SARS-CoV-2 N protein causes less lung inflammation in mice than SARS-CoV N protein during viral infection. (A) Schematic presentation of assessment of N variants function. RDPs were inoculated into K18-hACE2 KI mice expressing N variants via lentiviral transduction. Lung tissue and alveolar lavage fluid were collected 6 days after infection. (B) RDPs infected the lungs of K18-hACE2 KI mice expressing N variants for 6 days, the GFP expression analyzed by microscopy. Scale bars, 100 μm. (C–E) RDPs infected the lungs of K18-hACE2 KI mice expressing N variants for 6 days; the total RNA extracted from the lung tissue was evaluated by RT-qPCR, n = 6. (F) RDPs infected the lungs of K18-hACE2 KI mice expressing N variants for 6 days; Tnfα, Il-6, and Cxcl10 levels in the alveolar lavage fluid were evaluated by ELISA, n = 6. (G) RDPs infected the lungs of K18-hACE2 KI mice expressing N variants for 6 days. Histological abnormalities of the lung in mice were shown by HE staining. The arrows showed thickened alveolar walls, cellular infiltration, or exudate accumulation. Scale bar, 1,000 and 100 μm. (H) RDPs infected the lungs of K18-hACE2 KI mice expressing N variants for 6 days, Histological abnormalities of the lung in mice were shown by HE staining. The mice lungs were scored, n = 6. Graphs show mean ± SEM (n = 6 in C–F). ***P < 0.001, ****P < 0.0001, ns, not significant (unpaired, two-tailed Student’s t test).