Introduction
Primary Sjögren’s syndrome (pSS) is a complex and heterogeneous autoimmune disease that leads to secretory gland dysfunction. It causes dryness of the main mucosal surfaces such as the mouth, eyes, nose, pharynx, larynx, and vagina, mainly characterized by sicca symptoms (xerostomia and xerophthalmia) [
1], which can have a major impact on quality of life, including dry eye, reduced salivary flow rates, an increased risk of dental caries, and oral candidiasis [
2,
3]. Approximately 20–40% of patients with pSS may experience extraglandular involvement [
4], and among them lymphoma is the leading cause of death [
5]. In the 2016 American College of Rheumatology (ACR)/European League Against Rheumatism (EULAR) classification criteria, serological (Anti-Ro/SSA) and histological examinations (labial salivary gland biopsy) were assigned the highest specificity and highest values [
6]. However, anti-SSA antibodies can be indicative of a more advanced stage of the disease, and relying on them alone for diagnosis may result in inadequate recognition of very early pSS [
7]. Labial salivary gland biopsy is an invasive examination, which may possibly cause discomfort and complications [
8], and also may be affected by the subjective judgment of specimen observers [
9]. Therefore, it is necessary to develop sensitive and specific biomarkers to assist the early diagnosis of pSS.
Host inflammatory responses are essential for the development and progression of pSS and are regulated by various signaling pathways, such as pro-inflammatory cytokines and interferon [
10,
11], and the use of anti-inflammatory treatments has been reported to provide relief from symptoms associated with pSS [
12]. At present, local tear and saliva substitutes, systemic secretagogues and immunosuppressants (glucocorticoids, chloroquine/hydroxychloroquine chloroquine and methotrexate) are commonly used treatments for pSS; however, their effectiveness is rarely seen in practice [
13‐
15]. Targeted treatment for pSS is still unavailable despite continued research into the disease’s pathogenesis, which may be due to the lack of systematic research on targeted biomarkers. Previous studies have uncovered a substantial amount of differentially expressed genes in the SS peripheral blood sample dataset [
16]. Our study sought to investigate targeted inflammation-associated biomarkers through multiple bioinformatics pathways.
Given the limitations of anti-SSA antibody detection (delay and non-specificity) and labial salivary gland biopsy (invasiveness and subjectivity) in early pSS diagnosis, regulating the inflammatory response and employing anti-inflammatory therapy have emerged as crucial management strategies for pSS [
10‐
12]. Further, considering sensitivity and specificity of biomarker detection in serum, saliva, tears, or urine can potentially provide a more prompt and accurate reflection of the disease’s presence and progression, it has the potential to enhance the diagnostic accuracy of pSS and offer improved prospects for the treatment and intervention in early pSS. Hence, in this study, the inflammation-associated biomarkers with diagnostic value for pSS were filtered through two classical machine learning algorithms. The diagnostic value for the biomarkers was confirmed, and the biomarkers-related underlying mechanisms in pSS were initially investigate. Relevance analysis of inflammation-associated biomarkers and immune cell infiltration were performed. Moreover, the regulatory networks targeting the biomarkers were investigated, and the biomarkers-targeted drugs were predicted. Based on the five pSS-related online datasets containing the transcriptional expression profiles of whole peripheral blood samples, we make the case that the research could provide a basis for understanding disease pathogenesis and improving clinical diagnosis and treatment.
Discussion
pSS is characterized by chronic inflammation and is manifested by impaired function of the exocrine glands, and mononuclear cells infiltrate surrounding the ducts and replacing the secretory units of the involved glands [
26]. Due to the heterogeneities of clinical phenotypes and various causes, the identification of key biomarkers in pSS is critical to understanding the pathogenesis of this complex disease. Considering the biological significance of inflammatory response in pSS progress, the study was utilized to identify the potential inflammation-associated biomarkers through the bioinformatics methods based on the online datasets of pSS.
Using the differentially expressed analysis between pSS samples and healthy controls, as well as WCGNA in the GSE51092 datasets, nine pSS-related and inflammation-associated DEGs were identified in the present study, namely LY6E, EIF2AK2, IRF7, TNFAIP6, RTP4, IL15, CXCL10, LAMP3, and CCL2, which were mainly involved in the activation of the innate antiviral immunity process and inflammatory-related signaling pathways. Evidence has indicated that viral infections alter the clinical manifestations of various autoimmune diseases. On the other hand, protective effects can be achieved by suppressing autoimmune phenomena through regulatory immune responses [
27]. Influenza viruses and EBV infection were considered as central roles in the pathogenesis of pSS through the autoimmunity induced by different mechanisms in previous literature [
28‐
33].
Several functions relevant to the pSS-related inflammation-associated DEGs were related to immune and inflammation signaling pathways.
Toll-like receptors (TLRs) could sense nucleic acids derived from viruses and trigger antiviral innate immune responses as pattern-recognition receptors (PRRs) [
34,
35], where NF-kappaB, MAPK kinases, and IRFs that control the transcription of genes encoding type I interferon and other inflammatory cytokines were activated to eliminate viruses [
36]. Previous studies have shown that TLRs play an essential role in the pathogenesis of pSS [
37,
38]. They are elevated in salivary tissue [
39] and in the peripheral blood of pSS patients [
40]. Emerging data indicate that damage-associated molecular patterns (DAMPs) may be significant drivers of chronic and unremitting inflammation in pSS, although the ligands activating TLRs in pSS remain unknown [
41,
42]. Activating TLR signaling cascades likely reduce local and systemic inflammation, as shown in an animal study [
43]. There is no doubt that the interaction of the Toll-like signaling pathways and the viral defense response process may be important in pSS.
Nod-like receptor protein 3 (NLRP3) is a crucial player in regulating host immune responses to infection and cells stress [
44], and it was also found highly expressed in pSS patients than control [
45]. The NLRP3 inflammasome can be triggered by the P2X7 receptor (P2X7R), leading to acute inflammatory responses. Baldini et al. proposed the P2X7R-inflammasome axis as a novel potential pathway in both pSS exocrinopathy and lymphomagenesis [
46]. These results suggested that NLRP3 inflammasome-mediated inflammation might be implicated in the pathogenesis of pSS.
Interleukin-17 (IL-17) is a multifaceted cytokine with a well-recognized role in immune surveillance at mucosal and barrier surfaces [
47]. Previous research suggests that the IL-17 axis plays a pivotal role in the pathogenesis of several autoimmune disorders, including pSS [
48,
49]. Studies have demonstrated that IL-17 was overexpressed in the salivary glands (SGs) [
50], serum [
51], plasma [
52] and tears [
53] of pSS patients, and IL-17 mRNA levels in MSG biopsies seemed to be related to the degree of inflammation [
52,
54]. Different IL-17 family members may play several pathogenetic roles in the development of pSS. According to a recent study, IL-17F production in pSS patients is associated with a higher level of autoantibodies and EULAR SS disease activity index (ESSDAI) than IL-17A production in pSS patients [
55].
Next, four genes (
LY6E, EIF2AK2, IL15, and CXCL10) were authenticated as inflammation-associated pSS biomarkers, and the reliability of them in discriminating pSS samples from healthy control samples, suggesting a potential clinical diagnostic value. Functional enrichment results and immune infiltration analysis pointed to the involvement of the four genes in the immune process and inflammation-related pathways in pSS.
The Lymphocyte antigen 6E (LY6E) protein belongs to the Ly6/uPAR family of plasminogen activator receptors and is known as one of the IFN type I response genes. Recent studies have reported its essential role in immunological regulation, T cells physiology, oncogenesis, and viral infection [
56]. Our study found higher LY6E levels in the peripheral blood of pSS patients, which has been proven by previous clinical studies [
57‐
59]. These findings may reveal the importance of the peripheral blood LY6E levels and the monocyte IFN type I signature in pSS patients.
The eukaryotic translation initiation factor 2-α kinase 2 (EIF2AK2) gene is located on chromosome 2 and encodes modifying protein kinase R (PKR, interferon-induced, double-stranded RNA-activated protein kinase) [
60]. Recent studies have revealed that the coding gene PKR is associated with the treatment of pSS, which further confirms the role of EIF2AK2 in the progression of pSS [
39,
61,
62]. Although LY6E and EIF2AK2 have been found as pSS diagnostic genes in previous studies [
57], we further investigated the potential ceRNA regulatory network and related drugs of LY6E and EIF2AK2 in the context of inflammation, providing insight into the direction for future research.
Interleukin-15 (IL-15) is a crucial regulatory inflammatory cytokine that is upregulated in autoimmunity disorders [
63,
64]. Previous studies revealed a higher IL-15 expression level in the peripheral blood of pSS patients [
65], which is consistent with our results. Besides, based on gene and protein analysis and immunohistochemical results in minor salivary gland (MSG) biopsy specimens and human salivary gland epithelial cells (SGEC) obtained from patients with pSS, IL15 was documented a strong expression in acinar and duct cells of salivary glands with pSS, which may be related to TLR2/IL-15 signaling pathway [
66‐
69]. It’s consistent with our functional enrichment results (Toll-like receptor signaling pathway), which provides a theoretical basis for the detection of pSS by blood, but the protein levels in blood need further analyses.
C-X-C motif chemokine ligand 10 (CXCL10) protein is categorized functionally as a Th1-chemokine, and its secretion is regulated by interferon (IFN)-γ [
70]. The serum and/or tissue expressions of CXCL10 in various autoimmune diseases [
70‐
73]. A study that assessed CXCL10 plasma levels in pSS patients showed that the ratio of full-length (active) CXCL10 to truncated DPP4-truncated (inactive) CXCL10 was significantly increased in pSS patients and provided the highest correlation with disease activity [
74]. Elevated CXCL10 levels were also found in the salivary gland of pSS patients, which were associated with decreased circulating CXCR3 + helper cells, suggesting facilitating their concerted migration [
75]. These results guarantee the accuracy of our transcriptome analysis results.
The pathogenesis of pSS is multifactorial and complex. The process primarily encompasses antigen presentation, costimulation, B cell activation, and other related mechanisms [
11], in which the cytokine profiles of Th1, Th2, Th17, follicular helper T (Tfh) cells, and regulatory cells (Tregs/Bregs) play important roles [
76]. Studies have shown that the frequency of Foxp3 + regulatory T cells (Treg) in salivary glands may be correlated with glandular infiltration and the grade of local inflammation [
77], while B cell activation is generally associated with an increased risk of lymphoma [
78]. Lymphocytic infiltration in salivary and lacrimal glands and the deposition of autoantibodies, like anti-SS-A (anti-Ro) and anti-SS-B (anti-La), cause an autoimmune outbreak and chronic inflammation, leading to the destruction of the salivary gland architecture [
79].
In this study, we found that four key genes were significantly associated with regulatory T (Treg) cells and type 2 T helper (Th2) cells via immune infiltration and Pearson correlation analysis. Treg cell deficiency has been documented in pSS patients [
80], with peripheral blood levels significantly lower than those of healthy controls, suggesting that Treg cell deficiency may be involved in salivary gland destruction [
81]. Type 2 immune response which Th2 cells involved in has a regulatory relationship with autoinflammation [
82]. Th2 cells have been found to promote renal inflammation in patients with systemic lupus erythematosus [
83], and to play a part in the process of pSS by participating in costimulation and assisting B cell activation, with the cytokines they produce dominating the early stages of pSS [
84]. These findings demonstrate that significant changes occur in Treg cells and Th2 cells in pSS and other related autoimmune diseases.
The identified miRNAs in the present study exhibited consistency with other research on autoimmune or immune-mediated related diseases. The miR-26 expression level was downregulated in multiple sclerosis (MS) patients compared to controls [
85]. The neuroregulatory miRNA miR-9-5p was significantly upregulated in the peripheral blood samples of HLA-B27( +) radiographic axial spondyloarthropathy (rad-AxSpA) patients [
86]. Immuno-miRNAs miR-21-5p and let-7f-5p were significantly elevated in the serum of patients with acetylcholine receptor myasthenia gravis (AChR
+-MG) [
87‐
90], and miR-21-5p was also upregulated in type 1 autoimmune pancreatitis (AIP) [
91] and psoriatic arthritis (PsA) [
92]. Kim et al. found significantly downregulated expression of miR-30d-5p in the tear samples of pSS patients [
93]. These miRNAs may be involved in disease pathogenesis via immune-related processes.
In our study, three lncRNAs were identified as being associated with pSS, namely AL 136040.1, LINC02381, and AL157392.3. Previous reports have suggested that these three lncRNAs may be implicated in immunological disorders. The competitive binding of LINC02381 with miR-21 has been experimentally confirmed in previous studies. Zhao et al. demonstrated this interaction through luciferase reporter gene and RNA immunoprecipitation assays, indicating that LINC02381 sponged miR-21 to enhance KLF12 expression [
94]. However, the interaction between miR-21 and LINC02381/CXCL10 still requires further validation through additional functional experiments. Additionally, Jafarzadeh et al. also confirmed LINC02381 sponged miR-21 through dual luciferase assay [
95]. LINC02381/hsa-let-7f-5p/IL-6 competitive network in another immune-mediated connective tissue disease systemic sclerosis (SSc) was shown to be potentially involved in inflammatory and immune processes immune microenvironmental variation [
96]. Glycolysis-associated lncRNA AL157392.3 may influence immune-related signaling in pan-cancer analysis [
97].
Additionally, we predicted potential drugs based on drug-gene interaction pairs, which included glucocorticoids and immunosuppressive drugs. These drugs have been successfully used to treat autoimmune diseases. AMG-714 was used to treat celiac disease [
98], which is known as an associated autoimmune disease with pSS sharing a common genetic background [
99]. LEVODOPA is an effective and well-tolerated drug for the treatment of Parkinson’s disease [
100], which may have a potential association with pSS [
101], this suggests that IL-15 may be a potential target [
100]. ZIDOVUDINE for pSS has been cited in the manuscript, but studies have shown that antiretroviral therapy has a number of severe and life-threatening adverse drug reactions. For instance, taking ZIDOVUDINE was observed as a risk factor for anemia. STAVUDINE was utilized for the treatment of peripheral neuropathy, but among that, the use of nevirapine was identified as a risk factor for cutaneous reactions [
102].
Significantly, CYCLOSPORINE A has been found to be a potent inhibitor of IL-15 release in the context of acute rejection following heart transplantation in mice [
103]. However, varying doses of CYCLOSPORINE, which is a key immunosuppressive therapy for kidney transplant recipients, do not appear to have an impact on serum levels of IL-15 and IP-10 cytokines [
104]. While certain studies have proposed a potential role of TESTOSTERONE in modulating disease progression through the promotion of anti-inflammatory responses, the observed reduction in CXCL10 levels in male patients receiving TESTOSTERONE supplementation was not notably significant [
105]. IL-15 and IP-10, in conjunction with CYCLOSPORINE, have been identified as significant inflammatory biomarkers in rheumatoid arthritis [
106]. Given the notable upregulation of IL-15 and CXCL10 in pSS patients, it is postulated that pSS may contribute to the regulation of these cytokine levels via alternative mechanisms. Combined with the current research on the application of TESTOSTERONE and CYCLOSPORINE in autoimmune diseases [
107‐
109], we speculate that TESTOSTERONE and CYCLOSPORINE may regulate the abnormal activity of immune cells and reduce inflammation by targeting the inhibition of CXCL10, a proinflammatory cytokine, and IL15, an activator of immune cells. In turn, this will help improve the immune function of pSS patients and alleviate their symptoms and immune-mediated inflammation-related damage. However, it remains to be clinically verified in pSS patients.
However, there are still several limitations in our study: Verifying the reliability of transcriptional changes in gene expression establishes a theoretical foundation for the rapid evaluation of biomarkers expression in peripheral blood detection, while detecting gene expression at the protein level requires the further detection of specific proteins or cell surface markers, using techniques such as ELISA and flow cytometry. At the same time, the diagnostic efficacy of biomarkers, drug targeting results, and the regulatory networks are currently only preliminary findings from bioinformatics research and prediction, and it is necessary to conduct larger studies with a broader cohort of patients, as well as additional follow-up RNA-seq and animal studies, to validate their effectiveness, safety, and robustness. Furthermore, conducting clinical trials is necessary to verify the interaction mechanism between key genes and key immune cells using real data obtained from an increased number of clinical samples. Despite the challenges presented, the advances in genomics offer us a unique opportunity to gain a better understanding of the pathomechanism of pSS and develop novel therapeutic strategies. Further research into pSS could result in innovative treatments.
In conclusion, four genes (LY6E, EIF2AK2, IL15, CXCL10) that might be potential diagnostic inflammation-associated biomarkers of pSS in peripheral blood were identified by bioinformatics analysis, and their expression were validated by RT-qPCR. Given that the samples used in this study were all derived from peripheral blood for the pSS-datasets, we argue that leveraging peripheral blood tests for rapid evaluation of biomarker expression has the potential to improve the diagnostic accuracy of early pSS. Furthermore, the molecular mechanisms of these genes were preliminarily explored by generating a lncRNA-miRNA-mRNA regulatory network. And meanwhile, the predicted drugs, such as TESTOSTERONE targeting CXCL10 and CYCLOSPORINE targeting IL15, may potentially enhance immune function and alleviate symptoms and immune-mediated inflammation-related damage in patients with pSS. The results provided a basis for understanding the pathogenesis and improving clinical diagnosis and treatment for pSS.