TMPRSS2 and infection: Then, 24 h after transfection, HEK293 ACE2 cells were detached with trypsin and seeded onto a monolayer of Vero E6 cells overexpressing transmembrane serine protease 2 (Vero E6 TMPRSS2), which are highly susceptible and permissive to infection, serving as a strategy to enhance the isolation of rSARS-CoV-2 [50,51,52].