Mitogen-activated protein kinase), MCP (Monocyte chemoattractant protein), M-CSF (Macrophage colony-stimulating factor), MIP (Macrophage inflammatory protein), NK (Natural killer), nTreg (Natural regulatory T), ROR (Retinoic acid receptor–related orphan receptor), SCF (Stem cell factor), STAT (Signal transducer and activator of transcription), TACI (Transmembrane activator and calcium modulator and cyclophilin ligand interactor), T-bet (T-box expressed in T cells), Treg (Regulatory T), TSLP (Thymic stromal lymphopoietin), VCAM (Vascular cell adhesion molecule)
      Cytokines are secreted proteins with growth, differentiation, and activation functions that regulate and determine the nature of immune responses. For this review, cytokines are grouped according to those that are predominantly antigen-presenting cell (APC) or T lymphocyte derived; that predominantly mediate cytotoxic (antiviral and anticancer), humoral, cell-mediated (TH1 and TH17), or allergic immunity (TH2); or that are immunosuppressive (regulatory T [Treg]). This is followed by a discussion of the complementary family of secreted immune proteins, the chemokines. Cytokine families are summarized in Table I.
      Table ICytokine families
      FamilyMembers
      Hematopoietic
       Common γ chainIL-2, IL-4, IL-7, IL-9, IL-15, IL-21
       Shared β chain (CD131)IL-3, IL-5, GM-CSF
       Shared IL-2β chain (CD122)IL-2, IL-15
       Other hematopoieticIFN-γ, IL-7, IL-13, IL-21, IL-31, TSLP
      IL-1 familyIL-1α, IL-1β, IL-1ra, IL-18, IL-33
      gp130-utilizingIL-6, IL-11, IL-27, IL-31, ciliary neurotrophic factor (CNTF), cardiotrophin 1 (CT-1), leukemia inhibitory factor (LIF), oncostatin M (OSM), osteopontin
      IL-12IL-12, IL-23, IL-35
      IL-10 superfamilyIL-10, IL-19, IL-20, IL-22, IL-24, IL-26, IL-28, IL-29
      IL-17IL-17A-F, IL-25 (IL-17E)
      Interferons
       Type I interferonsIFN-α, IFN-β, IFN-ω
       Type II interferonIFN-γ (also a hematopoietic cytokine)
       Type III interferonsIFN-λ1 (IL-29), IFN-λ2 (IL-28A), IFN-λ3 (IL-28B)
      TNF superfamilyTNF-α, TNF-β, BAFF, APRIL

      Cytokine production by antigen-presenting cells

      Cytokines primarily derived from dendritic cells (DCs), mononuclear phagocytes, and other APCs are particularly effective in subserving the dual functions of generating a potent innate immune response and providing signals contributing to initiation and guidance of the nature of the adaptive immune response. The processing of antigens as they are taken up by APCs, metabolized, and presented to TH lymphocytes provides one pathway for this class of cytokine production. Alternatively, APCs are potently triggered to produce cytokines through their pattern recognition receptors. The cytokines predominantly produced by APCs include TNF, IL-1, IL-6 (and other glycoprotein 130 [gp130]–utilizing factors), CXCL8 (IL-8), and other members of the chemokine family (discussed later), as well as IL-12, IL-15, IL-18, IL-23, IL-27, and IL-32.

       TNF

      TNF represents 2 homologous proteins primarily derived from mononuclear phagocytes (TNF-α) and lymphocytes (TNF-β).
      • Beutler B.
      • Cerami A.
      The biology of cachectin/TNF—a primary mediator of the host response.
      TNF-α is also produced by neutrophils, lymphocytes, natural killer (NK) cells, endothelium, and mast cells. TNF-α is processed as a membrane-bound protein from which the soluble active factor is cleaved by using the enzyme TNF-α converting enzyme.
      • Perez C.
      • Albert I.
      • DeFay K.
      • Zachariades N.
      • Gooding L.
      • Kriegler M.
      A nonsecretable cell surface mutant of tumor necrosis factor (TNF) kills by cell-to-cell contact.
      TNF-β (also known as lymphotoxin α) can be synthesized and processed as a typical secreted protein but is usually linked to the cell surface by forming heterotrimers with a third membrane-associated member of this family, lymphotoxin β. TNF-α and TNF-β bind to the same 2 distinct cell-surface receptors, TNF receptor I (p75) and TNF receptor II (p55), with similar affinities and produce similar, although not identical, effects.
      • Tartaglia L.A.
      • Goeddel D.V.
      Two TNF receptors.
      Notably, the active form of both cytokines is a homotrimer. TNFs induce antitumor immunity through direct cytotoxic effects on cancerous cells and by stimulating antitumor immune responses. TNFs interact with endothelial cells to induce intercellular adhesion molecule (ICAM) 1, vascular cell adhesion molecule (VCAM) 1, and E-selectin, permitting the egress of granulocytes into inflammatory loci. TNFs are a potent activator of neutrophils, mediating adherence, chemotaxis, degranulation, and the respiratory burst. TNFs are responsible for the severe cachexia that occurs in chronic infections and cancer.
      • Beutler B.
      • Cerami A.
      The biology of cachectin/TNF—a primary mediator of the host response.
      Furthermore, TNFs induce vascular leakage and have negative inotropic effects, and because the most potent inducer of TNF is endotoxin, it is the primary mediator of septic shock.
      • Tracey K.J.
      • Fong Y.
      • Hesse D.G.
      • Manogue K.R.
      • Lee A.T.
      • Kuo G.C.
      • et al.
      Anti-cachectin/TNF monoclonal antibodies prevent septic shock during lethal bacteraemia.

       IL-1

      The IL-1 family represents 5 peptides (IL-1α, IL-1β, the IL-1 receptor antagonist [IL-1ra], IL-18, and IL-33).
      • Dinarello C.A.
      • Wolff S.M.
      The role of interleukin-1 in disease.
      IL-1α and IL-1β have similar biologic activities and, along with IL-1ra, have similar affinities for the 2 IL-1 receptors. Type I receptors transduce the biologic effects attributed to IL-1.
      • Sims J.E.
      • Gayle M.A.
      • Slack J.L.
      • Alderson M.R.
      • Bird T.A.
      • Giri J.G.
      • et al.
      Interleukin 1 signaling occurs exclusively via the type I receptor.
      Type II receptors have a minimal intracellular domain, and the capture and sequestration of IL-1 by these inactive receptors serves an anti-inflammatory function. The capacity of IL-1ra to bind IL-1 receptor without transducing activities is the basis for its antagonist function.
      • Arend W.P.
      Interleukin-1 receptor antagonist.
      IL-1ra is secreted in inflammatory processes in response to many cytokines, including IL-4, IL-6, IL-13, and TGF-β. Production of IL-1ra moderates the potentially deleterious effects of IL-1 in the natural course of inflammation.
      IL-1 is primarily produced by cells of the mononuclear phagocytic lineage but is also produced by endothelial cells, keratinocytes, synovial cells, osteoblasts, neutrophils, glial cells, and numerous other cells. IL-1 production is stimulated by a variety of agents, including endotoxin, that stimulate molecular pattern receptors. Both IL-1α and IL-1β, as well as the related proteins IL-18 and IL-33 (discussed later), are synthesized as inactive precursors without a secretory sequence. The mechanism for their secretion depends on cleavage by a specific converting enzyme, termed IL-1 converting enzyme or caspase-1, contained within a specialized intracellular complex termed the inflammasome, which cleaves the procytokines into their active secreted forms.
      • Cerretti D.P.
      • Kozlosky C.J.
      • Mosley B.
      • Nelson N.
      • Van Ness K.
      • Greenstreet T.A.
      • et al.
      Molecular cloning of the interleukin-1 beta converting enzyme.
      One of the most important biologic activities of IL-1 is its ability to activate T lymphocytes by enhancing the production of IL-2 and the expression of IL-2 receptors. In the absence of IL-1, a diminished immune response or tolerance develops. The production of IL-1 (and other APC-derived cytokines) during the immune response produces a spectrum of changes associated with being ill. IL-1 interacts with the central nervous system to produce fever, lethargy, sleep, and anorexia. An IL-1–hepatocyte interaction inhibits production of housekeeping proteins (eg, albumin) and stimulates the synthesis of acute-phase response peptides (eg, amyloid peptide, C-reactive peptide, and complement). IL-1 stimulates endothelial cell adherence of leukocytes through the upregulation of ICAM-1, VCAM-1, and E-selectin. IL-1 contributes to the hypotension of septic shock. TNF and IL-1 share numerous biologic activities, the major distinction being that TNF has no direct effect on lymphocyte proliferation.

       IL-6

      Mononuclear phagocytic cells are the most important source of IL-6
      • Akira S.
      • Taga T.
      • Kishimoto T.
      Interleukin-6 in biology and medicine.
      ; however, IL-6 is also produced by T and B lymphocytes, fibroblasts, endothelial cells, keratinocytes, hepatocytes, and bone marrow cells. IL-6 signals through a ligand-binding IL-6 receptor (IL-6R) α chain (CD126) and the signal-transducing component gp130 (CD130). CD130 is the common signal transducer for several cytokines in the IL-6 family and is ubiquitously expressed. In contrast, the expression of IL-6Rα is restricted. In addition to the membrane-bound receptor, a soluble form of IL-6R can capture circulating IL-6 and make it available to bind and activate gp130.
      • Muller-Newen G.
      • Kuster A.
      • Hemmann U.
      • Keul R.
      • Horsten U.
      • Martens A.
      • et al.
      Soluble IL-6 receptor potentiates the antagonistic activity of soluble gp130 on IL-6 responses.
      In contrast, soluble gp130 functions as an anti-inflammatory decoy receptor. Other cytokines that signal through gp130-containing receptors are IL-11, IL-27, IL-31, ciliary neurotrophic factor, leukemia inhibitory factor, oncostatin M, and osteopontin. These cytokines are referred to as the IL-6–like or gp130-utilizing cytokines (Table II).
      • Heinrich P.C.
      • Behrmann I.
      • Haan S.
      • Hermanns H.M.
      • Muller-Newen G.
      • Schaper F.
      Principles of interleukin (IL)-6-type cytokine signalling and its regulation.
      Table IIIL-6–like (gp130-utilizing) cytokines
      IL-6–like cytokineCharacteristics
      IL-31Primarily expressed by TH lymphocytes under TH2 conditions. Induces chemokines that recruit neutrophils, monocytes, T cells. Overexpression in mice leads to atopic dermatitis model. Increased IL-31 receptor levels in murine model of AHR.
      IL-11Increases production of acute-phase proteins. Induces lymphoid cell differentiation. Stimulatory factor for fibroblasts. Expression in severe asthma with remodeling.
      OsteopontinInduced by IFN-γ, IL-1β, and TNF-α. Expression inhibited by IL-4 and IL-13. Upregulated in patients with chronic sinusitis, nasal polyps, and asthma.
      OncostatinSynthesized by T cells and monocytes. Proinflammatory or anti-inflammatory functions. Roles in liver development, hematopoiesis, inflammation, and possibly CNS development. Signals through a shared type I receptor of gp130/LIFR-β and a specific type II receptor of gp130/OSMRβ.
      LIFInduces terminal differentiation of myeloid leukemia cells. Influences bone metabolism, cachexia, neural development, embryogenesis, and inflammation. Binds to the specific LIF receptor (gp130/LIFR-α).
      Under the influence of IL-6, B lymphocytes differentiate into mature plasma cells and secrete immunoglobulins. IL-6 mediates T-cell activation, growth, and differentiation. In addition to lymphocyte activation, IL-6 shares several activities with IL-1, including the induction of pyrexia and the production of acute-phase proteins. IL-6 is the most important inducer of acute-phase proteins. As discussed below, IL-6 has a primary role in TH17 immune deviation.

       IL-12, IL-18, and IL-23

      IL-12 and IL-23 are heterodimers that share a larger (IL-12p40) subunit. Both are primarily derived from DCs.
      • Brunda M.J.
      Interleukin-12.
      • Oppmann B.
      • Lesley R.
      • Blom B.
      • Timans J.C.
      • Xu Y.
      • Hunte B.
      • et al.
      Novel p19 protein engages IL-12p40 to form a cytokine, IL-23, with biological activities similar as well as distinct from IL-12.
      Their receptors are also heterodimers having distinct α chains and shared use of the IL-12 receptor (IL-12R) β1 chain. These cytokines are involved in T-cell activation and immune deviation of TH1 and TH17 cells, respectively (discussed later).
      IL-12 is derived most importantly from DCs but also from Langerhans cells, mononuclear phagocytic cells, B cells, PMNs, and mast cells. The biologically active form is a heterodimer. The larger subunit (IL-12p40) is homologous to the soluble receptor for IL-6, whereas the smaller subunit (IL-12p35) is homologous to IL-6. Homodimers of IL-12p40 are also functional (IL-12p80). IL-12 stimulates IFN-γ production and activates and induces proliferation, cytotoxicity, and cytokine production of NK cells. Other activities attributed to IL-12 include proliferation of TH and cytotoxic lymphocytes.
      IL-18, along with IL-12 and IL-23, is an inducer of IFN-γ.
      • Dinarello C.A.
      Interleukin-18, a proinflammatory cytokine.
      Similar to IL-1, IL-18 requires a specific converting enzyme (caspase-1) to permit secretion and activation. In contrast to most cytokines, IL-18 is constitutively expressed, and release of its active form is regulated through activation of this converting enzyme. IL-18 has an important role in cellular adhesion, being the final common pathway used by IL-1 and TNF that leads to ICAM-1 expression. IL-18 binds to a unique heterodimer receptor, the expression of which is upregulated by IL-12, and hence these 2 cytokines synergize to stimulate IFN-γ release.
      Finally, as noted, IL-23 is a heterodimer consisting of a larger subunit shared with IL-12 (IL-12p40) and a unique subunit (IL-23p19). Its inflammatory response includes induction of remodeling through activation of matrix metalloproteinases, increased angiogenesis, and reduced CD8 T-cell infiltration. Its important synergistic role in TH17 differentiation is discussed below.

       IL-15

      Mononuclear phagocytic cells are the main source of IL-15, whereas epithelium, fibroblasts, and placenta are additional sources. IL-15 is distinguished from IL-2 through its use of a unique α chain as part of its receptor signaling complex.
      • Grabstein K.H.
      • Eisenman J.
      • Shanebeck K.
      • Rauch C.
      • Srinivasan S.
      • Fung V.
      • et al.
      Cloning of a T cell growth factor that interacts with the beta chain of the interleukin-2 receptor.
      Both receptors share the use of the IL-2 receptor (IL-2R) β and common γ chain (Table I). IL-15, similar to IL-2, is a T-cell growth factor and is chemotactic for T lymphocytes. The most important activity of IL-15 might be its activation of NK cells. IL-2 and IL-15 are contrasted in their roles in adaptive immune responses in which IL-2, but not IL-15, is involved in the generation and maintenance of Treg cells, whereas IL-15 is necessary for maintaining the survival of CD8 memory T cells. IL-15 is also active as an accessory mast cell growth factor.

       IL-27

      The cells responsible for most of the production of IL-27 are macrophages and DCs. IL-27 is a heterodimer composed of IL-27B (EBV-induced gene B) and IL-27p28 (also known as IL-30).
      • Larousserie F.
      • Bardel E.
      • Pflanz S.
      • Arnulf B.
      • Lome-Maldonado C.
      • Hermine O.
      • et al.
      Analysis of interleukin-27 (EBI3/p28) expression in Epstein-Barr virus- and human T-cell leukemia virus type 1-associated lymphomas: heterogeneous expression of EBI3 subunit by tumoral cells.
      IL-27 subserves important functions in TH1 immunity, reflecting its ability to synergize with IL-12 to induce IFN-γ production from NK and TH cells (TH1 immune deviation). The effects of IL-27 are mediated through interaction with a receptor complex consisting of IL-27 receptor α and gp130.
      • Pflanz S.
      • Hibbert L.
      • Mattson J.
      • Rosales R.
      • Vaisberg E.
      • Bazan J.F.
      • et al.
      WSX-1 and glycoprotein 130 constitute a signal-transducing receptor for IL-27.

       IL-32

      IL-32 was discovered in a search for IL-18–inducible genes.
      • Kim S.H.
      • Han S.Y.
      • Azam T.
      • Yoon D.Y.
      • Dinarello C.A.
      Interleukin-32: a cytokine and inducer of TNFalpha.
      Its biologic activities include induction of proinflammatory cytokines (eg, TNF-α) and chemokines from differentiated macrophages. The highest levels of expression are observed in NK and T cells; however, expression can also be observed in epithelial cells in response to IFN-γ and IL-1β. IL-32 synergizes with nucleotide-binding oligomerization domain 1 and 2 ligands to stimulate IL-6 and IL-1β release in a caspase-1–dependent manner.
      • Netea M.G.
      • Azam T.
      • Ferwerda G.
      • Girardin S.E.
      • Walsh M.
      • Park J.S.
      • et al.
      IL-32 synergizes with nucleotide oligomerization domain (NOD) 1 and NOD2 ligands for IL-1beta and IL-6 production through a caspase 1-dependent mechanism.

      Cytotoxic immunity

      Immune responses directed against virus-infected and neoplastic cells are primarily mediated by CD8+ cytotoxic lymphocytes and NK cells. As discussed elsewhere, numerous cytokines contribute to cytotoxic immunity, as well as IL-11 and the interferons.

       IL-11

      In addition to its functions in promoting cytotoxic antiviral immune responses, IL-11 was originally described as a stimulatory factor for hematopoietic cells, synergizing with other growth factors to produce erythrocytes and platelets. IL-11 increases the production of acute-phase proteins and induces lymphoid cell differentiation. It is an important stimulatory factor for connective tissue cells, such as fibroblasts, that stimulate proliferation and collagen deposition. A role for IL-11 in asthma remodeling is suggested by studies demonstrating expression of IL-11 in patients with severe asthma.
      • Minshall E.
      • Chakir J.
      • Laviolette M.
      • Molet S.
      • Zhu Z.
      • Olivenstein R.
      • et al.
      IL-11 expression is increased in severe asthma: association with epithelial cells and eosinophils.
      • Tang W.
      • Geba G.P.
      • Zheng T.
      • Ray P.
      • Homer R.J.
      • Kuhn 3rd, C.
      • et al.
      Targeted expression of IL-11 in the murine airway causes lymphocytic inflammation, bronchial remodeling, and airways obstruction.

       Interferons

      Interferons derive their name from their ability to interfere with viral growth. There are 3 major classes of interferons. Type I interferons (IFN-α/β/ω) are primarily derived from monocytes, macrophages, B lymphocytes, and NK cells. An important source of IFN-α is plasmacytoid DCs, reflecting their activation by viral RNA acting through Toll-like receptors 3 and 7. The antiviral activity of type I interferons is mediated through their ability to inhibit viral replication within virus-infected cells, protect uninfected cells from infection, and stimulate antiviral immunity by cytotoxic (CD8+) lymphocytes and NK cells. IFN-α has other important biologic actions, including upregulation of class I MHC antigens and mediation of antitumor activity. IFN-ω
      • Adolf G.R.
      Monoclonal antibodies and enzyme immunoassays specific for human interferon (IFN) omega 1: evidence that IFN-omega 1 is a component of human leukocyte IFN.
      displays a high degree of homology with various IFN-α species, including positions of the cysteine residues involved in disulfide bonds
      • Adolf G.R.
      • Maurer-Fogy I.
      • Kalsner I.
      • Cantell K.
      Purification and characterization of natural human interferon omega 1. Two alternative cleavage sites for the signal peptidase.
      ; however, sequence divergence allows classification as a unique protein family. IFN-ω binds to the same receptors as IFN-α and IFN-β.
      • Flores I.
      • Mariano T.M.
      • Pestka S.
      Human interferon omega binds to the alpha/beta receptor.
      A sole member makes up the class of type II interferons: IFN-γ. IFN-γ is a homodimer primarily made by T cells and NK cells and to a lesser degree by macrophages. The biologic activities of IFN-γ include only modest antiviral activity, and its derivation primarily from T lymphocytes suggests that it is more of an interleukin than an interferon. IFN-γ and its role in cellular immunity are discussed below.
      The type III interferons consist of IFN-λ1, IFN-λ2, and IFN-λ3, also called IL-29, IL-28A, and IL-28B, respectively. Type III interferons share with type I interferons the same Janus kinase (JAK) and signal transducer and activator of transcription (STAT) signaling pathways. IFN-λs exhibit several other common features with type I interferons, including antiviral, antiproliferative, and antitumor activities. Despite amino acid homology with type I interferons, the intron-exon structure of the IFN-λ family more closely resembles that of IL-10.
      • Sheppard P.
      • Kindsvogel W.
      • Xu W.
      • Henderson K.
      • Schlutsmeyer S.
      • Whitmore T.E.
      • et al.
      IL-28, IL-29 and their class II cytokine receptor IL-28R.
      Moreover, IFN-λs act through a cell-surface heterodimer receptor, one chain being IFN-γ–specific (IFNLR1) and the second, IL-10 receptor (IL-10R) 2, being shared by IL-10, IL-22, and IL-26 (Table III). In addition to the full-length IFNLR1, 2 inhibitory splice variants have been identified, one variant deletes 29 amino acids in its intracytoplasmic portion, likely disabling its signaling capacity, and the second encodes a secreted (decoy) receptor.
      • Sheppard P.
      • Kindsvogel W.
      • Xu W.
      • Henderson K.
      • Schlutsmeyer S.
      • Whitmore T.E.
      • et al.
      IL-28, IL-29 and their class II cytokine receptor IL-28R.
      Although IL-10R2 is ubiquitously expressed, IFNLR1 is more tightly regulated. IFN-λ subtypes are induced on infection by multiple viruses, which is consistent with their antiviral activities,
      • Sheppard P.
      • Kindsvogel W.
      • Xu W.
      • Henderson K.
      • Schlutsmeyer S.
      • Whitmore T.E.
      • et al.
      IL-28, IL-29 and their class II cytokine receptor IL-28R.
      • Kotenko S.V.
      • Gallagher G.
      • Baurin V.V.
      • Lewis-Antes A.
      • Shen M.
      • Shah N.K.
      • et al.
      IFN-lambdas mediate antiviral protection through a distinct class II cytokine receptor complex.
      and pretreating hepatocellular cells prevents viral infection.
      • Sheppard P.
      • Kindsvogel W.
      • Xu W.
      • Henderson K.
      • Schlutsmeyer S.
      • Whitmore T.E.
      • et al.
      IL-28, IL-29 and their class II cytokine receptor IL-28R.
      One notable difference between IFN-λ and type I interferons is that IFN-λ shifts immature DCs toward a program characterized by the ability to produce forkhead box protein 3 (Foxp3)–expressing CD4+CD25+ Treg cells.
      • Mennechet F.J.
      • Uze G.
      Interferon-lambda-treated dendritic cells specifically induce proliferation of FOXP3-expressing suppressor T cells.
      Table IIIIL-10 superfamily
      Interleukin1° Cell sourceReceptorActivated signal transducerBiologic effectClinical association
      IL-10Monocytes, B cells, Treg cellsIL-10R1/IL-10R2JAK1, TYK2, STAT1, STAT3Immune suppression, anti-inflammatoryBurkitt lymphoma, malignant B-cell lymphomas
      IL-19MonocytesIL-20R1/IL-20R2STAT1, STAT3Skin development, immunoregulatoryPsoriasis, asthma
      IL-20Monocytes, skin keratinocytesIL-20R1/IL-20R2, IL-22R1/IL-10R2JAK/STATSkin development, innate immunity, hematopoiesisPsoriasis, atherosclerosis, angiogenesis
      IL-22Activated T cells, activated NK cells, TH17 cellsIL-22R1/IL-10R2STAT3Acute-phase response, innate immunityCrohn disease, interstitial lung disease, rheumatoid arthritis, psoriasis
      IL-24Melanocytes, monocytes, TH2 cellsIL-20R1/IL-20R2, IL-22R1/IL-20R2 (skin only)STAT3Proapoptosis, epidermal functions, inflammatory cascadeMelanoma, psoriasis, inflammation
      IL-26Monocytes, memory T cellsIL-20R1/IL-10R2STAT1, STAT3Mucosal and cutaneous immunityT-cell transformation
      IL-28, IL-29DCsIFNLR1/IL-10R2JAK1, STAT1, STAT2, STAT3, and STAT5Antiviral immunityHepatitis B/C infections

      Humoral immunity

      At least 2 cytokines contribute to B-lymphocyte maturation in the bone marrow: the lymphoid stem cell growth factors IL-7 and IL-11. IL-7 is critically important to the development of both B and T lymphocytes through its production by stromal tissue of the bone marrow and thymus, from which it interacts with lymphoid precursors. In addition, IL-7 stimulates the proliferation and differentiation of cytotoxic T and NK cells and stimulates the tumoricidal activity of monocytes and macrophages. The central importance of IL-7 to lymphoid maturation is reflected in severe combined immune deficiency resulting from the absence of either IL-7 or functional IL-7 receptors (IL-7 receptor α [CD127] or common γ chain).

       IL-21

      IL-21 is increasingly recognized as being central to B-cell proliferation, survival, and differentiation into immunoglobulin-producing plasma cells.
      • Konforte D.
      • Simard N.
      • Paige C.J.
      IL-21: an executor of B cell fate.
      Its induction of activation-induced cytidine deaminase contributes to class-switch recombination. IL-21 receptors are expressed on activated B, T, and NK cells. It shares numerous biologic activities with IL-2 and IL-15, with which it is homologous, including the capacity to activate NK cells and promote the proliferation of B and T lymphocytes.
      • Parrish-Novak J.
      • Dillon S.R.
      • Nelson A.
      • Hammond A.
      • Sprecher C.
      • Gross J.A.
      • et al.
      Interleukin 21 and its receptor are involved in NK cell expansion and regulation of lymphocyte function.
      Its receptor shares the common γ chain with IL-2, IL-4, IL-7, IL-9, and IL-15. Among T cells, it is preferentially expressed by TH17 cells and is involved in TH17 differentiation (discussed below).

       B-cell activation factor from the TNF family and a proliferation-inducing ligand

      Two other TNF family cytokines, B-cell activation factor from the TNF family (BAFF) and a proliferation-inducing ligand (APRIL), enhance the maturation and survival of transitional and mature B cells. BAFF and APRIL are expressed in bone marrow nonlymphoid cells, with low levels also in developing B cells. BAFF overexpression leads to an expanded B-cell compartment, and increased amounts of BAFF have been found in autoimmune patients. BAFF knockout mice have a severe block in B-cell development in the spleen, although not in bone marrow. Three receptors from the TNF receptor family bind to BAFF and APRIL: transmembrane activator and calcium modulator and cyclophilin ligand interactor (TACI), B-cell maturation antigen, and BAFF-R.
      • Gross J.A.
      • Johnston J.
      • Mudri S.
      • Enselman R.
      • Dillon S.R.
      • Madden K.
      • et al.
      TACI and BCMA are receptors for a TNF homologue implicated in B-cell autoimmune disease.
      • Thompson J.S.
      • Bixler S.A.
      • Qian F.
      • Vora K.
      • Scott M.L.
      • Cachero T.G.
      • et al.
      BAFF-R, a newly identified TNF receptor that specifically interacts with BAFF.
      BAFF-R binds specifically to BAFF, whereas TACI and BMCA bind primarily to APRIL. Similar to BAFF-deficient mice, BAFF-R–null mice show defective splenic B-cell maturation. Mutations in TACI have been identified as an important factor in common variable immunodeficiency.
      • Castigli E.
      • Wilson S.A.
      • Garibyan L.
      • Rachid R.
      • Bonilla F.
      • Schneider L.
      • et al.
      TACI is mutant in common variable immunodeficiency and IgA deficiency.
      • Salzer U.
      • Chapel H.M.
      • Webster A.D.
      • Pan-Hammarstrom Q.
      • Schmitt-Graeff A.
      • Schlesier M.
      • et al.
      Mutations in TNFRSF13B encoding TACI are associated with common variable immunodeficiency in humans.
      After B cells egress from the bone marrow, isotype switching, the activation of mature B cells into immunoglobulin-secreting B cells, and their final differentiation into plasma cells are processes that are under T-cell control.
      • Finkelman F.D.
      • Holmes J.
      • Katona I.M.
      • Urban Jr., J.F.
      • Beckmann M.P.
      • Park L.S.
      • et al.
      Lymphokine control of in vivo immunoglobulin isotype selection.
      Cytokines that trigger isotype switching include IL-4 and IL-13, which induces the IgE isotype TGF-β, which triggers IgA, and IL-10, which contributes to the generation of IgG4.

      Cellular immunity

       IL-2

      Stimulation of T cells by antigen (signal 1) in the presence of accessory signals provided by the cognate interaction of the B7 molecules (CD80 or CD86) with CD28 (signal 2) and the cytokines IL-1 and IL-6 (signal 3) induces the simultaneous secretion of IL-2 and the expression of high-affinity IL-2R by effector T cells. Subsequently, the binding of secreted IL-2 to these IL-2R–expressing T cells induces clonal T-cell proliferation. The requirement for both IL-2 production and IL-2R expression for T-cell proliferation ensures that only effector T cells specific for the antigen inciting the immune response become activated. This is in contrast to Treg cells, which constitutively express IL-2R and can thereby be spontaneously activated in the presence of IL-2. IL-2 is also necessary during Treg cell development in the thymus.
      • Thornton A.M.
      • Donovan E.E.
      • Piccirillo C.A.
      • Shevach E.M.
      Cutting edge: IL-2 is critically required for the in vitro activation of CD4 + CD25 + T cell suppressor function.
      IL-2 signals through a receptor complex consisting of IL-2–specific IL-2Rα (CD25), IL-2Rβ (CD122), and the common γ chain. In addition to its role as an effector and Treg cell growth factor, IL-2 is also involved in activation of NK cells, B cells, cytotoxic T cells, and macrophages. Many of the immunosuppressive drugs used in the treatment of autoimmune diseases, such as corticosteroids, cyclosporine, and tacrolimus, work, in part, by inhibiting the production of IL-2 by antigen-activated T cells, whereas others (eg, rapamycin) block IL-2R signaling.

       IFN-γ

      The most important cytokine responsible for cell-mediated immunity is IFN-γ.
      • Farrar M.A.
      • Schreiber R.D.
      The molecular cell biology of interferon-gamma and its receptor.
      It is the signature cytokine produced by TH1 cells but is also derived from cytotoxic T cells and NK cells. IFN-γ mediates increased MHC class I and II expression and stimulates antigen presentation and cytokine production by APCs. IFN-γ stimulates mononuclear phagocytic functions, including adherence, phagocytosis, secretion, respiratory burst, and nitric oxide production. The net result is the accumulation of macrophages at the site of cellular immune responses, with their activation into macrophages capable of killing intracellular pathogens. In addition to its effects on mononuclear phagocytes, IFN-γ stimulates killing by NK cells and neutrophils. It stimulates adherence of granulocytes to endothelial cells through the induction of ICAM-1, an activity shared with IL-1 and TNF. As with other interferons, IFN-γ inhibits viral replication. IFN-γ is critical for many aspects of innate and adaptive immunity, but its singular importance in the immune response to intracellular pathogens is shown by the enhanced susceptibility to tuberculosis observed in patients with mutations that result in defects in its synthesis or responsiveness.
      • Newport M.J.
      • Huxley C.M.
      • Huston S.
      • Hawrylowicz C.M.
      • Oostra B.A.
      • Williamson R.
      • et al.
      A mutation in the interferon-gamma-receptor gene and susceptibility to mycobacterial infection.
      IFN-γ is an inhibitor of TH2-mediated allergic inflammatory responses through its capacity to suppress many IL-4–mediated effects.
      Cellular responses to IFN-γ are activated through its interaction with a heterodimer receptor consisting of IFN-γ receptor (IFNGR) 1 and IFNGR2. Binding of IFN-γ to the receptor activates the JAK-STAT pathway. JAK1 and JAK2 constitutively associate with IFNGR1 and IFNGR2, respectively, and binding ultimately leads to phosphorylation of 2 STAT1 molecules, as discussed in greater detail below.
      • Bach E.A.
      • Aguet M.
      • Schreiber R.D.
      The IFN gamma receptor: a paradigm for cytokine receptor signaling.

       IL-16

      IL-16 is a T cell–derived product that is chemotactic for CD4+ lymphocytes, eosinophils, and monocytes and uses the CD4 molecule as its receptor.
      • Cruikshank W.W.
      • Center D.M.
      • Nisar N.
      • Wu M.
      • Natke B.
      • Theodore A.C.
      • et al.
      Molecular and functional analysis of a lymphocyte chemoattractant factor: association of biologic function with CD4 expression.
      The product of this gene undergoes proteolytic processing by caspase-3 and yields 2 functional proteins. The cytokine function is exclusively attributed to the secreted C-terminal peptide, whereas the N-terminal product might play a role in cell-cycle control.
      • Wilson K.C.
      • Center D.M.
      • Cruikshank W.W.
      The effect of interleukin-16 and its precursor on T lymphocyte activation and growth.

       IL-17

      Whereas IFN-γ is important in orchestrating the cellular immune response to intracellular pathogens, IL-17 generates T cell–mediated immune responses to extracellular pathogens. It is produced by a unique family of TH lymphocytes, termed TH17 cells. IL-17 comprises a structurally related family of 6 proteins (IL-17A through IL-17F) having no sequence similarity to any other cytokine.
      • Kawaguchi M.
      • Adachi M.
      • Oda N.
      • Kokubu F.
      • Huang S.K.
      IL-17 cytokine family.
      Because of its unique spectrum of activities, IL-17E is now termed IL-25 and is discussed separately. IL-17A (generally referred to as IL-17) is mainly expressed in CD4+ TH (TH17) cells and, to a lesser extent, neutrophils, eosinophils, and CD8 T cells. Similar to IL-17A, its most closely structurally related family member, IL-17F, is expressed by TH17 cells but also activated basophils and mast cells.
      • Kawaguchi M.
      • Adachi M.
      • Oda N.
      • Kokubu F.
      • Huang S.K.
      IL-17 cytokine family.
      The primary cellular sources for IL-17B and IL-17C have not been determined. IL-17D is expressed in resting CD4 T and B cells.
      IL-17 induces expression of a variety of cytokines and chemokines from stromal cells, fibroblasts, endothelium, and epithelium, including IL-6, IL-11, granulocyte colony-stimulating factor, GM-CSF, CXCL8, CXCL10 (IFN-inducible protein 10), and TGF-β, cytokines important to both fibroblast activation and neutrophil recruitment. Activation of fibroblasts by IL-17 might contribute to fibrotic autoimmune diseases, and a role for IL-17 has been proposed in inflammatory bowel disease and multiple sclerosis. IL-17 family members are also expressed in patients with asthma.
      • Molet S.
      • Hamid Q.
      • Davoine F.
      • Nutku E.
      • Taha R.
      • Page N.
      • et al.
      IL-17 is increased in asthmatic airways and induces human bronchial fibroblasts to produce cytokines.
      The tendency to induce neutrophil, but not eosinophil, migration makes it plausible that IL-17 plays a role in severe persistent asthma, in which accumulation of neutrophils is a hallmark. Both IL-17 and IL-17F induce goblet cell hyperplasia and mucus hypersecretion and activate epithelial innate immune responses. IL-17 could therefore plausibly contribute to the development of airway hyperreactivity (AHR), remodeling, neutrophilic infiltration, and subepithelial fibrosis.
      Induction of cytokines responsible for PMN recruitment and activation is central to its role in driving cellular immune responses to extracellular pathogens, as suggested by increased susceptibility to infection by Staphylococcus aureus and Citrobacter and Klebsiella species in IL-17–deficient mice.
      • Ishigame H.
      • Kakuta S.
      • Nagai T.
      • Kadoki M.
      • Nambu A.
      • Komiyama Y.
      • et al.
      Differential roles of interleukin-17A and -17F in host defense against mucoepithelial bacterial infection and allergic responses.
      In human subjects hyper-IgE syndrome has been characterized by a genetic deficiency in TH17 cell differentiation.
      • Milner J.D.
      • Brenchley J.M.
      • Laurence A.
      • Freeman A.F.
      • Hill B.J.
      • Elias K.M.
      • et al.
      Impaired T(H)17 cell differentiation in subjects with autosomal dominant hyper-IgE syndrome.
      • Ma C.S.
      • Chew G.Y.
      • Simpson N.
      • Priyadarshi A.
      • Wong M.
      • Grimbacher B.
      • et al.
      Deficiency of Th17 cells in hyper IgE syndrome due to mutations in STAT3.
      Increased susceptibility of these patients to infections with Candida species and S aureus is consistent with TH17 cells' role in immunity against these pathogens.
      • Minegishi Y.
      • Saito M.
      • Nagasawa M.
      • Takada H.
      • Hara T.
      • Tsuchiya S.
      • et al.
      Molecular explanation for the contradiction between systemic Th17 defect and localized bacterial infection in hyper-IgE syndrome.
      The IL-17 receptor (IL-17R) family consists of 5 broadly distributed receptors that have individual ligand specificities. IL-17RA is the best described and binds both IL-17A and IL-17F. IL-17RB binds both IL-17B and IL-17E, whereas the less well-described IL-17RC and IL-17RD might undergo alternate splicing to produce soluble (decoy) forms.
      • Kawaguchi M.
      • Adachi M.
      • Oda N.
      • Kokubu F.
      • Huang S.K.
      IL-17 cytokine family.
      The least described of these receptors, IL-17RE, is expressed in the pancreas, brain, and prostate.

       IL-34

      IL-34 is a newly discovered interleukin also having no homology to other cytokines.
      • Lin H.
      • Lee E.
      • Hestir K.
      • Leo C.
      • Huang M.
      • Bosch E.
      • et al.
      Discovery of a cytokine and its receptor by functional screening of the extracellular proteome.
      It is expressed in numerous tissues but is most abundant in the spleen. The receptor for IL-34 is colony-stimulating factor 1 receptor (CD115), a receptor also used by macrophage colony-stimulating factor (M-CSF), and like M-CSF, IL-34 stimulates monocyte proliferation and function.

      Allergic immunity

      An additional outcome of proinflammatory T-cell activation is the development of allergic (and presumably antiparasitic) immunity. Several features specifically associated with the allergic state are regulated by cytokines, including the regulation of IgE, eosinophilia, and mast cell proliferation, and these will be discussed separately.

       Regulation of IgE

      The inappropriate production of IgE in response to allergen defines atopy and is primarily mediated by IL-4 and IL-13.

       IL-4

      In addition to TH2 lymphocytes, IL-4
      • Paul W.E.
      • Ohara J.
      B-cell stimulatory factor-1/interleukin 4.
      is derived from basophils, NK T cells, eosinophils, and mast cells. In both eosinophils and basophils, IL-4 exists as a preformed, granule-associated peptide that can be rapidly released in allergic inflammatory responses. IL-4 stimulates MHC class II, B7 (CD80/CD86), CD40, surface IgM, and low-affinity IgE receptor (CD23) expression by B cells, thereby enhancing the antigen-presenting capacity of B cells. IL-4 induces the immunoglobulin isotype switch from IgM to IgE.
      • Coffman R.L.
      • Ohara J.
      • Bond M.W.
      • Carty J.
      • Zlotnik A.
      • Paul W.E.
      B cell stimulatory factor-1 enhances the IgE response of lipopolysaccharide-activated B cells.
      • Romagnani S.
      Regulation and deregulation of human IgE synthesis.
      IL-4 can be identified in the sera, bronchoalveolar lavage fluid, and lung tissue of asthmatic subjects and in nasal polyp tissue and nasal mucosa of subjects with allergic rhinitis.
      In addition to these effects on B cells, IL-4 has important influences on T lymphocytes. As will be discussed later, IL-4 contributes to the differentiation of naive TH0 lymphocytes toward a TH2 phenotype. IL-4 is also important in maintaining allergic immune responses by preventing apoptosis of TH2 lymphocytes.
      • Vella A.
      • Teague T.K.
      • Ihle J.
      • Kappler J.
      • Marrack P.
      Interleukin 4 (IL-4) or IL-7 prevents death of resting T cells: Stat-6 is probably not required for the effect of IL-4.
      • Enelow R.
      • Baramki D.F.
      • Borish L.C.
      Inhibition of effector T lymphocytes mediated through antagonism of IL-4.
      IL-4 renders TH2 cells refractory to the anti-inflammatory influences of corticosteroids.
      Another important activity of IL-4 in allergic inflammation is its ability to induce expression of VCAM-1. This produces enhanced adhesiveness of endothelium for T cells, eosinophils, basophils, and monocytes, but not neutrophils, as is characteristic of TH2-mediated allergic reactions.
      • Schleimer R.P.
      • Sterbinsky S.A.
      • Kaiser J.
      • Bickel C.A.
      • Klunk D.A.
      • Tomioka K.
      • et al.
      IL-4 induces adherence of human eosinophils and basophils but not neutrophils to endothelium. Association with expression of VCAM-1.
      IL-4 interacts with mast cells to stimulate IgE receptor expression and regulates expression of leukotriene C4 synthase, thereby determining their capacity to produce cysteinyl leukotrienes.
      • Hsieh F.H.
      • Lam B.K.
      • Penrose J.F.
      • Austen K.F.
      • Boyce J.A.
      T helper cell type 2 cytokines coordinately regulate immunoglobulin E-dependent cysteinyl leukotriene production by human cord blood-derived mast cells: profound induction of leukotriene C(4) synthase expression by interleukin 4.
      IL-4 contributes to the excessive mucous production in the asthmatic airway. Functional IL-4 receptors are heterodimers consisting of the IL-4 receptor (IL-4R) α chain interacting with either the shared γ chain or the IL-13 receptor (IL-13R) α1 chain.
      • Izuhara K.
      • Shirakawa T.
      Signal transduction via the interleukin-4 receptor and its correlation with atopy.
      This shared use of the IL-4Rα chain by IL-13 and IL-4 explains many of the common biologic activities of these cytokines.
      In contrast to these proinflammatory effects, IL-4 downregulates antibody-dependent cellular cytotoxicity by mononuclear phagocytes, inhibits their expression of Fcγ receptors and differentiation into macrophages, and downregulates production of nitric oxide, IL-1, IL-6, and TNF-α while stimulating production of IL-1ra and IL-10.
      • Steinke J.W.
      • Negri J.
      • Enelow R.
      • Baramki D.F.
      • Borish L.
      Proinflammatory effects of IL-4 antagonism.

       IL-13

      IL-13 shares much of IL-4's biologic activities on mononuclear phagocytic cells, endothelial cells, epithelial cells, and B cells. Thus IL-13 induces the IgE isotype switch and VCAM-1 expression.
      • Zurawski G.
      • de Vries J.E.
      Interleukin 13, an interleukin 4-like cytokine that acts on monocytes and B cells, but not on T cells.
      Functional IL-13 receptors are heterodimers containing the IL-4Rα chain and a unique IL-13Rα chain. The 2 IL-13Rα chains include the active form (IL-13Rα1) and a decoy (IL-13Rα2), which lacks the motif required for initiating intracellular signaling cascades.
      • Donaldson D.D.
      • Whitters M.J.
      • Fitz L.J.
      • Neben T.Y.
      • Finnerty H.
      • Henderson S.L.
      • et al.
      The murine IL-13 receptor alpha 2: molecular cloning, characterization, and comparison with murine IL-13 receptor alpha 1.
      IL-13Rα1 expression is more limited than IL-4 receptors and includes endothelial cells, B cells, mononuclear phagocytes, and basophils but not mast cells or T cells. This more limited distribution of IL-13Rα1 explains the unique ability of IL-4 to induce TH2 lymphocyte differentiation and mast cell activation. However, IL-13 is more widely produced than IL-4 and is more readily identified in allergic inflammatory tissue.
      • Zhu Z.
      • Homer R.J.
      • Wang Z.
      • Chen Q.
      • Geba G.P.
      • Wang J.
      • et al.
      Pulmonary expression of interleukin-13 causes inflammation, mucus hypersecretion, subepithelial fibrosis, physiologic abnormalities, and eotaxin production.
      In murine studies IL-13 has a singularly important role in causing mucus hypersecretion and nonspecific AHR, and its expression results in the characteristic airway metaplasia of asthma with the replacement of epithelial cells with goblet cells.
      • Zhu Z.
      • Homer R.J.
      • Wang Z.
      • Chen Q.
      • Geba G.P.
      • Wang J.
      • et al.
      Pulmonary expression of interleukin-13 causes inflammation, mucus hypersecretion, subepithelial fibrosis, physiologic abnormalities, and eotaxin production.

       Eosinophilia

      Another characteristic feature of allergic diseases is the presence of increased numbers of activated eosinophils.

       IL-5

      IL-5 is the most important eosinophilopoietin.
      • Clutterbuck E.J.
      • Hirst E.M.
      • Sanderson C.J.
      Human interleukin-5 (IL-5) regulates the production of eosinophils in human bone marrow cultures: comparison and interaction with IL-1, IL-3, IL-6, and GMCSF.
      In addition to stimulating eosinophil production and release from the bone marrow,
      • Mould A.W.
      • Ramsay A.J.
      • Matthaei K.I.
      • Young I.G.
      • Rothenberg M.E.
      • Foster P.S.
      The effect of IL-5 and eotaxin expression in the lung on eosinophil trafficking and degranulation and the induction of bronchial hyperreactivity.
      IL-5 is chemotactic for eosinophils and activates mature eosinophils, inducing eosinophil secretion and enhanced cytotoxicity. Another mechanism by which IL-5 promotes accumulation of eosinophils is through its ability to upregulate chemokine receptors and αDβ2 integrins, thereby promoting their adherence to VCAM-1–expressing endothelial cells. IL-5 prolongs eosinophil survival by blocking apoptosis.
      • Rothenberg M.E.
      • Petersen J.
      • Stevens R.L.
      • Silberstein D.S.
      • McKenzie D.T.
      • Austen K.F.
      • et al.
      IL-5-dependent conversion of normodense human eosinophils to the hypodense phenotype uses 3T3 fibroblasts for enhanced viability, accelerated hypodensity, and sustained antibody-dependent cytotoxicity.
      Administration of IL-5 causes mucosal eosinophilia and an increase in bronchial hyperreactivity. IL-5–dependent activation of eosinophils is now thought to be less central to the pathophysiology of asthma as a result of the disappointing results in trials using IL-5 antagonists, perhaps because of redundant cytokine profiles involving GM-CSF and heterogeneous presentations of asthma that are less dependent on eosinophils. Thus in asthmatic patients screened for sputum eosinophils, anti–IL-5 does have increased therapeutic benefit.
      • Haldar P.
      • Brightling C.E.
      • Hargadon B.
      • Gupta S.
      • Monteiro W.
      • Sousa A.
      • et al.
      Mepolizumab and exacerbations of refractory eosinophilic asthma.
      • Nair P.
      • Pizzichini M.M.
      • Kjarsgaard M.
      • Inman M.D.
      • Efthimiadis A.
      • Pizzichini E.
      • et al.
      Mepolizumab for prednisone-dependent asthma with sputum eosinophilia.
      Other activities of IL-5 include basophil differentiation. In addition to TH2-like lymphocytes, other sources for IL-5 include mast cells, NK T cells, and eosinophils themselves. IL-5 interacts with specific IL-5 receptors (IL-5Rs) that consist of a heterodimer containing IL-5Rα and a β chain (CD131) shared with GM-CSF receptor and IL-3 receptor (Table I).
      • Kitamura T.
      • Sato N.
      • Arai K.
      • Miyajima A.
      Expression cloning of the human IL-3 receptor cDNA reveals a shared beta subunit for the human IL-3 and GM-CSF receptors.

       IL-3 and GM-CSF

      In addition to IL-5, IL-3
      • Rothenberg M.E.
      • Owen Jr., W.F.
      • Silberstein D.S.
      • Woods J.
      • Soberman R.J.
      • Austen K.F.
      • et al.
      Human eosinophils have prolonged survival, enhanced functional properties, and become hypodense when exposed to human interleukin 3.
      and GM-CSF
      • Owen Jr., W.F.
      • Rothenberg M.E.
      • Silberstein D.S.
      • Gasson J.C.
      • Stevens R.L.
      • Austen K.F.
      • et al.
      Regulation of human eosinophil viability, density, and function by granulocyte/macrophage colony-stimulating factor in the presence of 3T3 fibroblasts.
      also strongly contribute to the activity of eosinophils in allergic inflammation through their capacities to prolong eosinophil survival and to generate activated eosinophils. IL-3 is an important factor that supports the growth of precursors for a variety of hematopoietic cells, including DCs, erythrocytes, granulocytes (especially basophils), macrophages, mast cells, and lymphoid cells. The major source of IL-3 is T lymphocytes, but in patients with allergic inflammation, it is also derived from eosinophils and mast cells.
      GM-CSF supports the maturation of DCs, neutrophils, and macrophages. GM-CSF synergizes with other colony-stimulating factors to support the production of platelets and erythrocytes. GM-CSF is an activating factor for mature granulocytes and mononuclear phagocytic cells. In the lungs GM-CSF is uniquely important in the maturation of alveolar macrophages, including their expression of matrix metalloproteinases and reactive oxygen species and their processing of surfactant proteins.
      • Ochs M.
      • Knudsen L.
      • Allen L.
      • Stumbaugh A.
      • Levitt S.
      • Nyengaard J.R.
      • et al.
      GM-CSF mediates alveolar epithelial type II cell changes, but not emphysema-like pathology, in SP-D-deficient mice.
      • Guth A.M.
      • Janssen W.J.
      • Bosio C.M.
      • Crouch E.C.
      • Henson P.M.
      • Dow S.W.
      Lung environment determines unique phenotype of alveolar macrophages.
      The role of GM-CSF in allergic immunity is derived from its shared ability with IL-3 and IL-5 to inhibit apoptosis of eosinophils and thereby prolong the survival of eosinophils at sites of allergic inflammation. GM-CSF is particularly important in the allergic airway because mature activated eosinophils lose their expression of IL-5Rs and responsiveness to IL-5 but instead upregulate GM-CSF receptors. Thus GM-CSF, and not IL-5, might be responsible for the persistent survival and function of eosinophils in the asthmatic airway. These observations provide one explanation for the failure of IL-5 antagonism in asthma trials. GM-CSF activates mature eosinophils, increasing their degranulation, cytotoxicity, and response to chemoattractants.

       Mast cell proliferation and activation

      Increased numbers of mast cells characterize allergic diseases, and this is a T cell–dependent process. The most important cytokine responsible for mast cell growth and proliferation from hematopoietic precursors is stem cell factor (SCF; or c-kit ligand).
      • Anderson D.M.
      • Lyman S.D.
      • Baird A.
      • Wignall J.M.
      • Eisenman J.
      • Rauch C.
      • et al.
      Molecular cloning of mast cell growth factor, a hematopoietin that is active in both membrane bound and soluble forms.
      SCF is derived from bone marrow stromal cells, endothelial cells, and fibroblasts. SCF induces histamine release from mast cells but inconsistently from basophils and remains the only cytokine with this property. In addition to being essential for mast cell differentiation, SCF interacts with other hematopoietic growth factors to stimulate myeloid, lymphoid, and erythroid progenitor cells. Several cytokines, including and especially IL-3, IL-5, IL-6, IL-9, IL-10, IL-11, and nerve growth factor, also contribute to mast cell proliferation.
      • Matsuda H.
      • Kannan Y.
      • Ushio H.
      • Kiso Y.
      • Kanemoto T.
      • Suzuki H.
      • et al.
      Nerve growth factor induces development of connective tissue-type mast cells in vitro from murine bone marrow cells.
      • Yanagida M.
      • Fukamachi H.
      • Ohgami K.
      • Kuwaki T.
      • Ishii H.
      • Uzumaki H.
      • et al.
      Effects of T-helper 2-type cytokines, interleukin-3 (IL-3), IL-4, IL-5, and IL-6 on the survival of cultured human mast cells.
      • Tsuji K.
      • Koike K.
      • Komiyama A.
      • Miyajima A.
      • Arai K.
      • Nakahata T.
      Synergistic action of interleukin-10 (IL-10) with IL-3, IL-4 and stem cell factor on colony formation from murine mast cells in culture.
      • Godfraind C.
      • Louahed J.
      • Faulkner H.
      • Vink A.
      • Warnier G.
      • Grencis R.
      • et al.
      Intraepithelial infiltration by mast cells with both connective tissue-type and mucosal-type characteristics in gut, trachea, and kidneys of IL-9 transgenic mice.
      • Gebhardt T.
      • Sellge G.
      • Lorentz A.
      • Raab R.
      • Manns M.P.
      • Bischoff S.C.
      Cultured human intestinal mast cells express functional IL-3 receptors and respond to IL-3 by enhancing growth and IgE receptor-dependent mediator release.
      In addition to the factors that stimulate mast cell proliferation, several cytokines induce histamine release from basophils, including several members of the chemokine family (discussed later).

       Other TH2 cell–derived cytokines involved in allergic inflammation: IL-9, IL-25, and IL-31

      IL-9 was originally described as a mast cell growth factor
      • Hultner L.
      • Druez C.
      • Moeller J.
      • Uyttenhove C.
      • Schmitt E.
      • Rude E.
      • et al.
      Mast cell growth-enhancing activity (MEA) is structurally related and functionally identical to the novel mouse T cell growth factor P40/TCGFIII (interleukin 9).
      and contributes to mast cell–mediated allergic responses through its ability to stimulate production of mast cell proteases. In addition, IL-9 increases expression of the IgE high-affinity receptor on mast cells. IL-9 synergizes with IL-4 to enhance the production of IgE and IL-5 to enhance the production of eosinophils. IL-9 supports the growth and survival of T lymphocytes. IL-9 has other important activities in allergic inflammation, including inducing expression of CCL11 (eotaxin-1), IL-5 receptors, and chemokine receptor 4. IL-9 is derived from eosinophils and TH2-like lymphocytes. Its selective production by TH2 cells supports a role in allergic inflammation. It appears to be primarily produced by a unique subfamily of TH2 cells termed TH9 lymphocytes (discussed below).
      • Veldhoen M.
      • Uyttenhove C.
      • van Snick J.
      • Helmby H.
      • Westendorf A.
      • Buer J.
      • et al.
      Transforming growth factor-beta “reprograms” the differentiation of T helper 2 cells and promotes an interleukin 9-producing subset.
      • Dardalhon V.
      • Awasthi A.
      • Kwon H.
      • Galileos G.
      • Gao W.
      • Sobel R.A.
      • et al.
      IL-4 inhibits TGF-beta-induced Foxp3 + T cells and, together with TGF-beta, generates IL-9 + IL-10 + Foxp3(-) effector T cells.
      IL-25 was originally described as a member of the IL-17 family (IL-17E) but has now been given its distinct nomenclature because of its unique spectrum of activities. Similar to IL-4, IL-5, IL-9, and IL-13, it is derived in part from TH2-like lymphocytes. It stimulates release of IL-4, IL-5, and IL-13 from nonlymphoid cells and from TH lymphocytes themselves, contributing to TH2 immune deviation. IL-25 enhances IgE secretion through its ability to stimulate IL-4 and IL-13 production.
      • Fort M.M.
      • Cheung J.
      • Yen D.
      • Li J.
      • Zurawski S.M.
      • Lo S.
      • et al.
      IL-25 induces IL-4, IL-5, and IL-13 and Th2-associated pathologies in vivo.
      IL-25 stimulation of IL-5 production promotes eosinophilopoiesis. IL-25 increases expression of CCL5 (RANTES) and CCL11, which further contribute to the homing of eosinophils to the lungs.
      • Kawaguchi M.
      • Adachi M.
      • Oda N.
      • Kokubu F.
      • Huang S.K.
      IL-17 cytokine family.
      IL-31 is a member of the subfamily of hematopoietin cytokines that also includes IL-3, IL-5, and GM-CSF. It is primarily expressed by TH2 lymphocytes. Its activities include induction of chemokines that are involved in recruitment of neutrophils, monocytes, and T cells. Overexpression of IL-31 in mice produces an inflammatory infiltrate suggestive of atopic dermatitis.
      • Dillon S.R.
      • Sprecher C.
      • Hammond A.
      • Bilsborough J.
      • Rosenfeld-Franklin M.
      • Presnell S.R.
      • et al.
      Interleukin 31, a cytokine produced by activated T cells, induces dermatitis in mice.
      • Neis M.M.
      • Peters B.
      • Dreuw A.
      • Wenzel J.
      • Bieber T.
      • Mauch C.
      • et al.
      Enhanced expression levels of IL-31 correlate with IL-4 and IL-13 in atopic and allergic contact dermatitis.
      • Sonkoly E.
      • Muller A.
      • Lauerma A.I.
      • Pivarcsi A.
      • Soto H.
      • Kemeny L.
      • et al.
      IL-31: a new link between T cells and pruritus in atopic skin inflammation.
      Similarly, the murine model of AHR demonstrates increased expression of the IL-31 receptor.

      Anti-inflammatory cytokines

      In addition to cytokines that stimulate cytotoxic, cellular, humoral, and allergic inflammation, several cytokines have predominantly anti-inflammatory effects, including, as previously discussed, IL-1ra, but also TGF-β and members of the IL-10 family.

       TGF-β

      TGF-β represents a family of peptides that are arguably the most pleiotropic of the cytokines, including having both stimulatory and inhibitory effects on numerous cell types.
      • Sporn M.B.
      • Roberts A.B.
      Transforming growth factor-beta: recent progress and new challenges.
      TGF-β is synthesized as an inactive precursor that requires cleavage for activation. It is produced by numerous cell types, including eosinophils, monocytes, and T cells. TGF-β is an important stimulant of fibrosis, inducing formation of the extracellular matrix and promoting wound healing and scar formation. In immunity it is largely inhibitory for B cells and TH/cytotoxic lymphocytes. In general, it inhibits proliferation and induces apoptosis. The production of TGF-β by apoptotic cells creates an immunosuppressive milieu and is one explanation for the absence of inflammation and autoimmunity as a consequence of apoptotic cell death.
      • Chen W.
      • Frank M.E.
      • Jin W.
      • Wahl S.M.
      TGF-beta released by apoptotic T cells contributes to an immunosuppressive milieu.
      It inhibits cytotoxicity of mononuclear phagocytes and NK cells. The primary TGF-β–producing TH lymphocytes are Treg cells (discussed below), and the expression of membrane-bound TGF-β mediates much of their suppressive activity. TGF-β production by mucosal (TH3) cells supports the α isotype switch and secretory IgA production by B cells
      • Sonoda E.
      • Matsumoto R.
      • Hitoshi Y.
      • Ishii T.
      • Sugimoto M.
      • Araki S.
      • et al.
      Transforming growth factor beta induces IgA production and acts additively with interleukin 5 for IgA production.
      and is also critical for the maintenance of immune nonresponsiveness to otherwise benign gut pathogens and food allergens. TGF-β is constitutively produced in the healthy lung and helps promote B- and T-cell nonresponsiveness and lessens allergic inflammation through inhibition of IgE synthesis and mast cell proliferation. In established allergic inflammation, eosinophils comprise the most important source of TGF-β,
      • Kay A.B.
      • Phipps S.
      • Robinson D.S.
      A role for eosinophils in airway remodelling in asthma.
      and their expression of TGF-β is a cause of the fibrosis observed in patients with asthma.
      In contrast to these largely anti-inflammatory influences, TGF-β is central to the differentiation of TH17 and IL-9–producing TH2 (TH9) lymphocytes. These conflicting proinflammatory and anti-inflammatory effects reflect the distinctive actions of TGF-β as a function of which cells are producing it, the stage of the immune response during which it is acting, different signaling pathways it engages, and other divergent influences.

       IL-10 family

      IL-10 is an important immunoregulatory cytokine with multiple biologic effects on different cell types. Although the primary T-cell source for IL-10 is regulatory T lymphocytes, monocytes and B cells are the major sources of IL-10 in human subjects.
      • Del Prete G.
      • De Carli M.
      • Almerigogna F.
      • Giudizi M.G.
      • Biagiotti R.
      • Romagnani S.
      Human IL-10 is produced by both type 1 helper (Th1) and type 2 helper (Th2) T cell clones and inhibits their antigen-specific proliferation and cytokine production.
      IL-10 forms a homodimer and exerts its biologic function through IL-10R1 and IL-10R2 receptor complex. IL-10 inhibits production of IFN-γ by TH1 lymphocytes; IL-4 and IL-5 by TH2 lymphocytes; IL-1β, IL-6, CXCL8, IL-12, and TNF-α by mononuclear phagocytes; and IFN-γ and TNF-α by NK cells. MHC class II expression by APCs is inhibited by IL-10, as is CD23 (low-affinity IgE receptor [FcεRII]) and ICAM-1. IL-10 inhibition of expression of the costimulatory molecules CD80 and CD86 by DCs and other APCs eliminates the ability of the APC to provide the accessory signals necessary for TH cell activation,
      • Ding L.
      • Linsley P.S.
      • Huang L.Y.
      • Germain R.N.
      • Shevach E.M.
      IL-10 inhibits macrophage costimulatory activity by selectively inhibiting the up-regulation of B7 expression.
      which is primarily responsible for the inhibition of cytokine production. However, IL-10 also functions directly on T cells to inhibit their cytokine production by suppressing expression of CD28 and inducible T-cell costimulator.
      • Taylor A.
      • Akdis M.
      • Joss A.
      • Akkoc T.
      • Wenig R.
      • Colonna M.
      • et al.
      IL-10 inhibits CD28 and ICOS costimulations of T cells via src homology 2 domain-containing protein tyrosine phosphatase 1.
      Constitutive expression of IL-10 in the respiratory tract of healthy subjects has a role in the maintenance of tolerance to allergens, whereas asthma and allergic rhinitis are associated with diminished IL-10 expression.
      • Borish L.
      • Aarons A.
      • Rumbyrt J.
      • Cvietusa P.
      • Negri J.
      • Wenzel S.
      Interleukin-10 regulation in normal subjects and patients with asthma.
      This diminished IL-10 expression contributes to the development of an inflammatory milieu, reflecting in part the presence of mature DCs.

       Other members of the IL-10 family: IL-19, IL-20, IL-22, IL-24, IL-26, IL-28, and IL-29

      These newer members of the IL-10 family cytokines and their receptors loosely share homologies with interferons/interferon receptors, and many display antiviral activity.
      • Conti P.
      • Kempuraj D.
      • Frydas S.
      • Kandere K.
      • Boucher W.
      • Letourneau R.
      • et al.
      IL-10 subfamily members: IL-19, IL-20, IL-22, IL-24 and IL-26.
      In contrast to IL-10, none of these cytokines significantly inhibit cytokine synthesis, an activity that remains unique for IL-10. Features of the IL-10 superfamily are summarized in Table III.
      IL-19 shares 21% amino acid identity with IL-10, but as with other members of the IL-10 superfamily, it is the exon-intron structure that primarily defines their homology. Within the immune system, IL-19 is primarily produced by monocytes, and its expression can be induced by LPS, IL-4, and GM-CSF. IL-19 signals through a receptor complex composed of the IL-20 receptor (IL-20R) 1 and IL-20R2 chains and activates monocytes to release IL-6, TNF-α, and reactive oxygen species. IL-19 contributes to TH2 immune deviation, as well as the development of airway inflammation, in murine models, and its increased expression has been observed in asthmatic patients.
      • Liao S.C.
      • Cheng Y.C.
      • Wang Y.C.
      • Wang C.W.
      • Yang S.M.
      • Yu C.K.
      • et al.
      IL-19 induced Th2 cytokines and was up-regulated in asthma patients.
      Similar to IL-19, IL-20
      • Blumberg H.
      • Conklin D.
      • Xu W.F.
      • Grossmann A.
      • Brender T.
      • Carollo S.
      • et al.
      Interleukin 20: discovery, receptor identification, and role in epidermal function.
      signals through the IL-20R1/IL-20R2 heterodimer; however, IL-20 also binds to the receptor complex composed of IL-22 receptor (IL-22R) 1/IL-20R2. IL-20 is predominantly expressed by monocytes and skin keratinocytes, and it is overexpressed in patients with psoriasis. It induces keratinocyte proliferation, and overexpression in mice is lethal, secondary to defective skin formation.
      IL-22 is derived from T lymphocytes, mast cells, and, at lower levels, activated NK cells.
      • Wolk K.
      • Kunz S.
      • Witte E.
      • Friedrich M.
      • Asadullah K.
      • Sabat R.
      IL-22 increases the innate immunity of tissues.
      Among T-lymphocyte subsets, IL-22 is preferentially expressed by TH17 cells. Notably, patients with psoriasis, Crohn disease, interstitial lung diseases, and rheumatoid arthritis all have evidence of increased levels of IL-22 that correlate with disease severity.
      • Wolk K.
      • Witte E.
      • Hoffmann U.
      • Doecke W.D.
      • Endesfelder S.
      • Asadullah K.
      • et al.
      IL-22 induces lipopolysaccharide-binding protein in hepatocytes: a potential systemic role of IL-22 in Crohn's disease.
      • Ikeuchi H.
      • Kuroiwa T.
      • Hiramatsu N.
      • Kaneko Y.
      • Hiromura K.
      • Ueki K.
      • et al.
      Expression of interleukin-22 in rheumatoid arthritis: potential role as a proinflammatory cytokine.
      • Whittington H.A.
      • Armstrong L.
      • Uppington K.M.
      • Millar A.B.
      Interleukin-22: a potential immunomodulatory molecule in the lung.
      The IL-22 receptor complex is a heterodimer consisting of IL-22R1/IL-10R2 chains. Neither resting nor stimulated immune cells express IL-22R1, and therefore despite its structural similarity to IL-10, immune cells are not the target cells of IL-22. The predominant biologic activity described for IL-22 is induction of acute-phase proteins by hepatocytes, including serum amyloid A protein, and it likely provides a protective role in liver injury. In addition, IL-22 leads to the production of antimicrobial peptides, and consistent with its expression by TH17 cells, it is presumed to play an important role in defense against extracellular pathogens.
      IL-24 is produced by both monocytes and TH2 lymphocytes in an IL-4–inducible fashion. Originally identified as a tumor-suppressor molecule (melanoma differentiation-associated gene 7) that was expressed in healthy melanocytes but not metastatic melanoma cells, it was subsequently discovered to share structural homology with IL-10 and to be located within the same locus on chromosome 1. IL-24 signals through a heterodimer consisting of IL-20R1/IL-20R2. Its potential role as a cancer therapeutic is derived from evidence that IL-24 induces antitumor immune responses with significant independent “bystander” antitumor effects.
      • Mumm J.B.
      • Ekmekcioglu S.
      • Poindexter N.J.
      • Chada S.
      • Grimm E.A.
      Soluble human MDA-7/IL-24: characterization of the molecular form(s) inhibiting tumor growth and stimulating monocytes.
      • Zheng M.
      • Bocangel D.
      • Doneske B.
      • Mhashilkar A.
      • Ramesh R.
      • Hunt K.K.
      • et al.
      Human interleukin 24 (MDA-7/IL-24) protein kills breast cancer cells via the IL-20 receptor and is antagonized by IL-10.
      Given the apparently ubiquitous apoptotic effect on malignant cells, the lack of an effect on normal cells, and the absence of significant side effects (eg, cytokine storm), IL-24 is a potential cancer therapeutic.
      IL-26 is located in a chromosomal cluster with IL-22 and IFN-γ in an area thought to contribute to allergic and autoimmune diseases; in contrast, IL-10, IL-19, IL-20, and IL-24 cluster separately. IL-26 is primarily generated by monocytes and T memory cells. IL-26 has a unique receptor consisting of a heterodimer of IL-20R1/IL-10R2.
      • Hor S.
      • Pirzer H.
      • Dumoutier L.
      • Bauer F.
      • Wittmann S.
      • Sticht H.
      • et al.
      The T-cell lymphokine interleukin-26 targets epithelial cells through the interleukin-20 receptor 1 and interleukin-10 receptor 2 chains.
      Binding of the IL-26 receptor leads to induction of CXCL8, IL-10, and ICAM-1.
      As previously discussed, the type III interferons IL-28 and IL-29 are closely related to the type I interferons, but their genomic organization and receptor use is more similar to that of members of the IL-10 family.

       IL-35

      IL-35 is a dimer composed of IL-12p35 and IL-27p28 (IL-30) chains. It is primarily secreted by Treg cells and suppresses inflammatory responses by causing proliferation of Treg cells while reducing the activity of TH17 cells.
      • Niedbala W.
      • Wei X.Q.
      • Cai B.
      • Hueber A.J.
      • Leung B.P.
      • McInnes I.B.
      • et al.
      IL-35 is a novel cytokine with therapeutic effects against collagen-induced arthritis through the expansion of regulatory T cells and suppression of Th17 cells.
      Studies using a murine model show that the absence of either IL-35 chain from Treg cells reduces their ability to suppress inflammation.
      • Collison L.W.
      • Workman C.J.
      • Kuo T.T.
      • Boyd K.
      • Wang Y.
      • Vignali K.M.
      • et al.
      The inhibitory cytokine IL-35 contributes to regulatory T-cell function.

      TH lymphocyte families

       TH1, TH2, and TH17 lymphocytes

      Subclasses of TH lymphocytes can be identified based on their repertoire of cytokines (Table IV).
      • Mosmann T.R.
      • Coffman R.L.
      TH1 and TH2 cells: different patterns of lymphokine secretion lead to different functional properties.
      Naive TH0 cells produce primarily IL-2 but might also synthesize cytokines characteristic of effector T lymphocytes. In contrast to murine studies, categorically distinct TH cytokine profiles are seldom apparent in human cells, although there remains an inverse relationship between the tendency of T lymphocytes to produce IFN-γ as opposed to IL-4/IL-5 or IL-17. In human subjects TH1 cells primarily produce IFN-γ and TNF-β but not IL-4 and IL-5. TH2 cells more prominently produce IL-4, IL-5, IL-9, and IL-13 but not IFN-γ. TH1 lymphocytes promote cell-mediated immune responses and are important in antibody-dependent immunity. TH17 cells are more important in the T cell–mediated immune response to extracellular pathogens and likely contribute to autoimmune diseases. TH2 lymphocytes produce IL-4, IL-5, and IL-13, which induce antiparasitic and allergic immune responses. A subclass of TH2 cells characterized by prominent IL-9 production has recently been described (TH9 cells).
      • Veldhoen M.
      • Uyttenhove C.
      • van Snick J.
      • Helmby H.
      • Westendorf A.
      • Buer J.
      • et al.
      Transforming growth factor-beta “reprograms” the differentiation of T helper 2 cells and promotes an interleukin 9-producing subset.
      • Dardalhon V.
      • Awasthi A.
      • Kwon H.
      • Galileos G.
      • Gao W.
      • Sobel R.A.
      • et al.
      IL-4 inhibits TGF-beta-induced Foxp3 + T cells and, together with TGF-beta, generates IL-9 + IL-10 + Foxp3(-) effector T cells.
      Table IVTH lymphocyte families
      FamilyCytokine repertoireCytokines involved in differentiationTranscription factors involved in differentiation
      TH1IFN-γ, TNF-α, TNF-β, GM-CSF, IL-2, IL-3IL-12: activates STAT4, leading to expression of T-bet; induces IL-18R expression IL-18: upregulates IL-12R, further induces IFN-γ expression IL-27: activates STAT4, leading to increased expression of T-bet and IFN-γ IFN-γ: increases expression of T-bet by increasing expression of STAT1; negative regulator of TH17 and TH2T-bet: master regulator of TH1 cells; potentiates production of IFN-γ and IL-12Rβ2; suppresses TH2 and TH17 differentiation STAT4: produced in response to IL-12 and potentiates production of IFN-γ STAT1: increases expression of T-bet; negative regulator of TH17
      TH2IL-2, IL-3, IL-4, IL-5, IL-9, IL-13, IL-24, IL-25, IL-31, TNF-α, GM-CSFIL-4: activates STAT6, leading to expression of GATA-3; negative regulator of TH17, IL-19, IL-25, IL-33 TSLP: promote differentiation and survival of TH2-like cellsGATA-3: master regulator of TH2 cells; potentiates IL-4 expression; suppresses expression of TH1 differentiation and cytokines expression (IFN-γ) MAF: contributes to IL-4 production once a TH2 program is established; inhibition of TH17 differentiation STAT6: promotes TH2 cell differentiation; negative regulator of T-bet expression and TH1 differentiation NFAT: increases transcription of IL-4
      TH9IL-4, IL-9TGF-β: induces the high IL-9 phenotype of TH2-like lymphocytes
      TH17IL-17 (IL-17A), IL-17F, IL-21, IL-22IL-6: differentiation factor for the generation of TH17 cells TGF-β, IL-21 IL-23: support the differentiation and function of TH17 cells in the additional presence of IL-6RORγt (retinoic acid–related orphan nuclear receptor) is the master regulator of TH17 cell differentiation STAT3: activated by IL-6 and essential for TH17 differentiation
      nTreg/iTregIL-10TGF-β: differentiation factor for the generation of nTreg cells IL-10: important for differentiation of peripheral iTreg cells, role in nTreg development uncertain IL-2: promotes survival, proliferation, and survival of nTreg cells through their constitutive expression of CD25FOXP3: master regulator of thymus-derived nTreg cells
      TH3TGF-β, IL-10

       Cytokines involved in TH1 differentiation

      One of the more important questions in understanding the cause of immune disorders is to determine the basis for effector T-cell differentiation in response to antigen. The most critical element in determining TH differentiation is the cytokine milieu in which the T lymphocyte is activated (Table IV). TH1 differentiation is induced and maintained through the influences of IL-12, IL-18, and IL-27, with IL-12 providing the most important role.
      • Manetti R.
      • Parronchi P.
      • Giudizi M.G.
      • Piccinni M.P.
      • Maggi E.
      • Trinchieri G.
      • et al.
      Natural killer cell stimulatory factor (interleukin 12 [IL-12]) induces T helper type 1 (Th1)-specific immune responses and inhibits the development of IL-4-producing Th cells.
      IL-12 interacts with naive TH lymphocytes to activate STAT4, leading to expression of the transcription factor T-box expressed in T cells (T-bet). T-bet is a nuclear transcription factor that is the master regulator responsible for the differentiation of TH1 cells. Actions of T-bet include production of IFN-γ and IL-12R. Simultaneously, it blocks alternative TH differentiation pathways by suppressing expression of TH2 cytokines, such as IL-4, and acting as a negative regulator of TH17 differentiation. Similar to IL-12, IL-27 also activates STAT4, leading to increased expression of T-bet and IFN-γ. Addition of recombinant IL-27 to naive T cells in culture under TH2-polarizing conditions results in decreased expression of GATA-3, the transcription factor that is the master regulator for TH2 development, along with a decrease in production of IL-4 and other TH2 cytokines.
      • Villarino A.V.
      • Huang E.
      • Hunter C.A.
      Understanding the pro- and anti-inflammatory properties of IL-27.
      Once TH1 cells become differentiated, newly synthesized IFN-γ, acting through STAT-1, also increases expression of T-bet and functions as a negative regulator of TH17 and TH2 differentiation. IL-18 upregulates IL-12R expression and is a growth factor for TH1 cells. IL-12–producing DCs are the most important mediator of TH1-like immune deviation. In addition, insofar as mononuclear phagocytes are an additional source of IL-12, this suggests a mechanism whereby antigens likely to be processed by macrophages, including obligate intracellular bacteria (eg, mycobacteria), produce TH1 responses.

       Cytokines involved in TH2 differentiation: IL-4, IL-19, IL-25, IL-33, and thymic stromal lymphopoietin

      One determinant of TH2 differentiation is IL-4 itself.
      • Seder R.A.
      • Paul W.E.
      • Davis M.M.
      Fazekas de St Groth B. The presence of interleukin 4 during in vitro priming determines the lymphokine-producing potential of CD4 + T cells from T cell receptor transgenic mice.
      IL-4 activates STAT6, which in turn promotes expression of GATA-3, the master regulator of TH2 cells, and suppresses expression of T-bet. GATA-3 potentiates IL-4 expression and suppresses expression of TH1 differentiation and cytokine (IFN-γ) production. IL-4 and GATA-3 similarly inhibit differentiation of TH17 lymphocytes. Other transcription factors, including especially MAF and NFAT, contribute to IL-4 and other TH2 signature cytokine production once TH2 differentiation is established. The original source of the IL-4 responsible for TH2 differentiation can be the naive TH0 lymphocytes themselves. Basophils, NK T cells, and mast cells are also capable of robust IL-4 secretion.
      • Bilenki L.
      • Yang J.
      • Fan Y.
      • Wang S.
      • Yang X.
      Natural killer T cells contribute to airway eosinophilic inflammation induced by ragweed through enhanced IL-4 and eotaxin production.
      • Akbari O.
      • Faul J.L.
      • Hoyte E.G.
      • Berry G.J.
      • Wahlstrom J.
      • Kronenberg M.
      • et al.
      CD4+ invariant T-cell-receptor+ natural killer T cells in bronchial asthma.
      Whatever the source is for the IL-4, the end result is that in a milieu in which allergic inflammation is present (eg, bronchial lymphatic tissue), more and more extensive allergenic responses against bystander antigens develop.
      IL-19, a member of the IL-10 family, is primarily produced by mononuclear phagocytic cells, and its expression is upregulated by IL-4 and downregulated by IFN-γ. IL-19 promotes TH2 immune deviation.
      • Gallagher G.
      • Eskdale J.
      • Jordan W.
      • Peat J.
      • Campbell J.
      • Boniotto M.
      • et al.
      Human interleukin-19 and its receptor: a potential role in the induction of Th2 responses.
      IL-19 expression is important to the development of airway inflammation in murine models, and its increased expression has been observed in asthmatic patients.
      • Liao S.C.
      • Cheng Y.C.
      • Wang Y.C.
      • Wang C.W.
      • Yang S.M.
      • Yu C.K.
      • et al.
      IL-19 induced Th2 cytokines and was up-regulated in asthma patients.
      As discussed, IL-25 induces expression of TH2 signature cytokines from numerous cell types but also specifically contributes to TH2 immune deviation.
      • Tamachi T.
      • Maezawa Y.
      • Ikeda K.
      • Kagami S.
      • Hatano M.
      • Seto Y.
      • et al.
      IL-25 enhances allergic airway inflammation by amplifying a TH2 cell-dependent pathway in mice.
      Its production by TH2 lymphocytes suggests a positive feedback cascade.
      Currently, the 2 most important cytokines responsible for TH2 immune deviation are considered to be IL-33 and thymic stromal lymphopoietin (TSLP). Similar to IL-18, IL-33
      • Schmitz J.
      • Owyang A.
      • Oldham E.
      • Song Y.
      • Murphy E.
      • McClanahan T.K.
      • et al.
      IL-33, an interleukin-1-like cytokine that signals via the IL-1 receptor-related protein ST2 and induces T helper type 2-associated cytokines.
      is an IL-1–like cytokine that signals through an IL-1 receptor–related protein.
      • Coyle A.J.
      • Lloyd C.
      • Tian J.
      • Nguyen T.
      • Erikkson C.
      • Wang L.
      • et al.
      Crucial role of the interleukin 1 receptor family member T1/ST2 in T helper cell type 2-mediated lung mucosal immune responses.
      As with IL-1 and IL-18, IL-33 is produced as an inactive precursor, and its secretion and activation are dependent on cleavage by caspase-1. IL-33 is expressed by bronchial epithelial cells, fibroblasts, smooth muscle cells, keratinocytes, macrophages, and DCs. IL-33 receptors are expressed on T cells (specifically nascent and mature TH2 cells), macrophages, hematopoietic stem cells, mast cells, and fibroblasts. Administration of IL-33 induces TH2 immune deviation and cytokine production, causes increased IgE levels, and generates profound mucosal eosinophilic inflammation in the lung and gastrointestinal tract.
      • Schmitz J.
      • Owyang A.
      • Oldham E.
      • Song Y.
      • Murphy E.
      • McClanahan T.K.
      • et al.
      IL-33, an interleukin-1-like cytokine that signals via the IL-1 receptor-related protein ST2 and induces T helper type 2-associated cytokines.
      • Xu D.
      • Chan W.L.
      • Leung B.P.
      • Huang F.
      • Wheeler R.
      • Piedrafita D.
      • et al.
      Selective expression of a stable cell surface molecule on type 2 but not type 1 helper T cells.
      Administration of an IL-33 receptor antagonist reduces production of TH2 cytokines and airway inflammation in murine asthma models.
      • Coyle A.J.
      • Lloyd C.
      • Tian J.
      • Nguyen T.
      • Erikkson C.
      • Wang L.
      • et al.
      Crucial role of the interleukin 1 receptor family member T1/ST2 in T helper cell type 2-mediated lung mucosal immune responses.
      • Lohning M.
      • Stroehmann A.
      • Coyle A.J.
      • Grogan J.L.
      • Lin S.
      • Gutierrez-Ramos J.C.
      • et al.
      T1/ST2 is preferentially expressed on murine Th2 cells, independent of interleukin 4, interleukin 5, and interleukin 10, and important for Th2 effector function.
      Its primary production by epithelial cells suggests a mechanism whereby the respiratory tract can generate a “danger signal” that will drive a subsequent TH2 immune response, arguably the initial trigger of asthma.
      The cytokine TSLP has also been suggested as a primary instigator of TH2 immune deviation.
      • Wang Y.H.
      • Ito T.
      • Homey B.
      • Watanabe N.
      • Martin R.
      • Barnes C.J.
      • et al.
      Maintenance and polarization of human TH2 central memory T cells by thymic stromal lymphopoietin-activated dendritic cells.
      TSLP is expressed by epithelial cells of the skin, gut, and lung and activates DCs in such a way as to promote TH2 cytokine production by their subsequently engaged effector T cells. The expression of TSLP in the lungs of mice produces severe AHR,
      • Al-Shami A.
      • Spolski R.
      • Kelly J.
      • Keane-Myers A.
      • Leonard W.J.
      A role for TSLP in the development of inflammation in an asthma model.
      • Zhou B.
      • Comeau M.R.
      • De Smedt T.
      • Liggitt H.D.
      • Dahl M.E.
      • Lewis D.B.
      • et al.
      Thymic stromal lymphopoietin as a key initiator of allergic airway inflammation in mice.
      and similarly, expression in the skin produces skin inflammation suggestive of atopic dermatitis.
      • Yoo J.
      • Omori M.
      • Gyarmati D.
      • Zhou B.
      • Aye T.
      • Brewer A.
      • et al.
      Spontaneous atopic dermatitis in mice expressing an inducible thymic stromal lymphopoietin transgene specifically in the skin.
      TSLP is highly expressed in the keratinocytes of patients with atopic dermatitis and the lungs of asthmatic patients.
      • Soumelis V.
      • Reche P.A.
      • Kanzler H.
      • Yuan W.
      • Edward G.
      • Homey B.
      • et al.
      Human epithelial cells trigger dendritic cell mediated allergic inflammation by producing TSLP.
      The TSLP receptor is a heterodimer composed of a unique TSLP-specific receptor and the IL-7 receptor α chain (CD127). TSLP receptors are primarily expressed by DCs, but their expression by mast cells also promotes secretion of TH2 signature cytokines. As with IL-33, its prominent expression by epithelium suggests an initial triggering event plausibly central to the development of allergic diseases of the skin and airways.
      TH9 lymphocytes are a recently described proposed subfamily of TH2 cells characterized by prominent production of IL-9 and relatively less IL-4. They result from the differentiation of TH2 cells in the concomitant presence of TGF-β.
      • Veldhoen M.
      • Uyttenhove C.
      • van Snick J.
      • Helmby H.
      • Westendorf A.
      • Buer J.
      • et al.
      Transforming growth factor-beta “reprograms” the differentiation of T helper 2 cells and promotes an interleukin 9-producing subset.
      • Dardalhon V.
      • Awasthi A.
      • Kwon H.
      • Galileos G.
      • Gao W.
      • Sobel R.A.
      • et al.
      IL-4 inhibits TGF-beta-induced Foxp3 + T cells and, together with TGF-beta, generates IL-9 + IL-10 + Foxp3(-) effector T cells.

       TH17 lymphocytes

      The selective production of IL-17 by clonal TH lymphocytes has led to the recognition of the TH17 cell as a distinct lymphocyte subset.
      • Dong C.
      Diversification of T-helper-cell lineages: finding the family root of IL-17-producing cells.
      The presence of distinct pathways involved in differentiation of IL-17–producing T lymphocytes (Table IV) and that counterregulate development of the alternative TH1- and TH2-like pathways further supports the concept that these TH17-producing TH lymphocytes comprise a distinct lineage. The mechanisms underlying TH17 differentiation in human subjects are not fully established. In mice IL-6 acting in the additional presence of TGF-β is the most important cytokine responsible for differentiation of TH17 cells.
      • Iwakura Y.
      • Ishigame H.
      The IL-23/IL-17 axis in inflammation.
      IL-21 and IL-23 further contribute to TH17 differentiation and expansion of established TH17 cells.
      • Mangan P.R.
      • Harrington L.E.
      • O'Quinn D.B.
      • Helms W.S.
      • Bullard D.C.
      • Elson C.O.
      • et al.
      Transforming growth factor-beta induces development of the T(H)17 lineage.
      Only in the absence of IL-6 does TGF-β promote differentiation into Treg cell pathways, as previously described. The highly pleiotropic cytokine TGF-β is therefore involved in the differentiation of Treg cells or, in the additional presence of IL-6 or IL-4, can be switched to induced TH17 or TH9 cells, respectively.
      • Bettelli E.
      • Carrier Y.
      • Gao W.
      • Korn T.
      • Strom T.B.
      • Oukka M.
      • et al.
      Reciprocal developmental pathways for the generation of pathogenic effector TH17 and regulatory T cells.
      The action of IL-6 in inducing TH17 is mediated through its activation of STAT3. The net result is activation of retinoic acid receptor–related orphan receptor (ROR) γt, the master regulating transcription factor for TH17 cells. Heterozygous mutations in STAT3 produce the hyper-IgE syndrome,
      • Holland S.M.
      • DeLeo F.R.
      • Elloumi H.Z.
      • Hsu A.P.
      • Uzel G.
      • Brodsky N.
      • et al.
      STAT3 mutations in the hyper-IgE syndrome.
      • Minegishi Y.
      • Saito M.
      • Tsuchiya S.
      • Tsuge I.
      • Takada H.
      • Hara T.
      • et al.
      Dominant-negative mutations in the DNA-binding domain of STAT3 cause hyper-IgE syndrome.
      a condition characterized by deficient TH17 lymphocytes.
      • Milner J.D.
      • Brenchley J.M.
      • Laurence A.
      • Freeman A.F.
      • Hill B.J.
      • Elias K.M.
      • et al.
      Impaired T(H)17 cell differentiation in subjects with autosomal dominant hyper-IgE syndrome.
      • Minegishi Y.
      • Saito M.
      • Nagasawa M.
      • Takada H.
      • Hara T.
      • Tsuchiya S.
      • et al.
      Molecular explanation for the contradiction between systemic Th17 defect and localized bacterial infection in hyper-IgE syndrome.

      Treg lymphocyte families: Natural Treg, induced Treg, and TH3 cells

      In addition to traditional TH subclasses, much progress has been made in the past several years in identifying and clarifying the characteristics of several families of regulatory T lymphocytes (Table V).
      • Sakaguchi S.
      Regulatory T cells: key controllers of immunologic self-tolerance.
      These include peripherally differentiated (induced) IL-10–producing lymphocytes, termed induced Treg (iTreg) cells; thymic-derived CD25+ natural Treg (nTreg) cells; and TGF-β–producing TH3 cells. Thymus-derived nTreg cells are characterized by their constitutive expression of IL-2Rα chains (CD25) and the transcription factor FOXP3. Similar to the role assumed by T-bet in TH1, GATA-3 in TH2, and RORγt in TH17 differentiation, FOXP3 serves as a master regulator of nTreg cells (Table IV). Although they secrete IL-10, membrane TGF-β appears to be primarily responsible for mediating their immune suppression, which is contact dependent. nTreg cells are produced in response to expression in the thymus of self-antigens and are thereby important for the prevention of autoimmunity. These nTreg cells are unlikely to be involved in tolerance to antigens not presented in the thymus (eg, in either tolerance to allergens in healthy subjects or in the immune benefits associated with allergen immunotherapy). TH3 cells are primarily gut derived and generate mucosal tolerance. Reflecting their prominent production of TGF-β, in addition to tolerance, they are relevant to secretory IgA production. In contrast to thymus-derived nTreg cells, an additional, less well-characterized class of adaptive Treg cells has been described that can develop in the periphery. These iTreg cells differentiate from pre-existing effector T lymphocytes or possibly circulating naive TH0 cells and are characterized by their prominent production of IL-10. iTreg expression of FOXP3 and CD25 is controversial but does occur. For example, it is unclear whether CD25 expression reflects the constitutive expression of this component of IL-2R, the signature characteristic of nTreg cells, or the derivation of iTreg cells from activated effector T cells that are transiently expressing CD25. The induction of IL-10–producing iTreg cells plays a key role in reducing allergen-specific T-cell responsiveness after immunotherapy.
      • Akdis C.A.
      • Blesken T.
      • Akdis M.
      • Wuthrich B.
      • Blaser K.
      Role of interleukin 10 in specific immunotherapy.
      • Francis J.N.
      • Till S.J.
      • Durham S.R.
      Induction of IL-10 + CD4 + CD25 + T cells by grass pollen immunotherapy.
      Table VCD4+ T cells with regulatory activity
      Treg cell subtypeCharacteristics
      nTreg (natural Treg cells)CD25+Foxp3+ thymus derived. Not dependent on IL-10 for their biologic activity. Mediate self-tolerance/prevent autoimmune disease. Not likely to be relevant to acquired tolerance to allergens.
      TH3Characterized by TGF-β (± IL-10) production. Mediate mucosal tolerance/antigen-specific IgA production. Not relevant to inhalant allergy or immunotherapy.
      iTreg (induced Treg cells)Peripheral-derived Treg cells. IL-10 responsible for their biologic activity (± TGF-β). Thought to be derived from TH1/TH2-like effector lymphocytes ±CD25 expression (reflecting their effector function/activation) ± FOXP3 expression. Induced in contact-dependent fashion by membrane TGF-β. Proposed mechanism of immunotherapy

      Signal transduction by cytokine receptors

      Two key events are required to initiate the intracellular signaling pathways activated by cytokines. First, binding of a cytokine to its receptor mediates the transduction of signals from the extracellular environment into the cytoplasm. Second, activation of tyrosine kinases results in phosphorylation of the receptor and signaling molecules, events that ultimately lead to delivery of intracellular signals. With the notable exceptions of the receptors for SCF (c-kit or CD117) and M-CSF (colony-stimulating factor 1 receptor [also used by IL-34]), cytokine receptors generally do not have cytoplasmic domains with intrinsic tyrosine kinase activity; however, cytokine receptors do activate cytoplasmic tyrosine kinases.
      Although numerous biochemical cascades are involved in cytokine signaling, this discussion will primarily focus on 2 families of protein tyrosine kinases, termed JAKs and STATs, which uniquely function in cytokine signaling (Fig 1, A).
      • Darnell Jr., J.E.
      • Kerr I.M.
      • Stark G.R.
      Jak-STAT pathways and transcriptional activation in response to IFNs and other extracellular signaling proteins.
      • Ihle J.N.
      • Witthuhn B.A.
      • Quelle F.W.
      • Yamamoto K.
      • Thierfelder W.E.
      • Kreider B.
      • et al.
      Signaling by the cytokine receptor superfamily: JAKs and STATs.
      The role for JAK family members in the pathway to gene activation was largely deduced from studies of signal transduction by the interferon receptors. The 2 chains of the IFN-α receptor bind JAK1 and TYK2, respectively, whereas the 2 chains of the IFN-γ receptor bind JAK1 and JAK2. The receptors and the JAKs themselves become phosphorylated, and this phosphorylated complex becomes the catalyst for the phosphorylation of cytoplasmic substrates. There are 4 JAKs, JAK1, JAK2, JAK3, and TYK2, and as such, receptor signaling is mediated by a surprisingly limited number of highly redundant tyrosine kinases. For example, JAK2 is involved in GM-CSF, granulocyte colony-stimulating factor, IL-6, and IL-3 signaling.
      Figure thumbnail gr1
      Fig 1Comparison of cytokine and chemokine signaling. A, Generalized cytokine signaling: a model of intracellular signaling pathways leading to transcription modulation by IL-4 and IL-12 (see text for details). B, Generalized chemokine signaling: a model of chemokine binding and activation of G proteins leading to induction of transcription factors and gene expression (see text for details). cAPK, cAMP-dependent protein kinase; CREB, cAMP response element binding protein.
      JAK1 and JAK3 are tyrosine phosphorylated in response to IL-2, IL-4, and all the other cytokines whose receptors are members of the shared γ chain family. This use of JAK3 by the shared γ chain is consistent with JAK3 deficiency sharing the severe combined immunodeficiency syndrome phenotype with γ chain deficiency. Once engagement of a cytokine receptor has led to tyrosine phosphorylation of the receptor and of receptor-associated JAKs, the next step in signal transduction involves the tyrosine phosphorylation of the STATs (Table VI).
      • Darnell Jr., J.E.
      • Kerr I.M.
      • Stark G.R.
      Jak-STAT pathways and transcriptional activation in response to IFNs and other extracellular signaling proteins.
      • Ihle J.N.
      • Witthuhn B.A.
      • Quelle F.W.
      • Yamamoto K.
      • Thierfelder W.E.
      • Kreider B.
      • et al.
      Signaling by the cytokine receptor superfamily: JAKs and STATs.
      After their activation, these proteins migrate to the nucleus, where they bind to specific regulatory sequences in the promoters of cytokine-responsive genes, thereby initiating gene transcription. As with the JAKs, the function of STATs was originally characterized with studies involving the biochemical events of interferon-induced gene transcription. Ligand binding of IFN-α/β induces the formation of a trimer composed of STAT1, STAT2, and a non-STAT protein, interferon regulatory factor protein p48. Evidence suggests that STAT2 is the crucial STAT in establishing type I interferon activity because it is specifically recruited to DNA sequences comprising interferon-stimulated response elements present in the promoters of type I interferon-responsive genes.
      • Ghislain J.J.
      • Wong T.
      • Nguyen M.
      • Fish E.N.
      The interferon-inducible Stat2:Stat1 heterodimer preferentially binds in vitro to a consensus element found in the promoters of a subset of interferon-stimulated genes.
      In contrast, the stimulation of cells with IFN-γ results in the tyrosine phosphorylation of STAT1 by JAK1 and JAK2 but not of STAT2. These homodimers of STAT2 recognize IFN-γ activation site DNA sequences in the promoters of IFN-γ–responsive genes. Similar to type I interferons, IL-28 and IL-29 (IFN-λs) induce the activation of the JAK/STAT signaling pathways.
      • Kotenko S.V.
      • Gallagher G.
      • Baurin V.V.
      • Lewis-Antes A.
      • Shen M.
      • Shah N.K.
      • et al.
      IFN-lambdas mediate antiviral protection through a distinct class II cytokine receptor complex.
      • Dumoutier L.
      • Lejeune D.
      • Hor S.
      • Fickenscher H.
      • Renauld J.C.
      Cloning of a new type II cytokine receptor activating signal transducer and activator of transcription (STAT)1, STAT2 and STAT3.
      • Dumoutier L.
      • Tounsi A.
      • Michiels T.
      • Sommereyns C.
      • Kotenko S.V.
      • Renauld J.C.
      Role of the interleukin (IL)-28 receptor tyrosine residues for antiviral and antiproliferative activity of IL-29/interferon-lambda 1: similarities with type I interferon signaling.
      JAK1 in particular is critical in mediating IFN-λ–induced STAT phosphorylation.
      • Dumoutier L.
      • Lejeune D.
      • Hor S.
      • Fickenscher H.
      • Renauld J.C.
      Cloning of a new type II cytokine receptor activating signal transducer and activator of transcription (STAT)1, STAT2 and STAT3.
      IFN-λ induces homodimers of STAT2 capable of recognizing both interferon-stimulated response element and IFN-γ activation site sequences. It is therefore not surprising that many genes whose expression is classically induced by both type I interferons and IFN-γ are also induced by IFN-λs.
      Table VISTAT family
      STAT proteinCytokine
      STAT1IFN-α/β
      IFN-α/β signaling complex (interferon-stimulated gene factor 3) consists of trimers of STAT1 (alternatively spliced α [p91] or β [p84] peptides), STAT2, and the non-STAT protein p48. IFN-γ signaling complex consists of dimers of STAT1.
      IFN-γ
      IFN-α/β signaling complex (interferon-stimulated gene factor 3) consists of trimers of STAT1 (alternatively spliced α [p91] or β [p84] peptides), STAT2, and the non-STAT protein p48. IFN-γ signaling complex consists of dimers of STAT1.
      Epidermal growth factor, platelet-derived growth factor, M-CSF, IL-6, IL-11
      STAT2IFN-α/β
      IFN-α/β signaling complex (interferon-stimulated gene factor 3) consists of trimers of STAT1 (alternatively spliced α [p91] or β [p84] peptides), STAT2, and the non-STAT protein p48. IFN-γ signaling complex consists of dimers of STAT1.
      IFN-λ
      STAT3IL-6 (IL-6 family cytokines, including IL-6, oncostatin M, and LIF) trigger STAT3 though the gp130 receptor) IL-5, IL-10, epidermal growth factor, human growth factor
      STAT4IL-12 (essential endogenous mediator of TH1 differentiation)
      STAT5A and STAT5B
      Two distinct genes that are 90% identical.
      Prolactin IL-2, IL-3, IL-7, GM-CSF, erythropoietin, thrombopoietin
      STAT6IL-4 (essential endogenous mediator of TH2 differentiation)
      IFN-α/β signaling complex (interferon-stimulated gene factor 3) consists of trimers of STAT1 (alternatively spliced α [p91] or β [p84] peptides), STAT2, and the non-STAT protein p48. IFN-γ signaling complex consists of dimers of STAT1.
      Two distinct genes that are 90% identical.
      There are 5 additional members of the STAT family. STAT3, STAT4, and STAT6 were identified as IL-6–, IL-12–, and IL-4–inducible peptides, respectively. Although important in cytokine signaling, STAT5 (consisting of 2 homologous genes, STAT5A and STAT5B) was originally defined as a prolactin-activated peptide. Engagement of the IL-4 receptor leads to the activation of JAK1, which in turn phosphorylates STAT6. STAT6 is necessary for IL-4–dependent expression of IL-4 receptor (IL-4R) α, the ε heavy chain, MHC class II, CD23, and mucin.
      • Takeda K.
      • Tanaka T.
      • Shi W.
      • Matsumoto M.
      • Minami M.
      • Kashiwamura S.
      • et al.
      Essential role of Stat6 in IL-4 signalling.
      An endogenous inhibitor of STAT6 is referred to as the suppressor of cytokine signaling 1.
      • Losman J.A.
      • Chen X.P.
      • Hilton D.
      • Rothman P.
      Cutting edge: SOCS-1 is a potent inhibitor of IL-4 signal transduction.
      Suppressor of cytokine signaling 1 inhibits IL-4–induced activation of JAK1 and STAT6 and thereby effectively inhibits IL-4 signaling.
      Compared with the number of cytokines, relatively few STATs exist, and therefore the signaling pathways of numerous distinct cytokines share common STAT proteins. For example, epidermal growth factor, platelet-derived growth factor, M-CSF, IL-6, IL-11, and the interferons all activate STAT1α. Mechanisms must exist that lead to the distinct responses to different cytokines. In part these reflect the activities of other signaling pathways stimulated by cytokine receptors. For example, the Ras-dependent pathway is also activated by members of the cytokine receptor families. In this cascade Ras, Raf-1, Map/Erk kinase kinase, and mitogen-activated protein kinases (MAPKs) are sequentially phosphorylated and activated. The MAPK pathway is associated with induction of several transcription factors, such as c-myc, c-fos, and nuclear factor–IL-6. This ras pathway is activated by several growth factors, as well as by the cytokines IL-2, IL-3, IL-5, and erythropoietin. An example of another complementary distinct pathway used for cytokine signaling is provided by IL-4, which activates the signaling protein insulin response substrate (IRS) 1 and its homologue, IRS-2. IRS-1 and IRS-2 regulate cellular proliferation and protection from apoptosis.

      Chemokines

      Chemokines are a group of small (8-12 kd) proteins that posses the ability to induce cell migration or chemotaxis in numerous cell types, including neutrophils, monocytes, lymphocytes, eosinophils, fibroblasts, and keratinocytes. Activity is regulated through binding to members of the 7-transmembrane, G protein–coupled receptor superfamily. This section uses the systematic nomenclature with the common names listed in parentheses the first time the chemokine is described. To date, 52 chemokines and 20 chemokine receptors have been described, which are listed in Table VII
      • Moser B.
      • Loetscher P.
      Lymphocyte traffic control by chemokines.
      • Zlotnik A.
      • Yoshie O.
      Chemokines: a new classification system and their role in immunity.
      along with the known chromosomal location and physiologic properties of each. Many of the chemokine receptors can bind more than 1 ligand, allowing extensive overlap and redundancy of chemokine function.
      Table VIICC, C, CXC, and CX3C chemokine/receptor families
      Systematic nameHuman chromosomeCommon nameReceptorPhysiologic features
      CC chemokine/receptor family
       CCL117q11.2I-309CCR8, R11Inflammation
       CCL217q11.2MCP-1, MCAFCCR2Inflammation
       CCL317q11-q21MIP-1α/LD78αCCR1, R5Inflammation, homeostasis
       CCL3L117q21.1LD78βCCR5Inflammation
       CCL417q11.2MIP-1βCCR5Inflammation
       CCL4L117q12NoneCCR5Inflammation
       CCL4L217q12NoneCCR5Inflammation
       CCL517q11.2RANTESCCR1, R3, R4, R5Inflammation
       CCL6(mouse)C-10CCR1, R2, R3Unknown
       CCL717q11.2MCP-3CCR1, R2, R3Inflammation
       CCL817q11.2MCP-2CCR1, R2, R5, R11Inflammation
       CCL9(mouse)MRP-2/MIP-1γCCR1Unknown
       CCL10(mouse)MRP-2/MIP-1γCCR1Unknown
       CCL1117q11.2EotaxinCCR3Inflammation, homeostasis
       CCL12(mouse)MCP-5CCR2Unknown
       CCL1317q11.2MCP-4CCR1, R2, R3, R11Inflammation
       CCL1417q11.2HCC-1CCR1Inflammation
       CCL1517q11.2HCC-2, Lkn-1CCR1, R3Inflammation
       CCL1617q11.2HCC-4, LECCCR1Inflammation
       CCL1716q13TARCCCR4Inflammation, homeostasis
       CCL1817q11.2DC-CK1, PARCUnknownHomeostasis
       CCL199p13MIP-3β, ELCCCR7, R11Homeostasis
       CCL202q33-q37MIP-3α, LARCCCR6Inflammation, homeostasis
       CCL219p136Ckine, SLCCCR7, R11Homeostasis
       CCL2216q13MDC, STCP-1CCR4Inflammation, homeostasis
       CCL2317q11,2MPIF-1CCR1Inflammation
       CCL247q11.23MPIF-2, Eotaxin-2CCR3Inflammation
       CCL2519p13.2TECKCCR9, R11Homeostasis
       CCL267q11.23Eotaxin-3CCR3Inflammation
       CCL279p13CTACK, ILCCCR2, R3, R10Homeostasis
       CCL285p12MECCCR3, R10Inflammation, homeostasis
      C chemokine/receptor family
       XCL11q23LymphotactinXCR1Inflammation
       XCL21q23SCM1-bXCR1Inflammation
      CXC chemokine/receptor family
       CXCL1 (ELR)4q12-q13GROα, MGSA-αCXCR2>R1Inflammation, homeostasis
       CXCL2 (ELR)4q12-q13GROβ, MGSA-βCXCR2Inflammation
       CXCL3 (ELR)4q12-q13GROγ, MGSA-γCXCR2Inflammation
       CXCL4 (non-ELR)4q12-q13PF4CXCR3Inflammation
       CXCL4L1 (non-ELR)4q12-q21PF4V1CRCR3Inflammation
       CXCL5 (ELR)4q12-q13ENA-78CXCR1, R2Inflammation
       CXCL6 (ELR)4q12-q13GCP-2CXCR1, R2Inflammation
       CXCL7 (ELR)4q12-q13NAP-2CXCR2Inflammation
       CXCL8 (ELR)4q12-q13IL-8CXCR1, R2Inflammation
       CXCL9 (non-ELR)4q21.21MigCXCR3Inflammation
       CXCL10 (non-ELR)4q21.21IP-10CXCR3Inflammation
       CXCL11 (non-ELR)4q21.21I-TACCXCR3Inflammation
       CXCL12 (non-ELR)10q11.1SDF-1α/βCXCR4, R7Inflammation, homeostasis
       CXCL13 (non-ELR)4q21BLC, BCA-1CXCR3, R5Inflammation, homeostasis
       CXCL14 (non-ELR)5q31BRAK, bolekineUnknownHomeostasis
       CXCL15 (ELR)(mouse)UnknownUnknown
       CXCL16 (non-ELR)17p13SR-PSOXCXCR6Inflammation
       CXCL17 (non-ELR)19q13.2VCC1, DMCUnknownInflammation, homeostasis
      CX3C chemokine/receptor family
       CXCCL116q13FractalkineCR3CR1Inflammation
      This table is an adaptation of the tables presented by Zlotnik and Yoshie
      • Zlotnik A.
      • Yoshie O.
      Chemokines: a new classification system and their role in immunity.
      and Moser and Loetscher.
      • Moser B.
      • Loetscher P.
      Lymphocyte traffic control by chemokines.
      The terms inflammation and homeostasis under the “Physiologic features” heading refer to inflammatory chemokines and homeostatic chemokines, respectively. The most common names for the human ligands are listed but are not all inclusive of ligand names found in the literature. ELR is a conserved amino acid motif (Glu-Leu-Arg) immediately preceding the first cysteine amino acid in the CXCL chemokine family.
      Originally, chemokines were described as inflammatory mediators produced at sites of infection or injury or in response to proinflammatory stimuli. Inflammatory chemokines recruit and activate leukocytes to mount an immune response and initiate wound healing. Although chemotaxis stands as the cardinal feature of chemokines, their physiologic role is more complex than originally described, with many having additional homeostatic or housekeeping functions. These functions range from trafficking of lymphocytes during hematopoiesis, antigen sampling in secondary lymphoid tissue, immune surveillance, and organ development.
      • Moser B.
      • Loetscher P.
      Lymphocyte traffic control by chemokines.
      In general, homeostatic chemokines are expressed in specific tissues or organs, whereas inflammatory chemokines are produced by many cell types in multiple locations.

       Classification

      Chemokines are characterized by the presence of 3 to 4 conserved cysteine residues and can be subdivided into 4 families based on the positioning of the N-terminal cysteine residues (Table VII). Within a subfamily, there exists 30% to 90% amino acid identity between members; however, across subfamilies, the amino acid identity decreases to less than 30%. The C-X-C subfamily is characterized by the separation of the first 2 cysteines by a variable amino acid. The CXC chemokines can be broken into 2 general subgroups: ELR and non-ELR containing. ELR is a conserved amino acid motif (Glu-Leu-Arg) immediately preceding the first cysteine residue. The ELR chemokines (CXCL1, CXCL2, CXCL3, CXCL5, CXCL6, CXCL7, and CXCL8) are angiogenic and act mainly through the CXCR2 receptor. In contrast, the non-ELR chemokines (CXCL4, CXCL9, CXCL10, CXCL11 and CXCL17) are angiostatic and act mainly through the CXCR3B receptor. This non-ELR group of chemokines can be induced by a variety of interferons. The exception to this is CXCL12, which is a non-ELR chemokine but is angiostatic and binds to the CXCR4 on endothelial cells. In the C-C subfamily the cysteine residues are adjacent to each other. The majority of the known chemokines are contained in these 2 families. Additionally, these groups can be distinguished by their primary target cell, with the C-X-C subfamily targeting neutrophils and the C-C family targeting eosinophils, monocytes, and T cells. A third family of chemokines, referred to as the C subfamily, lacks the first and third cysteines, containing only a single cysteine residue in the conserved position. This subfamily includes the lymphocyte-specific chemotactic peptide XCL1 (lymphotactin).
      • Kelner G.S.
      • Kennedy J.
      • Bacon K.B.
      • Kleyensteuber S.
      • Largaespada D.A.
      • Jenkins N.A.
      • et al.
      Lymphotactin: a cytokine that represents a new class of chemokine.
      A fourth subfamily (CX3C) has the 2 N-terminal cysteine residues separated by 3 variable amino acids.
      • Bazan J.F.
      • Bacon K.B.
      • Hardiman G.
      • Wang W.
      • Soo K.
      • Rossi D.
      • et al.
      A new class of membrane-bound chemokine with a CX3C motif.
      In human subjects this family only has 1 member, CX3CL1 (fractalkine), and it is unique in that it is has a mucin-like glycosylated stalk that allows it to exist as a soluble or membrane-bound chemokine.

       Receptors and signal transduction

      Receptor number on the cell surface varies from 3000 per cell on monocytes and lymphocytes for CCR1 and CCR2 to 40,000 to 50,000 per cell on eosinophils for CCR3.
      • Charo I.F.
      • Myers S.J.
      • Herman A.
      • Franci C.
      • Connolly A.J.
      • Coughlin S.R.
      Molecular cloning and functional expression of two monocyte chemoattractant protein-1 receptors reveals alternative splicing of the carboxy-terminal tails.
      • Ponath D.P.
      • Qin S.
      • Post T.W.
      • Wang J.
      • Wu L.
      • Gerard N.P.
      • et al.
      Molecular cloning and characterization of a human eotaxin receptor expressed selectively on eosinophils.
      • Daugherty B.L.
      • Siciliano S.J.
      • DeMartino J.A.
      • Malkowitz L.
      • Sirotina A.
      • Springer M.S.
      Cloning, expression, and characterization of the human eosinophil eotaxin receptor.
      Receptor numbers can be altered depending on the environmental milieu and the signals a cell receives. A given cell can express multiple chemokine receptors, each of which can induce specific signals, suggesting that each receptor can signal through different pathways. Additional complexities of receptor use are emerging through the recent demonstration that CXCR4 and CCR5 can heterodimerize and transmit a compound signal when stimulated with their respective ligands.
      • Isik N.
      • Hereld D.
      • Jin T.
      Fluorescence resonance energy transfer imaging reveals that chemokine-binding modulates heterodimers of CXCR4 and CCR5 receptors.
      The ability to signal through different pathways is due in part to the heptahelical transmembrane property of the receptors. A large surface area, allowing interactions with the α and βγ subunits of the heterotrimeric G proteins and other effector molecules, is created by looping of the receptor along the inner plasma membrane and the lateral orientation of the carboxy terminus.
      • Thelen M.
      Dancing to the tune of chemokines.
      Signaling is initiated after binding of the chemokine to the receptor, which activates guanine exchange factors, allowing replacement of guanine diphosphate with guanine triphosphate on the Gα subunit (Fig 1, B). The result is dissociation of the heterotrimeric G protein complex from the receptor and separation of the Gα and Gβγ subunits. The Gα subunit is able to directly activate the Src family kinases, leading to activation of the MAPKs and protein kinase B.
      • Ma Y.C.
      • Huang J.
      • Ali S.
      • Lowry W.
      • Huang X.Y.
      Src tyrosine kinase is a novel direct effector of G proteins.
      Signaling through the Gβγ subunit is more complex, involving at least 3 separate pathways. Gβγ can activate protein kinase B and the MAPKs through phosphatidylinositol 3–kinase γ and PKC through phospholipase C and Pyk-2.
      • Baggiolini M.
      • Dewald B.
      • Moser B.
      Human chemokines: an update.
      • Li Z.
      • Jiang H.
      • Xie W.
      • Zhang Z.
      • Smrcka A.
      • Wu D.
      Roles of PLC-b2 and -b3 and PI3Kg in chemoattractant-mediated signal transduction.
      • Lopez-Ilasaca M.
      • Crespo P.
      • Pellici P.G.
      • Gutkind J.S.
      • Wetzker R.
      Linkage of G protein-coupled receptors to the MAPK signaling pathway through PI3-kinase γ.
      Activation of phospholipase C increases the intracellular calcium ion concentration. Calcium influx activates many cellular processes, including degranulation of neutrophils, eosinophils, and basophils. Other pathways activated by chemokines include phospholipases A2 and D, protein tyrosine kinases, low-molecular-weight guanine triphosphatases, Rho, and Rac. Several other reviews cover chemokine signaling in more extensive detail.
      • Thelen M.
      Dancing to the tune of chemokines.
      • Balakin K.V.
      • Ivanenkov Y.A.
      • Tkachenko S.E.
      • Kiselyov A.S.
      • Ivachtchenko A.V.
      Regulators of chemokine receptor activity as promising anticancer therapeutics.
      Signaling through the G proteins ends when a phosphate group is removed from the guanine triphosphate bound to the Gα subunit reforming guanine diphosphate. This allows the Gα and Gβγ subunits to rejoin and terminate downstream signaling events. Chemokines can also activate signaling pathways, such as MAPK and protein tyrosine kinase, through G protein–independent mechanisms. Signaling through chemokine receptors can be dampened through several processes, including homologous and heterologous desensitization.
      Homologous desensitization occurs when G protein–coupled receptor kinases selectively phosphorylate chemokine-occupied receptors, leading to endocytic uptake of chemokine receptor complexes. Heterologous desensitization occurs when non–G protein–coupled receptor kinases phosphorylate ligand-free (nonengaged) chemokine receptors, preventing future G protein coupling and receptor activation.
      In addition to the receptors that activate cellular responses to chemokines, 3 other receptors bind chemokines: duffy antigen receptor for chemokines, D6, and CCX-CKR. These receptors bind chemokines but do not signal, leading to their designation as decoy receptors. Decoy receptors bind ligand and prevent the ligand from being able to act. In terms of chemokine action, decoy receptors play a role in dampening the immune response, leading to resolution of inflammation. Recently, this concept has been challenged by the finding that duffy antigen receptor for chemokines can mediate chemokine transcytosis, leading to apical retention of the chemokine and enhanced leukocyte migration across monolayers.
      • Pruenster M.
      • Muddle L.
      • Bombosi P.
      • Dimitrova S.
      • Zsak M.
      • Middleton J.
      • et al.
      The Duffy antigen receptor for chemokines transports chemokines and supports their promigratory activity.

       Chemokine function

      The original description of chemokines focused on their primary role in directing lymphocytes to sites of inflammation. A detailed examination of cell migration and recruitment is beyond the scope of this review and is covered elsewhere.
      • Zarbock A.
      • Ley K.
      Neutrophil adhesion and activation under flow.
      Briefly, in a process known as rolling, lymphocytes interact transiently with the vascular endothelium, searching for activating signals from chemokines. On binding of a chemokine to its receptor, integrins are expressed, which mediate high-affinity interactions and lead to firm arrest of the leukocytes. This has been demonstrated for the chemokines CCL19 (ELC), CCL21 (SLC), and CXCL12 (SDF-1), which rapidly induce a high-affinity state for the β2-intergrin lymphocyte function–associated antigen 1.
      • Constantin G.
      • Majeed M.
      • Giagulli C.
      • Piccio L.
      • Kim J.Y.
      • Butcher E.C.
      • et al.
      Chemokines trigger immediate beta2 integrin affinity and mobility changes: differential regulation and roles in lymphocyte arrest under flow.
      Once the cell has ceased rolling, it can cross the endothelium and will continue this process as it migrates along a concentration gradient and crosses the endothelial layer to the source of the generated chemokine. It is the expression of particular chemokines, receptors, and adhesion molecules that contribute to the selective migration and tissue specificity of leukocytes.
      Chemokines perform a variety functions aside from chemotaxis. Chemokines can have direct effects on T-cell differentiation through ligand-receptor interactions on the developing cell or indirectly by altering APC trafficking or cytokine secretion. Functioning through the CCR5 receptor, CCL3 (macrophage inflammatory protein [MIP] 1α), CCL4 (MIP-1β), and CCL5 can directly promote development of IFN-γ TH1 cells or indirectly by increasing IL-12 production from APCs. In contrast, CCL2 (monocyte chemoattractant protein [MCP] 1), CCL7 (MCP-3), CCL8 (MCP-2), and CCL13 (MCP-4) can inhibit IL-12 production from APCs and enhance IL-4 production from activated T cells, leading to a TH2 phenotype.
      • Luther S.A.
      • Cyster J.G.
      Chemokines as regulators of T cell differentiation.
      Chemokine receptor expression can serve as a marker for maturation and differentiation of lymphocytes. When monocytes and immature DCs exit blood in tissues and begin immune surveillance, they express the CCR1, CCR2, CCR5, CCR6, and CXCR2 receptors, which are classified as inflammatory receptors. As antigen is encountered and the DCs mature, the inflammatory receptors are downregulated and replaced by expression of CCR7. CCR7 expression allows the DCs to accumulate in the draining lymphatics and T-cell areas of the lymph nodes.
      • Mackay C.R.
      Chemokines: immunology's high impact factors.
      Expression of CXCR5 has been demonstrated on a distinct memory T-cell subset that displays B helper cell function. These cells respond to CXCL13 (BLC) and are directed to the B-cell follicle to help support production of antibodies.
      • Breitfeld D.
      • Ohl L.
      • Kremmer E.
      • Ellwart J.
      • Sallusto F.
      • Lipp M.
      • et al.
      Follicular B helper T cells express CXC chemokine receptor 5, localize to B cell follicles, and support immunoglobulin production.
      • Schaerli P.
      • Willimann K.
      • Lang A.B.
      • Lipp M.
      • Loetscher P.
      • Moser B.
      CXC chemokine receptor 5 expression defines follicular homing T cells with B cell helper function.
      Release of mature neutrophils from the bone marrow is regulated by binding of CXCL12 with its receptor, CXCR4.
      • von Vietinghoff S.
      • Ley K.
      Homeostatic regulation of blood neutrophil counts.
      Other examples include a role for CXCL1, CXCL12, and CCL3 in brain development; a role for CCL2 and CXCL8 in wound healing; and a role for CXCL12 in organogenesis.

       Clinical relevance

      Aberrant regulation of chemokine expression has been implicated in many diseases (Table VIII); however, the focus of this section will be on the role that chemokines play in allergic disorders. Many studies have demonstrated increased chemokine levels in asthmatic patients compared with control subjects, as measured in bronchoalveolar lavage and biopsy samples.
      • Ying S.
      • Merg Q.
      • Zeibecoglou K.
      • Robinson D.S.
      • Macfarlane A.
      • Humbert M.
      • et al.
      Eosinophil chemotactic chemokines (eotaxin, eotaxin-2, RANTES, monocyte chemoattractant protein-3 (MCP-3), and MCP-4), and C-C chemokine receptor 3 expression in bronchial biopsies from atopic and nonatopic (intrinsic) asthmatics.
      • Miotto D.
      • Christodoulopoulos P.
      • Olivenstein R.
      • Taha R.
      • Cameron L.
      • Tsicopoulos A.
      • et al.
      Expression of IFN-gamma-inducible protein; monocyte chemotactic proteins 1, 3, and 4; and eotaxin in TH1- and TH2-mediated lung diseases.
      These include CCL2, CCL3, CCL5, CCL7, CCL11, CCL13, CCL24, CXCL8, and CXCL10. Investigators have used murine models of asthma to understand the role that chemokines play in inducing AHR. CCL2, CCL5, CCL11, CXCL10, and CXCL12 all contribute to AHR and cellular emigration in these models of airway inflammation.
      • Medoff B.D.
      • Sauty A.
      • Tager A.M.
      • Maclean J.A.
      • Smith R.N.
      • Mathew A.
      • et al.
      IFN-γ-inducible protein 10 (CXCL10) contributes to airway hyperreactivity and airway inflammation in a mouse model of asthma.
      • Gonzalo J.A.
      • Lloyd C.M.
      • Peled A.
      • Delaney T.
      • Coyle A.J.
      • Gutierrez-Ramos J.C.
      Critical involvement of the chemotactic axis CXCR4/stromal cell-derived factor-1a in the inflammatory component of allergic airway disease.
      • Gerard C.
      • Rollins B.J.
      Chemokines and disease.
      Table VIIIChemokine/chemokine receptor involvement in human disease
      Chemokine/chemokine receptorDisease
      CCR5, CCL3L1, CCL4L1, CXCR4HIV/AIDS
      CXCR4WHIM syndrome
      CX3CR1, CX3CL1, CXCL1, CXCL8, CXCR2, CCL2Atherosclerosis
      CCL2, CCL5, CCL7, CCL11, CXCL8Asthma, allergic diseases
      CXCR4, CXCL1, CXCL12Cancer metastases
      CXCL4Heparin-induced thrombocytopenia
      CCL26Eosinophilic esophagitis
      CCR5Rheumatoid arthritis
      CCR5Renal allograft rejection
      CCR5West Nile virus infection
      Duffy antigen receptor for chemokinesMalaria (Plasmodium vivax infection)
      WHIM, Warts, hypogammaglobulinemia, infection, and myelokathexis.
      The C-C chemokine family has been extensively studied in allergic diseases because of its members' ability to recruit eosinophils, T cells, and monocytes to regions of inflammation. CCL5 and CCL11 are the most important eosinophil chemoattractants in allergic inflammation.
      • Venge J.
      • Lampinen M.
      • Hakansson L.
      • Rak S.
      • Venge P.
      Identification of IL-5 and RANTES as the major eosinophil chemoattractants in the asthmatic lung.
      This has been demonstrated in mice, in which instillation of CCL5 or CCL11 into the lungs results in an eosinophilic and mononuclear cell infiltrate in the absence of neutrophils.
      • Meurer R.
      • Van Riper G.
      • Feeney W.
      • Cunningham P.
      • Hora D.
      • Springer M.S.
      • et al.
      Formation of eosinophilic and monocytic intradermal inflammatory sites in the dog by injection of human RANTES but not human monocyte chemoattractant protein 1, human macrophage inflammatory protein 1 alpha, or human interleukin 8.
      Aside from production by eosinophils, macrophages, mast cells, and T cells, CCL5 and CCL11 are produced by structural cells of the airway, including airway smooth muscle and fibroblasts. In addition to lymphoid tissue, nasal epithelial cells express CCL17 (TARC), and expression of this chemokine and its receptor, CCR4, was higher in patients with allergic rhinitis compared with that seen in nonallergic control subjects. Both IL-4 and IL-13 promote CCL17 expression, leading to a TH2 response.
      • Terada N.
      • Nomura T.
      • Kim W.J.
      • Otsuka Y.
      • Takahashi R.
      • Kishi H.
      • et al.
      Expression of C-C chemokine TARC in human nasal mucosa and its regulation by cytokines.
      This is relevant in allergic bronchopulmonary aspergillosis (ABPA), in which increased serum levels of CCL17 predict ABPA exacerbations better than IgE levels.
      • Terada N.
      • Nomura T.
      • Kim W.J.
      • Otsuka Y.
      • Takahashi R.
      • Kishi H.
      • et al.
      Expression of C-C chemokine TARC in human nasal mucosa and its regulation by cytokines.
      CCL17 levels might serve as a marker of ABPA in patients with cystic fibrosis.
      • Hartl D.
      • Latzin P.
      • Zissel G.
      • Krane M.
      • Krauss-Etschmann S.
      • Griese M.
      Chemokines indicate allergic bronchopulmonary aspergillosis in patients with cystic fibrosis.
      CXCL8 is derived primarily from mononuclear phagocytes and endothelial and epithelial cells but also from T cells, eosinophils, neutrophils, fibroblasts, keratinocytes, hepatocytes, and chondrocytes. CXCL8 synthesis can be induced by LPS, IL-1, TNF, or viral infection.
      • Oppenheim J.J.
      • Zachariae C.O.
      • Mukaida N.
      • Matsushima K.
      Properties of the novel proinflammatory supergene “intercrine” cytokine family.
      • Horuk R.
      The interleukin-8-receptor family: from chemokines to malaria.
      On a molar basis, CXCL8 is one of the most potent chemoattractants for neutrophils in addition to stimulating the neutrophil respiratory burst and adherence to endothelial cells through CXCR1.
      • Leonard E.J.
      • Yoshimura T.
      Neutrophil attractant/activation protein-1 (NAP-1 [interleukin-8]).
      CXCL10 and CXCL13 are induced at different times after allergen exposure. CXCL10 is produced in the early phases after allergen exposure, whereas CXCL13 is only induced after secondary and subsequent allergen exposures.
      • Fulkerson P.C.
      • Zimmermann N.
      • Hassman L.M.
      • Finkelman F.D.
      • Rothenberg M.E.
      Pulmonary chemokine expression is coordinately regulated by STAT1, STAT6, and IFN-gamma.
      This might have to do with the cellular sources of these cytokines. Airway epithelial cells produce CXCL10, and contact with allergen might induce expression and thus explain the high levels early after allergen exposure. CXCL13 is produced by TH17, but not TH1 or TH2, cells. It is tempting to speculate that TH17 cells might play a role in asthma in later exposures after the allergic phenotype has already been established.
      T-cell subsets that might have regulatory activity are being identified, and chemokines and their receptors appear to have important roles in mediating activity and migration of these cells. Among CD4+CD25+Foxp3+ nTreg cells, there appears to be at least 2 subgroups that can be distinguished based on CCR6 expression. Those that are high in CCR6 seem to have regulatory activity, whereas those low in CCR6 secrete TH2 cytokines on stimulation with bacterial superantigen.
      • Reefer A.J.
      • Satinover S.M.
      • Solga M.D.
      • Lannigan J.A.
      • Nguyen J.T.
      • Wilson B.B.
      • et al.
      Analysis of CD25hiCD4+ “regulatory” T-cell subtypes in atopic dermatitis reveals a novel T(H)2-like population.
      Another group has demonstrated low levels of XCR1 on the surface of CD4+CD25hiCD127low T cells isolated from allergic asthmatic subjects compared with those from healthy control subjects.
      • Nguyen K.D.
      • Fohner A.
      • Booker J.D.
      • Dong C.
      • Krensky A.M.
      • Nadeau K.C.
      XCL1 enhances regulatory activities of CD4 + CD25(high) CD127(low/-) T cells in human allergic asthma.
      Although in the early stages, this emerging field of chemokine response and expression by Treg cells will hopefully clarify many of the questions about how these cells work.

      Conclusions

      It has been almost 25 years since the cloning of the first cytokine was described. Since that time, more than 300 cytokines, chemokines, and growth factors have been described, with varying functions on not just the immune system but on every organ system in the body. Despite the large number of articles concerning the role of these proteins, we are still in our infancy in understanding how these factors alone and in concert with other factors influence homeostatic and inflammatory events. Abnormal production of these factors can lead to diseases such as asthma and atopy, and continued research is needed to piece together how these can be balanced to eliminate disease processes without compromising the individual to other deleterious outcomes.

      References

        • Beutler B.
        • Cerami A.
        The biology of cachectin/TNF—a primary mediator of the host response.
        Annu Rev Immunol. 1989; 7: 625-655
        • Perez C.
        • Albert I.
        • DeFay K.
        • Zachariades N.
        • Gooding L.
        • Kriegler M.
        A nonsecretable cell surface mutant of tumor necrosis factor (TNF) kills by cell-to-cell contact.
        Cell. 1990; 63: 251-258
        • Tartaglia L.A.
        • Goeddel D.V.
        Two TNF receptors.
        Immunol Today. 1992; 13: 151-153
        • Tracey K.J.
        • Fong Y.
        • Hesse D.G.
        • Manogue K.R.
        • Lee A.T.
        • Kuo G.C.
        • et al.
        Anti-cachectin/TNF monoclonal antibodies prevent septic shock during lethal bacteraemia.
        Nature. 1987; 330: 662-664
        • Dinarello C.A.
        • Wolff S.M.
        The role of interleukin-1 in disease.
        N Engl J Med. 1993; 328: 106-113
        • Sims J.E.
        • Gayle M.A.
        • Slack J.L.
        • Alderson M.R.
        • Bird T.A.
        • Giri J.G.
        • et al.
        Interleukin 1 signaling occurs exclusively via the type I receptor.
        Proc Natl Acad Sci U S A. 1993; 90: 6155-6159
        • Arend W.P.
        Interleukin-1 receptor antagonist.
        Adv Immunol. 1993; 54: 167-227
        • Cerretti D.P.
        • Kozlosky C.J.
        • Mosley B.
        • Nelson N.
        • Van Ness K.
        • Greenstreet T.A.
        • et al.
        Molecular cloning of the interleukin-1 beta converting enzyme.
        Science. 1992; 256: 97-100
        • Akira S.
        • Taga T.
        • Kishimoto T.
        Interleukin-6 in biology and medicine.
        Adv Immunol. 1993; 54: 1-78
        • Muller-Newen G.
        • Kuster A.
        • Hemmann U.
        • Keul R.
        • Horsten U.
        • Martens A.
        • et al.
        Soluble IL-6 receptor potentiates the antagonistic activity of soluble gp130 on IL-6 responses.
        J Immunol. 1998; 161: 6347-6355
        • Heinrich P.C.
        • Behrmann I.
        • Haan S.
        • Hermanns H.M.
        • Muller-Newen G.
        • Schaper F.
        Principles of interleukin (IL)-6-type cytokine signalling and its regulation.
        Biochem J. 2003; 374: 1-20
        • Brunda M.J.
        Interleukin-12.
        J Leukoc Biol. 1994; 55: 280-288
        • Oppmann B.
        • Lesley R.
        • Blom B.
        • Timans J.C.
        • Xu Y.
        • Hunte B.
        • et al.
        Novel p19 protein engages IL-12p40 to form a cytokine, IL-23, with biological activities similar as well as distinct from IL-12.
        Immunity. 2000; 13: 715-725
        • Dinarello C.A.
        Interleukin-18, a proinflammatory cytokine.
        Eur Cytokine Netw. 2000; 11: 483-486
        • Grabstein K.H.
        • Eisenman J.
        • Shanebeck K.
        • Rauch C.
        • Srinivasan S.
        • Fung V.
        • et al.
        Cloning of a T cell growth factor that interacts with the beta chain of the interleukin-2 receptor.
        Science. 1994; 264: 965-968
        • Larousserie F.
        • Bardel E.
        • Pflanz S.
        • Arnulf B.
        • Lome-Maldonado C.
        • Hermine O.
        • et al.
        Analysis of interleukin-27 (EBI3/p28) expression in Epstein-Barr virus- and human T-cell leukemia virus type 1-associated lymphomas: heterogeneous expression of EBI3 subunit by tumoral cells.
        Am J Pathol. 2005; 166: 1217-1228
        • Pflanz S.
        • Hibbert L.
        • Mattson J.
        • Rosales R.
        • Vaisberg E.
        • Bazan J.F.
        • et al.
        WSX-1 and glycoprotein 130 constitute a signal-transducing receptor for IL-27.
        J Immunol. 2004; 172: 2225-2231
        • Kim S.H.
        • Han S.Y.
        • Azam T.
        • Yoon D.Y.
        • Dinarello C.A.
        Interleukin-32: a cytokine and inducer of TNFalpha.
        Immunity. 2005; 22: 131-142
        • Netea M.G.
        • Azam T.
        • Ferwerda G.
        • Girardin S.E.
        • Walsh M.
        • Park J.S.
        • et al.
        IL-32 synergizes with nucleotide oligomerization domain (NOD) 1 and NOD2 ligands for IL-1beta and IL-6 production through a caspase 1-dependent mechanism.
        Proc Natl Acad Sci U S A. 2005; 102: 16309-16314
        • Minshall E.
        • Chakir J.
        • Laviolette M.
        • Molet S.
        • Zhu Z.
        • Olivenstein R.
        • et al.
        IL-11 expression is increased in severe asthma: association with epithelial cells and eosinophils.
        J Allergy Clin Immunol. 2000; 105: 232-238
        • Tang W.
        • Geba G.P.
        • Zheng T.
        • Ray P.
        • Homer R.J.
        • Kuhn 3rd, C.
        • et al.
        Targeted expression of IL-11 in the murine airway causes lymphocytic inflammation, bronchial remodeling, and airways obstruction.
        J Clin Invest. 1996; 98: 2845-2853
        • Adolf G.R.
        Monoclonal antibodies and enzyme immunoassays specific for human interferon (IFN) omega 1: evidence that IFN-omega 1 is a component of human leukocyte IFN.
        Virology. 1990; 175: 410-417
        • Adolf G.R.
        • Maurer-Fogy I.
        • Kalsner I.
        • Cantell K.
        Purification and characterization of natural human interferon omega 1. Two alternative cleavage sites for the signal peptidase.
        J Biol Chem. 1990; 265: 9290-9295
        • Flores I.
        • Mariano T.M.
        • Pestka S.
        Human interferon omega binds to the alpha/beta receptor.
        J Biol Chem. 1991; 266: 19875-19877
        • Sheppard P.
        • Kindsvogel W.
        • Xu W.
        • Henderson K.
        • Schlutsmeyer S.
        • Whitmore T.E.
        • et al.
        IL-28, IL-29 and their class II cytokine receptor IL-28R.
        Nat Immunol. 2003; 4: 63-68
        • Kotenko S.V.
        • Gallagher G.
        • Baurin V.V.
        • Lewis-Antes A.
        • Shen M.
        • Shah N.K.
        • et al.
        IFN-lambdas mediate antiviral protection through a distinct class II cytokine receptor complex.
        Nat Immunol. 2003; 4: 69-77
        • Mennechet F.J.
        • Uze G.
        Interferon-lambda-treated dendritic cells specifically induce proliferation of FOXP3-expressing suppressor T cells.
        Blood. 2006; 107: 4417-4423
        • Konforte D.
        • Simard N.
        • Paige C.J.
        IL-21: an executor of B cell fate.
        J Immunol. 2009; 182: 1781-1787
        • Parrish-Novak J.
        • Dillon S.R.
        • Nelson A.
        • Hammond A.
        • Sprecher C.
        • Gross J.A.
        • et al.
        Interleukin 21 and its receptor are involved in NK cell expansion and regulation of lymphocyte function.
        Nature. 2000; 408: 57-63
        • Gross J.A.
        • Johnston J.
        • Mudri S.
        • Enselman R.
        • Dillon S.R.
        • Madden K.
        • et al.
        TACI and BCMA are receptors for a TNF homologue implicated in B-cell autoimmune disease.
        Nature. 2000; 404: 995-999
        • Thompson J.S.
        • Bixler S.A.
        • Qian F.
        • Vora K.
        • Scott M.L.
        • Cachero T.G.
        • et al.
        BAFF-R, a newly identified TNF receptor that specifically interacts with BAFF.
        Science. 2001; 293: 2108-2111
        • Castigli E.
        • Wilson S.A.
        • Garibyan L.
        • Rachid R.
        • Bonilla F.
        • Schneider L.
        • et al.
        TACI is mutant in common variable immunodeficiency and IgA deficiency.
        Nat Genet. 2005; 37: 829-834
        • Salzer U.
        • Chapel H.M.
        • Webster A.D.
        • Pan-Hammarstrom Q.
        • Schmitt-Graeff A.
        • Schlesier M.
        • et al.
        Mutations in TNFRSF13B encoding TACI are associated with common variable immunodeficiency in humans.
        Nat Genet. 2005; 37: 820-828
        • Finkelman F.D.
        • Holmes J.
        • Katona I.M.
        • Urban Jr., J.F.
        • Beckmann M.P.
        • Park L.S.
        • et al.
        Lymphokine control of in vivo immunoglobulin isotype selection.
        Annu Rev Immunol. 1990; 8: 303-333
        • Thornton A.M.
        • Donovan E.E.
        • Piccirillo C.A.
        • Shevach E.M.
        Cutting edge: IL-2 is critically required for the in vitro activation of CD4 + CD25 + T cell suppressor function.
        J Immunol. 2004; 172: 6519-6523
        • Farrar M.A.
        • Schreiber R.D.
        The molecular cell biology of interferon-gamma and its receptor.
        Annu Rev Immunol. 1993; 11: 571-611
        • Newport M.J.
        • Huxley C.M.
        • Huston S.
        • Hawrylowicz C.M.
        • Oostra B.A.
        • Williamson R.
        • et al.
        A mutation in the interferon-gamma-receptor gene and susceptibility to mycobacterial infection.
        N Engl J Med. 1996; 335: 1941-1949
        • Bach E.A.
        • Aguet M.
        • Schreiber R.D.
        The IFN gamma receptor: a paradigm for cytokine receptor signaling.
        Annu Rev Immunol. 1997; 15: 563-591
        • Cruikshank W.W.
        • Center D.M.
        • Nisar N.
        • Wu M.
        • Natke B.
        • Theodore A.C.
        • et al.
        Molecular and functional analysis of a lymphocyte chemoattractant factor: association of biologic function with CD4 expression.
        Proc Natl Acad Sci U S A. 1994; 91: 5109-5113
        • Wilson K.C.
        • Center D.M.
        • Cruikshank W.W.
        The effect of interleukin-16 and its precursor on T lymphocyte activation and growth.
        Growth Factors. 2004; 22: 97-104
        • Kawaguchi M.
        • Adachi M.
        • Oda N.
        • Kokubu F.
        • Huang S.K.
        IL-17 cytokine family.
        J Allergy Clin Immunol. 2004; 114: 1265-1274
        • Molet S.
        • Hamid Q.
        • Davoine F.
        • Nutku E.
        • Taha R.
        • Page N.
        • et al.
        IL-17 is increased in asthmatic airways and induces human bronchial fibroblasts to produce cytokines.
        J Allergy Clin Immunol. 2001; 108: 430-438
        • Ishigame H.
        • Kakuta S.
        • Nagai T.
        • Kadoki M.
        • Nambu A.
        • Komiyama Y.
        • et al.
        Differential roles of interleukin-17A and -17F in host defense against mucoepithelial bacterial infection and allergic responses.
        Immunity. 2009; 30: 108-119
        • Milner J.D.
        • Brenchley J.M.
        • Laurence A.
        • Freeman A.F.
        • Hill B.J.
        • Elias K.M.
        • et al.
        Impaired T(H)17 cell differentiation in subjects with autosomal dominant hyper-IgE syndrome.
        Nature. 2008; 452: 773-776
        • Ma C.S.
        • Chew G.Y.
        • Simpson N.
        • Priyadarshi A.
        • Wong M.
        • Grimbacher B.
        • et al.
        Deficiency of Th17 cells in hyper IgE syndrome due to mutations in STAT3.
        J Exp Med. 2008; 205: 1551-1557
        • Minegishi Y.
        • Saito M.
        • Nagasawa M.
        • Takada H.
        • Hara T.
        • Tsuchiya S.
        • et al.
        Molecular explanation for the contradiction between systemic Th17 defect and localized bacterial infection in hyper-IgE syndrome.
        J Exp Med. 2009; 206: 1291-1301
        • Lin H.
        • Lee E.
        • Hestir K.
        • Leo C.
        • Huang M.
        • Bosch E.
        • et al.
        Discovery of a cytokine and its receptor by functional screening of the extracellular proteome.
        Science. 2008; 320: 807-811
        • Paul W.E.
        • Ohara J.
        B-cell stimulatory factor-1/interleukin 4.
        Annu Rev Immunol. 1987; 5: 429-459
        • Coffman R.L.
        • Ohara J.
        • Bond M.W.
        • Carty J.
        • Zlotnik A.
        • Paul W.E.
        B cell stimulatory factor-1 enhances the IgE response of lipopolysaccharide-activated B cells.
        J Immunol. 1986; 136: 4538-4541
        • Romagnani S.
        Regulation and deregulation of human IgE synthesis.
        Immunol Today. 1990; 11: 316-321
        • Vella A.
        • Teague T.K.
        • Ihle J.
        • Kappler J.
        • Marrack P.
        Interleukin 4 (IL-4) or IL-7 prevents death of resting T cells: Stat-6 is probably not required for the effect of IL-4.
        J Exp Med. 1997; 186: 325-330
        • Enelow R.
        • Baramki D.F.
        • Borish L.C.
        Inhibition of effector T lymphocytes mediated through antagonism of IL-4.
        J Allergy Clin Immunol. 2004; 113: 560-562
        • Schleimer R.P.
        • Sterbinsky S.A.
        • Kaiser J.
        • Bickel C.A.
        • Klunk D.A.
        • Tomioka K.
        • et al.
        IL-4 induces adherence of human eosinophils and basophils but not neutrophils to endothelium. Association with expression of VCAM-1.
        J Immunol. 1992; 148: 1086-1092
        • Hsieh F.H.
        • Lam B.K.
        • Penrose J.F.
        • Austen K.F.
        • Boyce J.A.
        T helper cell type 2 cytokines coordinately regulate immunoglobulin E-dependent cysteinyl leukotriene production by human cord blood-derived mast cells: profound induction of leukotriene C(4) synthase expression by interleukin 4.
        J Exp Med. 2001; 193: 123-133
        • Izuhara K.
        • Shirakawa T.
        Signal transduction via the interleukin-4 receptor and its correlation with atopy.
        Int J Mol Med. 1999; 3: 3-10
        • Steinke J.W.
        • Negri J.
        • Enelow R.
        • Baramki D.F.
        • Borish L.
        Proinflammatory effects of IL-4 antagonism.
        J Allergy Clin Immunol. 2006; 118: 756-758
        • Zurawski G.
        • de Vries J.E.
        Interleukin 13, an interleukin 4-like cytokine that acts on monocytes and B cells, but not on T cells.
        Immunol Today. 1994; 15: 19-26
        • Donaldson D.D.
        • Whitters M.J.
        • Fitz L.J.
        • Neben T.Y.
        • Finnerty H.
        • Henderson S.L.
        • et al.
        The murine IL-13 receptor alpha 2: molecular cloning, characterization, and comparison with murine IL-13 receptor alpha 1.
        J Immunol. 1998; 161: 2317-2324
        • Zhu Z.
        • Homer R.J.
        • Wang Z.
        • Chen Q.
        • Geba G.P.
        • Wang J.
        • et al.
        Pulmonary expression of interleukin-13 causes inflammation, mucus hypersecretion, subepithelial fibrosis, physiologic abnormalities, and eotaxin production.
        J Clin Invest. 1999; 103: 779-788
        • Clutterbuck E.J.
        • Hirst E.M.
        • Sanderson C.J.
        Human interleukin-5 (IL-5) regulates the production of eosinophils in human bone marrow cultures: comparison and interaction with IL-1, IL-3, IL-6, and GMCSF.
        Blood. 1989; 73: 1504-1512
        • Mould A.W.
        • Ramsay A.J.
        • Matthaei K.I.
        • Young I.G.
        • Rothenberg M.E.
        • Foster P.S.
        The effect of IL-5 and eotaxin expression in the lung on eosinophil trafficking and degranulation and the induction of bronchial hyperreactivity.
        J Immunol. 2000; 164: 2142-2150
        • Rothenberg M.E.
        • Petersen J.
        • Stevens R.L.
        • Silberstein D.S.
        • McKenzie D.T.
        • Austen K.F.
        • et al.
        IL-5-dependent conversion of normodense human eosinophils to the hypodense phenotype uses 3T3 fibroblasts for enhanced viability, accelerated hypodensity, and sustained antibody-dependent cytotoxicity.
        J Immunol. 1989; 143: 2311-2316
        • Haldar P.
        • Brightling C.E.
        • Hargadon B.
        • Gupta S.
        • Monteiro W.
        • Sousa A.
        • et al.
        Mepolizumab and exacerbations of refractory eosinophilic asthma.
        N Engl J Med. 2009; 360: 973-984
        • Nair P.
        • Pizzichini M.M.
        • Kjarsgaard M.
        • Inman M.D.
        • Efthimiadis A.
        • Pizzichini E.
        • et al.
        Mepolizumab for prednisone-dependent asthma with sputum eosinophilia.
        N Engl J Med. 2009; 360: 985-993
        • Kitamura T.
        • Sato N.
        • Arai K.
        • Miyajima A.
        Expression cloning of the human IL-3 receptor cDNA reveals a shared beta subunit for the human IL-3 and GM-CSF receptors.
        Cell. 1991; 66: 1165-1174
        • Rothenberg M.E.
        • Owen Jr., W.F.
        • Silberstein D.S.
        • Woods J.
        • Soberman R.J.
        • Austen K.F.
        • et al.
        Human eosinophils have prolonged survival, enhanced functional properties, and become hypodense when exposed to human interleukin 3.
        J Clin Invest. 1988; 81: 1986-1992
        • Owen Jr., W.F.
        • Rothenberg M.E.
        • Silberstein D.S.
        • Gasson J.C.
        • Stevens R.L.
        • Austen K.F.
        • et al.
        Regulation of human eosinophil viability, density, and function by granulocyte/macrophage colony-stimulating factor in the presence of 3T3 fibroblasts.
        J Exp Med. 1987; 166: 129-141
        • Ochs M.
        • Knudsen L.
        • Allen L.
        • Stumbaugh A.
        • Levitt S.
        • Nyengaard J.R.
        • et al.
        GM-CSF mediates alveolar epithelial type II cell changes, but not emphysema-like pathology, in SP-D-deficient mice.
        Am J Physiol Lung Cell Mol Physiol. 2004; 287: L1333-L1341
        • Guth A.M.
        • Janssen W.J.
        • Bosio C.M.
        • Crouch E.C.
        • Henson P.M.
        • Dow S.W.
        Lung environment determines unique phenotype of alveolar macrophages.
        Am J Physiol Lung Cell Mol Physiol. 2009; 296: L936-L946
        • Anderson D.M.
        • Lyman S.D.
        • Baird A.
        • Wignall J.M.
        • Eisenman J.
        • Rauch C.
        • et al.
        Molecular cloning of mast cell growth factor, a hematopoietin that is active in both membrane bound and soluble forms.
        Cell. 1990; 63: 235-243
        • Matsuda H.
        • Kannan Y.
        • Ushio H.
        • Kiso Y.
        • Kanemoto T.
        • Suzuki H.
        • et al.
        Nerve growth factor induces development of connective tissue-type mast cells in vitro from murine bone marrow cells.
        J Exp Med. 1991; 174: 7-14
        • Yanagida M.
        • Fukamachi H.
        • Ohgami K.
        • Kuwaki T.
        • Ishii H.
        • Uzumaki H.
        • et al.
        Effects of T-helper 2-type cytokines, interleukin-3 (IL-3), IL-4, IL-5, and IL-6 on the survival of cultured human mast cells.
        Blood. 1995; 86: 3705-3714
        • Tsuji K.
        • Koike K.
        • Komiyama A.
        • Miyajima A.
        • Arai K.
        • Nakahata T.
        Synergistic action of interleukin-10 (IL-10) with IL-3, IL-4 and stem cell factor on colony formation from murine mast cells in culture.
        Int J Hematol. 1995; 61: 51-60
        • Godfraind C.
        • Louahed J.
        • Faulkner H.
        • Vink A.
        • Warnier G.
        • Grencis R.
        • et al.
        Intraepithelial infiltration by mast cells with both connective tissue-type and mucosal-type characteristics in gut, trachea, and kidneys of IL-9 transgenic mice.
        J Immunol. 1998; 160: 3989-3996
        • Gebhardt T.
        • Sellge G.
        • Lorentz A.
        • Raab R.
        • Manns M.P.
        • Bischoff S.C.
        Cultured human intestinal mast cells express functional IL-3 receptors and respond to IL-3 by enhancing growth and IgE receptor-dependent mediator release.
        Eur J Immunol. 2002; 32: 2308-2316
        • Hultner L.
        • Druez C.
        • Moeller J.
        • Uyttenhove C.
        • Schmitt E.
        • Rude E.
        • et al.
        Mast cell growth-enhancing activity (MEA) is structurally related and functionally identical to the novel mouse T cell growth factor P40/TCGFIII (interleukin 9).
        Eur J Immunol. 1990; 20: 1413-1416
        • Veldhoen M.
        • Uyttenhove C.
        • van Snick J.
        • Helmby H.
        • Westendorf A.
        • Buer J.
        • et al.
        Transforming growth factor-beta “reprograms” the differentiation of T helper 2 cells and promotes an interleukin 9-producing subset.
        Nat Immunol. 2008; 9: 1341-1346
        • Dardalhon V.
        • Awasthi A.
        • Kwon H.
        • Galileos G.
        • Gao W.
        • Sobel R.A.
        • et al.
        IL-4 inhibits TGF-beta-induced Foxp3 + T cells and, together with TGF-beta, generates IL-9 + IL-10 + Foxp3(-) effector T cells.
        Nat Immunol. 2008; 9: 1347-1355
        • Fort M.M.
        • Cheung J.
        • Yen D.
        • Li J.
        • Zurawski S.M.
        • Lo S.
        • et al.
        IL-25 induces IL-4, IL-5, and IL-13 and Th2-associated pathologies in vivo.
        Immunity. 2001; 15: 985-995
        • Dillon S.R.
        • Sprecher C.
        • Hammond A.
        • Bilsborough J.
        • Rosenfeld-Franklin M.
        • Presnell S.R.
        • et al.
        Interleukin 31, a cytokine produced by activated T cells, induces dermatitis in mice.
        Nat Immunol. 2004; 5: 752-760
        • Neis M.M.
        • Peters B.
        • Dreuw A.
        • Wenzel J.
        • Bieber T.
        • Mauch C.
        • et al.
        Enhanced expression levels of IL-31 correlate with IL-4 and IL-13 in atopic and allergic contact dermatitis.
        J Allergy Clin Immunol. 2006; 118: 930-937
        • Sonkoly E.
        • Muller A.
        • Lauerma A.I.
        • Pivarcsi A.
        • Soto H.
        • Kemeny L.
        • et al.
        IL-31: a new link between T cells and pruritus in atopic skin inflammation.
        J Allergy Clin Immunol. 2006; 117: 411-417
        • Sporn M.B.
        • Roberts A.B.
        Transforming growth factor-beta: recent progress and new challenges.
        J Cell Biol. 1992; 119: 1017-1021
        • Chen W.
        • Frank M.E.
        • Jin W.
        • Wahl S.M.
        TGF-beta released by apoptotic T cells contributes to an immunosuppressive milieu.
        Immunity. 2001; 14: 715-725
        • Sonoda E.
        • Matsumoto R.
        • Hitoshi Y.
        • Ishii T.
        • Sugimoto M.
        • Araki S.
        • et al.
        Transforming growth factor beta induces IgA production and acts additively with interleukin 5 for IgA production.
        J Exp Med. 1989; 170: 1415-1420
        • Kay A.B.
        • Phipps S.
        • Robinson D.S.
        A role for eosinophils in airway remodelling in asthma.
        Trends Immunol. 2004; 25: 477-482
        • Del Prete G.
        • De Carli M.
        • Almerigogna F.
        • Giudizi M.G.
        • Biagiotti R.
        • Romagnani S.
        Human IL-10 is produced by both type 1 helper (Th1) and type 2 helper (Th2) T cell clones and inhibits their antigen-specific proliferation and cytokine production.
        J Immunol. 1993; 150: 353-360
        • Ding L.
        • Linsley P.S.
        • Huang L.Y.
        • Germain R.N.
        • Shevach E.M.
        IL-10 inhibits macrophage costimulatory activity by selectively inhibiting the up-regulation of B7 expression.
        J Immunol. 1993; 151: 1224-1234
        • Taylor A.
        • Akdis M.
        • Joss A.
        • Akkoc T.
        • Wenig R.
        • Colonna M.
        • et al.
        IL-10 inhibits CD28 and ICOS costimulations of T cells via src homology 2 domain-containing protein tyrosine phosphatase 1.
        J Allergy Clin Immunol. 2007; 120: 76-83
        • Borish L.
        • Aarons A.
        • Rumbyrt J.
        • Cvietusa P.
        • Negri J.
        • Wenzel S.
        Interleukin-10 regulation in normal subjects and patients with asthma.
        J Allergy Clin Immunol. 1996; 97: 1288-1296
        • Conti P.
        • Kempuraj D.
        • Frydas S.
        • Kandere K.
        • Boucher W.
        • Letourneau R.
        • et al.
        IL-10 subfamily members: IL-19, IL-20, IL-22, IL-24 and IL-26.
        Immunol Lett. 2003; 88: 171-174
        • Liao S.C.
        • Cheng Y.C.
        • Wang Y.C.
        • Wang C.W.
        • Yang S.M.
        • Yu C.K.
        • et al.
        IL-19 induced Th2 cytokines and was up-regulated in asthma patients.
        J Immunol. 2004; 173: 6712-6718
        • Blumberg H.
        • Conklin D.
        • Xu W.F.
        • Grossmann A.
        • Brender T.
        • Carollo S.
        • et al.
        Interleukin 20: discovery, receptor identification, and role in epidermal function.
        Cell. 2001; 104: 9-19
        • Wolk K.
        • Kunz S.
        • Witte E.
        • Friedrich M.
        • Asadullah K.
        • Sabat R.
        IL-22 increases the innate immunity of tissues.
        Immunity. 2004; 21: 241-254
        • Wolk K.
        • Witte E.
        • Hoffmann U.
        • Doecke W.D.
        • Endesfelder S.
        • Asadullah K.
        • et al.
        IL-22 induces lipopolysaccharide-binding protein in hepatocytes: a potential systemic role of IL-22 in Crohn's disease.
        J Immunol. 2007; 178: 5973-5981
        • Ikeuchi H.
        • Kuroiwa T.
        • Hiramatsu N.
        • Kaneko Y.
        • Hiromura K.
        • Ueki K.
        • et al.
        Expression of interleukin-22 in rheumatoid arthritis: potential role as a proinflammatory cytokine.
        Arthritis Rheum. 2005; 52: 1037-1046
        • Whittington H.A.
        • Armstrong L.
        • Uppington K.M.
        • Millar A.B.
        Interleukin-22: a potential immunomodulatory molecule in the lung.
        Am J Respir Cell Mol Biol. 2004; 31: 220-226
        • Mumm J.B.
        • Ekmekcioglu S.
        • Poindexter N.J.
        • Chada S.
        • Grimm E.A.
        Soluble human MDA-7/IL-24: characterization of the molecular form(s) inhibiting tumor growth and stimulating monocytes.
        J Interferon Cytokine Res. 2006; 26: 877-886
        • Zheng M.
        • Bocangel D.
        • Doneske B.
        • Mhashilkar A.
        • Ramesh R.
        • Hunt K.K.
        • et al.
        Human interleukin 24 (MDA-7/IL-24) protein kills breast cancer cells via the IL-20 receptor and is antagonized by IL-10.
        Cancer Immunol Immunother. 2007; 56: 205-215
        • Hor S.
        • Pirzer H.
        • Dumoutier L.
        • Bauer F.
        • Wittmann S.
        • Sticht H.
        • et al.
        The T-cell lymphokine interleukin-26 targets epithelial cells through the interleukin-20 receptor 1 and interleukin-10 receptor 2 chains.
        J Biol Chem. 2004; 279: 33343-33351
        • Niedbala W.
        • Wei X.Q.
        • Cai B.
        • Hueber A.J.
        • Leung B.P.
        • McInnes I.B.
        • et al.
        IL-35 is a novel cytokine with therapeutic effects against collagen-induced arthritis through the expansion of regulatory T cells and suppression of Th17 cells.
        Eur J Immunol. 2007; 37: 3021-3029
        • Collison L.W.
        • Workman C.J.
        • Kuo T.T.
        • Boyd K.
        • Wang Y.
        • Vignali K.M.
        • et al.
        The inhibitory cytokine IL-35 contributes to regulatory T-cell function.
        Nature. 2007; 450: 566-569
        • Mosmann T.R.
        • Coffman R.L.
        TH1 and TH2 cells: different patterns of lymphokine secretion lead to different functional properties.
        Annu Rev Immunol. 1989; 7: 145-173
        • Manetti R.
        • Parronchi P.
        • Giudizi M.G.
        • Piccinni M.P.
        • Maggi E.
        • Trinchieri G.
        • et al.
        Natural killer cell stimulatory factor (interleukin 12 [IL-12]) induces T helper type 1 (Th1)-specific immune responses and inhibits the development of IL-4-producing Th cells.
        J Exp Med. 1993; 177: 1199-1204
        • Villarino A.V.
        • Huang E.
        • Hunter C.A.
        Understanding the pro- and anti-inflammatory properties of IL-27.
        J Immunol. 2004; 173: 715-720
        • Seder R.A.
        • Paul W.E.
        • Davis M.M.
        Fazekas de St Groth B. The presence of interleukin 4 during in vitro priming determines the lymphokine-producing potential of CD4 + T cells from T cell receptor transgenic mice.
        J Exp Med. 1992; 176: 1091-1098
        • Bilenki L.
        • Yang J.
        • Fan Y.
        • Wang S.
        • Yang X.
        Natural killer T cells contribute to airway eosinophilic inflammation induced by ragweed through enhanced IL-4 and eotaxin production.
        Eur J Immunol. 2004; 34: 345-354
        • Akbari O.
        • Faul J.L.
        • Hoyte E.G.
        • Berry G.J.
        • Wahlstrom J.
        • Kronenberg M.
        • et al.
        CD4+ invariant T-cell-receptor+ natural killer T cells in bronchial asthma.
        N Engl J Med. 2006; 354: 1117-1129
        • Gallagher G.
        • Eskdale J.
        • Jordan W.
        • Peat J.
        • Campbell J.
        • Boniotto M.
        • et al.
        Human interleukin-19 and its receptor: a potential role in the induction of Th2 responses.
        Int Immunopharmacol. 2004; 4: 615-626
        • Tamachi T.
        • Maezawa Y.
        • Ikeda K.
        • Kagami S.
        • Hatano M.
        • Seto Y.
        • et al.
        IL-25 enhances allergic airway inflammation by amplifying a TH2 cell-dependent pathway in mice.
        J Allergy Clin Immunol. 2006; 118: 606-614
        • Schmitz J.
        • Owyang A.
        • Oldham E.
        • Song Y.
        • Murphy E.
        • McClanahan T.K.
        • et al.
        IL-33, an interleukin-1-like cytokine that signals via the IL-1 receptor-related protein ST2 and induces T helper type 2-associated cytokines.
        Immunity. 2005; 23: 479-490
        • Coyle A.J.
        • Lloyd C.
        • Tian J.
        • Nguyen T.
        • Erikkson C.
        • Wang L.
        • et al.
        Crucial role of the interleukin 1 receptor family member T1/ST2 in T helper cell type 2-mediated lung mucosal immune responses.
        J Exp Med. 1999; 190: 895-902
        • Xu D.
        • Chan W.L.
        • Leung B.P.
        • Huang F.
        • Wheeler R.
        • Piedrafita D.
        • et al.
        Selective expression of a stable cell surface molecule on type 2 but not type 1 helper T cells.
        J Exp Med. 1998; 187: 787-794
        • Lohning M.
        • Stroehmann A.
        • Coyle A.J.
        • Grogan J.L.
        • Lin S.
        • Gutierrez-Ramos J.C.
        • et al.
        T1/ST2 is preferentially expressed on murine Th2 cells, independent of interleukin 4, interleukin 5, and interleukin 10, and important for Th2 effector function.
        Proc Natl Acad Sci U S A. 1998; 95: 6930-6935
        • Wang Y.H.
        • Ito T.
        • Homey B.
        • Watanabe N.
        • Martin R.
        • Barnes C.J.
        • et al.
        Maintenance and polarization of human TH2 central memory T cells by thymic stromal lymphopoietin-activated dendritic cells.
        Immunity. 2006; 24: 827-838
        • Al-Shami A.
        • Spolski R.
        • Kelly J.
        • Keane-Myers A.
        • Leonard W.J.
        A role for TSLP in the development of inflammation in an asthma model.
        J Exp Med. 2005; 202: 829-839
        • Zhou B.
        • Comeau M.R.
        • De Smedt T.
        • Liggitt H.D.
        • Dahl M.E.
        • Lewis D.B.
        • et al.
        Thymic stromal lymphopoietin as a key initiator of allergic airway inflammation in mice.
        Nat Immunol. 2005; 6: 1047-1053
        • Yoo J.
        • Omori M.
        • Gyarmati D.
        • Zhou B.
        • Aye T.
        • Brewer A.
        • et al.
        Spontaneous atopic dermatitis in mice expressing an inducible thymic stromal lymphopoietin transgene specifically in the skin.
        J Exp Med. 2005; 202: 541-549
        • Soumelis V.
        • Reche P.A.
        • Kanzler H.
        • Yuan W.
        • Edward G.
        • Homey B.
        • et al.
        Human epithelial cells trigger dendritic cell mediated allergic inflammation by producing TSLP.
        Nat Immunol. 2002; 3: 673-680
        • Dong C.
        Diversification of T-helper-cell lineages: finding the family root of IL-17-producing cells.
        Nat Rev Immunol. 2006; 6: 329-333
        • Iwakura Y.
        • Ishigame H.
        The IL-23/IL-17 axis in inflammation.
        J Clin Invest. 2006; 116: 1218-1222
        • Mangan P.R.
        • Harrington L.E.
        • O'Quinn D.B.
        • Helms W.S.
        • Bullard D.C.
        • Elson C.O.
        • et al.
        Transforming growth factor-beta induces development of the T(H)17 lineage.
        Nature. 2006; 441: 231-234
        • Bettelli E.
        • Carrier Y.
        • Gao W.
        • Korn T.
        • Strom T.B.
        • Oukka M.
        • et al.
        Reciprocal developmental pathways for the generation of pathogenic effector TH17 and regulatory T cells.
        Nature. 2006; 441: 235-238
        • Holland S.M.
        • DeLeo F.R.
        • Elloumi H.Z.
        • Hsu A.P.
        • Uzel G.
        • Brodsky N.
        • et al.
        STAT3 mutations in the hyper-IgE syndrome.
        N Engl J Med. 2007; 357: 1608-1619
        • Minegishi Y.
        • Saito M.
        • Tsuchiya S.
        • Tsuge I.
        • Takada H.
        • Hara T.
        • et al.
        Dominant-negative mutations in the DNA-binding domain of STAT3 cause hyper-IgE syndrome.
        Nature. 2007; 448: 1058-1062
        • Sakaguchi S.
        Regulatory T cells: key controllers of immunologic self-tolerance.
        Cell. 2000; 101: 455-458
        • Akdis C.A.
        • Blesken T.
        • Akdis M.
        • Wuthrich B.
        • Blaser K.
        Role of interleukin 10 in specific immunotherapy.
        J Clin Invest. 1998; 102: 98-106
        • Francis J.N.
        • Till S.J.
        • Durham S.R.
        Induction of IL-10 + CD4 + CD25 + T cells by grass pollen immunotherapy.
        J Allergy Clin Immunol. 2003; 111: 1255-1261
        • Darnell Jr., J.E.
        • Kerr I.M.
        • Stark G.R.
        Jak-STAT pathways and transcriptional activation in response to IFNs and other extracellular signaling proteins.
        Science. 1994; 264: 1415-1421
        • Ihle J.N.
        • Witthuhn B.A.
        • Quelle F.W.
        • Yamamoto K.
        • Thierfelder W.E.
        • Kreider B.
        • et al.
        Signaling by the cytokine receptor superfamily: JAKs and STATs.
        Trends Biochem Sci. 1994; 19: 222-227
        • Ghislain J.J.
        • Wong T.
        • Nguyen M.
        • Fish E.N.
        The interferon-inducible Stat2:Stat1 heterodimer preferentially binds in vitro to a consensus element found in the promoters of a subset of interferon-stimulated genes.
        J Interferon Cytokine Res. 2001; 21: 379-388
        • Dumoutier L.
        • Lejeune D.
        • Hor S.
        • Fickenscher H.
        • Renauld J.C.
        Cloning of a new type II cytokine receptor activating signal transducer and activator of transcription (STAT)1, STAT2 and STAT3.
        Biochem J. 2003; 370: 391-396
        • Dumoutier L.
        • Tounsi A.
        • Michiels T.
        • Sommereyns C.
        • Kotenko S.V.
        • Renauld J.C.
        Role of the interleukin (IL)-28 receptor tyrosine residues for antiviral and antiproliferative activity of IL-29/interferon-lambda 1: similarities with type I interferon signaling.
        J Biol Chem. 2004; 279: 32269-32274
        • Takeda K.
        • Tanaka T.
        • Shi W.
        • Matsumoto M.
        • Minami M.
        • Kashiwamura S.
        • et al.
        Essential role of Stat6 in IL-4 signalling.
        Nature. 1996; 380: 627-630
        • Losman J.A.
        • Chen X.P.
        • Hilton D.
        • Rothman P.
        Cutting edge: SOCS-1 is a potent inhibitor of IL-4 signal transduction.
        J Immunol. 1999; 162: 3770-3774
        • Moser B.
        • Loetscher P.
        Lymphocyte traffic control by chemokines.
        Nat Immunol. 2001; 2: 123-128
        • Zlotnik A.
        • Yoshie O.
        Chemokines: a new classification system and their role in immunity.
        Immunity. 2000; 12: 121-127
        • Kelner G.S.
        • Kennedy J.
        • Bacon K.B.
        • Kleyensteuber S.
        • Largaespada D.A.
        • Jenkins N.A.
        • et al.
        Lymphotactin: a cytokine that represents a new class of chemokine.
        Science. 1994; 266: 1395-1399
        • Bazan J.F.
        • Bacon K.B.
        • Hardiman G.
        • Wang W.
        • Soo K.
        • Rossi D.
        • et al.
        A new class of membrane-bound chemokine with a CX3C motif.
        Nature. 1997; 385: 640-644
        • Charo I.F.
        • Myers S.J.
        • Herman A.
        • Franci C.
        • Connolly A.J.
        • Coughlin S.R.
        Molecular cloning and functional expression of two monocyte chemoattractant protein-1 receptors reveals alternative splicing of the carboxy-terminal tails.
        Proc Natl Acad Sci U S A. 1994; 91: 2752-2756
        • Ponath D.P.
        • Qin S.
        • Post T.W.
        • Wang J.
        • Wu L.
        • Gerard N.P.
        • et al.
        Molecular cloning and characterization of a human eotaxin receptor expressed selectively on eosinophils.
        J Exp Med. 1996; 183: 2437-2448
        • Daugherty B.L.
        • Siciliano S.J.
        • DeMartino J.A.
        • Malkowitz L.
        • Sirotina A.
        • Springer M.S.
        Cloning, expression, and characterization of the human eosinophil eotaxin receptor.
        J Exp Med. 1996; 183: 2349-2354
        • Isik N.
        • Hereld D.
        • Jin T.
        Fluorescence resonance energy transfer imaging reveals that chemokine-binding modulates heterodimers of CXCR4 and CCR5 receptors.
        PLoS ONE. 2008; 3: e3424
        • Thelen M.
        Dancing to the tune of chemokines.
        Nat Immunol. 2001; 2: 129-134
        • Ma Y.C.
        • Huang J.
        • Ali S.
        • Lowry W.
        • Huang X.Y.
        Src tyrosine kinase is a novel direct effector of G proteins.
        Cell. 2000; 102: 635-646
        • Baggiolini M.
        • Dewald B.
        • Moser B.
        Human chemokines: an update.
        Annu Rev Immunol. 1997; 15: 675-705
        • Li Z.
        • Jiang H.
        • Xie W.
        • Zhang Z.
        • Smrcka A.
        • Wu D.
        Roles of PLC-b2 and -b3 and PI3Kg in chemoattractant-mediated signal transduction.
        Science. 2000; 287: 1046-1049
        • Lopez-Ilasaca M.
        • Crespo P.
        • Pellici P.G.
        • Gutkind J.S.
        • Wetzker R.
        Linkage of G protein-coupled receptors to the MAPK signaling pathway through PI3-kinase γ.
        Science. 1997; 275: 394-397
        • Balakin K.V.
        • Ivanenkov Y.A.
        • Tkachenko S.E.
        • Kiselyov A.S.
        • Ivachtchenko A.V.
        Regulators of chemokine receptor activity as promising anticancer therapeutics.
        Curr Cancer Drug Targets. 2008; 8: 299-340
        • Pruenster M.
        • Muddle L.
        • Bombosi P.
        • Dimitrova S.
        • Zsak M.
        • Middleton J.
        • et al.
        The Duffy antigen receptor for chemokines transports chemokines and supports their promigratory activity.
        Nat Immunol. 2009; 10: 101-108
        • Zarbock A.
        • Ley K.
        Neutrophil adhesion and activation under flow.
        Microcirculation. 2009; 16: 31-42
        • Constantin G.
        • Majeed M.
        • Giagulli C.
        • Piccio L.
        • Kim J.Y.
        • Butcher E.C.
        • et al.
        Chemokines trigger immediate beta2 integrin affinity and mobility changes: differential regulation and roles in lymphocyte arrest under flow.
        Immunity. 2000; 13: 759-769
        • Luther S.A.
        • Cyster J.G.
        Chemokines as regulators of T cell differentiation.
        Nat Immunol. 2001; 2: 102-107
        • Mackay C.R.
        Chemokines: immunology's high impact factors.
        Nat Immunol. 2001; 2: 95-101
        • Breitfeld D.
        • Ohl L.
        • Kremmer E.
        • Ellwart J.
        • Sallusto F.
        • Lipp M.
        • et al.
        Follicular B helper T cells express CXC chemokine receptor 5, localize to B cell follicles, and support immunoglobulin production.
        J Exp Med. 2000; 192: 1545-1552
        • Schaerli P.
        • Willimann K.
        • Lang A.B.
        • Lipp M.
        • Loetscher P.
        • Moser B.
        CXC chemokine receptor 5 expression defines follicular homing T cells with B cell helper function.
        J Exp Med. 2000; 192: 1553-1562