Interaction of complement-solubilized immune complexes with CR1 receptors on human erythrocytes. The binding reaction.

The binding of complement (C)-solubilized 125I bovine serum albumin (BSA) anti-BSA immune complexes (IC) to CR1 receptors on human red blood cells (RBC-CR1) was studied. The binding of IC to CR1 was strongly dependent on the molar antigen to antibody ratio, and IC formed in moderate antigen excess showed no binding. IC solubilized in 50% human serum in the presence of autologous RBC bound rapidly to RBC-CR1, with maximal binding within less than 1 min at 37 degrees C. Release of CR1-bound IC under these conditions occurred slowly, requiring more than 30 min. Only binding of 'partially' solubilized, e.g., anti C3c (C4c), and conglutinin-reactive IC occurred, whereas fully solubilized complexes (IC-C3dg, C4d) showed virtually no binding. Solubilization of IC in the presence of Mg-EGTA or in C2-deficient serum resulted in a markedly delayed binding of IC to RBC, indicating the importance of an intact classical pathway in preparing the IC for binding to RBC-CR1. C-solubilized IC could be absorbed to solid-phase conglutinin or antibody to C3c and C4c, and these ligands were able to inhibit the binding of solubilized IC to RBC. Heparin also exerted a marked, dose-dependent inhibitory effect on the binding of presolubilized IC to RBC-CR1, whereas the binding was unaffected by the addition of monosaccharides or by the concentration of Ca2+ or Mg2+ ions.