Release of immune complexes bound to CR1 on erythrocytes; suramin inhibits factor I in the presence of EDTA.

An experimental model was established in order to study the release of immune complexes (IC) bound by complement C3b receptors (CR1) on human erythrocytes (RBC). Soluble tetanus toxoid anti-tetanus toxoid complexes were incubated with RBC in the presence of autologous serum at optimal conditions for binding. The RBC carrying complement-opsonized complexes were incubated with appropriate serum reagents, and it was shown that factor I was required for release of the complexes, which occurred without loss of CR1. Suramin was, irrespective of factor I, found to induce release of CR1-bound IC in the absence of EDTA, whereas factor I-mediated release was inhibited by suramin in the presence of EDTA. EDTA probably interfered through a charge-dependent interaction. These observations are decisive for the interpretation of in vitro experiments involving these reagents. The combination of EDTA and suramin was found inappropriate for use in quantitative determination of in vivo CR1-bound IC.