Given the huge shortage of donor organs, researchers have been trying to find ways to transplant animal organs across different species (known as "xenotransplantation"), with the eventual aim of transplanting animal organs into humans. The major stumbling block, says Dr Muhammad Mohiuddin (US National Heart, Lung, and Blood Institute) in a paper in PLoS Medicine, is that the immune system in the animal receiving the organ tends to reject the transplant.

Nevertheless, the recent development of genetically modified pigs that are more compatible with humans, "has reinstated hope for the success of xenotransplantation," he says.

In his paper, Dr Mohiuddin discusses the reasons why xenotransplantation offers greater potential than other techniques for replacing diseases organs.

For example, mechanical devices that are aimed at replacing the function of an organ (like ventricular devices for treating heart failure) have a tendency to cause blood clots and are not yet proven suitable for replacing transplantation. And while the idea of growing organs in culture dishes has fascinated scientists for years, he says, there have been no major success stories yet.

In contrast, Dr Mohiuddin believes that there have been some promising reports that suggest that xenotransplantation may eventually benefit humans.

For example, insulin-producing cells (islets) from pigs were transplanted into monkeys with diabetes and this led to complete reversal of diabetes for over 100 days. But such research is still a long way off from being relevant to humans, says the author. For example, in the pig-to-monkey studies, very large doses of drugs called "immunosuppresants" were needed to stop the monkeys' immune system from rejecting the pig islet cells—such doses would be unacceptable in humans.

In addition to immune rejection, another concern about xenotransplantation is the risk of transmitting viruses and other pathogens from one species to another.

"Whether the risk of transmission of these pathogens will increase with xenotransplantation is not yet known," says the author. "But the risk can be anticipated, and thus prepared for. A long-term careful follow-up of transplanted patients will be required to monitor for infection by latent viruses and other pathogens. A timely intervention would be important to treat the infection and control its spread to other individuals."

Citation: Mohiuddin MM (2007) Clinical xenotransplantation of organs: Why aren’t we there yet? PLoS Med 4(3): e75.