Th17‐related cytokines contribute to recall‐like expansion/effector function of HMBPP‐specific Vγ2Vδ2 T cells after Mycobacterium tuberculosis infection or vaccination

Whether cytokines can influence the adaptive immune response by antigen‐specific γδ T cells during infections or vaccinations remains unknown. We previously demonstrated that, during BCG/Mycobacterium tuberculosis (Mtb) infections, Th17‐related cytokines markedly upregulated when phosphoantigen‐specific Vγ2Vδ2 T cells expanded. In this study, we examined the involvement of Th17‐related cytokines in the recall‐like responses of Vγ2Vδ2 T cells following Mtb infection or vaccination against TB. Treatment with IL‐17A/IL‐17F or IL‐22 expanded phosphoantigen 4‐hydroxy‐3‐methyl‐but‐enyl pyrophosphate (HMBPP)‐stimulated Vγ2Vδ2 T cells from BCG‐vaccinated macaques but not from naïve animals, and IL‐23 induced greater expansion than the other Th17‐related cytokines. Consistently, Mtb infection of macaques also enhanced the ability of IL‐17/IL‐22 or IL‐23 to expand HMBPP‐stimulated Vγ2Vδ2 T cells. When evaluating IL‐23 signaling as a prototype, we found that HMBPP/IL‐23‐expanded Vγ2Vδ2 T cells from macaques infected with Mtb or vaccinated with BCG or Listeria ΔactA prfA*‐ESAT6/Ag85B produced IL‐17, IL‐22, IL‐2, and IFN‐γ. Interestingly, HMBPP/IL‐23‐induced production of IFN‐γ in turn facilitated IL‐23‐induced expansion of HMBPP‐activated Vγ2Vδ2 T cells. Furthermore, HMBPP/IL‐23‐induced proliferation of Vγ2Vδ2 T cells appeared to require APC contact and involve the conventional and novel protein kinase C signaling pathways. These findings suggest that Th17‐related cytokines can contribute to recall‐like expansion and effector function of Ag‐specific γδ T cells after infection or vaccination.     

IL‐6 controls susceptibility to helminth infection by impeding Th2 responsiveness and altering the Treg phenotype in vivo

IL‐6 plays a pivotal role in favoring T‐cell commitment toward a Th17 cell rather than Treg‐cell phenotype, as established through in vitro model systems. We predicted that in the absence of IL‐6, mice infected with the gastrointestinal helminth Heligmosomoides polygyrus would show reduced Th17‐cell responses, but also enhanced Treg‐cell activity and consequently greater susceptibility. Surprisingly, worm expulsion was markedly potentiated in IL‐6‐deficient mice, with significantly stronger adaptive Th2 responses in both IL‐6^?/? mice and BALB/c recipients of neutralizing anti‐IL‐6 monoclonal Ab. Although IL‐6‐deficient mice showed lower steady‐state Th17‐cell levels, IL‐6‐independent Th17‐cell responses occurred during in vivo infection. We excluded the Th17 response as a factor in protection, as Ab neutralization did not modify immunity to H. polygyrus infection in BALB/c mice. Resistance did correlate with significant changes to the associated Treg‐cell phenotype however, as IL‐6‐deficient mice displayed reduced expression of Foxp3, Helios, and GATA‐3, and enhanced production of cytokines within the Treg‐cell population. Administration of an anti‐IL‐2:IL‐2 complex boosted Treg‐cell proportions in vivo, reduced adaptive Th2 responses to WT levels, and fully restored susceptibility to H. polygyrus in IL‐6‐deficient mice. Thus, in vivo, IL‐6 limits the Th2 response, modifies the Treg‐cell phenotype, and promotes host susceptibility following helminth infection.     

Role of CD4 T cell helper subsets in immune response and deviation of CD8 T cells in mice*

The ability of different CD4⁺ T cell subsets to help CD8⁺ T‐cell response is not fully understood. Here, we found using the murine system that Th17 cells induced by IL‐1β, unlike Th1, were not effective helpers for antiviral CD8 responses as measured by IFNγ‐producing cells or protection against virus infection. However, they skewed CD8 responses to a Tc17 phenotype. Thus, the apparent lack of help was actually immune deviation. This skewing depended on both IL‐21 and IL‐23. To overcome this effect, we inhibited Th17 induction by blocking TGF‐β. Anti‐TGF‐β allowed the IL‐1β adjuvant to enhance CD8⁺ T‐cell responses without skewing the phenotype to Tc17, thereby providing an approach to harness the benefit of common IL‐1‐inducing adjuvants like alum without immune deviation.     

c-Rel promotes type 1 and type 17 immune responses during Leishmania major infection

Recent studies demonstrated the crucial role of c‐Rel in directing Treg lineage commitment and its involvement in T helper 1 (Th1) cell‐mediated autoimmune inflammation. We thus wondered whether these opposite functions of c‐Rel influence the course of antiparasitic immune responses against Leishmania major, an accepted model for the impact of T‐cell subsets on disease outcome. Here we show that c‐Rel‐deficient (rel^?/?) mice infected with L. major displayed dramatically exacerbated leishmaniasis and enhanced parasite burdens. In contrast to WT mice, IFN‐γ and IL‐17 production in response to L. major antigens was severely impaired in rel^?/? mice. Reconstitution of Rag1^?/? T‐cell deficient mice with rel^?/? CD4⁺ T cells followed by L. major infection demonstrated that c‐Rel‐deficient T cells mount normal Th1 responses and are able to contain the infection. Similarly, Th1 differentiation of naïve CD4⁺ cells in vitro was normal. Notably, a selective defect in IL‐12 and IL‐23 production was observed in rel^?/? DCs compared with their WT counterparts. In conclusion, our data suggest that the expression of c‐Rel in myeloid cells is essential for clearance of L. major and that this c‐Rel‐mediated effect is dominant over the lack of Tregs.     

Induction, function and regulation of IL-17-producing T cells

Recent reports have provided convincing evidence that IL‐17‐producing T cells play a key role in the pathogenesis of organ‐specific autoimmune diseases, a function previously attributed exclusively to IFN‐γ‐secreting Th1 cells. Furthermore, it appears that IL‐17‐producing T cells can also function with Th1 cells to mediate protective immunity to pathogens. Although much of the focus has been on IL‐17‐secreting CD4⁺ T cells, termed Th17 cells, CD8⁺ T cells, γδ T cells and NKT cells are also capable of secreting IL‐17. The differentiation of Th17 cells from naïve T cells appears to involve signals from TGF‐β, IL‐6, IL‐21, IL‐1β and IL‐23. Furthermore, IL‐1α or IL‐1β in synergy with IL‐23 can promote IL‐17 secretion from memory T cells. The induction or function of Th17 cells is regulated by cytokines secreted by the other major subtypes of T cells, including IFN‐γ, IL‐4, IL‐10 and at high concentrations, TGF‐β. The main function of IL‐17‐secreting T cells is to mediate inflammation, by stimulating production of inflammatory cytokines, such as TNF‐α, IL‐1β and IL‐6, and inflammatory chemokines that promote the recruitment of neutrophils and macrophages.     

Interleukin‐23 promotes intestinal T helper type17 immunity and ameliorates obesity‐associated metabolic syndrome in a murine high‐fat diet model

We addressed the role of interleukin‐23 (IL‐23) in driving the intestinal T helper type 17 (Th17) response during obesity and metabolic syndrome progression induced by a high‐fat diet (HFD). Diet‐induced obese and lean mice received HFD or control diet (CTD), respectively, for 20 weeks. The nutritional, metabolic and immune parameters were examined at weeks 9 and 20. Gene and protein IL‐23p19 and IL‐23 receptor expression was increased in the ileum of obese wild‐type mice (WT) fed the HFD for 9 weeks. Mice lacking IL‐23 and fed the HFD exhibited greater weight gain, higher fat accumulation, adipocyte hypertrophy and hepatic steatosis. Notably, these mice had more glucose intolerance, insulin resistance and associated metabolic alterations, such as hyperinsulinaemia and hyperlipidaemia. IL‐23 deficiency also significantly reduced protein levels of IL‐17, CCL20 and neutrophil elastase in the ileum and reduced Th17 cell expansion in the mesenteric lymph nodes of the HFD mice. Of importance, IL‐23‐deficient mice exhibited increased gut permeability and blood bacterial translocation compared with WT mice fed HFD. Finally, metagenomics analysis of gut microbiota revealed a dramatic outgrowth of Bacteroidetes over Firmicutes phylum with the prevalence of Bacteroides genera in the faeces of IL‐23‐deficient mice after HFD. In summary, IL‐23 appears to maintain the Th17 response and neutrophil migration into the intestinal mucosa, minimizing the gut dysbiosis and protecting against obesity and metabolic disease development in mice.     

Antigen presenting cell‐derived IL‐6 restricts Th2‐cell differentiation

The identification of DC‐derived signals orchestrating activation of Th1 and Th17 immune responses has advanced our understanding on how these inflammatory responses develop. However, whether specific signals delivered by DCs also participate in the regulation of Th2 immune responses remains largely unknown. In this study, we show that administration of antigen‐loaded, IL‐6‐deficient DCs to naïve mice induced an exacerbated Th2 response, characterized by the differentiation of GATA‐3‐expressing T lymphocytes secreting high levels of IL‐4, IL‐5, and IL‐13. Coinjection of wild type and IL‐6‐deficient bone marrow‐derived dendritic cells (BMDCs) confirmed that IL‐6 exerted a dominant, negative influence on Th2‐cell development. This finding was confirmed in vitro, where exogenously added IL‐6 was found to limit IL‐4‐induced Th2‐cell differentiation. iNKT cells were required for optimal Th2‐cell differentiation in vivo although their activation occurred independently of IL‐6 secretion by the BMDCs. Collectively, these observations identify IL‐6 secretion as a major, unsuspected, mechanism whereby DCs control the magnitude of Th2 immunity.     

IL‐23 protection against Plasmodium berghei infection in mice is partially dependent on IL‐17 from macrophages

Although IL‐12 is believed to contribute to protective immune responses, the role played by IL‐23 (a member of the IL‐12 family) in malaria is elusive. Here, we show that IL‐23 is produced during infection with Plasmodium berghei NK65. Mice deficient in IL‐23 (p19KO) had higher parasitemia and died earlier than wild‐type (WT) controls. Interestingly, p19KO mice had lower numbers of IL‐17‐producing splenic cells than their WT counterparts. Furthermore, mice deficient in IL‐17 (17KO) suffered higher parasitemia than the WT controls, indicating that IL‐23‐mediated protection is dependent on induction of IL‐17 during infection. We found that macrophages were responsible for IL‐17 production in response to IL‐23. We observed a striking reduction in splenic macrophages in the p19KO and 17KO mice, both of which became highly susceptible to infection. Thus, IL‐17 appears to be crucial for maintenance of splenic macrophages. Adoptive transfer of macrophages into macrophage‐depleted mice confirmed that macrophage‐derived IL‐17 is required for macrophage accumulation and parasite eradication in the recipient mice. We also found that IL‐17 induces CCL2/7, which recruit macrophages. Our findings reveal a novel protective mechanism whereby IL‐23, IL‐17, and macrophages reduce the severity of infection with blood‐stage malaria parasites.     

IL‐23‐driven encephalo‐tropism and Th17 polarization during CNS‐inflammation in vivo

IL‐23 but not IL‐12 is essential for the development of autoimmune tissue inflammation in mice. Conversely, IL‐12 and IL‐23 impact on the polarization of Th1 and Th17 cells, respectively. While both polarized T helper populations can mediate autoimmune inflammation, their redundancy in the pathogenesis of EAE indicates that IL‐23 exerts its crucial influence on the disease independent of its T helper polarizing capacity. To study the impact of IL‐23 and IL‐12 on the behavior of encephalitogenic T cells in vivo, we generated BM‐chimeric mice in which we can trace individual populations of IL‐23 or IL‐12 responsive T helper cells during EAE. We observed that T cells, which lack IL‐12Rβ1 (no IL‐12 and IL‐23 signaling), fail to invade the CNS and do not acquire a Th17 phenotype. In contrast, loss of IL‐12 signaling prevents Th1 polarization but does not prevent T‐cell entry into the CNS. The loss of IL‐12R engagement does not appear to alter T‐cell expansion but leads to their accumulation in secondary lymphoid organs. We found that IL‐23 licenses T cells to invade the target tissue and to exert their effector function, whereas IL‐12 is critical for Th1 differentiation, but does not influence the pathogenic capacity of auto‐reactive T helper cells in vivo.     

IL‐6: Regulator of Treg/Th17 balance

IL‐6 is a pleiotropic cytokine involved in the physiology of virtually every organ system. Recent studies have demonstrated that IL‐6 has a very important role in regulating the balance between IL‐17‐producing Th17 cells and regulatory T cells (Treg). The two T‐cell subsets play prominent roles in immune functions: Th17 cell is a key player in the pathogenesis of autoimmune diseases and protection against bacterial infections, while Treg functions to restrain excessive effector T‐cell responses. IL‐6 induces the development of Th17 cells from naïve T cells together with TGF‐β; in contrast, IL‐6 inhibits TGF‐β‐induced Treg differentiation. Dysregulation or overproduction of IL‐6 leads to autoimmune diseases such as multiple sclerosis (MS) and rheumatoid arthritis (RA), in which Th17 cells are considered to be the primary cause of pathology. Given the critical role of IL‐6 in altering the balance between Treg and Th17 cells, controlling IL‐6 activities is potentially an effective approach in the treatment of various autoimmune and inflammatory diseases. Here, we review the role of IL‐6 in regulating Th17/Treg balance and describe the critical functions of IL‐6 and Th17 in immunity and immune‐pathology.     

IL‐12‐polarized Th1 cells produce GM‐CSF and induce EAE independent of IL‐23

CD4⁺ T‐helper (Th) cells reactive against myelin antigens mediate the mouse model experimental autoimmune encephalomyelitis (EAE) and have been implicated in the pathogenesis of multiple sclerosis (MS). It is currently debated whether encephalitogenic Th cells are heterogeneous or arise from a single lineage. In the current study, we challenge the dogma that stimulation with the monokine IL‐23 is universally required for the acquisition of pathogenic properties by myelin‐reactive T cells. We show that IL‐12‐modulated Th1 cells readily produce IFN‐γ and GM‐CSF in the CNS of mice and induce a severe form of EAE via an IL‐23‐independent pathway. Th1‐mediated EAE is characterized by monocyte‐rich CNS infiltrates, elicits a strong proinflammatory cytokine response in the CNS, and is partially CCR2 dependent. Conversely, IL‐23‐modulated, stable Th17 cells induce EAE with a relatively mild course via an IL‐12‐independent pathway. These data provide definitive evidence that autoimmune disease can be driven by distinct CD4⁺ T‐helper‐cell subsets and polarizing factors.     

Monocytes/macrophages and/or neutrophils are the target of IL‐10 in the LPS endotoxemia model

IL‐10 is a potent regulator of the innate and adaptive immune responses. Several cell types produce IL‐10 and its receptor chains and these may regulate different immune responses. Here we report that inactivation of the IL‐10 receptor (IL‐10R1) gene in mice leads to an increased susceptibility to chemically induced colitis as in the classical IL‐10‐deficient mutant. To identify the cells regulated by IL‐10 in immune responses, we generated several cell type specific IL‐10R1‐deficient mutants. We show that, in an IL‐10‐dependent LPS model of endotoxemia, dampening of the immune response requires expression of IL‐10R1 in monocytes/macrophages and/or neutrophils but not in T cells nor B cells. As the macrophage and/or neutrophil‐specific IL‐10‐deficient mutants also display the same phenotype, our results suggest that an autocrine loop in monocytes/macrophages is the most probable mechanism for the regulation of an LPS‐induced septic shock. In contrast, in an IL‐10‐regulated T‐cell response to Trichuris muris infection, IL‐10 acting on T cells or monocytes/macrophages/neutrophils is not critical for the control of the infection.     

IL‐17/Th17 in anti‐fungal immunity: What's new?

How the immune system tailors protective responses to suit the infectious challenge while limiting damage to the host is an emerging theme in T‐cell biology. Although many studies have focused on the pathological aspects of IL‐17‐producing T cells in many autoimmune diseases, their role in protective anti‐microbial immunity has also been increasingly recognized. This increased recognition also applies to their role in anti‐fungal immunity; however, the role of IL‐17‐producing T cells in protection versus pathology in fungal infections is still controversial. Although both positive and negative effects on immune resistance have been attributed to the IL‐23/Th17 axis in experimental models of fungal infections, defective Th17 cell differentiation has been linked to recurrent pneumonia by filamentous fungi and the occurrence of mucocutaneous candidiasis in patients with primary immunodeficiencies. Here we discuss how recent findings in experimental candidiasis and aspergillosis shed new lights on the contribution of Th17 cells to resistance and pathology to fungi.     

TGF‐β interactions with IL‐1 family members trigger IL‐4‐independent IL‐9 production by mouse CD4+ T cells

TGF‐β and IL‐4 were recently shown to selectively upregulate IL‐9 production by naïve CD4⁺ T cells. We report here that TGF‐β interactions with IL‐1α, IL‐1β, IL‐18, and IL‐33 have equivalent IL‐9‐stimulating activities that function even in IL‐4‐deficient animals. This was observed after in vitro antigenic stimulation of immunized or unprimed mice and after polyclonal T‐cell activation. Based on intracellular IL‐9 staining, all IL‐9‐producing cells were CD4⁺ and 80–90% had proliferated, as indicated by reduced CFSE staining. In contrast to IL‐9, IL‐13 and IL‐17 were strongly stimulated by IL‐1 and either inhibited (IL‐13) or were unaffected (IL‐17) by addition of TGF‐β. IL‐9 and IL‐17 production also differed in their dependence on IL‐2 and regulation by IL‐1/IL‐23. As IL‐9 levels were much lower in Th2 and Th17 cultures, our results identify TGF‐β/IL‐1 and TGF‐β/IL‐4 as the main control points of IL‐9 synthesis.     

CXCL4 is a novel inducer of human Th17 cells and correlates with IL‐17 and IL‐22 in psoriatic arthritis

CXCL4 regulates multiple facets of the immune response and is highly upregulated in various Th17‐associated rheumatic diseases. However, whether CXCL4 plays a direct role in the induction of IL‐17 production by human CD4⁺ T cells is currently unclear. Here, we demonstrated that CXCL4 induced human CD4⁺ T cells to secrete IL‐17 that co‐expressed IFN‐γ and IL‐22, and differentiated naïve CD4⁺ T cells to become Th17‐cytokine producing cells. In a co‐culture system of human CD4⁺ T cells with monocytes or myeloid dendritic cells, CXCL4 induced IL‐17 production upon triggering by superantigen. Moreover, when monocyte‐derived dendritic cells were differentiated in the presence of CXCL4, they orchestrated increased levels of IL‐17, IFN‐γ, and proliferation by CD4⁺ T cells. Furthermore, the CXCL4 levels in synovial fluid from psoriatic arthritis patients strongly correlated with IL‐17 and IL‐22 levels. A similar response to CXCL4 of enhanced IL‐17 production by CD4⁺ T cells was also observed in patients with psoriatic arthritis. Altogether, we demonstrate that CXCL4 boosts pro‐inflammatory cytokine production especially IL‐17 by human CD4⁺ T cells, either by acting directly or indirectly via myeloid antigen presenting cells, implicating a role for CXCL4 in PsA pathology.     

PD‐1 modulates steady‐state and infection‐induced IL‐10 production in vivo

Programmed death‐1 (PD‐1) plays an important role in mediating immune tolerance through mechanisms that remain unclear. Herein, we investigated whether PD‐1 prevents excessive host tissue damage during infection with the protozoan parasite, Toxoplasma gondii. Surprisingly, our results demonstrate that PD‐1‐deficient mice have increased susceptibility to T. gondii, with increased parasite cyst counts along with reduced type‐1 cytokine responses (IL‐12 and IFN‐γ). PD‐1^?/? DCs showed no cell intrinsic defect in IL‐12 production in vitro. Instead, PD‐1 neutralization via genetic or pharmacological approaches resulted in a striking increase in IL‐10 release, which impaired type‐1‐inflammation during infection. Our results indicate that the absence of PD‐1 increases IL‐10 production even in the absence of infection. Although the possibility that such increased IL‐10 protects against autoimmune damage is speculative, our results show that IL‐10 suppresses the development of protective Th1 immune response after T. gondii infection.     

IL‐22 neutralizing autoantibodies impair fungal clearance in murine oropharyngeal candidiasis model

Protection against mucocutaneous candidiasis depends on the T helper (Th)17 pathway, as gene defects affecting its integrity result in inability to clear Candida albicans infection on body surfaces. Moreover, autoantibodies neutralizing Th17 cytokines have been related to chronic candidiasis in a rare inherited disorder called autoimmune polyendocriopathy candidiasis ectodermal dystrophy (APECED) caused by mutations in autoimmune regulator (AIRE) gene. However, the direct pathogenicity of these autoantibodies has not yet been addressed. Here we show that the level of anti‐IL17A autoantibodies that develop in aged Aire‐deficient mice is not sufficient for conferring susceptibility to oropharyngeal candidiasis. However, patient‐derived monoclonal antibodies that cross‐react with murine IL‐22 increase the fungal burden on C. albicans infected mucosa. Nevertheless, the lack of macroscopically evident infectious pathology on the oral mucosa of infected mice suggests that additional susceptibility factors are needed to precipitate a clinical disease.