Discarded Thymus Tissue Provides Life for Infants with DiGeorge Syndrome
DURHAM, N.C. - Using thymus tissue that is normally thrown away after pediatric heart surgery, physicians at Duke University Medical Center have created a new T cell immune system for two children who would have otherwise died.
The children suffered from DiGeorge Syndrome, a rare disorder characterized by either a flawed thymus gland or no thymus at all - as in the case of these two children. To help them, physicians implanted thin strips of thymus tissue into their thighs when they were infants. The children are now 1 1/2 and 6 years old and their new gland has provided a normally functioning immune system that requires no long-term drug support or clinical assistance, the researchers reported in the Oct. 14 issue of the New England Journal of Medicine.
Even though the tissue was implanted from an unrelated donor, it cannot be rejected in patients who lack a thymus, the researchers said, because it is the thymus that produces immune cells that reject foreign tissue. After the transplant procedure, the new donor thymus tissue in the thighs of these children acted just like their own thymus organ would have.
"For these children at least, the transplant was a cure," said the lead investigator, pediatric immunologist Dr. Louise Markert, in an interview. "Early thymus transplantation prior to development of infectious complications can effectively restore immune function in these patients.
"The technique not only offers promise to some children born without a thymus, it demonstrates that medicine is now capable of restoring an immune system to those depleted of one," she said. "Today we have the tools to make this kind of technique work. We can slice thymus tissue thin enough to implant successfully and we can accurately measure immune response."
Three other children who received transplanted thymus tissue later died, the researchers said. Two were on a ventilator at the time they were treated and the third required a ventilator soon after transplant. They all developed infections in their lungs that would not clear, Markert said. Even so, T cell responses were seen in two of the three patients who died.
DiGeorge Syndrome is primarily a genetic disorder that arises before birth and can affect three organs that are developing in close proximity to each other -- the heart, the thymus gland and the parathyroid gland, which regulates the body's use of calcium. Children diagnosed with DiGeorge can show a constellation of disorders, from heart disease to impaired thymus function, but Markert and her team used the transplant technique only on babies born with no thymus tissue.
"We don't want to transplant tissue that will generate its own T cells into infants who have enough of their own thymus to put up a battle that would lead to rejection," she said.
While as many as one in 2,000 children in the United States are diagnosed each year with varying degrees of DiGeorge Syndrome, only between 5 and 10 of them are born without a thymus. This condition is called "complete DiGeorge Syndrome." Less than one-fourth of children with this severe form have a matched sibling who can be used as a source of donor T cells, which cures the disease. For the remaining infants, there is no long-term treatment short of the kind of thymus transplant the Markert team performed to replace T cell immunity, she said. Several previous reported attempts to replace thymus tissue in DiGeorge patients have been difficult to interpret because, Markert suspects, infants who already had low, but existing, levels of immune function were treated and these infants usually improve without therapy.
T cells, so named because they are immune cells that mature in the thymus gland, are made up of "helper" and "killer" cells that seek out and destroy infected cells in the body. Both T cells and B cells, which are specialized immune cells that produce antibodies to signal the presence of an infection, work together to protect a person against viruses, bacteria and other pathogens. Without T cells, an infection that would have little impact on a healthy person can be deadly, Markert said.
Markert first tried the technique in 1993 when a Tennessee family called, hoping for help for their infant. With guidance from co-authors Dr. Rebecca Buckley, an immunologist who pioneered treatment for children with SCID (severe combined immune deficiency), and Dr. Barton Haynes, an expert on thymus function, Markert retrieved a thumb-sized piece of thymus tissue that would have been discarded after pediatric heart surgery. Then, as always, Markert asked the parents of the child undergoing surgery to repair a heart problem for permission to use the tissue.
When a child is born, its thymus gland virtually covers its heart. "It sits on top of the heart and cascades down. And because the organ is so large, most surgeries on infant hearts result in removal of a portion of the gland," she said. Markert takes between 2 to 10 grams of the tissue back to a lab, slices it thinly and cultures it up to two weeks -- long enough to destroy most mature T cells in the gland.
All she needs for transplant are the gland's epithelial cells, whose function is to "educate" immature T cells that are sent to the thymus after stem cells in the bone marrow produce them. "These immature T cells go to the thymus to get trained," she says. "It's just like going to school. It takes about three months to learn what cells belong to the person, and what cells don't."
Because young thymus tissue is most active, Markert only uses donor thymus tissue from babies undergoing surgery who are less than 3 months old.
After the thymus has been in culture long enough, and blood samples from the donor baby have been collected and checked for infections, the pediatric surgeons take over. They lay the thymus strips side by side, "like bacon in a frying pan," deep down into the muscle of the DiGeorge baby's thigh. "It's the biggest muscle in their little bodies, and blood vessels in the thigh muscle grow into the strips of thymus, feed it and rejuvenate it," she said. That happened in four of the DiGeorge infants. In the surviving infants, follow-up biopsies showed normal appearing thymus tissue in their thigh muscle.
As people age, their thymus glands shrink and become hard to locate near the heart, Markert said. That's because T cells already educated by the gland are intended to provide life-long immunity by constantly dividing. But it may be possible to adapt this transplant technique to adults whose immune system have been completely depleted of T cells, such as those who have received a cord blood transplant to treat cancer - a procedure which can sometimes damage the patient's immune system - and in patients with HIV who have not developed T cells despite therapy.
The study, which covered a span of seven years, was funded by grants from the National Institutes of Health, the Max Kade Foundation and Centeon Pharmaceutics.