SPDL1 and COVID-19: Similar to its membrane-bound form, soluble PD-L1 (sPD-L1) in the blood is thought to exert inhibitory effects on immune function by binding to PD-1 on T cells (8, 9), and elevated blood levels of sPD-L1 are reportedly associated with various conditions, such as cancer (10, 11), aging (12), pregnancy (13), and diseases (8), e.g., acute coronary syndrome (14), severe viral infections, e.g., COVID-19 (15), and severe sepsis (16).