FIGURE 1.

A sensitive split luciferase assay enables the measurement of cell fusion. A, HEK239T were transfected with different amounts of spike and cLuc:N7 or ACE2 and nLuc:N8 plasmids. After 24 hours, the cells were mixed and seeded. After 3 hours, cell fusion was detected by measuring luciferase activity. B, Instead of split luciferase, EGFP and iRFP were transfected along with spike and ACE2 plasmids. Formation of syncytia was observed. C, HEK239T were transfected with spike and cLuc:N7 or ACE2 (and/or TMPRSS2) and nLuc:N8 plasmids. Camostat mesylate was added to ACE2‐expressing cells prior to mixing with spike‐expressing cells. After 3 hours, cell fusion was detected by measuring luciferase activity. Combined means from three (A and C) independent experiments are shown as mean ± SEM. Data from (B) are a representative of three independent experiments