Opacity-associated adhesin repertoire in hyperinvasive Neisseria meningitidis

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.

The Host-Pathogen Interactions and Epicellular Lifestyle of Neisseria meningitidis

Neisseria meningitidis is a gram-negative diplococcus and a transient commensal of the human nasopharynx. It shares and competes for this niche with a number of other Neisseria species including N. lactamica, N. cinerea and N. mucosa. Unlike these other members of the genus, N. meningitidis may become invasive, crossing the epithelium of the nasopharynx and entering the bloodstream, where it rapidly proliferates causing a syndrome known as Invasive Meningococcal Disease (IMD). IMD progresses rapidly to cause septic shock and meningitis and is often fatal despite aggressive antibiotic therapy. While many of the ways in which meningococci survive in the host environment have been well studied, recent insights into the interactions between N. meningitidis and the epithelial, serum, and endothelial environments have expanded our understanding of how IMD develops. This review seeks to incorporate recent work into the established model of pathogenesis. In particular, we focus on the competition that N. meningitidis faces in the nasopharynx from other Neisseria species, and how the genetic diversity of the meningococcus contributes to the wide range of inflammatory and pathogenic potentials observed among different lineages.

Investigation of putative invasion determinants of Actinobacillus species using comparative genomics

Actinobacillus spp. are Gram-negative bacteria associated with mucosal membranes. While some are commensals, others can cause important human and animal diseases. A. pleuropneumoniae causes severe fibrinous hemorrhagic pneumonia in swine but not systemic disease whereas other species invade resulting in septicemia and death. To understand the invasive phenotype of Actinobacillus spp., complete genomes of eight isolates were obtained and pseudogenomes of five isolates were assembled and annotated. Phylogenetically, A. suis isolates clustered by surface antigen type and were more closely related to the invasive A. ureae, A. equuli equuli, and A. capsulatus than to the other swine pathogen, A. pleuropneumoniae. Using the LS-BSR pipeline, 251 putative virulence genes associated with serum resistance and invasion were detected. To our knowledge, this is the first genome-wide study of the genus Actinobacillus and should contribute to a better understanding of host tropism and mechanisms of invasion of pathogenic Actinobacillus and related genera.

A deeper mining on the protein composition of VA-MENGOC-BC®: An OMV-based vaccine against N. meningitidis serogroup B and C

The protein composition of an Outer Membrane Vesicle (OMV) preparation that constitutes the active pharmaceutical ingredient of VA-MENGOC-BC®, an effective vaccine against Neisseria meningitidis serogroups B, and C is presented. This preparation has a high lipid content and five abundant membrane proteins (FetA, PorA, PorB, RmpM, and Opc), constituting approximately 70% of the total protein mass. The protein composition was determined by combining the use of the Hexapeptide Ligand Library and an orthogonal tandem fractionation of tryptic peptides by reverse-phase chromatography at alkaline and acid pH. This approach equalizes the concentration of tryptic peptides derived from low- and high-abundance proteins as well as considerably simplifying the number of peptides analyzed by LC-MS/MS, enhancing the possibility of identifying low-abundance species. Fifty-one percent of the proteins originally annotated as membrane proteins in the genome of the MC58 strain were identified. One hundred and sixty-eight low-abundance cytosolic proteins presumably occluded within OMV were also identified. Four (NadA, NUbp, GNA2091, and fHbp), out of the five antigens constituting the Bexsero® vaccine, were detected in this OMV preparation. In particular, fHbp is also the active principle of the Trumenba® vaccine developed by Pfizer. The HpuA and HpuB gene products (not annotated in the MC58 genome) were identified in the CU385 strain, a clinical isolate that is used to produce this OMV. Considering the proteins identified here and previous work done by our group, the protein catalogue of this OMV preparation was extended to 266 different protein species.

Antigenic Variation in Bacterial Pathogens

Antigenic variation is a strategy used by a broad diversity of microbial pathogens to persist within the mammalian host. Whereas viruses make use of a minimal proofreading capacity combined with large amounts of progeny to use random mutation for variant generation, antigenically variant bacteria have evolved mechanisms which use a stable genome, which aids in protecting the fitness of the progeny. Here, three well-characterized and highly antigenically variant bacterial pathogens are discussed: Anaplasma, Borrelia, and Neisseria. These three pathogens display a variety of mechanisms used to create the structural and antigenic variation needed for immune escape and long-term persistence. Intrahost antigenic variation is the focus; however, the role of these immune escape mechanisms at the population level is also presented.

Metabolic competition as a driver of bacterial population structure

Understanding the processes whereby diversity arises and is maintained in pathogen populations is pivotal for designing disease control interventions. A particular problem is the maintenance of strain structure in bacterial pathogen populations despite frequent genetic exchange. Although several theoretical frameworks have been put forward to explain this widespread phenomenon, few have focused on the role of genes encoding metabolic functions, despite an increasing recognition of their importance in pathogenesis and transmission. In this article, we review the literature for evidence of metabolic niches within the host and discuss theoretical frameworks which examine ecological interactions between metabolic genes. We contend that metabolic competition is an important phenomenon which contributes to the maintenance of population structure and diversity of many bacterial pathogens.

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.

Sterilizing immunity elicited by Neisseria meningitidis carriage shows broader protection than predicted by serum antibody cross-reactivity in CEACAM1-humanized mice

Neisseria meningitidis asymptomatically colonizes the human upper respiratory tract but is also the cause of meningitis and severe septicemia. Carriage or disease evokes an immune response against the infecting strain. Hitherto, we have known little about the breadth of immunity induced by natural carriage of a single strain or its implications for subsequent infectious challenge. In this study, we establish that transgenic mice expressing human CEACAM1 support nasal colonization by a variety of strains of different capsular types. Next, we nasally challenged these mice with either of the N. meningitidis strains H44/76 (serogroup B, ST-32) and 90/18311 (serogroup C, ST-11), while following the induction of strain-specific immunoglobulin. When these antisera were tested for reactivity with a diverse panel of N. meningitidis strains, very low levels of antibody were detected against all meningococcal strains, yet a mutually exclusive "fingerprint" of high-level cross-reactivity toward certain strains became apparent. To test the efficacy of these responses for protection against subsequent challenge, CEACAM1-humanized mice exposed to strain 90/18311 were then rechallenged with different N. meningitidis strains. As expected, the mice were immune to challenge with the same strain and with a closely related ST-11 strain, 38VI, while H44/76 (ST-32) could still colonize these animals. Notably, however, despite the paucity of detectable humoral response against strain 196/87 (ST-32), this strain was unable to colonize the 90/18311-exposed mice. Combined, our data suggest that current approaches may underestimate the actual breadth of mucosal protection gained through natural exposure to N. meningitidis strains.

Structure of the Neisserial outer membrane protein Opa₆₀: loop flexibility essential to receptor recognition and bacterial engulfment

The structure and dynamics of Opa proteins, which we report herein, are responsible for the receptor-mediated engulfment of Neisseria gonorrheae or Neisseria meningitidis by human cells and can offer deep understanding into the molecular recognition of pathogen-host receptor interactions. Such interactions are vital to understanding bacterial pathogenesis as well as the mechanism of foreign body entry to a human cell, which may provide insights for the development of targeted pharmaceutical delivery systems. The size and dynamics of the extracellular loops of Opa60 required a hybrid refinement approach wherein membrane and distance restraints were used to generate an initial NMR structural ensemble, which was then further refined using molecular dynamics in a DMPC bilayer. The resulting ensemble revealed that the extracellular loops, which bind host receptors, occupy compact conformations, interact with each other weakly, and are dynamic on the nanosecond time scale. We predict that this conformational sampling is critical for enabling diverse Opa loop sequences to engage a common set of receptors.

The role of variable DNA tandem repeats in bacterial adaptation

DNA tandem repeats (TRs), also designated as satellite DNA, are inter- or intragenic nucleotide sequences that are repeated two or more times in a head-to-tail manner. Because TR tracts are prone to strand-slippage replication and recombination events that cause the TR copy number to increase or decrease, loci containing TRs are hypermutable. An increasing number of examples illustrate that bacteria can exploit this instability of TRs to reversibly shut down or modulate the function of specific genes, allowing them to adapt to changing environments on short evolutionary time scales without an increased overall mutation rate. In this review, we discuss the prevalence and distribution of inter- and intragenic TRs in bacteria and the mechanisms of their instability. In addition, we review evidence demonstrating a role of TR variations in bacterial adaptation strategies, ranging from immune evasion and tissue tropism to the modulation of environmental stress tolerance. Nevertheless, while bioinformatic analysis reveals that most bacterial genomes contain a few up to several dozens of intra- and intergenic TRs, only a small fraction of these have been functionally studied to date.

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.

An analysis of the sequence variability of meningococcal fHbp, NadA and NHBA over a 50-year period in the Netherlands

Studies of meningococcal evolution and genetic population structure, including the long-term stability of non-random associations between variants of surface proteins, are essential for vaccine development. We analyzed the sequence variability of factor H-binding protein (fHbp), Neisserial Heparin-Binding Antigen (NHBA) and Neisseria adhesin A (NadA), three major antigens in the multicomponent meningococcal serogroup B vaccine 4CMenB. A panel of invasive isolates collected in the Netherlands over a period of 50 years was used. To our knowledge, this strain collection covers the longest time period of any collection available worldwide. Long-term persistence of several antigen sub/variants and of non-overlapping antigen sub/variant combinations was observed. Our data suggest that certain antigen sub/variants including those used in 4CMenB are conserved over time and promoted by selection.

Attachment and invasion of Neisseria meningitidis to host cells is related to surface hydrophobicity, bacterial cell size and capsule

We compared exemplar strains from two hypervirulent clonal complexes, strain NMB-CDC from ST-8/11 cc and strain MC58 from ST-32/269 cc, in host cell attachment and invasion. Strain NMB-CDC attached to and invaded host cells at a significantly greater frequency than strain MC58. Type IV pili retained the primary role for initial attachment to host cells for both isolates regardless of pilin class and glycosylation pattern. In strain MC58, the serogroup B capsule was the major inhibitory determinant affecting both bacterial attachment to and invasion of host cells. Removal of terminal sialylation of lipooligosaccharide (LOS) in the presence of capsule did not influence rates of attachment or invasion for strain MC58. However, removal of either serogroup B capsule or LOS sialylation in strain NMB-CDC increased bacterial attachment to host cells to the same extent. Although the level of inhibition of attachment by capsule was different between these strains, the regulation of the capsule synthesis locus by the two-component response regulator MisR, and the level of surface capsule determined by flow cytometry were not significantly different. However, the diplococci of strain NMB-CDC were shown to have a 1.89-fold greater surface area than strain MC58 by flow cytometry. It was proposed that the increase in surface area without changing the amount of anchored glycolipid capsule in the outer membrane would result in a sparser capsule and increase surface hydrophobicity. Strain NMB-CDC was shown to be more hydrophobic than strain MC58 using hydrophobicity interaction chromatography and microbial adhesion-to-solvents assays. In conclusion, improved levels of adherence of strain NMB-CDC to cell lines was associated with increased bacterial cell surface and surface hydrophobicity. This study shows that there is diversity in bacterial cell surface area and surface hydrophobicity within N. meningitidis which influence steps in meningococcal pathogenesis.

Construction of Opa-positive and Opa-negative strains of Neisseria meningitidis to evaluate a novel meningococcal vaccine

Neisseria meningitidis is a major global pathogen causing invasive disease with a mortality of 5-10%. Most disease in developed countries is caused by serogroup B infection, against which there is no universal vaccine. Opacity-associated adhesin (Opa) proteins are major meningococcal outer membrane proteins, which have shown recent promise as a potential novel vaccine. Immunisation of mice with different Opa variants elicited high levels of meningococcal-specific bactericidal antibodies, demonstrating proof in principle for this approach. Opa proteins are critical in meningococcal pathogenesis, mediating bacterial adherence to host cells, and modulating human cellular immunity via interactions with T cells and neutrophils, although there are conflicting data regarding their effects on CD4(+) T cells. We constructed Opa-positive and Opa-negative meningococcal strains to allow further evaluation of Opa as a vaccine component. All four opa genes from N. meningitidis strain H44/76 were sequentially disrupted to construct all possible combinations of N. meningitidis strains deficient in one, two, three, or all four opa genes. The transformations demonstrated that homologous recombination of exogenous DNA into the meningococcal chromosome can occur with as little as 80 bp, and that minor sequence differences are permissible. Anti-Opa bactericidal antibody responses following immunisation of mice with recombinant Opa were specific to the Opa variant used in immunisation. No immunomodulatory effects were observed when Opa was contained within meningococcal outer membrane vesicles (OMVs), compared to Opa-negative OMVs. These observations support the incorporation of Opa in meningococcal vaccines.

Lack of antigenic diversification of major outer membrane proteins during clonal waves of Neisseria meningitidis serogroup A colonization and disease

In particular in the 'meningitis belt' of sub-Saharan Africa, epidemic meningococcal meningitis is a severe public health problem. In the past decades, serogroup A lineages have been the dominant etiologic agents, but also other serogroups have caused outbreaks. A comprehensive vaccine based on subcapsular outer membrane proteins (OMPs) is not available. Here, we have investigated whether meningococcal populations overcome herd immunity by changing antigenic properties of their OMPs. Meningococcal isolates were collected in the context of longitudinal studies in Ghana between 2002 and 2008 and in Burkina Faso between 2006 and 2007. Serogroup A strains isolated during two clonal waves of colonization and disease showed no diversification in the genes encoding their PorA, PorB, and FetA proteins. However, we detected occasional allelic exchange of opa genes, as well as wide variation in the number of intragenic tandem repeats, showing that phase variation of Opa protein expression is a frequent event. Altogether we observed a remarkable antigenic stability of the PorA, PorB and FetA proteins over years. Our results indicate that while herd immunity may be responsible for the disappearance of meningococcal clones over time, it is not a strong driving force for antigenic diversification of the major OMPs analyzed here.

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 meningitidis: biology, microbiology, and epidemiology

Neisseria meningitidis (the meningococcus) causes significant morbidity and mortality in children and young adults worldwide through epidemic or sporadic meningitis and/or septicemia. In this review, we describe the biology, microbiology, and epidemiology of this exclusive human pathogen. N.meningitidis is a fastidious, encapsulated, aerobic gram-negative diplococcus. Colonies are positive by the oxidase test and most strains utilize maltose. The phenotypic classification of meningococci, based on structural differences in capsular polysaccharide, lipooligosaccharide (LOS) and outer membrane proteins, is now complemented by genome sequence typing (ST). The epidemiological profile of N. meningitidis is variable in different populations and over time and virulence of the meningococcus is based on a transformable/plastic genome and expression of certain capsular polysaccharides (serogroups A, B, C, W-135, Y and X) and non-capsular antigens. N. meningitidis colonizes mucosal surfaces using a multifactorial process involving pili, twitching motility, LOS, opacity associated, and other surface proteins. Certain clonal groups have an increased capacity to gain access to the blood, evade innate immune responses, multiply, and cause systemic disease. Although new vaccines hold great promise, meningococcal infection continues to be reported in both developed and developing countries, where universal vaccine coverage is absent and antibiotic resistance increasingly more common.

Mechanisms of meningococcal colonisation

Despite advances against infectious diseases over the past century, Neisseria meningitidis remains a major causative agent of meningitis and septicaemia worldwide. Its adaptation for survival in the human nasopharynx makes the meningococcus a highly successful commensal bacterium. Recent progress has been made in understanding the mechanisms that enable neisserial colonisation, in terms of the role of type IV pili, the impact of other adhesins, biofilm formation, nutrient acquisition and resistance to host immune defences. Refinements in cell-based and in vivo models will lead to improved understanding of the colonisation process, and hopefully to more effective vaccines and therapeutic strategies.

Potential of recombinant opa proteins as vaccine candidates against hyperinvasive meningococci

Neisseria meningitidis causes half a million cases of septicemia and meningitis globally each year. The opacity (Opa) integral outer membrane proteins from N. meningitidis are polymorphic and highly immunogenic. Particular combinations of Opa proteins are associated with the hyperinvasive meningococcal lineages that have caused the majority of serogroup B and C meningococcal disease in industrialized countries over the last 60 years. For the first time, this genetic structuring of a diverse outer membrane protein family has been used to select a novel combination of representative antigens for immunogenicity testing. Fourteen recombinant Opa variants were produced and used in murine immunizations inducing an increase in specific antimeningococcal total IgG levels. All 14 Opa proteins elicited bactericidal antibodies against at least one hyperinvasive meningococcal isolate, and most isolates from each hyperinvasive lineage were killed by at least one Opa antiserum at a titer of 1:16 or greater. Cross-reactive bactericidal antibody responses were observed among clonal complexes. A theoretical coverage of 90% can be achieved by using a particular combination of 6 Opa proteins against an isolate collection of 227 recent United Kingdom disease cases. This study indicates the potential of Opa proteins to provide broad coverage against multiple meningococcal hyperinvasive lineages.

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.

Variation in the Neisseria meningitidis FadL-like protein: an evolutionary model for a relatively low-abundance surface antigen

The molecular diversity of a novel Neisseria meningitidis antigen, encoded by the ORF NMB0088 of MC58 (FadL-like protein), was assessed in a panel of 64 diverse meningococcal strains. The panel consisted of strains belonging to different serogroups, serotypes, serosubtypes and MLST sequence types, of different clinical sources, years and countries of isolation. Based on the sequence variability of the protein, the FadL-like protein has been divided into four variant groups in this species. Antigen variants were associated with specific serogroups and MLST clonal complexes. Maximum-likelihood analyses were used to determine the relationships among sequences and to compare the selection pressures acting on the encoded protein. Furthermore, a model of population genetics and molecular evolution was used to detect natural selection in DNA sequences using the non-synonymous : synonymous substitution (d(N) : d(S)) ratio. The meningococcal sequences were also compared with those of the related surface protein in non-pathogenic commensal Neisseria species to investigate potential horizontal gene transfer. The N. meningitidis fadL gene was subject to only weak positive selection pressure and was less diverse than meningococcal major outer-membrane proteins. The majority of the variability in fadL was due to recombination among existing alleles from the same or related species that resulted in a discrete mosaic structure in the meningococcal population. In general, the population structuring observed based on the FadL-like membrane protein indicates that it is under intermediate immune selection. However, the emergence of a new subvariant within the hyperinvasive lineages demonstrates the phenotypic adaptability of N. meningitidis, probably in response to selective pressure.

Variation and molecular evolution of HmbR, the Neisseria meningitidis haemoglobin receptor

Meningococcal disease caused by serogroup B Neisseria meningitidis remains an important health problem in many parts of the world, and there are currently no comprehensive vaccines. Poor immunogenicity, combined with immunological identity to human sialic acids, have hindered the development of a serogroup B conjugate vaccine, resulting in the development of alternative vaccine candidates, including many outer-membrane protein (OMP)-based formulations. However, the design of protein-based meningococcal vaccines is complicated by the high level of genetic and antigenic diversity of the meningococcus. Knowledge of the extent and structuring of this diversity can have implications for the use of particular proteins as potential vaccine candidates. With this in mind, the diversity of the meningococcal OMP HmbR was investigated among N. meningitidis isolates representative of major hyper-invasive lineages. In common with other meningococcal antigens, the genetic diversity of hmbR resulted from a combination of intraspecies horizontal genetic exchange and de novo mutation. Furthermore, genealogical analysis showed an association of hmbR genes with clonal complexes and the occurrence of two hmbR families, A and B. Three variable regions (VR1–VR3), located in loops 2, 3 and 4, were observed with clonal complex structuring of VR types. A minority of codons (3.9 %), located within putative surface-exposed loop regions of a 2D model, were under diversifying selection, indicating regions of the protein likely to be subject to immune attack.

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.

Variation of the factor H-binding protein of Neisseria meningitidis

There is currently no comprehensive meningococcal vaccine, due to difficulties in immunizing against organisms expressing serogroup B capsules. To address this problem, subcapsular antigens, particularly the outer-membrane proteins (OMPs), are being investigated as candidate vaccine components. If immunogenic, however, such antigens are often antigenically variable, and knowledge of the extent and structuring of this diversity is an essential part of vaccine formulation. Factor H-binding protein (fHbp) is one such protein and is included in two vaccines under development. A survey of the diversity of the fHbp gene and the encoded protein in a representative sample of meningococcal isolates confirmed that variability in this protein is structured into two or three major groups, each with a substantial number of alleles that have some association with meningococcal clonal complexes and serogroups. A unified nomenclature scheme was devised to catalogue this diversity. Analysis of recombination and selection on the allele sequences demonstrated that parts of the gene are subject to positive selection, consistent with immune selection on the protein generating antigenic variation, particularly in the C-terminal region of the peptide sequence. The highest levels of selection were observed in regions corresponding to epitopes recognized by previously described bactericidal monoclonal antibodies.

Genealogical typing of Neisseria meningitidis

Despite the increasing popularity of multilocus sequence typing (MLST), the most appropriate method for characterizing bacterial variation and facilitating epidemiological investigations remains a matter of debate. Here, we propose that different typing schemes should be compared on the basis of their power to infer clonal relationships and investigate the utility of sequence data for genealogical reconstruction by exploiting new statistical tools and data from 20 housekeeping loci for 93 isolates of the bacterial pathogen Neisseria meningitidis. Our analysis demonstrated that all but one of the hyperinvasive isolates established by multilocus enzyme electrophoresis and MLST were grouped into one of six genealogical lineages, each of which contained substantial variation. Due to the confounding effect of recombination, evolutionary relationships among these lineages remained unclear, even using 20 loci. Analyses of the seven loci in the standard MLST scheme using the same methods reproduced this classification, but were unable to support finer inferences concerning the relationships between the members within each complex.

Challenges and progress in the development of a serogroup B meningococcal vaccine

Serogroup B meningococci cause the majority of the meningococcal disease burden in developed countries. Production of an effective and safe vaccine for serogroup B organisms has been hampered by the poor immunogenicity of the capsular polysaccharide that defines this group of bacteria. Previous efforts have focused on outer membrane vesicle vaccines, which have been implemented successfully during clonal outbreaks. However, the search for a universal vaccine against endemic polyclonal serogroup B meningococcal disease continues. In this review, we have highlighted recent development of outer membrane vesicle vaccines and progress in the evaluation of recombinant outer membrane protein vaccines.

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.

Meningococcal protein antigens and vaccines

The development of a comprehensive vaccine against meningococcal disease has been challenging. Recent developments in molecular genetics have provided both explanations for these challenges and possible solutions. Since genome sequence data became available there has been a marked increase in number of protein antigens that have been suggested as prospective vaccine components. This review catalogues the proposed vaccine candidates and examines the evidence for their inclusion in potential protein vaccine formulations.

Meningococcal carriage and disease--population biology and evolution

Meningococcal disease occurs worldwide with incidence rates varying from 1 to 1000 cases per 100,000. The causative organism, Neisseria meningitidis, is an obligate commensal of humans, which normally colonizes the mucosa of the upper respiratory tract without causing invasive disease, a phenomenon known as carriage. Studies using molecular methods have demonstrated the extensive genetic diversity of meningocococci isolated from carriers, in contrast to a limited number of genetic types, known as the hyperinvasive lineages, associated with invasive disease. Population and evolutionary models that invoke positive selection can be used to resolve the apparent paradox of virulent lineages persisting during the global spread of a non-clonal and normally commensal bacterium. The application of insights gained from studies of meningococcal population biology and evolution is important in understanding the spread of disease, as well as in vaccine development and implementation, especially with regard to the challenge of producing comprehensive vaccines based on sub-capsular antigens and measuring their effectiveness.

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.

Distribution and genetic variability of three vaccine components in a panel of strains representative of the diversity of serogroup B meningococcus

With the aim of studying the molecular diversity of the antigens of a new recombinant vaccine against meningococcus serogroup B, the three genes coding for the main vaccine components GNA (Genome-derived Neisseria Antigen) 1870 (fHbp, factor H Binding Protein), GNA1994 (NadA, Neisseria adhesin A) and GNA2132 were sequenced in a panel of 85 strains collected worldwide and selected as representative of the serogroup B meningococcal diversity. No correlations were found between vaccine antigen variability and serogroup, geographic area and year of isolation. Although a relevant clustering was found with MLST clonal complexes, each showing an almost specific antigen variant repertoire, the prediction of the antigen assortment was not possible on the basis of MLST alone. Therefore, classification of meningococcus on the basis of MLST only is not sufficient to predict vaccine antigens diversity. Sequencing each gene in the different strains will be important to evaluate antigen conservation and assortment and to allow a future prediction of potential vaccine coverage.

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.

Population genomics: diversity and virulence in the Neisseria

Advances in high-throughput nucleotide sequencing and bioinformatics make the study of genomes at the population level feasible. Preliminary population genomic studies have explored the relationships among three closely related bacteria, Neisseria meningitidis, Neisseria gonorrhoeae and Neisseria lactamica, which exhibit very different phenotypes with respect to human colonisation. The data obtained have been especially valuable in the establishing of the role of horizontal genetic exchange in bacterial speciation and shaping population structure. In the meningococcus, they have been used to define invasive genetic types, search for virulence factors and potential vaccine components and investigate the effects of vaccines on population structure. These are generic approaches and their application to the Neisseria provides a foretaste for their application to the wider bacterial world.

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.

An ecological and evolutionary perspective on human-microbe mutualism and disease

The microbial communities of humans are characteristic and complex mixtures of microorganisms that have co-evolved with their human hosts. The species that make up these communities vary between hosts as a result of restricted migration of microorganisms between hosts and strong ecological interactions within hosts, as well as host variability in terms of diet, genotype and colonization history. The shared evolutionary fate of humans and their symbiotic bacteria has selected for mutualistic interactions that are essential for human health, and ecological or genetic changes that uncouple this shared fate can result in disease. In this way, looking to ecological and evolutionary principles might provide new strategies for restoring and maintaining human health.

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.