Impact of IRG1 on lipid droplet formation and host immune response to control Mycobacterium tuberculosis infection
Arnaud Machelart 1, Imène Belhaouane 1, Nathalie Deboosere 1, Isabelle Poncin 2, Jean-Paul Saint-André 3, Carine Rouanet 1, Stéphane Canaan 2, Priscille Brodin 1,4,5,* and Eik Hoffmann 1,4,5,*
1 Institute Pasteur Lille, Center for Infection and Immunity of Lille, Lille University, INSERM U1019, CNRS UMR9017, Lille, France
2 Aix-Marseille University, CNRS UMR7255 - LISM, FR3479 - IMM, Marseille, France
3 University Hospital Center of Angers, Angers, France
4 European Regimen Accelerator for Tuberculosis (ERA4TB), Innovative Medicines Initiative 2 (grant agreement no. 853989)
5 This study was supported by the French government through the Program “Investissements d’avenir” (I-ULNE SITE / ANR-16-IDEX-0004 ULNE) managed by the National Research Agency.
Mycobacterium tuberculosis (Mtb), the pathogen causing human tuberculosis (TB), has evolved multiple strategies to successfully prevent clearance by immune cells and to establish dissemination and long-term survival in the host. The intracellular fate of Mtb within phagocytes is a pivotal question, since Mtb can persist for long periods of time and contribute to inadequate host control which can lead to active TB. Even though a large number of membrane and cytosolic host receptors contribute to the bacterial invasion process, key cellular parameters that modulate and orchestrate successful establishment of a replicative intracellular niche for Mtb are still largely unknown. In particular, the influence of immunometabolic host responses to Mtb infection remain to be uncovered. The modulation of host immunity to maximize pathogen elimination while minimizing inflammation-mediated tissue damage may provide another tool to fight drug-resistant Mtb strains. Metabolic reprogramming of immune cell populations can dramatically influence the outcome of immune responses and modulate antimicrobial properties of infected host cells, nicely demonstrating that metabolites are tightly linked to immune cell effector functions. One important endogenous metabolite of the rewired Krebs cycle during Mtb infection is itaconate, which has potent bactericidal activity by inhibiting isocitrate lyase and the glyoxylate shunt. Recent findings show that itaconate and the catalytic enzyme responsible for its generation in mammalian cells, i.e. IRG1 (immune-responsive gene 1), modify inflammatory signaling of infected cells enhancing host defense pathways.
Here, we demonstrate that IRG1 is recruited to Mtb-containing phagosomes and that it influences the host response dependent on the virulency of Mtb infection. IRG1-deficient mice were highly susceptible to infection by Mtb H37Rv via the intranasal route resulting in animal death three weeks post-infection, which was linked to exacerbated inflammation and high mycobacterial burden. The lungs of infected IRG1-deficient mice displayed large areas of necrotizing granulomatous inflammation and neutrophil infiltration, accompanied by reduced levels of B and T lymphocytes and increased levels of alveolar and interstitial macrophage populations, compared to their wild type counterparts. In contrast, exposure of IRG1-deficient mice to the attenuated strain Mtb H37Ra and M. bovis BCG induced neither lethality nor severe lung immunopathology, demonstrated by lower levels of neutrophils and higher levels of adaptive immune cells found in lungs and spleens of those mice. Importantly, administration of the vaccinal strain M. bovis BCG eight weeks before challenge with Mtb H37Rv was sufficient to reverse the susceptibility of IRG1-deficient mice to virulent Mtb infection. While IRG1 deficiency did not affect uptake of Mtb H37Rv by macrophages and dendritic cells (DCs) in vitro, it increased the intracellular replication of Mtb, observed and quantified by automated confocal microscopy. Concomitantly, in comparison to wild type cells, IRG1-deficient macrophages and DCs showed increased levels of lipid droplets, a correlate of inflammation. These intracellular organelles store triacylglycerol and phospholipids that are hijacked by Mtb as reservoir of host nutrients. When the generation of lipid droplets was impaired, e.g. by treatment with inhibitors of diacylglycerol acyltransferases (DGAT), we also observed reduced numbers of Mtb in macrophages. Our data demonstrate a potential role of IRG1 in the modulation of innate immunity and immunometabolic control of Mtb infection by regulating inflammation and availability of host nutrients to the pathogen, as well as an impact on the induction of adaptive immune responses, which are critical for the severity of Mtb-mediated pathologies.