Tumor-induced myeloid-derived suppressor cell function is independent of IFN-γ and IL-4Rα

Enhancing the efficacy of vaccinia-based oncolytic virotherapy by inhibiting CXCR2-mediated MDSC trafficking

Oncolytic virotherapy is an innovative approach for cancer treatment. However, recruitment of myeloid-derived suppressor cells (MDSCs) into the tumor microenvironment (TME) after oncolysis-mediated local inflammation leads to tumor resistance to the therapy. Using the murine malignant mesothelioma model, we demonstrated that the in situ vaccinia virotherapy recruited primarily polymorphonuclear MDSCs (PMN-MDSCs) into the TME, where they exhibited strong suppression of cytotoxic T lymphocytes in a reactive oxygen species-dependent way. Single-cell RNA sequencing analysis confirmed the suppressive profile of PMN-MDSCs at the transcriptomic level and identified CXCR2 as a therapeutic target expressed on PMN-MDSCs. Abrogating PMN-MDSC trafficking by CXCR2-specific small molecule inhibitor during the vaccinia virotherapy exhibited enhanced antitumor efficacy in 3 syngeneic cancer models, through increasing CD8+/MDSC ratios in the TME, activating cytotoxic T lymphocytes, and skewing suppressive TME into an antitumor environment. Our results warrant clinical development of CXCR2 inhibitor in combination with oncolytic virotherapy.

Roles of IL-4, IL-13, and Their Receptors in Lung Cancer

Interleukin (IL)-4 and IL-13 are the main effectors of innate lymphoid cells (ILC2) of the type 2 innate immune response, which can carry out specific signal transmission between multiple cells in the tumor immune microenvironment. IL-4 and IL-13 mediate signal transduction and regulate cellular functions in a variety of solid tumors through their shared receptor chain, the transmembrane heterodimer interleukin-4 receptor alpha/interleukin-13 receptor alpha-1 (type II IL-4 receptor). IL-4, IL-13, and their receptors can induce the formation of a variety of malignant tumors and play an important role in their progression, growth, and tumor immunity. In order to explore possible targets for lung cancer prediction and treatment, this review summarizes the characteristics and signal transduction pathways of IL-4 and IL-13, and their respective receptors, and discusses in depth their possible role in the occurrence and development of lung cancer.

Infection-Derived Monocytic MDSCs Require TGF-β to Suppress Filarial-Specific IFN-γ But Not IL-13 Release by Filarial-Specific CD4+ T Cells In Vitro

Lymphatic filariasis (LF) remains a major health problem with severe economic repercussions in endemic communities of Sub-saharan Africa, South-East Asia and South America. The rodent-specific nematode Litomosoides sigmodontis (Ls) is used to study the immunomodulatory potential of filariae and research has elucidated pathways involving regulatory T cells (Tregs), IL-10 producing cells and alternatively activated macrophages (AAMs) and that CD4+ T cells play a paramount role during infection. Myeloid-derived suppressor cells (MDSCs) have been identified and characterised in man in cancer and other pathologies. The hallmark of MDSC populations is the suppression of T and B cell responses using various mechanisms, which are mostly specific to the pathology or setting. However, until now, it remains unclear whether they play a role in filarial-specific responses. We report here that monocytic MDSCs (Mo-MDSCs, CD11b+Ly6C+Ly6G-) and polymorphonuclear MDSCs (PMN-MDSCs, CD11b+Ly6Cint/loLy6G+) expanded in the thoracic cavity (TC, the site of infection) and correlated positively with filarial life-stages in Ls-infected BALB/c mice. In vitro, only infection-derived Mo-MDSCs showed a suppressive nature by preventing IL-13 and IFN-γ secretion from filarial-specific CD4+ T cells upon co-culture with soluble worm extract. This suppression was not mediated by IL-10, IL-6 or TNF-α, and did not require cell-contact, nitric oxide (NO), IL-4/IL-5 signalling pathways or CCR2. Interestingly, neutralizing TGF-β significantly rescued IFN-γ but not IL-13 production by filarial-specific CD4+ T cells. In comparison to naive cells, PCR array data showed an overall down-regulation of inflammatory pathways in both infection-derived Mo-MDSCs and PMN-MDSCs. In conclusion, these primary data sets show activity and expansion of MDSCs during Ls infection adding this regulatory cell type to the complex milieu of host responses during chronic helminth infections.

A heparan-sulfate-bearing syndecan-1 glycoform is a distinct surface marker for intra-tumoral myeloid-derived suppressor cells

Myeloid-derived suppressor cells (MDSCs) infiltrate cancer tissue, promote tumor growth, and are associated with resistance to cancer therapies. However, there is no practical approach available to distinguish MDSCs from mature counterparts inside tumors. Here, we show that a recently isolated thioaptamer probe (T1) binds to MDSC subsets in colorectal and pancreatic tumors with high specificity. Whole transcriptome and functional analysis revealed that T1-binding cells contain polymorphonuclear (PMN)-MDSCs characterized by several immunosuppression pathways, ROS production, and T cell suppression activity, whereas T1-non-binding PMNs were mature and nonsuppressive. We identified syndecan-1 as the T1-interacting protein on MDSCs and chronic myelogenous leukemia K562 cell line. Heparan sulfate chains were essential in T1-binding. Inside tumors PMN-MDSCs expressed heparan sulfate biogenesis enzymes at higher levels. Tumor-cell-derived soluble factor(s) enhanced MDSCs' affinity for T1. Overall, we uncovered heparan-sulfate-dependent MDSC modulation in the tumor microenvironment and identified T1 as tool preferentially targeting tumor-promoting myeloid cell subsets.

Benzo[a]pyrene immunogenetics and immune archetype reprogramming of lung

Overexposure to carcinogenic precursor, benzo[a]pyrene [BaP], modulates the lung immune microenvironment. The present review seeks to elucidate novel pathways behind the tumor effect of BaP in the lungs, emphasizing immunomodulatory mediators and immune cells. In this review, BaP reprograms lung immune microenvironment through modulating transforming growth factor-beta (TGF-β), programmed cell death 1 (PD-1), cytotoxic T lymphocyte antigen-4 (CTLA-4), Interleukin 12 (IL-12), indoleamine 2,3 dioxygenase (IDO), forkhead box protein P3 (FOXP3) and interferon-gamma (IFN-γ) levels. Moreover, BaP modulated lung immune cellular architecture such as dendritic cells, T cells, Tregs, macrophages, neutrophils, and myeloid-derived suppressor cells (MDSCs). All mentioned changes in immune architecture and mediators lead to the induction of lung cancer.

Reactive myelopoiesis and the onset of myeloid-mediated immune suppression: Implications for adoptive cell therapy

Adoptive T cell therapy (ACT) in combination with lymphodepleting chemotherapy is an effective strategy to induce the eradication of cancer, providing long-term regressions in patients. However, only a minority of patients that receive ACT with tumor-specific T cells exhibit durable benefit. Thus, there is an urgent need to characterize mechanisms of resistance and define strategies to alleviate immunosuppression in the context of ACT in cancer. This article reviews the importance of lymphodepleting regimens in promoting the optimal engraftment and expansion of T cells in hosts after adoptive transfer. In addition, we discuss the role of concomitant immunosuppression and the accumulation of myeloid derived suppressor cells (MDSCs) during immune recovery after lymphodepleting regimens and mobilization regimens.

Myeloid-Derived Suppressor Cells: A New and Pivotal Player in Colorectal Cancer Progression

Colorectal cancer (CRC) remains a devastating human malignancy with poor prognosis. Of the various factors, immune evasion mechanisms play pivotal roles in CRC progression and impede the effects of cancer therapy. Myeloid-derived suppressor cells (MDSCs) constitute an immature population of myeloid cells that are typical during tumor progression. These cells have the ability to induce strong immunosuppressive effects within the tumor microenvironment (TME) and promote CRC development. Indeed, MDSCs have been shown to accumulate in both tumor-bearing mice and CRC patients, and may therefore become an obstacle for cancer immunotherapy. Consequently, numerous studies have focused on the characterization of MDSCs and their immunosuppressive capacity, as well as developing novel approaches to suppress MDSCs function with different approaches. Current therapeutic strategies that target MDSCs in CRC include inhibition of their recruitment and alteration of their function, alone or in combination with other therapies including chemotherapy, radiotherapy and immunotherapy. Herein, we summarize the recent roles and mechanisms of MDSCs in CRC progression. In addition, a brief review of MDSC-targeting approaches for potential CRC therapy is presented.

The Gut Microbiota, Kynurenine Pathway, and Immune System Interaction in the Development of Brain Cancer

Human gut microbiota contains a large, complex, dynamic microbial community of approximately 10¹⁴ microbes from more than 1,000 microbial species, i.e., equivalent to 4 × 10⁶ genes. Numerous evidence links gut microbiota with human health and diseases. Importantly, gut microbiota is involved in the development and function of the brain through a bidirectional pathway termed as the gut-brain axis. Interaction between gut microbiota and immune responses can modulate the development of neuroinflammation and cancer diseases in the brain. With respect of brain cancer, gut microbiota could modify the levels of antioxidants, amyloid protein and lipopolysaccharides, arginase 1, arginine, cytochrome C, granulocyte-macrophage colony-stimulating factor signaling (GM-CSF), IL-4, IL-6, IL-13, IL-17A, interferon gamma (IFN-γ), reactive oxygen species (ROS), reactive nitrogen species (e.g., nitric oxide and peroxynitrite), short-chain fatty acids (SCFAs), tryptophan, and tumor necrosis factor-β (TGF-β). Through these modifications, gut microbiota can modulate apoptosis, the aryl hydrocarbon receptor (AhR), autophagy, caspases activation, DNA integrity, microglia dysbiosis, mitochondria permeability, T-cell proliferation and functions, the signal transducer and activator of transcription (STAT) pathways, and tumor cell proliferation and metastasis. The outcome of such interventions could be either oncolytic or oncogenic. This review scrutinizes the oncogenic and oncolytic effects of gut microbiota by classifying the modification mechanisms into (i) amino acid deprivation (arginine and tryptophan); (ii) kynurenine pathway; (iii) microglia dysbiosis; and (iv) myeloid-derived suppressor cells (MDSCs). By delineating the complexity of the gut-microbiota-brain-cancer axis, this review aims to help the research on the development of novel therapeutic strategies that may aid the efficient eradication of brain cancers.

Differential Regulation of T-cell Immunity and Tolerance by Stromal Laminin Expressed in the Lymph Node

BACKGROUND

Stromal laminins α4 and α5 are differentially regulated in transplant tolerance and immunity, respectively, resulting in altered T-cell trafficking. We hypothesized that laminins directly regulated T-cell activation and polarization.

METHODS

Human and mouse CD4 T cells were activated in Th1, Th2, Th17, or regulatory T cell (Treg) environments with/without laminin α4 and/or α5. Laminin α5 receptors were blocked with anti-α6 integrin or anti-α-dystroglycan (αDG) monoclonal antibodies, and T-cell polarization was determined. T-cell receptor transgenic TEa CD4 cells that recognized donor alloantigen were transferred into C57BL/6 mice that received alloantigen or cardiac allografts. Laminin receptors were blocked, and TEa T-cell migration and differentiation were assessed. Laminin expression was measured in several models of immunity and tolerance.

RESULTS

In diverse models, laminins α4 and α5 were differentially regulated. Immunity was associated with decreased laminin α4:α5 ratio, while tolerance was associated with an increased ratio. Laminin α4 inhibited CD4+ T-cell proliferation and Th1, Th2, and Th17 polarization but favored Treg induction. Laminin α5 favored T-cell activation and Th1, Th2, and Th17 polarization and inhibited Treg. Laminin α5 was recognized by T cell integrin α6 and is important for activation and inhibition of Treg. Laminin α5 was also recognized by T cell α-DG and required for Th17 differentiation. Anti-α6 integrin or anti-DG prolonged allograft survival.

CONCLUSIONS

Laminins α4 and α5 are coinhibitory and costimulatory ligands for human and mouse CD4 T cells, respectively. Laminins and their receptors modulate immune responses by acting as one of the molecular switches for immunity or suppression.

Defective FasL expression is associated with increased resistance to melanoma liver metastases and enhanced natural killer cell activity

The objective was to determine if the absence of FasL signaling would affect melanoma liver metastases by influencing the antimelanoma properties of liver natural killer (NK) cells. Melanoma liver metastases were induced in wild-type C57BL/6 mice and the gld/gld mutant C57BL/6 mouse strain that expresses a defective form of FasL (CD95L) that fails to engage and signal via the Fas receptor (CD95). Liver metastases were produced by intrasplenic injection of B16LS9 melanoma cells. Liver NK cell activity directed against murine B16LS9 melanoma cells was determined in a 24 h in-vitro cytotoxicity assay. Liver NK cells, NK T cells, and the NK cell surface activation marker, NKG2D, were measured by flow cytometry. Mice expressing defective FasL displayed reduced, rather than enhanced, melanoma liver metastases that coincided with increased liver NK cell-mediated tumor cell cytotoxicity. Enhanced cytotoxicity was not mediated by perforin, tumor necrosis factor-α, or tumor necrosis-associated apoptosis-inducing ligand but was closely associated with elevated interferon-γ in the tumor-bearing liver. FasL-defective gld/gld mice also displayed reduced numbers of liver NK T cells, which have been previously implicated in suppression on liver NK cell activity. The absence of functional FasL in the liver correlates with a heightened, not diminished, resistance to melanoma liver metastases. The resistance to liver metastases coincides with a significant, albeit transient, increase in liver NK cytotoxicity and elevated levels of interferon-γ in the liver.

Functional Diversity of Myeloid-Derived Suppressor Cells: The Multitasking Hydra of Cancer

Myeloid-derived suppressor cells (MDSCs) are immature suppressive cells found in tumors and immunological niches. In this article, we highlight the ability of MDSCs to promote IL-17-producing T cells (Th17) and regulatory T cells in addition to suppressing cytotoxic T cells in different tumor models. These interactions between MDSCs and T cells support tumor growth because IL-17 is tumorigenic in many cancer types and regulatory T cells suppress antitumor T cells. Besides T cells, MDSCs promote regulatory B cells and suppress overall B cell function; however, tumor-evoked regulatory B cells also regulate MDSC function, suggesting cross-regulation between MDSCs and B cells. These multiple functions shed light on how MDSCs dysregulate several arms of host immune response. Moreover, MDSCs promote tumor cell survival and angiogenesis to support tumors. Therefore, the multifunctional feature of MDSCs make them attractive immunotherapeutic targets.

Myeloid-Derived Suppressor Cells as a Therapeutic Target for Cancer

The emergence of immunotherapy has been an astounding breakthrough in cancer treatments. In particular, immune checkpoint inhibitors, targeting PD-1 and CTLA-4, have shown remarkable therapeutic outcomes. However, response rates from immunotherapy have been reported to be varied, with some having pronounced success and others with minimal to no clinical benefit. An important aspect associated with this discrepancy in patient response is the immune-suppressive effects elicited by the tumour microenvironment (TME). Immune suppression plays a pivotal role in regulating cancer progression, metastasis, and reducing immunotherapy success. Most notably, myeloid-derived suppressor cells (MDSC), a heterogeneous population of immature myeloid cells, have potent mechanisms to inhibit T-cell and NK-cell activity to promote tumour growth, development of the pre-metastatic niche, and contribute to resistance to immunotherapy. Accumulating research indicates that MDSC can be a therapeutic target to alleviate their pro-tumourigenic functions and immunosuppressive activities to bolster the efficacy of checkpoint inhibitors. In this review, we provide an overview of the general immunotherapeutic approaches and discuss the characterisation, expansion, and activities of MDSCs with the current treatments used to target them either as a single therapeutic target or synergistically in combination with immunotherapy.

Myeloid-derived suppressor cells are bound and inhibited by anti-thymocyte globulin

Myeloid-derived suppressor cells (MDSCs) inhibit T cell responses and are relevant to cancer, autoimmunity and transplant biology. Anti-thymocyte globulin (ATG) is a commonly used T cell depletion agent, yet the effect of ATG on MDSCs has not been investigated. MDSCs were generated in Lewis Lung Carcinoma 1 tumor-bearing mice. MDSC development and function were assessed in vivo and in vitro with and without ATG administration. T cell suppression assays, RT-PCR, flow cytometry and arginase activity assays were used to assess MDSC phenotype and function. MDSCs increased dramatically in tumor-bearing mice and the majority of splenic MDSCs were of the polymorphonuclear subset. MDSCs potently suppressed T cell proliferation. ATG-treated mice developed 50% fewer MDSCs and these MDSCs were significantly less suppressive of T cell proliferation. In vitro, ATG directly bound 99.6% of MDSCs. CCR7, L-selectin and LFA-1 were expressed by both T cells and MDSCs, and binding of LFA-1 was inhibited by ATG pre-treatment. Arg-1 and PD-L1 transcript expression were reduced 30–40% and arginase activity decreased in ATG-pretreated MDSCs. MDSCs were bound and functionally inhibited by ATG. T cells and MDSCs expressed common Ags which were also targets of ATG. ATG may be helpful in tumor models seeking to suppress MDSCs. Alternatively, ATG may inadvertently inhibit important T cell regulatory events in autoimmunity and transplantation.

Bioprofiling TS/A Murine Mammary Cancer for a Functional Precision Experimental Model

The TS/A cell line was established in 1983 from a spontaneous mammary tumor arisen in an inbred BALB/c female mouse. Its features (heterogeneity, low immunogenicity and metastatic ability) rendered the TS/A cell line suitable as a preclinical model for studies on tumor-host interactions and for gene therapy approaches. The integrated biological profile of TS/A resulting from the review of the literature could be a path towards the description of a precision experimental model of mammary cancer.

MicroRNA-155 coordinates the immunological landscape within murine melanoma and correlates with immunity in human cancers

miR-155 has recently emerged as an important promoter of antitumor immunity through its functions in T lymphocytes. However, the impact of T cell-expressed miR-155 on immune cell dynamics in solid tumors remains unclear. In the present study, we used single-cell RNA sequencing to define the CD45+ immune cell populations at different time points within B16F10 murine melanoma tumors growing in either wild-type or miR-155 T cell conditional knockout (TCKO) mice. miR-155 was required for optimal T cell activation and reinforced the T cell response at the expense of infiltrating myeloid cells. Further, myeloid cells from tumors growing in TCKO mice were defined by an increase in wound healing genes and a decreased IFN-γ-response gene signature. Finally, we found that miR-155 expression predicted a favorable outcome in human melanoma patients and was associated with a strong immune signature. Moreover, gene expression analysis of The Cancer Genome Atlas (TCGA) data revealed that miR-155 expression also correlates with an immune-enriched subtype in 29 other human solid tumors. Together, our study provides an unprecedented analysis of the cell types and gene expression signatures of immune cells within experimental melanoma tumors and elucidates the role of miR-155 in coordinating antitumor immune responses in mammalian tumors.

Arginase-1 is neither constitutively expressed in nor required for myeloid-derived suppressor cell-mediated inhibition of T-cell proliferation

Although previous reports suggest that tumor-induced myeloid-derived suppressor cells (MDSC) inhibit T cells by L-arginine depletion through arginase-1 activity, we herein show that arginase-1 is neither inherently expressed in MDSC nor required for MDSC-mediated inhibition. Employing Percoll density gradients, large expansions of MDSC in the bone marrow of tumor-bearing mice were isolated and demonstrated potent inhibition in T-cell proliferation activated by TCR-ligation, Concanavalin A, PMA plus ionomycin, or IL-2. Despite demonstrating characteristic immunosuppressive capacity, these MDSC exhibit no arginase-1 expression and/or exert their inhibitory effects independent of arginase-1 activity. However, arginase-1 expression in MDSC can be induced by exposure to TCR-activated T cells or their culture medium, but not T cells activated by other means or growing tumor cells. Further investigation reveals multiple cytokines secreted by TCR-activated T cells as orchestrating two signaling-relay axes, IL-6-to-IL-4 and GM-CSF/IL-4-to-IL-10, leading to arginase-1 expression in MDSC. Specifically, IL-6 signaling increases IL-4R, enabling IL-4 to induce arginase-1 expression; similarly, GM-CSF in concert with IL-4 induces IL-10R, allowing IL-10-mediated induction. Surprisingly, our study indicates that induction of arginase-1 expression is not conducive to the critical MDSC-mediated inhibition toward T cells, which is rather dependent on direct cell contacts undiminished by PD-L1 blockade or SIRPα deficiency.

Soluble CD80 Protein Delays Tumor Growth and Promotes Tumor-Infiltrating Lymphocytes

Tumor cells use various immune-suppressive strategies to overcome antitumor immunity. One such method is tumor expression of programmed death ligand-1 (PD-L1), which triggers apoptotic death or anergy upon binding programmed death-1 (PD-1) on T cells. Our previous in vitro cellular studies with human and mouse PD-L1⁺ tumor cells demonstrated that a soluble form of the costimulatory molecule CD80 prevented PD-L1-mediated immune suppression and restored T-cell activation by binding PD-L1 and blocking interaction with PD-1. We now report that in vivo treatment of established syngeneic PD-L1⁺ CT26 colon carcinoma and B16F10 melanoma tumors with CD80-Fc delays tumor growth and promotes tumor-infiltrating T cells. Studies with PD-1^-/- and CD28^-/- mice demonstrate that soluble CD80 acts in vivo by simultaneously neutralizing PD-1 suppression and activating through CD28. We also report that soluble CD80 mediates its effects by activating transcription factors EGR1-4, NF-κB, and MAPK, downstream signaling components of the CD28 and T-cell receptor pathways. Soluble CD80 binds to CTLA-4 on activated human peripheral blood mononuclear cells. However, increasing quantities of CTLA-4 antagonist antibodies do not increase T-cell activation. These results indicate that soluble CD80 does not suppress T-cell function through CTLA-4 and suggest that CTLA-4 acts as a decoy receptor for CD80, rather than functioning as a suppressive signaling receptor. Collectively, these studies demonstrate that soluble CD80 has therapeutic efficacy in vivo in mouse tumor systems and that its effects are due to its ability to inhibit PD-1-mediated suppression while concurrently activating T cells through CD28. Cancer Immunol Res; 6(1); 59-68. ©2017 AACR.

IFN-γ-STAT1-iNOS Induces Myeloid Progenitors to Acquire Immunosuppressive Activity

Autoimmune diseases often induce dysregulated hematopoiesis with altered number and function of hematopoietic stem and progenitor cells (HSPCs). However, there are limited studies on the direct regulation of HSPCs on T cells, which are often detrimental to autoimmunity. Here, we found that in a murine model of Concanavalin A-induced autoimmune hepatitis, LSK (Lineage^-Sca-1⁺c-Kit⁺)-like cells accumulated in liver, spleen, and bone marrow (BM), which were myeloid progenitors (Lineage^-Sca-1^-c-Kit⁺) that upregulated Sca-1 expression upon T cell-derived IFN-γ stimulation. Strikingly, BM LSK-like cells from mice induced by Con A to develop autoimmune hepatitis or alternatively myeloid progenitors from wild-type mice possessed strong in vitro suppressive ability. Their suppressive function depended on T cell-derived IFN-γ in a paracrine fashion, which induced STAT1 phosphorylation, inducible nitric oxide synthase expression, and nitric oxide production. Blocking IFN-γ/IFN-γ receptor interaction, knockout of STAT1, or iNOS inhibition abrogated their suppressive function. In addition, the suppressive function was independent of differentiation; mitomycin C-treated myeloid progenitors maintained T cell suppressive ability in vitro. Our data demonstrate a mechanism of inflammation induced suppressive function of myeloid progenitors, which may participate directly in suppressing T cell-mediated immunopathology.

HMGB1 promotes myeloid-derived suppressor cells and renal cell carcinoma immune escape

Despite high immunogenicity and marked presence of immune cells in the RCC(renal cell carcinoma), immunotherapy fails to develop effective anti-tumor immune responses. This is due to the negative regulatory factors in the tumor microenvironment. As the main contributor of immunosuppression, myeloid-derived suppressor cells (MDSCs) inhibited anti-tumor immunity and promoted tumor progression. Meanwhile, it is confirmed that high mobility group box-1 protein (HMGB1) shows a high expression in many solid tumors and HMGB1 with high expression is involved in tumor immune escape. However, the mechanisms linking HMGB1 with tumor immune escape are unclear. The present study aimed to explore whether HMGB1 can promote RCC immune escape by inducing the generation of MDSCs. In this study, Renca mouse model was established and the influence of HMGB1 on MDSCs was investigated by using HMGB1 antibody to downregulate the expression of HMGB1 in tumor-bearing mice. The result showed that with the down-regulation of HMGB1, the tumor growth was inhibited significantly and the mice survival was prolonged greatly. Furthermore, the differentiation and proliferation of MDSCs were inhibited both in vitro and in vivo, and the inhibition rate showed a positive correlation with the degree of down-regulation of HMGB1. When MDSCs were eliminated with Gr-1 antibody in vivo, the ability of the HMGB1 to promote tumor growth was severely impaired. Thus, our findings indicated that HMGB1 might mediate tumor immune escape by promoting MDSCs cell proliferation, which provided a novel theoretical basis for preventing RCC using HMGB1 as the target.

Contrasting effects of cyclophosphamide on anti-CTL-associated protein 4 blockade therapy in two mouse tumor models

Immune checkpoint blockade is a promising anticancer therapy, but must be used in combination with other anticancer therapies to increase its therapeutic efficacy. Cyclophosphamide (CP) is a chemotherapeutic drug that shows immune‐modulating effects. In this study, we examined the effect of CP on anti‐CTL‐associated protein 4 (CTLA‐4) blockade therapy in two mouse tumor models. Drastic tumor regression was observed in the CT26 colon carcinoma model after i.p. injection of CP (100 mg/kg) followed by anti‐CTLA‐4 antibody. However, administration in the reverse order increased apoptosis in tumor‐specific CD8⁺ T cells. In the RENCA renal carcinoma model, the antitumor effect of combination therapy was marginal and the tumor‐bearing state reduced body weight with an increased serum level of interleukin‐6. Interestingly, although CP monotherapy increased myeloid‐derived suppressor cells (MDSCs) in the spleens of both models, subsequent anti‐CTLA‐4 therapy increased MDSCs only in RENCA‐bearing mice. Additionally, the serum levels of chemokine ligand 2 and C‐X‐C motif chemokine 10 were increased by the combination therapy only in RENCA‐bearing mice and in vivo depletion of Gr‐1⁺ cells augmented the antitumor effect to some degree. These results reveal a contrasting effect of CP on anti‐CTLA‐4 therapy between the two mouse tumor models. Cyclophosphamide augments the antitumor effect of anti‐CTLA‐4 therapy in CT26‐bearing hosts, whereas CP after anti‐CTLA‐4 therapy attenuates this effect through induction of apoptosis in tumor‐reactive T cells. Alternatively, CP‐induced MDSCs can be increased by anti‐CTLA‐4 therapy only in RENCA‐bearing hosts with an elevated level of interleukin‐6.

Immunomodulatory effects of myeloid-derived suppressor cells in diseases: Role in cancer and infections

Myeloid-derived suppressor cells (MDSCs) are heterogeneous cells capable of abrogating T and B cells responses and have been identified in numerous cancers. As with other regulatory cell populations, they aim to maintain balance between host-defence-associated inflammation and ensuing tissue pathology. MDSC accumulation and/or activation involve several growth factors and cytokines including Granulocyte Macrophage-Colony Stimulating Factor (GM-CSF) and Interleukin (IL)-6 and suppression has been linked to receptors such as IL-4Rα. Other immune pathways, such as Toll-like receptors (TLRs) have also been shown to interfere in MDSC activity adding to the complexity in clarifying their pathways. Monocytic- (Mo-MDSCs) and polymorphonuclear- (PMN-MDSCs) cells are two subsets of MDSCs that have been well characterized and have been shown to function through different mechanisms although both appear to require nitric oxide. In human and murine model settings, MDSCs have been shown to have inhibitory effects on T cell responses during bacterial, parasitic and viral pathologies and an increase of MDSC numbers has been associated with pathological conditions. Interestingly, the environment impacts on MDSC activity and regulatory T cells (Tregs), mast cells and a few cells that may help MDSC in order to regulate immune responses. Since the majority of pioneering data on MDSCs has stemmed from research on malignancies, this review will summarize MDSC biology and function in cancer and highlight current knowledge about these cells during infectious pathologies as well.

Stat6 Promotes Intestinal Tumorigenesis in a Mouse Model of Adenomatous Polyposis by Expansion of MDSCs and Inhibition of Cytotoxic CD8 Response

Intestinal tumorigenesis in the ApcMin/+ model is initiated by aberrant activation of Wnt pathway. Increased IL-4 expression in human colorectal cancer tissue and growth of colon cancer cell lines implied that IL-4–induced Stat6-mediated tumorigenic signaling likely contributes to intestinal tumor progression in ApcMin/+ mice. Stat6 also appears to promote expansion of myeloid-derived suppressor cells (MDSCs) cells. MDSCs promote polyp formation in the ApcMin/+ model. Hence, Stat6 could have a broad role in coordinating both polyp cell proliferation and MDSC expansion. We found that IL-4–induced Stat6-mediated proliferation of intestinal epithelial cells is augmented by platelet-derived growth factor–BB, a tumor-promoting growth factor. To determine whether polyp progression in ApcMin/+ mice is dependent on Stat6 signaling, we disrupted Stat6 in this model. Total polyps in the small intestine were fewer in ApcMin/+ mice lacking Stat6. Furthermore, proliferation of polyp epithelial cells was reduced, indicating that Stat6 in part controlled polyp formation. Stat6 also promoted expansion of MDSCs in the spleen and lamina propria of ApcMin/+ mice, implying regulation of antitumor T-cell response. More CD8 cells and reduced PD-1 expression on CD4 cells correlated with reduced polyps. In addition, a strong CD8-mediated cytotoxic response led to killing of tumor cells in Stat6-deficient ApcMin/+ mice. Therefore, these findings show that Stat6 has an oncogenic role in intestinal tumorigenesis by promoting polyp cell proliferation and immunosuppressive mediators, and preventing an active cytotoxic process.

Granulocytic myeloid-derived suppressor cells from human cord blood modulate T-helper cell response towards an anti-inflammatory phenotype

Infections are a leading cause of perinatal morbidity and mortality. The outstandingly high susceptibility to infections early in life is mainly attributable to the compromised state of the neonatal immune system. One important difference to the adult immune system is a bias towards T helper type 2 (Th2) responses in newborns. However, mechanisms regulating neonatal T-cell responses are incompletely understood. Granulocytic myeloid-derived suppressor cells (GR-MDSC) are myeloid cells with a granulocytic phenotype that suppress various functions of other immune cells and accumulate under physiological conditions during pregnancy in maternal and fetal blood. Although it has been hypothesized that GR-MDSC accumulation during fetal life could be important for the maintenance of maternal-fetal tolerance, the influence of GR-MDSC on the immunological phenotype of neonates is still unclear. Here, we investigated the impact of GR-MDSC isolated from cord blood (CB-MDSC) on the polarization of Th cells. We demonstrate that CB-MDSC inhibit Th1 responses and induced Th2 responses and regulatory T (Treg) cells. Th1 inhibition was cell-contact dependent and occurred independent of other cell types, while Th2 induction was mediated independently of cell contact through expression of ArgI and reactive oxygen species by CB-MDSC and partially needed the presence of monocytes. Treg cell induction by CB-MDSC also occurred cell-contact independently but was partially mediated through inducible nitric oxide synthase. These results point towards a role of MDSC in regulating neonatal immune responses. Targeting MDSC function in neonates could be a therapeutic opportunity to improve neonatal host defence.

Lack of STAT6 Attenuates Inflammation and Drives Protection against Early Steps of Colitis-Associated Colon Cancer

Colitis-associated colon cancer (CAC) is one of the most common malignant neoplasms and a leading cause of death. The immunologic factors associated with CAC development are not completely understood. Signal transducer and activator of transcription 6 (STAT6) is part of an important signaling pathway for modulating intestinal immune function and homeostasis. However, the role of STAT6 in colon cancer progression is unclear. Following CAC induction in wild-type (WT) and STAT6-deficient mice (STAT6^-/-), we found that 70% of STAT6^-/- mice were tumor-free after 8 weeks, whereas 100% of WT mice developed tumors. STAT6^-/- mice displayed fewer and smaller colorectal tumors than WT mice; this reduced tumorigenicity was associated with decreased proliferation and increased apoptosis in the colonic mucosa in the early steps of tumor progression. STAT6^-/- mice also exhibited reduced inflammation, diminished concentrations COX2 and nuclear β-catenin protein in the colon, and decreased mRNA expression of IL17A and TNFα, but increased IL10 expression when compared with WT mice. Impaired mucosal expression of CCL9, CCL25, and CXCR2 was also observed. In addition, the number of circulating CD11b⁺Ly6C^hiCCR2⁺ monocytes and CD11b⁺Ly6C^lowLy6G⁺ granulocytes was both decreased in a STAT6-dependent manner. Finally, WT mice receiving a STAT6 inhibitor in vivo confirmed a significant reduction in tumor load as well as less intense signs of CAC. Our results demonstrate that STAT6 is critical in the early steps of CAC development for modulating inflammatory responses and controlling cell recruitment and proliferation. Thus, STAT6 may represent a promising target for CAC treatment. Cancer Immunol Res; 5(5); 385-96. ©2017 AACR.

The myeloid immune signature of enterotoxigenic Bacteroides fragilis-induced murine colon tumorigenesis

Enterotoxigenic Bacteroides fragilis (ETBF), a human commensal and candidate pathogen in colorectal cancer (CRC), is a potent initiator of interleukin-17 (IL-17)-dependent colon tumorigenesis in Min^Apc+/- mice. We examined the role of IL-17 and ETBF on the differentiation of myeloid cells into myeloid-derived suppressor cells (MDSCs) and tumor-associated macrophages, which are known to promote tumorigenesis. The myeloid compartment associated with ETBF-induced colon tumorigenesis in Min mice was defined using flow cytometry and gene expression profiling. Cell-sorted immature myeloid cells were functionally assayed for inhibition of T-cell proliferation and inducible nitric oxide synthase expression to delineate MDSC populations. A comparison of ETBF infection with that of other oncogenic bacteria (Fusobacterium nucleatum or pks⁺Escherichia coli) revealed a specific, ETBF-associated colonic immune infiltrate. ETBF-triggered colon tumorigenesis is associated with an IL-17-driven myeloid signature characterized by subversion of steady-state myelopoiesis in favor of the generation of protumoral monocytic-MDSCs (MO-MDSCs). Combined action of the B. fragilis enterotoxin BFT and IL-17 on colonic epithelial cells promoted the differentiation of MO-MDSCs, which selectively upregulated Arg1 and Nos2, produced NO, and suppressed T-cell proliferation. Evidence of a pathogenic inflammatory signature in humans colonized with ETBF may allow for the identification of populations at risk for developing colon cancer.

Neutrophils in the tumor microenvironment: trying to heal the wound that cannot heal

Neutrophils are the first responders to infection and injury and are critical for antimicrobial host defense. Through the generation of reactive oxidants, activation of granular constituents and neutrophil extracellular traps, neutrophils target microbes and prevent their dissemination. While these pathways are beneficial in the context of trauma and infection, their off-target effects in the context of tumor are variable. Tumor-derived factors have been shown to reprogram the marrow, skewing toward the expansion of myelopoiesis. This can result in stimulation of both neutrophilic leukocytosis and the release of immature granulocytic populations that accumulate in circulation and in the tumor microenvironment. While activated neutrophils have been shown to kill tumor cells, there is growing evidence for neutrophil activation driving tumor progression and metastasis through a number of pathways, including stimulation of thrombosis and angiogenesis, stromal remodeling, and impairment of T cell-dependent anti-tumor immunity. There is also growing appreciation of neutrophil heterogeneity in cancer, with distinct neutrophil populations promoting cancer control or progression. In addition to the effects of tumor on neutrophil responses, anti-neoplastic treatment, including surgery, chemotherapy, and growth factors, can influence neutrophil responses. Future directions for research are expected to result in more mechanistic knowledge of neutrophil biology in the tumor microenvironment that may be exploited as prognostic biomarkers and therapeutic targets.

Age-associated impairment of antitumor immunity in carcinoma-bearing mice and restoration by oral administration of Lentinula edodes mycelia extract

Because cancer is associated with aging, immunological features in the aged should be considered in anticancer immunotherapy. In this study, we investigated antitumor immunity in aged mice using a CT26 colon carcinoma model. The tumor growth of CT26 was accelerated in aged mice compared with that in young mice, but this difference was not observed in nude mice. The serum levels of IL-6 and TNF-α were higher in aged mice than those in young mice, irrespective of the CT26-bearing state. The in vitro induction of CT26-specific CTLs from aged mice that were vaccinated with doxorubicin (DTX)-treated CT26 cells was impaired. In vivo neutralization of IL-6, but not TNF-α, showed a tendency to restore the in vitro induction of CT26-specific CTLs from vaccinated aged mice. Analyses on tumor-infiltrating immune cells as early as day 5 after CT26 inoculation revealed that monocytic and granulocytic MDSCs preferentially infiltrated into tumor sites in aged mice compared with young mice. Alternatively, oral administration of Lentinula edodes mycelia (L.E.M.) extract, which has the potential to suppress inflammation in tumor-bearing hosts, decreased the serum levels of IL-6 in aged mice. When administration of L.E.M. extract was started 1 week earlier, CT26 growth was retarded in aged mice and the in vivo priming of tumor-specific CTLs was improved in CT26-vaccinated aged mice. These results indicate early infiltration of MDSCs is related to impaired immunity of aged hosts and that oral administration of L.E.M. extract can mitigate the impairment.

Myeloid-Derived Suppressor Cell Survival and Function Are Regulated by the Transcription Factor Nrf2

Tumor-induced myeloid-derived suppressor cells (MDSC) contribute to immune suppression in tumor-bearing individuals and are a major obstacle to effective immunotherapy. Reactive oxygen species (ROS) are one of the mechanisms used by MDSC to suppress T cell activation. Although ROS are toxic to most cells, MDSC survive despite their elevated content and release of ROS. NF erythroid 2-related factor 2 (Nrf2) is a transcription factor that regulates a battery of genes that attenuate oxidative stress. Therefore, we hypothesized that MDSC resistance to ROS may be regulated by Nrf2. To test this hypothesis, we used Nrf2(+/+)and Nrf2(-/-)BALB/c and C57BL/6 mice bearing 4T1 mammary carcinoma and MC38 colon carcinoma, respectively. Nrf2 enhanced MDSC suppressive activity by increasing MDSC production of H2O2, and it increased the quantity of tumor-infiltrating MDSC by reducing their oxidative stress and rate of apoptosis. Nrf2 did not affect circulating levels of MDSC in tumor-bearing mice because the decreased apoptotic rate of tumor-infiltrating MDSC was balanced by a decreased rate of differentiation from bone marrow progenitor cells. These results demonstrate that Nrf2 regulates the generation, survival, and suppressive potency of MDSC, and that a feedback homeostatic mechanism maintains a steady-state level of circulating MDSC in tumor-bearing individuals.

CD33(+) /HLA-DR(neg) and CD33(+) /HLA-DR(+/-) Cells: Rare Populations in the Human Decidua with Characteristics of MDSC

PROBLEM

Human pregnancy needs a remarkable local immune tolerance toward the conceptus. Myeloid-derived suppressor cells (MDSC) are important players promoting cancer initiation and progression by suppressing T-cell functions and thus inducing immune tolerance. Therefore, MDSC were expected within decidua.

METHODS

Subpopulations of CD33(+) immune cells were isolated from human early pregnancy decidua and characterized phenotypically and functionally by microscopy, FACS analysis, RT-PCR, Western blotting and in the coculture with T cells.

RESULTS

Decidua harbors CD33(+) /HLA-DR(neg) and CD33(+) /HLA-DR(+/-) cells which both express arginase, iNOS and IDO and a typical cytokine profile. Both subtypes potently suppress T-cell proliferation and therefore fulfill the criteria of MDSC.

CONCLUSION

We characterized a new population of CD33(+) /HLA-DR(neg) and CD33(+) /HLA-DR(+/-) cells in human early pregnancy decidua with properties of classical MDSC and thus potentially being an important player in immune tolerance in pregnancy.

Dendritic cell defects in the colorectal cancer

Colorectal cancer (CRC) results from the accumulation of both genetic and epigenetic alterations of the genome. However, also the formation of an inflammatory milieu plays a pivotal role in tumor development and progression. Dendritic cells (DCs) play a relevant role in tumor by exerting differential pro-tumorigenic and anti-tumorigenic functions, depending on the local milieu. Quantitative and functional impairments of DCs have been widely observed in several types of cancer, including CRC, representing a tumor-escape mechanism employed by cancer cells to elude host immunosurveillance.

Understanding the interactions between DCs and tumors is important for comprehending the mechanisms of tumor immune surveillance and escape, and provides novel approaches to therapy of cancer. This review summarizes updated information on the role of the DCs in colon cancer development and/or progression.

Transcriptional regulation of myeloid-derived suppressor cells

Myeloid-derived suppressor cells are a heterogeneous group of pathologically activated immature cells that play a major role in the negative regulation of the immune response in cancer, autoimmunity, many chronic infections, and inflammatory conditions, as well as in the regulation of tumor angiogenesis, tumor cell invasion, and metastases. Accumulation of myeloid-derived suppressor cells is governed by a network of transcriptional regulators that could be combined into 2 partially overlapping groups: factors promoting myelopoiesis and preventing differentiation of mature myeloid cells and factors promoting pathologic activation of myeloid-derived suppressor cells. In this review, we discuss the specific nature of these factors and their impact on myeloid-derived suppressor cell development.

Tetraspanin CD81 promotes tumor growth and metastasis by modulating the functions of T regulatory and myeloid-derived suppressor cells

Tumor cells counteract innate and adaptive antitumor immune responses by recruiting regulatory T cells (Treg) and innate myeloid-derived suppressor cells (MDSC), which facilitate immune escape and metastatic dissemination. Here we report a role in these recruitment processes for CD81, a member of the tetraspanin family of proteins that have been implicated previously in cancer progression. We found that genetic deficiency in CD81 reduced tumor growth and metastasis in two genetic mouse backgrounds and multiple tumor models. Mechanistic investigations revealed that CD81 was not required for normal development of Treg and MDSC but was essential for immunosuppressive functions. Notably, adoptive transfer of wild-type Treg into CD81-deficient mice was sufficient to promote tumor growth and metastasis. Our findings suggested that CD81 modulates adaptive and innate immune responses, warranting further investigation of CD81 in immunomodulation in cancer and its progression.

MDSCs in cancer: Conceiving new prognostic and therapeutic targets

The incomplete clinical efficacy of anti-tumor immunotherapy can depend on the presence of an immunosuppressive environment in the host that supports tumor progression. Tumor-derived cytokines and growth factors induce an altered hematopoiesis that modifies the myeloid cell differentiation process, promoting proliferation and expansion of cells with immunosuppressive skills, namely myeloid derived suppressor cells (MDSCs). MDSCs promote tumor growth not only by shaping immune responses towards tumor tolerance, but also by supporting several processes necessary for the neoplastic progression such as tumor angiogenesis, cancer stemness, and metastasis dissemination. Thus, MDSC targeting represents a promising tool to eliminate host immune dysfunctions and increase the efficacy of immune-based cancer therapies.

Myeloid-Derived Suppressor Cells: Critical Cells Driving Immune Suppression in the Tumor Microenvironment

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of immature myeloid cells that suppress innate and adaptive immunity. MDSCs are present in many disease settings; however, in cancer, they are a major obstacle for both natural antitumor immunity and immunotherapy. Tumor and host cells in the tumor microenvironment (TME) produce a myriad of pro-inflammatory mediators that activate MDSCs and drive their accumulation and suppressive activity. MDSCs utilize a variety of mechanisms to suppress T cell activation, induce other immune-suppressive cell populations, regulate inflammation in the TME, and promote the switching of the immune system to one that tolerates and enhances tumor growth. Because MDSCs are present in most cancer patients and are potent immune-suppressive cells, MDSCs have been the focus of intense research in recent years. This review describes the history and identification of MDSCs, the role of inflammation and intracellular signaling events governing MDSC accumulation and suppressive activity, immune-suppressive mechanisms utilized by MDSCs, and recent therapeutics that target MDSCs to enhance antitumor immunity.

Complexity and challenges in defining myeloid-derived suppressor cells

Study of myeloid cells endowed with suppressive activity is an active field of research which has particular importance in cancer, in view of the negative regulatory capacity of these cells to the host's immune response. The expansion of these cells, called myeloid-derived suppressor cells (MDSCs), has been documented in many models of tumor-bearing mice and in patients with tumors of various origin, and their presence is associated with disease progression and reduced survival. For this reason, monitoring this type of cell expansion is of clinical importance, and flow cytometry is the technique of choice for their identification. Over the years, it has been demonstrated that MDSCs comprise a group of immature myeloid cells belonging both to monocytic and granulocytic lineages, with several stages of differentiation; their occurrence depends on tumor-derived soluble factors, which guide their expansion and determine their block of differentiation. Because of their heterogeneous composition, accurate phenotyping of these cells requires a multicolor approach, so that the expansion of all MDSC subsets can be appreciated. This review article focuses on identifying MDSCs and discusses problems associated with phenotyping circulating and tumor-associated MDSCs in humans and in mouse models.

Myeloid cells' evasion of melanoma immunity

An immune-suppressive role of myeloid-derived suppressor cells (MDSCs) in melanoma has long been speculated, whereas molecular mechanisms underlying this role are not well understood. Here, Chung and colleagues show that dendritic cell-associated, heparan sulfate proteoglycans-dependent integrin ligand (DC-HIL), a cell surface immune-modulatory molecule, is highly expressed on tumor-associated MDSCs. Genetic ablation or antibody blockade of DC-HIL delays the growth of transplantable B16 melanoma in syngeneic mice, which is accompanied by enhanced antitumor T-cell activities. These findings support a role for DC-HIL in immune evasion within the melanoma microenvironment.

Location, location, location: functional and phenotypic heterogeneity between tumor-infiltrating and non-infiltrating myeloid-derived suppressor cells

An increasing number of studies is focusing on the role of myeloid-derived suppressor cells (MDSCs) in the suppression of antitumor immune responses. Although the main site of action for MDSCs is most likely the tumor microenvironment, the study of these cells has been largely restricted to MDSCs derived from peripheral lymphoid organs. Only in a minority of studies MDSCs isolated from the tumor microenvironment have been characterized. This review will give an overview of the data available on the phenotypical and functional differences between tumor-derived MDSCs and MDSCs isolated from the spleen of tumor-bearing mice or from the peripheral blood of cancer patients.

Reduced Myeloid-derived Suppressor Cells in the Blood and Endometrium is Associated with Early Miscarriage

PROBLEM

The contribution of myeloid-derived suppressor cells (MDSC) in patients suffering from early or recurrent miscarriage is unknown. MDSC are implicated in modulation of T-cell response in healthy pregnancies; however, the role of MDSC in patients suffering from miscarriage has not been studied. We hypothesized that MDSC play major role in inducing maternal-fetal tolerance and this tolerance is compromised in patients suffering from miscarriage.

METHOD OF STUDY

MDSC level was assessed by flow cytometry and immunostaining in blood and endometrial decidua, respectively. Activation of T cells was determined by MTT proliferation and IL-2 ELISA assays.

RESULTS AND CONCLUSION

The miscarriage patients harbor reduced level of functionally suppressive MDSC in blood and endometrium as compared to healthy control women with successful pregnancies. These results suggest MDSC regulate maternal tolerance in healthy pregnancies and that drug inducing MDSC could have therapeutic implication in the miscarriage patients.

Hampering immune suppressors: therapeutic targeting of myeloid-derived suppressor cells in cancer

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of immature myeloid cells with suppressive properties that preferentially expand in cancer. Myeloid-derived suppressor cells mainly suppress T-cell proliferation and cytotoxicity, inhibit natural killer cell activation, and induce the differentiation and expansion of regulatory T cells. The wide spectrum of MDSC suppressive activity in cancer and its role in tumor progression have rendered these cells a promising target for effective cancer immunotherapy. In this review we briefly discuss the origin of MDSCs and their main mechanisms of suppression and focus more on the approaches developed up to date targeting of MDSCs in tumor-bearing animals and cancer patients.

Cross-talk among myeloid-derived suppressor cells, macrophages, and tumor cells impacts the inflammatory milieu of solid tumors

MDSC and macrophages are present in most solid tumors and are important drivers of immune suppression and inflammation. It is established that cross-talk between MDSC and macrophages impacts anti-tumor immunity; however, interactions between tumor cells and MDSC or macrophages are less well studied. To examine potential interactions between these cells, we studied the impact of MDSC, macrophages, and four murine tumor cell lines on each other, both in vitro and in vivo. We focused on IL-6, IL-10, IL-12, TNF-α, and NO, as these molecules are produced by macrophages, MDSC, and many tumor cells; are present in most solid tumors; and regulate inflammation. In vitro studies demonstrated that MDSC-produced IL-10 decreased macrophage IL-6 and TNF-α and increased NO. IL-6 indirectly regulated MDSC IL-10. Tumor cells increased MDSC IL-6 and vice versa. Tumor cells also increased macrophage IL-6 and NO and decreased macrophage TNF-α. Tumor cell-driven macrophage IL-6 was reduced by MDSC, and tumor cells and MDSC enhanced macrophage NO. In vivo analysis of solid tumors identified IL-6 and IL-10 as the dominant cytokines and demonstrated that these molecules were produced predominantly by stromal cells. These results suggest that inflammation within solid tumors is regulated by the ratio of tumor cells to MDSC and macrophages and that interactions of these cells have the potential to alter significantly the inflammatory milieu within the tumor microenvironment.

IFN-γ regulates survival and function of tumor-induced CD11b+ Gr-1high myeloid derived suppressor cells by modulating the anti-apoptotic molecule Bcl2a1

Myeloid derived suppressor cells (MDSCs) play a critical role in suppression of immune responses in cancer and inflammation. Here, we describe how regulation of Bcl2a1 by cytokines controls the suppressor function of CD11b(+) Gr-1(high) granulocytic MDSCs. Coculture of CD11b(+) Gr-1(high) granulocytic MDSCs with antigen-stimulated T cells and simultaneous blockade of IFN-γ by the use of anti-IFN-γ blocking antibody, IFN-γ(-/-) effector T cells, IFN-γR(-/-) MDSCs or STAT1(-/-) MDSCs led to upregulation of Bcl2a1 in CD11b(+) Gr-1(high) cells, improved survival, and enhanced their suppressor function. Molecular studies revealed that GM-CSF released by antigen-stimulated CD8(+) T cells induced Bcl2a1 upregulation, which was repressed in the presence of IFN-γ by a direct interaction of phosphorylated STAT-1 with the Bcl2a1 promotor. Bcl2a1 overexpressing granulocytic MDSCs demonstrated prolonged survival and enhanced suppressor function in vitro. Our data suggest that IFN-γ/ STAT1-dependent regulation of Bcl2a1 regulates survival and thereby suppressor function of granulocytic MDSCs.

Expansion of CD11b(+)Ly6G (+)Ly6C (int) cells driven by medroxyprogesterone acetate in mice bearing breast tumors restrains NK cell effector functions

The progesterone analog medroxyprogesterone acetate (MPA) is widely used as a hormone replacement therapy in postmenopausal women and as contraceptive. However, prolonged administration of MPA is associated with increased incidence of breast cancer through ill-defined mechanisms. Here, we explored whether exposure to MPA during mammary tumor growth affects myeloid-derived suppressor cells (MDSCs; CD11b(+)Gr-1(+), mostly CD11b(+)Ly6G(+)Ly6C(int) and CD11b(+)Ly6G(-)Ly6C(high) cells) and natural killer (NK) cells, potentially restraining tumor immunosurveillance. We used the highly metastatic 4T1 breast tumor (which does not express the classical progesterone receptor and expands MDSCs) to challenge BALB/c mice in the absence or in the presence of MPA. We observed that MPA promoted the accumulation of NK cells in spleens of tumor-bearing mice, but with reduced degranulation ability and in vivo cytotoxic activity. Simultaneously, MPA induced a preferential expansion of CD11b(+)Ly6G(+)Ly6C(int) cells in spleen and bone marrow of 4T1 tumor-bearing mice. In vitro, MPA promoted nuclear mobilization of the glucocorticoid receptor (GR) in 4T1 cells and endowed these cells with the ability to promote a preferential differentiation of bone marrow cells into CD11b(+)Ly6G(+)Ly6C(int) cells that displayed suppressive activity on NK cell degranulation. Sorted CD11b(+)Gr-1(+) cells from MPA-treated tumor-bearing mice exhibited higher suppressive activity on NK cell degranulation than CD11b(+)Gr-1(+) cells from vehicle-treated tumor-bearing mice. Thus, MPA, acting through the GR, endows tumor cells with an enhanced capacity to expand CD11b(+)Ly6G(+)Ly6C(int) cells that subsequently display a stronger suppression of NK cell-mediated anti-tumor immunity. Our results describe an alternative mechanism by which MPA may affect immunosurveillance and have potential implication in breast cancer incidence.

Myeloid-derived suppressor cells in the inflammatory bowel diseases

BACKGROUND

Myeloid cells are the most abundant and heterogeneous population of leukocytes. They are rapidly recruited from the blood to areas of inflammation and perform a number of important biological functions. Chronic inflammatory conditions contribute to generation of myeloid-derived suppressor cells (MDSCs). These pathologically activated cells are increasingly recognized as important players in cancer, transplantation, and autoimmunity for their abilities to modulate innate and adaptive immune responses.

METHODS

Since clinical data on MDSC accumulation in human patients affected with inflammatory bowel diseases (IBD) are relatively scarce, most of the information described in this review came from studies using experimental mouse models of IBD.

RESULTS

In this review, we discuss possible roles of these cells in chronic immune-mediated disorders focusing on studies conducted in IBD. We will review the available evidence on how MDSCs are involved in modulating T cell responses and look into the complex relationship between Th1, Th17 cells, and myeloid cells. Finally, we will review some recent successes and failures resulted from therapies aimed at manipulating myeloid cell numbers and/or their function.

CONCLUSIONS

Although MDSCs have been described in animal models of experimental colitis and in patients with IBD, their exact role in IBD pathogenesis is unclear and needs to be studied further. Information obtained from these studies will be useful to better understand the cross talk between myeloid cells in T cells during chronic inflammation and may identify novel pathways to be targeted therapeutically.