Zoe Berg

Yale researchers are exploring a new approach to treat a potentially deadly heart defect. They have begun designing an artificial blood vessel to grow in a patient’s heart.

After receiving over $2 million from the National Institutes of Health, bioengineers at the School of Medicine are looking to create artificial blood vessels to treat patients with single-ventricle congenital heart defects, or SVCHD, a heart condition that affects one out of every 1000 newborns. Though the defects can often be deadly without surgery, infants who are treated have a high likelihood of survival.

But the usual procedure, which involves rerouting blood from a weak heart to the lungs with synthetic tubing, isn’t a perfect fix: the grafts can become infected, can become clogged or the body might attack the synthetic tubing as a foreign object. The tubing also doesn’t grow with a patient, meaning that a child would require multiple surgeries to replace the tubes as they age. 

The Yale researchers believe that they have a solution: blood vessels made with cells, not synthetic materials like plastic. 

“In our case, we are making an artificial vessel, but it can actually grow with the age of the baby,” said Muhammad Riaz, a research scientist on the team who studies cardiology. “This will be the ultimate solution.” 

The Fontan operation

The human heart has four key components: two upper chambers, called atria, that receive blood arriving into the heart. The lower chambers, known as ventricles, pump blood out of it.

For patients with single ventricle congenital heart defects, only one of those ventricles is strong enough or large enough to pump blood out of the heart — including into the lungs, where blood flows to receive oxygen that it carries through the rest of the body.

 A newborn with defects, according to the American Heart Association, might have difficulty breathing or feeding. Their skin might also develop a blue tinge from a lack of oxygen — called cyanosis.

If left untreated, the defects can be fatal. Doctors get to work shortly after a baby is born, often using a type of open heart surgery called the Fontan operation. In the Fontan, surgeons use a synthetic graft to reroute oxygen-poor blood from the lower parts of the body directly into the pulmonary artery, bypassing the heart. Once the blood reaches the pulmonary artery, it travels to the lungs to replenish its cells’ oxygen supply. 

However, the synthetic tubes used in the Fontan operation have several shortcomings. Besides an inability to grow with the child, they’re susceptible to infection and clotting. Sometimes, the body’s immune system rejects the graft and attacks it, instead. 

With an artificial blood vessel made from cellular materials, the Yale team hopes to avoid those complications. A biological alternative can grow with a child, helping avoid repeat surgeries. It also minimizes the chance of rejection, the scientists said.

“[It is] a biological tube that can provide mechanical assistance to effectively transport blood,” said Peter Gruber, a professor of surgery at the School of Medicine and a pediatric heart surgeon at Yale New Haven Children’s Hospital. “[It] redirects blood flow away from the heart’s compromised chamber.”

“A universal vessel graft”

To do so, the team, led by Yibing Qyang, a cardiology expert and associate professor of medicine at the School of Medicine, is using cells called induced pluripotent stem cells. Unlike cells found in body parts like the muscles or liver, stem cells have not yet specialized to fulfill specific roles in the body. Under the right conditions, they can transform into virtually any type of cell.

Induced pluripotent stem cells, or iPSCs, are one example. And through a process of cellular reprogramming, scientists can convert most kinds of specialized, or differentiated, adult cells back into induced pluripotent stem cells. 

Qyang’s group hopes to use the iPSCs to generate the cellular building blocks that are essential components of blood vessels: specialized cells known as smooth muscle cells and endothelial cells. Since iPSCs can be reprogrammed from ordinary cells, the team hopes to be able to scale the production of those cell varieties with consistency.

“One advantage of using these stem cells is that they can proliferate all the time,” said Hangqi Luo, a postdoctoral associate in the lab. “So even one small cell can generate into thousands, tens of thousands of cells that we want.”

But using iPSCs is not always a smooth process, Riaz pointed out. Sometimes, the process of differentiating the stem cells into smooth muscle and endothelial cells yields other, unwanted cell types as well. 

“The first and foremost challenge has been … to get actually enough … cells that are required to engineer tissue and vessels,” Riaz said. “There have been other cell types that contaminated these functional cells. So we need to avoid those unrelated cell types, and it’s tough to get rid of the unwanted cells to have a pure population of smooth muscle cells that we need.”

Though they don’t yet have a perfect solution, Riaz said he and his colleagues are developing or improving methods to reduce the number of unwanted cells and to enhance the number of cells useful when building artificial blood vessels.

And once the researchers have isolated and cultivated the endothelial and smooth muscle cells from iPSCs, there’s another problem. Since the specialized cells used in the engineered blood vessels come from donors, they carry proteins on their surface called immune markers, Luo said — proteins that act like a unique immune fingerprint that the body can recognize.

Normally, those proteins are helpful: it’s the same mechanism that the body’s immune system uses to distinguish its own cells from pathogens, protecting a blood cell and attacking a virus. But since the engineered blood vessels come from a donor, they carry immune markers that the body doesn’t recognize. 

The result is an immune rejection, where the body’s immune system attacks the engineered graft, just as it might reject a plastic one.

“This is the biggest challenge,” Gruber added.

To get around that defense mechanism for the engineered blood vessels, the researchers use a gene editing technology called CRISPR Cas-9 to delete all the immune protein makers on the iPSC cells.

With a clean protein sheet, the final products are immune-compatible, “universal” cells that should skirt the body’s natural defense systems, Luo said.

“Our final goal is to generate a universal vessel graft that can be accepted by any patient, with no immune rejection,” said Luo.

From rats and rabbits to humans

The researchers said that they’ve tested the graft’s ability to withstand high blood pressure in the body and seamlessly integrate with the host’s immune system to minimize the risk of rejection. Part of their current work, Luo said, is to improve the vessel’s mechanical strength. 

“[The] mechanical property is not very strong, and we are working on making it much more strong and [so it] can tolerate higher blood pressure,” Luo said.

The devices are still far from being used in humans. So far, Luo said, they’ve had successful trials implanting the grafts into “humanized” rat models — which are genetically modified to carry specific human genes or tissues — and examining the rats’ immune response.

According to Luo, they hope to scale up the animal models: the plan is to implant the engineered vessel into a larger animal like a rabbit before moving onto a pig model. If the tests in pigs are successful, the team’s next step is clinical trials in humans — which Riaz estimates could occur within 5-10 years.

When that happens, Riaz added, the engineered blood vessels could be a promising solution for vascular diseases and injuries beyond congenital heart defects: with the ability to grow and avoid immune rejection, bioengineered blood vessels could be used to treat heart disease and repair end-stage heart failure.

“If we can use the stem cells to generate this graft, and finally, we can generate the graft without any immune rejection, then we can build all kinds of grafts that we want and implant them for every patient,” Riaz said.

According to the U.S. Centers for Disease Control and Prevention, congenital heart defects affect nearly 1 percent, or about 40,000, births per year in the United States.