KEAP1 and infection: To assess whether, HO-1 had any antiviral activity in our cellular system, Vero hTMPRSS2 and Calu-3 cells were either transfected with an overexpression plasmid encoding HO-1, or genetically silenced for KEAP1 and HO-1 by siRNA, respectively before infection with SARS-CoV-2.