Activation of human platelets by the membrane-expressed A1 domain of von Willebrand factor

Platelet activation and microthrombus formation are invariable features of xenograft rejection and the vascular injury observed when porcine organs are transplanted into primates. This pathological process could be mediated, at least in part, by aberrant interactions of von Willebrand Factor (vWF) associated with the donor vasculature with host platelets, Unlike human vWF, native porcine vWF (pvWF) interacts with human GPlb independently of shear stress or nonphysiological stimuli, eg, ristocetin. We therefore contrasted the potential of isolated human and porcine vWF-A(1)-domains to interact with human platelets in vitro. Both human and porcine vWF-A(1)-domains expressed as glycosyl phosphatidylinositol-linked FLAG fusion proteins on COS-7 cells induced GPlb-dependent aggregation and intracellular Ca++ uptake of platelets, independent of both the remainder of the vWF protein and additional modifying factors. Porcine A(1)-domains were more potent than human homologues, and in addition ristocetin could boost platelet aggregation only with the human A(1)-domain, Putative conformational changes in the porcine A(1)-domain could result in the heightened, ristocetin-independent interactions observed with human platelets and may be of importance for xenograft survival. (C) 1997 by The American Society of Hematology.