Fas/Fas ligand system mediates epithelial injury, but not pulmonary host defenses, in response to inhaled bacteria

Regulation of Mesenchymal Cell Fate by Transfer of Active Gasdermin-D via Monocyte-Derived Extracellular Vesicles

Fibrosis is characterized by inappropriately persistent myofibroblast accumulation and excessive extracellular matrix deposition with the disruption of tissue architecture and organ dysfunction. Regulated death of reparative mesenchymal cells is critical for normal wound repair, but profibrotic signaling promotes myofibroblast resistance to apoptotic stimuli. A complex interplay between immune cells and structural cells underlies lung fibrogenesis. However, there is a paucity of knowledge on how these cell populations interact to orchestrate physiologic and pathologic repair of the injured lung. In this context, gasdermin-D (GsdmD) is a cytoplasmic protein that is activated following cleavage by inflammatory caspases and induces regulated cell death by forming pores in cell membranes. This study was undertaken to evaluate the impact of human (Thp-1) monocyte-derived extracellular vesicles and GsdmD on human lung fibroblast death. Our data show that active GsdmD delivered by monocyte-derived extracellular vesicles induces caspase-independent fibroblast and myofibroblast death. This cell death was partly mediated by GsdmD-independent induction of cellular inhibitor of apoptosis 2 (cIAP-2) in the recipient fibroblast population. Our findings, to our knowledge, define a novel paradigm by which inflammatory monocytes may orchestrate the death of mesenchymal cells in physiologic wound healing, illustrating the potential to leverage this mechanism to eliminate mesenchymal cells and facilitate the resolution of fibrotic repair.

The anti-inflammatory activity of specific-sized hyaluronic acid oligosaccharides

Severe acute inflammatory conditions may cause tissue damage, sepsis, and death. As a critical component of the extracellular matrix, hyaluronic acid (HA) has been reported to possess pro- and anti-inflammatory properties via Toll-like receptors (TLRs). In this study, we prepared different sizes and structures of HA oligosaccharides and derivatives and investigated the effects on inflammation in vitro and in vivo. Our results showed that HA tetra-saccharide was the minimum fragment to enhance inflammation, whereas HA disaccharide competitively blocked TLR4-dependent inflammation. The enzymatic HA disaccharide (ΔHA2) inhibited lipopolysaccharide (LPS)-induced inflammation. Based on structure-activity relationship analysis, we observed that anti-inflammatory activity depended on HAs polymerization degree, acetyl group, and configuration. In addition, we demonstrated that ΔHA2 reduced LPS-induced pro-inflammatory cytokines production in vivo. ΔHA2, a native metabolite of HA polysaccharides, may have a potential role against LPS-mediated inflammatory diseases.

Cell Death in the Lung: The Apoptosis-Necroptosis Axis

Regulated cell death is a major mechanism to eliminate damaged, infected, or superfluous cells. Previously, apoptosis was thought to be the only regulated cell death mechanism; however, new modalities of caspase-independent regulated cell death have been identified, including necroptosis, pyroptosis, and autophagic cell death. As an understanding of the cellular mechanisms that mediate regulated cell death continues to grow, there is increasing evidence that these pathways are implicated in the pathogenesis of many pulmonary disorders. This review summarizes our understanding of regulated cell death as it pertains to the pathogenesis of chronic obstructive pulmonary disease, asthma, idiopathic pulmonary fibrosis, acute respiratory distress syndrome, and pulmonary arterial hypertension.

Cytokine-Ion Channel Interactions in Pulmonary Inflammation

The lungs conceptually represent a sponge that is interposed in series in the bodies’ systemic circulation to take up oxygen and eliminate carbon dioxide. As such, it matches the huge surface areas of the alveolar epithelium to the pulmonary blood capillaries. The lung’s constant exposure to the exterior necessitates a competent immune system, as evidenced by the association of clinical immunodeficiencies with pulmonary infections. From the in utero to the postnatal and adult situation, there is an inherent vital need to manage alveolar fluid reabsorption, be it postnatally, or in case of hydrostatic or permeability edema. Whereas a wealth of literature exists on the physiological basis of fluid and solute reabsorption by ion channels and water pores, only sparse knowledge is available so far on pathological situations, such as in microbial infection, acute lung injury or acute respiratory distress syndrome, and in the pulmonary reimplantation response in transplanted lungs. The aim of this review is to discuss alveolar liquid clearance in a selection of lung injury models, thereby especially focusing on cytokines and mediators that modulate ion channels. Inflammation is characterized by complex and probably time-dependent co-signaling, interactions between the involved cell types, as well as by cell demise and barrier dysfunction, which may not uniquely determine a clinical picture. This review, therefore, aims to give integrative thoughts and wants to foster the unraveling of unmet needs in future research.

Apoptotic cell death in rat lung following mustard gas inhalation

To investigate apoptosis as a mechanism of sulfur mustard (SM) inhalation injury in animals, we studied different caspases (caspase-8, -9, -3, and -6) in the lungs from a ventilated rat SM aerosol inhalation model. SM activated all four caspases in cells obtained from bronchoalveolar lavage fluid (BALF) as early as 6 h after exposure. Caspase-8, which is known to initiate the extrinsic Fas-mediated pathway of apoptosis, was increased fivefold between 6 and 24 h, decreasing to the unexposed-control level at 48 h. The initiator, caspase-9, in the intrinsic mitochondrial pathway of apoptosis as well as the executioner caspases, caspase-3 and -6, all peaked ( P < 0.01) at 24 h; caspase-3 and -6 remained elevated, but caspase-9 decreased to unexposed-control level at 48 h. To study further the Fas pathway, we examined soluble as well as membrane-bound Fas ligand (sFas-L and mFas-L, respectively) and Fas receptor (Fas-R) in both BALF cells and BALF. At 24 h after SM exposure, sFas-L increased significantly in both BALF cells ( P < 0.01) and BALF ( P < 0.05). However, mFas-L increased only in BALF cells between 24 and 48 h ( P < 0.1 and P < 0.001, respectively). Fas-R increased only in BALF cells by 6 h ( P < 0.01) after SM exposure. Apoptosis in SM-inhaled rat lung specimens was also confirmed by both immunohistochemical staining using cleaved caspase-3 and -9 antibodies and terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) staining as early as 6 h in the proximal trachea and bronchi, but not before 48 h in distal airways. These findings suggest pathogenic mechanisms at the cellular and molecular levels and logical therapeutic target(s) for SM inhalation injury in animals.

Group B Streptococcus Evades Host Immunity by Degrading Hyaluronan

In response to tissue injury, hyaluronan (HA) polymers are cleaved by host hyaluronidases, generating small fragments that ligate Toll-like receptors (TLRs) to elicit inflammatory responses. Pathogenic bacteria such as group B Streptococcus (GBS) express and secrete hyaluronidases as a mechanism for tissue invasion, but it is not known how this activity relates to immune detection of HA. We found that bacterial hyaluronidases secreted by GBS and other Gram-positive pathogens degrade pro-inflammatory HA fragments to their component disaccharides. In addition, HA disaccharides block TLR2/4 signaling elicited by both host-derived HA fragments and other TLR2/4 ligands, including lipopolysaccharide. Application of GBS hyaluronidase or HA disaccharides reduced pulmonary pathology and pro-inflammatory cytokine levels in an acute lung injury model. We conclude that breakdown of host-generated pro-inflammatory HA fragments to disaccharides allows bacterial pathogens to evade immune detection and could be exploited as a strategy to treat inflammatory diseases.

Enhanced expression of Fas and FasL modulates apoptosis in the lungs of severe P. falciparum malaria patients with pulmonary edema

Apoptosis mediated by Fas/FasL has been implicated in pulmonary disorders. However, little is known about the relationship between Fas and FasL in the process of lung injury during malaria infection. Paraffin-embedded lung tissues from malaria patients were divided into two groups: those with pulmonary edema (PE) and those without pulmonary edema (non-PE). Normal lung tissues were used as the control group. Cellular expression of Fas, FasL, and the markers of apoptotic caspases, including cleaved caspase-3 and cleaved caspase-8 in the lung tissues were investigated by the immunohistochemistry (IHC) method. Semi-quantitative analysis of IHC staining revealed that cellular expression of Fas, FasL, cleaved caspase-8, and cleaved caspase-3 were significantly increased in the lungs of patients with PE compared with the lungs of patients with non-PE and control groups (all P < 0.05). In addition, significant positive correlations were obtained between Fas and apoptosis (rs = 0.937, P < 0.001) and FasL and apoptosis (rs = 0.808, P < 0.001). Significant positive correlations were found between Fas and FasL expression (rs = 0.827, P < 0.001) and between cleaved caspase-8 and cleaved caspase-3 expression (rs = 0.823, P < 0.001), which suggests that Fas-dependent initiator and effector caspases, including cleaved caspase-8 and caspase-3, are necessary for inducing apoptosis in the lungs of patients with severe P. falciparum malaria. The Fas/FasL system and downstream activation of caspases are important mediators of apoptosis and may be involved in the pathogenesis of pulmonary edema in severe P. falciparum malaria patients. The proper regulation of the Fas/FasL pathway can be a potential treatment for pulmonary complications in falciparum malaria patients.

Fas ligand-expressing lymphocytes enhance alveolar macrophage apoptosis in the resolution of acute pulmonary inflammation

Apoptosis of alveolar macrophages and their subsequent clearance by neighboring phagocytes are necessary steps in the resolution of acute pulmonary inflammation. We have recently identified that activation of the Fas death receptor on the cell surface of macrophages drives macrophage apoptosis. However, the source of the cognate ligand for Fas (FasL) responsible for induction of alveolar macrophage apoptosis is not defined. Given their known role in the resolution of inflammation and ability to induce macrophage apoptosis ex vivo, we hypothesized that T lymphocytes represented a critical source of FasL. To address this hypothesis, C57BL/6J and lymphocyte-deficient (Rag-1(-/-)) mice were exposed to intratracheal lipopolysaccharide to induce pulmonary inflammation. Furthermore, utilizing mice expressing nonfunctional FasL, we adoptively transferred donor lymphocytes into inflamed lymphocyte-deficient mice to characterize the effect of lymphocyte-derived FasL on alveolar macrophage apoptosis in the resolution of inflammation. Herein, evidence is presented that lymphocytes expressing FasL enhance alveolar macrophage apoptosis during the resolution of LPS-induced inflammation. Moreover, lymphocyte induction of alveolar macrophage apoptosis results in contraction of the alveolar macrophage pool, which occurs in a FasL-dependent manner. Specifically, FasL-expressing CD8(+) T lymphocytes potently induce alveolar macrophage apoptosis and contraction of the alveolar macrophage pool. Together, these studies identify a novel role for CD8(+) T lymphocytes in the resolution of acute pulmonary inflammation.

Reprint of: A rapid increase in macrophage-derived versican and hyaluronan in infectious lung disease

The goals of this study were to characterize the changes in chondroitin sulfate proteoglycans and hyaluronan in lungs in acute response to gram-negative bacterial infection and to identify cellular components responsible for these changes. Mice were treated with intratracheal (IT) live Escherichia coli, E. coli lipopolysaccharide (LPS), or PBS. Both E. coli and LPS caused rapid selective increases in mRNA expression of versican and hyaluronan synthase (Has) isoforms 1 and 2 associated with increased immunohistochemical and histochemical staining for versican and hyaluronan in the lungs. Versican was associated with a subset of alveolar macrophages. To examine whether macrophages contribute to versican and hyaluronan accumulation, in vitro studies with primary cultures of bone marrow-derived and alveolar macrophages were performed. Unstimulated macrophages expressed very low levels of versican and hyaluronan synthase mRNA, with no detectible versican protein or hyaluronan product. Stimulation with LPS caused rapid increases in versican mRNA and protein, a rapid increase in Has1 mRNA, and concomitant inhibition of hyaluronidases 1 and 2, the major hyaluronan degrading enzymes. Hyaluronan could be detected following chloroquine pre-treatment, indicating rapid turnover and degradation of hyaluronan by macrophages. In addition, the effects of LPS, the M1 macrophage classical activation agonist, were compared to those of IL-4/IL-13 or IL-10, the M2a and M2c alternative activation agonists, respectively. Versican and Has1 increased only in response to M1 activation. Finally, the up-regulation of versican and Has1 in the whole lungs of wild-type mice following IT LPS was completely abrogated in TLR-4(-/-) mice. These findings suggest that versican and hyaluronan synthesis may play an important role in the innate immune response to gram-negative lung infection.

A rapid increase in macrophage-derived versican and hyaluronan in infectious lung disease

The goals of this study were to characterize the changes in chondroitin sulfate proteoglycans and hyaluronan in lungs in acute response to gram-negative bacterial infection and to identify cellular components responsible for these changes. Mice were treated with intratracheal (IT) live Escherichia coli, E. coli lipopolysaccharide (LPS), or PBS. Both E. coli and LPS caused rapid selective increases in mRNA expression of versican and hyaluronan synthase (Has) isoforms 1 and 2 associated with increased immunohistochemical and histochemical staining for versican and hyaluronan in the lungs. Versican was associated with a subset of alveolar macrophages. To examine whether macrophages contribute to versican and hyaluronan accumulation, in vitro studies with primary cultures of bone marrow-derived and alveolar macrophages were performed. Unstimulated macrophages expressed very low levels of versican and hyaluronan synthase mRNA, with no detectible versican protein or hyaluronan product. Stimulation with LPS caused rapid increases in versican mRNA and protein, a rapid increase in Has1 mRNA, and concomitant inhibition of hyaluronidases 1 and 2, the major hyaluronan degrading enzymes. Hyaluronan could be detected following chloroquine pre-treatment, indicating rapid turnover and degradation of hyaluronan by macrophages. In addition, the effects of LPS, the M1 macrophage classical activation agonist, were compared to those of IL-4/IL-13 or IL-10, the M2a and M2c alternative activation agonists, respectively. Versican and Has1 increased only in response to M1 activation. Finally, the up-regulation of versican and Has1 in the whole lungs of wild-type mice following IT LPS was completely abrogated in TLR-4(-/-) mice. These findings suggest that versican and hyaluronan synthesis may play an important role in the innate immune response to gram-negative lung infection.

Apoptotic and inflammatory signaling via Fas and tumor necrosis factor receptor I contribute to the development of chest trauma-induced septic acute lung injury

BACKGROUND

Direct acute lung injury (ALI) is still associated with a high mortality, whereas the underlying pathomechanisms are not yet fully understood. In this regard, epithelial cell death in the lungs has been attributed an important role in the pathogenesis of this clinical entity. Based on this background here, we hypothesized that signaling through Fas and tumor necrosis factor receptor 1 (TNFR-1) is involved in mediating apoptosis and inflammation in chest trauma induced septic ALI.

METHODS

Male C57BL/6 mice (wild-type [WT]), male mutant mice expressing nonfunctional Fas receptor (B6.MRL-Faslpr/J [lpr]) (lpr) and male TNFR-1-deficient mice (TNFR-1(-/-)) were subjected to a model of direct ALI consisting of blunt chest trauma followed by cecal ligation and puncture.Cytokine/chemokine concentrations of plasma, bronchoalveolar lavage (BAL) fluids, and lung tissue were investigated as well as BAL protein and lung myeloperoxidase. Lung histology was assessed; lung caspase 3, TUNEL-positive cells, and apoptotic polymorphonuclear neutrophil were measured, followed by a survival study.

RESULTS

Cytokine/chemokine levels in plasma, BAL, and lung tissue were markedly increased in WT animals following ALI, whereas lpr and TNFR-1((-/-) showed significantly decreased levels. BAL protein levels were substantially elevated following ALI, but lpr animals presented markedly diminished protein levels compared with WT and TNFR-1(-/-) animals. Lung myeloperoxidase level was only increased 12 hours after ALI in WT animals, whereas lung myeloperoxidase levels in lpr and TNFR-1(-/-) animals were not increased compared with sham. Lung histology revealed beneficial effects in lpr and TNFR-1(-/-). Lung active caspase 3 after ALI was substantially decreased in lpr and TNFR-1(-/-) mice compared with WT. Interestingly, an early but not persisting survival benefit was observed in lpr and TNFR-1 animals(-/-).

CONCLUSION

Pathomechanistically, Fas and TNFR-1 signaling contributed to the apoptotic and inflammatory response in a clinically relevant double-hit model of trauma-induced septic ALI. Moreover, this was associated with a temporary survival benefit.

The Fas/FasL pathway impairs the alveolar fluid clearance in mouse lungs

Alveolar epithelial damage is a critical event that leads to protein-rich edema in acute lung injury (ALI), but the mechanisms leading to epithelial damage are not completely understood. Cell death by necrosis and apoptosis occurs in alveolar epithelial cells in the lungs of patients with ALI. Fas activation induces apoptosis of alveolar epithelial cells, but its role in the formation of lung edema is unclear. The main goal of this study was to determine whether activation of the Fas/Fas ligand pathway in the lungs could alter the function of the lung epithelium, and the mechanisms involved. The results show that Fas activation alters the alveolar barrier integrity and impairs the ability of the lung alveolar epithelium to reabsorb fluid from the air spaces. This result was dependent on the presence of a normal Fas receptor and was not affected by inflammation induced by Fas activation. Alteration of the fluid transport properties of the alveolar epithelium was partially restored by β-adrenergic stimulation. Fas activation also caused apoptosis of alveolar endothelial cells, but this effect was less pronounced than the effect on the alveolar epithelium. Thus, activation of the Fas pathway impairs alveolar epithelial function in mouse lungs by mechanisms involving caspase-dependent apoptosis, suggesting that targeting apoptotic pathways could reduce the formation of lung edema in ALI.

Silencing of fas, fas-associated via death domain, or caspase 3 differentially affects lung inflammation, apoptosis, and development of trauma-induced septic acute lung injury

Activation of Fas signaling is a potentially important pathophysiological mechanism in the development of septic acute lung injury (ALI). However, so far the optimal targets within this signaling cascade remain elusive. Thus, we tested the hypothesis that in vivo gene silencing of Fas, Fas-associated via death domain (FADD), or caspase 3 by intratracheal administration of small interfering RNA would ameliorate ALI in a clinically relevant double-hit mouse model of trauma induced septic lung injury. Male C57Bl/6 mice received small interfering (Fas, FADD, caspase 3) or control RNA 24 h before and 12 h after blunt chest trauma or sham procedures. Polymicrobial sepsis was induced by cecal ligation and puncture 24 h after chest trauma. Twelve or 24 h later, lung tissue, plasma, and bronchoalveolar lavage fluid were harvested. During ALI, lung apoptosis (active caspase 3 Western blotting, TUNEL staining) was substantially increased when compared with sham. Silencing of caspase 3 or FADD both markedly reduced pulmonary apoptosis. Fas- and FADD-small interfering RNA administration substantially decreased lung cytokine concentration, whereas caspase 3 silencing did not reduce lung inflammation. In addition, Fas silencing markedly decreased lung neutrophil infiltration. Interestingly, only in response to caspase 3 silencing, ALI-induced lung epithelial barrier dysfunction was substantially improved, and histological appearance was beneficially affected. Taken together, downstream inhibition of lung apoptosis via caspase 3 silencing proved to be superior in mitigating ALI when compared with upstream inhibition of apoptosis via Fas or FADD silencing, even in the presence of additional anti-inflammatory effects. This indicates a major pathophysiological role of lung apoptosis and suggests the importance of other than Fas-driven apoptotic pathways in trauma-induced septic ALI.

A decoy receptor 3 analogue reduces localised defects in phagocyte function in pneumococcal pneumonia

Background

Therapeutic strategies to modulate the host response to bacterial pneumonia are needed to improve outcomes during community-acquired pneumonia. This study used mice with impaired Fas signalling to examine susceptibility to pneumococcal pneumonia and decoy receptor 3 analogue (DcR3-a) to correct factors associated with increased susceptibility.

Methods

Wild-type mice and those with varying degrees of impairment of Fas (lpr) or Fas ligand signalling (gld) were challenged with Streptococcus pneumoniae and microbiological and immunological outcomes measured in the presence or absence of DcR3-a.

Results

During established pneumonia, neutrophils became the predominant cell in the airway and gld mice were less able to clear bacteria from the lungs, demonstrating localised impairment of pulmonary neutrophil function in comparison to lpr or wild-type mice. T-cells from gld mice had enhanced activation and reduced apoptosis in comparison to wild-type and lpr mice during established pneumonia. Treatment with DcR3-a reduced T-cell activation and corrected the defect in pulmonary bacterial clearance in gld mice.

Conclusions

The results suggest that imbalance in tumour necrosis factor superfamily signalling and excessive T-cell activation can impair bacterial clearance in the lung but that DcR3-a treatment can reduce T-cell activation, restore optimal pulmonary neutrophil function and enhance bacterial clearance during S pneumoniae infection.

Inhibition of mammalian target of rapamycin augments lipopolysaccharide-induced lung injury and apoptosis

Acute lung injury during bacterial infection is associated with neutrophilic inflammation, epithelial cell apoptosis, and disruption of the alveolar-capillary barrier. TLR4 is required for lung injury in animals exposed to bacterial LPS and initiates proinflammatory responses in part via the transcription factor NF-κB. Ligation of TLR4 also initiates a proapoptotic response by activating IFN-β and STAT1-dependent genes. We recently demonstrated that mammalian target of rapamycin (mTOR), a key controller of cell growth and survival, can physically interact with STAT1 and suppress the induction of STAT1-dependent apoptosis genes. We therefore hypothesized that the mTOR inhibitor rapamycin would increase LPS-induced apoptosis and lung injury in vivo. Rapamycin increased lung injury and cellular apoptosis in C57BL/6J mice exposed to intratracheal LPS for 24 h. Rapamycin also augmented STAT1 activation, and the induction of STAT1-dependent genes that mediate cellular apoptosis (i.e., Fas, caspase-3). LPS-induced lung injury was attenuated in STAT1 knockout mice. In addition, LPS and IFN-β-induced apoptosis was absent in cultured cells lacking STAT1, and, unlike in wild-type cells, a permissive effect of rapamycin was not observed. In contrast to its effect on STAT1, rapamycin inhibited NF-κB activation in vivo and reduced selected markers of inflammation (i.e., neutrophils in the bronchoalveolar lavage fluid, TNF-α). Therefore, although it inhibits NF-κB and neutrophilic inflammation, rapamycin augments LPS-induced lung injury and apoptosis in a mechanism that involves STAT1 and the induction of STAT1-dependent apoptosis genes.

Apoptosis and the airway epithelium

The airway epithelium functions as a barrier and front line of host defense in the lung. Apoptosis or programmed cell death can be elicited in the epithelium as a response to viral infection, exposure to allergen or to environmental toxins, or to drugs. While apoptosis can be induced via activation of death receptors on the cell surface or by disruption of mitochondrial polarity, epithelial cells compared to inflammatory cells are more resistant to apoptotic stimuli. This paper focuses on the response of airway epithelium to apoptosis in the normal state, apoptosis as a potential regulator of the number and types of epithelial cells in the airway, and the contribution of epithelial cell apoptosis in important airways diseases.

Pathogenesis of indirect (secondary) acute lung injury

At present, therapeutic interventions to treat acute lung injury (ALI) or acute respiratory distress syndrome (ARDS) remain largely limited to lung-protective strategies, as no real molecular-pathophysiologic-driven therapeutic intervention has yet become available. This is in part the result of the heterogeneous nature of the etiological processes that contribute to the state of ALI/ARDS. This article sets out to understand the development of ALI resulting from indirect pulmonary insults, such as extrapulmonary sepsis and trauma, shock, burn injury or mass transfusion, as opposed to direct pulmonary challenges, such as pneumonia, aspiration or lung contusion. Here, we consider not only the experimental and clinical data concerning the roles of various immune (neutrophil, macrophage, lymphocyte and dendritic) as well as nonimmune (epithelial and endothelial) cells in orchestrating the development of ALI resulting from indirect pulmonary stimuli, but also how these cell populations might be targeted therapeutically.

Death receptors mediate the adverse effects of febrile-range hyperthermia on the outcome of lipopolysaccharide-induced lung injury

We have shown that febrile-range hyperthermia enhances lung injury and mortality in mice exposed to inhaled LPS and is associated with increased TNF-α receptor activity, suppression of NF-κB activity in vitro, and increased apoptosis of alveolar epithelial cells in vivo. We hypothesized that hyperthermia enhances lung injury and mortality in vivo by a mechanism dependent on TNF receptor signaling. To test this, we exposed mice lacking the TNF-receptor family members TNFR1/R2 or Fas (TNFR1/R2(-/-) and lpr) to inhaled LPS with or without febrile-range hyperthermia. For comparison, we studied mice lacking IL-1 receptor activity (IL-1R(-/-)) to determine the role of inflammation on the effect of hyperthermia in vivo. TNFR1/R2(-/-) and lpr mice were protected from augmented alveolar permeability and mortality associated with hyperthermia, whereas IL-1R(-/-) mice were susceptible to augmented alveolar permeability but protected from mortality associated with hyperthermia. Hyperthermia decreased pulmonary concentrations of TNF-α and keratinocyte-derived chemokine after LPS in C57BL/6 mice and did not affect pulmonary inflammation but enhanced circulating markers of oxidative injury and nitric oxide metabolites. The data suggest that hyperthermia enhances lung injury by a mechanism that requires death receptor activity and is not directly associated with changes in inflammation mediated by hyperthermia. In addition, hyperthermia appears to enhance mortality by generating a systemic inflammatory response and not by a mechanism directly associated with respiratory failure. Finally, we observed that exposure to febrile-range hyperthermia converts a modest, survivable model of lung injury into a fatal syndrome associated with oxidative and nitrosative stress, similar to the systemic inflammatory response syndrome.

Exudate macrophages attenuate lung injury by the release of IL-1 receptor antagonist in gram-negative pneumonia

RATIONALE

Exudate macrophages are key players in host defense toward invading pathogens. Their antiinflammatory and epithelial-protective potential in gram-negative pneumonia, however, remains elusive.

OBJECTIVES

We investigated whether exudate macrophages contributed to preservation of alveolar epithelial barrier integrity and analyzed the molecular pathways involved.

METHODS

We evaluated the antiinflammatory and epithelial-protective effects of exudate macrophages in a model of LPS- and Klebsiella pneumoniae-induced lung injury comparing wild-type and CC-chemokine receptor 2 (CCR2)-deficient mice with defective lung macrophage recruitment and in in vitro studies using primary alveolar epithelial cells.

MEASUREMENTS AND MAIN RESULTS

CCR2(-/-) mice exhibited enhanced alveolar epithelial cell apoptosis and lung leakage on intratracheal LPS treatment, which could be attributed to lack of exudate macrophage recruitment from the circulating pool as demonstrated in a model of wild-type/CCR2(-/-) bone-marrow chimeric mice. Among various antiinflammatory and proliferative mediators analyzed, the endogenous counterpart of resident macrophage-expressed IL-1β, IL-1 receptor antagonist (IL-1ra), was highly up-regulated in flow-sorted exudate macrophages in LPS-treated wild-type mice. LPS/IL-1β-induced impairment of alveolar epithelial cell integrity was antagonized by IL-1ra in vitro. Finally, intratracheal substitution of IL-1ra or intravenous adoptive transfer of IL-1ra(+/+) but not IL-1ra(-/-) blood mononuclear cells attenuated alveolar inflammation, epithelial apoptosis, and loss of barrier function in LPS-challenged or K. pneumoniae-infected CCR2(-/-) mice and enhanced survival after K. pneumoniae infection.

CONCLUSIONS

We conclude that recruited lung macrophages attenuate IL-1β-mediated acute lung injury in gram-negative pneumonia by release of IL-1ra.

The Role of Fas-FasL Signaling Pathway in Induction of Apoptosis in Patients with Sulfur Mustard-Induced Chronic Bronchiolitis

Sulfur mustard (SM) is an alkylating agent that induces apoptosis and necrosis in cells. Fas-Fas ligand (FasL) interaction could induce apoptosis as well. In this study, it was hypothesized that apoptosis might play an important role in the pathogenesis of SM-induced lung injury via Fas-FasL signaling pathway. In a case-control study, Fas and FasL levels, caspase-3 activity and percent of apoptotic cells were measured in bronchoalveolar lavage (BAL) fluid of patients 20 years after exposure to sulfur mustard and compared with the control group. Results show that Fas and FasL levels were significantly higher in BAL fluid cells in patients group compared with the control (P = .001). No significant differences were observed between mild and moderate-severe groups. BAL fluid cells caspase-3 activity was not significantly different among the mild, moderate-severe, and control groups. The data suggest that Fas-FasL-induced apoptosis was impaired in BAL fluid cells of SM-exposed patients which might be one of the initiators of pathogenesis in SM-induced lung injury in these patients.

Acute lung injury: epidemiology, pathogenesis, and treatment

Acute lung injury (ALI) remains a significant source of morbidity and mortality in the critically ill patient population. Defined by a constellation of clinical criteria (acute onset of bilateral pulmonary infiltrates with hypoxemia without evidence of hydrostatic pulmonary edema), ALI has a high incidence (200,000 per year in the US) and overall mortality remains high. Pathogenesis of ALI is explained by injury to both the vascular endothelium and alveolar epithelium. Recent advances in the understanding of pathophysiology have identified several biologic markers that are associated with worse clinical outcomes. Phase III clinical trials by the NHLBI ARDS Network have resulted in improvement in survival and a reduction in the duration of mechanical ventilation with a lung-protective ventilation strategy and fluid conservative protocol. Potential areas of future treatments include nutritional strategies, statin therapy, and mesenchymal stem cells.

Febrile-range hyperthermia augments lipopolysaccharide-induced lung injury by a mechanism of enhanced alveolar epithelial apoptosis

Fever is common in critically ill patients and is associated with worse clinical outcomes, including increased intensive care unit mortality. In animal models, febrile-range hyperthermia (FRH) worsens acute lung injury, but the mechanisms by which this occurs remain uncertain. We hypothesized that FRH augments the response of the alveolar epithelium to TNF-alpha receptor family signaling. We found that FRH augmented LPS-induced lung injury and increased LPS-induced mortality in mice. At 24 h, animals exposed to hyperthermia and LPS had significant increases in alveolar permeability without changes in inflammatory cells in bronchoalveolar lavage fluid or lung tissue as compared with animals exposed to LPS alone. The increase in alveolar permeability was associated with an increase in alveolar epithelial apoptosis and was attenuated by caspase inhibition with zVAD.fmk. At 48 h, the animals exposed to hyperthermia and LPS had an enhanced lung inflammatory response. In murine lung epithelial cell lines (MLE-15, LA-4) and in primary type II alveolar epithelial cells, FRH enhanced apoptosis in response to TNF-alpha but not Fas ligand. The increase in apoptosis was caspase-8 dependent and associated with suppression of NF-kappaB activity. The FRH-associated NF-kappaB suppression was not associated with persistence of IkappaB-alpha, suggesting that FRH-mediated suppression of NF-kappaB occurs by means other than alteration of IkappaB-alpha kinetics. These data show for the first time that FRH promotes lung injury in part by increasing lung epithelial apoptosis. The enhanced apoptotic response might relate to FRH-mediated suppression of NF-kappaB activity in the alveolar epithelium with a resultant increase in susceptibility to TNF-alpha-mediated cell death.

Effects of TNF-alpha-converting enzyme inhibition on acute lung injury induced by endotoxin in the rat

We studied the effects of TNF-converting enzyme inhibition with Y-41654, which down-regulates the production of soluble TNF-alpha (sTNF-alpha), on acute lung injury induced by intratracheal administration of LPS. We first verified in vitro that pretreatment of isolated alveolar macrophages from Sprague-Dawley male rats with 20 microL of 0.1-mM Y-41654, decreased significantly (P < 0.05) the concentration of sTNF-alpha in cell supernatants induced by 10 microg/mL of LPS. We then studied four groups of rats (each n = 10) including 1) a control group, 2) an LPS group (300 microg /kg, instilled intratracheally), 3) a Y-41654 group, and 4) a treatment group treated with Y-41654 after LPS instillation. Y-41654, 10 mg/kg in 0.7 mL of phosphate-buffered saline, was administered (i.v.), 15 min before and 0.5, 1.5, 2.5, and 3.5 h after saline or LPS instillation. The animals were observed for 4 h. In the animals treated with Y-41654, the concentrations of sTNF-alpha and protein in bronchoalveolar lavage fluid, and the number of neutrophils in lung tissue and bronchoalveolar lavage fluid were significantly lower at 4 h than in the LPS group (P < 0.05). In conclusion, sTNF-alpha plays an important role in the development of acute lung injury induced by intratracheal administration of LPS, in part modulating neutrophil kinetics.

Potential role of soluble TRAIL in epithelial injury in children with severe RSV infection

Lower respiratory tract infection by respiratory syncytial virus (RSV) is a frequent cause of acute lung injury in young children and infants. Studies in adults and animals suggest that tumor necrosis factor receptor (TNFR) ligands may mediate lung injury by causing apoptosis of epithelial cells. The main goal of the present study was to determine whether the TNF-related apoptosis-inducing ligand (Apo2L/TRAIL) pathway may be implicated in epithelial injury during severe RSV infection in children. We report elevated levels of soluble (s)TRAIL released by leukocytes in bronchoalveolar lavage fluid (BALF) of patients with RSV-associated respiratory failure (n = 22) as compared with mechanically ventilated patients without pulmonary illness (n = 7). Primary bronchial epithelial cells of children without pulmonary disease obtained by nonbronchoscopic cytobrushing expressed both death receptors TRAIL-R1 and -R2, and were found to be susceptible for cell death by human recombinant sTRAIL in vitro. Furthermore, BALF from a patient with RSV induced cell death in these cells, which was partly attenuated by inhibiting TRAIL signaling. These data suggest that the TRAIL pro-apoptotic pathway may contribute to lung epithelial injury in severe RSV infection in children.

Phosphodiesterase 2 inhibition diminished acute lung injury in murine pneumococcal pneumonia

OBJECTIVE

Severe pneumococcal pneumonia frequently causes respiratory failure. Both pathogen factors and an uncontrolled host response may contribute to acute lung injury by impairing microvascular barrier function. Phosphodiesterase 2 (PDE2) was examined as a potential target in pneumonia-induced lung microvascular hyperpermeability.

DESIGN

Controlled, in vitro, ex vivo, and in vivo laboratory study.

SUBJECTS

Female Balb/C and C57Bl/6 mice, 8-12 weeks old.

INTERVENTIONS

Human umbilical vein endothelial cells and isolated mouse lungs were challenged with the pneumococcal exotoxin pneumolysin in the presence or absence of the selective PDE2 inhibitors 9-(6-phenyl-2-oxohex-3-yl)-2-(3,4-dimethoxybenzyl)-purin-6one (PDP) or hydroxy-PDP. Transcellular electrical resistance or human serum albumin leakage in bronchoalveolar lavage fluid was determined, respectively. In addition, we induced pneumococcal pneumonia in mice and treated with hydroxy-PDP via continuous subcutaneous application by osmotic pumps. Human serum albumin leakage in bronchoalveolar lavage fluid was measured 48 hours after transnasal infection, and lung specimens were analyzed by TaqMan real-time polymerase chain reaction and Western blot for PDE2 gene and protein expression.

MEASUREMENTS AND MAIN RESULTS

In isolated perfused mouse lungs and in human umbilical vein endothelial cell monolayers, selective inhibition of PDE2 markedly decreased pneumolysin-induced hyperpermeability. Furthermore, in murine pneumococcal pneumonia, pulmonary PDE2-mRNA and -protein expression was significantly increased, and pneumonia-induced vascular permeability was distinctively reduced by PDE2 inhibition.

CONCLUSIONS

PDE2 inhibition diminished microvascular leakage in pneumococcal pneumonia, and pulmonary PDE2 upregulation may play a crucial role in this respect. Selective PDE2 inhibitors thus may offer a promising therapeutic approach in severe pneumococcal pneumonia.

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.

Fas-ligand-induced apoptosis of respiratory epithelial cells causes disruption of postcanalicular alveolar development

Premature infants are at risk for bronchopulmonary dysplasia, a complex condition characterized by impaired alveolar development and increased alveolar epithelial apoptosis. The functional involvement of pulmonary apoptosis in bronchopulmonary dysplasia- associated alveolar disruption remains undetermined. The aims of this study were to generate conditional lung-specific Fas-ligand (FasL) transgenic mice and to determine the effects of FasL-induced respiratory epithelial apoptosis on alveolar remodeling in postcanalicular lungs. Transgenic (TetOp)(7)-FasL responder mice, generated by pronuclear microinjection, were bred with Clara cell secretory protein (CCSP)-rtTA activator mice. Doxycycline (Dox) was administered from embryonal day 14 to postnatal day 7, and lungs were studied between embryonal day 19 and postnatal day 21. Dox administration induced marked respiratory epithelium-specific FasL mRNA and protein up-regulation in double-transgenic CCSP-rtTA(+)/(TetOp)(7)-FasL(+) mice compared with single-transgenic CCSP-rtTA(+) littermates. The Dox-induced FasL up-regulation was associated with dramatically increased apoptosis of alveolar type II cells and Clara cells, disrupted alveolar development, decreased vascular density, and increased postnatal lethality. These data demonstrate that FasL-induced alveolar epithelial apoptosis during postcanalicular lung remodeling is sufficient to disrupt alveolar development after birth. The availability of inducible lung-specific FasL transgenic mice will facilitate studies of the role of apoptosis in normal and disrupted alveologenesis and may lead to novel therapeutic approaches for perinatal and adult pulmonary diseases characterized by dysregulated apoptosis.

Animal models of human pneumonia

Pneumonia is a medical and public health priority, and advances against this disease will require improved knowledge of biological mechanisms. Human pneumonia is modeled with experimental infections of animals, most frequently mice. Mouse models are leading to important discoveries relevant to pneumonia, but their limitations must be carefully considered. Several approaches to establishing pneumonia in mice have been developed, and each has specific strengths and weaknesses. Similarly, procedures for characterizing microbial and host responses to infection have unique advantages and disadvantages. Mice are not small humans, and the applicability of results from murine models to human disease depends on understanding the similarities and differences between species. Additional considerations such as mouse strain, microbe strain, and prior mouse-microbe interactions also influence the design and interpretation of experiments. Results from studies of pneumonia in animals, combined with complementary basic and translational studies, are elucidating mechanisms responsible for susceptibility to and pathophysiology of lung infection.

Fas-induced pulmonary apoptosis and inflammation during indirect acute lung injury

RATIONALE

Indirect acute lung injury (ALI) is associated with high morbidity and mortality. No specific therapies have been developed, because the underlying pathophysiological processes remain elusive.

OBJECTIVES

To investigate the contribution of Fas-induced apoptotic and nonapoptotic/inflammatory signaling to the pathology of indirect ALI.

METHODS

A mouse model of indirect ALI, induced by successive exposure to hemorrhagic shock and cecal ligation and puncture, was used. Quantification of active caspase-3 and the short splice variant of FLICE-inhibitory protein, (FLIP)short, was performed by Western blotting and immunohistochemistry, and cytokines/chemokines were assessed by cytometric bead array or ELISA. M30 immunostaining was done to evaluate epithelial cell apoptosis. Lung injury was assessed on the basis of myeloperoxidase activity, bronchoalveolar lavage protein, and lung histology.

MEASUREMENTS AND MAIN RESULTS

Twelve hours after insult, lung monocyte chemoattractant protein-1, keratinocyte-derived chemokine, macrophage inflammatory protein-2, IL-6, tumor necrosis factor-alpha, and caspase-3 were increased and FLIP(short) was decreased. Fas- and Fas ligand-deficient mice showed marked protection from lung inflammation and apoptosis and decreased ALI. This was associated with a 10-day survival benefit. Similarly, 4 hours after pulmonary instillation of Fas-activating antibody in vivo, lung chemokines were markedly elevated in background mice and, interestingly, to a similar degree in macrophage-deficient animals. Fas activation on lung epithelial cells in vitro led to chemokine production that was dependent on extracellular signal-regulated kinase.

CONCLUSIONS

Activation of apoptotic and nonapoptotic/inflammatory Fas signaling is an early important pathophysiological event in the development of indirect ALI after hemorrhagic shock and sepsis, in which lung epithelial cells appear to play a central role.

Anti-KC autoantibody:KC complexes cause severe lung inflammation in mice via IgG receptors

We have shown previously that high concentrations of IL-8 associated with anti-IL-8 autoantibodies (anti-IL-8:IL-8 complexes) are present in lung fluids from patients with the acute respiratory distress syndrome (ARDS), and correlate both with the development and outcome of ARDS. We also detected deposition of these complexes in lung tissues from patients with ARDS but not in control tissues. Moreover, we determined that IgG receptors (FcgammaRs) mediate activity of anti-IL-8:IL-8 complexes. In the current study, we generated anti-KC (KC = chemokine (CXC motif) ligand 1 (CXCL1)) autoantibody:KC immune complexes (KC-functional IL-8) in lungs of mice to develop a mouse model of autoimmune complex-induced lung inflammation. Both wild-type (WT) and gamma-chain-deficient mice that lack receptors for immune complexes (FcgammaRs) were studied. First, the mice were immunized with KC to induce anti-KC autoantibodies. Then, KC was administered intratracheally to generate anti-KC:KC complexes in the lung. Presence of anti-KC:KC complexes was associated with development of severe pulmonary inflammation that was, however, dramatically suppressed in gamma-chain-deficient mice. Second, because sepsis is considered the major risk factor for development of ARDS, we evaluated LPS-treated WT as well as gamma-chain-deficient mice for the presence of anti-KC:KC complexes and pulmonary inflammatory responses. We detected complexes between anti-KC autoantibodies and KC in lung lavages and tissues of mice treated with LPS. Moreover, gamma-chain-deficient mice that lack receptors for immune complexes were protected from LPS-induced pulmonary inflammation. Our results suggest that immune complexes containing autoantibodies contribute to development of lung inflammation in LPS-treated mice.

Stroke Propagates Bacterial Aspiration to Pneumonia in a Model of Cerebral Ischemia

BACKGROUND AND PURPOSE

Bacterial pneumonia is the most common cause of death in patients sustaining acute stroke and is believed to result from an increased aspiration. Recently, stroke-induced immunodeficiency was described in a mouse model of cerebral ischemia, which is primarily caused by overactivation of sympathetic nervous system. We tested if stroke-induced immunodeficiency increases the risk of pneumonia after aspiration in a newly developed model of poststroke pneumonia.

METHODS

Experimental stroke in mice was induced by occlusion of the middle cerebral artery (MCAO) for 60 minutes. Aspiration pneumonia was induced by intranasal application of 20 microL of a defined suspension of Streptococcus pneumoniae in phosphate-buffered saline 4 or 14 days after MCAO. Treatment comprised moxifloxacin (100 mg/kg body weight, six times every 2 hours after operation) or propranolol (30 mg/kg body weight, immediately before as well as 4 and 8 hours after MCAO). Readout was lung histology and bacterial counts in lung and blood.

RESULTS

Nasal inoculation of only 200 colony-forming units of S. pneumoniae caused severe pneumonia and bacteremia after experimental stroke, whereas 200,000 colony-forming units are needed to induce comparable disease in sham animals. Aspiration pneumonia in stroke animals outlasted acute stroke state but was preventable by beta-adrenoreceptor blockade.

CONCLUSIONS

Experimental stroke propagates bacterial aspiration from harmless intranasal colonization to harmful pneumonia. Prevention of infections by beta-adrenoreceptor blockade suggests that immunodepression by sympathetic hyperactivity is essential for progression of bacterial aspiration to pneumonia.

Mechanical ventilation induces inflammation, lung injury, and extra-pulmonary organ dysfunction in experimental pneumonia

Mechanical ventilation (MV) is frequently employed for the management of critically ill patients with respiratory failure. A major complication of mechanical ventilation (MV) is the development of ventilator-associated pneumonia (VAP), in which Staphylococcus aureus is a prominent pathogen. Moreover, previous studies suggest that MV may be an important cofactor in the development of acute lung injury (ALI) and the acute respiratory distress syndrome (ARDS). S. aureus pulmonary infection was induced in spontaneously breathing mice (C57Bl/6) or mechanically ventilated mice to determine whether MV contributes to the development of ALI and/or systemic inflammation. The combination of MV and bacteria significantly increased the influx of neutrophils into bronchoalveolar lavage fluid (BALF), augmented pulmonary production of the proinflammatory cytokines KC, MIP-2, TNF-alpha, and IL-6, and increased alveolar-capillary permeability to proteins. MV also induced proinflammatory cytokine expression in peripheral blood, associated with extrapulmonary hepatic and renal dysfunction. Surprisingly, bacterial clearance in the lungs and extrapulmonary bacterial dissemination was not affected by MV. These data indicate that MV exacerbates both pulmonary and systemic inflammation in response to bacteria and contributes to the pathogenesis of both ALI and the multiple organ dysfunction syndrome, without necessarily affecting bacterial clearance or extra-pulmonary bacterial dissemination.

Role of pneumolysin for the development of acute lung injury in pneumococcal pneumonia

OBJECTIVE

Acute respiratory failure is a significant complication of severe pneumococcal pneumonia. In a mouse model, we observed early-onset lung microvascular leakage after pulmonary infection with Streptococcus pneumoniae, and we hypothesized that the important virulence factor pneumolysin may be the direct causative agent.

DESIGN

Controlled, in vivo, ex vivo, and in vitro laboratory study.

SETTING

Laboratory.

SUBJECTS

Female mice, 8-12 wks old.

INTERVENTIONS

Ventilated and blood-free perfused murine lungs were challenged with recombinant pneumolysin via the airways as well as via the vascular bed. In addition, we analyzed the impact of pneumolysin on electrical cell impedance and hydraulic conductivity of human umbilical vein endothelial cell (HUVEC) and alveolar epithelial cell (A549) monolayers.

MEASUREMENTS AND MAIN RESULTS

Aerosolized pneumolysin dose-dependently increased capillary permeability with formation of severe lung edema but did not affect pulmonary vascular resistance. Intravascular pneumolysin caused an impressive dose-dependent increase in pulmonary vascular resistance and in lung microvascular permeability. By immunohistochemistry, pneumolysin was detected mainly in endothelial cells of pulmonary arterial vessels, which concomitantly displayed strong vasoconstriction. Moreover, pneumolysin increased permeability of HUVEC and A549 monolayers. Interestingly, immunofluorescence of endothelial cell monolayers exposed to pneumolysin showed gap formation and moderate stress fiber generation.

CONCLUSIONS

Pneumolysin may play a central role for early-onset acute lung injury due to severe pneumococcal pneumonia by causing impairment of pulmonary microvascular barrier function and severe pulmonary hypertension.

Streptococcus pneumoniae: the role of apoptosis in host defense and pathogenesis

Programmed cell death or apoptosis is a recognised feature of infection with Streptococcus pneumoniae, and is observed during pneumococcal meningitis and pneumonia. The cholesterol-dependent cytolysin, pneumolysin, is a major trigger of apoptosis in the brain in association with pneumococcal production of hydrogen peroxide. Pneumococcal cell wall is also an important stimulus for apoptosis. Microbial factors and host factors combine in causing apoptosis in the brain, with hippocampal neurons being particularly susceptible. In pulmonary infection epithelial cell apoptosis contributes to tissue injury but macrophage apoptosis may benefit the host, aiding microbial killing and downregulating the inflammatory response. During sepsis lymphocyte apoptosis may be harmful to the host while dendritic cell apoptosis may limit the generation of an adaptive immune response during infection. Apoptosis induction may be harmful or potentially beneficial during pneumococcal infection and understanding its function in each setting is essential to allow specific therapeutic intervention.

Apoptosis and epithelial injury in the lungs

Epithelial injury is a critical event in the development of acute lung injury, but the mechanisms that cause death of the alveolar epithelium are not completely understood. Epithelial death occurs by necrosis and apoptosis; more information is needed about the balance between these two types of cell death in the lungs. Direct epithelial necrosis probably occurs in response to bacterial exotoxins and over-distension of alveolar units by mechanical ventilation. Apoptosis is a regulated form of cell death that is mediated by membrane death receptors and direct mitochondrial injury. Apoptosis pathways are activated in the lungs of patients with acute lung injury, in part by activation of the membrane Fas death receptor by soluble Fas ligand (sFasL), which accumulates in biologically active form at the onset of lung injury. Accumulating evidence in humans and experimental models links sFasL and Fas pathway with lung epithelial injury and fibrosis. New strategies to inhibit Fas-mediated epithelial apoptosis need to be developed in order to develop new ways to preserve epithelial function in patients who develop acute lung injury.

Silencing of Fas, but not caspase-8, in lung epithelial cells ameliorates pulmonary apoptosis, inflammation, and neutrophil influx after hemorrhagic shock and sepsis

Apoptosis and inflammation play an important role in the pathogenesis of direct/pulmonary acute lung injury (ALI). However, the role of the Fas receptor-driven apoptotic pathway in indirect/nonpulmonary ALI is virtually unstudied. We hypothesized that if Fas or caspase-8 plays a role in the induction of indirect ALI, their local silencing using small interfering RNA (siRNA) should be protective in hemorrhage-induced septic ALI. Initially, as a proof of principle, green fluorescent protein-siRNA was administered intratracheally into transgenic mice overexpressing green fluorescent protein. Twenty-four hours after siRNA delivery, lung sections revealed a significant decrease in green fluorescence. Intratracheally administered Cy-5-labeled Fas-siRNA localized primarily in pulmonary epithelial cells. Intratracheal instillation of siRNA did not induce lung inflammation via toll-like receptor or protein kinase PKR pathways as assessed by lung tissue interferon-alpha, tumor necrosis factor-alpha, and interleukin (IL)-6 levels. Mice subjected to hemorrhagic shock and sepsis received either Fas-, caspase-8-, or control-siRNA intratracheally 4 hours after hemorrhage. Fas- or caspase-8-siRNA significantly reduced lung tissue Fas or caspase-8 mRNA, respectively. Only Fas-siRNA markedly diminished lung tissue tumor necrosis factor-alpha, IL-6, IL-10, interferon-gamma, IL-12, and caspase-3 activity. Fas-siRNA also preserved alveolar architecture and reduced lung neutrophil infiltration and pulmonary epithelial apoptosis. These data indicate the pathophysiological significance of Fas activation in nonpulmonary/shock-induced ALI and the feasibility of intrapulmonary administration of anti-apoptotic siRNA in vivo.

Fas-mediated acute lung injury requires fas expression on nonmyeloid cells of the lung

Fas (CD95) is a membrane surface receptor, which, in the lungs, is expressed in macrophages, neutrophils, and epithelial cells. In mice, Fas activation leads to a form of lung injury characterized by increased alveolar permeability. We investigated whether Fas-mediated lung injury occurs primarily as a result of Fas activation in myeloid cells (such as macrophages) or in nonmyeloid cells (such as epithelial cells). Chimeric mice lacking Fas in either myeloid or nonmyeloid cells were generated by transplanting marrow cells from lpr mice (which lack Fas) into lethally irradiated C57BL/6 mice (MyFas(-) group) or vice versa (MyFas(+) group). Additional mice transplanted with marrow cells from their same strain served as controls (Fas(+) ctr and Fas(-) ctr groups). Sixty days after transplantation, the mice received intratracheal instillations of the Fas-activating mAb Jo2 (n = 10/group), or an isotype control Ab (n = 10/group), and were euthanized 24-h later. Only animals expressing Fas in nonmyeloid cells (Fas(+) ctr and MyFas(-)) showed significant increases in lung neutrophil content and in alveolar permeability. These same mice showed tissue evidence of lung injury and caspase-3 activation in cells of the alveolar walls. Despite differences in the neutrophilic response and lung injury, there was no statistical difference in the lung cytokine concentrations (KC and MIP-2) among groups. We conclude that Fas-mediated lung injury requires expression of Fas on nonmyeloid cells of the lungs. These findings suggest that the alveolar epithelium is the primary target of Fas-mediated acute lung injury, and demonstrate that apoptotic processes may be associated with neutrophilic inflammation.

Inflammatory cells as a source of airspace extracellular superoxide dismutase after pulmonary injury

Extracellular superoxide dismutase (EC-SOD) is an antioxidant abundant in the lung. Previous studies demonstrated depletion of lung parenchymal EC-SOD in mouse models of interstitial lung disease coinciding with an accumulation of EC-SOD in airspaces. EC-SOD sticks to the matrix by a proteolytically sensitive heparin-binding domain; therefore, we hypothesized that interstitial inflammation and matrix remodeling contribute to proteolytic redistribution of EC-SOD from lung parenchyma into the airspaces. To determine if inflammation limited to airspaces leads to EC-SOD redistribution, we examined a bacterial pneumonia model. This model led to increases in airspace polymorphonuclear leukocytes staining strongly for EC-SOD. EC-SOD accumulated in airspaces at 24 h without depletion of EC-SOD from lung parenchyma. This led us to hypothesize that airspace EC-SOD was released from inflammatory cells and was not a redistribution of matrix EC-SOD. To test this hypothesis, transgenic mice with lung-specific expression of human EC-SOD were treated with asbestos or bleomycin to initiate an interstitial lung injury. In these studies, EC-SOD accumulating in airspaces was entirely the mouse isoform, demonstrating an extrapulmonary source (inflammatory cells) for this EC-SOD. We also demonstrate that EC-SOD knockout mice possess greater lung inflammation in response to bleomycin and bacteria when compared with wild types. We conclude that the source of accumulating EC-SOD in airspaces in interstitial lung disease is inflammatory cells and not the lung and that interstitial processes such as those found in pulmonary fibrosis are required to remove EC-SOD from lung matrix.

Primary type II alveolar epithelial cells present microbial antigens to antigen-specific CD4+T cells

Type II alveolar epithelial cells (AEC) can produce various antimicrobial and proinflammatory effector molecules. This, together with their abundance and strategic location, suggests a role in host defense against pulmonary pathogens. We report that murine type II AEC, like their human counterparts, express class II major histocompatibility complex (MHC). Using a murine model of pulmonary tuberculosis, we find that type II AEC become activated and have increased cell surface expression of class II MHC, CD54, and CD95 following infection. Type II AEC use the class II MHC pathway to process and present mycobacterial antigens to immune CD4+T cells isolated from mice infected with Mycobacterium tuberculosis. Therefore, not only can type II AEC contribute to the pulmonary immunity by secreting chemokines that recruit inflammatory cells to the lung, but they can also serve as antigen-presenting cells. Although type II AEC are unlikely to prime naïve T cells, their ability to present antigens to T cells demonstrates that they can participate in the effector phase of the immune response. This represents a novel role for type II AEC in the immunological response to pulmonary pathogens.

TLR-4 pathway mediates the inflammatory response but not bacterial elimination in E. coli pneumonia

We examined the role of Toll-like receptor (TLR)-4 in modifying the lung inflammatory response and its effects on the bacterial recovery from the lungs following inhaled Escherichia coli in two different strains of TLR-4 mutant mice that are hyporesponsive to LPS. The C57BL/10ScN(tlr4(lps-del)) mice containing a deletion mutation in the TLR-4 gene showed lower proinflammatory cytokine levels, lower lung MPO activity, and less parenchymal and peribronchial inflammation compared with the C57BL/10ScSn mice, a related TLR-4 wild-type substrain. However, the C57BL/10ScN(tlr4(lps-del)) mutant showed lower bacterial recovery in the lungs following inhaled E. coli associated with a rapid but transient increase in air space neutrophil counts at 6 h. In comparison, the C3H/HeJ(tlr4(Lps-d)) mutant mice containing a Pro712His substitution in TLR-4 demonstrated lower proinflammatory cytokine levels, lower lung MPO activity, and lower neutrophil accumulation in the air spaces but showed no differences in the bacterial burden of inhaled E. coli at 6 h, when compared with the TLR-4 wild-type C3H/HeSnJ mice. Thus two different TLR-4 mutants showed attenuated inflammatory responses in the lungs, but the reduced inflammatory responses were not consistently associated with either improved or impaired bacterial elimination from the lungs. Our findings indicate that the inflammatory response to inhaled E. coli is TLR-4 dependent, but bacterial elimination depends on other factors in addition to TLR-4.

Tumor necrosis factor-alpha-dependent expression of phosphodiesterase 2: role in endothelial hyperpermeability

The pleiotropic cytokine tumor necrosis factor-alpha (TNF-alpha) and thrombin lead to increased endothelial permeability in sepsis. Numerous studies demonstrated the significance of intracellular cyclic nucleotides for the maintenance of endothelial barrier function. Actions of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) are terminated by distinct cyclic nucleotide phosphodiesterases (PDEs). We hypothesized that TNF-alpha could regulate PDE activity in endothelial cells, thereby impairing endothelial barrier function. In cultured human umbilical vein endothelial cells (HUVECs), we found a dramatic increase of PDE2 activity following TNF-alpha stimulation, while PDE3 and PDE4 activities remained unchanged. Significant PDE activities other than PDE2, PDE3, and PDE4 were not detected. TNF-alpha increased PDE2 expression in a p38 mitogen-activated protein kinase (MAPK)-dependent manner. Endothelial barrier function was investigated in HUVECs and in isolated mice lungs. Selective PDE2 up-regulation sensitized HUVECs toward the permeability-increasing agent thrombin. In isolated mice lungs, we demonstrated that PDE2 inhibition was effective in preventing thrombin-induced lung edema, as shown with a reduction in both lung wet-to-dry ratio and albumin flux from the vascular to bronchoalveolar compartment. Our findings suggest that TNF-alpha-mediated up-regulation of PDE2 may destabilize endothelial barrier function in sepsis. Inhibition of PDE2 is therefore of potential therapeutic interest in sepsis and acute respiratory distress syndrome (ARDS).

Sustained lipopolysaccharide-induced lung inflammation in mice is attenuated by functional deficiency of the Fas/Fas ligand system

To determine whether the Fas/Fas ligand (FasL) (CD95/CD178) system contributes to the development of an inflammatory response in vivo, 2.5 microg of bacterial lipopolysaccharide (LPS; endotoxin) per g was administered intranasally to healthy mice (C57BL/6) and mutant mice deficient in either Fas (lpr mice) or FasL (gld mice). Sustained LPS-induced neutrophilic inflammation in the lungs was attenuated in both lpr and gld mice. These observations provide further evidence of a proinflammatory role for the Fas/FasL system in the lungs.

Keratinocyte growth factor induces Akt kinase activity and inhibits Fas-mediated apoptosis in A549 lung epithelial cells

Acute respiratory distress syndrome (ARDS) is a syndrome characterized by the rapid influx of protein-rich edema fluid into the air spaces. The magnitude of alveolar epithelial cell injury is a key determinant of disease severity and an important predictor of patient outcome. The alveolar epithelium is positioned at the interface of the host response in the initiation, progression, and recovery phase of the disease. Keratinocyte growth factor (KGF) is a potent survival factor unique to the epithelium that promotes lung epithelial cell survival, accelerates wound closure, and reduces fibrosis. We therefore hypothesized that KGF preserves lung function by inhibiting apoptosis through activation of a signal transduction pathway responsible for cell survival. To test this hypothesis we determined that KGF inhibits death following Fas activation, a relevant apoptosis pathway, and then determined that cell survival is mediated through activation of the phosphatidylinositol 3'-kinase (PI3K)/Akt kinase signal transduction pathway. We found that KGF induces a dose- and time-dependent increase in Akt kinase activity and that, as expected, activation of Akt via KGF is PI3K dependent. KGF inhibited Fas-induced apoptosis as measured by a reduction in apoptotic cells and caspase-3 activity. This investigation supports our original hypothesis that KGF protects the lung epithelium by inhibiting apoptosis and that protection occurs through activation of PI3K/Akt-mediated cell survival pathway. Our results are in agreement with other reports that identify the PI3K/Akt axis as a key intracellular pathway in the lung epithelium that may serve as a therapeutic target to preserve epithelial integrity during inflammation.