Idiopathic pulmonary fibrosis (IPF) is a serious and complex lung disease characterized by repeated skin cell injury, fibroblast activation and a large amount of ECM deposition ultimately lead to lung function progressive loss, patients may even die from respiratory failure [21]. The prognosis of IPF is extremely poor [22]. Therefore, it is of great significance to clarify the pathogenesis of IPF, explore new treatment strategies and establish a prognosis model. In the process of efferocytosis, phagocytic cells will swallow apoptotic cells and form a large vacuole containing dead cells, which is called "efferocytosis body" [23]. At the early stage of apoptosis, the redistribution of phosphatidylserine (PS) to the outer leaves of the plasma membrane is a key signal for phagocytes to recognize apoptotic cells [24]. Recently, several receptors have been identified to recognize PS and thus mediate efferocytosis. The lack of these receptors will lead to the deterioration of inflammation and autoimmune diseases [25]. Efferocytosis can trigger signal transduction pathways downstream of the cell, such as anti-inflammatory, anti-protease and growth promotion effects. However, the damage of the mechanism of efferocytosis will lead to autoimmune diseases and tissue damage. Diseases related to the impairment of efferocytosis function include cystic fibrosis, bronchiectasis, chronic obstructive pulmonary disease, asthma, idiopathic pulmonary fibrosis, rheumatoid arthritis, lupus erythematosus, glomerulonephritis and atherosclerosis [26].
However, no bioinformatics analysis has been carried out on the genes related to IPF efferocytosis. In this study, we first screened DEGs between IPF samples and normal samples in GSE70866 data set, including 3543 differentially up-regulated genes and 221 differentially down-regulated genes. Among the top 10 up-regulated genes, some of them have been confirmed to be consistent with the results of this analysis, and some of them have not been studied, indicating that these genes may be new research targets for the pathogenesis of IPF. For example, SPP1 is up-regulated in the lower lung of IPF (late IPF) [27], CYTL1 has the function of promoting fibrosis and angiogenesis [28, 29], MMP7 promotes fibrosis [30, 31], and CAMP can inhibit IPF fibrosis [32]. After crossing DEGs with ECRGs, 18 IPF DEECRGs were obtained.
Then GO and KEGG enrichment analysis was carried out on these 18 DEECRGs to study the potential biological functions. GO and KEGG pathway analysis showed that these DEECRGs were mainly concentrated in “protein kinase B signal transduction”, “cytokine receptor binding”, and “interaction of viral proteins with cytokines and cytokine receptors”.
In order to further evaluate the prognostic significance of these genes, we used univariate Cox regression and Lasso regression to screen five genes related to the survival and prognosis of IPF patients from 18 potential genes related to efferocytosis, namely CXCR4, ODC1, AXL, DOCK5, MERTK. The overexpression of CXCR4 is related to the early death of IPF patients. The overexpression of CXCR4 is related to the early death of IPF patients. They recruit circulating fibroblasts (CFs) to the lungs and release extracellular matrix protein, collagen I and transforming growth factor β 1. Promote the proliferation of fibroblasts and their differentiation into myofibroblasts, thus promoting pulmonary fibrosis [33, 34]. In addition, CXCR4 can be down-regulated by small molecular antagonists (AMD3100, AMD070, BL−8040), among which AMD3100 has recently been proved to reduce pulmonary fibrosis after exposure to bleomycin (BLM) and paraquat [33, 35]. Ornithine decarboxylase (ODC1) metabolizes l-ornithine to polyamines, is a rate-limiting enzyme involved in polyamine metabolism, and is involved in cell differentiation, proliferation and migration [36]. In 2016, Lo H. C, Hardpower DM and others believed that ODC1 could cause pulmonary vasoconstriction, and then lead to capillary inflammation and leakage, leading to systemic inflammation and alveolar injury, and ODC1 could be up-regulated during bacterial infection [37, 38]. However, a document published in Nature in September 2022 said that ODC1 could inhibit the production of proinflammatory cytokines, reduce inflammatory cell death, and make it a potential therapeutic target for inflammatory diseases [39]. As the pathogenesis of IPF varies, so far, it is not known whether ODC1 plays a positive or negative role in IPF. However, the above evidence has shown that ODC1, as one of the ECRGs, may be related to inflammatory reaction and can be used as a new research target for the pathogenesis of IPF. AXL is a member of the TAM family. It is noteworthy that the Univariate COX regression results in this study show that only the HR of AXL is less than 1, which means that the high expression of AXL may delay the IPF process and reduce mortality. In 2014, Fujimori T and other scholars believed that AXL usually showed high expression on healthy airway macrophages, and inflammatory stimulation would induce high expression of AXL, while AXL led macrophages to phagocytosis of apoptotic cells (efferocytosis) under inflammatory conditions, which was the key step of inflammation regression [40]. However, there are 2021 literatures that believe that AXL is a smoke reaction molecule, which may interact with activated MARCKS to form a fibrosis molecular complex to drive the invasion of lung fibroblasts, thus aggravating the disease [41]. Not only that, AXL also shows high expression in various human malignant tumors, and AXL targeted drugs have also played a good therapeutic effect in cancer [42]. Therefore, whether AXL plays a positive role in delaying the IPF process needs further research and verification. There are few studies on Dock5 in respiratory diseases. It is known that Dock5 is highly up-regulated in the proliferative phase of wound repair and can affect the function of extracellular matrix (ECM) deposition [43]. Because the pathogenesis of IPF is related to ECM deposition, we speculate that the high expression of Dock5 may aggravate IPF, which can be used as a new target for further research. MerTK is a receptor of the complex of Gas6 or protein S and phosphatidylserine, which is often highly expressed on macrophages [44]. Blocking Gas6 can inhibit the marker of fibroblast activation [45]. The results of in vitro experiments also show that MERTK is highly expressed in IPF lung tissue [46]. In clinical studies, MERTK inhibitors may significantly reduce the activation of pro-fibrotic macrophages and fibroblasts in IPF lung [47]. Therefore, MERTK can promote IPF.
Lasso regression was used to construct a risk model based on five genes, and the patients in the training set and the validation set were divided into high risk group and low risk group with the median risk score as the critical value. K-M survival curve and ROC curve show certain prediction feasibility.
In order to analyze the different signal pathways and biological functions of the two risk groups, we used GSVA to explore the potential mechanism of how DEGs affect the prognosis of IPF. The full name of GSVA is Gene set variation analysis, which is a non-parametric and unsupervised algorithm. GSVA quantifies the results of gene enrichment, which can be more convenient for subsequent statistical analysis. After that, based on the grouping information, the difference analysis of GSVA results can find gene sets with significant differences between samples, which is more biological and more interpretable. We found that some pathways, such as tnfsf11 signal pathway and diall bacterial lipopeptide, were significantly enriched in IPF patients with high risk scores. Research has shown that TNFSF11 is also known as the nuclear factor- κ B ligand (RANKL) can reduce cell death, inhibit cell burial, and promote the proliferation and repair of lung epithelium. Therefore, we speculate that this pathway may be related to IPF caused by excessive proliferation and repair of lung epithelium[48].
In order to accurately evaluate the composition of immune cells in the microenvironment of diseased tissues, the immunoinfiltration analysis was carried out, and the abundance of M0 macrophages and monocytes accounted for the largest proportion. In addition, the infiltration degree and expression level of immune cells in the two groups were also different. In the high-risk group, activated mast cells (MCs) are higher, and MCs activate TGF-β Induce the differentiation of fibroblasts into myofibrin cells [49, 50], while in the low-risk group, immature B cells, resting dendritic cells, activated NK cells, M0 macrophages, and resting mast cells are abundant. Among them, mature B cells can promote the inflammation and fibrosis changes in IPF patients [51], and NK cell dysfunction may develop serious fibrotic lung disease [52]. As a professional phagocyte, M0 macrophages have high abundance in the low-risk group of IPF, indicating that the damage of efferocytosis aggravates IPF.
At the end of this study, our multivariate Cox analysis included age and sex as variables to predict the survival probability of IPF patients. As we all know, IPF can be seen all over the world, and it is more common in men than in women. Old age is one of the most important risk factors for IPF. Its incidence rate and prevalence rate increase significantly with age. Two thirds of IPF patients are more than 60 years old at the time of onset, and the average age at the time of diagnosis is 66 years old. Among people over 65 years old, the estimated prevalence rate may be as high as 400 cases per 100000 people [53, 54]. Toren et al found that longevity gene can resist IPF gene through animal experiments, and confirmed that age is an important risk factor for IPF [55].
In the construction of a prognostic model for predicting the survival of IPF patients, the expression trend of five model genes in the training set and the external validation set GSE10667 was compared. The results showed that four of the five genes had the same trend. In addition, the model genes that make up the risk score can also be used as the basis and direction of the next research.
Due to the poor prognosis, high variability and unpredictability of IPF progression, effective risk classification and treatment strategies are needed to develop personalized and targeted treatment. Compared with earlier research, our research is based on bioinformatics technology, which links the pathogenesis of IPF with the role of efferocytosis, which is innovative in IPF-related research so far and provides a good theoretical basis for IPF research.
However, our research also has several limitations. First, our results are based on bioinformatics analysis, so we need further experimental verification. Second, the data used for the study was retrieved from the public database, because there are not enough IPF patients in our clinical practice. Therefore, more detailed investigation is needed in the future.