Beyond sensing DNA:A role for cGAS in the detection of extracellular cyclic di-nucleotides

Cyclic GMP-AMP synthase (cGAS) is best known as an innate immune receptor that detects pathogen DNA in the cytosol. In this issue of EMBO Reports, Kaufmann and colleagues report that cGAS has an additional role in innate immunity: It can also bind cyclic di-nucleotides (CDNs), signalling molecules produced by bacteria. The authors show that when extracellular CDNs are taken up by endocytosis, they bind to cGAS, causing it to form a complex with the CDN receptor STING (STimulator of INterferon Genes), thereby enhancing its activation. As cGAS is dispensable for the detection of intracellular CDNs, this work exemplifies how the localisation of pathogen-associated molecular patterns (PAMPs) influences innate immune signalling

Camouflage and interception:How pathogens evade detection by intracellular nucleic acid sensors

Intracellular DNA and RNA sensors play a vital part in the innate immune response to viruses and other intracellular pathogens, causing the secretion of type I interferons, cytokines and chemokines from infected cells. Pathogen RNA can be detected by retinoic-acid inducible gene I-like receptors in the cytosol, whereas cytosolic DNA is recognized by DNA sensors such as cyclic GMP-AMP synthase (cGAS). The resulting local immune response, which is initiated within hours of infection, is able to eliminate many pathogens before they are able to establish an infection in the host. For this reason, all viruses, and some intracellular bacteria and protozoa, need to evade detection by nucleic acid sensors. Immune evasion strategies include the sequestration and modification of nucleic acids, and the inhibition or degradation of host factors involved in innate immune signalling. Large DNA viruses, such as herpesviruses, often use multiple viral proteins to inhibit signalling cascades at several different points; for instance herpes simplex virus 1 targets both DNA sensors cGAS and interferon-γ-inducible protein 16, as well as the adaptor protein STING (stimulator of interferon genes) and other signalling factors in the pathway. Viruses with a small genome encode only a few immunomodulatory proteins, but these are often multifunctional, such as the NS1 protein from influenza A virus, which inhibits RNA sensing in multiple ways. Intracellular bacteria and protozoa can also be detected by nucleic acid sensors. However, as the type I interferon response is not always beneficial for the host under these circumstances, some bacteria subvert, rather than evade, these signalling cascades for their own gain

Investigating the Innate Immune Systems of Bats and Their Roles as Zoonotic Viral Reservoirs

The zoonotic spillover of viral pathogens from wild animal reservoirs into human populations remains the leading cause of emerging and re-emerging infectious diseases globally. Bats represent important viral reservoirs, notorious for the diversity and richness of the viruses they host, several of which are highly pathogenic when transmitted to humans. Remarkably, bats appear to host an abundance of these viruses without exhibiting any clinical signs of disease. A dominant hypothesis for this ability suggests that bats can control viral replication early in the innate immune response, which acts as the first line of defence against infection. However, bat immunology remains fundamentally understudied, largely due to their high species diversity and the lack of accessible reagents required for bat research. Therefore, in this work we explored and characterised key components of bat innate immunity to gain a better understanding of bats as viral reservoirs and contribute to the currently limited literature. Here, we demonstrated the in vitro transcriptomic response of the bat model species, Pteropus alecto (P.alecto) upon stimulation with the bat henipavirus Cedar virus and also with a type III bat interferon (paIFNλ). These investigations highlighted key transcripts, some of which were immune-related, in the response of bats to the separate stimuli and presents a foundation for further research into significant genes concerned in bat viral infection. Building from genome-wide transcriptomics, three distinctive bat innate immune genes representative of different stages of interferon signalling were selected for comparative genomics and functional characterisation. Our work demonstrated the conservation of genes between bats and humans, including IRF7, IFIT5 and IFI35. Specific findings for IRF7 included its successful translocation to the cell nucleus upon stimulation. IFIT5 and IFI35 were specifically selected for exploration due to previous research demonstrating the respective antiviral and conflicting anti- or pro-viral roles of these genes in humans. Significantly, our research demonstrated the direct antiviral action of P.alecto IFIT5 against negative-sense RNA viruses. Collectively, our findings offer valuable contributions to the field of bat antiviral immunity and provide the framework for future investigative studies into the role and function of the bat innate immune system and bat viral tolerance mechanisms

Vaccinia Virus Protein C6 Is a Virulence Factor that Binds TBK-1 Adaptor Proteins and Inhibits Activation of IRF3 and IRF7

Recognition of viruses by pattern recognition receptors (PRRs) causes interferon-β (IFN-β) induction, a key event in the anti-viral innate immune response, and also a target of viral immune evasion. Here the vaccinia virus (VACV) protein C6 is identified as an inhibitor of PRR-induced IFN-β expression by a functional screen of select VACV open reading frames expressed individually in mammalian cells. C6 is a member of a family of Bcl-2-like poxvirus proteins, many of which have been shown to inhibit innate immune signalling pathways. PRRs activate both NF-κB and IFN regulatory factors (IRFs) to activate the IFN-β promoter induction. Data presented here show that C6 inhibits IRF3 activation and translocation into the nucleus, but does not inhibit NF-κB activation. C6 inhibits IRF3 and IRF7 activation downstream of the kinases TANK binding kinase 1 (TBK1) and IκB kinase-ε (IKKε), which phosphorylate and activate these IRFs. However, C6 does not inhibit TBK1- and IKKε-independent IRF7 activation or the induction of promoters by constitutively active forms of IRF3 or IRF7, indicating that C6 acts at the level of the TBK1/IKKε complex. Consistent with this notion, C6 immunoprecipitated with the TBK1 complex scaffold proteins TANK, SINTBAD and NAP1. C6 is expressed early during infection and is present in both nucleus and cytoplasm. Mutant viruses in which the C6L gene is deleted, or mutated so that the C6 protein is not expressed, replicated normally in cell culture but were attenuated in two in vivo models of infection compared to wild type and revertant controls. Thus C6 contributes to VACV virulence and might do so via the inhibition of PRR-induced activation of IRF3 and IRF7

cGAS-independent STING activation in response to DNA damage

Self-DNA has previously been thought to be protected from immune detection by compartmentalisation in the nucleus or mitochondria. Here, we describe the discovery of a signalling cascade that links the detection of DNA damage in the nucleus to the activation of the innate immune adaptor STING (STimulator of INterfern Genes)

SMG7 acts as a molecular link between mRNA surveillance and mRNA decay

Nonsense-mediated mRNA decay (NMD) is a surveillance mechanism that eliminates mRNAs containing premature termination codons (PTCs). The proteins UPF1, SMG5, SMG6, and SMG7 are essential NMD factors in metazoa. SMG5 and SMG7 form a complex with UPF1 and interact with each other via their N-terminal domains. Here we show that SMG5 and SMG7 colocalize in cytoplasmic mRNA decay bodies, while SMG6 forms separate cytoplasmic foci. When SMG7 is tethered to a reporter transcript, it elicits its degradation, bypassing the requirement for a PTC, UPF1, SMG5, or SMG6. This activity is mediated by the C-terminal domain of SMG7. In contrast, SMG5 requires SMG7 to trigger mRNA decay and to localize to decay bodies. Our findings indicate that SMG7 provides a link between the NMD and the mRNA degradation machinery by interacting with SMG5 and UPF1 via its N-terminal domain and targeting bound transcripts for decay via its C-terminal domain

The interplay between viruses and innate immune signaling:recent insights and therapeutic opportunities

The immediate response to viral infection relies on pattern-recognition receptors (PRRs), most prominently the Toll-like receptors (TLRs) and the RNA helicases RIG-I and MDA5, as well as double stranded RNA-dependent protein kinase (PKR) and the DNA receptor, DAI. These PRRs recognize pathogen-associated molecular patterns (PAMPs) such as viral proteins and nucleic acids. The engagement of these receptors then initiates intracellular signaling cascades which ultimately cause the activation of transcription factors and the expression of type I interferons and pro-inflammatory cytokines. This innate response establishes an anti-viral state in the infected cell and its neighbours and alerts immune cells to the danger. In order to establish a productive infection, viruses need to overcome this initial anti-viral response. Evasion of innate immune defences is achieved by means of viral proteins that inhibit the signaling cascades emanating from the PRRs. The same innate signal transduction pathways have been implicated in conditions of sterile inflammation, such as rheumatoid arthritis and multiple sclerosis, and in autoimmunity. Because viral proteins target crucial host proteins involved in these pathways, they can point the way to key drug targets. Further, the viral proteins themselves or derivatives of them may be of use therapeutically to curtail inflammation and autoimmunity