Rig-I-/- mice develop colitis associated with downregulation of G alpha i2

Chromatin as alarmins in necrotizing enterocolitis

Necrotizing enterocolitis (NEC) is a severe gastrointestinal disease primarily affecting premature neonates, marked by poorly understood pro-inflammatory signaling cascades. Recent advancements have shed light on a subset of endogenous molecular patterns, termed chromatin-associated molecular patterns (CAMPs), which belong to the broader category of damage-associated molecular patterns (DAMPs). CAMPs play a crucial role in recognizing pattern recognition receptors and orchestrating inflammatory responses. This review focuses into the realm of CAMPs, highlighting key players such as extracellular cold-inducible RNA-binding protein (eCIRP), high mobility group box 1 (HMGB1), cell-free DNA, neutrophil extracellular traps (NETs), histones, and extracellular RNA. These intrinsic molecules, often perceived as foreign, have the potential to trigger immune signaling pathways, thus contributing to NEC pathogenesis. In this review, we unravel the current understanding of the involvement of CAMPs in both preclinical and clinical NEC scenarios. We also focus on elucidating the downstream signaling pathways activated by these molecular patterns, providing insights into the mechanisms that drive inflammation in NEC. Moreover, we scrutinize the landscape of targeted therapeutic approaches, aiming to mitigate the impact of tissue damage in NEC. This in-depth exploration offers a comprehensive overview of the role of CAMPs in NEC, bridging the gap between preclinical and clinical insights.

Identification of pigeon mitochondrial antiviral signaling protein (MAVS) and its role in antiviral innate immunity

Pigeons can be infected with various RNA viruses, and their innate immune system responds to viral infection to establish an antiviral response. Mitochondrial antiviral signaling protein (MAVS), an important adaptor protein in signal transduction, plays a pivotal role in amplifying the innate immune response. In this study, we successfully cloned pigeon MAVS (piMAVS) and performed a bioinformatics analysis. The results showed that the caspase recruitment domain (CARD) and transmembrane (TM) domain are highly conserved in poultry and mammals but poorly conserved in other species. Furthermore, we observed that MAVS expression is upregulated both in pigeons and pigeon embryonic fibroblasts (PEFs) upon RNA virus infection. Overexpression of MAVS resulted in increased levels of β-interferon (IFN-β), IFN-stimulated genes (ISGs), and interleukin (ILs) mRNA and inhibited Newcastle disease virus (NDV) replication. We also found that piMAVS and human MAVS (huMAVS) induced stronger expression of IFN-β and ISGs when compared to chicken MAVS (chMAVS), and this phenomenon was also reflected in the degree of inhibition of NDV replication. Our findings demonstrate that piMAVS plays an important role in repressing viral replication by regulating the activation of the IFN signal pathway in pigeons. This study not only sheds light on the function of piMAVS in innate immunity but also contributes to a more comprehensive understanding of the innate immunity system in poultry. Our data also provide unique insights into the differences in innate immunity between poultry and mammal.

The NF-κB signaling system in the immunopathogenesis of inflammatory bowel disease

Inflammatory bowel disease (IBD) is an idiopathic, chronic condition characterized by episodes of inflammation in the gastrointestinal tract. The nuclear factor κB (NF-κB) system describes a family of dimeric transcription factors. Canonical NF-κB signaling is stimulated by and enhances inflammation, whereas noncanonical NF-κB signaling contributes to immune organogenesis. Dysregulation of NF-κB factors drives various inflammatory pathologies, including IBD. Signals from many immune sensors activate NF-κB subunits in the intestine, which maintain an equilibrium between local microbiota and host responses. Genetic association studies of patients with IBD and preclinical mouse models confirm the importance of the NF-κB system in host defense in the gut. Other studies have investigated the roles of these factors in intestinal barrier function and in inflammatory gut pathologies associated with IBD. NF-κB signaling modulates innate and adaptive immune responses and the production of immunoregulatory proteins, anti-inflammatory cytokines, antimicrobial peptides, and other tolerogenic factors in the intestine. Furthermore, genetic studies have revealed critical cell type-specific roles for NF-κB proteins in intestinal immune homeostasis, inflammation, and restitution that contribute to the etiopathology of IBD-associated manifestations. Here, we summarize our knowledge of the roles of these NF-κB pathways, which are activated in different intestinal cell types by specific ligands, and their cross-talk, in fueling aberrant intestinal inflammation. We argue that an in-depth understanding of aberrant immune signaling mechanisms may hold the key to identifying predictive or prognostic biomarkers and developing better therapeutics against inflammatory gut pathologies.

Microbial sensing in the intestine

The gut microbiota plays a key role in host health and disease, particularly through their interactions with the immune system. Intestinal homeostasis is dependent on the symbiotic relationships between the host and the diverse gut microbiota, which is influenced by the highly co-evolved immune-microbiota interactions. The first step of the interaction between the host and the gut microbiota is the sensing of the gut microbes by the host immune system. In this review, we describe the cells of the host immune system and the proteins that sense the components and metabolites of the gut microbes. We further highlight the essential roles of pattern recognition receptors (PRRs), the G protein-coupled receptors (GPCRs), aryl hydrocarbon receptor (AHR) and the nuclear receptors expressed in the intestinal epithelial cells (IECs) and the intestine-resident immune cells. We also discuss the mechanisms by which the disruption of microbial sensing because of genetic or environmental factors causes human diseases such as the inflammatory bowel disease (IBD).

Mutant RIG-I enhances cancer-related inflammation through activation of circRIG-I signaling

RIG-I/DDX58 plays a key role in host innate immunity. However, its therapeutic potential for inflammation-related cancers remains to be explored. Here we identify frameshift germline mutations of RIG-I occurring in patients with colon cancer. Accordingly, Rig-i^fs/fs mice bearing a frameshift mutant Rig-i exhibit increased susceptibility to colitis-related colon cancer as well as enhanced inflammatory response to chemical, virus or bacteria. In addition to interruption of Rig-i mRNA translation, the Rig-i mutation changes the secondary structure of Rig-i pre-mRNA and impairs its association with DHX9, consequently inducing a circular RNA generation from Rig-i transcript, thereby, designated as circRIG-I. CircRIG-I is frequently upregulated in colon cancers and its upregulation predicts poor outcome of colon cancer. Mechanistically, circRIG-I interacts with DDX3X, which in turn stimulates MAVS/TRAF5/TBK1 signaling cascade, eventually activating IRF3-mediated type I IFN transcription and aggravating inflammatory damage. Reciprocally, all-trans retinoic acid acts as a DHX9 agonist, ameliorates immunopathology through suppression of circRIG-I biogenesis. Collectively, our results provide insight into mutant RIG-I action and propose a potential strategy for the treatment of colon cancer.

RIG-I-like Receptor Regulation of Immune Cell Function and Therapeutic Implications

Retinoic acid-inducible gene I-like receptors (RLRs) are cytosolic RNA sensors critical for initiation of antiviral immunity. Activation of RLRs following RNA recognition leads to production of antiviral genes and IFNs for induction of broad antiviral immunity. Although the RLRs are ubiquitously expressed, much of our understanding of these molecules comes from their study in epithelial cells and fibroblasts. However, RLR activation is critical for induction of immune function and long-term protective immunity. Recent work has focused on the roles of RLRs in immune cells and their contribution to programming of effective immune responses. This new understanding of RLR function in immune cells and immune programming has led to the development of vaccines and therapeutics targeting the RLRs. This review covers recent advances in our understanding of the contribution of RLRs to immune cell function during infection and the emerging RLR-targeting strategies for induction of immunity against cancer and viral infection.

RIG-I acts as a tumor suppressor in melanoma via regulating the activation of the MKK/p38MAPK signaling pathway

Studies have indicated that RIG-I may act as a tumor suppressor and participate in the tumorigenesis of some malignant diseases. However, RIG-I induces distinct cellular responses via different downstream signaling pathways depending on the cell type. To investigate the biological function and underlying molecular mechanism of RIG-I in the tumorigenesis of melanoma, we constructed RIG-I knockout, RIG-I-overexpressing B16-F10 and RIG-I knockdown A375 melanoma cell lines, and analyzed the RIG-I-mediated change in the biological behavior of tumor cells in spontaneous and poly (I:C)-induced RIG-I activation. Cell proliferation, cell cycling, apoptosis and migration were detected by CCK-8 assay, BrdU incorporation assay, Annexin V-PI staining assay and Transwell assay, respectively. In vivo tumorigenicity was evaluated by tumor xenograft growth in nude mice and subsequently by Ki67 staining and TUNEL assays. Furthermore, Western blotting was utilized to explore the underlying mechanism of RIG-I in melanoma cells. Our data showed that RIG-I promotes apoptosis and inhibits proliferation by G1 phase cell cycle arrest in the melanoma cell lines. Mechanistically, RIG-I induced the phosphorylation of p38 MAPK and MAPK kinases MKK3 and MKK4. In conclusion, the current study demonstrated that RIG-I suppressed the development of melanoma by regulating the activity of the MKK/p38 MAPK signaling pathway, which is relevant to research on novel therapeutic targets for this malignant disease.

Human enteric viruses autonomously shape inflammatory bowel disease phenotype through divergent innate immunomodulation

Altered enteric microorganisms in concert with host genetics shape inflammatory bowel disease (IBD) phenotypes. However, insight is limited to bacteria and fungi. We found that eukaryotic viruses and bacteriophages (collectively, the virome), enriched from non-IBD, noninflamed human colon resections, actively elicited atypical anti-inflammatory innate immune programs. Conversely, ulcerative colitis or Crohn's disease colon resection viromes provoked inflammation, which was successfully dampened by non-IBD viromes. The IBD colon tissue virome was perturbed, including an increase in the enterovirus B species of eukaryotic picornaviruses, not previously detected in fecal virome studies. Mice humanized with non-IBD colon tissue viromes were protected from intestinal inflammation, whereas IBD virome mice exhibited exacerbated inflammation in a nucleic acid sensing-dependent fashion. Furthermore, there were detrimental consequences for IBD patient-derived intestinal epithelial cells bearing loss-of-function mutations within virus sensor MDA5 when exposed to viromes. Our results demonstrate that innate recognition of IBD or non-IBD human viromes autonomously influences intestinal homeostasis and disease phenotypes. Thus, perturbations in the intestinal virome, or an altered ability to sense the virome due to genetic variation, contribute to the induction of IBD. Harnessing the virome may offer therapeutic and biomarker potential.

Cyclic GMP-AMP synthase contributes to epithelial homeostasis in intestinal inflammation via Beclin-1-mediated autophagy

Inflammatory bowel disease (IBD) represents a set of idiopathic and chronic inflammatory diseases of the gastrointestinal tract. Central to the pathogenesis of IBD is a dysregulation of normal intestinal epithelial homeostasis. cGAS is a DNA-sensing receptor demonstrated to promote autophagy, a mechanism that removes dysfunctional cellular components. Beclin-1 is a crucial protein involved in the initiation of autophagy. We hypothesized that cGAS plays a key role in intestinal homeostasis by upregulating Beclin-1-mediated autophagy. We evaluated intestinal cGAS levels in humans with IBD and in murine colonic tissue after performing a 2% dextran sulfate sodium (DSS) colitis model. Autophagy and cell death mechanisms were studied in cGAS KO and WT mice via qPCR, WB analysis, H&E, IF, and TUNEL staining. Autophagy was measured in stimulated intestinal epithelial cells (IECs) via WB analysis. Our data demonstrates cGAS to be upregulated during human and murine colitis. Furthermore, cGAS deficiency leads to worsened colitis and decreased levels of autophagy proteins including Beclin-1 and LC3-II. Co-IP demonstrates a direct binding between cGAS and Beclin-1 in IECs. Transfection of cGAS in stimulated HCT-116 cells leads to increased autophagy. IECs isolated from cGAS KO have diminished autophagic flux. cGAS KO mice subjected to DSS have increased cell death and cleaved caspase-3. Lastly, treatment of cGAS KO mice with rapamycin decreased the severity of colitis. Our data suggest that cGAS maintains intestinal epithelial homeostasis during human IBD and murine colitis by upregulating Beclin-1-mediated autophagy and preventing IEC death. Rescue of autophagy can attenuate the severity of colitis associated with cGAS deficiency.

The RIG-I-NRF2 axis regulates the mesenchymal stromal niche for bone marrow transplantation

Bone marrow-derived mesenchymal stem cells (BMSCs) support bone formation and constitute the stromal niche in regulating hematopoietic stem cells (HSCs). Stromal niche dysfunction affects HSC engraftment during transplantation; however, the underlying mechanisms remain elusive. In the present study, we found that all-trans retinoic acid (ATRA) and inflammation stress upregulated retinoic acid-inducible gene I (RIG-I) in BMSCs. Excess RIG-I expression damaged the clonogenicity, bone-forming ability of BMSCs and, particularly, their stromal niche function that supports HSC expansion in vitro and engraftment in vivo. Mechanistically, RIG-I elevation promoted the degradation of NRF2, a checkpoint for antioxidant cellular response, by altering the RIG-I-Trim25-Keap1-NRF2 complex, leading to reactive oxygen species (ROS) accumulation and BMSC damage. Genetic inhibition of RIG-I sustained NRF2 protein levels and reduced ROS levels in ATRA-treated BMSCs, thus preserving their clonogenicity, bone-forming ability, and stromal niche function in supporting HSC engraftment in mice. More importantly, RIG-I inhibition recovered the ATRA-treated stromal niche function, to enhance HSC engraftment and emergency myelopoiesis for innate immunity against the bacterium Listeria monocytogenes during transplantation. Overall, we identified a non-canonical role of RIG-I in the regulation of the stromal niche for HSC transplantation.

Nucleic Acid Sensing in the Tumor Vasculature

Endothelial cells form a powerful interface between tissues and immune cells. In fact, one of the underappreciated roles of endothelial cells is to orchestrate immune attention to specific sites. Tumor endothelial cells have a unique ability to dampen immune responses and thereby maintain an immunosuppressive microenvironment. Recent approaches to trigger immune responses in cancers have focused on activating nucleic acid sensors, such as cGAS-STING, in combination with immunotherapies. In this review, we present a case for targeting nucleic acid-sensing pathways within the tumor vasculature to invigorate tumor-immune responses. We introduce two specific nucleic acid sensors-the DNA sensor TREX1 and the RNA sensor RIG-I-and discuss their functional roles in the vasculature. Finally, we present perspectives on how these nucleic acid sensors in the tumor endothelium can be targeted in an antiangiogenic and immune activation context. We believe understanding the role of nucleic acid-sensing in the tumor vasculature can enhance our ability to design more effective therapies targeting the tumor microenvironment by co-opting both vascular and immune cell types.

IFIH1 loss-of-function variants contribute to very early-onset inflammatory bowel disease

Genetic defects of innate immunity impairing intestinal bacterial sensing are linked to the development of Inflammatory Bowel Disease (IBD). Although much evidence supports a role of the intestinal virome in gut homeostasis, most studies focus on intestinal viral composition rather than on host intestinal viral sensitivity. To demonstrate the association between the development of Very Early Onset IBD (VEOIBD) and variants in the IFIH1 gene which encodes MDA5, a key cytosolic sensor for viral nucleic acids. Whole exome sequencing (WES) was performed in two independent cohorts of children with VEOIBD enrolled in Italy (n = 18) and USA (n = 24). Luciferase reporter assays were employed to assess MDA5 activity. An enrichment analysis was performed on IFIH1 comparing 42 VEOIBD probands with 1527 unrelated individuals without gastrointestinal or immunological issues. We identified rare, likely loss-of-function (LoF), IFIH1 variants in eight patients with VEOIBD from a combined cohort of 42 children. One subject, carrying a homozygous truncating variant resulting in complete LoF, experienced neonatal-onset, pan-gastrointestinal, IBD-like enteropathy plus multiple infectious episodes. The remaining seven subjects, affected by VEOIBD without immunodeficiency, were carriers of one LoF variant in IFIH1. Among these, two patients also carried a second hypomorphic variant, with partial function apparent when MDA5 was weakly stimulated. Furthermore, IFIH1 variants were significantly enriched in children with VEOIBD as compared to controls (p = 0.007). Complete and partial MDA5 deficiency is associated with VEOIBD with variable penetrance and expressivity, suggesting a role for impaired intestinal viral sensing in IBD pathogenesis.

Minor alterations in the intestinal microbiota composition upon Rotavirus infection do not affect susceptibility to DSS colitis

Viral triggers at the intestinal mucosa can have multiple global effects on intestinal integrity, causing elevated intestinal barrier strength and relative protection from subsequent inflammatory bowel disease (IBD) induction in various models. As viruses can interfere with the intestinal immune system both directly and indirectly through commensal bacteria, cause-effect relationships are difficult to define. Due to the complexity of putatively causative factors, our understanding of such virus-mediated protection is currently very limited. We here set out to better understand the impact that adult enteric infection with rotavirus (RV) might have on the composition of the intestinal microbiome and on the severity of IBD. We found that RV infection neither induced significant long-lasting microbiota community changes in the small or large intestine nor affected the severity of subsequent dextran sulfate sodium-induced colitis. Hence, adult murine RV infection does not exert lasting effects on intestinal homeostasis.

Immunogenetics of the Ocular Anterior Segment: Lessons from Inherited Disorders

Autoimmune and autoinflammatory diseases cause morbidity in multiple organ systems including the ocular anterior segment. Genetic disorders of the innate and adaptive immune system present an avenue to study more common inflammatory disorders and host-pathogen interactions. Many of these Mendelian disorders have ophthalmic manifestations. In this review, we highlight the ophthalmic and molecular features of disorders of the innate immune system. A comprehensive literature review was performed using PubMed and the Online Mendelian Inheritance in Man databases spanning 1973-2020 with a focus on three specific categories of genetic disorders: RIG-I-like receptors and downstream signaling, inflammasomes, and RNA processing disorders. Tissue expression, clinical associations, and animal and functional studies were reviewed for each of these genes. These genes have broad roles in cellular physiology and may be implicated in more common conditions with interferon upregulation including systemic lupus erythematosus and type 1 diabetes. This review contributes to our understanding of rare inherited conditions with ocular involvement and has implications for further characterizing the effect of perturbations in integral molecular pathways.

RDUR, a lncRNA, Promotes Innate Antiviral Responses and Provides Feedback Control of NF-κB Activation

Influenza A virus (IAV), a highly infectious respiratory pathogen, remains a major threat to global public health. Numerous long non-coding RNAs (lncRNAs) have been shown to be implicated in various cellular processes. Here, we identified a new lncRNA termed RIG-I-dependent IAV-upregulated noncoding RNA (RDUR), which was induced by infections with IAV and several other viruses. Both in vitro and in vivo studies revealed that robust expression of host RDUR induced by IAV was dependent on the RIG-I/NF-κB pathway. Overexpression of RDUR suppressed IAV replication and downregulation of RDUR promoted the virus replication. Deficiency of mouse RDUR increased virus production in lungs, body weight loss, acute organ damage and consequently reduced survival rates of mice, in response to IAV infection. RDUR impaired the viral replication by upregulating the expression of several vital antiviral molecules including interferons (IFNs) and interferon-stimulated genes (ISGs). Further study showed that RDUR interacted with ILF2 and ILF3 that were required for the efficient expression of some ISGs such as IFITM3 and MX1. On the other hand, we found that while NF-κB positively regulated the expression of RDUR, increased expression of RDUR, in turn, inactivated NF-κB through a negative feedback mechanism to suppress excessive inflammatory response to viral infection. Together, the results demonstrate that RDUR is an important lncRNA acting as a critical regulator of innate immunity against the viral infection.

Regulation of RIG-I-like receptor-mediated signaling: interaction between host and viral factors

Retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs) are RNA sensor molecules that play essential roles in innate antiviral immunity. Among the three RLRs encoded by the human genome, RIG-I and melanoma differentiation-associated gene 5, which contain N-terminal caspase recruitment domains, are activated upon the detection of viral RNAs in the cytoplasm of virus-infected cells. Activated RLRs induce downstream signaling via their interactions with mitochondrial antiviral signaling proteins and activate the production of type I and III interferons and inflammatory cytokines. Recent studies have shown that RLR-mediated signaling is regulated by interactions with endogenous RNAs and host proteins, such as those involved in stress responses and posttranslational modifications. Since RLR-mediated cytokine production is also involved in the regulation of acquired immunity, the deregulation of RLR-mediated signaling is associated with autoimmune and autoinflammatory disorders. Moreover, RLR-mediated signaling might be involved in the aberrant cytokine production observed in coronavirus disease 2019. Since the discovery of RLRs in 2004, significant progress has been made in understanding the mechanisms underlying the activation and regulation of RLR-mediated signaling pathways. Here, we review the recent advances in the understanding of regulated RNA recognition and signal activation by RLRs, focusing on the interactions between various host and viral factors.

Innate Immune Nod1/RIP2 Signaling Is Essential for Cardiac Hypertrophy but Requires Mitochondrial Antiviral Signaling Protein for Signal Transductions and Energy Balance

BACKGROUND

Cardiac hypertrophy is a key biological response to injurious stresses such as pressure overload and, when excessive, can lead to heart failure. Innate immune activation by danger signals, through intracellular pattern recognition receptors such as nucleotide-binding oligomerization domain 1 (Nod1) and its adaptor receptor-interacting protein 2 (RIP2), might play a major role in cardiac remodeling and progression to heart failure. We hypothesize that Nod1/RIP2 are major contributors to cardiac hypertrophy, but may not be sufficient to fully express the phenotype alone.

METHODS

To elucidate the contribution of Nod1/RIP2 signaling to cardiac hypertrophy, we randomized Nod1^-/-, RIP2^-/-, or wild-type mice to transverse aortic constriction or sham operations. Cardiac hypertrophy, fibrosis, and cardiac function were examined in these mice.

RESULTS

Nod1 and RIP2 proteins were upregulated in the heart after transverse aortic constriction, and this was paralleled by increased expression of mitochondrial proteins, including mitochondrial antiviral signaling protein (MAVS). Nod1^-/- and RIP2^-/- mice subjected to transverse aortic constriction exhibited better survival, improved cardiac function, and decreased cardiac hypertrophy. Downstream signal transduction pathways that regulate inflammation and fibrosis, including NF (nuclear factor) κB and MAPK (mitogen-activated protein kinase)-GATA4/p300, were reduced in both Nod1^-/- and RIP2^-/- mice after transverse aortic constriction compared with wild-type mice. Coimmunoprecipitation of extracted cardiac proteins and confocal immunofluorescence microscopy showed that Nod1/RIP2 interaction was robust and that this complex also included MAVS as an essential component. Suppression of MAVS expression attenuated the complex formation, NF κB signaling, and myocyte hypertrophy. Interrogation of mitochondrial function compared in the presence or ablation of MAVS revealed that MAVS serves to suppress mitochondrial energy output and mediate fission/fusion related dynamic changes. The latter is possibly linked to mitophagy during cardiomyocytes stress, which may provide an intriguing link between innate immune activation and mitochondrial energy balance under stress or injury conditions.

CONCLUSIONS

We have identified that innate immune Nod1/RIP2 signaling is a major contributor to cardiac remodeling after stress. This process is critically joined by and regulated through the mitochondrial danger signal adapter MAVS. This novel complex coordinates remodeling, inflammatory response, and mitochondrial energy metabolism in stressed cardiomyocytes. Thus, Nod1/RIP2/MAVS signaling complex may represent an attractive new therapeutic approach toward heart failure.

Inhibition of miR-93 promotes interferon effector signaling to suppress influenza A infection by upregulating JAK1

Type I interferons play a critical role in host defense against influenza virus infection. Interferon cascade induces the expression of interferon-stimulated genes then subsequently promotes antiviral immune responses. The microRNAs are important regulators of innate immunity, but microRNAs-mediated regulation of interferon cascade during influenza infection remains to be fully identified. Here we found influenza A virus (IAV) infection significantly inhibited miR-93 expression in alveolar epithelial type II cells through RIG-I/JNK pathway. IAV-induced downregulation of miR-93 was found to upregulate JAK1, the target of miR-93, and then feedback promote antiviral innate response by facilitating IFN effector signaling. Importantly, in vivo administration of miR-93 antagomiR markedly suppressed IAV infection, protecting mice form IAVs -associated death. Hence, the inducible downregulation of miR-93 feedback suppress IAV infection by strengthening IFN-JAK-STAT pathway via JAK1 upregulation, and in vivo inhibition of miR-93 bears considerable therapeutic potential for suppressing IAV infection.

RIG-I regulates myeloid differentiation by promoting TRIM25-mediated ISGylation

Retinoic acid-inducible gene I (RIG-I) is up-regulated during granulocytic differentiation of acute promyelocytic leukemia (APL) cells induced by all-trans retinoic acid (ATRA). It has been reported that RIG-I recognizes virus-specific 5'-ppp-double-stranded RNA (dsRNA) and activates the type I interferons signaling pathways in innate immunity. However, the functions of RIG-I in hematopoiesis remain unclear, especially regarding its possible interaction with endogenous RNAs and the associated pathways that could contribute to the cellular differentiation and maturation. Herein, we identified a number of RIG-I-binding endogenous RNAs in APL cells following ATRA treatment, including the tripartite motif-containing protein 25 (TRIM25) messenger RNA (mRNA). TRIM25 encodes the protein known as an E3 ligase for ubiquitin/interferon (IFN)-induced 15-kDa protein (ISG15) that is involved in RIG-I-mediated antiviral signaling. We show that RIG-I could bind TRIM25 mRNA via its helicase domain and C-terminal regulatory domain, enhancing the stability of TRIM25 transcripts. RIG-I could increase the transcriptional expression of TRIM25 by caspase recruitment domain (CARD) domain through an IFN-stimulated response element. In addition, RIG-I activated other key genes in the ISGylation pathway by activating signal transducer and activator of transcription 1 (STAT1), including the modifier ISG15 and several enzymes responsible for the conjugation of ISG15 to protein substrates. RIG-I cooperated with STAT1/2 and interferon regulatory factor 1 (IRF1) to promote the activation of the ISGylation pathway. The integrity of ISGylation in ATRA or RIG-I-induced cell differentiation was essential given that knockdown of TRIM25 or ISG15 resulted in significant inhibition of this process. Our results provide insight into the role of the RIG-I-TRIM25-ISGylation axis in myeloid differentiation.

New Horizons in the Genetic Etiology of Systemic Lupus Erythematosus and Lupus-Like Disease: Monogenic Lupus and Beyond

Systemic lupus erythematosus (SLE) is a clinically and genetically heterogeneous autoimmune disease. The etiology of lupus and the contribution of genetic, environmental, infectious and hormonal factors to this phenotype have yet to be elucidated. The most straightforward approach to unravel the molecular pathogenesis of lupus may rely on studies of patients who present with early-onset severe phenotypes. Typically, they have at least one of the following clinical features: childhood onset of severe disease (<5 years), parental consanguinity, and presence of family history for autoimmune diseases in a first-degree relative. These patients account for a small proportion of patients with lupus but they inform considerable knowledge about cellular pathways contributing to this inflammatory phenotype. In recent years with the aid of new sequencing technologies, novel or rare pathogenic variants have been reported in over 30 genes predisposing to SLE and SLE-like diseases. Future studies will likely discover many more genes with private variants associated to lupus-like phenotypes. In addition, genome-wide association studies (GWAS) have identified a number of common alleles (SNPs), which increase the risk of developing lupus in adult age. Discovery of a possible shared immune pathway in SLE patients, either with rare or common variants, can provide important clues to better understand this complex disorder, it's prognosis and can help guide new therapeutic approaches. The aim of this review is to summarize the current knowledge of the clinical presentation, genetic diagnosis and mechanisms of disease in patents with lupus and lupus-related phenotypes.

Genome-wide identification, characterization of RLR genes in Yesso scallop (Patinopecten yessoensis) and functional regulations in responses to ocean acidification

Retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs), are crucial sensors with a conserved structure in cytoplasm, inducing the production of cytokines, chemokines and host restriction factors which mediate a variety of intracellular activities to interfere with distinct PAMPs (pathogen-associated molecular patterns) for eliminating pathogens in innate immune system. Although RLR genes have been investigated in most vertebrates and some invertebrates, the systematic identification and characterization of RLR genes have not been reported in scallops. In this study, four RLR genes (PY-10413.4, PY-10413.5, PY-443.7 and PY-443.8, designated PyRLRs) were identified in Yesso scallop (Patinopecten yessoensis) through whole-genome scanning through in silico analysis, including two pairs of tandem duplicate genes located on the same scaffold (PY-10413.4 and PY-10413.5, PY-443.7 and PY-443.8, respectively). Phylogenetic and protein structural analyses were performed to determine the identities and evolutionary relationships of these genes. The expression profiles of PyRLRs were determined in all developmental stages, in healthy adult tissues, and in mantles that simulated ocean acidification (OA) exposure (pH = 6.5 and 7.5) at different time points (3, 6, 12 and 24 h). Spatiotemporal expression patterns suggested the functional roles of PyRLRs in all stages of development and growth of the scallop. Regulation expressions revealed PY-10413.4 and PY-10413.5 with one or two CARD(s) (caspase activation and recruitment domain) were up-regulated expressed at most time points, whereas PY-443.8 and PY-10413.4 without CARD were significantly down-regulated at each time points, suggesting functional differentiations in the two pairs of PyRLRs based on the structural differences in response to OA. Collectively, this study demonstrated gene duplication of RLR family genes and provide primary analysis for versatile roles in the response of the bivalve innate immune system to OA challenge.

Retinoic acid inducible gene-I slows down cellular senescence through negatively regulating the integrin β3/p38 MAPK pathway

Retinoic acid inducible gene-I (Rig-I) has been well documented as a cytosolic pattern recognition receptor that can sense viral RNA ligands to initiate the interferon-mediated antiviral immunity. However, little is known about the biological behaviors of Rig-I devoid of viral infection. Herein, we investigated the roles of Rig-I in the regulation of cellular senescence. In comparison to wild-type counterparts, Rig-I-/- mice displayed the accelerated loss of hair, less responsiveness to gentle physical stimuli and shorten survival time. Likewise, Rig-I deficiency rendered mouse embryonic fibroblasts (MEFs) more susceptible to the serial passages-associated replicative senescence. By performing a transcriptome analysis, we identified integrins at the intersections of biological pathways affected by Rig-I. Among these, integrin β3 was negatively regulated by Rig-I, and significantly upregulated with the occurrence of senescence. Gene silencing of Itgb3 (encoding integrin β3) retarded the progression of cellular senescence in both WT and Rig-I-/- MEFs. Notably, this effect was more prominent in Rig-I-/- MEFs. Furthermore, p38 MAPK was a key downstream molecule for integrin β3-mediated senescence, and overactivated in senescent Rig-I-/- MEFs. Taken together, Rig-I deficiency contributes to cellular senescence through amplifying integrin β3/p38 MAPK signaling. Our findings provide the evidence that Rig-I is a key regulator of cellular senescence, which will be helpful in better understanding its function without viral infection.Abbreviations: Rig-I: retinoic acid inducible gene-I; SASP: senescence-associated secretory phenotype; ECM: extracellular matrix; Itgb3: integrin beta 3; PRR: pattern recognition receptor; MEFs: mouse embryonic fibroblasts; Il-1β: interleukin-1 beta; Il-6: interleukin-6; Il-8: interleukin-8; Cxcl1: chemokine (C-X-C motif) ligand 1; Ccl2: chemokine (C-C motif) ligand 2; WT, wild type; BM: bone marrow; MAPK: mitogen-activated protein kinase; ERK: extracellular signal-regulated kinases; JNK: Jun N-terminal kinases; SA-β-gal: senescence-associated β-galactosidase; qPCR: quantitative reverse-transcription PCR; PBS: phosphate-buffered saline.

Syringeable immunotherapeutic nanogel reshapes tumor microenvironment and prevents tumor metastasis and recurrence

The low response rate of current cancer immunotherapy suggests the presence of few antigen-specific T cells and a high number of immunosuppressive factors in tumor microenvironment (TME). Here, we develop a syringeable immunomodulatory multidomain nanogel (iGel) that overcomes the limitation by reprogramming of the pro-tumoral TME to antitumoral immune niches. Local and extended release of immunomodulatory drugs from iGel deplete immunosuppressive cells, while inducing immunogenic cell death and increased immunogenicity. When iGel is applied as a local postsurgical treatment, both systemic antitumor immunity and a memory T cell response are generated, and the recurrence and metastasis of tumors to lungs and other organs are significantly inhibited. Reshaping of the TME using iGel also reverts non-responding groups to checkpoint blockade therapies into responding groups. The iGel is expected as an immunotherapeutic platform that can reshape immunosuppressive TMEs and synergize cancer immunotherapy with checkpoint therapies, with minimized systemic toxicity.

The limited efficacy of current immunotherapy suggests low antigen-specific T cells and immunosuppressive factors in tumor microenvironment (TME). Here, the authors develop a syringeable immunomodulatory multi-domain nanogel that can reprogram the TME and induce enhanced cancer immunotherapy.

Comparative Structure and Function Analysis of the RIG-I-Like Receptors: RIG-I and MDA5

RIG-I (Retinoic acid-inducible gene I) and MDA5 (Melanoma Differentiation-Associated protein 5), collectively known as the RIG-I-like receptors (RLRs), are key protein sensors of the pathogen-associated molecular patterns (PAMPs) in the form of viral double-stranded RNA (dsRNA) motifs to induce expression of type 1 interferons (IFN1) (IFNα and IFNβ) and other pro-inflammatory cytokines during the early stage of viral infection. While RIG-I and MDA5 share many genetic, structural and functional similarities, there is increasing evidence that they can have significantly different strategies to recognize different pathogens, PAMPs, and in different host species. This review article discusses the similarities and differences between RIG-I and MDA5 from multiple perspectives, including their structures, evolution and functional relationships with other cellular proteins, their differential mechanisms of distinguishing between host and viral dsRNAs and interactions with host and viral protein factors, and their immunogenic signaling. A comprehensive comparative analysis can help inform future studies of RIG-I and MDA5 in order to fully understand their functions in order to optimize potential therapeutic approaches targeting them.

KAT8 selectively inhibits antiviral immunity by acetylating IRF3

Optimal activation of IRF3 is crucial for maintaining immune homeostasis. Huai et al. demonstrate that KAT8 acetylates IRF3 at lysine 359, inhibits IRF3 recruitment to promoters of type I interferon genes, and then decreases type I interferon production to attenuate antiviral innate immune responses.

The transcription factor interferon regulatory factor 3 (IRF3) is essential for virus infection–triggered induction of type I interferons (IFN-I) and innate immune responses. IRF3 activity is tightly regulated by conventional posttranslational modifications (PTMs) such as phosphorylation and ubiquitination. Here, we identify an unconventional PTM of IRF3 that directly inhibits its transcriptional activity and attenuates antiviral immune response. We performed an RNA interference screen and found that lysine acetyltransferase 8 (KAT8), which is ubiquitously expressed in immune cells (particularly in macrophages), selectively inhibits RNA and DNA virus–triggered IFN-I production in macrophages and dendritic cells. KAT8 deficiency protects mice from viral challenge by enhancing IFN-I production. Mechanistically, KAT8 directly interacts with IRF3 and mediates IRF3 acetylation at lysine 359 via its MYST domain. KAT8 inhibits IRF3 recruitment to IFN-I gene promoters and decreases the transcriptional activity of IRF3. Our study reveals a critical role for KAT8 and IRF3 lysine acetylation in the suppression of antiviral innate immunity.

Therapeutic Targeting of RIG-I and MDA5 Might Not Lead to the Same Rome

RIG-I and MDA5 receptors are key sensors of pathogen-associated molecular pattern (PAMP)-containing viral RNA and transduce downstream signals to activate an antiviral and immunomodulatory response. Fifteen years of research have put them at the center of an ongoing hunt for novel pharmacological pan-antivirals, vaccine adjuvants, and antitumor strategies. Current knowledge testifies to the redundant, but also distinct, functions mediated by RIG-I and MDA5, opening opportunities for the use of specific and potent nucleic acid agonists. We critically discuss the evidence and remaining knowledge gaps that have an impact on the choice and design of optimal RNA ligands to achieve an appropriate immunostimulatory response, with limited adverse effects, for prophylactic and therapeutic interventions against viruses and cancer in humans.

Cytosolic Nucleic Acid Sensors in Inflammatory and Autoimmune Disorders

Innate immunity employs germline-encoded pattern recognition receptors (PRRs) to sense microbial pattern molecules. Recognition of pathogen-associated molecular patterns (PAMPs) by various PPRs located on the cell membrane or in the cytosol leads to the activation of cell signaling pathways and production of inflammatory mediators. Nucleic acids including DNA, RNA, and their derivatives are potent PAMPs which can be recognized by multiple PRRs to induce inflammatory responses. While nucleic acid sensors can also sense endogenous nucleic acids, they are capable of discriminating self from non-self. However, defects in nucleic acid sensing PRRs or dysregulation of nucleic acid sensing signaling pathways may cause excessive activation of the immune system resulting in the development of inflammatory and autoimmune diseases. This review will discuss the major pathways for sensing intracellular nucleic acids and how defects in these nucleic acid sensing are associated with different kinds of autoimmune and inflammatory disorders.

RIG-I Signaling via MAVS Is Dispensable for Survival in Lethal Influenza Infection In Vivo

Retinoic acid-inducible gene I (RIG-I) is an important regulator of virus-induced antiviral interferons (IFNs) and proinflammatory cytokines. It requires interaction with an adaptor molecule, mitochondrial antiviral-signaling protein (MAVS), to activate downstream signaling pathways. To elucidate the mechanism(s) by which RIG-I-dependent recognition of IAV infection in vivo triggers innate immune responses, we infected mutant mice lacking RIG-I or MAVS with influenza A virus (IAV) and measured their innate immune responses. As has previously been demonstrated with isolated deletion of the virus recognition receptors TLR3, TLR7, and NOD2, RIG-I or MAVS knockout (KO) did not result in higher mortality and did not reduce IAV-induced cytokine responses in mice. Infected RIG-I KO animals displayed similar lung inflammation profiles as did WT mice, in terms of the protein concentration, total cell count, and inflammatory cell composition in the bronchoalveolar lavage fluid. RNA-Seq results demonstrated that all types of mice exhibited equivalent antiviral and inflammatory gene responses following IAV infection. Together, the results indicated that although RIG-I is important in innate cytokine responses in vitro, individual deletion of the genes encoding RIG-I or MAVS did not change survival or innate responses in vivo after IAV infection in mice.

RIG-I is responsible for activation of type I interferon pathway in Seneca Valley virus-infected porcine cells to suppress viral replication

Background

Retinoic acid-inducible gene I (RIG-I) is a key cytosolic receptor of the innate immune system. Seneca valley virus (SVV) is a newly emerging RNA virus that infects pigs causing significant economic losses in pig industry. RIG-I plays different roles during different viruses infections. The role of RIG-I in SVV-infected cells remains unknown. Understanding of the role of RIG-I during SVV infection will help to clarify the infection process of SVV in the infected cells.

Methods

In this study, we generated a RIG-I knockout (KO) porcine kidney PK-15 cell line using the Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)/CRISPR-associated protein-9 nuclease (Cas9) genome editing tool. The RIG-I gene sequence of RIG-I KO cells were determined by Sanger sequencing method, and the expression of RIG-I protein in the RIG-I KO cells were detected by Western bloting. The activation status of type I interferon pathway in Sendai virus (SeV)- or SVV-infected RIG-I KO cells was investigated by measuring the mRNA expression levels of interferon (IFN)-β and IFN-stimulated genes (ISGs). The replicative state of SVV in the RIG-I KO cells was evaluated by qPCR, Western bloting, TCID₅₀ assay and indirect immunofluorescence assay.

Results

Gene editing of RIG-I in PK-15 cells successfully resulted in the destruction of RIG-I expression. RIG-I KO PK-15 cells had a lower expression of IFN-β and ISGs compared with wildtype (WT) PK-15 cells when stimulated by the model RNA virus SeV. The amounts of viral RNA and viral protein as well as viral yields in SVV-infected RIG-I WT and KO cells were determined and compared, which showed that knockout of RIG-I significantly increased SVV replication and propagation. Meanwhile, the expression of IFN-β and ISGs were considerably decreased in RIG-I KO cells compared with that in RIG-I WT cells during SVV infection.

Conclusion

Altogether, this study indicated that RIG-I showed an antiviral role against SVV and was essential for activation of type I IFN signaling during SVV infection. In addition, this study suggested that the CRISPR/Cas9 system can be used as an effective tool to modify cell lines to increase viral yields during SVV vaccine development.

The Role of Nucleic Acid Sensing in Controlling Microbial and Autoimmune Disorders

Innate immunity, the first line of defense against invading pathogens, is an ancient form of host defense found in all animals, from sponges to humans. During infection, innate immune receptors recognize conserved molecular patterns, such as microbial surface molecules, metabolites produces during infection, or nucleic acids of the microbe's genome. When initiated, the innate immune response activates a host defense program that leads to the synthesis proteins capable of pathogen killing. In mammals, the induction of cytokines during the innate immune response leads to the recruitment of professional immune cells to the site of infection, leading to an adaptive immune response. While a fully functional innate immune response is crucial for a proper host response and curbing microbial infection, if the innate immune response is dysfunctional and is activated in the absence of infection, autoinflammation and autoimmune disorders can develop. Therefore, it follows that the innate immune response must be tightly controlled to avoid an autoimmune response from host-derived molecules, yet still unencumbered to respond to infection. In this review, we will focus on the innate immune response activated from cytosolic nucleic acids, derived from the microbe or host itself. We will depict how viruses and bacteria activate these nucleic acid sensing pathways and their mechanisms to inhibit the pathways. We will also describe the autoinflammatory and autoimmune disorders that develop when these pathways are hyperactive. Finally, we will discuss gaps in knowledge with regard to innate immune response failure and identify where further research is needed.

Matrix-binding checkpoint immunotherapies enhance antitumor efficacy and reduce adverse events

Immune checkpoint blockade exhibits considerable antitumor activity, but previous studies have reported instances of severe treatment-related adverse events. We sought to explore local immune checkpoint blockade, with an antibody (Ab) form that would be retained intra- or peritumorally, limiting systemic exposure. To accomplish this, we conjugated the checkpoint blockade Abs to an extracellular matrix (ECM)-super-affinity peptide derived from placenta growth factor-2 (PlGF-2_123-144). We show enhanced tissue retention and lower Ab concentrations in blood plasma after PlGF-2_123-144 conjugation, reducing systemic side effects such as the risk of autoimmune diabetes. Peritumoral injections of PlGF-2_123-144-anti-CTLA4 (cytotoxic T lymphocyte antigen 4) and PlGF-2_123-144-anti-PD-L1 (programmed death ligand 1) Abs delayed tumor growth and prolonged survival compared to the unmodified Abs in genetically engineered murine tumor models of melanoma and breast cancer. The PlGF-2_123-144-Abs increased tumor-infiltrating activated CD8⁺ and CD4⁺ T cells, resulting in a delay of distant tumor growth as well. This simple and translatable approach of engineered ECM-binding Abs may present a viable and safer approach in checkpoint blockade.

Blocking IFNAR1 inhibits multiple myeloma-driven Treg expansion and immunosuppression

Despite significant advances in the treatment of multiple myeloma (MM), most patients succumb to disease progression. One of the major immunosuppressive mechanisms that is believed to play a role in myeloma progression is the expansion of regulatory T cells (Tregs). In this study, we demonstrate that myeloma cells drive Treg expansion and activation by secreting type 1 interferon (IFN). Blocking IFN α and β receptor 1 (IFNAR1) on Tregs significantly decreases both myeloma-associated Treg immunosuppressive function and myeloma progression. Using syngeneic transplantable murine myeloma models and bone marrow (BM) aspirates of MM patients, we found that Tregs were expanded and activated in the BM microenvironment at early stages of myeloma development. Selective depletion of Tregs led to a complete remission and prolonged survival in mice injected with myeloma cells. Further analysis of the interaction between myeloma cells and Tregs using gene sequencing and enrichment analysis uncovered a feedback loop, wherein myeloma-cell-secreted type 1 IFN induced proliferation and expansion of Tregs. By using IFNAR1-blocking antibody treatment and IFNAR1-knockout Tregs, we demonstrated a significant decrease in myeloma-associated Treg proliferation, which was associated with longer survival of myeloma-injected mice. Our results thus suggest that blocking type 1 IFN signaling represents a potential strategy to target immunosuppressive Treg function in MM.

Enteric Virome Sensing-Its Role in Intestinal Homeostasis and Immunity

Pattern recognition receptors (PRRs) sensing commensal microorganisms in the intestine induce tightly controlled tonic signaling in the intestinal mucosa, which is required to maintain intestinal barrier integrity and immune homeostasis. At the same time, PRR signaling pathways rapidly trigger the innate immune defense against invasive pathogens in the intestine. Intestinal epithelial cells and mononuclear phagocytes in the intestine and the gut-associated lymphoid tissues are critically involved in sensing components of the microbiome and regulating immune responses in the intestine to sustain immune tolerance against harmless antigens and to prevent inflammation. These processes have been mostly investigated in the context of the bacterial components of the microbiome so far. The impact of viruses residing in the intestine and the virus sensors, which are activated by these enteric viruses, on intestinal homeostasis and inflammation is just beginning to be unraveled. In this review, we will summarize recent findings indicating an important role of the enteric virome for intestinal homeostasis as well as pathology when the immune system fails to control the enteric virome. We will provide an overview of the virus sensors and signaling pathways, operative in the intestine and the mononuclear phagocyte subsets, which can sense viruses and shape the intestinal immune response. We will discuss how these might interact with resident enteric viruses directly or in context with the bacterial microbiome to affect intestinal homeostasis.

Bacterial Signaling at the Intestinal Epithelial Interface in Inflammation and Cancer

The gastrointestinal (GI) tract provides a compartmentalized interface with an enormous repertoire of immune and metabolic activities, where the multicellular structure of the mucosa has acquired mechanisms to sense luminal factors, such as nutrients, microbes, and a variety of host-derived and microbial metabolites. The GI tract is colonized by a complex ecosystem of microorganisms, which have developed a highly coevolved relationship with the host’s cellular and immune system. Intestinal epithelial pattern recognition receptors (PRRs) substantially contribute to tissue homeostasis and immune surveillance. The role of bacteria-derived signals in intestinal epithelial homeostasis and repair has been addressed in mouse models deficient in PRRs and signaling adaptors. While critical for host physiology and the fortification of barrier function, the intestinal microbiota poses a considerable health challenge. Accumulating evidence indicates that dysbiosis is associated with the pathogenesis of numerous GI tract diseases, including inflammatory bowel diseases (IBD) and colorectal cancer (CRC). Aberrant signal integration at the epithelial cell level contributes to such diseases. An increased understanding of bacterial-specific structure recognition and signaling mechanisms at the intestinal epithelial interface is of great importance in the translation to future treatment strategies. In this review, we summarize the growing understanding of the regulation and function of the intestinal epithelial barrier, and discuss microbial signaling in the dynamic host–microbe mutualism in both health and disease.

Nucleic acid sensing pattern recognition receptors in the development of colorectal cancer and colitis

Colorectal cancer (CRC) is a leading cause of cancer-related deaths that is often associated with inflammation initiated by activation of pattern recognition receptors (PRRs). Nucleic acid sensing PRRs are one of the major subsets of PRRs that sense nucleic acid (DNA and RNA), mainly including some members of Toll-like receptors (TLR3, 7, 8, 9), AIM2-like receptors (AIM2, IFI16), STING, cGAS, RNA polymerase III, and DExD/H box nucleic acid helicases (such as RIG-I like receptors (RIG-I, MDA5, LPG2), DDX1, 3, 5, 7, 17, 21, 41, 60, and DHX9, 36). Activation of these receptors eventually leads to the release of cytokines and activation of immune cells, which are well known to play crucial roles in host defense against intracellular bacterial and virus infection. However, the functions of these nucleic acid sensing PRRs in the other diseases such as CRC and colitis remain largely unknown. Recent studies indicated that nucleic acid sensing PRRs contribute to CRC and/or colitis development, and therapeutic modulation of nucleic acid sensing PRRs may reduce the risk of CRC development. However, until now, a comprehensive review on the role of nucleic acid sensing PRRs in CRC and colitis is still lacking. This review provided an overview of the roles as well as the mechanisms of these nucleic acid sensing PRRs (AIM2, STING, cGAS, RIG-I and its downstream molecules, DDX3, 5, 6,17, and DHX9, 36) in CRC and colitis, which may aid the diagnosis, therapy, and prognostic prediction of CRC and colitis.

Viral RNA-Unprimed Rig-I Restrains Stat3 Activation in the Modulation of Regulatory T Cell/Th17 Cell Balance

Innate immunity activation by viral RNA-primed retinoid acid inducible gene-I (Rig-I) in CD4⁺ T cells antagonizes TGFβ signaling to suppress the differentiation of regulatory T cells (Tregs). However, how viral RNA-unliganded Rig-I (apo-Rig-I) modulates Treg generation remains unclear. In this article, we show that, in the absence of viral infection, Treg differentiation of Rig-I^-/- CD4⁺ T cells was compromised, in the presence of increased generation of Th17 cells and overactivation of Stat3, a critical regulator tilting the Treg/Th17 cell balance. Mechanistically, apo-Rig-I physically associates with Stat3, thereby inhibiting Jak1's association with Stat3 while facilitating Shp2's association to inhibit p-Stat3 levels. Interestingly, inhibition of Stat3 ameliorates the Treg/Th17 imbalance and the colitis observed in Rig-I^-/- mice. Collectively, these results uncover an independent functional contribution of the apo-Rig-I/Stat3 interaction in the maintenance of Treg/Th17 cell balance.

RIG-I antiviral signaling drives interleukin-23 production and psoriasis-like skin disease

Retinoic acid inducible-gene I (RIG-I) functions as one of the major sensors of RNA viruses. DDX58, which encodes the RIG-I protein, has been newly identified as a susceptibility gene in psoriasis. Here, we show that the activation of RIG-I by 5'ppp-dsRNA, its synthetic ligand, directly causes the production of IL-23 and triggers psoriasis-like skin disease in mice. Repeated injections of IL-23 to the ears failed to induce IL-23 production and a full psoriasis-like skin phenotype, in either germ-free or RIG-I-deficient mice. RIG-I is also critical for a full development of skin inflammation in imiquimod (IMQ)-induced psoriasis-like mouse model. Furthermore, RIG-I-mediated endogenous IL-23 production was mainly confined to the CD11c+ dendritic cells (DCs) via nuclear factor-kappa B (NF-κB) signaling, and stimulated RIG-I expression in an auto-regulatory feedback loop. Thus, our data suggest that the dysregulation in the antiviral immune responses of hosts through the innate pattern recognition receptors may trigger the skin inflammatory conditions in the pathophysiology of psoriasis.

RIG-I/MAVS and STING signaling promote gut integrity during irradiation- and immune-mediated tissue injury

The molecular pathways that regulate the tissue repair function of type I interferon (IFN-I) during acute tissue damage are poorly understood. We describe a protective role for IFN-I and the RIG-I/MAVS signaling pathway during acute tissue damage in mice. Mice lacking mitochondrial antiviral-signaling protein (MAVS) were more sensitive to total body irradiation- and chemotherapy-induced intestinal barrier damage. These mice developed worse graft-versus-host disease (GVHD) in a preclinical model of allogeneic hematopoietic stem cell transplantation (allo-HSCT) than did wild-type mice. This phenotype was not associated with changes in the intestinal microbiota but was associated with reduced gut epithelial integrity. Conversely, targeted activation of the RIG-I pathway during tissue injury promoted gut barrier integrity and reduced GVHD. Recombinant IFN-I or IFN-I expression induced by RIG-I promoted growth of intestinal organoids in vitro and production of the antimicrobial peptide regenerating islet-derived protein 3 γ (RegIIIγ). Our findings were not confined to RIG-I/MAVS signaling because targeted engagement of the STING (stimulator of interferon genes) pathway also protected gut barrier function and reduced GVHD. Consistent with this, STING-deficient mice suffered worse GVHD after allo-HSCT than did wild-type mice. Overall, our data suggest that activation of either RIG-I/MAVS or STING pathways during acute intestinal tissue injury in mice resulted in IFN-I signaling that maintained gut epithelial barrier integrity and reduced GVHD severity. Targeting these pathways may help to prevent acute intestinal injury and GVHD during allogeneic transplantation.

RNA virus receptor Rig-I monitors gut microbiota and inhibits colitis-associated colorectal cancer

Background

Retinoic acid-inducible gene-I (Rig-I) is an intracellular viral RNA receptor, which specifically recognizes double-stranded viral RNA initiating antiviral innate immunity. Increasing evidences showed that Rig-I had broader roles in antibacterial immunity and cancer protection. However, the potential roles and mechanisms of Rig-I in gut flora regulation and colorectal cancer (CRC) progression remain unclear.

Methods

Immunohistochemistry was performed to detect Rig-I protein in 38 pairs of CRC tissue and matched adjacent mucosa, and immunofluorescence and western blot were also used to detect Rig-I protein expression in AOM/DSS-induced mice CRC samples. High-throughput sequencing was conducted to evaluate gut microbiota changes in Rig-I-deficient mice. Immunofluorescence and flow cytometry were used to detect IgA expression. Additionally, real-time quantitative PCR was performed to detect RNA expression in mouse intestines and cultured cells, and western blot was used to detect phosphorylation of STAT3 in IL-6-stimulated B cell line.

Results

Rig-I was downregulated in human and mouse CRC samples and Rig-I-deficient mice were more susceptible to AOM/DSS-induced colitis-associated colorectal cancer (CAC). Furthermore, Rig-I-deficient mice displayed gut microbiota disturbance compared to wild type mice. IgA, Reg3γ and Pdcd1 levels were decreased in intestines of Rig-I-deficient mice. Phosphorylation of STAT3 in IL-6-stimulated 1B4B6 was decreased.

Conclusion

Rig-I could regulate gut microbiota through regulating IgA and IL6-STAT3-dependent Reg3γ expression. Besides, Rig-I could inhibit CRC progression.

Electronic supplementary material

The online version of this article (doi:10.1186/s13046-016-0471-3) contains supplementary material, which is available to authorized users.

Combined Parthenolide and Balsalazide Have Enhanced Antitumor Efficacy Through Blockade of NF-κB Activation

Balsalazide is a colon-specific prodrug of 5-aminosalicylate that is associated with a reduced risk of colon cancer in patients with ulcerative colitis. Parthenolide, a strong NF-κB inhibitor, has recently been demonstrated to be a promising therapeutic agent, promoting apoptosis of cancer cells. In the current study, the antitumor effect of balsalazide combined with parthenolide in human colorectal cancer cells and colitis-associated colon cancers (CAC) was investigated. The results demonstrate that the combination of balsalazide and parthenolide markedly suppress proliferation, nuclear translocation of NF-κB, IκB-α phosphorylation, NF-κB DNA binding, and expression of NF-κB targets. Apoptosis via NF-κB signaling was confirmed by detecting expression of caspases, p53 and PARP. Moreover, treatment of a CAC murine model with parthenolide and balsalazide together resulted in significant recovery of body weight and improvement in histologic severity. Administration of parthenolide and balsalazide to CAC mice also suppressed carcinogenesis as demonstrated by uptake of 18F-fluoro-2-deoxy-D-glucose (FDG) using micro-PET/CT scans. These results demonstrate that parthenolide potentiates the efficacy of balsalazide through synergistic inhibition of NF-κB activation and the combination of dual agents prevents colon carcinogenesis from chronic inflammation.

IMPLICATIONS

This study represents the first evidence that combination therapy with balsalazide and parthenolide could be a new regimen for colorectal cancer treatment. Mol Cancer Res; 15(2); 141-51. ©2016 AACR.

Autophagy regulates MAVS signaling activation in a phosphorylation-dependent manner in microglia

Mitochondrial antiviral signaling (MAVS) protein has an important role in antiviral immunity and autoimmunity. However, the pathophysiological role of this signaling pathway, especially in the brain, remains elusive. Here we demonstrated that MAVS signaling existed and mediated poly(I:C)-induced inflammation in the brain. Along with the MAVS signaling activation, there was an induction of autophagic activation. Autophagy negatively regulated the activity of MAVS through direct binding of LC3 to the LIR motif Y(9)xxI(12) of MAVS. We also found that c-Abl kinase phosphorylated MAVS and regulated its interaction with LC3. Interestingly, tyrosine phosphorylation of MAVS was required for downstream signaling activation. Importantly, in vivo data showed that the deficiency of MAVS or c-Abl prevented MPTP-induced microglial activation and dopaminergic neuron loss. Together, our findings reveal the molecular mechanisms underlying the regulation of MAVS-dependent microglial activation in the nervous system, thus providing a potential target for the treatment of microglia-driven inflammatory brain diseases.

Discriminating self from non-self in nucleic acid sensing

Immune sensing of foreign nucleic acids among abundant self nucleic acids is a hallmark of virus detection and antiviral defence.

Efficient antiviral defence requires a balanced process of sensing foreign nucleic acids and ignoring self nucleic acids. This balance is accomplished by a multilevel, fail-safe system which combines immune sensing of pathogen-specific nucleic acid structures with specific labelling of self nucleic acids and nuclease-mediated degradation.

Cellular localization of nucleic acids, nucleic acid secondary structure, nucleic acid sequence and chemical modification all contribute to selective recognition of foreign nucleic acids.

Nucleic acid sensing occurs in immune cells and non-immune cells and results in antiviral responses that include the induction of antiviral effector proteins, the secretion of cytokines alarming neighbouring cells, the secretion of chemokines, which attract immune cells, and the induction of cell death.

Vertebrate cells cannot completely avoid the occurrence of endogenous self nucleic acid structures with immunostimulatory properties. Therefore, additional mechanisms involving self-nucleic acid modification and nuclease-mediated degradation are necessary to diminish uncontrolled immune activation.

Viruses have established sophisticated mechanisms to exploit and adopt endogenous tolerance mechanisms or to avoid the presentation of characteristic molecular features recognized by nucleic acid sensing receptors.

The detection of viruses by the immune system is mediated predominantly by the sensing of nucleic acids. Here, the authors review our current understanding of how this complex immune sensory system discriminates self from non-self nucleic acids to reliably detect pathogenic viruses, and discuss the future perspectives and implications for human disease.

Innate immunity against pathogens relies on an array of immune receptors to detect molecular patterns that are characteristic of the pathogens, including receptors that are specialized in the detection of foreign nucleic acids. In vertebrates, nucleic acid sensing is the dominant antiviral defence pathway. Stimulation of nucleic acid receptors results in antiviral immune responses with the production of type I interferon (IFN), as well as the expression of IFN-stimulated genes, which encode molecules such as cell-autonomous antiviral effector proteins. This Review summarizes the tremendous progress that has been made in understanding how this sophisticated immune sensory system discriminates self from non-self nucleic acids in order to reliably detect pathogenic viruses.

Nucleic acid sensing and innate immunity: signaling pathways controlling viral pathogenesis and autoimmunity

Innate immunity refers to the body's initial response to curb infection upon exposure to invading organisms. While the detection of pathogen-associated molecules is an ancient form of host defense, if dysfunctional, autoimmune disease may result. The innate immune response during pathogenic infection is initiated through the activation of receptors recognizing conserved molecular patterns, such as nucleic acids from a virus' genome or replicative cycle. Additionally, the host's own nucleic acids are capable of activating an immune response. Therefore, it follows that the nucleic acid-sensing pathways must be tightly controlled to avoid an autoimmune response from recognition of self, yet still be unimpeded to respond to viral infections. In this review, we will describe the nucleic acid sensing pathways and how they respond to virus infection. Moreover, we will discuss autoimmune diseases that develop when these pathways fail to signal properly and identify knowledge gaps that are prime for interrogation.

Prolonged Intake of Dietary Lipids Alters Membrane Structure and T Cell Responses in LDLr-/- Mice

Although it is recognized that lipids and membrane organization in T cells affect signaling and T cell activation, to what extent dietary lipids alter T cell responsiveness in the absence of obesity and inflammation is not known. In this study, we fed low-density lipoprotein receptor knockout mice a Western high-fat diet for 1 or 9 wk and examined T cell responses in vivo along with T cell lipid composition, membrane order, and activation ex vivo. Our data showed that high levels of circulating lipids for a prolonged period elevated CD4(+) and CD8(+) T cell proliferation and resulted in an increased proportion of CD4(+) central-memory T cells within the draining lymph nodes following induction of contact hypersensitivity. In addition, the 9-wk Western high-fat diet elevated the total phospholipid content and monounsaturated fatty acid level, but decreased saturated phosphatidylcholine and sphingomyelin within the T cells. The altered lipid composition in the circulation, and of T cells, was also reflected by enhanced membrane order at the activation site of ex vivo activated T cells that corresponded to increased IL-2 mRNA levels. In conclusion, dietary lipids can modulate T cell lipid composition and responses in lipoprotein receptor knockout mice even in the absence of excess weight gain and a proinflammatory environment.

Cardif (MAVS) Regulates the Maturation of NK Cells

Cardif, also known as IPS-1, VISA, and MAVS, is an intracellular adaptor protein that functions downstream of the retinoic acid-inducible gene I family of pattern recognition receptors. Cardif is required for the production of type I IFNs and other inflammatory cytokines after retinoic acid-inducible gene I-like receptors recognize intracellular antigenic RNA. Studies have recently shown that Cardif may have other roles in the immune system in addition to its role in viral immunity. In this study, we find that the absence of Cardif alters normal NK cell development and maturation. Cardif(-/-) mice have a 35% loss of mature CD27(-)CD11b(+) NK cells in the periphery. In addition, Cardif(-/-) NK cells have altered surface marker expression, lower cytotoxicity, decreased intracellular STAT1 levels, increased apoptosis, and decreased proliferation compared with wild-type NK cells. Mixed chimeric mice revealed that the defective maturation and increased apoptotic rate of peripheral Cardif(-/-) NK cells is cell intrinsic. However, Cardif(-/-) mice showed enhanced control of mouse CMV (a DNA β-herpesvirus) by NK cells, commensurate with increased activation and IFN-γ production by these immature NK cell subsets. These results indicate that the skewed differentiation and altered STAT expression of Cardif(-/-) NK cells can result in their hyperresponsiveness in some settings and support recent findings that Cardif-dependent signaling can regulate aspects of immune cell development and/or function distinct from its well-characterized role in mediating cell-intrinsic defense to RNA viruses.

Duck RIG-I CARD Domain Induces the Chicken IFN-β by Activating NF-κB

Retinoic acid-inducible gene I- (RIG-I-) like receptors (RLRs) have recently been identified as cytoplasmic sensors for viral RNA. RIG-I, a member of RLRs family, plays an important role in innate immunity. Although previous investigations have proved that RIG-I is absent in chickens, it remains largely unknown whether the chicken can respond to RIG-I ligand. In this study, the eukaryotic expression vectors encoding duRIG-I full length (duck RIG-I, containing all domains), duRIG-I N-terminal (containing the two caspase activation and recruitment domain, CARDs), and duRIG-I C-terminal (containing helicase and regulatory domains) labeled with 6∗His tags were constructed successfully and detected by western blotting. Luciferase reporter assay and enzyme-linked immunosorbent assay (ELISA) detected the duRIG-I significantly activated NF-κB and induced the expression of IFN-β when polyinosinic-polycytidylic acid (poly[I:C], synthetic double-stranded RNA) challenges chicken embryonic fibroblasts cells (DF1 cells), while the duRIG-I was inactive in the absence of poly[I:C]. Further analysis revealed that the CARDs (duRIG-I-N) induced IFN-β production regardless of the presence of poly[I:C], while the CARD-lacking duRIG-I (duRIG-I-C) was not capable of activating downstream signals. These results indicate that duRIG-I CARD domain plays an important role in the induction of IFN-β and provide a basis for further studying the function of RIG-I in avian innate immunity.

Expression of Blimp-1 in dendritic cells modulates the innate inflammatory response in dextran sodium sulfate-induced colitis

A single nucleotide polymorphism of PRDM1, the gene encoding Blimp-1, is strongly associated with inflammatory bowel disease. Here, we demonstrate that Blimp-1 in CD103(+) dendritic cells (DCs) critically contributes to the regulation of macrophage homeostasis in the colon. Dextran sodium sulfate (DSS)-exposed Blimp-1(cko) mice with a deletion of Blimp-1 in CD103(+) DCs and CD11c(hi) macrophages exhibited severe inflammatory symptoms, pronounced weight loss, high mortality, robust infiltration of neutrophils in epithelial regions of the colon, an increased expression of proinflammatory cytokines and a significant decrease in CD103(+) DCs in the colon compared with DSS exposed wild-type (WT) mice. Purified colonic macrophages from Blimp-1(cko) mice expressed increased levels of matrix metalloproteinase 8, 9 and 12 mRNA. WT macrophages cocultured with colonic DCs but not bone marrow-derived DCs from Blimp-1(cko) produced increased matrix metalloproteinases in an interleukin (IL)-1β- and IL-6-dependent manner. Treatment of Blimp-1(cko) mice with anti-IL-1β and anti-IL-6 abrogated the exaggerated clinical response. Overall, these data demonstrate that Blimp-1 expression in DCs can alter an innate inflammatory response by modulating the activation of myeloid cells. This is a novel mechanism of contribution of Blimp-1 for the pathogenesis of inflammatory bowel diseases, implicating another therapeutic target for the development of inflammatory bowel disease.

Parthenolide induces apoptosis in colitis-associated colon cancer, inhibiting NF-κB signaling

Recently, the nuclear factor (NF)-κB inhibitor parthenolide (PT) was identified as a promising anticancer agent for the promotion of cancer cell apoptosis. Additionally, our previous study demonstrated that PT administration suppresses tumor growth in a xenograft model of colorectal cancer cells via regulation of the B-cell lymphoma-2 (Bcl-2) family. However, the role of PT in the development of colitis-associated colon cancer (CAC) is poorly understood. Therefore, the aim of the present study was to investigate the effects of PT administration on CAC using a murine model. Azoxymethane (AOM) and dextran sulfate sodium (DSS) were administered to induce experimental CAC in the following three groups of treated mice: i) AOM and DSS plus vehicle; ii) AOM, DSS and 2 mg/kg PT; and iii) AOM, DSS and 4 mg/kg PT. It was demonstrated that the histological acuteness of AOM/DSS-induced CAC was significantly reduced following the administration of PT, resulting in decreased NF-κB p65 expression levels via a blockade of phosphorylation and subsequent degradation of inhibitor of κB-α (IκBα). Furthermore, PT administration appeared to enhance the process of carcinogenesis via the downregulation of the antiapoptotic proteins Bcl-2 and Bcl-extra large, mediated by inhibition of NF-κB activation. Apoptosis and caspase-3 expression were markedly increased in the PT-treated group. These findings indicate that PT inhibits IκBα phosphorylation and NF-κB activation, resulting in the initiation of apoptosis and the eventual suppression of CAC development. The beneficial effects of PT treatment observed in the experimental CAC model indicate the potential chemopreventive and therapeutic role of PT in CAC.