Which anti-platelet therapies might be beneficial in xenotransplantation?

Xenotransplantation could provide an unlimited and elective supply of grafts, once mechanisms of graft loss and vascular injury are better understood. The development of α-1,3-galactosyltransferase gene-knockout (GalT-KO) swine with the removal of a dominant xeno-antigen has been an important advance; however, delayed xenograft and acute vascular reaction in GalT-KO animals persist. These occur, at least in part, because of humoral reactions that result in vascular injury. Intrinsic molecular incompatibilities in the regulation of blood clotting and extracellular nucleotide homeostasis between discordant species may also predispose to thrombophilia within the vasculature of xenografts. Although limited benefits have been achieved with currently available pharmacological anti-thrombotics and anti-coagulants, the highly complex mechanisms of platelet activation and thrombosis in xenograft rejection also require potent immunosuppressive interventions. We will focus on recent thromboregulatory approaches while elucidating appropriate anti-platelet mechanisms. We will discuss potential benefits of additional anti-thrombotic interventions that are possible in transgenic swine and review recent developments in pharmacological anti-platelet therapy.

Coagulopathy in α-galactosyl transferase knockout pulmonary xenotransplants

BACKGROUND

After substantial progress on many fronts, one of the remaining barriers still opposing the clinical application of xenotransplantation is a disseminated intravascular coagulopathy (DIC) that is observed in the pre-clinical model of porcine-to-primate transplantation. The onset of DIC is particularly rapid in recipients of pulmonary xenografts, usually occurring within the first days or even hours of reperfusion.

METHODS

In this study, we describe the results of two porcine-to-baboon transplants utilizing porcine lungs depleted of macrophages, deficient in the α-1,3-galactosyltransferase gene, and with the expression of human decay-accelerating factor, a complement regulatory protein.

RESULTS

In both cases, evidence of DIC was observed within 48 h of reperfusion, with thrombocytopenia and increases in levels of thrombin-antithrombin complex evident in both cases. Depletion of fibrinogen was observed in one graft, whereas elevation of D-dimer levels was observed in the other. One graft, which showed focal lymphocytic infiltrates pre-operatively, failed within 3 h.

CONCLUSIONS

The results indicate that further efforts to address the coagulopathy associated with pulmonary xenotransplantation are needed. Further, evidence suggests that resident porcine immune cells can play an important role in the coagulopathy associated with xenotransplantation.

Therapeutic issues in the treatment of vascularized xenotransplants using gal-knockout donors in nonhuman primates

PURPOSE OF REVIEW

Solid organ xenotransplantation could be the future of transplantation, but improved outcomes are required in experimental models before clinical trials are justified. This review summarizes recent advances in solid organ xenotransplantation using organs from α1,3-galactosyltransferase gene-knockout (GTKO) pigs (with or without other genetic modifications) and novel therapeutic approaches.

RECENT FINDINGS

Work on the development of genetically engineered pigs has been considerable during the past few years, with many research institutes reporting the outcomes of research. Multiple gene modifications on a GTKO background have been reported, and the results of transplantation using organs from these pigs have been published. Progress, however, has been variable, and several obstacles, for example, coagulation dysregulation, have been identified. Heterotopic pig heart xenotransplantation has been associated with graft survival up to 8 months, but kidney graft survival has not improved significantly.

SUMMARY

The availability of GTKO pigs with additional genetic modifications aimed toward expression of multiple complement-regulatory proteins and/or human thromboregulatory genes, combined with novel immunosuppressive regimens, for example, the inclusion of B cell-depleting agents, should improve pig organ survival in the near future.