Endophilin A2 regulates B cell protein trafficking and humoral responses

Antigen-specific B cell responses require endosomal trafficking to regulate antigen uptake and presentation to helper T cells, and to control expression and signaling of immune receptors. However, the molecular composition of B cell endosomal trafficking pathways and their specific roles in B cell responses have not been systematically investigated. Here we report high-throughput identification of genes regulating B cell receptor (BCR)-mediated antigen internalization using genome-wide functional screens. We show that antigen internalization depends both on clathrin-coated pits and on clathrin-independent endocytosis mediated by endophilin A2. Although endophilin A2 is dispensable for presentation of the endocytosed antigen, it is required for metabolic support of germinal center (GC) B cell proliferation, in part through regulation of iron uptake. Consequently, endophilin A2 deficient mice show selective defects in GC B cell responses and production of high-affinity IgG. The requirement for endophilin A2 highlights a unique importance of clathrin-independent intracellular trafficking in GC B cell clonal expansion and antibody responses.