The role of nitric oxide synthase 2 (NOS2) in host defense on the lung

Chronic bacterial infection, mostly Pseudomonas aeruginosa, is the main cause of respiratory exacerbation in cystic fibrosis (CF), the most common genetic disease in Caucasians, and ultimately results in substantial morbidity. Respiratory epithelial cells are key inflammatory cells in the airway, and function in innate host defense in part by the continuous expression of nitric oxide synthase (NOS2). We propose that antiviral innate host defense mechanisms are defective in CF lung and that CF airway epithelial cells are defective in the signaling pathway that leads to NOS2 induction. To test the hypothesis, virus infection in human airway epithelial cells (HAEC) from CF lungs and from healthy control lungs was studied. Human parainfluenza virus 3 (HPIV3) was used to infect CF and normal HAEC. Fluorescent staining of HPIV3 and plaque assay 24 h postinfection showed that more viruses were present in CF than normal [pfu × 103/ml: CF, 37 ± 25, control, 9 ± 2]. Important antiviral proteins including dsRNA dependent protein kinase (PKR), RNase L, 2′-5′OA synthase, MxA and NOS2 were evaluated by Western Analysis. All proteins, except NOS2, which was absent in CF, were expressed at similar levels in CF and normal HAEC postinfection. The results indicate that NO is an essential and primary component of the antiviral host defense in the human airway. Investigation of the IFN-γ signaling pathway that lead to NOS2 expression in CF cells revealed that other IFN-γ regulated genes, such as MHCII and IRF-1 can be induced in CF cells as well as normal cells. However, the evaluation of the signal transducers and activators of transcription (STAT) 1 expression and activation showed that STAT1 expression is 2-fold less than normal in CF cells 24 hours after IFN-γ treatment. Overall, we show increased viral replication in CF airway cells, and provide a mechanism for understanding the susceptibility of CF children to viral infection. Increased viral replication is due to lack of nitric oxide synthase 2 (NOS2) in CF airway epithelial cells; other antiviral host defense pathways are intact. Our results also suggest that CF cells have a defect in NOS2 intracellular signaling pathway through STAT1 activation/production and/or the production of soluble mediator. (Abstract shortened by UMI.)...