Higher percentages of CD4+, CD4+CD26+ and CD8+CD26+ cells in CD26high group
For further analysis of the correlation of CD26 to T cell differentiation, after stimulation, CD26+cells were separated using MACS MicroBeads conjugated with anti-mouse IgG after binding of CD26+ -lymphocytes with anti-human CD26 mAb (Figure 3) . After separation, two groups of cells were obtained: CD26 low-expressing group (CD26low) and CD26 high-expressing group (CD26high). The expression profiles of CD4+ and CD8+ and their co-expression with CD26 on surfaces of cells in the CD26low and the CD26high group were analyzed. As shown in Figure 4 , the percentage of CD4+ cells in CD26high group was 2.2-fold of that in CD26low group (62.70±14%vs. 28.28±9%, p <0.005), while the percentage of CD8+ cells was lower in CD26highgroup compared to the CD26low group (32.24%±5%vs. 45.11±9%, p <0.05). Further analysis showed that the percentage of CD4+CD26+cells in the CD26high group was 6-fold of that in the CD26low group (44.27±15% vs. 7.13±7%,p <0.01) (Figure 4A, B) , while the percentage of CD8+CD26+ cells in the CD26high group was about 3.5-fold of that in the CD26low group (12.93±6% vs . 3.72±0.9%,p <0.05) (Figure 4C, D) . These results showed that the expression of CD26 occurred mostly in T helper cells (CD4+) and only a small part of T cytotoxic cells (CD8+) expressed CD26 after stimulation, indicating activation of most T helper cells (CD4+) but only a few T cytotoxic cells (CD8+). In consideration of the greatly increased percentages of CD4+ cells and CD4+CD26+ cells after stimulation, it is undoubtedly that CD26 is closely involved in the proliferation of T helper cells (CD4+).