Potent Immune Suppressive Activity Research Articles

Abstract P047: Inactivation of IFN signaling drive immunosuppressive MDSC function and can be therapeutically targeted

Abstract Myeloid-derived suppressor cells (MDSC) are pathologically activated neutrophils and monocytes with potent immune suppressive activity that promote tumor progression and impair the efficacy of anti-tumor therapeutics. Herein we show that type I interferons (IFN1) receptor dampen immune suppressive activity of MDSC. Downregulation of the IFNAR1 is found in MDSC from cancer patients and mouse tumor models. Downregulation of IFNAR1 depends on the activation of the p38 protein kinase and is required for activation of the immune suppressive phenotype moreover genetic stabilization of IFNAR1 in tumor bearing mice reduces suppressive activity of MDSC and promote antitumor effect. Stabilizing IFNAR1 using inhibitor of p38 combined with the interferon induction therapy elicits a robust anti-tumor effect that is completely dependent on IFNAR1 expression on MDSC. Thus, inhibition of MDSC immune suppressive function can be exploited therapeutically. Citation Format: Emilio Sanseviero, Kevin Alicea-Torres, Yulia Nefedova, Serge Y. Fuchs, Dmitry I. Gabrilovich. Inactivation of IFN signaling drive immunosuppressive MDSC function and can be therapeutically targeted [abstract]. In: Abstracts: AACR Virtual Special Conference: Tumor Immunology and Immunotherapy; 2021 Oct 5-6. Philadelphia (PA): AACR; Cancer Immunol Res 2022;10(1 Suppl):Abstract nr P047.

PH Low Insertion Peptide-Modified Programmed Cell Death-Ligand 1 Potently Suppresses T-Cell Activation Under Acidic Condition.

Programmed cell death-ligand 1 (PD-L1)/PD-1 axis is critical for maintenance of immune homeostasis by limiting overactivation of effector T-cell responses. The impairment of PD-L1/PD-1 signals play an important role in the pathogenesis of inflammatory diseases, making this pathway an ideal target for novel therapeutics to induce immune tolerance. Given weakly acidic environment as a putative hallmark of inflammation, in this study we designed a new cargo by linking the ectodomain of murine PD-L1 to the N terminus of pHLIPs, a low pH-responding and membrane-insertion peptide, and demonstrated its potent immune-suppressive activity. Specifically, PD-L1-pHLIP spanned the cellular membrane and perfectly recognized its ligand PD-1 in acidic buffer. Immobile PD-L1-pHLIP actively inhibited T-cell proliferation and IFN-γ production. Importantly, soluble PD-L1-pHLIP retained its function to dampen T-cell responses under acidic condition instead of neutral aqueous solution. Overall, these data suggest that PD-L1-pHLIP has potentials to be a novel therapeutic avenue for T-cell-mediated inflammatory diseases.

Expansion of myeloid-derived suppressor cells contributes to metabolic osteoarthritis through subchondral bone remodeling

BackgroundOsteoarthritis (OA) subsequent to acute joint injury accounts for a significant proportion of all arthropathies. Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of myeloid progenitor cells classically known for potent immune-suppressive activity; however, MDSCs can also differentiate into osteoclasts. In addition, this population is known to be expanded during metabolic disease. The objective of this study was to determine the role of MDSCs in the context of OA pathophysiology.MethodsIn this study, we examined the differentiation and functional capacity of MDSCs to become osteoclasts in vitro and in vivo using mouse models of OA and in MDSC quantitation in humans with OA pathology relative to obesity status.ResultsWe observed that MDSCs are expanded in mice and humans during obesity. MDSCs were expanded in peripheral blood of OA subjects relative to body mass index and in mice fed a high-fat diet (HFD) compared to mice fed a low-fat diet (LFD). In mice, monocytic MDSC (M-MDSC) was expanded in diet-induced obesity (DIO) with a further expansion after destabilization of the medial meniscus (DMM) surgery to induce post-traumatic OA (PTOA) (compared to sham-operated controls). M-MDSCs from DIO mice had a greater capacity to form osteoclasts in culture with increased subchondral bone osteoclast number. In humans, we observed an expansion of M-MDSCs in peripheral blood and synovial fluid of obese subjects compared to lean subjects with OA.ConclusionThese data suggest that MDSCs are reprogrammed in metabolic disease, with the potential to contribute towards OA progression and severity.

Immune suppressive activity of myeloid-derived suppressor cells in cancer requires inactivation of the type I interferon pathway

Myeloid-derived suppressor cells (MDSC) are pathologically activated neutrophils and monocytes with potent immune suppressive activity. These cells play an important role in accelerating tumor progression and undermining the efficacy of anti-cancer therapies. The natural mechanisms limiting MDSC activity are not well understood. Here, we present evidence that type I interferons (IFN1) receptor signaling serves as a universal mechanism that restricts acquisition of suppressive activity by these cells. Downregulation of the IFNAR1 chain of this receptor is found in MDSC from cancer patients and mouse tumor models. The decrease in IFNAR1 depends on the activation of the p38 protein kinase and is required for activation of the immune suppressive phenotype. Whereas deletion of IFNAR1 is not sufficient to convert neutrophils and monocytes to MDSC, genetic stabilization of IFNAR1 in tumor bearing mice undermines suppressive activity of MDSC and has potent antitumor effect. Stabilizing IFNAR1 using inhibitor of p38 combined with the interferon induction therapy elicits a robust anti-tumor effect. Thus, negative regulatory mechanisms of MDSC function can be exploited therapeutically.

Analysis of classical neutrophils and polymorphonuclear myeloid-derived suppressor cells in cancer patients and tumor-bearing mice.

In this study, using single-cell RNA-seq, cell mass spectrometry, flow cytometry, and functional analysis, we characterized the heterogeneity of polymorphonuclear neutrophils (PMNs) in cancer. We describe three populations of PMNs in tumor-bearing mice: classical PMNs, polymorphonuclear myeloid-derived suppressor cells (PMN-MDSCs), and activated PMN-MDSCs with potent immune suppressive activity. In spleens of mice, PMN-MDSCs gradually replaced PMNs during tumor progression. Activated PMN-MDSCs were found only in tumors, where they were present at the very early stages of the disease. These populations of PMNs in mice could be separated based on the expression of CD14. In peripheral blood of cancer patients, we identified two distinct populations of PMNs with characteristics of classical PMNs and PMN-MDSCs. The gene signature of tumor PMN-MDSCs was similar to that in mouse activated PMN-MDSCs and was closely associated with negative clinical outcome in cancer patients. Thus, we provide evidence that PMN-MDSCs are a distinct population of PMNs with unique features and potential for selective targeting opportunities.

Multifunctional Nanoregulator Reshapes Immune Microenvironment and Enhances Immune Memory for Tumor Immunotherapy.

Hypoxia leads to up‐regulation of PD‐L1 and decreases T lymphocyte infiltration, thus boosting immunotherapeutic resistance of tumors. Moreover, tumor‐infiltrating myeloid cells such as myeloid‐derived suppressor cells (MDSCs) correlate with potent immune suppressive activity and resistance to the immune checkpoint blocking (ICB) in tumor sites. Here, a multifunctional nanoregulator incorporating MnO2 particles and small molecular IPI549 is developed, which can reshape the tumor immune microenvironment (TIME) to unleash the immune system. The intravenously administered nanoregulator effectively accumulates in tumor sites to alleviate hypoxia via oxygen‐generating reduction of MnO2 and to inhibit PI3Kγ on MDSCs via IPI549 release in the tumor microenvironment (TME), which results in concurrent downregulation of PD‐L1 expression, polarization of tumor associated macrophages (TAMs) toward pro‐inflammatory M1‐like phenotype (tumor‐suppressive), enhanced infiltration of CD4+ helper T lymphocytes (Th cells), and cytotoxic CD8+ T lymphocytes (Tc cells), and suppressed infiltration of regulatory T lymphocytes (Treg cells) for effective tumor immunotherapy. Furthermore, the local generation of Mn2+ in TME allows tumor‐specific magnetic resonance imaging (MRI). More excitingly, the nanoregulator‐reshaped TIME is effectively reserved due to the synergistic effect of hypoxia alleviation and MDSC PI3Kγ inhibition, leading to remarkable post‐medication inhibition of tumor re‐growth and metastasis in an animal study.

Single cell RNA sequencing reveals distinct gene expression signatures of myeloidderived suppressor cells in breast cancer

Abstract Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of immature myeloid cells with potent immune suppressive activity. MDSCs regulate anti-tumor immunity by suppressing T cell proliferation. MDSCs can be further classified into granulocytic MDSCs (G-MDSCs) and monocytic MDSC (M-MDSCs). However, there is a lack of MDSC-specific markers since MDSCs, normal neutrophil granulocytes, and monocytes are defined by the same flow cytometry markers (CD11b+Gr1+). We used a breast cancer mouse model (MMTV-PyMT) to define cellular and molecular properties of MDSCs in single cell resolution. To test the capacity of MDSCs to inhibit immune responses, CD11b+Gr1+ cells from PyMT and wild type (WT) mice were sorted by fluorescence-activated cell sorting (FACS) from bone marrow, lung, and spleen, and then subjected to a T cell activation assay in co-culture with T cells. We found that predominantly spleen-derived CD11b+Gr1+ cells from PyMT significantly suppressed CD4 and CD8 T cell proliferation, while CD11b+Gr1+cells from bone marrow and lung showed no effect on T cell proliferation. Thus, we focused on spleen-derived CD11b+Gr1+ cells and utilized single cell RNA sequencing (scRNAseq) to compare breast cancer derived MDSCs to the respective cell populations harvested from normal, and non-tumor bearing hosts. Our computational analysis of 14,646 single cell transcriptomes revealed a separate G-MDSCs cluster from normal neutrophils and a M-MDSCs cluster from normal monocytes and a distinct gene expression signature for the entire MDSCs population. Our studies provide crucial insights into the biology of MDSCs, which may ultimately form the basis for novel marker and therapeutic avenues to improve cancer immunotherapy.

Abstract IA10: PMN-MDSC and neutrophils: Tale of two cells in cancer

Abstract Myeloid cells are a critical component of the tumor microenvironment. In cancer, the myeloid compartment is dramatically affected, which is now considered as one of the major immunological hallmarks of cancer. Accumulation of immunosuppressive macrophages (MΦ), defective dendritic cells (DC) function and expansion of pathologically activated immune suppressive immature myeloid cells – myeloid-derived suppressor cells (MDSC) are major changes in the myeloid compartment in cancer. The total population of MDSC consists of three groups of cells: the most abundant (>75%) immature, pathologically activated neutrophils (PMN-MDSC); the less abundant population of pathologically activated monocytes – (M-MDSC). Tumor associated macrophages (TAM) and DCs can persist in tissues for a long time, whereas PMN-MDSC have short lifespan (< 48 hours) and are constantly replaced from bone marrow (BM). The role of MDSC in mouse tumor models is well established. In most of the studies PMN-MDSC were cells that expanded the most. In recent years, the clinical role of MDSC has emerged. Results showed positive correlation of MDSC in peripheral blood with cancer stage and tumor burden. In a meta-analysis, elevated MDSC in the circulation was found to be an independent indicator of poor outcomes in patients with solid tumors and predicting response to chemotherapy therapy, radiotherapy failure, and immunotherapy. It is widely accepted that the population of myeloid cells in cancer is heterogenic. It consists of cells able to exert antitumor and pro-tumorigenic activity. The nature of this heterogeneity is highly controversial. Some data suggested that PMN-MDSC in tumors may undergo polarization in situ, similar to that observed in TAM. Based on available data on the very short lifespan of MDSC in animal models and cancer patients as well as on functional data, we have postulated that at any given moment, myeloid cells in tissues are comprised of classically activated neutrophils (PMN) and monocytes, with pro-inflammatory and antitumor activity, and pathologically activated MDSC with potent immune suppressive and pro-tumorigenic activity. The balance between these cells defines the tumor microenvironment as immunosuppressive. This balance depends on the stage of cancer. It is equally plausible that in tissues, MDSC are present not in a binary state but in a range of functional states that are characterized by different markers and different functional activities. We will discuss evidence supporting this hypothesis. This question has a direct implication for cancer therapy. It is not clinically feasible to deplete the entire population of PMN and monocytes in cancer patients. However, if a specific group of functionally defined M-MDSC and PMN-MDSC is determined, then much more precise methods of therapeutic targeting could be developed. Potential approaches to such targeting are currently being investigated. Citation Format: Dmitry I. Gabrilovich. PMN-MDSC and neutrophils: Tale of two cells in cancer [abstract]. In: Proceedings of the Fourth CRI-CIMT-EATI-AACR International Cancer Immunotherapy Conference: Translating Science into Survival; Sept 30-Oct 3, 2018; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2019;7(2 Suppl):Abstract nr IA10.

Myeloid-derived suppressor cells exacerbate Sjögren’s syndrome by inhibiting Th2 immune responses

Myeloid-derived suppressor cells (MDSCs) can regulate various aspects of immune responses based on their potent immune-suppressive activity. Studies reported that MDSCs participated in many autoimmune diseases. However, the role of MDSCs in Sjögren’s syndrome (SS) is unknown. In this study, we determined the frequencies and function of MDSCs in non-obese diabetic (NOD) mice and SS patients. The NOD mice were adoptively transferred with MDSCs or treated with anti-Gr1 antibody. Results showed that peripheral MDSCs increased significantly with the development of SS-like syndrome in NOD mice and the percentage of MDSCs was higher in SS patients than healthy controls. The SS-like syndrome aggravated after transfer of MDSCs in NOD mice. The deletion of MDSCs in NOD mice alleviated SS-like syndrome. Mechanistically, MDSCs down-regulated the percentages of Th2 cells in NOD mice and SS patients. In summary, our findings suggested that MDSCs exacerbated Sjögren’s syndrome by inhibiting Th2 cells.

Abstract 1729: Integrative immune profiling of syngeneic tumor models provides predictive immune signatures for treatment response with HMBD002, a novel anti-VISTA neutralizing antibody

Abstract Myeloid-derived suppressor cells (MDSCs) are a group of immature myeloid cells with potent immune suppressive activity. Accumulation of pathologically activated MDSCs within the tumor microenvironment has emerged as an important mechanism of primary and acquired resistance to immunotherapy. MDSCs have high surface expression levels of V-domain immunoglobulin (Ig)-containing suppressor of T-cell activation (VISTA), a co-inhibitory negative checkpoint regulator (NCR). Immune suppression by VISTA+ MDSCs has been implicated in the failure of, and resistance to, anti-PD1/PDL1 therapy and represents a unique axis for NCR targeting in the non-responder population. Elucidating the dynamics of VISTA+ MDSCs within tumors is imperative for predicting response and patient stratification. HMBD-002 is a novel anti-VISTA neutralizing monoclonal antibody, developed using Hummingbird Bioscience's proprietary Rational Antibody Discovery platform, against a specific functional epitope on VISTA predicted to be involved in ligand binding. HMBD002 binds human, cyno and murine VISTA protein with high affinity (kD <5nM) and demonstrated a strong, dose dependent inhibition for the interaction of VISTA with its putative ligand VSIG3. In ex vivo PBMC models of immune activation, HMBD002 treatment showed potent induction of cytokine release. Further, HMBD002 monotherapy in a CT26 syngeneic BALB/C model showed significant delay on tumor progression with no observable toxicity. To investigate predictive immune signatures for HMBD002 antibody efficacy, as a monotherapy and in combination with anti- PD-L1 therapy, we established multiple mouse models of tumor immune subtypes using syngeneic cell lines. Tumor biopsies were analyzed before and during treatment, to determine the evolution of immune response and its effect on tumor burden within responders and non-responders. The combination of HMBD-002 and anti-PD-L1 was more effective than either antibody alone, especially in tumors that showed abundant MDSC infiltration. Immune phenotyping revealed that this was associated with a significant increase in activated CD8+ T effector cells and concomitant increase in IFN-gamma. Profiling syngeneic models for deeper insights into the interplay between the tumor, immune system and therapy, will facilitate the clinical development of immune checkpoint and combination therapies. Hummingbird Bioscience anticipates commencing First-in-Human trials of HMBD002 in 2019. Citation Format: Jerome D. Boyd-Kirkup, Dipti Thakkar, Konrad Paszkiewicz, Piers J. Ingram. Integrative immune profiling of syngeneic tumor models provides predictive immune signatures for treatment response with HMBD002, a novel anti-VISTA neutralizing antibody [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 1729.

Plasticity of myeloid-derived suppressor cells in cancer

In recent years, myeloid-derived suppressor cells (MDSC) have emerged as one of the major inhibitors of immune effector cell function in cancer. MDSC represent a heterogeneous population of largely immature myeloid cells that are characterized by a pathological state of activation and display potent immune suppressive activity. Two major subsets of MDSC have been identified: monocytic (M-MDSC) and polymorphonuclear (PMN-MDSC). PMN-MSDC share phenotypic and morphologic features with neutrophils, whereas M-MDSC are similar to monocytes and are characterized by high plasticity. Differentiation of M-MDSC to macrophages and dendritic cells is shaped by tumor microenvironment. In recent years, the mechanisms of this process start to emerge.

Myeloid-Derived Suppressor Cells: Immune-Suppressive Cells That Impair Antitumor Immunity and Are Sculpted by Their Environment.

Myeloid-derived suppressor cells (MDSC) are a diverse population of immature myeloid cells that have potent immune-suppressive activity. Studies in both mice and humans have demonstrated that MDSC accumulate in most individuals with cancer, where they promote tumor progression, inhibit antitumor immunity, and are an obstacle to many cancer immunotherapies. As a result, there has been intense interest in understanding the mechanisms and in situ conditions that regulate and sustain MDSC, and the mechanisms MDSC use to promote tumor progression. This article reviews the characterization of MDSC and how they are distinguished from neutrophils, describes the suppressive mechanisms used by MDSC to mediate their effects, and explains the role of proinflammatory mediators and the tumor microenvironment in driving MDSC accumulation, suppressive potency, and survival.

C-C Chemokine Receptor Type 2-Dependent Migration of Myeloid-Derived Suppressor Cells in Protection of Islet Transplants.

Islet transplantation is a promising therapeutic approach to restore the physical response to blood glucose in type 1 diabetes. Current chronic use of immunosuppressive reagents for preventing islet allograft rejection is associated with severe complications. In addition, many of the immunosuppressive drugs are diabetogenic. The induction of transplant tolerance to eliminate the dependency on immunosuppression is ideal, but remains challenging. Addition of hepatic stellate cells allowed generation of myeloid-derived suppressor cells (MDSC) from precursors in mouse bone marrow. Migration of MDSC was examined in an islet allograft transplant model by tracking the systemic administered MDSC from CD45.1 congenic mice. The generated MDSC were expressed C-C chemokine receptor type 2 (CCR2), which was enhanced by exposure to interferon-γ. A single systemic administration of MDSC markedly prolonged survival of islet allografts without requirement of immunosuppression. Tracking the administered MDSC showed that they promptly migrated to the islet graft sites, at which point they exerted potent immune suppressive activity by inhibiting CD8 T cells, enhancing regulatory T cell activity. MDSC generated from CCR2 mice failed to be mobilized and lost tolerogenic activity in vivo, but sustained suppressive activity in vitro. MDSC migration was dependent on expression of CCR2, whereas CCR2 does not directly participate in immune suppression. Expression of CCR2 needs to be closely monitored for quality control purpose when MDSC are generated in vitro for immune therapy.

Myeloid derived suppressor cell: A new player in periodontal disease?

Periodontal disease can be initiated by a shift from a symbiotic to a dysbiotic microbial community. An increase in the recruitment of leukocytes and production of inflammatory cytokines, chemokines and oxidative stress are generated by this shift. In periodontitis, an exacerbated, poorly specific and effective inflammatory response is mounted. Moreover, failure in the inflammation resolving mechanism leads to establishment of a chronic inflammatory process, resulting in the progressive destruction of bone and soft tissue. In different diseases presenting chronic inflammation some important players of immune response are defectives. Thus, an immunosuppressive environment could be induced during chronic inflammation. Myeloid derived suppressor cells (MDSC), a heterogenic group of immature myeloid cells with potent immune suppressive activity, are increased in several acute and chronic inflammatory diseases. Dysbiosis-mediated inflammation can induce increased frequency of MDSC. In addition, mediators generated in diverse inflammatory diseases have demonstrated to promote expansion, activation and recruitment of MDSC, similar mediators have been described in periodontal disease. MDSC promote generation of nitric oxide (NO) and reactive oxygen species (ROS). Furthermore, MDSC can differentiate in functional osteoclasts. We hypothesize that MDSC are generated during periodontal disease. Review of literature evaluating this hypothesis and possible implications are assed in this work. It encourages the study of MDSC in this common disease.

CD45 phosphatase regulates the fate of myeloid cells in tumor microenvironment by inhibiting STAT3 activity

Abstract The abnormal differentiation of myeloid cells is one of the key features of altered immune response in cancer. Myeloid derived suppressor cells (MDSC) and tumor associated macrophages (TAM) are the major components of immune suppressive network. MDSC accumulate in large numbers in peripheral lymphoid organs and inside tumors. However, in contrast to peripheral lymphoid organs such as spleen, inside tumors, monocytic MDSC rapidly differentiate to TAM with potent immune suppressive activity. We have demonstrated that this phenomenon is mediated by dramatic down-regulation of STAT3 activity in MDSC in tumor site. In cancer patients, MDSC in tumor site also had substantially lower level of pSTAT3 than MDSC in peripheral blood. These results were unexpected, since high level of STAT3 activity is a hallmark of MDSC present in blood and lymphoid organs. The mice with constitutively active STAT3 had shown the expansion of MDSC and down-regulation of macrophage population in tumor microenvironment, emphasizing the crucial role of STAT3 in myeloid cell differentiation. Hypoxia is an intricate part of tumor microenvironment. The decreased STAT3 activity was caused by hypoxia and affected all myeloid cells but was not observed in tumor cells. We further demonstrate that the up-regulation of CD45 tyrosine phosphatase activity in MDSC exposed to hypoxia in tumor site was responsible for down-regulation of STAT3 activity. Moreover, the positive regulation of CD45 activity in hypoxia was mediated by the disruption of CD45 protein dimerization via sialylation. This suggests a novel role of STAT3 in the regulation of myeloid cells in tumors with potential implications for therapeutic targeting.

Tolerance and immune suppression in the tumor microenvironment

The concept of immunological tolerance has guided and permeated much of modern immunology. Ray Owen’s ground-breaking observations in twin cattle provided the first mechanistic explanation for tolerance to self-molecules and established tolerance as a beneficial process that protects the host against autoreactivity. However, his studies also opened the door to understanding that tolerance may be detrimental, such as occurs when cancer cells induce tolerance/immune suppression resulting in inhibition of anti-tumor immunity. This article briefly traces the early history of the field of tumor immunology with respect to tolerance, and then focuses on a relatively recently identified population of cells called myeloid-derived suppressor cells (MDSCs). MDSC are instrumental in causing tolerance/immune suppression in individuals with cancer. They are present in most individuals with cancer and because of their potent immune suppressive activity are a major deterrent to natural anti-tumor immunity and a significant obstacle to immunotherapy.

Abstract 2343: Tumor microenvironment regulates the fate of myeloid cells by controlling STAT3 activity

Abstract The abnormal differentiation of myeloid cells is one of the key features of altered immune response in cancer. Myeloid derived suppressor cells (MDSC) are a major component of immune suppressive network. MDSC accumulate in large numbers in peripheral lymphoid organs and inside tumors. Previous data indicated that the fate of MDSC depends on the localization of these cells. In contrast to peripheral lymphoid organs such as spleen, inside tumors, MDSC rapidly differentiate to tumor associated macrophages with potent immune suppressive activity. We have demonstrated that this phenomenon is mediated by dramatic down-regulation of STAT3 activity in MDSC at tumor site. In cancer patients, MDSC at tumor site also had substantially lower level of pSTAT3 than MDSC in peripheral blood. These results were unexpected, since high level of STAT3 activity is a hallmark of MDSC present in blood and lymphoid organs. The mice with constitutively active STAT3 had shown the dramatic expansion of MDSC and down-regulation of macrophage population in tumor microenvironment, emphasizing the crucial role of STAT3 in myeloid cell differentiation. Hypoxia is an intricate part of tumor microenvironment. We have found that hypoxia caused the down-regulation of STAT3 activity in MDSC. However, the effect of hypoxia on STAT3 activity was not observed in tumor cells. We further demonstrate that the up-regulation of CD45 tyrosine phosphatase activity in MDSC exposed to hypoxia in tumor site was responsible for down-regulation of STAT3 activity. Using KO model we have shown that MDSC devoid of CD45 failed to experience the effect of hypoxia on pSTAT3 down-regulation as well as their differentiation to macrophage at tumor site. Furthermore, the positive regulation of CD45 activity in hypoxia was mediated by the disruption of CD45 protein dimerization via sialylation. The treatment of sialidase abrogated the effect of hypoxia on STAT3 activity in MDSC and differentiation of MDSC to macrophages. This novel mechanism of regulation of myeloid cell differentiation by STAT3 may provide rationale for re-evaluation of therapeutic strategies in cancer. Citation Format: Vinit Kumar, Pingyan Cheng, Thomas Condamine, Dmitry Gabrilovich. Tumor microenvironment regulates the fate of myeloid cells by controlling STAT3 activity. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2343. doi:10.1158/1538-7445.AM2015-2343

Loss of Rb1 by epigenetic modification regulates expansion of MDSC in cancer

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous group of cells with potent immune suppressive activity in cancer and many other pathologic conditions. MDSCs consist of two major subsets: monocytic MDSCs (M-MDSCs) and polymorphonuclear MDSCs (PMN-MDSCs). Each subset of MDSCs is thought to be developed through the separate differentiation pathways. Here, we demonstrated that in a tumor environment a large proportion of M-MDSCs acquired phenotypic, morphological and functional features of PMN-MDSCs while the normal counterpart of M-MDSCs-Ly6C hi Ly6G - inflammatory monocytes (Mon) from tumor-free mice differentiate to macrophages (MF) and dendritic cells (DCs). This process was mediated by loss of the retinoblastoma (Rb1) protein in MDSCs. Inhibition of Rb1 expression in PMN-MDSCs correlated with the level of histone acetylation of Rb1 promoter. Treatment of PMN-MDSCs with inhibitors of histone deacetylases (HDAC) resulted in the increase in Rb1 expression. In addition, inhibition of HDAC abrogated differentiation of M-MDSCs to PMN-MDSCs, and restored M-MDSCs differentiation towards DCs and MF. These results suggest that down-regulation of Rb1 by epigenetic modification plays a major role in expansion of MDSCs in cancer by regulating PMN-MDSCs differentiation from M-MDSCs. Therefore, inhibition of MDSCs accumulation by treatment of HDAC inhibitors may be considered as a potential therapeutic tool in cancer immunotherapy.

Regulation of tumor metastasis by myeloid-derived suppressor cells.

Accumulation of pathologically activated immature myeloid cells with potent immune-suppressive activity is one of the major immunological hallmarks of cancer. In recent years, it became clear that in addition to their immune-suppressive activity, myeloid-derived suppressor cells (MDSCs) influence tumor progression in a variety of ways. They are directly implicated in the promotion of tumor metastases by participating in the formation of premetastatic niches, promoting angiogenesis and tumor cell invasion. In this review, we discuss recent data describing various roles of MDSCs in the formation of tumor metastases.

Abstract 3616: Hypoxia determines the fate of myeloid cell differentiation by controlling STAT3 activation in tumor site

Abstract The abnormal differentiation of myeloid cells is the key element of altered immune response in cancer. Myeloid derived suppressor cells (MDSC) are a major component of immune suppressive network. MDSC accumulate in large numbers in peripheral lymphoid organs and inside tumors. Recent data indicated that the fate of MDSC depends on the localization of these cells. In contrast to peripheral lymphoid organs, inside tumors, MDSC rapidly differentiate to tumor associated macrophages with potent immune suppressive activity. Hypoxia is intricate part of tumor microenvironment. We have found that hypoxia plays a major role in regulation of MDSC differentiation to macrophages in tumor site. Hypoxia caused dramatic reduction in phosphorylated STAT3 in MDSC. These results were unexpected, since high level of STAT3 activity is a hallmark of MDSC present in blood and lymphoid organs. In our experiments spleen MDSC in tumor-bearing mice had much higher level of STAT3 phosphorylation than their counterparts in tumor site from the same mice. In cancer patients MDSC in tumor site also had substantially lower level of pSTAT3 than MDSC in peripheral blood. In vitro culture of spleen MDSC in hypoxia led to the suppression of STAT3 activation. Furthermore, mice with constitutively active STAT3 had shown the dramatic expansion of MDSC and down-regulation of macrophage population in tumor microenvironment, suggesting that STAT3 is a downstream target for hypoxia driven myeloid cells differentiation in cancer. We found that CD45 protein tyrosine phosphatase was a molecule that regulated STAT3 activation in hypoxia. The activity but not expression of CD45 phosphatase was up-regulated in tumor MDSC as compared to MDSC in spleens. The exposure of spleen MDSC to hypoxia led to the significant increase in CD45 activity. The siRNA mediated silencing of CD45 abrogated the effect of hypoxia on the down-regulation of STAT3 activity in MDSC. This suggests that hypoxia driven up-regulation of CD45 phosphatase activity suppresses STAT3 activation, which in turn plays a key role in differentiation of myeloid cells in tumor site. These unexpected findings may provide rationale for re-evaluation of therapeutic strategies targeting myeloid cells in cancer. Citation Format: Vinit Kumar. Hypoxia determines the fate of myeloid cell differentiation by controlling STAT3 activation in tumor site. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3616. doi:10.1158/1538-7445.AM2014-3616