TH17 cells mediate steroid-resistant airway inflammation and airway hyperresponsiveness in mice

Apigenin inhibits allergen-induced airway inflammation and switches immune response in a murine model of asthma

Many flavonoids were demonstrated to possess the antiallergic effect. Here we detected whether apigenin, a flavonoid, can attenuate allergen-induced airway inflammation and what is the possible mechanism in a murine model of asthma. Apigenin decreased the degree of the inflammatory cell infiltration, airway hyperresponsiveness, and total immunoglobulin E levels compared with the ovalbumin group. In addition, apigenin triggered the switching of the immune response to allergens toward a T-helper type 1 (Th1) profile. Our data clearly demonstrated that apigenin exhibits an anti-inflammatory activity in a murine asthma model, and can switch the immune response to allergens toward the Th1 profile.

IL-17 and IL-22 in atopic allergic disease

A long standing paradigm is that antigen-specific Th2 cells and their cytokines such as IL-4, IL-5, and IL-13 orchestrate the characteristic features of atopic allergy. The discovery of a role for IL-17-producing (Th17) and IL-22-producing (Th22) T helper cells in inflammatory diseases has added an additional layer of complexity to the understanding of the pathogenesis of allergic diseases. Here we re-evaluate the role of T helper cells, with special focus on the Th17 and Th22 subsets in allergic asthma and atopic dermatitis. Whereas sparse data point to a protective role of the increasing amounts of Th22 cells that are found in chronic stages of both allergies, the data on Th17 cells paint different pictures for the contribution of Th17 cells during subsequent stages of these two forms of allergy.

Functions of T cells in asthma: more than just T(H)2 cells

Asthma has been considered a T helper 2 (T(H)2) cell-associated inflammatory disease, and T(H)2-type cytokines, such as interleukin-4 (IL-4), IL-5 and IL-13, are thought to drive the disease pathology in patients. Although atopic asthma has a substantial T(H)2 cell component, the disease is notoriously heterogeneous, and recent evidence has suggested that other T cells also contribute to the development of asthma. Here, we discuss the roles of different T cell subsets in the allergic lung, consider how each subset can contribute to the development of allergic pathology and evaluate how we might manipulate these cells for new asthma therapies.

Th17 cells and transplant acceptance

The discovery of Th17 cells has revealed a novel pathway of T-cell maturation. As with Th1 and Th2 lineages, Th17 cells promote graft pathology. However, a growing body of evidence indicates that Th17 cells may exhibit resistance to current methods of immunosuppression. Identification of this lineage provides an additional and challenging target for promoting graft acceptance.

Role of NKT cells in allergic asthma

T helper 2 (Th2) cells play crucial roles in the development of allergic asthma, while various distinct cell populations also contribute to the pathogenesis of the disease. Invariant natural killer T (iNKT) cells produce large amounts of cytokines such as IL-4 and IFNγ upon stimulation with a ligand, α-galactosylceramide, and regulate various immune responses. Recently, a critical role of iNKT cells in the mouse model of asthma and also in asthma patients has been reported, while some contradictory results have also been described. Here, we summarize the experimental results in mouse and human systems, and discuss the current understanding of the role of NKT cells in the pathogenesis of asthma, including a possible mechanism by which iNKT cells are activated in asthma patients.

A critical role for C5L2 in the pathogenesis of experimental allergic asthma

The complement fragment C5a plays dual roles in the development of experimental allergic asthma. It protects from pulmonary allergy by a regulatory effect on dendritic cells during allergen sensitization, but is proallergic during the effector phase. C5a can bind to two distinct receptors (i.e., C5a receptor and C5a receptor-like 2 [C5L2]). The functional role of C5L2 in vivo remains enigmatic. In this study, we show in two models of OVA- and house dust mite (HDM)-induced experimental allergic asthma that C5L2-deficient mice are protected from the development of airway hyperresponsiveness, Th2 cytokine production, eosinophilic airway inflammation, serum IgE, or mucus production. Surprisingly, HDM-induced experimental asthma in C5L2-deficient mice was associated with increased pulmonary IL-17A production and increased airway neutrophil numbers. To directly assess the role of C5L2 on myeloid dendritic cells (mDCs) during allergen sensitization, we performed single or repeated adoptive transfers of C5L2-deficient mDCs into wild-type mice. HDM-pulsed C5L2-deficient mDCs induced strong Th2 cytokine production, which was associated with marked IFN-γ and IL-17A production, decreased eosinophil numbers, and reduced IgE production as compared with HDM-pulsed mDCs from wild-type mice. HDM stimulation of C5L2(-/-) mDCs in vitro resulted in production of Th17-promoting cytokine IL-23, which was absent in wild-type mDCs. Our findings suggest that C5L2 acts at the mDC/T cell interface to control the development of Th1 and Th17 cells in response to airway HDM exposure. Furthermore, it drives Th2 immune responses independent of mDCs, suggesting a complex role for C5L2 in the development of experimental allergic asthma.

A viral PAMP double-stranded RNA induces allergen-specific Th17 cell response in the airways which is dependent on VEGF and IL-6

BACKGROUND

Innate immune response by a viral pathogen-associated molecular pattern dsRNA modulates the subsequent development of adaptive immune responses. Although virus-associated asthma is characterized by noneosinophilic inflammation, the role of Th17 cell response in the development of virus-associated asthma is still unknown.

OBJECTIVE

To evaluate the role of the Th17 cell response and its underlying polarizing mechanisms in the development of an experimental virus-associated asthma.

METHODS

An experimental virus-associated asthma was created via airway sensitization with ovalbumin (OVA, 75 μg) and a low (0.1 μg) or a high (10 μg) doses of synthetic dsRNA [polyinosine-polycytidylic acid; poly(I:C)]. Transgenic (IL-17-, IL-6-deficient mice) and pharmacologic [a vascular endothelial growth factor receptor (VEGFR) inhibitor] approaches were used to evaluate the roles of Th17 cell responses.

RESULTS

After cosensitization with OVA and low-dose poly(I:C), but not with high-dose poly(I:C), inflammation scores after allergen challenge were lower in IL-17-deficient mice than in wild-type (WT) mice. Moreover, inflammation enhanced by low-dose poly(I:C), but not by high-dose poly(I:C), was impaired in IL-6-deficient mice; this phenotype was accompanied by the down-regulation of IL-17 production from T cells from both lymph nodes and lung tissues. Airway exposure of low-dose poly(I:C) enhanced the production of VEGF and IL-6, and the production of IL-6 was blocked by treatment with a VEGFR inhibitor (SU5416). Moreover, the allergen-specific Th17 cell response and subsequent inflammation in the low-dose poly(I:C) model were impaired by the VEGFR inhibitor treatment during sensitization.

CONCLUSIONS

Airway exposure of low-level dsRNA induces an allergen-specific Th17 cell response, which is mainly dependent on VEGF and IL-6.

Chronic psychosocial stress promotes systemic immune activation and the development of inflammatory Th cell responses

Recent studies indicate that chronic psychosocial stress favors the development of generalized immune dysfunction. During stressor exposure neuroendocrine factors affect numbers and functionality of leukocytes. However, the exact mechanisms leading to systemic changes in immune functions during stress are still not clear. During chronic subordinate colony housing, a model of chronic psychosocial stress, mice developed spontaneous colonic inflammation. Decreased glucocorticoid signaling, induced by a combination of adrenal insufficiency and glucocorticoid resistance, was thought to prevent tempering of local immune cells, and to promote tissue inflammation. In this study we investigated changes in the systemic immune status after chronic subordinate colony housing and analyzed potential mechanisms underlying those alterations. Analysis of T helper cell subsets in peripheral lymph nodes revealed a reduction of regulatory T cells, accompanied by increased T cell effector functions. Generalized activation of T cells was shown by elevated cytokine production upon stimulation. In addition, we observed no apparent shift towards T helper type 2 responses. It is likely, that the previously reported hypocorticism in this stress model led to a steady production of inflammatory Th1, Th2, and Th17 cytokines and obstructed the shift towards an anti-inflammatory response. In conclusion, we established chronic subordinate colony housing as a model to investigate the outcome of stress on the systemic immune status. We also provide evidence that distinct T helper cell subtypes react differentially to the suppressive effect of glucocorticoids.

Anti-inflammatory effects of the neurotransmitter agonist Honokiol in a mouse model of allergic asthma

Chronic airway inflammation is a hallmark of asthma, an immune-based disease with great societal impact. Honokiol (HNK), a phenolic neurotransmitter receptor (γ-aminobutyric acid type A) agonist purified from magnolia, has anti-inflammatory properties, including stabilization of inflammation in experimentally induced arthritis. The present study tested the prediction that HNK could inhibit the chronic inflammatory component of allergic asthma. C57BL/6 mice sensitized to and challenged with OVA had increased airway hyperresponsiveness to methacholine challenge and eosinophilia compared with naive controls. HNK-treated mice showed a reduction in airway hyperresponsiveness as well as a significant decrease in lung eosinophilia. Histopathology studies revealed a marked drop in lung inflammation, goblet cell hyperplasia, and collagen deposition with HNK treatment. Ag recall responses from HNK-treated mice showed decreased proinflammatory cytokines in response to OVA, including TNF-α-, IL-6-, Th1-, and Th17-type cytokines, despite an increase in Th2-type cytokines. Regulatory cytokines IL-10 and TGF-β were also increased. Assessment of lung homogenates revealed a similar pattern of cytokines, with a noted increase in the number of FoxP3(+) cells in the lung. HNK was able to alter B and T lymphocyte cytokine secretion in a γ-aminobutyric acid type A-dependent manner. These results indicate that symptoms and pathology of asthma can be alleviated even in the presence of increased Th2 cytokines and that neurotransmitter agonists such as HNK have promise as a novel class of anti-inflammatory agents in the treatment of chronic asthma.

IL-27/IFN-γ induce MyD88-dependent steroid-resistant airway hyperresponsiveness by inhibiting glucocorticoid signaling in macrophages

Inflammation and airway hyperresponsiveness (AHR) are hallmark features of asthma and often correlate with the severity of clinical disease. Although these features of asthma can be effectively managed with glucocorticoid therapy, a subgroup of patients, typically with severe asthma, remains refractory to therapy. The mechanisms leading to steroid resistance in severe asthmatics are poorly understood but may be related to the activation of innate host defense pathways. Previously, we have shown that IFN-γ-producing cells and LPS, two factors that are associated with severe asthma, induce steroid-resistant AHR in a mouse model. We now demonstrate that cooperative signaling induced by IFN-γ and LPS results in the production of IL-27 by mouse pulmonary macrophages. IL-27 and IFN-γ uniquely cooperate to induce glucocorticoid-resistant AHR through a previously unknown MyD88-dependent mechanism in pulmonary macrophages. Importantly, integrated signaling by IL-27/IFN-γ inhibits glucocorticoid-induced translocation of the glucocorticoid receptor to the nucleus of macrophages. Furthermore, expression of both IL-27 and IFN-γ was increased in the induced sputum of steroid-refractory asthmatics. These results suggest that a potential mechanism for steroid resistance in asthma is the activation of MyD88-dependent pathways in macrophages that are triggered by IL-27 and IFN-γ, and that manipulation of these pathways may be a therapeutic target.

Heterogeneity of glucocorticoid resistance in patients with bronchial asthma

Bronchial asthma is assumed to be the result of excessive inflammation driven by an aberrant T-helper-2 (Th2) response. Recently, it has begun to be recognized that asthma is a heterogeneous disorder. Glucocorticoids (GCs) are effective treatment for bronchial asthma; however, the inflammation in bronchial asthma cannot always be fully controlled. A recent study demonstrated a new underlying mechanism of glucocorticoid resistance that acts in a Th2-independent manner. Thus, responses to GCs are highly heterogeneous.

Vitamin D3 attenuates Th2 responses to Aspergillus fumigatus mounted by CD4+ T cells from cystic fibrosis patients with allergic bronchopulmonary aspergillosis

Allergic bronchopulmonary aspergillosis (ABPA) is caused by a dominant Th2 immune response to antigens derived from the opportunistic mold Aspergillus, most commonly Aspergillus fumigatus. It occurs in 4%-15% of patients with cystic fibrosis (CF); however, not all patients with CF infected with A. fumigatus develop ABPA. Therefore, we compared cohorts of A. fumigatus-colonized CF patients with and without ABPA to identify factors mediating tolerance versus sensitization. We found that the costimulatory molecule OX40 ligand (OX40L) was critical in driving Th2 responses to A. fumigatus in peripheral CD4+ T cells isolated from patients with ABPA. In contrast, CD4+ T cells from the non-ABPA cohort did not mount enhanced Th2 responses in vitro and contained a higher frequency of TGF-beta-expressing regulatory T cells. Heightened Th2 reactivity in the ABPA cohort correlated with lower mean serum vitamin D levels. Further, in vitro addition of 1,25 OH-vitamin D3 substantially reduced DC expression of OX40L and increased DC expression of TGF-beta. This in vitro treatment also resulted in increased Treg TGF-beta expression and reduced Th2 responses by CD4+ T cells from patients with ABPA. These data provide rationale for a therapeutic trial of vitamin D to prevent or treat ABPA in patients with CF.

The role of the T cell in asthma

Since the initial detection of T(H)2 cytokines in asthmatic airways, our understanding of the complexity of T-cell subtypes and flexibility and of the potential role of airway structural cells in the immunopathology of asthma has increased. Cytokines derived from airway epithelium, including IL-25, IL-33, and thymic stromal lymphopoietin, might be important drivers of T(H)2-type inflammation in asthma. The balance between effector T(H)2 cells and suppressive regulatory T cells is skewed toward a proinflammatory T(H)2 response in atopy and asthma, and there is much interest in how to redress this equilibrium. Novel T-cell subsets, including T(H)17, T(H)9, and T(H)22, have been described, although their role in asthma remains unclear. Other T cells, including natural killer T cells, γδ T cells, and CD8 T cells, have also been implicated in asthma, although their importance remains to be confirmed. Therapeutic strategies aimed at T(H)2 cytokines are beginning to bear fruit in patients with asthma, although like many biologic agents, these might need specific targeting at subgroups of patients. Strategies directed specifically at the T cells are currently being evaluated, including novel forms of allergen immunotherapy. T cells remain an exciting potential target for new treatments in patients with asthma.

NO2 inhalation induces maturation of pulmonary CD11c+ cells that promote antigenspecific CD4+ T cell polarization

Background

Nitrogen dioxide (NO₂) is an air pollutant associated with poor respiratory health, asthma exacerbation, and an increased likelihood of inhalational allergies. NO₂ is also produced endogenously in the lung during acute inflammatory responses. NO₂ can function as an adjuvant, allowing for allergic sensitization to an innocuous inhaled antigen and the generation of an antigen-specific Th2 immune response manifesting in an allergic asthma phenotype. As CD11c⁺ antigen presenting cells are considered critical for naïve T cell activation, we investigated the role of CD11c⁺ cells in NO₂-promoted allergic sensitization.

Methods

We systemically depleted CD11c⁺ cells from transgenic mice expressing a simian diphtheria toxin (DT) receptor under of control of the CD11c promoter by administration of DT. Mice were then exposed to 15 ppm NO₂ followed by aerosolized ovalbumin to promote allergic sensitization to ovalbumin and were studied after subsequent inhaled ovalbumin challenges for manifestation of allergic airway disease. In addition, pulmonary CD11c⁺ cells from wildtype mice were studied after exposure to NO₂ and ovalbumin for cellular phenotype by flow cytometry and in vitro cytokine production.

Results

Transient depletion of CD11c⁺ cells during sensitization attenuated airway eosinophilia during allergen challenge and reduced Th2 and Th17 cytokine production. Lung CD11c⁺ cells from wildtype mice exhibited a significant increase in MHCII, CD40, and OX40L expression 2 hours following NO₂ exposure. By 48 hours, CD11c⁺MHCII⁺ DCs within the mediastinal lymph node (MLN) expressed maturation markers, including CD80, CD86, and OX40L. CD11c⁺CD11b^- and CD11c⁺CD11b⁺ pulmonary cells exposed to NO₂ in vivo increased uptake of antigen 2 hours post exposure, with increased ova-Alexa 647⁺ CD11c⁺MHCII⁺ DCs present in MLN from NO₂-exposed mice by 48 hours. Co-cultures of ova-specific CD4⁺ T cells from naïve mice and CD11c⁺ pulmonary cells from NO₂-exposed mice produced IL-1, IL-12p70, and IL-6 in vitro and augmented antigen-induced IL-5 production.

Conclusions

CD11c⁺ cells are critical for NO₂-promoted allergic sensitization. NO₂ exposure causes pulmonary CD11c⁺ cells to acquire a phenotype capable of increased antigen uptake, migration to the draining lymph node, expression of MHCII and co-stimulatory molecules required to activate naïve T cells, and secretion of polarizing cytokines to shape a Th2/Th17 response.

The other T helper cells in asthma pathogenesis

The complex phenotype of allergic bronchial asthma involves a variable degree of bronchoobstruction, increased mucus production, and airway remodeling. So far it is suggested that it arises from multiple interactions of infiltrating and structural cells in the context of chronic airway inflammation that is orchestrated by T helper 2 (TH2) cells. By secreting a plethora of typical mediators such as interleukin (IL) 4, IL-5, and IL-13, these cells hold a key position in asthma pathogenesis. However, therapeutic approaches targeting these TH2-type mediators failed to improve asthma symptoms and impressively showed that asthma pathogenesis cannot be reduced by TH2 cell functions. Recently, other T helper cells, that is, TH9 and TH17 cells, have been identified and these cells also contribute to asthma pathogenesis, the processes leading to formation or aggravation of asthma. Furthermore, TH25 cells, TH3 cells, and regulatory T cells have also been implicated in asthma pathogenesis. This paper aims at summarizing recent insights about these new T helper cells in asthma pathogenesis.

Natural killer T cells regulate the development of asthma

Studies in mice, monkeys and humans suggest that invariant natural killer (iNK) T cells play a very important role in the pathogenesis of asthma, a heterogeneous disease associated with airway inflammation and airway hyper-reactivity. The requirement for iNK T cells in multiple mouse models of asthma is novel and surprising, challenging the prevailing dogma that CD4(+) T cells responding to environmental allergens are the key cell type in asthma. In this article, we examine the recent studies of iNK T cells and asthma, and discuss how different subsets of NK T cells function in different forms of asthma, including forms that are independent of adaptive immunity and Th2 cells. Together, these studies suggest that iNK T cells, which can interact with many other cell types including Th2 cells, eosinophils and neutrophils, provide a unifying pathogenic mechanism for many distinct forms of asthma.

Interleukin-17 regulation: an attractive therapeutic approach for asthma

Interleukin (IL)-17 is recognized to play a critical role in numerous immune and inflammatory responses by regulating the expression of various inflammatory mediators, which include cytokines, chemokines, and adhesion molecules. There is growing evidence that IL-17 is involved in the pathogenesis of asthma. IL-17 orchestrates the neutrophilic influx into the airways and also enhances T-helper 2 (Th2) cell-mediated eosinophilic airway inflammation in asthma. Recent studies have demonstrated that not only inhibitor of IL-17 per se but also diverse regulators of IL-17 expression reduce antigen-induced airway inflammation, bronchial hyperresponsiveness, and Th2 cytokine levels in animal models of asthma. This review will summarize the role of IL-17 in the context of allergic airway inflammation and discuss the therapeutic potential of various strategies targeting IL-17 for asthma.

The overexpression of heparin-binding epidermal growth factor is responsible for Th17-induced airway remodeling in an experimental asthma model

Th17 cells that produce IL-17 have been found to participate in the development of allergy-triggered asthma. However, whether they play a causative role in the pathogenesis of airway remodeling in chronic asthma remains unclear. In this study, we investigated the role of Th17 cells in airway remodeling and the possible involvement of epidermal growth factor (EGF) receptor signals downstream of Th17. We established a C57BL/6 mouse model of prolonged allergen challenge that exhibits many characteristics of airway remodeling. Prolonged allergen challenge induced a progressive increase in the number of airway-infiltrating Th17 cells, and Th17 counts positively correlated with the severity of airway remodeling. Increases in mucus production, airway smooth muscle (ASM) mass, peribronchial collagen deposition, and airway heparin-binding EGF (HB-EGF) expression have been observed in sensitized mice following prolonged allergen exposure or adoptive Th17 transfer; remarkably, these effects can be abrogated by treatment with anti-IL-17 mAb. Both the EFGR inhibitor AG1478 and an anti-HB-EGF mAb ameliorated all of these effects, except for peribronchial collagen deposition in the presence of high levels of IL-17. In vitro, Th17 cells enhanced the airway epithelial expression of HB-EGF in a coculture of the two cells. The conditioned medium obtained from this coculture system effectively promoted ASM proliferation; this response was dramatically abolished by anti-HB-EGF mAb but not Abs against other EGF receptor ligands or IL-17. These observations demonstrated that overexpression of airway HB-EGF induced by IL-17 secreted from redundant expanding Th17 cells might contribute to excessive mucus expression and ASM proliferation in chronic asthma.

Regulatory T cells, inflammation and the allergic response-The role of glucocorticoids and Vitamin D

Regulatory T cells (TRegs) play a central role in the maintenance of peripheral tolerance. They prevent inappropriate immune responses to ubiquitous allergens in healthy individuals, and contribute to the maintenance of immune homeostasis in the airways. Both Foxp3+ and IL-10+ TReg have been implicated in these functions. Glucocorticoids represent the mainstay of treatment for asthma and other allergic conditions, and evidence that steroids influence TReg function will be reviewed. Growing bodies of epidemiological and immunological data suggest a role for endogenous Vitamin D in immune regulation. This review will discuss the role of glucocorticoids and Vitamin D, and their potential interactions in promoting tolerance in the context of allergic disease and asthma.

The role of the glucocorticoid receptor in inflammation and immunity

Glucocorticoids are potent immunosuppressive agents with complex actions on immune cells evoking the following effects: inducing apoptosis, changing differentiation fate, inhibition of cytokine release, inhibition of migration and other features. Distinct molecular mechanisms of the glucocorticoid receptor (GR) contribute to different anti-inflammatory effects. Recently inflammatory models have been investigated using conditional knockout and function selective mice shedding light on critical cell types and molecular mechanisms of endogenous and therapeutic GC actions. Here we review the multiple effects of GCs on major immune cells, dendritic cells, myeloid cells and B- and T-lymphocytes and give a summary of studies using conditional GR knockout mice.

Natural Foxp3(+) regulatory T cells inhibit Th2 polarization but are biased toward suppression of Th17-driven lung inflammation

nTregs prevent autoimmunity and modulate immune and inflammatory responses to foreign antigens. CD4(+)Foxp3(+) nTregs from DO11.10 mice were expanded ex vivo, and their effectiveness in suppressing the development of lung inflammatory responses, elicited by differentiated CD4(+) T cells following antigen inhalation, was examined. Effector DO11.10 CD4(+) Th2 cells, when adoptively transferred into BALB/c mice that subsequently inhaled OVA, elicited a pronounced pulmonary, eosinophilic inflammation. Surprisingly, the cotransfer of expanded nTregs failed to suppress the Th2-mediated airway inflammation. Nevertheless, expanded OVA-specific CD4(+)Foxp3(+) nTregs were highly effective at inhibiting the polarization of naïve CD4(+) T cells into a Th2 phenotype. This suppression was reversed by an antibody to GITR but was not affected by the presence of the soluble OX40L. Further analysis revealed that although nTregs also failed to inhibit the lung neutrophilic inflammation induced by effector CD4(+) Th1 cells, they markedly suppressed pulmonary inflammation elicited by CD4(+) Th17 cells but not AHR. The suppression of the Th17-mediated response was evident from a striking reduction in the proportion of OVA-specific T cells expressing IL-17 and the numbers of neutrophils present in the airways of Th17 recipient mice. Collectively, these results demonstrate that expanded nTregs clearly limit the Th2 polarization process and that Th17-mediated inflammatory responses are particularly prone to the immunoregulatory properties of nTregs. These findings thus indicate that expanded nTregs are restrictive in their ability to suppress airway inflammatory processes and AHR.

TH17 cells in asthma and COPD

Asthma and chronic obstructive pulmonary disease (COPD) represent two classes of chronic obstructive lung disorders that may share some similar immunologic mechanisms of disease. Asthma is a complex human disease characterized by airway hyperresponsiveness (AHR) and inflammation, whereas COPD is marked by progressive emphysematic changes in the lung. Recently it has been shown that advanced COPD is characterized by lymphoid follicles, drawing attention to immunological mechanisms in COPD. Despite numerous studies in mice to elucidate the immunologic mechanisms of asthma, sufficient current treatment options are limited. Clinically, many asthma patients fail to satisfactorily respond to standard steroid therapy, and this type of steroid-resistant, severe asthma has been linked to the presence of neutrophilic inflammation in the lung. The role of neutrophils, macrophages, and their secreted proteases in COPD needs to be better defined. Recently, the T lymphocyte subset T(H)17 was shown to play a role in regulating neutrophilic and macrophage inflammation in the lung, suggesting a potential role for T(H)17 cells in severe, steroid-insensitive asthma and COPD.

Regulatory T cells in many flavors control asthma

That regulatory T cells (Tregs) have a crucial role in controlling allergic diseases such as asthma is now undisputed. The cytokines most commonly implicated in Treg-mediated suppression of allergic asthma are transforming growth factor-beta (TGF-beta) and interleukin (IL)-10). In addition to naturally occurring Tregs, adaptive Tregs, induced in response to foreign antigens, have been shown in recent studies. The concept of inducible/adaptive Tregs (iTregs) has considerable significance in preventing asthma if generated early enough in life. This is because cytokines such as IL-4 and IL-6 inhibit Foxp3 induction in naive CD4+ T cells and therefore de novo generation of Tregs can be expected to be less efficient when it is concomitant with effector cell development in response to an allergen. However, if iTregs can be induced, the process of infectious tolerance would facilitate expansion of the iTreg pool as suggested in the recent literature. It is tempting to speculate that there is a window of opportunity in early life in the context of a relatively immature immune system that is permissive for the generation of iTregs specific to a spectrum of allergens that would regulate asthma for lifelong. The focus of this review is the relevance of nTregs and iTregs in controlling asthma from early life into adulthood, the mechanisms underlying Treg function, and the prospects for using our current concepts to harness the full potential of Tregs to limit disease development and progression.

Resolution of allergic airway inflammation and airway hyperreactivity is mediated by IL-17-producing {gamma}{delta}T cells

RATIONALE

gammadeltaT lymphocytes are enriched within the epithelial microenvironment, where they are thought to maintain homeostasis and limit immunopathology. gammadeltaT cells are postulated to exert a regulatory influence during acute allergic airway disease, but the mechanism is unknown. Although regulation of allergic airway disease has been attributed to IL-17-producing T helper (Th) 17 cells, we have found that gammadeltaT cells represent the major source of IL-17 in the allergic lung.

OBJECTIVES

The aim of this study was to determine the contribution of these IL-17-producing gammadeltaT cells to regulation of allergic airway inflammation.

METHODS

Flow cytometry revealed that IL-17-producing gammadeltaT cells are more prevalent than IL-17(+)alphabetaT cells (Th17) in a murine model of ovalbumin-induced allergic inflammation.

MEASUREMENTS AND MAIN RESULTS

Transfer of gammadeltaT cells at the peak of acute allergic responses ameliorated airway hyperresponsiveness with a corresponding acceleration in the resolution of eosinophilic and Th2-driven inflammation. Conversely, functional blockade of gammadeltaT cells led to exacerbation of injury. Neither treatment changed pulmonary Th17 cell numbers. Moreover, transfer of Th17 cells had no effect on disease outcome. Importantly, IL-17-deficient gammadeltaT cells were unable to promote resolution of injury. These data identify IL-17-producing gammadeltaT cells as key regulators of the allergic response in vivo.

CONCLUSIONS

This unfolds a new perspective for the understanding of gammadeltaT cell function with regard to innate regulation of the adaptive immune responses, emphasizing that resolution of responses are important in determining the outcome of acute inflammatory episodes as well as for maintenance of tissue integrity and homeostasis.

Perinatal stress and early life programming of lung structure and function

Exposure to environmental toxins during critical periods of prenatal and/or postnatal development may alter the normal course of lung morphogenesis and maturation, potentially resulting in changes that affect both structure and function of the respiratory system. Moreover, these early effects may persist into adult life magnifying the potential public health impact. Aberrant or excessive pro-inflammatory immune responses, occurring both locally and systemically, that result in inflammatory damage to the airway are a central determinant of lung structure-function changes throughout life. Disruption of neuroendocrine function in early development, specifically the hypothalamic-pituitary-adrenal (HPA) axis, may alter functional status of the immune system. Autonomic nervous system (ANS) function (sympathovagal imbalance) is another integral component of airway function and immunity in childhood. This overview discusses the evidence linking psychological factors to alterations in these interrelated physiological processes that may, in turn, influence childhood lung function and identifies gaps in our understanding.

Chlamydial respiratory infection during allergen sensitization drives neutrophilic allergic airways disease

Neutrophilic asthma is a prevalent, yet recently described phenotype of asthma. It is characterized by neutrophilic rather than eosinophilic airway inflammation and airways hyperresponsiveness (AHR) and may have an infectious origin. Chlamydial respiratory infections are associated with asthma, but how these Th1-inducing bacteria influence Th2-mediated asthma remains unknown. The effects of chlamydial infection on the development of asthma were investigated using a BALB/c mouse model of OVA-induced allergic airways disease (AAD). The effects of current and resolved Chlamydia muridarum infection during OVA sensitization on AAD were assessed and compared with uninfected and nonsensitized controls. Current, but not resolved, infection attenuated hallmark features of AAD: pulmonary eosinophil influx, T cell production of IL-5, mucus-secreting cell hyperplasia, and AHR. Current infection also induced robust OVA-driven neutrophilic inflammation and IFN-gamma release from T cells. The phenotype of suppressed but persistent Th2 responses in association with enhanced neutrophilia is reminiscent of neutrophilic asthma. This phenotype was also characterized by increased pulmonary IL-12 and IL-17 expression and activation of APCs, as well as by reduced thymus- and activation-regulated chemokine. Inhibition of pulmonary neutrophil influx during infection blocked OVA-induced neutrophilic inflammation and T cell IFN-gamma production and reversed the suppressive effects on mucus-secreting cell hyperplasia and AHR during AAD. These changes correlated with decreased IL-12 and IL-17 expression, increased thymus- and activation-regulated chemokine and altered APC activation. Blocking IFN-gamma and IL-17 during OVA challenge had no effect. Thus, active chlamydial respiratory infection during sensitization enhances subsequent neutrophilic inflammation and Th1/Th17 responses during allergen exposure and may have a role in the pathogenesis of neutrophilic asthma.

Role of IL-23 and Th17 Cells in Airway Inflammation in Asthma

Asthma is characterized by chronic airway inflammation with intense eosinophil and lymphocyte infiltration, mucus hyperproduction, and airway hyperresponsiveness. Accumulating evidence indicates that antigen-specific Th2 cells and their cytokines such as IL-4, IL-5, and IL-13 orchestrate these pathognomonic features of asthma. In addition, we and others have recently shown that IL-17-producing CD4⁺ T cells (Th17 cells) and IL-23, an IL-12-related cytokine that is essential for survival and functional maturation of Th17 cells, are involved in antigen-induced airway inflammation. In this review, our current understanding of the roles of IL-23 and Th17 cells in the pathogenesis of allergic airway inflammation will be summarized.

Narrative review: the role of Th2 immune pathway modulation in the treatment of severe asthma and its phenotypes

New therapeutic approaches are needed for patients with severe asthma who are refractory to standard therapy comprising high doses of inhaled corticosteroids plus long-acting beta(2)-agonists. Current treatment guidelines for patients with severe asthma from the National Asthma Education and Prevention Program recommend the addition of oral corticosteroids, which are associated with substantial morbidity, and, for those with allergic asthma, anti-IgE. Genetic and translational studies, as well as clinical trials, suggest that in a subgroup of patients, the pathobiology of severe asthma is mediated by immune pathways driven by T-helper 2 (Th2)-type CD4(+) T cells, which produce a characteristic repertoire of interleukins (ILs), including IL-4, IL-5, and IL-13. Therefore, biological modifiers of Th2-type ILs, such as monoclonal antibodies, soluble receptors, and receptor antagonists, are a rational strategy for developing new treatment approaches but will need to be targeted to selected patients in whom the appropriate Th2 immune pathway is "active." The benefits of immune-modifier therapies targeting Th2-type cytokines, however, need to be weighed against the toxicities associated with inhibition of key biological pathways, as well as the expense of future medications. Therefore, future clinical trials need to clearly establish the efficacy and safety of biological modifiers of Th2 immune pathways before these approaches can enter routine clinical practice for the treatment of severe asthma.

Narrative review: the role of Th2 immune pathway modulation in the treatment of severe asthma and its phenotypes

New therapeutic approaches are needed for patients with severe asthma who are refractory to standard therapy comprising high doses of inhaled corticosteroids plus long-acting beta(2)-agonists. Current treatment guidelines for patients with severe asthma from the National Asthma Education and Prevention Program recommend the addition of oral corticosteroids, which are associated with substantial morbidity, and, for those with allergic asthma, anti-IgE. Genetic and translational studies, as well as clinical trials, suggest that in a subgroup of patients, the pathobiology of severe asthma is mediated by immune pathways driven by T-helper 2 (Th2)-type CD4(+) T cells, which produce a characteristic repertoire of interleukins (ILs), including IL-4, IL-5, and IL-13. Therefore, biological modifiers of Th2-type ILs, such as monoclonal antibodies, soluble receptors, and receptor antagonists, are a rational strategy for developing new treatment approaches but will need to be targeted to selected patients in whom the appropriate Th2 immune pathway is "active." The benefits of immune-modifier therapies targeting Th2-type cytokines, however, need to be weighed against the toxicities associated with inhibition of key biological pathways, as well as the expense of future medications. Therefore, future clinical trials need to clearly establish the efficacy and safety of biological modifiers of Th2 immune pathways before these approaches can enter routine clinical practice for the treatment of severe asthma.

Harnessing regulatory T cells to suppress asthma: from potential to therapy

Regulatory T cells (Tregs) play an essential role in maintaining the homeostatic balance of immune responses. Asthma is an inflammatory condition of the airways that is driven by dysregulated immune responses toward normally innocuous antigens. Individuals with asthma have fewer and less functional Tregs, which may lead to uncontrolled effector cell responses and promote proasthmatic responses of T helper type 2, T helper 17, natural killer T, antigen-presenting, and B cells. Tregs have the capacity to either directly or indirectly suppress these responses. Hence, the induced expansion of functional Tregs in predisposed or individuals with asthma is a potential approach for the prevention and treatment of asthma. Infection by a number of micro-organisms has been associated with reduced prevalence of asthma, and many infectious agents have been shown to induce Tregs and reduce allergic airways disease in mouse models. The translation of the regulatory and therapeutic properties of infectious agents for use in asthma requires the identification of key modulatory components and the development and trial of effective immunoregulatory therapies. Further translational and clinical research is required for the induction of Tregs to be harnessed as a therapeutic strategy for asthma.

Preventive effects of mycobacteria and their culture supernatants against asthma development in BALB/c mice

Purpose

Live Mycobacterium bovis Bacille Calmette-Guérin (BCG) has a suppressive effect on asthma, but its use in clinical practice may be limited due to adverse reactions. To develop a product that is effective for suppressing asthma with minimal adverse reactions, we investigated whether the heat-killed body or culture supernatants of mycobacteria could also prevent asthma development.

Methods

Female BALB/c mice were treated with live BCG, the heat-killed body, or culture supernatants of BCG or Mycobacterium tuberculosis intraperitoneally, while sensitizing and provoking with ovalbumin. Then they underwent a methacholine bronchoprovocation test, and the peribronchial inflammatory cell numbers and cytokine levels in splenocyte culture supernatants were assessed.

Results

The airway sensitivity to methacholine decreased significantly after treatment with not only live BCG (30.8 versus 10.0 mg/mL, P<0.001) but also with the culture supernatant (BCG, 23.0 mg/mL, P<0.05; M. tuberculosis, 20.5 mg/mL, P<0.05). In contrast, heat-killed mycobacteria did not effectively decrease airway sensitivity. The peribronchial eosinophil counts and the goblet cell proportions in total epithelial cells decreased significantly in most of the groups. The interferon-γ/interleukin-5 ratios increased significantly in most of the treatment groups except for the heat-killed groups, and were significantly related to airway sensitivity (r=0.312, P<0.01) and peribronchial eosinophil counts (r=-0.416, P<0.001). Interleukin-17A level was inversely related to airway sensitivity (r=-0.212, P<0.05) and was significantly lower in the live BCG group than in the control (137±20 versus 308±57 pg/mL, P<0.05).

Conclusions

BCG and mycobacteria culture supernatants may effectively prevent the development of asthma associated with altered Th1/Th2 cytokines and interleukin-17A levels.

Mouse models of allergic diseases

The increasing prevalence of allergic diseases worldwide is posing significant socio-economic challenges. The pathogenesis of these diseases reflects a complex interaction of genetic and environmental factors. The heterogeneity of disease phenotypes challenges the concept of single mechanisms of disease. As human experimentation is limited, animal models have been developed to provide insights into pathogenesis and potential for discovery of novel therapeutics. Mice have served in models of many of the allergic diseases including asthma, allergic rhinitis, food allergy, atopic dermatitis (AD), and allergic conjunctivitis. Although much has been learned from these investigations, there are limitations when these models are translated to the human diseases.

Protease-activated receptors and prostaglandins in inflammatory lung disease

Protease-activated receptors (PARs) are a novel family of G protein-coupled receptors. Signalling through PARs typically involves the cleavage of an extracellular region of the receptor by endogenous or exogenous proteases, which reveals a tethered ligand sequence capable of auto-activating the receptor. A considerable body of evidence has emerged over the past 20 years supporting a prominent role for PARs in a variety of human physiological and pathophysiological processes, and thus substantial attention has been directed towards developing drug-like molecules that activate or block PARs via non-proteolytic pathways. PARs are widely expressed within the respiratory tract, and their activation appears to exert significant modulatory influences on the level of bronchomotor tone, as well as on the inflammatory processes associated with a range of respiratory tract disorders. Nevertheless, there is debate as to whether the principal response to PAR activation is an augmentation or attenuation of airways inflammation. In this context, an important action of PAR activators may be to promote the generation and release of prostanoids, such as prostglandin E(2), which have well-established anti-inflammatory effects in the lung. In this review, we primarily focus on the relationship between PARs, prostaglandins and inflammatory processes in the lung, and highlight their potential role in selected respiratory tract disorders, including pulmonary fibrosis, asthma and chronic obstructive pulmonary disease.

Vascular endothelial growth factor is a key mediator in the development of T cell priming and its polarization to type 1 and type 17 T helper cells in the airways

Chronic inflammatory airway diseases including asthma are characterized by immune dysfunction to inhaled allergens. Our previous studies demonstrated that T cell priming to inhaled allergens requires LPS, which is ubiquitously present in household dust allergens. In this study, we evaluated the role of vascular endothelial growth factor (VEGF) in the development of T cell priming and its polarization to Th1 or Th17 cells when exposed to LPS-contaminated allergens. An asthma mouse model was induced by airway sensitization with LPS-contaminated allergens and then challenged with allergens alone. Therapeutic intervention was performed during allergen sensitization. The present study showed that lung inflammation induced by sensitization with LPS-contaminated allergens was decreased in mice with homozygous disruption of the IL-17 gene; in addition, allergen-specific Th17 immune response was abolished in IL-6 knockout mice. Meanwhile, in vivo production of VEGF was up-regulated by airway exposure of LPS. In addition, airway sensitization of allergen plus recombinant VEGF induced both type 1 and type 17 Th cell (Th1 and Th17) responses. Th1 and Th17 responses induced by airway sensitization with LPS-contaminated allergens were blocked by treatment with a pan-VEGF receptor (VEGFR; VEGFR-1 plus VEGFR-2) inhibitor during sensitization. These effects were accompanied by inhibition of the production of Th1 and Th17 polarizing cytokines, IL-12p70 and IL-6, respectively. These findings indicate that VEGF produced by LPS plays a key role in activation of naive T cells and subsequent polarization to Th1 and Th17 cells.

Development of allergen-induced airway inflammation in the absence of T-bet regulation is dependent on IL-17

Dysfunctional expression of T-bet, a transcription factor that is critical for IFN-gamma production, has been implicated in the development of asthma. To investigate in detail the mechanisms responsible for exacerbated disease in the absence of T-bet expression, BALB/c wild-type (WT) and T-bet(-/-) mice were used in a murine model of OVA-induced allergic lung inflammation. Following OVA challenge, T-bet(-/-) mice displayed increased histological inflammation in the lungs as well as greater thickening of the bronchiole linings, increased numbers of eosinophils and neutrophils in the lung, and enhanced airway hyperresponsiveness, compared with WT mice. However, the production of Th2 cytokines in T-bet(-/-) mice did not appear to be significantly greater than in WT mice. Interestingly, a marked increase in the levels of the proinflammatory cytokine IL-17 was observed in T-bet(-/-) mice. Neutralization of pulmonary IL-17 in T-bet(-/-) mice by anti-IL-17 mAb treatment during OVA challenge resulted in decreased levels of neutrophilic infiltration into the airways and decreased airway inflammation, essentially reversing the development of allergic asthma development. These findings indicate that IL-17 is a key mediator of airway inflammation in the absence of T-bet. The results of this study suggest a possible target for therapeutic intervention of human asthma.

Chronic inflammation and asthma

Allergic asthma is a complex and chronic inflammatory disorder which is associated with airway hyper-responsiveness and tissue remodelling of the airway structure. Although originally thought to be a Th2-driven inflammatory response to inhaled innocuous allergen, the immune response in asthma is now considered highly heterogeneous. There are now various in vivo systems which have been designed to examine the pathways leading to the development of this chronic immune response and reflect, in part this heterogeneity. Furthermore, the emergence of endogenous immunoregulatory pathways and active pro-resolving mediators hold great potential for future therapeutic intervention. In this review, the key cellular and molecular mediators relating to chronic allergic airway disease are discussed, as well as emerging players in the regulation of chronic allergic inflammation.

Dexamethasone suppresses interleukin-22 associated with bacterial infection in vitro and in vivo

Interleukin (IL)-22 production triggered by innate immune mechanisms has been identified as key to efficient intestinal anti-bacterial host defence and preservation of homeostasis. We hypothesized that glucocorticoid therapy may impair IL-22 expression, which should promote intestinal epithelial damage with the potential of subsequent bacterial translocation. High-dose corticosteroid therapy in Crohn's disease has been associated with an increased rate of abscess formation and ultimately with a higher risk of developing postoperative infectious complications, including abdominal sepsis. Thus, we sought to investigate effects of the prototypic glucocorticoid dexamethasone on IL-22 production in the context of bacterial infection. Enhanced IL-22 plasma levels were detectable in rat sepsis. Moreover, heat-inactivated Staphylococcus epidermidis, used as a prototypic activator of innate immunity, induced robust production of IL-22 by human peripheral blood mononuclear cells (PBMC). Here, we report for the first time that dexamethasone mediates remarkable suppression of IL-22 as detected in S. epidermidis-activated PBMC and rat sepsis, respectively. The data presented herein suggest that insufficient IL-22 function may contribute to impaired intestinal host defence in the context of corticosteroid therapy.

ADAM8: a new therapeutic target for asthma

BACKGROUND

A proteinase with a disintegrin and a metalloproteinase domain-8 (ADAM8) has been linked to asthma.

OBJECTIVE

To explore whether ADAM8 is a therapeutic target for asthma.

METHODS

We reviewed literature on ADAM8's function and expression and activities in lungs of humans and mice with allergic airway inflammation (AAI). We used these data to generate hypotheses about the contributions of ADAM8 to asthma pathogenesis.

CONCLUSIONS

ADAM8 levels are increased in airway epithelium and airway inflammatory cells in mice with AAI and human asthma patients. Data from murine models of AAI indicate that ADAM8 dampens airway inflammation. It is not clear whether ADAM8 contributes directly to structural remodeling in asthmatic airways. Additional studies are required to validate ADAM8 as a therapeutic target for asthma.

Innate immunity and asthma

Asthma remains a major health problem worldwide that has increased in developed countries. Much of the focus in asthma research in the past has been on adaptive, antigen-dependent immune responses. Recent work suggests that the innate, non-antigen-dependent immune system plays a critical role in asthma pathogenesis. Here we will highlight innate receptors and cells in the context of allergic responses. Reviewing animal models and human studies, we focus on interactions of innate and adaptive immunity.

Animal models of asthma

Studies in animal models form the basis for much of our current understanding of the pathophysiology of asthma, and are central to the preclinical development of drug therapies. No animal model completely recapitulates all features of the human disease, however. Research has focused primarily on ways to generate allergic inflammation by sensitizing and challenging animals with a variety of foreign proteins, leading to an increased understanding of the immunological factors that mediate the inflammatory response and its physiological expression in the form of airways hyperresponsiveness. Animal models of exaggerated airway narrowing are also lending support to the notion that asthma may represent an abnormality of the airway smooth muscle. The mouse is now the species of choice for asthma research involving animals. This presents practical challenges for physiological study because the mouse is so small, but modern imaging methodologies, coupled with the forced oscillation technique for measuring lung mechanics, have allowed the asthma phenotype in mice to be precisely characterized.

IL-17-producing T cells in lung immunity and inflammation

T(H)17 cells are a recently described effector CD4 T-cell subset characterized by the production of IL-17A, IL-17F, and IL-22, which have been implicated in the pathogenesis of several autoimmune diseases. T(H)17 and other IL-17A-producing T cells, including a population of gammadelta T cells and natural killer T cells, have also been associated with the development of skin, intestinal, and lung inflammatory diseases, such as asthma, granulomatous disease, chronic obstructive pulmonary disease, and cystic fibrosis. On the other hand, IL-17-producing T cells play important roles in protective immunity against some bacterial infections, mainly through the recruitment and activation of neutrophils. Thus, their regulation appears to be critical, and excess or deficient IL-17 elaboration leads either to deficient responses or disease. This review will summarize T(H)17 cell differentiation and discuss the host beneficial and detrimental function of IL-17A and related cytokines produced by different subpopulations of T cells.

Development and function of TH17 cells in health and disease

T(H)17 cells are the newest member of the T(H) cell family and are characterized by their ability to produce specific cytokines such as IL-17, IL-22, IL-17F, and CCL20. In this review, conditions for the differentiation of T(H)17 cells are defined in both murine and human systems, with discussion of T(H)17-specific cytokines and transcription factors. Functionally, T(H)17 cells contribute to host defense as a new effector T(H) cell subset with a role in protection against extracellular bacteria through activities on immune and nonimmune cells. Their activities, however, are also pivotal in the development of autoimmune diseases under pathologic conditions. T(H)17 cells are also beginning to be associated with the development and pathophysiology of allergic diseases, such as allergic contact dermatitis, atopic dermatitis, and asthma. Lymphoid tissue inducer-like cells and natural killer-like cells, termed RORgammat(+)NKp46(+) or NK-22 cells, might also play a role in allergic diseases because of their propensity to produce IL-17 and IL-22.

Monoclonal antibodies and other biologic agents in the treatment of asthma

Asthma represents a syndrome of airway inflammatory diseases with complex pathology. The immunologic pathogenesis is being increasingly revealed and provides opportunity for targeted biological intervention. Current experience with immunomodulators as targeted therapy in asthma is described in this literature review. Targeted therapies have included strategies to activate dendritic cells through the TLR-9 receptors, to interrupt the action of T(H)2 cytokines with cytokine blockers and monoclonal antibodies, to promote development of T(H)1 responses, to block IgE mediated pathways and to block TNFalpha. Omalizumab is the only biological therapy that has an approved indication in asthma at this time. An improved understanding of the heterogeneity of asthma should allow for specific targeting of different disease phenotypes specific therapies including immunomodulators.

Pathogenesis of steroid-resistant airway hyperresponsiveness: interaction between IFN-gamma and TLR4/MyD88 pathways

Chronic inflammation and airway hyperresponsiveness (AHR) in asthmatics are usually managed effectively by treatment with glucocorticoids. However, a subset of patients remains refractory to therapy. The underlying mechanisms are largely unknown, although recruitment of neutrophils (rather than eosinophils) is strongly correlated, suggesting a role for nonallergic host defense responses. Increased levels of IFN-gamma and endotoxins are also related to severe asthma and because these activate host defense pathways, we investigated a possible common etiologic link to steroid-resistant disease. To be able to unravel the complexity of asthmatic inflammation, we used two model systems which permitted dissection of the relevant molecular pathways. In the first of these, we transferred transgenic OVA(323-339) peptide-specific IFN-gamma-producing Th1 cells into mice. These animals were subsequently challenge via the airways with OVA(323-339) peptide and/or LPS. Challenge with both components, but not with either one individually, induced AHR. Importantly, AHR was resistant to treatment with dexamethasone. Development of AHR was dependent on IFN-gamma, inhibited by depletion of pulmonary macrophages (but not neutrophils) and abrogated in TLR4- or MyD88-deficient mice. In contrast, in the second model in which we transferred OVA(323-339) peptide-activated Th2 cells, eosinophilic inflammation and AHR were induced, and both were suppressed by steroid treatment. We conclude that cooperative signaling between IFN-gamma and TLR4/MyD88 constitutes a previously unrecognized pathway that regulates macrophage-dependent steroid-resistant AHR.

Barrier immunity and IL-17

CD4+ T(H)17 cells display a featured role in barrier immunity. This effector population of T cells is important for clearance of microorganisms but can also promote autoimmunity at barrier sites. Recent work has indicated that these effector cells share a pathway with CD4+ regulatory T cells (T(R) cells) that also have a critical function in barrier protection and immune regulation. The development and function of T(H)17 cells, and their relationship with T(R) cells are discussed.

Regulation of TH17 cells in the mucosal surfaces

The mucosal surfaces represent the main intersection between jawed vertebrates and the environment. The mucosal surface of the intestine alone forms the largest surface that is exposed to exogenous antigens as well as the largest collection of lymphoid tissue in the body. Therefore, a protective immune activity must coexist with efficient regulatory mechanisms to maintain a health status of these organisms. The discovery of a new lineage of T(H) cells that produce IL-17 has provided valuable new insight into host defense and the pathogenesis of inflammatory diseases at the mucosal surfaces. Of particular interest for these surfaces, it has been reported that peripherally-induced regulatory T cells and T(H)17 effector cells arise in a mutually exclusive fashion, depending on whether they are activated in the presence of TGF-beta or TGF-beta plus inflammatory cytokines such as IL-6. This review addresses the protective and pathogenic roles of T(H)17 cells in the mucosal surfaces and potential regulatory mechanisms that control their development.