Platelet-erythrocyte interactions enhance αIIbβ3 integrin receptor activation and P-selectin expression during platelet recruitment: down-regulation by aspirin ex vivo

Abstract

Activated platelets release biologically active compounds, which then recruit additional platelets into an evolving thrombus. We studied activation of ␣ IIb ␤ 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 ␣ IIb ␤ 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 Pselectin and ␣ IIb ␤ 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 downregulated. These events represent an additional antithrombotic property of aspirin. We also examined the possibility that arachidonic acid, or eicosanoids derived therefrom, can induce a prothrombotic activity of erythrocytes. The TXA 2 -analog U46 619 and free arachidonate, but not PGI 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. IntroductionWhen platelets interact with the subendothelial matrix of an injured vessel, they become activated, release components of their intracellular granules, and generate metabolic products. These products include adenine nucleotides, eicosanoids, and serotonin. 1 In turn, the platelet releasate functions as agonist for recruitment of additional platelets into the evolving thrombus. [1][2][3][4][5] By using an experimental approach to evaluate platelet activation and recruitment independently, we previously demonstrated that cell-cell interactions between activated platelets and intact erythrocytes (RBCs) amplify both platelet activation and the proaggregatory capacity of cell-free releasates. [2][3][4] In contrast, we found that platelet-neutrophil interactions down-regulate platelet reactivity in our system. 5 One of the earliest events in platelet reactivity is activation of platelet ␣ IIb ␤ 3 integrin receptors. Such activation is a prerequisite for fibrinogen binding to platelets, which culminates ...

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