Pig kidneys vitrified at -150 °C, one for a year, work again after rewarming

Scientists say pig kidneys cooled to -150 °C and stored in a freezer have been rewarmed and shown to work again, which they describe as a first. The kidneys, similar in size to human ones, were left in a glass-like (vitrified) state, in one case for over a year. After rewarming, testing in machines and transplantation into pigs, they regained their color and made urine, key signs that they were working. The work was shared on the bioRxiv preprint server and has not been peer-reviewed. The team hopes to repeat it with human organs.

Erik Finger, a transplant surgeon at the University of Minnesota who co-led the work, said that ten years ago he would not have had high hopes it would work. After the last transplant, he says, he walked out of the room and said it was a big deal. The researchers have yet to show that the kidneys can sustain life over the long term, but others in the field call the result a milestone.

The motivation is the organ supply. Tens of thousands of transplants happen in the US every year, and they are thought to have saved 2 million years of life in the last 25 years. Yet organs are scarce, and an available organ must reach the right recipient within 24 to 36 hours, the current limit for preserving human organs. Recipients also take immune-suppressing drugs for life, which carry side effects and raise the risk of infections and cancer. Long-term banking could give doctors more time to match organs and possibly to prepare recipients' immune systems beforehand. In Finger's words, instead of patients waiting for the organ, the organs would wait for the patients.

Vitrification already works for eggs, sperm and embryos, but whole organs are much harder. Cryoprotectants, which work like antifreeze and stop damaging ice crystals, tend to be incredibly toxic. Large tissues are also hard to cool quickly and evenly, and just as hard to rewarm evenly. Finger and bioengineer John Bischof, also at the University of Minnesota, and colleagues tackle this with iron oxide nanoparticles. Once an organ is loaded with cryoprotectant, it is infused with the nanoparticles, which do nothing to the cells. On rewarming, an electromagnetic field is applied; when its radiofrequency switches between two states, the particles flip and realign with it, releasing heat throughout the organ. Adjusting the field's power controls the heating rate, which gets faster once the organ reaches -120 °C.

The team vitrified rat kidneys this way in work published in 2023, rewarming kidneys that had spent up to 100 days in a freezer before transplanting them into rats. Scaling up was hard: rat kidneys weigh about one gram, pig kidneys about 100. The cryoprotectant used for rat kidneys did not work as well in pig organs. Bischof says they had to rediscover and modify the cocktail, which took three years.

The procedure starts with removing a kidney from a donor pig, flushing it with a commonly used preservation solution to clear the blood, bagging it and carrying it on ice to the engineering lab. There the kidney is connected to a device that controls its temperature and pressure and loaded with the cryoprotectant cocktail, slowly enough to spread evenly but fast enough to limit toxicity; this takes around two hours. The nanoparticles are then pumped through for around seven to eight minutes, per Finger. Cold nitrogen vapor is pumped into a chamber around the organ to cool it, as fast as possible to avoid ice, with several people watching that it cools all the way through. At around -120 °C the organ reaches the vitrified state and "behaves like a glass", Finger says; he adds that it can stay stable for long periods, "we think indefinitely". The team pauses cooling so the organ is glassy throughout, since otherwise it could crack, then cools it to -150 °C. In the freezer it can stay for days, weeks or even months, and the team has kept a small number of kidneys banked for over a year.

After removal from the freezer, the kidneys are rewarmed and flushed of chemicals and nanoparticles. The team tested some on perfusion devices, which are already used in transplant surgery to feed organs and let doctors assess them outside the body. Others were transplanted into pigs and monitored for two hours. In both setups the kidneys seemed to recover; Finger says "they pink right up". A key test is urine output. The rewarmed kidneys produced around two-thirds of the amount normally expected, starting within a minute or so. Most had been banked for days or weeks, but one had been frozen for just over a year. Put on a perfusion machine, it produced urine "immediately" and, Finger says, looked better than many of the other kidneys. He says the team is comfortable saying it can store kidneys for a year and rewarm them with excellent function.

The outside reaction is warm but guarded. Peter Kilbride, a cryobiologist and cofounder of Kyros Kairos, who was not involved, says people have tried and failed to reach this step for years, and calls it one big breakthrough that solves half the problem. Korkut Uygun, a biomedical engineer and professor of surgery at Harvard Medical School, also not involved, says the work will enable truly indefinite organ banking. Kevin Myer, president and CEO of LifeGift, a Texas organ procurement organization, says going a year is a very big deal and imagines organ banks for on-demand use in emergencies such as some drug overdoses or gunshot wounds.

The procedure is not ready yet. Besides making less urine than expected, the rewarmed kidneys showed signs of injury, which the researchers say seems to be caused by the cryoprotectant chemicals. Greg Fahy, a cryobiologist considered a pioneer in the field, notes that blood flow returned, which is very important, but worries about the extent of the damage: from these experiments, he says, one cannot tell whether the kidneys might have supported life. Finger hopes to find ways to repair the damage, noting that organs can improve on perfusion machines and damaged kidneys might partly self-repair in a body. He also says human kidneys may cope better than pig kidneys. Next come longer tests in pigs, a continued search for less toxic cryoprotectants, and planned experiments with human kidneys.

Key facts

• Pig kidneys were vitrified with cryoprotectants and iron oxide nanoparticles, stored at -150 °C, then rewarmed with an electromagnetic field; one kidney had been frozen for just over a year, while most were banked for days or weeks.
• Rewarmed kidneys were tested on perfusion machines and transplanted into pigs, where they were monitored for two hours; they regained color and made urine, about two-thirds of the expected amount.
• The kidneys showed signs of injury that the researchers attribute to the toxic cryoprotectants, and the team has not shown they can sustain life over the long term.
• The work is a bioRxiv preprint and has not been peer-reviewed; the team plans longer pig tests and experiments with human kidneys.
• Today's limit for preserving human organs is 24 to 36 hours; outside experts call the results a milestone, and Kevin Myer says going a year is a very big deal.

Why it matters

Organs for transplant can currently be preserved for only 24 to 36 hours, which limits matching to recipients and leaves organs unused. Whole-organ vitrification is much harder than freezing eggs, sperm or embryos, and the team says this is the first time rewarmed vitrified pig kidneys have been shown to work. Building on its 2023 rat-kidney result, it scaled to organs about 100 times heavier. If banking works in people, doctors would have time for matching and could prepare recipients' immune systems beforehand, which might reduce rejection.

Who it affects

People waiting for transplants, and transplant surgeons and organ procurement organizations, are the main audience. Kevin Myer of LifeGift imagines banks of organs for emergencies where patients cannot wait, such as some drug overdoses or gunshot wounds. Cryobiologists working on less toxic cryoprotectants are also affected, since the source says many researchers will keep searching for them.

How to use it

Nothing here can be used clinically yet. The experiments were in pigs. The next steps are longer-term transplant tests in pigs and planned experiments with human kidneys; the source gives no timescale for human trials or clinical use. Readers can follow the bioRxiv preprint for peer review and follow-up data.

How solid is it

It is a preprint on bioRxiv and has not been peer-reviewed. The headline claim rests on one kidney stored for just over a year, which was tested on a perfusion machine and produced urine immediately; most others were banked for days or weeks. Transplanted kidneys were monitored for only two hours. Outside experts Peter Kilbride and Korkut Uygun, neither involved in the work, call it a breakthrough, while Greg Fahy says the experiments cannot show whether the kidneys could support life. The article does not give the number of pigs or kidneys tested.

Risks and caveats

The rewarmed kidneys made about two-thirds of the expected urine and showed injury that the researchers attribute to the cryoprotectant chemicals, which tend to be incredibly toxic. Long-term survival in animals has not been demonstrated. Finger's view that vitrified organs stay stable indefinitely is a belief, not a result, and his hope that human kidneys will cope better than pig kidneys is also untested. Finger suggests the damage might be repaired on perfusion machines or by self-repair in the body, but that is a hope rather than a finding.

“This is one big breakthrough that solves half the problem”

— Peter Kilbride, cryobiologist and cofounder of Kyros Kairos, who was not involved in the research