High-affinity CD8 variants enhance the sensitivity of pMHCI antigen recognition via low-affinity TCRs

The recognition of cell surface presented peptide-Major Histocompatibility Complex Class I (pMHCI) molecules by CD8 T-cells involves cooperative binding of the T-cell receptor (TCR) and CD8 co-receptor. CD8 T-cell antigen specificity is conferred by the TCR, whilst CD8 acts to stabilize the TCR/pMHCI complex and enhance T-cell antigen sensitivity. Earlier work has shown that the sensitivity of antigen recognition can be regulated in vitro by altering the strength of the pMHCI/CD8 interaction. Here, we characterize two CD8 variants with an enhanced affinity for MHCI that remains below the affinity threshold at which non-specific activation is observed. In model systems, expression of these CD8 variants preferentially enhanced pMHCI antigen recognition in the context of low-affinity TCRs. When combined with MHCI-restricted TCRs in primary CD4 T-cells, high affinity CD8 variants could improve T-cell functionality, without loss of antigen specificity. In primary CD8 T-cells, the introduction of high affinity CD8 enhanced T-cell activation compared to endogenous CD8 expression only, although we observed that the introduction of transgenic wild-type CD8 into primary CD8 T-cells also resulted in a similar T-cell effector function enhancement. Collectively, these findings could provide a generically applicable and immediately translatable strategy to augment the therapeutic efficacy of clinically relevant TCRs, which are already being delivered alongside wild-type CD8.