The complement cleavage product C3b was examined for its ability to influence the expression of Ia antigens on macrophages. Monoclonal anti-Ia antibodies were applied for quantitation of Ia antigens by using FACS analysis. Albumin-elicited guinea pig peritoneal macrophages were cultured for 48 hr in the presence of C3b. C3b at final concentrations varying from 10 to 80 micrograms/ml reduced the proportion of Ia-positive macrophages from 87 to 25%. The C3b-stimulated loss of Ia antigens from the macrophage surface was mediated by metabolites of the cyclooxygenase pathway as shown by the inhibitory effect of indomethacin. Exposure of macrophages to C3b for 5 min was sufficient to induce Ia reduction, seen after subsequent 48-hr incubation. C3b generated in vivo in the course of inflammatory reactions might therefore influence the interaction of immunocompetent cells by modulating macrophage Ia expression.

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