Neutrophil response in the upper and lower airways following influenza virus infection of mice. Groups of 5 mice were infected with 105 PFU of HKx31via the intranasal route. At the times indicated, mice were euthanized and neutrophils in the BAL (upper panels) and nasal tissues (lower panels) were identified as Gr-1high cells by flow cytometry and quantitated. (A) Representative dot plots of Gr-1high cells recovered from BAL and nasal tissues 3 days after infection with HKx31. Graphs show Gr-1 staining of CD45+ leukocytes and the gate used to define Gr-1high neutrophils. (B) The number of neutrophils in BAL and nasal tissues of naïve mice or mice at various times post-infection. A minimum of 50,000 living cells (PI-) were collected and analyzed from each mouse. (C) Viral loads in the lung (upper panel) and nasal tissues (lower panel) were determined by plaquing homogenates on MDCK monolayers. Data in panels (B) and (C) show the mean ± 1 SD (n = 5 mice/group). In panel (C), the dotted line represents the detection limit for the plaque assay. The data are representative of 2 or more independent experiments.