CD27 and infection: Fluorescence-activated cell sorting (FACS) phenotypic profiles of CTL differentiation subsets used in all assays corroborated surface protein changes during infection (e.g., PD-1, CD62L, CD44, KLRG1, CD25, CD27, CD127, and 2B4) (Figure 2C and data not shown), consistent with previously reported profiles of naive, effector, memory, and exhausted CTLs.42