Erythrocytes from young but not elderly donors can bind and degrade immune complex- and antibody-bound C3 in vitro.

Abstract

In situ aged erythrocytes (senescent erythrocytes from young donors and both young and old erythrocytes from elderly donors) demonstrate high levels of membrane-bound C3c and C3d and elevated susceptibility to in vitro phagocytosis. In this study we demonstrate that in situ aged erythrocytes are defective in their ability to degrade C3 fragments and clear them from the erythrocyte membrane. Erythrocytes from young donors bind complement-bearing immune complexes via CR1 and become susceptible to complement-mediated erythrophagocytosis ('innocent bystander' sequestration). Erythrocytes from elderly donors are defective in their ability to bind such immune complexes, as attested by the lack of an increment in membrane-associated C3 fragments as detected by flow cytometry and lack of an increment in in vitro sequestration. Factor I (serum)-dependent cleavage of C3 fragments and release of immune complexes from the erythrocytes of young donors lead to a drop in erythrocyte-associated C3 fragments and the disappearance of the 'innocent bystander' phenomenon. Inhibition of Factor I, and thus inhibition of C3b degradation and immune complex release for the erythrocyte membrane, enhances the levels of 'innocent bystander' sequestration of erythrocytes from young donors. Erythrocytes from elderly donors are defective in the dynamic process of CR1 binding of complement-bearing immune complexes and Factor I-associated release of membrane-associated C3 fragments. A defect in the ability of all in situ aged erythrocytes to clear their membranes of C3 fragments is also demonstrated for complement bound to the erythrocyte via IgM isoagglutinins. While erythrocytes from both young and elderly donors allow for the IgM-mediated binding of complement to the erythrocyte, only young erythrocytes from young donors demonstrate degradative activity with the release of membrane-associated C3c. Erythrocytes from elderly donors demonstrate lower levels of detectable CR1 (CD35), decay accelerating factor (DAF) (CD55) and Protectin (CD59) than do erythrocytes from young donors. Time course studies determine that the defect in handling of immune complexes observed for in situ aged erythrocytes is not due to differences in the kinetics of loading or releasing of immune complexes. These findings on the refractiveness of erythrocytes of elderly donors to uptake of immune complexes and the degradation of C3 fragments may be of importance not only in the understanding of erythrocyte sequestration, but also in the physiology of immune clearance in the elderly.