BAFF, IL‐4 and IL‐21 separably program germinal center‐like phenotype acquisition, BCL6 expression, proliferation and survival of CD40L‐activated B cells in vitro

A B cell culture system using BAFF, IL‐4 and IL‐21 was recently developed that generates B cells with phenotypic and functional characteristics of in vivo‐generated germinal center (GC) B cells. Here, we observe discrete influences of each exogenous signal on the expansion and differentiation of a CD40L‐activated B cell pool. IL‐4 was expressly necessary, but neither BAFF nor IL‐21 was required for B cell acquisition of the GC B cell phenotypes of peanut agglutinin binding and loss of CD38 and IgD expression. Both IL‐4 and IL‐21 enhanced cell cycle entry upon initial activation dose‐dependently, and did so additively. Importantly, while both cytokines acted in concert to increase overall BCL6 expression amounts, IL‐21 exposure uniquely caused a small proportion of cells to attain a higher level of BCL6 expression, reminiscent of in vivo GC B cells. In contrast, BAFF supported survival of a fraction of memory‐like B cells in extended cultures after removal of surrogate T cell‐help signals. Thus, by separably programming proliferation, survival and GC phenotype acquisition, IL‐4, BAFF and IL‐21 drive distinct components of activated B cell fate. This study dissects the roles of the individual factors involved in generating in vitro germinal center B cells. It describes the unique impacts of IL‐4, IL‐21 and BAFF on proliferation, differentiation and survival of in vitro cultured B cells.