Radical additions to aromatic residues in peptides facilitate unexpected side chain and backbone losses

Characterization of HLA-A*02:01 MHC Immunopeptide Antigens Enhanced by Ultraviolet Photodissociation Mass Spectrometry

Identifying major histocompatibility complex (MHC) class I immunopeptide antigens represents a key step in the development of immune-based targeted therapeutics and vaccines. However, the complete characterization of these antigens by tandem mass spectrometry remains challenging due to their short sequence length, high degree of hydrophobicity, and/or lack of sufficiently basic amino acids. This study seeks to address the potential for 193 nm ultraviolet photodissociation (UVPD) to improve the analysis of MHC class I immunopeptides by offering enhanced characterization of these sequences in lower charge states and differentiation of prominent isomeric leucine and isoleucine residues in the HLA-A*02:01 motif. Although electron transfer dissociation-higher energy collisional dissociation (EThcD) offered some success in the differentiation of leucine and isoleucine, 193 nm UVPD was able to confirm the identity of nearly 60% of leucine and isoleucine residues in a synthetic peptide mixture. Furthermore, 193 nm UVPD led to significantly more peptide identifications and higher scoring metrics than EThcD for peptides obtained from immunoprecipitation of MHC class I immunopeptides from in vitro cell culture. Additionally, 193 nm UVPD represents a promising complementary technique to higher-energy collisional dissociation (HCD), in which 424 of the 2593 peptides identified by 193 nm UVPD were not identified by HCD in HLA-A*02:01-specific immunoprecipitation and 804 of the 3300 peptides identified by 193 nm UVPD were not identified by HCD for pan HLA-A, -B, and -C immunoprecipitation. These results highlight that 193 nm UVPD offers an option for the characterization of immunopeptides, including differentiation of leucine and isoleucine residues.