Decreased IgG-FcyRII dissociation kinetics in the presence of a protein antigen

Total internal reflection fluorescence microscopy has been used to examine the interaction of a mouse monoclonal IgG2b, in the absence and presence of its protein antigen, with mouse FcyRII in substrate-supported planar membranes. Equilibrium association and kinetic dissociation constants were measured for the antibody S6-34.11, which is specific for bovine prothrombin fragment 1 (BF1). These measurements showed that BF1 induces a statistically significant decrease (30-40%) in the IgG-FcyRII dissociation kinetics. A corresponding increase in the equilibrium association constant was not observed, perhaps because the statistical accuracy of the equilibrium measurements is lower than that for the kinetic measurements. The consequences of these results for understanding the mechanism by which macrophages recognize and ingest opsonized targets are discussed.