To characterize the function of the CD3η subunit of the T cell receptor (TCR), we have used cDNAs encoding CD3ζ, CD3η, or both to reconstitute a variant of a cytochrome c-specific, I-Ek-restricted murine T cell hybridoma, termed MA5.8, which lacks CD3ζ and CD3η proteins. We provide direct evidence that assembly and surface expression of TCRs can be mediated by either of these subunits separately or together. However, the level of TCR expression on ζ transfectants is up to one order of magnitude greater than that on η transfectants, implying that CD3η is weakly associated with the pentameric Tiα-βCD3γδe complex and/or inefficient at salvaging the incomplete TCR from lysosomal degradation. As a component of the TCR, the CD3η subunit preferentially forms a heterodimer with CD3ζ, but is also able to form a CD3η-η homodimer. Crosslinking of Tiα-βCD3γδeζ-ζ, Tiα-βCD3γδeη-η, or Tiα-βCD3γδeζ-ζ/ζ-η-TCR isotypes with anti-CD3e monoclonal antibody or a cytochrome c peptide epitope on I-Ek antigen-presenting cells mediates signal transduction resulting in reversible cell-cycle arrest of transfected clones. Given the potential for diversity of signals generated by these functional TCR isotypes and the expression of the CD3 η gene product in the thymus, CD3η is likely to play a role in selection and/or activation of thymocytes during development.
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