SPDL1 and COVID-19: In 2022, Beserra et al. [22] correlated the high expression of programmed death protein 1 (PD-1) and sPD-L1 levels in TCD4+ and CD19+ to COVID-19 severity, promoting PD-1/PDL-1 axis as a severity-associated biomarker in SARS-CoV-2-infected patients.