 The study found that kefepsin L, CTSL, plays a critical role in the progression of COVID-19. It was discovered that CTSL levels were higher in patients with COVID-19 compared to healthy individuals, and this correlation was also seen between CTSL levels and disease severity. Additionally, CTSL was found to increase the ability of SARS-CoV-2 to infect human cells, and when CTSL was overexpressed or knocked down in human cells, it was observed that the virus could enter the cell more easily. Furthermore, the study showed that a mantidine, an approved influenza treatment, can reduce the activity of CTSL and thus decrease the ability of SARS-CoV-2 to infect cells. This suggests that CTSL may be a potential target for developing new treatments against COVID-19. This article was authored by Miamiao Zhao, Wei Li Yang, Fanyuan Yang, and others.