Recombinant pig TFPI efficiently regulates human tissue factor pathways

: Rejected pig‐to‐primate organ xenografts almost invariably exhibit significant microvascular thrombosis, believed to be due in part to several molecular incompatibilities affecting the regulation of coagulation. In this study, we tested one such proposed incompatibility: whether there is, at least in part, a functional incompatibility in pig tissue factor pathway inhibitor (TFPI) that impedes binding of human factor Xa and regulation of human tissue factor‐initiated coagulation. TFPIα cDNA was cloned from pig aortic endothelial cells and found to encode a 279‐residue mature protein with 79% overall identity to human TFPIα, increasing to 88 to 90% in the functional Kunitz‐1 and Kunitz‐2 domains. Transfected primate cells expressing equivalent levels of GPI‐linked pig or human TFPIα were assayed for binding of human factor Xa and inhibition of the human factor VIIa/tissue factor complex. The activity of the expressed pig anticoagulant was equivalent to that of the human protein in both measures of TFPI function in these systems. These data indicate that there are no apparent incompatibilities between recombinant pig TFPI and the human tissue factor pathway. Other factors must account for the thromboregulatory failure of pig endothelium and aberrant tissue factor activity in xenograft rejection.