PDCD1 and COVID-19: In contrast to the mild SARS-CoV-2 infection that harbors a successful lymphocyte-mediated virus clearance, T cells are functionally exhausted, a state that arises during many chronic infections (117), and express a high level of Programmed cell Death protein 1 (PD-1) and T-cell immunoglobulin mucin-3 in severe COVID-19 patients (117, 118).