Loss of N-terminal charged residues of mouse CD3ε chains generates isoforms modulating antigen T cell receptor-mediated signals and T cell receptor-CD3 interactions

Abstract

The antigen T cell receptor (TCR)-CD3 complexes present on the cell surface of CD4+ T lymphocytes and T cell lines express CD3ε chain isoforms with different isoelectric points (pI), with important structural and functional consequences. The pI values of the isoforms fit the predicted pI values of CD3ε chains lacking one, two, and three negatively charged amino acid residues present in the N-terminal region. Different T cells have different ratios of CD3ε chain isoforms. At a high pI, degraded CD3ε isoforms can be better recognized by certain anti-CD3 monoclonal antibodies such as YCD3-1, the ability of which to bind to the TCR-CD3 complex is directly correlated with the pI of CD3ε. The abundance of CD3ε isoforms can be modified by treatment of T cells with the proteinase inhibitor phenanthroline. In addition, these CD3ε isoforms have functional importance. This is shown, first, by the different structure of TCR-CD3 complexes in cells possessing different amounts of isoforms (as observed in surface biotinylation experiments), by their different antigen responses, and by the stronger interaction between low pI CD3ε isoforms and the TCR. Second, incubation of cells with phenanthroline diminished the proportion of degraded high pI CD3ε isoforms, but also the ability of the cells to deliver early TCR activation signals. Third, cells expressing mutant CD3ε chains lacking N-terminal acid residues showed facilitated recognition by antibody YCD3-1 and enhanced TCR-mediated activation. Furthermore, the binding avidity of antibody YCD3-1 was different in distinct thymus populations. These results suggest that changes in CD3ε N-terminal chains might help to fine-tune the response of the TCR to its ligands in distinct activation situations or in thymus selection. © 2007 by The American Society for Biochemistry and Molecular Biology, Inc.