Virus-infected or mock-treated mice were sacrificed at the indicated time point for histopathologic analysis. The lung tissues were fixed in 10% formalin and embedded in paraffin. Fivemicrometer-thick, paraffin-embedded sections were cut and stained with hematoxylin and eosin (H&E) to analyze histological lesions. Histopathologic score of lung tissues was examined by a board-certified pathologist blinded to the exposure status. Lung inflammatory changes were graded using a semiquantitative scoring system based on the following parameters: peribronchiolar and bronchial infiltrates, bronchiolar and bronchial luminal exudates, perivascular infiltrates, parenchymal pneumonia, and edema, as previously described [28] . Each parameter was graded on a scale of 0-4 with 0 as absent, 1 as slight, 2 as mild, 3 as moderate, and 4 as severe. The total lung inflammation score was expressed as the sum of the scores for each parameter. The degree of cell infiltration was independently scored on an increasing scale of 0-3 with 0 as no cells, 1 as few cells, 2 as moderate influx of cells, and 3 as extensive influx of cells [29] .