Deregulation of apoptotic factors Bcl-xL and Bax confers apoptotic resistance to myeloid-derived suppressor cells and contributes to their persistence in cancer

Understanding the immunosuppressive microenvironment of glioma: mechanistic insights and clinical perspectives

Glioblastoma (GBM), the predominant and primary malignant intracranial tumor, poses a formidable challenge due to its immunosuppressive microenvironment, thereby confounding conventional therapeutic interventions. Despite the established treatment regimen comprising surgical intervention, radiotherapy, temozolomide administration, and the exploration of emerging modalities such as immunotherapy and integration of medicine and engineering technology therapy, the efficacy of these approaches remains constrained, resulting in suboptimal prognostic outcomes. In recent years, intensive scrutiny of the inhibitory and immunosuppressive milieu within GBM has underscored the significance of cellular constituents of the GBM microenvironment and their interactions with malignant cells and neurons. Novel immune and targeted therapy strategies have emerged, offering promising avenues for advancing GBM treatment. One pivotal mechanism orchestrating immunosuppression in GBM involves the aggregation of myeloid-derived suppressor cells (MDSCs), glioma-associated macrophage/microglia (GAM), and regulatory T cells (Tregs). Among these, MDSCs, though constituting a minority (4-8%) of CD45+ cells in GBM, play a central component in fostering immune evasion and propelling tumor progression, angiogenesis, invasion, and metastasis. MDSCs deploy intricate immunosuppressive mechanisms that adapt to the dynamic tumor microenvironment (TME). Understanding the interplay between GBM and MDSCs provides a compelling basis for therapeutic interventions. This review seeks to elucidate the immune regulatory mechanisms inherent in the GBM microenvironment, explore existing therapeutic targets, and consolidate recent insights into MDSC induction and their contribution to GBM immunosuppression. Additionally, the review comprehensively surveys ongoing clinical trials and potential treatment strategies, envisioning a future where targeting MDSCs could reshape the immune landscape of GBM. Through the synergistic integration of immunotherapy with other therapeutic modalities, this approach can establish a multidisciplinary, multi-target paradigm, ultimately improving the prognosis and quality of life in patients with GBM.

5-Fluorouracil Suppresses Colon Tumor through Activating the p53-Fas Pathway to Sensitize Myeloid-Derived Suppressor Cells to FasL+ Cytotoxic T Lymphocyte Cytotoxicity

Myelosuppression is a major adverse effect of 5-fluorouracil (5-FU) chemotherapy. However, recent findings indicate that 5-FU selectively suppresses myeloid-derived suppressor cells (MDSCs), to enhance antitumor immunity in tumor-bearing mice. 5-FU-mediated myelosuppression may thus have a beneficial effect for cancer patients. The molecular mechanism underlying 5-FU's suppression of MDSCs is currently unknown. We aimed at testing the hypothesis that 5-FU suppresses MDSCs through enhancing MDSC sensitivity to Fas-mediated apoptosis. We observed that, although FasL is highly expressed in T cells, Fas is weakly expressed in myeloid cells in human colon carcinoma, indicating that downregulation of Fas is a mechanism underlying myeloid cell survival and accumulation in human colon cancer. 5-FU treatment upregulated expression of both p53 and Fas, and knocking down p53 diminished 5-FU-induced Fas expression in MDSC-like cells, in vitro. 5-FU treatment also increased MDSC-like cell sensitivity to FasL-induced apoptosis in vitro. Furthermore, we determined that 5-FU therapy increased expression of Fas on MDSCs, suppressed MDSC accumulation, and increased CTL tumor infiltration in colon tumor-bearing mice. In human colorectal cancer patients, 5-FU chemotherapy decreased MDSC accumulation and increased CTL level. Our findings determine that 5-FU chemotherapy activates the p53-Fas pathway, to suppress MDSC accumulation, to increase CTL tumor infiltration.

IRF8 Regulates Intrinsic Ferroptosis through Repressing p53 Expression to Maintain Tumor Cell Sensitivity to Cytotoxic T Lymphocytes

Ferroptosis has emerged as a cytotoxic T lymphocyte (CTL)-induced tumor cell death pathway. The regulation of tumor cell sensitivity to ferroptosis is incompletely understood. Here, we report that interferon regulatory factor 8 (IRF8) functions as a regulator of tumor cell intrinsic ferroptosis. Genome-wide gene expression profiling identified the ferroptosis pathway as an IRF8-regulated pathway in tumor cells. IRF8.KO tumor cells acquire resistance to intrinsic ferroptosis induction and IRF8-deficient tumor cells also exhibit decreased ferroptosis in response to tumor-specific CTLs. Irf8 deletion increased p53 expression in tumor cells and knocking out p53 in IRF8.KO tumor cells restored tumor cell sensitivity to intrinsic ferroptosis induction. Furthermore, IRF8.KO tumor cells grew significantly faster than WT tumor cells in immune-competent mice. To restore IRF8 expression in tumor cells, we designed and synthesized codon usage-optimized IRF8-encoding DNA to generate IRF8-encoding plasmid NTC9385R-mIRF8. Restoring IRF8 expression via a lipid nanoparticle-encapsulated NTC9385R-mIRF8 plasmid therapy suppressed established tumor growth in vivo. In human cancer patients, nivolumab responders have a significantly higher IRF8 expression level in their tumor cells as compared to the non-responders. Our data determine that IRF8 represses p53 expression to maintain tumor cell sensitivity to intrinsic ferroptosis.

Myeloid-derived suppressor cells in head and neck squamous cell carcinoma

Myeloid-derived suppressor cells (MDSCs), which originated from hematopoietic stem cells, are heterogeneous population of cells that have different differentiation patterns and widely presented in tumor microenvironment. For tumor research, myeloid suppressor cells have received extensive attention since their discovery due to their specific immunosuppressive properties, and the mechanisms of immunosuppression and therapeutic approaches for MDSCs have been investigated in a variety of different types of malignancies. To improve the efficacy of treatment for head and neck squamous cell carcinoma (HNSCC), a disease with a high occurrence, immunotherapy has gradually emerged in after traditional surgery and subsequent radiotherapy and chemotherapy, and has made some progress. In this review, we introduced the mechanisms on the development, differentiation, and elimination of MDSCs and provided a detailed overview of the mechanisms behind the immunosuppressive properties of MDSCs. We summarized the recent researches on MDSCs in HNSCC, especially for targeting-MDSCs therapy and combination with other types of therapy such as immune checkpoint blockade (ICB). Furthermore, we looked at drug delivery patterns and collected the current diverse drug delivery systems for the improvement that contributed to therapy against MDSCs in HNSCC. Most importantly, we made possible outlooks for the future research priorities, which provide a basis for further study on the clinical significance and therapeutic value of MDSCs in HNSCC.

Immunosuppressive tumor microenvironment modulation by chemotherapies and targeted therapies to enhance immunotherapy effectiveness

With the rapid clinical development of immune checkpoint inhibitors (ICIs), the standard of care in cancer management has evolved rapidly. However, immunotherapy is not currently beneficial for all patients. In addition to intrinsic tumor factors, other etiologies of resistance to ICIs arise from the complex interplay between cancer and its microenvironment. Recognition of the essential role of the tumor microenvironment (TME) in cancer progression has led to a shift from a tumor-cell-centered view of cancer development, to the concept of a complex tumor ecosystem that supports tumor growth and metastatic dissemination. The expansion of immunosuppressive cells represents a cardinal strategy deployed by tumor cells to escape detection and elimination by the immune system. Regulatory T lymphocytes (Treg), myeloid-derived suppressor cells (MDSCs), and type-2 tumor-associated macrophages (TAM2) are major components of these inhibitory cellular networks, with the ability to suppress innate and adaptive anticancer immunity. They therefore represent major impediments to anticancer therapies, particularly immune-based interventions. Recent work has provided evidence that, beyond their direct cytotoxic effects on cancer cells, several conventional chemotherapeutic (CT) drugs and agents used in targeted therapies (TT) can promote the elimination or inactivation of suppressive immune cells, resulting in enhanced antitumor immunity. In this review, we will analyze findings pertaining to this concept, discuss the possible molecular bases underlying the selective targeting of these immunosuppressive cells by antineoplastic agents (CT and/or TT), and consider current challenges and future prospects related to the integration of these molecules into more efficient anticancer strategies, in the era of immunotherapy.

Pharmacological modulation of myeloid-derived suppressor cells to dampen inflammation

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous cell population with potent suppressive and regulative properties. MDSCs’ strong immunosuppressive potential creates new possibilities to treat chronic inflammation and autoimmune diseases or induce tolerance towards transplantation. Here, we summarize and critically discuss different pharmacological approaches which modulate the generation, activation, and recruitment of MDSCs in vitro and in vivo, and their potential role in future immunosuppressive therapy.

IRF8: Mechanism of Action and Health Implications

Interferon regulatory factor 8 (IRF8) is a transcription factor of the IRF protein family. IRF8 was originally identified as an essentialfactor for myeloid cell lineage commitment and differentiation. Deletion of Irf8 leads to massive accumulation of CD11b⁺Gr1⁺ immature myeloid cells (IMCs), particularly the CD11b⁺Ly6C^hi/+Ly6G⁻ polymorphonuclear myeloid-derived suppressor cell-like cells (PMN-MDSCs). Under pathological conditions such as cancer, Irf8 is silenced by its promoter DNA hypermethylation, resulting in accumulation of PMN-MDSCs and CD11b⁺ Ly6G⁺Ly6C^lo monocytic MDSCs (M-MDSCs) in mice. IRF8 is often silenced in MDSCs in human cancer patients. MDSCs are heterogeneous populations of immune suppressive cells that suppress T and NK cell activity to promote tumor immune evasion and produce growth factors to exert direct tumor-promoting activity. Emerging experimental data reveals that IRF8 is also expressed in non-hematopoietic cells. Epithelial cell-expressed IRF8 regulates apoptosis and represses Osteopontin (OPN). Human tumor cells may use the IRF8 promoter DNA methylation as a mechanism to repress IRF8 expression to advance cancer through acquiring apoptosis resistance and OPN up-regulation. Elevated OPN engages CD44 to suppress T cell activation and promote tumor cell stemness to advance cancer. IRF8 thus is a transcription factor that regulates both the immune and non-immune components in human health and diseases.

Beyond immunosuppressive effects: dual roles of myeloid-derived suppressor cells in bone-related diseases

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of immature myeloid cells (IMCs) with immunosuppressive functions, whereas IMCs originally differentiate into granulocytes, macrophages, and dendritic cells (DCs) to participate in innate immunity under steady-state conditions. At present, difficulties remain in identifying MDSCs due to lacking of specific biomarkers. To make identification of MDSCs accurately, it also needs to be determined whether having immunosuppressive functions. MDSCs play crucial roles in anti-tumor, angiogenesis, and metastasis. Meanwhile, MDSCs could make close interaction with osteoclasts, osteoblasts, chondrocytes, and other stromal cells within microenvironment of bone and joint, and thereby contributing to poor prognosis of bone-related diseases such as cancer-related bone metastasis, osteosarcoma (OS), rheumatoid arthritis (RA), osteoarthritis (OA), and orthopedic trauma. In addition, MDSCs have been shown to participate in the procedure of bone repair. In this review, we have summarized the function of MDSCs in cancer-related bone metastasis, the interaction with stromal cells within the bone microenvironment as well as joint microenvironment, and the critical role of MDSCs in bone repair. Besides, the promising value of MDSCs in the treatment for bone-related diseases is also well discussed.

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

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

Myeloid-derived suppressor cells as immunosuppressive regulators and therapeutic targets in cancer

Myeloid-derived suppressor cells (MDSCs) are a heterogenic population of immature myeloid cells with immunosuppressive effects, which undergo massive expansion during tumor progression. These cells not only support immune escape directly but also promote tumor invasion via various non-immunological activities. Besides, this group of cells are proved to impair the efficiency of current antitumor strategies such as chemotherapy, radiotherapy, and immunotherapy. Therefore, MDSCs are considered as potential therapeutic targets for cancer therapy. Treatment strategies targeting MDSCs have shown promising outcomes in both preclinical studies and clinical trials when administrated alone, or in combination with other anticancer therapies. In this review, we shed new light on recent advances in the biological characteristics and immunosuppressive functions of MDSCs. We also hope to propose an overview of current MDSCs-targeting therapies so as to provide new ideas for cancer treatment.

IRF8 is a Reliable Monoblast Marker for Acute Monocytic Leukemias

Blast evaluation in patients with acute monocytic leukemias (AMoL) is notoriously difficult due to the lack of reliable surface markers and cytologic subtleties on the aspirate smears. While blasts of most nonmonocytic acute leukemias express CD34, available immunohistochemical antibodies to monocytic blasts also mark normal background mature monocytes. We searched for a potential biomarker candidate by surveying specific gene expression profiles of monocyte progenitors. Our investigations led us to IRF8, which is a lineage-specific transcription factor critical for the production of monocytic and dendritic cell progenitors. In this study, we tested and validated a monoclonal antibody to IRF8 as a novel immunohistochemical stain for trephine core biopsies of human bone marrow. We assessed the expression of IRF8 in 90 cases of AMoL, including posttherapy staging bone marrows, 23 cases of chronic myelomonocytic leukemia, 26 cases of other acute myeloid leukemia subtypes, and 18 normal control marrows. In AMoL, there was high correlation of IRF8-positive cells to aspirate blast count (R=0.95). Comparison of IRF8 staining to aspirate blast percentage in chronic myelomonocytic leukemia also showed good correlation (R=0.86). In contrast, IRF8-positive cells did not correlate with blast count in other subtypes of acute myeloid leukemia (R=0.56) and staining was <5% in all normal control marrows, even those with reactive monocytosis. We found that IRF8 was also weakly reactive in B cells and hematogones, with the latter accounting for rare cases of discrepancies. When IRF8 was used to categorize cases as AMoL, positive for residual leukemia or negative, the sensitivity was 98%, specificity was 82%, positive predictive value was 86%, and negative predictive value was 98%. These results demonstrate that IRF8 may serve as a clinically useful immunostain to diagnose and track AMoLs on bone marrow core biopsies. This can be particularly impactful in the setting of poor aspiration and focal blast increase. In the era of new targeted therapies that have been reported to induce monocytic outgrowths of leukemia, a marker for malignant monoblasts may prove even more critical.

Grass carp (Ctenopharyngodon idella) interferon regulatory factor 8 down-regulates interferon1 expression via interaction with interferon regulatory factor 2 in vitro

Interferon regulatory factor 8 (IRF8), also known as interferon consensus sequence-binding protein (ICSBP), is a negative regulatory factor of interferon (IFN) and plays an important role in cell differentiation and innate immunity in mammals. In recent years, some irf8 homologous genes have been cloned and confirmed to take part in innate immune response in fish, but the mechanism still remains unclear. In this paper, a grass carp (Ctenopharyngodon idella) irf8 gene (Ciirf8) was cloned and characterized. The deduced protein (CiIRF8) possesses a highly conserved N-terminal DNA binding domain but a less well-conserved C-terminal IRF association domain (IAD). Ciirf8 was widely expressed in all tested tissues of grass carp and up-regulated following poly(I:C) stimulation. Ciirf8 expression was also up-regulated in CIK cells upon treatment with poly(I:C). To explore the molecular mechanism of how fish IRF8 regulates ifn1 expression, the similarities and differences of grass carp IRF8 and IRF2 were compared and contrasted. Subcellular localization analysis showed that CiIRF8 is located both in the cytoplasm and nucleus; however, CiIRF2 is only located in the nucleus. The nuclear-cytoplasmic translocation of CiIRF8 was observed in CIK cells under stimulation with poly(I:C). The interaction of CiIRF8 and CiIRF2 was further confirmed by a co-immunoprecipitation assay in the nucleus. Dual-luciferase reporter assays showed that the promoter activity of Ciifn1 was significantly inhibited by co-transfection with CiIRF2 and CiIRF8. The transcription inhibition of Ciifn1 was alleviated by competitive binding of CiIRF2 and CiIRF8 to CiIRF1. In conclusion, CiIRF8 down-regulates Ciifn1 expression via interaction with CiIRF2 in cells.

Effects of low-dose bufalin combined with hydroxycamptothecin on human castration-resistant prostate cancer xenografts in nude mice

Prostate cancer is the most prevalent tumor found in men worldwide. Despite the efficiency of primary endocrine prostate cancer therapies, more efficient drugs are needed to tackle the most advanced and resistant forms of this condition. The present study investigated the antitumor effects of low-dose bufalin combined with hydroxycamptothecin on castration-resistant prostate cancer (CRPC) in mice, as well as the possible mechanisms of apoptosis induction. CRPC xenograft tumors were generated in mice and, subsequently, mice received appropriate doses of bufalin, hydroxycamptothecin or a combination of the two drugs. Tumors from each treatment group were removed, and the tumor volume, weight and inhibition rate of each group was determined. Hematoxylin and eosin staining was performed for pathological analysis and TUNEL staining was used to assess the level of apoptosis in the xenografts. Immunohistochemistry was used for the analysis of proliferating cell nuclear antigen expression and the expression of Bax, Bcl-XL, p53, programmed cell death 4 (PDCD4), phosphorylated (p)-AKT and glycogen synthase kinase (GSK)-3β was determined by western blotting. Treatment with bufalin significantly (P<0.05) reduced tumor volumes compared with the negative control group, reducing tumor volumes to lower levels when combined with hydroxycampothecin. The combination of bufalin (0.6 or 0.8 mg/kg) and hydroxycampothecin significantly (P<0.05) induced higher levels of cell apoptosis compared with the administration of bufalin or hydroxycampothecin alone. The combination of bufalin and hydroxycampothecin also increased the expression of apoptosis-related proteins Bax, p53, PDCD4 and GSK-3β, and decreased the expression of Bcl-XL and p-AKT compared with a single drug treatment. The present study suggested that the combination of bufalin and hydroxycampothecin improved the inhibitory effects of both drugs on CRPC tumors in vivo, potentially via the regulation of the PI3K/AKT/GSK-3β and p53-dependent apoptosis signaling pathways.

Myeloid-Derived Suppressive Cells Deficient in Liver X Receptor α Protected From Autoimmune Hepatitis

Myeloid-derived suppressor cells (MDSCs) emerge as a promising candidate for the immunotherapy of autoimmune hepatitis (AIH). However, targets for modulating MDSC in AIH are still being searched. Liver X receptors (LXRs) are important nuclear receptors linking lipid metabolism and immune responses. Despite the extensive studies of LXR in myeloid compartment, its role in MDSCs is currently less understood. Herein, expression of LXRα was found to be upregulated in AIH patients and colocalized with hepatic MDSCs. In ConA-induced hepatitis, deletion of LXRα led to increased expansion of MDSCs in the liver and alleviated the hepatic injury. MDSCs in LXRα-/- mice exhibited enhanced proliferation and survival comparing with WT mice. T-cell proliferation assay and adoptive cell transfer experiment validated the potent immunoregulatory role of MDSCs in vitro and in vivo. Mechanistically, MDSCs from LXRα-/- mice possessed significantly lower expression of interferon regulatory factor 8 (IRF-8), a key negative regulator of MDSC differentiation. Transcriptional activation of IRF-8 by LXRα was further demonstrated.

Conclusion

We reported that abrogation of LXRα facilitated the expansion of MDSCs via downregulating IRF-8, and thereby ameliorated hepatic immune injury profoundly. Our work highlights the therapeutic potential of targeting LXRα in AIH.

Combined Effects of Myeloid Cells in the Neuroblastoma Tumor Microenvironment

Despite multimodal treatment, survival chances for high-risk neuroblastoma patients remain poor. Immunotherapeutic approaches focusing on the activation and/or modification of host immunity for eliminating tumor cells, such as chimeric antigen receptor (CAR) T cells, are currently in development, however clinical trials have failed to reproduce the preclinical results. The tumor microenvironment is emerging as a major contributor to immune suppression and tumor evasion in solid cancers and thus has to be overcome for therapies relying on a functional immune response. Among the cellular components of the neuroblastoma tumor microenvironment, suppressive myeloid cells have been described as key players in inhibition of antitumor immune responses and have been shown to positively correlate with more aggressive disease, resistance to treatments, and overall poor prognosis. This review article summarizes how neuroblastoma-driven inflammation induces suppressive myeloid cells in the tumor microenvironment and how they in turn sustain the tumor niche through suppressor functions, such as nutrient depletion and generation of oxidative stress. Numerous preclinical studies have suggested a range of drug and cellular therapy approaches to overcome myeloid-derived suppression in neuroblastoma that warrant evaluation in future clinical studies.

Immunomodulation by targeted anticancer agents

At odds with conventional chemotherapeutics, targeted anticancer agents are designed to inhibit precise molecular alterations that support oncogenesis or tumor progression. Despite such an elevated degree of molecular specificity, many clinically employed and experimental targeted anticancer agents also mediate immunostimulatory or immunosuppressive effects that (at least in some settings) influence therapeutic efficacy. Here, we discuss the main immunomodulatory effects of targeted anticancer agents and explore potential avenues to harness them in support of superior clinical efficacy.

Osteopontin: A Key Regulator of Tumor Progression and Immunomodulation

Simple Summary

Anti-PD-1/PD-L1 and anti-CTLA-4-based immune checkpoint blockade (ICB) immunotherapy have recently emerged as a breakthrough in human cancer treatment. Durable efficacy has been achieved in many types of human cancers. However, not all human cancers respond to current ICB immunotherapy and only a fraction of the responsive cancers exhibit efficacy. Osteopontin (OPN) expression is highly elevated in human cancers and functions as a tumor promoter. Emerging data suggest that OPN may also regulate immune cell function in the tumor microenvironment. This review aims at OPN function in human cancer progression and new findings of OPN as a new immune checkpoint. We propose that OPN compensates PD-L1 function to promote tumor immune evasion, which may underlie human cancer non-response to current ICB immunotherapy.

Abstract

OPN is a multifunctional phosphoglycoprotein expressed in a wide range of cells, including osteoclasts, osteoblasts, neurons, epithelial cells, T, B, NK, NK T, myeloid, and innate lymphoid cells. OPN plays an important role in diverse biological processes and is implicated in multiple diseases such as cardiovascular, diabetes, kidney, proinflammatory, fibrosis, nephrolithiasis, wound healing, and cancer. In cancer patients, overexpressed OPN is often detected in the tumor microenvironment and elevated serum OPN level is correlated with poor prognosis. Initially identified in activated T cells and termed as early T cell activation gene, OPN links innate cells to adaptive cells in immune response to infection and cancer. Recent single cell RNA sequencing revealed that OPN is primarily expressed in tumor cells and tumor-infiltrating myeloid cells in human cancer patients. Emerging experimental data reveal a key role of OPN is tumor immune evasion through regulating macrophage polarization, recruitment, and inhibition of T cell activation in the tumor microenvironment. Therefore, in addition to its well-established direct tumor cell promotion function, OPN also acts as an immune checkpoint to negatively regulate T cell activation. The OPN protein level is highly elevated in peripheral blood of human cancer patients. OPN blockade immunotherapy with OPN neutralization monoclonal antibodies (mAbs) thus represents an attractive approach in human cancer immunotherapy.

Expression profiles and function of IL6 in polymorphonuclear myeloid-derived suppressor cells

IL6 is an inflammatory cytokine with pleiotropic functions in both immune and nonimmune cells, and its expression level is inversely correlated with disease prognosis in patients with cancer. However, blocking IL6 alone has only yielded minimal efficacy in human cancer patients. We aimed at defining IL6 expression profiles under inflammatory conditions and cancer, and elucidating the mechanism underlying IL6 intrinsic signaling in colon carcinoma. We report here that colonic inflammation induces IL6 expression primarily in the CD11b+Ly6G+Ly6Clo polymorphonuclear myeloid-derived suppressor cells (PMN-MDSC) in colon. Although both tumor cells, T cells and myeloid cells all express IL6, PMN-MDSCs are the primary cell type that express IL6 in colon carcinoma, suggesting that IL6 up-regulation is a response to inflammation in colon epithelium and tumor microenvironment. Furthermore, we determined that IL6 activates STAT3 to up-regulate DNMT1 and DNMT3b expression in colon tumor cells, thereby revealing an epigenetic mechanism that mediates the IL6-STAT3 signaling pathway in colon carcinoma. Surprisingly, knocking out IL6 in colon tumor cells did not significantly alter tumor growth in WT mice. Conversely, IL6-sufficient colon and pancreatic tumor grow at similar rate in WT and IL6-deficient mice. However, overexpression of IL6 in colon tumor cells significantly increases tumor growth in vivo. Our findings determine that a high tumor local IL6 threshold is essential for IL6 function in colon tumor promotion and targeting the IL6-expressing PMN-MDSCs is potentially an effective approach to suppress colon tumor growth in vivo.

Granulocytic Myeloid-Derived Suppressor Cells as Negative Regulators of Anticancer Immunity

The immune system plays a critical role in cancer progression and response to therapy. However, the immune system can be compromised during the neoplastic process. Notably, the myeloid lineage, which gives rise to granulocytic cells, including neutrophils, is a well-recognized target of tumor-mediated immune suppression. Ordinarily, granulocytic cells are integral for host defense, but in neoplasia the normal process of granulocyte differentiation (i.e., granulopoiesis) can be impaired leading instead to the formation of granulocytic (or PMN)-myeloid-derived suppressor cells (MDSCs). Such cells comprise various stages of myeloid differentiation and are defined functionally by their highly pro-tumorigenic and immune suppressive activities. Thus, considerable interest has been devoted to impeding the negative contributions of PMN-MDSCs to the antitumor response. Understanding their biology has the potential to unveil novel therapeutic opportunities to hamper PMN-MDSC production in the bone marrow, their mobilization, or their effector functions within the tumor microenvironment and, therefore, bolster anticancer therapies that require a competent myeloid compartment. In this review, we will highlight mechanisms by which the neoplastic process skews granulopoiesis to produce PMN-MDSCs, summarize mechanisms by which they execute their pro-tumorigenic activities and, lastly, underscore strategies to obstruct their role as negative regulators of antitumor immunity.

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

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

Upregulated TRAIL and Reduced DcR2 Mediate Apoptosis of Decidual PMN-MDSC in Unexplained Recurrent Pregnancy Loss

Myeloid-derived suppressor cells (MDSC), especially polymorphonuclear MDSC (PMN-MDSC), accumulate in maternal-fetal interface during pregnancy and are involved in the maintenance of immune tolerance. Decreased PMN-MDSC is associated with pregnancy complications such as unexplained recurrent pregnancy loss (URPL). In the present study we showed decreased PMN-MDSC in the URPL group compared with the normal pregnancy (NP) group, and PMN-MDSC was the major subset of MDSC in human decidua with potent immune suppression activity. We then performed gene expression profile and found that human decidual PMN-MDSC in the NP and URPL groups showed different gene and pathway signature, including apoptosis. Apoptosis of decidual PMN-MDSC was mediated by TNF-related apoptosis–induced ligand (TRAIL) in a Caspase 3 dependent manner. TRAIL was expressed in decidua and upregulated in decidua of the URPL group. Notably, of all the membrane TRAIL receptors, only DcR2 was down-regulated in PMN-MDSC in the URPL group. In vitro experiment demonstrated that DcR2 blockade sensitized PMN-MDSC to TRAIL-mediated apoptosis. Together, these data indicate that increased TRAIL and reduced DcR2 on PMN-MDSC sensitize PMN-MDSC response to TRAIL-induced apoptosis in the URPL group, which is responsible for decreased accumulation of PMN-MDSC in URPL.

Green Tea Polyphenol EGCG Attenuates MDSCs-mediated Immunosuppression through Canonical and Non-Canonical Pathways in a 4T1 Murine Breast Cancer Model

Several studies in the past decades have reported anti-tumor activity of the bioactive compounds extracted from tea leaves, with a focus on the compound epigallocatechin-3-gallate (EGCG). However, further investigations are required to unravel the underlying mechanisms behind the anti-tumor activity of EGCG. In this study, we demonstrate that EGCG significantly inhibits the growth of 4T1 breast cancer cells in vitro and in vivo. EGCG ameliorated immunosuppression by significantly decreasing the accumulation of myeloid-derived suppressor cells (MDSCs) and increasing the proportions of CD4⁺ and CD8⁺ T cells in spleen and tumor sites in 4T1 breast tumor-bearing mice. Surprisingly, a low dose of EGCG (0.5-5 μg/mL) effectively reduced the cell viability and increased the apoptosis rate of MDSCs in vitro. EGCG down-regulated the canonical pathways in MDSCs, mainly through the Arg-1/iNOS/Nox2/NF-κB/STAT3 signaling pathway. Moreover, transcriptomic analysis suggested that EGCG also affected the non-canonical pathways in MDSCs, such as ECM-receptor interaction and focal adhesion. qRT-PCR further validated that EGCG restored nine key genes in MDSCs, including Cxcl3, Vcan, Col4a1, Col8a1, Oasl2, Mmp12, Met, Itsnl and Acot1. Our results provide new insight into the mechanism of EGCG-associated key pathways/genes in MDSCs in the murine breast tumor model.

The functional activity of E-cadherin controls tumor cell metastasis at multiple steps

E-cadherin is a tumor suppressor protein, and the loss of its expression in association with the epithelial mesenchymal transition (EMT) occurs frequently during tumor metastasis. However, many metastases continue to express E-cadherin, and a full EMT is not always necessary for metastasis; also, positive roles for E-cadherin expression in metastasis have been reported. We hypothesize instead that changes in the functional activity of E-cadherin expressed on tumor cells in response to environmental factors is an important determinant of the ability of the tumor cells to metastasize. We find that E-cadherin expression persists in metastatic lung nodules and circulating tumor cells (CTCs) in two mouse models of mammary cancer: genetically modified MMTV-PyMT mice and orthotopically grafted 4T1 tumor cells. Importantly, monoclonal antibodies that bind to and activate E-cadherin at the cell surface reduce lung metastasis from endogenous genetically driven tumors and from tumor cell grafts. E-cadherin activation inhibits metastasis at multiple stages, including the accumulation of CTCs from the primary tumor and the extravasation of tumor cells from the vasculature. These activating mAbs increase cell adhesion and reduce cell invasion and migration in both cell culture and three-dimensional spheroids grown from primary tumors. Moreover, activating mAbs increased the frequency of apoptotic cells without affecting proliferation. Although the growth of the primary tumors was unaffected by activating mAbs, CTCs and tumor cells in metastatic nodules exhibited increased apoptosis. Thus, the functional state of E-cadherin is an important determinant of metastatic potential beyond whether the gene is expressed.

High expression of ID1 in monocytes is strongly associated with phenotypic and functional MDSC markers in advanced melanoma

The efficacy of immunotherapies for malignant melanoma is severely hampered by local and systemic immunosuppression mediated by myeloid-derived suppressor cells (MDSC). Inhibitor of differentiation 1 (ID1) is a transcriptional regulator that was shown to be centrally involved in the induction of immunosuppressive properties in myeloid cells in mice, while it was overexpressed in CD11b⁺ cells in the blood of late-stage melanoma patients. Therefore, we comprehensively assessed ID1 expression in PBMC from stage III and IV melanoma patients, and studied ID1 regulation in models for human monocyte differentiation towards monocyte-derived dendritic cells. A highly significant elevation of ID1 was observed in CD33⁺CD11b⁺CD14⁺HLA-DR^low monocytic MDSC in the blood of melanoma patients compared to their HLA-DR^high counterparts, while expression of ID1 correlated positively with established MDSC markers S100A8/9 and iNOS. Moreover, expression of ID1 in monocytes significantly decreased in PBMC samples taken after surgical removal of melanoma metastases, compared to those taken before surgery. Finally, maturation of monocyte-derived DC coincided with a significant downregulation of ID1. Together, these data indicate that increased ID1 expression is strongly associated with expression of phenotypic and immunosuppressive markers of monocytic MDSC, while downregulation is associated with a more immunogenic myeloid phenotype. As such, ID1 may be an additional phenotypic marker for monocytic MDSC. Investigation of ID1 as a pharmacodynamic biomarker or its use as a target for modulating MDSC is warranted.

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

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

The Tumor Microenvironment in Colorectal Cancer Therapy

The current standard-of-care for metastatic colorectal cancer (mCRC) includes chemotherapy and anti-angiogenic or anti-epidermal growth factor receptor (EGFR) monoclonal antibodies, even though the addition of anti-angiogenic agents to backbone chemotherapy provides little benefit for overall survival. Since the approval of anti-angiogenic monoclonal antibodies bevacizumab and aflibercept, for the management of mCRC over a decade ago, extensive efforts have been devoted to discovering predictive factors of the anti-angiogenic response, unsuccessfully. Recent evidence has suggested a potential correlation between angiogenesis and immune phenotypes associated with colorectal cancer. Here, we review evidence of interactions between tumor angiogenesis, the immune microenvironment, and metabolic reprogramming. More specifically, we will highlight such interactions as inferred from our novel immune-metabolic (IM) signature, which groups mCRC into three distinct clusters, namely inflamed-stromal-dependent (IM Cluster 1), inflamed-non stromal-dependent (IM Cluster 2), and non-inflamed or cold (IM Cluster 3), and discuss the merits of the IM classification as a guide to new immune-metabolic combinatorial therapeutic strategies in mCRC.

Direct Inhibition of IRF-Dependent Transcriptional Regulatory Mechanisms Associated With Disease

Interferon regulatory factors (IRFs) are a family of homologous proteins that regulate the transcription of interferons (IFNs) and IFN-induced gene expression. As such they are important modulating proteins in the Toll-like receptor (TLR) and IFN signaling pathways, which are vital elements of the innate immune system. IRFs have a multi-domain structure, with the N-terminal part acting as a DNA binding domain (DBD) that recognizes a DNA-binding motif similar to the IFN-stimulated response element (ISRE). The C-terminal part contains the IRF-association domain (IAD), with which they can self-associate, bind to IRF family members or interact with other transcription factors. This complex formation is crucial for DNA binding and the commencing of target-gene expression. IRFs bind DNA and exert their activating potential as homo or heterodimers with other IRFs. Moreover, they can form complexes (e.g., with Signal transducers and activators of transcription, STATs) and collaborate with other co-acting transcription factors such as Nuclear factor-κB (NF-κB) and PU.1. In time, more of these IRF co-activating mechanisms have been discovered, which may play a key role in the pathogenesis of many diseases, such as acute and chronic inflammation, autoimmune diseases, and cancer. Detailed knowledge of IRFs structure and activating mechanisms predisposes IRFs as potential targets for inhibition in therapeutic strategies connected to numerous immune system-originated diseases. Until now only indirect IRF modulation has been studied in terms of antiviral response regulation and cancer treatment, using mainly antisense oligonucleotides and siRNA knockdown strategies. However, none of these approaches so far entered clinical trials. Moreover, no direct IRF-inhibitory strategies have been reported. In this review, we summarize current knowledge of the different IRF-mediated transcriptional regulatory mechanisms and how they reflect the diverse functions of IRFs in homeostasis and in TLR and IFN signaling. Moreover, we present IRFs as promising inhibitory targets and propose a novel direct IRF-modulating strategy employing a pipeline approach that combines comparative in silico docking to the IRF-DBD with in vitro validation of IRF inhibition. We hypothesize that our methodology will enable the efficient identification of IRF-specific and pan-IRF inhibitors that can be used for the treatment of IRF-dependent disorders and malignancies.

STAT3 inhibition induces Bax-dependent apoptosis in liver tumor myeloid-derived suppressor cells

Immunosuppressive myeloid-derived suppressor cells (MDSC) subvert antitumor immunity and limit the efficacy of chimeric antigen receptor T cells (CAR-T). Previously, we reported that the GM-CSF/JAK2/STAT3 axis drives liver-associated MDSC (L-MDSC) proliferation and blockade of this axis rescued antitumor immunity. We extended these findings in our murine liver metastasis (LM) model, by treating tumor-bearing mice with STAT3 inhibitors (STATTIC or BBI608) to further our understanding of how STAT3 drives L-MDSC suppressive function. STAT3 inhibition caused significant reduction of tumor burden as well as L-MDSC frequencies due to decrease in pSTAT3 levels. L-MDSC isolated from STATTIC or BBI608-treated mice had significantly reduced suppressive function. STAT3 inhibition of L-MDSC was associated with enhanced antitumor activity of CAR-T. Further investigation demonstrated activation of apoptotic signaling pathways in L-MDSC following STAT3 inhibition as evidenced by an upregulation of the pro-apoptotic proteins Bax, cleaved caspase-3, and downregulation of the anti-apoptotic protein Bcl-2. Accordingly, there was also a decrease of pro-survival markers, pErk and pAkt, and an increase in pro-death marker, Fas, with activation of downstream JNK and p38 MAPK. These findings represent a previously unrecognized link between STAT3 inhibition and Fas-induced apoptosis of MDSCs. Our findings suggest that inhibiting STAT3 has potential clinical application for enhancing the efficacy of CAR-T cells in LM through modulation of L-MDSC.

S100A4 Protects Myeloid-Derived Suppressor Cells from Intrinsic Apoptosis via TLR4-ERK1/2 Signaling

Myeloid-derived suppressor cells (MDSCs) often expand during cancer or chronic inflammation and dampen immune responses. However, mechanisms underlying their capacity to escape intrinsic apoptosis in the inflammatory environment are still largely unknown. In this study, we investigated this in mouse tumor models with MDSC accumulation. Spontaneous rejection of tumors implanted into mice deficient for the small Ca²⁺-binding protein S100A4 (S100A4^-/-) was accompanied by low numbers of peripheral MDSCs. This was independent of S100A4 expression on tumor cells. In contrast, MDSCs from S100A4^-/- tumor-bearing mice showed a diminished resistance to the induction of intrinsic apoptosis. Further studies demonstrated that S100A4 protects MDSCs from apoptosis through toll-like receptor-4/extracellular signal-regulated kinase-dependent caspase-9 inhibition. The finding that S100A4 is critical for MDSC survival in inflammatory environments might have important implications for the clinical treatment of cancer or inflammation-related diseases.

Ceramide activates lysosomal cathepsin B and cathepsin D to attenuate autophagy and induces ER stress to suppress myeloid-derived suppressor cells

Myeloid-derived suppressor cells (MDSCs) are immune suppressive cells that are hallmarks of human cancer. MDSCs inhibit cytotoxic T lymphocytes (CTLs) and NK cell functions to promote tumor immune escape and progression, and therefore are considered key targets in cancer immunotherapy. Recent studies determined a key role of the apoptosis pathways in tumor-induced MDSC homeostasis and it is known that ceramide plays a key role in regulation of mammalian cell apoptosis. In this study, we aimed to determine the efficacy and underlying molecular mechanism of ceramide in suppression of MDSCs. Treatment of tumor-bearing mice with LCL521, a lysosomotropic inhibitor of acid ceramidase, significantly decreased MDSC accumulation in vivo. Using a MDSC-like myeloid cell model, we determined that LCL521 targets lysosomes and increases total cellular C16 ceramide level. Although MDSC-like cells have functional apoptosis pathways, LCL521-induced MDSC death occurs in an apoptosis- and necroptosis-independent mechanism. LCL521 treatment resulted in an increase in the number of autophagic vesicles, heterolysosomes and swollen ERs. Finally, concomitant inhibition of cathepsin B and cathepsin D was required to significantly decrease LCL521-induced cell death. Our observations indicate that LCL521 targets lysosomes to activate cathepsin B and cathepsin D, resulting in interrupted autophagy and ER stress that culminates in MDSC death. Therefore, a ceramidase inhibitor is potentially an effective adjunct therapeutic agent for suppression of MDSCs to enhance the efficacy of CTL-based cancer immunotherapy.

Relevance of Interferon Regulatory Factor-8 Expression in Myeloid-Tumor Interactions

Perturbations in myelopoiesis are a common feature in solid tumor biology, reflecting the central premise that cancer is not only a localized affliction but also a systemic disease. Because the myeloid compartment is essential for the induction of adaptive immunity, these alterations in myeloid development contribute to the failure of the host to effectively manage tumor progression. These "dysfunctional" myeloid cells have been coined myeloid-derived suppressor cells (MDSCs). Interestingly, such cells not only arise in neoplasia but also are associated with many other inflammatory or pathologic conditions. MDSCs affect disease outcome through multiple mechanisms, including their ability to mediate generalized or antigen-specific immune suppression. Consequently, MDSCs pose a significant barrier to effective immunotherapy in multiple disease settings. Although much interest has been devoted to unraveling mechanisms by which MDSCs mediate immune suppression, a large gap has remained in our understanding of the mechanisms that drive their development in the first place. Investigations into this question have identified an unrecognized role of interferon regulatory factor-8 (IRF-8), a member of the IRF family of transcription factors, in tumor-induced myeloid dysfunction. Ordinarily, IRF-8 is involved in diverse stages of myelopoiesis, namely differentiation and lineage commitment toward monocytes, dendritic cells, and granulocytes. Several recent studies now support the hypothesis that IRF-8 functions as a "master" negative regulator of MDSC formation in vivo. This review focuses on IRF-8 as a potential target suppressed by tumors to cripple normal myelopoiesis, redirecting myeloid differentiation toward the emergence of MDSCs. Understanding the bases by which neoplasia drives MDSC accumulation has the potential to improve the efficacy of therapies that require a competent myeloid compartment.

Pak2 regulates myeloid-derived suppressor cell development in mice

Myeloid-derived suppressor cells (MDSCs) are CD11b⁺Gr1⁺ cells that induce T-cell hyporesponsiveness, thus impairing antitumor immunity. We have previously reported that disruption of Pak2, a member of the p21-activated kinases (Paks), in hematopoietic stem/progenitor cells (HSPCs) induces myeloid lineage skewing and expansion of CD11b^highGr1^high cells in mice. In this study, we confirmed that Pak2-KO CD11b^highGr1^high cells suppressed T-cell proliferation, consistent with an MDSC phenotype. Loss of Pak2 function in HSPCs led to (1) increased hematopoietic progenitor cell sensitivity to granulocyte-macrophage colony-stimulating factor (GM-CSF) signaling, (2) increased MDSC proliferation, (3) decreased MDSC sensitivity to both intrinsic and Fas-Fas ligand-mediated apoptosis, and (4) promotion of MDSCs by Pak2-deficient CD4⁺ T cells that produced more interferon γ, tumor necrosis factor α, and GM-CSF. Pak2 disruption activated STAT5 while downregulating the expression of IRF8, a well-described myeloid transcription factor. Together, our data reveal a previously unrecognized role of Pak2 in regulating MDSC development via both cell-intrinsic and extrinsic mechanisms. Our findings have potential translational implications, as the efficacy of targeting Paks in cancer therapeutics may be undermined by tumor escape from immune control and/or acceleration of tumorigenesis through MDSC expansion.

Prostaglandin E2 Leads to the Acquisition of DNMT3A-Dependent Tolerogenic Functions in Human Myeloid-Derived Suppressor Cells

Myeloid-derived suppressor cells (MDSCs) and dendritic cells (DCs) arise from common progenitors. Tumor-derived factors redirect differentiation from immune-promoting DCs to tolerogenic MDSCs, an immunological hallmark of cancer. Indeed, in vitro differentiation of DCs from human primary monocytes results in the generation of MDSCs under tumor-associated conditions (PGE2 or tumor cell-conditioned media). Comparison of MDSC and DC DNA methylomes now reveals extensive demethylation with specific gains of DNA methylation and repression of immunogenic-associated genes occurring in MDSCs specifically, concomitant with increased DNA methyltransferase 3A (DNMT3A) levels. DNMT3A downregulation erases MDSC-specific hypermethylation, and it abolishes their immunosuppressive capacity. Primary MDSCs isolated from ovarian cancer patients display a similar hypermethylation signature in connection with PGE2-dependent DNMT3A overexpression. Our study links PGE2- and DNMT3A-dependent hypermethylation with immunosuppressive MDSC functions, providing a promising target for therapeutic intervention.

Neonatal myeloid derived suppressor cells show reduced apoptosis and immunosuppressive activity upon infection with Escherichia coli

Susceptibility to infection during the neonatal period and reduced control of inflammation in neonates are attributed to immunosuppression persisting from fetal life. Myeloid-derived suppressor cells (MDSCs) are immature myeloid progenitors with suppressive activity and increased numbers in cord blood. We hypothesized that MDSCs contribute to innate host defence in neonates, paralleled by anti-inflammatory signalling.Phagocytic activity, infection induced apoptosis, expression of B-cell lymphoma (Bcl)-2 family proteins, production of reactive oxygen species (ROS), cytokine production and T-cell suppression of neonatal granulocytic-MDSCs (G-MDSCs) after infection with Escherichia coli (E. coli) were compared to neonatal autologous mature polymorphonuclear leukocytes (PMNs). Phagocytic activity of G-MDSCs upon infection with E. coli was equal to that of mature PMNs, however, apoptosis of G-MDSCs was decreased. G-MDSCs showed enhanced Bcl-2-expression and lower ROS production compared to PMNs. Inhibition of Bcl-2 reduced apoptosis rates of G-MDSCs to that of mature PMNs. Induction of anti-inflammatory transforming growth factor beta (TGF-β) was enhanced, while pro-inflammatory IL-8 decreased in G-MDSCs compared to PMNs. Infected G-MDSCs strongly suppressed proliferation of T cells. We show a direct role of G-MDSCs for anti-bacterial host defence. Prolonged survival and anti-inflammatory capacity suggest that G-MDSCs are important for immune-regulation after bacterial infection.

A Schistosoma japonicum Infection Promotes the Expansion of Myeloid-Derived Suppressor Cells by Activating the JAK/STAT3 Pathway

Myeloid-derived suppressor cells (MDSCs), a heterogeneous group of immune cells from the myeloid lineage, play an important part in suppression of host immune responses during many pathologic conditions, including cancer and infectious diseases. Thus, understanding the functional diversity of these cells as well as the underlying mechanisms is crucial for the development of disease control strategies. The role of MDSCs during Schistosoma japonicum infection, however, is not clear, and there is a lack of systematic study so far. In this study, we provide strong evidence that the soluble egg Ag (SEA) and schistosome worm Ag (SWA) of S. japonicum enhance the accumulation of MDSCs. Ag-induced MDSCs have more potent suppressive effects on T cell responses than do control MDSCs in both in vivo S. japonicum infection and in vitro SEA- and SWA-treated mouse bone marrow cells experiments. Interestingly, the enhanced suppressive activity of MDSCs by Ag administration was coupled with a dramatic induction of the NADPH oxidase subunits gp91^phox and p47^phox and was dependent on the production of reactive oxygen species. Moreover, mechanistic studies revealed that the Ag effects are mediated by JAK/STAT3 signaling. Inhibition of STAT3 phosphorylation by the JAK inhibitor JSI-124 almost completely abolished the Ag effects on the MDSCs. In summary, this study sheds new light on the immune modulatory role of SEA and SWA and demonstrates that the expansion of MDSCs may be an important element of a cellular network regulating immune responses during S. japonicum infection.

SETD1B Activates iNOS Expression in Myeloid-Derived Suppressor Cells

Inducible nitric oxide synthase (iNOS) generates nitric oxide (NO) in myeloid cells that acts as a defense mechanism to suppress invading microorganisms or neoplastic cells. In tumor-bearing mice, elevated iNOS expression is a hallmark of myeloid-derived suppressor cells (MDSC). MDSCs use NO to nitrate both the T-cell receptor and STAT1, thus inhibiting T-cell activation and the antitumor immune response. The molecular mechanisms underlying iNOS expression and regulation in tumor-induced MDSCs are unknown. We report here that deficiency in IRF8 results in diminished iNOS expression in both mature CD11b⁺Gr1^- and immature CD11b⁺Gr1⁺ myeloid cells in vivo Strikingly, although IRF8 was silenced in tumor-induced MDSCs, iNOS expression was significantly elevated in tumor-induced MDSCs, suggesting that the expression of iNOS is regulated by an IRF8-independent mechanism under pathologic conditions. Furthermore, tumor-induced MDSCs exhibited diminished STAT1 and NF-κB Rel protein levels, the essential inducers of iNOS in myeloid cells. Instead, tumor-induced MDSCs showed increased SETD1B expression as compared with their cellular equivalents in tumor-free mice. Chromatin immunoprecipitation revealed that H3K4me3, the target of SETD1B, was enriched at the nos2 promoter in tumor-induced MDSCs, and inhibition or silencing of SETD1B diminished iNOS expression in tumor-induced MDSCs. Our results show how tumor cells use the SETD1B-H3K4me3 epigenetic axis to bypass a normal role for IRF8 expression in activating iNOS expression in MDSCs when they are generated under pathologic conditions. Cancer Res; 77(11); 2834-43. ©2017 AACR.

The Granulocyte Progenitor Stage Is a Key Target of IRF8-Mediated Regulation of Myeloid-Derived Suppressor Cell Production

Alterations in myelopoiesis are common across various tumor types, resulting in immature populations termed myeloid-derived suppressor cells (MDSCs). MDSC burden correlates with poorer clinical outcomes, credited to their ability to suppress antitumor immunity. MDSCs consist of two major subsets, monocytic and polymorphonuclear (PMN). Intriguingly, the latter subset predominates in many patients and tumor models, although the mechanisms favoring PMN-MDSC responses remain poorly understood. Ordinarily, lineage-restricted transcription factors regulate myelopoiesis that collectively dictate cell fate. One integral player is IFN regulatory factor (IRF)-8, which promotes monocyte/dendritic cell differentiation while limiting granulocyte development. We recently showed that IRF8 inversely controls MDSC burden in tumor models, particularly the PMN-MDSC subset. However, where IRF8 acts in the pathway of myeloid differentiation to influence PMN-MDSC production has remained unknown. In this study, we showed that: 1) tumor growth was associated with a selective expansion of newly defined IRF8^lo granulocyte progenitors (GPs); 2) tumor-derived GPs had an increased ability to form PMN-MDSCs; 3) tumor-derived GPs shared gene expression patterns with IRF8^-/- GPs, suggesting that IRF8 loss underlies GP expansion; and 4) enforced IRF8 overexpression in vivo selectively constrained tumor-induced GP expansion. These findings support the hypothesis that PMN-MDSCs result from selective expansion of IRF8^lo GPs, and that strategies targeting IRF8 expression may limit their load to improve immunotherapy efficacy.

Arsenic trioxide inhibits tumor-induced myeloid-derived suppressor cells and enhances T-cell activity

Myeloid-derived suppressor cells (MDSCs), one of the major orchestrators of the immunosuppressive network, are associated with immune suppression and considered a prime target for cancer immunotherapy. At present, various strategies have been explored to deplete and/or inactivate MDSCs in vivo. In this study, we investigated the effect of arsenic trioxide (ATO) on MDSCs derived from tumor-bearing mice. This study examined the in vitro and in vivo effects of ATO administration on MDSCs from C57/j mice bearing either the B16 or H22 tumor. The MDSCs were then characterized for phenotype, gene expression and function. Administration with ATO in vitro significantly induced MDSC differentiation, inhibited their proliferation and triggered apoptosis. Treatment with ATO in these murine tumor models significantly inhibited tumor growth and splenomegaly, decreased the percentages of MDSCs in the spleen, promoted their differentiation, reduced tumor necrosis factor-α and interleukin-10 levels and weakened the immune inhibition activity of MDSCs on T cells. In addition, we observed the underlying mechanism involved in the regulation of MDSCs by ATO, which included a panel of cytokines and signaling pathways. The findings showed the immunoregulatory effects of ATO by inducing apoptosis, promoting differentiation and inhibiting the function of MDSCs, suggesting that ATO has potential clinical benefit as it selectively attenuates MDSC-induced immunosuppression.

The expression profiles and regulation of PD-L1 in tumor-induced myeloid-derived suppressor cells

Programmed death-ligand 1 (PD-L1) is an inhibitory ligand that binds to PD-1 to suppress T cell activation. PD-L1 is constitutively expressed and inducible in tumor cells, but the expression profiles and regulatory mechanism of PD-L1 in myeloid-derived suppressor cells (MDSCs) are largely unknown. We report that PD-L1 is abundantly expressed in tumor-infiltrating leukocytes in human patients with both microsatellite instable and microsatellite stable colon cancer. About 60% CD11b⁺CD33⁺HLA-DR^- MDSCs from peripheral blood of human colon cancer patients are PD-L1⁺. PD-L1⁺ MDSCs are also significantly higher in tumor-bearing mice than in tumor-free mice. Interestingly, the highest PD-L1⁺ MDSCs were observed in the tumor microenvironment in which 56-71% tumor-infiltrating MDSCs are PD-L1⁺in vivo. In contrast, PD-L1⁺ MDSCs are significantly less in secondary lymphoid organs and peripheral blood as compared to the tumor tissues, whereas bone marrow MDSCs are essentially PD-L1^- in tumor-bearing mice. IFNγ is highly expressed in cells of the tumor tissues and IFNγ neutralization significantly decreased PD-L1⁺ MDSCs in the tumor microenvironment in vivo. However, IFNγ-activated pSTAT1 does not bind to the cd274 promoter in MDSCs. Instead, pSTAT1 activates expression of IRF1, IRF5, IRF7 and IRF8 in MDSCs, and only pSTAT1-activated IRF1 binds to a unique IRF-binding sequence element in vitro and chromatin in vivo in the cd274 promoter to activate PD-L1 transcription. Our data determine that PD-L1 is highly expressed in tumor-infiltrating MDSCs and in a lesser degree in lymphoid organs, and the pSTAT1-IRF1 axis regulates PD-L1 expression in MDSCs.

Thapsigargin sensitizes human esophageal cancer to TRAIL-induced apoptosis via AMPK activation

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a promising anticancer agent for esophageal squamous cell carcinoma (ESCC). Forced expression of CHOP, one of the key downstream transcription factors during endoplasmic reticulum (ER) stress, upregulates the death receptor 5 (DR5) levels and promotes oxidative stress and cell death. In this study, we show that ER stress mediated by thapsigargin promoted CHOP and DR5 synthesis thus sensitizing TRAIL treatment, which induced ESCC cells apoptosis. These effects were reversed by DR5 siRNA in vitro and CHOP siRNA both in vitro and in vivo. Besides, chemically inhibition of AMPK by Compound C and AMPK siRNA weakened the anti-cancer effect of thapsigargin and TRAIL co-treatment. Therefore, our findings suggest ER stress effectively sensitizes human ESCC to TRAIL-mediated apoptosis via the TRAIL-DR5-AMPK signaling pathway, and that activation of ER stress may be beneficial for improving the efficacy of TRAIL-based anti-cancer therapy.

Total cellular protein presence of the transcription factor IRF8 does not necessarily correlate with its nuclear presence

The transcription factor interferon regulatory factor-8 (IRF8) plays an essential role in myeloid differentiation and lineage commitment, based largely on molecular and genetic studies. The detection of IRF8 in specific cell populations by flow cytometry (FCM) has the potential to provide new insights into normal and pathologic myelopoiesis, but critical validation of this protein-based approach, particularly in human samples, is lacking. In this study, the assessment of total cellular IRF8 presence was compared to its specific nuclear presence as assessed by imaging flow cytometry (IFC) analysis. Peptide neutralization of the IRF8-specific antibody that has been predominantly used to date in the literature served as a negative control for the immunofluorescent labeling. Expression of total IRF8 was analyzed by total cellular fluorescence analogous to the mean fluorescence intensity readout of conventional FCM. Additionally, specific nuclear fluorescence and the similarity score between the nuclear image (DAPI) and the corresponding IRF8 image for each cell were analyzed as parameters for nuclear localization of IRF8. IFC showed that peptide blocking eliminated binding of the IRF8 antibody in the nucleus. It also reduced cytoplasmic binding of the antibody but not to the extent observed in the nucleus. In agreement with the similarity score data, the total cellular IRF8 as well as nuclear IRF8 intensities decreased with peptide blocking. In healthy donor peripheral blood subpopulations and a positive control cell line (THP-1), the assessment of IRF8 by total cellular presence correlated well with its specific nuclear presence and correlated with the known distribution of IRF8 in these cells. In clinical samples of myeloid-derived suppressors cells derived from patients with renal carcinoma, however, total cellular IRF8 did not necessarily correlate with its nuclear presence. Discordance was primarily associated with peptide blocking having a proportionally greater effect on the IRF8 nuclear localization versus total fluorescence assessment. The data thus indicate that IRF8 can have cytoplasmic presence and that during disease its nuclear-cytoplasmic distribution may be altered, which may provide a basis for potential myeloid defects during certain pathologies.

An Orthotopic Mouse Model of Spontaneous Breast Cancer Metastasis

Metastasis is the primary cause of mortality of breast cancer patients. The mechanism underlying cancer cell metastasis, including breast cancer metastasis, is largely unknown and is a focus in cancer research. Various breast cancer spontaneous metastasis mouse models have been established. Here, we report a simplified procedure to establish orthotopic transplanted breast cancer primary tumor and resultant spontaneous metastasis that mimic human breast cancer metastasis. Combined with the bioluminescence live tumor imaging, this mouse model allows tumor growth and progression kinetics to be monitored and quantified. In this model, a low dose (1 x 10(4) cells) of 4T1-Luc breast cancer cells was injected into BALB/c mouse mammary fat pad using a tuberculin syringe. Mice were injected with luciferin and imaged at various time points using a bioluminescent imaging system. When the primary tumors grew to the size limit as in the IACUC-approved protocol (approximately 30 days), mice were anesthetized under constant flow of 2% isoflurane and oxygen. The tumor area was sterilized with 70% ethanol. The mouse skin around the tumor was excised to expose the tumor which was removed with a pair of sterile scissors. Removal of the primary tumor extends the survival of the 4T-1 tumor-bearing mice for one month. The mice were then repeatedly imaged for metastatic tumor spreading to distant organs. Therapeutic agents can be administered to suppress tumor metastasis at this point. This model is simple and yet sensitive in quantifying breast cancer cell growth in the primary site and progression kinetics to distant organs, and thus is an excellent model for studying breast cancer growth and progression, and for testing anti-metastasis therapeutic and immunotherapeutic agents in vivo.

Ceramide mediates FasL-induced caspase 8 activation in colon carcinoma cells to enhance FasL-induced cytotoxicity by tumor-specific cytotoxic T lymphocytes

FasL-mediated cytotoxicity is one of the mechanisms that CTLs use to kill tumor cells. However, human colon carcinoma often deregulates the Fas signaling pathway to evade host cancer immune surveillance. We aimed at testing the hypothesis that novel ceramide analogs effectively modulate Fas function to sensitize colon carcinoma cells to FasL-induced apoptosis. We used rational design and synthesized twenty ceramide analogs as Fas function modulators. Five ceramide analogs, IG4, IG7, IG14, IG17, and IG19, exhibit low toxicity and potent activity in sensitization of human colon carcinoma cells to FasL-induced apoptosis. Functional deficiency of Fas limits both FasL and ceramide analogs in the induction of apoptosis. Ceramide enhances FasL-induced activation of the MAPK, NF-κB, and caspase 8 despite induction of potent tumor cell death. Finally, a sublethal dose of several ceramide analogs significantly increased CTL-mediated and FasL-induced apoptosis of colon carcinoma cells. We have therefore developed five novel ceramide analogs that act at a sublethal dose to enhance the efficacy of tumor-specific CTLs, and these ceramide analogs hold great promise for further development as adjunct agents in CTL-based colon cancer immunotherapy.

MMP3-mediated tumor progression is controlled transcriptionally by a novel IRF8-MMP3 interaction

Interferon regulatory factor-8 (IRF8), originally identified as a leukemic tumor suppressor, can also exert anti-neoplastic activities in solid tumors. We previously showed that IRF8-loss enhanced tumor growth, which was accompanied by reduced tumor-cell susceptibility to apoptosis. However, the impact of IRF8 expression on tumor growth could not be explained solely by its effects on regulating apoptotic response. Exploratory gene expression profiling further revealed an inverse relationship between IRF8 and MMP3 expression, implying additional intrinsic mechanisms by which IRF8 modulated neoplastic behavior. Although MMP3 expression was originally linked to tumor initiation, the role of MMP3 beyond this stage has remained unclear. Therefore, we hypothesized that MMP3 governed later stages of disease, including progression to metastasis, and did so through a novel IRF8-MMP3 axis. Altogether, we showed an inverse mechanistic relationship between IRF8 and MMP3 expression in tumor progression. Importantly, the growth advantage due to IRF8-loss was significantly compromised after silencing MMP3 expression. Moreover, MMP3-loss reduced spontaneous lung metastasis in an orthotopic mouse model of mammary carcinoma. MMP3 acted, in part, in a cell-intrinsic manner and served as a direct transcriptional target of IRF8. Thus, we identified a novel role of an IRF8-MMP3 axis in tumor progression, which unveils new therapeutic opportunities.

Fas Ligand Deficiency Impairs Tumor Immunity by Promoting an Accumulation of Monocytic Myeloid-Derived Suppressor Cells

The Fas receptor ligand FasL regulates immune cell levels by inducing apoptosis of Fas receptor-positive cells. Here, we studied the impact of host FasL on tumor development in mice. Genetically targeting FasL in naïve mice increased myeloid cell populations, but, in marked contrast, it reduced the levels of myeloid-derived suppressor cells (MDSC) in mice bearing Lewis lung carcinoma tumors. Analysis of the MDSC subset distribution revealed that FasL deficiency skewed cell populations toward the M-MDSC subset, which displays a highly immunosuppressive activity. Furthermore, tumor-bearing mice that were FasL-deficient displayed an enhanced proportion of tumor-associated macrophages and regulatory T cells. Overall, the immunosuppressive environment produced by FasL targeting correlated with reduced survival of tumor-bearing mice. These results disclose a new role for FasL in modulating immunosuppressive cells.

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.

Myeloid-derived suppressor cells in the tumor microenvironment: expect the unexpected

Our understanding of the role of myeloid-derived suppressor cells (MDSCs) in cancer is becoming increasingly complex. In addition to their eponymous role in suppressing immune responses, they directly support tumor growth, differentiation, and metastasis in a number of ways that are only now beginning to be appreciated. It is because of this increasingly complex role that these cells may become an important factor in the treatment of human cancer. In this Review, we discuss the most pertinent and controversial issues of MDSC biology and their role in promoting cancer progression and highlight how these cells may be used in the clinic, both as prognostic factors and as therapeutic targets.