In a recent study, we found that M2 AMs have a higher endosomal pH but a lower lysosomal pH.15 This increased pH inhibits the enzymatic activity of CTSL and impedes the release of viral RNA into the cytoplasm; in contrast, the virus is delivered from endosomes to lysosomes, where it is degraded by acidic lysosomal enzymes.15 Thus, M2 AMs possess the ability to degrade SARS-CoV-2 and limit viral infection and spread, which might explain the high percentage of asymptomatic SARS-CoV-2-infected people. Here, CTSL is linked to viral infectious disease.