Pig Hearts for People: The Science and Ethics Behind the First Clinical Xenotransplant Trials

For more than six million adults in the United States living with heart failure, the outlook has long depended on a number that never grows fast enough: fewer than 4,000 donor hearts become available for transplantation each year. Orthotopic heart transplantation remains the definitive treatment for advanced disease, delivering more than 90 percent one-year survival and meaningful gains in quality of life, but the therapy is constrained by an intractable supply problem. Now, in a perspective published in Frontiers in Transplantation, a team from the University of Alabama at Birmingham and Rowan-Virtua School of Osteopathic Medicine argues that the field of cardiac xenotransplantation has reached an inflection point, and that the barriers standing between experimental pig hearts and routine clinical use are as much ethical as they are biological.

The historical arc of the field is dramatic. Early cross-species heart transplants in the 1960s and 1970s yielded only hours to weeks of graft survival, and the field entered a prolonged dormancy after the 1984 Baby Fae baboon heart transplant triggered public backlash. What changed was the arrival of CRISPR-Cas9 gene editing, improved immunosuppressive strategies, and refined preclinical models. Multi-gene-edited porcine donors have now survived for more than 200 days in orthotopic non-human primate models, and two compassionate-use pig-to-human cardiac transplants at the University of Maryland in 2022 and 2023 demonstrated, for the first time, some feasibility for clinical cardiac xenotransplantation. In May 2026, the FDA granted clearance under an investigational new drug application for the EXPRESS study, the first prospective clinical trial of a gene-edited porcine heart.

The immunology explains why this problem was so hard. Unlike allotransplantation, where rejection is driven primarily by T-cell recognition of mismatched human leukocyte antigens, xenotransplant rejection involves multiple overlapping humoral, complement-mediated, cellular, and innate responses that must be addressed simultaneously. The most immediate threat is hyperacute rejection, mediated by preformed natural antibodies targeting carbohydrate antigens on the porcine vascular endothelium. Three antigens dominate: Galactose-alpha-1,3-galactose, produced by the GGTA1 gene; N-glycolylneuraminic acid, produced by CMAH; and the Sda antigen, encoded by B4GALNT2. Humans lack the first two and may carry preformed antibodies against all three, and anti-alpha-Gal antibodies can trigger complement-dependent cytotoxicity within minutes of reperfusion. CRISPR-Cas9 knockout of all three genes, the triple knockout, has become the foundation of modern porcine donor engineering.

But genetics alone was never going to be enough. Complement regulatory proteins are species-specific: porcine CD46, CD55, and CD59 do not effectively inhibit human complement, so the classical and alternative pathways amplify on the xenograft endothelium, producing membrane attack complex formation, endothelial activation, and microvascular thrombosis. Coagulation is similarly mismatched. Porcine thrombomodulin is a poor activator of human protein C, and porcine tissue factor pathway inhibitor does not adequately suppress human factor Xa, creating coagulation dysregulation at the graft surface. Donor pigs therefore carry inserted human transgenes encoding thrombomodulin, endothelial protein C receptor, and tissue factor pathway inhibitor to restore hemostatic balance across the species barrier. Even the innate immune system poses a problem: porcine CD47 engages human SIRPα inefficiently, leaving xenograft cells vulnerable to macrophage phagocytosis, which is why a human CD47 transgene is now standard in most multi-gene-edited pig lines.

The donors used in the first two human cardiac xenotransplants carried ten genetic modifications: knockout of GGTA1, CMAH, B4GALNT2, and the growth hormone receptor to limit overgrowth, plus six human transgenes, including the complement regulators CD46 and CD55, the anticoagulants thrombomodulin and endothelial protein C receptor, CD47, and the anti-inflammatory heme oxygenase-1. The first recipient, a 57-year-old man on venoarterial ECMO for 48 days, received his heart on January 7, 2022. The graft functioned well for 47 days, but porcine cytomegalovirus was detected on day 20 despite negative pre-transplant screening, and the patient developed diastolic dysfunction requiring ECMO on day 49; support was withdrawn on day 60. Retrospective analysis showed widespread endothelial injury consistent with antibody-mediated rejection, and several IVIG preparations were found to bind donor aortic endothelial cells, raising the possibility that the immunoglobulin therapy itself contributed to the injury.

The second recipient, transplanted on September 20, 2023, was a healthier 58-year-old, and the team deployed enhanced porcine cytomegalovirus surveillance combining quantitative PCR and serologic assays; no virus was detected at any point. Yet on day 13, biopsy showed endothelial activation with prominent capillary antibody and complement staining, and despite low donor-specific anti-pig antibodies the team initiated plasma exchange, giving fresh frozen plasma units that later tested positive for anti-pig antibodies. Acute hemodynamic decompensation occurred on day 29, and at autopsy the xenograft had nearly doubled in weight, from 273 to 480 grams, with widespread endothelial injury and myocyte necrosis. Together, the authors write, these cases answered a foundational question that had haunted the field since the 1960s: whether a genetically modified pig heart can physiologically sustain a human. The answer is yes, at least in the short term, and without hyperacute rejection.

What the cases did not settle is how to make the graft last. The authors identify three scientific priorities. First, the antibody targets driving xenograft antibody-mediated rejection must be defined at the epitope level; the non-Gal, non-Neu5Gc antigens responsible for graft loss remain unidentified, which means the next generation of gene edits, crossmatch assays, and biopsy interpretations cannot be rationally designed until they are characterized. Second, immunosuppression must be optimized for the xenogeneic context rather than carried over unchanged from allotransplantation, with strategies under investigation including enhanced B-cell depletion with rituximab, complement inhibition with eculizumab, and costimulation blockade with anti-CD40 or investigational anti-CD40L agents such as tegoprubart, which may avoid calcineurin-inhibitor nephrotoxicity. Third, virologic surveillance must be standardized. Porcine cytomegalovirus can be mitigated through rigorous multimodal donor screening, but porcine endogenous retrovirus is integrated into the porcine genome and cannot simply be screened away; its transmission risk and long-term consequences remain uncertain, shifting the surveillance question from the individual recipient to the public health.

A striking line of preclinical progress comes from pediatrics. At the 2026 meeting of the International Society for Heart and Lung Transplantation, a Children’s Hospital Los Angeles group reported an infant baboon that survived 734 days, more than two years, with a functioning heart from a genetically edited Yucatan miniature pig, far exceeding any prior orthotopic cardiac xenograft survival. The result matters beyond pediatrics: it suggests graft overgrowth can be controlled by choosing donor breeds with slower growth trajectories, and it demonstrates that durability is achievable. The neonatal immune window, the relative lack of preformed anti-pig antibodies, and the amenability of the infant immune system to tolerization may make congenital heart disease one of the most scientifically favorable settings for xenotransplantation, even as it remains among the most ethically demanding.

That ethical dimension, the authors insist, is not downstream of the science. The EXPRESS trial will enroll patients aged 50 or older with stage D heart failure and no remaining therapeutic options, individuals facing near-certain mortality, a population acutely vulnerable to therapeutic misconception, the tendency to conflate experimental intervention with individualized clinical care. Consent processes must therefore go beyond disclosure to actively counteract that misconception, explicitly stating that the primary purpose of early trials is knowledge generation rather than therapeutic benefit, with independent consent monitors and staged consent proposed to mitigate coercion. Zoonotic risk extends the ethical circle beyond the recipient entirely: recipients may be asked to accept lifelong surveillance, mandatory reporting, biobanking, and constraints on certain activities, obligations that complicate the conventional right to withdraw from research and that may extend to close contacts. Pediatric cases amplify these concerns, since a maturing recipient may ultimately object to living with a porcine xenograft accepted on their behalf.

Finally, the authors argue that equity must be designed in from the outset rather than retrofitted. Early xenotransplantation is resource-intensive, requiring designated-pathogen-free donor animals, specialized facilities, complex immunosuppression, and concentrated institutional expertise, features that left to market forces will localize access within a small number of elite centers and potentially exacerbate existing disparities in advanced heart failure care. Their central claim is that the scientific and ethical challenges are coupled and must be pursued in parallel: each unresolved biological problem generates a corresponding ethical obligation, and progress on one reduces the burden of the other. With the FDA clearance of the first prospective cardiac xenotransplantation trial, the field no longer faces a question of whether pig hearts can beat in humans, but whether the governance, consent frameworks, and allocation policies can mature fast enough to make that capability matter.

Subject of Research: Cardiac xenotransplantation using gene-edited pig hearts and the ethical, regulatory, and equity considerations for clinical integration

Article Title: Advancing cardiac xenotransplantation: overcoming challenges and ethical considerations for clinical integration

Article References: Advancing cardiac xenotransplantation: overcoming challenges and ethical considerations for clinical integration. (n.d.). https://doi.org/10.3389/frtra.2026.1971770

Image Credits: AI Generated

DOI: 10.3389/frtra.2026.1971770

Keywords: cardiac xenotransplantation, gene-edited pigs, CRISPR-Cas9, antibody-mediated rejection, porcine cytomegalovirus, porcine endogenous retrovirus, heart failure, EXPRESS trial, informed consent, health equity, pediatric transplantation, FDA