Vitronectin mediates internalization of Neisseria gonorrhoeae by Chinese hamster ovary cells

Galectins as potential therapeutic targets in STIs in the female genital tract

Every day, more than one million people worldwide acquire a sexually transmitted infection (STI). This public health problem has a direct effect on women's reproductive and sexual health as STIs can cause irreversible damage to fertility and can have negative consequences associated with discrimination and social exclusion. Infection with one sexually transmitted pathogen predisposes to co-infection with others, suggesting the existence of shared pathways that serve as molecular links between these diseases. Galectins, a family of β-galactoside-binding proteins, have emerged as endogenous mediators that facilitate cell-surface binding, internalization and cell invasion of many sexually transmitted pathogens, including Chlamydia trachomatis, Neisseria gonorrhoeae, Trichomonas vaginalis, Candida albicans, HIV and herpes simplex virus. The ability of certain galectins to dimerize or form multimeric complexes confers the capacity to interact simultaneously with glycosylated ligands on both the pathogen and the cervico-vaginal tissue on these proteins. Galectins can act as a bridge by engaging glycans from the pathogen surface and glycosylated receptors from host cells, which is a mechanism that has been shown to be shared by several sexually transmitted pathogens. In the case of viruses and obligate intracellular bacteria, binding to the cell surface promotes pathogen internalization and cell invasion. Inflammatory responses that occur in cervico-vaginal tissue might trigger secretion of galectins, which in turn control the establishment, evolution and severity of STIs. Thus, galectin-targeted therapies could potentially prevent or decrease STIs caused by a diverse array of pathogenic microorganisms; furthermore, anti-galectin agents might reduce treatment costs of STIs and reach the most vulnerable populations.

Complement interactions with the pathogenic Neisseriae: clinical features, deficiency states, and evasion mechanisms

Neisseria gonorrhoeae causes the sexually transmitted infection gonorrhea, while Neisseria meningitidis is an important cause of bacterial meningitis and sepsis. Complement is a central arm of innate immune defenses and plays an important role in combating Neisserial infections. Persons with congenital and acquired defects in complement are at a significantly higher risk for invasive Neisserial infections such as invasive meningococcal disease and disseminated gonococcal infection compared to the general population. Of note, Neisseria gonorrhoeae and Neisseria meningitidis can only infect humans, which in part may be related to their ability to evade only human complement. This review summarizes the epidemiologic and clinical aspects of Neisserial infections in persons with defects in the complement system. Mechanisms used by these pathogens to subvert killing by complement and preclinical studies showing how these complement evasion strategies may be used to counteract the global threat of meningococcal and gonococcal infections are discussed.

A Novel Factor H-Fc Chimeric Immunotherapeutic Molecule against Neisseria gonorrhoeae

Neisseria gonorrhoeae, the causative agent of the sexually transmitted infection gonorrhea, has developed resistance to almost every conventional antibiotic. There is an urgent need to develop novel therapies against gonorrhea. Many pathogens, including N. gonorrhoeae, bind the complement inhibitor factor H (FH) to evade complement-dependent killing. Sialylation of gonococcal lipooligosaccharide, as occurs in vivo, augments binding of human FH through its domains 18-20 (FH18-20). We explored the use of fusing FH18-20 with IgG Fc (FH18-20/Fc) to create a novel anti-infective immunotherapeutic. FH18-20 also binds to select host glycosaminoglycans to limit unwanted complement activation on host cells. To identify mutation(s) in FH18-20 that eliminated complement activation on host cells, yet maintained binding to N. gonorrhoeae, we created four mutations in domains 19 or 20 described in atypical hemolytic uremic syndrome that prevented binding of mutated fH to human erythrocytes. One of the mutant proteins (D to G at position 1119 in domain 19; FHD1119G/Fc) facilitated complement-dependent killing of gonococci similar to unmodified FH18-20/Fc but, unlike FH18-20/Fc, did not lyse human erythrocytes. FHD1119G/Fc bound to all (100%) of 15 sialylated clinical N. gonorrhoeae isolates tested (including three contemporary ceftriaxone-resistant strains), mediated complement-dependent killing of 10 of 15 (67%) strains, and enhanced C3 deposition (≥10-fold above baseline levels) on each of the five isolates not directly killed by complement. FHD1119G/Fc facilitated opsonophagocytic killing of a serum-resistant strain by human polymorphonuclear neutrophils. FHD1119G/Fc administered intravaginally significantly reduced the duration and burden of gonococcal infection in the mouse vaginal colonization model. FHD1119G/Fc represents a novel immunotherapeutic against multidrug-resistant N. gonorrhoeae.

Yersinia pestis uses the Ail outer membrane protein to recruit vitronectin

Yersinia pestis, the agent of plague, requires the Ail (attachment invasion locus) outer membrane protein to survive in the blood and tissues of its mammalian hosts. Ail is important for both attachment to host cells and for resistance to complement-dependent bacteriolysis. Previous studies have shown that Ail interacts with components of the extracellular matrix, including fibronectin, laminin and heparan sulfate proteoglycans, and with the complement inhibitor C4b-binding protein. Here, we demonstrate that Ail-expressing Y. pestis strains bind vitronectin - a host protein with functions in cell attachment, fibrinolysis and inhibition of the complement system. The Ail-dependent recruitment of vitronectin resulted in efficient cleavage of vitronectin by the outer membrane Pla (plasminogen activator protease). Escherichia coli DH5α expressing Y. pestis Ail bound vitronectin, but not heat-treated vitronectin. The ability of Ail to directly bind vitronectin was demonstrated by ELISA using purified refolded Ail in nanodiscs.

Identification and therapeutic potential of a vitronectin binding region of meningococcal msf

The human pathogen Neisseria meningitides (Nm) attains serum resistance via a number of mechanisms, one of which involves binding to the host complement regulator protein vitronectin. We have shown previously that the Meningococcal surface fibril (Msf), a trimeric autotransporter, binds to the activated form of vitronectin (aVn) to increase Nm survival in human serum. In this study, we aimed to identify the aVn-binding region of Msf to assess its potential as an antigen which can elicit antibodies that block aVn binding and/or possess bactericidal properties. Using several recombinant Msf fragments spanning its surface-exposed region, the smallest aVn-binding recombinants were found to span residues 1-86 and 39-124. The use of further deletion constructs and overlapping recombinant Msf fragments suggested that a region of Msf comprising residues 39-82 may be primarily important for aVn binding and that other regions may also be involved but to a lesser extent. Molecular modelling implicated K66 and K68, conserved in all available Msf sequences, to be involved in the interaction. Recombinant fragments which bound to aVn were able to reduce the survival advantage conveyed by aVn-interaction in serum bactericidal assays. Antibodies raised against one such fragment inhibited aVn binding to Msf. In addition, the antibodies enhanced specific killing of Msf-expressing Nm in a dose-dependent manner. Overall, this study identifies an aVn-binding region of Msf, an adhesin known to impart serum resistance properties to the pathogen; and shows that this region of Msf can elicit antibodies with dual properties which reduce pathogen survival within the host and thus has potential as a vaccine antigen.

Constitutively Opa-expressing and Opa-deficient neisseria gonorrhoeae strains differentially stimulate and survive exposure to human neutrophils

The Neisseria gonorrhoeae (the gonococcus [Gc]) opacity-associated (Opa) proteins mediate bacterial binding and internalization by human epithelial cells and neutrophils (polymorphonuclear leukocytes [PMNs]). Investigating the contribution of Opa proteins to gonococcal pathogenesis is complicated by high-frequency phase variation of the opa genes. We therefore engineered a derivative of Gc strain FA1090 in which all opa genes were deleted in frame, termed Opaless. Opaless Gc remained uniformly Opa negative (Opa(-)), whereas cultures of predominantly Opa(-) parental Gc and an intermediate lacking the "translucent" subset of opa genes (ΔopaBEGK) stochastically gave rise to Opa-positive (Opa(+)) bacterial colonies. Loss of Opa expression did not affect Gc growth. Opaless Gc survived exposure to primary human PMNs and suppressed the PMN oxidative burst akin to parental, Opa(-) bacteria. Notably, unopsonized Opaless Gc was internalized by adherent, chemokine-primed, primary human PMNs, by an actin-dependent process. When a non-phase-variable, in-frame allele of FA1090 opaD was reintroduced into Opaless Gc, the bacteria induced the PMN oxidative burst, and OpaD(+) Gc survived less well after exposure to PMNs compared to Opa(-) bacteria. These derivatives provide a robust system for assessing the role of Opa proteins in Gc biology.

Opa proteins and CEACAMs: pathways of immune engagement for pathogenic Neisseria

Neisseria meningitidis and Neisseria gonorrhoeae are globally important pathogens, which in part owe their success to their ability to successfully evade human immune responses over long periods. The phase-variable opacity-associated (Opa) adhesin proteins are a major surface component of these organisms, and are responsible for bacterial adherence and entry into host cells and interactions with the immune system. Most immune interactions are mediated via binding to members of the carcinoembryonic antigen cell adhesion molecule (CEACAM) family. These Opa variants are able to bind to different receptors of the CEACAM family on epithelial cells, neutrophils, and T and B lymphocytes, influencing the innate and adaptive immune responses. Increased epithelial cell adhesion creates the potential for prolonged infection, invasion and dissemination. Furthermore, Opa proteins may inhibit T-lymphocyte activation and proliferation, B-cell antibody production, and innate inflammatory responses by infected epithelia, in addition to conferring increased resistance to antibody-dependent, complement-mediated killing. While vaccines containing Opa proteins could induce adhesion-blocking and bactericidal antibodies, the consequence of CEACAM binding by a candidate Opa-containing vaccine requires further investigation. This review summarizes current knowledge of the immunological consequences of the interaction between meningococcal and gonococcal Opa proteins and human CEACAMs, considering the implications for pathogenesis and vaccine development.

Neisserial Opa Proteins: Impact on Colonization, Dissemination and Immunity

The pathogenic Neisseria sp. encode a family of phase-variable and antigenically distinct Opa proteins that allow bacterial attachment to virtually every cell type encountered during infection. Some Opa variants bind cell surface-expressed heparan sulfate proteoglycans, including members of the syndecan family of receptors, and extracellular matrix proteins such as fibronectin and vitronectin. Other variants bind members of the carcinoembryonic antigen family of cellular adhesion molecules. Depending on the Opa variant(s) expressed, these receptor interactions can allow neisserial entry and transcellular transcytosis across polarized epithelial cell monolayers, entry into endothelial cells, suppression of lymphocyte function and/or bacterial engulfment and killing by neutrophils. Recent advances in our understanding of how these Opa protein-mediated interactions influence the host cellular response are discussed in the context of their impact on various stages of neisserial infection.

Integrin-linked kinase is required for vitronectin-mediated internalization of Streptococcus pneumoniae by host cells

By interacting with components of the human host, including extracellular matrix (ECM) proteins, Streptococcus pneumoniae has evolved various strategies for colonization. Here, we characterized the interaction of pneumococci with the adhesive glycoprotein vitronectin and the contribution of this protein to pneumococcal uptake by host cells in an integrin-dependent manner. Specific interaction of S. pneumoniae with the heparin-binding sites of purified multimeric vitronectin was demonstrated by flow cytometry analysis. Host-cell-bound vitronectin promoted pneumococcal adherence to and invasion into human epithelial and endothelial cells. Pneumococci were trapped by microspike-like structures, which were induced upon contact of pneumococci with host-cell-bound vitronectin. Alphavbeta3 integrin was identified as the major cellular receptor for vitronectin-mediated adherence and uptake of pneumococci. Ingestion of pneumococci by host cells via vitronectin required a dynamic actin cytoskeleton and was dependent on integrin-linked kinase (ILK), phosphatidylinositol 3-kinase (PI3K), and protein kinase B (Akt), as demonstrated by gene silencing or in inhibition experiments. In conclusion, pneumococci exploit the vitronectin-alphavbeta3-integrin complex as a cellular receptor for invasion and this integrin-mediated internalization requires the cooperation between the host signalling molecules ILK, PI3K and Akt.

Pasteurella multocidaand bovine respiratory disease

Pasteurella multocidais a pathogenic Gram-negative bacterium that has been classified into three subspecies, five capsular serogroups and 16 serotypes.P. multocidaserogroup A isolates are bovine nasopharyngeal commensals, bovine pathogens and common isolates from bovine respiratory disease (BRD), both enzootic calf pneumonia of young dairy calves and shipping fever of weaned, stressed beef cattle.P. multocidaA:3 is the most common serotype isolated from BRD, and these isolates have limited heterogeneity based on outer membrane protein (OMP) profiles and ribotyping. Development ofP. multocida-induced pneumonia is associated with environmental and stress factors such as shipping, co-mingling, and overcrowding as well as concurrent or predisposing viral or bacterial infections. Lung lesions consist of an acute to subacute bronchopneumonia that may or may not have an associated pleuritis. Numerous virulence or potential virulence factors have been described for bovine respiratory isolates including adherence and colonization factors, iron-regulated and acquisition proteins, extracellular enzymes such as neuraminidase, lipopolysaccharide, polysaccharide capsule and a variety of OMPs. Immunity of cattle against respiratory pasteurellosis is poorly understood; however, high serum antibodies to OMPs appear to be important for enhancing resistance to the bacterium. Currently availableP. multocidavaccines for use in cattle are predominately traditional bacterins and a live streptomycin-dependent mutant. The field efficacy of these vaccines is not well documented in the literature.

The Mycoplasma gallisepticum OsmC-like protein MG1142 resides on the cell surface and binds heparin

Mycoplasma gallisepticum is an avian pathogen that causes a chronic respiratory disease of chickens and results in significant economic losses to the poultry industry worldwide. Colonization of the host and the establishment of chronic disease are initiated by the cytadherence of M. gallisepticum to the host respiratory epithelium. While several proteins involved in cytadhesion have been characterized, molecules that interact with components of the host extracellular matrix, a process that is central to pathogenesis, are only now being identified. In this study, M. gallisepticum whole cells were shown to bind heparin in a specific and saturable manner. Heparin also significantly inhibited the binding of M. gallisepticum to the human lung fibroblast cell line MRC-5, suggesting a potential role for glycosaminoglycans (GAGs) in cytadherence. M. gallisepticum protein MG1142 (encoded by mga 1142), which displays homology to the osmotically induced (OsmC) family of proteins, binds strongly to heparin, is highly expressed during in vitro culture, and is surface accessible. Recombinant MG1142 bound heparin in a dose-dependent and saturable manner with a dissociation constant (K(d)) of 10+/-1.8 nM, which is within a physiologically significant range, compared to that of other heparin-binding proteins. Binding to heparin was inhibited by the heavily sulfated polysaccharide fucoidan, but not by mucin or chondroitin sulfate A or B, suggesting that electrostatic interactions between the sulfate groups of heparin and the positively charged basic residues of the MG1142 protein are important in binding. The ability of M. gallisepticum to bind GAGs may contribute to host adherence and colonization.

Alpha-toxin interferes with integrin-mediated adhesion and internalization of Staphylococcus aureus by epithelial cells

Staphylococcus aureus is an important human and animal pathogen. During infection, this bacterium is able to attach to and enter host cells by using its cell surface-associated factors to bind to the host's extracellular matrix (ECM) proteins. In this study, we determined that a protein exported by S. aureus, alpha-toxin, can interfere with the integrin-mediated adhesion and internalization of S. aureus by human lung epithelial cells (A549). The downregulation of alpha-toxin production significantly increased bacterial adhesion and invasion into the epithelial cells. In contrast, bacterial adhesion and invasion was inhibited by both overproduction of alpha-toxin and the addition of alpha-toxin to the culture medium. Moreover, our results showed that the quantitative effects on invasion closely parallel those of adherence. This suggests that the effect on invasion is probably secondary to, and a consequence of, the reduced adherence caused by alpha-toxin exposure. Specifically, we demonstrated that alpha-toxin interacts with the hosts' ECM protein's receptor, beta1-integrin, which indicates that beta1-integrin may be a potential receptor of alpha-toxin on epithelial cells. Taken together, our results indicate that exported alpha-toxin inhibits the adhesion and internalization of S. aureus by interfering with integrin-mediated pathogen-host cell interactions.

Application of surface plasmon coupled emission to study of muscle

Muscle contraction results from interactions between actin and myosin cross-bridges. Dynamics of this interaction may be quite different in contracting muscle than in vitro because of the molecular crowding. In addition, each cross-bridge of contracting muscle is in a different stage of its mechanochemical cycle, and so temporal measurements are time averages. To avoid complications related to crowding and averaging, it is necessary to follow time behavior of a single cross-bridge in muscle. To be able to do so, it is necessary to collect data from an extremely small volume (an attoliter, 10(-18) liter). We report here on a novel microscopic application of surface plasmon-coupled emission (SPCE), which provides such a volume in a live sample. Muscle is fluorescently labeled and placed on a coverslip coated with a thin layer of noble metal. The laser beam is incident at a surface plasmon resonance (SPR) angle, at which it penetrates the metal layer and illuminates muscle by evanescent wave. The volume from which fluorescence emanates is a product of two near-field factors: the depth of evanescent wave excitation and a distance-dependent coupling of excited fluorophores to the surface plasmons. The fluorescence is quenched at the metal interface (up to approximately 10 nm), which further limits the thickness of the fluorescent volume to approximately 50 nm. The fluorescence is detected through a confocal aperture, which limits the lateral dimensions of the detection volume to approximately 200 nm. The resulting volume is approximately 2 x 10(-18) liter. The method is particularly sensitive to rotational motions because of the strong dependence of the plasmon coupling on the orientation of excited transition dipole. We show that by using a high-numerical-aperture objective (1.65) and high-refractive-index coverslips coated with gold, it is possible to follow rotational motion of 12 actin molecules in muscle with millisecond time resolution.

Low-phosphate-dependent invasion resembles a general way for Neisseria gonorrhoeae to enter host cells

Obligate human-pathogenic Neisseria gonorrhoeae expresses numerous variant surface proteins mediating adherence to and invasion of target cells. The invariant major outer membrane porin PorB of serotype A (P.IA) gonococci triggers invasion into Chang cells only if the medium is devoid of phosphate. Since gonococci expressing PorB(IA) are frequently isolated from patients with severe disseminating infections, the interaction initiated by the porin may be of major relevance for the development of this serious disease. Here, we investigated the low-phosphate-dependent invasion and compared it to the well-known pathways of entry initiated by Opa proteins. P.IA-triggered invasion requires clathrin-coated pit formation and the action of actin and Rho GTPases. However, in contrast to Opa-initiated invasion via heparan sulfate proteoglycans, microtubules, acidic sphingomyelinase, phosphatidylinositol 3-kinase, and myosin light chain kinase are not involved in this entry pathway. Nor are Src kinases required, as they are in invasion, e.g., via the CEACAM3 receptor. Invasion by PorB(IA) occurs in a wide spectrum of cell types, such as primary human epithelial and endothelial cells and in cancer cells of human and animal origin. Low-phosphate-dependent invasion is thus a pathway of gonococcal entry distinct from Opa-mediated invasion.

Recognition of saccharides by the OpcA, OpaD, and OpaB outer membrane proteins from Neisseria meningitidis

The adhesion of the pathogen Neisseria meningitidis to host cell surface proteoglycan, mediated by the integral outer membrane proteins OpcA and Opa, plays an important part in the processes of colonization and invasion by the bacterium. The precise specificities of the OpcA and Opa proteins are, however, unknown. Here we use a fluorescence-based binding assay to show that both proteins bind to mono- and disaccharides with high affinity. Binding of saccharides caused a quench in the intrinsic fluorescence emission of both proteins, and mutation of selected Tyr residues within the external loop regions caused a substantial decrease in fluorescence. We suggest that the intrinsic fluorescence arises from resonance energy transfer from Tyr to Trp residues in the beta-barrel portion of the structure. OpcA bound sialic acid with a Kd of 0.31 microM and was shown to be specific for pyranose saccharides. The binding specificities of two different Opa proteins were compared; unlike OpcA, neither protein bound to monosaccharides, but both bound to maltose, lactose, and sialic acid-containing oligosaccharides, with Kd values in the micromolar range. OpaB had a 10-fold higher affinity for sialic acid-containing ligands than OpaD as a result of the mutation Y165V, which was shown to restore this specificity to OpaD. Finally, the OpcA- and Opa-dependent adhesion of meningococci to epithelial cells was shown to be partially inhibited by exogenously added sialic acid and maltose. The results show that OpcA and the Opa proteins can be thought of as outer membrane lectins and that simple saccharides can modulate their recognition of complex proteoglycan receptors.

Role of alphavbeta5 integrins and vitronectin in Pseudomonas aeruginosa PAK interaction with A549 respiratory cells

Bacterial adherence to mammalian cells and their internalization are thought to participate in Pseudomonas aeruginosa pathogenicity. In this study, we explored the role of alpha5beta1 and alphavbeta5 integrins and their natural ligands, fibronectin (Fn) and vitronectin (Vn), in P. aeruginosa interaction with epithelial cells by using the PAK reference bacterial strain, A549 respiratory, and SKOV-3 human ovarian cell lines. The host cell cytoskeleton and cellular tyrosine kinases seem to be solicited during the PAK-respiratory cell interaction: cytochalasin D and genistein decreased the bacterial adherence and internalization. Blocking antibodies to alphavbeta5 integrins were the only antibodies tested to have inhibitory activity against PAK adherence to A549 cells. PAK internalization by A549 and SKOV-3 cells was markedly decreased in the presence of blocking antibodies to Vn and alphavbeta5 integrins. Addition of Vn in excess restored PAK invasion of both A549 and SKOV-3 cells in the presence of anti-Vn antibodies. Immunofluorescence experiments revealed that, in the presence of bacteria, the Vn fibrillar network disappeared, and alphavbeta5 staining was concentrated in sites where adherent bacteria were present. Taken together, these findings suggest that alphavbeta5 integrins, and their natural ligand Vn, are involved in PAK entry into human epithelial cells.

The molecular mechanisms used by Neisseria gonorrhoeae to initiate infection differ between men and women

The molecular mechanisms used by the gonococcus to initiate infection exhibit gender specificity. The clinical presentations of disease are also strikingly different upon comparison of gonococcal urethritis to gonococcal cervicitis. An intimate association occurs between the gonococcus and the urethral epithelium and is mediated by the asialoglycoprotein receptor. Gonococcal interaction with the urethral epithelia cell triggers cytokine release, which promotes neutrophil influx and an inflammatory response. Similarly, gonococcal infection of the upper female genital tract also results in inflammation. Gonococci invade the nonciliated epithelia, and the ciliated cells are subjected to the cytotoxic effects of tumor necrosis factor alpha induced by gonococcal peptidoglycan and lipooligosaccharide. In contrast, gonococcal infection of the lower female genital tract is typically asymptomatic. This is in part the result of the ability of the gonococcus to subvert the alternative pathway of complement present in the lower female genital tract. Gonococcal engagement of complement receptor 3 on the cervical epithelia results in membrane ruffling and does not promote inflammation. A model of gonococcal pathogenesis is presented in the context of the male and female human urogenital tracts.

Engulfment of Neisseria gonorrhoeae: revealing distinct processes of bacterial entry by individual carcinoembryonic antigen-related cellular adhesion molecule family receptors

Individual Neisseria gonorrhoeae colony opacity-associated (Opa) protein variants can bind up to four different carcinoembryonic antigen-related cellular adhesion molecule (CEACAM) receptors. Most human cells encountered by gonococci express a combination of CEACAM receptors, thereby complicating the elucidation of intracellular signaling pathways triggered by individual receptors. Here, we compare the process of bacterial engulfment by a panel of stably transfected HeLa epithelial cell lines expressing each CEACAM receptor in isolation. CEACAM1 and CEACAM3 each contain proteinaceous transmembrane and cytoplasmic domains; however, the processes of neisserial uptake mediated by these receptors differ with respect to their susceptibilities to both tyrosine kinase inhibitors and the actin microfilament-disrupting agent cytochalasin D. Neisserial uptake mediated by glycosylphosphatidylinositol (GPI)-anchored CEACAM5 and CEACAM6 was not significantly affected by any of a broad spectrum of inhibitors tested. However, cleavage of the GPI anchor by phosphatidylinositol-specific phospholipase C reduced bacterial uptake by HeLa cells expressing CEACAM5, consistent with a single zipper-like mechanism of uptake mediated by this receptor. Regardless of the CEACAM receptor expressed, internalized gonococci were effectively killed by a microtubule-dependent process that required acidification of the bacterium-containing phagosome. Given the phase-variable nature of neisserial Opa proteins, these results indicate that the mechanism of bacterial engulfment and the cellular response to gonococcal infection depend on both the receptor specificities of the neisserial Opa protein variants expressed and the spectrum of CEACAM receptors present on target cells, each of which determines the combination of receptors ultimately engaged.

Immunoreceptor tyrosine-based activation motif phosphorylation during engulfment of Neisseria gonorrhoeae by the neutrophil-restricted CEACAM3 (CD66d) receptor

Gonorrhea is characterized by a purulent urethral or cervical discharge consisting primarily of neutrophils associated with Neisseria gonorrhoeae. These interactions are facilitated by gonococcal colony opacity-associated (Opa) protein binding to host cellular CEACAM receptors. Of these, CEACAM3 is restricted to neutrophils and contains an immunoreceptor tyrosine-based activation motif (ITAM) reminiscent of that found within certain phagocytic Fc receptors. CEACAM3 was tyrosine phosphorylated by a Src family kinase-dependent process upon infection by gonococci expressing CEACAM-specific Opa proteins. This phosphorylation was necessary for efficient bacterial uptake; however, a less efficient uptake process became evident when kinase inhibitors or mutagenesis of the ITAM were used to prevent phosphorylation. Ligated CEACAM3 was recruited to a cytoskeleton-containing fraction, intense foci of polymerized actin were evident where bacteria attached to HeLa-CEACAM3, and disruption of polymerized actin by cytochalasin D blocked all bacterial uptake by these cells. These data support a model whereby CEACAM3 can mediate the Opa-dependent uptake of N. gonorrhoeae via either an efficient, ITAM phosphorylation-dependent process that resembles phagocytosis or a less efficient, tyrosine phosphorylation-independent mechanism.

Helicobacter pylori: an invading microorganism? A review

In this review we evaluate the pros and cons of Helicobacter pylori invasion of epithelial cells as part of the natural history of H. pylori infection. H. pylori is generally considered an extracellular microorganism. However, a growing body of evidence supports the controversial hypothesis that at least a subset of H. pylori microorganisms has an intracellular (intraepithelial) location. Most significant is the fact that H. pylori invades cultured epithelial cells with invasion frequencies similar to Yersinia enterocolitica and better than Shigella flexneri; furthermore, studies of invasion mechanisms suggest that H. pylori invasion of and survival within epithelial cells is not merely a passive event, but requires active participation of the microorganism. Although many studies of human gastric biopsy specimens have failed to demonstrate any intracellular H. pylori, some studies have revealed a minor fraction of H. pylori inside gastric epithelial cells, with possible linkage to peptic ulceration and epithelial cell damage. In conclusion, these data encourage further research to establish whether intracellular H. pylori does play a role in H. pylori colonization of the human stomach and in peptic ulcer pathogenesis.

The Haemophilus ducreyi serum resistance antigen DsrA confers attachment to human keratinocytes

Haemophilus ducreyi is the etiologic agent of the sexually transmitted genital ulcer disease chancroid. H. ducreyi serum resistance protein A (DsrA) is a member of a family of multifunctional outer membrane proteins that are involved in resistance to killing by human serum complement. The members of this family include YadA of Yersinia species, the UspA proteins of Moraxella catarrhalis, and the Eib proteins of Escherichia coli. The role of YadA, UspA1, and UspA2H as eukaryotic cell adhesins and the function of UspA2 as a vitronectin binder led to our investigation of the cell adhesion and vitronectin binding properties of DsrA. We found that DsrA was a keratinocyte-specific adhesin as it was necessary and sufficient for attachment to HaCaT cells, a keratinocyte cell line, but was not required for attachment to HS27 cells, a fibroblast cell line. We also found that DsrA was specifically responsible for the ability of H. ducreyi to bind vitronectin. We then theorized that DsrA might use vitronectin as a bridge to bind to human cells, but this hypothesis proved to be untrue as eliminating HaCaT cell binding of vitronectin with a monoclonal antibody specific to integrin alpha(v)beta(5) did not affect the attachment of H. ducreyi to HaCaT cells. Finally, we wanted to examine the importance of keratinocyte adhesion in chancroid pathogenesis so we tested the wild-type and dsrA mutant strains of H. ducreyi in our swine models of chancroid pathogenesis. The dsrA mutant was less virulent than the wild type in both the normal and immune cell-depleted swine models of chancroid infection.

Fibronectin mediates Opc-dependent internalization of Neisseria meningitidis in human brain microvascular endothelial cells

A central step in the pathogenesis of bacterial meningitis caused by Neisseria meningitidis (the meningococcus) is the interaction of the bacteria with cells of the blood-brain barrier. In the present study, we analysed the invasive potential of two strains representing hypervirulent meningococcal lineages of the ET-5 and ET-37 complex in human brain-derived endothelial cells (HBEMCs). In contrast to previous observations made with epithelial cells and human umbilical vein-derived endothelial cells (HUVECs), significant internalization of encapsulated meningococci by HBMECs was observed. However, this uptake was found only for the ET-5 complex isolate MC 58, and not for an ET-37 complex strain. Furthermore, the uptake of meningococci by HBMECs depended on the presence of human serum, whereas serum of bovine origin did not promote the internalization of meningococci in HBMECs. By mutagenesis experiments, we demonstrate that internalization depended on the expression of the opc gene, which is present in meningococci of the ET-5 complex, but absent in ET-37 complex meningococci. Chromatographic separation of human serum proteins revealed fibronectin as the uptake-promoting serum factor, which binds to HBMECs via alpha 5 beta 1 integrin receptors. These data provide evidence for unique molecular mechanisms of the interaction of meningococci with endothelial cells of the blood-brain barrier and contribute to our understanding of the pathogenesis of meningitis caused by meningococci of different clonal lineages.

CagA tyrosine phosphorylation and interleukin-8 induction by Helicobacter pylori are independent from alpAB, HopZ and bab group outer membrane proteins

In several studies Helicobacter pylori type I strains (cag-positive strains) have been described to translocate their CagA protein into epithelial cells, where it is tyrosine-phosphorylated. The intimate contact allows a Cag-dependent bacteria-to-cell signaling inducing the secretion of the chemokine interleukin-8. Although a contact between the bacterial and the eukaryotic cell is known to be necessary for these signal transduction events the bacterial adhesin and the cellular receptor are unknown, so far. In this study, we investigated the influence of several outer membrane proteins associated with adherence on CagA translocation and IL-8 induction. The quantitative assessment of a cag deletion mutant strain binding to epithelial cells revealed that the Cag secretion apparatus is not primarily necessary for attachment. In contrast, the knockout mutation of the adherence-associated alpAB locus significantly reduced the binding capacity in two independent strains. Despite this partial adherence defect, the alpAB mutation did not affect CagA translocation and IL-8 induction. The mutagenesis of the bab group genes hp317, hp896 and hp1243 in H. pylori 26695 did not influence the Cag-dependent signaling either. No causative linkage could be found between the production of the outer membrane proteins HopZ, OipA or seven additional outer membrane proteins and CagA translocation or IL-8 induction.

Nuclear factor-kappa B directs carcinoembryonic antigen-related cellular adhesion molecule 1 receptor expression in Neisseria gonorrhoeae-infected epithelial cells

The human-specific pathogen Neisseria gonorrhoeae expresses opacity-associated (Opa) protein adhesins that bind to various members of the carcinoembryonic antigen-related cellular adhesion molecule (CEACAM) family. In this study, we have analyzed the mechanism underlying N. gonorrhoeae-induced CEACAM up-regulation in epithelial cells. Epithelial cells represent the first barrier for the microbial pathogen. We therefore characterized CEACAM expression in primary human ovarian surface epithelial (HOSE) cells and found that CEACAM1-3 (L, S) and CEACAM1-4 (L, S) splice variants mediate an increased Opa(52)-dependent gonoccocal binding to HOSE cells. Up-regulation of these CEACAM molecules in HOSE cells is a direct process that takes place within 2 h postinfection and depends on close contact between microbial pathogen and HOSE cells. N. gonorrhoeae-triggered CEACAM1 up-regulation involves activation of the transcription factor nuclear factor kappaB (NF-kappaB), which translocates as a p50/p65 heterodimer into the nucleus, and an NF-kappaB-specific inhibitory peptide inhibited CEACAM1-receptor up-regulation in N. gonorrhoeae-infected HOSE cells. Bacterial lipopolysaccharides did not induce NF-kappaB and CEACAM up-regulation, which corresponds to our findings that HOSE cells do not express toll-like receptor 4. The ability of N. gonorrhoeae to up-regulate its epithelial receptor CEACAM1 through NF-kappaB suggests an important mechanism allowing efficient bacterial colonization during the initial infection process.

Interaction of an uuter membrane protein of enterotoxigenic Escherichia coli with cell surface heparan sulfate proteoglycans

We have previously shown that enterotoxigenic invasion protein A (Tia), a 25-kDa outer membrane protein encoded on an apparent pathogenicity island of enterotoxigenic Escherichia coli (ETEC) strain H10407, mediates attachment to and invasion into cultured human gastrointestinal epithelial cells. The epithelial cell receptor(s) for Tia has not been identified. Here we show that Tia interacts with cell surface heparan sulfate proteoglycans. Recombinant E. coli expressing Tia mediated invasion into wild-type epithelial cell lines but not invasion into proteoglycan-deficient cells. Furthermore, wild-type eukaryotic cells, but not proteoglycan-deficient eukaryotic cells, attached to immobilized polyhistidine-tagged recombinant Tia (rTia). Binding of epithelial cells to immobilized rTia was inhibited by exogenous heparan sulfate glycosaminoglycans but not by hyaluronic acid, dermatan sulfate, or chondroitin sulfate. Similarly, pretreatment of eukaryotic cells with heparinase I, but not pretreatment of eukaryotic cells with chrondroitinase ABC, inhibited attachment to rTia. In addition, we also observed heparin binding to both immobilized rTia and recombinant E. coli expressing Tia. Heparin binding was inhibited by a synthetic peptide representing a surface loop of Tia, as well as by antibodies directed against this peptide. Additional studies indicated that Tia, as a prokaryotic heparin binding protein, may also interact via sulfated proteoglycan molecular bridges with a number of mammalian heparan sulfate binding proteins. These findings suggest that the binding of Tia to host epithelial cells is mediated at least in part through heparan sulfate proteoglycans and that ETEC belongs on the growing list of pathogens that utilize these ubiquitous cell surface molecules as receptors.

Signal transduction pathways induced by virulence factors of Neisseria gonorrhoeae

The obligate human pathogen Neisseria gonorrhoeae infects a variety of human tissues. In recent years, several host cell receptors for the major bacterial adhesins have been identified. While the knowledge of the molecular mechanism of colonisation has helped to understand special aspects of the infection, like the explicit tropism of gonococci for human tissues, the long-term consequences of engaging these receptors are still unknown. A variety of signalling pathways initiated by the activated receptors and by bacterial proteins transferred to the infected cell have been defined which include lipid second messenger, protein kinases, proteases and GTPases. These pathways control important steps of the infection, such as tight adhesion and invasion, the induction of cytokine release, and apoptosis. The detailed knowledge of bacteria-induced signalling pathways could allow the design of new therapeutic approaches which might be advantageous over the classical antibiotics therapy.

Pathogenic Neisseria trigger expression of their carcinoembryonic antigen-related cellular adhesion molecule 1 (CEACAM1; previously CD66a) receptor on primary endothelial cells by activating the immediate early response transcription factor, nuclear factor-kappaB

Neisseria gonorrhoeae express opacity-associated (Opa) protein adhesins that mediate binding to various members of the carcinoembryonic antigen-related cellular adhesion molecule (CEACAM; previously CD66) receptor family. Although human umbilical vein endothelial cells express little CEACAM receptor in vitro, we found neisserial infection to induce expression of CEACAM1, CEACAM1-3L, and CECAM1-4L splice variants. This mediates an increased Opa(52)-dependent binding of gonococci by these cells. The induced receptor expression did not require bacterial Opa expression, but it was more rapid with adherent bacteria. Because the time course of induction was similar to that seen for induced proinflammatory cytokines, we tested whether CEACAM1 expression could be controlled by a similar mechanism. Gonococcal infection activated a nuclear factor-kappaB (NF-kappaB) heterodimer consisting of p50 and p65, and inhibitors that prevent the nuclear translocation of activated NF-kappaB complex inhibited CEACAM1 transcript expression. Each of these effects could be mimicked by using culture filtrates or purified lipopolysaccharide instead of intact bacteria. Together, our results support a model whereby the outer membrane "blebs" that are actively released by gonococci trigger a Toll-like receptor-4-dependent activation of NF-kappaB, which up-regulates the expression of CEACAM1 to allow Opa(52)-mediated neisserial binding. The regulation of CEACAM1 expression by NF-kappaB also implies a broader role for this receptor in the general inflammatory response to infection.

Interactions of pathogenic neisseriae with epithelial cell membranes

The closely related bacterial pathogens Neisseria gonorrhoeae (gonococci, GC) and N. meningitidis (meningococci, MC) initiate infection at human mucosal epithelia. Colonization begins at apical epithelial surfaces with a multistep adhesion cascade, followed by invasion of the host cell, intracellular persistence, transcytosis, and exit. These activities are modulated by the interaction of a panoply of virulence factors with their cognate host cell receptors, and signals are sent from pathogen to host and host to pathogen at multiple stages of the adhesion cascade. Recent advances place us on the verge of understanding the colonization process at a molecular level of detail. In this review we describe the Neisseria virulence factors in the context of epithelial cell biology, placing special emphasis on the signaling functions of type IV pili, pilus-based twitching motility, and the Opa and Opc outermembrane adhesin/invasin proteins. We also summarize what is known about bacterial intracellular trafficking and growth. With the accelerated integration of tools from cell biology, biochemistry, biophysics, and genomics, experimentation in the next few years should bring unprecedented insights into the interactions of Neisseriae with their host.

Implications of molecular contacts and signaling initiated by Neisseria gonorrhoeae

Neisseria gonorrhoeae employs diverse strategies with which to adhere to and invade host cells during the course of infection. These primary encounters provide means by which biologically active molecules can be efficiently targeted to disrupt or exploit normal host cell metabolism and immune response elements, which in turn leads to the pathological responses characteristic of gonococcal disease. Current studies have begun to elucidate in detail the molecular interactions orchestrating these processes and the signaling events that they provoke.

Role of strain type, AGS cells and fetal calf serum in Helicobacter pylori adhesion and invasion assays

In a human gastric biopsy specimen, 30% of adhering Helicobacter pylori strain AF4 (cagA and VacA positive) was associated with adhesion pedestals. In an AGS cell assay, only a few percent of this type I strain was found to be associated with adhesion pedestals. Nevertheless, a larger proportion of the type I strain was found to invade AGS cells (P < 0.03) and to attach with depressions in the AGS cell membrane (P < 0.03) than a type II strain (cagA and VacA negative). Incubation of AGS cells and H. pylori without adding fetal calf serum (FCS) to the culture medium increased actin accumulations (FITC-phalloidin stained) beneath adhering H. pylori, and decreased H. pylori invasion of AGS cells significantly (P < 0.01). However, no increase in the number of adhesion pedestals was observed by electron microscopy. Proteinase K treatment of FCS eliminated the H. pylori invasion promoting effect (P < 0.01). Our results suggest differences in the ability of H. pylori to induce adhesion pedestals in human gastric epithelial cells and in AGS cells, but a correlation between adhesion pedestal formation in vivo and H. pylori invasion in vitro can be speculated. In addition, H. pylori invasion into AGS cells was found to be mediated by proteins in FCS.

Host cell invasion by pathogenic Neisseriae

As outlined in this review, various experimental techniques have been employed in an attempt to understand neisserial pathogenesis. In vitro genetic analysis has been used to study the genetic basis for the structural variability of cell surface components. Transformed or primary epithelial cell cultures have provided the simplest model to analyze bacterial adherence and invasion, while the infection of polarized epithelial monolayers, fallopian tube and nasopharyngeal organ cultures, and ureteral tissue have each been used to more closely represent the events which occur in vivo. Finally, the in vivo infection of human volunteers with N. gonorrhoeae has provided a powerful means to confirm and expand the results obtained in vitro. By these various approaches, a number of neisserial adhesins (i.e. pilli, Opa, Opc and P36) and additional putative virulence determinants which affect bacterial adherence and invasion into host cells (i.e. LOS, capsule, PorB) have been identified. Clearly, neisserial surface variation serves as an adaptive mechanism which can modulate tissue tropism, immune evasion and survival in the changing host environment. Important progress has been made in recent years with respect to the host cellular receptors and subsequent signal transduction processes which are involved in neisserial adherence, invasion and transcytosis. This has led to the identification of (i) CD46 as a receptor for pilus which allows adherence to epithelial and endothelial cells, (ii) HSPGs, in cooperation with vitronectin and fibronectin, as receptors for a particular subset of Opa proteins and Opc, which may both mediate invasion into most epithelial and endothelial cells, and (iii) CD66 as the receptors for most Opa variants, potentially being involved in cellular interactions including adherence, invasion and transcytosis with epithelial, endothelial and phagocytic cells. As most of these data have been obtained using transformed cell lines growing in vitro, attempts must be made to translate these basic observations into a more natural situation. It can be expected that the successful ongoing integration of laboratory findings from the various infection models with human volunteer studies will further increase our understanding of the biology of neisserial infection. Perhaps the most difficult but also most rewarding challenge for the future will be to use volunteer studies to identify and understand the role of host factors which are important for the infectious process. Hopefully, insights gained from each of these studies will reveal new and useful strategies for the preventive and/or therapeutic intervention into infection and disease by these fascinating microbes.

Do pathogenic neisseriae need several ways to modify the host cell cytoskeleton?

Neisseria meningitidis and Neisseria gonorrhoeae are human pathogens which have to interact with mucosa and/or cellular barriers for their life cycle. Even though they both give rise to dramatically different diseases, most of the mechanisms mediating cellular interactions are common to N. meningitidis and N. gonorrhoeae. This suggests that bacterial cell interactions may be essential not only for pathogenesis but also for other aspects of the bacterial life cycle that are common to both N. meningitidis and N. gonorrhoeae. Opacity proteins and pili are two major components identified as transducing signals to host cells, thus leading to cytoskeleton modifications. This manuscript will review the recent developments concerning the mechanisms mediating cellular interactions of pathogenic Neisseria and will tentatively put them into the perspective of pathogenesis and bacterial life cycle.

Carcinoembryonic antigen family receptor specificity of Neisseria meningitidis Opa variants influences adherence to and invasion of proinflammatory cytokine-activated endothelial cells

The carcinoembryonic antigen (CEA) family member CEACAM1 (previously called biliary glycoprotein or CD66a) was previously shown to function as a receptor that can mediate the binding of Opa protein-expressing Neisseria meningitidis to both neutrophils and epithelial cells. Since neutrophils and polarized epithelia have both been shown to coexpress multiple CEACAM receptors, we have now extended this work to characterize the binding specificity of meningococcal Opa proteins with other CEA family members. To do so, we used recombinant Escherichia coli expressing nine different Opa variants from three meningococcal strains and stably transfected cell lines expressing single members of the CEACAM family. These infection studies demonstrated that seven of the nine Opa variants bound to at least one CEACAM receptor and that binding to each of these receptors is sufficient to trigger the Opa-dependent bacterial uptake by these cell lines. The other two Opa variants do not appear to bind to either CEACAM receptors or heparan sulfate proteoglycan receptors, which are bound by some gonococcal Opa variants, thus implying a novel class of Opa proteins. We have also extended previous studies by demonstrating induction of CEACAM1 expression after stimulation of human umbilical vein endothelial cells with the proinflammatory cytokine tumor necrosis factor alpha, which is present in high concentrations during meningococcal disease. This induced expression of CEACAM1 leads to an increased Opa-dependent bacterial binding and invasion into the primary endothelia, implying that these interactions may play an important role in the pathogenesis of invasive meningococcal disease.

Isolation of Neisseria gonorrhoeae mutants that show enhanced trafficking across polarized T84 epithelial monolayers

Initiation of a gonococcal infection involves attachment of Neisseria gonorrhoeae to the plasma membrane of an epithelial cell in the mucosal epithelium and its internalization, transepithelial trafficking, and exocytosis from the basal membrane. Piliation and expression of certain Opa proteins and the immunoglobulin A1 protease influence the transcytosis process. We are interested in identifying other genetic determinants of N. gonorrhoeae that play a role in transcellular trafficking. Using polarized T84 monolayers as a model epithelial barrier, we have assayed an N. gonorrhoeae FA1090 minitransposon (mTn) mutant bank for isolates that traverse the monolayer more quickly than the isogenic wild-type (WT) strain. From an initial screen, we isolated four mutants, defining three genetic loci, that traverse monolayers significantly more quickly than their WT parent strain. These mutants adhere to and invade cells normally and do not affect the integrity of the monolayer barrier. Backcrosses of the mutations into the WT FA1090 strain yielded mutants with a similar fast-trafficking phenotype. In two mutants, the mTns had inserted 370 bp apart into the same locus, which we have named fit, for fast intracellular trafficker. Backcrosses of one of these mutants into the MS11A genetic background also yielded a fast-trafficking mutant. The fit locus contains two overlapping open reading frames, fitA and fitB, whose deduced amino acid sequences have predicted molecular weights of 8.6 and 15.3, respectively. Neither protein contains a signal sequence. FitA has a potential helix-turn-helix motif, while the deduced sequence of FitB offers no clues to its function. fitA or fitB homologues are present in the genomes of Pseudomonas syringae and Rhizobium meliloti, but not Neisseria meningitidis. Replication of the MS11A fitA mutant in A431 and T84 cells is significantly accelerated compared to that of the isogenic WT strain. In contrast, growth of this mutant in liquid media is normal. Taken together, these results strongly suggest that traversal of N. gonorrhoeae across an epithelial barrier is linked to intracellular bacterial growth.

Syndecan-1 and syndecan-4 can mediate the invasion of OpaHSPG-expressing Neisseria gonorrhoeae into epithelial cells

Neisseria gonorrhoeae (Ngo) expressing the outer membrane protein OpaHSPG can adhere to and invade epithelial cells via binding to heparan sulphate proteoglycan (HSPG) receptors. In this study, we have investigated the role of syndecan-1 and syndecan-4, two members of the HSPG family, in the uptake of Ngo by epithelial cells. When overexpressed in HeLa cells, both syndecans co-localize with adherent Ngo on the host cell surface. This overexpression of syndecan-1 and syndecan-4 leads to a three- and sevenfold increase in Ngo invasion respectively. In contrast, transfection with the syndecan-1 and syndecan-4 mutant constructs lacking the intracellular domain results in an abrogation of the invasion process, characteristic of a dominant-negative mode of action. A concomitant loss of the capacity to mediate Ngo uptake was also observed with syndecan-4 mutant constructs carrying lesions in the dimerization motif necessary for the binding of protein kinase C (PKC) and phosphatidylinositol 4,5-bisphosphate (PIP2), and mutants that are deficient in a C-terminal EFYA amino acid motif responsible for binding to syntenin or CASK. We conclude that syndecan-1 and syndecan-4 can both mediate Ngo uptake into epithelial cells, and that their intracellular domains play a crucial role in this process, perhaps by mediating signal transduction or anchorage to the cytoskeleton.

Interaction of primary human endometrial cells with Neisseria gonorrhoeae expressing green fluorescent protein

Infection of the endometrium by Neisseria gonorrhoeae is a pivotal stage in the development of pelvic inflammatory disease in women. An ex vivo model of cultures of primary human endometrial cells was developed to study gonococcal-host cell interactions. To facilitate these studies, gonococci were transformed with a hybrid shuttle vector containing the gfp gene from Aequoria victoria, encoding the green fluorescent protein (GFP), to produce intrinsically fluorescent bacteria. The model demonstrated that both pili and Opa proteins were important for both mediating gonococcal interactions with endometrial cells and inducing the secretion of pro-inflammatory cytokines and chemokines. Pil+ gonococci showed high levels of adherence and invasion, regardless of Opa expression, which was associated with increased secretion of IL-8 chemokine and reduced secretion of IL-6 cytokine. Gonococcal challenge also caused increased secretion of TNF-alpha cytokine, but this did not correlate with expression of pili or Opa, suggesting that release of components from non-adherent bacteria may be involved in TNF-alpha induction. Thus, the use of cultured primary endometrial cells, together with gonococci expressing green fluorescent protein, has the potential to extend significantly our knowledge, at the molecular level, of the role of this important human pathogen in the immunobiology of pelvic inflammatory disease.

Critical determinants of host receptor targeting by Neisseria meningitidis and Neisseria gonorrhoeae: identification of Opa adhesiotopes on the N-domain of CD66 molecules

The human pathogens Neisseria meningitidis and Neisseria gonorrhoeae express a family of variable outer membrane opacity-associated (Opa) proteins that recognize multiple human cell surface receptors. Most Opa proteins target the highly conserved N-terminal domain of the CD66 family of adhesion molecules, although a few also interact with heparan sulphate proteoglycans. In this study, we observed that at least two Opa proteins of a N. meningitidis strain C751 have the dual capacity to interact with both receptors. In addition, all three Opa proteins of C751 bind equally well to HeLa cells transfected with cDNA encoding the carcinoembryonic antigen [CEA (CD66e)] subgroup of the CD66 family, but show distinct tropism for CGM1- (CD66d) and NCA (CD66c)-expressing cells. Because the C751 Opa proteins make up distinct structures via the surface-exposed hypervariable domains (HV-1 and HV-2), these combinations appear to be involved in tropism for the distinct CD66 subgroups. To define the determinants of receptor recognition, we used mutant proteins of biliary glycoprotein [BGP (CD66a)] carrying substitutions at several predicted exposed sites in the N-domain and compared their interactions with several Opa proteins of both N. meningitidis and N. gonorrhoeae. The observations applied to the molecular model of the BGP N-domain that we constructed show that the binding of all Opa proteins tested occurs at the non-glycosylated (CFG) face of the molecule and, in general, appears to require Tyr-34 and Ile-91. Further, efficient interaction of distinct Opa proteins depends on different non-adjacent amino acids. In the three-dimensional model, these residues lie in close proximity to Tyr-34 and Ile-91 at the CFG face, making continuous binding domains (adhesiotopes). The epitope of the monoclonal antibody YTH71.3 that inhibits Opa/CD66 interactions was also identified within the Opa adhesiotopes on the N-domain. These studies define the molecular basis that directs the Opa specificity for the CD66 family and the rationale for tropism of the Opa proteins for the CD66 subgroups.

Sulfated polysaccharide-directed recruitment of mammalian host proteins: a novel strategy in microbial pathogenesis

Fundamental to the virulence of microbial pathogens is their capacity for adaptation and survival within variable, and often hostile, environments encountered in the host. We describe a novel, extragenomic mechanism of surface modulation which may amplify the adaptive and pathogenic potential of numerous bacterial species, including Staphylococcus, Yersinia, and pathogenic Neisseria species, as well as Helicobacter pylori and Streptococcus pyogenes. The mechanism involves specific bacterial recruitment of heparin, glycosaminoglycans, or related sulfated polysaccharides, which in turn serve as universal binding sites for a diverse array of mammalian heparin binding proteins, including adhesive glycoproteins (vitronectin and fibronectin), inflammatory (MCP-3, PF-4, and MIP-1alpha) and immunomodulatory (gamma interferon) intermediates, and fibroblast growth factor. This strategy impacts key aspects of microbial pathogenicity as exemplified by increased bacterial invasion of epithelial cells and inhibition of chemokine-induced chemotaxis. Our findings illustrate a previously unrecognized form of parasitism that complements classical virulence strategies encoded within the microbial genome.

Molecular analysis of neisserial Opa protein interactions with the CEA family of receptors: identification of determinants contributing to the differential specificities of binding

The carcinoembryonic antigen (CEA) gene family members, CEACAM1, CEACAM3, CEACAM5 and CEACAM6, are bound by the Opa outer membrane proteins of pathogenic Neisseria spp., whereas CEACAM8 is not. In this study, we demonstrate that the closely related CEACAM4 and CEACAM7, which are also members of the CEA family, are not Opa receptors. We exploited the high conservation between CEACAM6 and CEACAM8 to generate an extensive set of chimeric receptors in order to delineate the sequences necessary for Opa binding. Using a transfection-based infection system, we showed that binding of Opa52 involves residues 27-42, which are predicted to form beta-strand C and short loops adjacent to it, and residues lying between amino acids 60 and 108 in the amino-terminal domain. The replacement of residues 27-29 in CEACAM6 with the CEACAM1 or CEACAM5 sequences generated recombinant CEACAM6 receptors that are bound by CEACAM1/CEACAM5-specific Opa variants. Together, our data demonstrate that Opa proteins bind to residues exposed on the GFCC' face of the N-terminal domain of CEACAM receptors, and identify an amino acid triplet sequence that is responsible for the differential binding of Opa proteins to CEACAM1, CEACAM5 and CEACAM6.

Fibronectin-binding protein acts as Staphylococcus aureus invasin via fibronectin bridging to integrin alpha5beta1

The ability of Staphylococcus aureus to invade mammalian cells may explain its capacity to colonize mucosa and to persist in tissues after bacteraemia. To date, the underlying molecular mechanisms of cellular invasion by S. aureus are unknown, despite its high prevalence and difficulties in treatment. Here, we show cellular invasion as a novel function for an S. aureus adhesin, previously implicated solely in attachment. S. aureus, but not S. epidermidis, invaded epithelial 293 cells in a temperature- and F-actin-dependent manner. Formaldehyde-fixed and live bacteria were equally invasive, suggesting that no active bacterial process was involved. All clinical S. aureus isolates analysed, but only a subset of laboratory strains, were invasive. Fibronectin-binding proteins (FnBPs) acted as S. aureus invasins, because: (i) FnBP deletion mutants of invasive laboratory strains lost invasiveness; (ii) expression of FnBPs in noninvasive strains conferred invasiveness; and (iii) the soluble isolated fibronectin-binding domain of FnBP (D1-D4) completely blocked invasion. Integrin alpha5beta1 served as host cell receptor, which interacted with staphylococcal FnBPs through cellular or soluble fibronectin. FnBP-deficient mutants lost invasiveness for epithelial cells, endothelial cells and fibroblasts. Thus, fibronectin-dependent bridging between S. aureus FnBPs and host cell integrin alpha5beta1 is a conserved mechanism for S. aureus invasion of human cells. This may prove useful in developing new therapeutic and vaccine strategies for S. aureus infections.

Cell adhesion molecules in the pathogenesis of and host defence against microbial infection

Eukaryotic cell adhesion molecules (CAMs) are used by various cells and extracellular molecules in host defence against infection. They are involved in many processes including recognition by circulating phagocytes of a site of inflammation, transmigration through the endothelial barrier, diapedesis through basement membrane and extracellular matrix, and release of effector mechanisms at the infected site. CAMs involved in leucocyte-endothelial cell interaction include the selectins, integrins, and members of the immunoglobulin superfamily. However, CAMs are also used by various microorganisms (protozoa, fungi, bacteria, and viruses) during their pathogenesis. For example, bacteria that utilise CAMs include Mycobacterium tuberculosis, Listeria monocytogenes, Yersinia spp, enteropathogenic Escherichia coli, Shigella spp, Neisseria spp, Bordetella spp, and Borrelia burgdorferi. In addition, CAMs are involved in the pathogenetic effects of the RTX toxins of Pasteurella haemolytica, Actinobacillus actinomycetemcomitans, and the superantigen exotoxins of Staphylococcus aureus and Streptococcus pyogenes. A recurrent and topical theme of potential importance within the bacterial group is the intimate relation between CAMs, bacterial protein receptors, and type III secretion systems. For example, the IpaBCD protein complex is secreted by the type III system of Shigella flexneri and interacts with alpha 5 beta 1 integrin on the eukaryotic cell surface, followed by Rho mediated internalisation; this illustrates the relevance of cellular microbiology. CAMs might prove to be novel therapeutic targets. Comparative genomics has provided the knowledge of shared virulence determinants among diverse bacterial genera, and will continue to deepen our understanding of microbial pathogenesis, particularly in the context of the interaction of prokaryotic and eukaryotic molecules.

Role of fibronectin-binding MSCRAMMs in bacterial adherence and entry into mammalian cells

Most bacterial infections are initiated by the adherence of microorganisms to host tissues. This process involves the interaction of specific bacterial surface structures, called adhesins, with host components. In this review, we discuss a group of microbial adhesins known as Microbial Surface Components Recognizing Adhesive Matrix Molecules (MSCRAMMs) which recognize and bind FN. The interaction of bacteria with FN is believed to contribute significantly to the virulence of a number of microorganisms, including staphylococci and streptococci. Several FN-binding MSCRAMMs of staphylococci and streptococci exhibit a similar structural organization and mechanism of ligand recognition. The ligand-binding domain consists of tandem repeats of a approximately 45 amino acid long unit which bind to the 29-kDa N-terminal region of FN. The binding mechanism is unusual in that the repeat units are unstructured and appear to undergo a conformational change upon ligand binding. Apart from supporting bacterial adherence, FN is also involved in bacterial entry into non-phagocytic mammalian cells. A sandwich model has been proposed in which FN forms a molecular bridge between MSCRAMMs on the bacterial surface and integrins on the host cell. However, the precise mechanism of bacterial invasion and the roles of FN and integrins in this process have yet to be fully elucidated.

Interactions of pathogenic Neisseria with host cells. Is it possible to assemble the puzzle?

Neisseria meningitidis and Neisseria gonorrhoeae are human pathogens that have to interact with mucosa and/or cellular barriers for their life cycles to progress. Even though they both give rise to dramatically different diseases, the use of in vitro models has shown that most of the mechanisms mediating cellular interactions are common to N. meningitidis and N. gonorrhoeae. This suggests that bacterial cell interactions may be essential not only for pathogenesis but also for other aspects of the bacterial life cycle that are common to both N. meningitidis and N. gonorrhoeae. This manuscript will review the most recent developments concerning the mechanisms mediating cellular interaction of pathogenic Neisseria and will then try to put them into the perspective of pathogenesis and bacterial life cycle.

Type IV pili of pathogenic Neisseriae elicit cortical plaque formation in epithelial cells

The pathogenic Neisseriae Neisseria meningitidis and Neisseria gonorrhoeae, initiate colonization by attaching to host cells using type IV pili. Subsequent adhesive interactions are mediated through the binding of other bacterial adhesins, in particular the Opa family of outer membrane proteins. Here, we have shown that pilus-mediated adhesion to host cells by either meningococci or gonococci triggers the rapid, localized formation of dramatic cortical plaques in host epithelial cells. Cortical plaques are enriched in both components of the cortical cytoskeleton and a subset of integral membrane proteins. These include: CD44v3, a heparan sulphate proteoglycan that may serve as an Opa receptor; EGFR, a receptor tyrosine kinase; CD44 and ICAM-1, adhesion molecules known to mediate inflammatory responses; f-actin; and ezrin, a component that tethers membrane components to the actin cytoskeleton. Genetic analyses reveal that cortical plaque formation is highly adhesin specific. Both pilE and pilC null mutants fail to induce cortical plaques, indicating that neisserial type IV pili are required for cortical plaque induction. Mutations in pilT, a gene required for pilus-mediated twitching motility, confer a partial defect in cortical plaque formation. In contrast to type IV pili, many other neisserial surface structures are not involved in cortical plaque induction, including Opa, Opc, glycolipid GgO4-binding adhesins, polysialic acid capsule or a particular lipooligosaccharide variant. Furthermore, it is shown that type IV pili allow gonococci to overcome the inhibitory effect of heparin, a soluble receptor analogue, on gonococcal invasion of Chang and A431 epithelial cells. These and other observations strongly suggest that type IV pili play an active role in initiating neisserial infection of the mucosal surface in vivo. The functions of type IV pili and other neisserial adhesins are discussed in the specific context of the mucosal microenvironment, and a multistep model for neisserial colonization of mucosal epithelia is proposed.

Proteoglycan receptor binding by Neisseria gonorrhoeae MS11 is determined by the HV-1 region of OpaA

The interaction of the OpaA protein of Neisseria gonorrhoeae MS11mk with heparan sulphate-containing proteoglycan receptors on Chang conjunctiva epithelial cells was examined using isolated receptor binding and cell adherence/internalization assays. OpaA deletion proteins, in which the four surface-exposed regions of the protein were deleted individually, and chimeric OpaA/B proteins, in which the surface-exposed regions of the OpaA and OpaB proteins were exchanged, were expressed in N. gonorrhoeae. The recombinant deletion proteins and the chimeric OpaA/B proteins were surface exposed in the outer membrane of N. gonorrhoeae. Isolated receptor-binding assays and Chang cell infection assays with OpaA deletion variants indicated that hypervariable region 1 was essential for the interaction of N. gonorrhoeae with the proteoglycan receptor. Expression of chimeric OpaA/B proteins confirmed the central role of hypervariable region 1 in receptor binding and demonstrated that this domain alone confers the invasive biological phenotype in a non-heparan sulphate proteoglycan-binding Opa protein. The other variable regions of OpaA enhanced receptor binding in the presence of region 1, but did not constitute binding domains on their own. The results indicate that proteoglycan receptor binding results from a hierarchical interaction between the variable domains of the OpaA protein of MS11mk.

Mutagenesis of the Neisseria gonorrhoeae porin reduces invasion in epithelial cells and enhances phagocyte responsiveness

Porin (PorB), the major outer membrane protein of Neisseria gonorrhoeae, has been implicated in pathogenesis previously. However, the fact that porin deletion mutants are not viable has complicated investigations. Here, we describe a method of manipulating the porin gene site-specifically. N. gonorrhoeae MS11, which harbours the porB1B (P.1B) porin allele, was used to generate mutants carrying deletions in the surface loops 1 and 5. An 11-amino-acid deletion in loop 1 impaired Opa50-dependent invasion into human Chang epithelial cells, whereas loop 5 deletion exhibited no apparent phenotype. In a second approach, the complete gonococcal porB1B was replaced by the porBNia gene of Neisseria lactamica. Such mutants were unable to induce efficient uptake by epithelial cells but induced an enhanced respiratory response in HL60 phagocytic cells. The increased respiratory burst was accompanied by an enhanced phagocytic uptake of the mutant compared with the wild-type strain. Our data extend previous evidence for multiple central functions of PorB in the infection process.

Interaction mechanisms of encapsulated meningococci with eucaryotic cells: what does this tell us about the crossing of the blood-brain barrier by Neisseria meningitidis?

An important feature of Neisseria meningitidis is its ability to invade the meninges. This requires that bacteria cross the blood-brain barrier (BBB), which is one of the tightest barriers of the body. N. meningitidis has, therefore, evolved very sophisticated means by which it circumvents the physical properties of this cellular barrier. Recent advances have allowed the identification of several steps that might occur in the interaction of N. meningitidis with the BBB and the transit of the bacteria to the meninges.

Host cell interactions and signalling with Neisseria gonorrhoeae

Neisseria gonorrhoeae is a highly adapted human pathogen that utilises multiple adhesins to interact with a variety of host cell receptors. Recently, substantial progress has been made in unravelling the signalling events induced by N. gonorrhoae that can lead to cytoskeletal reorganisation, invasion or phagocytic uptake, intraphagosomal accommodation, nuclear signalling, cytokine/chemokine release and apoptosis.