Fig. 2: Live virus fusion assay of SARS-CoV-2 variants.

a Huh7.5-A2T2 cells expressing GFP were seeded in 96-well plates. 24 h postseeding, the cells were infected with SARS-CoV-2 variants and incubated for 2 h. The cells were washed after 2 h and incubated for another 22 h followed by 4% paraformaldehyde (PFA) fixation. After incubation, DAPI was used for nuclei staining. GFP and DAPI signals were evaluated using CellSens fluorescence microscope. b For quantification, 20 fields were randomly selected to measure the fusion activity (n = 20). Fusogenicity was quantified by the number of DAPI nuclei divided by the number of GFP blobs. For counting the number of GFP blobs, one continuous GFP blob was counted one fused or unfused cell. When there is discontinuous GFP signal intensity or discontinuous circular blob-shaped GFP signal in the blob, then they were counted separately to distinguish unfused neighboring cells from fused cells. The average numbers of DAPI nuclei and GFP blobs per location are followed; Wuhan (DAPI: 103.7, GFP: 19.4), B.1.1.7 (DAPI: 98.3, GFP: 12.0), B.1.351 (DAPI: 86.4, GFP: 14.3), B.1.617.2 (DAPI: 122.6, GFP: 11.0), BA.1 (DAPI: 68.6, GFP: 23.0), Mock (DAPI: 68.2, GFP: 29.0). For statistical comparison, adjusted P values are shown (vs Wuhan, *P < 0.05; **P < 0.01; ***P < 0.001; ****P < 0.0001; ns: not significant). All results are representative of three independent experiments. Scale bar 50 µm.