Abstract 635: Inhibition of Platelet Aggregation by C-Reactive Protein is Mediated by an Interaction with GPIIb/IIIa
C-reactive protein (CRP) is a well established marker for inflammation, and a good predictor of coronary heart disease. It is also known to interact with platelet Fc gamma RIIa and to enhance the inhibition of platelet aggregation by aspirin by an unknown mechanism. We hypothesised that CRP was able to interact with platelets, and set out to characterize this mechanism. To determine if CRP could interact with platelets we coated a plate with recombinant CRP which was in the native pentameric form (confirmed by size exclusion chromatography). Platelets adhered to immobilized CRP and to immobilized fibrinogen to a similar extent. In both cases adhesion was inhibited by abciximab and tirofiban, but not by the anti-Fc gamma RIIa monoclonal antibody (IV.3), suggesting an interaction between GPIIb/IIIa and immobilized CRP. When viewed by confocal microscopy, the adherent platelets displayed pseudopodia, similar to that seen with platelets adhering to fibrinogen. However, CRP could not support platelet adhesion under shear conditions (200 s-1), while fibrinogen supported single platelet adhesion. Soluble CRP inhibited platelet adhesion to immobilized CRP suggesting that immobilization was not required for the interaction with GPIIb/IIIa. Soluble CRP did not lead to platelet activation as confirmed by measurement of p-selectin and CD63 expression by flow cytometry, but seemed to have an inhibitory role. Soluble CRP inhibited aggregation induced by low dose ADP (70% +/−8%, n=8), low dose collagen (95% +/− 2%, n=5), and low dose TRAP (72% +/− 14%, n=4). We propose that this interaction with GPIIb/IIIa may be taking place through the CRP surface exposed RGD-like sequence (RQD). In conclusion, immobilized CRP at a site of injury is likely to act to recruit platelets; however, while it binds to GPIIb/IIIa it cannot support platelet aggregation. Since it fails to support platelet adhesion under relatively low shear conditions, and inhibits platelet aggregation by soluble agonists, elevated levels of CRP are likely to reduce the overall thrombotic potential.