As an immune-mediated inflammatory disease, CSU is closely related to autoimmune process and systemic inflammatory response. The autoimmunity theory and Th1/Th2 cell imbalance hypothesis related to its pathogenesis have been discussed most in the past[6, 7]. In order to better understand its pathogenesis, this article explores from the molecular level. In this study, through protein-protein interaction (PPI) network analysis and GeneCards database, we identified four key genes, including IL6, TLR4, ICAM1, and PTGS2. Through the enrichment analysis of the core modules, three signal pathways, TNF signaling pathway, NF-κB signaling pathway and Jak-STAT signaling pathway, were found to be closely related to the occurrence and development of CSU. Based on these genes and pathways involved in a variety of immune responses and chemotaxis of immune cells, we used the xCell to analyze the immune cell infiltration of CSU patients with lesions. Results showed that immune cell infiltration of CSU was significantly different from that of the health control group.
Dysregulation of TNF-α signaling pathway is an important feature and pathogenic factor of various diseases, including sepsis, cancer, autoimmune and inflammatory diseases[8]. Previous studies reported that the upregulated expression of TNF-α was found in skin tissue biopsy of patients with different types of urticaria [9]. In addition, the concentration of TNF-α and its soluble receptor types 1 and 2 (sTNFR1 and sTNFR2, respectively) in serum of CSU patients increased significantly, suggesting that the activation of TNF-α signaling pathway is related to the development of CSU [10]. On the other hand, the significant effect of TNF-a inhibitors on refractory urticaria where other treatments were proved ineffective, also shows the importance of TNF-a signaling pathway in the development of CSU[11, 12]. However, control studies with larger scale and longer follow-up periods are needed to confirm the efficacy and safety of TNF-a inhibitors in the treatment of CSU patients.
The NF-κB signaling pathway plays an important role in regulating the survival and activation of T and B lymphocytes in the thymus, bone marrow, spleen, and periphery[13]. The dysregulation of NF-κB signal leads to its structural activation, which leads to autoimmunity and chronic inflammation. Many autoimmune diseases have been proved to be associated with dysregulation of NF-κB signaling, including type 1 diabetes, systemic lupus erythematosus, and rheumatoid arthritis[14]. TNF is a major inflammatory cytokine that activates NF-κB, with most of its inflammation mediated by TNF receptor 1 (TNFR1)[15, 16]. The positive effect of neutralizing TNFR1-NF-κB signaling in autoimmune and inflammatory syndrome has been confirmed[17]. Our GSEA enrichment results showed that the TNF-α signaling via NF-κB of CSU patients in the two data sets was highly expressed, which supports the autoimmunity theory of CSU.
The Jak/STAT3 signaling pathway, as a conduction pathway closely related to inflammatory reactions, is involved in the development of chronic inflammatory diseases such as atopic dermatitis and psoriasis. The Jak inhibitor JTE-052 has shown good efficacy in the treatment of chronic dermatitis induced by hapten in rats, and is expected to become a candidate drug for the treatment of chronic dermatitis [18]. Recent studies have shown that Jak/STAT3 signaling pathway is also involved in the pathogenesis of CSU. Luo et al. found that OSMR gene is highly expressed in skin lesions of CSU patients, which leads to the up-regulation of Jak/STAT3 signaling pathway related gene expression. In addition, OSMR gene silencing can significantly reduce the content of inflammatory factors, eosinophils, mRNA and protein expression of Jak/STAT3 signaling pathway related genes, promote cell proliferation and migration, and inhibit epithelial cell apoptosis to suppress CSU mouse model autoimmunity reaction [19]. Another study also found that IL9 and IL10 can promote the development of CSU by activating the Jak/STAT3 signaling pathway [20]. Interestingly, our GSEA enrichment results on two data sets showed high expression of IL6/Jak/STAT3 pathway in the CSU patient group. The abnormality of IL6/Jak/STAT3 signaling pathway is closely related to autoimmune diseases. IL-6 and its receptor IL-6R monoclonal antibody drugs have been approved for the treatment of rheumatoid arthritis and achieved good results[21, 22]. In addition, studies have confirmed that IL6/Jak/STAT3 signaling pathway is closely related to mast cell degranulation and allergic reactions[23]. Therefore, blocking IL6/Jak/STAT3 signaling pathway may be a new strategy for the treatment of refractory CSU.
IL6, a cytokine that plays an important role in inflammation and immune response, is involved in the development of various autoimmune and chronic inflammatory diseases, including rheumatoid arthritis (RA), systemic lupus erythematosus, Multiple sclerosis, adult Still's disease and psoriasis[24–26]. More and more evidences—the expression of IL6 in serum and skin lesion tissue of CSU patients significantly increased and its expression level can be significantly reduced after the symptoms are relieved, suggest that IL6 is also involved in the occurrence and development of CSU[27, 28]. IL6 mainly works in immune and inflammatory responses through two different pathways: one is that after binding to the low-affinity cell membrane IL6 receptor (IL6R), it interacts with the signal transduction component glycoprotein 130 ( gp130) to form a high-affinity complex that initiates intracellular signaling (classical receptor signaling pathway); the other is that IL6 binds to soluble IL6 receptor (sIL6R) and then forms a complex with gp130, resulting in a signal Transduction (trans signaling pathway)[29]. gp130 is commonly expressed in cells, but under normal circumstances, IL6R only exists on specific cell surfaces such as liver cells, neutrophils, monocytes, macrophages, and T and B lymphocytes. sIL6R is transported in body fluids and mediates IL6 signal transduction of various gp130-only cells such as nerve cells, smooth muscle cells and endothelial cells[30]. In fact, many IL6 effects are mediated by the IL6/slL6R/gp130 complex, especially in chronic inflammatory and autoimmune diseases[31]. Studies have confirmed that the increase of the plasma levels of IL6 and slL6R in CSU patients and the increase of the serum C-reactive protein (CRP) concentration suggest the activation of IL6 trans signaling pathway, which may be accompanied by the activation of acute phase response and enhancement of disease activity[32]. Specifically, on the one hand, the IL6/slL6R complex stimulates the synthesis of acute phase proteins (such as CRP) and promotes the synthesis and secretion of immunoglobulins by B lymphocytes, which thereby promotes the production of autoantibodies; on the other hand, it can inhibit the differentiation of Th1 and Treg cells, promote the differentiation of Th2 and Th17 cells, and finally cause the imbalance of Th1/Th2 cells[24, 33]. Th2 cells can not only promote B cells to produce IgE, but also participate in the pathogenesis of allergic diseases by producing cytokines such as IL4, IL10, IL13[34], which is verified in the results of Reactome enrichment. In addition, IL6 may activate eosinophils, leading to the expression of tissue factor (TF). TF can promote the formation of thrombin, which in turn causes mast cells to subsequently degranulate, increasing vascular permeability[35].
PTGS2, also known as cyclooxygenase 2 (COX2), is a key enzyme in the biosynthesis of prostaglandin D2 (PGD2) involved in inflammation. The high expression of PTGS2 can promote large amount of PGD2 synthesis and aggravate the inflammatory response in CSU patients. Besides, IL6 classical signaling can also enhance the expression of COX2 induced by FcεRI, which thereby enhances the production of IgE-dependent PGD2 by human tissue-derived mast cells[36]. It is well known that endothelial dysfunction may increase vascular permeability, leading to a pro-inflammatory response. ICAM1, as a biomarker of endothelial dysfunction, is detected in the skin biopsy of CSU patients whose expression level was up-regulated, reflecting the proinflammatory phenotype of its endothelium[37]. In addition, circulating soluble ICAM1 also plays a potential role in the pathogenesis of CSU, however, it is not parallel to disease activity, nor can it predict the efficacy of H1 antihistamine therapy[38]. TLR4, the earliest discovered Toll-like receptors (TLRs), can regulate the expression of various genes through NF-κB signaling after activation, like IL6, ICAM1, COX2, etc. At the same time, the massive production of Th2 cytokines will also lead to the imbalance of Th1/Th2 cells[39, 40], promoting the CSU to develop.
From the immune infiltration analysis, we found that CSU tissue generally contained a higher proportion of DC, Th2 cells, mast cells, MEP, preadipocytes, and macrophages M1. As we all know, type 1 allergy plays an important role in CSU. DC, as the main antigen presenting cell, transmits allergens to B cells and activates them to generate plasma cells, which synthesizes and secretes IgE. In addition, as previously mentioned, Th2 cells can help activate B cells to produce IgE, and also can further expand the inflammatory response by producing cytokines such as IL-4, IL-10, IL-13. As a key effector cell in the pathogenesis of CSU, mast cell can release inflammatory mediators such as histamine and PGD2 after being activated, resulting in increased permeability of vasculature and recruitment of inflammatory cells, further causing symptoms such as wheal, itching, and edema [41]. In addition, macrophage M1 can produce proinflammatory related factors, such as IL-6 and TNF, and participate in the inflammatory response of CSU[42]. Our analysis results are basically consistent with the previous reports about CSU immune cell infiltration, which in turns proves the accuracy of our study. However, studies on MEP, preadipocytes, and CSU have not been reported, and this potential connection is worth further exploration. In addition, we analyzed the correlation between IL6, TLR4, ICAM1, and PTGS2 and immune cells, and found that IL6 was positively correlated with activated Th2 cells, mv Endothelial cells, and preadipocytes; PTGS2 was positively correlated with neurons, macrophages M1, and Th2 cells; ICAM1 was positively correlated with Th2 cells, mast cells, MEP, macrophages M1, mv Endothelial cells, and preadipocytes; TLR4 was positively correlated with mv Endothelial cells, preadipocytes, macrophages M1, and MEP. This indicates that these key genes also play an important role in the immune infiltration of CSU. However, the specific impact of these differentially expressed genes on the immune invasion of CSU lesions needs further study.
We acknowledged that the study has some certain limitations. First, the sample size we analyzed is relatively small. Second, this is a retrospective study and all the data are from publicly available databases. Third, results require further confirmation such as by vivo or vitro experiments. However, it is important that no one has done similar research on CSU before. Our results may provide new insights into the occurrence and development of CSU.