NKG2C⁺ NK cells increase after CMV reactivation after HCT and produce high levels of IFNγ. PBMCs from HCT recipients were studied before viral infection, at diagnosis of viral reactivation, and after 2, 4, and 8 weeks of antiviral therapy. Cells were incubated in either medium alone (not shown) or with K562 cells for 5 hours. (A) After incubation, NKG2C expression was measured on CD56⁺CD3⁻ NK cells and the absolute number of NKG2C⁺ NK cells/μL was calculated. Aggregate data from recipients in the indicated groups is shown. Points represent the means ± SEM. Previral samples were compared with samples after infection using the Student t test. *P ≤ .05. (B) CD107a expression (left panel) and IFNγ production (right panel) was measured on NKG2C⁺ (black bars) or NKG2C⁻ (white bars) NK cells. Bars represent the means ± SEM. Previral samples were compared with samples after infection using the Student t test. NKG2C⁺ NK cells were compared with NKG2C⁻ NK cells using the paired t test. *P ≤ .05, **P < .01, and ***P < .001.