Human CD26 has dipeptidyl peptidase‐4 (DPP IV) enzyme activity and binds to adenosine deaminase (ADA). CD26 is costimulatory for lymphocytes and has a circulating soluble form (sCD26). DPP IV enzyme inhibition is a new successful type 2 diabetes therapy. We examined whether the ADA binding and catalytic functions of sCD26 contribute to its effects on T‐cell proliferation. Wildtype soluble recombinant human CD26 (srhCD26), an enzyme inactive mutant (srhCD26E‐) and an ADA non‐binding mutant (srhCD26A‐) were co‐incubated in in vitro T‐cell proliferation assays with peripheral blood mononuclear cells (PBMC) stimulated with phytohaemagglutinin (PHA), muromonab‐CD3 or Herpes simplex virus antigen (HSV Ag). Both srhCD26 and srhCD26E‐ enhanced PHA‐induced T‐cell proliferation dose‐dependently in all six subjects tested. srhCD26 and srhCD26A‐ had no overall effect on anti‐CD3‐stimulated PBMC proliferation in four of five subjects. srhCD26, srhCD26E‐ and srhCD26A‐ enhanced HSV Ag induced PBMC proliferation in low responders to HSV Ag, but had no effect or inhibited proliferation in HSV‐high responders. Thus, effects of soluble human CD26 on human T‐cell proliferation are mechanistically independent of both the enzyme activity and the ADA‐binding capability of sCD26.

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