Correlation between circulating fibrocytes, and activity and progression of interstitial lung diseases

Pulmonary Fibrosis as a Result of Acute Lung Inflammation: Molecular Mechanisms, Relevant In Vivo Models, Prognostic and Therapeutic Approaches

Pulmonary fibrosis is a chronic progressive lung disease that steadily leads to lung architecture disruption and respiratory failure. The development of pulmonary fibrosis is mostly the result of previous acute lung inflammation, caused by a wide variety of etiological factors, not resolved over time and causing the deposition of fibrotic tissue in the lungs. Despite a long history of study and good coverage of the problem in the scientific literature, the effective therapeutic approaches for pulmonary fibrosis treatment are currently lacking. Thus, the study of the molecular mechanisms underlying the transition from acute lung inflammation to pulmonary fibrosis, and the search for new molecular markers and promising therapeutic targets to prevent pulmonary fibrosis development, remain highly relevant tasks. This review focuses on the etiology, pathogenesis, morphological characteristics and outcomes of acute lung inflammation as a precursor of pulmonary fibrosis; the pathomorphological changes in the lungs during fibrosis development; the known molecular mechanisms and key players of the signaling pathways mediating acute lung inflammation and pulmonary fibrosis, as well as the characteristics of the most common in vivo models of these processes. Moreover, the prognostic markers of acute lung injury severity and pulmonary fibrosis development as well as approved and potential therapeutic approaches suppressing the transition from acute lung inflammation to fibrosis are discussed.

Review: Serum Biomarkers of Lung Fibrosis in Interstitial Pneumonia with Autoimmune Features-What Do We Already Know?

Interstitial pneumonia with autoimmune features (IPAF) belongs to a group of diseases called interstitial lung diseases (ILDs), which are disorders of a varied prognosis and course. Finding sufficiently specific and sensitive biomarkers would enable the progression to be predicted, the natural history to be monitored and patients to be stratified according to their treatment. To assess the significance of pulmonary fibrosis biomarkers studied thus far, we searched the PubMed, Medline and Cochrane Library databases for papers published between January 2015 and June 2021. We focused on circulating biomarkers. A primary review of the databases identified 38 articles of potential interest. Overall, seven articles fulfilled the inclusion criteria. This review aims to assess the diagnostic and prognostic value of molecules such as KL-6, SP-A, SP-D, circulating fibrocytes, CCL2, CXCL13, CXCL9, CXCL10 and CXCL11. All of these biomarkers have previously been studied in idiopathic pulmonary fibrosis (IPF) and connective tissue disease-associated interstitial lung disease (CTD-ILD). IPAF is a disorder of a heterogeneous nature. It explains the lack of coherent observations in terms of correlations with functional parameters. There is still no meta-analysis of pulmonary fibrosis biomarkers in IPAF. This is mainly due to the heterogeneity of the methodology and groups analysed in the research. More research in this area is needed.

Fibrocytes in early and long-standing rheumatoid arthritis: a 6-month trial with repeated synovial biopsy, imaging and lung function test

Objectives

To correlate the level of fibrocytes in peripheral blood, synovial tissue and in vitro culture in rheumatoid arthritis (RA) with changes in disease activity, imaging and pulmonary function.

Methods

Twenty patients with early RA (ERA) and 20 patients with long-standing RA (LRA) were enrolled in a 6-month prospective study. Sixteen patients undergoing wrist arthroscopy were healthy controls. Patients with RA underwent pulmonary function tests, ultrasound and synovial ultrasound-guided needle biopsy of the same wrist at baseline and 6 months. Wrist MRI was performed at baseline (all) and 6 months (ERA). Circulating fibrocytes were measured by flow cytometry, in vitro by the number of monocytes that were differentiated to fibrocytes and in synovial biopsies by counting in histological sections.

Results

Fibrocytes were primarily located around vessels and in the subintimal area in the synovium. Fibrocyte levels did not decline during the trial despite effective RA treatment. In the ERA group, increased synovitis assessed by ultrasound was moderate and strongly correlated with an increase in circulating and synovial fibrocyte levels, respectively. Increased synovitis assessed by MRI during the trial in the ERA group was moderately correlated with both increased numbers of circulating and cultured fibrocytes. Absolute diffusion capacity level was overall weakly negatively correlated with the level of circulating and synovial fibrocytes. The decline in diffusion capacity during the trial was moderately correlated with increased levels of synovial fibrocytes.

Conclusion

Our findings suggest that fibrocytes are involved in RA pathogenesis, both in the synovium and the reduction in lung function seen in a part of patients with RA.

Trial registration number

NCT02652299.

Circulating fibrocytes are not disease-specific prognosticators in idiopathic pulmonary fibrosis

A number of previous studies have observed greater levels of circulating fibrocytes in interstitial lung disease compared to healthy controls, and suggest a prognostic role in idiopathic pulmonary fibrosis (IPF) [1–4]. Fibrocytes are circulating mesenchymal progenitor cells that differentiate into tissue specific fibroblasts and contribute to multiple wound healing processes, including secretion of inflammatory cytokines, contractile wound closure and promotion of angiogenesis [5]. However, the contribution of fibrocytes to the pathogenesis of progressive pulmonary fibrosis remains unclear and clinical observations require independent validation in prospective cohorts.

In people with idiopathic pulmonary fibrosis, circulating fibrocytes ≥2.2% were associated with a greater risk of mortality over a median 3 years of follow-up, but were not associated with disease-related decline in lung function or short-term progression.https://bit.ly/3bWydwQ

Fibrotic extracellular matrix induces release of extracellular vesicles with pro-fibrotic miRNA from fibrocytes

RATIONALE

Extracellular vesicles (EVs) are small lipid vesicles, and EV-coupled microRNAs (miRNAs) are important modulators of biological processes. Fibrocytes are circulating bone marrow-derived cells that migrate into the injured lungs and contribute to fibrogenesis. The question of whether EV-coupled miRNAs derived from fibrocytes are able to regulate pulmonary fibrosis has not been addressed yet.

METHODS

Pulmonary fibrosis was induced in rats by intratracheal administration of an adenoviral gene vector encoding active transforming growth factor-β1 (TGF-β1) or control vector. Primary fibrocytes and fibroblasts were cultured from rat lungs and were sorted by anti-CD45 magnetic beads. Human circulating fibrocytes and fibrocytes in bronchoalveolar lavage fluid (BALF) were isolated by fibronectin-coated dishes. Fibrocytes were cultured on different stiffness plates or decellularised lung scaffolds. We also determined the effects of extracellular matrix (ECM) and recombinant TGF-β1 on the cellular and EV-coupled miRNA expression of fibrocytes.

RESULTS

The EVs of fibrocytes derived from fibrotic lungs significantly upregulated the expression of col1a1 of fibroblasts. Culturing on rigid plates or fibrotic decellularised lung scaffolds increased miR-21-5 p expression compared with soft plates or normal lung scaffolds. Dissolved ECM collected from fibrotic lungs and recombinant TGF-β1 increased miR-21-5 p expression on fibrocytes, and these effects were attenuated on soft plates. Fibrocytes from BALF collected from fibrotic interstitial pneumonia patients showed higher miR-21-5 p expression than those from other patients.

CONCLUSIONS

Our results indicate that ECM contributes to fibrogenesis through biomechanical and biochemical effects on miRNA expression in fibrocytes.

Implication of in vivo circulating fibrocytes ablation in experimental pulmonary hypertension murine model

BACKGROUND AND PURPOSE

Recruitment and involvement of bone-/blood-derived circulating fibrocytes (CF) in the promotion of fibrotic tissue remodelling processes have been shown. However, their direct contribution to pathological changes is not clear. The present study investigates the causal role of CF in the pathogenesis of pulmonary hypertension (PH).

EXPERIMENTAL APPROACH

For selective ablation of CF, we applied the suicidal gene strategy with herpes simplex virus thymidine kinase (HSV-TK) and ganciclovir. The transgenic mice were generated, having HSV-TK-GFP transgene under the collagen 1 promoter. To selectively target CF, HSV-TK-GFP+ bone marrow transplanted into irradiated wild type mice. These chimera mice were subjected to hypoxia for PH induction and ganciclovir for CF ablation.

KEY RESULTS

In vivo CF ablation reduced right ventricular hypertrophy and vascular remodelling with reduced total collagen content. We quantified the CF recruited in the perivascular area and arterial wall of small pulmonary arteries. There was significant recruitment of CF in the lung in response to hypoxia. The characterization of CF showed the expression of CD45 and collagen1 (GFP) along with α-smooth muscle actin (αSMA).

CONCLUSION AND IMPLICATIONS

Our data demonstrated that CF ablation has a potential impact on right ventricular hypertrophy and vascular remodelling in the setting of experimental pulmonary hypertension induced by hypoxia. The beneficial effects may be related to the direct contribution of fibrocytes or its paracrine effect on other resident cell types. Thus, clinical manipulation of CF may represent a novel therapeutic approach to ameliorate the disease state in pulmonary hypertension.

Protective effect of peptide DR8 on bleomycin-induced pulmonary fibrosis by regulating the TGF-β/MAPK signaling pathway and oxidative stress

Pulmonary fibrosis (PF) is a fatal and irreversible lung disease that eventually causes respiratory failure, lung dysfunction and death. The peptide DHNNPQIR-NH₂ (DR8) has been reported to possess potent antioxidant activity, and an imbalance of oxidation/antioxidation is a crucial mechanism that causes PF. Here, we studied the ability of DR8 to improve PF and further explored the pathway in which DR8 plays a critical role. We found that after prophylactic or therapeutic treatment with DR8, fibrosis-associated indices, including marker proteins, proinflammatory cytokines and profibrogenic cytokines, were significantly downregulated. Importantly, DR8 could reduce bleomycin-induced pathological changes and collagen deposition, especially collagen I content. Furthermore, DR8 prominently upregulated nonenzymatic antioxidants and enzymatic antioxidants. Consistent with the in vivo results, we observed that DR8 significantly inhibited the proliferation and reactive oxygen species (ROS) generation of A549 cells and NIH3T3 cells stimulated with transforming growth factor-β1 (TGF-β1), as well as decreased NADPH oxidase 4 (NOX4) levels under the same conditions. Moreover, DR8 reversed the TGF-β1-induced upregulation of phosphorylated ERK1/2 and p38 MAPK in cells and the bleomycin-induced upregulation of these indices in mice. Our results indicate that DR8 could prevent and treat PF by reducing oxidative damage and suppressing the TGF-β/MAPK pathway. Because of the high efficiency and low toxicity of DR8, we consider that DR8 could be a candidate drug for PF, and our studies establish a foundation for the development of a lead compound to be used as a therapy for fibrosis-related diseases.

Inhibitory effect of circulating fibrocytes on injury repair in acute lung injury/acute respiratory distress syndrome mice model

The study was aimed to explore the functions of circulating fibrocytes (CFs) on injury repair in acute lung injury/acute respiratory distress syndrome (ALI/ARDS) mice model and its clinical value as a biomarker for ALI/ARDS. ALI/ARDS mice model was established by intratracheal instillation of lipopolysaccharide (LPS). Mononuclear cells were isolated from peripheral blood of ALI/ARDS model and flow cytometry was used to measure CFs defined as cells positive for CD45 and collagen-1. Histological changes of lung tissues were evaluated by H&E staining and Masson's trichrome staining. The correlations of CFs counts with damnification of lung tissue and the severity of pulmonary fibrosis were evaluated by Pearson correlation analyses. Western blot was used to detect the protein expression of collagen-1. ELISA was applied to determine cytokine CXCL12 concentration. Clinical relevance between CFs and ALI/ARDS was investigated. The greater number of CFs in the ALI/ARDS group implied higher degree of lung injury and more severe pulmonary fibrosis. The protein expression of collagen-1 and concentration of cytokine CXCL12 in ALI/ARDS group were higher than that in control group. Clinical and prognostic analysis revealed the higher injury degree and death rates in ALI/ARDS group than those in control group, and identified a greater severity and mortality for patients with ARDS than those with ALI. ROC curve analysis indicated the counts of CFs greater than 5.85% can predict death rates with AUC = 0.928. CFs had an inhibitory effect on injury repair in ALI/ARDS mice model. This might be unfavorable as a clinical marker for progression of ALI/ARDS.

Fibrocytes are increased in lung and peripheral blood of patients with idiopathic pulmonary fibrosis

Background

Fibrocytes are implicated in Idiopathic Pulmonary Fibrosis (IPF) pathogenesis and increased proportions in the circulation are associated with poor prognosis. Upon tissue injury, fibrocytes migrate to the affected organ. In IPF patients, circulating fibrocytes are increased especially during exacerbations, however fibrocytes in the lungs have not been examined.

Therefore, we sought to evaluate if fibrocytes can be detected in IPF lungs and we compare percentages and phenotypic characteristics of lung fibrocytes with circulating fibrocytes in IPF.

Methods

First we optimized flow cytometric detection circulating fibrocytes using a unique combination of intra- and extra-cellular markers to establish a solid gating strategy. Next we analyzed lung fibrocytes in single cell suspensions of explanted IPF and control lungs and compared characteristics and numbers with circulating fibrocytes of IPF.

Results

Using a gating strategy for both circulating and lung fibrocytes, which excludes potentially contaminating cell populations (e.g. neutrophils and different leukocyte subsets), we show that patients with IPF have increased proportions of fibrocytes, not only in the circulation, but also in explanted end-stage IPF lungs. These lung fibrocytes have increased surface expression of HLA-DR, increased intracellular collagen-1 expression, and also altered forward and side scatter characteristics compared with their circulating counterparts.

Conclusions

These findings demonstrate that lung fibrocytes in IPF patients can be quantified and characterized by flow cytometry. Lung fibrocytes have different characteristics than circulating fibrocytes and represent an intermediate cell population between circulating fibrocytes and lung fibroblast. Therefore, more insight in their phenotype might lead to specific therapeutic targeting in fibrotic lung diseases.

Electronic supplementary material

The online version of this article (10.1186/s12931-018-0798-8) contains supplementary material, which is available to authorized users.

Personalised medicine in interstitial lung diseases

Interstitial lung diseases in general, and idiopathic pulmonary fibrosis in particular, are complex disorders with multiple pathogenetic pathways, various disease behaviour profiles and different responses to treatment, all facets that make personalised medicine a highly attractive concept. Personalised medicine is aimed at describing distinct disease subsets taking into account individual lifestyle, environmental exposures, genetic profiles and molecular pathways. The cornerstone of personalised medicine is the identification of biomarkers that can be used to inform diagnosis, prognosis and treatment stratification. At present, no data exist validating a personalised approach in individual diseases. However, the importance of the goal amply justifies the characterisation of genotype and pathway signatures with a view to refining prognostic evaluation and trial design, with the ultimate aim of selecting treatments according to profiles in individual patients.

The diversity of myeloid immune cells shaping wound repair and fibrosis in the lung

In healthy circumstances the immune system coordinates tissue repair responses in a tight balance that entails efficient inflammation for removal of potential threats, proper wound closure, and regeneration to regain tissue function. Pathological conditions, continuous exposure to noxious agents, and even ageing can dysregulate immune responses after injury. This dysregulation can lead to a chronic repair mechanism known as fibrosis. Alterations in wound healing can occur in many organs, but our focus lies with the lung as it requires highly regulated immune and repair responses with its continuous exposure to airborne threats. Dysregulated repair responses can lead to pulmonary fibrosis but the exact reason for its development is often not known. Here, we review the diversity of innate immune cells of myeloid origin that are involved in tissue repair and we illustrate how these cell types can contribute to the development of pulmonary fibrosis. Moreover, we briefly discuss the effect of age on innate immune responses and therefore on wound healing and we conclude with the implications of current knowledge on the avenues for future research.

Mechanisms of carbon nanotube-induced pulmonary fibrosis: a physicochemical characteristic perspective

Carbon nanotubes (CNTs) are engineered nanomaterials (ENMs) with numerous beneficial applications. However, they could pose a risk to human health from occupational or consumer exposures. Rodent models demonstrate that exposure to CNTs via inhalation, instillation, or aspiration results in pulmonary fibrosis. The severity of the fibrogenic response is determined by various physicochemical properties of the nanomaterial such as residual metal catalyst content, rigidity, length, aggregation status, or surface charge. CNTs are also increasingly functionalized post-synthesis with organic or inorganic agents to modify or enhance surface properties. The mechanisms of CNT-induced fibrosis involve oxidative stress, innate immune responses of macrophages, cytokine and growth factor production, epithelial cell injury and death, expansion of the pulmonary myofibroblast population, and consequent extracellular matrix accumulation. A comprehensive understanding of how physicochemical properties affect the fibrogenic potential of various types of CNTs should be considered in combination with genetic variability and gain or loss of function of specific genes encoding secreted cytokines, enzymes, or intracellular cell signaling molecules. Here, we cover the current state of the literature on mechanisms of CNT-exposed pulmonary fibrosis in rodent models with a focus on physicochemical characteristics as principal drivers of the mechanisms leading to pulmonary fibrosis. This article is categorized under: Therapeutic Approaches and Drug Discovery > Nanomedicine for Respiratory Disease Toxicology and Regulatory Issues in Nanomedicine > Toxicology of Nanomaterials.

Fibrocyte measurement in peripheral blood correlates with number of cultured mature fibrocytes in vitro and is a potential biomarker for interstitial lung disease in Rheumatoid Arthritis

Background

Interstitial lung disease (ILD) can be a severe extra-articular disease manifestation in Rheumatoid Arthritis (RA). A potential role of fibrocytes in RA associated ILD (RA-ILD) has not previously been described. We present a modified faster method for measuring circulating fibrocytes, without intracellular staining. The results are compared to the traditional culture method, where the number of monocytes that differentiate into mature fibrocytes in vitro are counted. The results are following compared to disease activity in patients with severe asthma, ILD, RA (without diagnosed ILD) and RA with verified ILD (RA-ILD).

Method

CD45⁺ CD34⁺ CD11b⁺ (7-AAD⁻ CD3⁻ CD19⁻ CD294⁻) cells were isolated by cell sorting and stained for pro-collagen type 1. Thirty-nine patients (10 RA, 9 ILD and 10 with severe asthma, 10 with RA-ILD) and 10 healthy controls (HC) were included. Current medication, disease activity, pulmonary function test and radiographic data were collected. Circulating fibrocytes were quantified by flow cytometry. Peripheral blood mononuclear cells were isolated and cultured for 5 days and the numbers of mature fibrocytes were counted.

Results

90.2% (mean, SD = 1.5%) of the sorted cells were pro-collagen type 1 positive and thereby fulfilled the criteria for being circulating fibrocytes. The ILD and RA-ILD groups had increased levels of circulating fibrocytes compared to HC (p < 0.05). Levels of circulating fibrocytes correlated overall to number of monocytes that subsequently in vitro differentiated to mature fibrocytes (r = 0.81, p < 0.001). RA patients with pathologically reduced diffusion capacity for carbon monoxide adjusted for hemoglobin (DLCO_c) in both the RA and in the combined RA + RA-ILD group, had significantly higher levels of both circulating and number of cultured mature fibrocytes (both p < 0.05). In both groups, the level of circulating fibrocytes and number of mature fibrocytes in culture also correlated to a reduction in DLCO_c (r = −0.61 an r = −0.58 both p < 0.05).

Conclusions

We presented a fast and valid method for measuring circulating fibrocytes using flow cytometry on lysed peripheral blood. Further, we showed for the first time, that the level of circulating fibrocytes correlated with the number of peripheral blood mononuclear cells, that differentiated into mature fibrocytes in vitro. Reduced DLCO_c was correlated with high levels of circulating and mature fibrocytes in RA, which have not been reported previously. In such, this study suggests that fibrocytes may exhibit an important role in the pathogenesis of RA-ILD, which requires further clarification in future studies.

Trial registration

ClinicalTrials.gov:NCT02711657, registered 13/3–2016, retrospectively registered.

Electronic supplementary material

The online version of this article (doi:10.1186/s12931-017-0623-9) contains supplementary material, which is available to authorized users.

Myeloid-derived suppressor cell-like fibrocytes are increased and associated with preserved lung function in chronic obstructive pulmonary disease

BACKGROUND

The role of fibrocytes in chronic obstructive pulmonary disease (COPD) is unknown. We sought to enumerate blood and tissue fibrocytes in COPD and determine the association of blood fibrocytes with clinical features of disease.

METHODS

Utilizing flow cytometry to identify circulating, collagen type 1⁺ cells, we found two populations: (i) CD45⁺ CD34⁺ (fibrocytes) and (ii) CD45⁺ CD34^- [myeloid-derived suppressor cell (MDSC)-like fibrocytes] cells in stable COPD (n = 41) and control (n = 29) subjects. Lung resection material from a separate group of subjects with (n = 11) or without (n = 11) COPD was collected for tissue fibrocyte detection. We examined circulating fibrocyte populations for correlations with clinical parameters including quantitative computed tomography (qCT) and determined pathways of association between correlated variables using a path analysis model.

RESULTS

Blood and tissue fibrocytes were not increased compared to control subjects nor were blood fibrocytes associated with lung function or qCT, but were increased in eosinophilic COPD. Myeloid-derived suppressor cell-like fibrocytes were increased in COPD compared to controls [2.3 (1.1-4.9), P = 0.038]. Our path analysis model showed that collagen type 1 intensity for MDSC-like fibrocytes was positively associated with lung function through associations with air trapping, predominately in the upper lobes.

CONCLUSION

We have demonstrated that two circulating populations of fibrocyte exist in COPD, with distinct clinical associations, but are not prevalent in proximal or small airway tissue. Blood MDSC-like fibrocytes, however, are increased and associated with preserved lung function through a small airway-dependent mechanism in COPD.

Hsp90 regulation of fibroblast activation in pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a severe fibrotic lung disease associated with fibroblast activation that includes excessive proliferation, tissue invasiveness, myofibroblast transformation, and extracellular matrix (ECM) production. To identify inhibitors that can attenuate fibroblast activation, we queried IPF gene signatures against a library of small-molecule-induced gene-expression profiles and identified Hsp90 inhibitors as potential therapeutic agents that can suppress fibroblast activation in IPF. Although Hsp90 is a molecular chaperone that regulates multiple processes involved in fibroblast activation, it has not been previously proposed as a molecular target in IPF. Here, we found elevated Hsp90 staining in lung biopsies of patients with IPF. Notably, fibroblasts isolated from fibrotic lesions showed heightened Hsp90 ATPase activity compared with normal fibroblasts. 17-N-allylamino-17-demethoxygeldanamycin (17-AAG), a small-molecule inhibitor of Hsp90 ATPase activity, attenuated fibroblast activation and also TGF-β-driven effects on fibroblast to myofibroblast transformation. The loss of the Hsp90AB, but not the Hsp90AA isoform, resulted in reduced fibroblast proliferation, myofibroblast transformation, and ECM production. Finally, in vivo therapy with 17-AAG attenuated progression of established and ongoing fibrosis in a mouse model of pulmonary fibrosis, suggesting that targeting Hsp90 represents an effective strategy for the treatment of fibrotic lung disease.

Circulating fibrocytes correlate with the asthma control test score

BACKGROUND

Bronchial asthma is characterised by airway inflammation and remodelling with a decline of lung function. Fibrocytes are bone marrow-derived mesenchymal progenitor cells that play important roles in the pathogenesis of airway remodelling. Several clinical parameters are currently being used in routine clinical practice to assess outcome of therapy in asthma including frequency of rescue with short-acting β2-agonist and the asthma control test. In this study, we hypothesised that asthma control test is associated with circulating levels of fibrocytes in bronchial asthma.

METHODS

There were 20 patients with asthma and seven healthy controls. The number of CD45(+)Collagen I(+) circulating fibrocytes was assessed in the peripheral blood by flow cytometry.

RESULTS

The number of circulating fibrocytes was significantly increased in asthma patients with moderate and severe disease compared to controls, and it was inversely correlated with % forced expiratory volume in one second and % forced vital capacity (%FVC). The frequency of inhalation of short-acting β2 agonist and the asthma control test score was significantly and inversely correlated with the number of circulating fibrocytes.

CONCLUSION

The results of this study showed that the number of circulating fibrocytes is inversely correlated with clinical asthma control parameters, further supporting the relevance of measuring circulating fibrocytes as a marker of clinical control in bronchial asthma.

Vaccinia vaccine-based immunotherapy arrests and reverses established pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a fatal disease without any cure. Both human disease and animal models demonstrate dysregulated wound healing and unregulated fibrogenesis in a background of low-grade chronic T lymphocyte infiltration. Tissue-resident memory T cells (Trm) are emerging as important regulators of the immune microenvironment in response to pathogens, and we hypothesized that they might play a role in regulating the unremitting inflammation that promotes lung fibrosis. Herein, we demonstrate that lung-directed immunotherapy, in the form of i.n. vaccination, induces an antifibrotic T cell response capable of arresting and reversing lung fibrosis. In mice with established lung fibrosis, lung-specific T cell responses were able to reverse established pathology - as measured by decreased lung collagen, fibrocytes, and histologic injury - and improve physiologic function. Mechanistically, we demonstrate that this effect is mediated by vaccine-induced lung Trm. These data not only have implications for the development of immunotherapeutic regimens to treat IPF, but also suggest a role for targeting tissue-resident memory T cells to treat other tissue-specific inflammatory/autoimmune disorders.

Pulmonary Macrophages: A New Therapeutic Pathway in Fibrosing Lung Disease?

Pulmonary fibrosis (PF) is a growing clinical problem which can result in breathlessness or respiratory failure and has an average life expectancy of 3 years from diagnosis. Therapeutic options for PF are limited and there is therefore a significant unmet clinical need. The recent resurgent interest in macrophage biology has led to a new understanding of lung macrophage origins, biology, and phenotypes. In this review we discuss fibrotic mechanisms and focus on the role of macrophages during fibrotic lung disease. Data from both human and murine studies are reviewed, highlighting novel macrophage-orientated biomarkers for disease diagnosis and potential targets for future anti-fibrotic therapies.

Fibrocytes Regulate Wilms Tumor 1-Positive Cell Accumulation in Severe Fibrotic Lung Disease

Collagen-producing myofibroblast transdifferentiation is considered a crucial determinant in the formation of scar tissue in the lungs of patients with idiopathic pulmonary fibrosis. Multiple resident pulmonary cell types and bone marrow-derived fibrocytes have been implicated as contributors to fibrotic lesions because of the transdifferentiation potential of these cells into myofibroblasts. In this study, we assessed the expression of Wilms tumor 1 (WT1), a known marker of mesothelial cells, in various cell types in normal and fibrotic lungs. We demonstrate that WT1 is expressed by both mesothelial and mesenchymal cells in idiopathic pulmonary fibrosis lungs but has limited or no expression in normal human lungs. We also demonstrate that WT1(+) cells accumulate in fibrotic lung lesions, using two different mouse models of pulmonary fibrosis and WT1 promoter-driven fluorescent reporter mice. Reconstitution of bone marrow cells into a TGF-α transgenic mouse model demonstrated that fibrocytes do not transform into WT1(+) mesenchymal cells, but they do augment accumulation of WT1(+) cells in severe fibrotic lung disease. Importantly, the number of WT1(+) cells in fibrotic lesions was correlated with severity of lung disease as assessed by changes in lung function, histology, and hydroxyproline levels in mice. Finally, inhibition of WT1 expression was sufficient to attenuate collagen and other extracellular matrix gene production by mesenchymal cells from both murine and human fibrotic lungs. Thus, the results of this study demonstrate a novel association between fibrocyte-driven WT1(+) cell accumulation and severe fibrotic lung disease.

Concise review: current status of stem cells and regenerative medicine in lung biology and diseases

Lung diseases remain a significant and devastating cause of morbidity and mortality worldwide. In contrast to many other major diseases, lung diseases notably chronic obstructive pulmonary diseases (COPDs), including both asthma and emphysema, are increasing in prevalence and COPD is expected to become the third leading cause of disease mortality worldwide by 2020. New therapeutic options are desperately needed. A rapidly growing number of investigations of stem cells and cell therapies in lung biology and diseases as well as in ex vivo lung bioengineering have offered exciting new avenues for advancing knowledge of lung biology as well as providing novel potential therapeutic approaches for lung diseases. These initial observations have led to a growing exploration of endothelial progenitor cells and mesenchymal stem (stromal) cells in clinical trials of pulmonary hypertension and COPD with other clinical investigations planned. Ex vivo bioengineering of the trachea, larynx, diaphragm, and the lung itself with both biosynthetic constructs as well as decellularized tissues have been used to explore engineering both airway and vascular systems of the lung. Lung is thus a ripe organ for a variety of cell therapy and regenerative medicine approaches. Current state-of-the-art progress for each of the above areas will be presented as will discussion of current considerations for cell therapy-based clinical trials in lung diseases.

Cooperation between human fibrocytes and endothelial colony-forming cells increases angiogenesis via the CXCR4 pathway

Fibrotic diseases of the lung are associated with a vascular remodelling process. Fibrocytes (Fy) are a distinct population of blood-borne cells that co-express haematopoietic cell antigens and fibroblast markers, and have been shown to contribute to organ fibrosis. The purpose of this study was to determine whether fibrocytes cooperate with endothelial colony-forming cells (ECFC) to induce angiogenesis. We isolated fibrocytes from blood of patient with idiopathic pulmonary fibrosis (IPF) and characterised them by flow cytometry, quantitative reverse transcriptase PCR (RTQ-PCR), and confocal microscopy. We then investigated the angiogenic interaction between fibrocytes and cord-blood-derived ECFC, both in vitro and in an in vivo Matrigel implant model. Compared to fibroblast culture medium, fibrocyte culture medium increased ECFC proliferation and differentiation via the SDF-1/CXCR4 pathway. IPF-Fy co-implanted with human ECFC in Matrigel plugs in immunodeficient mice formed functional microvascular beds, whereas fibroblasts did not. Evaluation of implants after two weeks revealed an extensive network of erythrocyte-containing blood vessels. CXCR4 blockade significantly inhibited this blood vessel formation. The clinical relevance of these data was confirmed by strong CXCR4 expression in vessels close to fibrotic areas in biopsy specimens from patients with IPF, by comparison with control lungs. In conclusion, circulating fibrocytes might contribute to the intense remodelling of the pulmonary vasculature in patients with idiopathic pulmonary fibrosis.

Stem cells, cell therapies, and bioengineering in lung biology and diseases. Comprehensive review of the recent literature 2010-2012

A conference, "Stem Cells and Cell Therapies in Lung Biology and Lung Diseases," was held July 25 to 28, 2011 at the University of Vermont to review the current understanding of the role of stem and progenitor cells in lung repair after injury and to review the current status of cell therapy and ex vivo bioengineering approaches for lung diseases. These are rapidly expanding areas of study that provide further insight into and challenge traditional views of mechanisms of lung repair after injury and pathogenesis of several lung diseases. The goals of the conference were to summarize the current state of the field, to discuss and debate current controversies, and to identify future research directions and opportunities for basic and translational research in cell-based therapies for lung diseases. The goal of this article, which accompanies the formal conference report, is to provide a comprehensive review of the published literature in lung regenerative medicine from the last conference report through December 2012.

Paracrine functions of fibrocytes to promote lung fibrosis

Fibrocytes are derived from the bone marrow and are found in the circulation. They can be recruited to sites of injury and contribute to repair/remodeling. In vitro evidence suggests that fibrocytes may differentiate into fibroblasts to promote lung fibrosis. However, in vivo evidence for this is sparse. This review summarizes recent literature which may suggest that fibrocytes function to promote fibrosis via paracrine actions. In this way, secretion of growth factors, proteases and matricellular proteins may strongly influence the actions of resident epithelial and mesenchymal cells to promote repair and resolution or to tip the scale toward pathologic remodeling.

Lysophosphatidic acid accelerates lung fibrosis by inducing differentiation of mesenchymal stem cells into myofibroblasts

Lung fibrosis is characterized by vascular leakage and myofibroblast recruitment, and both phenomena are mediated by lysophosphatidic acid (LPA) via its type-1 receptor (LPA1). Following lung damage, the accumulated myofibroblasts activate and secrete excessive extracellular matrix (ECM), and form fibrotic foci. Studies have shown that bone marrow-derived cells are an important source of myofibroblasts in the fibrotic organ. However, the type of cells in the bone marrow contributing predominantly to the myofibroblasts and the involvement of LPA-LPA1 signalling in this is yet unclear. Using a bleomycin-induced mouse lung-fibrosis model with an enhanced green fluorescent protein (EGFP) transgenic mouse bone marrow replacement, we first demonstrated that bone marrow derived-mesenchymal stem cells (BMSCs) migrated markedly to the bleomycin-injured lung. The migrated BMSC contributed significantly to α-smooth muscle actin (α-SMA)-positive myofibroblasts. By transplantation of GFP-labelled human BMSC (hBMSC) or EGFP transgenic mouse BMSC (mBMSC), we further showed that BMSC might be involved in lung fibrosis in severe combined immune deficiency (SCID)/Beige mice induced by bleomycin. In addition, using quantitative-RT-PCR, western blot, Sircol collagen assay and migration assay, we determined the underlying mechanism was LPA-induced BMSC differentiation into myofibroblast and the secretion of ECM via LPA1. By employing a novel LPA1 antagonist, Antalpa1, we then showed that Antalpa1 could attenuate lung fibrosis by inhibiting both BMSC differentiation into myofibroblast and the secretion of ECM. Collectively, the above findings not only further validate LPA1 as a drug target in the treatment of pulmonary fibrosis but also elucidate a novel pathway in which BMSCs contribute to the pathologic process.

Alveolar oxygen tension and angio-architecture of the distal adult lung

The present study demonstrates the fine structure of pulmonary capillaries first injured and then undergoing growth in response to a change in the ambient alveolar oxygen tension. Breathing a high fraction of inspired oxygen (FiO2 0.75) triggers restriction by endothelial cell injury and effacement leading to segment narrowing and shortening and segment loss as demonstrated by a fall in density. Subsequently, breathing a relatively low fraction (FiO2 0.21) triggers capillary assembly (angiogenesis), which reverses the changes. The data underscore the structural reprogramming (reduction and restoration) of pulmonary capillaries in response to significant shifts in oxygen tension.

Fibrocytes in the Pathogenesis of Chronic Fibrotic Lung Disease

Fibrocytes were initially described in 1999 and since that time there has been a growing body of literature to suggest their importance in a number of chronic lung diseases. It is now well established that fibrocytes derive from the bone marrow and circulate within the peripheral blood. However, when injury occurs, fibrocytes can travel to the site of damage via chemokine-mediated recruitment. Recent studies suggest that fibrocyte numbers increase within the lung or circulation during numerous disease processes. Although fibrocytes readily differentiate into fibroblasts in vitro, whether they do so in vivo is still unknown. The variety of pro-fibrotic mediators that are secreted by fibrocytes makes it likely that they act via paracrine functions to influence the behavior of resident lung cells. This review summarizes recent insights regarding fibrocytes in asthma, scleroderma and IPF.

Detection of alveolar fibrocytes in idiopathic pulmonary fibrosis and systemic sclerosis

Background

Fibrocytes are circulating precursors for fibroblasts. Blood fibrocytes are increased in patients with idiopathic pulmonary fibrosis (IPF). The aim of this study was to determine whether alveolar fibrocytes are detected in broncho-alveolar lavage (BAL), to identify their prognostic value, and their potential association with culture of fibroblasts from BAL.

Methods

We quantified fibrocytes in BAL from 26 patients with IPF, 9 patients with Systemic Sclerosis(SSc)-interstitial lung disease (ILD), and 11 controls. BAL cells were cultured to isolate alveolar fibroblasts.

Results

Fibrocytes were detected in BAL in 14/26 IPF (54%) and 5/9 SSc patients (55%), and never in controls. Fibrocytes were in median 2.5% [0.4–19.7] and 3.0% [2.7–3.7] of BAL cells in IPF and SSc-ILD patients respectively. In IPF patients, the number of alveolar fibrocytes was correlated with the number of alveolar macrophages and was associated with a less severe disease but not with a better outcome. Fibroblasts were cultured from BAL in 12/26 IPF (46%), 5/9 SSc-ILD (65%) and never in controls. The detection of BAL fibrocytes did not predict a positive culture of fibroblasts.

Conclusion

Fibrocytes were detected in BAL fluid in about half of the patients with IPF and SSc-ILD. Their number was associated with less severe disease in IPF patients and did not associate with the capacity to grow fibroblasts from BAL fluid.

Comparing the differential effects of LPA on the barrier function of human pulmonary endothelial cells

Lysophosphatidic acid (LPA) is a class of bioactive lyso-phospholipid that mediates most of its biological effects through a family of G protein-coupled receptors of which six have been identified. The role of the LPA pathway in driving chronic lung diseases such as idiopathic pulmonary fibrosis (IPF) has gained considerable academic and industry attention. Modulation of the pulmonary artery endothelial barrier function by the LPA1 receptor has been shown to drive pulmonary fibrosis in murine models of disease. The purpose of this study was (i) to assess the effect of LPA on the barrier function of human pulmonary arterial (HPAEC) and microvascular (HMVEC) endothelial cells and (ii) to identify the LPA receptor subtype(s) responsible for changes in human pulmonary endothelial cell permeability using LPA receptor antagonists and siRNA technology. Analysis of the LPA receptor subtype expression demonstrated predominant expression of LPA2 and LPA6 receptor subtypes in both HPAECs and HMVECs. HPAECs also exhibit low expression of LPA1, LPA3, and LPA4 receptor subtypes. Treatment of cells with increasing concentrations of LPA caused loss of barrier function in HPAECs but not HMVECs, despite both cell types exhibiting very similar LPA receptor expression profiles. The LPA-mediated loss of barrier function in HPAECs appears to be independent of the LPA1 receptor and likely to be mediated via the LPA6 receptor although we cannot exclude an additional role for the LPA2 and LPA4 receptors in mediating these effects. These results suggest cell-specific mechanisms exist in human pulmonary endothelial cells to permit regulation of barrier function downstream of LPA receptors. More importantly, our data indicate that selective LPA1 receptor antagonism may be insufficient for therapeutic use in pulmonary diseases where impaired endothelial barrier function is related to disease initiation and progression.