Lipooligosaccharide and polysaccharide capsule: virulence factors of Neisseria meningitidis that determine meningococcal interaction with human dendritic cells

Molecular typing of meningococci: recommendations for target choice and nomenclature

The diversity and dynamics of Neisseria meningitidis populations generate a requirement for high resolution, comprehensive, and portable typing schemes for meningococcal disease surveillance. Molecular approaches, specifically DNA amplification and sequencing, are the methods of choice for various reasons, including: their generic nature and portability, comprehensive coverage, and ready implementation to culture negative clinical specimens. The following target genes are recommended: (1) the variable regions of the antigen-encoding genes porA and fetA and, if additional resolution is required, the porB gene for rapid investigation of disease outbreaks and investigating the distribution of antigenic variants; (2) the seven multilocus sequence typing loci-these data are essential for the most effective national, and international management of meningococcal disease, as well as being invaluable in studies of meningococcal population biology and evolution. These targets have been employed extensively in reference laboratories throughout the world and validated protocols have been published. It is further recommended that a modified nomenclature be adopted of the form: serogroup: PorA type: FetA type: sequence type (clonal complex), thus: B: P1.19,15: F5-1: ST-33 (cc32).

Differential role of lipooligosaccharide of Neisseria meningitidis in virulence and inflammatory response during respiratory infection in mice

Meningococcal lipooligosaccharide (LOS) induces a strong proinflammatory response in humans during meningococcal infection. We analyzed the role of LOS in the inflammatory response and virulence during the early infectious process in a mouse model of meningococcal respiratory challenge. An lpxA mutant strain (serogroup B) devoid of LOS (strain Z0204) could not persist in the lungs and did not invade the blood. The persistence in the lungs and invasion of the bloodstream by a rfaD mutant expressing truncated LOS with only lipid A and 3-deoxy-d-manno-2-octulosonic acid molecules (strain Z0401) was intermediate between those of the wild-type and Z0204 strains. Both LOS mutants induced acute pneumonia with the presence of infiltrating polymorphonuclear leukocytes in lungs. Although tumor necrosis factor alpha production was reduced in mice infected with the mutant of devoid LOS, both LOS mutants induced production of other proinflammatory cytokines, such as interleukin-1beta (IL-1beta), IL-6, and the murine IL-8 homolog KC. Together, these results suggest that meningococcal LOS plays a role during the early infectious and invasive process, and they further confirm that other, nonlipopolysaccharide components of Neisseria meningitidis may significantly contribute to the inflammatory reaction of the host.

Outer membrane vesicles of the VA-MENGOC-BC vaccine against serogroup B of Neisseria meningitidis: Analysis of protein components by two-dimensional gel electrophoresis and mass spectrometry

Neisseria meningitidis is a Gram-negative bacterium responsible for significant mortality worldwide. While effective polysaccharides-based vaccines exist against serogroups A, C, W135, and Y, no similar vaccine is suitable for children under 4 years against disease caused by serogroup B strains. Therefore, major vaccine efforts against this serogroup are based on outer membrane vesicles (OMVs), containing major outer membrane proteins. The OMV-based vaccine produced by the Finlay Institute in Cuba (VA-MENGOC-BC) contributed to the rapid decline of the epidemic in this Caribbean island. While the content of major proteins in this vaccine has been discussed, no detailed work of an outer membrane proteomic map of this, or any other, commercially available OMV-derived product has been published so far. Since OMVs exhibit a large bias toward a few major proteins and usually contain a high content of lipids, establishing the adequate conditions for high resolution, 2-DE of this kind of preparation was definitely a technical challenge. In this work, 2-DE and MS have been used to generate a proteomic map of this product, detailing the presence of 31 different proteins, and it allows the identification of new putative protective protein components it contains.

Stable isotope metabolic labeling of Neisseria meningitidis lipooligosaccharide

The lipooligosaccharide (LOS) of a Neisseria meningitidis acetate auxotroph was metabolically labeled with either [2-13C]-sodium acetate or [1,2-13C2]-sodium acetate. In this study, we demonstrated that this label was efficiently incorporated into both the lipid A acyl moieties and the two N-acetylglucosamines present in the oligosaccharide branch of the LOS. The development of this efficient labeling protocol should prove useful in future structural studies analyzing the interactions between LOS and host proteins.

Carbohydrate composition of meningococcal lipopolysaccharide modulates the interaction of Neisseria meningitidis with human dendritic cells

Meningococcal lipopolysaccharide (LPS) is of crucial importance for the pathogenesis of invasive infection. We show that sialylation and elongation of the alpha-chain effectively shields viable unencapsulated Neisseria meningitidis from recognition by human dendritic cells (DC). In contrast, beta- and gamma- chain of the LPS carbohydrate moiety play only a minor role in the interaction with DC. The protective function of the LPS for the bacteria can be counteracted in vivo by phase variation of the lgtA gene encoding LPS glycosyltransferase A. Capsule expression protects N. meningitidis efficiently from recognition and phagocytosis by DC independent of the LPS structure. Despite the significant impact of LPS composition on the adhesion and phagocytosis of N. meningitidis no differences were found in terms of cytokine levels secreted by DC for IL1-beta, IL-6, IL-8, TNF-alpha, IFN-gamma and GM-CSF. However, significantly lower levels of the regulatory mediator IL-10 were induced by encapsulated strains in comparison to isogenic unencapsulated derivatives. IL-10 secretion was shown to depend on phagocytosis because poly alpha-2,8 sialic acid did not influence IL-10 secretion. The use of truncated LPS isoforms in vaccine preparations can therefore not only result in attenuation but also in more efficient targeting of DC.

Identification of a novel sialic acid transporter in Haemophilus ducreyi

Haemophilus ducreyi, the causative agent of chancroid, produces a lipooligosaccharide (LOS) which terminates in N-acetyllactosamine. This glycoform can be further extended by the addition of a single sialic acid residue to the terminal galactose moiety. H. ducreyi does not synthesize sialic acid, which must be acquired from the host during infection or from the culture medium when the bacteria are grown in vitro. However, H. ducreyi does not have genes that are highly homologous to the genes encoding known bacterial sialic acid transporters. In this study, we identified the sialic acid transporter by screening strains in a library of random transposon mutants for those mutants that were unable to add sialic acid to N-acetyllactosamine-containing LOS. Mutants that reacted with the monoclonal antibody 3F11, which recognizes the terminal lactosamine structure, and lacked reactivity with the lectin Maackia amurensis agglutinin, which recognizes alpha2,3-linked sialic acid, were further characterized to demonstrate that they produced a N-acetyllactosamine-containing LOS by silver-stained sodium dodecyl sulfate-polyacrylamide gel electrophoresis and mass spectrometric analyses. The genes interrupted in these mutants were mapped to a four-gene cluster with similarity to genes encoding bacterial ABC transporters. Uptake assays using radiolabeled sialic acid confirmed that the mutants were unable to transport sialic acid. This study is the first report of bacteria using an ABC transporter for sialic acid uptake.

Invasion of endothelial cells by Neisseria meningitidis requires cortactin recruitment by a phosphoinositide-3-kinase/Rac1 signalling pathway triggered by the lipo-oligosaccharide

Type-IV-pilus-mediated adhesion of Neisseria meningitidis (also known as meningococcus) to human endothelial cells induces the formation of membrane protrusions leading to bacterial uptake. We have previously shown that these protrusions result from a Rho- and Cdc42-dependent cortical actin polymerization, and from the activation of the ErbB2 tyrosine-kinase receptor and the Src kinase, leading to tyrosine phosphorylation of cortactin. We report here that N. meningitidis mutants expressing a deglycosylated lipo-oligosaccharide are poorly invasive. These mutants show structurally altered actin polymerization. Moreover, although they efficiently recruit and activate ErbB2 and Src, these mutants are defective in the recruitment and phosphorylation of cortactin. We demonstrate that phosphorylated cortactin controls the cortical actin polymerization, which leads to membrane protrusion formation. In addition, we show that cortactin recruitment is dependent on the activation of a phosphoinositide-3-kinase/Rac1-GTPase signalling pathway, which is required for actin polymerization and internalization of N. meningitidis, and is not activated by the mutant strains. Altogether, these results define a new role for the lipo-oligosaccharide in triggering a phosphoinositide-3-kinase/Rac1 signalling required to elicit an efficient uptake of N. meningitidis in non-phagocytic cells.

Neisserial porin-induced dendritic cell activation is MyD88 and TLR2 dependent

Neisserial porins have been shown to act as B cell mitogens and immune adjuvants. PorA and PorB are the major outer membrane porin proteins of the human pathogen Neisseria meningitidis. We have shown that the mechanism of the immunopotentiating capability of porin involves up-regulation of the T cell costimulatory ligand, CD86. Due to neisserial porin's ability to activate B cells and potentiate immune responses, we hypothesized that porin also employs the potent immune stimulatory function of dendritic cells (DC). We examined the ability of purified N. meningitidis PorB to induce maturation of murine splenic and bone marrow-derived DC. PorB treatment induced DC maturation, as demonstrated by increased expression of CD86 and class I and II MHC molecules. In addition, PorB not only enhanced the allostimulatory activity of DC, but also augmented the ability of DC to stimulate T cells in an Ag-specific manner. PorB-matured DC secreted the inflammatory cytokine IL-6, which may have implications for the adjuvant property of porin. Induction of IL-6 by PorB is also significant because IL-6 is one of a number of cytokines produced during infection with N. meningitidis and may be involved in the inflammatory process observed during infection and disease. We previously demonstrated the requirement of MyD88 and TLR2 for PorB-induced B cell activation. In the present study, MyD88 and TLR2 were also essential for PorB-induced DC activation. This work is significant for elucidating the mechanism(s) of neisserial porin's immune stimulatory activity.

Interaction of Neisseria meningitidis with human brain microvascular endothelial cells: role of MAP- and tyrosine kinases in invasion and inflammatory cytokine release

Neisseria meningitidis traversal across the blood-cerebrospinal fluid barrier is an essential step in the pathogenesis of bacterial meningitis. We have previously shown that invasion of human brain microvascular endothelial cells (HBMEC) by meningococci is mediated by bacterial outer membrane protein Opc that binds fibronectin, thereby anchoring the bacterium to the integrin alpha 5 beta 1-receptor on the endothelial cell surface. However, subsequent signal transduction mechanisms essential for or regulated by N. meningitidis adhesion and invasion, or HBMEC responses to N. meningitidis are unknown. In this report we investigated the role of c-Jun N-terminal kinases 1 and 2 (JNK1 and JNK2), p38 mitogen-activated (MAP) kinase and protein tyrosine kinases in endothelial-N. meningitidis interaction. Binding of meningococci to HBMEC phosphorylated and activated JNK1 and JNK2 and p38 MAPK as well as their direct substrates c-Jun and MAP kinase activated kinase-2 (MAPKAPK-2), respectively. Non-invasive meningococcal strains lacking opc gene (opc mutants and sequence type 11 complex meningococci) still activated p38 MAPK, however, failed to activate JNK. Inhibition of JNK1 and JNK2 significantly reduced internalization of N. meningitidis by HBMEC without affecting its adherence. Blocking the endothelial integrin alpha 5 beta 1 also decreased N. meningitidis-induced JNK activation in HBMEC. These findings indicate the crucial role of JNK signalling pathway in N. meningitidis invasion in HBMEC. In contrast, p38 MAPK pathway was important for the control of interleukin-6 (IL-6) and IL-8 release by HBMEC. Genistein, a protein tyrosine kinase inhibitor, decreased both invasion of N. meningitidis into HBMEC and IL-6 and IL-8 release, indicating that protein tyrosine kinases, which link signals from integrins to intracellular signalling pathways are essential for both bacterial internalization and cytokine secretion by HBMEC.

Human dendritic cell activation by Neisseria meningitidis: phagocytosis depends on expression of lipooligosaccharide (LOS) by the bacteria and is required for optimal cytokine production

Group B Neisseria meningitidis is a human pathogen, for which a universally effective vaccine is still not available. Immune responses to bacteria are initiated by dendritic cells (DC), which internalize and process bacterial antigens for presentation to T cells. We show here that optimal IL-12 and TNF-alpha production by human monocyte derived DC in response to killed serogroup B N. meningitidis depends on physical contact and internalization of the bacteria by DC. The majority of DC producing cytokines had internalized N. meningitidis while inhibition of bacterial internalization markedly impaired IL-12 and TNF-alpha, but not IL-6 production. Internalization of N. meningitidis was shown to depend on lipooligosaccharide (LOS) expressed by the bacteria with poor internalization of LOS deficient bacteria compared to wild-type bacteria. Restoration of LOS biosynthesis in a LOS regulatory strain also restored both internalization and cytokine production and was enhanced in the presence of LPS binding protein (LBP). These results suggest that DC phagocytosis depends on expression of LOS within the bacteria and that optimal cytokine production, particularly IL-12, requires internalization of the bacteria. These findings have important implications for designing vaccines that will induce protective immune responses to group B N. meningitidis.

The role of human dendritic cells in meningococcal and listerial meningitis

Few bacteria are capable of causing infections of the central nervous system (CNS), one of the most subtly shielded anatomical structures within the human body. Neisseria meningitidis is an important cause of bacterial meningitis and commonly affects otherwise healthy infants and adolescents. In contrast, Listeria monocytogenes is a cause of septicaemia and meningitis in neonates and immunocompromised adults. Dendritic cells (DCs) provide the physical link between the innate and adaptive immune system and play a crucial role in host defence against invading bacterial pathogens. The mechanisms of interaction of L. monocytogenes and N. meningitidis with DCs are entirely distinct. Whereas L. monocytogenes is readily phagocytosed by DCs by a serum-dependent mechanism, N. meningitidis is largely protected against phagocytotic uptake by its polysaccharide capsule. In addition, the pattern of secreted cytokines induced by L. monocytogenes is dominated by interleukin (IL)-12 and IL-18, capable of initiating a Th-1 response, whereas N. meningitidis induces high levels of proinflammatory cytokines. Therefore, we propose distinct functions of DCs in both types of bacterial meningitis.

Activation of human dendritic cells is modulated by components of the outer membranes of Neisseria meningitidis

Neisseria meningitidis serogroup B is a major cause of life-threatening meningitis and septicemia worldwide, and no effective vaccine is available. Initiation of innate and acquired immune responses to N. meningitidis is likely to be dependent on cellular responses of dendritic cells (DC) to antigens present in the outer membrane (OM) of the meningococcus. In this study, the responses of human monocyte-derived DC (mo-DC) to OM isolated from parent (lipopolysaccharide [LPS]-replete) meningococci and from a mutant deficient in LPS were investigated. Parent OM selectively up-regulated Toll-like receptor 4 (TLR4) mRNA expression and induced mo-DC maturation, as reflected by increased production of chemokines, proinflammatory cytokines, and CD83, CD80, CD86, CD40, and major histocompatibility complex (MHC) class II molecules. In contrast, LPS-deficient OM selectively up-regulated TLR2 mRNA expression and induced moderate increases in both cytokine production and expression of CD86 and MHC class II molecules. Preexposure to OM, with or without LPS, augmented the allostimulatory properties of mo-DC, which induced proliferation of naive CD4+ CD45RA+ T cells. In addition, LPS-replete OM induced a greater gamma interferon/interleukin-13 ratio in naive T cells, whereas LPS-deficient OM induced the reverse profile. These data demonstrate that components of the OM, other than LPS, are also likely to be involved in determining the levels of DC activation and the nature of the T-helper immune response.

Stimulus-dependent deacylation of bacterial lipopolysaccharide by dendritic cells

We describe here a previously unrecognized property of dendritic cells (DCs), the ability to deacylate the lipid A moiety of gram-negative bacterial LPSs. Both immature DCs of the XS52 cell line and bone marrow-derived DCs produce acyloxyacyl hydrolase, an enzyme that detoxifies LPS by selectively removing the secondary acyl chains from lipid A. Acyloxyacyl hydrolase expression decreased when DCs were incubated with IL-4, IL-1 beta, TNF alpha, and an agonistic CD40 antibody (maturation cocktail), and increased after treatment with LPS, CpG oligodeoxynucleotides, or a gram-positive bacterium (Micococcus luteus). Maturation cocktail treatment also diminished, whereas LPS treatment enhanced or maintained the cells' ability to kill Escherichia coli, deacylate LPS, and degrade bacterial protein. Enzymatic deacylation of LPS is an intrinsic, regulated mechanism by which DCs may modulate host responses to this potent bacterial agonist.