T Cell Receptor ζ Reconstitution Fails to Restore Responses of T Cells Rendered Hyporesponsive by Tumor Necrosis Factor α

Expression and function of the antigen T cell receptor (TCR) play a central role in regulating immune responsiveness. Accordingly, targeting the expression of TCRαβ or its associated CD3 subunits profoundly influences T cell development and adaptive immunity. Down-regulation of the invariant TCRζ chain has been documented in a wide variety of chronic inflammatory and infectious diseases, and is thought to contribute to the paradoxical immune suppression observed in these diseases. Previously, we reported that prolonged exposure of T cell hybridoma clones to tumor necrosis factor α (TNF) induces nondeletional and reversible hyporesponsiveness to TCR engagement, associated with down-regulation of TCRζ chain expression, impaired TCR/CD3 complex assembly, and attenuation of TCR-induced membrane proximal tyrosine phosphorylation. Here, we have tested whether receptor specific T cell responses are rescued in TNF-treated T cell hybridomas by retroviral-mediated expression of ζ-chimeric (C2ζ) receptors or wild-type TCRζ. Expression of C2ζ receptors at the cell surface is relatively refractory to chronic TNF stimulation. However, C2ζ receptor function depends on association with endogenous TCRζ chains, whose expression is down-regulated by TNF, and so C2 receptor specific responses are attenuated in TNF-treated T cells. Unexpectedly, overexpression of wild-type TCRζ maintains cell surface TCR/CD3 complex expression but fails to rescue receptor proximal signaling in TNF-treated T cells, suggesting the existence of hitherto unrecognized mechanisms through which TNF regulates T cell responsiveness. We provide additional evidence that TNF also uncouples distal TCR signaling pathways independently of its effects on TCRζ expression.