Macrophage–Fibroblast Crosstalk in Fibrosis: Key Regulator of Fibrosis Across Different Organs

Abstract

Fibrotic diseases encompass a range of pathological conditions characterized by the abnormal growth of connective tissue, involving various cell types and intricate signaling pathways. Central to the onset and development of fibrosis are macrophages and fibroblasts, whose interactions are a pivotal area of investigation. Macrophages facilitate the activation, growth, and collagen production of fibroblasts, doing so either directly or indirectly through the release of cytokines, chemokines, and growth factors. Conversely, fibroblasts boost macrophage activity and intensify local inflammatory responses by secreting cytokines and matrix proteins associated with fibrosis. Throughout the different phases of fibrosis, these two cell types communicate via cytokines and signaling pathways, thereby sustaining the pathological condition. This review emphasizes the interplay between macrophages and fibroblasts and their contributions to fibrosis in the lungs, liver, kidneys, and other organs. Furthermore, it delves into potential therapeutic targets within these interactions, with the aim of shedding light on future clinical research and treatment approaches for fibrotic diseases.

Graphical abstract

Keywords

fibrosis / macrophages / fibroblasts / extracellular matrix / tissue repair / inflammation / pulmonary fibrosis

1. Introduction

Fibrosis, a multifaceted pathological condition, is fundamentally characterized by the excessive buildup of extracellular matrix (ECM), leading to tissue scarring and, ultimately, compromised organ function [1]. This pathological transformation frequently emerges as the common endpoint in chronic diseases affecting critical organs, including the lungs, liver, and kidneys. Within the intricate web of cellular interactions and signaling pathways, macrophages and fibroblasts stand out as central players in driving the fibrotic process.

Macrophages, known for their remarkable phenotypic adaptability, exert both pro-inflammatory and anti-inflammatory effects [2]. This versatility directly impacts the activation, proliferation, and ECM production of fibroblasts. Once fibroblasts are activated, a self-perpetuating cycle ensues, marked by the reinforcement of inflammatory and fibrotic signals, making the condition notoriously difficult to reverse. Although significant strides have been made in deciphering the mechanisms of these cells, the intricate interplay between them and their underlying molecular processes remains largely enigmatic.

A wealth of research highlights the distinct roles played by different macrophage polarization states in fibrosis. Similarly, the diverse behaviors of fibroblasts and their interactions with immune cells have been closely linked to disease progression. However, how these cellular dynamics differ across organs and fibrosis stages, and how they might inform therapeutic approaches, continues to be an area of uncertainty and ongoing investigation.

Drawing on current insights into the mechanisms of macrophage–fibroblast interactions in fibrosis, this article delves into how these processes unfold in vital organs such as the lungs, liver, and kidneys. By thoroughly examining the key molecular mechanisms and cellular pathways involved, we aim not only to uncover potential therapeutic targets but also to spark innovative ideas for future clinical interventions. As we navigate the complexities of these interactions, we also grapple with unresolved scientific questions, striving to offer fresh perspectives that could propel the field forward.

2. Literature Review

2.1 Functional Changes in Macrophages in the Development of Fibrogenesis

2.1.1 Macrophage Classification

Macrophages in other tissues are typically classified into M1 (pro-inflammatory) and M2 (anti-inflammatory) phenotypes. However, with advancements in single-cell sequencing, the M1/M2 paradigm does not fully capture the complexity of pro-fibrotic macrophages. In contrast, monocyte-derived macrophages associated with fibrosis, as observed in idiopathic pulmonary fibrosis (IPF) and COVID-19-associated Acute Respiratory Distress Syndrome (ARDS), have acquired distinct transcriptional profiles [3].

In asbestos-induced pulmonary fibrosis, monocyte-derived macrophages are localized near fibroblasts within fibrotic areas, and their deletion reduces fibrosis without affecting tissue-resident alveolar macrophages (AMs) [4]. These findings underscore the pivotal role of monocyte-derived macrophages in fibrosis, with their pro-fibrotic properties often overlapping with those of tissue-resident macrophages.

The primary sources of macrophages in the kidneys are monocytes from the bone marrow and resident macrophages within the kidneys. Based on their origin and function, they are generally classified into two categories: one category consists of self-renewing resident macrophages in the kidneys, while the other category comprises macrophages that differentiate after migrating from circulating monocytes to the kidneys [5].

Macrophages in the liver are divided into tissue-resident macrophages and monocyte-derived macrophages. However, the resident macrophages in the liver have been given a new name: Kupffer cells [6].

2.1.2 SPP1⁺ Macrophages as Key Regulators in Fibrotic Progression Across Organs

With the advancement of single-cell RNA sequencing (scRNA-seq), numerous macrophage subpopulations associated with fibrosis have been identified across organs. In IPF, scRNA-seq of lung tissues revealed several distinct macrophage subsets, including Secreted Phosphoprotein 1 (SPP1⁺), Ficolin 1 (FCN1⁺), and Fatty Acid Binding Protein 4 (FABP4⁺) macrophages. Among these, FABP4⁺ macrophages, predominantly found in healthy lungs, are inferred to represent Alveolar Macrophages (AMs), while SPP1⁺ macrophages, co-expressing MAF BZIP Transcription Factor B (MAFB), CD163 Molecule (CD163), and Legumain (LGMN), likely derive from monocytes [7].

A similar classification was proposed in the study of Morse et al. [8], which identified three macrophage subsets in normal and IPF lungs: monocyte-like cells, FABP4hi/Inhibin Subunit Beta A (INHBA)⁺ macrophages, and SPP1hi/MER ProtoOncogene, Tyrosine Kinase (MERTK)⁺ macrophages. Notably, SPPhi macrophages exhibit limited proliferation in healthy lungs but strongly proliferate in IPF lungs, in contrast to the declining proliferative capacity of FABP4hi macrophages [8].

Importantly, SPP1⁺ macrophages appear to represent a conserved macrophages population across multiple fibrotic disorders. They are consistently expanded in Chronic Kidney Disease (CKD), heart failure, and other fibrotic conditions [9] (Fig. 1). In liver fibrosis, a related subpopulation known as Scar-Associated Macrophages (SAMs)—characterized by CD9⁺ Triggering Receptor Expressed on Myeloid Cells 2 (TREM2⁺) markers—also highly expresses SPP1, along with Glycoprotein Non-Metastatic Melanoma Protein B (GPNMB), FABP5, and CD63 [10, 11]. Similarly, in the Perivascular Adipose Tissue (PVAT) of patients with coronary atherosclerosis, elevated levels of SPP1⁺ macrophages were associated with greater fibrosis severity and higher coronary stenosis grades [12]. Together, these findings suggest that SPP1⁺ macrophages constitute a core fibrotic macrophage program conserved across organs, while other subtypes such as FABP4⁺ macrophages may play tissue-specific roles. However, the post-traumatic repair process does not solely involve a pro-fibrotic process but also includes tissue repair processes. Additionally, SPP1⁺ cells exist at different stages of maturation and exhibit diverse expression profiles, suggesting they may possess distinct functional states, potentially functioning as “remodeling” macrophages with dual roles in ECM regulation and repair [13]. While these findings highlight a conserved role for SPP1⁺ macrophages across various fibrotic diseases, it is important to interpret single-cell transcriptomic classifications with caution. Technical limitations such as batch effects, low RNA capture efficiency, and underrepresentation of rare cell types may affect the resolution and interpretation of macrophage subtypes, potentially overestimating their transcriptional discreteness [14].

2.1.3 Mechanisms by Which Macrophages Regulate Fibrosis

Fibrosis is a chronic inflammatory response induced by relatively severe or repetitive tissue damage, where ECM components continuously accumulate, leading to structural destruction and fibrosis of the tissue [15]. Macrophages play a pivotal role in initiating and sustaining inflammation and are key participants in the fibrotic process. Single-cell sequencing of human and mouse tissues has identified diverse macrophage subtypes involved in tissue inflammation and fibrosis, with distinct regulatory roles in these processes. In a 2005 study on liver injury and repair by Duffield et al. [16], macrophage depletion during the progression of hepatic fibrosis resulted in reduced scar formation and fewer myofibroblasts. Conversely, macrophage depletion during recovery impaired matrix degradation [16]. This suggests that macrophages not only promote fibrotic tissue repair but also contribute to post-injury tissue recovery by phagocytosing degraded matrix. These roles may be carried out by distinct macrophage subpopulations. Further research has revealed that certain macrophage subpopulations are more effective at triggering fibrosis, depending on their spatial localization and their capacity to activate other fibrosis-associated cells [4, 17].

Modulation of macrophages alone can significantly influence fibrotic outcomes, with diverse mechanisms observed across organ systems. In the liver, pharmacologic inhibition of macrophages has shown therapeutic potential. For instance, blocking dopamine receptor D2 signaling in macrophages—but not in hepatocytes—suppresses transcriptional co-activator activity and favors liver regeneration over fibrosis [18]. Additionally, the neurotrophic factor MANF, expressed in myeloid cells, interacts with S100A8 to disrupt its binding with A9, thereby inhibiting the TLR4–NF\kappaB pathway and attenuating liver fibrosis [19]. Moreover, hepatocyte ferroptosis-induced oxidative DNA damage can activate the STING pathway in macrophages, promoting hepatic inflammation, fibrosis, and even tumorigenesis [20].

In the kidney, hematopoietic cell kinase (HCK) drives macrophage polarization toward pro-inflammatory, proliferative, and migratory phenotypes via autophagy, thereby exacerbating fibrosis [21]. In contrast, the zinc-finger transcription factor KLF4 serves as a negative regulator of macrophage-mediated inflammation, protecting against TNF\alpha-induced kidney injury and fibrosis [22]. In the lung, macrophages contribute to fibrotic progression through adrenergic signaling initiated by the neuronal guidance protein netrin-1 [23, 24]. In parallel, keratin 8 regulates macrophage chemokine production and recruitment, further shaping the inflammatory environment that promotes pulmonary fibrosis.

In the skin, Sonic hedgehog (SHH) signaling enhances macrophage oxidative phosphorylation and cytophagocytic activity, promoting their polarization toward a pro-fibrotic phenotype and driving scar formation [25]. Beyond organ-specific mechanisms, macrophages have also been shown to directly produce ECM proteins. A 2020 zebrafish study demonstrated that macrophages can contribute to scar formation by expressing and depositing collagen both autonomously and in a cell-naïve manner. However, macrophage-derived collagen may differ in composition and organization compared to that of canonical myofibroblasts [26].

Because the original M1/M2 macrophage typing does not fully describe pro-fibrotic macrophage subtypes, there has been a gradual increase in studies on the regulation of SPP1⁺ macrophages alone. For instance, in hepatic fibrosis, SAMs are regulated by cytokines such as Granulocyte–Macrophage Colony-Stimulating Factor (GM-CSF), Interleukin 17A (IL-17A), and Transforming Growth Factor Beta 1 (TGF-\beta1), which drive the differentiation of human monocytes into macrophages expressing scar-associated markers. These differentiated cells degrade collagen IV, but not collagen I, and promote TGF-\beta1-induced deposition of collagen I by activated Mesenchymal Stem Cells (MSCs) [11]. Type I collagen is a major component of “restorative” fibrotic scar tissue and may play a supportive role in tissue repair in response to inflammation and other injuries. TGF-\beta1, a transforming growth factor, contributes to the formation of elastic fibers in connective tissue and is a critical inducer of fibrosis [27]. Chemokine (C-X-C motif) ligand 4 (CXCL4) is one of the most highly upregulated genes during SPP1⁺ macrophage differentiation; it promotes this macrophage subtype and facilitates macrophage–fibroblast crosstalk. CXCL4 deletion disrupts this interaction and significantly attenuates fibrosis [9]. Pirfenidone, a clinical antifibrotic agent, suppresses macrophage-mediated fibrosis by inhibiting integrin \alphaM\beta2 and Rho-associated kinase 2, thereby blocking mechanical activation of macrophages [28]. Similarly, inhibition of Macrophage Colony-Stimulating Factor Receptor (M-CSFR) signaling attenuates fibrosis by disrupting important fibrotic ecological niches formed by monocyte, AMs, and fibroblasts [4]. Ring Finger Protein 41 (RNF41) attenuates fibrosis and promotes liver regeneration by inhibiting the secretion of pro-inflammatory cytokines [29]. Conversely, FSTL1 promotes macrophage phenotypes toward pro-inflammatory activation and activates the NF-kappaB pathway, thereby accelerating the progression of liver fibrosis [30].

2.2 Functional Changes in Fibroblasts in the Development of Fibrogenesis

2.2.1 Fibroblast Classification

Fibroblasts are primarily derived from mesenchymal cells. Under normal physiological conditions, fibroblasts are a key cell type in connective tissues, and their main functions include the synthesis and secretion of collagen and other fibrous proteins as well as participation in wound healing and tissue repair processes. During fibrosis, fibroblasts not only increase in number, but their behavior also changes from supporting and repairing to promotion of hyperfibrosis and tissue sclerosis, a shift that is usually caused by a chronic inflammatory response in which pro-inflammatory factors released by immune cells activate and alter fibroblast function. These cells are not only involved in the inflammatory phase but also contribute significantly to the formation of type I collagen in the later stages of tissue remodeling [31]. Notably, type I collagen is the main component of scar tissue. In their review, Plikus et al. [32] summarized various fibroblast subpopulations in different tissues, including alveolar fibroblasts (PDGFR\alpha⁺COL1a1⁺NPNT⁺CES1D⁺), epithelial fibroblasts (PDGFR\alpha⁺COL1a1⁺Pi16⁺CCL11⁺), peribronchial fibroblasts (PDGFR\alpha⁺COL1a1⁺HHIP⁺), and myofibroblasts (FAP⁺CTHRC⁺COL1A1⁺POSTN⁺) (Fig. 1). Recent advances in single-cell sequencing have identified numerous fibroblast sub-phenotypes associated with fibrogenesis. For example, the pathological fibroblast subpopulation marked by Collagen Triple Helix Repeat Containing 1 (CTHRC1) expression produces collagen exclusively within fibrotic foci and serves as a specific marker for ECM-generating pathological fibroblasts in fibrotic tissues [33, 34, 35]. Mayr et al.’s study [36] on bleomycin-induced fibrotic lungs in mice identified transitional fibroblasts with high Sfrp1 expression in the early post-injury stages. These fibroblasts were located in peribronchiolar, peripheral, and alveolar regions and later differentiated into invasive CTHRC1⁺ myofibroblasts during advanced disease stages.

2.2.2 Major Molecules Regulating Fibroblast Activation

As with macrophages, certain fibroblast cells are more closely involved in the fibrotic process than others. In addition to fibroblasts, a variety of other cells can differentiate into myofibroblasts after acute tissue injury [37]. Fibroblasts contributing to fibrosis originate from diverse sources, and their activation represents a dynamic and context-dependent process. While Hhip⁺ lipofibroblasts have been identified as a fibroblast subtype enriched in regenerative regions and associated with anti-fibrotic signaling pathways in both mouse and human lungs [38], this trajectory represents only one of several potential differentiation pathways. However, to date, SCUBE2⁺ fibroblasts have primarily been characterized in mouse models, and their presence and relevance in human fibrotic diseases remain to be confirmed by future studies. Other resident cell populations, such as lipid fibroblasts, have been shown to be capable of mutual conversion with myofibroblasts during the progression and regression of fibrosis [39]. Adventitial fibroblasts are activated and migrate to the intimal region following arterial injury, differentiating into myofibroblasts and participating in ECM deposition and pathological remodeling [40]. This includes epithelial-mesenchymal transition (EMT), the process by which endothelial cells transform into mesenchymal fibroblasts, which is closely associated with wound healing and organ fibrosis [41]. However, EMT appears to play a more significant role in the kidney, while its role in other organs remains controversial, with complex mechanisms that require further validation. These cells have been shown to contribute to the myofibroblast pool depending on the type of injury, anatomical context, and species.

Notably, single-cell lineage tracing and transcriptomic analyses have identified transitional fibroblast states (e.g., SFRP1⁺ or Cthrc1^low) that precede full myofibroblastic differentiation and may retain partial lipogenic signatures. While these findings suggest potential plasticity, definitive in vivo evidence for their reversion to quiescence in established fibrosis remains limited. In particular, whether such transitions occur under pathological conditions or only in self-resolving models (e.g., young mice) is still under investigation. Therefore, fibroblast activation is better conceptualized as a context-dependent continuum, shaped by niche-derived cues, rather than a strictly linear or fully reversible process. However, this may also suggest that fibroblasts are not the end point of a single trajectory, but rather the result of the convergence of multiple input factors (developmental origin, spatial location, and signal exposure), and that functional heterogeneity exists even within subpopulations defined by markers.

Fibroblast activation plays a central role in tissue remodeling across multiple organs and is regulated by diverse, context-dependent mechanisms. In the heart, Ambra1 limits fibrosis by blocking the uptake of extracellular vesicles from cardiomyocytes and fibroblasts [42]. Additionally, Platelet-Derived Growth Factor AB (PDGF-AB) facilitates fibroblast migration and scar maturation without inducing proliferation, thus supporting functional repair [43].

Several pro-fibrotic pathways are shared across organs. Notably, Wnt/\beta-catenin signaling enhances TGF-\beta1- or IL-4-induced macrophage polarization, leading to downstream STAT3 activation and fibrosis in both the heart and kidney [44, 45]. In the kidney, Glutathione Peroxidase 3 (GPX3) deficiency increases NADPH Oxidase 4 (NOX4) and ROS production, activating fibroblasts [46], while CD44-binding exosomal proteins [47] and Fibroblast Growth Factor 18 (Fgf18) overexpression [48] further promote fibroblast proliferation and fibrotic progression.

In the lung, regulatory factors exert opposing roles: Platelet Endothelial Aggregation Receptor 1 (PEAR1) suppresses fibrogenic signaling via protein phosphatase 1, whereas CD147 promotes SARS-CoV-2-induced fibroblast activation [49, 50]. CD148, conversely, inhibits NF-kappaB-mediated necrotic gene expression, reducing fibrosis [51].

Epigenetic and transcriptional control also shape fibroblast behavior. In skin and epidural tissues, Histone Deacetylase 5 (HDAC5) silences Smad7 through Myocyte Enhancer Factor 2A (MEF2A), and Jun Proto-Oncogene, AP-1 Transcription Factor Subunit (JUN) activates CD36, both contributing to excessive collagen deposition and scar formation [52, 53]. In contrast, lncRNA-COX2 downregulates Early Growth Response 1 (EGR1), limiting fibroblast activation and epidural fibrosis [54], while p53 deletion causes fibroblast overproliferation and ECM accumulation, contributing to pathological fibrosis [55].

Across various tissues, fibroblast activation protein (FAP) serves as a consistent marker of activated fibroblasts. In the intestines and kidneys, FAP⁺ fibroblasts are key ECM producers [56, 57]. In both the heart and liver, FAP⁺ fibroblasts contribute to fibrosis, and targeting FAP—via genetic deletion or pharmacological inhibition—has been shown to attenuate fibrosis, reduce inflammation, and promote tissue repair [45, 58]. These findings underscore the organ-specific yet overlapping pathways that govern fibroblast activation in fibrotic diseases.

2.2.3 Regulation of Myofibroblast Differentiation

The differentiation of fibroblasts into myofibroblasts is a central step in the progression of fibrosis. In the heart, the Circular RNA SH3 Domain Containing Ring Finger 3 (Circ-sh3rf3)/GATA Binding Protein 4 (GATA-4)/microRNA-29a (miR-29a) axis has been identified as a regulatory pathway that modulates this process [59]. Mechanistically, Circ-sh3rf3 interacts with and downregulates GATA-4, a transcription factor that normally represses miR-29a. The resulting upregulation of miR-29a suppresses myofibroblast differentiation, thereby attenuating cardiac fibrosis. Although this pathway is currently characterized in cardiac tissue, further studies are needed to determine its relevance in other organs. Wnt binds Frizzled proteins, activating \beta-catenin to form T-cell factor/lymphoid enhancer-binding factor (TCF/LEF) conjugates, which drive the transcription of downstream genes to promote fibroblast differentiation [60] (Fig. 2). Additionally, the Lysophosphatidylcholine (LPC)/Autotaxin (ATX)/Lysophosphatidic Acid (LPA)/LPA Receptor 2 (LPA2) signaling pathway plays a significant role in the fibroblast-to-myofibroblast transition (FMT) [61].

Furthermore, the process of fibroblast-to-myofibroblast conversion is influenced by multiple factors. Secreted Frizzled-Related Protein 1 (SFRP1) is a key inhibitor of lung fibroblast invasion during the transition to myofibroblasts after lung injury. SFRP1⁺ transitional fibroblasts appear prior to the formation of invasive myofibroblasts, and TGF-\beta1 downregulates SFRP1 to drive this invasive shift [36]. Fibroblast Growth Factor 10 (FGF10) promotes cardiac regeneration after myocardial infarction by enhancing cardiomyocyte proliferation and reducing fibrosis. FGF10 activates major regenerative pathways while inhibiting myofibroblast activation, thereby reducing fibrosis and promoting cardiac regeneration after myocardial infarction [62]. GDF15 is a key driver of IPF; its upregulation in the ECM promotes fibroblast activation and myofibroblast differentiation [63]. In the heart, Muscleblind-Like Splicing Regulator 1 (MBNL1) regulates the transition of fibroblasts into myofibroblasts during cardiac wound healing. MBNL1 overexpression accelerates fibrotic scar formation, whereas MBNL1 deletion prevents myofibroblast activation, thereby reducing fibrosis [64]. Antiretroviruses induce the differentiation of cardiac fibroblasts into myofibroblasts via the JAK2/STAT3 and JNK/c-Jun signaling pathways, thereby promoting cardiac fibrosis [65]. Myofibroblast-specific overexpression of MBNL1 promotes therapeutic efficacy in cardiac fibrosis, whereas MBNL1 deletion limits scarring [64]. Simultaneously, BMP4 counteracts the effects of TGF-\beta1 by suppressing the transformation of fibroblasts into myofibroblasts and curbing the production of ECM. This is achieved through its ability to mitigate dysfunctional mitochondrial autophagy and reduce cellular aging in lung fibroblasts [66].

2.3 Mechanisms of Macrophage–Fibroblast Interaction

Both macrophages and fibroblasts act as important regulatory effector cells in tissue fibrosis, interacting through a macrophage–fibroblast axis that significantly influences fibrotic progression. Macrophage-conditioned medium has been shown to directly stimulate fibroblast proliferation and ECM protein production in vitro [67]. This interaction is mediated by multiple pathways and is influenced by the immunometabolic environment. Under conditions of inflammation, hypoxia, and immune modulation, macrophages undergo phenotypic changes that, in turn, activate fibroblasts and promote their differentiation into myofibroblasts. This dynamic interaction leads to excessive ECM deposition, further driving fibrosis [68].

2.3.1 Pathways by Which Macrophages Regulate Fibroblast Function

Monocyte-derived AMs are a key cell type in pulmonary fibrosis; they are located predominantly in fibrotic areas and stimulate fibroblast proliferation by secreting pro-fibrotic factors [4]. Macrophages produce pro-fibrotic factors that directly activate fibroblasts, including the transforming growth factor TGF-\beta1 and platelet-derived growth factor (PDGF) [69].

Macrophage–fibroblast interactions are stable in healthy tissues. However, after myocardial infarction, cardiac fibroblasts and macrophages function together to regulate tissue homeostasis and infarct repair. Cardiac fibroblasts and macrophages collaborate closely to regulate the fibrotic response. FAP⁺POSTN⁺ fibroblast subpopulations play a pivotal role in fibrosis progression, with their development and maturation promoted by IL-1\beta signaling [70]. Under cardiac pressure overload, macrophages can regulate fibroblast activation via miRNA-21, which drives fibroblast differentiation into myofibroblasts via the TGF-\beta/Smad signaling pathway, further promoting fibrosis [71]. In response to NOX4-mediated inflammation, resident macrophages are activated and exhibit a pro-fibrotic phenotype, secreting IL-6 and TGF-\beta, which directly promotes cardiac fibroblast activation and collagen deposition [72]. Hypoxia promotes the expression of V-Set and Immunoglobulin Domain Containing 4 (VSIG4) in macrophages, promoting scar formation and fibroblast transformation into myofibroblasts through TGF-\beta1 and IL-10 signaling [73]. WW Domain Containing E3 Ubiquitin Protein Ligase 2 (WWP2), an E3 ubiquitin ligase, regulates macrophage infiltration and activation mainly by regulating CCL5, which contributes to the transdifferentiation of myofibroblasts to regulate cardiac fibrosis [74]. The buildup of Vascular Endothelial Growth Factor Receptor 1 (VEGFR1)-positive perivascular macrophages following recurrent injury triggers abnormal activation of the Wnt/\beta-catenin signaling pathway. This activation subsequently boosts Notch signaling in perivascular fibroblasts, leading to increased fibrosis [75]. Additionally, SPP1⁺ macrophages gather in the adipose tissue adjacent to atherosclerotic coronary arteries, influenced by the Osteopontin (OPN)–Cluster of Differentiation 44 (CD44)/integrin interaction. This process facilitates the movement and growth of fibroblast progenitor cells, further intensifying fibrosis in the pericoronary adipose tissue [12]. Interestingly, mechanisms also exist to attenuate macrophage–fibroblast interactions in fibrosis. For example, Tumor Necrosis Factor-Stimulating Factor 6 (TSG6) can inhibit the interaction between macrophages and endothelial stromal fibroblasts during endothelial fibrosis, preventing stromal fibroblasts from differentiating into myofibroblasts, thus attenuating the degree of fibrosis [76].

In lung fibrosis, fibroblasts and macrophages are crucial for repair after tissue injury. Highly proliferating SPP1hi macrophages in fibrotic tissues play a crucial role in activating myofibroblasts, a process supported by co-localization studies and causality models [8]. Akt1 induces macrophage mitochondrial ROS and mitophagy, contributing to AM anti-apoptosis, which ultimately promotes TGF-\beta1 expression, fibroblast differentiation, and lung fibrosis [77]. In contrast, TGF-\beta1 has been found to induce global DNA hypermethylation in fibroblasts in a TGF-\beta/Smad3-dependent manner. Overexpression of Mbd2 inhibits this signaling and promotes fibroblast differentiation into myofibroblasts, exacerbating fibrosis [55].

In liver and kidney fibrosis, the macrophage surface receptor tyrosine kinase MERTK activates the ERK-TGF-\beta1 pathway, facilitating hepatic stellate cell (hepatic fibroblast) activation and promoting fibrosis [78]. NLRP3 inflammasomes modulate TGF-\beta signaling and macrophage phenotypes in CKD, promoting the development of inflammatory cells [79]. In a mouse model of CKD, macrophage-derived Vitronectin (Vtn)-rich ECM scaffolds promoted fibroblast activation and proliferation [80].

In the skin, a fibrotic signaling loop is formed between ECM-producing fibroblasts and pro-fibrotic SPP1⁺ myeloid cells, mediated through the SPP1 signaling pathway. Corticosteroids have been shown to disrupt this circuit by reversing the transcriptional programs of SPP1⁺ macrophages and POSTN⁺ fibroblasts [81]. Additionally, Wnt signaling enhances the expression of fibrosis-associated genes by activating \beta-linker proteins in fibroblasts and promoting ECM accumulation [82].

2.3.2 Effect of Fibroblasts on Macrophages

While most of the literature on macrophage–fibroblast interactions has focused on the effect of macrophages on fibroblasts, fibroblasts themselves also have a significant impact on macrophages during the course of fibrosis. One key interaction is the stimulation of bone marrow cell transformation. In this process, the proliferation of fibroblast-derived or autocrine growth factors, in addition to granulocyte–macrophage colony-stimulating factor (GM-CSF), is required [81]. Fibroblasts also regulate macrophage activation through signaling pathways. For example, fibroblast Smad7 expression is upregulated in the heart after pressure overload, and Smad7 protects cardiac function by reducing collagen deposition, inhibiting MMP2-mediated matrix degeneration and suppressing macrophage activation by inhibiting stromal cell proteins [83]. In addition, prostacyclin I2 (PGI2) inhibits fibroblast activation through activation of the cAMP/PKA signaling pathway via the prostaglandin I2 receptor (PTGIR). PGI2 is a key antifibrotic factor in the repair of renal injury, and attenuates renal fibrosis by inhibiting the over-activation of fibroblasts, whereas inhibition of stromal cell proteins inhibits macrophage activation and attenuates the progression of fibrosis [84]. Pro-fibrotic macrophages also produce matrix metalloproteinases (MMPs) and tissue inhibitors of metalloproteinases, which regulate inflammatory cell recruitment and ECM turnover [85]. CXCL14 promotes the activation and recruitment of bone marrow-derived macrophages [86]. FAP is expressed in a subpopulation of activated fibroblasts and promotes the pro-fibrotic activity of macrophages during liver fibrosis [58].

2.4 Temporal Characteristics of Macrophage–Fibroblast Interactions

The healing of organ tissues post-injury likely progresses through three stages: inflammation, tissue regeneration, and remodeling [87]. In the early inflammatory phase, macrophages rapidly accumulate at the injury site, promoting immune responses and clearing debris. If macrophages are depleted during this acute window—such as in models of myocardial infarction—this can lead to long-lasting adverse effects, including impaired tissue regeneration, persistent fibrosis, and functional decline [88]. Similarly, in a rat model of cerebral ischemia, macrophage infiltration in the acute phase was positively correlated with fibronectin expression, suggesting early involvement in fibrotic remodeling [67].

Fibroblasts respond to injury by differentiating into inflammatory fibroblasts during the early phase, which then give rise to pro-fibrotic fibroblasts during regeneration [89]. Macrophages regulate this transition: in macrophage-depleted models, premature activation of fibroblasts occurs, highlighting their role in restraining fibroblast activation during early repair [88]. This regulation is mediated by TGF-\beta1/SMAD3 and JAK/STAT signaling, which also promote fibroblast-to-myofibroblast differentiation [90].

Recent studies suggest that macrophage-fibroblast interactions are temporally dynamic. For example, CD38⁺ macrophages—enriched during chronic fibrosis—originate from resident macrophages under CSF1/CSF1R signaling and promote fibrosis by depleting NAD⁺ in renal tubular epithelial cells (RTECs), thereby inducing senescence and sustaining inflammation. CD38⁺ macrophages secrete SASP-related cytokines such as CCL2 and IL6, while their depletion—genetically or pharmacologically (e.g., 78c)—restores NAD⁺, reduces RTEC senescence, and attenuates fibrosis in AKI models. These findings highlight not only their pro-fibrotic role but also the importance of intervention timing, as delayed targeting yields limited benefit [91]. Importantly, the functional role of macrophages is shaped not only by timing but also by organ-specific microenvironments, the nature of the injury, and co-existing immune or stromal signals. The same macrophage subtype may exhibit pro-fibrotic, anti-inflammatory, or remodeling functions depending on these cues. Similarly, fibroblasts contribute to macrophage recruitment and polarization through CCL2 and CXCL14 during the repair phase, establishing a feedback loop that evolves with tissue context [86, 92].

While stage-specific depletion of macrophages in preclinical models has provided proof-of-concept for temporally guided antifibrotic interventions, translating this strategy into clinical practice remains challenging. Human fibrotic diseases often progress insidiously, with blurred transitions between inflammation, repair, and fibrosis, making the identification of precise intervention windows difficult.

Together, these findings underscore that macrophage–fibroblast crosstalk is not static but evolves across time and tissue context. Effective interventions must therefore consider not only cell identity but also timing and functional state to avoid disrupting beneficial repair processes while targeting pathogenic fibrosis (Fig. 3).

2.5 Targeted Therapy

The role of macrophages in the fibrotic microenvironment has received widespread attention, with modulation of their survival, differentiation, and function showing significant therapeutic potential. Since macrophages in fibrosis are primarily monocyte-derived, blocking SCUBE2⁺ to reduce monocyte-to-macrophage differentiation significantly reduces macrophage numbers and attenuates fibrosis. Inhibiting the M-CSF/M-CSFR pathway not only affects macrophage survival but also slows fibrosis progression by modifying the fibrotic microenvironment [4]. Darapladib, a Phospholipase A2 Group VII (PLA2G7) inhibitor, significantly reduces the number of PLA2G7-overexpressing macrophages, inhibits myofibroblast accumulation, and attenuates pulmonary fibrosis [61]. Local injection of corticosteroids greatly reduces the expression of SPP1 and POSTN genes, suggesting their potential as therapeutic targets for skin scar tissue [93]. Furthermore, restoring RNF41 expression in macrophages of fibrotic mice through a nanoparticle system improves liver function and reduces fibroblast activation, highlighting RNF41’s potential as an antifibrotic target [29]. Additionally, \alpha1-adrenoceptor blockade therapy inhibits macrophage-promoted fibrotic processes and could serve as a potential novel therapy for fibrosis [23].

However, regulating macrophage function alone may not be sufficient to completely reverse fibrosis. Fibroblasts are the main effector cells in fibrosis, and their proliferation, differentiation, and activation processes are equally important for fibrosis intervention. Poricoic acid A reduces renal fibroblast activation and interstitial fibrosis by upregulating Sirt3, a process that accelerates \beta-catenin degradation and inhibits pro-fibrotic gene expression [94]. TGF-\beta/Smad3 signaling in cardiac fibroblasts is critical for myofibroblast activation and ECM generation, driving cardiac fibrosis. The deletion of Smad3 significantly attenuates fibrosis, whereas Smad2 has a lesser effect. Targeting Smad3 may help treat heart failure without compromising cardiac function [95]. Microcystin-LR is effective in reducing fibrosis by blocking FMT through inhibition of CD206 macrophage differentiation [96]. In contrast, drug delivery systems such as liposomes loaded with clodronate (CLD) and hybrid systems utilizing fibroblast-derived exosomes (EL-CLD) are engineered to inhibit nonspecific phagocytosis and promote homing to fibroblasts, allowing them to preferentially target fibrotic lung tissue [97]. Dimethyl fumarate-loaded ROS-responsive DSPE-TK-PEG@DMF liposomes mitigate lung fibrosis by activating Nrf2 signaling, which impedes macrophage-driven FMT and ECM accumulation [98]. This approach enhances drug delivery to lesions, improving therapeutic efficiency.

Therapeutically, FAP-directed strategies such as CAR-T cells and peptide vaccines have demonstrated antifibrotic activity in preclinical models, though the magnitude of functional benefit is often modest and variable. Moreover, multiple translational barriers limit their immediate clinical applicability [99, 100]. These include on-target/off-tumor toxicity due to FAP expression in non-fibrotic tissues (e.g., peritumoral zones, inflamed or ischemic tissues), substantial inter-individual heterogeneity in FAP levels, and the context-dependent roles of FAP across different organ systems [101]. As such, there is a tangible risk of impairing physiological tissue remodeling or immune regulation. No FAP-targeted antifibrotic therapy has yet advanced to clinical approval.

Furthermore, the limited predictive value of many rodent models for human fibrosis, especially regarding chronic remodeling and immune heterogeneity, complicates the translation of preclinical findings. Future studies should prioritize biomarker-guided trial designs and mechanistic dissection of treatment windows. Given the dynamic, non-linear nature of fibrosis—with alternating active and quiescent phases—intervening at early pro-fibrotic transitions (e.g., monocyte-to-macrophage differentiation, fibroblast priming) may offer higher reversibility, whereas established fibrosis may require matrix-directed or epigenetic reprogramming approaches. Combining molecular imaging with selective delivery methods, such as nanoparticle or antibody-drug conjugates, may improve therapeutic precision and minimize systemic toxicity (Fig. 4).

3. Conclusions

Over the past decade, advances in single-cell technologies have expanded our view of the cellular ecosystem driving fibrosis. We now recognize that neither macrophages nor fibroblasts act as uniform cell types; instead, they comprise diverse subpopulations with distinct—and often reversible—functional states. However, identifying these subsets is only a first step. We still lack a clear understanding of how macrophage–fibroblast interactions vary across disease stages and tissue contexts, or how these signals shift from promoting repair to driving pathological remodeling.

While populations such as SPP1⁺ macrophages and SCUBE2⁺ fibroblasts have been described as key players, their exact functions—especially in human disease—remain to be validated. Most evidence still comes from acute injury models in mice, and the extent to which these findings apply to human fibrosis, which evolves more slowly and heterogeneously, is uncertain.

The field must now move beyond cell-type cataloging and focus on mechanistic questions: What signals determine whether macrophages restrain or promote fibroblast activation? When, and in which tissue niches, do fibroblasts retain the capacity to revert to non-pathogenic states? And critically, can we define intervention windows where antifibrotic therapy improves outcome without disrupting necessary repair?

Answering these questions will require functional validation in vivo, integration of spatial and temporal information, and comparative studies across organs and species. Without this, we risk mistaking descriptive complexity for understanding—and will continue to fall short of translating biological insights into effective treatments.

References

[1] Maimaitijiang A, Huang Q, Wu Y, Sun S, Chen Q. Transglutaminase 2 inhibition ameliorates cardiac fibrosis in myocardial infarction by inducing M2 macrophage polarization in vitro and in vivo. CytoJournal. 2024; 21: 58. https://doi.org/10.25259/cytojournal_32_2024.

[2] Wang S, Wang X, Lv Y, Zhang Z, He T, Hao X, et al. M2 Macrophage-Derived Exosomes Inhibit Atherosclerosis Progression by Regulating the Proliferation, Migration, and Phenotypic Transformation of Smooth Muscle Cells. Frontiers in Bioscience (Landmark Edition). 2024; 29: 288. https://doi.org/10.31083/j.fbl2908288.

[3] Wendisch D, Dietrich O, Mari T, von Stillfried S, Ibarra IL, Mittermaier M, et al. SARS-CoV-2 infection triggers profibrotic macrophage responses and lung fibrosis. Cell. 2021; 184: 6243–6261.e27. https://doi.org/10.1016/j.cell.2021.11.033.

[4] Joshi N, Watanabe S, Verma R, Jablonski RP, Chen CI, Cheresh P, et al. A spatially restricted fibrotic niche in pulmonary fibrosis is sustained by M-CSF/M-CSFR signalling in monocyte-derived alveolar macrophages. The European Respiratory Journal. 2020; 55: 1900646. https://doi.org/10.1183/13993003.00646-2019.

[5] Wen Y, Yan HR, Wang B, Liu BC. Macrophage Heterogeneity in Kidney Injury and Fibrosis. Frontiers in Immunology. 2021; 12: 681748. https://doi.org/10.3389/fimmu.2021.681748.

[6] Wen Y, Lambrecht J, Ju C, Tacke F. Hepatic macrophages in liver homeostasis and diseases-diversity, plasticity and therapeutic opportunities. Cellular & Molecular Immunology. 2021; 18: 45–56. https://doi.org/10.1038/s41423-020-00558-8.

[7] Bhattacharya M, Ramachandran P. Immunology of human fibrosis. Nature Immunology. 2023; 24: 1423–1433. https://doi.org/10.1038/s41590-023-01551-9.

[8] Morse C, Tabib T, Sembrat J, Buschur KL, Bittar HT, Valenzi E, et al. Proliferating SPP1/MERTK-expressing macrophages in idiopathic pulmonary fibrosis. The European Respiratory Journal. 2019; 54: 1802441. https://doi.org/10.1183/13993003.02441-2018.

[9] Hoeft K, Schaefer GJL, Kim H, Schumacher D, Bleckwehl T, Long Q, et al. Platelet-instructed SPP1⁺ macrophages drive myofibroblast activation in fibrosis in a CXCL4-dependent manner. Cell Reports. 2023; 42: 112131. https://doi.org/10.1016/j.celrep.2023.112131.

[10] Ramachandran P, Dobie R, Wilson-Kanamori JR, Dora EF, Henderson BEP, Luu NT, et al. Resolving the fibrotic niche of human liver cirrhosis at single-cell level. Nature. 2019; 575: 512–518. https://doi.org/10.1038/s41586-019-1631-3.

[11] Fabre T, Barron AMS, Christensen SM, Asano S, Bound K, Lech MP, et al. Identification of a broadly fibrogenic macrophage subset induced by type 3 inflammation. Science Immunology. 2023; 8: eadd8945. https://doi.org/10.1126/sciimmunol.add8945.

[12] Fu M, Shu S, Peng Z, Liu X, Chen X, Zeng Z, et al. Single-Cell RNA Sequencing of Coronary Perivascular Adipose Tissue From End-Stage Heart Failure Patients Identifies SPP1⁺ Macrophage Subpopulation as a Target for Alleviating Fibrosis. Arteriosclerosis, Thrombosis, and Vascular Biology. 2023; 43: 2143–2164. https://doi.org/10.1161/ATVBAHA.123.319828.

[13] King EM, Zhao Y, Moore CM, Steinhart B, Anderson KC, Vestal B, et al. GPNMB and SPP1 mark a conserved macrophage injury response masking fibrosis-specific programming in the lung. JCI Insight. 2024; 9: e182700. https://doi.org/10.1172/jci.insight.182700.

[14] Luecken MD, Theis FJ. Current best practices in single-cell RNA-seq analysis: a tutorial. Molecular Systems Biology. 2019; 15: e8746. https://doi.org/10.15252/msb.20188746.

[15] Henderson NC, Rieder F, Wynn TA. Fibrosis: from mechanisms to medicines. Nature. 2020; 587: 555–566. https://doi.org/10.1038/s41586-020-2938-9.

[16] Duffield JS, Forbes SJ, Constandinou CM, Clay S, Partolina M, Vuthoori S, et al. Selective depletion of macrophages reveals distinct, opposing roles during liver injury and repair. The Journal of Clinical Investigation. 2005; 115: 56–65. https://doi.org/10.1172/JCI22675.

[17] Aran D, Looney AP, Liu L, Wu E, Fong V, Hsu A, et al. Reference-based analysis of lung single-cell sequencing reveals a transitional profibrotic macrophage. Nature Immunology. 2019; 20: 163–172. https://doi.org/10.1038/s41590-018-0276-y.

[18] Qing J, Ren Y, Zhang Y, Yan M, Zhang H, Wu D, et al. Dopamine receptor D2 antagonism normalizes profibrotic macrophage-endothelial crosstalk in non-alcoholic steatohepatitis. Journal of Hepatology. 2022; 76: 394–406. https://doi.org/10.1016/j.jhep.2021.09.032.

[19] Hou C, Wang D, Zhao M, Ballar P, Zhang X, Mei Q, et al. MANF brakes TLR4 signaling by competitively binding S100A8 with S100A9 to regulate macrophage phenotypes in hepatic fibrosis. Acta Pharmaceutica Sinica. B. 2023; 13: 4234–4252. https://doi.org/10.1016/j.apsb.2023.07.027.

[20] Su W, Gao W, Zhang R, Wang Q, Li L, Bu Q, et al. TAK1 deficiency promotes liver injury and tumorigenesis via ferroptosis and macrophage cGAS-STING signalling. JHEP Reports: Innovation in Hepatology. 2023; 5: 100695. https://doi.org/10.1016/j.jhepr.2023.100695.

[21] Chen M, Menon MC, Wang W, Fu J, Yi Z, Sun Z, et al. HCK induces macrophage activation to promote renal inflammation and fibrosis via suppression of autophagy. Nature Communications. 2023; 14: 4297. https://doi.org/10.1038/s41467-023-40086-3.

[22] Wen Y, Lu X, Ren J, Privratsky JR, Yang B, Rudemiller NP, et al. KLF4 in Macrophages Attenuates TNFα-Mediated Kidney Injury and Fibrosis. Journal of the American Society of Nephrology. 2019; 30: 1925–1938. https://doi.org/10.1681/ASN.2019020111.

[23] Gao R, Peng X, Perry C, Sun H, Ntokou A, Ryu C, et al. Macrophage-derived netrin-1 drives adrenergic nerve-associated lung fibrosis. The Journal of Clinical Investigation. 2021; 131: e136542. https://doi.org/10.1172/JCI136542.

[24] Wang F, Ting C, Riemondy KA, Douglas M, Foster K, Patel N, et al. Regulation of epithelial transitional states in murine and human pulmonary fibrosis. The Journal of Clinical Investigation. 2023; 133: e165612. https://doi.org/10.1172/JCI165612.

[25] Zhang J, He Z, Xiong C, Yao Y, Zhang C, Yao W, et al. SHH induces macrophage oxidative phosphorylation and efferocytosis to promote scar formation. Cell Communication and Signaling. 2024; 22: 336. https://doi.org/10.1186/s12964-024-01692-w.

[26] Simões FC, Cahill TJ, Kenyon A, Gavriouchkina D, Vieira JM, Sun X, et al. Macrophages directly contribute collagen to scar formation during zebrafish heart regeneration and mouse heart repair. Nature Communications. 2020; 11: 600. https://doi.org/10.1038/s41467-019-14263-2.

[27] Shi M, Zhu J, Wang R, Chen X, Mi L, Walz T, et al. Latent TGF-β structure and activation. Nature. 2011; 474: 343–349. https://doi.org/10.1038/nature10152.

[28] Xu Y, Ying L, Lang JK, Hinz B, Zhao R. Modeling mechanical activation of macrophages during pulmonary fibrogenesis for targeted anti-fibrosis therapy. Science Advances. 2024; 10: eadj9559. https://doi.org/10.1126/sciadv.adj9559.

[29] Moreno-Lanceta A, Medrano-Bosch M, Fundora Y, Perramón M, Aspas J, Parra-Robert M, et al. RNF41 orchestrates macrophage-driven fibrosis resolution and hepatic regeneration. Science Translational Medicine. 2023; 15: eabq6225. https://doi.org/10.1126/scitranslmed.abq6225.

[30] Rao J, Wang H, Ni M, Wang Z, Wang Z, Wei S, et al. FSTL1 promotes liver fibrosis by reprogramming macrophage function through modulating the intracellular function of PKM2. Gut. 2022; 71: 2539–2550. https://doi.org/10.1136/gutjnl-2021-325150.

[31] Opalenik SR, Davidson JM. Fibroblast differentiation of bone marrow-derived cells during wound repair. FASEB Journal. 2005; 19: 1561–1563. https://doi.org/10.1096/fj.04-2978fje.

[32] Plikus MV, Wang X, Sinha S, Forte E, Thompson SM, Herzog EL, et al. Fibroblasts: Origins, definitions, and functions in health and disease. Cell. 2021; 184: 3852–3872. https://doi.org/10.1016/j.cell.2021.06.024.

[33] Ruiz-Villalba A, Romero JP, Hernández SC, Vilas-Zornoza A, Fortelny N, Castro-Labrador L, et al. Single-Cell RNA Sequencing Analysis Reveals a Crucial Role for CTHRC1 (Collagen Triple Helix Repeat Containing 1) Cardiac Fibroblasts After Myocardial Infarction. Circulation. 2020; 142: 1831–1847. https://doi.org/10.1161/CIRCULATIONAHA.119.044557.

[34] Tsukui T, Sun KH, Wetter JB, Wilson-Kanamori JR, Hazelwood LA, Henderson NC, et al. Collagen-producing lung cell atlas identifies multiple subsets with distinct localization and relevance to fibrosis. Nature Communications. 2020; 11: 1920. https://doi.org/10.1038/s41467-020-15647-5.

[35] Melms JC, Biermann J, Huang H, Wang Y, Nair A, Tagore S, et al. A molecular single-cell lung atlas of lethal COVID-19. Nature. 2021; 595: 114–119. https://doi.org/10.1038/s41586-021-03569-1.

[36] Mayr CH, Sengupta A, Asgharpour S, Ansari M, Pestoni JC, Ogar P, et al. Sfrp1 inhibits lung fibroblast invasion during transition to injury-induced myofibroblasts. The European Respiratory Journal. 2024; 63: 2301326. https://doi.org/10.1183/13993003.01326-2023.

[37] Younesi FS, Miller AE, Barker TH, Rossi FMV, Hinz B. Fibroblast and myofibroblast activation in normal tissue repair and fibrosis. Nature Reviews Molecular Cell Biology. 2024; 25: 617–638. https://doi.org/10.1038/s41580-024-00716-0.

[38] Guo JL, Griffin M, Yoon JK, Lopez DM, Zhu Y, Lu JM, et al. Histological signatures map anti-fibrotic factors in mouse and human lungs. Nature. 2025; 641: 993–1004. https://doi.org/10.1038/s41586-025-08727-3.

[39] Lingampally A, Truchi M, Mauduit O, Delcroix V, Vasquez-Pacheco E, Gautier-Isola M, et al. Evidence for a lipofibroblast-to-Cthrc1⁺ myofibroblast reversible switch during the development and resolution of lung fibrosis in young mice. The European Respiratory Journal. 2025; 65: 2300482. https://doi.org/10.1183/13993003.00482-2023.

[40] Michel JB, Thaunat O, Houard X, Meilhac O, Caligiuri G, Nicoletti A. Topological determinants and consequences of adventitial responses to arterial wall injury. Arteriosclerosis, Thrombosis, and Vascular Biology. 2007; 27: 1259–1268. https://doi.org/10.1161/ATVBAHA.106.137851.

[41] Marconi GD, Fonticoli L, Rajan TS, Pierdomenico SD, Trubiani O, Pizzicannella J, et al. Epithelial-Mesenchymal Transition (EMT): The Type-2 EMT in Wound Healing, Tissue Regeneration and Organ Fibrosis. Cells. 2021; 10: 1587. https://doi.org/10.3390/cells10071587.

[42] Zhang C, Hao H, Wang Y, Mu N, Jiang W, Zhang Z, et al. Intercellular mitochondrial component transfer triggers ischemic cardiac fibrosis. Science Bulletin. 2023; 68: 1784–1799. https://doi.org/10.1016/j.scib.2023.07.030.

[43] Hume RD, Deshmukh T, Doan T, Shim WJ, Kanagalingam S, Tallapragada V, et al. PDGF-AB Reduces Myofibroblast Differentiation Without Increasing Proliferation After Myocardial Infarction. JACC. Basic to Translational Science. 2023; 8: 658–674. https://doi.org/10.1016/j.jacbts.2022.11.006.

[44] Feng Y, Ren J, Gui Y, Wei W, Shu B, Lu Q, et al. Wnt/β-Catenin-Promoted Macrophage Alternative Activation Contributes to Kidney Fibrosis. Journal of the American Society of Nephrology. 2018; 29: 182–193. https://doi.org/10.1681/ASN.2017040391.

[45] Han M, Liu Z, Liu L, Huang X, Wang H, Pu W, et al. Dual genetic tracing reveals a unique fibroblast subpopulation modulating cardiac fibrosis. Nature Genetics. 2023; 55: 665–678. https://doi.org/10.1038/s41588-023-01337-7.

[46] Li L, Lu M, Peng Y, Huang J, Tang X, Chen J, et al. Oxidatively stressed extracellular microenvironment drives fibroblast activation and kidney fibrosis. Redox Biology. 2023; 67: 102868. https://doi.org/10.1016/j.redox.2023.102868.

[47] Chen S, Zhang M, Li J, Huang J, Zhou S, Hou X, et al. β-catenin-controlled tubular cell-derived exosomes play a key role in fibroblast activation via the OPN-CD44 axis. Journal of Extracellular Vesicles. 2022; 11: e12203. https://doi.org/10.1002/jev2.12203.

[48] Tsuchiya Y, Seki T, Kobayashi K, Komazawa-Sakon S, Shichino S, Nishina T, et al. Fibroblast growth factor 18 stimulates the proliferation of hepatic stellate cells, thereby inducing liver fibrosis. Nature Communications. 2023; 14: 6304. https://doi.org/10.1038/s41467-023-42058-z.

[49] Geng Y, Li L, Yan J, Liu K, Yang A, Zhang L, et al. Author Correction: PEAR1 regulates expansion of activated fibroblasts and deposition of extracellular matrix in pulmonary fibrosis. Nature Communications. 2022; 13: 7749. https://doi.org/10.1038/s41467-022-35543-4.

[50] Wu J, Chen L, Qin C, Huo F, Liang X, Yang X, et al. CD147 contributes to SARS-CoV-2-induced pulmonary fibrosis. Signal Transduction and Targeted Therapy. 2022; 7: 382. https://doi.org/10.1038/s41392-022-01230-5.

[51] Tsoyi K, Liang X, De Rossi G, Ryter SW, Xiong K, Chu SG, et al. CD148 Deficiency in Fibroblasts Promotes the Development of Pulmonary Fibrosis. American Journal of Respiratory and Critical Care Medicine. 2021; 204: 312–325. https://doi.org/10.1164/rccm.202008-3100OC.

[52] Gao Y, Liu Y, Zheng D, Ho C, Wen D, Sun J, et al. HDAC5-mediated Smad7 silencing through MEF2A is critical for fibroblast activation and hypertrophic scar formation. International Journal of Biological Sciences. 2022; 18: 5724–5739. https://doi.org/10.7150/ijbs.76140.

[53] Griffin MF, Borrelli MR, Garcia JT, Januszyk M, King M, Lerbs T, et al. JUN promotes hypertrophic skin scarring via CD36 in preclinical in vitro and in vivo models. Science Translational Medicine. 2021; 13: eabb3312. https://doi.org/10.1126/scitranslmed.abb3312.

[54] Yang L, Zheng S, Ge D, Xia M, Li H, Tang J. LncRNA-COX2 inhibits Fibroblast Activation and Epidural Fibrosis by Targeting EGR1. International Journal of Biological Sciences. 2022; 18: 1347–1362. https://doi.org/10.7150/ijbs.67974.

[55] Liu X, Burke RM, Lighthouse JK, Baker CD, Dirkx RA, Jr, Kang B, et al. p53 Regulates the Extent of Fibroblast Proliferation and Fibrosis in Left Ventricle Pressure Overload. Circulation Research. 2023; 133: 271–287. https://doi.org/10.1161/CIRCRESAHA.121.320324.

[56] Zhang Y, Wang J, Sun H, Xun Z, He Z, Zhao Y, et al. TWIST1⁺FAP⁺ fibroblasts in the pathogenesis of intestinal fibrosis in Crohn’s disease. The Journal of Clinical Investigation. 2024; 134: e179472. https://doi.org/10.1172/JCI179472.

[57] Zeng N, Guan X, Liu X, Shi H, Li N, Yang R, et al. Fibroblast activation protein-sensitive polymeric nanobeacon for early diagnosis of renal fibrosis. Biosensors & Bioelectronics. 2024; 253: 116144. https://doi.org/10.1016/j.bios.2024.116144.

[58] Yang AT, Kim YO, Yan XZ, Abe H, Aslam M, Park KS, et al. Fibroblast Activation Protein Activates Macrophages and Promotes Parenchymal Liver Inflammation and Fibrosis. Cellular and Molecular Gastroenterology and Hepatology. 2023; 15: 841–867. https://doi.org/10.1016/j.jcmgh.2022.12.005.

[59] Ma CX, Wei ZR, Sun T, Yang MH, Sun YQ, Kai KL, et al. Circ-sh3rf3/GATA-4/miR-29a regulatory axis in fibroblast-myofibroblast differentiation and myocardial fibrosis. Cellular and Molecular Life Sciences. 2023; 80: 50. https://doi.org/10.1007/s00018-023-04699-7.

[60] Blyszczuk P, Müller-Edenborn B, Valenta T, Osto E, Stellato M, Behnke S, et al. Transforming growth factor-β-dependent Wnt secretion controls myofibroblast formation and myocardial fibrosis progression in experimental autoimmune myocarditis. European Heart Journal. 2017; 38: 1413–1425. https://doi.org/10.1093/eurheartj/ehw116.

[61] Wang J, Jiang M, Xiong A, Zhang L, Luo L, Liu Y, et al. Integrated analysis of single-cell and bulk RNA sequencing reveals pro-fibrotic PLA2G7^high macrophages in pulmonary fibrosis. Pharmacological Research. 2022; 182: 106286. https://doi.org/10.1016/j.phrs.2022.106286.

[62] Hubert F, Payan SM, Pelce E, Bouchard L, Sturny R, Lenfant N, et al. FGF10 promotes cardiac repair through a dual cellular mechanism increasing cardiomyocyte renewal and inhibiting fibrosis. Cardiovascular Research. 2022; 118: 2625–2637. https://doi.org/10.1093/cvr/cvab340.

[63] Radwanska A, Cottage CT, Piras A, Overed-Sayer C, Sihlbom C, Budida R, et al. Increased expression and accumulation of GDF15 in IPF extracellular matrix contribute to fibrosis. JCI Insight. 2022; 7: e153058. https://doi.org/10.1172/jci.insight.153058.

[64] Bugg D, Bailey LRJ, Bretherton RC, Beach KE, Reichardt IM, Robeson KZ, et al. MBNL1 drives dynamic transitions between fibroblasts and myofibroblasts in cardiac wound healing. Cell Stem Cell. 2022; 29: 419–433.e10. https://doi.org/10.1016/j.stem.2022.01.012.

[65] Singh R, Kaundal RK, Zhao B, Bouchareb R, Lebeche D. Resistin induces cardiac fibroblast-myofibroblast differentiation through JAK/STAT3 and JNK/c-Jun signaling. Pharmacological Research. 2021; 167: 105414. https://doi.org/10.1016/j.phrs.2020.105414.

[66] Guan R, Yuan L, Li J, Wang J, Li Z, Cai Z, et al. Bone morphogenetic protein 4 inhibits pulmonary fibrosis by modulating cellular senescence and mitophagy in lung fibroblasts. The European Respiratory Journal. 2022; 60: 2102307. https://doi.org/10.1183/13993003.02307-2021.

[67] Huang JG, Ren JX, Chen Y, Tian MF, Zhou L, Wen J, et al. M2 macrophages mediate fibrotic scar formation in the early stages after cerebral ischemia in rats. Neural Regeneration Research. 2023; 18: 2208–2218. https://doi.org/10.4103/1673-5374.368299.

[68] Setten E, Castagna A, Nava-Sedeño JM, Weber J, Carriero R, Reppas A, et al. Understanding fibrosis pathogenesis via modeling macrophage-fibroblast interplay in immune-metabolic context. Nature Communications. 2022; 13: 6499. https://doi.org/10.1038/s41467-022-34241-5.

[69] Wynn TA, Barron L. Macrophages: master regulators of inflammation and fibrosis. Seminars in Liver Disease. 2010; 30: 245–257. https://doi.org/10.1055/s-0030-1255354.

[70] Amrute JM, Luo X, Penna V, Bredemeyer A, Yamawaki T, Yang S, et al. Targeting Immune-Fibroblast Crosstalk in Myocardial Infarction and Cardiac Fibrosis. Research Square. 2023. https://doi.org/10.21203/rs.3.rs-2402606/v1. (preprint)

[71] Ramanujam D, Schön AP, Beck C, Vaccarello P, Felician G, Dueck A, et al. MicroRNA-21-Dependent Macrophage-to-Fibroblast Signaling Determines the Cardiac Response to Pressure Overload. Circulation. 2021; 143: 1513–1525. https://doi.org/10.1161/CIRCULATIONAHA.120.050682.

[72] Vendrov AE, Xiao H, Lozhkin A, Hayami T, Hu G, Brody MJ, et al. Cardiomyocyte NOX4 regulates resident macrophage-mediated inflammation and diastolic dysfunction in stress cardiomyopathy. Redox Biology. 2023; 67: 102937. https://doi.org/10.1016/j.redox.2023.102937.

[73] Wang Y, Zhang Y, Li J, Li C, Zhao R, Shen C, et al. Hypoxia Induces M2 Macrophages to Express VSIG4 and Mediate Cardiac Fibrosis After Myocardial Infarction. Theranostics. 2023; 13: 2192–2209. https://doi.org/10.7150/thno.78736.

[74] Chen H, Chew G, Devapragash N, Loh JZ, Huang KY, Guo J, et al. The E3 ubiquitin ligase WWP2 regulates pro-fibrogenic monocyte infiltration and activity in heart fibrosis. Nature Communications. 2022; 13: 7375. https://doi.org/10.1038/s41467-022-34971-6.

[75] Cao Z, Lis R, Ginsberg M, Chavez D, Shido K, Rabbany SY, et al. Targeting of the pulmonary capillary vascular niche promotes lung alveolar repair and ameliorates fibrosis. Nature Medicine. 2016; 22: 154–162. https://doi.org/10.1038/nm.4035.

[76] Sun H, Dong J, Fu Z, Lu X, Chen X, Lei H, et al. TSG6-Exo@CS/GP Attenuates Endometrium Fibrosis by Inhibiting Macrophage Activation in a Murine IUA Model. Advanced Materials. 2024; 36: e2308921. https://doi.org/10.1002/adma.202308921.

[77] Larson-Casey JL, Deshane JS, Ryan AJ, Thannickal VJ, Carter AB. Macrophage Akt1 Kinase-Mediated Mitophagy Modulates Apoptosis Resistance and Pulmonary Fibrosis. Immunity. 2016; 44: 582–596. https://doi.org/10.1016/j.immuni.2016.01.001.

[78] Cai B, Dongiovanni P, Corey KE, Wang X, Shmarakov IO, Zheng Z, et al. Macrophage MerTK Promotes Liver Fibrosis in Nonalcoholic Steatohepatitis. Cell Metabolism. 2020; 31: 406–421.e7. https://doi.org/10.1016/j.cmet.2019.11.013.

[79] Anders HJ, Suarez-Alvarez B, Grigorescu M, Foresto-Neto O, Steiger S, Desai J, et al. The macrophage phenotype and inflammasome component NLRP3 contributes to nephrocalcinosis-related chronic kidney disease independent from IL-1-mediated tissue injury. Kidney International. 2018; 93: 656–669. https://doi.org/10.1016/j.kint.2017.09.022.

[80] Peng Y, Li L, Shang J, Zhu H, Liao J, Hong X, et al. Macrophage promotes fibroblast activation and kidney fibrosis by assembling a vitronectin-enriched microenvironment. Theranostics. 2023; 13: 3897–3913. https://doi.org/10.7150/thno.85250.

[81] Macmillan EM, Gonda TJ. Murine myeloid cells transformed by myb require fibroblast-derived or autocrine growth factors in addition to granulocyte-macrophage colony-stimulating factor for proliferation. Blood. 1994; 83: 209–216.

[82] Tian Y, Chen J, Huang W, Ren Q, Feng J, Liao J, et al. Myeloid-derived Wnts play an indispensible role in macrophage and fibroblast activation and kidney fibrosis. International Journal of Biological Sciences. 2024; 20: 2310–2322. https://doi.org/10.7150/ijbs.94166.

[83] Humeres C, Shinde AV, Tuleta I, Hernandez SC, Hanna A, Huang S, et al. Fibroblast Smad7 Induction Protects the Remodeling Pressure-Overloaded Heart. Circulation Research. 2024; 135: 453–469. https://doi.org/10.1161/CIRCRESAHA.123.323360.

[84] Li J, Guan Y, Xu Y, Cao Y, Xie Q, Harris RC, et al. Prostacyclin Mitigates Renal Fibrosis by Activating Fibroblast Prostaglandin I 2 Receptor. Journal of the American Society of Nephrology. 2024; 35: 149–165. https://doi.org/10.1681/ASN.0000000000000286.

[85] Wynn TA. Cellular and molecular mechanisms of fibrosis. The Journal of Pathology. 2008; 214: 199–210. https://doi.org/10.1002/path.2277.

[86] You Y, Yuan H, Min H, Li C, Chen J. Fibroblast-derived CXCL14 aggravates crystalline silica-induced pulmonary fibrosis by mediating polarization and recruitment of interstitial macrophages. Journal of Hazardous Materials. 2023; 460: 132489. https://doi.org/10.1016/j.jhazmat.2023.132489.

[87] D’Ambrosi N, Apolloni S. Fibrotic Scar in Neurodegenerative Diseases. Frontiers in Immunology. 2020; 11: 1394. https://doi.org/10.3389/fimmu.2020.01394.

[88] Godwin JW, Debuque R, Salimova E, Rosenthal NA. Heart regeneration in the salamander relies on macrophage-mediated control of fibroblast activation and the extracellular landscape. NPJ Regenerative Medicine. 2017; 2: 22. https://doi.org/10.1038/s41536-017-0027-y.

[89] Tsukui T, Wolters PJ, Sheppard D. Alveolar fibroblast lineage orchestrates lung inflammation and fibrosis. Nature. 2024; 631: 627–634. https://doi.org/10.1038/s41586-024-07660-1.

[90] Li G, Yang H, Zhang D, Zhang Y, Liu B, Wang Y, et al. The role of macrophages in fibrosis of chronic kidney disease. Biomedicine & Pharmacotherapy. 2024; 177: 117079. https://doi.org/10.1016/j.biopha.2024.117079.

[91] Yao W, Liu M, Li Z, Qu L, Sui S, Xiang C, et al. CD38^hi macrophages promote fibrotic transition following acute kidney injury by modulating NAD⁺ metabolism. Molecular Therapy. 2025; 33: 3434–3452. https://doi.org/10.1016/j.ymthe.2025.04.039.

[92] Ko KI, DerGarabedian BP, Chen Z, Debnath R, Ko A, Link BN, et al. Distinct fibroblast progenitor subpopulation expedites regenerative mucosal healing by immunomodulation. The Journal of Experimental Medicine. 2023; 220: e20221350. https://doi.org/10.1084/jem.20221350.

[93] Hong YK, Hwang DY, Yang CC, Cheng SM, Chen PC, Aala WJ, et al. Profibrotic Subsets of SPP1⁺ Macrophages and POSTN⁺ Fibroblasts Contribute to Fibrotic Scarring in Acne Keloidalis. The Journal of Investigative Dermatology. 2024; 144: 1491–1504.e10. https://doi.org/10.1016/j.jid.2023.12.014.

[94] Chen DQ, Chen L, Guo Y, Wu XQ, Zhao TT, Zhao HL, et al. Poricoic acid A suppresses renal fibroblast activation and interstitial fibrosis in UUO rats via upregulating Sirt3 and promoting β-catenin K49 deacetylation. Acta Pharmacologica Sinica. 2023; 44: 1038–1050. https://doi.org/10.1038/s41401-022-01026-x.

[95] Khalil H, Kanisicak O, Prasad V, Correll RN, Fu X, Schips T, et al. Fibroblast-specific TGF-β-Smad2/3 signaling underlies cardiac fibrosis. The Journal of Clinical Investigation. 2017; 127: 3770–3783. https://doi.org/10.1172/JCI94753.

[96] Wang J, Xu L, Xiang Z, Ren Y, Zheng X, Zhao Q, et al. Microcystin-LR ameliorates pulmonary fibrosis via modulating CD206⁺ M2-like macrophage polarization. Cell Death & Disease. 2020; 11: 136. https://doi.org/10.1038/s41419-020-2329-z.

[97] Sun L, Fan M, Huang D, Li B, Xu R, Gao F, et al. Clodronate-loaded liposomal and fibroblast-derived exosomal hybrid system for enhanced drug delivery to pulmonary fibrosis. Biomaterials. 2021; 271: 120761. https://doi.org/10.1016/j.biomaterials.2021.120761.

[98] Liu J, Wu Z, Liu Y, Zhan Z, Yang L, Wang C, et al. ROS-responsive liposomes as an inhaled drug delivery nanoplatform for idiopathic pulmonary fibrosis treatment via Nrf2 signaling. Journal of Nanobiotechnology. 2022; 20: 213. https://doi.org/10.1186/s12951-022-01435-4.

[99] Aghajanian H, Kimura T, Rurik JG, Hancock AS, Leibowitz MS, Li L, et al. Targeting cardiac fibrosis with engineered T cells. Nature. 2019; 573: 430–433. https://doi.org/10.1038/s41586-019-1546-z.

[100] Yoshida S, Hayashi H, Kawahara T, Katsuki S, Kimura M, Hino R, et al. A Vaccine Against Fibroblast Activation Protein Improves Murine Cardiac Fibrosis by Preventing the Accumulation of Myofibroblasts. Circulation Research. 2025; 136: 26–40. https://doi.org/10.1161/CIRCRESAHA.124.325017.

[101] Mori Y, Dendl K, Cardinale J, Kratochwil C, Giesel FL, Haberkorn U. FAPI PET: Fibroblast Activation Protein Inhibitor Use in Oncologic and Nononcologic Disease. Radiology. 2023; 306: e220749. https://doi.org/10.1148/radiol.220749.