CD3δ establishes a functional link between the T cell receptor and CD8

T cells expressing T cell receptor (TCR) complexes that lack CD3delta, either due to deletion of the CD3delta gene, or by replacement of the connecting peptide of the TCRalpha chain, exhibit severely impaired positive selection and TCR-mediated activation of CD8 single-positive T cells. Because the same defects have been observed in mice expressing no CD8beta or tailless CD8beta, we examined whether CD3delta serves to couple TCR-CD3 with CD8. To this end we used T cell hybridomas and transgenic mice expressing the T1 TCR, which recognizes a photoreactive derivative of the PbCS 252-260 peptide in the context of H-2K(d). We report that, in thymocytes and hybridomas expressing the T1 TCR-CD3 complex, CD8alphabeta associates with the TCR. This association was not observed on T1 hybridomas expressing only CD8alphaalpha or a CD3delta(-) variant of the T1 TCR. CD3delta was selectively coimmunoprecipitated with anti-CD8 antibodies, indicating an avid association of CD8 with CD3delta. Because CD8alphabeta is a raft constituent, due to this association a fraction of TCR-CD3 is raft-associated. Cross-linking of these TCR-CD8 adducts results in extensive TCR aggregate formation and intracellular calcium mobilization. Thus, CD3delta couples TCR-CD3 with raft-associated CD8, which is required for effective activation and positive selection of CD8(+) T cells.