Aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that mediates the toxic activity of many environmental xenobiotics. However, its role in innate immune responses during viral infection is not fully understood. Here we demonstrate that constitutive AHR signaling negatively regulates the type I interferon (IFN-I) response during infection with various types of virus. Virus-induced IFN-β production was enhanced in AHR- IFN-I-mediated innate response and, further, suggests that the AHR-TIPARP axis is a potential therapeutic target for enhancing antiviral responses.AHR was originally discovered as a xenobiotic sensor that mediates the toxicity of the persistent environmental contaminant, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), more commonly known as dioxin [1][2][3][4] . Activation of AHR induces its target genes, including those encoding cytochrome P4501A1, cytochrome P4501B1, AHR repressor, TCDD-inducible poly(ADP-ribose)polymerase (TIPARP) and aldehyde dehydrogenase 1A3 (refs. 1,2,5-9), which are involved in the adaptive metabolism of xenobiotic compounds. This property of AHR has been implicated in host defense against bacterial infection, as certain bacterial pigmented virulence factors are AHR agonists that are subsequently metabolized by AHR-regulated drug-metabolizing enzymes 10 . Studies of AHR-deficient mice have identified important physiological roles for AHR in response to endogenous ligands in cell cycle regulation, cell differentiation and immune responses 8,[11][12][13][14] . In relation to this, several putative endogenous ligands for the AHR have also been reported, including heme metabolites, arachidonic acids or leukotrienes and tryptophan metabolites, such as 6-formylindolo(3,2-b)carbazole (FICZ) and kynurenine (Kyn) 2,8,15 .There has been increased interest in understanding the role of AHR in immunity.Several reports, most of which are based mainly on experiments with dioxin treatment, have shown that the AHR is involved in the differentiation and/or function of T cells, macrophages and dendritic cells 7,9,11,[16][17][18][19][20][21] . AHR has been implicated in the control of acute graft-versus-host disease and autoimmunity 11,12,21 . Dioxin-activated AHR also reduces the survival rate of mice infected with influenza A virus 22,23 and indirectly suppresses the proliferation and differentiation of virus-specific CD8 + T cells via its regulatory role in dendritic cells 24 . FICZ and dioxin diminish CD8 + T cell responsiveness, whereas dioxin, but not FICZ, affects neutrophil recruitment or pulmonary inducible nitric oxide synthase (iNOS) induction in response to influenza virus infection 25 .Tryptophan metabolites such as Kyn are upregulated during inflammation and/or tumor progression in several types of immune and tumor cells through the catalytic activity of tryptophan dioxygenase (TDO) and indoleamine 2,3-dioxygenase (IDO), which catalyze the first step in the formation of Kyn from tryptophan 2,9 . This increase in Kyn leads to an increase in regulatory T c...

Japan. 8 These authors contributed equally to this work. Correspondence should be addressed to A.T. (takaoka@igm.hokudai.ac.jp). At least three DExD/H-box RNA helicases 4,5 , RIG-I, MDA5 and LGP2 are included in the RLR family. RIG-I is a key PRR for the detection of positive-and negative-stranded RNA viruses in the cytoplasm of cells 6,7 , and plays an important role in triggering responses against many viruses, such as orthomyxovirus (influenza A virus) and paramyxovirus (measles, mumps, and Sendai virus (SeV)) families, hepatitis C virus (HCV), and Japanese encephalitis virus (JEV) 8 . 5'-triphosphate modifications of RNA (3pRNA) are essential for RIG-I recognition and activation 9,10 . Ligand-binding activates the ATPase activity of RIG-I to change its structural conformation 6 , which in turn enables RIG-I to interact through its N-terminal tandem caspase recruitment domain (CARD) with the adaptor protein MAVS (for mitochondrial antiviral signaling protein, also known as IPS-1, VISA, or Cardif) [11][12][13][14] . MAVS then initiates the activation of interferon (IFN)-regulatory factor (IRF) 3, IRF7 and NF-κB transcriptional pathways for the subsequent production of type I IFNs and inflammatory cytokines, which are crucial for activating innate immune responses to viral infection 6,15 . Given the important role of the RIG-I pathway in the antiviral innate response, the mechanisms regulating RIG-I activation represent a topic of intense research [16][17][18] .Poly(ADP-ribose) polymerases (PARPs), a superfamily with least 17 members, are known to regulate not only cell survival and cell death programs triggered by DNA 3 damage, but also other biological functions as well as pathological processes, such as inflammatory and degenerative diseases, in a manner dependent or independent of their PARP activity [19][20][21][22] . Several PARP-superfamily members have a direct regulatory effect on replication of certain viruses 19,20,22,23 RESULTS 4PARPs contribute to the IFN response To investigate the role of the PARP-superfamily members in nucleic acid induced innate immune responses, we selected some of the PARP-superfamily members known to be involved in microbial infection, inflammation and immunity: PARP-1, PARP-2, PARP-7, PARP-9, 23,[31][32][33][34]. We then examined whether they have the ability to enhance the induction of IFNB mRNA in HEK293T cells in response to stimulation with three different types of nucleic acids-3pRNA, poly(rI:rC) and poly(dA-dT)•poly(dA-dT) (named poly(dA:dT) hereafter).Among the protein tested, PARP-13 uniquely showed a marked enhancing effect on IFNB mRNA expression induced by stimulation with 3pRNA, poly(rI:rC) and poly(dA:dT) ( Fig. 1a), all of which are known to activate the RIG-I-mediated pathway in HEK293T cells 9,10,[35][36][37] . A weak increase in IFNB mRNA expression was also detected in cells expressing PARP-1, PARP-2 and PARP-9. PARP-13 exists in at least two isoforms 22,26,38 . The amino-terminal 254-amino acid fragment of the rat homologue, which corresponds to the N...

Efficient immune responses against viral infection are determined by sufficient activation of nucleic acid sensor-mediated innate immunity 1,2 . Coronavirus disease 2019, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), remains an ongoing global pandemic. It is an urgent challenge to clarify the innate recognition mechanism to control this virus. Here we show that retinoic acid-inducible gene-I (RIG-I) sufficiently restrains SARS-CoV-2 replication in human lung cells in a type I/III interferon (IFN)-independent manner. RIG-I recognizes the 3′ untranslated region of the SARS-CoV-2 RNA genome via the helicase domains, but not the C-terminal domain. This new mode of RIG-I recognition does not stimulate its ATPase, thereby aborting the activation of the conventional mitochondrial antiviral-signaling protein-dependent pathways, which is in accordance with lack of cytokine induction. Nevertheless, the interaction of RIG-I with the viral genome directly abrogates viral RNA-dependent RNA polymerase mediation of the first step of replication. Consistently, genetic ablation of RIG-I allows lung cells to produce viral particles that expressed the viral spike protein. By contrast, the anti-SARS-CoV-2 activity was restored by all-trans retinoic acid treatment through upregulation of RIG-I protein expression in primary lung cells derived from patients with chronic obstructive pulmonary disease. Thus, our findings demonstrate the distinctive role of RIG-I as a restraining factor in the early phase of SARS-CoV-2 infection in human lung cells.More than 140 million people around the world have been affected by coronavirus disease 2019 (COVID-19) caused by SARS-CoV-2. COVID-19 varies from mild to severe, life-threatening respiratory infection with coagulopathy. Most (81%) people infected with SARS-CoV-2 show a mild and self-limited course 3 , while severe cases of disease are more likely to be present in older patients with underlying comorbidities, such as chronic obstructive pulmonary disease (COPD) 4-6 , compared to patients with mild disease. But even young healthy adults sometimes experience severe illness. Conversely, it has been reported that nearly 40-45% of patients with SARS-CoV-2 infections are asymptomatic 7 . The wide spectrum of clinical manifestations of COVID-19 suggests that individual immune responses to the underlying pathogen may play some crucial role in determining the clinical course. Currently, no efficient therapies and preventive measures exist for COVID-19, thus studies about host immune response against SARS-CoV-2 infection are required for a better understanding of the pathological processes for the rational development of countermeasures to control SARS-CoV-2 infection. There is also an urgent need to identify biomarkers that can predict which patients will deteriorate.Microbial invasion in our body is sensed by pattern-recognition receptors (PRRs) that are present in most types of cells, which initiate the activation of cell-intrinsic defense and innate immune responses. During ...

Type I and type III interferons are important anti-viral cytokines that are massively induced during viral infection. This dynamic process is regulated by many executors and regulators for efficient eradication of invading viruses and protection from harmful, excessive responses. An array of innate sensors recognizes virus-derived nucleic acids to activate their downstream signaling to evoke cytokine responses including interferons. In particular, a cytoplasmic RNA sensor RIG-I (retinoic acid-inducible gene I) is involved in the detection of multiple types of not only RNA viruses but also DNA viruses. Accumulating findings have revealed that activation of nucleic acid sensors and the related signaling mediators is regulated on the basis of post-translational modification such as ubiquitination, phosphorylation and ADP-ribosylation. In addition, long non-coding RNAs (lncRNAs) have been implicated as a new class of regulators in innate signaling. A comprehensive understanding of the regulatory mechanisms of innate sensor activation and its signaling in host–virus interaction will provide a better therapeutic strategy to efficiently control viral infection and maintain immune homeostasis.