Variability of low-molecular-weight, heat-modifiable outer membrane proteins of Neisseria meningitidis

Photoferrotrophs Produce a PioAB Electron Conduit for Extracellular Electron Uptake

Photoferrotrophy is a form of anoxygenic photosynthesis whereby bacteria utilize soluble or insoluble forms of ferrous iron as an electron donor to fix carbon dioxide using light energy. They can also use poised electrodes as their electron donor via phototrophic extracellular electron uptake (phototrophic EEU). The electron uptake mechanisms underlying these processes are not well understood. Using Rhodopseudomonas palustris TIE-1 as a model, we show that a single periplasmic decaheme cytochrome c, PioA, and an outer membrane porin, PioB, form a complex allowing extracellular electron uptake across the outer membrane from both soluble iron and poised electrodes. We observe that PioA undergoes postsecretory proteolysis of its N terminus to produce a shorter heme-attached PioA (holo-PioAC, where PioAC represents the C terminus of PioA), which can exist both freely in the periplasm and in a complex with PioB. The extended N-terminal peptide controls heme attachment, and its processing is required to produce wild-type levels of holo-PioAC and holo-PioACB complex. It is also conserved in PioA homologs from other phototrophs. The presence of PioAB in these organisms correlate with their ability to perform photoferrotrophy and phototrophic EEU.IMPORTANCE Some anoxygenic phototrophs use soluble iron, insoluble iron minerals (such as rust), or their proxies (poised electrodes) as electron donors for photosynthesis. However, the underlying electron uptake mechanisms are not well established. Here, we show that these phototrophs use a protein complex made of an outer membrane porin and a periplasmic decaheme cytochrome (electron transfer protein) to harvest electrons from both soluble iron and poised electrodes. This complex has two unique characteristics: (i) it lacks an extracellular cytochrome c, and (ii) the periplasmic decaheme cytochrome c undergoes proteolytic cleavage to produce a functional electron transfer protein. These characteristics are conserved in phototrophs harboring homologous proteins.

Structural basis of outer membrane protein biogenesis in bacteria

In Escherichia coli, a multicomponent BAM (β-barrel assembly machinery) complex is responsible for recognition and assembly of outer membrane β-barrel proteins. The functionality of BAM in protein biogenesis is mainly orchestrated through the presence of two essential components, BamA and BamD. Here, we present crystal structures of four lipoproteins (BamB-E). Monomeric BamB and BamD proteins display scaffold architectures typically implied in transient protein interactions. BamB is a β-propeller protein comprising eight WD40 repeats. BamD shows an elongated fold on the basis of five tetratricopeptide repeats, three of which form the scaffold for protein recognition. The rod-shaped BamC protein has evolved through the gene duplication of two conserved domains known to mediate protein interactions in structurally related complexes. By contrast, the dimeric BamE is formed through a domain swap and indicates fold similarity to the β-lactamase inhibitor protein family, possibly integrating cell wall stability in BAM function. Structural and biochemical data show evidence for the specific recognition of amphipathic sequences through the tetratricopeptide repeat architecture of BamD. Collectively, our data advance the understanding of the BAM complex and highlight the functional importance of BamD in amphipathic outer membrane β-barrel protein motif recognition and protein delivery.

Storage and stability of IgG and IgM monoclonal antibodies dried on filter paper and utility in Neisseria meningitidis serotyping by Dot-blot ELISA

Background

A simple filter paper method was developed for, the transport and storage of monoclonal antibodies (Mabs) at room temperature or -20°C after spotting on filter paper, for subsequent serotyping of outer membrane antigens of N.meningitidis by dot-blot ELISA.

Methods

Monoclonal antibodies (Mabs) were spotted within a 0.5–1 cm diameter area of Whatman grade 903 paper, which were stored individually at room temperature or at -20°C. These MAbs were stored and analyzed after periods of one week, 4 weeks, 12 months, or 13 years in the case of frozen Mab aliquots, or after 4 weeks at -20°C or at room temperature (RT) in the case of Mabs dried on filter paper strips. Assays were performed in parallel using dot-blot ELISA. In addition to the MAbs specific for serotyping class 1, 2 or 3, we used a larger number of Mabs for polysaccharides, lipooligosaccharides (LOS), class 5 and cross-reactive antigens for native outer membrane of N.meningitidis. The Mabs dried on filter paper were eluted with phosphate-buffered saline (PBS) containing 0.2% gelatin.

Results

Mabs of the isotypes IgG and IgM dried on filter papers were not affected by duration of storage. The detection by serotyping Mabs was generally consistent for dried filter paper MAb samples stored frozen for over 1 year at -20°C, and although decreased reactive antibody titers were found after storage, this did not interfere with the specificity of the Mabs used after 13 years as dry spots on filter paper.

Conclusion

The use of filter paper is an inexpensive and convenient method for collecting, storing, and transporting Mab samples for serotyping studies. In addition, the samples occupy little space and can be readily transported without freezing. The efficiency of using immunoglobulin G (IgG) or M (IgM) eluted was found to be consistent with measurement of IgG or IgM titers in most corresponding, ascites Mabs stored frozen for over 1 year. The application of meningococcal typing methods and designations depend on the question being asked.

Interactions of pathogenic neisseriae with epithelial cell membranes

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

Differences in surface expression of NspA among Neisseria meningitidis group B strains

NspA is a highly conserved membrane protein that is reported to elicit protective antibody responses against Neisseria meningitidis serogroups A, B and C in mice (D. Martin, N. Cadieux, J. Hanel, and B. R. Brodeur, J. Exp. Med. 185:1173-1183, 1997). To investigate the vaccine potential of NspA, we produced mouse anti-recombinant NspA (rNspA) antisera, which were used to evaluate the accessibility of NspA epitopes on the surface of different serogroup B strains by an immunofluorescence flow cytometric assay and by susceptibility to antibody-dependent, complement-mediated bacteriolysis. Among 17 genetically diverse strains tested, 11 (65%) were positive for NspA cell surface epitopes and 6 (35%) were negative. All six negative strains also were resistant to bactericidal activity induced by the anti-rNspA antiserum. In contrast, of the 11 NspA surface-positive strains, 8 (73%; P < 0.05) were killed by the antiserum and complement. In infant rats challenged with one of these eight strains, the anti-rNspA antiserum conferred protection against bacteremia, whereas the antiserum failed to protect rats challenged by one of the six NspA cell surface-negative strains. Neither NspA expression nor protein sequence accounted for differences in NspA surface accessibility, since all six negative strains expressed NspA in outer membrane preparations and since their predicted NspA amino acid sequences were 99 to 100% identical to those of three representative positive strains. However, the six NspA cell surface-negative strains produced, on average, larger amounts of group B polysaccharide than did the 11 positive strains (reciprocal geometric mean titers, 676 and 224, respectively; P < 0.05), which suggests that the capsule may limit the accessibility of NspA surface epitopes. Given these strain differences in NspA surface accessibility, an rNspA-based meningococcal B vaccine may have to be supplemented by additional antigens.

Analysis of the human Ig isotype response to lactoferrin binding protein A from Neisseria meningitidis

An effective vaccine for serogroup B meningococci has yet to be developed and attention has turned to subcapsular antigens of the meningococcus as possible vaccine candidates. Iron binding proteins are being studied, with most interest focused on the transferrin binding proteins (TbpA and TbpB) and the ferric binding protein (FbpA). This study describes the purification of lactoferrin binding protein A (LbpA) from two meningococcal strains and assesses the human isotype-specific serum antibody response to these proteins in patients with proven meningococcal disease due to a range of phenotypes. Overall, fewer than 50% of sera contained IgG that recognised LbpA isolated from either strain and this antibody response was not uniform between the two proteins. There was some evidence that the antibody response varied between meningococcal phenotypes. This study demonstrates that LbpA does not induce a highly cross-reactive antibody response, indicating that it is unlikely to be an effective vaccine antigen.

Deletion of porA by recombination between clusters of repetitive extragenic palindromic sequences in Neisseria meningitidis

PorA is an important component in a vaccine against infection with Neisseria meningitidis. However, porA-negative meningococci were isolated from patients, thereby potentially limiting the role of PorA-mediated immunity. To analyze the mechanism by which the porA deletion occurred, the regions upstream and downstream of porA from three meningococcal strains (H44/76, H355, and 860183) were sequenced. The porA upstream region in strain 860183 contains a cluster of 22 repetitive palindromic RS3 core sequences (ATTCCC-N8-GGGAAT) and 10 RS3 core sequences (ATTCCC) in direct orientation. The cluster is flanked by neisserial repeats, so-called Correia elements, and can be subdivided into three repeats of 518 bp followed by a truncated repeat. The porA upstream region of the other two strains showed deletions, probably caused by a recombination between RS3 core sequences. The porA downstream region of H44/76 and H355 contains the IS1106 element followed by a cluster of 10 palindromic RS3 core sequences, 4 RS3 core sequences, and 1 other RS3 core sequence (GGGAAT) and is followed by a Correia element. This cluster can be subdivided into four direct repeats of 370 bp. Strain 860183 had two such repeats instead of four. Sequence analysis of the porA-negative variants indicated that the deletion of porA occurred via a recombination between two copies of the 116-bp region, containing two palindromic RS3 core sequences and a single RS3 core sequence. This region is homologous in the upstream and downstream clusters.

Nonopsonic phagocytosis of group C Neisseria meningitidis by human neutrophils

Although complement-mediated bactericidal activity in serum has long been known to be very important in host defense against Neisseria meningitidis, recent studies have shown that opsonic phagocytosis by neutrophils is also important. The purpose of this study was to determine if endemic group C N. meningitidis strains were susceptible to nonopsonic (complement- and antibody-independent) phagocytosis by human neutrophils, which is a well-described phenomenon for Neisseria gonorrhoeae. Gonococci that possess one or more of a group of heat-modifiable outer membrane proteins (called opacity-associated [Opa] proteins) are phagocytosed by neutrophils in the absence of serum. We found that four serogroup C meningococcal strains bearing the lacto-N-neotetraose (LNnT) structure on lipooligosaccharide (LOS) were phagocytosed by neutrophils in the absence of antibody and active complement. Confocal microscopy confirmed that the organisms were internalized by neutrophils. This susceptibility was not restricted to carrier isolates, since two of the strains were cultured from blood or cerebrospinal fluid. All four strains expressed Opa protein and had relatively less endogenous LOS and capsule sialylation compared to six strains that were resistant to this type of phagocytosis. Nonopsonic phagocytosis of two of the four strains was inhibited by exogenous sialylation of LOS LNnT and the binding of monoclonal antibody to LNnT. However, an isogenic mutant that lacked the LNnT structure was fully susceptible to nonopsonic phagocytosis. We conclude that group C meningococci can be phagocytosed by neutrophils in the absence of antibody and active complement possibly by two different mechanisms. Expression of Opa protein and downregulation of endogenous surface sialic acids analogous to what is seen for N. gonorrhoeae might be necessary for N. meningitidis as well.

Structural and functional characterization of a recombinant PorB class 2 protein from Neisseria meningitidis. Conformational stability and porin activity

An outer membrane PorB class 2 protein from Neisseria meningitidis has been overexpressed in Escherichia coli, isolated from inclusion bodies, and refolded in the presence of zwitterionic detergent. The purified recombinant and native (strain M986) counterpart exhibit most of the typical functional and structural properties that are characteristic of bacterial porins. Channel forming activity has been monitored by incorporating class 2 into reconstituted liposomes and measuring the permeation rates of various oligosaccharides through the proteoliposomes to derive a pore diameter of approximately 1.6 nm. Structural studies employing a combination of spectroscopic and electrophoretic techniques reveal that recombinant and native class 2 are identical in terms of overall conformational stability. Both proteins form stable trimers in zwitterionic detergent and retain significant secondary and tertiary structure in the presence of SDS. The thermal unfolding of zwittergen-solubilized class 2 trimers (Tm = 88 degrees C) is reversible and characterized by solvent exposure of aromatic residues with concomitant disruption of tertiary and partial loss of secondary structures. SDS-induced destabilization and irreversible unfolding of the native trimeric assembly occurs at temperatures above 60 degrees C. Our physicochemical studies of PorB class 2 protein furnish significant insight regarding the structural and functional properties of this meningococcal outer membrane protein within the porin superfamily.

Human immune response to epitopes on the meningococcal outer membrane class 5 protein following natural infection

Two monoclonal antibodies (mAbs) were produced against a serogroup B Neisseria meningitidis strain. These mAbs recognized two epitopes in the class 5 outer membrane proteins (OMP), designated P5.7 and P5.Bm, and were able to kill the homologous strain through complement activation. Both epitopes were surface exposed and 68% of group B meningococcal clinical isolates had one or both epitopes present in their class 5 OMP. Antibodies to one or both epitopes were demonstrated in 17 patients with meningococcal meningitis using an ELISA inhibition assay. Of the 17 paired sera, 41% and 29% of the acute-phase sera had antibodies to the P5.7 and P5.Bm epitopes, respectively. Immunoglobulin G to P5.Bm were found in all 17 convalescent-phase sera while specific antibodies against P5.7 were only found in 6 of these sera. These results demonstrate the potential importance of the P5.Bm and P5.7 epitopes on the class 5 OMP as candidates for vaccine composition.

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

Former studies have shown that the class 5 outer membranes proteins (Opa and Opc proteins) of Neisseria meningitidis are at least as immunogenic as meningococcal porin proteins. High antibody titers to class 5 proteins have been observed in sera obtained during convalescence after meningococcal infection. A strong increase in anti-class 5 antibodies has also been observed in vaccinees who received a meningococcal outer membrane vesicle preparation. The enhanced B-cell response to class 5 proteins may be due to the presence of immunodominant helper T-cell epitopes in these proteins. In order to investigate this hypothesis, we tested purified Opa, Opc, and class 1 proteins for recognition by human T cells. a hierarchy of T-cell immunogenicity was observed among the outer membrane proteins, the Opa protein being more immunogenic than the other proteins. In most cases, the proliferative responses elicited by Opc were higher than the responses observed for the class 1 protein. The epitopes recognized by the immune T cells were identified by using overlapping synthetic peptides spanning the protein sequences of OpaB, Opa5d, and Opc.

Surface antigen analysis of group B Neisseria meningitis outer membrane by monoclonal antibodies: identification of bactericidal antibodies to class 5 protein

Twenty-four monoclonal antibodies (mAbs) against group B Neisseria meningitidis surface antigens were analyzed by immunoenzymatic assays and by a bactericidal test. Two mAbs were specific to polysaccharide B and one to lipopolysaccharide. The others were specific to polysaccharide B and one to lipopolysaccharide. The others were directed against outer membrane proteins ranging in molecular mass from 25 to 200 kDa. The outer membrane protein epitopes recognized by the mAbs were not conformational and were located on the outer surface of the microorganism. Linear epitopes on the class 5 protein, exposed on the surface of the membrane, were able to induce bactericidal antibodies to the homologous strain. The susceptibility of Neisseria meningitidis to these antibodies was unchanged when this organism was cultivated under conditions of iron depletion. These results demonstrate that peptides derived from class 5 proteins are potentially important in synthetic peptide or in recombinant protein vaccines containing linear bactericidal epitopes.

Variable expression of class 1 outer membrane protein in Neisseria meningitidis is caused by variation in the spacing between the -10 and -35 regions of the promoter

The class 1 outer membrane protein encoded by the porA gene of Neisseria meningitidis is a candidate for a vaccine against meningococcal infection. The expression of class 1 outer membrane protein displays phase variation between three expression levels. Northern (RNA) blot and primer extension analysis revealed that this phase variation is regulated at the transcriptional level. The start site for transcription is located 59 bp upstream of the translational initiation codon. Sequence analysis of the promoter region of the porA gene of a variant without class 1 protein expression revealed nine contiguous guanidine residues between the -10 and -35 domains. Comparison of promoter sequences of different phase variants indicated that the length of the polyguanidine stretch correlated with the expression level of the class 1 outer membrane protein; the presence of 11, 10, or 9 contiguous guanidine residues results in high levels, medium levels, or no expression of class 1 mRNA, respectively. These results suggest that the variable porA expression levels seen in different isolates are modulated by guanidine residue insertion and/or deletion due to slipped-strand mispairing on the polyguanidine stretch within the intervening sequence of the -35 and -10 regions of the promoter. The phase variation of class 1 outer membrane protein may provide a molecular mechanism to evade the host immune defense. Therefore, the protective efficacy of a vaccine based on class 1 outer membrane protein may be questioned.

Microevolution within a clonal population of pathogenic bacteria: recombination, gene duplication and horizontal genetic exchange in the opa gene family of Neisseria meningitidis

Opacity (Opa) proteins are a family of antigenically variable outer-membrane proteins of Neisseria meningitidis. Even among clonally related epidemic meningococcal isolates, there is greater variation of Opa protein expression than can be accounted for by the opa gene repertoire of any individual strain. We characterized the opa genes of eight closely related isolates of serogroup A N. meningitidis (subgroup IV-1) from a recent meningitis epidemic in West Africa. DNA sequence analysis and Southern blot experiments indicated that changes occurred in the opa genes of these bacteria as they spread through the human population, over a relatively short period of time. Such changes in one or a few loci within a clonal population are referred to as microevolution. The distribution of sequences present in hypervariable (HV) regions of the opa genes suggests that duplication of all or part of opa genes into other opa loci changed the repertoire of Opa proteins that could be expressed. Additional variability in this gene family appears to have been introduced by horizontal exchange of opa sequences from other meningococcal strains and from Neisseria gonorrhoeae. These results indicate that processes of recombination and genetic exchange contributed to variability in major surface antigens of this clonal population of pathogenic bacteria.

Molecular characterization of the 98-kilodalton iron-regulated outer membrane protein of Neisseria meningitidis

When grown under iron limitation, Neisseria meningitidis expresses several additional outer membrane proteins (OMPs), which were studied to assess their vaccine potential. Two monoclonal antibodies were obtained against a 98-kDa OMP of strain 2996 (B:2b:P1.2). Cross-reactivity studies revealed that the two antibodies reacted with 44 and 42 of 74 meningococcal strains, respectively. The antibodies did not block the binding of transferrin or lactoferrin to intact cells. The structural gene for the protein, tentatively designated iroA, was isolated and sequenced. Computer analysis revealed homology to the ferric siderophore receptors in the outer membrane of Escherichia coli and to gonococcal transferrin-binding protein 1 (TbpA). The high degree of cross-reactivity and the results of Southern blot analyses, which showed that the iroA gene is also present in strains that did not react with the monoclonal antibodies, suggest that the 98-kDa OMP is well conserved among meningococci and that it is a suitable vaccine candidate. However, the antibodies were not bactericidal in an in vitro assay with human complement.

Use of transformation to construct antigenic hybrids of the class 1 outer membrane protein in Neisseria meningitidis

The class 1 protein of Neisseria meningitidis is an important component of candidate outer membrane vaccines against meningococcal meningitis. This porin protein contains two variable regions which determine subtype specificity and provide binding sites for bactericidal monoclonal antibodies. To determine the contribution of each of these variable regions in the induction of bactericidal antibodies, a set of isogenic strains differing only in their class 1 epitopes was constructed. This was done by transformation of meningococcal strain H44/76 with cloned class 1 genes and selection of the desired epitope combinations in a colony blot with subtype-specific monoclonal antibodies. When used for the immunization of mice, outer membrane complexes induced bactericidal antibodies only against meningococcal strains sharing at least one of their class 1 epitopes. The results demonstrate that the P1.2 and P1.16 epitopes, normally located in the fourth exposed loop of the protein, efficiently induce bactericidal antibodies independently of the particular sequence in the first variable region. The P1.5 and P1.7 epitopes, normally located in the first exposed loop, were found to induce lower bactericidal titers. Hybrid class 1 outer membrane proteins were constructed by inserting oligonucleotides encoding the P1.7 and P1.16 epitopes into the porA gene. In this way, we obtained a set of strains which carry the P1.5 epitope in loop 1, P1.2 in loop 4, and P1.7 and P1.16 (separately or in combination) in either loop 5 or loop 6. The additional epitopes were found to be exposed at the cell surface. Outer membrane complexes from several of these strains were found to induce a bactericidal response in mice against the inserted epitopes. These results demonstrate that it is feasible to construct meningococcal strains carrying multivalent class 1 proteins in which multiple subtype-specific epitopes are present in different cell surface-exposed loops.

Construction of a multivalent meningococcal vaccine strain based on the class 1 outer membrane protein

Outer membrane complexes (OMCs) are promising vaccine candidates for protection against meningococcal disease. However, a major obstacle to this approach is the fact that the protective antibodies induced are generally type specific. In an attempt to overcome this problem, we have investigated the possibility of constructing a multivalent vaccine strain by insertion of an additional class 1 outer membrane protein-encoding gene. Starting with a derivative of strain H44/76 deficient in class 3 outer membrane protein, a second class 1 gene was inserted into the chromosome, through homologous recombination with a suicide plasmid carrying the class 1 gene from strain 2996 placed within a class 5 gene. In this way, a strain was obtained in which a class 3 protein was in effect replaced by a class 1 protein from another subtype, i.e. P1.5,2 in addition to the P1.7,16 protein of H44/76. Immunization of mice with such OMCs resulted in high bactericidal titers against both H44/76 and 2996, where normally only strain-specific antibodies are induced. Mutational removal of class 3 protein from the immunizing OMCs had no detectable effect on the bactericidal titer against H44/76, whereas removal of class 1 protein led to a strong reduction. These results demonstrate the dominant role of the subtype-specific sequences of class 1 protein in the induction of bactericidal antibodies and show that construction of a multivalent OMC-based vaccine should be feasible.

Cloning and expression in Escherichia coli of opc, the gene for an unusual class 5 outer membrane protein from Neisseria meningitidis (meningococci/surface antigen)

A genomic library was constructed in a lambda gt11 vector using chromosomal DNA from a meningococcal serogroup A strain and plaques expressing the class 5C protein were recognized by screening with specific monoclonal antibodies. The opc insert was subcloned into a multicopy plasmid which induced expression of that protein in Escherichia coli as a surface-exposed major outer membrane protein. The nucleotide sequence of opc is typical of an outer membrane protein with a promoter and terminator region, a leader peptide which is cleaved during expression and a complete open reading frame. Unlike other meningococcal class 5 proteins or gonococcal P.II proteins, the sequence did not contain any pentanucleotide repeats and the sequence showed little homology to these other functionally related proteins. However, the predicted amino acid sequence of the mature protein for opc showed 27% similarity to that for a second opa gene cloned from the same meningococcal strain. This is the first report of cloning and expression of a functional meningococcal gene encoding a class 5 outer membrane protein in E. coli.

Topology of outer membrane porins in pathogenic Neisseria spp

In Escherichia coli, membrane-spanning amphipathic beta-sheet structures are characteristic of many outer membrane proteins. By applying the principles that have been recognized for them to the four classes of neisserial porins, we have constructed a model for the topology of the porins within the outer membrane. This model predicts eight surface-exposed loops, both in the meningococcal class 1 and 2 proteins and in the gonococcal PIA and PIB proteins. The transmembrane sequences are highly conserved among these porins and are able to form an amphipathic beta-sheet structure. The surface-exposed hydrophilic loops show extensive variation in both length and sequence. Experimental evidence in support of this model has been obtained by using antisera against synthetic peptides which correspond to surface-exposed loops in class 1 and 2 proteins. Thus, binding to the cell surface was observed with antibodies against loops 1, 4, and 5 of class 1 and loops 1 and 5 of class 2. In class 1, these loops are the longest ones and show the highest sequence diversity among strains of different subtypes. Mapping of epitopes recognized by monoclonal antibodies with bactericidal activity has also provided strong support for the model. The epitopes are located in loops 1 and 4 of class 1 protein, loop 5 of PIB, and loop 6 of PIA. A nonbactericidal antibody that binds only weakly to whole cells was shown to recognize loop 3 of PIB. These results suggest that the longest loops are immunodominant, provide the binding sites for bactericidal antibodies, and display the greatest variation among different strains.

Characterization of serogroup A and B strains of Neisseria meningitidis with serotype 4 and 21 monoclonal antibodies and by multilocus enzyme electrophoresis

The reactions of serogroup A strains of Neisseria meningitidis with one monoclonal antibody specific for serotype 21 and three different monoclonal antibodies specific for serotype 4 were compared with those of serogroup B strains previously assigned to serotype 4. Antibody binding was studied by enzyme-linked immunosorbent assay (ELISA), dot blotting, and immunoblotting. Characterization of the isolates by the electrophoretic mobilities of 14 metabolic enzymes showed 50 multilocus enzyme genotypes. All except two genotypes fell into three distinct clusters: I, IIa and IIb. The enzyme genotypes of serogroup B strains were mainly in cluster I, and 88% of the serogroup A strains had genotypes in clusters IIa and IIb. Serogroup B strains generally reacted with all three serotype 4 monoclonal antibodies in ELISA and dot blotting but with only two in immunoblots. Serogroup A strains showed two different reactions in the blotting methods: either binding of the serotype 21 antibody only or binding of this and two of the three serotype 4 monoclonal antibodies. Strains of the first pattern were in clusters I and IIa, whereas all but two strains in cluster IIb were of the second pattern. In ELISA, an additional reaction of two of the serotype 4 monoclonal antibodies with serogroup A isolates was observed. The different binding of these two monoclonal antibodies in ELISA and the blotting methods appeared to result from heat inactivation of the meningococcal cells and use of detergent-containing reagents in ELISA. The results show that the serotype of serogroup A strains is distinct from serotype 4 of serogroup B strains.

Characterization of the opa (class 5) gene family of Neisseria meningitidis

Class 5 outer membrane proteins of Neisseria meningitidis show both phase- and antigenic variation of expression. The proteins are encoded by a family of opa genes that share a conserved framework interspersed with three variable regions, designated the semivariable (SV) region and hypervariable regions 1 (HV1) and 2 (HV2). In this study, we determined the number and DNA sequence of all of the opa genes of meningococcal strain FAM18, to assess the structural and antigenic variability in the family of proteins made by one strain. Pulsed field electrophoresis and Southern blotting showed that there are four opa genes in the FAM18 chromosome, and that they are not tightly clustered. DNA sequence analysis of the four cloned genes showed a modest degree of diversity in the SV region and more extensive differences in the HV1 and HV2 regions. There were four versions of HV1 and three versions of HV2 among the four genes. Each of the FAM18 opa loci contained a gene with a unique combination of SV, HV1, and HV2 sequences. We used lambda gt11 cloning and synthetic peptides to demonstrate that HV2 sequences completely encode the epitopes for two monoclonal antibodies specific for different class 5 proteins of FAM18.

Meningococcal vaccines--present and future

Over 20 years after the development of the meningococcal A and C vaccines, an effective vaccine against Neisseria meningitidis group B is still lacking. Major obstacles in the development of a B vaccine have been the remarkable capacity of the organism to evade the immune defences of the host and the lack of a predictive animal model. Three group B vaccines based on outer membrane proteins have been, or are currently being, evaluated in field trials. Nevertheless, a number of important questions remain such as the identity of the active components, the degree of efficacy against heterologous group B subtypes, and the duration of protection. In addition, work on a variety of alternative approaches to a group B vaccine is rapidly progressing. Among these are use of chemically modified group B polysaccharide, synthetic or natural lipopolysaccharide epitopes, synthetic peptides corresponding to bactericidal epitopes on the class 1 outer membrane protein, and iron binding proteins. Although each of these approaches has some problems associated with it, the prospects remain good for an effective solution to the group B problem.

Pathogenic neisseriae--a model of bacterial virulence and genetic flexibility

The outcome of the early stages of a neisserial infection is determined by receptor-mediated events that culminate in the attachment and invasion of human mucosal tissues. The factors participating in this process, including pili, opacity proteins (Opa), and perhaps lipopolysaccharide (LPS), are subject to complex genetic controls that allow these factors to be produced in multiple forms. Antigenic variation allows the pathogenic Neisseriae to evade the human immune response, and facilitates their interaction with a variety of different cells and tissues of the human host. One of the major genetic mechanisms causing antigenic variation is transformation, which allows virulence genes to be exchanged and recombined between independent Neisseria strains within multiply infected individuals. A number of other factors, such as IgA protease, alpha-factor, and the meningococcal capsule are also implicated in pathogenesis and render the pathogenic Neisseriae an excellent model for the investigation of bacterial virulence.

Stable expression of meningococcal class 1 protein in an antigenically reactive form in outer membranes of Escherichia coli

The entire gene encoding the class 1 outer membrane protein of Neisseria meningitidis is located on a 2.2kb fragment, obtained on digestion of chromosomal DNA with Xbal. This Xbal fragment from strain MC50 (subtype P1-16), which had previously been cloned in bacteriophage M13, has been transferred to the plasmid vector pMTL20. The resulting plasmid (pPORA100) was propagated in Escherichia coli (JM109) and cell lysates were subjected to SDS-PAGE. Western blotting with anti-class 1 protein antibodies revealed constitutive expression of a protein of 41 kD, corresponding to the class 1 protein of the parent meningococcal strain, which was absent in the E. coli control. Fractionation of E. coli cells carrying the recombinant plasmid revealed that the protein was exclusively located in the outer membrane, and N-terminal amino acid analysis of the expressed protein revealed that normal processing of the signal peptide had occurred. Immuno-gold electron microscopy showed that the protective epitope recognized by a P1-16 subtype-specific monoclonal antibody was exposed in an antigenically reactive form on the surface of E. coli cells carrying plasmid pPORA100. In contrast, expression in E. coli of a second plasmid (pPORA104) lacking the coding sequence for the first 15 amino acids of the signal peptide resulted in accumulation of recombinant class 1 protein only in the cytoplasm of the cells. Thus the presence of the meningococcal signal sequence ensures expression of this meningococcal porin protein in an antigenically native conformation in outer membranes of E. coli, while its absence results in expression of a soluble protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Isolation of Neisseria meningitidis mutants deficient in class 1 (porA) and class 3 (porB) outer membrane proteins

The class 1 major outer membrane protein of Neisseria meningitidis is a serious candidate for a meningococcal vaccine. To facilitate studies on the function of this protein, mutants were isolated that lacked this protein or the structurally related class 3 protein. These mutants were obtained by using the antibody-dependent bactericidal action of the complement system. The class 1 protein-deficient strain grew normally in vitro, whereas growth of the class 3 protein-deficient strain was slightly retarded. The class 3 protein-deficient strain displayed increased resistance to the antibiotics tetracycline and cefsulodin, which is consistent with the proposed role of the protein as a pore-forming protein. The class 1 protein was purified to homogeneity from the class 3 protein-deficient strain. Lipid bilayer experiments revealed that this protein also formed pores. The class 1 protein pores were cation selective.

Increased adherence to vaginal epithelial cells and phagocytic killing of gonococci and urogenital meningococci associated with heat modifiable proteins

Urogenital Neisseria meningitidis were characterized with regard to serogroup, colony morphology, the presence of heat modifiable proteins (HMP), attachment to human vaginal and buccal epithelial cells, and phagocytic killing by polymorphonuclear leukocytes. The findings were compared with those on gonococci, and with those on meningococci isolated from blood or cerebrospinal fluid, with regard to colony morphology, HMP and piliation. The opacity colony morphology characteristic could be used to predict the presence of HMP in gonococci but not in meningococci, and sodium dodecyl sulphate polyacrylamide gel electrophoresis had to be used to demonstrate this surface protein. The urogenital meningococci, serogroup Y, attached significantly more efficinetly to vaginal epithelial cells in the presence of HMP and behaved in this respect like those of gonococci. Gonococci and meningococci containing HMP were more sensitive to phagocytic killing than those without HMP. Meningococci from opaque and transparent colonies and isolated from patients with meningococcal disease had no demonstrable HMP. They showed low adherence to vaginal and buccal epithelial cells, with no difference between organisms from opaque or transparent colonies.

Human immune response to meningococcal outer membrane protein epitopes after natural infection or vaccination

Antibody levels in 41 sets of human acute- and convalescent-phase meningococcal sera were compared with those in 23 sets of human prevaccination and 2-week postvaccination sera. We used a modification of a solid-phase radioimmunoassay (SPRIA) technique to test each of the human serum samples as inhibitors of monoclonal antibodies (MAbs) that bind (HIMSPRIA) to the outer membrane complex from a 2a:P1.2:P5.1 strain. We used three murine MAbs specific for the 2a, P1.2, and P5.1 epitopes on meningococcal class 1, 2, and 5 proteins, respectively, to detect antibodies with similar specificities in human sera. Each of 40 available matching strains from patients were also screened with the three MAbs in a nitrocellulose spot blot assay. A total of 37 (92%) were positive for the 2a epitope, 36 (90%) were positive for the P1.2 epitope, and 16 (40%) were positive for the P5.1 epitope. Of 38 available convalescent-phase sera, 27 (71%) matched with these strains and had detectable inhibiting antibody for each of the MAb-defined protein epitopes of the infecting strain. Three convalescent-phase sera had no HIMSPRIA activity for MAb-defined epitopes that were present on the infecting strain; others had activity for one or two of the epitopes. The results were similar for pre- and postvaccination sera. The average level of HIMSPRIA activity for the P1.2 epitope was greater than fivefold higher in postvaccination sera compared with that in convalescent-phase sera. Sera with distinct patterns of HIMSPRIA activity also were tested by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblot analysis and showed a correlation between the HIMSPRIA activity for particular epitopes and the level of antibody binding to the immunoblotted proteins possessing those epitopes. A comparison of the HIMSPRIA and the bactericidal activity of selected postvaccination sera indicated a possible correlation between HIMSPRIA and bactericidal activity, but it also suggested the presence of bactericidal antibodies with specificities other than those defined by the MAbs.

The class 1 outer membrane protein of Neisseria meningitidis: gene sequence and structural and immunological similarities to gonococcal porins

The class 1 protein is a major protein of the outer membrane of Neisseria meningitidis, and an important immunodeterminant in humans. The complete nucleotide sequence for the structural gene of a class 1 protein has been determined. The sequence predicts a protein of 374 amino acids, preceded by a typical signal peptide of 19 residues. The hydropathy profile of the predicted protein sequence resembles that of the Escherichia coli and gonococcal porins. The predicted protein sequence of the class 1 protein exhibits considerable structural similarity to the gonococcal porins PIA and PIB. Western blot studies also reveal immunologically conserved domains between the class 1 protein, PIA and PIB. A restriction fragment from the class 1 gene hybridizes to gonococcal genomic fragments in Southern blots. In addition to the class 1 gene coding region there is a large open reading frame on the opposite strand.

Sero- and subtypes of group B meningococci causing invasive infections in Finland in 1976-87

Neisseria meningitidis group B (MenB) strains isolated from 1976 to 1987 in Finland in 339 patients with invasive infection were sero/subtyped by whole cell enzyme immunoassay using monoclonal antibodies to class 1 and 2/3 outer membrane proteins. 66.7% of the strains could be serotyped (class 2/3) and 70.2% subtyped (class 1). No single phenotype was clearly predominant. The most common serotypes were 4 (18.6%) and 14 (17.4%) and the most common subtypes P1.16 (20.1%) and P1.2 (12.1%). The Norwegian phenotype B:15:P1.16 was seen only rarely (a total of 18 strains). Strains from Northern Finland did not differ from those from Southern Finland: no single phenotype caused the slight increase seen in the incidence of MenB infections in the end of 1970s in the North.

Definition of meningococcal class 1 OMP subtyping antigens by monoclonal antibodies

The subtypes of meningococci are defined by antigenic determinants on the class 1 outer membrane proteins. The established subtypes, designated by P1 and a number according to the prototype reference strain on which they were first recognized by monoclonal antibodies, includes P1.2, P1.9, P1.15 and P1.16. We have investigated more prototype reference strains, using new monoclonal antibodies, and identified the new subtypes P1.1, P1.6 and P1.1,16. The P1.1,16 epitope is found on both the P1.1 and the P1.16 reference strains, but not on all P1.1 and P1.16 strains and can occur independently from the P1.1 and the P1.16 epitopes. It appears that class 1 outer membrane proteins contain at least two independent subtype-specific epitopes. For clarity, we now redefine P1.1,16 as P1.7, permitting thus the identification of strains of P1.1, P1.1,7, P1.7, P1.7,16 and P1.16 subtypes. It can clearly be expected that more class 1 outer membrane protein determinants will be recognized as more monoclonal typing antibodies are produced. The monoclonal antibodies now available to us can subtype 80-90% of group B and C meningococci; they also react with group A meningococci, but not with other Neisseriae. The immunological dissection of these subtyping antigens will improve our understanding of the relationship between components of the bacteria and the induction or prevention of disease.

Purification and characterization of eight class 5 outer membrane protein variants from a clone of Neisseria meningitidis serogroup A

Methods published for the purification of P.II proteins from Neisseria gonorrhoea have been modified to allow the purification of class 5 proteins from Neisseria meningitidis serogroup A bacteria. The five class 5 protein electrophoretic variants detected within an epidemic in the Gambia (a, b, c, d, and e) and three other variants (f, g, and h) found within other isolates of the same clone in West Africa have been purified with yields of 6-28 mg. The NH2-terminal amino acid sequence for variant c differs from those of the other class 5 proteins, whereas the latter are very similar to the sequence predicted for two class 5 proteins from DNA analyses of serogroup C meningococci and determined for 8 P.II proteins from gonococci. Numerous other regulatory, chemical, and serological differences were found between the c protein and the other class 5 proteins such that we recommend that the class 5 proteins be subdivided into two subclasses. mAbs have been isolated that distinguish between these two protein subclasses and Western blotting with these antibodies enabled us to conclude that both protein subclasses were found in bacteria isolated from different epidemics and pandemics of the last 50 yr.

Molecular cloning and expression of Neisseria meningitidis class 1 outer membrane protein in Escherichia coli K-12

A genomic library of meningococcal DNA from a clinical isolate of Neisseria meningitidis was constructed in the expression vector lambda gt11. Outer membrane complex was prepared from the same strain and used to immunize rabbits to raise polyclonal anti-outer membrane complex serum. The amplified library was probed with this polyclonal serum, and seven expressing recombinants were isolated; further investigations indicated these to be identical. The expressed meningococcal gene in these recombinants was fused to vector beta-galactosidase and shown to encode epitopes present on the 42-kilodalton class 1 outer membrane protein. Estimation of the size of the recombinant fusion protein suggests that up to 40 kilodaltons of protein-coding sequence is present. The lambda gt11 recombinant contains a 3.4-kilobase DNA insert, which has been recloned into a plasmid and characterized by restriction endonuclease analysis. A restriction fragment from the insert, representing the protein-coding region hybridizes to a single 2.2-kilobase XbaI fragment from the homologous strain and to similar-sized XbaI fragments in other strains of meningococci, expressing antigenically distinct class 1 proteins.

Comparison of meningococcal outer membrane protein vaccines solubilized with detergent or C polysaccharide

Outer membrane proteins (OMPs) were isolated from meningococcal strain H44/76 (B:15:P1.16) by detergent extraction of bacteria. A final product containing class 1 (P1.16), 3(15), 4 OMPs and 5% (w/w) lipooligosaccharide was obtained. Two experimental vaccines were prepared: OMP-detergent and OMP-C polysaccharide. The OMP-detergent vaccine tended to show a better bactericidal: ELISA ratio for the antibodies induced as compared to the OMP-C polysaccharide vaccine. The vaccine induced bactericidal antibodies appeared for the greater part to be directed against the class 1 OMP (P1.16). By comparison of cultures grown in Mueller Hinton Broth with and without 0.25% (w/v) glucose, it was found that monoclonal antibodies against the serotype OMP (class 2 or 3) were not bactericidal against meningococci grown in MHB without glucose. Antibodies against class 1 OMP and lipooligosaccharide were not influenced by this. A new major outer membrane protein (appr. 40 kd) is described that may function as a cation-specific porin.

SDS-PAGE analysis of membrane proteins of group A Neisseria meningitidis isolated before and during an epidemic of group A meningococcal disease in northern Nigeria

Meningococcal membrane protein patterns were studied by SDS-PAGE analysis of group A meningococci isolated before and during a large epidemic of group A meningococcal disease in northern Nigeria. No difference was found in the membrane protein patterns of strains isolated before or during the 3 years of the epidemic. Isolates obtained from cases and carriers had similar membrane protein patterns.

Meningococcal disease in The Netherlands, 1959-1981: the occurrence of serogroups and serotypes 2a and 2b of Neisseria meningitidis

By means of a filter radioimmunoassay and the use of monoclonal anti-2a and anti-2b antibodies, we have serotyped 3164 of 3688 strains of Neisseria meningitidis isolated from patients in The Netherlands between 1959 and 1981. Serotypes 2a and 2b were distributed differently among the major serogroups A, B, C, and W-135. Neither of the types was found among group A strains. Type 2b strains of serogroup B emerged in 1965, causing a country-wide epidemic which reached a peak incidence in March and April of 1966 and continued to predominate within group B until 1979. Type 2a strains of serogroup C were responsible for a substantial number of sporadic cases over a long period without any association with outbreaks or with a shift in the pattern of the serogroup. After the appearance of group W-135 in 1971, W-135 strains caused a small non-focal epidemic wave. The upsurge of disease due to virulent sub-populations of strains B:2b and C:2a appeared to be closely related to a basic pattern of regular cyclical waves with peak intervals which differed for serogroups A, B, and C. In recent years both serotype 2a and 2b strains within the different serogroups fell to insignificant numbers. Our results show that retrospective large-scale serotyping of collected strains provides insight into the epidemiological patterns of endemic meningococcal disease.

Human antibody response to a group B serotype 2a meningococcal vaccine determined by immunoblotting

The antibody response of 30 volunteers vaccinated with a complex of group B polysaccharide and outer membrane vesicles (OMV) from serotype 2a Neisseria meningitidis and of 3 individuals who received a placebo vaccine was determined by immunoblotting. OMV were separated by sodium dodecyl sulfate-gel electrophoresis and electrotransferred to nitrocellulose filters. Binding of immunoglobulin G (IgG), IgA, and IgM antibodies in the human sera to OMV components was detected with class-specific peroxidase-conjugated antibodies. The immunoblotting results were also related to the bactericidal activity of the sera and the meningococcal carrier status of the volunteers. Before vaccination weakly reactive bands in the molecular weight range of 140,000 to 10,000 were observed on the blots. Sera from carriers showed more marked bands. Individual patterns of increased reactivity were seen 6 weeks after vaccination. The main immunoreactive components of OMV corresponded to a molecular weight of 43,000 (class 1 protein), 30,000 (class 5 proteins), and 22,000. IgG antibodies in postvaccination sera of high bactericidal titers showed distinct binding to the 43,000-molecular-weight antigen. Meningococcal carriers had antibodies against an antigen of 22,000 molecular weight; in polyacrylamide gels this component did not stain with Coomassie brilliant blue or silver. The marked binding of IgG antibodies to the class 5 proteins decreased considerably between weeks 6 and 25 after vaccination. Periodate oxidation of OMV abolished the binding of IgG antibodies to the class 5 proteins, whereas the antigenicity of the 43,000-molecular-weight (class 1 protein) and 22,000-molecular-weight antigens was unaffected.

Identification and characterization of Vibrio cholerae surface proteins by radioiodination

Whole cells and isolated outer membrane from Vibrio cholerae (Classical, Inaba) were radiolabeled with Iodogen or Iodo-beads as catalyst. Radiolabeling of whole cells was shown to be surface specific by sodium dodecyl sulfate-urea polyacrylamide gel electrophoresis of whole cells and cell fractions. Surface-labeled whole cells regularly showed 16 distinguishable protein species, of which nine were found in radiolabeled outer membrane preparations obtained by a lithium chloride-lithium acetate procedure. Eight of these proteins were found in outer membranes prepared by sucrose density gradient centrifugation and Triton X-100 extraction of radiolabeled whole cells. The mobility of several proteins was shown to be affected by temperature, and the major protein species exposed on the cell surface was shown to consist of at least two different peptides.

Monoclonal antibodies to serotype 2 and serotype 15 outer membrane proteins of Neisseria meningitidis and their use in serotyping

A series of murine monoclonal antibodies to serotype 2 and serotype 15 strains of Neisseria meningitidis were produced which were specific for outer membrane proteins of classes 1, 2, 3, and 5. A panel of eight monoclonal antibodies that exhibited a high degree of serotype specificity when reacted with prototype strains of known serotype were selected for study. Each of the corresponding epitopes was localized on a specific outer membrane protein by means of immunoprecipitation, electroblotting, or both. The serotype 2a-, 2b-, and 2c-specific antibodies bound to the class 2 protein, the serotype 15-specific antibody bound to the class 3 protein, two antibodies (3-1-P1.2 and 3-1-P1.16) bound to class 1 proteins, and two antibodies (1-1-P5.1 and 3-1-P5.2) bound to class 5 proteins. Six of these monoclonal antibodies were used in a spot-blot procedure to survey 122 case isolates (groups B, C, Y, and W135) and 363 carrier isolates (all serogroups) for the presence of the 2a, 2b, 2c, 15, P1.2, and P1.16 epitopes. A total of 66% of the case isolates and 30% of the carrier isolates reacted with one or more of the monoclonal antibodies. The use of monoclonal antibodies for serotyping of meningococci appears to be feasible and easy and appears to have significant advantages over the use of polyclonal typing sera.

Sodium dodecyl sulfate-polyacrylamide gel typing system for characterization of Neisseria meningitidis isolates

Thirty to fifty percent of group B and group C Neisseria meningitidis carrier isolates are not serotypable with existing outer membrane protein typing sera. A typing system based on differences in the outer membrane protein profiles after sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) was therefore developed as an adjunct to existing serotyping methods. Although most N. meningitidis strains contain several outer membrane proteins visible by SDS-PAGE, there are only one to three predominant proteins. The SDS-PAGE profiles of these major proteins were used to establish 10 different PAGE types. Greater than 95% of all meningococcal isolates, regardless of serogroup, fit into 1 of the 10 PAGE types. The outer membrane protein profile of individual strains after SDS-PAGE was constant when outer membrane fractions were prepared from the same strain on several different days. A comparison of gel profiles of meningococcal isolates obtained from different sites of the same patient revealed no significant differences among both major and minor proteins for isolate sets thus far examined. Characterization of strains by PAGE type can be a valuable epidemiological tool in addition to serotyping and in the absence of specific serotype antisera.