The active form of vitamin D, 1,25-dihydroxyvitamin D (1,25(OH)₂D) enhances innate immunity by inducing the cathelicidin antimicrobial peptide (hCAP). In monocytes/macrophages, this occurs primarily in response to activation of TLR, that induce expression of the vitamin D receptor and localized synthesis of 1,25(OH)₂D from precursor 25-hydroxyvitamin D₃ (25OHD). To clarify the relationship between vitamin D and innate immunity, we assessed changes in hCAP expression in vivo and ex vivo in human subjects attending a bone clinic (n = 50). Of these, 38% were vitamin D-insufficient (<75 nM 25OHD) and received supplementation with vitamin D (50,000 IU vitamin D₂ twice weekly for 5 wk). Baseline 25OHD status or vitamin D supplementation had no effect on circulating levels of hCAP. Therefore, ex vivo changes in hCAP for each subject were assessed using peripheral blood monocytes cultured with 10% autologous serum (n = 28). Under these vitamin D “insufficient” conditions the TLR2/1 ligand 19 kDa lipopeptide or the TLR4 ligand LPS, monocytes showed increased expression of the vitamin D-activating enzyme CYP27b1 (5- and 5.5-fold, respectively, both p < 0.01) but decreased expression of hCAP mRNA (10-fold and 30-fold, both p < 0.001). Following treatment with 19 kDa, expression of hCAP: 1) correlated with 25OHD levels in serum culture supplements (R = 0.649, p < 0.001); 2) was significantly enhanced by exogenous 25OHD (5 nM); and 3) was significantly enhanced with serum from vivo vitamin D-supplemented patients. These data suggest that a key role of vitamin D in innate immunity is to maintain localized production of antibacterial hCAP following TLR activation of monocytes.

So much more to mucus: Mammals accommodate a dense community of metabolically active microorganisms in their gut. This is not a passive relationship, and host and microbe have antagonistic as well as mutualistic responses to each other. Using a whole-colon imaging method in mice, Bergstrom et al. looked at the role of colonic mucus in segregating the microbiota from host cells during elimination of feces (see the Perspective by Birchenough and Johansson). Host goblet cells synthesize two forms of mucin that differ in branched chain O-glycosylation and the site of production in the colon. A “thick” mucus in the proximal, ascending colon wraps the microbiota to form fecal pellets. Transit along the distal, descending colon is lubricated by “thin” mucus that transiently links with the thick mucus. Normal mucus encapsulation prevents inflammation and hyperplasia and thus is important for maintenance of a healthy gut. Science , this issue p. 467 ; see also p. 402

Vitamin D insufficiency is a global health issue. Although classically associated with rickets, low vitamin D levels have also been linked to aberrant immune function and associated health problems such as inflammatory bowel disease (IBD). To test the hypothesis that impaired vitamin D status predisposes to IBD, 8-wk-old C57BL/6 mice were raised from weaning on vitamin D-deficient or vitamin D-sufficient diets and then treated with dextran sodium sulphate (DSS) to induce colitis. Vitamin D-deficient mice showed decreased serum levels of precursor 25-hydroxyvitamin D₃ (2.5 ± 0.1 vs. 24.4 ± 1.8 ng/ml) and active 1,25-dihydroxyvitamin D₃ (28.8 ± 3.1 vs. 45.6 ± 4.2 pg/ml), greater DSS-induced weight loss (9 vs. 5%), increased colitis (4.71 ± 0.85 vs. 1.57 ± 0.18), and splenomegaly relative to mice on vitamin D-sufficient chow. DNA array analysis of colon tissue (n = 4 mice) identified 27 genes consistently (P < 0.05) up-regulated or down-regulated more than 2-fold in vitamin D-deficient vs. vitamin D-sufficient mice, in the absence of DSS-induced colitis. This included angiogenin-4, an antimicrobial protein involved in host containment of enteric bacteria. Immunohistochemistry confirmed that colonic angiogenin-4 protein was significantly decreased in vitamin D-deficient mice even in the absence of colitis. Moreover, the same animals showed elevated levels (50-fold) of bacteria in colonic tissue. These data show for the first time that simple vitamin D deficiency predisposes mice to colitis via dysregulated colonic antimicrobial activity and impaired homeostasis of enteric bacteria. This may be a pivotal mechanism linking vitamin D status with IBD in humans.

The vitamin D-activating enzyme 1α-hydroxylase (CYP27B1) and vitamin D receptor (VDR) support anti-inflammatory responses to vitamin D in many tissues. Given the high basal expression of CYP27B1 and VDR in trophoblastic cells from the placenta, we hypothesized that anti-inflammatory effects of vitamin D may be particularly important in this organ. Pregnant wild type (WT) mice i.p. injected with LPS showed elevated expression of mouse Cyp27b1 (4-fold) and VDR (6-fold). Similar results were also obtained after ex vivo treatment of WT placentas with LPS. To assess the functional impact of this, we carried out ex vivo studies using placentas −/− for fetal (trophoblastic) Cyp27b1 or VDR. Vehicle-treated −/− placentas showed increased expression of IFN-γ and decreased expression of IL-10 relative to +/+ placentas. LPS-treated −/− placentas showed increased expression of TLR2, IFN-γ, and IL-6. Array analyses identified other inflammatory factors that are dysregulated in Cyp27b1^−/− versus Cyp27b1^+/+ placentas after LPS challenge. Data highlighted enhanced expression of IL-4, IL-15, and IL-18, as well as several chemokines and their receptors, in Cyp27b1^−/− placentas. Similar results for IL-6 expression were observed with placentas −/− for trophoblastic VDR. Finally, ex vivo treatment of WT placentas with the substrate for Cyp27b1, 25-hydroxyvitamin D₃, suppressed LPS-induced expression of IL-6 and the chemokine Ccl11. These data indicate that fetal (trophoblastic) vitamin D plays a pivotal role in controlling placental inflammation. In humans, this may be a key factor in placental responses to infection and associated adverse outcomes of pregnancy.

The active form of vitamin D, 1,25-dihydroxyvitamin D₃, [1,25(OH)₂D₃] has potent actions on innate and adaptive immunity. Although endocrine synthesis of 1,25(OH)₂D₃ takes place in the kidney, the enzyme that catalyzes this, 25-hydroxyvitamin D-1α-hydroxylase (CYP27b1 in humans, Cyp27b1 in mice), is expressed at many extra-renal sites including the colon. We have shown previously that colonic expression of CYP27b1 may act to protect against the onset of colitis. To investigate this further, we firstly characterized changes in Cyp27b1 expression in a mouse model of colitis. Mice treated with dextran sodium sulfate (DSS) showed weight loss, histological evidence of colitis, and increased expression of inflammatory cytokines. This was associated with decreased renal expression of Cyp27b1 (5-fold, P = 0.013) and lower serum 1,25(OH)₂D₃ (51.8 ± 5.9 pg/nl vs. 65.1 ± 1.6 in controls, P < 0.001). However, expression of CYP27b1 was increased in the proximal colon of DSS mice (4-fold compared with controls, P < 0.001). Further studies were carried out using Cyp27b1 null (−/−) mice. Compared with +/− controls the Cyp27b1 −/− mice showed increased weight loss (4.9% vs. 22.8%, P < 0.001) and colitis. This was associated with raised IL-1 in the distal colon and IL-17 in the proximal and distal colon. Conversely, DSS-treated Cyp27b1−/− mice exhibited lower IL-10 in the proximal colon and toll-like receptors 2 and 4 in the distal colon. These data indicate that both local and endocrine synthesis of 1,25(OH)₂D₃ affect colitis in DSS-treated mice. Lack of Cyp27b1 exacerbates disease in this model, suggesting that similar effects may occur with vitamin D deficiency.