Foxp3+ follicular regulatory T cells control T follicular helper cells and the germinal center response

Follicular helper (TFH) cells provide crucial signals to germinal center B cells undergoing somatic hypermutation and selection that results in affinity maturation. Tight control of TFH numbers maintains self-tolerance. We describe a population of Foxp3+Blimp-1+CD4+ T cells constituting 10-25% of the CXCR5highPD-1highCD4+ T cells found in germinal center after immunization. These follicular regulatory T cells (TFR) share phenotypic characteristics with TFH and conventional Foxp3+ regulatory T cells (Treg) yet are distinct from either. Similar to TFH cells, TFR development depends on Bcl-6, SAP, CD28 and B cells; however TFR originate from thymic-derived Foxp3+ precursors, not naïve or TFH cells. TFR are suppressive in vitro and limit TFH and germinal center B cell numbers in vivo. In the absence of TFR, an outgrowth of non-antigen-specific B cells in germinal centers leads to fewer antigen-specific cells. Thus, Treg cells use the TFH differentiation pathway to produce specialized suppressor cells that control the germinal center response.