Abstract 5300: Targeting Ongoing Arterial Thrombosis With Platelet-Specific Microparticles

Abstract

In the arterial circulation with rapid blood flow, initial platelet adhesion at sites of vascular injury depends on the interaction between the amino terminal domain of platelet glycoprotein Ib (GPIb-NH2) and the A1 domain of von Willebrand Factor (VWF-A1) immobilized onto the vessel wall. Moreover, platelet aggregation responsible for subsequent thrombus growth is mediated by binding of soluble plasma VWF to GPIb on the platelet membrane. Thus, microparticles (MPs) presenting on their surface GPIb-NH2 or VWF-A1 should be capable of homing to sites of ongoing thrombus formation by mimicking platelets either without or with bound VWF, respectively. To test this hypothesis, we coated fluorescent MPs with recombinant VWF-A1 (residues 445–733 of the mature human VWF subunit) or GPIb-NH2 (residues -2–290). Both proteins were expressed as dimers in the attempt to mimic the corresponding native functional conformations. When added to a washed human red cell suspension and perfused over the appropriate substrate (i.e. MPs coated with VWF-A1 over surface-immobilized GPIb-NH2; or vice versa), both kinds of MPs adhered in equal number at shear rate as high as 10,000 s-1. This response was markedly decreased when MPs were coated with VWF-A1 carrying the loss-of-function mutation G561S. In vivo, derivatized MPs were used in mouse models of thrombosis in the carotid and femoral arteries and in the femoral vein in which a thrombogenic lesion was induce by superfusion of ferric chloride or with a pulsed nitrogen laser. Upon injection during the early stages of platelet response to injury, MPs coated with either VWF-A1 or GPIb-NH2 specifically adhered to the lesion, indicating that both GPIb and VWF were present on the surface of the forming thrombus. In contrast, MPs coated with VWF-A1 were remarkably more efficient than those coated with GPIb-NH2 in targeting nearly occluding thrombi, indicating that the surface of the latter presents GPIb (platelets) but not accessible VWF. These findings show that MPs derivatized with specific adhesive domains may be used to target areas of vascular injury with developing thrombi for diagnostic or therapeutic purposes, as well as to interrogate thrombus composition.