Application Of Xenotransplantation Research Articles

The Young Investigator Committee of the International Xenotransplantation Association-Perspective of advancements in the field in 2023.

The 2023 IXA conference, hosted in San Diego, CA, brimmed with excitement against the backdrop of recent innovations in both the pre-clinical and clinical realms with several first-in-human applications of xenotransplantation. The theme, "Pigs are flying," alluded to the adage that xenotransplantation would only become a clinical reality "when pigs fly," suggesting a day that might never come. The event witnessed significant attendance, with 600 participants-the highest in the history of an IXA-IPITA joint congress. Among the attendees were members of the Food and Drug Administration (FDA), the National Institutes of Health (NIH), and corporate sponsors deeply engaged in the field. We summarize the latest topics from the congress, ranging from the pros/cons of decedent models of xenotransplantation and genetic engineering of porcine heart valves, solid organs, and cells for clinical translation and their regulatory and ethical landscape.

The potential of cardiac xenotransplantation for management of infants with complex congenital heart disease.

Gene editing of the porcine genome has enabled the production of pigs that do not express the three known carbohydrate antigens that are associated with hyperacute rejection of a pig organ xenotransplant. In addition, it is now possible to insert a variety of human transgenes to protect against the human immune response, e.g., to protect from complement and coagulation activation. As a result, cardiac xenotransplantation of the gene-edited porcine heart is progressing towards clinical application. Many hope that it will definitively address the disparity between organ supply and demand. The role of cardiac xenotransplantation in pediatric care remains controversial but we believe there is an infant patient population with complex congenital heart disease (CHD) (not optimally managed by conventional surgical approaches) that is ideally suited to initial clinical application of this new technology. The most efficacious start would be to initiate clinical use as a short-term bridge to allotransplantation, particularly in infants with single ventricle pathology and significant risk factors for first stage Norwood palliation. Infants with end-stage heart failure after first stage palliation would represent a second target population. Infants experience unacceptably high mortality and morbidity when placed on mechanical circulatory support as a bridge to allotransplant. Effectively bridging these vulnerable populations could promote acceptance of cardiac xenotransplantation, allowing indications and use to expand, e.g., by (I) bridging patients with failed second and third stage single ventricle disease, or (II) with complex biventricular CHD, or (III) those with a restrictive or dilated cardiomyopathy. Finally, there is a reasonable expectation that the immunologic privilege of infants will allow porcine heart xenotransplantation to be destination therapy for some patients. In summary, heart allotransplantation in infants offers superior outcomes when compared to three-stage single ventricle palliation, but there is a continual shortage of deceased human donor organs. We should pursue research towards the application of xenotransplantation in patients with single ventricle pathology, in whom the results of staged palliation are likely to be suboptimal. There are many remaining issues to be resolved before cardiac xenotransplantation enters regular pediatric clinical use, but experience in this field is progressing rapidly.

In pursuit of xenoreactive antibodies: Where has it gotten us?

Numerous studies have aimed to overcome the barrier to xenotransplantation posed by xenoreactive antibodies and the antigens they recognize. Whether this work will eventually lead to the widespread clinical application of xenotransplantation remains unknown. However, the benefits of this research are already substantial, with research leading to dramatic new developments in fields other than xenotransplantation. Our understanding of natural immunity, particularly the nature and function of natural antibodies, has taken quantum leaps forward, with far-reaching implications. Our improved understanding of the immune response to xenografts has proven invaluable in the characterization of the human immune reaction to commonly used biological therapeutics of xenogeneic origin. Our understanding of cell surface carbohydrates and our ability to modify these carbohydrates in living animals has advanced substantially, with implications for diseases such as cancer and autoimmunity. With this in mind, it is argued that continued work in xenotransplantation is of great value, not only because of the great potential benefits of xenotransplantation, but also because of the more certain benefits that arise from setting our sights on a difficult challenge.

Genetic modification of xenografts.

For nearly a century, xenotransplantation has been seen as a potential approach to replacing organs and tissues damaged by disease. Until recently, however, the application of xenotransplantation has seemed only a remote possibility. What has changed this perspective is the advent of genetic engineering of large animals; that is, the ability to add genes to and remove genes from lines of animals that could provide an enduring source of tissues and organs for clinical application. Genetic engineering could address the immunologic, physiologic and infectious barriers to xenotransplantation, and could allow xenotransplantation to provide a source of cells with defined and even controlled expression of exogenous genes. This communication will consider one perspective on the application of genetic engineering in xenotransplantation.

Xenotransplantation of the kidney: a pediatric perspective.

The major factor limiting the application of allotransplantation for the treatment of renal failure is a severe shortage of donor organs. One approach to overcoming this limitation is the use of kidneys from animals for clinical transplantation, that is xenotransplantation. Although of interest for many years, the application of xenotransplantation has been prevented because of the devastating immune responses of the recipient against the graft. Research conducted in recent years has elucidated the immune mechanisms underlying the rejection of xenografts and has led to the development of incisive therapeutic strategies, including the genetic engineering of donor animals. Success in dealing with the xenogeneic immune response has encouraged the view that xenotransplantation might be feasible in the near future. It also raises consideration of two additional hurdles, the potential limitations of xenogeneic kidneys as physiological replacement for human kidneys and the possibility that the animal organ might transfer infectious organisms to the recipient and to the wider population. This paper reviews recent progress in the field of xenotransplantation of the kidney and offers a perspective on the hurdles to clinical application that remain.

Issues in xenotransplantation.

The shortage of organs for transplantation is especially severe for the critically ill newborn infant, for whom donors of the appropriate size are particularly scarce. One way to overcome this problem is to use animals in lieu of human as organ donors. The major limitation to using animals for this purpose is the susceptibility of animal organs to hyperacute rejection, a violent rejection reaction thought to be mediated by antidonor antibody and complement. To evaluate the potential application of xenotransplantation to newborns, we tested neonatal humans and neonatal baboons, and found that neither populatioh expressed significant levels of xenoreactive antipig antibodies. We transplanted heterotopically hearts from newborn pigs into unmanipulated newborn baboons (n = 4). There was no evidence of hyperacute rejection in any of the grafts; the animals were killed with functioning grafts at 15, 81, and 82 hour. This outcome contrasts with that of newborn pig-to-mature baboon and mature pig-to-mature baboon cardiac xenografts, which were rejected within one hour of transplantation. The histology of pig graft biopsies from the new born recipients was normal. Immunohistochemistry revealed only traces of IgM, C3, C4 and the membrane attack complex along graft endothelium. Fibrin deposition along endothelial surfaces was observed early after transplantation and became more extensive with time; in the absence of endothelial bound antibody or complement, this change may represent preservation injury. This study suggests that due to low levels of natural antibody, the newborn infant may permit prolonged xenograft survival.

The Application of Xenotransplantation in Humans--Reasons to Delay.

There is consensus, among those involved in the field of clinical transplantation, that a need exists for a larger pool of organ donors. Efforts to enlarge the current pool of cadaver organ donors have not alleviated what has now become a critical shortage of donors. As a result, substantial numbers of patients with end stage disease of vital organs, 2,359 in 1991—an increase of 20 percent over the previous year (according to the most recently available UNOS figures)—are dying while awaiting transplantation (DHHS 1993). Those on the waiting list are waiting longer for their organs, and even if efforts currently in place were able to maximize the pool of cadaver donors, there would still be a shortfall in meeting potential demand. This situation is particularly frustrating to clinicians given the high rate of successful outcome in those transplants that do occur. For these reasons interest is focused on xenogeneic tissue as an alternative for those requiring transplantation. The progress made in the field of experimental xenotransplantation over the past few years has further encouraged clinicians, particularly the measurable success that has been achieved in the survival of transplanted xenogeneic organs under certain circumstances, most notably in concordant rodent species (Hassan and others 1992). However, the barriers to successful clinical xenotransplantation are much greater than those to allogeneic transplantation, and clinically successful xenotransplantation has yet to be achieved.

PROTECTION OF PORCINE AORTIC ENDOTHELIAL CELLS FROM COMPLEMENT-MEDIATED CELL LYSIS AND ACTIVATION BY RECOMBINANT HUMAN CD59

Discordant xenogeneic organ transplantation is a potential solution to the critical shortage of suitable donor organs. However, clinical application of xenotransplantation with physiologically suitable organs such as those from the pig, is currently limited by the lack of agents to prevent antibody and complement-mediated hyperacute rejection of the transplanted organ. We have used retrovirus-mediated gene transfer to express the terminal complement inhibitor protein, human CD59, in neonatal porcine aortic endothelial cells (nPAEC). Human CD59 was constitutively expressed in nPAECs at levels similar to that of native CD59 in human umbilical vein endothelial cells. The protein was tethered to the cell surface by a glycosyl-phosphatidylinositol anchor, as demonstrated by its removal following treatment with phosphatidylinositol-specific phospholipase C. In a model of antibody-dependent complement activation, nPAECs expressing human CD59 were protected from membrane pore formation and cell lysis by complement derived from either human or baboon sera. Conversely, nPAECs expressing CD59 were not protected from lysis by rabbit or dog complement, indicating that recombinant CD59 retained its species-restricted inhibitory activity. Additionally, CD59 expressed on nPAECs inhibited the C5b-9-dependent generation of membrane prothrombinase activity. Collectively, these data establish that stable expression of human CD59 on xenotypic (porcine) endothelial cells renders these cells resistant to both the cytolytic and procoagulant effects of human complement. We propose that expression of recombinant human CD59 on porcine donor organs may prevent complement-mediated lysis and activation of endothelial cells that leads to hyperacute rejection.