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BY SOLVING THE CELLUAR PUZZLE OF BONE MARROW TRANSPLANTS, SUZANNE ILDSTAD OFFERS NEW TREATMENTS AND NEW HOPE.

OUR BODIES, OUR CELLS
WRITTEN BY LAURA SHEFLER
If you want to change the world, start by doubting the obvious truths. Your doubts may lead, as Suzanne Ildstad's did, to questions no one else has thought to ask. As a young surgeon, Ildstad asked herself a question about a scientific conclusion that other physicians had simply accepted. Ultimately, it was this single, bold question that determined the course of her career.

Specifically, she began to doubt the conventional thinking about bone marrow transplantation, a powerful medical tool whose use has been limited by its often deadly complications. Ildstad, now director of Pitt's Division of Cellular Therapeutics, has hit on a discovery that could offer a way past these complications. And if she can make bone marrow transplants easier and safer, then her research has the potential to save thousands of lives.

The unparalleled prospects for bone marrow transplants arise from bone marrow's unique role. Bone marrow produces all the cells in the bloodstream -- red cells, platelets, and the various cells that form the immune system. The forerunner of these cells is the immature "stem cell," which produces all the different branches of the blood cell family tree. The main requirement for a viable bone marrow transplant is getting the donor stem cells to take, or "engraft."

Bone marrow transplants could ultimately revolutionize the treatment of a remarkable array of diseases, from diabetes to sickle cell anemia to AIDS -- almost any condition that involves a malfunctioning immune system or abnormal blood cells.

"Never did I expect to be focusing on these kinds of diseases," says Ildstad, who started out as a transplant surgeon, operating on children who needed organs such as kidneys. Today, she heads a research group of 25 people whose work includes a broad range of clinical trials. These trials involve patients with sickle cell disease or leukemia, as well as patients with lesser known yet devastating diseases such as chronic granulomatous disease, an often-fatal enzyme deficiency. "If we succeed on even one, I'll be happy," she says.

And yet there's more. Marrow transplants could also transform Ildstad's own field of solid organ transplantation. Today, surgeons can replace livers, hearts, lungs -- but always at a price. Cells in the recipient's immune system seek out the foreign tissue and work to destroy it. This phenomenon, known as rejection, can kill the patient. In addition, many patients die waiting for organs that are genetically close enough to minimize the chances of rejection. Powerful drugs, such as FK506, which was developed at Pitt, can keep rejection in check, but they don't always work. They also dangerously reduce the recipient's natural resistance to disease.

The only kind of transplant that doesn't cause rejection is, you guessed it, bone marrow. Furthermore, any subsequent tissue from that same donor is accepted without the help of drugs.

Ildstad has already achieved such transplants in her lab. There, mice survive with rat skin transplanted onto their sides. The mice tolerate the rat skin without drugs because they have new immune systems composed of both rat cells and their own mouse cells.

These animals are known as chimeras -- the name of a mythical creature, part lion, part dragon, part goat. The mice are chimeras in that they provide a safe, drug-free home for genetically different tissue. If Ildstad can safely achieve chimerism in the human immune system, she could boost transplant patients' chances for survival. Her research could enable patients to tolerate genetically mismatched organs and could spare them the suffering associated with immunosuppressant drugs.

But until last year, when Ildstad made the discovery that changed everything, few would have proposed using bone marrow transplants in these ways. Today, such transplants are used only to treat desperate illnesses such as leukemia and advanced breast cancer. One reason is that bone marrow transplants involve the destruction, through drugs or radiation, of the patient's own immune system. Physicians kill the recipient's marrow, then "rescue" the patient with new marrow. If the rescue fails -- if stem cells don't engraft,then the patient usually dies. Even if the graft succeeds, many patients die anyway.

The problem is a life-threatening reaction known as graft vs. host disease. T cells -- immune cells,from the donated marrow turn and attack their new host. "Patients get very, very sick," Ildstad says. "It's a terrible disease." Recent newspaper stories have featured leukemia patients and their dire searches for genetically matched bone marrow donors. Such genetic matching lowers the chances of graft vs. host disease.

"Right now," explains Ildstad, "even if you have a perfect match, 40 percent of patients develop graft vs. host." With a mismatch of even one antigen -- proteins on the cell's surface that express genetic identity, 70 percent of patients develop graft vs. host, and half the victims die. "If you go on to two or more antigen mismatches," continues Ildstad, "100 percent develop graft vs. host, and 80 percent die."

In the late 1970s and early 1980s, many researchers hoped that by taking the T cells out of the bone marrow, they could elimate graft vs. host. Says Ildstad, "A number of centers did clinical trials in which they T-cell-depleted the bone marrow. Where the graft took, they were able to avoid graft vs. host disease. But 70 percent had failure of engraftment, and failure of engraftment almost always equals death."

The effect of these deaths was chilling. "The clinical trials were aborted," Ildstad says. And the researchers reached a conclusion that put an end to any hopes of eradicating graft vs. host disease. Says Ildstad, "They concluded that they had to have T cells in order for the bone marrow transplant to engraft."

AND THAT, OF COURSE, WAS the obvious assumption to make: that T cells were necessary in bone marrow transplants and that graft vs. host disease was inevitable. Yet Ildstad developed an alternate hypothesis in the early '80s. Ildstad has difficulty saying just how she reached this hypothesis. "It came to me while I was jogging," she admits.

What occurred to Ildstad was that the conclusions drawn from the disastrous clinical trials were not necessarily so. To Ildstad, it didn't seem logical that the T cell, which attacks foreign tissue, should be the agent that helps the stem cell to engraft in a foreign environment. What if the cell that helped engraftment was not a T cell, but rather some other, undiscovered cell that was being removed along with the T cells?

This was a question that Ildstad kept in the back of her mind for years. But it wasn't until in 1988, when she first came to Pitt, that Ildstad made an observation that spurred her to search for this hypothetical "facilitating" cell.

Early in her career, Ildstad had developed a keen interest in the intimate connections between transplantation and immunology. Immune system rejection, after all, was the major barrier to success in organ transplantation. "No matter how technically good you are as a surgeon" Ildstad says now, "if you can't control the rejection response, the graft dies."

In the early '80s, she spent several years as a research fellow at the National Institutes of Health (NIH), where she studied marrow transplants as a way to head off organ transplant rejection in mice. Scientists had long known that a bone marrow transplant could pave the way for the acceptance of other tissues from the donor, but only among newborns of the same species.

At the NIH, Ildstad and her mentor, David Sachs, tried to induce the same kind of "chimeric" organ tolerance in adult mice. They used a mixture of marrow from the donor and the recipient. To prevent graft vs. host, they filtered mature T cells out. Their hope was that immature immune cells in the mixed bone marrow would grow inside the mouse into a functioning immune system. If all went well, the hybrid immune system would tolerate both donor and recipient cells as "self."

Ildstad and Sachs were able to create hybrid immune systems when they transplanted bone marrow from one mouse to another. But when they ambitiously tried to cross the species barrier, transplanting a combination of rat and mouse marrow in to the mice, the bone marrow transplants failed.

When Ildstad was recruited by Pitt in 1988, she was eager to try cross-species transplantation again. But in the chaos of setting up her lab, she found that she lacked the chemical agent needed to remove T cells from the rat bone marrow. Ildstad decided she'd transplant it anyway, just to see what would happen.

To her astonishment, the mice accepted the rat marrow this time. "We had a huge level of engraftment," she says. Ildstad had cause to think that something in the bone marrow -- something ordinarily cleansed by the agent -- might account for the happy accident.

That's when Ildstad finally set out to determine exactly which cells were needed for the bone marrow to engraft. With a small research team, she tracked the cell down by examining protein "markers" on the cell surface. Using sophisticated cell sorting equipment, the researchers took marrow samples and removed cells with particular markers, then tested the samples in mice to see whether they would engraft.

Each test took more than a month. They had to wait to find out what would happen in the mice. But Ildstad accelerated the search by keeping several tests going at once. "We did 150 experiments," recalls researcher Christina Kaufman. "Suzanne isn't afraid to take a risk," she says.

At $50 an hour, cell sorting is expensive, Kaufman points out. Another investigator might have saved money by running fewer experiments, by waiting for the results of one test before starting the next. But, says Kaufman,"If we had done it the long way, we would still be in the process."

Instead, they relatively quickly narrowed the search to a single kind of cell -- the real helper, the facilitating cell. This cell isn't a T cell, but some of its markers are the same as the T cell's. Because it resembled the T cell -- and because no one knew it was there -- it had been removed from marrow by the same process that took the T cells out.

Ildstad was now able to remove T cells without removing the facilitating cell. Thus, graft vs. host disease, which had once been accepted as inevitable, turned out to be avoidable after all.

THE BIG QUESTION NOW, of course, is whether Ildstad's discoveries in the laboratory will work in people. The results with larger animals in "preclinical" trials have been promising enough for them to begin clinical -- i.e., human -- trials on a broad front of diseases. For instance, they recently received approval to have bone marrow transplants given to a group of patients suffering from sickle cell anemia, which, in the United States, kills more than half of its sufferers by age 40. Says Ildstad, "There's been a lot of debate about the use of conventional bone marrow transplant in sickle cell disease patients." Though life-threatening, sickle cell disease was deemed not dangerous enough to justify exposing a patient to the risk of graft vs. host. Now, with better prospects for avoiding graft vs. host, bone marrow transplantation can be considered after all.

And there's even better news for patients. Ildstad's research group believes that with powerful, purified doses of the facilitaing cell, they can achieve engraftment without completely destroying the patient's immune system -- a severe measure with the appropriately grim-sounding name of "lethal conditioning." They are hoping, instead, to prepare the patient for the graft with "sublethal" doses of radiation and drugs.

"In preclinical trials," says Ildstad, "we did tests to determine the minimum amount of conditioning to allow enough engraftment to treat the disease." Often, partial engraftment is sufficient. To remedy sickle cell anemia, for instance, they need at least 30 percent normal cells for the patient to recover. Even with only sublethal conditioning, Ildstad expects a considerable margin of victory: "We predict that the normal red blood cells will 'win,' outnumbering abnormal cells."

Ildstad's group is also planning to use the facilitating cell in clinical trials with leukemia. And they want to make transplants more widely available to leukemia patients. Today, only 20 percent of these patients have a donor whose bone marrow matches theirs closely enough. "Now we're going to address that 80 percent who don't have a suitable match," Ildstad says. The researchers will give bone marrow transplants to leukemia patients who would previously have been almost certain to get graft vs. host.

Ildstad's group is also collaborating with Nancy Ascher of the University of California at San Francisco, where they are studying patients who have had combined bone marrow and kidney transplants. "We have three patients," Ildstad says, "and it looks like the engraftment has succeeded in one patient, without graft vs. host disease." Preliminary results suggest that the patient's immune system accepts the kidney as "self" but continues to react against third-party invaders.

Beyond the current trials, Ildstad is looking toward the future. She hopes that her work will contribute to a permanent cure for diabetes, which affects 1.5 million people in the United States alone. Diabetes occurs when islet cells in the pancreas stop producing insulin. At this time, diabetes can only be managed, not cured.

Surgeons at Pitt and at other centers are now transplanting islet cells or entire pancreases in the hope of reversing the disease. Ildstad's research with facilitating cells may help squelch rejection, a major concern of this procedure. Using combined bone marrow and islet cell transplants, Ildstad is already able to cure diabetes safely in mice.

CHANCES ARE THAT Ildstad's research will influence modern medicine for decades to come. Ildstad envisions a day, 20 years or more down the line, when juvenile diabetes could be predicted by genetic testing and prevented by a marrow transplant given in the womb.

Despite her focus on the future, she also looks back in gratitude on the unexpected course of her professional life. She is thankful for the support of Richard Simmons, chief of surgery, and Thomas Starzl, chief of transplantation, who have had the vision to allow her to take several years off from her surgical practice to concentrate only on research. She recognizes that most laboratory investigators work their whole lives without ever seeing the kinds of tangible results that Ildstad has seen from her research. "It's not very often," she says,"that someone has the opportunity to see research through from the basic scientific observation to a clinical situation. I am very aware of that."

Ildstad is also aware of the radical nature of her work. "What we're doing is a paradigm shift. We're challenging the absolute beliefs right now. And I think some people find that very difficult to accept. We've seen it in the reviews of our papers -- that it's been a conceptual change that people have a hard time with," she says. On the other hand, researchers at other institutions have repeated her experiments and confirmed the results. "The data are unbelievably reproducible," she says.

Modestly, Ildstad attributes her success in part to luck and circumstance. "There are a lot of people who focus their efforts on ways to improve graft survival, and our group was just fortunate in the observations we made."

Fortunate, yes, but also persistent, willing to look past seeming truths to the hidden possibilities.


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