Physiologic homeostasis after pig-to-human kidney xenotransplantation

Genetically Modified Animal-Derived Products: From Regulations to Applications

Biotechnological advances applied to the generation of genetically modified (GM) animals have shown the potential to develop innovative solutions for different challenges in key areas such as agriculture and human medicine. Despite its enormous potential, the deployment of genetic modification in animals, and its subsequent commercialization, does not meet the same public acceptance as GM plant-derived products, which are currently widely adopted around the world. In this review, we highlight the main examples of GM and gene-edited animal-derived products already approved by the FDA and discuss the regulatory context inherent to such processes, including the risk-based assessment analysis based on a case-by-case evaluation. Moreover, cases of GM animals already approved by other jurisdictions around the world are also discussed.

Xenotransplantation of a Porcine Kidney for End-Stage Kidney Disease

Xenotransplantation offers a potential solution to the organ shortage crisis. A 62-year-old hemodialysis-dependent man with long-standing diabetes, advanced vasculopathy, and marked dialysis-access challenges received a gene-edited porcine kidney with 69 genomic edits, including deletion of three glycan antigens, inactivation of porcine endogenous retroviruses, and insertion of seven human transgenes. The xenograft functioned immediately. The patient's creatinine levels decreased promptly and progressively, and dialysis was no longer needed. After a T-cell-mediated rejection episode on day 8, intensified immunosuppression reversed rejection. Despite sustained kidney function, the patient died from unexpected, sudden cardiac causes on day 52; autopsy revealed severe coronary artery disease and ventricular scarring without evident xenograft rejection. (Funded by Massachusetts General Hospital and eGenesis.).

Construction and Validation of a Predictive Model for the Acceptance of Kidney Xenotransplantation by Medical Staff

With gene editing technology and immunosuppressive drugs, kidney xenotransplantation has developed rapidly in recent years. However, as the most cutting-edge medical personnel, there are few reports on the acceptance and awareness of kidney xenotransplantation. This study conducted an online questionnaire survey on medical staff and constructed the first predictive model for the acceptance of kidney xenotransplantation by medical staff through univariate and multivariate analysis of each variable. Their acceptance rate is 72.8%, and it was found that the willingness to donate organs, awareness of kidney xenotransplantation, and residential areas are independent factors affecting their acceptance rate. In addition, the study also found that although healthcare workers have a high acceptance and willingness to share, their awareness of kidney xenotransplantation is relatively low. This reminds us that in order to increase public acceptance of xenotransplantation, the first step is to start with promoting xenotransplantation among medical personnel.

Advances in Subclinical and Clinical Trials and Immunosuppressive Therapies in Xenotransplantation

Organ transplantation remains the foremost effective intervention for end-stage organ failure. Nevertheless, the scarcity of donors has resulted in prolonged waiting times for countless patients globally. The advent of xenografts presents a promising solution to the organ shortage crisis. Although the utilization of xenografts has a long history, it is only in recent years that breakthroughs in genetically modified pigs have rendered successful xenotransplantation a feasible option. In the past 4 years, numerous subclinical and clinical trials have involved xenotransplantation from genetically modified pigs to humans. However, the outcomes have been disappointing, necessitating a reassessment of basic and preclinical research to address the emerging challenges. Furthermore, immunosuppressive therapies remain essential in xenotransplantation. The range of immunosuppressive agents, encompassing traditional immunosuppressants and monoclonal antibodies such as anti-CD154/CD40 monoclonal antibodies, exhibits considerable diversity. However, the most effective drug combination for achieving optimal efficacy remains elusive. This review will offer a succinct overview of the results from recent clinical and subclinical xenotransplantation trials. Moreover, it will highlight recent advancements in immunosuppressive strategies and discuss potential future research directions in this field.

Pre-clinical xenotransplantation: physiology and pharmacy in human decedent and non-human primate models

Non-human primates and decedent humans have emerged as the two principal translational models in xenotransplantation. Each model has differing advantages and drawbacks. In this manuscript, we will compare and contrast the relative strengths of each model, focusing on the physiologic function of the xenograft in a human decedent or non-human primate. Additionally, we will discuss the pharmacologic agents typically employed in each model, highlighting both the ability of the decedent model to test clinically-relevant medication strategies that may be impossible in non-human primates due to species-specificity.

International Xenotransplantation Association (IXA) Position Paper on the History, Current Status, and Regulation of Xenotransplantation

Recent landmark clinical translation of xenotransplantation depended upon multiple innovations by the xenotransplant community, including the introduction of a variety of source pig genetic modifications, technical innovations, and novel immunosuppressive strategies, as well as the development of ethical and regulatory frameworks to support translation to the clinic. Each organ, tissue, or cell type intended for xenotransplantation will require application-specific preclinical milestones to be met in order to predict "success", as measured by ethical, safe, and efficacious translation to the clinic. Based on successful pre-clinical results and emerging evidence from decedent studies and initial clinical cases, evidence-based infectious disease, ethical, and regulatory considerations are emerging, and will be the foundations for the application-specific position papers that are currently under development. Here, we describe significant landmark events focusing upon safe and efficacious results underpinned by appropriate guidance documents developed over the past three decades that enabled recent translation to the clinic for heart and kidney xenografts. These steps have been undertaken over the past three decades by the xenotransplant community specifically led by the International Xenotransplantation Association (IXA) in consultation with the Transplantation Society (TTS) and the World Health Organization (WHO) to usher xenotransplantation to the clinic.

International Xenotransplantation Association (IXA) Position Paper on Kidney Xenotransplantation

Porcine kidney xenotransplantation for end-stage renal disease (ESRD) has reached the stage of clinical testing following major advances in donor pig genetic modifications and effective immunosuppressive strategies through decades of rigorous translational research. Reports of pig kidney xenograft survival beyond 1 year post-transplant in nonhuman primate (NHP) models justify optimism for its potential as an alternative to allotransplantation. In the United States, experimental transplantations of genetically engineered (GE) porcine kidneys into brain-dead subjects and a small number of ESRD patients have shown no evidence of hyperacute rejection and adequate pig kidney function for up to several months. Here we discuss pre-clinical/clinical results, infectious disease, ethical, and regulatory considerations, and propose evidence-based recommendations. For initial clinical trials in kidney xenotransplantation, we make the following recommendations: (i) transplantation with organs from a triple knockout (TKO) donor pig, preferably with added human transgenes, (ii) an immunosuppressive regimen with induction therapy to deplete T (and possibly B) cells, and maintenance therapy based on a cluster of differentiation (CD)40/CD154 co-stimulation pathway blockade, (iii) the patient should be fully acceptable as a candidate for allotransplantation but should be unlikely ever to receive an allograft. Patients aged 60-69 years (extendable to 40-75 years, if one of the criteria mentioned below is present), of blood group B or O, and with diabetes are most at risk in this regard. Other patients who could be considered are (i) those who have lost two or more previous kidney allografts from recurrent disease in the graft, (ii) those with broad human leukocyte antigen (HLA)-reactivity but no evidence of anti-pig antibodies, including swine leukocyte antigen (SLA), and (iii) those with failing vascular access. Clinical pilot studies in carefully and highly selected patients with no alternative therapy will provide the foundation upon which to base subsequent formal expanded clinical trials.

Xenotransplantation: future frontiers and challenges

PURPOSE OF REVIEW

Recent advancements in genetic engineering have propelled the field of xenotransplantation from preclinical models to early compassionate use cases. As first-in-human clinical trials (FIHCTs) approach, we examine recent developments, ethical and regulatory challenges, immunological considerations, and the clinical infrastructure necessary for successful xenotransplantation trials.

RECENT FINDINGS

Expanded access transplants of pig hearts, kidneys, and livers have identified key challenges. Heart xenotransplants revealed risks of antibody-mediated rejection and zoonotic infections, while kidney xenotransplants suggest that patient selection, rather than immune rejection, may have caused failures. While there has been a report of auxiliary liver transplantation conducted abroad, profound thrombocytopenia poses an obstacle. As FIHCTs draw near, critical clinical challenges include determining the optimal donor genetic constructs and immunosuppressive regimens. Enrollment criteria and patient selection pose additional complexity, alongside ethical concerns such as lifelong zoonosis monitoring. Only a limited number of centers have the expertise needed to conduct these complex trials.

SUMMARY

Xenotransplantation holds great promise as a solution to organ shortages, but success in FIHCTs will require careful design, multidisciplinary collaboration, and strong infrastructure. Addressing immunologic, ethical, and patient selection challenges will be critical. With proper preparation, xenotransplantation could transform organ transplantation.

Prediction of higher ceftazidime-avibactam concentrations in the human renal interstitium compared with unbound plasma using a minimal physiologically based pharmacokinetic model developed in rats and pigs through microdialysis

Last resort antibiotics, like ceftazidime-avibactam (CZA), were used to treat urinary tract infections caused by multidrug-resistant bacteria. However, no data on tissue distribution were available. Our aim was to describe the in vivo kidney distribution of CZA in healthy rats and pigs using a physiologically based pharmacokinetic model (PBPK). Microdialysis probes were inserted into the blood, muscle, and kidney of both species. The experiment started with a retrodialysis by drug period. An i.v. single dose of CZA was administered. Samples were collected for 3 h in rats and 7 h in pigs. A PBPK model was developed to describe tissue and blood CZA pharmacokinetics in animals and to predict human concentrations. The PBPK model adequately described CZA rat and pig data in each tissue and blood. In both species, the concentration profiles of CZA in muscle and blood were almost superimposed, with muscle-to-plasma area under the curve (AUC) ratios close to one. However, kidney CZA concentrations were higher than those in blood, as indicated by kidney-to-plasma AUC ratios exceeding one (respectively 2.27 in rats and 2.63 in pigs for ceftazidime [CAZ]; 2.7 in rats and 4.5 in pigs for avibacam [AVI]). Prediction of human concentrations led to same observations. This study demonstrated an excellent penetration of CZA into the renal parenchyma of rats and pigs. Our PBPK model adequately described the data, and AUCs were higher in the renal cortex interstitium compared with unbound plasma. Our data suggested that the joint PK/PD target for CZA in humans could be attained with reduced CZA doses.

The future of nephrology in 2050

As medicine advances at an unprecedented pace, the field of nephrology is poised for transformative change. By 2050, breakthroughs in kidney disease prevention, dialysis, transplantation, and omics-driven precision medicine could redefine patient care and outcomes. Here, we share our perspectives on the challenges faced and how changes in health policy, emerging technologies, novel therapies, and data-driven approaches might shape the future of nephrology. From innovative dialysis solutions to xenotransplantation and AI-powered diagnostics, we explore the possibilities that could revolutionise kidney health in the decades to come.

International Xenotransplantation Association (IXA) Position Paper on Kidney Xenotransplantation

Porcine kidney xenotransplantation for end-stage renal disease (ESRD) has reached the stage of clinical testing following major advances in donor pig genetic modifications and effective immunosuppressive strategies through decades of rigorous translational research. Reports of pig kidney xenograft survival beyond 1 year posttranplant in nonhuman primate (NHP) models justify optimism for its potential as an alternative to allotransplantation. In the United States, experimental transplantations of genetically engineered (GE) porcine kidneys into brain-dead subjects and a small number of ESRD patients have shown no evidence of hyperacute rejection and adequate pig kidney function for up to several months. Here we discuss pre-clinical/clinical results, infectious disease, ethical, and regulatory considerations, and propose evidence-based recommendations. For initial clinical trials in kidney xenotransplantation, we make the following recommendations: (i) transplantation with organs from a triple knockout (TKO) donor pig, preferably with added human transgenes, (ii) an immunosuppressive regimen with induction therapy to deplete T (and possibly B) cells, and maintenance therapy based on a cluster of differentiation (CD)40/CD154 co-stimulation pathway blockade, (iii) the patient should be fully acceptable as a candidate for allotransplantation but should be unlikely ever to receive an allograft. Patients aged 60-69 years (extendable to 40-75 years, if one of the criteria mentioned below is present), of blood group B or O, and with diabetes are most at risk in this regard. Other patients who could be considered are (i) those who have lost two or more previous kidney allografts from recurrent disease in the graft, (ii) those with broad human leukocyte antigen (HLA)-reactivity but no evidence of anti-pig antibodies, including swine leukocyte antigen (SLA), and (iii) those with failing vascular access. Clinical pilot studies in carefully and highly selected patients with no alternative therapy will provide the foundation upon which to base subsequent formal expanded clinical trials.

Detection of Anti-Non-α-Gal Xenoreactive Antibodies in Human Blood Products

BACKGROUND

Surgical bleeding is a risk in any solid organ transplant, and is commonly addressed with the transfusion of human blood products to replace or supplement coagulation factors. It is unknown if these blood products would harm xenotransplanted pig organs in human recipients demonstrating coagulopathy. The aim of this study was to investigate in vitro if blood products such as fresh frozen plasma (FFP) or cryoprecipitate (cryo) contain xenoantibodies capable of cytotoxicity to GTKO pig cells.

METHODS

We obtained 12 individual single-donor (7 FFP and 5 cryo) blood products from our institution's blood bank for testing. Peripheral blood mononuclear cells (PBMCs) were obtained from a GTKO/hCD55 pig for use as target cells. We performed a series of flow cytometry crossmatch (FCXM) and complement-dependent cytotoxicity (CDC) assays.

RESULTS

We found that all the tested blood products contained some degree of IgM and IgG xenoantibody. Tests using a 1:50 dilution revealed a significant decrease in IgM xenoantibody binding, but an increase in the detection of IgG binding. Multiple preparations were capable of GTKO PBMC cytotoxicity but the level of antibody binding and cell death varied by preparation.

CONCLUSIONS

Both FFP and cryo contain IgM and IgG non-galactose-α-1,3-galactose (αGal) xenoantibodies capable of killing GTKO PBMCs, though the level varies by preparation. Although some centers utilize a genetic background with mutations in the three enzymes responsible for the known xenoantigens, others are investigating the GTKO pig as a potential option. These results suggest that a center pursuing a human xenotransplantation study with a GTKO genetic background should pre-screen blood products prior to administration.

International Xenotransplantation Association (IXA) Position Paper on the History, Current Status, and Regulation of Xenotransplantation

Recent landmark clinical translation of xenotransplantation depended upon multiple innovations by the xenotransplant community, including the introduction of a variety of source pig genetic modifications, technical innovations, and novel immunosuppressive strategies, as well as the development of ethical and regulatory frameworks to support translation to the clinic. Each organ, tissue, or cell type intended for xenotransplantation will require application-specific preclinical milestones to be met in order to predict "success", as measured by ethical, safe, and efficacious translation to the clinic. Based on successful pre-clinical results and emerging evidence from decedent studies and initial clinical cases, evidence-based infectious disease, ethical, and regulatory considerations are emerging, and will be the foundations for the application-specific position papers that are currently under development. Here, we describe significant landmark events focusing upon safe and efficacious results underpinned by appropriate guidance documents developed over the past three decades that enabled recent translation to the clinic for heart and kidney xenografts. These steps have been undertaken over the past three decades by the xenotransplant community specifically led by the International Xenotransplantation Association (IXA) in consultation with the Transplantation Society (TTS) and the World Health Organization (WHO) to usher xenotransplantation to the clinic.

A chronological history of heart valve prostheses to offer perspectives of their limitations

Prosthetic heart valves (PHV) have been studied for around 70 years. They are the best alternative to save the life of patients with cardiac valve diseases. However, current PHVs may still cause significant disadvantages to patients. In general, native heart valves show complex structures and reproducing their functions challenges scientists. Valve repair and replacement are the options to heal heart valve diseases (VHDs), such as stenosis and regurgitation, which show high morbidity and mortality worldwide. Valve repair contributes to the performance of cardiac cycles. However, it fails to restore valve anatomy to its normal condition. On the other hand, replacement is the only alternative to treat valve degeneration. It may do so by mechanical or bioprosthetic valves. Although prostheses may restructure patients' cardiac cycle, both prostheses may show limitations and potential disadvantages, such as mechanical valves causing thrombogenicity or bioprosthetic valves, calcification. Thus, prostheses require constant improvements to remedy these limitations. Although the design of mechanical valve structures has improved, their raw materials cause great disadvantages, and alternatives for this problem remain scarce. Cardiac valve tissue engineering emerged 30 years ago and has improved over time, e.g., xenografts and fabricated heart valves serving as scaffolds for cell seeding. Thus, this review describes cardiac valve substitutes, starting with the history of valvular prosthesis transplants and ending with some perspectives to alleviate the limitations of artificial valves.

Recent progress in xenotransplantation and its application to pediatric kidney disease

Patients with kidney failure require dialysis or kidney transplantation. Kidney transplantation offers great benefits, including reduced mortality; however, many patients who wish to undergo kidney transplantation are unable to do so due to a shortage of donor organs. This shortage is a global issue, and xenotransplantation has emerged as a potential solution. The history of xenotransplantation is characterized by overcoming the immunological challenge of hyperacute rejection. Recently, breakthroughs such as gene-edited pigs and novel immunosuppressants have successfully lowered rejection rates. Recent clinical studies have reported transplants in patients diagnosed with brain death, and in March 2024, a gene-edited pig kidney was transplanted into a patient with kidney failure at Massachusetts General Hospital, marking the first instance of a gene-edited xenotransplantation into a living patient. Our research focuses on applying xenotransplantation in pediatric and obstetric fields, specifically exploring fetal therapy using pig fetal kidneys. We have long been researching the development of a novel kidney replacement therapy involving the transplantation of fetal pig kidneys. Fetal pig kidneys have the advantage of not requiring vascular anastomosis and are less likely to be rejected compared to adult pig kidneys. Currently, we are advancing nonhuman primate studies aimed at clinical trials of pig fetal kidney transplant therapy for fetuses diagnosed with Potter syndrome, characterized by bilateral kidney agenesis. We sincerely hope that xenotransplantation will soon become a viable treatment option for adult, pediatric, and fetal patients with kidney failure.

Systematic Review and Comparative Outcomes Analysis of NHP Liver Allotransplants and Xenotransplants

Patients with fulminant liver failure ineligible for transplantation have a high mortality rate. With recent progress in genetic modifications and clinical achievements, using pig livers as a bridge-to-transplant has regained popularity. Preclinical testing has been done in small cohorts of nonhuman primates (NHP), and maximum survival is limited to 1-month. We conducted a systematic review and comparative outcomes analysis of NHP-liver xenotransplantation and gathered 203 pig-to-NHP and NHP-to-NHP transplants reported in 23 studies. Overall, NHP survival after pig-liver xenotransplantation was limited (1, 3, 4 weeks: 18.0%, 5.6%, 1.1%), compared to NHPs after allotransplantation (1, 3, 4 weeks: 60.6%, 47.4%, 45.4%). A focus on pigs with genetic modifications evidenced some short-term survival benefits (1, 3, 4 weeks: 29.1%, 9.1%, 1.8%). The use of the auxiliary transplant technique was also associated with better short-term results (1, 3, 4 weeks: 40.9%, 9.1%, 4.5%). Causes of graft and animal loss were mostly rejection and liver failure in allotransplants, while bleeding, liver, and respiratory failure predominated in xenotransplants. Notably, the 1-month survival rate for NHP-allotransplants was significantly lower than the national > 98% rate for human liver transplants. This data confirms the short-term improvements brought by genetic modifications and auxiliary implantation in the NHP model, which remains imperfect.

Use of Brain Death Recipients in Xenotransplantation: A Double-Edged Sword

Organ transplants are used to treat many end-stage diseases, but a shortage of donors means many patients cannot be treated. Xenogeneic organs have become an important part of filling the donor gap. Many current studies of kidney, heart, and liver xenotransplantation have used gene-edited pig organs on brain-dead recipients. However, the endocrine system, immune system, and nervous system of brain-dead people are changed, which are different from that of real patients transplanted, and the current research results of brain death (BD) recipients are also different. So there are drawbacks to using brain-dead people for xenotransplantation. In addition, although the policy requires the use of non-human primate (NHP) experiments as the research standard for xenotransplantation, there are still differences between NHP and humans in terms of immunity. Therefore, to better study xenotransplantation, new models may be needed in addition to NHP and brain-dead individuals. Humanized animal models or organoids may be able to fill this gap.

What Genetic Modifications of Source Pigs Are Essential and Sufficient for Cell, Tissue, and Organ Xenotransplantation?

Xenotransplantation of porcine organs has made remarkable progress towards clinical application. A key factor has been the generation of genetically multi-modified source pigs for xenotransplants, protected against immune rejection and coagulation dysregulation. While efficient gene editing tools and multi-cistronic expression cassettes facilitate sophisticated and complex genetic modifications with multiple gene knockouts and protective transgenes, an increasing number of independently segregating genetic units complicates the breeding of the source pigs. Therefore, an optimal combination of essential genetic modifications may be preferable to extensive editing of the source pigs. Here, we discuss the prioritization of genetic modifications to achieve long-term survival and function of xenotransplants and summarise the genotypes that have been most successful for xenogeneic heart, kidney, and islet transplantation. Specific emphasis is given to the choice of the breed/genetic background of the source pigs. Moreover, multimodal deep phenotyping of porcine organs after xenotransplantation into human decedents will be discussed as a strategy for selecting essential genetic modifications of the source pigs. In addition to germ-line gene editing, some of these modifications may also be induced during organ preservation/perfusion, as demonstrated recently by the successful knockdown of swine leukocyte antigens in porcine lungs during ex vivo perfusion.

Opinions on the Future of Clinical Pig Kidney Xenotransplantation

Based on promising results obtained in primate models, pioneers in the US have now started to explore the new frontier of genetically-edited pig-to-human transplantation. The recent transition of xenotransplantation into clinical medicine has included transplants in brain-dead subjects and the compassionate use of xenotransplants in living recipients without options for allotransplantation. While the barrier of hyperacute rejection seems to be successfully overcome by gene editing of donor pigs, the occurrence of accelerated rejection could pose significant limitations to the success of the procedure. Ultimately, the establishment of efficient and safe strategies to overcome immunologic barriers will, among other critical factors, such as potential xenozoonotic disease transmission or physiological differences, determine whether and for which indications xenotransplantation will be viable. Considering preliminary outcomes of compassionate use xenotransplantions, which may raise questions about how faithfully data from non-human primate models translate into human outcomes, further research in decedents may be necessary before proceeding with additional clinical transplants. Looking ahead, designing systematic trials in xenotransplantation, including the definition of acceptable eligibility criteria for such high-risk transplants, will be an immense challenge, especially in kidney transplantation, where dialysis provides an effective alternative to transplantation in most cases.

Plasma exchange and intravenous immunoglobulin prolonged the survival of a porcine kidney xenograft in a sensitized, deceased human recipient

BACKGROUND

The primary limitation to kidney transplantation is organ shortage. Recent progress in gene editing and immunosuppressive regimens has made xenotransplantation with porcine organs a possibility. However, evidence in pig-to-human xenotransplantation remains scarce, and antibody-mediated rejection (AMR) is a major obstacle to clinical applications of xenotransplantation.

METHODS

We conducted a kidney xenotransplantation in a deceased human recipient using a porcine kidney with five gene edits (5GE) on March 25th, 2024 at Xijing Hospital, China. Clinical-grade immunosuppressive regimens were employed, and the observation period lasted 22 days. We collected and analyzed the xenograft function, ultrasound findings, sequential protocol biopsies, and immune surveillance of the recipient during the observation.

RESULTS

The combination of 5GE in the porcine kidney and clinical-grade immunosuppressive regimens prevented hyperacute rejection. The xenograft kidney underwent delayed graft function in the first week, but urine output increased later and the single xenograft kidney maintained electrolyte and pH homeostasis from postoperative day (POD) 12 to 19. We observed AMR at 24 h post-transplantation, due to the presence of pre-existing anti-porcine antibodies and cytotoxicity before transplantation; this AMR persisted throughout the observation period. Plasma exchange and intravenous immunoglobulin treatment mitigated the AMR. We observed activation of latent porcine cytomegalovirus toward the end of the study, which might have contributed to coagulation disorder in the recipient.

CONCLUSIONS

5GE and clinical-grade immunosuppressive regimens were sufficient to prevent hyperacute rejection during pig-to-human kidney xenotransplantation. Pre-existing anti-porcine antibodies predisposed the xenograft to AMR. Plasma exchange and intravenous immunoglobulin were safe and effective in the treatment of AMR after kidney xenotransplantation.

Genetically Modified Porcine Kidneys Have Sufficient Tissue Integrity for Use in Pig-to-Human Xenotransplantation

OBJECTIVE

We sought to determine if genetically modified porcine kidneys used for xenotransplantation had sufficient tissue integrity to support long-term function in a human recipient.

BACKGROUND

Kidney transplantation remains the best available treatment for patients with end-stage kidney disease. However, a shortage of available donor human kidneys prevents many patients from achieving the benefits of transplantation. Xenotransplantation is a potential solution to this shortage. Recent pre-clinical human studies have demonstrated kidneys from genetically modified pig donors can be transplanted without hyperacute rejection and are capable of providing creatinine and other solute clearance. It is unknown whether the porcine kidneys would tolerate the relatively higher resting blood pressure in an adult human recipient compared with the pig donor or non-human primate (NHP) recipients used in translational studies. Furthermore, previous experience in NHPs raised concerns about the tissue integrity of the porcine ureter and post-xenotransplant growth of the porcine kidney.

METHODS

Kidneys recovered from porcine donors with 10 gene edits were transplanted into decedent brain-dead recipients who were not eligible for organ donation. Decedents underwent bilateral native nephrectomy before transplant and were followed for 3 to 7 days. Standard induction and maintenance immunosuppression was used as previously reported. Vital signs, including blood pressure, were recorded frequently. Kidney xenografts were assessed daily, serially biopsied, and were measured at implantation and study completion.

RESULTS

Three decedents underwent successful xenotransplantation. Subcapsular hematomas developed, requiring incision of the xenograft capsules to prevent Page kidney. Blood pressures were maintained in a physiologic range for adult humans (median arterial pressures (MAP) 108.5 mm Hg (Interquartile Range (IQR): 97-114 mm Hg), 74 mm Hg (IQR: 71-78 mm Hg), and 95 mm Hg (IQR: 88-99 mm Hg, respectively) and no bleeding complications or aneurysm formation was observed. Serial biopsies were taken from the xenografts without apparent loss of tissue integrity despite the lack of a capsule. Ureteroneocystotomies remained intact without evidence of urine leak. Xenograft growth was observed, but plateaued, in 1 decedent with increased volume of the left and right xenografts by 25% and 26%, respectively, and in the context of human growth hormone levels consistently less <0.1 ng/ml and insulin-like growth factor 1 levels ranging from 34-50 ng/ml.

CONCLUSIONS

The findings of this study suggest kidneys from 10-gene edited porcine donors have sufficient tissue integrity to tolerate xenotransplantation into a living human recipient. There was no evidence of anastomotic complications, and the xenografts tolerated needle biopsy without issue. Xenograft growth occurred but plateaued by the study end; further observation and investigation will be required to confirm this finding and elucidate underlying mechanisms.

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.

Current challenges in xenotransplantation

PURPOSE OF REVIEW

In recent years, the xenotransplantation science has advanced tremendously, with significant strides in both preclinical and clinical research. This review intends to describe the latest cutting-edge progress in knowledge and methodologies developed to overcome potential obstacles that may preclude the translation and successful application of clinical xenotransplantation.

RECENT FINDINGS

Preclinical studies have demonstrated that it is now possible to extend beyond two years survival of primate recipients of life saving xenografts. This has been accomplished thanks to the utilization of genetic engineering methodologies that have allowed the generation of specifically designed gene-edited pigs, a careful donor and recipient selection, and appropriate immunosuppressive strategies.In this light, the compassionate use of genetically modified pig hearts has been authorized in two human recipients and xenotransplants have also been achieved in human decedents. Although encouraging the preliminary results suggest that several challenges have yet to be fully addressed for a successful clinical translation of xenotransplantation. These challenges include immunologic, physiologic and biosafety aspects.

SUMMARY

Recent progress has paved the way for the initial compassionate use of pig organs in humans and sets the scene for a wider application of clinical xenotransplantation.

A pig kidney supporting human physiology

Because of the global shortage of donor kidneys, xenotransplantation emerges as a potential solution for individuals with kidney failure who face challenges in securing a suitable donor kidney. A study featured in this month's issue of Kidney International assesses the kidney physiology of a porcine kidney transplanted into a brain-dead human with kidney failure, demonstrating life-sustaining physiological function for 7 days. Together with preclinical nonhuman primate studies, decedent models provide complementary data for development of clinical kidney xenotransplantation.