BRCA1 Regulates IFI16 Mediated Nuclear Innate Sensing of Herpes Viral DNA and Subsequent Induction of the Innate Inflammasome and Interferon-β Responses

Effect of HDAC9-induced deacetylation of glycolysis-related GAPDH lysine 219 on rotavirus replication in rotavirus-infected Caco-2 cells

Post-translational modifications (PTMs), as epigenetic modifications, are significant in the interaction between virus and its host. However, it is unclear whether rotavirus (RV) causes changes in both the host cell epigenetic protein modification and the regulatory mechanism of viral replication. Here, we analyzed the proteome of Caco-2 cells to determine if acetylation modification occurred within the cells after RV infection. We found that glyceraldehyde-3-phosphate dehydrogenase (GAPDH), a protein involved in glycolysis, was deacetylated at lysine 219 via histone deacetylase 9 (HDAC9) in 50 h after the RV infection. Remarkably, the deacetylation of GAPDH promoted RV replication. Finally, we found that glycolysis was alterable in Caco-2 cells by RV or the deacetylation of GAPDH lysine 219, using the Seahorse XF Glycolysis Stress Test. In conclusion, our results demonstrate for the first time that RV infection promoted deacetylation of GAPDH at lysine 219 in order to increase its own viral replication in Caco-2 cells.

The Interplay between KSHV Infection and DNA-Sensing Pathways

During viral infection, the innate immune system utilizes a variety of specific intracellular sensors to detect virus-derived nucleic acids and activate a series of cellular signaling cascades that produce type I IFNs and proinflammatory cytokines and chemokines. Kaposi's sarcoma-associated herpesvirus (KSHV) is an oncogenic double-stranded DNA virus that has been associated with a variety of human malignancies, including Kaposi's sarcoma, primary effusion lymphoma, and multicentric Castleman disease. Infection with KSHV activates various DNA sensors, including cGAS, STING, IFI16, and DExD/H-box helicases. Activation of these DNA sensors induces the innate immune response to antagonize the virus. To counteract this, KSHV has developed countless strategies to evade or inhibit DNA sensing and facilitate its own infection. This review summarizes the major DNA-triggered sensing signaling pathways and details the current knowledge of DNA-sensing mechanisms involved in KSHV infection, as well as how KSHV evades antiviral signaling pathways to successfully establish latent infection and undergo lytic reactivation.

MNDA, a PYHIN factor involved in transcriptional regulation and apoptosis control in leukocytes

Inflammation control is critical during the innate immune response. Such response is triggered by the detection of molecules originating from pathogens or damaged host cells by pattern-recognition receptors (PRRs). PRRs subsequently initiate intra-cellular signalling through different pathways, resulting in i) the production of inflammatory cytokines, including type I interferon (IFN), and ii) the initiation of a cascade of events that promote both immediate host responses as well as adaptive immune responses. All human PYRIN and HIN-200 domains (PYHIN) protein family members were initially proposed to be PRRs, although this view has been challenged by reports that revealed their impact on other cellular mechanisms. Of relevance here, the human PYHIN factor myeloid nuclear differentiation antigen (MNDA) has recently been shown to directly control the transcription of genes encoding factors that regulate programmed cell death and inflammation. While MNDA is mainly found in the nucleus of leukocytes of both myeloid (neutrophils and monocytes) and lymphoid (B-cell) origin, its subcellular localization has been shown to be modulated in response to genotoxic agents that induce apoptosis and by bacterial constituents, mediators of inflammation. Prior studies have noted the importance of MNDA as a marker for certain forms of lymphoma, and as a clinical prognostic factor for hematopoietic diseases characterized by defective regulation of apoptosis. Abnormal expression of MNDA has also been associated with altered levels of cytokines and other inflammatory mediators. Refining our comprehension of the regulatory mechanisms governing the expression of MNDA and other PYHIN proteins, as well as enhancing our definition of their molecular functions, could significantly influence the management and treatment strategies of numerous human diseases. Here, we review the current state of knowledge regarding PYHIN proteins and their role in innate and adaptive immune responses. Emphasis will be placed on the regulation, function, and relevance of MNDA expression in the control of gene transcription and RNA stability during cell death and inflammation.

Cellular RNA and DNA sensing pathways are essential for the dose-dependent response of human monocytes to ionizing radiation

Circulating monocytes are important players of the inflammatory response to ionizing radiation (IR). These IR-resistant immune cells migrate to radiation-damaged tissues and differentiate into macrophages that phagocytize dying cells, but also facilitate inflammation. Besides the effect of damage-associated molecular patterns, released from irradiated tissues, the inflammatory activation of monocytes and macrophages is largely dependent on IR-induced DNA damage and aberrant transcriptional activity, which may facilitate expression of type I interferons (IFN-I) and numerous inflammation-related genes. We analyzed the accumulation of dsRNA, dsDNA fragments, and RNA:DNA hybrids in the context of induction of RNA-triggered MAVS-mediated and DNA-triggered STING-mediated signaling pathways, in primary human monocytes and a monocytic cell line, THP1, in response to various doses of gamma IR. We found that exposure to lower doses (<7.5 Gy) led to the accumulation of dsRNA, along with dsDNA and RNA:DNA hybrids and activated both MAVS and STING pathway-induced gene expression and signaling activity of IFN-I. Higher doses of IR resulted in the reduced dsRNA level, degradation of RNA-sensing mediators involved in MAVS signaling and coincided with an increased accumulation of dsDNA and RNA:DNA hybrids that correlated with elevated STING signaling and NF-κB-dependent gene expression. While both pathways activate IFN-I expression, using MAVS- and STING-knockout THP1 cells, we identified differences in the spectra of interferon-stimulated genes (ISGs) that are associated with each specific signaling pathway and outlined a large group of STING signaling-associated genes. Using the RNAi technique, we found that increasing the dose of IR activates STING signaling through the DNA sensor cGAS, along with suppression of the DDX41 helicase, which is known to reduce the accumulation of RNA:DNA hybrids and thereby limit cGAS/STING signaling activity. Together, these results indicate that depending on the applied dose, IR leads to the activation of either dsRNA-induced MAVS signaling, which predominantly leads to the expression of both pro- and anti-inflammatory markers, or dsDNA-induced STING signaling that contributes to pro-inflammatory activation of the cells. While RNA:DNA hybrids boost both MAVS- and STING-mediated signaling pathways, these structures being accumulated upon high IR doses promote type I interferon expression and appear to be potent enhancers of radiation dose-dependent pro-inflammatory activation of monocytes.

Inflammasome-independent NLRP3 function enforces ATM activity in response to genotoxic stress

NLRP3 is a pattern recognition receptor with a well-documented role in inducing inflammasome assembly in response to cellular stress. Deregulation of its activity leads to many inflammatory disorders including gouty arthritis, Alzheimer disease, and cancer. Whereas its role in the context of cancer has been mostly explored in the immune compartment, whether NLRP3 exerts functions unrelated to immunity in cancer development remains unexplored. Here, we demonstrate that NLRP3 interacts with the ATM kinase to control the activation of the DNA damage response, independently of its inflammasome activity. NLRP3 down-regulation in both broncho- and mammary human epithelial cells significantly impairs ATM pathway activation, leading to lower p53 activation, and provides cells with the ability to resist apoptosis induced by acute genotoxic stress. Interestingly, NLRP3 expression is down-regulated in non-small cell lung cancers and breast cancers, and its expression positively correlates with patient overall survival. Our findings identify a novel non-immune function for NLRP3 in maintaining genome integrity and strengthen the concept of a functional link between innate immunity and DNA damage sensing pathways to maintain cell integrity.

The innate and T-cell mediated immune response during acute and chronic gammaherpesvirus infection

Immediately after entry into host cells, viruses are sensed by the innate immune system, leading to the activation of innate antiviral effector mechanisms including the type I interferon (IFN) response and natural killer (NK) cells. This innate immune response helps to shape an effective adaptive T cell immune response mediated by cytotoxic T cells and CD4+ T helper cells and is also critical for the maintenance of protective T cells during chronic infection. The human gammaherpesvirus Epstein-Barr virus (EBV) is a highly prevalent lymphotropic oncovirus that establishes chronic lifelong infections in the vast majority of the adult population. Although acute EBV infection is controlled in an immunocompetent host, chronic EBV infection can lead to severe complications in immunosuppressed patients. Given that EBV is strictly host-specific, its murine homolog murid herpesvirus 4 or MHV68 is a widely used model to obtain in vivo insights into the interaction between gammaherpesviruses and their host. Despite the fact that EBV and MHV68 have developed strategies to evade the innate and adaptive immune response, innate antiviral effector mechanisms still play a vital role in not only controlling the acute infection but also shaping an efficient long-lasting adaptive immune response. Here, we summarize the current knowledge about the innate immune response mediated by the type I IFN system and NK cells, and the adaptive T cell-mediated response during EBV and MHV68 infection. Investigating the fine-tuned interplay between the innate immune and T cell response will provide valuable insights which may be exploited to design better therapeutic strategies to vanquish chronic herpesviral infection.

HSV: The scout and assault for digestive system tumors

More than 25% of all malignant tumors are digestive system tumors (DSTs), which mostly include esophageal cancer, gastric cancer, pancreatic cancer, liver cancer, gallbladder cancer and cholangiocarcinoma, and colorectal cancer. DSTs have emerged as one of the prominent reasons of morbidity and death in many nations and areas around the world, posing a serious threat to human life and health. General treatments such as radiotherapy, chemotherapy, and surgical resection can poorly cure the patients and have a bad prognosis. A type of immunotherapy known as oncolytic virus therapy, have recently shown extraordinary anti-tumor effectiveness. One of the viruses that has been the subject of the greatest research in this field, the herpes simplex virus (HSV), has shown excellent potential in DSTs. With a discussion of HSV-1 based on recent studies, we outline the therapeutic effects of HSV on a number of DSTs in this review. Additionally, the critical function of HSV in the detection of cancers is discussed, and some HSV future possibilities are shown.

The Adaptability of Chromosomal Instability in Cancer Therapy and Resistance

Variation in chromosome structure is a central source of DNA damage and DNA damage response, together representinga major hallmark of chromosomal instability. Cancer cells under selective pressure of therapy use DNA damage and DNA damage response to produce newfunctional assets as an evolutionary mechanism. Recent efforts to understand DNA damage/chromosomal instability and elucidate its role in initiation or progression of cancer have also disclosed its vulnerabilities represented by inappropriate DNA damage response, chromatin changes, andinflammation. Understanding these vulnerabilities can provide important clues for predicting treatment response and for the development of novel strategies that prevent the emergence of therapy resistant tumors.

Nuclear antiviral innate responses at the intersection of DNA sensing and DNA repair

The coevolution of vertebrate and mammalian hosts with DNA viruses has driven the ability of host cells to distinguish viral from cellular DNA in the nucleus to induce intrinsic immune responses. Concomitant viral mechanisms have arisen to inhibit DNA sensing. At this virus-host interface, emerging evidence links cytokine responses and cellular homeostasis pathways, particularly the DNA damage response (DDR). Nuclear DNA sensors, such as the interferon (IFN)-γ inducible protein 16 (IFI16), functionally intersect with the DDR regulators ataxia telangiectasia mutated (ATM) and DNA-dependent protein kinase (DNA-PK). Here, we discuss accumulating knowledge for the DDR-innate immunity signaling axis. Through the lens of this infection-driven signaling axis, we present host and viral molecular strategies acquired to regulate autoinflammation and antiviral responses.

Insights into functional connectivity in mammalian signal transduction pathways by pairwise comparison of protein interaction partners of critical signaling hubs

Growth factors and cytokines activate signal transduction pathways and regulate gene expression in eukaryotes. Intracellular domains of activated receptors recruit several protein kinases as well as transcription factors that serve as platforms or hubs for the assembly of multi-protein complexes. The signaling hubs involved in a related biologic function often share common interaction proteins and target genes. This functional connectivity suggests that a pairwise comparison of protein interaction partners of signaling hubs and network analysis of common partners and their expression analysis might lead to the identification of critical nodes in cellular signaling. A pairwise comparison of signaling hubs across several related pathways might reveal novel signaling modules. Analysis of protein interaction connectome by Venn (PIC-Venn) of transcription factors STAT1, STAT3, NFKB1, RELA, FOS, and JUN, and their common interaction network suggested that BRCA1 and TSC22D3 function as critical nodes in immune responses by connecting the signaling hubs into signaling modules. Transcriptional regulation of critical hubs may play a major role in the lung epithelial cells in response to SARS-CoV-2 and in COVID-19 patients. Mutations and differential expression levels of these critical nodes and modules in pathological conditions might deregulate signaling pathways and their target genes involved in inflammation. Biological connectivity emerges from the structural connectivity of interaction networks across several signaling hubs in related pathways. The main objectives of this study are to identify critical hubs, critical nodes, and modules involved in the signal transduction pathways of innate and adaptive immunity. Application of PIC-Venn to several signaling hubs might reveal novel nodes and modules that can be targeted by small regulatory molecules to simultaneously activate or inhibit cell signaling in health and disease.

Context-dependent functions of pattern recognition receptors in cancer

The immune system plays a critical role in shaping all facets of cancer, from the early initiation stage through to metastatic disease and resistance to therapy. Our understanding of the importance of the adaptive arm of the immune system in antitumour immunity has led to the implementation of immunotherapy with immune checkpoint inhibitors in numerous cancers, albeit with differing efficacy. By contrast, the clinical utility of innate immunity in cancer has not been exploited, despite dysregulated innate immunity being a feature of at least one-third of all cancers associated with tumour-promoting chronic inflammation. The past two decades have seen innate immune pattern recognition receptors (PRRs) emerge as critical regulators of the immune response to microbial infection and host tissue damage. More recently, it has become apparent that in many cancer types, PRRs play a central role in modulating a vast array of tumour-inhibiting and tumour-promoting cellular responses both in immune cells within the tumour microenvironment and directly in cancer cells. Herein, we provide a comprehensive overview of the fast-evolving field of PRRs in cancer, and discuss the potential to target PRRs for drug development and biomarker discovery in a wide range of oncology settings.

Pathophysiological functions of self-derived DNA

Inflammation plays indispensable roles in building the immune responses such as acquired immunity against harmful pathogens. Furthermore, it is essential for maintaining biological homeostasis in ever-changing conditions. Pattern-recognition receptors (PRRs) reside in cell membranes, endosomes or cytoplasm, and function as triggers for inflammatory responses. Binding of pathogen- or self-derived components, such as DNA, to PRRs activates downstream signaling cascades, resulting in the production of a series of pro-inflammatory cytokines and type I interferons (IFNs). While these series of responses are essential for host defense, the unexpected release of DNA from the nucleus or mitochondria of host cells can lead to autoimmune and autoinflammatory diseases. In this review, we focus on DNA-sensing mechanisms via PRRs and the disorders and extraordinary conditions caused by self-derived DNA.

Core fucosylation involvement in the paracrine regulation of proteinuria-induced renal interstitial fibrosis evaluated with the use of a microfluidic chip

Proteinuria is a clinical manifestation of chronic kidney disease that aggravates renal interstitial fibrosis (RIF), in which injury of peritubular microvessels is an important event. However, the changes in peritubular microvessels induced by proteinuria and their molecular mechanisms remain unclear. Thus, we aimed to develop a co-culture microfluidic device that contains renal tubules and peritubular microvessels to create a proteinuria model. We found that protein overload in the renal tubule induced trans-differentiation and apoptosis of endothelial cells (ECs) and pericytes. Moreover, profiling of secreted proteins in this model revealed that a paracrine network between tubules and microvessels was activated in proteinuria-induced microvascular injury. Multiple cytokine receptors in this paracrine network were core-fucosylated. Inhibition of core fucosylation significantly reduced ligand-receptor binding ability and blocked downstream pathways, alleviating trans-differentiation and apoptosis of ECs and pericytes. Furthermore, the protective effect of genetic FUT8 deficiency on proteinuria overload-induced RIF and pericyte-myofibroblast trans-differentiation was validated in FUT8 knockout heterozygous mice. In conclusion, we constructed and used a multiple-unit integrated microfluidic device to uncover the mechanism of proteinuria-induced RIF. Furthermore, FUT8 may serve as a hub-like therapeutic target to alleviate peritubular microvascular injury in RIF. STATEMENT OF SIGNIFICANCE: In this study, we constructed a multiple-unit integrated renal tubule-vascular chip. We reproduced human proteinuria on the chip and found that multiple receptors were modified by FUT8-catalyzed core fucosylation (CF) involved in the cross-talk between renal tubules and peritubular microvessels in proteinuria-induced RIF, and inhibiting the FUT8 of receptors could block the tubule-microvessel paracrine network and reverse the damage of peritubular microvessels and renal interstitial fibrosis. This tubule-vascular chip may provide a prospective platform to facilitate future investigations into the mechanisms of kidney diseases, and target-FUT8 inhibition may be an innovative and potential therapeutic strategy for RIF induced by proteinuria.

Activation and Evasion of Innate Immunity by Gammaherpesviruses

Gammaherpesviruses are ubiquitous pathogens that establish lifelong infections in the vast majority of adults worldwide. Importantly, these viruses are associated with numerous malignancies and are responsible for significant human cancer burden. These virus-associated cancers are due, in part, to the ability of gammaherpesviruses to successfully evade the innate immune response throughout the course of infection. In this review, we will summarize the current understanding of how gammaherpesviruses are detected by innate immune sensors, how these viruses evade recognition by host cells, and how this knowledge can inform novel therapeutic approaches for these viruses and their associated diseases.

The Interplay between Viruses and Host DNA Sensors

DNA virus infections are often lifelong and can cause serious diseases in their hosts. Their recognition by the sensors of the innate immune system represents the front line of host defence. Understanding the molecular mechanisms of innate immunity responses is an important prerequisite for the design of effective antivirotics. This review focuses on the present state of knowledge surrounding the mechanisms of viral DNA genome sensing and the main induced pathways of innate immunity responses. The studies that have been performed to date indicate that herpesviruses, adenoviruses, and polyomaviruses are sensed by various DNA sensors. In non-immune cells, STING pathways have been shown to be activated by cGAS, IFI16, DDX41, or DNA-PK. The activation of TLR9 has mainly been described in pDCs and in other immune cells. Importantly, studies on herpesviruses have unveiled novel participants (BRCA1, H2B, or DNA-PK) in the IFI16 sensing pathway. Polyomavirus studies have revealed that, in addition to viral DNA, micronuclei are released into the cytosol due to genotoxic stress. Papillomaviruses, HBV, and HIV have been shown to evade DNA sensing by sophisticated intracellular trafficking, unique cell tropism, and viral or cellular protein actions that prevent or block DNA sensing. Further research is required to fully understand the interplay between viruses and DNA sensors.

Herpesviruses and Inflammasomes: One Sensor Does Not Fit All

Herpesviruses are ubiquitous double-stranded DNA viruses that cause lifelong infections and are associated with a variety of diseases. While they have evolved multiple mechanisms to evade the immune system, they are all recognized by the innate immune system, which can lead to both localized and systemic inflammation. A more recently appreciated mechanism of herpesvirus innate immune activation is through inflammasome signaling. The inflammasome is an intracellular multiprotein complex that, when activated, leads to the release of proinflammatory cytokines, including IL-1β and IL-18, and activation of the inflammatory programed cell death pathway known as pyroptosis. Despite the herpesviruses sharing a similar structure, their mechanisms of inflammasome activation and the consequences of inflammasome activation in cases of virus-associated disease are not uniform. This review will highlight the similarities and differences among herpesviruses with regard to their mechanisms of inflammasome activation and impacts on diseases caused by herpesviruses. Furthermore, it will identify areas where additional studies are warranted to better understand the impact of this important innate immune signaling program on the pathogenesis of these common viruses.

IFI16 inhibits DNA repair that potentiates type-I interferon-induced antitumor effects in triple negative breast cancer

Tumor DNA-damage response (DDR) has an important role in driving type-I interferon (IFN)-mediated host antitumor immunity, but it is not clear how tumor DNA damage is interconnected with the immune response. Here, we report the role of IFN-γ-inducible protein 16 (IFI16) in DNA repair, which amplifies the stimulator of IFN genes (STING)-type-I IFN signaling, particularly in triple-negative breast cancer (TNBC). IFI16 is rapidly induced and accumulated to the histone-evicted DNA at double-stranded breakage (DSB) sites, where it inhibits recruitment of DDR factors. Subsequently, IFI16 increases the release of DNA fragments to the cytoplasm and induces STING-mediated type-I IFN production. Synergistic cytotoxic and immunomodulatory effects of doxorubicin and type-I IFNs are decreased upon IFI16 depletion in vivo. Furthermore, IFI16 expression correlates with improved clinical outcome in patients with TNBC treated with chemotherapy. Together, our findings suggest that type-I IFNs and IFI16 could offer potential therapeutic strategies for TNBC.

Function and Molecular Mechanism of the DNA Damage Response in Immunity and Cancer Immunotherapy

The DNA damage response (DDR) is an organized network of multiple interwoven components evolved to repair damaged DNA and maintain genome fidelity. Conceptually the DDR includes damage sensors, transducer kinases, and effectors to maintain genomic stability and accurate transmission of genetic information. We have recently gained a substantially improved molecular and mechanistic understanding of how DDR components are interconnected to inflammatory and immune responses to stress. DDR shapes both innate and adaptive immune pathways: (i) in the context of innate immunity, DDR components mainly enhance cytosolic DNA sensing and its downstream STimulator of INterferon Genes (STING)-dependent signaling; (ii) in the context of adaptive immunity, the DDR is needed for the assembly and diversification of antigen receptor genes that is requisite for T and B lymphocyte development. Imbalances between DNA damage and repair impair tissue homeostasis and lead to replication and transcription stress, mutation accumulation, and even cell death. These impacts from DDR defects can then drive tumorigenesis, secretion of inflammatory cytokines, and aberrant immune responses. Yet, DDR deficiency or inhibition can also directly enhance innate immune responses. Furthermore, DDR defects plus the higher mutation load in tumor cells synergistically produce primarily tumor-specific neoantigens, which are powerfully targeted in cancer immunotherapy by employing immune checkpoint inhibitors to amplify immune responses. Thus, elucidating DDR-immune response interplay may provide critical connections for harnessing immunomodulatory effects plus targeted inhibition to improve efficacy of radiation and chemotherapies, of immune checkpoint blockade, and of combined therapeutic strategies.

ASC Speck Formation after Inflammasome Activation in Primary Human Keratinocytes

Chronic UV irradiation results in many changes in the skin, including hyperplasia, changes in dermal structures, and alteration of pigmentation. Exposure to UVB leads to cutaneous damage, which results in inflammation characterized by increased NF-κB activation and the induction of inflammatory cytokines, such as tumor necrosis factor (TNF), interleukin- (IL-) 1, or IL-8. IL-1 secretion is the result of inflammasome activation which is besides apoptosis, a result of acute UVB treatment. Inflammasomes are cytosolic protein complexes whose formation results in the activation of proinflammatory caspase-1. Key substrates of caspase-1 are IL-1β and IL-18, and the cytosolic protein gasdermin D (GSDMD), which is involved in inflammatory cell death. Here, we demonstrate that UVB-induced inflammasome activation leads to the formation of ASC specks. Our findings show that UVB provokes ASC speck formation in human primary keratinocytes prior to cell death, and that specks are, opposed to the perinuclear cytosolic localization in myeloid cells, formed in the nucleus. Additionally, we showed by RNAi that NLRP1 and not NLRP3 is the major inflammasome responsible for UVB sensing in primary human keratinocytes. Formation of ASC specks indicates inflammasome assembly and activation as their formation in hPKs depends on the presence of NLRP1 and partially on NLRP3. Nuclear ASC specks are not specific for NLRP1/NLRP3 inflammasome activation, as the activation of the AIM2 inflammasome by cytosolic DNA results in ASC specks too. These nuclear ASC specks putatively link cell death to inflammasome activation, possibly by binding of IFI16 (gamma-interferon-inducible protein) to ASC. ASC can interact upon UVB sensing via IFI16 with p53, linking cell death to ASC speck formation.

cGAS-STING-mediated sensing pathways in DNA and RNA virus infections: crosstalk with other sensing pathways

Viruses cause a variety of diseases in humans and other organisms. The most important defense mechanism against viral infections is initiated when the viral genome is sensed by host proteins, and this results in interferon production and pro-inflammatory cytokine responses. The sensing of the viral genome or its replication intermediates within host cells is mediated by cytosolic proteins. For example, cGAS and IFI16 recognize non-self DNA, and RIG-I and MDA5 recognize non-self RNA. Once these sensors are activated, they trigger a cascade of reactions activating downstream molecules, which eventually results in the transcriptional activation of type I and III interferons, which play a critical role in suppressing viral propagation, either by directly limiting their replication or by inducing host cells to inhibit viral protein synthesis. The immune response against viruses relies solely upon sensing of viral genomes and their downstream signaling molecules. Although DNA and RNA viruses are sensed by distinct classes of receptor proteins, there is a possibility of overlap between the viral DNA and viral RNA sensing mechanisms. In this review, we focus on various host sensing molecules and discuss the associated signaling pathways that are activated in response to different viral infections. We further highlight the possibility of crosstalk between the cGAS-STING and the RIG-I-MAVS pathways to limit viral infections. This comprehensive review delineates the mechanisms by which different viruses evade host cellular responses to sustain within the host cells.

Immune sensing of mouse polyomavirus DNA by p204 and cGAS DNA sensors

The mechanism by which DNA viruses interact with different DNA sensors and their connection with the activation of interferon (IFN) type I pathway are poorly understood. We investigated the roles of protein 204 (p204) and cyclic guanosine-adenosine synthetase (cGAS) sensors during infection with mouse polyomavirus (MPyV). The phosphorylation of IFN regulatory factor 3 (IRF3) and the stimulator of IFN genes (STING) proteins and the upregulation of IFN beta (IFN-β) and MX Dynamin Like GTPase 1 (MX-1) genes were detected at the time of replication of MPyV genomes in the nucleus. STING knockout abolished the IFN response. Infection with a mutant virus that exhibits defective nuclear entry via nucleopores and that accumulates in the cytoplasm confirmed that replication of viral genomes in the nucleus is required for IFN induction. The importance of both DNA sensors, p204 and cGAS, in MPyV-induced IFN response was demonstrated by downregulation of the IFN pathway observed in p204-knockdown and cGAS-knockout cells. Confocal microscopy revealed the colocalization of p204 with MPyV genomes in the nucleus. cGAS was found in the cytoplasm, colocalizing with viral DNA leaked from the nucleus and with DNA within micronucleus-like bodies, but also with the MPyV genomes in the nucleus. However, 2'3'-Cyclic guanosine monophosphate-adenosine monophosphate synthesized by cGAS was detected exclusively in the cytoplasm. Biochemical assays revealed no evidence of functional interaction between cGAS and p204 in the nucleus. Our results provide evidence for the complex interactions of MPyV and DNA sensors including the sensing of viral genomes in the nucleus by p204 and of leaked viral DNA and micronucleus-like bodies in the cytoplasm by cGAS.

Inflammasomes in Alveolar Bone Loss

Bone remodeling is tightly controlled by osteoclast-mediated bone resorption and osteoblast-mediated bone formation. Fine tuning of the osteoclast-osteoblast balance results in strict synchronization of bone resorption and formation, which maintains structural integrity and bone tissue homeostasis; in contrast, dysregulated bone remodeling may cause pathological osteolysis, in which inflammation plays a vital role in promoting bone destruction. The alveolar bone presents high turnover rate, complex associations with the tooth and periodontium, and susceptibility to oral pathogenic insults and mechanical stress, which enhance its complexity in host defense and bone remodeling. Alveolar bone loss is also involved in systemic bone destruction and is affected by medication or systemic pathological factors. Therefore, it is essential to investigate the osteoimmunological mechanisms involved in the dysregulation of alveolar bone remodeling. The inflammasome is a supramolecular protein complex assembled in response to pattern recognition receptors and damage-associated molecular patterns, leading to the maturation and secretion of pro-inflammatory cytokines and activation of inflammatory responses. Pyroptosis downstream of inflammasome activation also facilitates the clearance of intracellular pathogens and irritants. However, inadequate or excessive activity of the inflammasome may allow for persistent infection and infection spreading or uncontrolled destruction of the alveolar bone, as commonly observed in periodontitis, periapical periodontitis, peri-implantitis, orthodontic tooth movement, medication-related osteonecrosis of the jaw, nonsterile or sterile osteomyelitis of the jaw, and osteoporosis. In this review, we present a framework for understanding the role and mechanism of canonical and noncanonical inflammasomes in the pathogenesis and development of etiologically diverse diseases associated with alveolar bone loss. Inappropriate inflammasome activation may drive alveolar osteolysis by regulating cellular players, including osteoclasts, osteoblasts, osteocytes, periodontal ligament cells, macrophages, monocytes, neutrophils, and adaptive immune cells, such as T helper 17 cells, causing increased osteoclast activity, decreased osteoblast activity, and enhanced periodontium inflammation by creating a pro-inflammatory milieu in a context- and cell type-dependent manner. We also discuss promising therapeutic strategies targeting inappropriate inflammasome activity in the treatment of alveolar bone loss. Novel strategies for inhibiting inflammasome signaling may facilitate the development of versatile drugs that carefully balance the beneficial contributions of inflammasomes to host defense.

Systematic profiling of protein complex dynamics reveals DNA-PK phosphorylation of IFI16 en route to herpesvirus immunity

Dynamically shifting protein-protein interactions (PPIs) regulate cellular responses to viruses and the resulting immune signaling. Here, we use thermal proximity coaggregation (TPCA) mass spectrometry to characterize the on-off behavior of PPIs during infection with herpes simplex virus 1 (HSV-1), a virus with an ancient history of coevolution with hosts. Advancing the TPCA analysis to infer associations de novo, we build a time-resolved portrait of thousands of host-host, virus-host, and virus-virus PPIs. We demonstrate that, early in infection, the DNA sensor IFI16 recruits the active DNA damage response kinase, DNA-dependent protein kinase (DNA-PK), to incoming viral DNA at the nuclear periphery. We establish IFI16 T149 as a substrate of DNA-PK upon viral infection or DNA damage. This phosphorylation promotes IFI16-driven cytokine responses. Together, we characterize the global dynamics of PPIs during HSV-1 infection, uncovering the co-regulation of IFI16 and DNA-PK functions as a missing link in immunity to herpesvirus infection.

Pathogenic Role of Epstein-Barr Virus in Lung Cancers

Human oncogenic viruses account for at least 12% of total cancer cases worldwide. Epstein–Barr virus (EBV) is the first identified human oncogenic virus and it alone causes ~200,000 cancer cases and ~1.8% of total cancer-related death annually. Over the past 40 years, increasing lines of evidence have supported a causal link between EBV infection and a subgroup of lung cancers (LCs). In this article, we review the current understanding of the EBV-LC association and the etiological role of EBV in lung carcinogenesis. We also discuss the clinical impact of the knowledge gained from previous research, challenges, and future directions in this field. Given the high clinical relevance of EBV-LC association, there is an urgent need for further investigation on this topic.

Nucleic Acid Immunity and DNA Damage Response: New Friends and Old Foes

The maintenance of genomic stability in multicellular organisms relies on the DNA damage response (DDR). The DDR encompasses several interconnected pathways that cooperate to ensure the repair of genomic lesions. Besides their repair functions, several DDR proteins have emerged as involved in the onset of inflammatory responses. In particular, several actors of the DDR have been reported to elicit innate immune activation upon detection of cytosolic pathological nucleic acids. Conversely, pattern recognition receptors (PRRs), initially described as dedicated to the detection of cytosolic immune-stimulatory nucleic acids, have been found to regulate DDR. Thus, although initially described as operating in specific subcellular localizations, actors of the DDR and nucleic acid immune sensors may be involved in interconnected pathways, likely influencing the efficiency of one another. Within this mini review, we discuss evidences for the crosstalk between PRRs and actors of the DDR. For this purpose, we mainly focus on cyclic GMP-AMP (cGAMP) synthetase (cGAS) and Interferon Gamma Inducible Protein 16 (IFI16), as major PRRs involved in the detection of aberrant nucleic acid species, and components of the DNA-dependent protein kinase (DNA-PK) complex, involved in the repair of double strand breaks that were recently described to qualify as potential PRRs. Finally, we discuss how the crosstalk between DDR and nucleic acid-associated Interferon responses cooperate for the fine-tuning of innate immune activation, and therefore dictate pathological outcomes. Understanding the molecular determinants of such cooperation will be paramount to the design of future therapeutic approaches.

Making Sense of Intracellular Nucleic Acid Sensing in Type I Interferon Activation in Sjögren's Syndrome

Primary Sjögren's syndrome (pSS) is a systemic autoimmune rheumatic disease characterized by dryness of the eyes and mucous membranes, which can be accompanied by various extraglandular autoimmune manifestations. The majority of patients exhibit persistent systemic activation of the type I interferon (IFN) system, a feature that is shared with other systemic autoimmune diseases. Type I IFNs are integral to anti-viral immunity and are produced in response to stimulation of pattern recognition receptors, among which nucleic acid (NA) receptors. Dysregulated detection of endogenous NAs has been widely implicated in the pathogenesis of systemic autoimmune diseases. Stimulation of endosomal Toll-like receptors by NA-containing immune complexes are considered to contribute to the systemic type I IFN activation. Accumulating evidence suggest additional roles for cytosolic NA-sensing pathways in the pathogenesis of systemic autoimmune rheumatic diseases. In this review, we will provide an overview of the functions and signaling of intracellular RNA- and DNA-sensing receptors and summarize the evidence for a potential role of these receptors in the pathogenesis of pSS and the sustained systemic type I IFN activation.

Signaling Through Nucleic Acid Sensors and Their Roles in Inflammatory Diseases

Recognition of pathogen-derived nucleic acids by pattern-recognition receptors (PRRs) is essential for eliciting antiviral immune responses by inducing the production of type I interferons (IFNs) and proinflammatory cytokines. Such responses are a prerequisite for mounting innate and pathogen-specific adaptive immune responses. However, host cells also use nucleic acids as carriers of genetic information, and the aberrant recognition of self-nucleic acids by PRRs is associated with the onset of autoimmune or autoinflammatory diseases. In this review, we describe the mechanisms of nucleic acid sensing by PRRs, including Toll-like receptors, RIG-I-like receptors, and DNA sensor molecules, and their signaling pathways as well as the disorders caused by uncontrolled or unnecessary activation of these PRRs.

Emerging Role of PYHIN Proteins as Antiviral Restriction Factors

Innate immune sensors and restriction factors are cellular proteins that synergize to build an effective first line of defense against viral infections. Innate sensors are usually constitutively expressed and capable of detecting pathogen-associated molecular patterns (PAMPs) via specific pattern recognition receptors (PRRs) to stimulate the immune response. Restriction factors are frequently upregulated by interferons (IFNs) and may inhibit viral pathogens at essentially any stage of their replication cycle. Members of the Pyrin and hematopoietic interferon-inducible nuclear (HIN) domain (PYHIN) family have initially been recognized as important sensors of foreign nucleic acids and activators of the inflammasome and the IFN response. Accumulating evidence shows, however, that at least three of the four members of the human PYHIN family restrict viral pathogens independently of viral sensing and innate immune activation. In this review, we provide an overview on the role of human PYHIN proteins in the innate antiviral immune defense and on viral countermeasures.

Molecular and Structural Basis of DNA Sensors in Antiviral Innate Immunity

DNA viruses are a source of great morbidity and mortality throughout the world by causing many diseases; thus, we need substantial knowledge regarding viral pathogenesis and the host’s antiviral immune responses to devise better preventive and therapeutic strategies. The innate immune system utilizes numerous germ-line encoded receptors called pattern-recognition receptors (PRRs) to detect various pathogen-associated molecular patterns (PAMPs) such as viral nucleic acids, ultimately resulting in antiviral immune responses in the form of proinflammatory cytokines and type I interferons. The immune-stimulatory role of DNA is known for a long time; however, DNA sensing ability of the innate immune system was unraveled only recently. At present, multiple DNA sensors have been proposed, and most of them use STING as a key adaptor protein to exert antiviral immune responses. In this review, we aim to provide molecular and structural underpinnings on endosomal DNA sensor Toll-like receptor 9 (TLR9) and multiple cytosolic DNA sensors including cyclic GMP-AMP synthase (cGAS), interferon-gamma inducible 16 (IFI16), absent in melanoma 2 (AIM2), and DNA-dependent activator of IRFs (DAI) to provide new insights on their signaling mechanisms and physiological relevance. We have also addressed less well-understood DNA sensors such as DEAD-box helicase DDX41, RNA polymerase III (RNA pol III), DNA-dependent protein kinase (DNA-PK), and meiotic recombination 11 homolog A (MRE11). By comprehensive understanding of molecular and structural aspects of DNA-sensing antiviral innate immune signaling pathways, potential new targets for viral and autoimmune diseases can be identified.

IFI16 contributes to the pathogenesis of abdominal aortic aneurysm by regulating the caspase-1/IL-1β/MCPIP1 pathway

AIMS

Abdominal aortic aneurysm (AAA) is a multi-factorial progressive vascular disease characterized by chronic inflammatory cell infiltration. We investigated the roles played by IFI16 and ASC inflammasomes in AAA development and progression.

MATERIALS AND METHODS

Western blot and qRT-PCR studies were performed to analyze the expression of relative genes in AAA specimens and mouse vascular smooth muscle cells (VSMCs). The apoptosis rates and ROS levels of VSMCs were assessed by flow cytometry. Transwell assays were performed to analyze the migration ability of VSMCs. The levels of MCP-1, IL-1β, and IL-6 in the supernatants of cultured VSMCs were analyzed by ELISA.

KEY FINDINGS

Increased levels of IFI16 expression were found in AAA specimens and Ang-II-treated VSMCs. IFI16 and ASC silencing suppressed the apoptosis and migration ability of VSMCs undergoing Ang-II treatment, reduced elasticity damage to the aortic wall, and decreased the levels of MMP expression. The effect of IFI16 knockdown in Ang-II-induced VSMCs was reversed by MCPIP1 overexpression.

SIGNIFICANCE

Our data suggest that an up-regulation of IFI16 and ASC expression might promote the apoptosis of VSMCs, enhance the inflammatory response, and impairs vascular wall elasticity via a MCPIP1-related mechanism. The inflammasome components IFI16 and ASC might be involved in AAA progression and serve as target molecules for diagnosing and treating AAA.

Detection of Viral Infections by Innate Immunity

Pattern recognition receptors (PRRs) and inflammasomes are a key part of the anti-viral innate immune system as they detect conserved viral pathogen-associated molecular patterns (PAMPs). A successful host response to viral infections critically depend on the initial activation of PRRs by viruses, mainly by viral DNA and RNA. The signalling pathways activated by PRRs leads to the expression of pro-inflammatory cytokines, to recruit immune cells, and type I and type III interferons which leads to the induction of interferon stimulated genes (ISG), powerful virus restriction factors that establish the "antiviral state". Inflammasomes contribute to anti-viral responses through the maturation of interleukin (IL)-1 and IL-18 and through triggering pyroptotic cell death. The activity of the innate immune system along with the adaptive immune response normally leads to successful virus elimination, although disproportionate innate responses contribute to viral pathology. In this review we will discuss recent insights into the influence of PRR activation and inflammasomes on viral infections and what this means for the mammalian host. We will also comment on how specific PRRs and inflammasomes may be relevant to how SARS-CoV-2, the virus responsible for the current COVID-19 pandemic, interacts with host innate immunity.

Cytosolic DNA Sensors and CNS Responses to Viral Pathogens

Viral central nervous system (CNS) infections can lead to life threatening encephalitis and long-term neurological deficits in survivors. Resident CNS cell types, such as astrocytes and microglia, are known to produce key inflammatory and antiviral mediators following infection with neurotropic DNA viruses. However, the mechanisms by which glia mediate such responses remain poorly understood. Recently, a class of intracellular pattern recognition receptors (PRRs), collectively known as DNA sensors, have been identified in both leukocytic and non-leukocytic cell types. The ability of such DNA sensors to initiate immune mediator production and contribute to infection resolution in the periphery is increasingly recognized, but our understanding of their role in the CNS remains limited at best. In this review, we describe the evidence for the expression and functionality of DNA sensors in resident brain cells, with a focus on their role in neurotropic virus infections. The available data indicate that glia and neurons can constitutively express, and/or can be induced to express, various disparate DNA sensing molecules previously described in peripheral cell types. Furthermore, multiple lines of investigation suggest that these sensors are functional in resident CNS cells and are required for innate immune responses to viral infections. However, it is less clear whether DNA sensormediated glial responses are beneficial or detrimental, and the answer to this question appears to dependent on the context of the infection with regard to the identity of the pathogen, host cell type, and host species. Defining such parameters will be essential if we are to successfully target these molecules to limit damaging inflammation while allowing beneficial host responses to improve patient outcomes.

HPV E7 inhibits cell pyroptosis by promoting TRIM21-mediated degradation and ubiquitination of the IFI16 inflammasome

Human papillomavirus (HPV) is a DNA virus that causes sexually transmitted infections. The HPV oncoprotein E7 plays a critical role in the regulation of host immunity to promote the immune escape of HPV and the occurrence of cervical cancer or genital warts. Pyroptosis, a highly inflammatory form of programmed cell death, can be induced by inflammasomes and acts as a defense against pathogenic infection. However, whether HPV E7 can regulate cell pyroptosis to evade immune surveillance has not been determined. In this study, we found that HPV E7 could inhibit cell pyroptosis induced by transfection with dsDNA. The activation of the inflammasome, and the production of IL-18 and IL-1β were also restrained by HPV E7. Mass spectrometry and immunoprecipitation showed that HPV E7 interacted with IFI16 and TRIM21. We also discovered that HPV E7 recruited the E3 ligase TRIM21 to ubiquitinate and degrade the IFI16 inflammasome, leading to the inhibition of cell pyroptosis and self-escape from immune surveillance. Thus, our study reveals an important immune escape mechanism in HPV infection and may provide targets for the development of a novel immunotherapeutic strategy to effectively restore antiviral immunity.

Epigenetic Mechanisms of Inflammasome Regulation

The innate immune system represents the host's first-line defense against pathogens, dead cells or environmental factors. One of the most important inflammatory pathways is represented by the activation of the NOD-like receptor (NLR) protein family. Some NLRs induce the assembly of large caspase-1-activating complexes called inflammasomes. Different types of inflammasomes have been identified that can respond to distinct bacterial, viral or fungal infections; sterile cell damage or other stressors, such as metabolic imbalances. Epigenetic regulation has been recently suggested to provide a complementary mechanism to control inflammasome activity. This regulation can be exerted through at least three main mechanisms, including CpG DNA methylation, histones post-translational modifications and noncoding RNA expression. The repression or promotion of expression of different inflammasomes (NLRP1, NLRP2, NLRP3, NLRP4, NLRP6, NLRP7, NLRP12 and AIM2) through epigenetic mechanisms determines the development of pathologies with variable severity. For example, our team recently explored the role of microRNAs (miRNAs) targeting and modulating the components of the inflammasome as potential biomarkers in bladder cancer and during therapy. This suggests that the epigenetic control of inflammasome-related genes could represent a potential target for further investigations of molecular mechanisms regulating inflammatory pathways.

Understanding HCMV Latency Using Unbiased Proteomic Analyses

Human cytomegalovirus (HCMV) establishes either a latent (non-productive) or lytic (productive) infection depending upon cell type, cytokine milieu and the differentiation status of the infected cell. Undifferentiated cells, such as precursor cells of the myeloid lineage, support a latent infection whereas terminally differentiated cells, such as monocytes or dendritic cells are an environment conducive to reactivation and support a lytic infection. The mechanisms which regulate HCMV in either a latent or lytic infection have been the focus of intense investigation with a view to developing novel treatments for HCMV-associated disease which can have a heavy clinical burden after reactivation or primary infection in, especially, the immune compromised. To this end, a number of studies have been carried out in an unbiased manner to address global changes occurring within the latently infected cell to address the molecular changes associated with HCMV latency. In this review, we will concentrate on the proteomic analyses which have been carried out in undifferentiated myeloid cells which either stably express specific viral latency associated genes in isolation or on cells which have been latently infected with virus.

Interleukin-1β and Cancer

Within a tumor, IL-1β is produced and secreted by various cell types, such as immune cells, fibroblasts, or cancer cells. The IL1B gene is induced after "priming" of the cells and a second signal is required to allow IL-1β maturation by inflammasome-activated caspase-1. IL-1β is then released and leads to transcription of target genes through its ligation with IL-1R1 on target cells. IL-1β expression and maturation are guided by gene polymorphisms and by the cellular context. In cancer, IL-1β has pleiotropic effects on immune cells, angiogenesis, cancer cell proliferation, migration, and metastasis. Moreover, anti-cancer treatments are able to promote IL-1β production by cancer or immune cells, with opposite effects on cancer progression. This raises the question of whether or not to use IL-1β inhibitors in cancer treatment.

The intracellular DNA sensors cGAS and IFI16 do not mediate effective antiviral immune responses to HSV-1 in human microglial cells

Glia play a key role in immunosurveillance within the central nervous system (CNS) and can recognize a wide range of pathogen-associated molecular patterns (PAMPS) via members of multiple pattern recognition receptor (PRR) families. Of these, the expression of cytosolic/nuclear RNA and DNA sensors by glial cells is of particular interest as their ability to interact with intracellular nucleic acids suggests a critical role in the detection of viral pathogens. The recently discovered DNA sensors cyclic GMP-AMP synthase (cGAS) and interferon gamma-inducible protein 16 (IFI16) have been reported to be important for the recognition of DNA pathogens such as herpes simplex virus-1 (HSV-1) in peripheral human cell types, and we have recently demonstrated that human glia express cGAS and its downstream adaptor molecule stimulator of interferon genes (STING). Here, we have demonstrated that human microglial cells functionally express cGAS and exhibit robust constitutive IFI16 expression. While cGAS serves as a significant component in IRF3 activation and IFN-β production by human microglial cells in response to foreign intracellular DNA, IFI16 is not required for such responses. Surprisingly, neither of these sensors mediate effective antiviral responses to HSV-1 in microglia, and this may be due, at least in part, to viral suppression of cGAS and/or IFI16 expression. As such, this ability may represent an important HSV immune evasion strategy in glial cells, and approaches that mitigate such suppression might represent a novel strategy to limit HSV-1-associated neuropathology.

Replication Stress, DNA Damage, Inflammatory Cytokines and Innate Immune Response

Complete and accurate DNA replication is essential to genome stability maintenance during cellular division. However, cells are routinely challenged by endogenous as well as exogenous agents that threaten DNA stability. DNA breaks and the activation of the DNA damage response (DDR) arising from endogenous replication stress have been observed at pre- or early stages of oncogenesis and senescence. Proper detection and signalling of DNA damage are essential for the autonomous cellular response in which the DDR regulates cell cycle progression and controls the repair machinery. In addition to this autonomous cellular response, replicative stress changes the cellular microenvironment, activating the innate immune response that enables the organism to protect itself against the proliferation of damaged cells. Thereby, the recent descriptions of the mechanisms of the pro-inflammatory response activation after replication stress, DNA damage and DDR defects constitute important conceptual novelties. Here, we review the links of replication, DNA damage and DDR defects to innate immunity activation by pro-inflammatory paracrine effects, highlighting the implications for human syndromes and immunotherapies.

Role of inflammasomes in the pathogenesis of periodontal disease and therapeutics

Inflammasomes are a group of multimolecular intracellular complexes assembled around several innate immune proteins. Recognition of a diverse range of microbial, stress and damage signals by inflammasomes results in direct activation of caspase‐1, which subsequently induces the only known form of secretion of active interleukin‐1β and interleukin‐18. Although the importance of interleukin‐1β in the periodontium is not questioned, the impact of inflammasomes in periodontal disease and its potential for therapeutics in periodontology is still in its very early stages. Increasing evidence in preclinical models and human data strongly implicate the involvement of inflammasomes in a number of inflammatory, autoinflammatory and autoimmune disorders. Here we review: (a) the currently known inflammasome functions, (b) clinical/preclinical data supporting inflammasome involvement in the context of periodontal and comorbid diseases and (c) potential therapies targeting inflammasomes. To clarify further the inflammasome involvement in periodontitis, we present analyses of data from a large clinical study (n = 5809) that measured the gingival crevicular fluid‐interleukin‐1β and grouped the participants based on current periodontal disease classifications. We review data on 4910 European‐Americans that correlate 16 polymorphisms in the interleukin‐1B region with high gingival crevicular fluid‐interleukin‐1β levels. We show that inflammasome components are increased in diseased periodontal tissues and that the caspase‐1 inhibitor, VX‐765, inhibits ~50% of alveolar bone loss in experimental periodontitis. The literature review further supports that although patients clinically present with the same phenotype, the disease that develops probably has different underlying biological pathways. The current data indicate that inflammasomes have a role in periodontal disease pathogenesis. Understanding the contribution of different inflammasomes to disease development and distinct patient susceptibility will probably translate into improved, personalized therapies.

Herpes simplex virus type 1 inflammasome activation in proinflammatory human macrophages is dependent on NLRP3, ASC, and caspase-1

The proinflammatory cytokines interleukin (IL)-1β and IL-18 are products of activation of the inflammasome, an innate sensing system, and important in the pathogenesis of herpes simplex virus type 1 (HSV-1). The release of IL-18 and IL-1β from monocytes/macrophages is critical for protection from HSV-1 based on animal models of encephalitis and genital infection, yet if and how HSV-1 activates inflammasomes in human macrophages is unknown. To investigate this, we utilized both primary human monocyte derived macrophages and human monocytic cell lines (THP-1 cells) with various inflammasome components knocked-out. We found that HSV-1 activates inflammasome signaling in proinflammatory primary human macrophages, but not in resting macrophages. Additionally, HSV-1 inflammasome activation in THP-1 cells is dependent on nucleotide-binding domain and leucine-rich repeat-containing receptor 3 (NLRP3), apoptosis-associated speck-like molecule containing a caspase recruitment domain (ASC), and caspase-1, but not on absent in melanoma 2 (AIM2), or gamma interferon-inducible protein 16 (IFI16). In contrast, HSV-1 activates non-canonical inflammasome signaling in proinflammatory macrophages that results in IL-1β, but not IL-18, release that is independent of NLRP3, ASC, and caspase-1. Ultraviolet irradiation of HSV-1 enhanced inflammasome activation, demonstrating that viral replication suppresses inflammasome activation. These results confirm that HSV-1 is capable of activating the inflammasome in human macrophages through an NLRP3 dependent process and that the virus has evolved an NLRP3 specific mechanism to inhibit inflammasome activation in macrophages.

Nucleic Acid Sensors and Programmed Cell Death

Nucleic acids derived from microorganisms are powerful triggers for innate immune responses. Proteins called RNA and DNA sensors detect foreign nucleic acids and, in mammalian cells, include RIG-I, cGAS, and AIM2. On binding to nucleic acids, these proteins initiate signaling cascades that activate host defense responses. An important aspect of this defense program is the production of cytokines such as type I interferons and IL-1β. Studies conducted over recent years have revealed that nucleic acid sensors also activate programmed cell death pathways as an innate immune response to infection. Indeed, RNA and DNA sensors induce apoptosis, pyroptosis, and necroptosis. Cell death via these pathways prevents replication of pathogens by eliminating the infected cell and additionally contributes to the release of cytokines and inflammatory mediators. Interestingly, recent evidence suggests that programmed cell death triggered by nucleic acid sensors plays an important role in a number of noninfectious pathologies. In addition to nonself DNA and RNA from microorganisms, nucleic acid sensors also recognize endogenous nucleic acids, for example when cells are damaged by genotoxic agents and in certain autoinflammatory diseases. This review article summarizes current knowledge on the links between nucleic acid sensing and cell death and explores important open questions for future studies in this area.

Intracellular sensing of viral genomes and viral evasion

During viral infection, virus-derived cytosolic nucleic acids are recognized by host intracellular specific sensors. The efficacy of this recognition system is crucial for triggering innate host defenses, which then stimulate more specific adaptive immune responses against the virus. Recent studies show that signal transduction pathways activated by sensing proteins are positively or negatively regulated by many modulators to maintain host immune homeostasis. However, viruses have evolved several strategies to counteract/evade host immune reactions. These systems involve viral proteins that interact with host sensor proteins and prevent them from detecting the viral genome or from initiating immune signaling. In this review, we discuss key regulators of cytosolic sensor proteins and viral proteins based on experimental evidence.

HSV-2 Cellular Programming Enables Productive HIV Infection in Dendritic Cells

Genital herpes is a common sexually transmitted infection caused by herpes simplex virus type 2 (HSV-2). Genital herpes significantly enhances the acquisition and transmission of HIV-1 by creating a microenvironment that supports HIV infection in the host. Dendritic cells (DCs) represent one of the first innate cell types that encounter HIV-1 and HSV-2 in the genital mucosa. HSV-2 infection has been shown to modulate DCs, rendering them more receptive to HIV infection. Here, we investigated the potential mechanisms underlying HSV-2-mediated augmentation of HIV-1 infection. We demonstrated that the presence of HSV-2 enhanced productive HIV-1 infection of DCs and boosted inflammatory and antiviral responses. The HSV-2 augmented HIV-1 infection required intact HSV-2 DNA, but not active HSV-2 DNA replication. Furthermore, the augmented HIV infection of DCs involved the cGAS-STING pathway. Interestingly, we could not see any involvement of TLR2 or TLR3 nor suppression of infection by IFN-β production. The conditioning by HSV-2 in dual exposed DCs decreased protein expression of IFI16, cGAS, STING, and TBK1, which is associated with signaling through the STING pathway. Dual exposure to HSV-2 and HIV-1 gave decreased levels of several HIV-1 restriction factors, especially SAMHD1, TREX1, and APOBEC3G. Activation of the STING pathway in DCs by exposure to both HSV-2 and HIV-1 most likely led to the proteolytic degradation of the HIV-1 restriction factors SAMHD1, TREX1, and APOBEC3G, which should release their normal restriction of HIV infection in DCs. This released their normal restriction of HIV infection in DCs. We showed that HSV-2 reprogramming of cellular signaling pathways and protein expression levels in the DCs provided a setting where HIV-1 can establish a higher productive infection in the DCs. In conclusion, HSV-2 reprogramming opens up DCs for HIV-1 infection and creates a microenvironment favoring HIV-1 transmission.

Interferon-Responsive Genes Are Targeted during the Establishment of Human Cytomegalovirus Latency

Human cytomegalovirus (HCMV) latency is an active process which remodels the latently infected cell to optimize latent carriage and reactivation. This is achieved, in part, through the expression of viral genes, including the G-protein-coupled receptor US28. Here, we use an unbiased proteomic screen to assess changes in host proteins induced by US28, revealing that interferon-inducible genes are downregulated by US28. We validate that major histocompatibility complex (MHC) class II and two pyrin and HIN domain (PYHIN) proteins, myeloid cell nuclear differentiation antigen (MNDA) and IFI16, are downregulated during experimental latency in primary human CD14+ monocytes. We find that IFI16 is targeted rapidly during the establishment of latency in a US28-dependent manner but only in undifferentiated myeloid cells, a natural site of latent carriage. Finally, by overexpressing IFI16, we show that IFI16 can activate the viral major immediate early promoter and immediate early gene expression during latency via NF-κB, a function which explains why downregulation of IFI16 during latency is advantageous for the virus.IMPORTANCE Human cytomegalovirus (HCMV) is a ubiquitous herpesvirus which infects 50 to 100% of humans worldwide. HCMV causes a lifelong subclinical infection in immunocompetent individuals but is a serious cause of mortality and morbidity in the immunocompromised and neonates. In particular, reactivation of HCMV in the transplant setting is a major cause of transplant failure and related disease. Therefore, a molecular understanding of HCMV latency and reactivation could provide insights into potential ways to target the latent viral reservoir in at-risk patient populations.

Post-translational Control of Innate Immune Signaling Pathways by Herpesviruses

Herpesviruses constitute a large family of disease-causing DNA viruses. Each herpesvirus strain is capable of infecting particular organisms with a specific cell tropism. Upon infection, pattern recognition receptors (PRRs) recognize conserved viral features to trigger signaling cascades that culminate in the production of interferons and pro-inflammatory cytokines. To invoke a proper immune response while avoiding collateral tissue damage, signaling proteins involved in these cascades are tightly regulated by post-translational modifications (PTMs). Herpesviruses have developed strategies to subvert innate immune signaling pathways in order to ensure efficient viral replication and achieve persistent infection. The ability of these viruses to control the proteins involved in these signaling cascades post-translationally, either directly via virus-encoded enzymes or indirectly through the deregulation of cellular enzymes, has been widely reported. This ability provides herpesviruses with a powerful tool to shut off or restrict host antiviral and inflammatory responses. In this review, we highlight recent findings on the herpesvirus-mediated post-translational control along PRR-mediated signaling pathways.

IFI16, a nuclear innate immune DNA sensor, mediates epigenetic silencing of herpesvirus genomes by its association with H3K9 methyltransferases SUV39H1 and GLP

IFI16, an innate immune DNA sensor, recognizes the nuclear episomal herpes viral genomes and induces the inflammasome and interferon-β responses. IFI16 also regulates cellular transcription and act as a DNA virus restriction factor. IFI16 knockdown disrupted the latency of Kaposi’s sarcoma associated herpesvirus (KSHV) and induced lytic transcripts. However, the mechanism of IFI16’s transcription regulation is unknown. Here, we show that IFI16 is in complex with the H3K9 methyltransferase SUV39H1 and GLP and recruits them to the KSHV genome during de novo infection and latency. The resulting depositions of H3K9me2/me3 serve as a docking site for the heterochromatin-inducing HP1α protein leading into the IFI16-dependent epigenetic modifications and silencing of KSHV lytic genes. These studies suggest that IFI16’s interaction with H3K9MTases is one of the potential mechanisms by which IFI16 regulates transcription and establish an important paradigm of an innate immune sensor’s involvement in epigenetic silencing of foreign DNA.

From atoms to physiology: what it takes to really understand inflammasomes

Rapid inflammatory responses to cytosolic threats are mediated by inflammasomes - large macromolecular signalling complexes that control the activation of the pro-inflammatory cytokines interleukin (IL)-1β and IL-18, as well as cell death by pyroptosis. Different inflammasome sensors are activated by diverse direct and indirect signals, and subsequently nucleate the polymerization of the adaptor molecule ASC to form signalling platforms macroscopically observed as ASC specks. Caspase-1 is autocatalytically activated at these sites and subsequently matures pro-inflammatory cytokines and the pore-forming effector molecule gasdermin D. While most molecules and basic assembly principles have been deduced from reductionist experimental systems, we still lack fundamental information on the structure and regulation of these complexes in their physiological environment and in the interplay with other signalling pathways. In this review, novel experimental approaches are proposed, including some that rely on nanobodies and single domain antibodies, to understand inflammasome assembly and regulation in the context of the relevant tissues or cells.

Pharmacological modulation of nucleic acid sensors - therapeutic potential and persisting obstacles

Nucleic acid sensors, primarily TLR and RLR family members, as well as cGAS-STING signalling, play a critical role in the preservation of cellular and organismal homeostasis. Accordingly, deregulated nucleic acid sensing contributes to the origin of a diverse range of disorders, including infectious diseases, as well as cardiovascular, autoimmune and neoplastic conditions. Accumulating evidence indicates that normalizing aberrant nucleic acid sensing can mediate robust therapeutic effects. However, targeting nucleic acid sensors with pharmacological agents, such as STING agonists, presents multiple obstacles, including drug-, target-, disease- and host-related issues. Here, we discuss preclinical and clinical data supporting the potential of this therapeutic paradigm and highlight key limitations and possible strategies to overcome them.