Mass Spectrometry-Based Identification of MHC-Associated Peptides

Abstract

Neoantigen-based immunotherapies promise to improve patient outcomes over the current standard of care. However, detecting these cancer-specific antigens is one of the significant challenges in the field of mass spectrometry. Even though the first sequencing of the immunopeptides was done decades ago, today there is still a diversity of the protocols used for neoantigen isolation from the cell surface. This heterogeneity makes it difficult to compare results between the laboratories and the studies. Isolation of the neoantigens from the cell surface is usually done by mild acid elution (MAE) or immunoprecipitation (IP) protocol. However, limited amounts of the neoantigens present on the cell surface impose a challenge and require instrumentation with enough sensitivity and accuracy for their detection. Detecting these neopeptides from small amounts of available patient tissue limits the scope of most of the studies to cell cultures. Here, we summarize protocols for the extraction and identification of the major histocompatibility complex (MHC) class I and II peptides. We aimed to evaluate existing methods in terms of the appropriateness of the isolation procedure, as well as instrumental parameters used for neoantigen detection. We also focus on the amount of the material used in the protocols as the critical factor to consider when analyzing neoantigens. Beyond experimental aspects, there are numerous readily available proteomics suits/tools applicable for neoantigen discovery; however, experimental validation is still necessary for neoantigen characterization.

Keywords: Neoantigens; cancer; immunoprecipitation; mass spectrometry; mild acid elution.

Figure 1

Methods for major histocompatibility complex (MHC) peptide isolation. (A) Acid wash and (B) immunoprecipitation (IP)-based purification. Reprinted with permission from Schumacher et al. [25], copyright year 2017, John Wiley and Sons.

Figure 2

Derivatization reaction to increase sequence coverage and ionization efficiency of peptides. (A) Scheme of the derivatization and (B) tandem MS spectra of synthetic peptide (SVATITGV) in native, dimethylated, and alkylamidated form. Reprinted with permission from Chen et al. [39], copyright year 2018, American Chemical Society.