DNA sensor in virus infection 脱氧核糖核酸识别受体与病毒感染.doc

DNA sensor in virus infectionAbstract: In order to protect human from the detrimental effect of viral infection, immune system has evolved mechanisms to recognize and eradicate them. Recently, a novel antiviral tool of innate immune has been discoveredDNA sensors, which can recognize invading viral DNA and initiate innate antiviral responses, including the activation of type I interferons (IFNs) and proinammatory cytokines. These cytokines contributed to not only inhibiting viral replication in infected cells but also regulating the induction of adaptive immunity, leading to the swift eradication of viruses. All these indicate that DNA sensors play a pivotal role in antiviral response. Therefore, this article will provide an integral and comprehensive review of DNA sensor, focusing on sorts of DNA sensor, signaling pathway and the immune response triggered by them. Whats more, viral strategy against DNA sensors and negative regulation of host will be discussed as well.Key words: DNA sensor, virus infection, antiviral response, negative regulation of virus, negative regulation of host.1. IntroductionIn order to protect host from infection of virus, innate immune has developed mechanisms of how to detect virus to trigger immune response. Recently, DNA sensor has been found as a novel tool to detect virus. Toll-like receptor (TLR) 9, retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs) including RIG-I, melanoma differentiation-associated gene-5 (MDA5), DNA-dependent RNA polymerase III (Pol-III), DNA binding protein 1 (ZBP1) or DNA-dependent activator of IFN-regulatory factor (DAI), IFN-inducible p200-protein including p202，absent in melanoma 2 (AIM2)，interferon-inducible protein 16 (IFI16) have been identified as DNA sensor1-5. Whats more, DExDc helicase DDX41, DHX36 and DHX9, Leucine-rich repeat flightless-interacting protein 1 (LRRFIP1), Ku70 were recently found to be novel DNA sensors 6-9. Table 1 shows DNA sensors that have been discovered so far and which kind of DNA they recognize. Table 1 DNA sensors and DNAFamilyDNA sensorDNA be sensedReferenceTLRsTLR9DNA without methylation at CpG motifs10HelicaseDDX41cytosolic DNA6DHX36CpG-A DNA7DHX9CpG-B DNA7RLRsRIG-1cytosolic DNA3, 11MDA5cytosolic DNA3p200-proteinp202cytosolic dsDNA5AIM2cytosolic dsDNA12IFI16cytosolic as well as nuclear dsDNA13OthersKu70cytosolic DNA9Pol-IIIcytosolic poly(dA-dT) DNA2LRRFIP1ds DNA8DAI(ZBP1)Cytosolic dsDNA4DNA sensors were able to induce the production of cytokines such as IFNs, interleukins (ILs) and tumor necrosis factor (TNF) to promote defense response against microbial infection. Of these, types I IFNs played a pivotal role in antiviral immune response. So far, five subtypes of the type I IFN have been identified: IFN-, IFN-, IFN-, IFN-, and IFN- 14. Of these, IFN- and IFN- were not found in humans. Four main effector pathways of the IFN-mediated antiviral response:the Mx GTPase pathway, the 2,5-oligoadenylate-synthetase-directed ribonuclease L pathway, the protein kinase R pathway,the ISG15 ubiquitin-like pathway, which individually block viral transcription, degrade viral RNA, inhibit translation and modify protein function to control all steps of viral replication15. They were also positively linked to the activation and expansion of lymphocytes that were important for controlling intracellular infections 16. Type III IFNs, including IFN-1, functionally resembled type I IFNs and played an important role in antiviral response as well 17. But the type I and type III IFN antiviral systems did not merely duplicate each other for different antiviral potency, pattern of their induction and differential tissue expression of their corresponding receptor subunits 17. ILs induced by DNA sensor included IL-1, IL-6, IL-12 18, 19. IL-1 contributed to host defense against infection by augmenting antimicrobial properties of phagocytes such as monocytes, macrophages, and neutrophils and initiating Th1 and Th17 adaptive immune responses 20. IL-6 not only was characterized as a B cell growth factor and inducer of antibody production, but also induced neutrophil apoptosis and substantially contributed to the resolution of acute neutrophil infiltration and the CD4 T cell differentiation21, 22, suggesting that IL-6 played a pivotal role during the transition from innate to acquired immunity. IL-12 was a key cytokine in the development of Th1 cell polarization and has been shown to have potent immunomodulatory, antitumor, and anti-infection activity in vitro and in vivo 23. Finally, TNF played an important role in the defense against virus, which subverted the electron transport system or the mitochondria into production of oxygen radicals and killed the malignant cells that did not contain or produce protective enzymes 24. Thus how DNA sensors trigger all these cytokines will be discussed in this review. Whats more, this review will summarize DNA sensors that have been discovered and their interaction with virus. Besides these, how virus counteract and how host negatively regulate the effect of DNA sensors will be also considered in this article. 2. DNA sensors2.1. TLR 9 TLRs were one kind of pattern recognition receptor, which plays a critical role in innate immune. Among them, TLR9 has been identified as a DNA sensor. As the receptor for CpG containing bacterial and viral DNA, TLR9 was retained in the endoplasmic reticulum (ER) 25-27. It was found by Brinkmann MM et al that the interaction between the ER membrane protein UNC93B and TLR 9 was crucial for TLR9 signaling 28. Whats more, it has been reported that intracellular localization of TLR 9 prevented recognition of self DNA but facilitated access to viral DNA 29.2.1.1 DNA structureTLR 9 recognized viral or bacterial DNA without methylation at CpG motifs 10. In wild type (WT) mice, CpG- oligodeoxynucleotide (ODN) induced a strong activation of pulmonary NF-B as well as a significant increase in pulmonary TNF- and IL-1 mRNA/protein while the CpG-ODN-induced inflammatory response was abolished in TLR9- deficient mice30. In the study by Haas, T et al, the DNA sugar backbone homopolymeric, base-free phosphodiester(PD)2 deoxyribose acted as a basal TLR9 agonist as it bound to and activated TLR9, indicating that the PD 2 deoxyribose backbone as an important determinant of TLR9 activation by natural DNA31. CpG DNA moved into early endosomes and was subsequently transported to a tubular lysosomal compartment 26. And Yasuda, K et al found that macrophage activation by a DNA/cationic liposome complex required endosomal acidification 32. Concurrent with the movement of CpG DNA in cells, TLR9 redistributed from the ER to CpG DNA-containing structures through a model whereby modification of the cytosolic tail of TLR9, which also accumulated myeloid differentiation primary response gene 88 (MyD88) 25-27.2.1.2 Adenoviral (Ad) vectorAd vector, a DNA virus, has been widely used for gene therapy applications and as vaccine vehicles for treating infectious diseases. It was reported that Ad vector elicited innate immune responses via TLR9 and induced IL-6 and IL-12 secretion in primary macrophages and dendritic cells 19, 33. TLR9 deficiency attenuated the innate immune response to Ad, whereas TLR9 blockade reduced the acute inflammatory response after intravenous injection of the vector 19, 33. Interestingly, Ad upregulated TLR9 gene expression independent of TLR9 function for the intravenous injection of luciferase-expressing Ad vectors into MyD88- or TLR9-deficient mice resulted in almost comparable levels of IL-6 and IL-12 production and luciferase expression with wild-type mice, suggesting that additional innate signaling pathways work cooperatively with TLR9 19, 33. Cell type had an effect on the immune response induced by TLR9 after Ad injection. Plasmacytoid dendritic cells(pDCs) recognition of adenovirus was mediated by TLR9 and dependent on MyD88, which induced production of type I IFNs while Non-pDC recognition of adenovirus was TLR independent 34, 35.2.1.3 Baculovirus (BV)Autographa californica nucleopolyhedrovirus (AcNPV), a well-characterized baculovirus was found to activate TLR 9. The internalization of viral DNA via membrane fusion mediated by the viral envelope glycoprotein, as well as endosomal maturation, which released the viral genome into TLR9-expressing cellular compartments, was necessary for the induction of the innate immune response by AcNPV1. Mice inoculated intranasally with AcNPV were protected from a lethal challenge by influenza virus, which might be due to the innate immune response activated by TLR 9. It was found that high levels of TNF- and IL-6 production were detected in RAW264.7 cells treated with AcNPV; monocytes consisting of macrophages were strongly induced by preinoculation with AcNPV and these activated immunocompetent cells suppressed the spread of the influenza virus infection in lung tissue 36. 2.1.4 Myxoma virus (MV)The myxoma virus-induced innate immune response required the endosomal DNA sensor TLR9 and its adaptor MyD88, transcription factors IFN-regulatory factor (IRF)5 and IRF7, and the type I IFN positive-feedback loop mediated by the type I IFN receptor(IFNAR1), which led to the production of IFN-, IFN-, TNF, and IL-12p7037. This immune response was independent of the cytosolic RNA sensing pathway mediated by mitochondrial antiviral signaling protein (MAVS), the TLR3 adaptor Toll/IL-1 receptor domain-containing adaptor-inducing IFN-beta(TRIF), or the transcription factor IRF3 37. 2.1.5 Kilham rat virus (KRV)TLR9 signaling pathways might be involved in mediating autoimmune diabetes. KRV, an ssDNA parvovirus, induced diabetes in less than 1% of animals and deliberated infection by parenteral injection induced diabetes in about 2540% of treated rats 38. KRV upregulated STAT-1 in pancreatic lymph nodes and in spleen cells in vitro from infected BioBreeding diabetes-resistant (BBDR) rats and induced the production of the proinflammatory cytokines IL-6 and IL-12p40, which increased disease penetrance38, 39. These results showed that KRV induced proinflammatory innate immune responses in pancreatic lymph nodes and spleen cells early after virus infection. This activation could be blocked by TLR9 inhibitor, iCpG DNA, implicating TLR9-induced STAT-1 signaling pathways in KRV-induced innate immune activation in BBDR rat 39. Zipris, D also observed that KRV-induced up-regulation of B lymphocytes could be blocked by TLR9 antagonists including inhibitory CpG and chloroquine38.2.1.6 Herpes simplex virus (HSV)TLR9 played an important role in HSV infection. It has been shown that not only the secretion of type I IFN but also production of IL-12 in response to HSV-1 in vitro was mediated by TLR9/MyD88 pathway 40. When it comes to HSV-2, it induced cytokine expression through TLR9-IRF-3-MAPK p38 pathway in RAW264.7 cells 41. Lund et al found that HSV-2mediated activation of TLR9 required endocytosis and acidification of endosomes within the pDCs and then induced the expression of the adaptor molecule MyD88, which led to the secretion of IFN- from pDCs42. Besides TLR9, other pathways functioned during HSV infection as well. After HSV infection, early production of IFN in vivo was mediated through TLR9 in pDCs, whereas the subsequent IFN-/ response was derived from several cell types and induced independently of TLR943. Sorensen, L N et al also found that pDC and non-pDC produced IFN- in response to HSV-1 via both TLR9-independent and -dependent pathways 44. In the study conducted by Krug, A et al, mice lacking either MyD88 or TLR9 were capable of controlling HSV-1 replication in vivo after local infection, demonstrating that TLR9- and MyD88-independent pathways in cells could effectively compensate for defective responses to HSV-140. But Rasmussen, S.B et al demonstrated that TLRs were required to stimulate IFN expression in fibroblasts in response to HSV infection 41. Therefore, the role of TLR9 in anti HSV response needs to be further explored. 2.1.7 Cytomegalovirus (CMV)TLR9 was able to induce immune defense against murine CMV (mCMV). TLR9/ mice exhibited significantly decreased serum levels of IL-12p70 and, to a lesser extent, TNF- in response to mCMV infection, which might contribute to the higher susceptibility of TLR9/ mice to mCMV-induced mortality than WT mice 45. TLR9 signaling pathway could be found in different cell types. Interferon-producing cells (IPC) and DC recognized mCMV through TLR9, which mediated cytokine secretion and promoted viral clearance by NK cells that expressed the mCMV-specific receptor Ly49H 46. Although depletion of IPC led to a drastic reduction of the IFN- response, this allowed other cell types to secrete IL-12, ensuring normal IFN-gamma and NK cell responses to mCMV, indicating that TLR9/MyD88 pathway mediated antiviral cytokine responses not only by IPC, DC, but by possibly other cell types, which were coordinated to promote effective NK cell function and mCMV clearance 46.The infected tissue site uniquely contributed to the process of virus sensing and regulation of localized antiviral responses. In the liver, MyD88 deficiency markedly impaired secretion of IFN- and production of the cytokine was largely independent of TLR9 signaling while in the spleen MyD88 and TLR9 were both needed for IFN- production47.Figure 1 illustrates TLR9 pathways. TLR9 interacted with CpG DNA, and then activated the adaptor MyD88. The downstream pathway required IRKs, TRAF, NF- B, IRF. Finally, it leads to the production of typeIFN and ILs, TNF to activate immune response.2.1.8 TLR2, 3 and 7 help TLR 9Other TLRs especially TLR2, 3 and 7 cooperated with TLR 9 to protect host from viral infection. TLR2 and TLR9 synergistically stimulated innate antiviral activities and induced the production of TNF- and the IFN, thereby protecting against HSV infection in the brain 48. Ad-induced innate immune responses were dependent not only on TLR9 but also on TLR2 because the expression of innate immune response genes following Ad injection were TLR2 dependent in vivo49.The requirement of TLR2 also correlated with significantly altered adaptive immune responses for that the generation of Ad-neutralizing Abs, and anti-transgene-specific Abs elicited subsequent to Ad vector treatments, were both dependent upon TLR2 functionality49.TLR2 and CD14 recognized CMV virions and trigger inflammatory cytokine production, which was mediated via TLR2-dependent activation of NF-B50.TLR2 knockout mice had elevated levels of mCMV in the spleen and liver, which was associated with a decreased NK cell population in the spleen and liver and reduced amounts of interleukin-18 and / interferon secreted in the TLR2 knockout mice 51.Both the TLR 9-MyD88 and TLR3-TRIF signaling pathways contributed to innate defense against systemic mCMV infection by inducing type I IFN secretion, however, neither pathway offered full protection against mCMV infection in the absence of the other, indicating that TLR9 might work with TLR3 in antiviral immune response 52.Zucchini, N et al found that TLR7/TLR9/ mice showed an even more dramatic reduction in survival to the infection and both of TLR7 and TLR9 exerted redundant functions for type I IFNs, IL-12p40, and TNF- production by pDCs in vivo during mCMV infection45, suggesting overlapping functions of TLR7 and TLR9 for innate defense against CMV infection.In addition to TLRs, DNA triggered antiviral response including production of type I IFNs and chemokines by other DNA sensor, for that the cytokine production was not completely abolished after the knockdown of TLRs32, 53-57.2.2. RLRs: RIG-I MDA5RIG-I and MDA5, two members of RLRs, was originally characterized as RNA sensor 58, 59. RLRs sensed viral ribonucleic acid and signal through MAVS also called IFN- promoter stimulator-1（IPS-1）, virus-induced signaling adaptor(VISA), and Cardif, to trigger the production of type I IFNs and proinflammatory cytokines60, 61, which led to a strong inhibition of virus replication. TRIM25 binding, Lys-172 ubiquitination enabled RIG-I to form a stable complex with MAVS, thereby inducing IFN signal transduction, MAVS binding, and downstream signaling ability 62.Recently, it has been shown that they might also function as DNA sensors. Upon RIG-I or MDA5 overexpression, B-DNA stimulation could activate type I IFN responses while the induction of other cytokines remained unaffected, suggesting a selective contribution of RLRs to the activation of type I IFN responses upon B-DNA stimulation 3. Cheng, G et al also demonstrated that RIG-I played an essential role in the dsDNA signaling pathway: expression of both the RIG-I dominant-negative mutants completely blocked dsDNA-induced IFN- promoter activation,transfection of the RIG-I siRNAs into Huh-7 cells inhibited dsDNA-induced IFN- promoter activation dramatically,transfected dsDNA poly (dAT:dAT) failed to induce IFN- promoter activation in Huh-7.5.1 cells which contains an inactivating point mutation in RIG-I,overexpression of wild-type RIG-I was sufficient to restore the dsRNA and dsDNA signaling in Huh-7.5.1 cells63. B-DNA-stimulated IRF3 dimerization occurred normally in cells treated by actinomycin D which was an inhibitor of RNA synthesis, further in support of a direct activation of RLRs by DNA 3. And GST-RIG-ICARD was pulled down with B-DNA, which was inhibited by unconjugated B-DNA or poly (I: C), indicating that the RIG-I containing the helicase-like domain and C-terminal domain bound to both DNA and RNA 3.It might be also through MAVS RIG-I triggered immune response