Histopathological changes. (A) A lymphoid aggregate adjacent to a bronchiole, in lung tissue from an animal from the virus-infected control group at 11 weeks of age. (B) An eosinophil within the tracheal epithelium (arrow) from a mouse from the virus/OVA group. (C) Reticulin-stained trachea from a naïve animal demonstrating normal thickness of subepithelial collagenous zone (black arrow) and epithelial layer (red arrow). (D) Trachea from a mouse infected with PVM at birth, then sensitised to OVA and challenged (virus/OVA group) demonstrating significant thickening of both the subepithelial collagenous zone (black arrows) and the epithelium (red arrow). (E) Left main bronchus from a naïve animal stained with PAS, showing that no mucin-producing cells are present (grade 0). (F) Left main bronchus from a mouse from the virus/OVA group, demonstrating numerous PAS-positive goblet cells in the epithelium (arrows) (grade 4). Scale bar = 150 μm in A, 25 μm in B and 50 μm in C-F.