Consensus statement on heart xenotransplantation in children: Toward clinical translation.

Central MessageNeonates and infants with congenital heart disease may benefit from heart xenotransplantation. Successful clinical translation of heart xenotransplantation requires utmost ethical scrutiny.See Commentarry on page XXX. Neonates and infants with congenital heart disease may benefit from heart xenotransplantation. Successful clinical translation of heart xenotransplantation requires utmost ethical scrutiny. See Commentarry on page XXX. Gene-editing has made an enormous impact on our ability to create a bioengineered pig heart for clinical transplantation1Cooper D.K.C. Raza S.S. Chaban R. Pierson III, R.N. Shooting for the moon: genome editing for pig heart xenotransplantation.J Thorac Cardiovasc Surg. May 6, 2022; ([Epub ahead of print])Abstract Full Text Full Text PDF Scopus (3) Google Scholar (Figure 1). Survival of up to 9 months in experimental life-supporting pig heart xenotransplantation has been achieved,2Langin M. Mayr T. Reichart B. Michel S. Buchholz S. Guetholf S. et al.Consistent success in life-supporting porcine cardiac xenotransplantation.Nature. 2018; 564: 430-433Crossref PubMed Scopus (291) Google Scholar,3Mohiuddin M.M. Goerlich C.E. Singh A.K. Zhang T. Tatarov I. Lewis B. et al.Progressive genetic modifications of porcine cardiac xenografts extend survival to 9 months.Xenotransplantation. 2022; 29: e12744Crossref PubMed Scopus (39) Google Scholar and, following decades of collaborative research,4Rothblatt M. Commentary on achievement of first life-saving xenoheart transplant.Xenotransplantation. 2022; 29: e12746Crossref PubMed Scopus (13) Google Scholar the first transplantation of a genome-edited pig heart into a human became a reality.5Reardon S. First pig-to-human transplant: what can scientists learn?.Nature. 2022; 601: 305-306Crossref PubMed Scopus (63) Google Scholar, 6Cooper D.K.C. Pierson III, R.N. The future of cardiac xenotransplantation.Nat Rev Cardiol. 2022; 19: 281-282Crossref PubMed Scopus (10) Google Scholar, 7Griffith B.P. Goerlich C.E. Singh A.K. Rothblatt M. Lau C.L. Shah A. et al.Genetically modified porcine-to-human cardiac xenotransplantation.N Engl J Med. 2022; 387: 35-44Crossref PubMed Scopus (154) Google Scholar, 8Cooper D.K.C. Initial reflections on the world's first clinical genetically-engineered pig heart transplant.Xenotransplantation. 2022; 29: e12737https://doi.org/10.1111/xen.12737Crossref PubMed Scopus (3) Google Scholar This offered some promise and sparked renewed interest for clinical translation of cardiac xenotransplantation.9Platt J.L. Cascalho M. The future of transplantation.N Engl J Med. 2022; 387: 77-78Crossref PubMed Scopus (7) Google Scholar,10Phimister E.G. Genetic modification in pig-to-human Transplantation.N Engl J Med. 2022; 387: 79-82Crossref PubMed Scopus (6) Google Scholar This progress has largely been achieved by the combination of 2 approaches—(1) the transplantation of organs from pigs genetically engineered to protect their organs from the primate innate immune response11Cooper D.K.C. Hara H. Iwase H. Yamamoto T. Li Q. Ezzelarab M. et al.Justification of specific genetic modifications in pigs for clinical kidney or heart xenotransplantation.Xenotransplantation. 2019; 26: e12516https://doi.org/10.1111/xen.12516Crossref PubMed Scopus (97) Google Scholar and (2) the administration of novel immunosuppressive agents that prevent the adaptive immune response by blockade of the CD40/CD154 T cell costimulation pathway.12Cooper D.K.C. Moving xenotransplantation to the clinic. (The IXA Keith Reemtsma lecture).Xenotransplantation. 2021; 29: e12723https://doi.org/10.1111/xen.12723Crossref PubMed Scopus (7) Google Scholar Gene editing has included deletion of expression of 3 major carbohydrate xenoantigens and growth hormone receptor as well as addition of transgenic expression of human “protective” proteins, such as complement- and coagulation-regulatory proteins.11Cooper D.K.C. Hara H. Iwase H. Yamamoto T. Li Q. Ezzelarab M. et al.Justification of specific genetic modifications in pigs for clinical kidney or heart xenotransplantation.Xenotransplantation. 2019; 26: e12516https://doi.org/10.1111/xen.12516Crossref PubMed Scopus (97) Google Scholar Although the most suitable patient population for pig heart xenotransplantation is yet to be determined,6Cooper D.K.C. Pierson III, R.N. The future of cardiac xenotransplantation.Nat Rev Cardiol. 2022; 19: 281-282Crossref PubMed Scopus (10) Google Scholar,13Pierson III, R.N. Burdorf L. Madsen J.C. Lewis G.D. D'Alessandron D.A. Pig-to-human heart transplantation: who goes first?.Am J Transplant. 2020; 20: 2669-2674Abstract Full Text Full Text PDF PubMed Scopus (22) Google Scholar,14Chaban R. Cooper D.K.C. Pierson III., R.N. Pig heart and lung xenotransplantation: present status.J Heart Lung Transplant. 2022; 41: 1014-1022https://doi.org/10.1016/j.healun.2022.04.010Abstract Full Text Full Text PDF PubMed Scopus (10) Google Scholar neonates and infants with complex life-threatening congenital heart disease may benefit most.15Raza S.S. Hara H. Cleveland D.C. Cooper D.K.C. The potential of genetically engineered pig heart transplantation in infants with complex congenital heart disease.Pediatr Transplant. 2022; 26: e14260Crossref PubMed Scopus (5) Google Scholar, 16Cleveland D. Adam Banks C. Hara H. Carlo W.F. Mauchley D.C. Cooper D.K.C. The case for cardiac xenotransplantation in neonates: is now the time to reconsider xenotransplantation for hypoplastic left heart syndrome?.Pediatr Cardiol. 2019; 40: 437-444Crossref PubMed Scopus (31) Google Scholar, 17Cleveland D.C. Jagdale A. Carlo W.F. Iwase H. Crawford J. Walcott G.P. et al.The genetically engineered heart as a bridge to allotransplantation in infants: just around the corner?.Ann Thorac Surg. 2022; 114: 536-544Abstract Full Text Full Text PDF PubMed Scopus (19) Google Scholar Herein, we review the need and applicability of heart xenotransplantation in this vulnerable population of young children and discuss the prospects for clinical translation. This review is not aimed at discussing the ethical and regulatory issues but is rather focused on clinical aspects and feasibility. Despite significant progress in surgical and intensive care management, outcomes in neonates and infants with complex congenital heart disease who develop heart failure are poor, particularly in those with univentricular circulation. The Single Ventricle Reconstruction (SVR) trial demonstrated that 15% of patients who survived to hospital discharge after a Norwood procedure develop heart failure by 6 years of age.18Mahle W. Hu C.M.S. Trachtenberg F. Menteer J.D. Kindel S.J. Dipchand A. et al.Heart failure after the Norwood procedure: an analysis of the single ventricle reconstruction trial.J Heart Lung Transplant. 2018; 37: 879-885Abstract Full Text Full Text PDF PubMed Scopus (36) Google Scholar Mortality in children awaiting heart transplantation remains high.19Denflied S.W. Azeka E. Das B. Garcia-Guereta L. Irving C. Kemna M. et al.Pediatric cardiac waitlist mortality—Still too high.Pediatr Transplant. 2020; 24: e13671PubMed Google Scholar Newborns and infants have the greatest waiting-list mortality.20Almond C.S.D. Thiagarajan R.R. Piercey G.E. Gauvreau K. Blume E.D. Bastardi H.J. et al.Waiting list mortality among children listed for heart transplantation in the United States.Circulation. 2009; 119: 717-727Crossref PubMed Scopus (308) Google Scholar In particular, 1-year survival of infants placed on the cardiac transplantation waitlist was found in some studies to be as low as 55%, with especially high mortality for children weighing <2.5 kg at the time of listing or those on ventilatory or mechanical circulatory support.21Rizwan R. Zafar F. Chin C. Tweddell J. Bryant R. Morales D. Listing low-weight or ill infants for heart transplantation: is it prudent?.Ann Thorac Surg. 2018; 106: 1189-1196Abstract Full Text Full Text PDF PubMed Scopus (9) Google Scholar Although successful durable ventricular assist device (VAD) support in these children has been reported,22Bleiweis M.S. Philip J. Peek G.J. Fudge J.C. Sullivan K.J. Co-Vu J. et al.Palliation plus ventricular assist device insertion in 15 neonates and infants with functionally univentricular circulation.Ann Thorac Surg. March 15, 2022; (Online ahead of print)https://doi.org/10.1016/j.athoracsur.2022.02.051Abstract Full Text Full Text PDF Scopus (6) Google Scholar,23Weinstein S. Bello R. Pizarro C. Fynn-Thompson F. Kirklin J. Guleserian K. et al.The use of the Berlin Heart EXCOR in patients with functional single ventricle.J Thorac Cardiovasc Surg. 2014; 147 (discussion 704-5): 697-704Abstract Full Text Full Text PDF PubMed Scopus (176) Google Scholar such support is associated with high mortality and morbidity, which are, in turn, often associated with efforts made to maintain the difficult balance between the systemic and pulmonary circulations. Recent reviews suggested that survival of young children with univentricular circulation on VAD support was poor, with less than 50% survival by the 3-month mark.24Puri K. Adachi I. Mechanical support for the failing single ventricle at pre-Fontan stage: current state of the field and future directions.Semin Thorac Cardiovasc Surg Pediatr Card Surg Annu. 2021; 24: 10-18Abstract Full Text Full Text PDF PubMed Scopus (19) Google Scholar,25Rossano J.W. VanderPluym C.J. Peng D.M. Hollander S.A. Maeda K. Adachi I. et al.Fifth annual pediatric interagency registry for mechanical circulatory support (Pedimacs) report.Ann Thorac Surg. 2021; 112: 1763-1774Abstract Full Text Full Text PDF PubMed Scopus (47) Google Scholar Although VAD support in neonates and infants with failing univentricular circulation is feasible,26Villa C. Greenberg J.W. Morales D.L.S. Mechanical support for the failing single ventricle after Fontan.J Thorac Cardiovasc Surg Tech. 2022; 13: 174-181Scopus (2) Google Scholar bridging this subgroup to heart transplantation remains challenging. Furthermore, those surviving beyond infancy with staged univentricular palliations continue to need lifelong complex medical and surgical management that may eventually lead to heart transplantation.27Konstantinov I.E. Schulz A. Buratto E. Heart transplantation after Fontan operation.J Thorac Cardiovasc Surg Tech. 2022; 13: 182-191Scopus (6) Google Scholar,28Bleiweis M.S. Fudge J.C. Peek G.J. Vyas H.V. Cruz Beltran S. Pitkin A.D. et al.Ventricular assist device support in neonates and infants with a failing functionally univentricular circulation.J Thorac Cardiovasc Surg Tech. 2022; 13: 194-204Scopus (11) Google Scholar Thus, should a reliable source of “donor” hearts become available, primary heart transplantation would be an attractive option in neonates and infants with univentricular circulation. An International Society for Heart and Lung Transplantation report demonstrated that median survival for infants after heart transplantation was 22.3 years, which is substantially greater than in any other age group.29Rossano J.W. Cherikh W.S. Chambers D.C. Goldfarb S. Khush K. Kucheryavaya A.Y. et al.The Registry of the International Society for Heart and Lung Transplantation: twentieth pediatric heart transplantation report 2017; focus theme: allograft ischemic time.J Heart Lung Transplant. 2017; 36: 1060-1069Abstract Full Text Full Text PDF PubMed Scopus (90) Google Scholar In contrast, the SVR trial30Newburger J.W. Sleeper L.A. Gaynor J.W. Hollenbeck-Pringle D. Frommelt P.C. Li J.S. et al.Transplant-free survival and interventions at 6 years in the SVR trial.Circulation. 2018; 137: 2246-2253Crossref PubMed Scopus (151) Google Scholar demonstrated that survival at 6 years after multistage palliation for hypoplastic left heart syndrome (HLHS) was only 65%. Thus, survival in patients with HLHS after primary heart transplantation in the neonatal period appears to be greater than that after the Norwood operation.31Chinnock R.E. Bailey L.L. Heart transplantation for congenital heart disease in the first year of life.Curr Cardiol Rev. 2011; 7: 72-84Crossref PubMed Scopus (46) Google Scholar,32Wilder T.J. McCrindle B.W. Hickey E.J. Ziemer G. Tchervenkov C.I. Jacobs M.L. et al.Is a hybrid strategy a lower risk alternative to stage 1 Norwood palliation?.J Thorac Cardiovasc Surg. 2017; 153: 163-172Abstract Full Text Full Text PDF PubMed Scopus (50) Google Scholar Most importantly, long-term survival after neonatal heart transplantation for HLHS was reported to be 73.9%, 64.4%, and 55.8% at 10, 20, and 25 years, respectively.33John M.M. Razzouk A.J. Chinnock R.E. Bock M.J. Kuhn M.A. Martens T.P. et al.Primary transplantation for congenital heart disease in the neonatal period: long-term outcomes.Ann Thorac Surg. 2019; 108: 1857-1864Abstract Full Text Full Text PDF PubMed Scopus (16) Google Scholar It appears that the immature immune system of neonates and infants may bring important advantages.17Cleveland D.C. Jagdale A. Carlo W.F. Iwase H. Crawford J. Walcott G.P. et al.The genetically engineered heart as a bridge to allotransplantation in infants: just around the corner?.Ann Thorac Surg. 2022; 114: 536-544Abstract Full Text Full Text PDF PubMed Scopus (19) Google Scholar,34Bikhet M. Morsi M. Hara H. Rhodes L.A. Carlo W.F. Cleveland D. et al.The immune system in infants: relevance to xenotransplantation.Pediatr Transplant. 2020; 24: e13795Crossref PubMed Scopus (9) Google Scholar,35Li Q.I. Hara H. Banks Yamamoto T. Ayares D. Mauchley D.C. et al.Anti-pig antibody in infants: can genetically engineered pig heart bridge to allotransplantation?.Ann Thorac Surg. 2020; 109: 1268-1273Abstract Full Text Full Text PDF PubMed Scopus (18) Google Scholar Following the initial description of ABO-incompatible heart transplantation in infants,36West L.J. Pollock-Barziv S.M. Dipchand A.I. Lee K.J. Cardella C.J. Benson L.N. et al.ABO-incompatible heart transplantation in infants.N Engl J Med. 2001; 344: 793-800Crossref PubMed Scopus (369) Google Scholar several studies demonstrated that ABO-incompatible allotransplantation in young children (with immature immune systems) is not only possible but also provides good long-term results.37Dipchand A.I. Pollock-Barziv S.M. Manlhiot C. West L.J. VanderVliet M. McCrindle B.W. et al.Equivalent outcomes for pediatric heart transplantation recipients: ABO-blood group incompatible versus ABO-compatible.Am J Transplant. 2010; 10: 389-397Abstract Full Text Full Text PDF PubMed Scopus (92) Google Scholar, 38Urschel S. Larsen I.M. Kirk R. Flett J. Burch M. Shaw N. et al.ABO-incompatible heart transplantation in early childhood: an international multicenter study of clinical experiences and limits.J Heart Lung Transplant. 2013; 32: 285-292Abstract Full Text Full Text PDF PubMed Scopus (67) Google Scholar, 39Urschel S. Ballweg J.A. Cantor R.S. Koehl D.A. Reinhardt Z. Zuckerman W.A. et al.Clinical outcomes of children receiving ABO-incompatible versus ABO-compatible heart transplantation: a multicentre cohort study.Lancet Child Adolesc Health. 2021; 5: 341-349Abstract Full Text Full Text PDF PubMed Scopus (7) Google Scholar It appears that ABO-incompatible heart transplantation also results in persistent alteration of the immune response and tolerance to the donor blood group antigens.40Urschel S. Campbell P.M. Meyer S.R. Larsen I.M. Nuebel J. Birnbaum J. et al.Absence of donor-specific anti-HLA antibodies after ABO-incompatible heart transplantation in infancy: altered immunity or age?.Am J Transplant. 2010; 10: 149-156Abstract Full Text Full Text PDF PubMed Scopus (41) Google Scholar Interestingly, these changes in immune maturation are also associated with reduced response toward human leukocyte antigen epitopes and lower risk of posttransplantation infection.41Chen C.Y. Warner P. Albers E.L. Kemna M.S. Delaney M. Hong B.J. et al.Donor-specific anti-HLA antibody production following pediatric ABO-incompatible heart transplantation.Pediatr Transplant. 2019; 23: e13332Crossref PubMed Scopus (5) Google Scholar Thymectomy in neonates and infants alters T-cell responses.34Bikhet M. Morsi M. Hara H. Rhodes L.A. Carlo W.F. Cleveland D. et al.The immune system in infants: relevance to xenotransplantation.Pediatr Transplant. 2020; 24: e13795Crossref PubMed Scopus (9) Google Scholar Although the effects of thymectomy in young children undergoing heart transplantation are not yet fully understood,42Kim T.B. Ionescu L. Avdimiretz N. Murdoch F. Larsen I.M. Motyka B. et al.Alterations in the immune phenotype of thymectomized children and the development of atopic disorders after heart transplantation.Pediatr Transplant. 2022; 26: e14252Crossref PubMed Scopus (2) Google Scholar thymectomy may result in altered development of the T-cell profile, especially when combined with T-cell depletion treatments,43Eysteindottir I. Freysdottir J. Haraldsson A. Stefansdottir J. Skaftadottir I. Helgason H. et al.The influence of partial or total thymectomy during open heart surgery in infants on the immune function later in life.Clin Exp Immunol. 2004; 136: 349-355Crossref PubMed Scopus (86) Google Scholar, 44Halnon N.J. Jamieson B. Plunkett M. Kitchen C.M.R. Pham T. Krogstad P. Thymic function and impaired maintenance of peripheral T-cell populations in children with congenital heart disease and surgical thymectomy.Pediatr Res. 2005; 57: 42-48Crossref PubMed Scopus (76) Google Scholar, 45Applay V. Sauce D. Prelog M. The role of the thymus in immunosenescence: lessons from the study of thymectomized individuals.Aging. 2010; 2: 78-81Crossref PubMed Scopus (54) Google Scholar and play an important role in mitigating the immune response to an allograft (or xenograft) and in the induction of immunological tolerance.43Eysteindottir I. Freysdottir J. Haraldsson A. Stefansdottir J. Skaftadottir I. Helgason H. et al.The influence of partial or total thymectomy during open heart surgery in infants on the immune function later in life.Clin Exp Immunol. 2004; 136: 349-355Crossref PubMed Scopus (86) Google Scholar,46Pan H. Gazarian A. Dubernard J.M. Belot A. Michallet M.C. Michallet M. Transplant tolerance induction in newborn infants: mechanisms, advantages, and potential strategies.Front Immunol. 2016; 7: 116Crossref PubMed Scopus (13) Google Scholar However, this appears to be associated with atopic disorders after heart transplantation.47Avdimiretz N. Seitz S. Kim T. Murdoch F. Urschel S. Allergies and autoimmune disorders in children after heart transplantation.Clin Transplant. 2018; 32: e13400Crossref PubMed Scopus (8) Google Scholar In contrast to patients with DiGeorge syndrome, who lack normal thymic function, there has been no reported increase in infection in children after neonatal thymectomy, which may be related to the fact that some T cells are produced and matured during fetal and early neonatal life.48Ogle B.M. West L.J. Driscoll D.J. Strome S.E. Razonable R.R. Paya C.V. et al.Effacing of the T cell compartment by cardiac transplantation in infancy.J Immunol. 2006; 176: 1962-1967Crossref PubMed Scopus (45) Google Scholar Even after visually complete thymectomy, T cells are observed in peripheral blood.49Mengrelis K. Kucera F. Shahid N. Watt E. Ross S. Lau C.I. et al.T cell phenotype in paediatric heart transplant recipients.Pediatr Transplant. 2021; 25: e13930Crossref PubMed Scopus (5) Google Scholar After thymectomy in the neonatal period, the cotransplantation of pig thymic tissue could possibly contribute to induction of immunological tolerance to a bioengineered pig heart.50Yamada K. Scalea J. Thymic transplantation in pig-to-non-human primates for the induction of tolerance across xenogenetic barriers.Methods Mol Biol. 2012; 885: 191-212Crossref PubMed Scopus (12) Google Scholar,51Cooper D.K. Hara H. Iwase H. Banks C.A. Cleveland D. An approach to induction of tolerance to pig cardiac xenografts in neonates.Xenotransplantation. 2018; 25: e12454Crossref PubMed Scopus (12) Google Scholar The Markert technique, by which cultured thymic tissue is implanted into the quadriceps muscle, may induce tolerance52Kwun J. Li J. Rouse C. Park J.B. Farris A.B. Kuchibhatla M. et al.Cultured thymus tissue implantation promotes donor-specific tolerance to allogeneic heart transplants.JCI Insight. 2020; 5: e129983PubMed Google Scholar,53Fitch Z.W. Kang L. Li J. Knechtle S.J. Turek J.W. Kirk A.D. et al.Introducing thymus for promoting transplantation tolerance.Allergy Clin Immunol. 2022; 150: 549-556Abstract Full Text Full Text PDF Scopus (3) Google Scholar and has recently been applied in clinical heart transplantation.54Kang L. Markert M.L. Turek J.W. Induction of donor-specific tolerance to heart transplantation: from concept to clinical translation.J Thorac Cardiovasc Surg. January 14, 2022; ([Epub ahead of print])Abstract Full Text Full Text PDF Scopus (1) Google Scholar Rapid growth of porcine xenografts after transplantation in nonhuman primates was reported as long ago as 200055Soin B. Ostlie D. Cozzi E. Smith K.G. Bradley J.R. Vial C. et al.Growth of porcine kidneys in their native and xenograft environment.Xenotransplantation. 2000; 7: 96-100Crossref PubMed Scopus (30) Google Scholar and has been confirmed by several groups.56Iwase H. Liu H. Wijkstrom M. Zhou H. Singh J. Hara H. et al.Pig kidney graft survival in a baboon for 136 days: longest life-supporting organ graft survival to date.Xenotransplantation. 2015; 22: 302-309Crossref PubMed Scopus (174) Google Scholar, 57Iwase H. Hara H. Ezzelarab M. Li T. Zhang Z. Gao B. et al.Immunological and physiologic observations in baboons with life-supporting genetically-engineered pig kidney grafts.Xenotransplantation. 2017; 24https://doi.org/10.1111/xen.12293Crossref Scopus (161) Google Scholar, 58Tanabe T. Watanabe H. Shah J.A. Sahara H. Shimizu A. Nomura S. et al.Role of intrinsic (graft) versus extrinsic (host) factors in the growth of transplanted organs following allogeneic and xenogeneic transplantation.Am J Transplant. 2017; 17: 1778-1790Abstract Full Text Full Text PDF PubMed Scopus (75) Google Scholar It appears that the growth of the transplanted heart is multifactorial, and recipient's blood pressure may play an important role.59Goerlich C.E. Griffith B. Hanna P. Hong S.N. Ayeares D. Singh A.K. et al.The growth of xenotransplanted hearts can be reduced with growth hormone receptor knockout pig donors.J Thorac Cardiovasc Surg. September 4, 2021; ([Epub ahead of print])Abstract Full Text Full Text PDF PubMed Scopus (27) Google Scholar Rapid growth of a bioengineered pig heart after transplantation in neonates and infants may be problematic if heart growth is disproportionate to the recipient's somatic growth. Growth of pig organs can be reduced by deletion of the gene for the growth hormone receptors.59Goerlich C.E. Griffith B. Hanna P. Hong S.N. Ayeares D. Singh A.K. et al.The growth of xenotransplanted hearts can be reduced with growth hormone receptor knockout pig donors.J Thorac Cardiovasc Surg. September 4, 2021; ([Epub ahead of print])Abstract Full Text Full Text PDF PubMed Scopus (27) Google Scholar, 60Hinrichs H.A. Kesseler B. Kurome M. Blutke A. Kemter E. Bernau M. et al.Growth hormone receptor deficient pigs resemble the pathophysiology of human Laron syndrome and reveal altered activation of signaling cascades in liver.Mol Metab. 2018; 11: 113-128Crossref PubMed Scopus (60) Google Scholar, 61Iwase H. Ball S. Adams W.T. eyestone W. Walter A. Cooper D.K.C. Growth hormone receptor knockout: relevance to xenotransplantation.Xenotransplantation. 2020; 14: e12652Google Scholar An alternative would be to use a miniature pig as the source of the xenograft heart, eg, Auckland Island pigs or Yucatan pigs. Furthermore, there is evidence that inclusion of rapamycin in the immunosuppressive regimen appears to be associated with less rapid growth of the pig organ xenograft.2Langin M. Mayr T. Reichart B. Michel S. Buchholz S. Guetholf S. et al.Consistent success in life-supporting porcine cardiac xenotransplantation.Nature. 2018; 564: 430-433Crossref PubMed Scopus (291) Google Scholar,56Iwase H. Liu H. Wijkstrom M. Zhou H. Singh J. Hara H. et al.Pig kidney graft survival in a baboon for 136 days: longest life-supporting organ graft survival to date.Xenotransplantation. 2015; 22: 302-309Crossref PubMed Scopus (174) Google Scholar,57Iwase H. Hara H. Ezzelarab M. Li T. Zhang Z. Gao B. et al.Immunological and physiologic observations in baboons with life-supporting genetically-engineered pig kidney grafts.Xenotransplantation. 2017; 24https://doi.org/10.1111/xen.12293Crossref Scopus (161) Google Scholar Pigs, with their average life-expectancy of 12 to 18 years, do not live as long as humans. Thus, even if perfect immunological tolerance developed, it is possible that pig heart xenograft longevity might be limited (although a second size-matched porcine heart would be readily available). Thus, as an initial step in clinical xenotransplantation, the implantation of a pig heart as a bridge to human heart transplantation is compelling.16Cleveland D. Adam Banks C. Hara H. Carlo W.F. Mauchley D.C. Cooper D.K.C. The case for cardiac xenotransplantation in neonates: is now the time to reconsider xenotransplantation for hypoplastic left heart syndrome?.Pediatr Cardiol. 2019; 40: 437-444Crossref PubMed Scopus (31) Google Scholar,17Cleveland D.C. Jagdale A. Carlo W.F. Iwase H. Crawford J. Walcott G.P. et al.The genetically engineered heart as a bridge to allotransplantation in infants: just around the corner?.Ann Thorac Surg. 2022; 114: 536-544Abstract Full Text Full Text PDF PubMed Scopus (19) Google Scholar Xenotransplantation could be performed as a primary operation or after failed initial palliation. In children with previous cardiac surgery, previous testing for antibody binding to pig cells will be required to exclude the possibility of sensitization.62Oscherwitz M. Nguyen H.Q. Raza S.S. Cleveland D. Padilla L.A. Sorabella R. et al.Will previous palliative surgery for congenital heart disease be detrimental to subsequent pig heart xenotransplantation?.Transplant Immunol. 2022; 74: 101661Crossref PubMed Scopus (1) Google Scholar In contrast, sensitization to porcine xenoantigens, if it occurs, does not appear thus far to be detrimental to subsequent allotransplantation.63Li Q. Hara H. Zhang Z. Breimer M.E. Wang Y. Cooper D.K.C. Is sensitization to pig antigens detrimental to subsequent allotransplantation?.Xenotransplantation. 2018; 25: e12393Crossref PubMed Scopus (39) Google Scholar, 64Hara H. Nguyen H. Wang Z.-Y. Jagdale A. Bikhet M.H. Yamamoto T. et al.Evidence that sensitization to triple-knockout pig cells will not be detrimental to subsequent allotransplantation.Xenotransplantation. 2021; 28: e12701Crossref PubMed Scopus (11) Google Scholar, 65Cooper D.K.C. Habibabady Z. Kinoshita K. Hara H. Pierson III, R.N. The respective relevance of sensitization to alloantigens and xenoantigens in pig organ xenotransplantation.Hum Immunol. July 8, 2022; (Online ahead of print)https://doi.org/10.1016/j.humimm.2022.06.003Crossref PubMed Scopus (8) Google Scholar As an alternative, if tolerance to a pig heart develops, it might be possible to retransplant with a second size-matched pig heart, if retransplantation is required due to lack of growth of the organ. It is also tempting to speculate that eventually an autologous human bioengineered heart might be “manufactured” using blastocyst complementation or some other method.9Platt J.L. Cascalho M. The future of transplantation.N Engl J Med. 2022; 387: 77-78Crossref PubMed Scopus (7) Google Scholar,66Konstantinov I.E. King G. Porrello E.R. From genome editing to blastocyst complementation: a new horizon in heart transplantation?.J Thorac Cardiovasc Surg Tech. 2022; 12: 177-184Google Scholar Thus, in summary, if xenotransplantation is performed in neonates and infants, it could potentially be carried out as a bridge to human heart transplantation (either an allotransplantation or, if an autologous heart could be produced in the future using the patient's own tissues, autotransplantation) or to a second xenotransplantation using a size-matched pig heart. In the United States, organ-source pigs would need to be bred and housed in a designated pathogen-free (DPF) facility approved by the US Food and Drug Administration to prevent transmission of zoonotic microorganisms.67Denner J. Langin M. Reichart B. Kruger L. Fiebig U. Mokelke M. et al.Impact of porcine cytomegalovirus on long-term orthotopic cardiac xenotransplant survival.Sci Rep. 2020; 10: 17531Crossref PubMed Scopus (43) Google Scholar,68Kraebber K. Gray E. Addressing regulatory requirements for the organ-source pig—a pragmatic approach to facility design and pathogen prevention.in: Cooper D.K.C. Byrne G. Clinical Xenotransplantation. Springer, 2020: 141-153Crossref Scopus (6) Google Scholar The bioengineered heart from the pig that was transplanted into the patient at the University of Maryland at Baltimore carried latent porcine cytomegalovirus (pCMV)7Griffith B.P. Goerlich C.E. Singh A.K. Rothblatt M. Lau C.L. Shah A. et al.Genetically modified porcine-to-human cardiac xenotransplantation.N Engl J Med. 2022; 387: 35-44Crossref PubMed Scopus (154) Google Scholar that escaped the currently validated detection assay that was available and was later detected by using very sensitive nested polymerase chain reaction in donor spleen and transplanted heart. There are studies in progress to evaluate whether this latent virus replicated and caused damage to the transplanted heart. There is no definitive evidence so far that this virus caused any pathology in the patient. It should be noted that the pig donor used for transplantation was not raised in a DPF facility, although it was a clean facility at which the pigs are individually housed. Monitoring for potentially pathogenic microorganisms in the organ-source pig therefore requires meticulous attention.68Kraebber K. Gray E. Addressing regula