In January 2025, Tim Andrews received a kidney transplant from a pig at Massachusetts General Hospital in Boston. The patient, who was 66 years old at the time of the procedure, lived with that kidney for 271 days without needing the dialysis he had required for the previous two years to replace his failed kidneys. Those nine months are the record duration for an animal-to-human transplant.
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Furthermore, according to the team that performed the operation in the medical journal The Lancet this Thursday, in January of this year, Andrews received a kidney from a human donor that has allowed him to continue his life relatively normally. This transition, which envisions a possible future where organs taken from modified animals serve at least as a lifesaver while waiting for a human donor, is also the first successful one in history.
“The shortage of organs is the biggest crisis we currently have in the field of transplants,” said Leonardo Riella, medical director of the kidney transplant program at Massachusetts General Hospital and lead author of the trial. “We know that the best treatment option for patients with end-stage kidney disease who have developed kidney failure is transplantation, but we simply do not have enough human organs to perform transplants in time. Our vision is that xenotransplantation [transplanting cells, tissues, or organs from an animal species to a human being] could help cover this shortage: initially, as a bridge allowing patients to stop dialysis while waiting for a human donor kidney and, potentially, as we confirm its safety and long-term durability, as a definitive treatment in its own right,” he added.
In 2025, there were 53,343 patients on the waiting list for an organ transplant from a donor in the EU. In the US, despite having 25% less population, the number exceeds 100,000 people. Although dialysis allows survival in the case of the kidney, it does not fully replace the work of this organ and usually gradually deteriorates quality of life. That is why, for decades, scientists have tried to use animal organs for transplantation into humans.
Since the early last century, attempts were made to replace damaged human organs with those from animals, without success. Hearts from pigs, sheep, baboons, and even chimpanzees were transplanted into humans with survival times ranging from hours to months. In 1964, Edyth Parker died one day before reaching 9 months with a chimpanzee kidney, the same barrier reached by Andrews six decades later.
As with transplants between humans, the immune system rejects foreign bodies, but in the case of animals, with greater intensity given the greater immunological differences. The latest genetic editing techniques, such as CRISPR, have changed the landscape.
In the case of the study published today, the American company eGenesis created a genetically modified animal with 69 changes in its DNA so that Andrews’ body would not identify his kidney as something foreign. Some of these tweaks eliminated the instant rejection caused by contact of a pig organ with human blood, and others added human genes intended to control processes such as inflammation, coagulation, and immune system activation. Additionally, the researchers deactivated fragments of DNA known as porcine endogenous retroviruses, fragments of genetic material inserted in the ancestors of pigs that are inherited and could cause infections in humans.
The medical team reports that the initial results were very good. During the same surgical procedure, the kidney began producing urine and generated between four and five liters per day in the week following the operation. This blood-cleaning work freed the patient from dialysis and gave him independence he had not had in previous years.
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Two weeks later, the first problems appeared. Immune cells, specifically T lymphocytes, which defend the body from viruses or cancer, began attacking the kidney in a typical rejection episode. Then, the doctors were able to resolve it with the usual treatment, which includes drugs that block the immune response.
As is often the case with patients candidates for experimental treatments with many unknown risks, Andrews had very serious health problems, and the medical team had to face several crises in the following months, from a leg infection to angina that required another stent to be inserted in the heart, but the kidney continued to perform its function.
However, at six months, when they had to reduce immunosuppressive medication to fight a bacterial infection, a second type of problem appeared. Inflammation of the kidney vessels caused clots to accumulate in the organ, reducing its ability to filter blood. Interestingly, this damage was not caused by antibodies but seems to be the work of another branch of the immune system: macrophages and NK (natural killer) cells that are part of the innate immune system. At 271 days, the damage was already irreversible, and surgeons had to remove the pig kidney.
One of the highlights of the study, according to Beatriz Domínguez-Gil, director of the National Transplant Organization, is what happened afterward. Just over two months after the removal of the animal organ, Andrews was lucky, and a human donor with the rare characteristics he needed appeared. Seven months after the transplant, it was still functioning properly, and there has been no immune response caused by contact with the pig kidney. “This opens the future possibility that we talk about a bridge kidney transplant to human transplant while waiting for one to be available,” says Domínguez-Gil, who clarifies that this is still “an experimental field.”
The authors of the work warn that the study is only about one patient without a comparison group and that no definitive conclusions can be drawn. For the future, Pablo Ramírez, head of General Surgery and Transplants at the Arrixaca Hospital in Murcia and member of the Council of Europe’s Expert Committee on Xenotransplantation, comments on the most interesting points: “understanding what happens with those natural killer cells that have been key in the rejection of this kidney and do not appear in human transplants.” “Next, they will likely modulate with immunosuppression to control that rejection mechanism,” he predicts.
The authors also mention the need for improvements in drugs to protect the kidney’s blood vessels long-term and genetic modification of pigs to continue advancing toward the goal of making pig organs save human lives.
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