Hideo Negishi

IRF-7 is the master regulator of type-I interferon-dependent immune responses.

The type-I interferon (IFN-α/β) response is critical to immunity against viruses and can be triggered in many cell types by cytosolic detection of viral infection, or in differentiated plasmacytoid dendritic cells by the Toll-like receptor 9 (TLR9) subfamily, which generates signals via the adaptor MyD88 to elicit robust IFN induction. Using mice deficient in the Irf7 gene (Irf7-/- mice), we show that the transcription factor IRF-7 is essential for the induction of IFN-α/β genes via the virus-activated, MyD88-independent pathway and the TLR-activated, MyD88-dependent pathway. Viral induction of MyD88-independent IFN-α/β genes is severely impaired in Irf7-/- fibroblasts. Consistently, Irf7-/- mice are more vulnerable than Myd88-/- mice to viral infection, and this correlates with a marked decrease in serum IFN levels, indicating the importance of the IRF-7-dependent induction of systemic IFN responses for innate antiviral immunity. Furthermore, robust induction of IFN production by activation of the TLR9 subfamily in plasmacytoid dendritic cells is entirely dependent on IRF-7, and this MyD88–IRF-7 pathway governs the induction of CD8+ T-cell responses. Thus, all elements of IFN responses, whether the systemic production of IFN in innate immunity or the local action of IFN from plasmacytoid dendritic cells in adaptive immunity, are under the control of IRF-7.

DAI (DLM-1/ZBP1) is a cytosolic DNA sensor and an activator of innate immune response

Central to innate immunity is the sensing of pathogen-associated molecular patterns by cytosolic and membrane-associated receptors. In particular, DNA is a potent activator of immune responses during infection or tissue damage, and evidence indicates that, in addition to the membrane-associated Toll-like receptor 9, an unidentified cytosolic DNA sensor(s) can activate type I interferon (IFN) and other immune responses. Here we report on a candidate DNA sensor, previously named DLM-1 (also called Z-DNA binding protein 1 (ZBP1)), for which biological function had remained unknown; we now propose the alternative name DAI (DNA-dependent activator of IFN-regulatory factors). The artificial expression of otherwise IFN-inducible DAI (DLM-1/ZBP1) in mouse fibroblasts selectively enhances the DNA-mediated induction of type I IFN and other genes involved in innate immunity. On the other hand, RNA interference of messenger RNA for DAI (DLM-1/ZBP1) in cells inhibits this gene induction programme upon stimulation by DNA from various sources. Moreover, DAI (DLM-1/ZBP1) binds to double-stranded DNA and, by doing so, enhances its association with the IRF3 transcription factor and the TBK1 serine/threonine kinase. These observations underscore an integral role of DAI (DLM-1/ZBP1) in the DNA-mediated activation of innate immune responses, and may offer new insight into the signalling mechanisms underlying DNA-associated antimicrobial immunity and autoimmune disorders.

Integral role of IRF-5 in the gene induction programme activated by Toll-like receptors

The activation of Toll-like receptors (TLRs) is central to innate and adaptive immunity. All TLRs use the adaptor MyD88 for signalling, but the mechanisms underlying the MyD88-mediated gene induction programme are as yet not fully understood. Here, we demonstrate that the transcription factor IRF-5 is generally involved downstream of the TLR–MyD88 signalling pathway for gene induction of proinflammatory cytokines, such as interleukin-6 (IL-6), IL-12 and tumour-necrosis factor-α. In haematopoietic cells from mice deficient in the Irf5 gene (Irf5-/- mice), the induction of these cytokines by various TLR ligands is severely impaired, whereas interferon-α induction is normal. We also provide evidence that IRF-5 interacts with and is activated by MyD88 and TRAF6, and that TLR activation results in the nuclear translocation of IRF-5 to activate cytokine gene transcription. Consistently, Irf5-/- mice show resistance to lethal shock induced by either unmethylated DNA or lipopolysaccharide, which correlates with a marked decrease in the serum levels of proinflammatory cytokines. Thus, our study identifies IRF-5 as a new, principal downstream regulator of the TLR–MyD88 signalling pathway and a potential target of therapeutic intervention to control harmful immune responses.

Spatiotemporal regulation of MyD88-IRF-7 signalling for robust type-I interferon induction

Robust type-I interferon (IFN-α/β) induction in plasmacytoid dendritic cells, through the activation of Toll-like receptor 9 (TLR9), constitutes a critical aspect of immunity. It is absolutely dependent on the transcription factor IRF-7, which interacts with and is activated by the adaptor MyD88. How plasmacytoid dendritic cells, but not other cell types (such as conventional dendritic cells), are able to activate the MyD88–IRF-7-dependent IFN induction pathway remains unknown. Here we show that the spatiotemporal regulation of MyD88–IRF-7 signalling is critical for a high-level IFN induction in response to TLR9 activation. The IFN-inducing TLR9 ligand, A/D-type CpG oligodeoxynucleotide (CpG-A), is retained for long periods in the endosomal vesicles of plasmacytoid dendritic cells, together with the MyD88–IRF-7 complex. However, in conventional dendritic cells, CpG-A is rapidly transferred to lysosomal vesicles. We further show that conventional dendritic cells can also mount a robust IFN induction if CpG-A is manipulated for endosomal retention using a cationic lipid. This strategy also allows us to demonstrate endosomal activation of the IFN pathway by the otherwise inactive TLR9 ligand B/K-type oligodeoxynucleotide (CpG-B). Thus, our study offers insights into the regulation of TLR9 signalling in space, potentially suggesting a new avenue for therapeutic intervention.

Integration of interferon-alpha/beta signalling to p53 responses in tumour suppression and antiviral defence

Swift elimination of undesirable cells is an important feature in tumour suppression and immunity. The tumour suppressor p53 and interferon-α and -β (IFN-α/β) are essential for the induction of apoptosis in cancerous cells and in antiviral immune responses, respectively, but little is known about their interrelationship. Here we show that transcription of the p53 gene is induced by IFN-α/β, accompanied by an increase in p53 protein level. IFN-α/β signalling itself does not activate p53; rather, it contributes to boosting p53 responses to stress signals. We show examples in which p53 gene induction by IFN-α/β contributes to tumour suppression. Furthermore, we show that p53 is activated in virally infected cells to evoke an apoptotic response and that p53 is critical for antiviral defence of the host. Our study reveals a hitherto unrecognized link between p53 and IFN-α/β in tumour suppression and antiviral immunity, which may have therapeutic implications.

HMGB proteins function as universal sentinels for nucleic-acid-mediated innate immune responses.

The activation of innate immune responses by nucleic acids is crucial to protective and pathological immunities and is mediated by the transmembrane Toll-like receptors (TLRs) and cytosolic receptors. However, it remains unknown whether a mechanism exists that integrates these nucleic-acid-sensing systems. Here we show that high-mobility group box (HMGB) proteins 1, 2 and 3 function as universal sentinels for nucleic acids. HMGBs bind to all immunogenic nucleic acids examined with a correlation between affinity and immunogenic potential. Hmgb1-/- and Hmgb2-/- mouse cells are defective in type-I interferon and inflammatory cytokine induction by DNA or RNA targeted to activate the cytosolic nucleic-acid-sensing receptors; cells in which the expression of all three HMGBs is suppressed show a more profound defect, accompanied by impaired activation of the transcription factors interferon regulatory factor 3 (IRF3) and nuclear factor (NF)-κB. The absence of HMGBs also severely impairs the activation of TLR3, TLR7 and TLR9 by their cognate nucleic acids. Our results therefore indicate a hierarchy in the nucleic-acid-mediated activation of immune responses, wherein the selective activation of nucleic-acid-sensing receptors is contingent on the more promiscuous sensing of nucleic acids by HMGBs. These findings may have implications for understanding the evolution of the innate immune system and for the treatment of immunological disorders.

Regulation of innate immune responses by DAI (DLM-1/ZBP1) and other DNA-sensing molecules

DNA, whether it is microbe-derived or host-derived, evokes immune responses when exposed to the cytosol of a cell. We previously reported that DNA-dependent activator of IFN regulatory factors (DAI), also referred to as DLM-1/ZBP1, functions as a DNA sensor that activates the innate immune system. In the present study, we examined the regulation of the complex DNA-sensing system by DAI and other molecules. We first show that DAI directly interacts with DNA in vitro and that it requires three DNA-binding domains for full activation in vivo. We also show that the artificially induced dimerization of DAI results in the DNA-independent activation of type I IFN genes, thereby providing a better understanding for the molecular basis of DAI activation. Furthermore, we provide evidence for the presence of additional DNA sensors, either positively or negatively regulating cytosolic DNA-mediated innate immune responses. These results in toto provide insights into the mechanism of DAI activation and reveal the complex regulatory mechanisms underlying DNA-mediated protective and pathologic immune responses.

Essential role of IRF-3 in lipopolysaccharide-induced interferon-β gene expression and endotoxin shock

Type I interferons (IFN-alpha/beta) affect many aspects of immune responses. Many pathogen-associated molecules, including bacterial lipopolysaccharide (LPS) and virus-associated double-stranded RNA, induce IFN gene expression through activation of distinct Toll-like receptors (TLRs). Although much has been studied about the activation of the transcription factor IRF-3 and induction of IFN-beta gene by the LPS-mediated TLR4 signaling, definitive evidence is missing about the actual role of IRF-3 in LPS responses in vitro and in vivo. Using IRF-3 deficient mice, we show here that IRF-3 is indeed essential for the LPS-mediated IFN-beta gene induction. Loss of IRF-3 also affects the expression of profile of other cytokine/chemokine genes. We also provide evidence that the LPS/TLR4 signaling activates IRF-7 to induce IFN-beta, if IRF-7 is induced by IFNs prior to LPS simulation. Finally, the IRF-3-deficient mice show resistance to LPS-induced endotoxin shock. These results place IRF-3 as a molecule central to LPS/TLR4 signaling.

Evidence for licensing of IFN-γ-induced IFN regulatory factor 1 transcription factor by MyD88 in toll-like receptor-dependent gene induction program

The recognition of microbial components by Toll-like receptors (TLRs) initiates signal transduction pathways, which trigger the expression of a series of target genes. It has been reported that TLR signaling is enhanced by cytokines such as IFN-γ, but the mechanisms underlying this enhancement remain unclear. The MyD88 adaptor, which is essential for signaling by many TLRs, recruits members of the IFN regulatory factor (IRF) family of transcription factors, such as IRF5 and IRF7, to evoke the activation of TLR target genes. In this study we demonstrate that IRF1, which is induced by IFN-γ, also interacts with and is activated by MyD88 upon TLR activation. We provide evidence that MyD88-associated IRF1 migrates into the nucleus more efficiently than non-MyD88-associated IRF1 and that this IRF1 selectively participates in the TLR-dependent gene induction program. The critical role of MyD88-dependent “IRF1 licensing” is underscored by the observation that the induction of a specific gene subset downstream of the TLR–MyD88 pathway, such as IFN-β, inducible NO synthase, and IL-12p35, are impaired in Irf1-deficient cells. Thus, our present study places IRF1 as an additional member participating in MyD88 signaling and provides a mechanistic insight into the enhancement of the TLR-dependent gene induction program by IFN-γ.

A critical link between Toll-like receptor 3 and type II interferon signaling pathways in antiviral innate immunity

A conundrum of innate antiviral immunity is how nucleic acid-sensing Toll-like receptors (TLRs) and RIG-I/MDA5 receptors cooperate during virus infection. The conventional wisdom has been that the activation of these receptor pathways evokes type I IFN (IFN) responses. Here, we provide evidence for a critical role of a Toll-like receptor 3 (TLR3)-dependent type II IFN signaling pathway in antiviral innate immune response against Coxsackievirus group B serotype 3 (CVB3), a member of the positive-stranded RNA virus family picornaviridae and most prevalent virus associated with chronic dilated cardiomyopathy. TLR3-deficient mice show a vulnerability to CVB3, accompanied by acute myocarditis, whereas transgenic expression of TLR3 endows even type I IFN signal-deficient mice resistance to CVB3 and other types of viruses, provided that type II IFN signaling remains intact. Taken together, our results indicate a critical cooperation of the RIG-I/MDA5-type I IFN and the TLR3-type II IFN signaling axes for efficient innate antiviral immune responses.