IFN regulatory factor 8 represses GM-CSF expression in T cells to affect myeloid cell lineage differentiation

The multiple roles of interferon regulatory factor family in health and disease

Interferon Regulatory Factors (IRFs), a family of transcription factors, profoundly influence the immune system, impacting both physiological and pathological processes. This review explores the diverse functions of nine mammalian IRF members, each featuring conserved domains essential for interactions with other transcription factors and cofactors. These interactions allow IRFs to modulate a broad spectrum of physiological processes, encompassing host defense, immune response, and cell development. Conversely, their pivotal role in immune regulation implicates them in the pathophysiology of various diseases, such as infectious diseases, autoimmune disorders, metabolic diseases, and cancers. In this context, IRFs display a dichotomous nature, functioning as both tumor suppressors and promoters, contingent upon the specific disease milieu. Post-translational modifications of IRFs, including phosphorylation and ubiquitination, play a crucial role in modulating their function, stability, and activation. As prospective biomarkers and therapeutic targets, IRFs present promising opportunities for disease intervention. Further research is needed to elucidate the precise mechanisms governing IRF regulation, potentially pioneering innovative therapeutic strategies, particularly in cancer treatment, where the equilibrium of IRF activities is of paramount importance.

Plasmodium yoelii Infection Enhances the Expansion of Myeloid-Derived Suppressor Cells via JAK/STAT3 Pathway

Myeloid-derived suppressor cells (MDSCs), the negative immune regulators, have been demonstrated to be involved in immune responses to a variety of pathological conditions, such as tumors, chronic inflammation, and infectious diseases. However, the roles and mechanisms underlying the expansion of MDSCs in malaria remain unclear. In this study, the phenotypic and functional characteristics of splenic MDSCs during Plasmodium yoelii NSM infection are described. Furthermore, we provide compelling evidence that the sera from P. yoelii-infected C57BL/6 mice containing excess IL-6 and granulocyte-macrophage colony-stimulating factor promote the accumulation of MDSCs by inducing Bcl2 expression. Serum-induced MDSCs exert more potent suppressive effects on T cell responses than control MDSCs within both in vivo P. yoelii infection and in vitro serum-treated bone marrow cells experiments. Serum treatment increases the MDSC inhibitory effect, which is dependent on Arg1 expression. Moreover, mechanistic studies reveal that the serum effects are mediated by JAK/STAT3 signaling. By inhibiting STAT3 phosphorylation with the JAK inhibitor JSI-124, effects of serum on MDSCs are almost eliminated. In vivo depletion of MDSCs with anti-Gr-1 or 5-fluorouracil significantly reduces the parasitemia and promotes Th1 immune response in P. yoelii-infected C57BL/6 mice by upregulating IFN-γ expression. In summary, this study indicates that P. yoelii infection facilitates the accumulation and function of MDSCs by upregulating the expression of Bcl2 and Arg1 via JAK/STAT3 signaling pathway in vivo and in vitro. Manipulating the JAK/STAT3 signaling pathway or depleting MDSCs could be promising therapeutic interventions to treat malaria.

Hypermethylation Loci of ZNF671, IRF8, and OTX1 as Potential Urine-Based Predictive Biomarkers for Bladder Cancer

Bladder cancer (BCa) is a significant health issue and poses a healthcare burden on patients, highlighting the importance of an effective detection method. Here, we developed a urine DNA methylation diagnostic panel for distinguishing between BCa and non-BCa. In the discovery stage, an analysis of the TCGA database was conducted to identify BCa-specific DNA hypermethylation markers. In the validation phase, DNA methylation levels of urine samples were measured with real-time quantitative methylation-specific PCR (qMSP). Comparative analysis of the methylation levels between BCa and non-BCa, along with the receiver operating characteristic (ROC) analyses with machine learning algorithms (logistic regression and decision tree methods) were conducted to develop practical diagnostic panels. The performance evaluation of the panel shows that the individual biomarkers of ZNF671, OTX1, and IRF8 achieved AUCs of 0.86, 0.82, and 0.81, respectively, while the combined yielded an AUC of 0.91. The diagnostic panel using the decision tree algorithm attained an accuracy, sensitivity, and specificity of 82.6%, 75.0%, and 90.9%, respectively. Our results show that the urine-based DNA methylation diagnostic panel provides a sensitive and specific method for detecting and stratifying BCa, showing promise as a standard test that could enhance the diagnosis and prognosis of BCa in clinical settings.

Effective intravenous delivery of adenovirus armed with TNFα and IL-2 improves anti-PD-1 checkpoint blockade in non-small cell lung cancer

Lung cancer remains among the most difficult-to-treat malignancies and is the leading cause of cancer-related deaths worldwide. The introduction of targeted therapies and checkpoint inhibitors has improved treatment outcomes; however, most patients with advanced-stage non-small cell lung cancer (NSCLC) eventually fail these therapies. Therefore, there is a major unmet clinical need for checkpoint refractory/resistant NSCLC. Here, we tested the combination of aPD-1 and adenovirus armed with TNFα and IL-2 (Ad5-CMV-mTNFα/mIL-2) in an immunocompetent murine NSCLC model. Moreover, although local delivery has been standard for virotherapy, treatment was administered intravenously to facilitate clinical translation and putative routine use. We showed that treatment of tumor-bearing animals with aPD-1 in combination with intravenously injected armed adenovirus significantly decreased cancer growth, even in the presence of neutralizing antibodies. We observed an increased frequency of cytotoxic tumor-infiltrating lymphocytes, including tumor-specific cells. Combination treatment led to a decreased percentage of immunosuppressive tumor-associated macrophages and an improvement in dendritic cell maturation. Moreover, we observed expansion of the tumor-specific memory T cell compartment in secondary lymphoid organs in the group that received aPD-1 with the virus. However, although the non-replicative Ad5-CMV-mTNFα/mIL-2 virus allows high transgene expression in the murine model, it does not fully reflect the clinical outcome in humans. Thus, we complemented our findings using NSCLC ex vivo models fully permissive for the TNFα and IL-2- armed oncolytic adenovirus TILT-123. Overall, our data demonstrate the ability of systemically administered adenovirus armed with TNFα and IL-2 to potentiate the anti-tumor efficacy of aPD-1 and warrant further investigation in clinical trials.

MDSCs in sepsis-induced immunosuppression and its potential therapeutic targets

Sepsis is a life-threatening organ dysfunction caused by a dysregulated host response to infection. In sepsis, a complicated immune response is initiated, which varies over time with sustained excessive inflammation and immunosuppression. Identifying a promising way to orchestrate sepsis-induced immunosuppression is a challenge. Myeloid-derived suppressor cells (MDSCs) comprise pathologically activated neutrophils and monocytes with potent immunosuppressive activity. They play an important part in inhibiting innate and adaptive immune responses, and have emerged as part of the immune response in sepsis. MDSCs numbers are persistently high in sepsis patients, and associated with nosocomial infections and other adverse clinical outcomes. However, their characteristics and functional mechanisms during sepsis have not been addressed fully. Our review sheds light on the features and suppressive mechanism of MDSCs. We also review the potential applications of MDSCs as biomarkers and targets for clinical treatment of sepsis.

Stimulation of B-Lymphopoiesis by Administration of a Trimecaine-Based Ionic Compound in Cyclophosphamide-Induced Hematopoietic-Depressive Model

According to the WHO, the secondary form of hematopoietic-depressive status increases the risk of death in people with oncological, infectious, and hormonal diseases. The choice of drugs that stimulate the hematopoietic activity of B-lymphopoiesis is limited. The current leucopoiesis drugs have a number of side effects: thymic preparations stimulate the production of PGE2, which causes chronic inflammation and various autoimmune diseases through the differentiation of T helper 1 (Th1) cells, the proliferation of Th17 cells, and the production of IL-22 from Th22 cells through EP2 and EP4 receptors; cytokine preparations can cause uncontrolled immune reactions and impaired contractility of smooth and cardiac muscles; drugs based on nucleic acids can stimulate the division of all cells, including bacterial and cancerous ones. The use of oligonucleotides such as ribozymes and antisense oligodeoxynucleotides (AS-ODNs) shows promise as therapeutic moieties, but faces a number of challenges such as nuclease sensitivity, off-target effects, and efficient delivery. The search for substances that stimulate B-lymphopoiesis among ionic compounds was motivated by the discovery of the unique properties of lidocaine docusate, one of the first ionic liquid forms of the known drugs. The lidocaine docusate (protonated form of lidocaine (2-(diethylamino)-N-(2,6-dimethylphenyl) acetamide + docusate-anion (dioctylsulfosuccinate))) suppresses the division of pheochromocytoma cells and activates immunity in rats. The trimecaine-based ionic compound (TIC) demonstrates high B-lymphopoiesis-stimulating activity. The TIC compound stimulates an increase in the volume of transitional B cells, which play an important role for further differentiation and formation of a sufficient number of mature B1 cells and mature B2 cells, where mature B2 cells make up the bulk of the functional population of B lymphocytes. The TIC compound most strongly stimulated the restoration of the number of marginal zone B cells, follicular B cells, and activated germinal center B cells after the cytotoxic emptying of the follicular centers of the spleen induced cyclophosphamide. It significantly exceeds the activity of the comparison drug methyluracil. The TIC compound does not affect the level of pro-B, pre-B-I, or pre-B-II bone marrow cells, which prevents the risk of the formation of immature functionally defective cells.

Evaluation of porcine GM-CSF during PRRSV infection in vitro and in vivo indicating a protective role of GM-CSF related with M1 biased activation in alveolar macrophage during PRRSV infection

Granulocyte-macrophage colony stimulating factor (GM-CSF), participates in diverse biological processes associated with innate and adaptive immunity, has unknown effects during PRRSV infection. Here, a double-antibody sandwich ELISA for pGM-CSF was developed in-house for evaluation of pGM-CSF level during PRRSV infection both in vitro and in vivo. In in vitro assay, it was notable that PRRSV-infected porcine alveolar macrophages (PAMs) yielded inconsistent pGM-CSF protein- and mRNA-level, suggesting a post-transcriptional inhibition of pGM-CSF mRNA was employed by PRRSV. Meanwhile, concurrent analysis of pGM-CSF levels in serum samples from PRRSV-infected piglets suggested that effect of PRRSV infection demonstrated minimum effect on pGM-CSF levels regardless of PRRSV virulence phenotypes. Moreover, in vitro treatment of PAMs with pGM-CSF prior PRRSV inoculation did not inhibit PRRSV replication in PAMs although genes downstream of pGM-CSF in PAMs could be upregulated by pGM-CSF treatment. Meanwhile, knockdown of pGM-CSF using siRNA did not enhance PRRSV replication as well. Intriguingly, therapeutic antibody treatment of HP-PRRSV-infected piglets led to significantly increased serum pGM-CSF levels, thus aligning with low pneumonia incidence and low intracellular PRRSV-RNA levels in PAMs of therapeutic antibody treated piglets. Furthermore, transcriptome analysis of PAMs from infected piglets revealed increased serum pGM-CSF levels correlated with activation of downstream signal of pGM-CSF in PAMs as evidenced by a M1-like phenotypes of gene expression pattern, implying a potential host-protective role played by pGM-CSF for PRRSV infection in vivo. In conclusion, our results demonstrated developments of a highly sensitive and specific ELISA for pGM-CSF and revealed a potential protective role conferred by pGM-CSF during PRRSV infection.

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.

The role of myeloid-derived suppressor cells in lung cancer and targeted immunotherapies

INTRODUCTION

Lung cancer is the deadliest cancer in both sexes combined globally due to significant delays in diagnosis and poor survival. Despite advances in the treatment of lung cancer, the overall outcomes remain poor and traditional chemotherapy fails to provide long-term benefits for many patients. Therefore, new treatment strategies are needed to increase overall survival. Myeloid-derived suppressor cells (MDSCs) are immunosuppressive cells taking part in lung cancer, as has been described in other types of tumors. MDSCs immunosuppressive activity is mediated by arginases (ARG-1 and ARG-2), nitric oxide (NO), reactive oxygen species (ROS), peroxynitrite, PD-1/PD-L1 axis, and different cytokines. MDSCs can be a target for lung cancer immunotherapy by inducing their differentiation into mature myeloid cells, elimination, attenuation of their function, and inhibition of their accumulation.

AREAS COVERED

In this review, the immunosuppressive function of MDSCs, their role in lung cancer, and strategies to target them, which could result in increased efficacy of immunotherapy in patients with lung cancer, are discussed.

EXPERT OPINION

Identification of important mechanisms and upstream pathways involved in MDSCs functions paves the way for further preclinical and clinical lung cancer research, which could lead to the development of novel therapeutic approaches.

IRF4 and IRF8 expression are associated with clinical phenotype and clinico-hematological response to hydroxyurea in essential thrombocythemia

The morbidity and mortality of myeloproliferative neoplasms (MPNs) are primarily caused by arterial and venous complications, progression to myelofibrosis, and transformation to acute leukemia. However, identifying molecular-based biomarkers for risk stratification of patients with MPNs remains a challenge. We have previously shown that interferon regulatory factor-8 (IRF8) and IRF4 serve as tumor suppressors in myeloid cells. In this study, we evaluated the expression of IRF4 and IRF8 and the JAK2V617F mutant allele burden in patients with MPNs. Patients with decreased IRF4 expression were correlated with a more developed MPN phenotype in myelofibrosis (MF) and secondary AML (sAML) transformed from MPNs versus essential thrombocythemia (ET). Negative correlations between the JAK2V617F allele burden and the expression of IRF8 (P < 0.05) and IRF4 (P < 0.001) and between white blood cell (WBC) count and IRF4 expression (P < 0.05) were found in ET patients. IRF8 expression was negatively correlated with the JAK2V617F allele burden (P < 0.05) in polycythemia vera patients. Complete response (CR), partial response (PR), and no response (NR) were observed in 67.5%,10%, and 22.5% of ET patients treated with hydroxyurea (HU), respectively, in 12 months. At 3 months, patients in the CR group showed high IRF4 and IRF8 expression compared with patients in the PR and NR groups. In the 12-month therapy period, low IRF4 and IRF8 expression were independently associated with the unfavorable response to HU and high WBC count. Our data indicate that the expression of IRF4 and IRF8 was associated with the MPN phenotype, which may serve as biomarkers for the response to HU in ET.

IRF-8/miR-451a regulates M-MDSC differentiation via the AMPK/mTOR signal pathway during lupus development

Systemic lupus erythematosus (SLE) is a chronic systemic autoimmune disease. Myeloid-derived suppressor cells (MDSCs) have been found to be involved in the regulation of SLE development. However, little is known about the association between MDSC subsets and the factors that draw MDSCs into abnormal expansion. This study found that the percentage of M-MDSCs increased in mice with pristane-induced lupus. Toll-like receptor (TLR)7 signal activation and high interferon-α (IFN-α) level promoted M-MDSC differentiation in vitro. Moreover, both AMP-activated protein kinase (AMPK) agonist metformin and two mammalian targets of rapamycin (mTOR) inhibitors (INK128 and rapamycin) inhibited the percentage of M-MDSCs in lupus mice as well as in the TLR7- and IFN-α-induced bone marrow (BM) differentiation into MDSCs in vitro. In terms of mechanism, whole-genome transcriptome profiling was performed by RNA sequencing, revealing that the expression of the transcription factor IRF-8 was higher in M-MDSCs isolated from pristane-induced lupus mice, compared with control mice. IRF-8 was identified to be crucial for TLR7- and IFN-α-induced BM differentiation into MDSCs in vitro. Furthermore, interferon (IFN) regulatory factor8 (IRF-8) was targeted by miR-451a in M-MDSC differentiation. Of note, metformin-modified M-MDSCs could relieve lupus symptoms in pristane-induced lupus mice. The findings revealed a novel mechanism linking IRF-8/miR-451a to M-MDSC differentiation via the AMPK/mTOR signal pathway during lupus development. This study might provide an important reference for SLE therapy by targeting M-MDSCs.

COVID-19 is a systemic vascular hemopathy: insight for mechanistic and clinical aspects

Coronavirus disease 2019 (COVID-19) caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is presenting as a systemic disease associated with vascular inflammation and endothelial injury. Severe forms of SARS-CoV-2 infection induce acute respiratory distress syndrome (ARDS) and there is still an ongoing debate on whether COVID-19 ARDS and its perfusion defect differs from ARDS induced by other causes. Beside pro-inflammatory cytokines (such as interleukin-1 β [IL-1β] or IL-6), several main pathological phenomena have been seen because of endothelial cell (EC) dysfunction: hypercoagulation reflected by fibrin degradation products called D-dimers, micro- and macrothrombosis and pathological angiogenesis. Direct endothelial infection by SARS-CoV-2 is not likely to occur and ACE-2 expression by EC is a matter of debate. Indeed, endothelial damage reported in severely ill patients with COVID-19 could be more likely secondary to infection of neighboring cells and/or a consequence of inflammation. Endotheliopathy could give rise to hypercoagulation by alteration in the levels of different factors such as von Willebrand factor. Other than thrombotic events, pathological angiogenesis is among the recent findings. Overexpression of different proangiogenic factors such as vascular endothelial growth factor (VEGF), basic fibroblast growth factor (FGF-2) or placental growth factors (PlGF) have been found in plasma or lung biopsies of COVID-19 patients. Finally, SARS-CoV-2 infection induces an emergency myelopoiesis associated to deregulated immunity and mobilization of endothelial progenitor cells, leading to features of acquired hematological malignancies or cardiovascular disease, which are discussed in this review. Altogether, this review will try to elucidate the pathophysiology of thrombotic complications, pathological angiogenesis and EC dysfunction, allowing better insight in new targets and antithrombotic protocols to better address vascular system dysfunction. Since treating SARS-CoV-2 infection and its potential long-term effects involves targeting the vascular compartment and/or mobilization of immature immune cells, we propose to define COVID-19 and its complications as a systemic vascular acquired hemopathy.

High IRF8 expression correlates with CD8 T cell infiltration and is a predictive biomarker of therapy response in ER-negative breast cancer

Background

Characterization of breast cancer (BC) through the determination of conventional markers such as ER, PR, HER2, and Ki67 has been useful as a predictive and therapeutic tool. Also, assessment of tumor-infiltrating lymphocytes has been proposed as an important prognostic aspect to be considered in certain BC subtypes. However, there is still a need to identify additional biomarkers that could add precision in distinguishing therapeutic response of individual patients. To this end, we focused in the expression of interferon regulatory factor 8 (IRF8) in BC cells. IRF8 is a transcription factor which plays a well-determined role in myeloid cells and that seems to have multiple antitumoral roles: it has tumor suppressor functions; it acts downstream IFN/STAT1, required for the success of some therapeutic regimes, and its expression in neoplastic cells seems to depend on a cross talk between the immune contexture and the tumor cells.

The goal of the present study was to examine the relationship between IRF8 with the therapeutic response and the immune contexture in BC, since its clinical significance in this type of cancer has not been thoroughly addressed.

Methods

We identified the relationship between IRF8 expression and the clinical outcome of BC patients and validated IRF8 as predictive biomarker by using public databases and then performed in silico analysis. To correlate the expression of IRF8 with the immune infiltrate in BC samples, we performed quantitative multiplex immunohistochemistry.

Results

IRF8 expression can precisely predict the complete pathological response to monoclonal antibody therapy or to select combinations of chemotherapy such as FAC (fluorouracil, adriamycin, and cytoxan) in ER-negative BC subtypes. Analysis of immune cell infiltration indicates there is a strong correlation between activated and effector CD8⁺ T cell infiltration and tumoral IRF8 expression.

Conclusions

We propose IRF8 expression as a potent biomarker not only for prognosis, but also for predicting therapy response in ER-negative BC phenotypes. Its expression in neoplastic cells also correlates with CD8⁺ T cell activation and infiltration. Therefore, our results justify new efforts towards understanding mechanisms regulating IRF8 expression and how they can be therapeutically manipulated.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13058-021-01418-7.

The Monocytes That Repopulate in Mice After Cyclophosphamide Treatment Acquire a Neutrophil Precursor Gene Signature and Immunosuppressive Activity

Cyclophosphamide (CTX) is a major component of the chemotherapy conditioning regimens used in the clinic to prepare cancer patients for hematopoietic stem cell transplantation or adoptive T cell therapy. Previous studies have shown that CTX given at nonmyeloablative doses in mice and patients leads to expansion of myeloid cells within which the monocytic subset exhibits immunosuppressive activity. However, the ontogeny and gene expression signature of these CTX-induced monocytes are not well-defined. Here, we report that the expansion of myeloid cells is a default process intrinsic to hematopoietic recovery after chemotherapy. During this process, the monocytes repopulated in mice acquire immunosuppressive activity, which can persist long after cessation of chemotherapy. Moreover, monocytes acquire a gene signature characteristic of neutrophil precursors, marked by increased proliferative capability and elevated expressions of multiple primary and secondary granules. We provide evidence that CTX-induced myeloid cell expansion is regulated by DNA methyltransferase 1 (Dnmt1) and dependent on chemotherapy-induced microbial translocation. These findings help advance our understanding of the differentiation, heterogeneity, and function of myeloid cells repopulating after chemotherapy.

Developmental pathways of myeloid-derived suppressor cells in neoplasia

Immunotherapy has become a major weapon against the war on cancer. This has culminated from decades of seminal work that led to the discovery of innovative approaches to drive adaptive immunity. Notably, was the discovery of immune checkpoint inhibitory receptors on T cells, and the subsequent development of monoclonal antibodies that target those receptors, known as immune checkpoint inhibitors (ICIs). Blocking those receptors using ICIs leads to sustained effector function, which has translated to enhanced antitumor responses across multiple human cancer types. However, these treatments are effective in subsets of patients, implicating significant barriers limiting therapeutic potential. While numerous mechanisms may hinder immunotherapy potency, one prominent mechanism is the production of myeloid-derived suppressor cells (MDSCs). MDSCs comprise monocytic and granulocytic cell types and mediate pro-tumorigenic and immune suppressive activities. Here, we summarize several pathways by which MDSCs arise in cancer, providing a conceptual framework for identifying unique combination therapeutic interventions.

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.

Repeated stress induces a pro-inflammatory state, increases amygdala neuronal and microglial activation, and causes anxiety in adult male rats

A link exists between immune function and psychiatric conditions, particularly depressive and anxiety disorders. Psychological stress is a powerful trigger for these disorders and stress influences immune state. However, the nature of peripheral immune changes after stress conflicts across studies, perhaps due to the focus on few measures of pro-inflammatory or anti-inflammatory processes. The basolateral amygdala (BLA) is critical for emotion, and plays an important role in the effects of stress on anxiety. As such, it may be a primary central nervous system (CNS) mediator for the effects of peripheral immune changes on anxiety after stress. Therefore, this study aimed to delineate the influence of stress on peripheral pro-inflammatory and anti-inflammatory aspects, BLA immune activation, and its impact on BLA neuronal activity. To produce a more encompassing view of peripheral immune changes, this study used a less restrictive approach to categorize and group peripheral immune changes. We found that repeated social defeat stress in adult male Sprague-Dawley rats increased the frequencies of mature T-cells positive for intracellular type 2-like cytokine and serum pro-inflammatory cytokines. Principal component analysis and hierarchical clustering was used to guide grouping of T-cells and cytokines, producing unique profiles. Stress shifted the balance towards a specific set that included mostly type 2-like T-cells and pro-inflammatory cytokines. Within the CNS component, repeated stress caused an increase of activated microglia in the BLA, increased anxiety-like behaviors across several assays, and increased BLA neuronal firing in vivo that was prevented by blockade of microglia activation. Because repeated stress can trigger anxiety states by actions in the BLA, and altered immune function can trigger anxiety, these results suggest that repeated stress may trigger anxiety-like behaviors by inducing a pro-inflammatory state in the periphery and the BLA. These results begin to uncover how stress may recruit the immune system to alter the function of brain regions critical to emotion.

Blockade of myeloid-derived suppressor cell function by valproic acid enhanced anti-PD-L1 tumor immunotherapy

Regardless of the remarkable clinical success of immune checkpoint blockade (ICB) against PD-1/PD-L1 pathway, this approach has encountered drawbacks in most patients due to the activation of tumor immunosuppressive factors such as myeloid-derived suppressor cells (MDSCs). Histone deacetylase (HDAC) inhibitors combat ICB resistance by attenuating the immunosuppressive function of MDSCs and increasing PD-L1 expression on tumor cells. However, whether an HDAC inhibitor - valproic acid (VPA) suppression of MDSCs function could enhance PD-L1 blockade-mediated tumor immunotherapy remains unknown. Here we report that VPA and anti-PD-L1 antibody combined treatment promoted the polarization of bone marrow-derived precursor cells into M-MDSCs. Interestingly, the combination treatment of VPA and anti-PD-L1 antibody activated IRF1/IRF8 transcriptional axis in MDSCs leading to blockade of their immunosuppressive function by downregulating the expression of IL-10, IL-6, and ARG1 while re-activating CD8⁺ T-cells for the production of TNFα to further enhance anti-tumor immunity. These observations provide further rationale for the combination therapy of VPA with anti-PD-L1 antibody in preclinical settings.

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

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

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

GM-CSF Promotes the Expansion and Differentiation of Cord Blood Myeloid-Derived Suppressor Cells, Which Attenuate Xenogeneic Graft-vs.-Host Disease

Myeloid-derived suppressor cells (MDSCs) are increased in tumor patients. Studies have shown generation of MDSCs from human peripheral blood mononuclear cells (PBMCs) by various cytokine combinations. However, large scale expansion of human MDSCs has not been demonstrated or applied in clinic settings. We investigated which cytokine combinations among GM-CSF/SCF, G-CSF/SCF, or M-CSF/SCF efficiently expand and differentiate human MDSCs following culture CD34⁺ cells of umbilical cord blood (CB). GM-CSF/SCF showed the greatest expansion of MDSCs. Up to 10⁸ MDSCs (HLA-DR^lowCD11b⁺CD33⁺) could be produced from 1 unit of CB following 6 weeks of continuous culture. MDSCs produced from culture of CD34⁺ cells with GM-CSF/SCF for 6 weeks had the greatest suppressive function of T cell proliferation and had the highest expression of immunosuppressive molecules including iNOS, arginase 1 and IDO compared to those differentiated with G-CSF/SCF or M-CSF/SCF. MDSCs secreted IL-10, TGB-β, and VEGF. The infusion of expanded MDSCs significantly prolonged the survival and decreased the GVHD score in a NSG xenogeneic model of GVHD. Injected MDSCs increased IL-10 and TGF-β but decreased the level of TNF-α and IL-6 in the serum of treated mice. Notably, FoxP3 expressing regulatory T (Treg) cells were increased while IFN-γ (Th1) and IL-17 (Th17) producing T cells were decreased in the spleen of MDSC treated mice compared to untreated GVHD mice. Our results demonstrate that human MDSCs are generated from CB CD34⁺ cells using GM-CSF/SCF. These MDSCs exhibited potent immunosuppressive function, suggesting that they are useable as a treatment for inflammatory diseases such as GVHD.

Tumoral EHF predicts the efficacy of anti-PD1 therapy in pancreatic ductal adenocarcinoma

EHF transcriptionally inhibits the expressions of TGFβ1 and GM-CSF to decrease T reg cell and MDSC accumulation, making it a promising biomarker to evaluate the immune microenvironment in PDAC. EHF overexpression may improve the efficacy of checkpoint immunotherapy in PDAC.

Pancreatic ductal adenocarcinoma (PDAC) is a highly immune-suppressive tumor with a low response rate to single checkpoint blockade therapy. ETS homologous factor (EHF) is a tumor suppressor in PDAC. Here, we report a novel function of EHF in pancreatic cancer immune microenvironment editing and efficacy prediction for anti-PD1 therapy. Our findings support that the deficiency of tumoral EHF induced the accumulation of regulatory T (T reg) cells and myeloid-derived suppressor cells (MDSCs) and a decrease in the number of tumor-infiltrating CD8⁺ T cells. Mechanistically, EHF deficiency induced the conversion and expansion of T reg cells and MDSCs through inhibiting tumor TGFβ1 and GM-CSF secretion. EHF suppressed the transcription of TGFB1 and CSF2 by directly binding to their promoters. Mice bearing EHF overexpression tumors exhibited significantly better response to anti-PD1 therapy than those with control tumors. Our findings delineate the immunosuppressive mechanism of EHF deficiency in PDAC and highlight that EHF overexpression may improve PDAC checkpoint immunotherapy.

Polarization of granulocytic myeloid-derived suppressor cells by hepatitis C core protein is mediated via IL-10/STAT3 signalling

Myeloid-derived suppressor cells (MDSCs) have been described as suppressors of T-cell function in many malignancies. Impaired T-cell responses have been observed in patients with chronic hepatitis C virus infection (CHC), which is reportedly associated with the establishment of persistent HCV infection. Therefore, we hypothesized that MDSCs also play a role in chronic HCV infection. MDSCs in the peripheral blood of 206 patients with CHC and 20 healthy donors were analyzed by flow cytometry. Peripheral blood mononuclear cells (PBMCs) of healthy donors cultured with hepatitis C virus core protein (HCVc) were stimulated with or without interleukin 10 (IL-10). Compared to healthy donors and certain CHC patients with sustained viral response (SVR), CHC patients without SVR presented with a dramatic elevation of G-MDSCs with the HLA-DR^-/low CD33⁺ CD14^- CD11b⁺ phenotype in peripheral blood. The frequency of G-MDSCs in CHC patients was positively correlated with serum HCVc, and G-MDSCs were induced from healthy PBMCs by adding exogenous HCVc. Furthermore, we revealed a potential mechanism by which HCVc mediates G-MDSC polarization; activation of ERK1/2 resulting in IL-10 production and IL-10-activated STAT3 signalling. Finally, we confirmed that HCVc-induced G-MDSCs suppress the proliferation and production of IFN-γ in autologous T-cells. We also found that the frequency of G-MDSCs in serum was associated with CHC prognosis. HCVc maintains immunosuppression by promoting IL-10/STAT3-dependent differentiation of G-MDSCs from PBMCs, resulting in the impaired functioning of T-cells. G-MDSCs may thus be a promising biomarker for predicting prognosis of CHC patients.

Inhibition of Casein Kinase 2 Disrupts Differentiation of Myeloid Cells in Cancer and Enhances the Efficacy of Immunotherapy in Mice

The role of myeloid cells as regulators of tumor progression that significantly impact the efficacy of cancer immunotherapies makes them an attractive target for inhibition. Here we explore the effect of a novel, potent, and selective inhibitor of serine/threonine protein kinase casein kinase 2 (CK2) on modulating myeloid cells in the tumor microenvironment. Although inhibition of CK2 caused only a modest effect on dendritic cells in tumor-bearing mice, it substantially reduced the amount of polymorphonuclear myeloid-derived suppressor cells and tumor-associated macrophages. This effect was not caused by the induction of apoptosis, but rather by a block of differentiation. Our results implicated downregulation of CCAAT-enhancer binding protein-α in this effect. Although CK2 inhibition did not directly affect tumor cells, it dramatically enhanced the antitumor activity of immune checkpoint receptor blockade using anti-CTLA-4 antibody. These results suggest a potential role of CK2 inhibitors in combination therapies against cancer.Significance: These findings demonstrate the modulatory effects of casein kinase 2 inhibitors on myeloid cell differentiation in the tumor microenvironment, which subsequently synergize with the antitumor effects of checkpoint inhibitor CTLA4. Cancer Res; 78(19); 5644-55. ©2018 AACR.

Biallelic interferon regulatory factor 8 mutation: A complex immunodeficiency syndrome with dendritic cell deficiency, monocytopenia, and immune dysregulation

BACKGROUND

The homozygous K108E mutation of interferon regulatory factor 8 (IRF8) is reported to cause dendritic cell (DC) and monocyte deficiency. However, more widespread immune dysfunction is predicted from the multiple roles ascribed to IRF8 in immune cell development and function.

OBJECTIVE

We sought to describe the effect on hematopoiesis and immunity of the compound heterozygous R83C/R291Q mutation of IRF8, which is present in a patient with recurrent viral infection, granuloproliferation, and intracerebral calcification.

METHODS

Variant IRF8 alleles were identified by means of exome sequencing, and their function was tested by using reporter assays. The cellular phenotype was studied in detail by using flow cytometry, functional immunologic assay transcriptional profiling, and antigen receptor profiling.

RESULTS

Both mutations affected conserved residues, and R291Q is orthologous to R294, which is mutated in the BXH2 IRF8-deficient mouse. R83C showed reduced nuclear translocation, and neither mutant was able to regulate the Ets/IRF composite element or interferon-stimulated response element, whereas R291Q retained BATF/JUN interactions. DC deficiency and monocytopenia were observed in blood, dermis, and lung lavage fluid. Granulocytes were consistently increased, dysplastic, and hypofunctional. Natural killer cell development and maturation were arrested. TH1, TH17, and CD8+ memory T-cell differentiation was significantly reduced, and T cells did not express CXCR3. B-cell development was impaired, with fewer memory cells, reduced class-switching, and lower frequency and complexity of somatic hypermutation. Cell-specific gene expression was widely disturbed in interferon- and IRF8-regulated transcripts.

CONCLUSIONS

This analysis defines the clinical features of human biallelic IRF8 deficiency, revealing a complex immunodeficiency syndrome caused by DC and monocyte deficiency combined with widespread immune dysregulation.

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.

Neutrophils and PMN-MDSC: Their biological role and interaction with stromal cells

Neutrophils and polymorphonucler myeloid-derived suppressor cells (PMN-MDSC) share origin and many morphological and phenotypic features. However, they have different biological role. Neutrophils are one of the major mechanisms of protection against invading pathogens, whereas PMN-MDSC have immune suppressive activity and restrict immune responses in cancer, chronic infectious disease, trauma, sepsis, and many other pathological conditions. Although in healthy adult individuals, PMN-MDSC are not or barely detectable, in patients with cancer and many other diseases they accumulate at various degree and co-exist with neutrophils. Recent advances allow for better distinction of these cells and better understanding of their biological role. Accumulating evidence indicates PMN-MDSC as pathologically activated neutrophils, with important role in regulation of immune responses. In this review, we provide an overview on the definition and characterization of PMN-MDSC and neutrophils, their pathological significance in a variety of diseases, and their interaction with other stromal components.

Interleukin-27 Exerts Its Antitumor Effects by Promoting Differentiation of Hematopoietic Stem Cells to M1 Macrophages

The interleukin IL27 promotes expansion and differentiation of hematopoietic stem cells into myeloid progenitor cells. Many tumor-infiltrating myeloid cells exert immunosuppressive effects, but we hypothesized that the myeloid cells induced by IL27 would have antitumor activity. In this study, we corroborated this hypothesis as investigated in two distinct mouse transplantable tumor models. Malignant mouse cells engineered to express IL27 exhibited reduced tumor growth in vivo Correlated with this effect was a significant increase in the number of tumor-infiltrating CD11b⁺ myeloid cells exhibiting a reduced immunosuppressive activity. Notably, these CD11b⁺ cells were characterized by an activated M1 macrophage phenotype, on the basis of increased expression of inducible nitric oxide synthase and other M1 biomarkers. In vivo depletion of these cells by administering anti-Gr-1 eradicated the antitumor effects of IL27. When admixed with parental tumors, CD11b⁺ cells inhibited tumor growth and directly killed the tumor in a nitric oxide-dependent manner. Mechanistically, IL27 expanded Lineage^-Sca-1⁺c-Kit⁺ cells in bone marrow. Transplant experiments in Ly5.1/5.2 congenic mice revealed that IL27 directly acted on these cells and promoted their differentiation into M1 macrophages, which mobilized into tumors. Overall, our results illustrated how IL27 exerts antitumor activity by enhancing the generation of myeloid progenitor cells that can differentiate into antitumorigenic M1 macrophages.Significance: These findings show how the interleukin IL27 exerts potent antitumor activity by enhancing the generation of myeloid progenitor cells that can differentiate into antitumorigenic M1 macrophages.Cancer Res; 78(1); 182-94. ©2017 AACR.

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.

Role of interleukin (IL)-17 and T-helper (Th)17 cells in cancer

Interleukin-17 (IL-17), a pleiotropic proinflammatory cytokine, is reported to be significantly generated by a distinct subset of CD4⁺ T-cells, upgrading cancer-elicited inflammation and preventing cancer cells from immune surveillance. T-helper (Th)17 cells produced from naive CD4⁺ T cells have recently been renowned and generally accepted, gaining eminence in cancer studies and playing the effective role in context of cancer. Th17 cells are the main source of IL-17-secreting cells, It was found that other cell types produced this cytokine as well, including Group 3 innate lymphoid cells (ILC3), δγT cells, invariant natural killer T (iNKT) cells, lymphoid-tissue inducer (LTi)-like cells and Natural killer (NK) cells. Th17-associated cytokines give impetus to tumor progression, or inducing angiogenesis and metastasis. This review demonstrates an understanding on how the pro- or antitumor function of Th17 cells and IL-17 may change cancer progression, leading to the appearance of complex and pivotal biologic activities in tumor.

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.

Demethylating agent decitabine disrupts tumor-induced immune tolerance by depleting myeloid-derived suppressor cells

PURPOSE

The immunoregulatory effect of demethylating agent decitabine (DAC) has been recognized recently. However, little is known about its impact on immune tolerance. In this study, we aimed to determine the impact of DAC on the immune tolerance induced by tumor cells.

METHODS

The effects of DAC on immune cells in vivo were measured by flow cytometry. Myeloid-derived suppressor cells (MDSCs) were sorted using magnetic beads and cultured in vitro. The mixed lymphocyte reaction was used to determine the immunoregulatory effect of DAC in vitro. An adoptive transfusion mouse model was established to evaluate the effect in vivo.

RESULTS

We found that DAC treatment significantly depleted MDSCs in vivo by inducing MDSCs apoptosis. When given at a low dose, the immune effector cells were less affected by the treatment, except for MDSCs. The mixed lymphocyte reaction in vitro showed that T-cell responses were enhanced when MDSCs were depleted. Supplementation of MDSCs would attenuate this T-cell activation effect. Using an adoptive transfusion mouse model, we further demonstrated in vivo that DAC treatment could induce autologous anti-tumor immune response by depleting MDSCs.

CONCLUSIONS

This study is the first to illustrate DAC's immunoregulatory effect on immune tolerance. The disruption of immune tolerance is due to MDSCs depletion that induces an autologous immune response in vivo. By depleting MDSCs, DAC treatment removes one of the obstacles affecting anti-tumor immune activation and warrants further experimental and clinical studies to explore its potential utility in combination with various anti-tumor immunotherapies in the future.

Mechanisms overseeing myeloid-derived suppressor cell production in neoplastic disease

Perturbations in myeloid cell differentiation are common in neoplasia, culminating in immature populations known as myeloid-derived suppressor cells (MDSCs). MDSCs favor tumor progression due to their ability to suppress host immunity or promote invasion and metastasis. They are thought to originate from the bone marrow as a result of exposure to stromal- or circulating tumor-derived factors (TDFs). Although great interest has been placed on understanding how MDSCs function, less is known regarding how MDSCs develop at a transcriptional level. Our work explores the premise that MDSCs arise because cancer cells, through the production of certain TDFs, inhibit the expression of interferon regulatory factor-8 (IRF8) that is ordinarily essential for controlling fundamental properties of myeloid cell differentiation. Our interest in IRF8 has been based on the following rationale. First, it is well-recognized that IRF8 is a 'master regulator' of normal myelopoiesis, critical not only for producing monocytes, dendritic cells (DCs), and neutrophils, but also for controlling the balance of all three major myeloid cell types. This became quite evident in IRF8^-/- mice, whereby the loss of IRF8 leads to a disproportionate accumulation of neutrophils at the expense of monocytes and DCs. Second, we showed that such myeloid populations from IRF8^-/- mice exhibit similar characteristics to MDSCs from tumor-bearing mice. Third, in a reciprocal fashion, we showed that enforced expression of IRF8 in the myeloid system significantly mitigates tumor-induced MDSC accumulation and improves immunotherapy efficacy. Altogether, these observations support the hypothesis that IRF8 is an integral negative regulator of MDSC biology.

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.

Th17 cell differentiation proceeds independently of IRF8

The transcriptional repressor/activator interferon regulatory factor 8 (IRF8) modulates the differentiation of a multitude of hematopoietic lineages. However, the role of IRF8 in CD4(+) T-cell development is less well defined, with a recent study implicating IRF8 as an intrinsic repressor of interleukin-17 (IL-17) expressing T helper type 17 (Th17) cell differentiation. Using an IRF8-EGFP reporter strain we have confirmed that IRF8 is expressed in all T helper lineages, including Th17 cells. The loss of IRF8 did not affect Th17 differentiation in vitro, beyond a small increase in IL-22 expression. Moreover, IRF8 deficiency did not enhance the Th17 immune response in experimental T-cell transfer colitis. Together, these results suggest that IRF8 does not play an essential intrinsic role in Th17 cell differentiation.

TRAF3: a novel tumor suppressor gene in macrophages

Tumor necrosis factor receptor-associated factor 3 (TRAF3), a member of the TRAF family of cytoplasmic adaptor proteins with E3 ligase activity, is ubiquitously expressed in various cell types of the immune system. It is shared for signaling by a variety of adaptive and innate immune receptors as well as cytokine receptors. Previous studies examining conditional TRAF3-deficient mouse models that have the Traf3 gene specifically deleted in B lymphocytes or T lymphocytes have revealed the diverse and critical in vivo functions of TRAF3 in adaptive immunity. Although in vitro evidence points to a pivotal and indispensable role for TRAF3 in type I interferon production induced by pattern recognition receptors in macrophages and dendritic cells, the in vivo functions of TRAF3 in the innate immune system had long remained unclear. Three laboratories have recently addressed this gap in knowledge by investigating myeloid cell-specific TRAF3-deficient (genotype: TRAF3^flox/floxLysM^+/Cre) mice. The new evidence together demonstrates that specific ablation of TRAF3 in myeloid cells leads to inflammatory diseases, altered progression of diabetes, and spontaneous development of different types of tumors and infections in mice. These new findings indicate that TRAF3 acts as an anti-inflammatory factor and is required for optimal innate immunity in myeloid cells. Strikingly, the new evidence also identifies TRAF3 as a novel tumor suppressor gene in macrophages and other myeloid cells. In this review, we discuss and summarize the new findings and current knowledge about the multi-faceted regulatory roles and complex signaling mechanisms of myeloid cell TRAF3 in inflammation, innate immunity, and tumor development.

The NF-κB p65 and p50 homodimer cooperate with IRF8 to activate iNOS transcription

Background

Inducible nitric oxide synthase (iNOS) metabolizes L-arginine to produce nitric oxide (NO) which was originally identified in myeloid cells as a host defense mechanism against pathogens. Recent studies, however, have revealed that iNOS is often induced in tumor cells and myeloid cells in the tumor microenvironment. Compelling experimental data have shown that iNOS promotes tumor development in certain cellular context and suppresses tumor development in other cellular conditions. The molecular mechanisms underlying these contrasting functions of iNOS is unknown. Because iNOS is often induced by inflammatory signals, it is therefore likely that these contrasting functions of iNOS could be controlled by the inflammatory signaling pathways, which remains to be determined.

Methods

iNOS is expressed in colon carcinoma and myeloid cells in the tumor microenvironment. Colon carcinoma and myeloid cell lines were used to elucidate the molecular mechanisms underlying iNOS expression. Chromatin immunoprecipitation and electrophoretic mobility shift assay were used to determine the IFNγ-activated pSTAT1 and NF-κB association with the chromatin DNA of the nos2 promoter.

Results

We show here that iNOS is dramatically up-regulated in inflammed human colon tissues and in human colon carcinoma as compared to normal colon tissue. iNOS is expressed in either the colon carcinoma cells or immune cells within the tumor microenvironment. On the molecular level, the proinflammatory IFNγ and NF-κB signals induce iNOS expression in human colon cancer cells. We further demonstrate that NF-κB directly binds to the NOS2 promoter to regulate iNOS expression. Although neither the IFNγ signaling pathway nor the NF-κB signaling pathway alone is sufficient to induce iNOS expression in myeloid cells, IFNγ and NF-κB synergistically induce iNOS expression in myeloid cells. Furthermore, we determine that IFNγ up-regulates IRF8 expression to augment NF-κB induction of iNOS expression. More interestingly, we observed that the p65/p65 and p50/p50 homodimers, not the canonical p65/p50 heterodimer, directly binds to the nos2 promoter to regulate iNOS expression in myeloid cells.

Conclusions

IFNγ-induced IRF8 acts in concert with NF-κB to regulate iNOS expression in both colon carcinoma and myeloid cells. In myeloid cells, the NF-κB complexes that bind to the nos2 promoter are p65/p65 and p50/p50 homodimers.

Epigenetic and Immune Regulation of Colorectal Cancer Stem Cells

Colorectal cancer stem cells (CSCs) were initially considered to be a subset of undifferentiated tumor cells with well-defined phenotypic and molecular markers. However, emerging evidence indicates instead that colorectal CSCs are heterogeneous subsets of tumor cells that are continuously reshaped by the dynamic interactions between genetic, epigenetic, and immune factors in the tumor microenvironment. Thus, the colorectal CSC phenotypes and responsiveness to therapy may not only be a tumor cell-intrinsic feature, but also depend on tumor-extrinsic microenvironmental factors. Furthermore, emerging evidence also implicates colorectal CSCs in potential immune evasion. Therefore, understanding how colorectal CSC-intrinsic mechanisms cooperate with the extrinsic microenvironmental factors to dynamically shape colorectal CSC resistance to chemotherapy and immunotherapy holds great promise for development of targeted CSC therapies of advanced human CRC.

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.

Epigenetic Regulation of Apoptosis and Cell Cycle Regulatory Genes in Human Colon Carcinoma Cells

5-Fluorouracil (5-FU) is the standard chemotherapy for certain high risk stage 2 and all stages 3 and 4 human colorectal cancer patients. However, patients often develop chemoresistance to 5-FU. We have identified verticillin A from Verticillium-infected wild mushrooms as a potent anti-cancer agent that effectively suppresses 5-FU-resistant human colon cancer cells. Interestingly, a sublethal dose of verticillin A also acts as a potent sensitizer that overcomes human colon carcinoma cell resistance to FasL- and TRAIL-induced apoptosis. To identify verticillin A-regulated genes, we performed a genome-wide gene expression analysis and identified 1287 genes whose expression levels were either up- or down-regulated 1.5 fold. Forty-six of these genes have known function in regulation of apoptosis, and ninety genes have function in cell cycle regulation. Our recent study has identified verticillin A as a selective histone methyltransferase inhibitor. These identified genes are thus potential molecular targets for epigenetic-based therapy to overcome human colon cancer 5-FU resistance. The entire dataset is deposited in the NIH GEO database. Accession number GSE51262.

Special Conference on Tumor Immunology and Immunotherapy: A New Chapter

The overall objective of the fifth American Association for Cancer Research Special Conference, "Tumor Immunology and Immunotherapy: A New Chapter," organized by the Cancer Immunology Working Group, was to highlight multidisciplinary approaches of immunotherapy and mechanisms related to the ability of immunotherapy to fight established tumors. With the FDA approval of sipuleucel-T, ipilimumab (anti-CTLA-4; Bristol-Myers Squibb), and the two anti-PD-1 antibodies, pembrolizumab (formerly MK-3475 or lambrolizumab; Merck) and nivolumab (Bristol-Myers Squibb), immunotherapy has become a mainstream treatment option for some cancers. Many of the data presented at the conference and reviewed in this article showcase the progress made in determining the mechanistic reasons for the success of some treatments and the mechanisms associated with tolerance within the tumor microenvironment, both of which are potential targets for immunotherapy. In addition to combination and multimodal therapies, improvements in existing therapies will be needed to overcome the numerous ways that tumor-specific tolerance thwarts the immune system. This conference built upon the success of the 2012 conference and focused on seven progressing and/or emerging areas-new combination therapies, combination therapies and vaccine improvement, mechanisms of antibody therapy, factors in the tumor microenvironment affecting the immune response, the microbiomes effect on cancer and immunotherapy, metabolism in immunotherapy, and adoptive T-cell therapy. Cancer Immunol Res; 3(6); 1-8. ©2015 AACR.