Xenotransplantation: A historical-ethical account of viewpoints.

Formal clinical trials of pig-to-human organ transplant-known asxenotransplantation-may begin this decade, with the first trials likely to consist of either adult renal transplants or pediatric cardiac transplant patients. Xenotransplantation as a systematic scientific study only reaches back to the latter half of the 20th century, with episodic xenotransplantation events occurring prior to that. As the science of xenotransplantation has progressed in the 20th and 21st centuries, the public's knowledge of the potential therapy has also increased. With this, there have been shifting ethical stances toward xenotransplantation in key areas, such as religious and public viewpoints towards xenotransplantation, animal rights, and public health concerns. This review provides a historical-ethical account of xenotransplantation and details if or how viewpoints have shifted over time.

The attitudes of religious group leaders towards xenotransplantation: A focus group study.

Clinical trials of xenotransplantation (XTx) may start in coming years. Religious views have been mentioned as possible barriers to XTx acceptance. While there have been reports on perspectives of theologians in regard to XTx, no report has studied the perspectives of community religious leaders. A focus group was conducted with a sample of members of the following faith groups: Islam, Catholicism, and Protestantism. Qualitative content analysis was performed to identify interpretive themes. Four themes emerged. Participants were receptive to the idea of XTx and expressed no religious barriers to accepting a pig xenograft as a lifesaving therapy but did express certain concerns. Religious leaders accept the idea of XTx and do not see it as contradictory to their beliefs. However, some concerns were raised. Future studies addressing these concerns and exploring the potential role of religious leaders in educating the community on XTx are needed.

The impact of serum incubation time on IgM/IgG binding to porcine aortic endothelial cells.

The results of the assay for measuring anti-non-Gal antibodies (which affect pig xenograft survival) in recipients are important. Serum incubation time and concentration may be important factors in the extent of antibody binding to the graft. The aim of this in vitro study was to determine the optimal incubation time and serum concentration for measuring anti-non-Gal antibody binding to porcine aortic endothelial cells (pAECs). Pooled human, naive, and sensitized baboon sera were incubated with wild-type, α1,3-galactosyltransferase gene-knockout (GTKO), and GTKO/human CD55 pAECs. IgM/IgG binding to pAECs after varying serum incubation times (0.5, 1, 2, and 3 hour) and concentrations (5, 10, 20, and 40 µL) was determined by flow cytometry. An increase in incubation time from 30 minutes to 2 hour was associated with increases in anti-non-Gal IgM/IgG binding to GTKO and GTKO/hCD55 pAECs of pooled human, naive and sensitized baboon sera (P<.05). Pooled human serum showed a significant increase in anti-non-Gal IgM (1.5 times) and a minimal increase in anti-non-Gal IgG antibody binding. IgM/IgG binding of sensitized baboon serum to GTKO pAECs after 2-hour incubation was 1.5 times and 2 times greater than after 30-minutes incubation, respectively, whereas naïve baboon sera showed minimal (non-significant) increase in anti-non-Gal IgM/IgG antibody binding. With 2-hour incubation, increasing the serum concentration from 5 µL to 20 µL significantly increased antibody binding to non-Gal antigens in pooled human and sensitized baboon serum. With naïve baboon serum, only IgG was significantly increased. Increasing the serum incubation time contributed to improve the sensitivity of detecting anti-non-Gal antibodies, without affecting cell viability in vitro.

Incidence of Neoplasia in Pigs and Its Relevance to Clinical Organ Xenotransplantation.

As clinical pig organ xenotransplantation draws closer, more attention is being paid to diseases that affect pigs and those that provide a potential risk to human recipients of pig organs. Neoplasia arising from the pig organ graft is one such concern. Various tumors and other neoplastic diseases are well known to show increased incidence in organ allotransplant recipients receiving immunosuppressive therapy. Whether this effect will prove to be the case after xenotransplantation has not yet been established. Malignant tumors in young pigs are rare, with lymphosarcoma, nephroblastoma, and melanoma being the most common. The combination of noninvasive techniques and intraoperative examination of the pig organ likely will readily confirm that a pig organ graft is tumor-free before xenotransplantation. Posttransplantion lymphoproliferative disorder (PTLD) is a concern after allotransplantation, but the incidence after solid organ allotransplantation is low when compared with hematopoietic cell allotransplantation (for example, bone marrow transplantation), unless immunosuppressive therapy is particularly intensive. Organ-source pigs used for clinical xenotransplantation will be bred and housed under designated pathogen-free conditions and will be free of the γ-herpesvirus that is a key factor in the development of PTLD in pigs. Therefore if a recipient of a pig xenograft develops PTLD, it will almost certainly be of recipient origin. The increasing availability of organs from pigs genetically-engineered to protect them from the human immune response likely will diminish the need for intensive immunosuppressive therapy. Considering the low incidence of malignant disease in young pigs, donor-derived malignancy is likely to be rare in patients who receive pig organ grafts. However, if the graft remains viable for many years, the incidence of graft malignancy may increase.

Update: cardiac xenotransplantation.

PURPOSE OF REVIEW: To review the latest development in cardiac xenotransplantation in small and large animal models and related in-vitro studies. RECENT FINDINGS: With the recent introduction of alpha1,3-galactosyltransferase gene-knockout (GT-KO) pig organs for xenotransplantation, improved cardiac graft survival has been obtained. However, this experience has demonstrated the importance of pig antigens other than Galalpha1,3Gal (Gal) antigens (so-called nonGal antigens) as targets for primate anti-pig antibodies. Several in-vitro studies have confirmed that, although the incidence and levels of anti-nonGal antibodies in nonhuman primates and humans are significantly less when compared with total anti-pig antibodies (i.e., anti-Gal + anti-nonGal), they can result in complement-mediated lysis of GT-KO pig cells. More recently, it has been demonstrated that regulatory T cells suppress the cellular xenogeneic response, thus potentially preventing or reducing T-cell-mediated rejection. The importance of thrombotic microangiopathy as a feature of the immune/inflammatory response and incompatibilities between the coagulation-anticoagulation systems of pig and primate are receiving increasing attention. Development of GT-KO pigs transgenic for one or more 'antithrombotic' genes, for example, CD39 or tissue factor pathway inhibitor, may contribute to overcoming these problems. SUMMARY: Although GT-KO pigs have provided an advance over wild-type pigs as a source of organs for transplantation into primates, further genetic modification of GT-KO pigs is required to overcome the remaining immune barriers before a clinical trial of cardiac xenotransplantation can be contemplated.

Physiologic Aspects of Pig Kidney Transplantation in Nonhuman Primates.

Xenotransplantation can provide a solution to the current shortage of human organs for patients with terminal renal failure. The increasing availability of genetically engineered pigs, effective immunosuppressive therapy, and antiinflammatory therapy help to protect pig tissues from the primate immune response and can correct molecular incompatibilities. Life-supporting pig kidney xenografts have survived in NHP for more than 6 mo in the absence of markers of consumptive coagulopathy. However, few reports have focused on the physiologic aspects of life-supporting pig kidney xenografts. We have reviewed the literature regarding pig kidney xenotransplantation in NHP. The available data indicate (1) normal serum creatinine, (2) normal serum electrolytes, except for a trend toward increased calcium levels and a transient rise in phosphate followed by a fall to slightly subnormal values, (3) minimal or modest proteinuria without hypoalbuminemia (suggesting that previous reports of proteinuria likely were due to a low-grade immune response rather than physiologic incompatibilities), (4) possible discrepancies between pig erythropoietin and the primate erythropoietin receptor, and (5) significant early increase in kidney graft size, which might result from persistent effects of pig growth hormone. Further study is required regarding identification and investigation of physiologic incompatibilities. However, current evidence suggests that, in the absence of an immune response, a transplanted pig kidney likely would satisfactorily support a human patient.

The forgotten French: The 'heroic' era of kidney transplantation.

The efforts in the late 1940s to 1960s of two Parisian pioneers in kidney transplantation, René Kuss, a surgeon, and Jean Hamburger, a nephrologist, have largely been forgotten. Küss developed the operation that is basically unchanged today. Both groups initiated clinical transplant programs in January 1951, and both were among the first to carry out kidney transplantation (i) without immunosuppressive therapy, (ii) between living-related and unrelated donors, (iii) with organs from deceased donors, (iv) with irradiation and immunosuppressive drugs, and (v) with long-term survival. In the opinion of many, the French did not receive full credit for their work internationally.

Recent advances in understanding xenotransplantation: implications for the clinic.

The results of organ and cell allotransplantation continue to improve, but the field remains limited by a lack of deceased donor organs. Xenotransplantation, for example, between pig and human, offers unlimited organs and cells for clinical transplantation. The immune barriers include a strong innate immune response in addition to the adaptive T-cell response. The innate response has largely been overcome by the transplantation of organs from pigs with genetic modifications that protect their tissues from this response. T-cell-mediated rejection can be controlled by immunosuppressive agents that inhibit costimulation. Coagulation dysfunction between the pig and primate remains problematic but is being overcome by the transplantation of organs from pigs that express human coagulation-regulatory proteins. The remaining barriers will be resolved by the introduction of novel genetically-engineered pigs. Limited clinical trials of pig islet and corneal transplantation are already underway.

The optimal hormonal replacement modality selection for multiple organ procurement from brain-dead organ donors.

The management of brain-dead organ donors is complex. The use of inotropic agents and replacement of depleted hormones (hormonal replacement therapy) is crucial for successful multiple organ procurement, yet the optimal hormonal replacement has not been identified, and the statistical adjustment to determine the best selection is not trivial. Traditional pair-wise comparisons between every pair of treatments, and multiple comparisons to all (MCA), are statistically conservative. Hsu's multiple comparisons with the best (MCB) - adapted from the Dunnett's multiple comparisons with control (MCC) - has been used for selecting the best treatment based on continuous variables. We selected the best hormonal replacement modality for successful multiple organ procurement using a two-step approach. First, we estimated the predicted margins by constructing generalized linear models (GLM) or generalized linear mixed models (GLMM), and then we applied the multiple comparison methods to identify the best hormonal replacement modality given that the testing of hormonal replacement modalities is independent. Based on 10-year data from the United Network for Organ Sharing (UNOS), among 16 hormonal replacement modalities, and using the 95% simultaneous confidence intervals, we found that the combination of thyroid hormone, a corticosteroid, antidiuretic hormone, and insulin was the best modality for multiple organ procurement for transplantation.

Immunobiology of liver xenotransplantation.

Pigs are currently the preferred species for future organ xenotransplantation. With advances in the development of genetically modified pigs, clinical xenotransplantation is becoming closer to reality. In preclinical studies (pig-to-nonhuman primate), the xenotransplantation of livers from pigs transgenic for human CD55 or from α1,3-galactosyltransferase gene-knockout pigs+/- transgenic for human CD46, is associated with survival of approximately 7-9 days. Although hepatic function, including coagulation, has proved to be satisfactory, the immediate development of thrombocytopenia is very limiting for pig liver xenotransplantation even as a 'bridge' to allotransplantation. Current studies are directed to understand the immunobiology of platelet activation, aggregation and phagocytosis, in particular the interaction between platelets and liver sinusoidal endothelial cells, hepatocytes and Kupffer cells, toward identifying interventions that may enable clinical application.

Frankenswine, or bringing home the bacon: How close are we to clinical trials in xenotransplantation?

Xenotransplantation-specifically from pig into human-could resolve the critical shortage of organs, tissues and cells for clinical transplantation. Genetic engineering techniques in pigs are relatively well-developed and to date have largely been aimed at producing pigs that either (1) express high levels of one or more human complement-regulatory protein(s), such as decay-accelerating factor or membrane cofactor protein, or (2) have deletion of the gene responsible for the expression of the oligosaccharide, Galalpha1,3Gal (Gal), the major target for human anti-pig antibodies, or (3) have both manipulations. Currently the transplantation of pig organs in adequately-immunosuppressed baboons results in graft function for periods of 2-6 months (auxiliary hearts) and 2-3 months (life-supporting kidneys). Pig islets have maintained normoglycemia in diabetic monkeys for >6 months. The remaining immunologic barriers to successful xenotransplantation are discussed, and brief reviews made of (1) the potential risk of the transmission of an infectious microorganism from pig to patient and possibly to the public at large, (2) the potential physiologic incompatibilities between a pig organ and its human counterpart, (3) the major ethical considerations of clinical xenotransplantation, and (4) the possible alternatives that compete with xenotransplantation in the field of organ or cell replacement, such as mechanical devices, tissue engineering, stem cell biology and organogenesis. Finally, the proximity of clinical trials is discussed. Islet xenotransplantation is already at the stage where clinical trials are actively being considered, but the transplantation of pig organs will probably require further genetic modifications to be made to the organ-source pigs to protect their tissues from the coagulation/anticoagulation dysfunction that plays a significant role in pig graft failure after transplantation in primates.