Xenotransplantation involves transplanting living cells, tissues or organs from one species into another. The implantation of living animal cells, tissues or organs into the human body is a type of xenotransplantation.
Xenotransplantation involving people can take a number of forms, including:
| � | Transplantation of solid animal organs (such as hearts, kidneys, livers); |
| � | Cell therapies, such as the transplantation of pig neural cells and pig pancreatic islet cells; |
| � | Use as part of a medical device such as an extra-corporeal liver device utilizing viable animal cells; |
| � | The use of animal cells in gene therapies. |
Xenotransplantation is seen by some researchers as a means of overcoming the chronic shortage of donor organs. It is also being investigated as a possible cure for Type 1 diabetes, with clinical trials taking place involving pancreatic islet cells from pigs being implanted into diabetes patients. Research is also being conducted into xenotransplantation as a means of treating neurological diseases. Pigs are the preferred donor of tissue and organs because they are easy to breed, have the right size organs for humans and are less likely to transmit diseases than primates.
Researchers are working to overcome two major medical problems - the rejection of the animal tissue by the person's immune system, and the danger of cross-species infection. Pigs have now been produced that lack the gene which triggers an immune response in humans, and several companies have cloned these pigs successfully. Cross species infection, in particular the potential activation of pig retroviruses in humans, is a matter of fierce debate among researchers. (A retrovirus is a virus which replaces itself into the DNA of its host cell, and is copied with the cell's DNA when the cell replicates. The retrovirus is thus present in every cell in the organism, although it may cause no harm to its host organism. HIV, the virus which leads to AIDS, is a retrovirus thought to have originated in an animal population, and which became active when transmitted to the human population.)
In the United States of America and in the United Kingdom xenotransplantation research has been allowed under the control of regulatory authorities. In New Zealand in the mid 1990s Auckland company Diatranz carried out research into the use of porcine pancreatic cells as a means of treating Type 1 diabetes. In July 2001 a Diatranz application to carry out clinical trials in New Zealand was declined to by the Director-General of Health on the recommendation of GTAC (Gene Technology Advisory Committee). A Supplementary Order Paper was introduced into Parliament which imposed temporary controls on xenotranplantation pending further legislation. These moves angered diabetics, who considered that the Ministry of Health was being overly cautious. The temporary controls on xenotransplantation expire in June 2005, and the Bioethics Council will be initiating a public dialogue on xenotransplantation prior to this date.
There are complex ethical and legal issues involved in xenotransplantation. While an individual may benefit from xenotransplantation, there is a risk to public safety through the transmission of porcine retroviruses into the wider community. Xenotransplantation volunteers would probably have to agree to life-long rules which would limit their freedom, for example, never travelling outside their country, not having children, identifying their sexual partners to authorities, and being monitored by authorities. The standard model of informed consent would need to be modified for the recipients of xenotransplants, as the risks involve not just the patient but other people as well.
The Catholic Church has taken a cautious stand on xenotransplantation, with the Pontifical Academy for Life indicating that it sees xenotransplantation as morally acceptable in principle, provided certain conditions are observed. The Academy notes that xenotransplantation may be a means of addressing the shortage of donor organs in the world. However the acceptability of xenotransplantation is contingent upon safety issues relating to rejection and cross species infection being resolved; xenotransplants not affecting the identity of the person (for example, by being used in the brain or reproductive organs); the prevention of unnecessary suffering to animals; and a due regard for biodiversity and the balance of species.
A full explanation of the position of the Pontifical Academy for Life on xenotransplantation: Prospects for Xenotransplantation: Scientific Aspects and Ethical Considerations.