Interferon-gamma induces retinoic acid-inducible gene-I in endothelial cells

The Killer's Web: Interconnection between Inflammation, Epigenetics and Nutrition in Cancer

Inflammation is a key contributor to both the initiation and progression of tumors, and it can be triggered by genetic instability within tumors, as well as by lifestyle and dietary factors. The inflammatory response plays a critical role in the genetic and epigenetic reprogramming of tumor cells, as well as in the cells that comprise the tumor microenvironment. Cells in the microenvironment acquire a phenotype that promotes immune evasion, progression, and metastasis. We will review the mechanisms and pathways involved in the interaction between tumors, inflammation, and nutrition, the limitations of current therapies, and discuss potential future therapeutic approaches.

Endothelial Cells as a Key Cell Type for Innate Immunity: A Focused Review on RIG-I Signaling Pathway

The vascular endothelium consists of a highly heterogeneous monolayer of endothelial cells (ECs) which are the primary target for bacterial and viral infections due to EC’s constant and close contact with the bloodstream. Emerging evidence has shown that ECs are a key cell type for innate immunity. Like macrophages, ECs serve as sentinels when sensing invading pathogens or microbial infection caused by viruses and bacteria. It remains elusive how ECs senses danger signals, transduce the signal and fulfil immune functions. Retinoic acid-inducible gene-I (RIG-I, gene name also known as DDX58) is an important member of RIG-I-like receptor (RLR) family that functions as an important pathogen recognition receptor (PRR) to execute immune surveillance and confer host antiviral response. Recent studies have demonstrated that virus infection, dsRNA, dsDNA, interferons, LPS, and 25-hydroxycholesterol (25-HC) can increase RIG-1 expression in ECs and propagate anti-viral response. Of translational significance, RIG-I activation can be inhibited by Panax notoginseng saponins, endogenous PPARγ ligand 15-PGJ2, tryptanthrin and 2-animopurine. Considering the pivotal role of inflammation and innate immunity in regulating endothelial dysfunction and atherosclerosis, here we provided a concise review of the role of RIG-I in endothelial cell function and highlight future direction to elucidate the potential role of RIG-I in regulating cardiovascular diseases as well as virus infectious disease, including COVID-19. Furthered understanding of RIG-I-mediated signaling pathways is important to control disorders associated with altered immunity and inflammation in ECs.

Duck LGP2 Downregulates RIG-I Signaling Pathway-Mediated Innate Immunity Against Tembusu Virus

In mammals, the retinoic acid-inducible gene I (RIG-I)-like receptors (RLR) has been demonstrated to play a critical role in activating downstream signaling in response to viral RNA. However, its role in ducks' antiviral innate immunity is less well understood, and how gene-mediated signaling is regulated is unknown. The regulatory role of the duck laboratory of genetics and physiology 2 (duLGP2) in the duck RIG-I (duRIG-I)-mediated antiviral innate immune signaling system was investigated in this study. In duck embryo fibroblast (DEF) cells, overexpression of duLGP2 dramatically reduced duRIG-I-mediated IFN-promotor activity and cytokine expression. In contrast, the knockdown of duLGP2 led to an opposite effect on the duRIG-I-mediated signaling pathway. We demonstrated that duLGP2 suppressed the duRIG-I activation induced by duck Tembusu virus (DTMUV) infection. Intriguingly, when duRIG-I signaling was triggered, duLGP2 enhanced the production of inflammatory cytokines. We further showed that duLGP2 interacts with duRIG-I, and this interaction was intensified during DTMUV infection. In summary, our data suggest that duLGP2 downregulated duRIG-I mediated innate immunity against the Tembusu virus. The findings of this study will help researchers better understand the antiviral innate immune system's regulatory networks in ducks.

Immune Regulator Retinoic Acid-Inducible Gene I (RIG-I) in the Pathogenesis of Cardiovascular Disease

Retinoic acid-inducible gene I (RIG-I) is a cytosolic pattern recognition receptor that contains two CARD domains, an RNA helicase domain, and a C-terminal domain. RIG-I initiates antiviral innate immunity by recognizing exogenous viral RNAs/DNAs. However, some studies have reported that RIG-I activation leads to damage in various organs and tissues in diverse circumstances. Recent studies have shown that RIG-I is involved in cancer, lupus nephritis, immunoglobulin A nephropathy, Crohn's disease, and atherosclerosis. These reports indicate that RIG-I not only participates in antiviral signaling pathways but also exerts an influence on non-viral infectious diseases. RIG-I is widely expressed in immune and non-immune cells including smooth muscle cells, endothelial cells, and cardiomyocytes. A succinct overview of RIG-I and its signaling pathways, with respect to the cardiovascular system, will aid in the development of novel therapeutics for cardiovascular diseases. In this review, we summarize the structure, activation, signaling pathways, and role of RIG-I in cardiovascular diseases.

Free ISG15 and protein ISGylation emerging in SARS-CoV-2 infection

Interferon-simulated gene 15 (ISG15) belongs to the family of ubiquitin-like proteins. ISG15 acts as a cytokine and modifies proteins through ISGylation. This posttranslational modification has been associated with antiviral and immune response pathways. In addition, it is known that the genome of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) encodes proteases critical for viral replication. Consequently, these proteases are also central in the progression of coronavirus disease 2019 (COVID-19). Interestingly, the protease SARS-CoV-2-PLpro removes ISG15 from ISGylated proteins such as IRF3 and MDA5, affecting immune and antiviral defense from the host. Here, the implications of ISG15, ISGylation, and generation of SARS-CoV-2-PLpro inhibitors in SARS-CoV-2 infection are discussed.

Innate Immune Responses to Avian Influenza Viruses in Ducks and Chickens

Mallard ducks are important natural hosts of low pathogenic avian influenza (LPAI) viruses and many strains circulate in this reservoir and cause little harm. Some strains can be transmitted to other hosts, including chickens, and cause respiratory and systemic disease. Rarely, these highly pathogenic avian influenza (HPAI) viruses cause disease in mallards, while chickens are highly susceptible. The long co-evolution of mallard ducks with influenza viruses has undoubtedly fine-tuned many immunological host–pathogen interactions to confer resistance to disease, which are poorly understood. Here, we compare innate responses to different avian influenza viruses in ducks and chickens to reveal differences that point to potential mechanisms of disease resistance. Mallard ducks are permissive to LPAI replication in their intestinal tissues without overtly compromising their fitness. In contrast, the mallard response to HPAI infection reflects an immediate and robust induction of type I interferon and antiviral interferon stimulated genes, highlighting the importance of the RIG-I pathway. Ducks also appear to limit the duration of the response, particularly of pro-inflammatory cytokine expression. Chickens lack RIG-I, and some modulators of the signaling pathway and may be compromised in initiating an early interferon response, allowing more viral replication and consequent damage. We review current knowledge about innate response mediators to influenza infection in mallard ducks compared to chickens to gain insight into protective immune responses, and open questions for future research.

Interplay between interferon-stimulated gene 15/ISGylation and interferon gamma signaling in breast cancer cells

Interferon-stimulated gene 15 (ISG15) is a ubiquitin-like protein that conjugates to its target proteins to modify them through ISGylation, but the relevance of ISG15 expression and its effects have been not completely defined. Herein, we examined the interplay between ISG15/ISGylation and the interferon-gamma (IFN-γ) signaling pathway in mammary tumors and compared it with that in normal mammary tissues. Our results indicated that mammary tumors had higher levels of ISG15 mRNA and ISG15 protein than the adjacent normal mammary tissue. Furthermore, the expression of IFN-γ signaling components was altered in breast cancer. Interestingly, IFN-γ treatment induced morphological changes in MCF-7 and MDA-MB-231 breast cancer cell lines due to cytoskeletal reorganization. This cellular process seems to be related to the increase in ISGylation of cytoplasmic IQ Motif Containing GTPase Activating Protein 1 (IQGAP1). Interactome analysis also indicated that IFN-γ signaling and the ISGylation system are associated with several proteins implicated in cytoskeletal remodeling, including IQGAP1. Thus, ISG15 may present a potential biomarker for breast cancer, and IFN-γ signaling and protein ISGylation may participate in the regulation of the cytoskeleton in breast cancer cells.

Oncostatin M induces RIG-I and MDA5 expression and enhances the double-stranded RNA response in fibroblasts

Interleukin (IL)-6-type cytokines have no direct antiviral activity; nevertheless, they display immune-modulatory functions. Oncostatin M (OSM), a member of the IL-6 family, has recently been shown to induce a distinct number of classical interferon stimulated genes (ISG). Most of them are involved in antigen processing and presentation. However, induction of retinoic acid-inducible gene (RIG)-I-like receptors (RLR) has not been investigated. Here we report that OSM has the capability to induce the expression of the DExD/H-Box RNA helicases RIG-I and melanoma differentiation antigen 5 (MDA5) as well as of the transcription factors interferon regulatory factor (IRF)1, IRF7 and IRF9 in primary fibroblasts. Induction of the helicases depends on tyrosine as well as serine phosphorylation of STAT1. Moreover, we could show that the OSM-induced STAT1 phosphorylation is predominantly counter-regulated by a strong STAT3-dependent SOCS3 induction, as Stat3 as well as Socs3 knock-down results in an enhanced and prolonged helicase and IRF expression. Other factors involved in regulation of STAT1 or IRF1 activity, like protein tyrosine phosphatase, non-receptor type 2 (PTPN2), promyelocytic leukaemia protein (PML) or small ubiquitin-related modifier 1 (SUMO1), play a minor role in OSM-mediated induction of RLR. Remarkably, OSM and interferon-γ (IFN-γ) synergize to mediate transcription of RLR and pre-treatment of fibroblasts with OSM fosters the type I interferon production in response to a subsequent encounter with double-stranded RNA. Together, these findings suggest that the OSM-induced JAK/STAT1 signalling is implicated in virus protection of non-professional immune cells and may cooperate with interferons to enhance RLR expression in these cells.

Implications of Toll-like receptors in Ebola infection

INTRODUCTION

The potential roles of toll-like receptors (TLRs) in immunopathogenesis of Ebola virus disease should be unraveled to provoke possible prophylactic or therapeutic implications of TLRs for EVD. Areas covered: The Ebola virus (EBOV) infection virtually paralyses all the main mechanisms responsible for induction of type I interferon (IFN-I) response. To summarize, EBOV infection interferes with: a) the TIR-domain-containing adapter-inducing interferon-β (TRIF) pathway that is mediated by TLR3 and TLR4 signaling; b) the interferon regulatory factor 7 (IRF7) pathway that is stimulated by TLR7 and TLR9; c) the intracellular signaling that is induced by retinoic acid-inducible gene 1 (RIG-I)-like receptors (RLRs); and d) the autocrine/paracrine feedback loop that is mediated by the IFN-stimulated gene factor 3 (ISGF3) complex. Upon infection with EBOV infection, TLR4 plays a key role in production of proinflammatory mediators. Expert opinion: It is theoretically possible that use of TLRs 3, 4, 7, and 9 agonists would be beneficial to improve the IFN-I response, despite their systemic side effects. Also, antagonist of TLR4 can be utilized to prevent production of proinflammatory cytokines. Additionally, it is highly recommended to design future investigations aimed at determining if the utilization of IFN-I would be beneficial for prophylactic/therapeutic programs of Ebola.

Role of Innate Genes in HIV Replication

Cells use an elaborate innate immune surveillance and defense system against virus infections. Here, we discuss recent studies that reveal how HIV-1 is sensed by the innate immune system. Furthermore, we present mechanisms on the counteraction of HIV-1. We will provide an overview how HIV-1 actively utilizes host cellular factors to avoid sensing. Additionally, we will summarize effectors of the innate response that provide an antiviral cellular state. HIV-1 has evolved passive mechanism to avoid restriction and to regulate the innate response. We review in detail two prominent examples of these cellular factors: (i) NLRX1, a negative regulator of the innate response that HIV-1 actively usurps to block cytosolic innate sensing; (ii) SAMHD1, a restriction factor blocking the virus at the reverse transcription step that HIV-1 passively avoids to escape sensing.

Listeria monocytogenesInduces the Expression of Retinoic Acid-Inducible Gene-I

Retinoic acid-inducible gene-I (RIG-I) is considered to play a role in innate immunity against virus infections. We showed by immunohistochemical study that RIG-I expression is upregulated in vivo in hepatic Kupffer cells and in splenic reticular cells of mice infected with Listeria monocytogenes. Both heat-killed L. monocytogenes and live L. monocytogenes induced the expression of RIG-I in cultured RAW264.7 murine macrophage-like cells in vitro. RIG-I may also be involved in innate immunity against Listeria infection.

Passive carriage of rabies virus by dendritic cells

The rabies virus (RABV) is highly neurotropic and it uses evasive strategies to successfully evade the host immune system. Because rabies is often fatal, understanding the basic processes of the virus-host interactions, particularly in the initial events of infection, is critical for the design of new therapeutic approaches to target RABV. Here, we examined the possible role of dendritic cells (DCs) in the transmission of RABV to neural cells at peripheral site of exposure. Viral replication only occurred at a low level in the DC cell line, JAWS II, after its infection with either pathogenic RABV (CVS strain) or low-pathogenic RABV (ERA strain), and no progeny viruses were produced in the culture supernatants. However, both viral genomic RNAs were retained in the long term after infection and maintained their infectivity. The biggest difference between CVS and ERA was in their ability to induce type I interferons. Although the ERA-infected JAWS II cells exhibited cytopathic effect and were apparently killed by normal spleen cells in vitro, the CVS-infected JAWS II cells showed milder cytopathic effect and less lysis when cocultured with spleen cells. Strongly increased expression of major histocompatibility complex classes I, costimulatory molecules (CD80 and CD86), type I interferons and Toll- like receptor 3, and was observed only in the ERA-inoculated JAWS II cells and not in those inoculated with CVS. During the silencing of the cellular immune response in the DCs, the pathogenic CVS strain cryptically maintained an infectious viral genome and was capable of transmitting infectious RABV to permissive neural cells. These findings demonstrate that DCs may play a role in the passive carriage of RABV during natural rabies infections.

Viral double-stranded RNA sensors induce antiviral, pro-inflammatory, and pro-apoptotic responses in human renal tubular epithelial cells

Viral infection in the kidney is characterized by tubular injury induced directly by the virus and/or by cytotoxic lymphocytes. Previously, we found that human tubular epithelial cells express Toll-like receptor 3 (TLR3), melanoma differentiation-associated gene 5 (MDA5), and retinoic acid-inducible gene-I (RIG-I), all sensors of double-stranded RNA (dsRNA) and potent inducers of antiviral activity. Here, we demonstrate increased expression of these three dsRNA sensors in kidney transplant biopsies during cytomegalovirus or BK virus infection. In primary tubular epithelial cells, dsRNA sensor activation induced the production of pro-inflammatory TNF-α and antiviral IFN-β. Notably, dsRNA also enhanced the expression of pro-apoptotic proteins; however, dsRNA alone did not cause cell death due to the expression of anti-apoptotic proteins. The dsRNA sensitized tubular epithelial cells to apoptosis induced by an agonistic antibody against the Fas receptor (CD95), an apoptotic pathway that eliminates infected cells. These findings indicate that tubular epithelial cells require at least two signals to undergo apoptosis, which can help preserve tubular integrity even under inflammatory conditions. Thus, sensors of viral dsRNA promote antiviral, pro-inflammatory, and pro-apoptotic responses in tubular epithelial cells, which may orchestrate the control of viral infection in the kidney.

IFNγ inhibits the cytosolic replication of Shigella flexneri via the cytoplasmic RNA sensor RIG-I

The activation of host cells by interferon gamma (IFNγ) is essential for inhibiting the intracellular replication of most microbial pathogens. Although significant advances have been made in identifying IFNγ-dependent host factors that suppress intracellular bacteria, little is known about how IFNγ enables cells to recognize, or restrict, the growth of pathogens that replicate in the host cytoplasm. The replication of the cytosolic bacterial pathogen Shigella flexneri is significantly inhibited in IFNγ-stimulated cells, however the specific mechanisms that mediate this inhibition have remained elusive. We found that S. flexneri efficiently invades IFNγ-activated mouse embryonic fibroblasts (MEFs) and escapes from the vacuole, suggesting that IFNγ acts by blocking S. flexneri replication in the cytosol. This restriction on cytosolic growth was dependent on interferon regulatory factor 1 (IRF1), an IFNγ-inducible transcription factor capable of inducing IFNγ-mediated cell-autonomous immunity. To identify host factors that restrict S. flexneri growth, we used whole genome microarrays to identify mammalian genes whose expression in S. flexneri-infected cells is controlled by IFNγ and IRF1. Among the genes we identified was the pattern recognition receptor (PRR) retanoic acid-inducible gene I (RIG-I), a cytoplasmic sensor of foreign RNA that had not been previously known to play a role in S. flexneri infection. We found that RIG-I and its downstream signaling adaptor mitochondrial antiviral signaling protein (MAVS)—but not cytosolic Nod-like receptors (NLRs)—are critically important for IFNγ-mediated S. flexneri growth restriction. The recently described RNA polymerase III pathway, which transcribes foreign cytosolic DNA into the RIG-I ligand 5′-triphosphate RNA, appeared to be involved in this restriction. The finding that RIG-I responds to S. flexneri infection during the IFNγ response extends the range of PRRs that are capable of recognizing this bacterium. Additionally, these findings expand our understanding of how IFNγ recognizes, and ultimately restricts, bacterial pathogens within host cells.

Shigella flexneri, the major cause of bacillary dysentery worldwide, invades and replicates within the cytoplasm of intestinal epithelial cells, where it disseminates to neighboring cells and ultimately increases the likelihood of transmission to uninfected hosts. A hallmark of the mammalian immune system is its ability to inhibit the growth of such intracellular pathogens by upregulating intracellular resistance mechanisms in response to the cytokine IFNγ. We found that in non-myeloid host cells stimulated with IFNγ S. flexneri remains able to invade the cells efficiently and gain access to the host cytoplasm. Once in the cytoplasm of IFγ-activated cells, the RIG-I/ MAVS immunosurveillance pathway is activated, enabling the stimulated host cells to inhibit S. flexneri replication. Interestingly, RIG-I only played a minor role in the cellular response to this pathogen in the absence of IFNγ, suggesting that the IFNγ response ensures the recognition of the infection through an immunosurveillance pathway that is otherwise dispensable for controlling S. flexneri growth. Together, these findings implicate the RIG-I pathway as a crucial component in the cellular response to this devastating bacterial pathogen.

Endothelial RIG-I activation impairs endothelial function

BACKGROUND

Endothelial dysfunction is a crucial part of the chronic inflammatory atherosclerotic process and is mediated by innate and acquired immune mechanisms. Recent studies suggest that pattern recognition receptors (PRR) specialized in immunorecognition of nucleic acids may play an important role in endothelial biology in a proatherogenic manner. Here, we analyzed the impact of endothelial retinoic acid inducible gene I (RIG-I) activation upon vascular endothelial biology.

METHODS AND RESULTS

Wild type mice were injected intravenously with 32.5 μg of the RIG-ligand 3pRNA (RNA with triphosphate at the 5'end) or polyA control every other day for 7 days. In 3pRNA-treated mice, endothelium-depended vasodilation was significantly impaired, vascular oxidative stress significantly increased and circulating endothelial microparticle (EMP) numbers significantly elevated compared to controls. To gain further insight in RIG-I dependent endothelial biology, cultured human coronary endothelial cells (HCAEC) and endothelial progenitor cells (EPC) were stimulated in vitro with 3pRNA. Both cells types express RIG-I and react with receptor upregulation upon stimulation. Reactive oxygen species (ROS) formation is enhanced in both cell types, whereas apoptosis and proliferation is not significantly affected in HCAEC. Importantly, HCAEC release significant amounts of proinflammatory cytokines in response to RIG-I stimulation.

CONCLUSION

This study shows that activation of the cytoplasmatic nucleic acid receptor RIG-I leads to endothelial dysfunction. RIG-I induced endothelial damage could therefore be an important pathway in atherogenesis.

Interferon regulator factor 1/retinoic inducible gene I (IRF1/RIG-I) axis mediates 25-hydroxycholesterol-induced interleukin-8 production in atherosclerosis

AIMS

In this study, the role of retinoic inducible gene I (RIG-I)-mediated signalling in the inflammation of atherosclerosis was investigated to explain the pathology of atherosclerosis.

METHODS AND RESULTS

Human and mouse primary cells were exposed to 25-hydroxycholesterol followed by examination of gene expression and activation of the signal pathway with biochemical and molecular biological techniques. A mouse atherosclerotic model was also used. We found that RIG-I was induced in macrophages and endothelium by 25-hydroxycholesterol. Interferon regulatory factor 1 is a key transcription factor for the induction of RIG-I by 25-hydroxycholesterol. The induction of interleukin-8 and growth-regulated protein α, the mouse interleukin-8 homologue, by 25-hydroxycholesterol is mediated by RIG-I signalling. RIG-I transduces the signal to downstream molecules, mitochondrial antiviral-signalling protein, transforming growth factor-β-activated kinase 1, and mitogen-activated protein kinase, leading to the activation of nuclear factor κB, activator protein-1, and nuclear factor interleukin-6, all of which are required for the expression of interleukin-8. Finally, we observed that RIG-I is highly expressed in atherosclerotic lesions.

CONCLUSION

Our data demonstrate that RIG-I signalling mediates atherosclerotic inflammation. Targeting RIG-I signalling should provide a way to inhibit atherosclerotic inflammation, which holds potential for the therapy of atherosclerosis.

Retinoic acid inducible gene-I, more than a virus sensor

Retinoic acid inducible gene-I (RIG-I) is a caspase recruitment domain (CARD) containing protein that acts as an intracellular RNA receptor and senses virus infection. After binding to double stranded RNA (dsRNA) or 5'-triphosphate single stranded RNA (ssRNA), RIG-I transforms into an open conformation, translocates onto mitochondria, and interacts with the downstream adaptor mitochondrial antiviral signaling (MAVS) to induce the production of type I interferon and inflammatory factors via IRF3/7 and NF-κB pathways, respectively. Recently, accumulating evidence suggests that RIG-I could function in non-viral systems and participate in a series of biological events, such as inflammation and inflammation related diseases, cell proliferation, apoptosis and even senescence. Here we review recent advances in antiviral study of RIG-I as well as the functions of RIG-I in other fields.

Characterization of chicken Mda5 activity: regulation of IFN-β in the absence of RIG-I functionality

In mammals, Mda5 and RIG-I are members of the evolutionary conserved RIG-like helicase family that play critical roles in the outcome of RNA virus infections. Resolving influenza infection in mammals has been shown to require RIG-I; however, the apparent absence of a RIG-I homolog in chickens raises intriguing questions regarding how this species deals with influenza virus infection. Although chickens are able to resolve certain strains of influenza, they are highly susceptible to others, such as highly pathogenic avian influenza H5N1. Understanding RIG-like helicases in the chicken is of critical importance, especially for developing new therapeutics that may use these systems. With this in mind, we investigated the RIG-like helicase Mda5 in the chicken. We have identified a chicken Mda5 homolog (ChMda5) and assessed its functional activities that relate to antiviral responses. Like mammalian Mda5, ChMda5 expression is upregulated in response to dsRNA stimulation and following IFN activation of cells. Furthermore, RNA interference-mediated knockdown of ChMda5 showed that ChMda5 plays an important role in the IFN response of chicken cells to dsRNA. Intriguingly, although ChMda5 levels are highly upregulated during influenza infection, knockdown of ChMda5 expression does not appear to impact influenza proliferation. Collectively, although Mda5 is functionally active in the chicken, the absence of an apparent RIG-I-like function may contribute to the chicken's susceptibility to highly pathogenic influenza.

RA-inducible gene-I induction augments STAT1 activation to inhibit leukemia cell proliferation

RA-inducible gene I (RIG-I/DDX58) has been shown to activate IFN-β promoter stimulator 1 (IPS-1) on recognizing cytoplasmic viral RNAs. It is unclear how RIG-I functions within the IFN and/or RA signaling process in acute myeloid leukemia (AML) cells, however, where obvious RIG-I induction is observed. Here, we show that the RIG-I induction functionally contributes to IFN-α plus RA-triggered growth inhibition of AML cells. Interestingly, although RIG-I induction itself is under the regulation of STAT1, a major IFN intracellular signal mediator, under circumstances in which it does not stimulate IPS-1, it conversely augments STAT1 activation to induce IFN-stimulatory gene expression and inhibit leukemia cell proliferation. Thus, our results unveil a previously undescribed RIG-I activity in regulating the cellular proliferation of leukemia cells via STAT1, which is independent of its classic role of sensing viral invasion to trigger type I IFN transcription.

Function and regulation of retinoic acid-inducible gene-I

Antiviral innate immunity is triggered by sensing viral nucleic acids. RIG-I (retinoic acid-inducible gene-I) is an intracellular molecule that responds to viral nucleic acids and activates downstream signaling, resulting in the induction of members of the type I interferon (IFN) family, which are regarded among the most important effectors of the innate immune system. Although RIG-I is expressed ubiquitously in the cytoplasm, its levels are subject to transcriptional and post-transcriptional regulation. RIG-I belongs to the IFN-stimulated gene (ISG) family, but certain cells regulate its expression through IFN-independent mechanisms. Several lines of evidence indicate that deregulated RIG-I signaling is associated with autoimmune disorders. Further studies suggest that RIG-I has functions in addition to those directly related to its role in RNA sensing and host defense. We have much to learn and discover regarding this interesting cytoplasmic sensor so that we can capitalize on its properties for the treatment of viral infections, immune disorders, cancer, and perhaps other conditions.

Pregnancy and interferon tau regulate DDX58 and PLSCR1 in the ovine uterus during the peri-implantation period

Interferon τ (IFNT), the pregnancy recognition signal in ruminants, abrogates the luteolytic mechanism for maintenance of the corpus luteum for production of progesterone (P(4)). This study examined the expression of DEAD (Asp-Glu-Ala-Asp) box polypeptide 58 (DDX58) and phospholipid scramblase 1 (PLSCR1) mRNAs in the ovine uterus as these genes were increased most in 2fTGH (STAT1 positive) cells by IFNT. The results of this study indicated that IFNT regulates expression of DDX58 and PLSCR1 mRNAs in the ovine uterus, which confirmed the results of the in vitro transcriptional profiling experiment with the 2fTGH (parental STAT1 positive) and U3A (STAT1 null) cell lines. Steady-state levels of DDX58 and PLSCR1 mRNAs increased in cells of the ovine uterus between days 12 and 20 of pregnancy, but not between days 10 and 16 of the estrous cycle. The expression of DDX58 and PLSCR1 mRNAs was greatest in endometrial stromal cells, but there was transient expression in uterine luminal and superficial glandular epithelial cells. P(4) alone did not induce expression of DDX58 and PLSCR1 mRNAs; however, intrauterine injections of IFNT did induce expression of DDX58 and PLSCR1 mRNAs in the endometria of nonpregnant ewes independent of effects of P(4). These results indicate that IFNT induces expression of DDX58 and PLSCR1 in ovine endometrial cells via the classical STAT1-mediated cell signaling pathway. Based on their known biological effects, DDX58 and PLSCR1 are IFN-stimulated genes, which may increase the antiviral status of cells of the pregnant uterus to protect against viral infection and/or enhance secretion of type I IFNs that inhibit viral replication.

Retinoic acid-inducible gene-I is induced by double-stranded RNA and regulates the expression of CC chemokine ligand (CCL) 5 in human mesangial cells

BACKGROUND

Retinoic acid-inducible gene-I (RIG-I) is a putative RNA helicase involved in immune reactions against RNA viruses and various inflammatory and autoimmune diseases. The purpose of the present study was to investigate the role of RIG-I in glomerular diseases.

METHODS

We treated human mesangial cells in culture with polyinosinic-polycytidylic acid (poly IC), which is an authentic double-stranded RNA, and analysed the expression of RIG-I, CC chemokine ligand 5 (CCL5) and interferon (IFN)-β by western blotting, reverse transcriptase-polymerase chain reaction (RT-PCR) or enzyme-linked immunosorbent assay (ELISA). To elucidate the poly IC-signalling pathway, we subjected the cells to RNA interference (RNAi) against RIG-I, IFN-β or Toll-like receptor (TLR) 3. Furthermore, we studied the effects of IFN-β receptor blocking and IFN-β overexpression.

RESULTS

Poly IC induced the expression of RIG-I and CCL5 in human mesangial cells, and RNAi against RIG-I inhibited this poly IC-induced CCL5 expression. Poly IC-induced RIG-I expression was also inhibited by RNAi against IFN-β and by an antibody against the IFN-β receptor. IFN-β overexpression induced the expression of both RIG-I and CCL5. The knockdown of TLR3 abolished poly IC-induced RIG-I expression.

CONCLUSIONS

The TLR3/IFN-β/RIG-I/CCL5 signalling pathway may mediate immune and inflammatory responses against viral infection in mesangial cells, suggesting the role of this pathway in the aggravation of glomerulonephritis due to viral infection.

SUMOylation of RIG-I positively regulates the type I interferon signaling

Retinoic acid-inducible gene-I (RIG-I) functions as an intracellular pattern recognition receptor (PRR) that recognizes the 5'-triphosphate moiety of single-stranded RNA viruses to initiate the innate immune response. Previous studies have shown that Lys63-linked ubiquitylation is required for RIG-I activation and the downstream anti-viral type I interferon (IFN-I) induction. Herein we reported that, RIG-I was also modified by small ubiquitin-like modifier-1 (SUMO-1). Functional analysis showed that RIG-I SUMOylation enhanced IFN-I production through increased ubiquitylation and the interaction with its downstream adaptor molecule Cardif. Our results therefore suggested that SUMOylation might serve as an additional regulatory tier for RIG-I activation and IFN-I signaling.

TLR3-dependent upregulation of RIG-I leads to enhanced cytokine production from cells infected with the parainfluenza virus SV5

Here we address the role of RIG-I and TLR3 in differential cytokine responses against Simian Virus 5 (SV5) and two distinct cytokine inducing SV5 mutants. IFN-beta and IL-6 secretion was induced by infection with P/V-CPI-, an SV5 mutant with P/V substitutions, and were reduced by either siRNA-mediated knockdown of RIG-I expression or by expression of a dsRNA-binding protein. TLR3 overexpression did not alter cytokine secretion induced by P/V-CPI- or by Le-(U5C, A14G), an SV5 promoter mutant. TLR3 signaling by addition of exogenously added dsRNA was not blocked by WT SV5 or either SV5 mutant. Unexpectedly, TLR3 activation in infected cells led to enhanced IL-8 secretion, which correlated with increased RIG-I expression. Dominant negative RIG-I and TRIF supported a model whereby TLR3 activation upregulates RIG-I expression and in turn hypersensitizes cells to RIG-I-mediated cytokine secretion. Implications for crosstalk between different innate immunity pathways in mounting antiviral responses to paramyxoviruses are discussed.

Activation and evasion of innate antiviral immunity by herpes simplex virus

Herpes simplex virus (HSV), a human pathogenic virus, has evolved several strategies to evade the production and function of interferons (IFNs) and cytokines generated by the innate immune system to restrict the virus. Equilibrium exists between the virus and the immune response, and a shift in this delicate balance either restricts the virus or enhances virus spread and tissue damage. Therefore, understanding of the cytokine response generated after HSV infection and the underlying virus-cell interactions is essential to improve our understanding of viral pathogenesis. This review summarizes the current knowledge on induction and evasion of the innate immune response by HSV.

The viral RNA recognition sensor RIG-I is degraded during encephalomyocarditis virus (EMCV) infection

RNA helicase-like receptors MDA-5 but not RIG-I has been shown to be essential for triggering innate immune responses against picornaviruses. However, virus-host co-evolution has selected for viruses capable of replicating despite host cells antiviral defences. In this report, we demonstrate that RIG-I is degraded during encephalomyocarditis virus (EMCV) infection. This effect is mediated by both the viral-encoded 3C protease and caspase proteinase. In addition, we show that RIG-I overexpression confers IFN-beta promoter activation during EMCV infection, in MDA-5 knockout (MDA-5(-/-)) mouse embryo fibroblasts. This induction is followed by a strong inhibition reflecting the ability of EMCV to disrupt RIG-I signalling. Taken together, our data strongly suggest that during evolution RIG-I has been involved for triggering innate immune response to picornavirus infections.

Tumor-necrosis factor-alpha induces retinoic acid-inducible gene-I in rheumatoid fibroblast-like synoviocytes

Tumor-necrosis factor-alpha (TNF-alpha) is a potent proinflammtory cytokine and a key molecule in the pathogenesis of rheumatoid arthritis (RA). Retinoic acid-inducible gene-I (RIG-I) is a DExH box protein, which is known to play a role in the inflammatory and immune reactions. We previously reported about potential involvement of RIG-I in synovial inflammation in RA. In the present study, we demonstrated the expression of RIG-I in fibroblast-like synoviocytes stimulated with TNF-alpha. RNA interference against interferon (IFN)-beta abolished the TNF-alpha-induced RIG-I expression. In addition, knockdown of RIG-I partially inhibited the TNF-alpha-induced expression of CC chemokine ligand (CCL) 5, a chemokine with chemotactic activity toward lymphocytes and monocytes. These findings suggest that the TNF-alpha/IFN-beta/RIG-I/CCL5 pathway may be involved in the pathogenesis of synovial inflammation in RA.

Expression of interferon-stimulated gene 20 in vascular endothelial cells

ISG20 is an ribonuclease specific for single-stranded RNA and considered to play a role in innate immunity against virus infections. We herein show that both poly IC, an authentic double-stranded RNA, and IFN-gamma induced ISG20 expression in cultured HUVEC. Poly IC-induced ISG20 expression was inhibited by LY294002, an inhibitor of PI3K, or by RNA interference against IFN regulatory factor three. ISG20 expression was not induced by IFN-beta, loxoribine or CpG oligonucleotide. These results suggest that ISG20 induction by poly IC may not be dependent on the IRF-3-mediated type I IFN induction pathway in HUVEC. ISG20 may be involved in innate immunity against viral infection in vascular endothelial cells.

Role of retinoic acid inducible gene-I in human metapneumovirus-induced cellular signalling

Human metapneumovirus (HMPV) is a recently discovered pathogen that causes a significant proportion of respiratory infections in young infants, the elderly and immunocompromised patients. Very little is known regarding the cellular signalling elicited by this virus in airway epithelial cells, the target of HMPV infection. In this study, we investigated the role of the RNA helicases retinoic acid inducible gene-I (RIG-I) and melanoma differentiation-associated gene-5 (MDA-5) as the main pattern recognition receptors (PRRs) involved in viral detection and subsequent expression of proinflammatory and antiviral genes. HMPV infection readily induced RIG-I and MDA-5 gene and protein expression in A549 cells, a type II-like alveolar epithelial cell line. Expression of dominant-negative (DN) RIG-I or downregulation of RIG-I gene expression using small interfering RNA (siRNA) significantly decreased HMPV-induced beta interferon (IFN-beta), interleukin (IL)-8 and RANTES gene transcription, by inhibiting viral-induced activation of nuclear factor (NF)-kappaB and interferon regulatory factor (IRF), leading to enhanced viral replication. On the other hand, MDA-5 did not seem to play a significant role in HMPV-induced cellular responses. Mitochondrial antiviral signalling protein (MAVS), an adaptor protein linking both RIG-I and MDA-5 to downstream activation of IRF-3 and NF-kappaB, was also necessary for HMPV-induced cellular signalling. Expression of a DN MAVS significantly reduced IFN-beta and chemokine gene transcription, by inhibiting NF-kappaB- and IRF-dependent gene transcription, in response to HMPV infection. Our results show that HMPV activates the RIG-I-MAVS signalling pathway in airway epithelial cells, leading to the expression of important proinflammatory and antiviral molecules involved in the innate immune response to viruses.

Involvement of retinoic acid-inducible gene-I in inflammation of rheumatoid fibroblast-like synoviocytes

Interferon (IFN)-gamma is a major cytokine that regulates T helper 1-type immune reactions and serves as an important mediator in the pathogenesis of autoimmune diseases. Retinoic acid-inducible gene-I (RIG-I) is an IFN-gamma-inducible gene and known to be involved in the inflammatory and immune reactions. In the present study, we found high levels of RIG-I expression in synovial tissues of rheumatoid arthritis (RA), while the expression in osteoarthritis tissues was low. Treatment of cultured fibroblast-like synoviocytes with IFN-gamma markedly induced the expression of RIG-I. Knockdown of RIG-I in fibroblast-like synoviocytes, with specific siRNA, resulted in the inhibition of the IFN-gamma-induced expression of chemokine (C-X-C motif) ligand 10 (CXCL10)/IFN-gamma-inducible protein-10 (IP-10), a chemokine with chemotactic activity towards T cells. These findings suggest that RIG-I may play an important role in the pathogenesis of synovial inflammation in RA, at least in part, by regulating the IFN-gamma-induced expression of CXCL10/IP-10.

Regulation of IkappaB kinase-related kinases and antiviral responses by tumor suppressor CYLD

The IkappaB kinase (IKK)-related kinases, IKKepsilon and TBK1, participate in the induction of type I interferons (IFNs) during viral infections. Deregulated activation of IKKepsilon and TBK1 also contributes to the abnormal cell survival and transformation. However, how these kinases are negatively regulated remains unclear. We show here that the tumor suppressor CYLD has an essential role in preventing aberrant activation of IKKepsilon/TBK1. CYLD deficiency causes constitutive activation of IKKepsilon/TBK1, which is associated with hyper-induction of IFNs in virus-infected cells. We further show that CYLD targets a cytoplasmic RNA sensor, RIG-I, and inhibits the ubiquitination of this IKKepsilon/TBK1 stimulator. Consistent with the requirement of ubiquitination in RIG-I function, CYLD potently inhibits RIG-I-mediated activation of the IFN-beta promoter. These findings establish CYLD as a key negative regulator of IKKepsilon/TBK1 and suggest a role for CYLD in the control of RIG-I ubiquitination.

Retinoic acid-inducible gene-I is induced by interferon-gamma and regulates CXCL11 expression in HeLa cells

Retinoic acid-inducible gene-I (RIG-I) is a member of the DExH box family proteins, which have diverse roles in the regulation of gene expression and cellular functions. RIG-I is one of the factors regulated by interferon (IFN)-gamma and regarded as an intracellular signaling molecule for IFN-gamma. IFN-gamma is a major cytokine and also suggested to be involved in embryonal implantation and pregnancy. It is demonstrated that IFN-gamma stimulates endometrial epithelial cells to produce CXCL11, which is implicated in implantation. The aim of the present study was to investigate the effect of IFN-gamma on RIG-I expression in HeLa cells, a cell line derived from human uterine carcinoma. We found that RIG-I mRNA and protein were expressed in HeLa cells stimulated with IFN-gamma. The effect of IFN-gamma was observed in concentration- and time-dependent manners. The RNA interference against RIG-I resulted in the suppression of the IFN-gamma-induced CXCL11 expression. Immunohistochemical studies revealed the RIG-I expression in the normal human endometrium, suggesting a possible role of RIG-I in human reproductive organs.

Expression of mRNA for functional molecules in urinary sediment in glomerulonephritis

Recent studies have suggested that gene expression studies using urinary sediment might be a non-invasive approach to assessing activity and pathogenesis in glomerulonephritis. However, little information is available regarding the mRNA expression patterns of functional molecules, such as T-bet, GATA-3, FOXP3, and retinoic acid-inducible gene-I (RIG-I), in urinary sediment, from patients with immunocomplex-mediated glomerulonephritis. Fourteen lupus nephritis (LN) patients, 13 IgA nephropathy (IgAN) patients, and 12 healthy controls were enrolled in the study. The mRNA expressions of T-bet, GATA-3, FOXP3 and RIG-I in urinary sediment were measured using real time quantitative polymerase chain reaction. We also studied the expression of RIG-I in kidney tissue specimens obtained from LN and IgAN patients. Significant differences in the expression patterns of GATA-3, FOXP3 and RIG-I, and marginal differences in T-bet expression, were observed between the three study groups. Immunofluorescent staining for RIG-I was observed in the tissue specimens from the LN patients, but not in those from the IgAN patients. The mRNA expression patterns of T-bet, GATA-3, FOXP3 and RIG-I in urinary sediment differ according to diagnostic category. These results suggest that the measurement of these target gene expressions might be a useful, non-invasive method for clinical monitoring and studying of pathogenesis in glomerulonephritis.

Double-stranded RNA induces galectin-9 in vascular endothelial cells: involvement of TLR3, PI3K, and IRF3 pathway

Galectin-9 is a member of the galectin family, which induces various biological reactions such as chemotaxis of eosinophils and apoptosis of T cells. We previously reported that polyinosinic-polycytidylic acid (poly IC), an authentic double-stranded RNA (dsRNA), induces the expression of galectin-9 in human umbilical vein endothelial cells (HUVECs). In the present study, we addressed the possible involvement of two potential receptors for dsRNA, Toll-like receptor (TLR) 3 and retinoic acid-inducible gene-I (RIG-I), in the expression of galectin-9 in HUVECs. Poly IC-induced galectin-9 expression was almost completely suppressed by RNA interference (RNAi) against TLR3, but not against RIG-I. LY294002, an inhibitor of phosphatidylinositol 3-kinase (PI3K), inhibited the induction of galectin-9 by poly IC. RNAi against interferon regulatory factor 3 (IRF3) also inhibited poly IC-induced galectin-9 expression. We conclude that TLR3, PI3K, and IRF3 are involved in the poly IC-induced galectin-9 expression in HUVECs.

Expression of retinoic acid-inducible gene-I (RIG-I) in macrophages: possible involvement of RIG-I in atherosclerosis

AIM

Retinoic acid-inducible gene-I (RIG-I) is one of the genes induced by interferon (IFN)-gamma which plays an important role in atherosclerosis. The aim of this study is to examine if RIG-I is involved in atherosclerosis.

METHODS

The expression of RIG-I in atherosclerotic lesions in human aorta was examined by immunohistochemical analysis. The expression of RIG-I in THP-1 monocytic cell line or human monocyte-derived macrophages was studied by western blot and RT-PCR analyses.

RESULTS

Intense immunoreactivity for RIG-I was detected in intimal macrophages in atherosclerotic lesions. IFN-gamma slightly enhanced the RIG-I expression in THP-1 cells. Treatment of the cells with phorbol 12-myristate 13-acetate, which induces the differentiation of the cells into macrophage-like cells, significantly enhanced the IFN-gamma -induced RIG-I expression. IFN-gamma also stimulated the expression of RIG-I in monocyte-derived macrophages.

CONCLUSION

These results suggest that RIG-I may be involved in differentiation and activation of macrophages, playing a role in atherosclerosis.

Retinoic acid-inducible gene-I mediates RANTES/CCL5 expression in U373MG human astrocytoma cells stimulated with double-stranded RNA

Retinoic acid-inducible gene-I (RIG-I) mediates part of the cell signaling in response to viral infection. Polyinosinic-polycytidilic acid (poly IC) is a synthetic double-stranded RNA (dsRNA) and mimics viral infection when applied to cell cultures. The CC chemokine, RANTES (regulated on activation, normal T-cell expressed and secreted), is a potent attractant for inflammatory cells such as memory T-lymphocytes, monocytes and eosinophils. In the present study, we demonstrated that poly IC enhances the expression of RIG-I in U373MG human astrocytoma cells. The RNA interference of RIG-I resulted in the suppression of the poly IC-induced RANTES expression. Pretreatment of the cells with SB203580, an inhibitor of p38 mitogen-activated protein kinase, and dexamethasone inhibited the poly IC-induced expression of RIG-I. Furthermore, poly IC upregulated RIG-I in normal human astrocytes in culture and the in vivo injection of poly IC into the striatum of the mouse brain induced the expression of RIG-I in astrocytes. We conclude that RIG-I may be involved in immune reactions against viral infection, at least in part, through the regulation of RANTES expression in astrocytes.

Cytokine modulation of retinoic acid-inducible gene-I (RIG-I) expression in human epidermal keratinocytes

BACKGROUND

Retinoic acid-inducible gene-I (RIG-I) is a member of the DExH/D box family proteins and designated as a putative RNA helicase, which plays various roles in gene expression and cellular functions in response to a variety of RNA viruses.

OBJECTIVE

The present study was designed to investigate the effects of interferon (IFN)-gamma and tumor necrosis factor (TNF)-alpha on RIG-I expression in human keratinocytes, and the expression of RIG-I in skin lesions of psoriasis vulgaris, in which IFN-gamma and TNF-alpha are considered to be involved in its pathogenesis.

METHODS

The mRNA and protein expression of RIG-I was analyzed by reverse transcription-polymerase chain reaction (RT-PCR) and Western blotting. Immunohistochemical staining of RIG-I was examined on psoriatic skin section.

RESULTS

The levels of RIG-I mRNA and protein were upregulated in HaCaT keratinocytes in the presence of IFN-gamma or TNF-alpha. Immunohistochemically, RIG-I was detected in the cytoplasm of the spinous and basal layers of psoriatic skin.

CONCLUSION

Our results suggest that RIG-I might operate not only as a RNA helicase but also as a mediator of the cytokine network in the inflammatory skin diseases, such as psoriasis vulgaris.

Expression patterns and action analysis of genes associated with physiological responses during rat liver regeneration: cellular immune response

AIM

To study the cellular immune response during rat liver regeneration (LR) at a transcriptional level.

METHODS

Genes associated with the cellular immune response were obtained by collecting the data from databases and retrieving articles. Gene expression changes during LR were detected by rat genome 230 2.0 array.

RESULTS

A total of 127 genes were found to be associated with LR. The number of initially and totally expressing genes in the initial phase of LR [0.5-4 h after partial hepatectomy (PH)], transition from G(0)-G(1) (4-6 h after PH), cell proliferation (6-66 h after PH), cell differentiation and structure-function reconstruction (66-168 h after PH) was 54, 11, 34, 3 and 54, 49, 70, 49 respectively, illustrating that the associated genes were mainly triggered at the initiation of LR, and worked at different phases. According to their expression similarity, these genes were classified into 41 up-regulated, 21 predominantly up-regulated, 41 down-regulated, 14 predominantly down-regulated, 10 similarly up-regulated and down-regulated genes, respectively. The total up- and down-regulated expression times were 419 and 274, respectively, demonstrating that the expression of most genes was increased while the expression of a small number of genes was decreased. Their time relevance was classified into 14 groups, showing that the cellular physiological and biochemical activities were staggered during LR. According to the gene expression patterns, they were classified into 21 types, showing the activities were diverse and complicated during LR.

CONCLUSION

Antigen processing and presentation are enhanced mainly in the forepart, prophase and anaphase of LR. T-cell activation and antigen elimination are enhanced mainly in the forepart and prophase of LR. A total of 127 genes associated with LR play an important role in cellular immunity.

Polyinosinic-polycytidylic acid induces the expression of GRO-alpha in BEAS-2B cells

Growth-related oncogene protein-alpha (GRO-alpha)/CXCLl is a chemokine that activates neutrophils and plays an important role in inflammatory reactions. Polyinosinic-polycytidylic acid (poly IC) is a synthetic double-stranded RNA (dsRNA), which is a ligand for Toll-like receptor-3. Poly IC mimics viral infection when applied to cells and induces inflammatory and immune responses. In the present study, we found the induction of GRO-alpha in BEAS-2B bronchial epithelial cells treated with poly IC. Pretreatment of cells with 2-aminopurine, an inhibitor for dsRNA-dependent protein kinase (PKR), inhibited the expression of GRO-alpha-induced by poly IC. Overexpression of interferon-regulatory factor-3 (IRF-3) or retinoic-acid inducible gene-I (RIG-I) enhanced the induction of GRO-alpha by poly IC. PKR, IRF-3, and RIG-I may be involved in the poly IC-induced expression of GRO-alpha in BEAS-2B cells. Airway viral infection may elicit GRO-alpha expression in the bronchial epithelium, which may be implicated in inflammatory and immune reactions.

Expression of IP-10/CXCL10 Is Upregulated by Double-Stranded RNA in BEAS-2B Bronchial Epithelial Cells

BACKGROUND

Interferon (IFN)-gamma-inducible protein of 10 kDa (IP-10/CXCL10) is a potent chemoattractant for activated T and NK cells, and elevated levels of IP-10 are identified in bronchoalveolar lavage fluids from patients with pulmonary disorders related to Th-1-type immunity, which is a prerequisite for elimination of viral pathogens. Bronchial epithelial cells play an important role in respiratory infections as the initiator of airway inflammation by releasing chemokines and expressing cell surface membrane molecules involved in leukocyte adhesion. Polyinosinic-polycytidylic acid (poly IC) is a synthetic double-stranded RNA (dsRNA) and induces antiviral reactions in cells.

OBJECTIVES

We investigated the regulation of IP-10 in BEAS-2B bronchial epithelial cells in response to poly IC, and also addressed the possible role of retinoic-acid-inducible gene-I (RIG-I) and IFN-regulatory factor 3 (IRF-3), two genes involved in the signaling induced by viral infection.

METHODS

The expressions of IP-10 mRNA and protein were analyzed by reverse transcription-polymerase chain reaction and enzyme-linked immunosorbent assay. The overexpression of RIG-I or IRF-3 was performed by transfection of BEAS-2B cells with each cDNA.

RESULTS

Poly IC enhanced the expression of IP-10 mRNA and protein in concentration- and time-dependent manners. Overexpression of RIG-I or IRF-3 potentiated the poly-IC-induced upregulation of IP-10.

CONCLUSIONS

IP-10 may contribute to antiviral activity through the activation of Th-1-type immunity, and RIG-I and IRF-3 may be involved in this reaction.

Double-stranded RNA induces the synthesis of retinoic acid-inducible gene-I in vascular endothelial cells

Viral infection induces various responses in vascular endothelial cells. Polyinosinic-polycytidylic acid (poly IC) is a synthetic double-stranded RNA (dsRNA), and treatment of cells with poly IC mimics the viral infection to the cells. Retinoic acid-inducible gene-I (RIG-I) is a protein belonging to the DExH-box family and designated as a putative RNA helicase. RIG-I is considered to play a role in antiviral responses through the regulation of gene expressions. In the present study, the authors treated human umbilical vein endothelial cells (HUVECs) with poly IC and found that poly IC induced the expression of RIG-I. The poly IC-induced RIG-I expression was inhibited by the preincubation of the cells with 2-aminopurine, an inhibitor of dsRNA-dependent protein kinase (PKR). Immunohistochemical examination revealed high levels of RIG-I immunoreactivity in vascular endothelial cells in the thalamus from rats inoculated with hantavirus. Induction of RIG-I by poly IC may be involved in the antiviral responses in endothelial cells.

The RNA helicase Lgp2 inhibits TLR-independent sensing of viral replication by retinoic acid-inducible gene-I

The paramyxovirus Sendai (SV), is a well-established inducer of IFN-alphabeta gene expression. In this study we show that SV induces IFN-alphabeta gene expression normally in cells from mice with targeted deletions of the Toll-IL-1 resistance domain containing adapters MyD88, Mal, Toll/IL-1R domain-containing adaptor inducing IFN-beta (TRIF), and TRIF-related adaptor molecule TLR3, or the E3 ubiquitin ligase, TNFR-associated factor 6. This TLR-independent induction of IFN-alphabeta after SV infection is replication dependent and mediated by the RNA helicase, retinoic acid-inducible gene-I (RIG-I) and not the related family member, melanoma differentiation-associated gene 5. Furthermore, we characterize a RIG-I-like RNA helicase, Lgp2. In contrast to RIG-I or melanoma differentiation-associated gene 5, Lgp2 lacks signaling caspase recruitment and activation domains. Overexpression of Lgp2 inhibits SV and Newcastle disease virus signaling to IFN-stimulated regulatory element- and NF-kappaB-dependent pathways. Importantly, Lgp2 does not prevent TLR3 signaling. Like RIG-I, Lgp2 binds double-stranded, but not single-stranded, RNA. Quantitative PCR analysis demonstrates that Lgp2 is present in unstimulated cells at a lower level than RIG-I, although both helicases are induced to similar levels after virus infection. We propose that Lgp2 acts as a negative feedback regulator of antiviral signaling by sequestering dsRNA from RIG-I.

Delayed polarization of mononuclear phagocyte transcriptional program by type I interferon isoforms

Background

Interferon (IFN)-α is considered a key modulator of immunopathological processes through a signature-specific activation of mononuclear phagocytes (MPs). This study utilized global transcript analysis to characterize the effects of the entire type I IFN family in comparison to a broad panel of other cytokines on MP previously exposed to Lipopolysaccharide (LPS) stimulation in vitro.

Results

Immature peripheral blood CD14+ MPs were stimulated with LPS and 1 hour later with 42 separate soluble factors including cytokines, chemokines, interleukins, growth factors and IFNs. Gene expression profiling of MPs was analyzed 4 and 9 hours after cytokine stimulation. Four hours after stimulation, the transcriptional analysis of MPs revealed two main classes of cytokines: one associated with the alternative and the other with the classical pathway of MP activation without a clear polarization of type I IFNs effects. In contrast, after 9 hours of stimulation most type I IFN isoforms induced a characteristic and unique transcriptional pattern separate from other cytokines. These "signature" IFNs included; IFN-β, IFN-α2b/α2, IFN-αI, IFN-α2, IFN-αC, IFN-αJ1, IFN-αH2, and INF-α4B and induced the over-expression of 44 genes, all of which had known functional relationships with IFN such as myxovirus resistance (Mx)-1, Mx-2, and interferon-induced hepatitis C-associated microtubular aggregation protein. A second group of type I IFNs segregated separately and in closer association with the type II IFN-γ. The phylogenetic relationship of amino acid sequences among type I IFNs did not explain their sub-classification, although differences at positions 94 through 109 and 175 through 189 were present between the signature and other IFNs.

Conclusion

Seven IFN-α isoforms and IFN-β participate in the late phase polarization of MPs conditioned by LPS. This information broadens the previous view of the central role played by IFN-α in autoimmunity and tumor rejection by including and/or excluding an array of related factors likely to be heterogeneously expressed by distinct sub-populations of individuals in sickness or in response to biological therapy.