CD4 and CD8 molecules can physically associate with the same T-cell receptor.

The expression of the cell-surface glycoproteins CD4 and CD8 on functionally mature T cells is usually mutually exclusive and correlates with class II and class I major histocompatibility complex (MHC) restriction, respectively. CD4 and CD8 function by binding to class II and class I MHC molecules on the antigen-presenting cell (APC), thereby increasing the adhesion between the T cell and the APC. From antibody-blocking studies and from cocapping and comodulation experiments, CD4 and CD8 come into close physical contact with their appropriately restricted T-cell receptor (TCR) at the time of antigen recognition. By the use of affinity chromatography followed by two-dimensional diagonal gel electrophoresis, we have identified a Mr 43,000 disulfide-bonded heterodimer copurifying with CD4. This protein was identified as the TCR by its removal after preclearing with the anti-TCR antibody F23.1 and by its generation after protease digestion of the same peptides as the TCR from this clone. When CD4 and CD8 were similarly isolated from an unusual CD4+ CD8+ class II-restricted T-cell clone, the TCR was identified as associating with either accessory molecule in the absence of activation. Therefore, CD4 and CD8 do not distinguish between class I- and class II-restricted TCRs in their ability to form membrane complexes, indicating a need for both the TCR and its associated accessory molecule to recognize the same individual MHC molecule on the APC to optimize TCR triggering.