Abstract 1008: Peroxynitrite mediates the resistance of tumor cells to cytotoxic T cells by altering the MHC1-peptide repertoire on tumor cells
Authors: Tcyganov Evgenii N. , Gabrilovich Dmitry I. 
Published: August 2018

Therapeutic advances were able to significantly enhance antigen-specific T cell responses in cancer therapy. However, despite the obvious success, our ability to translate the generation of specific T-cell response into a clinical benefit remains limited. Recently, factors of tumor microenvironment such as hypoxia and oxidative agents were shown to dampen the efficiency of T-cell responses. Previously, we demonstrated that macrophages and myeloid-derived suppressor cells (MDSCs) produce a significant amount of highly oxidative agent, peroxynitrite (PNT), inside the tumor and revealed that PNT inhibited binding of processed peptides to tumor cell MHC class I (MHC I). As a result, tumor cells become resistant to antigen-specific cytotoxic T cells (CTLs). In other studies, it was shown that high levels of PNT (measured by nitrotyrosine level) in tumor tissues are associated with worse clinical outcome. We hypothesized that PNT affects peptide repertoire presented by tumor cell MHC I molecules and by this impairs the immune recognition of tumor cells by CTLs. To test this hypothesis we compared MHC I peptides isolated from PNT-treated and nontreated tumor cells. Quantitative LC-MS/MS analysis revealed that PNT treatment affected the peptide repertoire of MHC I bound peptides. A number of peptides were substantially underrepresented in PNT-treated cells (“nonstable” peptide subset), whereas others remained at the same level after PNT treatment (“stable” peptide subset). We generated CTLs specific to some peptides from “stable” or “nonstable” groups. PNT treatment significantly decreased the cytotoxic activity of T cells specific to “non-stable” but not to “stable” peptides, confirming our initial hypothesis. To further characterize “stable” and “nonstable” peptides we investigated their affinity to MHC I as well as the stability of the formed peptide-MHC I (pMHC) complexes. We found that peptides from both groups demonstrated equally high affinity to MHC I. However, we observed significantly higher dissociation rate of pMHC complexes formed by peptides from “nonstable” group than by “stable” peptides. PNT treatment disrupted proteasome function in tumor cells, suggesting that PNT may impair antigen-processing machinery and “nonstable” peptides quickly dissociating from MHC I were not recovered by malfunctioning proteasome. Consecutively, this could lead to the poorer peptide presentation on the tumor cell surface hindering tumor cell recognition and killing by CTLs. Our findings demonstrate a novel PNT-mediated mechanism of tumor cell resistance to specific CTLs and suggest potential therapeutic avenues for its neutralization.

Citation Format: Evgenii N. Tcyganov, Dmitry I. Gabrilovich. Peroxynitrite mediates the resistance of tumor cells to cytotoxic T cells by altering the MHC1-peptide repertoire on tumor cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1008.