Functional characterization of myeloid‐derived suppressor cell subpopulations during the development of experimental arthritis

Recent evidence indicates the existence of subpopulations of myeloid‐derived suppressor cells (MDSCs) with distinct phenotypes and functions. Here, we characterized the role of MDSC subpopulations in the pathogenesis of autoimmune arthritis in a collagen‐induced arthritis (CIA) mouse model. The splenic CD11b⁺Gr‐1⁺ MDSC population expanded in CIA mice, and these cells could be subdivided into polymorphonuclear (PMN) and mononuclear (MO) MDSC subpopulations based on Ly6C and Ly6G expression. During CIA, the proportion of splenic MO‐MDSCs was increased in association with the severity of joint inflammation, while PMN‐MDSCs were decreased. MO‐MDSCs expressed higher levels of surface CD40 and CD86 protein, but lower levels of Il10, Tgfb1, Ccr5, and Cxcr2 mRNA. PMN‐MDSCs exhibited a more potent capacity to suppress polyclonal T‐cell proliferation in vitro, compared with MO‐MDSCs. Moreover, the adoptive transfer of PMN‐MDSCs, but not MO‐MDSCs, decreased joint inflammation, accompanied by reduced levels of serum cytokine secretion and the frequencies of Th1 and Th17 cells in draining lymph nodes. These results suggest that there could be a shift from potently suppressive PMN‐MDSCs to poorly suppressive MO‐MDSCs during the development of experimental arthritis, which might reflect the failure of expanded MDSCs to suppress autoimmune arthritis.     

Neonatal myeloid derived suppressor cells show reduced apoptosis and immunosuppressive activity upon infection with Escherichia coli

Susceptibility to infection during the neonatal period and reduced control of inflammation in neonates are attributed to immunosuppression persisting from fetal life. Myeloid‐derived suppressor cells (MDSCs) are immature myeloid progenitors with suppressive activity and increased numbers in cord blood. We hypothesized that MDSCs contribute to innate host defence in neonates, paralleled by anti‐inflammatory signalling.Phagocytic activity, infection induced apoptosis, expression of B‐cell lymphoma (Bcl)‐2 family proteins, production of reactive oxygen species (ROS), cytokine production and T‐cell suppression of neonatal granulocytic‐MDSCs (G‐MDSCs) after infection with Escherichia coli (E. coli) were compared to neonatal autologous mature polymorphonuclear leukocytes (PMNs). Phagocytic activity of G‐MDSCs upon infection with E. coli was equal to that of mature PMNs, however, apoptosis of G‐MDSCs was decreased. G‐MDSCs showed enhanced Bcl‐2‐expression and lower ROS production compared to PMNs. Inhibition of Bcl‐2 reduced apoptosis rates of G‐MDSCs to that of mature PMNs. Induction of anti‐inflammatory transforming growth factor beta (TGF‐β) was enhanced, while pro‐inflammatory IL‐8 decreased in G‐MDSCs compared to PMNs. Infected G‐MDSCs strongly suppressed proliferation of T cells. We show a direct role of G‐MDSCs for anti‐bacterial host defence. Prolonged survival and anti‐inflammatory capacity suggest that G‐MDSCs are important for immune‐regulation after bacterial infection.     

Impaired antigen‐specific lymphocyte priming in mice after Toll‐like receptor 4 activation via induction of monocytic myeloid‐derived suppressor cells

In sepsis, the pathology involves a shift from a proinflammatory state toward an immunosuppressive phase. We previously showed that an agonistic anti‐TLR4 antibody induced long‐term endotoxin tolerance and suppressed antigen‐specific secondary IgG production when primed prior to immunization with antigen. These findings led us to speculate that TLR4‐induced innate tolerance due to primary infection causes an immunosuppressive pathology in sepsis. Therefore, the mechanism underlying impaired antigen‐specific humoral immunity by the TLR4 antibody was investigated. We showed, in a mouse model, that primary antigen‐specific IgG responses were impaired in TLR4 antibody‐induced tolerized mice, which was the result of reduced numbers of antigen‐specific GC B cells and plasma cells. Ovalbumin‐specific CD4 and CD8 T‐cell responses were impaired in TLR4 antibody‐injected OT‐I and ‐II transgenic mice ex vivo. Adoptive transfer studies demonstrated suppression of OVA‐specific CD4 and CD8 T‐cell responses by the TLR4 antibody in vivo. The TLR4 antibody induced Gr1⁺CD11b⁺ myeloid‐derived suppressor cell (MDSC) expansion with suppression of T‐cell activation. Monocytic MDSCs were more suppressive and exhibited higher expression of PD‐L1 and inducible nitric oxidase compared with granulocytic MDSCs. In conclusion, immune tolerance conferred by TLR4 activation induces the expansion of monocytic MDSCs, which impairs antigen‐specific T‐cell priming and IgG production.     

Human fibrocytic myeloid‐derived suppressor cells express IDO and promote tolerance via Treg‐cell expansion

By restraining T‐cell activation and promoting Treg‐cell expansion, myeloid‐derived suppressor cells (MDSCs) and tolerogenic DCs can control self‐reactive and antigraft effector T cells in autoimmunity and transplantation. Their therapeutic use and characterization, however, is limited by their scarce availability in the peripheral blood of tumor‐free donors. In the present study, we describe and characterize a novel population of human myeloid suppressor cells, named fibrocytic MDSC, which are differentiated from umbilical cord blood precursors by 4‐day culture with FDA‐approved cytokines (recombinant human‐GM‐CSF and recombinant human‐G‐CSF). This MDSC subset, characterized by the expression of MDSC‐, DC‐, and fibrocyte‐associated markers, promotes Treg‐cell expansion and induces normoglycemia in a xenogeneic mouse model of Type 1 diabetes. In order to exert their protolerogenic function, fibrocytic MDSCs require direct contact with activated T cells, which leads to the production and secretion of IDO. This new myeloid subset may have an important role in the in vitro and in vivo production of Treg cells for the treatment of autoimmune diseases, and in either the prevention or control of allograft rejection.     

Tumor‐induced myeloid‐derived suppressor cell subsets exert either inhibitory or stimulatory effects on distinct CD8+ T‐cell activation events

Tumor growth coincides with an accumulation of myeloid‐derived suppressor cells (MDSCs), which exert immune suppression and which consist of two main subpopulations, known as monocytic (MO) CD11b⁺CD115⁺Ly6G^?Ly6C^high MDSCs and granulocytic CD11b⁺CD115^?Ly6G⁺Ly6C^int polymorphonuclear (PMN)‐MDSCs. However, whether these distinct MDSC subsets hamper all aspects of early CD8⁺ T‐cell activation — including cytokine production, surface marker expression, survival, and cytotoxicity — is currently unclear. Here, employing an in vitro coculture system, we demonstrate that splenic MDSC subsets suppress antigen‐driven CD8⁺ T‐cell proliferation, but differ in their dependency on IFN‐γ, STAT‐1, IRF‐1, and NO to do so. Moreover, MO‐MDSC and PMN‐MDSCs diminish IL‐2 levels, but only MO‐MDSCs affect IL‐2Rα (CD25) expression and STAT‐5 signaling. Unexpectedly, however, both MDSC populations stimulate IFN‐γ production by CD8⁺ T cells on a per cell basis, illustrating that some T‐cell activation characteristics are actually stimulated by MDSCs. Conversely, MO‐MDSCs counteract the activation‐induced change in CD44, CD62L, CD162, and granzyme B expression, while promoting CD69 and Fas upregulation. Together, these effects result in an altered CD8⁺ T‐cell adhesiveness to the extracellular matrix and selectins, sensitivity to FasL‐mediated apoptosis, and cytotoxicity. Hence, MDSCs intricately influence different CD8⁺ T‐cell activation events in vitro, whereby some parameters are suppressed while others are stimulated.     

Granulocytic myeloid‐derived suppressor cells (GR‐MDSC) accumulate in cord blood of preterm infants and remain elevated during the neonatal period

Preterm delivery is the leading cause of perinatal morbidity and mortality. Among the most important complications in preterm infants are peri‐ or postnatal infections. Myeloid‐derived suppressor cells (MDSC) are myeloid cells with suppressive activity on other immune cells. Emerging evidence suggests that granulocytic MDSC (GR‐MDSC) play a pivotal role in mediating maternal–fetal tolerance. The role of MDSC for postnatal immune‐regulation in neonates is incompletely understood. Until the present time, nothing was known about expression of MDSC in preterm infants. In the present pilot study, we quantified GR‐MDSC counts in cord blood and peripheral blood of preterm infants born between 23 + 0 and 36 + 6 weeks of gestation (WOG) during the first 3 months of life and analysed the effect of perinatal infections. We show that GR‐MDSC are increased in cord blood independent of gestational age and remain elevated in peripheral blood of preterm infants during the neonatal period. After day 28 they drop to nearly adult levels. In case of perinatal or postnatal infection, GR‐MDSC accumulate further and correlate with inflammatory markers C‐reactive protein (CRP) and white blood cell counts (WBC). Our results point towards a role of GR‐MDSC for immune‐regulation in preterm infants and render them as a potential target for cell‐based therapy of infections in these patients.     

The impact of microRNAs on myeloid-derived suppressor cells in cancer

Inflammation promotes cancer development. To a large extent, this can be attributed to the recruitment of myeloid-derived suppressor cells (MDSCs) to tumors. These cells are known for establishing an immunosuppressive tumor microenvironment by suppressing T cell activities. However, MDSCs also promote metastasis and angiogenesis. Critically, as small non-coding RNAs that regulate gene expression, microRNAs (miRNAs) control MDSC activities. In this review, we discuss how miRNA networks regulate key MDSC signaling pathways, how they shape MDSC development, differentiation and activation, and how this impacts tumor development. By targeting the expression of miRNAs in MDSCs, we can alter their main signaling pathways. In turn, this can compromise their ability to promote multiple hallmarks of cancer. Therefore, this may represent a new powerful strategy for cancer immunotherapy.     

Myeloid-derived suppressor cells are increased in frequency but not maximally suppressive in peripheral blood of Type 1 Diabetes Mellitus patients

Type 1 Diabetes Mellitus (T1D) results from the destruction of insulin-producing beta cells in the pancreas by autoreactive T cells. Myeloid derived suppressor cells (MDSCs) are a recently identified immune cell subset that down-regulate T cells. Whether defects in MDSC numbers or function may contribute to T1D pathogenesis is not known. We report here that MDSCs are unexpectedly enriched in peripheral blood of both mice and patients with autoimmune diabetes. Peripheral blood MDSCs from T1D patients suppressed T cell proliferation in a contact-dependent manner; however, suppressive function could be enhanced with in vitro cytokine induction. These findings suggest that native T1D MDSCs are not maximally suppressive and that strategies to promote MDSC suppressive function may be effective in preventing or treating T1D.     

Granulocytic myeloid derived suppressor cells expand in human pregnancy and modulate T‐cell responses

Immune tolerance toward the semiallogeneic fetus plays a crucial role in the maintenance of pregnancy. Myeloid‐derived suppressor cells (MDSCs) are innate immune cells characterized by their ability to modulate T‐cell responses. Recently, we showed that MDSCs accumulate in cord blood of healthy newborns, yet their role in materno–fetal tolerance remained elusive. In the present study, we demonstrate that MDSCs with a granulocytic phenotype (GR‐MDSCs) are highly increased in the peripheral blood of healthy pregnant women during all stages of pregnancy compared with nonpregnant controls, whereas numbers of monocytic MDSCs were unchanged. GR‐MDSCs expressed the effector enzymes arginase‐I and iNOS, produced high amounts of ROS and efficiently suppressed T‐cell proliferation. After parturition, GR‐MDSCs decreased within a few days. In combination, our results show that GR‐MDSCs expand in normal human pregnancy and may indicate a role for MDSCs in materno–fetal tolerance.     

The roles of myeloid-derived suppressor cells in transplantation

CD11b⁺Gr1⁺ myeloid-derived suppressor cells (MDSCs) are an important regulatory innate cell population and have significant inhibitory effect on T cell-mediated responses. In addition to their negative role in cancer development, MDSCs also exert strong regulatory effects on transplantation and autoimmunity. In many transplantation models, such as bone marrow transplant, renal transplant, heart transplant and skin transplant settings, MDSCs accumulate and have inhibitory effect on graft rejection. However, the inducing factors, detailed phenotype and functional molecular mediators of MDSCs are significantly different in various transplant models. With their strong suppressive activity, MDSCs could become a potential clinical therapy during transplantation tolerance induction and the combination of the MDSCs with other immunoregulatory cells or immunosuppressive drugs is an intriguing protocol in the future. In this review, we will summarize MDSC expansion, activation and induction in different transplantation models and discuss the effects of immunoregulatory cells and immunosuppressive drugs on MDSCs in transplant settings.     

Hierarchy of immunosuppressive strength among myeloid‐derived suppressor cell subsets is determined by GM‐CSF

CD11b⁺/Gr‐1⁺ myeloid‐derived suppressor cells (MDSC) contribute to tumor immune evasion by restraining the activity of CD8⁺ T‐cells. Two major MDSC subsets were recently shown to play an equal role in MDSC‐induced immune dysfunctions: monocytic‐ and granulocytic‐like. We isolated three fractions of MDSC, i.e. CD11b⁺/Gr‐1^high, CD11b⁺/Gr‐1^int, and CD11b⁺/Gr‐1^low populations that were characterized morphologically, phenotypically and functionally in different tumor models. In vitro assays showed that CD11b⁺/Gr‐1^int cell subset, mainly comprising monocytes and myeloid precursors, was always capable to suppress CD8⁺ T‐cell activation, while CD11b⁺/Gr‐1^high cells, mostly granulocytes, exerted appreciable suppression only in some tumor models and when present in high numbers. The CD11b⁺/Gr‐1^int but not CD11b⁺/Gr‐1^high cells were also immunosuppressive in vivo following adoptive transfer. CD11b⁺/Gr‐1^low cells retained the immunosuppressive potential in most tumor models. Gene silencing experiments indicated that GM‐CSF was necessary to induce preferential expansion of both CD11b⁺/Gr‐1^int and CD11b⁺/Gr‐1^low subsets in the spleen of tumor‐bearing mice and mediate tumor‐induced tolerance whereas G‐CSF, which preferentially expanded CD11b⁺/Gr‐1^high cells, did not create such immunosuppressive environment. GM‐CSF also acted on granulocyte–macrophage progenitors in the bone marrow inducing local expansion of CD11b⁺/Gr‐1^low cells. These data unveil a hierarchy of immunoregulatory activity among MDSC subsets that is controlled by tumor‐released GM‐CSF.     

Serum inflammatory factors and circulating immunosuppressive cells are predictive markers for efficacy of radiofrequency ablation in non‐small‐cell lung cancer

In recent years, percutaneous radiofrequency ablation (RFA) has been developed as a new tool in the treatment of non‐small‐cell lung cancer (NSCLC) in non‐surgical patients. There is growing evidence that RFA‐mediated necrosis can modulate host immune responses. Here we analysed serum inflammatory factors as well as immunosuppressive cells in the peripheral blood to discover possible prognostic indicators. Peripheral blood and serum samples were collected before RFA and within 3 months after the treatment in a total of 12 patients. Inflammatory cytokines and growth factors were measured in serum by the Bio‐Plex assay. Myeloid‐derived suppressor cells (MDSCs) and regulatory T cells (T_regs) were evaluated in the peripheral blood via flow cytometry. In patients developing local or lymphogenic tumour relapse (n = 4), we found an early significant increase in the concentration of tumour necrosis factor (TNF)‐α as well as chemokine (C‐C motif) ligand (CCL)‐2 and CCL‐4 compared to patients without relapse (n = 4) and healthy donors (n = 5). These changes were associated with an elevated activity of circulating MDSC indicated by an increased nitric oxide (NO) production in these cells. Elevated serum levels of TNF‐α, CCL‐2 and CCL‐4 associated with an increased NO production in circulating MDSCs might be an early indicator of the incomplete RFA and subsequently a potential tumour relapse in NSCLC.     

High Frequency of Mononuclear Myeloid-Derived Suppressor Cells is Associated with Exacerbation of Inflammatory Bowel Disease

Exacerbation and relapse of inflammatory bowel disease (IBD) is associated with reduced antibacterial immunity and increased immune regulatory activity, but the source of increased immune regulation during episodes of disease activity is unclear. Myeloid-derived suppressor cells (MDSCs) are a cell type with a well-recognized role in limiting immune reactions. MDSC function in IBD and its relation to disease activity, however, remains unexplored. Here we show that patients with either ulcerative colitis (UC) or Crohn's disease (CD) have high peripheral blood levels of mononuclear MDSCs. Especially exacerbation of disease is associated with higher levels of mononuclear MDSC counts compared with those in remission of disease. Interestingly, chronic experimental colitis in mice coincides with increased MDCS mobilization. Thus, our results suggest that mononuclear MDCS are endogenous antagonists of immune system functionality in mucosal inflammation and the depression of antibacterial immunity associated with exacerbation of disease might involve increased activity of the MDSC compartment.     

MDSCs drive the process of endometriosis by enhancing angiogenesis and are a new potential therapeutic target

Endometriosis affects women of reproductive age via unclear immunological mechanism(s). Myeloid‐derived suppressor cells (MDSCs) are a heterogeneous group of myeloid cells with potent immunosuppressive and angiogenic properties. Here, we found MDSCs significantly increased in the peripheral blood of patients with endometriosis and in the peritoneal cavity of a mouse model of surgically induced endometriosis. Majority of MDSCs were granulocytic, produced ROS, and arginase, and suppressed T‐cell proliferation. Depletion of MDSCs by antiGr‐1 antibody dramatically suppressed development of endometrial lesions in mice. The chemokines CXCL1, 2, and 5 were expressed at sites of lesion while MDSCs expressed CXCR‐2. These CXC‐chemokines promoted MDSC migration toward endometriotic implants both in vitro and in vivo. Also, CXCR2‐deficient mice show significantly decreased MDSC induction, endometrial lesions, and angiogenesis. Importantly, adoptive transfer of MDSCs into CXCR2‐KO mice restored endometriotic growth and angiogenesis. Together, this study demonstrates that MDSCs play a role in the pathogenesis of endometriosis and identifies a novel CXC‐chemokine and receptor for the recruitment of MDSCs, thereby providing a potential target for endometriosis treatment.     

Myeloid-derived Suppressor Cells Adhere to Physiologic STAT3- vs STAT5-dependent Hematopoietic Programming,Establishing Diverse Tumor-Mediated Mechanisms of Immunologic Escape

The receptor tyrosine kinase inhibitor, sunitinib, is astonishingly effective in its capacity to reduce MDSCs in peripheral tissues such as blood (human) and spleen (mouse), restoring responsiveness of bystander T lymphocytes to TcR stimulation. Sunitinib blocks proliferation of undifferentiated MDSCs and decreases survival of more differentiated neutrophilic MDSC (n-MDSC) progeny. Ironically, sunitinib’s profound effects are observed even in a total absence of detectable anti-tumor therapeutic response. This is best explained by the presence of disparate MDSC-conditioning stimuli within individual body compartments, allowing sensitivity and resistance to sunitinib to coexist within the same mouse or patient. The presence or absence of GM-CSF is likely the major determinant in each compartment, given that GM-CSF’s capacity to preempt STAT3-dependent with dominant STAT5-dependent hematopoietic programming confers sunitinib resistance and redirects differentiation from the n-MDSC lineage to the more versatile monocytoid (m-MDSC) lineage. The clinical sunitinib experience underscores that strategies for MDSC and Treg depletions must be mindful of disparities among body compartments to avoid sanctuary effects. Ironically, m-MDSCs manifesting resistance to sunitinib also have the greatest potential to differentiate into tumoricidal accessory cells, by virtue of their capacity to respond to T cell-secreted IFN-γ or to TLR agonists with nitric oxide and peroxynitrate production.     

Role of myeloid-derived suppressor cells in mouse pre-sensitized cardiac transplant model

Harness of sensitized transplantation remains a clinical challenge particularly in parallel with prolonged cold ischemia time (PCI)-mediated injury. Our present study was to test the role of myeloid-derived suppressor cells (MDSCs) in mouse pre-sensitized transplantation. Our findings revealed that CD11b + Gr1^low MDSC was shown to have strong suppressive activity. MDSCs subsets from the tolerated mice exhibited higher suppressive capacities compared with counterparts from naive (untreated) mice. Depletion of Tregs could not affect splenic CD11b + Gr1^-low MDSC frequency, but increase peripheral and intragraft CD11b + Gr1^-low frequency. Intriguingly, boost of Tregs remarkably caused an increase of CD11b + Gr1^-low frequency in the graft, peripheral blood, and spleen. Furthermore, peripheral CD11b + Gr1^-low cells were massively accumulated at the early stage when allogeneic immune response was enhanced. Taken together, MDSCs could prevent grafts from PCI-mediated injury independent on Tregs in the pre-sensitized transplant recipients. Utilization of MDSC subset particularly CD11b + Gr1^-low might provide a novel insight into improving graft outcome under such clinical scenarios.     

IL-1β regulates a novel myeloid-derived suppressor cell subset that impairs NK cell development and function

Chronic inflammation is associated with promotion of malignancy and tumor progression. Many tumors enhance the accumulation of myeloid-derived suppressor cells (MDSC), which contribute to tumor progression and growth by suppressing anti-tumor immune responses. Tumor-derived IL-1β secreted into the tumor microenvironment has been shown to induce the accumulation of MDSC possessing an enhanced capacity to suppress T cells. In this study, we found that the enhanced suppressive potential of IL-1β-induced MDSC was due to the activity of a novel subset of MDSC lacking Ly6C expression. This subset was present at low frequency in tumor-bearing mice in the absence of IL-1β-induced inflammation; however, under inflammatory conditions, Ly6C(neg) MDSC were predominant. Ly6C(neg) MDSC impaired NK cell development and functions in vitro and in vivo. These results identify a novel IL-1β-induced subset of MDSC with unique functional properties. Ly6C(neg) MDSC mediating NK cell suppression may thus represent useful targets for therapeutic interventions.     

Immunosuppressive functions of hepatic myeloid-derived suppressor cells of normal mice and in a murine model of chronic hepatitis B virus

The immunosuppressive state of tumour-bearing hosts is attributable, at least in part, to myeloid-derived suppressor cells (MDSC). However, the role of MDSC in physiological conditions and diseases other than cancer has not been addressed. As the liver is a tolerogenic organ, the present study attempted to localize and assess functions of hepatic MDSC in a normal liver and in a murine model of chronic hepatitis B virus (HBV) infection. MDSC was identified in the liver of normal mice and HBV transgenic mice (TM) as CD11b(+) Gr1(+) cells by dual-colour flow cytometry. Highly purified populations of MDSC and their subtypes were isolated by fluorescence-activated cell sorting. The functions of MDSC and their subtypes were evaluated in allogenic mixed lymphocyte reaction (MLR) and hepatitis B surface antigen (HBsAg)-specific T cell proliferation assays. Normal mice-derived liver MDSC, but not other myeloid cells (CD11b(+) Gr1(-) ), suppressed T cell proliferation in allogenic MLR in a dose-dependent manner. Alteration of T cell antigens and impaired interferon-γ production seems to be related to MDSC-induced immunosuppression. In HBV TM, the frequencies of liver MDSC were about twice those of normal mice liver (13·6±3·2% versus 6·05±1·21%, n=5, P<0·05). Liver-derived MDSC from HBV TM also suppressed proliferative capacities of allogenic T cells and HBsAg-specific lymphocytes. Liver MDSC may have a critical role in maintaining homeostasis during physiological conditions. As liver MDSC had immunosuppressive functions in HBV TM, they may be a target of immune therapy in chronic HBV infection.     

Myeloid‐Derived Suppressor Cells in Cancers and Inflammatory Diseases: Angel or Demon?

Myeloid‐derived suppressor cell (MDSC) is an important heterogeneous cell population that regulates innate and adaptive immune. The immunosuppressive potentiality is widely known. However, more and more data indicated that the heterogeneous cell population had immunostimulatory activities rather than immunosuppression in some microenvironments. Chronic inflammatory conditions contribute to accumulation of MDSCs. These pathologically activated cells played critical roles in cancer and inflammatory diseases. In this review, we mainly discuss the immunoregulation potentiality of MDSCs in cancer and inflammatory diseases.