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 aggregat e small and large unilamellar lipid vesicles. Vesicles aggregated by the an tibody could be dispersed by trypsin digestion, Inclusion of unsaturated ph osphatidylethanolamine in the vesicle formulation lowered the relative amou nt of cholesterol necessary for aggregation, and prevented disaggregation b y trypsin treatment, Fluorimetric assays indicated that membrane mixing occ urred in aggregates resistant to trypsinization, but the vesicles did not m ix or leak their aqueous contents, Analysis of the kinetics of lipid-mixing showed an increase in the aggregation and fusion rate constants with incre asing antibody concentrations, indicating that the antibody reaction promot es both processes. An apparent inactivation process whose rate increased wi th antibody dose has been considered. We conclude that the simultaneous bin ding of antibodies to more than one vesicle at densities that allow the con tact of membrane surfaces, induces first aggregation followed by hemifusion , and with excess of antibody also results in inactivation of the latter pr ocess.