Viruses transfer the antiviral second messenger cGAMP between cells

Alert from a Distant Neighbor: Spread of Antiviral Immunity through Anion Channels

Cytosolic DNA detection via the DNA sensor cGAS initiates a major cell-intrinsic response to infection and malignancies. In this issue of Immunity, Zhou et al. (2020) report that the catalytic product of cGAS, cGAMP, can alert bystander cells over large distances through its cell-to-cell transmission via volume-regulated anion channels.

SAMHD1 Limits the Efficacy of Forodesine in Leukemia by Protecting Cells against the Cytotoxicity of dGTP

The anti-leukemia agent forodesine causes cytotoxic overload of intracellular deoxyguanosine triphosphate (dGTP) but is efficacious only in a subset of patients. We report that SAMHD1, a phosphohydrolase degrading deoxyribonucleoside triphosphate (dNTP), protects cells against the effects of dNTP imbalances. SAMHD1-deficient cells induce intrinsic apoptosis upon provision of deoxyribonucleosides, particularly deoxyguanosine (dG). Moreover, dG and forodesine act synergistically to kill cells lacking SAMHD1. Using mass cytometry, we find that these compounds kill SAMHD1-deficient malignant cells in patients with chronic lymphocytic leukemia (CLL). Normal cells and CLL cells from patients without SAMHD1 mutation are unaffected. We therefore propose to use forodesine as a precision medicine for leukemia, stratifying patients by SAMHD1 genotype or expression.

Enhanced Immunogenicity of Mitochondrial-Localized Proteins in Cancer Cells

Epitopes derived from mutated cancer proteins elicit strong antitumor T-cell responses that correlate with clinical efficacy in a proportion of patients. However, it remains unclear whether the subcellular localization of mutated proteins influences the efficiency of T-cell priming. To address this question, we compared the immunogenicity of NY-ESO-1 and OVA localized either in the cytosol or in mitochondria. We showed that tumors expressing mitochondrial-localized NY-ESO-1 and OVA proteins elicit significantdly higher frequencies of antigen-specific CD8+ T cells in vivo. We also demonstrated that this stronger immune response is dependent on the mitochondrial location of the antigenic proteins, which contributes to their higher steady-state amount, compared with cytosolic localized proteins. Consistent with these findings, we showed that injection of mitochondria purified from B16 melanoma cells can protect mice from a challenge with B16 cells, but not with irrelevant tumors. Finally, we extended these findings to cancer patients by demonstrating the presence of T-cell responses specific for mutated mitochondrial-localized proteins. These findings highlight the utility of prioritizing epitopes derived from mitochondrial-localized mutated proteins as targets for cancer vaccination strategies.

Research Advances in How the cGAS-STING Pathway Controls the Cellular Inflammatory Response

Double-stranded DNA (dsDNA) sensor cyclic-GMP-AMP synthase (cGAS) along with the downstream stimulator of interferon genes (STING) acting as essential immune-surveillance mediators have become hot topics of research. The intrinsic function of the cGAS-STING pathway facilitates type-I interferon (IFN) inflammatory signaling responses and other cellular processes such as autophagy, cell survival, senescence. cGAS-STING pathway interplays with other innate immune pathways, by which it participates in regulating infection, inflammatory disease, and cancer. The therapeutic approaches targeting this pathway show promise for future translation into clinical applications. Here, we present a review of the important previous works and recent advances regarding the cGAS-STING pathway, and provide a comprehensive understanding of the modulatory pattern of the cGAS-STING pathway under multifarious pathologic states.

Conserved strategies for pathogen evasion of cGAS-STING immunity

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Pathogens evade cytosolic DNA sensing using divergent and overlapping strategies to target cGAS, 2′3′-cGAMP, and STING.

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Studies of pathogen mechanisms of cGAS–STING evasion reveal new insights into cellular regulation of immune activation.

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Newly discovered regulatory features of cGAS–STING signaling suggest yet undiscovered strategies which pathogens may employ for immune evasion.

The cyclic GMP–AMP synthase (cGAS)– Stimulator of Interferon Genes (STING) pathway of cytosolic DNA sensing allows mammalian cells to detect and respond to infection with diverse pathogens. Pathogens in turn encode numerous factors that inhibit nearly all steps of cGAS–STING signal transduction. From masking of cytosolic DNA ligands, to post-translational modification of cGAS and STING, and degradation of the nucleotide second messenger 2′3′-cGAMP, pathogens have evolved convergent mechanisms to evade cGAS–STING sensing. Here we examine pathogen inhibitors of innate immunity in the context of newly discovered regulatory features controlling cellular cGAS–STING activation. Comparative analysis of these strategies provides insight into mechanisms of action and suggests aspects of cGAS–STING regulation and immune evasion that remain to be discovered.

Transfer of cGAMP into Bystander Cells via LRRC8 Volume-Regulated Anion Channels Augments STING-Mediated Interferon Responses and Anti-viral Immunity

The enzyme cyclic GMP-AMP synthase (cGAS) senses cytosolic DNA in infected and malignant cells and catalyzes the formation of 2'3'cGMP-AMP (cGAMP), which in turn triggers interferon (IFN) production via the STING pathway. Here, we examined the contribution of anion channels to cGAMP transfer and anti-viral defense. A candidate screen revealed that inhibition of volume-regulated anion channels (VRACs) increased propagation of the DNA virus HSV-1 but not the RNA virus VSV. Chemical blockade or genetic ablation of LRRC8A/SWELL1, a VRAC subunit, resulted in defective IFN responses to HSV-1. Biochemical and electrophysiological analyses revealed that LRRC8A/LRRC8E-containing VRACs transport cGAMP and cyclic dinucleotides across the plasma membrane. Enhancing VRAC activity by hypotonic cell swelling, cisplatin, GTPγS, or the cytokines TNF or interleukin-1 increased STING-dependent IFN response to extracellular but not intracellular cGAMP. Lrrc8e^-/- mice exhibited impaired IFN responses and compromised immunity to HSV-1. Our findings suggest that cell-to-cell transmission of cGAMP via LRRC8/VRAC channels is central to effective anti-viral immunity.

Cyclic dinucleotides at the forefront of innate immunity

Cyclic dinucleotides (CDNs) have emerged as ubiquitous signaling molecules in all domains of life. In eukaryotes, CDN signaling systems are evolutionarily ancient and have developed to sense and respond to pathogen infection. On the other hand, dysregulation of these pathways has been implicated in the pathogenesis of autoimmune diseases. Thus, CDNs have garnered major interest over recent years for their ability to elicit potent immune responses in the eukaryotic host. Similarly, ancestral CDN-based signaling systems also appear to confer immunological protection against infection in prokaryotes. Therefore, a better understanding of the host processes regulated by CDNs will be of tremendous value in many areas of research. Here, we aim to review the latest discoveries and recent trends in CDN research with a particular focus on the molecular mechanisms by which these small molecules mediate innate immunity.

Extracellular cGAMP is a cancer cell-produced immunotransmitter involved in radiation-induced anti-cancer immunity

2'3'-cyclic GMP-AMP (cGAMP) is an intracellular second messenger that is synthesized in response to cytosolic double-stranded DNA and activates the innate immune STING pathway. Our previous discovery of its extracellular hydrolase ENPP1 hinted at the existence of extracellular cGAMP. Here, we detected that cGAMP is continuously exported but then efficiently cleared by ENPP1, explaining why it has previously escaped detection. By developing potent, specific, and cell impermeable ENPP1 inhibitors, we found that cancer cells continuously export cGAMP in culture at steady state and at higher levels when treated with ionizing radiation (IR). In mouse tumors, depletion of extracellular cGAMP decreased tumor-associated immune cell infiltration and abolished the curative effect of IR. Boosting extracellular cGAMP with ENPP1 inhibitors synergized with IR to delay tumor growth. In conclusion, extracellular cGAMP is an anti-cancer immunotransmitter that could be harnessed to treat cancers with low immunogenicity.

Connexin-Dependent Transfer of cGAMP to Phagocytes Modulates Antiviral Responses

Activation of cyclic GMP-AMP (cGAMP) synthase (cGAS) plays a critical role in antiviral responses to many DNA viruses. Sensing of cytosolic DNA by cGAS results in synthesis of the endogenous second messenger cGAMP that activates stimulator of interferon genes (STING) in infected cells. Critically, cGAMP can also propagate antiviral responses to uninfected cells through intercellular transfer, although the modalities of this transfer between epithelial and immune cells remain poorly defined. We demonstrate here that cGAMP-producing epithelial cells can transactivate STING in cocultured macrophages through direct cGAMP transfer. cGAMP transfer was reliant upon connexin expression by epithelial cells and pharmacological inhibition of connexins blunted STING-dependent transactivation of the macrophage compartment. Macrophage transactivation by cGAMP contributed to a positive-feedback loop amplifying antiviral responses, significantly protecting uninfected epithelial cells against viral infection. Collectively, our findings constitute the first direct evidence of a connexin-dependent cGAMP transfer to macrophages by epithelial cells, to amplify antiviral responses.IMPORTANCE Recent studies suggest that extracellular cGAMP can be taken up by macrophages to engage STING through several mechanisms. Our work demonstrates that connexin-dependent communication between epithelial cells and macrophages plays a significant role in the amplification of antiviral responses mediated by cGAMP and suggests that pharmacological strategies aimed at modulating connexins may have therapeutic applications to control antiviral responses in humans.

Exosomal hsa-miR-21-5p derived from growth hormone-secreting pituitary adenoma promotes abnormal bone formation in acromegaly

Growth hormone-secreting pituitary adenoma (GHPA), a benign endocrine tumor located in the base of the skull, results in acromegaly. In addition to the mass effect of the tumor itself in the sellar region, GHPA can lead to the overgrowth of almost every organ. Previous findings indicated that the processes underlying acromegaly were partly attributable to hyperactivity of the growth hormone/insulin-like growth factor-1 (GH/IGF-1) axis. However, the mechanisms driving this syndrome remains largely unknown. Additionally, the roles of GHPA-derived exosomes, which contain functional microRNAs and proteins that manipulate target cell proliferation and differentiation in distal extremities, are also unknown. In this study, we demonstrated that GHPA exosomes promote bone formation in vitro and trabecula number in vivo. The mechanism of increased trabecula formation may be attributable to GHPA exosome-induced osteoblast proliferation via increased cell viability and DNA replication. We further discovered that exosomal hsa-miR-21-5p plays a distinct role from the GH/IGF-1 axis in these processes. Accordingly, the results of this study provide a novel mechanism whereby GHPA influences distal extremities and a new perspective for treating GHPA.

Regulation of cGAS- and RLR-mediated immunity to nucleic acids

Pathogen-derived nucleic acids are crucial signals for innate immunity. Despite the structural similarity between those and host nucleic acids, mammalian cells have been able to evolve powerful innate immune signaling pathways that originate from the detection of cytosolic nucleic acid species, one of the most prominent being the cGAS-STING pathway for DNA and the RLR-MAVS pathway for RNA, respectively. Recent advances have revealed a plethora of regulatory mechanisms that are crucial for balancing the activity of nucleic acid sensors for the maintenance of overall cellular homeostasis. Elucidation of the various mechanisms that enable cells to maintain control over the activity of cytosolic nucleic acid sensors has provided new insight into the pathology of human diseases and, at the same time, offers a rich and largely unexplored source for new therapeutic targets. This Review addresses the emerging literature on regulation of the sensing of cytosolic DNA and RNA via cGAS and RLRs.

Innate Immune Response to Cytoplasmic DNA: Mechanisms and Diseases

DNA has been known to be a potent immune stimulus for more than half a century. However, the underlying molecular mechanisms of DNA-triggered immune response have remained elusive until recent years. Cyclic GMP-AMP synthase (cGAS) is a major cytoplasmic DNA sensor in various types of cells that detect either invaded foreign DNA or aberrantly located self-DNA. Upon sensing of DNA, cGAS catalyzes the formation of cyclic GMP-AMP (cGAMP), which in turn activates the ER-localized adaptor protein MITA (also named STING) to elicit the innate immune response. The cGAS-MITA axis not only plays a central role in host defense against pathogen-derived DNA but also acts as a cellular stress response pathway by sensing aberrantly located self-DNA, which is linked to the pathogenesis of various human diseases. In this review, we summarize the spatial and temporal mechanisms of host defense to cytoplasmic DNA mediated by the cGAS-MITA axis and discuss the association of malfunctions of this axis with autoimmune and other diseases.

cGAS and CD-NTase enzymes: structure, mechanism, and evolution

Cyclic GMP-AMP synthase (cGAS) is a signaling enzyme in human cells that controls immune-sensing of cytosolic DNA. The recent discoveries of diverse structural homologs of cGAS in animals and bacteria reveal that cGAS-like signaling is surprisingly ancient and widespread in biology. Together with the Vibrio cholerae protein dinucleotide cyclase in Vibrio (DncV), cGAS and DncV homologs comprise a family of cGAS/DncV-like nucleotidyltransferase (CD-NTase) enzymes that synthesize noncanonical RNA signals including cyclic dinucleotides, cyclic trinucleotides, and linear oligonucleotides. Structural and biochemical breakthroughs provide a framework to understand how CD-NTase signaling allows cells to respond to changing environmental conditions. The CD-NTase family also includes uncharacterized human genes like MB21D2 and Mab21L1, highlighting emerging functions of cGAS-like signaling beyond innate immunity.

Interrupting cyclic dinucleotide-cGAS-STING axis with small molecules

The cyclic dinucleotide-cGAS-STING axis plays important roles in host immunity. Activation of this signaling pathway, via cytosolic sensing of bacterial-derived c-di-GMP/c-di-AMP or host-derived cGAMP, leads to the production of inflammatory interferons and cytokines that help resolve infection. Small molecule activators of the cGAS-STING axis have the potential to augment immune response against various pathogens or cancer. The aberrant activation of this pathway, due to gain-of-function mutations in any of the proteins that are part of the signaling axis, could lead to various autoimmune diseases. Inhibiting various nodes of the cGAS-STING axis could provide relief to patients with autoimmune diseases. Many excellent reviews on the cGAS-STING axis have been published recently, and these have mainly focused on the molecular details of the cGAS-STING pathway. This review however focuses on small molecules that can be used to modulate various aspects of the cGAS-STING pathway, as well as other parallel inflammatory pathways.

STING: a master regulator in the cancer-immunity cycle

The aberrant appearance of DNA in the cytoplasm triggers the activation of cGAS-cGAMP-STING signaling and induces the production of type I interferons, which play critical roles in activating both innate and adaptive immune responses. Recently, numerous studies have shown that the activation of STING and the stimulation of type I IFN production are critical for the anticancer immune response. However, emerging evidence suggests that STING also regulates anticancer immunity in a type I IFN-independent manner. For instance, STING has been shown to induce cell death and facilitate the release of cancer cell antigens. Moreover, STING activation has been demonstrated to enhance cancer antigen presentation, contribute to the priming and activation of T cells, facilitate the trafficking and infiltration of T cells into tumors and promote the recognition and killing of cancer cells by T cells. In this review, we focus on STING and the cancer immune response, with particular attention to the roles of STING activation in the cancer-immunity cycle. Additionally, the negative effects of STING activation on the cancer immune response and non-immune roles of STING in cancer have also been discussed.

HIV-2/SIV Vpx targets a novel functional domain of STING to selectively inhibit cGAS-STING-mediated NF-κB signalling

Innate immunity is the first line of host defence against pathogens. Suppression of innate immune responses is essential for the survival of all viruses. However, the interplay between innate immunity and HIV/SIV is only poorly characterized. We have discovered Vpx as a novel inhibitor of innate immune activation that associates with STING signalosomes and interferes with the nuclear translocation of NF-κB and the induction of innate immune genes. This new function of Vpx could be separated from its role in mediating degradation of the antiviral factor SAMHD1, and is conserved among diverse HIV-2/SIV Vpx. Vpx selectively suppressed cGAS-STING-mediated nuclear factor-κB signalling. Furthermore, Vpx and Vpr had complementary activities against cGAS-STING activity. Since SIV_MAC lacking both Vpx and Vpr was less pathogenic than SIV deficient for Vpr or Vpx alone, suppression of innate immunity by HIV/SIV is probably a key pathogenic determinant, making it a promising target for intervention.

Influenza A virus M2 protein triggers mitochondrial DNA-mediated antiviral immune responses

Cytosolic mitochondrial DNA (mtDNA) activates cGAS-mediated antiviral immune responses, but the mechanism by which RNA viruses stimulate mtDNA release remains unknown. Here we show that viroporin activity of influenza virus M2 or encephalomyocarditis virus (EMCV) 2B protein triggers translocation of mtDNA into the cytosol in a MAVS-dependent manner. Although influenza virus-induced cytosolic mtDNA stimulates cGAS- and DDX41-dependent innate immune responses, the nonstructural protein 1 (NS1) of influenza virus associates with mtDNA to evade the STING-dependent antiviral immunity. The STING-dependent antiviral signaling is amplified in neighboring cells through gap junctions. In addition, we find that STING-dependent recognition of influenza virus is essential for limiting virus replication in vivo. Our results show a mechanism by which influenza virus stimulates mtDNA release and highlight the importance of DNA sensing pathway in limiting influenza virus replication.

Cytosolic mitochondrial DNA (mtDNA) plays a role in innate antiviral immunity but how this is triggered during infection remains unclear. Here, the authors provide evidence that the Influenza virus protein M2 stimulates translocation of mtDNA into the cytosol in a MAVS-dependent manner.

Modern Techniques for the Isolation of Extracellular Vesicles and Viruses

Extracellular signaling is pivotal to maintain organismal homeostasis. A quickly emerging field of interest within extracellular signaling is the study of extracellular vesicles (EV), which act as messaging vehicles for nucleic acids, proteins, metabolites, lipids, etc. from donor cells to recipient cells. This transfer of biologically active material within a vesicular body is similar to the infection of a cell through a virus particle, which transfers genetic material from one cell to another to preserve an infection state, and viruses are known to modulate EV. Although considerable heterogeneity exists within EV and viruses, this review focuses on those that are small (< 200 nm in diameter) and of relatively low density (< 1.3 g/mL). A multitude of isolation methods for EV and virus particles exist. In this review, we present an update on methods for their isolation, purification, and phenotypic characterization. We hope that the information we provide will be of use to basic science and clinical investigators, as well as biotechnologists in this emerging field. Graphical Abstract.

The Role of Extracellular Vesicles in Viral Infection and Transmission

Extracellular vesicles (EVs) have been found to be released by any type of cell and can be retrieved in every circulating body fluid, namely blood (plasma, serum), saliva, milk, and urine. EVs were initially considered a cellular garbage disposal tool, but later it became evident that they are involved in intercellular signaling. There is evidence that viruses can use EV endocytic routes to enter uninfected cells and hijack the EV secretory pathway to exit infected cells, thus illustrating that EVs and viruses share common cell entry and biogenesis mechanisms. Moreover, EVs play a role in immune response against viral pathogens. EVs incorporate and spread both viral and host factors, thereby prompting or inhibiting immune responses towards them via a multiplicity of mechanisms. The involvement of EVs in immune responses, and their potential use as agents modulating viral infection, will be examined. Although further studies are needed, the engineering of EVs could package viral elements or host factors selected for their immunostimulatory properties, to be used as vaccines or tolerogenic tools in autoimmune diseases.

cGAS in action: Expanding roles in immunity and inflammation

DNA is highly immunogenic. It represents a key pathogen-associated molecular pattern (PAMP) during infection. Host DNA can, however, also act as a danger-associated molecular pattern (DAMP) and elicit strong inflammatory responses. The cGAS-STING pathway has emerged as a major pathway that detects intracellular DNA. Here, we highlight recent advances on how cGAS and STING mediate inflammatory responses and how these are regulated, allowing cells to readily respond to infections and noxious agents while avoiding the inappropriate sensing of self-DNA. A particular focus is placed on the role of cGAS in the context of sterile inflammatory conditions. Manipulating cGAS or STING may open the door for new therapeutic strategies for the treatment of acute and chronic inflammation relevant to many human diseases.

The Innate Antiviral Response in Animals: An Evolutionary Perspective from Flagellates to Humans

Animal cells have evolved dedicated molecular systems for sensing and delivering a coordinated response to viral threats. Our understanding of these pathways is almost entirely defined by studies in humans or model organisms like mice, fruit flies and worms. However, new genomic and functional data from organisms such as sponges, anemones and mollusks are helping redefine our understanding of these immune systems and their evolution. In this review, we will discuss our current knowledge of the innate immune pathways involved in sensing, signaling and inducing genes to counter viral infections in vertebrate animals. We will then focus on some central conserved players of this response including Toll-like receptors (TLRs), RIG-I-like receptors (RLRs) and cGAS-STING, attempting to put their evolution into perspective. To conclude, we will reflect on the arms race that exists between viruses and their animal hosts, illustrated by the dynamic evolution and diversification of innate immune pathways. These concepts are not only important to understand virus-host interactions in general but may also be relevant for the development of novel curative approaches against human disease.

Macrophages exposed to HIV viral protein disrupt lung epithelial cell integrity and mitochondrial bioenergetics via exosomal microRNA shuttling

Antiretroviral therapy extends survival but does not eliminate HIV from its cellular reservoirs. Between immune and stromal cells in the tissue microenvironment, a dynamic intercellular communication might influence host viral immune responses via intercellular transfer of extracellular vehicles (EVs) (microvesicles, exosome, or apoptotic bodies). It is increasingly recognized that HIV-infected macrophage-secreted nucleotide-rich exosomes might play a critical role in mediating communication between macrophages and other structural cells; however, molecular mechanisms underlying cell–cell crosstalk remain unknown. Here we show that HIV-1-infected macrophages and HIV-1 proteins Tat or gp120-treated macrophages express high levels of microRNAs, including miR-23a and miR-27a. Identical miRNAs expression patterns were detected in macrophage-secreted exosomes isolated from bronchoalveolar lavage fluid of HIV transgenic rats. Tat-treated macrophage-derived exosomal miR-23a attenuated posttranscriptional modulation of key tight junction protein zonula occludens (ZO-1) 3′-UTR in epithelial cells. In parallel, exosomal miR-27a released from Tat-treated macrophages altered the mitochondrial bioenergetics of recipient lung epithelial cells by targeting peroxisome proliferator-activated receptor gamma (PPARγ), while simultaneously stimulating glycolysis. Together, exosomal miRNAs shuttle from macrophages to epithelial cells and thereby explain in part HIV-mediated lung epithelial barrier dysfunction. These studies suggest that targeting miRNAs may be of therapeutic value to enhance lung health in HIV.

SLC19A1 Is an Importer of the Immunotransmitter cGAMP

2'3'-cyclic-GMP-AMP (cGAMP) is a second messenger that activates the antiviral stimulator of interferon genes (STING) pathway. We recently identified a novel role for cGAMP as a soluble, extracellular immunotransmitter that is produced and secreted by cancer cells. Secreted cGAMP is then sensed by host cells, eliciting an antitumoral immune response. Due to the antitumoral effects of cGAMP, other CDN-based STING agonists are currently under investigation in clinical trials for metastatic solid tumors. However, it is unknown how cGAMP and other CDNs cross the cell membrane to activate intracellular STING. Using a genome-wide CRISPR screen, we identified SLC19A1 as the first known importer of cGAMP and other CDNs, including the investigational new drug 2'3'-bisphosphosphothioate-cyclic-di-AMP (2'3'-CDAS). These discoveries will provide insight into cGAMP's role as an immunotransmitter and aid in the development of more targeted CDN-based cancer therapeutics.

Intercellular communication in the innate immune system through the cGAS-STING pathway

Type I interferons (IFNα/β) are pivotal to anti-viral defense. Although an absolute requirement for viral clearance, the presence of IFN in high doses sustained over time has been implicated to be the underlying cause of clinical outcomes such as interferonopathies. Therefore, tight regulation of cellular communication processes is required for tissue homeostasis. This review covers the cellular signaling pathways initiated by DNA sensing through cGAS in the cellular cytosol, with particular focus on the subsequent establishment of immune responses in bystander cells.

Intracellular bacteria engage a STING-TBK1-MVB12b pathway to enable paracrine cGAS-STING signalling

The innate immune system is crucial for eventual control of infections, but may also contribute to pathology. Listeria monocytogenes is an intracellular Gram-positive bacteria and a major cause of food-borne disease. However, important knowledge on the interactions between L. monocytogenes and the immune system is still missing. Here, we report that Listeria DNA is sorted into extracellular vesicles (EVs) in infected cells and delivered to bystander cells to stimulate the cyclic guanosine monophosphate-adenosine monophosphate synthase (cGAS)-stimulator of interferon genes (STING) pathway. This was also observed during infections with Francisella tularensis and Legionella pneumophila. We identify the multivesicular body protein MVB12b as a target for TANK-binding kinase 1 phosphorylation, which is essential for the sorting of DNA into EVs and stimulation of bystander cells. EVs from Listeria-infected cells inhibited T-cell proliferation, and primed T cells for apoptosis. Collectively, we describe a pathway for EV-mediated delivery of foreign DNA to bystander cells, and suggest that intracellular bacteria exploit this pathway to impair antibacterial defence.

cGAS facilitates sensing of extracellular cyclic dinucleotides to activate innate immunity

Cyclic dinucleotides (CDNs) are important second messenger molecules in prokaryotes and eukaryotes. Within host cells, cytosolic CDNs are detected by STING and alert the host by activating innate immunity characterized by type I interferon (IFN) responses. Extracellular bacteria and dying cells can release CDNs, but sensing of extracellular CDNs (eCDNs) by mammalian cells remains elusive. Here, we report that endocytosis facilitates internalization of eCDNs. The DNA sensor cGAS facilitates sensing of endocytosed CDNs, their perinuclear accumulation, and subsequent STING-dependent release of type I IFN Internalized CDNs bind cGAS directly, leading to its dimerization, and the formation of a cGAS/STING complex, which may activate downstream signaling. Thus, eCDNs comprise microbe- and danger-associated molecular patterns that contribute to host-microbe crosstalk during health and disease.

Roles of the cGAS-STING Pathway in Cancer Immunosurveillance and Immunotherapy

Cyclic GMP-AMP synthase (cGAS) is a cytosolic DNA sensor that initiates innate immune responses. DNA-bound cGAS produces cyclic GMP-AMP (cGAMP), which activates stimulator of interferon genes (STING) to induce inflammatory cytokines and other immune mediators. cGAS detects DNA without sequence specificity and responds to both cytosolic foreign DNA from pathogens and self-DNA leaked into the cytosol due to genome instability or cellular damage. Because of the diverse sources of cytosolic DNA, the cGAS-STING pathway plays a critical role during infection, autoimmune diseases, and senescence. Moreover, cGAS detects tumor-derived DNA and stimulates endogenous antitumor immunity. Thus, the cGAS-STING pathway is a promising target for cancer immunotherapy. Here, we review the role of the cGAS-STING pathway in various diseases and highlight various approaches targeting the cGAS-STING pathway for cancer therapy.

Coevolution pays off: Herpesviruses have the license to escape the DNA sensing pathway

Early detection of viral invasion by pattern recognition receptors (PRR) is crucial for the induction of a rapid and efficient immune response. Cytosolic DNA sensors are the most recently described class of PRR, and induce transcription of type I interferons (IFN) and proinflammatory cytokines via the key adaptor protein stimulator of interferon genes (STING). Herpesviruses are a family of large DNA viruses widely known for their immense arsenal of proteins dedicated to manipulating and evading host immune responses. Tantamount to the significant role played by DNA sensors and STING in innate immune responses, herpesviruses have in turn evolved a range of mechanisms targeting virtually every step of this key signaling pathway. Strikingly, some herpesviruses also take advantage of this pathway to promote their own replication. In this review, we will summarize the current understanding of DNA sensing and subsequent induction of signaling and transcription, and showcase the close adaptation of herpesviruses to their host reflected by the myriad of viral proteins dedicated to modulating this critical innate immune pathway.

cGAS/STING/TBK1/IRF3 Signaling Pathway Activates BMDCs Maturation Following Mycobacterium bovis Infection

Cyclic GMP-AMP synthase (cGAS) is an important cytosolic DNA sensor that plays a crucial role in triggering STING-dependent signal and inducing type I interferons (IFNs). cGAS is important for intracellular bacterial recognition and innate immune responses. However, the regulating effect of the cGAS pathway for bone marrow-derived dendritic cells (BMDCs) during Mycobacterium bovis (M. bovis) infection is still unknown. We hypothesized that the maturation and activation of BMDCs were modulated by the cGAS/STING/TBK1/IRF3 signaling pathway. In this study, we found that M. bovis promoted phenotypic maturation and functional activation of BMDCs via the cGAS signaling pathway, with the type I IFN and its receptor (IFNAR) contributing. Additionally, we showed that the type I IFN pathway promoted CD4⁺ T cells’ proliferation with BMDC during M. bovis infection. Meanwhile, the related cytokines increased the expression involved in this signaling pathway. These data highlight the mechanism of the cGAS and type I IFN pathway in regulating the maturation and activation of BMDCs, emphasizing the important role of this signaling pathway and BMDCs against M. bovis. This study provides new insight into the interaction between cGAS and dendritic cells (DCs), which could be considered in the development of new drugs and vaccines against tuberculosis.

Viral and metazoan poxins are cGAMP-specific nucleases that restrict cGAS-STING signalling

Cytosolic DNA triggers innate immune responses through the activation of cyclic GMP-AMP synthase (cGAS) and production of the cyclic dinucleotide second messenger 2',3'-cyclic GMP-AMP (cGAMP)1-4. 2',3'-cGAMP is a potent inducer of immune signalling; however, no intracellular nucleases are known to cleave 2',3'-cGAMP and prevent the activation of the receptor stimulator of interferon genes (STING)5-7. Here we develop a biochemical screen to analyse 24 mammalian viruses, and identify poxvirus immune nucleases (poxins) as a family of 2',3'-cGAMP-degrading enzymes. Poxins cleave 2',3'-cGAMP to restrict STING-dependent signalling and deletion of the poxin gene (B2R) attenuates vaccinia virus replication in vivo. Crystal structures of vaccinia virus poxin in pre- and post-reactive states define the mechanism of selective 2',3'-cGAMP degradation through metal-independent cleavage of the 3'-5' bond, converting 2',3'-cGAMP into linear Gp[2'-5']Ap[3']. Poxins are conserved in mammalian poxviruses. In addition, we identify functional poxin homologues in the genomes of moths and butterflies and the baculoviruses that infect these insects. Baculovirus and insect host poxin homologues retain selective 2',3'-cGAMP degradation activity, suggesting an ancient role for poxins in cGAS-STING regulation. Our results define poxins as a family of 2',3'-cGAMP-specific nucleases and demonstrate a mechanism for how viruses evade innate immunity.

Extracellular vesicles from Kaposi Sarcoma-associated herpesvirus lymphoma induce long-term endothelial cell reprogramming

Extracellular signaling is a mechanism that higher eukaryotes have evolved to facilitate organismal homeostasis. Recent years have seen an emerging interest in the role of secreted microvesicles, termed extracellular vesicles (EV) or exosomes in this signaling network. EV contents can be modified by the cell in response to stimuli, allowing them to relay information to neighboring cells, influencing their physiology. Here we show that the tumor virus Kaposi's Sarcoma-associated herpesvirus (KSHV) hijacks this signaling pathway to induce cell proliferation, migration, and transcriptome reprogramming in cells not infected with the virus. KSHV-EV activates the canonical MEK/ERK pathway, while not alerting innate immune regulators, allowing the virus to exert these changes without cellular pathogen recognition. Collectively, we propose that KSHV establishes a niche favorable for viral spread and cell transformation through cell-derived vesicles, all while avoiding detection.

Tumor cGAMP Awakens the Natural Killers

Type I interferon (IFN) production within the tumor microenvironment is important in shaping the immune response to the tumor. In this issue of Immunity, Marcus et al. (2018) reveal that tumor cells produce 2'3'-cGAMP, which activates the STING pathway in non-tumor cells and leads to type I IFN production and the priming of natural killer cells for tumor rejection.

Apoptosis-derived membrane vesicles drive the cGAS-STING pathway and enhance type I IFN production in systemic lupus erythematosus

OBJECTIVE

Despite the importance of type I interferon (IFN-I) in systemic lupus erythematosus (SLE) pathogenesis, the mechanisms of IFN-I production have not been fully elucidated. Recognition of nucleic acids by DNA sensors induces IFN-I and interferon-stimulated genes (ISGs), but the involvement of cyclic guanosine monophosphate (GMP)-AMP synthase (cGAS) and stimulator of interferon genes (STING) in SLE remains unclear. We studied the role of the cGAS-STING pathway in the IFN-I-producing cascade driven by SLE serum.

METHODS

We collected sera from patients with SLE (n=64), patients with other autoimmune diseases (n=31) and healthy controls (n=35), and assayed them using a cell-based reporter system that enables highly sensitive detection of IFN-I and ISG-inducing activity. We used Toll-like receptor-specific reporter cells and reporter cells harbouring knockouts of cGAS, STING and IFNAR2 to evaluate signalling pathway-dependent ISG induction.

RESULTS

IFN-I bioactivity and ISG-inducing activities of serum were higher in patients with SLE than in patients with other autoimmune diseases or healthy controls. ISG-inducing activity of SLE sera was significantly reduced in STING-knockout reporter cells, and STING-dependent ISG-inducing activity correlated with disease activity. Double-stranded DNA levels were elevated in SLE. Apoptosis-derived membrane vesicles (AdMVs) from SLE sera had high ISG-inducing activity, which was diminished in cGAS-knockout or STING-knockout reporter cells.

CONCLUSIONS

AdMVs in SLE serum induce IFN-I production through activation of the cGAS-STING pathway. Thus, blockade of the cGAS-STING axis represents a promising therapeutic target for SLE. Moreover, our cell-based reporter system may be useful for stratifying patients with SLE with high ISG-inducing activity.

Detection of Microbial Infections Through Innate Immune Sensing of Nucleic Acids

Microbial infections are recognized by the innate immune system through germline-encoded pattern recognition receptors (PRRs). As most microbial pathogens contain DNA and/or RNA during their life cycle, nucleic acid sensing has evolved as an essential strategy for host innate immune defense. Pathogen-derived nucleic acids with distinct features are recognized by specific host PRRs localized in endolysosomes and the cytosol. Activation of these PRRs triggers signaling cascades that culminate in the production of type I interferons and proinflammatory cytokines, leading to induction of an antimicrobial state, activation of adaptive immunity, and eventual clearance of the infection. Here, we review recent progress in innate immune recognition of nucleic acids upon microbial infection, including pathways involving endosomal Toll-like receptors, cytosolic RNA sensors, and cytosolic DNA sensors. We also discuss the mechanisms by which infectious microbes counteract host nucleic acid sensing to evade immune surveillance.

Cytoplasmic Mechanisms of Recognition and Defense of Microbial Nucleic Acids

Microbial nucleic acids are major signatures of invading pathogens, and their recognition by various host pattern recognition receptors (PRRs) represents the first step toward an efficient innate immune response to clear the pathogens. The nucleic acid-sensing PRRs are localized at the plasma membrane, the cytosol, and/or various cellular organelles. Sensing of nucleic acids and signaling by PRRs involve recruitment of distinct signaling components, and PRRs are intensively regulated by cellular organelle trafficking. PRR-mediated innate immune responses are also heavily regulated by posttranslational modifications, including phosphorylation, polyubiquitination, sumoylation, and glutamylation. In this review, we focus on our current understanding of recognition of microbial nucleic acid by PRRs, particularly on their regulation by organelle trafficking and posttranslational modifications. We also discuss how sensing of self nucleic acids and dysregulation of PRR-mediated signaling lead to serious human diseases.

Infection with a Brazilian isolate of Zika virus generates RIG-I stimulatory RNA and the viral NS5 protein blocks type I IFN induction and signaling

Zika virus (ZIKV) is a major public health concern in the Americas. We report that ZIKV infection and RNA extracted from ZIKV infected cells potently activated the induction of type I interferons (IFNs). This effect was fully dependent on the mitochondrial antiviral signaling protein (MAVS), implicating RIG-I-like receptors (RLRs) as upstream sensors of viral RNA. Indeed, RIG-I and the related RNA sensor MDA5 contributed to type I IFN induction in response to RNA from infected cells. We found that ZIKV NS5 from a recent Brazilian isolate blocked type I IFN induction downstream of RLRs and also inhibited type I IFN receptor (IFNAR) signaling. We defined the ZIKV NS5 nuclear localization signal and report that NS5 nuclear localization was not required for inhibition of signaling downstream of IFNAR. Mechanistically, NS5 blocked IFNAR signaling by both leading to reduced levels of STAT2 and by blocking phosphorylation of STAT1, two transcription factors activated by type I IFNs. Taken together, our observations suggest that ZIKV infection induces a type I IFN response via RLRs and that ZIKV interferes with this response by blocking signaling downstream of RLRs and IFNAR.

The cGAS-cGAMP-STING pathway connects DNA damage to inflammation, senescence, and cancer

The cGAS–cGAMP–STING pathway mediates immune and inflammatory responses to cytosolic DNA. This review summarizes recent findings on how genomic instability leads to cGAS activation and how this pathway critically connects DNA damage to autoinflammatory diseases, cellular senescence, and cancer.

Detection of microbial DNA is an evolutionarily conserved mechanism that alerts the host immune system to mount a defense response to microbial infections. However, this detection mechanism also poses a challenge to the host as to how to distinguish foreign DNA from abundant self-DNA. Cyclic guanosine monophosphate (GMP)–adenosine monophosphate (AMP) synthase (cGAS) is a DNA sensor that triggers innate immune responses through production of the second messenger cyclic GMP-AMP (cGAMP), which binds and activates the adaptor protein STING. However, cGAS can be activated by double-stranded DNA irrespective of the sequence, including self-DNA. Although how cGAS is normally kept inactive in cells is still not well understood, recent research has provided strong evidence that genomic DNA damage leads to cGAS activation to stimulate inflammatory responses. This review summarizes recent findings on how genomic instability and DNA damage trigger cGAS activation and how cGAS serves as a link from DNA damage to inflammation, cellular senescence, and cancer.

STING agonists enable antiviral cross-talk between human cells and confer protection against genital herpes in mice

In recent years, there has been an increasing interest in immunomodulatory therapy as a means to treat various conditions, including infectious diseases. For instance, Toll-like receptor (TLR) agonists have been evaluated for treatment of genital herpes. However, although the TLR7 agonist imiquimod was shown to have antiviral activity in individual patients, no significant effects were observed in clinical trials, and the compound also exhibited significant side effects, including local inflammation. Cytosolic DNA is detected by the enzyme cyclic GMP-AMP (2’3’-cGAMP) synthase (cGAS) to stimulate antiviral pathways, mainly through induction of type I interferon (IFN)s. cGAS is activated upon DNA binding to produce the cyclic dinucleotide (CDN) 2’3’-cGAMP, which in turn binds and activates the adaptor protein Stimulator of interferon genes (STING), thus triggering type I IFN expression. In contrast to TLRs, STING is expressed broadly, including in epithelial cells. Here we report that natural and non-natural STING agonists strongly induce type I IFNs in human cells and in mice in vivo, without stimulating significant inflammatory gene expression. Systemic treatment with 2’3’-cGAMP reduced genital herpes simplex virus (HSV) 2 replication and improved the clinical outcome of infection. More importantly, local application of CDNs at the genital epithelial surface gave rise to local IFN activity, but only limited systemic responses, and this treatment conferred total protection against disease in both immunocompetent and immunocompromised mice. In direct comparison between CDNs and TLR agonists, only CDNs acted directly on epithelial cells, hence allowing a more rapid and IFN-focused immune response in the vaginal epithelium. Thus, specific activation of the STING pathway in the vagina evokes induction of the IFN system but limited inflammatory responses to allow control of HSV2 infections in vivo.

Herpes simplex virus (HSV)-2 is the leading cause of genital ulcers, and HSV-2 infection has also been reported to amplify HIV-transmission. So far, all attempts at making an effective anti-HSV2 vaccine have failed. In recent years, there has been an increasing interest in immunomodulatory therapy as a means to treat infections. Although the TLR7 agonist imiquimod has been shown to have antiviral activity in individual patients, no significant effects were observed in clinical trials, and the compound also exhibited significant side effects including local inflammation. Type I interferon (IFN)s are key players in antiviral defense, and it is now known that the DNA sensor cyclic GMP-AMP synthase produces the cyclic di-nucleotide (CDN) 2’3’-cyclic GMP-AMP (cGAMP), which activates the adaptor protein STING to induce IFN expression. In this work we show that natural and non-natural CDNs activate strong type I IFN responses in vivo without stimulating significant expression of genes driven by the transcription factor NF-κB, which induces inflammation. Application of CDNs at epithelial surfaces gave rise to local IFN activity, but only limited systemic responses. Importantly, all tested treatment regimens, strongly reduced replication of HSV-2 in a model for genital herpes, and significantly reduced development of disease. Finally, when comparing to TLR agonists, CDNs showed the best profile with strong IFN response specifically in the epithelial cells and limited induction of inflammation.

STING dependent sensing - Does HIV actually care?

Sensing of DNA is essential for the innate immune system to detect threats, like viruses, intracellular bacteria or cellular DNA damage. At the centre of this conserved mammalian mechanism stands the adaptor protein STING. STING is highly regulated and is part of a complex signalling network. This network depends on the sensors cGAS and IFI16 to detect misplaced DNA in the cytoplasm as well as on the kinase TBK1 and the transcription factor IRF3. The DNA sensing machinery has been implicated in many diseases, among others HIV. Here we present a comprehensive review of current status on the STING pathway with all its components and regulations related to HIV pathogenesis. By this, we try to answer the question if STING-mediated DNA sensing plays a role in HIV infections.

Maintenance of cyclic GMP-AMP homeostasis by ENPP1 is involved in pseudorabies virus infection

In a previous study, we demonstrated that porcine cyclic GMP-AMP (cGAMP) synthase (cGAS) catalyzes cGAMP production and is an important DNA sensor for the pseudorabies virus (PRV)-induced activation of interferon β (IFN-β). Ectonucleotide pyrophosphatase phosphodiesterase 1 (ENPP1) has recently been identified as the hydrolase of cGAMP in rodents, but its role in porcine cells is not clear. Our recent study demonstrated that porcine ENPP1 is responsible for the homeostasis of cGAMP and is critical for PRV infection. Porcine ENPP1 mRNA is predominantly expressed in muscle. PRV infection was enhanced by ENPP1 overexpression and attenuated by silencing of ENPP1. During PRV infection, the activation of IFN-β and NF-κB was reduced in ENPP1 overexpressed cells and promoted in ENPP1 knockdown cells. Investigation of the molecular mechanisms of ENPP1 during PRV infection showed that ENPP1 hydrolyzed cGAMP in PRV-infected or cGAMP-transfected cells and inhibited IRF3 phosphorylation, reducing IFN-β secretion. These results, combined with those for porcine cGAS, demonstrate that ENPP1 acts coordinately with cGAS to maintain the reservoir of cGAMP and participates in PRV infection.

Intracellular Antiviral Immunity

Innate immunity is traditionally thought of as the first line of defense against pathogens that enter the body. It is typically characterized as a rather weak defense mechanism, designed to restrict pathogen replication until the adaptive immune response generates a tailored response and eliminates the infectious agent. However, intensive research in recent years has resulted in better understanding of innate immunity as well as the discovery of many effector proteins, revealing its numerous powerful mechanisms to defend the host. Furthermore, this research has demonstrated that it is simplistic to strictly separate adaptive and innate immune functions since these two systems often work synergistically rather than sequentially. Here, we provide a broad overview of innate pattern recognition receptors in antiviral defense, with a focus on the TRIM family, and discuss their signaling pathways and mechanisms of action with special emphasis on the intracellular antibody receptor TRIM21.

Tetraspanin blockage reduces exosome-mediated HIV-1 entry

HIV-1 is one of the most studied retroviruses. The role of exosomes in HIV-1 entry and pathogenesis are beginning to be appreciated. Exosomes can incorporate host proteins that are also contained in viruses (e.g., tetraspanins).

Multiple Inhibitory Factors Act in the Late Phase of HIV-1 Replication: a Systematic Review of the Literature

The use of lentiviral vectors for therapeutic purposes has shown promising results in clinical trials. The ability to produce a clinical-grade vector at high yields remains a critical issue. One possible obstacle could be cellular factors known to inhibit human immunodeficiency virus (HIV). To date, five HIV restriction factors have been identified, although it is likely that more factors are involved in the complex HIV-cell interaction. Inhibitory factors that have an adverse effect but do not abolish virus production are much less well described. Therefore, a gap exists in the knowledge of inhibitory factors acting late in the HIV life cycle (from transcription to infection of a new cell), which are relevant to the lentiviral vector production process. The objective was to review the HIV literature to identify cellular factors previously implicated as inhibitors of the late stages of lentivirus production. A search for publications was conducted on MEDLINE via the PubMed interface, using the keyword sequence "HIV restriction factor" or "HIV restriction" or "inhibit HIV" or "repress HIV" or "restrict HIV" or "suppress HIV" or "block HIV," with a publication date up to 31 December 2016. Cited papers from the identified records were investigated, and additional database searches were performed. A total of 260 candidate inhibitory factors were identified. These factors have been identified in the literature as having a negative impact on HIV replication. This study identified hundreds of candidate inhibitory factors for which the impact of modulating their expression in lentiviral vector production could be beneficial.

Emerging Concepts in Innate Immunity

This review introduces recent concepts in innate immunity highlighting some of the latest exciting findings. These include: the discovery of the initiator of pyroptosis, Gasdermin D, and mechanisms of inflammatory caspases in innate immune signaling; the formation of oligomeric signalosomes downstream of innate immune receptors; mechanisms that shape innate immune responses, such as cellular homeostasis, cell metabolism, and pathogen viability; rapid methods of cell-to-cell communication; the interplay between the host and its microbiome and the concept of innate immunological memory. Furthermore, we discuss open questions and illustrate how technological advances, such as CRISPR/Cas9, may provide important answers for outstanding questions in the field of innate immunity.

Reactive Astrocytes in Brain Metastasis

Brain metastasis, the secondary growth of malignant cells within the central nervous system (CNS), exceeds the incidence of primary brain tumors (i.e., gliomas) by tenfold and are seemingly on the rise owing to the emergence of novel targeted therapies that are more effective in controlling extracranial disease relatively to intracranial lesions. Despite the fact that metastasis to the brain poses a unmet clinical problem, with afflicted patients carrying significant morbidity and a fatal prognosis, our knowledge as to how metastatic cells manage to adapt to the tissue environment of the CNS remains limited. Answering this question could pave the way for novel and more specific therapeutic modalities in brain metastasis by targeting the specific makeup of the brain metastatic niche. In regard to this, astrocytes have emerged as the major host cell type that cancer cells encounter and interact with during brain metastasis formation. Similarly to other CNS disorders, astrocytes become reactive and respond to the presence of cancer cells by changing their phenotype and significantly influencing the outcome of disseminated cancer cells within the CNS. Here, we summarize the current knowledge on the contribution of reactive astrocytes in brain metastasis by focusing on the signaling pathways and types of interactions that play a crucial part in the communication with cancer cells and how these could be translated into innovative therapies.

Cyclic Dinucleotides in the Scope of the Mammalian Immune System

First discovered in prokaryotes and more recently in eukaryotes, cyclic dinucleotides (CDNs) constitute a unique branch of second messenger signaling systems. Within prokaryotes CDNs regulate a wide array of different biological processes, whereas in the vertebrate system CDN signaling is largely dedicated to activation of the innate immune system. In this book chapter we summarize the occurrence and signaling pathways of these small-molecule second messengers, most importantly in the scope of the mammalian immune system. In this regard, our main focus is the role of the cGAS-STING axis in the context of microbial infection and sterile inflammation and its implications for therapeutic applications.

Snapin promotes HIV-1 transmission from dendritic cells by dampening TLR8 signaling

HIV-1 traffics through dendritic cells (DCs) en route to establishing a productive infection in T lymphocytes but fails to induce an innate immune response. Within DC endosomes, HIV-1 somehow evades detection by the pattern-recognition receptor (PRR) Toll-like receptor 8 (TLR8). Using a phosphoproteomic approach, we identified a robust and diverse signaling cascade triggered by HIV-1 upon entry into human DCs. A secondary siRNA screen of the identified signaling factors revealed several new mediators of HIV-1 trans-infection of CD4+ T cells in DCs, including the dynein motor protein Snapin. Inhibition of Snapin enhanced localization of HIV-1 with TLR8+ early endosomes, triggered a pro-inflammatory response, and inhibited trans-infection of CD4+ T cells. Snapin inhibited TLR8 signaling in the absence of HIV-1 and is a general regulator of endosomal maturation. Thus, we identify a new mechanism of innate immune sensing by TLR8 in DCs, which is exploited by HIV-1 to promote transmission.

Are Evolution and the Intracellular Innate Immune System Key Determinants in HIV Transmission?

HIV-1 is the single most important sexually transmitted disease in humans from a global health perspective. Among human lentiviruses, HIV-1 M group has uniquely achieved pandemic levels of human-to-human transmission. The requirement to transmit between hosts likely provides the strongest selective forces on a virus, as without transmission, there can be no new infections within a host population. Our perspective is that evolution of all of the virus-host interactions, which are inherited and perpetuated from host-to-host, must be consistent with transmission. For example, CXCR4 use, which often evolves late in infection, does not favor transmission and is therefore lost when a virus transmits to a new host. Thus, transmission inevitably influences all aspects of virus biology, including interactions with the innate immune system, and dictates the biological niche in which the virus exists in the host. A viable viral niche typically does not select features that disfavor transmission. The innate immune response represents a significant selective pressure during the transmission process. In fact, all viruses must antagonize and/or evade the mechanisms of the host innate and adaptive immune systems that they encounter. We believe that viewing host-virus interactions from a transmission perspective helps us understand the mechanistic details of antiviral immunity and viral escape. This is particularly true for the innate immune system, which typically acts from the very earliest stages of the host-virus interaction, and must be bypassed to achieve successful infection. With this in mind, here we review the innate sensing of HIV, the consequent downstream signaling cascades and the viral restriction that results. The centrality of these mechanisms to host defense is illustrated by the array of countermeasures that HIV deploys to escape them, despite the coding constraint of a 10 kb genome. We consider evasion strategies in detail, in particular the role of the HIV capsid and the viral accessory proteins highlighting important unanswered questions and discussing future perspectives.