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CD4+ T-cells recognize peptide antigens, in the context of human leukocyte antigen (HLA) class II molecules (HLA-II), which through peptide-flanking residues (PFRs) can extend beyond the limits of the HLA binding. The role of the PFRs during antigen recognition is not fully understood; however, recent studies have indicated that these regions can influence T-cell receptor (TCR) affinity and pHLA-II stability. Here, using various biochemical approaches including peptide sensitivity ELISA and ELISpot assays, peptide-binding assays and HLA-II tetramer staining, we focused on CD4+ T-cell responses against a tumor antigen, 5T4 oncofetal trophoblast glycoprotein (5T4), which have been associated with improved control of colorectal cancer. Despite their weak TCR-binding affinity, we found that anti-5T4 CD4+ T-cells are polyfunctional and that their PFRs are essential for TCR recognition of the core bound nonamer. The high-resolution (1.95 Å) crystal structure of HLA-DR1 presenting the immunodominant 20-mer peptide 5T4111-130, combined with molecular dynamic simulations, revealed how PFRs explore the HLA-proximal space to contribute to antigen reactivity. These findings advance our understanding of what constitutes an HLA-II epitope and indicate that PFRs can tune weak affinity TCR-pHLA-II interactions.

Original publication

DOI

10.1074/jbc.ra119.009437

Type

Journal article

Journal

The Journal of biological chemistry

Publication Date

12/2019

Volume

294

Pages

20246 - 20258

Addresses

Division of Infection and Immunity and Systems Immunity Research Institute, Cardiff University, Cardiff CF14 4XN, United Kingdom.

Keywords

CD4-Positive T-Lymphocytes, Humans, Colorectal Neoplasms, Peptides, Membrane Glycoproteins, HLA-DR1 Antigen, Epitopes, Crystallography, X-Ray, Binding Sites, Amino Acid Sequence, Protein Structure, Secondary, Protein Structure, Tertiary, Protein Binding, Molecular Dynamics Simulation