Lung macrophages serve as obligatory intermediate between blood monocytes and alveolar macrophages

Characterization of IL-6R-expressing monocytes in the lung of patients with chronic obstructive pulmonary disease

BACKGROUND

Monocytes play a crucial role in innate immune responses for host defense, however, their involvement in chronic obstructive pulmonary disease (COPD) remains poorly understood. We previously identified a subset of monocytes in COPD lung tissues characterized by high interleukin-6 receptor (IL-6R) expression. This study aimed to characterize the phenotypes of IL-6Rhi monocytes in the lungs of COPD patients.

METHODS

Using flow cytometry, we assessed the abundance of pulmonary CD14+IL-6Rhi cells in never smokers (CNS), control ex-smokers (CES) and COPD patients. IL-6 expression in CD14+ monocytes isolated from the peripheral blood of patients with COPD was also examined. CD45+CD206-CD14+IL-6Rhi and CD45+CD206-CD14+IL-6R-/lo cells were isolated from COPD lung tissues for transcriptome analysis. A monocyte line THP1 cell with constitutive IL-6R expression was stimulated with recombinant IL-6, followed by RNA sequencing to evaluate the IL-6 responsiveness of IL-6R+ monocytes.

RESULTS

The number of pulmonary CD14+IL-6Rhi monocytes was elevated in COPD patients compared to CNS, whereas CD14+ monocytes in the peripheral blood of COPD patients did not express IL-6R. Upregulated mRNA expression in CD14+IL-6Rhi monocytes was associated with chemotaxis, monocyte differentiation, fatty acid metabolism and integrin-mediated signaling pathway. Stimulation of THP1 cells with recombinant IL-6 induced changes in the expression of genes linked to chemotaxis and organism development.

CONCLUSION

In patients with COPD, CD14+IL-6Rhi monocytes are increased in lung tissues compared to those in CNS. They exhibit a transcriptome profile different from that of CD14+IL-6R-/lo monocytes.

Noninvasive assessment of the lung inflammation-fibrosis axis by targeted imaging of CMKLR1

Precision management of fibrotic lung diseases is challenging due to their diverse clinical trajectories and lack of reliable biomarkers for risk stratification and therapeutic monitoring. Here, we validated the accuracy of CMKLR1 as an imaging biomarker of the lung inflammation-fibrosis axis. By analyzing single-cell RNA sequencing datasets, we demonstrated CMKLR1 expression as a transient signature of monocyte-derived macrophages (MDMφ) enriched in patients with idiopathic pulmonary fibrosis (IPF). Consistently, we identified MDMφ as the major driver of the uptake of CMKLR1-targeting peptides in a murine model of bleomycin-induced lung fibrosis. Furthermore, CMKLR1-targeted positron emission tomography in the murine model enabled quantification and spatial mapping of inflamed lung regions infiltrated by CMKLR1-expressing macrophages and emerged as a robust predictor of subsequent lung fibrosis. Last, high CMKLR1 expression by bronchoalveolar lavage cells identified an inflammatory endotype of IPF with poor survival. Our investigation supports the potential of CMKLR1 as an imaging biomarker for endotyping and risk stratification of fibrotic lung diseases.

Disulfiram, an Anti-alcoholic Drug, Targets Macrophages and Attenuates Acute Rejection in Rat Lung Allografts

Macrophages contribute to post-transplant lung rejection. Disulfiram (DSF), an anti-alcoholic drug, has an anti-inflammatory effect and regulates macrophage chemotactic activity. Here, we investigated DSF efficacy in suppressing acute rejection post-lung transplantation. Male Lewis rats (280-300 g) received orthotopic left lung transplants from Fisher 344 rats (minor histocompatibility antigen-mismatched transplantation). DSF (0.75 mg/h) monotherapy or co-solvent only (50% hydroxypropyl-β-cyclodextrin) as control was subcutaneously administered for 7 days (n = 10/group). No post-transplant immunosuppressant was administered. Grades of acute rejection, infiltration of immune cells positive for CD68, CD3, or CD79a, and gene expression of monocyte chemoattractant protein and pro-inflammatory cytokines in the grafts were assessed 7 days post-transplantation. The DSF-treated group had significantly milder lymphocytic bronchiolitis than the control group. The infiltration levels of CD68+ or CD3+ cells to the peribronchial area were significantly lower in the DSF than in the control groups. The normalized expression of chemokine ligand 2 and interleukin-6 mRNA in allografts was lower in the DSF than in the control groups. Validation assay revealed interleukin-6 expression to be significantly lower in the DSF than in the control groups. DSF can alleviate acute rejection post-lung transplantation by reducing macrophage accumulation around peripheral bronchi and suppressing pro-inflammatory cytokine expression.

Short-Chain Fatty Acid (SCFA) as a Connecting Link between Microbiota and Gut-Lung Axis-A Potential Therapeutic Intervention to Improve Lung Health

The microbiome is an integral part of the human gut, and it plays a crucial role in the development of the immune system and homeostasis. Apart from the gut microbiome, the airway microbial community also forms a distinct and crucial part of the human microbiota. Furthermore, several studies indicate the existence of communication between the gut microbiome and their metabolites with the lung airways, called "gut-lung axis". Perturbations in gut microbiota composition, termed dysbiosis, can have acute and chronic effects on the pathophysiology of lung diseases. Microbes and their metabolites in lung stimulate various innate immune pathways, which modulate the expression of the inflammatory genes in pulmonary leukocytes. For instance, gut microbiota-derived metabolites such as short-chain fatty acids can suppress lung inflammation through the activation of G protein-coupled receptors (free fatty acid receptors) and can also inhibit histone deacetylase, which in turn influences the severity of acute and chronic respiratory diseases. Thus, modulation of the gut microbiome composition through probiotic/prebiotic usage and fecal microbiota transplantation can lead to alterations in lung homeostasis and immunity. The resulting manipulation of immune cells function through microbiota and their key metabolites paves the way for the development of novel therapeutic strategies in improving the lung health of individuals affected with various lung diseases including SARS-CoV-2. This review will shed light upon the mechanistic aspect of immune system programming through gut and lung microbiota and exploration of the relationship between gut-lung microbiome and also highlight the therapeutic potential of gut microbiota-derived metabolites in the management of respiratory diseases.

Tissue-resident macrophages exacerbate lung injury after remote sterile damage

Remote organ injury, which is a common secondary complication of sterile tissue damage, is a major cause of poor prognosis and is difficult to manage. Here, we report the critical role of tissue-resident macrophages in lung injury after trauma or stroke through the inflammatory response. We found that depleting tissue-resident macrophages rather than disrupting the recruitment of monocyte-derived macrophages attenuated lung injury after trauma or stroke. Our findings revealed that the release of circulating alarmins from sites of distant sterile tissue damage triggered an inflammatory response in lung-resident macrophages by binding to receptor for advanced glycation end products (RAGE) on the membrane, which activated epidermal growth factor receptor (EGFR). Mechanistically, ligand-activated RAGE triggered EGFR activation through an interaction, leading to Rab5-mediated RAGE internalization and EGFR phosphorylation, which subsequently recruited and activated P38; this, in turn, promoted RAGE translation and trafficking to the plasma membrane to increase the cellular response to RAGE ligands, consequently exacerbating inflammation. Our study also showed that the loss of RAGE or EGFR expression by adoptive transfer of macrophages, blocking the function of RAGE with a neutralizing antibody, or pharmacological inhibition of EGFR activation in macrophages could protect against trauma- or stroke-induced remote lung injury. Therefore, our study revealed that targeting the RAGE-EGFR signaling pathway in tissue-resident macrophages is a potential therapeutic approach for treating secondary complications of sterile damage.

Diverse roles of lung macrophages in the immune response to influenza A virus

Influenza viruses are one of the major causes of human respiratory infections and the newly emerging and re-emerging strains of influenza virus are the cause of seasonal epidemics and occasional pandemics, resulting in a huge threat to global public health systems. As one of the early immune cells can rapidly recognize and respond to influenza viruses in the respiratory, lung macrophages play an important role in controlling the severity of influenza disease by limiting viral replication, modulating the local inflammatory response, and initiating subsequent adaptive immune responses. However, influenza virus reproduction in macrophages is both strain- and macrophage type-dependent, and ineffective replication of some viral strains in mouse macrophages has been observed. This review discusses the function of lung macrophages in influenza virus infection in order to better understand the pathogenesis of the influenza virus.

Tissue-resident innate lymphoid cells in asthma

Asthma is a chronic airway inflammatory disease whose global incidence increases annually. The role of innate lymphoid cells (ILCs) is a crucial aspect of asthma research with respect to different endotypes of asthma. Based on its pathological and inflammatory features, asthma is divided into type 2 high and type 2 low endotypes. Type-2 high asthma is distinguished by the activation of type 2 immune cells, including T helper 2 (Th2) cells and ILC2s; the production of cytokines interleukin (IL)-4, IL-5 and IL-13; eosinophilic aggregation; and bronchial hyper-responsiveness. Type-2 low asthma represents a variety of endotypes other than type 2 high endotype such as the IL-1β/ILC3/neutrophil endotype and a paucigranulocytic asthma, which may be insensitive to corticosteroid treatment and/or associated with obesity. The complexity of asthma is due to the involvement of multiple cell types, including tissue-resident ILCs and other innate immune cells including bronchial epithelial cells, dendritic cells, macrophages and eosinophils, which provide immediate defence against viruses, pathogens and allergens. On this basis, innate immune cells and adaptive immune cells combine to induce the pathological condition of asthma. In addition, the plasticity of ILCs increases the heterogeneity of asthma. This review focuses on the phenotypes of tissue-resident ILCs and their roles in the different endotypes of asthma, as well as the mechanisms of tissue-resident ILCs and other immune cells. Based on the phenotypes, roles and mechanisms of immune cells, the therapeutic strategies for asthma are reviewed.

Shaping of the alveolar landscape by respiratory infections and long-term consequences for lung immunity

Respiratory infections and especially viral infections, along with other extrinsic environmental factors, have been shown to profoundly affect macrophage populations in the lung. In particular, alveolar macrophages (AMs) are important sentinels during respiratory infections and their disappearance opens a niche for recruited monocytes (MOs) to differentiate into resident macrophages. Although this topic is still the focus of intense debate, the phenotype and function of AMs that recolonize the niche after an inflammatory insult, such as an infection, appear to be dictated in part by their origin, but also by local and/or systemic changes that may be imprinted at the epigenetic level. Phenotypic alterations following respiratory infections have the potential to shape lung immunity for the long-term, leading to beneficial responses such as protection against allergic airway inflammation or against other infections, but also to detrimental responses when associated with the development of immunopathologies. This review reports the persistence of virus-induced functional alterations in lung macrophages, and discusses the importance of this imprinting in explaining inter-individual and lifetime immune variation.

Single-cell RNA sequencing reveals unique monocyte-derived interstitial macrophage subsets during lipopolysaccharide-induced acute lung inflammation

Interstitial macrophages (IMs) reside in the lung tissue surrounding key structures including airways, vessels, and alveoli. Recent work has described IM heterogeneity during homeostasis, however, there are limited data on IMs during inflammation. We sought to characterize IM origin, subsets, and transcriptomic profiles during homeostasis and lipopolysaccharide (LPS) induced acute lung inflammation. During homeostasis, we used three complementary methods, spectral flow cytometry, single-cell RNA-sequencing, and gene regulatory network enrichment, to demonstrate that IMs can be divided into two core subsets distinguished by surface and transcriptional expression of folate receptor β (Folr2/FRβ). These subsets inhabited distinct niches within the lung interstitium. Within FRβ+ IMs we identified a subpopulation marked by coexpression of LYVE1. During acute LPS-induced inflammation, lung IM numbers expand. Lineage tracing revealed IM expansion was due to recruitment of monocyte-derived IMs. At the peak of inflammation, recruited IMs were comprised two unique subsets defined by expression of genes associated with interferon signaling and glycolytic pathways. As recruited IMs matured, they adopted the overall transcriptional state of FRβ- resident IMs but retained expression in several origin-specific genes, such as IL-1β. FRβ+ IMs were of near-pure resident origin. Taken together our data show that during LPS-induced inflammation, there are distinct populations of IMs that likely have unique functions. FRΒ+ IMs comprise a stable, resident population, whereas FRβ- ΙΜs represent a mixed population of resident and recruited IMs.

Immunopathological Mechanisms of Bird-Related Hypersensitivity Pneumonitis

Bird-related hypersensitivity pneumonitis (BRHP) is an interstitial lung disease induced by avian proteins. The immunopathological pathways involved in the disease are still unknown. This study assesses the cellular immune response and the cytokine pattern in a mouse model of BRHP. On days -3 and -1, mice were intraperitoneally sensitized with commercial pigeon serum (PS) or saline. Intranasal instillations with PS or saline were carried out on three consecutive days/week over either 3 weeks (Group 1) or 12 weeks (Group 2). Leukocyte and cytokine patterns in lung tissue and pulmonary inflammation in bronchoalveolar lavage (BAL) were analysed. Both groups presented increases in resident monocytes, interstitial macrophages and type 2 dendritic cells (DCs), but also reductions in inflammatory monocytes, alveolar macrophages and tolerogenic DCs compared with their control groups. Group 1 had increased levels of eosinophils and T cells with reductions in neutrophils and B cells, while Group 2 showed high levels of B cells. Both groups exhibited increases in Th1 and Th2 cytokines. Group 2 also showed increased levels of IL-23, a Th17 cytokine. Increased levels of neutrophils, eosinophils and lymphocytes were observed in BAL samples of both groups compared with controls. In the first stages of BRHP, there is a mixed Th1/Th2 immune response, while during the progression of the disease, although there is a Th1 response, the cytokine levels seem to indicate a switch towards a Th2/Th17 mixed response.

Airway microbiome-immune crosstalk in chronic obstructive pulmonary disease

Chronic Obstructive Pulmonary Disease (COPD) has significantly contributed to global mortality, with three million deaths reported annually. This impact is expected to increase over the next 40 years, with approximately 5 million people predicted to succumb to COPD-related deaths annually. Immune mechanisms driving disease progression have not been fully elucidated. Airway microbiota have been implicated. However, it is still unclear how changes in the airway microbiome drive persistent immune activation and consequent lung damage. Mechanisms mediating microbiome-immune crosstalk in the airways remain unclear. In this review, we examine how dysbiosis mediates airway inflammation in COPD. We give a detailed account of how airway commensal bacteria interact with the mucosal innate and adaptive immune system to regulate immune responses in healthy or diseased airways. Immune-phenotyping airway microbiota could advance COPD immunotherapeutics and identify key open questions that future research must address to further such translation.

Equine alveolar macrophages and monocyte-derived macrophages respond differently to an inflammatory stimulus

Alveolar macrophages (AMs) are the predominant innate immune cell in the distal respiratory tract. During inflammatory responses, AMs may be supplemented by blood monocytes, which differentiate into monocyte-derived macrophages (MDMs). Macrophages play important roles in a variety of common equine lower airway diseases, including severe equine asthma (SEA). In an experimental model, an inhaled mixture of Aspergillus fumigatus spores, lipopolysaccharide, and silica microspheres (FLS), induced SEA exacerbation in susceptible horses. However, whether equine AMs and MDMs have differing immunophenotypes and cytokine responses to FLS stimulation is unknown. To address these questions, alveolar macrophages/monocytes (AMMs) were isolated from bronchoalveolar lavage fluid and MDMs derived from blood of six healthy horses. Separately, AMMs and MDMs were cultured with and without FLS for six hours after which cell surface marker expression and cytokine production were analyzed by flow cytometry and a bead-based multiplex assay, respectively. Results showed that regardless of exposure conditions, AMMs had significantly higher surface expression of CD163 and CD206 than MDMs. Incubation with FLS induced secretion of IL-1β, IL-8, TNF-α and IFN-γ in AMMs, and IL-8, IL-10 and TNF-α in MDMs. These results suggest that AMMs have a greater proinflammatory response to in vitro FLS stimulation than MDMs, inferring differing roles in equine lung inflammation. Variability in recruitment and function of monocyte-macrophage populations warrant more detailed in vivo investigation in both homeostatic and diseased states.

Explaining the polarized macrophage pool during murine allergic lung inflammation

Introduction

Differentially polarized macrophages, especially YM1+ and MHCII+ macrophages, play an important role in asthma development. The origin of these polarized macrophages has not been elucidated yet. We therefore aimed to investigate how proliferation, monocyte recruitment, and/or switching of polarization states contribute to this specific pool of polarized interstitial and alveolar macrophages during development of house dust mite (HDM)-induced allergic lung inflammation in mice.

Methods

Male and female mice were first treated intranasally with PKH26 to label lung-resident macrophages and were then exposed to either HDM or phosphate-buffered saline (PBS) for two weeks. Different myeloid immune cell types were quantified in lung tissue and blood using flow cytometry.

Results

We found that macrophage polarization only starts up in the second week of HDM exposures. Before this happened, unpolarized alveolar and interstitial macrophages transiently increased in HDM-exposed mice. This transient increase was mostly local proliferation of alveolar macrophages, while interstitial macrophages also contained unlabeled macrophages suggesting monocyte contribution. After two weeks of exposures, the number of interstitial and alveolar macrophages was similar between HDM and PBS-exposed mice, but the distribution of polarization states was remarkably different. HDM-exposed mice selectively developed YM1+ alveolar macrophages and MHCII-hi interstitial macrophages while nonpolarized macrophages were lost compared to PBS-exposed mice.

Discussion

In this HDM model we have shown that development of a polarized macrophage pool during allergic inflammation is first dependent on proliferation of nonpolarized tissue-resident macrophages with some help of infiltrating unlabeled cells, presumably circulating monocytes. These nonpolarized macrophages then acquire their polarized phenotype by upregulating YM1 on alveolar macrophages and MHCII on interstitial macrophages. This novel information will help us to better understand the role of macrophages in asthma and designing therapeutic strategies targeting macrophage functions.

Gastrointestinal microbiota: A predictor of COVID-19 severity?

Coronavirus disease 2019 (COVID-19), caused by a severe acute respiratory syndrome coronavirus 2 infection, has raised serious concerns worldwide over the past 3 years. The severity and clinical course of COVID-19 depends on many factors (e.g., associated comorbidities, age, etc) and may have various clinical and imaging findings, which raises management concerns. Gut microbiota composition is known to influence respiratory disease, and respiratory viral infection can also influence gut microbiota. Gut and lung microbiota and their relationship (gut-lung axis) can act as modulators of inflammation. Modulating the intestinal microbiota, by improving its composition and diversity through nutraceutical agents, can have a positive impact in the prophylaxis/treatment of COVID-19.

The role of lung macrophages in acute respiratory distress syndrome

Acute respiratory distress syndrome (ARDS) is an acute and diffuse inflammatory lung injury in a short time, one of the common severe manifestations of the respiratory system that endangers human life and health. As an innate immune cell, macrophages play a key role in the inflammatory response. For a long time, the role of pulmonary macrophages in ARDS has tended to revolve around the polarization of M1/M2. However, with the development of single-cell RNA sequencing, fate mapping, metabolomics, and other new technologies, a deeper understanding of the development process, classification, and function of macrophages in the lung are acquired. Here, we discuss the function of pulmonary macrophages in ARDS from the two dimensions of anatomical location and cell origin and describe the effects of cell metabolism and intercellular interaction on the function of macrophages. Besides, we explore the treatments for targeting macrophages, such as enhancing macrophage phagocytosis, regulating macrophage recruitment, and macrophage death. Considering the differences in responsiveness of different research groups to these treatments and the tremendous dynamic changes in the gene expression of monocyte/macrophage, we discussed the possibility of characterizing the gene expression of monocyte/macrophage as the biomarkers. We hope that this review will provide new insight into pulmonary macrophage function and therapeutic targets of ARDS.

Blood monocyte counts as a prognostic biomarker and predictor in Chinese patients with idiopathic pulmonary fibrosis

OBJECTIVES

We sought to evaluate the prognostic value of blood routine parameters and biochemical parameters, especially inflammation-related biomarkers, and establish an inflammation-related prognostic model in Chinese patients with idiopathic pulmonary fibrosis (IPF).

MATERIAL/METHODS

Patients diagnosed as IPF at Beijing Chaoyang Hospital and aged 40 years and older were consecutively enrolled from June 2000 to March 2015, and finally, a total of 377 patients were enrolled in the derivation cohort. The follow-up ended in December 2016. We used Cox proportional hazard model to calculate the hazard ratio (HR) and establish the prognostic model. The discrimination and calibration of the prognostic model were evaluated in an independent validation cohort enrolled from China-Japan Friendship Hospital between January 2015 and December 2019.

RESULTS

Multivariate analysis revealed that patients with elevated monocyte-to-red blood cell count ratio (MRR) and monocyte counts showed increased risk of mortality. The clinical-physiological-biomarker (CPB) index and CPB stage we established in this study were a significant predictor, and the C-index for CPB index and CPB stage in the validation cohort was 0.635 (95% CI: 0.558-0.712) and 0.619 (95% CI: 0.544-0.694), respectively. Patients in CPB stage III had the poorest survival.

CONCLUSION

We developed and validated a new inflammation-related prognostic model (CPB index and CPB stage) which was integration of age, gender, FVC (%, predicted), DLCO (%, predicted), Charlson Comorbidity Index, and blood monocyte counts. This prediction model exhibited strong ability in predicting mortality in Chinese patients with IPF.

The Protective Effects of Nutraceutical Components in Methotrexate-Induced Toxicity Models—An Overview

There are multiple concerns associated with methotrexate (MTX), widely recognized for anti-neoplastic and anti-inflammatory effects in life-threatening disease conditions, i.e., acute lymphoblastic leukemia, non-Hodgkin’s lymphoma, psoriasis, and rheumatoid arthritis, due to long-term side effects and associated toxicity, which limits its valuable potential. MTX acts as an inhibitor of dihydrofolate reductase, leading to suppression of purine and pyrimidine synthesis in high metabolic and turnover cells, targeting cancer and dysregulated immune cells. Due to low discrimination between neoplastic cells and naturally high turnover cells, MTX is prone to inhibiting the division of all fast-dividing cells, causing toxicity in multiple organs. Nutraceutical compounds are plant-based or food-derived compounds, used for their preventive and therapeutic role, ascertained in multiple organ dysfunctions, including cardiovascular disease, ischemic stroke, cancer, and neurodegenerative diseases. Gut microbiota and microbiota-derived metabolites take part in multiple physiological processes, their dysregulation being involved in disease pathogenesis. Modulation of gut microbiota by using nutraceutical compounds represents a promising therapeutic direction to restore intestinal dysfunction associated with MTX treatment. In this review, we address the main organ dysfunctions induced by MTX treatment, and modulations of them by using nutraceutical compounds. Moreover, we revealed the protective mechanisms of nutraceuticals in MTX-induced intestinal dysfunctions by modulation of gut microbiota.

Uncovering the core principles of the gut-lung axis to enhance innate immunity in the chicken

Research in mammals has evidenced that proper colonization of the gut by a complex commensal microbial community, the gut microbiota (GM), is critical for animal health and wellbeing. It greatly contributes to the control of infectious processes through competition in the microbial environment while supporting proper immune system development and modulating defence mechanisms at distant organ sites such as the lung: a concept named ‘gut-lung axis’. While recent studies point to a role of the GM in boosting immunity and pathogen resilience also in poultry, the mechanisms underlying this role are largely unknown. In spite of this knowledge gap, GM modulation approaches are today considered as one of the most promising strategies to improve animal health and welfare in commercial poultry production, while coping with the societal demand for responsible, sustainable and profitable farming systems. The majority of pathogens causing economically important infectious diseases in poultry are targeting the respiratory and/or gastrointestinal tract. Therefore, a better understanding of the role of the GM in the development and function of the mucosal immune system is crucial for implementing measures to promote animal robustness in commercial poultry production. The importance of early gut colonization in the chicken has been overlooked or neglected in industrial poultry production systems, where chicks are hampered from acquiring a complex GM from the hen. Here we discuss the concept of strengthening mucosal immunity in the chicken through GM modulation approaches favouring immune system development and functioning along the gut-lung axis, which could be put into practice through improved farming systems, early-life GM transfer, feeding strategies and pre-/probiotics. We also provide original data from experiments with germ-free and conventional chickens demonstrating that the gut-lung axis appears to be functional in chickens. These key principles of mucosal immunity are likely to be relevant for a variety of avian diseases and are thus of far-reaching importance for the poultry sector worldwide.

Luteolin Inhibited the Self-Renewal and Altered the Polarization of Primary Alveolar Macrophages

Pure plant extract luteolin has been demonstrated to possess numerous biological and immunological effects. However, how luteolin affects mice alveolar macrophages' self-renewal and polarization closely related to inflammatory and immunomodulatory is still unknown. In our study, the transcriptomic analysis showed that several self-renewal-related pathways in luteolin-pretreated alveolar macrophages were inhibited compared to the granulocyte-macrophage colony-stimulating factor (GM-CSF)-treated group. Ki-67 staining and EdU assay indicated that luteolin inhibited GM-CSF-induced alveolar macrophage proliferation. Moreover, GM-CSF-induced expressions of c-Myc and KLF4 were significantly suppressed by luteolin at transcriptional and protein levels. Besides, we found that luteolin promoted M1 macrophage polarization induced by LPS plus IFN-γ. At the same time, it inhibited M2 macrophage polarization induced by IL-4 in both alveolar and bone marrow-derived macrophages by detecting macrophage polarization-related gene expressions at mRNA and protein levels. We found that luteolin inhibited self-renewal and altered the polarization of primary alveolar macrophages. Taken together, our data will aid in a better understanding of the immunomodulatory effects of luteolin on the primary alveolar macrophages.

Association of immune cell recruitment and BPD development

In the neonatal lung, exposure to both prenatal and early postnatal risk factors converge into the development of injury and ultimately chronic disease, also known as bronchopulmonary dysplasia (BPD). The focus of many studies has been the characteristic inflammatory responses provoked by these exposures. Here, we review the relationship between immaturity and prenatal conditions, as well as postnatal exposure to mechanical ventilation and oxygen toxicity, with the imbalance of pro- and anti-inflammatory regulatory networks. In these conditions, cytokine release, protease activity, and sustained presence of innate immune cells in the lung result in pathologic processes contributing to lung injury. We highlight the recruitment and function of myeloid innate immune cells, in particular, neutrophils and monocyte/macrophages in the BPD lung in human patients and animal models. We also discuss dissimilarities between the infant and adult immune system as a basis for the development of novel therapeutic strategies.

Dose-dependent immunomodulatory effects of metformin on human neonatal monocyte-derived macrophages

While the association of inflammation with bronchopulmonary dysplasia (BPD) has long been appreciated, M1 proinflammatory macrophage population is emerging as the key element in driving the BPD inflammatory environment. Previous study suggests that low-dose metformin elicits an anti-inflammatory response, possibly through modulating macrophages, to improve disease outcome in a rat BPD model. To investigate this concept further, we examined the dose-dependent immunomodulatory function of metformin directly on human macrophages derived from cord blood (CB) monocytes. We demonstrate that low-dose metformin promotes expansion of M2 anti-inflammatory macrophages, contrasted with high-dose treatment, which exacerbates inflammation by favoring M1 polarization and restricting M2 phenotype. These findings highlight that metformin hold immunomodulatory ability by regulating macrophage polarization in a dose-dependent manner, and only when applied at low dose, exhibiting potential for beneficial anti-inflammatory adjuvant in BPD setting.

Mesenchymal stem cells exert their anti-asthmatic effects through macrophage modulation in a murine chronic asthma model

Despite numerous previous studies, the full action mechanism of the pathogenesis of asthma remains undiscovered, and the need for further investigation is increasing in order to identify more effective target molecules. Recent attempts to develop more efficacious treatments for asthma have incorporated mesenchymal stem cell (MSC)-based cell therapies. This study aimed to evaluate the anti-asthmatic effects of MSCs primed with Liproxstatin-1, a potent ferroptosis inhibitor. In addition, we sought to examine the changes within macrophage populations and their characteristics in asthmatic conditions. Seven-week-old transgenic mice, constitutively overexpressing lung-specific interleukin (IL)-13, were used to simulate chronic asthma. Human umbilical cord-derived MSCs (hUC-MSCs) primed with Liproxstatin-1 were intratracheally administered four days prior to sampling. IL-13 transgenic mice demonstrated phenotypes of chronic asthma, including severe inflammation, goblet cell hyperplasia, and subepithelial fibrosis. Ly6C⁺M2 macrophages, found within the pro-inflammatory CD11c⁺CD11b⁺ macrophages, were upregulated and showed a strong correlation with lung eosinophil counts. Liproxstatin-1-primed hUC-MSCs showed enhanced ability to downregulate the activation of T helper type 2 cells compared to naïve MSCs in vitro and reduced airway inflammation, particularly Ly6C⁺M2 macrophages population, and fibrosis in vivo. In conclusion, intratracheal administration is an effective method of MSC delivery, and macrophages hold great potential as an additional therapeutic target for asthma.

The Beneficial Role of Probiotic Lactobacillus in Respiratory Diseases

Respiratory diseases cause a high incidence and mortality worldwide. As a natural immunobiotic, Lactobacillus has excellent immunomodulatory ability. Administration of some Lactobacillus species can alleviate the symptoms of respiratory diseases such as respiratory tract infections, asthma, lung cancer and cystic fibrosis in animal studies and clinical trials. The beneficial effect of Lactobacillus on the respiratory tract is strain dependent. Moreover, the efficacy of Lactobacillus may be affected by many factors, such as bacteria dose, timing and host background. Here, we summarized the beneficial effect of administered Lactobacillus on common respiratory diseases with a focus on the mechanism and safety of Lactobacillus in regulating respiratory immunity.

Tanreqing Injection Attenuates Macrophage Activation and the Inflammatory Response via the lncRNA-SNHG1/HMGB1 Axis in Lipopolysaccharide-Induced Acute Lung Injury

The etiology of acute lung injury (ALI) is not clear, and the treatment of ALI presents a great challenge. This study aimed to investigate the pathogenesis and potential therapeutic targets of ALI and to define the target gene of Tanreqing (TRQ), which is a traditional Chinese medicine formula composed of five medicines, scutellaria baicalensis, bear bile powder, goat horn powder, honeysuckle and forsythia. Macrophage activation plays a critical role in many pathophysiological processes, such as inflammation. Although the regulation of macrophage activation has been extensively investigated, there is little knowledge of the role of long noncoding RNAs (lncRNAs) in this process. In this study, we found that lncRNA-SNHG1 expression is distinctly regulated in differently activated macrophages in that it is upregulated in LPS. LncRNA-SNHG1 knockdown attenuates LPS-induced M1 macrophage activation. The SNHG1 promoter was bound by NF-κB subunit p65, indicative of SNHG1 being a direct transcriptional target of LPS-induced NF-κB activation. SNHG1 acts as a proinflammatory driver that leads to the production of inflammatory cytokines and the activation of macrophages and cytokine storms by physically interacting with high-mobility group box 1 (HMGB1) in ALI. TRQ inhibited NF-κB signaling activation and binding of NF-κB to the SNHG1 promoter. In conclusion, this study defined TRQ target genes, which can be further elucidated as mechanism(s) of TRQ action, and provides insight into the molecular pathogenesis of ALI. The lncRNA-SNHG1/HMGB1 axis is an ideal therapeutic for ALI treatment.

Mucosal immune responses to infection and vaccination in the respiratory tract

The lungs are constantly exposed to inhaled debris, allergens, pollutants, commensal or pathogenic microorganisms, and respiratory viruses. As a result, innate and adaptive immune responses in the respiratory tract are tightly regulated and are in continual flux between states of enhanced pathogen clearance, immune-modulation, and tissue repair. New single-cell-sequencing techniques are expanding our knowledge of airway cellular complexity and the nuanced connections between structural and immune cell compartments. Understanding these varied interactions is critical in treatment of human pulmonary disease and infections and in next-generation vaccine design. Here, we review the innate and adaptive immune responses in the lung and airways following infection and vaccination, with particular focus on influenza virus and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The ongoing SARS-CoV-2 pandemic has put pulmonary research firmly into the global spotlight, challenging previously held notions of respiratory immunity and helping identify new populations at high risk for respiratory distress.

Bitter taste signaling in tracheal epithelial brush cells elicits innate immune responses to bacterial infection

Constant exposure of the airways to inhaled pathogens requires efficient early immune responses protecting against infections. How bacteria on the epithelial surface are detected and first-line protective mechanisms are initiated are not well understood. We have recently shown that tracheal brush cells (BCs) express functional taste receptors. Here we report that bitter taste signaling in murine BCs induces neurogenic inflammation. We demonstrate that BC signaling stimulates adjacent sensory nerve endings in the trachea to release the neuropeptides CGRP and substance P that mediate plasma extravasation, neutrophil recruitment, and diapedesis. Moreover, we show that bitter tasting quorum-sensing molecules from Pseudomonas aeruginosa activate tracheal BCs. BC signaling depends on the key taste transduction gene Trpm5, triggers secretion of immune mediators, among them the most abundant member of the complement system, and is needed to combat P. aeruginosa infections. Our data provide functional insight into first-line defense mechanisms against bacterial infections of the lung.

Cadherin-11 Regulates Macrophage Development and Function

Cadherin-11 (CDH11) is a cell-cell adhesion protein that has previously been reported to play an important role in the pathogenesis of pulmonary fibrosis. It is expressed on macrophages in the fibrotic lung. However, the role of CDH11 on macrophage biology has not yet been studied. We show using immunophenotypic analyses that Cdh11^-/- mice have fewer recruited monocyte-derived macrophages and Ly6C^hi monocytes in the lungs compared to wild-type mice in the intraperitoneal bleomycin-induced pulmonary fibrosis model. Additionally, fewer Ly6C^hi monocytes were detected in the bone marrow and peripheral blood of naive Cdh11^-/- mice. Given that macrophages are derived from monocytes, we investigated the precursors of the monocyte/macrophage lineage in the bone marrow. We found increased numbers of CMPs and reduced numbers of GMPs and MPs/cMoPs in Cdh11^-/- mice compared to wild-type mice, suggesting decreased differentiation towards the myeloid lineage in Cdh11^-/- mice. Furthermore, we show using bone marrow cells that loss of CDH11 impaired monocyte to macrophage differentiation. We also demonstrate that CDH11 deficiency repressed the M2 program and impaired the phagocytic function of bone marrow-derived macrophages. Overall, our findings demonstrate a role for CDH11 in macrophage development, M2 polarization, and phagocytic function.

Targeted Accumulation of Macrophages Induced by Microbeam Irradiation in a Tissue-Dependent Manner

Radiation therapy (RT) is a vital component of multimodal cancer treatment, and its immunomodulatory effects are a major focus of current therapeutic strategies. Macrophages are some of the first cells recruited to sites of radiation-induced injury where they can aid in tissue repair, propagate radiation-induced fibrogenesis and influence tumour dynamics. Microbeam radiation therapy (MRT) is a unique, spatially fractionated radiation modality that has demonstrated exceptional tumour control and reduction in normal tissue toxicity, including fibrosis. We conducted a morphological analysis of MRT-irradiated normal liver, lung and skin tissues as well as lung and melanoma tumours. MRT induced distinct patterns of DNA damage, reflecting the geometry of the microbeam array. Macrophages infiltrated these regions of peak dose deposition at variable timepoints post-irradiation depending on the tissue type. In normal liver and lung tissue, macrophages clearly demarcated the beam path by 48 h and 7 days post-irradiation, respectively. This was not reflected, however, in normal skin tissue, despite clear DNA damage marking the beam path. Persistent DNA damage was observed in MRT-irradiated lung carcinoma, with an accompanying geometry-specific influx of mixed M1/M2-like macrophage populations. These data indicate the unique potential of MRT as a tool to induce a remarkable accumulation of macrophages in an organ/tissue-specific manner. Further characterization of these macrophage populations is warranted to identify their organ-specific roles in normal tissue sparing and anti-tumour responses.

Cell Origin and iNOS Function Are Critical to Macrophage Activation Following Acute Lung Injury

In the intratracheal bleomycin (ITB) model of acute lung injury (ALI), macrophages are recruited to the lung and participate in the inflammation and resolution that follows injury. Macrophage origin is influential in determining activation; however, the specific phenotype of recruited and resident macrophages is not known. Inducible nitric oxide synthase (iNOS) has been implicated in the pathogenesis of ALI; however, the effects of its inhibition are mixed. Here we examined how macrophage origin determines the phenotypic response to ALI. Further, we hypothesize cell specific iNOS is key to determining activation and recruitment. Using a chimeric mouse approach, we have identified recruited and resident macrophage populations. We also used the chimeric mouse approach to create either pulmonary or bone marrow NOS2^-/- mice and systemically inhibited iNOS via 1400 W. We evaluated macrophage populations at the peak of inflammation (8 days) and the beginning of resolution (15 days) following ITB. These studies demonstrate tissue resident macrophages adopt a M2 phenotype specifically, but monocyte originated macrophages activate along a spectrum. Additionally, we demonstrated that monocyte originating macrophage derived iNOS is responsible for recruitment to the lung during the inflammatory phase. Further, we show that macrophage activation is dependent upon cellular origin. Finally, these studies suggest pulmonary derived iNOS is detrimental to the lung following ITB. In conclusion, macrophage origin is a key determinant in response to ALI and iNOS is central to recruitment and activation.

Complex Involvement of Interleukin-26 in Bacterial Lung Infection

Pneumonia is a global cause of mortality, and this provides a strong incentive to improve the mechanistic understanding of innate immune responses in the lungs. Here, we characterized the involvement of the cytokine interleukin (IL)-26 in bacterial lung infection. We observed markedly increased concentrations of IL-26 in lower airway samples from patients with bacterial pneumonia and these correlated with blood neutrophil concentrations. Moreover, pathogen-associated molecular patterns (PAMPs) from both Gram-negative and -positive bacteria increased extracellular IL-26 concentrations in conditioned media from human models of alveolar epithelial cells, macrophages, and neutrophils in vitro. Stimulation with IL-26 inhibited the inherent release of neutrophil elastase and myeloperoxidase in unexposed neutrophils. This stimulation also inhibited the expression of activity makers in neutrophils exposed to Klebsiella pneumoniae. In addition, priming of human lung tissue ex vivo with exogenous IL-26 potentiated the endotoxin-induced increase in mRNA for other cytokines involved in the innate immune response, including the master Th17-regulator IL-23 and the archetype inhibitory cytokine IL-10. Finally, neutralization of endogenous IL-26 clearly increased the growth of Klebsiella pneumoniae in the macrophage culture. These findings suggest that IL-26 is involved in bacterial lung infection in a complex manner, by modulating critical aspects of innate immune responses locally and systemically in a seemingly purposeful manner and by contributing to the killing of bacteria in a way that resembles an antimicrobial peptide. Thus, IL-26 displays both diagnostic and therapeutic potential in pneumonia and deserves to be further evaluated in these respects.

Regulation of macrophage functions by FABP-mediated inflammatory and metabolic pathways

Macrophages are almost everywhere in the body, where they serve pivotal functions in maintaining tissue homeostasis, remodeling, and immunoregulation. Macrophages are traditionally thought to differentiate from bone marrow-derived hematopoietic stem cells (HSCs). Emerging studies suggest that some tissue macrophages at steady state originate from embryonic precursors in the yolk sac or fetal liver and are maintained in situ by self-renewal, but bone marrow-derived monocytes can give rise to tissue macrophages in pathogenic settings, such as inflammatory injuries and cancer. Macrophages are popularly classified as Th1 cytokine (e.g. IFNγ)-activated M1 macrophages (the classical activation) or Th2 cytokine (e.g. IL-4)-activated M2 macrophages (the alternative activation). However, given the myriad arrays of stimuli macrophages may encounter from local environment, macrophages exhibit notorious heterogeneity in their phenotypes and functions. Determining the underlying metabolic pathways engaged during macrophage activation is critical for understanding macrophage phenotypic and functional adaptivity under different disease settings. Fatty acid binding proteins (FABPs) represent a family of evolutionarily conserved proteins facilitating lipid transport, metabolism and responses inside cells. More specifically, adipose-FABP (A-FABP) and epidermal-FABP (E-FABP) are highly expressed in macrophages and play a central role in integrating metabolic and inflammatory pathways. In this review we highlight how A-FABP and E-FABP are respectively upregulated in different subsets of activated macrophages and provide a unique perspective in defining macrophage phenotypic and functional heterogeneity through FABP-regulated lipid metabolic and inflammatory pathways.

Nasal Priming with Lactobacillus rhamnosus CRL1505 Stimulates Mononuclear Phagocytes of Immunocompromised Malnourished Mice: Improvement of Respiratory Immune Response

The effect of Lactobacillus rhamnosus CRL1505 (Lr) on macrophages (Ma) and dendritic cells (DC) in the orchestration of anti-pneumococcal immunity was studied using malnutrition and pneumococcal infection mouse models. Monocytes (Mo), Ma, and DC in two groups of malnourished mice fed with balanced diet (BCD) were studied through flow cytometry; one group was nasally administered with Lr (BCD+Lr group), and the other group was not (BCD group). Well-nourished (WNC) and malnourished (MNC) mice were used as controls.Malnutrition affected the number of respiratory and splenic mononuclear phagocytes. The BCD+Lr treatment, unlike BCD, was able to increase and normalize lung Mo and Ma. The BCD+Lr mice were also able to upregulate the expression of the activation marker MHC II in lung DC and to improve this population showing a more significant effect on CD11b⁺ DC subpopulation. At post-infection, lung Mo values were higher in BCD+Lr mice than in BCD mice and similar to those obtained in WNC group. Although both repletion treatments showed similar values of lung Ma post-infection, the Ma activation state in BCD+Lr mice was higher than that in BCD mice. Furthermore, BCD+Lr treatment was able to normalize the number and activation of splenic Ma and DC after the challenge.Lr administration stimulates respiratory and systemic mononuclear phagocytes. Stimulation of Ma and DC populations would increase the microbicide activity and improve the adaptive immunity through its antigen-presenting capacity. Thus, Lr contributes to improved outcomes of pneumococcal infection in immunocompromised hosts.

A positive feedback loop reinforces the allergic immune response in human peanut allergy

Food allergies are a leading cause of anaphylaxis, and cellular mechanisms involving antigen presentation likely play key roles in their pathogenesis. However, little is known about the response of specific antigen-presenting cell (APC) subsets to food allergens in the setting of food allergies. Here, we show that in peanut-allergic humans, peanut allergen drives the differentiation of CD209+ monocyte-derived dendritic cells (DCs) and CD23+ (FcєRII) myeloid dendritic cells through the action of allergen-specific CD4+ T cells. CD209+ DCs act reciprocally on the same peanut-specific CD4+ T cell population to reinforce Th2 cytokine expression in a positive feedback loop, which may explain the persistence of established food allergy. In support of this novel model, we show clinically that the initiation of oral immunotherapy (OIT) in peanut-allergic patients is associated with a decrease in CD209+ DCs, suggesting that breaking the cycle of positive feedback is associated with therapeutic effect.

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.

Roles of lung-recruited monocytes and pulmonary Vascular Endothelial Growth Factor (VEGF) in resolving Ventilator-Induced Lung Injury (VILI)

Monocytes and vascular endothelial growth factor (VEGF) have profound effects on tissue injury and repair. In ventilator-induced lung injury (VILI), monocytes, the majority of which are Ly6C+high, and VEGF are known to initiate lung injury. However, their roles in post-VILI lung repair remain unclear. In this study, we used a two-hit mouse model of VILI to identify the phenotypes of monocytes recruited to the lungs during the resolution of VILI and investigated the contributions of monocytes and VEGF to lung repair. We found that the lung-recruited monocytes were predominantly Ly6C+low from day 1 after the insult. Meanwhile, contrary to inflammatory cytokines, pulmonary VEGF decreased upon VILI but subsequently increased significantly on days 7 and 14 after the injury. There was a strong positive correlation between VEGF expression and proliferation of alveolar epithelial cells in lung sections. The expression pattern of VEGF mRNA in lung-recruited monocytes was similar to that of pulmonary VEGF proteins, and the depletion of monocytes significantly suppressed the increase of pulmonary VEGF proteins on days 7 and 14 after VILI. In conclusion, during recovery from VILI, the temporal expression patterns of pulmonary growth factors are different from those of inflammatory cytokines, and the restoration of pulmonary VEGF by monocytes, which are mostly Ly6C+low, is associated with pulmonary epithelial proliferation. Lung-recruited monocytes and pulmonary VEGF may play crucial roles in post-VILI lung repair.

MAP3K8 Regulates Cox-2-Mediated Prostaglandin E2 Production in the Lung and Suppresses Pulmonary Inflammation and Fibrosis

Idiopathic pulmonary fibrosis (IPF) is characterized by exuberant deposition of extracellular matrix components, leading to the deterioration of lung architecture and respiratory functions. Profibrotic mechanisms are controlled by multiple regulatory molecules, including MAPKs, in turn regulated by multiple phosphorylation cascades. MAP3K8 is an MAPK kinase kinase suggested to pleiotropically regulate multiple pathogenic pathways in the context of inflammation and cancer; however, a possible role in the pathogenesis of IPF has not been investigated. In this report, MAP3K8 mRNA levels were found decreased in the lungs of IPF patients and of mice upon bleomycin-induced pulmonary fibrosis. Ubiquitous genetic deletion of Map3k8 in mice exacerbated the modeled disease, whereas bone marrow transfer experiments indicated that although MAP3K8 regulatory functions are active in both hematopoietic and nonhematopoietic cells, Map3k8 in hematopoietic cells has a more dominant role. Macrophage-specific deletion of Map3k8 was further found to be sufficient for disease exacerbation thus confirming a major role for macrophages in pulmonary fibrotic responses and suggesting a main role for Map3k8 in the homeostasis of their effector functions in the lung. Map3k8 deficiency was further shown to be associated with decreased Cox-2 expression, followed by a decrease in PGE₂ production in the lung; accordingly, exogenous administration of PGE₂ reduced inflammation and reversed the exacerbated fibrotic profile of Map3k8 ^-/- mice. Therefore, MAP3K8 has a central role in the regulation of inflammatory responses and Cox-2-mediated PGE₂ production in the lung, and the attenuation of its expression is integral to pulmonary fibrosis development.

Abundance of Non-Polarized Lung Macrophages with Poor Phagocytic Function in Chronic Obstructive Pulmonary Disease (COPD)

Lung macrophages are the key immune effector cells in the pathogenesis of Chronic Obstructive Pulmonary Disease (COPD). Several studies have shown an increase in their numbers in bronchoalveolar lavage fluid (BAL) of subjects with COPD compared to controls, suggesting a pathogenic role in disease initiation and progression. Although reduced lung macrophage phagocytic ability has been previously shown in COPD, the relationship between lung macrophages' phenotypic characteristics and functional properties in COPD is still unclear. (1) Methods: Macrophages harvested from bronchoalveolar lavage (BAL) fluid of subjects with and without COPD (GOLD grades, I-III) were immuno-phenotyped, and their function and gene expression profiles were assessed using targeted assays. (2) Results: BAL macrophages from 18 COPD and 10 (non-COPD) control subjects were evaluated. The majority of macrophages from COPD subjects were non-polarized (negative for both M1 and M2 markers; 77.9%) in contrast to controls (23.9%; p < 0.001). The percentages of these non-polarized macrophages strongly correlated with the severity of COPD (p = 0.006) and current smoking status (p = 0.008). Non-polarized macrophages demonstrated poor phagocytic function in both the control (p = 0.02) and COPD (p < 0.001) subjects. Non-polarized macrophages demonstrated impaired ability to phagocytose Staphylococcus aureus (p < 0.001). They also demonstrated reduced gene expression for CD163, CD40, CCL13 and C1QA&B, which are involved in pathogen recognition and processing and showed an increased gene expression for CXCR4, RAF1, amphiregulin and MAP3K5, which are all involved in promoting the inflammatory response. (3) Conclusions: COPD is associated with an abundance of non-polarized airway macrophages that is related to the severity of COPD. These non-polarized macrophages are predominantly responsible for the poor phagocytic capacity of lung macrophages in COPD, having reduced capacity for pathogen recognition and processing. This could be a key risk factor for COPD exacerbation and could contribute to disease progression.

IL-33-ST2 axis regulates myeloid cell differentiation and activation enabling effective club cell regeneration

Evidence points to an indispensable function of macrophages in tissue regeneration, yet the underlying molecular mechanisms remain elusive. Here we demonstrate a protective function for the IL-33-ST2 axis in bronchial epithelial repair, and implicate ST2 in myeloid cell differentiation. ST2 deficiency in mice leads to reduced lung myeloid cell infiltration, abnormal alternatively activated macrophage (AAM) function, and impaired epithelial repair post naphthalene-induced injury. Reconstitution of wild type (WT) AAMs to ST2-deficient mice completely restores bronchial re-epithelialization. Central to this mechanism is the direct effect of IL-33-ST2 signaling on monocyte/macrophage differentiation, self-renewal and repairing ability, as evidenced by the downregulation of key pathways regulating myeloid cell cycle, maturation and regenerative function of the epithelial niche in ST2^−/− mice. Thus, the IL-33-ST2 axis controls epithelial niche regeneration by activating a large multi-cellular circuit, including monocyte differentiation into competent repairing AAMs, as well as group-2 innate lymphoid cell (ILC2)-mediated AAM activation.

Signaling of IL-33 via its receptor, ST2, has been implicated in macrophage function in tissue repair. Here the authors show, using genetic mouse models and single-cell transcriptomic data, that the IL-33/ST2 axis regulates both ILC2-derived IL-13 and macrophage differentiation/reparative function required for club cell regeneration.

Macrophage TLR4 and PAR2 Signaling: Role in Regulating Vascular Inflammatory Injury and Repair

Macrophages play a central role in dictating the tissue response to infection and orchestrating subsequent repair of the damage. In this context, macrophages residing in the lungs continuously sense and discriminate among a wide range of insults to initiate the immune responses important to host-defense. Inflammatory tissue injury also leads to activation of proteases, and thereby the coagulation pathway, to optimize injury and repair post-infection. However, long-lasting inflammatory triggers from macrophages can impair the lung's ability to recover from severe injury, leading to increased lung vascular permeability and neutrophilic injury, hallmarks of Acute Lung Injury (ALI). In this review, we discuss the roles of toll-like receptor 4 (TLR4) and protease activating receptor 2 (PAR2) expressed on the macrophage cell-surface in regulating lung vascular inflammatory signaling.

Lung macrophages: current understanding of their roles in Ozone-induced lung diseases

Through the National Ambient Air Quality Standards (NAAQS), the Clean Air Act of the United States outlines acceptable levels of six different air pollutants considered harmful to humans and the environment. Included in this list is ozone (O₃), a highly reactive oxidant gas, respiratory health hazard, and common environmental air pollutant at ground level. The respiratory health effects due to O₃ exposure are often associated with molecular and cellular perturbations in the respiratory tract. Periodic review of NAAQS requires comprehensive scientific evaluation of the public health effects of these pollutants, which is formulated through integrated science assessment (ISA) of the most policy-relevant scientific literature. This review focuses on the protective and pathogenic effects of macrophages in the O₃-exposed respiratory tract, with emphasis on mouse model-based toxicological studies. Critical findings from 39 studies containing the words O₃, macrophage, mice, and lung within the full text were assessed. While some of these studies highlight the presence of disease-relevant pathogenic macrophages in the airspaces, others emphasize a protective role for macrophages in O₃-induced lung diseases. Moreover, a comprehensive list of currently known macrophage-specific roles in O₃-induced lung diseases is included in this review and the significant knowledge gaps that still exist in the field are outlined. In conclusion, there is a vital need in this field for additional policy-relevant scientific information, including mechanistic studies to further define the role of macrophages in response to O₃.

Decreased expression of transmembrane TNFR2 in lung leukocytes subpopulations of patients with non-fibrotic hypersensitivity pneumonitis compared with the fibrotic disease

Hypersensitivity pneumonitis (HP) is an interstitial lung disease, characterized by lung inflammation (non-fibrotic HP) that may often progresses to fibrosis (Fibrotic HP). The tumor necrosis factor (TNF) and its receptors (TNFR1 and TNFR2) can be found as soluble (sol) and transmembrane (tm) forms, playing pro-inflammatory functions but also has been related to immune regulatory functions. Bronchioalveolar lavage from fibrotic and non-fibrotic HP patients was obtained, and immune cells were characterized by flow cytometry, whereas soluble proteins were analyzed by ELISA. Compare to fibrotic HP patients, HP patients with non-fibrotic disease have accumulation of pro-inflammatory CD3+ myeloid cells, cell subpopulations that have decreased tmTNFR2 expression, and low frequency of regulatory-T cells. Whereas solTNF, solTNFR2, and IL-8 are increased. These findings suggest that the TNF pathway may explain, at least partially, the differences between both HP clinical forms. The evaluation of the TNF family molecules may help to develop new therapeutic approaches.

Lung Macrophage Functional Properties in Chronic Obstructive Pulmonary Disease

Chronic obstructive pulmonary disease (COPD) is caused by the chronic exposure of the lungs to toxic particles and gases. These exposures initiate a persistent innate and adaptive immune inflammatory response in the airways and lung tissues. Lung macrophages (LMs) are key innate immune effector cells that identify, engulf, and destroy pathogens and process inhaled particles, including cigarette smoke and particulate matter (PM), the main environmental triggers for COPD. The number of LMs in lung tissues and airspaces is increased in COPD, suggesting a potential key role for LMs in initiating and perpetuating the chronic inflammatory response that underpins the progressive nature of COPD. The purpose of this brief review is to discuss the origins of LMs, their functional properties (chemotaxis, recruitment, mediator production, phagocytosis and apoptosis) and changes in these properties due to exposure to cigarette smoke, ambient particulate and pathogens, as well as their persistent altered functional properties in subjects with established COPD. We also explore the potential to therapeutically modulate and restore LMs functional properties, to improve impaired immune system, prevent the progression of lung tissue destruction, and improve both morbidity and mortality related to COPD.

RPTPε promotes M2-polarized macrophage migration through ROCK2 signaling and podosome formation

Cysteinyl-leukotrienes (cys-LTs) have well-characterized physiopathological roles in the development of inflammatory diseases. We have previously found that protein tyrosine phosphatase ε (PTPε) is a signaling partner of CysLT₁R, a high affinity receptor for leukotriene D4 (LTD₄). There are two major isoforms of PTPε, receptor-like (RPTPε) and cytoplasmic (cyt-)PTPε, both of which are encoded by the PTPRE gene but from different promoters. In most cells, their expression is mutually exclusive, except in human primary monocytes, which express both isoforms. Here, we show differential PTPε isoform expression patterns between monocytes, M1 and M2 human monocyte-derived macrophages (hMDMs), with the expression of glycosylated forms of RPTPε predominantly in M2-polarized hMDMs. Using PTPε-specific siRNAs and expression of RPTPε and cyt-PTPε, we found that RPTPε is involved in monocyte adhesion and migration of M2-polarized hMDMs in response to LTD₄ Altered organization of podosomes and higher phosphorylation of the inhibitory Y-722 residue of ROCK2 was also found in PTPε-siRNA-transfected cells. In conclusion, we show that differentiation and polarization of monocytes into M2-polarized hMDMs modulates the expression of PTPε isoforms and RPTPε is involved in podosome distribution, ROCK2 activation and migration in response to LTD₄.

Intrapulmonary Cellular-Level Distribution of Inhaled Nanoparticles with Defined Functional Groups and Its Correlations with Protein Corona and Inflammatory Response

Concerns over the health risks associated with airborne exposure to ultrafine particles [PM0.1, or nanoparticles (NPs)] call for a comprehensive understanding in the interactions of inhaled NPs along their respiratory journey. We prepare a collection of polyethylene glycol-coated gold nanoparticles that bear defined functional groups commonly identified in atmospheric particulates (Au@PEG-X NPs, where X = OCH₃, COOH, NH₂, OH, or C₁₂H₂₅). Regardless of the functional group, these ∼50 nm NPs remain colloidally stable following aerosolization and incubation in bronchoalveolar lavage fluid (BALF), without pronouncedly crossing the air-blood barrier. The type of BALF proteins adhered onto the NPs is similar, but the composition of protein corona depends on functional group. By subjecting Balb/c mice to inhalation of Au@PEG-X NPs for 6 h, we demonstrate that the intrapulmonary distribution of NPs among the various types of cells (both found in BALF and isolated from the lavaged lung) and the acute inflammatory responses induced by inhalation are sensitive to the functional group of NPs and postinhalation period (0, 24, or 48 h). By evaluating the pairwise correlations between the three variables of "lung-nano" interactions (protein corona, intrapulmonary cellular-level distribution, and inflammatory response), we reveal strong statistical correlations between the (1) fractions of albumin or carbonyl reductase bound to NPs, (2) associations of inhaled NPs to neutrophils in BALF or macrophages in the lavaged lung, and (3) level of total protein in BALF. Our results provide insights into the effect of functional group on lung-nano interactions and health risks associated with inhalation of PM0.1.

Tissue-resident innate immunity in the lung

The lung is a unique organ that must protect against inhaled pathogens and toxins, without mounting a disproportionate response against harmless particulate matter and without compromising its vital function. Tissue-resident immune cells within the lung provide local immunity and protection from infection but are also responsible for causing disease when dysregulated. There is a growing appreciation of the importance of tissue-resident memory T cells to lung immunity, but non-recirculating, tissue-resident, innate immune cells also exist. These cells provide the first line of defence against pulmonary infection and are essential for co-ordinating the subsequent adaptive response. In this review, we discuss the main lung-resident innate immune subsets and their functions in common pulmonary diseases, such as influenza, bacterial pneumonia, asthma and inflammatory disorders.

Microbes, metabolites, and the gut-lung axis

The microbiota plays an essential role in the education, development, and function of the immune system, both locally and systemically. Emerging experimental and epidemiological evidence highlights a crucial cross-talk between the intestinal microbiota and the lungs, termed the 'gut-lung axis'. Changes in the constituents of the gut microbiome, through either diet, disease or medical interventions (such as antibiotics) is linked with altered immune responses and homeostasis in the airways. The importance of the gut-lung axis has become more evident following the identification of several gut microbe-derived components and metabolites, such as short-chain fatty acids (SCFAs), as key mediators for setting the tone of the immune system. Recent studies have supported a role for SCFAs in influencing hematopoietic precursors in the bone marrow-a major site of innate and adaptive immune cell development. Here, we review the current understanding of host-microbe cross-talk along the gut-lung axis. We highlight the importance of SCFAs in shaping and promoting bone marrow hematopoiesis to resolve airway inflammation and to support a healthy homeostasis.

Dudleya brittonii extract promotes survival rate and M2-like metabolic change in porcine 3D4/31 alveolar macrophages

Objective

Although alveolar macrophages play a key role in the respiratory immunity of livestock, studies on the mechanism of differentiation and survival of alveolar macrophages are lacking. Therefore, we undertook to investigate changes in the lipid metabolism and survival rate, using 3D4/31 macrophages and Dudleya brittonii which has been used as a traditional asthma treatment.

Methods

3D4/31 macrophages were used as the in vitro porcine alveolar macrophages model. The cells were activated by exposure to phorbol 12-myristate 13-acetate (PMA). Dudleya brittonii extraction was performed with distilled water. For evaluating the cell survival rate, we performed the water-soluble tetrazolium salt cell viability assay and growth curve analysis. To confirm cell death, cell cycle and intracellular reactive oxygen species (ROS) levels were measured using flow cytometric analysis by applying fluorescence dye dichlorofluorescein diacetate and propidium iodide. Furthermore, we also evaluated cellular lipid accumulation with oil red O staining, and fatty acid synthesis related genes expression levels using quantitative polymerase chain reaction (qPCR) with SYBR green dye. Glycolysis, fatty acid oxidation, and tricarboxylic acid (TCA) cycle related gene expression levels were measured using qPCR after exposure to Dudleya brittonii extract (DB) for 12 h.

Results

The ROS production and cell death were induced by PMA treatment, and exposure to DB reduced the PMA induced downregulation of cell survival. The PMA and DB treatments upregulated the lipid accumulation, with corresponding increase in the acetyl-CoA carboxylase alpha, fatty acid synthase mRNA expressions. DB-PMA co-treatment reduced the glycolysis genes expression, but increased the expressions of fatty acid oxidation and TCA cycle genes.

Conclusion

This study provides new insights and directions for further research relating to the immunity of porcine respiratory system, by employing a model based on alveolar macrophages and natural materials.