The EMT/ITK/TSK (EMT) tyrosine kinase is activated during TCR signaling: LCK is required for optimal activation of EMT

Functional T lymphocyte activation requires concurrent stimulation of the TCR complex and an accessory molecule, most frequently CD28. We have previously demonstrated that the TEC family tyrosine kinase EMT/ITK/TSK (EMT) is activated following cross-linking of CD28. We demonstrate herein that cross-linking of the CD3 component of the TCR complex also leads to EMT activation as indicated by a rapid and transient increase in EMT tyrosine phosphorylation and kinase activity in anti-EMT immunoprecipitates. However, although concurrent cross-linking of the TCR and CD28 results in a marked increase in production of the T cell growth factor IL-2, it does not result in a significant alteration in the magnitude or duration of EMT activation. Somatic cell mutants of the Jurkat T cell line, which lack the SRC family kinase LCK (JCaM1.6), fail to produce IL-2 when stimulated through the TCR complex. EMT activation, as evidenced by increased EMT tyrosine phosphorylation and EMT-associated kinase activity, was also greatly reduced following stimulation of the TCR in the JCaM1.6 Jurkat T cell mutants that lack LCK. In support of a role for LCK in EMT activation, reconstitution of the LCK-negative Jurkat T cell line by enforced expression of LCK restored TCR-mediated EMT activation. Taken together, the data indicate that the EMT tyrosine kinase is activated following cross-linking of the TCR, a process in which LCK likely plays an important role.