Cardiac Xenotransplantation: Progress in Preclinical Models and Prospects for Clinical Translation

Current status and prospects of genetically modified porcine-to-human cardiac xenotransplantation

Cardiac xenotransplantation utilizing genetically modified pigs presents a promising avenue for treating end-stage heart failure, a leading cause of mortality worldwide. This paper delineates the current landscape of heart failure treatment in Japan, emphasizing the limitations of existing therapies such as heart transplantation and implantable left ventricular assist devices. It discusses the history and advancements in the development of genetically modified pigs for xenotransplantation, highlighting recent breakthroughs and challenges. The manuscript also addresses the specific challenges facing the implementation of xenotransplantation in Japan, including the selection of suitable genetically modified pigs, ensuring organ safety, patient selection criteria, transplantation protocols, and immunosuppression strategies. Drawing from international experiences and ongoing research efforts, the paper emphasizes the potential of xenotransplantation while acknowledging the hurdles that must be overcome for widespread clinical adoption.

Anatomical and Physiological Considerations for Pig Cardiac Xenotransplantation

The escalating incidence of heart failure globally, and in the United States, necessitates innovative solutions beyond conventional human cardiac transplantation due to donor heart shortage. Recent measures to overcome this shortage include the novel idea of cardiac xenotransplantation, with the first procedure done in January 2022 at the University of Maryland. However, the patient did not survive in the postoperative phase, highlighting potential challenges in cardiac xenotransplantation. Trace amounts of research exist on the physiological impacts subsequent to innate anatomical differences of porcine hearts, regardless of genetic modifications in growth rates. As such, this review aims to explore and address the critical implications of utilizing genetically modified porcine hearts for cardiac xenotransplantation as it pertains to postoperative physiological function. An analysis of literature discussing multiple anatomical and physiological factors, such as differences in organ dimensions, vasculature, and cardiac conduction, was carried out. Although xenotransplantation offers a promising solution, the present analysis of relevant literature points out potentially important considerations relating to long-term survivability.

Tolerogenic Therapies in Cardiac Transplantation

Heart transplantation remains the gold-standard treatment for end-stage heart failure, yet long-term graft survival is hindered by chronic rejection and the morbidity and mortality caused by lifelong immunosuppression. While advances in medical and device-based therapies have reduced the overall need for transplantation, patients who ultimately require a transplant often present with more advanced disease and comorbidities. Recent advances in tolerance-inducing strategies offer promising avenues to improve allograft acceptance, while minimizing immunosuppressive toxicity. This review explores novel approaches aiming to achieve long-term immunological tolerance, including co-stimulation blockade, mixed chimerism, regulatory T-cell (Treg) therapies, thymic transplantation, and double-organ transplantation. These strategies seek to promote donor-specific unresponsiveness and mitigate chronic rejection. Additionally, expanding the donor pool remains a critical challenge in addressing organ shortages. Innovations such as ABO-incompatible heart transplantation are revolutionizing the field by increasing donor availability and accessibility. In this article, we discuss the mechanistic basis, clinical advancements, and challenges of these approaches, highlighting their potential to transform the future of heart transplantation with emphasis on clinical translation.

Transplantation of a genetically modified porcine heart into a live human

Following our previous experience with cardiac xenotransplantation of a genetically modified porcine heart into a live human, we sought to achieve improved results by selecting a healthier recipient and through more sensitive donor screening for potential zoonotic pathogens. Here we transplanted a 10-gene-edited pig heart into a 58-year-old man with progressive, debilitating inotrope-dependent heart failure due to ischemic cardiomyopathy who was not a candidate for standard advanced heart failure therapies. He was maintained on a costimulation (anti-CD40L, Tegoprubart) blockade-based immunomodulatory regimen. The xenograft initially functioned well, with excellent systolic and diastolic function during the first several weeks posttransplantation. Subsequently, the xenograft developed rapidly progressing diastolic heart failure, biventricular wall thickening and, ultimately, near-complete loss of systolic function necessitating initiation of extracorporeal membranous oxygenation on day 31. Given these setbacks, the patient chose to transition to comfort care after 40 days. As with our first patient, histology did not reveal substantial immune cell infiltration but suggested capillary endothelial injury with interstitial edema and early fibrosis. No evidence of porcine cytomegalovirus replication in the xenograft was observed. Strategies to overcome the obstacle of antibody-mediated rejection are needed to advance the field of xenotransplantation.

Genetically engineered pig heart transplantation in non-human primates

BACKGROUND

Improvement in gene modifications of donor pigs has led to the prevention of early cardiac xenograft rejection and significantly prolonged cardiac xenograft survival in both heterotopic and orthotopic preclinical non-human primate (NHP) models. This progress formed the basis for FDA approval for compassionate use transplants in two patients.

METHODS

Based on our earlier report of 9-month survival of seven gene-edited (7-GE) hearts transplanted (life-supporting orthotopic) in baboons, we transplanted 10 gene-edited pig hearts into baboons (n = 4) using non-ischemic continuous perfusion preservation (NICP) and immunosuppression regimen based on co-stimulation blockade by anti-CD40 monoclonal antibody. This pivotal study expands on the 7-GE backbone, with 3 additional gene edits, using 10-GE pigs as donors to baboon recipients.

RESULTS

10 GE cardiac xenografts provide life-supporting function up to 225 days (mean 128 ± 36 days) in a non-human primate model. Undetectable or latent porcine cytomegalovirus (PCMV) does not influence cardiac xenograft survival in this study but still needs more exploration with a larger cohort. Xenograft histology demonstrates adipose (Fat) deposition (n = 1), chronic vasculopathy (n = 1), micro and macro thrombosis, and acute cellular rejection (n = 1).

CONCLUSIONS

These data demonstrate that 10 GE cardiac xenografts have variable cardiac xenograft survival in NHP due to perhaps presence of 4th antigen and require further study. However, these 10GE organs may be suitable for clinical cardiac xenotransplantation and have already been utilized in two human cases.

Modified CD40L-Activated B-Cell Proliferation Model for Validating the Suppressive Activity of CD40-CD154 Pathway Inhibitors

BACKGROUND

CD40-CD154 pathway inhibitors are considered indispensable immunosuppressive drugs in xenotransplantation. At present, novel anti-CD154 and anti-CD40 monoclonal antibodies (mAbs) are continuously being developed. It is important to establish a simple and efficient in vitro method to evaluate the effectiveness of these therapeutic mAbs.

METHODS

A modified CD40L-activated B-cell proliferation model was established using irradiated NIH/3T3 cells transfected with human CD40 ligand (hCD40L-NIH/3T3) as stimulator cells and human or rhesus monkey peripheral blood mononuclear cells (PBMCs) as responder cells. After 8 days of culture, B-cell proliferation was detected by flow cytometry. Various concentrations of anti-CD40 or anti-CD154 mAbs were added to the co-culture system as an intervention. The inhibitory effects of anti-CD154 and anti-CD40 mAbs on the proliferation of B cells from humans and rhesus monkeys were studied and compared.

RESULTS

After 8 days of co-culture, the proliferation rate of B cells in both human and rhesus monkey PBMCs was more than 80%, and the expression of MHC-II and the co-stimulatory molecules CD80, CD86, and CD40 on B cells was significantly up-regulated. All three anti-CD154 mAbs showed a similar strong inhibitory effect on human B-cell proliferation, but the inhibitory effect on the proliferation of rhesus monkey B cells was weaker than that on human B cells, which showed a typical dose-dependent inhibition. The three anti-CD40 mAbs from different sources had different effects. One mAbs potently inhibited both human and monkey B-cell proliferation, whereas the other two mAbs exhibited strong or moderate inhibitory effects on human B-cell proliferation but had little inhibitory effect on monkey B-cell proliferation.

CONCLUSION

We have successfully established a modified CD40L-activated B-cell proliferation model for the in vitro evaluation of CD40-CD154 pathway inhibitors, which may provide important evidence for the selection of appropriate therapeutic antibodies and their dose determination for xenotransplantation.

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.

Risky first-in-human clinical trials on medically fragile persons: owning the moral cost

The purpose of a first-in-human (FIH) clinical trial is to gather information about how the drug or device affects and interacts with the human body: its safety, side effects, and (potential) dosage. As such, the primary goal of a FIH trial is not participant benefit but to gain knowledge of drug or device efficacy, i.e., baseline human safety knowledge. Some FIH clinical trials carry significant foreseeable risk to participants with little to no foreseeable participant benefit. Participation in such trials would be a bad deal for participants, and the research is considered justifiable because of the promise of significant potential social benefit. I argue that there is an ethical tension inherent in risky FIH research and that researchers should fairly compensate risky FIH trial participants. This does not make the risk-benefit outcome more favorable for participants; rather, it amounts to a collective reckoning with the ethical tension inherent in the research.

Extended survival of 9- and 10-gene-edited pig heart xenografts with ischemia minimization and CD154 costimulation blockade-based immunosuppression

BACKGROUND

Xenotransplantation has made significant advances recently using pigs genetically engineered to remove carbohydrate antigens, either alone or with addition of various human complement, coagulation, and anti-inflammatory ''transgenes''. Here we evaluated results associated with gene-edited (GE) pig hearts transplanted in baboons using an established costimulation-based immunosuppressive regimen and a cold-perfused graft preservation technique.

METHODS

Eight baboons received heterotopic abdominal heart transplants from 3-GE (GalKO.β4GalNT2KO.hCD55, n = 3), 9-GE (GalKO.β4GalNT2KO.GHRKO.hCD46.hCD55. TBM.EPCR.hCD47. HO-1, n = 3) or 10-G (9-GE+CMAHKO, n = 2) pigs using Steen's cold continuous perfusion for ischemia minimization. Immunosuppression (IS) included induction with anti-thymocyte globulin and αCD20, ongoing αCD154, MMF, and tapered corticosteroid.

RESULTS

All three 3-GE grafts functioned well initially, but failed within 5 days. One 9-GE graft was lost intraoperatively due to a technical issue and another was lost at POD 13 due to antibody mediated rejection (AMR) in a baboon with a strongly positive pre-operative cross-match. One 10-GE heart failed at POD113 with combined cellular and antibody mediated rejection. One 9-GE and one 10-GE hearts had preserved graft function with normal myocardium on protocol biopsies, but exhibited slowly progressive graft hypertrophy until elective necropsy at POD393 and 243 respectively. Elevated levels of IL-6, MCP-1, C-reactive protein, and human thrombomodulin were variably associated with conditioning, the transplant procedure, and clinically significant postoperative events.

CONCLUSION

Relative to reference genetics without thrombo-regulatory and anti-inflammatory gene expression, 9- or 10-GE pig hearts exhibit promising performance in the context of a clinically applicable regimen including ischemia minimization and αCD154-based IS, justifying further evaluation in an orthotopic model.

Cardiac Xenotransplantation: A Narrative Review

Cardiac xenotransplantation (cXT) has emerged as a solution to heart donor scarcity, prompting an exploration of its scientific, ethical, and regulatory facets. The review begins with genetic modifications enhancing pig hearts for human transplantation, navigating through immunological challenges, rejection mechanisms, and immune responses. Key areas include preclinical milestones, complement cascade roles, and genetic engineering to address hyperacute rejection. Physiological counterbalance systems, like human thrombomodulin and endothelial protein C receptor upregulation in porcine xenografts, highlight efforts for graft survival enhancement. Evaluating pig and baboon donors and challenges with non-human primates illuminates complexities in donor species selection. Ethical considerations, encompassing animal rights, welfare, and zoonotic disease risks, are critically examined in the cXT context. The review delves into immune control mechanisms with aggressive immunosuppression and clustered regularly interspaced palindromic repeats associated protein 9 (CRISPR/Cas9) technology, elucidating hyperacute rejection, complement activation, and antibody-mediated rejection intricacies. CRISPR/Cas9's role in creating pig endothelial cells expressing human inhibitor molecules is explored for rejection mitigation. Ethical and regulatory aspects emphasize the role of committees and international guidelines. A forward-looking perspective envisions precision medical genetics, artificial intelligence, and individualized heart cultivation within pigs as transformative elements in cXT's future is also explored. This comprehensive analysis offers insights for researchers, clinicians, and policymakers, addressing the current state, and future prospects of cXT.

The pig as an optimal animal model for cardiovascular research

Cardiovascular disease is a worldwide health problem and a leading cause of morbidity and mortality. Preclinical cardiovascular research using animals is needed to explore potential targets and therapeutic options. Compared with rodents, pigs have many advantages, with their anatomy, physiology, metabolism and immune system being more similar to humans. Here we present an overview of the available pig models for cardiovascular diseases, discuss their advantages over other models and propose the concept of standardized models to improve translation to the clinical setting and control research costs.

Genetic Engineering of Donor Pig for the First Human Cardiac Xenotransplantation: Combatting Rejection, Coagulopathy, Inflammation, and Excessive Growth

PURPOSE OF REVIEW

The first successful pig to human cardiac xenotransplantation in January 2022 represented a major step forward in the fields of heart failure, immunology, and applied genetic engineering, using a 10-gene edited (GE) pig. This review summarizes the evolution of preclinical modelling data which informed the use of each of the 10 genes modified in the 10-GE pig: GGTA1, Β4GalNT2, CMAH, CD46, CD55, TBM, EPCR, CD47, HO-1, and growth hormone receptor.

RECENT FINDINGS

The translation of the 10-GE pig from preclinical modelling to clinical compassionate xenotransplant use was the culmination of decades of research combating rejection, coagulopathy, inflammation, and excessive xenograft growth. Understanding these 10 genes with a view to their combinatorial effects will be useful in anticipated xenotransplant clinical trials.

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

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

Isolation of porcine adult cardiomyocytes: Comparison between Langendorff perfusion and tissue slicing-assisted enzyme digestion

Tissue slicing-assisted digestion (TSAD) of adult cardiomyocytes has shown significant improvements over conventional chunk methods. However, it remains unclear how this method compares to Langendorff perfusion, the current standard of adult cardiomyocyte isolation. Using adult Bama minipigs, we performed cardiomyocyte isolation via these two distinct methods, and compared the resulting cellular quality, including viability, cellular structure, gene expression, and electrophysiological properties, of cardiomyocytes from 3 distinct anatomical regions, namely the left ventricle, right ventricle, and left atrial appendage. Our results revealed largely indistinguishable cell quality in all of the measured parameters. These findings suggest that that TSAD can be reliably used to isolate adult mammalian cardiomyocytes as a reliable alternative to perfusion in cardiomyocyte isolation from larger mammals, particularly when Langendorff perfusion is not feasible.

Cardiac Surgeons Highlight the Need for Innovation Stewardship: Noteworthy in 2022

Modern cardiac surgery has rapidly evolved to treat complex cardiovascular disease. This past year boasted noteworthy advances in xenotransplantation, prosthetic cardiac valves, and endovascular thoracic aortic repair. Newer devices often offer incremental design changes while demanding significant cost increases that leave surgeons to decide if the benefit to patients justifies the increased cost. As innovations are introduced, surgeons must continuously aim to harmonize short- and long-term benefits with financial costs). We must also ensure quality patient outcomes while embracing innovations that will advance equitable cardiovascular care.