Analysis of CD161 expression by the activated CD107a⁺CD3^-CD56⁺ NK-cell subset. (A) Cell surface expression of CD161 was evaluated by 4-color flow cytometry after gating on CD107a⁺CD3⁻CD56⁺ NK cells. The percentage of CD161⁺ stained cells was analyzed within CD107a⁺CD3⁻CD56⁺ NK cells in absence of stimulation (unstim) and after 3 hours of K562 cell stimulation (K562) in exposed HIV-infected IDUs (HIV), exposed uninfected IDUs (EU), and unexposed controls (C). The y-axis values boxed represent 25th percentile, median, and 75th percentile. Horizontal bars indicate minimum and maximum values. (B) Representative flow cytometer analysis of percentage and mfi of CD161 staining among the CD107a⁺CD3⁻CD56⁺ NK-cell subset before exogenous stimulation (white: isotypic control staining, black, CD161 specific staining). (C) mfi of CD161 staining among the CD161⁺ unstimulated CD107a⁺CD3⁻CD56⁺ NK-cell subset. The y-axis values boxed represent 25th percentile, median, and 75th percentile. (D) Evaluation of mfi of CD161 expression within the CD107a⁺ and CD107a⁻CD3⁻CD56⁺NK-cell subsets after K562 cell stimulation. The y-axis values boxed represent 25th percentile, median, and 75th percentile.