CD26, the T cell activation molecule dipeptidyl peptidase IV (DPPIV), associates with a 43-kilodalton protein. Amino acid sequence analysis and immunoprecipitation studies demonstrated that this 43-kilodalton protein was adenosine deaminase (ADA). ADA was coexpressed with CD26 on the Jurkat T cell lines, and an in vitro binding assay showed that the binding was through the extracellular domain of CD26. ADA deficiency causes severe combined immunodeficiency disease (SCID) in humans. Thus, ADA and CD26 (DPPIV) interact on the T cell surface, and this interaction may provide a clue to the pathophysiology of SCID caused by ADA deficiency.
T cells have been shown to express CD26, a known ectoenzyme with dipeptidyl peptidase IV (DPPIV; EC 3.4.14.5) activity in its extracellular domain. CD26 can also deliver a second costimulatory signal and contribute to T-cell activation. In an earlier study, we established CD26-transfected Jurkat T-cell lines and demonstrated that monoclonal antibody-mediated crosslinking of CD26 and CD3 induced interleukin 2 (IL-2) production. To determine the contribution of DPPIV enzymatic activity to the costimulatory activity of CD26, human CD26 cDNA was mutated so that active-site serine was replaced by alanine. The mutant CD26 antigen lacked DPPIV enzyme activity but still retained reactivity with three anti-CD26 monoclonal antibodies directed against distinct epitopes of 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 IL-2 than mutant CD26 (DPPIVj)-transfected cells or CD26-control transfectants when triggered by stimuli not involving CD26, such as anti-CD3 and phorbol ester. These results suggest that the DPPIV activity of CD26 functions to augment the cellular responses of CD26-transfected Jurkat cells to external stimuli mediated by CD26 and/or the CD3/T-cell receptor complex, thus enhancing IL-2 production.
Adult T cell leukemia (ATL) is an aggressive neoplastic disease, in which a quarter of the patients develop opportunistic infections due to cellular immunodeficiency. However, the underlying mechanism responsible for the immunosuppression has remained unclear. Recent studies have demonstrated that the leukemia cells from a subset of patients with ATL express Foxp3, a specific marker for CD25+CD4+ regulatory T (Treg) cells, which regulate the immune response by suppressing CD4+ T cell functions. However, whether there is a functional resemblance between ATL cells that have Foxp3 expression and Treg cells is still unknown. In this report, we confirmed the high expression of Foxp3 in leukemia cells from 5 of 12 ATL patients and demonstrated that ATL cells from 3 patients suppressed the proliferation of CD4+ T cells. Similarly, one of six HTLV-I-infected cell lines showed both high Foxp3 expression and suppressive activity. Like Treg cells, the suppression induced by the ATL cells from two patients and the HTLV-infected cell line appeared to be mediated by a cell-cell contact-dependent mechanism. Nevertheless, among the ATL cells that strongly expressed Foxp3, those from two of the five patients showed no apparent suppressive activity. Furthermore, retroviral transfection of Foxp3 did not confer any suppressive function on low Foxp3-expressing HTLV-I-infected cell lines. These results indicate that Foxp3 may be essential but is not sufficient for the Treg-cell-like suppressive activity of ATL cells and HTLV-I-infected cell lines.
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