Published On May 3, 2009
TRANSPLANT SURGERY HAS COME A LONG WAY since 1954, when Joseph E. Murray, at what was then called the Peter Bent Brigham Hospital in Boston, removed a kidney from one identical twin and implanted it in the abdomen of the other. The recipient survived eight years, and in the decades since then such medical miracles have become almost routine, with hands and even faces entering the realm of remarkable possibilities. In a recent innovation, surgeons at Johns Hopkins University managed to remove a healthy kidney for an aunt-to-niece transplant through the donor’s vagina, avoiding long abdominal scars.
Yet despite such progress, twenty-first-century transplant patients still face the problem that has plagued recipients from the start: Sooner or later their bodies are likely to reject the donated organ. To prevent the immune system from attacking a new heart or liver just as it would a parasite, virus or bacteria, patients take powerful immunosuppressive drugs. But that daily regimen, by tamping down immunity, can let infections run rampant and increase the risk of many cancers, which may take hold when defenses are down. There are also unpleasant side effects—warts, excessive body hair—and the drugs can lead to reduced renal function, diabetes, hypertension, neurotoxicity and other chronic conditions. And even then, organ rejection can occur. The immune system’s T cells trigger inflammation while antibodies attack the organ until it’s damaged beyond repair. Ten years after surgery, only about half of transplanted organs still function.
One way to sidestep the need for immunosuppression and improve the success rate would be to persuade the body that the new tissue really isn’t foreign. That can sometimes be accomplished through a mechanism called chimerism, in which cells from the organ donor fool the recipient’s body into ?believing the transplanted tissue is its own and thus poses no danger. That possibility has intrigued scientists for decades, dating at least to the early 1950s, when British biologist Peter Medawar showed that adult cattle twins that became chimeric in utero because of interconnected blood vessels could receive skin grafts from each other without rejection.
Medawar’s research led to further studies, mostly in mice, of ways to induce chimerism. Certain complications make it difficult to attempt this method in humans, however, and relatively few treatments have been tried. But in early 2008, a team of scientists and clinicians led by David H. Sachs, director of the Transplantation Biology Research Center at the Massachusetts General Hospital, and A. Benedict Cosimi, chief of the MGH’s transplant unit, published the results of a clinical trial in which they were able to take four transplant recipients off immunosuppressant drugs—a remarkable achievement.
According to their report, published in The New England Journal of Medicine, the patients remain healthy and free from kidney disease and the tribulations of immunosuppressant therapy. The MGH team gave the patients bone marrow as well as kidneys from donors, and the recipients became part “other”—their bodies didn’t reject the organs because they didn’t consider the transplanted kidneys foreign. Scientists elsewhere are also trying to use chimerism to transplant a variety of tissues and have succeeded in taking some patients off immunosuppressants. Others, though, don’t consider chimerism a key to drug-free transplantation: They think other mechanisms are at play and need to be harnessed instead. Still, the MGH team’s results, though limited, are too intriguing to ignore.
edawar’s groundbreaking transplantation success owed much to a singular characteristic of cattle. When a cow gives birth to male and female twins, sometimes the sister tends to behave and look rather masculine—she acts like a bull and can’t reproduce. That’s apparently because, in utero, the twins’ blood vessels become intertwined, bathing the female, called a freemartin, with male hormones throughout her development. By some quirk of evolution, this phenomenon happens almost exclusively in cattle.
Through their intertwined vessels, these twin cattle also share something even more fundamental than hormones: hematopoietic (blood) stem cells. Hematopoietic stem cells can produce all the different types of blood cells, and those from one of the embryos infiltrate the other. The stem cells remain in the animals even after birth, so each twin continues to produce not just his or her own blood and immune cells (which also arise from hematopoietic stem cells) but also the other’s. That’s true chimerism—named for the creature Chimera in Greek mythology, which combined parts of a lion, a goat and a serpent.
David Sachs learned about chimerism in cattle—and about Medawar’s success in inducing tolerance of skin transplants—during a lecture at Harvard Medical School in 1965. Impressed that nature could arrange for tissue to be exchanged without immunosuppression, he wondered whether it would be possible to create similar conditions in humans. The field of transplantation was then just getting started, and Sachs thought inducing tolerance through chimerism might give transplant recipients a chance at a normal life.
IN THE EARLY 1950S, Medawar, who won a Nobel Prize in Physiology or Medicine in 1960, moved his work from freemartin cattle to mice. It was then that he found that if he exposed pups to foreign bone marrow cells just after birth, when their immune systems were still being formed and learning to distinguish their own tissue from foreign tissue, he could graft the donor mouse’s skin onto a recipient eight weeks after birth, and most grafts were accepted. But inducing tolerance through chimerism in adult mice proved much more difficult because their immune systems are well established and readily reject unfamiliar tissue. To achieve the same effect as in newborn mice, researchers first had to destroy the bone marrow—where hematopoietic stem cells and all immune cells originate—and introduce donor marrow. That creates full chimerism, in which the recipient mouse has a completely new immune system virtually identical to the donor’s. With full chimerism, skin transplants from the marrow donors normally succeed.
There is, however, a cost. Immune cells from the donor bone marrow may attack the recipient’s body, a condition known as graft-versus-host disease (GVHD). In the mid-1980s, when Sachs was at the National Institutes of Health, he and his colleagues proposed an alternative, which they called mixed chimerism, to deal with this counterattack. The technique derives from the way immune cells are created.
Hematopoietic stem cells in the bone marrow make not only red blood cells and platelets but also precursors to T cells, immune cells that kill invading organisms. The bone marrow sends the precursors to the thymus, and there the T cells mature and differentiate into cells that target the proteins in ?viruses or bacteria. Those proteins come in millions of shapes and sizes, and the body randomly generates T cells to kill cells that have been infected by these microbes. But inevitably, some T cells will go after proteins that are supposed to be in a person’s own body. To deal with the problem, the thymus has a system whereby it destroys all T cells that target “self.” (Problems with this system can cause autoimmune diseases such as lupus and rheumatoid arthritis.)
Sachs thought mixed chimerism could take advantage of this system. He proposed to replace only a portion of the host’s bone marrow so that donor and host bone marrow would coexist, supplying precursor T cells to the thymus for the rest of the patient’s life. That way the recipient T cells that attack a donated organ would be deleted, and organ rejection avoided, while the donor T cells that attack the recipient would also be eliminated, and GVHD avoided. In 1984 Sachs and his colleagues achieved mixed chimerism in mice.
BUT MICE ARE NOT MEN. “Something you can do in inbred mice turns out to be extremely difficult in humans,” says Northwestern University’s Joshua Miller, a pioneer in the field. There have been attempts over decades to induce chimerism in people, but they’ve met with only limited success, and the ultimate goal—avoiding organ rejection and immunosuppressant drugs—has rarely been achieved.
Sachs, rather than moving quickly to people, spent many years honing his technique, moving from mice to pigs and monkeys. He found that transplants were more often accepted when part of the recipient’s bone marrow was eliminated with radiation (and later with drugs) so that the donor’s marrow had space to grow and the immune cells of both donor and recipient could coexist. He also learned that killing all the recipient’s mature T cells, the main culprit in organ rejection, helped transplanted tissue survive.
After Sachs used mixed chimerism to achieve tolerance in mice and then pigs at the NIH, in 1991 he moved back to the MGH, where he had trained as a surgical intern and served as a research fellow. He began working with Cosimi, immunologist Megan Sykes, bone marrow transplant specialist Thomas Spitzer and transplant surgeon Tatsuo Kawai. With Cosimi and Kawai, he was able to perfect his procedure on nonhuman primates—the final step before moving on to people.
The team’s procedure is quite complex. Rather than irradiate the recipient’s body a few days before the transplant to eliminate part of the bone marrow—an approach used during some animal experiments—the patient is given the chemotherapy drug cyclophosphamide. Then, to prevent mature T cells from attacking the soon-to-be-transplanted organs, a three-pronged approach is used: First, an anti-CD2 monoclonal antibody drug destroys most of the T cells in the immune system’s periphery, such as the lymph nodes; another drug, cyclosporine A, suppresses any that remain; finally, radiation of the thymus kills any mature T cells left. Then surgeons take marrow from the donor’s hip bone and give it to the recipient intravenously. The kidney is transplanted at the same time, and the recipient receives traditional immunosuppressive drugs and steroids and rests in a sterile room for two weeks to prevent infection.
In 1999 the MGH team began applying this protocol to human patients. They chose to work with people who had multiple myeloma—cancer of the plasma cell—and resulting renal failure. These patients had not responded to other treatments, and because of their poor prognosis, were not eligible for kidney transplants, but because bone marrow transplants can sometimes cure myeloma, it was thought they might benefit from both new marrow and a new kidney, just as in Sachs’s protocol.
The first six patients all received closely matched kidneys from siblings. Overall, the procedure worked well. All six accepted the donated organs long-term (though one patient experienced a rejection episode, which was treated, and the kidney saved) and four achieved mixed chimerism and were able to go off immunosuppression. The other two progressed to full chimerism, which caused GVHD and forced them to continue immunosuppression. Three patients (two mixed chimeras and one full) seem to have been cured of both myeloma and renal failure, a remarkable development considering they otherwise would have died, says Sachs.
Intriguing as those results were, however, only a tiny fraction of patients—with cancer and access to a very closely matched kidney—could benefit. Soon, the MGH team began testing the protocol in people who didn’t have cancer but did need a kidney and lacked a closely matched donor. The first patient, in 2002, was a 22-year-old woman with severe kidney disease. Using the same basic protocol, the MGH team transplanted a kidney and bone marrow. During eight months, immunosuppressant drugs were tapered off and finally halted. More than six years later, the patient remains off immunosuppressive drugs and her kidney is working well. “That’s a really big deal,” says Ron Shapiro, a transplant surgeon at the University of Pittsburgh. Sachs’s team got similarly positive results in three more patients after they modified the procedure.
SACHS IS CONVINCED THAT INDUCING mixed chimerism was an important step in achieving transplant success without immunosuppressants. But other researchers have doubts, though the results of their trials have done little to settle the debate. Some researchers have reported that chimeric patients have been unable to stop taking immunosuppressants, whereas others have reported that patients, without ever achieving chimerism, have gone off the drugs. Some ?scientists even point to Sachs’s own trial as evidence that chimerism may not be the key to tolerance. Though all of his patients were initially chimeric, their chimerism disappeared after a few weeks. “So is chimerism really necessary to achieve tolerance? I find it more and more mystifying,” says Shapiro.
Based on his work in animals, Sachs thinks chimerism may be essential initially, but that a different mechanism of organ tolerance takes over once transplant recipients cease to be chimeric. He credits T regulatory cells, which live outside the thymus, for suppressing T cells that could target the donor organ. In fact, there’s evidence that chimerism may increase the number of T regulatory cells.
But Rodolfo Alejandro, an endocrinologist at the University of Miami, thinks T regulatory cells alone, rather than chimerism, are the reason for tolerance. “It may be that chimerism is just correlated with tolerance,” Alejandro says. He points to work by immunologist Maria Grazia Roncarolo at the Fondazione San Raffaele del Monte Tabor, in Milan, and others showing that T regulatory cells are essential for establishing and maintaining transplant tolerance, and to recent studies showing that the drug rapamycin, which improves survival of transplanted organs, may work in part by helping T regulatory cells tamp down T cells that would attack the organ.
Transplant pioneer Thomas Starzl at the University of Pittsburgh has yet another explanation: a mechanism he calls microchimerism. In 1992, Starzl, who performed the first liver transplant in 1963 and developed many of today’s immunosuppressive drugs, found several patients who had received kidney or liver transplants decades earlier but had gone off immunosuppressive drugs. (This happens in fewer than 1% of transplant recipients.) “Every recipient had microchimerism,” Starzl says. “Some level of chimerism is a necessary condition for long-term tolerance.”
As Starzl defines it, microchimerism occurs when fewer than 1% of the cells in a person’s blood are donor cells. Starzl thinks the cells of microchimerism migrate from the donor organ and then hide in the lymph nodes, heart or other internal niches. After the flood of cells exhausts the body’s initial antidonor response, these cells subsequently leave their protective niches, maintaining the weakened antidonor immune response and effectively preventing rejection of the donor organ, Starzl says.
Starzl thinks microchimerism could explain why some ?people who go off immunosuppressive drugs do not reject their organ. He has developed a protocol to induce microchimerism in transplant recipients by killing some of their immune cells and replacing them with donor immune cells before the transplant. His results, yet to be published or peer reviewed, show that some patients are able to reduce doses of immunosuppressants and possibly to stop them entirely, at least for a few months.
THOUGH MUCH ABOUT CHIMERISM and transplantation remains open to debate, most researchers agree that the MGH team’s protocol is the most innovative work under way. “It’s a landmark piece of work,” says Northwestern’s Miller, who is conducting a trial in which he gives patients stem cells after transplantation, at three-, six- and nine-month intervals, in an attempt to wean them off immunosuppression.
Sachs’s research is continuing, with a focus on extending tolerance to other organs. The MGH team is also beginning a trial of 15 patients who will receive kidneys from less-well-matched, unrelated donors. “Monkey studies suggest the results should be just as good with unrelated donors, but we won’t know for sure until we try it in humans,” Sachs says. Many organ recipients don’t have a closely matched sibling, so this work could change the prognosis—and quality of life—of many more organ recipients. “We’re hoping if we can reproduce these results and it turns out to be a protocol that will lead to tolerance on a broader scale,” Sachs says, “it would become a standard of care.”
“The Use of Skin Grafting to Distinguish Between Monozygotic and Dizygotic Twins in Cattle,” by D. Anderson et al., Heredity, 1951, and “Tolerance to Homografts, Twin Diagnosis, and the Freemartin Condition in Cattle,” by R. E. Billingham et al., Heredity, 1952. Two papers highlight Peter Medawar’s early exploration of chimerism, a foundation for the MGH team’s ability to remove patients from immunosuppressive drugs nearly 60 years later.
“HLA-Mismatched Renal Transplantation Without Maintenance Immunosuppression,” by Tatsuo Kawai et al., The New England Journal of Medicine, Jan. 24, 2008. The MGH team uses David Sachs’s strategy for taking human kidney recipients off all immunosuppressive drugs.
“Immunosuppressive Therapy and Tolerance of Organ Allografts,” by Thomas E. Starzl, The New England Journal of Medicine, Jan. 24, 2008. A transplant pioneer outlines the downsides of Sachs’s new procedure.
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