PDCD1 and infection: Reversal of T cell exhaustion by blockade of the PD-1:PD-L1 axis indicates that exhausted T cells span a spectrum of dysfunction with the least exhausted cells, characterized as Tim3−PD-1intTCF-1hiCXCR5hi cells in LMCV infection (21, 22), being more susceptible to functional resurrection after checkpoint inhibitor blockade.