Exposure to complement‐bearing immune complexes enhances the in vitro sequestration of erythrocytes from young but not elderly donors

SUMMARY Complement and immunoglobulin have each been claimed to be the major opsonins responsible for sequestration of the effete erythrocyte. Binding of immune complexes to the erythrocyte via CRI (CD35) provides a model for studying the effects of increments in metnbrane‐bound complemenl and immunoglobulin on the sequestration of the erythrocyte (‘innocent bystander’ sequestration). It is possible that C3b‐bearing immune complexes (IC C3b) bound to erythrocyte CRI contribute to the levels of immunoglobulin and complement fragments detectable on the human erythrocyte. We have, therefore, compared the capacity oferythrocytes from young and elderly donots to bind IC C3b and the effect of this binding on in vitro sequestration. Erythrocytes from young donors exposed to IC‐C3b bind these complexes, as attested by an increment in membrane‐bound C3, and undergo ‘innocent bystander’ sequestration. However., when density‐separated erythrocytes are so exposed, it is only the low density (young) erythrocytes from young donors which are susceptible to ‘innocent bystander’ sequestration. High density (old) erythrocytes from young donors and ail erythrocytes from elderly donors show initially high in vitro sequestration and are resistant to the “inniwent bystander’ effect. (Those erythrocytes which show initially high in vitro sequestration are referred to collectively as in situ aged’ erythrocytes.) There is a great similarity between the mechanisms of sequestration of in situ aged’ and “innocent bystander” erythrocytes in that they are both inhibited by the integrin binding peptide arginine‐glycine‐aspartic acid (RGD) and the /?‐galactosyl sugar n‐acetyl‐galactosamine, and unaffected by the Fe‐γ binding protein, Protein‐G. Complement is the major opsonin in Innocent bystander’ sequestration since this sequestration occurs whether the isotype of the antibody in the immune complex is IgM or IgG. and Protein‐G, which inhibits IgG‐dependent erythrophagocytosis. has no effect on ‘innocent bystander” sequestration. The present studies demonstrate that in vitro sequestration of ‘ in situ aged’ erythrocytes is similar to complement‐dependent “innocent bystander” sequestration, thus supporting the contention that complement recognition is eardinal in sequestration of ‘ in situ aged’ erythrocytes.