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International audience; The brain is no longer considered as an organ functioning in isolation; accumulating evidence suggests that changes in the peripheral immune system can indirectly shape brain function. At the interface between the brain and the systemic circulation, the choroid plexuses (CP), which constitute the blood-cerebrospinal fluid barrier, have been highlighted as a key site of periphery-to-brain communication. CP produce the cerebrospinal fluid, neurotrophic factors, and signaling molecules that can shape brain homeostasis. CP are also an active immunological niche. In contrast to the brain parenchyma, which is populated mainly by microglia under physiological conditions, the heterogeneity of CP immune cells recapitulates the diversity found in other peripheral organs. The CP immune cell diversity and activity change with aging, stress, and disease and modulate the activity of the CP epithelium, thereby indirectly shaping brain function. The goal of this protocol is to isolate murine CP and identify about 90% of the main immune subsets that populate them. This method is a tool to characterize CP immune cells and understand their function in orchestrating periphery-to-brain communication. The proposed protocol may help decipher how CP immune cells indirectly modulate brain function in health and across various disease conditions.

International audience; Tissue aging can be viewed as a loss of normal maintenance; in advanced age, the mechanisms which keep the tissue healthy on daily bases fail to manage the accumulating "wear and tear", leading to gradual loss of function. In the brain, maintenance is provided primarily by three components: the blood-brain barrier, which allows the influx of certain molecules into the brain while excluding others, the circulation of the cerebrospinal fluid, and the phagocytic function of microglia. Indeed, failure of these systems is associated with cognitive loss and other hallmarks of brain aging. Interestingly, all three mechanisms are regulated not only by internal conditions within the aging brain, but remain highly sensitive to the peripheral signals, such as cytokines or microbiome-derived molecules, present in the systemic circulation. In this article, we discuss the contribution of such peripheral factors to brain maintenance and its loss in aging.

We thank Dmitry Ershov from the Bioinformatics and Biostatistics Hub (Institut Pasteur, USR 3756CNRS) and the Image Analysis Hub for their help with the image and/or data analysis. We thank George M. Haustant and Thomas Cokelaer (Biomics Platform, C2RT, Institut Pasteur, Paris, France, supported by France Génomique (ANR-10-INBS-09-09) and IBISA) for 16S sequencing. We thank Claire Poyart or the BM110 GBS strain and Ivan Gladwyn-Ng and Laurent Nguyen for helpful discussions and preliminary experiments. We thank the members of the Biology of Infection Unit for their support, Sylvain Levallois for critical reading, and Rémy Dailleux and Thifaine Pouillon for help, and the Institut Pasteur Animalerie Centrale and Gnotobiology Center.; International audience; Neonates are highly susceptible to bacterial meningitis as compared to children and adults. Group B streptococcus (GBS) is a major cause of neonatal meningitis. Neonatal meningitis can result from GBS intestinal colonization and translocation across the intestinal barrier (IB). Here, we show that the immaturity of the neonatal intestinal microbiota results in low resistance to GBS intestinal colonization and higher permissiveness of the gut vascular barrier. Additionally, the age-dependent but microbiota-independent Wnt activity in intestinal and choroid plexus (CP) epithelia results in a lower degree of cell-cell junctions’ polarization which favors bacterial translocation. This study reveals that neonatal susceptibility to GBS meningitis results from the age-dependent immaturity of the microbiota and developmental pathways associated with neonatal tissue growth, which concur to GBS gut colonization, systemic dissemination and neuroinvasion. Whereas the activation of developmental pathways is intrinsic to neonates, interventions aimed at maturing the microbiota may help prevent neonatal meningitis.

International audience; The most prominent structural feature of the parasitophorous vacuole (PV) in which the intracellular parasite Toxoplasma gondii proliferates is a membranous nanotubular network (MNN), which interconnects the parasites and the PV membrane. The MNN function remains unclear. The GRA2 and GRA6 proteins secreted from the parasite dense granules into the PV have been implicated in the MNN biogenesis. Amphipathic alpha-helices (AAHs) predicted in GRA2 and an alpha-helical hydrophobic domain predicted in GRA6 have been proposed to be responsible for their membrane association, thereby potentially molding the MMN in its structure. Here we report an analysis of the recombinant proteins (expressed in detergent-free conditions) by circular dichroism, which showed that full length GRA2 displays an alpha-helical secondary structure while recombinant GRA6 and GRA2 truncated of its AAHs are mainly random coiled. Dynamic light scattering and transmission electron microscopy showed that recombinant GRA6 and truncated GRA2 constitute a homogenous population of small particles (6-8 nm in diameter) while recombinant GRA2 corresponds to 2 populations of particles (∼8-15 nm and up to 40 nm in diameter, respectively). The unusual properties of GRA2 due to its AAHs are discussed.

International audience; Listeria monocytogenes (Lm) is ubiquitous and widespread in the environment. It is responsible for one of the most severe human foodborne infection. Lm is a facultative intracellular bacterium that can cross the intestinal barrier, disseminate via the bloodstream and reach the liver, spleen, central nervous system and fetus. The bacterial surface protein ActA is one of the most critical and best characterized virulence factors of Lm. It fulfills many essential functions within host cells, allowing Lm escape from autophagy and recruiting an actin polymerization complex that promotes Lm actin-based motility, cell-to-cell spread and dissemination within host tissues. We have recently shown that ActA also acts extracellularly. It mediates Lm aggregation and biofilm formation in vitro and in vivo, and long-term colonization of the gut lumen. This new property of ActA favors Lm transmission and may participate in the selective pressure on Lm to maintain ActA.

BACKGROUND Little is known about Listeria monocytogenes-associated bone and joint infections. Only case reports of this infection have been published. METHODS Retrospective study of culture-proven bone and joint cases reported to the French National Reference Center for Listeria from 1992 to 2010. RESULTS Forty-three patients were studied: 61% were men, and the median age was 72 (range, 16-89); 24 patients exhibited comorbidities (56%). Thirty-six patients (84%) had orthopedic implant devices: prosthetic joints (n = 34) or internal fixation (n = 2); the median time after insertion was 9 years (0.1-22). Subacute infection was more frequent (median, 4 weeks [range, 2-100], 74%) than acute infection (

Gessain et al. explore how Listeria monocytogenes facilitates disseminated infection through host epithelial barriers using two of its surface proteins, InlA and InlB. By combining in vitro and in vivo approaches using intestinal and placental mouse and human tissues, they find a differential requirement for InlA and InlB in tissue invasiveness, with a critical role for PI3K activation., Invasion of nonphagocytic cells, a critical property of Listeria monocytogenes (Lm) that enables it to cross host barriers, is mediated by the interaction of two bacterial surface proteins, InlA and InlB, with their respective receptors E-cadherin and c-Met. Although InlA–E-cadherin interaction is necessary and sufficient for Lm crossing of the intestinal barrier, both InlA and InlB are required for Lm crossing of the placental barrier. The mechanisms underlying these differences are unknown. Phosphoinositide 3-kinase (PI3-K) is involved in both InlA- and InlB-dependent pathways. Indeed, InlA-dependent entry requires PI3-K activity but does not activate it, whereas InlB–c-Met interaction activates PI3-K. We show that Lm intestinal target cells exhibit a constitutive PI3-K activity, rendering InlB dispensable for InlA-dependent Lm intestinal barrier crossing. In contrast, the placental barrier does not exhibit constitutive PI3-K activity, making InlB necessary for InlA-dependent Lm placental invasion. Here, we provide the molecular explanation for the respective contributions of InlA and InlB to Lm host barrier invasion, and reveal the critical role of InlB in rendering cells permissive to InlA-mediated invasion. This study shows that PI3-K activity is critical to host barrier permissiveness to microbes, and that pathogens exploit both similarities and differences of host barriers to disseminate.

At present, little is known regarding Listeria monocytogenes-associated biliary tract infection, a rare form of listeriosis. In this article, we will study 12 culture-proven cases reported to the French National Reference Center for Listeria from 1996 to 2013 and review the 8 previously published cases. Twenty cases were studied: 17 cholecystitis, 2 cholangitis, and 1 biliary cyst infection. Half were men with a median age of 69 years (32–85). Comorbidities were present in 80%, including cirrhosis, rheumatoid arthritis, and diabetes. Five patients received immunosuppressive therapy, including corticosteroids and anti-tumor necrosis factor biotherapies. Half were afebrile. Blood cultures were positive in 60% (3/5). Gallbladder histological lesions were analyzed in 3 patients and evidenced acute, chronic, or necrotic exacerbation of chronic infection. Genoserogroup of the 12 available strains were IVb (n = 6), IIb (n = 5), and IIa (n = 1). Their survival in the bile was not enhanced when compared with isolates from other listeriosis cases. Adverse outcome was reported in 33% (5/15): 3 deaths, 1 recurrence; 75% of the patients with adverse outcome received inadequate antimicrobial therapy (P = 0.033). Biliary tract listeriosis is a severe infection associated with high mortality in patients not treated with appropriate therapy. This study provides medical relevance to in vitro and animal studies that had shown Listeria monocytogenes ability to survive in bile and induce overt biliary infections.

Antivirulence strategies targeting bacterial behavior, such as adhesion and biofilm formation, are expected to exert low selective pressure and have been proposed as alternatives to biocidal antibiotic treatments to avoid the rapid occurrence of bacterial resistance. Here, we tested this hypothesis using group 2 capsule polysaccharide (G2cps), a polysaccharidic molecule previously shown to impair bacterium-surface interactions, and we investigated the nature of bacterial resistance to a nonbiocidal antibiofilm strategy. We screened an Escherichia coli mutant library for an increased ability to form biofilm in the presence of G2cps, and we identified several mutants displaying partial but not total resistance to this antibiofilm polysaccharide. Our genetic analysis showed that partial resistance to G2cps results from multiple unrelated mutations leading to modifications in surface physicochemical properties that counteract the changes in ionic charge and Lewis base properties induced by G2cps. Moreover, some of the identified mutants harboring improved biofilm formation in the presence of G2cps were also partially resistant to other antibiofilm molecules. This study therefore shows that alterations of bacterial surface properties mediate only partial resistance to G2cps. It also experimentally validates the potential value of nonbiocidal antibiofilm strategies, since full resistance to antibiofilm compounds is rare and potentially unlikely to arise in clinical settings.

Listeria monocytogenes (Lm) is a ubiquitous bacterium able to survive and thrive within the environment and readily colonizes a wide range of substrates, often as a biofilm. It is also a facultative intracellular pathogen, which actively invades diverse hosts and induces listeriosis. So far, these two complementary facets of Lm biology have been studied independently. Here we demonstrate that the major Lm virulence determinant ActA, a PrfA-regulated gene product enabling actin polymerization and thereby promoting its intracellular motility and cell-to-cell spread, is critical for bacterial aggregation and biofilm formation. We show that ActA mediates Lm aggregation via direct ActA-ActA interactions and that the ActA C-terminal region, which is not involved in actin polymerization, is essential for aggregation in vitro. In mice permissive to orally-acquired listeriosis, ActA-mediated Lm aggregation is not observed in infected tissues but occurs in the gut lumen. Strikingly, ActA-dependent aggregating bacteria exhibit an increased ability to persist within the cecum and colon lumen of mice, and are shed in the feces three order of magnitude more efficiently and for twice as long than bacteria unable to aggregate. In conclusion, this study identifies a novel function for ActA and illustrates that in addition to contributing to its dissemination within the host, ActA plays a key role in Lm persistence within the host and in transmission from the host back to the environment., Author Summary Listeria monocytogenes (Lm) is a ubiquitous bacterium that survives and thrives within the environment, and a facultative intracellular pathogen that induces listeriosis. So far, these two complementary facets of Lm biology have been studied independently. Here we identify ActA, which is a major Lm virulence determinant mediating actin-based motility, as critical for bacterial aggregation and biofilm formation. ActA promotes Lm aggregation via direct ActA-ActA interaction and ActA C-terminal region, which is not involved in actin polymerization, is essential for aggregation. Whereas ActA-mediated Lm aggregation is not observed in infected tissues, it occurs in the gut lumen. Strikingly, ActA-dependent aggregating bacteria exhibit an increased ability to persist within the gut lumen, and are shed in the feces three order of magnitude more and for twice as long than bacteria unable to aggregate. This study identifies a novel function for ActA, which plays a key role in Lm persistence within the host and transmission.

Bacterial infections trigger the expression of type I and II interferon genes but little is known about their effect on type III interferon (IFN-lambda) genes, whose products play important roles in epithelial innate immunity against viruses. Here, we studied the expression of IFN-lambda genes in cultured human epithelial cells infected with different pathogenic bacteria and in the mouse placenta infected with Listeria monocytogenes. We first showed that in intestinal LoVo cells, induction of IFN-lambda genes by L. monocytogenes required bacterial entry and increased further during the bacterial intracellular phase of infection. Other Gram-positive bacteria, Staphylococcus aureus, Staphylococcus epidermidis and Enterococcus faecalis, also induced IFN-lambda genes when internalized by LoVo cells. In contrast, Gram-negative bacteria Salmonella enterica serovar Typhimurium, Shigella flexneri and Chlamydia trachomatis did not substantially induce IFN-lambda. We also found that IFN-lambda genes were up-regulated in A549 lung epithelial cells infected with Mycobacterium tuberculosis and in HepG2 hepatocytes and BeWo trophoblastic cells infected with L. monocytogenes. In a humanized mouse line permissive to fetoplacental listeriosis, IFN-lambda 2/lambda 3 mRNA levels were enhanced in placentas infected with L. monocytogenes. In addition, the feto-placental tissue was responsive to IFN-lambda 2. Together, these results suggest that IFN-lambda may be an important modulator of the immune response to Gram-positive intracellular bacteria in epithelial tissues.

International audience; The intracellular protozoan parasite Toxoplasma gondii develops within the parasitophorous vacuole (PV), an intracellular niche in which it secretes proteins from secretory organelles named dense granules and rhoptries. Here, we describe a new dense granule protein that should now be referred to as GRA12, and that displays no homology with other proteins. Immunofluorescence and immuno-electron microscopy showed that GRA12 behaves similarly to both GRA2 and GRA6. It is secreted into the PV from the anterior pole of the parasite soon after the beginning of invasion, transits to the posterior invaginated pocket of the parasite where a membranous tubulovesicular network is first assembled, and finally resides throughout the vacuolar space, associated with the mature membranous nanotubular network. GRA12 fails to localise at the parasite posterior end in the absence of GRA2. Within the vacuolar space, like the other GRA proteins, GRA12 exists in both a soluble and a membrane-associated form. Using affinity chromatography experiments, we showed that in both the parasite and the PV soluble fractions, GRA12 is purified with the complex of GRA proteins associated with a tagged version of GRA2 and that this association is lost in the PV membranous fraction.

Amphipathic alpha-helices have been proposed as the general means used by soluble proteins to induce membrane tubulation. Previous studies had shown that the GRA2 dense granule protein of Toxoplasma gondii would be a crucial protein for the formation of the intravacuolar membranous nanotubular network (MNN) and that one of the functions of the MNN is to organise the parasites within the parasitophorous vacuole. GRA2 is a small protein (185 amino acids), predicted to contain three amphipathic alpha-helices (alpha1: 70-92; alpha2: 95-110 and alpha3: 119-139) when using the standard programs of secondary structure prediction. To investigate the respective contribution of each alpha-helix in the GRA2 functions, we used DeltaGRA2-HXGPRT knock-out complementation: eight truncated forms of GRA2 were expressed in the deleted recipient and the phenotypes of these mutants were analysed. This study showed that: (i) alpha3, when associated with the N-terminal region (NT) and the C-terminal region (CT), is sufficient to target the protein to the parasite posterior end and to induce formation of membranous vesicles within the vacuole. However, when associated only with CT, alpha3 is not sufficient to provide the hydrophobicity required for membrane association; (ii) the alpha1alpha2 region is alone not sufficient to induce membrane tubulation within the PV; and (iii) only one mutant, NT-alpha1alpha2alpha3, restores most of the biochemical and functional properties of GRA2, including traffic to the dense granules, secretion into the vacuole, association with vacuolar membranes, induction of the MNN formation and organisation of the parasites within the vacuole.

Most Apicomplexa reside and multiply in the cytoplasm of their host cell, within a parasitophorous vacuole (PV) originating from both parasite and host cell components. Trafficking of parasite-encoded proteins destined to membrane compartments beyond the confine of the parasite plasma membrane is a process that offers a rich territory to explore novel mechanisms of protein-membrane interactions. Here, we focus on the PVs formed by the asexual stages of two pathogens of medical importance, Plasmodium and Toxoplasma. We compare the PVs of both parasites, with a particular emphasis on their evolutionary divergent compartmentalization within the host cell. We also discuss the existence of peculiar export mechanisms and/or sorting determinants that are potentially involved in the post-secretory targeting of parasite proteins to the PV subcompartments.

Introduction Bacterial infections trigger the expression of type I and II interferon genes but little is known about their effect on type III interferon (IFN-λ) genes, whose products play important roles in epithelial innate immunity against viruses. We have shown that the Gram-positive bacterium Listeria monocytogenes stimulates IFN-λ2 transcription during infection of epithelial cells and that this pathogen is able to tightly control the expression of the downstream responsive genes at chromatin level by hijacking a chromatin-silencing complex [1] , [2] . So far, no bacterial species other than L. monocytogenes has been examined for the induction of IFN-λ genes in epithelial cells. Because a wide range of bacterial pathogens colonize epithelia, it was of interest to compare the ability of diverse species to induce these genes in this cell type. Here, we further detailed L. monocytogenes-mediated stimulation of IFN-λ in human epithelial cells and in the mouse placenta. We also analyzed differences between bacterial species in the induction of IFN-λ genes in epithelial cells. Methods To analyze the mechanisms underlying L. monocytogenes stimulation of IFN-III expression, we determined bacterial loads and the kinetics of IFN-λ and IFN-s gene transcription (by qRT-PCR) and IFNs secretion (by ELISA) over a time-course infection with L. monocytogenes in LoVo epithelial cells. We also studied mutants deficient for internalization or phagosomal escape. We next determined whether other bacterial species, either Gram-positive or Gram-negative, could induce type III IFN. Finally, we examined whether type III IFN could be induced during in vivo infection with Listeria at the placental barrier. Results We report that IFN-III are the major interferons produced and secreted by epithelial cells in response to L. monocytogenes infection, by mechanisms involving both the internalization and cytosolic phases of infection. Other major bacterial pathogens, Mycobacterium tuberculosis and the Gram-positive cocci Staphylococcus aureus, also stimulate type III IFN expression in epithelial cells. In contrast, Gram-negative pathogens Salmonella enterica, Shigella flexneri and Chlamydia trachomatis have no or only weak effects. We also provide evidence for in vivo induction of these genes during infection of the placenta by L. monocytogenes in the mouse, and show that the feto-placental tissue is responsive to IFN-λ2. Conclusion Together, these results suggest that IFN-λ may be an important modulator of the immune response to Gram-positive intracellular bacteria in epithelial tissues. Most pathogenic bacteria target epithelia, in particular in skin, throat, gut, liver, lung, genital mucosa or placenta. We propose that some bacterial species allow epithelial cells to become a source of IFN-λ,acting as paracrine immunomodulators of mucosal surfaces.