P-Selectin Expression on Platelets Determines Size and Stability of Platelet Aggregates

BACKGROUND: P-selectin mediates rolling of platelets and leukocytes on activated endothelial cells. After platelet activation, P-selectin is translocated from intracellular granules to the external membrane, whereas fibrinogen aggregates platelets by bridging glycoprotein (GP) IIb/IIIa between adjacent platelets. METHODS AND RESULTS: In this study, we define a novel role for P-selectin in platelet aggregation. Expression of P-selectin on the platelet surface correlated strongly with the mean platelet aggregate size. Inhibition of P-selectin binding to its ligand by either monoclonal anti-P-selectin antibodies directed against the lectin domain or soluble human P-selectin reversed platelet aggregation even when added up to 5 minutes after activation; however, fibrinogen binding to platelets was not affected. This deaggregating effect significantly reduced the maximal size and number of platelet aggregates. When added 1 minute after platelet activation, anti-P-selectin antibody achieved 95% to 100% of the deaggregating effect of EDTA, whereas the anti-GP IIb/IIIa antibody abciximab had no effect. Monoclonal antibodies against known P-selectin ligands, such as P-selectin GP ligand-1 (PSGL-1) or GP Ib, had no effect on platelet aggregation, suggesting a different ligand for P-selectin in platelet aggregate stabilization. In kinetic studies, P-selectin was maximally expressed 10 minutes after platelet activation, whereas maximal activation of GP IIb/IIIa occurred within the first 10 seconds, suggesting that P-selectin operates after fibrinogen binding to activated GP IIb/IIIa. CONCLUSIONS: These results indicate that P-selectin interaction with a ligand, different from PSGL-1 or GP Ib, stabilizes initial GP IIb/IIIa-fibrinogen interactions, allowing the formation of large stable platelet aggregates.