Fig 4.

Modulation of viral growth and pathogenicity by the mutations downstream of S gene. (A) Scheme for the chimeric recombinant SARS-CoV-2 used in this study. The SARS-CoV-2 genome and its genes are shown. The template was SARS-CoV-2 strain WK-521 (PANGO lineage A, GISAID ID: EPI_ISL_408667) (22). A recombinant virus bearing S:D614G mutation (rB.1.1) was used in our previous study (3). (B and C) Animal experiment. Syrian hamsters (n = 4 per group) were intranasally inoculated with rB.1.1, rBA.2up, and rBA.2down (10,000 TCID₅₀ in 100 µL per animal). Hamsters of the same age were intranasally inoculated with 100 µL of saline (uninfected). (B) Body weight change of infected hamsters. (C) Viral RNA loads in the oral swabs of infected hamsters at 1, 3, and 5 d.p.i. (D–I) Viral growth assay. rB.1.1 (black), rBA.2down (red), rBA.2up (blue), or the rB.1.1 derivatives bearing the mutation indicated in the figure were inoculated into Vero cells (D, F, and H; m.o.i. = 0.1) and VeroE6/TMPRSS2 cells (E, G, and I; m.o.i. = 0.01). The copy numbers of viral RNA in the culture supernatant were routinely quantified by RT–qPCR. The presented data are expressed as the average ± SEM. In panels (D–I), assays were performed in quadruplicate, and the dashed black and red lines are the results of rB.1.1 and rBA.2 bottom, respectively. Statistically significant differences versus rB.1.1 across timepoints were determined by multiple regression. The FWERs calculated using the Holm method are indicated in the figures.