Why was PERV not transmitted during preclinical and clinical xenotransplantation trials and after inoculation of animals?

Monitoring for PERV Following Xenotransplantation

Porcine endogenous retroviruses (PERVs) are integrated in the genome of all pigs. PERV-A, PERV-B and PERV-C can be released as infectious virus particles and PERV-A and PERV-B can infect human cells in culture. PERV-C does not infect human cells, but high-titer recombinant PERV-A/C can infect them. Retroviruses are able to induce immunosuppression and/or tumors in the infected host. Numerous methods have been developed to study PERV in donor pigs. No PERV infections were observed in infection experiments as well as in preclinical and clinical xenotransplantation trials. Despite this, several strategies have been developed to prevent PERV infection of the recipient. PCR-based and immunological methods are required to screen xenotransplant recipients. Since the proviruses are integrated into the pig genome, PERV infection has to be distinguished from microchimerism, e.g., the presence of pig cells in the recipient, which is common in xenotransplantation. Sensitive PCR methods using pig short interspersed nuclear elements (SINE) sequences allow to detect pig cells easily. Virus infection can also be detected by an increase of viral genomic or mRNA in human cells. The method of choice, however, is to screen for specific antibodies against PERV using different recombinant PERV proteins, purified viruses or peptides.

How should cardiac xenotransplantation be initiated in Japan?

The world's first clinical cardiac xenotransplantation, using a genetically engineered pig heart with 10 gene modifications, prolonged the life of a 57-year-old man with no other life-saving options, by 60 days. It is foreseeable that xenotransplantation will be introduced in clinical practice in the United States. However, little clinical or regulatory progress has been made in the field of xenotransplantation in Japan in recent years. Japan seems to be heading toward a "device lag", and the over-importation of medical devices and technology in the medical field is becoming problematic. In this review, we discuss the concept of pig-heart xenotransplantation, including the pathobiological aspects related to immune rejection, coagulation dysregulation, and detrimental heart overgrowth, as well as genetic modification strategies in pigs to prevent or minimize these problems. Moreover, we summarize the necessity for and current status of xenotransplantation worldwide, and future prospects in Japan, with the aim of initiating xenotransplantation in Japan using genetically modified pigs without a global delay. It is imperative that this study prompts the initiation of preclinical xenotransplantation research using non-human primates and leads to clinical studies.

[Xenotransplantation of solid organs]

Transplantation of genetically modified porcine hearts and kidneys could become a solution to the persistent shortage of human organ donors. Progress has been made in genetic engineering of donor pigs, preservation techniques after organ harvesting and immunosuppression using co-stimulation blockade with anti-CD40/CD40L monoclonal antibodies. Progress has also been made in in the development of methods that detect pathogenic porcine viruses and prevent their transmission to the recipient. As normal land breed pig organs continue to grow in the recipient to their original size, different pig breeds (such as Auckland Island pigs) are now used which reach a final size suitable for humans. Alternatively, a knock-out of the growth hormone receptor gene has been established, e.g., in the 10GM genetically modified pigs from Revivicor/United Therapeutics, USA. The first clinical pilot studies including patients suffering from terminal heart failure are expected to start in Germany in about 2 years.

Physiological basis for xenotransplantation from genetically modified pigs to humans

The collective efforts of scientists over multiple decades have led to advancements in molecular and cellular biology-based technologies including genetic engineering and animal cloning that are now being harnessed to enhance the suitability of pig organs for xenotransplantation into humans. Using organs sourced from pigs with multiple gene deletions and human transgene insertions, investigators have overcome formidable immunological and physiological barriers in pig-to-nonhuman primate (NHP) xenotransplantation and achieved prolonged pig xenograft survival. These studies informed the design of Revivicor's (Revivicor Inc, Blacksburg, VA) genetically engineered pigs with 10 genetic modifications (10 GE) (including the inactivation of 4 endogenous porcine genes and insertion of 6 human transgenes), whose hearts and kidneys have now been studied in preclinical human xenotransplantation models with brain-dead recipients. Additionally, the first two clinical cases of pig-to-human heart xenotransplantation were recently performed with hearts from this 10 GE pig at the University of Maryland. Although this review focuses on xenotransplantation of hearts and kidneys, multiple organs, tissues, and cell types from genetically engineered pigs will provide much-needed therapeutic interventions in the future.

Enhancing Kidney Transplantation and the Role of Xenografts: Report of a Scientific Workshop Sponsored by the National Kidney Foundation

Chronic kidney disease affects an estimated 37 million people in the United States; of these,>800,000 have end-stage renal disease requiring chronic dialysis or a kidney transplant to survive. Despite efforts to increase the donor kidney supply, approximately 100,000 people are registered on the kidney transplant wait-list with no measurable decrease over the past 2 decades. The outcomes of kidney transplantation are significantly better than for chronic dialysis: kidney transplant recipients have lower rates of mortality and cardiovascular events and better quality of life, but wait-list time matters. Time on dialysis waiting for a deceased-donor kidney is a strong independent risk factor for outcomes after a kidney transplant. Deceased-donor recipients with wait-list times on dialysis of<6 months have graft survival rates equivalent to living-donor recipients with waitlist times on dialysis of>2 years. In 2021,>12,000 people had been on the kidney transplant waitlist for ≥5 years. As the gap between the demand for and availability of donor kidneys for allotransplantation continues to widen, alternative strategies are needed to provide a stable, sufficient, and timely supply. A strategy that is gaining momentum toward clinical application is pig-to-human kidney xenotransplantation. This report summarizes the proceedings of a meeting convened on April 11-12, 2022, by the National Kidney Foundation to review and assess the state of pig-to-human kidney xenotransplantation as a potential cure for end-stage renal disease.

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.

Pigs in Transplantation Research and Their Potential as Sources of Organs in Clinical Xenotransplantation

The pig has long been used as a research animal and has now gained importance as a potential source of organs for clinical xenotransplantation. When an organ from a wild-type (i. e., genetically unmodified) pig is transplanted into an immunosuppressed nonhuman primate, a vigorous host immune response causes hyperacute rejection (within minutes or hours). This response has been largely overcome by 1) extensive gene editing of the organ-source pig and 2) the administration to the recipient of novel immunosuppressive therapy based on blockade of the CD40/CD154 T cell costimulation pathway. Gene editing has consisted of 1) deletion of expression of the 3 known carbohydrate xenoantigens against which humans have natural (preformed) antibodies and 2) the introduction of human 'protective' genes. The combination of gene editing and novel immunosuppressive therapy has extended life-supporting pig kidney graft survival to greater than 1 y and of pig heart survival to up to 9 mo. This review briefly describes the techniques of gene editing, the potential risks of transfer of porcine endogenous retroviruses with the organ, and the need for breeding and housing of donor pigs under biosecure conditions.

Advancing kidney xenotransplantation with anesthesia and surgery - bridging preclinical and clinical frontiers challenges and prospects

Xenotransplantation is emerging as a vital solution to the critical shortage of organs available for transplantation, significantly propelled by advancements in genetic engineering and the development of sophisticated immunosuppressive treatments. Specifically, the transplantation of kidneys from genetically engineered pigs into human patients has made significant progress, offering a potential clinical solution to the shortage of human kidney supply. Recent trials involving the transplantation of these modified porcine kidneys into deceased human bodies have underscored the practicality of this approach, advancing the field towards potential clinical applications. However, numerous challenges remain, especially in the domains of identifying suitable donor-recipient matches and formulating effective immunosuppressive protocols crucial for transplant success. Critical to advancing xenotransplantation into clinical settings are the nuanced considerations of anesthesia and surgical practices required for these complex procedures. The precise genetic modification of porcine kidneys marks a significant leap in addressing the biological and immunological hurdles that have traditionally challenged xenotransplantation. Yet, the success of these transplants hinges on the process of meticulously matching these organs with human recipients, which demands thorough understanding of immunological compatibility, the risk of organ rejection, and the prevention of zoonotic disease transmission. In parallel, the development and optimization of immunosuppressive protocols are imperative to mitigate rejection risks while minimizing side effects, necessitating innovative approaches in both pharmacology and clinical practices. Furthermore, the post-operative care of recipients, encompassing vigilant monitoring for signs of organ rejection, infectious disease surveillance, and psychological support, is crucial for ensuring post-transplant life quality. This comprehensive care highlights the importance of a multidisciplinary approach involving transplant surgeons, anesthesiologists, immunologists, infectiologists and psychiatrists. The integration of anesthesia and surgical expertise is particularly vital, ensuring the best possible outcomes of those patients undergoing these novel transplants, through safe procedural practices. As xenotransplantation moving closer to clinical reality, establishing consensus guidelines on various aspects, including donor-recipient selection, immunosuppression, as well as surgical and anesthetic management of these transplants, is essential. Addressing these challenges through rigorous research and collective collaboration will be the key, not only to navigate the ethical, medical, and logistical complexities of introducing kidney xenotransplantation into mainstream clinical practice, but also itself marks a new era in organ transplantation.

Revisiting the Use of Ulysses Contracts in Xenotransplantation

BACKGROUND

Xenotransplantation clinical trials may begin soon. A persistent risk of xenotransplantation, known for decades, is the possibility that a xenozoonotic infection could be transferred from a xenograft to its recipient and then to other human contacts. Because of this risk, guidelines and commentators have advocated for xenograft recipients to agree to either long-term or lifelong surveillance mechanisms.

METHODS

For the past few decades, one solution that has been proposed to ensure that xenograft recipients will comply with surveillance protocols is the use of a heavily modified Ulysses contract, which we review.

RESULTS

These contracts are most often used in psychiatry, and their application to xenotransplantation has been espoused several times with minimal criticism.

CONCLUSIONS

In this article, we argue against the applicability of Ulysses contracts in xenotransplantation based upon (1) the telos of the advance directive that may not be applicable to this clinical context, (2) the suspect nature of enforcing Ulysses contracts in xenotransplantation, and (3) the ethical and regulatory hurdles that such enforcement would require. Although our focus is on the US regulatory landscape in preparation for clinical trials, there are applications globally.

Mitigating Cross-Species Viral Infections in Xenotransplantation: Progress, Strategies, and Clinical Outlook

Xenotransplantation holds great promise as a solution to address the critical shortage of organs, but it raises concerns regarding the potential transmission of porcine viruses to recipients, leading to infections and even zoonotic diseases. Data used in this review were mainly from literature of Pubmed database. Keywords included xenotransplantation, infection, virus, and epidemiology. The original articles and critical reviews selected were relevant to this review's theme. We review the major viral infections of concern in xenotransplantation, their risk of transmission, diagnosis, treatment, and ways to prevent infection. Then, we pivot to a comprehensive overview of the current status of xenotransplantation. In addition, we offer our own insights and recommendations for propelling xenotransplantation forward, transitioning from preclinical experiments to the critical phase of clinical trials. Viral infections pose considerable safety concerns within xenotransplantation, particularly with the possibility of emerging or currently unidentified viruses. Clinical trials serve as a crucial platform to progress the safety standards of xenotransplantation. However, further studies and dedicated efforts are required to effectively translate findings into practical applications that can improve safety measures in this field.

Evidence for Microchimerism in Baboon Recipients of Pig Hearts

Xenotransplantation, like allotransplantation, is usually associated with microchimerism, i.e., the presence of cells from the donor in the recipient. Microchimerism was reported in first xenotransplantation trials in humans, as well as in most preclinical trials in nonhuman primates (for review, see Denner, Viruses 2023, 15, 190). When using pigs as xenotransplantation donors, their cells contain porcine endogenous retroviruses (PERVs) in their genome. This makes it difficult to discriminate between microchimerism and PERV infection of the recipient. Here, we demonstrate the appropriate virological methods to be used for the identification of microchimerism, first by screening for porcine cellular genes, and then how to detect infection of the host. Using porcine short interspersed nuclear sequences (SINEs), which have hundreds of thousands of copies in the pig genome, significantly increased the sensitivity of the screening for pig cells. Second, absence of PERV RNA demonstrated an absence of viral genomic RNA or expression as mRNA. Lastly, absence of antibodies against PERV proteins conclusively demonstrated an absence of a PERV infection. When applying these methods for analyzing baboons after pig heart transplantation, microchimerism could be demonstrated and infection excluded in all animals. These methods can be used in future clinical trials.

Kidney xenotransplantation: Future clinical reality or science fiction?

There is a global shortage of organs for transplantation and despite many governments making significant changes to their organ donation systems, there are not enough kidneys available to meet the demand. This has led scientists and clinicians to explore alternative means of meeting this organ shortfall. One of the alternatives to human organ transplantation is xenotransplantation, which is the transplantation of organs, tissues, or cells between different species. The resurgence of interest in xenotransplantation and recent scientific breakthroughs suggest that genetically engineered pigs may soon present a realistic alternative as sources of kidneys for clinical transplantation. It is therefore important for healthcare professionals to understand what is involved in xenotransplantation and its future implications for their clinical practices. First, we explore the insufficiency of different organ donation systems to meet the kidney shortage. Second, we provide a background and a summary of the progress made so far in xenotransplantation research. Third, we discuss some of the scientific, technological, ethical, and public health issues associated with xenotransplantation. Finally, we summarize the literature on the attitudes of healthcare professionals toward xenotransplantation.

Milestones on the path to clinical pig organ xenotransplantation

Progress in pig organ xenotransplantation has been made largely through (1) genetic engineering of the organ-source pig to protect its tissues from the human innate immune response, and (2) development of an immunosuppressive regimen based on blockade of the CD40/CD154 costimulation pathway to prevent the adaptive immune response. In the 1980s, after transplantation into nonhuman primates (NHPs), wild-type (genetically unmodified) pig organs were rejected within minutes or hours. In the 1990s, organs from pigs expressing a human complement-regulatory protein (CD55) transplanted into NHPs receiving intensive conventional immunosuppressive therapy functioned for days or weeks. When costimulation blockade was introduced in 2000, the adaptive immune response was suppressed more readily. The identification of galactose-α1,3-galactose as the major antigen target for human and NHP anti-pig antibodies in 1991 allowed for deletion of expression of galactose-α1,3-galactose in 2003, extending pig graft survival for up to 6 months. Subsequent gene editing to overcome molecular incompatibilities between the pig and primate coagulation systems proved additionally beneficial. The identification of 2 further pig carbohydrate xenoantigens allowed the production of 'triple-knockout' pigs that are preferred for clinical organ transplantation. These combined advances enabled the first clinical pig heart transplant to be performed and opened the door to formal clinical trials.

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.

Microchimerism, PERV and Xenotransplantation

Microchimerism is the presence of cells in an individual that have originated from a genetically distinct individual. The most common form of microchimerism is fetomaternal microchimerism, i.e., cells from a fetus pass through the placenta and establish cell lineages within the mother. Microchimerism was also described after the transplantation of human organs in human recipients. Consequently, microchimerism may also be expected in xenotransplantation using pig cells or organs. Indeed, microchimerism was described in patients after xenotransplantations as well as in non-human primates after the transplantation of pig organs. Here, for the first time, a comprehensive review of microchimerism in xenotransplantation is given. Since pig cells contain porcine endogenous retroviruses (PERVs) in their genome, the detection of proviral DNA in transplant recipients may be misinterpreted as an infection of the recipient with PERV. To prevent this, methods discriminating between infection and microchimerism are described. This knowledge will be important for the interpretation of screening results in forthcoming human xenotransplantations.

The Influence of Interdisciplinary Work towards Advancing Knowledge on Human Liver Physiology

The knowledge accumulated throughout the years about liver regeneration has allowed a better understanding of normal liver physiology, by reconstructing the sequence of steps that this organ follows when it must rebuild itself after being injured. The scientific community has used several interdisciplinary approaches searching to improve liver regeneration and, therefore, human health. Here, we provide a brief history of the milestones that have advanced liver surgery, and review some of the new insights offered by the interdisciplinary work using animals, in vitro models, tissue engineering, or mathematical models to help advance the knowledge on liver regeneration. We also present several of the main approaches currently available aiming at providing liver support and overcoming organ shortage and we conclude with some of the challenges found in clinical practice and the ethical issues that have concomitantly emerged with the use of those approaches.

Progress in xenotransplantation: overcoming immune barriers

A major limitation of organ allotransplantation is the insufficient supply of donor organs. Consequently, thousands of patients die every year while waiting for a transplant. Progress in xenotransplantation that has permitted pig organ graft survivals of years in non-human primates has led to renewed excitement about the potential of this approach to alleviate the organ shortage. In 2022, the first pig-to-human heart transplant was performed on a compassionate use basis, and xenotransplantation experiments using pig kidneys in deceased human recipients provided encouraging data. Many advances in xenotransplantation have resulted from improvements in the ability to genetically modify pigs using CRISPR-Cas9 and other methodologies. Gene editing has the capacity to generate pig organs that more closely resemble those of humans and are hence more physiologically compatible and less prone to rejection. Despite such modifications, immune responses to xenografts remain powerful and multi-faceted, involving innate immune components that do not attack allografts. Thus, the induction of innate and adaptive immune tolerance to prevent rejection while preserving the capacity of the immune system to protect the recipient and the graft from infection is desirable to enable clinical xenotransplantation.

CRISPR/Cas-edited pigs for personalized medicine: more than preclinical test-system

Novel CRISPR-Cas-based genome editing tools made it feasible to introduce a variety of precise genomic modifications in the pig genome, including introducing multiple edits simultaneously, inserting long DNA sequences into specifically targeted loci, and performing nucleotide transitions and transversions. Pigs serve as a vital agricultural resource and animal model in biomedical studies, given their advantages over the other models. Pigs share high similarities to humans regarding body/organ size, anatomy, physiology, and a metabolic profile. The pig genome can be modified to carry the same genetic mutations found in humans to replicate inherited diseases to provide preclinical trials of drugs. Moreover, CRISPR-based modification of pigs antigen profile makes it possible to offer porcine organs for xenotransplantation with minimal transplant rejection responses. This review summarizes recent advances in endonuclease-mediated genome editing tools and research progress of genome-edited pigs as personalized test-systems for preclinical trials and as donors of organs with human-fit antigen profile.

Graphical abstract:

Genetically Engineered Porcine Organs for Human Xenotransplantation

Xenotransplantation holds a promising future for many patients, especially those with end-stage renal disease or uncontrollable serum glucose levels. Porcine organs are viewed as the perfect candidate for a source of xenografts. However, the recipient's immunity, incompatibility of biologic systems, and transfer of new pathogenic organisms are all obstacles to clinical xenotransplantation, in addition to the risk of zoonosis and xenoantigens. Genetic modification of pigs using clustered regularly interspaced short palindromic repeat (CRISPR)-CRISPR-associated protein 9 (Cas9) resulted in the production of porcine endogenous retrovirus (PERV)-free offsprings with the consequent removal of many clinical complications post-transplantation. Such as minimizing both acute and chronic inflammation, in addition to suppressing rejection reactions, which may prolong graft survival. To build on these recent successes, it is important to look at the limits of genetic engineering and develop ways to advance the field of xenotransplantation and reverse xenotransplantation clinical applications forward. Still, significant problems remain with clinical human xenotransplantation; future work should focus on developing an ideal genetically engineered swine donor source that can improve long-term graft survival and suppress the immune system in a clinically useful way.

Informed Consent for Potential Recipients of Pig Kidney Xenotransplantation in the United States

Clinical trials of kidney xenotransplantation are being considered in the United States. Before this novel procedure can take place, investigators will have to obtain approval from the institutional review board. The consent form that will be used for such a trial and that will receive approval from the institutional review board will be complex. Informed consent-the process by which a research participant provides his/her permission to participate in a clinical trial-is a staple of the research process and most commonly is in the form of a physical document. In the case of a novel procedure with uncertain benefits and risks and a participant population in acute need of a transplant, the consent process is crucial. These complexities may raise several ethical considerations for the initial pig kidney xenotransplantation recipients in the United States that will require adaptations of the required elements of the informed consent process by the US Department of Human and Health Services. The ethical issues include (1) a subject's ability to withdraw from the trial, (2) restrictions on their reproductive rights, and (3) the possibility of the need for quarantine if there is a perceived risk of xenozoonosis. This article aims to discuss ethical considerations that may challenge the general required elements of the informed consent form stipulated by the 45 Code of Federal Regulations 46 of the US Department of Health and Human Services and to suggest recommendations for deliberation.

Virus Safety of Xenotransplantation

The practice of xenotransplantation using pig islet cells or organs is under development to alleviate the shortage of human donor islet cells or organs for the treatment of diabetes or organ failure. Multiple genetically modified pigs were generated to prevent rejection. Xenotransplantation may be associated with the transmission of potentially zoonotic porcine viruses. In order to prevent this, we developed highly sensitive PCR-based, immunologicals and other methods for the detection of numerous xenotransplantation-relevant viruses. These methods were used for the screening of donor pigs and xenotransplant recipients. Of special interest are the porcine endogenous retroviruses (PERVs) that are integrated in the genome of all pigs, which are able to infect human cells, and that cannot be eliminated by methods that other viruses can. We showed, using droplet digital PCR, that the number of PERV proviruses is different in different pigs (usually around 60). Furthermore, the copy number is different in different organs of a single pig, indicating that PERVs are active in the living animals. We showed that in the first clinical trials treating diabetic patients with pig islet cells, no porcine viruses were transmitted. However, in preclinical trials transplanting pig hearts orthotopically into baboons, porcine cytomegalovirus (PCMV), a porcine roseolovirus (PCMV/PRV), and porcine circovirus 3 (PCV3), but no PERVs, were transmitted. PCMV/PRV transmission resulted in a significant reduction of the survival time of the xenotransplant. PCMV/PRV was also transmitted in the first pig heart transplantation to a human patient and possibly contributed to the death of the patient. Transmission means that the virus was detected in the recipient, however it remains unclear whether it can infect primate cells, including human cells. We showed previously that PCMV/PRV can be eliminated from donor pigs by early weaning. PERVs were also not transmitted by inoculation of human cell-adapted PERV into small animals, rhesus monkey, baboons and cynomolgus monkeys, even when pharmaceutical immunosuppression was applied. Since PERVs were not transmitted in clinical, preclinical, or infection experiments, it remains unclear whether they should be inactivated in the pig genome by CRISPR/Cas. In summary, by using our sensitive methods, the safety of xenotransplantation can be ensured.

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 potential of genetically engineered pig heart transplantation in infants with complex congenital heart disease

Despite advances in surgical and medical techniques, complex congenital heart disease in neonates and infants continues to be associated with significant mortality and morbidity. More than 500 infants in the USA are placed on the cardiac transplantation wait-list annually. However, there remains a critical shortage of deceased human donor organs for transplantation with a median wait-time of 4 months. Hence, infant mortality on the heart transplant wait-list in the USA is higher than for any other solid organ transplant group. Orthotopic transplantation of a pig heart as a bridge to allotransplantation might offer the best prospect of long-term survival of these patients. In recent years, there have been several advances in genetic engineering of pigs to mitigate the vigorous antibody-mediated rejection of a pig heart transplanted into a nonhuman primate. In this review, we briefly highlight (i) the history of clinical heart xenotransplantation, (ii) current advances and techniques of genetically engineering pigs, (iii) the current status of pig orthotopic cardiac graft survival in nonhuman primates, and (iv) progress toward pursuing clinical trials of cardiac xenotransplantation. Ultimately, we argue that pig heart xenotransplantation should initially be used as a bridge to cardiac allotransplantation in neonates and infants.

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.

Scan of the endogenous retrovirus sequences across the swine genome and survey of their copy number variation and sequence diversity among various Chinese and Western pig breeds

In pig-to-human xenotransplantation, the transmission risk of porcine endogenous retroviruses (PERVs) is of great concern. However, the distribution of PERVs in pig genomes, their genetic variation among Eurasian pigs, and their evolutionary history remain unclear. We scanned PERVs in the current pig reference genome (assembly Build 11.1), and identified 36 long complete or near-complete PERVs (lcPERVs) and 23 short incomplete PERVs (siPERVs). Besides three known PERVs (PERV-A, -B, and -C), four novel types (PERV-JX1, -JX2, -JX3, and -JX4) were detected in this study. According to evolutionary analyses, the newly discovered PERVs were more ancient, and PERV-Bs probably experienced a bottleneck ~0.5 million years ago (Ma). By analyzing 63 high-quality porcine whole-genome resequencing data, we found that the PERV copy numbers in Chinese pigs were lower (32.0±4.0) than in Western pigs (49.1±6.5). Additionally, the PERV sequence diversity was lower in Chinese pigs than in Western pigs. Regarding the lcPERV copy numbers, PERV-A and -JX2 in Western pigs were higher than in Chinese pigs. Notably, Bama Xiang (BMX) pigs had the lowest PERV copy number (27.8±5.1), and a BMX individual had no PERV-C and the lowest PERV copy number (23), suggesting that BMX pigs were more suitable for screening and/or modification as xenograft donors. Furthermore, we identified 451 PERV transposon insertion polymorphisms (TIPs), of which 86 were shared by all 10 Chinese and Western pig breeds. Our findings provide systematic insights into the genomic distribution, variation, evolution, and possible biological function of PERVs.

Clinical xenotransplantation seems close: Ethical issues persist

Scientific barriers that have prevented successful xenotransplantation are being breached, yet many ethical issues remain. Some are broad issues that accompany the adoption of novel and expensive technologies, and some are unique to xenotransplantation. Major ethical questions include areas such as: viral transmission; zoonoses and lifetime surveillance; interfering with nature; efficacy, access, and expense; treatment of animals; regulation and oversight.

First clinical-grade porcine kidney xenotransplant using a human decedent model

A radical solution is needed for the organ supply crisis, and the domestic pig is a promising organ source. In preparation for a clinical trial of xenotransplantation, we developed an in vivo pre-clinical human model to test safety and feasibility tenets established in animal models. After performance of a novel, prospective compatible crossmatch, we performed bilateral native nephrectomies in a human brain-dead decedent and subsequently transplanted two kidneys from a pig genetically engineered for human xenotransplantation. The decedent was hemodynamically stable through reperfusion, and vascular integrity was maintained despite the exposure of the xenografts to human blood pressure. No hyperacute rejection was observed, and the kidneys remained viable until termination 74 h later. No chimerism or transmission of porcine retroviruses was detected. Longitudinal biopsies revealed thrombotic microangiopathy that did not progress in severity, without evidence of cellular rejection or deposition of antibody or complement proteins. Although the xenografts produced variable amounts of urine, creatinine clearance did not recover. Whether renal recovery was impacted by the milieu of brain death and/or microvascular injury remains unknown. In summary, our study suggests that major barriers to human xenotransplantation have been surmounted and identifies where new knowledge is needed to optimize xenotransplantation outcomes in humans.

The 2021 IXA Keith Reemtsma Lecture: Moving xenotransplantation to the clinic

Keith Reemtsma was a pioneer in xenotransplantation, the Honorary Founding President of the International Xenotransplantation Association (in 1998), and a wonderful personality. It is a privilege to be invited to give this lecture in his memory. If he were alive today, he would be delighted to see the progress that has been made in pig organ transplantation into nonhuman primate recipients. This progress has largely resulted from two major advances: (i) the increasing availability of pigs with multiple genetic manipulations aimed at protecting the cells of the organ from the primate immune response and (ii) the introduction of novel immunosuppressive agents that block the CD40/CD154 costimulation pathway. There is strong evidence from numerous in vitro studies that the transplantation of a triple-knockout pig organ, particularly if expressing several human protective proteins, into a patient is likely to be significantly more successful than if that same organ is transplanted into a nonhuman primate recipient. With this fact in mind, and in view of the advances currently being made, the time has surely come when we need to consider moving from the laboratory to the clinic. However, there are still questions we need to definitively resolve: (i) What exact genetic modifications do we need in the organ-source pig? (ii) What exact immunosuppressive regimen will we choose? (iii) How will we monitor the immune response and diagnose and treat rejection? and (iv) How do we plan to prevent or treat potential infectious complications? Furthermore, when these matters have been resolved, which patients will be offered a pig organ in the first trial? We have suggested that patients who are very unlikely to survive until a suitable deceased human donor kidney becomes available are those who should be considered for the initial trials. Assessing public attitudes to xenotransplantation is also important before embarking on a clinical trial. I suggest that progress is much more likely to be made from a small clinical trial than if we persist in carrying out experiments in an animal model that no longer mimics the clinical situation.

Screening and Identification of the First Non-CRISPR/Cas9-Treated Chinese Miniature Pig With Defective Porcine Endogenous Retrovirus pol Genes

Pig to human xenotransplantation is considered to be a possible approach to alleviate the shortage of human allografts. Porcine endogenous retrovirus (PERV) is the most significant pathogen in xenotransplantation. We screened for pigs that consistently did not transmit human-tropic replication competent PERVs (HTRC PERVs), namely, non-transmitting pigs. Then, we conducted whole-genome resequencing and full-length transcriptome sequencing to further investigate the sequence characteristics of one non-transmitting pig. Using in vitro transmission assays, we found 5 (out of 105) pigs of the Chinese Wuzhishan minipig inbred line that did not transmit PERV to human cells, i.e., non-transmitting pigs. Whole-genome resequencing and full-length transcriptome sequencing of one non-transmitting pig showed that all of the pol genes were defective at both the genome and transcript levels. We speculate that the defective PERV pol genes in this pig might be attributable to the long-term inbreeding process. This discovery is promising for the development of a strain of highly homozygous and genetically stable pigs with defective PERV pol genes as a source animal species for xenotransplantation.

Current Barriers to Clinical Liver Xenotransplantation

Preclinical trials of pig-to-nonhuman primate liver xenotransplantation have recently achieved longer survival times. However, life-threatening thrombocytopenia and coagulation dysregulation continue to limit preclinical liver xenograft survival times to less than one month despite various genetic modifications in pigs and intensive pharmacological support. Transfusion of human coagulation factors and complex immunosuppressive regimens have resulted in substantial improvements in recipient survival. The fundamental biological mechanisms of thrombocytopenia and coagulation dysregulation remain incompletely understood. Current studies demonstrate that porcine von Willebrand Factor binds more tightly to human platelet GPIb receptors due to increased O-linked glycosylation, resulting in increased human platelet activation. Porcine liver sinusoidal endothelial cells and Kupffer cells phagocytose human platelets in an asialoglycoprotein receptor 1-dependent and CD40/CD154-dependent manner, respectively. Porcine Kupffer cells phagocytose human platelets via a species-incompatible SIRPα/CD47 axis. Key drivers of coagulation dysregulation include constitutive activation of the extrinsic clotting cascade due to failure of porcine tissue factor pathway inhibitor to repress recipient tissue factor. Additionally, porcine thrombomodulin fails to activate human protein C when bound by human thrombin, leading to a hypercoagulable state. Combined genetic modification of these key genes may mitigate liver xenotransplantation-induced thrombocytopenia and coagulation dysregulation, leading to greater recipient survival in pig-to-nonhuman primate liver xenotransplantation and, potentially, the first pig-to-human clinical trial.

Porcine Endogenous Retroviruses and Xenotransplantation, 2021

Porcine endogenous retroviruses (PERVs) are integrated in the genome of all pigs, and some of them are able to infect human cells. Therefore, PERVs pose a risk for xenotransplantation, the transplantation of pig cells, tissues, or organ to humans in order to alleviate the shortage of human donor organs. Up to 2021, a huge body of knowledge about PERVs has been accumulated regarding their biology, including replication, recombination, origin, host range, and immunosuppressive properties. Until now, no PERV transmission has been observed in clinical trials transplanting pig islet cells into diabetic humans, in preclinical trials transplanting pig cells and organs into nonhuman primates with remarkable long survival times of the transplant, and in infection experiments with several animal species. Nevertheless, in order to prevent virus transmission to the recipient, numerous strategies have been developed, including selection of PERV-C-free animals, RNA interference, antiviral drugs, vaccination, and genome editing. Furthermore, at present there are no more experimental approaches to evaluate the full risk until we move to the clinic.

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.

"You cannot stay in the laboratory forever"*: Taking pig kidney xenotransplantation from the laboratory to the clinic

Progress in life-supporting kidney transplantation in the genetically-engineered pig-to-nonhuman primate model has been encouraging, with pig kidneys sometimes supporting life for > 1 year. What steps need to be taken by (i) the laboratory team, and (ii) the clinical team to prepare for the first clinical trial? The major topics include (i) what currently-available genetic modifications are optimal to reduce the possibility of graft rejection, (ii) what immunosuppressive therapeutic regimen is optimal, and (iii) what steps need to be taken to minimize the risk of transfer of an infectious microorganism with the graft. We suggest that patients who are unlikely to live long enough to receive a kidney from a deceased human donor would benefit from the opportunity of a period of dialysis-free support by a pig kidney, and the experience gained would enable xenotransplantation to progress much more rapidly than if we remain in the laboratory.

Xenotransplantation of neonatal porcine bone marrow-derived mesenchymal stem cells improves murine hind limb ischemia through lymphangiogenesis and angiogenesis

BACKGROUND

The clinical utility of stem cell therapy for peripheral artery disease has not been fully discussed, and one obstacle is limited donor supplies. In this study, we attempted to rescue mouse ischemic hind limb by xenotransplantation of neonatal porcine bone marrow-derived mesenchymal stem cells (npBM-MSCs).

METHODS

Neonatal porcine bone marrow-derived mesenchymal stem cells were transplanted to ischemic hind limbs of male C57BL/6J mice (npBM-MSCs group). Mice with syngeneic transplantation of mouse BM-MSCs (mBM-MSCs group) were also prepared for comparison. The angiogenic effects were evaluated by recovery of blood flow on laser Doppler imaging, histologic findings, and genetic and protein levels of angiogenic factors.

RESULTS

Regarding laser Doppler assessments, blood flow in the hind limb was rapidly recovered in the npBM-MSCs group, compared with that in the mBM-MSCs group (P = .016). Compared with the mBM-MSCs group, the npBM-MSCs group had early and prominent lymphangiogenesis [P < .05 on both post-operative days (PODs) 3 and 7] but had similar angiogenesis. Regarding genomic assessments, xenotransplantation of npBM-MSCs enhanced the expressions of both porcine and murine Vegfc in the hind limbs by POD 3. Interestingly, the level of murine Vegfc expression was significantly higher in the npBM-MSCs group than in the mBM-MSCs group on PODs 3 and 7 (P < .001 for both). Furthermore, the secreted VEGFC protein level was higher from npBM-MSCs than from mBM-MSCs (P < .001).

CONCLUSION

Xenotransplantation of npBM-MSCs contributed to the improvement of hind limb ischemia by both angiogenesis and lymphangiogenesis, especially promotion of the latter. npBM-MSCs may provide an alternative to autologous and allogeneic MSCs for stem cell therapy of critical limb ischemia.

CRISPR/Cas Technology in Pig-to-Human Xenotransplantation Research

CRISPR/Cas (clustered regularly interspaced short palindromic repeats linked to Cas nuclease) technology has revolutionized many aspects of genetic engineering research. Thanks to it, it became possible to study the functions and mechanisms of biology with greater precision, as well as to obtain genetically modified organisms, both prokaryotic and eukaryotic. The changes introduced by the CRISPR/Cas system are based on the repair paths of the single or double strand DNA breaks that cause insertions, deletions, or precise integrations of donor DNA. These changes are crucial for many fields of science, one of which is the use of animals (pigs) as a reservoir of tissues and organs for xenotransplantation into humans. Non-genetically modified animals cannot be used to save human life and health due to acute immunological reactions resulting from the phylogenetic distance of these two species. This review is intended to collect and summarize the advantages as well as achievements of the CRISPR/Cas system in pig-to-human xenotransplantation research. In addition, it demonstrates barriers and limitations that require careful evaluation before attempting to experiment with this technology.

Clinical islet transplantation: Current progress and new frontiers

Islet transplantation (IT) is now a robust treatment for selected patients with type 1 diabetes suffering from recurrent hypoglycemia and impaired awareness of hypoglycemia. A global soar of clinical islet transplant programs attests to the commitment of many institutions and researchers to advance IT as a potential cure for this devastating disease. However, many challenges limiting the widespread applicability of clinical IT remain. In this review, we will touch on the milestones in the history of IT and its path to clinical success, discuss the current challenges around IT, propose some possible solutions, and elaborate on the frontiers envisioned in the future of clinical IT.

Attitudes toward xenotransplantation: A survey of parents and pediatric cardiac providers

BACKGROUND

Scientific advancements are occurring in cardiac xenotransplantation (XTx). However, there have been religious and social concerns surrounding this allotransplantation alternative. The purpose of this study was to explore the acceptance of XTx among stakeholders of the congenital heart disease (CHD) community.

METHODS

A Likert-scale anonymous survey was distributed to physicians and nurses who care for children with CHD and parents of children with CHD. Psychosocial and clinical attitudes were compared across all groups to identify differences, and regression analysis was performed to identify factors associated with XTx acceptance.

RESULTS

A total of 297 responded to the survey: 134 physicians, 62 nurses, and 101 parents. Potential acceptance of XTx if outcomes were similar to allotransplantation was high overall (75.3%), but different between the groups (physicians 86%; nurses 71%, parents 64%; P < .0001). Regression analysis showed respondents who reported religion would influence medical decision making (OR 0.48; 95%CI 0.24-0.97) and those who would not use a pig heart transplant as a bridge until a human heart became available were less likely to accept XTx (OR 0.09; 95%CI 0.04-0.21). Psychosocial concerns to XTx were minimal but were also associated with XTx acceptance particularly among parents (OR 0.17; 95%CI 0.03-0.80).

CONCLUSIONS

Potential acceptance of XTx is high, assuming results are similar to allotransplantation. Religious beliefs and attitudes toward the use of XTx as a bridge to allotransplant may present barriers to XTx acceptance. Future research is needed to assess potential attitude differences in light of ethical, psychosocial, and religious objections to XTx.

The origin of porcine endogenous retroviruses (PERVs)

Porcine endogenous retroviruses (PERVs) are integrated in the genome of all pigs, and they produce viral particles that are able to infect human cells and therefore pose a special risk for xenotransplantation. In contrast to other pig microorganisms that also pose a risk, such as porcine cytomegalovirus and hepatitis E virus, PERVs cannot be eliminated from pigs by vaccines, antiviral drugs, early weaning, or embryo transfer. Since PERVs are relevant for xenotransplantation, their biology and origin are of great interest. Recent studies have shown that PERVs are the result of a transspecies transmission of precursor retroviruses from different animals and further evolution in the pig genome. PERVs acquired different long terminal repeats (LTRs), and recombination took place. In parallel, it has been shown that the activity of the LTRs and recombination in the envelope are important for the transmissibility and pathogenesis of PERVs. Transspecies transmission of retroviruses is common, a well-known example being the transmission of precursor retroviruses from non-human primates to humans, resulting in human immunodeficiency virus (HIV). Here, recent findings concerning the origin of PERVs, their LTRs, and recombination events that occurred during evolution are reviewed and compared with other findings regarding transspecies transmission of retroviruses.

Kidney xenotransplantation: Recent progress in preclinical research

Kidney transplantation is the most effective treatment for end-stage renal disease, but is limited by the increasing shortage of deceased and living human donor kidneys. Xenotransplantation using pig organs provides the possibility to resolve the issue of organ supply shortage and is regarded as the next great medical revolution. In the past five years, there have been sequential advances toward the prolongation of life-supporting pig kidney xenograft survival in non-human primates, with the longest survival being 499 days. This progress is due to the growing availability of pigs with multi-layered genetic modifications to overcome the pathobiological barriers and the application of a costimulation blockade-based immunosuppressive regimen. These encouraging results bring the hope to initiate the clinical trials of pig kidney transplantation in the near future. In this review, we summarized the latest advances regarding pig kidney xenotransplantation in preclinical models to provide a basis for future investigation and potential clinical translation.

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.

Expression profile of Tripartite motif family (TRIM) in human fibroblasts (NHDF) infected with porcine endogenous retrovirus (PERV)

BACKGROUND

Understanding the interactions between the microRNA (miRNA) and mRNA of genes encoding restriction factors (RFs) can lead to the development of new antiretroviral strategies aimed at providing the resistance and reducing susceptibility of human cells to potential PERV infection. Among RFs TRIM family play an important role in shaping the immune response during various stages of infection. The aim of the study was to evaluate in vitro the transcriptional profile of TRIM family genes and identify complementary miRNAs in NHDF cells infected with PERVs and induced by gram negative lipopolysacharide (LPS).

METHODS

Human dermal fibroblasts cells were cultured in four separate conditions- 2 monocultures: control (N), treated with LPS (NL) and 2 co-cultures with porcine PK15 cells: without LPS (NP) and treated with LPS (NLP). Bacterial LPS was used in this study as an inducer of inflammation in NHDF cells. After extraction of DNA and RNA from cells PERV infection was confirmed in co-cultures by qPCR and RTqPCR. RNA extracts served as a matrix for HGU 133A 2.0 and miRNA 2.0 microarrays to evaluate the expression profile of the selected TRIM family genes and miRNAs adequately. TRIM 2, 14, 22, and 28 were selected for the validation of HGU 133A 2.0 results. Statistical analyses were performed with the use of REST^© 2009 and Genespring GX 13.0. Transcriptome Analysis Console 4.0 program (Affymetrix) was used to identify miRNAs that differentiate the studied genes in all conditions.

RESULTS

Porcine endogenous retrovirus infection at DNA and RNA level was confirmed in NHDF cells in each of the tested groups (NP and NLP). Contamination was excluded in N and NL groups. Based on the analysis of HGU 133A 2.0 results 93 mRNA IDs of TRIM family genes differentiating analyzed conditions were selected P < .05. HGU 133A 2.0 mRNA fluorescence profile was confirmed with RTqPCR of TRIM2, TRIM14, TRIM22 and TRIM28 P < .05. TRIM14 down regulation was specific only in PERV monoinfection (group NP). In miRNA 2.0 microarray 346 miRNAs were identified as differentiating NHDF cells in all analyzed conditions, P < .05. According to the analysis with mirTAR platform and Microrna.org datatbase none of the selected miRNAs had the potential to regulate the selected genes of the TRIM family.

CONCLUSION

Porcine endogenous retrovirus infection of NHDF cells is accompanied by TRIM14 down regulation suggesting TRIM14 as a possible marker of retroviral infection. None of the selected miRNAs had statistically significant potential to regulate the expression of the selected TRIMs in any of the analyzed conditions.

The resurgent landscape of xenotransplantation of pig organs in nonhuman primates

Organ shortage is a major bottleneck in allotransplantation and causes many wait-listed patients to die or become too sick for transplantation. Genetically engineered pigs have been discussed as a potential alternative to allogeneic donor organs. Although xenotransplantation of pig-derived organs in nonhuman primates (NHPs) has shown sequential advances in recent years, there are still underlying problems that need to be completely addressed before clinical applications, including (i) acute humoral xenograft rejection; (ii) acute cellular rejection; (iii) dysregulation of coagulation and inflammation; (iv) physiological incompatibility; and (v) cross-species infection. Moreover, various genetic modifications to the pig donor need to be fully characterized, with the aim of identifying the ideal transgene combination for upcoming clinical trials. In addition, suitable pretransplant screening methods need to be confirmed for optimal donor-recipient matching, ensuring a good outcome from xenotransplantation. Herein, we summarize the understanding of organ xenotransplantation in pigs-to-NHPs and highlight the current status and recent progress in extending the survival time of pig xenografts and recipients. We also discuss practical strategies for overcoming the obstacles to xenotransplantation mentioned above to further advance transplantation of pig organs in the clinic.

Transcription from a gene desert in a melanoma porcine model

The genetic mechanisms underlying cutaneous melanoma onset and progression need to be further understood to improve patients' care. Several studies have focused on the genetic determinism of melanoma development in the MeLiM pig, a biomedical model of cutaneous melanoma. The objective of this study was to better describe the influence of a particular genomic region on melanoma progression in the MeliM model. Indeed, a large region of the Sus scrofa chromosome 1 has been identified by linkage and association analyses, but the causal mechanisms have remained elusive. To deepen the analysis of this candidate region, a dedicated SNP panel was used to fine map the locus, downsizing the interval to less than 2 Mb, in a genomic region located within a large gene desert. Transcription from this locus was addressed using a tiling array strategy and further validated by RT-PCR in a large panel of tissues. Overall, the gene desert showed an extensive transcriptional landscape, notably dominated by repeated element transcription in tumor and fetal tissues. The transcription of LINE-1 and PERVs has been confirmed in skin and tumor samples from MeLiM pigs. In conclusion, although this study still does not identify a candidate mutation for melanoma occurrence or progression, it highlights a potential role of repeated element transcriptional activity in the MeLiM model.

Immunological response in cynomolgus macaques to porcine α-1,3 galactosyltransferase knockout viable skin xenotransplants-A pre-clinical study

BACKGROUND

Allogeneic skin recovered from human deceased donors (HDD) has been a mainstay interim treatment for severe burns, but unfortunately risk of infectious disease and availability limitations exist. Genetically engineered ɑ-1,3 galactosyltransferase knockout (GalT-KO) porcine source animals for viable skin xenotransplants may provide a promising clinical alternative.

METHODS

Four cynomolgus macaque recipients received full-thickness surgical wounds to model the defects arising from excision of full-thickness burn injury and were treated with biologically active skin xenotransplants derived from GalT-KO, Designated Pathogen Free (DPF) miniature swine. Evaluations were conducted for safety, tolerability, and recipient immunological response.

RESULTS

All skin xenotransplants demonstrated prolonged survival, vascularity, and persistent dermal adhesion until the study endpoint at post-operative day 30. No adverse outcomes were observed during the study. Varying levels of epidermolysis coincided with histologic detection of CD4+ and CD8+ T cells, and other cellular infiltrates in the epidermis. Recipient sera IgM and IgG demonstrated significant antibody immune response to non-α-1,3-galactose porcine xenoantigens. Separately, specific wound healing mediators were quantified. Neither porcine cell migration nor PERV were detected in circulation or any visceral organs.

CONCLUSIONS

These results provide a detailed analysis of vital skin xenotransplants utilizing a non-human primate model to predict the anticipated immunological response of human patients. The lack of adverse rejection even in the presence of elevated Ig indicates this is a prospective therapeutic option. The findings reported here directly supported regulatory clearance for a first-in-man, Phase I xenotransplantation clinical trial.

In silico mapping of microbial communities and stress responses in a porcine slaughterhouse and pork products through its production chain, and the efficacy of HLE disinfectant

The use of shotgun metagenomic sequencing to understand ecological-level spread of microbes and their genes has provided new insights for the prevention, surveillance and control of microbial contaminants in the slaughterhouse environment. Here, microbial samples were collected from products and surrounding areas though a porcine slaughter process; shotgun metagenomic DNA-sequencing of these samples revealed a high community diversity within the porcine slaughterhouse and pork products, in zones originating from animal arrival through to the sale zones. Bacteria were more prevalent in the first zones, such as arrival- and anesthesia-zones, and DNA viruses were prevalent in the scorching-and-whip zone, animal products and sale zone. Data revealed the dominance of Firmicutes and Proteobacteria phyla followed by Actinobacteria, with a clear shift in the relative abundance of lactic acid bacteria (mainly Lactobacillus sp.) from early slaughtering steps to Proteobacteria and then to viruses suggesting site-specific community compositions occur in the slaughterhouse. Porcine-type-C oncovirus was the main virus found in slaughterhouse, which causes malignant diseases in animals and humans. As such, to guarantee food safety in a slaughterhouse, a better decipher of ecology and adaptation strategies of microbes becomes crucial. Analysis of functional genes further revealed high abundance of diverse genes associated with stress, especially in early zones (animal and environmental surfaces of arrival zone with 57,710 and 40,806 genes, respectively); SOS responsive genes represented the most prevalent, possibly associated with genomic changes responsible of biofilm formation, stringent response, heat shock, antimicrobial production and antibiotic response. The presence of several antibiotic resistance genes suggests horizontal gene transfer, thus increasing the likelihood for resistance selection in human pathogens. These findings are of great concern, with the suggestion to focus control measures and establish good disinfection strategies to avoid gene spread and microbial contaminants (bacteria and viruses) from the animal surface into the food chain and environment, which was achieved by applying HLE disinfectant after washing with detergent.