Immunogenicity of non-canonical HLA-tumor ligands identified through proteogenomics
Tumor antigens are central to antitumor immunity. Recent evidence suggests that peptides from non-canonical (nonC) aberrantly translated proteins can be presented on HLA-I by tumor cells. Here, we investigated the immunogenicity of nonC tumor HLA-I ligands (nonC-TL) to better understand their contribution to cancer immunosurveillance and their therapeutic applicability. Using proteogenomics, we identified 517 nonC-TL from 9 patients with melanoma, gynecological, and head and neck cancer. We found no recognition of the 507 nonC-TL tested by autologous ex vivo expanded tumor reactive T-cell cultures while the same cultures demonstrated reactivity to mutated, cancer-germline, or melanocyte differentiation antigens. However, in vitro sensitization of donor peripheral blood lymphocytes against 170 selected nonC-TL, led to the identification of T-cell receptors (TCRs) specific to three nonC-TL, two of which mapped to the 5 UTR regions of HOXC13 and ZKSCAN1, and one mapping to a non-coding spliced variant of C5orf22C. T cells targeting these nonC-TL recognized cancer cell lines naturally presenting their corresponding antigens. Expression of the three immunogenic nonC-TL was shared across tumor types and barely or not detected in normal cells. Our findings predict a limited contribution of nonC-TL to cancer immunosurveillance but demonstrate they may be attractive novel targets for widely applicable immunotherapies.