IFN-γ ELISpot for HHV-6–specific T cells. (A) Ex vivo T-cell responses in HDs upon stimulation with the phytohemagglutinin control (PHA), CMV antigens, and HHV-6. Results are expressed as SFC/10⁵ PBMC. Lines indicate medians with interquartile range. (B) HHV-6–specific T-cell responses in HD at steady state (day 0) and 14 days after in vitro stimulation. (C) Frequencies of HHV-6–specific T cells at the indicated time points from the first day of viremia in HSCT recipients. (D) HHV-6–specific SFC/10⁵ PBMC measured at ≥3 or at ≥7 days from the first day of viremia in samples collected from the same patient. (E) Frequencies of HHV-6–specific T cells in different groups: reactivating patients (HHV6+, red), controls (nonreactivating patients; HHV6-, green) and HD (violet). (F) Frequencies of HHV-6–specific T cells in patients developing HHV-6 clinically relevant infection (red) and in patients experiencing HHV-6 subclinical reactivation (blue). (G) Absolute counts of polyclonal CD3+ cells in patients developing HHV-6 clinically relevant infection (red) and in patients experiencing HHV-6 subclinical reactivation (blue). Statistical analysis in panels B and D were performed by Wilcoxon t test. Statistical analyses in panel E were performed using Kruskal-Wallis test, and in panels A, C, F, and G using Mann-Whitney test. ^∗P < .05; ^∗∗P < .01; ^∗∗∗∗P < .0001. ns, not significant.