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In his more than 30 years as a surgeon, Robert Montgomery has transplanted hundreds of kidneys. But at four in the morning September 25, the director of NYU Langone’s Transplant Institute performed one unlike any he’d ever done before. The kidney — six inches long, bean-shaped, and pale pink — was excised overnight from a genetically engineered pig, and flown into New York by private plane and helicopter from hundreds of miles away. The “patient,” lying face-up on the operating table, had died the day before. Machines now kept her body in a state of suspended animation, long enough to undergo the two-hour procedure to attach the organ to blood vessels in the woman’s leg, and to study what happened after.

It was the first of a flurry of firsts over the last few months that have suddenly drawn attention to the niche field of xenotransplantation and its potential to solve the shortage of donated human organs.

Five days later, Jayme Locke, a surgeon who had trained under Montgomery, went one step further. Her team at the University of Alabama at Birmingham put two kidneys from a different herd of designer pigs into a man who had recently passed away. This time, they swapped his organs for the porcine ones, took off the clamps, and held their breaths. The man’s brain-dead body could still mount an immune attack. And if it did, blue splotches would begin to appear on the kidneys as clots would cripple it from the inside out, turning it to a hard black mass within minutes.

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Instead, they turned pink. Within 20 minutes, one of them was peeing. “It was exhilarating to say the least,” Locke told STAT this week.

The world first learned of the NYU operation last October, when it was reported by USA Today. Locke’s team waited for a peer-review of their own experiment, the results of which were published Thursday in the American Journal of Transplantation. In the meantime, Montgomery’s team performed a second kidney attachment to a brain-dead human, and a third team at the University of Maryland Medical Center transplanted a genetically engineered pig heart into a living patient. The 57-year-old man is recovering and doing well nearly two weeks out from the groundbreaking procedure, one of his doctors said.

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Xenotransplantation — putting animal organs into humans — is a centuries’ old idea that has been revived at multiple times throughout history as technological advances offer new hope of overcoming what has seemed like a never-ending parade of scientific hurdles. Norman Shumway, the pioneering Stanford surgeon considered the father of heart transplantation famously said that “xenotransplantation is the future, and always will be.”

For decades, that has certainly seemed to be the case. The field has long been stuck in the preclinical stage — testing organs in baboons and chimpanzees. But the gap is closing. And while it’s still too early to say exactly what new scientific knowledge has been gained, these recent experiments are generating new excitement for the possibility of an unlimited supply of organs that could relieve a supply shortage that leads to about 6,000 deaths each year in the U.S. alone. Now the race is on to build the sorts of biosecure facilities regulators are requiring for the pig organs to be tested in humans.

Millions of years of divergent evolution have made the human body a pretty inhospitable place for an organ grown inside a pig. When you transplant one into the other, a cascade of defensive maneuvers ensues. Anyone wishing to succeed at xenotransplantation has to come up with strategies for blocking or sidestepping them.

The first and most dangerous hurdle is hyperacute rejection. Within the first minutes to the first hours, human antibodies swarm over the new organ, glomming onto foreign sugars and other cell-surface proteins and triggering inflammation. This summons platelets, a type of blood cell that begins to form clots, hindering blood flow to the organ. Pig heart valves, which have become a stopgap in recent years, avoid these problems through a chemical processing step that removes immunogenic pig proteins (but also renders the tissue rigid, making it not suitable for whole organs).

The modern era of xenotransplantation dates back to the early 1960s, when surgeons at Tulane University transplanted 13 chimpanzee kidneys into humans and another team at the University of Colorado tried the procedure six times with baboon kidneys. Back then, the main tool available to these doctors were primitive immunosuppressive drugs. Only one patient survived for more than a few months; the others died either of rejection or infections.

It became clear that more nuanced methods were needed. They arrived in the ‘90s with the first generation of genetic engineering tools. Money and interest flowed into the field. One company even received approval from the Food and Drug Administration for clinical trials of pig livers altered to carry a handful of human genes. Then, a hiccup.

Pigs carry a number of viruses believed at the time to only transmit between members of their own species. These porcine endogenous retroviruses, or PERVs, embed copies of themselves in the DNA of pig cells, making them impossible to eradicate. In 1997, researchers in London discovered that PERVs could jump into human cells in culture. Later that year, the FDA put a halt on all xenotransplantation trials until researchers could prove that they had developed procedures for preventing PERV infection in human subjects. Although the moratorium was lifted the following year, the agency remained skittish, and commercial efforts sputtered out.

In the early 2000s, David Ayares, then COO of a Scottish company called PPL Therapeutics, used recombinant DNA technology and cloning to create pigs as a source of solid organs. The company had made one very important change to its animals’ DNA, disrupting a gene that made a cell-surface sugar called alpha-Gal.

The same sugar studs the cells of bacteria that live in the human gut. So our immune systems have evolved strong defenses for keeping them there, and not allowing infections to spread into our blood. These defenses are so strong, scientists realized, that almost 1% of all the antibodies we make are meant to recognize alpha-Gal — an order of magnitude greater than any other immune target. PPL Therapeutics spun out Revivicor as a standalone company to pursue xenotransplantation.

And for almost a decade, Revivicor was the only commercial outfit in the hunt. Then came the invention of even more precise gene editing tools like CRISPR. Paired with knowledge gleaned over decades from scientists studying the mechanisms the immune system uses to detect foreign invaders, these tools could be used to hoodwink it into regarding the pig as something more akin to a friendly tourist.

They could also be used to deal with that other pesky problem: PERVs. In 2015, Harvard University biologist George Church and members of his lab used CRISPR to snip out all traces of the viruses and make PERV-free piglets. They founded a company called eGenesis to further develop the technology. Organs from their animals are now being tested in monkeys at Duke University and Massachusetts General Hospital.

“When I first started, I thought I’d arrived just late enough for it not to be exciting,” said Joseph Tector, a clinical transplant surgeon at the University of Miami who for three decades has been pursuing xenotransplantation. “Then I thought, not only are we going to be there, but we might be there by ourselves. Now, all of a sudden it’s a race.”

In 2015, while at Indiana University, the surgeon-scientist made a triple-knockout pig that removed not just alpha-Gal, but two other immune-inflaming pig antigens. The changes made it possible to keep primates alive a year after receiving the modified kidneys. A company he co-founded called Makana, and which merged with genome engineering firm Recombinetics in 2020, is now working to test kidneys from those animals in humans.

Makana’s flashier competitors are betting that more editing will yield even better results. In 2020, scientists at eGenesis reported creating pigs carrying the triple knockout plus nine human genes that code for immune-dampening molecules. Revivicor has added six such transgenes and an additional knockout in a porcine growth hormone receptor gene, aimed at preventing organs from getting too big for their human recipients. It’s this “ten-gene” pig that Locke’s team in Alabama used in its kidney experiment and the Maryland group used for its heart transplant.

But there’s still  debate over how many changes are really necessary to achieve long-lasting xenotransplants. And some researchers say overengineering the animals can make it harder to produce consistent organs, which is something regulators are likely to consider when deciding what to greenlight for human testing. “The science of adding genes isn’t as advanced as the science of deleting things,” said Tector.

Montgomery told STAT that’s one reason he’s taking a “less is more” approach, using Revivicor’s original single-edit pig. Trademarked under the name “GalSafe,” these pigs were approved by the FDA for consumption and some biomedical research in late 2020.

Scientists won’t know which approach works best in humans until, well, they try xenotransplantation in humans. But according to Montgomery, momentum toward starting clinical trials, has picked up in recent months. “For a long time, there was a lack of forward inertia. Now we have it,” he said. “Even the skeptics are coming around.”

Xenotransplantation requires expertise from across many fields. You need genetic engineers to design pigs whose cells won’t trip a human’s immune’s system; animal scientists who understand the peculiarities of livestock species to raise them; immunologists to build tests that can predict if a patient will reject a pig organ and develop drugs to prevent that from happening over the long term; infectious disease experts to minimize the risks of pig viruses spilling over into human patients; and finally, a surgical team to do all the actual slicing and clipping and stitching. And one more thing: a decidedly non-sty-like home for the pigs.

“The hurdle now facing every group is building facilities to produce a pig that’s suitable for clinical transplantation,” Megan Sykes, a surgeon and immunologist at Columbia University told STAT. She was referring to a designated pathogen-free pig facility — a hermetically sealed building ventilated and pressurized to keep out bacteria, viruses, and fungi — the kind of place in which the FDA says any pigs destined to be human organ donors must be raised. “It’s a major undertaking,” said Sykes.

The University of Alabama at Birmingham began building theirs back in 2016, as part of a large grant it received from United Therapeutics, the pharmaceutical firm that acquired Revivicor in 2011. According to financial filings, the facility was federally certified in March of last year, and when Locke performed the kidney xenotransplant into a deceased individual last September, the organs came from an animal raised right there on the campus. “We wanted to make sure that we had documentation of that pig’s disease status throughout the course of its life,” said Locke. “And we were able to show that the transplant recipient did not have PERV-C, which is the one that people worry about being able to cause disease in humans. Those were important milestones for us as we think about how we are going to scale this up.”

Locke said her UAB team is now working on breeding pigs so that they can build up the herd to support a clinical trial, which she’s hopeful they could start as soon as the end of this year. “If everything goes off without a hitch and we can start Phase 1 later this year, then in theory the earliest we could be ready to offer this to the masses would be five years from now.”

Their first effort had mixed results. The kidneys weren’t immediately rejected. But they also didn’t work very well. The one that produced urine didn’t successfully filter out creatinine — a critical waste product. And the other one didn’t produce urine at all. Locke said she suspected it had to do with the fact that the recipient had been brain-dead for five days before the procedure.

“Brain death causes all sorts of pathological conditions in the body,” said David Cooper, a xenotransplantation researcher at Harvard Medical School’s Center for Transplant Sciences. “By day three this person was bleeding to death, essentially, as a consequence of an inflammatory response. We don’t know if that was because of the brain-death or the pig graft, so the results are very difficult to interpret.”

On the other hand, at least there’s data to analyze. The results from the two xenotransplants performed at NYU Langone have not yet been published. Montgomery told STAT they are currently going through peer review. As his group waits for publication, he’s planning another study, also with recently deceased individuals. It will also be with kidneys from Revivicor GalSafe pigs, but this time they intend to keep the bodies on life support longer to try to understand what happens two to four weeks post-transplantation.

“We know from the primate work that this is a critical time,” said Montgomery. In studies of monkeys, about half of the animals do just fine and half start to have problems, often fatal ones, he said. “Right now, no one fully understands that, so we think the additional information we might be able to get out of longer studies of the recently deceased would be very convincing to the regulators that this is ready for prime time.”

Sykes isn’t so sure there’s much to be gained from putting pig organs in brain-dead people. “I would describe them as small steps,” she said of the NYU Langone and University of Alabama at Birmingham experiments. Scientifically, they merely confirm what many studies in monkeys would have predicted, she said. Where their impact may have far wider implications is in making cross-species organ donation go mainstream. “The bigger impact is that it’s gotten the world accustomed to the notion that xenotransplantation is a real thing that’s going to be tried soon, and I think that’s a very positive outcome.”

She’s more excited about the transplant performed at the University of Maryland Medical Center earlier this month, in which a man received a heart from a “10-gene” pig. That effort was led by Muhammad Mohiuddin, who is best known for pioneering a 2016 trial in which his team kept baboons with transplanted pig hearts alive for over a year with a unique cocktail of immunosuppressants. One lived 945 days, a record. The next year he moved from the National Institutes of Health to start a cardiac xenotransplantation program there in Maryland as part of another United Therapeutics-funded project.

He told STAT this week that his team had approached the FDA about starting a human trial last year, and they were told they needed to show more consistently that they could keep a large group of primates alive for at least six months. They are in the process of running that study now. But in the meantime, they sought and received a one-time permission to try the procedure in a critically ill patient who wasn’t eligible for a human organ.

“I’ve been in this field for 30 years and I could not have imagined that this would happen in my lifetime,” said Mohiuddin. “Every time we’ve come close we’ve seen another problem pop up, like peeling the layer off an onion. “But now it’s like a dream-come-true moment.”

For now, both the Maryland and NYU teams have been getting their organs from Revivicor, which has a farm in Blacksburg, Va, and a herd of GalSafe pigs at a facility in Iowa. Mohiuddin told STAT that the company is building its own pathogen-free facility to supply organs for clinical trials.

Montgomery read about the Maryland team’s success with a particularly personal jolt of joy. He inherited a progressive genetic heart disorder, one that killed his father and an older brother. In 2018, he received a heart transplant of his own. “My stake in this is a little different,” he told STAT. “I really want to see this move forward so that my family has different options than I had and 6,000 people don’t have to die every year waiting for an organ.”

Cooper, who is an advisor to eGenesis, told STAT that the company not having a pathogen-free facility was holding up efforts to move forward into human testing. “I think we have reached the end of the road as far as animals go; we’ve done almost as much as we can possibly do,” he said. “We have the right pigs, very potent immunosuppressive drugs, and if we choose the right patient I think we have every chance of success. The one thing we need … is this clean facility.”

After this story published, a spokesperson for eGenesis told STAT via email that the company has secured an established biosecure animal facility in the Midwest from which it intends to supply pig organs—likely starting with the kidney—for human trials.

This story has been updated to include comment from eGenesis.

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