MMP9 and COVID-19: Additionally, a significant paucity of FOXP3+, Tbet+ and LEF1+ lymphocytes in COVID-19 suggest a depletion of Treg and TH1 equivalents (Figures 2A, B and 3); this is mirrored by the downregulation of key genes responsible for T-cell maturation and TCR signal transduction (Figure 4B, genes in green; Figure 6B) as well as ICAM3, ITGB2 and MMP9, which are involved in leucocyte adhesion and migration.