In 1964, Hardy and colleagues performed the first chimpanzee-to-human heart transplantation. However the transplanted heart was unable to support the recipient circulation and bypass was discontinued after one hour.
The prospect of transplanting animal organs, tissue and cells into humans is looking increasingly promising, as progress is made towards overcoming the formidable barriers of cross-species rejection.
Few doubt that xenotransplantation could eventually bring important medical benefits, but there is still heated debate about the circumstances under which it should be allowed to cross the Rubicon from animal studies into the clinic, if at all.
The dilemma is that, when one tests animal-to-human transplants, one is also carrying out another, unwanted, experiment testing the remote, but real danger that animal viruses might jump to humans and cause man-made pandemics.
This concern has come to the fore over the past two years, just as earlier concerns about animal welfare and the ethics of xenotransplantation have faded.
Optimism that breeding disease-free animals might overcome viral risks has been dealt a blow recently by the discovery that pigs, the current donor of choice, harbour endogenous retro viruses that can infect human cells in vitro.
Multiple copies of retro viruses are integrated in the pig genome which suggests that breeding clean pigs will be extremely difficult.
The discovery was made independently by virologists Robin Weiss, at the Institute of Cancer Research in London, and David Onions, at the University of Glasgow.
Regulation of xenotransplantation in the United States is at a turning point with the imminent release of guidelines that will give the go-ahead to clinical trials.
No country has yet judged that the risks of xenotransplantation so outweigh the potential benefits that they justify a permanent ban on clinical trials. Debate is advanced in the United Kingdom and the United States, but is only just beginning in most other countries and at the international level.
The United Kingdom has, for the moment, the most stringent position of any country, and the need for a regulatory framework was prompted by the announcement in 1995 that Imutran, a Cambridge-based biotechnology pioneering company in xenotransplantation, planned to proceed with clinical trials of pig hearts.
UK guidelines for xenotransplantation
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