The stress-response sensor chop regulates the function and accumulation of myeloid-derived suppressor cells in tumors

Characteristics and clinical significance of CD163+/CD206+M2 mono-macrophage in the bladder cancer microenvironment

The tumor microenvironment may recruit monocytes, with a protumoral macrophage phenotype (M2) that plays an important role in solid tumor progression and metastasis. Therefore, it is necessary to understand the characteristics of these cells for cancer prevention and treatment. Bladder cancer tissue samples and paracarcinoma tissues samples were collected, and the expression of CD163⁺ cells in tumor tissues was observed. Then, we observed the expression of infiltrating CD45⁺CD14⁺CD163⁺ cell subset and analyzed the molecular expressions related to immunity and angiogenesis. C57/BL6 mice were inoculated subcutaneously, and dynamic changes of CD11b⁺F4/80⁺CD206⁺ mononuclear macrophages expression for tumor-bearing mice were detected. The results showed that the proportion of CD45⁺CD14⁺CD163⁺ mono-macrophage subset infiltrated by tumor tissue was significantly higher than that in paracarcinoma tissues. In bladder cancer tissue, the expression rate of CD40 in CD45⁺CD14⁺CD163^- mono-macrophage subset was significantly lower than that in CD45⁺CD14⁺CD163⁺ mono-macrophage subset. Similar results were found in the paracarcinoma tissues. We found that, as the proportion of CD11b⁺F4/80⁺CD206⁺ mono-macrophages increased gradually, the difference was statistically significant. CD163⁺/CD206⁺ mono-macrophages in bladder cancer microenvironment are abnormally elevated, and these cells are closely related to tumor progression. CD40 may be an important molecule that exerts biological function in this subset.

A common approach for fighting tuberculosis and leprosy: controlling endoplasmic reticulum stress in myeloid-derived suppressor cells

Leprosy and tuberculosis are infectious diseases that are caused by bacteria, and both share primary risk factors. Mediators of these diseases are regulated by a heterogeneous immature population of myeloid cells called myeloid-derived suppressor cells (MDSCs) that exhibit immunosuppressive activity against innate and adaptive immunity. During pathological conditions, endoplasmic reticulum (ER) stress occurs in MDSCs, and high levels of ER stress affect MDSC-linked immunosuppressive activity. Investigating the role of ER stress in regulating immunosuppressive functions of MDSCs in leprosy and tuberculosis may lead to new approaches to treating these diseases. Here the authors discuss the immunoregulatory effects of ER stress in MDSCs as well as the possibility of targeting unfolded protein response elements of ER stress to diminish the immunosuppressive activity of MDSCs and reinvigorate diminished adaptive immune system responses that occur in leprosy and tuberculosis.

Endoplasmic Reticulum Quality Control in Immune Cells

The endoplasmic reticulum quality control (ERQC) system, including endoplasmic reticulum-associated degradation (ERAD), the unfolded protein response (UPR), and autophagy, presides over cellular protein secretion and maintains proteostasis in mammalian cells. As part of the immune system, a variety of proteins are synthesized and assembled correctly for the development, activation, and differentiation of immune cells, such as dendritic cells (DCs), macrophages, myeloid-derived-suppressor cells (MDSCs), B lymphocytes, T lymphocytes, and natural killer (NK) cells. In this review, we emphasize the role of the ERQC in these immune cells, and also discuss how the imbalance of ER homeostasis affects the immune response, thereby suggesting new therapeutic targets for immunotherapy.

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 (MDSCs) in brain cancer: challenges and therapeutic strategies

The most fatal malignancy of the central nervous system (CNS) is glioblastoma. Brain cancer is a 'cold' tumor because of fewer immunoregulatory cells and more immunosuppressive cells. Due to the cold nature of brain cancers, conventional treatments which are used to manage glioma patients show little effectiveness. Glioma patients even showed resistance to immune checkpoint blockade (ICB) and no significant efficacy. It has been shown that myeloid-derived suppressor cells (MDSCs) account for approximately 30-50% of the tumor mass in glioma. This study aimed to review MDSC function in brain cancer, as well as possible treatments and related challenges. In brain cancer and glioma, several differences in the context of MDSCs have been reported, including disagreements about the MDSC subtype that has the most inhibitory function in the brain, or inhibitory function of regulatory B cells (Bregs). There are also serious challenges in treating glioma patients. In addition to the cold nature of glioma, there are reports of an increase in MDSCs following conventional chemotherapy treatments. As a result, targeting MDSCs in combination with other therapies, such as ICB, is essential, and recent studies with the combination therapy approach have shown promising therapeutic effects in brain cancer.

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.

Group 2 innate lymphoid cells promote TNBC lung metastasis via the IL-13-MDSC axis in a murine tumor model

Group 2 innate lymphoid cells (ILC2s) are reportedly associated with the progression of many tumors. However, the role of ILC2s in triple-negative breast cancer (TNBC) lung metastasis remains unclear. In this study, we found that ILC2s may be a key element in the process of TNBC lung metastasis since the adoptive transfer of pulmonary ILC2s increased the numbers of metastatic lung nodules and reduced the survival of tumor-bearing mice. ILC2-promoted 4 T1 lung metastasis appears to be related to ILC2-derived IL-13. An expansion of IL-13-producing ILC2s and an elevated expression of IL-13 mRNA in pulmonary ILC2s were determined in tumor-bearing mice, in parallel with an increase in the levels of local IL-13 by ILC2 transfer. The neutralization of IL-13 reduced the increased pulmonary metastatic nodules and improved the decreased survival rate caused by ILC2-adoptive transfer. Interestingly, adoptive transfer of ILC2s elevated IL-13Ra1 expression in myeloid-derived suppressor cells (MDSCs). Treatment of ILC2-transferred tumor-bearing mice with anti-IL-13 antibodies significantly diminished the number of pulmonary MDSCs and inhibited MDSC activation. Moreover, when pulmonary MDSCs were cocultured with ILC2s in the presence of an anti-IL-13 mAb, the number and activation of MDSCs were reduced. Depletion of MDSCs may promote the proliferation of CD4+ T cells and CD8+ T cells, but reduce the expansion of regulatory T cells (Tregs) in the lungs of ILC2-transferred tumor-bearing mice. Our results suggest that pulmonary ILC2s may promote TNBC lung metastasis via the ILC2-derived IL-13-activated MDSC pathway.

A mechanism of cooling hot tumors: Lactate attenuates inflammation in dendritic cells

Turning non-inflamed (cold) tumors into inflamed (hot) tumors is important for maximizing the effect of immune checkpoint inhibitors (ICIs) against malignancies. We showed that lactate, a product of the Warburg effect, inhibited the efficacy of ICIs and suppressed IL-12 p40 expression in dendritic cells (DCs) through reducing NF-κB p65, p50, and c-Rel DNA-binding activity to the IL-12 p40 promoter. Additionally, lactate promoted the expression of early growth response protein 1 (EGR1), whose expression was increased in human invasive melanoma compared with non-invasive melanoma. We also found that EGR1 interacts with serum response factor (SRF) and represses the expression of CD80 in DCs. These findings suggest that lactate and its induced EGR1 are key factors that turn hot tumors into cold tumors and may represent targets in cancer treatment with ICIs.

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Lactate suppressed IL-12 p40 expression in dendritic cells

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Lactate promoted the expression of early growth response protein 1 (EGR1)

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EGR1 interacts with serum response factor (SRF) and represses the expression of CD80

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Lactate and EGR1 switch inflamed (hot) tumors to non-inflamed (cold) tumors

Novel Characterization of Myeloid-Derived Suppressor Cells in Tumor Microenvironment

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous group of cells generated in various pathologic conditions, which have been known to be key components of the tumor microenvironment (TME) involving in tumor immune tolerance. So MDSCs have been extensively researched recently. As its name suggests, immunosuppression is the widely accepted function of MDSCs. Aside from suppressing antitumor immune responses, MDSCs in the TME also stimulate tumor angiogenesis and metastasis, thereby promoting tumor growth and development. Therefore, altering the recruitment, expansion, activation, and immunosuppression of MDSCs could partially restore antitumor immunity. So, this view focused on the favorable TME conditions that promote the immunosuppressive effects of MDSCs and contribute to targeted therapies with increased precision for MDSCs.

Distinct mechanisms govern populations of myeloid-derived suppressor cells in chronic viral infection and cancer

Myeloid-derived suppressor cells (MDSCs) are major negative regulators of immune responses in cancer and chronic infections. It remains unclear if regulation of MDSC activity in different conditions is controlled by similar mechanisms. We compared MDSCs in mice with cancer and lymphocytic choriomeningitis virus (LCMV) infection. Chronic LCMV infection caused the development of monocytic MDSCs (M-MDSCs) but did not induce polymorphonuclear MDSCs (PMN-MDSCs). In contrast, both MDSC populations were present in cancer models. An acquisition of immune-suppressive activity by PMN-MDSCs in cancer was controlled by IRE1α and ATF6 pathways of the endoplasmic reticulum (ER) stress response. Abrogation of PMN-MDSC activity by blockade of the ER stress response resulted in an increase in tumor-specific immune response and reduced tumor progression. In contrast, the ER stress response was dispensable for suppressive activity of M-MDSCs in cancer and LCMV infection. Acquisition of immune-suppressive activity by M-MDSCs in spleens was mediated by IFN-γ signaling. However, it was dispensable for suppressive activity of M-MDSCs in tumor tissues. Suppressive activity of M-MDSCs in tumors was retained due to the effect of IL-6 present at high concentrations in the tumor site. These results demonstrate disease- and population-specific mechanisms of MDSC accumulation and the need for targeting different pathways to achieve inactivation of these cells.

Myeloid-derived suppressor cells in the era of increasing myeloid cell diversity

Myeloid-derived suppressor cells (MDSCs) are pathologically activated neutrophils and monocytes with potent immunosuppressive activity. They are implicated in the regulation of immune responses in many pathological conditions and are closely associated with poor clinical outcomes in cancer. Recent studies have indicated key distinctions between MDSCs and classical neutrophils and monocytes, and, in this Review, we discuss new data on the major genomic and metabolic characteristics of MDSCs. We explain how these characteristics shape MDSC function and could facilitate therapeutic targeting of these cells, particularly in cancer and in autoimmune diseases. Additionally, we briefly discuss emerging data on MDSC involvement in pregnancy, neonatal biology and COVID-19.

Myeloid-Derived Suppressor Cells: Implications in the Resistance of Malignant Tumors to T Cell-Based Immunotherapy

T lymphocytes function as major players in antigen-mediated cytotoxicity and have become powerful tools for exploiting the immune system in tumor elimination. Several types of T cell-based immunotherapies have been prescribed to cancer patients with durable immunological response. Such strategies include immune checkpoint inhibitors, adoptive T cell therapy, cancer vaccines, oncolytic virus, and modulatory cytokines. However, the majority of cancer patients still failed to take the advantage of these kinds of treatments. Currently, extensive attempts are being made to uncover the potential mechanism of immunotherapy resistance, and myeloid-derived suppressor cells (MDSCs) have been identified as one of vital interpretable factors. Here, we discuss the immunosuppressive mechanism of MDSCs and their contributions to failures of T cell-based immunotherapy. Additionally, we summarize combination therapies to ameliorate the efficacy of T cell-based immunotherapy.

Long Non-Coding RNAs in the Tumor Immune Microenvironment: Biological Properties and Therapeutic Potential

Cancer immunotherapy (CIT) is considered a revolutionary advance in the fight against cancer. The complexity of the immune microenvironment determines the success or failure of CIT. Long non-coding RNA (lncRNA) is an extremely versatile molecule that can interact with RNA, DNA, or proteins to promote or inhibit the expression of protein-coding genes. LncRNAs are expressed in many different types of immune cells and regulate both innate and adaptive immunity. Recent studies have shown that the discovery of lncRNAs provides a novel perspective for studying the regulation of the tumor immune microenvironment (TIME). Tumor cells and the associated microenvironment can change to escape recognition and elimination by the immune system. LncRNA induces the formation of an immunosuppressive microenvironment through related pathways, thereby controlling the escape of tumors from immune surveillance and promoting the development of metastasis and drug resistance. Using lncRNA as a therapeutic target provides a strategy for studying and improving the efficacy of immunotherapy.

Immune Surveillance by Myeloid-Derived Suppressor Cells in Liver Diseases

BACKGROUND

Myeloid-derived suppressor cells (MDSCs) are immunosuppressive in nature, originate in the bone marrow, and are mainly found in the blood, spleen, and liver. In fact, liver acts as an important organ for induction and accumulation of MDSCs, especially during infection, inflammation, and cancer. In humans and rodents, models of liver diseases revealed that MDSCs promote regeneration and drive the inflammatory processes, leading to hepatitis, fibrogenesis, and cirrhosis, ultimately resulting in hepatocellular carcinoma.

SUMMARY

This brief review is focused on the in-depth understanding of the key molecules involved in the expansion and regulation of MDSCs and their underlying immunosuppressive mechanisms in liver diseases.

KEY MESSAGE

Modulated MDSCs can be used for therapeutic purposes in inflammation, cancer, and sepsis.

The Unfolded Protein Response in Immune Cells as an Emerging Regulator of Neuroinflammation

Immune surveillance is an essential process that safeguards the homeostasis of a healthy brain. Among the increasing diversity of immune cells present in the central nervous system (CNS), microglia have emerged as a prominent leukocyte subset with key roles in the support of brain function and in the control of neuroinflammation. In fact, impaired microglial function is associated with the development of neurodegenerative diseases, including Alzheimer’s disease (AD) and Parkinson’s disease (PD). Interestingly, these pathologies are also typified by protein aggregation and proteostasis dysfunction at the level of the endoplasmic reticulum (ER). These processes trigger activation of the unfolded protein response (UPR), which is a conserved signaling network that maintains the fidelity of the cellular proteome. Remarkably, beyond its role in protein folding, the UPR has also emerged as a key regulator of the development and function of immune cells. However, despite this evidence, the contribution of the UPR to immune cell homeostasis, immune surveillance, and neuro-inflammatory processes remains largely unexplored. In this review, we discuss the potential contribution of the UPR in brain-associated immune cells in the context of neurodegenerative diseases.

ER stress and its PERK branch enhance TCR-induced activation in regulatory T cells

Although accumulating evidence indicates participation of endoplasmic reticulum (ER) stress pathway or the unfolded protein response (UPR) in immunity, there still exists little information linking ER stress to regulatory T cells (Tregs). To evaluate the potential contribution of the UPR, we tested the effects of thapsigargin (TG), an ER stress inducer, on the function of Tregs. Here we reported that TG stimulation induced the up-regulation of the endoplasmic reticulum (ER)-stress chaperon Glucose-Regulated Protein 78 (GRP78), which is a master regulator of the UPR, the phosphorylation of eukaryotic initiation factor 2 alpha (elF2α) and the induction of activating transcription factor 4 (ATF4), which are both protein kinase R (PKR)-like ER kinase (PERK) downstream targets in Tregs. Simultaneously, we demonstrated that, under ER stress conditions, Tregs presented enhanced functional activity upon TCR stimulation, as illustrated with forkhead box transcription factor (Foxp3) expression, interleukin-10 (IL-10) and transforming growth factor-β (TGF-β) production and suppressive functional analysis. Notably, pretreatment with GSK2656157, a potent and selective PERK inhibitor, markedly diminished the TG-induced hyperresponsiveness of Tregs upon T cell antigen receptor (TCR) stimulation. Therefore, our findings illustrated the inter-connection and coordination of the evolutionarily conserved ER stress response and TCR signaling in Tregs and uncover a critical new role of the PERK branch of UPR in the regulation of Tregs function.

Redox-responsive nanoassembly restrained myeloid-derived suppressor cells recruitment through autophagy-involved lactate dehydrogenase A silencing for enhanced cancer immunochemotherapy

Myeloid-derived suppressor cells (MDSCs) are the chief accomplices for assisting tumor's survival and suppressing anti-tumor immunity, which can be recruited by tumor-derived cytokines, such as granulocyte-colony stimulating factor (G-CSF) and granulocyte-macrophage colony stimulating factor (GM-CSF). The plentiful lactate dehydrogenase A (LDHA) in glycolysis is usually accompanied by abundant tumor-derived G-CSF and GM-CSF, further promoting MDSCs recruitment and immunosuppression. Herein, with the aim to achieve powerful anti-tumor immunity, an immunochemotherapy regimen basing on a redox-responsive nanoassembly (R-mPDV/PDV/DOX/siL) is developed, which integrates the combined strategy of restraining cytokines-mediated MDSCs recruitment through LDHA silencing and reinforcing tumor immunogenicity through anthracycline (DOX)-elicited immunogenic cell death (ICD) effects. This redox-responsive nanoassembly is self-assembled by three glutathione (GSH)-responsive polymers, which employ poly(δ-valerolactone) (PVL) as hydrophobic segment and 3, 3'-dithiodipropionic acid (DA) as linkage to connect hydrophilic segment. DOX is encapsulated in the core and LDHA siRNA (siL) is effectively compressed by cationic PAMAM. The cellular internalization and tumor-homing are strengthened by the specific recognition on integrin (α_vβ₃) by c(RGDfk) (RGD) ligand. After escaping from endosomes/lysosomes, R-mPDV/PDV/DOX/siL is disintegrated through GSH-elicited cleavage of DA, realizing burst release of drugs and high-efficient LDHA silencing. The reduced expression of LDHA suppresses the generation of G-CSF and GM-CSF cytokines, restrains MDSCs recruitment and reinforces anti-tumor immunity. Eventually, this therapeutic regimen of DOX and siL on R-mPDV/PDV/DOX/siL nanoassembly achieved powerful anti-tumor efficiency on 4 T1 orthotopic tumor, opening the new horizons for immunochemotherapy.

IDO1 Signaling through GCN2 in a Subpopulation of Gr-1+ Cells Shifts the IFNγ/IL6 Balance to Promote Neovascularization

In addition to immunosuppression, it is generally accepted that myeloid-derived suppressor cells (MDSC) also support tumor angiogenesis. The tryptophan-catabolizing enzyme indoleamine 2,3-dioxygenase (IDO1) has been implicated in promoting neovascularization through its positioning as a key regulatory node between the inflammatory cytokines IFNγ and IL6. Here, we report that within the heterogeneous expanse of Gr-1+ MDSCs, both IDO1 expression and the ability to elicit neovascularization in vivo were associated with a minor subset of autofluorescent, CD11blo cells. IDO1 expression was further restricted to a discrete, CD11c and asialo-GM1 double-positive subpopulation of these cells, designated here as IDVCs (IDO1-dependent vascularizing cells), due to the dominant role that IDO1 activity in these cells was found to play in promoting neovascularization. Mechanistically, the induction of IDO1 in IDVCs provided a negative-feedback constraint on the antiangiogenic effect of host IFNγ by intrinsically signaling for the production of IL6 through general control nonderepressible 2 (GCN2)-mediated activation of the integrated stress response. These findings reveal fundamental molecular and cellular insights into how IDO1 interfaces with the inflammatory milieu to promote neovascularization.

Tumor-related stress regulates functional plasticity of MDSCs

Myeloid-derived suppressor cells (MDSCs) impair protective anti-tumor immunity and remain major obstacles that stymie the effectiveness of promising cancer therapies. Diverse tumor-derived stressors galvanize the differentiation, intra-tumoral expansion, and immunomodulatory function of MDSCs. These tumor-associated 'axes of stress' underwrite the immunosuppressive programming of MDSCs in cancer and contribute to the phenotypic/functional heterogeneity that characterize tumor-MDSCs. This review discusses various tumor-associated axes of stress that direct MDSC development, accumulation, and immunosuppressive function, as well as current strategies aimed at overcoming the detrimental impact of MDSCs in cancer. To better understand the constellation of signals directing MDSC biology, we herein summarize the pivotal roles, signaling mediators, and effects of reactive oxygen/nitrogen species-related stress, chronic inflammatory stress, hypoxia-linked stress, endoplasmic reticulum stress, metabolic stress, and therapy-associated stress on MDSCs. Although therapeutic targeting of these processes remains mostly pre-clinical, intercepting signaling through the axes of stress could overcome MDSC-related immune suppression in tumor-bearing hosts.

MDSC: Markers, development, states, and unaddressed complexity

Myeloid-derived suppressor cells (MDSCs) are one of the most discussed biological entities in immunology. While the context and classification of this group of cells has evolved, MDSCs most commonly describe cells arising during chronic inflammation, especially late-stage cancers, and are defined by their T cell immunosuppressive functions. This MDSC concept has helped explain myeloid phenomena associated with disease outcome, but currently lacks clear definitions and a unifying framework across pathologies. Here, we propose such a framework to classify MDSCs as discrete cell states based on activation signals in myeloid populations leading to suppressive modes characterized by specific, measurable effects. Developing this level of knowledge of myeloid states across pathological conditions may ultimately transform how disparate diseases are grouped and treated.

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

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

Myeloid-Derived Suppressor Cells: Facilitators of Cancer and Obesity-Induced Cancer

Immature myeloid cells at varied stages of differentiation, known as myeloid-derived suppressor cells (MDSC), are present in virtually all cancer patients. MDSC are profoundly immune-suppressive cells that impair adaptive and innate antitumor immunity and promote tumor progression through nonimmune mechanisms. Their widespread presence combined with their multitude of protumor activities makes MDSC a major obstacle to cancer immunotherapies. MDSC are derived from progenitor cells in the bone marrow and traffic through the blood to infiltrate solid tumors. Their accumulation and suppressive potency are driven by multiple tumor- and host-secreted proinflammatory factors and adrenergic signals that act via diverse but sometimes overlapping transcriptional pathways. MDSC also accumulate in response to the chronic inflammation and lipid deposition characteristic of obesity and contribute to the more rapid progression of cancers in obese individuals. This article summarizes the key aspects of tumor-induced MDSC with a focus on recent progress in the MDSC field.

Interferon Regulatory Factor 4 Regulates the Development of Polymorphonuclear Myeloid-Derived Suppressor Cells Through the Transcription of c-Myc in Cancer

The accumulation of myeloid-derived suppressor cells (MDSCs) is one of the major obstacles to achieve an appropriate anti-tumor immune response and successful tumor immunotherapy. MDSCs in tumor-bearing hosts are primarily polymorphonuclear (PMN-MDSCs). However, the mechanisms regulating the development of MDSCs remain poorly understood. In this report, we showed that interferon regulatory factor 4 (IRF4) plays a key role in the development of PMN-MDSCs, but not monocytic MDSCs. IRF4 deficiency caused a significant elevation of PMN-MDSCs and enhanced the suppressive activity of PMN-MDSCs, increasing tumor growth and metastasis in mice. Mechanistic studies showed that c-Myc was up-regulated by the IRF4 protein. Over-expression of c-Myc almost abrogated the effects of IRF4 deletion on PMN-MDSCs development. Importantly, the IRF4 expression level was negatively correlated with the PMN-MDSCs frequency and tumor development but positively correlated with c-Myc expression in clinical cancer patients. In summary, this study demonstrated that IRF4 represents a novel regulator of PMN-MDSCs development in cancer, which may have predictive value for tumor progression.

Immunopathogenesis of Craniotomy Infection and Niche-Specific Immune Responses to Biofilm

Bacterial infections in the central nervous system (CNS) can be life threatening and often impair neurological function. Biofilm infection is a complication following craniotomy, a neurosurgical procedure that involves the removal and replacement of a skull fragment (bone flap) to access the brain for surgical intervention. The incidence of infection following craniotomy ranges from 1% to 3% with approximately half caused by Staphylococcus aureus (S. aureus). These infections present a significant therapeutic challenge due to the antibiotic tolerance of biofilm and unique immune properties of the CNS. Previous studies have revealed a critical role for innate immune responses during S. aureus craniotomy infection. Experiments using knockout mouse models have highlighted the importance of the pattern recognition receptor Toll-like receptor 2 (TLR2) and its adaptor protein MyD88 for preventing S. aureus outgrowth during craniotomy biofilm infection. However, neither molecule affected bacterial burden in a mouse model of S. aureus brain abscess highlighting the distinctions between immune regulation of biofilm vs. planktonic infection in the CNS. Furthermore, the immune responses elicited during S. aureus craniotomy infection are distinct from biofilm infection in the periphery, emphasizing the critical role for niche-specific factors in dictating S. aureus biofilm-leukocyte crosstalk. In this review, we discuss the current knowledge concerning innate immunity to S. aureus craniotomy biofilm infection, compare this to S. aureus biofilm infection in the periphery, and discuss the importance of anatomical location in dictating how biofilm influences inflammatory responses and its impact on bacterial clearance.

Immunotherapy with 4-1BBL-Expressing iPS Cell-Derived Myeloid Lines Amplifies Antigen-Specific T Cell Infiltration in Advanced Melanoma

We have established an immune cell therapy with immortalized induced pluripotent stem-cell-derived myeloid lines (iPS-ML). The benefits of using iPS-ML are the infinite proliferative capacity and ease of genetic modification. In this study, we introduced 4-1BBL gene to iPS-ML (iPS-ML-41BBL). The analysis of the cell-surface molecules showed that the expression of CD86 was upregulated in iPS-ML-41BBL more than that in control iPS-ML. Cytokine array analysis was performed using supernatants of the spleen cells that were cocultured with iPS-ML or iPS-ML-41BBL. Multiple cytokines that are beneficial to cancer immunotherapy were upregulated. Peritoneal injections of iPS-ML-41BBL inhibited tumor growth of peritoneally disseminated mouse melanoma and prolonged survival of mice compared to that of iPS-ML. Furthermore, the numbers of antigen-specific CD8+ T cells were significantly increased in the spleen and tumor tissues treated with epitope peptide-pulsed iPS-ML-41BBL compared to those treated with control iPS-ML. The number of CXCR6-positive T cells were increased in the tumor tissues after treatment with iPS-ML-41BBL compared to that with control iPS-ML. These results suggest that iPS-ML-41BBL could activate antigen-specific T cells and promote their infiltration into the tumor tissues. Thus, iPS-ML-41BBL may be a candidate for future immune cell therapy aiming to change immunological "cold tumor" to "hot tumor".

Endoplasmic reticulum stress signals in the tumour and its microenvironment

Protein handling, modification and folding in the endoplasmic reticulum (ER) are tightly regulated processes that determine cell function, fate and survival. In several tumour types, diverse oncogenic, transcriptional and metabolic abnormalities cooperate to generate hostile microenvironments that disrupt ER homeostasis in malignant and stromal cells, as well as infiltrating leukocytes. These changes provoke a state of persistent ER stress that has been demonstrated to govern multiple pro-tumoural attributes in the cancer cell while dynamically reprogramming the function of innate and adaptive immune cells. Aberrant activation of ER stress sensors and their downstream signalling pathways have therefore emerged as key regulators of tumour growth and metastasis as well as response to chemotherapy, targeted therapies and immunotherapy. In this Review, we discuss the physiological inducers of ER stress in the tumour milieu, the interplay between oncogenic signalling and ER stress response pathways in the cancer cell and the profound immunomodulatory effects of sustained ER stress responses in tumours.

Regulation of ROS in myeloid-derived suppressor cells through targeting fatty acid transport protein 2 enhanced anti-PD-L1 tumor immunotherapy

Despite the remarkable success and efficacy of immune checkpoint blockade (ICB) therapy against the PD-1/PD-L1 axis, it induces sustained responses in a sizeable minority of cancer patients due to the activation of immunosuppressive factors such as myeloid-derived suppressor cells (MDSCs). Inhibiting the immunosuppressive function of MDSCs is critical for successful cancer ICB therapy. Interestingly, lipid metabolism is a crucial factor in modulating MDSCs function. Fatty acid transport protein 2 (FATP2) conferred the function of PMN-MDSCs in cancer via the upregulation of arachidonic acid metabolism. However, whether regulating lipid accumulation in MDSCs by targeting FATP2 could block MDSCs reactive oxygen species (ROS) production and enhance PD-L1 blockade-mediated tumor immunotherapy remains unexplored. Here we report that FATP2 regulated lipid accumulation, ROS, and immunosuppressive function of MDSCs in tumor-bearing mice. Tumor cells-derived granulocyte macrophage-colony stimulating factor (GM-CSF) induced FATP2 expression in MDSCs by activation of STAT3 signaling pathway. Pharmaceutical blockade of FATP2 expression in MDSCs by lipofermata decreased lipid accumulation, reduced ROS, blocked immunosuppressive activity, and consequently inhibited tumor growth. More importantly, lipofermata inhibition of FATP2 in MDSCs enhanced anti-PD-L1 tumor immunotherapy via the upregulation of CD107a and reduced PD-L1 expression on tumor-infiltrating CD8⁺T-cells. Furthermore, the combination therapy blocked MDSC's suppressive role on T- cells thereby enhanced T-cell's ability for the production of IFN-γ. These findings indicate that FATP2 plays a key role in modulating lipid accumulation-induced ROS in MDSCs and targeting FATP2 in MDSCs provides a novel therapeutic approach to enhance anti-PD-L1 cancer immunotherapy.

Myeloid-Derived Suppressor Cell Differentiation in Cancer: Transcriptional Regulators and Enhanceosome-Mediated Mechanisms

Myeloid-derived Suppressor Cells (MDSCs) are a sub-population of leukocytes that are important for carcinogenesis and cancer immunotherapy. During carcinogenesis or severe infections, inflammatory mediators induce MDSCs via aberrant differentiation of myeloid precursors. Although several transcription factors, including C/EBPβ, STAT3, c-Rel, STAT5, and IRF8, have been reported to regulate MDSC differentiation, none of them are specifically expressed in MDSCs. How these lineage-non-specific transcription factors specify MDSC differentiation in a lineage-specific manner is unclear. The recent discovery of the c-Rel-C/EBPβ enhanceosome in MDSCs may help explain these context-dependent roles. In this review, we examine several transcriptional regulators of MDSC differentiation, and discuss the concept of non-modular regulation of MDSC signature gene expression by transcription factors such as c-Rel and C/EBPß.

[A Review of the Roles of Endoplasmic Reticulum Stress in Cancer Cell Metastasis]

Metastasis is a multistep and low-efficiency biological process driven by acquisition of genetic and/or epigenetic alterations within tumor cells. These evolutionary alterations enable tumor cells to thrive in the inhospitable microenvironment they encounter in the process of metastasis and eventually lead to macroscopic metastases in distant organs. The unfolded protein response (UPR) induced by endoplasmic reticulum (ER) stress is one of the most important mechanisms regulating cellular adaptation to an adverse microenvironment. UPR is involved in all stages of metastasis, playing an important role in tumor cell growth, survival, and differentiation and the process of maintaining protein hemostasis. Sustained activation of ER stress sensors endows tumor cells with better epithelial-mesenchymal transition (EMT), survival, immune escape, angiogenesis, cellular adhesion, dormancy-to reactivation capacity in the process of metastasis. Here, we discussed the role of UPR in regulating the above-mentioned abilities of tumor cells during metastasis, providing a reference for development of new targets for the treatment of tumor metastasis.UPR in regulating the above-mentioned characteristics and mechanisms of tumor cells during metastasis, providing a reference for development of new targets for the treatment of tumor metastasis.

B7-H3 Promotes Prostate Cancer Progression in Mice by Antagonizing Myeloid-Derived Suppressor Cell Apoptosis

Background:

B7-H3 is an important immunomodulatory molecule, and clinical studies have confirmed that its expression level is closely correlated with prostate cancer prognosis. However, the mechanism of its biological action is unclear.

Methods:

An engineered cell line overexpressing B7-H3 was constructed. Cell apoptosis, growth and proliferation assays in vitro and an animal model in vivo were performed to analyze the role and possible mechanism of B7-H3 in promoting prostate cancer progression.

Results:

Compared with the control cell line (Mock-RM-1), the B7-H3-overexpressing prostate cancer cell line (B7-H3-RM-1) showed no significant growth differences in vitro, whereas the in vivo tumorigenesis rate of B7-H3-RM-1 was significantly higher than that of Mock-RM-1. These results suggest that B7-H3indirectly, rather than directly, promotes prostate cancer progression. Further analysis revealed that significantly higher levels of myeloid-derived suppressor cells (MDSCs) accumulated in vivo in B7-H3-RM-1 tumor-bearing mice than in Mock-RM-1 mice. In vitro and in vivo experiments showed that B7-H3-RM-1 cells significantly antagonized MDSC apoptosis. To further confirm the role of MDSCs in B7-H3-mediated prostate cancer progression, model mice were pretreated with cyclophosphamide before inoculation to clear immune cells and achieve myelo suppression. The results showed no significant differences in tumor growth between the B7-H3-RM-1 group and the Mock-RM-1 group.

Conclusions:

We found, for the first time, that B7-H3 can antagonize MDSC apoptosis, leading to MDSC accumulation in the tumor microenvironment and thereby promoting prostate cancer progression.

A Highlight of the Mechanisms of Immune Checkpoint Blocker Resistance

In recent years, as our understanding of tumor immunology is continuously improved, immunotherapy has come to the center stage of cancer therapy and is deemed as the most promising approach for cancer control. Although immunotherapy, particularly immune checkpoint blockade (ICB), has achieved a milestone in several types of tumors, the majority of cancer patients do not benefit from immunotherapy. The dismal outcome of cancer immunotherapy is mainly due to primary or acquired resistance arising from tumor immune evasion. Exploring the mechanisms of tumor immune evasion in the course of immunotherapy may identify biological targets to conquer tumor resistance to immunotherapy. In this review, we highlight tumor cell-intrinsic and -extrinsic factors that may underlie tumor resistance to immune checkpoint blockers. Targeting these factors in combination with immune checkpoint blockers points to the future direction of cancer immunotherapy.

The role of non-coding genome in the behavior of infiltrated myeloid-derived suppressor cells in tumor microenvironment; a perspective and state-of-the-art in cancer targeted therapy

Cancer is one of the healthcare problems that affect many communities around the world. Many factors contribute to cancer development. Besides, these factors are counted as the main impediment in cancer immunotherapy. Myeloid-derived suppressor cells (MDSCs) are one of these impediments. MDSCs inhibit the immune responses through various mechanisms such as inhibitory cytokine release and nitric oxide metabolite production. Several factors are involved in forming these cells, including tumor secreted cytokine and chemokines, transcription factors, and non-coding RNA. In the meantime, micro-RNAs (miRNAs) and long non-coding RNAs (lncRNAs) are the vital gene regulatory elements that affect gene expression. In this study, we are going to discuss the role of miRNAs and lncRNAs in MDSCs development in a cancer situation. It is hoped that miRNA and lncRNAs targeting may prevent the growth and development of these inhibitory cells in the cancer environment.

Long noncoding RNA: a dazzling dancer in tumor immune microenvironment

Long noncoding RNAs (lncRNAs) are a class of endogenous, non-protein coding RNAs that are highly linked to various cellular functions and pathological process. Emerging evidence indicates that lncRNAs participate in crosstalk between tumor and stroma, and reprogramming of tumor immune microenvironment (TIME). TIME possesses distinct populations of myeloid cells and lymphocytes to influence the immune escape of cancer, the response to immunotherapy, and the survival of patients. However, hitherto, a comprehensive review aiming at relationship between lncRNAs and TIME is missing. In this review, we focus on the functional roles and molecular mechanisms of lncRNAs within the TIME. Furthermore, we discussed the potential immunotherapeutic strategies based on lncRNAs and their limitations.

The Unfolded Protein Response Mediator PERK Governs Myeloid Cell-Driven Immunosuppression in Tumors through Inhibition of STING Signaling

The primary mechanisms supporting immunoregulatory polarization of myeloid cells upon infiltration into tumors remain largely unexplored. Elucidation of these signals could enable better strategies to restore protective anti-tumor immunity. Here, we investigated the role of the intrinsic activation of the PKR-like endoplasmic reticulum (ER) kinase (PERK) in the immunoinhibitory actions of tumor-associated myeloid-derived suppressor cells (tumor-MDSCs). PERK signaling increased in tumor-MDSCs, and its deletion transformed MDSCs into myeloid cells that activated CD8⁺ T cell-mediated immunity against cancer. Tumor-MDSCs lacking PERK exhibited disrupted NRF2-driven antioxidant capacity and impaired mitochondrial respiratory homeostasis. Moreover, reduced NRF2 signaling in PERK-deficient MDSCs elicited cytosolic mitochondrial DNA elevation and, consequently, STING-dependent expression of anti-tumor type I interferon. Reactivation of NRF2 signaling, conditional deletion of STING, or blockade of type I interferon receptor I restored the immunoinhibitory potential of PERK-ablated MDSCs. Our findings demonstrate the pivotal role of PERK in tumor-MDSC functionality and unveil strategies to reprogram immunosuppressive myelopoiesis in tumors to boost cancer immunotherapy.

Transmissible Endoplasmic Reticulum Stress: A Novel Perspective on Tumor Immunity

As the first compartment of the protein secretory pathway, the endoplasmic reticulum (ER) acts as a protein synthesis factory, maintaining proteostasis and ER homeostasis. However, a variety of intrinsic and extrinsic perturbations, such as cancer, can disrupt the homeostasis and result in a large accumulation of misfolded/unfolded proteins in the ER lumen, thereby provoking a specific cellular state addressed as “ER stress”. Then the unfolded protein response (UPR), an adaptive signaling pathway, is triggered to address the stress and restore the homeostasis. A novel aspect of ER stress is that it can be transmitted from cancer cells to tumor-infiltrating myeloid cells through certain cancer cell-released soluble factors, which is termed as transmissible ER stress (TERS) or ER stress resonance (ERSR). In this review, we provide a comprehensive overview of the link between cancer and ER stress as well as the possible soluble factors mediating TERS. We further elaborate the cell-extrinsic effects of TERS on tumor immunity, and how it indirectly modulates cancer development and progression, which is expected to add a new dimension to anticancer therapy.

Reducing Stress PERKs up Anti-tumor Immunity

Myeloid-derived suppressor cells (MDSCs) can promote tumor progression. In this issue of Immunity, Mohamed et al. show that the unfolded protein response sensor, PERK, enhances MDSC-mediated immunosuppression through the NRF2 transcription factor, preventing oxidative damage, mitochondrial DNA release, and DNA sensor-STING-dependent type I interferon production.

Stress relief for cancer immunotherapy: implications for the ER stress response in tumor immunity

The solid tumor microenvironment is replete with factors that present a stress to infiltrating immune cells. Endoplasmic reticulum (ER) stress sensor PKR-like ER kinase (PERK) is primed to sense and respond to the burden of misfolded proteins in the ER lumen induced by cell stressors. PERK has documented roles as a master regulator of acute and chronic responses to cell stress as well as in the regulation of cell metabolism. Here, we provide an overview of the roles of PERK based on what is known and remains to be tested in immune cells in tumors and impacts on tumor control. PERK is one of several ER kinases able to preferentially induce activating transcription factor 4 (ATF4) as a response to cell stress. ATF4 orchestrates the oxidative stress response and governs amino acid metabolism. We discuss the tested role of ATF4 in tumor immunity and provide insight on the dueling protective and deleterious roles that ATF4 may play in the stress of solid tumors.

MicroRNAs and lncRNAs-A New Layer of Myeloid-Derived Suppressor Cells Regulation

Myeloid-derived suppressor cells (MDSCs) constitute an important component in regulating immune responses in several abnormal physiological conditions such as cancer. Recently, novel regulatory tumor MDSC biology modulating mechanisms, including differentiation, expansion and function, were defined. There is growing evidence that miRNAs and long non-coding RNAs (lncRNA) are involved in modulating transcriptional factors to become complex regulatory networks that regulate the MDSCs in the tumor microenvironment. It is possible that aberrant expression of miRNAs and lncRNA contributes to MDSC biological characteristics under pathophysiological conditions. This review provides an overview on miRNAs and lncRNAs epiregulation of MDSCs development and immunosuppressive functions in cancer.

Regulation of cellular immunity by activating transcription factor 4

Activating transcription factor 4 (ATF4) is a DNA binding transcription factor belonging to the family of basic Leucine zipper proteins. ATF4 can be activated in response to multiple cellular stress signals including endoplasmic reticulum stress in the event of improper protein folding or oxidative stress because of mitochondrial dysfunction as well as hypoxia. There are multiple downstream targets of ATF4 that can coordinate the regulation between survival and apoptosis of a cell based on time and exposure to stress. ATF4, therefore, has a broad range of control that results in the modulation of immune cells of the innate and adaptive responses leading to regulation of the cellular immunity. Studies provide evidence that ATF4 can regulate immune cells such as macrophages, T cells, B cells, NK cells and dendritic cells contributing to progression of disease. Immune cells can be exposed to stressed environment in the event of a pathogen attack, infection, inflammation, or in the tumor microenvironment leading to increased ATF4 activity to regulate these responses. ATF4 can further control differentiation and maturation of different immune cell types becoming a determinant of effective immune regulation. Additionally, ATF4 has been heavily implicated in rendering effector immune cells dysfunctional that are used to target tumorigenesis. Therefore, there is a need to evaluate where the literature stands in understanding the overall role of ATF4 in regulating cellular immunity to identify therapeutic targets and generalized mechanisms for different disease progressions.

TIPE2 specifies the functional polarization of myeloid-derived suppressor cells during tumorigenesis

Myeloid-derived suppressor cells (MDSCs) are "polarized" myeloid cells that effectively promote tumorigenesis by inhibiting antitumor immunity. How myeloid cells acquire the protumoral properties during tumorigenesis is poorly understood. We report here that the polarity protein TIPE2 (tumor necrosis factor-α-induced protein 8-like 2) mediates the functional polarization of murine and human MDSCs by specifying their pro- and antitumoral properties. Tumor cells induced the expression of TIPE2 in Gr1+CD11b+ cells through reactive oxygen species (ROS). TIPE2 in turn increased the expression of protumoral mediators such as CCAAT/enhancer-binding protein-β while inhibiting the expression of antitumoral mediators. Consequently, tumor growth in TIPE2-deficient mice was significantly diminished, and TIPE2-deficient MDSCs markedly inhibited tumor growth upon adoptive transfer. Pharmaceutical blockade of ROS inhibited TIPE2 expression in MDSCs and reduced tumor growth in mice. These findings indicate that TIPE2 plays a key role in the functional polarization of MDSCs and represents a new therapeutic target for cancer immunotherapy.

Endoplasmic Reticulum Stress in Bone Metastases

Metastases-the spreading of cancer cells from primary tumors to distant organs, including bone-is often incurable and is the major cause of morbidity in cancer patients. Understanding how cancer cells acquire the ability to colonize to bone and become overt metastases is critical to identify new therapeutic targets and develop new therapies against bone metastases. Recent reports indicate that the endoplasmic reticulum (ER) stress and, as its consequence, the unfolded protein response (UPR) is activated during metastatic dissemination. However, their roles in this process remain largely unknown. In this review, we discuss the recent progress on evaluating the tumorigenic, immunoregulatory and metastatic effects of ER stress and the UPR on bone metastases. We explore new opportunities to translate this knowledge into potential therapeutic strategies for patients with bone metastases.

Myeloid-Derived Suppressor Cells in Tumors: From Mechanisms to Antigen Specificity and Microenvironmental Regulation

Among the various immunological and non-immunological tumor-promoting activities of myeloid-derived suppressor cells (MDSCs), their immunosuppressive capacity remains a key hallmark. Effort in the past decade has provided us with a clearer view of the suppressive nature of MDSCs. More suppressive pathways have been identified, and their recognized targets have been expanded from T cells and natural killer (NK) cells to other immune cells. These novel mechanisms and targets afford MDSCs versatility in suppressing both innate and adaptive immunity. On the other hand, a better understanding of the regulation of their development and function has been unveiled. This intricate regulatory network, consisting of tumor cells, stromal cells, soluble mediators, and hostile physical conditions, reveals bi-directional crosstalk between MDSCs and the tumor microenvironment. In this article, we will review available information on how MDSCs exert their immunosuppressive function and how they are regulated in the tumor milieu. As MDSCs are a well-established obstacle to anti-tumor immunity, new insights in the potential synergistic combination of MDSC-targeted therapy and immunotherapy will be discussed.

Unraveling the regulatory role of endoplasmic-reticulum-associated degradation in tumor immunity

During malignant transformation and cancer progression, tumor cells face both intrinsic and extrinsic stress, endoplasmic reticulum (ER) stress in particular. To survive and proliferate, tumor cells use multiple stress response pathways to mitigate ER stress, promoting disease aggression and treatment resistance. Among the stress response pathways is ER-associated degradation (ERAD), which consists of multiple components and steps working together to ensure protein quality and quantity. In addition to its established role in stress responses and tumor cell survival, ERAD has recently been shown to regulate tumor immunity. Here we summarize current knowledge on how ERAD promotes protein degradation, regulates immune cell development and function, participates in antigen presentation, exerts paradoxical roles on tumorigenesis and immunity, and thus impacts current cancer therapy. Collectively, ERAD is a critical protein homeostasis pathway intertwined with cancer development and tumor immunity. Of particular importance is the need to further unveil ERAD's enigmatic roles in tumor immunity to develop effective targeted and combination therapy for successful treatment of cancer.

Reactive Myelopoiesis Triggered by Lymphodepleting Chemotherapy Limits the Efficacy of Adoptive T Cell Therapy

Adoptive T cell therapy (ACT) in combination with lymphodepleting chemotherapy is an effective strategy to induce the eradication of tumors, providing long-term regression in cancer patients. Despite that lymphodepleting regimens condition the host for optimal engraftment and expansion of adoptively transferred T cells, lymphodepletion concomitantly promotes immunosuppression during the course of endogenous immune recovery. In this study, we have identified that lymphodepleting chemotherapy initiates the mobilization of hematopoietic progenitor cells that differentiate to immunosuppressive myeloid cells, leading to a dramatic increase of peripheral myeloid-derived suppressor cells (MDSCs). In melanoma and lung cancer patients, MDSCs rapidly expanded in the periphery within 1 week after completion of a lymphodepleting regimen and infusion of autologous tumor-infiltrating lymphocytes (TILs). This expansion was associated with disease progression, poor survival, and reduced TIL persistence in melanoma patients. We demonstrated that the interleukin 6 (IL-6)-driven differentiation of mobilized hematopoietic progenitor cells promoted the survival and immunosuppressive capacity of post-lymphodepletion MDSCs. Furthermore, the genetic abrogation or therapeutic inhibition of IL-6 in mouse models enhanced host survival and reduced tumor growth in mice that received ACT. Thus, the expansion of MDSCs in response to lymphodepleting chemotherapy may contribute to ACT failure, and targeting myeloid-mediated immunosuppression may support anti-tumor immune responses.

Noncoding RNAs: the shot callers in tumor immune escape

Immunotherapy, designed to exploit the functions of the host immune system against tumors, has shown considerable potential against several malignancies. However, the utility of immunotherapy is heavily limited due to the low response rate and various side effects in the clinical setting. Immune escape of tumor cells may be a critical reason for such low response rates. Noncoding RNAs (ncRNAs) have been identified as key regulatory factors in tumors and the immune system. Consequently, ncRNAs show promise as targets to improve the efficacy of immunotherapy in tumors. However, the relationship between ncRNAs and tumor immune escape (TIE) has not yet been comprehensively summarized. In this review, we provide a detailed account of the current knowledge on ncRNAs associated with TIE and their potential roles in tumor growth and survival mechanisms. This review bridges the gap between ncRNAs and TIE and broadens our understanding of their relationship, providing new insights and strategies to improve immunotherapy response rates by specifically targeting the ncRNAs involved in TIE.

The "Janus" Role of C/EBPs Family Members in Cancer Progression

CCAAT/enhancer-binding proteins (C/EBPs) constitute a family of transcription factors composed of six members that are critical for normal cellular differentiation in a variety of tissues. They promote the expression of genes through interaction with their promoters. Moreover, they have a key role in regulating cellular proliferation through interaction with cell cycle proteins. C/EBPs are considered to be tumor suppressor factors due to their ability to arrest cell growth (contributing to the terminal differentiation of several cell types) and for their role in cellular response to DNA damage, nutrient deprivation, hypoxia, and genotoxic agents. However, C/EBPs can elicit completely opposite effects on cell proliferation and cancer development and they have been described as both tumor promoters and tumor suppressors. This "Janus" role of C/EBPs depends on different factors, such as the type of tumor, the isoform/s expressed in cells, the type of dimerization (homo- or heterodimerization), the presence of inhibitory elements, and the ability to inhibit the expression of other tumor suppressors. In this review, we discuss the implication of the C/EBPs family in cancer, focusing on the molecular aspects that make these transcription factors tumor promoters or tumor suppressors.

Myeloid Cell-Derived Arginase in Cancer Immune Response

Amino acid metabolism is a critical regulator of the immune response, and its modulating becomes a promising approach in various forms of immunotherapy. Insufficient concentrations of essential amino acids restrict T-cells activation and proliferation. However, only arginases, that degrade L-arginine, as well as enzymes that hydrolyze L-tryptophan are substantially increased in cancer. Two arginase isoforms, ARG1 and ARG2, have been found to be present in tumors and their increased activity usually correlates with more advanced disease and worse clinical prognosis. Nearly all types of myeloid cells were reported to produce arginases and the increased numbers of various populations of myeloid-derived suppressor cells and macrophages correlate with inferior clinical outcomes of cancer patients. Here, we describe the role of arginases produced by myeloid cells in regulating various populations of immune cells, discuss molecular mechanisms of immunoregulatory processes involving L-arginine metabolism and outline therapeutic approaches to mitigate the negative effects of arginases on antitumor immune response. Development of potent arginase inhibitors, with improved pharmacokinetic properties, may lead to the elaboration of novel therapeutic strategies based on targeting immunoregulatory pathways controlled by L-arginine degradation.

Metabolic Regulation of Myeloid-Derived Suppressor Cell Function in Cancer

Myeloid-derived suppressor cells (MDSCs) are a group of immunosuppressive cells that play crucial roles in promoting tumor growth and protecting tumors from immune recognition in tumor-bearing mice and cancer patients. Recently, it has been shown that the metabolic activity of MDSCs plays an important role in the regulation of their inhibitory function, especially in the processes of tumor occurrence and development. The MDSC metabolism, such as glycolysis, fatty acid oxidation and amino acid metabolism, is rewired in the tumor microenvironment (TME), which enhances the immunosuppressive activity, resulting in effector T cell apoptosis and suppressive cell proliferation. Herein, we summarized the recent progress in the metabolic reprogramming and immunosuppressive function of MDSCs during tumorigenesis.

ER stress activates immunosuppressive network: implications for aging and Alzheimer's disease

The endoplasmic reticulum (ER) contains stress sensors which recognize the accumulation of unfolded proteins within the lumen of ER, and subsequently these transducers stimulate the unfolded protein response (UPR). The ER sensors include the IRE1, PERK, and ATF6 transducers which activate the UPR in an attempt to restore the quality of protein folding and thus maintain cellular homeostasis. If there is excessive stress, UPR signaling generates alarmins, e.g., chemokines and cytokines, which activate not only tissue-resident immune cells but also recruit myeloid and lymphoid cells into the affected tissues. ER stress is a crucial inducer of inflammation in many pathological conditions. A chronic low-grade inflammation and cellular senescence have been associated with the aging process and many age-related diseases, such as Alzheimer’s disease. Currently, it is known that immune cells can exhibit great plasticity, i.e., they are able to display both pro-inflammatory and anti-inflammatory phenotypes in a context-dependent manner. The microenvironment encountered in chronic inflammatory conditions triggers a compensatory immunosuppression which defends tissues from excessive inflammation. Recent studies have revealed that chronic ER stress augments the suppressive phenotypes of immune cells, e.g., in tumors and other inflammatory disorders. The activation of immunosuppressive network, including myeloid-derived suppressor cells (MDSC) and regulatory T cells (Treg), has been involved in the aging process and Alzheimer’s disease. We will examine in detail whether the ER stress-related changes found in aging tissues and Alzheimer’s disease are associated with the activation of immunosuppressive network, as has been observed in tumors and many chronic inflammatory diseases.