Interleukin-17 is a negative regulator of established allergic asthma

IL-6 is required for airway mucus production induced by inhaled fungal allergens

Allergic asthma is caused by inhaled allergens and is characterized by airway eosinophilia, as well as mucus hypersecretion, which can lead to airflow obstruction. Despite the association of increased IL-6 levels with human atopic asthma, the contribution of IL-6 to the development of allergic airway inflammation triggered by inhaled allergens remains unclear. In this study, we examined the role of IL-6 in a mouse model of allergic airway inflammation induced by direct airway exposure to extracts of Aspergillus fumigatus, a common allergen in humans. We show that inhaled A. fumigatus extracts rapidly trigger the production of IL-6 in the airways. IL-6 appears to be dispensable for the recruitment of eosinophils to the lung during the development of allergic airway inflammation. However, IL-6 is essential for mucus hypersecretion by airway epithelial cells triggered in response to inhaled A. fumigatus Ags. Impaired mucus production caused by IL-6 deficiency correlates with a severe reduction in the levels of IL-13, a major inducer of mucin glycoproteins. Thus, IL-6 is a key regulator of specific hallmark features of allergic airway inflammation and it could be a potential target for pulmonary diseases that are associated with goblet cell metaplasia and mucus hypersecretion.

Absence of alpha 4 but not beta 2 integrins restrains development of chronic allergic asthma using mouse genetic models

OBJECTIVE

Chronic asthma is characterized by ongoing recruitment of inflammatory cells and airway hyperresponsiveness leading to structural airway remodeling. Although alpha 4 beta 1 and beta2 integrins regulate leukocyte migration in inflammatory diseases and play decisive roles in acute asthma, their role has not been explored under the chronic asthma setting. To extend our earlier studies with alpha 4(Delta/Delta) and beta2(-/-) mice, which showed that both alpha 4 and beta2 integrins have nonredundant regulatory roles in acute ovalbumin (OVA)-induced asthma, we explored to what extent these molecular pathways control development of structural airway remodeling in chronic asthma.

MATERIALS AND METHODS

Control, alpha 4(Delta/Delta), and beta2(-/-) mouse groups, sensitized by intraperitoneal OVA as allergen, received intratracheal OVA periodically over days 8 to 55 to induce a chronic asthma phenotype. Post-OVA assessment of inflammation and pulmonary function (airway hyperresponsiveness), together with airway modeling measured by goblet cell metaplasia, collagen content of lung, and transforming growth factor beta1 expression in lung homogenates, were evaluated.

RESULTS

In contrast to control and beta2(-/-) mice, alpha 4(Delta/Delta) mice failed to develop and maintain the composite chronic asthma phenotype evaluated as mentioned and subepithelial collagen content was comparable to baseline. These data indicate that beta2 integrins, although required for inflammatory migration in acute asthma, are dispensable for structural remodeling in chronic asthma.

CONCLUSION

alpha 4 integrins appear to have a regulatory role in directing transforming growth factor beta-induced collagen deposition and structural alterations in lung architecture likely through interactions of Th2 cells, eosinophils, or mast cells with endothelium, resident airway cells, and/or extracellular matrix.

Interleukin-23/Th17 pathways and inflammatory bowel disease

The IL-23/Th17 pathway has recently been identified to play a critical role in a number of chronic inflammatory diseases including inflammatory bowel disease (IBD). The identification in IBD patients of associations in IL23R and regions that include other genes in the IL-23/Th17 pathway has highlighted the importance of proper IL-23/Th17 pathway regulation in intestinal immune homeostasis. IL-23 plays a role in CD4+ Th17 lineage cells, characterized by IL-17 secretion and the expression of the transcription factor retinoic acid-related orphan receptor (ROR)gamma tau, and in other immune and nonimmune cells. The balance between effector T cell subsets, such as Th17 cells, and CD4+ T regulatory subsets is finely regulated; dysregulation of this balance can lead to inflammation and autoimmunity. As such, the IL-23/Th17 pathway contributes to immune responses that play a role in defenses to microbial infection, as well as in the intestinal inflammation observed in both animal models of colitis and human IBD.

Bacillus subtilis-specific poly-gamma-glutamic acid regulates development pathways of naive CD4(+) T cells through antigen-presenting cell-dependent and -independent mechanisms

Peripheral naive CD4(+) T cells selectively differentiate to type 1 T(h), type 2 T(h) and IL-17-producing T(h) (T(h)17) cells, depending on the priming conditions. Since these subsets develop antagonistically to each other to elicit subset-specific adaptive immune responses, balance between these subsets can regulate the susceptibility to diverse immune diseases. The present study was undertaken to determine whether poly-gamma-glutamic acid (gamma-PGA), an edible and safe exopolymer that is generated by microorganisms such as Bacillus subtilis, could modulate the development pathways of T(h) subsets. The presence of gamma-PGA during priming promoted the development of T(h)1 and T(h)17 cells but inhibited development of T(h)2 cells. gamma-PGA up-regulated the expression of T-bet and ROR-gammat, the master genes of T(h)1 and T(h)17 cells, respectively, whereas down-regulating the level of GATA-3, the master gene of T(h)2 cells. gamma-PGA induced the expression of IL-12p40, CD80 and CD86 in dendritic cells (DC) and macrophages in a Toll-like receptor-4-dependent manner, and the effect of gamma-PGA on T(h)1/T(h)2 development was dependent on the presence of antigen-presenting cells (APC). Furthermore, gamma-PGA-stimulated DC favored the polarization of naive CD4(+) T cells toward T(h)1 cells rather than T(h)2 cells. In contrast, gamma-PGA affected T(h)17 cell development, regardless of the presence or absence of APC. Thus, these data demonstrate that gamma-PGA has the potential to regulate the development pathways of naive CD4(+) T cells through APC-dependent and -independent mechanisms and to be applicable to treating T(h)2-dominated diseases.

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.

Regulatory T cells and asthma

Airway inflammation in asthma is characterized by activation of T helper type-2 (Th2) T cells, IgE production and eosinophilia. In many cases, this process is related to an inappropriate T cell response to environmental allergens, and other T cell-dependent pathways may also be involved (such as Th17). Regulatory T cells (Tregs) are T cells that suppress potentially harmful immune responses. Two major subsets of Treg are CD25(hi), Foxp3(+)Tregs and IL-10-producing Tregs. There is evidence that the numbers or function of both subsets may be deficient in patients with atopic allergic disease. Recent work has extended these findings into the airway in asthma where Foxp3 expression was reduced and CD25(hi) Treg-suppressive function was deficient. In animal models of allergic airways disease, Tregs can suppress established airway inflammation and airway hyperresponsiveness, and protocols to enhance the development, recruitment and function of Tregs have been described. Together with studies of patients and in vitro studies of human T cells, these investigations are defining potential interventions to enhance Treg function in the airway in asthma. Existing therapies including corticosteroids and allergen immunotherapy act on Tregs, in part to increase IL-10 production, while vitamin D3 and long-acting beta-agonists enhance IL-10 Treg function. Other possibilities may be enhancement of Treg function via histamine or prostanoid receptors, or by blocking pro-inflammatory pathways that prevent suppression by Tregs (activation of Toll-like receptors, or production of cytokines such as IL-6 and TNF-alpha). As Tregs can also suppress the potentially beneficial immune response important for controlling infections and cancer, a therapeutic intervention should target allergen- or site-specific regulation.

Epicutaneous sensitization with a protein antigen induces Th17 cells

BACKGROUND

Th17 is a newly identified effector T cell lineage which plays a central role in many human inflammatory diseases and experimental animal models. Epicutaneous sensitization with a protein antigen has been proven to induce a Th2-predominant immune response and lead to development of atopic diseases in a murine protein-patch model.

OBJECTIVE

We sought to assess the generation of Th17 cells in epicutaneous sensitization with a protein antigen and its regulation by environmental elements and genetic background.

METHODS

BALB/c, C57BL/6, and DO11.10 mice were epicutaneously immunized by patch application of the following: ovalbumin alone, or co-administration of one of TLR ligands, irritant, hapten or superantigens. IL-17 and IL-22 contents in supernatants of in vitro reactivation culture of lymph nodes cells were determined by ELISA. Frequency of IL-17-secreting CD4 T cells was measured by ELISPOT.

RESULTS

Small but significant amounts of IL-17 and IL-22 could be detected in supernatants of in vitro reactivation culture of lymph nodes cells of mice receiving patch application of ovalbumin. ELISPOT assay for IL-17 also revealed low frequency of IL-17-secreting CD4 T cells in lymph nodes cells in ovalbumin group. All TLR ligands tested including agonists for TLR2, TLR3, TLR4, TLR5, TLR7 and TLR9 as well as many environmental elements including irritant, hapten and superantigen could further promote the generation of Th17 cells. In addition, C57BL/6 mice generate less Th17 cells than BALB/c mice in epicutaneous sensitization.

CONCLUSION

This study demonstrates Th17 generation and its regulation by environmental elements and genetic background to a protein antigen by epicutaneous route.

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.

TGF-beta can leave you breathless

Transforming growth factor-beta (TGF-beta), a ubiquitous and multifunctional cytokine, is central to the evolution and modulation of host defense. Early on, TGF-beta was recognized for its chemotactic and pro-inflammatory properties, but then identification of its powerful suppressive activities focused attention on dissecting its mechanisms of immune inhibition. Just as quickly as TGF-beta-mediated regulation of a population of CD4(+)CD25(+)Foxp3(+) regulatory T cells became the rage, a surprising finding that TGF-beta was the impetus behind a subset of pro-inflammatory T helper (Th)17 cells brought back a re-emphasis on its broader ability to dictate inflammatory events. Emerging evidence indicates that much remains to be discovered regarding the complex and intertwined roles of TGF-beta in inflammation, T cell lineage commitment, antibody generation, immune suppression, and tolerance. While it may appear that TGF-beta has multiple, ill-defined, contradictory and overlapping modes of activity that are impossible to unravel, the current excitement for dissecting how TGF-beta controls immunity defines a challenge worthy of pursuit. The lung is particularly vulnerable to the influences of TGF-beta, which is produced by its immune and non-immune cell populations. In its absence, lung pathology becomes lethal, whereas TGF-beta overproduction also has untoward consequences, potentially leaving one breathless, and underscoring the paradoxical, but essential contribution of TGF-beta to tissue and immune homeostasis.

Critical role for STAT3 in IL-17A-mediated CCL11 expression in human airway smooth muscle cells

IL-17A has been shown to be expressed at higher levels in respiratory secretions from asthmatics and to correlate with airway hyperresponsiveness. Although these studies raise the possibility that IL-17A may influence allergic disease, the mechanism remains unknown. We previously demonstrated that IL-17A mediates CC chemokine (CCL11) production from human airway smooth muscle (ASM) cells. In this study, we demonstrate that STAT3 activation is critical in IL-17A-mediated CCL11 expression in ASM cells. IL-17A mediated a rapid phosphorylation of STAT3 but not STAT6 or STAT5 in ASM cells. Interestingly, transient transfection with wild-type or mutated CCL11 promoter constructs showed that IL-17A-mediated CCL11 expression relies on the STAT6 binding site. However, STAT3 but not STAT6 in vivo binding to the CCL11 promoter was detected following IL-17A stimulation of ASM cells. Overexpression of DN STAT3 (STAT3beta) abolishes IL-17A-induced CCL11 promoter activity. This effect was not observed with STAT6 DN or the STAT3 mutant at Ser(727). Interestingly, disruption of STAT3 activity with the SH2 domain binding peptide, but not with control peptide, results in a significant reduction of IL-17A-mediated STAT3 phosphorylation and CCL11 promoter activity. IL-17A-mediated CCL11 promoter activity and mRNA were significantly diminished in STAT3- but not STAT6-silenced ASM cells. Finally, IL-17A-induced STAT3 phosphorylation was sensitive to pharmacological inhibitors of JAK2 and ERK1/2. Taken together, our data provide the first evidence of IL-17A-mediated gene expression via STAT3 in ASM cells. Collectively, our results raise the possibility that the IL-17A/STAT3 signaling pathway may play a crucial role in airway inflammatory responses.

Preventive and therapeutic anti-inflammatory properties of the sesquiterpene alpha-humulene in experimental airways allergic inflammation

BACKGROUND AND PURPOSE

alpha-Humulene and trans-caryophyllene are plant sesquiterpenes with pronounced anti-inflammatory properties. Here, we evaluated the effects of these compounds in an experimental model of airways allergic inflammation.

EXPERIMENTAL APPROACH

Female BALB/c mice, sensitized to and challenged with ovalbumin received daily alpha-humulene or trans-caryophyllene (50 mg.kg(-1), orally) or alpha-humulene (1 mg.mL(-1), by aerosol) as either a preventive (for 22 days) or therapeutic (from the 18th to the 22nd day) treatment. Dexamethasone or budesonide was used as a positive control drug. Inflammation was determined on day 22 post-immunization by leukocyte recruitment, interleukin-5 (IL-5), CCL11, interferon-gamma (IFN-gamma) and leukotriene (LT)B(4) levels in bronchoalveolar lavage fluid (BALF). In addition, transcription factors [nuclear factor kappaB (NF-kappaB), activator protein 1 (AP-1)] and P-selectin in lung tissue were measured by immunohistochemistry and mucus secretion by histochemistry.

KEY RESULTS

Preventive or therapeutic treatments with alpha-humulene, but not with trans-caryophyllene, significantly reduced the eosinophil recruitment to the BALF. In addition, alpha-humulene recovery INF-gamma and reduced the IL-5, CCL11 and LTB(4) levels in BALF, as well as the IL-5 production in mediastinal lymph nodes (in vitro assay). Furthermore, alpha-humulene decreased the NF-kB and the AP-1 activation, the expression of P-selectin and the increased mucus secretion in the lung.

CONCLUSIONS AND IMPLICATIONS

alpha-Humulene, given either orally or by aerosol, exhibited marked anti-inflammatory properties in a murine model of airways allergic inflammation, an effect that seemed to be mediated via reduction of inflammatory mediators, adhesion molecule expression and transcription factors activation.

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.

The critical role of epithelial-derived Act1 in IL-17- and IL-25-mediated pulmonary inflammation

IL-25 initiates, promotes, and augments Th2 immune responses. In this study, we report that Act1, a key component in IL-17-mediated signaling, is an essential signaling molecule for IL-25 signaling. Although Act1-deficient mice showed reduced expression of KC (CXCL1) and neutrophil recruitment to the airway compared with wild-type mice in response to IL-17 stimulation, Act1 deficiency abolished IL-25-induced expression of IL-4, IL-5, IL-13, eotaxin-1 (CCL11), and pulmonary eosinophilia. Using a mouse model of allergic pulmonary inflammation, we observed diminished Th2 responses and lung inflammation in Act1-deficient mice compared with wild-type mice. Importantly, Act1 deficiency in epithelial cells reduced the phenotype of allergic pulmonary inflammation due to loss of IL-17-induced neutrophilia and IL-25-induced eosinophilia, respectively. These results demonstrate the essential role of epithelial-derived Act1 in allergic pulmonary inflammation through the distinct impact of the IL-17R-Act1 and IL-25R-Act1 axes. Such findings are crucial for the understanding of pathobiology of atopic diseases, including allergic asthma, which identifies Act1 as a potential therapeutic target.

The adaptor protein CIKS/Act1 is essential for IL-25-mediated allergic airway inflammation

IL-17 is the signature cytokine of recently discovered Th type 17 (Th17) cells, which are prominent in defense against extracellular bacteria and fungi as well as in autoimmune diseases, such as rheumatoid arthritis and experimental autoimmune encephalomyelitis in animal models. IL-25 is a member of the IL-17 family of cytokines, but has been associated with Th2 responses instead and may negatively cross-regulate Th17/IL-17 responses. IL-25 can initiate an allergic asthma-like inflammation in the airways, which includes recruitment of eosinophils, mucus hypersecretion, Th2 cytokine production, and airways hyperreactivity. We demonstrate that these effects of IL-25 are entirely dependent on the adaptor protein CIKS (also known as Act1). Surprisingly, this adaptor is necessary to transmit IL-17 signals as well, despite the very distinct biologic responses that these two cytokines elicit. We identify CD11c(+) macrophage-like lung cells as physiologic relevant targets of IL-25 in vivo.

Anti-inflammatory effects of IL-17A on Helicobacter pylori-induced gastritis

Helicobacter pylori-induced immune responses are skewed toward a T helper (Th) 1 phenotype. IL-17-producing Th17 cells have recently been discovered, and we examined the role of IL-17A in H. pylori-induced gastritis. Six months after inoculation with H. pylori, the mice received an intraperitoneal injection of recombinant IL-17A, anti-IL-17A antibody or irrelevant IgG(2a) for 3days. H. pylori infection markedly increased mRNA for IL-17A. Double immunofluorescence studies showed that IL-17A proteins were expressed on CD4(+) T cells, macrophages, and dendritic cells. H. pylori infection elevated mRNAs for IL-12, IFN-gamma, and TNF-alpha with increase in myeloperoxidase activity, whereas it did not affect mRNAs for IL-4 and IL-5. Neutralization of IL-17A elevated mRNAs for IFN-gamma and TNF-alpha, and myeloperoxidase activity, whereas recombinant IL-17A had a tendency to reduce these parameters. In conclusion, IL-17A exerts anti-inflammatory effects on H. pylori-induced gastritis through suppression of Th1 differentiation.

Toll/IL-1 signaling is critical for house dust mite-specific helper T cell type 2 and type 17 [corrected] responses

RATIONALE

One of the immunopathological features of allergic inflammation is the infiltration of helper T type 2 (Th2) cells to the site of disease. Activation of innate pattern recognition receptors such as Toll-like receptors (TLRs) plays a critical role in helper T type 1 cell differentiation, yet their contribution to the generation of Th2 responses to clinically relevant aeroallergens remains poorly defined.

OBJECTIVES

To determine the requirement for TLR2, TLR4, and the Toll/IL-1 receptor domain adaptor protein MyD88 in a murine model of allergic asthma.

METHODS

Wild-type and factor-deficient ((-/-)) mice were sensitized intranasally to the common allergen house dust mite (HDM) and challenged 2 weeks later on four consecutive days. Measurements of allergic airway inflammation, T-cell cytokine production, and airway hyperreactivity were performed 24 hours later.

MEASUREMENTS AND MAIN RESULTS

Mice deficient in MyD88 were protected from the cardinal features of allergic asthma, including granulocytic inflammation, Th2 cytokine production and airway hyperreactivity. Although HDM activated NF-kappaB in TLR2- or TLR4-expressing HEK cells, only in TLR4(-/-) mice was the magnitude of allergic airway inflammation and hyperreactivity attenuated. The diminished Th2 response present in MyD88(-/-) and TLR4(-/-) mice was associated with fewer OX40 ligand-expressing myeloid dendritic cells in the draining lymph nodes during allergic sensitization. Finally, HDM-specific IL-17 production and airway neutrophilia were attenuated in MyD88(-/-) but not TLR4(-/-) mice.

CONCLUSIONS

Together, these data suggest that Th2- and Th17-mediated inflammation generated on inhalational HDM exposure is differentially regulated by the presence of microbial products and the activation of distinct MyD88-dependent pattern recognition receptors.

The roles of IL-17A in inflammatory immune responses and host defense against pathogens

T-helper 17 (Th17) cells are a newly discovered CD4(+) helper T-cell subset that produces interleukin-17A (IL-17A) and IL-17F. IL-17A plays important roles in allergic responses such as delayed-type hypersensitivity, contact hypersensitivity, and allergic airway inflammation. IL-17A promotes inflammation by inducing various proinflammatory cytokines and chemokines, recruiting neutrophils, enhancing antibody production, and activating T cells. IL-17A expression is also augmented in autoimmune diseases such as multiple sclerosis and rheumatoid arthritis. Using mouse models of these diseases, we found that IL-17A plays a central role in their development. IL-6 is required for the development of Th17 cells and tumor necrosis factor functions downstream of IL-17A during the effector phase. IL-1 is important both for developing Th17 cells and eliciting inflammation. Th17 cells, like Th1 and Th2 cells, are involved in host defense against infections, but the contribution of these Th subsets to defense mechanisms differs among pathogens. The roles of IL-17F remain largely unknown. In this review, we introduce how IL-17A/IL-17F are involved in inflammatory immune responses and host defense mechanisms and discuss their relationship with other cytokines in the development of inflammatory and infectious diseases.

IL-17F: regulation, signaling and function in inflammation

The IL-17 cytokine family is composed of six members. IL-17F, discovered in 2001, recently has drawn increasing attention due to its greatest similarity to IL-17, a widely recognized inflammatory cytokine. The genes encoding IL-17 and IL-17F are localized in the same chromosomal region and are co-expressed by CD4+ and gammadelta T cells. IL-17F can be secreted as homodimers or heterodimers with IL-17. Similar to IL-17, IL-17F utilizes IL-17RA and IL-17RC as its receptor and employs Act1 and TRAF6 as its signal transducers to induce the expression of pro-inflammatory cytokines and chemokines in many different cell types. However, mice lacking either IL-17 or IL-17F exhibit distinct defects in experimental models of asthma and colitis. These results have laid the basis to understand the role of IL-17F in the pathogenesis of human diseases.

IL-23 promotes maintenance but not commitment to the Th17 lineage

IL-23 plays a critical role establishing inflammatory immunity and enhancing IL-17 production in vivo. However, an understanding of how it performs those functions has been elusive. In this report, using an IL-17-capture technique, we demonstrate that IL-23 maintains the IL-17-secreting phenotype of purified IL-17(+) cells without affecting cell expansion or survival. IL-23 maintains the Th17 phenotype over multiple rounds of in vitro stimulation most efficiently in conjunction with IL-1beta. However, in contrast to Th1 and Th2 cells, the Th17 phenotype is not stable and when long-term IL-23-stimulated Th17 cultures are exposed to Th1- or Th2-inducing cytokines, the Th17 genetic program is repressed and cells that previously secreted IL-17 assume the cytokine secreting profile of other Th subsets. Thus, while IL-23 can maintain the Th17 phenotype, it does not promote commitment to an IL-17-secreting lineage.

IL-17-producing alveolar macrophages mediate allergic lung inflammation related to asthma

IL-17 is a pivotal proinflammatory molecule in asthmatics. However, the cellular source of IL-17 in asthma has not been identified to date. In this study, we report that macrophages rather than Th17 cells are the main producer of IL-17 in allergic inflammation related to asthma. After OVA challenge in a mouse model mimicking allergic asthma, the increased IL-17(+) cells in the lung were mainly CD11b(+)F4/80(+) macrophages, instead of T cells or others. Importantly, IL-17(+) alveolar macrophages (AMs), but not IL-17(+) interstitial macrophages, were significantly increased after allergen challenge. The increase of IL-17(+) AMs was not due to the influx of IL-17(+) macrophages from circulation or other tissues, but ascribed to the activation of AMs by mediator(s) secreted by IgE/OVA-activated mast cells. Depleting alveolar macrophages or neutralizing IL-17 prevented the initiation of OVA-induced asthma-related inflammation by inhibiting the increase of inflammatory cells and inflammatory factors in bronchoalveolar lavage fluid. Th2 cytokine IL-10 could down-regulate IL-17 expression in alveolar macrophages. The increased IL-17 and the decreased IL-10 in bronchoalveolar lavage fluid were further confirmed in asthmatic patients. These findings suggest that IL-17 is mainly produced by macrophages but not Th17 cells in allergic inflammation related to asthma. Mast cell-released mediators up-regulate the expression of IL-17 by macrophages, whereas IL-10 down-regulates IL-17 expression.

Coexpression of TGF-beta1 and IL-10 enables regulatory T cells to completely suppress airway hyperreactivity

In allergic airway disease, Treg may play an important role in the modulation of airway hyperreactivity (AHR) and inflammation. We therefore investigated the therapeutic potential of Treg in an Ag-dependent murine asthma model. We here describe that AHR can be completely suppressed by adoptive transfer of Treg overexpressing active TGF-beta1. Using mice with impaired TGF-beta signaling in T cells, we could demonstrate that TGF-beta signaling in recipient effector T cells or transferred Treg themselves is not required for the protective effects on AHR. However, the expression of IL-10 by Treg was found to be essential for the suppression of AHR, since Treg overexpressing active TGF-beta1 but deficient in IL-10 lacked protective effects. Airway inflammation could not be significantly suppressed by wild-type or transgenic Treg. In conclusion, modulation of cytokine expression by Treg may have therapeutic potential for the treatment of AHR in asthma. The mechanisms of the effects of Treg on airway inflammation require further clarification.

Inflammatory mechanisms in the lung

Inflammation is the body's response to insults, which include infection, trauma, and hypersensitivity. The inflammatory response is complex and involves a variety of mechanisms to defend against pathogens and repair tissue. In the lung, inflammation is usually caused by pathogens or by exposure to toxins, pollutants, irritants, and allergens. During inflammation, numerous types of inflammatory cells are activated. Each releases cytokines and mediators to modify activities of other inflammatory cells. Orchestration of these cells and molecules leads to progression of inflammation. Clinically, acute inflammation is seen in pneumonia and acute respiratory distress syndrome (ARDS), whereas chronic inflammation is represented by asthma and chronic obstructive pulmonary disease (COPD). Because the lung is a vital organ for gas exchange, excessive inflammation can be life threatening. Because the lung is constantly exposed to harmful pathogens, an immediate and intense defense action (mainly inflammation) is required to eliminate the invaders as early as possible. A delicate balance between inflammation and anti-inflammation is essential for lung homeostasis. A full understanding of the underlying mechanisms is vital in the treatment of patients with lung inflammation. This review focuses on cellular and molecular aspects of lung inflammation during acute and chronic inflammatory states.

Allergen uptake, activation, and IL-23 production by pulmonary myeloid DCs drives airway hyperresponsiveness in asthma-susceptible mice

Maladaptive, Th2-polarized inflammatory responses are integral to the pathogenesis of allergic asthma. As regulators of T cell activation, dendritic cells (DCs) are important mediators of allergic asthma, yet the precise signals which render endogenous DCs “pro-asthmatic”, and the extent to which these signals are regulated by the pulmonary environment and host genetics, remains unclear. Comparative phenotypic and functional analysis of pulmonary DC populations in mice susceptible (A/J), or resistant (C3H) to experimental asthma, revealed that susceptibility to airway hyperresponsiveness is associated with preferential myeloid DC (mDC) allergen uptake, and production of Th17-skewing cytokines (IL-6, IL-23), whereas resistance is associated with increased allergen uptake by plasmacytoid DCs. Surprisingly, adoptive transfer of syngeneic HDM-pulsed bone marrow derived mDCs (BMDCs) to the lungs of C3H mice markedly enhanced lung IL-17A production, and rendered them susceptible to allergen-driven airway hyperresponsiveness. Characterization of these BMDCs revealed levels of antigen uptake, and Th17 promoting cytokine production similar to that observed in pulmonary mDCs from susceptible A/J mice. Collectively these data demonstrate that the lung environment present in asthma-resistant mice promotes robust pDC allergen uptake, activation, and limits Th17-skewing cytokine production responsible for driving pathologic T cell responses central to the development of allergen-induced airway hyperresponsiveness.

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

Steroid-resistant asthma comprises an important source of morbidity in patient populations. T(H)17 cells represent a distinct population of CD4(+) Th cells that mediate neutrophilic inflammation and are characterized by the production of IL-17, IL-22, and IL-6. To investigate the function of T(H)17 cells in the context of Ag-induced airway inflammation, we polarized naive CD4(+) T cells from DO11.10 OVA-specific TCR-transgenic mice to a T(H)2 or T(H)17 phenotype by culturing in conditioned medium. In addition, we also tested the steroid responsiveness of T(H)2 and T(H)17 cells. In vitro, T(H)17 cytokine responses were not sensitive to dexamethasone (DEX) treatment despite immunocytochemistry confirming glucocorticoid receptor translocation to the nucleus following treatment. Transfer of T(H)2 cells to mice challenged with OVA protein resulted in lymphocyte and eosinophil emigration into the lung that was markedly reduced by DEX treatment, whereas T(H)17 transfer resulted in increased CXC chemokine secretion and neutrophil influx that was not attenuated by DEX. Transfer of T(H)17 or T(H)2 cells was sufficient to induce airway hyperresponsiveness (AHR) to methacholine. Interestingly, AHR was not attenuated by DEX in the T(H)17 group. These data demonstrate that polarized Ag-specific T cells result in specific lung pathologies. Both T(H)2 and T(H)17 cells are able to induce AHR, whereas T(H)17 cell-mediated airway inflammation and AHR are steroid resistant, indicating a potential role for T(H)17 cells in steroid-resistant asthma.

Enforced expression of GATA3 allows differentiation of IL-17-producing cells, but constrains Th17-mediated pathology

The zinc-finger transcription factor GATA3 serves as a master regulator of T-helper-2 (Th2) differentiation by inducing expression of the Th2 cytokines IL-4, IL-5 and IL-13 and by suppressing Th1 development. Here, we investigated how GATA3 affects Th17 differentiation, using transgenic mice with enforced GATA3 expression. We activated naïve primary T cells in vitro in the presence of transforming growth factor-beta and IL-6, and found that enforced GATA3 expression induced co-expression of Th2 cytokines in IL-17-producing T cells. Although the presence of IL-4 hampered Th17 differentiation, transforming growth factor-beta/IL-6 cultures from GATA3 transgenic mice contained substantial numbers of IL-17(+) cells, partially because GATA3 supported Th17 differentiation by limiting IL-2 and IFN-gamma production. GATA3 additionally constrained Th17 differentiation in vitro through IL-4-independent mechanisms, involving downregulating transcription of STAT3, STAT4, NFATc2 and the nuclear factor RORgammat, which is crucial for Th17 differentiation. Remarkably, upon myelin oligodendrocyte glycoprotein immunization in vivo, GATA3 transgenic mice contained similar numbers of IL-17-producing T cells in their lymph nodes as wild-type mice, but were not susceptible to autoimmune encephalomyelitis, possibly due to concomitant production of IL-4 and IL-10 induction. We therefore conclude that although GATA3 allows Th17 differentiation, it acts as an inhibitor of Th17-mediated pathology, through IL-4-dependent and IL-4-independent pathways.

Impairment of T helper and T regulatory cell responses at birth

BACKGROUND

There is strong evidence that reduced exposures to microbial compounds triggering innate immune responses early in life are critical for the development of allergic illnesses. The underlying mechanisms remain unknown, but will include T-cell responses either along T helper type 1 (Th1)/Th2 pathways or via T regulatory and Th17 cells. Yet, little is known about innate immune responses and the function of T regulatory/Th17 cells at birth. The aim of this study was to investigate T-cell responses to innate (Lipid A/LpA, peptidoglycan/Ppg) and adaptive (phytohemagglutinin) stimuli at birth and to compare these findings with adult immune responses.

METHODS

Cord and peripheral blood mononuclear cells including T regulatory and Th17 cells from 25 neonates and 25 adults were examined for proliferation, cytokine secretion, surface, mRNA expression and functional suppression assays.

RESULTS

Proliferation and cytokine responses to innate stimuli were less mature at birth than in adulthood. T regulatory and Th17 cells were less expressed in cord than in adult blood (Ppg-induced Foxp3, P = 0.001, LpA-induced CD4(+) CD25(+) high, P = 0.02; Th17 : P < 0.0001). Mitogen-induced suppression of T-regulatory cells on T-effector cell function was less efficient in cord than in adult blood (P = 0.01). At both ages, Th17 cells were correlated with Th1/Th2 cells (P < 0.01), but not with interleukin-10 secretion following innate-stimulation.

CONCLUSION

Innate immune responses are immature at birth. Furthermore, the function of T regulatory and Th17 cells is impaired. Th17 cells in association with Th1/Th2 cells may be involved in early immuno-modulation. Potent innate immune stimulation early in life can potentially contribute to protection from allergic diseases.

Toll-like receptor 9 modulates immune responses to Aspergillus fumigatus conidia in immunodeficient and allergic mice

The role of Toll-like receptor 9 (TLR9) in antifungal responses in the immunodeficient and allergic host is unclear. We investigated the role of TLR9 in murine models of invasive aspergillosis and fungal asthma. Neutrophil-depleted TLR9 wild-type (TLR9(+/+)) and TLR9-deficient (TLR9(-/-)) mice were challenged with resting or swollen Aspergillus fumigatus conidia and monitored for survival and lung inflammatory responses. The absence of TLR9 delayed, but did not prevent, mortality in immunodeficient mice challenged with resting or swollen conidia compared to TLR9(+/+) mice. In a fungal asthma model, TLR9(+/+) and TLR9(-/-) mice were sensitized to soluble A. fumigatus antigens and challenged with resting or swollen A. fumigatus conidia, and both groups of mice were analyzed prior to and at days 7, 14, and 28 after the conidium challenge. When challenged with resting conidia, TLR9(-/-) mice exhibited significantly lower airway hyper-responsiveness compared to the TLR9(+/+) groups. In contrast, A. fumigatus-sensitized TLR9(-/-) mice exhibited pulmonary fungal growth at days 14 and 28 after challenge with swollen conidia, a finding never observed in their allergic wild-type counterparts. Increased fungal growth in allergic TLR9(-/-) mice correlated with markedly decreased dectin-1 expression in whole lung samples and isolated dendritic cell populations. Further, whole lung levels of interleukin-17 were lower in allergic TLR9(-/-) mice compared to similar TLR9(+/+) mice. Together, these data suggest that TLR9 modulates pulmonary antifungal immune responses to swollen conidia, possibly through the regulation of dectin-1 expression.

The IL-17 cytokine family and their role in allergic inflammation

Allergic diseases and asthma has long been hypothesized as the results of the dysregulation of type 2 immune responses to environmental allergens. Recent progresses in characterizing the proinflammatory IL-17 cytokine family have added additional layer of complexity on the regulation of allergic inflammation. The delineation of IL-17-producing CD4+ T cell subset (Th17) has led to the revision of Th1/Th2 paradigm and impacts our perspectives on the basis of chronic tissue inflammation. In addition, the distinctive expression patterns and biological activities of individual IL-17 cytokine member may play different roles in the regulation of the pathogenesis of allergic diseases. Understanding the cellular source and targeting cells of IL-17 cytokine family member will provide the basis to elucidate the cellular mechanism underlying allergic inflammation and improve our therapeutic approaches for allergy.

IL-23 and Th17 cells enhance Th2-cell-mediated eosinophilic airway inflammation in mice

RATIONALE

The IL-23-IL-17A-producing CD4(+) T-cell (Th17 cell) axis plays an important role in the development of chronic inflammatory diseases, including autoimmune diseases. However, the role of the IL-23-Th17 cell axis in the regulation of allergic airway inflammation is still largely unknown.

OBJECTIVES

To determine the role of IL-23 and Th17 cells in allergic airway inflammation.

METHODS

We examined the effect of anti-IL-23 antibody on antigen-induced airway inflammation. We also investigated the effect of enforced expression of IL-23 on allergic airway inflammation by generating lung-specific IL-23 transgenic mice. Moreover, we examined the effect of adoptive transfer of antigen-specific Th17 cells on allergic airway inflammation.

MEASUREMENTS AND MAIN RESULTS

IL-23 mRNA was expressed in the lung of sensitized mice upon antigen inhalation, and the neutralization of IL-23 decreased antigen-induced eosinophil recruitment and Th2 cytokine production in the airways. The enforced expression of IL-23 in the airways significantly enhanced antigen-induced eosinophil and neutrophil recruitment into the airways; Th2 cytokine, IL-17A, and tumor necrosis factor (TNF)-alpha production in the airways; goblet cell hyperplasia; and airway hyperresponsiveness. Moreover, IL-23-mediated enhancement of antigen-induced Th2 cytokine production and eosinophil recruitment in the airways was still observed in the mice lacking IL-17A. Furthermore, although adoptive transfer of antigen-specific Th17 cells alone induced neutrophil but not eosinophil recruitment into the airways upon antigen inhalation, cotransfer of Th17 cells with Th2 cells significantly enhanced antigen-induced Th2-cell-mediated eosinophil recruitment into the airways and airway hyperresponsiveness.

CONCLUSIONS

IL-23 and Th17 cells not only induce Th17-cell-mediated neutrophilic airway inflammation but also up-regulate Th2-cell-mediated eosinophilic airway inflammation.

B cell antigen presentation promotes Th2 responses and immunopathology during chronic allergic lung disease

Background

The role of B cells in allergic asthma remains undefined. One mechanism by which B cells clearly contribute to allergic disease is via the production of specific immunoglobulin, and especially IgE. Cognate interactions with specific T cells result in T cell help for B cells, resulting in differentiation and immunoglobulin secretion. Proximal to (and required for) T cell-dependent immunoglobulin production, however, is antigen presentation by B cells. While interaction with T cells clearly has implications for B cell function and differentiation, this study investigated the role that B cells have in shaping the T cell response during chronic allergic lung disease.

Methodology/Principal Findings

In these studies, we used a clinically relevant mouse model of chronic allergic lung disease to study the role of B cells and B cell antigen presentation in this disease. In these studies we present several novel findings: 1) Lung B cells from chronically allergen challenged mice up-regulated MHC II and costimulatory molecules CD40, CD80 and CD86. 2) Using in vitro studies, B cells from the lungs of allergen challenged mice could present antigen to T cells, as assessed by T cell proliferation and the preferential production of Th2 cytokines. 3) Following chronic allergen challenge, the levels of Th2 cytokines IL-4 and IL-5 in the lungs and airways were significantly attenuated in B cell −/− mice, relative to controls. 4) B cell driven Th2 responses and mucus hyper secretion in the lungs were dependent upon MHC II expression by B cells.

Conclusions/Significance

Collectively, these results provide evidence for antigen presentation as a novel mechanism by which B cells contribute to chronic allergic disease. These findings give new insight into the mechanisms by which B cells promote asthma and other chronic diseases.

Chronic inflammation in asthma: a contest of persistence vs resolution

Recent investigations have highlighted that endogenous anti-inflammatory mediators and immune regulating mechanisms are important for the resolution of inflammatory processes. A disruption of these mechanisms can be causally related not only to the initiation of unnecessary inflammation, but also to the persistence of several chronic inflammatory diseases. In asthma, chronic Th-2 driven eosinophilic inflammation of the airways is one of the central abnormalities. To date, elucidating the role of the different pro-inflammatory mediators involved in orchestrating the inflammatory processes in asthma has been the subject of intense research in both humans and animal models. However, the counter-regulatory mechanisms that co-determine the outcome in the contest of resolution vs persistence of the eosinophilic airway inflammation remain poorly understood. These are currently being investigated in animal models of chronic asthma. Elucidating these mechanisms is of relevance, since it can give rise to a new therapeutic approach in the treatment of chronic airway inflammation in asthmatics. This novel concept of treatment involves the stimulation of endogenous anti-inflammatory pathways, rather than solely antagonising the various pro-inflammatory mediators. Here, we review and discuss the current knowledge about these endogenous anti-inflammatory mediators in clinical and experimental asthma.

The roles of the classical and alternative nuclear factor-kappaB pathways: potential implications for autoimmunity and rheumatoid arthritis

Nuclear factor-κB (NF-κB) is an inducible transcription factor controlled by two principal signaling cascades, each activated by a set of signal ligands: the classical/canonical NF-κB activation pathway and the alternative/noncanonical pathway. The former pathway proceeds via phosphorylation and degradation of inhibitor of NF-κB (IκB) and leads most commonly to activation of the heterodimer RelA/NF-κB1(p50). The latter pathway proceeds via phosphorylation and proteolytic processing of NF-κB2 (p100) and leads to activation, most commonly, of the heterodimer RelB/NF-κB2 (p52). Both pathways play critical roles at multiple levels of the immune system in both health and disease, including the autoimmune inflammatory response. These roles include cell cycle progression, cell survival, adhesion, and inhibition of apoptosis. NF-κB is constitutively activated in many autoimmune diseases, including diabetes type 1, systemic lupus erythematosus, and rheumatoid arthritis (RA). In this review we survey recent developments in the involvement of the classical and alternative pathways of NF-κB activation in autoimmunity, focusing particularly on RA. We discuss the involvement of NF-κB in self-reactive T and B lymphocyte development, survival and proliferation, and the maintenance of chronic inflammation due to cytokines such as tumor necrosis factor-α, IL-1, IL-6, and IL-8. We discuss the roles played by IL-17 and T-helper-17 cells in the inflammatory process; in the activation, maturation, and proliferation of RA fibroblast-like synovial cells; and differentiation and activation of osteoclast bone-resorbing activity. The prospects of therapeutic intervention to block activation of the NF-κB signaling pathways in RA are also discussed.

Colonization with Heligmosomoides polygyrus suppresses mucosal IL-17 production

Helminth exposure appears to protect hosts from inappropriate inflammatory responses, such as those causing inflammatory bowel disease. A recently identified, strongly proinflammatory limb of the immune response is characterized by T cell IL-17 production. Many autoimmune type inflammatory diseases are associated with IL-17 release. Because helminths protect from these diseases, we examined IL-17 production in helminth-colonized mice. We colonized mice with Heligmosomoides polygyrus, an intestinal helminth, and analyzed IL-17 production by lamina propria mononuclear cells (LPMC) and mesenteric lymph node (MLN) cells. Colonization with H. polygyrus reduces IL-17A mRNA by MLN cells and inhibits IL-17 production by cultured LPMC and MLN cells. Helminth exposure augments IL-4 and IL-10 production. Blocking both IL-4 and IL-10, but not IL-10 alone, restores IL-17 production in vitro. Colonization of colitic IL-10-deficient mice with H. polygyrus suppresses LPMC IL-17 production and improves colitis. Ab-mediated blockade of IL-17 improves colitis in IL-10-deficient mice. Thus, helminth-associated inhibition of IL-17 production is most likely an important mechanism mediating protection from inappropriate intestinal inflammation.

A rush to judgment on Th17

Some immunologists have characterized T helper (Th)17 T cells as the master mediators of tissue damage in a variety of pathological conditions. New data now demonstrate that Th1 and Th17 T cells are independently capable of inducing disease in two established models of autoimmunity. Thus, the role of Th17 cytokines as the central mediators of pathological tissue damage seems to require clarification.

Endogenous IL-17 does not play a significant role in the development of experimental murine allergic conjunctivitis

BACKGROUND

Endogenous IL-17 is needed for the Ag sensitization that results in murine allergic asthma, a Th2-mediated disease. Here, we aimed to investigate the role IL-17 plays in the development of murine experimental allergic conjunctivitis (EC) which is also a Th2-mediated disease.

METHODS

To induce EC, wild-type (WT) and IL-17-deficient (IL-17 KO) mice on the BALB/c and C57BL/6 backgrounds were immunized with ragweed (RW) in alum and challenged with RW in eye drops. Alternatively, EC was induced by adoptively transferring RW-primed splenocytes followed by challenge with RW-containing eye drops. Twenty-four hours after the RW challenge, the conjunctivas and spleens were harvested for histological analyses and cytokine assays, respectively.

RESULTS

The WT and IL-17 KO mice on both backgrounds did not differ in terms of the severity of actively induced EC, as evaluated by the conjunctival eosinophil infiltration. They also did not differ with regard to the phenotypes of the inflammatory cells infiltrating the conjunctivas, although primed IL-17 KO splenocytes stimulated in vitro with RW extract did produce significantly higher amounts of IL-4, IL-13 and IFN-gamma than WT splenocytes. Reciprocal adoptive transfer experiments also demonstrated that the IL-17 from both the donor splenocytes and the recipient mice is not involved in the development of EC.

CONCLUSIONS

Endogenous IL-17 does not appear to play a significant role in the development of EC.

Absence of donor Th17 leads to augmented Th1 differentiation and exacerbated acute graft-versus-host disease

Th17 is a newly identified T-cell lineage that secretes proinflammatory cytokine IL-17. Th17 cells have been shown to play a critical role in mediating autoimmune diseases such as EAE, colitis, and arthritis, but their role in the pathogenesis of graft-versus-host disease (GVHD) is still unknown. Here we showed that, in an acute GVHD model of C57BL/6 (H-2(b)) donor to BALB/c (H-2(d)) recipient, IL-17(-/-) donor T cells manifested an augmented Th1 differentiation and IFN-gamma production and induced exacerbated acute GVHD. Severe tissue damage mediated by IL-17(-/-) donor T cells was associated with increased Th1 infiltration, up-regulation of chemokine receptors by donor T cells, and enhanced tissue expression of inflammatory chemokines. Administration of recombinant IL-17 and neutralizing IFN-gamma in the recipients given IL-17(-/-) donor cells ameliorated the acute GVHD. Furthermore, the regulation of Th1 differentiation by IL-17 or Th17 may be through its influence on host DCs. Our results indicate that donor Th17 cells can down-regulate Th1 differentiation and ameliorate acute GVHD in allogeneic recipients, and that treatments neutralizing proinflammatory cytokine IL-17 may augment acute GVHD as well as other inflammatory autoimmune diseases.

TH17 cells in development: an updated view of their molecular identity and genetic programming

Following activation, CD4+ T cells differentiate into different lineages of helper T (T(H)) cells that are characterized by distinct developmental regulation and biological functions. T(H)17 cells have recently been identified as a new lineage of effector T(H) cells, and they have been shown to be important in immune responses to infectious agents, as well as in various immune diseases. Over the past two to three years, there has been a rapid progress in our understanding of the differentiation programme of T(H)17 cells. Here, I summarize our current knowledge of the unique gene expression, cytokine-mediated regulation and transcriptional programming of T(H)17 cells, and provide my personal perspectives on the future studies that are required to elucidate this lineage in more detail.

Resolvin E1 regulates interleukin 23, interferon-gamma and lipoxin A4 to promote the resolution of allergic airway inflammation

Interleukin 23 (IL-23) is integral to the pathogenesis of chronic inflammation. The resolution of acute inflammation is an active process mediated by specific signals and mediators such as resolvin E1 (RvE1). Here we provide evidence that RvE1, in nanogram quantities, promoted the resolution of inflammatory airway responses in part by directly suppressing the production of IL-23 and IL-6 in the lung. Also contributing to the pro-resolution effects of RvE1 treatment were higher concentrations of interferon-gamma in the lungs of RvE1-treated mice. Our findings indicate a pivotal function for IL-23 and IL-6, which promote the survival and differentiation of IL-17-producing T helper cells, in maintaining inflammation and also identify an RvE1-initiated resolution program for allergic airway responses.

T cell trafficking in allergic asthma: the ins and outs

T cells are critical mediators of the allergic airway inflammation seen in asthma. Pathogenic allergen-specific T cells are generated in regional lymph nodes and are then recruited into the airway by chemoattractants produced by the asthmatic lung. These recruited effector T cells and their products then mediate the cardinal features of asthma: airway eosinophilia, mucus hypersecretion, and airway hyperreactivity. There has been considerable progress in delineating the molecular mechanisms that control T cell trafficking into peripheral tissue, including the asthmatic lung. In this review, we summarize these advances and formulate them into a working model that proposes that T cell trafficking into and out of the allergic lung is controlled by several discrete regulatory pathways that involve the collaboration of innate and acquired immune cells.