CD274 and COVID-19: For instance, in SARS-CoV-2 patients, it was demonstrated that overexpression of PD-1 and PD-L1 induces the activation of the PD-1/PD-L1 signaling pathway, which downregulates the activation of effector T-cell responses through a programmed T-cell death mechanism and predicts COVID-19 severity (52, 106).