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Genetically engineered pigs put xenotransplantation back in the spotlight

A flurry of firsts using organs from genetically engineered pigs – kidneys and a fortnight ago a heart – to save human lives, has drawn attention to xenotransplantation and its potential to solve donated human organ shortage, reports STAT.

In than 30 years as a surgeon, Robert Montgomery has transplanted hundreds of kidneys. But at 4am on 25 September 2021, the director of NYU Langone’s Transplant Institute performed one unlike any he’d done before. The kidney, 15cm long, and pale pink, was excised overnight from a genetically engineered pig. The “patient” on the operating table had died the day before. Machines 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 her leg, and to study what happened after.

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 died. They swopped his organs for the porcine ones, removed 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 appear on the kidneys as clots would cripple it from the inside out, turning it to a hard black mass within minutes.

Instead, they turned pink. Within 20 minutes, one of them was peeing. “It was exhilarating,” Locke told STAT.

The world first learned of the NYU operation last October, when it was reported by USA Today (MedicalBrief 20 October 2021). Locke’s team waited for a peer-review of their own experiment, the results of which were published last week in the American Journal of Transplantation (See MedicalBrief of 19 January 2022).

Meanwhile, 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 doing well two weeks after the groundbreaking procedure, doctors said. (MedicalBrief 12 January 2022).

Xenotransplantation, or putting animal organs into humans, is a centuries’ old idea revived multiple times throughout history as technological advances offer new hope of overcoming scientific hurdles. But the field has long been stuck in the preclinical stage, testing organs in baboons and chimpanzees.

However, these recent experiments are generating excitement for the possibility of an unlimited supply of organs to relieve a shortage leading to many thousands of deaths worldwide each year Now the race is on in the US for the biosecure facilities regulators say are imperative for the pig organs to be tested in humans.

Millions of years of divergent evolution have made the human body inhospitable for an organ grown inside a pig, and skills are required to help it sidestep its own defensive manoeuvres.

The most dangerous hurdle is hyperacute rejection. Within the first minutes and hours, human antibodies swarm over the new organ, glomming on to foreign sugars and other cell-surface proteins and triggering inflammation. This summons platelets that form clots, hindering blood flow to the organ. Pig heart valves, 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 doctors used primitive immunosuppressive drugs. Three decades later, the first generation of genetic engineering tools arrived. Money and interest flowed into the field, one company even receiving approval from the US Food and Drug Administration (FDA) for clinical trials of pig livers altered to carry a handful of human genes. Then, a hiccup.

Pigs carry 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, London researchers discovered that PERVs could jump into human cells in culture. Later that year, the FDA stopped all xenotransplantation trials until researchers could develop procedures for preventing PERV infection in human subjects. Although the moratorium was lifted the following year, 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 an 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 in the human gut. So our immune systems have evolved strong defences for keeping them there, and not allowing infections to spread into our blood. These defences are so strong, scientists realised, that almost 1% of all the antibodies we make are meant to recognise alpha-Gal, an order of magnitude greater than any other immune target. PPL Therapeutics spun out Revivicor as a standalone company to pursue xenotransplantation.

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.

They could also deal with PERVs. In 2015, Harvard University biologist George Church and his lab team 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.

Iin 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.

Competitors believe 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 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 necessary to achieve long-lasting xenotransplants. Some researchers say overengineering the animals can make it harder to produce consistent organs, which is something regulators might 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 Joseph Tector, a clinical transplant surgeon at the University of Miami who for three decades has been pursuing xenotransplantation.

Montgomery told STAT that’s why 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 biomedical research in 2020.

Scientists won’t know which approach works best in humans until, well, they try xenotransplantation in humans.

“The hurdle now is building facilities to produce a pig 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 pressurised to keep out bacteria, viruses, and fungi, in which the FDA says any pigs destined to be human organ donors must be raised.

The University of Alabama at Birmingham began building theirs in 2016, as part of a grant from United Therapeutics, the pharmaceutical firm that acquired Revivicor in 2011. When Locke performed the kidney xenotransplant into a deceased individual last September, the organs came from an animal raised on that campus.Her UAB team is now working on breeding pigs so they can build up the herd to support a clinical trial, to start at the end of 2022. “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 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 was because the recipient had been brain-dead for five days before the procedure.

On the other hand, at least there are data to analyse. 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, but he’s planning another study, also with recently deceased individuals. It will also be with kidneys from Revivicor GalSafe pigs, but this time they will keep the bodies on life support longer to try to understand what happens two to four weeks’ post-transplantation.

Sykes isn’t 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. “The bigger impact is that it’s got the world accustomed to the notion that xenotransplantation is a real thing that’s going to be tried soon.”

She’s more excited about the transplant performed at the University of Maryland Medical Center earlier this month, when 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 more than a year with a unique cocktail of immunosuppressants. One lived 945 days, a record.

He told STAT his team had approached the FDA about starting a human trial last year, and were told they needed to show they could keep a large group of primates alive for at least six months. They are running that study now. But meanwhile, they received a one-time permission to try the procedure in a critically ill patient ineligible 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.


STAT article – After a flurry of firsts, xenotransplantation is suddenly back in the spotlight (Open access)


See more from MedicalBrief archives:


World first: Transplant of pig heart on terminal US patient


First pig-to-human kidney transplant


Gene-edited piglets opening door to animal organ transplants



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