Some DM patients are insidious and frequently present to the respiratory department with pulmonary symptoms, making early diagnosis of DM-ILD difficult based on clinical symptoms, signs, and laboratory markers alone, which significantly impacts the prognosis and outcome of the disease [2]. Therefore, an active search for new markers of DM-ILD is urgent.
To the best of our knowledge, this is the first study to examine gene expression profiles common to DM and ILD at the transcriptome level. We found that the expression of SLAMF7, SPP1, TDO2, and VCAM1 was higher in tissue in both DM and ILD patients than in the corresponding normal tissue, and had some diagnostic value for the disease. Subsequently, GO and KEGG pathway enrichment analysis showed that these genes might affect the body’s immune response through the regulation of signalling pathways such as the PI3K-Akt signaling pathway or MAPK signaling pathway, leading to the development of the disease. Through further validation, we likewise found that the expression of SLAMF7, SPP1, TDO2, and VCAM1 was associated with the infiltration of activated CD4 memory T cells and M1 macrophages in DM and ILD. In addition, we found that 21 and 3 TFs may regulate the expression of these genes in lung and muscle tissues, respectively. Moreover, CTCT and SPI1, which are both TFs, have been shown to be involved in the regulation of the expression of common DEGs in the lung and muscles. Taking into account the diverse results and complexity of each of the 4 common DEGs, we will focus on the main analytical findings, potential clinical relevance, and current findings of each.
Signaling lymphocytic activation molecule (SLAM) family member 7 (SLAMF7) gene is located on chromosome 1, has 8 exons, and is an I type transmembrane protein. It expresses a subpopulation of myeloma cells and immune cells, capable of participating in the body's immune response[21]. It was found that in a mouse model of multiple sclerosis, the immune cell receptor SLAMF7 was expressed on various subsets of immune cells in the central nervous system (CNS) and that its lack of expression could activate specific subsets of B cells and T memory cells, thereby regulating the susceptibility of the CNS to autoimmunity leading to the development of disease[22]. Also, an analysis of RNA-seq data from the synovial macrophages of people with rheumatoid arthritis showed a close link between SLAMF7 and macrophage activation. This link was also seen in the intestinal macrophages of people with Crohn's disease and in the lung macrophages of people with severe COVID-19[23], consistent with our findings in DM and ILD that common DEGs may contribute to disease development by inducing infiltration of M1 macrophages. Although there are no results from studies correlating SLAMF7 with DM and ILD, this common potential biomarker may be a cue for immune cell infiltration, i.e., SLAMF7 expression may lead to the development of DM-ILD by inducing an immune response.
Secreted phosphoprotein 1 (SPP1) is involved in osteoclast attachment to mineralized bone matrix and is a cytokine that upregulates interferon-γ and interleukin-12 expression[24]. Interferon-γ signaling was upregulated in SPP1hi macrophages, cytotoxic T cells, and natural killer cells in idiopathic pulmonary fibrosis, whereas type I interferon signaling and production were upregulated in the corresponding systemic sclerosis-associated interstitial lung disease population. In addition, gene expression changes in normal macrophages during the transition to SPP1 macrophages may be one of the reasons for systemic sclerosis-associated interstitial lung disease[25]. In addition, the expression of SPP1 in bronchoalveolar lavage fluid of ILD patients was significantly higher than that of controls, and it regulated the occurrence and progression of ILD by affecting the expression of COL1A1[26]. Although SPP1 has not been reported in DM, to some extent, it is difficult to conclude the correlation between the abnormal expression of SPP1 in the two diseases. However, the above findings of systemic sclerosis-associated interstitial lung disease and our analysis may offer new insights into the pathogenesis of DM- ILD.
Tryptophan 2,3-dioxygenase (TDO2) encodes a haemoglobinase that plays a crucial role in tryptophan metabolism[27]. In vitro and in vivo assays found that TDO2 increased intracellular tryptophan metabolism levels in the kynurenine (Kyn) pathway and that the increase in the tryptophan metabolite Kyn led to sustained proliferation of glioma cells through AhR/AKT pro-survival signalling and immunosuppressive effects[28]. Similar to SLAMF7, no studies have reported a role for TDO2 in DM or ILD. However, in other diseases, such as hepatocellular carcinoma, it has been found that TDO2 expression contributes to the secretion of interleukin-6 (IL-6), which promotes tumour cell proliferation through STAT3 and NF-kB/TIM4 signalling[29]. In osteoarthritis patients, TDO2 levels were significantly and positively correlated with IL-1β and TNF-α levels, suggesting that high levels of TDO2 in the synovium may correlate with pro-inflammatory cytokines and the severity of osteoarthritis[30]. Among these, IL-6 plays a vital role in the inflammatory process, and studies have found higher levels of IL-6 in DM patients than in healthy populations[31]. Gono et al. also demonstrated higher serum IL-6 levels in DM patients, particularly DM-ILD. The IG superfamily gene vascular cell adhesion molecule 1 (VCAM1) has been reported to play an essential role in the development of systemic sclerosis-associated interstitial lung disease[32]. Although systemic sclerosis and DM are distinct diseases, they share similar pathogenesis. Furthermore, serum VCAM1 levels were significantly higher in DM-ILD patients compare to patients without ILD, suggesting that VCAM1 might be used as a biomarker to determine the severity of DM-related lung disease[33]. Our study found that high expression of VCAM1 was associated with infiltration of activated CD4 memory T cells in both DM and ILD, suggesting an essential role for activated CD4 memory T cells in DM-ILD. However, this remains a speculative hypothesis that requires further validation.
In conclusion, our bioinformatics and clinical data analysis revealed 4 potential biomarkers associated with immune regulation in the interconnection between DM and ILD. We found that common DEGs were associated with immune cell infiltration in both diseases, which supports the possibility that common DEGs may contribute to DM-ILD through modulation of the body's immune system. In addition, we found a decrease in the absolute number of CD8 + T cells and an increase in CD4+/CD8 + T cells in DM-RP-ILD serum, suggesting that the body is in a state of immune dysfunction, which should be further analysed for association with common DEGs and needs to be verified in a large sample. The inferences that have been made from the data are obviously hypotheses that need to be tested further. In addition, the specific mechanisms of common DEGs in the development of both diseases need to be further elucidated and are currently thought to be related to the PI3K-Akt signaling pathway or MAPK signaling pathway. In addition, it is unclear which gene is most relevant in the onset of the process between DM and ILD. A complex interplay between all these mechanisms appears to be the cause of the overlap between the two diseases. we have also explored the TFs that may regulate common DEGs in DM and ILD. This integrated bioinformatics approach has been shown to be reliable in various diseases. The present study will provide potential directions for the molecular mechanisms of DM-ILD.
Our study also has some limitations: (i) the main problem is the lack of validation of the findings by a large clinical sample. In addition, the DM in this study had a small number of healthy controls in the corresponding dataset, which needs to be considered when interpreting the results. (ii) In the case of DM-RP-ILD, we have limited the study to immune cells in serum, the sample size is very small, and further results in tissue are needed to explain this phenomenon.