By obliterating the broken immune systems of patients with severe forms of multiple sclerosis, then sowing fresh, defect-free systems with transplanted stem cells, researchers can thwart the degenerative autoimmune disease—but it comes at a price.

In a small phase II trial of 24 MS patients, the treatment halted or even reversed the disease in 70 percent of patients for three years after the transplant. Eight patients saw that improvement last for seven and a half years, researchers report in the Lancet. This means that some of those patients went from being wheelchair-bound to walking and being active again. But to reach that success, many suffered through severe side effects, such as life threatening infections and organ damage from toxicity brought on by the aggressive chemotherapy required to annihilate the body’s immune system. One patient died from complications of the treatment, which represents a four percent fatality rate.

Moreover, while the risks may be worthwhile to some patients with rapidly progressing forms of MS—a small percentage of MS patients—the researchers also caution that the trial was small and did not include a control group.

“Larger clinical trials will be important to confirm these results,” study coauthor Mark Freedman of University of Ottawa said in a statement. “Since this is an aggressive treatment, the potential benefits should be weighed against the risks of serious complications associated with [this stem cell transplant], and this treatment should only be offered in specialist centres experienced both in multiple sclerosis treatment and stem cell therapy, or as part of a clinical trial,” he added.

Similar treatments have been used before in other trials, which also showed positive—though not as dramatic—results. Generally, researchers start by harvesting a patient’s haematopoietic stem cells, which give rise to the body’s immune system. Then researchers use chemotherapy to knock back the patient’s misbehaving immune system. In MS patients, defective immune responses rip off the insulation from nerve cells in the brain and spinal cord, causing inflammation, lesions, and nerve damage that eventually lead to physical and mental disabilities. The disease can progress in bouts over decades or continuously over months.

With that defective immune system weakened, researchers can replace the patient's stem cells, which are distant enough predecessors that they don't carry the glitches that trigger MS. Thus, they can potentially spawn a flaw-free immune system.

Freedman and colleagues took this general treatment strategy a step further by not just knocking back the patient’s defective immune system, but by annihilating it completely with a cocktail of powerful drugs.

“It's important to stress that this is a very early study,” Stephen Minger, a stem cell biologist not involved with the study, told the BBC. “Nevertheless, the clinical results are truly impressive, in some cases close to being curative.”

Freedman added that future research will be geared not only to replicating the results in larger trials, but to figuring out how to make it safer for patients.

Lancet, 2016. DOI: 10.1016/S0140-6736(16)30169-6 (About DOIs).