Purified IgGs from heat-inactivated mouse anti-Spike serum mediate ADE of infection of Raji cells by SARS-CoVpp. Serial dilutions of purified IgGs (solid bars) and protein G-sepharose column flow-through (hatched bars) from either heat-inactivated mouse anti-Spike (dark grey) or control (light grey) serum were incubated with SARS-CoVpp for 1 hour and opsonized particles were then used to infect Raji cells as described under Materials and methods. SARS-CoVpp infection was assessed three days post infection by expressing luciferase activity as Relative Luminescence Units (RLU), as described under Materials and methods. IgG concentrations were used to calculate the dilution factor and adjust the amount of the corresponding flow-through (FT, hatched bars) from the purification columns. Results are shown as means ± SEM of 6 measurements from two independent experiments. *P < 0.05 (by the unpaired Student’s t test).