HLA class I binding predictions are widely used to identify candidate peptide targets of human CD8+ T cell responses. Many such approaches focus exclusively on a limited range of peptide lengths, typically 9 and sometimes 9-10 amino acids, despite multiple examples of dominant epitopes of other lengths. Here, we examined if epitope predictions can be improved by incorporating the natural length distribution of HLA class I ligands. We found that while different HLA alleles have diverse length binding preferences, the length profiles of ligands that are naturally presented by these alleles are much more homogeneous. We hypothesized that this is due to a defined length profile of peptides available for HLA binding in the endoplasmic reticulum. Based on this, we created a model of HLA allele specific ligand length profiles, and demonstrate how this model, in combination with HLA binding predictions, greatly improves comprehensive identification of CD8+ T cell epitopes.

Supplementary data are available at Bioinformatics online.

HLA class I presentation of pathogen-derived peptide ligands is essential for CD8+ T-cell recognition of Toxoplasma gondii infected cells. Currently, little data exist pertaining to peptides that are presented after T. gondii infection. Herein we purify HLA-A*02:01 complexes from T. gondii infected cells and characterize the peptide ligands using LCMS. We identify 195 T. gondii encoded ligands originating from both secreted and cytoplasmic proteins. Surprisingly, T. gondii ligands are significantly longer than uninfected host ligands, and these longer pathogen-derived peptides maintain a canonical N-terminal binding core yet exhibit a C-terminal extension of 1–30 amino acids. Structural analysis demonstrates that binding of extended peptides opens the HLA class I F’ pocket, allowing the C-terminal extension to protrude through one end of the binding groove. In summary, we demonstrate that unrealized structural flexibility makes MHC class I receptive to parasite-derived ligands that exhibit unique C-terminal peptide extensions.DOI: http://dx.doi.org/10.7554/eLife.12556.001