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Principal Role of Glycoprotein VI in &agr;2&bgr;1 and &agr;IIb&bgr;3 Activation During Collagen-Induced Thrombus Formation

Objective—High-shear perfusion of blood over collagen results in rapid platelet adhesion, aggregation, and procoagulant activity. We studied regulation of &agr;2&bgr;1 and &agr;IIb&bgr;3 integrin activation during thrombus formation on collagen. Methods and Results—Blockade of glycoprotein (GP) VI by 9O12 antibody or of P2Y purinergic receptors permitted platelet adhesion but reduced aggregate formation, fibrinogen binding, and activation of &agr;2&bgr;1 and &agr;IIb&bgr;3, as detected with antibodies IAC-1 and PAC1 directed against activation-dependent epitopes of these integrins. Combined blockade of GPVI and P2Y receptors and thromboxane formation abolished integrin activation but still allowed adhesion of morphologically unstimulated, nonprocoagulant platelets. Exogenous ADP partly restored the suppressive effect of GPVI blockade on integrin &agr;2&bgr;1 and &agr;IIb&bgr;3 activation. Adhesion was fully inhibited only with simultaneous blocking of GPVI and &agr;2&bgr;1, indicating that the integrin can support platelet–collagen binding in the absence of its activation. Blockade or absence of GPIb&agr; only moderately influenced integrin activation and adhesion unless GPVI was inhibited. Conclusions—GPVI- and autocrine-released ADP induce affinity changes of &agr;2&bgr;1 and &agr;IIb&bgr;3 during thrombus formation on collagen under flow. These integrin changes are dispensable for adhesion but strengthen platelet–collagen interactions and thereby collagen-induced platelet activation.