Immunoresponsive gene 1 (Irg1) is highly expressed in mammalian macrophages during inflammation, but its biological function has not yet been elucidated. Here, we identify Irg1 as the gene coding for an enzyme producing itaconic acid (also known as methylenesuccinic acid) through the decarboxylation of cis-aconitate, a tricarboxylic acid cycle intermediate. Using a gain-and-loss-of-function approach in both mouse and human immune cells, we found Irg1 expression levels correlating with the amounts of itaconic acid, a metabolite previously proposed to have an antimicrobial effect. We purified IRG1 protein and identified its cis-aconitate decarboxylating activity in an enzymatic assay. Itaconic acid is an organic compound that inhibits isocitrate lyase, the key enzyme of the glyoxylate shunt, a pathway essential for bacterial growth under specific conditions. Here we show that itaconic acid inhibits the growth of bacteria expressing isocitrate lyase, such as Salmonella enterica and Mycobacterium tuberculosis. Furthermore, Irg1 gene silencing in macrophages resulted in significantly decreased intracellular itaconic acid levels as well as significantly reduced antimicrobial activity during bacterial infections. Taken together, our results demonstrate that IRG1 links cellular metabolism with immune defense by catalyzing itaconic acid production.

These days it has been increasingly recognized that mast cells (MCs) are critical components of host defense against pathogens. In this study, we have provided the first evidence that MCs can kill bacteria by entrapping them in extracellular structures similar to the extracellular traps described for neutrophils (NETs). We took advantage of the ability of MCs to kill the human pathogen Streptococcus pyogenes by a phagocytosisindependent mechanism in order to characterize the extracellular antimicrobial activity of MCs. Close contact of bacteria and MCs was required for full antimicrobial activity. Immunofluorescence and electron microscopy revealed that S pyogenes was entrapped by extracellular structures produced by MCs ( MCs derived from bone-marrow progenitor cells circulate in the peripheral blood and migrate into vascularized tissue before undergoing final maturation under the influence of local factors. Maturated MCs are commonly found in tissues that interface with the external environment such as the skin and mucosa of the respiratory and gastrointestinal tract (reviewed by Mekori and Metcalf 2 ). Because these sites are also common portals of infection, MCs are likely to be among the first inflammatory cells to interact with invading pathogens.Several recent reports in the literature indicate that MCs can mediate a variety of antimicrobial activities following activation upon contact with pathogens. First, MCs have been shown to release preformed and newly synthesized inflammatory mediators, proteases, cytokines, and chemokines that recruit neutrophils to the site of infection. 3,4 They are the only cell type known to prestore TNF-␣ in their secretory granules, which can be released immediately upon activation by pathogens to initiate the early phase of the inflammatory response. 5,6 Secondly, there is increasing experimental evidence that MCs themselves can directly kill various Gramnegative and Gram-positive bacteria. [7][8][9][10] Finally, experiments using MC-deficient mice have clearly demonstrated that MCs are essential for mounting an effective immune response against bacterial infections such as Citrobacter rodentium, 9 Pseudomonas aeruginosa, 11 Klebsiella pneumoniae, 6 or enteropathogenic Escherichia coli. 12-15 Based on these observations, it has been proposed that MCs play a central role in the host defense against infectious pathogens (recently reviewed by Dawicki and Marshall 16 ).Regarding the direct antimicrobial activity of MCs, several studies have shown that MCs are capable of bacterial recognition and intracellular uptake. Bacteria endocytosed after opsoninmediated binding are internalized via a route involving the endosome-lysosome pathway, in which the bacteria are killed through a combination of oxidative and nonoxidative killing systems (reviewed in Féger et al 8 ). These observations suggest that MCs are able to eliminate bacteria through an intracellular bactericidal mechanism similar to that of professional phagocytes.However, recent investigations have reported that various p...

The release of extracellular traps (ETs) is a recently described mechanism of innate immune response to infection. Although ETs have been intensely investigated in the context of neutrophil antimicrobial effector mechanisms, other immune cells such as mast cells, eosinophils, and macrophages can also release these structures. The different ETs have several features in common, regardless of the type of cells from which they originated, including a DNA backbone with embedded antimicrobial peptides, proteases, and histones. However, they also exhibit remarkable individual differences such as the type of sub-cellular compartments from where the DNA backbone originates (e.g., nucleus or mitochondria), the proportion of responding cells within the pool, and/or the molecular mechanism/s underlying the ETs formation. This review summarizes the knowledge accumulated in recent years regarding the complex and expanding world of ETs and their role in immune function with particular emphasis on the role of other immune cells rather than on neutrophils exclusively.

Macrophages provide the first line of defense against invading pathogens. The aim of this study was to determine the role of macrophages during infection with group A streptococci (Streptococcus pyogenes) in mice. Here, we report that resident macrophages can efficiently take up and kill S. pyogenes during in vivo infection, as demonstrated by immunofluorescence and electron microscopy, as well as colony counts. To evaluate the contribution of macrophages to the resolution of experimental infection with S. pyogenes, we compared the susceptibility of BALB/c mice rendered macrophage deficient by treatment with carrageenan with that of intact mice. The results show that depletion of macrophages enhanced the susceptibility of BALB/c mice to S. pyogenes infection, as evidenced by 100% mortality of macrophage-depleted mice compared to 90% survival of nondepleted control animals. The in vivo depletion of macrophages strongly enhanced bacterial loads in the blood and systemic organs. Resistance to S. pyogenes can be restored in macrophage-depleted mice by adoptive transfer of purified macrophages. The in vivo blocking of the macrophage phagocytic function by treatment with gadolinium III chloride also resulted in enhanced susceptibility to S. pyogenes. Interestingly, depletion of macrophages prior to or during the first 24 h of infection decreased survival dramatically; in contrast, no mortality was observed in infected nondepleted animals or mice depleted after 48 h of infection. These results emphasize the important contribution of macrophages to the early control of S. pyogenes infection.

Mast cells (MCs), which are well known for their effector functions in T H 2-skewed allergic and also autoimmune inflammation, have become increasingly acknowledged for their role in protection of health. It is now clear that they are also key modulators of immune responses at interface organs, such as the skin or gut. MCs can prime tissues for adequate inflammatory responses and cooperate with dendritic cells in T-cell activation. They also regulate harmful immune responses in trauma and help to successfully orchestrate pregnancy. This review focuses on the beneficial effects of MCs on tissue homeostasis and elimination of toxins or venoms. MCs can enhance pathogen clearance in many bacterial, viral, and parasitic infections, such as through Toll-like receptor 2-triggered degranulation, secretion of antimicrobial cathelicidins, neutrophil recruitment, or provision of extracellular DNA traps. The role of MCs in tumors is more ambiguous; however, encouraging new findings show they can change the tumor microenvironment toward antitumor immunity when adequately triggered. Uterine tissue remodeling by a-chymase (mast cell protease [MCP] 5) is crucial for successful embryo implantation. MCP-4 and the tryptase MCP-6 emerge to be protective in central nervous system trauma by reducing inflammatory damage and excessive scar formation, thereby protecting axon growth. Last but not least, proteases, such as carboxypeptidase A, released by FcεRIactivated MCs detoxify an increasing number of venoms and endogenous toxins. A better understanding of the plasticity of MCs will help improve these advantageous effects and hint at ways to cut down detrimental MC actions.

Type I interferons (IFN-Is) are fundamental for antiviral immunity, but their role in bacterial infections is contradictory and incompletely described. Streptococcus pyogenes activates IFN-I production in innate immune cells, and IFN-I receptor 1 (Ifnar1)-deficient mice are highly susceptible to S. pyogenes infection. Here we report that IFN-I signaling protects the host against invasive S. pyogenes infection by restricting inflammation-driven damage in distant tissues. Lethality following infection in Ifnar1-deficient mice is caused by systemically exacerbated levels of the proinflammatory cytokine IL-1β. Critical cellular effectors of IFN-I in vivo are LysM+ and CD11c+ myeloid cells, which exhibit suppression of Il1b transcription upon Ifnar1 engagement. These cells are also the major source of IFN-β, which is significantly induced by S. pyogenes 23S rRNA in an Irf5-dependent manner. Our study establishes IL-1β and IFN-I levels as key homeostatic variables of protective, yet tuned, immune responses against severe invasive bacterial infection.

SummaryMacrophages are crucial components of the host defence against Streptococcus pyogenes. Here, we demonstrate the ability of S. pyogenes to kill macrophages through the activation of an inflammatory programmed cell death pathway. Macrophages exposed to S. pyogenes exhibited extensive cytoplasmic vacuolization, cellular and organelle swelling and rupture of the plasma membrane typical of oncosis. The cytotoxic effect of S. pyogenes on macrophages is mediated by the streptococcal cytolysins streptolysin S and streptolysin O and does not require bacterial internalization. S. pyogenes-induced death of macrophages was not affected by the addition of osmoprotectant, implicating the activation of an orchestrated cell death pathway rather than a simple osmotic lysis. This programme cell death pathway involves the loss of mitochondria transmembrane potential (Dy m) and was inhibited by the addition of exogenous glycine, which has been shown to prevent necrotic cell death by blocking the opening of death channels in the plasma membrane. The production of reactive oxygen species and activation of calpains were identified as mediators of the cell death process. We conclude that activation of the inflammatory programmed cell death pathway in macrophages could constitute an important pathogenic mechanism by which S. pyogenes evades host immune defences and causes disease.

In this study, we investigated the interactions of Staphylococcus aureus with mast cells, which are multifunctional sentinels lining the surfaces of the body. We found that bone marrow-derived murine mast cells (BMMC) exerted a powerful phagocytosis-independent antimicrobial activity against S. aureus. Both the release of extracellular traps as well as discharge of antimicrobial compounds were the mechanisms used by the BMMC to kill extracellular S. aureus. This was accompanied by the secretion of mediators such as TNF-α involved in the recruitment of effector cells. Interestingly, S. aureus subverted the extracellular antimicrobial activity of the BMMC by internalizing within these cells. S. aureus was also capable to internalize within human mast cells (HMC-1) and within murine skin mast cells during in vivo infection. Bacteria internalization was, at least in part, mediated by the α5β1 integrins expressed on the surface of the mast cell. In the intracellular milieu, the bacterium survived and persisted by increasing the cell wall thickness and by gaining access into the mast cell cytosol. The expression of α-hemolysin was essential for staphylococci intracellular persistence. By hiding within the long-life mast cells, staphylococci not only avoid clearance but also establish an infection reservoir that could contribute to chronic carriage.