Xenotransplantation - Nuffield Council on Bioethics

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Animal-to-Human Transplants : the ethics of xenotransplantation Assembling information about the risks of disease transmission 6.24 Experimental studies can be used to try to identify unknown infectious agents in animal tissue. 22 Transplantation of pig or baboon tissue into immunocompromised mice might allow the identification of previously unknown infectious organisms. It is also possible to test whether animal viruses will infect human cells grown in the laboratory, using co-cultivation techniques in which human cells and infected animal cells are grown in the same container. This technique has shown that baboon retroviruses can infect human cells. New molecular techniques allow virologists to search for the genetic material of previously unknown viruses. 23 6.25 Such research is not without its difficulties. Results obtained using human cells in tissue culture may not always be applicable to human recipients. But there is no doubt that much useful laboratory research can be done, particularly in the area of endogenous retroviruses which may present particular risks. Useful research could be done to identify the different retroviruses present in source animals, to assess whether the viruses are activated by transplantation, and to examine the rate of mutation shown by the viruses. Research involving animal xenograft recipients could be useful in this respect. Primate recipients of pig organs, for example, could be assessed in order to determine whether porcine retroviruses are activated after transplantation, and whether the viruses then infect the primate tissue. 6.26 Another area where research is needed is in improving the diagnostic tests used to identify infectious organisms. Many of the tests used to identify infectious organisms in animals were originally developed as tests for closely related organisms that infect human beings. When the tests are used with animals, the results are often less reliable, increasing the risk that an animal with a negative test result may in fact be infected with the organism. The technology is now available to develop tests which analyse the genetic material of infectious organisms. Such DNA-based analysis is often more reliable than tests which measure the antibody levels in the human or animal. Antibody tests have the drawback that they do not work well in patients who are immunosuppressed and therefore have low levels of antibody. Reliable, accurate and more sensitive methods of diagnosing infection in animals and human beings should be available and regularly used before clinical xenotransplantation trials go ahead. 22 Onions D and Lees G, Human herpesviruses and retroviruses: problems and solutions in the safety testing of biologicals in Horaud F and Brown F eds. (1990) Developments in Biological Standardisation, 75:145-158. 23 Representational difference analysis, for example, basically involves comparing the genetic material of uninfected organisms with that of potentially infected organisms: any extra material may be due to the presence of an infectious organism (Lisitsyn N et al. (1993) Cloning the difference between two complex genomes. Science, 259:946-51). Organisms can also be identified using polymerase chain reaction techniques with primers based on the sequence of organisms potentially related to the unknown organism (Compton T, Degenerate primers for DNA amplification. Chapter in Innis M A et al. eds. (1990) PCR Protocols: A guide to methods and applications. Academic Press). 76
Transmission of infectious diseases Producing animals free from known infectious organisms 6.27 Because of the potential dangers of disease transmission, xenotransplant teams have sought to produce source animals free from known infectious organisms. The animals are reared in captivity and maintained in a clean environment. Individual animals are tested regularly to check levels of infection. Precautions are also taken to ensure that human beings rearing the animals do not inadvertently infect them with human diseases, such as influenza. In order to eliminate other viruses, a longterm breeding programme and intensive screening would be required. This would be a particularly lengthy process for primates. 24 In addition, it might be necessary to use methods such as delivery of animals by Caesarean section, and rearing of animals in isolation. This would have implications for the welfare of the animals (Chapter 5). Moreover, it is essentially impossible to eliminate retroviruses that have become inserted into the genetic material of the host animal (paragraph 6.9.5). 6.28 There is currently no general agreement amongst xenograft teams as to precisely which organisms should be excluded from animal sources. Thus, some xenografts have already taken place which have used animals infected with known organisms. For example, the baboons used as sources for the two liver xenotransplant recipients in Pittsburgh were known to be infected with foamy virus, a member of the retrovirus family. 25 Neither patient showed evidence of foamy virus infection during the short time they lived after their operations. Baboon foamy virus is not known to infect human beings, but some foamy viruses have the ability to infect several species. For example, some mouse foamy viruses can infect human cells. While there is no evidence to date that foamy viruses cause diseases in their hosts, some virologists nevertheless hold the view that foamy viruses should be eliminated from animals used to provide xenografts. 26 6.29 The term specified-pathogen free is used to describe animals from which specified infectious organisms (pathogens) have been excluded. But, as the example above makes clear, there is no consensus about which organisms should be excluded from specified-pathogen free animals. The Working Party recommends that a code of practice should be drawn up specifying which organisms should be excluded from specified-pathogen free animals. <strong>Xenotransplantation</strong> teams should be required to exclude from source animals all the pathogens listed in the code of practice. Mechanisms should be in place to allow the list of organisms to be updated in the light of experience. The code of practice should recommend the diagnostic tests to be performed by accredited test centres. 24 Allan J S (1995) <strong>Xenotransplantation</strong> at a crossroads: Prevention versus progress. Nature Medicine, 2:18-21. 25 Michaels M (1995) Presentation at an Institute of Medicine conference on Xenograft transplantation: science, ethics and public policy,Washington DC. 26 Allan J (1995) <strong>Xenotransplantation</strong> and the infectious disease conundrum. Institute of Laboratory Animal Resources Journal, 37:37-48; Onions D (1995) Comments made at an Institute of Medicine conference on Xenograft transplantation: science, ethics and public policy,Washington DC. 77
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NUFFIELD COUNCIL ON BIOETHICS Anima
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Preface Preface Since the N
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Working Party on Xenografts Working
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Xenotransplantation - Nuffield Council on Bioethics

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