Vitamin D: modulator of the immune system
1,25-dihydroxyvitamin D3 (1,25(OH)2D3), the active form of vitamin D, is known to regulate calcium and phosphorus metabolism, thus being a key-player in bone-formation. However 1,25(OH)2D3 also has a physiological role beyond its well-known role in skeletal homeostasis. Here, we describe 1,25(OH)2D3 as an immunomodulator targeting various immune cells, including monocytes, macrophages, dendritic cells (DCs), as well as T-lymphocytes and B-lymphocytes, hence modulating both innate and adaptive immune responses. Besides being targets, immune cells express vitamin D-activating enzymes, allowing local conversion of inactive vitamin D into 1,25(OH)2D3 within the immune system. Taken together, these data indicate that 1,25(OH)2D3 plays a role in maintenance of immune homeostasis. Several epidemiological studies have linked inadequate vitamin D levels to a higher susceptibility of immune-mediated disorders, including chronic infections and autoimmune diseases. This review will discuss the complex immune-regulatory effects of 1,25(OH)2D3 on immune cells as well as its role in infectious and autoimmune diseases, more in particular in tuberculosis and type 1 diabetes (T1D).