New insights into mechanisms of immunoregulation in 2007

Recent developments in antigen presentation and immunoregulation 

Presentation of allergens by antigen-presenting cells or by other means and initiation of allergen-specific immune responses represent the first step in sensitization to allergens. New allergens and their cross-reactivities are continuously being identified, and types of immune responses to them are being demonstrated.4, 5 Allergic inflammation results from the activation of tissue-migrating hematopoietic and resident nonhematopoetic cells. This coordinated activation leads to increased production of a variety of soluble factors, including chemokines and cytokines. Direct or indirect effects of the innate immune response are decisive in the development of adoptive immunity to allergens.6 In principle, it is not only the protein allergen but also the adjuvants surrounding the allergens that are decisive for the type of immune response.7, 8 For example, pollen releases water-soluble phytoprostanes that display TH2-polarizing capacities in vivo.9 Thymic stromal lymphopoietin (TSLP) stimulates human CD11c+ dendritic cells (DCs) from peripheral blood with the capacity to induce proallergic T cells. TSLP is present at high levels in the epidermis of patients with atopic dermatitis (AD), where it appears to trigger emigration of epidermal Langerhans cells. It was recently demonstrated that TSLP converts human epidermal Langerhans cells into antigen-presenting cells that induce proallergic T cells.10

Exosomes are vesicles of 30 to 100 nm produced by inward budding of endosomal compartments and are released by a range of different cell types. Exosomes from antigen-presenting cells carry immunorelevant molecules, such as MHC class I and II and costimulatory molecules, and thus are suggested to have a role in immune modulation. Recently, exosomes were isolated from supernatants of B-cell lines derived from patients with birch pollen allergy.11 They showed expression of MHC, costimulatory molecules (eg, CD86), tetraspanin proteins (eg, CD81), and CD19. Furthermore, B cell–derived exosomes bound Bet v 1–derived peptides and subsequently induced a dose-dependent T-cell proliferation and IL-5 and IL-13 production. These results demonstrate that exosomes from B lymphocytes are an immunostimulatory factor in allergic immune responses.

Novel developments in TH1, TH2, and TH17 effector cells 

Activated effector T cells play an essential role in asthma. Originally interpreted within the framework of a binary TH1/TH2 paradigm, our knowledge of the pathogenesis of atopic diseases has broadened to incorporate the contribution of regulatory T (Treg) cells and the newly described proinflammatory TH17 cell lineage. The commitment of peripheral T-cell clones to undergo differentiation into one of those lineages is shaped by self-reinforcing transcriptional circuitries that center on key transcriptional regulators: T-box expressed in T cells (TH1), GATA-3 (TH2), forkhead box p3 (human, FOXP3; mice, Foxp3; Treg cells), and retinoid-related orphan receptor γt/retinoid-related orphan receptor α (TH17). Counterregulation between the 3 effector subsets has been continuously proposed.2, 12, 13, 14 The activation of T-bet as a key transcription factor of TH1 cells inhibits both TH2 cell–mediated eosinophil recruitment and TH17 cell–mediated neutrophil recruitment into the airways.15 An association between a specific T-bet haplotype and allergic asthma in children has been demonstrated.16

A predominant TH2 profile in atopic diseases might be a result of the increased tendency toward activation and apoptosis of high IFN-γ–producing TH1 cells.17 TH1 cells, particularly their high IFN-γ–producing fraction, and CXCR3+ T cells showed significantly increased apoptosis in atopic individuals. During their in vitro differentiation, significantly high apoptosis in TH1 cells was observed in atopic individuals compared with that seen in nonatopic individuals.17 It was recently studied whether asthmatic patients showed any change in cytokine-dependent signal transducer and activator of transcription (STAT) levels, focusing on the central/effector memory CD4+CD161+ subset, which represents 15% to 25% of circulating T cells.18 Oral corticosteroid treatment significantly decreased phospho-STAT6 and IL-4 levels, but not phospho-STAT1 levels, in these cells. It was suggested that combined phospho-STAT1 and phospho-STAT6 measurements showed predictive value for identifying (1) asthmatic patients versus healthy control subjects, (2) allergic versus nonallergic asthmatic patients, and (3) asthmatic patients taking versus those not taking oral corticosteroids.18

IL-23 has been thought to play a critical role in autoimmune disease through inducing the development of IL-17–producing CD4+ TH17 cells.13 Vogt-Koyanagi-Harada disease is a systemic refractory autoimmune disease characterized by granulomatous panuveitis, vitiligo, alopecia, and central nervous system signs.19 The patients with Vogt-Koyanagi-Harada disease with active uveitis showed an increased level of IL-23p19 mRNA in PBMCs, higher IL-23 levels in the serum and supernatants of PBMCs, and increased production of IL-17 by PBMCs and CD4+ T cells. IL-17 production was significantly increased in patients with active uveitis in the presence of rIL-23.19 IL-17F, a member of the IL-17 cytokine family, is expressed in activated CD4+ T cells, basophils, and mast cells. Recent reports demonstrated that IL-17F is produced by TH17 cells, a new and distinct lineage of CD4+ effector cells in mice.13 IL-17F induces the gene and protein expression of IFN-γ inducible protein-10, providing a novel and critical therapeutic target for airway inflammatory diseases.20

IL-6 is an essential cytokine in the development of TH17 cells. Two small chemical compounds, 3-O-formyl-20R,21-epoxyresibufogenin (TB-2-081) and 3-O-formyl-20S,21-epoxyresibufogenin (TB-2-082), known isolates from the Chinese toad skin extract drug Ch'an Su were tested on the IL-6–induced hepatic acute-phase reaction.21 It was found that these compounds inhibit IL-6–type cytokine action by attenuating the function of the common receptor subunit glycoprotein 130. IL-11 was initially discovered as an IL-6–like plasmacytoma proliferation stimulating activity in supernatants from transformed marrow fibroblasts. Studies have shown that IL-11 acts as a hematopoietic growth factor, stimulates the acute-phase response, augments immunoglobulin production, induces the expression of metalloproteinase inhibitors, regulates neural phenotype, regulates bone metabolism, and protects against the combined effects of radiation and chemotherapy. The upregulation of IL-11 by cysteinyl leukotrienes represents one of the molecular mechanisms of the role of cysteinyl leukotrienes in asthma.22 In addition, cysteinyl leukotrienes synergize with epidermal growth factor to induce proliferation of human bronchial fibroblasts.23 The effect of cysteinyl leukotrienes on tissue eosinophilia through cysteinyl leukotriene type 1 receptor is limited and does not involve IL-5 and eotaxin.24

The magnitude and duration of cytokine action determine the response to an allergen, either mounting a low-grade immunologic response or resulting in an exaggerated reaction, such as asthma or AD. The action of cytokines is tightly regulated both developmentally and within the cell. The suppressor of cytokine signaling (SOCS) protein family represents a novel group of cytoplasmic negative feedback regulators of type I and II cytokines. Several of the signaling pathways regulated by SOCS proteins are important in allergic immune responses.25

Inflammation is a key feature of asthma and allergic disease. The proinflammatory cytokines IL-4, IL-5, and IL-13 are clustered on chromosome 5q, with GM-CSF in close proximity, and each of these cytokines has been implicated in the pathogenesis of IgE- and eosinophilia-associated inflammations. A novel role for c-Maf in the transcriptional regulation of GM-CSF in human T cells has been proposed.26 IL-25 (IL-17E), a member of the IL-17 family of immunoregulatory cytokines, has been implicated in the regulation of type 2 immunity. Blocking IL-25 in an experimental model of allergic asthma prevented airway hyperreactivity (AHR) and reduced IL-5 and IL-13 production, eosinophil infiltration, goblet cell hyperplasia, and serum IgE secretion.27

TNF-α is an important player in AHR and airway inflammation in asthma.28 Although many cellular sources are important providers of TNF-α during inflammation, mast cell–derived TNF-α also contributes to AHR, inflammation, and TH2 cytokine production in an asthma model in mice.29 The development of novel TNF-α antagonists has allowed us to test the role of this cytokine in vivo.30 Studies have demonstrated an improvement in asthma quality of life, lung function, and airway hyperresponsiveness and a reduction in exacerbation frequency in patients treated with anti-TNF-α therapy.

T-cell response in drug allergy has been an interesting research focus. T-cell clones were investigated to better understand the molecular basis of T-cell responses and cross-reactivity and drug allergy. Clinically observed cross-reactivity between different contrast media is a result of the presence of contrast media–specific T cells, some of which show a broad cross-reactivity pattern.31 In addition, it was demonstrated that carbamazepine-specific CD4+, CD8+, and CD4+CD8+ T cells exist in the peripheral circulation of hypersensitive patients, often many years after the resolution of clinical manifestations.32

Chronic obstructive pulmonary disease (COPD) and asthma are both diseases of the lung characterized by airway inflammation and airway obstruction. Although COPD and asthma share several similarities, there are also significant differences. One recently reported difference is at the T-cell level. Subjects with COPD diagnosed on the basis of the presence of emphysema on computed tomographic scanning who have a bronchodilator response also have increased levels of bronchoalveolar lavage fluid (BALF) eosinophil cationic protein and eotaxin-1 but not TH2 cytokines.33

Advances in Treg cells and their cytokines 

The immune regulatory role of T cell–derived IL-10 in allergic disease has been extensively studied. In this context functionally different properties of other cells that produce IL-10 are being investigated. It was suggested that controlling alternatively activated macrophages in TH2-driven inflammatory processes might be a novel target for immune intervention. Compared with healthy control subjects, the percentage of IL-10–producing monocytes was significantly increased in atopic patients.34 IL-10–secreting monocytes were isolated by using an IL-10 secretion assay, and analysis of these sorted cells revealed that IL-10–secreting monocytes preferentially differentiate into SOCS3, expressing alternatively activated macrophages that perpetuate the TH2 immune response.34 Whether IL-10 released from macrophages directly plays a role in lung inflammation or is released to keep the level of the inflammation at a low level remains to be investigated. IL-10–treated DCs are potent suppressors of the development of AHR, inflammation, and TH2 cytokine production; these regulatory functions are at least in part through the induction of endogenous production of IL-10.35 In human cells and mouse models, IL-10 has been repeatedly shown to suppress not only allergic inflammation but also to play a role in transplantation tolerance, tumor cell tolerance, and suppression of autoimmunity. IL-10 inhibits CD28 and inducible costimulator (ICOS) costimulations of T cells through src homology 2 domain–containing protein tyrosine phosphatase (SHP) 1.36 The IL-10 receptor–associated tyrosine kinase Tyk-2 acts as a constitutive reservoir for SHP-1 in resting T cells and then tyrosine phosphorylates SHP-1 on IL-10 binding. SHP-1 rapidly binds to CD28 and ICOS costimulatory receptors and dephosphorylates them within minutes. In consequence, the binding of phosphatidylinositol 3-kinase to either costimulatory receptor no longer occurs, and downstream signaling is inhibited. Accordingly, spleen cells from SHP-1–deficient mice showed increased proliferation with CD28 and ICOS stimulation in comparison with that seen in wild-type mice, which was not suppressed by IL-10. Generation of dominant-negative SHP-1–overexpressing T cells or silencing of the SHP1 gene by small inhibitory RNA both altered SHP-1 functions and abolished the T cell–suppressive effect of IL-10. In conclusion, the rapid inhibition of the CD28 or ICOS costimulatory pathways by SHP-1 represents a novel mechanism for direct T-cell suppression by IL-10.36 Supporting these findings, Src homology 2 domain–containing inositol 5-phosphatase 1 inhibits allergic responses as a negative regulator of cytokine and immune receptor signaling. Its deficiency leads to a spontaneous development of allergic-like inflammation in the murine lung.37

In addition to IL-10, TGF-β is a key cytokine in immune tolerance. It was investigated whether orally administered TGF-β, such as TGF-β in human milk, retains and exerts its activity in the intestinal mucosa and can induce immune tolerance to dietary antigens. In a relevant mouse model, the oral administration of TGF-β increased activation and response in TGF-β–related responsive elements and increased serum TGF-β levels.38 BALB/c mice treated orally with ovalbumin (OVA) and TGF-β showed augmented reduction of OVA-specific IgE and IgG1 antibodies, T-cell reactivity, and immediate-type skin reactions when compared with that seen in mice treated orally with OVA alone.38 The data suggest that oral administration of TGF-β might become a potential strategy to prevent allergic diseases, such as food allergy.

CD4+CD25high Treg cells are essential for establishing tolerance to allergens. It was found that these cells increase in allergic children during the pollen season, which might represent systemic immune activation caused by pollen exposures.39 Whether these CD4+CD25high T cells directly contribute to inflammation or their increased levels keep the inflammation at low levels remains an important research question. Peripheral blood–circulating allergen-specific CD4+CD25highFoxp3+ Treg cells do not show a major difference between nonatopic and atopic individuals.40 However, it was demonstrated that FOXP3 expression shows a negative correlation with IgE, eosinophilia, and IFN-γ levels, and the FOXP3+/CD4+ ratio is significantly low in patients with asthma and AD.41 In addition, impaired CD4+CD25+FOXP3+ T-cell infiltration is observed in 3- to 6-day-old AD lesions, suggesting decreased skin infiltration of Treg cells during the early stage and development phase of AD.42 Similarly, CD4+CD25high T cells were decreased in BALF of asthmatic children compared with values in children with cough or control subjects.43 Inhaled corticosteroid treatment was associated with an increased percentage of CD4+CD25high T cells in peripheral blood and BALF. BALF CD4+CD25high T cells from asthmatic subjects failed to suppress proliferation and production of TH2-associated cytokines and chemokines by CD4+CD25− responder T cells, which was restored after use of inhaled corticosteroids.43 Together, these findings provide important evidence that tissue CD4+CD25+ Treg cells are impaired in patients with AD and asthma.

Immune dysregulation, polyendocrinopathy, enteropathy, X-linked (IPEX) syndrome results in systemic autoimmunity from birth and can be caused by mutations in the transcription factor FOXP3. A patient was described with clinical manifestations of IPEX syndrome who had a normal FOXP3 gene but who had CD25 deficiency caused by autosomal recessive mutations in this gene.44 This patient exhibited defective IL-10 expression from CD4 lymphocytes, whereas a FOXP3-deficient patient expressed normal levels of IL-10. These data show that CD25 deficiency results in an IPEX-like syndrome and suggests that although FOXP3 is not required for normal IL-10 expression by human CD4 lymphocytes, CD25 expression is important.

Immunoregulation by histamine receptors and other G protein–coupled receptors 

The diverse effects of histamine on immune regulation appear to be due to differential expression and regulation of 4 types of histamine receptors and their distinct intracellular signals. Differences in cellular expression and affinities of these receptors for histamine are highly decisive for the biologic effects of histamine and drugs that target histamine receptors. Histamine modulates several functions in human monocytes, macrophages, and DCs. Differentiation of monocytes into macrophages or DCs is associated with upregulation of histamine H1 receptors.45 The regulation of histamine receptor expression in different cells might play a role in the pathogenesis of allergic inflammation. Histamine, through histamine H1 receptor and TGF-β1, enhances the proliferation of lung fibroblasts and might play a role in airway remodeling.46 The expression of the histamine H4 receptor by monocytes suggests a role in immunomodulation. It might represent an interesting target for drug development.47 In addition, histamine H4 receptor has been reported to mediate mast cell migration toward CXCL12, a constitutive chemokine (ligand of CXCR4 and CXCR7) that is expressed in the skin and airway epithelium and plays a significant role in allergic airway diseases.48 These studies highlight novel roles for the histamine H4 receptor in the perpetuation of allergic inflammation and demonstrate it as a target in the treatment of allergic diseases.

β-Agonists and other G(s)-coupled receptor agonists have the potential to regulate T-cell development to affect disease pathogenesis or the efficacy of therapies, and variability of effect relates to the ability to stimulate protein kinase A activity. β-Agonists at physiologically and clinically relevant concentrations stimulate increased antigen-independent and cytokine-stimulated accumulation, specifically of TH2 cells from asthmatic subjects.49 The corticosteroid budesonide potently reverses this effect. In addition, β-agonists increased IL-2–induced accumulation of TH2 cells in patients with lupus, an effect attributable to differential activation of protein kinase A affecting regulation of cell proliferation and apoptosis.50

Regulation of IgE, B cells, and plasma cells 

Patients with atopic diseases are characterized by high levels of specific IgE production. Thus far, little is known about the composition of their allergen-specific T- and B-cell repertoires. The detailed analysis of the IgE-expressing B-cell and specific linear T-cell epitope-reactive T-cell receptor repertoires might be useful to follow changes during different therapy modalities.51, 52 During differentiation, B cells receive signals from antigens through the B-cell receptor and signals that induce isotype switching. B-cell receptor cross-linking delays activation-induced cytidine deaminase expression.53 This might interfere with class-switch recombination by disrupting the temporal coordination of signals that lead to class-switch recombination.

The bronchial mucosa in patients with asthma seems to be primed for class switching to IgE because of B-cell expression of variable ɛ germline transcripts and activation-induced cytidine deaminase mRNA.54 It takes place regardless of atopic status. Local IgE appears to be primarily a component of responses within the mucosal immune compartment and is related to cellular inflammation, lung function, and clinical outcomes in asthma. Recently, higher local IgE levels were associated with better lung function but also with more severe exacerbations of asthma.55 In another study it was revealed that the human IgE repertoire is produced by a limited number of highly related B-cell clones and is restricted in its recognition of a limited number of antigens.56 It seems that similar mechanisms take place in the periphery because serum IgE antibodies produced to specific antigens, such as dust mite–specific IgE, constitute a major fraction of serum total IgE levels found in children.57

TH2 cytokines play crucial roles in driving human B lymphocytes to produce IgE. However, it is unclear whether IL-4 and IL-13 have parallel or sequential roles in the development of B lymphocytes. There is lower constitutive expression and signaling of IL-13 receptor α1 in resting tonsillar B lymphocytes compared with that of IL-4 receptor α. IL4 receptor α was constitutively expressed on tonsillar B cells, and levels did not vary significantly after stimulation.58 B cells activated by CD40 ligation or B-cell receptor cross-linking, but not resting B cells, showed significant increases in STAT6 phosphorylation in response to IL-13. IL-13 is induced on both immature and mature B lymphocytes, which implies a higher priority of specific targeting of IL-13 in IgE-mediated diseases.58 IL-13 expression involves changes in transcript turnover and a change in phosphorylation of HuR and its association with the mRNA 3′ untranslated region.59

Soluble receptors of cytokines play an important role in neutralizing the effects of cytokines. In this context allergen-dependent solubilization of IL-13 receptor α2 reveals a novel mechanism to regulate the excess effects of IL-13.60 Recently, a polymorphic variant of the IL-13 receptor (R110Q) has been shown to be associated with atopy. Expression of R110Q and low IL-13 receptor α2 levels can result in important biologic differences that might have clinical relevance in an atopic environment.61 Polymorphisms in IL13 are associated with serum total IgE levels and eosinophil counts.62 Interestingly, mitogen-stimulated peripheral blood IL-5 production was reported to correlate with serum total IgE levels in infants and their parents.63 Similarly, IL-5 T-cell responses to aeroallergens, such as house dust mite, are associated with the development of allergen-specific IgE responses and asthma in the first 5 years of life.64

IL-9 has been shown to affect the differentiation pathway of different cell types. Its potential role in the maturation pathway of antigen-driven B-cell differentiation and its functional effects has been recently studied. IL-9 was unable to induce IgE secretion by itself; however, it potentiated IL-4–mediated IgE production from low-density tonsillar B cells. Increased IgE levels were paralleled by an upregulation of IL-9 receptor α with CD27, which is a memory B-cell marker implicated in increased IgE secretion.65 Transmembrane activator and calcium modulator and cyclophilin ligand interactor (TACI) is a receptor used by B cell–activating factor of the TNF family and a proliferation-inducing ligand, known as APRIL, to induce isotype switching independently of CD40.66 It is mutated in patients with common variable immunodeficiency. Recent findings suggest that TACI and CD40 signaling converge to promote B-cell differentiation into plasmablasts, and TACI dysfunction could contribute to the impaired antibody response to T-dependent antigens in common variable immunodeficiency.66

Several groups have demonstrated that a maternal history of atopy is an important modifier of the effects of various environmental factors on the development of asthma and atopy in the child, including early-life exposure to cat, day care, and endotoxin. The mitochondrial genome is a circular double-stranded DNA that replicates autonomously in mammalian mitochondria. Because spermatozoan mitochondria are not introduced into the ovum cytoplasm during fertilization, the mitochondrial genome is inherited exclusively through the maternal line in human subjects. A common mitochondrial haplogroup is associated with increased total serum IgE levels, suggesting a genetic mechanism of inheritance of maternal atopy in the offspring.67

Novel developments in immunoregulation in asthma 

Airway inflammation is a hallmark feature of asthma and a driver of airway hyperresponsiveness. Oxidative stress is a state of redox disequilibrium that is defined as a depletion of intracellular glutathione and an accumulation of glutathione disulfide. A decrease of the cellular glutathione/glutathione disulfide ratio initiates a series of biologic responses that are relevant to normal physiology, as well as the pathophysiology of disease.68 Reactive oxygen species that worsen the symptoms of asthma are induced by environmental exposure to diesel exhaust, cigarette smoke, and ozone. The inhibition of pollen-reduced nicotinamide adenine dinucleotide phosphate oxidase–induced signal by intrapulmonary administration of antioxidants blocks allergic airway inflammation.69 Ambient air pollution, including airborne particulate matter, might contribute to the increased incidence, morbidity, or mortality from asthma in adults and children in urban areas. Studies support the link between exposures to air pollution and the induction or exacerbation of asthma.70 Inhaled ambient particulate matter can act directly on DCs as a danger signal to direct a proallergic pattern of innate immune activation. Exposed DCs secreted less IL-12 and IL-6 but exhibited increased secretion of IL-18 and IL-10 compared with LPS stimulation. A TH2-like pattern of cytokine production was seen in cocultures of airborne particulate matter–stimulated DCs and alloreactive naive CD4+ T cells where the IL-13/IFN-γ ratio was reversed.70 Supporting these findings, fetal/infant exposure of rhesus monkeys to environmental tobacco smoke altered the normal maturation of mRNA levels of IFN-γ, IL-2, and IL-10, as well as the ratio of CD4 to CD8 lymphocytes compared with filtered-air control levels.71 IL-13 is a key inducer of airway inflammation in rodent models and has been found to be associated with several inflammatory features in human asthma. Using cynomolgus monkeys (Macaca fascicularis) that are sensitized to Ascaris suum through natural exposure, a primate model of acute airway inflammation has been developed. IL-13 neutralization in this model with an mAb significantly reduces lung inflammation.72

Organic cation transporters have been shown to affect the pharmacokinetics of diverse cationic drugs at the tissue and cellular level. Through organic cation transporter 3 inhibition, corticosteroids rapidly interfere with the disposal of cationic drugs by smooth muscle cells in the airway.73 The anti-inflammatory effect of glucocorticoids was recently shown to be mediated by glucocorticoid-induced leucine zipper in epithelial cells. Glucocorticoid-induced leucine zipper is constitutively expressed by human airway epithelial cells, and its levels are increased by dexamethasone and decreased by inflammatory cytokines. Knockdown of glucocorticoid-induced leucine zipper inhibited the ability of dexamethasone to suppress IL-1β–induced chemokine expression.74

Immune susceptibility to asthma is established very early during postnatal life. Low IFN-γ production in the first year of life was a predictor of wheeze during childhood.75 The Clara cell 10-kd protein gene (CC10) encoding an anti-inflammatory CC10 protein has been shown to be associated with asthma. CC10, also called CC16, uteroglobin, or secretoglobin 1A1, with primary expression in the uterus and nonciliated bronchiolar cells, has an anti-inflammatory effect on the urogenital and respiratory tracts. Infant frequent wheezing was interestingly reported to associate with the CC10 G+38A polymorphism and lower CC10 levels.76 The production of CC16 seems to peak early in life, and CC16 has an inhibitory effect on TH2 cell differentiation from human infants by affecting DCs.77

Respiratory tract viral infections in early childhood can enhance the development of airway allergen sensitization, and the mechanisms are being investigated.78 Airway allergen exposure during respiratory tract viral infections might lead to asthma development induced by specific T-cell response to inhaled allergens.79, 80 Biomarkers are being suggested to predict viral infection–induced exacerbations. Serum IFN-γ–induced protein 10 determination is one of them, which is predictive for rhinovirus-induced asthma exacerbations.81 CD4+ T cells expressing CD57 represent a memory effector T-cell subset with low proliferation capacity. CD57 expression correlates with alveolitis severity in subjects with beryllium-induced disease, and their role in asthma remains to be investigated.82

Airway remodeling and immunoregulation 

Severe asthma causes the majority of asthma morbidity,83 with more severe pathologic changes, including inflammation and remodeling, in the asthmatic airway. Multiple underlying factors contribute to airway dysfunction and severity.84 Steroid-resistant (SR) asthma is characterized by airway inflammation that fails to resolve despite treatment with corticosteroids. Bronchodilator reversibility is impaired in SR asthma and is associated with a shift in matrix metalloproteinase 9/tissue inhibitor of metalloproteinase 1 ratio caused by the inability of steroids to enhance tissue inhibitor of metalloproteinase 1 production, which potentially promotes proteolytic activity in the airways of patients with SR asthma and contributes to chronic airway remodeling.85 In addition, IL-13 augments TGF-β1–induced tissue inhibitor of metalloproteinase 1 expression through increased Smad phosphorylation.86 Furthermore, increased endothelin 1 synthesis by the bronchial epithelium characterizes severe refractory asthma and correlates with airway remodeling and airflow obstruction.87 In an interesting study, AHR decreased in TGF-β1 transgenic mice and was accompanied by the upregulation of basic fibroblast growth factor (FGF2) mRNA expression in lung tissues. AHR, mucus production, and lung inflammation were inhibited markedly by rFGF2 treatment. These data demonstrate FGF2 as a suppressor molecule in the downstream arm of TGF-β.88

The pathogenesis of exercise-induced bronchoconstriction (EIB) involves the release of mediators from several airway cells in response to exercise challenge, but the mechanism leading to airflow obstruction during EIB is incompletely understood. It was recently reported that the predominant gel-forming mucin expressed in the induced sputum of patients with asthma with EIB is MUC5AC. An increase in MUC5AC gene expression and release of MUC5AC protein occurs after exercise challenge, and MUC5AC release can occur through the cysteinyl leukotriene–associated activation of sensory airway nerves.89 Neurotrophins have been implicated in the pathogenesis of asthma because of their ability to induce airway inflammation and to promote hyperreactivity of sensory neurons. p75 neurotrophin receptor plays a dual role by promoting hyperreactivity of sensory nerves and airway inflammation.90 Interestingly, intracellular adhesion molecule 1 is expressed on inflamed airway parasympathetic nerves.91 Corticosteroids might prevent inflammatory cells from adhering to nerves by blocking intracellular adhesion molecule 1 expression on parasympathetic nerves.

The phospholipases and lysophospholipases expressed by eosinophils or other airway cells might represent novel therapeutic targets for blocking surfactant degradation, dysfunction, and peripheral airway closure in asthma.92 Different phases and phenotypes of allergic asthma are associated with distinct priming profiles of granulocytes.93, 94 Since their discovery, much data have been accumulated on eosinophil differentiation, morphology, trafficking, and anatomic location(s) in health and disease. “Classic” activation pathways (eg, cytokines, chemokines, proinflammatory components, and adhesion molecules) regulating eosinophil activation have been widely explored,95 and the expression and function of inhibitory receptors on eosinophils are being elucidated.3 In addition to eosinophils, neutrophils are also implicated in the pathogenesis of asthma. The presence of neutrophils is associated with an increase in the concentration of the proteolytic enzyme matrix metalloproteinase 9 in BALF, tissue, and sputum, suggesting a role of neutrophils in the remodeling process in asthma.93 Vascular remodeling is a feature of remodeling in asthma and nonasthmatic eosinophilic bronchitis and correlates with airway hyperresponsiveness.96 Chymase-positive mast cells are shown to be a relevant cellular source of vascular endothelial growth factor, and their specific targeting might be a tool for treating vascular remodeling in asthma.97

Advances in immunotherapy of allergic disease 

Many immunomodulators are currently under development for the therapy of asthma and allergic and immunologic diseases and are undergoing human clinical trials. The data from human clinical trials with immunomodulators, discussing the rationale for their use, efficacy results, and putative adverse events associated with them, have been summarized.98 There were also exciting novel developments in allergen specific immunotherapy (SIT).99 The mechanisms by which allergen SIT has its effects include the modulation of T-cell and B-cell responses and related antibody isotypes, as well as effector cells of allergic inflammation, such as eosinophils, basophils, and mast cells.99 The balance between allergen-specific Treg and TH2 cells appears to be decisive in the development of allergic and healthy immune responses against allergens. Treg cells consistently represent the dominant subset specific for common environmental allergens in sensitized healthy individuals. The induction of a tolerant state in peripheral T cells represents an essential step in allergen SIT. Peripheral T-cell tolerance is characterized mainly by generation of allergen-specific Treg cells, leading to suppressed T-cell proliferation and TH1 and TH2 cytokine responses against the allergen. The findings observed in sublingual SIT seem to be similar to those in injection SIT.100 Current understanding of mechanisms of allergen SIT, particularly the role of Treg cells in peripheral tolerance, might enable novel treatment strategies.

The combination of anti-IgE (omalizumab) therapy with ragweed injection immunotherapy for seasonal allergic rhinitis resulted in a significant reduction in systemic side effects and enhanced efficacy compared with immunotherapy alone. One of the proposed mechanisms is induction of serum regulatory antibodies, particularly IgG4, which partially blocks the binding of allergen–IgE complexes to B cells.101 The scope of anti-IgE therapy has now been extended to eosinophil-associated gastrointestinal disorders. Nine subjects have been treated with omalizumab, and the results suggest that anti-IgE therapy might be effective in these disorders.102

Recombinant allergens for SIT vaccines are continuously being produced and can be modified to reduce their allergenic activity and to foster certain advantageous immunologic properties. Recombinant allergens equaling the natural allergens are available for diagnostic and therapeutic purposes, and modified versions have been developed with the aim to reduce IgE-mediated side effects during immunotherapy.103, 104 During the development of novel allergen SIT vaccines, it has to be taken into consideration that the number of IgE epitopes on an allergen molecule and the concentration of allergen-specific IgE antibodies determine the extent of effector cell degranulation.105 The first injection immunotherapy trials conducted with recombinant vaccines for birch pollen and grass pollen allergy have shown that recombinant allergen–based immunotherapy holds promise to improve current immunotherapy practice and might open possibilities for new treatment strategies and possibly even strategies for prophylactic vaccination.

Other novel methods for allergen SIT or immunotherapy without allergens are being pursued. Peptide immunotherapy has been a promising alternative for treating allergic diseases. One way to enhance the efficacy of peptide immunotherapy is to use altered peptide ligands that contain amino acid substitutions compared with the natural peptide. An altered peptide ligand of an immunodominant Bos d 2 lipocalin allergen epitope was shown to induce a novel TH1-deviated T-cell population cross-reactive with the natural epitope in vitro.106 An interesting approach is using fusion proteins, with one arm targeting a cell-suppressor molecule.107, 108 A human bifunctional Fcγ-Fcɛ fusion protein cross-links FcγRIIb and FcɛRI on human mast cells and basophils and results in inhibition of FcɛRI-mediated degranulation.109 In addition, recent studies of microRNA function in specific immune response pathways indicate that specific microRNAs might be exploited as therapeutic targets to treat immune disorders, including autoimmunity, allergy, and hematopoietic cancers.110

The hygiene hypothesis and immune regulation 

The environment heavily influences the development of atopy, and several changes in lifestyle that have occurred over the past few decades are responsible for the increased prevalence of allergy in Western countries.2 After the discovery of Treg cells, their immune-suppressive role in the mechanism of the hygiene hypothesis was emphasized. Main arguments suggested to support the validity of such a mechanism are that (1) the prevalence of allergy is low in areas characterized by diffuse and chronic helminth infestations that induce strong TH2 responses, (2) high pathogen burden results in the production of increased amounts of suppressive cytokines (IL-10 and TGF-β), and (3) the prevalence of insulin-dependent diabetes mellitus, multiple sclerosis, and Crohn disease (autoimmune diseases thought to be TH1 dependent) is also increasing in developed countries.2 Several studies in rural populations have demonstrated that living on a farm provides a protective environment for the development of allergic diseases.111 An inverse association between farm milk consumption and allergic diseases is mediated by CD14-activated innate immune mechanisms.112 In addition, exposure to a farming environment has protective effects on TH2-dependent antibody isotype switching in response to common inhalant allergens in human subjects and reduces allergic reactions in mouse models.111, 113

Population studies have suggested that chronic and intense helminth infections, in contrast to acute and mild helminth infections, might suppress allergic airway inflammation. In mice during chronic, but not acute, Schistosoma mansoni infections, IL-10–dependent suppressive mechanisms are induced that regulate immune reactions to inhaled allergens. These data confirm human epidemiologic observations in a well-controlled animal model. As another supporting data on the hygiene hypothesis in human subjects, attendance to day care units before 3 months of age is inversely associated with asthma and atopy in later childhood through the reduction of IgE levels.114

Studies demonstrate a strong role for the innate immune system in the development of allergic symptoms. An interesting link for innate immune response stimulation of DCs and the development of distinct T-cell phenotypes has been recently proposed.8 Myeloid immature DCs constitutively expressed Jagged-1, which induces a TH2 polarization in CD4+ T cells. The TH2 polarization associated with high GATA-3/T-bet ratio was at least partly dependent on the early induction of IL-4. Maturation of DCs by Toll-like receptor (TLR) ligation resulted in the reduction of Jagged-1 and upregulation of Delta-4, which was at least in part responsible for the polarization of CD4+ T cells to the TH1 phenotype. In conclusion, CD4+ T-cell responses are usually characterized by a prevalent TH2 phenotype unless TLRs are triggered on DCs by microbial components.8 Monocytes, particularly the proinflammatory monocytes, from patients with AD are functionally defective in their capacity to produce proinflammatory cytokines on TLR2 stimuli, in part because of the high levels of their FcɛRI expression.115 An antibody to TLR2 partially blocked the effect of lipoteichoic acid but not peptidoglycan. Both lipoteichoic acid and peptidoglycan reduced mast cell degranulation caused by an antigen-specific IgE.116 Similarly, TLR4 ligation on oral Langerhans cells induced IL-10, TGF-β1, FOXP3, IFN-γ, and IL-2 production in T cells.117

Environmental influence and feeding habits can affect the signal transduction in T cells. Allergic disease, which develops after 1 and 2.5 years, is associated with altered expression of T-cell protein kinase C isozymes in the neonatal period.118 It has also been demonstrated that fish oil can modulate expression of protein kinase C isozymes in a potentially favorable direction.118 The study suggests that protein kinase Cζ should be explored further as an early marker and potential target for disease prevention.

Conclusions 

Substantial progress in understanding the mechanisms of immunoregulation in allergy, asthma, and other immunoregulation-related diseases, has been reported in the Journal of Allergy and Clinical Immunology (Table I). Recent developments in T-cell subsets, particularly the extension of the knowledge on reciprocal regulation and counterbalance between TH1 and TH2 cells to TH17 and Treg cells has increased our knowledge of the mechanisms of immunoregulation. Allergen-specific strategies for targeting immune responses have significantly evolved, and new insights into the mechanism of immunoregulation lead to novel approaches for allergen SIT vaccine development. Exciting advances in mechanisms of asthma, the hygiene hypothesis, and IgE regulation have been made, which will enable better understanding of the disease and novel treatment strategies.