Published by Harvard Medical School, Adapted from a Mass General news release and press conference.

A team at Massachusetts General Hospital led by Harvard Medical School physician-scientists has for the second time successfully transplanted a genetically edited pig kidney into a living person. The team performed the first surgery of its kind in the world in March 2024.

Results of the latest procedure, performed Jan. 25 and announced Feb. 7, highlight the promise of xenotransplantation — the transplanting of modified animal organs into humans — to alleviate human organ shortages and save lives. The approach could also help reduce health disparities in organ failure and access to transplantation.

“This second xenotransplant provides us with another excellent opportunity to learn how we can make genetically edited pig organs a viable, long-term solution for patients,” said lead surgeon Tatsuo Kawai, HMS professor of surgery and director of the Legorreta Center for Clinical Transplant Tolerance at Mass General. “Although we have a long way to go to make that a reality, this transplant is an important next step that has given us optimism to achieve that goal.”

About the recipient — and others who wait in hope

The patient, 66-year-old New Hampshire resident Tim Andrews, is the fourth person in the world to receive a pig kidney and one of two people currently living with one. (The world’s first pig-kidney recipient died two months after his transplant of unrelated causes.)

Andrews had been on dialysis for more than two years due to end-stage kidney disease. The treatment took a serious toll on his daily life, leaving him fatigued and unable to keep up with usual activities. Andrews also had severe complications, including a heart attack in July 2023.

His path to a kidney transplant was further complicated by his O-group blood type, which makes people universal donors but limited as recipients, able to accept organs only from donors with the same blood type. While most patients wait 3 to 5 years for a kidney, those with O-positive or O-negative blood types often wait 5 to 10 years, further diminishing their chances of receiving a donor organ.

Following his discharge from Mass General on Feb. 1, Andrews is off dialysis for the first time in more than two years. His new kidney is functioning as expected, his surgical team reported.

“As soon as I woke up after the surgery, the cloud of dialysis disappeared. I felt reenergized and revitalized. It was a miracle,” Andrews said. “The magnitude of what these doctors and nurses accomplished is unbelievable and I want to thank them for giving me a new lease on life.”

The United Network for Organ Sharing (UNOS) reports that more than 100,000 people in the United States are waiting for an organ for transplant, 17 of whom die each day waiting. About 90,000 people were waiting for a kidney as of Nov. 2024, according to the U.S. Organ Procurement and Transplantation Network.

Those on the list are just a fraction of the more than 800,000 Americans living with end-stage kidney disease, as estimated by the NIH’s National Institute of Diabetes and Digestive and Kidney Diseases.

Transplantation is the ideal treatment for kidney failure, said team co-lead Leonardo Riella, the HMS Harold and Ellen Danser Associate Professor of Surgery and medical director for kidney transplantation at Mass General. However, he explained, due to the severe organ shortage, the majority of patients remain on dialysis — a treatment that sustains life but raises risk of cardiovascular complications and infections, has poor long-term outcomes, and significantly reduces quality of life.

“Xenotransplantation represents a turning point by eliminating the organ shortage as a barrier to transplantation,” Riella said. “It offers a solution far superior to dialysis.”

“The announcement we have today is not just about science or medicine,” he added. “It is about our patients — patients with end-stage kidney disease like Tim who are not doing well on dialysis and face years on the wait list. We’re searching for a timely, life-saving alternative. Today we take another critical step forward.”

Andrews emphasized that while early advances in xenotransplantation may only affect individual recipients, in the long term such transplants have the potential to transform hundreds of thousands of lives.

“This transplant isn’t about me,” Andrews said. “It’s about all the people who I met at the dialysis clinic, and I saw what they were going through. There are more than 500,000 people on dialysis, and I want to inspire them to never give up hope because that’s what this transplant provides. It’s a glimmer of hope.”

The logistics of kidney xenotransplantation

As with the landmark 2024 surgery, the pig kidney was provided by eGenesis, a xenotransplantation therapy company co-founded by HMS geneticist George Church and former HMS postdoctoral fellow Luhan Yang.

The kidney had 69 genomic edits made with CRISPR/Cas9 technology. These edits removed harmful pig genes, added certain human genes to improve its compatibility with humans, and inactivated viruses present in all pig genomes, known as porcine endogenous retroviruses, to eliminate risk of infection in the recipient.

Two pharmaceutical companies provided medicines to suppress Andrews’ immune system and minimize the risk of organ rejection.

Surgeons performed the procedure under the FDA “compassionate use” or Expanded Access Protocol (EAP), which allows patients with serious, life-threatening illnesses or conditions to access experimental treatments when no comparable options are available.

The approval includes two more kidney xenotransplants scheduled to take place at Mass General in 2025 as part of a three-patient study to observe long-term viability of genetically edited pig kidneys in humans.