PDCD1 and viral infectious disease: Studies from chronic viral infection and tumor models have characterized two populations of epigenetically and spatially distinct populations of CD8+ T cells: TCF1+ PD-1+ TIM3- CD8+ TSL and their progeny, TCF1- PD-1+ CD8+ T transitory effector-like CD8+ T cells (TTE) (3–7, 9, 10).