Proceedings Of The National Academy Of Sciences Of The United States Of America
Th17 cells are major players in multiple autoimmune diseases and are developmentally contingent on reciprocal functionality between the transcription factor Retineic acid receptor-related orphan nuclear receptor gamma (RORt) and Forkhead box protein P3 (Foxp3). Here we deciphered a previously unappreciated role of Steroid receptor coactivator 1 (SRC1) in defining the lineage decision for the development of Th17 versus induced T-regulatory (iTreg) cells. We demonstrate that SRC1 functions as a critical coactivator for RORt in vivo to promote the functional dominance of RORt over Foxp3 and thus establishing an unopposed Th17 differentiation program. In the absence of SRC1, T cell polarization resulted in decreased IL-17+ and increased Foxp3+ cells during both in vitro differentiation and in vivo development of experimental autoimmune encephalomyelitis. Mechanistically, T cell receptor (TCR) signaling molecule protein kinase C theta (PKC-)-mediated phosphorylation of SRC1 is important for inducing enhanced RORt-SRC1 interaction, stable DNA binding, and resultant IL-17A transcription. Furthermore, phospho-SRC1-mediated recruitment of CARM1 induced prominent asymmetric dimethylation of H3R17 while preventing repressive H3K9 trimethylation and hence further modifying the IL-17 locus for optimal transcription. Moreover, binding of phospho-SRC1 to RORt displaced bound Foxp3, leading to prompt degradation of the dissociated Foxp3 via a ubiquitin-proteosomal pathway and hence reversing the inhibitory action of Foxp3 on RORt activity. Thus, SRC1 acts as a crucial molecular mediator to integrate positive PKC–dependent TCR signals to induce peak RORt activity and establish phenotypic dominance of Th17 over the iTreg pathway.