Abstract-Platelet adhesion in arterial blood flow is mainly supported by the platelet receptor glycoprotein (GP) Ib, which interacts with von Willebrand factor (vWF) that is bound to collagen at the site of vessel wall injury. Antibody 6B4 is a monoclonal antibody (MoAb) raised against purified human GPIb. MoAb 6B4 inhibits both ristocetin-and botrocetin-induced, vWF-dependent human platelet agglutination. MoAb 6B4 furthermore blocks shear-induced adhesion of human platelets to collagen I. We studied the antithrombotic effect of this inhibitory murine MoAb 6B4 in a baboon model of arterial thrombosis. When injected into baboons, intact IgG and its F(abЈ) 2 fragments caused almost immediate thrombocytopenia, whereas injection of the Fab fragments alone did not. Fab fragments were subsequently used to investigate their in vivo effect on platelet deposition onto a thrombogenic device, consisting of collagen-rich, glutaraldehyde-fixed bovine pericardium (0.6 cm 2 ), at a wall shear rate ranging from 700 to 1000 s Ϫ1 . Baboons were either pretreated with Fabs to study the effect of inhibition on platelet adhesion or treated 6 minutes after placement of the thrombogenic device to investigate the effect on interplatelet cohesion. Pretreatment of the animals with bolus doses ranging from 80 to 640 g/kg Fab fragments significantly reduced 111 In-labeled platelet deposition onto the collagen surface by Ϸ43% to 65%. Only the highest dose caused a significant prolongation (doubling) of the bleeding time. Ex vivo ristocetin-induced platelet agglutination was equally reduced. Treatment with a bolus of 110 g/kg Fab fragments after a thrombus was allowed to form for 6 minutes had no effect on further platelet deposition. We therefore conclude that Fab fragments or derivatives of inhibitory anti-GPIb antibodies may be useful compounds to prevent thrombosis.

We have developed an immunogold von Willebrand factor (VWF) detection method that permits almost complete coverage of individual VWF molecules, and by this unequivocal localization and morphologic analysis of collagen-bound VWF by atomic force microscopy (AFM). Perfusion of gel filtration-purified VWF in parallel plate perfusion chambers over glass coverslips coated with calf skin collagen, followed by AFM imaging in air, enabled us to assess possible morphologic differences between VWF bound at low (0.07 N/m 2 ‫؍‬ 0.7 dynes/ cm 2 ) and high (4.55 N/m 2 ‫؍‬ 45.5 dynes/ cm 2 ) shear stresses. No significant differences in VWF morphology were found, the molecules were oriented almost randomly, and there were no clear signs of VWF "uncoiling" either at a high or at a low shear regime. After perfusing 1 g/mL VWF for 5 minutes, surface coverage at high shear was almost twice the one seen at low shear, and some larger and more irregularly shaped VWF molecules could be seen at high shear. This difference disappeared, however, at 15 minutes of perfusion and was probably caused by diffusion kinetics. Moreover, the presence of 68 ؋ 10 9 /L washed fixed platelets in the perfusate did not have any visible effect on VWF morphology at high versus low shear stress. These findings suggest that shear stress does not influence significantly the overall molecular morphology of VWF during its binding to collagen-coated surface and are consistent with a constitutively expressed affinity of collagen-bound VWF for gly-

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