Vitamin D Counteracts an IL-23-Dependent IL-17A⁺IFN-γ⁺ Response Driven by Urban Particulate Matter

Urban particulate matter (UPM) air pollution and vitamin D deficiency are detrimentally associated with respiratory health. This is hypothesized to be due in part to regulation of IL-17A, which UPM is reported to promote. Here, we used a myeloid dendritic cell (DC)-memory CD4⁺ T cell co-culture system to characterize UPM-driven IL-17A⁺ cells, investigate the mechanism by which UPM-primed DCs promote this phenotype, and address evidence for cross-regulation by vitamin D. CD1c⁺ myeloid DCs were cultured overnight with or without a reference source of UPM and/or active vitamin D (1,25[OH]₂D₃) before they were co-cultured with autologous memory CD4⁺ T cells. Supernatants were harvested for cytokine analysis on Day 5 of co-culture, and intracellular cytokine staining was performed on Day 7. UPM-primed DCs increased the proportion of memory CD4⁺ T cells expressing the T helper 17 cell (Th17)-associated cytokines IL-17A, IL-17F, and IL-22, as well as IFN-γ, granulocyte-macrophage colony-stimulating factor, and granzyme B. Notably, a large proportion of the UPM-driven IL-17A⁺ cells co-expressed these cytokines, but not IL-10, indicative of a proinflammatory Th17 profile. UPM-treated DCs expressed elevated levels of il23 mRNA and increased secretion of IL-23p40. Neutralization of IL-23 in culture reduced the frequency of IL-17A⁺IFN-γ⁺ cells without affecting cell proliferation. 1,25(OH)₂D₃ counteracted the UPM-driven DC maturation and inhibited the frequency of IL-17A⁺IFN-γ⁺ cells, most prominently when DCs were co-treated with the corticosteroid dexamethasone, while maintaining antiinflammatory IL-10 synthesis. These data indicate that UPM might promote an inflammatory milieu in part by inducing an IL-23-driven proinflammatory Th17 response. Restoring vitamin D sufficiency may counteract these UPM-driven effects without obliterating important homeostatic immune functions.