The class 1 outer membrane protein of Neisseria meningitidis produced in Bacillus subtilis can give rise to protective immunity

Computational prediction of extracellular loops of the Por39 outer membrane porin of Rhodospirillum rubrum suitable for epitope surface display

Outer membrane porins from Gram-negative bacteria are established vehicles for the production of vaccines. Typically, one or more of the extracellular loops of a porin are replaced by a peptide encoding a foreign epitope, and recombinant porin is then used as a vaccine. However, many host strains are potentially pathogenic, and also produce toxic lipopolysaccharide (LPS), both of which are undesirable for safety reasons. In contrast, the outer membrane porins from photosynthetic, purple bacteria have no known human pathology and produce only weakly toxic LPS. The purple bacterium Rhodospirillum rubrum is well-suited for large-scale biotechnology, and expresses a major porin, Por39, which is a candidate for a vaccine platform. Unfortunately, the atomic structure of Por39 could not be determined so far, and Por39 shows only a weak homology to other porins of known structure, making the assignment of external loops difficult. Here, we construct a knowledge-based model of Por39 using secondary structure constraints from both the low sequence homology to the 2POR porin from Rhodobacter capsulatus, for which the X-ray structure is known, as well as those obtained using secondary structure prediction packages. The secondary structure predictions were used to constrain a three-dimensional model created using the I-TASSER package. The modelling procedure was validated by predicting the structure of 2POR using the same strategy, but excluding the 2POR X-ray structure from the I-TASSER database. The final Por39 model allows three external loops to be defined precisely, and could also be used to obtain an initial model for the closely related Por41 using molecular modelling. These structures provide a good starting point for the insertion of epitopes with vaccine potential.

Regulated protein aggregation: a mechanism to control the activity of the ClpXP adaptor protein YjbH

Bacteria use stress response pathways to activate diverse target genes to react to a variety of stresses. The Bacillus subtilis Spx protein is a global transcriptional regulator that controls expression of more than 140 genes and operons in response to thiol-specific oxidative stress. Under nonstress conditions the concentration of Spx is kept low by proteolysis catalyzed by the ClpXP complex. Spx protein levels increase in response to disulfide stress and decrease when the cells cope with the stress. The cytosolic adaptor protein YjbH is required to target Spx for efficient proteolysis by ClpXP. We demonstrate that YjbH aggregates in response to disulfide stress, that is, the YjbH protein is soluble under nonstressed conditions and destabilized during stress leading to aggregation. Stress conditions (heat and ethanol) that cause severe perturbations in protein stability/folding also induced aggregation of YjbH and led to induction of Spx. By heterologous expression of a less aggregation prone YjbH homolog Spx induction was abolished. Thus we show that moderation of YjbH solubility is an important mechanism of signal transduction and represents a new mechanism of controlling the activity of adaptor proteins.

Proteomics-identified Bvg-activated autotransporters protect against bordetella pertussis in a mouse model

Pertussis is a highly infectious respiratory disease of humans caused by the bacterium Bordetella pertussis. Despite high vaccination coverage, pertussis has re-emerged globally. Causes for the re-emergence of pertussis include limited duration of protection conferred by acellular pertussis vaccines (aP) and pathogen adaptation. Pathogen adaptations involve antigenic divergence with vaccine strains, the emergence of strains which show enhanced in vitro expression of a number of virulence-associated genes and of strains that do not express pertactin, an important aP component. Clearly, the identification of more effective B. pertussis vaccine antigens is of utmost importance. To identify novel antigens, we used proteomics to identify B. pertussis proteins regulated by the master virulence regulatory system BvgAS in vitro. Five candidates proteins were selected and it was confirmed that they were also expressed in the lungs of naïve mice seven days after infection. The five proteins were expressed in recombinant form, adjuvanted with alum and used to immunize mice as stand-alone antigens. Subsequent respiratory challenge showed that immunization with the autotransporters Vag8 and SphB1 significantly reduced bacterial load in the lungs. Whilst these antigens induced strong opsonizing antibody responses, we found that none of the tested alum-adjuvanted vaccines - including a three-component aP - reduced bacterial load in the nasopharynx, suggesting that alternative immunological responses may be required for efficient bacterial clearance from the nasopharynx.

Inhibition of the alternative pathway of nonhuman infant complement by porin B2 contributes to virulence of Neisseria meningitidis in the infant rat model

Neisseria meningitidis utilizes capsular polysaccharide, lipooligosaccharide (LOS) sialic acid, factor H binding protein (fHbp), and neisserial surface protein A (NspA) to regulate the alternative pathway (AP) of complement. Using meningococcal mutants that lacked all four of the above-mentioned molecules (quadruple mutants), we recently identified a role for PorB2 in attenuating the human AP; inhibition was mediated by human fH, a key downregulatory protein of the AP. Previous studies showed that fH downregulation of the AP via fHbp or NspA is specific for human fH. Here, we report that PorB2-expressing quadruple mutants also regulate the AP of baby rabbit and infant rat complement. Blocking a human fH binding region on PorB2 of the quadruple mutant of strain 4243 with a chimeric protein that comprised human fH domains 6 and 7 fused to murine IgG Fc enhanced AP-mediated baby rabbit C3 deposition, which provided evidence for an fH-dependent mechanism of nonhuman AP regulation by PorB2. Using isogenic mutants of strain H44/76 that differed only in their PorB molecules, we confirmed a role for PorB2 in resistance to killing by infant rat serum. The PorB2-expressing strain also caused higher levels of bacteremia in infant rats than its isogenic PorB3-expressing counterpart, thus providing a molecular basis for increased survival of PorB2 isolates in this model. These studies link PorB2 expression with infection of infant rats, which could inform the choice of meningococcal strains for use in animal models, and reveals, for the first time, that PorB2-expressing strains of N. meningitidis regulate the AP of baby rabbits and rats.

Quality control of inclusion bodies in Escherichia coli

Background

Bacterial inclusion bodies (IBs) are key intermediates for protein production. Their quality affects the refolding yield and further purification. Recent functional and structural studies have revealed that IBs are not dead-end aggregates but undergo dynamic changes, including aggregation, refunctionalization of the protein and proteolysis. Both, aggregation of the folding intermediates and turnover of IBs are influenced by the cellular situation and a number of well-studied chaperones and proteases are included. IBs mostly contain only minor impurities and are relatively homogenous.

Results

IBs of α-glucosidase of Saccharomyces cerevisiae after overproduction in Escherichia coli contain a large amount of (at least 12 different) major product fragments, as revealed by two-dimensional polyacrylamide gel electrophoresis (2D PAGE). Matrix-Assisted-Laser-Desorption/Ionization-Time-Of-Flight Mass-Spectrometry (MALDI-ToF MS) identification showed that these fragments contain either the N- or the C-terminus of the protein, therefore indicate that these IBs are at least partially created by proteolytic action. Expression of α-glucosidase in single knockout mutants for the major proteases ClpP, Lon, OmpT and FtsH which are known to be involved in the heat shock like response to production of recombinant proteins or to the degradation of IB proteins, clpP, lon, ompT, and ftsH did not influence the fragment pattern or the composition of the IBs. The quality of the IBs was also not influenced by the sampling time, cultivation medium (complex and mineral salt medium), production strategy (shake flask, fed-batch fermentation process), production strength (T5-lac or T7 promoter), strain background (K-12 or BL21), or addition of different protease inhibitors during IB preparation.

Conclusions

α-glucosidase is fragmented before aggregation, but neither by proteolytic action on the IBs by the common major proteases, nor during downstream IB preparation. Different fragments co-aggregate in the process of IB formation together with the full-length product. Other intracellular proteases than ClpP or Lon must be responsible for fragmentation. Reaggregation of protease-stable α-glucosidase fragments during in situ disintegration of the existing IBs does not seem to occur.

Localization of general and regulatory proteolysis in Bacillus subtilis cells

Protein degradation mediated by ATP-dependent proteases, such as Hsp100/Clp and related AAA+ proteins, plays an important role in cellular protein homeostasis, protein quality control and the regulation of, e.g. heat shock adaptation and other cellular differentiation processes. ClpCP with its adaptor proteins and other related proteases, such as ClpXP or ClpEP of Bacillus subtilis, are involved in general and regulatory proteolysis. To determine if proteolysis occurs at specific locations in B. subtilis cells, we analysed the subcellular distribution of the Clp system together with adaptor and general and regulatory substrate proteins, under different environmental conditions. We can demonstrate that the ATPase and the proteolytic subunit of the Clp proteases, as well as the adaptor or substrate proteins, form visible foci, representing active protease clusters localized to the polar and to the mid-cell region. These clusters could represent a compartmentalized place for protein degradation positioned at the pole close to where most of the cellular protein biosynthesis and also protein quality control are taking place, thereby spatially separating protein synthesis and degradation.

Bacillus subtilis as a tool for vaccine development: from antigen factories to delivery vectors

Bacillus subtilis and some of its close relatives have a long history of industrial and biotechnological applications. Search for antigen expression systems based on recombinant B. subtilis strains sounds attractive both by the extensive genetic knowledge and the lack of an outer membrane, which simplify the secretion and purification of heterologous proteins. More recently, genetically modified B. subtilis spores have been described as indestructible delivery vehicles for vaccine antigens. Nonetheless both production and delivery of antigens by B. subtilis strains face some inherent obstacles, as unstable gene expression and reduced immunogenicity that, otherwise, can be overcome by already available gene technology approaches. In the present review we present the status of B. subtilis-based vaccine research, either as protein factories or delivery vectors, and discuss some alternatives for a better use of genetically modified strains.

Bacillus subtilis as cell factory for pharmaceutical proteins: a biotechnological approach to optimize the host organism

Bacillus subtilis is a rod-shaped, Gram-positive soil bacterium that secretes numerous enzymes to degrade a variety of substrates, enabling the bacterium to survive in a continuously changing environment. These enzymes are produced commercially and this production represents about 60% of the industrial-enzyme market. Unfortunately, the secretion of heterologous proteins, originating from Gram-negative bacteria or from eukaryotes, is often severely hampered. Several bottlenecks in the B. subtilis secretion pathway, such as poor targeting to the translocase, degradation of the secretory protein, and incorrect folding, have been revealed. Nevertheless, research into the mechanisms and control of the secretion pathways will lead to improved Bacillus protein secretion systems and broaden the applications as industrial production host. This review focuses on studies that aimed at optimizing B. subtilis as cell factory for commercially interesting heterologous proteins.

Molecular cloning, sequencing and characterization of omp48, the gene encoding for an antigenic outer membrane protein from Aeromonas veronii

AIMS

To clone, sequence and characterize the gene encoding the Omp48, a major outer membrane protein from Aeromonas veronii.

METHODS AND RESULTS

A genomic library of Aer. veronii was constructed and screened to detect omp48 gene sequences, but no positive clones were identified, even under low stringency conditions. The cloned gene probably was toxic to the host Escherichia coli strain, so the cloning of omp48 was achieved by inverse PCR. The nucleotide sequence of omp48 consisted of an open reading frame of 1278 base pairs. The predicted primary protein is composed of 426 amino acids, with a 25-amino-acid signal peptide and common Ala-X-Ala cleavage site. The mature protein is composed of 401 amino acids with a molecular mass of 44,256 Da.

CONCLUSIONS

The omp48 gene from Aer. veronii was cloned, sequenced and characterized in detail. BLAST analysis of Omp48 protein showed sequence similarity (over 50%) to the LamB porin family from other pathogenic Gram-negative bacteria.

SIGNIFICANCE AND IMPACT OF THE STUDY

Bacterial diseases are a major economic problem for the fish farming industry. Outer membrane proteins are potentially important vaccine components. The characterization of omp48 gene will allow further investigation of the potential of Omp48 as recombinant or DNA vaccine component to prevent Aer. veronii and related species infections in reared fish.

Probing the ability of the coat and vertex protein of the membrane-containing bacteriophage PRD1 to display a meningococcal epitope

Bacteriophage PRD1 is an icosahedral dsDNA virus with a diameter of 740 A and an outer protein shell composed of 720 copies of major coat protein P3. Spike complexes at the vertices are composed of a pentameric base (protein P31) and a spike structure (proteins P5 and P2) where the N-terminal region of the trimeric P5 is associated with the base and the C-terminal region of P5 is associated with receptor-binding protein P2. The functionality of proteins P3 and P5 was investigated using insertions and deletions. It was observed that P3 did not tolerate changes whereas P5 tolerated changes much more freely. These properties support the hypothesis that viruses have core structures and functions, which remain stable over time, as well as other elements, responsible for host interactions, which are evolutionally more fluid. The insertional probe used was the apex of exposed loop 4 of group B meningococcal outer membrane protein PorA, a medically important subunit vaccine candidate. It was demonstrated that the epitope could be displayed on the virus surface as part of spike protein P5.

Production of Chlamydia pneumoniae proteins in Bacillus subtilis and their use in characterizing immune responses in the experimental infection model

Due to intracellular growth requirements, large-scale cultures of chlamydiae and purification of its proteins are difficult and laborious. To overcome these problems we produced chlamydial proteins in a heterologous host, Bacillus subtilis, a gram-positive nonpathogenic bacterium. The genes of Chlamydia pneumoniae major outer membrane protein (MOMP), the cysteine-rich outer membrane protein (Omp2), and the heat shock protein (Hsp60) were amplified by PCR, and the PCR products were cloned into expression vectors containing a promoter, a ribosome binding site, and a truncated signal sequence of the alpha-amylase gene from Bacillus amyloliquefaciens. C. pneumoniae genes were readily expressed in B. subtilis under the control of the alpha-amylase promoter. The recombinant proteins MOMP and Hsp60 were purified from the bacterial lysate with the aid of the carboxy-terminal histidine hexamer tag by affinity chromatography. The Omp2 was separated as an insoluble fraction after 8 M urea treatment. The purified proteins were successfully used as immunogens and as antigens in serological assays and in a lymphoproliferation test. The Omp2 and Hsp60 antigens were readily recognized by the antibodies appearing after pulmonary infection following intranasal inoculation of C. pneumoniae in mice. Also, splenocytes collected from mice immunized with MOMP or Hsp60 proteins proliferated in response to in vitro stimulation with the corresponding proteins.

Restored functional immunogenicity of purified meningococcal PorA by incorporation into liposomes

The impact of the conformation, lipooligosaccharide (LOS)-depletion and the presentation form of outer membrane protein PorA from Neisseria meningitidis (PorA) subtype P1.7-2,4 on the immune response in mice was studied. Native PorA was purified from outer membrane vesicles (OMVs) derived from meningococci and reconstituted into liposomes. The conformation of PorA after purification from OMVs and reconstitution in liposomes was monitored by use of electrophoretic and spectroscopic techniques and compared with the conformation of PorA in outer membrane complexes (OMCs) and heat-denatured PorA. The antigenicity of the PorA formulations was measured by ELISA by using a bactericidal anti-P1.4 monoclonal antibody. Immunogenicity was determined in Balb/c mice. PorA-specific IgG, isotype distribution and bactericidal activity were measured after subcutaneous immunization. In all formulations except in heat-denatured OMVs, PorA was present as trimers. The lipooligosaccharide (LOS) content was reduced by 96% in the purified protein with respect to the original OMVs. The antigenicity of purified PorA (i.e. ELISA response) was substantially higher as compared to PorA in liposomes, OMVs or OMCs. The results of the immunogenicity studies showed that all formulations were able to induce comparable IgG titers. However, whereas the antibodies raised by OMVs were bactericidal, the antibodies elicited by immunization with purified PorA were unable to kill meningococci. Remarkably, the ability to induce bactericidal antibodies was fully recovered by incorporation of the purified PorA into liposomes, in the absence of other adjuvants, as compared to LOS-containing OMVs.

Quantification of lipopolysaccharides in outer membrane vesicle vaccines against meningococcal disease. High-performance liquid chromatographic determination of the constituent 3-hydroxy-lauric acid

A high-performance liquid chromatographic (HPLC) assay for quantification of lipopolysaccharides (LPSs, endotoxins) in outer membrane vesicle vaccines against meningococcal disease has been developed. The LPS constituent, 3-hydroxy-lauric acid, served as marker substance for the quantification. LPS from the vaccine was precipitated by ethanol and the fatty acid constituents, including 3-hydroxy-lauric acid, were released by acidic hydrolysis, collected and purified by solid phase extraction on C18 disc-cartridges and converted into phenacyl esters for UV detection at 240 nm. Quantification of the derivatized 3-hydroxy-lauric acid was achieved by HPLC using a Brownlee RP-18 reversed phase column with acetonitrile/water (68:32, v/v) as mobile phase. The method was found to be linear over the range 3-49 microg LPS/ml with a sensitivity of 1.6 (microg/ml)(-1). The repeatability (within-day precision) of the method at three levels (3-49 microg LPS/ml) was 6-14% relative standard deviation and the intermediate (between-day) precision was 7% relative standard deviation (at level 15 microg LPS/ml). The method has been successfully used in the quality control of a meningococcal B outer membrane vesicle vaccine, containing 4-8% LPS relative to protein (w/w), in our laboratory for three years.

Passive immunization with Neisseria meningitidis PorA specific immune sera reduces nasopharyngeal colonization of group B meningococcus in an infant rat nasal challenge model

To examine the protective efficacy of specific immune sera generated by meningococcal vaccine candidates against nasopharyngeal colonization, we developed an infant rat nasal colonization model for group B meningococcus. In this model, Sprague-Dawley infant rats were challenged intranasally in with host adapted, piliated Neisseria meningitidis group B strains H355 or H44/76 administered concurrently with iron dextran. Colonization was assessed by quantitative culture of nasal homogenates and expressed as log(10) colony forming units (c.f.u.) per nose. Three to five log(10) c.f.u. of N. meningitidis were routinely recovered from the nasal tissue up to 4 days post-challenge. Passive immunization (i.p.) of the infant rats with either PorA or whole cell antisera 24 h prior to homologous challenge resulted in a significant reduction of N. meningitidis colonization in the nasal tissues of these animals. These results demonstrate that this model can be utilized to evaluate the role of antibody to prevent the initial nasopharyngeal colonization by group B meningococcus.

Immunogenicity of recombinant class 1 protein from Neisseria meningitidis refolded into phospholipid vesicles and detergent

The possibility of eliciting bactericidal antibodies against a recombinant class 1 protein (P1) from Neisseria meningitidis, joined to the first 45 amino acids of the neisserial LpdA protein (PM82), was examined. P1 was produced in Escherichia coli as intracellular inclusion bodies, from which it was purified and reconstituted by (a) inclusion into phospholipid vesicles and detergent and (b) refolding in 0.1% SDS. When Balb/c mice were immunised, high titres of subtype-specific bactericidal antibodies against P1 were obtained in both cases. These results suggest that in spite of being a denaturing agent, it is possible to use SDS to reconstitute the P1 protein in a conformation that exposes the immunodominat regions.

Monitoring of genes that respond to overproduction of an insoluble recombinant protein in Escherichia coli glucose-limited fed-batch fermentations

The cellular response of Escherichia coli to overproduction of the insoluble heterologous protein alpha-glucosidase of Saccharomyces cerevisiae during a glucose-limited fed-batch fermentation was analyzed on the transcriptional and the translational levels. After the induction of the tac-regulated overexpression of the recombinant model protein, a significant but transient increase of the mRNA levels of the heat shock genes lon and dnaK could be observed. The mRNA level of the gene coding for the inclusion body-associated protein IbpB showed the strongest increase and remained at a clearly higher level until the end of the fermentation. By contrast, the mRNA levels of htrA and ppiB were decreased after induction of the alpha-glucosidase overexpression. Analysis of the soluble cytoplasmic protein fraction 3 h after induction revealed increased levels of the chaperones GroEL, DnaK, and Tig and a decrease in the protein levels of the two ribosomal proteins S6 and L9, the peptidylprolyl-cis-trans-isomerase PpiB, and the sigma(38)-dependent protein Dps. Analysis of the aggregated protein fraction revealed a remarkably inhomogeneous composition of the alpha-glucosidase inclusion bodies. N-terminal sequencing and MALDI-TOF mass spectrometry identification showed that most of these spots are fragments of the heterologous alpha-glucosidase. Host stress proteins, like DnaK, GroEL, IbpA, IbpB, and OmpT, have been found to be associated with the alpha-glucosidase protein aggregates.

Immunogenicity of in vitro folded outer membrane protein PorA of Neisseria meningitidis

In vitro folded and the denatured form of PorA P1.6 from Neisseria meningitidis strain M990 were used for immunization studies in mice. Previously, the antigen was isolated from cytoplasmic inclusion bodies, folded and purified. Its immunogenicity without adjuvant appeared to be low. The addition of the adjuvant QuilA, but not of galE lipooligosaccharide, considerably enhanced the immunogenicity. Moreover, when immunized with folded PorA P1.6 plus QuilA, a clear switch towards the IgG2a subclass of antibodies and concomitantly, the appearance of serum bactericidal activity, which is believed to be important for protective immunity, was observed. Hence, a tool for preparing vaccines against serogroup B meningococci devoid of endotoxin is available.

The infant rat model adapted to evaluate human sera for protective immunity to group B meningococci

The infant rat infection model previously developed to evaluate protective ability of passively administered murine antibodies to group B meningococcal (MenB) surface antigens was adapted for human sera. Several challenge doses were tested, aiming at sensitive detection of protection with little interassay variability. Doses of 10(5) and 10(6) colony forming units of strain IH5341 (MenB:15:P1.7,16) injected intraperitoneally gave consistently high levels of bacteremia and meningitis developed in 6 h in 50-100% of the pups. A monoclonal antibody mAb735 to the MenB capsule, injected 1-2 h before bacterial challenge, gave full protection at a dose of 2 microg/pup. Sera from adult volunteers immunized with a MenB outer membrane vesicle vaccine reproducibly reduced bacterial counts in the blood and cerebrospinal fluid, whereas a normal human serum, lacking bactericidal and opsonophagocidal activity, was unprotective.

Immunization with recombinant class 1 outer-membrane protein from Neisseria meningitidis: influence of liposomes and adjuvants on antibody avidity, recognition of native protein and the induction of a bactericidal immune response against meningococci

The porA gene from Neisseria meningitidis was cloned into the pRSETA vector and recombinant class 1 outer-membrane protein expressed at high levels in Escherichia coli. The protein was readily purified by affinity chromatography on a Ni2+ matrix and used for immunization of mice with conventional AI(OH)3 adjuvant, with experimental adjuvants which have the potential for human use, and with liposomes. The resulting sera were analysed for the magnitude, subclass distribution and antigenic specificity of the immune response. In addition, surface plasmon resonance (SPR) was used to quantify antibody avidity by analysis of the kinetics of binding to native class 1 protein. Immunization with conventional and experimental adjuvants induced antibodies of low avidity that did not recognize native class 1 protein. In contrast, immunization with recombinant protein in liposomes induced antibodies of high avidity which recognized native class 1 protein, as measured by their ability to label meningococcal cells in immunofluorescence assays and to inhibit the binding of a protective mAb. These properties were associated with the presence in sera of high levels of antibodies with the ability to induce complement-mediated killing of meningococci. These data show that liposomes containing recombinant class 1 protein represent a potential basis of future vaccines, of defined composition, designed for the prevention of group B meningococcal infections.

Cloning and characterization of the major outer membrane protein gene (ompH) of Pasteurella multocida X-73

The major outer membrane protein (OmpH) of Pasteurella multocida X-73 was purified by selective extraction with detergents, followed by size exclusion chromatography. The planar lipid bilayer assay showed that OmpH has pore-forming function. The average single channel conductance in 1.0 M KCl was 0.62 nS. The gene (ompH) encoding OmpH has been isolated and sequenced by construction of a genomic library and PCR techniques. The coding region of this gene is 1,059 bp long. The predicted primary protein is composed of 353 amino acids, with a 20-amino-acid signal peptide. The mature protein is composed of 333 amino acids with a molecular mass of 36.665 kDa. The ompH gene encoding mature protein has been expressed in Escherichia coli by using a regulatable expression system. The ompH gene was distributed among 15 P. multocida serotypes and strain CU. Protection studies showed that OmpH was able to induce homologous protection in chickens. These findings demonstrate that OmpH is a protective outer membrane porin of strain X-73 and is conserved among P. multocida somatic serotypes.

Lipopolysaccharide heterogeneity and escape mechanisms of Neisseria meningitidis: possible consequences for vaccine development

We wanted to compare the potential protective capacity of antibodies to meningococcal lipopolysaccharides (LPS). The frequency of occurrence and degree of expression of the epitopes recognized by murine monoclonal antibodies (MAbs) to immunotypes L3,7,9 (9-2-L379) and L8 (2-1-L8) and to the LPS inner core (216-Lc and 217-Lc), were determined among 77 consecutive Norwegian meningococcal patient isolates from 1995. The immunotype L3,7,9 was strongly expressed by 95% of the isolates, whereas L8 was weakly to moderately expressed by 9%. The inner core epitopes, were widely distributed among the serogroup B organisms, but were proved weakly expressed. The bactericidal activity of the four MAbs to various selected strains, was found to correlate positively with the quantity of the LPS epitopes recognized by these four MAbs in the bacteria. When tested in the serum bactericidal assay (SBA), often a few percent of the colonies of the inocula survived high concentrations of the MAbs. The results indicate that escape from the bactericidal action could be achieved through: (i) selection of variants not expressing the LPS-epitope of the actual MAb, (ii) a relative reduction in the density of the LPS-epitope achieved by dilution with another LPS structure or (iii) other factors, not yet understood. In conclusion, antibodies to the L3,7,9 epitope seem to be of importance for protection, whereas antibodies to the epitopes of the LPS inner core or immunotype L8, are not likely to offer protection alone. However, in order to prevent escape through alteration of the LPS pattern of the microbes, various LPS structures should probably be present in the OMV vaccine.

Antigenic diversity of meningococcal outer membrane protein PorA has implications for epidemiological analysis and vaccine design

The currently used serological subtyping scheme for the pathogen Neisseria meningitidis is not comprehensive, a proportion of isolates are reported as not subtypeable (NST), and few isolates are fully characterized with two subtypes for each strain. To establish the reasons for this and to assess the effectiveness of DNA-based subtyping schemes, dot blot hybridization and nucleotide sequence analyses were used to characterize the genes encoding antigenic variants of the meningococcal subtyping antigen, the PorA protein. A total of 233 strains, including 174 serologically NST and 59 partially or completely subtyped meningococcal strains, were surveyed. The NST isolates were chosen to be temporally and geographically representative of NST strains, isolated in England and Wales, and submitted to the Meningococcal Reference Unit in the period 1989 to 1991. The DNA-based analyses demonstrated that all of the strains examined possessed a porA gene. Some of these strains were serologically NST because of a lack of monoclonal antibodies against certain PorA epitopes; in other cases, strains expressed minor variants of known PorA epitopes that did not react with monoclonal antibodies in serological assays. Lack of expression remained a possible explanation for serological typing failure in some cases. These findings have important implications for epidemiological analysis and vaccine design and demonstrate the need for genetic characterization, rather than phenotypic characterization using monoclonal antibodies, for the identification of meningococcal strains.

Purification and antigenic analysis of the major 25-kilodalton outer membrane protein of Brucella abortus

The major 25-kDa outer membrane protein (Omp25) of Brucella abortus was purified and antigenically characterized by use of monoclonal antibodies (mAbs). Purification was achieved from the sodium dodecyl sulphate-insoluble (SDS-I) cell wall (CW) fraction of vaccine strain B. abortus B19 which was shown by use of mAbs to contain the two major outer membrane proteins of 25 and 36 kDa linked to peptidoglycan, smooth lipopolysaccharide (S-LPS), and rough LPS (R-LPS). Purity of Omp25 was checked with a number of mAbs directed to the different components of the SDS-I fraction. In ELISA, five anti-Omp25 mAbs, which showed significant binding to B. abortus whole cells and which are probably directed to conformational epitopes well-exposed on the bacterial surface, reacted poorly or not at all with the purified Omp25. Addition of R-LPS to purified Omp25 restored the binding capacity of these mAbs, which suggested that R-LPS may play an important role in reconstitution and exposure of conformational epitopes of Omp25. Immunoelectron microscopy showed that Omp25 was inserted into the R-LPS vesicles. Four of these anti-Omp25 mAbs probably recognize the same or closely located epitopes on Omp25, since one of the mAbs conjugated to peroxidase was inhibited in its binding in ELISA by the three others. Other anti-Omp25 mAbs showed strong binding to purified denatured Omp25 and their binding capacity was not affected by the addition of R-LPS to the purified Omp25. Thus, these results confirmed, as defined by the mAbs, the presence of both sequential and at least one conformational epitope on Omp25.

Human antibody responses to meningococcal outer membrane antigens after three doses of the Norwegian group B meningococcal vaccine

The antibody kinetics in sera from 27 adults after three doses of the Norwegian group B meningococcal outer membrane vesicle (OMV) vaccine was studied. The vaccinees received the third dose 4 to 5 years after the first two. Antibody responses against outer membrane proteins (OMPs) and lipopolysaccharides were studied by enzyme-linked immunosorbent assay and immunoblotting and with serum bactericidal assays (SBA) with three variants of the vaccine strain, 44/76. Six weeks after the second injection, the geometric mean (GM) of the levels of immunoglobulin G (IgG) against OMVs was about sevenfold higher than that of prevaccination levels, and 74% of the vaccinees developed a greater-than-twofold rise in SBA titer. After 6 months, the GM of IgG levels declined to about threefold higher, and after 4 to 5 years it declined to about twofold higher, than that before vaccination. The third dose induced a rapid increase in SBA titers in 96% of the vaccinees, and the GM of levels of IgG against OMVs rose to about 14-fold the prevaccination level. One year later, the IgG antibody levels had dropped to 4.6-fold the prevaccination level, but 88% of the vaccinees still showed bactericidal activity. The response after the two first doses was higher in individuals with prevaccination antibodies, but no such effect was found after three doses. The use of defined mutants in SBA and linear multiple regression analyses indicated that among the major OMPs, antibodies to the Opc and class 1 proteins made the most important single contributions to the bactericidal activity against the vaccine strain, but it also demonstrated the importance of antibodies against other antigens. After three doses, 68% of the vaccinees showed a significant SBA response against a strain lacking both the Opc and the class 1 proteins. Three doses converted almost all subjects to SBA responders and gave higher antibody levels and relatively less serosubtype-specific bactericidal activity than did two doses, probably indicating a broader cross-protection against heterologous strains.

The antibody response to a prototype liposome vaccine containing Neisseria meningitidis outer membrane protein P1 produced in Bacillus subtilis

Monoclonal antibodies to the class 1 outer membrane protein P1 of Neisseria meningitidis B:15:P1.7,16 have been shown to be bactericidal and protective in an infant rat meningitis model. We have produced the P1 protein in Bacillus subtilis as inclusion bodies. When the purified and denatured protein (BacP1) was reconstituted with phosphatidylcholine into liposomes, native antigenic epitopes were formed. Such liposomes were reproducibly immunogenic in mice and guinea pigs at a low dose (1-10 micrograms of BacP1 protein) and without any other adjuvant. The resulting antisera contained high titers (enzyme immunoassay) of antibodies directed to native P1 epitopes exposed on the surface of meningococcal cells. The sera were also active with live N. meningitidis in bactericidal assays and protective in the infant rat meningitis model; all these activities were specific to the serosubtype of the P1 protein.

Human and guinea pig immune responses to Legionella pneumophila protein antigens OmpS and Hsp60

We studied the immune responses of guinea pigs and humans to two Legionella pneumophila antigens. Guinea pigs surviving a lethal intraperitoneal challenge dose of virulent L. pneumophila exhibited strong cutaneous delayed-type hypersensitivity (DTH) reactions to purified OmpS (28-kDa major outer membrane protein) and Hsp60 (heat shock protein or common antigen), while weak DTH reactions were noted for extracellular protease (major secretory protein [MSP] [ProA]) and no reaction was observed with an ovalbumin (OA) control. Lymphocyte proliferation responses (LPRs) were measured for peripheral blood and spleen lymphocytes from guinea pigs surviving sublethal and lethal challenge doses of L. pneumophila. Lymphocytes from uninfected animals showed no proliferation to Hsp60 or OmpS, while lymphocytes from sublethally and lethally challenged animals exhibited strong proliferative responses to Hsp60 and OmpS. Guinea pigs vaccinated with purified OmpS exhibited low antibody titers and strong DTH and LPRs to OmpS, whereas lymphocytes from animals vaccinated with Hsp60 exhibited weak DTH responses and high antibody titers to Hsp60. All guinea pigs immunized with OmpS survived experimental challenge with L. pneumophila (two of two in a pilot study and seven of seven in trial 2) versus zero of seven OA-immunized controls (P = 0.006 by Fisher's exact test). In three vaccine trials in which animals were vaccinated with Hsp60, only 1 guinea pig of 15 survived lethal challenge. Peripheral blood lymphocytes (PBLs) from humans with legionellosis showed stronger LPRs to OmpS than PBLs from humans with no history of legionellosis (P = 0.0002 by Mann-Whitney test). PBLs of humans surviving legionellosis exhibited a lower but highly significant proliferative response to Hsp60 (P < 0.0001 compared with controls by Mann-Whitney test). These studies indicate that OmpS and Hsp60 are important antigens associated with the development of protective cellular immunity. However, as determined in vaccine trial studies in the guinea pig model for legionellosis, the species-specific antigen OmpS proved much more effective than the genus-common Hsp60 antigen.

Expression of meningococcal epitopes in LamB of Escherichia coli and the stimulation of serosubtype-specific antibody responses

The class 1 outer membrane protein (OMP), a major variable surface antigen of Neisseria meningitidis, is a component of novel meningococcal vaccines currently in field trials. Serological variants of the protein are also used to serosubtype meningococci. Most of the amino acid changes that give rise to antigenic variants of the protein occur in two variable regions (VR1 and VR2) that are thought to form loops on the cell surface. The polymerase chain reaction (PCR) was used to amplify the nucleotide sequences encoding VR1 and VR2 from the chromosomal DNA of N. meningitidis strain M1080. These were cloned in frame into the lamB gene of the Escherichia coli expression vector pAJC264. Whole-cell enzyme-linked immunosorbent assays (ELISAs), using monoclonal antibodies, and SDS-PAGE confirmed that, upon induction, strains of E. coli carrying these constructs expressed hybrid LamB proteins containing the N. meningitidis surface loops. These strains were used to immunize rabbits and the resultant polyclonal antisera reacted specifically with the class 1 OMP of reference strain M1080 (P1.7). Immunogold labelling of meningococcal cells and whole-cell dot-blot analyses with these antisera showed that the variable epitopes were exposed on the cell surface and confirmed that this approach could be used to obtain serosubtype-specific antisera. The binding profiles of the antisera were determined from their reactions with overlapping synthetic peptides and their reactivity compared with that of relevant serosubtype-specific monoclonal antibodies. This approach was used successfully to raise antisera against two other class 1 OMP VR2s. A fourth antiserum raised against a VR2, including the P1.1 epitope, was not subtype specific.

Immunological properties of recombinant porin of Haemophilus influenzae type b expressed in Bacillus subtilis

The major surface-located, channel-forming protein in the outer membrane of Haemophilus influenzae type b (Hib) is porin (341 amino acids; M(r), 37,782). In order to generate Hib porin that is devoid of lipooligosaccharides and capsular polysaccharide, the Hib porin gene ompP2 was subcloned into a plasmid vector and recombinant Hib porin was expressed in Bacillus subtilis. Recombinant porin was produced in large quantities in B. subtilis and formed intracellular inclusion bodies. Recombinant porin was extracted from inclusion bodies and shown to be active in forming pores in synthetic black lipid membranes. However, these pores demonstrated different pore characteristics than wild-type Hib porin. Mouse hyperimmune sera against recombinant porin were generated and subjected to epitope scanning with a library of 336 overlapping synthetic hexapeptides that corresponded to the entire sequence of Hib porin. The epitope specificities of the anti-recombinant porin antibodies were similar to those of antibodies against Hib porin: selected regions near the amino terminus which include a buried loop in the native structure of Hib porin were more immunogenic than regions at the carboxy terminus. Although some mouse anti-recombinant porin antibodies mediated complement-dependent binding to Hib by polymorphonuclear leucocytes in opsonophagocytosis assays, the antibodies were not bactericidal, nor did they abrogate bacteremia in the infant rat model of infection. It was concluded that the native state of Hib porin is required for the generation of a protective immune response against the bacterium.

Assessment of non-protein impurities in potential vaccine proteins produced by Bacillus subtilis

The levels of non-protein impurities at different stages of purification of model vaccine proteins produced by Bacillus subtilis were assessed with special emphasis on peptidoglycan-wall teichoic acid and lipoteichoic acid. Intracytoplasmically produced proteins were purified by disrupting the lysozyme protoplasts using osmotic shock, depositing the inclusion bodies by low-speed centrifugation, and washing them with detergent. By this procedure most of the cell envelope-derived impurities could be removed. The final product contained less than 1% (w/w) of neutral sugars, fatty acids, phosphate, hexosamine, diaminopimelic acid and glycerol. A secreted protein was purified from the culture supernatant by successive ion-exchange and adsorption chromatography. The cell envelope-derived impurities were efficiently removed by the cation-exchanger, and the final product contained only minute amounts of non-protein components. The amounts of non-protein components such as peptidoglycan and lipoteichoic acid in proteins produced in either mode were shown to be negligible in relation to their potentially harmful biological effects.

Conformation of Escherichia coli outer membrane protein OmpA produced in Bacillus subtilis: influence of lipopolysaccharide

The conformation of the outer membrane protein OmpA of Escherichia coli produced in Bacillus subtilis and solubilized in Sarkosyl was studied by measuring its ability to bind OmpA-specific phage K3 and to inhibit F-mediated conjugation. The partially purified protein was inactive in both of these assays. Refolding of the protein in the presence of lipopolysaccharide resulted in preparations with full phage-binding and conjugation-inhibiting capacity, indicating the formation of surface-exposed loops of OmpA of native conformation. The finding is of importance for the potential use of outer membrane proteins of Gram-negative bacteria as vaccines.