Cellular injury and mesenchymal transition

Endothelial and epithelial cell injury is a key mechanism in the development of ARDS (Ref. Reference Matthay and Zemans16) and is also closely associated with the pathogenesis of PF (Ref. Reference Kuwano, Hagimoto and Nakanishi17). The interaction between epithelial cells and fibroblasts plays a critical role in the response to lung injury in adults. Epithelial cells can suppress the expression of ECM components such as type I collagen (COL1A1) and α-smooth muscle actin (α-SMA), as well as various growth factors, thereby limiting the progression of fibrosis following alveolar injury (Ref. Reference Correll, Edeen, Zemans, Redente, Serban, Curran-Everett, Edelman, Mikels-Vigdal and Mason18). In ARDS, multiple factors – including infection, inflammatory mediators, mechanical ventilation and oxidative stress – can lead to apoptosis, necrosis or impaired regenerative capacity, directly compromising the integrity and function of alveolar epithelial cells (AECs) (Ref. Reference Gorman, O’Kane and McAuley2). Damage to AECs and dysfunction in their repair mechanisms are closely linked to both early pathological injury and late alveolar structural remodelling in ARDS (Ref. Reference Tao, Xu and Zhang19). Furthermore, disruptions in cellular quality control processes – such as proteostasis, endoplasmic reticulum stress, aberrant autophagy, organelle dysfunction and cellular senescence – can drive epithelial cells towards a profibrotic phenotype, thereby promoting fibrotic remodelling of the lung (Ref. Reference Katzen and Beers20). Studies of SARS-CoV-2 infection have also shown that the virus can induce AECs to overexpress profibrotic cytokines such as transforming growth factor-β (TGF-β), connective tissue growth factor (CTGF) and ECM proteins such as fibronectin, contributing to the development of PF (Ref. Reference Xu, Xu, Jiang, Dua, Hansbro and Liu21). Additionally, endothelial cell injury and endothelial-to-mesenchymal transition (EndMT) have been shown to promote PF through various mechanisms, including immune cell recruitment, fibroblast activation and epithelial cell damage (Ref. Reference Zhao, Wang, Wang, Yuan, Zhao, Lian, Ma, Xu, Li and Yu22).

Studies on post-ALI/ARDS PF have revealed that AEC apoptosis, EMT (Ref. Reference Zheng, Wang, D’Souza, Bartis, Dancer, Parekh, Gao, Lian, Jin and Thickett23) and oxidative stress (Ref. Reference Dong, Zhang, Zhu, Mao, Sun, Liu and Jiang24) in lung tissue are closely associated with the development of fibrosis. The underlying mechanisms may involve various targets and signalling pathways, including secretory phospholipase A2 (Ref. Reference Preuß, Scheiermann, Stadelmann, Omam, Winoto-Morbach, Lex, von Bismarck, Adam-Klages, Knerlich-Lukoschus, Wesch, Held-Feindt, Uhlig, Schütze and Krause25), methyl-CpG binding domain protein 2 (MBD2) and Frizzled-2 (FZD2) (Ref. Reference Zhou, Yang, Liu, Yao, Jia, Tang, Gong, Wan, Wu, Zhao, Liang, Liu, Liu, Xie, Long, Xiang, Wang and Xiao26), pyruvate kinase M2 (PKM2) and the mitochondrial fission marker dynamin-related protein-1 (Ref. Reference Feng, Huang, Peng, Yang, Yang, Tang, Xu, Gao, He, Xing and Mei27), enhancer of zeste homologue 2 (Ref. Reference Bao, Liu, Liu, Zhuang, Yang, Ren, Zhao, Bai, Zhou and Tang28), miR-200b/c and zinc finger E-box-binding homeobox (ZEB) (Ref. Reference Cao, Liu, Ping, Yi, Zeng and Li29). Interventions such as dioleoyl-phosphatidylglycerol (Ref. Reference Preuß, Scheiermann, Stadelmann, Omam, Winoto-Morbach, Lex, von Bismarck, Adam-Klages, Knerlich-Lukoschus, Wesch, Held-Feindt, Uhlig, Schütze and Krause25), Resolvin D1 (Ref. Reference Zheng, Wang, D’Souza, Bartis, Dancer, Parekh, Gao, Lian, Jin and Thickett23), shikonin (Ref. Reference Feng, Huang, Peng, Yang, Yang, Tang, Xu, Gao, He, Xing and Mei27), 3-deazaneplanocin A (Ref. Reference Bao, Liu, Liu, Zhuang, Yang, Ren, Zhao, Bai, Zhou and Tang28), mesenchymal stem cell-derived microvesicles (MSC MVs) (Ref. Reference Zhang, Ye, Tang, Ji, Zheng, Chen, Ge and Huang30) and the free radical scavenger hydrogen-rich saline (Ref. Reference Dong, Zhang, Zhu, Mao, Sun, Liu and Jiang24) have been shown to modulate AEC apoptosis, EMT and oxidative stress, thereby attenuating post-ALI/ARDS PF. These findings underscore the significant role of AEC apoptosis and EMT in the pathogenesis of post-ALI/ARDS PF, suggesting their potential as predictive biomarkers and therapeutic targets. However, it remains unclear whether novel forms of regulated cell death – such as ferroptosis, pyroptosis and cuproptosis – also contribute to the development and progression of post-ALI/ARDS PF. Moreover, most current research on AECs in post-ALI/ARDS PF remains at the basic scientific stage. Translating these preclinical findings into clinical applications for patient diagnosis and treatment requires further clinical studies and comprehensive analysis.

Although the role of AECs in the development and progression of post-ALI/ARDS PF is widely recognised, the contribution of endothelial cells may be underestimated. Studies on idiopathic PF suggest that aberrant endothelial responses lead to loss of basement membrane integrity, vascular remodelling and altered vascular signalling, which further promote ECM deposition, fibrosis and parenchymal lung injury, indicating that endothelial cells also play a significant role in the initiation and progression of PF (Ref. Reference May, Mitchell and Jenkins31). Research on post-ALI/ARDS PF has also demonstrated that lipopolysaccharide (LPS) can induce EndMT in mouse pulmonary vascular endothelial cells. These EndMT-transformed endothelial cells exhibit greater proliferative and migratory capacities than mesenchymal cells do, thereby contributing to interstitial fibrosis (Ref. Reference Suzuki, Tada, Gladson, Nishimura, Shimomura, Karasawa, Tatsumi and West32). The underlying mechanisms may involve pathways such as DPP-4 signalling (Ref. Reference Suzuki, Tada, Gladson, Nishimura, Shimomura, Karasawa, Tatsumi and West32), TGF-β signalling (Ref. Reference Liu, Shi, Tang, Wang and Wang33), angiotensin-converting enzyme II (ACE2) (Ref. Reference Wang, Cong, Yin, Zhang, Zhu, Sun, Fan, Xue, Fan and Gong34) and MFGE8 (Ref. Reference Shi, Liu, Wang, Tang, Di and Wang35). Targeting EndMT represents a potential therapeutic strategy for mitigating post-ALI/ARDS PF. However, related research remains limited and further studies are needed to identify specific pharmacological interventions and the optimal timing for treatment.

In recent years, increasing attention has been given to the role of pericytes – microvascular wall cells in direct contact with endothelial cells – in addition to AECs and endothelial cells. Evidence suggests that pericytes may transform into fibroblasts during acute and chronic pulmonary inflammation, contributing to the development of PF (Ref. Reference Shammout and Johnson36). Studies in ALI/ARDS have also shown that LPS s can suppress NOTCH signalling in lung pericytes and downregulate the expression of its downstream target genes and angiogenic factors, thereby modulating lung injury repair (Ref. Reference Mierzejewski, Różycka, Stremińska, Brągiel-Pieczonka, Sidor, Hoser, Bartoszewicz, Gewartowska, Frontczak-Baniewicz, Ciemerych, Brzóska and Skirecki37). Research using porcine ALI/ARDS models further revealed a significant increase in the pericyte EMT marker PDGFRβ in lung regions with collagen deposition (Ref. Reference Wegrzyniak, Lechi, Rokka, Mitran, Zhang, Thelander, Löfblom, Frejd, Korsgren, Perchiazzi, Eriksson and Eriksson38), suggesting that pericyte dysfunction and transition may also play a role in post-ALI/ARDS collagen accumulation and the onset of PF. However, the specific mechanisms and regulatory approaches require further in-depth investigation (Figure 2).

Figure 2.

Pathogenic mechanisms and drivers of post-ALI/ARDS PF.

The fibrotic process is driven by a complex interplay of cellular injury, aberrant repair and dysregulated signalling, which can be initiated and exacerbated by distinct patient-specific and clinical factors.

Figure 2. Long description

At the center of the diagram, an arrow points from a pair of inflamed lungs labeled A L I / A R D S to fibrotic lungs labeled Pulmonary fibrosis. Surrounding this core are four quadrants.

1. Top-Left (Green): Patient-Specific and Clinical Management Factors. The inner arc shows icons for obesity, internal environmental homeostasis, high fever, and underlying diseases. The middle arc, labeled Diagnostic and Therapeutic, includes mechanical ventilation, hyperoxia, and other interventions. The outer perimeter lists upregulated factors: Piezo 1, angiotensin-converting enzyme, Midkine, Notch 2, T G F-beta 1, p-Smad 2, Wnt signaling, and H S F 1.

2. Top-Right (Blue/Purple): Inflammatory response and immune cell. The inner arc depicts monocytes, neutrophils, and macrophages. The middle arc, labeled Inflammatory response, shows chemokines, inflammatory factors, and sustained chronic inflammation. The outer perimeter lists molecular drivers: T G 2 up, M i R-141-3p up, L n C R N A H 19 up, F O X A 1 up, M i R-423-5p down, Keap 1 down, N r f 2 down, A R E down, P G C-1 alpha down, and P I 3 K / A K T / M T O R up.

3. Bottom-Left (Orange): Cellular injury and mesenchymal transition. The inner arc shows alveolar epithelial cells, endothelial cells, and other cells like pericytes. Processes include apoptosis, ferroptosis, cuproptosis, E M T, and End M T. Outer perimeter labels include M B D 2, D R P 1, P K M 2, F Z D 2, phospholipase A 2, Z E B, M F G E 8 up, A C E 2 down, T G F-beta 1 up, D P P-4 up, N O T C H signaling up, and P D G F R beta up.

4. Bottom-Right (Red): Fibroblast Activation and Abnormal Deposition of E C M. The inner arc shows fibroblast to myofibroblast transition, migration, and regulation. The middle arc, labeled Extracellular matrix degradation, shows versican, fibronectin, collagen, and E C M homeostasis. The outer perimeter lists: Activin-A up, I G F-1 up, P I 3 K / A K T / M T O R up, P T E N down, Lumican up, Arp 2 / 3 complex down, T L R 4 up, E R K / Slug up, P T P alpha up, T G F-beta 1 up, Versican up, and M M P s up.

Fibroblast activation and aberrant ECM deposition

Fibroblasts are key cells responsible for the synthesis and remodelling of the ECM and play crucial roles in the repair process following lung injury (Ref. Reference Wynn39). During lung injury repair, various cell types – including resident pulmonary fibroblasts, recruited circulating fibroblasts and epithelial or endothelial cells that have undergone mesenchymal transition – can become activated and differentiate into myofibroblasts expressing α-SMA. These myofibroblasts secrete ECM components to facilitate tissue repair. However, when the injury is too severe or the repair process becomes dysregulated, abnormal activation of myofibroblasts occurs. This leads to excessive deposition of ECM and the development of progressive fibrotic scarring within the lung parenchyma, ultimately contributing to the initiation and progression of PF (Refs Reference Jiang, Bu, Wang, Ruan, Shi, Yu, Huang, Xue, Tang, Zhao, Su and Cheng40, Reference Wu, Tang and Kapoor41).

Studies on post-ALI/ARDS PF have shown that LPS induces the proliferation of lung fibroblasts in a time-dependent manner (Refs Reference Zhou, Wang, Shao and Wang42, Reference Xie, Xu, Wan, Xing, Shang, Gao and He43). In mouse models of post-ALI/ARDS PF, the percentage of apoptotic fibroblasts is significantly reduced, and the induction of fibroblast apoptosis has been shown to attenuate lung tissue inflammation and ameliorate the extent of PF (Ref. Reference Zhu, An, Zhang, Wang and Duan44). Factors such as platelet-derived growth factor (PDGF), TGF-β (Ref. Reference Deng, Jin, Li, Gu, Liu and Zhou45) and insulin-like growth factor-I (IGF-I) (Ref. Reference Choi, Lee, Sunde, Huizar, Haugk, Thannickal, Vittal, Plymate and Schnapp46) promote fibroblast migration and drive their differentiation into myofibroblasts. These activated myofibroblasts upregulate the expression of COL1A1, type III collagen (COL3A1) and α-SMA, thereby facilitating excessive deposition and remodelling of the ECM (Refs Reference Deng, Jin, Li, Gu, Liu and Zhou45, Reference Wang, Wang, Cao, Wang, Zhou, Gao, Zeng, Cheng, Jin, Chen, Wen and Wang47). The resulting accumulation and structural alteration of ECM components increase matrix stiffness, which in turn further enhances fibroblast migration and α-SMA expression through positive feedback mechanisms, thereby accelerating the progression of post-ALI/ARDS PF (Ref. Reference Asano, Ito, Takahashi, Furuya, Kondo, Sokabe and Hasegawa48). The underlying mechanisms may involve multiple proteins and signalling pathways, including TLR4 and PTEN (Ref. Reference He, Gao, Deng, Li, Chen, Xing, Zhao, Ding and Wang49), PTPα (Ref. Reference Aschner, Correll, Beke, Foster, Roybal, Nelson, Meador, Strand, Anderson, Moore, Reynolds, Kopf, Burnham and Downey50), the PI3K-Akt–mTOR pathway (Ref. Reference Xie, Xu, Wan, Xing, Shang, Gao and He43), lumican and ERK/Slug signalling (Ref. Reference Wang, Wang, Cao, Wang, Zhou, Gao, Zeng, Cheng, Jin, Chen, Wen and Wang47) and Arp2/3 complex formation (Ref. Reference Mergault, Lisée, Tiroille, Magnien, Parent, Lenga Mabonda, Sizaret, Jaillet, Crestani, Marchand-Adam and Plantier51). Targeting these pathways to modulate fibroblast proliferation, migration (Ref. Reference Mu, Liu, Chen, Zhi, Li, Xu and Ma52) and ECM synthesis and deposition represents a promising therapeutic strategy for mitigating post-ALI/ARDS PF. However, clinical research in this area remains limited. Further studies involving the isolation or detection of relevant proteins in patient-derived samples – such as blood and bronchoalveolar lavage fluid (BALF) – are warranted to facilitate the translation of these basic research findings into clinical applications.

The ECM is involved in a variety of biological processes such as cellular scaffolding, adhesion, migration and survival, and plays a critical role in pulmonary homeostasis, injury repair and functional maintenance. In COVID-19-induced ARDS, significant deposition of collagen, fibronectin, versican and TGF-β has been observed alongside disrupted ECM architecture (Ref. Reference de Souza Xavier Costa, Ribeiro Júnior, de Brito, Antonangelo, Faria, Monteiro, Setubal, Pinho, Pereira, Seelaender, de Castro, Lima, de Almeida Monteiro, Duarte-Neto, Saldiva, Ferraz da Silva, Dolhnikoff and Mauad53), underscoring the importance of the ECM in the development of post-ALI/ARDS PF. During ALI repair, the production and resolution of the ECM are dynamically balanced. An increase in ECM production or a decrease in its clearance can lead to fibroproliferation and persistent inflammation. Matrix metalloproteinases (MMPs) and their tissue inhibitors (TIMPs) play key roles in ECM degradation and turnover (Ref. Reference Dos Santos54). Studies on idiopathic pulmonary fibrosis (IPF) also indicate that MMPs, which degrade both the ECM and numerous non-matrix proteins, represent potential therapeutic targets (Ref. Reference Craig, Zhang, Hagood and Owen55). Furthermore, studies on ALI have revealed that coagulation abnormalities and impaired fibrinolysis contribute to extravascular fibrin deposition. In injured lung tissue, upregulation of procoagulant pathways, suppression of fibrinolysis, extravascular fibrin deposition and dysregulation of fibrin turnover collectively contribute to excessive alveolar fibrin accumulation (Ref. Reference Idell56). These fibrin deposits can further stimulate fibroblast recruitment and collagen secretion, promoting the formation of hyaline membranes and collectively contributing to the development of PF (Ref. Reference Ferrer, Navas, Adda and Artigas57). In summary, modulating the production and degradation of the ECM represents a viable strategy for mitigating post-ALI/ARDS PF. Pharmacological agents such as MMP modulators (Ref. Reference Dos Santos54) and anticoagulants (Ref. Reference Chambers58) show therapeutic potential. However, further research is needed to determine how to effectively regulate the dynamic balance of ECM synthesis and degradation, restore normal ECM architecture and identify optimal drug interventions and timings for improving outcomes in post-ALI/ARDS PF (Figure 2).

Inflammatory response and immune cells

Inflammatory responses and immune cells are closely associated with the initiation and progression of ALI/ARDS and play significant roles in the development of post-ALI/ARDS PF. Studies on COVID-19 have revealed that pathways related to inflammation and neutrophil extracellular traps are significantly upregulated in patients with COVID-19-associated PF compared with those without fibrosis, and these alterations are correlated with prolonged ICU stays and increased mortality (Ref. Reference Kooistra, Dahm, van Herwaarden, Gerretsen, Nuesch Germano, Mauer, Smeets, van der Velde, van den Berg, van der Hoeven, Aschenbrenner, Schultze, Ulas, Kox and Pickkers59). SARS-CoV-2 infection promotes the accumulation of immune cells at sites of injury, where they release inflammatory cytokines and chemokines, leading to AEC damage and the secretion of proteolytic enzymes. This process disrupts the interaction between fibroblasts, collagen and the ECM, thereby exacerbating PF (Ref. Reference Carvajal, García-Castillo, Cuellar, Campillay-Véliz, Salazar-Ardiles, Avellaneda, Muñoz, Retamal-Díaz, Bueno, González, Kalergis and Lay60). Moreover, expression levels of NLRP3 inflammasome-related miRNAs (miR-17-5p, miR-146a-3p and miR-223-3p) in the plasma exosomes of COVID-19 patients have been associated with long-term radiological evidence of fibrotic changes following ARDS (Ref. Reference Curcio, Poli, Fabi, Sugoni, Pasticci, Ferranti, Rossi, Folletti, Sanesi, Santoni, Dominioni, Cavallo, Morgana, Mordeglia, Luca, Pucci, Brancorsini and Vaudo61). Additionally, persistent pulmonary inflammation driven by activin A promotes alveolar cell death and hyaline membrane formation, culminating in fibrotic remodelling and emphysema-like pathological changes resembling those observed in post-ALI/ARDS PF (Ref. Reference Apostolou, Stavropoulos, Sountoulidis, Xirakia, Giaglis, Protopapadakis, Ritis, Mentzelopoulos, Pasternack, Foster, Ritvos, Tzelepis, Andreakos and Sideras62). These findings collectively underscore the involvement of inflammatory mechanisms and immune cells in the pathogenesis and progression of post-ALI/ARDS PF.

In studies of post-ALI/ARDS PF, various inflammatory responses – including neutrophil and monocyte–macrophage infiltration (Ref. Reference Zambelli, Rizzi, Delvecchio, Bresciani, Molteni, Meanti, Pascal, Fehrentz, Omeljaniuk, Bellani and Torsello63), macrophage polarisation (Ref. Reference Kim, Kim, Kim, Bae, Kim, Jeong, Cho and Kang64) and increased release of inflammatory cytokines and chemokines (Ref. Reference Nguyen, Xu, Lam, Liao, Wong and Ge65) – have been shown to promote fibrin production and ECM deposition, thereby contributing to the development of fibrosis. Underlying mechanisms may involve protein targets and signalling pathways such as transglutaminase 2 (TG2) (Ref. Reference Kim, Kim, Kim, Bae, Kim, Jeong, Cho and Kang64), miR-141-3p and the Keap1/Nrf2/ARE axis (Ref. Reference Long, Zhang, Yang, Sun and Ji66), as well as long non-coding RNA (lncRNA) H19 and the miR-423-5p/FOXA1 pathway (Ref. Reference Mu, Wang and Li67). Agents including Isthmin 1 (Ref. Reference Nguyen, Xu, Lam, Liao, Wong and Ge65) and the TLQP-21 short analogue JMV5656 (Ref. Reference Zambelli, Rizzi, Delvecchio, Bresciani, Molteni, Meanti, Pascal, Fehrentz, Omeljaniuk, Bellani and Torsello63) can modulate inflammatory responses and immune cell infiltration, thereby attenuating post-ALI/ARDS PF. Furthermore, in SARS-CoV-induced ALI, dysregulated differentiation of T cells and macrophages impairs viral clearance and establishes a profibrotic microenvironment in the lungs, leading to the formation of profibrotic lesions in mice and ultimately promoting PF (Ref. Reference Zornetzer, Frieman, Rosenzweig, Korth, Page, Baric and Katze68). Oxidative stress in ALI/ARDS can induce the formation of immune complexes, which activate neutrophils and macrophages, cause widespread endothelial and epithelial damage and disrupt lung repair mechanisms, further driving fibrotic processes (Ref. Reference Ward69). In summary, inflammatory responses promote PF in ALI/ARDS through dual mechanisms: facilitating fibrin generation and ECM accumulation on the one hand and impairing tissue repair and pathogen clearance on the other hand, thereby perpetuating inflammation and collectively contributing to the development and persistence of fibrosis.

Studies on COVID-19 have revealed that SARS-CoV-2 induces aberrant expression of fibrosis-related genes in monocyte-derived macrophages, leading to the expansion of myofibroblasts and fibroblasts, strong upregulation of ECM protein-encoding genes and subsequent ECM deposition, thereby promoting PF (Ref. Reference Wendisch, Dietrich, Mari, von Stillfried, Ibarra, Mittermaier, Mache, Chua, Knoll, Timm, Brumhard, Krammer, Zauber, Hiller, Pascual-Reguant, Mothes, Bülow, Schulze, Leipold, Djudjaj, Erhard, Geffers, Pott, Kazmierski, Radke, Pergantis, Baßler, Conrad, Aschenbrenner, Sawitzki, Landthaler, Wyler, Horst, Hippenstiel, Hocke, Heppner, Uhrig, Garcia, Machleidt, Herold, Elezkurtaj, Thibeault, Witzenrath, Cochain, Suttorp, Drosten, Goffinet, Kurth, Schultze, Radbruch, Ochs, Eils, Müller-Redetzky, Hauser, Luecken, Theis, Conrad, Wolff, Boor, Selbach, Saliba and Sander70). Similarly, in the context of post-ALI/ARDS PF, oxidative stress has been shown to stimulate the secretion of proinflammatory and profibrotic mediators, which facilitate the recruitment of inflammatory cells and promote fibroblast proliferation and myofibroblast differentiation, ultimately contributing to fibrotic remodelling (Ref. Reference Makena, Kikalova, Prasad and Baxter71). In ALI/ARDS, reduced expression of PGC-1α in fibroblasts leads to mitochondrial dysfunction, which promotes the formation of migrasomes containing mitochondrial DNA during fibroblast migration. This process facilitates the macrophage-to-myofibroblast transition (Ref. Reference Peng, Mei, Qi, Tang, Yang, Feng, Zhou, Huang, Qian, Xing, Gao, Xu and He72). Additionally, bone marrow-derived macrophages can accumulate in the lungs and secrete activin A, establishing a profibrotic positive feedback loop and enhancing EMT, thereby driving the progression of fibrosis (Ref. Reference Pan, Feng, Li, Yang, Zhou and Song73). Activated mast cells further promote alveolar EMT via the PI3K/AKT/mTOR pathway, thereby accelerating fibrotic development in ALI/ARDS (Ref. Reference Tan, Zhou, Xiong, Xian, Liu, Zhang, Xu and Yao74). Moreover, granulocyte colony-stimulating factor exacerbates bleomycin-induced ALI and PF through mechanisms involving neutrophil activation (Ref. Reference Azoulay, Herigault, Levame, Brochard, Schlemmer, Harf and Delclaux75). These findings collectively demonstrate that, in addition to their classical roles in inflammation, immune cells contribute to the initiation and progression of post-ALI/ARDS PF through multiple mechanisms. These mechanisms include transdifferentiation into fibroblast-like cells, release of profibrotic factors and facilitation of mesenchymal transition programmes (Figure 2).

Patient-specific and clinical management factors

Patient-specific factors play a significant role in the onset and progression of disease. Studies have shown that under physiological conditions, obesity is associated with elevated plasma leptin levels, which increase pulmonary expression of cPLA2α/COX-2 and promote leukocyte recruitment via the NADPH oxidase/ROS/AP-1 pathway, thereby exacerbating lung inflammation (Ref. Reference Hsu, Lin, Chang, Wu, Sia, Lee, Huang and Lin76). However, research on post-ALI/ARDS PF has also indicated that under septic conditions, obesity may reduce monocyte phagocytic capacity, impair neutrophil migration, decrease the expression of inflammatory cytokines and chemokines, increase alveolar septal thickness and reduce collagen fibre content in animal models of ALI, ultimately attenuating both pulmonary inflammation and fibrosis (Ref. Reference Maia, Cruz, de Oliveira, Samary, Fernandes, Trivelin, Rocha, Gama de Abreu, Pelosi, Silva and Rocco77). Additionally, miRNA-122-5p in circulating exosomes under obese conditions can inhibit EndMT and oxidative stress by downregulating the TGF-β/TGF-β R1/Smad2 pathway, thereby mitigating pathological fibroproliferation in ALI/ARDS (Ref. Reference Qi, Deng, Chen, Fan, Peng, Tang, Wang and Yu78). These findings suggest that systemic metabolism may modulate inflammatory responses and EndMT, influencing both the development and resolution of post-ALI/ARDS PF. Further investigations into the regulation of systemic metabolism to ameliorate inflammatory responses and improve endothelial cell function may provide valuable insights for the clinical management of post-ALI/ARDS PF. It is also worth exploring whether other conditions affecting vascular endothelial function and metabolism – such as hypertension and diabetes – could similarly influence the onset and resolution of fibrosis in ALI/ARDS patients. Additionally, elevated temperature (39.5°C) has been found to activate the transcription factor HSF1, enhance Wnt signalling and induce genes associated with EMT in small airway epithelial cells, thereby promoting fibrotic progression during the proliferative phase of ARDS (Ref. Reference Potla, Tulapurkar, Luzina, Atamas, Singh and Hasday79). This highlights the importance of temperature management in patients with ALI/ARDS and suggests a link between the host internal environment and the development of PF. Further research into the specific mechanisms underlying these effects – as well as the roles of other internal environmental factors such as pH, lactate levels, the partial pressure of oxygen, the electrolyte balance and extrapulmonary organ function – could provide a stronger theoretical foundation for the clinical management of ARDS patients. Such insights may help reduce the incidence of post-ALI/ARDS PF and improve patient outcomes.

Oxygen therapy and mechanical ventilation are commonly used treatments for ARDS; however, under certain conditions, they may also contribute to the development of PF. As early as 1969, researchers reported that mechanical ventilation could exacerbate interstitial fibroblast proliferation, promote PF and worsen the pathology of hyaline membrane disease, suggesting that positive-pressure breathing should be reserved for patients refractory to other interventions (Ref. Reference Becker and Koppe80). Analysis of BALF from mechanically ventilated patients revealed the presence of activated fibroblasts with enhanced migratory capacity and increased COL1A1 production in ALI/ARDS patients (Ref. Reference Quesnel, Nardelli, Piednoir, Leçon, Marchal-Somme, Lasocki, Bouadma, Philip, Soler, Crestani and Dehoux81). Further studies have demonstrated that positive-pressure mechanical ventilation exerts stretch stress on AECs, regulating pathways such as the Piezo1 protein (Ref. Reference Fang, Li, Wang, Zhang, Sun, Xu, Yang, He, Yu, Li, Zhou, Reng, Sun, Shu, Yuan, Xu and Shang82), the Nox1-MK-Notch2-ACE pathway and the novel cytokine midkine (Ref. Reference Zhang, Pan, Fanelli, Wu, Luo, Islam, Han, Mao, Ghazarian, Zeng, Spieth, Wang, Khang, Mo, Liu, Uhlig, Liu, Laffey, Slutsky, Li and Zhang83), and innate immune mechanisms (Ref. Reference Heise, Stober, Cheluvaraju, Hollingsworth and Garantziotis84) to promote EMT. It also triggers calcium influx in fibroblasts, which acts as a second messenger to activate fibrotic functions, collectively driving the progression of PF (Ref. Reference Murata, Ito, Furuya, Takahara, Naruse, Aso, Kondo, Sokabe and Hasegawa85). Moreover, factors such as ventilation mode, tidal volume, positive end-expiratory pressure, transpulmonary pressure and recruitment manoeuvres can influence fibroblast activation, ECM production and degradation, thereby promoting ECM deposition and fibrosis in ALI/ARDS patients (Ref. Reference Al-Husinat, Azzam, Al Sharie, Al Sharie, Battaglini, Robba, Marini, Thornton, Cruz, Silva and PRM86). In addition, high-concentration oxygen therapy during mechanical ventilation can induce AEC apoptosis (Ref. Reference Liu, Jia, Ren, Yin, Xiao, Wu, Liu and Chen87) and impair macrophage autophagy (Ref. Reference Chen, Zhang, Li and Wang88), leading to further lung injury and fibrotic remodelling. These findings indicate that inappropriate oxygen therapy and mechanical ventilation strategies may facilitate the development of post-ALI/ARDS PF. Therefore, based on the current body of evidence, mechanical ventilation should be avoided in patients without a clear indication in clinical practice. For those who do require mechanical ventilation, the lowest feasible oxygen concentration should be used, and excessively high airway pressures should be avoided, in order to minimise the secondary lung injury caused by oxygen therapy and mechanical ventilation.

The primary challenge in the field remains the clinical translation of mechanistic insights into therapies that effectively improve patient outcomes. The surge of fibrosis cases following COVID-19-associated ARDS serves as a powerful test of disease heterogeneity (Ref. Reference Giacomelli, Piccarducci, Marchetti, Romei and Martini89). This finding implies that different inciting injuries can engage distinct profibrotic pathways. A critical future direction is to unravel this aetiologic diversity, which promises to uncover specific, targetable mechanisms and inform the development of personalised predictive biomarkers and therapeutic regimens (Figure 2).

Biomarkers

In the context of post-ALI/ARDS PF, circulating biomarkers have garnered increasing attention. These biomarkers not only facilitate early diagnosis but also provide valuable insights into disease progression and treatment response. In recent years, numerous studies have identified a range of potential circulating biomarkers, including growth factors that mediate mesenchymal transition (Ref. Reference Liu, Philp, Corte, Travis, Schilter, Hansbro, Burns, Eapen, Sohal, Burgess and Hansbro90), markers associated with ECM deposition (Ref. Reference Yu, Yin, Lei, Gong, Zheng, He, Lei and Sun91) and inflammatory mediators (Ref. Reference Gao, Li, Wang, Liao, Zhou, Wang, Hu, Cheng, Zeng, Wang and Wen92). These biomarkers show promise for predicting and monitoring the development and progression of post-ALI/ARDS PF and may guide therapeutic decision-making.

Growth factors are closely associated with mesenchymal transition and fibroblast activation in post-ALI/ARDS PF. Current studies have revealed that elevated levels of TGF-β in BALF may promote fibroproliferative ARDS and persistent fibrosis. During the early phase of ALI/ARDS, inflammatory signalling can stimulate TGF-β release from alveolar stromal cells, impair alveolar fluid reabsorption and induce oxidative stress. Moreover, certain microbes can exploit increased tissue TGF-β levels to evade host immune surveillance, enhancing infectivity and facilitating latent or chronic infection. However, complete absence of TGF-β may also exacerbate pulmonary inflammation (Ref. Reference Arguinchona, Zagona-Prizio, Joyce, Chan and Maloney93), indicating that TGF-β serves as a critical mediator in maintaining inflammatory balance. Both excessive upregulation and deficiency of TGF-β can contribute to the development of post-ALI/ARDS PF through profibrotic and proinflammatory mechanisms. Additionally, TGF-β can upregulate nuclear LOXL2 expression, driving the differentiation of fibroblasts into myofibroblasts and promoting the formation and progression of fibrotic scars. LOXL2 protein levels are significantly elevated in the lungs of patients during the proliferative and fibrotic phases of ARDS (Ref. Reference Matsuo, Tanida, Yanagi, Tsubouchi, Miura, Shigekusa, Matsumoto and Nakazato94), suggesting that TGF-β may serve as a potential biomarker for post-ALI/ARDS PF. Although existing clinical studies have reported that BALF TGF-β1 levels on day 3 are not associated with fibroproliferation or mortality in ARDS patients (Ref. Reference Forel, Guervilly, Farnarier, Donati, Hraiech, Persico, Allardet-Servent, Coiffard, Gainnier, Loundou, Sylvestre, Roch, Bourenne and Papazian95), these findings should be interpreted with caution. The study focused on primarily early-stage associations and included a cohort in which 85.5% of ARDS cases were of pulmonary origin, introducing potential selection bias. Notably, the TGF-β1 level in the BALF on day 7 was greater in the fibrosis group than in the non-fibrosis group, although the difference was not statistically significant (P = 0.107). These observations warrant further validation through rigorously designed studies with larger sample sizes, and future investigations should also determine the optimal timing and specimen type (e.g. peripheral blood or BALF) for measuring TGF-β1 as a predictive biomarker of PF. Other growth factors implicated in post-ALI/ARDS PF, such as PDGF (Ref. Reference Deng, Jin, Li, Gu, Liu and Zhou45), IGF-I (Ref. Reference Choi, Lee, Sunde, Huizar, Haugk, Thannickal, Vittal, Plymate and Schnapp46) and CTGF (Ref. Reference Xu, Xu, Jiang, Dua, Hansbro and Liu21), also represent promising candidate biomarkers worthy of further clinical investigation.

Abnormal deposition of the ECM is a key pathological feature of post-ALI/ARDS PF. As major components of the ECM, COL1A1, COL3A1 and α-SMA have been shown to be significantly correlated with the development of fibrosis (Refs. Reference Deng, Jin, Li, Gu, Liu and Zhou45, Reference Wang, Wang, Cao, Wang, Zhou, Gao, Zeng, Cheng, Jin, Chen, Wen and Wang47). Clinical studies have demonstrated pronounced activation of the type I procollagen promoter in the BALF of ARDS patients (Ref. Reference Budinger, Chandel, Donnelly, Eisenbart, Oberoi and Jain96). Additionally, the serum and alveolar levels of the N-terminal propeptide of type III procollagen (PIIINP) are elevated in patients with fibroproliferation and are associated with increased mortality, highlighting its potential utility in identifying fibrotic subtypes and guiding treatment (Ref. Reference Forel, Guervilly, Hraiech, Voillet, Thomas, Somma, Secq, Farnarier, Payan, Donati, Perrin, Trousse, Dizier, Chiche, Baumstarck, Roch and Papazian97). In patients with COVID-19-related PF, the serum levels of laminin (LN), type IV collagen (IV Col), PIIINP and hyaluronic acid (HA) were significantly greater than those in healthy controls. Among these, HA correlated positively with high-resolution computed tomography (HRCT) scores and LN, IV Col, PIIINP and HA were associated with the progression of COVID-19-related PF (Ref. Reference Yu, Yin, Lei, Gong, Zheng, He, Lei and Sun91), underscoring the potential of ECM-related biomarkers in predicting post-ALI/ARDS PF. However, a separate clinical study reported that longitudinal increases in PIIINP were correlated with 90-day mortality in COVID-19 patients but were not significantly associated with long-term fibrotic sequelae in survivors (Ref. Reference Zhang, Boers, de Brabander, van den Heuvel, Blok, Kullberg, Smids-Dierdorp, Dekker, Aberson, Meijboom, Vlaar, Heunks, Nossent, van der Poll, Bos and Duitman98). In summary, ECM-related molecules are closely linked to the pathogenesis of post-ALI/ARDS PF and represent promising candidate biomarkers. Nevertheless, specific cut-off values and predictive accuracy require further validation in large-scale, multicentre clinical trials. Furthermore, MMPs – key regulators of ECM degradation and turnover – have also been implicated in PF development and prognosis (Ref. Reference Khan, Stewart, Saini, Robinson and Jenkins99). However, their precise mechanistic roles and potential as biomarkers in post-ALI/ARDS PF warrant further investigation.

Inflammatory responses and immune cells play crucial roles in both the development of ARDS and the pathogenesis of post-ALI/ARDS PF, with significant differences observed between patients who develop fibrosis and those who do not (Ref. Reference Kooistra, Dahm, van Herwaarden, Gerretsen, Nuesch Germano, Mauer, Smeets, van der Velde, van den Berg, van der Hoeven, Aschenbrenner, Schultze, Ulas, Kox and Pickkers59). However, owing to the broad involvement of inflammatory cytokines and cells in a wide range of inflammatory conditions, they lack specificity for post-ALI/ARDS PF and are not widely accepted as standalone biomarkers. Nevertheless, other inflammation-related mediators show promise. For example, autotaxin (ATX) is correlated with inflammatory cytokines and established fibrotic biomarkers, such as MMP-7, oncostatin M, serum albumin and secreted protein acidic and rich in cysteine (SPARC), and has been shown to predict outcomes in ARDS more effectively than conventional indicators including the SOFA score, APACHE II score and PaO₂/FiO₂ ratio. The combination of serum ATX with clinical parameters further improves prognostic accuracy in ARDS patients (Ref. Reference Gao, Li, Wang, Liao, Zhou, Wang, Hu, Cheng, Zeng, Wang and Wen92), suggesting its potential utility as a biomarker for post-ALI/ARDS PF. Cortistatin, an endogenous inhibitor of pulmonary inflammation and fibrosis, suppresses inflammatory and fibrotic responses in lung macrophages and fibroblasts during ARDS. Its deficiency may serve as an indicator of poor prognosis in patients with inflammatory/fibrotic lung diseases (Ref. Reference Barriga, Benitez, Ferraz-de-Paula, Garcia-Frutos, Caro, Robledo, O’Valle, Campos-Salinas and Delgado100). Additionally, angiotensin II, which has both proinflammatory and profibrotic effects, has been significantly correlated with the development of PF (Ref. Reference Gupta, Kumar, Mandloi and Shenoy101), although its value as a biomarker in post-ALI/ARDS PF requires further investigation. In summary, while inflammatory cytokines and cells lack the specificity needed for reliable prediction of post-ALI/ARDS PF on their own, related mediators such as ATX, cortistatin and angiotensin II represent promising candidates worthy of further study as part of a biomarker panel for fibrotic progression after ALI/ARDS (Figure 3 and Table 1).

Figure 3.

Integrated framework for predicting post-ALI/ARDS PF.

A multimodal approach for the early prediction and risk stratification of post-ALI/ARDS PF. This framework integrates three complementary pillars of predictive data to enable proactive clinical management. Pillar 1: Circulating and biochemical biomarkers. Pillar 2: Advanced imaging markers. Pillar 3: Artificial intelligence (AI) and integrated data modelling.

Figure 3. Long description

At the center of the diagram is a white circle containing icons of a blood pressure monitor and a medical professional, with the text Prediction forward slash Early Warning. Surrounding this core are three large colored segments.

1. Biomarkers (Blue segment, top-left): This section contains various molecular illustrations and initialisms. At the top are question marks and labels for I G F-I, P D G F, A T X, and C T G F. Moving clockwise, it lists T G F-beta, O S M, M M P-7, I V C O L (represented by a double helix), H A (represented by a branched structure), S P A R C, A C E 2, L N, and P I I I N P.

2. Imaging Biomarkers (Pink segment, top-right): This section features a question mark at the top, followed by an Ultrasound probe icon. Below it is an H R C T forward slash Q C T scanner. Further down is an F M T image of a mouse and a line graph showing root-mean-squared difference of Q.

3. Novel A I-Based Predictive Biomarkers (Green segment, bottom): This section includes a question mark at the bottom center. To the left is a brain icon with A I text labeled Composite A I predictive models. In the center is a computer monitor displaying a neural network. To the right is a cross-sectional medical scan labeled Novel imaging biomarkers.

Table 1.

Promising predictive and early warning biomarkers for post-ALI/ARDS PF

Table 1. Long description

The table is organized into three main categories.

1. Biomarkers category:

- T G F beta: Studied in A R D S or C O V I D 19 patients via B A L F. It drives E M T and fibroblast activation. Elevated levels promote fibroproliferative A R D S.

- A T X: Studied in A R D S patients via serum and B A L F. Correlates with inflammation and predicts outcomes better than S O F A or A P A C H E 2 scores.

- S P A R C: Studied in A R D S patients via B A L F. Involved in E C M deposition and remodeling associated with fibrotic progression.

- M M P 7: Studied in A R D S patients via serum. Involved in E C M degradation and inflammation; implicated in P F development.

- I V Col, H A, P I I I N P, L N: Studied in C O V I D 19 associated P F patients via serum. Markers of aberrant E C M deposition that correlate with H R C T scores.

- A C E 2: Studied in fibrosis models via B A L F. Shows proinflammatory and profibrotic effects correlating with P F development.

2. Imaging markers category:

- H R C T or Q C T: Studied in I L D or P F patients via C T images. Identifies structural changes like reticulation and honeycombing. Q C T parameters correlate with severity.

- F M T T: Studied in I P F patients via M R I images. Assesses tissue perfusion; fibrotic areas show hypointensity and late enhancement.

3. A I markers category:

- Composite A I predictive models: Studied in A L I or A R D S patients using multimodal data. Uses integrated risk analysis for early identification and risk stratification.

- A I imaging markers: Studied in P F or C O V I D 19 patients via H R C T images. Identifies sub-visual imaging patterns and subtle fibrosis-related features.

Imaging markers

Imaging techniques provide non-invasive access to detailed structural and functional information of lung tissue and have become essential tools in clinical practice for monitoring ARDS progression and guiding treatment (Ref. Reference Giovanazzi, Nocera, Catozzi, Collino, Cressoni, Ball, Moerer, Quintel, Camporota and Gattinoni102). Current evidence suggests that HRCT features indicative of fibrosis include irreversible reticulation, traction bronchiectasis and honeycombing (Ref. Reference Rodriguez, Ashby, Varela and Sharma103). Longitudinal assessment via HRCT and quantitative computed tomography (QCT) correlates with increasing severity and progression of fibrosis and is associated with physiological impairment as well as elevated risks of disease progression and mortality in patients with interstitial lung disease (Ref. Reference Humphries, Chung, Swigris, Oh, Walsh, Lynch, Goldin and Kim104). Interstitial abnormalities on HRCT may indicate early fibrotic interstitial lung disease, and predictive accuracy can be improved by incorporating prior imaging studies (Ref. Reference Brixey, Oh, Alsamarraie and Chung9). Studies in IPF patients have also revealed heterogeneous lobar volume and airflow distribution on HRCT, with volume reduction in the lower lobes. The root-mean-square difference of Q (a measure derived from lobe volume change relative to total lung volume change between the inspiratory and expiratory phases) effectively distinguishes IPF patients from healthy individuals (Ref. Reference Sul, Flors, Cassani, Morris, Reifman, Altes and Wallqvist10), indicating that certain HRCT abnormalities may serve as valuable tools for predicting or early detection of PF.

Furthermore, MRI has demonstrated potential in differentiating fibrotic lesions from inflammatory lesions. Fibrosis-predominant regions often exhibit hypointense signals and late enhancement, helping distinguish quiescent fibrotic areas from actively inflamed interstitial tissue (Ref. Reference Lonzetti, Zanon, Pacini, Altmayer, Martins de Oliveira, Rubin, Gazzoni, Barros and Hochhegger105). The first moment transit time (FMTT) derived from dynamic contrast-enhanced MRI has also been shown to differentiate IPF patients from controls (Ref. Reference Torres, Lee, Barton, Hahn, Sandbo, Schiebler and Fain106), suggesting that MRI may contribute to the identification of PF and holds promise for early detection of post-ALI/ARDS PF.

In addition, critical care ultrasound is widely used for assessing ARDS patients and guiding therapeutic decisions (Ref. Reference Liu, Inchingolo, Suryawanshi, Guo, Kurepa, Cortés, Yan, Chi, Acosta, Jagła, Sharma, Sorantin, Hsieh, Graziani, Malta, Woods, Meng, You, Kruczek, Kneyber, Buda, Smargiassi, Lovrenski, Ren, Guo, Qiu, Razak and Feletti107). Although few studies have focused specifically on its role in evaluating post-ALI/ARDS PF, its advantages – such as ease of use, portability and ability to perform repeated bedside examinations – make it a promising tool for fibrosis assessment as research in this area advances.

Although specific imaging criteria and quantitative indicators require further validation, imaging modalities – already integral to the clinical evaluation of ARDS – have significant potential for the assessment and monitoring of post-ALI/ARDS PF (Figure 3 and Table 1).

Novel artificial intelligence-based predictive markers

With the advancements in big data and artificial intelligence (AI), the use of these technologies to construct predictive models has become a major approach for forecasting the onset and progression of diseases (Ref. Reference Smit, Hagens, Heijnen, Pisani, Cherpanath, Dongelmans, de Grooth, Pierrakos, Tuinman, Zimatore, Paulus, Schnabel, Schultz, Bergmans and Bos108). AI enables the comprehensive analysis of large-scale, multidimensional indicators and factors through sophisticated statistical methods, thereby building prediction or classification models that overcome the limitations of single-indicator specificity and sensitivity (Ref. Reference Maddali, Churpek, Pham, Rezoagli, Zhuo, Zhao, He, Delucchi, Wang, Wickersham, McNeil, Jauregui, Ke, Vessel, Gomez, Hendrickson, Kangelaris, Sarma, Leligdowicz, Liu, Matthay, Ware, Laffey, Bellani, Calfee, Sinha, Rios, van Haren, Sottiaux, Lora, Azevedo, Depuydt, Fan, Bugedo, Qiu, Gonzalez, Silesky, Cerny, Nielsen, Jibaja, Pham, Wrigge, Matamis, Ranero, Hashemian, Amin, Clarkson, Bellani, Kurahashi, Villagomez, Zeggwagh, Heunks, Laake, Palo, do Vale Fernandes, Sandesc, Arabi, Bumbasierevic, Nin, Lorente, Larsson, Piquilloud, Abroug, McAuley, McNamee, Hurtado, Bajwa, Démpaire, Francois, Sula, Nunci, Cani, Zazu, Dellera, Insaurralde, Alejandro, Daldin, Vinzio, Fernandez, Cardonnet, Bettini, Bisso, Osman, Setten, Lovazzano, Alvarez, Villar, Milstein, Pozo, Grubissich, Plotnikow, Vasquez, Ilutovich, Tiribelli, Chena, Pellegrini, Saenz, Estenssoro, Brizuela, Gianinetto, Gomez, Cerrato, Bezzi, Borello, Loiacono, Fernandez, Knowles, Reynolds, Inskip, Miller, Kong, Whitehead, Bihari, Seven, Krstevski, Rodgers, Millar, Mckenna, Bailey, Hanlon, Aneman, Lynch, Azad, Neal, Woods, Roberts, Kol, Wong, Riss, Staudinger, Wittebole, Berghe, Bulpa, Dive, Verstraete, Lebbinck, Depuydt, Vermassen, Meersseman, Ceunen, Rosa, Beraldo, Piras, Ampinelli, Nassar, Mataloun, Moock, Thompson, Gonçalves, Antônio, Ascoli, Biondi, Fontenele, Nobrega, Sales, Shindhe, Ismail, Laffey, Beloncle, Davies, Cirone, Manoharan, Ismail, Goligher, Jassal, Nishikawa, Javeed, Curley, Rittayamai, Parotto, Ferguson, Mehta, Knoll, Pronovost, Canestrini, Bruhn, Garcia, Aliaga, Farías, Yumha, Ortiz, Salas, Saez, Vega, Labarca, Martinez, Carreño, Lora, Liu, Qiu, Liu, Tang, Luo, An, Zhao, Gao, Zhai, Ye, Wang, Li, Li, Zheng, Yu, Shen, Li, Yu, Lu, Wu, Huang, He, Lu, Han, Zhang, Sun, Wang, Qin, Zhu, Zhao, Liu, Li, Liu, Zhou, Li, Zhang, Li-Xin, Xin-Hua, Jiang, Gao, Zhao, Li, Li, Wang, Yao, Yu, Chen, Shao, Qin, Huang, Zhu, Hang, Hua, Li, Xu, di, Ling, Qin, Wang, Qin, Han, Zhou, Vargas, Silesky Jimenez, González Rojas, Solis-Quesada, Ramirez-Alfaro, Máca, Sklienka, Gjedsted, Christiansen, Nielsen, Villamagua, Llano, Burtin, Buzancais, Beuret, Pelletier, Mortaza, Mercat, Chelly, Jochmans, Terzi, Daubin, Carteaux, de Prost, Chiche, Daviaud, Pham, Fartoukh, Barberet, Biehler, Dellamonica, Doyen, Arnal, Briquet, Hraiech, Papazian, Follin, Roux, Messika, Kalaitzis, Dangers, Combes, Au, Béduneau, Carpentier, Zogheib, Dupont, Ricome, Santoli, Besset, Michel, Gelée, Danin, Goubaux, Crova, Phan, Berkelmans, Badie, Tapponnier, Gally, Khebbeb, Herbrecht, Schneider, Declercq, Rigaud, Duranteau, Harrois, Chabanne, Marin, Bigot, Thibault, Ghazi, Boukhazna, Ould Zein, Richecoeur, Combaux, Grelon, le Moal, Sauvadet, Robine, Lemiale, Reuter, Dres, Demoule, Goldgran-Toledano, Baboi, Guérin, Lohner, Kraßler, Schäfer, Zacharowski, Meybohm, Reske, Simon, Hopf, Schuetz, Baltus, Papanikolaou, Papavasilopoulou, Zacharas, Ourailogloy, Mouloudi, Massa, Nagy, Stamou, Kiourtzieva, Oikonomou, Avila, Cortez, Citalán, Jog, Sable, Shah, Gurjar, Baronia, Memon, Muthuchellappan, Ramesh, Shenoy, Unnikrishnan, Dixit, Rhayakar, Ramakrishnan, Bhardwaj, Mahto, Sagar, Palaniswamy, Ganesan, Mohammadreza Hashemian, Jamaati, Heidari, Meaney, Nichol, Knapman, O’Croinin, Dunne, Breen, Clarkson, Jaafar, Dwyer, Amir, Ajetunmobi, O’Muircheartaigh, Black, Treanor, Collins, Altaf, Zani, Fusari, Spadaro, Volta, Graziani, Brunettini, Palmese, Formenti, Umbrello, Lombardo, Pecci, Botteri, Savioli, Protti, Mattei, Schiavoni, Tinnirello, Todeschini, Giarratano, Cortegiani, Sher, Rossi, Antonelli, Montini, Casalena, Scafetti, Panarello, Occhipinti, Patroniti, Pozzi, Biscione, Poli, Raimondi, Albiero, Crapelli, Beck, Pota, Schiavone, Molin, Tarantino, Monti, Frati, Mirabella, Cinnella, Fossali, Colombo, Terragni, Pattarino, Mojoli, Braschi, Borotto, Cracchiolo, Palma, Raponi, Foti, Vascotto, Coppadoro, Brazzi, Floris, Iotti, Venti, Yamaguchi, Takagi, Maeyama, Watanabe, Yamaji, Shimizu, Shiozaki, Futami, Ryosuke, Saito, Kameyama, Ueno, Izawa, Okuda, Suzuki, Harasawa, Nasu, Takada, Ito, Nunomiya, Koyama, Abe, Andoh, Kusumoto, Hirata, Takaba, Kimura, Matsumoto, Higashijima, Honda, Aoki, Imai, Ogino, Mizuguchi, Ichikado, Nitta, Mochizuki, Hashida, Tanaka, Nakamura, Niimi, Ueda, Kashiwa, Uchiyama, Sabelnikovs, Oss, Haddad, Liew, Ñamendys-Silva, Jarquin-Badiola, Sanchez-Hurtado, Gomez-Flores, Marin, Villagomez, Lemus, Fierro, Cervantes, Mejia, Gonzalez, Dector, Estrella, Sanchez-Medina, Ramirez-Gutierrez, George, Aguirre, Buensuseso, Poblano, Dendane, Zeggwagh, Balkhi, Elkhayari, Samkaoui, Ezzouine, Benslama, Amor, Maazouzi, Cimic, Beck, Bruns, Schouten, Rinia, Raaijmakers, Heunks, van Wezel, Heines, Buise, Simonis, Schultz, Goodson, rowne, Navarra, Hunt, Hutchison, Bailey, Newby, Mcarthur, Kalkoff, Mcleod, Casement, Hacking, Andersen, Dolva, Laake, Barratt-Due, Noremark, Søreide, Sjøbø, Guttormsen, Yoshido, Aguilar, Oscanoa, Alisasis, Robles, Pasanting-Lim, Tan, Andruszkiewicz, Jakubowska, Cox, Alvarez, Oliveira, Montanha, Barros, Pereira, Messias, Monteiro, Araujo, Catorze, Marum, Bouw, Gomes, Brito, Castro, Estilita, Barros, Serra, Martinho, Tomescu, Marcu, Bedreag, Papurica, Corneci, Negoita, Grigoriev, Gritsan, Gazenkampf, Almekhlafi, Albarrak, Mustafa, Maghrabi, Salahuddin, Aisa, al Jabbary, Tabhan, Arabi, Trinidad, al Dorzi, Tabhan, Bolon, Smith, Mancebo, Aguirre-Bermeo, Lopez-Delgado, Esteve, Rialp, Forteza, de Haro, Artigas, Albaiceta, de Cima-Iglesias, Seoane-Quiroga, Ceniceros-Barros, Ruiz-Aguilar, Claraco-Vega, Soler, Lorente, Hermosa, Gordo, Prieto-González, López-Messa, Perez, Pere, Allue, Roche-Campo, Ibañez-Santacruz, Temprano, Pintado, de Pablo, Gómez, Ruiz, Moles, Jurado, Arizmendi, Piacentini, Franco, Honrubia, Perez Cheng, Perez Losada, Blanco, Yuste, Carbayo-Gorriz, Cazorla-Barranquero, Alonso, Alda, Algaba, Navarro, Cereijo, Diaz-Rodriguez, Marcos, Montero, Para, Sanchez, Blasco Navalpotro, Abad, Montiel González, Toribio, Castro, Artiga, Penuelas, Roser, Olga, Curto, Sánchez, Imma, Elisabet, Claverias, Magret, Pellicer, Rodriguez, Sánchez-Ballesteros, González-Salamanca, Jimenez, Huerta, Diaz, Lopez, Moya, Alfonso, Eugenio Luis, Cesar, Rafael, Virgilio, Recio, Adamsson, Rylander, Holzgraefe, Broman, Wessbergh, Persson, Schiöler, Kedelv, Tibblin, Appelberg, Hedlund, Helleberg, Eriksson, Glietsch, Larsson, Nygren, Nunes, Morin, Kander, Adolfsson, Piquilloud, Zender, Leemann-Refondini, Elatrous, Bouchoucha, Chouchene, Ouanes, Ben Souissi, Kamoun, Demirkiran, Aker, Erbabacan, Ceylan, Girgin, Ozcelik, Ünal, Meco, Akyol, Derman, Kennedy, Parhar, Srinivasa, McNamee, McAuley, Steinberg, Hopkins, Mellis, Stansil, Kakar, Hadfield, Brown, Vercueil, Bhowmick, Humphreys, Ferguson, Mckee, Raj, Fawkes, Watt, Twohey, Thomas, Morton, Kadaba, Smith, Hormis, Kannan, Namih, Reschreiter, Camsooksai, Kumar, Rugonfalvi, Nutt, Oneill, Seasman, Dempsey, Scott, Ellis, Mckechnie, Hutton, di Tomasso, Vitale, Griffin, Dean, Cranshaw, Willett, Ioannou, Gillis, Csabi, Macfadyen, Dawson, Preez, Williams, Boyd, de Gordoa, Bramall, Symmonds, Chau, Wenham, Szakmany, Toth-Tarsoly, Mccalman, Alexander, Stephenson, Collyer, Chapman, Cooper, Allan, Sim, Wrathall, Irvine, Zantua, Adams, Burtenshaw, Sellors, Welters, Williams, Hessell, Oldroyd, Battle, Pillai, Kajtor, Sivashanmugave, Okane, Donnelly, Frigyik, Careless, May, Stewart, Trinder, Hagan, Wise, Cole, MacFie, Dowling, Hurtado, Hurtado, Nuñez, Pittini, Rodriguez, Imperio, Santos, França, Ebeid, Deicas, Serra, Uppalapati, Kamel, Banner-Goodspeed, Beitler, Mukkera, Kulkarni, Lee, Mesar, Shinn Iii, Gomaa, Tainter, Mesar, Cowley, Yeatts, Warren, Lanspa, Miller, Grissom, Brown, Bauer, Gosselin, Kitch, Cohen, Beegle, Gueret, Tulaimat, Choudry, Stigler, Batra, Huff, Lamb, Oetting, Mohr, Judy, Saito, Kheir, Schlichting, Delsing, Elmasri, Crouch, Ismail, Blakeman, Dreyer, Gomaa, Baron, Grijalba, Hou, Seethala, Aisiku, Henderson, Frendl, Hou, Owens, Schomer, Bumbasirevic, Jovanovic, Surbatovic and Veljovic109). Studies on PF have demonstrated that AI can identify subtle patterns in medical images that are often imperceptible to the human eye, leading to the discovery of novel imaging biomarkers (Ref. Reference Nan, Xing, Wang, Tang, Felder, Zhang, Ledda, Ding, Yu, Liu, Shi, Sun, Cao, Zhang, Gu, Zhang, Gao, Wang, Tang, Yu, Kang, Chen, Lu, Zhang, Mamalakis, Prinzi, Carlini, Cuneo, Banerjee, Xing, Zhu, Mesbah, Jain, Mayet, Yuan, Lyu, Qayyum, Mazher, Wells, Walsh and Yang110). Similarly, research on post-ALI/ARDS PF suggests that integrating biological and imaging markers can improve the prediction of disease severity and prognosis (Ref. Reference Liu, Zhao, Wang and Gong111). By leveraging big data and AI modelling, it is possible to synthesise diverse patient characteristics – including general health status (Ref. Reference Maia, Cruz, de Oliveira, Samary, Fernandes, Trivelin, Rocha, Gama de Abreu, Pelosi, Silva and Rocco77), history of smoking and alcohol use (Ref. Reference Sueblinvong, Kerchberger, Saghafi, Mills, Fan and Guidot112), vital signs during the disease course (Ref. Reference Potla, Tulapurkar, Luzina, Atamas, Singh and Hasday79), mechanical ventilation parameters (Ref. Reference Murata, Ito, Furuya, Takahara, Naruse, Aso, Kondo, Sokabe and Hasegawa85), imaging manifestations (Ref. Reference Humphries, Chung, Swigris, Oh, Walsh, Lynch, Goldin and Kim104) and biochemical indicators – can be synthesised to more accurately predict the development and progression of post-ALI/ARDS PF. Furthermore, such models may uncover additional risk factors associated with post-ALI/ARDS PF, such as metabolic disorders (e.g. hypertension and diabetes) and markers of internal homeostasis (e.g. pH, lactate and electrolytes). These insights could ultimately guide clinical management strategies aimed at preventing the occurrence of post-ALI/ARDS PF (Figure 3 and Table 1).

Anti-inflammatory therapies

Inflammation is a key mechanism driving the development of post-ALI/ARDS PF. Consequently, targeting inflammatory pathways to mitigate fibrosis has long attracted significant research interest. Preclinical studies suggest that several agents – including tranilast (Ref. Reference Cui, Tang, Zhan, Deng, Lai, Zhu, Xin, Li, Chen and Tong113), the synthetic amphipathic helical peptide L-37pA (Ref. Reference Chernov, Telegin, Minakov, Kazakov, Rodionov, Palikov, Kudriaeva and Belogurov114), inhaled hydrogen gas (Ref. Reference Aokage, Seya, Hirayama, Nojima, Iketani, Ishikawa, Terasaki, Taniguchi, Miyahara, Nakao, Ohsawa and Naito115), bilirubin nanoparticles (BRNPs) (Ref. Reference Keum, Kim, Kim, Kim, Whang, Jung and Jon116), the phosphodiesterase-4 inhibitor roflumilast (Ref. Reference Milara, Morcillo, Monleon, Tenor and Cortijo117), curcumin (Refs Reference Avasarala, Zhang, Liu, Wang, London and London118, Reference Venkatesan119), antioxidants such as N-acetylcysteine (NAC) (Ref. Reference Chen, Guan, Quinn and Ouyang120) and dimethyl sulfoxide (DMSO) (Ref. Reference Eberlein, Scheibner, Black, Collins, Chan-Li, Powell and Horton121), and low-dose dexamethasone (Ref. Reference Wang, Zhou, Zhou, Rong, Gao and Xu11) – can attenuate inflammatory responses and fibroblast activation by suppressing the release of proinflammatory cytokines and chemokines and reducing oxidative stress, thereby inhibiting the progression of post-ALI/ARDS PF. The herbal formulation hochu-ekki-to (TJ-41) has been shown to ameliorate bleomycin-induced ALI in mice by correcting the Th1/Th2 imbalance – specifically through the suppression of Th2 polarisation – resulting in reduced cellular infiltration in lung tissue, improved preservation of pulmonary architecture and attenuated fibrosis (Ref. Reference Tajima, Bando, Yamasawa, Ohno, Moriyama, Terada, Takada, Suzuki, Gejyo and Sugiyama122). Similarly, hexarelin, a synthetic hexapeptide and growth hormone secretagogue, reduces immune cell infiltration in ARDS models, thereby suppressing inflammation and collagen deposition and slowing pulmonary remodelling and fibrotic development (Ref. Reference Zambelli, Rizzi, Delvecchio, Bresciani, Rezoagli, Molteni, Meanti, Cuttin, Bovo, Coco, Omeljaniuk, Locatelli, Bellani and Torsello123).

Further supporting the role of immunomodulation, studies in COVID-19 suggest that interleukin-10 (IL-10), known for its antiviral properties, may also inhibit PF by activating CD8⁺ T cells and natural killer cells, suppressing proinflammatory cytokine expression in macrophages and neutrophils and antagonising profibrotic cytokine signalling (Ref. Reference Shih, Yang, Liao, Lu, Hu and Lin124). Additionally, alamandine – a peptide within the renin–angiotensin system (RAS) – exerts anti-inflammatory and antifibrotic effects during SARS-CoV-2 infection via the Mas-related G protein-coupled receptor D (MrgD) (Ref. Reference Soltani Hekmat and Javanmardi125). These findings collectively highlight that dampening proinflammatory cytokine and chemokine release and reducing immune cell infiltration represent promising therapeutic strategies for alleviating post-ALI/ARDS PF.

However, not all anti-inflammatory interventions show broad efficacy. For example, methylprednisolone (MPS) failed to modulate the levels of pro- and anti-inflammatory cytokines/chemokines or mitigate lung tissue damage in a model of reovirus 1/L-induced ALI/ARDS (Ref. Reference London, Majeski, Altman-Hamamdzic, Enockson, Paintlia, Harley and London126), underscoring that the response to anti-inflammatory therapy may vary considerably depending on the aetiology of ALI/ARDS.

Clinical studies have also indicated that inhalation of sodium thiosulfate (STS) combined with a heated helium–oxygen mixture can inhibit viral replication in vivo, aid in the restoration of immune responsiveness and suppress the development of postinfectious PF (Ref. Reference Onikienko, Vinokurov, Yurinskaya, Zemlyanoi, Abkin, Shaykhutdinova, Palikov, Ivanov, Smirnova, Fedyakina, Bychkova, Zatsepina, Garbuz and Evgen’ev127). MPS treatment has been shown to attenuate the increase in the levels of PINP and PIIINP in both the plasma and BALF of ARDS patients, thereby improving lung injury and reducing multiple organ dysfunction syndrome scores (Ref. Reference Meduri, Tolley, Chinn, Stentz and Postlethwaite128). However, studies in patients with COVID-19 have reported that early use of the anti-inflammatory agent dexamethasone does not significantly affect the incidence of PF, disease progression or clinical outcomes (Ref. Reference Kooistra, Dahm, van Herwaarden, Gerretsen, Nuesch Germano, Mauer, Smeets, van der Velde, van den Berg, van der Hoeven, Aschenbrenner, Schultze, Ulas, Kox and Pickkers59), suggesting that the timing and target population for anti-inflammatory therapies in mitigating post-ALI/ARDS PF require further investigation. In summary, anti-inflammatory treatment represents a potential strategy for improving the prognosis of post-ALI/ARDS PF patients. Nevertheless, further detailed research is essential to determine how to tailor the choice of agent and timing of intervention on the basis of individual patient characteristics and the specific aetiology of ALI/ARDS in order to generate more clinically actionable evidence (Figure 4 and Table 2).

Figure 4.

Evolving therapeutic landscape for post-ALI/ARDS PF.

Current and emerging therapeutic strategies for post-ALI/ARDS PF, highlighting a multipronged approach. Anti-inflammatory therapies; 2. Antifibrotic agents; cell-based and extracellular vesicle (EV)-based therapies; gene-based and RNA-based therapies.

Figure 4. Long description

A central vertical red arrow points from a pair of healthy lungs labeled A L I / A R D S down to a pair of scarred lungs labeled Pulmonary fibrosis. Four panels, labeled A through D, connect to this timeline via dashed lines.

Panel A, top-left, titled Antifibrotic: Features a green header listing Pirfenidone and Nintedanib. Arrows show these agents entering a blood vessel lined with endothelial cells, where they interact with triangular molecules.

Panel B, bottom-left, titled Anti-inflammatory: Features a purple header listing Tranilast, L-37pA, Hydrogen, B R N P s, Roflumilast, Curcumin, N A C, D M S O, Dexamethasone, T J-41, Hexarelin, S T S plus Helium, Alamandine, and Methylprednisolone. Various geometric shapes and H sub 2 circles cross the vessel wall into the bloodstream.

Panel C, top-right, titled Cell and E V s: Features a tan header listing B M M S C s, D P S C s, U C M S C s, A S C s, M S C s-exos, A T-exos, E n C s / A E C I I-exos, and I M R C s. Diverse cell types and circular extracellular vesicles are shown migrating into the vascular space.

Panel D, bottom-right, titled Gene and RNA: Features a blue header listing Inhibiting Wnt beta-catenin, Suppressing P P 2 A, and P I 3 K Inhibitors. Inside the vessel, a P I 3 K s i R N A molecule and a P P 2 A molecule interact with D N A strands.

Table 2.

Investigational therapies for post-ALI/ARDS PF

Table 2. Long description

The table contains 21 rows of data across 7 columns.

* Tranilast (Synthetic compound): Tested in rats exposed to pine sawdust smoke; administered via intraperitoneal injection (100, 200, and 300 mg/kg); anti-inflammatory and antifibrotic; targets T G F-beta 1 down and N F-kappa B down; lacks validation in other A R D S models.

* L-37pA (Synthetic peptide): Tested in male I C R mice with L P S; intravenous injection (10 mg/kg); anti-inflammatory; targets unspecified.

* Hydrogen gas: Tested in mice with bleomycin; inhalation of 3.2% H sub 2, 20% O sub 2, balance N sub 2; anti-inflammatory and antioxidant; targets T G F-beta down.

* B R N P s (Bilirubin nanoparticles): Tested in mice with bleomycin; intravenous injection (20 mg/kg); anti-inflammatory and antioxidant.

* Roflumilast (P D E 4 inhibitor): Tested in mice with bleomycin; oral gavage (1 and 5 mg/kg/day); anti-inflammatory and antifibrotic; targets P D E 4 down.

* Curcumin (Natural polyphenol): Tested in mice (reovirus) and rats (paraquat); administered via intraperitoneal injection (50 mg/kg) or oral (300 mg/kg); anti-inflammatory and antioxidant; targets T G F beta 1 down, N F kappa B down, and p 38 up.

* N A C (Antioxidant): Tested in rats with high-tidal-volume ventilation; intravenous injection (140 mg/kg); antioxidant and anti-inflammatory.

* Dexamethasone (Corticosteroid): Tested in rats with L P S; injection (5 mg/kg); anti-inflammatory; clinical trials in C O V I D-19 showed no significant benefit on P F incidence.

* T J-41 (Herbal formulation): Tested in mice with bleomycin; diet admixed (1 g/kg/day); anti-inflammatory.

* Hexarelin (Synthetic hexapeptide): Tested in mice with H Cl; intraperitoneal injection; anti-inflammatory.

* S T S plus Helium-oxygen: Tested in C O V I D-19 patients; inhalation and nebulization; anti-inflammatory; targets T L R 4 down.

* M P S (Corticosteroid): Tested in A R D S patients; intravenous injection; anti-inflammatory.

* Pirfenidone (Antifibrotic agent): Tested in mice and rats with bleomycin or L P S; oral administration; antifibrotic; targets p-G S K-3 beta down, G S K-3 beta up, beta-catenin down, T G F-beta R down, P-smad 2/3 down, M M P-9 down, and T I M P-1 up.

* Nintedanib (Tyrosine kinase inhibitor): Tested in C O V I D-19 patients; oral or nasogastric tube; antifibrotic.

* Cell Therapies (B M M S C s, D P S C s, A S C s, U C M S C s): Tested in mice models; administered via instillation or tail vein injection; mechanisms include alveolar epithelial protection and anti-inflammatory; targets include Hippo signaling pathway down and M M P-2 up with T I M P down.

* Vesicle Therapies (M S C s-M V, M S C-exos, A T-exos, En Cs/A E C I I-exos): Tested in mice models; intravenous or tail vein injection; mechanisms include epithelial cell protection and anti-inflammatory; targets include beta-Catenin down, N F-kappa B down, N L R P 3 down, T G F-beta 1 down, Smad 2 down, and R G S 1 down.

Antifibrotic therapies

PF is a major cause of poor prognosis in patients with ARDS and a frequent long-term complication. Consequently, the development of antifibrotic therapies has attracted considerable research attention. Pirfenidone (PFN) and nintedanib are currently recognised antifibrotic agents approved for the treatment of IPF (Ref. Reference Lee and Song129). PFN has notable anti-inflammatory and antifibrotic properties. It acts primarily through the TGF-β signalling pathway to inhibit the accumulation and recruitment of inflammatory cells, EMT, fibroblast proliferation and ECM deposition, thereby attenuating PF (Refs Reference Al-Kuraishy, Batiha, Faidah, Al-Gareeb, Saad and Simal-Gandara130, Reference Zhang, Tan, Yong, Jiao, Tang and Wang131). Nintedanib, a multitarget tyrosine kinase inhibitor, acts on various growth factor receptors to suppress the release of profibrotic mediators and inhibit EMT, ultimately preventing the development of PF (Ref. Reference Li, Kao, Liu, Lin, Chen, Lee, Wang and Yang132).

Studies on post-ALI/ARDS PF suggest that PFN can modulate transforming growth factor activity, suppress the expression of proinflammatory and profibrotic factors, reduce ECM deposition, promote lung tissue repair and inhibit fibrosis (Refs Reference Lv, Wang, Xu, Huang, Ruan and Dai133, Reference Paik, Lee, Ko, Kim, Kang, An, Kim, Kim, Hwang, Jin, Kim, Cha and Choi134). Case reports in COVID-19 patients have also indicated that PFN may ameliorate post-COVID-19 PF and improve quality of life (Ref. Reference Zhou, Yang, Kong, Wei, LvQiu, Wang, Lin, Yin, Zhou and Luo135). Similarly, nintedanib has been shown to attenuate fibrosis progression, improve respiratory function and enhance exercise tolerance in COVID-19 survivors (Ref. Reference Ogata, Nakagawa, Sakoda, Ishimatsu, Taguchi, Kadowaki, Moriwaki and Yoshida136). However, some clinical studies have reported that PFN mitigates only cytokine storm-induced fibrotic progression in COVID-19 patients and does not prevent the development of fibrosis during the acute phase of the disease. Additionally, it is associated with certain adverse effects, including hepatotoxicity and gastrointestinal symptoms (Ref. Reference Boshra, Abou Warda, Sayed, Elkomy, Alotaibi, Mohsen and Sarhan137). An ongoing multicentre, randomised, double-blind clinical trial is evaluating the efficacy of PFN in preventing post-ALI/ARDS PF, which may provide new evidence for antifibrotic strategies in these patients (Ref. Reference Monti, Marzaroli, Tucciariello, Ferrara, Meroi, Nakhnoukh, Zambon, Borghi, Guarracino, Manazza, Ajello, Belletti, Biuzzi, Plumari, Filippini, Cuffaro, Racanelli, Pontillo, Rauch, Oliva, Tescione, Baiardo Redaelli, Melegari, Maj, Navalesi, Gerardi, Caccioppola, Bruni, Ballotta, Ferri, Orso, di Benedetto, Baldassarri, Franceschini, Alamami, Pasin, Putzu, Romero Garcia, Chen, Noto, Yavorovskiy, Hajjar, Cortegiani, Likhvantsev, Konkayev, Finco, Sales, Brazzi, Paternoster, Bellomo, Zangrillo, Landoni, di Tomasso, Bonizzoni, Borello, Chiodi, D’Amico, de Piccoli, Fiorito, Fresilli, Giardina, Labanca, Lembo, Licheri, Losiggio, Marmiere, Nardelli, Notarianni, Nutta, Oriani, Ortalda, Pieri, Pruna, Ronca, Scquizzato, Turi, Vietri, Tomasi, Veneziano, Asiller, Azimova, Kabibulatov, Doroni, Villano, Berikashvili, Kadantseva, Piva, Carmela Izzi, Milano, Sannicandro, Petrun, Montrucchio, Sanna, Simonetti, Ollosu, Sardo, Bresssan, D’Andrea, Lugano, Roveri, Seraglio, Ippolito, Catalisano, Perna, Farinaccio, Flaminio, Scaini, Corradi, Cucciolini, Scopelliti, Vadalà, Ricciardi, Taniguchi, Mendes, Krzych and Cabrini12). Furthermore, clinical studies on nintedanib in COVID-19 patients have demonstrated its potential to reduce lung injury and shorten the duration of mechanical ventilation (Ref. Reference Umemura, Mitsuyama, Minami, Nishida, Watanabe, Okada, Yamakawa, Nochioka and Fujimi138), highlighting its therapeutic benefits.

In summary, both PFN and nintedanib are established antifibrotic agents with promising potential for treating post-ALI/ARDS PF. However, further large-scale multicentre clinical studies are needed to generate high-quality evidence supporting their clinical application (Figure 4 and Table 2).

Cell-based and EV-based therapies

Cell and EV-based therapies represent emerging treatment approaches for ALI/ARDS. These therapies exert protective effects by modulating immune and inflammatory cells, facilitating paracrine cytokine signalling, releasing beneficial exosomes, mitigating endoplasmic reticulum stress and attenuating PF (Ref. Reference Wang, Lei, Jiang, Gao, Hu, Zhao, Niu and Zhao139). Studies related to COVID-19 suggest that mesenchymal stem cells (MSCs) or their derived exosomes can promote angiogenesis and alveolar re-epithelialisation; reduce the levels of markers such as TNF-α, TGF-β, COL1A1 and COL3A1; inhibit myofibroblast proliferation; and enhance endothelial cell survival, thereby alleviating or even reversing PF (Ref. Reference Javed, Karki, Sami, Khan, Shree, Sah, Ghosh and Saxena140). Research on post-ALI/ARDS PF has demonstrated that various stem cell types – including bone marrow-derived MSCs (BMMSCs) (Ref. Reference Li, Dong, Zhang, Gao, Hui and Yan141), dental pulp stem cells (DPSCs) (Ref. Reference Geng, Zhang, Zhang, Dong, Yang, Zhu, Wu and Wang142), adipose-derived stem cells (ASCs) (Ref. Reference Jung, Park, Huh and Hong143) and umbilical cord MSCs (UCMSCs) (Refs Reference Moodley, Atienza, Manuelpillai, Samuel, Tchongue, Ilancheran, Boyd and Trounson144, Reference Xiao, Liu, Wang, Hou, Shi, Qian, Zhang, Deng and Xie145) – can suppress inflammatory responses and collagen deposition, promote collagen degradation and ameliorate fibrosis resulting from diverse insults such as bleomycin, LPS, paraquat and aspiration (Ref. Reference Mauri, Zambelli, Cappuzzello, Bellani, Dander, Sironi, Castiglioni, Doni, Mantovani, Biondi, Garlanda, D’amico and Pesenti146). These findings indicate that despite the heterogeneity in ALI/ARDS aetiology, stem cells – owing to their multipotent differentiation capacity and multifaceted mechanisms – have broad applicability across different fibrotic pathways. Clinical applications in COVID-19 have further shown that MSC administration improves gas exchange in patients, underscoring the potential of MSCs in treating ARDS and preventing PF (Ref. Reference Gazzaniga, Voltini, Carletti, Lenta, Meloni, Briganti, Avanzini, Comoli and Belliato147).

Notably, studies on BMMSCs revealed that compared with those cultured for 9 days, those cultured for 2 hours were smaller, exhibited greater proliferative capacity and expressed elevated levels of stem cell markers and chemokine receptors. Transplantation of these early-adherent BMMSCs resulted in comparable or superior amelioration of the inflammatory response, fibrotic lung pathology and mortality reduction (Ref. Reference Kumamoto, Nishiwaki, Matsuo, Kimura and Matsushima148), suggesting that stem cell properties can be influenced by culture conditions. Further research is needed to optimise cell sources and cultivation protocols for maximal therapeutic benefit.

EVs are nanoscale membrane-bound vesicles released into the extracellular space, which facilitate intercellular communication by transferring bioactive molecules and signals between cells or organs. These compounds represent promising therapeutic strategies for treating lung diseases (Ref. Reference Ding, Zhou, Ren, Li, Xiang, Li, Yu, Hong, Fu, Li, Pan and Liu149). Studies in the context of COVID-19 have indicated that EVs can attenuate inflammatory responses, promote AEC regeneration, restore microvascular permeability and reduce fibrin production, thereby preventing PF, highlighting their potential in treating COVID-19-associated ALI (Ref. Reference Khalaj, Figueira, Antounians, Lauriti and Zani150). Research on post-ALI/ARDS PF has demonstrated that MSC-EVs (Refs Reference Zhang, Ye, Tang, Ji, Zheng, Chen, Ge and Huang30, Reference Chen, Chen, Wu, Chen, Zhan, Wei, Ouyang, Jiang, Shen, Luo, Liu, Zhou and Qin151), adipose tissue-derived exosomal (AT-exos) miR-122-5p (Ref. Reference Qi, Deng, Chen, Fan, Peng, Tang, Wang and Yu78) and vascular endothelial cells (EnCs)- and type II AEC-derived exosomes (EnCs/AECII-exos) containing miR-223 and miR-27b-3p (Ref. Reference Feng, Zhou, Liu, Xia, Li, Yan, Chen, Chen, Jiang, Chao, Wang, Zhou, Zhang, Wang and Xia152) can modulate macrophage immune balance and inflammatory pathway activation. These mechanisms collectively alleviate inflammatory responses and inhibit EMT, thereby attenuating the development of post-ALI/ARDS PF. Furthermore, exosomes derived from 4 T1 cells have been shown to serve as effective carriers for resveratrol, enabling targeted delivery to the lung and significantly suppressing the expression of α-SMA and vimentin in mouse models. This approach effectively treats ARDS and inhibits PF, whereas resveratrol alone fails to achieve comparable effects (Ref. Reference Yang, Zhu, Jin, Wang, Song, Sun, Liu, Yin, Wang, Wu, Liu, Zhang, Zhao and Yu153). These findings suggest that in addition to their intrinsic cargo with anti-inflammatory and antifibrotic properties, EVs can also function as drug delivery vehicles, enhancing targeted therapy for injured lung tissues.

In terms of safety, human embryonic stem cell-derived immune and stromal regulatory cells highly express genes related to proliferation, immunomodulation and antifibrotic activity. Intravenous delivery of immunity- and matrix-regulatory cells (IMRCs) has been shown to suppress pulmonary inflammation and fibrosis in mouse models, demonstrating notable efficacy and an excellent safety profile in both mouse and non-human primate studies (Ref. Reference Wu, Song, Li, Guo, Xiao, Liu, Liang, Feng, Gao, Chen, Li, Wang, Wen, Yang, Liu, Wang, Wang, Peng, Stacey, Hu, Feng, Li, Huo, Jin, Shyh-Chang, Zhou, Wang, Hu, Dai and Hao154).

In summary, cell- and EV-based therapies show considerable promise for the treatment of post-ALI/ARDS PF, with encouraging efficacy and safety profiles. However, further research is needed to optimise the selection of cell sources, EV isolation methods, culture conditions, timing of administration and delivery strategies to maximise their clinical potential (Figure 4 and Table 2).

Gene-based and RNA-based therapies

Advances in genetic engineering and RNA interference technologies have enabled the modulation of pathological processes through pharmacological agents, small interfering RNAs, short hairpin RNAs and microRNAs, offering promising therapeutic options for respiratory diseases (Refs Reference Niu, Xu, Li, Sun and Liang155, Reference Zhang, Lu, Xie, Liu, Cun and Yang156). Studies on post-ALI/ARDS PF have demonstrated that inhibiting Wnt/β-catenin signalling (Ref. Reference Chen, Shi, Meng, Zhang, Li, Wang, Xiang, Hu and Han157), suppressing protein phosphatase 2A (PP2A) expression in bone marrow-derived macrophages (Ref. Reference Sun, Hult, Cornell, Kim, Shanley, Wilke, Agarwal, Gurczynski, Moore and Dahmer158) or targeting the PI3K signalling pathway (Ref. Reference Margaria, Moretta, Alves-Filho and Hirsch159) can attenuate inflammatory and fibrotic responses, thereby ameliorating fibrosis. These findings suggest that regulating protein expression through gene editing or RNA-based interventions represents a potential strategy for treating post-ALI/ARDS PF.

However, the clinical translation of gene editing and RNA interference technologies remains limited. Key concerns include the risk of unintended systemic effects owing to off-target genomic modifications, ethical issues related to alterations in the human genome and uncertainties regarding long-term safety. Future research should prioritise the development of safe, precise and efficient delivery systems that enable targeted gene modulation at disease sites without affecting healthy tissues (Figure 4).