In this study, we found that when sarcoidosis occurs, the upregulated DEGs of pulmonary sarcoidosis were mainly enriched in immune defense-related processes, interferon-γ-mediated signaling pathways, chemokine signaling pathways, positive regulation of the JAK-STAT cascade, chemokine activity, receptor activity, CXC chemokine receptor activity, CXCR3 chemokine receptor binding, protein homodimerization activity and CCR chemokine receptor binding. Similarly, for sarcoidosis involving the skin and lacrimal glands, the upregulated DEGs were also involved in various immune responses, chemokine signaling pathways, chemokine-mediated signaling pathways, interferon-γ-mediated signaling pathways and MHC class II receptor activity. For the DEGs involved in the biological process, there was no great difference among the three tissues. In the term of cellular components, the upregulated and downregulated DEGs of the three types of sarcoidosis were enriched in the membrane and intercellular spaces. And the biggest difference was that pulmonary sarcoidosis was not involved in MHC class II receptor activity.
Studies have shown that MHC, interferon-γ, chemokines were substances that played a key role in the immune process of the body. The results of this study indicated that chemokines and MHC class II can promote immune cells to act at inflammatory sites and perform actively in different involved tissues. Of course, DEGs in the three tissues were all related to the immune system, interferon-γ and vitamin D, which may be used as optional drugs for the treatment of sarcoidosis.
KEGG analysis revealed that pathways of the three different involved tissues were mainly related to cytokine-cytokine receptor interactions and chemokine signaling pathways. Cytokines and chemokines are important components of the pathology and physiology of sarcoidosis. Increasing evidence suggested that most of the events leading to inflammation, granuloma formation, and tissue damage could be regulated by these mediators and their receptors[19, 20]. Therefore, the pharmaceutical preparations developed according to the pathogenic mechanism of cytokines may become a powerful approach for the treatment of sarcoidosis.
If not limited to the top 20 meaningful KEGG pathways, it can be also found that sarcoidosis was also associated with NOD-like receptor signaling pathways, cell adhesion molecules, inflammatory bowel disease, leishmaniasis, Jak-STAT signaling pathway, ABC transporter, tuberculosis, Wnt signaling pathway. Researches have shown that abnormal regulation of NOD2 signaling might be related to the pathology of various inflammatory diseases, and its downstream signaling proteins such as inhibitor of apoptosis protein (IAP) have become potential therapeutic targets for sarcoidosis. Changes in cell adhesion molecules are also closely related to the inflammatory response in sarcoidosis. Recent studies suggested that overproduction of inflammatory cytokines, such as interferon gamma (IFN-γ), leading to constituent activation of the JAK-STAT pathway, may be a conserved feature of these diseases. The use of JAK inhibitors can stop these signals and improve symptoms in patients with sarcoidosis[23, 24]. Alveolar macrophage ABCG1 is an important regulator of the occurrence of lung granuloma and inflammation. As a granulomatous disease, pulmonary sarcoidosis is closely related to the ABC transporter pathway. The increased activation of Wnt signaling in sarcoidosis may promote the inflammatory process. In inflammatory bowel disease (IBD), immune diseases such as sarcoidosis are more common and IBD may share some common pathogenic mechanisms with sarcoidosis[27, 28]. Both leishmaniasis and tuberculosis have similar clinical manifestations to sarcoidosis and require to be differentiated. In particular, the relationship between tuberculosis and sarcoidosis has been controversial for a long time, and the studies of the connection between them will also help us to understand the mechanism of sarcoidosis[29, 30].
We also used the STRING database to construct the PPI network of the DEGs in the three datasets. To our surprise, the top 10 hub genes were entirely different, but more or less interrelated. Proinflammatory cytokines are the determinants of inflammatory events leading to sarcoidosis, and IL6 plays an important role in the occurrence of sarcoidosis. And studies have shown that progressive pulmonary sarcoidosis may be a fibroproliferative process triggered by IL-6[31, 32]. The role of CD28 and CD2 in pulmonary sarcoidosis has also been confirmed in studies[33, 34]. Microbial antigens can reduce the expression of tyrosine kinase LCK, which is related to the severity of sarcoidosis, therefore many studies have started to explore how to treat pulmonary sarcoidosis in this regard. The research by Prior C et al showed that elevated levels of IFNg are detectable in the majority of untreated patients with stage II/III pulmonary sarcoidosis, while those with the highest levels appear to have a better chance of being cured completely with corticosteroid therapy. In patients with sarcoidosis, IFNg and the receptor of CXCR3 have been turned out that they could induce elevated levels of Th1 chemokine in biopsy specimens and bronchoalveolar lavage fluid (BALF), demonstrating that IFNg is a biomarker for sarcoidosis. STAT1 plays an important role in innate immunity, and many sarcoidosis genomics studies have proved its functions in the immune response of sarcoidosis, and sarcoidosis may be a disease mediated by STAT1[38, 39]. CCR2 is a chemokine receptor whose polymorphism is associated with its susceptibility and protection of sarcoidosis. In all stages of sarcoidosis, CCL2 and CCL5 are important mediators that could induce migration of CCR1, CCR2, and CCR3-expressing monocytes, as well as CCR5-expressing mast cells. Human leukocyte antigen (HLA-G) may play an critical role in inducing and maintaining immune tolerance, and many studies have also suggested that HLA-G is associated with sarcoidosis. Agostini confirmed that CD86 can modulate T cell responses in sarcoidosis, explaining part of the mechanism of inflammatory responses in sarcoidosis. Stefan's studies have also reported that the formation of macrophages is associated with the upregulated expression of TYROBP, and the activation status of specific macrophage may be a crucial event in the development of sarcoidosis. The increasing activities such as matrix metalloproteinase-9 (MMP9) may influence the initiation and spread of inflammation and then induce the occurrence of sarcoidosis eventually.
In summary, we used bioinformatics to analyze the genome data of sarcoidosis from different tissue samples, and found the differences and similarities between the pathogenesis of sarcoidosis, which provided a new vision for exploring the possible mechanism of sarcoidosis and studying the diagnosis and treatment of sarcoidosis. Of course, our study still had limitations of lack of experimental verification and uncertain results, which could be affected by updating databases. For all this, our study still played a complementary and guiding role in the exploration of sarcoidosis. Subsequently, the results of bioinformatics analysis would be further verified by specific experiments.