Lipid Membrane-Based Antigen Presentation to B Cells Using a Fully Synthetic Ex Vivo Germinal Center Model.

High-affinity antigen-specific B cells are generated within specialized structures, germinal centers (GCs), inside lymphoid organs. In GCs, follicular dendritic cells (FDCs) present antigens on their membrane surface to cognate B cells, inducing rapid proliferation and differentiation of the B cells toward antibody-secreting cells. The FDC's fluid membrane surface allows B cells to "pull" the antigens into clusters and internalize them, a process that frequently involves tearing off and internalizing FDC membrane fragments. To study this process ex vivo, liposomal membranes are used as the antigen-presenting FDC-like fluid lipid surface to activate B cells. In a fully synthetic in vitro GC model (sGC), which uses the microbead-based presentation of the CD40 Ligand and a cytokine cocktail to mimic T follicular helper cell signals to B cells, liposomes presenting a model antigen mimic effectively engage B cell receptors (BCRs) and induce greater BCR clustering compared to soluble antigens, resulting in rapid antigen internalization and proliferation of the B cells. B cells showed GC-like reactions and undergo efficient IgG1 class-switching. Taken together, the results suggest that fluid membrane-bound antigen induces a strong GC response and provides a novel synthetic in vitro system for studying GC biology in health and diseases, and for expanding therapeutic B cells ex vivo.