Induction of aggregation and fusion of cholesterol-containing membrane vesicles by an anti-cholesterol monoclonal antibody.

A monoclonal IgM antibody that reacts with cholesterol was able to aggregate small and large unilamellar lipid vesicles. Vesicles aggregated by the antibody could be dispersed by trypsin digestion. Inclusion of unsaturated phosphatidylethanolamine in the vesicle formulation lowered the relative amount of cholesterol necessary for aggregation, and prevented disaggregation by trypsin treatment. Fluorimetric assays indicated that membrane mixing occurred in aggregates resistant to trypsinization, but the vesicles did not mix or leak their aqueous contents. Analysis of the kinetics of lipid-mixing showed an increase in the aggregation and fusion rate constants with increasing antibody concentrations, indicating that the antibody reaction promotes both processes. An apparent inactivation process whose rate increased with antibody dose has been considered. We conclude that the simultaneous binding of antibodies to more than one vesicle at densities that allow the contact of membrane surfaces, induces first aggregation followed by hemifusion, and with excess of antibody also results in inactivation of the latter process.