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Genetic Linkage

Pig Parts

According to a new review in The Lancet, when it comes to transplants from pigs, smaller is better. (“Clinical transplantation: the next medical revolution?” from David K. C. Cooper and colleagues at the Thomas E. Starzl Transplantation Institute of the University of Pittsburgh Medical Center.)

People have harbored pig parts for years, in the form of bladder linings, ligaments, and of course heart valves. But these are non-cellular. Using living cells would enable the transplant to actually partake of the physiology of the recipient. A precedent was the case of 19-year-old Robert Pennington who, in 1997, spent six hours hooked up to the disembodied yet still-living liver of Sweetie Pie, a 15-week-old, 118-pound porker, while awaiting a liver transplant. (And there's also Pigman, who terrified Kramer in an episode of Seinfeld.)

The organ shortage is fueling the search for xenotransplants, and pig organs are prime candidates because they are pretty close in size to human organs. The major challenge is circumventing the awakening of antibodies in primates that start a swift and powerful immune response against a certain sugar on the surfaces of pig cells, tearing open the blood vessels of the graft. In 2003, researchers cloned pigs that lack the sugar, and their organs nestled in monkeys lasted longer. Then researchers created pigs whose cells bore proteins found in the human immune system, and transplants did even better. Teamed with immune-suppressing drugs, pig parts worked well in monkeys.

Despite the reputation of pigs as filthy creatures that loll happily in mud, pigs bred for their parts are so clean that they cannot transmit some of the infections that can come from human organs, including Epstein-Barr and West Nile viruses, rabies, HIV, and cytomegalovirus. And the unfortunately-acronymed PERVS – pig endogenous retroviruses – don’t seem to infect humans, but researchers are applying gene-silencing technologies just in case.

Not all pig parts work equally well when ensconced in a monkey or, presumably, a human. Livers and lungs do particularly poorly. Pig liver works, but also gloms up the recipient’s platelets, causing widespread bleeding. Lungs last barely a day, but livers hang on for a week or so, making them possible bridges-to-transplant – keeping someone alive until a human organ becomes available. As for hearts and kidneys, left ventricular assist devices and hemodialysis, respectively, are not likely to give way to pig parts anytime soon.

But pancreatic islets are another story. We already know they can vanquish type 1 diabetes, at least for a few years, but human islets are in short supply. Enter the pig. The porcine variety of islets are in phase 2 clinical trials, in New Zealand, Russia, and Argentina, sponsored by Living Cell Technologies. The pig islets are encapsulated in an algae derivative, so that harsh immunosuppressant drugs aren’t necessary. They are delivered via laparoscopy into the peritoneal cavity. So far, so good! One recipient has had pig cells effectively churning out insulin for nearly a decade.

What’s next? Pig neurons and corneas. The Pittsburgh researchers suggest that pigs for xenotransplant be bred in developed nations, but used to help people in developing nations. I’m not sure whether religious Jews will be permitted to partake of living pig cell implants.
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