Combined expression of KIR2DS1 and HLA-C2 renders NK cells hyporesponsive to target cell stimulation. Paired analysis of CD107a cell-surface expression after stimulation with K562 cells within NKG2A⁻KIR⁻ and NKG2A⁻KIR2DS1sp NK cells cultured overnight in (A) complete medium, (B) 10⁴ U/mL IFN-α, or (C) 10 ng/mL IL-15. (D) Paired analysis of CD107a cell-surface expression after stimulation with P815 cells and anti-CD16 mAb by NKG2A⁻KIR⁻ and NKG2A⁻KIR2DS1sp NK cells. (A-D) Left panels represent responses in HLA-C1 homozygous (C1/C1) donors, and right panels represent responses in HLA-C2 homozygous (C2/C2) donors. P values indicate a statistically significant difference between the groups, tested by a Wilcoxon matched-pairs test.