Airway macrophage-intrinsic TGF-β1 regulates pulmonary immunity during early-life allergen exposure

E26 transformation-specific-1 controls asthma development by regulating the CD4+ Th2/Th17 immune response involved transcription factors in active genomic transcriptional regions

Asthma is a chronic respiratory disease characterized by airway inflammation and immune cell imbalance. The transcription factor E26 transformation-specific-1 regulates immune cell functions, but its role in asthma remains unclear. Here, we generated Ets1 heterozygous (Ets1+/-) mice by constitutively knocking out exons 7 and 8 of Ets1, confirmed significantly reduced Ets1 expression via western blot. Asthma models were then established in both wild-type and Ets1+/- mice, revealing more severe pulmonary inflammation in Ets1+/- mice. Then, we systematically explored the regulatory effects of Ets1 on immune cells function and inflammatory responses in asthma. Further analyses showed enhanced CD4+ helper T (Th) 2/Th17 cell responses and elevated interleukin-4 and interleukin-17A secretion in the asthma Ets1+/- mice. By combining chromatin immunoprecipitation sequencing and RNA sequencing analyses, we identified 17 transcription factors regulated by Ets1 and linked to the function of mitotic processes in asthma. These findings suggest that Ets1 mitigates asthma by modulating CD4+ Th2/Th17 immune responses and regulating transcription factors associated with cell cycle processes.

PI(3,4,5)P3-mediated Cdc42 activation regulates macrophage podosome assembly

Podosomes are adhesion structures with densely-polymerized F-actin. While PI(3,4,5)P3 and Cdc42-GTP are known factors to trigger WASP-mediated actin polymerization at the macrophage podosome, their causal mechanism to activate WASP remains unclear. Here, we demonstrate that spatially elevated Cdc42-GTP is a downstream effector of local PI(3,4,5)P3 production at the podosome. We further examine the expression and distribution of 19 Cdc42 guanine exchange factors (GEFs) and identify VAV1 as the key PI(3,4,5)P3-dependent Cdc42 GEF. VAV1 is spatially enriched at the macrophage podosome, and the association of VAV1 with the membrane plays a critical role in upregulating its GEF activity. Reintroduction of wildtype VAV1, rather than the PI(3,4,5)P3-binding deficient or catalytically dead mutants restores the matrix degradation and chemotactic migration of VAV1-knockdown macrophage. Thus, the biogenesis of PI(3,4,5)P3 acts as an upstream signal to locally recruit VAV1 and in turn triggers the guanine nucleotide exchange of Cdc42. Elevated levels of Cdc42-GTP then promote WASP-mediated podosome assembly and macrophage chemotaxis.

Airway macrophage glycolysis controls lung homeostasis and responses to aeroallergen

The lungs represent a dynamic microenvironment where airway macrophages (AMs) are the major lung-resident macrophages. AMs dictate the balance between tissue homeostasis and immune activation and thus have contradictory functions by maintaining tolerance and tissue homeostasis, as well as initiating strong inflammatory responses. Emerging evidence has highlighted the connection between macrophage function and cellular metabolism. However, the functional importance of these processes in tissue-resident specialized macrophage populations such as those found in the airways, remain poorly elucidated. Here, we reveal that glycolysis is a fundamental pathway in AMs which regulates both lung homeostasis and responses to inhaled allergen. Using macrophage specific targeting in vivo, and multi-omics approaches, we determined that glycolytic activity in AMs is necessary to restrain type 2 (T2) immunity during homeostasis. Exposure to a range of common aeroallergens, including house dust mite (HDM), drove AM-glycolysis and furthermore, AM-specific inhibition of glycolysis altered inflammation in the airways and HDM-driven airway metabolic adaptations in vivo. Additionally, allergen sensitised asthmatics had profound metabolic changes in the airways, compared to non-sensitised asthmatic controls. Finally, we found that allergen driven AM-glycolysis in mice was TLR2 dependent. Thus, our findings demonstrate a direct relationship between glycolysis in AMs, AM-mediated homeostatic processes, and T2 immune responses in the lungs. These data suggest that glycolysis is essential for the plasticity of AMs. Depending on the immunological context, AM-glycolysis is required to exert homeostatic activity but once activated by allergen, AM-glycolysis influences inflammatory responses. Thus, precise modulation of glycolytic activity in AMs is essential for preserving lung homeostasis and regulating airway inflammation.

TGFβ primes alveolar-like macrophages to induce type I IFN following TLR2 activation

Alveolar macrophages (AMs) are key mediators of lung function and are potential targets for therapies during respiratory infections. TGFβ is an important regulator of AM differentiation and maintenance, but how TGFβ directly modulates the innate immune responses of AMs remains unclear. This shortcoming prevents effective targeting of AMs to improve lung function in health and disease. Here we leveraged an optimized ex vivo AM model system, fetal-liver derived alveolar-like macrophages (FLAMs), to dissect the role of TGFβ in AMs. Using transcriptional analysis, we first globally defined how TGFβ regulates gene expression of resting FLAMs. We found that TGFβ maintains the baseline metabolic state of AMs by driving lipid metabolism through oxidative phosphorylation and restricting inflammation. To better understand inflammatory regulation in FLAMs, we next directly tested how TGFβ alters the response to TLR2 agonists. While both TGFβ (+) and TGFβ (-) FLAMs robustly responded to TLR2 agonists, we found an unexpected activation of type I interferon (IFN) responses in FLAMs and primary AMs in a TGFβ-dependent manner. Surprisingly, mitochondrial antiviral signaling protein and the interferon regulator factors 3 and 7 were required for IFN production by TLR2 agonists. Together, these data suggest that TGFβ modulates AM metabolic networks and innate immune signaling cascades to control inflammatory pathways in AMs.

Mononuclear phagocyte sub-types in vitro display diverse transcriptional responses to dust mite exposure

Mononuclear phagocytes (MNP), including macrophages and dendritic cells form an essential component of primary responses to environmental hazards and toxic exposures. This is particularly important in disease conditions such as asthma and allergic airway disease, where many different cell types are present. In this study, we differentiated CD34+ haematopoietic stem cells towards different populations of MNP in an effort to understand how different cell subtypes present in inflammatory disease microenvironments respond to the common allergen house dust mite (HDM). Using single cell mRNA sequencing, we demonstrate that macrophage subtypes MCSPP1+ and MLCMARCO+ display different patterns of gene expression after HDM challenge, noted especially for the chemokines CXCL5, CXCL8, CCL5 and CCL15. MLCCD206Hi alternatively activated macrophages displayed the greatest changes in expression, while neutrophil and monocyte populations did not respond. Further work investigated how pollutant diesel exhaust particles could modify these transcriptional responses and revealed that CXC but not CC type chemokines were further upregulated. Through the use of diesel particles with adsorbed material removed, we suggest that soluble pollutants on these particles are the active constituents responsible for the modifying effects on HDM. This study highlights that environmental exposures may influence tissue responses dependent on which MNP cell type is present, and that these should be considerations when modelling such events in vitro. Understanding the nuanced responsiveness of different immune cell types to allergen and pollutant exposure also contributes to a better understanding of how these exposures influence the development and exacerbation of human disease.

Single-cell profiling of bronchoalveolar cells reveals a Th17 signature in neutrophilic severe equine asthma

Severe equine asthma (SEA) is a complex respiratory condition characterized by chronic airway inflammation. It shares many clinical and pathological features with human neutrophilic asthma, making it a valuable model for studying this condition. However, the immune mechanisms driving SEA have remained elusive. Although SEA has been primarily associated with a Th2 response, there have also been reports of Th1, Th17, or mixed-mediated responses. To uncover the elusive immune mechanisms driving SEA, we performed single-cell mRNA sequencing (scRNA-seq) on cryopreserved bronchoalveolar cells from 11 Warmblood horses, 5 controls and 6 with SEA. We identified six major cell types, including B cells, T cells, monocytes-macrophages, dendritic cells, neutrophils, and mast cells. All cell types exhibited significant heterogeneity, with previously identified and novel cell subtypes. Notably, we observed monocyte-lymphocyte complexes and detected a robust Th17 signature in SEA, with CXCL13 upregulation in intermediate monocytes. Asthmatic horses exhibited expansion of the B-cell population, Th17 polarization of the T-cell populations, and dysregulation of genes associated with T-cell function. Neutrophils demonstrated enhanced migratory capacity and heightened aptitude for neutrophil extracellular trap formation. These findings provide compelling evidence for a predominant Th17 immune response in neutrophilic SEA, driven by dysregulation of monocyte and T-cell genes. The dysregulated genes identified through scRNA-seq have potential as biomarkers and therapeutic targets for SEA and provide insights into human neutrophilic asthma.

Effects of lysine deacetylase inhibitor treatment on LPS responses of alveolar-like macrophages

Macrophages are key immune cells that can adapt their metabolic phenotype in response to different stimuli. Lysine deacetylases are important enzymes regulating inflammatory gene expression and lysine deacetylase inhibitors have been shown to exert anti-inflammatory effects in models of chronic obstructive pulmonary disease. We hypothesized that these anti-inflammatory effects may be associated with metabolic changes in macrophages. To validate this hypothesis, we used an unbiased and a targeted proteomic approach to investigate metabolic enzymes, as well as liquid chromatography-mass spectrometry and gas chromatography-mass spectrometry, to quantify metabolites in combination with the measurement of functional parameters in primary murine alveolar-like macrophages after lipopolysaccharide-induced activation in the presence or absence of lysine deacetylase inhibition. We found that lysine deacetylase inhibition resulted in reduced production of inflammatory mediators such as tumor necrosis factor α and interleukin 1β. However, only minor changes in macrophage metabolism were observed, as only one of the lysine deacetylase inhibitors slightly increased mitochondrial respiration while no changes in metabolite levels were seen. However, lysine deacetylase inhibition specifically enhanced expression of proteins involved in ubiquitination, which may be a driver of the anti-inflammatory effects of lysine deacetylase inhibitors. Our data illustrate that a multiomics approach provides novel insights into how macrophages interact with cues from their environment. More detailed studies investigating ubiquitination as a potential driver of lysine deacetylase inhibition will help developing novel anti-inflammatory drugs for difficult-to-treat diseases such as chronic obstructive pulmonary disease.

Lung regeneration: diverse cell types and the therapeutic potential

Lung tissue has a certain regenerative ability and triggers repair procedures after injury. Under controllable conditions, lung tissue can restore normal structure and function. Disruptions in this process can lead to respiratory system failure and even death, causing substantial medical burden. The main types of respiratory diseases are chronic obstructive pulmonary disease (COPD), idiopathic pulmonary fibrosis (IPF), and acute respiratory distress syndrome (ARDS). Multiple cells, such as lung epithelial cells, endothelial cells, fibroblasts, and immune cells, are involved in regulating the repair process after lung injury. Although the mechanism that regulates the process of lung repair has not been fully elucidated, clinical trials targeting different cells and signaling pathways have achieved some therapeutic effects in different respiratory diseases. In this review, we provide an overview of the cell type involved in the process of lung regeneration and repair, research models, and summarize molecular mechanisms involved in the regulation of lung regeneration and fibrosis. Moreover, we discuss the current clinical trials of stem cell therapy and pharmacological strategies for COPD, IPF, and ARDS treatment. This review provides a reference for further research on the molecular and cellular mechanisms of lung regeneration, drug development, and clinical trials.

Combined Host-Pathogen Fate Mapping to Investigate Lung Macrophages in Viral Infection

Macrophage identity, as defined by epigenetic, transcriptional, proteomic, and functional programs, is greatly impacted by cues originating from the microenvironment. As a consequence, immunophenotyping based on surface marker expression is established and reliable in homeostatic conditions, whereas environmental challenges, in particular infections, severely hamper the determination of identity states. This has become more evident with recent discoveries that macrophage-inherent plasticity may go beyond limits of lineage-defining immunophenotypes. Therefore, transgenic fate mapping tools, such as the phage-derived loxP-cre-system, are essential for the analysis of macrophage adaptation in the tissue under extreme environmental conditions, for example, upon encounter with pathogens. In this chapter, we describe an advanced application of the loxP-cre-system during infection. Here, the host encodes a cell type-specific cre-recombinase, while the pathogen harbors a STOP-floxed fluorescent reporter gene. As an instructive example for the versatility of the system, we demonstrate that alveolar macrophages are predominantly targeted after respiratory tract infection with mouse cytomegalovirus (MCMV). Combined host-pathogen fate mapping not only enables to distinguish between infected and non-infected (bystander) macrophages but also spurs exploration of phenotypic adaptation and tracing of cellular localization in the context of MCMV infection. Moreover, we provide a gating strategy for resolving the diversity of pulmonary immune cell populations.

Macrophages in Lung Repair and Fibrosis

Macrophages are key regulators of tissue repair and fibrosis. Following injury, macrophages undergo marked phenotypic and functional changes to play crucial roles throughout the phases of tissue repair. Idiopathic Pulmonary Fibrosis, which is the most common fibrosing lung disease, has been described as an aberrant reparative response to repetitive alveolar epithelial injury in a genetically susceptible aging individual. The marked destruction of the lung architecture results from the excessive secretion of extracellular matrix by activated fibroblasts and myofibroblasts. Accumulating evidence suggests that macrophages have a pivotal regulatory role in pulmonary fibrosis. The origins and characteristics of macrophages in the lung and their role in regulating lung homeostasis, repair, and fibrosis are reviewed herein. We discuss recent studies that have employed single-cell RNA-sequencing to improve the identification and characterization of macrophage populations in the context of homeostatic and fibrotic conditions. We also discuss the current understanding of the macrophage-mediated mechanisms underlying the initiation and progression of pulmonary fibrosis, with a focus on the phenotypic and functional changes that aging macrophages acquire and how these changes ultimately contribute to age-related chronic lung diseases.

A conversation on allergy: recognizing the past and looking to the future

Allergy is an ever-evolving group of disorders, which includes asthma, atopic dermatitis, rhinitis and food allergies and that currently affects over 1 billion people worldwide. This group of disorders has exploded in incidence since around the start of the 20th century, implying that genetics is not solely responsible for its development but that environmental factors have an important role. Here, Fabio Luciani and Jonathan Coquet, in their role as editors at Immunology & Cell Biology, asked nine prominent researchers in the field of allergy to define the term 'allergy', discuss the role of genetics and the environment, nominate the most important discoveries of the past decade and describe the best strategies to combat allergy at the population level going forward.

Natural killer cells contribute to 'hot' tumor regression in the allergic inflammatory environment

Systemic immune status influences the elimination of tumor cells. However, it remains unclear how chronic inflammation in allergic diseases affects the tumor microenvironment and tumorigenesis. To investigate tumor progression in a state of heightened allergic inflammation, we established a mouse model of allergic inflammation. We used house dust mite extract to induce a hyper-reactive systemic immune response. Additionally, we subcutaneously inoculated two types of cancer cells (CT26 and 4T1 tumors). We conducted immune profiling of the ex-vivo tumor mass using multicolor flow cytometry staining and performed dynamic analysis of peripheral cytokines to explore the significant relationship between the development of allergic inflammation and tumorigenesis. We found that mice in a state of allergic inflammation were more susceptible to developing tumors. Interestingly, the growth of T cell-inflamed was inhibited in the allergic state, while growth of non-T cell-inflamed was promoted. Further research revealed that natural killer (NK) cells with enhanced tumor-killing or immune-regulating abilities were more active in " hot " tumors. Inhibiting NK cell activity can partially alleviate the impact of allergic inflammation on tumor growth. In summary, our results suggest that NK cells play significant role in suppressing tumor growth in an allergic inflammation mouse model. This phenomenon seems to be closely linked to both the inherent characteristics of the tumor and its interaction with the immune system. The innate immune system can be mobilized to synergize with the adaptive immune system to inhibit tumor growth, which opens a new way for a tumor immunotherapy.

Nafamostat has anti-asthmatic effects associated with suppressed pro-inflammatory gene expression, eosinophil infiltration and airway hyperreactivity

Introduction

Asthma is characterized by an imbalance between proteases and their inhibitors. Hence, an attractive therapeutic option could be to interfere with asthma-associated proteases. Here we exploited this option by assessing the impact of nafamostat, a serine protease inhibitor known to neutralize mast cell tryptase.

Methods

Nafamostat was administered in a mouse model for asthma based on sensitization by house dust mite (HDM) extract, followed by the assessment of effects on airway hyperreactivity, inflammatory parameters and gene expression.

Results

We show that nafamostat efficiently suppressed the airway hyperreactivity in HDM-sensitized mice. This was accompanied by reduced infiltration of eosinophils and lymphocytes to the airways, and by lower levels of pro-inflammatory compounds within the airway lumen. Further, nafamostat had a dampening impact on goblet cell hyperplasia and smooth muscle layer thickening in the lungs of HDM-sensitized animals. To obtain deeper insight into the underlying mechanisms, a transcriptomic analysis was conducted. This revealed, as expected, that the HDM sensitization caused an upregulated expression of numerous pro-inflammatory genes. Further, the transcriptomic analysis showed that nafamostat suppressed the levels of multiple pro-inflammatory genes, with a particular impact on genes related to asthma.

Discussion

Taken together, this study provides extensive insight into the ameliorating effect of nafamostat on experimental asthma, and our findings can thereby provide a basis for the further evaluation of nafamostat as a potential therapeutic agent in human asthma.

Alveolar macrophages and airway hyperresponsiveness associated with respiratory syncytial virus infection

Respiratory syncytial virus (RSV) is a ubiquitous pathogen of viral bronchiolitis and pneumonia in children younger than 2 years of age, which is closely associated with recurrent wheezing and airway hyperresponsiveness (AHR). Alveolar macrophages (AMs) located on the surface of the alveoli cavity are the important innate immune barrier in the respiratory tract. AMs are recognized as recruited airspace macrophages (RecAMs) and resident airspace macrophages (RAMs) based on their origins and roaming traits. AMs are polarized in the case of RSV infection, forming two macrophage phenotypes termed as M1-like and M2-like macrophages. Both M1 macrophages and M2 macrophages are involved in the modulation of inflammatory responses, among which M1 macrophages are capable of pro-inflammatory responses and M2 macrophages are capable of anti-proinflammatory responses and repair damaged tissues in the acute and convalescent phases of RSV infection. Polarized AMs affect disease progression through the alteration of immune cell surface phenotypes as well as participate in the regulation of T lymphocyte differentiation and the type of inflammatory response, which are closely associated with long-term AHR. In recent years, some progress have been made in the regulatory mechanism of AM polarization caused by RSV infection, which participates in acute respiratory inflammatory response and mediating AHR in infants. Here we summarized the role of RSV-infection-mediated AM polarization associated with AHR in infants.

A novel iridoid glycoside leonuride (ajugol) attenuates airway inflammation and remodeling through inhibiting type-2 high cytokine/chemokine activity in OVA-induced asthmatic mice

BACKGROUND

Asthma is a chronic airway disorder with a hallmark feature of airflow obstruction that associated with the remodeling and inflammation in the airway wall. Effective therapy for controlling both remodeling and inflammation is still urgently needed. Leonuride is the main pharmacological component identified from Bu-Shen-Yi-Qi-Tang (BSYQT) which has been traditionally used in treatment of lung diseases. However, no pharmacological effects of leonuride in asthma were reported.

PURPOSE

Here we aimed to investigated whether leonuride provided a therapeutic efficacy in reversing asthma airway remodeling and inflammation and uncover the underlying mechanisms.

STUDY DESIGN AND METHODS

Mouse models of chronic asthma were developed with ovalbumin (OVA) exposure for 8 weeks. Respiratory mechanics, lung histopathology and asthma-related cytokines were examined. Lung tissues were analyzed using RNA sequencing to reveal the transcriptional profiling changes.

RESULTS

After oral administration with leonuride (15 mg/kg or 30 mg/kg), mice exhibited a lower airway hyperresponsiveness in comparison to asthmatic mice. Leonuride suppressed airway inflammation evidenced by the significant reductions in accumulation of inflammatory cells around bronchi and vessels, leukocyte population counts and the abundance of type 2 inflammatory mediators (OVA specific IgE, IL-4, IL-5 and IL-13) in bronchoalveolar lavage fluid (BALF). On the other hand, leonuride slowed down the process of active remodeling as demonstrated by weaker goblet cell metaplasia and subepithelial fibrosis in lung histopathology and lower transforming growth factor (TGF)-β1 levels in serum and BALF in comparison to mice treated with OVA only. Furthermore, we uncovered transcriptional profiling alternations in lung tissue of mice after OVA exposure and leonuride treatment. Gene sets belonging to type-2 cytokine/chemokine activity stood out in leonuride target transcripts. Those upregulated (Bmp10, Ccl12, Ccl22, Ccl8, Ccl9, Cxcl15, Il13, Il33, Tnfrsf9, Il31ra, Il5ra, Il13ra2 and Ccl24) or downregulated (Acvr1c and Il18) genes in asthmatic mice, were all reversely regulated by leonuride treatment.

CONCLUSIONS

Our results revealed the therapeutic efficacy of leonuride in experimental chronic asthma for the first time, and implied that its anti-inflammatory and antifibrotic properties might be mediated by regulation of type-2 high cytokine/chemokines responses.

Biology of lung macrophages in health and disease

Tissue-resident alveolar and interstitial macrophages and recruited macrophages are critical players in innate immunity and maintenance of lung homeostasis. Until recently, assessing the differential functional contributions of tissue-resident versus recruited macrophages has been challenging because they share overlapping cell surface markers, making it difficult to separate them using conventional methods. This review describes how scRNA-seq and spatial transcriptomics can separate these subpopulations and help unravel the complexity of macrophage biology in homeostasis and disease. First, we provide a guide to identifying and distinguishing lung macrophages from other mononuclear phagocytes in humans and mice. Second, we outline emerging concepts related to the development and function of the various lung macrophages in the alveolar, perivascular, and interstitial niches. Finally, we describe how different tissue states profoundly alter their functions, including acute and chronic lung disease, cancer, and aging.

Roles of type 1 regulatory T (Tr1) cells in allergen-specific immunotherapy

Allergen-specific immunotherapy (AIT) is the only causative treatment for allergic diseases by modification of the immune response to allergens. A key feature of AIT is to induce immunotolerance to allergens by generating antigen-specific regulatory T (Treg) cells in allergic patients. Type 1 regulatory T (Tr1) cells and forkhead box protein 3 (Foxp3)-expressing Treg cells are well known among Treg cell subsets. Foxp3 was identified as a master transcription factor of Treg cells, and its expression is necessary for their suppressive activity. In contrast to Foxp3⁺ Treg cells, the master transcription factor of Tr1 cells has not been elucidated. Nevertheless, Tr1 cells are generally considered as a distinct subset of Treg cells induced in the periphery during antigen exposure in tolerogenic conditions and can produce large amounts of anti-inflammatory cytokines such as interleukin-10 and transforming growth factor-β, followed by down-regulation of the function of effector immune cells independently of Foxp3 expression. Since the discovery of Tr1 cells more than 20 years ago, research on Tr1 cells has expanded our understanding of the mechanism of AIT. Although the direct precursors and true identity of these cells continues to be disputed, we and others have demonstrated that Tr1 cells are induced in the periphery by AIT, and the induced cells are re-activated by antigens, followed by suppression of allergic symptoms. In this review, we discuss the immune mechanisms for the induction of Tr1 cells by AIT and the immune-suppressive roles of Tr1 cells in AIT.

CD116+ fetal precursors migrate to the perinatal lung and give rise to human alveolar macrophages

Despite their importance in lung health and disease, it remains unknown how human alveolar macrophages develop early in life. Here we define the ontogeny of human alveolar macrophages from embryonic progenitors in vivo, using a humanized mouse model expressing human cytokines (MISTRG mice). We identified alveolar macrophage progenitors in human fetal liver that expressed the GM-CSF receptor CD116 and the transcription factor MYB. Transplantation experiments in MISTRG mice established a precursor-product relationship between CD34-CD116+ fetal liver cells and human alveolar macrophages in vivo. Moreover, we discovered circulating CD116+CD64-CD115+ macrophage precursors that migrated from the liver to the lung. Similar precursors were present in human fetal lung and expressed the chemokine receptor CX3CR1. Fetal CD116+CD64- macrophage precursors had a proliferative gene signature, outcompeted adult precursors in occupying the perinatal alveolar niche, and developed into functional alveolar macrophages. The discovery of the fetal alveolar macrophage progenitor advances our understanding of human macrophage origin and ontogeny.

The impact of the lung environment on macrophage development, activation and function: diversity in the face of adversity

The last decade has been somewhat of a renaissance period for the field of macrophage biology. This renewed interest, combined with the advent of new technologies and development of novel model systems to assess different facets of macrophage biology, has led to major advances in our understanding of the diverse roles macrophages play in health, inflammation, infection and repair, and the dominance of tissue environments in influencing all of these areas. Here, we discuss recent developments in our understanding of lung macrophage heterogeneity, ontogeny, metabolism and function in the context of health and disease, and highlight core conceptual advances and key unanswered questions that we believe should be focus of work in the coming years.

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.

Respiratory Viral and Bacterial Factors That Influence Early Childhood Asthma

Asthma is a chronic respiratory condition characterised by episodes of shortness of breath due to reduced airway flow. The disease is triggered by a hyperreactive immune response to innocuous allergens, leading to hyper inflammation, mucus production, changes in structural cells lining the airways, and airway hyperresponsiveness. Asthma, although present in adults, is considered as a childhood condition, with a total of about 6.2 million children aged 18 and below affected globally. There has been progress in understanding asthma heterogeneity in adults, which has led to better patient stratification and characterisation of multiple asthma endotypes with distinct, but overlapping inflammatory features. The asthma inflammatory profile in children is not well-defined and heterogeneity of the disease is less described. Although many factors such as genetics, food allergies, antibiotic usage, type of birth, and cigarette smoke exposure can influence asthma development particularly in children, respiratory infections are thought to be the major contributing factor in poor lung function and onset of the disease. In this review, we focus on viral and bacterial respiratory infections in the first 10 years of life that could influence development of asthma in children. We also review literature on inflammatory immune heterogeneity in asthmatic children and how this overlaps with early lung development, poor lung function and respiratory infections. Finally, we review animal studies that model early development of asthma and how these studies could inform future therapies and better understanding of this complex disease.

Functional Contribution and Targeted Migration of Group-2 Innate Lymphoid Cells in Inflammatory Lung Diseases: Being at the Right Place at the Right Time

During the last decade, group-2 innate lymphoid cells (ILC2s) have been discovered and successfully established as crucial mediators of lung allergy, airway inflammation and fibrosis, thus affecting the pathogenesis and clinical course of many respiratory diseases, like for instance asthma, cystic fibrosis and chronic rhinosinusitis. As an important regulatory component in this context, the local pulmonary milieu at inflammatory tissue sites does not only determine the activation status of lung-infiltrating ILC2s, but also influences their motility and migratory behavior. In general, many data collected in recent murine and human studies argued against the former concept of a very strict tissue residency of innate lymphoid cells (ILCs) and instead pointed to a context-dependent homing capacity of peripheral blood ILC precursors and the inflammation-dependent capacity of specific ILC subsets for interorgan trafficking. In this review article, we provide a comprehensive overview of the so far described molecular mechanisms underlying the pulmonary migration of ILC2s and thereby the numeric regulation of local ILC2 pools at inflamed or fibrotic pulmonary tissue sites and discuss their potential to serve as innovative therapeutic targets in the treatment of inflammatory lung diseases.