Fig 1: U87 cells have no detectable endogenous ACE2 expression but high levels of Cathepsin B. A) Different cell lines were analyzed for ACE2 expression by immunoblotting, with ß-actin as loading control. B) Same as in (A) but with the stably ACE2 transduced U87 cells, tested at an early (# 1) or late (# 32) passage of the cells. The transduced U87 cells stably express ACE2 at a very high level as compared to endogenous ACE2 in the Vero E6 cells. The same immunoblot is used for panel A and B, but with a longer exposure time for panel A in order to visualize the faint band in the Calu-3 sample. C) Comparative qPCR analysis of TMPRSS2 mRNA levels in different cells. Graph shows individual copy numbers/µl of 3 technical replicates (n = 3) as calculated from a TMPRSS2 standard. The TMPRSS2 level in Vero E6 cells was below detection limit. D) Different cell lines were analyzed for Cathepsin L (CTSL; left) and Cathepsin B (CTSB; right) expression by immunoblotting, with clathrin as loading control. For CTSL, different species are detected (indicated by the bracket): Pro-CTSL (41 kDa), glycosylated mature CTSL (31 kDa) and non-glycosylated mature CTSL (25 kDa), whereas only the Pro-CTSB form (42 kDa) is visualized. M: molecular marker in kDa, UD: undetectable.