Constitutive and inducible activation of NK cells is revealed by the CD107 degranulation assay. (A) Comparative analysis of CD107a expression on CD3⁻CD56⁺ NK cells was performed in the 3 study groups, in the absence of stimulation (unstim) and after K562 target cells stimulation (K562). Boxes represent CD3⁻CD56⁺ cells expressing CD107a in exposed HIV-infected IDU (HIV), exposed HIV-uninfected IDU (EU), and unexposed control donors (C), y-axis refers to 25th percentile, median, 75th percentile values. (B) Comparative analysis of mfi of CD107a staining among CD107⁺CD3⁻CD56⁺ NK cells in exposed HIV-infected IDU (HIV), exposed uninfected IDU (EU), and controls (C) in the absence of stimulation (unstim) and on stimulation with K562 target cells (K562). Boxes represent the median mfi of CD107a staining among CD3⁻CD56⁺ NK cells (25th percentile, median, 75th percentile). Horizontal bars indicate minimum and maximum values. (C) Examples of flow cytometer analyses of CD107a staining on CD3⁻CD56⁺ NK cells in the absence of stimulation (white, isotypic control staining; black, NKR specific staining).