T helper 17 (Th17) cell development is programmed by the orphan nuclear receptor RORγt, but the underlying mechanism is not well understood. Nuclear receptor-mediated transcriptional activation depends on coactivators. Here, we show that steroid receptor coactivator-3 (SRC-3) critically regulates Th17 cell differentiation. Reduced incidence of experimental autoimmune encephalitis (EAE) associated with decreased Th17 cell generation in vivo was observed in mice with SRC-3 deletion specifically in T cells. In vitro, SRC-3 deficiency did not affect TGF-β/IL-6-induced Th17 cell generation but severely impaired pathogenic Th17 differentiation induced by IL-1/IL-6/IL-23. Microarray analysis revealed that SRC-3 not only regulates IL-17A but also IL-1R1 expression. SRC-3 bound to Il17a and Il1r1 loci in a RORγt-dependent manner and was required for recruitment of the p300 acetyltransferase. Thus, SRC-3 is critical for RORγt-dependent gene expression in Th17 cell-driven autoimmune diseases. The unique transcriptional programs in IL-1-induced inflammatory Th17 cells have remained unclear. Tanaka et al. demonstrate that SRC-3 in CD4+ T helper cells functions as a specific coactivator of RORγt, a master transcription factor of Th17 cells, by regulating IL-1-IL1R1 signaling.
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