Endogenous Nucleic Acids Research Articles

Spontaneous Type I IFN response in SAMHD1-deficient mice requires both, functional intracellular RNA and DNA sensing pathways

Nucleic acids are potent inducers of the antiviral type I interferons (IFN) and therefore represent prototypic PAMPs in viral infections. Innate nucleic acids sensors patrol the cytoplasm for the presence of RNA and DNA but have only a limited capacity to discriminate between endogenous and exogenous nucleic acids. Therefore mechanisms evolved that prevent the accumulation of endogenously derived nucleic acids. In Aicardi-Goutieres syndrome (AGS), which represents a rare monogenic variant of the prototypic autoimmune disease systemic lupus erythematosus, genetic defects of these mechanisms result in production of large amounts of type I IFN. SAMHD1 is an intracellular nuclease that degrades RNA and DNA and cleaves deoxynucleotides (dNTP) into nucleosides and inorganic triphosphate, mutation of which cause AGS. The nucleotide triphosphohydrolase also confers antiretroviral activity to SAMHD1, as dNTP degradation was shown to represent a major block to reverse transcription of retroviruses. How and why SAMHD1-deficent cells activate the type I IFN system is not known so far. Furthermore, a comprehensive analysis of the antiretroviral potential of mouse SAMHD1 is still lacking.

The regional function of cGAS/STING signal in multiple organs: One of culprit behind systemic lupus erythematosus?

The systemic dysregulation of adaptive and innate immunity have been identified as major hallmark of systemic lupus erythematosus (SLE) pathogenesis that predominantly affects women. Patients with SLE develop heterogeneous clinical manifestations which involve of multiple organ damage including renal, spleen, nervous system, joints and hematopoietic organs. A high rate of cell death, e.g., NETosis, and clearance deficiencies by myeloid cells led to increased cell debris and accumulation of endogenous nucleic acids, and the presence of anti-nuclear antibodies (ANAs) derived from immune response can break of self-tolerance and exacerbate SLE pathology. Currently, the nucleic acid receptors, such as Toll-like receptors, RIG-I-like receptors, AIM2-like receptors and IFI 200-family have been uncovered to be potential predisposing causes for SLE via triggering interferon (IFN) response and maturation of IL-1β. Notably, as the newly found DNA sensor, cyclic GMP-AMP synthase (cGAS) can activate the stimulator of interferon genes (STING), which plays a pivotal role in DNA/RNA sensing pathway, for type I IFN and other inflammatory cytokines induction including IL-6 and attributes to STING-associated inflammatory disorders. Interestingly, the elevated levels of IFN-α/β and IFN-stimulated genes were found in SLE patients than healthy individuals. Given this, we propose a hypothesis that the cGAS–STING pathway in multiple organs function versatile and can facilitate overall disease progression of SLE though impertinent cytosolic self-DNA sensing.

Genome-wide DNA hypomethylation and RNA:DNA hybrid accumulation in Aicardi-Goutières syndrome.

Aicardi-Goutières syndrome (AGS) is a severe childhood inflammatory disorder that shows clinical and genetic overlap with systemic lupus erythematosus (SLE). AGS is thought to arise from the accumulation of incompletely metabolized endogenous nucleic acid species owing to mutations in nucleic acid-degrading enzymes TREX1 (AGS1), RNase H2 (AGS2, 3 and 4), and SAMHD1 (AGS5). However, the identity and source of such immunogenic nucleic acid species remain undefined. Using genome-wide approaches, we show that fibroblasts from AGS patients with AGS1-5 mutations are burdened by excessive loads of RNA:DNA hybrids. Using MethylC-seq, we show that AGS fibroblasts display pronounced and global loss of DNA methylation and demonstrate that AGS-specific RNA:DNA hybrids often occur within DNA hypomethylated regions. Altogether, our data suggest that RNA:DNA hybrids may represent a common immunogenic form of nucleic acids in AGS and provide the first evidence of epigenetic perturbations in AGS, furthering the links between AGS and SLE.

The multifaceted biology of plasmacytoid dendritic cells.

Plasmacytoid dendritic cells (pDCs) are a unique DC subset that specializes in the production of type I interferons (IFNs). pDCs promote antiviral immune responses and have been implicated in the pathogenesis of autoimmune diseases that are characterized by a type I IFN signature. However, pDCs can also induce tolerogenic immune responses. In this Review, we summarize recent progress in the field of pDC biology, focusing on the molecular mechanisms that regulate the development and functions of pDCs, the pathways involved in their sensing of pathogens and endogenous nucleic acids, their functions at mucosal sites, and their roles in infection, autoimmunity and cancer.

Lupus erythematosus

Lupus erythematosus is an autoimmune disease with a broad spectrum of cutaneous manifestations. The pathogenesis of lupus is based on a loss of tolerance against self antigens and can be mediated by defects in apoptosis, defects in eliminating cellular remnants and increased activation of the innate as well as the adaptive immune system. The increased activation of the innate immune system can be mediated by sensing of endogenous or exogenous nucleic acids, genetic variants in the components of the receptor cascade or disturbances in restriction of self nucleic acids. The inflammatory milieu is characterized by type I interferon expression and autoantibody production. The main trigger factors of the disease are sun exposure and viral infections. Lupus erythematosus is effectively treated by glucocorticosteroids. Approved alternatives for long-term treatment are antimalarial agents and the B-cell inhibitor belimumab for patients with systemic lupus erythematosus. Future studies should more intensely analyse the effect of novel therapies on cutaneous manifestations to allow early detection of cutaneous lupus. Furthermore novel therapeutic strategies which specifically target the responsible pathogenetic mechanisms of the individual subtypes of lupus erythematosus are needed to improve the therapeutic success for this heterogeneous patient population.

Estrogen receptor-dependent modulation of dendritic cell biology: of mice and women

Autoimmune and infectious diseases differentially affect women from men. Women tend to develop stronger immune responses and thus in general men are more susceptible to infectious diseases whereas women are more likely to develop autoimmune diseases. These differences could be in part attributable to the pro-inflammatory role of the female sex hormone estrogen on immunity and particularly on dendritic cells (DCs), a key subset of innate immune cells. For several years now, we have undertaken studies to understand how estrogens influence the biology of murine and human DCs. We and others have demonstrated that estradiol (E2) was required for the optimal in vitro differentiation of murine DCs and acquisition of their effector functions. These effects on DC biology were dependent on the activation of the estrogen receptor a (ERa). More recently, we focused our interest on plasmacytoid dendritic cells (pDCs). Indeed, this subset that produces large amounts of IFN-a/b in response to viral or endogenous nucleic acids through activation of their TLR-7 and TLR-9 show gender differences with enhanced IFN-a production by pDCs from women, compared to men. We could establish, in Human and in mice, that in vivo treatment with E2 enhanced the TLR-dependent production of IFNa by pDCs. In mice, we demonstrated that the amplifying effect of endogenous and exogenous estrogens is dependent on the intrinsic activation of ERα by hormone in the pDCs. To further characterize the mechanisms underlying this sex-based difference in pDC innate functions, we investigated the respective contribution of X chromosome dosage versus sex hormones using a humanized mouse model in which male or female NOD-SCID-s2m -/- mice were transplanted with human progenitor cells (HPCs) purified from either male (XY) or female (XX) donors. We could show that cell-intrinsic ER-signaling and X chromosome complement both independently contribute to the enhanced TLR-7-mediated response of pDCs in women, which may account for the sex-based differences in autoimmune and infectious diseases. Altogether, our work demonstrates that estrogen-mediated activation of ER signaling is a key regulator of DC biology both in Human and in mouse.

SnapShot: Nucleic Acid Immune Sensors, Part 2

The innate immune system has evolved sensors that can detect specific molecular fingerprints of non-self RNA or DNA. At the same time, some receptors respond to nucleic acids of both exogenous and endogenous origin, yet they are spatially segregated from endogenous nucleic acids. This SnapShot schematizes families and individual members of nucleic acid sensors with a focus on their ligands and the signaling pathways they employ.

SnapShot: Nucleic Acid Immune Sensors, Part 1

The innate immune system has evolved sensors that can detect specific molecular fingerprints of non-self RNA or DNA. At the same time, some receptors respond to nucleic acids of both exogenous and endogenous origin, yet they are spatially segregated from endogenous nucleic acids. This SnapShot schematizes families and individual members of nucleic acid sensors with a focus on their ligands and the signaling pathways they employ.

Chemical shift assignments of DRB4 (1-153), a dsRNA binding protein in A. thaliana RNAi pathway.

RNA interference (RNAi) is a conserved biological response to dsRNA and regulates the expression of protein-coding genes to mediate resistance to both endogenous parasitic and exogenous pathogenic nucleic acids. In RNAi pathway, dsRNA binding proteins assists Dicer at various stages of RNAi. In plants, DRB4, is a multidomain protein containing two dsRNA binding domains that recognizes the long exogenous/endogenous dsRNA and presents it to Ribonuclease enzyme, Dicer like 4, resulting in the production of 21nt small interfering RNA. Here, we report nearly complete backbone and sidechain chemical shift assignments of N-terminus of DRB4 (1-153, ~18kDa), containing both double stranded RNA binding domains and the linker.

Advances in understanding the role of type I interferons in systemic lupus erythematosus.

Advances in understanding the genetic and molecular basis of innate immune system activation and function have supported the hypothesis that type I interferons (IFN-I), the essential mediators of antiviral host defense, are central contributors to the pathogenesis of systemic lupus erythematosus (SLE). This review addresses the recent data that support the rationale for therapeutic targeting of the IFN-I pathway in SLE. New insights into the mechanisms of cell-intrinsic innate immune system activation, driven by endogenous virus-like nucleic acids and potentially modified by environmental stressors, provide a model for the induction of IFN-I that may precede the clinically apparent autoimmunity in patients with lupus. Further amplification of IFN-α production, induced by nucleic-acid-containing immune complexes that activate endosomal Toll-like receptors, augments and sustains immune system activation, autoimmunity and tissue damage. As demonstrated in the murine studies of persistent virus infection accompanied by sustained production of IFN-I, blockade of the IFN-I pathway may reverse the immune dysregulation and tissue damage that are the essential features of the immunopathogenesis of SLE. Recent research progress has identified numerous therapeutic targets, and specific candidate therapeutics relevant to the IFN-I pathway are under investigation.

The Heparan Sulfate Mimetic, PG545, Induces a Potent Anti-Lymphoma Effect By Enhancing Innate Immune Activation By Endogenous Nucleic Acids.

The Heparan Sulfate Mimetic, PG545, Induces a Potent Anti-Lymphoma Effect By Enhancing Innate Immune Activation By Endogenous Nucleic Acids.

TREX1 deficiency triggers cell-autonomous immunity in a cGAS-dependent manner.

Cytosolic detection of DNA is crucial for the initiation of antiviral immunity but can also cause autoimmunity in the context of endogenous nucleic acids being sensed. Mutations in the human 3' repair exonuclease 1 (TREX1) have been linked to the type I IFN-associated autoimmune disease Aicardi-Goutières syndrome. The exact mechanisms driving unabated type I IFN responses in the absence of TREX1 are only partly understood, but it appears likely that accumulation of endogenous DNA species triggers a cell-autonomous immune response by activating a cytosolic DNA receptor. In this article, we demonstrate that knocking out the DNA sensor cyclic GMP-AMP synthase completely abrogates spontaneous induction of IFN-stimulated genes in TREX1-deficient cells. These findings indicate a key role of cyclic GMP-AMP synthase for the initiation of self-DNA-induced autoimmune disorders, thus providing important implications for novel therapeutic approaches.

AB0177 Toll-Like Receptors - Differences between Healthy Control, ANCA Associated Vasculitis and Lupus Kindeys - Preliminary Pilot Study

BackgroundSystemic lupus erythematosus (SLE) is characterized by a defective monocyte/macrophage-mediated clearance of the apoptotic (nucleic acid-containing) particles in vitro. The large amount of nucleic acids, free and/or complexed, are present...

AB0111 Role of Toll-Like Receptor 9 in the Pathogenesis of Inflammatory Autoimmune Arthritis and Osteoclast Activation

BackgroundThere is some evidence that release and insufficient removal of endogenous nucleic acids may be involved in triggering harmful autoimmune reactions that might be important in the initiation of RA....

The enemy within: endogenous retroelements and autoimmune disease

Inappropriate or chronic detection of self nucleic acids by the innate immune system underlies many human autoimmune diseases. We discuss here an unexpected source of endogenous immunostimulatory nucleic acids: the reverse-transcribed cDNA of endogenous retroelements. The interplay between innate immune sensing and clearance of retroelement cDNA has important implications for the understanding of immune responses to infectious retroviruses such as human immunodeficiency virus (HIV). Furthermore, the detection of cDNA by the innate immune system reveals an evolutionary tradeoff: selection for a vigorous, sensitive response to infectious retroviruses may predispose the inappropriate detection of endogenous retroelements. We propose that this tradeoff has placed unique constraints on the sensitivity of the DNA-activated antiviral response, with implications for the interactions of DNA viruses and retroviruses with their hosts. Finally, we discuss how better understanding of the intersection of retroelement biology and innate immunity can guide the way to novel therapies for specific autoimmune diseases.

Genotypic analysis of Asp299Gly and Thr399Ile polymorphisms of TLR4 in Egyptian patients with rheumatoid arthritis and systemic lupus erythematosus

Toll-like receptors (TLRs) are essential molecules of the innate immune system that stimulate numerous inflammatory pathways and harmonize systemic defense against a wide array of pathogens. TLRs may also recognize a number of self-proteins and endogenous nucleic acids. Inappropriate stimulation of the TLR pathway by endogenous or exogenous ligands or as a consequence of mutation in the TLR genes may lead to induction and/or prolongation of autoimmune response and tissue injury. This study was designed to detect the TLR4 (Asp299Gly and Thr399Ile) gene polymorphism in Egyptian patients with rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE) as well as its correlation with disease activity. This study enrolled 50 patients who were attending the internal medicine and immunology out-patient clinic of Beni-Suef University Hospital. They were classified into three groups: 20 patients with RA, 20 patients with SLE, and 10 healthy controls. All studied persons were subjected to complete clinical evaluation, laboratory investigations, and detection of mutation in the TLR4 (Asp299Gly and Thr399Ile) gene by PCR technique. No individual in the RA patients group, the SLE patients group, or control population was identified carrying the Asp299Gly and Thr399Ile polymorphisms; in other words, all RA patients, SLE patients, and controls were with the same wild genotype. The similarity in genotype between the patients group and control population concluded that these two missense polymorphisms do not contribute to RA and SLE in a group of Egyptian population.

4.8 Role of Toll-like receptor 9 in the pathogenesis of inflammatory autoimmune arthritis and osteoclast activation

Background and ObjectivesThere is some evidence that release and insufficient removal of endogenous nucleic acids may be involved in triggering harmful autoimmune reactions that might be important in the initiation...

Aicardi–Goutières syndrome: a model disease for systemic autoimmunity

Systemic autoimmunity is a complex disease process that results from a loss of immunological tolerance characterized by the inability of the immune system to discriminate self from non-self. In patients with the prototypic autoimmune disease systemic lupus erythematosus (SLE), formation of autoantibodies targeting ubiquitous nuclear antigens and subsequent deposition of immune complexes in the vascular bed induces inflammatory tissue injury that can affect virtually any organ system. Given the extraordinary genetic and phenotypic heterogeneity of SLE, one approach to the genetic dissection of complex SLE is to study monogenic diseases, for which a single gene defect is responsible. Considerable success has been achieved from the analysis of the rare monogenic disorder Aicardi-Goutières syndrome (AGS), an inflammatory encephalopathy that clinically resembles in-utero-acquired viral infection and that also shares features with SLE. Progress in understanding the cellular and molecular functions of the AGS causing genes has revealed novel pathways of the metabolism of intracellular nucleic acids, the major targets of the autoimmune attack in patients with SLE. Induction of autoimmunity initiated by immune recognition of endogenous nucleic acids originating from processes such as DNA replication/repair or endogenous retro-elements represents novel paradigms of SLE pathogenesis. These findings illustrate how investigating rare monogenic diseases can also fuel discoveries that advance our understanding of complex disease. This will not only aid the development of improved tools for SLE diagnosis and disease classification, but also the development of novel targeted therapeutic approaches.

TLR and NKG2D Signaling Pathways Mediate CS-Induced Pulmonary Pathologies

Long-term exposure to cigarette smoke (CS) can have deleterious effects on lung epithelial cells including cell death and the initiation of inflammatory responses. CS-induced cell injury can elaborate cell surface signals and cellular byproducts that stimulate immune system surveillance. Our previous work has shown that the expression of ligands for the cytotoxic lymphocyte activating receptor NKG2D is enhanced in patients with COPD and that the induction of these ligands in a mouse model can replicate COPD pathologies. Here, we extend these findings to demonstrate a role for the NKG2D receptor in CS-induced pathophysiology and provide evidence linking nucleic acid-sensing endosomal toll-like receptor (TLR) signaling to COPD pathology through NKG2D activation. Specifically, we show that mice deficient in NKG2D exhibit attenuated pulmonary inflammation and airspace enlargement in a model of CS-induced emphysema. Additionally, we show that CS exposure induces the release of free nucleic acids in the bronchoalveolar lavage and that direct exposure of mouse lung epithelial cells to cigarette smoke extract similarly induces functional nucleic acids as assessed by TLR3, 7, and 9 reporter cell lines. We demonstrate that exposure of mouse lung epithelial cells to TLR ligands stimulates the surface expression of RAET1, a ligand for NKG2D, and that mice deficient in TLR3/7/9 receptor signaling do not exhibit CS-induced NK cell hyperresponsiveness and airspace enlargement. The findings indicate that CS-induced airway injury stimulates TLR signaling by endogenous nucleic acids leading to elevated NKG2D ligand expression. Activation of these pathways plays a major role in the altered NK cell function, pulmonary inflammation and remodeling related to long-term CS exposure.

Sjögren's Syndrome

Sjögren's syndrome (SS) is a chronic autoimmune disorder that typically affects exocrine glands--mainly labial and lacrimal--leading to complaints of dry mouth and eyes. Given that periepithelial mononuclear cell infiltrates, both in exocrine glands and in other parenchymal organs (kidney, lung, and liver), are the histopathological disease hallmark, the term autoimmune epithelitis has been proposed. B cell hyperactivity is another cardinal SS feature manifested by the presence of autoantibodies and hypergammaglobulinemia, as well as clinical/serological phenotypes mediated by immune complexes, such as peripheral neuropathy, vasculitic lesions, and hypocomplementemia. These have been designated adverse predictors for lymphoma development in approximately 5% to 10% of patients. Activation of the type I interferon/B cell-activating factor axis in SS has recently attracted particular attention. Inappropriate overexpression of endogenous nucleic acids in a genetically susceptible individual might provide a plausible scenario for the immune activation observed in SS.