Alveolar-capillary Barrier Dysfunction Research Articles

The Complex Immune Cell Composition and Cellular Interaction in the Alveolar Compartment of Patients with Acute Respiratory Distress Syndrome.

Acute respiratory distress syndrome (ARDS) is characterized by protein rich edema due to alveolar-capillary barrier dysfunction caused by inflammatory processes. Currently, our understanding of the inflammatory response in patients with ARDS is mainly based on assessment of the systemic compartment and preclinical studies. Investigations into the intricate network of immune cells and their critical functions in the alveolar compartment remain limited. However, with recent improvements in single cell analyses, our comprehensive understanding of the interactions between immune cells in the lungs has improved. In this review, we summarize the current knowledge about the cellular composition and interactions of different immune cell types within the alveolar space of patients with ARDS. Neutrophils and macrophages are the predominant immune cells in the alveolar space of ARDS patients. Yet, all immune cells present, including lymphocytes, participate in complex interactions, coordinate recruitment, modulate the lifespan and control apoptosis through various signaling pathways. Moreover, the cellular composition of alveolar immune cells is associated with clinical outcomes of ARDS patients. In conclusion, this synthesis advances our understanding of ARDS immunology, emphasizing the crucial role of immune cells within the alveolar space. Associations between cellular composition and clinical outcomes highlight the significance of exploring distinct alveolar immune cell subsets. Such exploration holds promise for uncovering novel therapeutic targets in ARDS pathophysiology, presenting avenues for enhancing clinical management and treatment strategies for ARDS patients. This article is open access and distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives License 4.0 (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Ferritinophagy Mediated by Oxidative Stress-Driven Mitochondrial Damage Is Involved in the Polystyrene Nanoparticles-Induced Ferroptosis of Lung Injury.

Nanoplastics are a common type of contaminant in the air. However, no investigations have focused on the toxic mechanism of lung injury induced by nanoplastic exposure. In the present study, polystyrene nanoplastics (PS-NPs) caused ferroptosis in lung epithelial cells, which could be alleviated by ferrostatin-1, deferoxamine, and N-acetylcysteine. Further investigation found that PS-NPs disturbed mitochondrial structure and function and triggered autophagy. Mechanistically, oxidative stress-derived mitochondrial damage contributed to ferroptosis, and autophagy-dependent ferritinophagy was a pivotal intermediate link, resulting in ferritin degradation and iron ion release. Furthermore, inhibition of ferroptosis using ferrostatin-1 alleviated pulmonary and systemic toxicity to reverse the mouse lung injury induced by PS-NPs inhalation. Most importantly, the lung-on-a-chip was further used to clarify the role of ferroptosis in the PS-NPs-induced lung injury by visualizing the ferroptosis, oxidative stress, and alveolar-capillary barrier dysfunction at the organ level. In summary, our study indicated that ferroptosis was an important mechanism for nanoplastics-induced lung injury through different lung cells, mouse inhalation models, and three-dimensional-based lung-on-a-chip, providing an insightful reference for pulmonary toxicity assessment of nanoplastics.

Engineered Extracellular Vesicles Derived from Dermal Fibroblasts Attenuate Inflammation in a Murine Model of Acute Lung Injury.

Acute respiratory distress syndrome (ARDS) represents a significant burden to the healthcare system, with ≈200000 cases diagnosed annually in the USA. ARDS patients suffer from severe refractory hypoxemia, alveolar-capillary barrier dysfunction, impaired surfactant function, and abnormal upregulation of inflammatory pathways that lead to intensive care unit admission, prolonged hospitalization, and increased disability-adjusted life years. Currently, there is no cure or FDA-approved therapy for ARDS. This work describes the implementation of engineered extracellular vesicle (eEV)-based nanocarriers for targeted nonviral delivery of anti-inflammatory payloads to the inflamed/injured lung. The results show the ability of surfactant protein A (SPA)-functionalized IL-4- and IL-10-loaded eEVs to promote intrapulmonary retention and reduce inflammation, both in vitro and in vivo. Significant attenuation is observed in tissue damage, proinflammatory cytokine secretion, macrophage activation, influx of protein-rich fluid, and neutrophil infiltration into the alveolar space as early as 6 h post-eEVs treatment. Additionally, metabolomics analyses show that eEV treatment causes significant changes in the metabolic profile of inflamed lungs, driving the secretion of key anti-inflammatory metabolites. Altogether, these results establish the potential of eEVs derived from dermal fibroblasts to reduce inflammation, tissue damage, and the prevalence/progression of injury during ARDS via nonviral delivery of anti-inflammatory genes/transcripts.

Identification of a subset of lung neutrophils that promote Acute Lung Injury resolution in mice

Abstract The acute respiratory distress syndrome (ARDS) is a major cause of respiratory failure with limited therapeutic options. Unresolving inflammation in the lungs and alveolar-capillary barrier dysfunction are main features of ARDS. The sharp increase in the incidence of ARDS during COVID19 emphasized the need for a better understanding of immunological events in ARDS that inform novel therapeutics. Here, we used a model of self-limited acid-induced ALI by selectively instilling HCL to the left lung of adult mice. The time-course of barrier dysfunction was determined via quantification of lung consolidation by micro-computed tomography, while inflammation was assessed by histology and flow cytometry. Neutrophils, rapidly recruited into the lungs, decreased over time during resolution while macrophage numbers, reduced during initial stages, increased. Immunophenotyping of the leukocyte recruitment to the lungs during the resolution of injury uncovered a neutrophils’ subset that was expressing SiglecF. These cells were distinct from eosinophils and phenotypically, and functionally different from the classical SiglecF-negative neutrophils. The roles for SiglecF+ neutrophils were further assessed by in vitro functional studies, adoptive transfer experiments, and lipidomic analysis. Depletion of neutrophils impaired alveolar epithelial cell type II (AT2) repair, while the adoptive transfer of SiglecF+ neutrophils accelerated epithelial cell restitution. Among the mechanisms associated with the SiglecF+ neutrophils in lung repair was the production of specialized pro-resolving mediators and CSF1. In sum, we identified protective roles for a subset of tissue neutrophils during the resolution of lung injury in mice.

Complement as a vital nexus of the pathobiological connectome for acute respiratory distress syndrome: An emerging therapeutic target.

The hallmark of acute respiratory distress syndrome (ARDS) pathobiology is unchecked inflammation-driven diffuse alveolar damage and alveolar-capillary barrier dysfunction. Currently, therapeutic interventions for ARDS remain largely limited to pulmonary-supportive strategies, and there is an unmet demand for pharmacologic therapies targeting the underlying pathology of ARDS in patients suffering from the illness. The complement cascade (ComC) plays an integral role in the regulation of both innate and adaptive immune responses. ComC activation can prime an overzealous cytokine storm and tissue/organ damage. The ARDS and acute lung injury (ALI) have an established relationship with early maladaptive ComC activation. In this review, we have collected evidence from the current studies linking ALI/ARDS with ComC dysregulation, focusing on elucidating the new emerging roles of the extracellular (canonical) and intracellular (non-canonical or complosome), ComC (complementome) in ALI/ARDS pathobiology, and highlighting complementome as a vital nexus of the pathobiological connectome for ALI/ARDS via its crosstalking with other systems of the immunome, DAMPome, PAMPome, coagulome, metabolome, and microbiome. We have also discussed the diagnostic/therapeutic potential and future direction of ALI/ARDS care with the ultimate goal of better defining mechanistic subtypes (endotypes and theratypes) through new methodologies in order to facilitate a more precise and effective complement-targeted therapy for treating these comorbidities. This information leads to support for a therapeutic anti-inflammatory strategy by targeting the ComC, where the arsenal of clinical-stage complement-specific drugs is available, especially for patients with ALI/ARDS due to COVID-19.

Protective effect of genkwanin against lipopolysaccharide-induced acute lung injury in mice with p38 mitogen-activated protein kinase and nuclear factor-κB pathway inhibition

• Genkwanin inhibited the lung histopathological changes in LPS-induced ALI mice. • Genkwanin inhibited lung oedema, alveolar–capillary barrier, and leukocytes infiltration dysfunction in LPS-induced ALI mice. • Genkwanin reduced the secretion of IL-1β, IL-6, and TNF-α in LPS-induced ALI mice. • Genkwanin reduced IκB degradation and p65 phosphorylation in LPS-induced ALI mice. • Genkwanin reduced p38 MAPK phosphorylation in LPS-induced ALI mice. Acute lung injury (ALI) is a life-threatening clinical syndrome. Genkwanin is a nonglycosylated flavonoid that has several biopharmacological effects, such as anti-inflammatory, antioxidative, and immunomodulator effects. This study investigated the protective effect and mechanism of genkwanin in a lipopolysaccharide (LPS)-induced mouse model. Histopathological analysis indicated that genkwanin had a potential ameliorative effect on LPS-induced ALI. Genkwanin also inhibited several pathogenic features induced by LPS, including lung oedema, alveolar-capillary barrier dysfunction, and leukocyte and neutrophil infiltration. In addition, it inhibited NF-κB pathway activation, including phosphorylation of p65 and degradation of IκB, induced by LPS. Genkwanin also inhibited the phosphorylation of the p38 MAPK pathway induced by LPS, although it did not inhibit the phosphorylation of extracellular signal-regulated kinases or c-Jun N -terminal kinases induced by LPS. In conclusion, our results indicate that genkwanin has a protective effect against LPS-induced ALI that it exerts by inhibiting NF-κB pathway activation and p38 MAPK phosphorylation.

Yu-Ping-Feng Formula Ameliorates Alveolar-Capillary Barrier Injury Induced by Exhausted-Exercise via Regulation of Cytoskeleton.

Background: Yu-ping-feng powder (YPF) is a compound traditional Chinese medicine extensively used in China for respiratory diseases. However, the role of YPF in alveolar-capillary barrier dysfunction remains unknown. This study aimed to explore the effect and potential mechanism of YPF on alveolar-capillary barrier injury induced by exhausted exercise. Methods: Male Sprague–Dawley rats were used to establish an exhausted-exercise model by using a motorized rodent treadmill. YPF at doses of 2.18 g/kg was administrated by gavage before exercise training for 10 consecutive days. Food intake-weight/body weight, blood gas analysis, lung water percent content, BALF protein concentration, morphological observation, quantitative proteomics, real-time PCR, and Western blot were performed. A rat pulmonary microvascular endothelial cell line (PMVEC) subjected to hypoxia was applied for assessing the related mechanism. Results: YPF attenuated the decrease of food intake weight/body weight, improved lung swelling and hemorrhage, alleviated the increase of lung water percent content and BALF protein concentration, and inhibited the impairment of lung morphology. In addition, YPF increased the expression of claudin 3, claudin 18, occludin, VE-cadherin, and β-catenin, attenuated the epithelial and endothelial hyperpermeability in vivo and/or in vitro, and the stress fiber formation in PMVECs after hypoxia. Quantitative proteomics discovered that the effect of YPF implicated the Siah2-ubiquitin-proteasomal pathway, Gng12-PAK1-MLCK, and RhoA/ROCK, which was further confirmed by Western blot. Data are available via ProteomeXchange with identifier PXD032737. Conclusion: YPF ameliorated alveolar-capillary barrier injury induced by exhausted exercise, which is accounted for at least partly by the regulation of cytoskeleton.

Nano-metal–organic-frameworks for treating H2O2-Secreting bacteria alleviate pulmonary injury and prevent systemic sepsis

As a vital bacteria-secreted toxin, hydrogen peroxide (H2O2) can destroy infected tissues and increase vascular permeability, leading to life-threatening systemic bacteremia or sepsis. No strategy that can alleviate H2O2-induced injury and prevent systemic sepsis has been reported. Herein, as a proof of concept, we demonstrate the use of H2O2-reactive metal-organic framework nanosystems (MOFs) for treating H2O2-secreting bacteria. In mice infected with Streptococcus pneumoniae (S. pneumoniae) isolated from patients, MOFs efficiently accumulate in the lungs after systemic administration due to infection-induced alveolar-capillary barrier dysfunction. Moreover, MOFs sequester pneumococcal H2O2, reduce endothelial DNA damage, and prevent systemic dissemination of bacteria. In addition, this nanosystem exhibits excellent chemodynamic bactericidal effects against drug-resistant bacteria. Through synergistic therapy with the antibiotic ampicillin, MOFs eliminate over 98% of invading S. pneumoniae, resulting in a survival rate of greater than 90% in mice infected with a lethal dose of S. pneumoniae. This work opens up new paths for the clinical treatment of toxin-secreting bacteria.

The NLRP3 inflammasome in macrophages is stimulated by cell-free hemoglobin.

Cell‐free hemoglobin (CFH) is associated with severe lung injury in human patients and is sufficient to induce airspace inflammation and alveolar–capillary barrier dysfunction in an experimental model of acute lung injury. The mechanisms through which this occurs are unknown. One key pathway which regulates inflammation during acute lung injury is the NLRP3 inflammasome. Because CFH can act as a damage‐associated molecular pattern, we hypothesized that CFH may activate the NLRP3 inflammasome during acute lung injury. Primary mouse alveolar macrophages and cultured murine macrophages exposed to CFH (0–1 mg/ml) for 24 hr demonstrated robust upregulation of the NLRP3 inflammasome components NLRP3, caspase‐1, and caspase‐11. Maximal induction of the NLRP3 inflammasome by CFH required TLR4. Compared to wild‐type controls, mice lacking NLRP3 developed less airspace inflammation (2.7 × 105 cells/ml in bronchoalveolar lavage fluid versus. 1.1 × 105/ml, p = .006) after exposure to intratracheal CFH. Together, these data demonstrate that CFH can stimulate the NLRP3 inflammasome in macrophages and that this pathway may be important in the pathogenesis of CFH‐induced acute lung injury.

Structural Integrity of the Alveolar–Capillary Barrier in Cynomolgus Monkeys Challenged with Fully Virulent and Toxin-Deficient Strains of Bacillus anthracis

Inhalational anthrax, a disease caused by inhaling Bacillus anthracis spores, leads to respiratory distress, vascular leakage, high-level bacteremia, and often death within days. Anthrax lethal toxin and edema toxin, which are composed of protective antigen (PA) plus either lethal factor (LF) or edema factor (EF), respectively, play an important yet incompletely defined role in the pulmonary pathophysiology. To better understand their contribution, we examined the structural integrity of the alveolar-capillary barrier in archival formalin-fixed lungs of cynomolgus monkeys challenged with the fully virulent B. anthracis Ames wild-type strain or the isogenic toxin-deficient mutants ΔEF, ΔLF, and ΔPA. Pulmonary spore challenge with the wild-type strain caused high mortality, intra-alveolar hemorrhages, extensive alveolar septal sequestration of bacteria and neutrophils, diffuse destabilization of epithelial and endothelial junctions, increased markers of coagulation and complement activation (including tissue factor and C5a), and multifocal intra-alveolar fibrin deposition. ΔEF challenge was lethal and showed similar alveolar-capillary alterations; however, intra-alveolar hemorrhages, bacterial deposition, and markers of coagulation or complement were absent or markedly lower. In contrast, ΔLFor ΔPA challenges were nonlethal and showed no signs of alveolar bacterial deposition or alveolar-capillary changes. These findings provide evidence that lethal toxin plays a determinative role in bacterial dissemination and alveolar-capillary barrier dysfunction, and edema toxin may significantly exacerbate pulmonary pathologies in a systemic infection.

Human Lung Microvascular Endothelial Cell Death in Response to Cell‐free Hemoglobin

During sepsis, circulating levels of cell‐free hemoglobin (CFH) are often elevated due to the lysis of red blood cells, and higher concentrations are associated with worse clinical outcomes. Sepsis is caused by a widespread inflammation response, which leads to the release of pro‐inflammatory molecules and cytokines into the circulation. These compounds can prime and activate the endothelium rendering it more susceptible to damage from other circulating molecules. Sepsis is a leading cause of acute respiratory distress syndrome (ARDS), which is characterized by vascular leak and fluid accumulation in the lungs. In our previous studies in murine models of sepsis, lung endothelial cell death was observed, which may contribute to the alveolar capillary barrier dysfunction that characterizes ARDS. Here we investigate whether CFH can initiate cell death in vitro and whether this is potentiated by exposure to pro‐inflammatory signals. Primary human lung microvascular endothelial cells (HLMVECs) were cultured to confluence and exposed to various clinically relevant concentrations of CFH diluted in complete media for up to 24 hours or pre‐treated for 4 hours with 5 ng/ml Cytomix (equal parts IL‐1β, TNF‐α, and INFγ), followed by treatment with 1.0 mg/ml CFH for 24 hours. Cell viability was determined using Draq7, a cell membrane impermeable nuclear stain. Cell death was measured by terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) staining, which detects DNA fragmentation from late apoptosis or necrosis. While there was a time‐ and concentration‐dependent trend, no significant differences between CFH treated and untreated cells were seen in the absence of cytomix, even at the highest concentration of 1.0 mg/ml CFH at 24 hours for the TUNEL assay (57 TUNEL+ cells per FOV in CFH‐treated vs. 17 in control, p=0.8245). However, there was a significant increase in Draq7 staining with 1.0 mg/ml CFH treatment after 24 hours (7.9% Draq7+ cells in CFH‐treated vs. 2.3% in control, p=0.0240). Exposure to cytomix augmented the decrease in viability from CFH as measured by Draq7 assay (32.4% Draq7+ cells in Cyto + CFH‐treated cells vs. 14.5% in CFH‐treated cells, p<0.0001). These findings indicate that CFH alone has modest effects on lung endothelial cell viability. However, during sepsis when circulating cytokine levels are increased, the effect of CFH on endothelial cell death is enhanced, which may contribute to the microvascular leak and alveolar fluid accumulation that are characteristic of acute lung injury.Support or Funding InformationThis work is funded by NIH HL135849, HL103836, and HL094296.Figure 1

Glucagon-like peptide-1 receptor activation alleviates lipopolysaccharide-induced acute lung injury in mice via maintenance of endothelial barrier function

Glucagon-like peptide-1 (GLP-1), which is well known for regulating glucose homeostasis, exhibits multiple actions in cardiovascular disorders and renal injury. However, little is known about the effect of GLP-1 receptor (GLP-1R) activation on acute lung injury (ALI). In this study, we investigated the effect of GLP-1R on ALI and the potential underlying mechanisms with the selective agonist liraglutide. Our results show that GLP-1 levels decreased in serum, though they increased in bronchoalveolar lavage fluid (BALF) and lung tissue in a mouse model of lipopolysaccharide (LPS)-induced ALI. Liraglutide prevented LPS-induced polymorphonuclear neutrophil (PMN) extravasation, lung injury, and alveolar-capillary barrier dysfunction. In cultured human pulmonary microvascular endothelial cells (HPMECs), liraglutide protected against LPS-induced endothelial barrier injury by restoring intercellular tight junctions and adherens junctions. Moreover, liraglutide prevented PMN–endothelial adhesion by inhibiting the expression of intercellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1), and thereafter suppressed PMN transendothelial migration. Furthermore, liraglutide suppressed LPS-induced activation of Rho/NF-κB signaling in HPMECs. In conclusion, our results show that GLP-1R activation protects mice from LPS-induced ALI by maintaining functional endothelial barrier and inhibiting PMN extravasation. These results also suggest that GLP-1R may be a potential therapeutic target for the treatment of ALI.

Pseudomonas aeruginosa Protease IV Exacerbates Pneumococcal Pneumonia and Systemic Disease.

Pneumonia is a pulmonary disease affecting people of all ages and is consistently a leading cause of childhood mortality and adult hospitalizations. Streptococcus pneumoniae and Pseudomonas aeruginosa are major lung pathogens commonly associated with community-acquired and nosocomial pneumonia. Additionally, mixed lung infections involving these bacterial pathogens are increasing in prevalence and are frequently more severe than single infections. The cooperative interactions of these two pathogens that impact pulmonary disease severity are understudied. A major secreted virulence factor of P.aeruginosa, protease IV (PIV), cleaves interleukin 22 (IL-22), a cytokine essential for maintaining innate mucosal defenses against extracellular pathogens. Here, we investigate the ability of PIV to augment the virulence of a pneumococcal strain with limited virulence, S.pneumoniae EF3030, in a C57BL/6 murine model of pneumonia. We demonstrate that pulmonary coinfection involving P.aeruginosa 103-29 and S.pneumoniae EF3030 results in pneumococcal bacteremia that is abrogated during pneumococcal coinfection with a PIV-deficient strain. Furthermore, intratracheal administration of exogenous PIV and EF3030 resulted in abundant immune cell infiltration into the lung with large abscess formation, as well as severe bacteremia leading to 100% mortality. Heat-inactivated PIV did not worsen pneumonia or reliably induce bacteremia, suggesting that the specific activity of PIV is required. Our studies also show that PIV depletes IL-22 in vivo Moreover, PIV-mediated enhancement of pneumonia and disease severity was dependent on the expression of pneumolysin (Ply), a prominent virulence factor of S.pneumoniae Altogether, we reveal that PIV and Ply additively potentiate pneumonia in a murine model of lung infection.IMPORTANCES.pneumoniae remains the leading cause of bacterial pneumonia despite widespread use of pneumococcal vaccines, forcing the necessity for appropriate treatment to control pneumococcal infections. Coinfections involving S.pneumoniae with other bacterial pathogens threaten antibiotic treatment strategies and disease outcomes. Currently, there is not an effective treatment for alveolar-capillary barrier dysfunction that precedes bacteremia. An understanding of the dynamics of host-pathogen interactions during single and mixed pulmonary infections could elucidate proper treatment strategies needed to prevent or reduce invasive disease. Antibiotic treatment decreases bacterial burden in the lung but also increases acute pathology due to cytotoxins released via antibiotic-induced bacterial lysis. Therefore, targeted therapeutics that inhibit or counteract the effects of bacterial proteases and toxins are needed in order to limit pathology and disease progression. This study identifies the cooperative effect of PIV and Ply, products of separate lung pathogens that additively alter the lung environment and facilitate invasive disease.

Tyrosol attenuates lipopolysaccharide-induced acute lung injury by inhibiting the inflammatory response and maintaining the alveolar capillary barrier

Acute lung injury (ALI) is a life-threatening disease characterized by increased pulmonary vascular permeability because of alveolar capillary barrier dysfunction and increased immune responses. This study determined the anti-inflammatory effect of tyrosol on lipopolysaccharide (LPS)-induced ALI and its underlying mechanisms of action. BALB/c mice were orally administered with tyrosol (0.1, 1, and 10 mg/kg) 1 h before an intratracheal injection of LPS (25 μg/50 μL). Oral treatment with tyrosol inhibited lung vascular permeability, histopathological changes, wet/dry lung weight ratio, and pulmonary vascular cell infiltration. The LPS-induced imbalance in the activity of enzymes, such as superoxide dismutase and myeloperoxidase, was regulated by tyrosol. Pro-inflammatory cytokines, such as tumor necrosis factor-α, interleukin (IL)-1β, and IL-6, were reduced by tyrosol in bronchoalveolar lavage fluid and lung tissue. The activation of inflammatory molecules, including inducible nitric oxide synthase (iNOS), cyclooxygenase (COX)-2, and phosphorylated-IκBα, was suppressed by the presence of tyrosol in the lung tissue. In addition, tyrosol attenuated the production of NO, the expression of pro-inflammatory cytokines, the expression of iNOS and COX-2, and the nuclear translocation of nuclear factor-κB in LPS-stimulated RAW 264.7 macrophages. These results suggested that tyrosol is a potential therapeutic agent for treating inflammatory lung diseases.

Early disruption of the intercelullar junction proteins leads to alveolar-capillary barrier dysfunction and pulmonary edema in a mouse ricin intoxication model of Acute Respiratory Distress Syndrome.

Abstract Acute respiratory distress syndrome (ARDS), charactarized by an intense pulmonary inflammation involving neutrophil recruitment, is accompanied by a severe disruption of the alveolar-capillary barrier leading to life-threatening pulmonary edema. Ricin, a plant-toxin derived from the seeds of Ricinus communis, irreversibly inactivates ribosomes by site-specific depurination, thereby arresting cell protein synthesis, while its clinical manifestation following pulmonary exposure is that of rapidly developing severe ARDS. We evaluated the role of the members of tight, adherens and gap junction proteins in pulmonary edema formation in a clinically relevant mouse model of ARDS induced by ricin intoxication, since the respiratory endothelium and epithelium barrier function depends on the integrity of intercellular junctions. We found a significant reduction in Claudin5, Claudin18, VE-Cadherin and Connexin43 in mice lungs 3 hours after ricin intoxication. Neutrophil depletion markedly attenuated pulmonary edema and in combination with a matrix metalloproteinase (MMP) inhibitor restored the expression of VE-Cadherin and Connexin43, providing an evidence for MMP-dependent proteolysis of these proteins. Unlike the aforementioned proteins, disruption of Claudin18 following ricin intoxication was found to be neutrophil-independent and involved tyrosine phosphorylation. These results provide an in vivo mechanism for early impairment of the alveolar-capillary barrier by intercellular junction protein disruption, which at later time points, is enhanced by the specific loss of type II alveolar epithelial cells that no longer provide resistance to edema formation and altogether lead to respiratory failure and death.

Cell-free hemoglobin: a novel mediator of acute lung injury.

Patients with the acute respiratory distress syndrome (ARDS) have elevated levels of cell-free hemoglobin (CFH) in the air space, but the contribution of CFH to the pathogenesis of acute lung injury is unknown. In the present study, we demonstrate that levels of CFH in the air space correlate with measures of alveolar-capillary barrier dysfunction in humans with ARDS (r = 0.89, P < 0.001) and in mice with ventilator-induced acute lung injury (r = 0.89, P < 0.001). To investigate the specific contribution of CFH to ARDS, we studied the impact of purified CFH in the mouse lung and on cultured mouse lung epithelial (MLE-12) cells. Intratracheal delivery of CFH in mice causes acute lung injury with air space inflammation and alveolar-capillary barrier disruption. Similarly, in MLE-12 cells, CFH increases proinflammatory cytokine expression and increases paracellular permeability as measured by electrical cell-substrate impedance sensing. Next, to determine whether these effects are mediated by the iron-containing heme moiety of CFH, we treated mice with intratracheal hemin, the chloride salt of heme, and found that hemin was sufficient to increase alveolar permeability but failed to induce proinflammatory cytokine expression or epithelial cell injury. Together, these data identify CFH in the air space as a previously unrecognized driver of lung epithelial injury in human and experimental ARDS and suggest that CFH and hemin may contribute to ARDS through different mechanisms. Interventions targeting CFH and heme in the air space could provide a new therapeutic approach for ARDS.

Preconditioning of physiological cyclic stretch attenuated HMGB1 expression in pathologically mechanical stretch-activated A549 cells and ventilator-induced lung injury rats through inhibition of IL-6/STAT3/SOCS3

Previous studies have shown that physiologically cyclic stretch (5% CS) attenuated both oxidative- and LPS-induced increases in HMGB1 expression via STAT3. However, little information exists about the effect of precondition of physiological cyclic stretch (CS) on the expression of HMGB 1, which play a crucial role in ventilator-induced lung injury (VILI). We found that 5% CS-preconditioning significantly inhibited HMGB 1 expression, but not HMGB 1 receptors. 5% CS-preconditioning inhibits the IL-6/STAT3 pathway, and the inhibitory effect on the expression of HMGB 1 induced by 5% CS-preconditioning is abolished by additional treatment of rmIL-6. 5% CS-preconditioning also induces SOCS3 upregulation, and 5% CS-preconditioning fails to inhibit the IL-6/STAT3 pathway in cells transfected with SOCS3 siRNA. Moreover, low tidal volume ventilation preconditioning also decreases the severity of VILI evidenced by the markedly improved pulmonary alveolar-capillary barrier dysfunction, wet/dry weight ratio, and histological analysis. These results suggest that preconditioning of physiological 5% CS can reduce the expression of HMGB 1 induced by pathologically mechanical stretch through IL-6/STAT3 pathway associated with up-regulated SOCS3 expression.

New Pathogenetic Concepts and Pharmacological Studies on Adjuvant Therapy in Severe Pneumonia

Acute lung injury secondary to pneumonia results from inadequate activation of the innate immune system with hyperinflammation and alveolar-capillary barrier dysfunction. To date, effective strategies for prevention or treatment of acute lung injury in pneumonia besides antibiotics are lacking. In preclinical studies, promising therapeutic targets have been identified and novel strategies demonstrated to protect against lung failure in pneumonia. This review highlights some adjuvant therapeutic strategies for modulation of inflammation and stabilization of lung barrier function in pneumonia.

Experimental design of complement component 5a-induced acute lung injury (C5a-ALI): a role of CC-chemokine receptor type 5 during immune activation by anaphylatoxin

Excessive activation of the complement system is detrimental in acute inflammatory disorders. In this study, we analyzed the role of complement-derived anaphylatoxins in the pathogenesis of experimental acute lung injury/acute respiratory distress syndrome (ALI/ARDS) in C57BL/6J mice. Intratracheal administration of recombinant mouse complement component (C5a) caused alveolar inflammation with abundant recruitment of Ly6-G(+)CD11b(+) leukocytes to the alveolar spaces and severe alveolar-capillary barrier dysfunction (C5a-ALI; EC(50[C5a]) = 20 ng/g body weight). Equimolar concentrations of C3a or desarginated C5a (C5a(desArg)) did not induce alveolar inflammation. The severity of C5a-ALI was aggravated in C5-deficient mice. Depletion of Ly6-G(+) cells and use of C5aR1(-/-) bone marrow chimeras suggested an essential role of C5aR1(+) hematopoietic cells in C5a-ALI. Blockade of PI3K/Akt and MEK1/2 kinase pathways completely abrogated lung injury. The mechanistic description is that C5a altered the alveolar cytokine milieu and caused significant release of CC-chemokines. Mice with genetic deficiency of CC-chemokine receptor (CCR) type 5, the common receptor of chemokine (C-C motif) ligand (CCL) 3, CCL4, and CCL5, displayed reduced lung damage. Moreover, treatment with a CCR5 antagonist, maraviroc, was protective against C5a-ALI. In summary, our results suggest that the detrimental effects of C5a in this model are partly mediated through CCR5 activation downstream of C5aR1, which may be evaluated for potential therapeutic exploitation in ALI/ARDS.

Novel concepts of acute lung injury and alveolar-capillary barrier dysfunction

In this review we summarize recent major advances in our understanding on the molecular mechanisms, mediators, and biomarkers of acute lung injury (ALI) and alveolar-capillary barrier dysfunction, highlighting the role of immune cells, inflammatory and noninflammatory signaling events, mechanical noxae, and the affected cellular and molecular entities and functions. Furthermore, we address novel aspects of resolution and repair of ALI, as well as putative candidates for treatment of ALI, including pharmacological and cellular therapeutic means.