Emerging Concepts in Fibroblast Biology and Progressive Pulmonary Fibrosis

Abstract

Pulmonary fibrosis is characterized by scarring and thickening of the lung parenchyma due to excessive deposition of collagen and other extracellular matrix (ECM) proteins. This leads to disruption of gas exchange areas and ultimately respiratory failure, a pathology shared across multiple interstitial lung diseases (ILDs). Idiopathic pulmonary fibrosis (IPF) is a chronic and progressive ILD characterized by exertional dyspnea, dry cough, and restrictive lung defects. Clinical progression is marked by worsening lung function, declining exercise tolerance, and hypoxemia. High-resolution computed tomography in IPF typically shows reticular opacities and honeycombing, predominantly distributed in the subpleural regions and lower lobes of the lungs. The disease course is variable, with episodes of acute exacerbation associated with high mortality. Myofibroblasts and fibroblasts are central drivers of fibrogenesis through uncontrolled proliferation, migration, survival, senescence, myofibroblast differentiation, and ECM production. Myofibroblasts represent a heterogeneous population in both origin and function, arising from diverse precursor cells, including lung resident fibroblasts, endothelial cells, and mesothelial cells, and are shaped by tissue-specific niches. Persistent activation of (myo)fibroblasts is sustained by a complex network of profibrotic growth factors and their downstream transcriptional regulators. In this review, we comprehensively examine the cellular origins and molecular pathways underlying fibroblast activation, with an emphasis on mechanistic insights that may inform the development of targeted antifibrotic therapies to attenuate disease progression and improve patient outcomes.

Pulmonary fibrosis is a progressive and often fatal condition, pathologically characterized by the accumulation of (myo) fibroblasts and excessive deposition of extracellular matrix (ECM) proteins, resulting in irreversible scarring of the lung parenchyma. Pulmonary fibrosis can develop from a range of underlying interstitial lung diseases (ILDs), with idiopathic pulmonary fibrosis (IPF) representing the most severe and fatal form of lung fibrogenesis. IPF is defined by the presence of subpleural cystic airspaces with well-defined walls (honeycombing), peripheral alveolar septal thickening, and fibrotic foci. Fibrotic foci represent areas of active fibrosis, consisting of fibroblasts and myofibroblasts embedded within dense ECM and located immediately beneath the alveolar epithelium. IPF is a severe fibrotic lung disease that primarily affects older adults, with an estimated annual prevalence of three to nine cases per 100,000 individuals. The prognosis for patients with IPF is poor, with a median survival of 3 to 5 years from the time of diagnosis, despite current antifibrotic therapies. IPF progression varies widely, with some patients maintaining stable lung function for years, while others experience rapid decline, often measured by forced vital capacity and diffusion capacity for carbon monoxide (DLCO). Similar to IPF, other ILDs can progress to a phenotype known as progressive pulmonary fibrosis (PPF), characterized by gradual worsening of fibrosis and decline in lung function. Other ILDs that are associated with PPF include (1) connective tissue disease-associated ILD, such as systemic sclerosis-associated ILD, rheumatoid arthritis-associated ILD, idiopathic inflammatory myositis-associated ILD, (2) anti-synthetase syndrome-associated ILD, (3) fibrosing hypersensitivity pneumonitis-associated ILD, and (4) fibrosing occupational ILDs.

Several risk factors have been implicated in the pathogenesis of pulmonary fibrosis, including age, genetic variants, environmental exposures, smoking, and chronic viral infections. Advancing age and genetic mutations affecting telomere maintenance or surfactant production increase the risk of pulmonary fibrosis by impairing lung repair and promoting persistent epithelial injury.^1–4 Environmental pollutants, such as particulate matter, have been implicated in upregulating angiotensin-converting enzyme 2 and TMPRSS2, which may accelerate fibrosis progression, particularly in individuals with preexisting IPF.⁵ Smoking is a well-established risk factor that promotes oxidative stress and stimulates fibroblast growth factor (FGF) signaling, fostering fibroblast activation and ECM deposition.⁶ The COVID-19 pandemic has underscored thelink between viral pneumonia and subsequent pulmonary fibrosis, with studies showing that many post-COVID-19 survivors develop persistent fibrotic lung abnormalities driven by virus-induced inflammation.^5,7,8 Autoimmune diseases, particularly systemic sclerosis, are also known to induce pulmonary fibrosis, with specific autoantibodies serving as predictors of disease severity.^9,10 Overall, the complex interplay of environmental factors, genetic predisposition, immune cell dysregulation, and aging triggers aberrant fibroblast activation and severe fibrotic lung disease.

Myofibroblasts and fibroblasts are primarily responsible for producing and remodeling the ECM in the distal regions of fibrosing lungs in IPF. They are defined by the expression of markers such as PDGFRα, vimentin, ACTA2, and collagen genes, while lacking the expression of immune (CD45), epithelial (EPCAM), and endothelial (CD31) cell-specific markers. Also, these cells are critical to maintain lung architecture and regeneration as they secrete several growth factors along with ECM proteins to orchestrate alveolar repair and regeneration, immune responses, barrier function, and lung development. Fibroblasts communicate extensively through autocrine and paracrine signaling networks involving cytokines and growth factors, which regulate their activity and interactions with neighboring immune and epithelial cells.^11–13 During injury, fibroblasts transition from a quiescent to an activated state to proliferate, migrate, and transform into myofibroblasts. This differentiation is marked by the expression of alpha-smooth muscle actin (αSMA) and secretion of ECM proteins (such as collagens, fibronectin, elastin, periostin, and proteoglycans). This transition may be transient, involving dedifferentiation of myofibroblasts back to fibroblasts or their apoptotic clearance during the resolution of fibrotic lung lesions. Conversely, persistent activation and impaired clearance of myofibroblasts have been implicated in the progression of fibrosis. Consequently, the cellular and functional heterogeneity of fibroblasts plays a pivotal role in dictating the outcome between fibrosis resolution and maladaptive scarring within the lung parenchyma, highlighting fibroblasts as critical therapeutic targets. This review provides a comprehensive analysis of the molecular drivers of fibroblast activation in pulmonary fibrosis, with a major focus on their cellular origins, transcriptional programs, and profibrotic processes.

Cellular Origins of Myofibroblasts

The cellular origins of myofibroblasts have been the focus of extensive investigation and remain an area of active investigation and debate (Fig. 1). Traditionally, resident mesenchymal cells such as fibroblasts and pericytes were considered the primary sources of myofibroblasts in fibrosing lungs. However, accumulating evidence suggests that myofibroblasts may also arise from diverse cellular lineages through transdifferentiation processes, including mesothelial-to-mesenchymal transition (MMT) and endothelial-to-mesenchymal transition (EndMT).^14–17 Recent advances in lineage tracing, single-cell transcriptomics, and spatial profiling technologies have revealed an unappreciated degree of heterogeneity of myofibroblast precursors and the tissue-specific cues that govern their activation.^18,19

Fig. 1.

Cellular origins of myofibroblasts and fibroblasts in the pathogenesis of pulmonary fibrosis (Created in BioRender. Singh, P. (2025) https://BioRender.com/3pditmt).

Resident Fibroblasts

The lung epithelium originates from the endoderm, while mesenchymal cells, which differentiate into fibroblasts and produce ECM, are derived from the mesoderm. Lineage tracing studies reveal that pulmonary mesodermal lineages are derived from cardiopulmonary mesoderm progenitors, marked by the expression of Wnt2, Gli1, and Isl1.^20,21 T-box transcription factor 4 (TBX4) plays a critical role in the embryonic development of the respiratory system, particularly in the specification and patterning of the mesenchymal compartment that gives rise to lung fibroblasts.²² TBX4 is expressed in the lateral plate mesoderm and the mesenchyme surrounding the developing foregut endoderm (splanchnic mesoderm), contributing to early lung bud formation.^23,24 Lineage tracing studies using Tbx4-Cre drivers have demonstrated that TBX4-expressing mesodermal progenitors give rise to the majority of pulmonary mesenchymal cells, establishing TBX4 as a key determinant of fibroblast lineage specification in the lung.²⁵ Moreover, alterations in TBX4 expression or function are linked to developmental lung disorders such as acinar dysplasia and small patella syndrome.^26–28 They also influence postnatal fibroblast populations involved in disease susceptibility, fibrosis, or tissue repair.^25,29 Additionally, PDGFRα⁺ lineages are implicated in promoting alveolar septal formation and matrix fibroblast differentiation during lung development.^30,31 A recent study using PDGFRa^CreERT2/+ mediated lineage tracing combined with temporal scRNA sequencing revealed that following H1N1 infection, PDGFRa-expressing cells differentiate into PDGFRb⁺ alveolar fibroblasts.³² Using both bleomycin and silica-induced pulmonary fibrosis mouse models, Lepr⁺ and Scube2⁺ alveolar fibroblasts were identified as major sources of pathological fibroblasts characterized by CTHRC1 and POSTN expression. Integrated scRNA-seq and scATAC-seq analyses from this study further identified RUNX2 as a key transcriptional regulator driving the fibrotic gene program.³³

A study employing Scube2^CreER-mediated lineage tracing in bleomycin-injured mice demonstrated that alveolar fibroblasts (SCUBE2⁺, NPNT⁺, CES1D⁺, and WNT2⁺) differentiate into inflammatory fibroblasts (SAA3⁺, LCN2⁺, CCL2⁺, and CXCL12⁺) in response to early inflammatory cytokines. In the later stages of injury, exposure to pro-fibrotic signals such as TGFβ1 drives their further transition into fibrotic fibroblasts characterized by CTHRC1⁺, POSTN⁺, and COL1A1^high expression. Conditional deletion of Tgfbr2 using Scube2^CreER confirmed the requirement of this sequential fibroblast lineage progression for fibrosis development and underscored the essential role of TGF-β signaling in promoting fibroblast expansion and modulating the resolution of inflammation following lung injury.³⁴ Collectively, these findings suggest that alveolar fibroblasts, which play a crucial role in maintaining alveolar homeostasis, may also serve as a cellular source of emerging fibroblast subsets following fibrotic lung injury.

Lipofibroblasts (LIFs), defined by their lipid droplet inclusions, have traditionally been regarded as lipid-storing cells involved in surfactant metabolism. However, emerging evidence has revealed their diverse functions, highlighting active roles in lung development, repair, and fibrosis.^35,36 Rather than serving solely as passive lipid reservoirs, LIFs contribute to alveolar epithelial support and are increasingly recognized as dynamic regulators of fibrotic remodeling. Lineage-tracing studies have identified Tcf21⁺ mesenchymal progenitors as a source of LIFs, as well as other interstitial fibroblast subtypes in the developing lung. Specifically, these progenitors give rise to Limch1⁺ and Apoe⁺ LIFs and contribute to mesenchymal heterogeneity during lung development and homeostasis.³⁶ A recent study by Arun et al further elucidates the plasticity of LIFs in the context of fibrosis. They demonstrated that Acta2⁻, LIF^high alveolar fibroblasts are the principal precursors of Cthrc1⁺ myofibroblasts during lung injury. These cells progressively acquire a fibrotic transcriptional profile, transitioning through intermediate Cthrc1^low LIF^high states before differentiating into Cthrc1^high myofibroblasts. Notably, this differentiation trajectory is reversible; during fibrosis resolution, Cthrc1^high myofibroblasts gradually revert to Cthrc1⁻ LIF^high alveolar fibroblasts, underscoring the potential of LIFs as targets for pro-reparative therapies.³⁵

CTHRC1 has emerged as a prominent marker of fibrotic fibroblasts in multiple organs, including the lung, heart, liver, and tumor stroma.^37–42 In the lung, lineage tracing and ablation studies show that CTHRC1⁺ fibroblasts contribute to ECM deposition and disease progression.³⁵ Adding to the complexity, studies also suggest CTHRC1 functions as a negative regulator of transforming growth factor β (TGFβ) signaling by inhibiting Smad2/3 phosphorylation, and Cthrc1 knockout mice paradoxically exhibit increased fibrosis and worsened lung function following bleomycin injury.⁴³ This raises the possibility that CTHRC1 may be a marker of fibroblast activation rather than a direct driver of fibrosis. Therefore, further studies are needed to establish the functional role of Cthrc1 in fibroblasts, specifically to determine whether it acts as a pro-fibrotic or anti-fibrotic mediator in the context of lung injury and repair.

Fibrocytes

Fibrocytes are bone marrow-derived mesenchymal cells that circulate in the blood and migrate to the lung in response to injury and contribute to fibrogenesis. Fibrocytes co-express markers of hematopoietic cells (CD45 and CD34) and mesenchymal cells (collagen 1, vimentin, and fibronectin). In IPF, the number of fibrocytes is elevated, particularly during exacerbations.^44–46 This increase in fibrocyte levels is associated with worse prognosis and reduced survival in IPF patients.⁴⁴ However, fibrocyte contributions to the myofibroblast pool in pulmonary fibrosis remain a subject of significant debate. Lineage tracing and bone marrow transplantation studies in a mouse model of transforming growth factor α (TGF-α)-induced pulmonary fibrosis suggest that fibrocytes accumulate in fibrotic lesions, but are not a major source for myofibroblasts.⁴⁷ Nonetheless, fibrocytes promote the progression of fibrotic lung lesions through their paracrine effects on lung resident fibroblasts, including Wilms tumor protein 1 (WT1)-expressing fibroblasts in distal fibrotic lung lesions.^48–51

Pericytes

Pericytes are specialized mesenchymal cells that are closely associated with capillaries and venules. These cells are primarily characterized by their position within the capillary basement membrane and share a common basement membrane with endothelial cells. Pericytes express markers such as neural/glial antigen 2 (NG2) and platelet-derived growth factor receptor beta (PDGFRβ). Studies using pericyte line-age-tracing mice (NG2^CreER and Foxd1^CreER) support proliferation and accumulation of pericytes in lung parenchyma, but limited colocalization with αSMA during bleomycin-induced pulmonary fibrosis.⁵² In contrast, Hung et al showed that Foxd1-expressing pericytes do transform into myofibroblasts during bleomycin-induced pulmonary fibrosis.^53,54 Thus, pericyte contribution to the total myofibroblast pool is limited and needs further investigation using severe or progressive fibrotic lung disease models.

Mesothelial Cells

A single layer of cuboidal pleural mesothelial cells (PMCs) lines the lungs and expresses both epithelial and mesenchymal markers, such as calretinin, cytokeratin, collagen, and vimentin. These cells possess remarkable plasticity and can undergo MMT, a process by which they express αSMA and acquire a spindle-shaped myofibroblast-like phenotype. PMCs are increasingly recognized as a potential source of myofibroblasts in subpleural fibrotic lesions, contributing to the pathogenesis of fibrotic lung diseases.^55,56 Mesothelial cells originate from splanchnic mesoderm at the early developmental stage. Lineage-tracing studies using WT1^CreERT2 mice have demonstrated that, during embryonic development, WT1-positive mesothelial cells give rise to mesenchymal cells that localize to perivascular, peribronchial, and alveolar regions of the lung parenchyma. These findings suggest a developmental link between pleural mesothelium and the mesenchymal compartment of the lung. In adult mice, mesothelial cell-derived mesenchymal cells undergo reactivation and contribute to the majority of collagen-producing mesenchymal cells during bleomycin-induced pulmonary fibrosis. Indeed, our recent studies have shown that mesothelial cells can transform to myofibroblasts in adult mice during TGF-α-induced pleural thickening and pulmonary fibrosis.¹⁴ Understanding the complex molecular regulation of MMT and myofibroblast accumulation in distal fibrotic lung lesions in the pathogenesis of pulmonary fibrosis is essential to develop more efficacious therapeutics for IPF.

Endothelial Cells

Lung endothelial cells line the inner surface of blood vessels and play key roles in vascular homeostasis, permeability regulation, and angiogenesis. During lung development, endothelial cells arise upon vessel formation from mesodermal progenitors.⁵⁷ Structural alterations in the arterial layers, characterized by significant collagen and elastin deposition in the adventitia, have been observed in patients with IPF.¹⁵ Additionally, adventitial regions of IPF patients exhibit increased expression of myofibroblast markers, including N-cadherin, S100A4, and vimentin, and reduced endothelial VE-cadherin, with VE-cadherin shifting from junctions to the cytoplasm, the hallmark for weakening endothelial integrity. This study also demonstrated that N-cadherin and vimentin levels have negatively correlated with lung function (DLCO).⁵⁸ In a recent study, using a genetic lineage-tracing mouse model, it was demonstrated that lung endothelial cells undergo a limited EndMT following bleomycin treatment, characterized by elevated myofibroblast marker expression without a loss of endothelial markers.^59,60 This suggests that the EndMT is an active process in patients with pulmonary fibrosis and contributes to pulmonary arterial remodeling and fibrosis. EndMT-induced vascular changes have also been reported in pulmonary vascular diseases, intestinal fibrosis, and cardiac fibrosis.^16,61

Epithelial Cells

Alveolar epithelial cells (AECs) play a pivotal role in lung fibrosis by orchestrating mesenchymal responses following epithelial injury. Upon damage, AECs secrete chemotactic factors and mitogens that recruit and activate mesenchymal cells, leading to myofibroblast accumulation through migration, proliferation, and differentiation. While in vitro studies demonstrate that both human and murine AECs undergo epithelial-to-mesenchymal transition (EMT) upon TGF-β1 exposure, in vivo evidence for this process remains limited. In a TGF-β1 overexpression model, β-galactosidase-labeled lung epithelial cells exhibit minimal acquisition of mesenchymal markers even after 3 weeks (63).⁶² Similarly, lineage tracing studies using the Nkx2.1^Cre/mT/mG mice indicate a limited contribution of epithelial cells to myofibroblasts, with only approximately 4% expressing limited mesenchymal markers such as vimentin or ACTA2 following bleomycin injury. Furthermore, AECII lineage tracing with the Sftpc^CreERT2 Rosa^tdTomato mice demonstrated that these cells do not transdifferentiate into myofibroblasts during bleomycin-induced pulmonary fibrosis.⁵² Collectively, these findings suggest that AECII-mediated EMT is absent in murine fibrosis models, with AECs primarily influencing mesenchymal differentiation through paracrine signaling.

Molecular Regulation of Fibroblast Activation

Multiple molecular regulators have been implicated in the fibroblast activation, including proliferation, migration/invasion, myofibroblast transformation, and ECM production and organization.^63,64 Intricate cellular and molecular interactions, inflammatory responses, and the dysregulation of profibrotic signaling pathways drive fibroblast activation.

Transforming Growth Factor β

TGF-β is a pivotal cytokine in fibroblast activation that involves both Smad-dependent (canonical) and independent (noncanonical) pathways. The evidence supporting the role of TGF-β in pulmonary fibrosis is substantial, with various experimental studies demonstrating its effects on (myo) fibroblast accumulation and ECM production.^65–67 The deletion of Smad3 was sufficient to attenuate myofibroblast accumulation and ECM deposition during bleomycin-induced pulmonary fibrosis.^65–69

Transforming Growth Factor α

TGF-α plays a multifaceted role in the development of pulmonary fibrosis via EGFR-mediated pathways, promoting fibroblast activation and ECM accumulation. The overexpression of TGF-α in airway club cells was sufficient to activate fibroblasts and the induction of ECM proteins.^70–72 Specifically, inhibition of EGFR-signaling with rapamycin, an inhibitor of the mTOR pathway, was sufficient to attenuate TGF-α-induced fibrosis.⁷⁰ Several EGFR pathway inhibitors have been shown to mitigate TGF-α-induced pulmonary fibrosis.^47,73

Other Profibrotic Growth Factors

Platelet-derived growth factors (PDGFs) are polypeptides that bind to cell surface receptors and induce fibroblast proliferation and chemotaxis. PDGF exerts its effects in pulmonary fibrosis involves its interaction with PDGF receptors, particularly PDGFR-β.^74,75 PDGF induces fibroblast activation via autocrine and paracrine mechanisms, promoting the transition of fibroblasts to myofibroblasts, which is a hallmark of fibrotic lung disease.⁷⁶ PDGF synergizes with other pro-fibrotic factors such as TGF-β and CTGF to promote fibroblast activation.⁷⁷ FGFs are a family of signaling ligands that bind to and activate FGF receptors, which regulate various biological processes, including cell proliferation, differentiation, migration, and survival. Multiple FGF ligands, including FGF2, FGF9, and FGF18, are upregulated in IPF lungs.^78,79 FGFs signal through MAPK, PI3K, Smad3, and ERK1/2 pathways to promote fibroblast survival and ECM production. In the context of IPF, the role of FGF2 appears to be complex and induces anti-fibrotic effects under certain conditions. In particular, it antagonizes TGFβ1-induced expression of α-SMA, and FGF2 overexpression attenuated bleomycin-induced pulmonary fibrosis in vivo.^75–78 However, sustained or dysregulated FGF2 signaling may promote fibroblast expansion and ECM remodeling, contributing to the progression of fibrosis. Thus, FGF2 may play a dual role in IPF pathogenesis, balancing epithelial repair and fibroblast activation, with its net effect likely influenced by the cellular context, signaling crosstalk, and disease stage.

Wnt and Sonic Hedgehog Ligands

Wnt ligands, particularly Wnt3a, Wnt5a, and Wnt10a, are upregulated in fibrotic lungs and have been shown to promote fibroblast activation, proliferation, and survival.^80–84 Canonical Wnt/β-catenin signaling enhances the expression of profibrotic genes such as ACTA2, COL1A1, and TGF-β1, driving myofibroblast differentiation and ECM deposition.^83,85–87 Noncanonical Wnt signaling is mediated by ligands like Wnt5a, which also contributes to fibroblast migration and contractility through pathways involving RhoA and JNK.^81,86 Importantly, Wnt ligands are produced not only by fibroblasts themselves but also by epithelial cells, macrophages, and endothelial cells, establishing a complex network of paracrine and autocrine signaling that regulates epithelial repair, fibroblast activation, and fibrogenesis. Further, inhibiting Wnt signaling with a small molecule inhibitor of β-catenin/TCF/LEF complex, called ICG-001, has attenuated bleomycin-induced pulmonary fibrosis in vivo.⁸⁸

Hedgehog (HH) signaling is an evolutionarily conserved signaling pathway that regulates multiple biological processes, including embryonic development, tissue homeostasis, and wound repair.⁸⁹ In vertebrates, binding of one of the three HH ligands-Sonic Hedgehog (SHH), Indian Hedgehog, or Desert Hedgehog to the Patched1 receptor relieves Smoothened inhibition and activation of Gli transcription factors. Among the three Hh ligands, Shh is the most extensively studied and well-characterized.⁹⁰ The pharmacological inhibition of Shh signaling has been shown to attenuate α-SMA, fibronectin, and collagen expression.⁸⁹ Further, molecular crosstalk between SHH signaling and the Wnt pathway has been demonstrated in IPF.^86,89

Wilms Tumor Protein 1

WT1, a zinc finger transcription factor, plays a pivotal role in lung development and fibrosis. While expressed at low levels in adult lung mesothelial cells, WT1 is upregulated in IPF, particularly in mesothelial and mesenchymal cells. WT1 regulates fibroblast activation and myofibroblast transformation by binding directly to the promoter of α-SMA, a key marker of myofibroblasts. WT1 deletion reduces the expression of ECM proteins (Col1α1 and Col5α1) and proliferative genes (Grem1, Runx1, and Wnt4), significantly attenuating fibroblast proliferation, myofibroblast accumulation, and collagen deposition in TGF-α- and bleomycin-induced pulmonary fibrosis models. WT1-driven fibroproliferation involves paracrine signaling, suggesting complex cell-cell interactions that amplify fibrotic processes.^14,91 Targeting WT1 could be a novel approach to inhibit fibroblast activation and ECM deposition in IPF.

Sry-Box Transcription Factor 9

Sry-box transcription factor 9 (Sox9) is a member of the Sox family of transcription factors, defined by the presence of a highly conserved high mobility group DNA-binding domain homologous to that of the sex-determining region Y protein. The aberrant regulation of Sox9 has been implicated in the pathogenesis of various malignancies and fibrotic remodeling in several organs, including the kidney, heart, and liver. Recent studies from our group revealed that Sox9 is overexpressed in fibroblasts localized within the subpleural regions, peribronchial zones, and fibrotic foci of lungs from patients with IPF. Promoter-reporterluciferase assays further demonstrated that WT1 directly binds to the Sox9 promoter under conditions of TGFα stimulation, indicating that Sox9 expression in fibrotic lung fibroblasts is regulated by the TGFα–WT1 axis. Loss of Sox9 attenuates the expression of ECM genes and reduces fibroblast activation. Overexpression of Sox9 induces pro-fibrotic cytokines (TGFβ1, IL-6, and IL-13) and enhances mesenchymal cell differentiation.⁹² Sox9 is a promising target to mitigate fibroblast activation in IPF.

Conclusion

Myofibroblasts and fibroblasts are central mediators of pulmonary fibrosis, driving disease progression through their persistent activation, resistance to apoptosis, and excessive production of ECM proteins that disrupt normal lung architecture and impair gas exchange. While TGF-β is a well-established driver of fibroblast activation, emerging evidence highlights additional regulatory pathways, including Wnt/β-catenin signaling, YAP/TAZ mechanotransduction, and metabolic reprogramming. The convergence of these pathways suggests redundancy in fibroblast activation mechanisms, posing challenges for therapeutic targeting. Also, the growing recognition of fibroblast heterogeneity underscores the complexity of fibrosis, with distinct fibroblast subpopulations exhibiting diverse functional roles. Advances in single-cell transcriptomics have identified fibroblast subsets with pro-fibrotic, immunomodulatory, and even reparative properties. However, the origins, plasticity, and lineage relationships of these subpopulations remain incompletely understood. A detailed characterization of fibroblast heterogeneity may provide insights into how specific subsets contribute to fibrosis resolution versus progression, enabling the development of precision therapies that selectively target pathogenic fibroblast populations.

Despite recent advances in the understanding of critical drivers of fibroblast activation, fibrosis remains a major clinical challenge due to the lack of effective therapies that selectively target pathological facets of fibroblasts. Also, the lack of knowledge on the role of fibroblasts in immune modulation and their interactions with other stromal cells is a limitation to improving anti-fibrotic therapies. Moving forward, integrating multi-omics approaches, in vivo lineage and single-cell barcoding tracing, organoid models, and precision-cut lung slice culture methods will be crucial for unraveling the complexities of fibroblast biology and developing novel anti-fibrotic interventions.