A group of baboons at a research facility in Bethesda, Maryland, have been living with pig hearts in their abdomens for the past several years. The goal of the National Institutes of Health experiment was to see if pig organs would be viable for transplantation into humans.

Researchers recently revealed survival data for five transplanted pig hearts — one of the hearts remained healthy in a baboon for nearly three years. The baboons received large doses of immunosuppressive drugs to prevent organ rejection.

The pig hearts didn't function as a replacement heart for the baboons. The animals kept their original hearts, and the pig hearts were connected to blood vessels in the baboons' abdomens.

The research doesn't clear the way for testing pig organs in humans just yet. But it's an encouraging piece of evidence to support the field of xenotransplantation, or cross-species organ transplantation.

"People used to think that this was just some wild experiment and it has no implications," said Dr. Muhammad Mohiuddin, a cardiac transplant surgeon who led the study. "I think now we're all learning that [xenotransplantation in humans] can actually happen."

The problem with xenotransplantation is that if doctors move an organ from one animal species to another, the recipient's body will violently reject the organ almost immediately. In early experiments, survival rates were measured in minutes.

Pigs are the most likely candidates for human transplantation because the organs are of similar size. However, a carbohydrate called α 1,3-galactosyltransferase (gal) found on blood vessel cells would prompt the human body to create antibodies that cause blood clots in the transplanted organ.

Scientists developed a genetically engineered pig that lacks the gal gene in 2001, and the animal's organs began to survive for months at a time in nonhuman primates. The animals required a strict drug regimen that prevented organ rejection but left them at risk for infections.

Mohiuddin and his team have experimented with other drugs that would protect the transplant while keeping the recipient's immune system from experiencing too strong of a reaction. An antibody that blocks communication between immune cells by binding to a CD40 receptor on their surface is promising, he said.

In the latest experiment, researchers used the new anti-CD40 antibody as well as heparin to prevent clotting in the five baboons that housed hearts from genetically engineered pigs. The engineered hearts combined with immunosuppression exceeded the existing record for pig-to-baboon heart transplants, which had been 179 days. Results were reported in the online journal Nature Communications.

After the two-year mark, the researchers began to taper the baboons off the anti-CD40 antibody. At that point, the baboons rejected the hearts. However, the team did discover that two of the baboons lived with lower doses of the drug after being on immunosuppression for a year. Two other baboons who were tapered off the drugs at 100 days post-surgery began rejecting the pig hearts almost immediately.

Ultimately, the tapering experiments suggest a lower "maintenance dose" could be effective, but using genetically altered pig hearts would still require lifelong immune suppression, Mohiuddin said.

Mohiuddin and his team are preparing for true heart replacement surgeries in a new group of baboons.