HLA-I peptidomes and phosphopeptidomes (B*40, B*27, B*07, B*39)

As aberrant phosphorylation is a hallmark of tumor cells, the display of tumor-specific phosphopeptides by Human Leukocyte Antigen (HLA) class I molecules can be exploited in the treatment of cancer by T-cell-based immunotherapy. Yet, the characterization and prediction of HLA-I phospholigands is challenging as the molecular determinants of the presentation of such post-translationally modified peptides are not fully understood. Here, we employed a peptidomic workflow to identify phosphorylated ligands associated with HLA-B*40, -B*27, -B*39 and -B*07. Remarkably, these phosphopeptides showed similar molecular features. Besides the specific anchor motifs imposed by the binding groove of each allotype, the predominance of phosphorylation at peptide position 4 (P4) became strikingly evident, as was the enrichment of basic residues at P1. This molecular understanding of the presentation of phosphopeptides by HLA-B molecules can help in predicting tumor-specific neo-antigens that arise from aberrant phosphorylation in cancer cells.