CR3 receptor on platelets and its role in the prostaglandin metabolic pathway

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Abstract
The human C3R receptor, which binds C3bi, present on the surface of monocytes, granulocytes and natural killer cells, can be detected by several monoclonal antibodies, OKMI, Mol and Mac-1 and also by RM2.184 which detects a polymorphism of the receptor. Platelets have been considered to lack complement receptors on their cell surface; however, we now describe the detection of CR3 receptors on human platelets by radioimmunoassay using both OKM1 and RM2.184 antibodies. Using OKM1, immunoprecipitation studies with 125l-labelled platelets revealed the typical CR3 complex with an chain of 165,000 daltons and chain of 95,000 daltons. By immunofluorescence, megakaryocytes were also found to be OKM1+. However, platelet CR3 does not merely bind C3bi, but the binding of the OKM1 antibody to platelet CR3 selectively blocks platelet functions of aggregation and serotonin release induced by arachadonic acid but not by other ligands (ristocetin, ADP, L-epinephrine, collagen and thrombin). The studies demonstrate an important role of platelet CR3 in both complement binding and in prostaglandin metabolic pathways.