Xenotransplantation involves transplanting non-human tissues into human recipients. The Food and Drug Administration (FDA) defines this as any procedure that involves transplantation, implantation or infusion into a human recipient of:
- live cells, tissues or organs from a non-human animal source or
- human body fluids, cells, tissues or organs that have had ex vivo contact with live nonhuman animal cells, tissues or organs
The driving force behind xenotransplantation has been the acute shortage of human organs for the 118.000 individuals currently waiting for a life-saving organ transplant. Efforts at xenotransplantation date back to 1904 when Dr. Alexis Carrel conducted his experiments with dogs and developed vascular surgery techniques, some of which are still in use today. In 1906, Jaboulay transplanted kidneys from goats, sheep and monkeys into humans. Multiple other attempts at transplantation of different organs between animals and humans and other species took place in the first half of the 20th century but were met with very little success.
One major barrier to xenotransplantation is the prevention of rejection. The human immune system recognizes and attacks foreign cells and tissues. This is necessary for the health and well being of the human organism so that the bacteria, viruses and parasites that invade the human body are not allowed to proliferate and cause life-threatening infections. Similarly, a transplant from an animal source will also be rejected and any future success at xenotransplantation will depend on overcoming this huge barrier. Research is underway to develop drugs that can suppress the immune system and prevent the rejection process. Another method to prevent rejection is to remove antibodies from the recipient’s system that attack the donor cells, thus causing the host to not reject the organ. Genetically modified animals are also being developed that can donate organs and not be recognized as foreign by the human host and thus not be rejected.
Another risk to xenotransplantation is the ability of uniquely animal infections to spread in the human host. An example of this is HIV or bird flu.
How are animal donors chosen? Use of animals such as chimpanzees or baboons that are more similar to humans have run into difficulties because of greater risk of cross-species infections and some strong ethical objections by animal rights groups. Pigs are currently the preferred donor species. This is because their organs are similar in size to human organs, they have large litters and they are easy to rear. Also, pigs are being used as a food source for humans already, therefore there are fewer ethical issues. One concern though is the spread of PERV or porcine endogenous retrovirus, which to date has not shown to infect recipients of pig tissue but could cause a potential epidemic.