Fas determines differential fates of resident and recruited macrophages during resolution of acute lung injury

Surveying the Epigenetic Landscape of Tuberculosis in Alveolar Macrophages

Tuberculosis (TB) remains the leading cause of bacterial disease-related death and is among the top 10 overall causes of death worldwide. The complex nature of this infectious lung disease has proven difficult to treat, and significant research efforts are now evaluating the feasibility of host-directed, adjunctive therapies. An attractive approach in host-directed therapy targets host epigenetics, or gene regulation, to redirect the immune response in a host-beneficial manner. Substantial evidence exists demonstrating that host epigenetics are dysregulated during TB and that epigenetic-based therapies may be highly effective to treat TB. However, the caveat is that much of the knowledge that exists on the modulation of the host epigenome during TB has been gained using in vitro, small-animal, or blood-derived cell models, which do not accurately reflect the pulmonary nature of the disease. In humans, the first and major target cells of Mycobacterium tuberculosis are alveolar macrophages (AM). As such, their response to infection and treatment is clinically relevant and ultimately drives the outcome of disease. In this review, we compare the fundamental differences between AM and circulating monocyte-derived macrophages in the context of TB and summarize the recent advances in elucidating the epigenomes of these cells, including changes to the transcriptome, DNA methylome, and chromatin architecture. We will also discuss trained immunity in AM as a new and emerging field in TB research and provide some perspectives for the translational potential of targeting host epigenetics as an alternative TB therapy.

Pathogenic Mechanisms Underlying Idiopathic Pulmonary Fibrosis

The pathogenesis of idiopathic pulmonary fibrosis (IPF) involves a complex interplay of cell types and signaling pathways. Recurrent alveolar epithelial cell (AEC) injury may occur in the context of predisposing factors (e.g., genetic, environmental, epigenetic, immunologic, and gerontologic), leading to metabolic dysfunction, senescence, aberrant epithelial cell activation, and dysregulated epithelial repair. The dysregulated epithelial cell interacts with mesenchymal, immune, and endothelial cells via multiple signaling mechanisms to trigger fibroblast and myofibroblast activation. Recent single-cell RNA sequencing studies of IPF lungs support the epithelial injury model. These studies have uncovered a novel type of AEC with characteristics of an aberrant basal cell, which may disrupt normal epithelial repair and propagate a profibrotic phenotype. Here, we review the pathogenesis of IPF in the context of novel bioinformatics tools as strategies to discover pathways of disease, cell-specific mechanisms, and cell-cell interactions that propagate the profibrotic niche.

GM-CSF: Orchestrating the Pulmonary Response to Infection

This review summarizes the structure and function of the alveolar unit, comprised of alveolar macrophage and epithelial cell types that work in tandem to respond to infection. Granulocyte-macrophage colony-stimulating factor (GM-CSF) helps to maintain the alveolar epithelium and pulmonary immune system under physiological conditions and plays a critical role in restoring homeostasis under pathologic conditions, including infection. Given the emergence of novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and global spread of coronavirus disease 2019 (COVID-19), with subsequent acute respiratory distress syndrome, understanding basic lung physiology in infectious diseases is especially warranted. This review summarizes clinical and preclinical data for GM-CSF in respiratory infections, and the rationale for sargramostim (yeast-derived recombinant human [rhu] GM-CSF) as adjunctive treatment for COVID-19 and other pulmonary infectious diseases.

Gammaherpesvirus Alters Alveolar Macrophages According to the Host Genetic Background and Promotes Beneficial Inflammatory Control over Pneumovirus Infection

Human respiratory syncytial virus (hRSV) infection brings a wide spectrum of clinical outcomes, from a mild cold to severe bronchiolitis or even acute interstitial pneumonia. Among the known factors influencing this clinical diversity, genetic background has often been mentioned. In parallel, recent evidence has also pointed out that an early infectious experience affects heterologous infections severity. Here, we analyzed the importance of these two host-related factors in shaping the immune response in pneumoviral disease. We show that a prior gammaherpesvirus infection improves, in a genetic background-dependent manner, the immune system response against a subsequent lethal dose of pneumovirus primary infection notably by inducing a systematic expansion of the CD8+ bystander cell pool and by modifying the resident alveolar macrophages (AMs) phenotype to induce immediate cyto/chemokinic responses upon pneumovirus exposure, thereby drastically attenuating the host inflammatory response without affecting viral replication. Moreover, we show that these AMs present similar rapid and increased production of neutrophil chemokines both in front of pneumoviral or bacterial challenge, confirming recent studies attributing a critical antibacterial role of primed AMs. These results corroborate other recent studies suggesting that the innate immunity cells are themselves capable of memory, a capacity hitherto reserved for acquired immunity.

Effect of Acinetobacter lwoffii on the modulation of macrophage activation and asthmatic inflammation

BACKGROUND

Although lung macrophages are directly exposed to external stimuli, their exact immunologic roles in asthma are still largely unknown. The aim of this study was to investigate the anti-asthmatic effect of Acinetobacter lwoffii in terms of lung macrophage modulation.

METHODS

Six-week-old female BALB/c mice were sensitized and challenged with ovalbumin (OVA) with or without intranasal administration of A. lwoffii during the sensitization period. Airway hyperresponsiveness and inflammation were evaluated. Using flow cytometry, macrophages were subclassified according to their activation status. In the in vitro study, a murine alveolar macrophage cell line (MH-S) treated with or without A. lwoffii before IL-13 stimulation were analysed by quantitative RT-PCR.

RESULTS

In a murine asthma model, the number of inflammatory cells, including macrophages and eosinophils, decreased in mice treated with A. lwoffii (A. lwoffii/OVA group) compared with untreated mice (OVA group). The enhanced expression of MHCII in macrophages in the OVA group was decreased by A. lwoffii treatment. M2 macrophage subtypes were significantly altered. A. lwoffii treatment decreased CD11b⁺ M2a and CD11b⁺ M2c macrophages, which showed strong positive correlations with Th2 cells, ILC2 and eosinophils. In contrast, CD11b⁺ M2b macrophages were significantly increased by A. lwoffii treatment and showed strong positive correlations with ILC1 and ILC3. In vitro, A. lwoffii down-regulated the expression of M2 markers related but up-regulated those related to M2b macrophages.

CONCLUSIONS AND CLINICAL RELEVANCE

Intranasal A. lwoffii exposure suppresses asthma development by suppressing the type 2 response via modulating lung macrophage activation, shifting M2a and M2c macrophages to M2b macrophages.

The value of lymphocyte-to-monocyte ratio and neutrophil-to-lymphocyte ratio in differentiating pneumonia from upper respiratory tract infection (URTI) in children: a cross-sectional study

BACKGROUNDS

Early and accurate diagnosis of pediatric pneumonia in primary health care can reduce the chance of long-term respiratory diseases, related hospitalizations and mortality while lowering medical costs. The aim of this study was to assess the value of blood biomarkers, clinical symptoms and their combination in assisting discrimination of pneumonia from upper respiratory tract infection (URTI) in children.

METHODS

Both univariate and multivariate logistic regressions were used to build the pneumonia screening model based on a retrospective cohort, comprised of 5211 children (age ≤ 18 years). The electronic health records of the patients, who had inpatient admission or outpatient visits between February 15, 2012 to September 30, 2018, were extracted from the hospital information system of Zhejiang Provincial People's Hospital, Hangzhou, Zhejiang Province, China. The children who were diagnosed with pneumonia and URTI were enrolled and their clinical features and levels of blood biomarkers were compared. Using the area under the ROC curve, both two screening models were evaluated under 80% (training) versus 20% (test) cross-validation data split for their accuracy.

RESULTS

In the retrospective cohort, 2548 of 5211 children were diagnosed with the defined pneumonia. The univariate screening model reached predicted AUCs of 0.76 for lymphocyte/monocyte ratio (LMR) and 0.71 for neutrophil/lymphocyte ratio (NLR) when identified overall pneumonia from URTI, attaining the best performance among the biomarker candidates. In subgroup analysis, LMR and NLR attained AUCs of 0.80 and 0.86 to differentiate viral pneumonia from URTI, and AUCs of 0.77 and 0.71 to discriminate bacterial pneumonia from URTI respectively. After integrating LMR and NLR with three clinical symptoms of fever, cough and rhinorrhea, the multivariate screening model obtained increased predictive values, reaching validated AUCs of 0.84, 0.95 and 0.86 for distinguishing pneumonia, viral pneumonia and bacterial pneumonia from URTI respectively.

CONCLUSIONS

Our study demonstrated that combining LMR and NLR with critical clinical characteristics reached promising accuracy in differentiating pneumonia from URTI, thus could be considered as a useful screening tool to assist the diagnosis of pneumonia, in particular, in community healthcare centers. Further researches could be conducted to evaluate the model's clinical utility and cost-effectiveness in primary care scenarios to facilitate pneumonia diagnosis, especially in rural settings.

Dysregulated Inflammation During Obesity: Driving Disease Severity in Influenza Virus and SARS-CoV-2 Infections

Acute inflammation is a critical host defense response during viral infection. When dysregulated, inflammation drives immunopathology and tissue damage. Excessive, damaging inflammation is a hallmark of both pandemic influenza A virus (IAV) infections and Severe Acute Respiratory Syndrome-Coronavirus-2 (SARS-CoV-2) infections. Chronic, low-grade inflammation is also a feature of obesity. In recent years, obesity has been recognized as a growing pandemic with significant mortality and associated costs. Obesity is also an independent risk factor for increased disease severity and death during both IAV and SARS-CoV-2 infection. This review focuses on the effect of obesity on the inflammatory response in the context of viral respiratory infections and how this leads to increased viral pathology. Here, we will review the fundamentals of inflammation, how it is initiated in IAV and SARS-CoV-2 infection and its link to disease severity. We will examine how obesity drives chronic inflammation and trained immunity and how these impact the immune response to IAV and SARS-CoV-2. Finally, we review both medical and non-medical interventions for obesity, how they impact on the inflammatory response and how they could be used to prevent disease severity in obese patients. As projections of global obesity numbers show no sign of slowing down, future pandemic preparedness will require us to consider the metabolic health of the population. Furthermore, if weight-loss alone is insufficient to reduce the risk of increased respiratory virus-related mortality, closer attention must be paid to a patient’s history of health, and new therapeutic options identified.

RvD1 accelerates the resolution of inflammation by promoting apoptosis of the recruited macrophages via the ALX/FasL-FasR/caspase-3 signaling pathway

The uncontrolled inflammatory response caused by a disorder in inflammation resolution is one of the reasons for acute respiratory distress syndrome (ARDS). The macrophage pool markedly expands when inflammatory monocytes, known as recruited macrophages, migrate from the circulation to the lung. The persistent presence of recruited macrophages leads to chronic inflammation in the resolution phase of inflammation. On the contrary, elimination of the recruited macrophages at the injury site leads to the rapid resolution of inflammation. Resolvin D1 (RvD1) is an endogenous lipid mediator derived from docosahexaenoic acid. Mice were administered RvD1 via the tail vein 3 and 4 days after stimulation with lipopolysaccharide. RvD1 reduced the levels of the inflammatory factors in the lung tissue, promoted the anti-inflammatory M2 phenotype, and enhanced the phagocytic function of recruited macrophages to alleviate acute lung injury. We also found that the number of macrophages was decreased in BAL fluid after treatment with RvD1. RvD1 increased the apoptosis of recruited macrophages partly via the FasL-FasR/caspase-3 signaling pathway, and this effect could be blocked by Boc-2, an ALX/PRP2 inhibitor. Taken together, our findings reinforce the concept of therapeutic targeting leading to the apoptosis of recruited macrophages. Thus, RvD1 may provide a new therapy for the resolution of ARDS.

LIGHT of pulmonary NKT cells annihilates tissue protective alveolar macrophages in augmenting severe influenza pneumonia

CD1d-restricted natural killer T (NKT) cells are innate-like T lymphocytes with protective or pathogenic roles in the development of influenza pneumonia. Here, we show that lung-infiltrated and activated NKT cells are the major cellular source of LIGHT/TNFSF14, which determines the severity of pulmonary pneumonia by highly deteriorative influenza A virus (IAV) infection. Compared to wild-type mice, LIGHT^-/- mice exhibit much lower morbidity and mortality to IAV, due to alleviated lung damage and reduced apoptosis of alveolar macrophages (AMs). LIGHT preferentially promotes cell death of lymphotoxin β receptors positive (LTβR⁺) AMs but not herpesvirus entry mediator positive (HVEM⁺) AMs. Therefore, these results suggest that NKT-derived LIGHT augments cell death of the tissue protective AMs in exacerbating lung pathology and susceptibility to fatal influenza infection. Suppression of LIGHT signaling might be a viable option in the treatment of influenza-associated acute respiratory distress syndrome.

Diversity of Macrophages in Lung Homeostasis and Diseases

Lung macrophages play important roles in the maintenance of homeostasis, pathogen clearance and immune regulation. The different types of pulmonary macrophages and their roles in lung diseases have attracted attention in recent years. Alveolar macrophages (AMs), including tissue-resident alveolar macrophages (TR-AMs) and monocyte-derived alveolar macrophages (Mo-AMs), as well as interstitial macrophages (IMs) are the major macrophage populations in the lung and have unique characteristics in both steady-state conditions and disease states. The different characteristics of these three types of macrophages determine the different roles they play in the development of disease. Therefore, it is important to fully understand the similarities and differences among these three types of macrophages for the study of lung diseases. In this review, we will discuss the physiological characteristics and unique functions of these three types of macrophages in acute and chronic lung diseases. We will also discuss possible methods to target macrophages in lung diseases.

Transcriptomic Analysis of Polyhexamethyleneguanidine-Induced Lung Injury in Mice after a Long-Term Recovery

Polyhexamethyleneguanidine phosphate (PHMG-P) is one of the causative agents of humidifier disinfectant-induced lung injury. Direct exposure of the lungs to PHMG-P causes interstitial pneumonia with fibrosis. Epidemiological studies showed that patients with humidifier disinfectant-associated lung injuries have suffered from restrictive lung function five years after the onset of the lung injuries. We investigated whether lung damage was sustained after repeated exposure to PHMG-P followed by a long-term recovery and evaluated the adverse effects of PHMG-P on mice lungs. Mice were intranasally instilled with 0.3 mg/kg PHMG-P six times at two weeks intervals, followed by a recovery period of 292 days. Histopathological examination of the lungs showed the infiltration of inflammatory cells, the accumulation of extracellular matrix in the lung parenchyma, proteinaceous substances in the alveoli and bronchiolar–alveolar hyperplasia. From RNA-seq, the gene expression levels associated with the inflammatory response, leukocyte chemotaxis and fibrosis were significantly upregulated, whereas genes associated with epithelial/endothelial cells development, angiogenesis and smooth muscle contraction were markedly decreased. These results imply that persistent inflammation and fibrotic changes caused by repeated exposure to PHMG-P led to the downregulation of muscle and vascular development and lung dysfunction. Most importantly, this pathological structural remodeling induced by PHMG-P was not reversed even after long-term recovery.

In vitro and ex vivo models in inhalation biopharmaceutical research - advances, challenges and future perspectives

Oral inhalation results in pulmonary drug targeting and thereby reduces systemic side effects, making it the preferred means of drug delivery for the treatment of respiratory disorders such as asthma, chronic obstructive pulmonary disease or cystic fibrosis. In addition, the high alveolar surface area, relatively low enzymatic activity and rich blood supply of the distal airspaces offer a promising pathway to the systemic circulation. This is particularly advantageous when a rapid onset of pharmacological action is desired or when the drug is suffering from stability issues or poor biopharmaceutical performance following oral administration. Several cell and tissue-based in vitro and ex vivo models have been developed over the years, with the intention to realistically mimic pulmonary biological barriers. It is the aim of this review to critically discuss the available models regarding their advantages and limitations and to elaborate further which biopharmaceutical questions can and cannot be answered using the existing models.

Role of macrophages in fetal development and perinatal disorders

In the fetus and the neonate, altered macrophage function has been implicated not only in inflammatory disorders but also in developmental abnormalities marked by altered onset, interruption, or imbalance of key structural changes. The developmental role of macrophages were first noted nearly a century ago, at about the same time when these cells were being identified as central effectors in phagocytosis and elimination of microbes. Since that time, we have made considerable progress in understanding the diverse roles that these cells play in both physiology and disease. Here, we review the role of fetal and neonatal macrophages in immune surveillance, innate immunity, homeostasis, tissue remodeling, angiogenesis, and repair of damaged tissues. We also discuss the possibility of therapeutic manipulation of the relative abundance and activation status of macrophage subsets in various diseases. This article combines peer-reviewed evidence from our own studies with results of an extensive literature search in the databases PubMed, EMBASE, and Scopus. IMPACT: We have reviewed the structure, differentiation, and classification of macrophages in the neonatal period. Neonatal macrophages are derived from embryonic, hepatic, and bone marrow precursors. Macrophages play major roles in tissue homeostasis, innate immunity, inflammation, tissue repair, angiogenesis, and apoptosis of various cellular lineages in various infectious and inflammatory disorders. Macrophages and related inflammatory mediators could be important therapeutic targets in several neonatal diseases.

Electronic Cigarette Exposure Enhances Lung Inflammatory and Fibrotic Responses in COPD Mice

Although a few studies show that the use of electronic nicotine delivery systems (ENDS) may ameliorate objective and subjective outcomes in COPD smokers who switched to electronic cigarettes, it is unclear whether e-cigarette exposure alters lung pathological features and inflammatory response in COPD. Here, we employed βENaC-overexpressing mice bearing COPD-like pulmonary abnormality, and exposed them to ENDS. We found that ENDS exposure aggravated airspace enlargement and mucus production in βENaC-overexpressing mice, which was associated with increased MMP12 and Muc5ac, respectively. ENDS exposure to mice significantly increased the numbers of macrophages, particularly in M2 macrophages in bronchoalveolar lavage (BAL) fluid, despite ENDS did not induce M2 macrophage polarization in a cultured murine macrophage cell line (RAW264.7). There were no changes in neutrophils in BAL fluid by ENDS exposure. Multiple cytokine productions were increased including M-CSF, IL-1rα, IL-10, and TGF-β1, in BAL fluid from mice when exposed to ENDS. The Sirius Red staining and hydroxyproline assay showed ENDS-exposed mice displayed enhanced fibrotic phenotypes compared to control mice. In conclusion, ENDS exposure enhances airspace enlargement, mucus secretion, and fibrogenesis in COPD mice. This is associated with increased MMP12, inflammatory responses, and M2 macrophage phenotype. This study provides pre-clinical data implicating that electronic cigarette exposure is not safe in COPD patients who want to replace traditional cigarettes with ENDS.

Macrophages in lung fibrosis

Pulmonary fibrosis (PF) is a disease in which excessive extracellular matrix (ECM) accumulation occurs in the lungs, which induces thickening of the alveolar walls, ultimately leading to the destruction of alveolar structures and respiratory failure. Idiopathic PF, the cause of which is unknown, has a poor prognosis with a median survival of 2–4 years after diagnosis. There is currently no known curative treatment. The mechanism underlying PF is thought to be initiated by the dysfunction of type II alveolar epithelial cells, which leads to ECM overproduction through the activation of fibroblasts. In addition, it has been suggested that a variety of cells contribute to fibrotic processes. In particular, clinical and basic research findings examining the roles of macrophages suggest that they may be pivotal regulators of PF. In this review, we discuss the characteristics, functions and origins of subsets of macrophages involved in PF, including resident alveolar, interstitial and monocyte-derived macrophages.

The Role of Macrophages During Mammalian Tissue Remodeling and Regeneration Under Infectious and Non-Infectious Conditions

Several infectious pathologies in humans, such as tuberculosis or SARS-CoV-2, are responsible for tissue or lung damage, requiring regeneration. The regenerative capacity of adult mammals is limited to few organs. Critical injuries of non-regenerative organs trigger a repair process that leads to a definitive architectural and functional disruption, while superficial wounds result in scar formation. Tissue lesions in mammals, commonly studied under non-infectious conditions, trigger cell death at the site of the injury, as well as the production of danger signals favouring the massive recruitment of immune cells, particularly macrophages. Macrophages are also of paramount importance in infected injuries, characterized by the presence of pathogenic microorganisms, where they must respond to both infection and tissue damage. In this review, we compare the processes implicated in the tissue repair of non-infected versus infected injuries of two organs, the skeletal muscles and the lungs, focusing on the primary role of macrophages. We discuss also the negative impact of infection on the macrophage responses and the possible routes of investigation for new regenerative therapies to improve the recovery state as seen with COVID-19 patients.

Alveolar Macrophages Inherently Express Programmed Death-1 Ligand 1 for Optimal Protective Immunity and Tolerance

Macrophages play a central role in lung physiology and pathology. In this study, we show in mice that alveolar macrophages (AMs), unlike other macrophage types (interstitial, peritoneal, and splenic macrophages), constitutively express programmed death-1 ligand 1 (PD-L1), thereby possessing a superior phagocytic ability and the capacity to repress CTLs by cis- and trans-interacting with CD80 and programmed death-1 (PD-1), respectively. This extraordinary ability of AMs assures optimal protective immunity and tolerance within the lung. These findings uncover a unique characteristic of AMs and an innate immune function of PD-L1 and CD80 and therefore help in the understanding of lung physiology, diseases, and PD-L1/PD-1-based immunotherapy.

Dynamic Pneumococcal Genetic Adaptations Support Bacterial Growth and Inflammation during Coinfection with Influenza

Streptococcus pneumoniae (pneumococcus) is one of the primary bacterial pathogens that complicates influenza virus infections. These bacterial coinfections increase influenza-associated morbidity and mortality through a number of immunological and viral-mediated mechanisms, but the specific bacterial genes that contribute to postinfluenza pathogenicity are not known. Here, we used genome-wide transposon mutagenesis (Tn-Seq) to reveal bacterial genes that confer improved fitness in influenza virus-infected hosts. The majority of the 32 genes identified are involved in bacterial metabolism, including nucleotide biosynthesis, amino acid biosynthesis, protein translation, and membrane transport. We generated mutants with single-gene deletions (SGD) of five of the genes identified, SPD1414, SPD2047 (cbiO1), SPD0058 (purD), SPD1098, and SPD0822 (proB), to investigate their effects on in vivo fitness, disease severity, and host immune responses. The growth of the SGD mutants was slightly attenuated in vitro and in vivo, but each still grew to high titers in the lungs of mock- and influenza virus-infected hosts. Despite high bacterial loads, mortality was significantly reduced or delayed with all SGD mutants. Time-dependent reductions in pulmonary neutrophils, inflammatory macrophages, and select proinflammatory cytokines and chemokines were also observed. Immunohistochemical staining further revealed altered neutrophil distribution with reduced degeneration in the lungs of influenza virus-SGD mutant-coinfected animals. These studies demonstrate a critical role for specific bacterial genes and for bacterial metabolism in driving virulence and modulating immune function during influenza-associated bacterial pneumonia.

Viral Respiratory Pathogens and Lung Injury

Several viruses target the human respiratory tract, causing different clinical manifestations spanning from mild upper airway involvement to life-threatening acute respiratory distress syndrome (ARDS). As dramatically evident in the ongoing SARS-CoV-2 pandemic, the clinical picture is not always easily predictable due to the combined effect of direct viral and indirect patient-specific immune-mediated damage. In this review, we discuss the main RNA (orthomyxoviruses, paramyxoviruses, and coronaviruses) and DNA (adenoviruses, herpesviruses, and bocaviruses) viruses with respiratory tropism and their mechanisms of direct and indirect cell damage. We analyze the thin line existing between a protective immune response, capable of limiting viral replication, and an unbalanced, dysregulated immune activation often leading to the most severe complication. Our comprehension of the molecular mechanisms involved is increasing and this should pave the way for the development and clinical use of new tailored immune-based antiviral strategies.

Macrophage-derived cytokines in pneumonia: Linking cellular immunology and genetics

Macrophages represent the first line of anti-pathogen defense - they encounter invading pathogens to perform the phagocytic activity, to deliver the plethora of pro- and anti-inflammatory cytokines, and to shape the tissue microenvironment. Throughout pneumonia course, alveolar macrophages and infiltrated blood monocytes produce increasing cytokine amounts, which activates the antiviral/antibacterial immunity but can also provoke the risk of the so-called cytokine "storm" and normal tissue damage. Subsequently, the question of how the cytokine spectrum is shaped and balanced in the pneumonia context remains a hot topic in medical immunology, particularly in the COVID19 pandemic era. The diversity in cytokine profiles, involved in pneumonia pathogenesis, is determined by the variations in cytokine-receptor interactions, which may lead to severe cytokine storm and functional decline of particular tissues and organs, for example, cardiovascular and respiratory systems. Cytokines and their receptors form unique profiles in individual patients, depending on the (a) microenvironmental context (comorbidities and associated treatment), (b) lung monocyte heterogeneity, and (c) genetic variations. These multidisciplinary strategies can be proactively considered beforehand and during the pneumonia course and potentially allow the new age of personalized immunotherapy.

Global Trends in Research of Macrophages Associated With Acute Lung Injury Over Past 10 Years: A Bibliometric Analysis

Acute lung injury (ALI) is an intractable disorder associated with macrophages. This bibliometric analysis was applied to identify the characteristics of global scientific output, the hotspots, and frontiers about macrophages in ALI over the past 10 years. We retrieved publications published from 2011 to 2020 and their recorded information from Science Citation Index Expanded (SCI-expanded) of Web of Science Core Collection (WoSCC). Bibliometrix package was used to analyze bibliometric indicators, and the VOSviewer was used to visualize the trend and hotspots of researches on macrophages in ALI. Altogether, 2,632 original articles were reviewed, and the results showed that the annual number of publications (Np) concerning the role of macrophages in ALI kept increasing over the past 10 years. China produced the most papers, the number of citations (Nc) and H-index of the USA ranked first. Shanghai Jiaotong University and INT IMMUNOPHARMACOL were the most prolific affiliation and journal, respectively. Papers published by Matute-Bello G in 2011 had the highest local citation score (LCS). Recently, the keywords “NLRP3” and “extracellular vesicles” appeared most frequently. Besides, researches on COVID-19–induced ALI related to macrophages seemed to be the hotspot recently. This bibliometric study revealed that publications related to macrophages in ALI tend to increase continuously. China was a big producer and the USA was an influential country in this field. Most studies were mainly centered on basic researches in the past decade, and pathways associated with the regulatory role of macrophages in inhibiting and attenuating ALI have become the focus of attention in more recent studies. What is more, our bibliometric analysis showed that macrophages play an important role in COVID-19–induced ALI and may be a target for the treatment of COVID-19.

PAQR11 modulates monocyte-to-macrophage differentiation and pathogenesis of rheumatoid arthritis

During inflammation or tissue injury, pro-inflammatory mediators attract migratory monocytes to inflammatory sites and monocyte-to-macrophage differentiation occurs to activate macrophages. We report here that PAQR11, a member of the progesterone and AdipoQ receptor family, regulates monocyte-to-macrophage differentiation in vitro and in vivo. Paqr11 gene was highly induced during monocyte-to-macrophage differentiation. Knockdown or deletion of Paqr11 inhibited monocyte differentiation but had little effect on macrophage polarization. Mechanistically, PAQR11 promoted cell survival as apoptosis was increased by Paqr11 knockdown or deletion. Activation of the MAPK signalling pathway was involved in the regulatory role of PAQR11 on monocyte differentiation and cell survival. C/EBPβ regulated the expression of Paqr11 at the transcriptional level. In mice, deletion of Paqr11 gene alleviated progression of collagen-induced rheumatoid arthritis. Thus, these results provide strong evidence that PAQR11 has a function in monocyte-to-macrophage differentiation and such function is related to autoimmune disease in vivo.

Altered Macrophage Function Associated with Crystalline Lung Inflammation in Acid Sphingomyelinase Deficiency

Deficiency of ASM (acid sphingomyelinase) causes the lysosomal storage Niemann-Pick disease (NPD). Patients with NPD type B may develop progressive interstitial lung disease with frequent respiratory infections. Although several investigations using the ASM-deficient (ASMKO) mouse NPD model revealed inflammation and foamy macrophages, there is little insight into the pathogenesis of NPD-associated lung disease. Using ASMKO mice, we report that ASM deficiency is associated with a complex inflammatory phenotype characterized by marked accumulation of monocyte-derived CD11b+ macrophages and expansion of airspace/alveolar CD11c+ CD11b- macrophages, both with increased size, granularity, and foaminess. Both the alternative and classical pathways were activated, with decreased in situ phagocytosis of opsonized (Fc-coated) targets, preserved clearance of apoptotic cells (efferocytosis), secretion of Th2 cytokines, increased CD11c+/CD11b+ cells, and more than a twofold increase in lung and plasma proinflammatory cytokines. Macrophages, neutrophils, eosinophils, and noninflammatory lung cells of ASMKO lungs also exhibited marked accumulation of chitinase-like protein Ym1/2, which formed large eosinophilic polygonal Charcot-Leyden-like crystals. In addition to providing insight into novel features of lung inflammation that may be associated with NPD, our report provides a novel connection between ASM and the development of crystal-associated lung inflammation with alterations in macrophage biology.

Nutritional immunity: the impact of metals on lung immune cells and the airway microbiome during chronic respiratory disease

Nutritional immunity is the sequestration of bioavailable trace metals such as iron, zinc and copper by the host to limit pathogenicity by invading microorganisms. As one of the most conserved activities of the innate immune system, limiting the availability of free trace metals by cells of the immune system serves not only to conceal these vital nutrients from invading bacteria but also operates to tightly regulate host immune cell responses and function. In the setting of chronic lung disease, the regulation of trace metals by the host is often disrupted, leading to the altered availability of these nutrients to commensal and invading opportunistic pathogenic microbes. Similarly, alterations in the uptake, secretion, turnover and redox activity of these vitally important metals has significant repercussions for immune cell function including the response to and resolution of infection. This review will discuss the intricate role of nutritional immunity in host immune cells of the lung and how changes in this fundamental process as a result of chronic lung disease may alter the airway microbiome, disease progression and the response to infection.

Inhibiting OX40 Restores Regulatory T-Cell Function and Suppresses Inflammation in Pulmonary Sarcoidosis

BACKGROUND

Pulmonary sarcoidosis (PS) is a noncaseating granulomatous disease of unknown origin. Despite conflicting reports, it is considered that the regulatory T (Treg) cells are functionally impaired in PS, but the underlying mechanisms remain unclear. OX40, a pivotal costimulatory molecule, is essential for T-cell functions and memory development, but its impact on Treg cells is ambiguous.

RESEARCH QUESTION

Does the OX40 pathway influence the suppressive functions of Treg cells in PS?

STUDY DESIGN AND METHODS

Fifty treatment-naïve patients with PS and 30 healthy control participants were recruited for this study. Polychromatic flow cytometry-based immunologic assays were performed to enumerate effector T helper (Th) cells and Treg cells along with their functions. Using real-time polymerase chain reaction analysis, small interfering RNA, and pharmacologic inhibitors, the impact of OX40 on Treg cell function was investigated.

RESULTS

We observed enrichment of Th-9 cells perhaps for the first time along with Th-1, Th-17, and Treg cells in patients' BAL fluid (BALF) compared with peripheral blood. However, Treg cells were observed to be functionally defective at the pathological site. We observed higher expression of OX40 on both T effector (CD4⁺Foxp3^-) and Treg (CD4⁺Foxp3⁺) cells obtained from the BALF of patients with PS. However, OX40 exerted contrasting impact on these T-cell subsets, enhancing effector T-cell functions (interferon γ, tumor necrosis factor α) while inhibiting Treg cell function (IL-10, transforming growth factor β). OX40 silencing or blocking on Treg cells resulted in restoration of their impaired functions.

INTERPRETATION

We propose that inhibiting the OX40 pathway may constitute a therapeutic strategy for controlling inflammatory T cells by restoring Treg cell functions in patients with PS.

LXA4 Inhibits Lipopolysaccharide-Induced Inflammatory Cell Accumulation by Resident Macrophages in Mice

Introduction

Alveolar macrophages that regulate the inflammatory response in lungs are the main target cell for the treatment of inflammatory pulmonary pathologies, such as acute respiratory distress syndrome (ARDS). Yolk sac derived alveolar resident macrophages play an important role in the pulmonary inflammatory response. With regards to anti-inflammatory actions, lipoxin A4 (LXA4) has been identified as an inflammatory "braking signal".

Methods

In vivo, LXA4 (0.1 µg/mouse) was injected intraperitoneally after intratracheal (1 mg/kg) lipopolysaccharide (LPS) administration; flow cytometry was used to measure peripheral blood monocyte derived recruited macrophage and neutrophil numbers; resident alveolar macrophage was depleted by liposome clodronate; CXCL2, CCL2, MMP9 level was detected by RT-PCR and ELISA. In vitro, sorted resident macrophages (1×10⁶) were cultured with LPS (1 μg/mL) and LXA4 (100 nmol/mL) with or without BOC-2 (10 μM) for 24 h to gain a better understanding of the mechanisms of LXA4.

Results

LXA4 inhibited tumor necrosis factor-a (TNF-a) and interleukin-1β (IL-1β) production induced by LPS. LXA4 also mediated LPS-induced macrophage recruitment and showed that this was dependent on CCL2 secretion and release by resident macrophages. LXA4 protects lung tissue by inhibiting neutrophil recruitment, partly through the CXCL2/MMP-9 signaling pathway. CXCL2 and MMP-9 are mainly expressed by resident macrophages and neutrophils, respectively. Finally, LXA4's beneficial effects were abrogated by BOC-2, an LXA4 receptor inhibitor.

Conclusion

These results suggest that LXA4 may be a promising therapy for preventing and treating ARDS.

PDLIM2 repression by ROS in alveolar macrophages promotes lung tumorigenesis

One of the most fundamental and challenging questions in the field of cancer is how immunity is transformed from tumor immunosurveillance to tumor-promoting inflammation. Here, we identified the tumor suppressor PDZ-LIM domain–containing protein 2 (PDLIM2) as a checkpoint of alveolar macrophages (AMs) important for lung tumor suppression. During lung tumorigenesis, PDLIM2 expression in AMs is downregulated by ROS-activated transcription repressor BTB and CNC homology 1 (BACH1). PDLIM2 downregulation leads to constitutive activation of the transcription factor STAT3, driving AM protumorigenic polarization/activation and differentiation from monocytes attracted from the circulation to suppress cytotoxic T lymphocytes and promote lung cancer. PDLIM2 downregulation also decreases AM phagocytosis. These findings establish ROS/BACH1/PDLIM2/STAT3 as a signaling pathway driving AMs for lung tumor promotion.

Association of Monocyte Migration Marker CD11b With Pulmonary Function in People Living With HIV

BACKGROUND

Maladaptive immune responses contribute to the pathogenesis of many chronic lung diseases. Here, we tested hypotheses that CD4 and CD8 T-cell and monocyte phenotypes are associated with lung function in people living with HIV and those without HIV.

METHODS

Markers of T cell differentiation, activation, exhaustion and senescence, and markers of monocyte recruitment and migration were quantified in 142 HIV-positive and 73 HIV-negative participants of the Pittsburgh HIV Lung Cohort. All participants underwent lung function testing.

RESULTS

CD4 or CD8 T-cell phenotypes were not associated with measures of lung function in HIV-positive or HIV-negative participants after adjustment for multiple comparisons. In HIV-positive participants, however, the percentage of classical monocytes that were CD11b+ had positive associations at the Bonferroni-adjusted significance threshold of P = 0.05/63 with prebronchodilator and postbronchodilator forced expiratory volume in 1 second (FEV1)/forced vital capacity (FVC) ratio (β = 0.36; P = 0.00003 and β = 0.31; P = 0.0003, respectively). In stratified analyses of n = 87 participants with CD4 ≥ 500 cells/µL, associations of percentage of classical monocytes that were CD11b+ with prebronchodilator and postbronchodilator FEV1/FVC ratio were stronger (β = 0.48 and β = 0.41, for pre- and post-, respectively) than in the entire HIV-positive study population. Significant associations of monocyte phenotypes were not observed in HIV-negative participants after adjustment for multiple comparisons.

CONCLUSIONS

CD11b+ expression on classical monocytes is positively associated with FEV1/FVC ratio in people living with HIV including in those with CD4 T-cell recovery. Given the normal surveillance activity of monocytes, such association suggests this monocyte subset may play a role in preservation of pulmonary function in PLWH.

Immunological Feature and Transcriptional Signaling of Ly6C Monocyte Subsets From Transcriptome Analysis in Control and Hyperhomocysteinemic Mice

Background

Murine monocytes (MC) are classified into Ly6C^high and Ly6C^low MC. Ly6C^high MC is the pro-inflammatory subset and the counterpart of human CD14⁺⁺CD16⁺ intermediate MC which contributes to systemic and tissue inflammation in various metabolic disorders, including hyperhomocysteinemia (HHcy). This study aims to explore molecule signaling mediating MC subset differentiation in HHcy and control mice.

Methods

RNA-seq was performed in blood Ly6C^high and Ly6C^low MC sorted by flow cytometry from control and HHcy cystathionine β-synthase gene-deficient (Cbs^-/-) mice. Transcriptome data were analyzed by comparing Ly6C^high vs. Ly6C^low in control mice, Ly6C^high vs. Ly6C^low in Cbs^-/- mice, Cbs^-/- Ly6C^high vs. control Ly6C^high MC and Cbs^-/- Ly6C^low vs. control Ly6C^low MC by using intensive bioinformatic strategies. Significantly differentially expressed (SDE) immunological genes and transcription factor (TF) were selected for functional pathways and transcriptional signaling identification.

Results

A total of 7,928 SDE genes and 46 canonical pathways derived from it were identified. Ly6C^high MC exhibited activated neutrophil degranulation, lysosome, cytokine production/receptor interaction and myeloid cell activation pathways, and Ly6C^low MC presented features of lymphocyte immunity pathways in both mice. Twenty-four potential transcriptional regulatory pathways were identified based on SDE TFs matched with their corresponding SDE immunological genes. Ly6C^high MC presented downregulated co-stimulatory receptors (CD2, GITR, and TIM1) which direct immune cell proliferation, and upregulated co-stimulatory ligands (LIGHT and SEMA4A) which trigger antigen priming and differentiation. Ly6C^high MC expressed higher levels of macrophage (MΦ) markers, whereas, Ly6C^low MC highly expressed lymphocyte markers in both mice. HHcy in Cbs^-/- mice reinforced inflammatory features in Ly6C^high MC by upregulating inflammatory TFs (Ets1 and Tbx21) and strengthened lymphocytes functional adaptation in Ly6C^low MC by increased expression of CD3, DR3, ICOS, and Fos. Finally, we established 3 groups of transcriptional models to describe Ly6C^high to Ly6C^low MC subset differentiation, immune checkpoint regulation, Ly6C^high MC to MΦ subset differentiation and Ly6C^low MC to lymphocyte functional adaptation.

Conclusions

Ly6C^high MC displayed enriched inflammatory pathways and favored to be differentiated into MΦ. Ly6C^low MC manifested activated T-cell signaling pathways and potentially can adapt the function of lymphocytes. HHcy reinforced inflammatory feature in Ly6C^high MC and strengthened lymphocytes functional adaptation in Ly6C^low MC.

IFIH1 Contributes to M1 Macrophage Polarization in ARDS

Accumulated evidence has demonstrated that the macrophage phenotypic switch from M0 to M1 is crucial in the initiation of the inflammatory process of acute respiratory distress syndrome (ARDS). Better insight into the molecular control of M1 macrophages in ARDS may identify potential therapeutic targets. In the current study, 36 candidate genes associated with the severity of ARDS and simultaneously involved in M1-polarized macrophages were first screened through a weighted network algorithm on all gene expression profiles from the 26 ARDS patients and empirical Bayes analysis on the gene expression profiles of macrophages. STAT1, IFIH1, GBP1, IFIT3, and IRF1 were subsequently identified as hub genes according to connectivity degree analysis and multiple external validations. Among these candidate genes, IFIH1 had the strongest connection with ARDS through the RobustRankAggreg algorithm. It was selected as a crucial gene for further investigation. For in vitro validation, the RAW264.7 cell line and BMDMs were transfected with shIFIH1 lentivirus and plasmid expression vectors of IFIH1. Cellular experimental studies further confirmed that IFIH1 was a novel regulator for promoting M1 macrophage polarization. Moreover, gene set enrichment analysis (GSEA) and in vitro validations indicated that IFIH1 regulated M1 polarization by activating IRF3. In addition, previous studies demonstrated that activation of IFIH1-IRF3 was stimulated by viral RNAs or RNA mimics. Surprisingly, the current study found that LPS could also induce IFIH1-IRF3 activation via a MyD88-dependent mechanism. We also found that only IFIH1 expression without LPS or RNA mimic stimulation could not affect IRF3 activation and M1 macrophage polarization. These findings were validated on two types of macrophages, RAW264.7 cells and BMDMs, which expanded the knowledge on the inflammatory roles of IFIH1 and IRF3, suggesting IFIH1 as a potential target for ARDS treatment.

Tissue-Resident Macrophages in the Control of Infection and Resolution of Inflammation

ABSTRACT

Macrophage, as an integral component of the immune system and the first responder to local damage, is on the front line of defense against infection. Over the past century, the prevailing view of macrophage origin states that all macrophage populations resided in tissues are terminally differentiated and replenished by monocytes from bone-marrow progenitors. Nonetheless, this theory has been reformed by ground-breaking discoveries from the past decades. It is now believed that tissue-resident macrophages (TRMs) are originated from the embryonic precursors and seeded in tissue prenatally. They can replenish via self-renewal throughout the lifespan. Indeed, recent studies have demonstrated that tissue-resident macrophages should not be classified by the over-simplified macrophage polarization (M1/M2) dogma during inflammation. Moreover, multiple lines of evidence have indicated that tissue-resident macrophages play critical roles in maintaining tissue homeostasis and facilitating tissue repair through controlling infection and resolving inflammation. In this review, we summarize the properties of resident macrophages in the lung, spleen, and heart, and further highlight the impact of TRM populations on inflammation control and tissue repair. We also discuss the potential role of local proliferation in maintaining a physiologically stable TRM pool in response to acute inflammation.

Tissue-resident macrophage inflammaging aggravates homeostasis dysregulation in age-related diseases

Organs and tissues contain a large number of tissue-resident macrophages (MΦ-Ts), which are essential for regulating homeostasis and ensuring a rapid response to injury. However, the environmental signals shaping MΦ-Ts phenotypes and the contribution of MΦ-Ts to pathological processes are just starting to be identified. MΦ-Ts isolated from aged animals or patients show alterations in morphology and distribution, defects in phagocytosis and autophagy, and loss of tissue-repair capacity. These variations are closely associated with age-associated disorders, such as inflammaging, which is characterized by cell senescence and a senescence-associated secretory phenotype (SASP) and is frequently observed in patients afflicted with chronic diseases. It seems that the role of these resident populations cannot be avoided in the treatment of aging-related diseases. This review will describe the mechanism by which MΦ-Ts support immune homeostasis and will then discuss how MΦ-Ts facilitate inflammaging and age-related diseases, which will be helpful in the development of new interventions and treatments for chronic diseases of the elderly.

Inhibition of the CCL2 receptor, CCR2, enhances tumor response to immune checkpoint therapy

Immunotherapies targeting the PD-1/PD-L1 axis are now a mainstay in the clinical management of multiple cancer types, however, many tumors still fail to respond. CCL2 is highly expressed in various cancer types and has been shown to be associated with poor prognosis. Inhibition or blockade of the CCL2/CCR2 signaling axis has thus been an area of interest for cancer therapy. Here we show across multiple murine tumor and metastasis models that CCR2 antagonism in combination with anti-PD-1 therapy leads to sensitization and enhanced tumor response over anti-PD-1 monotherapy. We show that enhanced treatment response correlates with enhanced CD8+ T cell recruitment and activation and a concomitant decrease in CD4+ regulatory T cell. These results provide strong preclinical rationale for further clinical exploration of combining CCR2 antagonism with PD-1/PD-L1-directed immunotherapies across multiple tumor types especially given the availability of small molecule CCR2 inhibitors and antibodies.

Alveolar and lung interstitial macrophages: Definitions, functions, and roles in lung fibrosis

Mϕs are the main innate immune cells in the lung at homeostasis, with important roles in host defence and immune modulation. Alveolar Mϕs (AMs) and interstitial Mϕs (IMs) are the two lung Mϕ subsets, so called according to the sites they reside in. These subsets are also defined by their origins and immunological microenvironment, which endow these cells with distinct features and plasticity. This review summarizes the latest definitions and functions of lung Mϕs during homeostasis and provides exemplar of their divergent roles in lung fibrosis.

Carbon Monoxide-Releasing Molecule-3 Ameliorates Acute Lung Injury in a Model of Hemorrhagic Shock and Resuscitation: Roles of p38MAPK Signaling Pathway

OBJECTIVE

It was reported that carbon monoxide-releasing molecule-3 (CORM-3) administration immediately after hemorrhagic shock and resuscitation (HSR) ameliorates the HSR-induced acute lung injury (ALI); however, the specific mechanism of the protective effects against HSR-induced ALI remains unclear.

METHODS

To induce hemorrhagic shock, rats were bled to a mean arterial blood pressure of 30 mm Hg for 45 min and then resuscitated with shed blood via the left vein. CORM-3 (4 mg/kg or 8 mg/kg) was respectively administrated after HSR. Twelve  hours post-HSR, lung injury was assessed by wet/dry (W/D) ratio, hematoxylin-eosin staining staining, and lung ultrasound; the apoptotic and pyroptotic macrophages were measured by immunofluorescence staining; and the expression of phosphorylated p38 mitogen activated protein kinase (p-p38MAPK) and total p38MAPK was measured by western blotting. SB203580 (5 mg/kg), a special inhibitor of p-p38MAPK, was administrated by abdominal cavity to assess the roles of p38MAPK in HSR-induced ALI.

RESULTS

Increased B-line score, lung injury score, and W/D ratio indicated the fact of ALI after HSR. Twelve hours post-HSR, CORM-3 administration significantly decreased the B-line score, lung injury score, W/D ratio, apoptotic and pyroptotic macrophages, and the expressions of p-p38MAPK. Further, SB203580 not only reduced HSR-induced ALI, but also enhanced the protective effects of CORM-3 against ALI.

CONCLUSION

We identified the protective effects of CORM-3 against HSR-induced ALI. The mechanism might be related to the inhibition of p38MAPK signaling pathway in lung macrophages.

Cross-Talk Between Alveolar Macrophages and Lung Epithelial Cells is Essential to Maintain Lung Homeostasis

The main function of the lung is to perform gas exchange while maintaining lung homeostasis despite environmental pathogenic and non-pathogenic elements contained in inhaled air. Resident cells must keep lung homeostasis and eliminate pathogens by inducing protective immune response and silently remove innocuous particles. Which lung cell type is crucial for this function is still subject to debate, with reports favoring either alveolar macrophages (AMs) or lung epithelial cells (ECs) including airway and alveolar ECs. AMs are the main immune cells in the lung in steady-state and their function is mainly to dampen inflammatory responses. In addition, they phagocytose inhaled particles and apoptotic cells and can initiate and resolve inflammatory responses to pathogens. Although AMs release a plethora of mediators that modulate immune responses, ECs also play an essential role as they are more than just a physical barrier. They produce anti-microbial peptides and can secrete a variety of mediators that can modulate immune responses and AM functions. Furthermore, ECs can maintain AMs in a quiescent state by expressing anti-inflammatory membrane proteins such as CD200. Thus, AMs and ECs are both very important to maintain lung homeostasis and have to coordinate their action to protect the organism against infection. Thus, AMs and lung ECs communicate with each other using different mechanisms including mediators, membrane glycoproteins and their receptors, gap junction channels, and extracellular vesicles. This review will revisit characteristics and functions of AMs and lung ECs as well as different communication mechanisms these cells utilize to maintain lung immune balance and response to pathogens. A better understanding of the cross-talk between AMs and lung ECs may help develop new therapeutic strategies for lung pathogenesis.

Cigarette smoke induces endoplasmic reticulum stress and suppresses efferocytosis through the activation of RhoA

Impaired efferocytosis is a key mechanism of inflammatory lung diseases, including chronic obstructive pulmonary disease and cystic fibrosis. Cigarette smoking activates RhoA and impairs efferocytosis in alveolar macrophages, but the mechanism has not been fully elucidated. We investigated the role of endoplasmic reticulum (ER) stress induced by cigarette smoking in the disruption of efferocytosis. Both tunicamycin (10 μg/ml) and thapsigargin (0.1 and 1 μM), which are ER stress inducers, suppressed efferocytosis in J774 cells, and a Rho-associated coiled-coil-forming kinase (ROCK) inhibitor (Y27632) reversed this effect. We validated the effect of tunicamycin on efferocytosis in experiments using RAW264.7 cells. Then, we investigated the role of the unfolded protein response (UPR) in efferocytosis impaired by ER stress. A PERK inhibitor (GSK2606414) restored the efferocytosis that had been impaired by TM, and an eIF2α dephosphorylation inhibitor (salubrinal) suppressed efferocytosis. Cigarette smoke extract (CSE) induced ER stress in J774 macrophages and RhoA activation in J774 cells, and the CSE-induced ROCK activity was successfully reversed by GSK2606414 and tauroursodeoxycholic acid. Finally, we confirmed that ER stress suppresses efferocytosis in murine alveolar macrophages and that GSK2606414 could rescue this process. These data suggest that cigarette smoke-induced ER stress and the UPR play crucial roles in RhoA activation and suppression of efferocytosis in the lung.

Macrophage polarization and its role in the pathogenesis of acute lung injury/acute respiratory distress syndrome

PURPOSE

Macrophages are highly plastic cells. Under different stimuli, macrophages can be polarized into several different subsets. Two main macrophage subsets have been suggested: classically activated or inflammatory (M1) macrophages and alternatively activated or anti-inflammatory (M2) macrophages. Macrophage polarization is governed by a highly complex set of regulatory networks. Many recent studies have shown that macrophages are key orchestrators in the pathogenesis of acute lung injury (ALI)/acute respiratory distress syndrome (ARDS) and that regulation of macrophage polarization may improve the prognosis of ALI/ARDS. A further understanding of the mechanisms of macrophage polarization is expected to be helpful in the development of novel therapeutic targets to treat ALI/ARDS. Therefore, we performed a literature review to summarize the regulatory mechanisms of macrophage polarization and its role in the pathogenesis of ALI/ARDS.

METHODS

A computer-based online search was performed using the PubMed database and Web of Science database for published articles concerning macrophages, macrophage polarization, and ALI/ARDS.

RESULTS

In this review, we discuss the origin, polarization, and polarization regulation of macrophages as well as the role of macrophage polarization in various stages of ARDS. According to the current literature, regulating the polarized state of macrophages might be a potential therapeutic strategy against ALI/ARDS.

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.

The Impact of the Cancer Microenvironment on Macrophage Phenotypes

Within the tumor microenvironment, there is an intricate communication happening between tumor and stromal cells. This information exchange, in the form of cytokines, growth factors, extracellular vesicles, danger molecules, cell debris, and other factors, is capable of modulating the function of immune cells. The triggering of specific responses, including phenotypic alterations, can ultimately result in either immune surveillance or tumor cell survival. Macrophages are a well-studied cell lineage illustrating the different cellular phenotypes possible, depending on the tumor microenvironmental context. While our understanding of macrophage responses is well documented in vitro, surprisingly, little work has been done to confirm these observations in the cancer microenvironment. In fact, there are examples of opposing reactions of macrophages to cytokines in cell culture and in vivo tumor settings. Additionally, it seems that different macrophage lineages, for example tissue-resident and monocyte-derived macrophages, respond differently to cytokines and other cancer-derived signals. In this review article, we will describe and discuss the diverging reports on how cancer cells influence monocyte-derived and tissue-resident macrophage traits in vivo.

Alveolar macrophages are epigenetically altered after inflammation, leading to long-term lung immunoparalysis

Sepsis and trauma cause inflammation and elevated susceptibility to hospital-acquired pneumonia. As phagocytosis by macrophages plays a critical role in the control of bacteria, we investigated the phagocytic activity of macrophages after resolution of inflammation. After resolution of primary pneumonia, murine alveolar macrophages (AMs) exhibited poor phagocytic capacity for several weeks. These paralyzed AMs developed from resident AMs that underwent an epigenetic program of tolerogenic training. Such adaptation was not induced by direct encounter of the pathogen but by secondary immunosuppressive signals established locally upon resolution of primary infection. Signal-regulatory protein α (SIRPα) played a critical role in the establishment of the microenvironment that induced tolerogenic training. In humans with systemic inflammation, AMs and also circulating monocytes still displayed alterations consistent with reprogramming six months after resolution of inflammation. Antibody blockade of SIRPα restored phagocytosis in monocytes of critically ill patients in vitro, which suggests a potential strategy to prevent hospital-acquired pneumonia.

Pneumonia recovery reprograms the alveolar macrophage pool

Community-acquired pneumonia is a widespread disease with significant morbidity and mortality. Alveolar macrophages are tissue-resident lung cells that play a crucial role in innate immunity against bacteria that cause pneumonia. We hypothesized that alveolar macrophages display adaptive characteristics after resolution of bacterial pneumonia. We studied mice 1 to 6 months after self-limiting lung infections with Streptococcus pneumoniae, the most common cause of bacterial pneumonia. Alveolar macrophages, but not other myeloid cells, recovered from the lung showed long-term modifications of their surface marker phenotype. The remodeling of alveolar macrophages was (a) long-lasting (still observed 6 months after infection), (b) regionally localized (observed only in the affected lobe after lobar pneumonia), and (c) associated with macrophage-dependent enhanced protection against another pneumococcal serotype. Metabolomic and transcriptomic profiling revealed that alveolar macrophages of mice that recovered from pneumonia had new baseline activities and altered responses to infection that better resembled those of adult humans. The enhanced lung protection after mild and self-limiting bacterial respiratory infections includes a profound remodeling of the alveolar macrophage pool that is long-lasting; compartmentalized; and manifest across surface receptors, metabolites, and both resting and stimulated transcriptomes.

Inflammation-Induced Alternative Pre-mRNA Splicing in Mouse Alveolar Macrophages

Alveolar macrophages serve as central orchestrators of inflammatory responses in the lungs, both initiating their onset and promoting their resolution. However, the mechanisms that program macrophages for these dynamic responses are not fully understood. Over 95% of all mammalian genes undergo alternative pre-mRNA splicing. While alternative splicing has been shown to regulate inflammatory responses in macrophages in vitro, it has not been investigated on a genome-wide scale in vivo Here we used RNAseq to investigate alternative pre-mRNA splicing in alveolar macrophages isolated from lipopolysaccharide (LPS)-treated mice during the peak of inflammation and during its resolution. We found that lung inflammation induced substantial alternative pre-mRNA splicing in alveolar macrophages. The number of changes in isoform usage was greatest at the peak of inflammation and involved multiple classes of alternative pre-mRNA splicing events. Comparative pathway analysis of inflammation-induced changes in alternative pre-mRNA splicing and differential gene expression revealed overlap of pathways enriched for immune responses such as chemokine signaling and cellular metabolism. Moreover, alternative pre-mRNA splicing of genes in metabolic pathways differed in tissue resident vs. recruited (blood monocyte-derived) alveolar macrophages and corresponded to changes in core metabolism, including a switch to Warburg-like metabolism in recruited macrophages with increased glycolysis and decreased flux through the tricarboxylic acid cycle.

Mitochondrial quality control in pulmonary fibrosis

Mechanisms underlying the pathogenesis of pulmonary fibrosis remain incompletely understood. Emerging evidence suggests changes in mitochondrial quality control are a critical determinant in many lung diseases, including chronic obstructive pulmonary disease, asthma, pulmonary hypertension, acute lung injury, lung cancer, and in the susceptibility to pulmonary fibrosis. Once thought of as the kidney-bean shaped powerhouses of the cell, mitochondria are now known to form interconnected networks that rapidly and continuously change their size to meet cellular metabolic demands. Mitochondrial quality control modulates cell fate and homeostasis, and diminished mitochondrial quality control results in mitochondrial dysfunction, increased reactive oxygen species (ROS) production, reduced ATP production, and often induces intrinsic apoptosis. Here, we review the role of the mitochondria in alveolar epithelial cells, lung macrophages, and fibroblasts within the context of pulmonary fibrosis.

A spatially restricted fibrotic niche in pulmonary fibrosis is sustained by M-CSF/M-CSFR signalling in monocyte-derived alveolar macrophages

Ontologically distinct populations of macrophages differentially contribute to organ fibrosis through unknown mechanisms.We applied lineage tracing, single-cell RNA sequencing and single-molecule fluorescence in situ hybridisation to a spatially restricted model of asbestos-induced pulmonary fibrosis.We demonstrate that tissue-resident alveolar macrophages, tissue-resident peribronchial and perivascular interstitial macrophages, and monocyte-derived alveolar macrophages are present in the fibrotic niche. Deletion of monocyte-derived alveolar macrophages but not tissue-resident alveolar macrophages ameliorated asbestos-induced lung fibrosis. Monocyte-derived alveolar macrophages were specifically localised to fibrotic regions in the proximity of fibroblasts where they expressed molecules known to drive fibroblast proliferation, including platelet-derived growth factor subunit A. Using single-cell RNA sequencing and spatial transcriptomics in both humans and mice, we identified macrophage colony-stimulating factor receptor (M-CSFR) signalling as one of the novel druggable targets controlling self-maintenance and persistence of these pathogenic monocyte-derived alveolar macrophages. Pharmacological blockade of M-CSFR signalling led to the disappearance of monocyte-derived alveolar macrophages and ameliorated fibrosis.Our findings suggest that inhibition of M-CSFR signalling during fibrosis disrupts an essential fibrotic niche that includes monocyte-derived alveolar macrophages and fibroblasts during asbestos-induced fibrosis.

Origin and ontogeny of lung macrophages: from mice to humans

Macrophages are tissue-resident myeloid cells with essential roles in host defense, tissue repair, and organ homeostasis. The lung harbors a large number of macrophages that reside in alveoli. As a result of their strategic location, alveolar macrophages are critical sentinels of healthy lung function and barrier immunity. They phagocytose inhaled material and initiate protective immune responses to pathogens, while preventing excessive inflammatory responses and tissue damage. Apart from alveolar macrophages, other macrophage populations are found in the lung and recent single-cell RNA-sequencing studies indicate that lung macrophage heterogeneity is greater than previously appreciated. The cellular origin and development of mouse lung macrophages has been extensively studied, but little is known about the ontogeny of their human counterparts, despite the importance of macrophages for lung health. In this context, humanized mice (mice with a human immune system) can give new insights into the biology of human lung macrophages by allowing in vivo studies that are not possible in humans. In particular, we have created humanized mouse models that support the development of human lung macrophages in vivo. In this review, we will discuss the heterogeneity, development, and homeostasis of lung macrophages. Moreover, we will highlight the impact of age, the microbiota, and pathogen exposure on lung macrophage function. Altered macrophage function has been implicated in respiratory infections as well as in common allergic and inflammatory lung diseases. Therefore, understanding the functional heterogeneity and ontogeny of lung macrophages should help to develop future macrophage-based therapies for important lung diseases in humans.

Redistribution of EC-SOD resolves bleomycin-induced inflammation via increased apoptosis of recruited alveolar macrophages

A human single nucleotide polymorphism (SNP) in the matrix-binding domain of extracellular superoxide dismutase (EC-SOD), with arginine to glycine substitution at position 213 (R213G), redistributes EC-SOD from the matrix into extracellular fluids. We reported that, following bleomycin (bleo), knockin mice harboring the human R213G SNP (R213G mice) exhibit enhanced resolution of inflammation and protection against fibrosis, compared with wild-type (WT) littermates. In this study, we tested the hypothesis that the EC-SOD R213G SNP promotes resolution via accelerated apoptosis of recruited alveolar macrophage (AM). RNA sequencing and Ingenuity Pathway Analysis 7 d postbleo in recruited AM implicated increased apoptosis and blunted inflammatory responses in the R213G strain exhibiting accelerated resolution. We validated that the percentage of apoptosis was significantly elevated in R213G recruited AM vs. WT at 3 and 7 d postbleo in vivo. Recruited AM numbers were also significantly decreased in R213G mice vs. WT at 3 and 7 d postbleo. ChaC glutathione-specific γ-glutamylcyclotransferase 1 (Chac1), a proapoptotic γ-glutamyl cyclotransferase that depletes glutathione, was increased in the R213G recruited AM. Overexpression of Chac1 in vitro induced apoptosis of macrophages and was blocked by administration of cell-permeable glutathione. In summary, we provide new evidence that redistributed EC-SOD accelerates the resolution of inflammation through redox-regulated mechanisms that increase recruited AM apoptosis.-Allawzi, A., McDermott, I., Delaney, C., Nguyen, K., Banimostafa, L., Trumpie, A., Hernandez-Lagunas, L., Riemondy, K., Gillen, A., Hesselberth, J., El Kasmi, K., Sucharov, C. C., Janssen, W. J., Stenmark, K., Bowler, R., Nozik-Grayck, E. Redistribution of EC-SOD resolves bleomycin-induced inflammation via increased apoptosis of recruited alveolar macrophages.

Harnessing the Immunomodulatory Properties of Bacterial Ghosts to Boost the Anti-mycobacterial Protective Immunity

Tuberculosis (TB) pathogenesis is characterized by inadequate immune cell activation and delayed T cell response in the host. Recent immunotherapeutic efforts have been directed at stimulating innate immunity and enhancing interactions between antigen presenting cells and T cells subsets to improve the protective immunity against TB. In this study, we investigated the immunostimulatory properties of bacterial ghosts (BG) as a novel approach to potentiate the host immunity against mycobacterial infection. BG are intact cytoplasm-free Escherichia coli envelopes and have been developed as bacterial vaccines and adjuvant/delivery system in cancer immunotherapy. However, BG have yet to be exploited as immunopotentiators in the context of infectious diseases. Here, we showed that BG are potent inducers of dendritic cells (DC), which led to enhanced T cell proliferation and differentiation into effector cells. BG also induced macrophage activation, which was associated with enhanced nitric oxide production, a key anti-mycobacterial weapon. We further demonstrated that the immunostimulatory capability of BG far exceeds that of LPS and involves both TLR4-dependent and independent pathways. Consistently, BG treatment, but not LPS treatment, reduced the bacterial burden in infected mice, which correlated with increased influx of innate and adaptive effector immune cells and increased production of key cytokines in the lungs. Finally and importantly, enhanced bacilli killing was seen in mice co-administered with BG and second-line TB drugs bedaquiline and delamanid. Overall, this work paves the way for BG as potent immunostimulators that may be harnessed to improve mycobacteria killing at the site of infection.

Macrophage Polarization Favors Epithelial Repair During Acute Respiratory Distress Syndrome

OBJECTIVES

Alveolar macrophage polarization and role on alveolar repair during human acute respiratory distress syndrome remain unclear. This study aimed to determine during human acute respiratory distress syndrome: the alveolar macrophage polarization, the effect of alveolar environment on macrophage polarization, and the role of polarized macrophages on epithelial repair.

DESIGN

Experimental ex vivo and in vitro investigations.

SETTING

Four ICUs in three teaching hospitals.

PATIENTS

Thirty-three patients with early moderate-to-severe acute respiratory distress syndrome were enrolled for assessment of the polarization of alveolar macrophages.

INTERVENTIONS

Polarization of acute respiratory distress syndrome macrophages was studied by flow cytometry and quantitative polymerase chain reaction. Modulation of macrophage polarization was studied in vitro using phenotypic and functional readouts. Macrophage effect on repair was studied using alveolar epithelial cells in wound healing models.

MEASUREMENTS AND MAIN RESULTS

Ex vivo, alveolar macrophages from early acute respiratory distress syndrome patients exhibited anti-inflammatory characteristics with high CD163 expression and interleukin-10 production. Accordingly, early acute respiratory distress syndrome-bronchoalveolar lavage fluid drives an acute respiratory distress syndrome-specific anti-inflammatory macrophage polarization in vitro, close to that induced by recombinant interleukin-10. Culture supernatants from macrophages polarized in vitro with acute respiratory distress syndrome-bronchoalveolar lavage fluid or interleukin-10 and ex vivo acute respiratory distress syndrome alveolar macrophages specifically promoted lung epithelial repair. Inhibition of the hepatocyte growth factor pathway in epithelial cells and hepatocyte growth factor production in macrophages both reversed this effect. Finally, hepatocyte growth factor and soluble form of CD163 concentrations expressed relatively to macrophage count were higher in bronchoalveolar lavage fluid from acute respiratory distress syndrome survivors.

CONCLUSIONS

Early acute respiratory distress syndrome alveolar environment drives an anti-inflammatory macrophage polarization favoring epithelial repair through activation of the hepatocyte growth factor pathway. These results suggest that macrophage polarization may be an important step for epithelial repair and acute respiratory distress syndrome recovery.