STING-dependent type I IFN production inhibits cell-mediated immunity to Listeria monocytogenes

ER: a critical hub for STING signaling regulation

The Stimulator of Interferon Genes (STING) has a crucial role in mediating the immune response against cytosolic double-stranded DNA (dsDNA) and its activation is critically involved in various diseases. STING is synthesized, modified, and resides in the endoplasmic reticulum (ER), and its ER exit is intimately connected with its signaling. The ER, primarily known for its roles in protein folding, lipid synthesis, and calcium storage, has been identified as a pivotal platform for the regulation of a wide range of STING functions. In this review, we discuss the emerging factors that regulate STING in the ER and examine the interplay between STING signaling and ER pathways, highlighting the impacts of such regulations on immune responses and their potential implications in STING-related disorders.

Bacterial second messenger cyclic di-AMP in streptococci

Cyclic dimeric adenosine monophosphate (c-di-AMP) has been well studied in bacteria, including those of the genus Streptococcus, since the first recognition of this dinucleotide in 2008. Streptococci possess a sole diadenylate cyclase, CdaA, and distinct c-di-AMP phosphodiesterases. Interestingly, cdaA is required for viability of some streptococcal species but not all when streptococci are grown in standard laboratory media. Bacteria of this genus also have distinct c-di-AMP effector proteins, diverse c-di-AMP-signaling pathways, and subsequent biological outcomes. In streptococci, c-di-AMP may influence bacterial growth, morphology, biofilm formation, competence program, drug resistance, and bacterial pathogenesis. c-di-AMP secreted by streptococci has also been shown to interact with the mammalian host and induces immune responses including type I interferon production. In this review, we summarize the reported c-di-AMP networks in seven species of the genus Streptococcus, which cause diverse clinical manifestations, and propose future perspectives to investigate the signaling molecule in these streptococcal pathogens.

Characterization of STING from common carp (Cyprinus carpio L.) involved in spring viremia of carp virus infection

Stimulator of interferon genes (STING) serve as an endoplasmic reticulum (ER) protein and modulates innate immune responses to viral contagion. Most investigations involving teleost STING antiviral immunity have examined DNA viruses. Therefore, fish STING signaling events against RNA viruses require additional exploration. Here, common carp STING (named CcSTING) was cloned and characterized. The bioinformatics analyses of CcSTING showed evolutionary conservations and were most closely related to other cyprinid STINGs. Immunofluorescence staining discovered that the CcSTING was chiefly placed in the cytoplasm, specifically within the ER. CcSTING was ubiquitously generated in all analyzed organs, with especially strong expression in the gills and head kidney. Spring viremia of carp virus (SVCV) stimulation and poly(I:C) infection induced the generation of CcSTING in immune-associated organs, as well as in peripheral blood leukocytes. Additional investigations revealed that CcSTING overexpression strongly suppressed SVCV replication in EPC cells. Mechanistically, CcSTING enhanced IFN-1 and ISGs expression following SVCV infection. CcSTING also substantially increased both IFN and NF-κB promoter luciferase activity via a dosage-dependent fashion. Lastly, CcSTING significantly up-regulated both TBK1 and p65 phosphorylation. Collectively, these findings demonstrated the critical role and underlying mechanism of fish STING in response to RNA virus.

Recent trends in STING modulators: Structures, mechanisms, and therapeutic potential

The cyclic GMP-AMP synthase stimulator (cGAS)-stimulator of interferon gene (STING) signaling pathway has an integral role in the host immune response through DNA sensing followed by inducing a robust innate immune defense program. STING has become a promising therapeutic target associated with multiple diseases, including various inflammatory diseases, cancer, and infectious diseases, among others. Thus, modulators of STING are regarded as emerging therapeutic agents. Recent progress has been made in STING research, including recently identified STING-mediated regulatory pathways, the development of a new STING modulator, and the new association of STING with disease. In this review, we focus on recent trends in the development of STING modulators, including structures, mechanisms, and clinical application.

TRIM32 reduced the recruitment of innate immune cells and the killing capacity of Listeria monocytogenes by inhibiting secretion of chemokines

Listeria monocytogenes (Lm) is a facultative, intracellular Gram-positive pathogenic bacterium that causes sepsis, a condition characterized by persistent excessive inflammation and organ dysfunction. However, the pathogenesis of Lm-induced sepsis is unknown. In this research, we discovered that TRIM32 is required for innate immune regulation during Lm infection. Trim32 deficiency remarkably reduced bacteremia and proinflammatory cytokine secretion in mice with severe Lm infection, preventing sepsis. Trim32^-/- mice had a lower bacterial burden after Lm infection and survived significantly longer than wild-type (WT) mice, as well as lower serum levels of inflammatory cytokines TNF-α, IL-6, IL-18, IL-12p70, IFN-β, and IFN-γ at 1 day post infection (dpi) compared to WT mice. On the other hand, the chemokines CXCL1, CCL2, CCL7, and CCL5 were enhanced at 3 dpi in Trim32^-/- mice than WT mice, reflecting increased recruitment of neutrophils and macrophages. Furthermore, Trim32^-/- mice had higher levels of macrophage-associated iNOS to kill Lm. Collectively, our findings suggest that TRIM32 reduces innate immune cells recruitment and Lm killing capabilities via iNOS production.

Development of VHL-recruiting STING PROTACs that suppress innate immunity

STING acts as a cytosolic nucleotide sensor to trigger host defense upon viral or bacterial infection. While STING hyperactivation can exert anti-tumor effects by increasing T cell filtrates, in other contexts hyperactivation of STING can contribute to autoimmune and neuroinflammatory diseases. Several STING targeting agonists and a smaller subset of antagonists have been developed, yet STING targeted degraders, or PROTACs, remain largely underexplored. Here, we report a series of STING-agonist derived PROTACs that promote STING degradation in renal cell carcinoma (RCC) cells. We show that our STING PROTACs activate STING and target activated/phospho-STING for degradation. Locking STING on the endoplasmic reticulum via site-directed mutagenesis disables STING translocation to the proteasome and resultingly blocks STING degradation. We also demonstrate that PROTAC treatment blocks downstream innate immune signaling events and attenuates the anti-viral response. Interestingly, we find that VHL acts as a bona fide E3 ligase for STING in RCC; thus, VHL-recruiting STING PROTACs further promote VHL-dependent STING degradation. Our study reveals the design and biological assessment of VHL-recruiting agonist-derived STING PROTACs, as well as demonstrates an example of hijacking a physiological E3 ligase to enhance target protein degradation via distinct mechanisms.

The role of STING signaling in central nervous system infection and neuroinflammatory disease

The cyclic guanosine monophosphate-adenosine monophosphate (GMP-AMP) synthase-Stimulator of Interferon Genes (cGAS-STING) pathway is a critical innate immune mechanism for detecting the presence of double-stranded DNA (dsDNA) and prompting a robust immune response. Canonical cGAS-STING activation occurs when cGAS, a predominantly cytosolic pattern recognition receptor, binds microbial DNA to promote STING activation. Upon STING activation, transcription factors enter the nucleus to cause the production of Type I interferons, inflammatory cytokines whose primary function is to prime the host for viral infection by producing a number of antiviral interferon-stimulated genes. While the pathway was originally described in viral infection, more recent studies have implicated cGAS-STING signaling in a number of different contexts, including autoimmune disease, cancer, injury, and neuroinflammatory disease. This review focuses on how our understanding of the cGAS-STING pathway has evolved over time with an emphasis on the role of STING-mediated neuroinflammation and infection in the nervous system. We discuss recent findings on how STING signaling contributes to the pathology of pain, traumatic brain injury, and stroke, as well as how mitochondrial DNA may promote STING activation in common neurodegenerative diseases. We conclude by commenting on the current knowledge gaps that should be filled before STING can be an effective therapeutic target in neuroinflammatory disease. This article is categorized under: Neurological Diseases > Molecular and Cellular Physiology Infectious Diseases > Molecular and Cellular Physiology Immune System Diseases > Molecular and Cellular Physiology.

STING is a prognostic factor related to tumor necrosis, sarcomatoid dedifferentiation, and distant metastasis in clear cell renal cell carcinoma

STING is a molecule involved in immune reactions against double-stranded DNA fragments, released in infective and neoplastic diseases, whose role in the interactions between immune and neoplastic cells in clear cell renal cell carcinoma has not been studied yet. We investigated the immunohistochemical expression of STING in a series of 146 clear-cell renal cell carcinomas and correlated it with the main pathological prognostic factors. Furthermore, tumoral inflammatory infiltrate was evaluated and studied for the subpopulations of lymphocytes. Expression of STING was observed in 36% (53/146) of the samples, more frequently in high-grade (G3-G4) tumors (48%,43/90) and recurrent/metastatic ones (75%, 24/32) than in low grade (G1-G2) and indolent neoplasms (16%, 9/55). STING staining correlated with parameters of aggressive behavior, including coagulative granular necrosis (p = 0.001), stage (p < 0.001), and development of metastases (p < 0.001). Among prognostic parameters, STING immune expression reached an independent statistical significance (p = 0.029) in multivariable analysis, along with the stage and the presence of coagulative granular necrosis. About tumor immune-environment, no significant statistical association has been demonstrated between tumor-infiltrating lymphocytes and STING. Our results provide novel insights regarding the role of STING in aggressive clear cell renal cell carcinomas, suggesting its adoption as a prognostic marker and a potentially targetable molecule for specific immunotherapies.

CRISPR/Cas9-Induced Knockout of Sting Increases Susceptibility of Zebrafish to Bacterial Infection

Stimulator of interferon genes (STING) is an adapter protein that is activated when cyclic dinucleotides (CDNs) are present. CDNs originate from the cytosolic DNA of both pathogens and hosts. STING activation promotes efficient immune responses against viral infections; however, its impact in bacterial infections is unclear. In this study, we investigated the role of Sting in bacterial infections by successfully creating a sting-deficient (sting^(-/-) with a 4-bp deletion) knockout zebrafish model using CRISPR/Cas9. The transcriptional modulation of genes downstream of cGAS (cyclic GMP-AMP synthase)-Sting pathway-related genes was analyzed in seven-day-old wild-type (WT) and sting^(-/-) embryos, as well as in four-day-old LPS-stimulated embryos. The expression of downstream genes was higher in sting^(-/-) than in healthy WT fish. The late response was observed in sting^(-/-) larvae following LPS treatment, demonstrating the importance of Sting-induced immunity during bacterial infection by activating the cGAS-STING pathway. Furthermore, adult sting^(-/-) fish had a high mortality rate and significantly downregulated cGAS-STING pathway-related genes during Edwardsiella piscicida (E. piscicida) infection. In addition, we assessed NF-κB pathway genes following E. piscicida infection. Our results show fluctuating patterns of interleukin-6 (il6) and tumor necrosis factor-α (tnfα) expression, which is likely due to the influence of other NF-κB pathway-related immune genes. In summary, this study demonstrates the important role of Sting against bacterial infection.

STING controls T cell memory fitness during infection through T cell-intrinsic and IDO-dependent mechanisms

Stimulator of interferon genes (STING) signaling has been extensively studied in inflammatory diseases and cancer, while its role in T cell responses to infection is unclear. Using Listeria monocytogenes strains engineered to induce different levels of c-di-AMP, we found that high STING signals impaired T cell memory upon infection via increased Bim levels and apoptosis. Unexpectedly, reduction of TCR signal strength or T cell-STING expression decreased Bim expression, T cell apoptosis, and recovered T cell memory. We found that TCR signal intensity coupled STING signal strength to the unfolded protein response (UPR) and T cell survival. Under strong STING signaling, Indoleamine-pyrrole 2,3-dioxygenase (IDO) inhibition also reduced apoptosis and led to a recovery of T cell memory in STING sufficient CD8 T cells. Thus, STING signaling regulates CD8 T cell memory fitness through both cell-intrinsic and extrinsic mechanisms. These studies provide insight into how IDO and STING therapies could improve long-term T cell protective immunity.

STING Targeting in Lung Diseases

The cGAS-STING pathway displays important functions in the regulation of innate and adaptive immunity following the detection of microbial and host-derived DNA. Here, we briefly summarize biological functions of STING and review recent literature highlighting its important contribution in the context of respiratory diseases. Over the last years, tremendous progress has been made in our understanding of STING activation, which has favored the development of STING agonists or antagonists with potential therapeutic benefits. Antagonists might alleviate STING-associated chronic inflammation and autoimmunity. Furthermore, pharmacological activation of STING displays strong antiviral properties, as recently shown in the context of SARS-CoV-2 infection. STING agonists also elicit potent stimulatory activities when used as an adjuvant promoting antitumor responses and vaccines efficacy.

Pseudomonas aeruginosa Induces Interferon-β Production to Promote Intracellular Survival

Pseudomonas aeruginosa (PA) is known as one kind of extracellular pathogens. However, more evidence showed that PA encounters the intracellular environment in different mammalian cell types. Little is known of innate immune factors modulating intracellular PA survival. In the present study, we proposed that interferon-β (IFN-β) is beneficial to the survival of PA in the cytoplasm of macrophages. Furthermore, we found that interleukin-1β (IL-1β) induced by PA suppresses IFN-β response driven by the cGAS-STING-TBK1 pathway. Mechanistically, IL-1β decreased the production of cyclic GMP-AMP (cGAMP) by activating AKT kinase. cGAMP is necessarily sufficient to stimulate the transcription of IFN-β via the STING adaptor-TBK1 kinase-IRF3 transcription factor axis. Thus, our findings uncovered a novel module for PA intracellular survival involving IFN-β production restricted by IL-1β and provided a strong rationale for a potential clinical strategy against pulmonary PA infection patients. IMPORTANCE The link between innate immunity and intracellular Pseudomonas aeruginosa is unclear. Our studies illuminated the role of interferon-β (IFN-β) in remote intracellular PA infection. Furthermore, our experimental evidence also indicated that IL-1β is a negative regulator of IFN-β production and, in particular, P. aeruginosa infection. The inhibition of IFN-β may be used as a potential therapeutic method against pulmonary PA infection.

A stimulator of interferon gene (CgSTING) involved in antimicrobial immune response of oyster Crassostrea gigas

The stimulator of interferon gene (STING), an intracellular sensor of cyclic dinucleotides, is critical to the innate immune response, especially the induction of type I interferon (IFN) during pathogenic infection. A STING homologue (CgSTING) regulating the expression of IFN-like protein (CgIFNLP) was previously identified in the Pacific oyster Crassostrea gigas, and its involvement in antibacterial immunity was further investigated in the present study. The mRNA transcripts of CgSTING were ubiquitously detected in all the three subpopulations of haemocytes with the highest expression in semi-granulocytes. After the stimulation with Vibrio splendidus, the mRNA expression of CgSTING in haemocytes was significantly up-regulated and peaked at 72 h, which was 12.91-fold of that in control group (p < 0.01). The CgSTING protein was mainly located in the cytoplasm of haemocytes. After the expression of CgSTING was knocked down (0.12-fold of that in control group, p < 0.05) by RNAi, the mRNA expression levels of interleukin17-1 (CgIL17-1), interleukin17-3 (CgIL17-3), interleukin17-4 (CgIL17-4), defensins (Cgdefh1, Cgdefh2), big defensin (CgBigDef1), interferon-like protein (CgIFNLP), tumor necrosis factor (CgTNF) and nuclear factor-κB (CgRel) all decreased significantly at 12 h after V. splendidus stimulation, which was 0.12-fold-0.72-fold (p < 0.05) of that in control group, respectively. The positive signals of CgRel were observed in the haemocyte nucleus after V. splendidus stimulation. The nuclear translocation of CgRel was suppressed in CgSTING-RNAi oysters, and the green signals of CgRel were mainly observed in the haemocyte cytoplasm after V. splendidus stimulation. Furthermore, the number of V. splendidus in the haemolymph of CgSTING-RNAi oysters increased significantly, which was 26.78-fold (p < 0.01) of that in the control group at 12 h after V. splendidus stimulation. These results indicated that CgSTING played important role in the immune defense against bacterial infection by inducing the expressions of cytokines and defensins.

The role of bacterial cyclic di-adenosine monophosphate in the host immune response

Cyclic di-adenosine monophosphate (c-di-AMP) is a second messenger which is widely used in signal transduction in bacteria and archaea. c-di-AMP plays an important role in the regulation of bacterial physiological activities, such as the cell cycle, cell wall stability, environmental stress response, and biofilm formation. Moreover, c-di-AMP produced by pathogens can be recognized by host cells for the activation of innate immune responses. It can induce type I interferon (IFN) response in a stimulator of interferon genes (STING)-dependent manner, activate the nuclear factor kappa B (NF-κB) pathway, inflammasome, and host autophagy, and promote the production and secretion of cytokines. In addition, c-di-AMP is capable of triggering a host mucosal immune response as a mucosal adjuvant. Therefore, c-di-AMP is now considered to be a new pathogen-associated molecular pattern in host immunity and has become a promising target in bacterial/viral vaccine and drug research. In this review, we discussed the crosstalk between bacteria and host immunity mediated by c-di-AMP and addressed the role of c-di-AMP as a mucosal adjuvant in boosting evoked immune responses of subunit vaccines. The potential application of c-di-AMP in immunomodulation and immunotherapy was also discussed in this review.

Post-Translational Modifications of STING: A Potential Therapeutic Target

Stimulator of interferon genes (STING) is an endoplasmic-reticulum resident protein, playing essential roles in immune responses against microbial infections. However, over-activation of STING is accompanied by excessive inflammation and results in various diseases, including autoinflammatory diseases and cancers. Therefore, precise regulation of STING activities is critical for adequate immune protection while limiting abnormal tissue damage. Numerous mechanisms regulate STING to maintain homeostasis, including protein-protein interaction and molecular modification. Among these, post-translational modifications (PTMs) are key to accurately orchestrating the activation and degradation of STING by temporarily changing the structure of STING. In this review, we focus on the emerging roles of PTMs that regulate activation and inhibition of STING, and provide insights into the roles of the PTMs of STING in disease pathogenesis and as potential targeted therapy.

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.

An Immunomodulatory Transcriptional Signature Associated With Persistent Listeria Infection in Hepatocytes

Listeria monocytogenes causes severe foodborne illness in pregnant women and immunocompromised individuals. After the intestinal phase of infection, the liver plays a central role in the clearance of this pathogen through its important functions in immunity. However, recent evidence suggests that during long-term infection of hepatocytes, a subpopulation of Listeria may escape eradication by entering a persistence phase in intracellular vacuoles. Here, we examine whether this long-term infection alters hepatocyte defense pathways, which may be instrumental for bacterial persistence. We first optimized cell models of persistent infection in human hepatocyte cell lines HepG2 and Huh7 and primary mouse hepatocytes (PMH). In these cells, Listeria efficiently entered the persistence phase after three days of infection, while inducing a potent interferon response, of type I in PMH and type III in HepG2, while Huh7 remained unresponsive. RNA-sequencing analysis identified a common signature of long-term Listeria infection characterized by the overexpression of a set of genes involved in antiviral immunity and the under-expression of many acute phase protein (APP) genes, particularly involved in the complement and coagulation systems. Infection also altered the expression of cholesterol metabolism-associated genes in HepG2 and Huh7 cells. The decrease in APP transcripts was correlated with lower protein abundance in the secretome of infected cells, as shown by proteomics, and also occurred in the presence of APP inducers (IL-6 or IL-1β). Collectively, these results reveal that long-term infection with Listeria profoundly deregulates the innate immune functions of hepatocytes, which could generate an environment favorable to the establishment of persistent infection.

Phagocytes produce prostaglandin E2 in response to cytosolic Listeria monocytogenes

Listeria monocytogenes is an intracellular bacterium that elicits robust CD8+ T-cell responses. Despite the ongoing development of L. monocytogenes-based platforms as cancer vaccines, our understanding of how L. monocytogenes drives robust CD8+ T-cell responses remains incomplete. One overarching hypothesis is that activation of cytosolic innate pathways is critical for immunity, as strains of L. monocytogenes that are unable to access the cytosol fail to elicit robust CD8+ T-cell responses and in fact inhibit optimal T-cell priming. Counterintuitively, however, activation of known cytosolic pathways, such as the inflammasome and type I IFN, lead to impaired immunity. Conversely, production of prostaglandin E2 (PGE2) downstream of cyclooxygenase-2 (COX-2) is essential for optimal L. monocytogenes T-cell priming. Here, we demonstrate that vacuole-constrained L. monocytogenes elicit reduced PGE2 production compared to wild-type strains in macrophages and dendritic cells ex vivo. In vivo, infection with wild-type L. monocytogenes leads to 10-fold increases in PGE2 production early during infection whereas vacuole-constrained strains fail to induce PGE2 over mock-immunized controls. Mice deficient in COX-2 specifically in Lyz2+ or CD11c+ cells produce less PGE2, suggesting these cell subsets contribute to PGE2 levels in vivo, while depletion of phagocytes with clodronate abolishes PGE2 production completely. Taken together, this work demonstrates that optimal PGE2 production by phagocytes depends on L. monocytogenes access to the cytosol, suggesting that one reason cytosolic access is required to prime CD8+ T-cell responses may be to facilitate production of PGE2.

Genetic Modulators of Traumatic Brain Injury in Animal Models and the Impact of Sex-Dependent Effects

Traumatic brain injury (TBI) is a major health problem causing disability and death worldwide. There is no effective treatment, due in part to the complexity of the injury pathology and factors affecting its outcome. The extent of brain injury depends on the type of insult, age, sex, lifestyle, genetic risk factors, socioeconomic status, other co-injuries, and underlying health problems. This review discusses the genes that have been directly tested in TBI models, and whether their effects are known to be sex-dependent. Sex differences can affect the incidence, symptom onset, pathology, and clinical outcomes following injury. Adult males are more susceptible at the acute phase and females show greater injury in the chronic phase. TBI is not restricted to a single sex; despite variations in the degree of symptom onset and severity, it is important to consider both female and male animals in TBI pre-clinical research studies.

Spore-Associated Proteins Involved in c-di-GMP Synthesis and Degradation of Bacillus anthracis

Bis-(3'-5')-cyclic-dimeric GMP (c-di-GMP) is an important bacterial regulatory signaling molecule affecting biofilm formation, toxin production, motility, and virulence. The genome of Bacillus anthracis, the causative agent of anthrax, is predicted to encode ten putative GGDEF/EAL/HD-GYP-domain containing proteins. Heterologous expression in Bacillus subtilis hosts indicated that there are five active GGDEF domain-containing proteins and four active EAL or HD-GYP domain-containing proteins. Using an mCherry gene fusion-Western blotting approach, the expression of the c-di-GMP-associated proteins was observed throughout the in vitro life cycle. Of the six c-di-GMP-associated proteins found to be present in sporulating cells, four (CdgA, CdgB, CdgD, and CdgG) contain active GGDEF domains. The six proteins expressed in sporulating cells are retained in spores in a CotE-independent manner and thus are not likely to be localized to the exosporium layer of the spores. Individual deletion mutations involving the nine GGDEF/EAL protein-encoding genes and one HD-GYP protein-encoding gene did not affect sporulation efficiency, the attachment of the exosporium glycoprotein BclA, or biofilm production. Notably, expression of anthrax toxin was not affected by deletion of any of the cdg determinants. Three determinants encoding proteins with active GGDEF domains were found to affect germination kinetics. This study reveals a spore association of cyclic-di-GMP regulatory proteins and a likely role for these proteins in the biology of the B. anthracis spore. IMPORTANCE The genus Bacillus is composed of Gram-positive, rod shaped, soil-dwelling bacteria. As a mechanism for survival in the harsh conditions in soil, the organisms undergo sporulation, and the resulting spores permit the organisms to survive harsh environmental conditions. Although most species are saprophytes, Bacillus cereus and Bacillus anthracis are human pathogens and Bacillus thuringiensis is an insect pathogen. The bacterial c-di-GMP regulatory system is an important control system affecting motility, biofilm formation, and toxin production. The role of c-di-GMP has been studied in the spore-forming bacilli Bacillus subtilis, Bacillus amyloliquefaciens, B. cereus, and B. thuringiensis. However, this regulatory system has not heretofore been examined in the high-consequence zoonotic pathogen of this genus, B. anthracis.

Why is Listeria monocytogenes such a potent inducer of CD8+ T-cells?

Listeria monocytogenes is a rapidly growing, Gram-positive, facultative intracellular pathogen that has been used for over 5 decades as a model to study basic aspects of infection and immunity. In a murine intravenous infection model, immunisation with a sublethal infection of L. monocytogenes initially leads to rapid intracellular multiplication followed by clearance of the bacteria and ultimately culminates in the development of long-lived cell-mediated immunity (CMI) mediated by antigen-specific CD8+ cytotoxic T-cells. Importantly, effective immunisation requires live, replicating bacteria. In this review, we summarise the cell and immunobiology of L. monocytogenes infection and discuss aspects of its pathogenesis that we suspect lead to robust CMI. We suggest five specific features of L. monocytogenes infection that positively impact the development of CMI: (a) the bacteria have a predilection for professional antigen-presenting cells; (b) the bacteria escape from phagosomes, grow, and secrete antigens into the host cell cytosol; (c) bacterial-secreted proteins enter the major histocompatibility complex (MHC) class I pathway of antigen processing and presentation; (d) the bacteria do not induce rapid host cell death; and (e) cytosolic bacteria induce a cytokine response that favours CMI. Collectively, these features make L. monocytogenes an attractive vaccine vector for both infectious disease applications and cancer immunotherapy.

The analog of cGAMP, c-di-AMP, activates STING mediated cell death pathway in estrogen-receptor negative breast cancer cells

Immune adaptor protein like STING/MITA regulate innate immune response and plays a critical role in inflammation in the tumor microenvironment and regulation of metastasis including breast cancer. Chromosomal instability in highly metastatic cells releases fragmented chromosomal parts in the cytoplasm, hence the activation of STING via an increased level of cyclic dinucleotides (cDNs) synthesized by cGMP-AMP synthase (cGAS). Cyclic dinucleotides 2' 3'-cGAMP and it's analog can potentially activate STING mediated pathways leading to nuclear translocation of p65 and IRF-3 and transcription of inflammatory genes. The differential modulation of STING pathway via 2' 3'-cGAMP and its analog and its implication in breast tumorigenesis is still not well explored. In the current study, we demonstrated that c-di-AMP can activate type-1 IFN response in ER negative breast cancer cell lines which correlate with STING expression. c-di-AMP binds to STING and activates downstream IFN pathways in STING positive metastatic MDA-MB-231/MX-1 cells. Prolonged treatment of c-di-AMP induces cell death in STING positive metastatic MDA-MB-231/MX-1 cells mediated by IRF-3. c-di-AMP induces IRF-3 translocation to mitochondria and initiates Caspase-9 mediated cell death and inhibits clonogenicity of triple-negative breast cancer cells. This study suggests that c-di-AMP can activate and modulates STING pathway to induce mitochondrial mediated apoptosis in estrogen-receptor negative breast cancer cells.

Type I IFNs facilitate innate immune control of the opportunistic bacteria Burkholderia cenocepacia in the macrophage cytosol

The mammalian immune system is constantly challenged by signals from both pathogenic and non-pathogenic microbes. Many of these non-pathogenic microbes have pathogenic potential if the immune system is compromised. The importance of type I interferons (IFNs) in orchestrating innate immune responses to pathogenic microbes has become clear in recent years. However, the control of opportunistic pathogens-and especially intracellular bacteria-by type I IFNs remains less appreciated. In this study, we use the opportunistic, Gram-negative bacterial pathogen Burkholderia cenocepacia (Bc) to show that type I IFNs are capable of limiting bacterial replication in macrophages, preventing illness in immunocompetent mice. Sustained type I IFN signaling through cytosolic receptors allows for increased expression of autophagy and linear ubiquitination mediators, which slows bacterial replication. Transcriptomic analyses and in vivo studies also show that LPS stimulation does not replicate the conditions of intracellular Gram-negative bacterial infection as it pertains to type I IFN stimulation or signaling. This study highlights the importance of type I IFNs in protection against opportunistic pathogens through innate immunity, without the need for damaging inflammatory responses.

Carbomer-based adjuvant elicits CD8 T-cell immunity by inducing a distinct metabolic state in cross-presenting dendritic cells

There is a critical need for adjuvants that can safely elicit potent and durable T cell-based immunity to intracellular pathogens. Here, we report that parenteral vaccination with a carbomer-based adjuvant, Adjuplex (ADJ), stimulated robust CD8 T-cell responses to subunit antigens and afforded effective immunity against respiratory challenge with a virus and a systemic intracellular bacterial infection. Studies to understand the metabolic and molecular basis for ADJ's effect on antigen cross-presentation by dendritic cells (DCs) revealed several unique and distinctive mechanisms. ADJ-stimulated DCs produced IL-1β and IL-18, suggestive of inflammasome activation, but in vivo activation of CD8 T cells was unaffected in caspase 1-deficient mice. Cross-presentation induced by TLR agonists requires a critical switch to anabolic metabolism, but ADJ enhanced cross presentation without this metabolic switch in DCs. Instead, ADJ induced in DCs, an unique metabolic state, typified by dampened oxidative phosphorylation and basal levels of glycolysis. In the absence of increased glycolytic flux, ADJ modulated multiple steps in the cytosolic pathway of cross-presentation by enabling accumulation of degraded antigen, reducing endosomal acidity and promoting antigen localization to early endosomes. Further, by increasing ROS production and lipid peroxidation, ADJ promoted antigen escape from endosomes to the cytosol for degradation by proteasomes into peptides for MHC I loading by TAP-dependent pathways. Furthermore, we found that induction of lipid bodies (LBs) and alterations in LB composition mediated by ADJ were also critical for DC cross-presentation. Collectively, our model challenges the prevailing metabolic paradigm by suggesting that DCs can perform effective DC cross-presentation, independent of glycolysis to induce robust T cell-dependent protective immunity to intracellular pathogens. These findings have strong implications in the rational development of safe and effective immune adjuvants to potentiate robust T-cell based immunity.

Molecular and spatial mechanisms governing STING signalling

Detection of microbial nucleic acids via innate immune receptors is critical for establishing host defence against pathogens. The DNA-sensing cGAS-STING pathway has gained increasing attention in the last decade as a key pathway for combating viral and bacterial infections. cGAS-STING activation primarily promotes the secretion of antiviral type I IFNs via the key transcription factor, IRF3. In addition, cGAS-STING signalling also elicits proinflammatory cytokines through NF-κB activity. Activation of IRF3 and NF-κB is mediated by the chief signalling receptor protein STING. Interestingly, STING undergoes significant trafficking events across multiple subcellular locations, which regulates both the activation of downstream signalling pathways, as well as appropriate termination of the responses. Studies to date have provided a comprehensive view of the regulation and role of the IRF3-IFN pathway downstream of STING. However, many aspects of STING signalling remain relatively poorly defined. This review will explore the current understanding of the mechanisms through which STING elicits inflammatory and antimicrobial responses, focusing on the precise signalling and intracellular trafficking events that occur. We will also discuss exciting and emerging concepts in the field, including the importance of IFN-independent STING responses for host defence and during STING-related disease.

EGFR-mediated tyrosine phosphorylation of STING determines its trafficking route and cellular innate immunity functions

STING (STimulator of INterferon Genes) mediates protective cellular response to microbial infection and tissue damage, but its aberrant activation can lead to autoinflammatory diseases. Upon ligand stimulation, the endoplasmic reticulum (ER) protein STING translocates to endosomes for induction of interferon production, while an alternate trafficking route delivers it directly to the autophagosomes. Here, we report that phosphorylation of a specific tyrosine residue in STING by the epidermal growth factor receptor (EGFR) is required for directing STING to endosomes, where it interacts with its downstream effector IRF3. In the absence of EGFR-mediated phosphorylation, STING rapidly transits into autophagosomes, and IRF3 activation, interferon production, and antiviral activity are compromised in cell cultures and mice, while autophagic activity is enhanced. Our observations illuminate a new connection between the tyrosine kinase activity of EGFR and innate immune functions of STING and suggest new experimental and therapeutic approaches for selective regulation of STING functions.

Pathogenic Biohacking: Induction, Modulation and Subversion of Host Transcriptional Responses by Listeria monocytogenes

During infection, the foodborne bacterial pathogen Listeria monocytogenes dynamically influences the gene expression profile of host cells. Infection-induced transcriptional changes are a typical feature of the host-response to bacteria and contribute to the activation of protective genes such as inflammatory cytokines. However, by using specialized virulence factors, bacterial pathogens can target signaling pathways, transcription factors, and epigenetic mechanisms to alter host gene expression, thereby reprogramming the response to infection. Therefore, the transcriptional profile that is established in the host is delicately balanced between antibacterial responses and pathogenesis, where any change in host gene expression might significantly influence the outcome of infection. In this review, we discuss the known transcriptional and epigenetic processes that are engaged during Listeria monocytogenes infection, the virulence factors that can remodel them, and the impact these processes have on the outcome of infection.

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.

STING signaling and host defense against microbial infection

The first line of host defense against infectious agents involves activation of innate immune signaling pathways that recognize specific pathogen-associated molecular patterns (PAMPs). Key triggers of innate immune signaling are now known to include microbial-specific nucleic acid, which is rapidly detected in the cytosol of the cell. For example, RIG-I-like receptors (RLRs) have evolved to detect viral RNA species and to activate the production of host defense molecules and cytokines that stimulate adaptive immune responses. In addition, host defense countermeasures, including the production of type I interferons (IFNs), can also be triggered by microbial DNA from bacteria, viruses and perhaps parasites and are regulated by the cytosolic sensor, stimulator of interferon genes (STING). STING-dependent signaling is initiated by cyclic dinucleotides (CDNs) generated by intracellular bacteria following infection. CDNs can also be synthesized by a cellular synthase, cGAS, following interaction with invasive cytosolic self-DNA or microbial DNA species. The importance of STING signaling in host defense is evident since numerous pathogens have developed strategies to prevent STING function. Here, we review the relevance of STING-controlled innate immune signaling in host defense against pathogen invasion, including microbial endeavors to subvert this critical process.

Listeria monocytogenes cancer vaccines: bridging innate and adaptive immunity

Purpose of the Review

Immunotherapy has emerged as a promising cancer treatment, however success in only select clinical indications underscores the need for novel approaches. Recently Listeria monocytogenes-based vaccines have been developed to drive tumor specific T-cell responses. Here, we discuss recent preclinical studies using L. monocytogenes vaccines, innate immune pathways that influence T-cell priming, and new vaccine strategies in clinical trials.

Recent Findings

Recent studies indicate that in addition to inducing antigen specific T-cell responses, L. monocytogenes vaccines remodel the TME. In addition, several innate immune pathways influence adaptive immune responses to L. monocytogenes and modulating these pathways holds promise to enhance anti-tumor T-cell responses.

Summary

The interplay between innate and adaptive immune responses to L. monocytogenes is poorly understood. Understanding these interactions will facilitate the design of better anti-cancer vaccines and improved use of combination therapies.

Stimulator of interferon genes agonists attenuate type I diabetes progression in NOD mice

Reagents that activate the signaling adaptor stimulator of interferon genes (STING) suppress experimentally induced autoimmunity in murine models of multiple sclerosis and arthritis. In this study, we evaluated STING agonists as potential reagents to inhibit spontaneous autoimmune type I diabetes (T1D) onset in non-obese diabetic (NOD) female mice. Treatments with DNA nanoparticles (DNPs), which activate STING when cargo DNA is sensed, delayed T1D onset and reduced T1D incidence when administered before T1D onset. DNP treatment elevated indoleamine 2,3 dioxygenase (IDO) activity, which regulates T-cell immunity, in spleen, pancreatic lymph nodes and pancreas of NOD mice. Therapeutic responses to DNPs were partially reversed by inhibiting IDO and DNP treatment synergized with insulin therapy to further delay T1D onset and reduce T1D incidence. Treating pre-diabetic NOD mice with cyclic guanyl-adenyl dinucleotide (cGAMP) to activate STING directly delayed T1D onset and stimulated interferon-αβ (IFN-αβ), while treatment with cyclic diguanyl nucleotide (cdiGMP) did not delay T1D onset or induce IFN-αβ in NOD mice. DNA sequence analyses revealed that NOD mice possess a STING polymorphism that may explain differential responses to cGAMP and cdiGMP. In summary, STING agonists attenuate T1D progression and DNPs enhance therapeutic responses to insulin therapy.

Innate and Adaptive Immune Responses during Listeria monocytogenes Infection

It could be argued that we understand the immune response to infection with Listeria monocytogenes better than the immunity elicited by any other bacteria. L. monocytogenes are Gram-positive bacteria that are genetically tractable and easy to cultivate in vitro, and the mouse model of intravenous (i.v.) inoculation is highly reproducible. For these reasons, immunologists frequently use the mouse model of systemic listeriosis to dissect the mechanisms used by mammalian hosts to recognize and respond to infection. This article provides an overview of what we have learned over the past few decades and is divided into three sections: "Innate Immunity" describes how the host initially detects the presence of L. monocytogenes and characterizes the soluble and cellular responses that occur during the first few days postinfection; "Adaptive Immunity" discusses the exquisitely specific T cell response that mediates complete clearance of infection and immunological memory; "Use of Attenuated Listeria as a Vaccine Vector" highlights the ways that investigators have exploited our extensive knowledge of anti-Listeria immunity to develop cancer therapeutics.

DNA-stimulated cell death: implications for host defence, inflammatory diseases and cancer

The immune system detects disturbances in homeostasis that occur during infection, sterile tissue damage and cancer. This initiates immune responses that seek to eliminate the trigger of immune activation and to re-establish homeostasis. At the same time, these mechanisms can also play a crucial role in the progression of disease. The occurrence of DNA in the cytosol constitutes a potent trigger for the innate immune system, governing the production of key inflammatory cytokines such as type I interferons and IL-1β. More recently, it has become clear that cytosolic DNA also triggers other biological responses, including various forms of programmed cell death. In this article, we review the emerging literature on the pathways governing DNA-stimulated cell death and the current knowledge on how these processes shape immune responses to exogenous and endogenous challenges.

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

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

Gamma secretase dependent release of the CD44 cytoplasmic tail upregulates IFI16 in cd44-/- tumor cells, MEFs and macrophages

The adhesion molecule and co-receptor of receptor tyrosine kinases, CD44, is expressed in all cells of the immune system, but also in numerous non-immune cells. CD44 plays roles in the cellular response to different pathogens. The molecular actions of CD44 during these processes are by and large still unknown. The CD44 molecule undergoes a sequential proteolytic cleavage which leads to the release of a soluble intracellular domain (CD44-ICD). Previous reports had shown that the CD44-ICD is taken up into the nucleus where it enhances transcription of specific target genes. By RNA profiling we identified a CD44-dependent transcriptional increase of interferon-responsive genes, among them IFI16. IFI16 is important in the innate immune response. It senses and binds pathogenic DNA and, together with cGAS, activates the cGAS-cGAMP-STING pathway and induces the expression of genes relevant for the response, e.g. IFN-β. Our results show that the enhancement of IFI16 expression depended on CD44 cleavage. A CD44-negative tumor cell line, embryonic fibroblasts and bone marrow-derived macrophages from cd44^-/- mice were reduced in their response to IFN-γ, to viral DNA fragments and to Listeria monocytogenes infection. We could rescue the deficiency of CD44 negative RPM-MC cells and cd44^-/- MEFs by expressing only the soluble CD44-ICD in the absence of any other CD44 domain. Expression of the CD44-ICD carrying a mutation that prevented the uptake into the nucleus, could not rescue the absence of CD44. This molecular aspect of regulation by CD44 may explain part of the immune phenotypes of mice with cd44 gene disruption.

Cyclic di-AMP-mediated interaction between Mycobacterium tuberculosis ΔcnpB and macrophages implicates a novel strategy for improving BCG vaccination

Cyclic di-AMP (c-di-AMP) has been shown to play an important role in bacterial physiology and pathogen-host interactions. We previously reported that deletion of the sole c-di-AMP phosphodiesterase-encoding gene (cnpB) in Mycobacterium tuberculosis (Mtb) led to significant virulence attenuation. In this study, we found that ΔcnpB of M. bovisbacillus Calmette-Guerin (BCG) was unable to secrete c-di-AMP, which differs from Mtb ΔcnpB. We infected bone marrow-derived macrophages (BMDMs) with c-di-AMP-associated mutants generated from both Mtb and BCG. Our results showed that upon infection with Mtb ΔcnpB, BMDMs of wildtype mice secreted a large amount of interferon-β (IFN-β) post-infection similarly as we reported previously. In contrast, the response was less pronounced with BMDMs isolated from cGAS-/- mice and was nearly abolished with BMDMs prepared from STING-/- mice. Deletion of the region of difference 1 (RD1) locus in Mtb ΔcnpB did not alter the c-di-AMP secretion of ΔcnpB but eliminated the IFN-β production in the infected cells. In contrast, neither BCG ΔcnpB nor a recombinant BCG ΔcnpB with a pRD1 cosmid induced a type I interferon response. Interestingly, multiple studies have demonstrated that type I IFN enhances BCG's immunity. Thus, amending BCG based on our findings might improve BCG vaccination.

Staphylococcus aureus modulation of innate immune responses through Toll-like (TLR), (NOD)-like (NLR) and C-type lectin (CLR) receptors

Early recognition of pathogens by the innate immune system is crucial for bacterial clearance. Many pattern recognition receptors (PRRs) such as Toll-like (TLRs) and (NOD)-like (NLRs) receptors have been implicated in initial sensing of bacterial components. The intracellular signaling cascades triggered by these receptors result in transcriptional upregulation of inflammatory pathways. Although this step is crucial for bacterial elimination, it is also associated with the potential for substantial immunopathology, which underscores the need for tight control of inflammatory responses. The leading human bacterial pathogen Staphylococcus aureus expresses over 100 virulence factors that exert numerous effects upon host cells. In this manner, the pathogen seeks to avoid host recognition or perturb PRR-induced innate immune responses to allow optimal survival in the host. These immune system interactions may result in enhanced bacterial proliferation but also provoke systemic cytokine responses associated with sepsis. This review summarizes recent findings on the various mechanisms applied by S. aureus to modulate or interfere with inflammatory responses through PRRs. Detailed understanding of these complex interactions can provide new insights toward future immune-stimulatory therapeutics against infection or immunomodulatory therapeutics to suppress or correct dysregulated inflammation.

Listeria monocytogenes as a Vector for Cancer Immunotherapy: Current Understanding and Progress

Listeria monocytogenes, a Gram-positive facultative anaerobic bacterium, is becoming a popular vector for cancer immunotherapy. Indeed, multiple vaccines have been developed utilizing modified Listeria as a tool for generating immune responses against a variety of cancers. Moreover, over a dozen clinical trials testing Listeria cancer vaccines are currently underway, which will help to understand the utility of Listeria vaccines in cancer immunotherapy. This review aims to summarize current views on how Listeria-based vaccines induce potent antitumor immunity and the current state of Listeria-based cancer vaccines in clinical trials.

Stimulator of Interferon Genes Promotes Host Resistance Against Pseudomonas aeruginosa Keratitis

Pseudomonas aeruginosa (PA) is the leading cause of bacterial keratitis, especially in those who wear contact lens and who are immunocompromised. Once the invading pathogens are recognized by pattern recognition receptors expressed on the innate immune cells, the innate immune response is stimulated to exert host defense function, which is the first line to fight against PA infection. As a converging point of cytosolic DNA sense signaling, stimulator of interferon genes (STING) was reported to participate in host–pathogen interaction. However, the role of STING in regulating PA-induced corneal inflammation and bacterial clearance remains unknown. Our data demonstrated that STING was activated in murine model of PA keratitis and in in vitro-cultured macrophages, indicated by Western blot, immunostaining, and flow cytometry. To explore the role of STING in PA keratitis, we used siRNA to silence STING and 2′,3′-cGAMP to activate STING in vivo and in vitro, and the in vivo data found out that STING promoted host resistance against PA infection. To investigate the reason why STING played a protective role in PA keratitis, the inflammatory cytokine secretion and bacterial load were measured by using real-time PCR and bacterial plate count, respectively. Our data demonstrated that STING suppressed the production of inflammatory cytokines and enhanced bacterial elimination in murine model of PA keratitis and in PA-infected macrophages. To further investigate the mechanism beneath, the phosphorylation of mitogen-activated protein kinase, the nuclear translocation of nuclear factor-κB (NF-κB) and the bactericidal mechanism were measured by western-blot, immunofluorescence, and real-time PCR, respectively. Our data indicated that STING suppressed inflammatory cytokine expressing via restraining NF-κB activity and enhanced inducible NO synthase expression, an oxygen-dependent bactericidal mechanism. In conclusion, this study demonstrated that STING promoted host resistance against PA keratitis and played a protective role in PA-infected corneal disease, via inhibiting corneal inflammation and enhancing bacterial killing.

Cyclooxygenase-1 and -2 Play Contrasting Roles in Listeria-Stimulated Immunity

Nonsteroidal anti-inflammatory drugs (NSAIDs) inhibit cyclooxygenase (COX) activity and are commonly used for pain relief and fever reduction. NSAIDs are used following childhood vaccinations and cancer immunotherapies; however, how NSAIDs influence the development of immunity following these therapies is unknown. We hypothesized that NSAIDs would modulate the development of an immune response to Listeria monocytogenes-based immunotherapy. Treatment of mice with the nonspecific COX inhibitor indomethacin impaired the generation of cell-mediated immunity. This phenotype was due to inhibition of the inducible COX-2 enzyme, as treatment with the COX-2-selective inhibitor celecoxib similarly inhibited the development of immunity. In contrast, loss of COX-1 activity improved immunity to L. monocytogenes Impairments in immunity were independent of bacterial burden, dendritic cell costimulation, or innate immune cell infiltrate. Instead, we observed that PGE₂ production following L. monocytogenes is critical for the formation of an Ag-specific CD8⁺ T cell response. Use of the alternative analgesic acetaminophen did not impair immunity. Taken together, our results suggest that COX-2 is necessary for optimal CD8⁺ T cell responses to L. monocytogenes, whereas COX-1 is detrimental. Use of pharmacotherapies that spare COX-2 activity and the production of PGE₂ like acetaminophen will be critical for the generation of optimal antitumor responses using L. monocytogenes.

TNFα and Radioresistant Stromal Cells Are Essential for Therapeutic Efficacy of Cyclic Dinucleotide STING Agonists in Nonimmunogenic Tumors

The cGAS-STING cytosolic DNA sensing pathway may play an integral role in the initiation of antitumor immune responses. Studies evaluating the immunogenicity of various cyclic dinucleotide (CDN) STING agonists administered by intratumoral (i.t.) injection showed potent induction of inflammation, tumor necrosis, and, in some cases, durable tumor-specific adaptive immunity. However, the specific immune mechanisms underlying these responses remain incompletely defined. The majority of these studies have focused on the effect of CDNs on immune cells but have not conclusively interrogated the role of stromal cells in the acute rejection of the CDN-injected tumor. Here, we revealed a mechanism of STING agonist-mediated tumor response that relied on both stromal and immune cells to achieve tumor regression and clearance. Using knockout and bone marrow chimeric mice, we showed that although bone marrow-derived TNFα was necessary for CDN-induced necrosis, STING signaling in radioresistant stromal cells was also essential for CDN-mediated tumor rejection. These results provide evidence for crosstalk between stromal and hematopoietic cells during CDN-mediated tumor collapse after i.t. administration. These mechanistic insights may prove critical in the clinical development of STING agonists. Cancer Immunol Res; 6(4); 422-33. ©2018 AACR.

Listeria monocytogenes: The Impact of Cell Death on Infection and Immunity

Listeria monocytogenes has evolved exquisite mechanisms for invading host cells and spreading from cell-to-cell to ensure maintenance of its intracellular lifecycle. As such, it is not surprising that loss of the intracellular replication niche through induction of host cell death has significant implications on the development of disease and the subsequent immune response. Although L. monocytogenes can activate multiple pathways of host cell death, including necrosis, apoptosis, and pyroptosis, like most intracellular pathogens L. monocytogenes has evolved a series of adaptations that minimize host cell death to promote its virulence. Understanding how L. monocytogenes modulates cell death during infection could lead to novel therapeutic approaches. In addition, as L. monocytogenes is currently being developed as a tumor immunotherapy platform, understanding how cell death pathways influence the priming and quality of cell-mediated immunity is critical. This review will focus on the mechanisms by which L. monocytogenes modulates cell death, as well as the implications of cell death on acute infection and the generation of adaptive immunity.

Listeria monocytogenes: towards a complete picture of its physiology and pathogenesis

Listeria monocytogenes is a food-borne pathogen responsible for a disease called listeriosis, which is potentially lethal in immunocompromised individuals. This bacterium, first used as a model to study cell-mediated immunity, has emerged over the past 20 years as a paradigm in infection biology, cell biology and fundamental microbiology. In this Review, we highlight recent advances in the understanding of human listeriosis and L. monocytogenes biology. We describe unsuspected modes of hijacking host cell biology, ranging from changes in organelle morphology to direct effects on host transcription via a new class of bacterial effectors called nucleomodulins. We then discuss advances in understanding infection in vivo, including the discovery of tissue-specific virulence factors and the 'arms race' among bacteria competing for a niche in the microbiota. Finally, we describe the complexity of bacterial regulation and physiology, incorporating new insights into the mechanisms of action of a series of riboregulators that are critical for efficient metabolic regulation, antibiotic resistance and interspecies competition.

How to rewire the host cell: A home improvement guide for intracellular bacteria

Intracellular bacterial pathogens have developed versatile strategies to generate niches inside the eukaryotic cells that allow them to survive and proliferate. Making a home inside the host offers many advantages; however, intracellular bacteria must also overcome many challenges, such as disarming innate immune signaling and accessing host nutrient supplies. Gaining entry into the cell and avoiding degradation is only the beginning of a successful intracellular lifestyle. To establish these replicative niches, intracellular pathogens secrete various virulence proteins, called effectors, to manipulate host cell signaling pathways and subvert host defense mechanisms. Many effectors mimic host enzymes, whereas others perform entirely novel enzymatic functions. A large volume of work has been done to understand how intracellular bacteria manipulate membrane trafficking pathways. In this review, we focus on how intracellular bacterial pathogens target innate immune signaling, the unfolded protein response, autophagy, and cellular metabolism and exploit these pathways to their advantage. We also discuss how bacterial pathogens can alter host gene expression by directly modifying histones or hijacking the ubiquitination machinery to take control of several host signaling pathways.

Cyclic di-AMP in host-pathogen interactions

Cyclic di-AMP (c-di-AMP) is a bacterial signaling nucleotide synthesized by several human pathogens. This widespread and specific bacterial product is recognized by infected host cells to trigger an innate immune response. Detection of c-di-AMP in the host cytosol leads primarily to the induction of type I interferon via the STING-cGAS signaling axis, while being also entangled in the activation of the NF-κB pathway. During their long-standing interaction, host and pathogens have co-evolved to control c-di-AMP activation of innate immunity. On the bacterial side, the quantity of c-di-AMP released inside cells allows to manipulate the host response to exacerbate infection by avoiding immune recognition or, at the opposite, by overloading the STING-cGAS pathway.

Amplifying IFN-γ Signaling in Dendritic Cells by CD11c-Specific Loss of SOCS1 Increases Innate Immunity to Infection while Decreasing Adaptive Immunity

Although prophylactic vaccines provide protective humoral immunity against infectious agents, vaccines that elicit potent CD8 T cell responses are valuable tools to shape and drive cellular immunity against cancer and intracellular infection. In particular, IFN-γ-polarized cytotoxic CD8 T cell immunity is considered optimal for protective immunity against intracellular Ags. Suppressor of cytokine signaling (SOCS)1 is a cross-functional negative regulator of TLR and cytokine receptor signaling via degradation of the receptor-signaling complex. We hypothesized that loss of SOCS1 in dendritic cells (DCs) would improve T cell responses by accentuating IFN-γ-directed immune responses. We tested this hypothesis using a recombinant Listeria monocytogenes vaccine platform that targets CD11c⁺ DCs in mice in which SOCS1 is selectively deleted in all CD11c⁺ cells. Unexpectedly, in mice lacking SOCS1 expression in CD11c⁺ cells, we observed a decrease in CD8⁺ T cell response to the L. monocytogenes vaccine. NK cell responses were also decreased in mice lacking SOCS1 expression in CD11c⁺ cells but did not explain the defect in CD8⁺ T cell immunity. We found that DCs lacking SOCS1 expression were functional in driving Ag-specific CD8⁺ T cell expansion in vitro but that this process was defective following infection in vivo. Instead, monocyte-derived innate TNF-α and inducible NO synthase-producing DCs dominated the antibacterial response. Thus, loss of SOCS1 in CD11c⁺ cells skewed the balance of immune response to infection by increasing innate responses while decreasing Ag-specific adaptive responses to infectious Ags.

Activation of the Listeria monocytogenes Virulence Program by a Reducing Environment

Upon entry into the host cell cytosol, the facultative intracellular pathogen Listeria monocytogenes coordinates the expression of numerous essential virulence factors by allosteric binding of glutathione (GSH) to the Crp-Fnr family transcriptional regulator PrfA. Here, we report that robust virulence gene expression can be recapitulated by growing bacteria in a synthetic medium containing GSH or other chemical reducing agents. Bacteria grown under these conditions were 45-fold more virulent in an acute murine infection model and conferred greater immunity to a subsequent lethal challenge than bacteria grown in conventional media. During cultivation in vitro, PrfA activation was completely dependent on the intracellular levels of GSH, as a glutathione synthase mutant (ΔgshF) was activated by exogenous GSH but not reducing agents. PrfA activation was repressed in a synthetic medium supplemented with oligopeptides, but the repression was relieved by stimulation of the stringent response. These data suggest that cytosolic L. monocytogenes interprets a combination of metabolic and redox cues as a signal to initiate robust virulence gene expression in vivo.

Intracellular pathogens are responsible for much of the worldwide morbidity and mortality from infectious diseases. These pathogens have evolved various strategies to proliferate within individual cells of the host and avoid the host immune response. Through cellular invasion or the use of specialized secretion machinery, all intracellular pathogens must access the host cell cytosol to establish their replicative niches. Determining how these pathogens sense and respond to the intracellular compartment to establish a successful infection is critical to our basic understanding of the pathogenesis of each organism and for the rational design of therapeutic interventions. Listeria monocytogenes is a model intracellular pathogen with robust in vitro and in vivo infection models. Studies of the host-sensing and downstream signaling mechanisms evolved by L. monocytogenes often describe themes of pathogenesis that are broadly applicable to less tractable pathogens. Here, we describe how bacteria use external redox states as a cue to activate virulence.