Generation of SARS-CoV-2 pseudotyped variants
rVSV-ΔG-G* (G*: VSV glycoprotein) and VSV-ΔG-Spike (carrying Spike protein of coronavirus instead of VSV glycoprotein) virus were produced according to the protocol provided by Whitt in 2010 (Whitt, 2010). BHK-21/WI-2 cells (Baby Hamster Kidney-21/ Clone WI-2, Kerafast, EH1011) were infected with vaccinia virus (ATCC, strain vTF7-3, VR-2153TM) for 1 h, followed by co-transfection of five plasmids: VSV-ΔG, together with VSV accessory plasmids encoding for VSV-N, P, L, and G proteins (Kerafast), all of which were under T7 promoter control. The primary transfection was performed using Lipofectamine 2000. BHK-21/WI-2 cells were transfected with Spike plasmid to assist in creating passage 1 (P1). Forty-eight hours following primary transfection, supernatant containing the recovered VSV-Spike was collected, centrifuged at 1300 × g × 5 min to remove cell debris. Total supernatant was collected, centrifuged, and used for sequential passaging in Vero E6 cells to eliminate parental virus (Yahalom-Ronen et al., 2020), as shown in Figure 1 . VSV-ΔG-Spike was propagated in DMEM containing 5% FBS (reduced serum media), MEM NEAA, 2mM L-glutamine, and 1% P/S.