Role of vWf in thrombin-induced platelet procoagulant activity.

(A) Effect of MoAb against vWf (AVW1 and AVW3) and MoAb VM16d against the thrombin-binding site of GPIb; gel-filtered platelets were preincubated with Fab fragments of the MoAb IV.3 (20 μg/mL), treated with the MoAbs AVW1, AVW3, or VM16d (20 μg/mL) and were then activated with thrombin (0.05-2 U/mL); ● indicates 25 000 platelets/μL; ○, 25 000 platelets/μL in the presence of VM16d; ▵, 25 000 platelets/μL in presence of AVW1; ■, 25 000 platelets/μL in presence of AVW3. (B) vWf binding to platelets in the presence of MoAb VM16d, blocking the thrombin-binding site on GPIb; platelets from PRP were treated with ristocetin (200 μg-1 mg/mL) and vWf binding was measured with the FITC-labeled MoAb 4F9; ● indicates 25 000 platelets/μL; ○, 25 000 platelets/μL in the presence of MoAb VM16d. The fluorescence of 5000 platelets per measuring point was determined by flow cytometry. Data are the mean of 3 experiments using platelets from different donors.