Activation and development of lymphocytes is regulated by the engagement of cell surface immune cell antigen receptors. Following receptor engagement, these receptors transmit signals by the activation of cytoplasmic protein tyrosine kinases (PTKs). The Tec PTK belongs to a group of structurally related nonreceptor PTKs that also includes Btk, Itk (Emt), Rlk (Txk), and Bmx (Etk). We have found the contribution of Tec in the antigen receptor signaling in B-lymphoid cells in the previous studies. Despite the presence of evidences suggesting the involvement of Tec in T-lymphocyte activation pathway via T-cell receptor (TCR) and CD28, the role of Tec in T-lymphocyte still remains unclear because of the lack of apparent defects in T-lymphocyte function in Tec-deficient mice. In this study, we investigated the role of Tec in T-lymphocyte using Jurkat human T-lymphoid cell line stably transfected with a cDNA encoding Tec. We found that the expression of wild type Tec but not kinase-deleted Tec inhibited the expression of IL-2 receptor alpha (CD25) induced by TCR cross-linking. The percentage of CD25 expressing cells after TCR cross-linking was 41.7% in mock-transfected cells, 18.3% in the wild type Tec expressing cells and 41.3% in the kinase-deleted Tec expressing cells. Second, the selective inhibitor of Tec family PTK, LFM-A13, rescued the suppression of TCR-induced CD25 expression observed in the wild type Tec-expressing Jurkat cells. We conclude that Tec PTK mediates signals that negatively regulate CD25 expression induced by TCR cross-linking, implying that this PTK plays a role in the attenuation of the IL-2 activity in human T-lymphocytes.

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