Partially Phosphorylated T Cell Receptor ζ Molecules Can Inhibit T Cell Activation

The T cell receptor complex (TCR) zeta chain is constitutively tyrosine phosphorylated specifically at two of the six zeta immunoreceptor tyrosine-based activation motif (ITAM) tyrosine residues in resting peripheral T cells. Further phosphorylation of zeta is induced by both agonist and antagonist ligands of the TCR, with agonists inducing complete phosphorylation of the zeta ITAM tyrosines. After antagonist stimulation, zeta phosphorylation is incomplete and generates discrete forms of partially phosphorylated ITAMs. Here, we mutate specific tyrosines in chimeric human CD8-zeta molecules to reflect phosphorylation in resting T cells as well as phosphorylation induced by agonist and antagonist ligands. We demonstrate that such partially phosphorylated TCR-zeta species can inhibit IL-2 production in T cell hybridomas and proliferation in T cell clones. This reveals a previously unrecognized, inhibitory function of partially phosphorylated ITAMs. These findings support the concept that TCR antagonism can arise through the generation of an inhibitory signal within the TCR complex and that constitutive zeta phosphorylation in resting T cells is an inhibitory signaling environment.