Monocytes recruited to the lungs of mice during immune inflammation ingest apoptotic cells poorly

Cholesterol 25-hydroxylase promotes efferocytosis and resolution of lung inflammation

Alveolar macrophages (AM) play a central role in initiation and resolution of lung inflammation, but the integration of these opposing core functions is poorly understood. AM expression of cholesterol 25-hydroxylase (CH25H), the primary biosynthetic enzyme for 25-hydroxycholesterol (25HC), far exceeds the expression of macrophages in other tissues, but no role for CH25H has been defined in lung biology. As 25HC is an agonist for the antiinflammatory nuclear receptor, liver X receptor (LXR), we speculated that CH25H might regulate inflammatory homeostasis in the lung. Here, we show that, of natural oxysterols or sterols, 25HC is induced in the inflamed lung of mice and humans. Ch25h-/- mice fail to induce 25HC and LXR target genes in the lung after LPS inhalation and exhibit delayed resolution of airway neutrophilia, which can be rescued by systemic treatment with either 25HC or synthetic LXR agonists. LXR-null mice also display delayed resolution, suggesting that native oxysterols promote resolution. During resolution, Ch25h is induced in macrophages upon their encounter with apoptotic cells and is required for LXR-dependent prevention of AM lipid overload, induction of Mertk, efferocytic resolution of airway neutrophilia, and induction of TGF-β. CH25H/25HC/LXR is, thus, an inducible metabolic axis that programs AMs for efferocytic resolution of inflammation.

Nox2-derived oxidative stress results in inefficacy of antibiotics against post-influenza S. aureus pneumonia

Phagocyte oxidative burst is the primary source of lethal lung injury during influenza and MRSA coinfection.

Clinical post-influenza Staphylococcus aureus pneumonia is characterized by extensive lung inflammation associated with severe morbidity and mortality even after appropriate antibiotic treatment. In this study, we show that antibiotics rescue nicotinamide adenine dinucleotide phosphate (NADPH) oxidase 2 (Nox2)–deficient mice but fail to fully protect WT animals from influenza and S. aureus coinfection. Further experiments indicate that the inefficacy of antibiotics against coinfection is attributable to oxidative stress–associated inflammatory lung injury. However, Nox2-induced lung damage during coinfection was not associated with aggravated inflammatory cytokine response or cell infiltration but rather caused by reduced survival of myeloid cells. Specifically, oxidative stress increased necrotic death of inflammatory cells, thereby resulting in lethal damage to surrounding tissue. Collectively, our results demonstrate that influenza infection disrupts the delicate balance between Nox2-dependent antibacterial immunity and inflammation. This disruption leads to not only increased susceptibility to S. aureus infection, but also extensive lung damage. Importantly, we show that combination treatment of antibiotic and NADPH oxidase inhibitor significantly improved animal survival from coinfection. These findings suggest that treatment strategies that target both bacteria and oxidative stress will significantly benefit patients with influenza-complicated S. aureus pneumonia.

Coordinated expression of tyro3, axl, and mer receptors in macrophage ontogeny

The TAM receptors (Tyro3, Axl, and Mer) are a family of homologous receptor-tyrosine kinases that inhibit Toll-like receptor signaling to regulate downstream pathways and restore homeostasis. TAM triple mutant mice (Tyro3^−/−, Axl^−/−, Mer^−/−) have elevated levels of pro-inflammatory cytokines and are prone to developing lymphoproliferative disorders and autoimmunity. Understanding differential expression of TAM receptors among human subjects is critical to harnessing this pathway for therapeutic interventions. We have quantified changes in TAM expression during the ontogeny of human macrophages using paired samples of monocytes and macrophages to take advantage of characteristic expression within an individual. No significant differences in levels of Tyro3 were found between monocytes and macrophages (flow cytometry: p=0.652, immunoblot: p=0.231, qPCR: p=0.389). Protein levels of Axl were reduced (flow cytometry: p=0.049, immunoblot: p<0.001) when monocytes matured to macrophages. No significant differences in the levels of Axl mRNA transcripts were found (qPCR: p=0.082), however, Tyro3 and Axl were proportionate. The most striking difference was upregulation of expression of Mer with both protein and mRNA being significantly increased when monocytes developed into macrophages (flow cytometry: p<0.001, immunoblot: p<0.001, qPCR: p=0.004). A fuller characterization of TAM receptor expression in macrophage ontogeny informs our understanding of their function and potential therapeutic interventions.

TRPV4 Mechanosensitive Ion Channel Regulates Lipopolysaccharide-Stimulated Macrophage Phagocytosis

Macrophage phagocytosis of particles and pathogens is an essential aspect of innate host defense. Phagocytic function requires cytoskeletal rearrangements that depend on the interaction between macrophage surface receptors, particulates/pathogens, and the extracellular matrix. In the present study we determine the role of a mechanosensitive ion channel, transient receptor potential vanilloid 4 (TRPV4), in integrating the LPS and matrix stiffness signals to control macrophage phenotypic change for host defense and resolution from lung injury. We demonstrate that active TRPV4 mediates LPS-stimulated murine macrophage phagocytosis of nonopsonized particles (Escherichia coli) in vitro and opsonized particles (IgG-coated latex beads) in vitro and in vivo in intact mice. Intriguingly, matrix stiffness in the range seen in inflamed or fibrotic lung is required to sensitize the TRPV4 channel to mediate the LPS-induced increment in macrophage phagocytosis. Furthermore, TRPV4 is required for the LPS induction of anti-inflammatory/proresolution cytokines. These findings suggest that signaling through TRPV4, triggered by changes in extracellular matrix stiffness, cooperates with LPS-induced signals to mediate macrophage phagocytic function and lung injury resolution. These mechanisms are likely to be important in regulating macrophage function in the context of pulmonary infection and fibrosis.

TAM receptor signaling in immune homeostasis

The TAM receptor tyrosine kinases (RTKs)-TYRO3, AXL, and MERTK-together with their cognate agonists GAS6 and PROS1 play an essential role in the resolution of inflammation. Deficiencies in TAM signaling have been associated with chronic inflammatory and autoimmune diseases. Three processes regulated by TAM signaling may contribute, either independently or collectively, to immune homeostasis: the negative regulation of the innate immune response, the phagocytosis of apoptotic cells, and the restoration of vascular integrity. Recent studies have also revealed the function of TAMs in infectious diseases and cancer. Here, we review the important milestones in the discovery of these RTKs and their ligands and the studies that underscore the functional importance of this signaling pathway in physiological immune settings and disease.

Assessing anti-fungal activity of isolated alveolar macrophages by confocal microscopy

The lung is an interface where host cells are routinely exposed to microbes and microbial products. Alveolar macrophages are the first-line phagocytic cells that encounter inhaled fungi and other microbes. Macrophages and other immune cells recognize Aspergillus motifs by pathogen recognition receptors and initiate downstream inflammatory responses. The phagocyte NADPH oxidase generates reactive oxygen intermediates (ROIs) and is critical for host defense. Although NADPH oxidase is critical for neutrophil-mediated host defense1-3, the importance of NADPH oxidase in macrophages is not well defined. The goal of this study was to delineate the specific role of NADPH oxidase in macrophages in mediating host defense against A. fumigatus. We found that NADPH oxidase in alveolar macrophages controls the growth of phagocytosed A. fumigatus spores4. Here, we describe a method for assessing the ability of mouse alveolar macrophages (AMs) to control the growth of phagocytosed Aspergillus spores (conidia). Alveolar macrophages are stained in vivo and ten days later isolated from mice by bronchoalveolar lavage (BAL). Macrophages are plated onto glass coverslips, then seeded with green fluorescent protein (GFP)-expressing A. fumigatus spores. At specified times, cells are fixed and the number of intact macrophages with phagocytosed spores is assessed by confocal microscopy.

Advances in gene therapy technologies to treat retinitis pigmentosa

Retinitis pigmentosa (RP) is a class of diseases that leads to progressive degeneration of the retina. Experimental approaches to gene therapy for the treatment of inherited retinal dystrophies have advanced in recent years, inclusive of the safe delivery of genes to the human retina. This review is focused on the development of gene therapy for RP using recombinant adenoassociated viral vectors, which show a positive safety record and have so far been successful in several clinical trials for congenital retinal disease. Gene therapy for RP is under development in a variety of animal models, and the results raise expectations of future clinical application. Nonetheless, the translation of such strategies to the bedside requires further understanding of the mutations and mechanisms that cause visual defects, as well as thorough examination of potential adverse effects.

Monocyte- and macrophage-targeted NADPH oxidase mediates antifungal host defense and regulation of acute inflammation in mice

Chronic granulomatous disease, an inherited disorder of the NADPH oxidase in which phagocytes are defective in the generation of superoxide anion and downstream reactive oxidant species, is characterized by severe bacterial and fungal infections and excessive inflammation. Although NADPH oxidase isoforms exist in several lineages, reactive oxidant generation is greatest in neutrophils, where NADPH oxidase has been deemed vital for pathogen killing. In contrast, the function and importance of NADPH oxidase in macrophages are less clear. Therefore, we evaluated susceptibility to pulmonary aspergillosis in globally NADPH oxidase-deficient mice versus transgenic mice with monocyte/macrophage-targeted NADPH oxidase activity. We found that the lethal inoculum was >100-fold greater in transgenic versus globally NADPH oxidase-deficient mice. Consistent with these in vivo results, NADPH oxidase in mouse alveolar macrophages limited germination of phagocytosed Aspergillus fumigatus spores. Finally, globally NADPH oxidase-deficient mice developed exuberant neutrophilic lung inflammation and proinflammatory cytokine responses to zymosan, a fungal cell wall-derived product composed principally of particulate β-glucans, whereas inflammation in transgenic and wild-type mice was mild and transient. Taken together, our studies identify a central role for monocyte/macrophage NADPH oxidase in controlling fungal infection and in limiting acute lung inflammation.

Virus-like particle-induced protection against MRSA pneumonia is dependent on IL-13 and enhancement of phagocyte function

The importance of the priming of the lung environment by past infections is being increasingly recognized. Exposure to any given antigen can either improve or worsen the outcome of subsequent lung infections, depending on the immunological history of the host. Thus, an ability to impart transient alterations in the lung environment in anticipation of future insult could provide an important novel therapy for emerging infectious diseases. In this study, we show that nasal administration of virus-like particles (VLPs) before, or immediately after, lethal challenge with methicillin-resistant Staphylococcus aureus (MRSA) of mice i) ensures complete recovery from lung infection and near absolute clearance of bacteria within 12 hours of challenge, ii) reduces host response-induced lung tissue damage, iii) promotes recruitment and efficient bacterial clearance by neutrophils and CD11c⁺ cells, and iv) protects macrophages from MRSA-induced necrosis. VLP-mediated protection against MRSA relied on innate immunity. Complete recovery occurred in VLP-dosed mice with severe combined immunodeficiency, but not in wild-type mice depleted of either Ly6G⁺ or CD11c⁺ cells. Early IL-13 production associated with VLP-induced CD11c⁺ cells was essential for VLP-induced protection. These results indicate that VLP-induced alteration of the lung environment protects the host from lethal MRSA pneumonia by enhancing phagocyte recruitment and killing and by reducing inflammation-induced tissue damage via IL-13–dependent mechanisms.

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.

Ozone inhalation promotes CX3CR1-dependent maturation of resident lung macrophages that limit oxidative stress and inflammation

Inhalation of ambient ozone alters populations of lung macrophages. However, the impact of altered lung macrophage populations on the pathobiology of ozone is poorly understood. We hypothesized that subpopulations of macrophages modulate the response to ozone. We exposed C57BL/6 mice to ozone (2 ppm × 3 h) or filtered air. At 24 h after exposure, the lungs were harvested and digested and the cells underwent flow cytometry. Analysis revealed a novel macrophage subset present in ozone-exposed mice, which were distinct from resident alveolar macrophages and identified by enhanced Gr-1(+) expression [Gr-1 macrophages (Gr-1 Macs)]. Further analysis showed that Gr-1(+) Macs exhibited high expression of MARCO, CX3CR1, and NAD(P)H:quinone oxioreductase 1. Gr-1(+) Macs were present in the absence of CCR2, suggesting that they were not derived from a CCR2-dependent circulating intermediate. Using PKH26-PCL to label resident phagocytic cells, we demonstrated that Gr-1 Macs were derived from resident lung cells. This new subset was diminished in the absence of CX3CR1. Interestingly, CX3CR1-null mice exhibited enhanced responses to ozone, including increased airway hyperresponsiveness, exacerbated neutrophil influx, accumulation of 8-isoprostanes and protein carbonyls, and increased expression of cytokines (CXCL2, IL-1β, IL-6, CCL2, and TNF-α). Our results identify a novel subset of lung macrophages, which are derived from a resident intermediate, are dependent upon CX3CR1, and appear to protect the host from the biological response to ozone.

Blunt chest trauma induces mediator-dependent monocyte migration to the lung

OBJECTIVE

This study was designed to determine whether lung contusion induces an increased pulmonary recruitment of monocytes as a source of alveolar macrophages and which mediators are involved.

SETTING AND DESIGN

Prospective animal study.

SUBJECTS AND INTERVENTIONS

Male Sprague-Dawley rats were subjected to chest trauma by a single blast wave.

MEASUREMENTS

Chemokine concentrations in bronchoalveolar lavage fluids and supernatants of alveolar macrophages, chemokine and chemokine receptor mRNA expressions in monocytes, pulmonary interstitial macrophages, and alveolar macrophages isolated after trauma or sham procedure were evaluated. Immigration of monocytes was determined by staining alveolar macrophages with the fluorescent marker PKH26 before chest trauma. Chemotaxis of naïve monocytes in response to bronchoalveolar lavage or supernatants from alveolar macrophages isolated after trauma or sham procedure and the migratory response of monocytes isolated after trauma/sham to recombinant chemokines were measured.

MAIN RESULTS

Chemokine levels in bronchoalveolar lavage and alveolar macrophage supernatants and the percentage of monocytes migrated to the lungs were increased after chest trauma. Lung contusion enhanced the mRNA expression for CCR2 in monocytes and interstitial macrophages and for monocyte chemotactic protein-1 in alveolar macrophages. Migration of naïve monocytes vs. bronchoalveolar lavage or alveolar macrophage supernatants from traumatized animals was increased when compared with samples from shams. Monocytes isolated 2 hrs after trauma showed a reduced migration to CINC-1 or monocyte chemotactic protein-1 compared with sham.

CONCLUSIONS

Alveolar macrophages seem to contribute to increased chemokine concentrations in alveoli of animals subjected to blunt chest trauma. Mediators released by alveolar macrophage are potent stimuli for monocyte migration. Monocytes alter their chemokine receptor expression and are recruited to the lungs.

Amelioration of emphysema in mice through lentiviral transduction of long-lived pulmonary alveolar macrophages

Directed gene transfer into specific cell lineages in vivo is an attractive approach for both modulating gene expression and correcting inherited mutations such as emphysema caused by human alpha1 antitrypsin (hAAT) deficiency. However, somatic tissues are mainly comprised of heterogeneous, differentiated cell lineages that can be short lived and difficult to specifically transfect. Here, we describe an intratracheally instilled lentiviral system able to deliver genes selectively to as many as 70% of alveolar macrophages (AMs) in the mouse lung. Following a single in vivo lentiviral transduction, genetically tagged AMs persisted in lung alveoli and expressed transferred genes for the lifetime of the adult mouse. A prolonged macrophage lifespan, rather than precursor cell proliferation, accounted for the surprisingly sustained presence of transduced AMs. We utilized this long-lived population to achieve localized secretion of therapeutic levels of hAAT protein in lung epithelial lining fluid. In an established mouse model of emphysema, lentivirally delivered hAAT ameliorated the progression of emphysema, as evidenced by attenuation of increased lung compliance and alveolar size. After 24 weeks of sustained gene expression, no humoral or cellular immune responses to hAAT protein were detected. Our results challenge the dogma that AMs are short lived and suggest that these differentiated cells may be a possible target cell population for in vivo gene therapy applications, including the sustained correction of hAAT deficiency.

Tyro3 receptor tyrosine kinases in the heterogeneity of apoptotic cell uptake

Mononuclear phagocytes comprise a mobile, broadly dispersed and highly adaptable system that lies at the very epicenter of host defense against pathogens and the interplay of the innate and adaptive arms of immunity. Understanding the molecular mechanisms that control the response of mononuclear phagocytes to apoptotic cells and the anti-inflammatory consequences of that response is an important goal with implications for multiple areas of biomedical sciences. This review details current understanding of the heterogeneity of apoptotic cell uptake by different members of the mononuclear phagocyte family in humans and mice. It also recounts the unique role of the Tyro3 family of receptor tyrosine kinases, best characterized for Mertk, in the signal transduction leading both to apoptotic cell ingestion and the anti-inflammatory effects that result.

Surfactant proteins A and D suppress alveolar macrophage phagocytosis via interaction with SIRP alpha

RATIONALE

Efficient removal of apoptotic cells is essential for the resolution of acute pulmonary inflammation. Alveolar macrophages ingest apoptotic cells less avidly than other professional phagocytes at rest but overcome this defect during acute inflammation. Surfactant protein (SP)-A and SP-D are potent modulators of macrophage function and may suppress clearance of apoptotic cells through activation of the transmembrane receptor signal inhibitory regulatory protein alpha (SIRP alpha).

OBJECTIVES

To investigate whether binding of SP-A and SP-D to SIRP alpha on alveolar macrophages suppresses apoptotic cell clearance.

METHODS

Phagocytosis of apoptotic cells was assessed using macrophages pretreated with SP-A, SP-D, or the collectin-like molecule C1q. Binding of SP-A and SP-D to SIRP alpha was confirmed in vitro using blocking antibodies and fibroblasts transfected with active and mutant SIRP alpha. The effects of downstream molecules SHP-1 and RhoA on phagocytosis were studied using SHP-1-deficient mice, sodium stibogluconate, and a Rho kinase inhibitor. Lipopolysaccharide was given to chimeric mice to study the effects of SP-A and SP-D binding on inflammatory macrophages.

MEASUREMENTS AND MAIN RESULTS

Preincubation of macrophages with SP-A or SP-D suppressed apoptotic cell clearance. Surfactant suppression of macrophage phagocytosis was reversed by blocking SIRP alpha and inhibiting downstream molecules SHP-1 and RhoA. Macrophages from inflamed lungs ingested apoptotic cells more efficiently than resting alveolar macrophages. Recruited mononuclear phagocytes with low levels of SP-A and SP-D mediated this effect.

CONCLUSIONS

SP-A and SP-D tonically inhibit alveolar macrophage phagocytosis by binding SIRP alpha. During acute pulmonary inflammation, defects in apoptotic cell clearance are overcome by recruited mononuclear phagocytes.

The role of the macrophage in lung disease mediated by bacteria

Respiratory infections are a major cause of human morbidity and a leading cause of death. The lower respiratory tract is a sterile environment and host defense is well developed to clear bacteria. This response includes both humeral factors and resident and recruited cells. The alveolar macrophage is an integral component and its long-lifespan aids function. Following low-dose challenge alveolar macrophages clear bacteria from the lung, employing an over-lapping set of microbicidal strategies. At a higher-dose the phagocytic capacity of alveolar macrophages is overwhelmed but alveolar macrophages help orchestrate the inflammatory response. In the resolution phase of infection alveolar macrophages contribute to apoptosis induction and clearance of recruited cells. This process down-regulates pro-inflammatory cytokine production. Macrophage function is controlled by induction of apoptosis. Delayed-onset macrophage apoptosis contributes both to bacterial clearance and to resolution of the inflammatory response. Mcl-1, an anti-apoptotic protein with a very short half-life, is a key regulator of macrophage survival and therefore of host responses to common bacterial pathogens in the lung. Studies involving Streptococcus pneumoniae and other respiratory bacteria are discussed to illustrate these points and ephasise that the timing of macrophage apoptosis is important in determining its overall effect on the host pathogen interaction.

Increased inflammatory properties of adipose tissue macrophages recruited during diet-induced obesity

Although recent studies show that adipose tissue macrophages (ATMs) participate in the inflammatory changes in obesity and contribute to insulin resistance, the properties of these cells are not well understood. We hypothesized that ATMs recruited to adipose tissue during a high-fat diet have unique inflammatory properties compared with resident tissue ATMs. Using a dye (PKH26) to pulse label ATMs in vivo, we purified macrophages recruited to white adipose tissue during a high-fat diet. Comparison of gene expression in recruited and resident ATMs using real-time RT-PCR and cDNA microarrays showed that recruited ATMs overexpress genes important in macrophage migration and phagocytosis, including interleukin-6 (IL-6), inducible nitric oxide synthase (iNOS), and C-C chemokine receptor 2 (CCR2). Many of these genes were not induced in ATMs from high-fat diet-fed CCR2 knockout mice, supporting the importance of CCR2 in regulating recruitment of inflammatory ATMs during obesity. Additionally, expression of Apoe was decreased, whereas genes important in lipid metabolism, such as Pparg, Adfp, Srepf1, and Apob48r, were increased in the recruited macrophages. In agreement with this, ATMs from obese mice had increased lipid content compared with those from lean mice. These studies demonstrate that recruited ATMs in obese animals represent a subclass of macrophages with unique properties.

Decreased Alveolar Macrophage Apoptosis Is Associated with Increased Pulmonary Inflammation in a Murine Model of Pneumococcal Pneumonia

Regulation of the inflammatory infiltrate is critical to the successful outcome of pneumonia. Alveolar macrophage apoptosis is a feature of pneumococcal infection and aids disease resolution. The host benefits of macrophage apoptosis during the innate response to bacterial infection are incompletely defined. Because NO is required for optimal macrophage apoptosis during pneumococcal infection, we have explored the role of macrophage apoptosis in regulating inflammatory responses during pneumococcal pneumonia, using inducible NO synthase (iNOS)-deficient mice. iNOS(-/-) mice demonstrated decreased numbers of apoptotic macrophages as compared with wild-type C57BL/6 mice following pneumococcal challenge, greater recruitment of neutrophils to the lung and enhanced expression of TNF-alpha. Pharmacologic inhibition of iNOS produced similar results. Greater pulmonary inflammation was associated with greater levels of early bacteremia, IL-6 production, lung inflammation, and mortality within the first 48 h in iNOS(-/-) mice. Labeled apoptotic alveolar macrophages were phagocytosed by resident macrophages in the lung and intratracheal instillation of exogenous apoptotic macrophages decreased neutrophil recruitment in iNOS(-/-) mice and decreased TNF-alpha mRNA in lungs and protein in bronchial alveolar lavage, as well as chemokines and cytokines including IL-6. These changes were associated with a lower probability of mice becoming bacteremic. This demonstrates the potential of apoptotic macrophages to down-regulate the inflammatory response and for the first time in vivo demonstrates that clearance of apoptotic macrophages decreases neutrophil recruitment and invasive bacterial disease during pneumonia.

Resolution of airway disease: removal of inflammatory cells through apoptosis, egression or both?

Pathogenic granulocytes (eosinophils and neutrophils) infiltrate airway tissues in asthma and chronic obstructive pulmonary disease. Granulocytes release tissue-toxic and inflammatory mediators, making their removal an important pharmacological goal. Removal is thought to be accomplished through apoptosis followed by engulfment by macrophages. Thus, the molecular mechanisms of granulocyte apoptosis have been unravelled and pro-apoptotic actions that target granulocytes have been proposed as desirable features of future airway drugs. However, observations in vitro and in airway lumen that support this role of granulocyte apoptosis translate poorly to airway tissues in vivo. Either apoptosis cannot be demonstrated, even at the resolution of airway inflammation, or, when significant granulocyte apoptosis is induced in airway tissues in vivo, there is insufficient engulfment of apoptotic granulocytes. Therefore, apoptotic eosinophils and neutrophils in airway tissues undergo secondary necrosis, causing inflammation. As an alternative or complement to the apoptosis hypothesis, in vivo work indicates that egression to the airway lumen can produce swift non-injurious removal of tissue granulocytes. Once in the airway lumen, granulocytes can undergo apoptosis and engulfment, be trapped by secretions and plasma exudates and be removed by mucociliary escalator mechanisms. In this article, we propose that egression into the airway lumen is an effective mode of inflammatory cell disposal that connotes novel drug opportunities.

CCR2 and CCR6, but not endothelial selectins, mediate the accumulation of immature dendritic cells within the lungs of mice in response to particulate antigen

Dendritic cells (DC) migrate from sites of inflammation to lymph nodes to initiate primary immune responses, but the molecular mechanisms by which DC are replenished in the lungs during ongoing pulmonary inflammation are unknown. To address this question, we analyzed the secondary pulmonary immune response of Ag-primed mice to intratracheal challenge with the particulate T cell-dependent Ag sheep erythrocytes (SRBC). We studied wild-type C57BL/6 mice and syngeneic gene-targeted mice lacking either both endothelial selectins (CD62E and CD62P), or the chemokine receptors CCR2 or CCR6. DC, defined as non-autofluorescent, MHC class II(+)CD11c(mod) cells, were detected in blood, enzyme-digested minced lung, and bronchoalveolar lavage fluid using flow cytometry and immunohistology. Compared with control mice, Ag challenge increased the frequency and absolute numbers of DC, peaking at day 1 in peripheral blood (6.5-fold increase in frequency), day 3 in lung mince (20-fold increase in total DC), and day 4 in bronchoalveolar lavage fluid (55-fold increase in total DC). Most lung DC expressed CD11c, CD11b, and low levels of MHC class II, CD40, CD80, and CD86, consistent with an immature myeloid phenotype. DC accumulation depended in part upon CCR2 and CCR6, but not endothelial selectins. Thus, during lung inflammation, immature myeloid DC from the bloodstream replace emigrating immature DC and transiently increase total intrapulmonary APC numbers. Early DC recruitment depends in part on CCR2 to traverse vascular endothelium, plus CCR6 to traverse alveolar epithelium. The recruitment of circulating immature DC represents a potential therapeutic step at which to modulate immunological lung diseases.