Ethical and legislative advances in xenotransplantation for clinical translation: focusing on cardiac, kidney and islet cell xenotransplantation

In this state-of-the-art review we detail the journey of xenotransplantation from its infancy, detailing one of the first published cases and the subsequent journey the field took in its inception and development. With a focus on the science, technological advances, precautions required along with the potential limitations in application, the ethics, guidance's, and legislative advances that are required to reach the safe and efficacious clinical application of xenotransplantation. Along with a view over the past several decades with the overall significant advancements in pre-clinical study outcomes particularly in islet, kidney, and heart xenotransplantation, to ultimately reach the pinnacle of successful clinical heart and kidney xenotransplants. It outlines the importance for the appropriate guidance's required to have been developed by experts, scientists, clinicians, and other players who helped develop the field over the past decades. It also touches upon patient advocacy along with perspectives and expectations of patients, along with public opinion and media influence on the understanding and perception of xenotransplantation. It discusses the legislative environment in different jurisdictions which are reviewed in line with current clinical practices. All of which are ultimately based upon the guidance's developed from a strong long-term collaboration between the International Xenotransplantation Association, the World Health Organisation and The Transplantation Society; each having constantly undertaken consultation and outreach to help develop best practice for clinical xenotransplantation application. These clearly helped forge the legislative frameworks required along with harmonization and standardization of regulations which are detailed here. Also, in relation to the significant advances in the context of initial xeno-kidney trials and the even greater potential for clinical xeno-islet trials to commence we discuss the significant advantages of xenotransplantation and the ultimate benefit to our patients.

Xenotransplantation: back to the future?

The field of xenotransplantation has fluctuated between great optimism and doubts over the last 50 years. The initial clinical attempts were extremely ambitious but faced technical and ethical issues that prompted the research community to go back to preclinical studies. Important players left the field due to perceived xenozoonotic risks and the lack of progress in pig-to-nonhuman-primate transplant models. Initial apparently unsurmountable issues appear now to be possible to overcome due to progress of genetic engineering, allowing the generation of multiple-xenoantigen knockout pigs that express human transgenes and the genomewide inactivation of porcine endogenous retroviruses. These important steps forward were made possible by new genome editing technologies, such as CRISPR/Cas9, allowing researchers to precisely remove or insert genes anywhere in the genome. An additional emerging perspective is the possibility of growing humanized organs in pigs using blastocyst complementation. This article summarizes the current advances in xenotransplantation research in nonhuman primates, and it describes the newly developed genome editing technology tools and interspecific organ generation.

Gene editing advance re-ignites debate on the merits and risks of animal to human transplantation

In Australia, and internationally, the shortage of organ and tissue donors significantly limits the number of patients with critical organ or tissue failure who are able to receive a transplant each year. The rationale for xenotransplantation - the transplantation of living cells, tissues or organs from one species to another - is to meet this shortfall in human donor material. While early clinical trials showed promise, particularly in patients with type I diabetes whose insulin dependence could be temporarily reversed by the transplantation of porcine islet cells, these benefits have been balanced with scientific, clinical and ethical concerns revolving around the risks of immune rejection and the potential transmission of porcine endogenous retroviruses or other infectious agents from porcine grafts to human recipients. However, the advent of CRISPR/Cas9, a revolutionary gene editing technology, has reignited interest in the field with the possibility of genetically engineering porcine organs and tissues that are less immunogenic and have virtually no risk of transmission of porcine endogenous retroviruses. At the same time, CRISPR/Cas9 may also open up a myriad of possibilities for tissue engineering and stem cell research, which may complement xenotransplantation research by providing an additional source of donor cells, tissues and organs for transplantation into patients. The recent international symposium on gene editing, organised by the US National Academy of Sciences, highlights both the enormous therapeutic potential of CRISPR/Cas9 and the raft of ethical and regulatory challenges that may follow its utilisation in transplantation and in medicine more generally.

Long-term safety from transmission of porcine endogenous retrovirus after pig-to-non-human primate corneal transplantation

BACKGROUND

The risk of xenozoonosis mainly by porcine endogenous retrovirus (PERV) has been considered as one of the main hurdles in xenotransplantation and therefore should be elucidated prior to the clinical use of porcine corneal grafts. Accordingly, an investigation was performed to analyze the infectivity of PERVs from porcine keratocytes to human cells, and the long-term risk of transmission of PERVs was determined using pig-to-non-human primate (NHP) corneal transplantation models.

METHODS

The infectivity of PERVs from the SNU miniature pig keratocytes was investigated by coculture with a human embryonic kidney cell line. Twenty-two rhesus macaques underwent xenocorneal transplantation as follows: (i) group 1 (n=4): anterior lamellar keratoplasty (LKP) with freshly preserved porcine corneas, (ii) group 2 (n=5): anterior LKP with decellularized porcine corneas followed by penetrating keratoplasty (PKP) with allografts, (iii) group 3 (n=3): PKP under steroid-based immunosuppression, (iv) group 4 (n=4): PKP under anti-CD154 antibody-based immunosuppression, (v) group 5 (n=4): deep anterior LKP with freshly preserved porcine corneas under anti-CD40 antibody-based immunosuppression, and (vi) group 6 (n=2): PKP under anti-CD40 antibody-based immunosuppression. Postoperative blood samples were serially collected, and tissue samples were obtained from thirteen different organs at the end of each experiment. The existence of PERV DNA and RNA was investigated using PCR and RT-PCR.

RESULTS

Using two independent in vitro infectivity tests, neither PERV pol nor pig mitochondrial cytochrome oxidase II was detected after 41 and 92 days of coculture, respectively. After xenocorneal transplantation, a total of 257 serial peripheral blood mononuclear cell samples, 34 serial plasma samples, and 282 tissue samples were obtained from the NHP recipients up to 1176 days post-transplantation. No PERV transmission was evident in any samples.

CONCLUSIONS

Within the limits of this study, there is no evidence to support any risk of PERV transmission from porcine corneal tissues to NHP recipients, despite the existence of PERV-expressing cells in porcine corneas.

An approach to the control of disease transmission in pig-to-human xenotransplantation

Although several major immunologic hurdles need to be overcome, the pig is currently considered the most likely source animal of cells, tissues and organs for transplantation into humans. Concerns have been raised with regard to the potential for the transfer of infectious agents with the transplanted organ to the human recipient. This risk is perceived to be increased as it is likely that the patient will be iatrogenically immunocompromised and the organ-source pig may be genetically engineered in such a way to render its organs particularly susceptible to infection with human viruses. Furthermore, the risk may not be restricted to the recipient, but may have consequences for the health of others in the community. The identification of porcine endogenous retroviruses and of hitherto unknown viruses have given rise to the most concern. We document here the agents we believe should be excluded from the organ-source pigs. We discuss the likelihood of achieving this aim and outline the potential means by which it may best be achieved.