Comparison of the enzymatic sensitivities of the platelet receptor for human C-reactive protein and its functional relationship to the platelet IgG Fc receptor.

Thermally modified human C-reactive protein (H-CRP) and IgG (AHGG) each activate isolated human platelets to reactions of aggregation and secretion. As these molecules exhibit many functional similarities, we questioned whether they might also share a receptor on the platelet membrane. Neither plasmin nor phospholipase C altered the platelet response to H-CRP or AHGG, although these reagents enhanced the platelet expression to acid soluble collagen (ASC). Conversely, chymotrypsin treatment of platelets resulted in an elevated response to each H-CRP and AHGG, but not to ASC. These data suggest that the H-CRP and AHGG platelet receptors share characteristics which contrast with those of the receptor for collagen. However, monomeric IgG, which can bind with the platelet and inhibit the response to AHGG, exerted no effect on the platelet response to H-CRP. Further, a functional receptor for thermally modified human or rabbit CRP was detected on rabbit platelets in the absence of a demonstrable Fc receptor for aggregated IgG. These data indicate that the platelet receptors for the modified forms of CRP and IgG are distinct.