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 ha ve previously demonstrated that the TEC family tyrosine kinase EMT/ITK /TSK (EMT) is activated following cross-linking of CD28. We demonstrat e herein that cross-linking of the CD3 component of the TCR complex al so leads to EMT activation as indicated by a rapid and transient incre ase in EMT tyrosine phosphorylation and kinase activity in anti-EMT im munoprecipitates. However, although concurrent cross-linking of the TC R and CD28 results in a marked increase in production of the T cell gr owth factor IL-2, it does not result in a significant alteration in th e magnitude or duration of EMT activation. Somatic cell mutants of the Jurkat T cell line, which lack the SRC family kinase LCK (JCaM1.6), f ail to produce IL-2 when stimulated through the TCR complex. EMT activ ation, as evidenced by increased EMT tyrosine phosphorylation and EMT- associated kinase activity, was also greatly reduced following stimula tion 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 LC K-negative Jurkat T cell line by enforced expression of LCK restored T CR-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.