CD161 expression by regulatory T cells is enriched in the inflammatory environment. (A) CD161 status of Treg was analyzed on T cells from peripheral blood (PB) and joint (SF) of children with JIA. Representative FACS plots of FoxP3 expression on CD4+ lymphocytes and CD161 staining on FoxP3+CD4+ lymphocytes from JIA PB and JIA SF; summary plot (right) shows data for healthy adult PB (n = 39), healthy child PB (n = 12), JIA PB (n = 24), and JIA SF (n = 41) samples. Data in dot plots are gated on CD4+ lymphocytes (left plots) and then (right plots) on FoxP3+ T cells; horizontal lines represent median with Kruskal-Wallis test with Dunn’s multiple comparison test. ***P < .001; **P < .01; *P < .05; ns, not significant. (B) Frequency of CD161+ Treg in different clinical JIA phenotypes was analyzed by flow cytometry. Frequency of CD161– Treg (clear) and CD161+ Treg (black) within CD4+ T cells; data are divided into clinical subgroups, persistent O-JIA (pers), extended O-JIA (ext), and P-JIA (poly) for PB (n = 8, n = 8, and n = 5, respectively) and SF (n = 15, n = 11, and n = 11, respectively) samples. Bars represent mean ± SEM. (C) The inverse relationship between Th17 and Treg is specific to the CD161– Treg population. Correlation between SF CD161– Treg (left, clear symbols) and CD161+ Treg (right, filled symbols) with IL-17A+ CD4+ T cells within the same sample (n = 28).