Trans-splicing of class I HLA bound peptides diversifies the immunopeptidome

The diversity of peptides displayed by class I HLA plays an essential role in T cell immunity. The peptide repertoire is extended by various post-translational modifications, including cis-splicing of peptides from the same protein. Here, we have applied a novel bioinformatic workflow and demonstrate that spliced-peptides are also generated through trans-splicing (fusion of peptide segments from distinct antigens) and their abundance challenges current models of proteasomal-splicing that predict cis-splicing as the most probable outcome. These trans-spliced peptides display canonical HLA binding motif sequence features. These results highlight the unanticipated diversity of the immunopeptidome and have important implications for autoimmunity, vaccine design and immunotherapy.