Increased percentages of CD4+-, CD4+CD26+- and CD8+CD26+-HPBLS after stimulation
In order to clarify the role of CD26 in lymphocyte differentiation, the percentages of CD4+ T lymphocytes (T helper cells) and CD8+ T lymphocytes (T cytotoxic cells), as well as the percentage of cells that were co-expressing each of these two subpopulation markers with CD26 were analyzed after stimulation. As shown in Figure 2 , after stimulation, the percentage of CD4+ cells was increased from 32.57±8.91% to 54.72±12.85% of total HPBLs while the percentage of CD8+ cells did not increase significantly. This result suggests a strong proliferation of the T helper subpopulation (CD4+) of T lymphocytes after stimulation. Further analysis revealed that after stimulation the percentage of cells that were co-expressing CD4 and CD26 (CD4+CD26+) in total HBPLs was 2.8-fold of that in the control group (39.98% vs. 14.43%). In the stimulated CD4+ subpopulation, about 73% of the CD4+ cells were co-expressed with CD26, while in the control CD4+ subpopulation only 40% of the CD4+ cells were co-expressed with CD26 (Figure 2A, B ). As previously known, CD26 is a co-stimulator of T cell activation; the increased T helper cells (CD4+) after stimulation were mostly co-expressed with CD26 observed in the present work, indicating that the activation and proliferation of CD4+ cells are closely related to CD26 expression.
While the percentage of CD8+ cells did not increase significantly after stimulation, we found that the percentage of CD8+CD26+ cells in the stimulated group was about 2.1 times that of the control group (14.28±3.35%vs. 6.72±4.21%). In the stimulated group approx. 40% of CD8+ cells were co-expressing CD26, compared with 21% of the CD8 cells in the control group (Figure 2C, D) . The increased CD8+CD26+ cells suggest that CD26 is also related to the activation of CD8+cells. Interestingly, the percentage of total CD8+cells was not increased significantly. Since cell survival analysis showed that almost no dead lymphocytes were observed after stimulation (data not shown), it suggests that T cytotoxic CD8+cells hardly proliferated, or their proliferation rate was much slower than that of CD4+ cells.