Both expansion of regulatory GR1+ CD11b+ myeloid cells and anergy of T lymphocytes participate in hyporesponsiveness of the lung-associated immune system during acute toxoplasmosis

Polyamines: the pivotal amines in influencing the tumor microenvironment

Cellular proliferation, function and survival is reliant upon maintaining appropriate intracellular polyamine levels. Due to increased metabolic needs, cancer cells elevate their polyamine pools through coordinated metabolism and uptake. High levels of polyamines have been linked to more immunosuppressive tumor microenvironments (TME) as polyamines support the growth and function of many immunosuppressive cell types such as MDSCs, macrophages and regulatory T-cells. As cancer cells and other pro-tumorigenic cell types are highly dependent on polyamines for survival, pharmacological modulation of polyamine metabolism is a promising cancer therapeutic strategy. This review covers the roles of polyamines in various cell types of the TME including both immune and stromal cells, as well as how competition for nutrients, namely polyamine precursors, influences the cellular landscape of the TME. It also details the use of polyamines as biomarkers and the ways in which polyamine depletion can increase the immunogenicity of the TME and reprogram tumors to become more responsive to immunotherapy.

Role of myeloid-derived suppressor cells during Trypanosoma cruzi infection

Chagas disease (CD), caused by the protozoan parasite Trypanosoma cruzi, is the third largest parasitic disease burden globally. Currently, more than 6 million people are infected, mainly in Latin America, but international migration has turned CD into an emerging health problem in many nonendemic countries. Despite intense research, a vaccine is still not available. A complex parasite life cycle, together with numerous immune system manipulation strategies, may account for the lack of a prophylactic or therapeutic vaccine. There is substantial experimental evidence supporting that T. cruzi acute infection generates a strong immunosuppression state that involves numerous immune populations with regulatory/suppressive capacity. Myeloid-derived suppressor cells (MDSCs), Foxp3+ regulatory T cells (Tregs), regulatory dendritic cells and B regulatory cells are some of the regulatory populations that have been involved in the acute immune response elicited by the parasite. The fact that, during acute infection, MDSCs increase notably in several organs, such as spleen, liver and heart, together with the observation that depletion of those cells can decrease mouse survival to 0%, strongly suggests that MDSCs play a major role during acute T. cruzi infection. Accumulating evidence gained in different settings supports the capacity of MDSCs to interact with cells from both the effector and the regulatory arms of the immune system, shaping the outcome of the response in a very wide range of scenarios that include pathological and physiological processes. In this sense, the aim of the present review is to describe the main knowledge about MDSCs acquired so far, including several crosstalk with other immune populations, which could be useful to gain insight into their role during T. cruzi infection.

The role of polyamine metabolism in remodeling immune responses and blocking therapy within the tumor immune microenvironment

The study of metabolism provides important information for understanding the biological basis of cancer cells and the defects of cancer treatment. Disorders of polyamine metabolism is a common metabolic change in cancer. With the deepening of understanding of polyamine metabolism, including molecular functions and changes in cancer, polyamine metabolism as a new anti-cancer strategy has become the focus of attention. There are many kinds of polyamine biosynthesis inhibitors and transport inhibitors, but not many drugs have been put into clinical application. Recent evidence shows that polyamine metabolism plays essential roles in remodeling the tumor immune microenvironment (TIME), particularly treatment of DFMO, an inhibitor of ODC, alters the immune cell population in the tumor microenvironment. Tumor immunosuppression is a major problem in cancer treatment. More and more studies have shown that the immunosuppressive effect of polyamines can help cancer cells to evade immune surveillance and promote tumor development and progression. Therefore, targeting polyamine metabolic pathways is expected to become a new avenue for immunotherapy for cancer.

Cleaved CD95L perturbs in vitro macrophages responses to Toxoplasma gondii

Toxoplasma gondii infects approximately 1-2 billion people, and manipulation of the macrophage response is critical to host and parasite survival. A cleaved (cl)-CD95L form can promote cellular migration and we have previously shown that cl-CD95L aggravates inflammation and pathology in systemic lupus erythematosus (SLE). Findings have shown that CD95L is upregulated during human infection, therefore we examined the effect of cl-CD95L on the macrophage response to T. gondii. . We find that cl-CD95L promotes parasite replication in macrophages, associated with increased arginase-1 levels, mediated by signal transducer and activator of transcription (STAT)6. Inhibition of both arginase-1 and STAT6 reversed the effects of cl-CD95L. Phospho-kinase array showed that cl-CD95L alters Janus Kinases (JAK)/STAT, mammalian target of rapamycin (mTOR), and Src kinase signals. By triggering changes in JAK/STAT cl-CD95L may limit anti-parasite effectors.

YAP1 induces marrow derived suppressor cell recruitment in Chlamydia trachomatis infection

Chlamydia trachomatis (C. trachomatis) is the most commonly diagnosed bacterial sexually transmitted infection (STI) worldwide. Marrow derived suppressor cells (MDSCs) are a heterogeneous population of immature monocytes and granulocytes, which are effective inhibitors for T cell activation. This study explores the role of MDSCs in the immune escape mechanism of C. trachomatis. We established a vaginal infection model of a BALB/c-Chlamydia trachomatis mouse pneumonia strain (MoPn), and compared the percentages of MDSCs, CD4⁺T, and CD8⁺T cells in the spleen and cervix of mice before and after infection. The expression levels of arginase-1 (Arg-1) and inducible nitric oxide synthase (iNOS) in MDSCs, and the expression level of transcriptional co-activator yes-associated protein 1 (YAP1) in the cervix were also compared. The results show that the proportion of MDSCs increases, while the proportion of CD4⁺T and CD8⁺T cells decreases after C. trachomatis-infection. The expression of Arg-1 and iNOS in MDSCs and YAP1 in host cells is up-regulated. C. trachomatis growth is inhibited after the inhibition of YAP1 in host cells. The proportion of MDSCs decreases after in vivo pharmacological inhibition of YAP1 in the C. trachomatis-infected mouse model. These results demonstrate, for the first time, the participation of MDSC in the immune escape of C. trachomatis under the action of YAP1.

Immunosuppressive CD14+/HLA-DRlow/‒ monocytes in patients with Chagas Disease

Chagas Disease (CD) is a neglected illness whose immunopathological mechanisms have not yet been plainly elucidated. The asymptomatic (indeterminate) form of CD is a long-term condition and approximately 20% to 35% of the individuals with this form evolve into one of the three chronic symptomatic clinical forms of CD, namely: cardiac, digestive or cardio-digestive (mixed). A variant of blood monocytes characterized by low expression of the HLA-DR antigen (CD14⁺/HLA-DR^low/‒) constitutes a subtype of myeloid-derived suppressor cells (MDSCs) whose main function is to regulate exacerbated inflammatory processes. The development of the symptomatic forms of CD can be related to the interaction between the host's immune system and the CD14⁺/HLA-DR^low/‒ immunosuppressive monocytes. Here, we evaluated, by flow cytometry, the absolute number and the HLA-DR antigenic density of this population of MDSCs in 57 patients with the diagnosis of CD: 34 with the symptomatic clinical forms (26 cardiac and 8 mixed) and 23 with the asymptomatic (indeterminate) form. The asymptomatic form exhibited a greater number of CD14⁺/HLA-DR^low/‒ monocytes and, accordingly, a low HLA-DR antigenic density, when compared to the symptomatic forms. It is possible to speculate that the predominance of CD14⁺/HLA-DR^low/- monocytes in the patients with the asymptomatic (indeterminate) form might have been a factor that could delay or even prevent the evolution of the asymptomatic form to the symptomatic forms of Chagas Disease.

STAT1-Dependent Recruitment of Ly6ChiCCR2+ Inflammatory Monocytes and M2 Macrophages in a Helminth Infection

Signal Transducer and Activator of Transcription (STAT) 1 signaling is critical for IFN-γ-mediated immune responses and resistance to protozoan and viral infections. However, its role in immunoregulation during helminth parasitic infections is not fully understood. Here, we used STAT1^-/- mice to investigate the role of this transcription factor during a helminth infection caused by the cestode Taenia crassiceps and show that STAT1 is a central molecule favoring susceptibility to this infection. STAT1^-/- mice displayed lower parasite burdens at 8 weeks post-infection compared to STAT1^+/+ mice. STAT1 mediated the recruitment of inflammatory monocytes and the development of alternatively activated macrophages (M2) at the site of infection. The absence of STAT1 prevented the recruitment of CD11b⁺Ly6C^hiLy6G^- monocytic cells and therefore their suppressive activity. This failure was associated with the defective expression of CCR2 on CD11b⁺Ly6C^hiLy6G^- cells. Importantly, CD11b⁺Ly6C^hiLy6G^- cells highly expressed PDL-1 and suppressed T-cell proliferation elicited by anti-CD3 stimulation. PDL-1⁺ cells were mostly absent in STAT1^-/- mice. Furthermore, only STAT1^+/+ mice developed M2 macrophages at 8 weeks post-infection, although macrophages from both T. crassiceps-infected STAT1^+/+ and STAT1^-/- mice responded to IL-4 in vitro, and both groups of mice were able to produce the Th2 cytokine IL-13. This suggests that CD11b⁺CCR2⁺Ly6C^hiLy6G^- cells give rise to M2 macrophages in this infection. In summary, a lack of STAT1 resulted in impaired recruitment of CD11b⁺CCR2⁺Ly6C^hiLy6G^- cells, failure to develop M2 macrophages, and increased resistance against T. crassiceps infection.

Myeloid-Derived Suppressor Cells in Trypanosoma cruzi Infection

Myeloid-derived suppressor cells (MDSCs) are immature heterogeneous myeloid cells that expand in pathologic conditions as cancer, trauma, and infection. Although characterization of MDSCs is continuously revisited, the best feature is their suppressor activity. There are many markers for MDSC identification, it is distinctive that they express inducible nitric oxide synthase (iNOS) and arginase 1, which can mediate immune suppression. MDSCs can have a medullary origin as a result of emergency myelopoiesis, but also can have an extramedullary origin. Early studies on Trypanosoma cruzi infection showed severe immunosuppression, and several mechanisms involving parasite antigens and host cell mediators were described as inhibition of IL-2 and IL-2R. Another mechanism of immunosuppression involving tumor necrosis factor/interferon γ-dependent nitric oxide production by inducible nitric oxide synthase was also described. Moreover, other studies showed that nitric oxide was produced by CD11b⁺ Gr-1⁺ MDSCs in the spleen, and later iNOS and arginase 1 expressed in CD11b⁺Ly6C⁺Ly6G^lo monocytic MDSC were found in spleen and heart of T. cruzi infected mice that suppressed T cell proliferation. Uncontrolled expansion of monocytic MDSCs leads to L-arginine depletion which hinders nitric oxide production leading to death. Supplement of L-arginine partially reverts L-arginine depletion and survival, suggesting that L-arginine could be administered along with anti-parasitical drugs. On the other hand, pharmacological inhibition of MDSCs leads to death in mice, suggesting that some expansion of MDSCs is needed for an efficient immune response. The role of signaling molecules mediating immune suppression as reactive oxygen species, reactive nitrogen species, as well as prostaglandin E2, characteristics of MDSCs, in T. cruzi infection is not fully understood. We review and discuss the role of these reactive species mediators produced by MDSCs. Finally, we discuss the latest results that link the SLAMF1 immune receptor with reactive oxygen species. Interaction of the parasite with the SLAMF1 modulates parasite virulence through myeloid cell infectivity and reactive oxygen species production. We discuss the possible strategies for targeting MDSCs and SLAMF1 receptor in acute Trypanosoma cruzi infection in mice, to evaluate a possible translational application in human acute infections.

Recent advances in myeloid-derived suppressor cell biology

In recent years, studying the role of myeloid-derived suppressor cells (MDSCs) in many pathological inflammatory conditions has become a very active research area. Although the role of MDSCs in cancer is relatively well established, their role in non-cancerous pathological conditions remains in its infancy resulting in much confusion. Our objectives in this review are to address some recent advances in MDSC research in order to minimize such confusion and to provide an insight into their function in the context of other diseases. The following topics will be specifically focused upon: (1) definition and characterization of MDSCs; (2) whether all MDSC populations consist of immature cells; (3) technical issues in MDSC isolation, estimation and characterization; (4) the origin of MDSCs and their anatomical distribution in health and disease; (5) mediators of MDSC expansion and accumulation; (6) factors that determine the expansion of one MDSC population over the other; (7) the Yin and Yang roles of MDSCs. Moreover, the functions of MDSCs will be addressed throughout the text.

Myeloid-derived suppressor cells exert immunosuppressive function on the T helper 2 in mice infected with Echinococcus granulosus

Cystic echinococcosis (CE) is a worldwide hazardous zoonotic parasitosis caused by Echinococcus granulosus. CE development involves complex immunological mechanisms, including participation of multiple immune cells and effector molecules. Myeloid-derived suppressor cells (MDSCs) are known to be involved in chronic and acute inflammatory conditions. In this study, we aimed to characterize the immune function of MDSCs in CE to improve the understanding, prevention and treatment of CE. Our results indicated that MDSCs overexpressing Ly6C and Ly6G inhibit the formation and activity of T helper 2 cells in a NO-dependent manner during E. granulosus infection.

Proliferation of Resident Macrophages Is Dispensable for Protection during Giardia duodenalis Infections

Infection with the intestinal parasite Giardia duodenalis is one of the most common causes of diarrheal disease in the world. Previous work has demonstrated that the cells and mechanisms of the adaptive immune system are critical for clearance of this parasite. However, the innate system has not been as well studied in the context of Giardia infection. We have previously demonstrated that Giardia infection leads to the accumulation of a population of CD11b⁺, F4/80⁺, ARG1⁺, and NOS2⁺ macrophages in the small intestinal lamina propria. In this report, we sought to identify the accumulation mechanism of duodenal macrophages during Giardia infection and to determine if these cells were essential to the induction of protective Giardia immunity. We show that F4/80⁺, CD11b⁺, CD11c^int, CX3CR1⁺, MHC class II⁺, Ly6C⁻, ARG1⁺, and NOS2⁺ macrophages accumulate in the small intestine during infections in mice. Consistent with this resident macrophage phenotype, macrophage accumulation does not require CCR2, and the macrophages incorporate EdU, indicating in situ proliferation rather than the recruitment of monocytes. Depletion of macrophages using anti-CSF1R did not impact parasite clearance nor development of regulatory T cell or Th17 cellular responses, suggesting that these macrophages are dispensable for protective Giardia immunity.

mTOR and Aging: An Old Fashioned Dress

Aging is a physiologic/pathologic process characterized by a progressive impairment of cellular functions, supported by the alterations of several molecular pathways, leading to an increased cell susceptibility to injury. This deterioration is the primary risk factor for several major human pathologies. Numerous cellular processes, including genomic instability, telomere erosion, epigenetic alterations, loss of proteostasis, deregulated nutrient-sensing, mitochondrial dysfunction, stem cell exhaustion, and altered intercellular signal transduction represent common denominators of aging in different organisms. Mammalian target of rapamycin (mTOR) is an evolutionarily conserved nutrient sensing protein kinase that regulates growth and metabolism in all eukaryotic cells. Studies in flies, worms, yeast, and mice support the hypothesis that the mTOR signalling network plays a pivotal role in modulating aging. mTOR is emerging as the most robust mediator of the protective effects of various forms of dietary restriction, which has been shown to extend lifespan and slow the onset of age-related diseases across species. Herein we discuss the role of mTor signalling network in the development of classic age-related diseases, focused on cardiovascular system, immune response, and cancer.

Modulation of MS-like disease by a multi epitope protein is mediated by induction of CD11c+CD11b+Gr1+ myeloid-derived dendritic cells

Specific neutralization of the pathogenic autoimmune cells is the ultimate goal in therapy of Multiple Sclerosis (MS). However, the pathogenic autoimmunity in MS, can be directed against several major target antigens, and therefore targeting pathogenic T-cells directed against a single target antigen is unlikely to be effective. To overcome this multiplicity and the potential complexity of pathogenic autoreactivities in MS, we have put forward the concept of concomitant multi-antigen/multi-epitope targeting as, a conceivably more effective approach to immunotherapy of MS. We constructed an (Experimental Autoimmune Encephalomeylitis (EAE)/MS-related synthetic human Target Autoantigen Gene (MS-shMultiTAG) designed to encode in tandem only EAE/MS related epitopes of all known encephalitogenic proteins. The MS-related protein product (designated Y-MSPc) was immunofunctional and upon tolerogenic administration, it effectively suppressed and reversed EAE induced by a single encephalitogenic protein. Furthermore, Y-MSPc also fully abrogated the development of "complex EAE" induced by a mixture of five encephalitogenic T-cell lines, each specific for a different encephalitogenic epitope of MBP, MOG, PLP, MOBP and OSP. Strikingly, Y-MSPc was consistently more effective than treatment with the single disease-specific peptide or with the peptide cocktail, both in suppressing the development of "classical" or "complex" EAE and in ameliorating ongoing disease. Overall, the modulation of EAE by Y-MSPc was associated with anergizing the pathogenic autoreactive T-cells, downregulation of Th1/Th17 cytokine secretion and upregulation of TGF-β secretion. Moreover, we show that both suppression and treatment of ongoing EAE by tolerogenic administration of Y-MSPc is associated also with a remarkable increase in a unique subset of dendritic-cells (DCs), CD11c+CD11b+Gr1+-myeloid derived DCs in both spleen and CNS of treated mice. These DCs, which are with strong immunoregulatory characteristics and are functional in down-modulation of MS-like-disease displayed increased production of IL-4, IL-10 and TGF-β and low IL-12. Functionally, these myeloid DCs suppress the in-vitro proliferation of myelin-specific T-cells and more importantly, the cells were functional in-vivo, as their adoptive transfer into EAE induced mice resulted in strong suppression of the disease, associated with a remarkable induction of CD4 + FoxP3+ regulatory cells. These results, which highlight the efficacy of "multi-epitope-targeting" agent in induction of functional regulatory CD11c+CD11b+Gr1+myeloid DCs, further indicate the potential role of these DCs in maintaining peripheral tolerance and their involvement in downregulation of MS-like-disease.

MDSCs in infectious diseases: regulation, roles, and readjustment

Many pathogens, ranging from viruses to multicellular parasites, promote expansion of MDSCs, which are myeloid cells that exhibit immunosuppressive features. The roles of MDSCs in infection depend on the class and virulence mechanisms of the pathogen, the stage of the disease, and the pathology associated with the infection. This work compiles evidence supported by functional assays on the roles of different subsets of MDSCs in acute and chronic infections, including pathogen-associated malignancies, and discusses strategies to modulate MDSC dynamics to benefit the host.

The role of myeloid-derived suppressor cells in chronic infectious diseases and the current methodology available for their study

An effective pathogen has the ability to evade the immune response. The strategies used to achieve this may be based on the direct action of virulence factors or on the induction of host factors. Myeloid-derived suppressor cells (MDSCs) are immune cells with an incredible ability to suppress the inflammatory response, which makes them excellent targets to be exploited by pathogenic bacteria, viruses, or parasites. In this review, we describe the origin and suppressive mechanisms of MDSCs, as well as their role in chronic bacterial, viral, and parasitic infections, where their expansion seems to be essential in the chronicity of the disease. We also analyze the disadvantages of current MDSC depletion strategies and the different in vitro generation methods, which can be useful tools for the deeper study of these cells in the context of microbial infections.

Role of Polyamines in Immune Cell Functions

The immune system is remarkably responsive to a myriad of invading microorganisms and provides continuous surveillance against tissue damage and developing tumor cells. To achieve these diverse functions, multiple soluble and cellular components must react in an orchestrated cascade of events to control the specificity, magnitude and persistence of the immune response. Numerous catabolic and anabolic processes are involved in this process, and prominent roles for l-arginine and l-glutamine catabolism have been described, as these amino acids serve as precursors of nitric oxide, creatine, agmatine, tricarboxylic acid cycle intermediates, nucleotides and other amino acids, as well as for ornithine, which is used to synthesize putrescine and the polyamines spermidine and spermine. Polyamines have several purported roles and high levels of polyamines are manifest in tumor cells as well in autoreactive B- and T-cells in autoimmune diseases. In the tumor microenvironment, l-arginine catabolism by both tumor cells and suppressive myeloid cells is known to dampen cytotoxic T-cell functions suggesting there might be links between polyamines and T-cell suppression. Here, we review studies suggesting roles of polyamines in normal immune cell function and highlight their connections to autoimmunity and anti-tumor immune cell function.

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.

Dual role of monocyte-derived dendritic cells in Trypanosoma cruzi infection

Pathogens can cause inflammation when inoculated into the skin. The vector-transmitted protozoan parasite Trypanosoma cruzi induces poor cellular-infiltration and disseminates, causing high mortality in the experimental model. Here, we characterized the inflammatory foci at the parasite inoculation site and secondary lymphoid organs using a murine model. While no macrophages and few neutrophils and monocytes (Mo) were recruited into the skin, T. cruzi infection elicited the mobilization of Ly6C⁺ Mo to draining lymph nodes and spleen. Over time, this population became enriched in CD11b⁺ Ly6C⁺ CD11c⁺ MHCII⁺ CD86⁺ cells resembling inflammatory dendritic cells (DCs). Adoptive transfer of Ly6C⁺ Mo purified from the bone marrow of CD11c-GFP transgenic mice confirmed the monocytic origin of Ly6C⁺ DCs found in the spleen of infected animals. Isolated Mo-derived cells not only produced TNF-α and nitric oxide, but also IL-10 and displayed a poor capacity to induce lymphoproliferation. Ablation of Mo-derived cells by 5-fluorouracil confirmed their dual role during infection, limiting the parasite load by inducible nitric oxide synthase-related mechanisms and negatively affecting the development of anti-parasite T-cell response. This study demonstrated that consistent with their antagonistic properties, these cells not only control the parasite spreading but also its persistence in the host.

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.

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.

Immature myeloid cells in the tumor microenvironment: Implications for immunotherapy

Various preclinical studies have demonstrated that the success of immunotherapeutic strategies in inhibiting tumor progression in animal models of Glioblastoma multiforme (GBM). It is also evident that tumor-induced immune suppression drastically impacts the efficacy of immune based therapies. Among the mechanisms employed by GBM to induce immunosuppression is the accumulation of regulatory T cells (Tregs) and Myeloid derived suppressor cells (MDSCs). Advancing our understanding about the pathways regulating the expansion, accumulation and activity of MDSCs will allow for the development of therapies aimed at abolishing the inhibitory effect of these cells on immunotherapeutic approaches. In this review, we have focused on the origin, expansion and immunosuppressive mechanisms of MDSCs in animal models and human cancer, in particular GBM.

mTOR inhibitors effects on regulatory T cells and on dendritic cells

The mammalian target of rapamycin (mTOR), a cytoplasmic serine/threonine kinase, represents a key biologic "switch" modulating cell metabolisms in response to environmental signals and is now recognized as a central regulator of the immune system. There is an increasing body of evidence supporting the hypothesis that mTOR inhibitors exhibit several biological properties in addition to immunosuppression, including anti-neoplastic effects, cardio-protective activities, and an array of immunomodulatory actions facilitating the development of an operational graft tolerance. The biological mechanisms explaining how mTOR inhibition can enable a tolerogenic state are still largely unclear. The induction of transplant tolerance might at the same time decrease rejection rate and minimize immunosuppression-related side effects, leading to an improvement in long-term graft outcome. In this scenario, T cell immunoregulation has been defined as the hallmark of peripheral tolerance. Two main immunologic cell populations have been reported to play a central role in this setting: regulatory T cells (Tregs) and dendritic cells (DCs). In this review we focus on mTOR inhibitors effects on Treg and DCs differentiation, activation, and function in the transplantation setting.

Oocyst-Derived Extract of Toxoplasma Gondii Serves as Potent Immunomodulator in a Mouse Model of Birch Pollen Allergy

Introduction

Previously, we have shown that oral infection with Toxoplasma gondii oocysts prevented type I allergy in mice. Here we investigated whether the application of a T. gondii oocyst lysate antigen (OLA) could also reduce allergy development. BALB/c mice were immunised twice with OLA followed by sensitisation with the major birch pollen (BP) allergen Bet v 1 and an aerosol challenge with BP extract.

Methods

First, we tested OLA in vitro. Stimulation of splenocytes and bone marrow-derived dendritic cells (BMDC) with OLA led to the production of pro-inflammatory and regulatory cytokines such as IL-6, IFN-γ and IL-10. Moreover, BMDC exposed to OLA upregulated the maturation markers CD40, CD80, CD86, and MHCII. Furthermore, OLA was recognised by TLR2-transfected human embryonic kidney cells.

Results

Immunisation of mice with OLA induced high levels of Toxoplasma-specific IgG antibodies in sera along with increased production of IFN-γ and IL-10 in Toxoplasma-antigen restimulated splenocytes. OLA reduced allergic airway inflammation as manifested by significant reduction of eosinophils in bronchoalveolar fluids, decreased cellular infiltrates and mucus production in the lungs. Accordingly, Bet v 1-specific IgE was decreased in OLA-pretreated mice. The reduced allergic immune responses were accompanied by increased numbers of CD4⁺CD25^highFoxp3⁺ regulatory T cells in spleens as well as by increased numbers of granulocytic myeloid-derived suppressor cells in lungs when compared to sensitised controls suggesting that these two cell populations might be involved in the suppression of the allergic immune responses.

Conclusion

Our data demonstrate that pretreatment with the oocyst extract can exert anti-allergic effects comparable to T. gondii infection. Thus, the immunomodulatory properties of the parasite extract indicate that this extract and in the future defined molecules thereof might serve as immunomodulatory adjuvants in allergy treatment and prophylaxis.

Tolerance and immune suppression in the tumor microenvironment

The concept of immunological tolerance has guided and permeated much of modern immunology. Ray Owen's ground-breaking observations in twin cattle provided the first mechanistic explanation for tolerance to self-molecules and established tolerance as a beneficial process that protects the host against autoreactivity. However, his studies also opened the door to understanding that tolerance may be detrimental, such as occurs when cancer cells induce tolerance/immune suppression resulting in inhibition of anti-tumor immunity. This article briefly traces the early history of the field of tumor immunology with respect to tolerance, and then focuses on a relatively recently identified population of cells called myeloid-derived suppressor cells (MDSCs). MDSC are instrumental in causing tolerance/immune suppression in individuals with cancer. They are present in most individuals with cancer and because of their potent immune suppressive activity are a major deterrent to natural anti-tumor immunity and a significant obstacle to immunotherapy.

Reciprocal relationship of T regulatory cells and monocytic myeloid-derived suppressor cells in LP-BM5 murine retrovirus-induced immunodeficiency

Immunomodulatory cellular subsets, including myeloid-derived suppressor cells (MDSCs) and T regulatory cells (Tregs), contribute to the immunosuppressive tumour microenvironment and are targets of immunotherapy, but their role in retroviral-associated immunosuppression is less well understood. Due to known crosstalk between Tregs and MDSCs in the tumour microenvironment, and also their hypothesized involvement during human immunodeficiency virus/simian immunodeficiency virus infection, studying the interplay between these immune cells during LP-BM5 retrovirus-induced murine AIDS is of interest. IL-10-producing FoxP3+ Tregs expanded after LP-BM5 infection. Following in vivo adoptive transfer of natural Treg (nTreg)-depleted CD4+T-cells, and subsequent LP-BM5 retroviral infection, enriched monocytic MDSCs (M-MDSCs) from these nTreg-depleted mice displayed altered phenotypic subsets. In addition, M-MDSCs from LP-BM5-infected nTreg-depleted mice exhibited increased suppression of T-cell, but not B-cell, responses, compared with M-MDSCs derived from non-depleted LP-BM5-infected controls. Additionally, LP-BM5-induced M-MDSCs modulated the production of IL-10 by FoxP3+ Tregs in vitro. These collective data highlight in vitro and for the first time, to the best of our knowledge, in vivo reciprocal modulation between retroviral-induced M-MDSCs and Tregs, and may provide insight into the immunotherapeutic targeting of such regulatory cells during retroviral infection.

Tofacitinib facilitates the expansion of myeloid-derived suppressor cells and ameliorates arthritis in SKG mice

OBJECTIVE

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of cells that have the ability to suppress T cell responses. The aim of this study was to evaluate the effects of the JAK inhibitor tofacitinib on MDSCs in a mouse model of rheumatoid arthritis.

METHODS

Arthritis was induced in SKG mice by zymosan A (ZyA) injection. MDSCs isolated from the bone marrow (BM) of donor SKG mice with arthritis were adoptively transferred to recipient mice with arthritis. In a separate experiment, tofacitinib was administered to arthritic SKG mice subcutaneously via osmotic pump, in some cases followed by injection of an anti-Gr-1 monoclonal antibody (mAb). BM cells from untreated mice were cultured for 5 days with granulocyte-macrophage colony-stimulating factor, with or without tofacitinib, and then analyzed by flow cytometry.

RESULTS

The numbers of MDSCs and polymorphonuclear MDSCs (PMN-MDSCs) were significantly increased in the spleens of SKG mice following ZyA injection. Adoptive transfer of MDSCs to recipient arthritic mice reduced the severity of arthritis compared to that in untreated control mice. Treatment with tofacitinib also ameliorated the progression of arthritis in SKG mice and induced significantly higher numbers of MDSCs and PMN-MDSCs in the BM of these animals. Furthermore, administration of an anti-Gr-1 mAb reduced the antiarthritic effect of tofacitinib in SKG mice. In vitro, tofacitinib facilitated the differentiation of BM cells to MDSCs, and inhibited their differentiation to dendritic cells.

CONCLUSION

Tofacitinib facilitates the expansion of MDSCs and ameliorates arthritis in SKG mice.

Myeloid-Derived Suppressor Cells: Critical Cells Driving Immune Suppression in the Tumor Microenvironment

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of immature myeloid cells that suppress innate and adaptive immunity. MDSCs are present in many disease settings; however, in cancer, they are a major obstacle for both natural antitumor immunity and immunotherapy. Tumor and host cells in the tumor microenvironment (TME) produce a myriad of pro-inflammatory mediators that activate MDSCs and drive their accumulation and suppressive activity. MDSCs utilize a variety of mechanisms to suppress T cell activation, induce other immune-suppressive cell populations, regulate inflammation in the TME, and promote the switching of the immune system to one that tolerates and enhances tumor growth. Because MDSCs are present in most cancer patients and are potent immune-suppressive cells, MDSCs have been the focus of intense research in recent years. This review describes the history and identification of MDSCs, the role of inflammation and intracellular signaling events governing MDSC accumulation and suppressive activity, immune-suppressive mechanisms utilized by MDSCs, and recent therapeutics that target MDSCs to enhance antitumor immunity.

Differential induction of Ly6G and Ly6C positive myeloid derived suppressor cells in chronic kidney and liver inflammation and fibrosis

CD11b⁺Gr1⁺ myeloid derived suppressor cells (MDSC) are known to be very potent suppressors of T cell immunity and can be further stratified into granulocytic MDSC and monocytic MDSC in mice based on expression of Ly6G or Ly6C, respectively. Here, using these markers and functional assays, we aimed to identify whether MDSC are induced during chronic inflammation leading to fibrosis in both kidney and liver and whether additional markers could more specifically identify these MDSC subsets. In an adenine-induced model of kidney inflammation/fibrosis suppressive Ly6G^pos MDSC were induced. The suppressive function within the Ly6G⁺ MDSC population was exclusively present in IFNγRβ expressing cells. In contrast, in chronic inflammation in the liver induced by bile duct ligation, suppressive capacity was exclusively present in the Ly6C^pos MDSC subset. Gene expression analyses confirmed the differential origins and regulation of those MDSC subsets. Additionally, depletion of MDSC in either kidney or liver fibrosis enhanced fibrosis markers, indicating a protective role for MDSC in organ fibrosis. Thus, our data demonstrate that during liver inflammation and kidney fibrosis MDSC with similar function arise bearing a distinct marker profile and arising from different cell populations.

A major role for myeloid-derived suppressor cells and a minor role for regulatory T cells in immunosuppression during Staphylococcus aureus infection

Staphylococcus aureus can cause difficult-to-treat chronic infections. We recently reported that S. aureus chronic infection was associated with a profound inhibition of T cell responses. In this study, we investigated the mechanisms responsible for the suppression of T cell responses during chronic S. aureus infection. Using in vitro coculture systems, as well as in vivo adoptive transfer of CFSE-labeled OT-II cells, we demonstrated the presence of immunosuppressive mechanisms in splenocytes of S. aureus-infected mice that inhibited the response of OT-II cells to cognate antigenic stimulation. Immunosuppression was IL-10/TGF-β independent but required cell-cell proximity. Using DEREG and Foxp3(gfp) mice, we demonstrated that CD4(+)CD25(+)Foxp3(+) regulatory T cells contributed, but only to a minor degree, to bystander immunosuppression. Neither regulatory B cells nor tolerogenic dendritic cells contributed to immunosuppression. Instead, we found a significant expansion of granulocytic (CD11b(+)Ly6G(+)Ly6C(low)) and monocytic (CD11b(+)Ly6G(-)Ly6C(high)) myeloid-derived suppressor cells (MDSC) in chronically infected mice, which exerted a strong immunosuppressive effect on T cell responses. Splenocytes of S. aureus-infected mice lost most of their suppressive activity after the in vivo depletion of MDSC by treatment with gemcitabine. Furthermore, a robust negative correlation was observed between the degree of T cell inhibition and the number of MDSC. An increase in the numbers of MDSC in S. aureus-infected mice by adoptive transfer caused a significant exacerbation of infection. In summary, our results indicate that expansion of MDSC and, to a minor degree, of regulatory T cells in S. aureus-infected mice may create an immunosuppressive environment that sustains chronic infection.

Contextual functions of antigen-presenting cells in the gastrointestinal tract

The immune system of the gastrointestinal tract must be tightly regulated to limit pathologic responses toward innocuous antigens while simultaneously allowing for rapid development of effector responses against invading pathogens. Highly specialized antigen-presenting cell (APC) subsets present in the gut play a dominant role in balancing these seemingly disparate functions. In this review, we discuss new findings associated with the function of gut APCs and particularly the contextual role of these cells in both establishing tolerance to orally acquired antigens in the steady state and regulating acute inflammation during infection.

Host immune response to infection and cancer: unexpected commonalities

Both microbes and tumors activate innate resistance, tissue repair, and adaptive immunity. Unlike acute infection, tumor growth is initially unapparent; however, inflammation and immunity affect all phases of tumor growth from initiation to progression and dissemination. Here, we discuss the shared features involved in the immune response to infection and cancer including modulation by commensal microbiota, reactive hematopoiesis, chronic immune responses and regulatory mechanisms to prevent collateral tissue damage. This comparative analysis of immunity to infection and cancer furthers our understanding of the basic mechanisms underlying innate resistance and adaptive immunity and their translational application to the design of new therapeutic approaches.

Impairment of T cell function in parasitic infections

In mammals subverted as hosts by protozoan parasites, the latter and/or the agonists they release are detected and processed by sensors displayed by many distinct immune cell lineages, in a tissue(s)-dependent context. Focusing on the T lymphocyte lineage, we review our present understanding on its transient or durable functional impairment over the course of the developmental program of the intracellular parasites Leishmania spp., Plasmodium spp., Toxoplasma gondii, and Trypanosoma cruzi in their mammalian hosts. Strategies employed by protozoa to down-regulate T lymphocyte function may act at the initial moment of naïve T cell priming, rendering T cells anergic or unresponsive throughout infection, or later, exhausting T cells due to antigen persistence. Furthermore, by exploiting host feedback mechanisms aimed at maintaining immune homeostasis, parasites can enhance T cell apoptosis. We will discuss how infections with prominent intracellular protozoan parasites lead to a general down-regulation of T cell function through T cell anergy and exhaustion, accompanied by apoptosis, and ultimately allowing pathogen persistence.

Engineered silica nanoparticles act as adjuvants to enhance allergic airway disease in mice

Background

With the increase in production and use of engineered nanoparticles (NP; ≤ 100 nm), safety concerns have risen about the potential health effects of occupational or environmental NP exposure. Results of animal toxicology studies suggest that inhalation of NP may cause pulmonary injury with subsequent acute or chronic inflammation. People with chronic respiratory diseases like asthma or allergic rhinitis may be even more susceptible to toxic effects of inhaled NP. Few studies, however, have investigated adverse effects of inhaled NP that may enhance the development of allergic airway disease.

Methods

We investigated the potential of polyethylene glycol coated amorphous silica NP (SNP; 90 nm diameter) to promote allergic airway disease when co-exposed during sensitization with an allergen. BALB/c mice were sensitized by intranasal instillation with 0.02% ovalbumin (OVA; allergen) or saline (control), and co-exposed to 0, 10, 100, or 400 μg of SNP. OVA-sensitized mice were then challenged intranasally with 0.5% OVA 14 and 15 days after sensitization, and all animals were sacrificed a day after the last OVA challenge. Blood and bronchoalveolar lavage fluid (BALF) were collected, and pulmonary tissue was processed for histopathology and biochemical and molecular analyses.

Results

Co-exposure to SNP during OVA sensitization caused a dose-dependent enhancement of allergic airway disease upon challenge with OVA alone. This adjuvant-like effect was manifested by significantly greater OVA-specific serum IgE, airway eosinophil infiltration, mucous cell metaplasia, and Th2 and Th17 cytokine gene and protein expression, as compared to mice that were sensitized to OVA without SNP. In saline controls, SNP exposure did cause a moderate increase in airway neutrophils at the highest doses.

Conclusions

These results suggest that airway exposure to engineered SNP could enhance allergen sensitization and foster greater manifestation of allergic airway disease upon secondary allergen exposures. Whereas SNP caused innate immune responses at high doses in non-allergic mice, the adjuvant effects of SNP were found at lower doses in allergic mice and were Th2/Th17 related. In conclusion, these findings in mice suggest that individuals exposed to SNP might be more prone to manifest allergic airway disease, due to adjuvant-like properties of SNP.

Anaphylatoxins coordinate innate and adaptive immune responses in allergic asthma

Allergic asthma is a chronic disease of the airways in which maladaptive Th2 and Th17 immune responses drive airway hyperresponsiveness (AHR), eosinophilic and neutrophilic airway inflammation and mucus overproduction. Airway epithelial and pulmonary vascular endothelial cells in concert with different resident and monocyte-derived dendritic cells (DC) play critical roles in allergen sensing and consecutive activation of TH cells and their differentiation toward TH2 and TH17 effector or regulatory T cells (Treg). Further, myeloid-derived regulatory cells (MDRC) act on TH cells and either suppress or enhance their activation. The complement-derived anaphylatoxins (AT) C3a and C5a are generated during initial antigen encounter and regulate the development of maladaptive immunity at allergen sensitization. Here, we will review the complex role of ATs in activation and modulation of different DC populations, MDRCs and CD4⁺ TH cells. We will also discuss the potential impact of ATs on the regulation of the pulmonary stromal compartment as an important means to regulate DC functions.

Chronic but not acute virus infection induces sustained expansion of myeloid suppressor cell numbers that inhibit viral-specific T cell immunity

Resolution of acute and chronic viral infections requires activation of innate cells to initiate and maintain adaptive immune responses. Here we report that infection with acute Armstrong (ARM) or chronic Clone 13 (C13) strains of lymphocytic choriomeningitis virus (LCMV) led to two distinct phases of innate immune response. During the first 72 hr of infection, dendritic cells upregulated activation markers and stimulated antiviral CD8(+) T cells, independent of viral strain. Seven days after infection, there was an increase in Ly6C(hi) monocytic and Gr-1(hi) neutrophilic cells in lymphoid organs and blood. This expansion in cell numbers was enhanced and sustained in C13 infection, whereas it occurred only transiently with ARM infection. These cells resembled myeloid-derived suppressor cells and potently suppressed T cell proliferation. The reduction of monocytic cells in Ccr2(-/-) mice or after Gr-1 antibody depletion enhanced antiviral T cell function. Thus, innate cells have an important immunomodulatory role throughout chronic infection.

Infection and cancer: revaluation of the hygiene hypothesis

Several studies have shown that persistent infections and inflammation can favor carcinogenesis. At the same time, certain types of pathogens and antitumor immune responses can decrease the risk of tumorigenesis or lead to cancer regression. Infectious agents and their products can orchestrate a wide range of host immune responses, through which they may positively or negatively modulate cancer development and/or progression. The factors that direct this dichotomous influence of infection-mediated immunity on carcinogenesis are not well understood. Even though not universal, several previous reports have investigated the inverse link of pathogen-induced "benign" inflammation to carcinogenesis and various other pathologies, ranging from autoimmune diseases to allergy and cancer. Several models and ideas are discussed in this review, including the impact of decreased exposure to pathogens, as well as the influence of pathogen load, the timing of infection, and the type of instigated immune response on carcinogenesis. These phenomena should guide future investigations into identifying novel targets within the microbial and host proteome, which will assist in the development of cancer therapeutics and vaccine remedies, analogous to earlier efforts based on helminthic components for the prevention and/or treatment of several pathologies.

Surveillance on the status of immune cells after Echinnococcus granulosus protoscoleces infection in Balb/c mice

Background

Cystic echinococcosis is a global parasitic disease caused by infection with Echinococcus granulosus larvae with potentially life-threatening complications in humans. To date, the status of the immune cells believed to be associated with the pathogenicity of E. granulosus infection has not been demonstrated clearly.

Methodology/Principal Findings

In this study, we developed a multiplex flow cytometry assay to investigate the systemic immune status of innate and adaptive immunity at 30, 180, 360 days post-infection (dpi) in mice infected with E. granulousus. At 30 dpi, an increase in the number of CD11b⁺ and CD11c⁺ antigen-presenting cells (APCs) was observed. This was accompanied by the slight down-regulated expression of the co-stimulatory molecule MHC-II, indicating the impairment of APCs in early infection through the release of secretory-excretory products. In all infected groups, we observed a significant increase in innate immune cells, including APCs and GR-1⁺ cells, and a dramatic increase in the myeloid-derived suppressor cells (MDSC) expressing CD11b⁺/GR-1⁺. Moreover, the upregulation of the activated markers CD69, CD44, CD40L, and the downregulation of CD62L were observed in the CD4⁺ and CD8⁺ T cells following infection. Regulatory T cells expressing CD4⁺/CD25⁺/FoxP₃⁺ increased significantly over the course of infection.

Conclusions

Our findings demonstrate that the microenvironment in the peripheral immune system after E. granulosus infection changes in subtle but detectably ways, especially during the persistent period of infection. We found that T cells were activated following infection, but observed that the significant increase of immunosuppressive cells such as MDSC and Treg cells could inhibit T cell response to E. granulosus antigens. We suggest these cells may play a neglected but key role in the downregulation of the immune response in long-term parasitic infection. Understanding the basic functions and temporal interactions of these immunosuppressive cells will pave the way for new strategies of parasite vaccine design.

In vivo ablation of plasmacytoid dendritic cells inhibits autoimmunity through expansion of myeloid-derived suppressor cells

Autoimmunity ensues upon breakdown of tolerance mechanism and priming of self-reactive T cells. Plasmacytoid dendritic cells (pDCs) constitute a unique cell subset that participates in the activation of autoreactive T cells but also has been shown to be critically involved in the induction of self-tolerance. However, their functional importance during the priming phase of an organ-specific autoimmune response remains unclear. In this study, we demonstrate that absence of pDCs during myelin antigenic challenge resulted in amelioration of experimental autoimmune encephalomyelitis and reduced disease severity. This was accompanied by significantly decreased frequency of myelin-specific T cells in the draining lymph nodes and inhibition of Th1 and Th17 immune responses. Unexpectedly, in vivo ablation of pDCs increased myelopoiesis in the bone marrow and specifically induced the generation of CD11b(hi)Gr1(+) myeloid-derived suppressor cells (MDSCs). Furthermore, we demonstrate that pDC depletion enhanced the mobilization of MDSCs in the spleen, and that sorted MDSCs could potently suppress CD4(+) T cell responses in vitro. Importantly, pDC-depleted mice showed increased levels of MCP-1 in the draining lymph nodes, and in vivo administration of MCP-1 increased the frequency and absolute numbers of MDSCs in the periphery of treated mice. Together, our results reveal that absence of pDCs during the priming of an autoimmune response leads to increased mobilization of MDSCs in the periphery in an MCP-1-dependent manner and subsequent amelioration of autoimmunity.

All-trans retinoic acid in combination with primaquine clears pneumocystis infection

Pneumocystis pneumonia (PcP) develops in immunocompromised patients. Alveolar macrophages play a key role in the recognition, phagocytosis, and degradation of Pneumocystis, but their number is decreased in PcP. Our study of various inflammatory components during PcP found that myeloid-derived suppressor cells (MDSCs) accumulate in the lungs of mice and rats with Pneumocystis pneumonia (PcP). We hypothesized that treatment with all-trans retinoic acid (ATRA), a metabolite of vitamin A, may effectively control Pneumocystis (Pc) infection by inducing MDSCs to differentiate to AMs. In rodent models of PcP, we found that 5 weeks of ATRA treatment reduced the number of MDSCs in the lungs and increased the number of AMs which cleared Pc infection. We also found that ATRA in combination with primaquine was as effective as the combination of trimethoprim and sulfamethaxazole for treatment of PcP and completely eliminated MDSCs and Pc organisms in the lungs in two weeks. No relapse of PcP was seen after three weeks of the ATRA-primaquine combination treatment. Prolonged survival of Pc-infected animals was also achieved by this regimen. This is the very first successful development of a therapeutic regimen for PcP that combines an immune modulator with an antibiotic, enabling the hosts to effectively defend the infection. Results of our study may serve as a model for development of novel therapies for other infections with MDSC accumulation.

Transplantation tolerance

Tolerance has been defined as graft-specific survival in the absence of continued immunosuppression. The mechanisms of central and peripheral tolerance are discussed in this review, as well as the barriers and limitations in achieving graft-specific tolerance. The need remains for definitive laboratory assays to determine the presence of a tolerant state. Genetic biomarker analysis pre-transplant may allow for better donor: recipient matching, lessening the need for immunosuppression, while post-transplant analysis of biomarkers, certain cytokines, and regulatory leukocytes may permit minimally invasive assessment of graft function and potentially, of graft-specific tolerance.

Myeloid-derived suppressor cells in murine retrovirus-induced AIDS inhibit T- and B-cell responses in vitro that are used to define the immunodeficiency

Myeloid-derived suppressor cells (MDSCs) have been characterized in several disease settings, especially in many tumor systems. Compared to their involvement in tumor microenvironments, however, MDSCs have been less well studied in their responses to infectious disease processes, in particular to retroviruses that induce immunodeficiency. Here, we demonstrate for the first time the development of a highly immunosuppressive MDSC population that is dependent on infection by the LP-BM5 retrovirus, which causes murine acquired immunodeficiency. These MDSCs express a cell surface marker signature (CD11b(+) Gr-1(+) Ly6C(+)) characteristic of monocyte-type MDSCs. Such MDSCs profoundly inhibit immune responsiveness by a cell dose- and substantially inducible nitric oxide synthase (iNOS)-dependent mechanism that is independent of arginase activity, PD-1-PD-L1 expression, and interleukin 10 (IL-10) production. These MDSCs display levels of immunosuppressive function in parallel with the extent of disease in LP-BM5-infected wild-type (w.t.) versus knockout mouse strains that are differentially susceptible to pathogenesis. These MDSCs suppressed not only T-cell but also B-cell responses, which are an understudied target for MDSC inhibition. The MDSC immunosuppression of B-cell responses was confirmed by the use of purified B responder cells, multiple B-cell stimuli, and independent assays measuring B-cell expansion. Retroviral load measurements indicated that the suppressive Ly6G(low/±) Ly6C(+) CD11b(+)-enriched MDSC subset was positive for LP-BM5, albeit at a significantly lower level than that of nonfractionated splenocytes from LP-BM5-infected mice. These results, including the strong direct MDSC inhibition of B-cell responsiveness, are novel for murine retrovirus-induced immunosuppression and, as this broadly suppressive function mirrors that of the LP-BM5-induced disease syndrome, support a possible pathogenic effector role for these retrovirus-induced MDSCs.

Myeloid-derived suppressor cells regulate natural killer cell response to adenovirus-mediated gene transfer

The attendant innate and adaptive immune responses to viral vectors have posed a significant hurdle for clinical application of viral vector-mediated gene therapy. Previous studies have shown that natural killer (NK) cells play a critical role in innate immune elimination of adenoviral vectors in the liver. However, it is not clear how the NK cell response to adenoviral vectors is regulated. In this study, we identified a role for granulocytic myeloid-derived suppressor cells (G-MDSCs) in this process. We show that in vivo administration of adenoviral vectors results in rapid accumulation of G-MDSCs early during adenoviral infection. In vivo depletion of both MDSC populations, but not monocytic MDSCs (M-MDSCs) alone, resulted in accelerated clearance of adenoviral vectors in the liver. This was accompanied by enhanced NK cell proliferation and activation, suggesting a role for MDSCs, probably G-MDSCs, in suppressing NK cell activation and function in vivo. We further demonstrate in vitro that G-MDSCs, but not M-MDSCs, are responsible for the suppression of NK cell activation. In addition, we show that adenoviral infection activated G-MDSCs to produce higher levels of reactive oxygen species (ROS) and that G-MDSC-mediated suppression of NK cells is mediated by ROS, specifically, H(2)O(2). This study demonstrates for the first time that the NK cell response to adenoviral vectors is negatively regulated by G-MDSCs and suggests that G-MDSC-based strategies could potentially improve the outcome of viral vector-mediated gene therapy.

Regulatory cells and immunosuppressive cytokines: parasite-derived factors induce immune polarization

Parasitic infections are prevalent in both tropical and subtropical areas. Most of the affected and/or exposed populations are living in developing countries where control measures are lacking or inadequately applied. Although significant progress has been made in our understanding of the immune response to parasites, no definitive step has yet been successfully done in terms of operational vaccines against parasitic diseases. Evidence accumulated during the past few years suggests that the pathology observed during parasitic infections is in part due to deregulation of normal components of the immune system, mainly cytokines, antibodies, and immune effector cell populations. A large number of studies that illustrate how parasites can modify the host immune system for their own benefit have been reported in both metazoan and protozoan parasites. The first line of defense against foreign organisms is barrier tissue such as skin, humoral factors, for instance the complement system and pentraxin, which upon activation of the complement cascade facilitate pathogen recognition by cells of innate immunity such as macrophages and DC. However, all the major groups of parasites studied have been shown to contain and/or to release factors, which interfere with both arms of the host immune system. Even some astonishing observations relate to the production by some parasites of orthologues of mammalian cytokines. Furthermore, chronic parasitic infections have led to the immunosuppressive environment that correlates with increased levels of myeloid and T suppressor cells that may limit the success of immunotherapeutic strategies based on vaccination. This minireview briefly analyzes some of the current data related to the regulatory cells and molecules derived from parasites that affect cellular function and contribute to the polarization of the immune response of the host. Special attention is given to some of the data from our laboratory illustrating the role of immunomodulatory factors released by protozoan parasites, in the induction and perpetuation of chronic disease.

Accumulation of myeloid-derived suppressor cells in the lungs during Pneumocystis pneumonia

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of hematopoietic precursors with the ability to adversely affect host immunity. They have been shown to accumulate in pathological conditions, such as cancer and some microbial diseases. In the mouse and rat models of Pneumocystis pneumonia (PcP), we found a distinct population of cells with MDSC-like morphology in the bronchoalveolar lavage (BAL) fluid, constituting up to 50% of the total cells in BAL fluid. These cells were not seen in the BAL fluid from normal animals or from Pneumocystis-infected animals that had been successfully treated for PcP with a combination of trimethoprim and sulfamethoxazole. With flow cytometry, these cells were found to express the characteristic MDSC surface markers Gr-1 and CD11b in mice or CD11bc and His48 in rats. Using reverse transcription-PCR, we demonstrated that these cells produced high levels of arginase-1 and inducible nitric oxide synthase (iNOS) mRNA. These cells were shown to suppress CD4(+) T-cell proliferation in response to stimulation by anti-CD3 and anti-CD28 antibodies. Adoptive transfer of these cells to normal mice caused lung damage, as indicated by elevated levels of albumin and lactate dehydrogenase in the BAL fluid. These experiments provide evidence of the presence of MDSCs in the lungs during PcP. Further studies on the roles of MDSCs in PcP are warranted in order to develop treatment strategies which can reduce the number of MDSCs and the damage caused by these cells.

NK cell response to vaccinia virus is regulated by myeloid-derived suppressor cells

NK cells are critical for the innate immune control of poxviral infections. Previous studies have shown that NK cells are efficiently activated in response to infection with vaccinia virus (VV), the most studied member of the poxvirus family. However, it remains unknown whether the activation of NK cells in response to VV infection is tightly regulated. In this study, we showed that myeloid-derived suppressor cells (MDSCs) rapidly accumulated at the site of VV infection. In vivo depletion of MDSCs led to enhanced NK cell proliferation, activation, and function in response to VV infection. This was accompanied by an increase in mortality and systemic IFN-γ production. We further demonstrated that the granulocytic-MDSC (G-MDSC) subset was responsible for the suppression on NK cells and that this suppression was mediated by reactive oxygen species. These results indicate that G-MDSCs can negatively regulate NK cell activation and function in response to VV infection and suggest that manipulation of G-MDSCs could represent an attractive strategy for regulating NK cell activities for potential therapeutic benefits.

On the armament and appearances of human myeloid-derived suppressor cells

Myeloid-derived suppressor cells (MDSC) have frequently been observed in patients with cancer. This heterogeneous population of myeloid cells can exert potent suppression of lymphocyte function and thereby poses a significant hurdle to natural or therapeutically induced anti-tumor immunity. On the other hand, the natural function of MDSC is not yet well understood and their role in infection, inflammation and autoimmune disease is still puzzling. Understanding MDSC biology will provide the tools necessary for therapeutic targeting of this population, but also permit exploitation of their strong tolerogenic function in the treatment of inflammatory conditions and the prevention of graft rejection.