LC-MSMS of OVCAR-3 class I immunopeptidome

Endogenous retroelements (ERE) constitute about 42% of the human genome and have been implicated in common human diseases such as autoimmunity and cancer. Despite evidence that some EREs can be expressed at the RNA and protein levels in specific contexts, a systems-level evaluation of their expression in human tissues is lacking. We report that all human tissues express EREs but the breadth and magnitude of ERE expression are very heterogeneous from one tissue to another. ERE expression was particularly high in two MHC-I-deficient tissues (ESCs and testis) and one MHC-I-expressing tissue, mTECs. In mutant mice, we report that the exceptional expression of EREs in mTECs was AIRE-independent. MS analyses identified 103 non-redundant ERE-derived MAPs (ereMAPs) in B-LCLs. These ereMAPs preferentially derived from sense translation of intronic EREs. Notably, detailed analyses of their amino acid composition revealed that ERE-derived MAPs presented homology to viral MAPs. This study shows that ERE expression in somatic tissues is more pervasive and heterogeneous than anticipated. The high and diversified expression of EREs in mTECs and their ability to generate MAPs suggest that EREs may play an important role in the establishment of self-tolerance. The viral-like properties of ERE-derived MAPs suggest that those not expressed in mTECs can be highly immunogenic.