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

Prolonged xenokidney graft survival in sensitized NHP recipients by expression of multiple human transgenes in a triple knockout pig

Genetic modification of porcine donors, combined with optimized immunosuppression, has been shown to improve outcomes of experimental xenotransplant. However, little is known about outcomes in sensitized recipients, a population that could potentially benefit the most from the clinical implementation of xenotransplantation. Here, five highly allosensitized rhesus macaques received a porcine kidney from GGTA1 (α1,3-galactosyltransferase) knockout pigs expressing the human CD55 transgene (1KO.1TG) and were maintained on an anti-CD154 monoclonal antibody (mAb)-based immunosuppressive regimen. These recipients developed de novo xenoreactive antibodies and experienced xenograft rejection with evidence of thrombotic microangiopathy and antibody-mediated rejection (AMR). In comparison, three highly allosensitized rhesus macaques receiving a kidney from GGTA1, CMAH (cytidine monophospho-N-acetylneuraminic acid hydroxylase), and b4GNT2/b4GALNT2 (β-1,4-N-acetyl-galactosaminyltransferase 2) knockout pigs expressing seven human transgenes including human CD46, CD55, CD47, THBD (thrombomodulin), PROCR (protein C receptor), TNFAIP3 (tumor necrosis factor-α-induced protein 3), and HMOX1 (heme oxygenase 1) (3KO.7TG) experienced significantly prolonged graft survival and reduced AMR, associated with dampened post-transplant humoral responses, early monocyte and neutrophil activation, and T cell repopulation. After withdrawal of all immunosuppression, recipients who received kidneys from 3KO.7TG pigs rejected the xenografts via AMR. These data suggest that allosensitized recipients may be suitable candidates for xenografts from genetically modified porcine donors and could benefit from an optimized immunosuppression regimen designed to target the post-transplant humoral response, thereby avoiding AMR.

Current Status of Cardiac Xenotransplantation: Report of a Workshop of the German Heart Transplant Centers, Martinsried, March 3, 2023

This report comprises the contents of the presentations and following discussions of a workshop of the German Heart Transplant Centers in Martinsried, Germany on cardiac xenotransplantation. The production and current availability of genetically modified donor pigs, preservation techniques during organ harvesting, and immunosuppressive regimens in the recipient are described. Selection criteria for suitable patients and possible solutions to the problem of overgrowth of the xenotransplant are discussed. Obviously microbiological safety for the recipient and close contacts is essential, and ethical considerations to gain public acceptance for clinical applications are addressed. The first clinical trial will be regulated and supervised by the Paul-Ehrlich-Institute as the National Competent Authority for Germany, and the German Heart Transplant Centers agreed to cooperatively select the first patients for cardiac xenotransplantation.

Pediatric Cardiac Xenotransplantation: Recommendations for the Ethical Design of Clinical Trials

For children with complex congenital heart problems, cardiac allotransplantation is sometimes the best therapeutic option. However, availability of hearts for pediatric patients is limited, resulting in a long and growing waitlist, and a high mortality rate while waiting. Cardiac xenotransplantation has been proposed as one therapeutic alternative for neonates and infants, either in lieu of allotransplantation or as a bridge until an allograft becomes available. Scientific and clinical developments in xenotransplantation appear likely to permit cardiac xenotransplantation clinical trials in adults in the coming years. The ethical issues around xenotransplantation of the heart and other organs and tissues have recently been examined, but to date, only limited literature is available on the ethical issues that are attendant with pediatric heart xenotransplantation. Here, we summarize the ethical issues, focusing on (1) whether cardiac xenotransplantation should proceed in adults or children first, (2) pediatric recipient selection for initial xenotransplantation trials, (3) special problems regarding informed consent in this context, and (4) related psychosocial and public perception considerations. We conclude with specific recommendations regarding ethically informed design of pediatric heart xenotransplantation trials.

Heart transplantation: advances in expanding the donor pool and xenotransplantation

Approximately 65 million adults globally have heart failure, and the prevalence is expected to increase substantially with ageing populations. Despite advances in pharmacological and device therapy of heart failure, long-term morbidity and mortality remain high. Many patients progress to advanced heart failure and develop persistently severe symptoms. Heart transplantation remains the gold-standard therapy to improve the quality of life, functional status and survival of these patients. However, there is a large imbalance between the supply of organs and the demand for heart transplants. Therefore, expanding the donor pool is essential to reduce mortality while on the waiting list and improve clinical outcomes in this patient population. A shift has occurred to consider the use of organs from donors with hepatitis C virus, HIV or SARS-CoV-2 infection. Other advances in this field have also expanded the donor pool, including opt-out donation policies, organ donation after circulatory death and xenotransplantation. We provide a comprehensive overview of these various novel strategies, provide objective data on their safety and efficacy, and discuss some of the unresolved issues and controversies of each approach.

A Standardized Approach to Orthotopic (Life-supporting) Porcine Cardiac Xenotransplantation in a Nonhuman Primate Model

Cardiac xenotransplantation from swine has been proposed to "bridge the gap" in supply for heart failure patients requiring transplantation. Recent preclinical success using genetically modified pig donors in baboon recipients has demonstrated survival greater than 6 mo, with a modern understanding of xenotransplantation immunobiology and continued experience with large animal models of cardiac xenotransplantation. As a direct result of this expertise, the Food and Drug Administration approved the first in-human transplantation of a genetically engineered cardiac xenograft through an expanded access application for a single patient. This clinical case demonstrated the feasibility of xenotransplantation. Although this human study demonstrated proof-of-principle application of cardiac xenotransplantation, further regulatory oversight by the Food and Drug Administration may be required with preclinical trials in large animal models of xenotransplantation with long-term survival before approval of a more formalized clinical trial. Here we detail our surgical approach to pig-to-primate large animal models of orthotopic cardiac xenotransplantation, and the postoperative care of the primate recipient, both in the immediate postoperative period and in the months thereafter. We also detail xenograft surveillance methods and common issues that arise in the postoperative period specific to this model and ways to overcome them. These studies require multidisciplinary teams and expertise in orthotopic transplantation (cardiac surgery, anesthesia, and cardiopulmonary bypass), immunology, genetic engineering, and experience in handling large animal donors and recipients, which are described here. This article serves to reduce the barriers to entry into a field with ever-growing enthusiasm, but demands expertise knowledge and experience to be successful.

Cardiac xenotransplantation: from concept to clinic

For many patients with terminal/advanced cardiac failure, heart transplantation is the most effective, durable treatment option, and offers the best prospects for a high quality of life. The number of potentially life-saving donated human organs is far fewer than the population who could benefit from a new heart, resulting in increasing numbers of patients awaiting replacement of their failing heart, high waitlist mortality, and frequent reliance on interim mechanical support for many of those deemed among the best candidates but who are deteriorating as they wait. Currently, mechanical assist devices supporting left ventricular or biventricular heart function are the only alternative to heart transplant that is in clinical use. Unfortunately, the complication rate with mechanical assistance remains high despite advances in device design and patient selection and management, and the quality of life of the patients even with good outcomes is only moderately improved. Cardiac xenotransplantation from genetically multi-modified (GM) organ-source pigs is an emerging new option as demonstrated by the consistent long-term success of heterotopic (non-life-supporting) abdominal and life-supporting orthotopic porcine heart transplantation in baboons, and by a recent 'compassionate use' transplant of the heart from a GM pig with 10 modifications into a terminally ill patient who survived for 2 months. In this review, we discuss pig heart xenotransplantation as a concept, including pathobiological aspects related to immune rejection, coagulation dysregulation, and detrimental overgrowth of the heart, as well as GM strategies in pigs to prevent or minimize these problems. Additional topics discussed include relevant results of heterotopic and orthotopic heart transplantation experiments in the pig-to-baboon model, microbiological and virologic safety concepts, and efficacy requirements for initiating formal clinical trials. An adequate regulatory and ethical framework as well as stringent criteria for the selection of patients will be critical for the safe clinical development of cardiac xenotransplantation, which we expect will be clinically tested during the next few years.

Preclinical rationale and current pathways to support the first human clinical trials in cardiac xenotransplantation

Recent initiation of the first FDA-approved cardiac xenotransplantation suggests xenotransplantation could soon become a therapeutic option for patients unable to undergo allotransplantation. Until xenotransplantation is widely applied in clinical practice, consideration of benefit versus risk and approaches to management of clinical xenografts will based at least in part on observations made in experimental xenotransplantation in non-human primates. Indeed, the decision to proceed with clinical trials reflects significant progress in last few years in experimental solid organ and cellular xenotransplantation. Our laboratory at the NIH and now at University of Maryland contributed to this progress, with heterotopic cardiac xenografts surviving more than two years and life-supporting cardiac xenografts survival up to 9 months. Here we describe our contributions to the understanding of the mechanism of cardiac xenograft rejection and development of methods to overcome past hurdles, and finally we share our opinion on the remaining barriers to clinical translation. We also discuss how the first in human xenotransplants might be performed, recipients managed, and graft function monitored.

Pig heart and lung xenotransplantation: Present status

The recent pig heart transplant in a patient at the University of Maryland Medical Center has stimulated renewed interest in the xenotransplantation of organs from genetically engineered pigs. The barriers to the use of pigs as sources of organs have largely been overcome by 2 approaches - (1) the deletion of expression of the three known pig carbohydrate xenoantigens against which humans have preformed antibodies, and (2) the transgenic introduction of human 'protective' proteins, such as complement-regulatory proteins. These gene modifications, coupled with immunosuppressive therapy based on blockade of the CD40/CD154 costimulation pathway, have resulted in survival of baboons with life-supporting pig heart grafts for almost 9 months. The initial clinical success at the University of Maryland reinforces encouraging preclinical results. It suggests that pig hearts are likely to provide an effective bridge to an allotransplant, but their utility for destination therapy remains uncertain. Because of additional complex immunobiological problems, the same approach has been less successful in preclinical lung xenograft transplantation, where survival is still measured in days or weeks. The first formal clinical trials of pig heart transplantation may include patients who do not have access to an allotransplant, those with contraindications for mechanical circulatory support, those in need of retransplantation or with a high level of panel-reactive antibodies. Infants with complex congenital heart disease, should also be considered.

The immunobiology and clinical use of genetically engineered porcine hearts for cardiac xenotransplantation

A summary of the scientific rationale of the advancements that led to the first genetically modified pig-to-human cardiac xenotransplantation is lacking in a complex and rapidly evolving field. Here, we aim to aid the general readership in the understanding of the gradual progression of cardiac (xeno)transplantation research, the immunobiology of cardiac xenotransplantation (including the latest immunosuppression, cardiac preservation and genetic engineering required for successful transplantation) and the regulatory landscape related to the clinical application of cardiac xenotransplantation for people with end-stage heart failure. Finally, we provide an overview of the outcomes and lessons learned from the first genetically modified pig-to-human cardiac heart xenotransplantation.

Evolution of Xenotransplantation as an Alternative to Shortage of Donors in Heart Transplantation

This review aims to show and illustrate the history, current, ethical considerations, and limitations concerning xenotransplantation. Due to the current shortage of available donor organs for transplantation, many alternative sources are being examined to solve the donor shortage. One of them is xenotransplantation which refers to the transplantation of organs from one species to another. Compared to other nonhuman primates (NHP), pigs are ideal species for organ harvesting as they rapidly grow to human size in a handful of months. There is much advancement in the genetic engineering of pigs, which have hearts structurally and functionally similar to the human heart. The role of genetic engineering is to overcome the immune barriers in xenotransplantation and can be used in hyperacute rejection and T cell-mediated rejection. It is technically difficult to use large animal models for orthotopic, life-sustaining heart transplantation. Despite the fact that some religious traditions, such as Jewish and Muslim, prohibit the ingestion of pork products, few religious leaders consider that donating porcine organs is ethical because it saves human life. Although recent technologies have lowered the risk of a xenograft producing a novel virus that causes an epidemic, the risk still exists. It has major implications for the informed consent procedure connected with clinical research on heart xenotransplantation.

Ethical considerations during a pioneering surgical procedure: porcine cardiac xenotransplantation

Preclinical advances in life-sustaining porcine cardiac xenotransplantation from donor pigs to baboons have paved the way for the performance of porcine cardiac xenotransplantation in a human. This procedure was performed with emergency use authorisation granted by the United States Food and Drug Administration under the umbrella of investigational new drug use on compassionate grounds. The patient was denied candidacy for durable mechanical circulatory support and heart transplantation as a result of non-adherence to medical advice.

Successful porcine cardiac xenotransplantation in humans will significantly increase the availability of potential donor organs for long-term management of end-stage heart failure. Human porcine cardiac xenotransplantation is associated with ethical conflicts encompassing multiple ethical principles which are not mutually exclusive and are sometimes conflicting. This article focuses on some of the ethical conflicts encountered in relation to the use of mechanical circulatory support, pretransplant evaluation, shared decision making during informed consent, infectious disease risk, preclinical and clinical testing, and the role of regulatory bodies during performance of the first human porcine cardiac xenotransplantation.

An increase in human trials of xenotransplantation procedures is imminent. Potential ethical conflicts associated with xenotransplantation should be addressed appropriately.

Signaling cascades in the failing heart and emerging therapeutic strategies

Chronic heart failure is the end stage of cardiac diseases. With a high prevalence and a high mortality rate worldwide, chronic heart failure is one of the heaviest health-related burdens. In addition to the standard neurohormonal blockade therapy, several medications have been developed for chronic heart failure treatment, but the population-wide improvement in chronic heart failure prognosis over time has been modest, and novel therapies are still needed. Mechanistic discovery and technical innovation are powerful driving forces for therapeutic development. On the one hand, the past decades have witnessed great progress in understanding the mechanism of chronic heart failure. It is now known that chronic heart failure is not only a matter involving cardiomyocytes. Instead, chronic heart failure involves numerous signaling pathways in noncardiomyocytes, including fibroblasts, immune cells, vascular cells, and lymphatic endothelial cells, and crosstalk among these cells. The complex regulatory network includes protein-protein, protein-RNA, and RNA-RNA interactions. These achievements in mechanistic studies provide novel insights for future therapeutic targets. On the other hand, with the development of modern biological techniques, targeting a protein pharmacologically is no longer the sole option for treating chronic heart failure. Gene therapy can directly manipulate the expression level of genes; gene editing techniques provide hope for curing hereditary cardiomyopathy; cell therapy aims to replace dysfunctional cardiomyocytes; and xenotransplantation may solve the problem of donor heart shortages. In this paper, we reviewed these two aspects in the field of failing heart signaling cascades and emerging therapeutic strategies based on modern biological techniques.

Organ transplants of the future: planning for innovations including xenotransplantation

The future clinical application of animal-to-human transplantation (xenotransplantation) is of importance to society as a whole. Favourable preclinical data relevant to cell, tissue and solid organ xenotransplants have been obtained from many animal models utilizing genetic engineering and protocols of pathogen-free husbandry. Findings have reached a tipping point, and xenotransplantation of solid organs is approaching clinical evaluation, the process of which now requires close deliberation. Such discussions include considering when there is sufficient evidence from preclinical animal studies to start first-in-human xenotransplantation trials. The present article is based on evidence and opinions formulated by members of the European Society for Organ Transplantation who are involved in the Transplantation Learning Journey project. The article includes a brief overview of preclinical concepts and biology of solid organ xenotransplantation, discusses the selection of candidates for first-in-human studies and considers requirements for study design and conduct. In addition, the paper emphasizes the need for a regulatory framework for xenotransplantation of solid organs and the essential requirement for input from public and patient stakeholders.

Pathways to Clinical Cardiac Xenotransplantation

Heart transplantation is the only long-lasting lifesaving option for patients with terminal cardiac failure. The number of available human organs is however far below the actual need, resulting in substantial mortality of patients while waiting for a human heart. Mechanical assist devices are used to support cardiac function but are associated with a high risk of severe complications and poor quality of life for the patients. Consistent success in orthotopic transplantation of genetically modified pig hearts into baboons indicates that cardiac xenotransplantation may become a clinically applicable option for heart failure patients who cannot get a human heart transplant. In this overview, we project potential paths to clinical cardiac xenotransplantation, including the choice of genetically modified source pigs; associated requirements of microbiological, including virological, safety; optimized matching of source pig and recipient; and specific treatments of the donor heart after explantation and of the recipients. Moreover, selection of patients and the regulatory framework will be discussed.

Don't pig(!) the wrong heart!

Cardiac xenotransplantation is believed to have approached clinical application. However, this approach to advanced heart failure is burdened with a multitude of ethical issues. These issues need to be addressed openly and be broadly discussed in public. Only through an honest and transparent approach, it will be possible to engage the lay audience in the evaluation of pig to human transplant.

Clinical trials in cardiac xenotransplantation: Are we ready to overcome barriers?

Heart allotransplantation has become one of the methods of choice in the treatment of severe heart failure. In the face of its difficulties, such as the unmet balance between organ supply and demand, the use of xenotransplantation (XTx) might be an attractive option shortly, even more with the ongoing progress achieved regarding the avoidance of hyperacute rejection and primary organ disfunction, maintenance of xenograft function and control of xenograft growth. To make possible this translational challenge, some points must be taken into account indeed, and they are the equipoise of human benefit and animal suffering, the risk of unknown infections, a well prepared informed consent, ethical and religious beliefs, and the role of cardiac XTx in a ventricular assistance device era.

Corneal xenotransplantation: Where are we standing?

The search for alternatives to allotransplants is driven by the shortage of corneal donors and is demanding because of the limitations of the alternatives. Indeed, current progress in genetically engineered (GE) pigs, the introduction of gene-editing technology by clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9, and advanced immunosuppressants have made xenotransplantation a possible option for a human trial. Porcine corneal xenotransplantation is considered applicable because the eye is regarded as an immune-privileged site. Furthermore, recent non-human primate studies have shown long-term survival of porcine xenotransplants in keratoplasty. Herein, corneal immune privilege is briefly introduced, and xenogeneic reactions are compared with allogeneic reactions in corneal transplantation. This review describes the current knowledge on special issues of xenotransplantation, xenogeneic rejection mechanisms, current immunosuppressive regimens of corneal xenotransplantation, preclinical efficacy and safety data of corneal xenotransplantation, and updates of the regulatory framework to conduct a clinical trial on corneal xenotransplantation. We also discuss barriers that might prevent xenotransplantation from becoming common practice, such as ethical dilemmas, public concerns on xenotransplantation, and the possible risk of xenozoonosis. Given that the legal definition of decellularized porcine cornea (DPC) lies somewhere between a medical device and a xenotransplant, the preclinical efficacy and clinical trial data using DPC are included. The review finally provides perspectives on the current standpoint of corneal xenotransplantation in the fields of regenerative medicine.

Clinical impact of high-profile animal-based research reported in the UK national press

OBJECTIVES

We evaluated animal-based biomedical 'breakthroughs' reported in the UK national press in 1995 (25 years prior to the conclusion of this study). Based on evidence of overspeculative reporting of biomedical research in other areas (eg, press releases and scientific papers), we specifically examined animal research in the media, asking, 'In a given year, what proportion of animal research "breakthroughs"' published in the UK national press had translated, more than 20 years later, to approved interventions?'

METHODS

We searched the Nexis media database (LexisNexis.com) for animal-based biomedical reports in the UK national press. The only restrictions were that the intervention should be specific, such as a named drug, gene, biomedical pathway, to facilitate follow-up, and that there should be claims of some clinical promise.

MAIN OUTCOME MEASURES

Were any interventions approved for human use? If so, when and by which agency? If not, why, and how far did development proceed? Were any other, directly related interventions approved? Did any of the reports overstate human relevance?

RESULTS

Overspeculation and exaggeration of human relevance was evident in all the articles examined. Of 27 unique published 'breakthroughs', only one had clearly resulted in human benefit. Twenty were classified as failures, three were inconclusive and three were partially successful.

CONCLUSIONS

The results of animal-based preclinical research studies are commonly overstated in media reports, to prematurely imply often-imminent 'breakthroughs' relevant to human medicine.

Progress Toward Cardiac Xenotransplantation

Consistent survival of life-supporting pig heart xenograft recipients beyond 90 days was recently reported using genetically modified pigs and a clinically applicable drug treatment regimen. If this remarkable achievement proves reproducible, published benchmarks for clinical translation of cardiac xenografts appear to be within reach. Key mechanistic insights are summarized here that informed recent pig design and therapeutic choices, which together appear likely to enable early clinical translation.

Pig-to-human heart transplantation: Who goes first?

Cardiac xenotransplantation has recently taken an important step towards clinical reality. In anticipation of the "first-in-human" heart xenotransplantation trial, we propose a set of patient characteristics that define potential candidates. Our premise is that, to be ethically justified, the risks posed by current state-of-the-art options must outweigh the anticipated risks of a pioneering xenotransplant procedure. Suitable candidates include patients who are at high immunologic risk because of sensitization to alloantigens, including those who have exhibited early onset or accelerated cardiac allograft vasculopathy. In addition, patients should be considered (1) for whom mechanical circulatory support would be prohibitively risky due to a hypercoagulable state, a contraindication to anticoagulation, or restrictive physiology; (2) with severe biventricular dysfunction predicting unsuccessful univentricular left heart support; and (3) adults with complex congenital heart disease. In conclusion, because the published preclinical benchmark for clinical translation of heart xenotransplantation appears within reach, carefully and deliberately defining appropriate trial participants is timely as the basis for ethical clinical trial design.

Cold non-ischemic heart preservation with continuous perfusion prevents early graft failure in orthotopic pig-to-baboon xenotransplantation

BACKGROUND

Successful preclinical transplantations of porcine hearts into baboon recipients are required before commencing clinical trials. Despite years of research, over half of the orthotopic cardiac xenografts were lost during the first 48 hours after transplantation, primarily caused by perioperative cardiac xenograft dysfunction (PCXD). To decrease the rate of PCXD, we adopted a preservation technique of cold non-ischemic perfusion for our ongoing pig-to-baboon cardiac xenotransplantation project.

METHODS

Fourteen orthotopic cardiac xenotransplantation experiments were carried out with genetically modified juvenile pigs (GGTA1- KO/hCD46/hTBM) as donors and captive-bred baboons as recipients. Organ preservation was compared according to the two techniques applied: cold static ischemic cardioplegia (IC; n = 5) and cold non-ischemic continuous perfusion (CP; n = 9) with an oxygenated albumin-containing hyperoncotic cardioplegic solution containing nutrients, erythrocytes and hormones. Prior to surgery, we measured serum levels of preformed anti-non-Gal-antibodies. During surgery, hemodynamic parameters were monitored with transpulmonary thermodilution. Central venous blood gas analyses were taken at regular intervals to estimate oxygen extraction, as well as lactate production. After surgery, we measured troponine T and serum parameters of the recipient's kidney, liver and coagulation functions.

RESULTS

In porcine grafts preserved with IC, we found significantly depressed systolic cardiac function after transplantation which did not recover despite increasing inotropic support. Postoperative oxygen extraction and lactate production were significantly increased. Troponin T, creatinine, aspartate aminotransferase levels were pathologically high, whereas prothrombin ratios were abnormally low. In three of five IC experiments, PCXD developed within 24 hours. By contrast, all nine hearts preserved with CP retained fully preserved systolic function, none showed any signs of PCXD. Oxygen extraction was within normal ranges; serum lactate as well as parameters of organ functions were only mildly elevated. Preformed anti-non-Gal-antibodies were similar in recipients receiving grafts from either IC or CP preservation.

CONCLUSIONS

While standard ischemic cardioplegia solutions have been used with great success in human allotransplantation over many years, our data indicate that they are insufficient for preservation of porcine hearts transplanted into baboons: Ischemic storage caused severe impairment of cardiac function and decreased tissue oxygen supply, leading to multi-organ failure in more than half of the xenotransplantation experiments. In contrast, cold non-ischemic heart preservation with continuous perfusion reliably prevented early graft failure. Consistent survival in the perioperative phase is a prerequisite for preclinical long-term results after cardiac xenotransplantation.

Update and breakthrough in cardiac xenotransplantation

PURPOSE OF REVIEW

Considerable advancements have been made in the field of cardiac xenotransplantation in the recent years, achieving prolonged survival of the life-supporting cardiac xenograft and paving the way toward first clinical implications.

RECENT FINDINGS

The combination of genetic modifications and novel immunosuppression with costimulation blockade, as well as supporting therapy with antiinflammatory treatment, growth prevention, and adaptation of the heart procurement system to reduce myocardial ischemia and reperfusion injury improves the overall cardiac xenograft function and overall survival in nonhuman primates. Through the newly identified xenoantigens and novel gene-editing techniques, further genetic modification of the porcine xenografts should be explored, to ensure clinical safety.

SUMMARY

With continuous progress in all fields of cardiac xenotransplantation, first clinical use in humans seems accomplishable. To ensure the clinical safety and to conform to the ethical regulations, further investigation of the infectious and immunological implications on humans should be explored prior to first clinical use. The first clinical use of cardiac xenotransplantation will be limited to only highly selected patients.

Pig-to-non-human primate heart transplantation: The final step toward clinical xenotransplantation?

BACKGROUND

The demand for donated human hearts far exceeds the number available. Xenotransplantation of genetically modified porcine organs provides an alternative. In 2000, an Advisory Board of the International Society for Heart and Lung Transplantation set the benchmark for commencing clinical cardiac xenotransplantation as consistent 60% survival of non-human primates after life-supporting porcine heart transplantations. Recently, we reported the stepwise optimization of pig-to-baboon orthotopic cardiac xenotransplantation finally resulting in consistent success, with 4 recipients surviving 90 (n = 2), 182, and 195 days. Here, we report on 4 additional recipients, supporting the efficacy of our procedure.

RESULTS

The first 2 additional recipients succumbed to porcine cytomegalovirus (PCMV) infections on Days 15 and 27, respectively. In 2 further experiments, PCMV infections were successfully avoided, and 3-months survival was achieved. Throughout all the long-term experiments, heart, liver, and renal functions remained within normal ranges. Post-mortem cardiac diameters were slightly increased when compared with that at the time of transplantation but with no detrimental effect. There were no signs of thrombotic microangiopathy. The current regimen enabled the prolonged survival and function of orthotopic cardiac xenografts in altogether 6 of 8 baboons, of which 4 were now added. These results exceed the threshold set by the Advisory Board of the International Society for Heart and Lung Transplantation.

CONCLUSIONS

The results of our current and previous experimental cardiac xenotransplantations together fulfill for the first time the pre-clinical efficacy suggestions. PCMV-positive donor animals must be avoided.

Xenotransplantation: Current Status in Preclinical Research

The increasing life expectancy of humans has led to a growing numbers of patients with chronic diseases and end-stage organ failure. Transplantation is an effective approach for the treatment of end-stage organ failure; however, the imbalance between organ supply and the demand for human organs is a bottleneck for clinical transplantation. Therefore, xenotransplantation might be a promising alternative approach to bridge the gap between the supply and demand of organs, tissues, and cells; however, immunological barriers are limiting factors in clinical xenotransplantation. Thanks to advances in gene-editing tools and immunosuppressive therapy as well as the prolonged xenograft survival time in pig-to-non-human primate models, clinical xenotransplantation has become more viable. In this review, we focus on the evolution and current status of xenotransplantation research, including our current understanding of the immunological mechanisms involved in xenograft rejection, genetically modified pigs used for xenotransplantation, and progress that has been made in developing pig-to-pig-to-non-human primate models. Three main types of rejection can occur after xenotransplantation, which we discuss in detail: (1) hyperacute xenograft rejection, (2) acute humoral xenograft rejection, and (3) acute cellular rejection. Furthermore, in studies on immunological rejection, genetically modified pigs have been generated to bridge cross-species molecular incompatibilities; in the last decade, most advances made in the field of xenotransplantation have resulted from the production of genetically engineered pigs; accordingly, we summarize the genetically modified pigs that are currently available for xenotransplantation. Next, we summarize the longest survival time of solid organs in preclinical models in recent years, including heart, liver, kidney, and lung xenotransplantation. Overall, we conclude that recent achievements and the accumulation of experience in xenotransplantation mean that the first-in-human clinical trial could be possible in the near future. Furthermore, we hope that xenotransplantation and various approaches will be able to collectively solve the problem of human organ shortage.

Consistent success in life-supporting porcine cardiac xenotransplantation

Heart transplantation is the only cure for patients with terminal cardiac failure, but the supply of allogeneic donor organs falls far short of the clinical need^1-3. Xenotransplantation of genetically modified pig hearts has been discussed as a potential alternative⁴. Genetically multi-modified pig hearts that lack galactose-α1,3-galactose epitopes (α1,3-galactosyltransferase knockout) and express a human membrane cofactor protein (CD46) and human thrombomodulin have survived for up to 945 days after heterotopic abdominal transplantation in baboons⁵. This model demonstrated long-term acceptance of discordant xenografts with safe immunosuppression but did not predict their life-supporting function. Despite 25 years of extensive research, the maximum survival of a baboon after heart replacement with a porcine xenograft was only 57 days and this was achieved, to our knowledge, only once⁶. Here we show that α1,3-galactosyltransferase-knockout pig hearts that express human CD46 and thrombomodulin require non-ischaemic preservation with continuous perfusion and control of post-transplantation growth to ensure long-term orthotopic function of the xenograft in baboons, the most stringent preclinical xenotransplantation model. Consistent life-supporting function of xenografted hearts for up to 195 days is a milestone on the way to clinical cardiac xenotransplantation⁷.

Brief Summary Report From the 14th Biennial Meeting of the International Xenotransplantation Association

The fourteenth meeting of the International Xenotransplantation Association (IXA) 2017, was held at University of Maryland, Baltimore, between September 19th and 23rd, 2017. This monograph summarizes reported progress in the field.

Porcine to Human Heart Transplantation: Is Clinical Application Now Appropriate?

Cardiac xenotransplantation (CXTx) is a promising solution to the chronic shortage of donor hearts. Recent advancements in immune suppression have greatly improved the survival of heterotopic CXTx, now extended beyond 2 years, and life-supporting kidney XTx. Advances in donor genetic modification (B4GALNT2 and CMAH mutations) with proven Gal-deficient donors expressing human complement regulatory protein(s) have also accelerated, reducing donor pig organ antigenicity. These advances can now be combined and tested in life-supporting orthotopic preclinical studies in nonhuman primates and immunologically appropriate models confirming their efficacy and safety for a clinical CXTx program. Preclinical studies should also allow for organ rejection to develop xenospecific assays and therapies to reverse rejection. The complexity of future clinical CXTx presents a substantial and unique set of regulatory challenges which must be addressed to avoid delay; however, dependent on these prospective life-supporting preclinical studies in NHPs, it appears that the scientific path forward is well defined and the era of clinical CXTx is approaching.