CD161 expression by regulatory T cells. Expression of CD161 on CD4+FoxP3+ T cells was assessed by flow cytometry. Representative fluorescence-activated cell sorter (FACS) plots of FoxP3 expression gated on CD4+ lymphocytes (left plot) and CD161 staining gated on FoxP3+CD4+ lymphocytes (right plot) in healthy adult (n = 39; left panels) and healthy child (n = 12; right panels) blood samples (summary plot far right) (A), and in cord blood (UCB; n = 5; left panels) and mature CD4 single positive thymocytes, defined by CD1a–CD3+CD4+CD8– (n = 4; right panels) (B). (C) No difference in FoxP3 protein levels between CD161–FoxP3+ and CD161+FoxP3+ Treg. Representative histograms overlay, left (white, CD161–FoxP3+; black, CD161+FoxP3+), with summary plot, right, of FoxP3 staining intensity (MFI) on CD161–FoxP3+ (clear symbols) and CD161+FoxP3+ (filled symbols) CD4+ lymphocytes for healthy adult (n = 37) and healthy child (n = 12) samples. (D) Nonlinear regression between CD161+ Treg and age in healthy child and adult samples (n = 31), 1-site–binding hyperbola. Horizontal bars represent the medians in all summary plots. Statistical analysis by Wilcoxon matched pairs test. **P < .01; *P < .05.