Functional LCK Is Required for Optimal CD28-mediated Activation of the TEC Family Tyrosine Kinase EMT/ITK

Activation of CD28 on T lymphocytes initiates a cascade of intracellular events, which in concert with activation of the T cell receptor, culminates in production of cytokines and a functional immune response. One of the earliest biochemical changes observed following stimulation of CD28 is tyrosine phosphorylation. We have demonstrated that both the LCK and the EMT/ITK/TSK (EMT) intracellular tyrosine kinases are activated following cross-linking of CD28. Utilizing somatic cell mutants lacking LCK, we demonstrate that functional LCK is required for CD28-induced activation of EMT as evidenced by increased tyrosine phosphorylation and kinase activity. In support of a role for LCK in EMT activation, reconstitution of a LCK-negative Jurkat T cell line by transfection with normal LCK recreates CD28-mediated EMT activation. Furthermore, co-transfection of LCK and EMT into COS-7 cells showed that EMT becomes phosphorylated in the presence of LCK. In addition, increases in EMT association with CD28 were eliminated in a LCK-negative Jurkat cell line, but were restored following transfection of wild type LCK. The data are most compatible with a model in which LCK, either directly or indirectly, initiates EMT activation and association with CD28 following ligation of CD28.