Stat 6-dependent induction of myeloid derived suppressor cells after physical injury regulates nitric oxide response to endotoxin

Modulation of Myeloid-Derived Suppressor Cells in the Tumor Microenvironment by Natural Products

During carcinogenesis, the microenvironment plays a fundamental role in tumor progression and resistance. This tumor microenvironment (TME) is characterized by being highly immunosuppressive in most cases, which makes it an important target for the development of new therapies. One of the most important groups of cells that orchestrate immunosuppression in TME is myeloid-derived suppressor cells (MDSCs), which have multiple mechanisms to suppress the immune response mediated by T lymphocytes and thus protect the tumor. In this review, we will discuss the importance of modulating MDSCs as a therapeutic target and how the use of natural products, due to their multiple mechanisms of action, can be a key alternative for modulating these cells and thus improve response to therapy in cancer patients.

Targeting myeloid-derived suppressor cells in tumor immunotherapy: Current, future and beyond

Myeloid-derived suppressor cells (MDSCs) are one of the major negative regulators in tumor microenvironment (TME) due to their potent immunosuppressive capacity. MDSCs are the products of myeloid progenitor abnormal differentiation in bone marrow, which inhibits the immune response mediated by T cells, natural killer cells and dendritic cells; promotes the generation of regulatory T cells and tumor-associated macrophages; drives the immune escape; and finally leads to tumor progression and metastasis. In this review, we highlight key features of MDSCs biology in TME that are being explored as potential targets for tumor immunotherapy. We discuss the therapies and approaches that aim to reprogram TME from immunosuppressive to immunostimulatory circumstance, which prevents MDSC immunosuppression activity; promotes MDSC differentiation; and impacts MDSC recruitment and abundance in tumor site. We also summarize current advances in the identification of rational combinatorial strategies to improve clinical efficacy and outcomes of cancer patients, via deeply understanding and pursuing the mechanisms and characterization of MDSCs generation and suppression in TME.

Delicaflavone reactivates anti-tumor immune responses by abrogating monocytic myeloid cell-mediated immunosuppression

BACKGROUND

Myeloid cell-mediated immunosuppression is a major obstacle to checkpoint blockade immunotherapy. We previously reported that total biflavonoids extract from Selaginella doederleinii (TBESD) and a flavone monomer isolated from TBESD, named Delicaflavone, have favorable anti-tumor activity. However, whether TBESD and Delicaflavone could affect the tumor microenvironment (TME) remains unclear.

PURPOSE

In this study, we focused on the TME to determine whether TBESD and Delicaflavone could restore anti-tumor immune response.

METHODS

4T1 tumor-bearing immunocompetent BALB/c mice and T cell-deficient nude mice were used to examine the effect of TBESD on T cell-mediated immunity in vivo. Multi-parameter flow cytometry was conducted to evaluate the impacts of TBESD on TME. Primary cells, including murine CD8⁺ T cells, tumor associated macrophages (TAMs) and myeloid-derived suppressor cells (MDSCs) were prepared to investigate the modulatory activities of TBESD on immune cells. It was further determined whether Delicaflavone or Amentoflavone, two typical functional biflavones from TBESD, mediated those effects of TBESD. Finally, the impacts of TBESD and Delicaflavone on Jak1/STAT6 signaling pathway were explored via western blot.

RESULTS

We found that TBESD significantly reduced 4T1 tumor growth in immunocompetent BALB/c mice, but not in nude mice. This effect was associated with the regulation of TME, shown as an increase in functional T cells and M1 phenotype TAMs (M1-TAMs), and a decrease in M2 phenotype TAMs (M2-TAMs), monocytic-MDSCs (M-MDSCs) and regulatory T cells (Tregs) in TBESD-treated BALB/c mouse 4T1 tumors. It was found ex vivo that TBESD restrained the viability and immunosuppressive properties of M2-TAMs and M-MDSCs, especially for the loss of arginase-1 expression. Additionally, TBESD re-educated M2-TAMs to an M1 like phenotype. Further investigations determined that Delicaflavone predominantly mediated the immuno-modulatory activities of TBESD both ex vivo and in vivo. Finally, Delicaflavone and TBESD blocked Jak1/STAT6 signaling pathway in M2-TAMs and MDSCs.

CONCLUSION

The present study suggests Delicaflavone as a potent natural inhibitor of M2-TAMs and MDSCs, which fills the gap in knowledge on the immuno-modulatory effects of TBESD and Delicaflavone, and could have translational implications to improve the efficacy of cancer immunotherapy.

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

INTRODUCTION

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

AREAS COVERED

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

EXPERT OPINION

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

Myeloid-Derived Suppressor Cells (MDSCs) and the Immunoinflammatory Response to Injury (Mini Review)

ABSTRACT

Myeloid-derived suppressor cells (MDSCs) are a heterogenous population of immature myeloid cells hallmarked by their potent immunosuppressive function in a vast array of pathologic conditions. MDSCs have recently been shown to exhibit marked expansion in acute inflammatory states including traumatic injury, burn, and sepsis. Although MDSCs have been well characterized in cancer, there are significant gaps in our knowledge of their functionality in trauma and sepsis, and their clinical significance remains unclear. It is suggested that MDSCs serve an important role in quelling profound inflammatory responses in the acute setting; however, MDSC accumulation may also predispose patients to developing persistent immune dysregulation with increased risk for nosocomial infections, sepsis, and multiorgan failure. Whether MDSCs may serve as the target for novel therapeutics or an important biomarker in trauma and sepsis is yet to be determined. In this review, we will discuss the current understanding of MDSCs within the context of specific traumatic injury types and sepsis. To improve delineation of their functional role, we propose a systemic approach to MDSC analysis including phenotypic standardization, longitudinal analysis, and expansion of clinical research.

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.

Actors on the Scene: Immune Cells in the Myeloma Niche

Two mechanisms are involved in the immune escape of cancer cells: the immunoediting of tumor cells and the suppression of the immune system. Both processes have been revealed in multiple myeloma (MM). Complex interactions between tumor plasma cells and the bone marrow (BM) microenvironment contribute to generate an immunosuppressive milieu characterized by high concentration of immunosuppressive factors, loss of effective antigen presentation, effector cell dysfunction, and expansion of immunosuppressive cell populations, such as myeloid-derived suppressor cells, regulatory T cells and T cells expressing checkpoint molecules such as programmed cell death 1. Considering the great immunosuppressive impact of BM myeloma microenvironment, many strategies to overcome it and restore myeloma immunosurveillance have been elaborated. The most successful ones are combined approaches such as checkpoint inhibitors in combination with immunomodulatory drugs, anti-monoclonal antibodies, and proteasome inhibitors as well as chimeric antigen receptor (CAR) T cell therapy. How best to combine anti-MM therapies and what is the optimal timing to treat the patient are important questions to be addressed in future trials. Moreover, intratumor MM heterogeneity suggests the crucial importance of tailored therapies to identify patients who might benefit the most from immunotherapy, reaching deeper and more durable responses.

MicroRNA 449c Mediates the Generation of Monocytic Myeloid-Derived Suppressor Cells by Targeting STAT6

Myeloid-derived suppressor cells (MDSCs) promote tumour progression by contributing to angiogenesis, immunosuppression, and immunotherapy resistance. Although recent studies have shown that microRNAs (miRNAs) can promote the expansion of MDSCs in the tumour environment, the mechanisms involved in this process are largely unknown. Here, we report that microRNA 449c (miR-449c) expression was upregulated in myeloid progenitor cells upon activation of C-X-C motif chemokine receptor 2 (CXCR2) under tumour conditions. MiR-449c upregulation increased the generation of monocytic MDSCs (mo-MDSCs). The increased expression of miR-449c could target STAT6 mRNA in myeloid progenitor cells to shift the differentiation balance of myeloid progenitor cells and lead to an enhancement of the mo-MDSCs population in the tumour environment. Thus, our results demonstrate that the miR-449c/STAT6 axis is involved in the expansion of mo-MDSCs from myeloid progenitor cells upon activation of CXCR2, and thus, inhibition of miR-449c/STAT6 signalling may help to attenuate tumour progression.

High-density neutrophils in MGUS and multiple myeloma are dysfunctional and immune-suppressive due to increased STAT3 downstream signaling

To understand neutrophil impairment in the progression from MGUS through active MM, we investigated the function of mature, high-density neutrophils (HDNs), isolated from peripheral blood. In 7 MM, 3 MGUS and 3 healthy subjects by gene expression profile, we identified a total of 551 upregulated and 343 downregulated genes in MM-HDN, involved in chemokine signaling pathway and FC-gamma receptor mediated phagocytosis conveying in the activation of STAT proteins. In a series of 60 newly diagnosed MM and 30 MGUS patients, by flow-cytometry we found that HDN from MM, and to a lesser extend MGUS, had an up-regulation of the inducible FcγRI (also known as CD64) and a down-regulation of the constitutive FcγRIIIa (also known as CD16) together with a reduced phagocytic activity and oxidative burst, associated to increased immune-suppression that could be reverted by arginase inhibitors in co-culture with lymphocytes. In 43 consecutive newly-diagnosed MM patients, who received first-line treatment based on bortezomib, thalidomide and dexamethasone, high CD64 could identify at diagnosis patients with inferior median overall survival (39.5 versus 86.7 months, p = 0.04). Thus, HDNs are significantly different among healthy, MGUS and MM subjects. In both MGUS and MM neutrophils may play a role in supporting both the increased susceptibility to infection and the immunological dysfunction that leads to tumor progression.

A Call for Epidemiological Research on Myeloid-Derived Suppressor Cells in Ovarian Cancer: A Review of the Existing Immunological Evidence and Suggestions for Moving Forward

Recently, there have been encouraging findings suggesting that myeloid-derived suppressor cells (MDSCs) may be a good target for studying immune suppression in ovarian cancer. MDSCs are an abundance of immature myeloid cells that have demonstrated the ability to decrease tumor-infiltrating immune cells, increase the accrual of tumor-associated macrophages and regulatory T cells, as well as secrete various pro-inflammatory mediators and growth stimulating cytokines. Most studies on this topic utilized murine models, but there are limited reports in human subjects which have important limitations. With the majority of ovarian cancer patients presenting with distant metastases and a corresponding 5-year relative survival rate of < 30%, continued efforts are obligatory toward identifying potential prognostic factors. Given the difficulty of studying exposures in this patient population, as well as the existing immunologic characteristics of this cancer, there is growing interest in further identifying genetic and immunologic associations with patient survival. Furthermore, prognostic factors that may necessitate therapeutic intervention may significantly alter disease outlook. In this review paper, we address the current literature on MDSCs and their immunosuppressive behavior in ovarian cancer patients. While the previous studies on these cells in ovarian cancer have demonstrated some potential prognostic significance, there are many limitations to such studies including small sample sizes, inconsistent staging and histology, as well as inconsistent surface markers for the identification of MDSCs. Additionally, such studies include minimal patient characteristics involved with the clinical course of ovarian cancer. Here, we have proposed improving on studies analyzing MDSCs as a potential prognostic factor in ovarian cancer patients, as well as further identifying the potential of this novel prognostic factor in future care, through the use of a comprehensive epidemiologic model.

Targeting myeloid-derived suppressor cells for cancer immunotherapy

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of immature myeloid cells with an immune suppressive phenotype. They represent a critical component of the immune suppressive niche described in cancer, where they support immune escape and tumor progression through direct effects on both the innate and adaptive immune responses, largely by contributing to maintenance of a high oxidative stress environment. The number of MDSCs positively correlates with protumoral activity, and often diminishes the effectiveness of immunotherapies, which is particularly problematic with the emergence of personalized medicine. Approaches targeting MDSCs showed promising results in preclinical studies and are under active investigation in clinical trials in combination with various immune checkpoint inhibitors. In this review, we discuss MDSC targets and therapeutic approaches targeting MDSC that have the aim of enhancing the existing tumor therapies.

Tumor-Induced Myeloid-Derived Suppressor Cells

Myeloid-derived suppressor cells (MDSCs) represent a heterogeneous, immune-suppressive leukocyte population that develops systemically and infiltrates tumors. MDSCs can restrain the immune response through different mechanisms including essential metabolite consumption, reactive oxygen and nitrogen species production, as well as display of inhibitory surface molecules that alter T-cell trafficking and viability. Moreover, MDSCs play a role in tumor progression, acting directly on tumor cells and promoting cancer stemness, angiogenesis, stroma deposition, epithelial-to-mesenchymal transition, and metastasis formation. Many biological and pharmaceutical drugs affect MDSC expansion and functions in preclinical tumor models and patients, often reversing host immune dysfunctions and allowing a more effective tumor immunotherapy.

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

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

Myeloid-derived suppressor cells-a new therapeutic target to overcome resistance to cancer immunotherapy

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of immature myeloid cells that accumulate during pathologic conditions, such as cancer. Patients diagnosed with advanced metastatic cancers have an average survival of 12-24 mo, a survival time that hasn't changed significantly in the past 30 yr. Despite some encouraging improvements in response rates and overall survival in patients receiving immunotherapies, such as immune checkpoint inhibitors, most patients will ultimately progress. MDSCs contribute to immunotherapeutic resistance by actively inhibiting antitumor T cell proliferation and cytotoxic activity as well as by promoting expansion of protumorigenic T regulatory cells, thereby, dampening the host immune responses against the tumor. In addition, MDSCs promote angiogenesis, tumor invasion, and metastasis. Thus, MDSCs are potential therapeutic targets in cases of multiple cancers. This review focuses on the phenotypic and functional characteristics of MDSCs and provides an overview of the mono- and combinatorial-therapeutic strategies that target MDSCs with an objective of enhancing the efficacy of cancer immunotherapies.

Improving cancer immunotherapy by targeting the STATe of MDSCs

Cancer immunotherapy is a promising therapeutic avenue; however, in practice its efficacy is hampered by an immunosuppressive tumor microenvironment that consists of suppressive cell types like myeloid-derived suppressor cells (MDSCs). Eradication or reprogramming of MDSCs could therefore enhance clinical responses to immunotherapy. Here, we review clinically available drugs that target MDSCs, often through inhibition of STAT signaling, which is essential for MDSC accumulation and suppressive functions. Interestingly, several drugs used for non-cancerous indications and natural compounds similarly inhibit MDSCs by STAT inhibition, but have fewer side effects than anticancer drugs. Therefore, they show great potential for combination strategies with immunotherapy.

Myeloid-Derived Suppressor Cells in Bacterial Infections

Myeloid-derived suppressor cells (MDSCs) comprise monocytic and granulocytic innate immune cells with the capability of suppressing T- and NK-cell responses. While the role of MDSCs has been studied in depth in malignant diseases, the understanding of their regulation and function in infectious disease conditions has just begun to evolve. Here we summarize and discuss the current view how MDSCs participate in bacterial infections and how this knowledge could be exploited for potential future therapeutics.

The immunobiology of myeloid-derived suppressor cells in cancer

The tumor microenvironment is a complex and heterogeneous milieu in which multiple interactions occur between tumor and host cells. Immunosuppressive cells which are present in this microenvironment, such as regulatory T (Treg) cells and myeloid-derived suppressor cells (MDSCs), play an important role in tumor progression, via down-regulation of antitumor responses. MDSCs represent a heterogeneous group of cells originated from the myeloid lineage that are in the immature state. These cells markedly accumulate under pathologic conditions, such as cancer, infection, and inflammation, and use various mechanisms to inhibit both adaptive and innate immune responses. These immunosuppressive mechanisms include deprivation of T cells from essential amino acids, induction of oxidative stress, interference with viability and trafficking of T cells, induction of immunosuppressive cells, and finally polarizing immunity toward a tumor-promoting type 2 phenotype. In addition to suppression of antitumor immune responses, MDSCs can also enhance the tumor metastasis and angiogenesis. Previous studies have shown that increased frequency of MDSCs is related to the tumor progression. Moreover, various drugs that directly target these cells or reverse their suppressive activity can improve antitumor immune responses as well as increase the efficacy of immunotherapeutic intervention. In this review, we will first discuss on the immunobiology of MDSCs in an attempt to find the role of these cells in tumor progression and then discuss about therapeutic approaches to target these cells.

Immature myeloid Gr-1+ CD11b+ cells from lipopolysaccharide-immunosuppressed mice acquire inhibitory activity in the bone marrow and migrate to lymph nodes to exert their suppressive function

Secondary infections due to post-sepsis immunosuppression are a major cause of death in patients with sepsis. Repetitive inoculation of increasing doses of lipopolysaccharide (LPS) into mice mimics the immunosuppression associated with sepsis. Myeloid-derived suppressor cells (MDSCs, Gr-1(+) CD11b(+)) are considered a major component of the immunosuppressive network, interfering with T-cell responses in many pathological conditions. We used LPS-immunosuppressed (IS) mice to address whether MDSCs acquired their suppressive ability in the bone marrow (BM) and whether they could migrate to lymph nodes (LNs) to exert their suppressive function. Our results showed that Gr-1(+) CD11b(+) cells of IS mice already had the potential to inhibit T-cell proliferation in the BM. Moreover, soluble factors present in the BM from IS mice were responsible for inducing this inhibitory ability in control BM cells. In addition, migration of Gr-1(+) CD11b(+) to LNs in vivo was maximal when cells obtained from the BM of IS mice were inoculated into an IS context. In this regard, we found chemoattractant activity in cell-free LN extracts (LNEs) from IS mice and an increased expression of the LN-homing chemokine receptor C-C chemokine receptor type 7 (CCR7) in IS BM Gr-1(+) CD11b(+) cells. These results indicate that Gr-1(+) CD11b(+) cells found in BM from IS mice acquire their suppressive activity in the same niche where they are generated, and migrate to LNs to exert their inhibitory role. A better understanding of MDSC generation and/or regulation of factors able to induce their inhibitory function may provide new and more effective tools for the treatment of sepsis-associated immunosuppression.

Myeloid derived suppressor cells in inflammatory conditions of the central nervous system

The knowledge of the immune system elements and their relationship with other tissues, organs and systems are key approximations for the resolution of many immune-related disorders. The control of the immune response and/or its modulation from the pro-inflammatory to the anti-inflammatory response is being deeply studied in the field. In the last years, the study of myeloid-derived suppressor cells (MDSCs), a group of immature myeloid cells with a high suppressive activity on T cells has been extensively addressed in cancer. In contrast, their role in neuroimmune diseases is far from being totally understood. In this review, we will summarize data about MDSCs coming from the study of neuroinflammatory diseases in general and their potential role in multiple sclerosis, in order to introduce the putative use of this extraordinary promising cell type for future cell-based therapies. This article is part of a Special Issue entitled: Neuro Inflammation edited by Helga E. de Vries and Markus Schwaninger.

Transcriptional regulation of myeloid-derived suppressor cells

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

MDSCs in cancer: Conceiving new prognostic and therapeutic targets

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

Anti-HMGB1 monoclonal antibody ameliorates immunosuppression after peripheral tissue trauma: attenuated T-lymphocyte response and increased splenic CD11b (+) Gr-1 (+) myeloid-derived suppressor cells require HMGB1

Although tissue-derived high mobility group box 1 (HMGB1) is involved in many aspects of inflammation and tissue injury after trauma, its role in trauma-induced immune suppression remains elusive. Using an established mouse model of peripheral tissue trauma, which includes soft tissue and fracture components, we report here that treatment with anti-HMGB1 monoclonal antibody ameliorated the trauma-induced attenuated T-cell responses and accumulation of CD11b⁺Gr-1⁺ myeloid-derived suppressor cells in the spleens seen two days after injury. Our data suggest that HMGB1 released after tissue trauma contributes to signaling pathways that lead to attenuation of T-lymphocyte responses and enhancement of myeloid-derived suppressor cell expansion.

Myeloid expression of angiotensin-converting enzyme facilitates myeloid maturation and inhibits the development of myeloid-derived suppressor cells

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of immature myeloid cells which accumulate in cancer, infection and chronic inflammation. These cells suppress T-cell function and the immune response. Angiotensin-converting enzyme (ACE) is a peptidase that is now known to regulate aspects of myelopoiesis. Here, we show that ACE expression correlates with myeloid maturation in vitro. Forced ACE overexpression in monocytic cells reduces the generation of MDSCs. In vivo, mice with a genetic change resulting in myeloid cell ACE overexpression have reduced numbers of blood and splenic MDSCs in a tumor model and in a model of chronic inflammation induced by complete Freund's adjuvant. In contrast, ACE-null mice produce large numbers of MDSCs during chronic inflammation. Macrophages from mice with myeloid ACE overexpressing are more pro-inflammatory and have more tumor-killing activity than cells from wild-type mice. Thus, manipulating myeloid ACE activity can interfere with MDSC development and the maturation of myeloid cells.

The central role of arginine catabolism in T-cell dysfunction and increased susceptibility to infection after physical injury

OBJECTIVE

To explore the hypothesis that decreased arginine availability by myeloid-derived suppressor cells (MDSCs) is a cause of T-cell dysfunction after physical injury (PI).

BACKGROUND

Arginine is an essential amino acid for normal T-cell function whose availability becomes limited after PI. MDSCs expressing arginase 1 are induced by PI. T-cell dysfunction after PI seems to increase the risk of infection but the mechanisms that cause it are unclear.

METHODS

PI was created using a standard laparotomy model. Phenotypical and functional alterations in T cells were evaluated in vivo. MDSCs expressing arginase 1 were measured by flow cytometry. Infection after PI was created by intraperitoneal injection of Listeria monocytogenes. N-Hydroxy-Nor-L-arginine (Nor-NOHA) was used as an arginase inhibitor. The effect of arginine depletion on T-cell function and susceptibility to infection was assessed through adoptive transfer of MDSC or injection of arginase into noninjured mice.

RESULTS

PI caused a decrease in intracellular arginine in T cells, loss of the T-cell receptor (TCR) CD3-ζ chain, inhibition of in vivo T-cell proliferation, memory, and cytotoxicity. PI exponentially increased bacterial growth and mortality to L. monocytogenes. T-cell dysfunction and increased infection were reversed by arginase inhibitor Nor-NOHA but were reproduced by adoptively transferring MDSC or injecting arginase 1 to noninjured mice.

CONCLUSIONS

Arginine availability is decreased after PI coinciding with an induction of MDSC expressing arginase 1. Decreased arginine may inhibit T-cell function and increase susceptibility to infection after injury.

Reciprocal relationship between myeloid-derived suppressor cells and T cells

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous group of myeloid cells that play a major role in the regulation of immune responses in many pathological conditions. These cells have a common myeloid origin, relatively immature state, common genetic and biochemical profiles, and, most importantly, the ability to inhibit immune responses. Although initial studies of MDSCs were almost exclusively performed in tumor-bearing mice or cancer patients, in recent years, it became clear that MDSCs play a critical role in the regulation of different types of inflammation that are not directly associated with cancer. In this review we discuss the nature of the complex relationship between MDSCs and the different populations of CD4(+) T cells.

miR-142-3p prevents macrophage differentiation during cancer-induced myelopoiesis

Tumor progression is accompanied by an altered myelopoiesis causing the accumulation of immunosuppressive cells. Here, we showed that miR-142-3p downregulation promoted macrophage differentiation and determined the acquisition of their immunosuppressive function in tumor. Tumor-released cytokines signaling through gp130, the common subunit of the interleukin-6 cytokine receptor family, induced the LAP∗ isoform of C/EBPβ transcription factor, promoting macrophage generation. miR-142-3p downregulated gp130 by canonical binding to its messenger RNA (mRNA) 3' UTR and repressed C/EBPβ LAP∗ by noncanonical binding to its 5' mRNA coding sequence. Enforced miR expression impaired macrophage differentiation both in vitro and in vivo. Mice constitutively expressing miR-142-3p in the bone marrow showed a marked increase in survival following immunotherapy with tumor-specific T lymphocytes. By modulating a specific miR in bone marrow precursors, we thus demonstrated the feasibility of altering tumor-induced macrophage differentiation as a potent tool to improve the efficacy of cancer immunotherapy.

Shaping the murine macrophage phenotype: IL-4 and cyclic AMP synergistically activate the arginase I promoter

Arginase I is a marker of murine M2 macrophages and is highly expressed in many inflammatory diseases. The basis for high arginase I expression in macrophages in vivo is incompletely understood but likely reflects integrated responses to combinations of stimuli. Our objective was to elucidate mechanisms involved in modulating arginase I induction by IL-4, the prototypical activator of M2 macrophages. IL-4 and 8-bromo-cAMP individually induce arginase I, but together they rapidly and synergistically induce arginase I mRNA, protein, and promoter activity in murine macrophage cells. Arginase I induction by IL-4 requires binding of the transcription factors STAT6 and C/EBPβ to the IL-4 response element of the arginase I gene. Chromatin immunoprecipitation showed that the synergistic response involves binding of both transcription factors to the IL-4 response element at levels significantly greater than in response to IL-4 alone. The results suggest that C/EBPβ is a limiting factor for the level of STAT6 bound to the IL-4 response element. The enhanced binding in the synergistic response was not due to increased expression of either STAT6 or C/EBPβ but was correlated primarily with increased nuclear abundance of C/EBPβ. Our findings also suggest that induction of arginase I expression is stochastic; that is, differences in induction reflect differences in probability of transcriptional activation and not simply differences in rate of transcription. Results of the present study also may be useful for understanding mechanisms underlying regulated expression of other genes in macrophages and other myeloid-derived cells in health and disease.

Melanoma-educated CD14+ cells acquire a myeloid-derived suppressor cell phenotype through COX-2-dependent mechanisms

Tumors can suppress the host immune system by employing a variety of cellular immune modulators, such as regulatory T cells, tumor-associated macrophages, and myeloid-derived suppressor cells (MDSC). In the peripheral blood of patients with advanced stage melanoma, there is an accumulation of CD14(+)HLA-DR(lo/-) MDSC that suppress autologous T cells ex vivo in a STAT-3-dependent manner. However, a precise mechanistic basis underlying this effect is unclear, particularly with regard to whether the MDSC induction mechanism relies on cell-cell contact of melanoma cells with CD14(+) cells. Here, we show that early-passage human melanoma cells induce phenotypic changes in CD14(+) monocytes, leading them to resemble MDSCs characterized in patients with advanced stage melanoma. These MDSC-like cells potently suppress autologous T-cell proliferation and IFN-γ production. Notably, induction of myeloid-suppressive functions requires contact or close proximity between monocytes and tumor cells. Further, this induction is largely dependent on production of cyclooxygenase-2 (COX-2) because its inhibition in these MDSC-like cells limits their ability to suppress T-cell function. We confirmed our findings with CD14(+) cells isolated from patients with advanced stage melanoma, which inhibited autologous T cells in a manner relying up prostaglandin E2 (PGE2), STAT-3, and superoxide. Indeed, PGE2 was sufficient to confer to monocytes the ability to suppress proliferation and IFN-γ production by autologous T cells ex vivo. In summary, our results reveal how immune suppression by MDSC can be initiated in the tumor microenvironment of human melanoma.

Radiation combined with thermal injury induces immature myeloid cells

The continued development of nuclear weapons and the potential for thermonuclear injury necessitates the further understanding of the immune consequences after radiation combined with injury (RCI). We hypothesized that sublethal ionization radiation exposure combined with a full-thickness thermal injury would result in the production of immature myeloid cells. Mice underwent either a full-thickness contact burn of 20% total body surface area or sham procedure followed by a single whole-body dose of 5-Gy radiation. Serum, spleen, and peripheral lymph nodes were harvested at 3 and 14 days after injury. Flow cytometry was performed to identify and characterize adaptive and innate cell compartments. Elevated proinflammatory and anti-inflammatory serum cytokines and profound leukopenia were observed after RCI. A population of cells with dual expression of the cell surface markers Gr-1 and CD11b were identified in all experimental groups, but were significantly elevated after burn alone and RCI at 14 days after injury. In contrast to the T-cell-suppressive nature of myeloid-derived suppressor cells found after trauma and sepsis, myeloid cells after RCI augmented T-cell proliferation and were associated with a weak but significant increase in interferon γ and a decrease in interleukin 10. This is consistent with previous work in burn injury indicating that a myeloid-derived suppressor cell-like population increases innate immunity. Radiation combined injury results in the increase in distinct populations of Gr-1CD11b cells within the secondary lymphoid organs, and we propose these immature inflammatory myeloid cells provide innate immunity to the severely injured and immunocompromised host.

Endogenous glucocorticoids promote the expansion of myeloid-derived suppressor cells in a murine model of trauma

Stress-dose of glucocorticoid has been demonstrated to be beneficial for trauma patients in clinical studies. Recently, a heterogeneous population of myeloid cells with immunosuppressive activity named myeloid-derived suppressor cells (MDSCs) has been found to accumulate in the trauma host and can be induced by glucocorticoids in vitro. In order to explore the effect of endogenous glucocorticoids on MDSCs under trauma conditions, we blocked the glucocorticoid signal in a murine trauma model using the antagonist of the glucocorticoid receptor RU486 (mifepristone). We found for the first time that RU486 not only blunted MDSC expansion induced by trauma in the spleen, peripheral blood and bone marrow especially at 6 h after traumatic stress but also decreased the survival rate from 100 to 20% in traumatic mice within 7 days. Moreover, neither MDSCs producing arginase-1 nor the morphological characterization of trauma-induced MDSCs was affected by the blockage of the glucocorticoid receptor. Our results suggest that endogenous glucocorticoids may promote MDSCs expansion in a murine trauma model and MDSCs may be beneficial for the trauma host.

Myeloid-derived suppressor cells control microbial sepsis

PURPOSE

To investigate the role of myeloid-derived suppressor cells (MDSCs) during sepsis in mice. MDSCs are a heterogeneous population of cells that expand during cancer, inflammation and infection. These cells, by their ability to suppress T lymphocyte proliferation, regulate immune responses during various diseases. Their role during microbial infections is scarcely known.

METHODS

Septic shock was induced by caecal ligation and puncture in adult male BALB/c mice; sham-operated animals served as controls. Animals were killed under anaesthesia to harvest blood and organs.

RESULTS

Polymicrobial sepsis induced a progressive accumulation of MDSCs in spleens that were found to be enlarged in surviving mice. MDSCs harvested at day 10 after the onset of infection were highly responsive to LPS in terms of cytokines secretion, NF-kB activation, ROS production and arginase I activity, whereas early-appearing (day 3) MDSCs poorly responded to this stimulus. By contrast, both day 3 and day 10 MDSCs were able to inhibit T cell proliferation. Adoptive transfer of day 10 MDSCs to septic mice attenuated peritoneal cytokine production, increased bacterial clearance and dramatically improved survival rate.

CONCLUSION

These results provide new information on the role of MDSCs, suggesting a protective effect during sepsis. Pharmacologic agents known to promote the expansion of MDSCs should thus be further studied for sepsis treatment.

Immunonutrition: does it have a role in improving recovery in patients receiving a stem cell transplant?

Numerous clinical trials have demonstrated that immunomodulating diets (IMDs) reduce treatment complications such as the risk of acquired infections, length of hospital stay, and wound complications in patients receiving planned surgery. These complications are possibly exacerbated by malnutrition at the time of surgery, resulting in decreased cell-mediated and humoral immune responses, which can be improved with the utilization of IMDs both prior to and following surgery. Although numerous randomized studies have investigated IMDs in the surgical setting, IMDs have not been well studied to evaluate whether their use improves outcomes for other patient groups with high incidence of malnutrition and acquired infections. Patients receiving stem cell transplantation following preparative myeloablative chemotherapy for treatment of a hematologic malignancy would be a prime example of another patient group who would share these characteristics. Given the proposed mechanism of action by which IMDs have aided recovery after surgery, it is reasonable to expect that IMDs may aid recovery after stem cell transplantation, and current preclinical and clinical data support the need for further clinical studies.

Coordinated regulation of myeloid cells by tumours

Myeloid cells are the most abundant nucleated haematopoietic cells in the human body and are a collection of distinct cell populations with many diverse functions. The three groups of terminally differentiated myeloid cells - macrophages, dendritic cells and granulocytes - are essential for the normal function of both the innate and adaptive immune systems. Mounting evidence indicates that the tumour microenvironment alters myeloid cells and can convert them into potent immunosuppressive cells. Here, we consider myeloid cells as an intricately connected, complex, single system and we focus on how tumours manipulate the myeloid system to evade the host immune response.

Systemic arginine depletion after a murine model of surgery or trauma

BACKGROUND

Arginine metabolism and availability after surgery or trauma (ST) is an important modulator of immune responses. Arginine levels are significantly depleted in human trauma patients. Diets containing arginine administered to surgery patients have restored immune function. We hypothesized an arginase-dependent depletion of arginine in a murine model of ST. In addition, we hypothesized a systemic arginase release in human trauma patients.

METHODS

Male mice were anesthetized and a laparotomy with bowel manipulation was used as a model of ST. Plasma was collected after ST for analysis of arginase activity and arginine, ornithine, and citrulline. Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) in plasma were measured after ST. Also, arginase activity was determined in human plasma from 4 healthy controls and 8 trauma patients.

RESULTS

Arginase activity increased maximally at 2-4 hours after ST, and arginine was significantly reduced after ST. Citrulline was significantly decreased at 8 and 12 hours after ST. Plasma AST and ALT did not significantly vary from control mice after ST. In addition, on day 1 after intensive care unit admission, human trauma patients exhibited a significant increase in arginase activity.

CONCLUSIONS

The biological consequences of arginine depletion remain incompletely understood. These data are consistent with data showing that patients given arginine-containing diets experience reduced morbidity. Understanding of arginine metabolism after ST may lead to therapies aimed at improving clinical outcome after ST.

Myeloid-derived suppressor cells--their role in haemato-oncological malignancies and other cancers and possible implications for therapy

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of immature myeloid cells at different stages of maturation that play a role in cancer tolerance and function as an immune-suppressive cell subpopulation. They utilize different mechanisms to block both innate and adaptive arms of anti-tumour immunity, mostly through inhibition of T cell activation and expansion. Further advances in our understanding of this cell population in both murine models and humans has enabled more accurate characterization of their phenotype and the recognition of two major classes of MDSCs: granulocytic and monocytic. Recently, the mechanism of action and clinical importance of MDSCs has been more clearly defined and their interactions with cancer cells have been shown to be among the factors influencing tumour development and induction of tolerance. Most of the earlier studies were performed using murine models, but recent clinical investigations have shown their potential role in human cancers. Here, we review the origin of MDSCs, their mechanisms of action, the factors influencing their production and related signalling pathways. We focus on their role in human solid tumours and haemato-oncological malignancies, and relate to possible novel therapeutic approaches targeting MDSCs which could be considered together with other anticancer strategies in the not too distant future.

Molecular mechanisms regulating myeloid-derived suppressor cell differentiation and function

Myeloid-derived suppressor cells (MDSCs) are one of the main cell populations responsible for regulating immune responses. MDSCs accumulate during tumor progression, autoimmunity, chronic infection and other pathological conditions, and can potently suppress T cell function. Recent studies have demonstrated the ability of MDSCs to modulate the activity of NK and myeloid cells and have implicated MDSCs in the induction of regulatory T cells. Here, we discuss recent findings that describe the molecular mechanisms that regulate the expansion and function of MDSCs, as well as recent attempts to use MDSCs in cell therapy for different pathologic conditions.

Characterization of cytokine-induced myeloid-derived suppressor cells from normal human peripheral blood mononuclear cells

Tumor immune tolerance can derive from the recruitment of suppressor cell populations, including myeloid-derived suppressor cells (MDSCs). In cancer patients, increased MDSCs correlate with more aggressive disease and a poor prognosis. Expression of 15 immune factors (TGFbeta, IL-1beta, IL-4, IL-6, IL-10, GM-CSF, M-CSF, IDO, fms-related tyrosine kinase 3 ligand, c-kit ligand, inducible NO synthase, arginase-1, TNF-alpha, cyclo-oxygenase 2, vascular endothelial growth factor [VEGF]) by MDSC-inducing human solid tumor cell lines was evaluated by RT-PCR. Based upon these data, cytokine mixtures were then tested for their ability to generate suppressive CD33(+) cells from healthy donor PBMCs in vitro by measuring their ability to inhibit the proliferation of, and IFN-gamma production by, fresh autologous human T cells after CD3/CD28 stimulation. Induced MDSCs were characterized with respect to their morphology, surface phenotype, and gene expression profile. MDSC-inducing cancer cell lines demonstrated multiple pathways for MDSC generation, including overexpression of IL-6, IL-1beta, cyclo-oxygenase 2, M-CSF, and IDO. CD33(+) cells with potent suppressive capacity were best generated in vitro by GM-CSF and IL-6, and secondarily by GM-CSF + IL-1beta, PGE(2), TNF-alpha, or VEGF. Characterization studies of cytokine-induced suppressive cells revealed CD33(+)CD11b(+)CD66b(+)HLA-DR(low)IL-13R alpha2(int) large mononuclear cells with abundant basophilic cytoplasm. Expression of inducible NO synthase, TGFbeta, NADPH oxidase, VEGF, and/or arginase-1 was also upregulated, and Transwell studies showed suppression of autologous T cells to be contact dependent. Suppressive CD33(+) cells generated from PBMCs by GM-CSF and IL-6 were consistent with human MDSCs. This study suggests that these cytokines are potential therapeutic targets for the inhibition of MDSC induction in cancer patients.