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 D3 [1,25(OH)2D3] from 25-hydroxyvitamin D3 [25(OH)D3] by 1α-hydroxylase (1α-OHase). Based on human studies, it was hypothesized that bovine monocytes could produce 1,25(OH)2D3 upon activation and 1,25(OH)2D3 would regulate expression of vitamin D-responsive genes in monocytes. First, the effects of 1,25(OH)2D3 on bovine monocytes isolated from peripheral blood were tested. Treatment of nonstimulated monocytes with 1,25(OH)2D3 increased expression of the gene for the vitamin D 24-hydroxylase (24-OHase) enzyme by 51 ± 13 fold, but 1,25(OH)2D3 induction of 24-OHase expression was blocked by lipopolysaccharide (LPS) stimulation. In addition, 1,25(OH)2D3 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 1α-OHase and produce 1,25(OH)2D3 was tested. Activation of monocytes with LPS, tripalmitoylated lipopeptide (Pam3CSK4), or peptidoglycan caused 43 ± 9, 17 ± 3, and 19 ± 3 fold increases in 1α-OHase gene expression, respectively. Addition of 25(OH)D3 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)D3 to 1,25(OH)2D3. In conclusion, bovine monocytes produce 1,25(OH)2D3 in response to toll-like receptor signaling, and 1,25(OH)2D3 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)2D3 production in activated monocytes and subsequent upregulation of inducible nitric oxide synthase and RANTES expression was dependent on 25(OH)D3 availability.