Myeloid-derived suppressor cells promote B-cell production of IgA in a TNFR2-dependent manner

The Role of Myeloid-Derived Suppressor Cells in Multiple Sclerosis and Its Animal Model

Myeloid-derived suppressor cells (MDSCs), a heterogeneous cell population that consists of mostly immature myeloid cells, are immunoregulatory cells mainly characterized by their suppressive functions. Emerging findings have revealed the involvement of MDSCs in multiple sclerosis (MS) and its animal model experimental autoimmune encephalomyelitis (EAE). MS is an autoimmune and degenerative disease of the central nervous system characterized by demyelination, axon loss, and inflammation. Studies have reported accumulation of MDSCs in inflamed tissues and lymphoid organs of MS patients and EAE mice, and these cells display dual functions in EAE. However, the contribution of MDSCs to MS/EAE pathogenesis remains unclear. This review aims to summarize our current understanding of MDSC subsets and their possible roles in MS/EAE pathogenesis. We also discuss the potential utility and associated obstacles in employing MDSCs as biomarkers and cell-based therapies for MS.

Myeloid-derived suppressor cells from tumour-bearing mice induce the population expansion of CD19hiFcγRIIbhi regulatory B cells via PD-L1

Myeloid-derived suppressor cells (MDSCs) increase in number and gain immunosuppressive functions in tumours and many other pathological conditions. MDSCs are characterized by their strong T-cell immunosuppressive capacity. The effects that MDSCs may have on B cells, especially within the tumour microenvironment, are less well understood. Here, we report that either monocytic MDSCs or polymorphonuclear MDSCs can promote increases in interleukin (IL)-10-expressing CD19hiFcγRIIbhi regulatory B cells in vitro and in vivo. Splenic transitional-1, -2, and -3 cells and marginal zone B cells, but not follicular B cells, differentiate into IL-10-expressing CD19hiFcγRIIbhi regulatory B cells. The adoptive transfer of CD19hiFcγRIIbhi regulatory B cells via tail vein injection can promote subcutaneous 3LL tumour growth in mice. The expression of programmed death-ligand 1 on MDSCs was found to be strongly associated with CD19hiFcγRIIbhi regulatory B cell population expansion. Furthermore, the frequency of circulating CD19+FcγRIIhi regulatory B cells was significantly increased in advanced-stage lung cancer patients. Our results unveil a critical role of MDSCs in regulatory B-cell differentiation and population expansion in lung cancer patients.

Combinational delivery of TLR4 and TLR7/8 agonist enhanced the therapeutic efficacy of immune checkpoint inhibitors to colon tumor

Immunotherapy is regarded as a potent cancer treatment, with DC vaccines playing a crucial role. Although clinical trials have demonstrated the safety and efficacy of DC vaccines, loading antigens in vitro is challenging, and their therapeutic effects remain unpredictable. Moreover, the diverse subtypes and maturity states of DCs in the body could induce both immune responses and immune tolerance, potentially affecting the vaccine's efficacy. Hence, the optimization of DC vaccines remains imperative. Our study discovered a new therapeutic strategy by using CT26 and MC38 mouse colon cancer models, as well as LLC mouse lung cancer models. The strategy involved the synergistic activation of DCs through intertumoral administration of TLR4 agonist high-mobility group nucleosome binding protein 1 (HMGN1) and TLR7/8 agonist (R848/resiquimod), combined with intraperitoneal administration of TNFR2 immunosuppressant antibody. The experimental results indicated that the combined use of HMGN1, R848, and α-TNFR2 had no effect on LLC cold tumors. However, it was effective in eradicating CT26 and MC38 colon cancer and inducing long-term immune memory. The combination of these three drugs altered the TME and promoted an increase in anti-tumor immune components. This may provide a promising new treatment strategy for colon cancer.

Modulation of T-cell function by myeloid-derived suppressor cells in hematological malignancies

Myeloid-derived suppressor cells (MDSCs) are pathologically activated neutrophils and monocytes that negatively regulate the immune response to cancer and chronic infections. Abnormal myelopoiesis and pathological activation of myeloid cells generate this heterogeneous population of myeloid-derived suppressor cells. They are characterized by their distinct transcription, phenotypic, biochemical, and functional features. In the tumor microenvironment (TME), myeloid-derived suppressor cells represent an important class of immunosuppressive cells that correlate with tumor burden, stage, and a poor prognosis. Myeloid-derived suppressor cells exert a strong immunosuppressive effect on T-cells (and a broad range of other immune cells), by blocking lymphocyte homing, increasing production of reactive oxygen and nitrogen species, promoting secretion of various cytokines, chemokines, and immune regulatory molecules, stimulation of other immunosuppressive cells, depletion of various metabolites, and upregulation of immune checkpoint molecules. Additionally, the heterogeneity of myeloid-derived suppressor cells in cancer makes their identification challenging. Overall, they serve as a major obstacle for many cancer immunotherapies and targeting them could be a favorable strategy to improve the effectiveness of immunotherapeutic interventions. However, in hematological malignancies, particularly B-cell malignancies, the clinical outcomes of targeting these myeloid-derived suppressor cells is a field that is still to be explored. This review summarizes the complex biology of myeloid-derived suppressor cells with an emphasis on the immunosuppressive pathways used by myeloid-derived suppressor cells to modulate T-cell function in hematological malignancies. In addition, we describe the challenges, therapeutic strategies, and clinical relevance of targeting myeloid-derived suppressor cells in these diseases.

Myeloid-derived suppressor cells cross-talk with B10 cells by BAFF/BAFF-R pathway to promote immunosuppression in cervical cancer

Immunosuppression induced by myeloid-derived suppressor cells (MDSCs) is one of the main obstacles to the efficacy of immunotherapy for cervical cancer. Recent studies on the immunosuppressive ability of MDSCs have primarily focused on T cells, but the effect of MDSCs on B cells function is still unclear. In a study of clinical specimens, we found that the accumulation of MDSCs in patients with cervical cancer was accompanied by high expression of B cell activating factor (BAFF) on the surface and high expression of interleukin (IL)-10-producing B cells (B10) in vivo. We found that the absence of BAFF could significantly inhibit tumor growth in a cervical cancer model using BAFF KO mice. Further studies showed that abundant MDSCs in cervical cancer induced B cells to differentiate into B10 cells by regulating BAFF which acted on the BAFF receptor (BAFF-R) of them. In this process, we found that a large amount of IL-10 secreted by B10 cells can activate STAT3 signaling pathway in MDSCs, and then form a positive feedback loop to promote the differentiation of B10 cells. Therefore, this study reveals a new mechanism of BAFF-mediated mutual immune regulation between MDSCs and B cells in the occurrence and development of cervical cancer.

The Function and Therapeutic Implications of TNF Signaling in MDSCs

Myeloid-derived suppressor cells (MDSCs) are a group of immature and heterogeneous myeloid cells with immunosuppressive functions. MDSCs play important roles in the pathogenesis of cancer, chronic inflammatory diseases, and many autoimmune disorders. The accumulation and activation of MDSCs can be regulated by tumor necrosis factor α (TNF-α). In this review, we summarize the roles played by TNF-α in the recruitment, immunosuppressive functions, and chemotaxis of MDSCs, and discuss the potential therapeutic effects of TNF-α upon these cells in tumor growth and some inflammatory disorders.

Conditioned media of pancreatic cancer cells and pancreatic stellate cells induce myeloid-derived suppressor cells differentiation and lymphocytes suppression

As pancreatic cancer cells (PCCs) and pancreatic stellate cells (PSCs) are the two major cell types that comprise the immunosuppressive tumor microenvironment of pancreatic cancer, we aimed to investigate the role of conditioned medium derived from PCCs and PSCs co-culture on the viability of lymphocytes. The conditioned medium (CM) collected from PCCs and/or PSCs was used to treat peripheral blood mononuclear cells (PBMCs) to determine CM ability in reducing lymphocytes population. A proteomic analysis has been done on the CM to investigate the differentially expressed protein (DEP) expressed by two PCC lines established from different stages of tumor. Subsequently, we investigated if the reduction of lymphocytes was directly caused by CM or indirectly via CM-induced MDSCs. This was achieved by isolating lymphocyte subtypes and treating them with CM and CM-induced MDSCs. Both PCCs and PSCs were important in suppressing lymphocytes, and the PCCs derived from a metastatic tumor appeared to have a stronger suppressive effect than the PCCs derived from a primary tumor. According to the proteomic profiles of CM, 416 secreted proteins were detected, and 13 DEPs were identified between PANC10.05 and SW1990. However, CM was found unable to reduce lymphocytes viability through a direct pathway. In contrast, CM that contains proteins secreted by PCC and/or PSC appear immunogenic as they increase the viability of lymphocytes subtypes. Lymphocyte subtype treated with CM-induced MDSCs showed reduced viability in T helper 1 (Th1), T helper 2 (Th2), and T regulatory (Treg) cells, but not in CD8+ T cells, and B cells. As a conclusion, the interplay between PCCs and PSCs is important as their co-culture displays a different trend in lymphocytes suppression, hence, their co-culture should be included in future studies to better mimic the tumor microenvironment.

TFNR2 in Ischemia-Reperfusion Injury, Rejection, and Tolerance in Transplantation

Tumor necrosis factor receptor 2 (TNFR2) has been shown to play a crucial role in CD4+ T regulatory cells (CD4+Tregs) expansion and suppressive function. Increasing evidence has also demonstrated its role in a variety of immune regulatory cell subtypes such as CD8+ T regulatory cells (CD8+ Tregs), B regulatory cells (Bregs), and myeloid-derived suppressor cells (MDSCs). In solid organ transplantation, regulatory immune cells have been associated with decreased ischemia-reperfusion injury (IRI), improved graft survival, and improved overall outcomes. However, despite TNFR2 being studied in the context of autoimmune diseases, cancer, and hematopoietic stem cell transplantation, there remains paucity of data in the context of solid organ transplantation and islet cell transplantation. Interestingly, TNFR2 signaling has found a clinical application in islet transplantation which could guide its wider use. This article reviews the current literature on TNFR2 expression in immune modulatory cells as well as IRI, cell, and solid organ transplantation. Our results highlighted the positive impact of TNFR2 signaling especially in kidney and islet transplantation. However, further investigation of TNFR2 in all types of solid organ transplantation are required as well as dedicated studies on its therapeutic use during induction therapy or treatment of rejection.

The Role of Myeloid-Derived Suppressor Cells in Tumor Growth and Metastasis

Myeloid-derived suppressor cells (MDSCs) are immature bone marrow-derived suppressive cells that are an important component of the pathological immune response associated with cancer. Expansion of MDSCs has been linked to poor disease outcome and therapeutic resistance in patients with various malignancies, making these cells potential targets for next-generation treatment strategies. MDSCs are classified into monocytic (M-MDSC) and polymorphonuclear/granulocytic (PMN-MDSC) subtypes that undertake distinct and numerous roles in the tumor microenvironment or systemically to drive disease progression. In this chapter, we will discuss how MDSC subsets contribute to the growth of primary tumors and induce metastatic spread by suppressing the antitumor immune response, supporting cancer stem cell (CSC)/epithelial-to-mesenchymal transition (EMT) phenotypes and promoting angiogenesis. We will also summarize the signaling networks involved in the crosstalk between cancer cells and MDSCs that could represent putative immunotherapy targets.

Role of fibrosarcoma-induced CD11b+ myeloid cells and tumor necrosis factor-α in B cell responses

The role of B cells in the anti-tumor immune response remains controversial. An increase in the number of B cells in the peripheral blood of some tumor patients has been associated with poor immunotherapy efficacy. However, the mechanism leading to the generation of these cells is not well-described. Using a fibrosarcoma model, we show that intraperitoneal administration of a xenogeneic antigen in tumor-bearing mice evokes large increases in antigen-specific serum immunoglobulin formation compared to tumor-naïve mice. An inability of tumor-bearing mice to induce enhanced antibody production after myeloid cell depletion suggests the antibody responses are CD11b⁺ myeloid cell-dependent. In vitro, CD11b⁺ myeloid cells promoted B cell proliferation, activation, and survival. High levels of tumor necrosis factor (TNF)-α were produced by CD11b⁺ cells, and TNF-α blockade inhibited B cell responses. CD11b⁺ cells appear to be important promoters of B cell responses and targeting B cells may increase the efficacy of immunotherapy in tumor-bearing hosts.

Loss-of-function of the hippo transducer TAZ reduces mammary tumor growth through a myeloid-derived suppressor cell-dependent mechanism

TAZ, one of the key effectors in the Hippo pathway, is often dysregulated in breast cancer, leading to cancer stemness, survival, and metastasis. However, the mechanistic bases of these tumor outcomes are incompletely understood and even less is known about the potential role played by the non-malignant cellular constituents of the tumor microenvironment (TME). Here, we revealed an inverse correlation between TAZ expression and survival in triple-negative breast cancer (TNBC), but not other subtypes of breast cancer. We found that TAZ knockdown in two murine TNBC tumor cell line models significantly inhibited tumor growth and metastasis in immune competent but not immune deficient hosts. RNA-seq analyses identified substantial alterations in immune components in TAZ knockdown tumors. Using mass cytometry analysis, we found that TAZ-deficiency altered the immune landscape of the TME leading to significant reductions in immune suppressive populations, namely myeloid-derived suppressor cells (MDSCs) and macrophages accompanied by elevated CD8⁺ T cell/myeloid cell ratios. Mechanistic studies demonstrated that TAZ-mediated tumor growth was MDSC-dependent in that MDSC depletion led to reduced tumor growth in control, but not TAZ-knockdown tumor cells. Altogether, we identified a novel non-cancer cell-autonomous mechanism by which tumor-intrinsic TAZ expression aids tumor progression. Thus, our findings advance an understanding of the crosstalk between tumor-derived TAZ expression and the immune contexture within the TME, which may lead to new therapeutic interventions for TNBC or other TAZ-driven cancers.

The dual role of neutrophils in cancer

For the past decade, the role and importance of neutrophils in cancer is being increasingly appreciated. Research has focused on the ability of cancer-related neutrophils to either support tumor growth or interfere with it, showing diverse mechanisms through which the effects of neutrophils take place. In contrast to the historic view of neutrophils as terminally differentiated cells, mounting evidence has demonstrated that neutrophils are a plastic and diverse population of cells. These dynamic and plastic abilities allow them to perform varied and sometimes opposite functions simultaneously. In this review, we summarize and detail clinical and experimental evidence for, and underlying mechanisms of, the dual impact of neutrophils' functions, both supporting and inhibiting cancer development. We first discuss the effects of various basic functions of neutrophils, namely direct cytotoxicity, secretion of reactive oxygen species (ROS), nitric oxide (NO) and proteases, NETosis, autophagy and modulation of other immune cells, on tumor growth and metastatic progression. We then describe the clinical evidence for pro- vs anti-tumor functions of neutrophils in human cancer. We believe and show that the "net" impact of neutrophils in cancer is the sum of a complex balance between contradicting effects which occur simultaneously.

Pro- and Anti- Effects of Immunoglobulin A- Producing B Cell in Tumors and Its Triggers

B cells are well known as key mediators of humoral immune responses via the production of antibodies. Immunoglobulin A (IgA) is the most abundantly produced antibody isotype and provides the first line of immune protection at mucosal surfaces. However, IgA has long been a divisive molecule with respect to tumor progression. IgA exerts anti- or pro-tumor effect in different tumor types. In this review, we summarize emerging evidence regarding the production and effects of IgA and IgA⁺ cells in the tumor microenvironment (TME). Moreover, we discuss that the TME cytokines, host diet, microbiome, and metabolites play a pivotal role in controlling the class-switch recombination (CSR) of IgA. The analysis of intratumoral Ig repertoires and determination of metabolites that influence CSR may help establish novel therapeutic targets for the treatment of cancers.

Integrin CD11b Negatively Regulates B Cell Receptor Signaling to Shape Humoral Response during Immunization and Autoimmunity

Our previous work has revealed the ability of CD11b to regulate BCR signaling and control autoimmune disease in mice. However, how CD11b regulates the immune response under normal conditions remains unknown. Through the use of a CD11b knockout model on a nonautoimmune background, we demonstrated that CD11b-deficient mice have an elevated Ag-specific humoral response on immunization. Deletion of CD11b resulted in elevated low-affinity and high-affinity IgG Ab and increases in Ag-specific germinal center B cells and plasma cells (PCs). Examination of BCR signaling in CD11b-deficient mice revealed defects in association of negative regulators pLyn and CD22 with the BCR, but increases in colocalizations between positive regulator pSyk and BCR after stimulation. Using a CD11b-reporter mouse model, we identified multiple novel CD11b-expressing B cell subsets that are dynamically altered during immunization. Subsequent experiments using a cell-specific CD11b deletion model revealed this effect to be B cell intrinsic and not altered by myeloid cell CD11b expression. Importantly, CD11b expression on PCs also impacts on BCR repertoire selection and diversity in autoimmunity. These studies describe a novel role for CD11b in regulation of the healthy humoral response and autoimmunity, and reveal previously unknown populations of CD11b-expressing B cell subsets, suggesting a complex function for CD11b in B cells during development and activation.

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.

Tumor-Associated Neutrophils Drive B-cell Recruitment and Their Differentiation to Plasma Cells

A major mechanism through which neutrophils have been suggested to modulate tumor progression involves the interaction and subsequent modulation of other infiltrating immune cells. B cells have been found to infiltrate various cancer types and play a role in tumor immunity, offering new immunotherapy opportunities. Nevertheless, the specific impact of tumor-associated neutrophils (TAN) on B cells has largely been overlooked. In the current study, we aimed to characterize the role of TANs in the recruitment and modulation of B cells in the tumor microenvironment (TME). We showed that TANs actively participate in the recruitment of B cells to the TME and identified TNFα as the major cytokine mediating B-cell chemotaxis by TANs. The recruitment of CD45⁺B220⁺CD138^- splenic B cells by TANs in vitro resulted in B-cell phenotypic modulation, with 68.6% ± 2.1% of the total migrated B cells displaying a CD45^-B220⁺CD138⁺ phenotype, which is typical for plasma cells. This phenotype mirrored the large proportion (54.0% ± 6.1%) of CD45^-B220⁺CD138⁺ intratumoral B cells (i.e., plasma cells) in Lewis lung carcinoma tumors. We next confirmed that the differentiation of CD45⁺B220⁺CD138^- B cells to functionally active CD45^-B220⁺CD138⁺ plasma cells required contact with TANs, was independent of T cells, and resulted in IgG production. We further identified membranal B-cell activating factor (BAFF) on TANs as a potential contact mechanism mediating B-cell differentiation, as blocking BAFF-receptor (BAFF-R) significantly reduced IgG production by 20%. Our study, therefore, demonstrates that TANs drive the recruitment and modulation of B cells into plasma cells in the TME, hence opening new avenues in the targeting of the immune system in cancer.

The diverse roles of myeloid derived suppressor cells in mucosal immunity

The mucosal immune system plays a vital role in protecting the host from the external environment. Its major challenge is to balance immune responses against harmful and harmless agents and serve as a 'homeostatic gate keeper'. Myeloid derived suppressor cells (MDSCs) are a heterogeneous population of undifferentiated cells that are characterized by an immunoregulatory and immunosuppressive phenotype. Herein we postulate that MDSCs may be involved in shaping immune responses related to mucosal immunity, due to their immunomodulatory and tissue remodeling functions. Until recently, MDSCs were investigated mainly in cancerous diseases, where they induce and contribute to an immunosuppressive and inflammatory environment that favors tumor development. However, it is now becoming clear that MDSCs participate in non-cancerous conditions such as chronic infections, autoimmune diseases, pregnancy, aging processes and immune tolerance to commensal microbiota at mucosal sites. Since MDSCs are found in the periphery only in small numbers under normal conditions, their role is highlighted during pathologies characterized by acute or chronic inflammation, when they accumulate and become activated. In this review, we describe several aspects of the current knowledge characterizing MDSCs and their involvement in the regulation of the mucosal epithelial barrier, their crosstalk with commensal microbiota and pathogenic microorganisms, and their complex interactions with a variety of surrounding regulatory and effector immune cells. Finally, we discuss the beneficial and harmful outcomes of the MDSC regulatory functions in diseases affecting mucosal tissues. We wish to illuminate the pivotal role of MDSCs in mucosal immunity, the limitations in our understanding of all the players and the intricate challenges stemming from the complex interactions of MDSCs with their environment.

An Inflammatory Loop Between Spleen-Derived Myeloid Cells and CD4+ T Cells Leads to Accumulation of Long-Lived Plasma Cells That Exacerbates Lupus Autoimmunity

Splenic long-lived plasma cells are abnormally numerous and deleterious in systemic autoimmune diseases, yet how they accumulate remains poorly understood. We demonstrate here that a pathological role of spleen-derived CD11b⁺Gr-1⁺ myeloid cells (SDMCs) underpins the accumulation of splenic long-lived plasma cells in a lupus-prone model named sanroque. We found that SDMCs were progressively accumulated in sanroque mice from the early clinical phase. Transcriptome profiles revealed that SDMCs have a predominant shift toward an inflammatory phenotype relative to the bone marrow-derived counterparts and are distinct from neutrophils and monocytes. SDMCs were expanded in situ via splenic extramedullary myelopoiesis under the proinflammatory cytokine milieu during lupus progression. SDMCs promoted the development of IFN-γ-secreting Th1 and follicular helper T cells, thereby licensing CD4⁺ T cells to be pathologic activators of SDMCs and plasma cells. SDMCs also directly promoted the survival of plasma cells by providing B-cell activating factor of the TNF family. The frequency of SDMCs correlated with that of splenic long-lived plasma cells. Selective depletion of CD11b⁺Gr-1⁺ cells reduced autoantibody production in sanroque mice. Thus, our findings suggest that SDMCs expanded in situ establish a positive feedback loop with CD4⁺ T cells, leading to accumulation of long-lived plasma cells which exacerbates lupus autoimmunity.

The neglected brothers come of age: B cells and cancer

The opposing roles of innate and adaptive immune cells in suppressing or supporting cancer initiation, progression, metastasis and response to therapy has been long debated. The mechanisms by which different monocyte and T cell subtypes affect and modulate cancer have been extensively studied. However, the role of B cells and their subtypes have remained elusive, perhaps partially due to their heterogeneity and range of actions. B cells can produce a variety of cytokines and present tumor-derived antigens to T cells in combination with co-stimulatory or inhibitory ligands based on their phenotype. Unlike most T cells, B cells can be activated by innate immune stimuli, such as endotoxin. Furthermore, the isotype and specificity of the antibodies produced by plasma cells regulate distinct immune responses, including opsonization, antibody-mediated cellular cytotoxicity (ADCC) and complement activation. B cells are shaped by the tumor environment (TME), with the capability to regulate the TME in return. In this review, we will describe the mechanisms of B cell action, including cytokine production, antigen presentation, ADCC, opsonization, complement activation and how they affect tumor development and response to immunotherapy. We will also discuss how B cell fate within the TME is affected by tumor stroma, microbiome and metabolism.

Use of Cocultures for the Study of Cellular Interactions Influencing B-Cell Regulatory Functions

Although IL-10-producing B cells have been shown to play key roles in regulating immune responses involved in autoimmunity, inflammation, and cancer, the mechanisms at the base of the generation and maintenance of the pool of regulatory B cells are still poorly characterized. Several evidences show that the cross talk between B cells and other immune cell types promotes IL-10 production by B lymphocytes. Soluble mediators released into the microenvironment, together with direct cell-cell contact, are key signals in the process of regulatory B-cell development and differentiation. Here we describe the methods required to follow IL-10-producing B cells in MC- and MDSC-B-cell cocultures as examples of in vitro systems that induce the expansion of the regulatory B-cell population. These protocols can be also adapted for the study of other immune cell systems.

Targeting Myeloid-Derived Suppressor Cells in Cancer Immunotherapy

Simple Summary

Myeloid-Derived Suppressor Cells (MDSCs) have been regarded as the main promoters of cancer development in recent years. They can protect tumor cells from being eliminated by neutralizing the anti-tumor response mediated by T cells, macrophages and dendritic cells (DCs). Therefore, different treatment methods targeting MDSCs, including chemotherapy, radiotherapy and immunotherapy, have been developed and proven to effectively inhibit tumor expansion. Herein, we summarize the immunosuppressive role of MDSCs in the tumor microenvironment and some effective treatments targeting MDSCs, and discuss the differences between different therapies.

Abstract

Myeloid-derived suppressor cells (MDSCs), which are activated under pathological conditions, are a group of heterogeneous immature myeloid cells. MDSCs have potent capacities to support tumor growth via inhibition of the antitumoral immune response and/or the induction of immunosuppressive cells. In addition, multiple studies have demonstrated that MDSCs provide potential therapeutic targets for the elimination of immunosuppressive functions and the inhibition of tumor growth. The combination of targeting MDSCs and other therapeutic approaches has also demonstrated powerful antitumor effects. In this review, we summarize the characteristics of MDSCs in the tumor microenvironment (TME) and current strategies of cancer treatment by targeting MDSCs.

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.

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

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

Myeloid-derived suppressor cells control B cell accumulation in the central nervous system during autoimmunity

Polymorphonuclear myeloid-derived suppressor cells (PMN-MDSCs) have been characterized in the context of malignancies. Here we show that PMN-MDSCs can restrain B cell accumulation during central nervous system (CNS) autoimmunity. Ly6G⁺ cells were recruited to the CNS during experimental autoimmune encephalomyelitis (EAE), interacted with B cells that produced the cytokines GM-CSF and interleukin-6 (IL-6), and acquired properties of PMN-MDSCs in the CNS in a manner dependent on the signal transducer STAT3. Depletion of Ly6G⁺ cells or dysfunction of Ly6G⁺ cells through conditional ablation of STAT3 led to the selective accumulation of GM-CSF-producing B cells in the CNS compartment, which in turn promoted an activated microglial phenotype and lack of recovery from EAE. The frequency of CD138⁺ B cells in the cerebrospinal fluid (CSF) of human subjects with multiple sclerosis was negatively correlated with the frequency of PMN-MDSCs in the CSF. Thus PMN-MDSCs might selectively control the accumulation and cytokine secretion of B cells in the inflamed CNS.

mTOR-mediated glycolysis contributes to the enhanced suppressive function of murine tumor-infiltrating monocytic myeloid-derived suppressor cells

Immune cell activation occurs concurrently with metabolic reprogramming. As important components of the tumor microenvironment, monocytic myeloid-derived suppressor cells (M-MDSCs) are featured by their potent immunosuppressive abilities on anti-tumor effector cells. However, little is known about the contribution of metabolic adaptations to their suppressive roles. In this study, we found that tumor-infiltrating M-MDSCs had the same phenotype with splenic M-MDSCs. Compared with splenic M-MDSCs, tumor-infiltrating M-MDSCs exhibited stronger suppressive activities which was accompanied by higher glycolysis. Inhibition of glycolysis impaired the suppressive function of tumor M-MDSCs. Meanwhile, the results demonstrated that mTOR was responsible for this function regulation. mTOR inhibition by rapamycin decreased the glycolysis and reduced the suppressive activities of these cells. Furthermore, rapamycin treatment inhibited the tumor growth and reduced the percentage of M-MDSCs in 3LL tumor bearing mice. These results demonstrated that modulation of metabolism in immune cells can be an effective way to enhance anti-tumor effects.

TNF Receptor 2 Makes Tumor Necrosis Factor a Friend of Tumors

Tumor necrosis factor (TNF) is widely accepted as a tumor-suppressive cytokine via its ubiquitous receptor TNF receptor 1 (TNFR1). The other receptor, TNFR2, is not only expressed on some tumor cells but also on suppressive immune cells, including regulatory T cells and myeloid-derived suppressor cells. In contrast to TNFR1, TNFR2 diverts the tumor-inhibiting TNF into a tumor-advocating factor. TNFR2 directly promotes the proliferation of some kinds of tumor cells. Also activating immunosuppressive cells, it supports immune escape and tumor development. Hence, TNFR2 may represent a potential target of cancer therapy. Here, we focus on expression and role of TNFR2 in the tumor microenvironment. We summarize the recent progress in understanding how TNFR2-dependent mechanisms promote carcinogenesis and tumor growth and discuss the potential value of TNFR2 in cancer treatment.

Immunomodulatory Function of Myeloid-Derived Suppressor Cells during B Cell-Mediated Immune Responses

Myeloid-derived suppressor cells (MDSCs) play roles in immune regulation during neoplastic and non-neoplastic inflammatory responses. This immune regulatory function is directed mainly toward T cells. However, MDSCs also regulate other cell populations, including B cells, during inflammatory responses. Indeed, B cells are essential for antibody-mediated immune responses. MDSCs regulate B cell immune responses directly via expression of effector molecules and indirectly by controlling other immune regulatory cells. B cell-mediated immune responses are a major component of the overall immune response; thus, MDSCs play a prominent role in their regulation. Here, we review the current knowledge about MDSC-mediated regulation of B cell responses.

A novel MDSC-induced PD-1-PD-L1+ B-cell subset in breast tumor microenvironment possesses immuno-suppressive properties

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous group of myeloid cells that suppress T-cell activity in a tumor microenvironment. However, the suppressive function of MDSCs on B cells and its underlying mechanism remain unclear. Here, we show that in 4T1 breast cancer mice, a significantly increased number of MDSCs, in parallel with splenic B cells, are accumulated when compared to normal mice. In the presence of MDSCs, the surface molecules of B cells are remolded, with checkpoint-related molecules such as PD-1 and PD-L1 changing prominently. MDSCs also emerge as vital regulators in B-cell immune functions such as proliferation, apoptosis and the abilities to secrete antibodies and cytokines. Our study further identifies that MDSCs can transform normal B cells to a subtype of immuno- regulatory B cells (Bregs) which inhibit T-cell response. Furthermore, we identified a novel kind of Bregs with a specific phenotype PD-1^-PD-L1⁺CD19⁺, which exert the greatest suppressive effects on T cells in comparison with the previously reported Bregs characterized as CD1d⁺CD5⁺CD19⁺, CD5⁺CD19⁺ and Interleukin (IL)-10-secreting B cells. Our results highlight that MDSCs regulate B-cell response and may serve as a therapeutic approach in anti-tumor treatment. Investigation of this new Breg subtype extends our understanding of regulation of T-cell response and sheds new light on anti-tumor immunity and immune therapy.

Monocytic Myeloid-Derived Suppressor Cells in Chronic Infections

Heterogeneous populations of myeloid regulatory cells (MRC), including monocytes, macrophages, dendritic cells, and neutrophils, are found in cancer and infectious diseases. The inflammatory environment in solid tumors as well as infectious foci with persistent pathogens promotes the development and recruitment of MRC. These cells help to resolve inflammation and establish host immune homeostasis by restricting T lymphocyte function, inducing regulatory T cells and releasing immune suppressive cytokines and enzyme products. Monocytic MRC, also termed monocytic myeloid-derived suppressor cells (M-MDSC), are bona fide phagocytes, capable of pathogen internalization and persistence, while exerting localized suppressive activity. Here, we summarize molecular pathways controlling M-MDSC genesis and functions in microbial-induced non-resolved inflammation and immunopathology. We focus on the roles of M-MDSC in infections, including opportunistic extracellular bacteria and fungi as well as persistent intracellular pathogens, such as mycobacteria and certain viruses. Better understanding of M-MDSC biology in chronic infections and their role in antimicrobial immunity, will advance development of novel, more effective and broad-range anti-infective therapies.

A functional characteristic of cysteine-rich protein 61: Modulation of myeloid-derived suppressor cells in liver inflammation

There is increasing awareness of the immunologic roles of liver mononuclear populations, including myeloid-derived suppressor cells (MDSCs). We took advantage of a large well-defined cohort of 148 patients with liver inflammation and 45 healthy controls to focus on the qualitative and quantitative characteristics of MDSCs. We investigated the frequency, phenotype, and functional capacities of MDSCs by using peripheral blood MDSCs in a cohort of 55 patients with primary biliary cholangitis (PBC), 40 with autoimmune hepatitis, 39 with chronic hepatitis B, 14 with nonalcoholic fatty liver disease, and 45 healthy controls. This was followed by a liver-targeted determination in 27 patients with PBC, 27 with autoimmune hepatitis, 20 with chronic hepatitis B, 14 with nonalcoholic fatty liver disease, and 6 controls. We then focused on mechanisms of this expansion with PBC as an example, using both ursodeoxycholic acid-naive and treated patients. HLA-DR^-/low CD33⁺ CD11b⁺ CD14⁺ CD15^- monocytic MDSCs were elevated in diseases characterized by liver inflammation compared to healthy controls. Using PBC as a focus, there was a significant correlation between levels of circulating MDSCs and disease-related biochemical markers (alkaline phosphatase, total bilirubin). We found higher amounts of MDSCs in patients with PBC who were responsive to ursodeoxycholic acid. MDSCs from PBC were found to manifest a potent immunosuppressive function. There was a significant correlation in the accumulation of hepatic MDSCs in the inflamed lesions of PBC with histologic changes, such as fibrosis. We also found that cysteine-rich protein 61 (CCN1), a highly expressed protein in impaired cholangiocytes and hepatocytes, contributes to MDSC expansion and MDSC inducible nitric oxide synthase-associated immune suppression.

CONCLUSION

CCN1 modulates expansion and a suppressive function of MDSCs. Our data highlight the potential functions of CCN1 on MDSCs and suggest therapeutic implications in inflammatory liver diseases. (Hepatology HEPATOLOGY 2018;67:232-246).

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

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

Reciprocal influence of B cells and tumor macro and microenvironments in the ApcMin/+ model of colorectal cancer

One of the most fascinating aspects of the immune system is its dynamism, meant as the ability to change and readapt according to the organism needs. Following an insult, we assist to the spontaneous organization of different immune cells which cooperate, locally and at distance, to build up an appropriate response. Throughout tumor progression, adaptations within the systemic tumor environment, or macroenvironment, result in the promotion of tumor growth, tumor invasion and metastasis to distal organs, but also to dramatic changes in the activity and composition of the immune system. In this work, we show the changes of the B-cell arm of the immune system following tumor progression in the Apc^Min/+ model of colorectal cancer. Tumor macroenvironment leads to an increased proportion of total and IL-10-competent B cells in draining LNs while activates a differentiation route that leads to the expansion of IgA⁺ lymphocytes in the spleen and peritoneum. Importantly, serum IgA levels were significantly higher in Apc^Min/+ than Wt mice. The peculiar involvement of IgA response in the adenomatous transformation had correlates in the gut-mucosal compartment where IgA-positive elements increased from normal mucosa to areas of low grade dysplasia while decreasing upon overt carcinomatous transformation. Altogether, our findings provide a snapshot of the tumor education of B lymphocytes in the Apc^Min/+ model of colorectal cancer. Understanding how tumor macroenvironment affects the differentiation, function and distribution of B lymphocytes is pivotal to the generation of specific therapies, targeted to switching B cells to an anti-, rather than pro-, tumoral phenotype.