T-cell responses to outer membrane proteins of Neisseria meningitidis: comparative study of the Opa, Opc, and PorA proteins

Immune responses of meningococcal B outer membrane vesicles in middle-aged mice

The elderly are more likely to die when infected with Neisseria meningitidis. Aging is associated with immune system dysfunctions that impair responses to vaccines and infections. Therefore, immunization of middle-aged individuals could be beneficial. This study aims to evaluate the immunogenicity of N. meningitidis B outer membrane vesicles (OMVs) complexed to two different adjuvants. Middle-aged BALB/c and A/Sn mice were immunized and subsequent immune response was assessed by ELISA, immunoblotting and ELISpot. IgG levels were similar between the animals immunized with OMVs complexed to adjuvants. A total of 235 days after the last immunization only A/Sn mice presented higher IgG levels than those observed in the baseline, especially the group immunized with OMVs and aluminum hydroxide. The predominant IgG subclasses were IgG2a and IgG2b. Immunization with the three-dose regimen generated IgG antibodies that recognized a variety of antigens present in the homologous and heterologous meningococcal OMVs evaluated. There was an increase in the frequency of antigen-specific IFN-γ secreting splenocytes, after in vitro stimulation, in mice immunized with OMVs and adjuvants compared to the control group, almost 1 year after the last immunization. Both adjuvants showed similar performance. Immunization of middle-aged mice has generated a robust immune response and it appears to be advantageous.

Detecting HLA-infectious disease associations for multi-strain pathogens

Human Leukocyte Antigen (HLA) molecules play a vital role helping our immune system to detect the presence of pathogens. Previous work to try and ascertain which HLA alleles offer advantages against particular pathogens has generated inconsistent results. We have constructed an epidemiological model to understand why this may occur. The model captures the epidemiology of a multi strain pathogen for which the host's ability to generate immunological memory responses to particular strains depends on that host's HLA genotype. We find that an HLA allele's ability to protect against infection, as measured in a case control study, depends on the population frequency of that HLA allele. Furthermore, our capability to detect associations between HLA alleles and infection with a multi strain pathogen may be affected by the properties of the pathogen itself (i.e R₀ and length of infectious period). Both host and pathogen genetics must be considered in order to identify true HLA associations. However, in the absence of detailed pathogen genetic information, a negative correlation between the frequency of an HLA type and its apparent protectiveness against disease caused by multi strain pathogen is a strong indication that the HLA type in question is well adapted to a subset of strains of that pathogen.

Transcriptome analysis of human brain microvascular endothelial cells response to Neisseria meningitidis and its antigen MafA using RNA-seq

Interaction of Neisseria meningitidis (NM) with human brain microvascular endothelial cells (hBMECs) initiates of multiple cellular processes, which allow bacterial translocation across the blood-brain barrier (BBB). NM is equipped with several antigens, which interacts with the host cell receptors. Recently we have shown that adhesin MafA (UniProtKB-X5EG71), relatively less studied protein, is one of those surface exposed antigens that adhere to hBMECs. The present study was designed to comprehensively map the undergoing biological processes in hBMECs challenged with NM or MafA using RNA sequencing. 708 and 726 differentially expressed genes (DEGs) were identified in hBMECs exposed to NM and MafA, respectively. Gene ontology analysis of the DEGs revealed that several biological processes, which may alter the permeability of BBB, were activated. Comparative analysis of DEGs revealed that MafA, alike NM, might provoke TLR-dependent pathway and augment cytokine response. Moreover, both MafA and NM were able to induce genes involved in cell surface modifications, endocytosis, extracellular matrix remodulation and anoikis/apoptosis. In conclusion, this study for the first time describes effect of NM on the global gene expression in hBMECs using high-throughput RNA-seq. It also presents ability of MafA to induce gene expression, which might aid NM in breaching the BBB.

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

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

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.

Cellular Immune Responses in Humans Induced by Two Serogroup B Meningococcal Outer Membrane Vesicle Vaccines Given Separately and in Combination

MenBvac and MeNZB are safe and efficacious outer membrane vesicle (OMV) vaccines against serogroup B meningococcal disease. Antibody responses have previously been investigated in a clinical trial with these two OMV vaccines given separately (25 μg/dose) or in combination (12.5 and 12.5 μg/dose) in three doses administered at 6-week intervals. Here, we report the results from analyzing cellular immune responses against MenBvac and MeNZB OMVs in terms of antigen-specific CD4(+)T cell proliferation and secretion of cytokines. The proliferative CD4(+)T cell responses to the combined vaccine were of the same magnitude as the homologous responses observed for each individual vaccine. The results also showed cross-reactivity in the sense that both vaccine groups receiving separate vaccines responded to both homologous and heterologous OMV antigen when assayed for antigen-specific cellular proliferation. In addition, a multiplex bead array assay was used to analyze the presence of Th1 and Th2 cytokines in cell culture supernatants. The results showed that gamma interferon, interleukin-4 (IL-4), and IL-10 responses could be detected as a result of vaccination with both the MenBvac and the MeNZB vaccines given separately, as well as when given in combination. With respect to cross-reactivity, the cytokine results paralleled the observations made for proliferation. In conclusion, the results demonstrate that cross-reactive cellular immune responses involving both Th1 and Th2 cytokines can be induced to the same extent by different tailor-made OMV vaccines given either separately or in combination with half the dose of each vaccine.

Evaluation of truncated NhhA protein as a candidate meningococcal vaccine antigen

NhhA (Neisseria hia homologue) is an outer membrane protein from Neisseria meningitidis, the causative agent of meningococcal disease. The protein is surface exposed and its expression in a wide range of meningococcal strains suggests it is a promising vaccine candidate. In addition, immunization of mice with outer membrane vesicles of strains that overexpress NhhA in conjunction with one of TbpA, Omp85 or NspA results in synergistic bactericidal responses. We previously showed that the NhhA sequence is highly conserved between strains, with the majority of the differences localized to four distinct variable regions located in the amino-terminal region of the mature protein. In this study, N. meningitidis strains were constructed that over-express wild-type NhhA. Strains expressing truncated versions of NhhA, with deletions from the amino-terminal region that removed the most variable regions, were also made. These expression strains were also modified so that immunodominant, phase- and antigenically-variable outer membrane proteins were not expressed, truncated lipooligosaccharide (LOS) expression was genetically fixed (no phase variability), and capsular polysaccharide expression abolished. Outer membrane vesicles derived from these strains were used to immunize mice. As previously observed, a synergistic effect involving another antigen, TbpA, was required to demonstrate bactericidal activity. The highest bactericidal response against a heterologous strain was obtained with a truncated variant of NhhA. These results indicate that removal of (a) variable region(s) does not reduce bactericidal responses against NhhA, and that bactericidal targets exist in regions other than the variable N-teminus. This provides the basis for future examination of responses against truncated NhhA in protecting against heterologous NhhA strains, and further evaluation of truncated NhhA as a candidate for inclusion in a vaccine against all serogroups of N. meningitidis.

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.

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.

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.

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.

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.

Absence of mucosal immunity in the human upper respiratory tract to the commensal bacteria Neisseria lactamica but not pathogenic Neisseria meningitidis during the peak age of nasopharyngeal carriage

The normal flora that colonizes the mucosal epithelia has evolved diverse strategies to evade, modulate, or suppress the immune system and avoid clearance. Neisseria lactamica and Neisseria meningitidis are closely related obligate inhabitants of the human upper respiratory tract. N. lactamica is a commensal but N. meningitidis is an opportunistic pathogen that occasionally causes invasive disease such as meningitis and septicemia. We demonstrate that unlike N. meningitidis, N. lactamica does not prime the development of mucosal T or B cell memory during the peak period of colonization. This cannot be explained by the induction of peripheral tolerance or regulatory CD4(+)CD25(+) T cell activity. Instead, N. lactamica mediates a B cell-dependent mitogenic proliferative response that is absent to N. meningitidis. This mitogenic response is associated with the production of T cell-independent polyclonal IgM that we propose functions by shielding colonizing N. lactamica from the adaptive immune system, maintaining immunological ignorance in the host. We conclude that, in contrast to N. meningitidis, N. lactamica maintains a commensal relationship with the host in the absence of an adaptive immune response. This may prolong the period of susceptibility to colonization by both pathogenic and nonpathogenic Neisseria species.

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.

Mucosal immunity and optimizing protection with meningococcal serogroup B vaccines

Candidate Neisseria meningitidis serogroup B vaccines that are based on outer-membrane vesicles induce protective immunity in adults but provide neither crossprotection for infants nor long-lasting immunity. We suggest that this lack of vaccine efficacy is not solely because the best antigens are yet to be identified but also results from inappropriate programming of the immune response. Natural carriage of N. meningitidis and related bacteria leads to the development of protective immunity both at the mucosal surface and in the circulation. We propose that vaccine strategies that mimic this natural immunization process would better-optimize vaccine-induced protective immunity. Thus, mucosal immunization before a systemic booster vaccination could provide the solution and reduce the necessity for multiple injections to achieve immunity.

Mass tag-assisted identification of naturally processed HLA class II-presented meningococcal peptides recognized by CD4+ T lymphocytes

The meningococcal class I outer membrane protein porin A plays an important role in the development of T cell-dependent protective immunity against meningococcal serogroup B infection and is therefore a major component of candidate meningococcal vaccines. T cell epitopes from porin A are poorly characterized because of weak in vitro memory T cell responses against purified Ag and strain variation. We applied a novel strategy to identify relevant naturally processed and MHC class II-presented porin A epitopes, based on stable isotope labeling of Ag. Human immature HLA-DR1-positive dendritic cells were used for optimal uptake and MHC class II processing of (14)N- and (15)N-labeled isoforms of the neisserial porin A serosubtype P1.5-2,10 in bacterial outer membrane vesicles. HLA-DR1 bound peptides, obtained after 48 h of Ag processing, contained typical spectral doublets in mass spectrometry that could easily be assigned to four porin A regions, expressed at diverging densities ( approximately 30-4000 copies/per cell). Epitopes from two of these regions are recognized by HLA-DR1-restricted CD4(+) T cell lines and are conserved among different serosubtypes of meningococcal porin A. This mass tag-assisted approach provides a useful methodology for rapid identification of MHC class II presented bacterial CD4(+) T cell epitopes relevant for vaccine development.

Evidence for Naturally Acquired T Cell-Mediated Mucosal Immunity toNeisseria meningitidis

Naturally acquired protective immunity against Neisseria meningitidis is thought to partially explain the disparity between the high levels of carriage in the human nasopharynx and the rare incidence of disease. To investigate this immunity to Neisseria meningitidis at the mucosal level, in vitro cellular responses to outer membrane vesicle preparations derived from this pathogen were examined using mononuclear cells from the palatine tonsils of adults and children. Characterization of these responses was achieved by depletion of CD45RA(+), CD45RO(+), and CD19(+) populations and outer membrane vesicles derived from isogenic mutants expressing different serosubtypes of the major outer membrane protein, porin A (PorA), no PorA and membrane preparations from a mutant with no LPS (LpxA(-)). The magnitude of cellular proliferative responses against the outer membrane vesicles were strongly associated with age and were largely T cell mediated, involving both CD45RO(+) and CD45RA(+) T cell phenotypes. Responses were not dependent on LPS but consisted of both PorA cross-specific and non-PorA-dependent responses. Cellular immunity against Neisseria meningitidis was found to be frequently associated with systemic IgG Abs but was not associated with serum bactericidal Abs. For the first time our results demonstrate an age-associated acquisition of mucosal T effector/memory cell responses to Neisseria meningitidis. This mucosal cellular immunity can be present in the absence of serum bactericidal Abs, a classical marker of protective immunity.

Analysis of Neisseria lactamica antigens putatively implicated in acquisition of natural immunity to Neisseria meningitidis

Sera from healthy human volunteers, patients convalescent from meningococcal meningitis, and mice immunized with outer membrane proteins from Neisseria meningitidis and Neisseria lactamica strains were used to analyze and identify antigens cross-reactive to both neisserial species. All classes of meningococcal proteins except class 1 (PorA) and class 5 cross-reacted with N. lactamica proteins and two other proteins of 65 and 55 kDa (an iron-regulated protein). Results obtained with the mouse sera demonstrate that cross-reactive antibodies can be elicited by either N. meningitidis or N. lactamica. These results support the suggestion that N. lactamica contributes to the development of natural immunity against N. meningitidis during the first years of life. The use of vaccines containing proteins other than PorA could interfere in colonization of mucosal surfaces by N. lactamica, hampering the natural mechanisms of immunity acquisition in humans. Only convalescent sera reacted with the 55 and 65 kDa proteins, which suggests that they might be relevant for pathogenicity.

Neisserial immunoglobulin A1 protease induces specific T-cell responses in humans

We have previously shown that immunoglobulin A1 (IgA1) protease, an exoenzyme of pathogenic neisseriae, can trigger the release of proinflammatory cytokines from human monocytic subpopulations. Here, we demonstrate a dose-dependent T-cell response to recombinant gonococcal IgA1 protease (strain MS11) in healthy human blood donors. This response was delayed in comparison to the immune response against tetanus toxoid. Stimulation with IgA1 protease led to the activation of CD4(+) and CD8(+) T cells, as well as CD19(+) B cells and CD56(+) NK cells, indicated by de novo expression of CD69. Only CD4(+) T cells proliferated and stained positive for intracellular gamma interferon (IFN-gamma). Both proliferation and IFN-gamma production were dependent on antigen presentation via major histocompatibility complex class II. Peripheral blood mononuclear cells stimulated with IgA1 protease produce IFN-gamma and tumor necrosis factor alpha but no, or very low amounts of, interleukin-10 (IL-10) or IL-4, indicating a Th1-based proinflammatory immune response. These findings support the significance of IgA1 protease as a virulence determinant of bacterial meningitis and its function as a dominant proinflammatory T-cell antigen.

Characterization of four members of a multigene family encoding outer membrane proteins of Helicobacter pylori and their potential for vaccination

In search of protective antigens which can be used in a vaccine to prevent Helicobacter pylori infection, we report on the identification of four genes, hopV, hopW, hopX and hopY, and the characterization of the corresponding proteins which belong to the H. pylori outer membrane protein (Hop) family containing 32 homologous members, some of which were shown to function as porins. Sequence analysis of 16 different H. pylori strains revealed that the proteins HopV, HopW, HopX and HopY are highly conserved. Localization of HopV, HopW, HopX and HopY at the surface of the bacteria was investigated by immunofluorescence. Using a planar lipid bilayer system the proteins HopV and HopX were shown to form pores with single-channel conductances of 1.4 and 3.0 nS, respectively.

Development of natural immunity to Neisseria meningitidis

Although meningococcal disease is rare in industrialized nations, Neisseria meningitidis holds a prominent position amongst pediatric infections because of the dramatic clinical presentation of the disease, high mortality, epidemic potential and the recent disappearance of many other important infectious diseases in developed countries through improvements in public health and vaccination. The precise nature of natural immunity to meningococci remains unknown, although a complex interaction between the organism and nasopharyngeal mucosal barrier, innate immune mechanisms and acquired immunity is involved. Study of the mechanisms of natural immunity may provide the key to development of vaccines that can reduce the burden of disease in early childhood.

Auto-transporter A protein of Neisseria meningitidis: a potent CD4+ T-cell and B-cell stimulating antigen detected by expression cloning

A meningococcal genomic expression library was screened for potent CD4+ T-cell antigens, using patients' peripheral blood lymphocytes (PBLs). One of the most promising positive clones was fully characterized. The recombinant meningococcal DNA contained a single, incomplete, open reading frame (ORF), which was fully reconstructed with reference to available genomic sequence data. The gene was designated autA (auto-transporter A) as its peptide sequence shares molecular characteristics of the auto-transporter family of proteins. Only a single copy of this gene was detected in the meningococcal, and none in the gonococcal, genomic sequence databases. The complete autA gene, when cloned into an expression vector, expressed a protein of approximately 68 kDa. Purified rAutA recalled strong secondary T-cell responses in PBLs of patients and some healthy donors, and induced strong primary T-cell responses in healthy donors. The human B-cell immunogenicity and cross-reactivity of AutA, purified under native conditions, was confirmed in dot immunoblot experiments. Immunoblots with rabbit polyclonal antibodies to rAutA demonstrated the conserved nature, antigenicity and cross-reactivity of AutA amongst meningococci of different serogroups and strains representing different hypervirulent lineages. AutA showed homology with another meningococcal and gonococcal ORF (designated AutB). AutB was cloned and expressed and used to raise an autB-specific antiserum. Immunoblot experiments indicated that AutB is not expressed in meningococci and does not cross-react with AutA. Thus, AutA, being a potent CD4+ T-cell and B-cell-stimulating antigen, which is highly conserved, deserves further investigation as a potential vaccine candidate.

Human T-cell response to meningococcal immunoglobulin A1 protease associated alpha-proteins

A unique feature of the immunoglobulin A1 (IgA1) protease from pathogenic Neisseriae, i.e. N. meningitidis and N. gonorrhoeae, is its co-secretion with an amphipathic a-protein. Polymerase chain reaction (PCR) analysis of the respective iga(alpha). gene region in 48 meningococcal strains revealed that this protein domain is conserved throughout all isolates in four different principal variants. Despite strain-dependent size and sequence variations, sequence analysis showed common structural characteristics. More than 80% of the amino acid sequence of all a-proteins is dependent on the five amino acids Q, E, A, K and R, resulting in a pI> 10. The sequences are highly conserved at the N-terminus and the C-terminus and contain long amphipathic alpha-helical stretches. These stretches have a strong probability of forming coiled coil conformations and comprise short repetitive sequence modules with pronounced similarities to T-cell epitopes. We therefore analyzed the T-cell response of 20 volunteer blood donors to four peptides, representing such predicted epitopes, and a recombinant meningococcal alpha-protein. Sixteen donors reacted against at least one peptide after culture of peripheral blood mononuclear cells in interleukin (IL)-2-rich medium, while two individuals showed a positive reaction only against an IgA1 protease-derived control peptide. From one donor, we established and maintained T-cell clones specific for purified alpha-protein. Characterization of the T-cell clones revealed a CD3- and a CD4-positive phenotype and the secretion of IL-2 and interferon-gamma (IFN-gamma),

Identification and characterization of TspA, a major CD4(+) T-cell- and B-cell-stimulating Neisseria-specific antigen

In search for novel T-cell immunogens involved in protection against invasive meningococcal disease, we screened fractionated proteins of Neisseria meningitidis (strain SD, B:15:P1.16) by using peripheral blood mononuclear cells (PBMCs) and specific T-cell lines obtained from normal individuals and patients convalescing from N. meningitidis infection. Proteins of iron-depleted meningococci produced higher PBMC proliferation indices than proteins of iron-replete organisms, indicating that iron-regulated proteins are T-cell immunogens. Insoluble proteins of the iron-depleted cells, which produced better T-cell stimulation than soluble ones, were fractionated by using sodium dodecyl sulfate-polyacrylamide gels and recovered as five fractions (F1 to F5) corresponding to decreasing molecular weight ranges. The proteins were purified (by elution and precipitation) or electroblotted onto nitrocellulose membranes (dissolved and precipitated) before use in further T-cell proliferation assays. One of the fractions (F1), containing high-molecular-mass proteins (>130 kDa), consistently showed the strongest T-cell proliferation responses in all of the T-cell lines examined. F1 proteins were subdivided into four smaller fractions (F1A to F1D) which were reexamined in T-cell proliferation assays, and F1C induced the strongest responses in patients' T-cell lines. Rabbit polyclonal antibodies to F1C components were used to screen a genomic expression library of N. meningitidis. Two major clones (C1 and C24) of recombinant meningococcal DNA were identified and fully sequenced. Sequence analysis showed that C24 (1,874 bp) consisted of a single open reading frame (ORF), which was included in clone C1 (2, 778 bp). The strong CD4(+) T-cell-stimulating effect of the polypeptide product of this ORF (named TspA) was confirmed, using a patient T-cell line. Immunogenicity for B cells was confirmed by showing that convalescent patients' serum antibodies recognized TspA on Western blots. Additional genetic sequence downstream of C24 was obtained from the meningococcal genomic sequence database (Sanger Centre), enabling the whole gene of 2,761 bp to be reconstructed. The DNA and deduced amino acid sequence data for tspA failed to show significant homology to any known gene, except for a corresponding (uncharacterized) gene in Neisseria gonorrhoeae genome sequences, suggesting that tspA is unique to the genus Neisseria. The DNA and deduced amino acid sequence of the second ORF of clone C1 showed significant homology to gloA, encoding glyoxalase I enzyme, of Salmonella typhimurium and Escherichia coli. Thus, we have identified a novel neisserial protein (TspA) which proved to be a strong CD4(+) T-cell- and B-cell-stimulating immunogen with potential as a possible vaccine candidate.

Cellular immune responses to Neisseria meningitidis in children

There is an urgent need for effective vaccines against serogroup B Neisseria meningitidis. Current experimental vaccines based on the outer membrane proteins (OMPs) of this organism provide a measure of protection in older children but have been ineffective in infants. We postulated that the inability of OMP vaccines to protect infants might be due to age-dependent defects in cellular immunity. We measured proliferation and in vitro production of gamma interferon (IFN-gamma), tumor necrosis factor alpha, and interleukin-10 (IL-10) in response to meningococcal antigens by peripheral blood mononuclear cells (PBMCs) from children convalescing from meningococcal disease and from controls. After meningococcal infection, the balance of cytokine production by PBMCs from the youngest children was skewed towards a TH1 response (low IL-10/IFN-gamma ratio), while older children produced more TH2 cytokine (higher IL-10/IFN-gamma ratio). There was a trend to higher proliferative responses by PBMCs from older children. These responses were not influenced by the presence or subtype of class 1 (PorA) OMP or by the presence of class 2/3 (PorB) or class 4 OMP. Even young infants might be expected to develop adequate cellular immune responses to serogroup B N. meningitidis vaccines if a vaccine preparation can be formulated to mimic the immune stimulus of invasive disease, which may include stimulation of TH2 cytokine production.

Human T-cell responses after vaccination with the Norwegian group B meningococcal outer membrane vesicle vaccine

We have analyzed human T-cell responses in parallel with serum immunoglobulin G (IgG) antibody levels after systemic vaccination with the Norwegian group B Neisseria meningitidis outer membrane vesicle (OMV) vaccine. Ten adult volunteers, with no or very low levels of serum IgG antibodies against meningococci, received three doses intramuscularly of the OMV vaccine (at weeks 0, 6, and 46). T-cell proliferation against the OMV vaccine, purified outer membrane proteins (PorA and PorB), and control antigens (Mycobacterium bovis BCG vaccine and tetanus toxoid) was measured by thymidine incorporation of peripheral blood mononuclear cells before and after vaccination. The results showed that vaccination with OMV elicits strong primary and booster T-cell responses specific to OMV as well as the PorA (class 1) protein and significant, but markedly lower, responses against the PorB (class 3) protein. The median responses to OMV and PorA were 26 and 16 times the prevaccination levels, respectively. Most of the vaccinees showed low T-cell responses against OMV and PorA before vaccination, and the maximum T-cell responses to all vaccine antigens were usually obtained after the second vaccine dose. We found a positive correlation between T-cell responses and anti-OMV IgG antibody levels (r = 0.50, P < 0.0001, for OMV and PorA). In addition, we observed a progressive increase in the percentage of CD45R0+ (memory) CD4-positive T cells (P = 0.002). In conclusion, we have shown that the Norwegian OMV vaccine against meningococcal B disease induced antigen-specific T-cell responses, kinetically accompanied by serum IgG responses, and that vaccination increased the proportion of memory T-helper cells.

T-Cell epitope mapping the PorB protein of serogroup B Neisseria meningitidis in B10 congenic strains of mice

T-cell epitope mapping the meningococcal serotype 15 PorB protein performed in this study in three congenic strains of mice with B10 genetic background revealed at least three murine T-cell epitopes (55-72, 163-180, and 226-261), located in the highly conserved putative transmembrane regions of Neisserial porins. Proliferation assays with popliteal lymph node cells derived from mice immunized with the PorB protein or with synthetic 18-mer peptides showed that epitope 163-180 immunized only in the H-2d haplotype, epitope 55-72 could be presented by both H-2f and H-2s molecules, while the 226-261 region covered by three overlapping peptides could be efficiently recognized in context of all three MHC class II haplotypes studied. Inhibition experiments with blocking I-Aalpha- and I-Ealpha-specific mAb showed that peptide 163-180 was presented by I-Ad and peptide 244-261 was presented by both I-Af and I-As. In addition, evidence was obtained that peptide 226-243 was presented in context of H-2d or I-As haplotypes and peptide 55-72 was presented in context of I-Af and I-As loci. Finally, the Norwegian outer membrane vesicle vaccine, but not the purified PorB protein, could recall responses in mice immunized with synthetic peptides corresponding to the 226-261 region. Altogether, these results suggest that T-cell epitopes identified on the serotype 15 PorB protein, particularly those presented by several MHC class II molecules (e.g., 226-261), could have important implications for the development of meningococcal vaccines.

Use of synthetic peptides to identify measles nucleoprotein T-cell epitopes in vaccinated and naturally infected humans

Recombinant measles nucleoprotein (N) and synthetic peptides spanning the length of the N-protein-coding region were used with a proliferation assay to identify human T-cell epitopes in vaccinated and naturally infected adults. A number of epitopes were mapped to specific regions of the measles virus N. The proliferative response of at least two donors was mediated by CD4(+) T cells in association with HLA DR antigens. Over 70% of all donors tested responded to peptides representing amino acids 271-290, 367-386, 400-420, and 483-502, suggesting that these peptides may be broadly recognized within an HLA diverse population. The most frequently recognized T-cell epitopes in both naturally infected and vaccinated donors were located in the genetically heterogeneous carboxy-terminal half of the N. Analysis of patterns of peptide reactivity among vaccinated and naturally infected subjects identified several regions of potential difference between these two groups. Peptides 221-240 and 237-256 were recognized among 100% of naturally infected donors but among only 37.5% of vaccinated donors and therefore may be of further interest in studies to investigate induction of lifelong versus transient immunity to measles. Use of chimeric molecules containing multiple well-characterized T- and B-cell epitopes or genetic alteration of attenuated vaccine virus to enhance critical T-cell responses may eventually lead to the development of a vaccine candidate that can more closely model the patterns of immune response elicited by wild-type virus.

Antigenic characterization and analysis of the human immune response to outer membrane protein E of Branhamella catarrhalis

Outer membrane protein E (OMP E) is a 50-kDa major OMP of Branhamella catarrhalis. Polyclonal antisera and four monoclonal antibodies (MAbs) to OMP E were generated to study its antigenic structure. All antibodies recognized epitopes in all 19 B. catarrhalis strains tested by immunoblot assays. By flow cytometry, it was determined that MAbs 1B3 and 9G10d recognized epitopes which are expressed on the surface of the intact bacterium, while MAbs IC11 and 7C10 recognized epitopes which were buried within the outer membrane. A competitive enzyme-linked immunosorbent assay showed that MAbs 1B3 and 9G10d recognize the same or closely related epitopes. Proteinase K treatment of whole bacterial cells revealed that MAbs 1B3 and 9G10d recognize a surface-exposed epitope located in the 17-kDa region towards the amino terminus of OMP E. The human serum and mucosal antibody responses to OMP E in adults with chronic bronchitis were studied. A majority of these patients had immunoglobulin A to OMP E in sputum supernatants. None of ten adults who experienced lower respiratory tract infections due to B. catarrhalis demonstrated a clear-cut rise in antibody titer to OMP E in serum or sputum supernatant. This study has demonstrated that OMP E has at least one surface-exposed epitope which is highly conserved among strains of B. catarrhalis and which is located in the amino-terminal 184 amino acids of the molecule.

Production of a T-cell clone which reacts with membrane proteins of Acholeplasma laidlawii

The role of cellular immunity in mycoplasma infection is not completely understood. In this study, we established mycoplasma-specific T-cell clones to evaluate cellular immunity in mycoplasma infection. We developed a T-cell clone (G-10) which was stimulated with Acholeplasma laidlawii. The T-cell clone G-10, CD4+ and T-cell receptor (TCR) alpha beta- recognized the 42- and 65-kilodalton (kDa) membrane proteins of A. laidlawii and responded to A. hippikon. Hence, the application of mycoplasma-specific T cells such as G-10 in animal models may allow the assessment of cellular immune response to mycoplasma infection.