Virus Infection and Determination of Fluorescence Formation Units (FFU) and Tissue Culture Infectious Dose 50 (TCID50)

For virus infection, the required dose of the virus was diluted in the medium used to culture the various types of cells and incubated with the cells at 37°C for 1 h. The cells were then washed twice with PBS and maintained in the corresponding medium supplemented with or without 2% FBS and 1% penicillin-streptomycin. The cell culture supernatants were collected and titrated to determine the TCID₅₀ on DEFs for DTMUV, DPV, and DPV-GFP, according to our previously described standardized methods (He et al., 2018; Chen et al., 2019; Li et al., 2019), or FFU on BHK21 for VSV-GFP. All viruses were handled under biosafety level 2 laboratory conditions. For GFP expression observation and FFU determination, a fluorescence microscope was used (excitation wavelength was 460–550 nm, IX71, Olympus Corporation, Tokyo, Japan).