Georgios Nikitas

Bio: Georgios Nikitas is an academic researcher from French Institute of Health and Medical Research. The author has contributed to research in topics: Listeria monocytogenes & Virulence. The author has an hindex of 5, co-authored 5 publications receiving 1420 citations. Previous affiliations of Georgios Nikitas include World Health Organization & Pasteur Institute.

The Listeria transcriptional landscape from saprophytism to virulence

Abstract: The bacterium Listeria monocytogenes is ubiquitous in the environment and can lead to severe food-borne infections. It has recently emerged as a multifaceted model in pathogenesis. However, how this bacterium switches from a saprophyte to a pathogen is largely unknown. Here, using tiling arrays and RNAs from wild-type and mutant bacteria grown in vitro, ex vivo and in vivo, we have analysed the transcription of its entire genome. We provide the complete Listeria operon map and have uncovered far more diverse types of RNAs than expected: in addition to 50 small RNAs (<500 nucleotides), at least two of which are involved in virulence in mice, we have identified antisense RNAs covering several open-reading frames and long overlapping 5' and 3' untranslated regions. We discovered that riboswitches can act as terminators for upstream genes. When Listeria reaches the host intestinal lumen, an extensive transcriptional reshaping occurs with a SigB-mediated activation of virulence genes. In contrast, in the blood, PrfA controls transcription of virulence genes. Remarkably, several non-coding RNAs absent in the non-pathogenic species Listeria innocua exhibit the same expression patterns as the virulence genes. Together, our data unravel successive and coordinated global transcriptional changes during infection and point to previously unknown regulatory mechanisms in bacteria.

Conjugated action of two species-specific invasion proteins for fetoplacental listeriosis

Abstract: Listeriosis and other microbial infections in pregnancy can affect the fetus as well as the mother, but little is known about how pathogens cross the placental barrier. Disson et al. investigated the process using two complementary animal models infected by Listeria monocytogenes. They show that two virulence factors or invasion proteins, InlA and InlB, are required for the transfer of pathogen to the placenta. Thus by blocking one or both of these pathways it may be possible to stop microbes passing into the fetus. Conversely, it may be possible to exploit these pathways to target therapeutic molecules across the same barrier. Listeria monocytogenes can cross the placental barrier and may result in fetal or neonatal mortality. Using two complementary animal models, it is now shown that virulence factors InlA and InlB are both required for this process in vivo. The ability to cross host barriers is an essential virulence determinant of invasive microbial pathogens. Listeria monocytogenes is a model microorganism that crosses human intestinal and placental barriers, and causes severe maternofetal infections by an unknown mechanism1. Several studies have helped to characterize the bacterial invasion proteins InlA and InlB2. However, their respective species specificity has complicated investigations on their in vivo role3,4. Here we describe two novel and complementary animal models for human listeriosis: the gerbil, a natural host for L. monocytogenes, and a knock-in mouse line ubiquitously expressing humanized E-cadherin. Using these two models, we uncover the essential and interdependent roles of InlA and InlB in fetoplacental listeriosis, and thereby decipher the molecular mechanism underlying the ability of a microbe to target and cross the placental barrier.

Transcytosis of Listeria monocytogenes across the intestinal barrier upon specific targeting of goblet cell accessible E-cadherin

Abstract: Listeria monocytogenes (Lm) is a foodborne pathogen that crosses the intestinal barrier upon interaction between its surface protein InlA and its species-specific host receptor E-cadherin (Ecad). Ecad, the key constituent of adherens junctions, is typically situated below tight junctions and therefore considered inaccessible from the intestinal lumen. In this study, we investigated how Lm specifically targets its receptor on intestinal villi and crosses the intestinal epithelium to disseminate systemically. We demonstrate that Ecad is luminally accessible around mucus-expelling goblet cells (GCs), around extruding enterocytes at the tip and lateral sides of villi, and in villus epithelial folds. We show that upon preferential adherence to accessible Ecad on GCs, Lm is internalized, rapidly transcytosed across the intestinal epithelium, and released in the lamina propria by exocytosis from where it disseminates systemically. Together, these results show that Lm exploits intrinsic tissue heterogeneity to access its receptor and reveal transcytosis as a novel and unanticipated pathway that is hijacked by Lm to breach the intestinal epithelium and cause systemic infection.

Identity, regulation and in vivo function of gut NKp46+RORγt+ and NKp46+RORγt- lymphoid cells.

Abstract: The gut is a major barrier against microbes and encloses various innate lymphoid cells (ILCs), including two subsets expressing the natural cytotoxicity receptor NKp46. A subset of NKp46(+) cells expresses retinoic acid receptor-related orphan receptor γt (RORγt) and produces IL-22, like lymphoid tissue inducer (LTi) cells. Other NKp46(+) cells lack RORγt and produce IFN-γ, like conventional Natural Killer (cNK) cells. The identity, the regulation and the in vivo functions of gut NKp46(+) ILCs largely remain to be unravelled. Using pan-genomic profiling, we showed here that small intestine (SI) NKp46(+)RORγt(-) ILCs correspond to SI NK cells. Conversely, we identified a transcriptional programme conserved in fetal LTi cells and adult SI NKp46(+)RORγt(+) and NKp46(-)RORγt(+) ILCs. We also demonstrated that the IL-1β/IL-1R1/MyD88 pathway, but not the commensal flora, drove IL-22 production by NKp46(+)RORγt(+) ILCs. Finally, oral Listeria monocytogenes infection induced IFN-γ production in SI NK and IL-22 production in NKp46(+)RORγt(+) ILCs, but only IFN-γ contributed to control bacteria dissemination. NKp46(+) ILC heterogeneity is thus associated with subset-specific transcriptional programmes and effector functions that govern their implication in gut innate immunity.

Modeling human listeriosis in natural and genetically engineered animals

Abstract: Listeria monocytogenes causes listeriosis, a human foodborne infection leading to gastroenteritis, meningoencephalitis and maternofetal infections. InlA and InlB, two L. monocytogenes surface proteins, interact with their respective receptors E-cadherin and Met and mediate bacterial entry into human cultured cells. Here, we present protocols for studying listeriosis in three complementary animal models: (i) the human E-cadherin (hEcad) transgenic mouse line; (ii) the knock-in E16P mouse line; and (iii) the gerbil, in which both InlA–E-cadherin and InlB–Met species-specific interactions occur as in humans. Two routes of infection are described: oral inoculation, the natural route for infection; and intravenous inoculation that bypasses the intestinal barrier. We describe how to monitor L. monocytogenes infection, both qualitatively by imaging techniques and quantitatively by bacterial enumeration. The advantage of these methods over the classical intravenous inoculation of L. monocytogenes in wild-type mice (in which the InlA–E-cadherin interaction does not occur) is that it allows the pathophysiology of listeriosis to be studied in animal models relevant to humans, as they are permissive to the interactions that are thought to mediate L. monocytogenes crossing of human host barriers. The whole procedure (inoculation, in vivo imaging, bacterial enumeration, histopathology) takes one full week to complete, including 3 d of actual experiments.

How colonization by microbiota in early life shapes the immune system

Abstract: Microbial colonization of mucosal tissues during infancy plays an instrumental role in the development and education of the host mammalian immune system. These early-life events can have long-standing consequences: facilitating tolerance to environmental exposures or contributing to the development of disease in later life, including inflammatory bowel disease, allergy, and asthma. Recent studies have begun to define a critical period during early development in which disruption of optimal host-commensal interactions can lead to persistent and in some cases irreversible defects in the development and training of specific immune subsets. Here, we discuss the role of early-life education of the immune system during this “window of opportunity,” when microbial colonization has a potentially critical impact on human health and disease.

Regional specialization within the intestinal immune system

Abstract: The intestine represents the largest compartment of the immune system. It is continually exposed to antigens and immunomodulatory agents from the diet and the commensal microbiota, and it is the port of entry for many clinically important pathogens. Intestinal immune processes are also increasingly implicated in controlling disease development elsewhere in the body. In this Review, we detail the anatomical and physiological distinctions that are observed in the small and large intestines, and we suggest how these may account for the diversity in the immune apparatus that is seen throughout the intestine. We describe how the distribution of innate, adaptive and innate-like immune cells varies in different segments of the intestine and discuss the environmental factors that may influence this. Finally, we consider the implications of regional immune specialization for inflammatory disease in the intestine.

The mucus and mucins of the goblet cells and enterocytes provide the first defense line of the gastrointestinal tract and interact with the immune system

Abstract: The gastrointestinal tract is covered by mucus that has different properties in the stomach, small intestine, and colon. The large highly glycosylated gel-forming mucins MUC2 and MUC5AC are the major components of the mucus in the intestine and stomach, respectively. In the small intestine, mucus limits the number of bacteria that can reach the epithelium and the Peyer's patches. In the large intestine, the inner mucus layer separates the commensal bacteria from the host epithelium. The outer colonic mucus layer is the natural habitat for the commensal bacteria. The intestinal goblet cells secrete not only the MUC2 mucin but also a number of typical mucus components: CLCA1, FCGBP, AGR2, ZG16, and TFF3. The goblet cells have recently been shown to have a novel gate-keeping role for the presentation of oral antigens to the immune system. Goblet cells deliver small intestinal luminal material to the lamina propria dendritic cells of the tolerogenic CD103+ type. In addition to the gel-forming mucins, the transmembrane mucins MUC3, MUC12, and MUC17 form the enterocyte glycocalyx that can reach about a micrometer out from the brush border. The MUC17 mucin can shuttle from a surface to an intracellular vesicle localization, suggesting that enterocytes might control and report epithelial microbial challenge. There is communication not only from the epithelial cells to the immune system but also in the opposite direction. One example of this is IL10 that can affect and improve the properties of the inner colonic mucus layer. The mucus and epithelial cells of the gastrointestinal tract are the primary gate keepers and controllers of bacterial interactions with the host immune system, but our understanding of this relationship is still in its infancy.

The Listeria transcriptional landscape from saprophytism to virulence

Abstract: The bacterium Listeria monocytogenes is ubiquitous in the environment and can lead to severe food-borne infections. It has recently emerged as a multifaceted model in pathogenesis. However, how this bacterium switches from a saprophyte to a pathogen is largely unknown. Here, using tiling arrays and RNAs from wild-type and mutant bacteria grown in vitro, ex vivo and in vivo, we have analysed the transcription of its entire genome. We provide the complete Listeria operon map and have uncovered far more diverse types of RNAs than expected: in addition to 50 small RNAs (<500 nucleotides), at least two of which are involved in virulence in mice, we have identified antisense RNAs covering several open-reading frames and long overlapping 5' and 3' untranslated regions. We discovered that riboswitches can act as terminators for upstream genes. When Listeria reaches the host intestinal lumen, an extensive transcriptional reshaping occurs with a SigB-mediated activation of virulence genes. In contrast, in the blood, PrfA controls transcription of virulence genes. Remarkably, several non-coding RNAs absent in the non-pathogenic species Listeria innocua exhibit the same expression patterns as the virulence genes. Together, our data unravel successive and coordinated global transcriptional changes during infection and point to previously unknown regulatory mechanisms in bacteria.