Costimulation of CD4+ and CD8+ T cells through CD26: The ADA-binding epitope is not essential for complete signaling

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Abstract
It was previously shown that CD26 (DPP IV, EC 3.4.14.5) is a binding site for adenosine deaminase (ADA, EC 3.5.4.4) on T cells and that costimulation by some anti-CD26 monoclonal antibodies (mAbs) and anti-CD3 induces CD4+ T cell proliferation. The CD26 epitopes involved in costimulation, the precise sequence of the events preceding proliferation, and the response of CD8+ compared with CD4+ T cells to CD26 were not extensively studied. We therefore compared the effects of the novel TA5.9 anti-CD26 mAb, recognizing an ADA-binding epitope, and the clearly distinct anti-Ta1 reference anti-CD26 mAb for their costimulatory properties in various T cell subsets. Both purified CD4+ and CD8+ T cells proliferated upon costimulation with anti-CD3 and either anti-CD26 mAb, but anti-TA5.9 mAb induced a more potent response than anti-Tal. Either anti-CD26 mAb, together with anti-CD3, caused a similar sequential up-regulation of CD69, CD25 (IL-2Rα), and CD71 (transferrin receptor) expression on CD4+ and CD8+ T cells. The activation markers appeared faster on the CD45R0+ than on the CD45R0− subsets. After costimulation, CD4+ T cell cultures contained significant amounts of the Th1 cytokines EL-2, interferon-γ (IFN-γ), and tumor necrosis factor-α (TNF-α). In CD8+ T cell cultures relatively more IFN-γ and TNF-α but almost no IL-2 was measured after triggering of CD3 and CD26. Our data demonstrate that the recognition of the ADA-binding epitope is not a prerequisite for the costimulatory capacity of anti-CD26 mAbs. Both CD4+ and CD8+ T cells and their CD45R0−and CD45R0+ subsets are sensitive to various aspects of activation via CD26, but the magnitude and/or kinetics differ according to the anti-CD26 used and the T cell subset studied.

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