Macrophages inhibit Coxiella burnetii by the ACOD1-itaconate pathway for containment of Q fever

IRG1/itaconate alleviates acute liver injury in septic mice by suppressing NLRP3 expression and its mediated macrophage pyroptosis via regulation of the Nrf2 pathway

Sepsis, a systemic inflammatory response triggered by infection, has a considerably high mortality rate. However, effective prevention and intervention measures against sepsis remain insufficient. Therefore, this study aimed to investigate the mechanisms underlying the protective properties of immune response gene-1 (IRG1) and 4-Octyl itaconate (OI) during acute liver damage in mice with sepsis. A sepsis mouse model was established to compare wild-type and IRG1-/- groups. The impact of IRG1/Itaconate on pro- and anti-inflammatory cytokines was evaluated using J774A.1 cells. IRG1/Itaconate substantially reduced pro-inflammatory cytokines and increased the release of anti-inflammatory cytokines. It reduced pathological damage to liver tissues, preserved normal liver function, decreased the release of reactive oxygen species (ROS) and LDH, and enhanced the GSH/GSSG ratio. Moreover, IRG1 and itaconic acid activated the Nrf2 signaling pathway, regulating the expression of its downstream antioxidative stress-related proteins. Additionally, they inhibited the activity of NLRP3 inflammatory vesicles to suppress the expression of macrophage-associated pyroptosis signaling molecules. Our findings demonstrate that IRG1/OI inhibits NLRP3 inflammatory vesicle activation and macrophage pyroptosis by modulating the Nrf2 signaling pathway, thereby attenuating acute liver injury in mice with sepsis. These findings could facilitate the clinical application of IRG1/Itaconate to prevent sepsis-induced acute liver injury.

IRG1/Itaconate inhibits proliferation and promotes apoptosis of CD69+CD103+CD8+ tissue-resident memory T cells in autoimmune hepatitis by regulating the JAK3/STAT3/P53 signalling pathway

To investigate the protective role of immune response gene 1 (IRG1) and exogenous itaconate in autoimmune hepatitis (AIH) and elucidate the underlying mechanisms. Wild-type and IRG1-/- AIH mouse models were established, and samples of liver tissue and ocular blood were collected from each group of mice to assess the effects of IRG1/itaconate on the expression of pro- and anti-inflammatory cytokines. The levels of liver enzymes and related inflammatory factors were determined using enzyme-linked immunosorbent assay and real-time quantitative polymerase chain reaction (PCR). Liver histomorphology was detected through hematoxylin and eosin staining and then scored for liver injury, and the infiltration levels of tissue-resident memory T (TRM) cells and related molecules in the liver tissue were detected through immunofluorescence staining in vitro. RNA sequencing and gene enrichment analysis were conducted to identify the corresponding molecules and pathways, and lentiviral transfection was used to generate TRM cell lines with IRG1, Jak3, Stat3, and p53 knockdown. Real-time quantitative PCR and western blot were performed to detect the expression levels of relevant mRNAs and proteins in the liver tissue and cells. The percentage of apoptotic cells was determined using flow cytometry. IRG1/itaconate effectively reduced the release of pro-inflammatory cytokines and the pathological damage to liver tissue, thereby maintaining normal liver function. At the same time, IRG1/itaconate inhibited the JAK3/STAT3 signaling pathway, regulated the expression of related downstream proteins, and inhibited the proliferation and promoted the apoptosis of CD69+CD103+CD8+ TRM cells. For the first time, P53 was found to act as a downstream molecule of the JAK3/STAT3 pathway and was regulated by IRG1/itaconate to promote the apoptosis of CD8+ TRM cells. IRG1/itaconate can alleviate concanavalin A-induced autoimmune hepatitis in mice by inhibiting the proliferation and promoting the apoptosis of CD69+CD103+CD8+ TRM cells via the JAK3/STAT3/P53 pathway.

Dual proteomics of infected macrophages reveal bacterial and host players involved in the Francisella intracellular life cycle and cell to cell dissemination by merocytophagy

Bacterial pathogens adapt and replicate within host cells, while host cells develop mechanisms to eliminate them. Using a dual proteomic approach, we characterized the intra-macrophage proteome of the facultative intracellular pathogen, Francisella novicida. More than 900 Francisella proteins were identified in infected macrophages after a 10-h infection. Biotin biosynthesis-related proteins were upregulated, emphasizing the role of biotin-associated genes in Francisella replication. Conversely, proteins encoded by the Francisella pathogenicity island (FPI) were downregulated, supporting the importance of the F. tularensis Type VI Secretion System for vacuole escape, not cytosolic replication. In the host cell, over 300 proteins showed differential expression among the 6200 identified during infection. The most upregulated host protein was cis-aconitate decarboxylase IRG1, known for itaconate production with antimicrobial properties in Francisella. Surprisingly, disrupting IRG1 expression did not impact Francisella's intracellular life cycle, suggesting redundancy with other immune proteins or inclusion in larger complexes. Over-representation analysis highlighted cell-cell contact and actin polymerization in macrophage deregulated proteins. Using flow cytometry and live cell imaging, we demonstrated that merocytophagy involves diverse cell-to-cell contacts and actin polymerization-dependent processes. These findings lay the groundwork for further exploration of merocytophagy and its molecular mechanisms in future research.Data are available via ProteomeXchange with identifier PXD035145.

Itaconate in host inflammation and defense

Immune cells undergo rapid and extensive metabolic changes during inflammation. In addition to contributing to energetic and biosynthetic demands, metabolites can also function as signaling molecules. Itaconate (ITA) rapidly accumulates to high levels in myeloid cells under infectious and sterile inflammatory conditions. This metabolite binds to and regulates the function of diverse proteins intracellularly to influence metabolism, oxidative response, epigenetic modification, and gene expression and to signal extracellularly through binding the G protein-coupled receptor (GPCR). Administration of ITA protects against inflammatory diseases and blockade of ITA production enhances antitumor immunity in preclinical models. In this article, we review ITA metabolism and its regulation, discuss its target proteins and mechanisms, and conjecture a rationale for developing ITA-based therapeutics to treat inflammatory diseases and cancer.

The evolutionary tug-of-war of macrophage metabolism during bacterial infection

The function and phenotype of macrophages are intimately linked with pathogen detection. On sensing pathogen-derived signals and molecules, macrophages undergo a carefully orchestrated process of polarization to acquire pathogen-clearing properties. This phenotypic change must be adequately supported by metabolic reprogramming that is now known to support the acquisition of effector function, but also generates secondary metabolites with direct microbicidal activity. At the same time, bacteria themselves have adapted to both manipulate and take advantage of macrophage-specific metabolic adaptations. Here, we summarize the current knowledge on macrophage metabolism during infection, with a particular focus on understanding the 'arms race' between host immune cells and bacteria during immune responses.

Control of immune cell signaling by the immuno-metabolite itaconate

Immune cell activation triggers signaling cascades leading to transcriptional reprogramming, but also strongly impacts on the cell's metabolic activity to provide energy and biomolecules for inflammatory and proliferative responses. Macrophages activated by microbial pathogen-associated molecular patterns and cytokines upregulate expression of the enzyme ACOD1 that generates the immune-metabolite itaconate by decarboxylation of the TCA cycle metabolite cis-aconitate. Itaconate has anti-microbial as well as immunomodulatory activities, which makes it attractive as endogenous effector metabolite fighting infection and restraining inflammation. Here, we first summarize the pathways and stimuli inducing ACOD1 expression in macrophages. The focus of the review then lies on the mechanisms by which itaconate, and its synthetic derivatives and endogenous isomers, modulate immune cell signaling and metabolic pathways. Multiple targets have been revealed, from inhibition of enzymes to the post-translational modification of many proteins at cysteine or lysine residues. The modulation of signaling proteins like STING, SYK, JAK1, RIPK3 and KEAP1, transcription regulators (e.g. Tet2, TFEB) and inflammasome components (NLRP3, GSDMD) provides a biochemical basis for the immune-regulatory effects of the ACOD1-itaconate pathway. While the field has intensely studied control of macrophages by itaconate in infection and inflammation models, neutrophils have now entered the scene as producers and cellular targets of itaconate. Furthermore, regulation of adaptive immune responses by endogenous itaconate, as well as by exogenously added itaconate and derivatives, can be mediated by direct and indirect effects on T cells and antigen-presenting cells, respectively. Taken together, research in ACOD1-itaconate to date has revealed its relevance in diverse immune cell signaling pathways, which now provides opportunities for potential therapeutic or preventive manipulation of host defense and inflammation.

Caspase-8 activity mediates TNFα production and restricts Coxiella burnetii replication during murine macrophage infection

Coxiella burnetii is an obligate intracellular bacteria which causes the global zoonotic disease Q Fever. Treatment options for infection are limited, and development of novel therapeutic strategies requires a greater understanding of how C. burnetii interacts with immune signaling. Cell death responses are known to be manipulated by C. burnetii, but the role of caspase-8, a central regulator of multiple cell death pathways, has not been investigated. In this research, we studied bacterial manipulation of caspase-8 signaling and the significance of caspase-8 to C. burnetii infection, examining bacterial replication, cell death induction, and cytokine signaling. We measured caspase, RIPK, and MLKL activation in C. burnetii-infected TNFα/CHX-treated THP-1 macrophage-like cells and TNFα/ZVAD-treated L929 cells to assess apoptosis and necroptosis signaling. Additionally, we measured C. burnetii replication, cell death, and TNFα induction over 12 days in RIPK1-kinase-dead, RIPK3-kinase-dead, or RIPK3-kinase-dead-caspase-8-/- BMDMs to understand the significance of caspase-8 and RIPK1/3 during infection. We found that caspase-8 is inhibited by C. burnetii, coinciding with inhibition of apoptosis and increased susceptibility to necroptosis. Furthermore, C. burnetii replication was increased in BMDMs lacking caspase-8, but not in those lacking RIPK1/3 kinase activity, corresponding with decreased TNFα production and reduced cell death. As TNFα is associated with the control of C. burnetii, this lack of a TNFα response may allow for the unchecked bacterial growth we saw in caspase-8-/- BMDMs. This research identifies and explores caspase-8 as a key regulator of C. burnetii infection, opening novel therapeutic doors.

TNF and type I IFN induction of the IRG1-itaconate pathway restricts Coxiella burnetii replication within mouse macrophages

The intracellular Gram-negative bacterium Coxiella burnetii replicates within macrophages and causes a zoonotic disease known as Q fever. In murine macrophages, the cytokine tumor necrosis factor (TNF) is critical for restriction of intracellular C. burnetii replication. Here, we show that TNF collaborates with type I interferon (IFN) signaling for maximal control of C. burnetii. We found that TNF and type I IFN upregulate the expression of the metabolic enzyme immune responsive gene 1 (IRG1), also known as cis-aconitate decarboxylase 1 (ACOD1), and that IRG1 is required to restrict C. burnetii T4SS translocation and replication within macrophages. Further, we show that itaconic acid, the metabolic product of IRG1, restricts C. burnetii replication both intracellularly and in axenic culture. These data reveal that TNF and type I IFN upregulate the IRG1-itaconate pathway to restrict intracellular C. burnetii replication within murine macrophages.

Itaconate family-based host-directed therapeutics for infections

Itaconate is a crucial anti-infective and anti-inflammatory immunometabolite that accumulates upon disruption of the Krebs cycle in effector macrophages undergoing inflammatory stress. Esterified derivatives of itaconate (4-octyl itaconate and dimethyl itaconate) and its isomers (mesaconate and citraconate) are promising candidate drugs for inflammation and infection. Several itaconate family members participate in host defense, immune and metabolic modulation, and amelioration of infection, although opposite effects have also been reported. However, the precise mechanisms by which itaconate and its family members exert its effects are not fully understood. In addition, contradictory results in different experimental settings and a lack of clinical data make it difficult to draw definitive conclusions about the therapeutic potential of itaconate. Here we review how the immune response gene 1-itaconate pathway is activated during infection and its role in host defense and pathogenesis in a context-dependent manner. Certain pathogens can use itaconate to establish infections. Finally, we briefly discuss the major mechanisms by which itaconate family members exert antimicrobial effects. To thoroughly comprehend how itaconate exerts its anti-inflammatory and antimicrobial effects, additional research on the actual mechanism of action is necessary. This review examines the current state of itaconate research in infection and identifies the key challenges and opportunities for future research in this field.

The Coxiella burnetii effector EmcB is a deubiquitinase that inhibits RIG-I signaling

Eukaryotes have cytosolic surveillance systems to detect invading microorganisms and initiate protective immune responses. In turn, host-adapted pathogens have evolved strategies to modulate these surveillance systems, which can promote dissemination and persistence in the host. The obligate intracellular pathogen Coxiella burnetii infects mammalian hosts without activating many innate immune sensors. The Defect in Organelle Trafficking/Intracellular Multiplication (Dot/Icm) protein secretion system is necessary for C. burnetii to establish a vacuolar niche inside of host cells, which sequesters these bacteria in a specialized organelle that could evade host surveillance systems. However, bacterial secretion systems often introduce agonists of immune sensors into the host cytosol during infection. For instance, nucleic acids are introduced to the host cytosol by the Dot/Icm system of Legionella pneumophila, which results in type I interferon production. Despite host infection requiring a homologous Dot/Icm system, C. burnetii does not induce type I interferon production during infection. Here, it was found that type I interferons are detrimental to C. burnetii infection and that C. burnetii blocks type I interferon production mediated by retionic acid inducible gene I (RIG-I) signaling. Two Dot/Icm effector proteins, EmcA and EmcB, are required for C. burnetii inhibition of RIG-I signaling. EmcB is sufficient to block RIG-I signaling and is a ubiquitin-specific cysteine protease capable of deconjugating ubiquitin chains from RIG-I that are necessary for signaling. EmcB preferentially cleaves K63-linked ubiquitin chains of three or more monomers, which represent ubiquitin chains that potently activate RIG-I signaling. Identification of a deubiquitinase encoded by C. burnetii provides insights into how a host-adapted pathogen antagonizes immune surveillance.

Bovine blood derived macrophages are unable to control Coxiella burnetii replication under hypoxic conditions

Background

Coxiella burnetii is a zoonotic pathogen, infecting humans, livestock, pets, birds and ticks. Domestic ruminants such as cattle, sheep, and goats are the main reservoir and major cause of human infection. Infected ruminants are usually asymptomatic, while in humans infection can cause significant disease. Human and bovine macrophages differ in their permissiveness for C. burnetii strains from different host species and of various genotypes and their subsequent host cell response, but the underlying mechanism(s) at the cellular level are unknown.

Methods

C. burnetii infected primary human and bovine macrophages under normoxic and hypoxic conditions were analyzed for (i) bacterial replication by CFU counts and immunofluorescence; (ii) immune regulators by westernblot and qRT-PCR; cytokines by ELISA; and metabolites by gas chromatography-mass spectrometry (GC-MS).

Results

Here, we confirmed that peripheral blood-derived human macrophages prevent C. burnetii replication under oxygen-limiting conditions. In contrast, oxygen content had no influence on C. burnetii replication in bovine peripheral blood-derived macrophages. In hypoxic infected bovine macrophages, STAT3 is activated, even though HIF1α is stabilized, which otherwise prevents STAT3 activation in human macrophages. In addition, the TNFα mRNA level is higher in hypoxic than normoxic human macrophages, which correlates with increased secretion of TNFα and control of C. burnetii replication. In contrast, oxygen limitation does not impact TNFα mRNA levels in C. burnetii-infected bovine macrophages and secretion of TNFα is blocked. As TNFα is also involved in the control of C. burnetii replication in bovine macrophages, this cytokine is important for cell autonomous control and its absence is partially responsible for the ability of C. burnetii to replicate in hypoxic bovine macrophages. Further unveiling the molecular basis of macrophage-mediated control of C. burnetii replication might be the first step towards the development of host directed intervention measures to mitigate the health burden of this zoonotic agent.