Fig. 1: Mouse-adapted SARS-CoV-2 induces lethal disease.

a Illustration of mouse adaptation process with SARS-CoV-2 Alpha variant by serial passages (pass.) to obtain SARS-CoV-2 MA20. Severity score (b), weight loss curves (c, left) and survival curves (c, right) of 2 m BALB/c, 2 m C57BL/6 and 8 m C57BL/6 mice infected with indicated viral titres of MA20(n = 4 or 6). d Persistent (light orange) and acquired (dark orange) mutations in SARS-CoV-2 isolates during mouse adaptation as compared to SARS-CoV-2 Wuhan strain. e Location of mutations within genes of SARS-CoV-2 MA20, acquired mutations depicted in bold. f (a–d) Depiction of predicted interactions with H-bonds (black) and CH-π interactions (magenta) of Alpha Spike RBD (cyan) in complex with hACE2 (green) (a), MA20 Spike RBD (cyan) in complex with hACE2 (green) (b), Alpha Spike RBD (cyan) in complex with mACE2 (green) (c), MA20 Spike RBD (cyan) in complex with mACE2 (green) (d). Only interactions up to 3.5 Å are shown. For H-bonds indicated distances correspond to distance between the corresponding hydrogen atom and acceptor oxygen atoms of either D38 or E35. g ELISA-style binding assay of the RBD of Spike proteins from Alpha (blue) and MA20 (orange) with mACE2 (n = 3). h Experimental design for sample collection after infection with MA20. Viral titres of infected 2 m BALB/c (i), 2 m C57BL/6 and 8 m C57BL/6 (j) mice (n = 4 to 6). p values were determined by One-way ANOVA with post-hoc Tukey.*p < 0.0332, **p < 0.0021, ***p < 0.0002, ****p < 0.0001. IHC staining of lung sections of SARS-CoV-2 Spike protein of infected 2 m BALB/c (k), 2 m C57BL/6 and 8 m C57BL/6 (l) mice. Values represent mean ± SEM. Scale bars indicate 500 µm. 2 m:2-month-old. 8 m: 8-month-old.