The study, published in the Proceedings of the National Academy of Sciences, offers new insight into the widely purported benefits of vitamin D supplementation in people with MS.
Led by Dr Anne Groke from the Johns Hopkins University School of Medicine, the research team systematically examined the effects of vitamin D in the form of 1,25(OH)2D3 on T lymphocytes at each step of their journey from the lymph nodes to the CNS - finding that the sunshine vitamin does not affect generation of these damaging autoimmune cells, but does prevent their presence in the brain and CNS.
"With this research, we learned vitamin D might be working not by altering the function of damaging immune cells but by preventing their journey into the brain," explained Gocke. "If we are right, and we can exploit Mother Nature's natural protective mechanism, an approach like this could be as effective as and safer than existing drugs that treat MS."
The sunshine vitamin and MS
The quest to understand the role of vitamin D in MS began with the observation that the disease is more prevalent in regions of the world farthest from the equator where there is less sunshine - which is the main natural source of vitamin D.
Multiple sclerosis is believed to be an autoimmune disorder that is caused when the immune system wrongly attacks a person's own cells; in this case, the fatty protein called myelin that insulates nerves and helps them send electrical signals that control movement, speech and other functions.
The immune system primes T cells in the body's lymph nodes, preparing them to seek out and destroy myelin, a process that can lead to debilitating symptoms such as blurred vision, weakness and numbness.
Many previous studies have suggested that additional vitamin D from sunshine or dietary supplementation can have benefits in terms of preventing and easing the symptoms of MS - however very little is currently known about the mechanism behind these effects.
In their new study, Gocke and her colleagues simultaneously gave mice the rodent form of MS and a high dose of vitamin D3 - finding that this protected the mice from showing symptoms of the disease.
When the team then looked at the levels of T cells, they found a large number of them primed and in the bloodstream of the mice, but very few in their brains and spinal cords.
"Vitamin D doesn't seem to cause global immunosuppression," Gocke said. "What's interesting is that the T cells are primed, but they are being kept away from the places in the body where they can do the most damage."
Gocke added that an important thing to consider with vitamin D supplemenation is that its immunosuppressive effects appear to be fleeting - once vitamin D is withdrawn, MS-like flare-ups in mice can occur very quickly.
The upside of this, she said, is that if a patient develops an infection and the body appears too immune-compromised to fight it, discontinuing the vitamin D temporarily could quickly allow the immune system to recover and attack the infection - while current popular immune-suppressing medications for MS can take six to 12 weeks to be cleared from the body.
The team noted that with a clinical trial on vitamin D supplementation ongoing, no one is certain whether it will actually work to prevent or slow the progression of MS in humans. However, they noted that the current research may offer the opportunity to study samples taken from participants to see whether vitamin D is having the same effect on human cells as it appears to be having in mice.