Blimp-1 functions as a molecular switch to prevent inflammatory activity in Foxp3⁺ ROR gamma t⁺regulatory T cells.

Foxp3(+) regulatory T cells (Treg) are essential modulators of immune responses, but the molecular mechanisms underlying their function are not fully understood. Here we show that the transcription factor Blimp-1 is a crucial regulator of the Foxp3(+)ROR gamma t(+) Treg subset. The intrinsic expression of Blimp-1 in these cells is required to prevent production of Th17-associated cytokines. Direct binding of Blimp-1 to the II17 locus in Treg is associated with inhibitory histone modifications but unaltered binding of ROR gamma t. In the absence of Blimp-1, the II17 locus is activated, with increased occupancy of the co-activator p300 and abundant binding of the transcriptional regulator IRF4, which is required, along with ROR gamma t, for IL-17 expression in the absence of Blimp-1. We also show that despite their sustained expression of Foxp3, Blimp-1(-/-) ROR gamma t(+)IL-17-producing Treg lose suppressor function and can promote intestinal inflammation, indicating that repression of Th17-associated cytokines by Blimp-1 is a crucial requirement for ROR gamma t(+) Treg function