The role of macrophages in renal fibrosis and therapeutic prospects

Macrophage origins and phenotypic diversity

Renal macrophages encompass resident and infiltrating cell populations. Resident macrophages in the kidney originate from diverse sources, including yolk sac-derived erythromyeloid progenitors (EMPs), fetal liver EMP-derived macrophages, and hematopoietic stem cells (HSCs)-derived macrophages (Ma et al., 2022; Chen, Liu & Zhuang, 2022). Notably, a subset of HSCs migrates to the bone marrow and spleen and subsequently differentiates into circulating monocytes that enter the bloodstream. These monocytes contribute to the pool of tissue-resident macrophages within the kidneys (Cheung, Agarwal & George, 2022; Cheung et al., 2022).

Macrophages can be classified based on their origin into bone marrow-derived macrophages and tissue-resident macrophages, and based on their function, activation state, and secreted factors, are categorized into M1 and M2 types (according to the level of lymphocyte antigen 6C [Ly6C]). Furthermore, these types can be divided into three subtypes: CD11b+/Ly6Chigh, CD11b+/Ly6Cintermediate, and CD11b+/Ly6Clow (Table 1) (Fu et al., 2022; Duffield, 2011).

M1 and M2 macrophages play opposing roles in renal inflammation (Fig. 1). During the initial phase of kidney injury, macrophages are activated by pathogen-associated molecular patterns (PAMPs), damage-associated molecular patterns (DAMPs), interferon-gamma (IFN-γ), and pro-inflammatory cytokines such as interleukin (IL)-1 and tumor necrosis factor (TNF)-α (Cho et al., 2014; You et al., 2022; Fu et al., 2023; Zhou et al., 2019; Zhang et al., 2012). This activation drives their differentiation into proinflammatory M1 macrophages, which respond to infections or cellular damage. Simultaneously, circulating monocytes (CD11b+Ly6Chigh) are recruited to the kidney, where they differentiate into proinflammatory M1 macrophages. Proinflammatory macrophages are the first responders to injury. These macrophages engulf cellular debris and secrete cytotoxic agents such as inducible nitric oxide synthase (iNOS) and reactive oxygen species (ROS) (Fang et al., 2021; Tran & Mills, 2024; West et al., 2011), both of which can induce mitochondrial damage and cell apoptosis (Alexander et al., 2020). M1 macrophages release pro-inflammatory cytokines (TNF-α, IL-1β, and IL-6), which exacerbate tissue inflammation and injury (Hirani et al., 2022; Beyranvand Nejad et al., 2021; Tang, Nikolic-Paterson & Lan, 2019).

M2 macrophages play a dual role in kidney repair. M2 macrophages secrete a variety of pro-fibrotic factors (such as TGF-β1, FGF-2, and PDGF) (Luo et al., 2023), which stimulate the activation and proliferation of myofibroblasts and lead to excessive deposition of collagen in the extracellular matrix (ECM), thereby promoting fibrosis. In contrast, they release anti-inflammatory cytokines, such as IL-10, which help suppress renal inflammation (Sun et al., 2024b; Liao et al., 2023).

M2 macrophages can be further categorized into four subtypes: M2a, M2b, M2c, and M2d (Yao, Xu & Jin, 2019; Shapouri-Moghaddam et al., 2018). Among these, M2a macrophages express high levels of CD206, which are induced by IL-4 and IL-13 (Luo et al., 2023; Chen et al., 2019; Rao et al., 2021). M2a macrophages secrete pro-fibrotic mediators, including transforming growth factor-β (TGF-β), insulin-like growth factor (IGF), and fibronectin, thereby to contribute to tissue repair and wound healing (Sindrilaru & Scharffetter-Kochanek, 2013). M2b macrophages are induced by dual activation of Toll-like receptor (TLRs) or IL-1 receptor (IL-1R) ligands. This subtype plays a key role in immunoregulation and promotes T helper 2 (Th2)-like activation (Wang et al., 2019). M2c macrophages are polarized by IL-10, TGF-β, and glucocorticoids. This subtype exhibits potent immunosuppressive properties while actively participating in extracellular matrix remodeling and facilitating tissue repair (Lurier et al., 2017). M2d macrophages, which are activated by TLR ligands and A2 adenosine receptor agonists, play a pivotal role in regulating tumor progression, angiogenesis, and metastasis (Hao et al., 2012; Li et al., 2023a).

The role of macrophages in renal fibrosis

Anti-inflammatory macrophages facilitate tubular reepithelialization by secreting trophic factors. However, persistent or severe inflammatory responses can also trigger renal fibrosis. Macrophage depletion alleviates renal fibrosis, highlighting the pro-fibrotic role of macrophages in various kidney pathologies (Cohen et al., 2024; Ma et al., 2022).

Recent single-cell sequencing studies have shown that macrophage infiltration increases significantly on the first day after ischemia/reperfusion, reaching a second peak by the 14th day. Spatial transcriptomic analysis revealed that during the early stages of AKI, macrophages extensively infiltrate the corticomedullary junction region, where tubular damage is severe, whereas in the chronic phase, they exhibit spatial proximity to fibroblasts (Zhang et al., 2024b).

Further, pseudotime analysis has identified two distinct macrophage lineages during the transition from AKI to CKD: kidney-resident macrophages can self-renew and differentiate into pro-repair subtypes, whereas monocyte-derived macrophages are involved in chronic renal inflammation and fibrosis. A novel subset of monocyte-derived macrophages, EAMs, has been identified and characterized by their ability to promote extracellular matrix remodeling. EAMs infiltrate the kidneys during the early stages of AKI and persist during the fibrotic phase. This subset facilitates chronic renal inflammation and fibrosis through intercellular communication with fibroblasts via an insulin-like growth factor (IGF) signaling pathway (Zhang et al., 2024b; Du et al., 2019).

Single-cell sequencing analysis of left ventricular myocardial infarction in mice revealed a distinct population of macrophages characterized by expression of Spp1, Fn1, and Arg1. This macrophage subset is upregulated in response to injury not only in the heart, but also in other organs, including the kidney, where it actively promotes the progression of fibrotic processes (Hoeft et al., 2023). Differential gene expression analysis demonstrated that the most significantly altered factor in Spp1+ macrophages was chemokine ligand 4 (CXCL4). Genetic ablation of CXCL4 suppresses the activation of this macrophage population and attenuates fibrosis following renal injury (Yang et al., 2022).

Increased macrophage infiltration during renal aging leads to chronic low-grade inflammation, with enhanced communication between macrophages and renal tubular cells (Li et al., 2024b; Sun et al., 2024a). Ferroptosis inhibition can alleviate macrophage-mediated partial epithelial-mesenchymal transition in the renal tubules in vitro, thereby reducing the expression of fibrosis-related genes (Cheng et al., 2023). The natural small-molecule compound, rutin, which can inhibit macrophage senescence and ferroptosis by preserving Pcbp1, is a potential therapeutic agent for mitigating age-related chronic low-grade inflammation and fibrosis in kidneys (Wu et al., 2024). In addition, an interaction between hematopoietic cell kinase (HCK), an Src family kinase member, and ATG2A and CBL (two autophagy-related proteins) was found in unilateral ureteral obstruction and unilateral ureteral obstruction (UUO) models. Macrophage activation induced by autophagy inhibition promotes renal fibrosis (Chen et al., 2023).

Single-cell RNA sequencing analysis of a murine glomerulonephritis model revealed that VISTA-positive macrophages exert protective effects against glomerulonephritis and subsequent renal fibrosis. Mechanistically, these macrophages attenuate disease progression by suppressing IFN-γ production in infiltrating T cells, which in turn reduces IL-9 cytokine production in downstream parenchymal response cells, ultimately alleviating glomerulonephritis-associated renal fibrosis (Kim et al., 2022). Recent studies have provided compelling evidence of the pivotal role of macrophages in diabetic nephropathy progression. Pathological analysis of renal autopsy specimens from 88 patients with type 2 diabetes and confirmed diabetic nephropathy revealed significant correlations; glomerular CD163+ macrophage infiltration was associated with disease severity, including interstitial fibrosis, tubular atrophy, and glomerulosclerosis, whereas interstitial CD68+ macrophages showed an inverse correlation with glomerular filtration rate and a positive association with albuminuria (Klessens et al., 2017). Advancements in single-cell RNA sequencing (scRNA-seq) technology have further elucidated macrophage dynamics in diabetic kidneys. Analysis of streptozotocin-induced diabetic mice demonstrated increased immune cell infiltration into glomeruli, with macrophage populations being particularly prominent (Fu et al., 2019). Furthermore, longitudinal single-cell transcriptomic profiling of CD45+ kidney immune cells in OVE26 type 1 diabetic mice revealed the temporal expansion of macrophage subsets accompanied by upregulated pro-inflammatory gene signatures (Fu et al., 2022).

The relationship between macrophages and epithelial cells

Thymic epithelial cells (TECs) or invading pathogens initiate immune responses by releasing damage-associated molecular patterns (DAMPs) and pathogen-associated molecular patterns (PAMPs). These molecular signatures are primarily detected in tissue-resident macrophages, which serve as the first line of immune surveillance. Macrophages recognize danger signals through their pattern recognition receptors (PRRs) and initiate a cytokine cascade. This inflammatory response not only activates local macrophages but also recruits additional monocyte-derived macrophages from the bone marrow, amplifying the immune reaction.

Macrophage-derived exosomes carrying miR-155 can be internalized by TECs, where the microRNA exerts its regulatory function by binding to the 3′-UTR region of TRF1. This interaction leads to downregulation of TRF1 expression, subsequently promoting telomere shortening and functional impairment in TECs. The resulting cellular senescence of TECs and exacerbation of renal fibrosis highlights the critical role of this macrophage-mediated exosomal pathway in kidney pathology (Yin et al., 2024). FKBP5 deficiency in TECs can reduce apoptosis, promote the proliferation of TECs, and inhibit M1 polarization and chemotaxis of macrophages to alleviate CaOx kidney stone injury in mice (Song et al., 2023).

Macrophage infiltration around lipotoxic TECs is a hallmark of diabetic nephropathy. Despite their clinical significance, the precise molecular mechanisms governing bidirectional communication between these cell populations remain poorly understood. Recent research has revealed that TECs can release specialized extracellular vesicles (EVs) enriched with LGR4, which serve as potent activators of macrophage functions. Conversely, activated macrophages can reciprocate by secreting distinct EV populations that initiate apoptotic pathways in the TECs, thereby establishing a vicious cycle of renal injury (Jiang et al., 2022).

In a murine model of adriamycin (ADR)-induced chronic proteinuric nephropathy, exosomal miR-19b-3p was derived from injured tubular epithelial cells. TECs are critical mediators of intercellular communication between damaged TECs and resident macrophages. This exosome-mediated signaling pathway specifically promotes polarization and activation of M1 macrophages, exacerbating renal inflammation and tissue injury (Lv et al., 2020).

The relationship between macrophages and fibroblasts

The pathogenesis of renal fibrosis is characterized by the activation, proliferation, and phenotypic transformation of multiple renal cell types, including resident fibroblasts, epithelial cells, podocytes, and macrophages, into myofibroblasts. These activated myofibroblasts exhibit a distinctive secretory profile, characterized by the excessive production of α-smooth muscle actin (α-SMA), various collagen subtypes, vimentin, platelet-derived growth factor receptor-β (PDGFR-β), and fibroblast-specific protein 1 (FSP-1). This aberrant extracellular matrix deposition and growth factor signaling creates a self-perpetuating fibrotic microenvironment that progressively compromises renal architecture and function.

Emerging evidence from recent investigations has fundamentally reshaped our understanding of the pathogenesis of renal fibrosis, highlighting the pivotal role of macrophages in CKD progression. Notably, lineage-tracing studies have demonstrated that bone marrow-derived macrophages can undergo phenotypic transformation into myofibroblasts through a well-defined process called macrophage-to-myofibroblast transition (MMT) (Chen et al., 2022; Zhuang et al., 2024; Wang et al., 2017). This cellular trans-differentiation process contributes to pathological extracellular matrix (ECM) deposition and serves as a key driver of fibrotic progression in various CKD models (Chen et al., 2022; Wei, Xu & Yan, 2022).

Investigations into MMT have primarily focused on characterizing intermediate transitional cell populations that exhibit dual expression of both myofibroblast and macrophage markers, specifically α-SMA and CD68. Notably, these α-SMA+CD68+ double-positive cells within fibrotic lesions co-express CD206, a well-established marker of M2 macrophage polarization, suggests a potential preferential involvement of M2-like macrophages in the MMT process (Wang et al., 2022; Han et al., 2022). In UUO mouse models, sophisticated lineage-tracing studies utilizing fluorescent protein labeling techniques have demonstrated that approximately 50% of α-smooth muscle actin-positive (α-SMA+) myofibroblasts are derived from myeloid lineage cells. Notably, a significant subset of these α-SMA+CD68+ transitional cells co-express CD206, a characteristic marker of M2 macrophage polarization (Meng et al., 2016). These findings suggest a potential differentiation cascade in which circulating myeloid cells migrate to sites of renal injury, initially adopting an M2-like macrophage phenotype before ultimately undergoing MMT to become fully differentiated myofibroblasts. This proposed multistep differentiation pathway (involving sequential phenotypic transitions from myeloid cells to M2 macrophages and finally to myofibroblasts) may represent a critical mechanism driving fibrotic progression in renal disease (Wang et al., 2017).

The estrogen membrane receptor, GPER1, alleviates the progression of renal fibrosis by inhibiting macrophage polarization towards both M1 and M2 phenotypes. GPER1 activation in M1 macrophages suppresses inflammatory signaling pathways, thereby protecting tubular epithelial cells (TECs) from immune-mediated activation and injury. Furthermore, GPER1 activation in M2 macrophages inhibits the transformation of resident fibroblasts into myofibroblasts, ultimately mitigating renal fibrosis (Xie et al., 2023).

Epigenetic modifications play a pivotal role in fibrotic progression in kidney diseases. Specifically, METTL3 promotes M2 macrophage-driven macrophage-to-myofibroblast transition (MMT) via modulation of the TGF-β1/Smad3 pathway, thereby exacerbating renal fibrosis in chronic renal transplant rejection (Yao et al., 2025). Macrophage Dectin-1 exacerbates Ang II-induced renal fibrosis by modulating TGF-β1 production and mediating macrophage-renal fibroblast interactions (Ye et al., 2023). Endogenous metabolites promote tubulointerstitial fibrosis progression through aryl hydrocarbon receptor (AhR) signaling. The metabolite 1-methoxypyrene (MP) is a crucial metabolite that activates the AhR signaling pathway, thereby promoting tubulointerstitial fibrosis through both epithelial-mesenchymal transition and macrophage-myofibroblast transition (Cao et al., 2022). Consequently, targeting the cellular origins of myofibroblasts and elucidating the molecular mechanisms governing MMT have emerged as promising therapeutic strategies for modulating fibrotic progression (Yuan et al., 2023; Xu et al., 2023; Zeng et al., 2022).

The relationship between macrophages and podocyte

Podocytes are highly specialized, terminally differentiated glomerular epithelial cells that play a crucial role in maintaining the kidney filtration barrier (Nagata, 2016). In diabetic nephropathy (DN), activated macrophages contribute to podocyte injury through multiple mechanisms including apoptosis induction and cytoskeletal disruption. Podocyte depletion directly correlates with the development and progression of proteinuria, a hallmark clinical manifestation of DN. Given their limited regenerative capacity, the degree of podocyte injury serves as a critical prognostic indicator to assess disease severity and predict renal outcomes in patients with DN (Li et al., 2023b; Barutta, Bellini & Gruden, 2022).

Single-cell RNA sequencing (scRNA-seq) analysis has demonstrated significant upregulation of TNF-α signaling pathways in human podocytes following growth hormone stimulation (Wu et al., 2019). Conditioned media collected from growth hormone-treated podocytes potently induced monocyte-to-macrophage differentiation. Notably, this differentiation capacity was substantially attenuated when the media were pretreated with a TNF-α neutralizing antibody, suggesting a TNF-α-dependent mechanism. In vivo studies using growth hormone-treated mice have revealed a pathological triad of increased macrophage infiltration, podocyte injury, and proteinuria development, further supporting the critical role of this signaling axis in glomerular pathology. Podocytes exposed to high-glucose culture conditions demonstrate a marked increase in their capacity to induce macrophage migration, with approximately 2.5-fold higher chemotactic activity than that of podocytes maintained under normal glucose conditions (You et al., 2013). Under diabetic conditions, podocytes undergo significant phenotypic alterations, leading to the sustained release of pro-inflammatory mediators including TNF-α and monocyte chemoattractant protein-1 (MCP-1). These cytokines orchestrate a complex inflammatory cascade by promoting macrophage chemotaxis and upregulating T-cell immunoglobulin and mucin domain-3 (TIM-3) expression in renal macrophages through activation of the NF-κB/TNF-α signaling pathway. M1 macrophage-derived exosomal miR-21a-5p and miR-25-3p promote podocyte apoptosis by directly targeting Tnpo1 and Atxn3, respectively (Zhuang et al., 2022). This self-perpetuating inflammatory cycle exacerbates podocyte injury and drives the progression of diabetic nephropathy, ultimately contributing to the deterioration of renal function in diabetic kidney disease (Yang et al., 2019).

Persistently high proteinuria levels are associated with poor long-term renal outcomes in patients with lupus nephritis (LN). Podocyte injury is responsible for substantial proteinuria. Thus, podocytes are key targets for LN therapy. Piezo1 knockout significantly reduced glomerulonephritis, tubulointerstitial injury, and podocyte foot process fusion, and improved renal function and proteinuria in MRL/lpr mice (Fu et al., 2024). In IgAN, dysglycosylated IgA1 induces NLRP3 expression in podocytes, initiates podocyte macrophage transdifferentiation, and contributes to the inflammatory cascade and renal fibrosis changes associated with IgAN (Qi, 2024).

The relationship between macrophages and mesangial cells

Mesangial cells, derived from mesenchymal stromal progenitors, form a critical structural and functional component of the glomerular tuft, where they interact with the mesangial matrix to maintain the glomerular architecture and regulate the glomerular microvascular network. These specialized perivascular cells play a central role in glomerular homeostasis and their dysfunction contributes significantly to the pathogenesis of various glomerular diseases. Upon pathological stimulation, mesangial cells are activated through the MAPK and PKC signaling cascades, triggering a series of molecular events that drive both inflammatory responses and fibrotic processes within the glomerulus (Zhao, 2019; Hu et al., 2024). Injured mesangial cells actively recruit circulating monocytes and macrophages to the sites of glomerular damage through the secretion of chemotactic factors. Notably, these infiltrating macrophages predominantly exhibit an M1-polarized phenotype, a phenomenon potentially mediated by intricate crosstalk between the NOTCH signaling pathway and NF-κB activation, which collectively establishes a proinflammatory microenvironment within the glomerulus (Ma et al., 2022).

Exosomes serve as crucial mediators of intercellular communication between macrophages and mesangial cells in the renal microenvironment (Zhu et al., 2020; Phu et al., 2022). Specifically, exosomes secreted by macrophages exposed to high glucose conditions have been shown to potently induce mesangial cell activation and proliferation, leading to pathological mesangial expansion and the subsequent secretion of pro-inflammatory cytokines. Mechanistic studies have revealed that these macrophage-derived exosomes contain elevated levels of TGF-β1, which activates the TGF-β1/Smad3 signaling cascade in recipient mesangial cells, ultimately driving excessive extracellular matrix deposition in both in vitro and in vivo models of diabetic nephropathy (Zhu et al., 2020). Macrophages regulate mesangial cell proliferation and migration through CXC motif chemokine ligand 12 (CXCL12)/dipeptidyl peptidase 4 (DPP4) axis interaction, leading to disruption of the LN filtration barrier and impaired renal function (Li et al., 2024c).

The relationship between macrophages and endothelial cells

As integral components of the glomerular filtration barrier, glomerular endothelial cells maintain a direct interface with the circulatory system, making them particularly vulnerable to physiological and pathological blood-borne factors (Zhang et al., 2024a). Renal-resident macrophages are predominantly localized in the periglomerular interstitial space and are strategically positioned to monitor glomerular function. The pro-inflammatory cytokine, TNF-α, upregulates endothelial adhesion molecule expression, thereby promoting leukocyte extravasation and subsequent infiltration into renal parenchymal tissue (Summers et al., 2011).

In a recent study on septic mice, the authors utilized single-cell sequencing revealed that F4/80hi macrophages undergo the most significant changes, with a notable reduction in the proportion of this macrophage subset following sepsis. Compared to other macrophage subsets, F4/80hi macrophages highly express anti-inflammatory genes, such as Socs3, Il1r2, and Il1rn. Ablation of F4/80hi macrophages exacerbated sepsis-induced AKI. Mechanistically, F4/80hi macrophages inhibit endothelial cell expression of IL-6 through IL1ra (an IL-1 receptor antagonist), ultimately mitigating sepsis-induced kidney damage (Privratsky et al., 2023).

Under hyperglycemic conditions, endothelial cells exhibit significant upregulation of the hypoxia-inducible factor-1α (HIF-1α)/Notch1 signaling pathway, which orchestrates the recruitment of pro-inflammatory M1 macrophages to renal tissue, thereby exacerbating renal injury in db/db diabetic mice (Torres et al., 2020). Therapeutic intervention with fenofibrate, a peroxisome proliferator-activated receptor alpha (PPAR-α) agonist, effectively attenuates this pathological process by suppressing HIF-1α/Notch1 signaling, resulting in reduced M1 macrophage infiltration and subsequent protection against diabetic nephropathy progression (Torres et al., 2020).

Macrophage-renal cell crosstalk in AKI-to-CKD transition

Survey methodology

Literature searches were conducted using PubMed and Web of Science. In addition to articles published in 2019, earlier articles were considered. The following keywords were used: chronic kidney disease, macrophages, M1 and M2 phenotypes, interstitial fibrosis, crosstalk, and therapeutic potential. As our work gradually unfolded, we then searched literature by keywords macrophages and renal cells, macrophages and epithelial cells, macrophages and fibroblasts, macrophages and podocyte, macrophages and mesangial cells, macrophages and endothelial cells, and therapeutic potentials of macrophages in CKD after removing duplicate articles and the articles with little relevance, 118 articles were selected for this review.