Fig. 1

Cell-free and cell-to-cell transmission of SARS-CoV-2 in lung cells. a Schematic viral transmission assay. b–d Cell-free infection: SARS-CoV-2 was preincubated with neutralizing antibody (NAb) or control antibody (Ctrl Ab) and then used to infect hACE2-RFP-A549 cells (R) for 24 h. Coculture system: SARS-CoV-2-infected hACE2-A549 cells (MOI = 0.2, 24 hpi) were used as virus donor cells (D) and cocultured with hACE2-RFP-A549 cells with NAb or Ctrl Ab for 24 h. NSP3 expression was analyzed using IFA (b and c) and the infection of R cells was quantified with a high-content image analysis system (d). The supernatant was harvested for the virus infectivity assay (e). f mCherry-A549 cells (without exogenous hACE2 expression, exhACE2-) and hACE2-RFP-A549 cells (with exogenous hACE2 expression, exhACE2 +) were used as R cells for cell-free infection and cell-to-cell transmission assay. The infection of R cells was quantified with a high-content image analysis system at 24 hpi. Arrowhead, SARS-CoV-2-infected R cells; scale bar, 50 μm; Ctrl Ab groups (d and e) and exhACE2 + groups (f) were defined as 100%; all data indicated means with standard deviation (SD) (n = 3) of each group; ***p < 0.001, determined using two-tailed unpaired Student’s t test