The Middle East respiratory syndrome coronavirus (MERS-CoV) utilizes host
proteases for virus entry into lung cells. In the current study, Vero cells constitutively expressing type II transmembrane
serine protease (Vero-TMPRSS2 cells) showed larger syncytia at 18 h after
infection with MERS-CoV than after
infection with other coronaviruses. Furthermore, the susceptibility of Vero-TMPRSS2 cells to MERS-CoV was 100-fold higher than that of non-TMPRSS2-expressing parental Vero cells. The
serine protease inhibitor camostat, which inhibits TMPRSS2 activity, completely blocked syncytium formation but only partially blocked virus entry into Vero-TMPRSS2 cells. Importantly, the coronavirus is thought to enter cells via two distinct pathways, one mediated by TMPRSS2 at the cell surface and the other mediated by
cathepsin L in the endosome. Simultaneous treatment with inhibitors of
cathepsin L and TMPRSS2 completely blocked virus entry into Vero-TMPRSS2 cells, indicating that MERS-CoV employs both the cell surface and the endosomal pathway to infect Vero-TMPRSS2 cells. In contrast, a single
camostat treatment suppressed MERS-CoV entry into human bronchial submucosal gland-derived Calu-3 cells by 10-fold and virus growth by 270-fold, although treatment with both
camostat and (23,25)-trans-epoxysuccinyl-L-leucylamindo-3-methylbutane ethyl
ester, a
cathepsin inhibitor, or treatment with
leupeptin, an inhibitor of
cysteine,
serine, and
threonine peptidases, was no more efficacious than treatment with
camostat alone. Further, these inhibitors were not efficacious against MERS-CoV
infection of MRC-5 and WI-38 cells, which were derived from lung, but these characters differed from those of mature pneumocytes. These results suggest that a single treatment with
camostat is sufficient to block MERS-CoV entry into a well-differentiated lung-derived cell line.