Human eosinophil toll-like receptor 7-induced nitric oxide is antiviral against parainfluenza

Abstract

BACKGROUND: Asthma is an inflammatory disease of the airways often characterized by eosinophils and elevated exhaled nitric oxide. Respiratory viruses exacerbate asthma and are detected in airways by the innate immune receptor Toll-like receptor 7 (TLR7). Airway eosinophilia reduces virus titers in mouse lungs suggesting eosinophils have antiviral effects. However, the mechanisms responsible for eosinophil antiviral activity remain unclear. OBJECTIVE: To determine whether nitric oxide from human eosinophils inhibits parainfluenza virus 1 infection. METHODS: Human eosinophils isolated from peripheral blood of healthy volunteers were inoculated with parainfluenza and evaluated for infectivity by hemadsorption assay. Nitric oxide was measured with the nitric oxide-detecting fluorophore CuFL using confocal microscopy. Some cultures were treated with interferon-gamma, nitric oxide synthase inhibitor (L-NAME), TLR7 agonist (R837) or with TLR7 antagonist (IRS661). TLR7 expression was evaluated by immunofluorescence and RT-PCR. RESULTS: Eosinophils reduced parainfluenza virus titers. Eosinophils produced nitric oxide when exposed to virus and this was blocked by L-NAME. Furthermore, eosinophils9 antiviral effect was reduced when nitric oxide production was blocked. Similarly, eosinophils produced nitric oxide in response to the TLR7 agonist R837, and this effect was blocked by L-NAME and by TLR7 antagonist IRS661. Interferon-gamma upregulated TLR7 expression and potentiated TLR7-induced nitric oxide production in eosinophils. CONCLUSIONS: Eosinophils produce nitric oxide in response to parainfluenza virus and TLR7 agonist. Eosinophil-derived nitric oxide is antiviral against parainfluenza.