3’-5’-nucleic acid repair exonuclease 1 (TREX1) is an important endogenous DNA exonuclease in cells. It can remove DNA fragments that need to be processed by hydrolyzing damaged DNA, thereby avoiding excessive immune activation and AID. Researchers have found that Trex1^-/- mice develop fatal inflammatory diseases around the 8th week after birth. If Irf3 is knocked out at the same time, it can effectively avoid the generation of high IFN-I levels and autoantibodies and prevent the early death of Trex1^-/- mice. This result indicates that the autoimmune symptoms of Trex1^-/- animals are associated with the generation of IRF3 dependent IFN-I (42). Meanwhile, Trex1^-/- mice exhibited an autoimmune inflammatory phenotype associated with increased expression of ISGs. All detectable pathological and molecular phenotypes of Trex1^-/- cGAS^-/- mice were alleviated, including ISGs induction, the production of autoantibody, abnormal activation of T cell. Even if only one cGAS allele is missing, it can greatly salvage the phenotype of Trex1^-/- mice. These results suggested that cGAS inhibition may help prevent and treat certain self-DNA-induced AID (43). Other studies have shown that TREX1 has a lower degradation efficiency for DNA molecules with oxidative damage. In lupus-prone mice, injection of oxidized DNA into the skin can cause injuries like the observation in AID patients (36). TREX1 functional deletion mutations can cause IFN-I dependent AID such as familial chilblain lupus (FCL) and AGS. The manifestations of autoimmune disorders are related to the inability of cells to clear their own DNA fragments normally (44, 45).

Deoxynucleotide ribonuclease II (DNase II) is an acid endonuclease located in lysosomes, nucleus, and cell secretions. The DNase II family is involved in various functions in the human body, such as degradation of the phagocytic dead cells DNA and participation in the growth and differentiation of red blood cells. The function of phagocytes in DNase II^-/- mice is damaged, which can cause serious immune deficiency and affect growth and development (46). It is reported that DNase II^-/- mice die in the embryonic stage of uterus, and the mouse embryonic liver contains many macrophages with undigested DNA, which indicates that DNase II^-/- mice cannot degrade DNA derived from erythroid Precursor cell, leading to the generation of IFN-I, inducing the specific ISGs expression, and leading to embryonic death of DNase II^-/- mice (5, 43, 47). However, adult DNase II^-/- mice can die from chronic polyarthritis similar to RA [43]. For DNase II^-/- cGAS^-/- mice, the mice were able to develop normally without obvious arthritis symptoms and showed low levels of anti-dsDNA antibodies. In DNase II^-/- cGAS^-/- mice, cGAMP expression was not observed, while cGAMP was highly expressed in DNase II^-/- Sting^-/- mice. Therefore, it can be considered that cGAS is essential for the cGAMP generation in response to the accumulation of their own DNA in the cytoplasm in DNase II^-/- cells (43). Through systematic screening of ISGs, studies have identified two siblings and a single male patient with severe membranoproliferative glomerulonephritis, neonatal anemia, and liver fibrosis, all of which are accompanied by an increase in anti-DNA antibodies. A biallelic mutation in DNASE2 was found in these two families, which was related to the loss activity of DNase II endonuclease (48).

Ribonuclease H2 (RNase H2) is a non-nucleic acid sequence-specific DNA repair enzyme to remove unnecessary DNA ribonucleotides. Changes in RNase H2 enzyme function are a common cause of AGS in children and are related to SLE (49). A study using Rnaseh2b^A174T/A174T mice as a subclinical disease model found that the transcription of ISGs in mice was upregulated, which is similar to the characteristics of patients with AGS. This inflammatory response depends on the nucleic acid sensor cGAS and its adapter STING and is related to decreased cell RNaseH2 enzyme activity and increased DNA damage (50, 51). Cgas ^-/- and Sting^-/- mice rescued autoimmune phenotypes and inflammation in Rnaseh2b^A174T/A174T mice (50).

STING proteins play an essential role in immune defense. Although STING gene-deficient mice survive, they are susceptible to infection by various pathogenic microorganisms. For example, after knocking out the STING gene in mouse embryonic fibroblasts, macrophages, dendritic cells, herpes simplex virus 1 (HSV1), and gram-positive Listeria monocytogenes, bacterial genomes or plasmids cannot effectively induce these cells to produce type I interferons (60, 61). After using in vitro short hairpin RNA to interfere with cGAS and STING in human monocyte-macrophage lines, the type I interferons expression level significantly decreased following infection with Mycobacterium tuberculosis in the presence of restricted cGAS or STING expression. This conclusion was also confirmed in mice with cGAS or STING gene deletion mutations; M. tuberculosis could activate STING-related autophagy processes, clearing M. tuberculosis in mice through the lysosome degradation pathway (62). In addition, murine C. trachomatis can activate the STING-related signaling pathway, inducing an anti-C. trachomatis immune response in mice (63, 64). In mice infected with chronic hepatitis B virus (HBV), and the cGAS-STING pathway could inhibit HBV replication in the liver cells of mice infected with chronic HBV. Activating STING protein in mouse liver cells and surrounding immune cells significantly inhibits viral replication in mice, whereas knocking out STING protein in liver-related cells significantly increases the copy number of HBV DNA in mice (65). Moreover, the expression level of STING in chronic hepatitis B patients is lower than that in normal individuals and is correlated negatively with the viral replication level (66). STING not only exerts an essential role in the host’s resistance to dsDNA microbial infections but may also play a significant role in the body’s immune response induced by negative- or positive-stranded RNA viruses. Studies have shown that mice with downregulated STING protein expression in embryonic fibroblasts are extremely sensitive to vesicular stomatitis and murine respiratory virus infections (67). Studies have shown that after HIV (HIV) invades host cells, the body can induce anti-HIV immune effects by activating cGAS-STING-related signaling pathways, thus effectively inhibiting HIV replication in the host. However, when cells are restricted to STING expression, the addition of the synthetic double-stranded RNA (dsRNA) analog polyinosinic: polycytidylic acid does not affect the capbility of the cells to generate type I interferon. In contrast to double-stranded DNA microorganisms, RNA pathogens not only did not significantly induce STING-dependent autophagy, but also did not significantly activate the expression of its associated innate immune genes (24, 68). Therefore, the anti-pathogenic mechanism of STING in RNA virus replication may differ from its mechanism of inhibition of DNA virus replication. In conclusion, the mechanism for STING to regulate RNA viral replication remains further study; however, STING molecules may assume a variety of functions in host cells, including the control of transposon-related tasks, and may affect the post-translational modification of viral proteins. In comparison to wild-type mice, the central nervous system of STING gene-deficient mice can easily cause herpes simplex virus encephalitis when infected with HSV1, and the deficient mice easily develop HSV1 venous infection, but there is no obvious difference in the survival ratio of the deficient and wild-type (WT) mice infected with mucosal infection. During mucosal HSV1 infection, clearance of the virus in mice may not require the involvement of STING, as its impact on the generation of type I interferon is minimal (69, 70). As significant heterogeneity and racial differences exist in human STING genes, further clarification is needed regarding the key role of STING in human pathogenic microorganisms invasion.

The release of endogenous substances from myocardial cells during myocardial infarction will initiate the cGAS-STING signaling pathway to induce the generation of a good deal of inflammatory factors and inflammatory cells, and intensifies the necrosis of myocardial cells. In comparison to WT mice, the early survival rate of myocardial infarction mice lacking cGAS, STING, and IRF3 was significantly improved (71). STING-related signaling pathways also exert a pivotal role in the inflammation of cardiovascular tissues induced by high glucose and high-fat diet in mice. In the above model, weight gain, cardiovascular inflammatory response, reduced glucose tolerance, and insulin resistance of STING-deficient mice improved, suggesting that STING is also involved in metabolic-related inflammatory diseases (72). The STING pathway activation may be an essential regulatory factor in renal injury. In cisplatin-induced acute renal damage in mice, cisplatin can induce the leakage of mtDNA into the cytoplasm through Bcl-2 like protein 4 on the outer layer mitochondrial membrane tubules and activate the cGAS-STING pathway, thus triggering inflammation and exacerbating acute renal failure in mice. However, this process is upregulated in STING gene-deficient mice, and in renal tubular cells in vitro, which inhibit the activity of STING protein, can improve the inflammatory response of renal tubular cells caused by cisplatin (73). In mice with acute pancreatitis, the STING-related signaling pathway can sense the DNA of dying acinar cells in the mouse pancreas, further increasing the inflammatory factors generation. Simultaneously, macrophages exacerbate the inflammatory response of the mouse pancreas by overexpressing STING protein (74). A physiological barrier formed by intestinal epithelial cells can separate the intestinal lumen from the internal environment in the host. The STING signaling pathway is associated with the intestinal inflammatory and immune responses of intestinal epithelial cells, leading to fatal sepsis by promoting intestinal epithelial cell apoptosis and destroying the intestinal barrier and the STING protein expression level in mononuclear cells of peripheral blood in septic patients (75). In sepsis mice induced by cecal ligation and perforation, the STING signaling pathway in the intestine is significantly activated, whereas STING gene knockout mice show a reduced intestinal inflammatory response, weakened intestinal permeability, and reduced bacterial translocation. The STING protein was activated in mice by the flavonoid vascular disruptor 5,6-dimethyloxanthenone-4-acetic acid and more significant intestinal cell apoptosis and severe systemic immune inflammatory responses were observed in mice (76).

SLE is a prototypic AID with high autoantibodies titers and various organ lesion. Although the exact cause of SLE is currently unclear, accumulating evidence has shown that IFN-I is a primary pathogenic factor in SLE and an important element in the family of cytokines to activate both the innate and adaptive immune systems (77). Moreover, Existing research has shown that SLE is related to nuclease activity deficiency (78, 79). DNase II^-/- mice die in the embryonic stage because of the excessive accumulation of IFN-I in their bodies (80), whereas mice with both DNase II and STING deletions can survive without developing lupus (5). These results suggest that lupus caused by DNase deficiency rely on the STING signaling pathway.

The cGAS-STING signaling pathway is associated with the development and occurrence of SLE. Serum dsDNA levels in SLE patients are significantly higher than those in healthy controls (81). Some SLE patients have elevated serum levels of cGAMP, cGAS, and immune-stimulating factors (82), indicating that cGAS may be involved in the process of recognizing and binding to dsDNA to produce cGAMP in patients with SLE, thereby inducing inflammation. However, the subsequent research carried out by Kato et al. shows that the STING agonist 2’3’-cGAMP does not exist in the serum of SLE patients, and dsDNA is stored in apoptosis derived membrane vesicle, which can effectively prevent the degradation of extracellular nuclease and continuously activate the cGAS-STING signal pathway, promoting the release of some inflammatory factors, including IFN and interleukin-6 (IL-6) (81). The STING pathway activation by dsDNA is not necessarily mediated solely by cGAS, there exists a possibility that dsDNA directly activates STING, and the sustained activation of the cGAS-STING signaling pathway is likely involved in cell apoptosis. A study has shown that calcium-related signaling proteins are also associated with regulating the STING pathway in patients with SLE, thereby regulating their immune response (83). The role of cGAS-STING signaling in SLE remains controversial. In a SLE mouse model induced by pristane (TMPD), cGAS and STING defects failed to prevent the mice from getting sick,and led to upregulated generation of autoantibodies and elevated proteinuria levels in mice (84). Overall, there is considerable heterogeneity in the underlying mechanisms of SLE, and the effect of the cGAS-STING signaling pathway in different stages of SLE pathogenesis varies largely depending on the type of SLE animal model being studied and the stage of disease development.

Administration of the calcium ion inhibitor, dipyridamole, the levels of inflammatory factors mediated by T cells in patients with SLE significantly decreased. SLE animal model experiments also supported this result, suggesting a common molecular pathway between calcium ions and IFN in patients with SLE (85). Using calcium ion chelating agents, Zhang et al. found that inhibition of the calcium signal transduction using calcium ion chelating agents was able to suppress macrophage activation induced by the DNA-dependent activator of interferon regulatory factors (DAI) in SLE (86). These results suggest that calcium-related signals are associated with the STING pathway and modulate the immune responses in patients with SLE.

SLE pathogenesis is associated with enhanced generation of IFNα from lupus monocytes induced by augmented activation of STING pathway, and enhanced expression of STING and subsequent production of IFNα by monocytes were downregulated by the suppression of the mTOR pathway (87). We propose that the cGAS-STING pathway is versatile in multiple organs and can promote the overall SLE progression through impertinent sensing of cytosolic self-DNA (8). These suggest that IFIT3 may contribute to the overactivation of cGAS-STING signaling pathway in monocytes of human SLE, laying the foundation for IFIT3 to be a new therapeutic candidate for blocking type I IFN and other proinflammatory cytokines generation via the cGAS-STING signaling pathway in SLE patients (88). Further investigation will disclose the importance of these molecules in SLE development and progression.

RA, a chronic AID character by destructive and symmetrical joint lesions and synovitis (89). The expression of dsDNA in the cytoplasm of fibroblast-like synovial cells in RA patients increases and that the dsDNA expression and cGAS is related to the severity of rheumatoid synovitis. After knocking out cGAS or STING in RA patient cells, the expression of cytokines decreases (90), indicating that the cGAS-STING signaling pathway regulates the inflammatory response in RA. Another study found that cGAS promoted the inflammatory response of RA fibroblast-like synovial cells by activating extracellular regulated protein kinase (ERK) and protein kinase B (Akt) signaling pathways (91). Although the AKT signaling pathway activation may inhibit downstream activation of the STING pathway (92), whether cGAS can activate of the ERK and AKT signaling pathways in RA is related to the regulation of activation of cGAS-STING signaling pathway. In summary, the cGAS-STING signaling pathway is associate with the occurrence and development of RA. Further study will investigate RA pathogenesis and clinical treatment options.

Aicardi-Goutières syndrome (AGS) is an autoinflammatory disorder connected with spontaneous IFN production without virus infection (93). Studies have shown that the pathological mechanism of AGS is tightly associated with the cGAS-STING signaling pathway (94). AGS is caused by the mutation and deletion of some nuclease genes, which leads to the reduction or loss of nuclease activity, and a good deal of nucleic acids accumulate STING-TBK1-IRF3/NF-κB in the cytoplasm, the excessive activation of these signaling pathways ultimately results in a significant increase in IFN-I levels (43). It has been confirmed that patients with pulmonary and vascular syndrome have mutations in exons of STING (34) and that primary biliary liver disease is also related to the cGAS-STING signaling pathway activation (95).

SAVI is a chronic inflammation and vascular disease caused by increased IFN-I signaling because of mutations in genes associated with STING activation. Reports of SAVI cases, both domestically and internationally, are generally related to mutations in the STING coding gene (TMEM173), with clinical manifestations including neonatal inflammatory pulmonary disease, skin vascular disease, and decreased activity tolerance (96). In related SAVI mouse model, the excessive cGAS-STING signaling pathway activation can result in immune system dysfunction, aseptic inflammation, and severe pneumonia in mice, similar to the symptoms observed in patients with SAVI (97). Mutations in the STING coding gene result in the overactivation of STING, inducing a large amount of IFN-I production. After binding to interferon receptors, IFN-I is further cascaded and activated through the Janus kinase (JAK)-STAT (JAK-STAT) signaling pathway, leading to inflammatory storms and causing severe and persistent damage to STING overexpressing tissues (98, 99).

Psoriasis, a chronic inflammatory skin disorder, is usually accompanied by complications, including RA, autoimmune Thyroiditis and other AIDs and is related to adaptive and congenital immune responses (100). DNA oxidative insult in the skin of psoriasis patients worsens, and levels of circulating free DNA (cfDNA)in the plasma of psoriasis like mice increase. Additionally, STING and its associated genes are increased in the skin of patients (101–103). Preliminary mechanism research shows that STING, as an autoDNA sensor, induces inflammatory response in macrophages and keratinocyte of psoriatic skin and mediates TNF-α or H₂O₂ release by immune cells, generated TNF-α, H₂O₂ can promote the discharge of DNA into the cytoplasm of keratinocytes and inhibit the degradation of STING protein induced by dsDNA (102). In a study on the in vivo and in vitro anti-psoriasis effects of H-151, a STING antagonist, it was found that local administration of H-151 can alleviate imiquimod induced skin damage in psoriasis and alleviate inflammation by inhibiting the STING/NF-κB in keratinocyte and signal transduction of immune cells, suppresses skin inflammation, and alleviates the symptoms of psoriasis (103).

The current view is that non-alcoholic steatohepatitis (NASH) is an autoimmune disease caused by abnormal mitochondrial DNA released by liver Kupffer cells against necrotic liver cells (104). Cell necrosis releases mitochondrial DNA, which leads to the generation of TNF-α and proinflammatory factors, through the cGAS-STING signaling pathway, is a committed step for the disease to produce persistent chronic inflammation. WT mice treated with STING activators were more sensitive to steatohepatitis than STING-/- mice, indicating that the cGAS-STING signaling pathway exerts an essential role in the NASH induction. However, it is a long way to go for us to better understand the NASH pathophysiology and underlying molecular mechanisms.

IBD includes ulcerative colitis and Crohn’s disease, and its pathogenesis is not totally clear. Currently, the mainstream view is that many factors such as genetics, environment, gut microbiota imbalance, and host mucosal immune dysfunction jointly promote the development and occurrence of IBD (105). The special functions of STING in the human gut remain unclear, but STING-deficient mice are prone to experimental colitis (106). Another study suggested that the incidence of spontaneous colitis in STING-deficient mice was obviously shorter than that in the control group (107). Ahn et al. (107) and Balci et al. (108) observed that IL-10 deficient mice showed severe spontaneous colitis and intestinal polyps within 10 weeks, whereas IL-10 and STING dual-gene-deficient mice didn’t show a similar phenotype after 19 weeks. Further research found that only 10% of IL-10 and cGAS dual-gene knockout mice formed intestinal polyps, indicating that the cGAS-STING signaling pathway exerts an indispensable role in IL-10 related colitis in mice.

In a colitis model induced by sodium dextran sulfate (DSS) in mice, compared with WT mice, the colitis symptoms of STING mutant and cGAS^-/- mice were obviously reduced, and that the expression level of STING protein is increased in colon M1 macrophages of both DSS-treated WT mice and IBD patients. STING agonists are capable of promoting the transformation of M2 macrophages to M1 macrophages, suggesting that the cGAS-STING signaling pathway may regulate intestinal immunity by regulating colon macrophages (109).