Fig 1.

Virological and histopathological characterization of SARS-CoV-2 lung infection time course. (a) SARS-CoV-2 kinetics in hamster lungs. Viral RNA copy number (Log₁₀) (black, left axis) and replication-competent virus (Log₁₀ TCID₅₀/mL; magenta, right axis) detected in the lung samples. Each point represents the data from approximately 1.5 mm³ sample from the right cranial lobe of one animal. The dotted line indicates the detection limit for replication-competent virus. (b) Immunohistochemical analysis of SARS-CoV-2 NP antigen distribution in lung tissue. Viral antigen score in main bronchi, distal bronchi, and alveolar epithelial cells (AEC), control or 1–14 days post-infection (dpi). Dots, individual animals; bar, median. Brown—NP. Scores correspond to 0 = no antigen, 1 = rare, <5% antigen labeling per slide; 2 = multifocal, 6%–40%; 3 = coalescing, 41%–80%; 4 = diffuse, >80%. (c) 1—Viral antigen distribution at 1 dpi showing strong labeling in bronchi (green asterisk) and AEC. Bar 100 µm. 2 —Viral antigen distribution at 5 dpi, showing labeling restricted to AEC (green arrow), no labeling in bronchi (green asterisk). Bar 100 µm. 3—Viral antigen in cells morphologically consistent with ciliated bronchial epithelium at 1 dpi. Bar 25 µm. 4—Viral antigen in cells morphologically consistent with type 2 pneumocytes (green arrow) at 1 dpi. Bar 25 µm. 5—Target cell, viral antigen in cells morphologically consistent with type 1 pneumocytes (green arrow), also intravascular cells (blue arrow) are labeled, interpreted to be monocytes/macrophages taking up viral antigen at 1 dpi. Bar 25 µm. 6—Viral antigen clearing, scattered between proliferating pneumocytes and infiltrating immune cells, and the antigen is found at 7 dpi. Bar 50 µm. n = 3 animals per time point.