To analyze the contribution of individual T-cell antigen receptor (TCR) components to the structure and function of the complete complex, variants of an antigen-specific T-cell hybridoma that survived TCR-mediated stimulation in vivo were cloned and grown in vitro. In addition to cells that had lost the CD3-zeta chain, examples of which we have previously described, clones were isolated that had very low levels of the CD3-zeta heterodimer, a pairing that is present in approximately 20% of the TCR complexes of a given cell. Whereas stimulation of these clones with antigen or antibodies to the TCR yielded normal levels of tyrosine kinase activation in these cells, the levels of phosphatidylinositol hydrolysis were dramatically reduced. Comparison of the wild type T-cell hybridoma with six different variants yielded a remarkably good positive correlation between CD3-zeta eta expression and the ability to generate inositol phosphates upon stimulation. These results have been interpreted to indicate that TCR's that contain the CD3-zeta eta heterodimer interact with a phospholipase C, leading to the hydrolysis of phosphatidylinositol. TCR's that do not contain CD3-zeta eta (i.e., contain a CD3-zeta 2 homodimer) interact with a tyrosine kinase pathway. These data suggest a novel biological mechanism whereby a single ligand-binding molecule can initiate independent intracellular pathways due to an association with different signal transducing elements.
