Platelet thrombus is required for enhanced endothelial cell activation in the presence of anti-β2GP1 autoantibodies. (A) Infusion of eptifibatide (10 μg/g mouse) initially inhibits platelet interaction with the injured vessel wall but its effect is gone by 60 minutes. Platelets were labeled using anti-CD42 antibody conjugated to Dylight 488 (0.1 μg/g) and platelet fluorescence (green) imaged at 488 nm. Images were obtained with the high-resolution CMOS camera that resolves individual platelets. (Lane 1) Image at time 0. (Lane 2) Image at time 60 minutes after eptifibatide infusion. (B) Representative images of ICAM-1 and platelets in the developing thrombus from time 0 to 120 seconds obtained in the presence of eptifibatide (10 μg/g mouse) (lane 1); human anti-β2GP1 autoantibodies (10 μg) and eptifibatide (10 μg/g mouse) (lane 2); human anti-β2GP1 autoantibodies (10 μg) in the absence of eptifibatide (lane 3). ICAM-1, green; platelets, red. (C) The median integrated ICAM-1 fluorescence (F ICAM-1) associated with thrombus formation in wild-type mice infused with human anti-β2GP1 autoantibodies (27 thrombi, 4 mice; red); human anti-β2GP1 autoantibodies and eptifibatide (27 thrombi, 3 mice; blue); and eptifibatide (18 thrombi, 3 mice; black).