The cGAS/STING Pathway Is Important for Dendritic Cell Activation but Is Not Essential to Induce Protective Immunity against Mycobacterium tuberculosis Infection

The cGAS-STING pathway drives neuroinflammation and neurodegeneration via cellular and molecular mechanisms in neurodegenerative diseases

Neurodegenerative diseases (NDs) are a type of common chronic progressive disorders characterized by progressive damage to specific cell populations in the nervous system, ultimately leading to disability or death. Effective treatments for these diseases are still lacking, due to a limited understanding of their pathogeneses, which involve multiple cellular and molecular pathways. The triggering of an immune response is a common feature in neurodegenerative disorders. A critical challenge is the intricate interplay between neuroinflammation, neurodegeneration, and immune responses, which are not yet fully characterized. In recent years, the cyclic GMP-AMP synthase (cGAS)-stimulator of interferon gene (STING) pathway, a crucial immune response for intracellular DNA sensing, has gradually gained attention. However, the specific roles of this pathway within cellular types such as immune cells, glial and neuronal cells, and its contribution to ND pathogenesis, remain not fully elucidated. In this review, we systematically explore how the cGAS-STING signaling links various cell types with related cellular effector pathways under the context of NDs for multifaceted therapeutic directions. We emphasize the discovery of condition-dependent cellular heterogeneity in the cGAS-STING pathway, which is integral for understanding the diverse cellular responses and potential therapeutic targets. Additionally, we review the pathogenic role of cGAS-STING activation in Parkinson's disease, ataxia-telangiectasia, and amyotrophic lateral sclerosis. We focus on the complex bidirectional roles of the cGAS-STING pathway in Alzheimer's disease, Huntington's disease, and multiple sclerosis, revealing their double-edged nature in disease progression. The objective of this review is to elucidate the pivotal role of the cGAS-STING pathway in ND pathogenesis and catalyze new insights for facilitating the development of novel therapeutic strategies.

Mechanisms of immune evasion by Mycobacterium tuberculosis: the impact of T7SS and cell wall lipids on host defenses

Mycobacterium tuberculosis (M. tb) is one of the most successful human pathogens, causing a severe and widespread infectious disease. The frequent emergence of multidrug-resistant (MDR) strains has exacerbated this public health crisis, particularly in underdeveloped regions. M. tb employs a sophisticated array of virulence factors to subvert host immune responses, both innate and adaptive. It utilizes the early secretory antigenic target (ESAT6) secretion system 1 (ESX-1) type VII secretion system (T7SS) and cell wall lipids to disrupt phagosomal integrity, inhibiting phagosome maturation, and fusion with lysosomes. Although host cells activate mechanisms such as ubiquitin (Ub), Ub-ligase, and cyclic GMP-AMP synthase-stimulator of interferon genes 1 (CGAS-STING1)-mediated autophagy to inhibit M. tb survival within macrophages, the pathogen counteracts these defenses with its own virulence factors, thereby inhibiting autophagy and dampening host-directed responses. T7SSs are critical for transporting proteins across the complex mycobacterial cell envelope, performing essential functions, including metabolite uptake, immune evasion, and conjugation. T7SS substrates fall into two main families: ESAT-6 system proteins, which are found in both Firmicutes and Actinobacteria, and proline-glutamic acid (PE) and proline-proline-glutamic acid (PPE) proteins, which are unique to mycobacteria. Recent studies have highlighted the significance of T7SSs in mycobacterial growth, virulence, and pathogenesis. Understanding the mechanisms governing T7SSs could pave the way for novel therapeutic strategies to combat mycobacterial diseases, including tuberculosis (TB).

Regulation of Type I Interferon and Autophagy in Immunity against Mycobacterium Tuberculosis: Role of CGAS and STING1

Mycobacterium tuberculosis (M. tb) is a significant intracellular pathogen responsible for numerous infectious disease-related deaths worldwide. It uses ESX-1 T7SS to damage phagosomes and to enter the cytosol of host cells after phagocytosis. During infection, M. tb and host mitochondria release dsDNA, which activates the CGAS-STING1 pathway. This pathway leads to the production of type I interferons and proinflammatory cytokines and activates autophagy, which targets and degrades bacteria within autophagosomes. However, the role of type I IFNs in immunity against M. tb is controversial. While previous research has suggested a protective role, recent findings from cgas-sting1 knockout mouse studies have contradicted this. Additionally, a study using knockout mice and non-human primate models uncovered a new mechanism by which neutrophils recruited to lung infections form neutrophil extracellular traps. Activating plasmacytoid dendritic cells causes them to produce type I IFNs, which interfere with the function of interstitial macrophages and increase the likelihood of tuberculosis. Notably, M. tb uses its virulence proteins to disrupt the CGAS-STING1 signaling pathway leading to enhanced pathogenesis. Investigating the CGAS-STING1 pathway can help develop new ways to fight tuberculosis.

Activation of the lysosomal damage response and selective autophagy: the coordinated actions of galectins, TRIM proteins, and CGAS-STING1 in providing immunity against Mycobacterium tuberculosis

Autophagy is a crucial immune defense mechanism that controls the survival and pathogenesis of M. tb by maintaining cell physiology during stress and pathogen attack. The E3-Ub ligases (PRKN, SMURF1, and NEDD4) and autophagy receptors (SQSTM1, TAX1BP1, CALCOCO2, OPTN, and NBR1) play key roles in this process. Galectins (LGALSs), which bind to sugars and are involved in identifying damaged cell membranes caused by intracellular pathogens such as M. tb, are essential. These include LGALS3, LGALS8, and LGALS9, which respond to endomembrane damage and regulate endomembrane damage caused by toxic chemicals, protein aggregates, and intracellular pathogens, including M. tb. They also activate selective autophagy and de novo endolysosome biogenesis. LGALS3, LGALS9, and LGALS8 interact with various components to activate autophagy and repair damage, while CGAS-STING1 plays a critical role in providing immunity against M. tb by activating selective autophagy and producing type I IFNs with antimycobacterial functions. STING1 activates cGAMP-dependent autophagy which provides immunity against various pathogens. Additionally, cytoplasmic surveillance pathways activated by ds-DNA, such as inflammasomes mediated by NLRP3 and AIM2 complexes, control M. tb. Modulation of E3-Ub ligases with small regulatory molecules of LGALSs and TRIM proteins could be a novel host-based therapeutic approach for controlling TB.

Activation of host nucleic acid sensors by Mycobacterium: good for us or good for them?

Although the importance of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) sensors in controlling viral infection is well established, their role in promoting an effective immune response to pathogens other than viruses is less clear. This is particularly true for infections with mycobacteria, as studies point to both protective and detrimental roles for activation of nucleic acid sensors in controlling a mycobacterial infection. Some of the contradiction likely stems from the use of different model systems and different mycobacterial species/strains as well as from which nucleic acid sensors were studied and what downstream effectors were evaluated. In this review, we will describe the different nucleic acid sensors that have been studied in the context of mycobacterial infections, and how the different studies compare. We conclude with a section on how nucleic acid sensor agonists have been used therapeutically and what further information is needed to enhance their potential as therapeutic agents.

Hypoxic glioblastoma-cell-derived extracellular vesicles impair cGAS-STING activity in macrophages

BACKGROUND

Solid tumors such as glioblastoma (GBM) exhibit hypoxic zones that are associated with poor prognosis and immunosuppression through multiple cell intrinsic mechanisms. However, release of extracellular vesicles (EVs) has the potential to transmit molecular cargos between cells. If hypoxic cancer cells use EVs to suppress functions of macrophages under adequate oxygenation, this could be an important underlying mechanism contributing to the immunosuppressive and immunologically cold tumor microenvironment of tumors such as GBM.

METHODS

EVs were isolated by differential ultracentrifugation from GBM cell culture supernatant. EVs were thoroughly characterized by transmission and cryo-electron microscopy, nanoparticle tracking analysis (NTA), and EV marker expression by Western blot and fluorescent NTA. EV uptake by macrophage cells was observed using confocal microscopy. The transfer of miR-25/93 as an EV cargo to macrophages was confirmed by miRNA real-time qPCR. The impact of miR-25/93 on the polarization of recipient macrophages was shown by transcriptional analysis, cytokine secretion and functional assays using co-cultured T cells.

RESULTS

We show that indirect effects of hypoxia can have immunosuppressive consequences through an EV and microRNA dependent mechanism active in both murine and human tumor and immune cells. Hypoxia enhanced EV release from GBM cells and upregulated expression of miR-25/93 both in cells and in EV cargos. Hypoxic GBM-derived EVs were taken up by macrophages and the miR-25/93 cargo was transferred, leading to impaired cGAS-STING pathway activation revealed by reduced type I IFN expression and secretion by macrophages. The EV-treated macrophages downregulated expression of M1 polarization-associated genes Cxcl9, Cxcl10 and Il12b, and had reduced capacity to attract activated T cells and to reactivate them to release IFN-γ, key components of an efficacious anti-tumor immune response.

CONCLUSIONS

Our findings suggest a mechanism by which immunosuppressive consequences of hypoxia mediated via miRNA-25/93 can be exported from hypoxic GBM cells to normoxic macrophages via EVs, thereby contributing to more widespread T-cell mediated immunosuppression in the tumor microenvironment.

TRAF6 triggers Mycobacterium-infected host autophagy through Rab7 ubiquitination

Tumor necrosis factor receptor-associated factor 6 (TRAF6) is an E3 ubiquitin ligase that is extensively involved in the autophagy process by interacting with diverse autophagy initiation and autophagosome maturation molecules. However, whether TRAF6 interacts with lysosomal proteins to regulate Mycobacterium-induced autophagy has not been completely characterized. Herein, the present study showed that TRAF6 interacted with lysosomal key proteins Rab7 through RING domain which caused Rab7 ubiquitination and subsequently ubiquitinated Rab7 binds to STX17 (syntaxin 17, a SNARE protein that is essential for mature autophagosome), and thus promoted the fusion of autophagosomes and lysosomes. Furthermore, TRAF6 enhanced the initiation and formation of autophagosomes in Mycobacterium-induced autophagy in both BMDMs and RAW264.7 cells, as evidenced by autophagic flux, colocalization of LC3 and BCG, autophagy rates, and autophagy-associated protein expression. Noteworthy to mention, TRAF6 deficiency exacerbated lung injury and promoted BCG survival. Taken together, these results identify novel molecular and cellular mechanisms by which TRAF6 positively regulates Mycobacterium-induced autophagy.

Santos R, Brito CB, Ferrero MC, Ortiz Wilczyñski JM, Silva EAC, C. Oliveira S, Baldi PC. Role of the cGAS/STING pathway in the control of Brucella abortus infection acquired through the respiratory route

Despite the importance of the respiratory route for Brucella transmission, the lung immune response to this pathogen is scarcely characterized. We investigated the role of the cGAS/STING pathway of microbial DNA recognition in the control of respiratory Brucella infection. After in vitro B. abortus infection, CFU numbers were significantly higher in alveolar macrophages (AM) and lung explants from STING KO mice than in samples from wild type (WT) mice, but no difference was observed for cGAS KO samples. CFU were also increased in WT AM and lung epithelial cells preincubated with the STING inhibitor H151. Several proinflammatory cytokines (TNF-α, IL-1β, IL-6, IP-10/CXCL10) were diminished in Brucella-infected lung explants and/or AM from STING KO mice and cGAS KO mice. These cytokines were also reduced in infected AM and lung epithelial cells pretreated with H151. After intratracheal infection with B. abortus, STING KO mice exhibited increased CFU in lungs, spleen and liver, a reduced expression of IFN-β mRNA in lungs and spleen, and reduced levels of proinflammatory cytokines and chemokines in bronchoalveolar lavage fluid (BALF) and lung homogenates. Increased lung CFU and reduced BALF cytokines were also observed in cGAS KO mice. In summary, the cGAS/STING pathway induces the production of proinflammatory cytokines after respiratory Brucella infection, which may contribute to the STING-dependent control of airborne brucellosis.

Engineering an active immunotherapy for personalized cancer treatment and prevention of recurrence

Breast cancer has been shown to be resistant to immunotherapies. To overcome this challenge, we developed an active immunotherapy for personalized treatment based on a smart nanovesicle. This is achieved by anchoring membrane-bound bioactive interleukin 2 (IL2) and enriching T cell-promoting costimulatory factors on the surface of the dendritic cell-derived small extracellular vesicles. This nanovesicle also displays major histocompatibility complex-bound antigens inherited from tumor lysate-pulsed dendritic cell. When administrated, the surface-bound IL2 is able to guide the nanovesicle to lymphoid organs and activate the IL2 receptor on lymphocytes. Furthermore, it is able to perform antigen presentation in the replacement of professional antigen-presenting cells. This nanovesicle, named IL2-ep13nsEV, induced a strong immune reaction to rescue 50% of the mice in our humanized patient-derived xenografts, sensitized cancer cells to immune checkpoint inhibitor treatment, and prevented the recurrence of resected tumors. This paradigm presents a feasible strategy for the treatment and prevention of metastatic breast cancer.

Recent Developments in the Understanding of Immunity, Pathogenesis and Management of COVID-19

Coronaviruses represent a diverse family of enveloped positive-sense single stranded RNA viruses. COVID-19, caused by Severe Acute Respiratory Syndrome Coronavirus-2, is a highly contagious respiratory disease transmissible mainly via close contact and respiratory droplets which can result in severe, life-threatening respiratory pathologies. It is understood that glutathione, a naturally occurring antioxidant known for its role in immune response and cellular detoxification, is the target of various proinflammatory cytokines and transcription factors resulting in the infection, replication, and production of reactive oxygen species. This leads to more severe symptoms of COVID-19 and increased susceptibility to other illnesses such as tuberculosis. The emergence of vaccines against COVID-19, usage of monoclonal antibodies as treatments for infection, and implementation of pharmaceutical drugs have been effective methods for preventing and treating symptoms. However, with the mutating nature of the virus, other treatment modalities have been in research. With its role in antiviral defense and immune response, glutathione has been heavily explored in regard to COVID-19. Glutathione has demonstrated protective effects on inflammation and downregulation of reactive oxygen species, thereby resulting in less severe symptoms of COVID-19 infection and warranting the discussion of glutathione as a treatment mechanism.

Interval- and cycle-dependent combined effect of STING agonist loaded lipid nanoparticles and a PD-1 antibody

Programmed cell death 1 (PD-1) blockade combination to other drugs have attracted the interest of scientists for treating tumors resistant to PD-1 blockade. In this study, the impact of the interval, order of administration, and number of cycles of immunotherapeutic combination of stimulator of interferon genes (STING) pathway agonist loaded lipid nanoparticle (STING-LNP) and PD-1 antibody for inducing the optimal combined antitumor activity against a melanoma lung metastasis is reported. One cycle had no effect, but two and three cycles resulted in a combinedantitumor effect. The interval between the administration was found to influence the induction of the combined effect. The second and third doses increased the gene expression of the NK cell activation marker, interferon γ (IFN-γ), PD-1 and a ligand of PD-1 (PD-L1), whereas the first dose failed. NK cells in the lung showed an increase in the expression of the activation markers and PD-1 after the second dose. The combined antitumor effect of this combination therapy against melanoma lung metastasis model could be dependent on the interval as well as the number of doses of STING-LNP.These findings suggest the importance of the protocol setting when combining a nano system loaded with an immune adjuvant and PD-1 antibody.

Detecting DNA: An Overview of DNA Recognition by Inflammasomes and Protection against Bacterial Respiratory Infections

The innate immune system plays a key role in modulating host immune defense during bacterial disease. Upon sensing pathogen-associated molecular patterns (PAMPs), the multi-protein complex known as the inflammasome serves a protective role against bacteria burden through facilitating pathogen clearance and bacteria lysis. This can occur through two mechanisms: (1) the cleavage of pro-inflammatory cytokines IL-1β/IL-18 and (2) the initiation of inflammatory cell death termed pyroptosis. In recent literature, AIM2-like Receptor (ALR) and Nod-like Receptor (NLR) inflammasome activation has been implicated in host protection following recognition of bacterial DNA. Here, we review current literature synthesizing mechanisms of DNA recognition by inflammasomes during bacterial respiratory disease. This process can occur through direct sensing of DNA or indirectly by sensing pathogen-associated intracellular changes. Additionally, DNA recognition may be assisted through inflammasome-inflammasome interactions, specifically non-canonical inflammasome activation of NLRP3, and crosstalk with the interferon-inducible DNA sensors Stimulator of Interferon Genes (STING) and Z-DNA Binding Protein-1 (ZBP1). Ultimately, bacterial DNA sensing by inflammasomes is highly protective during respiratory disease, emphasizing the importance of inflammasome involvement in the respiratory tract.

Precision Vaccine Development: Cues From Natural Immunity

Traditional vaccine development against infectious diseases has been guided by the overarching aim to generate efficacious vaccines normally indicated by an antibody and/or cellular response that correlates with protection. However, this approach has been shown to be only a partially effective measure, since vaccine- and pathogen-specific immunity may not perfectly overlap. Thus, some vaccine development strategies, normally focused on targeted generation of both antigen specific antibody and T cell responses, resulting in a long-lived heterogenous and stable pool of memory lymphocytes, may benefit from better mimicking the immune response of a natural infection. However, challenges to achieving this goal remain unattended, due to gaps in our understanding of human immunity and full elucidation of infectious pathogenesis. In this review, we describe recent advances in the development of effective vaccines, focusing on how understanding the differences in the immunizing and non-immunizing immune responses to natural infections and corresponding shifts in immune ontogeny are crucial to inform the next generation of infectious disease vaccines.

Impact of STING Inflammatory Signaling during Intracellular Bacterial Infections

The early detection of bacterial pathogens through immune sensors is an essential step in innate immunity. STING (Stimulator of Interferon Genes) has emerged as a key mediator of inflammation in the setting of infection by connecting pathogen cytosolic recognition with immune responses. STING detects bacteria by directly recognizing cyclic dinucleotides or indirectly by bacterial genomic DNA sensing through the cyclic GMP-AMP synthase (cGAS). Upon activation, STING triggers a plethora of powerful signaling pathways, including the production of type I interferons and proinflammatory cytokines. STING activation has also been associated with the induction of endoplasmic reticulum (ER) stress and the associated inflammatory responses. Recent reports indicate that STING-dependent pathways participate in the metabolic reprogramming of macrophages and contribute to the establishment and maintenance of a robust inflammatory profile. The induction of this inflammatory state is typically antimicrobial and related to pathogen clearance. However, depending on the infection, STING-mediated immune responses can be detrimental to the host, facilitating bacterial survival, indicating an intricate balance between immune signaling and inflammation during bacterial infections. In this paper, we review recent insights regarding the role of STING in inducing an inflammatory profile upon intracellular bacterial entry in host cells and discuss the impact of STING signaling on the outcome of infection. Unraveling the STING-mediated inflammatory responses can enable a better understanding of the pathogenesis of certain bacterial diseases and reveal the potential of new antimicrobial therapy.

Brucella spp. Omp25 Promotes Proteasome-Mediated cGAS Degradation to Attenuate IFN-β Production

Type I interferons (IFN), a family of cytokines widely expressed in various tissues, play important roles in anti-infection immunity. Nevertheless, it is not known whether Brucella spp. could interfere with IFN-I production induced by other pathogens. This study investigated the regulatory roles of Brucella outer membrane protein (Omp)25 on the IFN-I signaling pathway and found that Omp25 inhibited the production of IFN-β and its downstream IFN-stimulated genes induced by various DNA viruses or IFN-stimulatory DNA in human, murine, porcine, bovine, and ovine monocyte/macrophages or peripheral blood mononuclear cells. Brucella Omp25 suppressed the phosphorylation of stimulator of IFN genes (STINGs) and IFN regulatory factor 3 and nuclear translocation of phosphorylated IFN regulatory factor 3 in pseudorabies virus- or herpes simplex virus-1-infected murine, human, or porcine macrophages. Furthermore, we found that Brucella Omp25 promoted cyclic guanosine monophosphate-adenosine monophosphate synthase (cGAS) degradation via the proteasome-dependent pathway, resulting in a decreased cyclic guanosine monophosphate-adenosine monophosphate production and downstream signaling activation upon DNA virus infection or IFN-stimulatory DNA stimulation. Mapping the predominant function domain of Omp25 showed that the amino acids 161 to 184 of Omp25 were required for Omp25-induced cGAS degradation, among which five amino acid residues (R176, Y179, R180, Y181, and Y184) were required for the inhibitory effect of Omp25 on IFN-β induction. Altogether, our results demonstrated that Brucella Omp25 inhibits cGAS STING signaling pathway-induced IFN-β via facilitating the ubiquitin-proteasome-dependent degradation of cGAS in various mammalian monocyte/macrophages.

Underwhelming or Misunderstood? Genetic Variability of Pattern Recognition Receptors in Immune Responses and Resistance to Mycobacterium tuberculosis

Human genetic control is thought to affect a considerable part of the outcome of infection with Mycobacterium tuberculosis (Mtb). Most of us deal with the pathogen by containment (associated with clinical "latency") or sterilization, but tragically millions each year do not. After decades of studies on host genetic susceptibility to Mtb infection, genetic variation has been discovered to play a role in tuberculous immunoreactivity and tuberculosis (TB) disease. Genes encoding pattern recognition receptors (PRRs) enable a consistent, molecularly direct interaction between humans and Mtb which suggests the potential for co-evolution. In this review, we explore the roles ascribed to PRRs during Mtb infection and ask whether such a longstanding and intimate interface between our immune system and this pathogen plays a critical role in determining the outcome of Mtb infection. The scientific evidence to date suggests that PRR variation is clearly implicated in altered immunity to Mtb but has a more subtle role in limiting the pathogen and pathogenesis. In contrast to 'effectors' like IFN-γ, IL-12, Nitric Oxide and TNF that are critical for Mtb control, 'sensors' like PRRs are less critical for the outcome of Mtb infection. This is potentially due to redundancy of the numerous PRRs in the innate arsenal, such that Mtb rarely goes unnoticed. Genetic association studies investigating PRRs during Mtb infection should therefore be designed to investigate endophenotypes of infection - such as immunological or clinical variation - rather than just TB disease, if we hope to understand the molecular interface between innate immunity and Mtb.

Intracellular Routing and Recognition of Lipid-Based mRNA Nanoparticles

Messenger RNA (mRNA) is being extensively used in gene therapy and vaccination due to its safety over DNA, in the following ways: its lack of integration risk, cytoplasmic expression, and transient expression compatible with fine regulations. However, clinical applications of mRNA are limited by its fast degradation by nucleases, and the activation of detrimental immune responses. Advances in mRNA applications, with the recent approval of COVID-19 vaccines, were fueled by optimization of the mRNA sequence and the development of mRNA delivery systems. Although delivery systems and mRNA sequence optimization have been abundantly reviewed, understanding of the intracellular processing of mRNA is mandatory to improve its applications. We will focus on lipid nanoparticles (LNPs) as they are the most advanced nanocarriers for the delivery of mRNA. Here, we will review how mRNA therapeutic potency can be affected by its interactions with cellular proteins and intracellular distribution.

Mycobacterium tuberculosis MmsA (Rv0753c) Interacts with STING and Blunts the Type I Interferon Response

It is unclear how the type I IFN response is regulated by mycobacterial determinants. Here, we characterized the previously unreported role of M. tuberculosis MmsA in immunological regulation of type I IFN response by targeting the central adaptor STING in the DNA sensing pathway. We identified STING-interacting MmsA by coimmunoprecipitation-mass spectrometry-based (IP-MS) proteomic analysis and showed MmsA interacting with STING and autophagy receptor p62 via its N terminus and C terminus, respectively. We also showed that MmsA downregulated type I IFN by promoting p62-mediated STING degradation. Moreover, the MmsA mutant R138W is potentially associated with the virulence of M. tuberculosis clinical strains owing to the modulation of STING protein. Our results provide novel insights into the regulatory mechanism of type I IFN response manipulated by mycobacterial MmsA and the additional cross talk between autophagy and STING in M. tuberculosis infection, wherein a protein from microbial pathogens induces autophagic degradation of host innate immune molecules.

Type I interferon (IFN) plays an important role in Mycobacterium tuberculosis persistence and disease pathogenesis. M. tuberculosis has evolved a number of mechanisms to evade host immune surveillance. However, it is unclear how the type I IFN response is tightly regulated by the M. tuberculosis determinants. Stimulator of interferon genes (STING) is an essential adaptor for type I IFN production triggered by M. tuberculosis genomic DNA or cyclic dinucleotides upon infection. To investigate how the type I IFN response is regulated by M. tuberculosis determinants, immunoprecipitation-mass spectrometry-based (IP-MS) proteomic analysis was performed to screen proteins interacting with STING in the context of M. tuberculosis infection. Among the many predicted candidates interacting with STING, the M. tuberculosis coding protein Rv0753c (MmsA) was identified. We confirmed that MmsA binds and colocalizes with STING, and the N-terminal regions of MmsA (amino acids [aa] 1 to 251) and STING (aa 1 TO 190) are responsible for MmsA-STING interaction. Type I IFN production was impaired with exogenous expression of MmsA in RAW264.7 cells. MmsA inhibited the STING-TBK1-IRF3 pathway, as evidenced by reduced STING levelS and subsequent IRF3 activation. Furthermore, MmsA facilitated p62-mediated STING autophagic degradation by binding p62 with its C terminus (aa 252 to 455), which may account for the negative regulation of M. tuberculosis MmsA in STING-mediated type I IFN production. Additionally, the M. tuberculosismmsA R138W mutation, detected in a hypervirulent clinical isolate, enhanced the degradation of STING, implying the important relevance of MmsA in disease outcome. Together, we report a novel mechanism where M. tuberculosis MmsA serves as an antagonist of type I IFN response by targeting STING with p62-mediated autophagic degradation.

Cytosolic Sensors for Pathogenic Viral and Bacterial Nucleic Acids in Fish

Recognition of the non-self signature of invading pathogens is a crucial step for the initiation of the innate immune mechanisms of the host. The host response to viral and bacterial infection involves sets of pattern recognition receptors (PRRs), which bind evolutionarily conserved pathogen structures, known as pathogen-associated molecular patterns (PAMPs). Recent advances in the identification of different types of PRRs in teleost fish revealed a number of cytosolic sensors for recognition of viral and bacterial nucleic acids. These are DExD/H-box RNA helicases including a group of well-characterized retinoic acid inducible gene I (RIG-I)-like receptors (RLRs) and non-RLR DExD/H-box RNA helicases (e.g., DDX1, DDX3, DHX9, DDX21, DHX36 and DDX41) both involved in recognition of viral RNAs. Another group of PRRs includes cytosolic DNA sensors (CDSs), such as cGAS and LSm14A involved in recognition of viral and intracellular bacterial dsDNAs. Moreover, dsRNA-sensing protein kinase R (PKR), which has a role in antiviral immune responses in higher vertebrates, has been identified in fish. Additionally, fish possess a novel PKR-like protein kinase containing Z-DNA binding domain, known as PKZ. Here, we review the current knowledge concerning cytosolic sensors for recognition of viral and bacterial nucleic acids in teleosts.

STING Mediates Lupus via the Activation of Conventional Dendritic Cell Maturation and Plasmacytoid Dendritic Cell Differentiation

Signaling through stimulator of interferon genes (STING) leads to the production of type I interferons (IFN-Is) and inflammatory cytokines. A gain-of-function mutation in STING was identified in an autoinflammatory disease (STING-associated vasculopathy with onset in infancy; SAVI). The expression of cyclic GMP-AMP, DNA-activated cGAS-STING pathway, increased in a proportion of patients with SLE. The STING signaling pathway may be a candidate for targeted therapy in SLE. Here, we demonstrated that disruption of STING signaling ameliorated lupus development in Fcgr2b-deficient mice. Activation of STING promoted maturation of conventional dendritic cells and differentiation of plasmacytoid dendritic cells via LYN interaction and phosphorylation. The inhibition of LYN decreased the differentiation of STING-activated dendritic cells. Adoptive transfer of STING-activated bone marrow-derived dendritic cells into the FCGR2B and STING double-deficiency mice restored lupus phenotypes. These findings provide evidence that the inhibition of STING signaling may be a candidate targeted treatment for a subset of patients with SLE.

New insights into the evasion of host innate immunity by Mycobacterium tuberculosis

Mycobacterium tuberculosis (Mtb) is an extremely successful intracellular pathogen that causes tuberculosis (TB), which remains the leading infectious cause of human death. The early interactions between Mtb and the host innate immune system largely determine the establishment of TB infection and disease development. Upon infection, host cells detect Mtb through a set of innate immune receptors and launch a range of cellular innate immune events. However, these innate defense mechanisms are extensively modulated by Mtb to avoid host immune clearance. In this review, we describe the emerging role of cytosolic nucleic acid-sensing pathways at the host-Mtb interface and summarize recently revealed mechanisms by which Mtb circumvents host cellular innate immune strategies such as membrane trafficking and integrity, cell death and autophagy. In addition, we discuss the newly elucidated strategies by which Mtb manipulates the host molecular regulatory machinery of innate immunity, including the intranuclear regulatory machinery, the ubiquitin system, and cellular intrinsic immune components. A better understanding of innate immune evasion mechanisms adopted by Mtb will provide new insights into TB pathogenesis and contribute to the development of more effective TB vaccines and therapies.

Interferon-independent STING signaling promotes resistance to HSV-1 in vivo

The Stimulator of Interferon Genes (STING) pathway initiates potent immune responses upon recognition of DNA. To initiate signaling, serine 365 (S365) in the C-terminal tail (CTT) of STING is phosphorylated, leading to induction of type I interferons (IFNs). Additionally, evolutionary conserved responses such as autophagy also occur downstream of STING, but their relative importance during in vivo infections remains unclear. Here we report that mice harboring a serine 365-to-alanine (S365A) mutation in STING are unexpectedly resistant to Herpes Simplex Virus (HSV)-1, despite lacking STING-induced type I IFN responses. By contrast, resistance to HSV-1 is abolished in mice lacking the STING CTT, suggesting that the STING CTT initiates protective responses against HSV-1, independently of type I IFNs. Interestingly, we find that STING-induced autophagy is a CTT- and TBK1-dependent but IRF3-independent process that is conserved in the STING S365A mice. Thus, interferon-independent functions of STING mediate STING-dependent antiviral responses in vivo.

Innate type 1 immune response, but not IL-17 cells control tuberculosis infection

The role of the innate immune response and host resistance to Mycobacterium tuberculosis infection (TB) is reviewed. Based on our data and the abundant literature, an early type 1 immune response is critical for infection control, while ILC3 and Th17 cells seem to be dispensable. Indeed, in M. tuberculosis infected mice, transcriptomic levels of Il17, Il17ra, Il22 and Il23a were not significantly modified as compared to controls, suggesting a limited role of IL-17 and IL-22 pathways in TB infection control. Neutralization of IL-17A or IL-17F did not affect infection control either. Ongoing clinical studies with IL-17 neutralizing antibodies show high efficacy in patients with psoriasis without increased incidence of TB infection or reactivation. Therefore, both experimental studies in mice and clinical trials in human patients suggest no risk of TB infection or reactivation by therapeutic IL-17 antibodies, unlike by TNF.

Sodium Butyrate Abrogates the Growth and Pathogenesis of Mycobacterium bovis via Regulation of Cathelicidin (LL37) Expression and NF-κB Signaling

Mycobacterium bovis is the causative agent of bovine tuberculosis, has been identified a serious threat to human population. It has been found that sodium butyrate (NaB), the inhibitor of histone deacetylase, can promote the expression of cathelicidin (LL37) and help the body to resist a variety of injuries. In the current study, we investigate the therapeutic effect of NaB on the regulation of host defense mechanism against M. bovis infection. We found an increased expression of LL37 in M. bovis infected THP-1 cells after NaB treatment. In contrast, NaB treatment significantly down-regulated the expression of Class I HDAC in THP-1 cells infected with M. bovis. Additionally, NaB reduced the expression of phosphorylated P65 (p-P65) and p-IκBα, indicating the inhibition of nuclear factor-κB (NF-κB) signaling. Furthermore, we found that NaB treatment reduced the production of inflammatory cytokines (IL-1β, TNF-α, and IL-10) and a key anti-apoptotic marker protein Bcl-2 in THP-1 cell infected with M. bovis. Notably, mice showed high resistance to M. bovis infection after NaB treatment. The reduction of viable M. bovis bacilli indicates that NaB-induced inhibition of M. bovis infection mediated by upregulation of LL37 and inhibition of NF-κB signaling pathway. These observations illustrate that NaB mediate protective immune responses against M. bovis infection. Overall, these results suggest that NaB can be exploited as a therapeutic strategy for the control of M. bovis in animals and human beings.

miR29a and miR378b Influence CpG-Stimulated Dendritic Cells and Regulate cGAS/STING Pathway

The Cytosine–phosphate–guanosine (CpG) motif, which is specifically recognized intracellularly by dendritic cells (DCs), plays a crucial role in regulating the innate immune response. MicroRNAs (miRNAs) can strongly influence the antigen-presenting ability of DCs. In this study, we examine the action of miRNAs on CpG-stimulated and control DCs, as well as their effect on cyclic guanosine monophosphate-adenosine monophosphate (GMP–AMP) synthase (cGAS) and the stimulator of interferon genes (STING) signal pathway. Firstly, we selected miRNAs (miR-29a and miR-378b) based on expression in CpG-stimulated mouse bone marrow-derived dendritic cells (BMDCs). Secondly, we investigated the functions of miR-29a and miR-378b on CpG-stimulated and unstimulated BMDCs. The results showed that miR-29a and miR-378b increased expression of both the immunoregulatory DC surface markers (CD86 and CD40) and the immunosuppressive molecule CD273 by DCs. Thirdly, cytokine detection revealed that both miR-29a and miR-378b enhanced interferon-β (IFN-β) expression while suppressing tumor necrosis factor-α (TNF-α) production. Finally, our results suggest that miR-378b can bind TANK-binding kinase binding protein 1 (TBKBP1) to activate the cGAS/STING signaling pathway. By contrast, miR-29a targeted interferon regulatory factor 7 (IRF7) and promoted the expression of STING. Together, our results provide insight into the molecular mechanism of miRNA induction by CpG to regulate DC function.

Type I interferon induced by DNA of nontypeable Haemophilus influenza modulates inflammatory cytokine profile to promote susceptibility to this bacterium

Type I interferon (IFN) is indispensable for antiviral immunity, but its role in bacterial infections is controversial and not fully described. Nontypeable Haemophilus influenzae (NTHi) is one of the most common bacterial pathogens in patients with chronic obstructive pulmonary disease (COPD). NTHi-DNA activates type I IFN production in macrophages, but the function of type I IFN in host-pathogen interactions, in the context of NTHi infection, is still unclear. Here, we showed that type I IFN, induced by NTHi-DNA, restrained bacterial killing in vitro and promoted COPD development in vivo in response to NTHi. Mice deficient for type I IFN receptor (IFNAR) exhibited improved resistance to NTHi infection. Moreover, similar to exogenous IFN-β, NTHi-DNA-induced type I IFN increased the production of IL-6, IL-1β, IL-12 and CXCL10 via p38 MAPK activation. Our findings demonstrated that NTHi-DNA-induced type I IFN signaling played a negative role in host defense against NTHi infection and identified potential targets for future therapeutic management of COPD.

A SNP upstream of the cyclic GMP-AMP synthase (cGAS) gene protects from relapse and extra-pulmonary TB and relates to BCG vaccination status in an Indian cohort

Tuberculosis (TB) caused by Mycobacterium tuberculosis (M.tb) is a major health care threat worldwide causing over a million deaths annually. Host-pathogen interaction is complex, and a strong genetic contribution to disease susceptibility has been proposed. We have investigated single-nucleotide polymorphisms (SNPs) within cGAS/STING in Indian TB patients and healthy cohorts from India and Germany by Lightcycler®480 genotyping technique. The cGAS/STING pathway is an essential defense pathway within the cytosol after M.tb is internalized and mycobacterial DNA is released inducing the production of type I IFNs. We found that the rs311686 SNP upstream of cGAS provides protection from getting TB overall and is differently distributed in pulmonary TB patients compared with extra-pulmonary and particularly relapse cases. This SNP furthermore differs in distribution when comparing individuals with respect to BCG vaccination status. Taken together, our results show that the presence of the rs311686 SNP influences the course of TB significantly. However, structural conformation changes were found only for the cGAS rs610913 SNP. These findings underscore the importance of M.tb DNA recognition for TB pathogenesis and may eventually help in risk stratification of individuals. This may ultimately help in prevention of disease and aid in developing new vaccination and treatment strategies.