The regulation of B cell activation by membrane damage and antigen density

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Antibodies generated by B cells neutralize pathogens and pathogen-secreted toxins and flag them for immune clearance. Antibody responses are initiated via binding of cognate antigen to B-cell receptors (BCR). This induces BCR aggregation in lipid rafts, promoting BCR signaling and internalization of antigen for processing and presentation to T helper cells, which is essential for generating high affinity and long-lasting antibody responses. The ability of an immunogen to activate BCR signaling and internalization is necessary for efficient vaccines. To capture these immunogens, B cells circulate and migrate through blood and lymphoid tissues. During circulation, the plasma membrane of B cells may be damaged by mechanical forces and membrane-perforating toxins. The impact of plasma membrane damage on B-cell activation is unknown.

The first part of this thesis investigated the mechanism of plasma membrane repair in B cells and the effects of repair on BCR activation. My research reveals that B cells rapidly repair membrane wounds provoked by streptolysin O, a pore forming bacterial toxin. Similar to the mechanism reported for fibroblasts and muscle cells, B cells repair by Ca2+ triggered lysosome exocytosis, which releases acid sphingomyelinase (ASM) to the plasma membrane to induce endocytosis of damaged membrane. Different from previous reports, ASM induces direct endocytosis of lipid rafts in the absence of the membrane invaginating lipid raft protein, caveolin. Importantly, it was discovered that BCR activation interferes with plasma membrane repair, while wounding inhibits BCR signaling and internalization by segregating BCRs from lipid rafts. These data suggest that plasma membrane repair and B cell activation interfere with one another due to competition for lipid rafts.

The second part of my thesis established and characterized a membrane-bound antigen system where all antigenic molecules are optimally oriented for BCR binding. Using the new system, we investigated the role of the density and valency of membrane-bound antigen on BCR activation. The results show that increases in the density but not valency of antigen on membranes significantly enhance the magnitudes of the early events of BCR activation, including BCR self-clustering, cell spreading on antigen-presenting surface, and protein tyrosine phosphorylation. The enhanced signaling is correlated with greater actin dynamics required for BCR aggregation, B-cell spreading and signaling. These results indicate that this model antigen will benefit quantitative studies of the molecular mechanisms underlying BCR activation, and also suggest that manipulations of molecular configuration and density can be applied to enhance the immunogenicity of vaccines.