Binding of Affinity Cross-Linked Oligomers of IgG to Cells Bearing Fc Receptors

The binding of covalently cross-linked oligomers of rabbit IgG antibodies to tumor cells resembling macrophages and lymphocytes and to normal spleen cells has been measured. With all cells trimeric IgG binds with greater affinity than does the dimer, which in turn binds more tightly than does the monomer. However, as the oligomers increase in size, the individual monomeric subunits bind with decreasing energy. The macrophage tumor line. P388D1, binds oligomers with greater affinity than does the lymphocyte line, AKTB-1, but the differences in affinities are not great, differing by at most a factor of 5 in equilibrium constant. Normal spleen cells bind oligomers in the same concentration range as the tumor cells. The kinetics of binding do not occur as single first or second order reactions and suggest a multistage mechanism for oligomer binding. The presence of large concentrations of monomeric IgG tends to weaken oligomer binding and increases the exchange rate of bound oligomer from the cells. Since plasma and extracellular fluid also contain large concentrations of monomeric IgG, it is suggested that many immune complexes will bind weakly to the types of cells examined here and will rapidly exchange with IgG from the external medium.