Mapping the binding domains on meningococcal Opa proteins for CEACAM1 and CEA receptors

Magic-angle spinning NMR structure of Opa60 in lipid bilayers

Here we report the structure of Opa60 in lipid bilayers using proton-detected magic-angle spinning nuclear magnetic resonance (MAS NMR). Preparations including near-native oligosaccharide lipids reveal a consistent picture of a stable transmembrane beta barrel with a minor increase in the structured region as compared with the previously reported detergent structure. The large variable loops known to interact with host proteins could not be detected, confirming their dynamic nature even in a lipid bilayer environment. The structure provides a starting point for investigation of the functional role of Opa60 in gonococcal infection, which is understood to involve interaction with host proteins. At the same time, it demonstrates the recent advances in proton-detected methodology for membrane protein structure determination at atomic resolution by MAS NMR.

Controling the cytoskeleton during CEACAM3-mediated phagocytosis

Phagocytosis, an innate defense mechanism of multicellular animals, is initiated by specialized surface receptors. A phagocytic receptor expressed by human polymorphonuclear granulocytes, the major professional phagocytes in our body, is one of the fastest evolving human proteins implying a special role in human biology. This receptor, CEACAM3, is a member of the CarcinoEmbryonic Antigen-related Cell Adhesion Molecule (CEACAM) family and dedicated to the immediate recognition and rapid internalization of human-restricted pathogens. In this focused contribution, we will review the special adaptations of this protein, which co-evolves with different species of mucosa-colonizing bacteria. While the extracellular Immunoglobulin-variable (IgV)-like domain recognizes various bacterial adhesins, an Immunoreceptor Tyrosine-based Activation Motif (ITAM)-like sequence in the cytoplasmic tail of CEACAM3 constitutes the central signaling hub to trigger actin rearrangements needed for efficient phagocytosis. A major emphasis of this review will be placed on recent findings, which have revealed the multi-level control of this powerful phagocytic device. As tyrosine phosphorylation and small GTPase activity are central for CEACAM3-mediated phagocytosis, the counterregulation of CEACAM3 activity involves the receptor-type protein tyrosine phosphatase J (PTPRJ) as well as the Rac-GTP scavenging protein Cyri-B. Interference with such negative regulatory circuits has revealed that CEACAM3-mediated phagocytosis can be strongly enhanced. In principle, the knowledge gained by studying CEACAM3 can be applied to other phagocytic systems and opens the door to treatments, which boost the phagocytic capacity of professional phagocytes.

Microevolution and Its Impact on Hypervirulence, Antimicrobial Resistance, and Vaccine Escape in Neisseria meningitidis

Neisseria meningitidis is commensal of the human pharynx and occasionally invades the host, causing the life-threatening illness invasive meningococcal disease. The meningococcus is a highly diverse and adaptable organism thanks to natural competence, a propensity for recombination, and a highly repetitive genome. These mechanisms together result in a high level of antigenic variation to invade diverse human hosts and evade their innate and adaptive immune responses. This review explores the ways in which this diversity contributes to the evolutionary history and population structure of the meningococcus, with a particular focus on microevolution. It examines studies on meningococcal microevolution in the context of within-host evolution and persistent carriage; microevolution in the context of meningococcal outbreaks and epidemics; and the potential of microevolution to contribute to antimicrobial resistance and vaccine escape. A persistent theme is the idea that the process of microevolution contributes to the development of new hyperinvasive meningococcal variants. As such, microevolution in this species has significant potential to drive future public health threats in the form of hypervirulent, antibiotic-resistant, vaccine-escape variants. The implications of this on current vaccination strategies are explored.

Imaging Flow Cytometry Analysis of CEACAM Binding to Opa-Expressing Neisseria gonorrhoeae

Human carcinoembryonic antigen-related cell adhesion molecules (CEACAMs) are a family of receptors that mediate intercellular interactions. Pathogenic bacteria have ligands that bind CEACAMs on human cells. Neisseria gonorrhoeae (Gc) encodes numerous unique outer membrane opacity-associated (Opa) proteins that are ligands for one or more CEACAMs. CEACAMs that are expressed on epithelial cells facilitate Gc colonization, while those expressed on neutrophils affect phagocytosis and consequent intracellular survival of Gc. Since Opa protein expression is phase-variable, variations in receptor tropism affect how individual bacteria within a population interact with host cells. Here we report the development of a rapid, quantitative method for collecting and analyzing fluorescence intensity data from thousands of cells in a population using imaging flow cytometry to detect N-CEACAM bound to the surface of Opa-expressing Gc. We use this method to confirm previous findings regarding Opa-CEACAM interactions and to examine the receptor-ligand interactions of Gc expressing other Opa proteins, as well as for other N-CEACAM proteins. © 2020 International Society for Advancement of Cytometry.

CEACAM3-A Prim(at)e Invention for Opsonin-Independent Phagocytosis of Bacteria

Phagocytosis is one of the key innate defense mechanisms executed by specialized cells in multicellular animals. Recent evidence suggests that a particular phagocytic receptor expressed by human polymorphonuclear granulocytes, the carcinoembryonic antigen-related cell adhesion molecule 3 (CEACAM3), is one of the fastest-evolving human proteins. In this focused review, we will try to resolve the conundrum why a conserved process such as phagocytosis is conducted by a rapidly changing receptor. Therefore, we will first summarize the biochemical and structural details of this immunoglobulin-related glycoprotein in the context of the human CEACAM family. The function of CEACAM3 for the efficient, opsonin-independent detection and phagocytosis of highly specialized, host-restricted bacteria will be further elaborated. Taking into account the decisive role of CEACAM3 in the interaction with pathogenic bacteria, we will discuss the evolutionary trajectory of the CEACAM3 gene within the primate lineage and highlight the consequences of CEACAM3 polymorphisms in human populations. From a synopsis of these studies, CEACAM3 emerges as an important component of human innate immunity and a prominent example of a dedicated receptor for professional phagocytosis.

Fusobacterium spp. target human CEACAM1 via the trimeric autotransporter adhesin CbpF

Neisseria meningitidis, Haemophilus influenzae, and Moraxella catarrhalis are pathogenic bacteria adapted to reside on human respiratory mucosal epithelia. One common feature of these species is their ability to target members of the carcinoembryonic antigen-related cell adhesion molecule (CEACAM) family, especially CEACAM1, which is achieved via structurally distinct ligands expressed by each species. Beside respiratory epithelial cells, cells at the dentogingival junction express high levels of CEACAM1. It is possible that bacterial species resident within the oral cavity also utilise CEACAM1 for colonisation and invasion of gingival tissues. From a screen of 59 isolates from the human oral cavity representing 49 bacterial species, we identified strains from Fusobacterium bound to CEACAM1. Of the Fusobacterium species tested, the CEACAM1-binding property was exhibited by F. nucleatum (Fn) and F. vincentii (Fv) but not F. polymorphum (Fp) or F. animalis (Fa) strains tested. These studies identified that CEACAM adhesion was mediated using a trimeric autotransporter adhesin (TAA) for which no function has thus far been defined. We therefore propose the name CEACAM binding protein of Fusobacterium (CbpF). CbpF was identified to be present in the majority of unspeciated Fusobacterium isolates confirming a subset of Fusobacterium spp. are able to target human CEACAM1.

Characterisation of the Immunomodulatory Effects of Meningococcal Opa Proteins on Human Peripheral Blood Mononuclear Cells and CD4+ T Cells

Opa proteins are major surface-expressed proteins located in the Neisseria meningitidis outer membrane, and are potential meningococcal vaccine candidates. Although Opa proteins elicit high levels of bactericidal antibodies following immunisation in mice, progress towards human clinical trials has been delayed due to previous findings that Opa inhibits T cell proliferation in some in vitro assays. However, results from previous studies are conflicting, with different Opa preparations and culture conditions being used. We investigated the effects of various Opa+ and Opa- antigens from N. meningitidis strain H44/76 in a range of in vitro conditions using peripheral blood mononuclear cells (PBMCs) and purified CD4+ T cells, measuring T cell proliferation by CFSE dilution using flow cytometry. Wild type recombinant and liposomal Opa proteins inhibited CD4+ T cell proliferation after stimulation with IL-2, anti-CD3 and anti-CD28, and these effects were reduced by mutation of the CEACAM1-binding region of Opa. These effects were not observed in culture with ex vivo PBMCs. Opa+ and Opa- OMVs did not consistently exert a stimulatory or inhibitory effect across different culture conditions. These data do not support a hypothesis that Opa proteins would be inhibitory to T cells if given as a vaccine component, and T cell immune responses to OMV vaccines are unlikely to be significantly affected by the presence of Opa proteins.

Outer membrane protein P1 is the CEACAM-binding adhesin of Haemophilus influenzae

Haemophilus influenzae is a Gram-negative pathogen colonizing the upper respiratory tract mucosa. H. influenzae is one of several human-restricted bacteria, which bind to carcinoembryonic antigen-related cell adhesion molecules (CEACAMs) on the epithelium leading to bacterial uptake by the eukaryotic cells. Adhesion to CEACAMs is thought to be mediated by the H. influenzae outer membrane protein (OMP) P5. However, CEACAMs still bound to H. influenzae lacking OMP P5 expression, and soluble CEACAM receptor ectodomains failed to bind to OMP P5, when heterologously expressed in Escherichia coli. Screening of a panel of H. influenzae OMP mutants revealed that lack of OMP P1 completely abrogated CEACAM binding and supressed CEACAM-mediated engulfment of H. influenzae by epithelial cells. Moreover, ectopic expression of OMP P1 in E. coli was sufficient to induce CEACAM binding and to promote attachment to and internalization into CEACAM-expressing cells. Interestingly, OMP P1 selectively recognizes human CEACAMs, but not homologs from other mammals and this binding preference is preserved upon expression in E. coli. Together, our data identify OMP P1 as the bona fide CEACAM-binding invasin of H. influenzae. This is the first report providing evidence for an involvement of the major OMP P1 of H. influenzae in pathogenesis.

Opa+ Neisseria gonorrhoeae exhibits reduced survival in human neutrophils via Src family kinase-mediated bacterial trafficking into mature phagolysosomes

During gonorrhoeal infection, there is a heterogeneous population of Neisseria gonorrhoeae (Gc) varied in their expression of opacity-associated (Opa) proteins. While Opa proteins are important for bacterial attachment and invasion of epithelial cells, Opa+ Gc has a survival defect after exposure to neutrophils. Here, we use constitutively Opa- and OpaD+ Gc in strain background FA1090 to show that Opa+ Gc is more sensitive to killing inside adherent, chemokine-treated primary human neutrophils due to increased bacterial residence in mature, degradative phagolysosomes that contain primary and secondary granule antimicrobial contents. Although Opa+ Gc stimulates a potent oxidative burst, neutrophil killing of Opa+ Gc was instead attributable to non-oxidative components, particularly neutrophil proteases and the bactericidal/permeability-increasing protein. Blocking interaction of Opa+ Gc with carcinoembryonic antigen-related cell adhesion molecules (CEACAMs) or inhibiting Src family kinase signalling, which is downstream of CEACAM activation, enhanced the survival of Opa+ Gc in neutrophils. Src family kinase signalling was required for fusion of Gc phagosomes with primary granules to generate mature phagolysosomes. Conversely, ectopic activation of Src family kinases or coinfection with Opa+ Gc resulted in decreased survival of Opa- Gc in neutrophils. From these results, we conclude that Opa protein expression is an important modulator of Gc survival characteristics in neutrophils by influencing phagosome dynamics and thus bacterial exposure to neutrophils' full antimicrobial arsenal.

Expression of human CEACAM1 in transgenic mice limits the Opa-specific immune response against meningococcal outer membrane vesicles

Outer membrane vesicles (OMVs) have been extensively investigated as meningococcal vaccine candidates. Among their major components are the opacity (Opa) proteins, a family of surface-exposed outer membrane proteins important for bacterial adherence and entry into host cells. Many Opa-dependent interactions are mediated through the carcinoembryonic antigen-related cell adhesion molecule (CEACAM) family of receptors. Importantly, binding of Opa to CEACAM1 has been reported to suppress human CD4 T cell proliferation in vitro in response to OMV preparations. This raises the question whether OMV vaccines should contain Opa proteins at all. Until now it has been difficult to answer this question, as the proposed immunosuppressive effect was only demonstrated with human cells in vitro, while immunization experiments in mice are not informative because the Opa interaction is specific for human CEACAM1. In the present study we have used Opa+ and Opa- OMVs for immunization experiments in a human CEACAM1 transgenic mouse model. OMVs were prepared from a meningococcal strain H44/76 variant expressing the CEACAM1-binding OpaJ protein, and from an isogenic variant in which all opa genes have been inactivated. Both the CEACAM1 expressing transgenic mice and their congenic littermates lacking it were immunized twice with the OMV preparations, and the sera were analyzed for bactericidal activity and ELISA antibody titres. Total IgG antibodies against the OMVs were similar in both mouse strains. Yet the titres for IgG antibodies specific for purified OpaJ protein were significantly lower in the mice expressing human CEACAM1 than in the nontransgenic mice. No significant differences were found in bactericidal titres among the four groups. Overall, these data indicate that expression of human CEACAM1 confers a reduced Opa-specific antibody response in vivo without affecting the overall immune response against other OMV antigens.

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.

Innate recognition by neutrophil granulocytes differs between Neisseria gonorrhoeae strains causing local or disseminating infections

Members of the carcinoembryonic antigen-related cell adhesion molecule (CEACAM) family serve as cellular receptors for Neisseria gonorrhoeae. More specifically, neisserial colony opacity (OpaCEA)) proteins bind to epithelial CEACAMs (CEACAM1, CEA, CEACAM6) to promote bacterial colonization of the mucosa. In contrast, recognition by CEACAM3, expressed by human granulocytes, results in uptake and destruction of Opa(CEA)-expressing bacteria. Therefore, CEACAM3-mediated uptake might limit the spread of gonococci. However, some strains can cause disseminating gonococcal infections (DGIs), and it is currently unknown how these strains escape detection by granulocyte CEACAM3. Therefore, the opa gene loci from N. gonorrhoeae strain VP1, which was derived from a patient with disseminated gonococcal disease, were cloned and constitutively expressed in Escherichia coli. Similar to Opa proteins of the nondisseminating strain MS11, the majority of Opa proteins from strain VP1 bound epithelial CEACAMs and promoted CEACAM-initiated responses by epithelial cells. In sharp contrast to the Opa proteins of strain MS11, the Opa proteins of strain VP1 failed to interact with the human granulocyte receptor CEACAM3. Accordingly, bacteria expressing VP1 Opa proteins were not taken up by primary human granulocytes and did not trigger a strong oxidative burst. Analysis of Opa variants from four additional clinical DGI isolates again demonstrated a lack of CEACAM3 binding. In summary, our results reveal that particular N. gonorrhoeae strains express an Opa protein repertoire allowing engagement of epithelial CEACAMs for successful mucosal colonization, while avoiding recognition and elimination via CEACAM3-mediated phagocytosis. A failure of CEACAM3-mediated innate immune detection might be linked to the ability of gonococci to cause disseminated infections.

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.

Outer membrane protein A of bovine and ovine isolates of Mannheimia haemolytica is surface exposed and contains host species-specific epitopes

Mannheimia haemolytica is the etiological agent of pneumonic pasteurellosis of cattle and sheep; two different OmpA subclasses, OmpA1 and OmpA2, are associated with bovine and ovine isolates, respectively. These proteins differ at the distal ends of four external loops, are involved in adherence, and are likely to play important roles in host adaptation. M. haemolytica is surrounded by a polysaccharide capsule, and the degree of OmpA surface exposure is unknown. To investigate surface exposure and immune specificity of OmpA among bovine and ovine M. haemolytica isolates, recombinant proteins representing the transmembrane domain of OmpA from a bovine serotype A1 isolate (rOmpA1) and an ovine serotype A2 isolate (rOmpA2) were overexpressed, purified, and used to generate anti-rOmpA1 and anti-rOmpA2 antibodies, respectively. Immunogold electron microscopy and immunofluorescence techniques demonstrated that OmpA1 and OmpA2 are surface exposed, and are not masked by the polysaccharide capsule, in a selection of M. haemolytica isolates of various serotypes and grown under different growth conditions. To explore epitope specificity, anti-rOmpA1 and anti-rOmpA2 antibodies were cross-absorbed with the heterologous isolate to remove cross-reacting antibodies. These cross-absorbed antibodies were highly specific and recognized only the OmpA protein of the homologous isolate in Western blot assays. A wider examination of the binding specificities of these antibodies for M. haemolytica isolates representing different OmpA subclasses revealed that cross-absorbed anti-rOmpA1 antibodies recognized OmpA1-type proteins but not OmpA2-type proteins; conversely, cross-absorbed anti-rOmpA2 antibodies recognized OmpA2-type proteins but not OmpA1-type proteins. Our results demonstrate that OmpA1 and OmpA2 are surface exposed and could potentially bind to different receptors in cattle and sheep.

NafA negatively controls Neisseria meningitidis piliation

Bacterial auto-aggregation is a critical step during adhesion of N. meningitidis to host cells. The precise mechanisms and functions of bacterial auto-aggregation still remain to be fully elucidated. In this work, we characterize the role of a meningococcal hypothetical protein, NMB0995/NMC0982, and show that this protein, here denoted NafA, acts as an anti-aggregation factor. NafA was confirmed to be surface exposed and was found to be induced at a late stage of bacterial adherence to epithelial cells. A NafA deficient mutant was hyperpiliated and formed bundles of pili. Further, the mutant displayed increased adherence to epithelial cells when compared to the wild-type strain. In the absence of host cells, the NafA deficient mutant was more aggregative than the wild-type strain. The in vivo role of NafA in sepsis was studied in a murine model of meningococcal disease. Challenge with the NafA deficient mutant resulted in lower bacteremia levels and mortality when compared to the wild-type strain. The present study reveals that meningococcal NafA is an anti-aggregation factor with strong impact on the disease outcome. These data also suggest that appropriate bacterial auto-aggregation is controlled by both aggregation and anti-aggregation factors during Neisseria infection in vivo.

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.

Tracing phylogenomic events leading to diversity of Haemophilus influenzae and the emergence of Brazilian Purpuric Fever (BPF)-associated clones

Here we report the use of a multi-genome DNA microarray to elucidate the genomic events associated with the emergence of the clonal variants of Haemophilus influenzae biogroup aegyptius causing Brazilian Purpuric Fever (BPF), an important pediatric disease with a high mortality rate. We performed directed genome sequencing of strain HK1212 unique loci to construct a species DNA microarray. Comparative genome hybridization using this microarray enabled us to determine and compare gene complements, and infer reliable phylogenomic relationships among members of the species. The higher genomic variability observed in the genomes of BPF-related strains (clones) and their close relatives may be characterized by significant gene flux related to a subset of functional role categories. We found that the acquisition of a large number of virulence determinants featuring numerous cell membrane proteins coupled to the loss of genes involved in transport, central biosynthetic pathways and in particular, energy production pathways to be characteristics of the BPF genomic variants.

Cellular and molecular biology of Neisseria meningitidis colonization and invasive disease

The human species is the only natural host of Neisseria meningitidis, an important cause of bacterial meningitis globally, and, despite its association with devastating diseases, N. meningitidis is a commensal organism found frequently in the respiratory tract of healthy individuals. To date, antibiotic resistance is relatively uncommon in N. meningitidis isolates but, due to the rapid onset of disease in susceptible hosts, the mortality rate remains approx. 10%. Additionally, patients who survive meningococcal disease often endure numerous debilitating sequelae. N. meningitidis strains are classified primarily into serogroups based on the type of polysaccharide capsule expressed. In total, 13 serogroups have been described; however, the majority of disease is caused by strains belonging to one of only five serogroups. Although vaccines have been developed against some of these, a universal meningococcal vaccine remains a challenge due to successful immune evasion strategies of the organism, including mimicry of host structures as well as frequent antigenic variation. N. meningitidis express a range of virulence factors including capsular polysaccharide, lipopolysaccharide and a number of surface-expressed adhesive proteins. Variation of these surface structures is necessary for meningococci to evade killing by host defence mechanisms. Nonetheless, adhesion to host cells and tissues needs to be maintained to enable colonization and ensure bacterial survival in the niche. The aims of the present review are to provide a brief outline of meningococcal carriage, disease and burden to society. With this background, we discuss several bacterial strategies that may enable its survival in the human respiratory tract during colonization and in the blood during infection. We also examine several known meningococcal adhesion mechanisms and conclude with a section on the potential processes that may operate in vivo as meningococci progress from the respiratory niche through the blood to reach the central nervous system.

Opa+ and Opa- isolates of Neisseria meningitidis and Neisseria gonorrhoeae induce sustained proliferative responses in human CD4+ T cells

T cells may interact with a number of bacterial surface antigens, an encounter which has the potential to downmodulate host immune responses. Neisseria meningitidis, a human colonizer and an agent of septicemia and meningitis, expresses Opa proteins which interact with the CEACAM1 receptor expressed on activated T cells. Since CEACAM1 can act as an inhibitory receptor and T cells in subepithelial tissues may encounter whole bacteria, which often express Opa proteins in vivo, this study assessed primarily if Opa proteins expressed on meningococci affect T-cell functions. In addition, Opa-containing outer membrane vesicles (OMV) have been used as vaccine antigens, and therefore Opa+ and Opa- OMV were also studied. While Opa+ bacteria adhered to CEACAM-expressing T cells, both the Opa+ and Opa- phenotypes induced no to a small transient depression, followed by a prolonged increase in proliferation as well as cytokine production. Such responses were also observed with heat-killed bacteria or OMV. In addition, while anti-CEACAM antibodies alone inhibited proliferation, on coincubation of T cells with bacteria and the antibodies, bacterial effects predominated and were Opa independent. Thus, while Opa proteins of N. meningitidis can bind to T-cell-expressed CEACAM1, this is not sufficient to overcome the T-cell recognition of bacterial factors, which results in a proliferative and cytokine response, an observation consistent with the ability of the host to establish lasting immunity to Opa-expressing meningococci that it frequently encounters. The data also imply that Opa-proficient vaccine preparations may not necessarily inhibit T-cell functions via CEACAM1 binding.

Meningococcal interactions with the host

Neisseria meningitidis interacts with host tissues through hierarchical, concerted and co-ordinated actions of a number of adhesins; many of which undergo antigenic and phase variation, a strategy that helps immune evasion. Three major structures, pili, Opa and Opc predominantly influence bacterial adhesion to host cells. Pili and Opa proteins also determine host and tissue specificity while Opa and Opc facilitate efficient cellular invasion. Recent studies have also implied a role of certain adhesin-receptor pairs in determining increased host susceptibility to infection. This chapter examines our current knowledge of meningococcal adhesion and invasion mechanisms particularly related to human epithelial and endothelial cells which are of primary importance in the disease process.

Relative contributions of recombination and mutation to the diversification of the opa gene repertoire of Neisseria gonorrhoeae

To understand the rates and mechanisms of Neisseria gonorrhoeae opa gene variation, the 11 opa genes were amplified independently so that an opa allelic profile could be defined for any isolate from the sequences at each locus. The opa allelic profiles from 14 unrelated isolates were all different, with no opa alleles shared between isolates. Examination of very closely related isolates from sexual contacts and sexual networks showed that these typically shared most opa alleles, and the mechanisms by which recent changes occurred at individual opa loci could be determined. The great majority of changes were due to recombination among existing alleles that duplicated an opa allele present at another locus or resulted in a mosaic of existing opa alleles. Single nucleotide changes or insertion/deletion of a single codon also occurred, but few of these events were assigned to mutation, the majority being assigned to localized recombination. Introduction of novel opa genes from coinfecting strains was rare, and all but one were observed in the same sexual network. Changes at opa loci occurred at a greater rate than those at the porin locus, and the opa11 locus changed more rapidly than other opa loci, almost always differing even between recent sexual contacts. Examination of the neighboring pilE gene showed that changes at opa11 and pilE often occurred together, although this linkage may not be a causal one.

Reverse transcriptase-PCR differential display analysis of meningococcal transcripts during infection of human cells: up-regulation of priA and its role in intracellular replication

BACKGROUND

In vitro studies with cell line infection models are beginning to disclose the strategies that Neisseria meningitidis uses to survive and multiply inside the environment of the infected host cell. The goal of this study was to identify novel virulence determinants that are involved in this process using an in vitro infection system.

RESULTS

By using reverse transcriptase-PCR differential display we have identified a set of meningococcal genes significantly up-regulated during residence of the bacteria in infected HeLa cells including genes involved in L-glutamate transport (gltT operon), citrate metabolism (gltA), disulfide bond formation (dsbC), two-partner secretion (hrpA-hrpB), capsulation (lipA), and DNA replication/repair (priA). The role of PriA, a protein that in Escherichia coli plays a central role in replication restart of collapsed or arrested DNA replication forks, has been investigated. priA inactivation resulted in a number of growth phenotypes that were fully complemented by supplying a functional copy of priA. The priA-defective mutant exhibited reduced viability during late logarithmic growth phase. This defect was more severe when it was incubated under oxygen-limiting conditions using nitrite as terminal electron acceptors in anaerobic respiration. When compared to wild type it was more sensitive to hydrogen peroxide and the nitric oxide generator sodium nitroprusside. The priA-defective strain was not affected in its ability to invade HeLa cells, but, noticeably, exhibited severely impaired intracellular replication and, at variance with wild type and complemented strains, it co-localized with lysosomal associated membrane protein 1.

CONCLUSION

In conclusion, our study i.) demonstrates the efficacy of the experimental strategy that we describe for discovering novel virulence determinants of N. meningitidis and ii.) provides evidence for a role of priA in preventing both oxidative and nitrosative injury, and in intracellular meningococcal replication.

Opa protein repertoires of disease-causing and carried meningococci

The meningococcal Opa proteins play an important role in pathogenesis by mediating invasion of human cells. The aim of this investigation was to determine whether carried and disease-associated meningococci possess different Opa repertoires and whether the diversity of these proteins is associated with clinical severity of disease. Opa repertoires in 227 disease-associated meningococci, isolated in the United Kingdom over a period of 6 years, were compared to the repertoires in 190 asymptomatically carried meningococci isolated in the United Kingdom from a contemporary, nonepidemic period. Multidimensional scaling (MDS) was employed to investigate the association between Opa repertoires and multilocus sequence typing (MLST) genotypes. Associations with clinical severity were also analyzed statistically. High levels of diversity were observed in opa alleles, variable regions, and repertoires, and MDS revealed that MLST genotypes were strongly associated with particular Opa repertoires. Individual Opa proteins or repertoires were not associated with clinical severity, though there was a trend toward an association with the opaD locus. Meningococcal Opa repertoire is strongly linked to MLST genotype irrespective of epidemiological sampling and therefore correlates with invasiveness. It is not, however, strongly associated with severity of meningococcal disease.

The effect of immune selection on the structure of the meningococcal opa protein repertoire

The opa genes of the Gram negative bacterium Neisseria meningitidis encode Opacity-associated outer membrane proteins whose role is to promote adhesion to the human host tissue during colonisation and invasion. Each meningococcus contains 3–4 opa loci, each of which may be occupied by one of a large number of alleles. We analysed the Opa repertoire structure in a large, well-characterised collection of asymptomatically carried meningococci. Our data show an association between Opa repertoire and meningococcal lineages similar to that observed previously for meningococci isolated from cases of invasive disease. Furthermore, these Opa repertoires exhibit discrete, non-overlapping structure at a population level, and yet low within-repertoire diversity. These data are consistent with the predictions of a mathematical model of strong immune selection upon a system where identical alleles may occupy different loci.

Neisseria meningitidis is a globally important pathogen that causes 2,000–3,000 cases of invasive meningococcal disease annually in the United Kingdom. The meningococcal Opa proteins are important in mediating adhesion to and invasion of human tissues, and are important for evasion of the host immune response. They are encoded by a repertoire of 3–4 genomic loci in each meningococcus and exhibit high levels of sequence diversity. Here we analyzed the Opa repertoires of a large, well-characterised, asymptomatically carried meningococcal isolate collection. We found that the Opa repertoires were specific to individual meningococcal genotypes, similar to that observed in isolates from cases of invasive disease. These repertoires exhibited discrete, non-overlapping structure at a population level, and yet low within-repertoire diversity. These data were consistent with the predictions of a mathematical model of strong immune selection, suggesting that the collective immune response of the host population shapes the antigenic diversity of the meningococcal Opa repertoire. This study provides new insights into Opa-mediated meningococcal pathogenesis and the effect of host population immunity on the biodiversity and population structure of bacterial pathogens. These data may also have implications for the design of new meningococcal vaccines based on surface proteins.

Haplotypic diversity in human CEACAM genes: effects on susceptibility to meningococcal disease

Adhesion between the opacity-associated adhesin (Opa) proteins of Neisseria meningitidis and human carcino-embryonic antigen cell adhesion molecule (CEACAM) proteins is an important stage in the pathogenesis of meningococcal disease, a globally important bacterial infection. Most disease is caused by a small number of meningococcal genotypes known as hyperinvasive lineages. As these are also carried asymptomatically, acquisition of them alone cannot explain why only some hosts develop meningococcal disease. Our aim was to determine whether genetic diversity in CEACAM is associated with susceptibility to meningococcal disease. Frequency distributions of alleles, genotypes and haplotypes were compared in four CEACAM genes in 384 case samples and 190 controls. Linkage disequilibrium among polymorphic sites, haplotype structures and relationships were also analysed. A number of polymorphisms were observed in CEACAM genes but the diversity of CEACAM1, to which most Opa proteins bind, was lower, and a small number of high-frequency haplotypes were detected. Dose-dependent associations of three CEACAM haplotypes with meningococcal disease were observed, with the effect of carrying these haplotypes amplified in homozygous individuals. Two haplotypes were protective while one haplotype in CEACAM6 was associated with a twofold increase in disease susceptibility. These data imply that human CEACAM may be one determinant of human susceptibility to meningococcal disease.

The Moraxella catarrhalis outer membrane protein CD contains two distinct domains specifying adherence to human lung cells

Most Moraxella catarrhalis isolates express a highly-conserved outer membrane protein of 453 residues designated OMPCD, which has been previously shown to mediate binding to A549 human lung cells. Here, it is reported that two distinct domains of the M. catarrhalis strain O35E OMPCD protein specify adherence. Truncated proteins were expressed in Escherichia coli to demonstrate that OMPCD residues 1-240 as well as 241-400 are important for attachment to A549 cells, and database searches indicated that amino acids 285-299 resemble an adhesive motif found in eukaryotic proteins termed thrombospondin-type 3 repeat (TT3R). Cellular enzyme-linked immunosorbent assay using His-tagged proteins demonstrated that residues 236-300 of OMPCD, containing the TT3R motif, specify adhesive properties. Furthermore, these assays revealed that a purified protein encompassing residues 16-236 binds to A549 cells. The two cell-binding domains of OMPCD were further defined to amino acids 16-150 and 261-300 by utilizing a surface-display system, which was constructed from the M. catarrhalis autotransporter protein McaP, to express foreign peptides on the surface of recombinant bacteria.

Opacity-associated adhesin repertoire in hyperinvasive Neisseria meningitidis

The opacity (Opa) proteins mediate a variety of interactions between the bacterium Neisseria meningitidis and its human host. These interactions are thought to be of central importance in both the asymptomatic colonization of the nasopharynx and the sporadic occurrence of meningococcal disease. The receptor specificities of a limited number of Opa protein variants have been explored, but the high level of amino acid sequence diversity among variants has complicated the assignment of specific roles to individual Opa variants or combinations of variants. In addition, the distribution of Opa protein variants among diverse meningococci, information that is potentially informative for studies of Opa function, is poorly understood. A systematic survey of the genetic diversity in the four opa gene loci in each of 77 meningococcal isolates was undertaken. These isolates were representative of the seven hyperinvasive meningococcal clonal complexes that caused the majority of meningococcal disease over the last 50 years. Consistent with previous studies, a high level of sequence diversity was observed among the opa genes and the proteins that they encoded; however, particular sets of Opa protein variants were consistently associated with each of the clonal complexes over time periods often spanning decades and during global spread. These observations were consistent with the postulate that particular combinations of Opa proteins confer fitness advantages to individual clonal complexes and have implications for studies of Opa function and the inclusion of Opa proteins in novel meningococcal vaccines.

Mutational analysis of human CEACAM1: the potential of receptor polymorphism in increasing host susceptibility to bacterial infection

A common overlapping site on the N-terminal IgV-like domain of human carcinoembryonic antigen (CEA)-related cell adhesion molecules (CEACAMs) is targeted by several important human respiratory pathogens. These include Neisseria meningitidis (Nm) and Haemophilus influenzae (Hi) that can cause disseminated or persistent localized infections. To define the precise structural features that determine the binding of distinct pathogens with CEACAMs, we have undertaken molecular modelling and mutation of the receptor molecules at previously implicated key target residues required for bacterial binding. These include Ser-32, Tyr-34, Val-39, Gln-44 and Gln-89, in addition to Ile-91, the primary docking site for the pathogens. Most, but not all, of these residues located adjacent to each other in a previous N-domain model of human CEACAM1, which was based on REI, CD2 and CD4. In the current studies, we have refined this model based on the mouse CEACAM1 crystal structure, and observe that all of the above residues form an exposed continuous binding region on the N-domain. Examination of the model also suggested that substitution of two of these residues 34 and 89 could affect the accessibility of Ile-91 for ligand binding. By introducing selected mutations at the positions 91, 34 and 89, we confirmed the primary importance of Ile-91 in all bacterial binding to CEACAM1 despite the inter- and intraspecies structural differences between the bacterial CEACAM-binding ligands. The studies further indicated that the efficiency of binding was significantly enhanced for specific strains by mutations such as Y34F and Q89N, which also altered the hierarchy of Nm versus Hi strain binding. These studies imply that distinct polymorphisms in human epithelial CEACAMs have the potential to decrease or increase the risk of infection by the receptor-targeting pathogens.

Critical determinants of the interactions of capsule-expressing Neisseria meningitidis with host cells: the role of receptor density in increased cellular targeting via the outer membrane Opa proteins

Neisseria meningitidis capsule is an important virulence determinant required for survival in the blood but is reportedly involved in inhibiting cellular interactions mediated by meningococcal outer membrane adhesins. However, evidence from our previous studies suggested that target receptor density on host cells may determine whether or not capsulate bacteria can adhere via outer membrane proteins such as Opa. To confirm this and evaluate the impact of capsulation on bacterial interactions, we used Opa(+) and Opa(-) derivatives of capsulate and acapsulate meningococcal isolates and transfected cell lines expressing CEACAM1, a receptor targeted by Opa proteins. To assess the extent and rate of cell association, subpopulations of stably transfected Chinese hamster ovary cells with different receptor levels were derived. A quantitative correlation of CEACAM1 levels and Opa-dependent binding of both capsulate and acapsulate bacteria was demonstrated, which was accelerated at high receptor densities. However, it appears that invasion by Opa(+) capsulate bacteria only occurs when a threshold level of CEACAM density has been reached. Target cells expressing high levels of CEACAM1 (MFI c. 400) bound threefold more, but internalized 20-fold more Opa(+) capsulate bacteria than those with intermediate expression (MFI c. 100). No overall selection of acapsulate phenotype was observed in the internalized population. These observations confirm that capsule may not be an adequate barrier for cellular interactions and demonstrate the role of a host factor that may determine capsulate bacterial invasion potential. Upregulation of CEACAMs, which can occur in response to inflammatory cytokines, could lead to translocation of a small number of fully capsulate bacteria across mucosal epithelium into the bloodstream sufficient to cause a rapid onset of disseminated disease. Thus the data also suggest a novel rationale for the epidemiological observations that individuals with prior infectious/inflammatory conditions carry a high risk of invasive meningococcal disease.

Carcinoembryonic antigen-related cell adhesion molecule (CEACAM)-binding recombinant polypeptide confers protection against infection by respiratory and urogenital pathogens

The human-specific pathogens Neisseria meningitidis, N. gonorrhoea, Haemophilus influenzae and Moraxella catarrhalis share the property of targeting the carcinoembryonic antigen (CEA)-related cell adhesion molecules (CEACAMs) expressed on human epithelia. CEACAMs are signalling receptors implicated in cell adhesion and regulation of several physiological functions. Their targeting by pathogens can lead to tissue invasion. Although the CEACAM-binding ligands of the bacteria are structurally diverse, they target a common site on the receptor. We have generated a recombinant polypeptide that blocks the interactions of the mucosal pathogens with human epithelial cells and antibodies against it inhibit M. catarrhalis interactions with the receptor. As such, it is a potential antimicrobial agent to prevent infection via a strategy unlikely to promote bacterial resistance and a vaccine candidate against M. catarrhalis. In addition, it could serve more widely as a novel research tool and as a potential therapeutic agent in CEACAM-based physiological disorders.