This information comes to you by way of JenAus of the Psoriatic Arthritis group, who has a Science background and she states that vitamin D3 works in an immunomodulatory role as a hormone, interacting with the HPA axis, rather than just as a vitamin.
So, as much as this non-science background can translate that, after looking it up, is that it supresses or prevents the production of inflammatory t-cells. The studies were used in conjunction with high calcium diets, and calcium should be balanced with magnesium.
These studies were found successful in patients with lupus, RA, IBD, MS and others.