Mediation of CRP-Dependent Phagocytosis through Mouse Macrophage Fc-Receptors

The purified acute phase serum constituent, C-reactive protein (CRP), was examined for its ability to interact with mouse macrophage Fc-receptors when serving as an opsonin for C-polysaccharide (CPS)-coated erythrocytes (E.CPS). The ingestion of E.CPS-CRP by macrophage monolayers was dependent on the presence of CRP on the erythrocyte surface and on the treatment of the CRP-sensitized cells with human complement (C). The ingestion of E.CPS-CRP-C was inhibited by exposing the macrophages to either 2-deoxyglucose, a selective inhibitor of Fc-receptor activity, or to aggregated γ-globulin. Inhibition was also brought about by plating macrophages on glass-bound immune complexes of BSA/anti-BSA (IgG) but not on complexes formed with the F(ab′)2 fragment of the anti-BSA. Substrate-bound complexes of CPS-CRP selectively inhibited the uptake of IgG-coated E, EA(IgG); the inhibition was proportional to the concentration of CRP used to form the complex. The opsonin-independent ingestion of latex particles was not altered by these Fc-receptor blocking procedures. These findings coupled with the recent demonstration of a sequence homology between CRP and the CH3 domain of human IgG lead us to propose shared opsonic functions for antibody and CRP.