Concerns over pig-human fusion cells

US scientists have found that human cells injected into a pig fetus can fuse with pig cells, forming 'hybrid' cells that contain genetic material from both species. The surprising findings have implications for research into the use of animal organs for human transplants (xenotransplantation). They might also help explain how viruses such as those that cause the diseases AIDS (acquired immune deficiency syndrome) and SARS (sudden acute respiratory syndrome) could have jumped from animal species to humans.

Researchers at the Mayo Clinic in Rochester, Minnesota, injected human blood stem cells into fetal pigs, around a third of the way through gestation. They grew into adult pigs that seemed completely normal, but the researchers found some hybrid cells, as well as human and pig cells, in their blood and some of their organs. 'What we found was completely unexpected. We found that the human and pig cells had totally fused in the animals' bodies' said Jeffrey Platt, director of the Mayo Clinic Transplant Biology Program. The researchers found that the hybrid cells each had one nucleus, which contained a mixture of pig and human genetic material.

Importantly, the team also found that the porcine endogenous retrovirus (PERV), which is found in most pigs, was present in the hybrid cells. Like all retroviruses, PERV inserts itself into the host cell's DNA, where it is copied along with the cell's own genetic material, after which it can infect other cells. If hybrid cells prove to be a common occurrence, they could present a serious problem for researchers hoping to create 'humanised' pig organs to address the shortage of donated human organs for transplant operations. It has long been recognised that people receiving animal organ transplants might be at risk of contracting animal diseases, although it is not known if PERV itself would cause any symptoms in humans. No human clinical trials in this experimental area of medicine have yet taken place, although another US group reported some success in transplanting pig kidneys into baboons last year.

The new study, published in the journal of the Federation of the American Societies for Experimental Biology, also suggests a possible way in which some viruses might have 'jumped' from animals to humans: 'Perhaps HIV (human immunodeficiency virus) managed to jump from primates to humans through infected blood from a bite, which allowed the stem cells from the two species to fuse' said Platt. The researchers are now carrying out further studies, to find out how the cell fusion happened, and what implications it might have for other viruses apart from PERV.