Co-expression of CD26 with Th1- or Th17-typical cytokines in cells of CD26high group
The association of CD26 expression to the differentiation of Th1 or Th17 subset was further analyzed by determination of the co-expression of CD26 with each of Th1 typical cytokines (IL-2 or IFN-γ), Th17 typical cytokines (IL-6, IL-17, IL-22), or Th17 specific surface marker (IL-23R). In comparison to CD26 low group, the percentages of cells that were co-expressing CD26 with each of these cytokines were obviously higher in the CD26high group(Figure 6A) . The percentages of cells that were co-expressing CD26 with IL-2 (CD26+ IL-2+ ) or IFN-γ (CD26+IFN-γ+ ) in the CD26highgroup were 3.5- and 3-fold of that in the CD26lowgroup (20.31% vs. 5.83% and 15.66% vs. 5.18%), respectively. Notably, the percentages of cells that were co-expressing CD26 with IL-17 (CD26+ IL-17+ ), IL-6 (CD26+ IL-6+ ) or IL-22 (CD26+ IL-22+ ) in the CD26high group were nearly 6-fold, 5-fold and 6.5-fold of that in the CD26low group (20.14%vs. 3.43%, 14.81% vs. 3% and 18.64% vs.2.86%), respectively. Also, the percentage of cells that were co-expressing CD26 with Th17 marker IL-23R (CD26+ IL-23R+ ) in CD26high group was 6-fold compared to the CD26low group (23.14% vs. 3.7%)(Figure 6A) .
Fluorescence microscopy detected that the CD26 protein was predominantly located on the cell plasma membrane, while IL-2, IFN-γ, IL-17, and IL-22 were present in the cytosol, and IL-23R was also mainly located on the cell surface. After merging the photos, CD26 was found to be co-expressed with IL-2, IFN-γ, IL-17, IL-22, or IL-23R (Figure 6B) in the same lymphocytes. Since IL-2 and IFN-γ are typical Th1 cytokines, the co-expression of Th1-cytokines with CD26 suggests a correlation of CD26 to the differentiation and function of Th1 cells. Similarly, IL-17 and IL-22 are typical Th17 cytokines, and IL-23R is a typical Th17 cell surface marker. Therefore, the co-expression of Th17-cytokines or -markers with CD26 suggests a correlation of CD26 to the differentiation and function of Th17 cells.