Transplants of organs from non-human animals to human recipients could transform medicine and potentially save countless lives each year as many die awaiting transplants. Past experiments have seen pig hearts and kidneys transplanted into humans, but this marks the first instance of an animal liver being transplanted into a living person.

“This is truly groundbreaking,” remarks Heiner Wedemeyer from Hannover Medical School in Germany, who was not involved in the procedure. “The patient was critically ill, but thanks to the transplant, he survived for six months.”

The complexities of the liver have prevented previous surgeries of this kind. Earlier studies were conducted on brain-dead individuals, but indications of success were observed. “The heart acts merely as a muscle for pumping blood,” Wedemeyer explains. “Kidneys are simpler as they filter waste. The liver, however, is unique as it synthesizes a variety of proteins essential for numerous metabolic functions.”

Similar early successes were noted in heart and kidney transplants, although subsequent complications arose. In the realm of heart transplantation, risks potentially include the spread of swine viruses.

Recently, Hokujo Taiyo and colleagues at Anhui Medical University reported a pig liver transplant performed on a 71-year-old man. His liver was deemed too damaged for a traditional transplant due to severe tumor growth and significant scarring from hepatitis B. Thousands perish annually awaiting liver transplants, so each surgical case must be meticulously justified, according to Sun.

However, Sun indicated that the man required some form of transplant as there was a risk of the tumor rupturing, which could be life-threatening. With the patient’s consent, Sun and his team replaced the affected portion of the liver with one harvested from an 11-month-old minipig in May 2024. During a five-hour procedure, they connected the blood vessels of the pig liver to those of the left side of the recipient’s own liver.

To mitigate the risk of rejection by the immune system, three pig genes were disabled while seven human genes were introduced, enhancing compatibility. The patient was also administered immunosuppressants while the team diligently examined his liver to ensure it was free from swine viruses.

Almost immediately post-surgery, the new liver began to produce bile. Bile is crucial for the digestion of fats. Within weeks, levels of bile and albumin (a protein that retains fluid within blood vessels) in the patient rose to healthy ranges, as reported by Sun.

Nevertheless, about a month post-transplant, a life-threatening blood clot formed in a blood vessel, necessitating the removal of the graft. This complication likely stemmed from an overactive immune response, leading to abnormal blood-clotting protein levels—a challenge that may be common in pig transplants given the biological differences between species.

The patient lived for roughly five additional months with only the left side of his liver remaining before succumbing to gastrointestinal bleeding, a frequent issue associated with liver scarring, according to Sun. Both Sun and Wedemeyer believe this bleeding was probably not related to the transplant.

Despite the outcome, the operation is seen as a partial success because the patient would likely have died very soon after the tumor’s removal, noted Wedemeyer. Furthermore, he added that the patient’s liver may have partially regenerated during the successful functioning of the transplant, enabling survival for several months after the graft removal.

Wedemeyer emphasized that this procedure enhanced the understanding of xenotransplantation and opened up the possibility of pig livers providing temporary solutions for patients awaiting human transplants. There may even be a chance that the remaining liver tissue could grow sufficiently to negate the need for further treatment, indicated Sun.

However, Sun cautioned that it may take at least ten years before pig livers can replace human livers permanently. He stressed the need to minimize potential complications through further genetic advancements.