T-cell cytokines differentially control human monocyte antimicrobial responses by regulating vitamin D metabolism

Abstract

We investigated the mechanisms by which T-cell cytokines are able to influence the Toll-like receptor (TLR)-induced, vitamin Ddependent antimicrobial pathway in human monocytes. T-cell cytokines differentially influenced TLR2/1-induced expression of the antimicrobial peptides cathelicidin and DEFB4, being upregulated by IFN-γ, down-regulated by IL-4, and unaffected by IL-17. The Th1 cytokine IFN-γ up-regulated TLR2/1 induction of 25-hydroxyvitamin D-1α-hydroxylase (i.e., CYP27B1), leading to enhanced bioconversion of 25-hydroxyvitamin D 3 (25D 3 ) to its active metabolite 1,25D 3 . In contrast, the Th2 cytokine IL-4, by itself and in combination with the TLR2/1 ligand, induced catabolism of 25D 3 to the inactive metabolite 24,25D 3 , and was dependent on expression of vitamin D-24-hydroxylase (i.e., CYP24A1). Therefore, the ability of T-cell cytokines to differentially control monocyte vitamin D metabolism represents a mechanism by which cell-mediated immune responses can regulate innate immune mechanisms to defend against microbial pathogens. Mycobacterium tuberculosisT he ability of Toll-like receptors (TLRs) to trigger a direct antimicrobial activity is a key aspect of their role in innate immunity. In mouse monocytes, activation of the TLR2/1 heterodimer by microbial lipoproteins (1-3), induces an antimicrobial activity against Mycobacterium tuberculosis that is nitric oxide (NO)-dependent, but in human monocytes is NO-independent (4). Instead, a key antimicrobial mechanism for TLR-activated human monocytes involves induction of the 25-hydroxyvitamin D-1α-hydroxylase (i.e., CYP27B1), which enzymatically converts the major circulating form of vitamin D, 25-hydroxyvitamin D3 (25D 3 ) into the active form of vitamin D, 1,25D 3 . Parallel TLRmediated up-regulation of the vitamin D receptor (VDR) and activation of this receptor by 1,25D 3 leads to downstream induction of the genes encoding the antimicrobial peptides cathelicidin and DEFB4 (5-10). Here, we tested the hypothesis that adaptive T-cell cytokines, including key cytokines of the Th1, Th2, and Th17 pattern, regulate the TLR2/1-induced, vitamin Ddependent antimicrobial pathway. ResultsEffect of T-Cell Cytokines on TLR2/1 Induction of Cathelicidin and DEFB4. To determine the role of individual cytokines on the TLRtriggered vitamin D-dependent induction of antimicrobial peptides, monocytes were treated with TLR2/1L with or without a specific T-cell cytokine, and cathelicidin and DEFB4 mRNAs measured at 24 h. IFN-γ by itself up-regulated cathelicidin and DEFB4 mRNA levels by twofold ( Fig. 1A; P < 0.05 and P < 0.001). Consistent with previous findings, TLR2/1L induced both cathelicidin and DEFB4 mRNAs (8, 10). However, whereas IFN-γ augmented TLR2/1L-triggered induction of cathelicidin by 4.1-fold (P < 0.01), it had no effect on TLR2/1L-mediated induction of DEFB4 (Fig. 1A). The addition of IL-17 had no effect on induction of antimicrobial peptide gene expression in the presence or absence of TLR2/1L (Fig. 1B).Whereas IFN-γ augmented TLR2/1 induction of cath...

Hypoxia Triggers the Expression of Human β Defensin 2 and Antimicrobial Activity against Mycobacterium tuberculosis in Human Macrophages

“…In contrast, cathelicidin, which was shown to mediate VDR-dependent killing of M. tuberculosis (17), was downregulated under oxygen-limiting conditions. Intriguingly, this indicates a differential regulation of vitamin D-dependent antimicrobial peptides, possibly shaped by the surrounding cytokine milieu (33). Because both antimicrobial peptides are active against M. tuberculosis (16,18), it was remarkable that antimicrobial activity was almost completely abrogated by silencing of cathelicidin in a human monocytic cell line (17).…”

Vitamin D accelerates resolution of inflammatory responses during tuberculosis treatment

“…In contrast to these suppressive actions, vitamin D also attenuated treatment-induced falls in antigen-stimulated CCL5, IL-4, and IFN-α. IL-4 has recently been reported to induce expression of CYP24A, the principal catabolic enzyme of both calcidiol and calcitriol (38); the increase in antigenstimulated IL-4 secretion observed in the intervention arm of the study may therefore represent part of a negative-feedback loop via which calcitriol regulates its own concentration at the site of disease. The finding that administration of vitamin D enhanced antigen-stimulated IFN-α responses is of particular interest, given the pivotal role of type 1 interferons in antiviral responses (39), and the clinical observation of a sixfold reduction in upper respiratory tract infections among patients in the intervention arm of the trial (19).…”

Vitamin D [1,25(OH)2D3] Differentially Regulates Human Innate Cytokine Responses to Bacterial versus Viral Pattern Recognition Receptor Stimuli

“…Our understanding of the impact of vitamin D on immune regulation, specifically cytokine production by the innate immune response that programs subsequent adaptive immune responses, is much more limited. To date, the majority of mechanistic studies examining these responses have focused on antibacterial (i.e., Mycobacterium tuberculosis) or, when using purified pattern recognition receptor (PRR) ligands, TLR4-or TLR2-driven responses (24,26,27). Vitamin D metabolites clearly decrease proinflammatory cytokines stimulated by bacteria or bacterial PRR in a wide variety of murine and human systems (28)(29)(30).…”