*Purpose: Transplantation of organs from animals to humans (xenotransplantation) has been proposed to address the critical shortage of organs for transplantation. The use of genetically engineered organ xenografts, including knock-out (KO) of alpha 1-3 galactosyltransferase (GTKO) along with insertion of a human complement regulator genes (hCD46-human CD46, hDAF-human decay accelerating factor) and other cell surface carbohydrate KOs, has been helpful in circumventing acute and subacute rejection. However, thrombotic complications still remain. Xenografts with additional human transgenes, including human thrombomodulin (hTBM), have been generated to address this barrier. Here, we studied the importance of hTBM in preventing microvascular and intramural thrombosis by comparing two transplanted groups utilizing GE donors with or without hTBM.

*Methods: Specific pathogen-free baboons of either sex weighing 15-30 kg (2-3 years of age) were used as recipients. 6 to 8-week-old genetically modified pigs with hTBM (GTKO.hCD46.hTBM) or without hTBM (triple knock out-GTKO.B4KO.CMAHKO, with or without hCD46 and hDAF) were used as donors. Expression of transgenes were consistent and high level across all pigs. All animals were used in compliance with guidelines provided by the Institutional Animal Care and Use Committee (IACUC).

*Results: GTKO.hCD46.hTBM donors (n=3) were found to have no intracardiac thrombus with survival up to 30 days post-transplant. However, all non-hTBM donors (n=2) were found to have a large intracardiac thrombus burden with propagation into major vascular structures such as the aorta, pulmonary artery and coronary sinus. Gross examination of thrombi indicated acute and subacute components, suggesting early formation of the thrombus postoperatively. Additionally, early clinical signs of thrombus formation included profound hypocalcemia requiring continuous calcium infusions in the first 48-72 hours postoperatively. These phenomena were not prevented by continuous heparin infusion. Histologic examination revealed no signs of rejection.

*Conclusions: hTBM prevents intracardiac thrombus in this model. While immunologic rejection has been circumvented with novel immunosuppression regimens and GTKO, it is likely that hTBM or other transgenes involved in regulation of coagulation may be needed for prevention of thrombotic complications.

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