Molecular control of monocyte development

High frequency of circulating non-classical monocytes is associated with stable remission in relapsing-remitting multiple sclerosis

'No evidence of disease activity (NEDA)', judged by clinical and radiological findings, is a therapeutic goal in patients with multiple sclerosis (MS). It is, however, unclear if distinct biological mechanisms contribute to the maintenance of NEDA. To clarify the immunological background of long-term disease stability defined by NEDA, circulating immune cell subsets in patients with relapsing-remitting MS (RRMS) were analyzed using flow cytometry. Patients showing long-term NEDA (n = 31) had significantly higher frequencies of non-classical monocytes (NCMs) (6.1% vs 1.4%) and activated regulatory T cells (Tregs; 2.1% vs 1.6%) than those with evidence of disease activity (n = 8). The NCM frequency and NCMs to classical monocytes ratio (NCM/CM) positively correlated with activated Treg frequency and duration of NEDA. Co-culture assays demonstrated that NCMs could increase the frequency of activated Tregs and the expression of PD-L1, contributing to development of Tregs, was particularly high in NCMs from patients with NEDA. Collectively, NCMs contribute to stable remission in patients with RRMS, possibly by increasing activated Treg frequency. In addition, the NCM frequency and NCM/CM ratio had high predictive values for disease stability (AUC = 0.97 and 0.94, respectively), suggesting these markers are potential predictors of a long-term NEDA status in RRMS.

Harnessing the potential of monocytes/macrophages to regenerate tissue-engineered vascular grafts

Cell-free tissue-engineered vascular grafts provide a promising alternative to treat cardiovascular disease, but timely endothelialization is essential for ensuring patency and proper functioning post-implantation. Recent studies from our lab showed that blood cells like monocytes (MCs) and macrophages (Mϕ) may contribute directly to cellularization and regeneration of bioengineered arteries in small and large animal models. While MCs and Mϕ are leucocytes that are part of the innate immune response, they share common developmental origins with endothelial cells (ECs) and are known to play crucial roles during vessel formation (angiogenesis) and vessel repair after inflammation/injury. They are highly plastic cells that polarize into pro-inflammatory and anti-inflammatory phenotypes upon exposure to cytokines and differentiate into other cell types, including EC-like cells, in the presence of appropriate chemical and mechanical stimuli. This review focuses on the developmental origins of MCs and ECs; the role of MCs and Mϕ in vessel repair/regeneration during inflammation/injury; and the role of chemical signalling and mechanical forces in Mϕ inflammation that mediates vascular graft regeneration. We postulate that comprehensive understanding of these mechanisms will better inform the development of strategies to coax MCs/Mϕ into endothelializing the lumen and regenerate the smooth muscle layers of cell-free bioengineered arteries and veins that are designed to treat cardiovascular diseases and perhaps the native vasculature as well.

A novel immune‑related lncRNA as a prognostic biomarker in HER2+ breast cancer

Human epidermal growth factor receptor 2 (HER2)+ breast cancer is characterized by high malignancy and poor prognosis. Long non-coding (lnc)RNAs are crucial in breast cancer progression and prognosis, especially in tumor-associated immune processes. The present study aimed to elucidate novel lncRNAs related to immune function that could serve as biomarkers for both diagnosis and prognosis of this cancer subtype. Using data from The Cancer Genome Atlas and The Immunology Database and Analysis Portal, correlation analysis was performed to identify differentially expressed lncRNAs and immune-related genes. Through receiver operating characteristic analysis, the diagnostic value of specific lncRNAs was identified and evaluated, with a focus on their capacity to distinguish between cancerous and non-cancerous states. The present research revealed 22 differentially expressed lncRNAs and 23 differentially expressed immune-related genes, with 19 immune-related lncRNAs. A total of 13 of these lncRNAs demonstrated diagnostic relevance. In particular, it was demonstrated that the expression of lncRNA CTC-537E7.2 was significantly correlated with patient survival, suggesting its potential as a prognostic marker. Additionally, the expression of lncRNA CTC-537E7.2 was significantly correlated with clinical parameters, such as hormone receptor status and patient demographics. Moreover, it exhibited associations with four distinct immune cell types and demonstrated involvement in the Janus kinase-signal transducer and activator of transcription pathway. Further assessment by in situ hybridization confirmed the increased expression of lncRNA CTC-537E7.2 in samples from HER2+ patients, reinforcing its significance. In summary, the present study uncovered a novel prognostic biomarker for HER2+ breast cancer, thereby laying the groundwork for investigating the underlying molecular mechanisms driving the development of this subtype of breast cancer.

Communication molecules (ncRNAs) mediate tumor-associated macrophage polarization and tumor progression

Non-coding RNAs play important roles in tumor cells and macrophages and participate in their communication as messengers. Non-coding RNAs have an impact in tumor cell proliferation, migration, and apoptosis, and they also regulate the differentiation and regulation of immune cells. In macrophages, they stimulate the polarization of macrophages into M1 or M2 by regulating proteins related to signaling pathways; in tumor cells, non-coding RNAs can enter macrophages through exosomes and affect the latter polarization. The polarization of macrophages further regulates the biological functions of cancer cells. The direction of macrophage polarization determines tumor progression, angiogenesis and drug resistance. This often creates a feedback loop. Non-coding RNAs act as bridges between tumor cells and macrophages to regulate the balance of the tumor microenvironment. We reviewed the signaling pathways related to macrophage polarization and the regulatory mechanisms of non-coding RNA in tumor-associated macrophages M1 and M2, and discussed the potential applications and prospects of exosome engineering.

Mercury chloride activates the IFNγ-IRF1 signaling in myeloid progenitors and promotes monopoiesis in mice

Inorganic mercury (Hg2+) is a highly toxic heavy metal in the environment. To date, the impacts of Hg2+ on the development of monocytes, or monopoiesis, have not been fully addressed. The aim of the present study was to investigate the impact of Hg2+ on monopoiesis. In this study, we treated B10.S mice and DBA/2 mice with 10 μM or 50 μM HgCl2 via drinking water for 4 wk, and we then evaluated the development of monocytes. Treatment with 50 μM HgCl2, but not 10 μM HgCl2, increased the number of monocytes in the blood, spleen and bone marrow (BM) of B10.S mice. Accordingly, treatment with 50 μM HgCl2, but not 10 μM HgCl2, increased the number of common myeloid progenitors (CMP) and granulocyte-macrophage progenitors (GMP) in the BM. Functional analyses indicated that treatment with 50 μM HgCl2 promoted the differentiation of CMP and GMP to monocytes in the BM of B10.S mice. Mechanistically, treatment with 50 μM HgCl2 induced the production of IFNγ, which activated the Jak1/3-STAT1/3-IRF1 signaling in CMP and GMP and enhanced their differentiation potential for monocytes in the BM, thus likely leading to increased number of mature monocytes in B10.S mice. Moreover, the increased monopoiesis by Hg2+ was associated with the increased inflammatory status in B10.S mice. In contrast, treatment with 50 μM HgCl2 did not impact the monopoiesis in DBA/2 mice. Our study reveals the impact of Hg on the development of monocytes.

Acute murine cytomegalovirus infection boosts cell-type specific response and lipid metabolism changes in the liver of infant mice

Introduction

Human cytomegalovirus (HCMV) infection in infants can lead to severe diseases, including neonatal hepatitis. The single-cell dimensional changes in immune cells after the initial CMV infection remain elusive, as do the effects of CMV infection on hepatic lipid metabolism.

Methods

We employed single-cell RNA-sequencing to investigate the changes in liver cell types and immune responses in infant mice following murine CMV (MCMV) infection. Additionally, we examined alterations in protein expression profiles related to lipid metabolism in hepatocytes and the role of the key transcription factor PPAR-γ in hepatocytes during CMV infection.

Results

Our study revealed that MCMV infects most liver cell types in infant mice, leading to an increase in the proportion of proliferating CD8 effector T cells and a subset of Nos2+ monocytes, potentially playing an essential role in early anti-viral responses. Furthermore, MCMV infection resulted in altered protein expression of lipid metabolism in hepatocytes. Knocking down the transcription factor PPAR-γ in hepatocytes effectively inhibited CMV infection.

Discussion

Our findings underscore the immune system's response to early-stage MCMV infection and the subsequent impact on hepatic lipid metabolism in infant mice. This research provides new insights into the mechanisms of CMV infection and could pave the way for novel therapeutic strategies.

Heterogeneity of tumor immune microenvironment in malignant and metastatic change in LUAD is revealed by single-cell RNA sequencing

Lung adenocarcinoma (LUAD) is the most common type of non-small cell lung cancer and accounts for approximately 40% of all lung cancer cases. Multiple distant metastases are the major cause of mortality in lung cancer. In this study, single-cell sequencing datasets of LUAD were utilized to depict the transcriptome characteristic of LUAD based on the bioinformatic method. Firstly, the transcriptome landscape of heterogeneous cell types in LUAD was analyzed and memory T cells, NK cells, and helper T cells were revealed to be the common immune cells in tumor, normal, and metastasis tissue, respectively. Then, marker genes were calculated and 709 genes were identified to play a vital role in the microenvironment of LUAD. While macrophages were reported to act as one of the cells in LUAD, enrichment analysis of macrophage marker genes revealed the important role of macrophages in the activation of neutrophils. Next, the results of cell-cell communication analysis suggested that pericytes interact with broad immune cells via MDK-NCL pathways in metastasis samples, MIF-(CD74+CXCR4) and MIF-(CD74+CC44) interaction especially occurred between different cell types in tumor and normal samples. Finally, bulk RNA-seq was integrated to validate the prognosis effect of the marker gene and the maker gene of M2 macrophage, CCL20, showed the most related to LUAD prognosis. Besides, ZNF90 (Helper T cells), FKBP4 (memory T, helper T, Cytotoxic T, and B cells), CD79A (B cells), TPI1 (pericyte), and HOPX (epithelial cells, pericytes) were also pivotal in the pathology of LUAD, helping researchers understand the molecular insight of microenvironment in LUAD.

Fibronectin leucine-rich transmembrane protein 2 drives monocyte differentiation into macrophages via the UNC5B-Akt/mTOR axis

Upon migrating into the tissues, hematopoietic stem cell (HSC)-derived monocytes differentiate into macrophages, playing a crucial role in determining innate immune responses towards external pathogens and internal stimuli. However, the regulatory mechanisms underlying monocyte-to-macrophage differentiation remain largely unexplored. Here we divulge a previously uncharacterized but essential role for an axon guidance molecule, fibronectin leucine-rich transmembrane protein 2 (FLRT2), in monocyte-to-macrophage maturation. FLRT2 is almost undetectable in human monocytic cell lines, human peripheral blood mononuclear cells (PBMCs), and mouse primary monocytes but significantly increases in fully differentiated macrophages. Myeloid-specific deletion of FLRT2 (Flrt2ΔMyel) contributes to decreased peritoneal monocyte-to-macrophage generation in mice in vivo, accompanied by impaired macrophage functions. Gain- and loss-of-function studies support the promoting effect of FLRT2 on THP-1 cell and human PBMC differentiation into macrophages. Mechanistically, FLRT2 directly interacts with Unc-5 netrin receptor B (UNC5B) via its extracellular domain (ECD) and activates Akt/mTOR signaling. In vivo administration of mTOR agonist MYH1485 reverses the impaired phenotypes observed in Flrt2ΔMyel mice. Together, these results identify FLRT2 as a novel pivotal endogenous regulator of monocyte differentiation into macrophages. Targeting the FLRT2/UNC5B-Akt/mTOR axis may provide potential therapeutic strategies directly relevant to human diseases associated with aberrant monocyte/macrophage differentiation.

Identification of Scoparone from Chinese Olive Fruit as a Modulator of Macrophage Polarization

Chinese olive (Canarium album L.) has been highlighted for its remarkable health benefits. We previously showed that the ethyl acetate fraction of Chinese olive (COE) is an effective anti-inflammatory agent. In this study, we used a luciferase-based RAW 264.7 cell platform to detect the transcriptional activity of NF-κB, a key mediator of inflammation, and the promoter activity of its downstream target, COX-2. Through functional-oriented screening using these platforms, we further divided COE into several subfractions. Subsequently, we used silica gel column chromatography for purification, and the active compounds were separated and isolated by thin-layer chromatography (TLC) and high-performance liquid chromatography (HPLC). The structure of the resulting compound with high anti-inflammatory activity was then identified as scoparone. Our results showed that scoparone not only inhibited lipopolysaccharide (LPS)-induced secretion of nitric oxide and suppressed M1 macrophage markers (iNOS, Il-6, Ccl2, and Tnf-α) but also markedly decreased the production of pro-inflammatory cytokines (IL-6, CCL2, and TNF-α). Treatment with scoparone significantly reduced the protein level of TNF-α in LPS-treated bone-marrow-derived macrophages (BMDMs). In addition, scoparone promoted macrophages toward an M2 anti-inflammatory phenotype, as determined by the significantly increased gene expression of M2 macrophage markers (Arg1, Ym1, Mrc1, Il-10, and Cd206) and the protein level of Arg1. This study indicates that COE fruit has high therapeutic potential for various inflammatory diseases as a result of switching the macrophage phenotype from pro-inflammatory M1 to anti-inflammatory M2.

Recapitulation of pro-inflammatory signature of monocytes with ACVR1A mutation using FOP patient-derived iPSCs

BACKGROUND

Fibrodysplasia ossificans progressiva (FOP) is a rare genetic disease characterized by progressive heterotopic ossification (HO) in soft tissues due to a heterozygous mutation of the ACVR1A gene (FOP-ACVR1A), which erroneously transduces the BMP signal by Activin-A. Although inflammation is known to trigger HO in FOP, the role of FOP-ACVR1A on inflammatory cells remains to be elucidated.

RESULTS

We generated immortalized monocytic cell lines from FOP-iPSCs (FOP-ML) and mutation rescued iPSCs (resFOP-ML). Cell morphology was evaluated during the monocyte induction and after immortalization. Fluorescence-activated cell sorting (FACS) was performed to evaluate the cell surface markers CD14 and CD16 on MLs. MLs were stimulated with lipopolysaccharide or Activin-A and the gene expression was evaluated by quantitative PCR and microarray analysis. Histological analysis was performed for HO tissue obtained from wild type mice and FOP-ACVR1A mice which conditionally express human mutant ACVR1A gene by doxycycline administration. Without any stimulation, FOP-ML showed the pro-inflammatory signature of CD16+ monocytes with an upregulation of INHBA gene, and treatment of resFOP-ML with Activin-A induced an expression profile mimicking that of FOP-ML at baseline. Treatment of FOP-ML with Activin-A further induced the inflammatory profile with an up-regulation of inflammation-associated genes, of which some, but not all, of which were suppressed by corticosteroid. Experiments using an inhibitor for TGFβ or BMP signal demonstrated that Activin-A-induced genes such as CD16 and CCL7, were regulated by both signals, indicating Activin-A transduced dual signals in FOP-ML. A comparison with resFOP-ML identified several down-regulated genes in FOP-ML including LYVE-1, which is known to suppress matrix-formation in vivo. The down-regulation of LYVE-1 in HO tissues was confirmed in FOP model mice, verifying the significance of the in vitro experiments.

CONCLUSION

These results indicate that FOP-ML faithfully recapitulated the phenotype of primary monocytes of FOP and the combination with resFOP-ML is a useful tool to investigate molecular events at the initial inflammation stage of HO in FOP.

Stem Cells in the Tumor Immune Microenvironment -Part of the Cure or Part of the Disease? Ontogeny and Dichotomy of Stem and Immune Cells has Led to better Understanding

Stem cells are at the basis of tissue homeostasis, hematopoiesis and various regenerative processes. Epigenetic changes in their somatically imprinted genes, prolonged exposure to mutagens/carcinogens or alteration of their niche can lead to the development of an enabling environment for tumor growth and progression. The involvement of stem cells in both health and disease becomes even more compelling with ontogeny as embryonic and extraembryonic stem cells which persist into adulthood in well established and specific niche may have distinct implications in tumorigenesis. Immune surveillance plays an important role in this interplay since the response of immune cells toward the oncogenic process can range from reactivity to placidity and even complicity, being orchestrated by intercellular molecular dialogues with the other key players of the tumor microenvironment. With the current understanding that every developing and adult tissue contains inherent stem and progenitor cells, in this manuscript we review the most relevant interactions carried out between the stem cells, tumor cells and immune cells in a bottom-up incursion through the tumor microenvironment beginning from the perivascular niche and going through the tumoral parenchyma and the related stroma. With the exploitation of various factors that influence the behavior of immune effectors toward stem cells and other resting cells in their niche, new therapeutic strategies to tackle the polarization of immune effectors toward a more immunogenic phenotype may arise.

Interferon-γ-Producing CD4+ T Cells Drive Monocyte Activation in the Bone Marrow During Experimental Leishmania donovani Infection

Ly6C^hi inflammatory monocytes develop in the bone marrow and migrate to the site of infection during inflammation. Upon recruitment, Ly6C^hi monocytes can differentiate into dendritic cells or macrophages. According to the tissue environment they can also acquire different functions. Several studies have described pre-activation of Ly6C^hi monocytes in the bone marrow during parasitic infection, but whether this process occurs during experimental visceral leishmaniasis and, if so, the mechanisms contributing to their activation are yet to be established. In wild type C57BL/6 (B6) mice infected with Leishmania donovani, the number of bone marrow Ly6C^hi monocytes increased over time. Ly6C^hi monocytes displayed a highly activated phenotype from 28 days to 5 months post infection (p.i), with >90% expressing MHCII and >20% expressing iNOS. In comparison, in B6.Rag2^-/- mice <10% of bone marrow monocytes were MHCII⁺ at day 28 p.i., an activation deficiency that was reversed by adoptive transfer of CD4⁺ T cells. Depletion of CD4⁺ T cells in B6 mice and the use of mixed bone marrow chimeras further indicated that monocyte activation was driven by IFNγ produced by CD4⁺ T cells. In B6.Il10^-/- mice, L. donovani infection induced a faster but transient activation of bone marrow monocytes, which correlated with the magnitude of CD4⁺ T cell production of IFNγ and resolution of the infection. Under all of the above conditions, monocyte activation was associated with greater control of parasite load in the bone marrow. Through reinfection studies in B6.Il10^-/- mice and drug (AmBisome^®) treatment of B6 mice, we also show the dependence of monocyte activation on parasite load. In summary, these data demonstrate that during L. donovani infection, Ly6C^hi monocytes are primed in the bone marrow in a process driven by CD4⁺ T cells and whereby IFNγ promotes and IL-10 limits monocyte activation and that the presence of parasites/parasite antigen plays a crucial role in maintaining bone marrow monocyte activation.

Alcohol Consumption Accumulation of Monocyte Derived Macrophages in Female Mice Liver Is Interferon Alpha Receptor Dependent

Monocytes develop in the bone marrow from the hematopoietic stem cells and represent heterogeneous phagocyte cells in the circulation. In homeostatic and inflammatory conditions, after recruitment into tissues, monocytes differentiate into macrophages and dendritic cells. Alcohol use causes about 3.3 million worldwide deaths per year, which is about 5.9% of all deaths. In the United States and Europe, alcohol use disorders represent the fifth leading cause of death. Females are more susceptible to alcoholic liver injury in both humans and mice. Strikingly, we still do not know how much of this difference in tissue injury is due to the differential effect of alcohol and its toxic metabolites on a) parenchymal or resident cells and/or b) immune response to alcohol. Therefore, we used a model of chronic alcohol exposure in mice to investigate the dynamics of monocytes, an innate immune cell type showed to be critical in alcoholic liver injury, by using immunophenotypic characterization. Our data reveal a sex-dimorphism of alcohol response of hepatic monocytes in female mice that is interferon receptor alpha dependent. This dimorphism could shed light on potential cellular mechanism(s) to explain the susceptibility of females to alcoholic immunopathogenesis and suggests an additional targetable pathway for alcoholic liver injury in females.

Dwellers and Trespassers: Mononuclear Phagocytes at the Borders of the Central Nervous System

The central nervous system (CNS) parenchyma is enclosed and protected by a multilayered system of cellular and acellular barriers, functionally separating glia and neurons from peripheral circulation and blood-borne immune cells. Populating these borders as dynamic observers, CNS-resident macrophages contribute to organ homeostasis. Upon autoimmune, traumatic or neurodegenerative inflammation, these phagocytes start playing additional roles as immune regulators contributing to disease evolution. At the same time, pathological CNS conditions drive the migration and recruitment of blood-borne monocyte-derived cells across distinct local gateways. This invasion process drastically increases border complexity and can lead to parenchymal infiltration of blood-borne phagocytes playing a direct role both in damage and in tissue repair. While recent studies and technical advancements have highlighted the extreme heterogeneity of these resident and CNS-invading cells, both the compartment-specific mechanism of invasion and the functional specification of intruding and resident cells remain unclear. This review illustrates the complexity of mononuclear phagocytes at CNS interfaces, indicating how further studies of CNS border dynamics are crucially needed to shed light on local and systemic regulation of CNS functions and dysfunctions.

The chemokine CX3CL1/fractalkine regulates immunopathogenesis during fungal-associated allergic airway inflammation

Individuals that present with difficult-to-control asthma and sensitivity to one or more fungal species are categorized as a subset of severe asthma patients belonging to a group herein referred to as severe asthma with fungal sensitization (SAFS). We have previously reported the identification of numerous cytokines and chemokines that were elevated in human asthmatics that were sensitized to fungi vs. nonfungal sensitized asthmatics. Here, we show that the unique chemokine CX3CL1 (fractalkine) is elevated in both bronchoalveolar lavage fluid and sputum from human asthmatics sensitized to fungi, implicating an association with CX3CL1 in fungal asthma severity. In an experimental model of fungal-associated allergic airway inflammation, we demonstrate that the absence of CX3CR1 signaling unexpectedly resulted in a profound impairment in lung function. Histological assessment of lung tissue revealed an unrestricted inflammatory response that was subsequently characterized by enhanced levels of neutrophils, eosinophils, and inflammatory monocytes. Neutrophilic inflammation correlated with elevated IL-17A, proinflammatory cytokines (TNF-α, IL-1α, and IL-1β), neutrophil survival factors (granulocyte colony-stimulating factor), and neutrophil-targeting chemokines (CCL3 and CCL4). Eosinophilia correlated with elevated type 2 responses (IL-5 and IL-13) whereas inflammatory monocyte levels correlated with elevated type 1 responses (IFN-γ and CXCL9) and survival factors (macrophage colony-stimulating factor). Despite enhanced inflammatory responses, the immunoregulatory cytokine IL-10 and the natural inhibitor of IL-1 signaling, IL-1RA, were significantly elevated rather than impaired. Regulatory T-cell levels were unchanged, as were levels of the anti-inflammatory cytokines IL-35 and IL-38. Taken together, the CX3CL1/CX3CR1 axis preserves lung function during fungal-associated allergic airway inflammation through a nonclassical immunoregulatory mechanism.

S100A8/A9 in Myocardial Infarction: A Promising Biomarker and Therapeutic Target

Myocardial infarction (MI), the main cause of cardiovascular-related deaths worldwide, has long been a hot topic because of its threat to public health. S100A8/A9 has recently attracted an increasing amount of interest as a crucial alarmin that regulates the pathogenesis of cardiovascular disease after its release from myeloid cells. However, the role of S100A8/A9 in the etiology of MI is not well understood. Here, we elaborate on the critical roles and potential mechanisms of S100A8/A9 driving the pathogenesis of MI. First, cellular source of S100A8/A9 in infarcted heart is discussed. Then we highlight the effect of S100A8/A9 heterodimer in the early inflammatory period and the late reparative period of MI as well as myocardial ischemia/reperfusion (I/R) injury. Moreover, the predictive value of S100A8/A9 for the risk of recurrence of cardiovascular events is elucidated. Therefore, this review focuses on the molecular mechanisms of S100A8/A9 in MI pathogenesis to provide a promising biomarker and therapeutic target for MI.

Androgen Receptor Signaling Positively Regulates Monocytic Development

Myeloid cells are critical cells involved in the orchestration of innate and adaptive immune responses. Most myeloid cells derive from the adult bone marrow in a process called myelopoiesis, a tightly controlled process that ensures constant production of myeloid cells. Sex differences in myeloid cell development have been observed; males exhibit greater monocytic differentiation in the bone marrow, and men have increased blood monocyte numbers when compared to women. Here we use a genetic mouse model of myeloid androgen receptor (AR) knockout (MARKO) and pharmacological inhibition of AR to investigate the role of androgen signaling in monocytic differentiation. We observe that although myeloid AR signaling does not influence total bone marrow cell numbers, it does affect the composition of the bone marrow myeloid population in both homeostatic and emergency settings. Genetic deletion of AR in myeloid cells led to reduced monocytic development in vivo. Similarly, pharmacologic inhibition of AR signaling in vitro reduced monocytic development. However, alteration in monocytic differentiation in the absence of AR signaling did not lead to reduced numbers of circulating myeloid cells, although MARKO male mice display reduced ratio of classical to non-classical monocytes in the blood, implying that blood monocyte subsets are skewed upon myeloid AR deletion. Our results suggest that the sex differences observed in monocytic differentiation are partly attributed to the positive role of the androgen-AR axis in regulating monocytic development directly at the myeloid cell level. Furthermore, we have identified a novel role for AR in regulating blood mature monocyte subset turnover. Investigating how androgen signaling affects monocytic development and monocyte subset heterogeneity will advance our understanding of sex differences in monocytic function at homeostasis and disease and can ultimately impact future therapeutic design targeting monocytes in the clinic.

Immunopathogenesis and therapeutic potential of macrophage influx in diffuse parenchymal lung diseases

INTRODUCTION

The diffuse parenchymal lung diseases (DPLD)/interstitial lung diseases (ILD) are progressive lung disorders with usually unclear etiology, poor long-term survival and no effective treatment. Their pathogenesis is characterized by alveolar epithelial cell injury, inflammation, epithelial-mesenchymal transition, and parenchymal fibrosis. Macrophages play diverse roles in their development, both in the acute phase and in tissue repair.

AREAS COVERED

In this review, we summarize the current state of knowledge regarding the role of macrophages and their phenotypes in the immunopathogenesis of DPLDs; CVD-ILD, UIP, NSIP, DIP, RB-ILD, AIP, HP, Sarcoidosis, etc. Our goal is to update the understanding of the immune mechanisms underlying the initiation and progression of fibrosis in DPLDs. This will help in identification of biomarkers and in developing novel therapeutic strategies for DPLDs. A thorough literature search of the published studies in PubMed (from 1975 to 2020) was done.

EXPERT OPINION

The macrophage associated inflammatory markers needs to be explored for their potential as biomarkers of disease activity and progression. Pharmacological targeting of macrophage activation may reduce the risk of macrophage activation syndrome (MAS) and help improving the survival and prognosis of these patients.

Fate Mapping via Ms4a3-Expression History Traces Monocyte-Derived Cells

Most tissue-resident macrophage (RTM) populations are seeded by waves of embryonic hematopoiesis and are self-maintained independently of a bone marrow contribution during adulthood. A proportion of RTMs, however, is constantly replaced by blood monocytes, and their functions compared to embryonic RTMs remain unclear. The kinetics and extent of the contribution of circulating monocytes to RTM replacement during homeostasis, inflammation, and disease are highly debated. Here, we identified Ms4a3 as a specific gene expressed by granulocyte-monocyte progenitors (GMPs) and subsequently generated Ms4a3^TdT reporter, Ms4a3^Cre, and Ms4a3^CreERT2 fate-mapping models. These models traced efficiently monocytes and granulocytes, but no lymphocytes or tissue dendritic cells. Using these models, we precisely quantified the contribution of monocytes to the RTM pool during homeostasis and inflammation. The unambiguous identification of monocyte-derived cells will permit future studies of their function under any condition.

S100A9 Links Inflammation and Repair in Myocardial Infarction

RATIONALE

The alarmin S100A9 has been identified as a potential therapeutic target in myocardial infarction. Short-term S100A9 blockade during the inflammatory phase post-myocardial infarction inhibits systemic and cardiac inflammation and improves cardiac function long term.

OBJECTIVE

To evaluate the impact of S100A9 blockade on postischemic cardiac repair.

METHODS AND RESULTS

We assessed cardiac function, hematopoietic response, and myeloid phagocyte dynamics in WT (wild type) C57BL/6 mice with permanent coronary artery ligation, treated with the specific S100A9 blocker ABR-238901 for 7 or 21 days. In contrast to the beneficial effects of short-term therapy, extended S100A9 blockade led to progressive deterioration of cardiac function and left ventricle dilation. The treatment reduced the proliferation of Lin^-Sca-1⁺c-Kit⁺ hematopoietic stem and progenitor cells in the bone marrow and the production of proreparatory CD150⁺CD48^-CCR2⁺ hematopoietic stem cells. Monocyte trafficking from the spleen to the myocardium and subsequent phenotype switching to reparatory Ly6C^loMerTK^hi macrophages was also impaired, leading to inefficient efferocytosis, accumulation of apoptotic cardiomyocytes, and a larger myocardial scar. The transcription factor Nur77 (Nr4a1 [nuclear receptor subfamily 4 group A member 1]) mediates the transition from inflammatory Ly6C^hi monocytes to reparatory Ly6C^lo macrophages. S100A9 upregulated the levels and activity of Nur77 in monocytes and macrophages in vitro and in Ly6C^hi/int monocytes in vivo, and S100A9 blockade antagonized these effects. Finally, the presence of reparatory macrophages in the myocardium was also impaired in S100A9^-/- mice with permanent myocardial ischemia, leading to depressed cardiac function long term.

CONCLUSIONS

We show that S100A9 plays an important role in both the inflammatory and the reparatory immune responses to myocardial infarction. Long-term S100A9 blockade negatively impacts cardiac recovery and counterbalances the beneficial effects of short-term therapy. These results define a therapeutic window targeting the inflammatory phase for optimal effects of S100A9 blockade as potential immunomodulatory treatment in acute myocardial infarction.

MSCs Contribute to the Conversion of Ly6Chigh Monocytes into Ly6Clow Subsets under AMI

Background

Ly6C^high monocytes are inflammatory cells that accumulate in an infarcted myocardium, and Ly6C^low monocytes are believed to be reparative and curb myocardial remodeling. NR4A1 is a novel target for modulating the inflammatory phenotype of monocytes during atherogenesis.

Objectives

We aimed to investigate whether MSCs can contribute to the heterogeneity of Ly6C^high monocytes differentiated into Ly6C^low monocytes and whether this regulation is related to nuclear receptor NR4A1.

Methods

Ly6C^high/low monocytes were first cocultured with MSCs. C57BL/6^CX3CR1-/- mice and C57BL/6 wild-type mice were then used to construct AMI models, and survival functions in the two groups were further compared. Ly6C^high/low monocytes in circulation and in MI tissue of C57BL/6^CX3CR1-/- AMI mice with or without MSC transplantation were determined by flow cytometry at day 1 and day 3. NR4A1 expression was further determined by Western blot. Apoptosis of cardiac myocytes in the infarct border zone at day 3 and day 7 was identified by TUNEL kits. Angiogenesis in the AMI heart at day 7 and day 21 was determined through immunohistochemistry by CD31.

Results

We first demonstrated that the percentage of Ly6C^low monocytes increased greatly after 3 days of coculture with MSCs (12.8% ± 3.77% vs. 3.69% ± 0.74%, p < 0.001). The expression of NR4A1 in Ly6C^high/low monocytes was also significantly elevated at that time (1.81 ± 0.46 vs. 0.43 ± 0.09, p < 0.001). Following AMI, the percentage of circulating Ly6C^low monocytes in C57BL/6^CX3CR1-/- mice was significantly lower than that in C57BL/6 wild-type mice (4.36% ± 1.27% vs. 12.17% ± 3.81%, p < 0.001). The survival rate of C57BL/6^CX3CR1-/- mice (25%) was significantly lower than that of C57BL/6 wild-type mice (56.3%) after AMI (χ² = 4.343, p = 0.037). After MSCs were transplanted, we observed a significant increase in Ly6C^low monocytes both in circulation (16.7% ± 3.67% vs. 3.22% ± 0.44%, p < 0.001) and in the MI heart (3.31% ± 0.69% vs. 0.42% ± 0.21%, p < 0.001) of C57BL/6^CX3CR1-/- mice. Western blot analysis further showed that the expression level of NR4A1 in the MI hearts of C57BL/6^CX3CR1-/- mice increased significantly under MSC transplantation (0.39 ± 0.10 vs. 0.11 ± 0.04, p < 0.001). We also found significantly decreased TUNEL⁺ cardiac myocytes (15.45% ± 4.42% vs. 22.78% ± 6.40%, p < 0.001) in mice with high expression levels of NR4A1 compared to mice with low expression levels. Meanwhile, we further identified increased capillary density in the infarct zones of mice with high expression levels of NR4A1 (0.193 ± 0.036 vs. 0.075 ± 0.019, p < 0.001) compared to mice with low expression levels 21 days after AMI.

Conclusions

MSCs can control the heterogeneity of Ly6C^high monocyte differentiation into Ly6C^low monocytes and further reduce inflammation after AMI. The underlying mechanism might be that MSCs contribute to the increased expression of NR4A1 in Ly6C^high/low monocytes.

Towards the better understanding of myelopoiesis using single-cell technologies

Myeloid cells and their progenitors have historically been characterized based on their expression of a well-defined set of surface proteins, transcription factors and cytokines they depend on. These traditional analyses on "bulk" myeloid cell populations led to valuable early insights into the ontogeny of dendritic cells, granulocytes, monocytes and macrophages - a process called myelopoiesis. However, bulk approaches have limitations: they are unable to discern the individual stages and functions of progenitors and may be compromised by contaminating cells of non-myeloid lineages with similar or overlapping phenotypes. In recent years the emergence of high dimensional technologies to interrogate single cells at the molecular level, including single-cell mRNA sequencing and mass cytometry, has revolutionised our understanding of immune cell development and differentiation. Here, we highlight how the use of single-cell technologies has advanced our understanding of myelopoiesis and the emerging opportunities for it to continue to do so.

The Role of miRNAs in Immune Cell Development, Immune Cell Activation, and Tumor Immunity: With a Focus on Macrophages and Natural Killer Cells

The tumor microenvironment (TME) is the primary arena where tumor cells and the host immune system interact. Bidirectional communication between tumor cells and the associated stromal cell types within the TME influences disease initiation and progression, as well as tumor immunity. Macrophages and natural killer (NK) cells are crucial components of the stromal compartment and display either pro- or anti-tumor properties, depending on the expression of key regulators. MicroRNAs (miRNAs) are emerging as such regulators. They affect several immune cell functions closely related to tumor evasion of the immune system. This review discusses the role of miRNAs in the differentiation, maturation, and activation of immune cells as well as tumor immunity, focusing particularly on macrophages and NK cells.

Hemogram study of an artificially feeding tree shrew (Tupaia belangeri chinensis)

Systematic classification and determination of various cells in normal peripheral blood of artificially feeding Tupaia belangeri chinensis of different ages and genders and evaluation of the effectiveness of an automatic blood cell classification counter for measuring tree shrew blood cells. Child, young and adult tree shrews (forty for each group) were randomly selected, half male and half female. After the animals were stable, the peripheral blood of each group was collected through the femoral vein, and the morphology of various blood cells of the tree shrew was observed and classified by the manual microscopic counting method and by an automatic blood cell classification counter. The Reference intervals of the normal peripheral blood cell absolute count, cell diameter and white blood cell percentage in tree shrews of different ages and genders has been calculated. White blood cell count and neutrophil relative count increased with age, while lymphocyte relative count decreased. The white blood cell count, neutrophil relative count, and lymphocyte relative count in the child group, as well as lymphocyte relative count in the young group, significantly differed according to gender (P<0.05), and the differences in other indicators were not significant. The Bland-Altman plot and the Passing-Bablok scattergram showed that the change trend of each indicator was consistent but exhibited large systematic differences between methods. Differences in peripheral blood cells exist among different age groups and different genders. An automatic blood cell classification counter is not suitable for the absolute count of blood cells in the tree shrew.

Macrophage Polarization in Leishmaniasis: Broadening Horizons

Leishmaniasis is a vector-borne neglected tropical disease that affects more than 700,000 people annually. Leishmania parasites cause the disease, and different species trigger a distinct immune response and clinical manifestations. Macrophages are the final host cells for the proliferation of Leishmania parasites, and these cells are the key to a controlled or exacerbated response that culminates in clinical manifestations. M1 and M2 are the two main macrophage phenotypes. M1 is a pro-inflammatory subtype with microbicidal properties, and M2, or alternatively activated, is an anti-inflammatory/regulatory subtype that is related to inflammation resolution and tissue repair. The present review elucidates the roles of M1 and M2 polarization in leishmaniasis and highlights the role of the salivary components of the vector and the action of the parasite in the macrophage plasticity.

Transcriptional Regulation of Macrophages Polarization by MicroRNAs

Diversity and plasticity are the hallmarks of cells from the monocyte-macrophage lineage. Macrophages undergo classical M1 or alternative M2 activation in response to the microenvironment signals. Several transcription factors, such as peroxisome proliferator-activated receptors, signal transducers and activators of transcription, CCAAT-enhancer-binding proteins, interferon regulatory factors, Kruppel-like factors, GATA binding protein 3, nuclear transcription factor-κB, and c-MYC, were found to promote the expression of specific genes, which dictate the functional polarization of macrophages. Importantly, these transcription factors can be regulated by microRNAs (miRNAs), a group of small non-coding RNAs, which regulate gene expression through translation repression or mRNA degradation. Recent studies have also revealed that miRNAs control macrophage polarization by regulating transcription factors in response to the microenvironment signals. This review will summarize recent progress of miRNAs in the transcriptional regulation of macrophage polarization and provide the insights into the development of macrophage-centered diagnostic and therapeutic strategies.

Blockade of TGF-β/Smad signaling by the small compound HPH-15 ameliorates experimental skin fibrosis

Background

Transforming growth factor-β (TGF-β)/Smad signaling is well known to play a critical role in the pathogenesis of systemic sclerosis (SSc). We previously developed an artificial molecule, the histidine-pyridine-histidine ligand derivative HPH-15, which may have an antifibrotic effect. The purpose of the present study was to clarify the effects of this drug in human skin fibroblasts and in a preclinical model of SSc.

Methods

The effects of HPH-15 on expression of extracellular matrix components and TGF-β signaling in human dermal fibroblasts were analyzed. The antifibrotic properties of HPH-15 and its mechanisms were also examined in a bleomycin-induced skin fibrosis mouse model.

Results

HPH-15 suppressed the TGF-β-induced phosphorylation of Smad3 and inhibited the expression of collagen I, fibronectin 1, connective tissue growth factor, and α-smooth muscle actin induced by TGF-β in cultured human skin fibroblasts. In the bleomycin-induced skin fibrosis model, oral administration of HPH-15 protected against the development of skin fibrosis and ameliorated established skin fibrosis. Additionally, HPH-15 suppressed the phosphorylation of Smad3 in various cells, including macrophages in the bleomycin-injected skin. Further, in the treated mice, dermal infiltration of proinflammatory macrophages (CD11b⁺Ly6C^hi) and M2 profibrotic macrophages (CD11b⁺CD204⁺ or CD11b⁺CD206⁺) was significantly decreased during the early and late stages, respectively. HPH-15 treatment resulted in decreased messenger RNA (mRNA) expression of the M2 macrophage markers arginase 1 and Ym-1 in the skin, whereas it inversely augmented expression of Friend leukemia integration 1 and Krüppel-like factor 5 mRNAs, the transcription factors that repress collagen synthesis. No apparent adverse effects of HPH-15 were found during the treatment.

Conclusions

HPH-15 may inhibit skin fibrosis by inhibiting the phosphorylation of Smad3 in dermal fibroblasts and possibly in macrophages. Our results demonstrate several positive qualities of HPH-15, including oral bioavailability, a good safety profile, and therapeutic effectiveness. Thus, this TGF-β/Smad inhibitor is a potential candidate therapeutic for SSc clinical trials.

Electronic supplementary material

The online version of this article (10.1186/s13075-018-1534-y) contains supplementary material, which is available to authorized users.

IL-32θ: a recently identified anti-inflammatory variant of IL-32 and its preventive role in various disorders and tumor suppressor activity

Interleukin-32 theta (IL-32θ) is newly identified isoform of IL-32 which plays a vital role in inflammatory responses. Like IL-32α and IL-32β, IL-32θ isoform acts as an intracellular inflammatory modulator. It results in reduction of IL-1β production by attenuating the expression of PU.1 and inhibition of monocytes differentiation into macrophages. IL-32θ hinders TNF-α expression by inhibiting p38 MAPK and inhibitor of κB (IκB) as well. It also reserved STAT3-ZEB1 pathway leading to the inhibition of epithelial-mesenchymal transition (EMT) and stemness. Hence, it can be concluded that IL-32θ is an anti-inflammatory cytokine that can act as a tumor suppressor and can play vital role in colon cancer therapies. IL-32θ also plays a crucial role in immune system responses and cellular differentiation during disease pathogenesis. To our best knowledge this is the first ever review to condense the importance, precise mode of action in disease progression and latent remedial implications of IL-32θ in several inflammatory disorders.

The impact of interferon-regulatory factors to macrophage differentiation and polarization into M1 and M2

The mononuclear phagocytes control the body homeostasis through the involvement in resolving tissue injury and further wound healing. Indeed, local tissue microenvironmental changes can significantly influence the functional behavior of monocytes and macrophages. Such microenvironmental changes for example occur in an atherosclerotic plaque during all progression stages. In response to exogenous stimuli, macrophages show a great phenotypic plasticity and heterogeneity. Exposure of monocytes to inflammatory or anti-inflammatory conditions also induces predominant differentiation to proinflammatory (M1) or anti-inflammatory (M2) macrophage subsets and phenotype switch between macrophage subsets. The phenotype transition is accompanied with great changes in the macrophage transcriptome and regulatory networks. Interferon-regulatory factors (IRFs) play a key role in hematopoietic development of monocytes, their differentiation to macrophages, and regulating macrophage maturation, phenotypic polarization, phenotypic switch, and function. Of 9 IRFs, at least 3 (IRF-1, IRF-5, and IRF-8) are involved in the commitment of proinflammatory M1 whereas IRF-3 and IRF-4 control M2 polarization. The role of IRF-2 is context-dependent. The IRF impact on macrophage phenotype plasticity and heterogeneity is complex and involves activating and repressive function in triggering transcription of target genes.

Murine Monocytes: Origins, Subsets, Fates, and Functions

Monocytes are short-lived mononuclear phagocytes that circulate in the bloodstream and comprise two main subpopulations that in the mouse are best defined by the Ly6C marker. Intravascular functions of "classical" Ly6C+ monocytes and their interactions with other lymphoid and myeloid leukocytes in the circulation remain poorly understood. Rather, these cells are known to efficiently extravasate into tissues. Indeed, Ly6C+ monocytes and their descendants have emerged as a third, highly plastic and dynamic cellular system that complements the two classical, tissue-resident mononuclear phagocyte compartments, i.e., macrophages and dendritic cells, on demand. Following recruitment to injured tissue, Ly6C+ monocytes respond to local cues and can critically contribute to the initiation and resolution of inflammatory reactions. The second main murine monocyte subset, Ly6C- cells, derive in steady state from Ly6C+ monocytes and remain in the vasculature, where the cells act as scavengers. Moreover, a major fraction of Ly6C- monocytes adheres to the capillary endothelium and patrols the vessel wall for surveillance. Given the central role of monocytes in homeostasis and pathology, in-depth study of this cellular compartment can be highly informative on the health state of the organism and provides an attractive target for therapeutic intervention.

Genome-Wide Approaches to Defining Macrophage Identity and Function

Macrophages play essential roles in the response to injury and infection and contribute to the development and/or homeostasis of the various tissues they reside in. Conversely, macrophages also influence the pathogenesis of metabolic, neurodegenerative, and neoplastic diseases. Mechanisms that contribute to the phenotypic diversity of macrophages in health and disease remain poorly understood. Here we review the recent application of genome-wide approaches to characterize the transcriptomes and epigenetic landscapes of tissue-resident macrophages. These studies are beginning to provide insights into how distinct tissue environments are interpreted by transcriptional regulatory elements to drive specialized programs of gene expression.

Galectin-3, histone deacetylases, and Hedgehog signaling: Possible convergent targets in schistosomiasis-induced liver fibrosis

Schistosomiasis affects approximately 240 million people in the world. Schistosoma mansoni eggs in the liver induce periportal fibrosis and hepatic failure driven by monocyte recruitment and macrophage activation, resulting in robust Th2 response. Here, we suggested a possible involvement of Galectin-3 (Gal-3), histone deacetylases (HDACs), and Hedgehog (Hh) signaling with macrophage activation during Th1/Th2 immune responses, fibrogranuloma reaction, and tissue repair during schistosomiasis. Gal-3 is highly expressed by liver macrophages (Kupffer cells) around Schistosoma eggs. HDACs and Hh regulate macrophage polarization and hepatic stellate cell activation during schistosomiasis-associated fibrogenesis. Previously, we demonstrated an abnormal extracellular matrix distribution in the liver that correlated with atypical monocyte-macrophage differentiation in S. mansoni-infected, Gal-3-deficient (Lgals3-/-) mice. New findings explored in this review focus on the chronic phase, when wild-type (Lgals3+/+) and Lgals3-/- mice were analyzed 90 days after cercariae infection. In Lgals3-/- infected mice, there was significant inflammatory infiltration with myeloid cells associated with egg destruction (hematoxylin and eosin staining), phagocytes (specifically Kupffer cells), numerically reduced and diffuse matrix extracellular deposition in fibrotic areas (Gomori trichrome staining), and severe disorganization of collagen fibers surrounding the S. mansoni eggs (reticulin staining). Granuloma-derived stromal cells (GR cells) of Lgals3-/- infected mice expressed lower levels of alpha smooth muscle actin (α-SMA) and eotaxin and higher levels of IL-4 than Lgals3+/+ mice (real-time PCR). The relevant participation of macrophages in these events led us to suggest distinct mechanisms of activation that culminate in defective fibrosis in the liver of Lgals3-/- infected mice. These aspects were discussed in this review, as well as the possible interference between Gal-3, HDACs, and Hh signaling during progressive liver fibrosis in S. mansoni-infected mice. Further studies focused on macrophage roles could elucidate these questions and clear the potential utility of these molecules as antifibrotic targets.

Targeting the Monocyte-Macrophage Lineage in Solid Organ Transplantation

There is an unmet clinical need for immunotherapeutic strategies that specifically target the active immune cells participating in the process of rejection after solid organ transplantation. The monocyte–macrophage cell lineage is increasingly recognized as a major player in acute and chronic allograft immunopathology. The dominant presence of cells of this lineage in rejecting allograft tissue is associated with worse graft function and survival. Monocytes and macrophages contribute to alloimmunity via diverse pathways: antigen processing and presentation, costimulation, pro-inflammatory cytokine production, and tissue repair. Cross talk with other recipient immune competent cells and donor endothelial cells leads to amplification of inflammation and a cytolytic response in the graft. Surprisingly, little is known about therapeutic manipulation of the function of cells of the monocyte–macrophage lineage in transplantation by immunosuppressive agents. Although not primarily designed to target monocyte–macrophage lineage cells, multiple categories of currently prescribed immunosuppressive drugs, such as mycophenolate mofetil, mammalian target of rapamycin inhibitors, and calcineurin inhibitors, do have limited inhibitory effects. These effects include diminishing the degree of cytokine production, thereby blocking costimulation and inhibiting the migration of monocytes to the site of rejection. Outside the field of transplantation, some clinical studies have shown that the monoclonal antibodies canakinumab, tocilizumab, and infliximab are effective in inhibiting monocyte functions. Indirect effects have also been shown for simvastatin, a lipid lowering drug, and bromodomain and extra-terminal motif inhibitors that reduce the cytokine production by monocytes–macrophages in patients with diabetes mellitus and rheumatoid arthritis. To date, detailed knowledge concerning the origin, the developmental requirements, and functions of diverse specialized monocyte–macrophage subsets justifies research for therapeutic manipulation. Here, we will discuss the effects of currently prescribed immunosuppressive drugs on monocyte/macrophage features and the future challenges.

Identification of an atypical monocyte and committed progenitor involved in fibrosis

Monocytes and macrophages comprise a variety of subsets with diverse functions. It is thought that these cells play a crucial role in homeostasis of peripheral organs, key immunological processes and development of various diseases. Among these diseases, fibrosis is a life-threatening disease of unknown aetiology. Its pathogenesis is poorly understood, and there are few effective therapies. The development of fibrosis is associated with activation of monocytes and macrophages. However, the specific subtypes of monocytes and macrophages that are involved in fibrosis have not yet been identified. Here we show that Ceacam1⁺Msr1⁺Ly6C^-F4/80^-Mac1⁺ monocytes, which we term segregated-nucleus-containing atypical monocytes (SatM), share granulocyte characteristics, are regulated by CCAAT/enhancer binding protein β (C/EBPβ), and are critical for fibrosis. Cebpb deficiency results in a complete lack of SatM. Furthermore, the development of bleomycin-induced fibrosis, but not inflammation, was prevented in chimaeric mice with Cebpb^-/- haematopoietic cells. Adoptive transfer of SatM into Cebpb^-/- mice resulted in fibrosis. Notably, SatM are derived from Ly6C^-FcεRI⁺ granulocyte/macrophage progenitors, and a newly identified SatM progenitor downstream of Ly6C^-FcεRI⁺ granulocyte/macrophage progenitors, but not from macrophage/dendritic-cell progenitors. Our results show that SatM are critical for fibrosis and that C/EBPβ licenses differentiation of SatM from their committed progenitor.

Functions of interleukin-34 and its emerging association with rheumatoid arthritis

Rheumatoid arthritis (RA) is a systemic autoimmune disease characterized by chronic, synovial inflammation affecting multiple joints, finally leading to extra-articular lesions for which limited effective treatment options are currently available. Interleukin-34 (IL-34), recently discovered as the second colony-stimulating factor-1 receptor (CSF-1R) ligand, is a newly discovered cytokine. Accumulating evidence has disclosed crucial roles of IL-34 in the proliferation and differentiation of mononuclear phagocyte lineage cells, osteoclastogenesis and inflammation. Recently, IL-34 was detected at high levels in patients with active RA and in experimental models of inflammatory arthritis. Blockade of functional IL-34 with a specific monoclonal antibody can reduce the severity of inflammatory arthritis, suggesting that targeting IL-34 or its receptors may constitute a novel therapeutic strategy for autoimmune diseases such as RA. Here, we have comprehensively discussed the structure and biological functions of IL-34, and reviewed recent advances in our understanding of the emerging role of IL-34 in the development of RA as well as its potential utility as a therapeutic target.

Proteomic and bioinformatics profile of paired human alveolar macrophages and peripheral blood monocytes

Little is known about proteomic differences between pluripotent human peripheral blood monocytes (MN) and their terminally-differentiated pulmonary counterparts, alveolar macrophages (AM). To better characterize these cell populations, we performed a label-free shotgun proteomics assessment of matched AM and MN preparations from eight healthy volunteers. With an FDR of less than 0.45%, we identified 1754 proteins within AM and 1445 from MN. Comparison of the two proteomes revealed that 1239 of the proteins found in AM were shared with MN, whereas 206 proteins were uniquely identified in MN and 515 were unique to AM. Molecular and cellular functions, protein classes, development associations, and membership in physiological systems and canonical pathways were identified among the detected proteins. Analysis of biologic processes represented by these proteomes indicated that MN were most prominently enriched for proteins involved in cellular movement and immune cell trafficking. In contrast, AM were enriched for proteins involved in protein trafficking, molecular transport, and cellular assembly and organization. These findings provide a baseline proteomic resource for further studies aimed at better understanding of the functional differences between MN and AM in both health and disease.

Myeloid Growth Factors Promote Resistance to Mycobacterial Infection by Curtailing Granuloma Necrosis through Macrophage Replenishment

The mycobacterial ESX-1 virulence locus accelerates macrophage recruitment to the forming tuberculous granuloma. Newly recruited macrophages phagocytose previously infected apoptotic macrophages to become new bacterial growth niches. Granuloma macrophages can then necrose, releasing mycobacteria into the extracellular milieu, which potentiates their growth even further. Using zebrafish with genetic or pharmacologically induced macrophage deficiencies, we find that global macrophage deficits increase susceptibility to mycobacterial infection by accelerating granuloma necrosis. This is because reduction in the macrophage supply below a critical threshold decreases granuloma macrophage replenishment to the point where apoptotic infected macrophages, failing to get engulfed, necrose. Reducing macrophage demand by removing bacterial ESX-1 offsets the susceptibility of macrophage deficits. Conversely, increasing macrophage supply in wild-type fish by overexpressing myeloid growth factors induces resistance by curtailing necrosis. These findings may explain the susceptibility of humans with mononuclear cytopenias to mycobacterial infections and highlight the therapeutic potential of myeloid growth factors in tuberculosis.

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Myeloid deficiencies increase innate immune susceptibility to mycobacterial infection

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Depletion of macrophage supply below a critical threshold hastens granuloma necrosis

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Increasing macrophage supply to the granuloma delays granuloma necrosis

Transcriptional Control of Dendritic Cell Development

The dendritic cells (DCs) of the immune system function in innate and adaptive responses by directing activity of various effector cells rather than serving as effectors themselves. DCs and closely related myeloid lineages share expression of many surface receptors, presenting a challenge in distinguishing their unique in vivo functions. Recent work has taken advantage of unique transcriptional programs to identify and manipulate murine DCs in vivo. This work has assigned several nonredundant in vivo functions to distinct DC lineages, consisting of plasmacytoid DCs and several subsets of classical DCs that promote different immune effector modules in response to pathogens. In parallel, a correspondence between human and murine DC subsets has emerged, underlying structural similarities for the DC lineages between these species. Recent work has begun to unravel the transcriptional circuitry that controls the development and diversification of DCs from common progenitors in the bone marrow.

IL-32θ inhibits monocytic differentiation of leukemia cells by attenuating expression of transcription factor PU.1

PU.1 is a key transcription factor regulating the myeloid differentiation. PU.1-induced monocytic differentiation into macrophage is also important for blood cancer development. Therefore, we chose THP-1 monocytic leukemia cells to investigate the function of a recently discovered IL-32θ. Genetic analyses identified differences in the sequences of IL-32θ and IL-32β. Using previously established cell lines that stably express IL-32θ and IL-32β and cell lines transiently expressing IL-32θ, we observed that expression of IL-32θ inhibited phorbol 12-myristate 13-acetate (PMA)-induced monocytic differentiation in both THP-1 and HL-60 cells. IL-32θ also suppressed expression of the macrophage cell surface markers, CD11b, CD18, and CD36. Interestingly, expression of IL-32β or IL-32θ had no effect on the expression levels of cell cycle related factors. As a result, we concluded that these isoforms did not contribute to PMA-induced cell cycle arrest. IL-32θ was found to modulate expression of PU.1, a transcription factor necessary for myeloid lineage commitment. Transient expression of PU.1 in THP-1/IL-32θ cells rescued the observed differentiation defect. Additionally, transient expression of both CCAAT-enhancer-binding protein α (C/EBPα) and PU.1 in THP-1/IL-32θ cells exhibited synergistic effects in rescuing the differentiation defect. These observations indicate that intracellular IL-32θ inhibits the differentiation of monocytes into macrophages by attenuating PU.1 expression.

Transcriptional Regulation of Mononuclear Phagocyte Development

Mononuclear phagocytes (MP) are a quite unique subset of hematopoietic cells, which comprise dendritic cells (DC), monocytes as well as monocyte-derived and tissue-resident macrophages. These cells are extremely diverse with regard to their origin, their phenotype as well as their function. Developmentally, DC and monocytes are constantly replenished from a bone marrow hematopoietic progenitor. The ontogeny of macrophages is more complex and is temporally linked and specified by the organ where they reside, occurring early during embryonic or perinatal life. The functional heterogeneity of MPs is certainly a consequence of the tissue of residence and also reflects the diverse ontogeny of the subsets. In this review, we will highlight the developmental pathways of murine MP, with a particular emphasis on the transcriptional factors that regulate their development and function. Finally, we will discuss and point out open questions in the field.

Role of Erk1/2 signaling in the regulation of neutrophil versus monocyte development in response to G-CSF and M-CSF

Lineage specification in the hematopoietic system depends on the expression of lineage specific transcription factors. However, the role of hematopoietic cytokines in this process has been controversial and little is known about the intracellular signaling mechanisms by which cytokines instruct lineage choice. G-CSF and M-CSF are two lineage-specific cytokines that play a dominant role in granulopoiesis and monopoiesis, respectively. We show here that a G-CSFR mutant in which tyrosine 729 had been mutated to phenylalanine (Y729F) promoted monocyte rather than neutrophil development in myeloid precursors, which was associated with prolonged activation of Erk1/2 and augmented activation of downstream targets c-Fos and Egr1. Inhibition of Erk1/2 activation or knockdown of c-Fos or Egr1 largely rescued neutrophil development in cells expressing G-CSFR Y729F. We also show that M-CSF, but not G-CSF, stimulated strong and sustained activation of Erk1/2 in mouse lineage marker negative (Lin(-)) bone marrow cells. Significantly, inhibition of Erk1/2 signaling in these cells favored neutrophil over monocyte development in response to M-CSF. Thus, prolonged Erk1/2 activation resulted in monocyte development following G-CSF induction whereas inhibition of Erk1/2 signaling promoted neutrophil development at the expense of monocyte formation in response to M-CSF. These results reveal an important mechanism by which G-CSF and M-CSF instruct neutrophil versus monocyte lineage choice, i.e. differential activation of Erk1/2 pathway.

Klf4 expression in conventional dendritic cells is required for T helper 2 cell responses

The two major lineages of classical dendritic cells (cDCs) express and require either IRF8 or IRF4 transcription factors for their development and function. IRF8-dependent cDCs promote anti-viral and T-helper 1 (Th1) cell responses, whereas IRF4-expressing cDCs have been implicated in controlling both Th2 and Th17 cell responses. Here, we have provided evidence that Kruppel-like factor 4 (Klf4) is required in IRF4-expressing cDCs to promote Th2, but not Th17, cell responses in vivo. Conditional Klf4 deletion within cDCs impaired Th2 cell responses during Schistosoma mansoni infection, Schistosoma egg antigen (SEA) immunization, and house dust mite (HDM) challenge without affecting cytotoxic T lymphocyte (CTL), Th1 cell, or Th17 cell responses to herpes simplex virus, Toxoplasma gondii, and Citrobacter rodentium infections. Further, Klf4 deletion reduced IRF4 expression in pre-cDCs and resulted in selective loss of IRF4-expressing cDCs subsets in several tissues. These results indicate that Klf4 guides a transcriptional program promoting IRF4-expressing cDCs heterogeneity.

Differential requirement for irf8 in formation of embryonic and adult macrophages in zebrafish

Interferon regulatory factor 8 (Irf8) is critical for mammalian macrophage development and innate immunity, but its role in teleost myelopoiesis remains incompletely understood. In particular, genetic tools to analyze the role of Irf8 in zebrafish macrophage development at larval and adult stages are lacking. We generated irf8 null mutants in zebrafish using TALEN-mediated targeting. Our analysis defines different requirements for irf8 at different stages. irf8 is required for formation of all macrophages during primitive and transient definitive hematopoiesis, but not during adult-phase definitive hematopoiesis starting at 5-6 days postfertilization. At early stages, irf8 mutants have excess neutrophils and excess cell death in pu.1-expressing myeloid cells. Macrophage fates were recovered in irf8 mutants after wildtype irf8 expression in neutrophil and macrophage lineages, suggesting that irf8 regulates macrophage specification and survival. In juvenile irf8 mutant fish, mature macrophages are present, but at numbers significantly reduced compared to wildtype, indicating an ongoing requirement for irf8 after embryogenesis. As development progresses, tissue macrophages become apparent in zebrafish irf8 mutants, with the possible exception of microglia. Our study defines distinct requirement for irf8 in myelopoiesis before and after transition to the adult hematopoietic system.

Functional significance of mononuclear phagocyte populations generated through adult hematopoiesis

Tissue homeostasis requires a complete repertoire of functional macrophages in peripheral tissues. Recent evidence indicates that many resident tissue macrophages are seeded during embryonic development and persist through adulthood as a consequence of localized proliferation. Mononuclear phagocytes are also produced during adult hematopoiesis; these cells are then recruited to sites throughout the body, where they function in tissue repair and remodeling, resolution of inflammation, maintenance of homeostasis, and disease progression. The focus of this review is on mononuclear phagocytes that comprise the nonresident monocyte/macrophage populations in the body. Key features of monocyte differentiation are presented, focusing primarily on the developmental hierarchy that is established through this process, the markers used to identify discrete cell populations, and novel, functional attributes of these cells. These features are then explored in the context of the tumor microenvironment, where mononuclear phagocytes exhibit extensive plasticity in phenotype and function.

Regulation of Macrophage Polarization by RON Receptor Tyrosine Kinase Signaling

The M1 and M2 states of macrophage polarization are the two extremes of a physiologic/phenotypic continuum that is dynamically influenced by environmental signals. The M1/M2 paradigm is an excellent framework to understand and appreciate some of the diverse functions that macrophages perform. Molecular analysis of mouse and human macrophages indicated that they gain M1 and M2-related functions after encountering specific ligands in the tissue environment. In this perspective, I discuss the function of recepteur d’origine nantais (RON) receptor tyrosine kinase in regulating the M2-like state of macrophage activation Besides decreasing pro-inflammatory cytokine production in response to toll-like receptor-4 activation, macrophage-stimulating protein strongly suppresses nitric oxide synthase and at the same time upregulates arginase, which is the rate limiting enzyme in the ornithine biosynthesis pathway. Interestingly, RON signaling preserved some of the characteristics of the M1 state, while still promoting the hallmarks of M2 polarization. Therefore, therapeutic modulation of RON activity can shift the activation state of macrophages between acute and chronic inflammatory states.

The innate immune response

The innate immune response is of prime importance in the immediate recognition and elimination of invading micro-organisms. However, deregulation of this system is clearly associated with the pathogenesis of a wide range of inflammatory diseases. Innate immunity consists of a humoral and a cellular branch, which are closely interacting. An additional level of control is found at the level of neuronal reflexes that can fine-tune these immunological mechanisms.

RNA sequencing of microglia and monocyte-derived macrophages from mice with experimental autoimmune encephalomyelitis illustrates a changing phenotype with disease course

The role of microglia and monocyte-derived macrophages in experimental autoimmune encephalomyelitis pathogenesis has been controversial. To gain insight into their respective roles, we developed a method for differentiating between microglia and monocyte-derived macrophages in the CNS by flow cytometry utilizing anti-CD44 antibodies. We used this system to monitor changes in cell number, activation status, and gene expression by RNA sequencing over the course of disease. This in vivo characterization and RNA-Seq dataset improves our understanding of macrophage biology in the brain under inflammatory conditions and may lead to strategies to identify therapies for neuroinflammatory diseases.