Invariant NKT cells reduce the immunosuppressive activity of influenza A virus-induced myeloid-derived suppressor cells in mice and humans

Invariant Natural Killer T Cell Subsets-More Than Just Developmental Intermediates

Invariant natural killer T (iNKT) cells are a CD1d-restricted T cell population that can respond to lipid antigenic stimulation within minutes by secreting a wide variety of cytokines. This broad functional scope has placed iNKT cells at the frontlines of many kinds of immune responses. Although the diverse functional capacities of iNKT cells have long been acknowledged, only recently have distinct iNKT cell subsets, each with a marked functional predisposition, been appreciated. Furthermore, the subsets can frequently occupy distinct niches in different tissues and sometimes establish long-term tissue residency where they can impact homeostasis and respond quickly when they sense perturbations. In this review, we discuss the developmental origins of the iNKT cell subsets, their localization patterns, and detail what is known about how different subsets specifically influence their surroundings in conditions of steady and diseased states.

Advances in NKT cell Immunotherapy for Glioblastoma

Type I or invariant natural killer T cells belong to a unique lineage of innate T cells, which express markers of both T lymphocytes and NK cells, namely T cell receptor (TCR) and NK1.1 (CD161C), respectively. Thus, apart from direct killing of target cells like NK cells, and they also produce a myriad of cytokines which modulate the adaptive immune responses. Unlike traditional T cells which carry a conventional αβ TCR, NKT cells express semi-invariant TCR - Vα14-Jα18, coupled with Vβ8, Vβ7 and Vβ2 in mice. In humans, the invariant TCR is composed of Vα24-Jα18, coupled with Vβ11.

Activation and Regulation of B Cell Responses by Invariant Natural Killer T Cells

CD1d-restricted invariant natural killer T (iNKT) cells play central roles in the activation and regulation of innate and adaptive immunity. Cytokine-mediated and CD1d-dependent interactions between iNKT cells and myeloid and lymphoid cells enable iNKT cells to contribute to the activation of multiple cell types, with important impacts on host immunity to infection and tumors and on the prevention of autoimmunity. Here, we review the mechanisms by which iNKT cells contribute to B cell maturation, antibody and cytokine production, and antigen presentation. Cognate interactions with B cells contribute to the rapid production of antibodies directed against conserved non-protein antigens resulting in rapid but short-lived innate humoral immunity. iNKT cells can also provide non-cognate help for the generation of antibodies directed against protein antigens, by promoting the activation of follicular helper T cells, resulting in long-lasting adaptive humoral immunity and B cell memory. iNKT cells can also regulate humoral immunity by promoting the development of autoreactive B cells into regulatory B cells. Depletions and functional impairments of iNKT cells are found in patients with infectious, autoimmune and malignant diseases associated with altered B cell function and in murine models of these conditions. The adjuvant and regulatory activities that iNKT cells have for B cells makes them attractive therapeutic targets for these diseases.

Integrated analysis of mRNA-seq and miRNA-seq for host susceptibilities to influenza A (H7N9) infection in inbred mouse lines

Host genetic factors play an important role in diverse host outcomes after influenza A (H7N9) infection. Studying differential responses of inbred mouse lines with distinct genetic backgrounds to influenza virus infection could substantially increase our understanding of the contributory roles of host genetic factors to disease severity. Here, we utilized an integrated approach of mRNA-seq and miRNA-seq to investigate the transcriptome expression and regulation of host genes in C57BL/6J and DBA/2J mouse strains during influenza virus infection. The differential pathogenicity of influenza virus in C57BL/6J and DBA/2J has been fully demonstrated through immunohistochemical staining, histopathological analyses, and viral replication assessment. A transcriptional molecular signature correlates to differential host response to infection has been uncovered. With the introduction of temporal expression pattern analysis, we demonstrated that host factors responsible for influenza virus replication and host-virus interaction were significantly enriched in genes exhibiting distinct temporal dynamics between different inbred mouse lines. A combination of time-series expression analysis and temporal expression pattern analysis has provided a list of promising candidate genes for future studies. An integrated miRNA regulatory network from both mRNA-seq and miRNA-seq revealed several regulatory modules responsible for regulating host susceptibilities and disease severity. Overall, a comprehensive framework for analyzing host susceptibilities to influenza infection was established by integrating mRNA-seq and miRNA-seq data of inbred mouse lines. This work suggests novel putative molecular targets for therapeutic interventions in seasonal and pandemic influenza.

CD1-Restricted T Cells During Persistent Virus Infections: "Sympathy for the Devil"

Some of the clinically most important viruses persist in the human host after acute infection. In this situation, the host immune system and the viral pathogen attempt to establish an equilibrium. At best, overt disease is avoided. This attempt may fail, however, resulting in eventual loss of viral control or inadequate immune regulation. Consequently, direct virus-induced tissue damage or immunopathology may occur. The cluster of differentiation 1 (CD1) family of non-classical major histocompatibility complex class I molecules are known to present hydrophobic, primarily lipid antigens. There is ample evidence that both CD1-dependent and CD1-independent mechanisms activate CD1-restricted T cells during persistent virus infections. Sophisticated viral mechanisms subvert these immune responses and help the pathogens to avoid clearance from the host organism. CD1-restricted T cells are not only crucial for the antiviral host defense but may also contribute to tissue damage. This review highlights the two edged role of CD1-restricted T cells in persistent virus infections and summarizes the viral immune evasion mechanisms that target these fascinating immune cells.

Liposomal formulation of glycosphingolipids from Sphingomonas paucimobilis induces antitumour immunity in mice

Natural Killer T (NKT) cells play an important role in host's anti-tumour immune response. Glycosphingolipids (GSLs) isolated from Sphingomonas paucimobilis have the ability to stimulate NKT cells. In this study, the activity of free GSLs or GSLs-incorporated liposomes (glycosphingosomes) was investigated against dimethyl-α-benzanthracene (DMBA)-induced tumours in mice. The anti-tumour immunity of GSLs- or glycosphingosomes-loaded bone marrow-derived dendritic cells (BMDCs) was investigated in tumour-bearing mice. The Immunotherapeutic potential of co-administration of liposomal doxorubicin (Lip-Dox) and GSLs or glycosphingosomes was assessed by measuring cytokine levels and VEGF in the tumour tissues. Pretreatment with glycosphingosomes significantly delayed the frequency of tumour formation. Immunotherapy with glycosphingosomes-loaded BMDCs increased serum IFN-γ level and survival rate in mice. The effect of immunotherapy was dependent on effector functions of NK cells because the depletion of NK cells abolished the effects of immunotherapy. There was reduced tumour growth with low expression of VEGF in the group of mice treated with glycosphingosomes and Lip-Dox combination. Moreover, the splenocytes secreted higher levels of IFN-γ, IL-12 and lower TGF-β level. The results of this study indicate that glycosphingosomes can induce better antitumour immunity and may be considered a novel formulation in antitumour 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.

Natural Killer T Cells: An Ecological Evolutionary Developmental Biology Perspective

Type I natural killer T (NKT) cells are innate-like T lymphocytes that recognize glycolipid antigens presented by the MHC class I-like protein CD1d. Agonistic activation of NKT cells leads to rapid pro-inflammatory and immune modulatory cytokine and chemokine responses. This property of NKT cells, in conjunction with their interactions with antigen-presenting cells, controls downstream innate and adaptive immune responses against cancers and infectious diseases, as well as in several inflammatory disorders. NKT cell properties are acquired during development in the thymus and by interactions with the host microbial consortium in the gut, the nature of which can be influenced by NKT cells. This latter property, together with the role of the host microbiota in cancer therapy, necessitates a new perspective. Hence, this review provides an initial approach to understanding NKT cells from an ecological evolutionary developmental biology (eco-evo-devo) perspective.

Harnessing the Power of Invariant Natural Killer T Cells in Cancer Immunotherapy

Invariant natural killer T (iNKT) cells are a distinct subset of innate-like lymphocytes bearing an invariant T-cell receptor, through which they recognize lipid antigens presented by monomorphic CD1d molecules. Upon activation, iNKT cells are capable of not only having a direct effector function but also transactivating NK cells, maturing dendritic cells, and activating B cells, through secretion of several cytokines and cognate TCR-CD1d interaction. Endowed with the ability to orchestrate an all-encompassing immune response, iNKT cells are critical in shaping immune responses against pathogens and cancer cells. In this review, we examine the critical role of iNKT cells in antitumor responses from two perspectives: (i) how iNKT cells potentiate antitumor immunity and (ii) how CD1d+ tumor cells may modulate their own expression of CD1d molecules. We further explore hypotheses to explain iNKT cell activation in the context of cancer and how the antitumor effects of iNKT cells can be exploited in different forms of cancer immunotherapy, including their role in the development of cancer vaccines.

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

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

Initiation of Antiviral B Cell Immunity Relies on Innate Signals from Spatially Positioned NKT Cells

B cells constitute an essential line of defense from pathogenic infections through the generation of class-switched antibody-secreting cells (ASCs) in germinal centers. Although this process is known to be regulated by follicular helper T (TfH) cells, the mechanism by which B cells initially seed germinal center reactions remains elusive. We found that NKT cells, a population of innate-like T lymphocytes, are critical for the induction of B cell immunity upon viral infection. The positioning of NKT cells at the interfollicular areas of lymph nodes facilitates both their direct priming by resident macrophages and the localized delivery of innate signals to antigen-experienced B cells. Indeed, NKT cells secrete an early wave of IL-4 and constitute up to 70% of the total IL-4-producing cells during the initial stages of infection. Importantly, the requirement of this innate immunity arm appears to be evolutionarily conserved because early NKT and IL-4 gene signatures also positively correlate with the levels of neutralizing antibodies in Zika-virus-infected macaques. In conclusion, our data support a model wherein a pre-TfH wave of IL-4 secreted by interfollicular NKT cells triggers the seeding of germinal center cells and serves as an innate link between viral infection and B cell immunity.

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NKT cells promote B cell immunity upon viral infection

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NKT cells are primed by lymph-node-resident macrophages

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NKT cells produce early IL-4 wave at the follicular borders

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Early IL-4 wave is required for efficient seeding of germinal centers

CD1d is a novel cell-surface marker for human monocytic myeloid-derived suppressor cells with T cell suppression activity in peripheral blood after allogeneic hematopoietic stem cell transplantation

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of cells that regulate immune responses in cancer and various pathological conditions. However, the phenotypic and functional heterogeneity of human MDSCs represents a major hurdle for the development of therapeutic strategies targeting or regulating MDSCs in tumor progression, inflammation, and graft-versus-host disease (GVHD). We previously shown that circulating HLA-DR^-CD14⁺ monocytic MDSCs are a major contributor to clinical outcomes after allogeneic hematopoietic stem cell transplantation (allo-HSCT). In this study, we identified, using high-throughput screening, a set of surface markers that are strongly expressed in HLA-DR^-CD14⁺ monocytic MDSCs isolated from the peripheral blood (PB) of patients receiving allo-HSCT. Subsequent experiments showed the consistent dominant expression of CD1d in monocytic MDSCs of allo-HSCT PB in comparison with granulocytic MDSCs. In addition, CD1d-expressing cells isolated from PB of allo-HSCT patients showed the suppressive activity of T cell proliferation and higher expression of MyD88 and IDO compared with CD1d^- cells. Our results suggest that CD1d could be a valuable marker for further therapeutic evaluation of human monocytic MDSCs for immune-related diseases, including GVHD.

Natural Killer T Cells in Cancer Immunotherapy

Natural killer T (NKT) cells are specialized CD1d-restricted T cells that recognize lipid antigens. Following stimulation, NKT cells lead to downstream activation of both innate and adaptive immune cells in the tumor microenvironment. This has impelled the development of NKT cell-targeted immunotherapies for treating cancer. In this review, we provide a brief overview of the stimulatory and regulatory functions of NKT cells in tumor immunity as well as highlight preclinical and clinical studies based on NKT cells. Finally, we discuss future perspectives to better harness the potential of NKT cells for cancer therapy.

IFN-α-based treatment of patients with chronic HCV show increased levels of cells with myeloid-derived suppressor cell phenotype and of IDO and NOS

INTRODUCTION

Hepatitis C virus (HCV) infection causes chronic hepatitis, which is often associated with suppressed anti-HCV immune responses. We have recently reported accumulation of myeloid-derived suppressor cells (MDSCs) and suppressed immunity in cancer patients.

AIM

The main aim of this study was to determine whether chronic HCV patients harbor high of MDSCs in general and in nonresponders to IFN-based therapy in particular as well as to analyze the immune suppressive molecules.

METHODS

Peripheral blood samples withdrawn from 154 patients with chronic HCV infection and were categorized into responders and nonresponders based on viral titer upon IFN-α treatment.

RESULTS

The relative and absolute numbers of MDSCs defined as Lin^-/HLA-DR^-/CD33⁺/CD11b⁺ increased in all HCV patients, where they were higher in nonresponders than in responders. Additionally, the levels of MDSCs after 4-6 months of treatment in responders were lower than during the course of treatment. The responders also showed higher levels of IL-2 coincided with increased numbers of dendritic cells (DCs), CD4⁺ and CD8⁺ T cells. The levels of total NOS and IDO were also higher in nonresponders as compared to responders and healthy controls, while the expression levels of CD3ζ was lower in responders as compared to nonresponders and healthy volunteers.

CONCLUSION

Chronic HCV patients harbor high numbers of MDSCs, which are higher in nonresponders than in responders. The higher numbers of MDSCs associated with increases in the suppressing factors.

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.

Contribution of innate immune cells to pathogenesis of severe influenza virus infection

Influenza A viruses (IAVs) cause respiratory illness of varying severity based on the virus strains, host predisposition and pre-existing immunity. Ultimately, outcome and recovery from infection rely on an effective immune response comprising both innate and adaptive components. The innate immune response provides the first line of defence and is crucial to the outcome of infection. Airway epithelial cells are the first cell type to encounter the virus in the lungs, providing antiviral and chemotactic molecules that shape the ensuing immune response by rapidly recruiting innate effector cells such as NK cells, monocytes and neutrophils. Each cell type has unique mechanisms to combat virus-infected cells and limit viral replication, however their actions may also lead to pathology. This review focuses how innate cells contribute to protection and pathology, and provides evidence for their involvement in immune pathology in IAV infections.

Macrophage NOS2 in Tumor Leukocytes

SIGNIFICANCE

Leukocytes and especially macrophages are a major cellular constituent of the tumor mass. The tumor microenvironment not only determines their activity but in turn these cells also contribute to tumor initiation and progression. Recent Advances: Proinflammatory stimulated macrophages upregulate inducible nitric oxide synthase (NOS2) and produce high steady-state NO concentrations. NO provokes tumor cell death by initiating apoptosis and/or necrosis. Mechanisms may comprise p53 accumulation, immunestimulatory activities, and an increased efficacy of chemo- and/or radiotherapy. However, the potential cytotoxic activity of macrophages often is compromised in the tumor microenvironment and instead a protumor activity of macrophages dominates. Contributing factors are signals generated by viable and dying tumor cells, attraction and activation of myeloid-derived suppressor cells, and hypoxia. Limited oxygen availability not only attenuates NOS2 activity but also causes accumulation of hypoxia-inducible factors 1 and 2 (HIF-1/HIF-2). Activation of the HIF system is tightly linked to NO formation and affects the expression of macrophage phenotype markers that in turn add to tumor progression.

CRITICAL ISSUES

To make use of the cytotoxic arsenal of activated macrophages directed against tumor cells, it will be critical to understand how, when, and where these innate immune responses are blocked and whether it will be possible to reinstall their full capacity to kill tumor cells.

FUTURE DIRECTIONS

Low-dose irradiation or proinflammatory activation of macrophages in the tumor microenvironment may open options to boost NOS2 expression and activity and to initiate immunestimulatory features of NO that may help to restrict tumor growth. Antioxid. Redox Signal. 26, 1023-1043.

Reciprocal Crosstalk between Dendritic Cells and Natural Killer T Cells: Mechanisms and Therapeutic Potential

Natural killer T cells carrying a highly conserved, semi-invariant T cell receptor (TCR) [invariant natural killer T (iNKT) cells] are a subset of unconventional T lymphocytes that recognize glycolipids presented by CD1d molecules. Although CD1d is expressed on a variety of hematopoietic and non-hematopoietic cells, dendritic cells (DCs) are key presenters of glycolipid antigen in vivo. When stimulated through their TCR, iNKT cells rapidly secrete copious amounts of cytokines and induce maturation of DCs, thereby facilitating coordinated stimulation of innate and adaptive immune responses. The bidirectional crosstalk between DCs and iNKT cells determines the functional outcome of iNKT cell-targeted responses and iNKT cell agonists are used and currently being evaluated as adjuvants to enhance the efficacy of antitumor immunotherapy. This review illustrates mechanistic underpinnings of reciprocal DCs and iNKT cell interactions and discusses how those can be harnessed for cancer therapy.

Monocytic myeloid-derived suppressor cells regulate T-cell responses against vaccinia virus

Vaccinia virus (VV) can potently activate NK- and T-cell responses, leading to efficient viral control and generation of long-lasting protective immunity. However, immune responses against viral infections are often tightly controlled to avoid collateral damage and systemic inflammation. We have previously shown that granulocytic myeloid-derived suppressor cells (g-MDSCs) can suppress the NK-cell response to VV infection. It remains unknown what regulates T-cell responses to VV infection in vivo. In this study, we first showed that monocytic MDSCs (m-MDSCs), but not g-MDSCs, from VV-infected mice could directly suppress CD4⁺ and CD8⁺ T-cell activation in vitro. We then demonstrated that defective recruitment of m-MDSCs to the site of VV infection in CCR2^-/- mice enhanced VV-specific CD8⁺ T-cell response and that adoptive transfer of m-MDSCs into VV-infected mice suppressed VV-specific CD8⁺ T-cell activation, leading to a delay in viral clearance. Mechanistically, we further showed that T-cell suppression by m-MDSCs is mediated by indication of iNOS and production of NO upon VV infection, and that IFN-γ is required for activation of m-MDSCs. Collectively, our results highlight a critical role for m-MDSCs in regulating T-cell responses against VV infection and may suggest potential strategies using m-MDSCs to modulate T-cell responses during viral infections.

M1-like monocytes are a major immunological determinant of severity in previously healthy adults with life-threatening influenza

In each influenza season, a distinct group of young, otherwise healthy individuals with no risk factors succumbs to life-threatening infection. To better understand the cause for this, we analyzed a broad range of immune responses in blood from a unique cohort of patients, comprising previously healthy individuals hospitalized with and without respiratory failure during one influenza season, and infected with one specific influenza A strain. This analysis was compared with similarly hospitalized influenza patients with known risk factors (total of n = 60 patients recruited). We found a sustained increase in a specific subset of proinflammatory monocytes, with high TNF-α expression and an M1-like phenotype (independent of viral titers), in these previously healthy patients with severe disease. The relationship between M1-like monocytes and immunopathology was strengthened using murine models of influenza, in which severe infection generated using different models (including the high-pathogenicity H5N1 strain) was also accompanied by high levels of circulating M1-like monocytes. Additionally, a raised M1/M2 macrophage ratio in the lungs was observed. These studies identify a specific subtype of monocytes as a modifiable immunological determinant of disease severity in this subgroup of severely ill, previously healthy patients, offering potential novel therapeutic avenues.

In a cohort of influenza patients, previously healthy and young patients who succumbed to life-threatening disease were defined by high levels of circulating M1-like, TNF-α^hi monocytes.

Influenza A virus-induced release of interleukin-10 inhibits the anti-microbial activities of invariant natural killer T cells during invasive pneumococcal superinfection

During influenza A virus (IAV) infection, changes in the lung's physical and immunological defenses predispose the host to bacterial superinfections. Invariant natural killer T (iNKT) cells are innate-like T lymphocytes that have beneficial or harmful functions during infection. We investigated the iNKT cells' role in a model of invasive pneumococcal superinfection. The use of Jα18^-/- mice indicated that iNKT cells limited susceptibility to influenza-pneumococcal infection and reduced the lethal synergism. This role did not depend on immune-based anti-bacterial mechanisms. At the time of bacterial exposure, iNKT cells from IAV-experienced mice failed to produce antipneumococcal interferon-γ and adoptive transfer of fresh iNKT cells before Streptococcus pneumoniae challenge did not restore anti-bacterial host defenses. Impaired iNKT cell activation in superinfected animals was related to the IAV-induced immunosuppressive cytokine interleukin-10 (IL-10), rather than to an intrinsic functional defect. IL-10 dampened the activation of iNKT cells in response to pneumococci by inhibiting the production of IL-12 by pulmonary monocyte-derived dendritic cells. Neutralization of IL-10 restored iNKT cell activation and tends to increase resistance to secondary bacterial infection. Overall, iNKT cells have a beneficial role (upstream of bacterial colonization) in controlling influenza-pneumococcal superinfection, although they represent novel targets of immunosuppression at the time of bacterial challenge.

NNKTT120, an anti-iNKT cell monoclonal antibody, produces rapid and sustained iNKT cell depletion in adults with sickle cell disease

Invariant NKT (iNKT) cells can be activated to stimulate a broad inflammatory response. In murine models of sickle cell disease (SCD), interruption of iNKT cell activity prevents tissue injury from vaso-occlusion. NKTT120 is an anti-iNKT cell monoclonal antibody that has the potential to rapidly and specifically deplete iNKT cells and, potentially, prevent vaso-occlusion. We conducted an open-label, multi-center, single-ascending-dose study of NKTT120 to determine its pharmacokinetics, pharmacodynamics and safety in steady-state patients with SCD. Doses were escalated in a 3+3 study design over a range from 0.001 mg/kg to 1.0 mg/kg. Twenty-one adults with SCD were administered NKTT120 as part of 7 dose cohorts. Plasma levels of NKTT120 predictably increased with higher doses. Median half-life of NKTT120 was 263 hours. All subjects in the higher dose cohorts (0.1 mg/kg, 0.3 mg/kg, and 1 mg/kg) demonstrated decreased iNKT cells below the lower limit of quantification within 6 hours after infusion, the earliest time point at which they were measured. In those subjects who received the two highest doses of NKTT120 (0.3, 1 mg/kg), iNKT cells were not detectable in the peripheral blood for a range of 2 to 5 months. There were no serious adverse events in the study deemed to be related to NKTT120. In adults with SCD, NKTT120 produced rapid, specific and sustained iNKT cell depletion without any infusional toxicity or attributed serious adverse events. The next step is a trial to determine NKTT120’s ability to decrease rate of vaso-occlusive pain episodes.

Trial Registration: clinicaltrials.gov NCT01783691.

Adjuvant use of the NKT cell agonist alpha-galactosylceramide leads to enhancement of M2-based DNA vaccine immunogenicity and protective immunity against influenza A virus

DNA vaccines can induce both humoral and cellular immune responses in animals. However, DNA vaccines suffer from limited vaccine potency due to low immunogenicity. Therefore, different strategies are required for significant improvement of DNA vaccine efficacy such as inclusion of strong adjuvants. The aim of the present study was to investigate the effects of using α-Galactosylceramide (α-GalCer) as an adjuvant to enhance the immune responses induced by a DNA vaccine, encoding influenza A virus matrix protein 2 (M2), against influenza A challenge. BALB/c mice were immunized three times by intramuscular inoculations of DNA vaccine encoding M2 alone or in combination with α-GalCer adjuvant. The adjuvant effect was evaluated by measuring the serum antibody titers, using ELISA, lymphocyte proliferation, using MTT assay as well as Th1 (IFN-γ and IL-12) and Th2 (IL-4) cytokines. The results showed that co-administration of α-GalCer with the vaccine exert protective effects by influencing the magnitude and quality of humoral responses. Adjuvanted DNA-vaccinated mice revealed a higher IgG titer and IgG2a/IgG1 ratio than mice vaccinated with DNA alone. Furthermore, analysis of M2-specific responses revealed that the DNA vaccine triggered predominately IgG1 and IL-4 responses indicating a Th2 bias. The data also showed that α-GalCer is a potent adjuvant for activation of cellular immune responses to DNA vaccine. This was supported by a higher IgG2a/IgG1 ratio, significantly increased IFN-γ and IL-4 production and CD4+ proliferation, compared with mice receiving the DNA vaccine alone, suggesting a mixed Th1/Th2-type cellular immune response with a Th1 bias. The findings of this study indicate that α-GalCer has the potential to be used as a potent adjuvant for a DNA vaccine encoding M2, since it enhances humoral and cellular immune response and improves immune protection against influenza challenge in mice.

Expansion of CD11b+Ly-6C+ myeloid-derived suppressor cells (MDSCs) driven by galectin-9 attenuates CVB3-induced myocarditis

Galectin-9 is known to play a role in the modulation of innate and adaptive immunity to ameliorate CVB3-induced myocarditis. In the present study, we found that galectin-9 induced the expansion of CD11b⁺Ly-6C⁺ myeloid-derived suppressor cells (MDSCs) in the heart from CVB3-infected mice. Adoptive transfer of CD11b⁺Ly-6C⁺ MDSCs significantly alleviated myocarditis accompanied by increased Th2 and Treg frequency and anti-inflammatory cytokines expression in the heart tissue. Moreover, Ly6C⁺ MDSCs, but not Ly6G⁺ cells, expressed Arg-1 and NOS2, and suppressed CD4⁺ T cell proliferation in vitro in an Arg-1-dependent mechanism; an event that was reversed with treatment of either an Arg-1 inhibitor or addition of excess l-arginine. Furthermore, Ly6C⁺ MDSCs co-expressed higher levels of F4/80, Tim-3, and IL-4Rα, and had the plasticity to up-regulate NOS2 or Arg-1 in response to IFN-γ or IL-4 treatment. The present results indicate that galectin-9 expands CD11b⁺Ly-6C⁺ MDSCs to ameliorate CVB3-induced myocarditis.

The Role of Myeloid-Derived Suppressor Cells in Viral Infection

Myeloid-derived suppressor cells (MDSCs) are heterogeneous immature myeloid cells that are well described as potent immune regulatory cells during human cancer and murine tumor models. Reports of MDSCs during viral infections remain limited, and their association with immunomodulation of viral diseases is still being defined. Here, we provide an overview of MDSCs or MDSC-like cells identified during viral infections, including murine viral models and human viral diseases. Understanding the similarities and/or differences of virally induced versus tumor-derived MDSCs will be important for designing future immunotherapeutic approaches.

Improved Antitumor Activity of a Therapeutic Melanoma Vaccine through the Use of the Dual COX-2/5-LO Inhibitor Licofelone

Immune-suppressive cell populations impair antitumor immunity and can contribute to the failure of immune therapeutic approaches. We hypothesized that the non-steroidal anti-inflammatory drug licofelone, a dual cyclooxygenase-2/5-LO inhibitor, would improve therapeutic melanoma vaccination by reducing immune-suppressive cell populations. Therefore, licofelone was administered after tumor implantation, either alone or in combination with a peptide vaccine containing a long tyrosinase-related protein 2-peptide and the adjuvant α-galactosylceramide, all formulated into cationic liposomes. Mice immunized with the long-peptide vaccine and licofelone showed delayed tumor growth compared to mice given the vaccine alone. This protection was associated with a lower frequency of immature myeloid cells (IMCs) in the bone marrow (BM) and spleen of tumor-inoculated mice. When investigating the effect of licofelone on IMCs in vitro, we found that the prostaglandin E₂-induced generation of IMCs was decreased in the presence of licofelone. Furthermore, pre-incubation of BM cells differentiated under IMC-inducing conditions with licofelone reduced the secretion of cytokines interleukin (IL)-10 and -6 upon lipopolysaccharides (LPS) stimulation as compared to untreated cells. Interestingly, licofelone increased IL-6 and IL-10 secretion when administered after the LPS stimulus, demonstrating an environment-dependent effect of licofelone. Our findings support the use of licofelone to reduce tumor-promoting cell populations.

Rapid control of pandemic H1N1 influenza by targeting NKT-cells

Swine influenza A viruses (IAV) are a major cause of respiratory disease in pigs and humans. Currently approved anti-influenza therapies directly target the virus, but these approaches are losing effectiveness as new viral strains quickly develop drug resistance. To over come this challenge, there is an urgent need for more effective antiviral drugs. Here we tested the anti-influenza efficacy of the invariant natural killer T (NKT) cell superagonist, α-galactosylceramide (α-GalCer), which stimulates a wide array of anti-viral immune responses. We show that intranasal but not systemic administration of α-GalCer to piglets infected with pandemic A/California/04/2009 (CA04) H1N1 IAV ameliorated disease symptoms and resulted in the restoration of weight gain to the level of uninfected pigs. Correspondingly, viral titers in the upper-and lower-respiratory tract were reduced only in piglets that had received intranasal α-GalCer. Most significantly, lung inflammation as a consequence of virus persistence was largely prevented when NKT-cells were targeted via the respiratory route. Thus, targeting mucosal NKT-cells may provide a novel and potent platform for improving the course of disease in swine infected with seasonal and pandemic influenza viruses, and leads to the suggestion that this may also be true in humans and therefore deserves further study.

Respiratory viral infections and host responses; insights from genomics

Respiratory viral infections are a leading cause of disease and mortality. The severity of these illnesses can vary markedly from mild or asymptomatic upper airway infections to severe wheezing, bronchiolitis or pneumonia. In this article, we review the viral sensing pathways and organizing principles that govern the innate immune response to infection. Then, we reconstruct the molecular networks that differentiate symptomatic from asymptomatic respiratory viral infections, and identify the underlying molecular drivers of these networks. Finally, we discuss unique aspects of the biology and pathogenesis of infections with respiratory syncytial virus, rhinovirus and influenza, drawing on insights from genomics.

The inflammatory response triggered by Influenza virus: a two edged sword

Influenza A virus (IAV) is a relevant respiratory tract pathogen leading to a great number of deaths and hospitalizations worldwide. Secondary bacterial infections are a very common cause of IAV associated morbidity and mortality. The robust inflammatory response that follows infection is important for the control of virus proliferation but is also associated with lung damage, morbidity and death. The role of the different components of immune response underlying protection or disease during IAV infection is not completely elucidated. Overall, in the context of IAV infection, inflammation is a 'double edge sword' necessary to control infection but causing disease. Therefore, a growing number of studies suggest that immunomodulatory strategies may improve disease outcome without affecting the ability of the host to deal with infection. This review summarizes recent aspects of the inflammatory responses triggered by IAV that are preferentially involved in causing severe pulmonary disease and the anti-inflammatory strategies that have been suggested to treat influenza induced immunopathology.

Distinct gene expression patterns correlate with developmental and functional traits of iNKT subsets

Invariant natural killer T (iNKT) cells comprise a subpopulation of innate lymphocytes developing in thymus. A new model proposes subdividing murine iNKT cells into iNKT1, 2 and 17 cells. Here, we use transcriptome analyses of iNKT1, 2 and 17 subsets isolated from BALB/c and C57BL/6 thymi to identify candidate genes that may affect iNKT cell development, migration or function. We show that Fcɛr1γ is involved in generation of iNKT1 cells and that SerpinB1 modulates frequency of iNKT17 cells. Moreover, a considerable proportion of iNKT17 cells express IL-4 and IL-17 simultaneously. The results presented not only validate the usefulness of the iNKT1/2/17-concept but also provide new insights into iNKT cell biology.

Myeloid-derived suppressor cells are elevated in patients with psoriasis and produce various molecules

Psoriasis is a debilitating chronic inflammatory disease. In addition to the characteristic effects on the skin, chronic inflammation associated with the disease is recognized to contribute to cardiovascular, hepatic and renal comorbidities. Immature myeloid regulatory cells, known as myeloid‑derived suppressor cells (MDSCs), have been demonstrated to accumulate in various diseases and chronic inflammatory states, including inflammatory bowel disease and various types of cancer. The results of the present study, obtained using flow cytometry and cell culture analysis of peripheral blood mononuclear cells from psoriasis and healthy patients, revealed that MDSC levels are significantly increased in the blood of patients with psoriasis compared with healthy controls. Furthermore, these cells are capable of producing various molecules, including matrix metalloproteinase‑9 and‑1, interleukin‑8, growth‑related oncogene, and monocyte chemoattractant protein 1. These molecules may recruit additional immune cells involved in the pathogenesis of the disease, and contribute to the chronic inflammatory state in these patients. Therefore, MDSCs, which have various immune regulatory functions, may contribute to the pathogenesis of psoriasis as a systemic inflammatory disease.

IL-33 inhibits the differentiation and immunosuppressive activity of granulocytic myeloid-derived suppressor cells in tumor-bearing mice

Myeloid-derived suppressor cells (MDSCs) contribute to tumor-mediated immune escape by suppressing antitumor immune responses. Interleukin-33 (IL-33) is capable of regulating various immune cell populations; however, the effects of IL-33 on the differentiation of MDSCs have not been well characterized. In this study, we evaluated the effects of IL-33 on MDSCs and found that IL-33 significantly reduced the differentiation of lineage-negative bone marrow progenitor cells into granulocytic MDSCs (G-MDSCs). IL-33-treated MDSCs exhibited diminished immunosuppressive capacity; reduced inhibition on T-cell proliferation and interferon-γ production, and diminished production of reactive oxygen species. However, IL-33 treatment did not affect the frequency of monocytic MDSCs (M-MDSCs) or their production of nitric oxide and expression of arginase-1. Additionally, compared with control MDSCs, IL-33-treated MDSCs had reduced capacity to induce the differentiation or expansion of Treg cells. Moreover, in vivo IL-33 administration significantly decreased MDSCs and G-MDSCs accumulation in the spleen and tumor microenvironment. Also, despite increasing CD4⁺ and CD8⁺ T-cell infiltration, IL-33 administration markedly decreased Treg-cell population in tumor microenvironment. Taken together, our findings indicate that IL-33 reduces the frequency and immunosuppressive activity of G-MDSCs and ultimately the extent of tumor growth.

Activated NKT cells facilitated functional switch of myeloid-derived suppressor cells at inflammation sites in fulminant hepatitis mice

Myeloid-derived suppressor cells (MDSCs) confer immunosuppressive properties, but their roles in fulminant hepatitis have not been well defined. In this study, we systematically examined the distribution of MDSCs in bone marrow (BM), liver and spleen, and their functional and differentiation status in an acute fulminant hepatitis mouse model induced by lipopolysaccharide and D-galactosamine (LPS-GalN). Moreover, the interaction between NKT cells and MDSCs was determined. Our study revealed that BM contained the largest pool of MDSCs during pathogenesis of fulminant hepatitis compared with liver and spleen. MDSCs in liver/spleen expressed higher levels of chemokine receptors such as CCR2, CX3CR1 and CXCR2. At inflamed tissues such as liver or spleen, activated NKT cells induced differentiation of MDSCs through cell-cell interaction, which markedly dampened the immunosuppressive effects and promoted MDSCs to produce pro-inflammatory cytokines and activate inflammatory cells. Our findings thus demonstrated an unexpected pro-inflammatory state for MDSCs, which was mediated by the activated NKT cells that precipitated the differentiation and functional evolution of these MDSCs at sites of inflammation.

Activation strategies for invariant natural killer T cells

Invariant natural killer T (iNKT) cells are a specialized T cell subset that plays an important role in host defense, orchestrating both innate and adaptive immune effector responses against a variety of microbes. Specific microbial lipids and mammalian self lipids displayed by the antigen-presenting molecule CD1d can activate iNKT cells through their semi-invariant αβ T cell receptors (TCRs). iNKT cells also constitutively express receptors for inflammatory cytokines typically secreted by antigen-presenting cells (APCs) after recognition of pathogen-associated molecular patterns (PAMPs), and they can be activated through these cytokine receptors either in combination with TCR signals, or in some cases even in the absence of TCR signaling. During infection, experimental evidence suggests that both TCR-driven and cytokine-driven mechanisms contribute to iNKT cell activation. While the relative contributions of these two signaling mechanisms can vary widely depending on the infectious context, both lipid antigens and PAMPs mediate reciprocal activation of iNKT cells and APCs, leading to downstream activation of multiple other immune cell types to promote pathogen clearance. In this review, we discuss the mechanisms involved in iNKT cell activation during infection, focusing on the central contributions of both lipid antigens and PAMP-induced inflammatory cytokines, and highlight in vivo examples of activation during bacterial, viral, and fungal infections.

The Immunology of CD1- and MR1-Restricted T Cells

CD1- and MHC-related molecule-1 (MR1)-restricted T lymphocytes recognize nonpeptidic antigens, such as lipids and small metabolites, and account for a major fraction of circulating and tissue-resident T cells. They represent a readily activated, long-lasting population of effector cells and contribute to the early phases of immune response, orchestrating the function of other cells. This review addresses the main aspects of their immunological functions, including antigen and T cell receptor repertoires, mechanisms of nonpeptidic antigen presentation, and the current evidence for their participation in human and experimental diseases.

Non-glycosidic compounds can stimulate both human and mouse iNKT cells

Invariant natural killer T (iNKT) cells recognize CD1d/glycolipid complexes and upon activation with synthetic agonists display immunostimulatory properties. We have previously described that the non-glycosidic CD1d-binding lipid, threitolceramide (ThrCer) activates murine and human iNKT cells. Here, we show that incorporating the headgroup of ThrCer into a conformationally more restricted 6- or 7-membered ring results in significantly more potent non-glycosidic analogs. In particular, ThrCer 6 was found to promote strong anti-tumor responses and to induce a more prolonged stimulation of iNKT cells than does the canonical α-galactosylceramide (α-GalCer), achieving an enhanced T-cell response at lower concentrations compared with α-GalCer both in vitro, using human iNKT-cell lines and in vivo, using C57BL/6 mice. Collectively, these studies describe novel non-glycosidic ThrCer-based analogs that have improved potency in iNKT-cell activation compared with that of α-GalCer, and are clinically relevant iNKT-cell agonists.

Natural killer T cells in liver injury, inflammation and cancer

The liver is an organ that has the largest amount of natural killer T(NKT) cells, which play critical roles in the pathogenesis of liver diseases. In this article, the authors summarize recent findings about the roles of NKT cells in liver injury, inflammation, fibrosis, regeneration and cancer. In brief, NKT cells accelerate liver injury by producing pro-inflammatory cytokines and directly killing hepatocytes. NKT cells are involved in complex roles in liver fibrogenesis. For instance, NKT cells inhibit liver fibrosis via suppressing hepatic stellate cell activation and can also promote liver fibrosis via enhancing liver inflammation and injury. Inactivated or weakly activated NKT cells play a minimal role in controlling liver regeneration, whilst activated NKT cells have an inhibitory effect on liver regeneration. In liver cancer, NKT cells play both pro-tumor and anti-tumor roles in controlling tumor progress.

Does an NKT-cell-based immunotherapeutic approach have a future in multiple myeloma?

Natural killer T (NKT) cells constitute a unique subset of innate-like T lymphocytes which differ from conventional T cells by recognizing lipid antigens presented by the non-polymorphic major histocompatibility complex (MHC) I-like molecule CD1d. Despite being a relatively infrequent population of lymphocytes, NKT cells can respond rapidly upon activation with glycosphingolipids by production of cytokines which aim to polarize different axes of the immune system. Due to their dual effector capacities, NKT cells can play a vital role in cancer immunity, infection, inflammation and autoimmune diseases. It is believed that modulation of their activity towards immune activation can be a useful tool in anti-tumor immunotherapeutic strategies. Here we summarize the characteristics of NKT cells and discuss their involvement in immunosurveillance. Furthermore, an update is given about their role and the progress that has been made in the field of multiple myeloma (MM). Finally, some challenges are discussed that are currently hampering further progress.

The burgeoning family of unconventional T cells

While most studies of T lymphocytes have focused on T cells reactive to complexes of peptide and major histocompatibility complex (MHC) proteins, many other types of T cells do not fit this paradigm. These include CD1-restricted T cells, MR1-restricted mucosal associated invariant T cells (MAIT cells), MHC class Ib-reactive T cells, and γδ T cells. Collectively, these T cells are considered 'unconventional', in part because they can recognize lipids, small-molecule metabolites and specially modified peptides. Unlike MHC-reactive T cells, these apparently disparate T cell types generally show simplified patterns of T cell antigen receptor (TCR) expression, rapid effector responses and 'public' antigen specificities. Here we review evidence showing that unconventional T cells are an abundant component of the human immune system and discuss the immunotherapeutic potential of these cells and their antigenic targets.

α-Galactosylceramide protects swine against influenza infection when administered as a vaccine adjuvant

Natural killer T (NKT) -cells activated with the glycolipid ligand α-galactosylceramide (α-GalCer) stimulate a wide array of immune responses with many promising immunotherapeutic applications, including the enhancement of vaccines against infectious diseases and cancer. In the current study, we evaluated whether α-GalCer generates protective immunity against a swine influenza (SI) virus infection when applied as an intramuscular vaccine adjuvant. Immunization of newly weaned piglets with UV-killed pandemic H1N1 A/California/04/2009 (kCA04) SI virus and α-GalCer induced high titers of anti-hemagglutinin antibodies and generated virus-specific T cells that localized in intrapulmonary airways and in alveolar walls. Vaccination with α-GalCer resulted in a systemic increase in NKT-cell concentrations, including in the respiratory tract, which was associated with complete inhibition of viral replication in the upper and lower respiratory tract and much reduced viral shedding. These results indicate that NKT-cell agonists could be used to improve swine vaccine formulations in order to reduce the clinical signs of SI infection and limit the spread of influenza viruses amongst commercial pigs.

Monocytic myeloid-derived suppressor cells from females, but not males, alleviate CVB3-induced myocarditis by increasing regulatory and CD4(+)IL-10(+) T cells

Coxsackievirus group B type 3 (CVB3) is a common etiologic agent of viral myocarditis and often causes sexually dimorphic myocarditis with increased incidence and mortality in male. So far, the underlying mechanism for the high male prevalence is not well elucidated. In this study, we deciphered the role of myeloid-derived suppressor cells (MDSCs) in the gender bias in murine CVB3-induced myocarditis by comparing their frequencies, subsets as well as immune suppressive functions. We found that much more myocardial MDSCs were enriched in infected females than males, with dramatically higher percentage ratio of CD11b⁺Ly6G-Ly6C^high monocytic subset (M-MDSCs) to CD11b⁺Ly6G⁺Ly6C^low granulocytic subset (G-MDSCs). Interestingly, more potent suppression on T cell proliferation was also evidenced in female-derived M-MDSCs. Consistently, adoptive transfer of female- but not male-derived M-MDSCs efficiently alleviated CVB3-induced myocarditis in male recipient mice, and this protection could be ascribed to the increased induction of regulatory and CD4⁺IL-10⁺ T cells. Our study suggested that myocardial MDSCs were distinctively induced not only in quantities but also in phenotypes and immune suppressive functions in CVB3-infected males and females; and female-derived more suppressive M-MDSCs contributed to their insensitivity to CVB3-induced myocarditis.

Invariant NKT cells are resistant to circulating CD15+ myeloid-derived suppressor cells in patients with head and neck cancer

Myeloid-derived suppressor cells (MDSC) are a heterogeneous population of immature and progenitor myeloid cells with an immunosuppressive role in various types of cancer, including head and neck squamous cell carcinoma (HNSCC). However, the effect on the host immune system, especially on invariant NKT (iNKT) cells with potent anti-tumor activity, remains unclear. In this study, we investigated the effects of circulating MDSC subsets on the peripheral lymphocytes of patients with head and neck tumors. A significant accumulation of CD15+ granulocytic MDSC (G-MDSC) and CD14+ monocytic MDSC (M-MDSC) was demonstrated in HNSCC patients. The percentage of G-MDSC showed an inverse correlation with the percentage of T cells in the peripheral blood. The increased G-MDSC was significantly associated with advanced clinical stage and poor prognosis of HNSCC patients. The proliferation and viability of T cells were suppressed by CD15+ cells, and the suppression was reversed by adding the hydrogen peroxide scavenger catalase. However, iNKT cell activation upon α-galactosylceramide (αGalCer) stimulation was not affected by the presence or absence of CD15+ G-MDSC. These results indicate that increased G-MDSC negatively affects peripheral T cell immunity, but not iNKT cells, in HNSCC patients, and that T cells are more sensitive to hydrogen peroxide produced by G-MDSC than iNKT cells. Cancer immunotherapy designed to enhance the antitumor activity of iNKT cells by stimulation with αGalCer may remain effective in the presence of G-MDSC.

The emerging role of myeloid-derived suppressor cells in lung diseases

Myeloid-derived suppressor cells (MDSCs) are innate immune cells characterised by their potential to control T-cell responses and to dampen inflammation. While the role of MDSCs in cancer has been studied in depth, our understanding of their relevance for infectious and inflammatory disease conditions has just begun to evolve. Recent studies highlight an emerging and complex role for MDSCs in pulmonary diseases. In this review, we discuss the potential contribution of MDSCs as biomarkers and therapeutic targets in lung diseases, particularly lung cancer, tuberculosis, chronic obstructive pulmonary disease, asthma and cystic fibrosis.

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.