Neutrophil margination, sequestration, and emigration in the lungs of L-selectin-deficient mice

Single-cell landscape of functionally cured chronic hepatitis B patients reveals activation of innate and altered CD4-CTL-driven adaptive immunity

BACKGROUND & AIMS

Hepatitis B surface antigen (HBsAg) loss or functional cure (FC) is considered the optimal therapeutic outcome for patients with chronic hepatitis B (CHB). However, the immune-pathological biomarkers and underlying mechanisms of FC remain unclear. In this study we comprehensively interrogate disease-associated cell states identified within intrahepatic tissue and matched PBMCs (peripheral blood mononuclear cells) from patients with CHB or after FC, at the resolution of single cells, to provide novel insights into putative mechanisms underlying FC.

METHODS

We combined single-cell transcriptomics (single-cell RNA sequencing) with multiparametric flow cytometry-based immune phenotyping, and multiplexed immunofluorescence to elucidate the immunopathological cell states associated with CHB vs. FC.

RESULTS

We found that the intrahepatic environment in CHB and FC displays specific cell identities and molecular signatures that are distinct from those found in matched PBMCs. FC is associated with the emergence of an altered adaptive immune response marked by CD4 cytotoxic T lymphocytes, and an activated innate response represented by liver-resident natural killer cells, specific Kupffer cell subtypes and marginated neutrophils. Surprisingly, we found MHC class II-expressing hepatocytes in patients achieving FC, as well as low but persistent levels of covalently closed circular DNA and pregenomic RNA, which may play an important role in FC.

CONCLUSIONS

Our study provides conceptually novel insights into the immuno-pathological control of HBV cure, and opens exciting new avenues for clinical management, biomarker discovery and therapeutic development. We believe that the discoveries from this study, as it relates to the activation of an innate and altered immune response that may facilitate sustained, low-grade inflammation, may have broader implications in the resolution of chronic viral hepatitis.

IMPACT AND IMPLICATIONS

This study dissects the immuno-pathological cell states associated with functionally cured chronic hepatitis B (defined by the loss of HBV surface antigen or HBsAg). We identified the sustained presence of very low viral load, accessory antigen-presenting hepatocytes, adaptive-memory-like natural killer cells, and the emergence of helper CD4 T cells with cytotoxic or effector-like signatures associated with functional cure, suggesting previously unsuspected alterations in the adaptive immune response, as well as a key role for the innate immune response in achieving or maintaining functional cure. Overall, the insights generated from this study may provide new avenues for the development of alternative therapies as well as patient surveillance for better clinical management of chronic hepatitis B.

Neutrophil Infiltration and Function in the Pathogenesis of Inflammatory Airspace Disease

Neutrophils are an important cell type often considered the body's first responders to inflammatory insult or damage. They are recruited to the tissue of the lungs in patients with inflammatory airspace diseases and have unique and complex functions that range from helpful to harmful. The uniqueness of these functions is due to the heterogeneity of the inflammatory cascade and retention in the vasculature. Neutrophils are known to marginate, or remain stagnant, in the lungs even in nondisease conditions. This review discusses the ways in which the recruitment, presence, and function of neutrophils in the airspace of the lungs are unique from those of other tissues, and the complex effects of neutrophils on pathogenesis. Inflammatory mediators produced by neutrophils, such as neutrophil elastase, proresolving mediators, and neutrophil extracellular traps, dramatically affect the outcomes of patients with disease of the lungs.

Multicompartmental analysis of the murine pulmonary immune response by spectral flow cytometry

Studies of pulmonary inflammation require unique considerations due to the complex structure and composition of the lungs. The lungs have multiple compartments and diverse immune cell populations, with inherently high autofluorescence, and are involved in the host response to pulmonary pathogens. We describe a protocol that accounts for these factors through a novel combination of methodologies-in vivo compartmental analysis and spectral flow cytometry with a broad panel of antibodies. In vivo compartmental analysis enables the precise localization of immune cells within the marginated vasculature, lung interstitium, nonlavageable airways, and lavageable airways of the lungs, as well as the pulmonary lymph nodes. Spectral flow cytometry with a broad panel of antibodies supports an unbiased exploratory approach to investigating diverse immune cell populations during pulmonary inflammation. Most importantly, spectral flow uses cellular autofluorescence to aid in the resolution and identification of immune cell populations. This methodology enables the acquisition of high-quality data compatible with informed gating and dimensionality reduction algorithms. In addition, our protocol emphasizes considerations for compartmentalization of the inflammatory response, spectral flow panel design, and autofluorescence spectra analysis. These methodologies are critical for increasing the rigor of pulmonary research. We apply this protocol for the precise characterization and localization of leukocytes in the pulmonary host response to influenza A virus in C57BL/6J mice. In particular, we demonstrate that this protocol improves the quantification and localization of alveolar macrophages within the airways. The methodology is modifiable and expandable to allow for further characterization of leukocyte populations of special interest.NEW & NOTEWORTHY We describe a novel combination of methodologies that incorporates dual in vivo compartmental analysis using intravascular and intratracheal CD45 labeling, a broad panel of antibodies for identifying lymphoid and nonlymphoid cells, and spectral flow cytometry that uses cellular autofluorescence to aid in resolving and identifying immune cell populations. This methodology allows precise localization of immune cells in the lavageable airways, nonlavageable airways, interstitial lung tissue, and marginated in the lung vasculature.

The Neonatal Immune System and Respiratory Pathogens

Neonates are more susceptible to some pathogens, particularly those that cause infection in the respiratory tract. This is often attributed to an incompletely developed immune system, but recent work demonstrates effective neonatal immune responses to some infection. The emerging view is that neonates have a distinctly different immune response that is well-adapted to deal with unique immunological challenges of the transition from a relatively sterile uterus to a microbe-rich world, tending to suppress potentially dangerous inflammatory responses. Problematically, few animal models allow a mechanistic examination of the roles and effects of various immune functions in this critical transition period. This limits our understanding of neonatal immunity, and therefore our ability to rationally design and develop vaccines and therapeutics to best protect newborns. This review summarizes what is known of the neonatal immune system, focusing on protection against respiratory pathogens and describes challenges of various animal models. Highlighting recent advances in the mouse model, we identify knowledge gaps to be addressed.

Early mechanisms of neutrophil activation and transmigration in acute lung injury

Introduction:Neutrophil transmigration is multifactorial and primarily driven by selectins and β₂-integrins (CD11b/CD18), whose expression are dependent on the underlying stimulus. Ventilator-induced lung injury (VILI) results in a predominantly CD18-independent mechanism of neutrophil recruitment, while direct endotoxin-induced lung injury results from a CD18-dependent mechanism. We previously observed that lack of NADPH oxidases DUOX1 and DUOX2 resulted in reduced neutrophil influx in a VILI model of lung injury but had no influence on neutrophil influx after LPS exposure. Based on these observations, we hypothesized that DUOX1/DUOX2 are an important component of CD18-independent mechanisms of neutrophil recruitment in the lung. Methods:We exposed Duoxa ^-/- (KO) mice and Duoxa ^+/+ (WT) mice to either an intratracheal exposure of lipopolysaccharide (LPS/endotoxin)-or high tidal volume ventilation and compared expression of neutrophil markers between groups. WT mice (129S6/SvEvTac) were obtained from Taconic Biosciences (One Discovery Drive Suite 304; Rensselaer, NY 1244) and were allowed to acclimatize for one week prior to study enrollment. KO mice were generated as previously described [Grasberger 2012] and bred in-house on a 129S6 background. We provided positive-pressure ventilation at a tidal volume of 10 ml/kg with 2 cmH20 positive end-expiratory pressure (PEEP). Mice were assigned to groups consisting of KO (n = 5) and WT (n = 5) in each group and divided into non-ventilated, positive-pressure ventilation, or LPS IT exposure groups. Positive-pressure ventilation was instituted for 4-h using a FlexiVent (Flexiware 8.1, Scireq, Montreal, QC, Canada). Lipopolysaccharide (Salmonella enterica serotype tryphimurium L6143, Millipore Sigma) was administered via an intratracheal (IT) route at a dose of 0.1 mg/kg. Mice were humanely euthanized at 4-h post-injection consistent with the UC Davis IAUCAC-approved protocol. Results:As previously observed, neutrophilic influx into the airways was significantly impaired in the Duoxa ^-/- (KO) mice after VILI, but not after LPS exposure. LPS-induced lung injury resulted in upregulation of CD11b⁺ neutrophils and shedding of CD62L and CD162 regardless of DUOX expression, whereas VILI resulted in upregulation of CD49⁺ neutrophils in the Duoxa ^+/+ (WT) mice but not the Duoxa ^-/- (KO) mice. Conclusion:Our data suggest DUOX is required for CD18-independent mechanisms of neutrophil recruitment in the lung induced by acute lung injury, but not for canonical CD18depedent mechanisms after LPS exposure.

Defining the versican interactome in lung health and disease

The extracellular matrix (ECM) imparts critical mechanical and biochemical information to cells in the lungs. Proteoglycans are essential constituents of the ECM and play a crucial role in controlling numerous biological processes, including regulating cellular phenotype and function. Versican, a chondroitin sulfate proteoglycan required for embryonic development, is almost absent from mature, healthy lungs and is reexpressed and accumulates in acute and chronic lung disease. Studies using genetically engineered mice show that the versican-enriched matrix can be pro- or anti-inflammatory depending on the cellular source or disease process studied. The mechanisms whereby versican develops a contextual ECM remain largely unknown. The primary goal of this review is to provide an overview of the interaction of versican with its many binding partners, the "versican interactome," and how through these interactions, versican is an integrator of complex extracellular information. Hopefully, the information provided in this review will be used to develop future studies to determine how versican and its binding partners can develop contextual ECMs that control select biological processes. Although this review focuses on versican and the lungs, what is described can be extended to other proteoglycans, tissues, and organs.

Neutrophil Recruitment in Pneumococcal Pneumonia

Streptococcus pneumoniae (Spn) is the primary agent of community-acquired pneumonia. Neutrophils are innate immune cells that are essential for bacterial clearance during pneumococcal pneumonia but can also do harm to host tissue. Neutrophil migration in pneumococcal pneumonia is therefore a major determinant of host disease outcomes. During Spn infection, detection of the bacterium leads to an increase in proinflammatory signals and subsequent expression of integrins and ligands on both the neutrophil as well as endothelial and epithelial cells. These integrins and ligands mediate the tethering and migration of the neutrophil from the bloodstream to the site of infection. A gradient of host-derived and bacterial-derived chemoattractants contribute to targeted movement of neutrophils. During pneumococcal pneumonia, neutrophils are rapidly recruited to the pulmonary space, but studies show that some of the canonical neutrophil migratory machinery is dispensable. Investigation of neutrophil migration is necessary for us to understand the dynamics of pneumococcal infection. Here, we summarize what is known about the pathways that lead to migration of the neutrophil from the capillaries to the lung during pneumococcal infection.

Supramolecular arrangement of protein in nanoparticle structures predicts nanoparticle tropism for neutrophils in acute lung inflammation

This study shows that the supramolecular arrangement of proteins in nanoparticle structures predicts nanoparticle accumulation in neutrophils in acute lung inflammation (ALI). We observed homing to inflamed lungs for a variety of nanoparticles with agglutinated protein (NAPs), defined by arrangement of protein in or on the nanoparticles via hydrophobic interactions, crosslinking and electrostatic interactions. Nanoparticles with symmetric protein arrangement (for example, viral capsids) had no selectivity for inflamed lungs. Flow cytometry and immunohistochemistry showed NAPs have tropism for pulmonary neutrophils. Protein-conjugated liposomes were engineered to recapitulate NAP tropism for pulmonary neutrophils. NAP uptake in neutrophils was shown to depend on complement opsonization. We demonstrate diagnostic imaging of ALI with NAPs; show NAP tropism for inflamed human donor lungs; and show that NAPs can remediate pulmonary oedema in ALI. This work demonstrates that structure-dependent tropism for neutrophils drives NAPs to inflamed lungs and shows NAPs can detect and treat ALI.

Regulatory mechanisms of neutrophil migration from the circulation to the airspace

The neutrophil, a short-lived effector leukocyte of the innate immune system best known for its proteases and other degradative cargo, has unique, reciprocal physiological interactions with the lung. During health, large numbers of ‘marginated’ neutrophils reside within the pulmonary vasculature, where they patrol the endothelial surface for pathogens and complete their life cycle. Upon respiratory infection, rapid and sustained recruitment of neutrophils through the endothelial barrier, across the extravascular pulmonary interstitium, and again through the respiratory epithelium into the airspace lumen, is required for pathogen killing. Overexuberant neutrophil trafficking to the lung, however, causes bystander tissue injury and underlies several acute and chronic lung diseases. Due in part to the unique architecture of the lung’s capillary network, the neutrophil follows a microanatomic passage into the distal airspace unlike that observed in other end-organs that it infiltrates. Several of the regulatory mechanisms underlying the stepwise recruitment of circulating neutrophils to the infected lung have been defined over the past few decades; however, fundamental questions remain. In this article, we provide an updated review and perspective on emerging roles for the neutrophil in lung biology, on the molecular mechanisms that control the trafficking of neutrophils to the lung, and on past and ongoing efforts to design therapeutics to intervene upon pulmonary neutrophilia in lung disease.

Cell-Cell Interaction Mechanisms in Acute Lung Injury

ABSTRACT

Acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) are caused by an exaggerated inflammatory response arising from a wide variety of pulmonary and systemic insults. Lung tissue is composed of a variety of cell populations, including parenchymal and immune cells. Emerging evidence has revealed that multiple cell populations in the lung work in concert to regulate lung inflammation in response to both direct and indirect stimulations. To date, the question of how different types of pulmonary cells communicate with each other and subsequently regulate or modulate inflammatory cascades remains to be fully addressed. In this review, we provide an overview of current advancements in understanding the role of cell-cell interaction in the development of ALI and depict molecular mechanisms by which cell-cell interactions regulate lung inflammation, focusing on inter-cellular activities and signaling pathways that point to possible therapeutic opportunities for ALI/ARDS.

Emerging roles of infiltrating granulocytes and monocytes in homeostasis

The infiltration of naïve tissues by myeloid cells has been long related to their clearance and the physiological cell turnover, however, increasing evidence shows that they can additionally fulfill specific, non-immune functions in different tissues. There is also growing evidence to support that infiltrated granulocytes and monocytes respond to different environments by modulating gene expression and cytokine production, which in turn contribute to the normal function of the host tissue. This review will address the roles of immigrated myeloid cells in different tissues and their crosstalk with the host tissue environments.

A Tissue-Specific Rhythmic Recruitment Pattern of Leukocyte Subsets

The circulating of leukocytes in the vasculature to reach various organs is a crucial step that allows them to perform their function. With a sequence of interaction with the endothelial cells, the leukocytes emigrate from the circulation either by firm attachment to vascular beds or by trafficking into the tissues. Recent findings reveal that the leukocyte recruitment shows time as well as tissue specificity depending on the cell type and homing location. This spatiotemporal distribution of leukocyte subsets is driven by the circadian expression of pro-migratory molecules expressed on the leukocytes and the endothelium. Both the systemic circadian signals and the cell's intrinsic molecule clock contribute to the oscillatory expression of pro-migratory molecules. The rhythmic recruitment of leukocytes plays an important role in the time-dependency of immune responses. It also helps to update blood components and maintain the tissue circadian microenvironment. In this review, we discuss the current knowledge about the mechanisms of the circadian system regulating the leukocyte rhythmic migration, the recruitment pattern of leukocyte subsets into different tissue/organs, and the time-dependent effects behind this process.

Neutrophil Adaptations upon Recruitment to the Lung: New Concepts and Implications for Homeostasis and Disease

Neutrophils have a prominent role in all human immune responses against any type of pathogen or stimulus. The lungs are a major neutrophil reservoir and neutrophilic inflammation is a primary response to both infectious and non-infectious challenges. While neutrophils are well known for their essential role in clearance of bacteria, they are also equipped with specific mechanisms to counter viruses and fungi. When these defense mechanisms become aberrantly activated in the absence of infection, this commonly results in debilitating chronic lung inflammation. Clearance of bacteria by phagocytosis is the hallmark role of neutrophils and has been studied extensively. New studies on neutrophil biology have revealed that this leukocyte subset is highly adaptable and fulfills diverse roles. Of special interest is how these adaptations can impact the outcome of an immune response in the lungs due to their potent capacity for clearing infection and causing damage to host tissue. The adaptability of neutrophils and their propensity to influence the outcome of immune responses implicates them as a much-needed target of future immunomodulatory therapies. This review highlights the recent advances elucidating the mechanisms of neutrophilic inflammation, with a focus on the lung environment due to the immense and growing public health burden of chronic lung diseases such as cystic fibrosis (CF) and chronic obstructive pulmonary disease (COPD), and acute lung inflammatory diseases such as transfusion-related acute lung injury (TRALI).

Circadian Expression of Migratory Factors Establishes Lineage-Specific Signatures that Guide the Homing of Leukocyte Subsets to Tissues

The number of leukocytes present in circulation varies throughout the day, reflecting bone marrow output and emigration from blood into tissues. Using an organism-wide circadian screening approach, we detected oscillations in pro-migratory factors that were distinct for specific vascular beds and individual leukocyte subsets. This rhythmic molecular signature governed time-of-day-dependent homing behavior of leukocyte subsets to specific organs. Ablation of BMAL1, a transcription factor central to circadian clock function, in endothelial cells or leukocyte subsets demonstrated that rhythmic recruitment is dependent on both microenvironmental and cell-autonomous oscillations. These oscillatory patterns defined leukocyte trafficking in both homeostasis and inflammation and determined detectable tumor burden in blood cancer models. Rhythms in the expression of pro-migratory factors and migration capacities were preserved in human primary leukocytes. The definition of spatial and temporal expression profiles of pro-migratory factors guiding leukocyte migration patterns to organs provides a resource for the further study of the impact of circadian rhythms in immunity.

Organ-Specific Mechanisms of Transendothelial Neutrophil Migration in the Lung, Liver, Kidney, and Aorta

Immune responses are dependent on the recruitment of leukocytes to the site of inflammation. The classical leukocyte recruitment cascade, consisting of capture, rolling, arrest, adhesion, crawling, and transendothelial migration, is thoroughly studied but mostly in model systems, such as the cremasteric microcirculation. This cascade paradigm, which is widely accepted, might be applicable to many tissues, however recruitment mechanisms might substantially vary in different organs. Over the last decade, several studies shed light on organ-specific mechanisms of leukocyte recruitment. An improved awareness of this matter opens new therapeutic windows and allows targeting inflammation in a tissue-specific manner. The aim of this review is to summarize the current understanding of the leukocyte recruitment in general and how this varies in different organs. In particular we focus on neutrophils, as these are the first circulating leukocytes to reach the site of inflammation. Specifically, the recruitment mechanism in large arteries, as well as vessels in the lungs, liver, and kidney will be addressed.

Transporters MRP1 and MRP2 Regulate Opposing Inflammatory Signals To Control Transepithelial Neutrophil Migration during Streptococcus pneumoniae Lung Infection

Streptococcus pneumoniae is a Gram-positive bacterium that normally inhabits the human nasopharynx asymptomatically. However, it is also a major cause of pneumonia, bacteremia, and meningitis. The transition from pneumonia to bacteremia is critical, as patients that develop septicemia have ~20% mortality rates. Previous studies have shown that while neutrophils, a major bacterium-induced leukocyte, aid in S. pneumoniae elimination, they also contribute to pathology and may mediate the lung-to-blood passage of the bacteria. Herein, we show that epithelium-derived MRP1 and MRP2 efflux immunomodulatory agents that assist in controlling passage of neutrophils during infection and that limiting neutrophil infiltration produced less bacteremia and better survival during murine infection. The importance of our work is twofold: ours is the first to identify an MRP1/MRP2 axis of neutrophil control in the lung. The second is to provide possible therapeutic targets to reduce excess inflammation, thus reducing the chances of developing bacteremia during pneumococcal pneumonia.

Streptococcus pneumoniae remains a source of morbidity and mortality in both developed and underdeveloped nations of the world. Disease can manifest as pneumonia, bacteremia, and meningitis, depending on the localization of infection. Interestingly, there is a correlation in experimental murine infections between the development of bacteremia and influx of neutrophils into the pulmonary lumen. Reduction of this neutrophil influx has been shown to improve survivability during infection. In this study, we use in vitro biotinylation and neutrophil transmigration and in vivo murine infection to identify a system in which two epithelium-localized ATP-binding cassette transporters, MRP1 and MRP2, have inverse activities dictating neutrophil transmigration into the lumen of infected mouse lungs. MRP1 effluxes an anti-inflammatory molecule that maintains homeostasis in uninfected contexts, thus reducing neutrophil infiltration. During inflammatory events, however, MRP1 decreases and MRP2 both increases and effluxes the proinflammatory eicosanoid hepoxilin A3. If we then decrease MRP2 activity during experimental murine infection with S. pneumoniae, we reduce both neutrophil infiltration and bacteremia, showing that MRP2 coordinates this activity in the lung. We conclude that MRP1 assists in depression of polymorphonuclear cell (PMN) migration by effluxing a molecule that inhibits the proinflammatory effects of MRP2 activity.

IMPORTANCEStreptococcus pneumoniae is a Gram-positive bacterium that normally inhabits the human nasopharynx asymptomatically. However, it is also a major cause of pneumonia, bacteremia, and meningitis. The transition from pneumonia to bacteremia is critical, as patients that develop septicemia have ~20% mortality rates. Previous studies have shown that while neutrophils, a major bacterium-induced leukocyte, aid in S. pneumoniae elimination, they also contribute to pathology and may mediate the lung-to-blood passage of the bacteria. Herein, we show that epithelium-derived MRP1 and MRP2 efflux immunomodulatory agents that assist in controlling passage of neutrophils during infection and that limiting neutrophil infiltration produced less bacteremia and better survival during murine infection. The importance of our work is twofold: ours is the first to identify an MRP1/MRP2 axis of neutrophil control in the lung. The second is to provide possible therapeutic targets to reduce excess inflammation, thus reducing the chances of developing bacteremia during pneumococcal pneumonia.

Nephrogenic acute respiratory distress syndrome: A narrative review on pathophysiology and treatment

The kidneys have a close functional relationship with other organs especially the lungs. This connection makes the kidney and the lungs as the most organs involved in the multi-organ failure syndrome. The combination of acute lung injury (ALI) and renal failure results a great clinical significance of 80% mortality rate. Acute kidney injury (AKI) leads to an increase in circulating cytokines, chemokines, activated innate immune cells and diffuse of these agents to other organs such as the lungs. These factors initiate pathological cascade that ultimately leads to ALI and acute respiratory distress syndrome (ARDS). We comprehensively searched the English medical literature focusing on AKI, ALI, organs cross talk, renal failure, multi organ failure and ARDS using the databases of PubMed, Embase, Scopus and directory of open access journals. In this narrative review, we summarized the pathophysiology and treatment of respiratory distress syndrome following AKI. This review promotes knowledge of the link between kidney and lung with mechanisms, diagnostic biomarkers, and treatment involved ARDS induced by AKI.

Neutrophil Extracellular Traps in Pulmonary Diseases: Too Much of a Good Thing?

Neutrophil extracellular traps (NETs) arise from the release of granular and nuclear contents of neutrophils in the extracellular space in response to different classes of microorganisms, soluble factors, and host molecules. NETs are composed by decondensed chromatin fibers coated with antimicrobial granular and cytoplasmic proteins, such as myeloperoxidase, neutrophil elastase (NE), and α-defensins. Besides being expressed on NET fibers, NE and MPO also regulate NET formation. Furthermore, histone deimination by peptidylarginine deiminase 4 (PAD4) is a central step to NET formation. NET formation has been widely demonstrated to be an effective mechanism to fight against invading microorganisms, as deficiency in NET release or dismantling NET backbone by bacterial DNases renders the host susceptible to infections. Therefore, the primary role of NETs is to prevent microbial dissemination, avoiding overwhelming infections. However, an excess of NET formation has a dark side. The pathogenic role of NETs has been described for many human diseases, infectious and non-infectious. The detrimental effect of excessive NET release is particularly important to lung diseases, because NETs can expand more easily in the pulmonary alveoli, causing lung injury. Moreover, NETs and its associated molecules are able to directly induce epithelial and endothelial cell death. In this regard, massive NET formation has been reported in several pulmonary diseases, including asthma, chronic obstructive pulmonary disease, cystic fibrosis, respiratory syncytial virus bronchiolitis, influenza, bacterial pneumonia, and tuberculosis, among others. Thus, NET formation must be tightly regulated in order to avoid NET-mediated tissue damage. Recent development of therapies targeting NETs in pulmonary diseases includes DNA disintegration with recombinant human DNase, neutralization of NET proteins, with anti-histone antibodies and protease inhibitors. In this review, we summarize the recent knowledge on the pathophysiological role of NETs in pulmonary diseases as well as some experimental and clinical approaches to modulate their detrimental effects.

Adhesion Molecules: Master Controllers of the Circulatory System

This manuscript will review our current understanding of cellular adhesion molecules (CAMs) relevant to the circulatory system, their physiological role in control of vascular homeostasis, innate and adaptive immune responses, and their importance in pathophysiological (disease) processes such as acute lung injury, atherosclerosis, and pulmonary hypertension. This is a complex and rapidly changing area of research that is incompletely understood. By design, we will begin with a brief overview of the structure and classification of the major groups of adhesion molecules and their physiological functions including cellular adhesion and signaling. The role of specific CAMs in the process of platelet aggregation and hemostasis and leukocyte adhesion and transendothelial migration will be reviewed as examples of the complex and cooperative interplay between CAMs during physiological and pathophysiological processes. The role of the endothelial glycocalyx and the glycobiology of this complex system related to inflammatory states such as sepsis will be reviewed. We will then focus on the role of adhesion molecules in the pathogenesis of specific disease processes involving the lungs and cardiovascular system. The potential of targeting adhesion molecules in the treatment of immune and inflammatory diseases will be highlighted in the relevant sections throughout the manuscript.

PSGL-1 on Leukocytes is a Critical Component of the Host Immune Response against Invasive Pneumococcal Disease

Bacterial uptake by phagocytic cells is a vital event in the clearance of invading pathogens such as Streptococcus pneumoniae. A major role of the P-selectin glycoprotein ligand-1 (PSGL-1) on leukocytes against invasive pneumococcal disease is described in this study. Phagocytosis experiments using different serotypes demonstrated that PSGL-1 is involved in the recognition, uptake and killing of S. pneumoniae. Co-localization of several clinical isolates of S. pneumoniae with PSGL-1 was demonstrated, observing a rapid and active phagocytosis in the presence of PSGL-1. Furthermore, the pneumococcal capsular polysaccharide and the main autolysin of the bacterium ―the amidase LytA― were identified as bacterial ligands for PSGL-1. Experimental models of pneumococcal disease including invasive pneumonia and systemic infection showed that bacterial levels were markedly increased in the blood of PSGL-1^−/− mice. During pneumonia, PSGL-1 controls the severity of pneumococcal dissemination from the lung to the bloodstream. In systemic infection, a major role of PSGL-1 in host defense is to clear the bacteria in the systemic circulation controlling bacterial replication. These results confirmed the importance of this receptor in the recognition and clearance of S. pneumoniae during invasive pneumococcal disease. Histological and cellular analysis demonstrated that PSGL-1^−/− mice have increased levels of T cells migrating to the lung than the corresponding wild-type mice. In contrast, during systemic infection, PSGL-1^−/− mice had increased numbers of neutrophils and macrophages in blood, but were less effective controlling the infection process due to the lack of this functional receptor. Overall, this study demonstrates that PSGL-1 is a novel receptor for S. pneumoniae that contributes to protection against invasive pneumococcal disease.

S. pneumoniae is one of the most important and devastating human pathogens worldwide, mainly affecting young children, elderly people and immunocompromised patients. In terms of host immune defense against invasive pneumococcal isolates, professional phagocytes require receptor-mediated recognition of certain ligands on the bacterial surface for the uptake and clearance of the microorganism. In this study, we demonstrate that the P-selectin glycoprotein ligand-1 (PSGL-1) on leukocytes is involved in the phagocytosis process of S. pneumoniae by targeting the capsule and the surface protein LytA as pathogen-associated molecular patterns. To explore this process in more detail, we have used wild-type mice and mice deficient in PSGL-1 demonstrating that lack of PSGL-1 is detrimental for the host by increasing the susceptibility to the infection and the severity of the pneumococcal invasive disease. Overall, these data show the importance of PSGL-1 on leukocytes in host defense against S. pneumoniae and confirm that PSGL-1 plays a critical protective role against invasive bacterial disease.

Cellular softening mediates leukocyte demargination and trafficking, thereby increasing clinical blood counts

Leukocytes normally marginate toward the vascular wall in large vessels and within the microvasculature. Reversal of this process, leukocyte demargination, leads to substantial increases in the clinical white blood cell and granulocyte count and is a well-documented effect of glucocorticoid and catecholamine hormones, although the underlying mechanisms remain unclear. Here we show that alterations in granulocyte mechanical properties are the driving force behind glucocorticoid- and catecholamine-induced demargination. First, we found that the proportions of granulocytes from healthy human subjects that traversed and demarginated from microfluidic models of capillary beds and veins, respectively, increased after the subjects ingested glucocorticoids. Also, we show that glucocorticoid and catecholamine exposure reorganizes cellular cortical actin, significantly reducing granulocyte stiffness, as measured with atomic force microscopy. Furthermore, using simple kinetic theory computational modeling, we found that this reduction in stiffness alone is sufficient to cause granulocyte demargination. Taken together, our findings reveal a biomechanical answer to an old hematologic question regarding how glucocorticoids and catecholamines cause leukocyte demargination. In addition, in a broader sense, we have discovered a temporally and energetically efficient mechanism in which the innate immune system can simply alter leukocyte stiffness to fine tune margination/demargination and therefore leukocyte trafficking in general. These observations have broad clinically relevant implications for the inflammatory process overall as well as hematopoietic stem cell mobilization and homing.

Intravital imaging of mesenchymal stem cell trafficking and association with platelets and neutrophils

Early events of mesenchymal stem/stromal cell (MSC) adhesion to and transmigration through the vascular wall following systemic infusion are important for MSC trafficking to inflamed sites, yet are poorly characterized in vivo. Here, we used intravital confocal imaging to determine the acute extravasation kinetics and distribution of culture-expanded MSC (2-6 hours postinfusion) in a murine model of dermal inflammation. By 2 hours postinfusion, among the MSC that arrested within the inflamed ear dermis, 47.8% ± 8.2% of MSC had either initiated or completed transmigration into the extravascular space. Arrested and transmigrating MSCs were equally distributed within both small capillaries and larger venules. This suggested existence of an active adhesion mechanism, since venule diameters were greater than those of the MSC. Heterotypic intravascular interactions between distinct blood cell types have been reported to facilitate the arrest and extravasation of leukocytes and circulating tumor cells. We found that 42.8% ± 24.8% of intravascular MSC were in contact with neutrophil-platelet clusters. A role for platelets in MSC trafficking was confirmed by platelet depletion, which significantly reduced the preferential homing of MSC to the inflamed ear, although the total percentage of MSC in contact with neutrophils was maintained. Interestingly, although platelet depletion increased vascular permeability in the inflamed ear, there was decreased MSC accumulation. This suggests that increased vascular permeability is unnecessary for MSC trafficking to inflamed sites. These findings represent the first glimpse into MSC extravasation kinetics and microvascular distribution in vivo, and further clarify the roles of active adhesion, the intravascular cellular environment, and vascular permeability in MSC trafficking.

Live imaging of the lung

Live lung imaging has spanned the discovery of capillaries in the frog lung by Malpighi to the current use of single and multiphoton imaging of intravital and isolated perfused lung preparations incorporating fluorescent molecular probes and transgenic reporter mice. Along the way, much has been learned about the unique microcirculation of the lung, including immune cell migration and the mechanisms by which cells at the alveolar-capillary interface communicate with each other. In this review, we highlight live lung imaging techniques as applied to the role of mitochondria in lung immunity, mechanisms of signal transduction in lung compartments, studies on the composition of alveolar wall liquid, and neutrophil and platelet trafficking in the lung under homeostatic and inflammatory conditions. New applications of live lung imaging and the limitations of current techniques are discussed.

Systemic disease during Streptococcus pneumoniae acute lung infection requires 12-lipoxygenase-dependent inflammation

Acute pulmonary infection by Streptococcus pneumoniae is characterized by high bacterial numbers in the lung, a robust alveolar influx of polymorphonuclear cells (PMNs), and a risk of systemic spread of the bacterium. We investigated host mediators of S. pneumoniae-induced PMN migration and the role of inflammation in septicemia following pneumococcal lung infection. Hepoxilin A3 (HXA3) is a PMN chemoattractant and a metabolite of the 12-lipoxygenase (12-LOX) pathway. We observed that S. pneumoniae infection induced the production of 12-LOX in cultured pulmonary epithelium and in the lungs of infected mice. Inhibition of the 12-LOX pathway prevented pathogen-induced PMN transepithelial migration in vitro and dramatically reduced lung inflammation upon high-dose pulmonary challenge with S. pneumoniae in vivo, thus implicating HXA3 in pneumococcus-induced pulmonary inflammation. PMN basolateral-to-apical transmigration in vitro significantly increased apical-to-basolateral transepithelial migration of bacteria. Mice suppressed in the expression of 12-LOX exhibited little or no bacteremia and survived an otherwise lethal pulmonary challenge. Our data suggest that pneumococcal pulmonary inflammation is required for high-level bacteremia and systemic infection, partly by disrupting lung epithelium through 12-LOX-dependent HXA3 production and subsequent PMN transepithelial migration.

Neutrophil mobilization via plerixafor-mediated CXCR4 inhibition arises from lung demargination and blockade of neutrophil homing to the bone marrow

The CXCR4 antagonist plerixafor augments frequency of circulating neutrophils via release from the lung and prevents neutrophil homing to the bone marrow.

Blood neutrophil homeostasis is essential for successful host defense against invading pathogens. Circulating neutrophil counts are positively regulated by CXCR2 signaling and negatively regulated by the CXCR4–CXCL12 axis. In particular, G-CSF, a known CXCR2 signaler, and plerixafor, a CXCR4 antagonist, have both been shown to correct neutropenia in human patients. G-CSF directly induces neutrophil mobilization from the bone marrow (BM) into the blood, but the mechanisms underlying plerixafor-induced neutrophilia remain poorly defined. Using a combination of intravital multiphoton microscopy, genetically modified mice and novel in vivo homing assays, we demonstrate that G-CSF and plerixafor work through distinct mechanisms. In contrast to G-CSF, CXCR4 inhibition via plerixafor does not result in neutrophil mobilization from the BM. Instead, plerixafor augments the frequency of circulating neutrophils through their release from the marginated pool present in the lung, while simultaneously preventing neutrophil return to the BM. Our study demonstrates for the first time that drastic changes in blood neutrophils can originate from alternative reservoirs other than the BM, while implicating a role for CXCR4–CXCL12 interactions in regulating lung neutrophil margination. Collectively, our data provides valuable insights into the fundamental regulation of neutrophil homeostasis, which may lead to the development of improved treatment regimens for neutropenic patients.

Hydrogen sulfide reduces neutrophil recruitment in hind-limb ischemia-reperfusion injury in an L-selectin and ADAM-17-dependent manner

BACKGROUND

Reperfusion following ischemia leads to neutrophil recruitment into injured tissue. Selectins and β2-integrins regulate neutrophil interaction with the endothelium during neutrophil rolling and firm adhesion. Excessive neutrophil infiltration into tissue is thought to contribute to ischemia-reperfusion injury damage. Hydrogen sulfide mitigates the damage caused by ischemia-reperfusion injury. This study's objective was to determine the effect of hydrogen sulfide on neutrophil adhesion receptor expression.

METHODS

Human neutrophils were either left untreated or incubated in 20 μM hydrogen sulfide and/or 50 μg/ml pharmacologic ADAM-17 inhibitor TAPI-0; activated by interleukin-8, fMLP, or TNF-α; and labeled against P-selectin glycoprotein ligand-1, leukocyte function associated antigen-1, Mac-1 α, L-selectin, and β2-integrin epitopes CBRM1/5 or KIM127 for flow cytometry. Cohorts of three C57BL/6 mice received an intravenous dose of saline vehicle or 20 μM hydrogen sulfide with or without 50 μg/ml TAPI-0 before unilateral tourniquet-induced hind-limb ischemia for 3 hours followed by 3 hours of reperfusion. Bilateral gastrocnemius muscles were processed for histology before neutrophil infiltration quantification.

RESULTS

Hydrogen sulfide treatment significantly increased L-selectin shedding from human neutrophils following activation by fMLP and interleukin-8 in an ADAM-17-dependent manner. Mice treated with hydrogen sulfide to raise bloodstream concentration by 20 μM before ischemia or reperfusion showed a significant reduction in neutrophil recruitment into skeletal muscle tissue following tourniquet-induced hind-limb ischemia-reperfusion injury.

CONCLUSIONS

Hydrogen sulfide administration results in the down-regulation of L-selectin expression in activated human neutrophils. This leads to a reduction in neutrophil extravasation and tissue infiltration and may partially account for the protective effects of hydrogen sulfide seen in the setting of ischemia-reperfusion injury.

The endothelial glycocalyx: an important regulator of the pulmonary vascular barrier

Once thought to be a structure of small size and uncertain significance, the endothelial glycocalyx is now known to be an important regulator of endothelial function. Studies of the systemic vasculature have demonstrated that the glycocalyx forms a substantial in vivo endothelial surface layer (ESL) critical to inflammation, barrier function and mechanotransduction. The pulmonary ESL is significantly thicker than the systemic ESL, suggesting unique physiologic function. We have recently demonstrated that the pulmonary ESL regulates exposure of endothelial surface adhesion molecules, thereby serving as a barrier to neutrophil adhesion and extravasation. While the pulmonary ESL is not a critical structural component of the endothelial barrier to fluid and protein, it serves a major role in the mechanotransduction of vascular pressure, with impact on the active regulation of endothelial permeability. It is likely that the ESL serves numerous additional functions in vascular physiology, representing a fertile area for future investigation.

On, around, and through: neutrophil-endothelial interactions in innate immunity

This manuscript will review our current understanding of neutrophilic polymorphonuclear leukocyte (neutrophil) interactions with the endothelium during immune and inflammatory responses, focusing on the molecular mechanisms regulating neutrophil adhesion to and migration through the endothelium in response to infection or tissue injury. This is a complex and dynamic area of research and one that has been the topic of several recent comprehensive reviews to which the interested reader is referred (64, 118, 131). By design, this review will begin with a brief review of some basic aspects of neutrophil biology and endothelial adhesion to provide a foundation. The remainder of the review will focus on selected areas of this complex field, specifically the role of the endothelial glycocalyx in regulating neutrophil adhesion and the mechanisms and consequences of migration of neutrophils between (paracellular) and through (transcellular) endothelial cells during egress from the vasculature.

Real-time imaging reveals endothelium-mediated leukocyte retention in LPS-treated lung microvessels

Endotoxemia, a major feature of sepsis, is a common cause of acute lung injury and initiates rapid accumulation of leukocytes in the lung vasculature. Endothelial mechanisms that underlie this accumulation remain unclear, as current experimental models of endotoxemia are less suitable for targeted activation of the endothelium. Toward elucidating this, we used the isolated blood-perfused rat lung preparation. With a microcatheter inserted through a left atrial cannula, we cleared blood cells from a small lung region and then infused lipopolysaccharide (LPS) into microvessels. After a Ringer's wash to remove residual LPS, we infused fluorescently-labeled autologous leukocytes and imaged their transit through the treated microvessels. Image analysis revealed that leukocytes infused 90 min after LPS treatment were retained more in treated venules and capillaries than untreated vessels. Further, pretreatment with either the intercellular adhesion molecule-1 (ICAM-1) mAb or polymyxin-B blunted LPS-induced leukocyte retention in both microvessel segments. In addition, retention of leukocytes treated ex vivo with LPS in LPS-treated microvessels was higher compared to retention of untreated leukocytes. In situ immunofluorescence experiments revealed that LPS significantly increased microvessel ICAM-1 expression at 90 min post treatment. Polymyxin pretreatment inhibited this increase. Taken together, the data suggest that LPS increased leukocyte retention in both venules and capillaries and this response was mediated by the increased expression of endothelial ICAM-1. Thus, endothelial mechanisms may themselves play a major role in LPS-induced leukocyte retention in lung microvessels. Blunting the endothelial responses may mitigate endotoxin-induced morbidity.

Leukocyte compartments in the mouse lung: distinguishing between marginated, interstitial, and alveolar cells in response to injury

We developed a flow cytometry-based assay to simultaneously quantify multiple leukocyte populations in the marginated vascular, interstitial, and alveolar compartments of the mouse lung. An intravenous injection of a fluorescently labeled anti-CD45 antibody was used to label circulating and marginated vascular leukocytes. Following vascular flushing to remove non-adherent cells and collection of broncho-alveolar lavage (BAL) fluid, lungs were digested and a second fluorescent anti-CD45 antibody was added ex vivo to identify cells not located in the vascular space. In the naïve mouse lung, we found about 11 million CD45+ leukocytes, of which 87% (9.5 million) were in the vascular marginated compartment, consisting of 17% NK cells, 17% neutrophils, 57% mononuclear myeloid cells (monocytes, macrophage precursors and dendritic cells), and 10% T cells (CD4+, CD8+, and invariant NKT cells). Non-vascular compartments including the interstitial compartment contained 7.7×10(5)cells, consisting of 49% NK cells, 25% dendritic cells, and 16% other mononuclear myeloid cells. The alveolar compartment was overwhelmingly populated by macrophages (5.63×10(5)cells, or 93%). We next studied leukocyte margination and extravasation into the lung following acid injury, a model of gastric aspiration. At 1 h after injury, neutrophils were markedly elevated in the blood while all other circulating leukocytes declined by an average of 79%. At 4 h after injury, there was a peak in the numbers of marginated neutrophils, NK cells, CD4+ and CD8+ T cells and a peak in the number of alveolar NK cells. Most interstitial cells consisted of DCs, neutrophils, and CD4+ T cells, and most alveolar compartment cells consisted of macrophages, neutrophils, and NK cells. At 24 h after injury, there was a decline in the number of all marginated and interstitial leukocytes and a peak in alveolar neutrophils. In sum, we have developed a novel assay to study leukocyte margination and trafficking following pulmonary inflammation and show that marginated cells comprise a large fraction of lung leukocytes that increases shortly after lung injury. This assay may be of interest in future studies to determine if leukocytes become activated upon adherence to the endothelium, and have properties that distinguish them from interstitial and circulating cells.

The acute respiratory distress syndrome: pathogenesis and treatment

The acute respiratory distress syndrome (ARDS) causes 40% mortality in approximately 200,000 critically ill patients annually in the United States. ARDS is caused by protein-rich pulmonary edema that causes severe hypoxemia and impaired carbon dioxide excretion. The clinical disorders associated with the development of ARDS include sepsis, pneumonia, aspiration of gastric contents, and major trauma. The lung injury is caused primarily by neutrophil-dependent and platelet-dependent damage to the endothelial and epithelial barriers of the lung. Resolution is delayed because of injury to the lung epithelial barrier, which prevents removal of alveolar edema fluid and deprives the lung of adequate quantities of surfactant. Lymphocytes may play a role in resolution of lung injury. Mortality has been markedly reduced with a lung-protective ventilatory strategy. However, there is no effective pharmacologic therapy, although cell-based therapy and other therapies currently being tested in clinical trials may provide novel treatments for ARDS.

Role of c-Abl in L-selectin shedding from the neutrophil surface

L-selectin is a key molecule that participates in neutrophil tethering and subsequent rolling. It is cleaved from the surface of neutrophils activated in the presence of lipopolysaccharides, N-formyl-methionine-leucine-phenylalanine (fMLP), or Interleukin-8 (IL-8). We previously showed that L-selectin is also shed from the neutrophil surface during rolling on sialyl Lewis-x coated surfaces in a force-, ADAM-17 sheddase-, and p38 MAP kinase-dependent manner under flow. c-Abl tyrosine kinase is phosphorylated when L-selectin on the surface of neutrophils is cross-linked with anti-L-selectin antibodies. Here, we study the effect of c-Abl inhibition on L-selectin shedding from primary human neutrophils in static conditions following exposure to fMLP, IL-8, and hypotonic buffer and under flow through sialyl Lewis-x coated microtubes. Results indicate that c-Abl inhibition by STI571 significantly affects neutrophil adhesion via L-selectin, by decreasing the average rolling velocity and increasing the flux of rolling cells. The change in surface receptor expression was verified by flow cytometry. Interestingly, other forms of L-selectin shedding induced by fMLP, IL-8 or osmotic swelling were unaffected by STI571 treatment. These findings implicate the c-Abl signaling molecule in regulating L-selectin mechanical shedding in response to shear stress, setting this type of signaling apart from those triggered by the presence of a hypotonic environment, fMLP, or IL-8. This study sheds light on the role of c-Abl in neutrophil adhesion not previously reported in the literature.

Fractionation of swine barn dust and assessment of its impact on the respiratory tract following repeated airway exposure

The effects of repeated exposure to a range of doses of swine barn dust (SBD) on airway hyperresponsiveness (AHR) and inflammation were evaluated using a mouse model system. A number of components, including endotoxin and a number of feed proteins, were identified in SBD, and mice were exposed 20 min/d for 14 d to a log dilution series of nebulized SBD suspensions. AHR to methacholine was measured using head-out whole-body plethysmography, and the methacholine concentration inducing a 20% decrease in pulmonary airflow (PC(20) MCh) was calculated. At the end of the 14-d exposure period, bronchoalveolar lavage (BAL) fluids were recovered, cytokines (interleukin [IL]-1beta, IL-6, keratinocyte-derived chemokine [KC], and tumor necrosis factor [TNF]) in BAL were measured by enzyme-linked immunosorbent assay (ELISA), and leukocytes in BAL were counted. The PC(20) MCh was significantly lower in the group of mice that were exposed to the highest concentration of SBD than in controls or the group exposed to the lowest level of dust. Likewise, the group that was exposed to the highest level of SBD had significantly higher levels of IL-1beta, KC, and TNF than controls and some other groups. There were substantially more lymphocytes and monocytes in the BAL from mice that were exposed to the higher levels of SBD for the 14-d period, but neutrophils were not a part of this response. The SBD exposures used in these experiments induced chronic inflammatory phenotype responses, as indicated by the predominance of lymphocytes and monocytes, but not neutrophils, in BAL and by inflammatory cytokines detected. The association between the PC(20)MCh and dose of SBD suggests that a threshold of susceptibility occurs after a relatively low, chronic exposure to SBD.

Epithelial cell apoptosis and neutrophil recruitment in acute lung injury-a unifying hypothesis? What we have learned from small interfering RNAs

In spite of protective ventilatory strategies, Acute Lung Injury (ALI) remains associated with high morbidity and mortality. One reason for the lack of therapeutic options might be that ALI is a co-morbid event associated with a diverse family of diseases and, thus, may be the result of distinct pathological processes. Among them, activated neutrophil- (PMN-) induced tissue injury and epithelial cell apoptosis mediated lung damage represent two potentially important candidate pathomechanisms that have been put forward. Several approaches have been undertaken to test these hypotheses, with substantial success in the treatment of experimental forms of ALI. With this in mind, we will summarize these two current hypotheses of ALI briefly, emphasizing the role of apoptosis in regulating PMN and/or lung epithelial cell responses. In addition, the contribution that Fas-mediated inflammation may play as a potential biological link between lung cell apoptosis and PMN recruitment will be considered, as well as the in vivo application of small interfering RNA (siRNA) as a novel approach to the inhibition of ALI and its therapeutic implications.

Hypertonic saline resuscitation from hemorrhagic shock does not impair the neutrophil response to intraabdominal infection

BACKGROUND

Hypertonic saline (HTS) has been proposed as a resuscitation strategy following trauma based on its ability to prevent organ dysfunction by exerting immunosuppressive effects on inflammatory cells, including neutrophils. Because these cells are central to the innate response to bacteria, we hypothesized that hypertonic treatment for hemorrhagic shock might alter the host response to bacterial contamination of the peritoneal cavity and therefore render the host more susceptible to invasive infection.

METHODS

Male Sprague-Dawley rats were subjected to hemorrhagic shock and resuscitated with either lactated Ringer solution (RL) or HTS. After intraperitoneal injection of feces, Escherichia coli, or lipopolysaccharide, peritoneal neutrophil accumulation and bacterial clearance were studied. In some studies, lipopolysaccharide as an inflammatory stimulus was injected into both the peritoneal cavity and the lungs.

RESULTS

Peritoneal neutrophil accumulation in response to each of the stimuli did not differ between RL- and HTS-resuscitated animals. Whereas emigration into the peritoneum activated neutrophils, there was no difference between resuscitation strategies, consistent with the finding that bacterial clearance did not differ between groups. Although peritoneal neutrophil sequestration was unaffected by resuscitation type, HTS still was able to prevent lung neutrophil accumulation compared to RL treatment.

CONCLUSIONS

HTS resuscitation did not impair the host response to bacterial contamination of the peritoneal cavity. However, the ability of HTS to prevent lung neutrophil accumulation in this setting persisted. These findings suggest that peritoneal bacterial contamination should not be considered a contraindication to the use of HTS in the trauma setting associated with hemorrhagic shock.

CD59a deficiency exacerbates influenza-induced lung inflammation through complement-dependent and -independent mechanisms

Influenza-specific immune activity not only promotes virus clearance but also causes immunopathology, thereby underlining the importance of mounting a measured anti-viral immune response. Since complement bridges both the innate and adaptive immune systems and has been implicated in defence against influenza, the role of the complement regulator CD59a in modulating the response to influenza was explored. For this purpose, immune responses to influenza virus, strain E61-13-H17, in mice deficient in the complement regulator protein CD59a (Cd59a^–/– mice) were compared to those in wild-type mice. The severity of lung inflammation was significantly enhanced in the lungs of Cd59a^–/– mice with increased numbers of infiltrating neutrophils and CD4⁺ T cells. When complement was inhibited using soluble complement receptor1, the frequency of lung-infiltrating neutrophils in influenza-infected Cd59a^–/– mice was much reduced whilst numbers of CD4⁺ T cells remained unchanged. These results demonstrate that CD59a, previously defined as a complement regulator, modulates both the innate and adaptive immune response to influenza virus by both complement-dependent and-independent mechanisms.

Acute uremia but not renal inflammation attenuates aseptic acute lung injury: a critical role for uremic neutrophils

Acute renal failure (ARF) remains a major clinical challenge, especially in the intensive care setting. Mortality of ARF combined with acute lung injury (ALI) is even higher and may reach 80%. Recent studies have suggested a remote effect of ARF on pulmonary homeostasis. However, it is unknown whether and to what extent ARF clinically affects pulmonary function, in particular oxygenation. For elucidation of the impact of ARF on aseptic ALI, a murine two-hit model that consists of acute uremia (AU) and subsequent ALI was developed. AU was induced by renal ischemia-reperfusion (inflammatory AU) or bilateral nephrectomy (noninflammatory AU). ALI was initiated by intratracheal HCl instillation and characterized by severe, PMN-dependent decrease in arterial partial pressure of O(2) (>70%) in nonuremic mice. Uremic mice, by contrast, showed a significant protection from ALI (decrease in arterial partial pressure of O(2) <40%); this was independent of the type of AU. Reconstitution experiments, in which uremic neutrophils were injected into nonuremic mice and vice versa, identified uremic neutrophils as the primary mediators. Between normal and uremic neutrophils, there were no differences in apoptosis or superoxide production. Pulmonary recruitment of uremic neutrophils, however, was significantly attenuated compared with that of normal neutrophils. This defect was associated with altered surface expression of L-selectin; sialyl Lewis(x), an L-selectin counterreceptor, previously was proved to be critical in aseptic ALI. In conclusion, it is shown that AU but not renal inflammation attenuates aseptic, neutrophil-dependent ALI and exerts an anti-inflammatory effect by attenuating pulmonary neutrophil recruitment.

Neutrophil cytoskeletal rearrangements during capillary sequestration in bacterial pneumonia in rats

RATIONALE

Neutrophils accumulate in pulmonary capillaries during acute inflammation. Initial events in injury recognition and sequestration do not occur through selectin-mediated rolling. Cytoskeletal rearrangements, as assessed by submembrane F-actin rims, result in poorly deformable neutrophils that may not pass through capillaries.

OBJECTIVE

To test the hypothesis that neutrophils sequestering during pneumonia contain F-actin rims and to determine the roles of CD11/CD18, L-selectin expression, and neutrophil-platelet adhesion in neutrophil sequestration.

METHODS

Neutrophils were compared in blood obtained simultaneously from venous and arterial sites before and 4 h after instillation of Streptococcus pneumoniae or Escherichia coli in rats.

MEASUREMENTS AND MAIN RESULTS

At 4 h of pneumonia, the number of neutrophils was greater in the venous blood entering the lungs than in the arterial blood leaving the lungs, indicating that neutrophil sequestration was occurring. More neutrophils entering the lungs contained F-actin rims than did neutrophils exiting, and the venous-arterial difference in F-actin-rimmed neutrophil counts completely accounted for sequestration. In E. coli pneumonia, in which neutrophil adhesion is mediated by CD11/CD18, CD18 blockade 15 min before blood samples were obtained did not prevent this sequestration of F-actin-rimmed neutrophils. Neutrophils expressing high or low levels of L-selectin or of neutrophils that bound platelets while circulating did not preferentially sequester.

CONCLUSIONS

Neutrophils with cytoskeletal rearrangements preferentially sequester within the lungs during pneumonia, and this sequestration is not due to CD11/CD18-mediated adhesion, L-selectin expression, or platelet adhesion to neutrophils, suggesting that cytoskeletal rearrangements result in sequestration of neutrophils.

Lung NF-kappaB activation and neutrophil recruitment require IL-1 and TNF receptor signaling during pneumococcal pneumonia

Pulmonary inflammation is an essential component of the host defense against Streptococcus pneumoniae infection of the lungs. The early response cytokines, TNF-alpha and IL-1, are rapidly induced upon microbial exposure. Mice deficient in all TNF- and IL-1-dependent signaling receptors were used to determine the roles of these cytokines during pneumococcal pneumonia. The deficiency of signaling receptors for TNF and IL-1 decreased bacterial clearance. Neutrophil recruitment to alveolar air spaces was impaired by receptor deficiency, as was pulmonary expression of the neutrophil chemokines KC and MIP-2. Because NF-kappaB mediates the expression of both chemokines, we assessed NF-kappaB activation in the lungs. During pneumococcal pneumonia, NF-kappaB proteins translocate to the nucleus and activate gene expression; these functions were largely abrogated by the deficiency of receptors for TNF-alpha and IL-1. Thus, the combined deficiency of TNF and IL-1 signaling reduces innate immune responses to S. pneumoniae in the lungs, probably due to essential roles for these receptors in activating NF-kappaB.

Transfusion-related acute lung injury

Transfusion-related acute lung injury (TRALI) is a life-threatening adverse event of transfusion, which has an increasing incidence in the United States and is the leading cause of transfusion-related death. TRALI and acute lung injury (ALI) share a common clinical definition except that TRALI is temporally- and mechanistically-related to transfusion of blood or blood components. A number of different models have been proposed to explain the pathogenesis. The first is an antibody-mediated event whereby transfusion of anti-HLA, class I or class II, or anti-granulocyte antibodies into patients whose leukocytes express the cognate antigens. The antibody:antigen interaction causes complement-mediated pulmonary sequestration and activation of neutrophils (PMNs) resulting in TRALI. The second is a two-event model: the first event is the clinical condition of the patient resulting in pulmonary endothelial activation and PMN sequestration, and the second event is the transfusion of a biologic response modifier (including anti-granulocyte antibodies, lipids, and CD40 ligand) that activates these adherent PMNs resulting in endothelial damage, capillary leak, and TRALI. These hypotheses are discussed with respect to animal models and human studies that provide the experimental and clinical relevance. The definition of TRALI, patient predisposition, treatment, prevention and reporting guidelines are also examined.

Protease-activated receptor-2 activation induces acute lung inflammation by neuropeptide-dependent mechanisms

Protease-activated receptors (PARs) and tachykinin-immunoreactive fibers are located in the lung as sentries to respond to a variety of pathological stimuli. The effects of PAR activation on the lung have not been adequately studied. We report on the effects of instilling PAR-activating peptides (PAR-APs, including PAR1-, PAR2-, and PAR4-AP) into the lungs of ventilated or spontaneously breathing mice. PAR2-AP, but not PAR1-AP or PAR4-AP, caused a sharp increase in lung endothelial and epithelial permeability to protein, extravascular lung water, and airway tone. No responses to PAR2-AP were detected in PAR2 knockout mice. In bronchoalveolar lavage, PAR2 activation caused 8- and 5-fold increase in MIP-2 and substance P levels, respectively, and a 12-fold increase in the number of neutrophils. Ablation of sensory neurons (by capsaicin) markedly decreased the PAR2-mediated airway constriction, and virtually abolished PAR2-mediated pulmonary inflammation and edema, as did blockade of NK1 or NK2 receptors. Thus, PAR2 activation in the lung induces airway constriction, lung inflammation, and protein-rich pulmonary edema. These effects were either partly or completely neuropeptide dependent, suggesting that PAR2 can cause lung inflammation by a neurogenic mechanism.

Respiratory syncytial virus infection of human lung endothelial cells enhances selectively intercellular adhesion molecule-1 expression

Respiratory syncytial virus (RSV) is worldwide the most frequent cause of bronchiolitis and pneumonia in infants requiring hospitalization. In the present study, we supply evidence that human lung microvascular endothelial cells, human pulmonary lung aorta endothelial cells, and HUVEC are target cells for productive RSV infection. All three RSV-infected endothelial cell types showed an enhanced cell surface expression of ICAM-1 (CD54), which increased in a time- and RSV-dose-dependent manner. By using noninfectious RSV particles we verified that replication of RSV is a prerequisite for the increase of ICAM-1 cell surface expression. The up-regulated ICAM-1 expression pattern correlated with an increased cellular ICAM-1 mRNA amount. In contrast to ICAM-1, a de novo expression of VCAM-1 (CD106) was only observed on RSV-infected HUVEC. Neither P-selectin (CD62P) nor E-selectin (CD62E) was up-regulated by RSV on human endothelial cells. Additional experiments performed with neutralizing Abs specific for IL-1alpha, IL-1beta, IL-6, and TNF-alpha, respectively, excluded an autocrine mechanism responsible for the observed ICAM-1 up-regulation. The virus-induced ICAM-1 up-regulation was dependent on protein kinase C and A, PI3K, and p38 MAPK activity. Adhesion experiments using polymorphonuclear neutrophil granulocytes (PMN) verified an increased ICAM-1-dependent adhesion rate of PMN cocultured with RSV-infected endothelial cells. Furthermore, the increased adhesiveness resulted in an enhanced transmigration rate of PMN. Our in vitro data suggest that human lung endothelial cells are target cells for RSV infection and that ICAM-1 up-regulated on RSV-infected endothelial cells might contribute to the enhanced accumulation of PMN into the bronchoalveolar space.

Reduced interleukin-8 response to Streptococcus pneumoniae by alveolar macrophages from adults with HIV/AIDS

BACKGROUND

HIV-infected adults are highly susceptible to pneumococcal disease.

OBJECTIVE

To examine if alveolar macrophages from HIV-infected subjects exhibited a failure of cytokine production in response to Streptococcus pneumoniae in vitro.

DESIGN

Case-control comparison of alveolar macrophages from 11 HIV-infected and 13 non-infected adults.

METHODS

Type 1 opsonized S. pneumoniae were used to challenge the alveolar macrophages in vitro. Cell supernatant fluid was collected from unstimulated cells, and cells challenged with bacteria for 0, 6, 12 and 24 h. Cytokine production (interleukins 1beta, 6 and 8) was measured in all fluids using an enzyme-linked immunosorbent assay.

RESULTS

All the cytokines tested increased over time in both HIV-infected and uninfected subjects. Interleukin-8 release was significantly lower in HIV-infected than in non-HIV-infected subjects (P = 0.02).

CONCLUSION

Reduced interleukin-8 production may result in decreased neutrophil recruitment, and hence increased susceptibility to pneumococcal infection in HIV-infected subjects.