Peroxynitrite in the tumor microenvironment changes the profile of antigens allowing escape from cancer immunotherapy

Cancer immunotherapy often depends on recognition of peptide epitopes by cytotoxic T lymphocytes (CTLs). The tumor microenvironment (TME) is enriched for peroxynitrite (PNT), a potent oxidant produced by infiltrating myeloid cells and some tumor cells. We demonstrate that PNT alters the profile of MHC class I bound peptides presented on tumor cells. Only CTLs specific for PNT-resistant peptides have a strong antitumor effect in vivo, whereas CTLs specific for PNT-sensitive peptides are not effective. Therapeutic targeting of PNT in mice reduces resistance of tumor cells to CTLs. Melanoma patients with low PNT activity in their tumors demonstrate a better clinical response to immunotherapy than patients with high PNT activity. Our data suggest that intratumoral PNT activity should be considered for the design of neoantigen-based therapy and also may be an important immunotherapeutic target.

Abstract 5610: Targeting Rb1 in experimental pancreatic cancer remodels immune cell landscape and suppresses tumor progression

Background: Pancreatic ductal adenocarcinoma (PDA) remains among the cancers with the poorest prognosis. Currently, no effective treatment available for PDA. Immunotherapy that has been shown effective for several cancer types is not successful for PDA treatment overall. The possible reasons of this PDA immunotherapy resistance include lack of neoantigen mutations, low T cell responses and overall immunosuppression state in the tumor microenvironment mediated by myeloid cells and tumor stroma. Recent studies suggest that targeting tumor microenvironment can overcome this immunotherapy resistance and significantly improve the disease outcome. Retinoblastoma (Rb1) protein is a known tumor suppressor protein. In addition, there is accumulating evidence for its role in immune cell response as well, including regulation of apoptosis in myeloid cells. However, the mechanisms of this phenomenon remain not clear. Here we show that targeting Rb1 with a novel molecule AP-3-84 developed in our lab causes apoptosis of tumor-associated macrophages (TAMs) in the model of PDA, induces T cell infiltration of the tumor and decreases tumor burden.

Results: Recently our lab has developed the new small molecule AP-3-84 that is able to bind Rb1 and modulate its activity. Originally, we found that this Rb1 modulator can induce cell death of TAMs and thioglycolate-induced macrophages, but not of the other cell types (neither tumor nor T cells). Gene expression changes upon AP-3-84 treatment showed the induction of oxidative and ER stress programs with a clear activation of p53-dependent genes and caspases. We tested the use of AP-3-84 as immunotherapy against PDA cancer growth using 2 PDA cell lines intrinsically characterized by low T cell and high T cell infiltration (T-low-PDA and T-high-PDA, respectively, originally generated and characterized by J. Li et al., Immunity 2018). In accordance with our in vitro observation, AP-3-84 was able to reshape immune cell landscape in mice with T-high-PDA by depleting macrophages and inducing T cell infiltration. Importantly, those changes were accompanied by significant reduction of tumor growth. Addition of AP-3-84 to immunotherapy (anti-CD40/PD1/CTL4) was also able to decrease tumor burden in T-low-PDA model, which is otherwise resistant to this immunotherapy. Thus, AP-3-84 treatment was able to significantly improve the outcome of experimental PDA by altering immune cell subsets and represent a prospective clinical approach for the clinic.

Statistical analysis of the data was conducted using unpaired Student t-test with additional false discovery rate adjustment for multiple comparisons for gene expression analysis.

Conclusions: Modulation of Rb1 in myeloid cells is a novel promising target for PDA treatment. This approach reduces PDA tumor burden by shifting the immune cell landscape towards greater T cell infiltration upon tumor macrophage depletion.

Citation Format: Evgenii N. Tcyganov, Taekyoung Kwak, Adi Narayana Reddy Poli, Laxminarasimha Donthireddy, Joel Cassel, Andrew Kossenkov, Rahul Shinde, Ben Z. Stanger, Joseph Salvino, Luis J. Montaner. Targeting Rb1 in experimental pancreatic cancer remodels immune cell landscape and suppresses tumor progression [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 5610.

Distinct mechanisms govern populations of myeloid-derived suppressor cells in chronic viral infection and cancer

Myeloid-derived suppressor cells (MDSCs) are major negative regulators of immune responses in cancer and chronic infections. It remains unclear if regulation of MDSC activity in different conditions is controlled by similar mechanisms. We compared MDSCs in mice with cancer and lymphocytic choriomeningitis virus (LCMV) infection. Chronic LCMV infection caused the development of monocytic MDSCs (M-MDSCs) but did not induce polymorphonuclear MDSCs (PMN-MDSCs). In contrast, both MDSC populations were present in cancer models. An acquisition of immune-suppressive activity by PMN-MDSCs in cancer was controlled by IRE1α and ATF6 pathways of the endoplasmic reticulum (ER) stress response. Abrogation of PMN-MDSC activity by blockade of the ER stress response resulted in an increase in tumor-specific immune response and reduced tumor progression. In contrast, the ER stress response was dispensable for suppressive activity of M-MDSCs in cancer and LCMV infection. Acquisition of immune-suppressive activity by M-MDSCs in spleens was mediated by IFN-γ signaling. However, it was dispensable for suppressive activity of M-MDSCs in tumor tissues. Suppressive activity of M-MDSCs in tumors was retained due to the effect of IL-6 present at high concentrations in the tumor site. These results demonstrate disease- and population-specific mechanisms of MDSC accumulation and the need for targeting different pathways to achieve inactivation of these cells.

Myeloid-Derived Suppressor Cells Are a Major Source of Wnt5A in the Melanoma Microenvironment and Depend on Wnt5A for Full Suppressive Activity

Metastatic dissemination remains a significant barrier to successful therapy for melanoma. Wnt5A is a potent driver of invasion in melanoma and is believed to be secreted from the tumor microenvironment (TME). Our data suggest that myeloid-derived suppressor cells (MDSC) in the TME are a major source of Wnt5A and are reliant upon Wnt5A for multiple actions. Knockdown of Wnt5A specifically in the myeloid cells demonstrated a clear decrease in Wnt5A expression within the TME in vivo as well as a decrease in intratumoral MDSC and regulatory T cell (Treg). Wnt5A knockdown also decreased the immunosuppressive nature of MDSC and decreased expression of TGFβ1 and arginase 1. In the presence of Wnt5A-depleted MDSC, tumor-infiltrating lymphocytes expressed decreased PD-1 and LAG3, suggesting a less exhausted phenotype. Myeloid-specific Wnt5A knockdown also led to decreased lung metastasis. Tumor-infiltrating MDSC from control animals showed a strong positive correlation with Treg, which was completely ablated in animals with Wnt5A-negative MDSC. Overall, our data suggest that while MDSC contribute to an immunosuppressive and less immunogenic environment, they exhibit an additional function as the major source of Wnt5A in the TME. SIGNIFICANCE: These findings demonstrate that myeloid cells provide a major source of Wnt5A to facilitate metastatic potential in melanoma cells and rely on Wnt5A for their immunosuppressive function.

Abstract 6615: Peroxynitrite mediates immune escape of tumor cells from cytotoxic T cells in situ

Despite improved ability to generate strong tumor-specific T cell responses, clinical benefits remain relatively moderate. Only a small proportion of predicted neoepitopes was shown to induce specific cytotoxic T cells (CTLs) or serve as an efficient target for T cell-based therapy. We suggest here that low efficacy of CTLs could be at least partially explained by the fact that tumor microenvironment (TME) modulates antigen presentation and epitope profile expressed by tumor. As a result, tumor escapes recognition by CTLs. TME is enriched in peroxynitrite (PNT), a potent oxidant. It is mostly produced by infiltrating myeloid cells, but also by some tumors (as melanoma). We hypothesized that PNT is able to affect antigen presentation on tumor cells and facilitates tumor immune escape.

To check this hypothesis, we treated murine EG7 tumor cells with PNT, isolated MHC I peptides and compared their expression on treated versus non-treated cells using SILAC mass spectrometry. We found that PNT treatment significantly decreased expression of a portion of MHC I peptides presented by tumor cells, whereas other peptides remained relatively intact. We found that PNT-sensitive and resistant peptides demonstrated similar binding capacity to MHC I. However, the off-rate of PNT-sensitive peptides was significantly higher. PNT strongly affected proteasomal activity of tumor cells suggesting the mechanism for under-representation of MHC I peptides with high off-rate level. To investigate the biological role of this effect, we generated CTLs specific to a pool of PNT-sensitive or resistant peptides. In vitro we observed that both types of CTLs effectively killed tumor cells. Pre-treatment of tumor cells with PNT significantly reduced killing only by CTLs specific to PNT-sensitive peptides. By ELISPOT we showed that most of antitumor-specific CTL responses in mice with different tumors were induced against PNT-resistant, but not PNT-sensitive species. Moreover, the adoptive transfer of CTLs specific to PNT-resistant peptides combined with immune checkpoint inhibition (ICI) was able to induce tumor rejection in 70% of mice, whereas similar setting for CTLs specific to PNT-sensitive peptides didn't affect tumor growth. PNT inhibition combined with ICI significantly delayed the tumor growth in mice. In addition, we showed that PNT had similar effect on MHC I peptide profile for human melanoma cells. Finally, we measured nitrotyrosine (NT) level (marker of PNT presence) in tumor tissues resected from melanoma patients before the start of ICI therapy. We retrospectively correlated NT levels with patient ability to respond to ICI therapy. We found that high NT level was a predictor of poor prognosis and patient un-responsiveness to ICI.

Overall, our study demonstrates PNT ability to affect antigen presentation in tumor site. In addition, we suggest PNT as a promising target for ICI-combined therapy as well as a biomarker for resistance to ICI.

Citation Format: Evgenii N. Tcyganov, Dmitry I. Gabrilovich. Peroxynitrite mediates immune escape of tumor cells from cytotoxic T cells in situ [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 6615.

Polymorphonuclear myeloid-derived suppressor cells limit antigen cross-presentation by dendritic cells in cancer

DCs are a critical component of immune responses in cancer primarily due to their ability to cross-present tumor-associated antigens. Cross-presentation by DCs in cancer is impaired, which may represent one of the obstacles for the success of cancer immunotherapies. Here, we report that polymorphonuclear myeloid-derived suppressor cells (PMN-MDSC) blocked cross-presentation by DCs without affecting direct presentation of antigens by these cells. This effect did not require direct cell-cell contact and was associated with transfer of lipids. Neutrophils (PMN) and PMN-MDSC transferred lipid to DCs equally well; however, PMN did not affect DC cross-presentation. PMN-MDSC generate oxidatively truncated lipids previously shown to be involved in impaired cross-presentation by DCs. Accumulation of oxidized lipids in PMN-MDSC was dependent on myeloperoxidase (MPO). MPO-deficient PMN-MDSC did not affect cross-presentation by DCs. Cross-presentation of tumor-associated antigens in vivo by DCs was improved in MDSC-depleted or tumor-bearing MPO-KO mice. Pharmacological inhibition of MPO in combination with checkpoint blockade reduced tumor progression in different tumor models. These data suggest MPO-driven lipid peroxidation in PMN-MDSC as a possible non-cell autonomous mechanism of inhibition of antigen cross-presentation by DCs and propose MPO as potential therapeutic target to enhance the efficacy of current immunotherapies for patients with cancer.

HDAC6 Inhibition Synergizes with Anti-PD-L1 Therapy in ARID1A-Inactivated Ovarian Cancer

ARID1A, encoding a subunit of the SWI/SNF complex, is the most frequently mutated epigenetic regulator in human cancers and is mutated in more than 50% of ovarian clear cell carcinomas (OCCC), a disease that currently has no effective therapy. Inhibition of histone deacetylase 6 (HDAC6) suppresses the growth of ARID1A-mutated tumors and modulates tumor immune microenvironment. Here, we show that inhibition of HDAC6 synergizes with anti-PD-L1 immune checkpoint blockade in ARID1A-inactivated ovarian cancer. ARID1A directly repressed transcription of CD274, the gene encoding PD-L1. Reduced tumor burden and improved survival were observed in ARID1A^flox/flox/PIK3CA^H1047R OCCC mice treated with the HDAC6 inhibitor ACY1215 and anti-PD-L1 immune checkpoint blockade as a result of activation and increased presence of IFNγ-positive CD8 T cells. We confirmed that the combined treatment limited tumor progression in a cytotoxic T-cell-dependent manner, as depletion of CD8⁺ T cells abrogated these antitumor effects. Together, these findings indicate that combined HDAC6 inhibition and immune checkpoint blockade represents a potential treatment strategy for ARID1A-mutated cancers. SIGNIFICANCE: These findings offer a mechanistic rationale for combining epigenetic modulators and existing immunotherapeutic interventions against a disease that has been so far resistant to checkpoint blockade as a monotherapy.See related commentary by Prokunina-Olsson, p. 5476.

Identification of monocyte-like precursors of granulocytes in cancer as a mechanism for accumulation of PMN-MDSCs

Mastio et al. describe monocytic precursors of granulocytes. These precursors are barely detectable in steady state conditions and are not consequential for differentiation of granulocytes. However, they accumulate in cancer and substantially contribute to PMN-MDSC expansion.

We have identified a precursor that differentiates into granulocytes in vitro and in vivo yet belongs to the monocytic lineage. We have termed these cells monocyte-like precursors of granulocytes (MLPGs). Under steady state conditions, MLPGs were absent in the spleen and barely detectable in the bone marrow (BM). In contrast, these cells significantly expanded in tumor-bearing mice and differentiated to polymorphonuclear myeloid-derived suppressor cells (PMN-MDSCs). Selective depletion of monocytic cells had no effect on the number of granulocytes in naive mice but decreased the population of PMN-MDSCs in tumor-bearing mice by 50%. The expansion of MLPGs was found to be controlled by the down-regulation of Rb1, but not IRF8, which is known to regulate the expansion of PMN-MDSCs from classic granulocyte precursors. In cancer patients, putative MLPGs were found within the population of CXCR1⁺CD15⁻CD14⁺HLA-DR^−/lo monocytic cells. These findings describe a mechanism of abnormal myelopoiesis in cancer and suggest potential new approaches for selective targeting of MDSCs.

Abstract 1008: Peroxynitrite mediates the resistance of tumor cells to cytotoxic T cells by altering the MHC1-peptide repertoire on tumor cells

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.