Abstract

Human influenza viruses attach to sialic acid with an α2,6linkage (SAα2,6Gal) on the airway epithelial cells, and the entry of the viruses into the cells and uncoating of the viruses require low pH of endosomes. Bafilomycin A₁, a macrolide antibiotic and a specific inhibitor of vacuolar H⁺-ATPase, inhibits growth of type A and type B human influenza viruses in Madin-Darby canine kidney cells. However, the inhibitory effects of clinically used macrolide antibiotics on influenza virus infection in human airways have not been studied. To examine the effects of clarithromycin on seasonal human influenza virus infection, cultured human tracheal epithelial cells were infected with type A influenza virus (H₃N₂). Influenza virus infection increased viral titers and the content of cytokines, including interleukin (IL)-1β and IL-6, in supernatant fluids, and viral RNA in the cells. Clarithromycin reduced viral titers and the content of cytokines in supernatant fluids, viral RNA in the cells, and the susceptibility to virus infection. Clarithromycin reduced the expression of SAα2,6Gal, a receptor for human influenza virus, on the mucosal surface of human tracheae, and the number and fluorescence intensity of acidic endosomes in the cells from which viral ribonucleoproteins enter into the cytoplasm. Furthermore, clarithromycin reduced nuclear factor-κB (NF-κB) proteins, including p50 and p65, in the nuclear extracts. These results suggest that clarithromycin may inhibit seasonal human influenza virus infection by reducing SAα2,6Gal partly through the inhibition of NF-κB, and increasing pH in endosomes in airway epithelial cells. Clarithromycin may modulate airway inflammation in influenza virus infection.