Memory B cells contribute to rapid Bcl6 expression by memory follicular helper T cells

In primary humoral responses, B-cell lymphoma 6 (Bcl6) is a master regulator of follicular helper T (T FH) cell differentiation; however, its activation mechanisms and role in memory responses remain unclear. Here we demonstrate that survival of CXCR5 ⁺ T FH memory cells, and thus subsequent recall antibody response, require Bcl6 expression. Furthermore, we show that, upon rechallenge with soluble antigen Bcl6 in memory T FH cells is rapidly induced in a dendritic cell-independent manner and that peptide:class II complexes (pMHC) on cognate memory B cells significantly contribute to this induction. Given the previous evidence that antigen-specific B cells residing in the follicles acquire antigens within minutes of injection, our results suggest that memory B cells present antigens to the cognate T FH memory cells, thereby contributing to rapid Bcl6 reexpression and differentiation of the T FH memory cells during humoral memory responses.