Fig. 4: SARS-CoV-2 BA.5 acquired fusogenicity during chronic persistent infection.

Cytopathogenic effects (CPEs) in VeroE6/TMPRSS2 caused by various isolates (CH-LT1, CH-LT1m, CH-LT2m, CH-LT3m, CH-LT4, CH-LT6, and CH-LT7) are shown. The isolates obtained on earlier days (from CH-LT1 to CH-LT3m) caused similar cell-rounding CPEs (a–d). Remarkably, the viral properties of CH-LT4 obtained on Day 62 appeared to change drastically and become more fusogenic (e). In addition to cell-rounding, large clumps of cells that merged with the adjacent cells were observed frequently. This fusogenic nature was maintained in the later isolates (CH-LT6 and CH-LT7, Days 78 and 83, respectively) (f, g). We performed three independent experiments, and obtained basically the similar results. Cell images were captured using a ZEISS Prim overt inverted microscope equipped with an Axiocam 208 colour camera at 3 d.p.i. Scale bars denote 100 μm. a: CH-LT1, isolated on Day 22, b: CH-LT1m, another variant with a V792I mutant in nsp12, isolated on Day 22, c: CH-LT2m, a variant with a C799F mutant in nsp12, isolated on Day 29, d: CH-LT3m, a variant with a M794I mutant in nsp12, isolated on Day 36, e: CH-LT4, a variant with C799F mutant in nsp12, isolated on Day 62, f, g: CH-LT6 and 7, variants with C799F mutant in nsp12, isolated on Day 78 and 83, respectively, h: control.