ABSTRACTAntigen-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.HIGHLIGHTSGenome-wide CRISPR screens comprehensively identify genes regulating antigen uptake in B cellsB cell receptor-mediated antigen internalization is mediated by both epsin1- dependent clathrin-coated pits and a novel fast endophilin A2-mediated endocytosis.Endophilin A2 is required for peripheral B cell development, antigen-specific germinal center responses and high-affinity IgG production.Endophilin A2 is broadly essential for B cell intracellular trafficking pathways providing metabolic support of germinal center B cell proliferation, in part through regulation of iron uptake.
Engagement of the B cell receptor for antigen differentially affects B cell responses to Toll-like receptor-7 agonists and antagonists in BXSB mice