Patients who got bone marrow stem-cell transplants - similar to those given to leukemia patients -- have enjoyed a mysterious remission of their disease.
And Dr Mark Freedman of the University of Ottawa is not sure why.
"Not a single patient, and it's almost seven years, has ever had a relapse,'' Dr Freedman said.
Multiple sclerosis or MS affects an estimated 1 million people globally. There is no cure.It can cause mild illness in some people while causing permanent disability in others.
Symptoms may include numbness or weakness in one or more limbs, partial or complete loss of vision, and an unsteady gait.
Dr Freedman, who specialises in treating MS, wanted to study how the disease unfolds.
He set up an experiment in which doctors destroyed the bone marrow and thus the immune systems of MS patients.
Then stem cells known as hematopoeitic stem cells, blood-forming cells taken from the bone marrow, were transplanted back into the patients.
"We weren't looking for improvement,'' Dr Freedman told a stem cell seminar at the US National Institutes of Health.
"The actual study was to reboot the immune system.''
Once MS is diagnosed, Dr Freedman said, "you've already missed the boat. We figured we would reboot the immune system and watch the disease evolve. It failed.''
They had thought that destroying the bone marrow would improve symptoms within a year.
MS is believed to be an autoimmune disease, in which immune system cells mistakenly attack the fatty myelin sheath that protects nerve strands.
Patients lose the ability to move as the thin strands that connect one nerve
cell to another wither.
Instead, improvements began two years after treatment.
Dr Freedman reported to the seminar about 17 of the patients he has given the transplants to.
"We have yet to get the disease to restart,'' he said.
Patients are not developing some of the characteristic brain lesions seen in MS. ``But we are seeing this repair.''
MS patients often have hard-to-predict changes in their symptoms and disease course, so Freedman says his team must study the patients longer before they can say precisely what is going on.
"We are trying to find out what is happening and what could possibly be the source of repair,'' Dr Freedman said.
But he has found some hints that may help doctors who treat MS by using drugs to suppress the immune system.
"Those with a lot of inflammation going on were the most likely to benefit (from the treatment),'' he said.
"We need some degree of inflammation.
'' While inflammation may be the process that destroys myelin, it could be that the body needs some inflammation to make repair," Dr Freedman said.
Immune cells secrete compounds known as cytokines. While these are linked with inflammation, they may also direct cells, perhaps even the stem cells, to regenerate.
The treatment itself is dangerous - one patient died when the chemicals used to destroy his bone marrow also badly damaged his liver.