Interleukin-23 (IL-23)-IL-17 cytokine axis in murine Pneumocystis carinii infection

Th-17 cells in the lungs?

Naive CD4 cells are capable of integrating signals from antigen-activated cells of the innate immune system and differentiating into effector CD4 cells, also termed T helper (Th) cells. According to the traditional paradigm explaining adaptive CD4 cell responses, there are two subsets of Th cells: the Th-1 and Th-2 subset. Each of these subsets undergoes a distinct differentiation pathway (a pathway that is characterized by a unique profile of cytokine production and has specific immunoregulatory functions). However, recent studies in mouse models have forwarded evidence of a third subset of Th cells: the Th-17 subset. As indicated predominantly in studies on mice, the Th-17 subset is characterized by an ability to produce the neutrophil-mobilizing cytokine IL-17 in response to stimulation with the cytokine IL-23, an IL-12-related cytokine released from antigen-presenting cells. There is now a growing body of evidence from animal models that the Th-17 subset plays an important role in host defence in the lungs and other organs. Altered IL-17 levels have also been demonstrated in human patients with asthma, exacerbations of cystic fibrosis or following lung transplantation. There is now also evidence that the Th-17 subset is functionally distinct from the Th-2 subset but little is known of the functional inter-relationship between the Th-1 and Th-17 cell subsets; this is particularly true in human lungs. It has been proposed that the Th-17 subset plays a unique role by linking the arms of innate and adaptive immunity. Thus, an improved understanding of the human correlate to the Th-17 subset may reveal new targets for pharmacotherapy against lung disorders that are characterized by aberrant innate responses in host defense.

Cellular and cytokine changes in the alveolar environment among immunocompromised patients during Pneumocystis jirovecii infection

Limited data exist on the cytokine and cellular changes in the alveolar environment in immunocompromised patients during Pneumocystis jirovecii infection. A cellular and a cytokine analysis were performed on bronchoalveolar lavage (BAL) samples from three groups of patients, i.e., an initial study group of 64 immunocompromised P. jirovecii-positive individuals and two control groups of P. jirovecii-negative patients who had been or not immunosuppressed (65 patients). The results were related to alveolar dilution as determined by urea measurement. Compared with non-infected groups, P. jirovecii-infected patients had a lower level of alveolar macrophages (AM), particularly those with high burdens of P. jirovecii. Alveolar macrophages over-expressed the Dectin-1 receptor, which was largely implicated in P. jirovecii clearance. The alveolar CD8+T and CD4+T lymphocyte counts were increased and an inverse correlation was observed between the alveolar CD4+ cell count and the P. jirovecii burden. Although the alveolar IL-6 level was considerably increased, alveolar IL-17, IL-10, TNF-α, TGF-β concentrations of P. jirovecii patients were not different from the control groups. Changes in the pulmonary environment were also highlighted during P. jirovecii colonization. Our study suggests that there is a correlation between the P. jirovecii burden in the alveolus (from colonization to a high P. jirovecii burden), and the degree of impairment of the alveolar immune response.

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.

Helicobacter pylori-induced Th17 responses modulate Th1 cell responses, benefit bacterial growth, and contribute to pathology in mice

CD4(+) T cell responses are critical for the pathogenesis of Helicobacter pylori infection. The present study evaluated the role of the Th17 subset in H. pylori infection. H. pylori infection induced significant expression of IL-17 and IFN-gamma in mouse gastric tissue. IL-23 and IL-12 were increased in the gastric tissue and in H. pylori-stimulated macrophages. Cell responses were examined by intracellular staining for IFN-gamma, IL-4, and IL-17. Mice infected with H. pylori developed a mixed Th17/Th1 response; Th17 responses preceded Th1 responses. Treatment of mice with an anti-IL-17 Ab but not a control Ab significantly reduced the H. pylori burden and inflammation in the stomach. H. pylori colonization and gastric inflammation were also lower in IL-17(-/-) mice. Furthermore, administration of recombinant adenovirus encoding mouse IL-17 increased both H. pylori load and inflammation. Further analysis showed that the Th1 cell responses to H. pylori were downregulated when IL-17 is deficient. These results together suggest that H. pylori infection induces a mixed Th17/Th1 cell response and the Th17/IL-17 pathway modulates Th1 cell responses and contributes to pathology.

CCR5 dictates the equilibrium of proinflammatory IL-17+ and regulatory Foxp3+ T cells in fungal infection

CCR5 is a chemotactic mediator for inflammatory cells as well as regulatory T cells (Tregs). In this study, we investigated the role of CCR5 during infection with the fungal pathogen Histoplasma capsulatum. Mice lacking CCR5 or treated with an mAb to CCL4 had impaired infiltration of inflammatory cells to the lungs. Despite displaying an elevated fungal burden prior to activation of an adaptive immune response, CCL4-neutralized and CCR5(-/-) mice resolved infection more efficiently than controls. Accelerated fungal clearance was associated with a reduced number of Tregs in the lungs. Impaired trafficking was not solely responsible for the paucity of Tregs in the lungs, as proliferation of both CD4(+) T cells and Tregs was diminished in CCR5(-/-) lungs during infection. A reduced number of Tregs in CCR5(-/-) lungs was associated with a selective increase of Th17 cytokines, and neutralization of IL-17 increased Treg proliferation and consequently fungal burden in CCR5(-/-) mice. Thus, CCR5 dictates pathogen persistence by tightly regulating the balance between Treg and Th17 cells in H. capsulatum infection.

Susceptibility of human Th17 cells to human immunodeficiency virus and their perturbation during infection

BACKGROUND

Identification of the Th17 T cell subset as important mediators of host defense and pathology prompted us to determine their susceptibility to human immunodeficiency virus (HIV) infection.

METHODS AND RESULTS

We found that a sizeable portion of Th17 cells express HIV coreceptor CCR5 and produce very low levels of CCR5 ligands macrophage inflammatory protein (MIP)-1alpha and MIP-1beta. Accordingly, CCR5(+) Th17 cells were efficiently infected with CCR5-tropic HIV and were depleted during viral replication in vitro. Remarkably, HIV-infected individuals receiving treatment had significantly reduced Th17 cell counts, compared with HIV-uninfected subjects, regardless of viral load or CD4 cell count, whereas treatment-naive subjects had normal levels. However, there was a preferential reduction in CCR5(+) T cells that were also CCR6 positive, which is expressed on all Th17 cells, compared with CCR6(-)CCR5(+) cells, in both treated and untreated HIV-infected subjects. This observation suggests preferential targeting of CCR6(+)CCR5(+) Th17 cells by CCR5-tropic viruses in vivo. Th17 cell levels also inversely correlated with activated CD4(+) T cells in HIV-infected individuals who are receiving treatment.

CONCLUSIONS

Our findings suggest a complex perturbation of Th17 subsets during the course of HIV disease potentially through both direct viral infection and virus indirect mechanisms, such as immune activation.

Th17 cells and HIV infection

PURPOSE OF REVIEW

This review summarizes the recent literature about the potential perturbation and role of Th17 cells in HIV pathogenesis. We discuss the recent findings on Th17 deficiency in HIV/simian immunodeficiency virus (SIV) infection and how this deficiency may impact the mucosal host defenses, potentially contributing to chronic immune activation.

RECENT FINDINGS

Th17 cells have been implicated in host defense against a variety of pathogens and are involved in the pathogenesis of autoimmune diseases. Recently, Th17 cells were shown to be perturbed during HIV infection in humans and SIV infection in nonhuman primates. Th17 cells were found to be infected in vitro by HIV and SIV and are significantly depleted in the gastrointestinal tract of HIV-infected individuals. In monkeys, Th17 cells are only depleted in the pathogenic SIV infection of rhesus macaques, which correlates with the progression to AIDS in these primates, whereas they remain intact in the nonpathogenic SIV infection of African green monkeys or sooty mangabeys.

SUMMARY

Th17 cells appear to be perturbed during HIV and SIV infection. This finding could have important implications in understanding the disruption of mucosal defenses in the gastrointestinal tract and potentially in predicting opportunistic infections during the course of HIV disease.

Th17 cytokines in mucosal immunity and inflammation

PURPOSE OF REVIEW

Compelling evidence suggests that the Th17 lineage and other IL-17-producing cells play critical roles in host defense against pathogens at mucosal sites. However, IL-17 can also contribute to inflammatory responses at mucosal sites. In this review, we will discuss the recent progress in our understanding of the role of Th17 and other IL-17-producing cells in defining the fine balance between immunity and inflammation at different mucosal sites. RECENT FNDINGS: Recent findings have highlighted that Th17 cytokines are important for the induction of innate and adaptive host responses and contribute to host defense against pathogens at mucosal sites. More recent developments have probed how the Th17 responses are generated in vivo in response to infections and their requirement in maintaining barrier function at mucosal sites. Most importantly, it is becoming apparent that there is a fine balance between protective and pathological manifestation of Th17 responses at mucosal sites that defines immunity or inflammation.

SUMMARY

In this review, we have summarized the recent advances in our understanding of Th17 cytokines and how they contribute to immunity versus inflammation at mucosal sites.

Current understanding of Pneumocystis immunology

Pneumocystis jirovecii is the opportunistic fungal organism that causes Pneumocystis pneumonia (PCP) in humans. Similar to other opportunistic pathogens, Pneumocystis causes disease in individuals who are immunocompromised, particularly those infected with HIV. PCP remains the most common opportunistic infection in patients with AIDS. Incidence has decreased greatly with the advent of HAART. However, an increase in the non-HIV immunocompromised population, noncompliance with current treatments, emergence of drug-resistant strains and rise in HIV(+) cases in developing countries makes Pneumocystis a pathogen of continued interest and a public health threat. A great deal of research interest has addressed therapeutic interventions to boost waning immunity in the host to prevent or treat PCP. This article focuses on research conducted during the previous 5 years regarding the host immune response to Pneumocystis, including innate, cell-mediated and humoral immunity, and associated immunotherapies tested against PCP.

The IL-17 family cytokines in immunity and disease

INTRODUCTION

Accumulating evidence suggests that the interleukin (IL)-17 cytokines are major players in the immune response to foreign pathogens. In addition, the pathogeneses of a number of inflammatory diseases have been linked to uncontrolled expression of these cytokine pathways.

DISCUSSION

Genetic and biochemical analyses have elucidated the cellular and molecular events triggered by these proteins during an inflammatory response. While significant efforts have been placed on understanding the functions of IL-17A, IL-17F, and IL-17E, the significance of the other family members, IL-17B-D, in inflammation remains to be determined.

CONCLUSION

This review will focus on the cellular sources, target cell/receptors that are utilized by these cytokines to control pathogenesis, and the therapeutic potential of targeting these pathways to treat inflammatory disorders.

The innate immune response to uropathogenic Escherichia coli involves IL-17A in a murine model of urinary tract infection

Uropathogenic Escherichia coli is the causative agent for >80% of uncomplicated urinary tract infections (UTIs). Uropathogenic E. coli strains express a number of virulence and fitness factors that allow successful colonization of the mammalian bladder. To combat this, the host has distinct mechanisms to prevent adherence to the bladder wall and to detect and kill uropathogenic E. coli in the event of colonization. In this study, we investigated the role of IL-17A, an innate-adaptive immunomodulatory cytokine, during UTI using a murine model. Splenocytes isolated from mice infected by the transurethral route robustly expressed IL-17A in response to in vitro stimulation with uropathogenic E. coli Ags. Transcript expression of IL-17A in the bladders of infected mice correlated with a role in the innate immune response to UTI, and gammadelta cells seem to be a key source of IL-17A production. Although IL-17A seems to be dispensable for the generation of a protective response to uropathogenic E. coli, its importance in innate immunity is demonstrated by a defect in acute clearance of uropathogenic E. coli in IL-17A(-/-) mice. This clearance defect is likely a result of deficient cytokine and chemokine transcripts and impaired macrophage and neutrophil influx during infection. These results show that IL-17A is a key mediator for the innate immune response to UTIs.

Th17 cytokines and vaccine-induced immunity

T helper type 17 (Th17) cells are a distinct lineage of T cells that produce the effector molecules IL-17, IL-17F, IL-21, and IL-22. Although the role of Th17 cells in primary immune responses against infections is well documented, there is growing evidence that the Th17 lineage maybe critical for vaccine-induced memory immune responses against infectious diseases. Here, we summarize recent progress in our understanding of the role of IL-17 in vaccine-induced immunity.

Altered peptide ligands inhibit arthritis induced by glucose-6-phosphate isomerase peptide

INTRODUCTION

Immunosuppressants, including anti-TNFalpha antibodies, have remarkable effects in rheumatoid arthritis; however, they increase infectious events. The present study was designed to examine the effects and immunological change of action of altered peptide ligands (APLs) on glucose-6-phosphate isomerase (GPI) peptide-induced arthritis.

METHODS

DBA/1 mice were immunized with hGPI325-339, and cells of draining lymph node (DLN) were stimulated with hGPI325-339 to investigate the T-cell receptor (TCR) repertoire of antigen-specific CD4+ T cells by flow cytometry. Twenty types of APLs with one amino acid substitution at a TCR contact site of hGPI325-339 were synthesized. CD4+ T cells primed with human GPI and antigen-presenting cells were co-cultured with each APL and cytokine production was measured by ELISA to identify antagonistic APLs. Antagonistic APLs were co-immunized with hGPI325-339 to investigate whether arthritis could be antigen-specifically inhibited by APL. After co-immunization, DLN cells were stimulated with hGPI325-339 or APL to investigate Th17 and regulatory T-cell population by flow cytometry, and anti-mouse GPI antibodies were measured by ELISA.

RESULTS

Human GPI325-339-specific Th17 cells showed predominant usage of TCRVbeta8.1 8.2. Among the 20 synthesized APLs, four (APL 6; N329S, APL 7; N329T, APL 12; G332A, APL 13; G332V) significantly reduced IL-17 production by CD4+ T cells in the presence of hGPI325-339. Co-immunization with each antagonistic APL markedly prevented the development of arthritis, especially APL 13 (G332V). Although co-immunization with APL did not affect the population of Th17 and regulatory T cells, the titers of anti-mouse GPI antibodies in mice co-immunized with APL were significantly lower than in those without APL.

CONCLUSIONS

We prepared antagonistic APLs that antigen-specifically inhibited the development of experimental arthritis. Understanding the inhibitory mechanisms of APLs may pave the way for the development of novel therapies for arthritis induced by autoimmune responses to ubiquitous antigens.

Precarious balance: Th17 cells in host defense

Lineage-specific responses from the effector T-cell repertoire form a critical component of adaptive immunity. The recent identification of Th17 cells-a third, distinct lineage of helper T cells-collapses the long-accepted paradigm in which Th1 and Th2 cells distinctly mediate cellular and humoral immunity, respectively. In this minireview, we discuss the involvement of the Th17 lineage during infection by extracellular bacteria, intracellular bacteria, and fungi. Emerging trends suggest that the Th17 population bridges innate and adaptive immunity to produce a robust antimicrobial inflammatory response. However, because Th17 cells mediate both host defense and pathological inflammation, elucidation of mechanisms that attenuate but do not completely abolish the Th17 response may have powerful implications for therapy.

Transforming growth factor beta is dispensable for the molecular orchestration of Th17 cell differentiation

Interleukin (IL)-17–producing T helper (Th17) cells play a critical role in the pathophysiology of several autoimmune disorders. The differentiation of Th17 cells requires the simultaneous presence of an unusual combination of cytokines: IL-6, a proinflammatory cytokine, and transforming growth factor (TGF) β, an antiinflammatory cytokine. However, the molecular mechanisms by which TGF-β exerts its effects on Th17 cell differentiation remain elusive. We report that TGF-β does not directly promote Th17 cell differentiation but instead acts indirectly by blocking expression of the transcription factors signal transducer and activator of transcription (STAT) 4 and GATA-3, thus preventing Th1 and Th2 cell differentiation. In contrast, TGF-β had no effect on the expression of retinoic acid receptor–related orphan nuclear receptor γt, a Th17-specific transcription factor. Interestingly, in Stat-6^−/−T-bet^−/− mice, which are unable to generate Th1 and Th2 cells, IL-6 alone was sufficient to induce robust differentiation of Th17 cells, whereas TGF-β had no effect, suggesting that TGF-β is dispensable for Th17 cell development. Consequently, BALB/c Stat-6^−/−T-bet^−/− mice, but not wild-type BALB/c mice, were highly susceptible to the development of experimental autoimmune encephalomyelitis, which could be blocked by anti–IL-17 antibodies but not by anti–TGF-β antibodies. Collectively, these data provide evidence that TGF-β is not directly required for the molecular orchestration of Th17 cell differentiation.

Interleukins 17 and 23 influence the host response to Histoplasma capsulatum

Host defenses against Histoplasma capsulatum require the action of several cytokines. Here, we explored the influence of interleukin (IL)-17 and IL-23 on immunity to H. capsulatum infection in mice. In lungs, synthesis of IL-17 was up-regulated during acute infection, and the cells producing it were predominantly CD3(+). Neutralization of IL-17A blunted fungal clearance but did not promote progressive infection. Decreased inflammatory cell recruitment and increased levels of IL-6 and IL-10 were associated with impaired clearance. To determine whether the elevated cytokine levels were important in the action of IL-17A, IL-6(-/-) or IL-10(-/-) mice were treated with anti-IL-17A; neutralization of IL-17A did not alter fungal burden in either group of knockout mice. We explored the relationship between IL-17 and IL-23 because they have been reported to form a regulatory network. IL-23 transcription and protein level were increased in the lungs of infected mice. Mice producing IL-23 in the absence of IL-12 manifested prolonged survival that was IL-17 dependent. Thus, IL-17 is requisite for the generation of optimal inflammatory and protective responses. Generation of functional IL-17(+) cells is dependent on IL-6 and IL-10. Our findings also establish the existence a regulatory IL-17/IL-23 axis in histoplasmosis.

IFN-gamma deficiency worsen Pneumocystis pneumonia with Th17 development in nude mice

Pneumocystis pneumonia (PCP) occurs frequently in patients with immunodeficiency syndromes, especially AIDS. In order to investigate the role of IFN-gamma on PCP, nude mice deficient in IFN-gamma (GKO nude) and their wild-type ones (WT nude) were infected with murine Pneumocystis. Nine weeks later they were sacrificed, and cytokines in BALF and lung histopathology were compared between them. Cyst burden was greater in GKO than in WT nude mice. Histopathology in the lung was severer and granulomatous lesions were observed more frequently in GKO nude mice. Levels of IL-17 were higher in BALF of GKO than in that of WT nude mice. Greater number of CD4(+) T cells from lungs of infected GKO nude mice produced IL-17 than those from WT ones. These results suggest that deficiency in IFN-gamma induces the differentiation of Th17 and that IL-17 is responsible for inflammatory response in PCP.

Th17 cells at the crossroads of innate and adaptive immunity against infectious diseases at the mucosa

T helper type 17 (Th17) cells are a distinct lineage of T cells that produce the effector molecules IL-17, IL-17F, IL-21, and IL-22. Although the role of Th17 cells in autoimmunity is well documented, there is growing evidence that the Th17 lineage and other interleukin (IL)-17-producing cells are critical for host defense against bacterial, fungal, and viral infections at mucosal surfaces. Here we summarize recent progress in our understanding of the function of IL-17-producing cells as a bridge between innate and adaptive immunity against infectious diseases at the mucosa.

A MyD88-dependent early IL-17 production protects mice against airway infection with the obligate intracellular pathogen Chlamydia muridarum

We found that IL-17, a signature cytokine of Th17, was produced early in the innate immunity phase after an intranasal infection with the obligate intracellular pathogen Chlamydia muridarum. The airway IL-17, which peaked at 48 h after infection, was dependent on live chlamydial organism replication and MyD88-mediated signaling pathways. Treatment with antibiotics or knockout of the MyD88 gene, but not Toll/IL receptor domain-containing adapter-inducing IFN-beta, can block the early IL-17 production. Treatment of mice with an anti-IL-17-neutralizing mAb enhanced growth of chlamydial organisms in the lung, dissemination to other organs, and decreased mouse survival, whereas treatment with an isotype-matched control IgG had no effect. Although IL-17 did not directly affect chlamydial growth in cell culture, it enhanced the production of other inflammatory cytokines and chemokines by Chlamydia-infected cells and promoted neutrophil infiltration in mouse airways during chlamydial infection, which may contribute to the antichlamydial effect of IL-17. These observations suggest that an early IL-17 response as an innate immunity component plays an important role in initiating host defense against infection with intracellular bacterial pathogens in the airway.

Th17 cells in human disease

SUMMARY

Our understanding of the role of T cells in human disease is undergoing revision as a result of the discovery of T-helper 17 (Th17) cells, a unique CD4(+) T-cell subset characterized by production of interleukin-17 (IL-17). IL-17 is a highly inflammatory cytokine with robust effects on stromal cells in many tissues. Recent data in humans and mice suggest that Th17 cells play an important role in the pathogenesis of a diverse group of immune-mediated diseases, including psoriasis, rheumatoid arthritis, multiple sclerosis, inflammatory bowel disease, and asthma. Initial reports also propose a role for Th17 cells in tumorigenesis and transplant rejection. Important differences, as well as many similarities, are emerging when the biology of Th17 cells in the mouse is compared with corresponding phenomena in humans. As our understanding of human Th17 biology grows, the mechanisms underlying many diseases are becoming more apparent, resulting in a new appreciation for both previously known and more recently discovered cytokines, chemokines, and feedback mechanisms. Given the strong association between excessive Th17 activity and human disease, new therapeutic approaches targeting Th17 cells are highly promising, but the potential safety of such treatments may be limited by the role of these cells in normal host defenses against infection.

Th17 cells in human disease

SUMMARY

Our understanding of the role of T cells in human disease is undergoing revision as a result of the discovery of T-helper 17 (Th17) cells, a unique CD4(+) T-cell subset characterized by production of interleukin-17 (IL-17). IL-17 is a highly inflammatory cytokine with robust effects on stromal cells in many tissues. Recent data in humans and mice suggest that Th17 cells play an important role in the pathogenesis of a diverse group of immune-mediated diseases, including psoriasis, rheumatoid arthritis, multiple sclerosis, inflammatory bowel disease, and asthma. Initial reports also propose a role for Th17 cells in tumorigenesis and transplant rejection. Important differences, as well as many similarities, are emerging when the biology of Th17 cells in the mouse is compared with corresponding phenomena in humans. As our understanding of human Th17 biology grows, the mechanisms underlying many diseases are becoming more apparent, resulting in a new appreciation for both previously known and more recently discovered cytokines, chemokines, and feedback mechanisms. Given the strong association between excessive Th17 activity and human disease, new therapeutic approaches targeting Th17 cells are highly promising, but the potential safety of such treatments may be limited by the role of these cells in normal host defenses against infection.

Th17 cells: from precursors to players in inflammation and infection

Upon activation, naive CD4(+) T cells differentiate into different lineages of effector T(h) subsets. Each subset is characterized by its unique cytokine profile and biological functions. T(h)17, a newly described T(h) subset that produces IL-17, IL-17F and IL-22 in preference to other cytokines, has been shown to play an important role in clearing specific pathogens and in inducing autoimmune tissue inflammations. Over the last 2-3 years, significant progress has been made to understand the development and biological functions of T(h)17 subset. Transforming growth factor beta (TGF) together with IL-6 or IL-21 initiates the differentiation while IL-23 stabilizes the generation of T(h)17 cells. The transcription factors of T(h)17 cells [retinoid-related orphan receptor (ROR) gammat, ROR-alpha and signal transducer and activator of transcription-3] have been described recently. Since TGF-beta is essential for the generation of both T(h)17 and regulatory T (T(reg)) cells from naive T cells, which suggests a developmental link between T(h)17 and T(reg) cells. Functions of these two subsets of T cells are, however, opposite to each other; T(h)17 cells are highly pathogenic during the inflammatory process while T(reg) cells are crucial for inhibiting tissue inflammation and maintaining self-tolerance. Here, we review the recent information on differentiation and effector functions of T(h)17 cells during inflammatory conditions.

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.

Interleukin-17 in host defence against bacterial, mycobacterial and fungal pathogens

The mammalian immune system is intricately regulated, allowing for potent pathogen-specific immunity to be rapidly activated in response to infection with a broad and diverse array of potential pathogens. As a result of their ability to differentiate into distinct effector lineages, CD4 T cells significantly contribute to pathogen-specific adaptive immune responses. Through the production of effector cytokines, CD4 T helper (Th) cells orchestrate the precise mobilization of specific immune cells to eradicate infection. The protective effects of the newly identified lineage of Th17 cells against pathogens like Klebsiella pneumoniae, Citrobacter rodentium and Candida albicans indicate the capacity of Th17 cells to confer protection against extracellular bacterial and fungal pathogens, filling a critical void in host immunity not covered by the classically described Th1 lineage that activates immunity to intracellular pathogens or the Th2 lineage that is important in protection against mucosal parasitic pathogens. Host defence by Th17 cells extends beyond protection against extracellular bacterial and fungal pathogens, as demonstrated in infections against intracellular bacteria like Listeria monocytogenes and Salmonella enterica, as well as Mycobacterium tuberculosis. Herein, we summarize both experimental data from mouse infection models and epidemiological studies in humans that demonstrate the protective effects of interleukin-17 and Th17 CD4 T cells in immunity to bacterial, mycobacterial and fungal pathogens.

IL-17 and Th17 Cells

CD4+ T cells, upon activation and expansion, develop into different T helper cell subsets with different cytokine profiles and distinct effector functions. Until recently, T cells were divided into Th1 or Th2 cells, depending on the cytokines they produce. A third subset of IL-17-producing effector T helper cells, called Th17 cells, has now been discovered and characterized. Here, we summarize the current information on the differentiation and effector functions of the Th17 lineage. Th17 cells produce IL-17, IL-17F, and IL-22, thereby inducing a massive tissue reaction owing to the broad distribution of the IL-17 and IL-22 receptors. Th17 cells also secrete IL-21 to communicate with the cells of the immune system. The differentiation factors (TGF-β plus IL-6 or IL-21), the growth and stabilization factor (IL-23), and the transcription factors (STAT3, RORγt, and RORα) involved in the development of Th17 cells have just been identified. The participation of TGF-β in the differentiation of Th17 cells places the Th17 lineage in close relationship with CD4+CD25+Foxp3+ regulatory T cells (Tregs), as TGF-β also induces differentiation of naive T cells into Foxp3+ Tregs in the peripheral immune compartment. The investigation of the differentiation, effector function, and regulation of Th17 cells has opened up a new framework for understanding T cell differentiation. Furthermore, we now appreciate the importance of Th17 cells in clearing pathogens during host defense reactions and in inducing tissue inflammation in autoimmune disease.

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.

Cell mediated immunity to fungi: a reassessment

Protective immunity against fungal pathogens is achieved by the integration of two distinct arms of the immune system, the innate and adaptive responses. Innate and adaptive immune responses are intimately linked and controlled by sets of molecules and receptors that act to generate the most effective form of immunity for protection against fungal pathogens. The decision of how to respond will still be primarily determined by interactions between pathogens and cells of the innate immune system, but the actions of T cells will feed back into this dynamic equilibrium to regulate the balance between tolerogenic and inflammatory responses. In the last two decades, the immunopathogenesis of fungal infections and fungal diseases was explained primarily in terms of Th1/Th2 balance. Although Th1 responses driven by the IL-12/IFN-gamma axis are central to protection against fungi, other cytokines and T cell-dependent pathways have come of age. The newly described Th17 developmental pathway may play an inflammatory role previously attributed to uncontrolled Th1 responses and serves to accommodate the seemingly paradoxical association of chronic inflammatory responses with fungal persistence in the face of an ongoing inflammation. Regulatory T cells in their capacity to inhibit aspects of innate and adaptive antifungal immunity have become an integral component of immune resistance to fungi, and provide the host with immune defense mechanisms adequate for protection, without necessarily eliminating fungal pathogens which would impair immune memory--or causing an unacceptable level of tissue damage. The enzyme indoleamine 2,3-dioxygenase and tryptophan metabolites contribute to immune homeostasis by inducing Tregs and taming overzealous or heightened inflammatory responses.

Th17 cells and IL-17 receptor signaling are essential for mucosal host defense against oral candidiasis

The commensal fungus Candida albicans causes oropharyngeal candidiasis (OPC; thrush) in settings of immunodeficiency. Although disseminated, vaginal, and oral candidiasis are all caused by C. albicans species, host defense against C. albicans varies by anatomical location. T helper 1 (Th1) cells have long been implicated in defense against candidiasis, whereas the role of Th17 cells remains controversial. IL-17 mediates inflammatory pathology in a gastric model of mucosal candidiasis, but is host protective in disseminated disease. Here, we directly compared Th1 and Th17 function in a model of OPC. Th17-deficient (IL-23p19(-/-)) and IL-17R-deficient (IL-17RA(-/-)) mice experienced severe OPC, whereas Th1-deficient (IL-12p35(-/-)) mice showed low fungal burdens and no overt disease. Neutrophil recruitment was impaired in IL-23p19(-/-) and IL-17RA(-/-), but not IL-12(-/-), mice, and TCR-alphabeta cells were more important than TCR-gammadelta cells. Surprisingly, mice deficient in the Th17 cytokine IL-22 were only mildly susceptible to OPC, indicating that IL-17 rather than IL-22 is vital in defense against oral candidiasis. Gene profiling of oral mucosal tissue showed strong induction of Th17 signature genes, including CXC chemokines and beta defensin-3. Saliva from Th17-deficient, but not Th1-deficient, mice exhibited reduced candidacidal activity. Thus, the Th17 lineage, acting largely through IL-17, confers the dominant response to oral candidiasis through neutrophils and antimicrobial factors.

Dendritic cells--why can they help and hurt us

Dendritic cells (DCs) show a Janus-like functional behavior. They help us by their orchestration of numerous immune responses to defend our body against invading pathogenic micro-organisms and also induce regulatory T cells to inhibit immune reactions against autoantigens as well as diverse harmless environmental antigens. However, DCs can also be of harm to us when misguided by their microenvironment as in allergic and autoimmune diseases or when DCs are targeted and exploited by microbes and cancer cells to evade the immune defense. This huge and diverse functional repertoire of DCs requires complex decision-making processes and the integration of multiple stimulatory and inhibitory signals. Although a given DC type has an extensive functionally plasticity, DCs are heterogeneous and individual DC subtypes are differentially distributed in tissues, express distinct sets of pattern recognition receptors and differ in their capacity to program naive T cells. With the help of transgenic mouse models and selective ablation of individual DC subtypes, we are just at the beginning of understanding the DC system in its complexity. Obtaining a more detailed knowledge of the DC system in mice and men holds strong promise for the successful induction of immunity and tolerance in therapeutic trials. This review presents the recent advances in the understanding of DC biology and discusses why and how DC can help and hurt us.

IL-17 and Th17 Cells

CD4+ T cells, upon activation and expansion, develop into different T helper cell subsets with different cytokine profiles and distinct effector functions. Until recently, T cells were divided into Th1 or Th2 cells, depending on the cytokines they produce. A third subset of IL-17-producing effector T helper cells, called Th17 cells, has now been discovered and characterized. Here, we summarize the current information on the differentiation and effector functions of the Th17 lineage. Th17 cells produce IL-17, IL-17F, and IL-22, thereby inducing a massive tissue reaction owing to the broad distribution of the IL-17 and IL-22 receptors. Th17 cells also secrete IL-21 to communicate with the cells of the immune system. The differentiation factors (TGF-beta plus IL-6 or IL-21), the growth and stabilization factor (IL-23), and the transcription factors (STAT3, RORgammat, and RORalpha) involved in the development of Th17 cells have just been identified. The participation of TGF-beta in the differentiation of Th17 cells places the Th17 lineage in close relationship with CD4+CD25+Foxp3+ regulatory T cells (Tregs), as TGF-beta also induces differentiation of naive T cells into Foxp3+ Tregs in the peripheral immune compartment. The investigation of the differentiation, effector function, and regulation of Th17 cells has opened up a new framework for understanding T cell differentiation. Furthermore, we now appreciate the importance of Th17 cells in clearing pathogens during host defense reactions and in inducing tissue inflammation in autoimmune disease.

Th17 cytokines and mucosal immunity

The T-helper 17 (Th17) lineage is a recently described subset of memory T cells that is characterized by its CD4(+) status and its ability to make a constellation of cytokines including interleukin-17A (IL-17A), IL-17F, IL-22, and, in humans, IL-26. Although most extensively described in the autoimmunity literature, there is growing evidence that the Th17 lineage plays a significant role in mediating host mucosal immunity to a number of pulmonary pathogens. This review highlights our current understanding of the role of the Th17 lineage and Th17 cytokines in mediating mucosal immunity to both pulmonary and gastrointestinal pathogens. While we have the strongest evidence that the Th17 lineage is centrally involved in mediating the host response to Gram-negative extracellular pulmonary pathogens, this literature is rapidly evolving and demonstrates a central role for Th17 cytokines both in primary infection and in recall responses seen in vaccine studies. In this review, we summarize the current state of this literature and present possible applications of Th17-targeted immunotherapy in the treatment and prevention of infection.

Regulation and function of proinflammatory TH17 cells

Naïve CD4(+) helper T (TH) cells, upon activation by antigen-presenting cells (APC), differentiate into different types of effector cells that are characterized by their distinct cytokine production profiles and immune regulatory functions. In addition to TH1 and TH2 cells, a third subset of effector TH cells has recently been described and termed TH17. Since their identification, TH17 cells have emerged as crucial players in infectious, inflammatory, and autoimmune diseases, and cancer. In this review, we summarize the latest discoveries on the cytokine-mediated regulation and transcriptional programming of TH17 cells and their roles in different immune responses and diseases.

LPS and poly I:C induce chromatin modifications at a novel upstream region of the IL-23 p19 promoter

IL-23, a heterodimer of IL-12 p40 and IL-23 p19, is critical for an effective immune response to many infections and has been implicated in several autoimmune diseases, however, little is known about the regulation of IL-23 gene expression in monocytes. We found that poly I:C, LPS, flagellin, and zymogen activated significant IL-23 production in primary human monocytes. Using chromatin immunoprecipitation, we found that a distal upstream region of the IL-23 p19 promoter at -601 to -521 underwent extensive histone modifications in response to stimuli. This distal region of the promoter is not highly conserved between species and has not been previously implicated in the regulation of IL-23 expression. Knockdown of CBP markedly decreased IL-23 p19 responses to poly I:C but had a less dramatic effect on LPS responses, confirming different chromatin responses to these two stimuli. Our data suggest that one of the mechanisms regulating IL-23 expression is the regulation of histone modifications at this distal upstream region of the promoter.

Increased numbers of circulating polyfunctional Th17 memory cells in patients with seronegative spondylarthritides

OBJECTIVE

A distinct subset of proinflammatory CD4+ T cells that produce interleukin-17 was recently identified. These cells are implicated in different autoimmune disease models, such as experimental autoimmune encephalomyelitis and collagen-induced arthritis, but their involvement in human autoimmune disease has not yet been clearly established. The purpose of this study was to assess the frequency and functional properties of Th17 cells in healthy donors and in patients with different autoimmune diseases.

METHODS

Peripheral blood was obtained from 10 psoriatic arthritis (PsA), 10 ankylosing spondylitis (AS), 10 rheumatoid arthritis (RA), and 5 vitiligo patients, as well as from 25 healthy donors. Synovial tissue samples from a separate group of patients were also evaluated (obtained as paraffin-embedded sections). Peripheral blood cells were analyzed by multiparameter flow cytometry and immunohistochemistry. Cytokine production was examined by enzyme-linked immunosorbent assay and intracellular cytokine staining using specific monoclonal antibodies. Synovial tissue was examined for infiltrating T cells by immunohistochemical analysis.

RESULTS

We found increased numbers of circulating Th17 cells in the peripheral blood of patients with seronegative spondylarthritides (PsA and AS), but not in patients with RA or vitiligo. In addition, Th17 cells from the spondylarthritis patients showed advanced differentiation and were polyfunctional in terms of T cell receptor-driven cytokine production.

CONCLUSION

These observations suggest a role of Th17 cells in the pathogenesis of certain human autoimmune disorders, in particular the seronegative spondylarthritides.

The interleukin-23/interleukin-17 axis in spondyloarthritis

PURPOSE OF REVIEW

To inform readers of recent advances in our understanding of the development and function of Th17 T cells and emerging data suggesting that the interleukin-23/interleukin-17 axis may be involved in the pathogenesis of spondyloarthritis.

RECENT FINDINGS

The discovery of CD4+ Th17 T cells and the interleukin-23/interleukin-17 axis has challenged existing paradigms and the role of Th1 T cells in many autoimmune diseases. The development and cytokine profile of Th17 T cells differs in mice and humans. In humans, interleukin-23 synergizes with interleukin-6 and interleukin-1 to promote Th17 development. In mice, transforming growth factor-beta and interleukin-6 are critical, whereas interleukin-23 is more important at later stages promoting interleukin-17 production. In mice, CD4+ cells producing interferon-gamma appear to be distinct from interleukin-17-producing cells, while in humans cells secreting both cytokines have been observed. Growing evidence from animal models, cytokine analyses of patient fluids, and whole-genome association studies suggest that the interleukin-23/interleukin-17 axis plays an important role in spondyloarthritis pathogenesis. Possible links between an HLA-B27-induced unfolded protein response and activation of the interleukin-23/interleukin-17 axis have been observed in animal models and may contribute to the development of the spondyloarthritis phenotype.

SUMMARY

Activation of the interleukin-23/interleukin-17 axis in spondyloarthritis has important therapeutic implications.

Novel immune regulatory pathways and their role in immune reconstitution syndrome in organ transplant recipients with invasive mycoses

Immune regulatory pathways involving the newly discovered T regulatory (Treg) and Th17 cells are amongst the principal targets of immunosuppressive agents employed in transplant recipients and key mediators of host inflammatory responses in fungal infections. These novel signaling pathways, in concert with or independent of Th1/Th2 responses, have potentially important implications for yielding valuable insights into the pathogenesis of immune reconstitution syndrome (IRS) in transplant recipients, for aiding the diagnosis of this entity, and for achieving a balance of immune responses that enhance host immunity while curbing unfettered inflammation in IRS.

Interleukin-12 and host defense against murine Pneumocystis pneumonia

Little is known about the role of the cytokine interleukin-12 (IL-12) in Pneumocystis pneumonia or its potential use as immunotherapy. We asked whether release of IL-12 is part of the normal host response to this infection and whether local treatment with IL-12 or gene transfer of IL-12 could accelerate clearance of infection. IL-12 was assayed by enzyme-linked immunosorbent assay in normal mice and in mice deficient in IL-12 after inoculation of Pneumocystis carinii. P. carinii-infected mice were treated with local instillation of IL-12 and gene transfer of the IL-12 gene. Inoculation of P. carinii into normal mice evoked a brisk release of IL-12 into lung tissue, and IL-12 P35-deficient mice showed delayed clearance of infection measured by PCR for P. carinii rRNA. In control mice, intranasal recombinant IL-12 accelerated clearance of infection, and this was associated with increased recruitment of inflammatory cells into lavage fluid and increased release of tumor necrosis factor alpha, IL-12, and gamma interferon. Similar results were observed in infected mice depleted of CD4+ lymphocytes by using in vivo transfer of the IL-12 gene in a replication-deficient adenoviral vector. IL-12 is part of the normal host response to infection with P. carinii. IL-12 therapy can enhance host resistance to infection in both normal mice and mice depleted of CD4+ T lymphocytes. A treatment effect of IL-12 is mediated through enhanced inflammatory cell recruitment into lung tissue and increased tissue concentrations of proinflammatory cytokines.

Emergence of the Th17 pathway and its role in host defense

Recently, a paradigm shift has emerged in T-cell-mediated adaptive immunity. On the heels of the discovery of T cells with immunosuppressive function, so-called regulatory T cells (Tregs), the diversity of effector cells has expanded to include a third helper T cell, termed Th17. The appreciation that Th17 cells are products of a distinct effector pathway depended critically on observations made during investigations of mouse models of autoimmunity, advanced by discovery of the cytokines IL-17 and IL-23. These studies understandably led investigators to highlight the role played by Th17 cells in autoimmunity. Yet while the dysfunctional behavior of this phenotype as a contributor to inflammatory disease remains a central issue, this pathway evolved to meet a need for host protection against potential pathogens. It has become apparent that the Th17 pathway promotes host defense against certain extracellular bacteria and fungi, but more recent studies also implicate a role in protection against some protozoa and viruses. Here we review the experimental history that ultimately uncovered the existence and nature of Th17 cells, and then turn the reader's attention to what is currently known about Th17 cells as a bulwark against pathogens.