Inhibition of platelet thrombus formation with eptifibatide prevents anti-β2GP1 autoantibody–mediated enhancement of fibrin generation. Platelet and fibrin imaging were performed using anti-CD42 antibody labeled with Dylight 488 (0.1 μg/g mouse) and anti-fibrin antibody labeled with Alexa 647 (0.5 μg/g mouse). Platelet thrombus size and fibrin generation at the site of laser-induced injury were determined by calculating median fluorescence values at 488 nm and 647 nm over 3 minutes, respectively. Anti-β2GP1 autoantibody–induced changes in platelet thrombus size and fibrin generation were observed in thrombi performed upstream in a single arteriole before (16 thrombi, 3 mice) and 15 minutes after (20 thrombi, 3 mice) infusion of 10 μg of anti-β2GP1 antibodies. Subsequently, platelet accumulation at the site of injury was prevented by infusion of eptifibatide (10 μg/g mouse) every 15 minutes, and platelet thrombus size and fibrin generation were observed in thrombi (19 thrombi, 3 mice) performed upstream in a single arteriole in the presence of anti-β2GP1 autoantibodies. (A) Representative images of the developing thrombus obtained (a) without antibody, (b) with 10 μg anti-β2GP1 IgG, and (c) with 10 μg anti-β2GP1 IgG and eptifibatide. Platelets, green; fibrin, red; merge, yellow. (B) Platelet accumulation. (C) Fibrin generation. No antibody (a), black; anti-β2GP1 autoantibody (b), light gray; anti-β2GP1 autoantibody and eptifibatide (c), dark gray.