Heart – Current progress in xenogeneic heart transplantation

David K.C. Cooper¹, Hayato Iwase¹, Hidetaka Hara¹, Mohamed Ezzelarab¹, David Ayares².

¹Thomas E. Starzl Transplantation Institute, University of Pittsburgh, Pittsburgh, PA, USA; ²Revivicor, Blacksburg, VA, USA.

The availability of pigs expressing a human complement-regulatory protein, e.g., CD55, CD46, CD59, and, subsequently, of pigs in which the Gal antigen is no longer expressed (GTKO pigs), has largely overcome the problems of hyperacute and acute humoral xenograft rejection. When these two genetic manipulations have been combined (i.e., GTKO.CD46 pigs), and adequate immunosuppressive therapy has been administered, prolonged survival of heterotopically-placed pig hearts can be obtained in baboons.

However, features of thrombotic microangiopathy (TM) develop within the graft, and can be associated with the late development of a life-threatening consumptive coagulopathy. TM is probably initiated by activation of the graft vascular endothelium by anti-nonGal antibody, complement, and innate immune cells, but is exacerbated by discrepancies between primate coagulation and pig anticoagulation factors. Anticoagulation (e.g., with heparin) and/or anti-platelet agents (e.g., aspirin, ketorolac, clopidrogrel), have proved unsuccessful in preventing it. The recent availability of pigs expressing a coagulation-regulatory protein, e.g., thrombomodulin (TBM), endothelial protein C receptor, tissue factor pathway inhibitor, CD39, particularly when these transgenes are combined, may prevent TM. Initial experience with the transplantation (Tx) of hearts from GTKO.CD46.TBM pigs indicates that, although TM develops, it may be delayed. Mohiuddin et al have documented function of a GTKO.CD46.TBM pig heart in a baboon for >10 months (the longest on record).

McGregor et al have explored orthotopic heart Tx in baboons using GTKO.CD55 pigs. Baboon survival of a maximum of 57 days has been achieved to date. Although this experience is valuable, it is doubtful whether grafts from pigs not expressing one or more coagulation-regulatory proteins will function long-term.

As pig heart Tx in nonhuman primates is associated with coagulation dysfunction (which may be exacerbated by administration of an anti-CD154mAb), this sometimes makes determination and treatment of the adaptive immune response difficult. We have therefore developed a simple technique in which a patch of pig artery is sutured into the wall of the baboon abdominal aorta, exposing the blood to pig antigens. Costimulation blockade with a combination of anti-CD40mAb + belatacept successfully prevents an adaptive response, and is not associated with the thrombogenic effects of an anti-CD154mAb. The same regimen has proved equally successful in pig heart Tx.

Currently, suppression of the primate T cell response to a pig graft requires exogenous immunosuppression. However, there is an unanticipated, but welcome, reduced primate T cell response to GTKO and CD46 pig cells. Furthermore, the expression of a human mutant MHC class II gene suppresses swine leukocyte antigen (SLA) class II expression and also reduces its upregulation when the endothelial cells are activated, resulting in reduced human T cell proliferation in vitro on MLR and reduced sensitization to pig antigens (i.e., a reduced elicited IgG response) after pig artery patch Tx in baboons.

Thus, genetic engineering of the organ-source pig may not only protect from the innate response, but also allow a reduction in the immunosuppressive therapy required to prevent an adaptive response.

Sources of funding: NIH, Revivicor.