Pig-to-Human Xenotransplants Take Another Step Forward

The Parsons Model

In early 2022 (3), Locke and her team published the results of their first feasibility study of transplanting a genetically engineered pig kidney in a human patient who was declared dead based on neurologic criteria but maintained mechanical life support with consent from his family. The team coined the model The Parsons Model after the decedent recipient whose family generously agreed to let his body be used for the experiment. Montgomery published the results of two similar, short-term, experimental pig transplants using the model in May 2022 (4). Montgomery has since performed two more short-term pig kidney transplants, and his colleagues at NYU Langone Health have also published the results of two short-term pig heart xenotransplants using this experimental model (5).

Before these experiments, decades of research on primates suggested that xenotransplants may be feasible. However, The Parsons Model provided a path to preliminary human studies. Those initial human experiments demonstrated that the transplanted organs did not trigger hyperacute rejection and that the transplanted organs produced urine. Locke said the model can provide the public and the FDA with critical safety information about xenotransplantation. “I am confident we are all going to learn a ton,” Locke said.

So far, that is proving to be true. A detailed molecular and genetic analysis of tissue from the first two pig-to-human kidney transplants, co-led by author Valentin Goutaudier, MD, MSc, a transplant nephrologist and researcher at the Paris Institute for Transplantation and Organ Regeneration in France, found early signs of antibody-mediated rejection, including capillary inflammation and activation of innate immune cells (6). The changes occurred primarily in the glomeruli—tiny blood vessels or capillaries that help filter urine. In an interview with Kidney News in late August, Goutaudier said that the immune changes and their location in the glomeruli differ from that typically seen in human-to-human transplants. Alexandre Loupy, MD, professor at Necker Hospital and head of the Paris Institute for Transplantation and Organ Regeneration, and Alessia Giarraputo, MS, PhD, a post-doctoral researcher at the Paris Institute for Transplantation and Organ Regeneration, were co-lead authors of the study.

Based on the results from the first two kidneys, Goutaudier recommended adding a few genetic modifications to the pig kidneys used for transplant and modifying the immunosuppressive treatment given to the recipient. Montgomery said his team has not yet implemented the recommendations because they want to see if the changes persist in longer-term experiments. He noted that the changes reflect the innate immune system's response, which provides an immediate, non-specific immune response. He is also interested in learning from longer-term experiments whether the organs trigger an adaptive immune response, which can take 10 to 14 days to develop.

“We do not know the significance of [the changes],” Montgomery explained. Goutaudier and his colleagues are now analyzing samples from Montgomery's longer-term pig-to-human kidney transplant experiments to see if similar changes occurred in those organs, which may provide additional information. Locke and her colleagues are conducting similar analyses on samples from their pig kidney xenotransplants.

Both groups are monitoring their recipients carefully for signs of infections transmitted from the donor organ, but they have not yet detected any.

Organ choice

Locke and Montgomery currently use pig kidneys with two different sets of genetic modifications for these preliminary human experiments. Having multiple teams working and testing different approaches may help the field identify best practices, Goutaudier said.

“Each team uses different protocols, so everybody is not doing the same thing,” he said. “It will allow us to learn the best protocols for xenotransplants.”

The pig kidneys transplanted by Locke's group have 10 genetic modifications. The pigs were genetically engineered to knock out three genes encoding carbohydrate antigens found to cause hyperacute rejection in non-human primate studies. “Knocking out those three gives you the best cross-match with a human,” Locke explained.

Locke said that the scientists who developed the pig knocked out a fourth gene encoding a growth hormone receptor to prevent the pig organ from continuing to grow after transplant. Six other human genes were also inserted into the pig's genomes to modulate the human immune system. “Those edits have allowed us to do xenotransplants with immunosuppression regimens we routinely use in human-to-human transplants,” she explained. “All medications are FDA-approved, and we have been using them for decades.”

Montgomery and his colleagues used a pig kidney with just one genetic alteration. The gene encoding an enzyme that produces a sugar, called alpha-gal, which causes hyperacute rejection in humans and other primates, has been knocked out. He said he chose a simplified version because there are FDA-approved medications available to treat many of the problems targeted by the additional genetic modifications of the other pig kidney. He explained that many of the genetic modifications used in the 10-gene edited version were designed to tackle problems that could not be treated with medication in primates.

“Let us take a stripped-down version that we know will prevent hyperacute rejection, and then by using drugs that work well in humans, let us see if we get a good outcome,” Montgomery explained. The plan, he said, is to start over in this new human model to determine which of the genetic alterations are necessary and add them back in as needed.

Montgomery said one advantage of this approach is that the alpha-gal pigs are already FDA-approved as a food source for individuals allergic to alpha-gal. This condition has recently become more prevalent because of the spread in the United States of Lone Star ticks whose bites trigger the condition. The pig producers must meet strict standards for monitoring diseases that could be transmitted to humans, a key concern for xenotransplantation. Montgomery said that single, gene-edited animals are bred and can produce large numbers of offspring, which could help meet the demand for organs more quickly.

The 10-gene altered pigs, by contrast, must be cloned and produce smaller litters, Montgomery said. They also receive close monitoring for disease. Locke noted that some of the immune changes detected by Goutaudier and colleagues are consistent with what would be predicted without knocking out the three carbohydrate antigens. “When we first move into living people, going across a negative cross-match is going to be important,” Locke said. However, she also stated that having multiple organ options may be helpful. She noted that about one-third to one-half of patients would have a negative cross-match with the 10-gene edited pig. For those patients, the alpha-gal knockout pig may be an alternative. “Having pig organs with different genetic knockout backgrounds is going to be critical to solve the organ shortage for everyone,” she said.

The following steps for Locke's team are to learn more about how genetically engineered pig kidneys function differently than human kidneys and to develop ways to manage those differences clinically. They are also continuing to work toward getting FDA authorization to conduct clinical trials of pig xenotransplant.

Montgomery's team plans to continue studying pig heart and kidney transplants in decedent patients. The heart transplants have used pigs with the 10 genetic alterations, and Montgomery plans to test adding some additional genetic edits to the pigs used for donor kidneys. “We want to develop both heart and kidney transplants simultaneously,” he said.

Locke said that using multiple organs from the same donor animal would honor the animals used for these transplants. “How remarkable would it be to have one genetically edited pig that could provide a heart, a pair of lungs, a liver, and two kidneys?” she asked. “I think it speaks to the commitment to trying to do this in an ethically responsible way where we really are trying to study and understand how we can optimize that gift even of the pig itself.”

If the work by Montgomery, Locke, Goutaudier, and others in the field is successful, it could have a tremendous impact on the shortage of organs for transplant that leads to patients dying while still on the wait list. It may also create a sustainable supply to extend transplants as an option to patients with end stage kidney disease who are not currently on the transplant wait list. “It would be transformative,” Montgomery said. “If we had another sustainable source of organs, we could prevent those deaths.”