Adherence of pilus- Opa+ gonococci to epithelial cells in vitro involves heparan sulfate

Gonococcal PorB: a multifaceted modulator of host immune responses

Neisseria gonorrhoeae is a human-specific pathogen responsible for the sexually transmitted infection, gonorrhoea. N. gonorrhoeae promotes its survival by manipulating both innate and adaptive immune responses. The most abundant gonococcal outer-membrane protein is PorB, an essential porin that facilitates ion exchange. Importantly, gonococcal PorB has several immunomodulatory properties. To subvert the innate immune response, PorB suppresses killing mechanisms of macrophages and neutrophils, and recruits negative regulators of complement to the gonococcal cell surface. For manipulation of adaptive immune responses, gonococcal PorB suppresses the capability of dendritic cells to stimulate proliferation of T cells. As gonococcal PorB is highly abundant in outer-membrane vesicles, consideration of the immunomodulatory properties of this porin is critical when designing gonococcal vaccines.

Harnessing Human Papillomavirus’ Natural Tropism to Target Tumors

Human papillomaviruses (HPV) are small non-enveloped DNA tumor viruses established as the primary etiological agent for the development of cervical cancer. Decades of research have elucidated HPV’s primary attachment factor to be heparan sulfate proteoglycans (HSPG). Importantly, wounding and exposure of the epithelial basement membrane was found to be pivotal for efficient attachment and infection of HPV in vivo. Sulfation patterns on HSPG’s become modified at the site of wounds as they serve an important role promoting tissue healing, cell proliferation and neovascularization and it is these modifications recognized by HPV. Analogous HSPG modification patterns can be found on tumor cells as they too require the aforementioned processes to grow and metastasize. Although targeting tumor associated HSPG is not a novel concept, the use of HPV to target and treat tumors has only been realized in recent years. The work herein describes how decades of basic HPV research has culminated in the rational design of an HPV-based virus-like infrared light activated dye conjugate for the treatment of choroidal melanoma.

PgtE Enzyme of Salmonella enterica Shares the Similar Biological Roles to Plasminogen Activator (Pla) in Interacting With DEC-205 (CD205), and Enhancing Host Dissemination and Infectivity by Yersinia pestis

Yersinia pestis, the cause of plague, is a newly evolved Gram-negative bacterium. Through the acquisition of the plasminogen activator (Pla), Y. pestis gained the means to rapidly disseminate throughout its mammalian hosts. It was suggested that Y. pestis utilizes Pla to interact with the DEC-205 (CD205) receptor on antigen-presenting cells (APCs) to initiate host dissemination and infection. However, the evolutionary origin of Pla has not been fully elucidated. The PgtE enzyme of Salmonella enterica, involved in host dissemination, shows sequence similarity with the Y. pestis Pla. In this study, we demonstrated that both Escherichia coli K-12 and Y. pestis bacteria expressing the PgtE-protein were able to interact with primary alveolar macrophages and DEC-205-transfected CHO cells. The interaction between PgtE-expressing bacteria and DEC-205-expressing transfectants could be inhibited by the application of an anti-DEC-205 antibody. Moreover, PgtE-expressing Y. pestis partially re-gained the ability to promote host dissemination and infection. In conclusion, the DEC-205-PgtE interaction plays a role in promoting the dissemination and infection of Y. pestis, suggesting that Pla and the PgtE of S. enterica might share a common evolutionary origin.

Neisseria gonorrhoeae physiology and pathogenesis

Neisseria gonorrhoeae is an obligate human pathogen that is the cause of the sexually transmitted disease gonorrhoea. Recently, there has been a surge in gonorrhoea cases that has been exacerbated by the rapid rise in gonococcal multidrug resistance to all useful antimicrobials resulting in this organism becoming a significant public health burden. Therefore, there is a clear and present need to understand the organism's biology through its physiology and pathogenesis to help develop new intervention strategies. The gonococcus initially colonises and adheres to host mucosal surfaces utilising a type IV pilus that helps with microcolony formation. Other adhesion strategies include the porin, PorB, and the phase variable outer membrane protein Opa. The gonococcus is able to subvert complement mediated killing and opsonisation by sialylation of its lipooligosaccharide and deploys a series of anti-phagocytic mechanisms. N. gonorrhoeae is a fastidious organism that is able to grow on a limited number of primary carbon sources such as glucose and lactate. The utilization of lactate by the gonococcus has been implicated in a number of pathogenicity mechanisms. The bacterium lives mainly in microaerobic environments and can grow both aerobically and anaerobically with the aid of nitrite. The gonococcus does not produce siderophores for scavenging iron but can utilize some produced by other bacteria, and it is able to successful chelate iron from host haem, transferrin and lactoferrin. The gonococcus is an incredibly versatile human pathogen; in the following chapter, we detail the intricate mechanisms used by the bacterium to invade and survive within the host.

Establishing an invertebrate Galleria mellonella greater wax moth larval model of Neisseria gonorrhoeae infection

Neisseria gonorrhoeae (gonococcus) causes the human sexually transmitted disease gonorrhea. Studying gonococcal pathogenesis and developing new vaccines and therapies to combat the increasing prevalence of multi-antibiotic resistant bacteria has made use of many ex vivo models based on human cells and tissues, and in vivo vertebrate models, for example, rodent, pig and human. The focus of the current study was to examine the utility of the invertebrate greater wax moth Galleria mellonella as an in vivo model of gonococcal infection. We observed that a threshold of ~106 - 107 gonococci/larva was required to kill >50% of larvae (P < 0.05), and increased toxicity correlated with reduced health index scores and pronounced histopathological changes such as increases in the total lesion grade, melanized nodules, hemocyte reaction, and multifocal adipose body degeneration. Larval death was independent of the expression of pilus or Opa protein or LOS sialylation within a single gonococcal species studied, but the model could demonstrate relative toxicity of different isolates. N. meningitidis, N. lacatamica and gonococci all killed larvae equally, but were significantly less toxic (P > 0.05) than Pseudomonas aeruginosa. Larvae primed with nontoxic doses of gonococci were more susceptible to subsequent challenge with homologous and heterologous bacteria, and larval survival was significantly reduced (P < 0.05) in infected larvae after depletion of their hemocytes with clodronate-liposomes. The model was used to test the anti-gonococcal properties of antibiotics and novel antimicrobials. Ceftriaxone (P < 0.05) protected larvae from infection with different gonococcal isolates, but not azithromycin or monocaprin or ligand-coated silver nanoclusters (P > 0.05).

Loss of the virulence plasmid by Shigella sonnei promotes its interactions with CD207 and CD209 receptors

Introduction. Shigella sonnei, the cause of bacillary dysentery, belongs to Gram-negative enteropathogenic bacteria. S. sonnei contains a 210 kb virulence plasmid that encodes an O-antigen gene cluster of LPSs. However, this virulence plasmid is frequently lost during replication. It is well-documented that after losing the O-antigen and becoming rough strains, the Gram-negative bacteria may express an LPS core on its surface. Previous studies have suggested that by using the LPS core, Gram-negative bacteria can interact with several C-type lectin receptors that are expressed on antigen-presenting cells (APCs).Hypothesis/Gap Statement. S. sonnei by losing the virulence plasmid may hijack APCs via the interactions of LPS-CD209/CD207.Aim. This study aimed to investigate if the S. sonnei rough strain, by losing the virulence plasmid, interacted with APCs that express C-type lectins of human CD207, human CD209a and mouse CD209b.Methodology. SDS-PAGE silver staining was used to examine the O-antigen expression of S. sonnei WT and its rough strain. Invasion assays and inhibition assays were used to examine the ability of S. sonnei WT and its rough strain to invade APCs and investigate whether CD209 and CD207 are receptors for phagocytosis of rough S. sonnei. Animal assays were used to observe the dissemination of S. sonnei.Results. S. sonnei did not express O-antigens after losing the virulence plasmid. The S. sonnei rough strain invades with APCs, including human dendritic cells (DCs) and mouse macrophages. CD209 and CD207 are receptors for phagocytosis of rough S. sonnei. Expression of the O-antigen reduces the ability of the S. sonnei rough strain to be disseminated to mesenteric lymph nodes and spleens.Conclusion. This work demonstrated that S. sonnei rough strains - by losing the virulence plasmid - invaded APCs through interactions with CD209 and CD207 receptors.

Anti-Virulence Therapeutic Approaches for Neisseria gonorrhoeae

While antimicrobial resistance (AMR) is seen in both Neisseria gonorrhoeae and Neisseria meningitidis, the former has become resistant to commonly available over-the-counter antibiotic treatments. It is imperative then to develop new therapies that combat current AMR isolates whilst also circumventing the pathways leading to the development of AMR. This review highlights the growing research interest in developing anti-virulence therapies (AVTs) which are directed towards inhibiting virulence factors to prevent infection. By targeting virulence factors that are not essential for gonococcal survival, it is hypothesized that this will impart a smaller selective pressure for the emergence of resistance in the pathogen and in the microbiome, thus avoiding AMR development to the anti-infective. This review summates the current basis of numerous anti-virulence strategies being explored for N. gonorrhoeae.

Proteus mirabilis Targets Atherosclerosis Plaques in Human Coronary Arteries via DC-SIGN (CD209)

Bacterial DNAs are constantly detected in atherosclerotic plaques (APs), suggesting that a combination of chronic infection and inflammation may have roles in AP formation. A series of studies suggested that certain Gram-negative bacteria were able to interact with dendritic cell-specific intercellular adhesion molecule-3-grabbing non-integrin [DC-SIGN; cluster of differentiation (CD) 209] or langerin (CD207), thereby resulting in deposition of CD209s at infection sites. We wondered if Proteus mirabilis (a member of Proteobacteria family) could interact with APs through CD209/CD207. In this study, we first demonstrated that CD209/CD207 were also receptors for P. mirabilis that mediated adherence and phagocytosis by macrophages. P. mirabilis interacted with fresh and CD209s/CD207-expressing APs cut from human coronary arteries, rather than in healthy and smooth arteries. These interactions were inhibited by addition of a ligand-mimic oligosaccharide and the coverage of the ligand, as well as by anti-CD209 antibody. Finally, the hearts from an atherosclerotic mouse model contained higher numbers of P. mirabilis than that of control mice during infection-challenging. We therefore concluded that the P. mirabilis interacts with APs in human coronary arteries via CD209s/CD207. It may be possible to slow down the progress of atherosclerosis by blocking the interactions between CD209s/CD207 and certain atherosclerosis-involved bacteria with ligand-mimic oligosaccharides.

Therapeutic strategies against autophagic escape by pathogenic bacteria

Growing multidrug-resistant (MDR) strains of various infectious bacterial species are hindering research aiming to eliminate such infections. During a bacterial infection, the host response eliminates the pathogen via fusion of the endocytic vesicles with lysosomes, called xenophagy. However, MDR bacteria have evolved strategies to escape xenophagy. In this review, we propose novel therapeutics for overcoming such escape, including chimeric antibiotics, nanoformulations for the induction of autophagy in infected cells, and small interfering (si)RNA-mediated silencing of genes to inhibit the host-pathogen interaction. We also discuss the role of combinations of antibiotics showing synergy, the administrative routes of differentially capped nanoparticles (NPs), and the use of different types of nanoformulations for eliminating pathogenic bacteria from the host.

Folliculin Controls the Intracellular Survival and Trans-Epithelial Passage of Neisseria gonorrhoeae

Neisseria gonorrhoeae, a Gram-negative obligate human pathogenic bacterium, infects human epithelial cells and causes sexually transmitted diseases. Emerging multi-antibiotic resistant gonococci and increasing numbers of infections complicate the treatment of infected patients. Here, we used an shRNA library screen and next-generation sequencing to identify factors involved in epithelial cell infection. Folliculin (FLCN), a 64 kDa protein with a tumor repressor function was identified as a novel host factor important for N. gonorrhoeae survival after uptake. We further determined that FLCN did not affect N. gonorrhoeae adherence and invasion but was essential for its survival in the cells by modulating autophagy. In addition, FLCN was also required to maintain cell to cell contacts in the epithelial layer. In an infection model with polarized cells, FLCN inhibited the polarized localization of E-cadherin and the transcytosis of gonococci across polarized epithelial cells. In conclusion, we demonstrate here the connection between FLCN and bacterial infection and in particular the role of FLCN in the intracellular survival and transcytosis of gonococci across polarized epithelial cell layers.

Invasiveness of the Yersinia pestis ail protein contributes to host dissemination in pneumonic and oral plague

Yersinia pestis, a Gram-negative bacterium, is the etiologic agent of plague. A hallmark of Y. pestis infection is the organism's ability to rapidly disseminate through an animal host. Y. pestis expresses the outer membrane protein, Ail (Attachment invasion locus), which is associated with host invasion and serum resistance. However, whether Ail plays a role in host dissemination remains unclear. In this study, C57BL/6J mice were challenged with a defined Y. pestis strain, KimD27, or an isogenic ail-deleted mutant derived from KimD27 via metacarpal paw pad inoculation, nasal drops, orogastric infection, or tail vein injection to mimic bubonic, pneumonic, oral, or septicemic plague, respectively. Our results showed that ail-deleted Y. pestis KimD27 lost the ability to invade host cells, leading to failed host dissemination in the pneumonic and oral plague models but not in the bubonic or septicemic plague models, which do not require invasiveness. Therefore, this study demonstrated that whether Ail plays a role in Y. pestis pathogenesis depends on the infection route.

Murine SIGNR1 (CD209b) Contributes to the Clearance of Uropathogenic Escherichia coli During Urinary Tract Infections

Uropathogenic Escherichia coli (UPEC), a Gram-negative bacterial pathogen, is a major causative agent of urinary tract infections (UTIs). However, the molecular mechanisms of how UPEC causes infections have not been determined. Recent studies indicated that certain enteric Gram-negative bacteria interact with and hijack innate immune receptors DC-SIGN (CD209a) and SIGNR1 (CD209b), often expressed by antigen-presenting cells (APCs), such as macrophages, leading to dissemination and infection. It was not known whether UPEC could utilize DC-SIGN receptors to promote its infection and dissemination similarly to the enteric pathogens. The results of this study reveal that UPEC interacts with CD209-expressing macrophages and transfectants. This interaction is inhibited by anti-CD209 antibody, indicating that CD209s are receptors for UPEC. Additionally, in contrast to the results of previous studies, mice lacking SIGNR1 are more susceptible to infection of this uropathogen, leading to prolonged bacterial persistence. Overall, the results of our study indicate that the innate immune receptor CD209s participate in the clearance of UPEC during UTIs.

Neisseria gonorrhoeae infects the heterogeneous epithelia of the human cervix using distinct mechanisms

Sexually transmitted infections are a critical public health issue. However, the mechanisms underlying sexually transmitted infections in women and the link between the infection mechanism and the wide range of clinical outcomes remain elusive due to a lack of research models mimicking human infection in vivo. We established a human cervical tissue explant model to mimic local Neisseria gonorrhoeae (GC) infections. We found that GC preferentially colonize the ectocervix by activating integrin-β1, which inhibits epithelial shedding. GC selectively penetrate into the squamocolumnar junction (TZ) and endocervical epithelia by inducing β-catenin phosphorylation, which leads to E-cadherin junction disassembly. Epithelial cells in various cervical regions differentially express carcinoembryonic antigen-related cell adhesion molecules (CEACAMs), the host receptor for GC opacity-associated proteins (Opa_CEA). Relatively high levels were detected on the luminal membrane of ecto/endocervical epithelial cells but very low levels intracellularly in TZ epithelial cells. CEACAM-Opa_CEA interaction increased ecto/endocervical colonization and reduced endocervical penetration by increasing integrin-β1 activation and inhibiting β-catenin phosphorylation respectively, through CEACAM downstream signaling. Thus, the intrinsic properties of cervical epithelial cells and phase-variation of bacterial surface molecules both play a role in controlling GC infection mechanisms and infectivity, preferential colonization or penetration, potentially leading to asymptomatic or symptomatic infection.

Gonorrhea, caused by Neisseria gonorrhoeae (GC), is a common sexually transmitted infection and has become a public health crisis due to a steady increase in antibiotic-resistant cases. How GC infect the female reproductive tract (FRT) and cause various clinical outcomes is still unknown. This study used human cervical explants to examine the mechanism by which GC infect the heterogeneous mucosa of the human cervix, the gate of the FRT. We show that GC preferentially colonize stratified epithelial cells in the vaginal-cervical region by enhancing epithelial cell adherence. GC selectively penetrate into tissues of the squamocolumnar junction and cervical-endometrial regions but not the vaginal-cervical region by loosening epithelial cell-cell junctions. The expression of opacity-associated proteins on GC regulates the infection patterns depending on the expression of the host receptors on various types of epithelia and receptor downstream signaling. These data suggest that both intrinsic properties of cervical epithelial cells and phase-variation of bacterial surface molecules play a role in determining GC infectivity. Our findings can explain how GC infection leads to asymptomatic or symptomatic infection.

Salmonella enterica Serovar Typhimurium Interacts with CD209 Receptors To Promote Host Dissemination and Infection

Salmonella enterica serovar Typhimurium, a Gram-negative bacterium, can cause infectious diseases ranging from gastroenteritis to systemic dissemination and infection. However, the molecular mechanisms underlying this bacterial dissemination have yet to be elucidated. A study indicated that using the lipopolysaccharide (LPS) core as a ligand, S Typhimurium was able to bind human dendritic cell-specific intercellular adhesion molecule-3-grabbing nonintegrin (hCD209a), an HIV receptor that promotes viral dissemination by hijacking antigen-presenting cells (APCs). In this study, we showed that S Typhimurium interacted with CD209s, leading to the invasion of APCs and potentially the dissemination to regional lymph nodes, spleen, and liver in mice. Shielding of the exposed LPS core through the expression of O-antigen reduces dissemination and infection. Thus, we propose that similar to HIV, S Typhimurium may also utilize APCs via interactions with CD209s as a way to disseminate to the lymph nodes, spleen, and liver to initiate host infection.

Glycointeractome of Neisseria gonorrhoeae: Identification of Host Glycans Targeted by the Gonococcus To Facilitate Adherence to Cervical and Urethral Epithelial Cells

Multidrug-resistant strains of Neisseria gonorrhoeae are emerging worldwide, and novel treatment and prevention strategies are needed. Glycans are ubiquitously expressed by all human cells and can be specifically targeted by pathogens to facilitate association with host cells. Here we identify and characterize the N. gonorrhoeae host-glycan binding profile (glycointeractome), which revealed numerous interactions, including high-affinity binding to mannosyl glycans. We identify gonococcal potential mannose-binding proteins and show that N. gonorrhoeae uses mannosyl glycans expressed on the surface of cervical and urethral epithelia to facilitate adherence. Furthermore, a mannose-binding lectin or a mannoside compound was able to reduce this adherence. By characterizing the glycointeractome of N. gonorrhoeae, we were able to elucidate a novel mechanism used by this important pathogen to interact with human cells, and this interaction could be exploited to develop novel therapeutics to treat antibiotic-resistant gonorrhea.

Neisseria gonorrhoeae is a significant threat to global health for which a vaccine and novel treatment options are urgently needed. Glycans expressed by human cells are commonly targeted by pathogens to facilitate interactions with the host, and thus characterization of these interactions can aid identification of bacterial receptors that can be exploited as vaccine and/or drug targets. Using glycan array analysis, we identified 247 specific interactions between N. gonorrhoeae and glycans representative of those found on human cells. Interactions included those with mannosylated, fucosylated, and sialylated glycans, glycosaminoglycans (GAGs), and glycans terminating with galactose (Gal), N-acetylgalactosamine (GalNAc), and N-acetylglucosamine (GlcNAc). By investigating the kinetics of interactions with selected glycans, we demonstrate that whole-cell N. gonorrhoeae has a high affinity for mannosylated glycans (dissociation constant [K_D], 0.14 to 0.59 μM), which are expressed on the surface of cervical and urethral epithelial cells. Using chromatography coupled with mass spectrometric (MS) analysis, we identified potential mannose-binding proteins in N. gonorrhoeae. Pretreatment of cells with mannose-specific lectin (concanavalin A) or free mannose competitor (α-methyl-d-mannopyranoside) substantially reduced gonococcal adherence to epithelial cells. This suggests that N. gonorrhoeae targets mannosyl glycans to facilitate adherence to host cells and that mannosides or similar compounds have the potential to be used as a novel treatment option for N. gonorrhoeae.

Yersinia pestis Interacts With SIGNR1 (CD209b) for Promoting Host Dissemination and Infection

Yersinia pestis, a Gram-negative bacterium and the etiologic agent of plague, has evolved from Yersinia pseudotuberculosis, a cause of a mild enteric disease. However, the molecular and biological mechanisms of how Y. pseudotuberculosis evolved to such a remarkably virulent pathogen, Y. pestis, are not clear. The ability to initiate a rapid bacterial dissemination is a characteristic hallmark of Y. pestis infection. A distinguishing characteristic between the two Yersinia species is that Y. pseudotuberculosis strains possess an O-antigen of lipopolysaccharide (LPS) while Y. pestis has lost the O-antigen during evolution and therefore exposes its core LPS. In this study, we showed that Y. pestis utilizes its core LPS to interact with SIGNR1 (CD209b), a C-type lectin receptor on antigen presenting cells (APCs), leading to bacterial dissemination to lymph nodes, spleen and liver, and the initiation of a systemic infection. We therefore propose that the loss of O-antigen represents a critical step in the evolution of Y. pseudotuberculosis into Y. pestis in terms of hijacking APCs, promoting bacterial dissemination and causing the plague.

Neisseria gonorrhoeae evades autophagic killing by downregulating CD46-cyt1 and remodeling lysosomes

The Gram-negative human pathogen N. gonorrhoeae (Ngo) quickly attaches to epithelial cells, and large numbers of the bacteria remain on the cell surface for prolonged periods. Ngo invades cells but few viable intracellular bacteria are recovered until later stages of infection, leading to the assumption that Ngo is a weak invader. On the cell surface, Ngo quickly recruits CD46-cyt1 to the epithelial cell cortex directly beneath the bacteria and causes its cleavage by metalloproteinases and Presenilin/γSecretease; how these interactions affect the Ngo lifecycle is unknown. Here, we show Ngo induces an autophagic response in the epithelial cell through CD46-cyt1/GOPC, and this response kills early invaders. Throughout infection, the pathogen slowly downregulates CD46-cyt1 and remodeling of lysosomes, another key autophagy component, and these activities ultimately promote intracellular survival. We present a model on the dynamics of Ngo infection and describe how this dual interference with the autophagic pathway allows late invaders to survive within the cell.

Yersinia pseudotuberculosis Exploits CD209 Receptors for Promoting Host Dissemination and Infection

Yersinia pseudotuberculosis is a Gram-negative enteropathogen and causes gastrointestinal infections. It disseminates from gut to mesenteric lymph nodes (MLNs), spleen, and liver of infected humans and animals. Although the molecular mechanisms for dissemination and infection are unclear, many Gram-negative enteropathogens presumably invade the small intestine via Peyer's patches to initiate dissemination. In this study, we demonstrate that Y. pseudotuberculosis utilizes its lipopolysaccharide (LPS) core to interact with CD209 receptors, leading to invasion of human dendritic cells (DCs) and murine macrophages. These Y. pseudotuberculosis-CD209 interactions result in bacterial dissemination to MLNs, spleens, and livers of both wild-type and Peyer's patch-deficient mice. The blocking of the Y. pseudotuberculosis-CD209 interactions by expression of O-antigen and with oligosaccharides reduces infectivity. Based on the well-documented studies in which HIV-CD209 interaction leads to viral dissemination, we therefore propose an infection route for Y. pseudotuberculosis where this pathogen, after penetrating the intestinal mucosal membrane, hijacks the Y. pseudotuberculosis-CD209 interaction antigen-presenting cells to reach their target destinations, MLNs, spleens, and livers.

Neisseria gonorrhoeae infects the human endocervix by activating non-muscle myosin II-mediated epithelial exfoliation

Colonization and disruption of the epithelium is a major infection mechanism of mucosal pathogens. The epithelium counteracts infection by exfoliating damaged cells while maintaining the mucosal barrier function. The sexually transmitted bacterium Neisseria gonorrhoeae (GC) infects the female reproductive tract primarily from the endocervix, causing gonorrhea. However, the mechanism by which GC overcome the mucosal barrier remains elusive. Using a new human tissue model, we demonstrate that GC can penetrate into the human endocervix by inducing the exfoliation of columnar epithelial cells. We found that GC colonization causes endocervical epithelial cells to shed. The shedding results from the disassembly of the apical junctions that seal the epithelial barrier. Apical junction disruption and epithelial exfoliation increase GC penetration into the endocervical epithelium without reducing bacterial adherence to and invasion into epithelial cells. Both epithelial exfoliation and junction disruption require the activation and accumulation of non-muscle myosin II (NMII) at the apical surface and GC adherent sites. GC inoculation activates NMII by elevating the levels of the cytoplasmic Ca²⁺ and NMII regulatory light chain phosphorylation. Piliation of GC promotes, but the expression of a GC opacity-associated protein variant, OpaH that binds to the host surface proteins CEACAMs, inhibits GC-induced NMII activation and reorganization and Ca²⁺ flux. The inhibitory effects of OpaH lead to reductions in junction disruption, epithelial exfoliation, and GC penetration. Therefore, GC phase variation can modulate infection in the human endocervix by manipulating the activity of NMII and epithelial exfoliation.

Neisseria gonorrhoeae (GC) infects human genital epithelium causing gonorrhea, a common sexually transmitted infection. Gonorrhea is a critical public health issue due to increased prevalence of antibiotic-resistant strains. Because humans are the only host for GC, a lack of a human infection model has been a major obstacle to our understanding of GC infection. Here we use a human tissue explant model to examine the mechanism by which GC infect the human endocervix, the primary site for GC infection in women. We show that GC penetrate into the human endocervix by activating the actin motor myosin and epithelial shedding. Myosin activation causes the disruption of the endocervical epithelial barrier by inducing apical junction disassembly and epithelial cell shedding, allowing GC penetration into the human endocervical tissue. GC activate myosin by inducing Ca²⁺-dependent phosphorylation of myosin light chain. We further show that GC can enhance and reduce the penetration by expressing pili and the opacity-associated protein that promotes and inhibits myosin activation, respectively. Our study is the first demonstration of GC penetration into the human endocervix. Our results provide new insights into the mechanism by which GC manipulate signaling and cytoskeletal apparatus in epithelial cells to achieve penetrating and non-penetrating infection.

Cellulose nanocrystals are effective in inhibiting host cell bacterial adhesion

The use of cellulose nanocrystals (CNCs) as a biomaterial able to inhibit host cell bacterial adhesion is described. Pre-incubation ofE. coliwith a suspension of CNCs affords a significant reduction of bacterial adhesion to intestinal cell monolayer HT29, without exerting a bactericidal effect.

Different Use of Cell Surface Glycosaminoglycans As Adherence Receptors to Corneal Cells by Gram Positive and Gram Negative Pathogens

The epithelium of the cornea is continuously exposed to pathogens, and adhesion to epithelial cells is regarded as an essential first step in bacterial pathogenesis. In this article, the involvement of glycosaminoglycans in the adhesion of various pathogenic bacteria to corneal epithelial cells is analyzed. All microorganisms use glycosaminoglycans as receptors, but arranged in different patterns depending on the Gram-type of the bacterium. The heparan sulfate chains of syndecans are the main receptors, though other molecular species also seem to be involved, particularly in Gram-negative bacteria. Adherence is inhibited differentially by peptides, including heparin binding sequences, indicating the participation of various groups of Gram-positive, and -negative adhesins. The length of the saccharides produces a major effect, and low molecular weight chains inhibit the binding of Gram-negative microorganisms but increase the adherence of Gram-positives. Pathogen adhesion appears to occur preferentially through sulfated domains, and is very dependent on N- and 6-O-sulfation of the glucosamine residue and, to a lesser extent, 2-O sulfation of uronic acid. These data show the differential use of corneal receptors, which could facilitate the development of new anti-infective strategies.

Neisserial Heparin Binding Antigen (NHBA) Contributes to the Adhesion of Neisseria meningitidis to Human Epithelial Cells

Neisserial Heparin Binding Antigen (NHBA) is a surface-exposed lipoprotein ubiquitously expressed by Neisseria meningitidis strains and an antigen of the Bexsero^® vaccine. NHBA binds heparin through a conserved Arg-rich region that is the target of two proteases, the meningococcal NalP and human lactoferrin (hLf). In this work, in vitro studies showed that recombinant NHBA protein was able to bind epithelial cells and mutations of the Arg-rich tract abrogated this binding. All N-terminal and C-terminal fragments generated by NalP or hLf cleavage, regardless of the presence or absence of the Arg-rich region, did not bind to cells, indicating that a correct positioning of the Arg-rich region within the full length protein is crucial. Moreover, binding was abolished when cells were treated with heparinase III, suggesting that this interaction is mediated by heparan sulfate proteoglycans (HSPGs). N. meningitidis nhba knockout strains showed a significant reduction in adhesion to epithelial cells with respect to isogenic wild-type strains and adhesion of the wild-type strain was inhibited by anti-NHBA antibodies in a dose-dependent manner. Overall, the results demonstrate that NHBA contributes to meningococcal adhesion to epithelial cells through binding to HSPGs and suggest a possible role of anti-Bexsero^® antibodies in the prevention of colonization.

Gonorrhea - an evolving disease of the new millennium

Etiology, transmission and protection: Neisseria gonorrhoeae (the gonococcus) is the etiological agent for the strictly human sexually transmitted disease gonorrhea. Infections lead to limited immunity, therefore individuals can become repeatedly infected. Pathology/symptomatology: Gonorrhea is generally a non-complicated mucosal infection with a pustular discharge. More severe sequellae include salpingitis and pelvic inflammatory disease which may lead to sterility and/or ectopic pregnancy. Occasionally, the organism can disseminate as a bloodstream infection. Epidemiology, incidence and prevalence: Gonorrhea is a global disease infecting approximately 60 million people annually. In the United States there are approximately 300, 000 cases each year, with an incidence of approximately 100 cases per 100,000 population. Treatment and curability: Gonorrhea is susceptible to an array of antibiotics. Antibiotic resistance is becoming a major problem and there are fears that the gonococcus will become the next "superbug" as the antibiotic arsenal diminishes. Currently, third generation extended-spectrum cephalosporins are being prescribed. Molecular mechanisms of infection: Gonococci elaborate numerous strategies to thwart the immune system. The organism engages in extensive phase (on/off switching) and antigenic variation of several surface antigens. The organism expresses IgA protease which cleaves mucosal antibody. The organism can become serum resistant due to its ability to sialylate lipooligosaccharide in conjunction with its ability to subvert complement activation. The gonococcus can survive within neutrophils as well as in several other lymphocytic cells. The organism manipulates the immune response such that no immune memory is generated which leads to a lack of protective immunity.

Positive Selection Pressure Drives Variation on the Surface-Exposed Variable Proteins of the Pathogenic Neisseria

Pathogenic species of Neisseria utilize variable outer membrane proteins to facilitate infection and proliferation within the human host. However, the mechanisms behind the evolution of these variable alleles remain largely unknown due to analysis of previously limited datasets. In this study, we have expanded upon the previous analyses to substantially increase the number of analyzed sequences by including multiple diverse strains, from various geographic locations, to determine whether positive selective pressure is exerted on the evolution of these variable genes. Although Neisseria are naturally competent, this analysis indicates that only intrastrain horizontal gene transfer among the pathogenic Neisseria principally account for these genes exhibiting linkage equilibrium which drives the polymorphisms evidenced within these alleles. As the majority of polymorphisms occur across species, the divergence of these variable genes is dependent upon the species and is independent of geographical location, disease severity, or serogroup. Tests of neutrality were able to detect strong selection pressures acting upon both the opa and pil gene families, and were able to locate the majority of these sites within the exposed variable regions of the encoded proteins. Evidence of positive selection acting upon the hypervariable domains of Opa contradicts previous beliefs and provides evidence for selection of receptor binding. As the pathogenic Neisseria reside exclusively within the human host, the strong selection pressures acting upon both the opa and pil gene families provide support for host immune system pressure driving sequence polymorphisms within these variable genes.

Neisserial Opa Protein-CEACAM Interactions: Competition for Receptors as a Means of Bacterial Invasion and Pathogenesis

Carcino-embryonic antigen-like cellular adhesion molecules (CEACAMs), members of the immunoglobulin superfamily, are responsible for cell-cell interactions and cellular signaling events. Extracellular interactions with CEACAMs have the potential to induce phagocytosis, as is the case with pathogenic Neisseria bacteria. Pathogenic Neisseria species express opacity-associated (Opa) proteins, which interact with a subset of CEACAMs on human cells, and initiate the engulfment of the bacterium. We demonstrate that recombinant Opa proteins reconstituted into liposomes retain the ability to recognize and interact with CEACAMs in vitro but do not maintain receptor specificity compared to that of Opa proteins natively expressed by Neisseria gonorrhoeae. We report that two Opa proteins interact with CEACAMs with nanomolar affinity, and we hypothesize that this high affinity is necessary to compete with the native CEACAM homo- and heterotypic interactions in the host. Understanding the mechanisms of Opa protein-receptor recognition and engulfment enhances our understanding of Neisserial pathogenesis. Additionally, these mechanisms provide insight into how human cells that are typically nonphagocytic can utilize CEACAM receptors to internalize exogenous matter, with implications for the targeted delivery of therapeutics and development of imaging agents.

Opacity proteins of neisseria gonorrhoeae in lipooligosaccharide mutants lost ability to interact with neutrophil-restricted CEACAM3 (CD66d)

Lipooligosacharide (LOS) of Neisseria gonorrhoeae (gonococci, GC) is involved in the interaction of GC with host cells. Deletion of the alpha-oligosaccharide (alpha-OS) moiety of LOS (lgtF mutant) significantly impairs invasion of GC into epithelial cell lines. GC opacity (Opa) proteins, such as OpaI, mediate phagocytosis and stimulate chemiluminescence responses in neutrophils in part through interaction with members of the carcinoembryonic antigen (CEA) family, which includes CEACAM3 (CD66d), a human neutrophil specific receptor for phagocytosis of bacteria. In the present work, we examined the effects of OpaI-expressing lgtF mutant on phagocytosis by HeLa-CEACAM3 cells and chemiluminescence responses in neutrophils. The results showed that lgtF mutant even expressing OpaI completely lost the ability to promote either phagocytosis mediated by CEACAM3 interaction in HeLa cells or chemiluminescence responses in neutrophils. These data indicated that Opa proteins in the lgtF mutant, which might result from the conformational change, cannot be functional.

Host Langerin (CD207) is a receptor for Yersinia pestis phagocytosis and promotes dissemination

Yersinia pestis is a Gram-negative bacterium that causes plague. After Y. pestis overcomes the skin barrier, it encounters antigen-presenting cells (APCs), such as Langerhans and dendritic cells. They transport the bacteria from the skin to the lymph nodes. However, the molecular mechanisms involved in bacterial transmission are unclear. Langerhans cells (LCs) express Langerin (CD207), a calcium-dependent (C-type) lectin. Furthermore, Y. pestis possesses exposed core oligosaccharides. In this study, we show that Y. pestis invades LCs and Langerin-expressing transfectants. However, when the bacterial core oligosaccharides are shielded or truncated, Y. pestis propensity to invade Langerhans and Langerin-expressing cells decreases. Moreover, the interaction of Y. pestis with Langerin-expressing transfectants is inhibited by purified Langerin, a DC-SIGN (DC-specific intercellular adhesion molecule 3 grabbing nonintegrin)-like molecule, an anti-CD207 antibody, purified core oligosaccharides and several oligosaccharides. Furthermore, covering core oligosaccharides reduces the mortality associated with murine infection by adversely affecting the transmission of Y. pestis to lymph nodes. These results demonstrate that direct interaction of core oligosaccharides with Langerin facilitates the invasion of LCs by Y. pestis. Therefore, Langerin-mediated binding of Y. pestis to APCs may promote its dissemination and infection.

Selection for a CEACAM receptor-specific binding phenotype during Neisseria gonorrhoeae infection of the human genital tract

Infections by Neisseria gonorrhoeae are increasingly common, are often caused by antibiotic-resistant strains, and can result in serious and lasting sequelae, prompting the reemergence of gonococcal disease as a leading global health concern. N. gonorrhoeae is a human-restricted pathogen that primarily colonizes urogenital mucosal surfaces. Disease progression varies greatly between the sexes: men usually present with symptomatic infection characterized by a painful purulent urethral discharge, while in women, the infection is often asymptomatic, with the most severe pathology occurring when the bacteria ascend from the lower genital tract into the uterus and fallopian tubes. Classical clinical studies demonstrated that clinically infectious strains uniformly express Opa adhesins; however, their specificities were unknown at the time. While in vitro studies have since identified CEACAM proteins as the primary target of Opa proteins, the gonococcal specificity for this human family of receptors has not been addressed in the context of natural infection. In this study, we characterize a collection of low-passage-number clinical-specimen-derived N. gonorrhoeae isolates for Opa expression and assess their CEACAM-binding profiles. We report marked in vivo selection for expression of phase-variable Opa proteins that bind CEACAM1 and CEACAM5 but selection against expression of Opa variants that bind to the neutrophil-restricted decoy receptor CEACAM3. This is the first study showing phenotypic selection for distinct CEACAM-binding phenotypes in vivo, and it supports the opposing functions of CEACAMs that facilitate infection versus driving inflammation within the genital tract.

Surface glycosaminoglycans mediate adherence between HeLa cells and Lactobacillus salivarius Lv72

Background

The adhesion of lactobacilli to the vaginal surface is of paramount importance to develop their probiotic functions. For this reason, the role of HeLa cell surface proteoglycans in the attachment of Lactobacillus salivarius Lv72, a mutualistic strain of vaginal origin, was investigated.

Results

Incubation of cultures with a variety of glycosaminoglycans (chondroitin sulfate A and C, heparin and heparan sulfate) resulted in marked binding interference. However, no single glycosaminoglycan was able to completely abolish cell binding, the sum of all having an additive effect that suggests cooperation between them and recognition of specific adhesins on the bacterial surface. In contrast, chondroitin sulfate B enhanced cell to cell attachment, showing the relevance of the stereochemistry of the uronic acid and the sulfation pattern on binding. Elimination of the HeLa surface glycosaminoglycans with lyases also resulted in severe adherence impairment. Advantage was taken of the Lactobacillus-glycosaminoglycans interaction to identify an adhesin from the bacterial surface. This protein, identify as a soluble binding protein of an ABC transporter system (OppA) by MALDI-TOF/(MS), was overproduced in Escherichia coli, purified and shown to interfere with L. salivarius Lv72 adhesion to HeLa cells.

Conclusions

These data suggest that glycosaminoglycans play a fundamental role in attachment of mutualistic bacteria to the epithelium that lines the cavities where the normal microbiota thrives, OppA being a bacterial adhesin involved in the process.

The biology of Neisseria adhesins

Members of the genus Neisseria include pathogens causing important human diseases such as meningitis, septicaemia, gonorrhoea and pelvic inflammatory disease syndrome. Neisseriae are found on the exposed epithelia of the upper respiratory tract and the urogenital tract. Colonisation of these exposed epithelia is dependent on a repertoire of diverse bacterial molecules, extending not only from the surface of the bacteria but also found within the outer membrane. During invasive disease, pathogenic Neisseriae also interact with immune effector cells, vascular endothelia and the meninges. Neisseria adhesion involves the interplay of these multiple surface factors and in this review we discuss the structure and function of these important molecules and the nature of the host cell receptors and mechanisms involved in their recognition. We also describe the current status for recently identified Neisseria adhesins. Understanding the biology of Neisseria adhesins has an impact not only on the development of new vaccines but also in revealing fundamental knowledge about human biology.

Pilus phase variation switches gonococcal adherence to invasion by caveolin-1-dependent host cell signaling

Many pathogenic bacteria cause local infections but occasionally invade into the blood stream, often with fatal outcome. Very little is known about the mechanism underlying the switch from local to invasive infection. In the case of Neisseria gonorrhoeae, phase variable type 4 pili (T4P) stabilize local infection by mediating microcolony formation and inducing anti-invasive signals. Outer membrane porin PorB(IA), in contrast, is associated with disseminated infection and facilitates the efficient invasion of gonococci into host cells. Here we demonstrate that loss of pili by natural pilus phase variation is a prerequisite for the transition from local to invasive infection. Unexpectedly, both T4P-mediated inhibition of invasion and PorB(IA)-triggered invasion utilize membrane rafts and signaling pathways that depend on caveolin-1-Y14 phosphorylation (Cav1-pY14). We identified p85 regulatory subunit of PI3 kinase (PI3K) and phospholipase Cγ1 as new, exclusive and essential interaction partners for Cav1-pY14 in the course of PorBIA-induced invasion. Active PI3K induces the uptake of gonococci via a new invasion pathway involving protein kinase D1. Our data describe a novel route of bacterial entry into epithelial cells and offer the first mechanistic insight into the switch from local to invasive gonococcal infection.

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.

Association of Neisseria gonorrhoeae Opa(CEA) with dendritic cells suppresses their ability to elicit an HIV-1-specific T cell memory response

Infection with Neisseria gonorrhoeae (N. gonorrhoeae) can trigger an intense local inflammatory response at the site of infection, yet there is little specific immune response or development of immune memory. Gonococcal surface epitopes are known to undergo antigenic variation; however, this is unlikely to explain the weak immune response to infection since individuals can be re-infected by the same serotype. Previous studies have demonstrated that the colony opacity-associated (Opa) proteins on the N. gonorrhoeae surface can bind human carcinoembryonic antigen-related cellular adhesion molecule 1 (CEACAM1) on CD4⁺ T cells to suppress T cell activation and proliferation. Interesting in this regard, N. gonorrhoeae infection is associated with impaired HIV-1 (human immunodeficiency virus type 1)-specific cytotoxic T-lymphocyte (CTL) responses and with transient increases in plasma viremia in HIV-1-infected patients, suggesting that N. gonorrhoeae may also subvert immune responses to co-pathogens. Since dendritic cells (DCs) are professional antigen presenting cells (APCs) that play a key role in the induction of an adaptive immune response, we investigated the effects of N. gonorrhoeae Opa proteins on human DC activation and function. While morphological changes reminiscent of DC maturation were evident upon N. gonorrhoeae infection, we observed a marked downregulation of DC maturation marker CD83 when the gonococci expressing CEACAM1-specific Opa(CEA), but not other Opa variants. Consistent with a gonococcal-induced defect in maturation, Opa(CEA) binding to CEACAM1 reduced the DCs' capacity to stimulate an allogeneic T cell proliferative response. Moreover, Opa(CEA)-expressing N. gonorrhoeae showed the potential to impair DC-dependent development of specific adaptive immunity, since infection with Opa(CEA)-positive gonococci suppressed the ability of DCs to stimulate HIV-1-specific memory CTL responses. These results reveal a novel mechanism to explain why infection of N. gonorrhoeae fails to trigger an effective specific immune response or develop immune memory, and may affect the potent synergy between gonorrhea and HIV-1 infection.

Bacterial infections and ceramide

Ceramide is released from sphingomyelin primarily by the activity of acid, neutral, or alkaline sphingomyelinases or is synthesized de novo. Several bacteria, viruses, and even parasites infect mammalian cells by exploiting the acid sphingomyelinase or the neutral sphingomyelinase-ceramide system, or both. Sphingomyelinases and ceramide have been shown to be crucially involved in the internalization of pathogens, the induction of apoptosis in infected cells, the intracellular activation of signaling pathways, and the release of cytokines. The diverse functions of ceramide in infections suggest that the sphingomyelinase-ceramide system is a key player in the host response to many pathogens.

Bacterial invasion factors: tools for crossing biological barriers and drug delivery?

The oral route is the preferential route of drug delivery in humans. However, effective delivery through the gastrointestinal tract is often hampered by the low permeability of the intestinal epithelium. One possibility to overcome this problem is the encapsulation of drugs inside nanoparticulate systems, containing targeting moieties with cell invasive properties. The bioinvasive features of the delivery system could be provided by the attachment of bacterial invasion factors, which promote efficient uptake into host cells and mediate rapid transcytosis of the pathogen through the intestinal epithelium. This review gives an overview of bacterial invasion systems. The molecular structure and function of suitable bacterial invasins, their relative values as targeting agents and possible pitfalls of their use are described. The potential of bioinvasive drug delivery systems is mainly presented on the basis of the well-characterized Yersinia invasin protein, which enters M cells to gain access to subepithelial layers of the gastrointestinal tract, but alternative approaches and future prospects for oral drug delivery are also discussed.

CEA-Related CAMs

The carcinoembryonic antigen (CEA) family comprises a large number of cellular surface molecules, the CEA-related cell adhesion molecules (CEACAMs), which belong to the Ig superfamily. CEACAMs exhibit a complex expression pattern in normal and malignant tissues. The majority of the CEACAMs are cellular adhesion molecules that are involved in a great variety of distinct cellular processes, for example in the integration of cellular responses through homo- and heterophilic adhesion and interaction with a broad selection of signal regulatory proteins, i.e., integrins or cytoskeletal components and tyrosine kinases. Moreover, expression of CEACAMs affects tumor growth, angiogenesis, cellular differentiation, immune responses, and they serve as receptors for commensal and pathogenic microbes. Recently, new insights into CEACAM structure and function became available, providing further elucidation of their kaleidoscopic functions.

Meningococcal ligands and molecular targets of the host

Meningococcal mechanisms of adhesion are complex, involving multiple adhesins and their respective target receptors on host cells. Three major surface structures--pili, Opa, and Opc--have been known for some time to mediate meningococcal adhesion to target human cells. More recently, several other relatively minor adhesins have also come to light. The literature on bacterial adhesion mechanisms provides numerous examples of various adhesins acting cooperatively in an apparently hierarchical and sequential manner; in other instances, adhesins may act in concert leading to high avidity interactions, often a prelude to cellular invasion and tissue penetration. Such examples are also present in the case of meningococci, although our knowledge of adhesin cooperation and synergy is far from complete. Meningococcal mechanisms used to target the host, which are often specific for the host or a tissue within the host, include both lectin-like interactions and protein-protein interactions; the latter tend to determine specificity in general. Understanding (a) what determines specificity (i.e. molecular features of adhesins and receptors), (b) encourages cellular penetration (i.e. adhesin pairs, which act in concert or synergistically to deliver effective signals for invasion and induce other cellular responses), (c) level of redundancy (more than one mechanisms of targeting host receptors), (d) host situations that encourage tissue penetration (inflammatory situations during which circulating cytokines upregulate target cell receptors, effectively encouraging greater adhesion/invasion), and (e) down-stream effects on host functions in general are all clearly important in our future strategies of controlling meningococcal pathogenesis.

Neisseria gonorrhoeae-induced transactivation of EGFR enhances gonococcal invasion

Neisseria gonorrhoeae, the causative agent of the sexually transmitted infection gonorrhoea, adheres to and invades into genital epithelial cells. Here, we investigate host components that are used by the bacteria for their entry into epithelial cells. We found that gonococcal microcolony formation on the surface of HEC-1-B cells disrupted the polarized, basolateral distribution of both epidermal growth factor receptor (EGFR) and ErbB2, a related family member, and induced their accumulation under the microcolonies at the apical membrane. Gonococcal infection increased EGFR and ErbB2 phosphorylation. The EGFR kinase inhibitor, AG1478, reduced gonococcal invasion by 80%, but had no effect on adherence or the recruitment of EGFR and ErbB2 to the microcolonies. Gonococcal inoculation upregulated the mRNA levels of several ligands of EGFR. Prevention of EGFR ligand shedding by blocking matrix metalloproteinase activation reduced gonococcal invasion without altering their adherence, while the addition of the EGFR ligand, HB-EGF, was able to restore invasion to 66% of control levels. These data indicate that N. gonorrhoeae modulates the activity and cellular distribution of host EGFR, facilitating their invasion. EGFR activation does not appear to be due to direct gonococcal binding to EGFR, but instead by its transactivation by gonococcal induced increases in EGFR ligands.

Neisseria meningitidis has two independent modes of recognizing its human receptor CEACAM1

BACKGROUND

Several human-restricted gram-negative bacteria exploit carcinoembryonic antigen-related cell adhesion molecules (CEACAMs) for host colonization. For example, Neisseria meningitidis engages these human receptors via outer membrane proteins of the colony opacity-associated (Opa) protein family triggering internalization into non-phagocytic cells.

PRINCIPAL FINDINGS

We report that a non-opaque strain of N. meningitidis selectively interacts with CEACAM1, but not other CEACAM family members. Using functional assays of bacterial adhesion and internalisation, microscopic analysis, and a panel of CEACAM1 deletion mutants we demonstrate that the engagement of CEACAM1 by non-opaque meningococci occurs in a manner distinct from Opa protein-mediated association. In particular, the amino-terminal domain of CEACAM1 is necessary, but not sufficient for Opa protein-independent binding, which requires multiple extracellular domains of the human receptor in a cellular context. Knock-down of CEACAM1 interferes with binding to lung epithelial cells, whereas chemical or pharmacological disruption of host protein glycosylation does not abrogate CEACAM1 recognition by non-opaque meningococci. The previously characterized meningococcal invasins NadA or Opc do not operate in a CEACAM1-dependent manner.

CONCLUSIONS

The results demonstrate a mechanistically distinct, Opa protein-independent interaction between N. meningitidis and human CEACAM1. Our functional investigations suggest the presence of a second CEACAM1-binding invasin on the meningococcal surface that associates with the protein backbone and not the carbohydrate structures of CEACAM1. The redundancy in meningococcal CEACAM1-binding factors further highlights the important role of CEACAM recognition in the biology of this human-adapted pathogen.

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.

Human-restricted bacterial pathogens block shedding of epithelial cells by stimulating integrin activation

Colonization of mucosal surfaces is the key initial step in most bacterial infections. One mechanism protecting the mucosa is the rapid shedding of epithelial cells, also termed exfoliation, but it is unclear how pathogens counteract this process. We found that carcinoembryonic antigen (CEA)-binding bacteria colonized the urogenital tract of CEA transgenic mice, but not of wild-type mice, by suppressing exfoliation of mucosal cells. CEA binding triggered de novo expression of the transforming growth factor receptor CD105, changing focal adhesion composition and activating beta1 integrins. This manipulation of integrin inside-out signaling promotes efficient mucosal colonization and represents a potential target to prevent or cure bacterial infections.

Lp95, a novel leptospiral protein that binds extracellular matrix components and activates e-selectin on endothelial cells

OBJECTIVES

The study of a predicted outer membrane leptospiral protein encoded by the gene LIC12690 in mediating the adhesion process.

METHODS

The gene was cloned and expressed in Escherichia coli BL21 (SI) strain by using the expression vector pAE. The recombinant protein tagged with N-terminal hexahistidine was purified by metal-charged chromatography and used to assess its ability to activate human umbilical vein endothelial cells (HUVECs).

RESULTS

The recombinant leptospiral protein of 95kDa, named Lp95, activated E-selectin in a dose-dependent fashion but not the intercellular adhesion molecule 1 (ICAM-1). In addition, we show that pathogenic and non-pathogenic Leptospira are both capable to stimulate endothelium E-selectin and ICAM-1, but the pathogenic L. interrogans serovar Copenhageni strain promotes a statistically significant higher activation than the non-pathogenic L. biflexa serovar Patoc (P<0.01). The Lp95 was identified in vivo in the renal tubules of animal during experimental infection with L. interrogans. The whole Lp95 as well as its fragments, the C-terminal containing the domain of unknown function (DUF), the N-terminal and the central overlap regions bind laminin and fibronectin ECM molecules, being the binding stronger with the DUF containing fragment.

CONCLUSION

This is the first leptospiral protein capable to mediate the adhesion to ECM components and the activation of HUVECS, thus suggesting its participation in the pathogenesis of Leptospira.

Contacting the Host: Insights and Implications of Pathogenic Neisseria Cell Interactions

Neisseria is a highly adapted human specific pathogen that initiates infection at the mucosal epithelia by using multiple adhesins to interact with host cell receptors. Colonization begins at the apical cell surface with a multi-step adhesion cascade, followed by invasion and persistence within the cell and finally transcytosis at the basolateral surface. The type IV pill are implicated in mediating the initial attachment of both meningococci and gonococci, and this association has been shown to involve contact with the cellular receptor CD46. In this review we describe the initial events in the adhesion, invasion and signaling of pathogenic Neisseria focusing on the initial attachment and signaling induced by the interaction of the type IV pili with CD46.

Host iron binding proteins acting as niche indicators for Neisseria meningitidis

Neisseria meningitidis requires iron, and in the absence of iron alters its gene expression to increase iron acquisition and to make the best use of the iron it has. During different stages of colonization and infection available iron sources differ, particularly the host iron-binding proteins haemoglobin, transferrin, and lactoferrin. This study compared the transcriptional responses of N. meningitidis, when grown in the presence of these iron donors and ferric iron, using microarrays.

Specific transcriptional responses to the different iron sources were observed, including genes that are not part of the response to iron restriction. Comparisons between growth on haemoglobin and either transferrin or lactoferrin identified changes in 124 and 114 genes, respectively, and 33 genes differed between growth on transferrin or lactoferrin. Comparison of gene expression from growth on haemoglobin or ferric iron showed that transcription is also affected by the entry of either haem or ferric iron into the cytoplasm. This is consistent with a model in which N. meningitidis uses the relative availability of host iron donor proteins as niche indicators.

Growth in the presence of haemoglobin is associated with a response likely to be adaptive to survival within the bloodstream, which is supported by serum killing assays that indicate growth on haemoglobin significantly increases survival, and the response to lactoferrin is associated with increased expression of epithelial cell adhesins and oxidative stress response molecules. The transferrin receptor is the most highly transcribed receptor and has the fewest genes specifically induced in its presence, suggesting this is the favoured iron source for the bacterium. Most strikingly, the responses to haemoglobin, which is associated with unrestricted growth, indicates a low iron transcriptional profile, associated with an aggressive phenotype that may be adaptive to access host iron sources but which may also underlie the lethal features of meningococcal septicaemia, when haemoglobin may become a major source of iron.

Pathogenic neisseriae: surface modulation, pathogenesis and infection control

Although renowned as a lethal pathogen, Neisseria meningitidis has adapted to be a commensal of the human nasopharynx. It shares extensive genetic and antigenic similarities with the urogenital pathogen Neisseria gonorrhoeae but displays a distinct lifestyle and niche preference. Together, they pose a considerable challenge for vaccine development as they modulate their surface structures with remarkable speed. Nonetheless, their host-cell attachment and invasion capacity is maintained, a property that could be exploited to combat tissue infiltration. With the primary focus on N. meningitidis, this Review examines the known mechanisms used by these pathogens for niche establishment and the challenges such mechanisms pose for infection control.

Neisseria meningitidis Opc invasin binds to the cytoskeletal protein alpha-actinin

Neisseria meningitidis Opc protein is an effective invasin for human endothelial cells. We have investigated novel human endothelial receptors targeted by Opc and observed that Opc-expressing bacteria interacted with a 100 kDa protein in whole-cell lysates of human endothelial and epithelial cells. The identity of the protein was established as alpha-actinin by mass spectrometry. Opc expression was essential for the recognition of alpha-actinin whether provided in a purified form or in cell extracts. The interaction of the two proteins did not involve intermediate molecules. As there was no demonstrable expression of alpha-actinin on the surfaces of any of the eight cell lines studied, the likelihood of the interactions after meningococcal internalization was examined. Confocal imaging demonstrated considerable colocalization of N. meningitidis with alpha-actinin especially after a prolonged period of internalization. This may imply that bacteria and alpha-actinin initially occur in separate compartments and co-compartmentalization occurs progressively over the 8 h infection period used. In conclusion, these studies have identified a novel and an intracellular target for the N. meningitidis Opc invasin. Since alpha-actinin is a modulator of a variety of signalling pathways and of cytoskeletal functions, its targeting by Opc may enable bacteria to survive/translocate across endothelial barriers.

Plasminogen activator Pla of Yersinia pestis utilizes murine DEC-205 (CD205) as a receptor to promote dissemination

Yersinia pestis, a Gram-negative bacterium that causes bubonic and pneumonic plague, is able to rapidly disseminate to other parts of its mammalian hosts. Y. pestis expresses plasminogen activator (PLA) on its surface, which has been suggested to play a role in bacterial dissemination. It has been speculated that Y. pestis hijacks antigen-presenting cells, such as macrophages (MPhis) and dendritic cells, to be delivered to lymph nodes to initiate dissemination and infection. Both alveolar MPhis and pulmonary dendritic cells express a C-type lectin receptor, DEC-205 (CD205), which mediates antigen uptake and presentation. However, no ligand has been identified for DEC-205. In this study, we show that the invasion of alveolar MPhisby Y. pestis depends both in vitro and in vivo on the expression of PLA. DEC-205-expressing MPhis and transfectants, but not their negative counterparts, phagocytosed PLA-expressing Y. pestis and Escherichia coli K12 more efficiently than PLA-negative controls. The interactions between PLA-expressing bacteria and DEC-205-expressing transfectants or alveolar MPhis could be inhibited by an anti-DEC-205 antibody. Importantly, the blockage of the PLA-DEC-205 interaction reduced the dissemination of Y. pestis in mice. In conclusion, murine DEC-205 is a receptor for PLA of Y. pestis, and this host-pathogen interaction appears to play a key role in promoting bacterial dissemination.

CEACAM1 dynamics during neisseria gonorrhoeae suppression of CD4+ T lymphocyte activation

Neisseria gonorrhoeae colony opacity-associated (Opa) proteins bind to human carcinoembryonic antigen cellular adhesion molecules (CEACAM) found on host cells including T lymphocytes. Opa binding to CEACAM1 suppresses the activation of CD4(+) T cells in response to a variety of stimuli. In this study, we use primary human CD4(+) T cells isolated from peripheral blood to define the molecular events occurring subsequent to Opa-CEACAM1 binding. We establish that, in contrast to other cell types, T cells do not engulf N. gonorrhoeae upon CEACAM1 binding. Instead, the bacteria recruit CEACAM1 from intracellular stores and maintain it on the T cell surface. Upon TCR ligation, the co-engaged CEACAM1 becomes phosphorylated on tyrosine residues within the ITIMs apparent in the cytoplasmic domain. This allows the recruitment and subsequent activation of the src homology domain 2-containing tyrosine phosphatases SHP-1 and SHP-2 at the site of bacterial attachment, which prevents the normal tyrosine phosphorylation of the CD3zeta-chain and ZAP-70 kinase in response to TCR engagement. Combined, this dynamic response allows the bacteria to effectively harness the coinhibitory function of CEACAM1 to suppress the adaptive immune response at its earliest step.