Pulmonary fibrosis occurs in a variety of clinical settings, constitutes a major cause of morbidity and mortality, and represents an enormous unmet medical need. However, the disease is heterogeneous, and the failure to accurately discern between forms of fibrosing lung diseases leads to inaccurate treatments. Pulmonary fibrosis occurring in the context of connective tissue diseases is often characterized by a distinct pattern of tissue pathology and may be amenable to immunosuppressive therapies. In contrast, idiopathic pulmonary fibrosis (IPF) is a progressive and lethal form of fibrosing lung disease that is recalcitrant to therapies that target the immune system. Although animal models of fibrosis imperfectly recapitulate IPF, they have yielded numerous targets for therapeutic intervention. Understanding the heterogeneity of these diseases and elucidating the final common pathways of fibrogenesis are critical for the development of efficacious therapies for severe fibrosing lung diseases.
The lung is a complex organ with multiple functions; in addition to facilitating gas exchange, it also serves as the first line of defense against inhaled environmental pathogens and toxins. Given these critical roles, disruption of normal cell function or cell-cell interactions can have devastating health consequences. The articles of this Review Series highlight recent progress in understanding the pathophysiology of several pulmonary diseases and suggest how these insights are leading to the development of new therapeutic strategies.
The chemokine, CXCL10, and its cognate receptor, CXCR3, are important mediators of the pathobiology of lung fibrosis. Macrophages are a known source of CXCL10, but their specific source in the lung is poorly defined due to incomplete characterization of macrophage subpopulations. We recently developed a novel flow cytometric approach that discriminates resident alveolar macrophages from recruited exudative macrophages (ExMacs) after infectious lung injury. We hypothesized that ExMacs are present after noninfectious lung injury with bleomycin, and are a source of CXCL10. We found that ExMacs are recruited to the lung after injury, peaking at Day 7, then maintained through Day 28. ExMac recruitment was significantly reduced, but not abolished, in CCR2 null mice. ExMacs, but not alveolar macrophages, produce CXCL10, both constitutively and after stimulation with hyaluronan (HA) fragments. Interestingly, ExMac stimulation with LPS resulted in complete suppression of CXCL10. In contrast, ExMacs produced TNF-α and CXCL2/MIP-2 (Macrophage Inflammatory Protein-2) after stimulation with both HA and LPS. ExMacs were present in CXCR3 null mice after bleomycin, but produced minimal CXCL10. This impairment was overcome by administration of exogenous IFN-γ or IFN-γ with HA. Collectively, these data suggest that ExMacs are recruited and maintained in the lung after noninfectious lung injury, are a source of a variety of cytokines, but importantly, are essential for the production of antifibrotic CXCL10. Understanding the contribution of ExMacs to the pathobiology of lung injury and repair could lead to new treatment options for fibrosing lung diseases.
macrophage; bleomycin; pulmonary fibrosis; CXCR3; CXCL10
Gas exchange in the lung occurs within alveoli, air-filled sacs composed of type 2 and type 1 epithelial cells (AEC2s and AEC1s), capillaries, and various resident mesenchymal cells. Here, we use a combination of in vivo clonal lineage analysis, different injury/repair systems, and in vitro culture of purified cell populations to obtain new information about the contribution of AEC2s to alveolar maintenance and repair. Genetic lineage-tracing experiments showed that surfactant protein C–positive (SFTPC-positive) AEC2s self renew and differentiate over about a year, consistent with the population containing long-term alveolar stem cells. Moreover, if many AEC2s were specifically ablated, high-resolution imaging of intact lungs showed that individual survivors undergo rapid clonal expansion and daughter cell dispersal. Individual lineage-labeled AEC2s placed into 3D culture gave rise to self-renewing “alveolospheres,” which contained both AEC2s and cells expressing multiple AEC1 markers, including HOPX, a new marker for AEC1s. Growth and differentiation of the alveolospheres occurred most readily when cocultured with primary PDGFRα+ lung stromal cells. This population included lipofibroblasts that normally reside close to AEC2s and may therefore contribute to a stem cell niche in the murine lung. Results suggest that a similar dynamic exists between AEC2s and mesenchymal cells in the human lung.
Accumulation and turnover of extracellular matrix components are the hallmarks of tissue injury. Fragmented hyaluronan stimulates the expression of inflammatory genes by a variety of immune cells at the injury site. Hyaluronan binds to a number of cell surface proteins on a variety of cell types. Hyaluronan fragments signal through both Toll-like receptor (TLR) 4 and TLR2 as well as CD44 to stimulate inflammatory genes in inflammatory cells. Hyaluronan is also present on the cell surface of epithelial cells and provides protection against tissue damage by interacting with TLR2 and TLR4 on these parenchymal cells. Hyaluronan and hyaluronan-binding proteins regulate inflammation, tissue injury and repair through regulating inflammatory cell recruitment, release of inflammatory cytokines, and stem cell migration. This review focuses on the role of hyaluronan as an immune regulator in human diseases.
Chemokines and chemokine receptors have been implicated in the pathogenesis of bronchiolitis. CXCR3 ligands (CXCL10, CXCL9, and CXCL11) were elevated in patients with bronchiolitis obliterans syndrome (BOS) and chronic allorejection. Studies also suggested that blockage of CXCR3 or its ligands changed the outcome of T-cell recruitment and airway obliteration. We wanted to determine the role of the chemokine CXCL10 in the pathogenesis of bronchiolitis and BOS. In this study, we found that CXCL10 mRNA levels were significantly increased in patients with BOS. We generated transgenic mice expressing a mouse CXCL10 cDNA under control of the rat CC10 promoter. Six-month-old CC10-CXCL10 transgenic mice developed bronchiolitis characterized by airway epithelial hyperplasia and developed peribronchiolar and perivascular lymphocyte infiltration. The airway hyperplasia and T-cell inflammation were dependent on the presence of CXCR3. Therefore, long-term exposure of the chemokine CXCL10 in the lung causes bronchiolitis-like inflammation in mice.
bronchiolitis; chemokine CXCL10; inflammation; airway inflammation
An important hallmark of tissue remodeling is the dynamic turnover of extracellular matrix (ECM). ECM performs a variety of functions in tissue repair including scaffold formation, modulation of fluid dynamics, and regulating cell behavior. During non-infectious tissue injury ECM degradation products are generated that acquire signaling functions not attributable to the native precursor molecules. Hyaluronan (HA) is a non-sulfated glycosaminoglycan which is produced in great abundance following tissue injury. It exists both in a soluble form and as side chains on proteoglycans. HA has critical roles in development as well as a variety of biological processes including wound healing, tumor growth and metastasis, and inflammation. HA fragments share structural similarities with pathogens and following tissue injury can be recognized by innate immune receptors. Elucidating the protean roles of HA in tissue injury, inflammation, and repair will generate new insights into mechanisms of disease characterized by chronic inflammation and tissue remodeling.
extracellular matrix; glycosaminoglycan; lung injury
Asthma remains an important cause of morbidity and mortality with an incidence that continues to rise. Despite the importance of this disease, the mechanisms by which the host develops allergic airways disease remain poorly understood. The development of allergic airways disease appears to be contingent on activation of both the innate and adaptive immune system, but little is known about the cross-talk between these two systems. The extracellular matrix protein mindin (Spondin 2) has been previously demonstrated to have functional roles in both the innate and adaptive immunological responses. Previous work supports that pulmonary challenge with fungal-associated allergenic proteinase (FAP) induces an innate allergic response. We hypothesized that mindin would modify the biological response to FAP. Saline or FAP was administered by oropharyngeal aspiration to C57BL/6 wild type or mindin-null mice every 4 days for a total of five exposures. FAP exposed C57BL/6 mice developed enhanced airway hyperresponsiveness (AHR) to methacholine challenge and increased neutrophils and eosinophils in the bronchoalveolar lavage as compared to saline exposed controls. These responses were significantly reduced in mindin-null mice exposed to FAP. FAP challenge was associated with a broad induction of cytokines (IL-1β, TNFα, Th1, Th2, and IL-17), chemokines, and growth factors, which were reduced in mindin-null mice exposed to FAP. RNA expression in lung monocytes for representative M1 and M2 activation markers were increased by FAP, but were independent of mindin. Our observations support that challenge with FAP results in activation of both innate and adaptive immune signaling pathways in a manner partially dependent on mindin. These findings suggest a potential role for the extracellular matrix protein mindin in cross-talk between the innate and adaptive immune systems.
Environment; Asthma; Reactive airways disease; Extracellular matrix; Allergy; Aspergillus
The molecular mechanisms of acute lung injury are incompletely understood. MicroRNAs (miRNAs) are crucial biological regulators that act by suppressing their target genes and are involved in a variety of pathophysiologic processes. miR-127 appears to be down-regulated during lung injury. We set out to investigate the role of miR-127 in lung injury and inflammation. Expression of miR-127 significantly reduced cytokine release by macrophages. Looking into the mechanisms of regulation of inflammation by miR-127, we found that IgG Fcγ Receptor I (FcγRI/CD64) was a target of miR-127, as evidenced by reduced CD64 protein expression in macrophages over-expressing miR-127. Furthermore, miR-127 significantly reduced the luciferase activity with a reporter construct containing the native 3′-UTR of CD64. Importantly, we demonstrated that miR-127 attenuated lung inflammation in an IgG immune complex (IgG IC) model in vivo. Collectively, these data show that miR-127 targets macrophage CD64 expression and promotes the reduction of lung inflammation. Understanding how miRNAs regulate lung inflammation may represent an attractive way to control inflammation induced by infectious or non-infectious lung injury.
Rationale: Surfactant protein (SP)-D and SP-A have been implicated in immunomodulation in the lung. It has been reported that patients with idiopathic pulmonary fibrosis (IPF) often have elevated serum levels of SP-A and SP-D, although their role in the disease is not known.
Objectives: The goal of this study was to test the hypothesis that SP-D plays an important role in lung fibrosis using a mouse model of fibrosis induced by bleomycin (BLM).
Methods: Triple transgenic inducible SP-D mice (iSP-D mice), in which rat SP-D is expressed in response to doxycycline (Dox) treatment, were administered BLM (100 U/kg) or saline subcutaneously using miniosmotic pumps.
Measurements and Main Results: BLM-treated iSP-D mice off Dox (SP-D off) had increased lung fibrosis compared with mice on Dox (SP-D on). SP-D deficiency also increased macrophage-dominant cell infiltration and the expression of profibrotic cytokines (transforming growth factor [TGF]-β1, platelet-derived growth factor-AA). Alveolar macrophages isolated from BLM-treated iSP-D mice off Dox (SP-D off) secreted more TGF-β1. Fibrocytes, which are bone marrow–derived mesenchymal progenitor cells, were increased to a greater extent in the lungs of the BLM-treated iSP-D mice off Dox (SP-D off). Fibrocytes isolated from BLM-treated iSP-D mice off Dox (SP-D off) expressed more of the profibrotic cytokine TGF-β1 and more CXCR4, a chemokine receptor that is important in fibrocyte migration into the lungs. Exogenous SP-D administered intratracheally attenuated BLM-induced lung fibrosis in SP-D−/− mice.
Conclusions: These data suggest that alveolar SP-D regulates numbers of macrophages and fibrocytes in the lungs, profibrotic cytokine expression, and fibrotic lung remodeling in response to BLM injury.
surfactant; lung fibrosis; macrophage; fibrocyte; growth factor
Inhalation of ambient ozone alters populations of lung macrophages. However, the impact of altered lung macrophage populations on the pathobiology of ozone is poorly understood. We hypothesized that sub-populations of macrophages modulate the response to ozone. We exposed C57BL/6 mice to ozone (2 ppm × 3h) or filtered air. 24 h after the exposure, the lungs were harvested and digested and the cells underwent flow cytometry. Analysis revealed a novel macrophage subset present in ozone exposed mice, which were distinct from resident alveolar macrophages (AM) and identified by enhanced Gr-1+ expression (Gr-1 Macs). Further analysis identified that Gr-1+ Macs exhibited high expression of MARCO, CX3CR1, and NQO1. Gr-1+ Macs were present in the absence of CCR2, suggesting that they were not derived from a CCR2-dependent circulating intermediate. Using PKH26-PCL to label resident phagocytic cells, we demonstrated that Gr-1 Macs were derived from resident lung cells. This new subset was diminished in the absence of CX3CR1. Interestingly, CX3CR1-null mice exhibited enhanced responses to ozone, including increased airway hyperresponsiveness (AHR), exacerbated neutrophil influx, accumulation of 8-isoprostanes and protein carbonyls, and increased expression of cytokines (CXCL2, IL-1β, IL-6, CCL2, and TNF-α). Our results identify a novel subset of lung macrophages, which are derived from a resident intermediate, dependent upon CX3CR1, and appear to protect the host from the biological response to ozone.
The characteristics of human asthma are chronic inflammation and airway remodeling. Hyaluronan (HA), a major extracellular matrix component, accumulates during inflammatory lung diseases including asthma. Hyaluronan fragments stimulate macrophages to produce inflammatory cytokines. We hypothesized that HA and its receptors would play a role in human asthma.
To investigate the role of HA and HA binding proteins in human asthma.
Twenty-one subjects with asthma and 25 normal control subjects underwent bronchoscopy with endobronchial biopsy and bronchoalveolar lavage (BAL). Fibroblasts were cultured, HA and HA synthase expression was determined at baseline and after exposure to several mediators relevant to asthma pathobiology. The expression of HA binding proteins, CD44, TLR2 and TLR4 on BAL macrophages was determined by flow cytometry. IL-8 production by macrophages in response to HA fragment stimulation was compared.
Airway fibroblasts from asthma patients produced significantly increased concentrations of lower molecular weight HA compared to those of normal fibroblasts. Hyaluronan synthase 2 mRNA was markedly increased in asthmatic fibroblasts. Asthmatic macrophages showed a decrease in cell surface CD44 expression and an increase in TLR2 and TLR4 expression. Macrophages from asthmatic subjects showed an increase in responsiveness to low molecular weight HA stimulation, as demonstrated by increased IL-8 production.
HA homeostasis is deranged in asthma with increased production by fibroblasts and decreased CD44 expression on alveolar macrophages. Upregulation of TLR2 and TLR4 on macrophages with increased sensitivity to HA fragments suggests a novel pro-inflammatory mechanism by which persistence of HA fragments could contribute to chronic inflammation and airway remodeling in asthma.
Asthma; Hyaluronan; Cytokines; Fibroblasts; Macrophages
Rationale: Invasive cell phenotypes have been demonstrated in malignant transformation, but not in other diseases, such as asthma. Cellular invasiveness is thought to be mediated by transforming growth factor (TGF)-β1 and matrix metalloproteinases (MMPs). IL-13 is a key TH2 cytokine that directs many features of airway remodeling through TGF-β1 and MMPs.
Objectives: We hypothesized that, in human asthma, IL-13 stimulates increased airway fibroblast invasiveness via TGF-β1 and MMPs in asthma compared with normal controls.
Methods: Fibroblasts were cultured from endobronchial biopsies in 20 subjects with mild asthma (FEV1: 90 ± 3.6% pred) and 17 normal control subjects (FEV1: 102 ± 2.9% pred) who underwent bronchoscopy. Airway fibroblast invasiveness was investigated using Matrigel chambers. IL-13 or IL-13 with TGF-β1 neutralizing antibody or pan-MMP inhibitor (GM6001) was added to the lower chamber as a chemoattractant. Flow cytometry and immunohistochemistry were performed in a subset of subjects to evaluate IL-13 receptor levels.
Measurements and Main Results: IL-13 significantly stimulated invasion in asthmatic airway fibroblasts, compared with normal control subjects. Inhibitors of both TGF-β1 and MMPs blocked IL-13–induced invasion in asthma, but had no effect in normal control subjects. At baseline, in airway tissue, IL-13 receptors were expressed in significantly higher levels in asthma, compared with normal control subjects. In airway fibroblasts, baseline IL-13Rα2 was reduced in asthma compared with normal control subjects.
Conclusions: IL-13 potentiates airway fibroblast invasion through a mechanism involving TGF-β1 and MMPs. IL-13 receptor subunits are differentially expressed in asthma. These effects may result in IL-13–directed airway remodeling in asthma.
airway remodeling; interleukin-13; transforming growth factor-β; matrix metalloproteinase
Pulmonary graft-versus-host disease (GVHD) after hematopoietic cell transplant (HCT) and allograft rejection after lung transplant are parallel immunologic processes that lead to significant morbidity and mortality. Our murine model of pulmonary GVHD after inhaled lipopolysaccharide (LPS) suggests that innate immune activation potentiates pulmonary transplant-related alloimmunity. We hypothesized that the CXCR3 receptor is necessary for development of LPS-induced pulmonary GVHD.
Recipient mice underwent allogeneic or syngeneic HCT followed by inhaled LPS. CXCR3 receptor inhibition was performed by using either CXCR3-knockout donors or systemic anti-CXCR3 antibody blockade. Pulmonary histopathology, cellular sub-populations, cytokine proteins and transcripts were analyzed.
In comparison to lungs of LPS-unexposed and syngeneic controls, lungs of LPS-exposed allogeneic HCT mice demonstrated prominent lymphocytic perivascular and peribronchiolar infiltrates. This pathology was associated with increased CD4+ and CD8+ T cells as well as an increase in CXCR3 expression on T cells, a 2-fold upregulation of CXCR3 transcript and a 4-fold increase in its ligand CXCL10/IP10. CXCR3 inhibition using gene-knockout strategy or antibody blockade did not change the severity of pulmonary pathology (mean pathology score 6.5 for sufficient vs. 6.5 knockout, p=1.00; mean score 6.8 for antibody blockade vs. 7.4 control, p=0.46). CXCR3 inhibition did not prevent CD3 infiltration, nor prevent production of IL-12p40, nor significantly change other Th1, Th2, or Th17 cytokines in the lung.
In the setting of allogeneic HCT, innate immune activation by LPS potentiates pulmonary GVHD through CXCR3-independent mechanisms. Clinical strategies focused on inhibition of CXCR3 may prove insufficient to ameliorate transplant-related lung disease.
Pulmonary graft-versus-host disease; Lung rejection; CXCR3; Lipopolysaccharide; Innate immunity
Hyaluronan synthase 2 and CD44 are required for severe lung fibrosis in response to bleomycin.
Tissue fibrosis is a major cause of morbidity, and idiopathic pulmonary fibrosis (IPF) is a terminal illness characterized by unremitting matrix deposition in the lung. The mechanisms that control progressive fibrosis are unknown. Myofibroblasts accumulate at sites of tissue remodeling and produce extracellular matrix components such as collagen and hyaluronan (HA) that ultimately compromise organ function. We found that targeted overexpression of HAS2 (HA synthase 2) by myofibroblasts produced an aggressive phenotype leading to severe lung fibrosis and death after bleomycin-induced injury. Fibroblasts isolated from transgenic mice overexpressing HAS2 showed a greater capacity to invade matrix. Conditional deletion of HAS2 in mesenchymal cells abrogated the invasive fibroblast phenotype, impeded myofibroblast accumulation, and inhibited the development of lung fibrosis. Both the invasive phenotype and the progressive fibrosis were inhibited in the absence of CD44. Treatment with a blocking antibody to CD44 reduced lung fibrosis in mice in vivo. Finally, fibroblasts isolated from patients with IPF exhibited an invasive phenotype that was also dependent on HAS2 and CD44. Understanding the mechanisms leading to an invasive fibroblast phenotype could lead to novel approaches to the treatment of disorders characterized by severe tissue fibrosis.
Rationale: Surfactant protein A (SP-A) is a collectin family member that has multiple immunomodulatory roles in lung host defense. SP-A levels are altered in the bronchoalveolar lavage (BAL) fluid and serum of patients with acute lung injury and acute respiratory distress syndrome, suggesting the importance of SP-A in the pathogenesis of acute lung injury.
Objectives: Investigate the role of SP-A in the murine model of noninfectious lung injury induced by bleomycin treatment.
Methods: Wild-type (WT) or SP-A deficient (SP-A−/−) mice were challenged with bleomycin, and various indices of lung injury were analyzed.
Measurements and Main Results: On challenge with bleomycin, SP-A−/− mice had a decreased survival rate as compared with WT mice. SP-A−/− mice had a higher degree of neutrophil-dominant cell recruitment and the expression of the inflammatory cytokines in BAL fluid than did WT mice. In addition, SP-A−/− mice had increased lung edema as assessed by the increased levels of intravenously injected Evans blue dye leaking into the lungs. Terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling and active caspase-3 staining suggested the increased apoptosis in the lung sections from SP-A−/− mice challenged with bleomycin. SP-A also specifically reduced bleomycin-induced apoptosis in mouse lung epithelial 12 cells in vitro. Moreover, intratracheal administration of exogenous SP-A rescued the phenotype of SP-A−/− mice in vivo.
Conclusions: These data suggest that SP-A plays important roles in modulating inflammation, apoptosis, and epithelial integrity in the lung in response to acute noninfectious challenges.
collectin; noninfectious lung injury; apoptosis; surfactant replacement therapy
Idiopathic pulmonary fibrosis (IPF) is a progressive disease causing unremitting extracellular matrix deposition with resultant distortion of pulmonary architecture and impaired gas exchange. β-arrestins regulate G-protein-coupled receptors through receptor desensitization while acting as signaling scaffolds that facilitate numerous effector pathways. Here we examine the role of β-arrestin1 and β-arrestin2 in the pathobiology of pulmonary fibrosis. In the bleomycin-induced mouse lung fibrosis model, loss of eitherβ-arrestin1 or β-arrestin2 results in protection from mortality, inhibition of matrix deposition, and protected lung function. Fibrosis is prevented despite preserved recruitment of inflammatory cells and fibroblast chemotaxis. However, isolated lung fibroblasts from bleomycin-treated β-arrestin null mice fail to invade extracellular matrix while displaying altered expression of genes involved in matrix production and degradation. Furthermore, knockdown of β-arrestin2 in fibroblasts from IPF patients attenuated the invasive phenotype. These data implicate β-arrestins as mediators of fibroblast invasion and development of pulmonary fibrosis, thus representing a potential target for therapeutic intervention for patients with IPF.
Pulmonary fibrosis is a progressive, dysregulated response to injury culminating in compromised lung function due to excess extracellular matrix production. The heparan sulfate proteoglycan syndecan-4 is important in mediating fibroblast-matrix interactions, but its role in pulmonary fibrosis has not been explored. To investigate this issue, we used intratracheal instillation of bleomycin as a model of acute lung injury and fibrosis. We found that bleomycin treatment increased syndecan-4 expression. Moreover, we observed a marked decrease in neutrophil recruitment and an increase in both myofibroblast recruitment and interstitial fibrosis in bleomycin-treated syndecan-4–null (Sdc4–/–) mice. Subsequently, we identified a direct interaction between CXCL10, an antifibrotic chemokine, and syndecan-4 that inhibited primary lung fibroblast migration during fibrosis; mutation of the heparin-binding domain, but not the CXCR3 domain, of CXCL10 diminished this effect. Similarly, migration of fibroblasts from patients with pulmonary fibrosis was inhibited in the presence of CXCL10 protein defective in CXCR3 binding. Furthermore, administration of recombinant CXCL10 protein inhibited fibrosis in WT mice, but not in Sdc4–/– mice. Collectively, these data suggest that the direct interaction of syndecan-4 and CXCL10 in the lung interstitial compartment serves to inhibit fibroblast recruitment and subsequent fibrosis. Thus, administration of CXCL10 protein defective in CXCR3 binding may represent a novel therapy for pulmonary fibrosis.
Rationale: The etiology and pathogenesis of angiogenesis in idiopathic pulmonary fibrosis (IPF) is poorly understood. Inter-α-trypsin inhibitor (IaI) is a serum protein that can bind to hyaluronan (HA) and may contribute to the angiogenic response to tissue injury.
Objectives: To determine whether IaI promotes HA-mediated angiogenesis in tissue injury.
Methods: An examination was undertaken of angiogenesis in IaI-sufficient and -deficient mice in the bleomycin model of pulmonary fibrosis and in angiogenesis assays in vivo and in vitro. IaI and HA in patients with IPF were examined.
Measurements and Main Results: IaI significantly enhances the angiogenic response to short-fragment HA in vivo and in vitro. lal deficiency Ieads to decreased angiogenesis in the matrigel model, and decreases lung angiogenesis after bleomycin exposure in mice. IaI is found in fibroblastic foci in IPF, where it colocalizes with HA. The colocalization is particularly strong in vascular areas around fibroblastic foci. Serum levels of IaI and HA are significantly elevated in patients with IPF compared with control subjects. High serum IaI and HA levels are associated with decreased lung diffusing capacity, but not FVC.
Conclusions: Our findings indicate that serum IaI interacts with HA, and promotes angiogenesis in lung injury. IaI appears to contribute to the vascular response to lung injury and may lead to aberrant angiogenesis.
Clinical trial registered with www.clinicaltrials.gov (NCT00016627).
inter–α-trypsin inhibitor; hyaluronan; angiogenesis; pulmonary fibrosis
Idiopathic pulmonary fibrosis (IPF) is a non-neoplastic pulmonary disease that is characterized by the formation of scar tissue within the lungs in the absence of any known provocation. IPF is a rare disease which affects approximately 5 million persons worldwide. The prevalence is estimated to be slightly greater in men (20.2/100,000) than in women (13.2/100,000). The mean age at presentation is 66 years. IPF initially manifests with symptoms of exercise-induced breathless and dry coughing. Auscultation of the lungs reveals early inspiratory crackles, predominantly located in the lower posterior lung zones upon physical exam. Clubbing is found in approximately 50% of IPF patients. Cor pulmonale develops in association with end-stage disease. In that case, classic signs of right heart failure may be present. Etiology remains incompletely understood. Some environmental factors may be associated with IPF (cigarette smoking, exposure to silica and livestock). IPF is recognized on high-resolution computed tomography by peripheral, subpleural lower lobe reticular opacities in association with subpleural honeycomb changes. IPF is associated with a pathological lesion known as usual interstitial pneumonia (UIP). The UIP pattern consists of normal lung alternating with patches of dense fibrosis, taking the form of collagen sheets. The diagnosis of IPF requires correlation of the clinical setting with radiographic images and a lung biopsy. In the absence of lung biopsy, the diagnosis of IPF can be made by defined clinical criteria that were published in guidelines endorsed by several professional societies. Differential diagnosis includes other idiopathic interstitial pneumonia, connective tissue diseases (systemic sclerosis, polymyositis, rheumatoid arthritis), forme fruste of autoimmune disorders, chronic hypersensitivity pneumonitis and other environmental (sometimes occupational) exposures. IPF is typically progressive and leads to significant disability. The median survival is 2 to 5 years from the time of diagnosis. Medical therapy is ineffective in the treatment of IPF. New molecular therapeutic targets have been identified and several clinical trials are investigating the efficacy of novel medication. Meanwhile, pulmonary transplantation remains a viable option for patients with IPF. It is expected that, during the next decade, considerable progress will be made toward the understanding and treatment of this devastating illness.
Extracellular superoxide dismutase (EC-SOD) is expressed at high levels in lungs. EC-SOD has a polycationic matrix-binding domain that binds to polyanionic constituents in the matrix. Previous studies indicate that EC-SOD protects the lung in both bleomycin- and asbestos-induced models of pulmonary fibrosis. Although the mechanism of EC-SOD protection is not fully understood, these studies indicate that EC-SOD plays an important role in regulating inflammatory responses to pulmonary injury. Hyaluronan is a polyanionic high molecular mass polysaccharide found in the extracellular matrix that is sensitive to oxidant-mediated fragmentation. Recent studies found that elevated levels of low molecular mass hyaluronan are associated with inflammatory conditions. We hypothesize that EC-SOD may inhibit pulmonary inflammation in part by preventing superoxide-mediated fragmentation of hyaluronan to low molecular mass fragments. We found that EC-SOD directly binds to hyaluronan and significantly inhibits oxidant-induced degradation of this glycosaminoglycan. In vitro human polymorphic neutrophil chemotaxis studies indicate that oxidative fragmentation of hyaluronan results in polymorphic neutrophil chemotaxis and that EC-SOD can completely prevent this response. Intratracheal injection of crocidolite asbestos in mice leads to pulmonary inflammation and injury that is enhanced in EC-SOD knock-out mice. Notably, hyaluronan levels are increased in the bronchoalveolar lavage fluid after asbestos-induced pulmonary injury, and this response is markedly enhanced in EC-SOD knock-out mice. These data indicate that inhibition of oxidative hyaluronan fragmentation probably represents one mechanism by which EC-SOD inhibits inflammation in response to lung injury.
Mechanisms that regulate host defense after noninfectious tissue injury are incompletely understood. Our laboratory is interested in the role of the extracellular matrix glycosaminoglycan hyaluronan in the regulation of lung inflammation and fibrosis. We have identified key roles for two cell surface receptor systems that interact with hyaluronan to control lung inflammation and tissue repair. Hematopoietic CD44 is necessary to clear hyaluronan fragments that are produced after lung injury. Failure to clear hyaluronan fragments leads to unremitting inflammation. However, in the absence of CD44, alveolar macrophages continue to produce chemokines in response to hyaluronan fragments, implicating another receptor system in controlling macrophage effector function. We found that Toll-like receptors 2 and 4 (TLR2 and TLR4) are responsible for macrophage inflammatory gene expression in response to hyaluronan fragments. Although TLR2 and TLR4 initiate the innate immune response in noninfectious inflammation, they have a protective role against lung injury on alveolar epithelial cells.
CD44; hyaluronan; innate immunity; tissue injury; Toll-like receptors
Fibrosis and apoptosis are juxtaposed in pulmonary disorders such as asthma and the interstitial diseases, and transforming growth factor (TGF)-β1 has been implicated in the pathogenesis of these responses. However, the in vivo effector functions of TGF-β1 in the lung and its roles in the pathogenesis of these responses are not completely understood. In addition, the relationships between apoptosis and other TGF-β1–induced responses have not been defined. To address these issues, we targeted bioactive TGF-β1 to the murine lung using a novel externally regulatable, triple transgenic system. TGF-β1 produced a transient wave of epithelial apoptosis that was followed by mononuclear-rich inflammation, tissue fibrosis, myofibroblast and myocyte hyperplasia, and septal rupture with honeycombing. Studies of these mice highlighted the reversibility of this fibrotic response. They also demonstrated that a null mutation of early growth response gene (Egr)-1 or caspase inhibition blocked TGF-β1–induced apoptosis. Interestingly, both interventions markedly ameliorated TGF-β1–induced fibrosis and alveolar remodeling. These studies illustrate the complex effects of TGF-β1 in vivo and define the critical role of Egr-1 in the TGF-β1 phenotype. They also demonstrate that Egr-1–mediated apoptosis is a prerequisite for TGF-β1–induced fibrosis and remodeling.
asthma; pulmonary fibrosis; fibrosis reversibility; airway remodeling
CXC chemokine receptor 3 (CXCR3) is the receptor for the IFN-γ–inducible C-X-C chemokines MIG/CXCL9, IP-10/CXCL10, and I-TAC/CXCL11. CXCR3 is expressed on activated immune cells and proliferating endothelial cells. The role of CXCR3 in fibroproliferation has not been investigated. We examined the role of CXCR3 in pulmonary injury and repair in vivo. CXCR3-deficient mice demonstrated increased mortality with progressive interstitial fibrosis relative to WT mice. Increased fibrosis occurred without increased inflammatory cell recruitment. CXCR3 deficiency resulted in both a reduced early burst of IFN-γ production and decreased expression of CXCL10 after lung injury. We identified a relative deficiency in lung NK cells in the unchallenged CXCR3-deficient lung and demonstrated production of IFN-γ by WT lung NK cells in vivo following lung injury. The fibrotic phenotype in the CXCR3-deficient mice was significantly reversed following administration of exogenous IFN-γ or restoration of endogenous IFN-γ production by adoptive transfer of WT lymph node and spleen cells. Finally, pretreatment of WT mice with IFN-γ–neutralizing Ab’s enhanced fibrosis following lung injury. These data demonstrate a nonredundant role for CXCR3 in limiting tissue fibroproliferation and suggest that this effect may be mediated, in part, by the innate production of IFN-γ following lung injury.
These discussions are selected from the weekly staff conferences in the Department of Medicine, University of California, San Francisco. Taken from transcriptions, they are prepared by Homer A. Boushey, MD, Professor of Medicine, and John G. Fitz, MD, Assistant Professor of Medicine, under the direction of Lloyd H. Smith, Jr, MD, Professor of Medicine and Associate Dean in the School of Medicine. Requests for reprints should be sent to the Department of Medicine, University of California, San Francisco, School of Medicine, San Francisco, CA 94143.