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-specifi c 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 deliver y of the second signal. Signaling via CD28 and the T-cell receptor res ults (i) in their costimulation of T-cells to produce numerous lymphok ines including interleukin 2 and (ii) in the prevention of anergy indu ction. Little is known about the pathway by which CD28 mediates its si gnals except that protein-tyrosine phosphorylation is involved. We sho w here in human Jurkat cells that the Tec-family protein-tyrosine kina se ITK/EMT (p72(ITK/EMT)) is associated with CD28 and becomes tyrosine -phosphorylated and activated within seconds of CD28 ligation. This ty rosine phosphorylation of p72(ITK/EMT) is rapid (within 30 sec), occur s in the absence of LCK activation, and precedes tyrosine phosphorylat ion of the guanine nucleotide exchange factor VAV. Secondary crosslink ing of CD28 is unnecessary for the induced tyrosine phosphorylation of p72(ITK/EMT). Thus, tyrosine phosphorylation of p72(ITK/EMT) may repr esent one of the earliest events in CD28 signaling. This demonstrates that a member of the Tec family of protein tyrosine kinases, similar t o 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 b y analogy other members of the Tec family, responds to extracellularly generated signals.