Fig. 7.

Antibody-mediated infection is independent of acidic pH and cysteine-protease activity of the endosomal/lysosomal compartment. (A, C, and D) pH and protease requirements for ACE2- versus antibody-mediated entry of SARS-CoVpp were investigated with different pharmacological treatments. Prior to infection, VeroE6 and Raji cells were preincubated with the indicated concentrations of ammonium chloride (NH₄Cl) (A) for 1 h or the cysteine protease inhibitor E-64d (C) and cathepsin L inhibitor III (D) for 3 h. SARS-CoVpp, with or without anti-Spike serum (1/2,000), were then added to the cells in the continuous presence of the drugs. At 3 days postinfection, luciferase substrate reagent was added to wells, and the luminescence was measured. The data were normalized to the appropriate control conditions (taken as unity) with SARS-CoVpp with or without anti-Spike serum for Raji and VeroE6 cells, respectively. The results are shown as means ± the SD of nine measurements from three independent experiments. Statistical differences between VeroE6 and Raji cells were assessed by the unpaired Student t test. **, P < 0.005. (B) Cell surface expression of FcγRII on Raji cells after NH₄Cl treatment. Cells were incubated with the indicated concentrations of the drug for 1, 3, and 5 h, labeled with anti-huFcγRII antibody, and subjected to flow cytometry. The data were normalized to the mean fluorescence intensity of controls (no drug). The results are shown as means ± the SD of six measurements from two independent experiments. When not visible, the SD values were contained within the size of the symbols.