CTSL and infection: To evaluate the effects of the endosomal pathway on cell-to-cell and non-cell-to-cell infection with PDCoV, a panel of endosomal protease inhibitors, including the pan-spectrum protease inhibitor leupeptin, the lysosomal acidification inhibitor Baf-A1, the pan-cysteine cathepsin inhibitor E64d, the cathepsin L-specific inhibitor Z-FY-CHO, and the cathepsin B-specific inhibitor CA-074, were used in parallel.