In a lab at Yale University, a rat inhales. Every breath this rodent takes is a sign of important medical advances looming on the horizon, for only one of its lungs comes from the pair it was born with. The other was built in a laboratory.

This transplanted lung is the work of Thomas Petersen and a large team of US scientists. Their technique isn’t a way of growing a lung from scratch. Instead it takes an existing lung, strips away all the cells and blood vessels to leave behind a scaffold of connective tissues, and re-grows the missing cells in a vat. It’s the medical equivalent of stripping a house down to a frame of beams and struts and rebuilding the rest from scratch. The whole process only took a few days and when the reconstituted lung was transplanted into a rat, it worked.

This is important because the lungs are notoriously bad at regenerating and repairing themselves. If a person’s lungs are severely damaged, the only real solution is a lung transplant. But that’s easier said than done. The procedure is expensive, only 20% of patients at most are still alive ten years later, and the demand for donor lungs far exceeds their supply.

Peterson’s ultimate vision is to solve these problems by fitting patients with a transplanted lung grown using their own stem cells. The scaffold would come from a dead donor, or possibly even a primate or pig. Its own cells would be stripped away and the patient’s stem cells would give the scaffold a personalised makeover, seeding it with the various types of cells in the lungs. The whole process should only take around 1-2 weeks. Laura Niklason, who led the study, says, “The value here is that the resultant lung would not reject, which is the key that limits survival of lung transplant patients right now.”

The team’s latest success in rats is a proof-of-concept – it shows that the technique should eventually be possible. But as Petersen notes, there are many technical hurdles to overcome before it could ever used in humans. That achievement is still years of hard work away. “I think that 20 to 25 years is not a bad time frame,” says Niklason. “I previously developed an engineered artery that will be ready for patients next year. It was first published in 1999. If an artery takes 12 years from first report to patients, then a lung will take 20-25.”

First, the team used detergents to strip away all the cells and blood vessels from freshly harvested lungs, leaving behind the ‘extracellular matrix’. This scaffold of connective tissues keeps the lung’s physical properties, as well as its three-dimensional structure. Right down to the microscopic level, every branch was preserved. So were the structures of the alveoli, the little spheres through which our lungs exchange gas with our blood.

The complicated nature of the scaffold explains why the team ruled out the possibility of simply growing a lung from scratch. “We grow arteries from scratch all the time in my group,” says Niklason, “but lungs are harder because of the enormous surface area that is required for adequate gas exchange.” You’d need to provide a template for that surface area and a man-made material is unlikely to do the trick. “That is, technologically, a very tall order.”Other groups have tried this approach and failed to produce anything that can actually exchange gases as a real lung can. For this reason, the team decided to use nature’s template – the lung’s own matrix.

Having exposed the matrix, they marinated it in a cocktail of lung cells taken from newborn rats. The added cells stuck to the matrix in the right places and started reproducing quickly, in a way that they normally struggle to do on standard plastic surfaces. The conditions certainly helped – the team incubated the lung matrix in a ‘bioreactor’ designed to mimic the conditions inside a growing foetus. Different tubes imitated the flow of blood and air into the developing organ, with everything was maintained at just the right pressure. All of these conditions proved to be essential for getting the lungs to re-grow in the right way.

Within just four days, the lungs were once again full of alveoli, blood vessels and small airways, all containing the right types of cells. Then, the big test: Petersen transplanted four of these brand-new lungs (just the left ones) into living rats. Within seconds, the lungs became suffused with blood, which rapidly turned from dark to bright red as it started taking up oxygen. When the team took samples of blood from the major vessels, they confirmed that the new lungs were indeed exchanging gas as they were meant to.

Their biggest challenge now is to find a good source of cells to seed the empty frames they expose. The re-fitted lungs will be immediately rejected by the immune system unless Petersen can grow them using lung stem cells derived from the patients themselves. These aren’t available yet, although techniques that reprogram adult cells into stem-like ones may help to solve this problem in the future. “The stem cell biology will be the biggest hurdle,” says Niklason. “Making the cells and growing them is not so bad, but controlling their fate within the lung matrix will be a substantial issue.”

The team have already shown that the technique works on human lung samples taken from a tissue bank. But even in rats, the results are far from perfect. Chest X-rays revealed that the fresh left lungs were indeed inflating with air but to a lesser extent than the native right ones. There were also signs of minor bleeding into the airways and some clots after a few hours. The matrix had probably become slightly damaged during the process of removing its cells. These leaks will have to be addressed before the procedure can be used in the clinic, but other scientists regard them as a sign that Petersen’s group have rushed ahead too quickly.

Joaquin Cortiella, who works on lung tissue engineering at the University of Texas Medical Branch, says, “I believe that they did not wait long enough with their cultured lung before they implanted it in the animal.” His colleague Joan Nichols agrees. “The big problem with tissue engineering is that because of the clinical need very often researchers have rushed to implant tissues before they had really produced materials worthy of transplantation,” she says.

Nichols’s own own group is working with engineered tissues that are two months old, and they only plan on implanting them into animals in 4-6 months, after careful evaluation. In particular, they want to see if the lung’s blood vessels form a proper junction with the alveoli and the matrix, something that Petersen’s group haven’t established. These junctions are the places where gas exchange takes place. If they aren’t formed properly, gas will probably still diffuse through the blood vessels because the whole organ is sitting in an oxygen-rich environment, but you get blood leaks.

Nonetheless, both researchers say that the technique used to actually produce the lung was “careful, well planned and beautifully presented”. Cortiella says that it “shows the importance of using the organ’s own extracellular matrix”, while Nichols notes that it “advances our view of what a bioreactor needs to look like in order to both grow and mature lung tissue”.

And Nichols is particularly excited about the fact that other researchers are making significant headway in engineering a lung. “It is hard to make headway in a field when so few people have tried to engineer a lung,” she says. “Good science does not take place in a vacuum. You need a critical mass to move the field along.”

Lung engineering may not be a competitive field, but it’s clear that similar approaches are being tested for other organs. Just last week, another team from Massachussetts General Hospital achieved the same trick for livers, stripping them down to a scaffold, re-growing them, and transplanting them back into rats. Again, we’re a long way off from the clinic but the fact that progress is being made at all makes this a very exciting time to be alive.

Reference: Science http://dx.doi.org/10.1126/science.1189345

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