Animal-to-Human Transplants
Conclusions and Recommendations.
10.1 There is considerable interest in the transplantation of animal organs or tissue as a means of reducing the shortage of human organs and tissue for transplantation. Transplantation is an important and successful procedure in modern health care. It provides significant benefits to patients, both extending life expectancy and improving quality of life (Chapter 1). Debate continues about how to reduce the gap between the demand for transplantation and the shortage of human organs and tissue (Chapter 2). The benefits of preventive measures to improve health and prevent the diseases requiring treatment by transplantation are likely to be long-term ones (paragraphs 2.2 - 2.3). Some of the suggested measures for increasing the supply of human organs, such as changing the consent requirements, are not without ethical and practical difficulties (paragraphs 2.4 - 2.10). Even radically innovative measures to increase rates of human organ donation are likely to be insufficient to bridge the gap between supply and demand. The gap is, if anything, likely to widen, not least because improvements in surgical and medical techniques will permit a larger range of patients to be treated by transplantation.
10.2 Thus, there is interest in alternative ways of meeting the organ shortage. Mechanical devices, notably battery-powered hearts, have made great advances in recent years, and may be expected to develop further (paragraphs 2.11 - 2.21). They are prone to problems, however, associated with increased risks of infection and of blood clotting. Mechanical devices are also unlikely to be able to take the place of some human organs, such as the liver, which has sophisticated biochemical functions. Tissue engineering techniques, in which living cells are used to produce replacement organs and tissue, are promising, but still in the early stages of development (paragraphs 2.22 - 2.31). Skin can now be replaced by tissue engineering, but the development of bioengineered heart valves, much less functioning organs, is a more distant prospect.
10.3 Xenotransplantation of animal organs and tissues is an alternative option for reducing the shortage of human organs and tissue for transplantation. The strong immune response to animal organs or tissue, however, means that transplant rejection is a major problem and, to date, xenotransplantation involving human recipients has not been successful (Table 3.1). An exception is the routine transplantation of pig heart valves. These can be treated so they do not cause such a strong immune response. Two main strategies have been used to try and prevent xenograft rejection. In the US, attempts have been made to use organs and tissue from primates, such as baboons, for xenotransplantation. Since, biologically speaking, primates are closely related to human beings, the immune response to a primate xenograft is not that much stronger than the response to a poorly matched human transplant. In December 1995, an AIDS sufferer from the US received a transplant of baboon bone marrow in the hope that this would restore the function of his bone marrow (paragraphs 3.18 - 3.21).
10.4 Ethical concerns about the use of primates for xenotransplantation have led to attempts to develop non-primates as sources of organs and tissue. Attention has focused in particular on pigs, since their organs are comparable in size to human ones, and they breed rapidly and could thus be used to supply transplant material on a large scale. Since pigs are less closely related to human beings than primates, the immune response to pig xenografts is rapid and severe. Attempts are being made to modify pigs genetically so that their organs do not cause such a strong immune response when transplanted into human beings (paragraphs 3.24 - 3.32). Hearts from these transgenic pigs last longer than unmodified organs when they are transplanted into monkeys. The UK company Imutran Ltd has announced its intention to start transplanting hearts from transgenic pigs into human recipients in 1996. In the US, Parkinson's sufferers have undergone xenotransplantation of fetal neural tissue from unmodified pigs in an attempt to treat their condition (paragraph 3.35).