The issue is simple: there are simply not enough hearts for all the children who need them. So 17 percent of all children who need a heart transplant die while waiting; this translates to 20 percent to 25 percent of infants.
The University of Alabama at Birmingham (UAB) and Children’s of Alabama aim to change those dismal statistics with one of the most revolutionary approaches since the first heart was transplanted from one human to another in 1967 – xenotransplantation.
Thanks to a $19.5 million grant from biotechnology magnate United Therapeutics Corporation, UAB and Children’s have launched one of the top programs in the world dedicated to developing genetically modified solid organs from pig models for transplantation.
The idea isn’t new. Pig tissue has been used to replace heart valves for years, said cardiothoracic surgeon David Cleveland, M.D., MBA, who leads the program at Children’s. The greatest challenge with solid organs, he said, is overcoming immunological and physiological barriers.
If they can do that, “We believe that there’s huge potential to improve the lives of children,” he said.
Supporting Evidence
Earlier this year, Cleveland presented preliminary results from a study showing little reactivity in an infant’s blood to cells from a triple-knockout (TKO) pig. The pig had been genetically modified to delete the three major antigens that react with natural human anti-pig antibodies. Even those human cells that did react demonstrated a very mild reaction.
“We found that very promising,” Cleveland said.
Another area of interest is producing immune tolerance by transplanting porcine thymus tissue to “re-educate” the immune system to accept the pig heart, said cardiac intensivist Leslie Rhodes, M.D. The idea comes from the fact that children can develop an immune system via a human thymus transplant. “We wonder if we could we train their immune system to be tolerant to the pig thymus transplant,” she said.
Infants are the ideal starting place, Cleveland said, not only because they have the highest wait list mortality of any other demographic waiting for a solid organ transplant, but because their immune systems are still naïve. Indeed, they do not develop antibodies to pig glycans during at least the first three months of life, Cleveland and his team wrote in a recent journal article, providing a “window of opportunity” for the transplant.
The next step is a transplant in a non-human primate. “The FDA won’t even consider it until we can prove consistent survival in a non-human primate,” Rhodes said. They hope to perform their first transplant later this year.
Societal Concerns Addressed
The team is also aware of the societal issues around xenotransplantation. To address that, they surveyed the families of patients on the transplant list and the nurses and physicians who will care for these children.
”I was surprised by how positive they were,” Cleveland said. “I thought there would be more pushback than there was.” Still, he said, “I think there has to be major education,” once xenotransplantation becomes a reality. “The idea of replacing a heart with a pig heart will take some people a little time to get over.”
He’s confident it will happen, though. “UAB is going to be one of the centers in the world with the potential to make this happen,” he said. “We have children living in our ICU because there’s not enough cardiac function; they are having their birthdays here. It totally changes entire families to have a child in the hospital forever. There has to be another way.“
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