Cytofluorographic evidence that thymocyte dipeptidyl peptidase IV (CD26) activity is altered with stage of ontogeny and apoptotic status

CD26 is a multifunctional molecule implied to have a variety of roles in the immune response including its activity as a membrane exopeptidase (Dipeptidyl peptidase IV) which cleaves several protein molecules. In order to further define the expression and functiorial activity of CD26 in the developing thymus, we utilized a nondisruptive, cytofluorogenic assay which allowed simultaneous measurement of DPP IV activity with a fluorochrome-conjugated peptide substrate and surface staining of the T lymphocyte lineage antigens CD4 and CD8. Neonatal and adult murine thymi were examined using the three-color assay and significant differences in DPP IV activity were found among the thymocyte subsets defined by their CD4/ CD8 phenotype. Single-positive cells bore higher activity than CD4^-/CD8^- cells and neonates had higher activity than adults. Thymocytes with characteristics consistent with apoptotic cells expressed higher DPP TV activity. Thus, DPP IV appears to be upregulated both as thymocytes mature and among thymocytes which are undergoing programmed cell death. These results suggest that CD26 is ontogenically controlled during T cell maturation and may play a role in thymic deletion of emerging clones.