Stimulation of GPVI receptor with the mAb HGP4C9 induces platelet activation. P-selectin (CD62P), activated integrin αIIbβ3 (PAC-1), and CD40L surface expression was determined on isolated human platelets after stimulation with ADP (20μM), different concentrations of mAb HGP4C9 (0.01-5 μg/mL), or control IgG1 (5 μg/mL) for 10 minutes (n = 4-6, mean ± SEM, *P < .001 for HGP4C9 1 μg/mL vs control). Results are given as mean fluorescence intensity (arbitrary units, AU).