"Cure" is not a word doctors like to use regarding cancer or genetic diseases. The word makes a promise they fear they cannot keep. But now, only 14 years after Hopkins oncologist Curt Civin first discovered how to isolate the "stem" cells that give rise to the entire blood and immune system, experimental stem cell transplants are in use. So is the word "cure," albeit in a whisper.
The beauty of it is, stem cells are the active agents in bone marrow, so by refining marrow down to stem cells, doctors get a graft that helps just as marrow does--to repopulate the marrow after chemotherapy, for instance. Implanted normal stem cells can also cure genetic diseases of the blood and immune system, because they are the ultimate source of all the body's blood cells: red ones, white ones (including immune cells), and platelets.
The big difference: transplanted stem cells are safer than the same donor's bone marrow, because stem cells do not trigger immune rejection. By using them, doctors can transplant marrow without the deadly threat of graft-versus-host disease (GVHD), in which a donor's immune cells attack the patient's body. Stem cell patients have less need for the toxic (and costly) drugs that prevent GVHD. The donor need not be as impeccably matched.
That's why stem cell grafts will soon be helping people for whom bone marrow transplant would work but has been too risky--people with not-always-fatal genetic diseases such as thalassemia or sickle cell anemia. Autoimmune diseases like lupus and rheumatoid arthritis should be curable. And for patients with a cancer type that tends to metastasize to the marrow (solid pediatric tumors, breast cancer, small cell lung cancer, lymphoma, multiple myeloma, and ovarian cancer), Civin predicts that stem cell grafts will be used pre-emptively, helping doctors zap occult tumors before they spread further. "At Hopkins, we already do it all the time with pediatric solid tumors."
The technology that makes these transplants possible is already in use in Canada and Europe, and more than 1,000 people have been treated. The FDA's Oncology Advisory Committee has unanimously approved a device and reagents for isolating the cells, and a decision is expected soon. The FDA normally gives great weight to its advisory committees.
After FDA approval, autologous (self-to-self) stem cell transplants will be something any doctor can prescribe, with other uses in due course. "So here's my discovery finally getting out to large numbers of people," says Civin. "I must say, that's very gratifying."
Meanwhile, clinical trials continue.
At Hopkins, 13-year-old Wade Poet and his family are living through hell, all in hopes of a cure. Wade began limping last June, because of what he thought was a soccer injury, and was diagnosed in November: he had a primitive neuroectodermal tumor, one that grew misguidedly from fetal cells that should have developed into nervous tissue. Before he got to Hopkins and began chemotherapy, Wade had a lump in his belly the size of a football. He also had multiple metastases in several bones, including his back and hip; his bone marrow undoubtedly harbors more tumor cells, ready to spread.
This type of cancer is aggressive, and as these words are written, Wade is a very sick boy. The skin around his eyes is pale brown in a face the color of skim milk. He raises one hand to greet his visitor, but feels too tired to talk just now. When he's up and around, he uses a walker. "Once up and down the hall is a lot for him," says his mother, Betsy Harner.
The chemotherapy has helped. Wade is in remission, and his hip is better. His mother says that sometimes he lifts the walker and walks a few steps without it. "He's like a kid learning to walk again. He says, 'Look what I'm doing, Mom!'" At the same time, whatever complications a person can get from chemo--nausea, mouth sores, infections--Wade got. Chemotherapy made him so sick he spent about half the winter in Hopkins Hospital. In March, as he begins a series of 25 jolts of radiation to shrink the tumors further, he's back in because now he has shingles: pain and a ferocious itch, as if he didn't have enough trouble.
As Wade and his parents know, the prognosis is poor--a fact that gives paradoxical reason for hope, however, because it makes Wade eligible for an experimental protocol, begun in April 1995 by Hopkins pediatric oncologist Allen Chen. (A specialist in immunology and cytokine biology, Chen recently came to Hopkins from Seattle's Fred Hutchinson Cancer Research Center.) Briefly, the new protocol involving stem cells is a significant improvement on bone marrow transplant.
After the initial radiation, Wade may undergo surgery to take out remnants of solid tumor; then chemotherapy at "supralethal doses." "Supra" means "more than enough": more than enough, it is hoped, to wipe out every last tumor cell in Wade's body, and definitely more than enough to kill his marrow.
Without marrow to make fresh blood cells as his old ones died, Wade would die within weeks--except for a "button" (as the researchers call it) of cells, frozen at -180øC in a liquid nitrogen freezer at Hopkins Hospital. This button, just a fraction of a teaspoon of Wade's own stem cells, should be enough to repopulate and "rescue" his marrow.
Until now, for solid tumors like Wade's, the standard has been "autologous transplant," using a patient's own marrow for the rescue graft. That can work. With untreated marrow, however, most tumors recur, partly because of cells hiding in the marrow. So in the past two years, using a procedure Civin pioneered on a child with very advanced cancer in 1992, many sophisticated medical centers have switched to grafting patients with selected stem cells --no tumor stowaways, it is hoped. Certainly there must be fewer than in marrow.
In all, Civin transplanted 10 children with stem cells. Eight of them died or relapsed over the next two years, possibly due to residual tumor outside the marrow. But two may be cured; they have been in remission for nearly three years since the transplant. That's a good showing, given that all 10 children had extensive cancers, judged as sure to be fatal without the procedure.
The latest protocol, the one Wade is on under Allen Chen, goes one step further: Wade's stem cell harvest was taken from his circulating blood, not from his marrow. Circulating blood generally carries less tumor than does marrow. Moreover, by purifying the harvest, says Chen, the tumor cells are reduced "by three to four orders of magnitude." Are the tumors gone? Chen won't go so far as to say that, although a journalist might. He will say that the team has been unable to culture any tumor cells from their transplant-ready material.
Chen expects that the experimental protocol may prove easier on Wade than his winter has been. The final five days of intensive chemotherapy will be rough, but once the stem cells are transfused into his blood stream, they will "home" to the marrow and start producing new blood cells. With that help, the boy should recover faster than he did from earlier treatments. Hope lies in the button, and the more rigorous chemotherapy it allows.
Wade sat with his mother when she signed the informed consent, listening as Chen explained the risks and benefits. "They had, I think, six pages of all the bad things that could happen," says Harner. "There were about two lines of the good. But the good is, you're cancer-free and you're going to live."
At first Wade didn't want his mother to sign. "He said, 'Mom, I could die. Didn't you hear that?' I said, 'Wade, you've already been through so much. If we stop now, we've done all this for nothing.'" So Wade is suffering through some long, long days. If all goes well, by June he will have received his stem cells back, and he'll be feeling better. He'll be at home, in Hanover, Pennsylvania, facing a summer with some gentle swimming in it, and the prospect of many years to come. Food will taste good.
Genetic repairs are another use for stem cells, already in clinical trials. Take the genetic disease SCID, severe combined immunodeficiency. Patients with SCID are the so-called "bubble children," who formerly had to live in isolation because their bodies did not produce adenosine deaminase, an enzyme without which the immune system shuts down.
In the last few years, a number of these children have received stem cell grafts with apparent success. The grafts colonized the bone marrow, then set to work keeping the children supplied with the missing enzyme (among other blood products). Though booster treatments might some day be needed, in theory the treatment should last for the rest of their lives. Stem cells are "essentially immortal," says Civin.
With SCID, researchers have two possible approaches. One involves donor cells, replacing the patient's abnormal stem cells with cells containing normal genes. Or doctors can work with the patient's own stem cells, using molecular techniques to insert a functional gene for the missing enzyme. Judging from experience with SCID, either way will work for diseases where something is missing.
More difficult are ailments like sickle cell anemia, where something is not just absent, but actively misbehaving. In sickle cell, which affects 1 percent of African Americans and can be fatal, patients have red blood cells with abnormal hemoglobin. The cells contort and interlock, creating exquisitely painful logjams in the blood vessels. Here a molecular correction would need to remove the mutant gene, as well as insert a good one. Most researchers believe it will be better to graft such patients with healthy donor cells.
Unhappily, patients in for genetic corrections have to undergo intensive chemotherapy, just as cancer patients do, to wipe out their marrow. Otherwise, their defective stem cells would "outshout" the normal replacements.
The trade-off can make sense, of course. Compared with a lifetime of sickle cell (it shortens life, and its pain is said to be worse than childbirth), a few wretched weeks may not seem so bad. Most serious genetic diseases are diagnosed when the patients are very young, therefore quick to recover. And the patient's marrow need not always be obliterated to the last cell. Civin says that for many diseases, eliminating symptoms requires a high level of donor engraftment, but not 100 percent.
Still, prevention would be best--and stem cell technology promises even that. Recently, a team at Hopkins transferred stem cells into the abdominal cavity of a first-trimester fetus, in utero. They hoped to correct the baby's globoid cell leuko-dystrophy (GCLD), a "storage" disease in which the lack of an enzyme causes fatty acids to accumulate in the nervous system. Like most storage diseases, GCLD progresses rapidly after birth; the children become brain-damaged and die before age 10, often by age 1. Other than bone marrow transplantation, no specific treatment exists.
Although the fetus died two months after the procedure (the cell dose was too high), some good did result: for the first time anywhere, a graft in utero took, replacing all original marrow. That means the baby would not have had GCLD.
The results are preliminary, emphasizes the principal investigator, oncologist Richard Jones. That said, he believes the outcome ratifies his basic approach--giving a concentrated dose of stem cells, earlier. "It really does have to be the first trimester," he says, while the fetal immune system is too immature to reject donor cells. And he now knows what dose of cells to give next time. Both he and the parents, a couple who had previously refused abortion and carried a GCLD baby to term, are excited and hopeful.
"It has shown us that full engraftment is possible," says Jones, "and it shows that genetic diseases should be curable in utero--and I don't mean it's a decade away." He means curable within two to five years, for disorders affecting as many as 10,000 U.S. newborns each year.
As well as sickle cell and SCID, target diseases include thalassemia and other metabolic storage diseases--basically, any inherited disease of the blood or immune system.
Compared with a $100,000 transplant, inserting stem cells in utero will be economical--about $10,000, Jones estimates. "It's simple. It's a one-day procedure. You take marrow from the father's hip, isolate the stem cells, and put them in the baby. Then mom goes home and is followed. That's it."
Best of all, says Jones, "in utero you're treating so early there is no damage to the children." No damage from chemotherapy-and-transplant, no damage from the disease.
Curt Civin observes this booming progress with a certain bemusement. In the early '80s, when he first tackled the problem of stem cells, he had trouble getting the work funded. Too many untested assumptions, people said. The problem was just too hard, and anyway, established methods could do the same thing. Why waste money fooling with it? Civin laughs. "It seemed like this experiment was fraught with the possibility of failure."
It was, if only because the stem cell he sought was so scarce. It numbers perhaps one in 10,000 (or even 100,000) marrow cells, for Civin sought the Adam of stem cells, the stem cell as classically defined: undifferentiated, pluripotent, and most primitive, the beginning of the line. "It's all possibility," Civin says. "It's the ancestor, the parent of all. It's much like a fertilized egg, only it has fewer choices. It can divide and reproduce itself"--self-renew--"or it can differentiate into two types of cells, branching like a tree."
Each progenitor generation thereafter makes a similar choice, the cells guided by chemical/electrical signals from their neighbors. In effect, if things are crowded, they leave well enough alone. If there's space, they fill it. With every branching, progenitor cells become more specialized, until finally they can produce only mature, working blood cells.
In that way, over a period of weeks, a single stem cell can generate all the different kinds of blood cells (that's what makes it pluripotent), in proportions that change to meet the body's need: more stem cells to repopulate a bone marrow blasted by chemotherapy, perhaps. Or the cell might push on to a generation able to differentiate into white cells and red cells, as needed. Those cells and their descendants would then turn out extra red blood cells after bleeding, or white blood cells to combat infection. Whatever is needed, progenitor cells provide, in quantity.
Because they can self-renew, a very few of the classical stem cells are all it takes to provide a lifetime's supply of blood cells--and that's a lot of cells, because mature blood cells age and die. Granulocytes live only about five hours, Civin explains. "They're kamikazes! They ingest bacteria and die." Platelets live two weeks. Red blood cells, four months. Always new ones are needed, of every type. "You have millions of blood cells in every drop of blood. In a week, your bone marrow must produce, literally, pounds of cells."
Stem cells live in the bone marrow, says Civin, protected there until called to self-renew or create a fresh crop of cells. Meanwhile, the cells are dormant, like the spore of a bacterium. They are therefore resistant to damage, even to chemotherapy.
That trait is important, because body cells mutate all the time in the course of replication; the average human adult has suffered about 1015 mutations (that's a 10 followed by 15 zeroes). But since most stem cells are dormant at any given time, stem cells don't take their share of the hit. That's good engineering on Nature's part, since stem cells are the single ancestor for so many crucial elements of blood.
In the late 1970s, little was known about these cells. Scientists used to hunt for them, Civin recounts, by harvesting bone marrow cells and placing them in culture. "Then if you got a colony, you could say, 'Aha! Two weeks ago there was a progenitor cell here.' But you couldn't study it."
Wishing to study it, Civin and his small research group--he was then the most junior of assistant professors--took a long shot, based on educated guesses. They were making monoclonal antibodies, cloning antibodies that could pick out various white blood cells. "So I said, 'Let's make one to find the needle in the haystack. And let's make the hypothesis--we didn't know this was true--that stem cells would have their own antigens,'" a target for monoclonal antibodies to zero in on.
They started with leukemia cells, which are often cartoons of normal cells, as if they'd frozen during differentiation. "So our second hypothesis was, that we could find some leukemia cell that would have this stem cell antigen that we didn't know existed in the first place."
Unable to get outside support for the work, Civin's group began anyway, using startup funds that Hopkins's Oncology Center provides for new labs. They immunized mouse after mouse, each with a different leukemia cell. Then they harvested the resulting antibodies, which were cloned and tested on marrow. "Anything that reacted strongly," says Civin, chuckling, "we threw away--because what did we know about the stem cell? That it was rare. So anything that bound to a lot of cells, we knew we didn't want."
The first success came in 1982, after months of screening clones, when the thousandy-oomph monoclonal antibody bound to just 1 percent of marrow cells. One percent was too many to be exclusively the stem cells. However, the cells later differentiated into the full range of specialized blood cells, so they were clearly progenitor cells, and very early ones.
The antigen that Civin's clone binds is now called CD34 (CD for cluster designation). Other stem cell antigens have since been discovered, but CD34 remains central, the likely workhorse for rescuing marrow after chemotherapy. The device under FDA review relies on CD34.
The National Institutes of Health began funding Civin's work in 1982. Under the requirements of a federal law designed to promote technology transfer, in 1984 he patented the CD34 antibody and others that might copy it, as well as the antigen itself, for all the obvious research and clinical purposes.
And the hunt was on. Faced with actual stem cells, skepticism evaporated. Immediately, industrial and academic labs all over the world began to investigate these cells and the technology that could pick them out. It didn't take a Louis Pasteur to see that stem cells would be important to all diseases involving the blood and immune system--in a way, almost every disease, because blood cells go everywhere in the body. It didn't take a Warren Buffett to see there was lots of money to be made: $500 million a year, according to one estimate.
Despite the inevitable legal squabbling, the community of bone marrow specialists followed a conscious policy not to let competition slow down the work. They developed standards for such things as ways to count cells, so they could compare research results without setting apples against eggplants. They continued to publish basic and clinical research, to share CD34 antibodies and other reagents, and to stay in touch with one another and with colleagues in industry. More, they specifically dealt industry in.
"Biotechnology was a key player," says Hopkins oncologist Steven Noga, medical/scientific director of the Graft Engineering Laboratory. "It's one thing to do something in a lab, and another to scale it up for clinical use. We needed better technology." To date, companies have spent close to a billion dollars developing the robust, user-friendly equipment that labs and hospitals will need not very long from now.
In the 1990s, the work of the previous decade is bearing fruit, and discoveries are tumbling forth pellmell. At Hopkins alone:
A crucial piece of information came in 1990, when Rick Jones and Saul Sharkis published results in Nature that explain the importance of small cells. It seems, and Jones says most would agree, that as a lineage of blood cells specializes, cells get progressively larger. Conversely, pluripotent stem cells are among the smallest of blood cells.
Steve Noga's group has developed a workable protocol for cancer patients, using CD34 to "engineer" donor grafts. First they mechanically sort the marrow cells by size and weight, then keep the large ones. Large marrow cells include all types of mature blood cells, plus progenitors to make more quickly. Short-term result: The patient lives.
By taking out the small cells, the researchers eliminate T-cells, the tiny lymphocytes that orchestrate the body's immune attack. Result: No acute GVHD.
Then they give the small cells a chance to bond with CD34 antibodies, thereby picking out the stem cells. These, too, go into the patient's graft, more than doubling the number of stem cells. Result: a durable graft that provides patients with blood and immune cells for years to come.
Since early 1992, the Hopkins bone marrow team has transplanted about 100 cancer patients with such "engineered" grafts. On average, the patients spend 11 fewer days in the hospital than is usual with untreated marrow, saving 40 percent of costs. "And their quality of life is improved," says Noga. They go home sooner, feeling better. They need fewer transfusions of platelets and white cells, only 30 days on cyclosporine (as opposed to six months). Their doctors see no acute GVHD, and no increase in relapse.
In a nutshell:
Large marrow cells - T-cells + CD34+ cells = 40 percent less cost and suffering.
Allen Chen's approach is more direct than Noga's, because Chen performs autologous transplants. He therefore has no need to worry about GVHD or to take out T-cells. Chen simply refines the patient's marrow to only those cells that are CD34 positive. That excludes solid tumor cells, while providing a durable graft and plenty of progenitors to pull the patient through the first few weeks.
One major stumbling block remains: Neither Chen nor Noga's approach will work for patients with malignancies of the blood. Many leukemia cells express CD34, so the bad guys would get selected with the good ones. Moreover, leukemia patients do better when they do get a light case of GVHD. The graft's immune bombardment has an anti-tumor effect.
Jones, however, has a line of research growing out of stem cell work that shows preliminary, emphasis preliminary, promise for leukemia. With chronic myelogenous leukemia (CML), a fatal disease in which too many white cells clog the circulation, he has found that the cells are "exquisitely sensitive" to hematopoetic growth factor.
In vitro, he has given leukemia progenitor cells a dose of growth factor that makes stem cells rush out into the blood stream and proliferate. The progenitor cells for leukemia rush too, so much so they stop self-renewing. "They all differentiate into end-stage blood cells without the capacity for further growth," says Jones. In other words, the leukemic cells die.
In the past 11 months, Jones has administered the growth factor to four leukemia patients. Each of the four patients, as is usual for early experimental subjects, had a poor prognosis. Yet so far, none has relapsed. "It's beginning to be... interesting," Jones concedes.
At this point in the stem cell story, we are so far out on the cutting edge of research that you should multiply every "maybe" by three, then add four "ifs."
That said, a central question remains open: Are CD34+ cells in fact the most primitive of stem cells? They are pluripotent, able to give rise to all types of blood cells. They start producing blood cells within weeks, and the effect is long-lasting. But will the effect last for a lifetime?
Only time will tell, says Civin. "Over the years, how patients do will tell the tale." Some others say no. In mice, unpublished (as of April) work in Japan and the U.S. indicates that transplanted CD34+ cells may poop out partway through the animal's lifetime; Jones finds these results convincing, and he believes they probably apply to people, "because mice have almost never led us astray when it comes to blood."
For people with cancer, the durability issue does not matter, because researchers agree that at least a few of one's own most primitive stem cells survive chemotherapy. "Really," says Jones, "at a survivable dose, it's almost impossible to ablate them all." So if transplanted CD34+ cells age and die after some years, the patient's own stem cells will repopulate the marrow. No problem. A few is all that's needed.
For postnatal genetic therapy, however, poor durability would be disastrous. "For a lifetime, you absolutely, positively need to give the earliest bone marrow stem cell," says Jones. The human body was not designed to tolerate intensive chemotherapy, far less a second time. "This is not something we want to do twice."
Enter some work Jones and Sharkis will publish in Blood in July: they have discovered a group of very small blood cells that express little if any CD34 antigen, but a lot of aldehyde dehydrogenase (ALDH). ALDH+ cells give rise to stem cells that do produce CD34, however, which implies that ALDH+ cells are more primitive than CD34-expressers. ALDH+ cells graft slowly.
If CD34+ cells have limits for genetic corrections, then, perhaps this new group of cells can do the job. Jones reports that 20 female mice grafted with as few as 5 to 10 ALDH+ cells from males successfully regrew their marrow, then lived up to 17 months--extreme old age, in mice. Autopsy showed that their marrows were more than two-thirds male cells, therefore derived from the graft.
At the least, ALDH+ cells will be a useful addition to the CD34+ armamentarium. They will surely provide another way to select discrete populations of very primitive stem cells with which to tailor grafts--useful as the researchers move on toward custom-tailored grafts, the next frontier.
Civin's group (and others all over the world) now struggle to find out how to signal stem cells. They want to be able to make stem cells at every phase self-renew or differentiate, on command. Then one could produce a stem cell graft with any desired composition.
For example, "you could manipulate your stem cells so they would attack your own particular cancer," says Civin dreamily. That would be important because when people with cancer relapse, they almost always get the same cancer again, from a cell that got away. So an immune system primed to attack one's own particular mutation would be a permanent protection against relapse.
In particular, Civin wants to know what triggers self-renewal, when stem cells clone themselves. Once scientists can duplicate the process, he foresees harvesting just a syringe of blood, then growing stem cells "kind of like you make beer in a big fermentor."
Blood from placentas and umbilical cords could then be collected from as many childbirths as possible, to build up a national bank of cryopreserved blood stem cells. Civin says fetal stem cells are ideal because it's thought they are "more stemmy, more proliferative than adult cells--plus the material is normally thrown away. So here is a waste product that could be used to help people." Computers would keep track of molecular types, and the cells would be available to anybody who was a match.
Although some commercial blood banks are now collecting cords and placentas, Civin believes the material should remain non-profit, as organs are. It is illegal to sell a kidney or any other organ.
"If we could enroll the immune system to fight cancer..." muses Civin. "My hope is, once we get stem cells to reproduce, we could also program them. No more transfusions! You'd grow your own red blood cells. You could grow your own T-lymphocytes. You could get back your own immune cells programmed to attack your own cancer--to supplement surgery or chemo, or to replace it."
Elise Hancock is the magazine's senior editor.