CTSS and infection: Calu-3 are human lung epithelial cells that endogenously express both the ACE2 receptor and the host serine protease TMPRSS2, which is required for SARS-CoV-2 Spike protein processing and viral entry into host cells19, while the robust infection in HeLa-ACE2 cells, which lack TMPRSS2 expression, is likely dependent on endosomal, cathepsin-mediated viral entry pathway that has been a generally recognized mechanism for coronaviruses20.