Link, Patrick A.

Abstract A population of pathologic fibroblasts has been identified in several human fibrotic lung diseases, including IPF. These cells, marked by high expression of collagen triple-helix repeat containing 1 (CTHRC1), display an invasive phenotype and aggressively deposit extracellular matrix. The goal of this study was to identify transcriptional regulators of the CTHRC1+ pathologic fibroblast state. We analyzed published single-cell RNA sequencing data of IPF and control lungs to predict transcription factor activity in CTHRC1+ fibroblasts, which led us to hypothesize that transcription factors in the nuclear factor of activated T-cells (NFAT) family may contribute to the pathologic fibroblast state. To test whether NFAT family members promote and/or maintain pathologic fibroblasts, we silenced each cytosol-localized NFAT (NFATc1-4) in lung fibroblasts from IPF donors (passage 3-6, N = 5 donors) and measured expression of key NFAT target genes related to fibrosis using qPCR. NFATC1 silencing significantly reduced expression of CTHRC1, SERPINE1, FN1, HAS2, and TNC, whereas other computationally predicted regulators showed little effect. Our computational analysis also predicted high activator protein 1 (AP-1) activity in pathologic fibroblasts, which interestingly has been found to act in concert with NFATs to promote an effector response in T-cells. Thus, we hypothesized that NFAT:AP-1 signaling may be involved in lung fibroblast activation. To test this hypothesis, we employed a small molecule inhibitor of NFAT:AP-1 complex formation on DNA (“Compound 10”, 25-50 µM). NFAT:AP-1 inhibition significantly reduced expression of fibrillar collagens COL1A1 and COL3A1, as well as CTHRC1 in primary IPF lung fibroblasts. Additionally, NFAT:AP-1 inhibition prevented TGF-β-mediated increase of several fibrosis-related NFATc1 targets. Aside from CTHRC1, NFAT:AP-1 inhibition led to decrease in a different set of NFATc1 targets than NFATc1 silencing, suggesting a potentially complex relationship between NFATc1 and AP-1 in fibroblast gene regulation. Finally, in mouse precision cut lung slices (PCLS) stimulated with a fibrosis-inducing cocktail ex vivo, NFAT:AP-1 inhibition prevented increased expression of Col1a1 and Cthrc1. Taken together, these studies suggest that NFATc1, together with AP-1, is involved in fibrogenic activation of pathological fibroblasts. Ongoing and future experiments will determine the roles of these signaling mechanisms in matrix deposition and contraction, as well as fibroblast invasion using human fibroblasts and PCLS models. Furthermore, we aim to determine how mechanical forces and soluble factors encountered in IPF lungs promote NFAT signaling in fibroblasts. This abstract is funded by: NHLBI, Pulmonary Fibrosis Foundation

Abstract Pulmonary fibrosis (PF) involves excessive collagen accumulation, yet mechanisms shifting the balance of synthesis and degradation toward net deposition remain unclear. Myeloperoxidase (MPO) inversely correlates with survival in PF. Using the bleomycin model, we found MPO knockout (MPOko) mice were protected from fibrosis, and pharmacological MPO inhibition after peak inflammation (day 7) recapitulated this protection. MPO persisted in lung tissue 21 days post-injury despite neutrophil efflux, linking acute inflammation to sustained remodeling. Mechanistically, we identified that MPO inhibits Cathepsin K (CatK), a potent collagenolytic enzyme involved in fibrosis resolution. Notably, CatK gene expression ( CTSK ) is elevated in PF, suggesting post-translational inhibition of CatK. MPOko and inhibitor-treated mice exhibited elevated CatK activity after bleomycin; exogenous addition of pathophysiologic concentrations of MPO reduced CatK activity in mouse precision-cut lung slices and human fibroblasts. Biochemically, MPO reduced CatK activity to 33% of control. In two distinct cohorts of PF patients, we observed significantly increased MPO protein levels in platelet poor plasma and in lung tissue. In PF patients, 62% had MPO levels in platelet poor plasma exceeding healthy controls, while lung tissue from other PF patients showed significantly elevated MPO staining. Plasma levels were inversely correlated with decreased survival, FVC, and DLCO. These findings establish MPO as a post-translational inhibitor of CatK-mediated collagenolysis, revealing a mechanism linking acute inflammation to sustained fibrosis and suggest a patient subpopulation that may benefit from MPO-targeted therapy. Graphical Abstract Myeloperoxidase persists in lung tissue after injury and inhibits cathepsin K activity, impairing collagen degradation and promoting extracellular matrix accumulation during pulmonary fibrosis.

Abstract Repetitive injury is hypothesized to lead to progressive tissue fibrosis and end-stage organ failure. Whether tissue-resident mesenchymal cell populations retain epigenetic memory of prior injuries that contribute to this pathological process is unknown. Here we used a genetic lineage labeling approach to mark the lung mesenchyme prior to injury, then performed multimodal analyses on isolated lung mesenchyme during the initiation, progression, and resolution of the fibrotic response. Our results demonstrate the remarkable epigenetic and transcriptional plasticity of the lung mesenchyme during fibrotic activation and de-activation. Despite this plasticity, we also find that the lung mesenchyme exhibits an enhanced fibrotic program upon reinjury. We identify RUNX1 as a critical driver of both fibrotic activation and fibrotic memory. Comparison of fresh isolated and cultured lung mesenchyme demonstrates that RUNX1 is spontaneously activated in standard culture conditions, previously masking these roles of RUNX1. Targeted knockdown of RUNX1 dampens fibrotic mesenchymal cell activation immediately after cell isolation, but with reduced efficacy after only days of culture, confirming its functional importance to both early activation and long-term memory. Collectively, our findings implicate RUNX1 in the initiation and memory of fibrotic mesenchymal cell activation that together prime enhanced mesenchymal cell responses upon repeated injury.

Pulmonary fibrosis is a devastating and progressive disease marked by replacement of gas-exchanging tissue with collagen-rich scar. The mechanical environment is profoundly altered in pulmonary fibrosis and contributes to disease progression via feedback relationships between cells, the extracellular matrix, and the evolving mechanical environment. Targeting these mechanobiological feedback loops has emerged as a promising approach to interrupt disease progression, though with challenges in how to intervene selectively, safely, and effectively. We posit that further delineation of cell–matrix mechanobiological interactions will be pivotal to promoting fibrosis resolution and should guide efforts to discover and implement new approaches that can preserve or even restore lung function. To set the stage for these advances, we first review the mechanobiology of the healthy lung and the feedback loops that promote fibrosis progression. We then lay out the challenges and opportunities for targeting the fibrotic matrix as an essential element for protecting or restoring lung function.

Recent studies highlight transcriptionally distinct fibroblast subpopulations in human lungs. We observed the loss of these native transcriptional programs as fresh isolated cells are maintained in traditional culture conditions. Identifying the signals defining native fibroblast identities will be pivotal to creating culture models that preserve unique subpopulations. The screening system developed here will allow the investigation of a broad selection of cues, leading to better culture models for studying human lung fibroblast function and plasticity.

Abstract Pulmonary fibrosis is increasingly understood to involve dysfunction within and across multiple cellular compartments, with recent attention highlighting the involvement of pulmonary vascular dysfunction in failed repair and progression of fibrosis. Formulation and delivery of lung-targeting lipid nanoparticles (LNPs) may provide a means to selectively target the lung but not systemic vasculature. However, the feasibility and efficacy of such approaches in the fibrotic lung are unknown. We sought to test whether intravenously administered lung-targeting LNPs can safely deliver mRNA to the healthy and fibrotic lung vasculature in young and aged mice and whether delivery of mRNA encoding a matricellular protein could promote fibrosis resolution. We used a selective organ targeting LNP formulation and characterized cell-specificity of delivery after bleomycin-induced lung fibrosis. We then delivered Ccn3 mRNA (encoding cellular communication network factor 3) to aged mice in the setting of established lung fibrosis and evaluated fibrotic regression and vascular repair. The matricellular protein encoded by Ccn3 was previously identified by our group as an important regulator of lung endothelial function. We found that LNP delivery was lung specific and predominantly endothelial targeting in the setting of lung fibrosis. Delivery of Ccn3 mRNA to aged mice via LNPs modestly reduced fibrosis and improved microvascular density in the lungs. Our results support the concept that cell-specific and repair-promoting cargos delivered via lung-targeting LNPs may have utility for treatment of established fibrosis.

This study demonstrates that circulating extracellular vesicles (cEVs) isolated from young mice reverse the transcriptional alterations of the aged mouse pulmonary vasculature, leading to a more youthful endothelial transcriptional phenotype. As a result of this vascular phenotype, aged mice are protected from bleomycin-induced pulmonary fibrosis. These findings highlight the therapeutic potential of targeting vascular aging to alleviate pulmonary fibrosis.

Abstract Rationale Extracellular matrix (ECM) accumulation results from an imbalance between ECM deposition and degradation, due to persistently increased ECM synthesis or reduced resorption. Fibroblasts, as key regulators of ECM homeostasis in both health and disease, are essential targets to rebalance ECM turnover. Recent findings indicate that fibroblasts are regulated by immune cells, which can influence ECM deposition. Specifically, myeloperoxidase (MPO), an enzyme produced by neutrophils, has been shown to enhance fibroblast-driven ECM accumulation in various diseases. In patients with radiographic Interstitial Lung Disease (ILD), elevated MPO levels have been correlated with reduced lung capacity (Cantin et al., 1987), while recent studies report increased MPO levels in the bronchoalveolar lavage fluid of ILD patients (Ngo et al., EJROR 2024). Additionally, MPO and related immune dysregulation proteins are found in fibrotic alveoli in Idiopathic Pulmonary Fibrosis (IPF) (Herrera et al., JCI Insight 2022). These findings suggest an association between MPO and lung fibrosis, yet it remains uncertain whether MPO directly influences ECM accumulation in fibrotic progression. Methods Patient tissue and serum samples were collected with consent, and MPO levels were quantified using ELISA and immunofluorescence techniques. Biochemical assays were performed using MPO isolated from human leukocytes, recombinant cathepsin K, and hydrogen peroxide to investigate whether MPO influences cathepsin K activity. Precision cut lung slices (PCLS) were prepared from untreated C57BL/6 mice and treated with transforming growth factor-beta (TGFβ) and/or MPO for four days, with cathepsin K activity assessed using fluorescence assays. Additionally, MPO-knockout and wild-type mice received intratracheal bleomycin, and after 14 days, cathepsin K activity was quantified via in vivo imaging. Collagen content, as an indicator of fibrosis, was measured by quantifying hydroxyproline in harvested lung tissue. Results/Discussion/Conclusion Increased MPO levels were observed in immunofluorescent-stained samples (57%; 4/7) and serum samples (30%; 3/10) from IPF patients. Biochemical assays demonstrated that physiological levels of MPO (0.2 µg/ml) combined with H2O2 reduced recombinant cathepsin K activity by 85%. In PCLS treated with MPO, cathepsin K activity decreased by 50% with or without TGFβ. After bleomycin treatment, wild-type mice showed reduced cathepsin K activity, while MPO-knockout mice maintained activity levels. Both survival and weight loss were similar between wild-type and MPO-knockout mice; however, collagen content was significantly higher in wild-type mice, whereas MPO-knockout mice were protected from bleomycin-induced collagen accumulation. These findings suggest that MPO contributes to ECM accumulation in lung fibrosis, potentially through inhibition of collagenolytic enzymes such as cathepsin K.

Abstract Idiopathic pulmonary fibrosis (IPF) is an aggressive and, thus far, incurable disease characterized by aberrant fibroblast-mediated extracellular matrix deposition. Our understanding of the disease etiology is incomplete; however, there is consensus that a reduction-oxidation (redox) imbalance plays a role. In this study, we use the autofluorescent properties of two redox molecules, NAD(P)H and FAD, to quantify changes in their relative abundance in living lung tissue of mice with experimental lung fibrosis and in freshly isolated cells from mouse lungs and humans with IPF. Our results identify cell population–specific intracellular redox changes in the lungs in experimental and human fibrosis. We focus particularly on redox changes within collagen-producing cells, where we identified a bimodal distribution of NAD(P)H concentrations, establishing NAD(P)Hhigh and NAD(P)Hlow subpopulations. NAD(P)Hhigh fibroblasts exhibited elevated profibrotic gene expression and decreased collagenolytic protease activity relative to NAD(P)Hlow fibroblasts. The NAD(P)Hhigh population was present in healthy lungs but expanded with time after bleomycin injury, suggesting a potential role in fibrosis progression. We identified a similar increased abundance of NAD(P)Hhigh cells in freshly dissociated lungs of subjects with IPF relative to control subjects, as well as similar reductions in collagenolytic activity in this cell population. These data highlight the complexity of redox state changes in experimental and human pulmonary fibrosis and the need for selective approaches to restore redox imbalances in the fibrotic lung.