Lung Histopathology, RSV antigenic distribution and Correlations with blood RSV RNA loads. Fig 6A [1–8]: From days 1 to 7 post-inoculation, changes included perivascular edema and margination of neutrophils (day 1), which progressed to a sparse perivascular infiltrate and later to dense perivascular and peribronchial/peribronchiolar inflammatory infiltrates composed of lymphocytes, macrophages and scattered neutrophils. At its peak (days 5 to 7), these inflammatory infiltrates extended into surrounding alveolar septa in a stellate manner with patchy involvement of the parenchyma and abundant macrophages, occasional lymphocytes and neutrophils in alveolar spaces. On days 10 and 14 the persistent changes included inflammatory infiltrates located around vessels and airways, consisting of lymphocytes and macrophages, but no neutrophils or eosinophils. Fig 6A. [A-H]. By immunohistochemical staining (IHC) the alveolar epithelium showed a diffuse faint linear staining on day 1, was stronger and widespread on days 3-4, and decreased by days 6-7. In contrast, alveolar macrophages' IHC was negative on day 1, occasionally positive on day 3, strong and diffusely stained on days 4 to 6 and decreased by day 7. Both the alveolar epithelium and alveolar macrophages were negative for RSV IHC on days 10 and 14 post-infection. Fig 6B. Histopathologic scores significantly increased in mice infected with live-RSV from day 1 (HPS mean ± SD; 5.7 ± 0.5) to day 5 (HPS 11.2 ± 2.1; p = 0.005), day 6 (HPS = 13.6 ± 2.3; p = 0.001) and day 7 (HPS = 10.7 ± 3.3; p = 0.02). There was no correlation between the peak of blood RSV RNA loads and lung inflammation.