FIG 2.
Inflammation of the SARS-CoV-2-infected hamster lung. Male 5- to 6-week-old golden Syrian hamsters were intratracheally infected with SARS-CoV-2 isolate USA-WA-1/2020 or mock challenged with vehicle (PBS), and lungs were excised at days 2, 4, and 6 postinfection. (A and B) Hematoxylin and eosin staining of lung sections on days 2 and 6 postinfection, respectively (A, 100×, scale bar is 100 mm; B, 200×, scale bar is 50 mm). (C) Histology with low magnification shows the mixture of dilated air space in alveoli (right side) and solid part (left side) filled with mononuclear inflammatory cells in intra-alveolar space, alveolar septa, and peribronchial and perivascular areas. Day 2 postinfection, H&E, 40×. (D) Pulmonary edema and pneumocyte desquamation with inflammatory cells infiltration were commonly observed. Dilated alveoli, perivasculitis, and thickening of alveolar septa with inflammatory cells infiltration were also presented. Some cases showed the mononuclear cell aggregation and adherence to endothelial cells in the vessels. Day 4 postinfection, H&E, 100×. (E) Different degrees of congestion were commonly observed, and intra-alveolar hemorrhage was often associated in the area where prominent congestion occurred. Bronchial epithelial hyperplasia, perivascular inflammation and edema, organized pneumonia-like lesion with inflammatory cells, and giant cells were also presented. Day 6 postinfection, H&E, 100×. (F) Reactive mesothelial hyperplasia with inflammatory cell infiltration and subpleural edema were observed in the advanced stages. Alveolar space and septa were filled by inflammatory cells and giant cells with interstitial thickening, which resembles organized pneumonia. Day 6 postinfection, H&E, 100×. (G) Inflammatory cytokine expression. ELISA determined the concentration of IFN-γ, TNF-α, and interleukin-6 (IL-6) in the BALF or serum of uninfected controls or hamsters infected i.t. with 9 × 105 PFU SARS-CoV-2 for 2 or 4 days. Data were evaluated by Kruskal-Wallis test with Dunn’s multiple-comparison posttest. Asterisks denote the level of significance observed, as follows: **, P ≤ 0.005; ****, P ≤ 0.00005. n= 6 hamsters per time point.