Endogenous retroelements and autoimmune disease

Senolytic Vaccines from the Central and Peripheral Tolerance Perspective

Preventive medicine has proven its long-term effectiveness and economic feasibility. Over the last century, vaccination has saved more lives than any other medical technology. At present, preventative measures against most infectious diseases are successfully used worldwide; in addition, vaccination platforms against oncological and even autoimmune diseases are being actively developed. At the same time, the development of medicine led to an increase in both life expectancy and the proportion of age-associated diseases, which pose a heavy socio-economic burden. In this context, the development of vaccine-based approaches for the prevention or treatment of age-related diseases opens up broad prospects for extending the period of active longevity and has high economic potential. It is well known that the development of age-related diseases is associated with the accumulation of senescent cells in various organs and tissues. It has been demonstrated that the elimination of such cells leads to the restoration of functions, rejuvenation, and extension of the lives of experimental animals. However, the development of vaccines against senescent cells is complicated by their antigenic heterogeneity and the lack of a unique marker. In addition, senescent cells are the body's own cells, which may be the reason for their low immunogenicity. This mini-review discusses the mechanisms of central and peripheral tolerance that may influence the formation of an anti-senescent immune response and be responsible for the accumulation of senescent cells with age.

STING signaling in the brain: Molecular threats, signaling activities, and therapeutic challenges

Stimulator of interferon genes (STING) is an innate immune signaling protein critical to infections, autoimmunity, and cancer. STING signaling is also emerging as an exciting and integral part of many neurological diseases. Here, we discuss recent advances in STING signaling in the brain. We summarize how molecular threats activate STING signaling in the diseased brain and how STING signaling activities in glial and neuronal cells cause neuropathology. We also review human studies of STING neurobiology and consider therapeutic challenges in targeting STING to treat neurological diseases.

New Insights into Endogenous Retrovirus-K Transcripts in Amyotrophic Lateral Sclerosis

Retroviral reverse transcriptase activity and the increased expression of human endogenous retroviruses (HERVs) are associated with amyotrophic lateral sclerosis (ALS). We were interested in confirming HERVK overexpression in the ALS brain, its use as an accessory diagnostic marker for ALS, and its potential interplay with neuroinflammation. Using qPCR to analyze HERVK expression in peripheral blood mononuclear cells (PBMCs) and in postmortem brain samples from ALS patients, no significant differences were observed between patients and control subjects. By contrast, we report alterations in the expression patterns of specific HERVK copies, especially in the brainstem. Out of 27 HERVK copies sampled, the relative expression of 17 loci was >1.2-fold changed in samples from ALS patients. In particular, the relative expression of two HERVK copies (Chr3-3 and Chr3-5) was significantly different in brainstem samples from ALS patients compared with controls. Further qPCR analysis of inflammation markers in brain samples revealed a significant increase in NLRP3 levels, while TNFA, IL6, and GZMB showed slight decreases. We cannot confirm global HERVK overexpression in ALS, but we can report the ALS-specific overexpression of selected HERVK copies in the ALS brain. Our data are compatible with the requirement for better patient stratification and support the potential importance of particular HERVK copies in ALS.

Clinical spectrum and currently available treatment of type I interferonopathy Aicardi-Goutières syndrome

BACKGROUND

Aicardi-Goutières syndrome (AGS) is a genetically determined disorder with a variable phenotype. Since the original description of AGS, advances in gene sequencing techniques have resulted in a significant broadening of the phenotypic spectrum associated with AGS genes, and new clinical pictures have emerged beyond the classic presentation. The aim of this review is to provide a comprehensive analysis of the clinical spectrum of AGS and report currently available treatments and new immunosuppressive strategies.

DATA SOURCES

Literature reviews and original research articles were collected from databases, including PubMed and ClinicalTrials.gov. Relevant articles about AGS were included.

RESULTS

The involvement of the nervous system certainly represents the major cause of mortality and morbidity in AGS patients. However, other clinical manifestations, such as chilblains, hepatosplenomegaly, and hematological disturbances, may lead to the diagnosis and considerably impact the prognosis and overall quality of life of these patients. Therapeutic approaches of AGS are limited to interventions aimed at specific symptoms and the management of multiple comorbidities. However, advances in understanding the pathogenesis of AGS could open new and more effective therapies.

CONCLUSIONS

The over-activation of innate immunity due to upregulated interferon production plays a critical role in AGS, leading to multi-organ damage with the main involvement of the central nervous system. To date, there is no specific and effective treatment for AGS. New drugs specifically targeting the interferon pathway may bring new hope to AGS patients.

The Role of Viral Infections in the Onset of Autoimmune Diseases

Autoimmune diseases (AIDs) are the consequence of a breach in immune tolerance, leading to the inability to sufficiently differentiate between self and non-self. Immune reactions that are targeted towards self-antigens can ultimately lead to the destruction of the host's cells and the development of autoimmune diseases. Although autoimmune disorders are comparatively rare, the worldwide incidence and prevalence is increasing, and they have major adverse implications for mortality and morbidity. Genetic and environmental factors are thought to be the major factors contributing to the development of autoimmunity. Viral infections are one of the environmental triggers that can lead to autoimmunity. Current research suggests that several mechanisms, such as molecular mimicry, epitope spreading, and bystander activation, can cause viral-induced autoimmunity. Here we describe the latest insights into the pathomechanisms of viral-induced autoimmune diseases and discuss recent findings on COVID-19 infections and the development of AIDs.

Involvement of retroelements in the autoimmune response in humans

Retroelements are mobile genomic components requiring an RNA intermediate which is reverse-transcribed into complementary DNA for transposition. Human genome contains a vast amount of retroelements including retrotransposons and endogenous retroviruses. These elements are categorized according to presence or absence of long terminal repeats, LTRs or non-LTRs, as well as autonomous and non-autonomous according to involvement of reverse transcriptase. The retroelements have been accumulated in mammalian genomes over all evolutionary times through vertical transmission, and many of them were inactivated through accumulation of mutations. However, the retroelements entered into genome within the last 200,000 years are mostly functional. Some of the active retroelements are associated with varying autoimmune diseases because anti-retroelement antibodies might cross-react with other proteins in the human body. For instance, autoimmunity and inflammation could be stimulated by increased expression of long interspersed element 1 (LINE-1 or L1) or decreased L1 degradation. Different regulation of L1 expression might be related to the genetic and sex-related variations or environmental factors. Activation of retroelements is also controlled by epigenetic silencing mechanisms such as histone modification. Elevated levels of L1 retroelements could trigger the production of type I interferon, a crucial innate defense mechanism in mammals against viruses, and systemic autoimmune response is induced. Loss-of-function in some deoxyribonucleases (DNases) such as three prime repair exonuclease 1 that degrades reverse-transcribed DNA is also related to autoimmune diseases. Additionally, human endogenous retroviruses also play a role in autoimmune diseases. Involvement of retroelements in autoimmune disorders is exemplified with three diseases, i.e. systemic lupus erythematosus, Aicardi–Goutières syndrome, and multiple sclerosis.

How Pyroptosis Contributes to Inflammation and Fibroblast-Macrophage Cross-Talk in Rheumatoid Arthritis

About thirty years ago, a new form of pro-inflammatory lytic cell death was observed and termed pyroptosis. Only in 2015, gasdermins were defined as molecules that create pores at the plasma membrane and drive pyroptosis. Today, we know that gasdermin-mediated death is an important antimicrobial defence mechanism in bacteria, yeast and mammals as it destroys the intracellular niche for pathogen replication. However, excessive and uncontrolled cell death also contributes to immunopathology in several chronic inflammatory diseases, including arthritis. In this review, we discuss recent findings where pyroptosis contributes to tissue damage and inflammation with a main focus on injury-induced and autoimmune arthritis. We also review novel functions and regulatory mechanisms of the pyroptotic executors gasdermins. Finally, we discuss possible models of how pyroptosis may contribute to the cross-talk between fibroblast and macrophages, and also how this cross-talk may regulate inflammation by modulating inflammasome activation and pyroptosis induction.

Red blood cell-derived phosphatidylserine positive extracellular vesicles are associated with past thrombotic events in patients with systemic erythematous lupus

Background

Extracellular vesicles (EVs) released by blood cells have proinflammation and procoagulant action. Patients with systemic lupus erythematosus (SLE) present high vascular inflammation and are prone to develop cardiovascular diseases. Therefore, we postulated that the EV populations found in blood, including platelet EVs (PEVs) and red blood cell EVs (REVs), are associated with SLE disease activity and SLE-associated cardiovascular accidents.

Method

We assessed autotaxin (ATX) plasma levels by ELISA, the platelet activation markers PAC1 and CD62P, ATX bound to platelets and the amounts of plasma PEVs and REVs by flow cytometry in a cohort of 102 patients with SLE, including 29 incident cases of SLE and 30 controls. Correlation analyses explored the associations with the clinical parameters.

Result

Platelet activation markers were increased in patients with SLE compared with healthy control, with the marker CD62P associated with the SLE disease activity index (SLEDAI). The incident cases show additional associations between platelet markers (CD62P/ATX and PAC1/CD62P) and the SLEDAI. Compared with controls, patients with SLE presented higher levels of PEVs, phosphatidylserine positive (PS⁺) PEVs, REVs and PS⁺ REVs, but there is no association with disease activity. When stratified according to the plasma level of PS⁺ REVs, the group of patients with SLE with a high level of PS⁺ REVs presented a higher number of past thrombosis events and higher ATX levels.

Conclusion

Incident and prevalent forms of SLE cases present similar levels of platelet activation markers, with CD62P correlating with disease activity. Though EVs are not associated with disease activity, the incidence of past thrombotic events is higher in patients with a high level of PS⁺ REVs.

Factors Regulating the Activity of LINE1 Retrotransposons

LINE-1 (L1) is a class of autonomous mobile genetic elements that form somatic mosaicisms in various tissues of the organism. The activity of L1 retrotransposons is strictly controlled by many factors in somatic and germ cells at all stages of ontogenesis. Alteration of L1 activity was noted in a number of diseases: in neuropsychiatric and autoimmune diseases, as well as in various forms of cancer. Altered activity of L1 retrotransposons for some pathologies is associated with epigenetic changes and defects in the genes involved in their repression. This review discusses the molecular genetic mechanisms of the retrotransposition and regulation of the activity of L1 elements. The contribution of various factors controlling the expression and distribution of L1 elements in the genome occurs at all stages of the retrotransposition. The regulation of L1 elements at the transcriptional, post-transcriptional and integration into the genome stages is described in detail. Finally, this review also focuses on the evolutionary aspects of L1 accumulation and their interplay with the host regulation system.

Expression of Retroelements in Mammalian Gametes and Embryos

Retroelements are genetic mobile elements, expressed during male and female gamete differentiation. Retrotransposons are normally regulated by the methylation machinery, chromatin modifications, non-coding RNAs, and transcription factors, while retrotransposition control is of vital importance in cellular proliferation and differentiation process. Retrotransposition requires a transcription step, by a cellular RNA polymerase, followed by reverse transcription of an RNA intermediate to cDNA and its integration into a new genomic locus. Long interspersed elements (LINEs), human endogenous retroviruses (HERVs), short interspersed elements (SINEs) and SINE-VNTR-Alu elements (SVAs) constitute about half of the human genome, play a crucial role in genome organization, structure and function and interfere with several biological procedures. In this mini review, we discuss recent data regarding retroelement expression (LINE-1, HERVK-10, SVA and VL30) and retrotransposition events in mammalian oocytes and spermatozoa, as well as the importance of their impact on human and mouse preimplantation embryo development.

Implications of Endogenous Retroelements in the Etiopathogenesis of Systemic Lupus Erythematosus

Systemic lupus erythematosus (SLE) is a heterogeneous autoimmune disease. While its etiology remains elusive, current understanding suggests a multifactorial process with contributions by genetic, immunologic, hormonal, and environmental factors. A hypothesis that combines several of these factors proposes that genomic elements, the L1 retrotransposons, are instrumental in SLE pathogenesis. L1 retroelements are transcriptionally activated in SLE and produce two proteins, ORF1p and ORF2p, which are immunogenic and can drive type I interferon (IFN) production by producing DNA species that activate cytosolic DNA sensors. In addition, these two proteins reside in RNA-rich macromolecular assemblies that also contain well-known SLE autoantigens like Ro60. We surmise that cells expressing L1 will exhibit all the hallmarks of cells infected by a virus, resulting in a cellular and humoral immune response similar to those in chronic viral infections. However, unlike exogenous viruses, L1 retroelements cannot be eliminated from the host genome. Hence, dysregulated L1 will cause a chronic, but perhaps episodic, challenge for the immune system. The clinical and immunological features of SLE can be at least partly explained by this model. Here we review the support for, and the gaps in, this hypothesis of SLE and its potential for new diagnostic, prognostic, and therapeutic options in SLE.

The role of nucleic acid sensors and type I IFNs in patient populations and animal models of autoinflammation

A spectrum of human autoinflammatory conditions result from defects in cytosolic nucleic acid clearance or overexpression of the nucleic acid sensor STING. These patients often develop severely debilitating lesions and invariably show robust IFN signatures that have been attributed to the cGAS/STING signaling cascade and type I IFN. However, murine models that recapitulate major features of these syndromes have now shown that autoinflammation is more likely to depend on type II IFN/IFNgamma or type III IFN/IFNlambda, and further revealed a critical role for Th1 cells in tissue damage and the persistence of inflammation. These studies provide important insights about the types of IFNs, and the interplay of the innate and adaptive immune systems mediated by these IFNs, that can initiate and maintain the corresponding human diseases. They further point to type II/III IFNs and effector T cells as targets for more effective therapeutic strategies in the treatment of these patient populations.

Structural basis for overhang excision and terminal unwinding of DNA duplexes by TREX1

Three prime repair exonuclease 1 (TREX1) is an essential exonuclease in mammalian cells, and numerous in vivo and in vitro data evidenced its participation in immunity regulation and in genotoxicity remediation. In these very complicated cellular functions, the molecular mechanisms by which duplex DNA substrates are processed are mostly elusive because of the lack of structure information. Here, we report multiple crystal structures of TREX1 complexed with various substrates to provide the structure basis for overhang excision and terminal unwinding of DNA duplexes. The substrates were designed to mimic the intermediate structural DNAs involved in various repair pathways. The results showed that the Leu24-Pro25-Ser26 cluster of TREX1 served to cap the nonscissile 5′-end of the DNA for precise removal of the short 3′-overhang in L- and Y-structural DNA or to wedge into the double-stranded region for further digestion along the duplex. Biochemical assays were also conducted to demonstrate that TREX1 can indeed degrade double-stranded DNA (dsDNA) to a full extent. Overall, this study provided unprecedented knowledge at the molecular level on the enzymatic substrate processing involved in prevention of immune activation and in responses to genotoxic stresses. For example, Arg128, whose mutation in TREX1 was linked to a disease state, were shown to exhibit consistent interaction patterns with the nonscissile strand in all of the structures we solved. Such structure basis is expected to play an indispensable role in elucidating the functional activities of TREX1 at the cellular level and in vivo.

Three prime repair exonuclease 1 (TREX1) was shown to participate in various cellular events such as DNA repair, immunity regulation, and viral infection. In addition to relating to autoimmune diseases, this exonuclease also acts as a potential protein target for anticancer or antiviral therapies. A key for such broad attendance of TREX1 is the activities of precise trimming of the 3′-overhang in a double-stranded (dsDNA) and breaking of the terminal base pairing of the duplex. Here, we designed a series of structural DNA substrates and activity assays to delineate the underlying mechanisms. The structures newly resolved in this work indicated that the Leu24-Pro25-Ser26 cluster of TREX1 is essential for the enzyme to carry out the aforementioned activities. Together, our results established an integrated structure view into the versatile exonuclease functions of TREX1 and illuminated the molecular origin for the unique catalytic properties of TREX1 in processing various DNA intermediates in DNA repair and in cytosolic immunity regulation.

Neonatal detection of Aicardi Goutières Syndrome by increased C26:0 lysophosphatidylcholine and interferon signature on newborn screening blood spots

BACKGROUND

Aicardi Goutières Syndrome (AGS) is a heritable interferonopathy associated with systemic autoinflammation causing interferon (IFN) elevation, central nervous system calcifications, leukodystrophy and severe neurologic sequelae. An infant with TREX1 mutations was recently found to have abnormal C26:0 lysophosphatidylcholine (C26:0 Lyso-PC) in a newborn screening platform for X-linked adrenoleukodystrophy, prompting analysis of this analyte in retrospectively collected samples from individuals affected by AGS.

METHODS

In this study, we explored C26:0 Lyso-PC levels and IFN signatures in newborn blood spots and post-natal blood samples in 19 children with a molecular and clinical diagnosis of AGS and in the blood spots of 22 healthy newborns. We used Nanostring nCounter™ for IFN-induced gene analysis and a high-performance liquid chromatography with tandem mass spectrometry (HPLC MS/MS) newborn screening platform for C26:0 Lyso-PC analysis.

RESULTS

Newborn screening cards from patients across six AGS associated genes were collected, with a median disease presentation of 2months. Thirteen out of 19 (68%) children with AGS had elevations of first tier C26:0 Lyso-PC (>0.4μM), that would have resulted in a second screen being performed in a two tier screening system for X-linked adrenoleukodystrophy (X-ALD). The median (95%CI) of first tier C26:0 Lyso-PC values in AGS individuals (0.43μM [0.37-0.48]) was higher than that seen in controls (0.21μM [0.21-0.21]), but lower than X-ALD individuals (0.72μM [0.59-0.84])(p<0.001). Fourteen of 19 children had elevated expression of IFN signaling on blood cards relative to controls (Sensitivity 73.7%, 95%CI 51-88%, Specificity 95%, 95% CI 78-99%) including an individual with delayed disease presentation (36months of age). All five AGS patients with negative IFN signature at birth had RNASEH2B mutations. Consistency of agreement between IFN signature in neonatal and post-natal samples was high (0.85).

CONCLUSION

This suggests that inflammatory markers in AGS can be identified in the newborn period, before symptom onset. Additionally, since C26:0 Lyso-PC screening is currently used in X-ALD newborn screening panels, clinicians should be alert to the fact that AGS infants may present as false positives during X-ALD screening.

STING-associated vasculopathy develops independently of IRF3 in mice

Patients with stimulator of interferon genes (STING)-associated vasculopathy with onset in infancy (SAVI) develop systemic inflammation characterized by vasculopathy, interstitial lung disease, ulcerative skin lesions, and premature death. Autosomal dominant mutations in STING are thought to trigger activation of IRF3 and subsequent up-regulation of interferon (IFN)-stimulated genes (ISGs) in patients with SAVI. We generated heterozygous STING N153S knock-in mice as a model of SAVI. These mice spontaneously developed inflammation within the lung, hypercytokinemia, T cell cytopenia, skin ulcerations, and premature death. Cytometry by time-of-flight (CyTOF) analysis revealed that the STING N153S mutation caused myeloid cell expansion, T cell cytopenia, and dysregulation of immune cell signaling. Unexpectedly, we observed only mild up-regulation of ISGs in STING N153S fibroblasts and splenocytes and STING N154S SAVI patient fibroblasts. STING N153S mice lacking IRF3 also developed lung disease, myeloid cell expansion, and T cell cytopenia. Thus, the SAVI-associated STING N153S mutation triggers IRF3-independent immune cell dysregulation and lung disease in mice.

STING Contributes to Abnormal Bone Formation Induced by Deficiency of DNase II in Mice

OBJECTIVE

Cytosolic DNA sensors detect microbial DNA and promote type I interferon (IFN) and proinflammatory cytokine production through the adaptor stimulator of IFN genes (STING) to resolve infection. Endogenous DNA also engages the STING pathway, contributing to autoimmune disease. This study sought to identify the role of STING in regulating bone formation and to define the bone phenotype and its pathophysiologic mechanisms in arthritic mice double deficient in DNase II and IFN-α/β/ω receptor (IFNAR) (DNase II^-/- /IFNAR^-/- double-knockout [DKO] mice) compared with controls.

METHODS

Bone parameters were evaluated by micro-computed tomography and histomorphometry in DKO mice in comparison with mice triple deficient in STING, DNase II, and IFNAR and control mice. Cell culture techniques were employed to determine the parameters of osteoclast and osteoblast differentiation and function. NanoString and Affymetrix array analyses were performed to identify factors promoting ectopic bone formation.

RESULTS

Despite the expression of proinflammatory cytokines that would be expected to induce bone loss in the skeleton of DKO mice, the results, paradoxically, demonstrated an accumulation of bone in the long bones and spleens, sites of erythropoiesis and robust DNA accrual. In addition, factors promoting osteoblast recruitment and function were induced. Deficiency of STING significantly inhibited bone accrual.

CONCLUSION

These data reveal a novel role for cytosolic DNA sensor pathways in bone in the setting of autoimmune disease. The results demonstrate the requirement of an intact STING pathway for bone formation in this model, a finding that may have relevance to autoimmune diseases in which DNA plays a pathogenic role. Identification of pathways linking innate immunity and bone could reveal novel targets for the treatment of bone abnormalities in human autoimmune diseases.

Type 1 diabetes pathogenesis is modulated by spontaneous autoimmune responses to endogenous retrovirus antigens in NOD mice

Secreted microvesicles (MVs) are potent inflammatory triggers that stimulate autoreactive B and T cells, causing Type 1 Diabetes in non-obese diabetic (NOD) mice. Proteomic analysis of purified MVs released from islet cells detected the presence of endogenous retrovirus (ERV) antigens, including Env and Gag sequences similar to the well-characterized murine leukemia retroviruses. This raises the possibility that ERV antigens may be expressed in the pancreatic islets via MV secretion. Using virus-like particles produced by co-expressing ERV Env and Gag antigens, and a recombinant gp70 Env protein, we demonstrated that NOD but not diabetes-resistant mice developed anti-Env autoantibodies that increase in titer as disease progresses. A lentiviral-based RNA interference knockdown of Gag revealed that Gag contributes to the MV-induced T-cell response, whose diabetogenic function can be demonstrated via cell-transfer into immune-deficient mice. Finally, we observed that Gag and Env are expressed in NOD islet-derived primary mesenchymal stem cells (MSCs). However, MSCs derived from the islets of diabetes-resistant mice do not express the antigens. Taken together, abnormal ERV activation and secretion of MVs may induce anti-retroviral responses to trigger autoimmunity.

Cytosolic RNA:DNA Duplexes Generated by Endogenous Reverse Transcriptase Activity as Autonomous Inducers of Skin Inflammation in Psoriasis

Psoriasis is a chronic skin disease of unknown ætiology. Recent studies suggested that a large amount of cytosolic DNA (cyDNA) in keratinocytes is breaking keratinocytes DNA tolerance and promotes self-sustained inflammation in the psoriatic lesion. We investigated the origin of this cyDNA. We show that, amongst all the possible DNA structures, the cyDNA could be present as RNA:DNA duplexes in keratinocytes. We further show that endogenous reverse transcriptase activities generate such duplexes and consequently activate the production of Th1-inflammatory cytokines. These observations open a new research avenue related to endogenous retroelements for the aetiology of psoriasis and probably of other human chronic inflammatory diseases.

Two case reports of severe myocarditis associated with the initiation of dolutegravir treatment in HIV patients

RATIONALE

The integrase inhibitor dolutegravir is now recommended as first-line treatment for HIV. A single case of myocarditis after treatment with dolutegravir was reported in the FLAMINGO trial. We present here 2 cases of severe myocarditis that occurred shortly after the initiation of dolutegravir treatment.

PATIENTS CONCERNS

The first case is a 45-year-old female who developed severe congestive heart failure and died, weeks after the initiation of dolutegravir treatment (for simplification of her antiretroviral regimen). The second case was a 51-year-old male who presented with effort dyspnea 3 weeks after the initiation of dolutegravir treatment and was later diagnosed as severe congestive heart failure. The treatment was changed and the patient survived, but he still suffers from severe heart failure with functional impairment.

DIAGNOSIS AND OUTCOME

Patient 1 died, patient 2 suffers from severe heart failure.

LESSONS

We discuss here the possible relationship between the initiation of dolutegravir treatment and the development of lymphocytic myocarditis in our patients, and we suggest a possible mechanism.

Systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is an autoimmune disease that can affect many organs, including the skin, joints, the central nervous system and the kidneys. Women of childbearing age and certain racial groups are typically predisposed to developing the condition. Rare, inherited, single-gene complement deficiencies are strongly associated with SLE, but the disease is inherited in a polygenic manner in most patients. Genetic interactions with environmental factors, particularly UV light exposure, Epstein-Barr virus infection and hormonal factors, might initiate the disease, resulting in immune dysregulation at the level of cytokines, T cells, B cells and macrophages. Diagnosis is primarily clinical and remains challenging because of the heterogeneity of SLE. Classification criteria have aided clinical trials, but, despite this, only one drug (that is, belimumab) has been approved for use in SLE in the past 60 years. The 10-year mortality has improved and toxic adverse effects of older medications such as cyclophosphamide and glucocorticoids have been partially offset by newer drugs such as mycophenolate mofetil and glucocorticoid-sparing regimes. However, further improvements have been hampered by the adverse effects of renal and neuropsychiatric involvement and late diagnosis. Adding to this burden is the increased risk of premature cardiovascular disease in SLE together with the risk of infection made worse by immunosuppressive therapy. Challenges remain with treatment-resistant disease and symptoms such as fatigue. Newer therapies may bring hope of better outcomes, and the refinement to stem cell and genetic techniques might offer a cure in the future.

Synergy between Hematopoietic and Radioresistant Stromal Cells Is Required for Autoimmune Manifestations of DNase II-/-IFNaR-/- Mice

Detection of endogenous nucleic acids by cytosolic receptors, dependent on STING, and endosomal sensors, dependent on Unc93b1, can provoke inflammatory responses that contribute to a variety of autoimmune and autoinflammatory diseases. In DNase II-deficient mice, the excessive accrual of undegraded DNA leads to both a STING-dependent inflammatory arthritis and additional Unc93b1-dependent autoimmune manifestations, including splenomegaly, extramedullary hematopoiesis, and autoantibody production. In this study, we use bone marrow chimeras to show that clinical and histological inflammation in the joint depends upon DNase II deficiency in both donor hematopoietic cells and host radioresistant cells. Additional features of autoimmunity in these mice, known to depend on Unc93b1 and therefore endosomal TLRs, also require DNase II deficiency in both donor and host compartments, but only require functional TLRs in the hematopoietic cells. Collectively, our data demonstrate a major role of both stromal and hematopoietic cells in all aspects of DNA-driven autoimmunity. These findings further point to the importance of cytosolic nucleic acid sensors in creating an inflammatory environment that facilitates the development of Unc93b1-dependent autoimmunity.

Nucleic Acid-Sensing Receptors: Rheostats of Autoimmunity and Autoinflammation

Distinct families of germline-encoded pattern recognition receptors can sense both microbial and endogenous nucleic acids. These DNA and RNA sensors include endosomal TLRs and cytosolic sensors upstream of stimulator of type I IFN genes (STING) and MAVS. The existence of overlapping specificities for both foreign and self nucleic acids suggests that, under optimal conditions, the activity of these receptors is finely tuned to effectively mediate host defense yet constrain pathogenic self-reactivity. This equilibrium becomes disrupted with the loss of either TLR9 or STING. To maintain immune protection, this loss can be counterbalanced by the elevated response of an alternative receptor(s). Unfortunately, this adjustment can lead to an increased risk for the development of systemic autoimmunity, as evidenced by the exacerbated clinical disease manifestations of TLR9-deficient and STING-deficient autoimmune-prone mice. These studies underscore the delicate balance normally maintained by tonic signals that prevent unchecked immune responses to nucleic acids released during infections and cellular duress or death.

Emerging treatments for pediatric leukodystrophies

The leukodystrophies are a heterogeneous group of inherited disorders with broad clinical manifestations and variable pathologic mechanisms. Improved diagnostic methods have allowed identification of the underlying cause of these diseases, facilitating identification of their pathologic mechanisms. Clinicians are now able to prioritize treatment strategies and advance research in therapies for specific disorders. Although only a few of these disorders have well-established treatments or therapies, a number are on the verge of clinical trials. As investigators are able to shift care from symptomatic management of disorders to targeted therapeutics, the unmet therapeutic needs could be reduced for these patients.

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

PURPOSE OF REVIEW

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.

RECENT FINDINGS

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.

SUMMARY

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.

Disease specific therapies in leukodystrophies and leukoencephalopathies

Leukodystrophies are a heterogeneous, often progressive group of disorders manifesting a wide range of symptoms and complications. Most of these disorders have historically had no etiologic or disease specific therapeutic approaches. Recently, a greater understanding of the pathologic mechanisms associated with leukodystrophies has allowed clinicians and researchers to prioritize treatment strategies and advance research in therapies for specific disorders, some of which are on the verge of pilot or Phase I/II clinical trials. This shifts the care of leukodystrophy patients from the management of the complex array of symptoms and sequelae alone to targeted therapeutics. The unmet needs of leukodystrophy patients still remain an overwhelming burden. While the overwhelming consensus is that these disorders collectively are symptomatically treatable, leukodystrophy patients are in need of advanced therapies and if possible, a cure.

Cytosolic RNA:DNA hybrids activate the cGAS-STING axis

Intracellular recognition of non-self and also self-nucleic acids can result in the initiation of potent pro-inflammatory and antiviral cytokine responses. Most recently, cGAS was shown to be critical for the recognition of cytoplasmic dsDNA. Binding of dsDNA to cGAS results in the synthesis of cGAMP(2'-5'), which then binds to the endoplasmic reticulum resident protein STING. This initiates a signaling cascade that triggers the induction of an antiviral immune response. While most studies on intracellular nucleic acids have focused on dsRNA or dsDNA, it has remained unexplored whether cytosolic RNA:DNA hybrids are also sensed by the innate immune system. Studying synthetic RNA:DNA hybrids, we indeed observed a strong type I interferon response upon cytosolic delivery of this class of molecule. Studies in THP-1 knockout cells revealed that the recognition of RNA:DNA hybrids is completely attributable to the cGAS-STING pathway. Moreover, in vitro studies showed that recombinant cGAS produced cGAMP upon RNA:DNA hybrid recognition. Altogether, our results introduce RNA:DNA hybrids as a novel class of intracellular PAMP molecules and describe an alternative cGAS ligand next to dsDNA.

Stress and the dynamic genome: Steroids, epigenetics, and the transposome

Stress plays a substantial role in shaping behavior and brain function, often with lasting effects. How these lasting effects occur in the context of a fixed postmitotic neuronal genome has been an enduring question for the field. Synaptic plasticity and neurogenesis have provided some of the answers to this question, and more recently epigenetic mechanisms have come to the fore. The exploration of epigenetic mechanisms recently led us to discover that a single acute stress can regulate the expression of retrotransposons in the rat hippocampus via an epigenetic mechanism. We propose that this response may represent a genomic stress response aimed at maintaining genomic and transcriptional stability in vulnerable brain regions such as the hippocampus. This finding and those of other researchers have made clear that retrotransposons and the genomic plasticity they permit play a significant role in brain function during stress and disease. These observations also raise the possibility that the transposome might have adaptive functions at the level of both evolution and the individual organism.

Spontaneous activation of RNA-sensing pathways in autoimmune disease

Multiple intracellular RNA sensing innate immune pathways have been linked to autoimmune disease. RNA-related ligands taken up by the endocytic pathway activate TLRs, and affect primarily immune cells. This type of activation is enhanced by nucleic acid-specific antibodies and induces an inflammatory program. In contrast, spontaneous activation of cytoplasmic RNA sensing pathways targets mostly non-hematopoietic tissues and their effect on autoimmune disease is secondary to the release of interferon in the circulation. The fact that pathologies result from spontaneous activation of innate pathways implies that endogenous RNA ligands that might be sensed as pathogenic are commonly found in both immune and non-immune cells.

Gain-of-function mutations in IFIH1 cause a spectrum of human disease phenotypes associated with upregulated type I interferon signaling

The type I interferon system is integral to human antiviral immunity. However, inappropriate stimulation or defective negative regulation of this system can lead to inflammatory disease. We sought to determine the molecular basis of genetically uncharacterized cases of the type I interferonopathy Aicardi-Goutières syndrome, and of other patients with undefined neurological and immunological phenotypes also demonstrating an upregulated type I interferon response. We found that heterozygous mutations in the cytosolic double-stranded RNA receptor gene IFIH1 (MDA5) cause a spectrum of neuro-immunological features consistently associated with an enhanced interferon state. Cellular and biochemical assays indicate that these mutations confer a gain-of-function - so that mutant IFIH1 binds RNA more avidly, leading to increased baseline and ligand-induced interferon signaling. Our results demonstrate that aberrant sensing of nucleic acids can cause immune upregulation.

Assessment of interferon-related biomarkers in Aicardi-Goutières syndrome associated with mutations in TREX1, RNASEH2A, RNASEH2B, RNASEH2C, SAMHD1, and ADAR: a case-control study

BACKGROUND

Aicardi-Goutières syndrome (AGS) is an inflammatory disorder caused by mutations in any of six genes (TREX1, RNASEH2A, RNASEH2B, RNASEH2C, SAMHD1, and ADAR). The disease is severe and effective treatments are urgently needed. We investigated the status of interferon-related biomarkers in patients with AGS with a view to future use in diagnosis and clinical trials.

METHODS

In this case-control study, samples were collected prospectively from patients with mutation-proven AGS. The expression of six interferon-stimulated genes (ISGs) was measured by quantitative PCR, and the median fold change, when compared with the median of healthy controls, was used to create an interferon score for each patient. Scores higher than the mean of controls plus two SD (>2·466) were designated as positive. Additionally, we collated historical data for interferon activity, measured with a viral cytopathic assay, in CSF and serum from mutation-positive patients with AGS. We also undertook neutralisation assays of interferon activity in serum, and looked for the presence of autoantibodies against a panel of interferon proteins.

FINDINGS

74 (90%) of 82 patients had a positive interferon score (median 12·90, IQR 6·14-20·41) compared with two (7%) of 29 controls (median 0·93, IQR 0·57-1·30). Of the eight patients with a negative interferon score, seven had mutations in RNASEH2B (seven [27%] of all 26 patients with mutations in this gene). Repeat sampling in 16 patients was consistent for the presence or absence of an interferon signature on 39 of 41 occasions. Interferon activity (tested in 147 patients) was negatively correlated with age (CSF, r=-0·604; serum, r=-0·289), and was higher in CSF than in serum in 104 of 136 paired samples. Neutralisation assays suggested that measurable antiviral activity was related to interferon α production. We did not record significantly increased concentrations of autoantibodies to interferon subtypes in patients with AGS, or an association between the presence of autoantibodies and interferon score or serum interferon activity.

INTERPRETATION

AGS is consistently associated with an interferon signature, which is apparently sustained over time and can thus be used to differentiate patients with AGS from controls. If future studies show that interferon status is a reactive biomarker, the measurement of an interferon score might prove useful in the assessment of treatment efficacy in clinical trials.

FUNDING

European Union's Seventh Framework Programme; European Research Council.

What role (if any) does the highly conserved CSB-PGBD3 fusion protein play in Cockayne syndrome?

The PGBD3 piggyBac transposon inserted into CSB intron 5 early in the primate lineage. As a result of alternative splicing, the human CSB gene now encodes three proteins: CSB, a CSB-PGBD3 fusion protein that joins the N-terminal CSB domain to the C-terminal PGBD3 transposase domain, and PGBD3 transposase. The fusion protein is as highly conserved as CSB, suggesting that it is advantageous in health; however, expression of the fusion protein in CSB-null cells induces a constitutive interferon (IFN) response. The fusion protein binds in vivo to PGBD3-related MER85 elements, but is also tethered to c-Jun, TEAD1, and CTCF motifs by interactions with the cognate transcription factors. The fusion protein regulates nearby genes from the c-Jun (and to a lesser extent TEAD1 and CTCF) motifs, but not from MER85 elements. We speculate that the fusion protein interferes with CSB-dependent chromatin remodeling, generating double-stranded RNA (dsRNA) that induces an IFN response through endosomal TLR or cytoplasmic RIG-I and/or MDA5 RNA sensors. We suggest that the fusion protein was fixed in primates because an elevated IFN response may help to fight viral infection. We also speculate that an inappropriate IFN response may contribute to the clinical presentation of CS.