Myeloid-derived Suppressor Cells Survival Research Articles

Myeloid-derived suppressor cell mitochondrial fitness governs chemotherapeutic efficacy in hematologic malignancies

Myeloid derived suppressor cells (MDSCs) are key regulators of immune responses and correlate with poor outcomes in hematologic malignancies. Here, we identify that MDSC mitochondrial fitness controls the efficacy of doxorubicin chemotherapy in a preclinical lymphoma model. Mechanistically, we show that triggering STAT3 signaling via β2-adrenergic receptor (β2-AR) activation leads to improved MDSC function through metabolic reprograming, marked by sustained mitochondrial respiration and higher ATP generation which reduces AMPK signaling, altering energy metabolism. Furthermore, induced STAT3 signaling in MDSCs enhances glutamine consumption via the TCA cycle. Metabolized glutamine generates itaconate which downregulates mitochondrial reactive oxygen species via regulation of Nrf2 and the oxidative stress response, enhancing MDSC survival. Using β2-AR blockade, we target the STAT3 pathway and ATP and itaconate metabolism, disrupting ATP generation by the electron transport chain and decreasing itaconate generation causing diminished MDSC mitochondrial fitness. This disruption increases the response to doxorubicin and could be tested clinically.

Abstract 4503: Polyamine blockade therapy: A strategy to block immunosuppression in pancreatic cancer

Abstract Pancreatic ductal adenocarcinoma (PDAC) is characterized by dense desmoplasia which contributes to poor prognosis by hindering tumor entry of immune cells and therapeutics. This leads to PDAC being difficult to treat with gemcitabine, a current standard-of-care, with only a 22% 12-month survival rate. Therapeutic modalities are needed to tackle the PDAC tumor microenvironment (TME). An alternative target is polyamine metabolism which is required for tumor growth, survival, metastasis, and also for modulating generation, survival, and activity of immune cells. Myeloid-derived suppressor cells (MDSCs) are immune cells that have a key role in PDAC metastasis, invasion, progression, and in impeding anti-tumor immune response to therapeutics in PDAC. MDSCs include monocyte-like (M-MDSC) and neutrophil-like (PMN-MDSC) cells. Previous studies have shown that polyamine blockade therapy (PBT), consisting of difluoromethylornithine (DFMO) to block polyamine biosynthesis and a polyamine transport inhibitor (Trimer44NMe), elicits anti-tumor immune response, improved survival, and a decrease in MDSCs in other tumor types. This is of interest given our previous findings that PBT improved survival of PDAC-bearing mice. Here, we tested PBT in altering PDAC-stimulated MDSC survival and whether it shifts their immunosuppressive phenotype. Studies were conducted using CD11b+ bone marrow cells that were differentiated into MDSCs via GM-CSF and tumor-conditioned media from PDAC cells derived from a genetic mouse model for pancreatic cancer (KPC-mice). Bulk RNA-sequencing was conducted on MDSCs treated with PBT, gemcitabine or combination. MDSCs exhibited widespread downregulation of expression in the gemcitabine and combination treatment, but PBT alone exhibited more targeted downregulation. Specifically, Gene Ontology (GO) analysis revealed a decrease in genes associated with immune cell migration and chemotaxis, and KEGG analysis showed downregulation of inflammatory pathways. These findings indicate that PBT may modify MDSCs from their tumor-associated immunosuppressive and pro-inflammatory behavior. MDSC subtype-specific markers were investigated by flow cytometry. Results show subsets of immune cells, such as PMN-MDSCs, are reduced upon PBT treatment alone or in combination with gemcitabine, thereby highlighting the dependence of this subtype on polyamine metabolism. In conclusion, PBT alters the expression of MDSC immunomodulatory genes, decreases MDSC survival and supports the use of PBT for treatment of PDAC in combination with other immunomodulatory therapeutics. Citation Format: Joseph A. Goode, Sai Preethi Nakkina, Manav Gandhi, Otto Phanstiel, Deborah A. Altomare. Polyamine blockade therapy: A strategy to block immunosuppression in pancreatic cancer. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 4503.

Hydroxyphenylpyruvate Dioxygenase Is a Metabolic Immune Checkpoint for UTX-deficient Colorectal Cancer

Aberrant epigenetic events mediated by histone methyltransferases and demethylases contribute to malignant progression of colorectal cancer (CRC). However, the role of the histone demethylase ubiquitously transcribed tetratricopeptide repeat on chromosome X (UTX) in CRC remains poorly understood. UTX conditional knockout mice and UTX-silenced MC38 cells were used to investigate UTX function in tumorigenesis and development of CRC. We performed time of flight mass cytometry to clarify the functional role of UTX in remodeling immune microenvironment of CRC. To investigate metabolic interaction between myeloid-derived suppressor cells (MDSCs) and CRC, we analyzed metabolomics data to identify metabolites secreted by UTX-deficient cancer cells and taken up by MDSCs. We unraveled a tyrosine-mediated metabolic symbiosis between MDSC and UTX-deficient CRC. Loss of UTX in CRC resulted in methylation of phenylalanine hydroxylase, preventing its degradation and subsequently increasing tyrosine synthesis and secretion. Tyrosine taken up by MDSCs was metabolized to homogentisic acid by hydroxyphenylpyruvate dioxygenase. Homogentisic acid modified protein inhibitor of activated STAT3 via carbonylation of Cys 176, and relieved the inhibitory effect of protein inhibitor of activated STAT3 on signal transducer and activator of transcription 5 transcriptional activity. This in turn, promoted MDSC survival and accumulation, enabling CRC cells to acquire invasive and metastatic traits. Collectively, these findings highlight hydroxyphenylpyruvate dioxygenase as a metabolic checkpoint to restrict immunosuppressive MDSCs and to counteract malignant progression of UTX-deficient CRC.

Myeloid-derived suppressor cells promote the formation of abdominal aortic aneurysms through the IL-3-ICOSL-ICOS axis

Th17 cells are powerful inflammation promoters in the pathogenesis of abdominal aortic aneurysms (AAAs). Myeloid-derived suppressor cells (MDSCs) can promote the differentiation of Th17 cells in chronic inflammatory autoimmune injury. Here, we aim to examine whether MDSCs regulate the differentiation of Th17 cells to participate in the development of AAA. We demonstrated an abnormal accumulation of MDSCs in AAA patients, which was positively associated with Th17 cells. We established angiotensin II-induced apolipoprotein E knockout mice and found the impaired immunosuppressive function of M-MDSCs. After systemic injection of anti-Gr-1 antibody in AAA mice to deplete circulating MDSCs, AAA formation and the differentiation of Th17 cells were abolished, and the overexpression of inducible T-cell costimulator (ICOS) on Th17 cells was reversed accordingly. Regulating the expression of ICOS ligand (ICOSL) on MDSCs affects the differentiation of Th17 cells. The adoptive transfer of ICOSLlowMDSCs in AAA mice inhibited the differentiation of Th17 cells and the development of AAA. Meanwhile, rIL-3 promoted the survival and immunosuppressive dysfunction of MDSCs, upregulated ICOSL expression on MDSCs by inhibiting activation of the PI3K/AKT signaling pathway, and regulated MDSCs to promote the differentiation of Th17 cells via the ICOSL-ICOS axis. An increase in serum IL-3, ICOSL+MDSCs, and ICOS+Th17 cells was detected in AAA patients, and IL-3 levels were positively correlated with the proportion of ICOSL+MDSC cells. In conclusion, we uncovered a pivotal role of MDSCs in promoting the differentiation of Th17 cells through the IL-3-ICOSL-ICOS axis during AAA, providing an important theoretical basis for understanding the pathogenesis of AAA.

β2-AR Signaling Enhances MDSC Survival through Metabolic Reprograming and Impairs Therapeutic Efficacy in Hematologic Malignancies

Nerve-driven chronic adrenergic stress can suppress anti-tumor immunity. However, the mechanistic details of how this occurs are still poorly understood. Catecholamines (epinephrine, norepinephrine) released by the autonomic nervous system (ANS) can activate adrenergic receptors (AR) present on nearly every cell type, including myeloid derived suppressor cells (MDSCs). MDSCs are important regulatory cells that hinder anti-tumor immune responses in hematological malignancies by inhibiting numerous cell types, including effector NK cells, CD8+ T cells, CD4+ T cells, and processes, such as blocking cytokine secretion. Because first line treatment for most hematologic malignancies includes chemotherapy, and MDSCs are known to be detrimental to its efficacy, understanding their role, including through chronic stress pathways, is relevant. Recently, using preclinical models of solid tumors, we have shown β2 adrenergic receptor (β2-AR) signaling, activated by chronic stress, increases the accumulation of MDSCs in the tumor microenvironment (TME). However, the role of β2-AR in MDSC survival and accumulation in hematological malignancies after chemotherapy, and the precise mechanisms by which β2-AR increases MDSC accumulation, remain unresolved. We have discovered β2-AR signaling drives MDSC survival in tumor bearing mice treated with chemotherapy. We have also shown the combination of propranolol, a non-selective β-AR blocker, with doxorubicin prolongs survival in mice bearing EL4 and C1498 tumors. Examination of immune cell compartments demonstrated a significant reduction of both M-MDSC and PMN-MDSC abundance in mice receiving the combination therapy, indicating the key effect of stress induced β2-AR signaling on MDSC accumulation. Further, we implanted EL4 tumors into β2-AR-/- and β2ARfl/fl LysmCre mice and confirmed doxorubicin increases survival in both β2-AR-/- and β2ARfl/fl LysmCre mice compared to WT control mice, suggesting β2-AR signaling in MDSCs is the main receptor driving the effects of propranolol in combination therapy. Using both in vitro and in vivo systems, we showed β2-AR signaling increased MDSC survival by the induction of BCL-2 expression and the suppression of caspase-3 and caspase-8. Seahorse metabolic studies further showed β2-AR signaling induced a metabolic shift to increased mitochondrial respiration (OCR) and ATP production in MDSCs. We then applied single cell RNA sequencing (scRNAseq) pathway analysis and multiplexed D7-Glc plus 13C5-Gln tracing in Stable Isotope-Resolved Metabolomics (SIRM). We found β2-AR signaling induces a metabolic shift toward higher ATP generation and decreased mitochondrial reactive oxygen species (mROS) that serves as a survival signal for MDSCs. We also found β2-AR signaling suppressed the expression of ATPase Inhibitory Factor 1 (ATPIF1) in MDSCs, which is responsible for increased ATP generation. These results show β2-AR signaling induces MDSC survival through metabolic reprogramming in mitochondria by enhancing ATP generation and modulating key survival signals to increase the ratio of ATP/mROS. Altogether, these studies reveal β2-AR signaling plays an important role in mitochondrial metabolic fitness in MDSCs, priming them to survive chemotherapy-induced cytotoxicity. The combination of the stress blocker propranolol, with chemotherapy regimens, could potentially improve the current therapeutic regimens used for hematological malignancies. Figure 1View largeDownload PPTFigure 1View largeDownload PPT Close modal

S-1 eliminates MDSCs and enhances the efficacy of PD-1 blockade via regulation of tumor-derived Bv8 and S100A8 in thoracic tumor.

Myeloid-derived suppressor cells (MDSCs) have been known to play a pivotal role in the induction of immune tolerance, which limits the benefits of immune checkpoint inhibitors (ICIs). Recent studies revealed that several chemotherapeutic agents decreased tumor-infiltrating MDSCs. Therefore, combination therapy with cytotoxic chemotherapeutic agents and ICIs was approved for first-line treatment for lung cancer. However, the impact of chemotherapeutic agents on MDSCs and an optimal partner of ICIs has not been fully investigated in thoracic tumors, including lung cancer and malignant pleural mesothelioma. In the present study, we found that treatment with 5-FU and its oral formulation, S-1, suppressed tumor progression and inhibited the accumulation of MDSCs in thoracic tumor-bearing mice. Tumor-infiltrating T cells and dendritic cells were significantly expanded in S-1-treated mice. 5-FU suppressed the ability of tumor cells to recruit MDSCs, while it did not suppress the survival and differentiation of mouse MDSCs in vitro. We also revealed that 5-FU or S-1 significantly downregulated the expression of tumor-derived Bv8 and S100A8. The knockdown of Bv8 or S100A8 in tumor cells suppressed tumor growth and MDSC recruitment in vivo. Furthermore, in comparison with pemetrexed, administration of S-1 improved the synergistic therapeutic efficacy of anti-PD-1 antibodies with or without carboplatin. Our findings revealed a novel mechanism wherein S-1 primed a favorable tumor microenvironment to provide the rationale for combination therapy with S-1 and ICIs as the optimal therapy for thoracic cancer.

Abstract 239: Assessment of immunotherapeutic efficacy of entinostat by using the 3d-explore ex vivo platform utilizing tumoroids of fresh patient tumor samples with intact tumor microenvironment

Abstract Background: Entinostat, a selective histone deacetylase 1/3 inhibitor has been shown to promote a robust anti-tumor response and increased neoantigen expression and antigen-specific T cell activation in preclinical tumor models. Entinostat plays a role in neutralizing myeloid derived suppressor cells (MDSC) to improve anti-tumor immune response. Entinostat is currently being tested in later-stage clinical trials in various types of cancer. Using a novel 3D ex-vivo platform with fresh patient tumor samples we assessed the therapeutic efficacy of Entinostat with intact stromal components and tumor immune microenvironment. Methods: All tumor samples were obtained with patient consent and relevant IRB approval. Unpropagated 3D tumoroids with intact TME measuring 150 µm in size were prepared from fresh tumor samples from renal cell carcinoma (RCC) patients using a proprietary technology developed at Nilogen Oncosystems. Tumoroids prepared from each patient’s tumor sample were pooled to represent the tumor heterogeneity and treated ex vivo with Entinostat for 48h to detect treatment-mediated changes in tumor immune cell composition including lymphoid and myeloid markers and their activation status. Results: Tumoroids treated with Entinostat increased NK, NKT, CD4 and CD8+T cells compared to untreated control. Ex vivo treatment with Entinostat led to an increase in CD69 and Granzyme B expression in tumor resident CD4+ T cells. Immune checkpoint receptors, 41BB and PD-1 expression was increased in CD8+T cells followed by Entinostat treatment. Decreased monocytic MDSC was seen in Entinostat treated tumoroids compared to untreated control and this was associated with decreased GM-CSF, IL-4, IL-6 cytokine levels and increased IFN-γ and IL-2, quantified by multiplex cytokine assay. Interestingly, the tumor cell killing study revealed a higher percentage of death in early stage MDSC (eMDSC) indicating a direct effect of Entinostat on MDSC survival. Conclusions: These results demonstrate that Entinostat treatment leads to a broad range of immune cell activation and MDSC inhibition in RCC. These data provide a mechanistic rationale utilizing our 3D tumoroid platform for discovery of potential biomarkers of drug sensitivity and to identify novel immunotherapeutic approaches. Clinical applications of 3D-EXplore may help to identify RCC patients who may likely benefit from Entinostat treatment. Citation Format: Krithika Nandakumar Kodumudi, Brittany Bunch, Jared Ehrhart, Matt Weitzman, Olivia MacIntosh, Kelly Sussman, Soner Altiok. Assessment of immunotherapeutic efficacy of entinostat by using the 3d-explore ex vivo platform utilizing tumoroids of fresh patient tumor samples with intact tumor microenvironment [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 239.

Cytarabine-induced TNFα promotes the expansion and suppressive functions of myeloid-derived suppressor cells in acute myeloid leukaemia.

Acute myeloid leukaemia (AML) is very common haematopoietic malignancies with poor prognosis. Chemotherapy is still a mainstay therapy for AML patients. AML microenvironment plays critical roles in therapy response. However, the role of chemotherapy in AML microenvironment is poorly understood. In this study, we report that cytarabine (AraC)-triggered TNFα from AML cells expanded myeloid-derived suppressor cells (MDSCs) and enhanced MDSC functions and survival through activating IL-6/STAT3 and NFκB pathways. Blockade of TNFα in conditioned medium-derived AraC-treated AML cells (AraC_CM) impaired MDSC expansion and functions, reduced IL-6 secretion and the level of activated STAT3. Inhibiting IL6 or STAT3 abrogated AraC_CM-mediated MDSC suppressive function. Additionally, inhibiting TNFα also impaired AraC_CM-mediated NFκB activation. Blocking NFκB activation reduced MDSC viability induced by AraC_CM. Together, these results provided a role of AraC-induced TNFα in MDSC expansion and functions and suggest that targeting TNFα may benefit AML patients to current anticancer strategies by blocking MDSC-mediated immunosuppression.

Mannan-Binding Lectin Deficiency Limits Inflammation-induced Myeloid-Derived Suppressor Cells Expansion via Modulating Tumor Necrosis Factor Alpha-triggered Apoptosis.

Background: Mannan-binding lectin (MBL), a soluble pattern recognition molecule in the innate immune system, is reported to be associated with the function of immune cells. Myeloid-derived suppressor cells (MDSCs) are mainly characterized by immunosuppressive activities involving several inflammatory diseases such as cancer, infection, and arthritis. Some of the factors inducing their apoptosis are known, however, mechanisms have not been identified. The underlying impact of MBL on the MDSCs especially under inflammatory conditions remains unknown. This study was designed to investigate whether MBL affects MDSCs survival during inflammation conditions.Methods: WT and MBL-deficient (MBL-/-) mice were induced on day 0 of the experiment by subcutaneous injection of complete Freund's adjuvant and then injected with incomplete Freund's adjuvant into the knee joint space under general anesthesia on day 14 to induce inflammatory arthritis. The proportions of MDSCs in the spleen and blood and the serum level of the inflammatory cytokines were measured. In vitro study, MDSCs were isolated from the bone marrow of WT and MBL-/- mice and cultured in the presence of interleukin-6 (IL-6) and granulocyte-macrophage colony-stimulating factor (GM-CSF) for 5 days with or without tumor necrosis factor-alpha (TNF-α).Results: After adjuvant treatment, MBL-/- mice had a significantly lower frequency of MDSCs as well as elevated serum inflammatory cytokines levels compared to WT mice. MBL deficiency markedly inhibited the MDSCs frequency from mice bone marrow induced by IL-6 and GM-CSF in the presence of TNF-α in vitro. Mechanistic studies established that MBL inhibited MDSCs apoptosis via down-regulation of TNF-α/tumor necrosis factor-alpha receptor 1 (TNFR1) signaling pathway and subsequent caspase 3-dependent manner.Conclusion: Mannan-binding lectin deficiency inhibits myeloid-derived suppressor cells expansion via modulating TNF-α triggered apoptosis.

Immunosuppressive triangle depletion through the combination punches strategy for enhanced immunotherapy

Tumor immunotherapy non-responders often have high levels of tumor-associated macrophages (TAMs), circulating myeloid-derived suppressor cells (MDSCs), and regulatory T cells (Tregs), which form a vicious “immunosuppressive triangle” to promote tumor growth and metastasis. Rather than single immunosuppressive cell modulation in some reports, it is more meaningful to precisely target as well as overall deplete the immunosuppressive triangle for enhanced immunotherapy. To achieve this, a pH/matrix metalloproteinase-2 (MMP2) dual responsive nanoplatform was constructed. The acidic sensitive poly β-amino esters-polyethylene glycol nanoparticles (PAE-PEG NPs) loaded with liver-X-receptor agonist GW3965 were prepared, and then were decorated with MMP2 sensitive M2-TAMs targeted apoptotic peptides (PEP) to obtain GW3965@PAE-PEG-PEP (GW@PPP). After reaching tumor microenvironment, the combination punches strategy based on this nanoplatform was implemented. As one punch, the MMP2 cleaved PEP segments induced M2-TAMs apoptosis specifically. As another punch, the acidic responsive GW3965 release promoted immunoregulatory apolipoprotein E (ApoE) transcription, further inhibiting the survival of MDSCs, tumor cells and endothelial recruitment. In addition, Tregs, the formation and recruitment of which was related to MDSC and TAMs, were suppressed subsequently. Overall, the GW@PPP directly drove immunosuppressive triangle depletion and further immune response, which effectively prevented tumor growth and metastasis for enhanced immunotherapy in an all-round way.

A Complex Metabolic Network Confers Immunosuppressive Functions to Myeloid-Derived Suppressor Cells (MDSCs) within the Tumour Microenvironment.

Myeloid-derived suppressor cells (MDSCs) constitute a plastic and heterogeneous cell population among immune cells within the tumour microenvironment (TME) that support cancer progression and resistance to therapy. During tumour progression, cancer cells modify their metabolism to sustain an increased energy demand to cope with uncontrolled cell proliferation and differentiation. This metabolic reprogramming of cancer establishes competition for nutrients between tumour cells and leukocytes and most importantly, among tumour-infiltrating immune cells. Thus, MDSCs that have emerged as one of the most decisive immune regulators of TME exhibit an increase in glycolysis and fatty acid metabolism and also an upregulation of enzymes that catabolise essential metabolites. This complex metabolic network is not only crucial for MDSC survival and accumulation in the TME but also for enhancing immunosuppressive functions toward immune effectors. In this review, we discuss recent progress in the field of MDSC-associated metabolic pathways that could facilitate therapeutic targeting of these cells during cancer progression.

Evaluation of autophagy mediators in myeloid-derived suppressor cells during human tuberculosis

Myeloid-derived suppressor cells (MDSC) are induced during active TB disease to restore immune homeostasis but instead exacerbate disease outcome due to chronic inflammation. Autophagy, in conventional phagocytes, ensures successful clearance of M.tb. However, autophagy has been demonstrated to induce prolonged MDSC survival. Here we investigate the relationship between autophagy mediators and MDSC in the context of active TB disease and during anti-TB therapy. We demonstrate a significant increase in MDSC frequencies in untreated active TB cases with these MDSC expressing TLR4 and significantly more mTOR and IL-6 than healthy controls, with mTOR levels decreasing during anti-TB therapy. Finally, we show that HMGB1 serum concentrations decrease in parallel with mTOR. These findings suggest a complex interplay between MDSC and autophagic mediators, potentially dependent on cellular localisation and M.tb infection state.

Dual roles of myeloid-derived suppressor cells induced by Toll-like receptor signaling in cancer.

Myeloid-derived suppressor cells (MDSCs) are one of the major components of the tumor microenvironment (TME), and are the main mediators of tumor-induced immunosuppression. Recent studies have reported that the survival, differentiation and immunosuppressive activity of MDSCs are affected by the Toll-like receptor (TLR) signaling pathway. However, the regulatory effect of TLR signaling on MDSCs remains controversial. TLR-induced MDSC can acquire different immunosuppressive activities to influence the immune response that can be either beneficial or detrimental to cancer immunotherapy. The present review summarizes the effects of TLR signals on the number, phenotype and inhibitory activity of MDSCs, and their role in cancer immunotherapy, which cannot be ignored if effective cancer immunotherapies are to be developed for the immunosuppression of the TME.

Autocrine IL6-Mediated Activation of the STAT3-DNMT Axis Silences the TNFα-RIP1 Necroptosis Pathway to Sustain Survival and Accumulation of Myeloid-Derived Suppressor Cells.

Although accumulation of myeloid-derived suppressor cells (MDSC) is a hallmark of cancer, the underlying mechanism of this accumulation within the tumor microenvironment remains incompletely understood. We report here that TNFα-RIP1-mediated necroptosis regulates accumulation of MDSCs. In tumor-bearing mice, pharmacologic inhibition of DNMT with the DNA methyltransferease inhibitor decitabine (DAC) decreased MDSC accumulation and increased activation of antigen-specific cytotoxic T lymphocytes. DAC-induced decreases in MDSC accumulation correlated with increased expression of the myeloid cell lineage-specific transcription factor IRF8 in MDSCs. However, DAC also suppressed MDSC-like cell accumulation in IRF8-deficient mice, indicating that DNA methylation may regulate MDSC survival through an IRF8-independent mechanism. Instead, DAC decreased MDSC accumulation by increasing cell death via disrupting DNA methylation of RIP1-dependent targets of necroptosis. Genome-wide DNA bisulfite sequencing revealed that the Tnf promoter was hypermethylated in tumor-induced MDSCs in vivo. DAC treatment dramatically increased TNFα levels in MDSC in vitro, and neutralizing TNFα significantly increased MDSC accumulation and tumor growth in tumor-bearing mice in vivo. Recombinant TNFα induced MDSC cell death in a dose- and RIP1-dependent manner. IL6 was abundantly expressed in MDSCs in tumor-bearing mice and patients with human colorectal cancer. In vitro, IL6 treatment of MDSC-like cells activated STAT3, increased expression of DNMT1 and DNMT3b, and enhanced survival. Overall, our findings reveal that MDSCs establish a STAT3-DNMT epigenetic axis, regulated by autocrine IL6, to silence TNFα expression. This results in decreased TNFα-induced and RIP1-dependent necroptosis to sustain survival and accumulation. SIGNIFICANCE: These findings demonstrate that targeting IL6 expression or function represent potentially effective approaches to suppress MDSC survival and accumulation in the tumor microenvironment.

Autocrine IL6 activates the STAT3-DNMT axis to silence the TNFa-RIP1 necroptosis pathway to sustain myeloid-derived suppressor cell survival and accumulation

Abstract Accumulation of myeloid-derived suppressor cells (MDSCs) is a hallmark of cancer. However, the underlying mechanism of MDSC accumulation in the tumor microenvironment (TME) remain incompletely understood. We report that MDSC accumulation is regulated by the TNFα-RIP1-mediated necroptosis. We determined that inhibition of DNMTs with Decitabine (DAC) abolished MDSC accumulation and increased activation of antigen-specific cytotoxic T lymphocytes (CTLs) in tumor-bearing mice. DAC-induced decrease of MDSC accumulation is correlated with increased IRF8 expression in MDSCs. However, DAC also abolished MDSC-like cell accumulation in IRF8 KO mice, indicating that DNA methylation does not regulate MDSC lineage differentiation but mediates MDSC accumulation at post differentiation stage. We determined that DAC decreased MDSC accumulation through increasing cell death and identified RIP1-dependent necroptosis as target of DNA methylation in MDSCs. Genome-wide DNA bisulfite sequencing revealed that the Tnf promoter is hypermethylated in tumor-induced MDSCs in vivo. Consequently, DAC dramatically increased TNFα level in MDSCs and neutralizing TNFα significantly decreased MDSC cell death. Furthermore, recombinant TNFα induced MDSC cell death in a does- and RIP1-dependent manner. IL6 which is expressed in MDSCs in tumor-bearing mice and human colorectal cancer patients. Our data shows that the autocrine IL6 activates the STAT3-DNMT axis to epigenetically silence the TNFα-RIP1 necroptosis pathway to sustain MDSC survival and accumulation in cancer. Targeting the TNFα-RIP1 necroptosis is potentially an effective approach to supress MDSCs to activate tumor-reactive CTLs in the TME.

Abstract P3-01-02: B2 adrenergic receptor-mediated signaling regulates the immunosuppressive potential of myeloid-derived suppressor cells

Abstract Chronic stress is thought to suppress immunity; however, details about how chronic stress affects tumor growth and anti-tumor immune responses are still missing or poorly understood. Catecholamines (epinephrine, norepinephrine) released during sympathetic nerve stimulation can activate adrenergic receptors present on nearly every cell type including myeloid derived suppressor cells (MDSCs). MDSCs are increasingly recognized as critical players in tumor immunology because, in addition to their fundamental roles in suppressing effector NK cells, CD8+ T cells and CD4+ T cells, they also play a direct role in promoting tumor growth, differentiation and metastasis. Here, we demonstrate that the chronic adrenergic stress (as manipulated by a) housing temperature (which affects norepinephrine (NE) production, b) use of b-AR agonists or antagonists, or c) using β2-AR knockout mice regulates the accumulation and the pro-tumor phenotype of MDSCs. This same result was seen using two different murine models of breast cancer (4T1 and AT-3) and through the use of β2-AR knockout mice, we found that these effects are dependent on β2-adrenergic receptor (β2-AR) signaling. Using an agonist of β-AR signaling, we found that the activation of β2-AR signaling increases the expression of immunosuppressive molecules such as arginase-I and PDL-1 in MDSCs and enhances their immune-regulatory functions. β2-AR expression in MDSCs was induced by tumor promoting cytokines such as granulocyte-macrophage colony-stimulating factor (GM-CSF). The survival of MDSCs is increased upon the activation of β2-AR on MDSCs in a Fas-FasL dependent manner. The regulation by β-AR signaling on MDSCs is dependent upon phosphorylation of STAT3 as a pivotal transcription factor in the function and expansion of MDSCs while MDSCs lacking β2-AR had poorer survival in both spleen and tumor. In murine tumor models, we found that the tumor promoting effects of β2-AR activation can be prevented by simple treatment of mice with β-blockers or by peripheral sympathectomy using 6-hydroxydopamine (6-OHDA). Furthermore, treatment with β-AR agonist increases the generation and immunosuppressive functions of MDSCs derived from human PBMCs in vitro. These data reveal a major, physiologically relevant role for nervous activity and adrenergic stress in MDSC-tumor biology and underscore the clinical potential for blockade of a previously unrecognized β2-AR-MDSC axis to improve immunotherapy. Keywords: β2 adrenergic receptor, MDSC, Chronic Stress, Tumor, STAT3 Citation Format: Hemn Mohammadpour, Cameron R. MacDonald, Guanxi Qiao, Minhui Chen, Bowen Dong, Bonnie L. Hylander, Philip L. McCarthy, Scott I. Abrams, Elizabeth A. Repasky. B2 adrenergic receptor-mediated signaling regulates the immunosuppressive potential of myeloid-derived suppressor cells [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P3-01-02.

β2 adrenergic receptor-mediated signaling regulates the immunosuppressive potential of myeloid-derived suppressor cells.

Catecholamines released by sympathetic nerves can activate adrenergic receptors present on nearly every cell type, including myeloid-derived suppressor cells (MDSCs). Using in vitro systems, murine tumor models in wild-type and genetically modified (β2-AR-/-) mice, and adoptive transfer approaches, we found that the degree of β2-AR signaling significantly influences MDSC frequency and survival in tumors and other tissues. It also modulates their expression of immunosuppressive molecules such as arginase-I and PD-L1 and alters their ability to suppress the proliferation of T cells. The regulatory functions of β2-AR signaling in MDSCs were also found to be dependent upon STAT3 phosphorylation. Moreover, we observed that the β2-AR-mediated increase in MDSC survival is dependent upon Fas-FasL interactions, and this is consistent with gene expression analyses, which reveal a greater expression of apoptosis-related genes in β2-AR-/- MDSCs. Our data reveal the potential of β2-AR signaling to increase the generation of MDSCs from both murine and human peripheral blood cells and that the immunosuppressive function of MDSCs can be mitigated by treatment with β-AR antagonists, or enhanced by β-AR agonists. This strongly supports the possibility that reducing stress-induced activation of β2-ARs could help to overcome immune suppression and enhance the efficacy of immunotherapy and other cancer therapies.

Survival of the fittest: how myeloid-derived suppressor cells survive in the inhospitable tumor microenvironment.

Myeloid-derived suppressor cells (MDSC) are present in most cancer patients where they are significant contributors to the immune suppressive tumor microenvironment (TME). The TME is a hostile locale due to deficiencies in oxygen (hypoxia) and nutrients, and the presence of reactive oxygen species (ROS). The survival of tumor cells within the TME is partially governed by two mechanisms: (1) Activation of the transcription factor Nuclear Factor Erythroid-derived 2-like 2 (Nrf2) which turns on genes that attenuate oxidative stress; and (2) The presence of High Mobility Group Box Protein-1 (HMGB1), a damage-associated molecular pattern molecule (DAMP) that induces autophagy and protects against apoptosis. Because Nrf2 and HMGB1 promote tumor cell survival, we speculated that Nrf2 and HMGB1 may facilitate MDSC survival. We tested this hypothesis using Nrf2+/+ and Nrf2-/- BALB/c and C57BL/6 mice and pharmacological inhibitors of HMGB1. In vitro and in vivo studies demonstrated that Nrf2 increased the suppressive potency and quantity of tumor-infiltrating MDSC by up-regulating MDSC production of H2O2 and decreasing MDSC apoptosis. Decreased apoptosis was accompanied by a decrease in the production of MDSC, demonstrating that MDSC levels are homeostatically regulated. Pharmacological inhibition of autophagy increased MDSC apoptosis, indicating that autophagy increases MDSC half-life. Inhibition of HMGB1 also increased MDSC apoptosis and reduced MDSC autophagy. These results combined with our previous findings that HMGB1 drives the accumulation of MDSC demonstrate that HMGB1 maintains MDSC viability by inducing autophagy. Collectively, these findings identify Nrf2 and HMGB1 as important factors that enable MDSC to survive in the TME.

TRAF3 is a critical regulator of the expansion of myeloid-derived suppressor cells in mice

Abstract TRAF3 is an adaptor protein and E3 ligase that regulates the signal transduction pathways of the TNFR superfamily, pattern recognition receptors, and cytokine receptors. We previously showed that specific deletion of TRAF3 in myeloid cells leads to spontaneous inflammation and tumor development in aging M-TRAF3−/− mice, which is consistently associated with a striking expansion of CD11b+Gr1+ myeloid cells. Here we report that these CD11b+Gr1+ cells were myeloid-derived suppressor cells (MDSCs), as they exhibited a strong suppressive activity on CD8 T cell proliferation and also displayed a nuclear morphology of immature myeloid cells. Using a chronic inflammation model induced by repeated injections of heat-killed Bacillus Calmette-Guérin (BCG), we showed that young adult M-TRAF3−/− mice also exhibited hyperexpansion of MDSCs after BCG injections. Our in vivo BrdU labeling experiments revealed that MDSC hyperexpansion in M-TRAF3−/− mice was mainly due to the increased proliferation of MDSCs or progenitor cells in the spleen but not in the bone marrow (BM). Interestingly, TRAF3 does not appear to be an intrinsic regulator of the development, survival or proliferation of MDSCs, as suggested by the results of cultured BM progenitor cells and purified splenic MDSCs, in the absence or presence of CSFs or agonists of TLRs or CD40. Furthermore, we identified environmental factors that induced the aberrant expansion of MDSCs in M-TRAF3−/− mice using cytokine protein array analyses, which were further verified by macrophage cultures and in vivo neutralization studies. Taken together, our findings identify TRAF3 as a new regulator of MDSC expansion, and therefore also a critical regulator of chronic inflammation and immunosuppression.

Human T cells modulate myeloid-derived suppressor cells through a TNF-α-mediated mechanism

Myeloid-derived suppressor cells (MDSC) represent an innate immune cell subset capable of suppressing T-cell responses in cancer and chronic inflammation. While the effect of MDSC on T cells has been defined thoroughly, the reciprocal impact of T cells on MDSC homeostasis remains poorly understood. Therefore, we comprehensively analyzed the effect of different T-cell subsets on the generation and survival of human MDSC. Using an in vitro MDSC generation assay, we demonstrate that unstimulated CD4+, but not CD8+ T cells, induce polymorphonuclear MDSC (PMN-MDSC) from CD33+ myeloid cells. This effect was dependent on direct cell-cell contact and required TNF-α signaling. Soluble TNF-α was dispensable for PMN-MDSC generation, suggesting that transmembrane TNF-α is involved in that trans-cellular process. Stimulated human CD3+ T cells delayed the apoptosis of PMN-MDSC, which was independent of TNF-α signaling or direct cell-cell contact, but was recapitulated by IL-2. Taken together, our study shows that human T cells modulate MDSC generation and survival through two distinct mechanisms and thereby fine-tune the homeostasis of human MDSC in a regulated manner.