Evolution: Its Complexity and Impact on Xenotransplantation

Liver xenografts for the treatment of acute liver failure: clinical and experimental experience and remaining immunologic barriers

A critical element restricting the application of liver transplantation is the shortage of human deceased donor organs. Xenotransplantation using pig organs might be a solution to this shortage. Although the problems that still require resolution include the immunologic barrier, the potential risk of transferring infectious agents with the transplanted organ, and uncertainty about whether the transplanted organ will function satisfactorily in the human environment, recent progress in the genetic manipulation of pigs has led to the prospect that clinical xenografting, at least as a bridge to allotransplantation, may be possible in the foreseeable future. Experience with clinical auxiliary and orthotopic liver xenotransplantation and experimental liver xenotransplantation in nonhuman primate and other large animal models is reviewed, and the remaining immunologic problems are discussed. Evidence suggests that, in patients with hepatic failure, the pig liver may be less susceptible to antibody-mediated injury than other pig organs, such as the heart or kidney. Pig Kupffer cells and other macrophages will recognize and phagocytose primate red blood cells, but this problem should be overcome by pretransplant depletion of macrophages from the organ-source pig. From the evidence currently available, it does not seem unduly optimistic to anticipate that a liver from an alpha1,3-galactosyltransferase gene-knockout pig would survive at least long enough to function as a successful bridge to allotransplantation.

Human ABO blood group is important in survival and function of porcine working hearts

Pig organs express alphaGal antigen and thus are hyperacutely rejected if perfused by human blood. Human B/A antigens are similar to pig alphaGal antigen, suggesting that the corresponding antibodies may cross-react. Our purpose was to determine if there is a human ABO blood-group difference in porcine-human xenotransplantation. Plasma from six A, five B, seven AB, and six O individuals pooled by blood group were tested in an ex-vivo porcine working heart model. Blood-group A plasma-perfused hearts survived 20 +/- 14 min (n = 5), B 241 +/- 9 min (n = 3), AB 151 +/- 37 min (n = 5), and O 9 +/- 1 min (n = 8). A and O were different (p < 0.001) from B and AB. Function was significantly better in group B. Edema accumulation and creatine kinase change was highest in A and O. All groups had comparable levels of anti-alphaGal antibody, as well as comparable perfusion and operative conditions. Multivariate linear regression analysis showed the anti-B antibody levels to be predictive of survival (p < 0.001). At higher plasma concentrations, hearts perfused with B plasma survived longer (p = 0.01) than AB (218 +/- 45 min, n = 4 vs. 6 +/- 0 min, n = 3). These results suggest a human ABO blood-group difference in porcine-to-human xenotransplantation, which may be mediated by the anti-A and anti-B antibodies.

Will the pig solve the transplantation backlog?

The increasing shortage of human cadaveric organs for purposes of transplantation has become the critical limiting factor in the number of transplants performed each year. Some of this deficit is being met by the use of organs or partial organs from living donors, but this source is insufficient. Xenotransplantation-the transplantation of organs between species, namely from the pig to human-could provide a solution if immunologic and other associated problems could be solved. When a pig organ is transplanted into a primate, hyperacute rejection, induced by anti-pig antibody and mediated by complement and the coagulation system, develops rapidly. This immediate problem can now be overcome, but the return or persistence of anti-pig antibody leads to a delayed form of humoral rejection, acute humoral xenograft rejection, which leads to destruction of the organ within days or weeks. We review the various approaches being investigated to overcome this barrier. Whether they will also prevent subsequent acute cellular rejection remains unknown. Brief mention is made of the potential physiologic incompatibilities between pig and human organs, as well as the microbiologic safety aspects of xenotransplantation. Finally, the question of patient and societal acceptance of xenotransplantation is discussed.

The pig as a source of cardiac xenografts

The inadequate availability of human donor hearts and other organs has inspired interest in the field of xenotransplantation. Historically, ten attempts to transplant animal hearts into human recipients have been reported. Those who received hearts from nonhuman primates (i.e., baboons and chimpanzees) survived rather longer than did those who received hearts from nonprimates (i.e., sheep and pigs). Nevertheless, current opinion is that the pig is the best candidate as a source of hearts for humans despite the considerable immunologic disparity between the two species. Pigs are available in large numbers and can be bred easily and rapidly. They grow to appropriate sizes and their cardiovascular system is similar to that of humans. Substantial knowledge has been accumulated regarding both genetic engineering and tolerance induction in pigs, two strategies that may help to overcome the existing immunologic barriers. Concern has been raised, however, with regard to the potential for the transfer of a porcine infection with the pig organ to the human recipient. This brief review addresses these and other aspects of the use of the pig as a source of hearts for patients with end-stage cardiac disease.

Report of the Xenotransplantation Advisory Committee of the International Society for Heart and Lung Transplantation: the present status of xenotransplantation and its potential role in the treatment of end-stage cardiac and pulmonary diseases

An urgent and steadily increasing need exists world-wide for a greater supply of donor thoracic organs. Xenotransplantation offers the possibility of an unlimited supply of hearts and lungs that could be available electively when required. However, anti-body- mediated mechanisms cause the rejection of pig organs transplanted into non-human primates, and these mechanisms provide major immunologic barriers that have not yet been overcome. Having reviewed the literature on xenotransplantation, we present a number of conclusions on its present status with regard to thoracic organs, and we make a number of recommendations relating to eventual clinical trials. Although pig hearts have functioned in heterotopic sites in non-human primates for periods of several weeks, median survival of orthotopically transplanted hearts is currently ,1 month. No transplanted pig lung has functioned for even 24 hours. Current experimental results indicate that a clinical trial would be premature. A potential risk exists, hitherto undetermined, of transferring infectious organisms along with the donor pig organ to the recipient, and possibly to other members of the community. A clinical trial of xeno-transplantation should not be undertaken until experts in microbiology and the relevant regulatory authorities consider this risk to be minimal. A clinical trial should be considered when approximately 60% survival of life-supporting pig organs in non-human primates has been achieved for a minimum of 3 months, with at least 10 animals surviving for this minimum period. Furthermore, evidence should suggest that longer survival (.6 months) can be achieved. These results should be achieved in the absence of life-threatening complications caused by the immunosuppressive regimen used. The relationship between the presence of anti-HLA antibody and anti-pig antibody and their cross-reactivity, and the outcome of pig-organ xenotransplantation in recipients previously sensitized to HLA antigens require further investigation. We recommend that the patients who initially enter into a clinical trial of cardiac xenotransplantation be unacceptable for allotransplantation, or acceptable for allotransplantation but unlikely to survive until a human cadaveric organ becomes available, and in whom mechanical assist-device bridging is not possible. National bodies that have wide-reaching government-backed control over all aspects of the trials should regulate the initial clinical trial and all subsequent clinical xenotransplantation procedures for the foreseeable future. We recommend coordination and monitoring of these trials through an international body, such as the International Society for Heart and Lung Transplantation, and setting up a registry to record and widely disperse the results of these trials. Xenotransplantation has the potential to solve the problem of donor-organ supply, and therefore research in this field should be actively encouraged and supported.