down-regulation by aspirin ex vivo activation and P-selectin expression during platelet recruitment: Platelet-erythrocyte interactions enhance alpha IIbbeta 3 integrin receptor

Activated platelets release biologically active compounds, which then recruit additional platelets into an evolving thrombus. We studied activation of alpha(IIb)beta(3) and exposure of P-selectin on platelets recruited by releasates obtained from collagen-treated platelets and evaluated modifications in prothrombotic effects of releasates induced by platelet-erythrocyte interactions and aspirin treatment. Releasates from collagen-stimulated platelets induced alpha(IIb)beta(3) activation and P-selectin exposure (monitored by flow cytometry using fluorescein isothiocyanate-PAC-1 and phycoerythrin-CD62 antibodies). These responses were markedly amplified by releasates from combined platelet-erythrocyte suspensions. This finding demonstrates a novel mechanism(s) by which erythrocytes intensify platelet aggregability and mediate increased platelet recruitment. Because P-selectin and alpha(IIb)beta(3) are potential sites for platelet-leukocyte interactions, erythrocytes may also modulate leukocyte recruitment. Following aspirin ingestion both the recruiting capacity of platelet releasates and erythrocyte-induced amplification of platelet recruitment were down-regulated. These events represent an additional antithrombotic property of aspirin. We also examined the possibility that arachidonic acid, or eicosanoids derived there from, can induce a prothrombotic activity of erythrocytes. The TXA(2)-analog U46 619 and free arachidonate, but not PGl(2) or 12-HETE, Induced increases in cytosolic Ca++ and promoted phosphatidylserine (PS) exposure on a subpopulation of erythrocytes. PS exposure and increases in erythrocyte [Ca++](i) are associated with enhanced procoagulant activity, Increased endothelial adhesion, and reduced erythrocyte deformability. Our findings, therefore, suggest that TXA(2) and arachidonic acid, derived from activated platelets, Induce a prothrombotic phenotype on erythrocytes In proximity. We conclude that by these mechanisms, erythrocytes can actively contribute to platelet-driven thrombogenesis and microvascular occlusion. (C) 2002 by The American Society of Hematology.