Generation of pathogenic TH17 cells in the absence of TGF-β signalling

T-helper 17 (TH17) cells are a subset of T-helper cells that produce interleukin (IL)-17 and are critical for host immunity. IL-6 and transforming growth factor-β (TGF-β) had been thought of as the principal inducers of TH17 differentiation, but this work provides further support for an alternative TGF-β-independent pathway of TH17 cell differentiation in mice. TH17 cells can be generated in the absence of TGF-β signalling by using IL-23 in combination with IL-6 and IL-1β. The resulting TH17 cells express not only RORγ-t, but also T-bet, and are more pathogenic than TH17 cells generated in the presence of TGF-β. These TH17 cells, generated independently of TGF-β, could be potential targets for the treatment of autoimmune disease. CD4+ T cells that selectively produce interleukin (IL)-17 (TH17 cells) are essential for host defence and autoimmunity. It has been thought that IL-6 and transforming growth factor (TGF)-β1 are the factors responsible for initiating the specification of TH17 cells. Here, however, it is shown that TH17 differentiation can occur in the absence of TGF-β signalling. IL-6, IL-23 and IL-1β effectively induced IL-17 production in naive precursors. These data reveal an alternative mode for TH17 differentiation and the importance of IL-23. CD4+ T-helper cells that selectively produce interleukin (IL)-17 (TH17), are critical for host defence and autoimmunity1,2,3,4. Although crucial for TH17 cells in vivo5,6, IL-23 has been thought to be incapable of driving initial differentiation. Rather, IL-6 and transforming growth factor (TGF)-β1 have been proposed to be the factors responsible for initiating specification7,8,9,10. Here we show that TH17 differentiation can occur in the absence of TGF-β signalling. Neither IL-6 nor IL-23 alone efficiently generated TH17 cells; however, these cytokines in combination with IL-1β effectively induced IL-17 production in naive precursors, independently of TGF-β. Epigenetic modification of the Il17a, Il17f and Rorc promoters proceeded without TGF-β1, allowing the generation of cells that co-expressed RORγt (encoded by Rorc) and T-bet. T-bet+RORγt+ TH17 cells are generated in vivo during experimental allergic encephalomyelitis, and adoptively transferred TH17 cells generated with IL-23 without TGF-β1 were pathogenic in this disease model. These data indicate an alternative mode for TH17 differentiation. Consistent with genetic data linking IL23R with autoimmunity, our findings re-emphasize the importance of IL-23 and therefore may have therapeutic implications.