1159 Identifying cross-presented glioblastoma antigens for improved immunotherapy design

Abstract

Background

Although immune checkpoint inhibitors (ICIs) have demonstrated success in several solid tumors, response among patients with glioblastoma (GBM) remains limited. Challenges in developing effective immunotherapy include defective T cell priming by antigen presenting cells (APCs), lack of tumor cell-specific targets, and T cell exhaustion in the tumor microenvironment. MHC-I associated peptides on tumors provide a rich source of targetable antigens, but antigen presentation abundance, cell of origin specificity, or targeted therapeutic efficacy are not well understood.

Methods

We used Liquid Chromatography - Tandem Mass spectrometry (LC-MS/MS) based immunopeptidomics to characterize glioblastoma tumor antigens that are cross-presented (XPT) on bone marrow-derived dendritic cells (BMDCs), splenic DCs (sDCs), and bone marrow-derived macrophages (BMDMs) (figure 1). We also performed absolute quantification of XPT peptide abundance using SureQuant MHC, estimating the copies/cell cross-presentation on BMDCs or BMDMs.

Results

We discovered distinct cell type specificity of MHC-I peptides by comprehensively profiling the immunopeptidome of two GBM cell lines GL261 and CT2A, and cross-presenting APCs. We categorized XPT peptides as 1) shared antigens that are also presented endogenously on tumor cells and APCs, 2) tumor-specific antigens that are presented on immune cells, and 3) XPT-specific peptides that are not already endogenously presented on tumor or APCs. Shared XPT antigens cross-presentation abundance was positively correlated with the endogenous presentation abundance, reflecting APC-specific antigen processing machinery. Surprisingly, there were few tumor-specific XPT antigens, which should be important for priming an adaptive immune response against the tumor. Additionally, more than 50% of the profiled XPT repertoire was comprised of XPT-specific peptides, which may generate a T cell response incapable of recognizing tumor targets.

Conclusions

Overall, we uncovered the GBM XPT peptide repertoire on APCs and quantified XPT peptide abundance. The profiled XPT peptides and their cell of origin restriction may inform novel immunotherapy targets, and the relationship between endogenous and XPT peptide presentation may instruct generalizable computational models to predict XPT antigens across other cancers.

Abstract 1159 Figure 1

Identifying cross-presented peptides Tumor cells were cultured in stable isotope labeled amino acids and fed to antigen presenting cells. Cross-presented peptides were identified by immunopeptidomics analysis

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