How does Vitamin D help with autoimmune diseases such as MS, and rheumatoid arthritis?

How does Vitamin D help with autoimmune diseases such as MS, and rheumatoid arthritis?

Information about lack of sun exposure and vitamin D deficiency leading to MS.

There is something called the Hygiene Hypothesis which "postulates that higher levels of cleanliness and improper exposure to microorganisms early in childhood could disturb the intestinal microbiome resulting in abnormal immune responses" (1). This idea developed in 1989 regarding asthma and hay fever has been expanded to involve autoimmune diseases like MS, and has added the idea that hyper-clean environments also lack sunlight exposure (thus vitamin D deficiency) (1).  

Taking this one step further the authors of this paper in source 1 link Vitamin D with the gut microbiome and the development of autoimmune diseases. As your post points out, one role of vitamin D involves the immune system where immune cells express VDR and active vitamin D, seemingly enhancing the immune response. Conversely, a deficiency of vitamin D seems to blunt the immune response. From week 1 we read that one of the largest immune centers of the body exists in the GI tract as GALT. As such, it makes sense to look at vitamin D's influence on GI immune function.

This thorough review paper looks at the hypothesis that Vitamin D is involved in GI immune regulation and finds that "evidence is emerging that VDR is a key component in maintaining gut intestinal barrier function and preventing dysbiosis, which can attenuate inflammation" (1). Here is a helpful diagram for us from this paper:

Source: 

• Clark, A., Mach, N., 2016. Role of Vitamin D in the Hygiene Hypothesis: The Interplay between Vitamin D, Vitamin D Receptors, Gut Microbiota, and Immune Response. Frontiers in Immunology 7. https://doi.org/10.3389/fimmu.2016.00627 (Links to an external site.)Links to an external site.