Vitamin D plays multiple roles in regulation of protective and maladaptive immunity. Although epidemiologic studies link poor in vivo 25(OH)D status to increased viral respiratory infections, we poorly understand how vitamin D affects viral pattern recognition receptor (PRR)-driven cytokine production. In this study, we hypothesized that the biologically active metabolite of vitamin D, 1,25(OH)2D3, inhibits human proinflammatory and anti-inflammatory innate cytokine responses stimulated by representative bacterial or viral PRR ligands. Fresh PBMCs or CD14+ monocytes were stimulated with TLR4, TLR7/8-selective ligands, or respiratory syncytial virus (RSV) ± 1,25(OH)2D3. Proinflammatory and anti-inflammatory responses resulting from TLR4 stimulation were inhibited ∼50% in the presence of 1,25(OH)2D3. Conversely, its usage at physiologic through pharmacologic concentrations inhibited neither proinflammatory nor anti-inflammatory responses evoked by viral PRR ligands or infectious RSV. This differential responsiveness was attributed to the finding that TLR7/8, but not TLR4, stimulation markedly inhibited vitamin D receptor mRNA and protein expression, selectively reducing the sensitivity of viral PRR responses to modulation. 1,25(OH)2D3 also enhanced expression of IkBa, a potent negative regulator of NF-κB and cytokine production, in TLR4-stimulated monocytes while not doing so upon TLR7/8 stimulation. Thus, 1,25(OH)2D3 inhibits both proinflammatory and a broad panel of anti-inflammatory responses elicited by TLR4 stimulation, arguing that the common view of it as an anti-inflammatory immune response modifier is an oversimplification. In viral responses, it consistently fails to modify TLR7/8- or RSV-stimulated innate cytokine production, even at supraphysiologic concentrations. Collectively, the data call into question the rationale for increasingly widespread self-medication with vitamin D supplements.

Population health studies demonstrate that insufficient/deficient Vitamin D status is strongly associated with increased upper respiratory tract infections. As low vitamin D status is widespread, it is imperative to understand the roles it plays during an immune response. Here we determine the effects of active vitamin D (1,25(OH)2D3) on peripheral blood mononuclear cells (PBMC) cytokine responses elicited by bacterial or viral ligands and explore mechanisms by which vitamin D regulates innate immune responses. Results: PBMC stimulated with TLR-4L and exogenous 1,25(OH)2D3 exhibited decreased pro-inflammatory cytokine production; whereas cultures stimulated with TLR-8L did not exhibit alterations in cytokine production patterns in the presence of supplemental 1,25(OH)2D3. 1,25(OH)2D3 alone slightly reduced TLR-4 but decreased TLR-8 by >75%. Moreover, while TLR-4 ligand stimulation had no effect on VDR levels, TLR-8L stimulation reduced VDR mRNA by 60% (p<0.005). Conclusions: Exogenous 1,25(OH)2D3 markedly reduces inflammatory cytokine production elicited by TLR-4 but not by TLR-8 or RSV activation. TLR-8L strikingly reduce VDR while 1,25(OH)2D3 reduces viral PRR expression, resulting in a synergistic decrease in the capacity of viral stimulated cells to have responses modulated by exogenous vitamin D. The results suggest a need for closer examination of the benefits of exogenous vitamin D supplementation in bacterial versus viral infection.

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