Vitamin D Induces Interleukin-1β Expression: Paracrine Macrophage Epithelial Signaling Controls M. tuberculosis Infection

Abstract

Although vitamin D deficiency is a common feature among patients presenting with active tuberculosis, the full scope of vitamin D action during Mycobacterium tuberculosis (Mtb) infection is poorly understood. As macrophages are the primary site of Mtb infection and are sites of vitamin D signaling, we have used these cells to understand the molecular mechanisms underlying modulation of the immune response by the hormonal form of vitamin D, 1,25-dihydroxyvitamin D (1,25D). We found that the virulent Mtb strain H37Rv elicits a broad host transcriptional response. Transcriptome profiling also revealed that the profile of target genes regulated by 1,25D is substantially altered by infection, and that 1,25D generally boosts infection-stimulated cytokine/chemokine responses. We further focused on the role of 1,25D- and infection-induced interleukin 1β (IL-1β) expression in response to infection. 1,25D enhanced IL-1β expression via a direct transcriptional mechanism. Secretion of IL-1β from infected cells required the NLRP3/caspase-1 inflammasome. The impact of IL-1β production was investigated in a novel model wherein infected macrophages were co-cultured with primary human small airway epithelial cells. Co-culture significantly prolonged survival of infected macrophages, and 1,25D/infection-induced IL-1β secretion from macrophages reduced mycobacterial burden by stimulating the anti-mycobacterial capacity of co-cultured lung epithelial cells. These effects were independent of 1,25D-stimulated autophagy in macrophages but dependent upon epithelial IL1R1 signaling and IL-1β-driven epithelial production of the antimicrobial peptide DEFB4/HBD2. These data provide evidence that the anti-microbial actions of vitamin D extend beyond the macrophage by modulating paracrine signaling, reinforcing its role in innate immune regulation in humans. In 2010 there were ∼9 million cases of tuberculosis and 1.4 million deaths, representing the second largest cause of death worldwide and the leading cause of death from a curable disease. M. tuberculosis (Mtb) replicates within cells of the immune system called macrophages over an approximate 72 hour period, ultimately inducing cell death. Notably, macrophages are sites of vitamin D signaling, and there is broad evidence that vitamin D modulates macrophage responses to Mtb. Elevated levels of TB have long been associated with vitamin D deficiency, strongly suggesting that vitamin D supplementation may be of therapeutic benefit. In this study we profile the host macrophage response to Mtb infection through the use of high-throughput genomics techniques. From this we have discovered that the dominant function of vitamin D is the modulation of the levels of specific cytokines, mediators of immune cell to cell signaling. Of particular interest was the increase in IL-1β signaling, which we show to be directly regulated by vitamin D. We also show that this increase in IL-1β is critical for driving a signaling cascade between macrophages and lung epithelial cells leading to epithelial antimicrobial peptide production that helps to contain Mtb infection in our model culture system.