Transforming growth factor-β (TGF-β) has been shown to be required for Th17 cell differentiation via Smad-independent mechanisms. The molecular mechanism underlying this pathway remains to be clarified, however. We searched for genes regulated by TGF-β through the Smad-independent pathway by using Smad2 and Smad3 double-deficient T cells and identified the transcription factor Eomesodermin (Eomes), whose expression was suppressed by TGF-β via the c-Jun N-terminal kinase (JNK)-c-Jun signaling pathway. Inhibition of JNK strongly suppressed disease in an in vivo EAE model as well as in vitro Th17 cell induction. Overexpression of Eomes substantially suppressed Th17 cell differentiation, whereas ablation of Eomes expression could substitute for TGF-β in Th17 cell induction in primary T cells. Eomes suppressed Rorc and Il17a promoters by directly binding to the proximal region of these promoters. In conclusion, the suppression of Eomes by TGF-β via the JNK pathway is an important mechanism for Smad-independent Th17 cell differentiation.
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