Coagulation and the xenograft endothelium.

Acute humoral rejection remains the major barrier to long-term pig-to-primate xenograft survival, and microvascular thrombosis is a critical element of the rejection process. It appears that persistent endothelial cell activation and injury, by even low levels of anti-graft antibodies, eventually overwhelm the cellular anticoagulant defences and promote the development of thrombotic microangiopathy. Porcine endothelium may be particularly vulnerable because of cross-species molecular incompatibilities affecting the function of thrombomodulin and possibly TFPI. Recent data from small animal models suggest that transgenic overexpression of anti-thrombotic molecules on xenograft endothelium is capable of inhibiting intravascular thrombosis and preventing acute humoral rejection. In conjunction with existing genetic modifications (e.g. Gal KO, hDAF), this is a promising strategy to move xenotransplantation to the clinic.