S100A9 and COVID-19: In addition, other factors like the immunomodulatory calcium binding protein S100A9 as well as the tissue inhibitor of metalloproteinases-1 (TIMP-1), both of which were described to correlate with poor prognosis in COVID-19 patients [46, 47], were also significantly decreased by treatment with mFas-Fc (Supplementary information, Fig. S5b).