CD28 IS ASSOCIATED WITH AND INDUCES THE IMMEDIATE TYROSINE PHOSPHORYLATION AND ACTIVATION OF THE TEC FAMILY KINASE ITK/EMT IN THE HUMAN JURKAT LEUKEMIC T-CELL LINE

T lymphocytes require two signals to be activated. The antigen-specific T-cell receptor can deliver the first signal, while ligation of the T-cell surface molecule CD28 by antibodies or its cognate ligands B7-1 (CD8O) or B7-2 has been demonstrated to be sufficient for the delivery of the second signal. Signaling via CD28 and the T-cell receptor results (i) in their costimulation of T-cells to produce numerous lymphokines including interleukin 2 and (ii) in the prevention of anergy induction. Little is known about the pathway by which CD28 mediates its signals except that protein-tyrosine phosphorylation is involved. We show here in human Jurkat cells that the Tec-family protein-tyrosine kinase ITK/EMT (p72(ITK/EMT)) is associated with CD28 and becomes tyrosine-phosphorylated and activated within seconds of CD28 ligation. This tyrosine phosphorylation of p72(ITK/EMT) is rapid (within 30 sec), occurs in the absence of LCK activation, and precedes tyrosine phosphorylation of the guanine nucleotide exchange factor VAV. Secondary crosslinking of CD28 is unnecessary for the induced tyrosine phosphorylation of p72(ITK/EMT). Thus, tyrosine phosphorylation of p72(ITK/EMT) may represent one of the earliest events in CD28 signaling. This demonstrates that a member of the Tec family of protein tyrosine kinases, similar to members of the Src and Syk families, plays a role in the activation of T cells. Furthermore, the data demonstrate that p72(ITK/EMT), and by analogy other members of the Tec family, responds to extracellularly generated signals.