CL-14

ACE2/TMPRSS2 Expression is Insufficient to Predict SARS-CoV-2 Permissiveness

To systematically identify robust cellular models for SARS-CoV-2 infection, researchers screened the Cancer Cell Line Encyclopedia (CCLE) RNA-seq database for expression of the receptor ACE2and priming protease TMPRSS2, followed by rigorous experimental validation of viral permissiveness in vitro. Though these two factors are canonical mediators of viral entry, their sufficiency and the possible involvement of alternative receptors are still discussed. In order to better understand the correlation between receptor expression and actual infection outcome, the study defined permissiveness as the ability of a host cell to support the complete viral replication cycle and the release of mature virions.

Experimental infection with wild-type SARS-CoV-2 across the screened panel showed remarkable discordance between receptor expression and viral replication. Unexpectedly, most cell lines (15 of 29) failed to support detectable virus replication in spite of expressing both ACE2and TMPRSS2 (Fig. 1A). An additional 11 lines gave rise to infectious titers which never exceeded the input inoculum. Only 3 cell lines, i.e. the colon carcinoma lines CL-14 and CL-40 and the breast carcinoma line CAL-51, were found to be highly permissive with an increase of 2 to 500-fold in the initial viral load at 1-4 days post infection (dpi). The infectious titers obtained in CL-14 and CAL-51 were similar to those in gold-standard cell lines including the lung carcinoma CALU-3, colon carcinoma CACO-2 and African green monkey kidney VERO cells (Fig. 1B). Interestingly, CAL-51 and CACO-2 showed a steady increase of viral titers until day three while VERO cells and CL-14 peaked within 24 hours. The newly identified CL-40 line, expressing high levels of NRP1 in addition to ACE2and TMPRSS2 showed titers close to the inoculum level despite clear replication. These results underscore that ACE2and TMPRSS2expression alone is not sufficient for productive infection and reveal novel cell-type-specific replication kinetics in cancer cell lines.