The Role of Vitamin D in Autoimmune Diseases

: Vitamin D is a steroid hormone whose relevant immunomodulatory role has been widely described. Therefore, its contribution to the pathogenesis of immune-mediated diseases is an important and ongoing matter of research. Specifically, the active form of vitamin D, i.e., 1,25-dihydroxyvitamin D, through the interaction with its receptor, exerts different activities on the innate and adaptive immune system, among which are suppression of inflammation and promotion of tolerogenic responses. Indeed, vitamin D insufficiency/deficiency has been related to the pathogenesis and/or disease activity of several autoimmune diseases, including, amongst others, multiple sclerosis, rheumatoid arthritis, systemic lupus erythematosus, and type 1 diabetes mellitus. Based on these premises, in this review, we will describe the main molecular mechanisms modulated by vitamin D in the regulation of immune responses, including the induction of immune tolerance. Moreover, we will focus on the current knowledge regarding the contribution of vitamin D depletion to the aforementioned autoimmune diseases, seeking to provide evidence as to why its supplementation in the context of these immune-mediated disorders may potentially ameliorate disease activity and its related clinical manifestations.