Mast cells release cytokines in response to mediators produced by virus-infected epithelial cellss

Abstract

Background: Mast cells have long been recognized for their involvement in allergic diseases. In the last decade, the importance of mast cells in innate responses against bacteria has been established, but little is known about their contribution in viral infections. Mast cells are abundant at mucosal surfaces such as the lungs in close proximity to the epithelium. In the lung, the epithelium is a primary target for viral infections. Mast cells are secondarily exposed to newly formed virions released from epithelial cells. Mast cells have toll-like receptors (TLRs) that detect various pathogen components. Our first hypothesis is that viral TLR agonists will induce mast cells to release cytokines thought to be involved in viral infections. Both the pro-inflammatory cytokine IL-6 and the chemokine IL-8 are produced during viral infections. TGF-β is an immunoregulatory cytokine that modulates the activity of various immune cells and could also play a role in viral infections. 
 Methods: We used polyI:C, a synthetic double-stranded RNA (dsRNA), as a TLR3 agonist, loxoribine as a TLR7 agonist, and unmethylated CpG DNA as a TLR9 agonist. We treated mast cells from the cell line HMC-1 (Human Mast cell-1) for 0.5 – 24hr with the TLR agonists and performed dose response studies for all stimuli. Supernatants from treated mast cells were measured for IL-6, IL-8, and TGF-β by ELISA. 
 Results: Cytokine release was highest at the 24hr time point. Mast cells released IL-6, IL-8, and TGF-β in response to the TLR3 agonist polyI:C in a dose-dependent manner, but not to the other viral TLR agonists. PolyI:C (10μg/mL) versus unstimulated controls significantly increased mast cell release of IL-6 (224.7±57.4 vs. 39.0±5.7, p≤0.001) and TGF-β (240.2±28.9 vs. 116.1±16.7, p≤0.05). PolyI:C induced release of IL-8 from mast cells was increased but not significant. Viral exposure also induces epithelial cells to produce type I interferons, IFNα and β. These interferons have potent antiviral activity, but also have effects on mast cells, decreasing mast cell adhesion to extracellular matrix and reducing co-stimulatory activity on T cells. Our second hypothesis is that IFNα and β will induce mast cells to release cytokines similar to stimulation with polyI:C. Our preliminary data showed that IFNα, IFNβ , and IFNα and β in combination induced a low level of IL-8 and TGF-β release from mast cells, but had no effect on IL-6 release.
 Conclusion: HMC-1 responds to dsRNA, a TLR3 agonist produced in epithelial cells during viral replication, by releasing IL-6 and TGF-β. HMC-1 also responds to IFNα and β by releasing IL-8 and TGF-β, indicating that human mast cells respond to epithelial mediators produced during viral infections. Our results show that mast cells contribute to the innate response against viruses by responding to mediators released by virus-infected epithelial cells.