T Cells Produce IFN-α in the TREX1 D18N Model of Lupus-like Autoimmunity

Mouse models of type I interferonopathies

Type I interferonopathies are severe monogenic diseases caused by mutations that result in chronically upregulated production of type I interferon. They present with a broad variety of symptoms, the mechanisms of which are being extensively studied. Mouse models of type I interferonopathies are an important resource for this purpose, and in this context, we review several key molecular and phenotypic findings that are advancing our understanding of the respective diseases. We focus on genotypes related to nucleic acid metabolism, sensing by cytosolic receptors and downstream signalling.

cGAS-STING pathway in systemic lupus erythematosus: biological implications and therapeutic opportunities

The cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) signaling pathway has been identified as a significant modulator of inflammation in various clinical contexts, including infection, cellular stress, and tissue injury. The extensive participation of the cGAS-STING pathway can be attributed to its ability to detect and control the cellular reaction to DNAs originating from both microorganisms and hosts. These DNAs are well recognized as molecules linked with potential risks. At physiological levels, the STING signaling system exhibits protective effects. However, prolonged stimulation of this pathway contributes to autoimmune disorder pathogenesis. The present paper provides an overview of the activation mechanism of the cGAS-STING signaling pathways and their associated significant functions, as well as therapeutic interventions in the context of systemic lupus erythematosus (SLE). The primary objective is to enhance our comprehension of SLE and facilitate more effective diagnosis and treatment strategies for this condition.

The prototypical interferonopathy: Aicardi-Goutières syndrome from bedside to bench

Aicardi-Goutières syndrome (AGS) is a progressive genetic encephalopathy caused by pathogenic mutations in genes controlling cellular anti-viral responses and nucleic acid metabolism. The mutations initiate autoinflammatory processes in the brain and systemically that are triggered by chronic overproduction of type I interferon (IFN), including IFN-alpha. Emerging disease-directed therapies aim to dampen autoinflammation and block cellular responses to IFN production, creating an urgent and unmet need to understand better which cells, compartments, and mechanisms underlying disease pathogenesis. In this review, we highlight existing pre-clinical models of AGS and our current understanding of how causative genetic mutations promote disease in AGS, to promote new model development and a continued focus on improving and directing future therapies.

STING-STAT6 Signaling Pathway Promotes IL-4+ and IFN-α+ Fibrotic T Cell Activation and Exacerbates Scleroderma in SKG Mice

Systemic sclerosis (SS) is an autoimmune disease and pathological mechanisms of SS are unclear. In this study, we investigated the role of T cells in the progression of SS using SKG mice and humanized mice. SKG mice have a spontaneous point mutation in ZAP70. We induced scleroderma in SKG mice and a humanized SS mouse model to assess whether T cell-mediated immune responses induce SS. As a result, we found increased dermal thickness, fibrosis, and lymphocyte infiltration in skin tissue in SKG SS mice compared to BALB/c mice (control). Also, blood cytokine level, including IL-4- and IFN-α which are produced by CD4+ T cells via STIM1/STING/STAT6/IRF3 signaling pathways, were increased in SKG mice. Interestingly, skin fibrosis was reduced by inhibiting STING pathway in skin fibroblast. Next, we demonstrated the pathophysiological role of IL-4 and IFN-α in skin fibrosis using a humanized SS mouse model and found increased IL-4- and IFN-α-producing CD4+ T cells and fibrosis. In this study, we found that STING-induced production of IL-4- and type I IFN by CD4+ T cells is a key factor in mouse model and humanized mouse model of SS. Our findings suggest that the STING/STAT6/IRF3 signaling pathways are potential therapeutic targets in SS.

Subset-specific mitochondrial stress and DNA damage shape T cell responses to fever and inflammation

Heat is a cardinal feature of inflammation, yet its impacts on immune cells remain uncertain. We show that moderate-grade fever temperatures (39°C) increased murine CD4 T cell metabolism, proliferation, and inflammatory effector activity while decreasing regulatory T cell suppressive capacity. However, heat-exposed T helper 1 (TH1) cells selectively developed mitochondrial stress and DNA damage that activated Trp53 and stimulator of interferon genes pathways. Although many TH1 cells subjected to such temperatures died, surviving TH1 cells exhibited increased mitochondrial mass and enhanced activity. Electron transport chain complex 1 (ETC1) was rapidly impaired under fever-range temperatures, a phenomenon that was specifically detrimental to TH1 cells. TH1 cells with elevated DNA damage and ETC1 signatures were also detected in human chronic inflammation. Thus, fever-relevant temperatures disrupt ETC1 to selectively drive apoptosis or adaptation of TH1 cells to maintain genomic integrity and enhance effector functions.

A novel TREX1 inhibitor, VB-85680, upregulates cellular interferon responses

Activation of the cGAS-STING pathway plays a key role in the innate immune response to cancer through Type-1 Interferon (IFN) production and T cell priming. Accumulation of cytosolic double-stranded DNA (dsDNA) within tumor cells and dying cells is recognized by the DNA sensor cyclic GMP-AMP synthase (cGAS) to create the secondary messenger cGAMP, which in turn activates STING (STimulator of INterferon Genes), resulting in the subsequent expression of IFN-related genes. This process is regulated by Three-prime Repair EXonuclease 1 (TREX1), a 3' → 5' exonuclease that degrades cytosolic dsDNA, thereby dampening activation of the cGAS-STING pathway, which in turn diminishes immunostimulatory IFN secretion. Here, we characterize the activity of VB-85680, a potent small-molecule inhibitor of TREX1. We first demonstrate that VB-85680 inhibits TREX1 exonuclease activity in vitro in lysates from both human and mouse cell lines. We then show that treatment of intact cells with VB-85680 results in activation of downstream STING signaling, and activation of IFN-stimulated genes (ISGs). THP1-Dual™ cells cultured under low-serum conditions exhibited an enhanced ISG response when treated with VB-85680 in combination with exogenous DNA. Collectively, these findings suggest the potential of a TREX1 exonuclease inhibitor to work in combination with agents that generate cytosolic DNA to enhance the acquisition of the anti-tumor immunity widely associated with STING pathway activation.

Harnessing cGAS-STING axis for therapeutic benefits in systemic lupus erythematosus

The cyclic GMP-AMP synthase (cGAS), a prominent intracellular DNA sensor in mammalian cells, controls the innate immune response and the stimulator of interferon genes (STING)-mediated synthesis of pro-inflammatory cytokines, such as type-I interferon (IFN-I). For decades, IFN-I has been hypothesized to be essential in the development of systemic lupus erythematosus (SLE), a chronic multisystem autoimmunity characterized by immune complex (IC) deposition in small vessels. Recent findings revealed that the activation of the cGAS-STING pathway by self-DNA would propagate the autoimmune responses via upregulating IFN-I production in SLE. In this review, we aimed to provide a comprehensive outlook of the role of the cGAS-STING pathway in SLE pathobiology, as well as, a better understanding of current therapeutic opportunities targeting this axis.

Intratumoral TREX1 Induction Promotes Immune Evasion by Limiting Type I IFN

Chromosomal instability is a hallmark of human cancer that is associated with aggressive disease characteristics. Chromosome mis-segregations help fuel natural selection, but they risk provoking a cGAS-STING immune response through the accumulation of cytosolic DNA. The mechanisms of how tumors benefit from chromosomal instability while mitigating associated risks, such as enhanced immune surveillance, are poorly understood. Here, we identify cGAS-STING-dependent upregulation of the nuclease TREX1 as an adaptive, negative feedback mechanism that promotes immune evasion through digestion of cytosolic DNA. TREX1 loss diminishes tumor growth, prolongs survival of host animals, increases tumor immune infiltration, and potentiates response to immune checkpoint blockade selectively in tumors capable of mounting a type I IFN response downstream of STING. Together, these data demonstrate that TREX1 induction shields chromosomally unstable tumors from immune surveillance by dampening type I IFN production and suggest that TREX1 inhibitors might be used to selectively target tumors that have retained the inherent ability to mount an IFN response downstream of STING. See related article by Lim et al., p. 663.

TREX1 531C/T Polymorphism and Autoantibodies Associated with the Immune Status of HIV-1-Infected Individuals

Autoimmune diseases can develop during HIV-1 infection, mainly related to the individual's immune competence. The study investigated the association of the TREX1 531C/T polymorphism and antinuclear antibodies (ANA) in HIV-1 infection and the time of antiretroviral therapy (ART) used. Cross-sectional and longitudinal assessments were carried out in 150 individuals, divided into three groups: ART-naïve, 5 years and 10 years on ART; ART-naïve individuals were evaluated for 2 years after initiation of treatment. The individuals' blood samples were submitted to indirect immunofluorescence tests, real-time PCR and flow cytometry. The TREX1 531C/T polymorphism was associated with higher levels of TCD4⁺ lymphocytes and IFN-α in individuals with HIV-1. Individuals on ART had a higher frequency of ANA, higher levels of T CD4⁺ lymphocytes, a higher ratio of T CD4⁺/CD8⁺ lymphocytes and higher levels of IFN-α than therapy-naïve individuals (p < 0.05). The TREX1 531C/T polymorphism was associated with better maintenance of the immune status of individuals with HIV-1 and ANA with immune restoration in individuals on ART, indicating the need to identify individuals at risk of developing an autoimmune disease.

RELA tunes innate-like interferon I/III responses in human T cells

In innate immune cells, intracellular sensors such as cGAS-STING stimulate type I/III interferon (IFN) expression, which promotes antiviral defense and immune activation. However, how IFN-I/III expression is controlled in adaptive cells is poorly understood. Here, we identify a transcriptional rheostat orchestrated by RELA that confers human T cells with innate-like abilities to produce IFN-I/III. Despite intact cGAS-STING signaling, IFN-I/III responses are stunted in CD4+ T cells compared with dendritic cells or macrophages. We find that lysine residues in RELA tune the IFN-I/III response at baseline and in response to STING stimulation in CD4+ T cells. This response requires positive feedback driven by cGAS and IRF7 expression. By combining RELA with IRF3 and DNA demethylation, IFN-I/III production in CD4+ T cells reaches levels observed in dendritic cells. IFN-I/III production provides self-protection of CD4+ T cells against HIV infection and enhances the elimination of tumor cells by CAR T cells. Therefore, innate-like functions can be tuned and leveraged in human T cells.

TREX1 cytosolic DNA degradation correlates with autoimmune disease and cancer immunity

The N-terminal domain of Three Prime Repair Exonuclease 1 (TREX1) is catalytically active and can degrade dsDNA or ssDNA in the cytosol, whereas the C-terminal domain is primarily involved in protein localization. TREX1 deficiency induces cytosolic DNA accumulation as well as activation of the cGAS-STING-IFN signaling pathway, which results in tissue inflammation and autoimmune diseases. Furthermore, TREX1 expression in cancer immunity can be adaptively regulated to promote tumor proliferation, making it a promising therapeutic target.

Targeting Type I IFN/STAT1 signaling inhibited and reversed corneal squamous metaplasia in Aire-deficient mouse

Corneal transparency and integrity are essential for obtaining good vision; nevertheless, squamous metaplasia (SQM) of ocular epithelium is a kind of serious blinding corneal diseases, without therapeutic medication in clinic. Here, we found that deficiency of the autoimmune regulator (AIRE) in corneas spontaneously developed corneal plaques. Using corneal abrasion model, we revealed that deletion of Aire not only resulted in delayed corneal re-epithelialization, but also promoted a cell-fate transition from transparent corneal epithelium to keratinized epithelium, histopathologically characterized with SQM based on the transcriptomic analysis. Mechanistically, Aire-deficient corneas led to the heightened Type I interferon (IFN-I)/STAT1 signaling after abrasion. Pharmacological blockade of IFN-I/JAK/STAT1 signaling in Aire-knockout (KO) corneas not only accelerated epithelial wound healing, but also alleviated corneal plaques and SQM. Collectively, our findings revealed critical roles of AIRE in governing corneal epithelial homeostasis and pathologic keratinization, and further identified IFN-I/STAT1 signaling as a potential target for treating ocular surface diseases with SQM, and even for treating pathological scenarios related to SQM in other tissues.

Role of the cGAS-STING pathway in systemic and organ-specific diseases

Cells are equipped with numerous sensors that recognize nucleic acids, which probably evolved for defence against viruses. Once triggered, these sensors stimulate the production of type I interferons and other cytokines that activate immune cells and promote an antiviral state. The evolutionary conserved enzyme cyclic GMP-AMP synthase (cGAS) is one of the most recently identified DNA sensors. Upon ligand engagement, cGAS dimerizes and synthesizes the dinucleotide second messenger 2',3'-cyclic GMP-AMP (cGAMP), which binds to the endoplasmic reticulum protein stimulator of interferon genes (STING) with high affinity, thereby unleashing an inflammatory response. cGAS-binding DNA is not restricted by sequence and must only be >45 nucleotides in length; therefore, cGAS can also be stimulated by self genomic or mitochondrial DNA. This broad specificity probably explains why the cGAS-STING pathway has been implicated in a number of autoinflammatory, autoimmune and neurodegenerative diseases; this pathway might also be activated during acute and chronic kidney injury. Therapeutic manipulation of the cGAS-STING pathway, using synthetic cyclic dinucleotides or inhibitors of cGAMP metabolism, promises to enhance immune responses in cancer or viral infections. By contrast, inhibitors of cGAS or STING might be useful in diseases in which this pro-inflammatory pathway is chronically activated.

Genotype-Phenotype Correlation and Functional Insights for Two Monoallelic TREX1 Missense Variants Affecting the Catalytic Core

The TREX1 exonuclease degrades DNA to prevent aberrant nucleic-acid sensing through the cGAS-STING pathway, and dominant Aicardi–Goutières Syndrome type 1 (AGS1) represents one of numerous TREX1-related autoimmune diseases. Monoallelic TREX1 mutations were identified in patients showing early-onset cerebrovascular disease, ascribable to small vessel disease, and CADASIL-like neuroimaging. We report the clinical-neuroradiological features of two patients with AGS-like (Patient A) and CADASIL-like (Patient B) phenotypes carrying the heterozygous p.A136V and p.R174G TREX1 variants, respectively. Genetic findings, obtained by a customized panel including 183 genes associated with monogenic stroke, were combined with interferon signature testing and biochemical assays to determine the mutations’ effects in vitro. Our results for the p.A136V variant are inconsistent with prior biochemistry-pathology correlates for dominant AGS-causing TREX1 mutants. The p.R174G variant modestly altered exonuclease activity in a manner consistent with perturbation of substrate interaction rather than catalysis, which represents the first robust enzymological data for a TREX1 variant identified in a CADASIL-like patient. In conclusion, functional analysis allowed us to interpret the impact of TREX1 variants on patients’ phenotypes. While the p.A136V variant is unlikely to be causative for AGS in Patient A, Patient B’s phenotype is potentially related to the p.R174G variant. Therefore, further functional investigations of TREX1 variants found in CADASIL-like patients are warranted to determine any causal link and interrogate the molecular disease mechanism(s).

Regulation and function of the cGAS-MITA/STING axis in health and disease

The innate immune systems detect pathogens via pattern-recognition receptors including nucleic acid sensors and non-nucleic acid sensors. Cyclic guanosine monophosphate-adenosine monophosphate (cGAMP) synthase (cGAS, also known as MB21D1) is a cytosolic DNA sensor that recognizes double-stranded DNA (dsDNA) and catalyzes the synthesis of 2',3'-cGAMP. Subsequently, 2',3'-cGAMP binds to the adaptor protein mediator of IRF3 activation (MITA, also known as STING, MPYS, ERIS, and TMEM173) to activate downstream signaling cascades. The cGAS-MITA/STING signaling critically mediates immune responses against DNA viruses, retroviruses, bacteria, and protozoan parasites. In addition, recent discoveries have extended our understanding of the roles of the cGAS-MITA/STING pathway in autoimmune diseases and cancers. Here, we summarize the identification and activation of cGAS and MITA/STING, present the updated functions and regulatory mechanisms of cGAS-MITA/STING signaling and provide a comprehensive understanding of the cGAS-MITA/STING axis in autoimmune diseases and cancers.

TREX1 as a Novel Immunotherapeutic Target

Mutations in the TREX1 3' → 5' exonuclease are associated with a spectrum of autoimmune disease phenotypes in humans and mice. Failure to degrade DNA activates the cGAS-STING DNA-sensing pathway signaling a type-I interferon (IFN) response that ultimately drives immune system activation. TREX1 and the cGAS-STING DNA-sensing pathway have also been implicated in the tumor microenvironment, where TREX1 is proposed to degrade tumor-derived DNA that would otherwise activate cGAS-STING. If tumor-derived DNA were not degraded, the cGAS-STING pathway would be activated to promote IFN-dependent antitumor immunity. Thus, we hypothesize TREX1 exonuclease inhibition as a novel immunotherapeutic strategy. We present data demonstrating antitumor immunity in the TREX1 D18N mouse model and discuss theory surrounding the best strategy for TREX1 inhibition. Potential complications of TREX1 inhibition as a therapeutic strategy are also discussed.

ER-directed TREX1 limits cGAS activation at micronuclei

Micronuclei are aberrant nuclear compartments that can form as a result of chromosome mis-segregation. Frequent loss of micronuclear envelope integrity exposes DNA to the cytoplasm, leading to chromosome fragmentation and immune activation. Here, we use micronuclei purification to show that the endoplasmic reticulum (ER)-associated nuclease TREX1 inhibits cGAS activation at micronuclei by degrading micronuclear DNA upon micronuclear envelope rupture. We demonstrate that the ER accesses ruptured micronuclei and plays a critical role in enabling TREX1 nucleolytic attack. TREX1 mutations, previously implicated in immune disease, untether TREX1 from the ER, disrupt TREX1 localization to micronuclei, diminish micronuclear DNA damage, and enhance cGAS activation. These results establish ER-directed resection of micronuclear DNA by TREX1 as a critical regulator of cytosolic DNA sensing in chromosomally unstable cells and provide a mechanistic basis for the importance of TREX1 ER tethering in preventing autoimmunity.

The Trinity of cGAS, TLR9, and ALRs Guardians of the Cellular Galaxy Against Host-Derived Self-DNA

The immune system has evolved to protect the host from the pathogens and allergens surrounding their environment. The immune system develops in such a way to recognize self and non-self and develops self-tolerance against self-proteins, nucleic acids, and other larger molecules. However, the broken immunological self-tolerance leads to the development of autoimmune or autoinflammatory diseases. Pattern-recognition receptors (PRRs) are expressed by immunological cells on their cell membrane and in the cytosol. Different Toll-like receptors (TLRs), Nod-like receptors (NLRs) and absent in melanoma-2 (AIM-2)-like receptors (ALRs) forming inflammasomes in the cytosol, RIG (retinoic acid-inducible gene)-1-like receptors (RLRs), and C-type lectin receptors (CLRs) are some of the PRRs. The DNA-sensing receptor cyclic GMP–AMP synthase (cGAS) is another PRR present in the cytosol and the nucleus. The present review describes the role of ALRs (AIM2), TLR9, and cGAS in recognizing the host cell DNA as a potent damage/danger-associated molecular pattern (DAMP), which moves out to the cytosol from its housing organelles (nucleus and mitochondria). The introduction opens with the concept that the immune system has evolved to recognize pathogens, the idea of horror autotoxicus, and its failure due to the emergence of autoimmune diseases (ADs), and the discovery of PRRs revolutionizing immunology. The second section describes the cGAS-STING signaling pathway mediated cytosolic self-DNA recognition, its evolution, characteristics of self-DNAs activating it, and its role in different inflammatory conditions. The third section describes the role of TLR9 in recognizing self-DNA in the endolysosomes during infections depending on the self-DNA characteristics and various inflammatory diseases. The fourth section discusses about AIM2 (an ALR), which also binds cytosolic self-DNA (with 80–300 base pairs or bp) that inhibits cGAS-STING-dependent type 1 IFN generation but induces inflammation and pyroptosis during different inflammatory conditions. Hence, this trinity of PRRs has evolved to recognize self-DNA as a potential DAMP and comes into action to guard the cellular galaxy. However, their dysregulation proves dangerous to the host and leads to several inflammatory conditions, including sterile-inflammatory conditions autoinflammatory and ADs.

TREX1 - Apex predator of cytosolic DNA metabolism

The cytosolic Three prime Repair EXonuclease 1 (TREX1) is a powerful DNA-degrading enzyme required for clearing cytosolic DNA to prevent aberrant inflammation and autoimmunity. In the absence of TREX1 activity, cytosolic DNA pattern recognition receptors of the innate immune system are constitutively activated by undegraded TREX1 substrates. This triggers a chronic inflammatory response in humans expressing mutant TREX1 alleles, eliciting a spectrum of rare autoimmune diseases dependent on the nature of the mutation. The precise origins of cytosolic DNA targeted by TREX1 continue to emerge, but DNA emerging from the nucleus or taken up by the cell could represent potential sources. In this Review, we explore the biochemical and immunological data supporting the role of TREX1 in suppressing cytosolic DNA sensing, and discuss the possibility that TREX1 may contribute to maintenance of genome integrity.