The binding of soluble immune complexes of guinea pig IgG2 to homologous peritoneal macrophages. Determination of the avidity constants at 4 degrees C.

Soluble immune complexes of guinea pig IgG2 anti-DNP (2,4-dinitrophenyl) antibodies and DNP19BSA (bovine serum albumin) were prepared at a variety of antibody-to-antigen ratios. The mean sizes of the complexes were determined by gel filtration, and equilibrium binding of the complexes to homologous peritoneal macrophages was measured at 4 degrees C. Scatchard plots of the binding data were linear indicating that the complexes were binding to a uniform population of functionally independent membrane receptors. The avidity constants for complex binding to macrophages increases from 15.4 X 10(7) M-1 for complexes containing an average of two antibody molecules to 59.0 X 10(7) M-1 for complexes containing an average of four, compared with an association constant for monomeric IgG2 of 0.21 X 10(7) M-1. Calculation of the molar-free energy changes accompanying monomer and immune complex binding revealed not only that the intrinsic binding activity of mononer was sufficient to account for the complex binding activities by simple cooperation of linked antibodies, but also that the cross-linking of antibodies with antigen imposed a strain on the antibody-receptor interaction, resulting in complex binding energies that were lower than the values expected from a simple cooperative mechanism. Complex inhibition by monomeric IgG2 of unrelated antibody specificity was studied, and it was observed that high concentrations of monomer led to an increase in macrophage discrimination between complexes of different size. The relevance of this finding to the in vivo clearance of circulating complexes is discussed.