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Stem-cell scientists certainly aren't expecting to answer all these questions with this first round of trials. In fact, the initial patients are part of important safety tests to determine if stem-cell-based tissues are safe and robust enough to live and grow in human patients. As in any situation involving a completely novel treatment with no precedent in medical history, the scientists are hoping for the best but bracing for the worst.
They have good reason to be cautious. Just eight months after it won approval from the Food and Drug Administration, the spinal-cord trial, led by Geron, was suspended for nearly a year after ongoing animal studies found that the transplanted nerve cells started to form odd clusters in the spines of the animals. Scientists eventually determined that the so-called rosettes weren't tumors and allowed the trial to continue, but the experience highlights the vigor and unpredictability of the cells. Learning from that incident, Lanza decided to take no chances and developed a test that would detect a single stray stem cell, with the potential of developing into a teratoma, that might have escaped into a preparation of over a million RPE cells. "Our cells are 99% to 100% pure," he says.
The retinal cells may have an advantage when it comes to immune rejection. The space beneath the retina where the cells are injected is generally free of the body's patrolling immune sentries. But in the patients in the trial, the RPE cells have been so damaged by disease that it's not clear whether they continue to maintain their immune-protected cocoon. So just to be safe, the volunteers will be taking drugs to suppress their immune system, in much the same way that patients receiving organ transplants from unrelated donors do.
Looking ahead, such immune protection may not be necessary. Researchers can now make embryo-like stem cells from a patient's own skin cells, which means that the concern about immune mismatch between donor and recipient cells may become moot. There's another benefit to these embryo-free stem cells, known as induced pluripotent stem cells, or iPS cells. Because they can be made from patients suffering from diseases like diabetes or Lou Gehrig's, scientists can watch how these cells develop and better understand how a motor neuron in a patient with such a disease starts to go awry.
So while a lot hangs on these first trials of embryonic-stem-cell therapies, they are, says Schwartz, only the beginning. Realistically, he doesn't anticipate that early participants will regain their vision completely, nor do the spinal-cord experts expect their patients to walk again after getting the treatments. But if the therapies are safe, then scientists can start figuring out when to intervene with the cells to do the most good. "We can start thinking about striking at diseases like macular degeneration before central vision is completely gone," says Schwartz. And that would indeed be something worth seeing.
Adapted from The Stem Cell Hope (Hudson Street Press, 2011) by Alice Park