Type I IFN expression is stimulated by cytosolic Mt DNA released from pneumolysin‐damaged mitochondria via the STING signaling pathway in macrophages

Bacterial cholesterol-dependent cytolysins and their interaction with the human immune response

PURPOSE OF REVIEW

Many cholesterol-dependent cytolysin (CDC)-producing pathogens pose a significant threat to human health. Herein, we review the pore-dependent and -independent properties CDCs possess to assist pathogens in evading the host immune response.

RECENT FINDINGS

Within the last 5 years, exciting new research suggests CDCs can act to inhibit important immune functions, disrupt critical cell signaling pathways, and have tissue-specific effects. Additionally, recent studies have identified a key region of CDCs that generates robust immunity, providing resources for the development of CDC-based vaccines.

SUMMARY

This review provides new information on how CDCs alter host immune responses to aid bacteria in pathogenesis. These studies can assist in the design of more efficient vaccines and therapeutics against CDCs that will enhance the immune response to CDC-producing pathogens while mitigating the dampening effects CDCs have on the host immune response.

Melatonin-induced upregulation of telomerase activity interferes with macrophage mitochondrial metabolism and suppresses NLRP3 inflammasome activation in the treatment of Pneumonia

Objective

This study aims to investigate the effects of melatonin-induced upregulation of telomerase activity on mitochondrial metabolism and NLRP3 inflammasome activation in macrophages, with the ultimate goal of elucidating potential therapeutic implications for pneumonia treatment.

Materials and methods

Macrophages were treated with melatonin to assess its impact on telomerase activity. Mitochondrial function was evaluated through the measurement of reactive oxygen species (ROS) levels and cellular energy production. NLRP3 inflammasome activation was assessed by examining the production of inflammatory cytokines, such as interleukin-1β (IL-1β). The expression levels of key proteins involved in mitochondrial metabolism and NLRP3 inflammasome signaling were also analyzed.

Results

Our findings demonstrated that melatonin treatment significantly upregulated telomerase activity in macrophages. This was associated with a reduction in ROS levels and enhanced cellular energy production, indicating improved mitochondrial function. Moreover, melatonin treatment suppressed NLRP3 inflammasome activation, resulting in reduced secretion of IL-1β. The expression levels of proteins involved in mitochondrial metabolism and NLRP3 inflammasome signaling were modulated by melatonin.

Conclusion

These results suggest that melatonin-induced upregulation of telomerase activity can interfere with mitochondrial metabolism and inhibit NLRP3 inflammasome activation in macrophages. This indicates a potential therapeutic role for melatonin in the treatment of pneumonia. Understanding the molecular mechanisms underlying these effects may lead to the development of novel therapeutic strategies targeting mitochondria and NLRP3 inflammasome activation for the management of pneumonia. Further investigations are warranted to fully uncover the therapeutic potential of melatonin and its implications for pneumonia treatment.

Epitope-level profiling in children with mitochondrial disease reveals limitations in the antibacterial antibody repertoire

INTRODUCTION

Immunometabolic studies in mice have suggested the importance of oxidative phosphorylation (OXPHOS) in humoral immunity. However, there are important distinctions between murine and human immunity. Furthermore, translational studies on the role of OXPHOS in humoral immunity are nearly absent from the biomedical literature. Children with primary OXPHOS deficiency (i.e., mitochondrial disease, MtD), are an important patient population for demonstrating the functional effects of this bioenergetic defect on humoral immunity.

METHODS

To define whether OXPHOS deficiency extended to human B cells, we performed extracellular flux analysis on lymphoblastoid B cell lines from children with MtD and controls (N = 4/group). To expand the immune phenotype of B cell OXPHOS deficiency, we conducted a cross-sectional multiplex serology study of the antibacterial antibody repertoire in children with MtD (N = 16) and controls (N = 16) using phage display and immunoprecipitation sequencing (PhIPseq). The PhIPseq library contained >3000 peptides (i.e., epitopes) covering >40 genera and > 150 species of bacteria that infect humans.

RESULTS

B cell lymphoblastoid cell lines from children with MtD displayed depressed baseline oxygen consumption, ATP production and reserve capacity, indicating that OXPHOS deficiency extended to these key cells in humoral immunity. Characterization of the bacterial exposome revealed comparable bacterial species between the two groups, mostly Streptococcus and Staphylococcus. The most common species of bacteria was S. pneumoniae. By interrogating the antibacterial antibody repertoire, we found that children with MtD had less robust antibody fold changes to common epitopes. Furthermore, we also found that children with MtD failed to show a direct relationship between the number of bacterial epitopes recognized and age, unlike controls. OXPHOS deficiency extends to B cells in children with MtD, leading to limitations in the antibacterial antibody repertoire. Furthermore, the timing of bacterial exposures was asynchronous, suggesting different periods of increased exposure or susceptibility.

CONCLUSIONS

Overall, the antibacterial humoral response is distinctive in children with MtD, suggesting an important role for OXPHOS in B cell function.

STING-Triggered CNS Inflammation in Human Neurodegenerative Diseases

BACKGROUND

Some neurodegenerative diseases have an element of neuroinflammation that is triggered by viral nucleic acids, resulting in the generation of type I interferons. In the cGAS-STING pathway, microbial and host-derived DNA bind and activate the DNA sensor cGAS, and the resulting cyclic dinucleotide, 2'3-cGAMP, binds to a critical adaptor protein, stimulator of interferon genes (STING), which leads to activation of downstream pathway components. However, there is limited work demonstrating the activation of the cGAS-STING pathway in human neurodegenerative diseases.

METHODS

Post-mortem CNS tissue from donors with multiple sclerosis (n = 4), Alzheimer's disease (n = 6), Parkinson's disease (n = 3), amyotrophic lateral sclerosis (n = 3) and non-neurodegenerative controls (n = 11) were screened by immunohistochemistry for STING and relevant protein aggregates (e.g., amyloid-β, α-synuclein, TDP-43). Human brain endothelial cells were cultured and stimulated with the STING agonist palmitic acid (1-400 μM) and assessed for mitochondrial stress (release of mitochondrial DNA into cytosol, increased oxygen consumption), downstream regulator factors, TBK-1/pIRF3 and inflammatory biomarker interferon-β release and changes in ICAM-1 integrin expression.

RESULTS

In neurodegenerative brain diseases, elevated STING protein was observed mainly in brain endothelial cells and neurons, compared to non-neurodegenerative control tissues where STING protein staining was weaker. Interestingly, a higher STING presence was associated with toxic protein aggregates (e.g., in neurons). Similarly high STING protein levels were observed within acute demyelinating lesions in multiple sclerosis subjects. To understand non-microbial/metabolic stress activation of the cGAS-STING pathway, brain endothelial cells were treated with palmitic acid. This evoked mitochondrial respiratory stress up to a ~2.5-fold increase in cellular oxygen consumption. Palmitic acid induced a statistically significant increase in cytosolic DNA leakage from endothelial cell mitochondria (Mander's coefficient; p < 0.05) and a significant increase in TBK-1, phosphorylated transcription factor IFN regulatory factor 3, cGAS and cell surface ICAM. In addition, a dose response in the secretion of interferon-β was observed, but it failed to reach statistical significance.

CONCLUSIONS

The histological evidence shows that the common cGAS-STING pathway appears to be activated in endothelial and neural cells in all four neurodegenerative diseases examined. Together with the in vitro data, this suggests that the STING pathway might be activated via perturbation of mitochondrial stress and DNA leakage, resulting in downstream neuroinflammation; hence, this pathway may be a target for future STING therapeutics.

Salmonella Induces the cGAS-STING-Dependent Type I Interferon Response in Murine Macrophages by Triggering mtDNA Release

Salmonella enterica serovar Typhimurium (S. Typhimurium) elicited strong innate immune responses in macrophages. To activate innate immunity, pattern recognition receptors (PRRs) in host cells can recognize highly conserved pathogen-associated molecular patterns (PAMPs). Here, we showed that S. Typhimurium induced a robust type I interferon (IFN) response in murine macrophages. Exposure of macrophages to S. Typhimurium activated a Toll-like receptor 4 (TLR4)-dependent type I IFN response. Next, we showed that type I IFN and IFN-stimulated genes (ISGs) were elicited in a TBK1-IFN-dependent manner. Furthermore, cytosolic DNA sensor cyclic GMP-AMP synthase (cGAS) and immune adaptor protein stimulator of interferon genes (STING) were also required for the induction of type I IFN response during infection. Intriguingly, S. Typhimurium infection triggered mitochondrial DNA (mtDNA) release into the cytosol to activate the type I IFN response. In addition, we also showed that bacterial DNA was enriched in cGAS during infection, which may contribute to cGAS activation. Finally, we showed that cGAS and STING deficient mice and cells were more susceptible to S. Typhimurium infection, signifying the critical role of the cGAS-STING pathway in host defense against S. Typhimurium infection. In conclusion, in addition to TLR4-dependent innate immune response, we demonstrated that S. Typhimurium induced the type I IFN response in a cGAS-STING-dependent manner and the S. Typhimurium-induced mtDNA release was important for the induction of type I IFN. This study elucidated a new mechanism by which bacterial pathogen activated the cGAS-STING pathway and also characterized the important role of cGAS-STING during S. Typhimurium infection.

The Yin and Yang of Pneumolysin During Pneumococcal Infection

Pneumolysin (PLY) is a pore-forming toxin produced by the human pathobiont Streptococcus pneumoniae, the major cause of pneumonia worldwide. PLY, a key pneumococcal virulence factor, can form transmembrane pores in host cells, disrupting plasma membrane integrity and deregulating cellular homeostasis. At lytic concentrations, PLY causes cell death. At sub-lytic concentrations, PLY triggers host cell survival pathways that cooperate to reseal the damaged plasma membrane and restore cell homeostasis. While PLY is generally considered a pivotal factor promoting S. pneumoniae colonization and survival, it is also a powerful trigger of the innate and adaptive host immune response against bacterial infection. The dichotomy of PLY as both a key bacterial virulence factor and a trigger for host immune modulation allows the toxin to display both "Yin" and "Yang" properties during infection, promoting disease by membrane perforation and activating inflammatory pathways, while also mitigating damage by triggering host cell repair and initiating anti-inflammatory responses. Due to its cytolytic activity and diverse immunomodulatory properties, PLY is integral to every stage of S. pneumoniae pathogenesis and may tip the balance towards either the pathogen or the host depending on the context of infection.

Biochemistry, Cell Biology, and Pathophysiology of the Innate Immune cGAS-cGAMP-STING Pathway

In the decade since the discovery of the innate immune cyclic GMP-AMP synthase (cGAS)- 2'3'-cyclic GMP-AMP (cGAMP)- stimulator of interferon genes (STING) pathway, its proper activation and dysregulation have been rapidly implicated in many aspects of human disease. Understanding the biochemical, cellular, and regulatory mechanisms of this pathway is critical to developing therapeutic strategies that either harness it to boost defense or inhibit it to prevent unwanted inflammation. In this review, we first discuss how the second messenger cGAMP is synthesized by cGAS in response to double-stranded DNA and cGAMP's subsequent activation of cell-type-dependent STING signaling cascades with differential physiological consequences. We then review how cGAMP as an immunotransmitter mediates tightly controlled cell-cell communication by being exported from producing cells and imported into responding cells via cell-type-specific transporters. Finally, we review mechanisms by which the cGAS-cGAMP-STING pathway responds to different sources of mislocalized double-stranded DNA in pathogen defense, cancer, and autoimmune diseases. Expected final online publication date for the Annual Review of Biochemistry, Volume 91 is June 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Role of Mitochondrial Nucleic Acid Sensing Pathways in Health and Patho-Physiology

Mitochondria, in symbiosis with the host cell, carry out a wide variety of functions from generating energy, regulating the metabolic processes, cell death to inflammation. The most prominent function of mitochondria relies on the oxidative phosphorylation (OXPHOS) system. OXPHOS heavily influences the mitochondrial-nuclear communication through a plethora of interconnected signaling pathways. Additionally, owing to the bacterial ancestry, mitochondria also harbor a large number of Damage Associated Molecular Patterns (DAMPs). These molecules relay the information about the state of the mitochondrial health and dysfunction to the innate immune system. Consequently, depending on the intracellular or extracellular nature of detection, different inflammatory pathways are elicited. One group of DAMPs, the mitochondrial nucleic acids, hijack the antiviral DNA or RNA sensing mechanisms such as the cGAS/STING and RIG-1/MAVS pathways. A pro-inflammatory response is invoked by these signals predominantly through type I interferon (T1-IFN) cytokines. This affects a wide range of organ systems which exhibit clinical presentations of auto-immune disorders. Interestingly, tumor cells too, have devised ingenious ways to use the mitochondrial DNA mediated cGAS-STING-IRF3 response to promote neoplastic transformations and develop tumor micro-environments. Thus, mitochondrial nucleic acid-sensing pathways are fundamental in understanding the source and nature of disease initiation and development. Apart from the pathological interest, recent studies also attempt to delineate the structural considerations for the release of nucleic acids across the mitochondrial membranes. Hence, this review presents a comprehensive overview of the different aspects of mitochondrial nucleic acid-sensing. It attempts to summarize the nature of the molecular patterns involved, their release and recognition in the cytoplasm and signaling. Finally, a major emphasis is given to elaborate the resulting patho-physiologies.

The cGAS-STING Pathway in Bacterial Infection and Bacterial Immunity

Cyclic guanosine monophosphate (GMP)-adenosine monophosphate (AMP) (cGAMP) synthase (cGAS), along with the adaptor stimulator of interferon genes (STING), are crucial components of the innate immune system, and their study has become a research hotspot in recent years. Many biochemical and structural studies that have collectively elucidated the mechanism of activation of the cGAS-STING pathway with atomic resolution have provided insights into the roles of the cGAS-STING pathway in innate immunity and clues to the origin and evolution of the modern cGAS-STING signaling pathway. The cGAS-STING pathway has been identified to protect the host against viral infection. After detecting viral dsDNA, cGAS synthesizes a second messenger to activate STING, eliciting antiviral immune responses by promoting the expression of interferons (IFNs) and hundreds of IFN-stimulated genes (ISGs). Recently, the cGAS-STING pathway has also been found to be involved in response to bacterial infections, including bacterial pneumonia, melioidosis, tuberculosis, and sepsis. However, compared with its functions in viral infection, the cGAS-STING signaling pathway in bacterial infection is more complex and diverse since the protective and detrimental effects of type I IFN (IFN-I) on the host depend on the bacterial species and infection mode. Besides, STING activation can also affect infection prognosis through other mechanisms in different bacterial infections, independent of the IFN-I response. Interestingly, the core protein components of the mammalian cGAS-STING signaling pathway have been found in the bacterial defense system, suggesting that this widespread signaling pathway may have originated in bacteria. Here, we review recent findings related to the structures of major molecules involved in the cGAS-STING pathway and the effects of the cGAS-STING pathway in various bacterial infections and bacterial immunity, which may pave the way for the development of new antibacterial drugs that specifically kill bacteria without harmful effects on the host.

Streptococcus pneumoniae autolysin LytA inhibits ISG15 and ISGylation through decreasing bacterial DNA abnormally accumulated in the cytoplasm of macrophages

Interferon stimulated gene 15 (ISG15) is one of the most robustly upregulated interferon stimulated genes (ISGs) and also a ubiquitin-like modifier which has been reported to play an important role in host defense against pathogens. Cytosolic nucleic acids detected by DNA sensors induce type Ⅰ interferons (IFN-Ⅰs) and ISGs in host cells. Streptococcus pneumoniae (S. pn) autolysin LytA triggers bacterial lysis and then S. pn-derived genomic DNA (hereafter referred to as S. pn-DNA) can be released and accumulates in the cytoplasm of host cells. However, it remains elusive whether LytA-mediated S. pn-DNA release is involved in ISG15 induction. Here we verified that ISG15 conjugation system can be widely activated by S. pn and cytosolic S. pn-DNA in host cells. Moreover, the phagocytosis of macrophages to the mutant strain S. pn D39 ΔlytA was enhanced when compared to S. pn D39, which in turn increased S. pn-DNA uptake into macrophages and augmented ISG15 expression. ISG15 might upregulate proinflammatory cytokines tumor necrosis factor α (TNF-α) and interleukin 1β (IL-1β) in macrophages and further promoted the clearance of S. pn in the absence of LytA. These results indicate that S. pn autolysis blunts ISG15 induction through preventing bacteria internalization and reducing cytosolic S. pn-DNA accumulation in macrophages, revealing a new strategy of S. pn for avoiding elimination. This study will help us to further understand the role of ISG15 during S. pn infection as well as the regulatory mechanisms of immune responses mediated by bacterial autolysis and bacterial DNA.

Mitochondrial Nucleic Acid as a Driver of Pathogenic Type I Interferon Induction in Mendelian Disease

The immune response to viral infection involves the recognition of pathogen-derived nucleic acids by intracellular sensors, leading to type I interferon (IFN), and downstream IFN-stimulated gene, induction. Ineffective discrimination of self from non-self nucleic acid can lead to autoinflammation, a phenomenon implicated in an increasing number of disease states, and well highlighted by the group of rare genetic disorders referred to as the type I interferonopathies. To understand the pathogenesis of these monogenic disorders, and polyfactorial diseases associated with pathogenic IFN upregulation, such as systemic lupus erythematosus and dermatomyositis, it is important to define the self-derived nucleic acid species responsible for such abnormal IFN induction. Recently, attention has focused on mitochondria as a novel source of immunogenic self nucleic acid. Best appreciated for their function in oxidative phosphorylation, metabolism and apoptosis, mitochondria are double membrane-bound organelles that represent vestigial bacteria in the cytosol of eukaryotic cells, containing their own DNA and RNA enclosed within the inner mitochondrial membrane. There is increasing recognition that a loss of mitochondrial integrity and compartmentalization can allow the release of mitochondrial nucleic acid into the cytosol, leading to IFN induction. Here, we provide recent insights into the potential of mitochondrial-derived DNA and RNA to drive IFN production in Mendelian disease. Specifically, we summarize current understanding of how nucleic acids are detected as foreign when released into the cytosol, and then consider the findings implicating mitochondrial nucleic acid in type I interferonopathy disease states. Finally, we discuss the potential for IFN-driven pathology in primary mitochondrial disorders.

SS-31 ameliorates hepatic injury in rats subjected to severe burns plus delayed resuscitation via inhibiting the mtDNA/STING pathway in Kupffer cells

Hepatic injury is common in patients who suffer from severe burns plus delayed resuscitation (B + DR). Stimulator of interferon genes (STING) is primarily expressed in Kupffer cells (KCs). We demonstrated that B + DR caused hepatic injury and oxidative stress. Reactive oxygen species (ROS) damage mitochondrial membranes in hepatocytes, leading to the release of mitochondrial DNA (mtDNA) into the hepatocyte cytosol and the circulation. The damaged hepatocytes then activate the mtDNA/STING pathway in KCs and trigger KCs polarization towards pro-inflammatory phenotype. SS-31 is a strong antioxidant that specifically concentrates in the inner mitochondrial membrane. SS-31 prevented hepatic injury by neutralizing ROS, inhibiting the release of mtDNA, protecting hepatocyte mitochondria, suppressing the activation of the mtDNA/STING pathway and inhibiting KCs polarization into pro-inflammatory phenotype.

An in vivo atlas of host-pathogen transcriptomes during Streptococcus pneumoniae colonization and disease

Streptococcus pneumoniae (Spn) colonizes the nasopharynx and can cause pneumonia. From the lungs it spreads to the bloodstream and causes organ damage. We characterized the in vivo Spn and mouse transcriptomes within the nasopharynx, lungs, blood, heart, and kidneys using three Spn strains. We identified Spn genes highly expressed at all anatomical sites and in an organ-specific manner; highly expressed genes were shown to have vital roles with knockout mutants. The in vivo bacterial transcriptome during colonization/disease was distinct from previously reported in vitro transcriptomes. Distinct Spn and host gene-expression profiles were observed during colonization and disease states, revealing specific genes/operons whereby Spn adapts to and influences host sites in vivo. We identified and experimentally verified host-defense pathways induced by Spn during invasive disease, including proinflammatory responses and the interferon response. These results shed light on the pathogenesis of Spn and identify therapeutic targets.

The cGAS-STING Pathway in Hematopoiesis and Its Physiopathological Significance

Cytosolic DNA sensing is a fundamental mechanism by which organisms handle various stresses, including infection and genotoxicity. The hematopoietic system is sensitive to stresses, and hematopoietic changes are often rapid and the first response to stresses. Based on the transcriptome database, cytosolic DNA sensing pathways are widely expressed in the hematopoietic system, and components of these pathways may be expressed at even higher levels in hematopoietic stem and progenitor cells (HSPCs) than in their certain progeny immune cells. Recent studies have described a previously unrecognized role for cytosolic DNA sensing pathways in the regulation of hematopoiesis under both homeostatic and stress conditions. In particular, the recently discovered cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) pathway is a critical modulator of hematopoiesis. Perturbation of the cGAS-STING pathway in HSPCs may be involved in the pathogenesis of hematopoietic disorders, autoimmune diseases, and inflammation-related diseases and may be candidate therapeutic targets. In this review, we focus on the recent findings of the cGAS-STING pathway in the regulation of hematopoiesis, and its physiopathological significance including its implications in diseases and therapeutic potential.

Antiviral system of innate immunity: COVID-19 pathogenesis and treatment

Antiviral system of innate immunity includes two main components: the mitochondrial antiviral sensor — the mitochondrial outer membrane protein and peripheral blood neutrophils capable of forming neutrophilic extracellular traps. Depending on the activation pathway of the mitochondrial antiviral sensor (MAVS), two possible variants of cells death, apoptosis or cellular degeneration with necrotic changes, develop during cell infection with an RNA-containing virus. The development of virus-induced apoptosis of infected cells causes the formation of neutrophilic extracellular traps, the secretion of inflammatory cytokines, ROS generation, tissue damage, hemocoagulation and the development of an acute inflammatory process with the development of COVID-19 pneumonia. Violation of the prion-like reaction of MAVS in response to viral infection of the cell triggers an alternative pathway for activating autophagy. Cells under conditions of prolonged activation of autophagy experience necrotic changes and are eliminated from the organism by monocytes/macrophages that secrete anti-inflammatory cytokines. This type of reaction of the antiviral system of innate immunity corresponds to the asymptomatic course of the disease. From the most significant aspects of the pathogenesis of the coronavirus infection COVID-19 given, recommendations for the prophylactic treatment of this dangerous disease follow. The proposed treatment can significantly decrease the severity of COVID-19 disease and reduce mortality.

The cGAS-STING pathway: The role of self-DNA sensing in inflammatory lung disease

The presence of DNA in the cytosol is usually a sign of microbial infections, which alerts the host innate immune system to mount a defense response. Cyclic GMP-AMP synthase (cGAS) is a critical cytosolic DNA sensor that elicits robust innate immune responses through the production of the second messenger, cyclic GMP-AMP (cGAMP), which binds and activates stimulator of interferon genes (STING). However, cGAS binds to DNA irrespective of DNA sequence, therefore, self-DNA leaked from the nucleus or mitochondria can also serve as a cGAS ligand to activate this pathway and trigger extensive inflammatory responses. Dysregulation of the cGAS-STING pathway is responsible for a broad array of inflammatory and autoimmune diseases. Recently, evidence has shown that self-DNA release and cGAS-STING pathway over-activation can drive lung disease, making this pathway a promising therapeutic target for inflammatory lung disease. Here, we review recent advances on the cGAS-STING pathway governing self-DNA sensing, highlighting its role in pulmonary disease.

Cytosolic mtDNA released from pneumolysin-damaged mitochondria triggers IFN-β production in epithelial cells

Pneumolysin (Ply) is a major virulence factor of Streptococcus pneumoniae. Ply-induced interferon-β (IFN-β) expression in host macrophages has been shown to be due to the accumulation of mitochondrial deoxyribonucleic acid (mtDNA) in the cytoplasm during S. pneumoniae infection. Our findings extend this work to show human bronchial epithelial cells that reside at the interface of inflammatory injury, BEAS-2B, adapt to local cues by altering mitochondrial states and releasing excess mtDNA. The results in this research showed that purified Ply induced the expression of IFN-β in human epithelial cells, which was accompanied by mitochondrial damage both in vivo and in vitro. The observations also were supported by the increased mtDNA concentrations in the bronchial lavage fluid of mice infected with S. pneumoniae. In summary, our study demonstrated that Ply triggered the production of IFN-β in epithelial cells, and this response was mediated by mtDNA released from Ply-damaged mitochondria. It displayed an impressive modulation of IFN-β response to S. pneumoniae in epithelial cells.