Superantigen-induced T Cell : B cell conjugation is mediated by LFA-1 and requires signaling through Lck, but not ZAP-70

The formation of a conjugate between a T cell and an APC requires the activ ation of integrins on the T cell surface and remodeling of cytoskeletal ele ments at the cell-cell contact site via inside-out signaling. The early eve nts in this signaling pathway are not well understood, and may differ from the events involved in adhesion to immobilized ligands. We find that conjug ate formation between Jurkat T cells and EBV-B cells presenting superantige n is mediated by LFA-1 and absolutely requires Lck. Mutations in the Lek ki nase, Src homology 2 or 3 domains, or the myristoylation site all inhibit c onjugation to background levels, and adhesion cannot be restored by the exp ression of Fyn. However, ZAP-70-deficient cells conjugate normally, indicat ing that Lek is required for LFA-1-dependent adhesion via other downstream pathways. Several drugs that inhibit T cell adhesion to ICAM-1 immobilized on plastic, including inhibitors of mitogen-activated protein/extracellular signal-related kinase kinase, phosphatidylinositol-3 kinase, and calpain, do not inhibit conjugation. Inhibitors of phospholipase C and protein kinas e C block conjugation of both wild-type and ZAP-70-deficient cells, suggest ing that a phospholipase C that does not depend on ZAP-70 for its activatio n is involved. These results are not restricted to Jurkat T cells; Ag-speci fic primary T cell blasts behave similarly. Although the way in which Lek s ignals to enhance LFA-1-dependent adhesion is not clear, Nye find that cell s lacking functional Lek fail to recruit F-actin and LFA-1 to the T cell:AP C contact site, whereas ZAP-70-deficient cells show a milder phenotype char acterized by disorganized actin and LFA-1 at the contact site.