4. Conclusions
Permissiveness for infection with SARS-CoV-2 was confirmed for Vero
cells, but was absent for CHO, HT-1080 and MRC-5 cells. HEK-293 cells
did not show productive amplification of SARS-CoV-2 although some
minimal and self-limiting replication may be possible. This study also
showed that in CHO cells, HT-1080 cells and MRC-5 cells, no “silent”
infection of SARS-CoV-2 did occur, i.e. virus replication but no
development of detectable CPE, as a “silent” virus replication would
have been detected by the titration of supernatants on Vero cells. In
routine AAT assays, SARS-CoV-2 was confidently detected by CPE in Vero
cells and HAD with human erythrocytes, i.e. two different readouts that
revealed the presence of SARS-CoV-2, which underlines its reliable
detection by AAT if it were to occur as a contaminant in a
biomanufacturing cell culture process.
The detection of SARS-CoV-2 in AAT when Vero cells are part of the
indicator cell line panel was reassuring, even in the hypothetical case
that SARS-CoV-2 would replicate in the production cell line but would
not produce a CPE and not be detected by cell parameter screening, for
example by measurement of cell viability, oxygen consumption etc.
Furthermore, SARS-CoV-2 would represent a contaminant for which highly
effective clearance capacities can be expected from widely used
dedicated down-stream virus reduction steps: for example, as a
lipid-enveloped virus SARS-CoV-2 would be robustly inactivated by
solvent/detergent (S/D) treatment (Dichtelmueller et al., 2009) and due
to its large size of about 120 nm SARS-CoV-2 can be expected to be
completely retained by nanofiltration, even when using larger pore sizes
such as 35 nm (Burnouf, Radosevich, Goubran, & Willkommen, 2005).