PDCD1 and infection: Blackburn et al. [73] demarcated TEX into PD-1INT CD44HI and PD-1HI CD44INT subset, with the PD-1INT CD44HI subset being enriched for cells responsive to anti-PD-1 in clone 13 infection [73,74] and only the PD-1HI CD44INT subset displaying co-expression of PD-1, CD244, CD160 and LAG-3 [74].