CD26 and CD126/IL-6Rα display a coordinated expression on effector CD4+ T cells, but only CD26/sCD26 levels are altered in asthma. (a) Gating strategy in flow cytometry assays to identify relevant CD4+ T cell subsets. Lymphocytes were gated based on the forward (FSC-Height) and side-scatter (SSC-Height) parameters (left dot plot). Then, TH lymphocytes were selected according to their high CD4 expression (middle dot plot). Finally, levels of CD25 and CD127 were used to identify three different TH subsets (right dot plot): “conventional” Teff cells (Teff), regulatory T cells (Treg), and “triple low” Teff cells (Tlow). (b,c) Correlation between the expression (% of positive cells) of CD26 and CD126 on CD4+ (red) or CD4− (blue) lymphocytes (b) or within different CD4+ subsets (c): “conventional” Teff (red), Treg (green), and Tlow cells (black) (data from all study subjects; N = 268). (d) Expression of CD26 (median ± IQR1-3) presented as number of antibody molecules bound per cell (ABC; see Material and Methods section) in healthy controls (HC; N = 32), rhinitis (R; N = 44), allergic asthma (AA; N = 100), and non-allergic asthma (NAA; N = 92). (e) Serum levels of sCD26 (ng/mL) measured by ELISA. (f) Percentage of Tlow cells within the CD4+ T cell gate. (d–f) Statistically significant differences between groups are indicated (Kruskal-Wallis test: P < 0.05).