THE COSTIMULATORY ACTIVITY OF THE CD26 ANTIGEN REQUIRES DIPEPTIDYL PEPTIDASE-IV ENZYMATIC-ACTIVITY

T cells have been shown to express CD26, a known ectoenzyme with dipep tidyl peptidase IV (DPPIV; EC 3.4.14.5) activity in its extracellular domain. CD26 can also deliver a second costimulatory signal and contri bute to T-cell activation. In an earlier study, we established CD26-tr ansfected Jurkat T-cell lines and demonstrated that monoclonal antibod y-mediated crosslinking of CD26 and CD3 induced interleukin 2 (IL-2) p roduction. To determine the contribution of DPPIV enzymatic activity t o the costimulatory activity of CD26, human CD26 cDNA was mutated so t hat active-site serine was replaced by alanine. The mutant CD26 antige n lacked DPPIV enzyme activity but still retained reactivity with thre e anti-CD26 monoclonal antibodies directed against distinct epitopes o f CD26. After stimulation with a combination of anti-CD26 and anti-CD3 antibodies, wild-type CD26 (DPPIV+)-transfected Jurkat cells produced substantially more IL-2 than did mutant CD26 (DPPIV-) or CD26-control transfectants. Nevertheless, the mutant CD26-transfected cells still produced significantly more IL-2 than did CD26- control transfectants. These results suggest that DPPIV activity plays an important but not absolute role in the costimulatory activity of CD26 in this system. We also found that wild-type CD26 (DPPIV+) transfectants produced more I L-2 than mutant CD26 (DPPIV-)-transfected cells or CD26- control trans fectants when triggered by stimuli not involving CD26, such as anti-CD 3 and phorbol ester. These results suggest that the DPPIV activity of CD26 functions to augment the cellular responses of CD26-transfected J urkat cells to external stimuli mediated by CD26 and/or the CD3/T-cell receptor complex, thus enhancing IL-2 production.