Cytokine (tumor necrosis factor, IL-6, and IL-8) production by respiratory syncytial virus-infected human alveolar macrophages

Human alveolar macrophages (AM) are susceptible to infection with respiratory syncytial virus (RSV), but the infection is abortive after the initial cycles of virus replication. We have investigated if RSV infection of AM results in the production of cytokines TNF, IL-6, and IL-8, all of which may modulate inflammatory and immune responses to the virus, as well as may directly protect respiratory epithelial cells against spread of infection. Within 1 h after interaction with RSV, increased mRNA levels were found for all three cytokines. Peak expression of the mRNAs occurred at 3 to 6 h. The virus most effectively induced TNF mRNA expression greater than IL-6 mRNA greater than IL-8 mRNA, as compared to cytokine mRNA expression induced by bacterial endotoxin. Inactivated virus was almost as effective as live virus in inducing and maintaining increased IL-6 and IL-8 mRNA over 16 h, whereas live infectious RSV was necessary for maintaining TNF mRNA expression over the same time. Protein concentrations of the different cytokines in the supernatants of infected AM reflected the increased levels of mRNA in the cells. Despite the high levels of cytokines with possible antiviral activity (TNF and IL-6) in the AM supernatants, neither supernatants nor rTNF when added to bronchial epithelial cells protected them from infection with RSV. However, TNF, IL-1, and RSV, but not IL-6, induced IL-8 and IL-6 mRNA expression by the bronchial epithelial cells suggesting that cytokines produced by RSV-infected AM may be more important in modulating the inflammatory response in infection than directly interfering with virus infection/replication of airway epithelium.