Interleukin-10 from CD4+ follicular regulatory T cells promotes the germinal center response

CD4+ follicular regulatory T cells (Tfr cells) suppress B cell responses through modulation of follicular helper T cells (Tfh cells) and germinal center (GC) development. We found that Tfr cells also can promote the GC response through provision of IL-10 following acute infection with lymphocytic choriomeningitis virus (LCMV). Sensing of IL-10 by B cells was necessary for optimal development of the GC response. GC B cells formed in the absence of Treg-cell derived IL-10 displayed an altered dark zone state and decreased expression of the transcription factor FOXO1. IL-10 promoted nuclear translocation of FOXO1 in activated B cells. These data indicate that Tfr cells play a multifaceted role in the fine-tuning of the GC response and identify IL-10 as an important mediator by which Tfr cells support the GC reaction.