Reactive oxygen species and 12/15-lipoxygenase contribute to the antiproliferative capacity of alternatively activated myeloid cells elicited during helminth infection

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.

Monocytes maintain central nervous system homeostasis following helminth-induced inflammation

Neuroimmune interactions are crucial for regulating immunity and inflammation. Recent studies have revealed that the central nervous system (CNS) senses peripheral inflammation and responds by releasing molecules that limit immune cell activation, thereby promoting tolerance and tissue integrity. However, the extent to which this is a bidirectional process, and whether peripheral immune cells also promote tolerance mechanisms in the CNS remains poorly defined. Here we report that helminth-induced type 2 inflammation promotes monocyte responses in the brain that are required to inhibit excessive microglial activation and host death. Mechanistically, infection-induced monocytes express YM1 that is sufficient to inhibit tumor necrosis factor production from activated microglia. Importantly, neuroprotective monocytes persist in the brain, and infected mice are protected from subsequent lipopolysaccharide-induced neuroinflammation months after infection-induced inflammation has resolved. These studies demonstrate that infiltrating monocytes promote CNS homeostasis in response to inflammation in the periphery and demonstrate that a peripheral infection can alter the immunologic landscape of the host brain.

Myeloid-Derived Suppressor Cells: The Expanding World of Helminth Modulation of the Immune System

Infection with helminths or parasitic worms are highly prevalent worldwide especially in developing regions. Helminths cause chronic infections that are associated with suppression of immune responses to unrelated pathogens, vaccines, and by-stander antigens responsible for dysregulated immune responses as occurs in diseases such as allergies. Helminths use multiple mechanisms to modulate the immune system to evade the highly polarized type 2 immune response required to expel adult worms and for immunity to reinfection. Anthelmintic drugs are efficient in reducing adult worm burdens in helminth-infected individuals, but resistance to these drugs is rapidly increasing and vaccines against these pathogens are not available. Emerging evidence indicate that helminths induce myeloid-derived suppressor cells (MDSC), originally described in tumor-bearing mice and cancer patients. MDSC are a heterogenous population of immature cells that consist of two distinct sub-populations, polymorphonuclear (PMN)-MDSC and monocytic (M)-MDSC based on morphology and phenotype. MDSC suppress the function of T cells and other innate and adaptive immune cells including NK cells and B cells. During cancer or infection with bacteria or viruses, there is marked expansion of MDSC. Furthermore, the frequencies of MDSC correlate inversely with the prognosis and survival of tumor-bearing hosts as well as bacterial and viral burdens, persistence, and outcome in infected hosts. Currently, there is a paucity of data on MDSC and helminth infections. Here, we provide a survey of the evidence accumulated so far that overall support a role for MDSC in modulating immune responses during helminth infections. We review data from studies in various helminths, including those that infect humans. Finally, we summarize the progress to date in understanding the role of MDSC in helminth infections and briefly discuss potential host-directed strategies to target MDSC-mediated suppression of immune responses to helminths in favor of development of immunity to eliminate adult worms and possibly induce protection against reinfection.

Acute and chronic immunomodulatory response mechanisms against Toxocara canis larvae infection in mice

Abstract The objective of this work was to evaluate the early and late immunological modulation of an experimental infection of T. canis larvae in mice. Mice were infected with 100 infective larvae and euthanized at different period: 24, 48 hours post infection (HPI), 15- and 30 days post infection (DPI). The humoral response was evaluated by indirect ELISA. Quantitative RT–PCR (qPCR) was used to quantify the mRNA transcription of cytokines IL4, IL10, IL12 and Ym1 in the early and late infection periods. Infection with T. canis was able to generate specific total IgG at 15- and 30- DPI. Analyzing the IgG isotype revealed a significant differentiation for IgG1 compared with IgG2a, IgG2b and IgG3, characterizing a Th-2 response. Evaluating the gene transcription at the early phase of infection, higher transcription levels of IL10, IL4 and Ym1 and a downregulation of IL12 were observed. By the late phase, increased transcription levels of IL4, Ym1 and IL12 were observed, and downregulation of IL-10 transcription was observed. The data obtained suggest that during experimental infection with T. canis, the participation of the IL4, IL10, IL12 cytokines and Ym1 can play an important role in T. canis immunomodulation.

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.

Clinical and immunologic implications of COVID-19 in patients with melanoma and renal cell carcinoma receiving immune checkpoint inhibitors

The clinical and immunologic implications of the SARS-CoV-2 pandemic for patients with cancer receiving systemic anticancer therapy have introduced a multitude of clinical challenges and academic controversies. This review summarizes the current evidence, discussion points, and recommendations regarding the use of immune checkpoint inhibitors (ICIs) in patients with cancer during the SARS-CoV-2 pandemic, with a focus on patients with melanoma and renal cell carcinoma (RCC). More specifically, we summarize the theoretical concepts and available objective data regarding the relationships between ICIs and the antiviral immune response, along with recommended clinical approaches to the management of melanoma and RCC patient cohorts receiving ICIs throughout the course of the COVID-19 pandemic. Additional insights regarding the use of ICIs in the setting of current and upcoming COVID-19 vaccines and broader implications toward future pandemics are also discussed.

Arginine-dependent immune responses

A growing body of evidence indicates that, over the course of evolution of the immune system, arginine has been selected as a node for the regulation of immune responses. An appropriate supply of arginine has long been associated with the improvement of immune responses. In addition to being a building block for protein synthesis, arginine serves as a substrate for distinct metabolic pathways that profoundly affect immune cell biology; especially macrophage, dendritic cell and T cell immunobiology. Arginine availability, synthesis, and catabolism are highly interrelated aspects of immune responses and their fine-tuning can dictate divergent pro-inflammatory or anti-inflammatory immune outcomes. Here, we review the organismal pathways of arginine metabolism in humans and rodents, as essential modulators of the availability of this semi-essential amino acid for immune cells. We subsequently review well-established and novel findings on the functional impact of arginine biosynthetic and catabolic pathways on the main immune cell lineages. Finally, as arginine has emerged as a molecule impacting on a plethora of immune functions, we integrate key notions on how the disruption or perversion of arginine metabolism is implicated in pathologies ranging from infectious diseases to autoimmunity and cancer.

Arginase 1+ IL-10+ polymorphonuclear myeloid-derived suppressor cells are elevated in patients with active pemphigus and correlate with an increased Th2/Th1 response

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of immature myeloid cells, which are characterized by their capability to suppress T-cell responses. While MDSCs have been traditionally associated with cancer diseases, their role as regulators of autoimmune diseases is emerging. Pemphigus is a chronic autoimmune blistering skin disease characterized by dysregulated T-cell responses and autoantibody production. The role of MDSCs in pemphigus disease has not been defined yet. The aim of this study was to characterize MDSCs in pemphigus patients and to dissect their relationship with CD4⁺ T-cell subsets and clinical disease assessments. For this purpose, we performed a cross-sectional analysis of 20 patients with pemphigus. Our results indicate that a population of CD66b⁺ CD11b⁺ polymorphonuclear-like MDSCs (PMN-MDSCs) is expanded in the peripheral blood mononuclear cell fraction of pemphigus patients compared to age-matched healthy donors. These PMN-MDSCs have the capability of suppressing allogeneic T-cell proliferation in vitro and show increased expression of characteristic effector molecules such as arginase I and interleukin-10. We further demonstrate that PMN-MDSCs are especially expanded in patients with active pemphigus, but not in patients in remission. Moreover, MDSC frequencies correlate with an increased Th2/Th1 cell ratio. In conclusion, the identification of a functional PMN-MDSC population suggests a possible role of these cells as regulators of Th cell responses in pemphigus.

Expression patterns of the immunomodulatory enzyme arginase 1 in blood, lymph nodes and tumor tissue of early-stage breast cancer patients

Arginase 1 (ARG1) is an important enzyme in amino acid metabolism that also exerts immunoregulatory function. High ARG1 expression, which is associated with cell cycle arrest and functional unresponsiveness in T cells, has been observed after trauma, infections and in cancer patients. We studied ARG1 expression in early-stage breast cancer patients (stage 1, n = 20; stage 2, n = 23) by multi-parametric flow cytometry and immunohistochemistry. Despite a low tumor burden, ARG1 expression was significantly increased in blood-derived myeloid cells of breast cancer patients compared with healthy controls. The ARG1^hi myeloid population in the blood of cancer patients contained a high frequency of CD14⁺ cells and was, therefore, distinct from the granulocytic ARG1⁺ population observed in control individuals. Expression of ARG1 in patient blood cells correlated with tumor grade and was significantly reduced after surgical tumor removal. ARG1⁺ myeloid cells could also be detected in tumors and tumor-draining lymph nodes, where ARG1 expression levels exceeded those measured in the blood. We conclude that even patients with early-stage breast cancer exhibit tumor-related changes of ARG1 expression. The level of ARG1-mediated immunomodulation at this early stage remains to be determined. However, high ARG1 expression is likely to interfere with antitumor T-cell responses and immunotherapeutic interventions, making ARG1 or its downstream effector interesting therapeutic targets.

Cellular and humoral peritoneal immunity to Mesocestoides vogae metacestode infection in mice

BACKGROUND

Here, Mesocestoides (M.) vogae infection in mice is proposed as a suitable experimental model for studying the immunity in the peritoneal cavity of mice.

METHODS

To investigate the kinetics of immune parameters in M. vogae-infected mice, we detected, using flow cytometry, the expression of selected lymphoid and myeloid markers within the peritoneal cell population at day 0, 3, 6, 10, 14, 19, 25, 30 and 35 post-infection. Then, using ELISA, we analyzed the cytokine IFN-γ, TGF-β, IL-4 and IL-10 responses and the levels of anti-M. vogae IgG and IgM antibodies in the peritoneal lavage fluid. Cells isolated from the peritoneal cavity were subjected to further molecular analysis. To assess cell activation, peritoneal cells were exposed to LPS, and culture supernatants were collected and assayed for the level of cytokines and production of nitrite. Ly6C+ and Ly6G+ cells were isolated using MACS from the peritoneal cells at day 35 post-infection. Both MACS-isolated subsets were co-cultured with preactivated T cells to measure their suppressive capacity. Next, the role of parasite excretory-secretory antigens in induction of CD11b+ myeloid cells with the suppressive phenotype and the production of IL-10 was examined.

RESULTS

In the peritoneal cavity an initial increase of CD11b+Gr-1+F4/80^highMHC II^high cells, NK, NKT cells and CD8+ cytotoxic T cells was observed in the first week of infection. At day 14 post-infection, an increase in the number of myeloid CD11b+Gr-1+ cells was detected, and most of this cell population expressed low levels of F4/80 and MHC II in later stages of infection, suggesting the impairment of antigen-presenting cell functions, probably through the excretory-secretory molecules. Moreover, we confirmed that peritoneal Gr1+ cells (Ly6C+ and Ly6G+ population) are phenotypically and functionally consistent with myeloid-derived suppressor cells. Metacestode infection elicited high levels of IL-10 and upregulated STAT-3 in peritoneal cells. A higher level of IgM suggests that this isotype may be predominant and is involved in the host protection.

CONCLUSIONS

Mesocestoides vogae tetrathyridia induced the recruitment of immunosuppressive cell subsets, which may play a key role in the downregulation of immune response in long-term parasitic diseases, and excretory-secretory antigens seem to be the main regulatory factor.

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.

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.

Proteomic analysis reveals a protective role of specific macrophage subsets in liver repair

Macrophages are a heterogeneous population of immune cells that play central roles in a broad range of biological processes, including the resolution of inflammation. Although diverse macrophage subpopulations have been identified, the characterization and functional specialization of certain macrophage subsets in inflamed tissues remain unclear. Here we uncovered a key role of specific macrophage subsets in tissue repair using proteomics, bioinformatics and functional analysis. We isolated two hepatic monocyte-derived macrophage subpopulations: Ly6C^hiCX₃CR1^lo macrophages and Ly6C^loCX₃CR1^hi macrophages during distinct phases of acute liver injury and employed label-free proteomics approach to profile the proteome of these cells. We found that the endocytosis- and apoptotic cell clearance-related proteins were specifically enriched in Ly6C^loCX₃CR1^hi macrophages at the resolution phase. Intriguingly, 12/15-lipoxygenase (Alox15), the most strongly up-regulated protein in Ly6C^loCX₃CR1^hi macrophages, was identified as a specific marker for these macrophages. In co-culture systems, Ly6C^loCX₃CR1^hi macrophages specifically induced hepatocyte proliferation. Furthermore, selective depletion of this population in CD11b-diphtheria toxin receptor mice significantly delayed liver repair. Overall, our studies shed light on the functional specialization of distinct macrophage subsets from different phases in the resolution of inflammation.

Roles of Myeloid-Derived Suppressor Cell Subpopulations in Autoimmune Arthritis

Emerging evidence suggests the promise of the use of myeloid-derived suppressor cells (MDSCs) in inflammatory disorders based on their unique immune-intervention properties. However, the roles of MDSCs in autoimmune arthritis are not completely understood. Indeed, their immunosuppressive functions in arthritic conditions remain controversial, with heterogeneity among MDSCs and differential effects among subpopulations receiving much attention. As a result, it is necessary to determine the roles of MDSC subpopulations in autoimmune arthritis to clarify their diagnostic and therapeutic potential. Interestingly, in the inflammation niche of autoimmune arthritis, each MDSC subpopulation can exhibit both alternatives of a given characteristic. Moreover, polymorphonuclear MDSCs (PMN-MDSCs) are likely to be more suppressive and stable compared with monocytic MDSCs (MO-MDSCs). Although various important cytokines associated with the differentiation of MDSCs or MDSC subpopulations from immature myeloid precursors, such as granulocyte colony-stimulating factor (G-CSF), have been largely applied in external inductive systems, their roles are not entirely clear. Moreover, MDSC-based clinical treatments in rheumatoid arthritis (RA) continue to represent a significant challenge, as also reported for other autoimmune diseases. In this review, we describe the effects and actions of MDSC subpopulations on the development of autoimmune arthritis and analyze several types of MDSC-based therapeutic strategies to provide comprehensive information regarding immune networks and a foundation for more effective protocols for autoimmune arthritis.

Combined administration of anti-IL-13 and anti-IL-17A at individually sub-therapeutic doses limits asthma-like symptoms in a mouse model of Th2/Th17 high asthma

BACKGROUND

Recent studies have demonstrated that Th2 responses have the ability to antagonize Th17 responses. In mouse models of allergic asthma, blockade of Th2-effector cytokines results in elaboration of Th17 responses and associated increases in pulmonary neutrophilia. While these can be controlled by simultaneous blockade of Th17-associated effector cytokines, clinical trials of anti-IL-17/IL-17RA blocking therapies have demonstrated increased of risk of bacterial and fungal infections. Identification of minimally effective doses of cytokine-blocking therapies with the goal of reducing the potential emergence of infection-related complications is a translationally relevant goal.

OBJECTIVE

In the current report, we examine whether combined blockade of IL-13 and IL-17A, at individually sub-therapeutic levels, can limit the development of allergic asthma while sparing expression of IL-17A-associated anti-microbial effectors.

METHODS

House dust mite was given intratracheally to A/J mice. Anti-IL-13 and anti-IL-17A antibodies were administered individually, or concomitantly at sub-therapeutic doses. Airway hyper-reactivity, lung inflammation, magnitude of Th2- and Th17-associated cytokine production and expression of IL-13- and IL-17A-induced genes in the lungs was assessed.

RESULTS

Initial dosing studies identified sub-therapeutic levels of IL-13 and IL-17A blocking mAbs that have a limited effect on asthma parameters and do not impair responses to microbial products or infection. Subsequent studies demonstrated that combined sub-therapeutic dosing with IL-13 and IL-17A blocking mAbs resulted in significant improvement in airway hyperresponsiveness (AHR) and expression of IL-13-induced gene expression. Importantly, these doses neither exacerbated nor inhibited production of Th17-associated cytokines, or IL-17A-associated gene expression.

CONCLUSION

This study suggests that combining blockade of individual Th2 and Th17 effector cytokines, even at individually sub-therapeutic levels, may be sufficient to limit disease development while preserving important anti-microbial pathways. Such a strategy may therefore have reduced potential for adverse events associated with blockade of these pathways.

Neutrophils as myeloid-derived suppressor cells

Neutrophils form the first line of defence against invading pathogens, such as bacteria and fungi, as part of the innate immune response. Recently, neutrophils have also been discovered as repressors of adaptive immune responses. Under certain conditions, such as cancer and severe injury, an expansion of immature and mature neutrophils has been observed to induce suppression of T-cell proliferation. These suppressing cells are known as so-called myeloid-derived suppressor cells (MDSCs), a heterogeneous population of granulocytic-MDSCs and monocytic-MDSCs. Initially, MDSCs were believed to be a specific immature type of myeloid immune cell released from the bone marrow, but mature neutrophils have also been proposed to have suppressive capacity. However, granulocytic-MDSCs show a similar morphology and expression of cell surface markers as mature neutrophils. The only characteristic that discriminates granulocytic (g)-MDSCs from mature neutrophils is their suppressive capacity, raising the question whether human g-MDSCs and neutrophils are actually different cell types or whether they are one plastic cell type that can functionally polarize from microbial killers to immunosuppressor cells, depending on local conditions. In this review, we will focus on the MDSC activity of circulating mature neutrophils.

Analysis of the Trichuris suis excretory/secretory proteins as a function of life cycle stage and their immunomodulatory properties

Parasitic worms have a remarkable ability to modulate host immune responses through several mechanisms including excreted/secreted proteins (ESP), yet the exact nature of these proteins and their targets often remains elusive. Here, we performed mass spectrometry analyses of ESP (TsESP) from larval and adult stages of the pig whipworm Trichuris suis (Ts) and identified ~350 proteins. Transcriptomic analyses revealed large subsets of differentially expressed genes in the various life cycle stages of the parasite. Exposure of bone marrow-derived macrophages and dendritic cells to TsESP markedly diminished secretion of the pro-inflammatory cytokines TNFα and IL-12p70. Conversely, TsESP exposure strongly induced release of the anti-inflammatory cytokine IL-10, and also induced high levels of nitric oxide (NO) and upregulated arginase activity in macrophages. Interestingly, TsESP failed to directly induce CD4+ CD25+ FoxP3+ regulatory T cells (Treg cells), while OVA-pulsed TsESP-treated dendritic cells suppressed antigen-specific OT-II CD4+ T cell proliferation. Fractionation of TsESP identified a subset of proteins that promoted anti-inflammatory functions, an activity that was recapitulated using recombinant T. suis triosephosphate isomerase (TPI) and nucleoside diphosphate kinase (NDK). Our study helps illuminate the intricate balance that is characteristic of parasite-host interactions at the immunological interface, and further establishes the principle that specific parasite-derived proteins can modulate immune cell functions.

Regulatory Lymphoid and Myeloid Cells Determine the Cardiac Immunopathogenesis of Trypanosoma cruzi Infection

Chagas disease is a multisystemic disorder caused by the protozoan parasite Trypanosoma cruzi, which affects ~8 million people in Latin America, killing 7,000 people annually. Chagas disease is one of the main causes of death in the endemic area and the leading cause of infectious myocarditis in the world. T. cruzi infection induces two phases, acute and chronic, where the infection is initially asymptomatic and the majority of patients will remain clinically indeterminate for life. However, over a period of 10–30 years, ~30% of infected individuals will develop irreversible, potentially fatal cardiac syndromes (chronic chagasic cardiomyopathy [CCC]), and/or dilatation of the gastro-intestinal tract (megacolon or megaesophagus). Myocarditis is the most serious and frequent manifestation of chronic Chagas heart disease and appears in about 30% of infected individuals several years after infection occurs. Myocarditis is characterized by a mononuclear cell infiltrate that includes different types of myeloid and lymphoid cells and it can occur also in the acute phase. T. cruzi infects and replicates in macrophages and cardiomyocytes as well as in other nucleated cells. The pathogenesis of the chronic phase is thought to be dependent on an immune-inflammatory reaction to a low-grade replicative infection. It is known that cytokines produced by type 1 helper CD4+ T cells are able to control infection. However, the role that infiltrating lymphoid and myeloid cells may play in experimental and natural Chagas disease pathogenesis has not been completely elucidated, and several reports indicate that it depends on the mouse genetic background and parasite strain and/or inoculum. Here, we review the role that T cell CD4+ subsets, myeloid subclasses including myeloid-derived suppressor cells may play in the immunopathogenesis of Chagas disease with special focus on myocarditis, by comparing results obtained with different experimental animal models.

Infective Larvae of Brugia malayi Induce Polarization of Host Macrophages that Helps in Immune Evasion

Filarial parasites suppress, divert, or polarize the host immune response to aid their survival. However, mechanisms that govern the polarization of host MΦs during early filarial infection are not completely understood. In this study, we infected BALB/c mice with infective larvae stage-3 of Brugia malayi (Bm-L3) and studied their effect on the polarization of splenic MΦs. Results showed that MΦs displayed M2-phenotype by day 3 p.i. characterized by upregulated IL-4, but reduced IL-12 and Prostaglandin-D2 secretion. Increased arginase activity, higher arginase-1 but reduced NOS2 expression and poor phagocytic and antigen processing capacity was also observed. M2 MΦs supported T-cell proliferation and characteristically upregulated p-ERK but downregulated NF-κB-p65 and NF-κB-p50/105. Notably, Bm-L3 synergized with host regulatory T-cells (Tregs) and polarized M2 MΦs to regulatory MΦs (Mregs) by day 7 p.i., which secreted copious amounts of IL-10 and prostaglandin-E2. Mregs also showed upregulated expression levels of MHC-II, CD80, and CD86 and exhibited increased antigen-processing capacity but displayed impaired activation of NF-κB-p65 and NF-κB-p50/105. Neutralization of Tregs by anti-GITR + anti-CD25 antibodies checked the polarization of M2 MΦs to Mregs, decreased accumulation of regulatory B cells and inflammatory monocytes, and reduced secretion of IL-10, but enhanced IL-4 production and percentages of eosinophils, which led to Bm-L3 killing. In summary, we report hitherto undocumented effects of early Bm-L3 infection on the polarization of splenic MΦs and show how infective larvae deftly utilize the functional plasticity of host MΦs to establish themselves inside the host.

Cell Type-Specific Immunomodulation Induced by Helminthes: Effect on Metainflammation, Insulin Resistance and Type-2 Diabetes

Recent epidemiological studies have documented an inverse relationship between the decreasing prevalence of helminth infections and the increasing prevalence of metabolic diseases ("metabolic hygiene hypothesis"). Chronic inflammation leading to insulin resistance (IR) has now been identified as a major etiological factor for a variety of metabolic diseases other than obesity and Type-2 diabetes (metainflammation). One way by which helminth infections such as filariasis can modulate IR is by inducing a chronic, nonspecific, low-grade, immune suppression mediated by modified T-helper 2 (Th2) response (induction of both Th2 and regulatory T cells) which can in turn suppress the proinflammatory responses and promote insulin sensitivity (IS). This article provides evidence on how the cross talk between the innate and adaptive arms of the immune responses can modulate IR/sensitivity. The cross talk between innate (macrophages, dendritic cells, natural killer cells, natural killer T cells, myeloid derived suppressor cells, innate lymphoid cells, basophils, eosinophils, and neutrophils) and adaptive (helper T [CD4⁺] cells, cytotoxic T [CD8⁺] cells and B cells) immune cells forms two opposing circuits, one associated with IR and the other associated with IS under the conditions of metabolic syndrome and helminth-mediated immunomodulation, respectively.

Adjuvants and myeloid-derived suppressor cells: enemies or allies in therapeutic cancer vaccination

Adjuvants are a critical but largely overlooked and poorly understood component included in vaccine formulations to stimulate and modulate the desired immune responses to an antigen. However, unlike in the protective infectious disease vaccines, adjuvants for cancer vaccines also need to overcome the effect of tumor-induced suppressive immune populations circulating in tumor-bearing individuals. Myeloid-derived suppressor cells (MDSC) are considered to be one of the key immunosuppressive populations that inhibit tumor-specific T cell responses in cancer patients. This review focuses on the different signals for the activation of the immune system induced by adjuvants, and the close relationship to the mechanisms of recruitment and activation of MDSC. This work explores the possibility that a cancer vaccine adjuvant may either strengthen or weaken the effect of tumor-induced MDSC, and the crucial need to address this in present and future cancer vaccines.

Immunity and immune modulation elicited by the larval cestode Mesocestoides vogae and its products

Larval cestodes (metacestodes) induce long-lasting infections leading to considerable pathology in humans and livestock. Their survival is typically associated with Th2-biased immune responses and immunosuppressive effects and depends on the parasite's ability to excrete/secrete antigens with immunoregulatory properties. Here, Mesocestoides vogae, a natural parasite of mice, is proposed as a new model species for the identification and characterization of cestode-derived immunomodulatory factors. We followed the kinetics of immune parameters after infection with M. vogae or exposure to their excretory/secretory (ES) products in a permissive strain of mice. Besides, a dominant IL-10 production and accumulation of macrophages and eosinophils expressing mRNA for Fizz-1, YM1 and Arg-1, mice showed minimal IFN-γ and transient IL-4 production at early time points with a complete loss at later stages of infection. We found that serum-free ES products without host contamination directly induced M2 macrophages and suppressed IFN-γ production in vivo and in vitro. This study highlights the use of the M. vogae as a cestode infection model and its ES products as a valuable tool for the identification of new therapeutic targets for the control of larval cestodiasis.

E-cadherin expression in macrophages dampens their inflammatory responsiveness in vitro, but does not modulate M2-regulated pathologies in vivo

IL-4/IL-13-induced alternatively activated macrophages (M(IL-4/IL-13), AAMs or M2) are known to express E-cadherin, enabling them to engage in heterotypic cellular interactions and IL-4-driven macrophage fusion in vitro. Here we show that E-cadherin overexpression in Raw 264.7 macrophages inhibits their inflammatory response to LPS stimulation, as demonstrated by a reduced secretion of inflammatory mediators like interleukin (IL)-6, tumor necrosis factor (TNF) and nitric oxide (NO). To study the function of E-cadherin in M(IL-4/IL-13) macrophages in vivo, we generated macrophage-specific E-cadherin-deficient C57BL/6 mice. Using this new tool, we analyzed immunological parameters during two typical AAM-associated Th2-driven diseases and assessed Th2-associated granuloma formation. Although E-cadherin is strongly induced in AAMs during Taenia crassiceps helminth infections and allergic airway inflammation, its deletion in macrophages does not affect the course of both Th2 cytokine-driven diseases. Moreover, macrophage E-cadherin expression is largely redundant for granuloma formation around Schistosoma mansoni ova. Overall, we conclude that E-cadherin is a valuable AAM marker which suppresses the inflammatory response when overexpressed. Yet E-cadherin deletion in macrophages does not affect M(LPS+IFNγ) and M(IL-4) polarization in vitro, nor in vivo macrophage function, at least in the conditions tested.

Macrophage proliferation, provenance, and plasticity in macroparasite infection

Macrophages have long been center stage in the host response to microbial infection, but only in the past 10–15 years has there been a growing appreciation for their role in helminth infection and the associated type 2 response. Through the actions of the IL-4 receptor α (IL-4Rα), type 2 cytokines result in the accumulation of macrophages with a distinctive activation phenotype. Although our knowledge of IL-4Rα-induced genes is growing rapidly, the specific functions of these macrophages have yet to be established in most disease settings. Understanding the interplay between IL-4Rα-activated macrophages and the other cellular players is confounded by the enormous transcriptional heterogeneity within the macrophage population and by their highly plastic nature. Another level of complexity is added by the new knowledge that tissue macrophages can be derived either from a resident prenatal population or from blood monocyte recruitment and that IL-4 can increase macrophage numbers through proliferative expansion. Here, we review current knowledge on the contribution of macrophages to helminth killing and wound repair, with specific attention paid to distinct cellular origins and plasticity potential.

Circulating CD14(+)HLA-DR(-/low) myeloid-derived suppressor cell is an indicator of poor prognosis in patients with ESCC

Accumulating evidences demonstrate that a population of suppressive cells known as myeloid-derived suppressor cells (MDSCs) is key immune modulators which suppress antitumor immunity. In this study, we found that the level of circulating CD14(+)HLA-DR(-/low) cells in patients was significantly higher than that of healthy donors and was correlated with tumor burden, lymph node metastasis, and tumor, node, and metastasis (TNM) clinical stage. More importantly, we for the first time find the level of CD14(+)HLA-DR(-/low) is a biological indicator of poor prognosis through the analysis of 3-year overall survival. Furthermore, we evidenced that the proportion of CD14(+)HLA-DR(-/low) cells in the tumor metastatic tumor-draining lymph nodes (TDLNs) was notably higher compared to tumor-free TDLNs. Additionally, CD14(+)HLA-DR(-/low) cells from esophageal squamous cell carcinoma (ESCC) patients expressed dramatically increased programmed death ligand 1 (PD-L1) comparing to that from healthy control. Subsequently, blocking PD-L1 pathway by antibody could effectively reverse the suppressive effect on autologous T cell proliferation mediated by CD14(+)HLA-DR(-/low) cells in vitro. In conclusion, our data revealed CD14(+)HLA-DR(-/low) MDSCs which increase in ESCC patients is a novel poor prognostic indicator and may exert immunosuppressive properties through PD-L1/PD-1 pathway.

Critical Role of Mast Cells and Peroxisome Proliferator-Activated Receptor γ in the Induction of Myeloid-Derived Suppressor Cells by Marijuana Cannabidiol In Vivo

Cannabidiol (CBD) is a natural nonpsychotropic cannabinoid from marijuana (Cannabis sativa) with anti-epileptic and anti-inflammatory properties. Effect of CBD on naive immune system is not precisely understood. In this study, we observed that administering CBD into naive mice triggers robust induction of CD11b(+)Gr-1(+) myeloid-derived suppressor cells (MDSC) in the peritoneum, which expressed functional arginase 1, and potently suppressed T cell proliferation ex vivo. Furthermore, CBD-MDSC suppressed LPS-induced acute inflammatory response upon adoptive transfer in vivo. CBD-induced suppressor cells were comprised of CD11b(+)Ly6-G(+)Ly6-C(+) granulocytic and CD11b(+)Ly6-G(-)Ly6-C(+) monocytic subtypes, with monocytic MDSC exhibiting higher T cell-suppressive function. Induction of MDSC by CBD was markedly attenuated in Kit-mutant (Kit(W/W-v)) mast cell-deficient mice. MDSC response was reconstituted upon transfer of wild-type bone marrow-derived mast cells in Kit(W/W-v) mice, suggesting the key role of cKit (CD117) as well as mast cells. Moreover, mast cell activator compound 48/80 induced significant levels of MDSC in vivo. CBD administration in mice induced G-CSF, CXCL1, and M-CSF, but not GM-CSF. G-CSF was found to play a key role in MDSC mobilization inasmuch as neutralizing G-CSF caused a significant decrease in MDSC. Lastly, CBD enhanced the transcriptional activity of peroxisome proliferator-activated receptor γ in luciferase reporter assay, and PPAR-γ selective antagonist completely inhibited MDSC induction in vivo, suggesting its critical role. Together, the results suggest that CBD may induce activation of PPAR-γ in mast cells leading to secretion of G-CSF and consequent MDSC mobilization. CBD being a major component of Cannabis, our study indicates that marijuana may modulate or dysregulate the immune system by mobilizing MDSC.

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.

The significance of macrophage phenotype in cancer and biomaterials

Macrophages have long been known to exhibit heterogeneous and plastic phenotypes. They show functional diversity with roles in homeostasis, tissue repair, immunity and disease. There exists a spectrum of macrophage phenotypes with varied effector functions, molecular determinants, cytokine and chemokine profiles, as well as receptor expression. In tumor microenvironments, the subset of macrophages known as tumor-associated macrophages generates byproducts that enhance tumor growth and angiogenesis, making them attractive targets for anti-cancer therapeutics. With respect to wound healing and the foreign body response, there is a necessity for balance between pro-inflammatory, wound healing, and regulatory macrophages in order to achieve successful implantation of a scaffold for tissue engineering. In this review, we discuss the multitude of ways macrophages are known to be important in cancer therapies and implanted biomaterials.

Myeloid-derived suppressor cells: paradoxical roles in infection and immunity

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of immature suppressor cells that are generated due to aberrant myelopoiesis under pathological conditions. Although MDSCs have been recognized for more than 20 years under the guise of different monikers, these particular populations of myeloid cells gained more attention recently due to their immunosuppressive properties, which halt host immune responses to growing cancers or overwhelming infections. While MDSCs may contribute to immune homeostasis after infection or tissue injury by limiting excessive inflammatory processes, their expansion may be at the expense of pathogen elimination and thus may lead to disease persistence. Therefore, MDSCs may be either damaging or obliging to the host by attenuating, for example, antitumor or anti-infectious immune responses. In this review, we recapitulate the biological and immunological aspects of MDSCs, including their generation, distribution, trafficking and the factors involved in their activation, expansion, suppressive functions, and interplay between MDSCs and regulatory T cells, with a focus on the perspectives of infection and inflammation.

Dynamic changes in macrophage activation and proliferation during the development and resolution of intestinal inflammation

Macrophages (Mφs) accumulate at sites of inflammation, and, because they can assume several functionally distinct states of activation, they can either drive or restrain inflammatory responses. Once believed to depend on the recruitment of blood monocytes, it is now clear that the accumulation of Mφs in some tissues can result from the proliferation of resident Mφs in situ. However, little is known about the proliferation and activation state of Mφ subsets in the gut during the development and resolution of intestinal inflammation. We show that inflammatory Mφs accumulate in the large intestine of mice during the local inflammatory response to infection with the gastrointestinal nematode parasite Trichuris muris. Classically activated Mφs predominate initially (as the inflammation develops) and then, following worm expulsion (as the inflammation resolves), both the resident and inflammatory populations of Mφs become alternatively activated. A small but significant increase in the proliferation of inflammatory Mφs is seen but only during the resolution phase of the inflammatory response following both worm expulsion and the peak in Mφ accumulation. In contrast to recent studies in the pleural and peritoneal cavities, the proliferation of resident and alternatively activated Mφs does not increase during the inflammatory response. Furthermore, in CCR2^−/− mice, monocyte recruitment to the gut is impeded, and the accumulation of alternatively activated Mφs is greatly reduced. In conclusion, the recruitment of blood monocytes is the principle mechanism of Mφ accumulation in the large intestine. This study provides a novel insight into the phenotype and behavior of intestinal Mφ during infection-driven inflammation.

Myeloid derived suppressor cells in physiological and pathological conditions: the good, the bad, and the ugly

Myeloid derived suppressor cells (MDSCs), a heterogeneous population of myeloid progenitors, are recognized as a key element in tumor escape and progression. The importance of MDSCs in human malignancies has been demonstrated in recent years, and new approaches targeting their suppressive/tolerogenic action are currently being tested in both preclinical model and clinical trials. However, emerging evidence suggests that MDSCs may play a prominent role as regulator of the physiologic, the chronic, and the pathologic immune responses. This review will focus on the biology of MDSC in light of these new findings and the possible role of this myeloid population not only in the progression of the tumor but also in its initiation.

Alternatively Activated Macrophages Revisited: New Insights into the Regulation of Immunity, Inflammation and Metabolic Function following Parasite Infection

The role of macrophages in homeostatic conditions and the immune system range from clearing debris to recognizing and killing pathogens. While classically activated macrophages (CAMacs) are induced by T helper type 1 (Th1) cytokines and exhibit microbicidal properties, Th2 cytokines promote alternative activation of macrophages (AAMacs). AAMacs contribute to the killing of helminth parasites and mediate additional host-protective processes such as regulating inflammation and wound healing. Yet, other parasites susceptible to Th1 type responses can exploit alternative activation of macrophages to diminish Th1 immune responses and prolong infection. In this review, we will delineate the factors that mediate alternative activation (e.g. Th2 cytokines and chitin) and the resulting downstream signaling events (e.g. STAT6 signaling). Next, the specific AAMac-derived factors (e.g. Arginase1) that contribute to resistance or susceptibility to parasitic infections will be summarized. Finally, we will conclude with the discussion of additional AAMac functions beyond immunity to parasites, including the regulation of inflammation, wound healing and the regulation of metabolic disorders.

CpG oligodeoxynucleotide induces bone marrow precursor cells into myeloid-derived suppressor cells

Dendritic cells (DCs) and myeloid-derived suppressor cells (MDSCs) perform a number of functions in different immunological settings. In standard in vitro experiments, DCs are produced from mouse bone marrow (BM) cells in the presence of granulocyte-macrophage colony-stimulating factor (GM-CSF) and IL-4. Our previous study demonstrated that BM precursor cells could differentiate into MDSCs when co-cultured with poly (I:C). In the present study, BM precursor cells cultured in GM-CSF and IL-4 were treated with CpG oligodeoxynucleotide (CpG ODN). We observed that Gr1+CD11b+ cells exhibiting MDSC functions accumulated in the co-culture system. A similar phenomenon was also observed in Listeria monocytogenes-infected mice. In conclusion, we demonstrated that prolonged CpG ODN stimulation could inhibit the development of DCs and induce the differentiation of BM precursor cells into MDSCs.

LTβR expression on hematopoietic cells regulates acute inflammation and influences maturation of myeloid subpopulations

Lymphotoxin beta-receptor (LTβR) is involved in the formation and maintenance of secondary lymphoid structures, as well as in the regulation of inflammatory responses. Because LTβR lymphoid structure formation continues to develop in infants, we compared two different chimera models: one using adult mice and the other using a transplantation model of neonatal mice. To elucidate the function of LTβR on lymphoid and non-lymphoid cells, we generated bone marrow chimeras on the wild type C57Bl/6 and the LTβR-deficient (LTβR(-/-)) background, and reconstituted the mice with bone marrow cells reciprocally. These chimeric mice were analyzed in the experimental model of acute dextran sulfate sodium-induced colitis. Interestingly, both models revealed not only equal reconstitution levels but also similar immunological responses: LTβR expression on stromal cells is essential for lymph node formation, whereas LTBR on hematopoietic cells is crucial for a decrease in inflammation. In addition, mice lacking LTβR on hematopoietic cells revealed (a) an increase of immature granulocytic cells in the spleen and (b) a reduced proportion of myeloid cells in peripheral blood and spleen expressing CD11b(+)Ly6C(+)Ly6G(-) (myeloid-derived suppressor cells expression profile). In conclusion, LTβR expression on hematopoietic cells seems to be involved in the down-regulation of acute inflammatory reactions paralleled by the appearance of immature myeloid cells.

Myeloid-derived suppressor cells and anti-tumor T cells: a complex relationship

Myeloid-Derived Suppressor Cells (MDSC) are immature myeloid cells that are potent inhibitors of immune cell function and which accumulate under conditions of inflammation, especially cancer. MDSC are suggested to promote the growth of cancer by both enhancement of tumor angiogenesis and metastasis and also inhibition of antitumor immune responses. The presence of deficient and/or defective antitumor adaptive and innate immune responses, coincident with accumulation of MDSC in lymphoid organs and tumor parenchyma, supports the notion of a causal relationship. The potent ability of MDSC to inhibit several components and phases of immune response highlights the likelihood that targeting the inhibitory functions of MDSC may maximize the therapeutic potential of antitumor immunotherapy. In order to guide the rational development of immunotherapeutic strategies that incorporate inhibition of MDSC activity and enzymatic functions, thorough understanding of the role of MDSC in antitumor immune responses is required. In this manuscript we review the multifaceted inhibitory functions of MDSC and consider the role of MDSC-induced inhibition of antitumor T cell effector phase. Support for this research is from NIH R01 CA108573.

Reactive oxygen species and thiol redox signaling in the macrophage biology of atherosclerosis

SIGNIFICANCE

Despite the recent decline in the prevalence of cardiovascular diseases, atherosclerosis remains the leading cause of death in industrialized countries. Monocyte recruitment into the vessel wall is a rate-limiting step in atherogenesis. Death of macrophage-derived foam cells promotes lesion progression and the majority of acute complications of atherosclerotic disease (e.g., myocardial infarction) occur in lesions that are intensely infiltrated with monocyte-derived macrophages, underlining the critical roles monocytes and macrophages play in this complex chronic inflammatory disease.

RECENT ADVANCES

A rapidly growing body of literature supports a critical role for reactive oxygen species (ROS) in the regulation of monocyte and macrophage (dys)function associated with atherogenesis and macrophage death in atherosclerotic plaque.

CRITICAL ISSUES

In this review we highlight the important roles of NADHP oxidase 4 recently identified in monocytes and macrophages and the role of ROS and (thiol) redox signaling in different aspects of monocytes and macrophage biology associated with atherosclerosis.

FUTURE DIRECTIONS

Studies aimed at identifying the intracellular targets of ROS involved in redox signaling in macrophages and at elucidating the redox signaling mechanisms that control differentiation, activation, polarization, and death of monocytes and macrophages may ultimately lead to the development of novel preventive and therapeutic strategies for atherosclerosis.

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.

Macrophage plasticity and polarization in tissue repair and remodelling

Mononuclear phagocyte plasticity includes the expression of functions related to the resolution of inflammation, tissue repair and remodelling, particularly when these cells are set in an M2 or an M2-like activation mode. Macrophages are credited with an essential role in remodelling during ontogenesis. In extraembryonic life, under homeostatic conditions, the macrophage trophic and remodelling functions are recapitulated in tissues such as bone, mammary gland, decidua and placenta. In pathology, macrophages are key components of tissue repair and remodelling that occur during wound healing, allergy, parasite infection and cancer. Interaction with cells bearing stem or progenitor cell properties is likely an important component of the role of macrophages in repair and remodelling. These properties of cells of the monocyte-macrophage lineage may represent a tool and a target for therapeutic exploitation.

Amino acid metabolism related to immune tolerance by MDSCs

Myeloid-derived suppressor cells (MDSCs) are present in most cancer patients. Due to their significant role in blocking immune responses, MDSCs are strategic obstacles to immunotherapy that require activation of the host's cell-mediated and innate immune responses. Following a brief description of the immunosuppressive MDSCs, the authors review the discovered mechanisms of amino acid metabolism that MDSCs use to suppress the activation of T cells. Given the heterogeneity of MDSCs and the variety of suppressive mechanisms employed by MDSCs, it is essential to understand which group of these cells and mechanisms are dominant.

The CD20 homolog Ms4a8a integrates pro- and anti-inflammatory signals in novel M2-like macrophages and is expressed in parasite infection

Recently, we identified the CD20 homolog Ms4a8a as a novel molecule expressed by tumor-associated macrophages that directly enhances tumor growth. Here, we analyzed Ms4a8a(+) macrophages in M2-associated infectious pathologies. In late-stage Trypanosoma congolense and Taenia crassiceps infections, Ms4a8a expression was detected in hepatic and peritoneal macrophages respectively. Innate immunity in these infections is modulated by Toll-like receptor (TLR) signaling and TLR2/4/7 agonists strongly induced Ms4a8a expression in bone marrow derived macrophages (BMDMs) treated with M2 mediators (glucocorticoids/IL-4). LPS/dexamethasone/IL-4-induced Ms4a8a(+) BMDMs were characterized by strong expression of mRNA of mannose receptor (Mmr), arginase 1, and CD163, and by decreased iNOS expression. Coinduction of Ms4a8a by M2 mediators and TLR agonists involved the classical TLR signaling cascade via activation of MyD88/TRIF and NF-κB. Forced overexpression of Ms4a8a modulated the TLR4 response of RAW264.7 cells as shown by gene expression profiling. Upregulation of Hdc, Tcfec, and Sla was confirmed both in primary LPS/dexamethasone/IL-4-stimulated Ms4a8a(+) BMDMs and in peritoneal macrophages from late-stage Taenia crassiceps infection. In conclusion, we show that TLR signaling skews the typical alternative macrophage activation program to induce a special M2-like macrophage subset in vitro that also occurs in immunomodulatory immune reactions in vivo, a process directly involving the CD20 homolog Ms4a8a.

Claudin-1, claudin-2 and claudin-11 genes differentially associate with distinct types of anti-inflammatory macrophages in vitro and with parasite- and tumour-elicited macrophages in vivo

Macrophages altered by various Th2-associated and anti-inflammatory mediators--including IL-4 and IL-13 [inducing alternatively activated macrophages (AAMs)], IL-10 and TGF-β--were generically termed M2. However, markers that discriminate between AAMs and other M2 remain scarce. We previously described E-cadherin as a marker for AAMs, permitting these macrophages to fuse upon IL-4 stimulation. To identify novel potential contributors to macrophage fusion, we assessed the effect of IL-4 on other adherens and tight junction-associated components. We observed an induction of claudin-1 (Cldn1), Cldn2 and Cldn11 genes by IL-4 in different mouse macrophage populations. Extending our findings to other stimuli revealed Cldn1 as a mainly TGF-β-induced gene and showed that Cldn11 is predominantly associated with IL-4-induced AAMs. Cldn2 is upregulated by diverse stimuli and is not associated with a specific macrophage activation state in vitro. Interestingly, different claudin genes preferentially associate with M2 from distinct diseases. While Cldn11 is predominantly expressed in AAMs from helminth-infected mice, Cldn1 is the major macrophage claudin during chronic trypanosomiasis and Cldn2 dominates in tumour-associated macrophages. Overall, we identified Cldn1, Cldn2 and Cldn11 as genes that discriminate between diverse types of M2.

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.

Allergic airway disease is unaffected by the absence of IL-4Rα-dependent alternatively activated macrophages

BACKGROUND

Markers of alternatively activated macrophages (AAMs) are upregulated in the lungs of asthmatic patients and in mice with allergic airway disease. AAMs are thought to contribute to the pathogenesis of allergic airway disease by virtue of their decreased NO production and increased production of proline and polyamines, which are important in the synthesis of connective tissues such as collagen.

OBJECTIVE

We aimed to define the role of AAMs in the pathogenesis of allergic airway disease.

METHODS

The IL-4 receptor alpha (IL-4Rα) gene is genetically abrogated in macrophages in LysM(cre)IL-4Rα(-/lox) mice, which therefore have impaired IL-4/IL-13 activation of AAMs through IL-4R types 1 and 2. Responses of LysM(cre)IL-4Rα(-/lox) mice and IL-4Rα(-/lox) littermate controls were examined in ovalbumin- and house dust mite-induced allergic airway disease.

RESULTS

IL-4Rα expression was shown to be efficiently depleted from alveolar macrophages, interstitial macrophages, and CD11b(+)MHCII(+) inflammatory macrophages. Although the expression of markers of AAMs such as Ym-1, arginase and found in inflammatory zone 1 was decreased in macrophages of LysM(cre)IL-4Rα(-/lox) mice in chronic ovalbumin-induced allergic airway disease, airway hyperreactivity, T(H)2 responses, mucus hypersecretion, eosinophil infiltration, and collagen deposition were not significantly reduced. LysM(cre)IL-4Rα(-/lox) mice and littermate controls also developed similar responses in acute ovalbumin- and house dust mite-induced allergic airway disease.

CONCLUSION

Our results suggest that the presence of AAMs in allergic airway disease may be only an association, as a result of the increased T(H)2 responses present during disease, and that IL-4Rα-dependent AAMs do not play an important role in the pathology of disease.

12/15-lipoxygenase orchestrates the clearance of apoptotic cells and maintains immunologic tolerance

Noninflammatory clearance of apoptotic cells (ACs) is crucial to maintain self-tolerance. Here, we have reported a role for the enzyme 12/15-lipoxygenase (12/15-LO) as a central factor governing the sorting of ACs into differentially activated monocyte subpopulations. During inflammation, uptake of ACs was confined to a population of 12/15-LO-expressing, alternatively activated resident macrophages (resMΦ), which blocked uptake of ACs into freshly recruited inflammatory Ly6C(hi) monocytes in a 12/15-LO-dependent manner. ResMΦ exposed 12/15-LO-derived oxidation products of phosphatidylethanolamine (oxPE) on their plasma membranes and thereby generated a sink for distinct soluble receptors for ACs such as milk fat globule-EGF factor 8, which were essential for the uptake of ACs into inflammatory monocytes. Loss of 12/15-LO activity, in turn, resulted in an aberrant phagocytosis of ACs by inflammatory monocytes, subsequent antigen presentation of AC-derived antigens, and a lupus-like autoimmune disease. Our data reveal an unexpected key role for enzymatic lipid oxidation during the maintenance of self-tolerance.

Macrophage plasticity and polarization: in vivo veritas

Diversity and plasticity are hallmarks of cells of the monocyte-macrophage lineage. In response to IFNs, Toll-like receptor engagement, or IL-4/IL-13 signaling, macrophages undergo M1 (classical) or M2 (alternative) activation, which represent extremes of a continuum in a universe of activation states. Progress has now been made in defining the signaling pathways, transcriptional networks, and epigenetic mechanisms underlying M1-M2 or M2-like polarized activation. Functional skewing of mononuclear phagocytes occurs in vivo under physiological conditions (e.g., ontogenesis and pregnancy) and in pathology (allergic and chronic inflammation, tissue repair, infection, and cancer). However, in selected preclinical and clinical conditions, coexistence of cells in different activation states and unique or mixed phenotypes have been observed, a reflection of dynamic changes and complex tissue-derived signals. The identification of mechanisms and molecules associated with macrophage plasticity and polarized activation provides a basis for macrophage-centered diagnostic and therapeutic strategies.

Changes in cyst's nuclear chromatin resulting after experimental manipulation of Taenia crassiceps mice infections: biological implications

During some estimations of the nuclear DNA content, based on determinations propidium iodide (PI) binding through fluorocytometry for Taenia crassiceps cysticerci, significant variation in the results were found. This initial observation led to a series of experiments designed to explain the variation. These changes could be induced by the diameter of the needles in the syringes used for the mouse to mouse transfer of the cysts. Nuclei from cysts transferred through 27-gauge needles showed 30% less PI staining than those transferred through 21 gauge needles, after 2 months infections. Reduction in PI capture induced by 27-gauge needle was reversible when the cysts were maintained in their mice hosts during 5 months. Moreover, variation in PI binding to cysticercal DNA was also found when comparing parasites grown in male versus female mice. The use of agents that homogenize the chromatin structure during PI staining, allowed demonstrating that variation were entirely due to differences in the chromatin relaxation/compaction. Additional experiments demonstrated that the higher compaction is accompanied by a reduced ability of cysts to grow in the peritoneal cavity of BALB/cAnN mice. Furthermore, proteomic analysis also showed that these changes in chromatin relaxation/compaction resulted in different levels and patterns of protein expression. Our results strongly suggest that chromatin is involved in several well characterized phenomena of the T. crassiceps murine model, and open new avenues for a detailed approach to understand such a complex host-parasite relationship.