Graft-derived VWF drives platelet activation and thrombocytopenia during porcine liver xenotransplantation to brain-dead human recipients.

BACKGROUND: Genetically engineered porcine livers are being developed as a bridge therapy for acute liver failure, providing detoxification and restoration of hepatic protein synthesis. Severe xenograft-associated severe thrombocytopenia remains a major limitation, and human mechanistic data are scarce. METHODS: Platelet kinetics were characterized in three human decedents undergoing extracorporeal cross-circulation with transgenic porcine livers. Platelet counts, transfusion requirements, and clearance patterns were assessed to distinguish consumption from marrow suppression or hypersplenism. Antibody- and complement-directed inhibitors were administered to test immune-mediated mechanisms. Mechanistic studies focused on porcine von Willebrand factor (pVWF)-dependent platelet activation, including ex vivo blockade with the anti-VWF nanobody caplacizumab, a vWF-directed antibody fragment that prevents vWF-platelet binding. A fourth decedent received caplacizumab during porcine liver perfusion. RESULTS: In all three initial cases, 80%-90% of circulating and transfused platelets were rapidly cleared, a pattern inconsistent with marrow suppression or hypersplenism. Antibody and complement inhibition failed to ameliorate thrombocytopenia. Recipient plasma induced robust pVWF-mediated platelet activation analogous to human Type IIb von Willebrand disease, which was completely abrogated ex vivo by caplacizumab. In a fourth decedent treated with caplacizumab, aberrant platelet activation was prevented, though full hematologic recovery was limited by pre-existing disseminated intravascular coagulation (DIC). CONCLUSIONS: Early thrombocytopenia during porcine liver xenotransplantation appears to be primarily driven by pVWF-mediated platelet activation rather than by classical immune or splenic mechanisms. Targeted VWF blockade with agents such as caplacizumab may mitigate platelet loss and improve the safety profile of extracorporeal porcine liver support in acute liver failure.