T cell receptor engagement of peptide-major histocompatibility complex class I does not modify CD8 binding

Activation of cytotoxic T cells is initiated by engagement of the T-cell receptor (TCR) with peptide-major histocompatibility class I complexes (pMHCI). The CD8 co-receptor also binds to pMHCI, but at a distinct site, and allows the potential for tripartite TCR/pMHCI/CD8 interactions, which can increase T cell antigen sensitivity. There has been a substantial interest in the effect of the pMHCI/CD8 interaction upon TCR/pMHCI engagement, and several conflicting studies have examined this event, using the soluble extracellular domains of CD8 and the TCR, by surface plasmon resonance. However, the evidence to date suggests that the TCR engages cognate pMHCI before CD8 recruitment, so the question of whether TCR engagement alters CD8 binding is likely to be more relevant to the biological order of T cell antigen encounter. Here, we have examined the binding of CD8 to several variants of the HLA A2-restricted telomerase(540-548) antigen (ILAKFLHWL) and the HLA A2-restricted NY-ESO-1(157-165) antigen (SLLMWITQC) that bind to their cognate TCRs with distinct affinities and kinetics. These interactions represent a range of agonists that exhibit different CD8 dependency for activation of their respective T cells. By using engineered affinity enhanced TCRs to these ligands, which have extended off-rates of approximately 1h compared to seconds for the wildtype TCRs, we have examined pMHCI/CD8 binding before and during TCR-engagement. Here we show that the binding of the extracellular domain of the TCR to pMHCI does not transmit structural changes to the pMHCI-CD8 binding site that would alter the subsequent pMHCI/CD8 interaction.