We sought to understand the intracellular dynamics of SARS-CoV-2 infection in vitro to determine the growth rate of SARS-CoV-2 and dynamics of antibody neutralisation.
VeroE6-TMPRSS2 cells were inoculated with SARS-CoV-2 (Ancestral variant, MOI 0.001) for 1 hour. To block virus growth, cells were treated with a cocktail of two neutralising SARS-CoV-2 monoclonal antibodies (mAbs 222 and 96) or control mAb (HIV-specific PGT121) at 10µg/ml applied 1 hour before infection or 1-, 6-, 12- or 24-hours post-infection (hpi). To model neutralisation dynamics, cells were treated with three-fold serial dilutions (10µg/ml to 0.04µg/ml) neutralising mAb cocktail or control mAb 1 hour before or after infection. Virus release was quantified by qPCR and infectivity assay; live and infected cells were quantified by flow cytometry at 6, 12, 24, 48 and 72hpi.
Infectious virus and viral RNA were first detected at 12hpi in control-treated cells, peaking at 48 hpi. Cell viability remained unchanged from control (uninfected) wells until 48 and 72hpi where we found 45% and 98% reductions in viability. Infected cells rose from 0.3% at 1 and 6hpi, to 4.2% at 12hpi, 42.6% at 24hpi and 75.2% at 48hpi. Antibody neutralisation was effective when 0.37 and 1.11µg/ml of the mAb cocktail was applied 1hpi, reducing percent of infected cells at 48hpi from 79.4% to 9.8% and 9.4%, respectively. In contrast, the same concentrations applied before inoculation and removed after the first hour did not reduce percent of infected cells at 48hpi.
SARS-CoV-2 exhibits rapid growth in VeroE6-TMPRSS2 cells, with infectious virus and RNA release detected within 12hpi, peaking at 48hpi. Pretreatment and removal of neutralising mAb with the same concentration did not reduce infected cells at 48hpi, whereas addition of mAb at 1hpi did. This supports a role for mAbs in blocking in vitro viral growth rather than neutralising the inoculum.