Protective adaptive immune responses rely on TCR-mediated recognition of Ag-derived peptides presented by self-MHC molecules. However, self-Ag (tumor)-specific TCRs are often of too low affinity to achieve best functionality. To precisely assess the relationship between TCR–peptide–MHC binding parameters and T cell function, we tested a panel of sequence-optimized HLA-A* 0201/NY–ESO-1 157–165–specific TCR variants with affinities lying within physiological boundaries to preserve antigenic specificity and avoid cross-reactivity, as well as two outliers (ie, a very high-and a low-affinity TCR). Primary human CD8 T cells transduced with these TCRs demonstrated robust correlations between binding measurements of TCR affinity and avidity and the biological response of the T cells, such as TCR cell-surface clustering, intracellular signaling, proliferation, and target cell lysis. Strikingly, above a defined TCR–peptide–MHC affinity threshold (K D<∼ 5 μM), T cell function could not be further enhanced, revealing a plateau of maximal T cell function, compatible with the notion that multiple TCRs with slightly different affinities participate equally (codominantly) in immune responses. We propose that rational design of improved self-specific TCRs may not need to be optimized beyond a given affinity threshold to achieve both optimal T cell function and avoidance of the unpredictable risk of cross-reactivity.

Protective immune responses rely on the specific TCR-dependent recognition of Ag-derived peptides bound to self-MHC molecules, and the strength of TCR-mediated Ag recognition is a major correlate of protection from disease (1–3). In patients with melanoma, Ag-specific CD8+ T cell responses often develop, but their protective activity is limited. It has been shown that T lymphocytes directed against tumor Ags express TCRs of lower affinity/avidity for their antigenic ligands than pathogen-specific T cells (4). Apparently, mechanisms of self-tolerance shape self-Ag–specific T cell repertoires with a relative lack of high-affinity TCRs. Indeed, many of the tumor Ags recognized by CTLs are in fact self-Ags that are expressed by germ line cells and selected adult tissues or by lineages of normal cells (5). Therefore, the relative lack of high-affinity/avidity TCRs may be a major reason why immune responses toward self-Ags (tumor) are often nonprotective.