Responses of CD57-defined NK cell subsets and impact of HCMV exposure. (A) Gating strategy for CD57-defined NK cell subsets. PBMCs were restimulated with H1N1 and IFN-γ (B and E), CD107a (C and F), and CD25 (D and G) responses were analyzed for each CD56/CD57-defined NK cell subset. (B–D) Responses for cells collected 2 wk after vaccination are stratified by HCMV infection status: open circles, HCMV− (n = 33) individuals; closed circles, HCMV+ (n = 19) individuals. Bars represent mean values. Significant values for trends across CD56dim NK cell subsets (repeated-measures ANOVA) and comparisons between HCMV− and HCMV+ individuals (Mann–Whitney U test) are shown. (E–G) For each subset, baseline (0) responses are compared with postvaccination (2 wk) by Wilcoxon signed-rank test; paired data for each individual are joined by a solid line. *p < 0.05, **p < 0.01, ***p < 0.001, ****p < 0.0001.