Response of the lower respiratory tract to injury. Mechanisms of repair of the parenchymal cells of the alveolar wall

Imaging of the Spectrum of Acute Lung Injury

Organizing pneumonia, acute fibrinous and organizing pneumonia, and diffuse alveolar damage, represent multi-compartment patterns of lung injury. The initial region of injury in all remains the same and is centered on the fused basement membrane (BM) between the capillary endothelium and type I pneumocyte. Injury leads to cellular death, BM denudation, increased cellular permeability, and BM structural damage, which leads to exudation, organization, and attempts at repair. When acute lung injury does lead to fibrosis, in some instances it can lead to histologic and/or radiologic usual interstitial pneumonia or nonspecific interstital pneumonia patterns suggesting that lung injury is the primary mechanism for the development of fibrosis.

Advances in cellular senescence in idiopathic pulmonary fibrosis (Review)

Idiopathic pulmonary fibrosis (IPF) is a progressive, irreversible and fatal interstitial lung disease of unknown cause, with a median survival of 2-3 years. Its pathogenesis is unclear and there is currently no effective treatment for IPF. Approximately two-thirds of patients with IPF are >60 years old, with a mean age of 66 years, suggesting a link between aging and IPF. However, the mechanism by which aging promotes development of PF remains unclear. Senescence of alveolar epithelial cells and lung fibroblasts (LFs) and their senescence-associated secretion phenotype (SASP) may be involved in the occurrence and development of IPF. The present review focus on senescence of LFs and epithelial and stem cells, as well as SASP, the activation of profibrotic signaling pathways and potential treatments for pathogenesis of IPF.

Potential contribution of alveolar epithelial type I cells to pulmonary fibrosis

Pulmonary fibrosis (PF) is characterized by inflammation and fibrosis of the interstitium and destruction of alveolar histoarchitecture ultimately leading to a fatal impairment of lung function. Different concepts describe either a dominant role of inflammatory pathways or a disturbed remodeling of resident cells of the lung parenchyma during fibrogenesis. Further, a combination of both the mechanisms has been postulated. The present review emphasizes the particular involvement of alveolar epithelial type I cells in all these processes, their contribution to innate immune/inflammatory functions and maintenance of proper alveolar barrier functions. Amongst the different inflammatory and repair events the purinergic receptor P2X7, an ATP-gated cationic channel that regulates not only apoptosis, necrosis, autophagy, and NLPR3 inflammosome activation, but also the turnover of diverse tight junction (TJ) and water channel proteins, seems to be essential for the stability of alveolar barrier integrity and for the interaction with protective factors during lung injury.

Hepatocyte growth factor is an attractive target for the treatment of pulmonary fibrosis

INTRODUCTION

Pulmonary fibrosis (PF) is a progressive fatal disorder and is characterized by alveolar epithelial injury, myofibroblast proliferation, and extracellular matrix remodeling, resulting in irreversible distortion of lung's architecture. Available therapies are associated with side effects and show restricted efficacy. Therefore, there is an urgent need to find a therapeutic solution to PF. Therapeutic strategies interfering myofibroblast expansion, apoptosis of epithelial and endothelial cells might be beneficial for treatment of PF. Hepatocyte growth factor (HGF), a pleiotropic growth factor, plays an important role in lung development, inflammation, repair, and regeneration. In animal model of PF, administration of recombinant HGF protein or ectopic HGF expression ameliorates fibrosis.

AREAS COVERED

The focus of this review is to highlight HGF as a promising therapeutic approach for the treatment of PF. The review discusses the currently available treatment option for PF as well as highlights the possible beneficial effect of HGF as a drug target.

EXPERT OPINION

HGF with its anti-fibrotic effect provides a promising new therapeutic approach by protecting lung from fibrotic remodeling and also promoting normal regeneration of lung. The development of HGF mimetics may provide a potential attractive therapy for treatment of this devastating and complex disease.

Characterization of the lung epithelium of wild-type and TLR9(-/-) mice after single and repeated exposures to chicken barn air

Exposure to chicken barn air causes lung injury resulting in lower and upper respiratory symptoms in the poultry workers, and mechanisms of which are not fully understood. The lung injury can initiate modifications such as proliferation of the airway epithelial cells such as Clara cells, type II alveolar (T2) cells and mucus producing goblet cells as part of the innate immune response. Toll-like receptors (TLR) have been suggested to play a role in cell division and proliferation. To understand the effect of TLR9 on Clara cells, T2 and mucus-producing goblet cells, we quantified the numbers of these cells in the lungs of wild-type (WT) and TLR9(-/-) mice exposed to chicken barn air. The mice were exposed for either one day or five or 20 days for 8 h/day. Clara cells and T2 cells were labelled with antibodies, and the mucus cells were identified with Periodic-acid Schiff stain, and quantified in per unit tissue section area. The data show decrease in the number of Clara cells and increase in mucus-producing goblet cells after exposure to chicken barn air in both WT and TLR9(-/-) mice. Numbers of T2 cells increased and decreased in WT and TLR9(-/-) mice, respectively, after exposure to poultry barn air. These data show that exposure to chicken barn air can affect major lung epithelial cells, and allude to the role of TLR9 in regulation of some of these responses.

Potential role for ESAT6 in dissemination of M. tuberculosis via human lung epithelial cells

ESAT6 has recently been demonstrated to cause haemolysis and macrophage lysis. Our studies demonstrate that ESAT6 causes cytolysis of type 1 and type 2 pneumocytes. Both types of pneumocytes express membrane laminin, and ESAT6 exhibits dose-dependent binding to both cell types and to purified human laminin. While minimal ESAT6 was detected on the surface of Mycobacterium tuberculosis grown in vitro, exogenously provided ESAT6 specifically associated with the bacterial cell surface, and the bacterium-associated ESAT6 retained its cytolytic ability. esat6 transcripts were upregulated approximately 4- to approximately 13-fold in bacteria replicating in type 1 cells, and approximately 3- to approximately 5 fold in type 2 cells. In vivo, laminin is primarily concentrated at the basolateral surface of pneumocytes where they rest on the basement membrane, which is composed primarily of laminin and collagen. The upregulation of esat6 transcripts in bacteria replicating in pneumocytes, the specific association of ESAT6 with the bacterial surface, the binding of ESAT6 to laminin and the lysis of pneumocytes by free and bacterium-associated ESAT6 together suggest a scenario wherein Mycobacterium tuberculosis replicating in pneumocytes may utilize surface ESAT6 to anchor onto the basolateral laminin-expressing surface of the pneumocytes, and damage the cells and the basement membrane to directly disseminate through the alveolar wall.

Cultured lung fibroblasts from ovalbumin-challenged "asthmatic" mice differ functionally from normal

Asthmatic airway remodeling is characterized by goblet cell hyperplasia, angiogenesis, smooth muscle hypertrophy, and subepithelial fibrosis. This study evaluated whether acquired changes in fibroblast phenotype could contribute to this remodeling. Airway and parenchymal fibroblasts from control or chronically ovalbumin (OVA)-sensitized and challenged "asthmatic" mice were assessed for several functions related to repair and remodeling +/- exogenous transforming growth factor (TGF)-beta. All OVA-challenged mouse fibroblasts demonstrated augmented gel contraction (P < 0.05) and chemotaxis (P < 0.05); increased TGF-beta(1) (P < 0.05), fibronectin (P < 0.05), and vascular endothelial growth factor (P < 0.05) release; and expressed more alpha-smooth muscle actin (P < 0.05). TGF-beta(1) stimulated both control and asthmatic fibroblasts, which retained all differences from control fibroblasts for all features(P < 0.05, all comparisons). Parenchymal fibroblasts proliferated more rapidly (P < 0.05), while airway fibroblasts proliferated similarly compared with control fibroblasts (P = 0.25). Thus, in this animal model, OVA-challenged mouse fibroblasts acquire a distinct phenotype that differs from control fibroblasts. The augmented profibrotic activity and mediator release of asthmatic fibroblasts could contribute to airway remodeling in asthma.

Epithelial-mesenchymal interactions in pulmonary fibrosis

Pulmonary fibrosis represents the sequelae of a variety of acute and chronic lung injuries of known and unknown etiologies. Tissue specimens obtained from patients with pulmonary fibrosis, regardless of the etiology, consistently show evidence of an ongoing wound-repair response. Epithelial-mesenchymal interactions have critical roles in normal lung development, tissue repair processes, and fibrosis. Current hypotheses propose that dysregulated function of, and impaired communication between, epithelial and mesenchymal cells prevent resolution of the wound-repair response and contribute to the pathobiology of pulmonary fibrosis. This hypothesis is supported by abundant evidence from patients, animal models, and cell-culture studies demonstrating abnormalities in epithelial cell and mesenchymal cell activities including proliferation, differentiation, and survival. This article reviews the aberrant epithelial and mesenchymal cellular phenotypes found in the context of pulmonary fibrosis and discusses the mechanisms that perpetuate these cellular phenotypes.

Keratinocyte Growth Factor Stimulates Alveolar Type II Cell Proliferation through the Extracellular Signal–Regulated Kinase and Phosphatidylinositol 3-OH Kinase Pathways

Keratinocyte growth factor (KGF or FGF-7) stimulates alveolar type II cell proliferation, but little is known about the signaling pathways involved. We investigated the role of the ERK (p42/44 mitogen activated protein [MAP] kinase) and phosphatidylinositol 3-OH kinase (PI3 kinase) pathways on alveolar type II cell proliferation and differentiation. Rat type II cells were cultured on tissue culture plastic and Matrigel in the presence or absence of KGF and specific chemical inhibitors PD98059, LY294002, and rapamycin at various concentrations. Proliferation was measured by thymidine incorporation and DNA quantitation, and differentiation was measured by expression of surfactant protein A and alkaline phosphatase. We demonstrate that KGF activates distal effectors of the PI3 kinase pathway, PKB/Akt, and p70S6 kinase, as well as p42/44 MAP kinase proteins. Inhibition of these pathways with PD98059, LY294002, or rapamycin inhibited type II cell proliferation but had no significant effect on differentiation. KGF did not activate the c-Jun kinase or p38 MAP kinase pathways. We conclude that the p42/44 MAP kinase and PI3 kinase pathways are important in regulating alveolar type II cell proliferation in response to KGF.

Mechanisms of pulmonary fibrosis

Tissue injury evokes highly conserved, tightly regulated inflammatory responses and less well-understood host repair responses. Both inflammation and repair involve the recruitment, activation, apoptosis, and eventual clearance of key effector cells. In this review, we propose the concept of pulmonary fibrosis as a dysregulated repair process that is perpetually "turned on" even though classical inflammatory pathways may be dampened or "switched off." Significant regional heterogeneity, with varied histopathological patterns of inflammation and fibrosis, has been observed in individual patients with idiopathic pulmonary fibrosis. We discuss environmental factors and host response factors, such as genetic susceptibility and age, that may influence these varied manifestations. Better understanding of the mechanisms of lung repair, which include alveolar reepithelialization, myofibroblast differentiation/activation, and apoptosis, should offer more effective therapeutic options for progressive pulmonary fibrosis.

Matrilysin expression and function in airway epithelium

We report that matrilysin, a matrix metalloproteinase, is constitutively expressed in the epithelium of peribronchial glands and conducting airways in normal lung. Matrilysin expression was increased in airway epithelial cells and was induced in alveolar type II cells in cystic fibrosis. Other metalloproteinases (collagenase-1, stromelysin-1, and 92-kD gelatinase) were not produced by normal or injured lung epithelium. These observations suggest that matrilysin functions in injury-mediated responses of the lung. Indeed, matrilysin expression was increased in migrating airway epithelial cells in wounded human and mouse trachea. In human tissue, epithelial migration was reduced by > 80% by a hydroxamate inhibitor, and in mouse tissue, reepithelialization in trachea from matrilysin-null mice was essentially blocked. In vivo observations and cell culture studies demonstrated that matrilysin was secreted lumenally by lung epithelium, but upon activation or while migrating over wounds, some matrilysin was released basally. The constitutive production of matrilysin in conducting airways, its upregulation after injury, its induction by alveolar epithelium, and its release into both lumenal and matrix compartments suggest that this metalloproteinase serves multiple functions in intact and injured lung, one of which is to facilitate reepithelialization.

Prenatal dexamethasone administration to premature rats exposed to prolonged hyperoxia: a new rat model of pulmonary fibrosis (bronchopulmonary dysplasia)

OBJECTIVE

To evaluate the postnatal effects of prenatal dexamethasone treatment of preterm rats and to test the hypothesis that prenatal dexamethasone treatment projects against pulmonary oxygen toxicity in the preterm rats and stimulates lung antioxidant enzyme levels in response to hyperoxia.

STUDY DESIGN

We administered dexamethasone (0.4 mg/kg, intraperitoneally), or equivolume saline solution to pregnant rats at 48 and 24 hours before premature delivery at gestation day 21. Both groups of prematurely delivered rat pups were randomly assigned to other > 95% O2 or room air immediately after birth and brief resuscitation.

RESULTS

The hyperoxic survival rates from day 1 through day 14 were similar in both dexamethasone-treated and control preterm O2 groups. At 7 days of hyperoxia, the preterm pups demonstrated similar lung antioxidant enzyme activity and sufactant content responses to high O2 in the dexamethasone-treated and control groups. Lung quantitative morphometry changes were similar (equal degree of inhibition of normal alveolar development) in both groups. Unexpectedly, the lungs of the preterm O2 control rats showed evidence of septal fibrosis and the pups that received dexamethasone-O2 showed even greater severity of septal fibrosis and a greater increase (+50%) of lung hydroxyproline compared with the O2 groups control rats.

CONCLUSIONS

In preterm animals, prenatal dexamethasone administration does not show any of the hypothesized protective effects against hyperoxia or protective biochemical lung changes during prolonged O2 exposure. However, prenatal dexamethasone administration with prolonged exposure of the preterm rat to hyperoxia results in a pulmonary pathologic picture quite similar to bronchopulmonary dysplasia.

Compensatory lung growth occurs in adult dogs after right pneumonectomy

We investigated the structural changes in the left lung of five adult male foxhounds 5 mo (n = 2) or 16 mo (n = 3) after right pneumonectomy (approximately 54% of lung resected) and five sex- and age-matched foxhounds 15-16 mo after right thoracotomy without pneumonectomy. Lungs were fixed by intratracheal instillation of glutaraldehyde and analyzed by standard morphometric techniques. After right pneumonectomy, volume of the left lung increased by 72%. Volumes of all septal structures increased significantly and were more pronounced at 5 than at 16 mo after pneumonectomy. At 16 mo, the relative increases in volume with respect to the control left lung were as follows: epithelium 73%, interstitium 100%, endothelium 55%, and capillary blood volume 43%. Surface areas of alveoli and capillary increased significantly by 52% and 34%, respectively. At 5 mo after pneumonectomy, harmonic mean thickness of the tissue-plasma barrier was significantly greater but at 16 mo it was not different from controls. There was a significant increase in diffusing capacity for oxygen (33% above controls) at 16 mo after pneumonectomy. These data suggest that, in contrast to previous findings after left pneumonectomy, compensatory lung growth does occur in adult dogs after resection of > 50% of lung.

Interstitial lung disease in children

Interstitial lung disease in children is a heterogeneous group of disorders of both known and unknown causes that share a common histologic characteristic (i.e., inflammation of the pulmonary interstitium that may resolve completely, partially, or progress to derangement of alveolar structures with varying degrees of fibrosis). The inflammatory process, evoked as a result of injury to alveolar epithelium and/or the endothelium, is responsible for alveolar wall thickening that is the histologic marker of ILD. This article extrapolates some of the known pathogenic mechanisms of ILD from adult and animal models and applies this information for a better understanding of the pathogenesis of ILD in children. The clinical manifestations vary and are often subtle and nonspecific. There is no consensus on specific criteria for the clinical diagnosis of ILD in children. There are no pathognomonic laboratory criteria for the diagnosis of ILD in children other than the characteristic findings on histologic examination of the lung. It is important to make the diagnosis early to minimize lung damage. Therapy is directed toward the reduction of the inflammatory response to minimize or prevent the progression to fibrosis. ILD suffers from lack of uniform guidelines for diagnostic evaluation, therapy, and prognostic indicators essential for critical monitoring of disease activity. No one medical center has enough cases of ILD in children to allow objective evaluation of a significant number of cases with adequate longitudinal follow-up to determine guidelines for optimal management and to identify accurate prognostic indicators. The organization of a multicenter approach will guide us towards a better understanding of ILD in children.

Absence of long term pulmonary sequelae after mild meconium aspiration syndrome

To assess residual damage from meconium aspiration syndrome (MAS), we studied 12 children, ages 6 to 9 years, an average of 7.4 years after injury. Our sample consisted of mildly affected patients, only one having required intubation and mechanical ventilation in the acute phase of illness. In each child we measured FVC, FEV1, PEFR, MMEFR, VisoV, lung volumes by plethysmography, DLCO and calculated FEV1/FVC and RV/TLC. Methacholine bronchial challenge was performed and the PD20 was calculated. These results were compared with those obtained from a group of 12 normal children studied in our laboratory. No significant difference was found for pulmonary function parameters studied, except for VisoV which was greater in MAS patients than normals (P less than 0.02). Given the inherent high degree of variability with VisoV, we were not able to ascribe this isolated finding to be indicative of small airway disease in these asymptomatic patients. We conclude that patients with mild to moderate initial insult from MAS show an absence of pulmonary sequelae when tested at an average of 7.4 years of age.

Pathogenesis of "fibrosis" in interstitial pneumonia: an electron microscopic study

Seven cases in which interstitial fibrosis developed in patients who had acute interstitial pneumonia were studied ultrastructurally to elucidate the pathogenesis of the interstitial thickening seen by light microscopy. Interstitial fibrosis is generally thought to result from fibroblast proliferation and collagen deposition, and this mechanism was confirmed. However, two additional mechanisms that also contributed to the interstitial thickening were identified. One, which was not described previously, involves folding of portions of alveolar septa or collapse of entire alveoli and permanent apposition of their walls. This process occurred in areas that had been denuded of alveolar lining epithelium. Granular pneumocytes attempting to re-epithelialize the denuded basal lamina proliferated over the surface of apposed septa, thereby combining the folded or collapsed alveoli and forming a single thickened septum. The second mechanism involves incorporation of intra-alveolar exudates into alveolar septa. It occurred when granular pneumocytes re-epithelialized along the luminal surface of intra-alveolar debris overlying denuded alveolar epithelial basal laminae. The relative importance of each of these mechanisms in the pathogenesis of interstitial fibrosis and their role in the more common chronic interstitial pneumonias are unknown. However, their recognition may inspire new approaches for the prevention and treatment of interstitial fibrosis.

Changes in the pulmonary glucocorticoid receptor content in the course of interstitial disease

The level of cytosolic glucocorticoid receptors (GR) was measured in 12 open-chest lung biopsies of interstitial pulmonary diseases. The results showed an increase in the GR content in the diseased lungs correlated to the degree of septal cellularity in nine cases. Two pulmonary sarcoidosis and one end-stage idiopathic pulmonary fibrosis patients presented higher levels of GR than those predicted by the septal cellularity. It was concluded that the GR content of the lungs increases in the course of interstitial diseases, reflecting the number of cells that express cytosolic GR in pulmonary parenchyma.

Clinical features to stage alveolitis in asbestos workers

To analyze the clinical features of asbestos-induced alveolitis and stage its activity, we evaluated 217 asbestos workers by the usual clinical, radiological, and functional parameters and computerized gallium 67(Ga) lung scan; we obtained bronchoalveolar lavage (BAL) in 33 and lung biopsy in 6. In addition, we scored the profusion of lung rales and correlated it with other parameters of severity of asbestosis. In the 55 workers without asbestosis and normal 67Ga scan, BAL analyses were comparable to those of controls. Of the 56 without asbestosis but increased 67Ga lung uptake, BAL analyses in 8 documented a predominantly macrophagic alveolitis (confirmed on lung biopsy in 3), with the highest levels of BAL fibronectin. In the 106 workers with asbestosis, 67Ga lung uptake was increased in 75; BAL in 17 demonstrated a macrophagic and neutrophilic alveolitis with elevated fibronectin levels. Lung biopsy in 3 of the latter workers documented peribronchiolar fibrosing alveolitis. Rale scores in all workers or in those without asbestosis did not correlate with 67Ga scores; they correlated fairly well with profusion of parenchymal opacities (Rs = 0.42) and rigidity of the lung pressure-volume curve (Rs = 0.39). Thus, 67Ga lung uptake is an early indicator of chronic macrophagic alveolitis in asbestos workers, which usually progresses to asbestosis. In the disease, profusion of lung rales constitutes a simple clinical mode of assessment of disease severity that correlates better with radiological and functional parameters than with parameters of alveolitis.