Prognostic value of anti-IFI16 autoantibodies in pulmonary arterial hypertension and mortality in patients with systemic sclerosis

OBJECTIVES

To determine the diagnostic value of anti-interferon gamma inducible protein 16 (IFI16) autoantibodies in systemic sclerosis (SSc) patients negative for all tested SSc-specific autoantibodies (SSc-seronegative patients) and to evaluate the clinical significance of these autoantibodies, whether isolated or in the presence of anti-centromere autoantibodies (ACA).

METHODS

Overall, 58 SSc-seronegative and 66 ACA-positive patients were included in the study. All patients were tested for anti-IFI16 autoantibodies by an in-house direct ELISA. Associations between clinical parameters and anti-IFI16 autoantibodies were analysed.

RESULTS

Overall, 17.2% of SSc-seronegative and 39.4% of ACA-positive patients were positive for anti-IFI16 autoantibodies. Anti-IFI16 autoantibodies were found only in patients within the limited cutaneous SSc (lcSSc) subset. A positive association between anti-IFI16 positivity and isolated pulmonary arterial hypertension (PAH) was found (odds ratio [OR]=5.07; p=0.014) even after adjusting for ACA status (OR=4.99; p=0.019). Anti-IFI16-positive patients were found to have poorer overall survival than negative patients (p=0.032). Cumulative survival rates at 10, 20 and 30 years were 96.9%, 92.5% and 68.7% for anti-IFI16-positive patients vs. 98.8%, 97.0% and 90.3% for anti-IFI16-negative-patients, respectively. Anti-IFI16-positive patients also had worse overall survival than anti-IFI16-negative patients after adjusting for ACA status in the multivariate Cox analysis (hazard ratio [HR]=3.21; p=0.043).

CONCLUSION

Anti-IFI16 autoantibodies were associated with isolated PAH and poorer overall survival. Anti-IFI16 autoantibodies could be used as a supplementary marker of lcSSc in SSc-seronegative patients and for identifying ACA-positive patients with worse clinical outcome.

IL-17 Imbalance Promotes the Pyroptosis in Immune-Mediated Liver Injury Through STAT3-IFI16 Axis

Autoimmune hepatitis (AIH) affects all age group and occurs mainly in women. Pyroptosis is a novel programmed cell death featured with cell bursting and release of proinflammatory cytokines. A deeper understanding of AIH pathogenesis will contribute to novel therapy for AIH patients. Here, we aimed to investigate the role of IL-17 in immune-mediated liver injury. The levels of cytokines were measured by ELISA, and mRNA levels of STAT3 and IFN gamma-inducible protein 16 (IFI16) were detected by PCR. Expressions of STAT3, IFI16, gasdermin D and cleaved caspase-1 were measured by western-blotting. Immunohistochemical staining and transmission electron microscopy were applied to evaluate liver histopathological changes of the treated mice. Our results showed that the levels of IFI16 was increased in hepatocytes treated with IL-17 protein, and further elevated after STAT3-overexpressed (STAT3-OE) lentivirus treatment. The levels of IFI16 were reduced in hepatocytes treated with IL-17 neutralizing Ab (nAb), but were significantly increased after STAT3-OE treatment. Pyroptosis was observed in hepatocytes treated with IL-17 protein, and further cell damage was observed after STAT3-OE lentivirus treatment. Liver damage was alleviated in mice treated with IL-17 nAb, however sever damage was experienced after STAT3-OE lentivirus treatment. A binding interaction between IFI16 and STAT3 was detected in IL-17 treated hepatocytes. Glutathione transaminase activity was enhanced in concanavalin A-induced AIH mice compared to the control group (p<0.01). IL-17 plays an important role in activating STAT3 and up-regulating IFI16, which may promote the pyroptosis in AIH-related liver injury through STAT3-IFI16 axis.

IFI16 Induced by Direct Interaction between Esophageal Squamous Cell Carcinomas and Macrophages Promotes Tumor Progression via Secretion of IL-1α

Tumor-associated macrophages (TAMs), one of the major components of the tumor microenvironment, contribute to the progression of esophageal squamous cell carcinoma (ESCC). We previously established a direct co-culture system of human ESCC cells and macrophages and reported the promotion of malignant phenotypes, such as survival, growth, and migration, in ESCC cells. These findings suggested that direct interactions between cancer cells and macrophages contribute to the malignancy of ESCC, but its underlying mechanisms remain unclear. In this study, we compared the expression levels of the interferon-induced genes between mono- and co-cultured ESCC cells using a cDNA microarray and found that interferon-inducible protein 16 (IFI16) was most significantly upregulated in co-cultured ESCC cells. IFI16 knockdown suppressed malignant phenotypes and also decreased the secretion of interleukin-1α (IL-1α) from ESCC cells. Additionally, recombinant IL-1α enhanced malignant phenotypes of ESCC cells through the Erk and NF-κB signaling. Immunohistochemistry revealed that high IFI16 expression in human ESCC tissues tended to be associated with disease-free survival and was significantly associated with tumor depth, lymph node metastasis, and macrophage infiltration. The results of this study reveal that IFI16 is involved in ESCC progression via IL-1α and imply the potential of IFI16 as a novel prognostic factor for ESCC.

Renal interferon-inducible protein 16 expression is associated with disease activity and prognosis in lupus nephritis

BACKGROUND

Lupus nephritis (LN) is one of the most severe complications of systemic lupus erythematosus (SLE). However, the current management of LN remains unsatisfactory due to sneaky symptoms during early stages and lack of reliable predictors of disease progression.

METHODS

Bioinformatics and machine learning algorithms were initially used to explore the potential biomarkers for LN development. Identified biomarker expression was evaluated by immunohistochemistry (IHC) and multiplex immunofluorescence (IF) in 104 LN patients, 12 diabetic kidney disease (DKD) patients, 12 minimal change disease (MCD) patients, 12 IgA nephropathy (IgAN) patients and 14 normal controls (NC). The association of biomarker expression with clinicopathologic indices and prognosis was analyzed. Gene Set Enrichment Analysis (GSEA) and Gene Set Variation Analysis (GSVA) were utilized to explore potential mechanisms.

RESULTS

Interferon-inducible protein 16 (IFI16) was identified as a potential biomarker for LN. IFI16 was highly expressed in the kidneys of LN patients compared to those with MCD, DKD, IgAN or NC. IFI16 co-localized with certain renal and inflammatory cells. Glomerular IFI16 expression was correlated with pathological activity indices of LN, while tubulointerstitial IFI16 expression was correlated with pathological chronicity indices. Renal IFI16 expression was positively associated with systemic lupus erythematosus disease activity index (SLEDAI) and serum creatinine while negatively related to baseline eGFR and serum complement C3. Additionally, higher IFI16 expression was closely related to poorer prognosis of LN patients. GSEA and GSVA suggested that IFI16 expression was involved in adaptive immune-related processes of LN.

CONCLUSION

Renal IFI16 expression is a potential biomarker for disease activity and clinical prognosis in LN patients. Renal IFI16 levels may be used to shed light on predicting the renal response and develop precise therapy for LN.

Interferon and autoantigens: intersection in autoimmunity

Interferon (IFN) is a key component of the innate immune response. For reasons that remain incompletely understood, the IFN system is upregulated in several rheumatic diseases, particularly those that feature autoantibody production, such as SLE, Sjögren's syndrome, myositis and systemic sclerosis. Interestingly, many of the autoantigens targeted in these diseases are components of the IFN system, representing IFN-stimulated genes (ISGs), pattern recognition receptors (PRRs), and modulators of the IFN response. In this review, we describe features of these IFN-linked proteins that may underlie their status as autoantigens. Note is also made of anti-IFN autoantibodies that have been described in immunodeficiency states.

Sirt1 Negatively Regulates Cellular Antiviral Responses by Preventing the Cytoplasmic Translocation of Interferon-Inducible Protein 16 in Human Cells

Interferon-inducible protein 16 (IFI16) plays a critical role in antiviral innate immune responses against DNA viruses. Although the acetylation of IFI16 is crucial to its cytoplasmic translocation and downstream signal transduction, the regulation of IFI16 acetylation remains unclear. In this study, we demonstrated that the NAD-dependent deacetylase silent information regulatory 1 (Sirtuin1, Sirt1) interacted with IFI16 and decreased the acetylation of IFI16, resulting in the inhibition of IFI16 cytoplasmic localization and antiviral responses against DNA virus and viral DNA in human cells. Meantime, Sirt1 could not inhibit RNA virus-triggered signal transduction. Interestingly, even p204, the murine ortholog of human IFI16, barely interacted with Sirt1. Thus, Sirt1 could not negatively regulate the acetylation of p204 and subsequent signal transduction upon herpes simplex virus 1 (HSV-1) infection in mouse cells. Taken together, our research work showed a new mechanism by which Sirt1 manipulated IFI16-mediated host defense. Our study also demonstrated a difference in the regulation of antiviral host defense between humans and mice, which might be considered in preclinical studies for antiviral treatment. IMPORTANCE DNA viruses, such as hepatitis B virus (HBV), human papillomavirus (HPV), human cytomegalovirus (HCMV), Epstein-Barr virus (EBV), and herpes simplex virus (HSV), can cause a wide range of diseases and are considered a global threat to human health. Interferon-inducible protein 16 (IFI16) binds virus DNA and triggers antiviral innate immune responses to restrict viral infection. In this study, we identified that silent information regulatory 1 (Sirtuin1, Sirt1) interacted with IFI16 and regulated IFI16-mediated innate host defense. Therefore, the activator or inhibitor of Sirt1 may have the potential to be used as a novel strategy to treat DNA virus-associated diseases. We also found that Sirt1 barely interacted with p204, the murine ortholog of human IFI16, and could not negatively regulate innate immune responses upon HSV-1 infection in mouse cells. This difference between humans and mice in the regulation of antiviral host defense might be considered in preclinical studies for antiviral treatment.

AIM2-inflammasome role in systemic lupus erythematous and rheumatoid arthritis

The inflammasome AIM2 regulates multiple aspects of innate immune functions and serves as a critical mediator of inflammatory responses. AIM2 inflammasome activation leads to the production of pro-inflammatory cytokines, IL-1β and IL-18 and participates triggering a pyroptosis response needed to counteract excessive cell proliferation. In addition, AIM2 expression and activation is wide regulated since alteration in its activity may derived in pathological consequences. Consequently, deregulated AIM2 activation contributes to the pathogenic processes of various inflammatory diseases. In this review, we will discuss the activation and function of AIM2 inflammasome, as well as its contribution in rheumatoid arthritis and systemic lupus erythematous pathology. Finally, we highlight the participation of the AIM2-inflammasome at the level of joint in rheumatoid arthritis and at kidney in systemic lupus erythematous. The development of therapeutic strategies based on modulation of AIM2-inflammasome activity should have a tissue-specific focus.

The Type I Interferon Pathway Is Upregulated in the Cutaneous Lesions and Blood of Multibacillary Leprosy Patients With Erythema Nodosum Leprosum

In leprosy patients, acute inflammatory episodes, known as erythema nodosum leprosum (ENL), are responsible for high morbidity and tissue damage that occur during the course of Mycobacterium leprae infection. In a previous study, we showed evidence implicating DNA-sensing via TLR9 as an important inflammatory pathway in ENL. A likely important consequence of TLR9 pathway activation is the production of type I interferons (IFN-I) by plasmacytoid dendritic cells (pDCs), also implicated in the pathogenesis of several chronic inflammatory diseases. In this study, we investigated whether the IFN-I pathway is activated during ENL. Blood samples and skin lesions from multibacillary patients diagnosed with ENL were collected and the expression of genes of the IFN-I pathway and interferon-stimulated genes were compared with samples collected from non-reactional multibacillary (NR) patients. Whole blood RNAseq analysis suggested higher activation of the IFN-I pathway in ENL patients, confirmed by RT-qPCR. Likewise, significantly higher mRNA levels of IFN-I-related genes were detected in ENL skin biopsies when compared to NR patient lesions. During thalidomide administration, the drug of choice for ENL treatment, a decrease in the mRNA and protein levels of some of these genes both in the skin and blood was observed. Indeed, in vitro assays showed that thalidomide was able to block the secretion of IFN-I by peripheral blood mononuclear cells in response to M. leprae sonicate or CpG-A, a TLR9 ligand. Finally, the decreased frequencies of peripheral pDCs in ENL patients, along with the higher TLR9 expression in ENL pDCs and the enrichment of CD123⁺ cells in ENL skin lesions, suggest the involvement of these cells as IFN-I producers in this type of reaction. Taken together, our data point to the involvement of the pDC/type I IFN pathway in the pathogenesis of ENL, opening new avenues in identifying biomarkers for early diagnosis and new therapeutic targets for the better management of this reactional episode.

The role of cyclic GMP-AMP synthase and Interferon-I-inducible protein 16 as candidatebiomarkers of systemic lupus erythematosus

BACKGROUND

Diverse clinical and serological manifestations of systemic lupus erythematosus (SLE) compromise its diagnosis and treatment. A more reliable biomarker for SLE, which can play a critical role in either diagnosis, monitoring the disease progress or evaluating the response to treatment for individualized therapeutic, is necessary. DNA sensor is an important mediator of inflammation in systemic autoimmune diseases. However, the potential role for DNA sensor as disease activity biomarkers for SLE remained obscure. We detected the aberrant activation of DNA sensors and the corresponding IFN-β response in SLE patients, and to evaluate their potential role as disease biomarkers for SLE.

METHODS

We quantified the expressions of IFN-I and DNA sensor, such as cGAS, IFI16, DDX41, DAI and their down-stream adaptor STING in PBMC derived from patients with SLE (n = 100), healthy controls (HCs) (n = 62) by real-time PCR. The relationships between the expression of cGAS or IFI16 and clinical features in SLE patients were investigated. ROC curve analysis was performed to examine the predictive value of cGAS and IFI16 in SLE diagnosis, disease activity monitoring, specific organ manifestation and therapeutic response. RNA interference-mediated depletion of IFI16 or cGAS was conducted to evaluate their impact on IFN-I response.

RESULTS

The expressions of cGAS and IFI16 were significantly higher in PBMC from SLE patients, closely correlated with the SLEDAI scores and high anti-dsDNA antibody titers. While the AUC for cGAS (0.767) was less than that of IFI16 and IFN-β, the AUC for IFI16 (0.856) and IFN-β (0.856) were similar. Expression of cGAS and IFI16 combine with IFN-β in PBMC showed high sensitivity (89.2%) and specificity (89.1%) for discrimination between mild and moderate/severe disease activity in SLE. Higher expression of IFI16 was association with ocular disorder in SLE patients. Neither IFI16 nor cGAS was a reliable indicator of therapeutic response. RNA interference-mediated depletion of IFI16 or cGAS prevented active SLE serum-induced upregulating in both IFN-α and IFN-β.

CONCLUSIONS

High expression levels of cGAS and IFI16 in PBMC from SLE patients correlated strongly with disease activity. Both cGAS and IFI16 mediated signaling pathway were account for the robust production of IFN-β. Expression of cGAS and IFI16 combined with IFN-β in PBMC might serve as potential biomarkers for early diagnosis and monitoring disease activity in SLE.

Interrogating Host Antiviral Environments Driven by Nuclear DNA Sensing: A Multiomic Perspective

Nuclear DNA sensors are critical components of the mammalian innate immune system, recognizing the presence of pathogens and initiating immune signaling. These proteins act in the nuclei of infected cells by binding to foreign DNA, such as the viral genomes of nuclear-replicating DNA viruses herpes simplex virus type 1 (HSV-1) and human cytomegalovirus (HCMV). Upon binding to pathogenic DNA, the nuclear DNA sensors were shown to initiate antiviral cytokines, as well as to suppress viral gene expression. These host defense responses involve complex signaling processes that, through protein–protein interactions (PPIs) and post-translational modifications (PTMs), drive extensive remodeling of the cellular transcriptome, proteome, and secretome to generate an antiviral environment. As such, a holistic understanding of these changes is required to understand the mechanisms through which nuclear DNA sensors act. The advent of omics techniques has revolutionized the speed and scale at which biological research is conducted and has been used to make great strides in uncovering the molecular underpinnings of DNA sensing. Here, we review the contribution of proteomics approaches to characterizing nuclear DNA sensors via the discovery of functional PPIs and PTMs, as well as proteome and secretome changes that define a host antiviral environment. We also highlight the value of and future need for integrative multiomic efforts to gain a systems-level understanding of DNA sensors and their influence on epigenetic and transcriptomic alterations during infection.

Toll-like receptor 4-mediated inflammation triggered by extracellular IFI16 is enhanced by lipopolysaccharide binding

Damage-associated molecular patterns (DAMPs) are endogenous molecules activating the immune system upon release from injured cells. Here we show that the IFI16 protein, once freely released in the extracellular milieu of chronically inflamed tissues, can function as a DAMP either alone or upon binding to lipopolysaccharide (LPS). Specifically, using pull-down and saturation binding experiments, we show that IFI16 binds with high affinity to the lipid A moiety of LPS. Remarkably, IFI16 DAMP activity is potentiated upon binding to subtoxic concentrations of strong TLR4-activating LPS variants, as judged by TLR4-MD2/TIRAP/MyD88-dependent IL-6, IL-8 and TNF-α transcriptional activation and release in stimulated monocytes and renal cells. Consistently, using co-immunoprecipitation (co-IP) and surface plasmon resonance (SPR) approaches, we show that IFI16 is a specific TLR4-ligand and that IFI16/LPS complexes display a faster stimulation turnover on TLR4 than LPS alone. Altogether, our findings point to a novel pathomechanism of inflammation involving the formation of multiple complexes between extracellular IFI16 and subtoxic doses of LPS variants, which then signal through TLR4.

PYHIN Proteins and HPV: Role in the Pathogenesis of Head and Neck Squamous Cell Carcinoma

In the last decades, the human papillomavirus (HPV) emerged as an etiological cause of head and neck squamous cell carcinoma (HNSCC), especially in the oropharynx. The role of two intracellular DNA sensors, which belong to the PYHIN family (interferon-inducible protein 16 (IFI16) and absent in melanoma 2 protein (AIM2)), has been analyzed in relation to HPV infection and head and neck carcinogenesis. In particular, IFI16 and AIM2 expression depends on HPV infection in HNSCC. They represent viral restriction factors and are key components of the intrinsic immunity activated against different viruses, including HPV. This review analyzed and summarized the recent findings about the role of PYHIN proteins in HPV⁺ and HPV⁻ HNSCC.

Serum IFI16 and anti-IFI16 antibodies in psoriatic arthritis

Nuclear interferon-inducible protein 16 (IFI16) and anti-IFI16 antibodies have been detected in subjects with several rheumatic diseases, often correlating with disease severity, and in this study we investigated their prevalence and clinical associations in psoriatic arthritis (PsA) compared to psoriasis (Pso). We tested sera and synovial fluids of patients with PsA for IFI16 protein levels by capture enzyme-linked immunosorbent assay (ELISA) and for anti-IFI16 immunoglobulin (Ig)G and IgA by ELISA, protein radio-immunoprecipitation and immunoprecipitation-Western blot of IgG. Sera from patients with Pso and healthy subjects were used as controls, and in a subgroup of patients with PsA we also studied sera after treatment with etanercept. IFI16 was detectable in the sera of 66% of patients with Pso, 46% with PsA and 19% of controls. Among PsA cases, 51% of IFI16-positive cases had elevated levels of C-reactive protein (CRP) compared to 31% of patients with undetectable IFI16. Anti-IFI16 of both IgG and IgA isoforms were detected with significantly higher frequency in PsA and Pso compared to healthy controls, with higher IgG titres in patients with elevated C-reactive protein (CRP) (P = 0·015). Immunoprecipitation confirmed the presence of anti-IFI16 IgG antibodies and these preferentially recognized epitopes outside the N-terminus of the protein. Lastly, IFI16 was detected in one of seven and anti-IFI16 in three of seven synovial fluids from patients with PsA. Therefore, IFI16 and anti-IFI16 are detectable in serum and synovial fluid of PsA patients, especially in cases of elevated CRP.

Nuclear hnRNPA2B1 initiates and amplifies the innate immune response to DNA viruses

DNA viruses typically eject genomic DNA into the nuclei of host cells after entry. It is unclear, however, how nuclear pathogen-derived DNA triggers innate immune responses. We report that heterogeneous nuclear ribonucleoprotein A2B1 (hnRNPA2B1) recognizes pathogenic DNA and amplifies interferon-α/β (IFN-α/β) production. Upon DNA virus infection, nuclear-localized hnRNPA2B1 senses viral DNA, homodimerizes, and is then demethylated at arginine-226 by the arginine demethylase JMJD6. This results in hnRNPA2B1 translocation to the cytoplasm where it activates the TANK-binding kinase 1-interferon regulatory factor 3 (TBK1-IRF3) pathway, leading to IFN-α/β production. Additionally, hnRNPA2B1 facilitates N ⁶-methyladenosine (m⁶A) modification and nucleocytoplasmic trafficking of CGAS, IFI16, and STING messenger RNAs. This, in turn, amplifies the activation of cytoplasmic TBK1-IRF3 mediated by these factors. Thus, hnRNPA2B1 plays important roles in initiating IFN-α/β production and enhancing stimulator of interferon genes (STING)-dependent cytoplasmic antiviral signaling.

Sex Differences in Correlation with Gene Expression Levels between Ifi200 Family Genes and Four Sets of Immune Disease-Relevant Genes

Background

The HIN-200 family genes in humans have been linked to several autoimmune diseases—particularly to systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA). Recently, its human counterpart gene cluster, the Ifi200 family in mice, has been linked to spontaneous arthritis disease (SAD). However, many immune-mediated diseases (including RA and SLE) show gender difference. Understanding whether or not and how these genes play a role in sex difference in immune-mediated diseases is essential for diagnosis/treatment.

Methods

This study takes advantage of the whole genome gene expression profiles of recombinant inbred (RI) strain populations from female and male mice to analyze potential sex differences in a variety of genes in disease pathways. Expression levels and regulatory QTL of Ifi200 family genes between female and male mice were first examined in a large mouse population, including RI strains derived from C57BL/6J, DBA/2J (BXD), and classic inbred strains. Sex similarities and differences were then analyzed for correlations with gene expression levels between genes in the Ifi200 family and four selected gene sets: known immune Ifi200 pathway-related genes, lupus-relevant genes, osteoarthritis- (OA-) and RA-relevant genes, and sex hormone-related genes.

Results

The expression level of Ifi202b showed the most sex difference in correlation with known immune-related genes (the P value for Ifi202b is 0.0004). Ifi202b also showed gender difference in correlation with selected sex hormone genes, with a P value of 0.0243. When comparing coexpression levels between Ifi200 genes and lupus-relevant genes, Ifi203 and Ifi205 showed significant sex difference (P values: 0.0303 and 0.002, resp.). Furthermore, several key genes (e.g., Csf1r, Ifnb1, IL-20, IL-22, IL-24, Jhdm1d, Csf1r, Ifnb1, IL-20, IL-22, IL-24, and Tgfb2 that regulate sex differences in immune diseases) were discovered.

Conclusions

Different genes in the Ifi200 family play different roles in sex difference among dissimilar pathways of these four gene groups.

DNA Damage and Deficiencies in the Mechanisms of Its Repair: Implications in the Pathogenesis of Systemic Lupus Erythematosus

Systemic lupus erythematosus (SLE) is a perplexing and potentially severe disease, the pathogenesis of which is yet to be understood. SLE is considered to be a multifactorial disease, in which genetic factors, immune dysregulation, and environmental factors, such as ultraviolet radiation, are involved. Recently, the description of novel genes conferring susceptibility to develop SLE even in their own (monogenic lupus) has raised the interest in DNA dynamics since many of these genes are linked to DNA repair. Damage to DNA induces an inflammatory response and eventually triggers an immune response, including those targeting self-antigens. We review the evidence that indicates that patients with SLE present higher levels of DNA damage than normal subjects do and that several proteins involved in the preservation of the genomic stability show polymorphisms, some of which increase the risk for SLE development. Also, the experience from animal models reinforces the connection between DNA damage and defective repair in the development of SLE-like disease including characteristic features such as anti-DNA antibodies and nephritis. Defining the role of DNA damage response in SLE pathogenesis might be strategic in the quest for novel therapies.

The Absent in Melanoma 2-Like Receptor IFN-Inducible Protein 16 as an Inflammasome Regulator in Systemic Lupus Erythematosus: The Dark Side of Sensing Microbes

Absent in melanoma 2 (AIM2)-like receptors (ALRs) are a newly characterized class of pathogen recognition receptors (PRRs) involved in cytosolic and nuclear pathogen DNA recognition. In recent years, two ALR family members, the interferon (IFN)-inducible protein 16 (IFI16) and AIM2, have been linked to the pathogenesis of various autoimmune diseases, among which systemic lupus erythematosus (SLE) has recently gained increasing attention. SLE patients are indeed often characterized by constitutively high serum IFN levels and increased expression of IFN-stimulated genes due to an abnormal response to pathogens and/or incorrect self-DNA recognition process. Consistently, we and others have shown that IFI16 is overexpressed in a wide range of autoimmune diseases where it triggers production of specific autoantibodies. In addition, evidence from mouse models supports a model whereby ALRs are required for IFN-mediated host response to both exogenous and endogenous DNA. Following interaction with cytoplasmic or nuclear nucleic acids, ALRs can form a functional inflammasome through association with the adaptor ASC [apoptosis-associated speck-like protein containing a caspase recruitment domain (CARD)] and with procaspase-1. Importantly, inflammasome-mediated upregulation of IL-1β and IL-18 production positively correlates with SLE disease severity. Therefore, targeting ALR sensors and their downstream pathways represents a promising alternative therapeutic approach for SLE and other systemic autoimmune diseases.

STING dependent sensing - Does HIV actually care?

Sensing of DNA is essential for the innate immune system to detect threats, like viruses, intracellular bacteria or cellular DNA damage. At the centre of this conserved mammalian mechanism stands the adaptor protein STING. STING is highly regulated and is part of a complex signalling network. This network depends on the sensors cGAS and IFI16 to detect misplaced DNA in the cytoplasm as well as on the kinase TBK1 and the transcription factor IRF3. The DNA sensing machinery has been implicated in many diseases, among others HIV. Here we present a comprehensive review of current status on the STING pathway with all its components and regulations related to HIV pathogenesis. By this, we try to answer the question if STING-mediated DNA sensing plays a role in HIV infections.

Distinct Anti-IFI16 and Anti-GP2 Antibodies in Inflammatory Bowel Disease and Their Variation with Infliximab Therapy

BACKGROUND

Inflammatory bowel disease (IBD) is characterized by a chronic inflammation of the gut, partly driven by defects in the expression and function of pattern recognition receptors, including the IFI16 protein. Because this protein is a target for autoantibodies and its aberrant expression was reported in colonic mucosa from active patients with ulcerative colitis, we studied its expression and specific seroresponse in patients with IBD before and after infliximab (IFX) therapy.

METHODS

Anti-IFI16 antibodies (IgG and IgA subtypes) were measured in the sera of 74 patients with IBD: 48 patients with Crohn's disease (CD) and 26 patients with ulcerative colitis, prospectively harvested before and after IFX therapy. Anti-GP2 antibodies (both IgG and IgA subtypes) were also tested for comparison. The patient antibody statuses were qualitatively and quantitatively associated with disease phenotype and response to IFX therapy.

RESULTS

Significantly higher titers of anti-IFI16 IgG were found in both CD and ulcerative colitis patients compared with healthy controls. Anti-IFI16 IgA titers were also present in patients with CD. Anti-GP2 IgG subtype titers were significantly increased in patients with CD, as were IgA subtype titers. Significant changes in anti-IFI16 IgG subtype titers were observed after IFX in patients with CD who correlated with clinical remission or response.

CONCLUSIONS

Our results highlight the importance of IFI16 in IBD pathogenesis showing that its de novo overexpression in the gut epithelial cells leads to a breakdown in immune tolerance and the subsequent development of specific autoantibodies. Anti-IFI16 IgG antibodies hold the potential to serve as a biomarker of response to IFX therapy.

Association of Antibodies to Interferon-Inducible Protein-16 With Markers of More Severe Disease in Primary Sjögren's Syndrome

OBJECTIVE

Interferon-inducible protein-16 (IFI16) is an intracellular DNA receptor involved in innate immunity. We evaluated the frequency, phenotypic characteristics, and clinical associations of anti-IFI16 antibodies in patients with primary Sjögren's syndrome (SS), and quantitated expression levels of IFI16 in SS and control salivary gland lysates.

METHODS

Anti-IFI16 antibodies were assayed by enzyme-linked immunosorbent assay using sera from patients with primary SS (n = 133) and from healthy controls (n = 47). Sera from systemic lupus erythematosus (SLE) patients (n = 132) were included as disease controls. Immunoprecipitation of in vitro transcription-translated IFI16 was used to determine which portion of IFI16 the antibodies recognized. Expression of IFI16 in salivary gland lysates was quantitated by immunoblotting.

RESULTS

Anti-IFI16 antibodies were present in the sera of 38 of 133 SS patients (29%) compared to 1 of 47 healthy controls (2.1%) (SS versus controls; P < 0.0002) and in 31 of 132 SLE controls (24%). In SS, anti-IFI16 antibodies were associated with an abnormal Schirmer's test (P = 0.003), hyperglobulinemia (P = 0.02), antinuclear antibody ≥1:320 (P = 0.01), germinal center-like structures in labial salivary gland lymphoid infiltrates (P = 0.01), and higher focus scores (3.4 versus 2.4; P = 0.005). High-titer IFI16 antibodies were directed against an epitope outside the N-terminus in 9 of 13 SS patients (69%). IFI16 was expressed in 4 of 5 (80%) of SS and 1 of 6 (17%) of control labial salivary glands.

CONCLUSION

Anti-IFI16 antibodies are a prominent specificity in primary SS and are associated with markers of severe disease. IFI16 is expressed at higher levels in SS salivary glands compared to controls. These high levels in disease target tissue may contribute to the ongoing anti-IFI16 immune response.

Interferon gamma-inducible protein 16 in primary Sjögren's syndrome: a novel player in disease pathogenesis?

Introduction

There is evidence that interferon is involved in the pathogenesis of primary Sjögren’s syndrome (pSS). The interferon-inducible IFI16 protein, normally expressed in cell nuclei, may be overexpressed, mislocalized in the cytoplasm and secreted in the extracellular milieu in several autoimmune disorders. This leads to tolerance breaking to this self-protein with consequent development of anti-IFI16 antibodies. The aim of this study was to identify the pathogenic and clinical significance of IFI16 and anti-IFI16 in pSS.

Methods

IFI16 and anti-IFI16 were assessed in the serum of 67 pSS patients and over 100 healthy donors by enzyme-linked immunosorbent assay. IFI16 was also evaluated by immunohistochemistry in minor salivary glands of 15 pSS patients and 10 subjects with sicca symptoms but without any clinical, serological or histological features of pSS.

Results

pSS patients display higher serum levels of both IFI16 and anti-IFI16 compared to healthy donors. IFI16 concentration was directly correlated with disease duration and focus score and inversely correlated with age at diagnosis. Moreover, IFI16 positivity was associated with concurrent positivity for rheumatoid factor. Interestingly, the direct correlation between IFI16 positivity and focus score was independent of disease duration and age at diagnosis. pSS minor salivary glands display marked expression and cytoplasmic mislocalization of IFI16 by acinar and ductal epithelial cells as well as infiltrating lymphocytes and peri/intralesional endothelium compared to minor salivary glands with normal architecture or nonspecific chronic sialadenitis. Within the mononuclear cell infiltrate, IFI16 expression appears to parallel the distribution of T lymphocytes.

Conclusion

Our data suggest that the IFI16 protein may be involved in the pathogenesis of glandular inflammation occurring in pSS.

The roles of interferon-inducible p200 family members IFI16 and p204 in innate immune responses, cell differentiation and proliferation

p204 is a member of the interferon-inducible p200 family proteins in mice. The p200 family has been reported to be multifunctional regulators of cell proliferation, differentiation, apoptosis and senescence. Interferon-inducible protein 16 (IFI16) is regarded as the human ortholog of p204 in several studies. This is possibly due to the similarity of their structures. However the consistency of their functions is still elusive. Currently, an emerging focus has been placed upon the role of the p200 proteins as sensors for microbial DNA in innate immune responses and provides new insights into infections as well as autoimmune diseases. This review specially focuses on IFI16 and p204, the member of p200 family in human and murine respectively, and their pathophysiological roles in innate immune responses, cell differentiation and proliferation.

Mislocalization of the interferon inducible protein IFI16 by environmental insults: implications in autoimmunity

The nuclear DNA sensor IFI16, a member of PYHIN family of proteins, was previously studied for its role in cell cycle regulation, tumor suppression, apoptosis and DNA damage signaling. Autoantibodies against IFI16 are prevalent in the sera of patients with systemic autoimmunity, thus depicting physiological significance as an autoantigen. At present, the nuclear IFI16 protein has been thoroughly investigated for its role as an innate immune sensor involved in inflammasome signaling and viral restriction. While the sub-cellular localization of IFI16 during such events has been known, very little knowledge about its presence and significance in the extracellular space is available. Recently our group has discovered the presence of circulating IFI16 in the sera from systemic autoimmune patients indicating that in this setting it may be mislocalized form its nuclear site and secreted in the extracellular milieu. In this review, we will discuss the leakage of endogenous IFI16 that has been experimentally proved using in vivo and in vitro models. Also we will comment on the significance of mislocalized inflammasome components in the extracellular space and how it can be responsible for chronic inflammation.

Intracellular sensing of viral DNA by the innate immune system

Recent years have seen a great advance in knowledge of how a host senses infection. Nucleic acids, as a common denominator to all pathogens, are at the centre of several of the sensing pathways, especially those involved with the recognition of viruses. In this review we discuss the current knowledge on how intracellular DNA is sensed by the mammalian host.

The emerging role of human PYHIN proteins in innate immunity: implications for health and disease

The innate immune response depends on the ability of immune cells to detect pathogens through germline-encoded pattern recognition receptors (PRRs). Recently discovered PRRs include some members of the Pyrin and HIN domain (PYHIN) family, which are encoded on an interferon-inducible gene cluster located on chromosome 1q23. There are five human PYHIN proteins; Absent in melanoma 2 (AIM2), IFN-γ inducible protein 16 (IFI16), Myeloid cell nuclear differentiation antigen (MNDA), Pyrin and HIN domain family member 1 (PYHIN1) and the recently identified Pyrin domain only protein 3 (POP3). Early studies reported roles for these proteins in cell cycle control, tumour suppression and transcriptional regulation. AIM2 and IFI16 have now been shown to be immune sensors of non-self DNA, such as that produced by viruses in infected cells. AIM2 binds DNA to activate the inflammasome, while IFI16 detection of DNA can lead to the up-regulation of type I interferons or inflammasome activation. Recent studies have shown how IFI16 senses DNA viruses, and also how viruses evade detection by IFI16, while structural studies have greatly advanced our understanding of how AIM2 and IFI16 bind DNA to activate these immune responses. Furthermore, following the identification of POP3, interplay between members of this gene cluster has been established, with POP3 acting as a negative regulator of the AIM2 and IFI16 inflammasomes. In this review we discuss the current understanding of how PYHIN proteins function in innate immunity, their role in disease and the therapeutic possibilities that arise as a result.

The interferon response to intracellular DNA: why so many receptors?

The detection of intracellular DNA has emerged to be a key event in the innate immune response to viruses and intracellular bacteria, and during conditions of sterile inflammation and autoimmunity. One of the consequences of the detection of DNA as a 'stranger' and a 'danger' signal is the production of type I interferons and pro-inflammatory cytokines. Much work has been dedicated to the elucidation of the signalling cascades that activate this DNA-induced gene expression programme. However, while many proteins have been proposed to act as sensors for intracellular DNA in recent years, none has been met with universal acceptance, and a theory linking all the recent observations is, as yet, lacking. This review presents the evidence for the various interferon-inducing DNA receptors proposed to date, and examines the hypotheses that might explain why so many different receptors appear to be involved in the innate immune recognition of intracellular DNA.