Lab-created heart valves grow with the recipient

A groundbreaking new study led by the University of Minnesota, Twin Cities has shown for the first time that lab-created heart valves implanted in young lambs for a year were capable of growth within the recipient.

The valves also showed reduced calcification and improved blood flow function compared to animal-derived valves currently used when tested in the same growing lamb model.

Currently, researchers have not been able to develop a heart valve that can grow and maintain function for paediatric patients. The only accepted options for these children with heart defects are valves made from chemically treated animal tissues that often become dysfunctional due to calcification and require replacement because they don’t grow with the child. These children will often need to endure up to five (or more) open heart surgeries until a mechanical valve is implanted in adulthood. This requires them to take blood thinners the rest of their lives.

Minnesota Professor Robert Tranquillo and his colleagues used a hybrid of tissue engineering and regenerative medicine to create the growing heart valves. Over an eight-week period, they used a specialised tissue engineering technique they previously developed to generate vessel-like tubes in the lab from a post-natal donor’s skin cells. To develop the tubes, the researchers combined the donor sheep skin cells in a gelatine-like material, called fibrin, in the form of a tube and then provided nutrients necessary for cell growth using a bioreactor.

The researchers then used special detergents to wash away all the sheep cells from the tissue-like tubes, leaving behind a cell-free collagenous matrix that does not cause immune reaction when implanted. This means the tubes can be stored and implanted without requiring customised growth using the recipient’s cells.

The next step was to precisely sew three of these tubes (about 16 mm in diameter) together into a closed ring. The researchers then trimmed them slightly to create leaflets to replicate a structure similar to a heart valve about 19 mm in diameter.

“After these initial steps, it looked like a heart valve, but the question then became if it could work like a heart valve and if it could grow,” Prof Tranquillo said. “Our findings confirmed both.”

This second generation of tri-tube valves was implanted into the pulmonary artery of three lambs. After 52 weeks, the valve regenerated as its matrix became populated by cells from the recipient lamb, and the diameter increased from 19 mm to a physiologically normal valve about 25 mm. The researchers also saw a 17 to 34% increase in the length of the valve leaflets as measured from ultrasound images. In addition, researchers showed that the tri-tube valves worked better than current animal-derived valves, with almost none of the calcification or blood clotting that the other valves showed after being implanted in lambs of the same age.

“We knew from previous studies that the engineered tubes have the capacity to regenerate and grow in a growing lamb model, but the biggest challenge was how to maintain leaflet function in a growing valved conduit that goes through 40 million cycles in a year,” said Zeeshan Syedain, a senior research associate in Prof Tranquillo’s lab. “When we saw how well the valves functioned for an entire year from young lamb to adult sheep, it was very exciting.”

“This is a huge step forward in paediatric heart research,” Tranquillo said. “This is the first demonstration that a valve implanted into a large animal model, in our case a lamb, can grow with the animal into adulthood.”

If confirmed in humans, the new heart valves could prevent the need for repeated valve replacement surgeries in thousands of children born each year with congenital heart defects. The valves can also be stored for at least six months, which means they could provide surgeons with an ‘off the shelf’ option for treatment.

The study has now been published in the journal Science Translational Medicine, while the valve-making procedure has been patented and licensed to University of Minnesota start-up company Vascudyne. Prof Tranquillo said the next steps are to implant the tri-tube valve directly into the right ventricle of the heart to emulate the most common surgical repair and then start the process of requesting approval from the US Food and Drug Administration (FDA) for human clinical trials over the next few years.

“If we can get these valves approved someday for children, it would have such a big impact on the children who suffer from heart defects and their families who have to deal with the immense stress of multiple surgeries,” Prof Tranquillo said. “We could potentially reduce the number of surgeries these children would have to endure from five to one. That’s the dream.”

Image credit: Syedain, et al, Tranquillo Lab, University of Minnesota.

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