Interstitial lung disease (ILD) is a heterogeneous disease characterized by gradual decline in lung function due to impaired gas exchange and decreased lung compliance. Among the various types of ILD, idiopathic pulmonary fibrosis (IPF) is the most common and the largest lethal group of fibrotic ILD[1]. Pulmonary fibrosis (PF) is the final outcome of various types of ILD, characterized by the excessive accumulation of scar tissue in the lung parenchyma. IPF is a progressive, no clear etiological fibrotic respiratory disorder with poor prognosis[2]. Patients diagnosed with IPF may encounter a sudden, life-threatening event termed an acute exacerbation of IPF (AE-IPF) that firstly documented in 1993 and was recognized as the onset of acute lung injury/acute respiratory distress syndrome (ALI/ARDS)[3]. These acute exacerbations arise in about 4 % to 20 % of cases, significantly impacting life expectancy to around 3 to 4 months[4], [5]. Additionally, AE-IPF patients had a poor outcome, with an overall short-term mortality rate exceeding 50 %, necessitating ventilator as high as 90 % to 100 %[6]. Meanwhile, AE‐IPF-related deaths make up 46 % of all fatalities among patients diagnosed with IPF[7]. As of now, little effective prophylactic treatments for AE-IPF patients are available, however, high-dose steroids have been recommended for AE-IFP therapy despite their unverified benefits[8]. Thus, there is an urgent need to further unveil the potential mechanism of AE-IPF pathobiology to identify suitable targets for intervention.

Basic animal model researches serve as a vital link between patients and laboratory bench. Although IPF is a complex and progressive disease and animal models don’t fully mirror physiologic findings of IPF, they do enable mechanistic investigations relevant to fibrogenesis[9]. For instance, there is a consensus regarding the pathological changes characteristics of AE-IPF i.e., ALI/ARDS occurs on the basis of established pulmonary fibrosis between patients and animal research[10]. Recently, several studies have indicated that common triggers for acute exacerbations of IPF in patients include respiratory infection, gastroesophageal reflux, exposure to air pollution, invasive operations, and medications[11], [12]; However, it is increasingly recognized that infections are the most common trigger for acute exacerbations in IFP patients. Bacterial infections, including both gram-positive and gram-negative bacteria, along with fungal infections, are recognized as potential contributors to acute exacerbations in IPF patients[13]. Most of bacteria were Gram-negative in the sputum of AE-IPF, only about 10 % were Gram-positive.[14] Meanwhile, extensive studies animal models demonstrated that endotoxin, particularly LPS that derived from the cell walls of gram-negative bacteria[15], is considered a pivotal pathogen implicated in the onset of acute exacerbation in pre-existing pulmonary fibrosis mice model[16], [17], [18]. Its effects include the recruitment of inflammatory cells into the lungs, leading to subsequent rises in capillary permeability and the accumulation of fluid in the alveoli (alveolar edema) and histology shows diffuse alveolar damage or organizing pneumonia plus usual interstitial pneumonitis[19].

Numerous existing studies reveal that comprehending the importance of repetitive alveolar epithelial cells (AECs) injuries, excessive pulmonary alveolar inflammatory responses, and increased deposition of extracellular matrix proteins are crucial steps in the development of AE-IPF[13], [20], [21]. For example, alteration of alveolar epithelial barrier function, leads to the influx of protein-rich fluid into alveolar spaces, further enhances inflammatory responses and subsequent devastates lung fibrosis[22]. This cascade results in reduced lung function, disrupted air exchange, and eventual respiratory failure and mortality[23]. Although the destruction of alveolar epithelial barrier integrity contributes to the onset and advancement of AE-IPF, the precise molecular mechanism remains ambiguous. Thus, further investigating the underlying molecular mechanism to AE-IPF by targeting alveolar epithelial barrier integrity is of great importance.

Colony-stimulating factor 3 (CSF3), also termed granulocyte colony-stimulating factor (G-CSF), functions as a hematopoietic growth factor that specifically stimulates the production and maturation of neutrophils in the bone marrow[24]. However, akin to various cytokines, the impacts of CSF3 extend beyond merely influencing neutrophil function. For instance, treating with CSF3 can result in elevated levels of circulating lymphocytes and monocytes and trigger increased concentrations of γ-glutamyl transferase, alkaline phosphatase, interleukin-6 (IL-6), and interleukin-1β (IL-1β)[25], [26]. Additionally, CSF3 is the most upregulated gene after SARS-CoV2-infected genes, suggesting that CSF3 is a significant target after infection and it may also be a potential target for drug therapy[27]. Another bioinformatics analyses reveal that EGOT and CSF3 are the only two elevated all 3 cell lines upon SARS-CoV2 infection, which seems related to lung epithelial barrier junction modulations[28]. Indeed, the expression of CSF3 in neutrophils can compromise the function of the alveolar-capillary barrier during influenza infection[29]. AE-IPF patients exhibited similar pulmonary pathophysiological features to those of SARS-CoV-2-infected patients[30]. Thus, it’s reasonable to hypothesize that CSF3 involves in acute exacerbation of IPF.

In the present study, we established a murine model of acute exacerbation of pulmonary fibrosis by BLM injection and subsequent LPS administration. Transcriptomic profiling identified CSF3 as one of the top 10 upregulated DEGs in AE-PF mice. Additionally, administration of exogenous CSF3 protein exacerbated AE-PF in mice. Mechanistically, CSF3 disrupted alveolar epithelial barrier integrity and permeability by regulating tight junctions (TJs) and adherens junctions (AJs) via PI3K/p-Akt/Snail pathway, contributing to the aggravation of AE-PF in mice. Finally, a notable increase in CSF3 levels of IPF patients during the exacerbation of the disease. Sera CSF3 levels in IPF patient positive associations with and KL-6 levels, LDH levels, CRP levels, respectively. The insights from our study could pave the way for identifying suitable targets to manage AE-IPF.