Fig. 4.

Flow cytometric analysis of cell surface S glycoprotein. COS-7 cells transiently expressing the wild-type and HTVR mutant S glycoproteins were stained using either (top) MAb F26G18 (Berry et al., 2004) and a secondary fluorescein isothiocyanate (FITC)-conjugated anti-mouse antibody or (bottom) a recombinant soluble ACE2 receptor, biotinylated goat anti-ACE2 antibody and FITC-conjugated streptavidin. Cells were analyzed using a FACSCalibur flow cytometer (BD Biosciences), and propidium-iodide-staining (dead) cells were excluded. Background staining of mock-transfected cells is shown in the left panel; non-expressing cells are also evident in the transfected cell populations. Expressing cells were defined using a gate of ≥30 (<0.1% of the mock-transfected population).