Induction and regulation of nitric oxide synthase in airway epithelial cells by respiratory syncytial virus

In this study, we evaluated the effects of respiratory syncytial virus (RSV) infection on nitric oxide (NO) production in human airway epithelial cells. In addition, we evaluated whether T-helper type 1 (Th1)- and Th2-type cytokines modulate the release of NO in response to RSV infection. To do this, we infected monolayers of A549 cells with RSV and determined nitrite levels in the supernatant fluids. We also measured nitrite levels in human small-airway epithelial cells (SAEC) in primary culture and in the bronchoalveolar lavage fluid (BALF) obtained from Balb/c mice after RSV infection. To further support our observations in these analyses, we performed immunocytochemistry and Western blot analysis for inducible nitric oxide synthase (iNOS) in A549 cells. To evaluate the regulation of NO production in response to RSV, we performed experiments in the absence and presence of the Th1 and Th2 type cytokines: interferon (IFN)-gamma, interleukin (IL)-4, and IL-13. In addition, we assessed the inhibitory effect of dexamethasone on iNOS in RSV infected A549 cells. Results were expressed in terms of nmol/mg protein and shown as percents of control values (mean +/- SE). RSV increased the release of nitrites in A549 cells, SAEC, and BALE. The increase in nitrite levels was supported by immunocytochemistry and Western blot analysis for iNOS protein in A549 cells, indicating activation of iNOS in response to RSV infection. IFN-gamma and IL-13 did not affect the RSV-induced increase in NO production. By contrast, IL-4 and dexamethasone suppressed the release of NO in response to RSV infection. These observations show that RSV infection leads to activation of iNOS within the airway epithelium and that IL-4 and dexamethasone inhibit the production of NO in response to RSV infection.