FIG. 4.
Lung pathology and virus replication on day 4 p.i. Histopathology and immunohistochemistry analyses were performed 4 days after infection with pH1N1. Representative slides were selected for the protected animals from group 2 (immunized with sVacMF59 and pVacMF59). For intermediate protection, representative slides were chosen from group 1 (immunized with sVacMF59 and pVac) and represent group 1 and groups 3 to 6. The unprotected animals were from group 7; these were immunized twice with PBS and developed severe pneumonia upon challenge infection. Based on the histopathology of the alveoli (first row, H&E stained) there was only mild inflammation in the sVacMF59-pVacMF59-immunized animals (group 2) with few inflammatory cells. The inflammation was intermediate in animals of group 1 and groups 3 to 6, with thickening of the septa with moderate infiltration of neutrophils, pneumocyte type II hyperplasia, and mild necrosis, and a severe pneumonia was seen in the PBS control group with severe necrosis and intra-alveolar flooding with edema fluid and inflammatory cells. Based on immunohistochemistry of the alveoli (second row), there was no antigen expression in sVacMF59-pVacMF59-immunized animals, intermediate expression of predominantly pneumocytes type II in group 1 and groups 3 to 6, and abundant expression in the PBS control group. In the bronchioles (third row) there was a mild to moderate bronchiolitis with neutrophils and macrophages and necrosis in sVacMF59-pVacMF59-immunized animals and in the animals in group 1 and groups 3 to 6, and there was severe bronchiolitis with severe necrosis in the animals in group 7. Based upon immunohistochemistry (fourth row), there was no antigen expression in sVacMF59-pVacMF59-immunized animals, mild expression in bronchiolar epithelial cells in group 1 and groups 3 to 6, and moderate expression in the PBS control group. Magnifications: ×400, alveoli; ×200, bronchioles.