Prolonged Exposure of T Cells to TNF Down-Regulates TCRζ and Expression of the TCR/CD3 Complex at the Cell Surface

A role for TNF-α in the pathogenesis of chronic inflammatory disease is now firmly established. Paradoxically, TNF also has potent immunomodulatory effects on CD4+ T lymphocytes, because Ag-specific proliferative and cytokine responses are suppressed following prolonged exposure to TNF. We explored whether TNF attenuated T cell activation by uncoupling proximal TCR signal transduction pathways using a mouse T cell hybridoma model. Chronic TNF exposure induced profound, but reversible, T cell hyporesponsiveness, with TNF-treated T cells requiring TCR engagement with higher peptide concentrations for longer periods of time for commitment to IL-2 production. Subsequent experiments revealed that chronic TNF exposure led to a reversible loss of TCRζ chain expression, in part through a reduction in gene transcription. Down-regulation of TCRζ expression impaired TCR/CD3 assembly and expression at the cell surface and uncoupled membrane-proximal tyrosine phosphorylation events, including phosphorylation of the TCRζ chain itself, CD3e, ZAP-70 protein tyrosine kinase, and linker for activation of T cells (LAT). Intracellular Ca2+ mobilization was also suppressed in TNF-treated T cells. We propose that TNF may contribute to T cell hyporesponsiveness in chronic inflammatory and infectious diseases by mechanisms that include down-regulation of TCRζ expression. We speculate that by uncoupling proximal TCR signals TNF could also interrupt mechanisms of peripheral tolerance that are dependent upon intact TCR signal transduction pathways.