1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)), 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)(2)D(3) also has a physiological role beyond its well-known role in
skeletal homeostasis. Here, we describe 1,25(OH)(2)D(3) 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)(2)D(3) within
the immune system. Taken together, these data indicate that 1,25(OH)(2)D(3) 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)(2)D(3) on immune cells as well as its
role in infectious and autoimmune diseases, more in particular in tuberculosis and
type 1 diabetes (T1D).
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