Review of xenografts in organ transplantation

Xenografts may provide one approach for resolving organ procurement problems for clinical transplantation. Also, they would allow surgical procedures to be done in an elective, highly organized fashion. The process of xenograft rejection qualitatively resembles allograft rejection in that both cellular and humoral immune mechanisms are involved, but differs quantitatively depending on the genetic disparity between the donor and the recipient. Thus, with minor disparity rejection is mediated primarily by cellular mechanisms whereas with major disparity the process is a violent one, mediated primarily by humoral mechanisms. Immunosuppressive regimen in most of the clinical studies has included azathioprine, steroids, antilymphocyte globulin (ALG), Actinomycin-C, and local graft irradiation. Other modalities including cyclosporin, which have shown improved xenograft survival in animal models, have not yet been tested critically in a clinical setting. It is doubtful that the present battery of immunosuppressive agents would improve xenograft survival comparable to that observed with allografts. The clinical experience in xenografts to date has involved primarily renal and to some extent liver and heart transplants from chimpanzee and baboon donors. These studies demonstrated clearly that the transplanted organ was capable of function in the human recipient; however, the patient and graft survival in general have been poor. On the other hand, the one case of renal xenograft surviving for nine months is intriguing and certainly provides rationale for future investigations.