Regulation of Constitutive TCR Internalization by the ζ-Chain

Abstract

The ability of a T cell to be activated is critically regulated by the number of TCRs expressed on the plasma membrane. Cell surface TCR expression is influenced by dynamic processes such as synthesis and transport of newly assembled receptors, endocytosis of surface TCR, and recycling to the plasma membrane of internalized receptors. In this study, the internalization of fluorescently labeled anti-TCR Abs was used to analyze constitutive endocytosis of TCRs on T cells, and to investigate the role of the ζ-chain in this process. We found that cell surface TCRs lacking ζ were endocytosed more rapidly than completely assembled receptors, and that reexpression of full-length ζ led to a dose-dependent decrease in the rate of TCR internalization. Rapid TCR internalization was also observed with CD4+CD8+ thymocytes from ζ-deficient mice, whereas TCR internalization on thymocytes from CD3-δ deficient animals was slow, similar to that of wild-type thymocytes. This identifies a specific role for ζ in the regulation of constitutive receptor internalization. Furthermore, chimeric ζ molecules containing non-native intracellular amino acid sequences also led to high levels of TCR expression and reduced TCR cycling. These effects were dependent solely on the length of the intracellular tail, ruling out a role for intracellular ζ-specific interactions with other molecules as a mechanism for regulating TCR internalization. Rather, these findings strongly support a model in which the ζ-chain stabilizes TCR residency on the cell surface, and functions to maintain cell surface receptor expression by sterically blocking internalization sequences in other TCR components.