SARS-CoV Pathogenesis Is Regulated by a STAT1 Dependent but a Type I, II and III Interferon Receptor Independent Mechanism

Frieman, Matthew B.; Chen, Jun; Morrison, Thomas E.; Whitmore, Alan; Funkhouser, William; Ward, Jerrold M.; Lamirande, Elaine W.; Roberts, Anjeanette; Heise, Mark; Subbarao, Kanta; Baric, Ralph S.

We determined the localization of virus in infected lungs during the course of infection by in situ hybridization. We used a riboprobe complementary to the SARS-CoV nucleocapsid RNA that was labeled with radioactive nucleotides to visualize viral RNA in the tissue. In situ hybridization was performed on lungs from 129 WT, IFNAR12/2, IFNGR2/2 and STAT12/2 mice harvested at days 2, 5, and 9 post-infection. In all 4 strains of mice, the probe signal was predominantly localized in the airway epithelial cells at day 2 post-infection ( Figure 3A ). This correlated with the pathologic lesions in airway epithelial cells at day 2 postinfection. By day 5 post-infection, viral RNA was virtually eliminated from the control 129, and IFNAR12/2 mice and to a slightly lesser extent in IFNGR2/2 mice. Occasionally, a few cells with viral RNA were noted in the periphery, consistent with the low titers of virus in these animals at day 5. By day 9, there was no viral RNA signal found in the lungs of 129 WT, IFNAR12/2 and IFNGR2/2 mice, corresponding to the lack of viral replication at this time point. In contrast, lungs of STAT12/2 mice showed significant viral RNA signal throughout the lungs, including prominent distribution into the periphery of the lung at early and late times post-infection. Surprisingly, but consistent with the findings on histopathology, viral RNA signal was lost from the airway epithelial cells in the bronchioles by day 9 in STAT12/2 mice and was found throughout the periphery of the lungs, prominently focused in large focal compactions of cells. These foci correspond to the prominent focal lesions noted in H&E stained sections of the lung, which are predominantly composed of fibroblasts, macrophages and lymphocytes ( Figure 3B ). Interestingly, cellular debris can be seen in these foci that may represent lysed airway epithelial cells.