Search

Priming of macrophages with interferon-gamma (IFNγ) or interleukin-4 (IL-4) leads to polarisation into pro-inflammatory or anti-inflammatory subtypes, which produce key enzymes such as inducible nitric oxide synthase (iNOS) and arginase 1 (ARG1), respectively, and in this way determine host responses to infection. Importantly, L-arginine is the substrate for both enzymes. ARG1 upregulation is associated with increased pathogen load in different infection models. However, while differentiation of macrophages with IL-4 impairs host resistance to the intracellular bacterium Salmonella enterica serovar Typhimurium ( S .tm), little is known on the effects of IL-4 on unpolarised macrophages during infection. Therefore, bone-marrow-derived macrophages (BMDM) from C57BL/6N, Tie2Cre +/- ARG1 fl/fl (KO), Tie2Cre -/- ARG1 fl/fl (WT) mice were infected with S .tm in the undifferentiated state and then stimulated with IL-4 or IFNγ. In addition, BMDM of C57BL/6N mice were first polarised upon stimulation with IL-4 or IFNγ and then infected with S .tm. Interestingly, in contrast to polarisation of BMDM with IL-4 prior to infection, treatment of non-polarised S .tm-infected BMDM with IL-4 resulted in improved infection control whereas stimulation with IFNγ led to an increase in intracellular bacterial numbers compared to unstimulated controls. This effect of IL-4 was paralleled by decreased ARG1 levels and increased iNOS expression. Furthermore, the L-arginine pathway metabolites ornithine and polyamines were enriched in unpolarised cells infected with S .tm and stimulated with IL-4. Depletion of L-arginine reversed the protective effect of IL-4 toward infection control. Our data show that stimulation of S .tm-infected macrophages with IL-4 reduced bacterial multiplication via metabolic re-programming of L-arginine-dependent pathways.

Arginase 1 (ARG1) is a cytosolic enzyme that cleaves L-arginine, the substrate of inducible nitric oxide synthase (iNOS), and thereby impairs the control of various intracellular pathogens. Herein, we investigated the role of ARG1 during infection with Salmonella enterica serovar Typhimurium ( S .tm). To study the impact of ARG1 on Salmonella infections in vitro, bone marrow-derived macrophages (BMDM) from C57BL/6N wild-type, ARG1-deficient Tie2Cre +/- ARG1 fl/fl and NRAMP G169 C57BL/6N mice were infected with S .tm. In wild-type BMDM, ARG1 was induced by S .tm and further upregulated by the addition of interleukin (IL)-4, whereas interferon-γ had an inhibitory effect. Deletion of ARG1 did not result in a reduction in bacterial numbers. In vivo, Arg1 mRNA was upregulated in the spleen, but not in the liver of C57BL/6N mice following intraperitoneal S .tm infection. The genetic deletion of ARG1 (Tie2Cre +/- ARG1 fl/fl ) or its pharmacological inhibition with CB-1158 neither affected the numbers of S .tm in spleen, liver and blood nor the expression of host response genes such as iNOS, IL-6 or tumour necrosis factor (TNF). Furthermore, ARG1 was dispensable for pathogen control irrespective of the presence or absence of the phagolysosomal natural resistance-associated macrophage protein 1 (NRAMP1). Thus, unlike the detrimental function of ARG1 seen during infections with other intraphagosomal microorganisms, ARG1 did not support bacterial survival in systemic salmonellosis, indicating differential roles of arginine metabolism for host immune response and microbe persistence depending on the type of pathogen.

Iron is an essential co-factor for several metabolic processes, including the Krebs cycle and mitochondrial oxidative phosphorylation. Therefore, maintaining an appropriate iron balance is essential to ensure sufficient energy production and to avoid excessive reactive oxygen species formation. Iron overload impairs mitochondrial fitness; however, little is known about the associated metabolic changes. Here we aimed to characterize the metabolic signature triggered by dietary iron overload over time in a mouse model, where mice received either a standard or a high-iron diet. Metabolic profiling was assessed in blood, plasma and liver tissue. Peripheral blood was collected by means of volumetric absorptive microsampling (VAMS). Extracted blood and tissue metabolites were analyzed by liquid chromatography combined to high resolution mass spectrometry. Upon dietary iron loading we found increased glucose, aspartic acid and 2-/3-hydroxybutyric acid levels but low lactate and malate levels in peripheral blood and plasma, pointing to a re-programming of glucose homeostasis and the Krebs cycle. Further, iron loading resulted in the stimulation of the urea cycle in the liver. In addition, oxidative stress was enhanced in circulation and coincided with increased liver glutathione and systemic cysteine synthesis. Overall, iron supplementation affected several central metabolic circuits over time. Hence, in vivo investigation of metabolic signatures represents a novel and useful tool for getting deeper insights into iron-dependent regulatory circuits and for monitoring of patients with primary and secondary iron overload, and those ones receiving iron supplementation therapy.