Receptor-ligand interactions between serum amyloid P component and model soluble immune complexes.

We report here that isolated human serum amyloid P (SAP) binds aggregates of human IgG (AAg), which are model soluble immune complexes. The binding interaction was specific with regard to AAg compared to monomeric IgG, saturable, of high affinity, and reversible. With SAP adsorbed to microtiter plate wells, protein binding assays of AAg preparations with m.w. values of 1.05 to 2.25 x 10(6) were performed. The assay results yielded binding isotherms and estimates of the dissociation constant values of SAP:AAg binding ranging between 0.60 and 1.9 nM. Using the same method, the dissociation constant of SAP binding of biotinylated AAg was estimated to be approximately 3.0 nM. Competitive protein binding assays using biotinylated AAg and unlabeled AAg or monomeric IgG were performed to determine the relative affinity (IC50) of SAP binding. The IC50 values ranged from 22 to 51 nM for different sizes of AAg. The IC50 determined for monomeric IgG was 10 microM, demonstrating a relative specificity of SAP binding for AAg. The binding interaction was reversible, the SAP:AAg complexes dissociating with a t1/2 of 125 min. AAg binding to solid phase-adsorbed SAP was not dependent upon the presence of several cations, including calcium, but was not supported by cuprous ion. The differential binding of AAg vs monomeric IgG reported here for SAP is similar to the other human immune complex binding proteins C1q, rheumatoid factor, and cellular Ig C-region receptors. These results indicate that SAP:AAg binding has the hallmarks of a receptor:ligand interaction and may have implications for the disposal of immune complexes in vivo.