Xenotransplantation: panacea or poisoned chalice?

This essay was written by Jonathan P. Stoye and was first published in the 1997 Mill Hill Essays.

Your heart is failing. You go to the doctor and after multiple tests you are placed on the waiting list for a heart transplant. You learn that the progress which has been made over the last thirty years in transplant surgery and immunology means that if you receive a new heart your chances of making a full recovery are excellent. However you also learn that the shortage of donated organs makes your chance of actually receiving a new heart much lower and dependent upon, among other things, the rate of serious car accidents. So you sit and wait and possibly wish for the traumatic death of another. Or consider another scenario. Your pancreas is being destroyed and you are developing diabetes. Transplanting insulin-producing cells from the pancreas of a dead foetus offers a possible solution but success is likely to require harvesting these cells from a number of foetuses and these are unlikely to be available for reasons both ethical and practical. You therefore face the prospect of attempting to control glucose levels and prevent the numerous complications of diabetes, by insulin injection and dietary restriction for the rest of your life. Is there no alternative? Xenotransplantation, the transplantation of organs and tissues from one species to another, may offer solutions to problems like these. However, there are a number of possible complications to consider.

The first successful human to human organ transplant was performed in 1954 when a kidney was transferred between identical twins. In the succeeding years improvements in transplant surgery and advances in controlling organ rejection by tissue matching and administration of immunosuppressive drugs have made transplantation the treatment of choice for an ever increasing number of people with kidney, liver, heart and lung failure, offering many a new lease on life. The popularity of the biennial World Transplant Games is eloquent testimony to the success of these procedures. It should also be noted that, contrary to popular perception, transplantation is a relatively cost-effective treatment with, for example, the expense of a kidney transplant being recoverable in about two and a half years as a result of savings in dialysis costs.

However, with the success of transplantation have come steadily lengthening transplant waiting lists. In 1990 twenty-two thousand patients were waiting for transplants in the United States and by 1995 this number had doubled. Despite improvements in organ recovery and collection, supply has not kept pace with demand. More than six thousand patients in the United Kingdom were waiting for solid organ transplants at the end of 1995, but in the course of that year only about half this number of transplants had been performed. Not only have the numbers on waiting lists gone up but so has the waiting time, from twofold in the case of kidney transplants to eightfold for livers. A significant fraction of all patients on transplant waiting lists will die from the consequences of organ failure before receiving a transplant.

The majority of human organs for transplantation in the UK come from people who have died in an intensive care unit as a result of injuries received in a road accident or following a stroke. There are also a few living donors of kidneys, mainly donated to family members. Several factors limit the number of available organs. First, the success of road safety campaigns and changes in the management of strokes have reduced the number of potential donors. Second, permission for organ removal from brain-dead donors may not be given by bereaved relatives. Third, not all organs can be recovered in usable condition, or used before they begin to deteriorate. Significant progress has been made in improving the yield of organs from donors who have died, for example by co-ordinating the use of several organs from the same donor. Further improvements in donation rates could undoubtedly be made. However a shortage of suitable organs for transplantation would still remain as indicated by calculations from the United States that even if completely efficient organ recovery could take place the number of patients starting dialysis would still exceed the number of available kidneys. It seems highly likely that future medical advances will actually cause an increase in the organ shortage. For example, many of the conditions best treated by transplantation occur in the elderly, who tolerate immunosuppression poorly and are not currently candidates for transplantation. If improved procedures for preventing organ rejection were developed then the number of potential recipients would increase markedly. An alternative source of organs therefore seems desirable.

This is particularly true for situations where the use of human tissue and organs may not be possible for ethical, cultural or legal reasons. In many countries, for example until very recently in Japan, the use of organs from recently-dead donors was not acceptable. The first surgeon to attempt a heart transplant in Japan was accused of murder. Further problems are associated with the use of foetal material. There are good reasons to believe that transplantation of foetal insulin-producing cells could provide a means of treating type I diabetes and that foetal brain tissues could be used to try to restore some aspects of control of the involuntary movements suffered by patients with Parkinson’s disease. But in both cases tissues from multiple donors are likely to be required and ethical considerations preclude the widespread use of foetal tissue.

For all these reasons the use of animal rather than human organs is being seriously considered and researched. If we accept that ethical concerns over using animal material for treating humans can be met, there are three major challenges which can be identified as crucial for the ultimate success of xenotransplantation. These challenges can be summarised in the following three questions. Can one overcome the problem of immunological rejection of a xenograft? Will there be physiological incompatibilities between xenograft and human recipients, or in other words, will the transplanted organ function? Will transmission of donor pathogens from the graft to the human host or vice versa give rise to disease or to destruction of the graft?

In considering these issues it has become clear that the choice of source animals will be of prime importance. Ideally the donor animals would show immunological, anatomical and physiological similarities to humans, possess organs of similar size to humans, would breed fast and produce large litters under farmed conditions and would show minimal risk of transmitting infectious agents to humans. Two groups of animals have been considered as donors: non-human primates and large non-primates such as pigs. As we shall see, each group has some, but not all, of the desired characteristics.

The primary barrier to transplantation is immunological rejection. Grafts from other humans, just like viruses and bacterial infections, induce immune responses that cause acute rejection starting a few days after transplantation. In general such immunity can be controlled safely by mixtures of immunosuppressive drugs, although in some cases chronic rejection by a poorly understood mechanism will eventually occur and this can be treated only by further transplantation. Xenotransplantation can lead to an additional mechanism of rejection called hyperacute rejection which occurs when the recipient’s blood already contains antibodies to the foreign tissue. Binding of these antibodies to the graft activates a set of human serum-proteins, called complement. This results in breakdown of the blood vessels in the graft and coagulation of the patient’s blood throughout the transplanted organ, which then dies within minutes. Hyperacute rejection cannot be controlled by immunosuppressive drugs.

Human to human transplantation will generally not result in hyperacute rejection. Similarly, organs from closely related species such as non-human primates will also not induce hyperacute reactions. For this reason, the majority of xenotransplantation trials attempted to date in humans have used chimpanzee or baboon organs. However, the problem will occur upon xenotransplantation into humans of organs from more distantly related species such as pigs or sheep. Nevertheless, enthusiasm for using organs from pigs has been rekindled by progress in controlling hyperacute reaction. Experimentally, pigs have been modified by introduction into their genetic makeup of the specific human genes which regulate the activation of human complement. Organs from these transgenic pigs do not undergo hyperacute reactions when transplanted into baboons or if they are perfused with human blood. Using similar transgenic procedures human genes which will cause masking of the normal targets for hyperacute rejection have also been introduced into pigs. Further genes to control the problems of degeneration of the blood supply could also be introduced into pigs. It therefore appears that the problem of hyperacute rejection can or will soon be overcome. This has led to proposals for human trials using organs from the appropriate transgenic pigs in combination with the immunosuppressive protocols established for human to human transplants.

Does this mean that all rejection problems have been solved? Probably not yet. There are indications that human immune responses to material from pigs will be stronger than responses to material from unrelated humans. If this is the case then higher doses of immunosuppressive drugs than currently used for human to human transplants might be required to prevent xenograft rejection. However one complication of current regimens to prevent rejection is over-immunosuppression leaving the patient susceptible to infectious diseases, or sometimes even cancer. There are therefore fears that an acceptable balance between combating infection and preventing rejection may not be found. In the long run it would be highly desirable if alternative strategies to prevent rejection could be established. Ironically, such approaches might prove easier with xenotransplants. For example, it might be possible to generate in the transplant patient a state of immunological non-responsiveness, since studies using animals have shown that it is sometimes possible to induce such a state by introducing bone marrow cells from the donor before the transplant operation. If this were possible with humans one could imagine situations where, before a transplant operation, a patient would first be made unresponsive to cells of the very animal which would subsequently provide the organ for transplantation. Such forward planning would not be possible with human donors! An alternative strategy, more appropriate for transplantation of cells and tissues than solid organs, would be to encapsulate the foreign cells in a membrane which would allow passage of nutrients and oxygen but exclude antibodies or cells of the immune system. Clinical studies of just such a strategy for the transplantation of insulin-producing cells to treat diabetics are likely in the near future.

Assuming that organs and cells can be transplanted without rejection, will they function correctly? At the present time we have insufficient evidence to be able to answer this question. Heart and lung, which serve mainly a mechanical function, might be expected to work provided that they are of the correct size. The kidney and liver serve more complicated biochemical roles and there is real concern that the proteins produced by pig livers will turn out to be unable to work properly in humans. Insulin obtained from pigs has been used for many years to treat human diabetes, but there are concerns as to whether the insulin-producing cells of pigs would respond correctly to the slightly different levels of glucose present in human blood. In practice these questions can only be resolved by trials on living patients. There is only limited evidence from the human trials conducted to date. In one study, in the early 1960s a small number of transplants were performed using chimpanzee and baboon kidneys. Different survival times were seen but one patient survived for nine months indicating that xenotransplanted primate organs can function for prolonged times. However, at the time of her death from bacterial infection, that patient was also found to be showing symptoms which implied that she was suffering a degree of kidney failure. In 1992 a patient survived for two months with a baboon liver before succumbing to infection. However, the liver never achieved completely normal function. Other trials were even less successful; in particular, all organ transplants from discordant species resulted in rapid rejection. The apparent solution of the problems of hyperacute rejection has led to calls for further human trials of organs from pigs. Other, more cautious, voices have argued that the efficacy of such transplanted organs must be demonstrated in pig to primate transplants before further human trials are undertaken.

One reason for delaying human trials is the fear that such procedures would facilitate the introduction of new infectious agents, particularly viruses, into the human population. This fear is coloured by the realisation that human diseases such as AIDS and Creutzfeldt-Jakob disease have their origins in the transfer of pathogens from animals to humans. To complicate matters, many infectious agents can be essentially harmless in their natural host but cause fatal disease after transfer to another species. Thus an animal could appear perfectly healthy but carry, say, an unknown virus which could cause lethal disease in humans. Under these circumstances it would be easy to exaggerate the potential risks associated with the transmission of infectious disease. How real are these concerns?

Unquestionably transplantation provides an ideal means for transmitting infectious agents if they are present in the donor organ. It will circumvent such normal physical barriers to infection such as the skin and the acid environment of the stomach. Added to this, since a transplant patient’s immune system is deliberately suppressed to avoid organ rejection, an effective immune response to the pathogen cannot take place. In human to human transplants serious infections causing AIDS and CJD have already been shown to be associated with the donors. Clearly then, one important step to minimise the risk of transplant-associated infections is to ensure that the donated organs are as free as possible from infectious agents. This is not easy using human donors; obtaining a detailed history about a potential donor to determine possible exposure to disease will usually not be feasible and only limited testing will be possible before a decision must be made whether to use the organ. In theory with xenotransplants the situation should be simpler. Herds of animals to be used as organ donors could be screened for all agents known to be infectious for humans and steps taken to eliminate these agents. In practice this may be possible for pigs, where over a period of many years the demand for extremely healthy animals has led to the development of strategies for the breeding of pigs free from specific pathogens. However, this would be much more difficult for non-human primates such as baboons, requiring both a very sizeable financial investment as well as a considerable period of time. Primates breed slowly and have small litters and are thus considerably less suitable in these respects than pigs. Furthermore, non-human primates carry a significant number of viruses known to infect humans and presumably more will be identified in the future. Given the fact that both human immunodeficiency virus, which causes AIDS, and viruses which cause some leukaemias, come originally from primates, it would seem sensible to rule out the use of non-human primates for xenotransplantation simply on these grounds unless colonies of specific pathogen-free baboons can be developed. However, the political will and financial investment necessary for such developments are unlikely given the ethical considerations involved in such widespread use of primates.

Some groups of virus will probably prove difficult to eliminate. One class of such agents are the so-called endogenous retroviruses. These are viruses that have inserted their genetic material into the host’s DNA. Essentially they have become part of the host species and are carried as unobtrusive passengers for generation after generation in that species. Sometimes they can be activated and cause cancers. Such viruses with the ability to infect human cells have been found in the DNA of pigs but it remains to be seen whether they will cause any harm. The design of primate trials for efficacy of transplanted organs should also include tests for such viruses to see whether they are activated and if so whether they cause disease. Such trials might also reveal the presence of previously unknown agents of disease.

Ultimately we will only learn of the threats posed by such infections with properly designed human trials. In considering these risks we must consider both the threat to the individual transplant patient as well as the threat to society as a whole if the infectious agent should spread from transplant recipients to other people. Ironically, an infection which poses the greatest risk to an organ recipient, that is one which causes a rapidly fatal disease, would pose only a very small risk to society. Such an infection would become immediately apparent and could be avoided or eliminated. The greatest danger would come from something causing disease with a very long incubation period. In such a case, even though the risks resulting from spread of the agent must be considered more remote than the risk to individual recipients the hazards may be much greater. For example, a one in ten probability of developing lymphoma five years after transplant might be perfectly acceptable for a patient whose life expectancy otherwise is only a year, but this treatment could not be contemplated if there was any chance that the agent responsible for the much increased risk of lymphoma could enter the general population. Overall, we should take every opportunity to learn more about the risks posed by infectious agents such as retroviruses; any trial designed to examine whether the transplanted organs work properly should also include experiments to look at transmission of infectious diseases.

What of the future? While it should be remembered that xenotransplantation is a broad term covering a wide range of procedures, and factors which will be important in one area might be much less so in another, it seems to me that a number of general conclusions can be drawn. First, xenotransplantation shows great promise for treating a variety of conditions. Second, at the present time, emphasis should be placed on the use of pigs as source animals since ethical, supply and safety considerations argue against the use of primates. Third, we need to know more about the immunological and physiological factors controlling the fate of transplanted organs. Before organ transplants are attempted, organs from transgenic animals should be shown to fulfil the role for which they were designed, consistently out-performing mechanical devices in primate transplantation. Finally, limited clinical trials of transplants from pigs have already started and it seems inevitable that more will take place, if not in the United Kingdom then in other parts of the world. However, before these trials proceed, it would seem advisable to draw up strict guidelines for the monitoring of transplant patients for infection, as well as preparing contingency plans for preventing the further spread of any new disease.

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