Targeted macrophage phagocytosis by Irg1/itaconate axis improves the prognosis of intracerebral hemorrhagic stroke and peritonitis

BACKGROUND

Macrophages are innate immune cells whose phagocytosis function is critical to the prognosis of stroke and peritonitis. cis-aconitic decarboxylase immune-responsive gene 1 (Irg1) and its metabolic product itaconate inhibit bacterial infection, intracellular viral replication, and inflammation in macrophages. Here we explore whether itaconate regulates phagocytosis.

METHODS

Phagocytosis of macrophages was investigated by time-lapse video recording, flow cytometry, and immunofluorescence staining in macrophage/microglia cultures isolated from mouse tissue. Unbiased RNA-sequencing and ChIP-sequencing assays were used to explore the underlying mechanisms. The effects of Irg1/itaconate axis on the prognosis of intracerebral hemorrhagic stroke (ICH) and peritonitis was observed in transgenic (Irg1flox/flox; Cx3cr1creERT/+, cKO) mice or control mice in vivo.

FINDINGS

In a mouse model of ICH, depletion of Irg1 in macrophage/microglia decreased its phagocytosis of erythrocytes, thereby exacerbating outcomes (n = 10 animals/group, p < 0.05). Administration of sodium itaconate/4-octyl itaconate (4-OI) promoted macrophage phagocytosis (n = 7 animals/group, p < 0.05). In addition, in a mouse model of peritonitis, Irg1 deficiency in macrophages also inhibited phagocytosis of Staphylococcus aureus (n = 5 animals/group, p < 0.05) and aggravated outcomes (n = 9 animals/group, p < 0.05). Mechanistically, 4-OI alkylated cysteine 155 on the Kelch-like ECH-associated protein 1 (Keap1), consequent in nuclear translocation of nuclear factor erythroid 2-related factor 2 (Nrf2) and transcriptional activation of Cd36 gene. Blocking the function of CD36 completely abolished the phagocytosis-promoting effects of Irg1/itaconate axis in vitro and in vivo.

INTERPRETATION

Our findings provide a potential therapeutic target for phagocytosis-deficiency disorders, supporting further development towards clinical application for the benefit of stroke and peritonitis patients.

FUNDING

The National Natural Science Foundation of China (32070735, 82371321 to Q. Li, 82271240 to F. Yang) and the Beijing Natural Science Foundation Program and Scientific Research Key Program of Beijing Municipal Commission of Education (KZ202010025033 to Q. Li).

Immuno-Responsive Gene-1: A mitochondrial gene regulates pathogenic Th17 in CNS autoimmunity mouse model

Pathogenic Th17 cells are crucial to CNS autoimmune diseases like multiple sclerosis (MS), though their control by endogenous mechanisms is unknown. RNAseq analysis of brain glial cells identified immuno-responsive gene 1 (Irg1), a mitochondrial-related enzyme-coding gene, as one of the highly upregulated gene under inflammatory conditions which were further validated in the spinal cord of animals with experimental autoimmune encephalomyelitis (EAE), an animal model of MS. Moreover, Irg1 mRNA and protein levels in myeloid, CD4, and B cells were higher in the EAE group, raising questions about its function in CNS autoimmunity. We observed that Irg1 knockout (KO) mice exhibited severe EAE disease and greater mononuclear cell infiltration, including triple-positive CD4 cells expressing IL17a, GM-CSF, and IFNγ. Lack of Irg1 in macrophages led to higher levels of Class II expression and polarized myelin primed CD4 cells into pathogenic Th17 cells through the NLRP3/IL1β axis. Our findings show that Irg1 in macrophages plays an important role in the formation of pathogenic Th17 cells, emphasizing its potential as a therapy for autoimmune diseases, including MS.

Itaconate Suppresses the Activation of Mitochondrial NLRP3 Inflammasome and Oxidative Stress in Allergic Airway Inflammation

Itaconate has emerged as a novel anti-inflammatory and antioxidative endogenous metabolite, yet its role in allergic airway inflammation (AAI) and the underlying mechanism remains elusive. Here, the itaconate level in the lung was assessed by High Performance Liquid Chromatography (HPLC), and the effects of the Irg1/itaconate pathway on AAI and alveolar macrophage (AM) immune responses were evaluated using an ovalbumin (OVA)-induced AAI model established by wild type (WT) and Irg1^-/- mice, while the mechanism of this process was investigated by metabolomics analysis, mitochondrial/cytosolic protein fractionation and transmission electron microscopy in the lung tissues. The results demonstrated that the Irg1 mRNA/protein expression and itaconate production in the lung were significantly induced by OVA. Itaconate ameliorated while Irg1 deficiency augmented AAI, and this may be attributed to the fact that itaconate suppressed mitochondrial events such as NLRP3 inflammasome activation, oxidative stress and metabolic dysfunction. Furthermore, we identified that the Irg1/itaconate pathway impacted the NLRP3 inflammasome activation and oxidative stress in AMs. Collectively, our findings provide evidence for the first time, supporting the conclusion that in the allergic lung, the itaconate level is markedly increased, which directly regulates AMs' immune responses. We therefore propose that the Irg1/itaconate pathway in AMs is a potential anti-inflammatory and anti-oxidative therapeutic target for AAI.

Mycobacterium tuberculosis Induces Irg1 in Murine Macrophages by a Pathway Involving Both TLR-2 and STING/IFNAR Signaling and Requiring Bacterial Phagocytosis

Irg1 is an enzyme that generates itaconate, a metabolite that plays a key role in the regulation of inflammatory responses. Previous studies have implicated Irg1 as an important mediator in preventing excessive inflammation and tissue damage in Mycobacterium tuberculosis (Mtb) infection. Here, we investigated the pattern recognition receptors and signaling pathways by which Mtb triggers Irg1 gene expression by comparing the responses of control and genetically deficient BMDMs. Using this approach, we demonstrated partial roles for TLR-2 (but not TLR-4 or -9), MyD88 and NFκB signaling in Irg1 induction by Mtb bacilli. In addition, drug inhibition studies revealed major requirements for phagocytosis and endosomal acidification in Irg1 expression triggered by Mtb but not LPS or PAM3CSK4. Importantly, the Mtb-induced Irg1 response was highly dependent on the presence of the bacterial ESX-1 secretion system, as well as host STING and Type I IFN receptor (IFNAR) signaling with Type II IFN (IFN-γ) signaling playing only a minimal role. Based on these findings we hypothesize that Mtb induces Irg1 expression in macrophages via the combination of two independent triggers both dependent on bacterial phagocytosis: 1) a major signal stimulated by phagocytized Mtb products released by an ESX-1-dependent mechanism into the cytosol where they activate the STING pathway leading to Type I-IFN production, and 2) a secondary TLR-2, MyD88 and NFκB dependent signal that enhances Irg1 production independently of Type I IFN induction.

Blockage of citrate export prevents TCA cycle fragmentation via Irg1 inactivation

The metabolism of activated macrophages relies on aerobic glycolysis, while mitochondrial oxidation is disrupted. In lipopolysaccharide-activated macrophages, the citrate carrier (CIC) exports citrate from mitochondria to enhance glycolytic genes through histone acetylation. CIC inhibition or Slc25a1 knockdown reduces the occupancy of H3K9ac to hypoxia-inducible factor-1α (HIF-1α) binding sites in promoters of glycolytic genes to restrain glycolysis. HIF-1α also transcriptionally upregulates immune-responsive gene 1 for itaconate production, which is inhibited by CIC blocking. Isotopic tracing of [U-13C6] glucose shows that CIC blockage prevents citrate accumulation and itaconate production by reducing glycolytic flux and facilitating metabolic flux in the TCA cycle. Isotopic tracing of [U-13C5] glutamine reveals that CIC inhibition reduces succinate accumulation from glutaminolysis and the gamma-aminobutyric acid shunt by enhancing mitochondrial oxidation. By restraining glycolysis, CIC inhibition increases NAD+ content to ensure mitochondrial biogenesis for oxidative phosphorylation. Furthermore, blockage of citrate export reduces cerebral thrombosis by inactivation of peripheral macrophages.

Aconitate decarboxylase 1 suppresses cerebral ischemia-reperfusion injury in mice

Immunometabolic changes have been shown to be a key factor in determining the immune cell response in disease models. The immunometabolite, itaconate, is produced by aconitate decarboxylase 1 (Acod1) and has been shown to inhibit inflammatory signaling in macrophages. In this study, we explore the role of Acod1 and itaconate in cerebral ischemia/reperfusion injury. We assessed the effect of global Acod1 knockout (Acod1KO, loss of endogenous itaconate) in a transient ischemia/reperfusion occlusion stroke model. Mice received a transient 90-min middle cerebral artery occlusion followed with 24-h of reperfusion. Stroke lesion volume was measured by MRI analysis and brain tissues were collected for mRNA gene expression analysis. Acod1KO mice showed significant increases in lesion volume compared to control mice, however no differences in pro-inflammatory mRNA levels were observed. Cell specific knockout of Acod1 in myeloid cells (LysM-Cre), microglia cells (CX3CR1, Cre-ERT2) and Endothelial cells (Cdh5(PAC), Cre-ERT2) did not reproduce lesion volume changes seen in global Acod1KO, indicating that circulating myeloid cells, resident microglia and endothelial cell populations are not the primary contributors to the observed phenotype. These effects however do not appear to be driven by changes in inflammatory gene regulation. These data suggests that endogenous Acod1 is protective in cerebral ischemia/reperfusion injury.

Integrative metabolomics and transcriptomics identifies itaconate as an adjunct therapy to treat ocular bacterial infection

The eye is highly susceptible to inflammation-mediated tissue damage evoked during bacterial infection. However, mechanisms regulating inflammation to protect the eye remain elusive. Here, we used integrated metabolomics and transcriptomics to show that the immunomodulatory metabolite itaconate and immune-responsive gene 1 (Irg1) are induced in bacterial (Staphylococcus aureus)-infected mouse eyes, bone-marrow-derived macrophages (BMDMs), and Müller glia. Itaconate levels are also elevated in the vitreous of patients with bacterial endophthalmitis. Irg1 deficiency in mice led to increased ocular pathology. Conversely, intraocular administration of itaconate protects both Irg1-/- and wild-type mice from bacterial endophthalmitis by reducing inflammation, bacterial burden, and preserving retinal architecture and visual function. Notably, itaconate exerts synergistic effects with antibiotics. The protective, anti-inflammatory effects of itaconate are mediated via activation of NRF2/HO-1 signaling and inhibition of NLRP3 inflammasome. Collectively, our study demonstrates the Irg1/itaconate axis is a regulator of intraocular inflammation and provides evidence for using itaconate, along with antibiotics, to treat bacterial infections.

Establishment, Validation, and Initial Application of a Sensitive LC-MS/MS Assay for Quantification of the Naturally Occurring Isomers Itaconate, Mesaconate, and Citraconate

Itaconate is derived from the tricarboxylic acid (TCA) cycle intermediate cis-aconitate and links innate immunity and metabolism. Its synthesis is altered in inflammation-related disorders and it therefore has potential as clinical biomarker. Mesaconate and citraconate are naturally occurring isomers of itaconate that have been linked to metabolic disorders, but their functional relationships with itaconate are unknown. We aimed to establish a sensitive high performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) assay for the quantification of itaconate, mesaconate, citraconate, the pro-drug 4-octyl-itaconate, and selected TCA intermediates. The assay was validated for itaconate, mesaconate, and citraconate for intra- and interday precision and accuracy, extended stability, recovery, freeze/thaw cycles, and carry-over. The lower limit of quantification was 0.098 µM for itaconate and mesaconate and 0.049 µM for citraconate in 50 µL samples. In spike-in experiments, itaconate remained stable in human plasma and whole blood for 24 and 8 h, respectively, whereas spiked-in citraconate and mesaconate concentrations changed during incubation. The type of anticoagulant in blood collection tubes affected measured levels of selected TCA intermediates. Human plasma may contain citraconate (0.4-0.6 µM, depending on the donor), but not itaconate or mesaconate, and lipopolysaccharide stimulation of whole blood induced only itaconate. Concentrations of the three isomers differed greatly among mouse organs: Itaconate and citraconate were most abundant in lymph nodes, mesaconate in kidneys, and only citraconate occurred in brain. This assay should prove useful to quantify itaconate isomers in biomarker and pharmacokinetic studies, while providing internal controls for their effects on metabolism by allowing quantification of TCA intermediates.

Irg1-itaconate axis protects against acute kidney injury via activation of Nrf2

Acute kidney injury (AKI) is a common clinical implication with increased tissue damage, uncontrolled immune responses, and risk of mortality, in which ischemia-reperfusion injury (IRI) is one of the leading causes. As critical role for metabolic remodeling in inflammation, Irg1-itaconate axis has received much attention for its immunomodulation in the control of the inflammation. However, its role in the AKI and IRI remains unknown. Here, we found that Irg1 expression was negatively correlated with the expression of inflammatory cytokines during ischemia-reperfusion injury. And Irg1 deficiency promotes renal inflammation and ischemia-reperfusion injury in vivo. Itaconate treatment promoted the survival of WT mice from lethal ischemia and protected against renal IRI and systemic inflammation. Mechanistically, dimethyl itaconate protected renal cells from oxidative stress and prevented macrophage activation by enhancing the translocation of Nrf2 into the nuclei. Our study highlighted the importance of the Irg1-itaconate axis in the protecting against ischemia-reperfusion injury and acute kidney injury, providing potential therapeutic targets to control AKI.