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Here, we develop PEPSeek, a web-server based software to allow higher performance in the identification of pathogen-derived epitope candidates detected via mass spectrometry in MHC class I immunopeptidomes. We apply it to human and mouse cell lines infected with either SARS-CoV-2, Listeria monocytogenesor Chlamydia trachomatis, thereby identifying a large number of novel antigens and epitopes that we prove to be recognized by CD8+T cells. In infected cells, we identified antigenic peptide features that suggested how processing and presentation of pathogenic antigens differ between pathogens. The quantitative tools of PEPSeek also helped to define how C. trachomatisinfection cycle could impact on the antigenic landscape of the host human cell system, likely reflecting metabolic changes occurred in the infected cells.

Rescoring of mass spectrometry (MS) search results using spectral predictors can strongly increase peptide spectrum match (PSM) identification rates. This approach is particularly effective when aiming to search MS data against large databases, for example, when dealing with nonspecific cleavage in immunopeptidomics or inflation of the reference database for noncanonical peptide identification. Here, we present inSPIRE (in silicoSpectral Predictor Informed REscoring), a flexible and performant open-source rescoring pipeline built on Prosit MS spectral prediction, which is compatible with common database search engines. inSPIRE allows large-scale rescoring with data from multiple MS search files, increases sensitivity to minor differences in amino acid residue position, and can be applied to various MS sample types, including tryptic proteome digestions and immunopeptidomes. inSPIRE boosts PSM identification rates in immunopeptidomics, leading to better performance than the original Prosit rescoring pipeline, as confirmed by benchmarking of inSPIRE performance on ground truth datasets. The integration of various features in the inSPIRE backbone further boosts the PSM identification in immunopeptidomics, with a potential benefit for the identification of noncanonical peptides.