The interaction of monoclonal antibodies with MHC class I antigens on mouse spleen cells. I. Analysis of the mechanism of binding.

We studied the mechanism of binding of radiolabeled, monoclonal anti-H-2 antibodies to mouse spleen cells to determine the number of H-2 antigen molecules per cell. Equilibrium and kinetic data were analyzed in detail according to theoretical models developed for different modes of antibody binding. The results of binding experiments from three monoclonal IgG antibodies (36-7-5, anti-Kk; 27-11-13, anti-DbDd; and 11-4-1, anti-Kk) and their F(ab')2 and F(ab') fragments show that for the IgG and F(ab')2 from all three antibodies, the monovalently and bivalently bound states of the antibody co-exist in rapid equilibrium with one another on the cell surface, with the bivalent state predominating. We show that the relative proportions of the monovalently and bivalently bound species can be estimated from dissociation kinetics experiments, and that once the mode of antibody binding has been established, the density of H-2 determinants on the cell surface can be estimated from equilibrium-binding data. We conclude that the average numbers of H-2K and H-2D molecules on B10.A spleen cells are 5 X 10(4) and 1.1 X 10(5) molecules/cell, respectively.