Expression of CD26 (dipeptidyl peptidase IV) on memory and naive T lymphocytes

It has been suggested that human CD26-positive T lymphocytes represent the memory pool of the cellular immune system. For proof of this suggestion we analysed the responsiveness of CD26-positive and CD26-negative T lymphocytes after antigenic stimulation in limiting dilution experiments. After stimulation with tetanus toxoid (TT) the number of proliferating cells within the CD26-positive subset was two- to sixfold higher than that within the CD26-negative subset. These differences in responsiveness were also detectable between CD4+/CD26+ and CD4+/CD26- T cells. To further investigate the memory character of the cells, human peripheral blood mononuclear cells were stimulated with TT for 7 or 14 days. Thereafter, CD26+ and CD26- T cells were isolated and restimulated with TT in limiting dilution experiments. Responding cells were found not only within the CD26-positive subset but also within the CD26-negative subset, and their number increased with time. Surface marker analysis of freshly isolated human T lymphocytes or T-cell clones indicated that CD26 is not a stable cell surface marker. Furthermore, CD26 is both absent and present on CD29-positive or CD45RA-positive cells, indicating no association of CD26 with surface markers for memory or naive T cells, respectively. These results strongly argue against the hypothesis that CD26-positive T cells represent the memory pool. We conclude that CD26 is an activation marker of T lymphocytes, which is associated with reactivity on naive and memory cells.