The Janus kinase (JAK)–signal transducer of activators of transcription (STAT) pathway is now recognized as an evolutionarily conserved signaling pathway employed by diverse cytokines, interferons, growth factors, and related molecules. This pathway provides an elegant and remarkably straightforward mechanism whereby extracellular factors control gene expression. It thus serves as a fundamental paradigm for how cells sense environmental cues and interpret these signals to regulate cell growth and differentiation. Genetic mutations and polymorphisms are functionally relevant to a variety of human diseases, especially cancer and immune-related conditions. The clinical relevance of the pathway has been confirmed by the emergence of a new class of therapeutics that targets JAKs.

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.