Transcription factor RORα enforces stability of the Th17 cell effector program by binding to a Rorc cis-regulatory element

T helper 17 (Th17) cells regulate mucosal barrier defenses, but also promote multiple autoinflammatory diseases. Although many molecular determinants of Th17 cell differentiation have been elucidated, the transcriptional programs that sustain Th17 cells in vivo remain obscure. The transcription factor RORγt is critical for Th17 cell differentiation; however, it is not clear whether the closely-related RORα, which is co-expressed in Th17 cells, has a distinct role. Here, we demonstrated that, although dispensable for Th17 cell differentiation, RORα was necessary for optimal Th17 responses in peripheral tissues. The absence of RORα in T cells led to reductions in both RORγt expression and effector function amongst Th17 cells. Cooperative binding of RORα and RORγt to a previously unidentified Rorc cis-regulatory element was essential for Th17 lineage maintenance in vivo. These data point to a non- redundant role of RORα in Th17 lineage maintenance via reinforcement of the RORγt transcriptional program.