The structural characteristics of autoreactive-T cell receptor (TCR) engagement of major histocompatability (MHC) class II-restricted self-antigens is established, but how autoimmune-TCRs interact with self-MHC class I has been unclear. We examined how CD8+ T cells kill human islet β-cells, in Type-1 diabetes, via autoreactive-TCR (1E6) recognition of an HLA-A*0201-restricted glucose-sensitive preproinsulin peptide. Rigid ‘lock-and-key’ binding underpinned the 1E6-HLA-A*0201-peptide interaction, whereby 1E6 docked similarly to most MHCI-restricted TCRs. However, this interaction was extraordinarily weak, due to limited contacts with MHCI. TCR binding was highly peptide-centric, dominated by two CDR3-loop-encoded residues, acting as an ‘aromatic-cap’, over the peptide MHCI (pMHCI). Thus, highly focused peptide-centric interactions associated with suboptimal TCR-pMHCI binding affinities might lead to thymic escape and potential CD8+ T cell-mediated autoreactivity.

Background: The molecular principles governing T-cell specificity are poorly understood.Results: High affinity binding of a melanoma-specific T-cell receptor (TCR) is mediated through new MHC contacts and distinct thermodynamics.Conclusion: A novel thermodynamic mechanism upholds TCR-peptide specificity.Significance: TCRs can maintain peptide specificity using a mechanism that may enable widespread, safe enhancement of TCR binding affinity in therapeutic applications.

MHC anchor residue-modified “heteroclitic” peptides have been used in many cancer vaccine trials and often induce greater immune responses than the wild-type peptide. The best-studied system to date is the decamer MART-1/Melan-A26–35 peptide, EAAGIGILTV, where the natural alanine at position 2 has been modified to leucine to improve human leukocyte antigen (HLA)-A*0201 anchoring. The resulting ELAGIGILTV peptide has been used in many studies. We recently showed that T cells primed with the ELAGIGILTV peptide can fail to recognize the natural tumor-expressed peptide efficiently, thereby providing a potential molecular reason for why clinical trials of this peptide have been unsuccessful. Here, we solved the structure of a TCR in complex with HLA-A*0201-EAAGIGILTV peptide and compared it with its heteroclitic counterpart, HLA-A*0201-ELAGIGILTV. The data demonstrate that a suboptimal anchor residue at position 2 enables the TCR to “pull” the peptide away from the MHC binding groove, facilitating extra contacts with both the peptide and MHC surface. These data explain how a TCR can distinguish between two epitopes that differ by only a single MHC anchor residue and demonstrate how weak MHC anchoring can enable an induced-fit interaction with the TCR. Our findings constitute a novel demonstration of the extreme sensitivity of the TCR to minor alterations in peptide conformation.

Background: Current peptide vaccines may select suboptimal antigen-specific T-cells from polyclonal populations.Results: A combinatorial peptide library screen was used to generate an optimal ligand that could preferentially activate a known effective T-cell clonotype.Conclusion: Rationally designed altered peptide ligands may enable the preferential selection of high quality, antigen-sensitive T-cell clonotypes.Significance: This proof-of-principle study could facilitate the development of more effective peptide vaccination strategies.

Background: TCR recognition of bipartite ligands composed of self (MHC) and non-self (peptide) maintains T-cell specificity.Results: Mutation of residues in the cognate peptide override TCR mutations that enhance MHC binding.Conclusion: TCR-pMHC binding affinity requires specific TCR-peptide interactions.Significance: Stabilization of TCR-pMHC engagement by TCR-peptide interactions maintains T-cell specificity and prevents recognition of self-pMHC in the periphery.