Advertisement

Molecular and cellular mechanisms of food allergy and food tolerance

  • Author Footnotes
    ∗ These authors contributed equally to this work.
    R. Sharon Chinthrajah
    Footnotes
    ∗ These authors contributed equally to this work.
    Affiliations
    Department of Medicine, Stanford University School of Medicine, Stanford, Calif

    Department of Pediatrics, Stanford University School of Medicine, Stanford, Calif

    Sean N. Parker Center for Allergy & Asthma Research, Stanford University School of Medicine, Stanford, Calif
    Search for articles by this author
  • Author Footnotes
    ∗ These authors contributed equally to this work.
    Joseph D. Hernandez
    Footnotes
    ∗ These authors contributed equally to this work.
    Affiliations
    Department of Pediatrics, Stanford University School of Medicine, Stanford, Calif

    Department of Pathology, Stanford University School of Medicine, Stanford, Calif

    Sean N. Parker Center for Allergy & Asthma Research, Stanford University School of Medicine, Stanford, Calif
    Search for articles by this author
  • Scott D. Boyd
    Affiliations
    Department of Pathology, Stanford University School of Medicine, Stanford, Calif

    Sean N. Parker Center for Allergy & Asthma Research, Stanford University School of Medicine, Stanford, Calif
    Search for articles by this author
  • Author Footnotes
    ‡ These authors contributed equally to this work as co-senior authors.
    Stephen J. Galli
    Footnotes
    ‡ These authors contributed equally to this work as co-senior authors.
    Affiliations
    Department of Pathology, Stanford University School of Medicine, Stanford, Calif

    Department of Microbiology and Immunology, Stanford University School of Medicine, Stanford, Calif

    Sean N. Parker Center for Allergy & Asthma Research, Stanford University School of Medicine, Stanford, Calif
    Search for articles by this author
  • Author Footnotes
    ‡ These authors contributed equally to this work as co-senior authors.
    Kari C. Nadeau
    Correspondence
    Corresponding author: Kari C. Nadeau, MD, PhD, Division of Pulmonary and Critical Care Medicine, Department of Medicine, Sean N. Parker Center for Allergy and Asthma Research, Stanford University, Stanford University School of Medicine, 269 Campus Dr, CCSR 3215, MC 5366, Stanford, CA 94305-5101.
    Footnotes
    ‡ These authors contributed equally to this work as co-senior authors.
    Affiliations
    Department of Medicine, Stanford University School of Medicine, Stanford, Calif

    Department of Pediatrics, Stanford University School of Medicine, Stanford, Calif

    Sean N. Parker Center for Allergy & Asthma Research, Stanford University School of Medicine, Stanford, Calif
    Search for articles by this author
  • Author Footnotes
    ∗ These authors contributed equally to this work.
    ‡ These authors contributed equally to this work as co-senior authors.
      Ingestion of innocuous antigens, including food proteins, normally results in local and systemic immune nonresponsiveness in a process termed oral tolerance. Oral tolerance to food proteins is likely to be intimately linked to mechanisms that are responsible for gastrointestinal tolerance to large numbers of commensal microbes. Here we review our current understanding of the immune mechanisms responsible for oral tolerance and how perturbations in these mechanisms might promote the loss of oral tolerance and development of food allergies. Roles for the commensal microbiome in promoting oral tolerance and the association of intestinal dysbiosis with food allergy are discussed. Growing evidence supports cutaneous sensitization to food antigens as one possible mechanism leading to the failure to develop or loss of oral tolerance. A goal of immunotherapy for food allergies is to induce sustained desensitization or even true long-term oral tolerance to food allergens through mechanisms that might in part overlap with those associated with the development of natural oral tolerance.

      Key words

      Abbreviations used:

      APC (Antigen-presenting cell), DC (Dendritic cell), DNFB (2,4-Dinitrofluorobenzene), EPIT (Epicutaneous immunotherapy), Foxp3 (Forkhead box protein 3), GALT (Gut-associated lymphoid tissue), GPR (G protein–coupled receptor), M cell (Microfold cell), MLN (Mesenteric lymph node), OIT (Oral immunotherapy), OVA (Ovalbumin), SCFA (Short-chain fatty acid), SLIT (Sublingual immunotherapy), Treg (Regulatory T)

       α4β7

      An integrin expressed on lymphocytes that is shown to promote T-cell homing into gut-associated lymphoid tissues through its binding to mucosal addressin cell adhesion molecule, which is present on high endothelial venules of mucosal lymphoid organs.

       ANTIGEN-PRESENTING CELLS (APCs)

      Cells that present antigens through MHCs on their surfaces to T-cell receptors on T cells.

       Ara h 1, Ara h 2

      Proteins found in peanuts that are known to be food antigens.

       αvβ8

      A member of the integrin family of transmembrane proteins that mediates cell-cell and cell–extracellular matrix adhesion.

       B220

      A CD45 isoform and a commonly used B-cell marker predominantly expressed on all mouse B lymphocytes.

       BUTYRATE

      A short-chain fatty acid and major microbial fermentation metabolite in the lumen of the colon that has been shown to be a critical mediator of the colonic inflammatory response. Without butyrates for energy, colon epithelial cells undergo autophagy and die.

       CCR7

      A chemokine receptor involved in the adhesion and migration of immune cells. Signals mediated by this receptor regulate T-cell homeostasis in lymph nodes and facilitate DC migration (eg, from the gut to the mesenteric lymph nodes).

       CCR9

      A chemokine receptor involved in the adhesion and migration of immune cells. CCR9 has also been shown to promote the migration of T lymphocytes (T cells) to the gastrointestinal tract.

       CD11c

      A cell-surface molecule expressed on many immune cells, with especially high abundance on many dendritic cells.

       CD14

      A coreceptor for bacterial LPS and other pathogen-associated molecular patterns, such as lipoteichoic acid, which is expressed on subsets of monocytes, dendritic cells, and other hematopoietic cells.

       CD45

      A receptor-linked protein tyrosine phosphatase that is expressed on all leukocytes.

       CHOLERA TOXIN

      A highly toxic protein secreted by the bacterium Vibrio cholerae, which causes severe gastric inflammation in animals. It is often used as an adjuvant to induce an immune response in biological experiments.

       CpG SITES

      Regions of DNA where a cytosine nucleotide occurs next to a guanine nucleotide separated by only 1 phosphate. Methylation of the cytosine within CpG sites of a gene can turn the gene off through epigenetic regulation.

       CX3CR1

      A chemokine receptor involved in the adhesion and migration of immune cells. It is expressed on a subset of phagocytic cells in the small intestine.

       DENDRITIC CELLS (DCs)

      Professional antigen-presenting cells that link the innate and adaptive immune systems by capturing and then presenting antigens to T cells.

       FOLLICULAR HELPER T (TFH) CELLS

      A specific subset of effector T cells that traffic to the B-cell areas of secondary lymphoid tissues, such as through interactions mediated by the chemokine receptor CXCR5 and its ligand, CXCL13. TFH cells can regulate antigen-specific B-cell development and antibody production.

       HAPTENS

      Small molecules that elicit an immune response only when covalently bound to a large carrier, typically a protein antigen.

       IgA

      The main immunoglobulin found in mucous secretions. Secretory IgA is resistant to degradation by proteolytic enzymes in the gastrointestinal tract, where it provides protection against pathogens.

       IgE

      An antibody (immunoglobulin) associated with type 2 immunity, including allergic responses. Found only in mammals, IgE antibodies bind allergens and can help to enhance host resistance to parasites (eg, helminths and protozoans) and increase resistance to venoms in mice. When bound to allergens and FcεRI on basophils and mast cells, antigen- and IgE-induced aggregation of FcεRI can trigger release of histamine, proteases, prostaglandins, leukotrienes, chemokines, and cytokines.

       IgG4

      A subtype of immunoglobulin IgG, IgG4 can be produced in part to dampen inflammation by helping to curtail Fc receptor (FcR)–mediated processes.

       IL-5

      A major maturation and differentiation cytokine expressed by TH2 cells and eosinophils in mice and human subjects. IL-5 has been shown to play an instrumental role in eosinophilic inflammation in patients with allergic diseases.

       IL-6

      A cytokine implicated in a wide variety of inflammation-associated disease states, it is involved in the maturation of B cells and has been shown to be an endogenous pyrogen capable of inducing fever in patients with autoimmune diseases or infections.

       IL-10

      A cytokine produced primarily by monocytes and, to a lesser extent, by lymphocytes (particularly Treg cells) and mast cells that has pleiotropic effects in immunoregulation and inflammation by limiting the immune response to pathogens and thereby limiting damage to the host.

       IL-22

      A cytokine that has important functions in host defense both at mucosal surfaces and in tissue repair. It appears to be unique in that it is produced by immune cells, including T-helper cell subsets and innate lymphocytes, but acts mostly on nonhematopoietic stromal cells, in particular epithelial cells, keratinocytes, and hepatocytes.

       IL-25

      A cytokine known to be involved in mucosal immunity. It induces production of the type 2 cytokines IL-4, IL-5, and IL-13.

       IL-33

      Belonging to the IL-1 family of cytokines, IL-33 potently drives production of type 2 cytokines. It is a ligand for IL-33 receptor (IL1RL1), an IL-1 family receptor that is selectively expressed on TH2 cells and mast cells.

       INHIBITORY Fcγ RECEPTORS

      Receptors that downregulate the immune complex–mediated inflammatory responses on phagocytes and IgE- and antigen-induced activation of mast cells and basophils when cross-linked with stimulatory Fcγ receptors (FcγRs).

       INNATE LYMPHOID CELLS (ILCs)

      Innate immune cells that belong to the lymphoid lineage but cannot respond in an antigen-specific manner because they lack a B- or T-cell receptor. ILCs are a recently described group of cells with physiologic functions analogous in some ways to helper T cells and cytotoxic natural killer cells. They have a role in protective immunity and the regulation of homeostasis and inflammation. Their dysregulation has been shown to contribute to immune pathology and diseases, such as allergy and autoimmune disease.

       MHC CLASS II

      A complex that presents antigen derived from extracellular proteins to CD4+ T cells.

       MHC TETRAMERS

      Fluorescently labeled tetrameric MHC-peptide complexes that enable the direct detection, quantification, and phenotypic characterization of antigen-specific T cells by using flow cytometry.

       MICROFOLD CELLS (M CELLS)

      Specialized epithelial cells of the gastrointestinal tract that sample antigens.

       OVALBUMIN

      The most abundant protein found in egg white, ovalbumin is a well-characterized allergen used in immunologic studies in mice.

       OX40–OX40 LIGAND

      Members of the TNF superfamily expressed on a variety of cells, including activated CD4+ and CD8+ T cells. The OX40-OX40 ligand (OX40L) complex has been shown to regulate cytokine production from T cells (including differentiation to TH2 cells), antigen-presenting cells, natural killer cells, and natural killer T cells and also modulate cytokine receptor signaling. In mice Treg cells can directly inhibit the FcεRI-dependent degranulation of mast cells through cell-cell contact involving OX40-OX40L interactions between Treg cells and mast cells, respectively. The OX40-OX40L complex plays a central role in the development of multiple inflammatory and autoimmune diseases.

       PROPIONATE

      A short-chain fatty acid and a major microbial fermentation metabolite in the human gut with putative health effects that extend beyond the gut epithelium.

       RETINOIC ACID

      A metabolite derived from retinol (vitamin A) that plays important roles in cell growth and differentiation, including differentiation of Treg cells.

       STAPHYLOCOCCAL ENTEROTOXIN B

      A superantigen produced by the bacterium Staphylococcus aureus that elicits cytokine release. Staphylococcal enterotoxin B–induced inflammation can promote allergic inflammation.

       TGF-β

      A cytokine secreted by many cell types, including macrophages and mast cells, which controls proliferation, cellular differentiation, and other functions in most cells. It also promotes differentiation of Treg cells and IgA-secreting B cells.

       THYMIC STROMAL LYMPHOPOIETIN (TSLP)

      A cytokine produced mainly by nonhematopoietic cells (eg, epithelial cells), which stimulates the maturation of T cells through activation of antigen-presenting cells, such as dendritic cells and macrophages.

       TYPE 1 REGULATORY (TR1) CELLS

      A subset of regulatory T cells that are Foxp3 and induced by chronic activation of CD4+ T cells by antigen in the presence of IL-10 and that mediate their suppressive effects through secretion of IL-10.

       TYPE 2 INNATE LYMPHOID CELLS (ILC2s)

      ILC2s can produce the TH2 cytokines IL-4, IL-5, IL-9, and IL-13 in response to helminth infections. They also have been implicated in the development of allergic inflammation. They require IL-7 for their development, which activates 2 transcription factors, RORα and GATA3.
      The Editors wish to acknowledge Kristina Bielewicz, MS, for preparing this glossary.
      Discuss this article on the JACI Journal Club blog: www.jaci-online.blogspot.com.
      To maintain immune tolerance, the immune system must not only be able to distinguish self from nonself antigens but also to discriminate between innocuous nonself and threatening nonself antigens. The gastrointestinal tract represents a unique challenge to the immune system in making these distinctions and in maintaining tolerance for several reasons. It is the largest interface between the body and the external environment, with the intestinal mucosa having a surface area of more than 300 m2.
      • Moog F.
      The lining of the small intestine.
      As such, it encounters a huge quantity and diversity of foreign antigens representing nonself antigens (ie, >30 kg of food proteins each year),
      • Brandtzaeg P.
      Development and basic mechanisms of human gut immunity.
      as well as the products of trillions of resident bacteria representing more than 1000 species.
      • Lozupone C.A.
      • Stombaugh J.I.
      • Gordon J.I.
      • Jansson J.K.
      • Knight R.
      Diversity, stability and resilience of the human gut microbiota.
      Maintaining tolerance requires complex interactions between nonimmune cells and cells making up the gut-associated lymphoid tissue (GALT), which contains 1012 lymphoid cells per meter of gut and more immunoglobulin-producing cells than the rest of the body.
      • Mestecky J.
      • McGhee J.R.
      Immunoglobulin A (IgA): molecular and cellular interactions involved in IgA biosynthesis and immune response.
      • van der Heijden P.J.
      • Stok W.
      • Bianchi A.T.
      Contribution of immunoglobulin-secreting cells in the murine small intestine to the total ‘background’ immunoglobulin production.
      These cells must act in concert to limit inflammatory responses to resident bacteria and food proteins that could lead to tissue injury, keep microbes confined to the gut, and recognize and respond to pathogens that can cause tissue injury or disease. Failure to achieve an appropriate balance in these roles can lead to a loss of tolerance, resulting in inflammatory diseases, such as inflammatory bowel disease, or responses to innocuous food antigens, such as those occurring in patients with celiac disease and IgE-mediated food allergies.
      This review will focus on the complex mechanisms underlying the development of natural tolerance to food antigens and how these might break down in subjects who have IgE-mediated food allergies and anaphylaxis. Also, we will discuss how experimental immunotherapeutic approaches, some of them currently in clinical trials, have the potential to restore food tolerance.

      Antigen uptake, dissemination, and presentation

      Potentially immunogenic proteins are first subject to denaturation and degradation by means of digestion in the gut. The fact that these processes might play a role in preventing sensitization to food antigens has been shown in several models. Coadministration of antacids with fish proteins to mice resulted in increased levels of IgE reactive with fish proteins and increased T-cell reactivity compared with that seen in mice administered fish proteins alone,
      • Untersmayr E.
      • Scholl I.
      • Swoboda I.
      • Beil W.J.
      • Forster-Waldl E.
      • Walter F.
      • et al.
      Antacid medication inhibits digestion of dietary proteins and causes food allergy: a fish allergy model in BALB/c mice.
      implying that acid either directly decreases protein antigenicity by denaturing the protein or indirectly influences antigenicity by affecting protein proteolysis or uptake.
      • Untersmayr E.
      • Jensen-Jarolim E.
      The role of protein digestibility and antacids on food allergy outcomes.
      Similar observations were made in a mouse model of hazelnut allergy, and there is a positive correlation in human subjects with antacid use and sensitization to food allergens.
      • Untersmayr E.
      • Bakos N.
      • Scholl I.
      • Kundi M.
      • Roth-Walter F.
      • Szalai K.
      • et al.
      Anti-ulcer drugs promote IgE formation toward dietary antigens in adult patients.
      Antigens placed in acrylic microspheres and thereby protected from both acid denaturation and enzymatic proteolysis can induce allergy in animals previously tolerant to ovalbumin (OVA).
      • Barone K.S.
      • Reilly M.R.
      • Flanagan M.P.
      • Michael J.G.
      Abrogation of oral tolerance by feeding encapsulated antigen.
      Proteins and peptides that survive denaturation and digestion in the gut can pass through the epithelial barrier through several potential mechanisms, including paracellular diffusion, transcytosis through intestinal epithelial cells, endocytosis by microfold cells (M cells), and sampling by luminal processes of CX3CR1+ cells (Fig 1, A).
      • Rescigno M.
      • Urbano M.
      • Valzasina B.
      • Francolini M.
      • Rotta G.
      • Bonasio R.
      • et al.
      Dendritic cells express tight junction proteins and penetrate gut epithelial monolayers to sample bacteria.
      • Menard S.
      • Cerf-Bensussan N.
      • Heyman M.
      Multiple facets of intestinal permeability and epithelial handling of dietary antigens.
      • Niess J.H.
      • Brand S.
      • Gu X.
      • Landsman L.
      • Jung S.
      • McCormick B.A.
      • et al.
      CX3CR1-mediated dendritic cell access to the intestinal lumen and bacterial clearance.
      • Pabst O.
      • Mowat A.M.
      Oral tolerance to food protein.
      • Mabbott N.A.
      • Donaldson D.S.
      • Ohno H.
      • Williams I.R.
      • Mahajan A.
      Microfold (M) cells: important immunosurveillance posts in the intestinal epithelium.
      Intestinal epithelial cells might also directly present antigens to T cells in the gut because they can express MHC class II on basolateral surfaces under some conditions.
      • Chehade M.
      • Mayer L.
      Oral tolerance and its relation to food hypersensitivities.
      • Hershberg R.M.
      • Cho D.H.
      • Youakim A.
      • Bradley M.B.
      • Lee J.S.
      • Framson P.E.
      • et al.
      Highly polarized HLA class II antigen processing and presentation by human intestinal epithelial cells.
      • Scott C.L.
      • Aumeunier A.M.
      • Mowat A.M.
      Intestinal CD103+ dendritic cells: master regulators of tolerance?.
      It is unclear at this point which mechanisms are the most important in promoting oral tolerance and food sensitization.
      Figure thumbnail gr1
      Fig 1Model of how the gut promotes tolerance or sensitization. Protein antigens pass through the epithelial barrier through multiple mechanisms, including capture by transluminal processes of CX3CR1+ cells. CD103+ DCs then capture antigens, migrate to the MLNs, and present antigens to naive T cells. A, In tolerance this interaction promotes the generation of Treg cells through (1) production of retinoic acid by MLNs, (2) DC expression of indoleamine 2,3-dioxygenase, (3) DC secretion of TGF-β, and (4) DC upregulation of αvβ8 to activate latent TGF-β. The gut-homing receptors CCR9 and α4β7 are upregulated on newly formed Treg cells. Retinoic acid and DC interactions also stimulate differentiation of IgA-producing B cells. AMPs, Antimicrobial peptides. B, In sensitization epithelial disruption allows increased antigen penetration and promotes production/release of epithelial cytokines (IL-33, thymic stromal lymphopoietin [TSLP], and IL-25) that upregulate OX40 ligand (OX40L) on DCs. DCs then promote differentiation of naive T cells to TH2 cells producing cytokines that recruit eosinophils (IL-5) and promote IgE class-switching in B cells (IL-4 and IL-13). IgE can facilitate antigen uptake through CD23.
      Certain specialized cells are implicated in antigen sampling from the gut through distinct mechanisms. M cells are a type of epithelial cell overlying the GALT (including Peyer patches) that have a reduced glycocalyx, irregular brush border, and reduced microvilli. M cells actively engage in phagocytosis and transcytosis of particulate antigens (including microbes) and, less efficiently, soluble macromolecules from the gut lumen.
      • Chehade M.
      • Mayer L.
      Oral tolerance and its relation to food hypersensitivities.
      • Macpherson A.J.
      • Uhr T.
      Induction of protective IgA by intestinal dendritic cells carrying commensal bacteria.
      • Sicinski P.
      • Rowinski J.
      • Warchol J.B.
      • Jarzabek Z.
      • Gut W.
      • Szczygiel B.
      • et al.
      Poliovirus type 1 enters the human host through intestinal M cells.
      Although one study demonstrated that targeting of soluble protein antigens to M cells facilitated tolerance induction to OVA,
      • Suzuki H.
      • Sekine S.
      • Kataoka K.
      • Pascual D.W.
      • Maddaloni M.
      • Kobayashi R.
      • et al.
      Ovalbumin-protein sigma 1 M-cell targeting facilitates oral tolerance with reduction of antigen-specific CD4+ T cells.
      other studies have demonstrated that tolerance to soluble antigens could be induced even in the absence of Peyer patches found beneath M cells, implying that M cell–facilitated transport to Peyer patches does not play an essential role in oral tolerance.
      • Kraus T.A.
      • Brimnes J.
      • Muong C.
      • Liu J.H.
      • Moran T.M.
      • Tappenden K.A.
      • et al.
      Induction of mucosal tolerance in Peyer's patch-deficient, ligated small bowel loops.
      • Spahn T.W.
      • Fontana A.
      • Faria A.M.
      • Slavin A.J.
      • Eugster H.P.
      • Zhang X.
      • et al.
      Induction of oral tolerance to cellular immune responses in the absence of Peyer's patches.
      • Spahn T.W.
      • Weiner H.L.
      • Rennert P.D.
      • Lugering N.
      • Fontana A.
      • Domschke W.
      • et al.
      Mesenteric lymph nodes are critical for the induction of high-dose oral tolerance in the absence of Peyer's patches.
      A population of CD11c+ myeloid cells in the lamina propria that express CX3CR1 can extend cellular processes into the intestinal lumen and sample antigens without compromising tight junctions or epithelial integrity.
      • Rescigno M.
      • Urbano M.
      • Valzasina B.
      • Francolini M.
      • Rotta G.
      • Bonasio R.
      • et al.
      Dendritic cells express tight junction proteins and penetrate gut epithelial monolayers to sample bacteria.
      • Niess J.H.
      • Brand S.
      • Gu X.
      • Landsman L.
      • Jung S.
      • McCormick B.A.
      • et al.
      CX3CR1-mediated dendritic cell access to the intestinal lumen and bacterial clearance.
      These cells do not migrate to mesenteric lymph nodes (MLNs) and cannot activate naive T cells but might pass antigens to neighboring migratory dendritic cells (DCs).
      • Pabst O.
      • Mowat A.M.
      Oral tolerance to food protein.
      • Bain C.C.
      • Mowat A.M.
      Intestinal macrophages—specialised adaptation to a unique environment.
      • Bogunovic M.
      • Ginhoux F.
      • Helft J.
      • Shang L.
      • Hashimoto D.
      • Greter M.
      • et al.
      Origin of the lamina propria dendritic cell network.
      • Schulz O.
      • Jaensson E.
      • Persson E.K.
      • Liu X.
      • Worbs T.
      • Agace W.W.
      • et al.
      Intestinal CD103+, but not CX3CR1+, antigen sampling cells migrate in lymph and serve classical dendritic cell functions.
      It has been estimated that 2% of gut luminal proteins can pass through the epithelial barrier intact
      • Warshaw A.L.
      • Walker W.A.
      • Isselbacher K.J.
      Protein uptake by the intestine: evidence for absorption of intact macromolecules.
      and then be disseminated locally or systemically through blood or lymph. Food proteins can be detected in the blood of mice and human subjects shortly after eating.
      • Husby S.
      • Jensenius J.C.
      • Svehag S.E.
      Passage of undegraded dietary antigen into the blood of healthy adults. Quantification, estimation of size distribution, and relation of uptake to levels of specific antibodies.
      • Walker W.A.
      • Isselbacher K.J.
      Uptake and transport of macromolecules by the intestine. Possible role in clinical disorders.
      Food antigens can then be presented by conventional antigen-presenting cells (APCs; eg, DCs) or unconventional APCs (eg, liver-sinusoidal endothelial cells, Kupffer cells, or plasmacytoid DCs), where, in the absence of costimulatory signals, antigen is likely to induce tolerance.
      • Pabst O.
      • Mowat A.M.
      Oral tolerance to food protein.
      • Goubier A.
      • Dubois B.
      • Gheit H.
      • Joubert G.
      • Villard-Truc F.
      • Asselin-Paturel C.
      • et al.
      Plasmacytoid dendritic cells mediate oral tolerance.
      • Thomson A.W.
      • Knolle P.A.
      Antigen-presenting cell function in the tolerogenic liver environment.
      A potential role for systemic dissemination of antigens in tolerance is supported by studies demonstrating that transfer of serum from fed mice can induce tolerance
      • Peng H.J.
      • Turner M.W.
      • Strobel S.
      The generation of a ‘tolerogen’ after the ingestion of ovalbumin is time-dependent and unrelated to serum levels of immunoreactive antigen.
      and that shunting of the portal blood flow can inhibit development of oral tolerance.
      • Callery M.P.
      • Kamei T.
      • Flye M.W.
      The effect of portacaval shunt on delayed-hypersensitivity responses following antigen feeding.
      • Yang R.
      • Liu Q.
      • Grosfeld J.L.
      • Pescovitz M.D.
      Intestinal venous drainage through the liver is a prerequisite for oral tolerance induction.
      Locally, in the gut CD11c+CD103+ DCs migrate from the lamina propria to the MLNs in a CCR7-dependent manner, carrying antigens that appear critical for the development of oral tolerance.
      • Mazzini E.
      • Massimiliano L.
      • Penna G.
      • Rescigno M.
      Oral tolerance can be established via gap junction transfer of fed antigens from CX3CR1(+) macrophages to CD103(+) dendritic cells.
      • Milling S.
      • Yrlid U.
      • Cerovic V.
      • MacPherson G.
      Subsets of migrating intestinal dendritic cells.
      • Worbs T.
      • Bode U.
      • Yan S.
      • Hoffmann M.W.
      • Hintzen G.
      • Bernhardt G.
      • et al.
      Oral tolerance originates in the intestinal immune system and relies on antigen carriage by dendritic cells.
      Inhibition of normal lymph node drainage and lymphocyte trafficking through mesenteric lymphadenectomy, small-bowel transplantation (without ligation of lymphatic vessels), or CCR7 deficiency all prevented induction of oral tolerance in mice.
      • Worbs T.
      • Bode U.
      • Yan S.
      • Hoffmann M.W.
      • Hintzen G.
      • Bernhardt G.
      • et al.
      Oral tolerance originates in the intestinal immune system and relies on antigen carriage by dendritic cells.
      How antigen uptake, dissemination, and/or presentation can differ in those with food allergy rather than food tolerance and the importance of such differences in the development of the disorder are not well understood. Patients with food allergy are known to have increased gut permeability at baseline,
      • Ventura M.T.
      • Polimeno L.
      • Amoruso A.C.
      • Gatti F.
      • Annoscia E.
      • Marinaro M.
      • et al.
      Intestinal permeability in patients with adverse reactions to food.
      which can be further exacerbated by cytokines and chemokines produced during an allergic reaction (eg, TNF-α), which can act to reduce tight junction integrity or otherwise increase gut permeability.
      • Clayburgh D.R.
      • Musch M.W.
      • Leitges M.
      • Fu Y.X.
      • Turner J.R.
      Coordinated epithelial NHE3 inhibition and barrier dysfunction are required for TNF-mediated diarrhea in vivo.
      • Perrier C.
      • Corthesy B.
      Gut permeability and food allergies.
      • Wang F.
      • Graham W.V.
      • Wang Y.
      • Witkowski E.D.
      • Schwarz B.T.
      • Turner J.R.
      Interferon-gamma and tumor necrosis factor-alpha synergize to induce intestinal epithelial barrier dysfunction by up-regulating myosin light chain kinase expression.
      In rodent models of food allergy, increased specific antigen uptake can occur through IgE binding to antigen in the lumen, followed by transcytosis through intestinal epithelia caused by the low-affinity IgE receptor CD23.
      • Yang P.C.
      • Berin M.C.
      • Yu L.C.
      • Conrad D.H.
      • Perdue M.H.
      Enhanced intestinal transepithelial antigen transport in allergic rats is mediated by IgE and CD23 (FcepsilonRII).
      • Yu L.C.
      • Yang P.C.
      • Berin M.C.
      • Di Leo V.
      • Conrad D.H.
      • McKay D.M.
      • et al.
      Enhanced transepithelial antigen transport in intestine of allergic mice is mediated by IgE/CD23 and regulated by interleukin-4.
      Notably, although CD103+ DC migration is necessary for induction of oral tolerance, adoptive transfer of CD11c+B220 splenic and Peyer patch cells (including DC populations) from mice with cow's milk allergy to naive recipient mice was sufficient to induce milk-specific IgE production.
      • Chambers S.J.
      • Bertelli E.
      • Winterbone M.S.
      • Regoli M.
      • Man A.L.
      • Nicoletti C.
      Adoptive transfer of dendritic cells from allergic mice induces specific immunoglobulin E antibody in naive recipients in absence of antigen challenge without altering the T helper 1/T helper 2 balance.
      These observations indicate that DCs are likely to play critical roles in the induction of both oral tolerance and allergic sensitization.
      Even before introduction to complementary or solid foods, infants can be exposed to food proteins (eg, in household dust) through cutaneous contact.
      • Brough H.A.
      • Simpson A.
      • Makinson K.
      • Hankinson J.
      • Brown S.
      • Douiri A.
      • et al.
      Peanut allergy: effect of environmental peanut exposure in children with filaggrin loss-of-function mutations.
      • Makinen-Kiljunen S.
      • Mussalo-Rauhamaa H.
      Casein, an important house dust allergen.
      Like the gut, the skin is one of the largest immune organs and provides not only a physical and chemical barrier but also serves as a protective immunologic barrier in maintaining immune homeostasis between the environment and the host's deeper tissues. The ability of the skin to constitute a protective barrier to environmental insults and antigen exposure is the result of a complex constellation of its properties, including: proper epidermal cell differentiation; a hydrolipidic milieu caused by lipids, sebum, sweat, and antimicrobial peptides; and features of the normal dermis (Fig 2, A).
      • Di Meglio P.
      • Perera Gayathri K.
      • Nestle Frank O.
      The multitasking organ: recent insights into skin immune function.
       Healthy skin normally represents a noninflammatory environment with numerous resident APCs. One such APC, the CD14+ DC, shares features of both CX3CR1+ and CD103+ cells in the lamina propria of the gut. It phagocytoses large quantities of antigens (similar to CX3CR1+ cells) but produces large amounts of IL-10 and effectively induces regulatory T (Treg) cell differentiation.
      • Klechevsky E.
      • Morita R.
      • Liu M.
      • Cao Y.
      • Coquery S.
      • Thompson-Snipes L.
      • et al.
      Functional specializations of human epidermal Langerhans cells and CD14+ dermal dendritic cells.
      Loss of epithelial integrity and skin inflammation can predispose subjects to allergic sensitization (Fig 2, B), as discussed below.
      Figure thumbnail gr2
      Fig 2Model of how the skin promotes tolerance or sensitization. A, Keratinocyte differentiation, an intact uppermost stratum corneum layer with epidermal proteins that maintain barrier function (eg, filaggrin), antimicrobial peptides, and tolerogenic APCs, such as CD14+ DCs producing IL-10, are important for promoting tolerance in the skin barrier. Loss of barrier function in the stratum corneum, allowing increased antigen penetration, can occur as a result of genetically determined defects in factors necessary for keratinocyte differentiation (eg, mutations in filaggrin) or as a result of inflammatory skin diseases (eg, atopic dermatitis). RA, Retinoic acid. B, In response to injury, activation by microbial or food antigens, or inflammatory signals, thymic stromal lymphopoietin (TSLP), IL-33, and/or IL-25 produced by keratinocytes can upregulate OX40L on APCs to promote TH2 differentiation.

      Oral tolerance

       DCs in oral tolerance

      It is still unclear whether there is a critical time period for encountering antigen and an ideal type of antigen exposure for the development of tolerance. Many believe that first encountering antigen at the gastrointestinal mucosa or GALT can promote tolerogenic responses to food proteins.
      • Pabst O.
      • Mowat A.M.
      Oral tolerance to food protein.
      Indeed, a recent clinical trial has indicated that earlier exposure in infancy to one potentially allergenic food (ie, peanut) can decrease rates of clinical food allergy.
      • Du Toit G.
      • Roberts G.
      • Sayre P.H.
      • Plaut M.
      • Bahnson H.T.
      • Mitchell H.
      • et al.
      Identifying infants at high risk of peanut allergy: the Learning Early About Peanut Allergy (LEAP) screening study.
      Tolerance can be driven largely by APCs within the lamina propria that sample antigens in the lumen and promote T-cell differentiation. Intriguingly, diet can influence the development of APCs in the lamina propria because mice fed an elemental diet exhibited differences in lamina propria DC subsets.
      • Kim K.S.
      • Hong S.W.
      • Han D.
      • Yi J.
      • Jung J.
      • Yang B.G.
      • et al.
      Dietary antigens limit mucosal immunity by inducing regulatory T cells in the small intestine.
      As mentioned above, mouse studies have revealed that at least 2 distinct CD11c+ APC populations exist: CX3CR1+CD103 and CX3CR1CD103+ phagocytes. CX3CR1+CD103 phagocytes are derived from monocytes and extend processes through the epithelium to sample antigens. They do not migrate or activate naive T cells, but they influence early immunologic responses to antigens and are involved in the restimulation of T cells.
      • Swiatczak B.
      • Rescigno M.
      How the interplay between antigen presenting cells and microbiota tunes host immune responses in the gut.
      In contrast, CX3CR1CD103+ DCs capture antigens in the lamina propria and migrate to draining MLNs to present antigen to T cells.
      In the MLNs migratory CD103+ DCs from the lamina propria can promote the development of gut-homing Treg cells through multiple mechanisms. CD103+ DCs produce TGF-β and retinoic acid (derived from vitamin A), driving Treg cell differentiation.
      • Coombes J.L.
      • Siddiqui K.R.
      • Arancibia-Carcamo C.V.
      • Hall J.
      • Sun C.M.
      • Belkaid Y.
      • et al.
      A functionally specialized population of mucosal CD103+ DCs induces Foxp3+ regulatory T cells via a TGF-beta and retinoic acid-dependent mechanism.
      Moreover, upregulation of αvβ8 on CD103+ DCs is important for activating latent TGF-β and generating Treg cells during the induction of tolerance to intestinal antigens in mice.
      • Paidassi H.
      • Acharya M.
      • Zhang A.
      • Mukhopadhyay S.
      • Kwon M.
      • Chow C.
      • et al.
      Preferential expression of integrin alphavbeta8 promotes generation of regulatory T cells by mouse CD103+ dendritic cells.
      • Worthington J.J.
      • Czajkowska B.I.
      • Melton A.C.
      • Travis M.A.
      Intestinal dendritic cells specialize to activate transforming growth factor-beta and induce Foxp3+ regulatory T cells via integrin alphavbeta8.
      Additionally, CD103+ DCs can express indoleamine 2,3-dioxygenase, an enzyme involved in tryptophan catabolism. Inhibition of indoleamine 2,3-dioxygenase diminishes Treg cell conversion and favors TH1 and TH17 induction.
      • Matteoli G.
      • Mazzini E.
      • Iliev I.D.
      • Mileti E.
      • Fallarino F.
      • Puccetti P.
      • et al.
      Gut CD103+ dendritic cells express indoleamine 2,3-dioxygenase which influences T regulatory/T effector cell balance and oral tolerance induction.
      Cooperation of CD103+ DCs and MLN stromal cells is important for inducing expression of gut-homing receptors on activated T cells.
      • Molenaar R.
      • Greuter M.
      • van der Marel A.P.
      • Roozendaal R.
      • Martin S.F.
      • Edele F.
      • et al.
      Lymph node stromal cells support dendritic cell-induced gut-homing of T cells.
      CD103+ DCs induce expression of the gut-homing receptors CCR9 and α4β7 on T cells primed in MLNs to facilitate migration of T cells to the small intestine.
      • Jaensson E.
      • Uronen-Hansson H.
      • Pabst O.
      • Eksteen B.
      • Tian J.
      • Coombes J.L.
      • et al.
      Small intestinal CD103+ dendritic cells display unique functional properties that are conserved between mice and humans.
      However, other mechanisms might also be capable of inducing gut tropism. In vitro activation of T cells by intestinal DCs, retinoic acid alone, and stromal cells isolated from MLNs was sufficient for induction of gut tropism. High levels of retinoic acid–producing enzymes are unique to MLNs (compared with peripheral lymph nodes) and support induction of the chemokine receptor CCR9 on activated T cells; CCR9 expression is further enhanced by bone marrow–derived DCs in vitro.
      • Hammerschmidt S.I.
      • Ahrendt M.
      • Bode U.
      • Wahl B.
      • Kremmer E.
      • Forster R.
      • et al.
      Stromal mesenteric lymph node cells are essential for the generation of gut-homing T cells in vivo.
      In addition to driving gut tropism of T cells, retinoic acid derived from GALT-associated DCs has also been shown to imprint gut tropism on B cells and to act synergistically with DC-derived IL-6 or IL-5 to induce IgA secretion; mice deprived of vitamin A and thus retinoic acid lacked IgA-secreting cells in the small intestine.
      • Mora J.R.
      • Iwata M.
      • Eksteen B.
      • Song S.Y.
      • Junt T.
      • Senman B.
      • et al.
      Generation of gut-homing IgA-secreting B cells by intestinal dendritic cells.
      Clearly, such studies indicate that retinoic acid, as derived from DCs and MLN stromal cells, is important for inducing tolerogenic responses in B and T cells and for directing these cells to the small intestine in mice.

       Treg cells in oral tolerance

      Treg cells play a central role in oral tolerance. In patients with immunodysregulation, polyendocrinopathy, enteropathy, X-linked syndrome, a rare disease linked to dysfunction of the transcription factor forkhead box protein 3 (Foxp3), which is essential for Treg cell development, there is an increased incidence of food allergies.
      • Torgerson T.R.
      • Linane A.
      • Moes N.
      • Anover S.
      • Mateo V.
      • Rieux-Laucat F.
      • et al.
      Severe food allergy as a variant of IPEX syndrome caused by a deletion in a noncoding region of the FOXP3 gene.
      Several mouse models support a role for Foxp3+ T cells in oral tolerance. In a model using the hapten 2,4-dinitrofluorobenzene (DNFB), antibody depletion of CD25+ cells (a marker for Treg cells) impaired the oral tolerance normally induced by feeding DNFB.
      • Dubois B.
      • Chapat L.
      • Goubier A.
      • Papiernik M.
      • Nicolas J.F.
      • Kaiserlian D.
      Innate CD4+CD25+ regulatory T cells are required for oral tolerance and inhibition of CD8+ T cells mediating skin inflammation.
      Transfer of CD4+CD25+ cells to CD4+ T cell–deficient mice, which do not normally have oral tolerance after DNFB feeding, is sufficient to restore oral tolerance induced by feeding.
      • Dubois B.
      • Chapat L.
      • Goubier A.
      • Papiernik M.
      • Nicolas J.F.
      • Kaiserlian D.
      Innate CD4+CD25+ regulatory T cells are required for oral tolerance and inhibition of CD8+ T cells mediating skin inflammation.
      Similarly, in a model of OVA-induced allergic diarrhea, Foxp3+ antigen-specific cells proliferated in the lamina propria during oral tolerance induction, and depletion of Foxp3+ cells abrogated oral tolerance.
      • Hadis U.
      • Wahl B.
      • Schulz O.
      • Hardtke-Wolenski M.
      • Schippers A.
      • Wagner N.
      • et al.
      Intestinal tolerance requires gut homing and expansion of FoxP3+ regulatory T cells in the lamina propria.
      Dietary antigens can promote differentiation of Treg cells and development of oral tolerance. Mice fed an elemental diet had reduced numbers of lamina propria Treg cells, increased proliferation of antigen-specific T cells on antigen feeding, and increased susceptibility to a model of allergic diarrhea when compared with control mice fed normal chow.
      • Kim K.S.
      • Hong S.W.
      • Han D.
      • Yi J.
      • Jung J.
      • Yang B.G.
      • et al.
      Dietary antigens limit mucosal immunity by inducing regulatory T cells in the small intestine.
      Although it is now appreciated that there are multiple subtypes of Treg cells
      • Sakaguchi S.
      • Yamaguchi T.
      • Nomura T.
      • Ono M.
      Regulatory T cells and immune tolerance.
      • Sakaguchi S.
      • Vignali D.A.
      • Rudensky A.Y.
      • Niec R.E.
      • Waldmann H.
      The plasticity and stability of regulatory T cells.
      and that these populations can exhibit phenotypic plasticity, the roles of individual Treg cell subtypes in oral tolerance is less well defined. Both Foxp3+ and Foxp3 Treg cells producing IL-10 can be found in the gut, and many of the Foxp3 Treg cells are likely peripherally induced type 1 regulatory cells that can produce large amounts of IL-10 and TGF-β.
      • Yang R.
      • Liu Q.
      • Grosfeld J.L.
      • Pescovitz M.D.
      Intestinal venous drainage through the liver is a prerequisite for oral tolerance induction.
      • Maynard C.L.
      • Harrington L.E.
      • Janowski K.M.
      • Oliver J.R.
      • Zindl C.L.
      • Rudensky A.Y.
      • et al.
      Regulatory T cells expressing interleukin 10 develop from Foxp3+ and Foxp3- precursor cells in the absence of interleukin 10.
      • Battaglia M.
      • Gianfrani C.
      • Gregori S.
      • Roncarolo M.G.
      IL-10-producing T regulatory type 1 cells and oral tolerance.
      In 2 models of oral tolerance to OVA, peripheral conversion of naive T cells to Foxp3+ inducible Treg cells was necessary for tolerance induction.
      • Hadis U.
      • Wahl B.
      • Schulz O.
      • Hardtke-Wolenski M.
      • Schippers A.
      • Wagner N.
      • et al.
      Intestinal tolerance requires gut homing and expansion of FoxP3+ regulatory T cells in the lamina propria.
      • Curotto de Lafaille M.A.
      • Kutchukhidze N.
      • Shen S.
      • Ding Y.
      • Yee H.
      • Lafaille J.J.
      Adaptive Foxp3+ regulatory T cell-dependent and -independent control of allergic inflammation.
      Deficiency of CCR9 or α4β7 integrin on T cells or deficiency of the α4β7 ligand mucosal addressin cell adhesion molecule 1 on gut endothelial cells inhibited Treg cell homing to the gut, which is essential for induction of oral tolerance.
      • Hadis U.
      • Wahl B.
      • Schulz O.
      • Hardtke-Wolenski M.
      • Schippers A.
      • Wagner N.
      • et al.
      Intestinal tolerance requires gut homing and expansion of FoxP3+ regulatory T cells in the lamina propria.
      • Cassani B.
      • Villablanca E.J.
      • Quintana F.J.
      • Love P.E.
      • Lacy-Hulbert A.
      • Blaner W.S.
      • et al.
      Gut-tropic T cells that express integrin alpha4beta7 and CCR9 are required for induction of oral immune tolerance in mice.
      These observations indicate that Treg cells can act locally in the gut and GALT rather than (or in addition to) acting at peripheral sites. As a prominent source of TGF-β, Treg cells that have homed to the GALT might also promote B-cell production of noninflammatory IgA.
      • Tsuji M.
      • Komatsu N.
      • Kawamoto S.
      • Suzuki K.
      • Kanagawa O.
      • Honjo T.
      • et al.
      Preferential generation of follicular B helper T cells from Foxp3+ T cells in gut Peyer's patches.

      Allergic sensitization: A breakdown in oral tolerance

      The inciting events leading to the breakdown of oral tolerance, allergic sensitization, and development of food allergies in a subset of sensitized subjects are poorly understood. It is likely that multiple pathways could ultimately lead to a failure to develop or loss of oral tolerance (Fig 1, B). Epithelial cells produce thymic stromal lymphopoietin, IL-25, and/or IL-33 in response to injury, inflammation, and innate immune activation, and these cytokines can drive TH2 inflammation.
      • Hammad H.
      • Lambrecht B.N.
      Barrier epithelial cells and the control of type 2 immunity.
      Such epithelial cytokines promote TH2 inflammation through activating innate lymphoid cells, mast cells, basophils, and DCs to produce cytokines that drive TH2 immunity.
      • Hammad H.
      • Lambrecht B.N.
      Barrier epithelial cells and the control of type 2 immunity.
      IL-33 was shown to be critical in the development of allergy in a cholera toxin–induced mouse model of peanut allergy.
      • Chu D.K.
      • Llop-Guevara A.
      • Walker T.D.
      • Flader K.
      • Goncharova S.
      • Boudreau J.E.
      • et al.
      IL-33, but not thymic stromal lymphopoietin or IL-25, is central to mite and peanut allergic sensitization.
      Intriguingly, both cholera toxin and IL-33 upregulate OX40 ligand (OX40L) on DCs, driving differentiation of naive CD4+ T cells to TH2 cells.
      • Chu D.K.
      • Llop-Guevara A.
      • Walker T.D.
      • Flader K.
      • Goncharova S.
      • Boudreau J.E.
      • et al.
      IL-33, but not thymic stromal lymphopoietin or IL-25, is central to mite and peanut allergic sensitization.
      • Blazquez A.B.
      • Berin M.C.
      Gastrointestinal dendritic cells promote Th2 skewing via OX40L.
      We are only beginning to understand the full spectrum of factors that interact to regulate epithelial cell production of cytokines in food allergy.

       More than your average food protein: A role for allergens in sensitization

      Although ample evidence supports a role for aeroallergens in promoting allergic sensitization and type 2 immunity, similar evidence for food allergens is more limited. Aeroallergens have been shown to foster allergic sensitization and production of epithelial cell cytokines through multiple mechanisms, including activation of innate pattern recognition receptors, activation of protease-activated receptors, and through direct injury to epithelia.
      • Lambrecht B.N.
      • Hammad H.
      Allergens and the airway epithelium response: gateway to allergic sensitization.
      For example, the proteolytic activity of the dust mite protein Der p 1 has been extensively studied and shown to disrupt bronchial epithelial integrity and enhance allergen uptake from the lumen,
      • Herbert C.A.
      • King C.M.
      • Ring P.C.
      • Holgate S.T.
      • Stewart G.A.
      • Thompson P.J.
      • et al.
      Augmentation of permeability in the bronchial epithelium by the house dust mite allergen Der p1.
      cleave receptors from the surfaces of immune cells (CD25 and CD23), and enhance IgE production.
      • Shakib F.
      • Schulz O.
      • Sewell H.
      A mite subversive: cleavage of CD23 and CD25 by Der p 1 enhances allergenicity.
      Among food antigens, Ara h 1 binds to CD209 on DCs, and milk sphingomyelin activates invariant natural killer T cells, effectively acting as adjuvants that enhance type 2 cytokine production.
      • Jyonouchi S.
      • Abraham V.
      • Orange J.S.
      • Spergel J.M.
      • Gober L.
      • Dudek E.
      • et al.
      Invariant natural killer T cells from children with versus without food allergy exhibit differential responsiveness to milk-derived sphingomyelin.
      • Shreffler W.G.
      • Castro R.R.
      • Kucuk Z.Y.
      • Charlop-Powers Z.
      • Grishina G.
      • Yoo S.
      • et al.
      The major glycoprotein allergen from Arachis hypogaea, Ara h 1, is a ligand of dendritic cell-specific ICAM-grabbing nonintegrin and acts as a Th2 adjuvant in vitro.
      Many have tried to identify other characteristics of proteins that promote loss of tolerance, allergic sensitization, or both. Features of proteins, such as disulfide bonds, resistance to enzymatic proteolysis or thermal degradation, biological functional activity, and protein glycosylation, can contribute to allergenicity and elicit more avid IgE binding.
      • Huby R.D.
      • Dearman R.J.
      • Kimber I.
      Why are some proteins allergens?.
      Disulfide bonds preserve protein structure and stability. For example, among aeroallergens, disrupting the disulfide bonds in the dust mite proteins Der p 1 and Lep d 2 reduced binding of IgE derived from allergic patients.
      • Smith A.M.
      • Chapman M.D.
      Reduction in IgE binding to allergen variants generated by site-directed mutagenesis: contribution of disulfide bonds to the antigenic structure of the major house dust mite allergen Der p 2.
      • Olsson S.
      • van Hage-Hamsten M.
      • Whitley P.
      Contribution of disulphide bonds to antigenicity of Lep d 2, the major allergen of the dust mite Lepidoglyphus destructor.
      Similarly, common food allergens resisted proteolysis in gastric fluid,
      • Astwood J.D.
      • Leach J.N.
      • Fuchs R.L.
      Stability of food allergens to digestion in vitro.
      although there are many proteins resistant to digestion that are not ordinarily allergenic. Sensitization to food proteins that are resistant to thermal degradation during cooking is associated with more severe and persistent milk and egg allergies,
      • Nowak-Wegrzyn A.
      • Fiocchi A.
      Rare, medium, or well done? The effect of heating and food matrix on food protein allergenicity.
      • Nowak-Wegrzyn A.
      • Bloom K.A.
      • Sicherer S.H.
      • Shreffler W.G.
      • Noone S.
      • Wanich N.
      • et al.
      Tolerance to extensively heated milk in children with cow's milk allergy.
      • Kim J.S.
      • Nowak-Węgrzyn A.
      • Sicherer S.H.
      • Noone S.
      • Moshier E.L.
      • Sampson H.A.
      Dietary baked milk accelerates the resolution of cow's milk allergy in children.
      • Leonard S.A.
      • Sampson H.A.
      • Sicherer S.H.
      • Noone S.
      • Moshier E.L.
      • Godbold J.
      • et al.
      Dietary baked egg accelerates resolution of egg allergy in children.
      whereas sensitization to heat-labile pathogenesis-related 10 proteins in patients with oral allergy syndrome is associated with symptoms that are generally more mild and rarely systemic.
      • Webber C.M.
      • England R.W.
      Oral allergy syndrome: a clinical, diagnostic, and therapeutic challenge.
      Finally, glycosylation of antigens can prevent proteolysis and form neoantigens, potentially affecting food protein allergenicity. The Maillard reaction is a nonenzymatic chemical reaction between amino acids and reducing sugars occurring at high temperatures, which leads to the “browning” of food. Studies of peanut allergenicity have shown that dry roasting can increase sensitization to peanut in mouse models.
      • Moghaddam A.E.
      • Hillson W.R.
      • Noti M.
      • Gartlan K.H.
      • Johnson S.
      • Thomas B.
      • et al.
      Dry roasting enhances peanut-induced allergic sensitization across mucosal and cutaneous routes in mice.
      Indeed, when the major peanut epitopes Ara h 1 and Ara h 2 were subjected to Maillard reactions, they bound higher levels of IgE from patients with peanut allergy and were resistant to digestion.
      • Maleki S.J.
      • Chung S.-Y.
      • Champagne E.T.
      • Raufman J.-P.
      The effects of roasting on the allergenic properties of peanut proteins.
      It might be that the allergenicity of various proteins from the environment and in food operate in concert to induce allergenic responses.

       Strange encounters: Food allergen sensitization through skin

      Atopic dermatitis is a chronic inflammatory disorder of the skin in which defects in the epidermal epithelium can lead to systemic allergen sensitization often preceding other atopic diseases, such as food allergy, asthma, and allergic rhinitis, a phenomenon known as the atopic march. Recent studies have highlighted the effect of both defective skin epithelium and the presence of food antigens in household dust in leading to sensitization to food allergens. Filaggrin is an epidermal protein involved in maintenance of skin barrier function, and patients with loss-of-function filaggrin mutations are at higher risk of eczema and atopy.
      • Palmer C.N.
      • Irvine A.D.
      • Terron-Kwiatkowski A.
      • Zhao Y.
      • Liao H.
      • Lee S.P.
      • et al.
      Common loss-of-function variants of the epidermal barrier protein filaggrin are a major predisposing factor for atopic dermatitis.
      • Marenholz I.
      • Nickel R.
      • Rüschendorf F.
      • Schulz F.
      • Esparza-Gordillo J.
      • Kerscher T.
      • et al.
      Filaggrin loss-of-function mutations predispose to phenotypes involved in the atopic march.
      Patients with loss-of-function filaggrin mutations had a higher prevalence of food sensitization and development of food allergy by age 10 years.
      • Venkataraman D.
      • Soto-Ramírez N.
      • Kurukulaaratchy R.J.
      • Holloway J.W.
      • Karmaus W.
      • Ewart S.L.
      • et al.
      Filaggrin loss-of-function mutations are associated with food allergy in childhood and adolescence.
      Given this observation, Brough et al
      • Brough H.A.
      • Simpson A.
      • Makinson K.
      • Hankinson J.
      • Brown S.
      • Douiri A.
      • et al.
      Peanut allergy: effect of environmental peanut exposure in children with filaggrin loss-of-function mutations.
      investigated exposure of peanut protein in the dust of households consuming peanut and the development of peanut allergy in patients with skin barrier defects. In children with filaggrin loss-of-function mutations, peanut allergy was 3-fold higher in 8- to 11-year-olds compared with those without filaggrin mutations. As with filaggrin, polymorphisms of SPINK5, a protease regulating keratinocyte differentiation and affecting epithelial integrity, are associated with severe atopic dermatitis and increased incidence of food allergy.
      • Kusunoki T.
      • Okafuji I.
      • Yoshioka T.
      • Saito M.
      • Nishikomori R.
      • Heike T.
      • et al.
      SPINK5 polymorphism is associated with disease severity and food allergy in children with atopic dermatitis.
      Further supporting a role for cutaneous sensitization in food allergies, increased exposure to peanut protein in dust increased the risk of peanut sensitization in children, especially those with atopic dermatitis.
      • Brough H.A.
      • Liu A.H.
      • Sicherer S.
      • Makinson K.
      • Douiri A.
      • Brown S.J.
      • et al.
      Atopic dermatitis increases the effect of exposure to peanut antigen in dust on peanut sensitization and likely peanut allergy.
      Because atopic dermatitis has been identified as an instigating event predisposing to further sensitization, many have tried to find therapies to prevent eczema and potentially food sensitization. Simply applying moisturizers or emollients from birth has been shown to reduce the development of atopic dermatitis in 2 studies,
      • Simpson E.L.
      • Chalmers J.R.
      • Hanifin J.M.
      • Thomas K.S.
      • Cork M.J.
      • McLean W.H.I.
      • et al.
      Emollient enhancement of the skin barrier from birth offers effective atopic dermatitis prevention.
      • Horimukai K.
      • Morita K.
      • Narita M.
      • Kondo M.
      • Kitazawa H.
      • Nozaki M.
      • et al.
      Application of moisturizer to neonates prevents development of atopic dermatitis.
      one of which also examined whether emollient use would influence allergic sensitization to egg but observed no differences between those receiving emollient and control groups.
      • Horimukai K.
      • Morita K.
      • Narita M.
      • Kondo M.
      • Kitazawa H.
      • Nozaki M.
      • et al.
      Application of moisturizer to neonates prevents development of atopic dermatitis.
      Larger long-term studies are needed to determine whether this approach can prevent the further development of additional atopic diseases in patients with atopic dermatitis.
      Just as some allergens can have an adjuvant effect in the respiratory and gastrointestinal tracts, some allergens can have adjuvant effects in the skin. Peanut extracts, specifically Ara h 2, were shown to have adjuvant activity in mouse skin.
      • Tordesillas L.
      • Goswami R.
      • Benede S.
      • Grishina G.
      • Dunkin D.
      • Jarvinen K.M.
      • et al.
      Skin exposure promotes a Th2-dependent sensitization to peanut allergens.
      In vitro peanut extract induced IL-33 and IL-6 expression in keratinocytes and upregulated OX40L expression on bone marrow–derived DCs, whereas in vivo peanut extract enhanced cutaneous responses to a bystander antigen, OVA, and promoted TH2 T-cell development.
      • Tordesillas L.
      • Goswami R.
      • Benede S.
      • Grishina G.
      • Dunkin D.
      • Jarvinen K.M.
      • et al.
      Skin exposure promotes a Th2-dependent sensitization to peanut allergens.
      These observations suggest that future studies examining food allergenicity and the propensity to develop clinical food allergies might need to examine both the skin and gut.

      The microbiome in tolerance and allergy

      The communities of bacteria comprising the gut microbiome are complex and dynamic. They are influenced by the environment in which subjects live, and they evolve as persons age from infancy to adulthood.
      • Yatsunenko T.
      • Rey F.E.
      • Manary M.J.
      • Trehan I.
      • Dominguez-Bello M.G.
      • Contreras M.
      • et al.
      Human gut microbiome viewed across age and geography.
      • Mackie R.I.
      • Sghir A.
      • Gaskins H.R.
      Developmental microbial ecology of the neonatal gastrointestinal tract.
      • Palmer C.
      • Bik E.M.
      • DiGiulio D.B.
      • Relman D.A.
      • Brown P.O.
      Development of the human infant intestinal microbiota.
      Living in a rural versus urban environment likely influences the composition of an individual's microbiome,
      • Yatsunenko T.
      • Rey F.E.
      • Manary M.J.
      • Trehan I.
      • Dominguez-Bello M.G.
      • Contreras M.
      • et al.
      Human gut microbiome viewed across age and geography.
      • Schnorr S.L.
      • Candela M.
      • Rampelli S.
      • Centanni M.
      • Consolandi C.
      • Basaglia G.
      • et al.
      Gut microbiome of the Hadza hunter-gatherers.
      but the underlying causes of these differences are not fully understood. An increased diversity of bacteria in household dust in farm homes inversely correlated with the risk of asthma and atopy,
      • Ege M.J.
      • Mayer M.
      • Normand A.C.
      • Genuneit J.
      • Cookson W.O.
      • Braun-Fahrlander C.
      • et al.
      Exposure to environmental microorganisms and childhood asthma.
      but it is unclear whether this was related to any differences in the subjects' microbiomes. It is possible that additional factors can contribute to variations in microbial communities between urban and rural dwellers, such as diet.
      • David L.A.
      • Weil A.
      • Ryan E.T.
      • Calderwood S.B.
      • Harris J.B.
      • Chowdhury F.
      • et al.
      Gut microbial succession follows acute secretory diarrhea in humans.
      • Wu G.D.
      • Chen J.
      • Hoffmann C.
      • Bittinger K.
      • Chen Y.Y.
      • Keilbaugh S.A.
      • et al.
      Linking long-term dietary patterns with gut microbial enterotypes.
      For instance, plant-based diets promote growth of phyla capable of fermenting plant polysaccharides.
      • David L.A.
      • Weil A.
      • Ryan E.T.
      • Calderwood S.B.
      • Harris J.B.
      • Chowdhury F.
      • et al.
      Gut microbial succession follows acute secretory diarrhea in humans.
      As factors affecting the diversity and development of the gut microbiome are being elucidated, it is also becoming clear that the microbiome can dramatically influence the development of immune responses in the gut, including those to food antigens (Fig 3).
      Figure thumbnail gr3
      Fig 3Microbial mechanisms contributing to oral tolerance and allergic sensitization in the colon. A, Microbial diversity and abundance promote tolerance. Microbes ferment fiber to produce SCFAs that bind GPRs on (1) intestinal epithelial cells to activate inflammasome production of IL-18 that promotes epithelial barrier integrity, (2) DCs to drive naive T cells to become Treg cells, and (3) Treg cells to induce proliferation. Additionally, SCFAs promote acetylation of histone H3 to preserve or induce Foxp3+ Treg cells. Microbe-induced IL-22 production by RORγt+ innate lymphocytes and CD4+ T cells promotes barrier integrity and intestinal epithelial cell synthesis of antimicrobial peptides and mucus. Tolerogenic colonic DCs and lymphocytes likely migrate to MLNs. B, In allergic sensitization changes in microbial abundance and diversity (eg, after antibiotic exposure) decrease SCFA, IL-18, and IL-22 levels, compromising epithelial integrity and thereby facilitating epithelial passage of microbial and food antigens. DC activation promotes inflammation, development of TH2 cell–associated immune responses (including production of allergen-specific IgE antibodies), and allergic sensitization.
      Data suggest that particular bacteria, most notably from the Clostridia class, can promote the development of tolerance in the gut. Colonization of antibiotic-treated mice with Clostridia-enriched microbiota prevented allergen absorption and allergic sensitization, restoring oral tolerance.
      • Stefka A.T.
      • Feehley T.
      • Tripathi P.
      • Qiu J.
      • McCoy K.
      • Mazmanian S.K.
      • et al.
      Commensal bacteria protect against food allergen sensitization.
      Clostridia can promote tolerance in the gut through several mechanisms. Colonization of germ-free mice with Clostridia-enriched microbiota promoted IgA production and Foxp3+ cell numbers in the colon.
      • Stefka A.T.
      • Feehley T.
      • Tripathi P.
      • Qiu J.
      • McCoy K.
      • Mazmanian S.K.
      • et al.
      Commensal bacteria protect against food allergen sensitization.
      IgA in the intestinal lumen (Fig 3, A) can regulate the composition of the microbiome and inhibit inflammation induced by the bacterial species and antigens to which it binds.
      • Macpherson A.J.
      • Koller Y.
      • McCoy K.D.
      The bilateral responsiveness between intestinal microbes and IgA.
      • Suzuki K.
      • Meek B.
      • Doi Y.
      • Muramatsu M.
      • Chiba T.
      • Honjo T.
      • et al.
      Aberrant expansion of segmented filamentous bacteria in IgA-deficient gut.
      Although the mechanisms underlying these changes are unclear, Clostridium species clusters IV, XIVa, and XVIII are known to promote a TGF-β– and IL-10–rich environment in the mouse colon.
      • Atarashi K.
      • Tanoue T.
      • Oshima K.
      • Suda W.
      • Nagano Y.
      • Nishikawa H.
      • et al.
      Treg induction by a rationally selected mixture of Clostridia strains from the human microbiota.
      • Atarashi K.
      • Tanoue T.
      • Shima T.
      • Imaoka A.
      • Kuwahara T.
      • Momose Y.
      • et al.
      Induction of colonic regulatory T cells by indigenous Clostridium species.
      In addition, Clostridia promote IL-22 production by RAR-related orphan receptor γ–positive innate lymphoid cells and T cells in the intestinal lamina propria, promoting epithelial integrity and upregulating expression of antimicrobial peptides, including REG3β and mucus (Fig 3, A).
      • Stefka A.T.
      • Feehley T.
      • Tripathi P.
      • Qiu J.
      • McCoy K.
      • Mazmanian S.K.
      • et al.
      Commensal bacteria protect against food allergen sensitization.
      Injection of anti–IL-22 in mice enhanced absorption of peanut antigens but did not lead to significantly increased levels of peanut-specific IgE or IgG.
      • Stefka A.T.
      • Feehley T.
      • Tripathi P.
      • Qiu J.
      • McCoy K.
      • Mazmanian S.K.
      • et al.
      Commensal bacteria protect against food allergen sensitization.
      Exactly how Clostridia promote these effects is unknown, but evidence now supports a role for bacterial metabolites in regulating epithelial integrity and immune responses in the gut. The clostridial families Lachnospiraceae and Ruminococcaceae are among prominent bacterial groups in the proximal colon that ferment dietary fiber to produce short-chain fatty acids (SCFAs), including acetate, propionic acid, and most notably butyric acid, which can have multiple effects on the immune response (Fig 3, A).
      • Berni Canani R.
      • Gilbert J.A.
      • Nagler C.R.
      The role of the commensal microbiota in the regulation of tolerance to dietary allergens.
      • Cao S.
      • Feehley T.J.
      • Nagler C.R.
      The role of commensal bacteria in the regulation of sensitization to food allergens.
      • Thorburn A.N.
      • Macia L.
      • Mackay C.R.
      Diet, metabolites, and “western-lifestyle” inflammatory diseases.
      • Vital M.
      • Howe A.C.
      • Tiedje J.M.
      Revealing the bacterial butyrate synthesis pathways by analyzing (meta)genomic data.
      SCFAs bind to the G protein–coupled receptors (GPRs) GPR43 and GPR109A on mouse enterocytes, activating the inflammasome and promoting production of IL-18; IL-18 fosters epithelial integrity, repair, and homeostasis.
      • Macia L.
      • Tan J.
      • Vieira A.T.
      • Leach K.
      • Stanley D.
      • Luong S.
      • et al.
      Metabolite-sensing receptors GPR43 and GPR109A facilitate dietary fibre-induced gut homeostasis through regulation of the inflammasome.
      • Singh N.
      • Gurav A.
      • Sivaprakasam S.
      • Brady E.
      • Padia R.
      • Shi H.
      • et al.
      Activation of Gpr109a, receptor for niacin and the commensal metabolite butyrate, suppresses colonic inflammation and carcinogenesis.
      SCFAs, particularly butyric acid, can increase numbers of colonic Foxp3+ Treg cells when administered to mice in drinking water, as an enema, or as dietary precursors.
      • Arpaia N.
      • Campbell C.
      • Fan X.
      • Dikiy S.
      • van der Veeken J.
      • deRoos P.
      • et al.
      Metabolites produced by commensal bacteria promote peripheral regulatory T-cell generation.
      • Furusawa Y.
      • Obata Y.
      • Fukuda S.
      • Endo T.A.
      • Nakato G.
      • Takahashi D.
      • et al.
      Commensal microbe-derived butyrate induces the differentiation of colonic regulatory T cells.
      • Smith P.M.
      • Howitt M.R.
      • Panikov N.
      • Michaud M.
      • Gallini C.A.
      • Bohlooly Y.M.
      • et al.
      The microbial metabolites, short-chain fatty acids, regulate colonic Treg cell homeostasis.
      Inhibition of histone deacetylase activity by SCFAs can promote acetylation of histone H3 at the Foxp3 promoter, thereby promoting Treg cell differentiation.
      • Arpaia N.
      • Campbell C.
      • Fan X.
      • Dikiy S.
      • van der Veeken J.
      • deRoos P.
      • et al.
      Metabolites produced by commensal bacteria promote peripheral regulatory T-cell generation.
      • Furusawa Y.
      • Obata Y.
      • Fukuda S.
      • Endo T.A.
      • Nakato G.
      • Takahashi D.
      • et al.
      Commensal microbe-derived butyrate induces the differentiation of colonic regulatory T cells.
      Signaling through GPR43 by propionate might promote expansion of Treg cells,
      • Smith P.M.
      • Howitt M.R.
      • Panikov N.
      • Michaud M.
      • Gallini C.A.
      • Bohlooly Y.M.
      • et al.
      The microbial metabolites, short-chain fatty acids, regulate colonic Treg cell homeostasis.
      whereas there is conflicting evidence for whether GPR109a expression on APCs has a role in promoting Treg cell differentiation.
      • Singh N.
      • Gurav A.
      • Sivaprakasam S.
      • Brady E.
      • Padia R.
      • Shi H.
      • et al.
      Activation of Gpr109a, receptor for niacin and the commensal metabolite butyrate, suppresses colonic inflammation and carcinogenesis.
      • Arpaia N.
      • Campbell C.
      • Fan X.
      • Dikiy S.
      • van der Veeken J.
      • deRoos P.
      • et al.
      Metabolites produced by commensal bacteria promote peripheral regulatory T-cell generation.
      How the gut microbiome, which is largely resident in the colon, affects immune responses in the small intestine is unclear. Possible mechanisms include migration of colonic immune cells (APCs, T cells, and B cells) to the MLNs, where they interact with cells from the small intestine or transport bacterial metabolites or cytokines to distant sites through blood or lymph, where they can exert their effects; there also might be direct effects of the less abundant microbiota in the small intestine.
      • Cao S.
      • Feehley T.J.
      • Nagler C.R.
      The role of commensal bacteria in the regulation of sensitization to food allergens.
      Consistent with observations that the gut microbiome can normally promote oral tolerance, growing evidence suggests that perturbations of the microbiome might correlate with or even predispose to food allergy (Fig 3, B). Most notably early antibiotic use in human subjects has been linked to alterations in microbiome composition and development of food allergies. Intrapartum antibiotics were associated with changes in infant microbiome composition at 3 and 12 months.
      • Azad M.B.
      • Konya T.
      • Guttman D.S.
      • Field C.J.
      • Sears M.R.
      • HayGlass K.T.
      • et al.
      Infant gut microbiota and food sensitization: associations in the first year of life.
      Similarly, other studies have shown that effects of antibiotics on human microbiome composition can be persistent, even in older subjects.
      • Dethlefsen L.
      • Huse S.
      • Sogin M.L.
      • Relman D.A.
      The pervasive effects of an antibiotic on the human gut microbiota, as revealed by deep 16S rRNA sequencing.
      • Jakobsson H.E.
      • Jernberg C.
      • Andersson A.F.
      • Sjolund-Karlsson M.
      • Jansson J.K.
      • Engstrand L.
      Short-term antibiotic treatment has differing long-term impacts on the human throat and gut microbiome.
      Dysbiosis has also been observed in neonatal mice treated with antibiotics, and these perturbations also can persist.
      • Stefka A.T.
      • Feehley T.
      • Tripathi P.
      • Qiu J.
      • McCoy K.
      • Mazmanian S.K.
      • et al.
      Commensal bacteria protect against food allergen sensitization.
      • Cox L.M.
      • Yamanishi S.
      • Sohn J.
      • Alekseyenko A.V.
      • Leung J.M.
      • Cho I.
      • et al.
      Altering the intestinal microbiota during a critical developmental window has lasting metabolic consequences.
      Maternal use of antibiotics during human pregnancy or infant antibiotic use in the first month of life is associated with increased risk of cow's milk allergy,
      • Metsala J.
      • Lundqvist A.
      • Virta L.J.
      • Kaila M.
      • Gissler M.
      • Virtanen S.M.
      Mother's and offspring's use of antibiotics and infant allergy to cow's milk.
      and higher urinary levels of triclosan are found in children sensitized to food allergens and aeroallergens.
      • Savage J.H.
      • Matsui E.C.
      • Wood R.A.
      • Keet C.A.
      Urinary levels of triclosan and parabens are associated with aeroallergen and food sensitization.
      In mice it has been similarly observed that neonatal antibiotic administration promotes allergic sensitization to peanut.
      • Stefka A.T.
      • Feehley T.
      • Tripathi P.
      • Qiu J.
      • McCoy K.
      • Mazmanian S.K.
      • et al.
      Commensal bacteria protect against food allergen sensitization.
      In one study disturbances in the microbiome occurred at doses of antibiotics of only 1/50th to 1/100th of treatment doses,
      • Cox L.M.
      • Yamanishi S.
      • Sohn J.
      • Alekseyenko A.V.
      • Leung J.M.
      • Cho I.
      • et al.
      Altering the intestinal microbiota during a critical developmental window has lasting metabolic consequences.
      raising the possibility that even exposure to low doses of antibiotics can affect human microbiome composition.
      Given the large number of other factors that can affect microbial composition, as discussed above, identification of consistent patterns of microbiome features in patients with food allergy has been difficult. In a prospective study an increased Enterobacteriaceae to Bacteroidaceae ratio and low Ruminococcaceae abundance in the context of low microbiota richness at 3 months was associated with sensitization to 1 or more foods at 12 months of age.
      • Azad M.B.
      • Konya T.
      • Guttman D.S.
      • Field C.J.
      • Sears M.R.
      • HayGlass K.T.
      • et al.
      Infant gut microbiota and food sensitization: associations in the first year of life.
      In contrast, 4-month-old infants with milk allergy identified by means of double-blind oral food challenge were found to have increased microbial diversity with an increased abundance of Ruminococcaceae and Lachnospiraceae compared with that seen in healthy age-matched control subjects.
      • Berni Canani R.
      • Sangwan N.
      • Stefka A.T.
      • Nocerino R.
      • Paparo L.
      • Aitoro R.
      • et al.
      Lactobacillus rhamnosus GG-supplemented formula expands butyrate-producing bacterial strains in food allergic infants.
      In that study the microbiota of healthy infant control subjects had lower diversity dominated by Bifidobacteriaceae, Enterobacteriaceae, and Enterococaceae.
      • Berni Canani R.
      • Sangwan N.
      • Stefka A.T.
      • Nocerino R.
      • Paparo L.
      • Aitoro R.
      • et al.
      Lactobacillus rhamnosus GG-supplemented formula expands butyrate-producing bacterial strains in food allergic infants.
      Chinese infants with both IgE- and non–IgE-mediated food allergy had similar overall microbial diversity when compared with control infants but showed alterations in particular phylotypes.
      • Ling Z.
      • Li Z.
      • Liu X.
      • Cheng Y.
      • Luo Y.
      • Tong X.
      • et al.
      Altered fecal microbiota composition associated with food allergy in infants.
      Notably, infants with IgE-mediated allergy had decreased levels of Bacteroides and Clostridium species XVIII.
      • Ling Z.
      • Li Z.
      • Liu X.
      • Cheng Y.
      • Luo Y.
      • Tong X.
      • et al.
      Altered fecal microbiota composition associated with food allergy in infants.
      Given the many factors that can influence microbiome composition in the gut, many more large longitudinal studies will be needed to identify whether reproducible patterns are associated with type I food allergy.

       Skin microbiota in patients with food allergy

      With growing evidence that food sensitization can occur through the skin, as discussed above, it is possible that dysbiosis of skin microbiomes also can contribute to food sensitization. Alterations of the skin microbiome have been observed in patients with atopic dermatitis and can drive atopic dermatitis in a mouse model;
      • Kobayashi T.
      • Glatz M.
      • Horiuchi K.
      • Kawasaki H.
      • Akiyama H.
      • Kaplan D.H.
      • et al.
      Dysbiosis and Staphylococcus aureus colonization drives inflammation in atopic dermatitis.
      • Williams M.R.
      • Gallo R.L.
      The role of the skin microbiome in atopic dermatitis.
      therefore it is reasonable to hypothesize that such changes can promote food sensitization through the skin. Staphylococcal enterotoxin B acts as an adjuvant in the skin to drive TH2 responses and follicular helper T cell development.
      • Tordesillas L.
      • Goswami R.
      • Benede S.
      • Grishina G.
      • Dunkin D.
      • Jarvinen K.M.
      • et al.
      Skin exposure promotes a Th2-dependent sensitization to peanut allergens.
      Indeed, staphylococcal enterotoxin B is used in animal models of food allergy as an oral adjuvant to promote allergic sensitization in the gut and can enhance TH2 responses to peanut in mouse skin.
      • Forbes-Blom E.
      • Camberis M.
      • Prout M.
      • Tang S.C.
      • Le Gros G.
      Staphylococcal-derived superantigen enhances peanut induced Th2 responses in the skin.
      • Ganeshan K.
      • Neilsen C.V.
      • Hadsaitong A.
      • Schleimer R.P.
      • Luo X.
      • Bryce P.J.
      Impairing oral tolerance promotes allergy and anaphylaxis: a new murine food allergy model.
      Clearly, the relationship between dysbiosis of the skin microbiota and food allergy needs to be more closely examined.

       Probiotic therapy

      Probiotic administration for the prevention or treatment of allergic disease has yielded conflicting results to date. A meta-analysis of trials of prenatal and neonatal probiotic treatment found reduced total IgE levels and atopic sensitization but no reductions in asthma or wheezing.
      • Elazab N.
      • Mendy A.
      • Gasana J.
      • Vieira E.R.
      • Quizon A.
      • Forno E.
      Probiotic administration in early life, atopy, and asthma: a meta-analysis of clinical trials.
      Few trials of probiotics in the prevention or treatment of food challenge–verified food allergies have been published. In patients with food challenge–proved cow's milk allergy, treatment with Lactobacillus casei and Bifidobacterium lactis for 12 months did not affect rates of milk allergy resolution; however, treatment with Lactobacillus rhamnosus in combination with extensively hydrolyzed casein formula increased rates of milk allergy resolution compared with those in a control group receiving hydrolyzed formula alone.
      • Berni Canani R.
      • Nocerino R.
      • Terrin G.
      • Coruzzo A.
      • Cosenza L.
      • Leone L.
      • et al.
      Effect of Lactobacillus GG on tolerance acquisition in infants with cow's milk allergy: a randomized trial.
      • Berni Canani R.
      • Nocerino R.
      • Terrin G.
      • Frediani T.
      • Lucarelli S.
      • Cosenza L.
      • et al.
      Formula selection for management of children with cow's milk allergy influences the rate of acquisition of tolerance: a prospective multicenter study.
      • Hol J.
      • van Leer E.H.
      • Elink Schuurman B.E.
      • de Ruiter L.F.
      • Samsom J.N.
      • Hop W.
      • et al.
      The acquisition of tolerance toward cow's milk through probiotic supplementation: a randomized, controlled trial.
      Notably, treatment with L rhamnosus correlated with increased levels of fecal butyrate.
      • Berni Canani R.
      • Sangwan N.
      • Stefka A.T.
      • Nocerino R.
      • Paparo L.
      • Aitoro R.
      • et al.
      Lactobacillus rhamnosus GG-supplemented formula expands butyrate-producing bacterial strains in food allergic infants.
      Similarly, coadministration of peanut oral immunotherapy (OIT) with L rhamnosus for 18 months resulted in nonresponsiveness in 82% of treated subjects at 2 to 5 weeks after cessation of OIT versus 3% of those receiving placebo.
      • Tang M.L.
      • Ponsonby A.L.
      • Orsini F.
      • Tey D.
      • Robinson M.
      • Su E.L.
      • et al.
      Administration of a probiotic with peanut oral immunotherapy: a randomized trial.
      However, because no OIT-only or probiotic-only control groups were included, it is unclear what the benefit of probiotic plus OIT would be over that of OIT alone or probiotic alone. It is likely that the benefits of probiotic supplementation are phyla specific, but insufficient data are currently available to support the use of probiotics containing particular phyla at this time. Additional data on microbiota dysbiosis in patients with food allergy will allow the design of appropriate randomized controlled trials of probiotics and prebiotics (dietary substances promoting the growth of beneficial microorganisms) in patients with food allergy. Given current data discussed above, it will be important to evaluate critically whether probiotic supplementation with specific Clostridia can have benefits for treating or preventing food allergy.

      Immunotherapy: Mechanisms of desensitization and long-term tolerance

      Multiple approaches have been attempted to regain or induce tolerance to foods. OIT was first reported by Schofield
      • Schofield A.
      A case of egg poisoning.
      in 1908, with successful desensitization through incorporation of egg into a child's diet. Note that we are using the term “desensitized” here to refer to the ability of a subject to ingest the offending food without clinical reactivity to it but requiring continued consumption of that food to maintain this state of nonreactivity. Nelson et al
      • Nelson H.S.
      • Lahr J.
      • Rule R.
      • Bock A.
      • Leung D.
      Treatment of anaphylactic sensitivity to peanuts by immunotherapy with injections of aqueous peanut extract.
      attempted subcutaneous desensitization to peanut in 1997 in a small cohort and noted significantly increased systemic reactions requiring epinephrine during buildup and maintenance periods, directing others to find alternative routes and safer approaches. Further attempts at desensitization have involved trials of OIT, sublingual immunotherapy (SLIT), or epicutaneous immunotherapy (EPIT). Although SLIT and EPIT are regarded as safer approaches compared with OIT because subjects typically only experience mild local oral and cutaneous reactions, respectively,
      • Dupont C.
      • Kalach N.
      • Soulaines P.
      • Legoué-Morillon S.
      • Piloquet H.
      • Benhamou P.-H.
      Cow's milk epicutaneous immunotherapy in children: a pilot trial of safety, acceptability, and impact on allergic reactivity.

      Dupont C. Peanut epicutaneous immunotherapy (EPIT) in peanut-allergic children: 18 months treatment in the ARACHILD Study. In: 2014 American Academy of Allergy, Asthma & Immunology Annual Meeting, San Diego, CA; 2014.

      • De Boissieu D.
      • Dupont C.
      Sublingual immunotherapy for cow's milk protein allergy: a preliminary report.
      • Kim E.H.
      • Bird J.A.
      • Kulis M.
      • Laubach S.
      • Pons L.
      • Shreffler W.
      • et al.
      Sublingual immunotherapy for peanut allergy: clinical and immunologic evidence of desensitization.
      1 subject undergoing SLIT for peanut allergy required epinephrine for urticaria and coughing.
      • Fleischer D.M.
      • Burks A.W.
      • Vickery B.P.
      • Scurlock A.M.
      • Wood R.A.
      • Jones S.M.
      • et al.
      Sublingual immunotherapy for peanut allergy: a randomized, double-blind, placebo-controlled multicenter trial.
      OIT can be done safely, but subjects experience increased reactions (the majority are mild gastrointestinal complaints) with daily dosing during buildup and maintenance periods.
      • Bégin P.
      • Winterroth L.C.
      • Dominguez T.
      • Wilson S.P.
      • Bacal L.
      • Mehrotra A.
      • et al.
      Safety and feasibility of oral immunotherapy to multiple allergens for food allergy.
      However, these approaches differ significantly in the amount of food to which the subject is effectively desensitized, with OIT achieving desensitization to serving sizes and SLIT and EPIT achieving desensitization to amounts substantially smaller than a typical serving size.

      Dupont C. Peanut epicutaneous immunotherapy (EPIT) in peanut-allergic children: 18 months treatment in the ARACHILD Study. In: 2014 American Academy of Allergy, Asthma & Immunology Annual Meeting, San Diego, CA; 2014.

      • Kim E.H.
      • Bird J.A.
      • Kulis M.
      • Laubach S.
      • Pons L.
      • Shreffler W.
      • et al.
      Sublingual immunotherapy for peanut allergy: clinical and immunologic evidence of desensitization.
      • Bégin P.
      • Winterroth L.C.
      • Dominguez T.
      • Wilson S.P.
      • Bacal L.
      • Mehrotra A.
      • et al.
      Safety and feasibility of oral immunotherapy to multiple allergens for food allergy.
      • Keet C.A.
      • Frischmeyer-Guerrerio P.A.
      • Thyagarajan A.
      • Schroeder J.T.
      • Hamilton R.G.
      • Boden S.
      • et al.
      The safety and efficacy of sublingual and oral immunotherapy for milk allergy.
      • Enrique E.
      • Pineda F.
      • Malek T.
      • Bartra J.
      • Basagaña M.
      • Tella R.
      • et al.
      Sublingual immunotherapy for hazelnut food allergy: a randomized, double-blind, placebo-controlled study with a standardized hazelnut extract.
       Recently, omalizumab, an mAb against IgE, has been explored as an adjunctive therapy with OIT, with multiple studies showing safe and faster desensitization rates to milk, peanut, or multiple foods simultaneously.
      • Nadeau K.C.
      • Schneider L.C.
      • Hoyte L.
      • Borras I.
      • Umetsu D.T.
      Rapid oral desensitization in combination with omalizumab therapy in patients with cow's milk allergy.
      • Schneider L.C.
      • Rachid R.
      • LeBovidge J.
      • Blood E.
      • Mittal M.
      • Umetsu D.T.
      A pilot study of omalizumab to facilitate rapid oral desensitization in high-risk peanut-allergic patients.
      • Begin P.
      • Dominguez T.
      • Wilson S.P.
      • Bacal L.
      • Mehrotra A.
      • Kausch B.
      • et al.
      Phase 1 results of safety and tolerability in a rush oral immunotherapy protocol to multiple foods using omalizumab.
      • Wood R.A.
      • Kim J.S.
      • Lindblad R.
      • Nadeau K.
      • Henning A.K.
      • Dawson P.
      • et al.
      A randomized, double-blind, placebo-controlled study of omalizumab combined with oral immunotherapy for the treatment of cow's milk allergy.
      Although many questions remain in optimizing OIT regarding the optimal dose of the offending food allergen to be used for maintenance, the maintenance time period, and the sustainability of the desensitization process, OIT can achieve average desensitization rates of 80% to 85%.
      • Begin P.
      • Chinthrajah R.S.
      • Nadeau K.C.
      Oral immunotherapy for the treatment of food allergy.
      Whether these therapies produce long-lasting nonreactivity to the offending allergen (either with or without the continued intentional ingestion of those allergens) has only been analyzed in a few studies. Rechallenge after varying periods of avoidance has shown rates of sustained unresponsiveness (defined here as nonreactivity to a food challenge after avoidance of the offending allergen for periods of 1 week to 6 months) ranging from 13% to 36%.
      • Begin P.
      • Chinthrajah R.S.
      • Nadeau K.C.
      Oral immunotherapy for the treatment of food allergy.
      Although these rates seem suboptimal, most subjects still maintained a state of desensitization to a threshold level higher than their screening challenge. Whether any of these patients will maintain “long-term tolerance” to that allergen, which we propose to define as experiencing years of unresponsiveness to the food in the absence of intentional ingestion of the offending allergens, remains to be seen. It also will be important to determine the extent to which the mechanisms underlying “desensitization” versus “long-term tolerance” are similar or different and to develop tests that can reliably determine the immune status of patients with food allergy.

       Early responses to antigen-specific immunotherapy

      Mechanisms of action in allergen-specific immunotherapy have been explored for allergic rhinitis and stinging insect hypersensitivity
      • Akdis M.
      • Akdis C.A.
      Mechanisms of allergen-specific immunotherapy: multiple suppressor factors at work in immune tolerance to allergens.
      and are likely to be similar in food allergy immunotherapy. Protection from reactions in the early stages of immunotherapy is associated with decreased activation of mast cells and basophils, which has been seen as early as in the first 3 to 4 months of OIT.
      • Keet C.A.
      • Frischmeyer-Guerrerio P.A.
      • Thyagarajan A.
      • Schroeder J.T.
      • Hamilton R.G.
      • Boden S.
      • et al.
      The safety and efficacy of sublingual and oral immunotherapy for milk allergy.
      • Syed A.
      • Garcia M.A.
      • Lyu S.C.
      • Bucayu R.
      • Kohli A.
      • Ishida S.
      • et al.
      Peanut oral immunotherapy results in increased antigen-induced regulatory T-cell function and hypomethylation of forkhead box protein 3 (FOXP3).
      • Burks A.W.
      • Jones S.M.
      • Wood R.A.
      • Fleischer D.M.
      • Sicherer S.H.
      • Lindblad R.W.
      • et al.
      Oral immunotherapy for treatment of egg allergy in children.
      • Jones S.M.
      • Pons L.
      • Roberts J.L.
      • Scurlock A.M.
      • Perry T.T.
      • Kulis M.
      • et al.
      Clinical efficacy and immune regulation with peanut oral immunotherapy.
      This might in part be due to reduced levels of antigen-specific IgE on the surfaces of these effector cells.
      • Khodoun M.V.
      • Kucuk Z.Y.
      • Strait R.T.
      • Krishnamurthy D.
      • Janek K.
      • Clay C.D.
      • et al.
      Rapid desensitization of mice with anti-FcgammaRIIb/FcgammaRIII mAb safely prevents IgG-mediated anaphylaxis.
      • Oka T.
      • Rios E.J.
      • Tsai M.
      • Kalesnikoff J.
      • Galli S.J.
      Rapid desensitization induces internalization of antigen-specific IgE on mouse mast cells.
      OIT in a mouse model of egg allergy did not result in desensitization of blood basophils or peritoneal mast cells or protection from challenge by means of intraperitoneal injection despite protection from oral challenge, implying that desensitization can occur locally in the gastrointestinal tract.
      • Leonard S.A.
      • Martos G.
      • Wang W.
      • Nowak-Wegrzyn A.
      • Berin M.C.
      Oral immunotherapy induces local protective mechanisms in the gastrointestinal mucosa.
      However, other mechanisms also might be involved. It has long been speculated that desensitization might deplete certain mediators of effector cells (eg, by inducing the release of histamine from granules) and stimulate leukotriene release but in amounts that are small and less than the threshold for causing anaphylaxis. In patients undergoing rush desensitization for venom allergy, decreased histamine levels were observed in whole blood, suggesting degranulation of basophils or decreased basophil numbers.
      • Jutel M.
      • Muller U.
      • Ericker M.
      • Rihs S.
      • Pichler W.
      • Dahinden C.
      Influence of bee venom immunotherapy on degranulation and leukotriene generation in human blood basophils.
      In contrast, patients undergoing standard (not rush) immunotherapy for venom allergy had normal histamine content in blood leukocytes.
      • Eberlein-Konig B.
      • Ullmann S.
      • Thomas P.
      • Przybilla B.
      Tryptase and histamine release due to a sting challenge in bee venom allergic patients treated successfully or unsuccessfully with hyposensitization*.
      Notably, mouse studies of oral desensitization for penicillin demonstrated antigen-specific desensitization with no evidence for mediator depletion.
      • Leonard S.A.
      • Martos G.
      • Wang W.
      • Nowak-Wegrzyn A.
      • Berin M.C.
      Oral immunotherapy induces local protective mechanisms in the gastrointestinal mucosa.
      • Woo H.Y.
      • Kim Y.S.
      • Kang N.I.
      • Chung W.C.
      • Song C.H.
      • Choi I.W.
      • et al.
      Mechanism for acute oral desensitization to antibiotics.
      In addition to basophils and mast cells, many other cell types can contribute to early immunotherapy responses. During venom desensitization, monocytes increased expression of immunoglobulin-like transcript 3 and 4 receptors, which are crucial to the tolerogenic function of monocytes.
      • Bussmann C.
      • Xia J.
      • Allam J.P.
      • Maintz L.
      • Bieber T.
      • Novak N.
      Early markers for protective mechanisms during rush venom immunotherapy.
      Tolerogenic APCs, such as CD103+ DCs, as described above, can aid in the differentiation of T cells into Treg cells. Oral mucosal Langerhans cells have been shown to bind grass pollen in an ex vivo model, which enhanced their migratory capacity and promoted the secretion of the tolerogenic cytokines TFG-β1 and IL-10.
      • Allam J.-P.
      • Würtzen P.A.
      • Reinartz M.
      • Winter J.
      • Vrtala S.
      • Chen K.-W.
      • et al.
      Phl p 5 resorption in human oral mucosa leads to dose-dependent and time-dependent allergen binding by oral mucosal Langerhans cells, attenuates their maturation, and enhances their migratory and TGF-β1 and IL-10–producing properties.
      Grass pollen SCIT has been shown to mitigate seasonal increases in the numbers of peripheral type 2 innate lymphoid cells,
      • Lao-Araya M.
      • Steveling E.
      • Scadding G.W.
      • Durham S.R.
      • Shamji M.H.
      Seasonal increases in peripheral innate lymphoid type 2 cells are inhibited by subcutaneous grass pollen immunotherapy.
      potent potential producers of the type 2 cytokines IL-4, IL-13, and IL-5 that can enhance inflammation in patients with asthma, allergic rhinitis, and atopic dermatitis through activation of mast cells, basophils, and eosinophils and promotion of B-cell class-switching to produce IgE antibodies.
      • Licona-Limón P.
      • Kim L.K.
      • Palm N.W.
      • Flavell R.A.
      TH2, allergy and group 2 innate lymphoid cells.
      • Salimi M.
      • Barlow J.L.
      • Saunders S.P.
      • Xue L.
      • Gutowska-Owsiak D.
      • Wang X.
      • et al.
      A role for IL-25 and IL-33–driven type-2 innate lymphoid cells in atopic dermatitis.
      • Doherty T.A.
      • Scott D.
      • Walford H.H.
      • Khorram N.
      • Lund S.
      • Baum R.
      • et al.
      Allergen challenge in allergic rhinitis rapidly induces increased peripheral blood type 2 innate lymphoid cells that express CD84.
      • Yu S.
      • Kim H.Y.
      • Chang Y.-J.
      • DeKruyff R.H.
      • Umetsu D.T.
      Innate lymphoid cells and asthma.
      Skin-derived APCs are important in directing the initial immune response. When OVA is applied to the intact skin of mice, it is taken up in the superficial layers of the stratum corneum and transported to the draining lymph nodes. After repeated epicutaneous delivery of OVA, local and systemic TH2 responses are downregulated, with associated upregulation of Treg cells.
      • Dioszeghy V.
      • Mondoulet L.
      • Dhelft V.
      • Ligouis M.
      • Puteaux E.
      • Benhamou P.-H.
      • et al.
      Epicutaneous immunotherapy results in rapid allergen uptake by dendritic cells through intact skin and downregulates the allergen-specific response in sensitized mice.
      Consistent with this observation in mice, EPIT requires an intact stratum corneum layer.
      • Mondoulet L.
      • Dioszeghy V.
      • Puteaux E.
      • Ligouis M.
      • Dhelft V.
      • Letourneur F.
      • et al.
      Intact skin and not stripped skin is crucial for the safety and efficacy of peanut epicutaneous immunotherapy (EPIT) in mice.

       T-cell responses to antigen-specific immunotherapy

      Induction of peripheral T-cell tolerance is a crucial step induced by immunotherapy, and in different models various changes in antigen-specific T-cell populations correlated with tolerance, including increased Treg cell numbers,
      • Akdis M.
      • Verhagen J.
      • Taylor A.
      • Karamloo F.
      • Karagiannidis C.
      • Crameri R.
      • et al.
      Immune responses in healthy and allergic individuals are characterized by a fine balance between allergen-specific T regulatory 1 and T helper 2 cells.
      decreased TH2 cell numbers,
      • Smaldini P.L.
      • Delgado M.L.O.
      • Fossati C.A.
      • Docena G.H.
      Orally-induced intestinal CD4+ CD25+ FoxP3+ Treg controlled undesired responses towards oral antigens and effectively dampened food allergic reactions.
      and increased anergic T-cell numbers.
      • Aslam A.
      • Chan H.
      • Warrell D.A.
      • Misbah S.
      • Ogg G.S.
      Tracking antigen-specific T-cells during clinical tolerance induction in humans.
      The proportion of allergen-specific T-cell subsets and the change in the dominant subset might skew toward allergy versus tolerance.
      • Akdis M.
      • Verhagen J.
      • Taylor A.
      • Karamloo F.
      • Karagiannidis C.
      • Crameri R.
      • et al.
      Immune responses in healthy and allergic individuals are characterized by a fine balance between allergen-specific T regulatory 1 and T helper 2 cells.
      In a cholera toxin–induced mouse model of milk allergy, treatment with milk OIT increased levels of IL-10 and TGF-β in the jejunum, which were likely produced by Treg cells within the gut.
      • Smaldini P.L.
      • Delgado M.L.O.
      • Fossati C.A.
      • Docena G.H.
      Orally-induced intestinal CD4+ CD25+ FoxP3+ Treg controlled undesired responses towards oral antigens and effectively dampened food allergic reactions.
      Using allergen-MHC tetramers to track allergen-specific T cells during the course of wasp venom immunotherapy, clinical tolerance was associated with a loss of IL-4–producing T cells and increased IL-10–producing Foxp3+ antigen-specific T cells that might share a common precursor with IL-4–producing T cells specific for the same epitope.
      • Aslam A.
      • Chan H.
      • Warrell D.A.
      • Misbah S.
      • Ogg G.S.
      Tracking antigen-specific T-cells during clinical tolerance induction in humans.
      Recently, we used peanut-MHC dextramers to sort peanut-specific T cells from patients with peanut allergy and analyzed changes in gene expression of individual CD4+ T cells during the course of OIT. Our evidence indicated that increased length of treatment with OIT induced peanut-specific T cells to shift toward an anergic, memory T-cell phenotype (CD28loKi67lo). Moreover, sustained nonresponsiveness to peanut, even after a 3-month period of withdrawal from peanut, was associated with induction and maintenance of naive and memory peanut-specific T cells that were detectable even 3 months after therapy.
      • Ryan J.F.
      • Hovde R.
      • Glanville J.
      • Lyu S.C.
      • Ji X.
      • Gupta S.
      • et al.
      Successful immunotherapy induces previously unidentified allergen-specific CD4+ T-cell subsets.
      The failure to maintain tolerance to the offending allergen might be due to the induction of Treg cells that are short lived or epigenetically modified. In our cohort of subjects who completed 24 months of peanut OIT (20/23 subjects), we have shown that 7 of 20 subjects were still “immune tolerant” after a 3-month period of withdrawal, and 3 of 7 remained “immune tolerant” after an additional 3-month withdrawal period (totaling 6 months of withdrawal from therapy).
      • Syed A.
      • Garcia M.A.
      • Lyu S.C.
      • Bucayu R.
      • Kohli A.
      • Ishida S.
      • et al.
      Peanut oral immunotherapy results in increased antigen-induced regulatory T-cell function and hypomethylation of forkhead box protein 3 (FOXP3).
      All subjects were found to have an increase in numbers of peanut-specific Treg cells after 12 months of OIT; in those who were immune tolerant, there was significant hypomethylation of CpG sites in peanut-specific Treg cells at 24 and 27 months.
      • Syed A.
      • Garcia M.A.
      • Lyu S.C.
      • Bucayu R.
      • Kohli A.
      • Ishida S.
      • et al.
      Peanut oral immunotherapy results in increased antigen-induced regulatory T-cell function and hypomethylation of forkhead box protein 3 (FOXP3).
      In the 4 subjects who “lost” their tolerant status after 6 months of peanut withdrawal, there was increased methylation of their peanut-specific Treg cells.
      • Syed A.
      • Garcia M.A.
      • Lyu S.C.
      • Bucayu R.
      • Kohli A.
      • Ishida S.
      • et al.
      Peanut oral immunotherapy results in increased antigen-induced regulatory T-cell function and hypomethylation of forkhead box protein 3 (FOXP3).
      These findings suggest that epigenetic changes of antigen-specific immune cells might, at least in part, explain desensitization and tolerance. However, achieving sustained responses to therapy might depend on whether such epigenetic changes can be maintained.

       B-cell responses to antigen-specific immunotherapy

      OIT can induce changes in immunoglobulin subsets in patients with peanut allergy. Patients undergoing peanut OIT for a median of 41 months exhibited increased levels of peanut-specific IgG4 with de novo specificities associated with reduced serum levels of peanut IgE.
      • Vickery B.P.
      • Lin J.
      • Kulis M.
      • Fu Z.
      • Steele P.H.
      • Jones S.M.
      • et al.
      Peanut oral immunotherapy modifies IgE and IgG 4 responses to major peanut allergens.
      In our own cohort of patients, peanut OIT was associated with increases in the frequency of peanut-specific B cells in the blood.
      • Hoh R.A.
      • Joshi S.A.
      • Liu Y.
      • Wang C.
      • Roskin K.M.
      • Lee J.-Y.
      • et al.
      Single B-cell deconvolution of peanut-specific antibody responses in allergic patients.
      The allergen-specific B cells were mainly of the memory phenotype, with lower numbers of plasmablasts, and predominantly expressed somatically mutated class-switched antibodies of IgG and IgA subtypes, with lower numbers of IgM-expressing cells also noted.
      • Hoh R.A.
      • Joshi S.A.
      • Liu Y.
      • Wang C.
      • Roskin K.M.
      • Lee J.-Y.
      • et al.
      Single B-cell deconvolution of peanut-specific antibody responses in allergic patients.
      Antibodies from these cells recognized both conformational and linear epitopes.
      • Hoh R.A.
      • Joshi S.A.
      • Liu Y.
      • Wang C.
      • Roskin K.M.
      • Lee J.-Y.
      • et al.
      Single B-cell deconvolution of peanut-specific antibody responses in allergic patients.
      Notably, during the course of OIT, more highly mutated IgG4-expressing members of a peanut-specific clone were observed; in contrast, the somatic mutation levels in IgE members of the clone did not increase, suggesting that ongoing somatic mutation of IgG4-expressing B cells might contribute to the increased effectiveness of peanut OIT over time, perhaps by increasing IgG4 affinity for allergen.
      • Hoh R.A.
      • Joshi S.A.
      • Liu Y.
      • Wang C.
      • Roskin K.M.
      • Lee J.-Y.
      • et al.
      Single B-cell deconvolution of peanut-specific antibody responses in allergic patients.
      Although increasing evidence supports a role for antigen-specific IgG4 in directly promoting tolerance, IgG4 levels can also correlate with other mechanisms responsible for inducing tolerance. Ratios of peanut-specific IgG4 to peanut-specific IgE were higher in sensitized (ie, positive skin test or specific IgE results) but clinically tolerant patients than in patients with clinical peanut allergy.
      • Santos A.F.
      • James L.K.
      • Bahnson H.T.
      • Shamji M.H.
      • Couto-Francisco N.C.
      • Islam S.
      • et al.
      IgG4 inhibits peanut-induced basophil and mast cell activation in peanut-tolerant children sensitized to peanut major allergens.
      Sera from sensitized but clinically tolerant patients or patients undergoing peanut OIT inhibited activation of peanut-sensitized mast cells or basophils by peanut extract, and the inhibitory activity of sera was decreased if IgG4 was depleted.
      • Santos A.F.
      • James L.K.
      • Bahnson H.T.
      • Shamji M.H.
      • Couto-Francisco N.C.
      • Islam S.
      • et al.
      IgG4 inhibits peanut-induced basophil and mast cell activation in peanut-tolerant children sensitized to peanut major allergens.
      • Burton O.T.
      • Logsdon S.L.
      • Zhou J.S.
      • Medina-Tamayo J.
      • Abdel-Gadir A.
      • Noval Rivas M.
      • et al.
      Oral immunotherapy induces IgG antibodies that act through FcgammaRIIb to suppress IgE-mediated hypersensitivity.
      The mechanisms by which IgG4 inhibited mast cell or basophil activation might include IgG4 activity as a “blocking antibody,” binding allergen before it encounters IgE bound to the surfaces of basophils or mast cells, or IgG4-dependent activation of inhibitory Fcγ receptors.
      • Akdis M.
      • Akdis C.A.
      Mechanisms of allergen-specific immunotherapy: multiple suppressor factors at work in immune tolerance to allergens.
      • Strait R.T.
      • Morris S.C.
      • Finkelman F.D.
      IgG-blocking antibodies inhibit IgE-mediated anaphylaxis in vivo through both antigen interception and Fc gamma RIIb cross-linking.
      Studies using blocking antibodies for CD32 with human basophils or in a mouse model of peanut OIT demonstrated that IgG binding to inhibitory Fcγ receptors is at least partially responsible for the observed inhibitory effects.
      • Burton O.T.
      • Logsdon S.L.
      • Zhou J.S.
      • Medina-Tamayo J.
      • Abdel-Gadir A.
      • Noval Rivas M.
      • et al.
      Oral immunotherapy induces IgG antibodies that act through FcgammaRIIb to suppress IgE-mediated hypersensitivity.
      In a study of egg OIT, children with egg allergy had lower egg white–specific IgA levels compared with healthy control subjects; in most who became tolerant to egg, there was a significant increase of greater than 28% in egg white–specific IgA levels over time, suggesting a role for allergen-specific IgA in food tolerance.
      • Konstantinou G.N.
      • Nowak-Węgrzyn A.
      • Bencharitiwong R.
      • Bardina L.
      • Sicherer S.H.
      • Sampson H.A.
      Egg-white-specific IgA and IgA2 antibodies in egg-allergic children: Is there a role in tolerance induction?.
      Analysis of tolerant beekeepers and patients after venom immunotherapy demonstrated that venom-specific IgG4 production was predominately through IL-10–secreting regulatory B1 cells, a population that was observed to increase after immunotherapy and to express high levels of IL-10 on a per-cell basis.
      • van de Veen W.
      • Stanic B.
      • Yaman G.
      • Wawrzyniak M.
      • Söllner S.
      • Akdis D.G.
      • et al.
      IgG 4 production is confined to human IL-10–producing regulatory B cells that suppress antigen-specific immune responses.
      Large amounts of IL-10 produced by B cells could suppress effector T-cell function or promote differentiation of Treg cells. Overall, global epitope-specific shifts from IgE to IgG4 binding occur over the course of immunotherapy and can often be attributed to B cells of a regulatory type. Such changes can act in concert with Treg cell alterations and tolerogenic APC functions to promote long-term tolerance.

      Summary

      Oral tolerance to food is a result of complicated interactions among antigens in the food we consume, the microbiome inhabiting our guts, nonimmune cells in the gut, and specialized APCs and lymphocytes found in the gut and associated lymphatic tissues. A failure to develop or breakdown in tolerance leading to food allergy could occur at multiple points and possibly in multiple tissues, including the gut or skin. Future progress in our understanding of oral tolerance in human subjects will be greatly advanced by new techniques allowing detailed analysis of innate and antigen-specific responses in the blood and in small samples of affected tissues complemented by using genetic models in experimental animals to analyze in mechanistic detail immune responses in the gut that contribute to sensitization, desensitization, and tolerance. Such studies will offer crucial insights into factors determining development of natural or therapy-induced long-lasting tolerance.

      References

        • Moog F.
        The lining of the small intestine.
        Sci Am. 1981; 245 (160, 162): 154-158
        • Brandtzaeg P.
        Development and basic mechanisms of human gut immunity.
        Nutr Rev. 1998; 56: S5-S18
        • Lozupone C.A.
        • Stombaugh J.I.
        • Gordon J.I.
        • Jansson J.K.
        • Knight R.
        Diversity, stability and resilience of the human gut microbiota.
        Nature. 2012; 489: 220-230
        • Mestecky J.
        • McGhee J.R.
        Immunoglobulin A (IgA): molecular and cellular interactions involved in IgA biosynthesis and immune response.
        Adv Immunol. 1987; 40: 153-245
        • van der Heijden P.J.
        • Stok W.
        • Bianchi A.T.
        Contribution of immunoglobulin-secreting cells in the murine small intestine to the total ‘background’ immunoglobulin production.
        Immunology. 1987; 62: 551-555
        • Untersmayr E.
        • Scholl I.
        • Swoboda I.
        • Beil W.J.
        • Forster-Waldl E.
        • Walter F.
        • et al.
        Antacid medication inhibits digestion of dietary proteins and causes food allergy: a fish allergy model in BALB/c mice.
        J Allergy Clin Immunol. 2003; 112: 616-623
        • Untersmayr E.
        • Jensen-Jarolim E.
        The role of protein digestibility and antacids on food allergy outcomes.
        J Allergy Clin Immunol. 2008; 121: 1301-1310
        • Untersmayr E.
        • Bakos N.
        • Scholl I.
        • Kundi M.
        • Roth-Walter F.
        • Szalai K.
        • et al.
        Anti-ulcer drugs promote IgE formation toward dietary antigens in adult patients.
        FASEB J. 2005; 19: 656-658
        • Barone K.S.
        • Reilly M.R.
        • Flanagan M.P.
        • Michael J.G.
        Abrogation of oral tolerance by feeding encapsulated antigen.
        Cell Immunol. 2000; 199: 65-72
        • Rescigno M.
        • Urbano M.
        • Valzasina B.
        • Francolini M.
        • Rotta G.
        • Bonasio R.
        • et al.
        Dendritic cells express tight junction proteins and penetrate gut epithelial monolayers to sample bacteria.
        Nat Immunol. 2001; 2: 361-367
        • Menard S.
        • Cerf-Bensussan N.
        • Heyman M.
        Multiple facets of intestinal permeability and epithelial handling of dietary antigens.
        Mucosal Immunol. 2010; 3: 247-259
        • Niess J.H.
        • Brand S.
        • Gu X.
        • Landsman L.
        • Jung S.
        • McCormick B.A.
        • et al.
        CX3CR1-mediated dendritic cell access to the intestinal lumen and bacterial clearance.
        Science. 2005; 307: 254-258
        • Pabst O.
        • Mowat A.M.
        Oral tolerance to food protein.
        Mucosal Immunol. 2012; 5: 232-239
        • Mabbott N.A.
        • Donaldson D.S.
        • Ohno H.
        • Williams I.R.
        • Mahajan A.
        Microfold (M) cells: important immunosurveillance posts in the intestinal epithelium.
        Mucosal Immunol. 2013; 6: 666-677
        • Chehade M.
        • Mayer L.
        Oral tolerance and its relation to food hypersensitivities.
        J Allergy Clin Immunol. 2005; 115: 3-13
        • Hershberg R.M.
        • Cho D.H.
        • Youakim A.
        • Bradley M.B.
        • Lee J.S.
        • Framson P.E.
        • et al.
        Highly polarized HLA class II antigen processing and presentation by human intestinal epithelial cells.
        J Clin Invest. 1998; 102: 792-803
        • Scott C.L.
        • Aumeunier A.M.
        • Mowat A.M.
        Intestinal CD103+ dendritic cells: master regulators of tolerance?.
        Trends Immunol. 2011; 32: 412-419
        • Macpherson A.J.
        • Uhr T.
        Induction of protective IgA by intestinal dendritic cells carrying commensal bacteria.
        Science. 2004; 303: 1662-1665
        • Sicinski P.
        • Rowinski J.
        • Warchol J.B.
        • Jarzabek Z.
        • Gut W.
        • Szczygiel B.
        • et al.
        Poliovirus type 1 enters the human host through intestinal M cells.
        Gastroenterology. 1990; 98: 56-58
        • Suzuki H.
        • Sekine S.
        • Kataoka K.
        • Pascual D.W.
        • Maddaloni M.
        • Kobayashi R.
        • et al.
        Ovalbumin-protein sigma 1 M-cell targeting facilitates oral tolerance with reduction of antigen-specific CD4+ T cells.
        Gastroenterology. 2008; 135: 917-925
        • Kraus T.A.
        • Brimnes J.
        • Muong C.
        • Liu J.H.
        • Moran T.M.
        • Tappenden K.A.
        • et al.
        Induction of mucosal tolerance in Peyer's patch-deficient, ligated small bowel loops.
        J Clin Invest. 2005; 115: 2234-2243
        • Spahn T.W.
        • Fontana A.
        • Faria A.M.
        • Slavin A.J.
        • Eugster H.P.
        • Zhang X.
        • et al.
        Induction of oral tolerance to cellular immune responses in the absence of Peyer's patches.
        Eur J Immunol. 2001; 31: 1278-1287
        • Spahn T.W.
        • Weiner H.L.
        • Rennert P.D.
        • Lugering N.
        • Fontana A.
        • Domschke W.
        • et al.
        Mesenteric lymph nodes are critical for the induction of high-dose oral tolerance in the absence of Peyer's patches.
        Eur J Immunol. 2002; 32: 1109-1113
        • Bain C.C.
        • Mowat A.M.
        Intestinal macrophages—specialised adaptation to a unique environment.
        Eur J Immunol. 2011; 41: 2494-2498
        • Bogunovic M.
        • Ginhoux F.
        • Helft J.
        • Shang L.
        • Hashimoto D.
        • Greter M.
        • et al.
        Origin of the lamina propria dendritic cell network.
        Immunity. 2009; 31: 513-525
        • Schulz O.
        • Jaensson E.
        • Persson E.K.
        • Liu X.
        • Worbs T.
        • Agace W.W.
        • et al.
        Intestinal CD103+, but not CX3CR1+, antigen sampling cells migrate in lymph and serve classical dendritic cell functions.
        J Exp Med. 2009; 206: 3101-3114
        • Warshaw A.L.
        • Walker W.A.
        • Isselbacher K.J.
        Protein uptake by the intestine: evidence for absorption of intact macromolecules.
        Gastroenterology. 1974; 66: 987-992
        • Husby S.
        • Jensenius J.C.
        • Svehag S.E.
        Passage of undegraded dietary antigen into the blood of healthy adults. Quantification, estimation of size distribution, and relation of uptake to levels of specific antibodies.
        Scand J Immunol. 1985; 22: 83-92
        • Walker W.A.
        • Isselbacher K.J.
        Uptake and transport of macromolecules by the intestine. Possible role in clinical disorders.
        Gastroenterology. 1974; 67: 531-550
        • Goubier A.
        • Dubois B.
        • Gheit H.
        • Joubert G.
        • Villard-Truc F.
        • Asselin-Paturel C.
        • et al.
        Plasmacytoid dendritic cells mediate oral tolerance.
        Immunity. 2008; 29: 464-475
        • Thomson A.W.
        • Knolle P.A.
        Antigen-presenting cell function in the tolerogenic liver environment.
        Nat Rev Immunol. 2010; 10: 753-766
        • Peng H.J.
        • Turner M.W.
        • Strobel S.
        The generation of a ‘tolerogen’ after the ingestion of ovalbumin is time-dependent and unrelated to serum levels of immunoreactive antigen.
        Clin Exp Immunol. 1990; 81: 510-515
        • Callery M.P.
        • Kamei T.
        • Flye M.W.
        The effect of portacaval shunt on delayed-hypersensitivity responses following antigen feeding.
        J Surg Res. 1989; 46: 391-394
        • Yang R.
        • Liu Q.
        • Grosfeld J.L.
        • Pescovitz M.D.
        Intestinal venous drainage through the liver is a prerequisite for oral tolerance induction.
        J Pediatr Surg. 1994; 29: 1145-1148
        • Mazzini E.
        • Massimiliano L.
        • Penna G.
        • Rescigno M.
        Oral tolerance can be established via gap junction transfer of fed antigens from CX3CR1(+) macrophages to CD103(+) dendritic cells.
        Immunity. 2014; 40: 248-261
        • Milling S.
        • Yrlid U.
        • Cerovic V.
        • MacPherson G.
        Subsets of migrating intestinal dendritic cells.
        Immunol Rev. 2010; 234: 259-267
        • Worbs T.
        • Bode U.
        • Yan S.
        • Hoffmann M.W.
        • Hintzen G.
        • Bernhardt G.
        • et al.
        Oral tolerance originates in the intestinal immune system and relies on antigen carriage by dendritic cells.
        J Exp Med. 2006; 203: 519-527
        • Ventura M.T.
        • Polimeno L.
        • Amoruso A.C.
        • Gatti F.
        • Annoscia E.
        • Marinaro M.
        • et al.
        Intestinal permeability in patients with adverse reactions to food.
        Dig Liver Dis. 2006; 38: 732-736
        • Clayburgh D.R.
        • Musch M.W.
        • Leitges M.
        • Fu Y.X.
        • Turner J.R.
        Coordinated epithelial NHE3 inhibition and barrier dysfunction are required for TNF-mediated diarrhea in vivo.
        J Clin Invest. 2006; 116: 2682-2694
        • Perrier C.
        • Corthesy B.
        Gut permeability and food allergies.
        Clin Exp Allergy. 2011; 41: 20-28
        • Wang F.
        • Graham W.V.
        • Wang Y.
        • Witkowski E.D.
        • Schwarz B.T.
        • Turner J.R.
        Interferon-gamma and tumor necrosis factor-alpha synergize to induce intestinal epithelial barrier dysfunction by up-regulating myosin light chain kinase expression.
        Am J Pathol. 2005; 166: 409-419
        • Yang P.C.
        • Berin M.C.
        • Yu L.C.
        • Conrad D.H.
        • Perdue M.H.
        Enhanced intestinal transepithelial antigen transport in allergic rats is mediated by IgE and CD23 (FcepsilonRII).
        J Clin Invest. 2000; 106: 879-886
        • Yu L.C.
        • Yang P.C.
        • Berin M.C.
        • Di Leo V.
        • Conrad D.H.
        • McKay D.M.
        • et al.
        Enhanced transepithelial antigen transport in intestine of allergic mice is mediated by IgE/CD23 and regulated by interleukin-4.
        Gastroenterology. 2001; 121: 370-381
        • Chambers S.J.
        • Bertelli E.
        • Winterbone M.S.
        • Regoli M.
        • Man A.L.
        • Nicoletti C.
        Adoptive transfer of dendritic cells from allergic mice induces specific immunoglobulin E antibody in naive recipients in absence of antigen challenge without altering the T helper 1/T helper 2 balance.
        Immunology. 2004; 112: 72-79
        • Brough H.A.
        • Simpson A.
        • Makinson K.
        • Hankinson J.
        • Brown S.
        • Douiri A.
        • et al.
        Peanut allergy: effect of environmental peanut exposure in children with filaggrin loss-of-function mutations.
        J Allergy Clin Immunol. 2014; 134: 867-875.e1
        • Makinen-Kiljunen S.
        • Mussalo-Rauhamaa H.
        Casein, an important house dust allergen.
        Allergy. 2002; 57: 1084-1085
        • Di Meglio P.
        • Perera Gayathri K.
        • Nestle Frank O.
        The multitasking organ: recent insights into skin immune function.
        Immunity. 2011; 35: 857-869
        • Klechevsky E.
        • Morita R.
        • Liu M.
        • Cao Y.
        • Coquery S.
        • Thompson-Snipes L.
        • et al.
        Functional specializations of human epidermal Langerhans cells and CD14+ dermal dendritic cells.
        Immunity. 2008; 29: 497-510
        • Du Toit G.
        • Roberts G.
        • Sayre P.H.
        • Plaut M.
        • Bahnson H.T.
        • Mitchell H.
        • et al.
        Identifying infants at high risk of peanut allergy: the Learning Early About Peanut Allergy (LEAP) screening study.
        J Allergy Clin Immunol. 2013; 131 (135-43.e1-12)
        • Kim K.S.
        • Hong S.W.
        • Han D.
        • Yi J.
        • Jung J.
        • Yang B.G.
        • et al.
        Dietary antigens limit mucosal immunity by inducing regulatory T cells in the small intestine.
        Science. 2016; 351: 858-863
        • Swiatczak B.
        • Rescigno M.
        How the interplay between antigen presenting cells and microbiota tunes host immune responses in the gut.
        Semin Immunol. 2012; 24: 43-49
        • Coombes J.L.
        • Siddiqui K.R.
        • Arancibia-Carcamo C.V.
        • Hall J.
        • Sun C.M.
        • Belkaid Y.
        • et al.
        A functionally specialized population of mucosal CD103+ DCs induces Foxp3+ regulatory T cells via a TGF-beta and retinoic acid-dependent mechanism.
        J Exp Med. 2007; 204: 1757-1764
        • Paidassi H.
        • Acharya M.
        • Zhang A.
        • Mukhopadhyay S.
        • Kwon M.
        • Chow C.
        • et al.
        Preferential expression of integrin alphavbeta8 promotes generation of regulatory T cells by mouse CD103+ dendritic cells.
        Gastroenterology. 2011; 141: 1813-1820
        • Worthington J.J.
        • Czajkowska B.I.
        • Melton A.C.
        • Travis M.A.
        Intestinal dendritic cells specialize to activate transforming growth factor-beta and induce Foxp3+ regulatory T cells via integrin alphavbeta8.
        Gastroenterology. 2011; 141: 1802-1812
        • Matteoli G.
        • Mazzini E.
        • Iliev I.D.
        • Mileti E.
        • Fallarino F.
        • Puccetti P.
        • et al.
        Gut CD103+ dendritic cells express indoleamine 2,3-dioxygenase which influences T regulatory/T effector cell balance and oral tolerance induction.
        Gut. 2010; 59: 595-604
        • Molenaar R.
        • Greuter M.
        • van der Marel A.P.
        • Roozendaal R.
        • Martin S.F.
        • Edele F.
        • et al.
        Lymph node stromal cells support dendritic cell-induced gut-homing of T cells.
        J Immunol. 2009; 183: 6395-6402
        • Jaensson E.
        • Uronen-Hansson H.
        • Pabst O.
        • Eksteen B.
        • Tian J.
        • Coombes J.L.
        • et al.
        Small intestinal CD103+ dendritic cells display unique functional properties that are conserved between mice and humans.
        J Exp Med. 2008; 205: 2139-2149
        • Hammerschmidt S.I.
        • Ahrendt M.
        • Bode U.
        • Wahl B.
        • Kremmer E.
        • Forster R.
        • et al.
        Stromal mesenteric lymph node cells are essential for the generation of gut-homing T cells in vivo.
        J Exp Med. 2008; 205: 2483-2490
        • Mora J.R.
        • Iwata M.
        • Eksteen B.
        • Song S.Y.
        • Junt T.
        • Senman B.
        • et al.
        Generation of gut-homing IgA-secreting B cells by intestinal dendritic cells.
        Science. 2006; 314: 1157-1160
        • Torgerson T.R.
        • Linane A.
        • Moes N.
        • Anover S.
        • Mateo V.
        • Rieux-Laucat F.
        • et al.
        Severe food allergy as a variant of IPEX syndrome caused by a deletion in a noncoding region of the FOXP3 gene.
        Gastroenterology. 2007; 132: 1705-1717
        • Dubois B.
        • Chapat L.
        • Goubier A.
        • Papiernik M.
        • Nicolas J.F.
        • Kaiserlian D.
        Innate CD4+CD25+ regulatory T cells are required for oral tolerance and inhibition of CD8+ T cells mediating skin inflammation.
        Blood. 2003; 102: 3295-3301
        • Hadis U.
        • Wahl B.
        • Schulz O.
        • Hardtke-Wolenski M.
        • Schippers A.
        • Wagner N.
        • et al.
        Intestinal tolerance requires gut homing and expansion of FoxP3+ regulatory T cells in the lamina propria.
        Immunity. 2011; 34: 237-246
        • Sakaguchi S.
        • Yamaguchi T.
        • Nomura T.
        • Ono M.
        Regulatory T cells and immune tolerance.
        Cell. 2008; 133: 775-787
        • Sakaguchi S.
        • Vignali D.A.
        • Rudensky A.Y.
        • Niec R.E.
        • Waldmann H.
        The plasticity and stability of regulatory T cells.
        Nat Rev Immunol. 2013; 13: 461-467
        • Maynard C.L.
        • Harrington L.E.
        • Janowski K.M.
        • Oliver J.R.
        • Zindl C.L.
        • Rudensky A.Y.
        • et al.
        Regulatory T cells expressing interleukin 10 develop from Foxp3+ and Foxp3- precursor cells in the absence of interleukin 10.
        Nat Immunol. 2007; 8: 931-941
        • Battaglia M.
        • Gianfrani C.
        • Gregori S.
        • Roncarolo M.G.
        IL-10-producing T regulatory type 1 cells and oral tolerance.
        Ann N Y Acad Sci. 2004; 1029: 142-153
        • Curotto de Lafaille M.A.
        • Kutchukhidze N.
        • Shen S.
        • Ding Y.
        • Yee H.
        • Lafaille J.J.
        Adaptive Foxp3+ regulatory T cell-dependent and -independent control of allergic inflammation.
        Immunity. 2008; 29: 114-126
        • Cassani B.
        • Villablanca E.J.
        • Quintana F.J.
        • Love P.E.
        • Lacy-Hulbert A.
        • Blaner W.S.
        • et al.
        Gut-tropic T cells that express integrin alpha4beta7 and CCR9 are required for induction of oral immune tolerance in mice.
        Gastroenterology. 2011; 141: 2109-2118
        • Tsuji M.
        • Komatsu N.
        • Kawamoto S.
        • Suzuki K.
        • Kanagawa O.
        • Honjo T.
        • et al.
        Preferential generation of follicular B helper T cells from Foxp3+ T cells in gut Peyer's patches.
        Science. 2009; 323: 1488-1492
        • Hammad H.
        • Lambrecht B.N.
        Barrier epithelial cells and the control of type 2 immunity.
        Immunity. 2015; 43: 29-40
        • Chu D.K.
        • Llop-Guevara A.
        • Walker T.D.
        • Flader K.
        • Goncharova S.
        • Boudreau J.E.
        • et al.
        IL-33, but not thymic stromal lymphopoietin or IL-25, is central to mite and peanut allergic sensitization.
        J Allergy Clin Immunol. 2013; 131 (187-200.e1-8)
        • Blazquez A.B.
        • Berin M.C.
        Gastrointestinal dendritic cells promote Th2 skewing via OX40L.
        J Immunol. 2008; 180: 4441-4450
        • Lambrecht B.N.
        • Hammad H.
        Allergens and the airway epithelium response: gateway to allergic sensitization.
        J Allergy Clin Immunol. 2014; 134: 499-507
        • Herbert C.A.
        • King C.M.
        • Ring P.C.
        • Holgate S.T.
        • Stewart G.A.
        • Thompson P.J.
        • et al.
        Augmentation of permeability in the bronchial epithelium by the house dust mite allergen Der p1.
        Am J Respir Cell Mol Biol. 1995; 12: 369-378
        • Shakib F.
        • Schulz O.
        • Sewell H.
        A mite subversive: cleavage of CD23 and CD25 by Der p 1 enhances allergenicity.
        Immunol Today. 1998; 19: 313-316
        • Jyonouchi S.
        • Abraham V.
        • Orange J.S.
        • Spergel J.M.
        • Gober L.
        • Dudek E.
        • et al.
        Invariant natural killer T cells from children with versus without food allergy exhibit differential responsiveness to milk-derived sphingomyelin.
        J Allergy Clin Immunol. 2011; 128: 102-109.e13
        • Shreffler W.G.
        • Castro R.R.
        • Kucuk Z.Y.
        • Charlop-Powers Z.
        • Grishina G.
        • Yoo S.
        • et al.
        The major glycoprotein allergen from Arachis hypogaea, Ara h 1, is a ligand of dendritic cell-specific ICAM-grabbing nonintegrin and acts as a Th2 adjuvant in vitro.
        J Immunol. 2006; 177: 3677-3685
        • Huby R.D.
        • Dearman R.J.
        • Kimber I.
        Why are some proteins allergens?.
        Toxicol Sci. 2000; 55: 235-246
        • Smith A.M.
        • Chapman M.D.
        Reduction in IgE binding to allergen variants generated by site-directed mutagenesis: contribution of disulfide bonds to the antigenic structure of the major house dust mite allergen Der p 2.
        Mol Immunol. 1996; 33: 399-405
        • Olsson S.
        • van Hage-Hamsten M.
        • Whitley P.
        Contribution of disulphide bonds to antigenicity of Lep d 2, the major allergen of the dust mite Lepidoglyphus destructor.
        Mol Immunol. 1998; 35: 1017-1023
        • Astwood J.D.
        • Leach J.N.
        • Fuchs R.L.
        Stability of food allergens to digestion in vitro.
        Nat Biotechnol. 1996; 14: 1269-1273
        • Nowak-Wegrzyn A.
        • Fiocchi A.
        Rare, medium, or well done? The effect of heating and food matrix on food protein allergenicity.
        Curr Opin Allergy Clin Immunol. 2009; 9: 234-237
        • Nowak-Wegrzyn A.
        • Bloom K.A.
        • Sicherer S.H.
        • Shreffler W.G.
        • Noone S.
        • Wanich N.
        • et al.
        Tolerance to extensively heated milk in children with cow's milk allergy.
        J Allergy Clin Immunol. 2008; 122 (347.e1-2): 342-347
        • Kim J.S.
        • Nowak-Węgrzyn A.
        • Sicherer S.H.
        • Noone S.
        • Moshier E.L.
        • Sampson H.A.
        Dietary baked milk accelerates the resolution of cow's milk allergy in children.
        J Allergy Clin Immunol. 2011; 128: 125-131.e2
        • Leonard S.A.
        • Sampson H.A.
        • Sicherer S.H.
        • Noone S.
        • Moshier E.L.
        • Godbold J.
        • et al.
        Dietary baked egg accelerates resolution of egg allergy in children.
        J Allergy Clin Immunol. 2012; 130: 473-480.e1
        • Webber C.M.
        • England R.W.
        Oral allergy syndrome: a clinical, diagnostic, and therapeutic challenge.
        Ann Allergy Asthma Immunol. 2010; 104: 101-108
        • Moghaddam A.E.
        • Hillson W.R.
        • Noti M.
        • Gartlan K.H.
        • Johnson S.
        • Thomas B.
        • et al.
        Dry roasting enhances peanut-induced allergic sensitization across mucosal and cutaneous routes in mice.
        J Allergy Clin Immunol. 2014; 134: 1453-1456
        • Maleki S.J.
        • Chung S.-Y.
        • Champagne E.T.
        • Raufman J.-P.
        The effects of roasting on the allergenic properties of peanut proteins.
        J Allergy Clin Immunol. 2000; 106: 763-768
        • Palmer C.N.
        • Irvine A.D.
        • Terron-Kwiatkowski A.
        • Zhao Y.
        • Liao H.
        • Lee S.P.
        • et al.
        Common loss-of-function variants of the epidermal barrier protein filaggrin are a major predisposing factor for atopic dermatitis.
        Nat Genet. 2006; 38: 441-446
        • Marenholz I.
        • Nickel R.
        • Rüschendorf F.
        • Schulz F.
        • Esparza-Gordillo J.
        • Kerscher T.
        • et al.
        Filaggrin loss-of-function mutations predispose to phenotypes involved in the atopic march.
        J Allergy Clin Immunol. 2006; 118: 866-871
        • Venkataraman D.
        • Soto-Ramírez N.
        • Kurukulaaratchy R.J.
        • Holloway J.W.
        • Karmaus W.
        • Ewart S.L.
        • et al.
        Filaggrin loss-of-function mutations are associated with food allergy in childhood and adolescence.
        J Allergy Clin Immunol. 2014; 134: 876-882.e4
        • Kusunoki T.
        • Okafuji I.
        • Yoshioka T.
        • Saito M.
        • Nishikomori R.
        • Heike T.
        • et al.
        SPINK5 polymorphism is associated with disease severity and food allergy in children with atopic dermatitis.
        J Allergy Clin Immunol. 2005; 115: 636-638
        • Brough H.A.
        • Liu A.H.
        • Sicherer S.
        • Makinson K.
        • Douiri A.
        • Brown S.J.
        • et al.
        Atopic dermatitis increases the effect of exposure to peanut antigen in dust on peanut sensitization and likely peanut allergy.
        J Allergy Clin Immunol. 2015; 135: 164-170
        • Simpson E.L.
        • Chalmers J.R.
        • Hanifin J.M.
        • Thomas K.S.
        • Cork M.J.
        • McLean W.H.I.
        • et al.
        Emollient enhancement of the skin barrier from birth offers effective atopic dermatitis prevention.
        J Allergy Clin Immunol. 2014; 134: 818-823
        • Horimukai K.
        • Morita K.
        • Narita M.
        • Kondo M.
        • Kitazawa H.
        • Nozaki M.
        • et al.
        Application of moisturizer to neonates prevents development of atopic dermatitis.
        J Allergy Clin Immunol. 2014; 134: 824-830.e6
        • Tordesillas L.
        • Goswami R.
        • Benede S.
        • Grishina G.
        • Dunkin D.
        • Jarvinen K.M.
        • et al.
        Skin exposure promotes a Th2-dependent sensitization to peanut allergens.
        J Clin Invest. 2014; 124: 4965-4975
        • Yatsunenko T.
        • Rey F.E.
        • Manary M.J.
        • Trehan I.
        • Dominguez-Bello M.G.
        • Contreras M.
        • et al.
        Human gut microbiome viewed across age and geography.
        Nature. 2012; 486: 222-227
        • Mackie R.I.
        • Sghir A.
        • Gaskins H.R.
        Developmental microbial ecology of the neonatal gastrointestinal tract.
        Am J Clin Nutr. 1999; 69: 1035S-1045S
        • Palmer C.
        • Bik E.M.
        • DiGiulio D.B.
        • Relman D.A.
        • Brown P.O.
        Development of the human infant intestinal microbiota.
        PLoS Biol. 2007; 5: e177
        • Schnorr S.L.
        • Candela M.
        • Rampelli S.
        • Centanni M.
        • Consolandi C.
        • Basaglia G.
        • et al.
        Gut microbiome of the Hadza hunter-gatherers.
        Nat Commun. 2014; 5: 3654
        • Ege M.J.
        • Mayer M.
        • Normand A.C.
        • Genuneit J.
        • Cookson W.O.
        • Braun-Fahrlander C.
        • et al.
        Exposure to environmental microorganisms and childhood asthma.
        N Engl J Med. 2011; 364: 701-709
        • David L.A.
        • Weil A.
        • Ryan E.T.
        • Calderwood S.B.
        • Harris J.B.
        • Chowdhury F.
        • et al.
        Gut microbial succession follows acute secretory diarrhea in humans.
        MBio. 2015; 6: e00381-e003815