Human Platelet Aggregation and Thrombotic Microangiopathy (TM) in Pig Cardiac Xenografts Is Reduced by Expression of Human Thrombomodulin (TBM)

Abstract

Purpose Platelet aggregation plays a key role in dysregulation of coagulation and development of TM in pig cardiac xenografts transplanted into baboon recipients. The present study aimed to evaluate the influence of pig genetic modification on human platelet aggregation induced by pig endothelial cells (EC). Methods and Materials Using a Chrono-log Whole Blood Aggregometer, we studied platelet aggregation after incubation of human blood with human (h) or pig (p) aortic endothelial cells (AEC) using wild-type (WT), α1,3-galactosyltransferase gene-knockout (GTKO), and various transgenic pigs expressing human complement- or coagulation-regulatory proteins. We also studied the development of TM after transplantation of hearts from two types of genetically-engineered pigs, one of which expressed hTBM. Results Platelet aggregation induced by pAEC from GTKO pigs or by pAEC expressing complement-regulatory (CD46 and/or CD55) and/or coagulation-regulatory (TFPI or TBM) proteins was significantly reduced, particularly by GTKO.CD46.TBM pAEC (WT 53% vs GTKO.CD46.TBM 28%; p 3 /mm 3 or no reduction in platelet count or fibrinogen was observed. Two experiments are ongoing, but one baboon was euthanized after 52 days; there was no TM on histology. Conclusions The absence of the Gal antigen, and transgenic expression of human complement- and/or coagulation-regulatory proteins on pAEC are all associated with reduced human platelet aggregation in vitro. Transplantation of hearts expressing TBM was associated with no (or possibly delayed) TM. The barrier of coagulation dysregulation may be overcome.