Flow Cytometric Analysis of Mononuclear Phagocytes in Nondiseased Human Lung and Lung-Draining Lymph Nodes

The inflammatory cell landscape in the lungs of patients with idiopathic pulmonary arterial hypertension

Increasing evidence points towards an inflammatory component underlying pulmonary hypertension. However, the conclusive characterisation of multiple inflammatory cell populations in the lung is challenging due to the complexity of marker specificity and tissue inaccessibility. We used an unbiased computational flow cytometry approach to delineate the inflammatory landscape of idiopathic pulmonary arterial hypertension (IPAH) and healthy donor lungs.

Donor and IPAH samples were discriminated clearly using principal component analysis to reduce the multidimensional data obtained from single-cell flow cytometry analysis. In IPAH lungs, the predominant CD45⁺ cell type switched from neutrophils to CD3⁺ T-cells, with increases in CD4⁺, CD8⁺ and γδT-cell subsets. Additionally, diversely activated classical myeloid-derived dendritic cells (CD14⁻HLA-DR⁺CD11c⁺CD1a^+/−) and nonclassical plasmacytoid dendritic cells (pDCs; CD14⁻CD11c⁻CD123⁺HLA-DR⁺), together with mast cells and basophils, were more abundant in IPAH samples. We describe, for the first time, the presence and regulation of two cell types in IPAH, γδT-cells and pDCs, which link innate and adaptive immunity.

With our high-throughput flow cytometry with multidimensional dataset analysis, we have revealed the interactive interplay between multiple inflammatory cells is a crucial part of their integrative network. The identification of γδT-cells and pDCs in this disease potentially provides a missing link between IPAH, autoimmunity and inflammation.

Computational flow cytometry details the complex inflammatory cell landscape in patients with pulmonary hypertensionhttp://ow.ly/rjFZ30g1tew

A Consistent Method to Identify and Isolate Mononuclear Phagocytes from Human Lung and Lymph Nodes

Mononuclear phagocytes (MP) consist of macrophages, dendritic cells (DCs), and monocytes. In all organs, including the lung, there are multiple subtypes within these categories. The existence of all these cell types suggest that there is a clear division of labor and delicate balance between the MPs under steady state and inflammatory conditions. Although great strides have been made to understand MPs in the mouse lung, and human blood, little is known about the MPs that exist in the human lung and lung-draining lymph nodes (LNs), and even less is known about their functional roles, studies of which will require a large number of sorted cells. We have comprehensively examined cell surface markers previously used in a variety of organs to identify human pulmonary MPs. In the lung, we consistently identify five extravascular pulmonary MPs and three LN MPs. These MPs were present in over 100 lungs regardless of age or gender. Notably, the human blood CD141⁺ DCs, as described in the literature, were not observed in non-diseased lungs or their draining LNs. In the lung and draining LNs, expression of CD141 was only observed on HLADR⁺ CD11c⁺ CD14⁺ extravascular monocytes (often confused in the LN as resident DCs based on the level of HLADR expression and mouse LN data). In the human lung and LNs there are at least two DC subtypes expressing HLADR, DEC205 and CD1c, along with circulating monocytes that behave as either antigen-presenting cells or macrophages. Furthermore, we demonstrate how to distinguish between alveolar macrophages and interstitial macrophage subtypes. It still remains unclear how the human pulmonary MPs identified here align with mouse MPs. Clearly, we are now past the stage of cell surface marker characterization, and future studies will need to move toward understanding what these cell types are and how they function. Our hope is that the strategy described here can help the pulmonary community take this next step.

Isolation and Characterization of Human Lung Myeloid Cells

Multiparameter flow cytometry of human lungs allows for characterization, isolation, and examination of human pulmonary immune cell composition, phenotype, and function. Here we describe an approach to process lung tissues and then utilize a base antibody panel to define all of the major immune cell types in a single staining condition. This base antibody panel can also be used to identify major immune cell types in human blood and bronchoalveolar lavage (BAL) fluid.

Macrophage phagocytosis cracking the defect code in COPD

In the normal non-diseased lung, various macrophage populations maintain homeostasis and sterility by ingesting and clearing inhaled particulates, pathogens and apoptotic cells from the local environment. This process of phagocytosis leads to the degradation of the internalized material, coordinated induction of gene expression, antigen presentation and cytokine production, implicating phagocytosis as a central regulator of innate immunity. Phagocytosis is extremely efficient and any perturbation of this function is deleterious. In inflammatory lung diseases such as chronic obstructive pulmonary disease (COPD), despite their increased numbers, macrophages demonstrate significantly reduced phagocytic capacity of bacteria and apoptotic cells. This defect could play a role in dysbiosis of the lung microbiome contributing to disease pathophysiology. In this review, we will discuss lung macrophages, describe phagocytosis and its related downstream processes and the reported phagocytosis defects in COPD. Finally, we will briefly examine current strategies that focus on restoring the phagocytic capabilities of lung macrophages that may have utility in COPD.

Enhanced antigen presenting and T cell functions during late-phase allergic responses in the lung

BACKGROUND

Allergic inflammation is a common feature of asthma and may contribute to both development and perpetuation of disease. The interaction of antigen-presenting cells (APC) with sensitized helper T lymphocytes (TC) producing Th2 cytokines may determine the inflammatory response. Recruitment of APC and TC to the lung during allergic responses has been demonstrated, but functional studies in humans have been limited.

OBJECTIVE

This study examined the function of APC and TC accumulating at sites of inflammation after segmental allergen challenge (SAC).

METHODS

Fifteen allergic patients underwent SAC, and cells from bronchoalveolar lavage (BAL) were collected after 24 hours. APC and TC from the blood and BAL were purified based on expression of the monocyte marker, CD14; the plasmacytoid dendritic cell (pDC) marker, BDCA4, identifying neuropilin-1 (NRP1); and the helper T cell marker, CD4. Functional activity was assessed using allergen-induced T cell proliferation. Flow cytometry identified cells expressing CD14 and NRP1.

RESULTS

SAC resulted in a 12-fold increase in mononuclear cells having the morphologic appearance of blood monocytes. Most of these cells co-expressed CD14 and NRP1. After saline challenge, BAL mononuclear cells demonstrated little APC function. Following SAC, BAL mononuclear cells showed function equal to pDC from blood and greater than blood monocytes. Purified NRP1⁺ cells from BAL had even greater function than pDC cells from blood (P = .008). Using consistent sources of APC, enhanced proliferation of TC from lung compared to blood was also demonstrated (P = .002).

CONCLUSIONS

The marked increase in APC function for allergen-specific TC proliferation during allergic inflammation is largely due to the recruitment of monocytes and dendritic cells. There is also an enhanced response in the lung TC population, consistent with recruitment of allergen-specific T cells. Interactions between recruited APC and TC may occur as an early event promoting allergic airway inflammation.

Update on Dendritic Cell-Induced Immunological and Clinical Tolerance

Dendritic cells (DCs) as highly efficient antigen-presenting cells are at the interface of innate and adaptive immunity. As such, they are key mediators of immunity and antigen-specific immune tolerance. Due to their functional specialization, research efforts have focused on the characterization of DCs subsets involved in the initiation of immunogenic responses and in the maintenance of tissue homeostasis. Tolerogenic DCs (tolDCs)-based therapies have been designed as promising strategies to prevent and control autoimmune diseases as well as allograft rejection after solid organ transplantation (SOT). Despite successful experimental studies and ongoing phase I/II clinical trials using autologous tolDCs in patients with type 1 diabetes, rheumatoid arthritis, multiple sclerosis, and in SOT recipients, additional basic research will be required to determine the optimal DC subset(s) and conditioning regimens for tolDCs-based treatments in vivo. In this review, we discuss the characteristics of human DCs and recent advances in their classification, as well as the role of DCs in immune regulation and their susceptibility to in vitro or in vivo manipulation for the development of tolerogenic therapies, with a focus on the potential of tolDCs for the treatment of autoimmune diseases and the prevention of allograft rejection after SOT.

Dendritic Cell Trafficking and Function in Rare Lung Diseases

Dendritic cells (DCs) are highly specialized immune cells that capture antigens and then migrate to lymphoid tissue and present antigen to T cells. This critical function of DCs is well defined, and recent studies further demonstrate that DCs are also key regulators of several innate immune responses. Studies focused on the roles of DCs in the pathogenesis of common lung diseases, such as asthma, infection, and cancer, have traditionally driven our mechanistic understanding of pulmonary DC biology. The emerging development of novel DC reagents, techniques, and genetically modified animal models has provided abundant data revealing distinct populations of DCs in the lung, and allow us to examine mechanisms of DC development, migration, and function in pulmonary disease with unprecedented detail. This enhanced understanding of DCs permits the examination of the potential role of DCs in diseases with known or suspected immunological underpinnings. Recent advances in the study of rare lung diseases, including pulmonary Langerhans cell histiocytosis, sarcoidosis, hypersensitivity pneumonitis, and pulmonary fibrosis, reveal expanding potential pathogenic roles for DCs. Here, we provide a review of DC development, trafficking, and effector functions in the lung, and discuss how alterations in these DC pathways contribute to the pathogenesis of rare lung diseases.

Dendritic Cells Display Subset and Tissue-Specific Maturation Dynamics over Human Life

Maturation and migration to lymph nodes (LNs) constitutes a central paradigm in conventional dendritic cell (cDC) biology but remains poorly defined in humans. Using our organ donor tissue resource, we analyzed cDC subset distribution, maturation, and migration in mucosal tissues (lungs, intestines), associated lymph nodes (LNs), and other lymphoid sites from 78 individuals ranging from less than 1 year to 93 years of age. The distribution of cDC1 (CD141^hiCD13^hi) and cDC2 (Sirp-α⁺CD1c⁺) subsets was a function of tissue site and was conserved between donors. We identified cDC2 as the major mature (HLA-DR^hi) subset in LNs with the highest frequency in lung-draining LNs. Mature cDC2 in mucosal-draining LNs expressed tissue-specific markers derived from the paired mucosal site, reflecting their tissue-migratory origin. These distribution and maturation patterns were largely maintained throughout life, with site-specific variations. Our findings provide evidence for localized DC tissue surveillance and reveal a lifelong division of labor between DC subsets, with cDC2 functioning as guardians of the mucosa.

Pandemic 2009 H1N1 Influenza Venus reporter virus reveals broad diversity of MHC class II-positive antigen-bearing cells following infection in vivo

Although it is well established that Influenza A virus infection is initiated in the respiratory tract, the sequence of events and the cell types that become infected or access viral antigens remains incompletely understood. In this report, we used a novel Influenza A/California/04/09 (H1N1) reporter virus that stably expresses the Venus fluorescent protein to identify antigen-bearing cells over time in a mouse model of infection using flow cytometry. These studies revealed that many hematopoietic cells, including subsets of monocytes, macrophages, dendritic cells, neutrophils and eosinophils acquire influenza antigen in the lungs early post-infection. Surface staining of the viral HA revealed that most cell populations become infected, most prominently CD45^neg cells, alveolar macrophages and neutrophils. Finally, differences in infection status, cell lineage and MHC class II expression by antigen-bearing cells correlated with differences in their ability to re-stimulate influenza-specific CD4 T cells ex vivo. Collectively, these studies have revealed the cellular heterogeneity and complexity of antigen-bearing cells within the lung and their potential as targets of antigen recognition by CD4 T cells.

Smoking-Dependent Distal-to-Proximal Repatterning of the Adult Human Small Airway Epithelium

RATIONALE

Small airways are the primary site of pathologic changes in chronic obstructive pulmonary disease (COPD), the major smoking-induced lung disorder.

OBJECTIVES

On the basis of the concept of proximal-distal patterning that determines regional specialization of the airway epithelium during lung development, we hypothesized that a similar program operates in the adult human lung being altered by smoking, leading to decreased regional identity of the small airway epithelium (SAE).

METHODS

The proximal and distal airway signatures were identified by comparing the transcriptomes of large and small airway epithelium samples obtained by bronchoscopy from healthy nonsmokers. The expression of these signatures was evaluated in the SAE of healthy smokers and smokers with COPD compared with that of healthy nonsmokers. The capacity of airway basal stem cells (BCs) to maintain region-associated phenotypes was evaluated using the air-liquid interface model.

MEASUREMENTS AND MAIN RESULTS

The distal and proximal airway signatures, containing 134 and 233 genes, respectively, were identified. These signatures included known developmental regulators of airway patterning, as well as novel regulators such as epidermal growth factor receptor, which was associated with the proximal airway phenotype. In the SAE of smokers with COPD, there was a dramatic smoking-dependent loss of the regional transcriptome identity with concomitant proximalization. This repatterning phenotype was reproduced by stimulating SAE BCs with epidermal growth factor, which was up-regulated in the SAE of smokers, during differentiation of SAE BCs in vitro.

CONCLUSIONS

Smoking-induced global distal-to-proximal reprogramming of the SAE represents a novel pathologic feature of COPD and is mediated by exaggerated epidermal growth factor/epidermal growth factor receptor signaling in SAE BCs.

Characterisation of lung macrophage subpopulations in COPD patients and controls

Lung macrophage subpopulations have been identified based on size. We investigated characteristics of small and large macrophages in the alveolar spaces and lung interstitium of COPD patients and controls. Alveolar and interstitial cells were isolated from lung resection tissue from 88 patients. Macrophage subpopulation cell-surface expression of immunological markers and phagocytic ability were assessed by flow cytometry. Inflammatory related gene expression was measured. Alveolar and interstitial macrophages had subpopulations of small and large macrophages based on size and granularity. Alveolar macrophages had similar numbers of small and large cells; interstitial macrophages were mainly small. Small macrophages expressed significantly higher cell surface HLA-DR, CD14, CD38 and CD36 and lower CD206 compared to large macrophages. Large alveolar macrophages showed lower marker expression in COPD current compared to ex-smokers. Small interstitial macrophages had the highest pro-inflammatory gene expression levels, while large alveolar macrophages had the lowest. Small alveolar macrophages had the highest phagocytic ability. Small alveolar macrophage CD206 expression was lower in COPD patients compared to smokers. COPD lung macrophages include distinct subpopulations; Small interstitial and small alveolar macrophages with more pro-inflammatory and phagocytic function respectively, and large alveolar macrophages with low pro-inflammatory and phagocytic ability.

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.

Monocyte-derived alveolar macrophages drive lung fibrosis and persist in the lung over the life span

Misharin et al. elucidate the fate and function of monocyte-derived alveolar macrophages during the course of pulmonary fibrosis. These cells persisted throughout the life span, were enriched for the expression of profibrotic genes, and their genetic ablation ameliorated development of pulmonary fibrosis.

Little is known about the relative importance of monocyte and tissue-resident macrophages in the development of lung fibrosis. We show that specific genetic deletion of monocyte-derived alveolar macrophages after their recruitment to the lung ameliorated lung fibrosis, whereas tissue-resident alveolar macrophages did not contribute to fibrosis. Using transcriptomic profiling of flow-sorted cells, we found that monocyte to alveolar macrophage differentiation unfolds continuously over the course of fibrosis and its resolution. During the fibrotic phase, monocyte-derived alveolar macrophages differ significantly from tissue-resident alveolar macrophages in their expression of profibrotic genes. A population of monocyte-derived alveolar macrophages persisted in the lung for one year after the resolution of fibrosis, where they became increasingly similar to tissue-resident alveolar macrophages. Human homologues of profibrotic genes expressed by mouse monocyte-derived alveolar macrophages during fibrosis were up-regulated in human alveolar macrophages from fibrotic compared with normal lungs. Our findings suggest that selectively targeting alveolar macrophage differentiation within the lung may ameliorate fibrosis without the adverse consequences associated with global monocyte or tissue-resident alveolar macrophage depletion.

Diversity of Human and Macaque Airway Immune Cells at Baseline and during Tuberculosis Infection

Immune cells of the distal airways serve as "first responders" of host immunity to the airborne pathogen Mycobacterium tuberculosis (Mtb). Mtb infection of cynomolgus macaques recapitulates the range of human outcomes from clinically silent latent tuberculosis infection (LTBI) to active tuberculosis of various degrees of severity. To further advance the application of this model to human studies, we compared profiles of bronchoalveolar lavage (BAL) cells of humans and cynomolgus macaques before and after Mtb infection. A simple gating strategy effectively defined BAL T-cell and phagocyte populations in both species. BAL from Mtb-naive humans and macaques showed similar differential cell counts. BAL T cells of macaques were composed of fewer CD4⁺cells but more CD8⁺ and CD4⁺CD8⁺ double-positive cells than were BAL T cells of humans. The most common mononuclear phagocyte population in BAL of both species displayed coexpression of HLA-DR, CD206, CD11b, and CD11c; however, multiple phagocyte subsets displaying only some of these markers were observed as well. Macaques with LTBI displayed a marked BAL lymphocytosis that was not observed in humans with LTBI. In macaques, the prevalence of specific mononuclear phagocyte subsets in baseline BAL correlated with ultimate outcomes of Mtb infection (i.e., LTBI versus active disease). Overall, these findings demonstrate the comparability of studies of pulmonary immunity to Mtb in humans and macaques. They also indicate a previously undescribed complexity of airway mononuclear phagocyte populations that suggests further lines of investigation relevant to understanding the mechanisms of both protection from and susceptibility to the development of active tuberculosis within the lung.

Human hantavirus infection elicits pronounced redistribution of mononuclear phagocytes in peripheral blood and airways

Hantaviruses infect humans via inhalation of virus-contaminated rodent excreta. Infection can cause severe disease with up to 40% mortality depending on the viral strain. The virus primarily targets the vascular endothelium without direct cytopathic effects. Instead, exaggerated immune responses may inadvertently contribute to disease development. Mononuclear phagocytes (MNPs), including monocytes and dendritic cells (DCs), orchestrate the adaptive immune responses. Since hantaviruses are transmitted via inhalation, studying immunological events in the airways is of importance to understand the processes leading to immunopathogenesis. Here, we studied 17 patients infected with Puumala virus that causes a mild form of hemorrhagic fever with renal syndrome (HFRS). Bronchial biopsies as well as longitudinal blood draws were obtained from the patients. During the acute stage of disease, a significant influx of MNPs expressing HLA-DR, CD11c or CD123 was detected in the patients’ bronchial tissue. In parallel, absolute numbers of MNPs were dramatically reduced in peripheral blood, coinciding with viremia. Expression of CCR7 on the remaining MNPs in blood suggested migration to peripheral and/or lymphoid tissues. Numbers of MNPs in blood subsequently normalized during the convalescent phase of the disease when viral RNA was no longer detectable in plasma. Finally, we exposed blood MNPs in vitro to Puumala virus, and demonstrated an induction of CCR7 expression on MNPs. In conclusion, the present study shows a marked redistribution of blood MNPs to the airways during acute hantavirus disease, a process that may underlie the local immune activation and contribute to immunopathogenesis in hantavirus-infected patients.

Inhalation of hantavirus-infected rodent droppings can cause a wide range of disease ranging from mild symptoms to deaths in humans. Central to hantavirus disease is vascular leakage that can manifest in different organs, including the lungs. Although the virus can infect endothelial cells lining the blood vessels, it does not cause cell death. Instead, activation of the immune system in response to viral infection has been implicated in causing vascular leakage. In this study, we investigated how monocytes and dendritic cells (DCs) are involved in hantavirus disease, given their capacity to activate other immune cells. We obtained unique clinical material from 17 Puumala virus-infected patients including mucosal biopsies from the airways as well as multiple blood draws over the course of disease. In the airways of these patients, we observed an infiltration of monocytes and DCs. In parallel, there was a dramatic depletion in peripheral blood—more than ten-fold—of monocytes and DCs that was sustained throughout the first two weeks of disease. Taken together, this study provides novel insights into immune mediated processes underlying human hantavirus pathogenesis.

Dendritic Cell-Airway Epithelial Cell Cross-Talk Changes with Age and Contributes to Chronic Lung Inflammatory Diseases in the Elderly

Age-associated dysregulated immune and inflammatory responses are one of the major factors responsible for the prevalence of chronic respiratory diseases in the older population. Pulmonary dendritic cells (DCs) are present below the airway epithelial cells (AECs) and are critical in initiating effective immune responses to harmful pathogens while maintaining tolerance against harmless antigens. The interaction between DCs and AECs plays a crucial role in lung immunity at homeostasis and during infections. The functions of both DCs and AECs are impacted with age. The present report reviews how the potential crosstalk between pulmonary DCs and AECs is dysregulated in the elderly impairing the capacity to maintain tolerance at the respiratory surfaces, which results in severe and chronic respiratory inflammatory diseases. We also discuss how such DC-AECs crosstalk will provide insight into the mechanisms underlying the increased susceptibility of the elderly to pulmonary inflammatory diseases.

COPD monocytes demonstrate impaired migratory ability

Background

Increased lung macrophage numbers in COPD may arise from upregulation of blood monocyte recruitment into the lungs. CCR5 is a monocyte chemokine receptor regulated by interleukin-6 (IL-6); the concentration of CCR5 ligands are known to be elevated in COPD lungs. The objective of this study was to investigate mechanisms of monocyte recruitment to the lung in COPD, including the role of CCR5 signalling.

Methods

Ninety one COPD patients, 29 smokers (S) and 37 non-smokers (NS) underwent sputum induction, plasma sampling (to measure IL-6 and soluble IL-6 receptor [sIL-6R] by immunoassay), monocyte characterization (by flow cytometry) and monocyte isolation for cell migration and quantitative polymerase chain reaction studies. Lung tissue was used for immunohistochemistry.

Results

Plasma IL-6 and sIL-6R levels were increased in COPD. Greater proportions of COPD CD14⁺⁺CD16⁺ monocytes expressed CCR5 compared to controls. Monocyte stimulation with IL-6 and sIL-6R increased CCR5 gene expression. COPD monocytes demonstrated impaired migration towards sputum supernatant compared to NS (% migration, 4.4 vs 11.5, respectively; p < 0.05). Pulmonary microvessels showed reduced monocyte recruitment (% marginated cells) in COPD compared to NS, (9.3% vs 83.1%, respectively). The proportion of replicating Ki67⁺ alveolar macrophages was reduced in COPD compared to NS. All alveolar macrophages from COPD and S expressed the anti-apoptosis marker BCL2; this protein was not present in non-smokers or COPD ex-smokers.

Conclusion

COPD monocytes show decreased migratory ability despite increased CCR5 expression. Increased COPD lung macrophage numbers may be due to delayed apoptosis.

Electronic supplementary material

The online version of this article (doi:10.1186/s12931-017-0569-y) contains supplementary material, which is available to authorized users.

Innate Immune Landscape in Early Lung Adenocarcinoma by Paired Single-Cell Analyses

To guide the design of immunotherapy strategies for patients with early stage lung tumors, we developed a multiscale immune profiling strategy to map the immune landscape of early lung adenocarcinoma lesions to search for tumor-driven immune changes. Utilizing a barcoding method that allows a simultaneous single-cell analysis of the tumor, non-involved lung, and blood cells, we provide a detailed immune cell atlas of early lung tumors. We show that stage I lung adenocarcinoma lesions already harbor significantly altered T cell and NK cell compartments. Moreover, we identified changes in tumor-infiltrating myeloid cell (TIM) subsets that likely compromise anti-tumor T cell immunity. Paired single-cell analyses thus offer valuable knowledge of tumor-driven immune changes, providing a powerful tool for the rational design of immune therapies. VIDEO ABSTRACT.

Human Lung Mononuclear Phagocytes in Health and Disease

The lungs are vulnerable to attack by respiratory insults such as toxins, allergens, and pathogens, given their continuous exposure to the air we breathe. Our immune system has evolved to provide protection against an array of potential threats without causing collateral damage to the lung tissue. In order to swiftly detect invading pathogens, monocytes, macrophages, and dendritic cells (DCs)-together termed mononuclear phagocytes (MNPs)-line the respiratory tract with the key task of surveying the lung microenvironment in order to discriminate between harmless and harmful antigens and initiate immune responses when necessary. Each cell type excels at specific tasks: monocytes produce large amounts of cytokines, macrophages are highly phagocytic, whereas DCs excel at activating naïve T cells. Extensive studies in murine models have established a division of labor between the different populations of MNPs at steady state and during infection or inflammation. However, a translation of important findings in mice is only beginning to be explored in humans, given the challenge of working with rare cells in inaccessible human tissues. Important progress has been made in recent years on the phenotype and function of human lung MNPs. In addition to a substantial population of alveolar macrophages, three subsets of DCs have been identified in the human airways at steady state. More recently, monocyte-derived cells have also been described in healthy human lungs. Depending on the source of samples, such as lung tissue resections or bronchoalveolar lavage, the specific subsets of MNPs recovered may differ. This review provides an update on existing studies investigating human respiratory MNP populations during health and disease. Often, inflammatory MNPs are found to accumulate in the lungs of patients with pulmonary conditions. In respiratory infections or inflammatory diseases, this may contribute to disease severity, but in cancer patients this may improve clinical outcomes. By expanding on this knowledge, specific lung MNPs may be targeted or modulated in order to attain favorable responses that can improve preventive or treatment strategies against respiratory infections, lung cancer, or lung inflammatory diseases.

Human Bronchial Epithelial Cells Induce CD141/CD123/DC-SIGN/FLT3 Monocytes That Promote Allogeneic Th17 Differentiation

Little is known about monocyte differentiation in the lung mucosal environment and about how the epithelium shapes monocyte function. We studied the role of the soluble component of bronchial epithelial cells (BECs) obtained under basal culture conditions in innate and adaptive monocyte responses. Monocytes cultured in bronchial epithelial cell-conditioned media (BEC-CM) specifically upregulate CD141, CD123, and DC-SIGN surface levels and FLT3 expression, as well as the release of IL-1β, IL-6, and IL-10. BEC-conditioned monocytes stimulate naive T cells to produce IL-17 through IL-1β mechanism and also trigger IL-10 production by memory T cells. Furthermore, monocytes cultured in an inflammatory environment induced by the cytokines IL-6, IL-8, IL-1β, IL-15, TNF-α, and GM-CSF also upregulate CD123 and DC-SIGN expression. However, only inflammatory cytokines in the epithelial environment boost the expression of CD141. Interestingly, we identified a CD141/CD123/DC-SIGN triple positive population in the bronchoalveolar lavage fluid (BALF) from patients with different inflammatory conditions, demonstrating that this monocyte population exists in vivo. The frequency of this monocyte population was significantly increased in patients with sarcoidosis, suggesting a role in inflammatory mechanisms. Overall, these data highlight the specific role that the epithelium plays in shaping monocyte responses. Therefore, the unraveling of these mechanisms contributes to the understanding of the function that the epithelium may play in vivo.

Intravascular donor monocytes play a central role in lung transplant ischaemia-reperfusion injury

Rationale

Primary graft dysfunction in lung transplant recipients derives from the initial, largely leukocyte-dependent, ischaemia-reperfusion injury. Intravascular lung-marginated monocytes have been shown to play key roles in experimental acute lung injury, but their contribution to lung ischaemia-reperfusion injury post transplantation is unknown.

Objective

To define the role of donor intravascular monocytes in lung transplant-related acute lung injury and primary graft dysfunction.

Methods

Isolated perfused C57BL/6 murine lungs were subjected to warm ischaemia (2 hours) and reperfusion (2 hours) under normoxic conditions. Monocyte retention, activation phenotype and the effects of their depletion by intravenous clodronate-liposome treatment on lung inflammation and injury were determined. In human donor lung transplant samples, the presence and activation phenotype of monocytic cells (low side scatter, 27E10+, CD14+, HLA-DR+, CCR2+) were evaluated by flow cytometry and compared with post-implantation lung function.

Results

In mouse lungs following ischaemia-reperfusion, substantial numbers of lung-marginated monocytes remained within the pulmonary microvasculature, with reduced L-selectin and increased CD86 expression indicating their activation. Monocyte depletion resulted in reductions in lung wet:dry ratios, bronchoalveolar lavage fluid protein, and perfusate levels of RAGE, MIP-2 and KC, while monocyte repletion resulted in a partial restoration of the injury. In human lungs, correlations were observed between pre-implantation donor monocyte numbers/their CD86 and TREM-1 expression and post-implantation lung dysfunction at 48 and 72 hours.

Conclusions

These results indicate that lung-marginated intravascular monocytes are retained as a ‘passenger’ leukocyte population during lung transplantation, and play a key role in the development of transplant-associated ischaemia-reperfusion injury.

Dual polarization of human alveolar macrophages progressively increases with smoking and COPD severity

Background

It is known that tissue macrophages derive not only from blood monocytes but also from yolk sac or fetal liver, and the tissue of residence guides their function. When isolated, they lose tissue specific signatures, hence studies of human macrophages should be ideally done directly in the tissue. The aim of this study was to investigate directly in human lung tissue the polarization of alveolar macrophage (AM), classic (M1) or alternative (M2), in health and disease, using COPD as a model.

Methods

Surgical lungs from 53 subjects were studied: 36 smokers whose FEV₁ varied from normal to severe COPD, 11 non-smokers and 6 normal donors. iNOS and CD206 immunohistochemistry was used to quantify the percentage of AM polarized as M1 or M2 in lung sections.

Results and Discussion

The percentage of M1 and M2 increased progressively with smoking and COPD severity, from 26% to 84% for M1 and from 7% to 78% for M2. In donors 74% of AM were negative for M1 and 93% for M2. Confocal microscopy showed co-localization of M1 and M2 in the same AM in severe COPD.

Conclusion

In normal lungs alveolar macrophages were mostly non-polarized. With smoking and COPD severity, M1 and M2 polarization increased significantly and so did the co-expression of M1 and M2 in the same alveolar macrophage.

Electronic supplementary material

The online version of this article (doi:10.1186/s12931-017-0522-0) contains supplementary material, which is available to authorized users.

Transcriptional Classification and Functional Characterization of Human Airway Macrophage and Dendritic Cell Subsets

The respiratory system is a complex network of many cell types, including subsets of macrophages and dendritic cells that work together to maintain steady-state respiration. Owing to limitations in acquiring cells from healthy human lung, these subsets remain poorly characterized transcriptionally and phenotypically. We set out to systematically identify these subsets in human airways by developing a schema of isolating large numbers of cells by whole-lung bronchoalveolar lavage. Six subsets of phagocytic APC (HLA-DR+) were consistently observed. Aside from alveolar macrophages, subsets of Langerin+, BDCA1-CD14+, BDCA1+CD14+, BDCA1+CD14-, and BDCA1-CD14- cells were identified. These subsets varied in their ability to internalize Escherichia coli, Staphylococcus aureus, and Bacillus anthracis particles. All subsets were more efficient at internalizing S. aureus and B. anthracis compared with E. coli Alveolar macrophages and CD14+ cells were overall more efficient at particle internalization compared with the four other populations. Subsets were further separated into two groups based on their inherent capacities to upregulate surface CD83, CD86, and CCR7 expression levels. Whole-genome transcriptional profiling revealed a clade of "true dendritic cells" consisting of Langerin+, BDCA1+CD14+, and BDCA1+CD14- cells. The dendritic cell clade was distinct from a macrophage/monocyte clade, as supported by higher mRNA expression levels of several dendritic cell-associated genes, including CD1, FLT3, CX3CR1, and CCR6 Each clade, and each member of both clades, was discerned by specific upregulated genes, which can serve as markers for future studies in healthy and diseased states.

Macrophage Dysfunction in Respiratory Disease

In the healthy lung, macrophages maintain homeostasis by clearing inhaled particles, bacteria, and removing apoptotic cells from the local pulmonary environment. However, in respiratory diseases including chronic obstructive pulmonary disease (COPD), asthma, and cystic fibrosis, macrophages appear to be dysfunctional and may contribute to disease pathogenesis. In COPD, phagocytosis of bacterial species and apoptotic cells by both alveolar macrophages and monocyte-derived macrophages is significantly reduced, leading to colonization of the lung with pathogenic bacteria. COPD macrophages also release high levels of pro-inflammatory cytokines and chemokines, including CXCL8, TGFβ, and CCL2, driving recruitment of other inflammatory cells including neutrophils and monocytes to the lungs and promoting disease progression.In asthma, defective phagocytosis and efferocytosis have also been reported, and macrophages appear to have altered cell surface receptor expression; however, it is as yet unclear how this contributes to disease progression but may be important in driving Th2-mediated inflammation. In cystic fibrosis, macrophages also display defective phagocytosis, and reduced bacterial killing, which may be driven by the pro-inflammatory environment present in the lungs of these patients.The mechanisms behind defective macrophage function in lung diseases are not currently understood, but potential mechanisms include alterations in phagocytic receptor expression levels, oxidative stress, but also the possibility that specific diseases are associated with a specific, altered, macrophage phenotype that displays reduced function. Identification of the mechanisms responsible may present novel therapeutic opportunities for treatment.

Myeloid Cells in Asthma

Asthma is a heterogeneous chronic inflammatory disorder of the airways, and not surprisingly, many myeloid cells play a crucial role in pathogenesis. Antigen-presenting dendritic cells are the first to recognize the allergens, pollutants, and viruses that are implicated in asthma pathogenesis, and subsequently initiate the adaptive immune response by migrating to lymph nodes. Eosinophils are the hallmark of type 2 inflammation, releasing toxic compounds in the airways and contributing to airway remodeling. Mast cells and basophils control both the early- and late-phase allergic response and contribute to alterations in smooth muscle reactivity. Finally, relatively little is known about neutrophils and macrophages in this disease. Although many of these myeloid cells respond well to treatment with inhaled steroids, there is now an increasing armamentarium of targeted biologicals that can specifically eliminate only one myeloid cell population, like eosinophils. It is only with those new tools that we will be able to fully understand the role of myeloid cells in chronic asthma in humans.

Dendritic Cells in Human Lung Disease: Recent Advances

Dendritic cells (DCs) are potent antigen-presenting cells. Because of their particular ability to initiate and regulate cell mediated and humoral immune responses, there is considerable interest in the role that DCs play in the pathogenesis of various lung diseases, especially those in which there is an excessive immune response to specific antigens (as in asthma) or a deficient immune response (as in lung cancer). A number of DC subpopulations have been defined in the lungs, including myeloid or conventional DCs that initiate T-cell immunity and antibody production and plasmacytoid DCs that have an important role in antiviral immunity and immune tolerance. Although an extensive body of literature has documented the role that DCs play in experimental models of lung disease, this review will highlight recent advances in our understanding of DC function in human disease, including asthma, COPD, antimicrobial immunity, and lung cancer. The future is likely to see new approaches whereby antigens and small molecules are targeted to receptors on particular DC subpopulations in order to modify pulmonary immune responses.

Control of lung defence by mucins and macrophages: ancient defence mechanisms with modern functions

Owing to the need to balance the requirement for efficient respiration in the face of tremendous levels of exposure to endogenous and environmental challenges, it is crucial for the lungs to maintain a sustainable defence that minimises damage caused by this exposure and the detrimental effects of inflammation to delicate gas exchange surfaces. Accordingly, epithelial and macrophage defences constitute essential first and second lines of protection that prevent the accumulation of potentially harmful agents in the lungs, and under homeostatic conditions do so effectively without inducing inflammation. Though epithelial and macrophage-mediated defences are seemingly distinct, recent data show that they are linked through their shared reliance on airway mucins, in particular the polymeric mucin MUC5B. This review highlights our understanding of novel mechanisms that link mucus and macrophage defences. We discuss the roles of phagocytosis and the effects of factors contained within mucus on phagocytosis, as well as newly identified roles for mucin glycoproteins in the direct regulation of leukocyte functions. The emergence of this nascent field of glycoimmunobiology sets forth a new paradigm for considering how homeostasis is maintained under healthy conditions and how it is restored in disease.

Dendritic cells in lung immunopathology

Dendritic cells (DCs) lie at the heart of the innate immune system, specialised at recognising danger signals in many forms including foreign material, infection or tissue damage and initiating powerful adaptive immune and inflammatory responses. In barrier sites such as the lung, the instrumental role that DCs play at the interface between the environment and the host places them in a pivotal position in determining the severity of inflammatory disease. The past few years has seen a significant increase in our fundamental understanding of the subsets of DCs involved in pulmonary immunity, as well as the mechanisms by which they are activated and which they may use to coordinate downstream inflammation and pathology. In this review, we will summarise current understanding of the multi-faceted role that DCs play in the induction, maintenance and regulation of lung immunopathology, with an emphasis on allergic pulmonary disease.

Ly6C(+) monocyte efferocytosis and cross-presentation of cell-associated antigens

Recently it was shown that circulating Ly6C(+) monocytes traffic from tissue to the draining lymph nodes (LNs) with minimal alteration in their overall phenotype. Furthermore, in the steady state, Ly6C(+) monocytes are as abundant as classical dendritic cells (DCs) within the draining LNs, and even more abundant during inflammation. However, little is known about the functional roles of constitutively trafficking Ly6C(+) monocytes. In this study we investigated whether Ly6C(+) monocytes can efferocytose (acquire dying cells) and cross-present cell-associated antigen, a functional property particularly attributed to Batf3(+) DCs. We demonstrated that Ly6C(+) monocytes intrinsically efferocytose and cross-present cell-associated antigen to CD8(+) T cells. In addition, efferocytosis was enhanced upon direct activation of the Ly6C(+) monocytes through its corresponding TLRs, TLR4 and TLR7. However, only ligation of TLR7, and not TLR4, enhanced cross-presentation by Ly6C(+) monocytes. Overall, this study outlines two functional roles, among others, that Ly6C(+) monocytes have during an adaptive immune response.

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.

Dendritic Cells and Monocytes with Distinct Inflammatory Responses Reside in Lung Mucosa of Healthy Humans

Every breath we take contains potentially harmful pathogens or allergens. Dendritic cells (DCs), monocytes, and macrophages are essential in maintaining a delicate balance of initiating immunity without causing collateral damage to the lungs because of an exaggerated inflammatory response. To document the diversity of lung mononuclear phagocytes at steady-state, we performed bronchoscopies on 20 healthy subjects, sampling the proximal and distal airways (bronchial wash and bronchoalveolar lavage, respectively), as well as mucosal tissue (endobronchial biopsies). In addition to a substantial population of alveolar macrophages, we identified subpopulations of monocytes, myeloid DCs (MDCs), and plasmacytoid DCs in the lung mucosa. Intermediate monocytes and MDCs were highly frequent in the airways compared with peripheral blood. Strikingly, the density of mononuclear phagocytes increased upon descending the airways. Monocytes from blood and airways produced 10-fold more proinflammatory cytokines than MDCs upon ex vivo stimulation. However, airway monocytes were less inflammatory than blood monocytes, suggesting a more tolerant nature. The findings of this study establish how to identify human lung mononuclear phagocytes and how they function in normal conditions, so that dysregulations in patients with respiratory diseases can be detected to elucidate their contribution to immunity or pathogenesis.