Release of soluble immune complexes from immune adherence receptors on human erythrocytes is mediated by C3b inactivator independently of β1H and is accompanied by generation of C3c

Antigen.antibody complexes (Ag.Ab) prepared from (125)I-labeled bovine serum albumin and guinea pig anti-albumin were incubated at 37 degrees C for 30 min with normal human serum diluted optimally for binding (1:16) and then with autologous erythrocytes (RBC). After washing, RBC-bearing antigen.antibody.complement complexes (Ag.Ab.C) were resuspended in serum reagents or solutions of purified complement components, and the kinetics of dissociation were analyzed. Ag.Ab.C dissociated in serum heated at 56 degrees C for 30 min (SDelta30) but not in serum heated for 120 min (SDelta120). Dissociation in SDelta30 markedly decreased after adsorption with anti-C3b inactivator but not anti-beta1H or anti-C4 binding protein (C4bp), and dissociation in SDelta120 markedly increased after addition of C3b inactivator. Hemolytic assays revealed that SDelta30 retained inactivator activity whereas SDelta120 lacked significant activity. Ag.Ab.C dissociated in the presence of purified inactivator or C3b but not beta1H or C3. Dissociation was more rapid with inactivator than with C3b and occurred at 0 degrees C as well as at 37 degrees C. Treatment with inactivator inhibitor abolished dissociation in SDelta30; dissociation in inactivator deficient serum was markedly reduced. Addition of beta1H did not enhance inactivator-mediated dissociation at limiting dilutions of inactivator, and adsorption of Ag.Ab.C with anti-beta1H or preparation of Ag.Ab.C with serum adsorbed with anti-beta1H did not diminish dissociation. After dissociation with inactivator, Ag.Ab.C were unchanged in size but were no longer able to bind to fresh RBC and gave enhanced binding to Raji and Daudi lymphoblastoid cells. NaDodSO(4)/polyacrylamide gel electrophoresis of Ag.Ab.C prepared with (125)I-labeled C3 revealed that, after binding to RBC, dissociation with inactivator was accompanied by generation of a C3 fragment the size of C3c. Preincubation of Ag.Ab.C with excess inactivator did not prevent subsequent binding of Ag.Ab.C to RBC but, immediately after binding, Ag.Ab.C dissociated rapidly. These findings indicate that C3b inactivator can release immune complexes from immune adherence receptors on human RBC, that release occurs independently of beta1H, alters cell binding properties of immune complexes, and involves multiple cleavages of the C3b alpha' chain, and that receptors in human RBC membrane are required for this C3b inactivator-mediated breakdown.