Modulation of the bovine innate immune response by production of 1α,25-dihydroxyvitamin D3 in bovine monocytes

In cattle, the kidney has been the only known site for production of 1,25-dihydroxyvitamin D-3 [1,25(OH)(2)D-3] from 25-hydroxyvitamin D-3 [25(OH)D-3] by 1 alpha-hydroxylase (1 alpha-OHase). Based on human studies, it was hypothesized that bovine monocytes could produce 1,25(OH)(2)D-3 upon activation and 1,25(OH)(2)D-3 would regulate expression of vitamin D-responsive genes in monocytes. First, the effects of 1,25(OH)(2)D-3 on bovine monocytes isolated from peripheral blood were tested. Treatment of nonstimulated monocytes with 1,25(OH)(2)D-3 increased expression of the gene for the vitamin D 24-hydroxylase (24-OHase) enzyme by 51 +/- 13 fold, but 1,25(OH)(2)D-3 induction of 24-OHase expression was blocked by lipopolysaccharide (LPS) stimulation. In addition, 1,25(OH)(2)D-3 increased the gene expression of inducible nitric oxide synthase and the chemokine RANTES (regulated upon activation, normal T-cell expressed and secreted) in LPS-stimulated monocytes 69 +/- 13 and 40 +/- 12 fold, respectively. Next, the ability of bovine monocytes to express 1 alpha-OHase and produce 1,25(OH)(2)D-3 was tested. Activation of monocytes with LPS, tripalmitoylated lipopeptide (Pam3CSK4), or peptidoglycan caused 43 +/- 9, 17 +/- 3, and 19 +/- 3 fold increases in 1 alpha-OHase gene expression, respectively. Addition of 25(OH)D-3 to LPS-stimulated monocytes enhanced expression of inducible nitric oxide synthase and RANTES and nitric oxide production in a dose-dependent manner, giving evidence that activated monocytes convert 25(OH)D-3 to 1,25(OH)(2)D-3. In conclusion, bovine monocytes produce 1,25(OH)(2)D-3 in response to toll-like receptor signaling, and 1,25(OH)(2)D-3 production in monocytes increased the expression of genes involved in the innate immune system. Vitamin D status of cattle might be important for optimal innate immune function because 1,25(OH)(2)D-3 production in activated monocytes and subsequent upregulation of inducible nitric oxide synthase and RANTES expression was dependent on 25(OH)D-3 availability.