A closer look at the first pig-to-human kidney transplants shows signs of rejection but also repair

The first two pig-to-human kidney xenotransplants were performed in 2021 when researchers at the NYU Langone Transplant Institute transplanted kidneys from genetically modified pigs into two brain-dead human recipients. Though the initial clinical report did not detect physical evidence of tissue rejection, the new cellular analysis detected early signs of antibody-mediated rejection and showed that human immune cells rapidly infiltrated the transplanted kidneys. Both transplanted pig kidneys showed molecular biomarkers of tissue damage, but, surprisingly, there were also signs that they were beginning to repair themselves.

The transplants were only maintained for around 3 days, so further research is needed to unravel the long-term implications of pig-to-human xenotransplantation. Ultimately, understanding how pig and human cells respond to xenotransplantation will help optimize the procedure, the researchers say.

“These studies could help to design a better pig down the road if they identify particular pig genes that are causing a deleterious response,” says senior author Jef Boeke (@JefBoeke), a geneticist at NYU Langone Health. “Similarly, by helping us identify human responses that are deleterious to the maintenance of the xenotransplant, we can take measures to control those responses.”

The possibility of transplanting pig organs into humans has been around for over 100 years, but its reality is becoming increasingly feasible thanks to genetic engineering techniques. In this study, the transplanted kidneys came from pigs genetically engineered to remove a single gene, the alpha-1,3-galactosyltransferase gene, which is responsible for “alpha-gal,” a sugar that coats the outside of most non-human mammalian cells and to which most humans have pre-existing antibodies. The alpha-gal knockout strain was developed, and FDA approved, as a meat source for people with red meat allergies, which are also due to the alpha-gal sugar.

While the previously published clinical report showed that the kidneys were functional and did not show signs of hyperacute rejection for the 54 hours that they were maintained, little was known at the cellular level about how xenotransplanted organs respond to the human physiological environment or how the human immune system responds to xenotransplants.

“Our goal was to investigate both the short-term response of the organ to being transplanted and how the decedents’ blood cells reacted to this hookup of a foreign organ from another species,” says Boeke.

To do this, the researchers used single-cell RNA sequencing to analyze the gene expression of individual cells in the xenotransplanted kidneys and the recipients’ blood. They collected kidney and blood samples prior to transplantation; 12, 24, and 48 hours post-transplantation; and after 54 hours, when the trials were concluded.

They found molecular evidence of reciprocal interactions between the human and pig cells. Human immune cells were detectable within the pig kidney after only 12 hours, and there was a second wave of immune activation after 48 hours, when the recipients’ immune cells began producing pro-inflammatory cytokines.

“Human immune cells began infiltrating the pig tissue very rapidly, only 12 hours after transplantation,” says corresponding author Bo Xia (@BoXia7) of the Broad Institute of MIT and Harvard. “This kind of swift immune response was not obvious from the earlier clinical study, which lacked sufficient sensitivity and cellular resolution to distinguish human versus pig cells.”

Within the transplanted kidneys, the researchers detected early signs of antibody-mediated rejection, likely due to pre-existing antibodies.

“The source of the antibody-mediated rejection would have to be pre-formed antibody that the person already had prior to transplantation, because you can't have an immune response in 54 hours,” says Boeke. “The alpha-gal knockout takes out the most important antibody-mediated rejection signal, but there are other sugar types displayed on the surface of cells.”

The pig cells within the transplanted kidneys also showed molecular signs of tissue damage and inflammation. This damage was not evident in the pigs’ non-transplanted kidneys, which the researchers analyzed as a control, suggesting that it resulted from exposure to the human environment and the physical trauma involved in transplantation.

However, the pig cells showed signs that they were bouncing back. Within 24 hours of transplantation, the transplanted kidneys began expressing genes associated with cell proliferation, and this gene expression continued to increase in intensity until 48 hours post-transplantation. The researchers speculate that this proliferation represents the kidneys’ attempt to repair themselves, though it’s possible that the proliferation could be maladaptive.

“Whether this rapid proliferation is helping to maintain a good, functional transplanted kidney or whether it relates to maladaptive tissue repair will require longer term follow-up,” says Xia.

Longer-term studies are already underway—in September 2023, the same team at NYU Langone performed a third kidney xenotransplant of the same type, which they were able to maintain for 61 days.

“We're really looking at the very initial response here, and we will be putting that into a much bigger context in subsequent studies,” says Boeke. “By studying that whole trajectory, we’ll have a much better picture of whether these effects are transient or if they persist over time.”

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This research was supported by the National Human Genome Research Institute and the National Institutes of Health.

Med, Pan, Zhang, and Zheng et al. “Cellular dynamics in pig-to-human kidney xenotransplantation” https://www.cell.com/med/fulltext/S2666-6340(24)00207-1

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