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Enhanced degradation of soluble immunoglobulin aggregates by macrophages in the presence of complement.

The role of complement in the processing of soluble immune complexes by guinea-pig peritoneal macrophages was studied in a homologous system in vitro, using isolated stable IgG2 aggregates as a model for immune complexes. Under serum-free conditions, peritoneal macrophages were already able to degrade substantial amounts of the available immunoglobulin aggregates. Addition of fresh serum to the incubation mixtures caused a marked increase in the rate of degradation. The stimulating effect of fresh serum was complement-mediated, because it was abolished by heat treatment or CoVF treatment of the serum and was not seen when C4 or C3-deficient sera were tested. Reconstitution of C4 and C3-deficient sera with purified C4 or C3 restored the stimulating effect of serum on the degradation of the aggregates. The results suggest that activation of the complement system by immune aggregates enhances binding of the aggregates to the cells, which results in the increased degradation observed.