Structural basis for ineffective T-cell responses to MHC anchor residue-improved "heteroclitic" peptides.

Florian Madura,Andrew K Sewell,Christopher J Holland,David K Cole,Andrew J Godkin,Andrea J Schauenburg,Anna Fuller,Anna Bulek,Pierre J Rizkallah

doi:10.1002/eji.201445114

Abstract

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.

Highlights

CD8+ T cells can exploit the major histocompatibility complex class I (MHCI) peptide presentation pathway to interrogate the cellular proteome and identify aberrant gene expression within cancer cells
An effective CD8+ T-cell response requires that the cell surface expressed TCR binds to its cognate peptide-MHCI molecule with sufficient affinity/duration to induce cellular activation [2, 3]
In order to determine the mechanism for how the MEL5 TCR bound to these peptides with over twofold difference in affinity, we performed a thermodynamic analyses of the MEL5-A/MART-126–35 (EAAGIGILTV) antigen (A2-EAA) interaction and compared it to data for A2-ELA [25] (Fig. 1C)

Summary

Introduction

CD8+ T cells can exploit the major histocompatibility complex class I (MHCI) peptide presentation pathway to interrogate the cellular proteome and identify aberrant gene expression within cancer cells. To date there is no atomic structure of the natural A2-EAA peptide in complex with a TCR We fill this important knowledge gap by solving the atomic structure of the MEL5-A2-EAA complex and performing an in-depth biophysical analysis to determine the molecular mechanism for altered T-cell recognition between A2-ELA and the tumor expressed A2-EAA antigen by the MEL5 T cell. These data demonstrate the extreme sensitivity of the TCR to minor conformational changes in the peptide, extend our understanding of CD8+ T-cell recognition of a prominent tumor target and have important implications for the design of altered peptide ligands for use in cancer vaccination

Results

Discussion

Materials and methods