The key problem for transplant scientists has always been to overcome the body's natural defence mechanism whereby a transplanted organ is rejected. Most of the animal research directed towards this end has relied on rodents, with rats by far the most commonly used species.(1) Yet scientists have discovered important tissue differences which mean their results are of questionable relevance to people, and could be misleading.
For instance, if experiments with rats were used as a guide, patients receiving heart or kidney grafts would only need a very brief period of immunosuppression with drugs like cyclosporin, after which they would never reject their new organ.(2,3) In fact, such a course would be disastrous, for unlike rats, human patients need lifelong immunosuppression to prevent organ rejection.
The reason, scientists suspect, is that within a few days of transplantation, the rat's kidney has no cells to stimulate the immune system, so the animal does not reject a transplant when immunosuppressive drug treatment is stopped. In contrast, the human kidney does have these cells as an integral part of its structure and transplant patients must therefore have lifelong drug treatment to suppress the cells' immune-stimulating effects.(4)
During the 1960s and 1970s much research focussed on rat "models" of kidney and heart transplants, and according to John Fabre of Oxford University's Nuffield Department of Surgery, "The many encouraging results raised hopes that a major advance in clinical immunosuppression for transplantation was in the offing, but these hopes have now faded and nothing of the great mass of work has been translated into clinical practice." Fabre suggests that the tissue differences between people and rats may be responsible.(2)
1) According to British figures for 1986, 66% of experiments performed in transplant research used rats, 26% used mice, 7% used rabbits, dogs, primates or other species. Source: Statistics of Experiments on Living Animals, Great Britain, 1986 (HMSO,1987)
2) J.W.Fabre, Transplantation, 1982, vol.34, 223-224.
3) D.J.Cohen et al, Annals of Internal Medicine, 1984, vol.101, 667-682.
4) P.J.Morris (Ed.), Tissue Transplantation (Churchill Livingstone, 1982). See also ref .2.
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