To explain a little more about vitamin D and its connection to regulatory T cells, I would first like to summarize some facts about Vitamin D.
1. Vitamin D is not a vitamin. It is a steroid hormone. You cannot get natural amounts of vitamin D from foods. For example, a healthy person in mid-day summer sun can synthesis 10,000 IU's of vitamin D in 1 hour. Compare this to the amount that is fortified in a glass of milk which is about 200 IU's.
2. There are three stages to vitamin D synthesis in the body. The first is called Cholecalciferol which is generated by UV exposure. This is the same as Vitamin D3 that you can purchase at any drug store.
Cholecalciferol is then hydroxylated in the liver to become calcifediol (25-hydroxyvitamin D3, otherwise known as Calcidiol). This is the main circulating form in your blood and is what should be checked by your doctor.
Finally, calcifediol is again hydroxylated, this time in the kidney but also by immune cells, and becomes calcitriol (1,25-dihydroxyvitamin D3). Calcitriol is the most active hormone form of vitamin D3.
3. There is mounting evidence that Vitamin D3 has a direct effect on regulatory T-cells. These Treg-cells restrain the immune system to prevent autoimune diseases and maintain self tolerance.
It appears from the mounting evidence that Tregs are defective in their supressive function in people with MS as well as other autoimune diseases.
I believe this is why there is such great success with the HSCT procedure, because it wipes out these defective Tregs and enables normal Tregs to perform their proper function in people with auto-immune diseases.
The key thing to remember is that all people generate self-reactive immune cells, but it appears that people with autoimmune diseases lack the function to shut-down an autoimmune response because of the lack of suppressing ability of their Tregs.
Here is a recent article that describes this:
So to me, it is a no brainer to follow the recent studies of high doses of vitamin D (Phase I/II) on people with MS. I cannot understand why the MS community is not pushing for more research on vitamin D to find the optimum levels. There has been suspicion of a connection of vitamin D's role in preventing MS for more than 30 years, but little has been done to investigate this.