Flow cytometry analysis was also performed to detect the binding between SARS-CoV-2 RBD and hACE2, or bat-ACE2 (bACE2), receptor in transiently transfected 293T cells. Briefly, 293T cells were transfected with hACE2 or bACE2 plasmid using the calcium phosphate method, and 48 h later, they were incubated with SARS-CoV-2 RBD protein at various concentrations for 30 min at room temperature. SARS-CoV and MERS-CoV RBDs were included as controls. After staining with FITC-conjugated goat anti-human IgG antibody (1:500; Thermo Fisher Scientific), the mixture was analyzed by flow cytometry as described above.