The role of macrophages in healing the wounded lung
Abstract
Acute tissue injury is often considered in the context of a wound. The host response to wounding is an orchestrated series of events, the fundamentals of which are preserved across all multicellular organisms. In the human lung, there are a myriad of causes of injury, but only a limited number of consequences: complete resolution, persistent and/or overwhelming inflammation, a combination of resolution/remodelling with fibrosis or progressive fibrosis. In all cases where complete resolution does not occur, there is the potential for significant ongoing morbidity and ultimately death through respiratory failure. In this review, we consider the elements of injury, resolution and repair as they occur in the lung. We specifically focus on the role of the macrophage, long considered to have a pivotal role in regulating the host response to injury and tissue repair.
... AMs can also act as accessory cells by presenting antigens to T lymphocytes to facilitate adaptive immune responses [11,12]. Finally, upon pathogen clearance, AMs play a critical role in the repair and remodeling of connective tissue in the lung parenchyma [13]. Therefore, AMs are the main gatekeepers orchestrating pulmonary immune balance and tissue repair processes during homeostasis and inflammation. ...
... 23 Some studies have proposed that latent intracellular mycobacteria infection can raise a dysfunction of macrophages, resulting in persistent and excessive activation of inflammatory immunity. 24,25 The aforementioned processes are pivotal in the airway remodeling process that leads to chronic airflow obstruction. 26 Age is often listed as a risk factor for COPD because there is a physiologic decline in lung function with age. ...
Background
Prior pulmonary tuberculosis (PTB) might be associated with the development of chronic obstructive pulmonary disease (COPD). However, the impact of prior PTB on the risk of incident COPD has not been studied in a large prospective cohort study of the European population.
Objectives
This study aimed to investigate the association of prior PTB with the risk of COPD.
Design
Prospective cohort study.
Methods
A multivariable Cox proportional model was used to estimate the hazard ratio (HR) and 95% confidence interval (95% CI) for the association of prior PTB with COPD. Subgroup analyses were further conducted among individuals stratified by age, sex, body mass index, smoking status, drinking status, physical activity, and polygenic risk score (PRS).
Results
The study involved a total of 216,130 participants, with a median follow-up period of 12.6 years and 2788 incident cases of COPD. Individuals with a prior history of PTB at baseline had an 87% higher risk of developing incident COPD compared to those without such history [adjusted hazard ratio (aHR) = 1.87; 95% confidence interval (CI): 1.26–2.77; p = 0.002]. Subgroup analysis revealed that individuals having prior PTB history presented a higher risk of incident COPD among individuals who were aged from 50 to 59 years with aHR of 2.47 (1.02–5.95, p = 0.044), older than 59 years with aHR of 1.81 (1.16–2.81, p = 0.008), male with aHR of 2.37 (1.47–3.83, p < 0.001), obesity with aHR of 3.35 (2.16–5.82, p < 0.001), previous smoking with aHR of 2.27 (1.39–3.72, p < 0.001), current drinking with aHR of 1.98 (1.47–3.83, p < 0.001), low physical activity with aHR of 2.62 (1.30–5.26, p = 0.007), and low PRS with aHR of 3.24 (1.61–6.53, p < 0.001), as well as high PRS with aHR of 2.43 (1.15–5.14, p = 0.019).
Conclusion
A history of PTB is an important independent risk factor for COPD. Clinical staff should be aware of this risk factor in patients with prior PTB, particularly in countries or regions with high burdens of PTB.
... Due to their multiscale geometry with a set of characteristic distances ranging from a few micrometers lattice spacing down to 500 nm features [12], fractal-like inorganic architectures arrayed over a substrate can serve as biomimetics, such as tissue and wound healing applications. In fact, the most significant representation of fractal geometries in the human tissues is the lungs [13,14], where lung-resident macrophages play a key role in regulating both injury and tissue repair [15]. Among these macrophages, two main populations exist: M1, which inhibits cell proliferation, and M2, which promotes cell proliferation and tissue repair. ...
In vitro cellular models denote a crucial part of drug discovery programs as they aid in identifying successful drug candidates based on their initial efficacy and potency. While tremendous headway has been achieved in improving 2D and 3D culture techniques, there is still a need for physiologically relevant systems that can mimic or alter cellular responses without the addition of external biochemical stimuli. A way forward to alter cellular responses is using physical cues, like 3D topographical inorganic substrates, to differentiate macrophage-like cells. Herein, protein secretion and gene expression markers for various macrophage subsets cultivated on a 3D topographical substrate are investigated. The results show that macrophages differentiate into anti-inflammatory M2-type macrophages, secreting increased IL-10 levels compared to the controls. Remarkably, these macrophage cells are differentiated into the M2d subset, making up the main component of tumour-associated macrophages (TAMs), as measured by upregulated Il-10 and Vegf mRNA. M2d subset differentiation is attributed to the topographical substrates with 3D fractal-like geometries arrayed over the surface, else primarily achieved by tumour-associated factors in vivo. From a broad perspective, this work paves the way for implementing 3D topographical inorganic surfaces for drug discovery programs, harnessing the advantages of in vitro assays without external stimulation and allowing the rapid characterisation of therapeutic modalities in physiologically relevant environments.
... Macrophages are phenotypically plastic cells, with their transcriptional make-up being highly dependent on their environment and the external stimulant. Consistent with this, their function and role in fibrosis changes with disease progression (Alber et al. 2012;Zhang et al. 2018;Ogawa et al. 2021). At the early stages of the disease, characterized by the inflammation of epithelial cells lining the airways, 'classically activated' macrophages are observed (Fig. 4A). ...
Comparing molecular features, including the identification of genes with differential expression (DE) between conditions, is a powerful approach for characterising disease-specific phenotypes. When testing for DE in single-cell RNA sequencing data, current pipelines first assign cells into discrete clusters (or cell types), followed by testing for differences within each cluster. Consequently, the sensitivity and specificity of DE testing are limited and ultimately dictated by the granularity of the cell type annotation, with discrete clustering being especially suboptimal for continuous trajectories. To overcome these limitations, we present miloDE - a cluster-free framework for differential expression testing. We build on the Milo approach, introduced for differential cell abundance testing, which leverages the graph representation of single-cell data to assign relatively homogenous, neighbouring cells into overlapping neighbourhoods. We address key differences between differential abundance and expression testing at the level of neighbourhood assignment, statistical testing, and multiple testing correction. To illustrate the performance of miloDE we use both simulations and real data, in the latter case identifying a transient haemogenic endothelia-like state in chimeric mouse embryos lacking Tal1 as well as uncovering distinct transcriptional programs that characterise changes in macrophages in patients with Idiopathic Pulmonary Fibrosis. miloDE is available as an open-source R package at https://github.com/MarioniLab/miloDE.
... Once the lung microbiota is unbalanced, it will result in aberrant immunological signal transduction. On the one hand, the above biological effects will cause the reproduction of pathogenic bacteria in a vicious cycle, further damage the pulmonary microbiota, and participate in the occurrence and development of lung fibrosis (5,6). On the other hand, the unbalanced microbiota will cause the antigen-presenting cell (mainly macrophage), B cell, T cell, and natural killer cell to participate in pathological biological effects. ...
Pulmonary fibrosis is an irreversible disease, and its mechanism is unclear. The lung is a vital organ connecting the respiratory tract and the outside world. The changes in lung microbiota affect the progress of lung fibrosis. The latest research showed that lung microbiota differs in healthy people, including idiopathic pulmonary fibrosis (IPF) and acute exacerbation-idiopathic pulmonary fibrosis (AE-IPF). How to regulate the lung microbiota and whether the potential regulatory mechanism can become a necessary targeted treatment of IPF are unclear. Some studies showed that immune response and lung microbiota balance and maintain lung homeostasis. However, unbalanced lung homeostasis stimulates the immune response. The subsequent biological effects are closely related to lung fibrosis. Core fucosylation (CF), a significant protein functional modification, affects the lung microbiota. CF regulates immune protein modifications by regulating key inflammatory factors and signaling pathways generated after immune response. The treatment of immune regulation, such as antibiotic treatment, vitamin D supplementation, and exosome micro-RNAs, has achieved an initial effect in clearing the inflammatory storm induced by an immune response. Based on the above, the highlight of this review is clarifying the relationship between pulmonary microbiota and immune regulation and identifying the correlation between the two, the impact on pulmonary fibrosis, and potential therapeutic targets.
... Conversely, M2 macrophages produce anti-inflammatory cytokines to resolve ongoing inflammation while initiating tissue repair [66]. In the lungs, this polarization shift from inflammatory M1 to reparative M2 is key to resolving pulmonary inflammation and inducing tissue repair after infection [67]. Combined preclinical and clinical evidence indicates increased ACh/cholinergic signal transmission alters inflammatory cytokine expression, limits pulmonary damage and decreases overall morbidity and mortality during respiratory injury and infection. ...
The lungs are continuously exposed to diverse array of microbes, organic and inorganic particulate materials. Pulmonary immunity determines the outcome of infections and failure of immune defenses may lead to development of pneumonia. Predisposing factors weaken the lung defenses and lead to infection with any pathogens including viruses, bacteria, parasites, and fungi. Pneumonia is an inflammation of the lungs that can cause mild to severe illness in animals of all ages. Pneumonia is one of the leading causes of death due to infection in animals in younger ages worldwide. Very young, older animals and animals with underlying health problems or immunosuppression were at high risk for pneumonia. Pathogens could interfere with different respiratory defenses, allowing two or more bacteria to colonize the lungs. Inhalants like microbes and environmental particles need to be eliminated quickly by the immune system, as failure to do so can lead to inflammatory responses that result in swelling that closes the airways results in increased incidence of infections. Most viruses and bacteria cause ciliary dysfunction and alteration of the mucosal surfaces that predisposes the bacterial and/or viral pneumonia. Further, virus and bacterial infections cause alteration of alveolar macrophage function, suppression of lymphocyte proliferation, induces apoptosis, and modified cytokine and other inflammatory mediator release resulted in modification of innate and adaptive immune responses. Viscosity of the mucus interferes with clearance, leading to airway obstruction, colonization with bacteria, and impaired migration and bactericidal activity of neutrophils. Elevated ammonia levels are associated with overcrowding and reduced ventilation results in ciliary dysfunction, degeneration and inflammation of the nasal epithelium, and increased susceptibility to pneumonia.
The understanding of pathogenesis underlying idiopathic pulmonary fibrosis (IPF) is still limited presently. Monocytes or macrophages are involved in progression of the pulmonary injury and repair. The aim of this study is to investigate the roles of CD11b+ monocytes/macrophages in the progression of pulmonary fibrosis. In this study, the expression levels of CD11B gene and inflammatory genes in the IPF patients are evaluated using the available datasets. CD11b cells are conditionally depleted in a CD11b-diptheria toxin receptor (CD11b-DTR) mouse by administration of diptheria toxin (DT). Pulmonary fibrosis in mice is induced using intranasalbleomycin. The mRNAs and proteins expression in lung tissues are determined by quantitative real-time polymerase chain reaction (qRT-PCR), immunofluorescence (IF) staining and Western-blot assays. It shows that the expression of CD11B mRNA is up-regulated in fibrotic lungs and alveolar macrophages of IPF patients and bleomycin-treated rodents. Selective depletion of CD11b+ monocytes/macrophages in CD11b-DTR mice potently halts bleomycin-induced pulmonary fibrosis progression. CD11b depletion inhibits the polarization of macrophages in the fibrotic lungs. Mechanically, CD11b deficiency represses the activation of sphingosine 1-phosphate receptor 2 (S1PR2)/sphingosine kinase 2 (SphK2) signaling during pulmonary fibrosis. In conclusion, our data suggest that CD11b+ monocytes/macrophages contribute to pulmonary fibrosis and represent a potential therapeutic target for IPF.
The coronavirus disease 2019 (COVID-19) pandemic has caused more than 6.3 million deaths to date. Despite great efforts to curb the spread of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), vaccines and neutralizing antibodies are in the gloom due to persistent viral mutations and antiviral compounds face challenges of specificity and safety. In addition, vaccines are unable to treat already-infected individuals, and antiviral drugs cannot be used prophylactically. Therefore, exploration of unconventional strategies to curb the current pandemic is highly urgent. Alveolar macrophages (AMs) residing on the surface of alveoli are the first immune cells that dispose of alveoli-invading viruses. Our findings demonstrate that M1 AMs have an acidic endosomal pH, thus favoring SARS-CoV-2 to leave endosomes and release into the cytosol where the virus initiates replication; in contrast, M2 AMs have an increased endosomal pH, which dampens the viral escape and facilitates delivery of the virus for lysosomal degradation. In this review, we propose that AMs are the Achilles’ heel of SARS-CoV-2 infection and that modulation of the endosomal pH of AMs has the potential to eliminate invaded SARS-CoV-2; the same strategy might also be suitable for other lethal respiratory viruses.
Lung macrophages play a crucial role in the maintenance of homeostasis of lungs by combating the invaded foreign particles. However, the physiology of macrophages was exploited by the pathogens where they can cloak and thrive inside; subsequently the treatment for such conditions will become a challenging task. To combat this condition, specialized engineered medicines are essential. Therefore, this chapter is focused on the lung macrophages along with their particle uptake mechanism, followed by passive targeting of the lung macrophages via manipulation of particle properties and active targeting approach via tethering with different kinds of ligands.
Lung macrophages are long living cells with broad differentiation potential, which reside in the lung interstitium and alveoli or are organ-recruited upon inflammatory stimuli. A role of resident and recruited macrophages in initiating and maintaining pulmonary inflammation in lung infection or injury has been convincingly demonstrated. More recent reports suggest that lung macrophages are main orchestrators of termination and resolution of inflammation. They are also initiators of parenchymal repair processes that are essential for return to homeostasis with normal gas exchange. In this review we will discuss cellular cross-talk mechanisms and molecular pathways of macrophage plasticity which define their role in inflammation resolution and in initiation of lung barrier repair following lung injury.
The co-ordinated recruitment of monocyte subpopulations, neutrophils and regulatory T-cells (Tregs) during the early stages of human acute lung inflammation remains poorly understood. We therefore performed a detailed characterisation of these lineages in the blood and lungs in a model of human acute lung inflammation. Healthy volunteers inhaled lipopolysaccharide (LPS) or saline (n=6 for each group). Blood was collected at 0, 2, 4, 6 and 8 h and bronchoscopy with bronchoalveolar lavage (BAL) performed at 8 h. Multiparameter flow cytometry was used to characterise monocyte subpopulations, neutrophils and Tregs in the blood and lung. Inhalation of LPS was associated with significant blood and BAL fluid neutrophilia. Blood populations of monocyte subpopulations and Tregs were unaltered by LPS. In contrast, LPS induced an accumulation of a pulmonary monocyte-like cell (PMLC) population, which was further subdivided into "inducible" CD14(++)CD16(-) and "resident" CD14(++)CD16(+) subsets. Inducible PMLCs were significantly increased following LPS inhalation (p=0.0046), whereas resident PMLCs were unchanged. In addition, we noted a significant decrease in Tregs in BAL fluid with LPS inhalation (p=0.027). The early stages of LPS-induced inflammation in humans is characterised by pulmonary accumulation of a novel inducible monocyte-like subpopulation, accompanied by significant changes in both neutrophil and Treg numbers.
Helminths induce potent T helper 2 (TH2)-type immune responses that can mediate worm expulsion, but the role of this response in controlling the acute tissue damage caused by migrating multicellular parasites through vital tissues remains uncertain. We used a helminth infection model in which parasitic nematode larvae migrate transiently through the lung, resulting in hemorrhage and inflammation. We found that IL-17 initially contributed to inflammation and lung damage, whereas subsequent IL-4 receptor (IL-4R) signaling reduced elevations in IL-17 mRNA levels, enhanced the expression of insulin-like growth factor 1 (IGF-1) and IL-10 and stimulated the development of M2 macrophages, all of which contributed to the rapid resolution of tissue damage. These studies indicate an essential role for TH2-type immune responses in mediating acute wound healing during helminth infection.
We recently demonstrated that ω-3-polyunsaturated fatty acids ameliorate obesity-induced adipose tissue inflammation and insulin resistance. In this study, we report novel mechanisms underlying ω-3-polyunsaturated fatty acid actions on adipose tissue, adipocytes, and stromal vascular cells (SVC). Inflamed adipose tissue from high-fat diet-induced obese mice showed increased F4/80 and CD11b double-positive macrophage staining and elevated IL-6 and MCP-1 levels. Docosahexaenoic acid (DHA; 4 μg/g) did not change the total number of macrophages but significantly reduced the percentage of high CD11b/high F4/80-expressing cells in parallel with the emergence of low-expressing CD11b/F4/80 macrophages in the adipose tissue. This effect was associated with downregulation of proinflammatory adipokines in parallel with increased expression of IL-10, CD206, arginase 1, resistin-like molecule α, and chitinase-3 like protein, indicating a phenotypic switch in macrophage polarization toward an M2-like phenotype. This shift was confined to the SVC fraction, in which secretion of Th1 cytokines (IL-6, MCP-1, and TNF-α) was blocked by DHA. Notably, resolvin D1, an anti-inflammatory and proresolving mediator biosynthesized from DHA, markedly attenuated IFN-γ/LPS-induced Th1 cytokines while upregulating arginase 1 expression in a concentration-dependent manner. Resolvin D1 also stimulated nonphlogistic phagocytosis in adipose SVC macrophages by increasing both the number of macrophages containing ingested particles and the number of phagocytosed particles and by reducing macrophage reactive oxygen species production. No changes in adipocyte area and the phosphorylation of hormone-sensitive lipase, a rate-limiting enzyme regulating adipocyte lipolysis, were observed. These findings illustrate novel mechanisms through which resolvin D1 and its precursor DHA confer anti-inflammatory and proresolving actions in inflamed adipose tissue.
TLRs are a family of receptors that mediate immune system pathogen recognition. In the respiratory system, TLR activation has both beneficial and deleterious effects in asthma. For example, clinical data indicate that TLR6 activation exerts protective effects in asthma. Here, we explored the mechanism or mechanisms through which TLR6 mediates this effect using mouse models of Aspergillus fumigatus-induced and house dust mite antigen-induced (HDM antigen-induced) chronic asthma. Tlr6-/- mice with fungal- or HDM antigen-induced asthma exhibited substantially increased airway hyperresponsiveness, inflammation, and remodeling compared with WT asthmatic groups. Surprisingly, whole-lung levels of IL-23 and IL-17 were markedly lower in Tlr6-/- versus WT asthmatic mice. Tlr6-/- DCs generated less IL-23 upon activation with lipopolysaccharide, zymosan, or curdlan. Impaired IL-23 generation in Tlr6-/- mice also corresponded with lower levels of expression of the pathogen-recognition receptor dectin-1 and expansion of Th17 cells both in vivo and in vitro. Exogenous IL-23 treatment of asthmatic Tlr6-/- mice restored IL-17A production and substantially reduced airway hyperresponsiveness, inflammation, and lung fungal burden compared with that in untreated asthmatic Tlr6-/- mice. Together, our data demonstrate that TLR6 activation is critical for IL-23 production and Th17 responses, which both regulate the allergic inflammatory response in chronic fungal-induced asthma. Thus, therapeutics targeting TLR6 activity might prove efficacious in the treatment of clinical asthma.
Macrophages are strategically located throughout the body tissues, where they ingest and process foreign materials, dead cells and debris and recruit additional macrophages in response to inflammatory signals. They are highly heterogeneous cells that can rapidly change their function in response to local microenvironmental signals. In this Review, we discuss the four stages of orderly inflammation mediated by macrophages: recruitment to tissues; differentiation and activation in situ; conversion to suppressive cells; and restoration of tissue homeostasis. We also discuss the protective and pathogenic functions of the various macrophage subsets in antimicrobial defence, antitumour immune responses, metabolism and obesity, allergy and asthma, tumorigenesis, autoimmunity, atherosclerosis, fibrosis and wound healing. Finally, we briefly discuss the characterization of macrophage heterogeneity in humans.
PHD2 serves as an oxygen sensor that rescues blood supply by regulating vessel formation and shape in case of oxygen shortage. However, it is unknown whether PHD2 can influence arteriogenesis. Here we studied the role of PHD2 in collateral artery growth by using hindlimb ischaemia as a model, a process that compensates for the lack of blood flow in case of major arterial occlusion. We show that Phd2 (also known as Egln1) haplodeficient (Phd2(+/-)) mice displayed preformed collateral arteries that preserved limb perfusion and prevented tissue necrosis in ischaemia. Improved arteriogenesis in Phd2(+/-) mice was due to an expansion of tissue-resident, M2-like macrophages and their increased release of arteriogenic factors, leading to enhanced smooth muscle cell (SMC) recruitment and growth. Both chronic and acute deletion of one Phd2 allele in macrophages was sufficient to skew their polarization towards a pro-arteriogenic phenotype. Mechanistically, collateral vessel preconditioning relied on the activation of canonical NF-κB pathway in Phd2(+/-) macrophages. These results unravel how PHD2 regulates arteriogenesis and artery homeostasis by controlling a specific differentiation state in macrophages and suggest new treatment options for ischaemic disorders.
Macrophages are widely distributed immune system cells that play an indispensable role in homeostasis and defense. They can be phenotypically polarized by the microenvironment to mount specific functional programs. Polarized macrophages can be broadly classified in two main groups: classically activated macrophages (or M1), whose prototypical activating stimuli are IFNgamma and LPS, and alternatively activated macrophages (or M2), further subdivided in M2a (after exposure to IL-4 or IL-13), M2b (immune complexes in combination with IL-1beta or LPS) and M2c (IL-10, TGFbeta or glucocorticoids). M1 exhibit potent microbicidal properties and promote strong IL-12-mediated Th1 responses, whilst M2 support Th2-associated effector functions. Beyond infection M2 polarized macrophages play a role in resolution of inflammation through high endocytic clearance capacities and trophic factor synthesis, accompanied by reduced pro-inflammatory cytokine secretion. Similar functions are also exerted by tumor-associated macrophages (TAM), which also display an alternative-like activation phenotype and play a detrimental pro-tumoral role. Here we review the main functions of polarized macrophages and discuss the perspectives of this field.
The repair of wounds is one of the most complex biological processes that occur during human life. After an injury, multiple biological pathways immediately become activated and are synchronized to respond. In human adults, the wound repair process commonly leads to a non-functioning mass of fibrotic tissue known as a scar. By contrast, early in gestation, injured fetal tissues can be completely recreated, without fibrosis, in a process resembling regeneration. Some organisms, however, retain the ability to regenerate tissue throughout adult life. Knowledge gained from studying such organisms might help to unlock latent regenerative pathways in humans, which would change medical practice as much as the introduction of antibiotics did in the twentieth century.
The observation that human monocytes cultured in the presence of the chemokine CCL18 showed increased survival, led us to profile cytokine expression in CCL18-stimulated versus control cultures. CCL18 caused significantly increased expression of chemokines (CXCL8, CCL2, CCL3 and CCL22), interleukin-10 (IL-10) and platelet-derived growth factor, but no up-regulation of M1 cytokines IL-1β or IL-12. CCL18-stimulated monocytes matured into cells with morphological resemblance to IL-4-stimulated macrophages, and expressed the monocyte marker CD14 as well the M2 macrophage markers CD206 and 15-lipoxygenase, but no mature dendritic cell markers (CD80, CD83 or CD86). Functionally, CCL18-stimulated macrophages showed a high capacity for unspecific phagocytosis and for pinocytosis, which was not associated with an oxidative burst. These findings suggest that CCL18-activated macrophages stand at the cross-roads between inflammation and its resolution. The chemokines that are produced in response to CCL18 are angiogenic and attract various leucocyte populations, which sustain inflammation. However, the capacity of these cells to remove cellular debris without causing oxidative damage and the production of the anti-inflammatory IL-10 will initiate termination of the inflammatory response. In summary, CCL18 induces an M2 spectrum macrophage phenotype in the absence of IL-4.