P-selectin expression on platelets determines size and stability of platelet aggregates

Background-P-selectin mediates rolling of platelets and leukocytes on activ ated endothelial cells. After platelet activation, P-selectin is translocat ed from intracellular granules to the external membrane, whereas fibrinogen aggregates platelets by bridging glycoprotein (GP) IIb/IIIa between adjace nt 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 cor related strongly with the mean platelet aggregate size. Inhibition of P-sel ectin binding to its Ligand by either monoclonal anti-P-selectin antibodies directed against the lectin domain or soluble human P-selectin reversed pl atelet aggregation even when added up to 5 minutes after activation; howeve r, fibrinogen binding to platelets was not affected. This deaggregating eff ect significantly reduced the maximal size and number of platelet aggregate s. When added I minute after platelet activation, anti-P-selectin antibody achieved 95% to 100% of the deaggregating effect of EDTA, whereas the anti- GP IIb/lIIa 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-select in was maximally expressed 10 minutes after platelet activation, whereas ma ximal activation of GP IIb/IIIa occurred within the first 10 seconds, sugge sting that P-selectin operates after fibrinogen binding to activated GP IIb /IIIa. Conclusions-These results indicate that P-selectin interaction with a ligan d, different from PSGL-1 or GP Ib, stabilizes initial GP IIb/IIIa-fibrinoge n interactions, allowing the formation of large stable platelet aggregates.