Emerging new roles of Th17 cells (2012)

Just about 20 years after the establishment of the Th1 and Th2 paradigm 1, and about 7 years ago now, a new effector CD4+ T-cell subset was described, called Th17 cells 2, 3. Upon activation these cells produce a panel of cytokines that are distinct from Th1 and Th2 cells. Th17 cells produce IL-17A, IL-17F, IL-21, and IL-22 and are considered to be the dominant proinflammatory T cells involved in multiple autoimmune diseases (reviewed in 4). Since the initial discovery of this T-cell subset, progress in defining Th17 cells has moved at an unprecedented rate. The differentiation factors for Th17 cells have been identified, the transcription factors that mediate differentiation and function have been uncovered and their role in human disease has been elucidated 4. From being the new kids on the block, Th17 cells have now been established as critical players in human disease. Since the publication in 2009 of the Viewpoint series on Th17 cells in the European Journal of Immunology, prefaced by Casey Weaver 5, even more impressive progress has been made and new functions of Th17 cells have been identified as highlighted in this latest review series. Similar to their Th1 and Th2 siblings, Th17 cells are differentiated by specific cytokines. Although initial studies suggested that IL-23 is the differentiation factor for Th17 cells, IL-23 cannot act on naïve T cells to induce differentiation of Th17 cells. This observation provided the impetus for identifying the factors responsible for the differentiation of Th17 cells. A series of studies later showed that Th17 cells differentiate from naïve CD4+ T cell in response to TGF-β plus IL-6 6-8. An emerging model for the development of Th17 cells includes three overlapping steps: differentiation, amplification, and stabilization 9. TGF-β plus IL-6 induces differentiation, IL-21 produced by developing Th17 cells mediates amplification, and IL-23 expands and stabilizes previously differentiated Th17 cells (reviewed in 9). Although there is some debate about whether TGF-β is required for the differentiation of human Th17 cells (as discussed in the review by Sallusto et al. 10 in this issue), emerging data suggests that TGF-β1 may be required for the differentiation of different subtypes of Th17 cells (see below). In the series of reviews discussing Th17 cells in this issue, Mills et al. 11 reprise that, in addition to Th17 cells, other lymphocyte populations are the innate source of IL-17. Among the cell types, γδ T cells and innate lymphoid cells (ILCs) produce an early burst of IL-17 and related cytokines such as IL-21 and IL-22 11. The IL-17 production by these cells is not due to TCR signaling. It is, in fact, primarily induced by IL-23 in combination with other proinflammatory cytokines that play a major role in anti-bacterial and anti-fungal immune responses. Moreover, accumulating literature suggests that both γδ T cell and ILC-derived IL-17, along with other unknown factors, may play a crucial role in the development of autoimmune inflammation such as experimental autoimmune encephalomyelitis (EAE) and inflammatory bowel disease (IBD) 12. Although IL-23 is one of the critical cytokines that is required for stabilization and development of pathogenic Th17 cells (see below), IL-23R signaling is now shown to play a crucial role in regulating an IL-17 response in γδ T cells and ILCs (as discussed in 11). Whether IL-23 is needed just for eliciting IL-17 or is required for expansion or maintenance of IL-17 producing γδ and ILCs has not yet been addressed. The development of the Th17-cell lineage is instructed by a specific master transcription factor called RORγt that, together with STAT-3, BATF3, and IRF4, mediates transactivation of the IL-17 promoter (as reviewed in this issue by Huh and Littman 13). In addition to RoRγt, another family member RoRα also plays a role in Th17 differentiation. Loss of RoRα, however, has only minimal effects on Th17 differentiation 14, supporting the concept that the RoRγt orphan nuclear receptor plays the most dominant role in the differentiation of Th17 cells and production of IL-17 from γδ T cells and ILCs. Because of its importance in generating proinflammatory Th17 cells, extensive efforts have been made to develop small molecular weight inhibitors that could be used to suppress the differentiation of Th17 cells and inhibit the development of autoimmunity. In their review article, Huh and Littman also discuss the novel strategy by which they identified a small molecular weight compound called Digioxin (and its derivatives) that inhibits Th17 cells differentiation and suppresses development of autoimmunity 13. In addition to their role in inducing autoimmune disease in experimental models, IL-17 and Th17 cells have been implicated in multiple human autoimmune and infectious diseases. Th17 cells infiltrate the affected joints in RA patients at a very high frequency and IL-17-activated osteoclasts can induce bone resorption and bone erosion 15. IL-17 has also been identified in lesions in multiple sclerosis, rheumatoid arthritis, psoria-sis vulgaris, Sjogren's syndrome, and IBD 16-19. Additional support for the role of Th17 cells in inducing autoimmune diseases comes from genetic analysis of human autoimmune diseases. A genome wide association scan for single nucleotide polymorphisms (SNPs) associated with human autoimmune diseases revealed that a specific coding variant of the IL23R gene (arginine 381 to glutamine) conferred strong protection from Crohn's disease, whereas several variants in the noncoding region of this gene were associated with increased susceptibility to Crohn's disease 20. The coding SNP, arginine to glutamine at position 381 in the cytoplasmic domain of IL-23R, has now been shown to have a significant genetic association with many other autoimmune diseases including psoriasis, psoriatic arthritis, Crohn's disease, rheumatoid arthritis, and ankylosing spondylitis 20-23. In contrast to the overexuberant Th17 response in autoimmune diseases, many human immunodeficiencies have been associated with inborn errors in IL-17 and Th17 cells associated immunity often leading to the development of unrelenting chronic mucocutaneous candidiasis (CMC) in affected individuals (reviewed in this issue by Cypowyj et al. 24). In autosomal dominant hyper IgE syndrome, in which patients have recurrent infections with Stapholococcus aureus and Candida albicans, multiple different mutations in STAT-3 have been identified. Mutations in the DNA-binding, Src-homology, and transactivating domains of STAT-3 result in an almost complete loss of Th17 cells development, an observation that supports the crucial role of STAT-3 in Th17 cells differentiation and underscores the value of Th17 cells immunity in clearing fungal infections 25-28. In contrast to decreased STAT-3 function, a gain of function mutation in the STAT-1 coiled-coil domain results in stronger STAT-1 activation, which interferes with the development of Th17 cells. These patients also develop CMC 29. Similarly, CMC is the primary infection observed in patients with autoimmune polyendocrino-pathy syndrome-1. These patients have circulating neutralizing antibodies against IL-17A, IL-17F, and IL-22. Collectively, these data suggest that CMC is due to “inborn errors of IL-17 immunity,” and that IL-17 and Th17 cells have a critical role in clearing fungal infections 24. Although TGF-β1 together with IL-6 was shown to induce Th17 cells 6, it became increasingly clear that, even though these T cells produced IL-17, they were not efficient in transferring autoimmunity 30. Becher et al. in this issue describe that exposure to IL-23 is critical for Th17 cells to acquire a pathogenic phenotype 31. This observation is consistent with the results that IL-23- and IL-23R-deficient mice are highly resistant to multiple autoimmune diseases, and that genetic linkage to IL-23R has been observed in multiple human autoimmune diseases (as reviewed in 31). As well as its role in Th17 cells, Becher et al. further describe the role of the IL-23–IL-23R pathway in various immune cells such as γδ T cells, ILCs, and other innate immune cells 31. In addition, Becher et al. describe how IL-23-induced GM-CSF is a critical cytokine required for the pathogenicity of Th17 cells in EAE 31. Furthermore, it has been shown that the loss of GM-CSF makes mice resistant to EAE, and Th17 cells that lack the ability to produce GM-CSF do not transfer autoimmune disease 32; however, the expression of a number of molecules besides GM-CSF is regulated by IL-23, one such molecule being the transcription factor T-bet. Of note, loss of T-bet in Th17 cells inhibits their ability to induce autoimmune diseases 33. Emerging data suggest that Th17 cells may come in different flavors, each with the ability to clear different types of infections. This variability might also translate into their ability to become pathogenic or nonpathogenic in autoimmune diseases. Th17 cells that are present in the intestine at steady state appear to be distinct from those that are found at the sites of tissue inflammation and autoimmunity because intestinal Th17 cells coproduce IL-17 and IL-10. Flavell et al. discuss in their review this and the role of commensal microbiota-derived metabolites in maintaining immune homeostasis in both the induction of intestinal Th17 cells and in mediating defense against gut pathogens 34. It has been debated whether TGF-β1 is absolutely required for the differentiation of human Th17 cells (as discussed in the review by Sallusto et al. 10), with some studies demonstrating no need for exogenous TGF-β1 for human Th17 differentiation 35. Consistent with this observation, a recent study showed that, as for human Th17 cells, naïve murine T cells can also be differentiated into Th17 cells with IL-1 plus IL-6 without any requirement for exogenous TGF-β in the process 36. These Th17 cells co-express both RoRγt and T-bet and are highly pathogenic in inducing EAE. The two types of Th17 cells induced by TGF-β1+IL-6 and IL-1+IL-6 appear to be functionally and transcriptionally distinct. For this reason, they may also have different roles in autoimmunity and in clearing infections. Consistent with this observation, in humans, two different types of Th17 cells have been described (i) Th17 cells that produce IL-17 with IFN-γ and have specificity for Candida albicans and (ii) Th17 cells that coproduce IL-17 with IL-10 and have specificity for Staphylococcus aureus 37. In their review, Sallusto et al. further emphasize the heterogeneity, functions, and migration of human Th17 cells, illustrating how IL-1β and IL-12 contribute to the existence of IL-17/IFN-γ double producing T cells in various inflammatory conditions in humans diseases 10. In addition to inducing tissue inflammation and organ-specific autoimmune diseases, Th17 cells have now been shown to help B cells 38 and to play a critical role in forming ectopic lymphoid follicles in target organs 39-41. The review in this issue by Grogan and Ouyang 42 emphasizes that Th17 cells share many developmental and effector markers with lymphoid tissue inducer (LTi) cells. The cytokines IL-17 and IL-22, and the cell surface molecules (Podoplanin/gp38) that are uniquely expressed by Th17 cells play a crucial role in organizing the formation of tertiary lymphoid follicles in several chronically inflamed tissues including the lung, gut and the central nervous system of the mice 39-41. In their review Wahren-Herlenius et al. describe the functions of Th17 cells and other IL-17-producing cells in the pathogenesis of type-1 interferon-driven systemic autoimmune tissue inflammation in lupus, Sjogren's syndrome and systemic sclerosis 43. Recent data suggests that IL-17 and Th17 cells may be responsible for an exaggerated autoreactive B-cell response and increased autoantibody production along with the development of SLE in both humans and mouse models of lupus. The role of IL-23 and Th17 was further established in an experimental model of SLE as deletion of IL-23R in C57BL6-Faslpr/lpr mice abolished the development of pathogenic double negative (DN) T cells and also abrogated the development of lupus nephritis 44. Similarly, deletion of IL-17 in spontaneous autoimmune prone lupus BXD2 mice was shown to decrease germinal center formation and suppress autoantibody production 45. Since their discovery 7 years ago, Th17 cells have hogged the center stage in the literature because they are so often incriminated in inducing autoimmunity and tissue inflammation. The eight review articles published in this issue of the European Journal of Immunology, however, emphasize the additional features of Th17 cells that have gone unnoticed: Th17 cells are not all pathogenic, they may, in fact, have an important role in tissue homeostasis; innate cells (γδ and iLCs) are the major producers of IL-17 initially; Th17 cells are effective B-cell helpers; and RoRγt, the Th17 transcription factor, is targetable with small molecular weight compounds. With this emerging knowledge, IL-17 and Th17 cells are being targeted for drug development as evidenced by the number of biologics being tested in human clinical trials. The new phase of this development would be the generation of orally available small molecular weight compounds that will affect pathogenic Th17 cells and suppress tissue inflammation in multiple autoimmune and inflammatory diseases. The authors thank many colleagues for their helpful discussions. The work in the Kuchroo lab is funded by NINDS, NIAID, National MS society, Guthy Jackson Foundation, and JDRF. The authors declare no financial or commercial conflict of interest.