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Frequently asked questions (FAQs): Xenotransplantation
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- » What is xenotransplantation?
- » Why is xenotransplantation being considered?
- » What sort of disease or health problems could xenotransplantation help to cure or alleviate?
- » What does xenotransplantation research involve?
- » Why use pigs?
- » What are the risks of xenotransplantation to the recipient?
- » What are the risks of xenotransplantation to the wider community?
- » What viruses are we most concerned about in xenotransplantation?
- » What steps would be taken to minimise the risks from xenotransplantation?
- » Further information
What is xenotransplantation?
Xenotransplantation is transplantation from one species to another; for example, from a pig to a human. The term covers transplantation of solid organs (such as kidney or heart), tissues (such as skin) or clusters of specialised cells (such as brain cells or the pancreatic cells that produce insulin).
Xenotransplantation can also involve the infusion or transplantation of body fluids, tissues or cells that have been in contact with the tissues or cells of another species outside the recipient's body. For example, a person might be treated for liver failure by having their blood passed through an artificial device containing pig liver cells.
Some nonliving animal devices, such as pig heart valves, have been used in humans for many years. Xenotransplants differ from these devices in that they are alive and can perform the same functions as the organ, tissue or cells that they replace.
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Why is xenotransplantation being considered?
Transplantation between members of the same species is known as allotransplantation, and in humans this is a very successful way to treat a variety of illnesses. However, very few human tissues and organs are available for transplantation, so that many patients who could benefit from a transplant wait in vain for a suitable donor. Transplant specialists are therefore considering animals as a possible source of organs and tissues for human transplantation. The greatest benefit of xenotransplantation would be a potentially unlimited supply of cells, tissues and organs for use in humans.
Recent advances in technology have increased the possibility of successful xenotransplantation and stimulated research in this area. For example, genetic engineering has allowed human genes to be inserted into pigs so that their cells, tissues and organs are less likely to be rejected when transplanted into humans.
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What sort of disease or health problems could xenotransplantation help to cure or alleviate?
Xenotransplantation has the potential to treat a wide range of life-threatening or debilitating conditions. For example, it is possible that isolated cells could be transplanted to treat diseases such as diabetes, Parkinson's disease, Huntingdon's disease or strokes.
Another possibility is that xenotransplantation could be used to 'buy time' while potential transplant patients wait for a suitable donor. There have been promising results from overseas trials in which isolated pig liver cells contained in a bioreactor were used to treat acute liver failure.
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What does xenotransplantation research involve?
In order for xenotransplantation to become an option for human therapy, research is needed that includes:
- animal-to-animal studies (preclinical studies) - in which proposed xenotransplantation procedures are tested on animals (eg pig-to-baboon kidney transplant); and
- animal-to-human trials (clinical trials) - in which animal products are used for xenotransplantation procedures on human beings (eg pig-to-human pancreatic islet cell transplantation to treat Type I diabetes).
Animal-to-animal studies are covered by existing regulations for research involving animals. The NHMRC recommended in December 2004 that there should be no animal-to-human clinical trials in Australia for a period of 5 years.
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Why use pigs?
Pigs are considered the most suitable species as a source of material for xenotransplantation for several reasons:
- they reproduce quickly and have large litters;
- their organs are similar in size to those of humans;
- they are easy to rear in conditions free of particular pathogens (disease-causing organisms);
- the risk that they will carry pathogens that can infect humans is smaller than with nonhuman primates (apes and monkeys); and
- they can be genetically manipulated to reduce the risk of rejection.
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What are the risks of xenotransplantation to the recipient?
The main risk to the recipient of a transplant is rejection due to the patient's immune response. In human-to-human transplantation (allotransplantation), rejection has been largely overcome by tissue matching of donors and recipients, and by giving the recipient drugs that suppress their immune response.
The risk of rejection in xenotransplantation is more severe because the differences between the donor and the recipient are much greater. The most promising approach at this stage is to genetically modify the source animals so that they do not cause such a strong immune response. Scientists have already produced several genetically modified strains of pig that show promising results.
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What are the risks of xenotransplantation to the wider community?
Xenotransplantation carries some risks for the wider community. The major concern for public health is that xenotransplantation might transmit an infectious agent (such as a virus) from animals to humans. Retroviruses are the chief concern, because there are many examples of such viruses moving from one species to become infectious in another.
However, retroviruses do not always cause obvious signs of disease initially. If a retrovirus present in a xenotransplant were to infect the recipient of the transplant, it may spread to close contacts, carers and even the general population before it became obvious that an infection had occurred.
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What viruses are we most concerned about in xenotransplantation?
Nonhuman primates (apes and monkeys) are not being considered as the source for animal-to-human transplants because their close relationship to humans increases the risk of a virus being transmitted across species.
The virus that is of most concern in xenotransplantation using pigs as the donor species is the porcine endogenous retrovirus (PERV). PERV is present in almost all strains of pigs and cannot be removed by raising pigs in sterile conditions. Although PERV is inactive, and therefore harmless, in pigs, there are concerns that transplantation into humans may activate the virus, creating a new human disease that could spread to those close to the transplant recipient and eventually to the wider community. PERV can infect human cells in the laboratory, suggesting that it could infect humans through xenotransplantation. However, studies of around 150 people worldwide who have been transplanted with pig tissue or had their blood pass through pig cells have shown no evidence of infection with a virus or any other infectious agent originating from pigs.
Approval of an animal-to-human trial would depend on there being an appropriate policy for testing the xenotransplant recipient and their close contacts for PERV and any other organisms that may emerge as a result of the transplant.
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What steps would be taken to minimise the risks from xenotransplantation?
Although most pigs carry PERV, the pig endogenous retrovirus that is of concern as a possible infectious agent in humans, at least one strain of 'minipigs' does not. Researchers are therefore investigating the use of this strain (or breeding others) for xenotransplantation in order to reduce the risk of infection in the recipient.
Most importantly, animal-to-human trials will not be approved unless there is an appropriate infection control policy in the hospital where the transplant is taking place, to prevent transmission of infections from the xenotransplant recipient to hospital contacts. In addition, because the long-term consequences of xenotransplantation will not be fully understood for some years, anyone transplanted with cells, tissues or organs from another species will need to be carefully monitored. Therefore, anyone receiving a transplant would be informed about the potential infectious disease risks to themselves and their close contacts and asked to support such long-term monitoring.
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Further information
For more information on the issues about animal to human transplantation see our overview of xenotransplantation.» Return to the animal to human transplantation research page
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