T cell antigen receptor zeta chain down-regulation and impaired in vitro T cell function have been described in cancer and autoimmune and infectious diseases. However, the immunological basis for this phenomenon is unknown. Sustained exposure to antigen and chronic systemic inflammation, factors shared by the various pathologies, might account for this phenomenon. We developed an in vivo experimental system that mimics these conditions and show that sustained exposure of mice to bacterial antigens was sufficient to induce T cell antigen receptor zeta chain down-regulation and impair T cell function, provided an interferon-gamma-dependent T helper type 1 immune response developed. This indicates zeta chain down-regulation could be a physiological response that attenuates an exacerbated immune response. However, it can act as a 'double-edged sword', impairing immune responses to chronic diseases.

The multisubunit T cell antigen receptor (TCR) is involved in antigen recognition and signal transduction, leading to T cell activation and rapid down-modulation of the cell surface expressed TCRs. Although the levels of TCR cell surface expression are pivotal to the efficiency and duration of the immune response, the molecular mechanisms controlling TCR down-modulation and re-expression upon activation, remain obscure. Here, we provide a biochemical characterization of the regulatory mechanisms governing TCR expression following long-term T cell activation. We focused primarily on the TCR chain, as this is considered the limiting factor in TCR complex formation and transport to the cell surface. We found that following TCR-mediated activation mRNA is up-regulated by a transcription-dependent mechanism. Concomitantly, protein levels are modified according to a biphasic pattern: rapid degradation coinciding with TCR cell surface down-regulation, followed by a rebound to normal levels 24 h subsequent to T cell activation. Even though there are adequate levels of all the TCR subunits within the cell following 24 h of activation, TCR cell surface expression remained very low, provided the activating antibody is continuously present. Correlative with the latter, we detected a previously undescribed monomeric form of the chain. This form could be indicative of adverse endoplasmic reticulum conditions affecting correct protein folding, dimerization, and TCR assembly, all critical for optimal receptor surface re-expression. Cumulatively, our results indicate that the levels of TCR expression following activation, are tightly controlled at several checkpoints.The T cell antigen receptor (TCR) 1 is a multisubunit complex composed of the clonotypic ␣/␤ heterodimer that is involved in the recognition and binding of the antigen-major histocompatibility complex, as well as of the invariant CD3 chains (␥, ␦, ⑀) and the / homodimer that couple antigen recognition to intracellular signal transduction pathways. Optimal T cell activation is achieved by the delivery of two signals: one mediated via the TCR upon binding of the antigen-major histocompatibility complex presented by antigen-presenting cells, and the other is delivered through costimulatory receptors (1-3). Subsequent to TCR engagement, long lasting TCR down-regulation has been observed (4), resulting in a state of sustained desensitization and unresponsiveness to renewed antigenic stimuli (5, 6). Interestingly, it was recently shown that distinct modes of T cell activation have a differential effect on the fate of cell surface expressed TCRs: whereas TCR ligation induces rapid TCR internalization and degradation (7,8), T cell stimulation with PMA, which by-passes the TCR, leads to TCR internalization and recycling (9). These observations suggest that activation-induced TCR down-regulation may trigger the termination of an immune response and/or induce tolerance (10). Eventually, the TCRs are re-expressed on the cell surface and the cells regain their responsiveness to ...