Posted by tom cavanagh on October 16, 1997 at 14:46:25:
In Reply to: Porcine Virus posted by SJSANTEN on October 16, 1997 at 08:16:09:
The presence of retroviruses in an islet or organ donor animal (pig) is a serious concern to islet researchers, and was one of the topics discussed at the International Xenotransplantation Congress In France last month. Although this meeting dealt primarily with whole organ transplants, many of the concerns for whole organ xenotransplant carry over to islets. One reason xenotransplant researchers are concerned is that AIDS is caused by a retrovirus. There is considerable evidence to support the widely held belief that the retrovirus which causes AIDS found its way into the human population from monkeys. This concern about retrovirus transmission from monkeys to humans prompted a group of scientists to recently issue a warning, strongly urging that monkeys not be used as donors for xenotransplantation into humans. It was felt that the relative dissimilarity between pigs and humans makes pigs a better choice for xenotransplantation, but not one which is guaranteed to be risk-free. For this reason, researchers are now looking for pig retroviruses, and attempting to understand their prevalence in different pig populations. In a perfect world, we would fully understand the risks before transplanting pig cells into a human patient. We will never fully understand the risks, but we must have a better understanding before this procedure will be routinely performed. Obviously, nobody wants to be responsible for creating a new epidemic. On the other hand, many people are working very hard to cure diabetes. So, enthusiasm and optimism must be tempered with at least some caution.
Porcine retrovirses are not a concern in purified insulin. Manufacturers must demonstrate that the process of deriving a pharmaceutical from animal tissue actually removes any contaminating viruses (this is called viral clearance). In islet or organ transplantation, the donor cells are alive and contain DNA. The retroviruses are able to hide by incorporating their DNA into the DNA of the host cell. When the islet is transplanted, the retrovirus could get out into the islet recipient, and infect cells of the recipient. The Nature paper is significant because it shows that in a laboratory, these retroviruses can infect human cells. However, there are substantial differences between this particular laboratory experiment and what might actually happen during transplantation into a human recipient.
Several people have been transplanted with pig pancreas tissue in Europe, and they will be studied to see if they show any evidence of porcine retrovirus. Hopefully, the people who were transplanted with pig cells in New Zealand will also be tested.
A study underway by the Centers for Disease Control examined almost 400 monkey handlers in the US and Canada who were either bit by monkeys or stuck with needles which had been used to draw blood from monkeys. They found a low (1.8%) incidence of a monkey retrovirus in that group of handlers, but the retrovirus did not seem to cause any frank disease, and was not apparently transmitted to others by blood donation or sexual contact. It is this type of study which will help us understand the risks of disease transmission associated with porcine islet and organ transplants.
Of benefit to the diabetic community is that several companies are spending many millions of dollars on the future of organ xenotransplantation, and this research will spill over into the field of islet xenotransplantation. Porcine endogenous retroviruses represent an additional hurdle to islet xenotransplantation, but not an insurmountable one.