High doses of intravenous Ig inhibit in vitro uptake of C4 fragments onto sensitized erythrocytes

We have recently reported that intravenous Ig (IVIg) inhibits uptake of activated C3 fragments onto antibody-sensitized red blood cells (RBCs). To elucidate the mechanism by which IVIg exerts its effect on the complement system, we examined the possible interference with the C4 step of the classical complement cascade. We examined the capacity of autologous serum containing high concentrations of human IVIg to deposit C4 fragments onto model targets (guinea pig and/or human erythrocytes sensitized with rabbit anti-guinea pig/human erythrocytes IgG antibody). C4 binding was quantified with radiolabeled anti-C4. Guinea pig serum with added IVIg suppressed C4 uptake onto IgG- sensitized guinea pig erythrocytes at all time points (0, 5, 15, and 30 minutes). Using sera of guinea pigs treated with increasing doses of IVIg, this effect was shown to be dose-responsive. Serum from a patient treated with IVIg showed reduced C4 uptake onto sensitized homologous RBCs. In comparison with the serum from the same patient before IVIg therapy was administered, levels were decreased almost to background. C4 functional titers in those two samples were not different. C3 uptake was studied in parallel with C4 to compare the degree of inhibition using sera with increasing doses of IVIg in both the human and guinea pig system. C3 and C4 inhibition curves completely overlapped. Our findings suggest that IVIg is an effective inhibitor of deposition of early complement activation products (C4b, C3b) onto target surfaces and may indicate interference of IVIg with multiple sites of complement activation.