CD274 and COVID-19: In all children with acute COVID-19 (ward and ICU), expression of the cell surface marker programmed death-ligand 1 (PD-L1) was almost exclusively identified on G-MDSC compared to monocytic (M)-MDSC (100% [74, 100] vs. 1.4% [0.6, 6.3], p < 0.0001).