PDCD1 and COVID-19: In COVID-19 patients, TNF-α and IFN-γ expression is reduced in CD4+ T cells (99); high levels of failure markers are expressed in CD8+ T cells (100); and programmed cell death protein-1 (PD-1) and T cell immunoglobulin structural domain and mucin structural domain-3 (TIM-3) expression are increased (101).