The role of donor versus recipient tissue factor in coagulopathy during pig-to-primate xenotransplantation

Abstract

The increasing demand for organs has stimulated the necessity in xenotransplantion, which promises an unlimited supply of organs for clinical use. However, coagulopathy of xenografts remains a major hurdle to successful pig-to-primate xenotransplantation. The ability to generate pigs expressing a human complement-regulatory protein (hCRP) and/or pigs in which the α1,3-Galactosyltransferase gene has been knocked-out (GT-KO) has largely overcome the barrier of hyperacute rejection (HAR) of a pig organ transplanted into a primate. However, acute humoral xenograft rejection (AHXR), presenting as microvascular thrombosis with/without consumptive coagulopathy (CC), ensures and results in graft failure. The causes of coagulopathy were believed to be humoral response-dependent. Xenoreactive antibodies (Abs) and activation of complement provoke porcine endothelial cells (ECs) from an anticoagulant to a procoagulant phenotype. In this study, I demonstrated that recipient platelets and monocytes were activated to express tissue factor (TF), an initiator of the coagulation cascade, after incubation with porcine ECs through humoral immune response-independent process. These observations were mirrored in the animal studies. Kidneys or livers from GT-KO pigs that express a hCRP transplanted into nonhuman primates were not susceptible to HAR. Nevertheless, most recipients developed CC, even when the grafts were still functioning. Activation of graft ECs and the measurable immune response were minimal. TF expression on recipient platelets plays a pivotal role in initiating CC. Therefore, understanding the interactions between porcine ECs and primate platelets may be crucial to prevent coagulopathy. On the other hand, the generation of GT-KO pigs has directed interest to the role of anti-nonGal Abs in intravascular thrombosis. My study revealed that anti-nonGal Abs activated porcine ECs to express TF, independent of complement activation. I also demonstrated that anti-P-selectin and vWF Abs and some anti-platelet agents, such as clopidogrel and eptifibatide, prevented TF expression on platelets after incubation with porcine ECs. Porcine ECs from pigs that expressed tissue factor pathway inhibitor (TFPI) were resistant to the activation induced by primate serum even with high titre anti-nonGal Abs. Atorvastatin not only inhibited this activation of platelets but also prevented the activation of porcine ECs induced by primate serum. Coagulopathy is increasingly recognized as barriers to successful xenotransplantation, many mechanisms of which are not associated with humoral immune response. Further manipulation of the immune response alone, with the risk of inducing infection and other complications, does not appear likely to resolve the challenge of xenograft coagulopathy. My results provide evidence for further genetic manipulation or systemic pharmaceutical treatment to prevent coagupolpathy seen after pig-primate xenotransplantation.