From our Spring 2000 issue. See all our past issues here.
Thomas Starzl once took 16 transplant recipients off of immunosuppressants on a hunch. His peers thought he was crazy to do it, though they knew he was a maverick, and several decades into a career built on the fringe. Starzl, University of Pittsburgh distinguished service professor of the health sciences and director of the Thomas E. Starzl Transplantation Institute, had proven himself time and time again, but this seemed utter lunacy.
Starzl’s hunch started with something he’d seen in animal models: some only needed enough immunosuppressants to overcome the body’s initial immune reaction, then the organ mysteriously induced a state called tolerance—peaceful coexistence of donor and recipient tissues.
To other surgeons, his theory seemed unlikely. After all, everyone knows that after any successful transplant, a patient’s blood, rich with cells programmed to distinguish self from non-self, floods into a foreign organ full of foreign cells. This point, where the recipient’s immune system meets its new organ, is a moment of potentially fatal clash. Mature cells from the donor and recipient recognize each other as foreign and mount their immune attacks. Surgeons keep a handle on this initial reaction with immunosuppressive drugs, but there’s a natural process of cell death that must accompany transplantation: All mature immune cells must die. What’s left, from both donor and recipient, are young cells, those not yet learned in self versus nonself. As long as their cellular cohabitation is peaceful, the organ isn’t rejected, and the patient has achieved tolerance. The field of transplantation is pretty much in agreement on that. What might happen next, however, now is hotly debated.
If you’re a transplant surgeon, and you’re not Starzl, when your patient reaches this initial state of tolerance, chances are you’ll exhale a sigh of relief and write a prescription regime for immunosuppressants that he’ll be on for, oh, the rest of his life. The belief has been that it takes an arsenal of medication to maintain the necessary peace between the donor organ and its recipient. Transplant surgeons have done this for years, and the resulting amity is still called tolerance. But if you ask Starzl and his colleagues, this is not true tolerance.
“We can’t go on thinking that immune suppression is the best solution,” says Massimo Trucco, a professor of pediatrics at Pitt. “It’s the worst solution, but right now it’s the only solution we have.” It’s the worst solution because humans lead active, sometimes destructive, lives: they get drunk, they get the flu, and they need an immune system that can keep up. Unfortunately, immunosuppression doesn’t just inhibit an immune response to the foreign organ, it inhibits all immune responses, even those necessary to withstand day-to-day exposures. And for a transplant patient on immunosuppressants, one case of the flu or another infection can kick off organ rejection. And that’s saying nothing of the kidney failure, lymphomas, and other side effects that can come with immunosuppressants.
According to Starzl and John Fung, an MD, PhD and Pitt professor of surgery, by chemically inhibiting the immune system of the recipient, physicians induce a state that mimics tolerance, but actually prevents true tolerance from developing. True tolerance, if you ask them, is a state of perfect natural harmony. No drugs. And it starts with those two populations of young cells that remain after the initial immune response. If all goes well, and they’re allowed to rebuild an immune system together, the young cells mature, never learning the other is truly foreign, and in doing so, train the body that the transplanted organ is not foreign. And that, say Fung and Starzl, is true tolerance. So, if you’re Starzl, once your patient passed through that initial immune response, you might take him off medications to see if his body could take over.
Of the 16 patients Starzl took off immunosuppressants on a hunch, 11 started to reject—he quickly put them back on medication. But five made his hunch seem far from lunacy: They had no immune reaction. To this day, they’re walking around with transplanted organs and aren’t taking immunosuppressants. According to Starzl this true tolerance has only one explanation: chimerism.
***
According to the ancient Greeks, a creature once roamed the earth with the body of a lion, tail of a serpent, and head of a goat. They called it a chimera. The beast actually had a number of configurations in ancient myth, and the term “chimera” lingers today with many meanings. If you look up “chimera” in the dictionary, you’ll see that it’s more than a mythical creature. It’s an unrealizable dream, a fabrication of the mind, or a person with tissues of diverse genetic composition. And with a Starzl-sized step further, “chimera” could mean the end of immunosuppressants and organ rejection, and the beginning of true tolerance.
“By definition,” says Fung, “if you get a transplant, you’re chimeric.” In the transplant world, a chimera is a patient with two cell populations, one from her own body, one from that of an organ donor.
But no one besides Starzl thought too much about what chimerism could mean until a Festschrift in 1990, when he and a colleague were each asked to write a chapter for a book on heart and lung transplantation. Starzl was to write a chapter on the mechanisms for whole organ acceptance; the other chapter was to cover tolerance.
When Starzl caught wind of this idea he looked at his colleagues and said, “Well hell, they’re the same thing.” They all looked at Starzl, told him he’d lost his mind, for real this time, and dared him to prove it.
Starzl went home and began searching for his chimera. He called Trucco, an expert in polymerase chain reaction, which can be used to test cellular identity based on DNA, and Pitt’s Anthony Demetris, whose expertise was in identifying cells through HLA antibody testing, which utilizes dyes that fluoresce in the presence of certain cellular proteins. And he called 30 of his longest-term patients. He needed them for samples, he said, and they came from around the world to help.
One patient, we’ll call her Susan, received an organ from a 4-year-old boy when she herself was 4. Susan was a wife and mother by the time Starzl called. She and patients like her, those who had transgender transplants, offered particularly hard-hitting evidence for Starzl. When Starzl, Trucco, and Demetris presented their lab results, complete with images of Susan’s cells, there was nothing much their opponents could say:
Susan had Y chromosomes on approximately .1 percent of the cells in her body. The donor’s cells had actually migrated and remained in her tissue.
“That means,” says Starzl, “she’s closer, in a physical sense, to that donor than she’s been to anyone in her life, with only one exception: when she was a baby in her mother’s womb.” And in case that wasn’t evidence enough, Starzl offered many other examples.
“I always knew those cells were there, I had just never looked for them myself,” he says.
To Starzl, the lingering of donor cells was a key to understanding how his early animal models developed true tolerance. These cells, as Starzl and his colleagues discovered, don’t live just in the donated organs. They can live peacefully throughout recipients’ bodies: in their lymph organs, in their skin, and in other sites. And it’s not that the donor gives cells along with, say, a liver, it’s that the donor’s cells actually move from that liver, or lung, or any other organ that was transplanted, out into the surrounding tissues. Once this happens, these patients are chimeric, or more accurately, microchimeric. At this point, the highest level of chimerism anyone has seen in a transplant patient is about what Susan reached—.1 percent. But most importantly, even if only on a micro-level, these patients have accepted some donor cells as self. This means, to a degree, that they’ve accepted the donor organ as self also. If only this microlevel of acceptance could be increased, says Starzl, true tolerance could be reached.
***
According to Demetris, it’s clear that Starzl was on to something with his hunch about chimerism: “Microchimerism gives you a form of tolerance;” he says, “it’s just not quite robust enough because there are not enough donor cells [in a microchimeric’s body].” So Starzl, Demetris, and Fung are all trying to find ways to move from micro to macro.
They started by infusing large quantities of donor blood cells, and later infused about 2 billion donor bone marrow cells, at the time of transplantation to increase chimerism. But without fail, patients came out microchimeric at best. Researchers have tried irradiating the recipients to suppress their immune systems and increase chimerism. But in the end, that’s no step-up from immunosuppressive drug therapy: “You can irradiate heavily and give donor bone marrow,” says Demetris, “but you pay a price for that irradiation down the road. And that’s not quite acceptable.” So, researchers are trying to boost chimerism through less morbid procedures.
Some Pitt researchers are trying to identify which donor cells remain in recipients’ bodies and have tolerance-inducing properties. Once they’ve succeeded, they hope to grow those cells in culture to be used in new infusion therapy trials. Another group is developing what they call “co-stimulatory molecule blockers,” which are designed to thwart the body’s immune response, preventing it from killing the cells deemed essential for tolerance.
“Figuring out how to induce tolerance is the Holy Grail in transplantation,” says Fung.
He, Starzl, and a few other Pitt researchers say they found the map to the Holy Grail back in 1991 after the Festschrift, though no one believed them at the time. But that was then. Today, there’s essentially no question as to whether chimerism plays a role in tolerance. Some argue that the role is unclear, that there’s no way to know which came first, the tolerance or the chimerism. Starzl hears these objections, and has one reply: You have to prove me wrong, then I’ll believe you. And he’ll tell you, so far, no one has even come close.