Expression of the T Cell Antigen Receptor ζ Chain following Activation Is Controlled at Distinct Checkpoints

Abstract

The multisubunit T cell antigen receptor (TCR) is in-volved 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 effi-ciency and duration of the immune response, the molec-ular mechanisms controlling TCR down-modulation and re-expression upon activation, remain obscure. Here, we provide a biochemical characterization of the regula-tory 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 sur-face. We found that following TCR-mediated activation ␨ mRNA is up-regulated by a transcription-dependent mechanism. Concomitantly, ␨ protein levels are modi-fied according to a biphasic pattern: rapid degradation coinciding with TCR cell surface down-regulation, fol-lowed 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 pre-viously undescribed monomeric form of the ␨ chain. This form could be indicative of adverse endoplasmic reticu-lum conditions affecting correct protein folding, dimer-ization, and TCR assembly, all critical for optimal recep-tor surface re-expression. Cumulatively, our results indicate that the levels of TCR expression following ac-tivation, are tightly controlled at several checkpoints.