Figure 1.

SARS-CoV-2 B.1.1.7 virus-infected wild-type C57BL/6N mice and replicated effectively in the upper and lower respiratory tissues of aged mice. Mice were grouped according to their age and inoculated with 10³PFUs of B.1.1.7 virus via the intranasal route. Body weight and signs of disease were monitored for 14 days after virus infection. (A) Body weight changes in young and aged mice. (B) Clinical scores of disease signs after virus infection. During daily monitoring of the infected mice, one score was given to each disease sign, including ruffled fur, hunched posture and laboured breathing. Highest total score = 3 per mouse. Data represent mean ± SD. n = 6 for each group. ****p < 0.0001 by Student’s t-test. (C and D) Real-time RT-PCR determined viral RdRp gene copies in the nasal turbinate (NT) (C) and lung tissues (D) of infected mice at 2 or 4 days post-virus infection (dpi). Data presented as copies of RdRp gene per copy of β-actin in log scale. Horizontal dashed lines indicate the detection limit of the assay. (E) Infectious virus titre in the lung tissues determined by 50% tissue culture infection dose (TCID₅₀) assay on Vero E6 cells. Data represent mean ± SD. n = 6 for each group. *P < 0.05, **P < 0.01, ***P < 0.001, ****P < 0.0001 by two-way ANOVA. (F and G) Representative images of immunofluorescence staining of SARS-CoV-2 nucleocapsid protein (NP) in nasal turbinate (NT) and lung tissues of young and aged mice at 2 dpi (F), and 4 dpi (G). SARS-CoV-2 NP was stained green and indicated with white arrows. Cell nuclei were stained blue by 4’, 6-diamidino-2-phenylindole (DAPI). Scale bars = 100 µm.