Listeria monocytogenes infects human endothelial cells by two distinct mechanisms

Identification of the agr locus of Listeria monocytogenes: role in bacterial virulence

Listeria monocytogenes is a gram-positive facultative intracellular food-borne pathogen that can cause severe infections in humans and animals. We have recently adapted signature-tagged transposon mutagenesis (STM) to identify genes involved in the virulence of L. monocytogenes. A new round of STM allowed us to identify a new locus encoding a protein homologous to AgrA, the well-studied response regulator of Staphylococcus aureus and part of a two-component system involved in bacterial virulence. The production of several secreted proteins was modified in the agrA mutant of L. monocytogenes grown in broth, indicating that the agr locus influenced protein secretion. Inactivation of agrA did not affect the ability of the pathogen to invade and multiply in cells in vitro. However, the virulence of the agrA mutant was attenuated in the mouse (a 10-fold increase in the 50% lethal dose by the intravenous route), demonstrating for the first time a role for the agr locus in the virulence of L. monocytogenes.

Modification of the signal sequence cleavage site of listeriolysin O does not affect protein secretion but impairs the virulence of Listeria monocytogenes

Listeriolysin O (LLO, hly-encoded), a major virulence factor secreted by the bacterial pathogen Listeria monocytogenes, is synthesized as a precursor of 529 residues. To impair LLO secretion, the four residues of the predicted signal sequence cleavage site (EA-KD) were deleted and the mutant LLO protein was expressed in a hly-negative derivative of L. monocytogenes. Unexpectedly, the mutant protein was secreted in normal amounts in the culture supernatant and was fully haemolytic. N-terminal sequencing of the secreted LLO molecule revealed that N-terminal processing of the preprotein occurred three residues downstream of the natural cleavage site. L. monocytogenes expressing this truncated LLO showed a reduced capacity to disrupt the phagosomal membranes of bone marrow macrophages and of hepatocytes; and the mutant strain showed a 100-fold decrease in virulence in the mouse model. These results suggest that the first N-terminal residues of mature LLO participate directly in phagosomal escape and bacterial infection.

Antibodies present in normal human serum inhibit invasion of human brain microvascular endothelial cells by Listeria monocytogenes

Listeria monocytogenes causes meningitis and encephalitis in humans and crosses the blood-brain barrier by yet unknown mechanisms. The interaction of the bacteria with different types of endothelial cells was recently analyzed, and it was shown that invasion into, but not adhesion to, human brain microvascular endothelial cells (HBMEC) depends on the product of the inlB gene, the surface molecule InlB, which is a member of the internalin multigene family. In the present study we analyzed the role of the medium composition in the interaction of L. monocytogenes with HBMEC, and we show that invasion of HBMEC is strongly inhibited in the presence of adult human serum. The strong inhibitory activity, which is not present in fetal calf serum, does not inhibit uptake by macrophage-like J774 cells but does also inhibit invasion of Caco-2 epithelial cells. The inhibitory component of human serum was identified as being associated with L. monocytogenes-specific antibodies present in the human serum. Human newborn serum (cord serum) shows only a weak inhibitory activity on the invasion of HBMEC by L. monocytogenes.

Interaction of human hematopoietic stem cells with bacterial pathogens

Primitive hematopoietic stem cells (HSCs) in the bone marrow are rare pluripotent cells with the capacity to give rise to all lineages of blood cells. During commitment, progenitor cells are composed mainly of cells with the potential for differentiation into 1 or 2 lineages. This commitment involves the acquisition of specific growth factor receptors and the loss of others. Viral and bacterial infections may lead to profound disturbance of hematopoiesis, which is possibly due to different susceptibility of HSCs to infectious agents. Here, we show that quiescent human HSCs are fully resistant to infection by the intracellular bacteria, Listeria monocytogenes and Salmonella enterica serovariation typhimurium, and the extracellular pathogen Yersinia enterocolitica. During myeloid/monocytic differentiation induced by incubation with stem cell factor, thrombopoietin, and flt-3 ligand, partially differentiated HSCs emerge, which readily take up these pathogens and also latex beads by macropinocytosis. After further monocytic differentiation, bacterial uptake by macropinocytosis still occurs but internalization of the pathogens is now mainly achieved by receptor-mediated phagocytosis. These results suggest that in the case of HSCs uptake mechanisms for bacteria develop sequentially.

Inducible control of virulence gene expression in Listeria monocytogenes: temporal requirement of listeriolysin O during intracellular infection

We have constructed a lac repressor/operator-based system to tightly regulate expression of bacterial genes during intracellular infection by Listeria monocytogenes. An L. monocytogenes strain was constructed in which expression of listeriolysin O was placed under the inducible control of an isopropyl-beta-D-thiogalactopyranoside (IPTG)-dependent promoter. Listeriolysin O (LLO) is a pore-forming cytolysin that mediates lysis of L. monocytogenes-containing phagosomes. Using hemolytic-activity assays and Western blot analysis, we demonstrated dose-dependent IPTG induction of LLO during growth in broth culture. Moreover, intracellular growth of the inducible-LLO (iLLO) strain in the macrophage-like cell line J774 was strictly dependent upon IPTG. We have further shown that iLLO bacteria trapped within primary phagocytic vacuoles can be induced to escape into the cytosol following addition of IPTG to the cell culture medium, thus yielding the ability to control bacterial escape from the phagosome and the initiation of intracellular growth. Using the iLLO strain in plaque-forming assays, we demonstrated an additional requirement for LLO in facilitating cell-to-cell spread in L2 fibroblasts, a nonprofessional phagocytic cell line. Furthermore, the efficiency of cell-to-cell spread of iLLO bacteria in L2 cells was IPTG dose dependent. The potential use of this system for determining the temporal requirements of additional virulence determinants of intracellular pathogenesis is discussed.

Critical role of the N-terminal residues of listeriolysin O in phagosomal escape and virulence of Listeria monocytogenes

A putative PEST sequence was recently identified close to the N-terminus of listeriolysin O (LLO), a major virulence factor secreted by the pathogenic Listeria monocytogenes. The deletion of this motif did not affect the secretion and haemolytic activity of LLO, but abolished bacterial virulence. Here, we first tested whether the replacement of the PEST motif of LLO by two different sequences, with either a very high or no PEST score, would affect phagosomal escape, protein stability and, ultimately, the virulence of L. monocytogenes. Then, we constructed LLO mutants with an intact PEST sequence but carrying mutations on either side, or on both sides, of the PEST motif. The properties of these mutants prompted us to construct three LLO mutants carrying single amino acid substitutions in the distal portion of the PEST region (P49A, K50A and P52A; preprotein numbering). Our data demonstrate that the susceptibility of LLO to intracellular proteolytic degradation is not related to the presence of a high PEST score sequence and that the insertion of two residues immediately downstream of the intact PEST sequence is sufficient to impair phagosomal escape and abolish bacterial virulence. Furthermore, we show that single amino acid substitutions in the distal portion of the PEST motif are sufficient to attenuate bacterial -virulence significantly, unravelling the critical role of this region of LLO in the pathogenesis of L. -monocytogenes.

Colony-stimulating factor 1-dependent cells protect against systemic infection with Listeria monocytogenes but facilitate neuroinvasion

By using mice genomically lacking the mononuclear phagocytic growth factor colony-stimulating factor 1 and thereby deficient in macrophage and dendritic cell populations, we show that these cells play a dual role: they constitute a major defense against systemic infection but also facilitate cerebral bacterial invasion by Listeria monocytogenes.

Listeriolysin O secreted by Listeria monocytogenes induces NF-kappaB signalling by activating the IkappaB kinase complex

Listeriolysin O (LLO) is a pore-forming cytolysin secreted by the pathogen Listeria monocytogenes and is required for its intracellular survival. We recently demonstrated that in endothelial cells, LLO activates the NF-kappaB signalling pathway. In this work, we studied the LLO-induced molecular cascade of NF-kappaB activation with a cellular model extensively used to analyse the signalling pathway of NF-kappaB activation, i.e. the human embryonic kidney HEK-293 cell line and its derivatives (transfectants or mutants). When the stably transfected derivative HEK-293 cells expressing IL-1RI were exposed to LLO, a strong NF-kappaB activation was detected, contrasting with other cell lines (HEK-293 wild type, HEK-293.T and COS) expressing a very low level of IL-1RI. Although a delayed kinetics of LLO-dependent NF-kappaB activation suggests an autocrine or paracrine IL-1-dependent pathway, we found that LLO-dependent NF-kappaB activation did not require the IL-1 protein synthesis nor the interaction with the IL-1RI specific receptor. Herein, we demonstrated that LLO-dependent NF-kappaB activation requires the activation of the IkappaB kinase beta (IKKbeta) subunit of IKK complex to phosphorylate and degrade cytoplasmic IkappaBalpha, a natural inhibitor of NF-kappaB. The activation induced by LLO does not require the adapters MyD88 and IL-1R-associated kinase (IRAK). We suggested that LLO induces a distinct signalling pathway from that of IL-1 and its receptor.

A role for ActA in epithelial cell invasion by Listeria monocytogenes

We assessed the role of the actin-polymerizing protein, ActA, in host cell invasion by Listeria monocytogenes. An in frame DeltaactA mutant was constructed in a hyperinvasive strain of prfA* genotype, in which all genes of the PrfA-dependent virulence regulon, including actA, are highly expressed in vitro. Loss of ActA production in prfA* bacteria reduced entry into Caco-2, HeLa, MDCK and Vero epithelial cells to basal levels. Reintroduction of actA into the DeltaactA prfA* mutant fully restored invasiveness, demonstrating that ActA is involved in epithelial cell invasion. ActA did not contribute to internalization by COS-1 fibroblasts and Hepa 1-6 hepatocytes. Expression of actA in Listeria innocua was sufficient to promote entry of this non-invasive species into epithelial cell lines, but not into COS-1 and Hepa 1-6 cells, indicating that ActA directs an internalization pathway specific for epithelial cells. Scanning electron microscopy of infected Caco-2 human enterocytes suggested that this pathway involves microvilli. prfA* bacteria, but not wild-type bacteria (which express PrfA-dependent genes very weakly in vitro) or prfA* DeltaactA bacteria, efficiently invaded differentiated Caco-2 cells via their apical surface. Microvilli played an active role in the phagocytosis of the prfA* strain, and actA was required for their remodelling into pseudopods mediating bacterial uptake. Thus, ActA appears to be a multifunctional virulence factor involved in two important aspects of Listeria pathogenesis: actin-based motility and host cell tropism and invasion.

Functional assembly of two membrane-binding domains in listeriolysin O, the cytolysin of Listeria monocytogenes

Listeriolysin O (LLO) is a major virulence factor secreted by the pathogenic Listeria monocytogenes and acts as pore-forming cytolysin. Based on sequence similarities between LLO and perfringolysin (PFO), the cytolysin from Clostridium perfringens of known crystallographic structure, two truncated LLO proteins were produced: LLO-d123, comprising the first three predicted domains, and LLO-d4, the last C-terminal domain. The two proteins were efficiently secreted into the culture supernatant of L. monocytogenes and were able to bind to cell membranes. Strikingly, when expressed simultaneously, the two secreted domains LLO-d123 and LLO-d4 reassembled into a haemolytically active form. Two in-frame linker insertions were generated in the hinge region between the d123 and d4 domains. In both cases, the insertion created a major cleavage site for proteolytic degradation and abolished cytolytic activity, which might suggest that the region connecting d123 and d4 participates in the interaction between the two portions of the monomer.

Listeria pathogenesis and molecular virulence determinants

The gram-positive bacterium Listeria monocytogenes is the causative agent of listeriosis, a highly fatal opportunistic foodborne infection. Pregnant women, neonates, the elderly, and debilitated or immunocompromised patients in general are predominantly affected, although the disease can also develop in normal individuals. Clinical manifestations of invasive listeriosis are usually severe and include abortion, sepsis, and meningoencephalitis. Listeriosis can also manifest as a febrile gastroenteritis syndrome. In addition to humans, L. monocytogenes affects many vertebrate species, including birds. Listeria ivanovii, a second pathogenic species of the genus, is specific for ruminants. Our current view of the pathophysiology of listeriosis derives largely from studies with the mouse infection model. Pathogenic listeriae enter the host primarily through the intestine. The liver is thought to be their first target organ after intestinal translocation. In the liver, listeriae actively multiply until the infection is controlled by a cell-mediated immune response. This initial, subclinical step of listeriosis is thought to be common due to the frequent presence of pathogenic L. monocytogenes in food. In normal individuals, the continual exposure to listerial antigens probably contributes to the maintenance of anti-Listeria memory T cells. However, in debilitated and immunocompromised patients, the unrestricted proliferation of listeriae in the liver may result in prolonged low-level bacteremia, leading to invasion of the preferred secondary target organs (the brain and the gravid uterus) and to overt clinical disease. L. monocytogenes and L. ivanovii are facultative intracellular parasites able to survive in macrophages and to invade a variety of normally nonphagocytic cells, such as epithelial cells, hepatocytes, and endothelial cells. In all these cell types, pathogenic listeriae go through an intracellular life cycle involving early escape from the phagocytic vacuole, rapid intracytoplasmic multiplication, bacterially induced actin-based motility, and direct spread to neighboring cells, in which they reinitiate the cycle. In this way, listeriae disseminate in host tissues sheltered from the humoral arm of the immune system. Over the last 15 years, a number of virulence factors involved in key steps of this intracellular life cycle have been identified. This review describes in detail the molecular determinants of Listeria virulence and their mechanism of action and summarizes the current knowledge on the pathophysiology of listeriosis and the cell biology and host cell responses to Listeria infection. This article provides an updated perspective of the development of our understanding of Listeria pathogenesis from the first molecular genetic analyses of virulence mechanisms reported in 1985 until the start of the genomic era of Listeria research.

Dendritic cells are early cellular targets of Listeria monocytogenes after intestinal delivery and are involved in bacterial spread in the host

We studied the sequence of cellular events leading to the dissemination of Listeria monocytogenes from the gut to draining mesenteric lymph nodes (MLNs) by confocal microscopy of immunostained tissue sections from a rat ligated ileal loop system. OX-62-positive cells beneath the epithelial lining of Peyer's patches (PPs) were the first Listeria targets identified after intestinal inoculation. These cells had other features typical of dendritic cells (DCs): they were large, pleiomorphic and major histocompatibility complex class II(hi). Listeria were detected by microscopy in draining MLNs as early as 6 h after inoculation. Some 80-90% of bacteria were located in the deep paracortical regions, and 100% of the bacteria were present in OX-62-positive cells. Most infected cells contained more than five bacteria each, suggesting that they had arrived already loaded with bacteria. At later stages, the bacteria in these areas were mostly present in ED1-positive mononuclear phagocytes. These cells were also infected by an actA mutant defective in cell-to-cell spreading. This suggests that Listeria are transported by DCs from PPs to the deep paracortical regions of draining MLNs and are then transmitted to other cell populations by mechanisms independent of ActA. Another pathway of dissemination to MLNs was identified, probably involving free Listeria and leading to the infection of ED3-positive mononuclear phagocytes in the subcapsular sinus and adjacent paracortical areas. This study provides evidence that DCs are major cellular targets of L. monocytogenes in PPs and that DCs may be involved in the early dissemination of this pathogen. DCs were not sites of active bacterial replication, making these cells ideal vectors of infection.

Antibodies against listerial protein 60 act as an opsonin for phagocytosis of Listeria monocytogenes by human dendritic cells

Human-monocyte-derived dendritic cells (MoDC) are very efficient in the uptake of Listeria monocytogenes, a gram-positive bacterium which is an important pathogen in humans and animals causing systemic infections with symptoms such as septicemia and meningitis. In this work, we analyzed the influence of blood plasma on the internalization of L. monocytogenes into human MoDC and compared the uptake of L. monocytogenes with that of Salmonella enterica serovar Typhimurium and Yersinia enterocolitica. While human plasma did not significantly influence the uptake of serovar Typhimurium and Y. enterocolitica by human MoDC, the efficiency of the uptake of L. monocytogenes by these phagocytes was strongly enhanced by human plasma. In plasma-free medium the internalization of L. monocytogenes was very low, whereas the addition of pooled human immunoglobulins resulted in the internalization of these bacteria to a degree comparable to the highly efficient uptake observed with human plasma. All human plasma tested contained antibodies against the 60-kDa extracellular protein of L. monocytogenes (p60), and anti-p60 antibodies were also found in the commercially available pooled immunoglobulins. Strikingly, in contrast to L. monocytogenes wild type, an iap deletion mutant (totally deficient in p60) showed only a minor difference in the uptake by human MoDC in the presence or the absence of human plasma. These results support the assumption that antibodies against the listerial p60 protein may play an important role in Fc-receptor-mediated uptake of L. monocytogenes by human MoDC via opsonization of the bacteria. This process may have a major impact in preventing systemic infection in L. monocytogenes in immunocompetent humans.

Identification of new genes involved in the virulence of Listeria monocytogenes by signature-tagged transposon mutagenesis

Listeria monocytogenes is a gram-positive, facultative intracellular pathogen that can cause severe food-born infections in humans and animals. We have adapted signature-tagged transposon mutagenesis to L. monocytogenes to identify new genes involved in virulence in the murine model of infection. We used transposon Tn1545 carried on the integrative vector pAT113. Forty-eight tagged transposons were constructed and used to generate banks of L. monocytogenes mutants. Pools of 48 mutants were assembled, taking one mutant from each bank, injected into mice, and screened for those affected in their multiplication in the brains of infected animals. From 2,000 mutants tested, 18 were attenuated in vivo. The insertions harbored by these mutants led to the identification of 10 distinct loci, 7 of which corresponded to previously unknown genes. The properties of four loci involving putative cell wall components were further studied in vitro and in vivo. The data suggested that these components are involved in bacterial invasion and multiplication in the brain.

Identification of a PEST-like motif in listeriolysin O required for phagosomal escape and for virulence in Listeria monocytogenes

The hly-encoded listeriolysin O (LLO) is a major virulence factor secreted by the intracellular pathogen Listeria monocytogenes, which plays a crucial role in the escape of bacteria from the phagosomal compartment. Here, we identify a putative PEST sequence close to the N-terminus of LLO and focus on the role of this motif in the biological activities of LLO. Two LLO variants were constructed: a deletion mutant protein, lacking the 19 residues comprising this sequence (residues 32-50), and a recombinant protein of wild-type size, in which all the P, E, S or T residues within this motif have been substituted. The two mutant proteins were fully haemolytic and were secreted in culture supernatants of L. monocytogenes in quantities comparable with that of the wild-type protein. Strikingly, both mutants failed to restore virulence to a hly-negative strain in vivo. In vitro assays showed that L. monocytogenes expressing the LLO deletion mutant was strongly impaired in its ability to escape from the phagosomal vacuole and, subsequently, to divide in the cytosol of infected cells. This work reveals for the first time that the N-terminal portion of LLO plays an important role in the development of the infectious process of L. monocytogenes.

Listeria monocytogenes-infected phagocytes can initiate central nervous system infection in mice

Listeria monocytogenes-infected phagocytes are present in the bloodstream of experimentally infected mice, but whether they play a role in central nervous system (CNS) invasion is unclear. To test whether bacteria within infected leukocytes could establish CNS infection, experimentally infected mice were treated with gentamicin delivered by surgically implanted osmotic pumps. Bacterial inhibitory titers in serum and plasma ranged from 1:16 to 1:256, and essentially all viable bacteria in the bloodstream of treated mice were leukocyte associated. Nevertheless, CNS infection developed in gentamicin-treated animals infected intraperitoneally or by gastric lavage, suggesting that intracellular bacteria could be responsible for neuroinvasion. This was supported by data showing that 43.5% of bacteria found with blood leukocytes were intracellular and some colocalized with F-actin, indicating productive intracellular parasitism. Experiments using an L. monocytogenes strain containing a chromosomal actA-gfpuv-plcB transcriptional fusion showed that blood leukocytes were associated with intracellular and extracellularly bound green fluorescent protein-expressing (GFP+) bacteria. Treatment with gentamicin decreased the numbers of extracellularly bound GFP+ bacteria significantly but did not affect the numbers of intracellular GFP+ bacteria, suggesting that the latter were the result of intercellular spread of GFP+ bacteria to leukocytes. These data demonstrate that infected leukocytes and the intracellular L. monocytogenes harbored within them play key roles in neuroinvasion. Moreover, they suggest that phagocytes recruited to infected organs such as the liver or spleen are themselves parasitized by intercellular spread of L. monocytogenes and then reenter the bloodstream and contribute to the systemic dissemination of bacteria.

Dithiothreitol enhances Listeria monocytogenes mediated cell cytotoxicity

The hybridoma Ped-2E9 based cytotoxicity assay was developed to distinguish virulent from avirulent Listeria species in 6 hr. The cytotoxicity effect on Ped-2E9 was reported to be primarily due the cytolytic action of listeriolysin O (LLO), produced by L. monocytogenes. In this study, the effect of a reducing agent, dithiothreitol (DTT, 0-2 mM) that is known to activate LLO was investigated to make the Ped-2E9 based cytotoxicity assay an even more sensitive and rapid. Also, we examined the effect of fetal bovine serum (FBS, 0-50%), a common ingredient of tissue culture media on cytotoxicity. A DTT concentration of 0.5 mM gave an optimum cytotoxicity effect, which could be measured by both alkaline phosphatase (AP) and lactate dehydrogenase (LDH) assays in just 1.5-2 hr. FBS, at levels between 10 to 50%, significantly inhibited Listeria-mediated cytotoxicity. Concentrated culture filtrates from L. monocytogenes or LLO producing recombinant L. innocua (prfA+ hlyA+) strain also caused cytotoxicity effects, which were observed by scanning electron microscopy or a cytotoxicity assay in 2-3 hr. Interestingly, addition of DTT to culture filtrates produced 100% cell cytotoxicity in just 15 min. This indicated that LLO activity, which is responsible for Ped-2E9 cytotoxicity, was augmented several folds with the addition of a reducing agent. Examination of Listeria isolates belonging to different serogroups from clinical sources or naturally contaminated meat products with DTT gave cytotoxicity results in 2 hr, which were comparable to the 5-hr assay analyzed concurrently without DTT. These results indicated that DTT, which activated the LLO, could be used in the cytotoxicity assay to enhance Listeria-mediated Ped-2E9 cell cytotoxicity. This knowledge will greatly assist us to develop a user-friendly rapid assay to screen cytopathogenic properties of Listeria species.

Human endothelial cell activation and mediator release in response to Listeria monocytogenes virulence factors

The interaction of Listeria monocytogenes with endothelial cells represents a crucial step in the pathogenesis of listeriosis. Incubation of human umbilical vein endothelial cells (HUVEC) with wild-type L. monocytogenes (EGD) provoked immediate strong NO synthesis, attributable to listerial presentation of listeriolysin O (LLO), as the NO release was missed upon employment of a deletion mutant for LLO (EGD hly mutant) and was reproduced by purified LLO. Studies of conditions lacking extracellular Ca(2+) suggested LLO-elicited Ca(2+) flux as the underlying mechanism. In addition, HUVEC incubation with EGD turned out to be a potent stimulus for sustained (>12-h) upregulation of proinflammatory cytokine generation (interleukin 6 [IL-6], IL-8, and granulocyte-macrophage colony-stimulating factor). Use of deletion mutants for LLO (EGD hly mutant), listerial phosphatidylinositol-specific phospholipase C (EGD plcA mutant), broad-spectrum phospholipase C (EGD plcB mutant) and internalin B (EGD inlB mutant), as well as purified LLO, identified LLO as largely responsible for the cytokine response. Endothelial cells responded with diacylglycerole and ceramide generation as well as nuclear translocation of NF-kappa B to the stimulation with the LLO-producing strains EGD and Listeria innocua. The endothelial PC-phospholipase C inhibitor tricyclodecan-9-yl-xanthogenate as well as two independent inhibitors of NF-kappa B activation, pyrolidine dithiocarbamate and caffeic acid phenethyl ester, suppressed both the NF-kappa B translocation and the upregulation of cytokine synthesis. We conclude that L. monocytogenes is a potent stimulus of NO release and sustained upregulation of proinflammatory cytokine synthesis in human endothelial cells, both events being largely attributable to LLO presentation. LLO-induced transmembrane Ca(2+) flux as well as a sequence of endothelial phospholipase activation and the appearance of diacylglycerole, ceramide, and NF-kappa B are suggested as underlying host signaling events. These endothelial responses to L. monocytogenes may well contribute to the pathogenic sequelae in severe listerial infection and sepsis.

ClpC ATPase is required for cell adhesion and invasion of Listeria monocytogenes

We studied the role of two members of the 100-kDa heat shock protein family, the ClpC and ClpE ATPases, in cell adhesion and invasion of the intracellular pathogen Listeria monocytogenes. During the early phase of infection, a clpC mutant failed to disseminate to hepatocytes in the livers of infected mice whereas the invasive capacity of a clpE mutant remained unchanged. This was confirmed by a confocal microscopy study on infected cultured hepatocyte and epithelial cell lines, showing a strong reduction of cell invasion only by the clpC mutant. Western blot analysis with specific antisera showed that the absence of ClpC, but not that of ClpE, reduced expression of the virulence factors InlA, InlB, and ActA. ClpC-dependent modulation of these factors occurs at the transcriptional level with a reduction in the transcription of inlA, inlB, and actA in the clpC mutant, in contrast to the clpE mutant. This work provides the first evidence that, in addition to promoting escape from the phagosomes, ClpC is required for adhesion and invasion and modulates the expression of InlA, InlB, and ActA, further supporting the major role of the Clp chaperones in the virulence of intracellular pathogens.

Leukocyte-facilitated entry of intracellular pathogens into the central nervous system

Microbes use numerous strategies to invade the central nervous system. Leukocyte-facilitated entry is one such mechanism whereby intracellular pathogens establish infection by taking advantage of leukocyte trafficking to the central nervous system. Key components of this process include peripheral infection and activation of leukocytes, activation of cerebral endothelial cells with or without concomitant infection, and trafficking of infected leukocytes to and through the blood-brain or blood-cerebrospinal fluid barrier.

The contribution of both oxygen and nitrogen intermediates to the intracellular killing mechanisms of C1q-opsonized Listeria monocytogenes by the macrophage-like IC-21 cell line

Listeria monocytogenes is a facultative intracellular pathogen which is internalized by host mammalian cells upon binding to their surface. Further listerial growth occurs in the cytosol after escape from the phagosomal-endosomal compartment. We have previously reported that C1q is able to potentiate L. monocytogenes phagocytosis upon bacterial opsonization by ingestion through C1q-binding structures. In this report, we analysed the post-phagocytic events upon internalization of C1q-opsonized L. monocytogenes and found an induction of macrophage (Mphi)-like IC-21 cell bactericidal mechanisms displayed by the production of oxygen and nitrogen metabolites. Both types of molecules are effective in L. monocytogenes killing. Further analysis of the cellular responses promoted by interaction of C1q with its surface binding structures, leads us to consider C1q as a collaborative molecule involved in Mphi activation. Upon interaction with surface binding structures, C1q was able to trigger and/or amplify the production of reactive oxygen and nitrogen intermediates induced by stimuli such as interferon-gamma and L. monocytogenes phagocytosis.

Critical role of neutrophils in eliminating Listeria monocytogenes from the central nervous system during experimental murine listeriosis

Neutrophils are the main inflammatory cell present in lesions involving the central nervous system (CNS) during human and murine listeriosis. In this study, administration of the neutrophil-depleting monoclonal antibody RB6-8C5 during experimental murine listeriosis facilitated the multiplication of Listeria monocytogenes in the CNS. These data suggest that neutrophils play a key role in eliminating bacteria that gain access to the CNS compartment. In addition, we provide evidence that their migration into the CNS may be necessary for the subsequent recruitment of macrophages and activated lymphocytes.

Virulent rough filaments of Listeria monocytogenes from clinical and food samples secreting wild-type levels of cell-free p60 protein

Atypical rough cell filaments of Listeria monocytogenes (designated FR variants), isolated from clinical and food samples, form long filaments up to 96 microm in length and demonstrated wild-type levels of adherence, invasion, and cytotoxicity to human epithelial HEp-2, Caco-2, and HeLa cells. Unlike previously described avirulent rough mutants of L. monocytogenes that secrete diminished levels of the major extracellular protein p60 and that form long chains that consist of multiple cells of similar size (designated MCR variants), FR variants secreted wild-type or greater levels of p60. This study shows that virulent filamentous forms of L. monocytogenes occur in clinical and food environments and have atypical morphological characteristics compared to those of the wild-type form.

Interaction of Listeria monocytogenes with human brain microvascular endothelial cells: an electron microscopic study

Internalization of Listeria monocytogenes into human brain microvascular endothelial cells (HBMEC) has recently been demonstrated to be dependent upon the inlB gene. In the present scanning electron microscopic study we show that L. monocytogenes efficiently interacts with the surface of HBMEC in an inlB-independent manner which is also different from invasion. The inlB-dependent invasion of HBMEC by L. monocytogenes is accompanied by intracellular multiplication, movement, and production of bacterium-containing protrusions. These protrusions extend from the cell surface without perturbation of any adjacent cellular membrane.

Listeria monocytogenes-infected human dendritic cells: uptake and host cell response

Dendritic cells (DCs) are potent antigen-presenting cells and play a crucial role in initiation and modulation of specific immune responses. Various pathogens are able to persist inside DCs. However, internalization of the gram-positive bacterium Listeria monocytogenes into human DCs has not yet been shown. In the present study, we demonstrate that human monocyte-derived immature DCs can efficiently phagocytose L. monocytogenes. This uptake is independent of listerial adhesion factors internalin A and internalin B but requires cytoskeletal motion and factors present in human plasma. A major portion of internalized bacteria is found in membrane-bound phagosomes and is rarely free in the cytosol, as shown by transmission electron microscopy and by using an L. monocytogenes strain expressing green fluorescent protein when in the host cell cytosol. The infection caused maturation of the immature DCs into mature DCs displaying high levels of CD83, CD25, major histocompatibility complex class II, and the CD86 costimulator molecule. This effect appeared to be largely mediated by listerial lipoteichoic acid. Although L. monocytogenes infection is known to induce death in other cell types, infection of human DCs was found to induce necrotic but not apoptotic death in fewer than 20% of DCs. Therefore, the ability of DCs to act as effective antigen-presenting cells for listerial immunity is probably enhanced by their resistance to cell death, as well as their ability to rapidly differentiate into mature, immunostimulatory DCs upon encountering bacteria.

Association of Mycobacterium leprae with human endothelial cells in vitro

Endothelial cell infection by Mycobacterium leprae has long been described histologically in all types of leprosy and in some of the acute reactions occurring in this disease. Recent evidence from experimental lepromatous neuritis indicates that M. leprae colonizes endothelial cells of epineural blood vessels even in sites of minimal infection, suggesting that interaction between these cells and M. leprae may play an important role in the selective localization of this organism to peripheral nerve. To begin to study the mechanisms involved, we have examined the interaction between M. leprae and human umbilical vein endothelial cells (HUVEC) in vitro using light microscopy, scanning and transmission electron microscopy, and confocal laser scanning microscopy. When M. leprae were added to confluent monolayers of HUVEC, uptake increased slowly to a maximum at 24 hours. Maximal percentages of infected cells were similar at ratios of organisms:cell over a range of 25:1 to 100:1. The bacilli appeared to lie within membrane-bound vacuoles at all time points. The kinetics of association of M. leprae with HUVEC are much slower than has previously been observed with macrophages, possibly due to differences in the binding of M. leprae. Compared with other pathogens that infect endothelial cells, M. leprae also appear to be ingested more slowly, and to a more limited degree. The receptors involved in M. leprae binding to endothelial cells and the impact of intracellular infection by M. leprae on these cells remain to be determined.

Beryllium-stimulated production of tumor necrosis factor-alpha by a mouse hybrid macrophage cell line

Chronic beryllium disease (CBD) results from exposure to the light-weight metal beryllium (Be). In vitro stimulation of bronchoalveolar lavage cells from CBD subjects causes the production of high levels of TNF-alpha, IFN-gamma and IL-6. We tested the hypothesis that Be-stimulation might induce the production of TNF-alpha by macrophage cell lines. We observed that H36.12j cells (12j), a mouse hybrid macrophage cell line, but not other mouse and human macrophage cell lines, produced TNF-alpha upon Be-stimulation. The response was maximal at 100 microM BeSO4 and did not occur when 12j cells were stimulated with either aluminum sulfate or cobalt sulfate. Beryllium-stimulated the production of 725+/-25 pg/ml (mean +/- SEM) TNF-alpha protein by 12j cells as measured by ELISA of culture supernatants after 24 h. As measured by RT-PCR, Be-stimulated 12j cell TNF-alpha protein production was accompanied by an increased intracellular TNF-alpha mRNA at 3 and 24 h. The addition of 10U or 100U of rMu-IFN-gamma to Be-stimulated 12j cells further increased TNF-alpha production 1.5-4 fold (1.6+/-0.1 ng/ml) respectively. Bacterial lipopolysaccharide (LPS, 1 microg/ml) stimulated production of TNF-alpha in 12j culture supernatants after 6 h (515+/-151 pg/ml). This early versus late TNF-alpha production suggests that LPS and Be both stimulate 12j cell TNF-alpha synthesis, but through different pathways. We report for the first time, the direct effects of Be stimulation on the ability of 12j cells to produce TNF-alpha. The 12j cell line, contrasted with other macrophage hybrids that do not respond to Be-stimulation, may provide a useful tool to evaluate the mechanisms by which Be stimulates macrophage cytokine production, and by which T cell derived IFN-gamma amplifies TNF-alpha production in granulomatous diseases.

Identification of new loci involved in adhesion of Listeria monocytogenes to eukaryotic cells. European Listeria Genome Consortium

Insertional mutagenesis was performed with Tn1545 in the genetic background of an inIAB deletion mutant to identify new adhesion determinants in Listeria monocytogenes. Four insertion mutants defective in adhesion to eukaryotic cells were identified. Insertion sites were cloned by inverse-PCR and sequenced. The genetic organization of insertion regions was further analysed by screening and sequencing DNA fragments from a HindIII library and by searching databases. Three adhesion-defective mutants each had one copy of Tn1545 inserted into their chromosome. The insertion sites were different in the three mutants: (i) upstream from two ORFs in tandem, similar to dfp and priA of Bacillus subtilis, respectively; (ii) within an ORF encoding a putative 126 amino-acid-polypeptide with no significant similarity to any known protein; (iii) within an ORF similar to a B. subtilis ORF with no known function, just upstream from an operon similar to an ABC (ATP-binding cassette) transporter operon from B. subtilis. The excisants obtained from these mutants using the excision reporter plasmid pTCR9 recovered full adhesion capacity. A fourth mutant was the most severely defective in adhesion. It had five Tn1545 insertions, one of which was upstream from dfp and priA, and another of which was upstream from ami, a gene encoding a surface-exposed autolysin with a C terminus similar to that of InIB. Ami was clearly involved because an ami null mutant constructed in an EGDdeltainIA-F background was adhesion-defective. Thus new regions involved in the adhesion of L. monocytogenes to eukaryotic cells were identified. Further study is required to define more accurately the roles of these regions in the adhesion process itself.

Mutants of Listeria monocytogenes defective in In vitro invasion and cell-to-cell spreading still invade and proliferate in hepatocytes of neutropenic mice

Listeria monocytogenes mutants defective in the actA gene, the plcB gene, and the inlA and inlB genes were less virulent when injected intravenously into BALB/c mice. The growth of these strains as well as of the virulent wild-type strains was increased by treating mice with a neutrophil-specific depleting monoclonal antibody, RB6-8C5. Histologic examination of the livers of the treated animals showed intrahepatocytic proliferation of the listeriae in all cases. Our data show that more than one pathway exists that allows L. monocytogenes to invade parenchymal cells. One pathway most likely involves the actA and plcB gene products, and a second one probably involves the internalins.

Listeria monocytogenes possesses adhesins for fibronectin

Listeria monocytogenes is a gram-positive, nonsporulating, food-borne pathogen of humans and animals that is able to invade many eukaryotic cells. Several listerial surface components have been reported to interact with eukaryotic cell receptors, but the complete mechanism by which the bacteria interact with all of these cell types remains largely unknown. In this work, we found that L. monocytogenes binds to human fibronectin, a 450,000-Da dimeric glycoprotein found in body fluids, on the surface of cells and in an insoluble component of the extracellular matrix. The binding of fibronectin to L. monocytogenes was found to be saturable and dependent on proteinaceous receptors. Five fibronectin-binding proteins of 55.3, 48.6, 46.7, 42.4, and 26.8 kDa were identified. The 55.3-kDa protein was proved to be present at the bacterial cell surface. The binding of L. monocytogenes to fibronectin adds to the number of molecules to which the bacterium is able to adhere and emphasizes the complexity of host-pathogen interactions.

Comparative efficacies of antibiotics in a rat model of meningoencephalitis due to Listeria monocytogenes

The antibacterial activities of amoxicillin-gentamicin, trovafloxacin, trimethoprim-sulfamethoxazole (TMP-SMX) and the combination of trovafloxacin with TMP-SMX were compared in a model of meningoencephalitis due to Listeria monocytogenes in infant rats. At 22 h after intracisternal infection, the cerebrospinal fluid was cultured to document meningitis, and the treatment was started. Treatment was instituted for 48 h, and efficacy was evaluated 24 h after administration of the last dose. All tested treatment regimens exhibited significant activities in brain, liver, and blood compared to infected rats receiving saline (P < 0.001). In the brain, amoxicillin plus gentamicin was more active than all of the other regimens, and trovafloxacin was more active than TMP-SMX (bacterial titers of 4.1 +/- 0.5 log10 CFU/ml for amoxicillin-gentamicin, 5.0 +/- 0.4 log10 CFU/ml for trovafloxacin, and 5.8 +/- 0.5 log10 CFU/ml for TMP-SMX; P < 0.05). In liver, amoxicillin-gentamicin and trovafloxacin were similarly active (2.8 +/- 0.8 and 2.7 +/- 0.8 log10 CFU/ml, respectively) but more active than TMP-SMX (4.4 +/- 0. 6 log10 CFU/ml; P < 0.05). The combination of trovafloxacin with TMP-SMX did not alter the antibacterial effect in the brain, but it did reduce the effect of trovafloxacin in the liver. Amoxicillin-gentamicin was the most active therapy in this study, but the activity of trovafloxacin suggests that further studies with this drug for the treatment of Listeria infections may be warranted.

Dissemination of Listeria monocytogenes by infected phagocytes

In vitro data suggest that blood-borne Listeria monocytogenes organisms enter the central nervous system (CNS) by direct invasion of endothelial cells or by cell-to-cell spread from infected phagocytes to endothelial cells. However, a role for infected phagocytes in neuroinvasion and dissemination of L. monocytogenes in vivo has not been confirmed experimentally. Experiments described here tested whether L. monocytogenes-infected peripheral blood leukocytes (PBL) circulated in bacteremic mice and could establish organ infection in vivo. A mean of 30.5% of bacteria cultured from whole blood were PBL associated, and microscopy showed that 22.2% of monocytes and 1.6% of neutrophils were infected. PBL-associated bacteria spread to endothelial cells in vitro, indicating their potential for virulence in vivo. To test this possibility, mice were injected intravenously with infected PBL and CFU of bacteria in liver, spleen, and brain were quantified and compared with values for mice injected with broth-grown bacteria and in vitro-infected macrophage cell lines. An inoculum of infected macrophage cell lines led to greater numbers of bacteria in the liver than the numbers produced by a similar inoculum of broth-grown bacteria. In contrast, brain infection was best established by infected PBL. Results of intraperitoneal injection of infected peritoneal cells compared with results of injection with infected J774A.1 cells suggested that unrestricted intracellular bacterial replication within J774A.1 cells contributed to excessive liver infection in those mice. These data show dissemination of intracellular L. monocytogenes and indicate that phagocyte-facilitated invasion has a role in CNS infection in vivo. Heterogeneity with regard to bactericidal activity as well as to other phagocyte characteristics is a critical feature of this mechanism.

Mechanism of the intracellular killing and modulation of antibiotic susceptibility of Listeria monocytogenes in THP-1 macrophages activated by gamma interferon

Listeria monocytogenes, a facultative intracellular pathogen, readily enters cells and multiplies in the cytosol after escaping from phagosomal vacuoles. Macrophages exposed to gamma interferon, one of the main cellular host defenses against Listeria, become nonpermissive for bacterial growth while containing Listeria in the phagosomes. Using the human myelomonocytic cell line THP-1, we show that the combination of L-monomethyl arginine and catalase restores bacterial growth without affecting the phagosomal containment of Listeria. A previous report (B. Scorneaux, Y. Ouadrhiri, G. Anzalone, and P. M. Tulkens, Antimicrob. Agents Chemother. 40:1225-1230, 1996) showed that intracellular Listeria was almost equally sensitive to ampicillin, azithromycin, and sparfloxacin in control cells but became insensitive to ampicillin and more sensitive to azithromycin and sparfloxacin in gamma interferon-treated cells. We show here that these modulations of antibiotic activity are largely counteracted by L-monomethyl arginine and catalase. In parallel, we show that gamma interferon enhances the cellular accumulation of azithromycin and sparfloxacin, an effect which is not reversed by addition of L-monomethyl arginine and catalase and which therefore cannot account for the increased activity of these antibiotics in gamma interferon-treated cells. We conclude that (i) the control exerted by gamma interferon on intracellular multiplication of Listeria in THP-1 macrophages is dependent on the production of nitric oxide and hydrogen peroxide; (ii) intracellular Listeria may become insensitive to ampicillin in macrophages exposed to gamma interferon because the increase in reactive oxygen and nitrogen intermediates already controls bacterial growth; and (iii) azithromycin and still more sparfloxacin cooperate efficiently with gamma interferon, one of the main cellular host defenses in Listeria infection.

Listeriolysin O-dependent activation of endothelial cells during infection with Listeria monocytogenes: activation of NF-kappa B and upregulation of adhesion molecules and chemokines

The facultative intracellular bacterium Listeria monocytogenes is an invasive pathogen that crosses the vascular endothelium and disseminates to the placenta and the central nervous system. Its interaction with endothelial cells is crucial for the pathogenesis of listeriosis. By infecting in vitro human umbilical vein endothelial cells (HUVEC) with L. monocytogenes, we found that wild-type bacteria induced the expression of the adhesion molecules (ICAM-1 and E-selectin), chemokine secretion (IL-8 and monocyte chemotactic protein-1) and NF-kappa B nuclear translocation. The activation of HUVEC required viable bacteria and was abolished in prfA-deficient mutants of L. monocytogenes, suggesting that virulence genes are associated with endothelial cell activation. Using a genetic approach with mutants of virulence genes, we found that listeriolysin O (LLO)-deficient mutants inactivated in the hly gene did not induce HUVEC activation, as opposed to mutants inactivated in the other virulence genes. Adhesion molecule expression, chemokine secretion and NF-kappa B activation were fully restored by a strain of Listeria innocua transformed with the hly gene encoding LLO. The relevance in vivo of endothelial cell activation for listerial pathogenesis was investigated in transgenic mice carrying an NF-kappa B-responsive lacZ reporter gene. NF-kappa B activation was visualized by a strong lacZ expression in endothelial cells of capillaries of mice infected with a virulent haemolytic strain, but was not seen in those infected with a non-haemolytic isogenic mutant. Direct evidence that LLO is involved in NF-kappa B activation in transgenic mice was provided by injecting intravenously purified LLO, thus inducing stimulation of NF-kappa B in endothelial cells of blood capillaries. Our results demonstrate that functional listeriolysin O secreted by bacteria contributes as a potent inflammatory stimulus to inducing endothelial cell activation during the infectious process.

Phosphatidylcholine-specific phospholipase C from Listeria monocytogenes is an important virulence factor in murine cerebral listeriosis

Meningoencephalitis is a serious and often fatal complication of Listeria monocytogenes infection. The aim of the present study was to analyze the role of internalin A (InlA) and B, which are involved in the invasion of L. monocytogenes into cultivated host tissue cells, and that of phosphatidylcholine-specific phospholipase C (PlcB), which mainly promotes the direct cell-to-cell spread of L. monocytogenes, in murine cerebral listeriosis by use of an InlA/B (DeltainlAB2)- and a PlcB (DeltaplcB2)-deficient isogenic deletion mutant strain and the wild-type (WT) L. monocytogenes EGD. Listeria strains were directly applied to the brain, a technique which has been employed previously to study the pathogenesis of cerebral listeriosis (D. Schlüter, S. B. Oprisiu, S. Chahoud, D. Weiner, O. D. Wiestler, H. Hof, and M. Deckert-Schlüter, Eur. J. Immunol. 25:2384-2391, 1995). We demonstrated that PlcB, but not InlA or InlB, is an important virulence factor in cerebral listeriosis. Nonimmunized mice infected intracerebrally with the DeltaplcB2 strain survived significantly longer and had a reduced intracerebral bacterial load compared to mice infected with the DeltainlAB2 strain or WT bacteria. In addition, immunization with the WT prior to intracerebral infection significantly increased the survival rate of mice challenged intracerebrally with the DeltaplcB2 strain compared to that of mice infected with the WT or DeltainlAB2 strain. Histopathology revealed that the major difference between the various experimental groups was a significantly delayed intracerebral spread of the DeltaplcB2 mutant strain, indicating that cell-to-cell spread is an important pathogenic feature of cerebral listeriosis. Interestingly, irrespective of the Listeria mutant used, the apoptosis of hippocampal and cerebellar neurons and an internal hydrocephalus developed in surviving mice, indicating that these complications are not dependent on the virulence factors InlA/B and PlcB. In conclusion, this study points to PlcB as a virulence factor important for the intracerebral pathogenesis of murine L. monocytogenes meningoencephalitis.

Interaction of Listeria monocytogenes with human brain microvascular endothelial cells: InlB-dependent invasion, long-term intracellular growth, and spread from macrophages to endothelial cells

Invasion of endothelial tissues may be crucial in a Listeria monocytogenes infection leading to meningitis and/or encephalitis. Internalization of L. monocytogenes into endothelial cells has been previously demonstrated by using human umbilical vein endothelial cells as a model system. However, during the crossing of the blood-brain barrier, L. monocytogenes most likely encounters brain microvascular endothelial cells which are strikingly different from macrovascular or umbilical vein endothelial cells. In the present study human brain microvascular endothelial cells (HBMEC) were used to study the interaction of L. monocytogenes with endothelial cells, which closely resemble native microvascular endothelial cells of the brain. We show that L. monocytogenes invades HBMEC in an InlB-dependent and wortmannin-insensitive manner. Once within the HBMEC, L. monocytogenes replicates efficiently over a period of at least 18 h, moves intracellularly by inducing actin tail formation, and spreads from cell to cell. Using a green fluorescent protein-expressing L. monocytogenes strain, we present direct evidence that HBMEC are highly resistant to damage by intracellularly growing L. monocytogenes. Infection of HBMEC with L. monocytogenes results in foci of heavily infected, but largely undamaged endothelial cells. Heterologous plaque assays with L. monocytogenes-infected P388D1 macrophages as vectors demonstrate efficient spreading of L. monocytogenes into HBMEC, fibroblasts, hepatocytes, and epithelial cells, and this phenomenon is independent of the inlC gene product.

Two Distinct Phospholipases C of Listeria monocytogenes Induce Ceramide Generation, Nuclear Factor-κB Activation, and E-Selectin Expression in Human Endothelial Cells

Infection of endothelial cells by Listeria monocytogenes is an essential step in the pathogenesis of listeriosis. We recently reported that L. monocytogenes induces up-regulation of E-selectin and other endothelial adhesion molecules and subsequent polymorphonuclear leukocyte (PMN) adhesion into cultured human endothelial cells. In the present study, we characterized the mechanisms of enhanced E-selectin expression using L. monocytogenes wild type (EGD), the isogenic in-frame deletion mutants for phosphatidylcholine (PC)- and phosphatidylinositol (PI)-specific phospholipases EGDΔplcA and EGDΔplcB, as well as the nonvirulent control strain Listeria innocua. Infection of endothelial cells with EGDΔplcA or EGDΔplcB for 6 h induced, as compared with EGD wild type, intermediate levels of E-selectin mRNA and protein as well as PMN rolling and adhesion at a shear rate of 1 dyne/cm2, indicating that both bacterial phospholipases are required for a maximal effect. Similarly, ceramide content and NF-κB activity were increased in L. monocytogenes-exposed endothelial cells, but only to intermediate levels for PC- or PI-phospholipase C (PLC)-deficient listerial mutants. Phospholipase effects could be mimicked by exogenously added ceramides or bacterial sphingomyelinase. The data presented indicate that PI-PLC and PC-PLC are important virulence factors for L. monocytogenes infections that induce accumulation of ceramides that in turn may act as second messengers to control host cell signal-transduction pathways leading to persistent NF-κB activation, increased E-selectin expression, and enhanced PMN rolling/adhesion. The ability of L. monocytogenes to stimulate PMN adhesion to endothelial cells may be an important mechanism in the pathogenesis of severe listeriosis.

Entry of Listeria monocytogenes into neurons occurs by cell-to-cell spread: an in vitro study

Listeria monocytogenes is an intracellular pathogen that causes severe central nervous system infection in humans and animals. The ability of this bacterium to penetrate nerve cells was investigated by using rat spinal cell cultures. Entry into distinct cell types, i. e., glial cells and neurons, was monitored by a differential immunofluorescence technique with antibodies against cell type-specific markers and the bacterial pathogen. L. monocytogenes was detected predominantly within macrophages constituting the microglia. Astrocytes and oligodendrocytes, the major components of macroglia, were infected to a lesser extent. Surprisingly, Listeria innocua, a noninvasive and nonpathogenic species, also has the capacity to enter into these three types of glial cells. Entry into neurons was a very rare event. In contrast, we found that L. monocytogenes could efficiently invade neurons when these latter cells were cocultivated with Listeria-infected mouse macrophages. In this case, infection of neurons occurs by cell-to-cell spread via an actA-dependent mechanism. These data support the notion that infected phagocytes can be vectors by which L. monocytogenes gains access to privileged niches such as the central nervous system.

Internalin B is essential for adhesion and mediates the invasion of Listeria monocytogenes into human endothelial cells

Listeria monocytogenes causes rhombencephalitis in humans and animals and also affects the fetus in utero, causing disseminated sepsis. In both instances, the infection occurs by the crossing of endothelial cells lining a physiological barrier, the blood-brain barrier or the transplacental barrier. In this study, the ability of L. monocytogenes wild-type EGD to invade human umbilical vein endothelial cells (HUVECs) was evaluated using wild-type bacteria and isogenic Listeria mutants. Here, we show that invasion of HUVECs by L. monocytogenes is dependent on the expression of the internalin B gene product. This was demonstrated in several ways. First, L. monocytogenes strains lacking the inlB gene did not invade HUVECs. Secondly, avid invasion was obtained when a strain deleted for inlAB was complemented with a plasmid harbouring inlB only, whereas strains expressing inlA did not enter HUVECs. Thirdly, entry of wild-type EGD could be blocked effectively with antibodies to InlB. Fourthly, cell binding assays and flow cytometry with HUVECs showed binding of purified InlB, but not InlA, suggesting a tropism of InlB for this cell type. Finally, physical association of purified native InlB with the surface of non-invasive mutants dramatically increased their ability to invade HUVECs. In laser-scanning confocal microscopy, binding of InlB was observed as focal and localized patches on the cell surface of HUVECs. Qualitative examination of the entry process by scanning electron microscopy revealed that both wild-type EGD and a recombinant strain overexpressing only InlB enter HUVECs in a similar fashion. The entry process was polarized, involved single bacteria and occurred over the entire surface of endothelial cells.

The microtubule depolymerizing drugs nocodazole and colchicine inhibit the uptake of Listeria monocytogenes by P388D1 macrophages

Uptake of Listeria monocytogenes by different mammalian cells like macrophages and epithelial cells is dependent on functional actin filaments and hence susceptible to inhibition by cytochalasin. Here we show that phagocytic uptake of L. monocytogenes by P388D1 macrophages is also highly sensitive to treatment with the microtubule depolymerizing drugs nocodazole and colchicine. This sensitivity is cell type specific and much less pronounced in bone marrow-derived macrophages and Caco-2 epithelial cells. In contrast to nocodazole and colchicine, the microtubule stabilizing drug taxol has no significant effect on the uptake of L. monocytogenes by all three cell types tested.

Listeria monocytogenes virulence factors that stimulate endothelial cells

Listeria monocytogenes infection of endothelial cells upregulates surface expression of adhesion molecules and stimulates neutrophil adhesion to infected cell monolayers. The experiments presented here tested the roles of specific bacterial virulence factors as triggers for this inflammatory phenotype and function. Human umbilical vein endothelial cell (HUVEC) monolayers were infected with wild-type L. monocytogenes or L. monocytogenes mutants; then surface expression of E-selectin and neutrophil adhesion were measured. The results showed that delta hly and prfA mutants were the most crippled, requiring 100-fold more mutant bacteria than wild-type bacteria for analogous stimulation. By comparison, L. monocytogenes mutants with deletions of actA, inlA, inlB, inlAB, plcA, and plcB resembled their parent strains, and a delta plcA delta plcB mutant displayed decreased intracellular growth rate but only a minor decrease in stimulation of E-selectin or neutrophil adhesion. Other experiments showed that cytochalasin D-treated HUVEC monolayers bound bacteria, but internalization and increased surface E-selectin and intercellular adhesion molecule-1 expression were profoundly inhibited. However, cytochalasin D had no effect on the HUVEC response to stimulation with lipopolysaccharide or tumor necrosis factor alpha. These data suggest that listeriolysin O production by infecting L. monocytogenes contributes to increased expression of surface E-selectin and intercellular adhesion molecule-1, but neither it nor intracellular replication are directly responsible for this event. Nonetheless it is possible that listeriolysin O potentiates the effect(s) of an other molecule(s) that directly triggers this response. Additionally, cellular invasion by L. monocytogenes appears to be critical for initiating the HUVEC response, potentially by providing a signal which results in upregulation of the necessary bacterial genes.

Listeria monocytogenes-infected human umbilical vein endothelial cells: internalin-independent invasion, intracellular growth, movement, and host cell responses

The interaction of Listeria monocytogenes with human umbilical vein endothelial cells was studied. We show that L. monocytogenes invades human umbilical vein endothelial cells independently of internalin A, internalin B, internalin C, and ActA. L. monocytogenes replicates efficiently inside the cells and moves intracellularly by the induction of actin polymerization. We further show that L. monocytogenes-infection of human umbilical vein endothelial cells induces interleukin-6 and interleukin-8 expression during the first 6 h of infection. The expression of MCP-1 and the adhesion molecules VCAM-1 and ICAM-1 was not altered under the experimental conditions used here.

Some macrophages kill Listeria monocytogenes while others do not

It is not known why some macrophages can kill certain microbes, such as the facultative intracellular bacterium Listeria monocytogenes (L. monocytogenes), while other macrophages cannot. Perhaps listericidal activity is a property of macrophages at specific stages of differentiation; may be the ability to kill this bacterium is regulated by the microenvironment of the cell: or it is possible that other regulatory forces are important. We describe here three characteristics that distinguish macrophages which can kill L. monocytogenes from those which cannot. First, listericidal macrophages must have neither too much nor too little intracellular iron-they must have an intermediate amount. Second, the receptor a macrophage uses to phagocytose L. monocytogenes seems to influence the intracellular fate of this bacterium. And third, macrophages which have cell-surface interleukin-10 (IL-10), a known downregulator of macrophage function, cannot kill L. monocytogenes. These traits of macrophages and their effects on listericidal activity are reviewed here, and the possibility that these properties might interact to control macrophage bactericidal activity is discussed.

Host cell heparan sulfate proteoglycans mediate attachment and entry of Listeria monocytogenes, and the listerial surface protein ActA is involved in heparan sulfate receptor recognition

The mechanisms by which the intracellular pathogen Listeria monocytogenes interacts with the host cell surface remain largely unknown. In this study, we investigated the role of heparan sulfate proteoglycans (HSPG) in listerial infection. Pretreatment of bacteria with heparin or heparan sulfate (HS), but not with other glycosaminoglycans, inhibited attachment and subsequent uptake by IC-21 murine macrophages and CHO epithelial-like cells. Specific removal of HS from target cells with heparinase III significantly impaired listerial adhesion and invasion. Mutant CHO cells deficient in HS synthesis bound and internalized significantly fewer bacteria than wild-type cells did. Pretreatment of target cells with the HS-binding proteins fibronectin and platelet factor 4, or with heparinase III, impaired listerial infectivity only in those cells expressing HS. Moreover, a synthetic peptide corresponding to the HS-binding ligand in Plasmodium falciparum circumsporozoite protein (pepPf1) inhibited listerial attachment to IC-21 and CHO cells. A motif very similar to the HS-binding site of pepPf1 was found in the N-terminal region of ActA, the L. monocytogenes surface protein responsible for actin-based bacterial motility and cell-to-cell spread. In the same region of ActA, several clusters of positively charged amino acids which could function as HS-binding domains were identified. An ActA-deficient mutant was significantly impaired in attachment and entry due to altered HS recognition functions. This work shows that specific interaction with an HSPG receptor present on the surface of both professional and nonprofessional phagocytes is involved in L. monocytogenes cytoadhesion and invasion and strongly suggests that the bacterial surface protein ActA may be a ligand mediating HSPG receptor recognition.

Neutrophils are involved in the non-specific resistance to listeriosis induced by mycobacterial infections

A major role has been recently ascribed to the neutrophil in the resistance to infection by Listeria monocytogenes (L. monocytogenes). Here we evaluated whether such neutrophils played a role in the non-specific resistance to listeriosis that develops in hosts infected by mycobacteria. We found that the depletion of neutrophils completely abrogated the resistance conferred by the activated macrophages induced during the mycobacterial infection. The lack of killing by activated Kupffer cells and the visualization of bacteria proliferating inside peritoneal macrophages in neutrophil-depleted mice allowed us to postulate a role for the cooperation between neutrophils and macrophages in the killing of L. monocytogenes. We also found listerial proliferation in hepatocytes of neutrophil-depleted, mycobacteria-infected mice showing that the neutrophils may be involved in the control of listeria infection of parenchymal cells.

Expression of the inlAB operon by Listeria monocytogenes is not required for entry into hepatic cells in vivo

Listeria monocytogenes injected intravenously into mice is taken up in the liver, where hepatocytes serve as the principal site of intracellular replication. The factors effecting entry of L. monocytogenes into hepatic cells remain to be determined. Others have shown that the protein products of the inlAB (internalin) operon are required for maximum entry of L. monocytogenes into a number of cell lines in vitro. Likewise, we report here that expression of the inlAB operon was required for maximum uptake of L. monocytogenes by primary cultures of mouse hepatocytes. Uptake of an inlAB mutant strain of L. monocytogenes was approximately 10-fold less than that of the isogenic wild-type control. In contrast, inlAB expression was not a factor in (i) clearance of L. monocytogenes injected intravenously into mice and taken up in the liver, (ii) the distribution of L. monocytogenes among hepatocytes and nonparenchymal cells in the liver, or (iii) internalization of L. monocytogenes by hepatic cells in vivo. These latter findings suggest that infection of hepatic cells by L. monocytogenes in vivo does not require the protein products of the inlAB operon.