Naturally occurring anti-band 3 antibodies and complement in phagocytosis of oxidatively-stressed and in clearance of senescent red cells.

Treatment of human red blood cells with diamide and opsonization with whole serum enhanced their phagocytosis by mononuclear phagocytes. Opsonization of diamide-treated red cells with whole serum containing 20-100 times the physiologic concentration of naturally occurring anti-band 3 antibodies further increased the extent of phagocytosis. Enhanced phagocytosis was due to an anti-band 3 mediated binding of C3b to red cells via the alternative pathway. Red cell-bound anti-band 3 was slightly elevated on diamide-treated cells and elicited a C3 binding that exceeded the amount of bound antibody by two orders of magnitude. Pretreatment of red cells with a monoclonal anti-CR1 did not significantly inhibit opsonization and phagocytosis if cells were opsonized at elevated anti-band 3 concentrations. On the other hand, phagocytosis of mildly oxidized (20 microM diamide) red cells was completely inhibited by blocking CR1 if cells were opsonized with serum containing physiologic concentrations of anti-band 3. The results suggest that two types of opsonization mediate in vitro phagocytosis: one operating at physiologic anti-band 3 concentrations with mildly oxidized red cells (IC-like mechanism) and one that operates with either heavily oxidized (greater than 200 microM diamide) red cells at physiologic anti-band 3 concentrations, or with mildly oxidized cells opsonized at elevated concentration of anti-band 3. The latter mechanism is relevant in vivo. It is most likely that it starts by Fab-dependent binding of anti-band 3 to diamide-induced band 3 protein oligomers. Complement activation may occur by assembly of an alternative convertase on C3b covalently bound to red cell-associated anti-band 3. This mechanism is also likely to mediate clearance of senescent red cells, as it was primarily from senescent red cells that we could isolate complexes containing IgG covalently bound to C3b.