Background— We studied the role of glycoprotein (GP) VI in platelet adhesion and thrombus formation on the immobilized collagen and von Willebrand factor (vWF) surface under flow conditions.

Methods and Results— Whole blood obtained from 2 patients with GP VI–deficient platelets and the effects of the Fab of anti–GP VI antibody (Fab/anti–GP VI) were tested. Blood containing platelets rendered fluorescent by mepacrine was perfused on immobilized type I collagen or vWF under controlled wall shear rate. Platelet adhesion and thrombus formation were detected by epifluorescent videomicroscopy. The percentage of surface coverage by the platelets was calculated. Fc receptor γ-chain and spleen tyrosine kinase (Syk) were immunoprecipitated from the lysate of platelets stimulated by vWF plus ristocetin and then analyzed by antiphosphotyrosine immunoblotting. No platelet attachment was seen on the surface of collagen even after 9 minutes of perfusion of blood at relatively low (100 s⁻¹) or high (1500 s⁻¹) wall shear rate, either in the case of blood containing GP VI–deficient platelets or in the presence of Fab/anti–GP VI, whereas significant platelet thrombus formation was noted after control blood perfusion. Such interference with the actions of GP VI also reduced firm platelet adhesion on immobilized vWF. vWF-induced tyrosine phosphorylation of GP VI–associated Fc receptor γ-chain followed by Syk activation occurred in normal platelets, but little activation of Syk occurred in GP VI–deficient platelets.

Conclusions— GP VI plays crucial roles in platelet thrombus formation on the surface of collagen under flow conditions in humans and is also involved in the process of firm platelet adhesion on the surface of vWF.