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Revision as of 10:32, 13 January 2006
Xenotransplantation is the transplantation of cells, tissues or organs from one species to another such as from pigs to humans. Such cells, tissues or organs are called xenografts (xenotransplants). The terms heterograft and heterotransplant are also sometimes used, while the term homograft refers to a same-species transplant.
Xenotransplantation offers a potential treatment for end-stage organ failure, one of the most important health problems facing the industrialized world today. It also raises many novel medical, legal and ethical issues. Disease transmission (xenozoonosis), and possible long-term effects of xenotransplantation on the human gene pool and permanent alteration to the genetic code of animals are a cause for concern. A continuing concern is that cows and pigs have different lifespans than humans and their tissues age at a different rate.
Because there is a worldwide shortage of organs for clinical transplantation about 60% of patients needing new organs die on the waiting list. In many cases there is so little chance of a person actually receiving a transplant doctors do not even add the person to the list, causing an underrepresentation of the shortage. Recent advances in understanding the mechanisms of transplant organ rejection have brought science to a stage where it is reasonable to consider that organs from other species, probably pigs, may soon be engineered to minimize the risk of serious rejection and used as an alternative to human tissues, possibly ending organ shortages.
Other procedures, some of which are being investigated in early clinical trials, aim to use cells or tissues from other species to treat life-threatening and debilitating illnesses such as cancer, diabetes, liver failure and Parkinson's disease. If vitrification can be perfected it could allow for long-term storage of xenogenic cells, tissues and organs so they would be more readily available for transplant.
There are only a few published successful xenotransplantations. Some patients who were in need of liver transplants were able to use pig livers that were on a trolley by their bedside successfully until a proper donor liver was available. Some recipients of pig neural cells with paralysis due to stroke (CVA) and Parkinson's disease have experienced dramatic improvements.
The first xenografts involving human beings were performed by Dr. Keith Reemtsma between 1963 and 1964, in which thirteen chimpanzee kidneys were transplanted into humans. Twelve of the thirteen recipients died within two months, while the thirteenth survived for nine months after the procedure. The first cross-species heart transplant was performed in 1964, in which a 68-year-old man received a chimpanzee heart. He survived only two hours. Several other attempts have been made to transplant primate hearts into humans, with no patients surviving more than twenty-one days.
While whole-organ xenografts have thus far been unsuccessful, less radical transplants have demonstrated great success. Hundreds of thousands of patients have received pig heart valves since 1975, when the procedure first became commercially available. Cow heart valves have likewise been used in humans since 1981.
Immune rejection remains the biggest challenge for xenotransplantation. The problem exists even for human to human transplants (known as allotransplantation), but is more serious for transplants between different species. Nearly all mammalian cells have markers which enable the immune system to recognise them not being foreign. The more different the genetic code between the donor organ and recipient, the greater the difference between a "self" marker and a "foreign" marker. Some companies are currently developing transgenic animals such as pigs, that produce human markers.
Cross-species transplants are more likely to produce host-vs-graft or graft-vs-host reactions than same-species transplants, because of the lack of antigenic similarity. Organisms which have been genetically engineered to reduce this lack of similarity have been produced but are not yet used to any significant degree in medical care.
A worrisome element of xenotransplantation is the potential for infectious disease to spread from the donor animal, which is called xenozoonosis. One example is porcine endogenous retroviruses (PERVs) which are viruses within pigs that pigs are immune to, but can infect humans. Some recipients of pig neural cell transplants have had to agree to never donate blood, take frequent blood tests and use safe sex methods for the rest of their lives due to the risk of spreading such viruses. However, the patients who have received these pig cell transplants have yet to show any PERV-type infection. The situation with other animals is currently unknown.
Xenografts have been a controversial procedure since they were first attempted. Many, including animal rights groups, strongly oppose killing animals in order to harvest their organs for human use. Legitimate medical concerns exist about possible disease transfer between animals and humans, such as the porcine endogenous retrovirus found in pig tissues.
In general, however, the use of pig and cow tissue in humans has met with little resistance. The tissue is harvested from agricultural animals that were already being butchered, which is less offensive to most people than the idea of raising a primate solely as an organ donor. Similarly, while some individual Jews may not wish to receive a pig valve based on their personal beliefs, the rabbinical view is that the use of pig valves in humans is not a violation of kashrut law. In fact, killing a pig to save a human life is not against the Jewish Faith.