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.

Cigarette smoke-induced airspace disease in mice develops independently of HIF-1α signaling in leukocytes

The pathogenesis of chronic obstructive pulmonary disease (COPD), a prevalent disease primarily caused by cigarette smoke exposure, is incompletely elucidated. Studies in humans and mice have suggested that hypoxia-inducible factor-1α (HIF-1α) may play a role. Reduced lung levels of HIF-1α are associated with decreased vascular density, whereas increased leukocyte HIF-1α may be responsible for increased inflammation. To elucidate the specific role of leukocyte HIF-1α in COPD, we exposed transgenic mice with conditional deletion or overexpression of HIF-1α in leukocytes to cigarette smoke for 7 mo. Outcomes included pulmonary physiology, aerated lung volumes via microcomputed tomography, lung morphometry and histology, and cardiopulmonary hemodynamics. On aggregate, cigarette smoke increased the aerated lung volume, quasi-static lung compliance, inspiratory capacity of all strains while reducing the total alveolar septal volume. Independent of smoke exposure, mice with leukocyte-specific HIF-1α overexpression had increased quasi-static compliance, inspiratory capacity, and alveolar septal volume compared with mice with leukocyte-specific HIF-1α deletion. However, the overall development of cigarette smoke-induced lung disease did not vary relative to control mice for either of the conditional strains. This suggests that the development of murine cigarette smoke-induced airspace disease occurs independently of leukocyte HIF-1α signaling.

Polyamine import and accumulation causes immunomodulation in macrophages engulfing apoptotic cells

Phagocytosis of apoptotic cells, termed efferocytosis, is critical for tissue homeostasis and drives anti-inflammatory programming in engulfing macrophages. Here, we assess metabolites in naive and inflammatory macrophages following engulfment of multiple cellular and non-cellular targets. Efferocytosis leads to increases in the arginine-derived polyamines, spermidine and spermine, in vitro and in vivo. Surprisingly, polyamine accumulation after efferocytosis does not arise from retention of apoptotic cell metabolites or de novo synthesis but from enhanced polyamine import that is dependent on Rac1, actin, and PI3 kinase. Blocking polyamine import prevents efferocytosis from suppressing macrophage interleukin (IL)-1β or IL-6. This identifies efferocytosis as a trigger for polyamine import and accumulation, and imported polyamines as mediators of efferocytosis-induced immune reprogramming.

Isolation and Analysis of Macrophage Subsets from the Mouse and Human Lung

Pulmonary macrophages are heterogeneous. Distinct populations of resident tissue macrophages exist in the lung airspace and tissue compartments during homeostasis. During inflammation, these are joined by monocyte-derived recruited macrophages. Flow cytometry can be used to identify and purify lung macrophage subsets. Here, we describe methods for identifying and isolating macrophages from bronchoalveolar lavage and digested lung tissues from mouse and human. We also describe basic staining for flow cytometry analysis of different macrophage subsets.

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

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

Airspace Macrophages and Monocytes Exist in Transcriptionally Distinct Subsets in Healthy Adults

Rationale: Macrophages are the most abundant immune cell in the alveoli and small airways and are traditionally viewed as a homogeneous population during health. Whether distinct subsets of airspace macrophages are present in healthy humans is unknown. Single-cell RNA sequencing allows for examination of transcriptional heterogeneity between cells and between individuals. Understanding the conserved repertoire of airspace macrophages during health is essential to understanding cellular programing during disease.Objectives: We sought to determine the transcriptional heterogeneity of human cells obtained from BAL of healthy adults.Methods: Ten subjects underwent bronchoscopy with BAL. Cells from lavage were subjected to single-cell RNA sequencing. Unique cell populations and putative functions were identified. Transcriptional profiles were compared across individuals.Measurements and Main Results: We identify two novel subgroups of resident airspace macrophages-defined by proinflammatory and metallothionein gene expression profiles. We define subsets of monocyte-like cells and compare them with peripheral blood mononuclear cells. Finally, we compare global macrophage and monocyte programing between males and females.Conclusions: Healthy human airspaces contain multiple populations of myeloid cells that are highly conserved between individuals and between sexes. Resident macrophages make up the largest population and include novel subsets defined by inflammatory and metal-binding profiles. Monocyte-like cells within the airspaces are transcriptionally aligned with circulating blood cells and include a rare population defined by expression of cell-matrix interaction genes. This study is the first to delineate the conserved heterogeneity of airspace immune cells during health and identifies two previously unrecognized macrophage subsets.

CD73 Promotes Age-Dependent Accretion of Atherosclerosis

OBJECTIVE

CD73 is an ectonucleotidase which catalyzes the conversion of AMP (adenosine monophosphate) to adenosine. Adenosine has been shown to be anti-inflammatory and vasorelaxant. The impact of ectonucleotidases on age-dependent atherosclerosis remains unclear. Our aim was to investigate the role of CD73 in age-dependent accumulation of atherosclerosis. Approach and results: Mice doubly deficient in CD73 and ApoE (apolipoprotein E; (cd73^-/-/apoE^-/-) were generated, and the extent of aortic atherosclerotic plaque was compared with apoE^-/- controls at 12, 20, 32, and 52 weeks. By 12 weeks of age, cd73^-/-/apoE^-/- mice exhibited a significant increase in plaque (1.4±0.5% of the total vessel surface versus 0.4±0.1% in apoE^-/- controls, P<0.005). By 20 weeks of age, this difference disappeared (2.9±0.4% versus 3.3±0.7%). A significant reversal in phenotype emerged at 32 weeks (9.8±1.2% versus 18.3±1.4%; P<0.0001) and persisted at the 52 week timepoint (22.4±2.1% versus 37.0±2.1%; P<0.0001). The inflammatory response to aging was found to be comparable between cd73^-/-/apoE^-/- mice and apoE^-/- controls. A reduction in lipolysis in CD73 competent mice was observed, even with similar plasma lipid levels (cd73^-/-/apoE^-/- versus apoE^-/- at 12 weeks [16.2±0.7 versus 9.5±1.4 nmol glycerol/well], 32 weeks [24.1±1.5 versus 7.4±0.4 nmol/well], and 52 weeks [13.8±0.62 versus 12.7±2.0 nmol/well], P<0.001).

CONCLUSIONS

At early time points, CD73 exerts a subtle antiatherosclerotic influence, but with age, the pattern reverses, and the presence of CD73 promoted suppression of lipid catabolism.

Deletion of c-FLIP from CD11bhi Macrophages Prevents Development of Bleomycin-induced Lung Fibrosis

Idiopathic pulmonary fibrosis is a progressive lung disease with complex pathophysiology and fatal prognosis. Macrophages (MΦ) contribute to the development of lung fibrosis; however, the underlying mechanisms and specific MΦ subsets involved remain unclear. During lung injury, two subsets of lung MΦ coexist: Siglec-F^hi resident alveolar MΦ and a mixed population of CD11b^hi MΦ that primarily mature from immigrating monocytes. Using a novel inducible transgenic system driven by a fragment of the human CD68 promoter, we targeted deletion of the antiapoptotic protein cellular FADD-like IL-1β-converting enzyme-inhibitory protein (c-FLIP) to CD11b^hi MΦ. Upon loss of c-FLIP, CD11b^hi MΦ became susceptible to cell death. Using this system, we were able to show that eliminating CD11b^hi MΦ present 7-14 days after bleomycin injury was sufficient to protect mice from fibrosis. RNA-seq analysis of lung MΦ present during this time showed that CD11b^hi MΦ, but not Siglec-F^hi MΦ, expressed high levels of profibrotic chemokines and growth factors. Human MΦ from patients with idiopathic pulmonary fibrosis expressed many of the same profibrotic chemokines identified in murine CD11b^hi MΦ. Elimination of monocyte-derived MΦ may help in the treatment of fibrosis. We identify c-FLIP and the associated extrinsic cell death program as a potential pathway through which these profibrotic MΦ may be pharmacologically targeted.

Modulation of Myeloid Cell Function Using Conditional and Inducible Transgenic Approaches

Transgenic mice have emerged as a central tool in the study of lung myeloid cells during homeostasis and disease. The use of Cre/Lox site-specific recombination allows for conditional deletion of a gene of interest in a spatially controlled manner. The basic Cre/Lox system can be further refined to include an inducible trigger, enabling conditional deletion of a gene of interest in a spatially and temporally controlled manner. Here we provide an overview of commercially available conditional and inducible conditional mouse strains that target lung myeloid cells and describe the appropriate breeding schemes and controls for transgenic animal systems that can be used to modulate myeloid cell function.

Promoter Specificity and Efficacy in Conditional and Inducible Transgenic Targeting of Lung Macrophages

Conditional and inducible Cre-loxP systems are used to target gene deletion to specific cell lineages and tissues through promoter-restricted expression of the bacterial DNA recombinase, Cre. Although Cre-loxP systems are widely used to target gene deletion in lung macrophages, limited data are published on the specificity and efficiency of “macrophage targeting” Cre lines. Using R26-stop^fl/fl-TdTomato and tetOn-GFP reporter lines, we assessed the specificity and efficiency of four commercially available Cre driver lines that are often considered “macrophage specific.” We evaluated two conditional (Csf1r-Cre and LysM-Cre) and two inducible [CX₃CR1-estrogen receptor-Cre (ERCre) and CD68-rtTA] lines. We assessed Cre activation in six resident lung myeloid populations, as well as activation in lung leukocytes, lung epithelial and endothelial cells, peripheral blood leukocytes, and tissue macrophages of the spleen, bone marrow, and peritoneal cavity. Although Csf1r-Cre and LysM-Cre target resident alveolar macrophages (ResAM) and interstitial macrophages (IM) with high efficiency, neither line is specific for macrophages. Csf1r-Cre targets all leukocyte populations, while LysM-Cre targets dendritic cell, neutrophils, monocytes, and a quarter of lung epithelial cells. CX₃CR1-ERCre and CD68-rtTA both target IM, but do not target ResAM. Further, although neither line is specific for macrophages, a pulse-wait administration of tamoxifen or doxycycline can be used to significantly improve IM specificity in these inducible lines. In summary, while Cre-loxP remains a powerful tool to study macrophage function, numerous pitfalls exist. Herein, we document strengths and weaknesses of Csf1r-Cre, LysM-Cre, CX₃CR1-ERCre, and CD68-rtTA systems for targeting specific macrophage populations in the lungs and provide data that will aid investigators in selecting the proper strain.

Cell Origin Dictates Programming of Resident versus Recruited Macrophages during Acute Lung Injury

Two populations of alveolar macrophages (AMs) coexist in the inflamed lung: resident AMs that arise during embryogenesis, and recruited AMs that originate postnatally from circulating monocytes. The objective of this study was to determine whether origin or environment dictates the transcriptional, metabolic, and functional programming of these two ontologically distinct populations over the time course of acute inflammation. RNA sequencing demonstrated marked transcriptional differences between resident and recruited AMs affecting three main areas: proliferation, inflammatory signaling, and metabolism. Functional assays and metabolomic studies confirmed these differences and demonstrated that resident AMs proliferate locally and are governed by increased tricarboxylic acid cycle and amino acid metabolism. Conversely, recruited AMs produce inflammatory cytokines in association with increased glycolytic and arginine metabolism. Collectively, the data show that even though they coexist in the same environment, inflammatory macrophage subsets have distinct immunometabolic programs and perform specialized functions during inflammation that are associated with their cellular origin.

Three Unique Interstitial Macrophages in the Murine Lung at Steady State

The current paradigm in macrophage biology is that some tissues mainly contain macrophages from embryonic origin, such as microglia in the brain, whereas other tissues contain postnatal-derived macrophages, such as the gut. However, in the lung and in other organs, such as the skin, there are both embryonic and postnatal-derived macrophages. In this study, we demonstrate in the steady-state lung that the mononuclear phagocyte system is comprised of three newly identified interstitial macrophages (IMs), alveolar macrophages, dendritic cells, and few extravascular monocytes. We focused on similarities and differences between the three IM subtypes, specifically, their phenotype, location, transcriptional signature, phagocytic capacity, turnover, and lack of survival dependency on fractalkine receptor, CX₃CR1. Pulmonary IMs were located in the bronchial interstitium but not the alveolar interstitium. At the transcriptional level, all three IMs displayed a macrophage signature and phenotype. All IMs expressed MER proto-oncogene, tyrosine kinase, CD64, CD11b, and CX₃CR1, and were further distinguished by differences in cell surface protein expression of CD206, Lyve-1, CD11c, CCR2, and MHC class II, along with the absence of Ly6C, Ly6G, and Siglec F. Most intriguingly, in addition to the lung, similar phenotypic populations of IMs were observed in other nonlymphoid organs, perhaps highlighting conserved functions throughout the body. These findings promote future research to track four distinct pulmonary macrophages and decipher the division of labor that exists between them.

Three unique interstitial macrophages in the murine lung at steady state

Rationale

The current paradigm in macrophage biology is that some tissues mainly contain macrophages from embryonic origin such as microglia in the brain, while other tissues contain postnatal-derived macrophages, such as the gut. However, in the lung and in other organs such as the skin, there are both embryonic and postnatal-derived macrophages.

Objectives

In this study, we demonstrate in the steady-state lung that the mononuclear phagocyte system is comprised of three newly identified interstitial macrophages (IMs), alveolar macrophages (AMs), dendritic cells and few extravascular monocytes.

Methods

We focused on similarities and differences between the three IM subtypes, specifically, their phenotype, location, transcriptional signature, phagocytic capacity, turnover and lack of survival dependency on CX3CR1.

Measurements and Main Results

Pulmonary IMs were located in the bronchial interstitium but not the alveolar interstitium. At the transcriptional level, all three IMs displayed a macrophage signature. All IMs expressed MerTK+CD64+ CD11b+ CX CR1+ and were furthermore distinguished by differences in cell surface protein expression of CD206, Lyve-1, CD11c, CCR2 and MHCII, along with the absence of Ly6C, Ly6G, and Siglec F. Ex vivo analysis revealed that all three IMs were highly phagocytic compared to AMs. Finally, similar phenotypic populations of IMs were present in other non-lymphoid organs, suggesting that these IMs may not be unique to the lung.

Conclusions

These findings promote future research to track four distinct pulmonary macrophages and decipher the division of labor that exists between them.

Selective and inducible targeting of CD11b+ mononuclear phagocytes in the murine lung with hCD68-rtTA transgenic systems

During homeostasis two distinct macrophage (Mø) populations inhabit the lungs: tissue Mø (often called interstitial Mø) and resident alveolar Mø (resAMø). During acute lung inflammation, monocytes from the circulation migrate to areas of injury where they mature into a third Mø population: recruited Mø. Resident AMø uniquely express low levels of CD11b and high levels of CD11c. In comparison, recruited Mø and tissue Mø express high levels of CD11b and low levels of CD11c. It is likely that these three Mø subpopulations play distinct roles in injury and disease states; however, tools with which to individually target or track these populations are lacking. Here we demonstrate the utility of an hCD68-rtTA transgenic system for specific, robust, and inducible targeting of CD11b(+) recruited Mø and tissue Mø in the murine lung with negligible activation in resAMø. Using hCD68rtTA-GFP reporter mice, we show both during homeostasis and inflammation that administration of doxycycline induces tet-On reporter expression in recruited Mø and tissue Mø but not in resident AMø. We further demonstrate how hCD68-rtTA can be effectively combined with tet-On Cre to target these same recMø and tissue Mø. Accordingly, the hCD68-rtTA system is a powerful new tool that can be used for lineage tracing, fate mapping, and gene deletion in a variety of murine models, thereby enabling sophisticated investigation of the unique role of these CD11b(+) Mø during lung heath and disease.

MicroRNA-34a Negatively Regulates Efferocytosis by Tissue Macrophages in Part via SIRT1

Apoptotic cell (AC) clearance (efferocytosis) is an evolutionarily conserved process essential for immune health, particularly to maintain self-tolerance. Despite identification of many recognition receptors and intracellular signaling components of efferocytosis, its negative regulation remains incompletely understood and has not previously been known to involve microRNAs (miRs). In this article, we show that miR-34a (gene ID 407040), well recognized as a p53-dependent tumor suppressor, mediates coordinated negative regulation of efferocytosis by resident murine and human tissue macrophages (Mø). The miR-34a expression varied greatly between Mø from different tissues, correlating inversely with their capacity for AC uptake. Transient or genetic knockdown of miR-34a increased efferocytosis, whereas miR-34a overexpression decreased efferocytosis, without altering recognition of live, necrotic, or Ig-opsonized cells. The inhibitory effect of miR-34a was mediated both by reduced expression of Axl, a receptor tyrosine kinase known to recognize AC, and of the deacetylase silent information regulator T1, which had not previously been linked to efferocytosis by tissue Mø. Exposure to AC downregulated Mø miR-34a expression, resulting in a positive feedback loop that increased subsequent capacity to engulf AC. These findings demonstrate that miR-34a both specifically regulates and is regulated by efferocytosis. Given the ability of efferocytosis to polarize ingesting Mø uniquely and to reduce their host-defense functions, dynamic negative regulation by miR-34a provides one means of fine-tuning Mø behavior toward AC in specific tissue environments with differing potentials for microbial exposure.

Glucocorticoid-Augmented Efferocytosis Inhibits Pulmonary Pneumococcal Clearance in Mice by Reducing Alveolar Macrophage Bactericidal Function

Inhaled corticosteroids (ICS) increase community-acquired pneumonia (CAP) incidence in patients with chronic obstructive pulmonary disease (COPD) by unknown mechanisms. Apoptosis is increased in the lungs of COPD patients. Uptake of apoptotic cells (ACs) ("efferocytosis") by alveolar macrophages (AMøs) reduces their ability to combat microbes, including Streptococcus pneumoniae, the most common cause of CAP in COPD patients. Having shown that ICS significantly increase AMø efferocytosis, we hypothesized that this process, termed glucocorticoid-augmented efferocytosis, might explain the association of CAP with ICS therapy in COPD. To test this hypothesis, we studied the effects of fluticasone, AC, or both on AMøs of C57BL/6 mice in vitro and in an established model of pneumococcal pneumonia. Fluticasone plus AC significantly reduced TLR4-stimulated AMø IL-12 production, relative to either treatment alone, and decreased TNF-α, CCL3, CCL5, and keratinocyte-derived chemoattractant/CXCL1, relative to AC. Mice treated with fluticasone plus AC before infection with viable pneumococci developed significantly more lung CFUs at 48 h. However, none of the pretreatments altered inflammatory cell recruitment to the lungs at 48 h postinfection, and fluticasone plus AC less markedly reduced in vitro mediator production to heat-killed pneumococci. Fluticasone plus AC significantly reduced in vitro AMø killing of pneumococci, relative to other conditions, in part by delaying phagolysosome acidification without affecting production of reactive oxygen or nitrogen species. These results support glucocorticoid-augmented efferocytosis as a potential explanation for the epidemiological association of ICS therapy of COPD patients with increased risk for CAP, and establish murine experimental models to dissect underlying molecular mechanisms.

Basal gene expression by lung CD4+ T cells in chronic obstructive pulmonary disease identifies independent molecular correlates of airflow obstruction and emphysema extent

Lung CD4+ T cells accumulate as chronic obstructive pulmonary disease (COPD) progresses, but their role in pathogenesis remains controversial. To address this controversy, we studied lung tissue from 53 subjects undergoing clinically-indicated resections, lung volume reduction, or transplant. Viable single-cell suspensions were analyzed by flow cytometry or underwent CD4+ T cell isolation, followed either by stimulation with anti-CD3 and cytokine/chemokine measurement, or by real-time PCR analysis. In lung CD4+ T cells of most COPD subjects, relative to lung CD4+ T cells in smokers with normal spirometry: (a) stimulation induced minimal IFN-γ or other inflammatory mediators, but many subjects produced more CCL2; (b) the T effector memory subset was less uniformly predominant, without correlation with decreased IFN-γ production. Analysis of unstimulated lung CD4+ T cells of all subjects identified a molecular phenotype, mainly in COPD, characterized by markedly reduced mRNA transcripts for the transcription factors controlling T_H1, T_H2, T_H17 and FOXP3+ T regulatory subsets and their signature cytokines. This mRNA-defined CD4+ T cell phenotype did not result from global inability to elaborate mRNA; increased transcripts for inhibitory CD28 family members or markers of anergy; or reduced telomerase length. As a group, these subjects had significantly worse spirometry, but not DLCO, relative to subjects whose lung CD4+ T cells expressed a variety of transcripts. Analysis of mRNA transcripts of unstimulated lung CD4+ T cell among all subjects identified two distinct molecular correlates of classical COPD clinical phenotypes: basal IL-10 transcripts correlated independently and inversely with emphysema extent (but not spirometry); by contrast, unstimulated IFN-γ transcripts correlated independently and inversely with reduced spirometry (but not reduced DLCO or emphysema extent). Aberrant lung CD4+ T cells polarization appears to be common in advanced COPD, but also exists in some smokers with normal spirometry, and may contribute to development and progression of specific COPD phenotypes.

Efferocytosis and lung disease

In healthy individuals, billions of cells die by apoptosis each day. Clearance of these apoptotic cells, termed "efferocytosis," must be efficient to prevent secondary necrosis and the release of proinflammatory cell contents that disrupt tissue homeostasis and potentially foster autoimmunity. During inflammation, most apoptotic cells are cleared by macrophages; the efferocytic process actively induces a macrophage phenotype that favors tissue repair and suppression of inflammation. Several chronic lung diseases, particularly airways diseases such as chronic obstructive lung disease, asthma, and cystic fibrosis, are characterized by an increased lung burden of uningested apoptotic cells. Alveolar macrophages from individuals with these chronic airways diseases have decreased efferocytosis relative to alveolar macrophages from healthy subjects. These two findings have led to the hypothesis that impaired apoptotic cell clearance may contribute causally to sustained lung inflammation and that therapies to enhance efferocytosis might be beneficial. This review of the English-language scientific literature (2006 to mid-2012) explains how such existing therapies as corticosteroids, statins, and macrolides may act in part by augmenting apoptotic cell clearance. However, efferocytosis can also impede host defenses against lung infection. Thus, determining whether novel therapies to augment efferocytosis should be developed and in whom they should be used lies at the heart of efforts to differentiate specific phenotypes within complex chronic lung diseases to provide appropriately personalized therapies.

Smoking decreases the response of human lung macrophages to double-stranded RNA by reducing TLR3 expression

BACKGROUND

Cigarette smoking is associated with increased frequency and duration of viral respiratory infections, but the underlying mechanisms are incompletely defined. We investigated whether smoking reduces expression by human lung macrophages (Mø) of receptors for viral nucleic acids and, if so, the effect on CXCL10 production.

METHODS

We collected alveolar macrophages (AMø) by bronchoalveolar lavage of radiographically-normal lungs of subjects undergoing bronchoscopies for solitary nodules (n = 16) and of volunteers who were current or former smokers (n = 7) or never-smokers (n = 13). We measured expression of mRNA transcripts for viral nucleic acid receptors by real-time PCR in those AMø and in the human Mø cell line THP-1 following phorbol myristate acetate/vitamin D3 differentiation and exposure to cigarette smoke extract, and determined TLR3 protein expression using flow cytometry and immunohistochemistry. We also used flow cytometry to examine TLR3 expression in total lung Mø from subjects undergoing clinically-indicated lung resections (n = 25). Of these, seven had normal FEV1 and FEV1/FVC ratio (three former smokers, four current smokers); the remaining 18 subjects (14 former smokers; four current smokers) had COPD of GOLD stages I-IV. We measured AMø production of CXCL10 in response to stimulation with the dsRNA analogue poly(I:C) using Luminex assay.

RESULTS

Relative to AMø of never-smokers, AMø of smokers demonstrated reduced protein expression of TLR3 and decreased mRNA for TLR3 but not TLR7, TLR8, TLR9, RIG-I, MDA-5 or PKR. Identical changes in TLR3 gene expression were induced in differentiated THP-1 cells exposed to cigarette smoke-extract in vitro for 4 hours. Among total lung Mø, the percentage of TLR3-positive cells correlated inversely with active smoking but not with COPD diagnosis, FEV1% predicted, sex, age or pack-years. Compared to AMø of never-smokers, poly(I:C)-stimulated production of CXCL10 was significantly reduced in AMø of smokers.

CONCLUSIONS

Active smoking, independent of COPD stage or smoking duration, reduces both the percent of human lung Mø expressing TLR3, and dsRNA-induced CXCL10 production, without altering other endosomal or cytoplasmic receptors for microbial nucleic acids. This effect provides one possible mechanism for increased frequency and duration of viral lower respiratory tract infections in smokers.

TRIAL REGISTRATION

ClinicalTrials.gov NCT00281190, NCT00281203 and NCT00281229.

Lung CD8+ T cells in COPD have increased expression of bacterial TLRs

Background

Toll-like receptors (TLRs) on T cells can modulate their responses, however, the extent and significance of TLR expression by lung T cells, NK cells, or NKT cells in chronic obstructive pulmonary disease (COPD) is unknown.

Methods

Lung tissue collected from clinically-indicated resections (n = 34) was used either: (a) to compare the expression of TLR1, TLR2, TLR2/1, TLR3, TLR4, TLR5, TLR6 and TLR9 on lung CD8+ T cells, CD4+ T cells, NK cells and NKT cells from smokers with or without COPD; or (b) to isolate CD8+ T cells for culture with anti-CD3ε without or with various TLR ligands. We measured protein expression of IFN-γ, TNF-α, IL-13, perforin, granzyme A, granzyme B, soluble FasL, CCL2, CCL3, CCL4, CCL5, CCL11, and CXCL9 in supernatants.

Results

All the lung subsets analyzed demonstrated low levels of specific TLR expression, but the percentage of CD8+ T cells expressing TLR1, TLR2, TLR4, TLR6 and TLR2/1 was significantly increased in COPD subjects relative to those without COPD. In contrast, from the same subjects, only TLR2/1 and TLR2 on lung CD4+ T cells and CD8+ NKT cells, respectively, showed a significant increase in COPD and there was no difference in TLR expression on lung CD56+ NK cells. Production of the Tc1 cytokines IFN-γ and TNF-α by lung CD8+ T cells were significantly increased via co-stimulation by Pam3CSK4, a specific TLR2/1 ligand, but not by other agonists. Furthermore, this increase in cytokine production was specific to lung CD8+ T cells from patients with COPD as compared to lung CD8+ T cells from smokers without COPD.

Conclusions

These data suggest that as lung function worsens in COPD, the auto-aggressive behavior of lung CD8+ T cells could increase in response to microbial TLR ligands, specifically ligands against TLR2/1.

Glucocorticoids relieve collectin-driven suppression of apoptotic cell uptake in murine alveolar macrophages through downregulation of SIRPα

The lung environment actively inhibits apoptotic cell (AC) uptake by alveolar macrophages (AMøs) via lung collectin signaling through signal regulatory protein α (SIRPα). Even brief glucocorticoid (GC) treatment during maturation of human blood monocyte-derived or murine bone marrow-derived macrophages (Møs) increases their AC uptake. Whether GCs similarly impact differentiated tissue Møs and the mechanisms for this rapid response are unknown and important to define, given the widespread therapeutic use of inhaled GCs. We found that the GC fluticasone rapidly and dose-dependently increased AC uptake by murine AMøs without a requirement for protein synthesis. Fluticasone rapidly suppressed AMø expression of SIRPα mRNA and surface protein, and also activated a more delayed, translation-dependent upregulation of AC recognition receptors that was not required for the early increase in AC uptake. Consistent with a role for SIRPα suppression in rapid GC action, murine peritoneal Møs that had not been exposed to lung collectins showed delayed, but not rapid, increase in AC uptake. However, pretreatment of peritoneal Møs with the lung collectin surfactant protein D inhibited AC uptake, and fluticasone treatment rapidly reversed this inhibition. Thus, GCs act not only by upregulating AC recognition receptors during Mø maturation but also via a novel rapid downregulation of SIRPα expression by differentiated tissue Møs. Release of AMøs from inhibition of AC uptake by lung collectins may, in part, explain the beneficial role of inhaled GCs in inflammatory lung diseases, especially emphysema, in which there is both increased lung parenchymal cell apoptosis and defective AC uptake by AMøs.