Platelet thrombus formation on collagen at high shear rates is mediated by von Willebrand factor-glycoprotein Ib interaction and inhibited by von Willebrand factor-glycoprotein IIb/IIIa interaction

We studied the role of von Willebrand Factor (VWF) in platelet thrombus formation in flowing blood by using a perfusion system and mutant forms of VWF lacking either interaction with glycoprotein Tt, (GpIb) or with glycoprotein IIb/lIIa (alpha IIb-beta 3). These mutants were added to the blood of patients with severe von Willebrand's disease (vWD) or to normal blood reconstituted with a human albumin solution instead of plasma. This blood was then perfused over collagen type III spray-coated on a glass surface and preincubated for 2 hours with 20 mu g/mL plasma VWF. In this way, the adhesion step was mediated by the preincubated plasma VWF bound to collagen type III, whereas thrombus formation was mediated by mutant VWF added to the perfusate. Thrombus formation was absent at all 3 shear rates studied (300, 800, and 2600 s(-1)) when Delta Al-VWF, lacking interaction with GpIb, was added to the perfusate, indicating the importance of GpIb-vWF interaction for thrombus formation. The interaction of vWF and GpIb is currently thought to be possible under physiological conditions in which the conformation of VWF has been changed by adsorption to a surface, Our results regarding the role of GpIb-VWF interaction in thrombus formation suggest that a second mechanism may operate by which a change may occur in GpIb on the surface of adhered platelets either by activation of the molecule or as a consequence of shear stress. Increased thrombus formation was observed when the Arg-Gly-Gly-Ser-VWF, which does not interact with alpha IIb-beta 3, was added to vWD blood and perfused at 2600 s(-1). This increase was not observed in vWD blood at lower shear rates or after addition of Arg-Gly-Gly-Ser-vWF to reconstituted normal blood. Thrombus formation at a high sheer rate was largest when either VWF or fibrinogen was present as a single ligand for alpha IIb-beta 3 at a high shear rate, When both were present, thrombus formation was decreased, We postulate that thrombus formation is less efficient because of incomplete bridge formation when VWF and fibrinogen are both present as ligands for alpha IIb-beta 3.