Surgeons have now published the first report of a gene-edited pig liver transplanted into a person.

The liver, which came from a genetically modified pig, appeared to stay active, producing bile and liver proteins inside the brain-dead transplant recipient, researchers reported March 26 in Nature.

Such a transplant could one day buy time for people waiting on the liver transplant list. Doctors could potentially use the pig liver as bridge until a human liver is available or the patient’s liver has recovered, Lin Wang, a surgeon at Xijing hospital in Xi-an, China, said in a March 25 press briefing. “It is our dream to achieve this,” he said. Earlier this year his team also performed a different pig-to-human liver transplant, though the results from that surgery have not yet been published. 

The feats are the latest in a string of advances in xenotransplantation, the transfer of living organs or tissues from one species into another. Doctors have already seen success with pig kidneys and hearts. One woman, a 53-year-old from Alabama, received a gene-edited kidney in November and is still doing well more than 100 days after her surgery. And in February, the U.S. Food and Drug Administration gave two companies the green-light to begin clinical trials with gene-edited pig kidneys in people with kidney failure. 

But pig liver transplants pose a particular challenge, Wang said: “The liver is so complicated.” Unlike the heart, which pumps blood, and the kidneys, which produce urine, the liver is somewhat of an overachiever. The lobed organ juggles many jobs, including detoxifying the blood, making bile to help with digestion, weeding out old red blood cells, storing energy and producing molecules that help the blood clot.

Getting a pig organ to successfully take over all those roles will be really difficult, says Adam Griesemer, a liver transplant surgeon at NYU Langone Health. The functions that the liver performs are so vital to our bodies, he says. If there’s even a tiny mismatch between how pig and human organs work, “I think we’re going to have problems.”

But there’s certainly a need for new solutions for liver-failure patients, Griesemer says. In the United States, around 10,000 people are on the national transplant list waiting for a liver. And unlike dialysis for patients waiting for kidneys, there’s no long-term way to keep liver-failure patients alive. Liver dialysis exists, but it’s only a temporary solution, says Parsia Vagefi, a transplant surgeon at UT Southwestern Medical Center in Dallas. “People still die waiting for a liver transplant,” he says. 

In 2023, researchers at the University of Pennsylvania tried using a gene-edited pig liver as a kind of backup device, hooking up the organ externally to a person who had recently died. The team was able to circulate the person’s blood through the pig liver, a step toward using the organ to temporarily take on the liver’s duties in patients with liver failure. In the new case, surgeons brought the liver inside the body. The team started with a Bama miniature pig that had some pig genes knocked out and some human genes pasted in. These genes play a role in transplant rejection; the goal of the edits was to lessen the chance the transplant would fail.

Surgeons placed the modified liver inside the recipients’ body, connected it to their blood vessels, and then monitored it for 10 days. They pig liver retained its functions, blood flowed smoothly, and Wang’s team did not see inflammatory cells accumulate, a sign that the human recipient was tolerating the transplant. After the experiment, the recipient’s family collected the body and returned it back home.

Vagefi points out that the surgery was not a typical transplant. The team didn’t replace the person’s liver, as would usually occur in a liver transplant. Instead, the pig liver was auxiliary, existing inside the body alongside the original liver. More recently, Wang’s team has performed a full pig-to-human liver transplant, swapping out the liver of a different brain-dead person with one from a genetically modified pig. That transplant took place in January, and the researchers plan to report the results of the investigation in a future publication, Wang said at the briefing. 

Still, Griesmer suspects “that the pig liver is not going to be a long-term solution for patients with liver disease.” Previous work in primates has showcased problems with pig liver transplants, he says. The animals that receive the livers don’t tend to live that long.

It’s also too early to say whether an auxiliary pig organ, like the one reported in the study, could help patients by serving as a bridge for people on the transplant waiting list, Vagefi says. But, he notes, work like this is important because there’s a lot left to be learned. “This is a starting point.”