DETERMINATION OF ADENOSINE-DEAMINASE BINDING DOMAIN ON CD26 AND ITS IMMUNOREGULATORY EFFECT ON T-CELL ACTIVATION

CD26, a 110-kDa cell surface glycoprotein, exhibits dipeptidyl peptida se IV enzyme activity and plays an important role in T cell costimulat ion. In the present study, we examined both the exact adenosine deamin ase (ADA) binding domain on CD26 and the functional consequences of mu tated CD26 transfectants that were deficient for cell surface ADA. Usi ng CD26 deletion, human-rat swap, and point mutations, we found that t he residues of L-340, V-341, A(342), and R-343, on the CD26 molecule w ere essential amino acids for ADA binding. When these amino acids were mutated and transfected into jurkat cells, the resultant CD26 transfe ctants expressed only CD26, not ADA, on the cell surface. The amount o f IL-2 produced by wild-type and mutated CD26 transfectants was almost the same following stimulation with anti-CD3 plus PMA. However, the m utated CD26 transfectants were much more sensitive to the inhibitory e ffect of adenosine on IL-2 production than were the wild CD26 transfec tants. These data suggest that ADA on the cell surface does not direct ly involve T cell activation. Conversely, CD26 alone does not result i n modulating the inhibitory effect of adenosine. Only the ADA bound to CD26 on the cell surface was functional and could counteract the inhi bitory effect of elevated extracellular adenosine.