ENHANCEMENT OF ANTIGEN-INDUCED T-CELL PROLIFERATION BY SOLUBLE CD26 DIPEPTIDYL PEPTIDASE-IV

The addition of a soluble recombinant CD26 (sCD26) enhanced proliferat ion of peripheral blood lymphocytes induced by the recall antigen teta nus toxoid. sCD26 itself did not provide a mitogenic signal and did no t augment the proliferative response of T cells to other mitogenic sti muli such as phytohemagglutinin and anti-CD3. Dipeptidyl peptidase IV- negative sCD26 did not have this enhancement effect, implying a requir ement for enzyme activity. It was found that there exists a large vari ation in the levels of human plasma sCD26/dipeptidyl peptidase IV in v ivo which may regulate T-cell activity. Peripheral blood lymphocytes f rom individuals whose plasma sCD26 was high and responded strongly to tetanus toxoid stimulation were insensitive to the enhancing effects o f exogenously added sCD26. This suggests that plasma sCD26 had modulat ed the responsiveness of T cells of these individuals in vivo and that the endogenous plasma sCD26 regulates immune responses by allowing an tigen-specific T cells to exert a maximal response to their specific a ntigen.