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Tuesday, June 9, 2009

Hybrid hearts could solve transplant shortage

A "decellularised" pig's heart

Video: Hybrid heart

"IT'S amazing, absolutely beautiful," says Doris Taylor, describing the latest addition to an array of tiny thumping hearts that sit in her lab, hooked up to an artificial blood supply.

The rat hearts beat just as if there were inside a live animal, but even more remarkable is how each one has been made: by coating the stripped-down "scaffolding" of one rat's heart with tissue grown from another rat's stem cells.

Taylor, a stem cell scientist at the University of Minnesota in Minneapolis, now wants to repeat the achievement on a much larger scale, by "decellularising" hearts, livers and other organs taken either from human cadavers or from larger animals such as pigs, and coating them in stem cells harvested from people.

This could lead to a virtually limitless supply of organs for transplantation that are every bit as intricate as those that grow naturally, except that they don't provoke the catastrophic immune response that obstructs the use of traditional "xenotransplants".

The organs don't provoke the immune response that prevents traditional xenotransplants

"We're already working with heart, kidney, liver, lung, pancreas, gallbladder and muscle," Taylor says. Rival groups are using similar procedures to create new livers and muscle too.

Human organs for transplant are scarce. One option is to engineer organs from scratch in the lab, using artificial scaffolds. While bladders and skin can be grown in the lab, growing more complex organs and their intricate blood-vessel networks, has proved tricky.

Xenotransplants from pigs are another possibility, though fraught with problems. You have to prevent the recipient's immune system from destroying the organ, and also ensure the transplant is free of pig viruses that could be passed on.

Taylor's organs avoid these problems. For starters, building an intricate scaffold from scratch is unnecessary. "It's letting nature do most of the work," she says. What's more, because the stem cells that "clothe" the naked scaffold are taken from the patient, the organ stands a higher chance of being accepted by their immune system.

The idea is fairly simple: take an organ from a human donor or animal (see image), and use a mild detergent to strip away flesh, cells and DNA (see image) so that all is left is the inner "scaffold" of collagen, an "immunologically inert" protein (see image). Add stem cells from the relevant patient to this naked shell of an organ and they will differentiate into all the cells the organ needs to function without inducing an immune response after transplant, or any new infections.

The idea has already worked with simple organs. Last year Claudia Castillo received a transplant made a stripped-down windpipe from a dead human donor. Researchers cut it to size and seeded the scaffold with her stem cells, which grew into the right tissues and gave her a new windpipe. Anthony Hollander of the University of Bristol, UK, a member of the team, says Castillo no longer needs to take drugs and is back at her job.

Taylor's team is using the same technique to create much more complex organs such as hearts, and extending it to using animal, as well as human, scaffolds.

A big challenge with complex organs is ensuring that all their cells are infused with blood. Without blood, cells in the centre of the organ would be starved of oxygen and die after transplantation. Taylor says her method overcomes this problem.

A big breakthrough came in January 2008, when her team produced a beating heart by filling a rat heart scaffold with heart cells from newborn rats (Nature Medicine, vol 14, p 213). These hearts kept their 3D shape, including spaces for all the blood vessels. When they were seeded with new cells (see image), some grew into blood vessel lining (see image).

Since then, Taylor says they have managed to "pretty much repopulate the whole vascular tree" with cells, which includes veins, arteries and capillaries. "Because we've retained the blood vessels, we can take the plumbing and hook it up to the recipient's natural blood supply," says Taylor. "That's the beauty of this."

Although Taylor only added stem cells to the hearts, these cells differentiated into many different cells, in all the correct places, which is the best part of using decellularised scaffolds. The stem cells transformed into endothelial cells in the ventricles and atria, for example, and into vascular and smooth-muscle cells in the spaces for blood vessels, just as in a natural heart. Taylor thinks this happened because she pumped blood and nutrients through the organ, producing pressure in each zone which helps to determine how cells differentiate there.

But chemical, as well as mechanical, cues seem to have guided differentiation. Taylor has evidence that growth factors and peptides remained anchored to the scaffold even after the flesh was washed off. These chemicals likely signalled to the stem cells, indicating how many should migrate to which areas and what to change into in each zone. "Our mantra is to give nature the tools and get out of the way," she says.

Her team has implanted the reclothed hearts into the abdomens of rats, where they survived temporarily and were not rejected. The next step is to see if the transplants can replace an existing heart and keep the animal alive and healthy. To do this, Taylor says they will need to come up with ways to grow more muscle tissue on the hearts. "We've built the vasculature but we don't think we've built enough muscle to keep animals alive."

The next step is to see if the transplants can replace an existing heart and keep the rat alive and healthy

She is also gearing up to repeat the rat experiments with pig hearts and livers. This could be easier because pig organs are larger and easier to handle than tiny rat hearts. Decellularised livers could also appear in humans before hearts because it may not be necessary to recreate entire livers for them to be useful.

Others are also working on livers. Steven Badylak says he has unpublished "proof of concept" that liver recellularisation works in rats and mice. A team lead by Martin Yarmush at Massachusetts General Hospital in Boston has manufactured recellularised rat grafts that provide liver function "in the lab and when transplanted", according to team member Korkut Uygun. But he stresses that the team's ultimate goal is to decellularise human, not animal, organs for transplantation.

Not everyone believes that turning decellularised tissue into a complex, functional organ is as simple as it sounds. "We're a long way from being able to make functional tissues and organs," says Alan Colman of the Singapore Stem Cell Consortium. "We'll be able to make structures that look like the organ, but with almost none of the correct functionality."

David Cooper of the University of Pittsburgh School of Medicine in Pennsylvania, a leading developer of xenotransplants, says that "naked" pig hearts would still carry traces of alpha-Gal, which the human immune system recognises and will attack.

But Chris Mason, professor of regenerative medicine at University College London points out that many decellularised pig components have been used in people without the need for immunosuppressive drugs (see "Pig parts"). He says sufficiently rigorous sterilisation destroys these residues. Otherwise, says Mason, millions of people would already have had adverse reactions to the pig heart valves and tissues they've received.

Taylor says people who find the idea of pig parts unacceptable should consider their current uses in humans. "We're not ready for prime time yet, but we're moving in the right direction," she says.

Pig parts already commonplace

IMPLANTING organs made from the scaffold of a pig organ may sound off-putting and even dangerous, but millions of patients have already been treated with decellularised pig parts without being infected by stowaway pig viruses or suffering disastrous immunological reactions.

Pig heart valves are often used to replace faulty ones in people. In the past, patients who got such valves had to take immunosuppressive drugs. But this isn't necessary with newer pig valves, made by the company AutoTissue in Berlin, which have been thoroughly decellularised.

For years, companies have also been selling decellularised pig gut to produce patches that help the healing of diabetic ulcers, hernias and strained ligaments. Cook Biotech of West Lafayette, Indianapolis, sells patches made from pig sub-mucosal collagen membrane, which provides mechanical strength to the small intestine. "Since 1998, we've treated more than a million patients," says the company's Michael Hiles. Meanwhile, Tissue Regenix of Leeds, UK, is about to start testing tissue from pig heart membranes for patching up holes in arteries.

Chris Mason, professor of regenerative medicine at University College London, says the work of these companies bodes well for the idea of one day implanting much more complicated decellularised pig organs into people.

Available thumbnails

A "decellularised" pig's heart A pig's heart before the process of decellularisation (Image: courtesy of the University of Minnesota) A pig's heart undergoing decellularisation in the lab (Image: courtesy of the University of Minnesota) A re-celled rat's heart (Image: courtesy of the University of Minnesota) A rat heart undergoing decellularisation (top three images), and during recellularisation (bottom) (Image: courtesy of the University of Minnesota)