BCL6 corepressor contributes to Th17 cell formation by inhibiting Th17 fate suppressors

CD4(+) T helper 17 (Th17) cells protect vertebrate hosts from extracellular pathogens at mucosal surfaces. Th17 cells form from naive precursors when signals from the T cell antigen receptor (TCR) and certain cytokine receptors induce the expression of the ROR.t transcription factor, which activates a set of Th17-specific genes. Using T cell-specific loss-of-function experiments, we find that two components of the Polycomb repressive complex 1.1 (PRC1.1), BCL6 corepressor (BCOR) and KDM2B, which helps target the complex to unmethylated CpG DNA islands, are required for optimal Th17 cell formation in mice after Streptococcus pyogenes infection. Genome-wide expression and BCOR chromatin immunoprecipitation studies revealed that BCOR directly represses Lef1, Runx2, and Dusp4, whose products inhibit Th17 differentiation. Together, the results suggest that the PRC1.1 components BCOR and KDM2B work together to enhance Th17 cell formation by repressing Th17 fate suppressors.