A Xenotransplant Performed Using a Pig Liver into a Brain-dead Recipient – Hurdles, Ethical Dilemmas and Future Perspectives

Article information Mallapaty S. First pig liver transplanted into a person lasts for 10 days. Nature 2024;627:710-1. SUMMARY A recent news item published in Nature reported a groundbreaking achievement in xenotransplantation when a genetically modified whole liver from a pig, which had been maintained in a pathogen-free facility and screened for all the pathogens, was transplanted into a brain-dead, heartbeating patient on 10th March 2024. This was performed by the transplant team at the Xijing Hospital of the Air Force Medical University in Xi'an, China. After obtaining approval from the recipient's family and several university committees, the procedure was carried out according to national and international regulations. The 700 g liver was retrieved from a miniature pig, which had had six genetic modifications, three to prevent rejection and three to prevent virus transmission, and was transplanted heterotopically. After 10 days, a normally functioning pig liver was removed. COMMENTARY Although clinically brain-dead individuals provide a valuable model for evaluating xenotransplantation' potential, in living recipients, their utility is constrained by physiological changes that occur once brain activity ceases. Specifically, hormonal alterations occur after brain death, complicating the assessment of the long-term viability of the transplanted organ. In the present case the post transplantation liver structure and function were assessed with liver biopsy and serial blood tests to rule out acute rejection. The colour of the liver was normal, bile production of around 30 mL/day and there was an absence of evidence of rejection on biopsy. Controversy still remains regarding the long-term sustainability of pig liver transplantation, as the liver has complex functions, including detoxification of unwanted substances and production of an array of complex proteins. Till now, it can be used as a temporary solution or as a bridge therapy for a failing liver or for acute liver failure in persons waiting for allotransplantation. However, xenotransplantation has opened a new horizon in this field and provided new hope to persons dying because of the lack of available organs. Calne et al. performed the first pig to non-human primate experimental liver xenotransplant in 1968.[1] In September 2021, at the NYU Langone Health Hospital in New York, a gene-edited kidney was transplanted into a brain-dead human. It showed function with a good urine output. Starzl et al. performed the first baboon liver-human recipient on 28 June 1992, with a survival of 70 days.[2] The patient passed away from severe sepsis as a complication of over immunosuppression. The first clinical pig liver to human xenotransplant in history was performed by Makowka et al. in 1993 in a 26-year-old woman with autoimmune hepatitis who was admitted with grade III encephalopathy. The patient survived the following 34 h with improved liver function.[3] Since then, research has evolved from transplanting porcine organs to non-human primates to gene-edited organs as bridge therapy before human allotransplantation. A very recent milestone was achieved on 16 March 2024, when doctors at the Massachusetts General Hospital in Boston, USA, successfully transplanted the world's first genetically engineered pig kidney into a living human being who had recurrent chronic kidney disease 7 years after he had undergone a previous allotransplant.[4] The patient was discharged from the hospital on 3rd April, perfectly healthy. In this case, because of the long waiting period, and the non-availability of organs, the transplant team discussed the risks and benefits, and ethical aspects of the procedure with him and then proceeded to perform the operation (This procedure was performed under the US FDA compassionate use provision, which is applied in patients with life-threatening illnesses, who may benefit from new treatment modalities, yet to be approved). These recent successes may change the lives of patients with chronic kidney disease as it will abolish haemodialysis, which is uncomfortable and expensive as a treatment modality and promises to herald a new era in organ transplantation. Chronic or acute liver failure accounts for around 2 million deaths annually worldwide.[5] The only cure is a transplant, for which many people die waiting or cannot afford the procedure, which costs between 20 and 40 lakhs here. In India, there are currently over 60,000 individuals waiting for a liver transplant. Due to their clinical decline or financial situation, many people die before they can have a transplant. This number may be even greater if it included patients too sick to be enlisted and patients with hepatocellular carcinoma beyond Milan's criteria. If an unlimited number of organs from other species are available, the procedure will become accessible and affordable and many of these deaths can possibly be prevented. The easy availability of pigs, and the similarity in their size, anatomy and physiology to humans, makes them the species of choice for xenotransplantation compared to non-human primates in spite of the latter being more genetically matched. Pigs have more children per litter (10 vs. 1) and mature and achieve human size faster (6 months vs. 15 years for a maximum size of 4 ft 11 in) compared with non-human primates.[6] Pigs can be bred in a domesticated environment, whereas non-human primates are mostly wild animals that have a high risk of bearing unknown pathogens. Some aspects to be considered before xenotransplantation are: Pigs are used because they are easier to breed, house and develop. They are preferred to non-human primates because these are largely caught in the wild and there is much less information about the viruses and bacteria they carry. Pigs also have a lower risk of zoonosis, but the risk for infectious transmission is yet to be determined. The secret to preventing zoonosis is to breed and maintain 'designated pathogen-free' pigs in a biosecure, 'super-clean' facility.[7] Prophylaxis is used against pneumocystis infection as well as non-human primate CMV activation. When it comes to pig CMV, ganciclovir and valganciclovir are comparatively ineffective. Prophylaxis has to be given for the prevention of human CMV reactivation in recent clinical trials. There are also less ethical concerns as many people use pigs for food Genetic editing is necessary to reduce the possibility of rejection of the xenograft. During the past three decades, pig genetic engineering techniques have evolved, ranging from microinjection of randomly-integrating transgenes (1992), somatic cell nuclear transfer (2000) to CRISPR/Cas9(2014).[8] To address organ scarcity, SCNT and genome editing may make it possible to produce pigs whose tissues might be utilised in clinical xenotransplantation. To develop transgenic pigs, CRISPRs can be employed to generate targeted gene insertions or knock out genes. In essence, CRISPRs are enzymes that using a guide RNA, can locate and cut certain DNA regions. The plan is to knock out the pig genes that make sugars or enzymes against which the human response is directed. THE FUTURE It has been suggested that if researchers can make 10 baboon recipients survive for 3 months after pig liver donations, this will justify considering a clinical trial in humans. The deepest fear, of course, is that some undetected pig virus can infect a human population which has no resistance to it and we will end up having another AIDS epidemic. However, if xenotransplantation is successful, it will save the lives of hundreds and thousands of people who cannot be treated for want of a functioning organ or because they cannot afford this costly procedure. The recent reports of small successes have provided hope that this will occur in the not-too-distant future. Financial support and sponsorship Nil. Conflicts of interest There are no conflicts of interest.