Translocation of influenza virus by migrating neutrophils

Influenza, a predominantly upper respiratory tract infection, replicates in the respiratory epithelia and spreads by an unknown mechanism to the regional lymph nodes. Neutrophils, which accumulate during the early stages of the infection, may be involved in this process. An in vitro model system was used to examine the effect of migrating neutrophils on the permeability of the infected epithelium and on the spread of virus. Epithelial cells (MDCK) infected with influenza virus (WSN H1N1) maintained a stable transepithelial electrical resistance (a measure of epithelial permeability) for 12 hrs. However, when neutrophils migrated across the epithelium toward the virus budding on the apical surface of the epithelium (6 hrs. after infection), the transepithelial electrical resistance fell 24% (P less than 0.001). Neutrophils adhered specifically to the virus and to hemagglutinin expressed exclusively on the apical surface of the cells and phagocytized the free virions. In response to a chemotactic gradient, the infected neutrophils were able to leave the lumenal surface of the infected epithelium, and were able to migrate across the epithelium in equal numbers and at the same rate as uninfected neutrophils. Migration across infected monolayers from the lumenal to the ablumenal surface also caused a fall in resistance (21%, P less than 0.01). Electron microscopic examination of emigrating neutrophils revealed that the leukocytes transported the influenza virions within phagocytic vacuoles and on their surface to the ablumenal side of the monolayer. The results of these studies suggest that the passage of leukocytes across influenza-infected epithelia increases the permeability of the epithelium and provides a route for viral spread.