Polyoma virus middle T antigen (MT): A novel tool for the dissection of T cell antigen receptor (TCR)-Linked signaling pathways

Ligation of the TCR triggers signaling events leading to the activation of IL-2 gene transcription. We have developed a genetically manipulatable model system that provides a TCR-like set of activation signals in Jurkat T cells. MT Is a membranelocalized, viral oncoprotein that binds signaling proteins implicated in IL-2 gene transcription, including She, phosphatidylinositol 3-kinase(PI3-K), phospholipase C-?1 (PLCyl), and the src-family tyrosine kinases. Our results demonstrate that the MT antigen transiently expressed in Jurkat cells leads to the activation of an IL-2-promoter-driven luciferase reporter construct. Mutants of MT that are unable to associate with She or PI3-K show a dramatic increase in IL-2 promoter activity over wildtype MT; however, reporter activity is absent with a MT mutant that fails to bind PLC-y1. Similar results were obtained with an (NFAT)3-driven luciferase construct. Treatment of wildtype MTexpressing Jurkat cells with the PI3-K inhibitor, wortmannin, enhances both IL-2 promoter- and NFAT-mediated transcription. These results suggest that whereas PLC-γ1 activation is essential for IL-2 gene transcription, PI3-K and She may downmodulate this response.