Cloning, nucleotide sequence, and expression of the Brucella melitensis omp31 gene coding for an immunogenic major outer membrane protein

Development and evaluation as vaccines in mice of Brucella melitensis Rev.1 single and double deletion mutants of the bp26 and omp31 genes coding for antigens of diagnostic significance in ovine brucellosis

The live attenuated Brucella melitensis Rev.1 strain is considered the best vaccine available for the prophylaxis of brucellosis in sheep caused by either B. melitensis or Brucella ovis. However, its application stimulates antibody responses in vaccinated animals indistinguishable by the current conventional serological tests from those observed in infected animals. The periplasmic protein BP26 and the outer membrane protein (OMP) Omp31 are immunodominant antigens in the serological responses of B. melitensis and B. ovis infected sheep, respectively. Accordingly, vaccine strain Rev.1 single and double deletion mutants of the bp26 and omp31 genes were developed, based on the principle that the use of such mutants as vaccines in association with diagnostic tests based on BP26 and Omp31 antigens would allow the serological differentiation between infected and vaccinated animals. The deletion mutants obtained were indistinguishable from the parental Rev.1 strain by conventional bacteriological and typing tests. The expression of their major surface antigens, as determined by reactivity with specific monoclonal antibodies (MAbs), remained unaffected, i.e. smooth-lipopolysaccharide (S-LPS) and OMPs besides in the expression of the antigens whose respective genes were deleted. The bp26 and omp31 deletions did not modify the kinetics of splenic infection nor the residual virulence of Rev.1 in the BALB/c mouse model. Vaccination of BALB/c mice with the deletion mutants conferred significant protective immunity against B. melitensis strain H38 or B. ovis strain PA challenges, to the same extent as that induced by parental Rev.1 strain. Thus, these Rev.1 bp26 or omp31 deletion mutants are promising vaccine candidates against B. melitensis and B. ovis infections and will be further evaluated in sheep.

Immunogenicity of recombinant Omp31 from Brucella melitensis in rams and serum bactericidal activity against B. ovis

Detergent-extracted recombinant Omp31 (rOmp31 extract) from Brucella melitensis produced in Escherichia coli was previously identified as a protective immunogen against B. ovis in mice. In this study, we evaluated the immunogenicity of rOmp31extract in rams. This immunogen was emulsified in an oil adjuvant and administered three times with 4 and 8 weeks intervals. Antibody response was measured in serum by whole B. ovis ELISA. Specific antibodies to purified rOmp31 (pET-Omp31) were detected by Western blotting and indirect ELISA. In addition, isotype specific antibodies were measured in tears. Serum bactericidal activity against B. ovis in the presence of complement was measured in vitro. Cellular immune response was explored by intradermal testing with purified rOmp31. Immunization with rOmp31 extract induced IgG specific antibodies in serum able to bind to whole B. ovis cells. Furthermore, strong inhibition in a competitive ELISA (with an Omp31-specific monoclonal antibody) suggested that a proportion of Omp31-specific antibodies were directed against a loop containing a protective epitope. Serum antibodies killed efficiently B. ovis in vitro in the presence of either guinea pig or ovine serum. Tears had both IgG and IgA antibodies to equivalent titers. Finally, immunized rams showed skin reactivity to Omp31. These data demonstrate that B. melitensis Omp31, a protective antigen identified in the mouse model, induces antibody and cellular immune mechanisms in sheep.

Antibody reactivity to Omp31 from Brucella melitensis in human and animal infections by smooth and rough Brucellae

Group 3 of outer membrane proteins (OMPs) of Brucella includes Omp25 and Omp31, which share 34% identity. Omp25 is highly conserved in Brucella species, and Omp31 is present in all Brucella species, except Brucella abortus. Antibodies to Brucella melitensis Omp31 have been sought only in infected sheep, and Western blotting of sera from infected sheep did not reveal anti-Omp31 reactivity. We obtained recombinant purified Omp31 (B. melitensis) and tested its recognition by sera from humans and animals suffering from brucellosis by an indirect enzyme-linked immunosorbent assay (ELISA). Serum samples from 74 patients, 57 sheep, and 47 dogs were analyzed; brucellosis was confirmed by bacteriological isolation in all ovine and canine cases and 31 human cases of brucellosis. Thirty-five patients (47%) were positive for antibodies to Omp31, including seven cases of Brucella suis infection, two cases of B. abortus infection, and three cases of B. melitensis infection. Of 39 sheep naturally infected with B. melitensis (biovars 1 and 3), 23 (59%) were positive for antibodies to Omp31. Anti-Omp31 antibodies were also detected in 12 of 18 rams (67%) in which Brucella ovis was isolated from semen. Antibodies to Omp31 were also found in 41 (87%) of the 47 dogs, including 13 with recent infection. These results suggest that an indirect ELISA using recombinant purified Omp31 from B. melitensis would be of limited value for the diagnosis of human and animal brucellosis. Nevertheless, the potential usefulness of this antigen in combination with other recombinant proteins from Brucella should not be dismissed.

Influence of the co-encapsulation of different excipients on the properties of polyester microparticle-based vaccine against brucellosis

This work evaluates the influence of different pharmaceutical auxiliaries (Pluronic F68, polyvinylpyrrolidone [PVP] or Tween 20), when mixed with an antigenic extract from Brucella ovis (hot saline; HS), on the characteristics of the resulting poly(epsilon-caprolactone) (PEC) and poly(lactide-co-glycolide) (PLGA) microparticles. In all cases, PEC microparticles were smaller than PLGA ones. Concerning the HS loading, PLGA microparticles were highly dependent on the type of the excipient used, whereas all the PEC formulations displayed similar encapsulation efficiencies. For both types of microparticles, the presence of PVP induced a burst release effect. On the contrary, the use of Tween 20 or Pluronic F68 dramatically modified this profile. For PLGA-Tween 20 and PEC-Pluronic F68 microparticles, the HS was released in a pulsatil way during the first 7 days followed by a continuous release for at least 3 weeks. The antigenicity of the HS components was kept in all cases. Phagocytosis by murine monocytes showed a clear difference based just on the hydrophobicity of the polymer, being PEC microparticles better engulfed. Cell activation quantified by the release of H2O2 did not showed major differences between batches, however, microparticles of PEC and Pluronic F68 induced the highest nitric oxide production. Together, these results confirm the advantageous qualities of the "HS-PEC-Pluronic F68 microparticles" as favorable candidate for vaccine purposes against brucellosis.

Engineering of a polymeric bacterial protein as a scaffold for the multiple display of peptides

Protein assemblies with a high degree of repetitiveness and organization are known to induce strong immune responses. For that reason they have been postulated for the design of subunit vaccines by means of protein engineering. The enzyme lumazine synthase from Brucella spp. (BLS) is highly immunogenic, presumably owing to its homodecameric arrangement and remarkable thermodynamic stability. Structural analysis has shown that it is possible to insert foreign peptides at the ten amino terminus of BLS without disrupting its general folding. These peptides would be displayed to the immune system in a highly symmetric three-dimensional array. In the present work, BLS has been used as a protein carrier of foreign peptides. We have established a modular system to produce chimeric proteins decorated with ten copies of a desired peptide as long as 27 residues and have shown that their folding and stability is similar to that of the wild-type protein. The knowledge about the mechanisms of dissociation and unfolding of BLS allowed the engineering of polyvalent chimeras displaying different predefined peptides on the same molecular scaffold. Moreover, the reassembly of mixtures of chimeras at different steps of the unfolding process was used to control the stoichiometry and spatial arrangement for the simultaneous display of different peptides on BLS. This strategy would be useful for vaccine development and other biomedical applications.

The recombinant Omp31 from Brucella melitensis alone or associated with rough lipopolysaccharide induces protection against Brucella ovis infection in BALB/c mice

Immunogenicity and protective activity against Brucella ovis of detergent-extracted recombinant Omp31 (rOmp31 extract) from Brucella melitensis produced in Escherichia coli, purified rough lipopolysaccharide from B. ovis (R-LPS) and a mixture of rOmp31 extract and R-LPS (rOmp31 extract + R-LPS) were assessed in BALB/c mice. The experimental vaccines were compared with a hot saline extract (HS extract) from B. ovis mainly composed of outer membrane proteins (OMPs) and R-LPS, and known to be protective in mice against a B. ovis infection. Serum antibodies to Omp31 and R-LPS were detected in the corresponding mice using Western blotting with B. ovis whole-cell lysates and ELISA with purified antigens. Protection was evaluated by comparing the levels of infection in the spleens of vaccinated mice challenged with B. ovis. A significantly lower number of B. ovis colony-forming units in spleens relative to unimmunized (saline injected) controls were considered as protection. Mice immunized with rOmp31 extract or rOmp31 extract mixed with R-LPS developed antibodies that bound to the B. ovis surface with similar titers. Vaccination with rOmp31 extract plus R-LPS provided the best protection level, which was comparable with that given by HS extract. Similar protection was also obtained with rOmp31 extract alone and, to a lesser degree, with R-LPS. Comparisons between groups showed that an extract from E. coli-pUC19 (devoid of Omp31) provided no protection relative to either HS extract, rOmp31 extract or rOmp31 extract mixed with R-LPS. In conclusion, the recombinant Omp31 associated or not with B. ovis R-LPS, could be an interesting candidate for a subcellular vaccine against B. ovis infection.

Five-member gene family of Bartonella quintana

Bartonella quintana, the agent of trench fever and an etiologic agent of bacillary angiomatosis, has an extraordinarily high hemin requirement for growth compared to other bacterial pathogens. We previously identified the major hemin receptor of the pathogen as a 30-kDa surface protein, termed HbpA. This report describes four additional homologues that share approximately 48% amino acid sequence identity with hbpA. Three of the genes form a paralagous cluster, termed hbpCAB, whereas the other members, hbpD and hbpE, are unlinked. Secondary structure predictions and other evidence suggest that Hbp family members are beta-barrels located in the outer membrane and contain eight transmembrane domains plus four extracellular loops. Homologs from a variety of gram-negative pathogens were identified, including Bartonella henselae Pap31, Brucella Omp31, Agrobacterium tumefaciens Omp25, and neisserial opacity proteins (Opa). Family members expressed in vitro-synthesized proteins ranging from ca. 26.5 to 35.1 kDa, with the exception of HbpB, an approximately 55.9-kDa protein whose respective gene has been disrupted by a approximately 510 GC-rich element containing variable-number tandem repeats. Transcription analysis by quantitative reverse transcriptase-PCR (RT-PCR) indicates that all family members are expressed under normal culture conditions, with hbpD and hbpB transcripts being the most abundant and the rarest, respectively. Mutagenesis of hbpA by allelic exchange produced a strain that exhibited an enhanced hemin-binding phenotype relative to the parental strain, and analysis by quantitative RT-PCR showed elevated transcript levels for the other hbp family members, suggesting that compensatory expression occurs.

Major outer membrane proteins of Brucella spp.: past, present and future

The major outer membrane proteins (OMPs) of Brucella spp. were initially identified in the early 1980s and characterised as potential immunogenic and protective antigens. They were classified according to their apparent molecular mass as 36-38 kDa OMPs or group 2 porin proteins and 31-34 and 25-27 kDa OMPs which belong to the group 3 proteins. The genes encoding the group 2 porin proteins were identified in the late 1980s and consist of two genes, omp2a and omp2b, which are closely linked in the Brucella genome, and which share a great degree of identity (>85%). In the 1990s, two genes were identified coding for the group 3 proteins and were named omp25 and omp31. The predicted amino acid sequences of omp25 and omp31 share 34% identity. The recent release of the genome sequence of B. melitensis 16 M has revealed the presence of five additional gene products homologous to Omp25 and Omp31. The use of recombinant protein technology and monoclonal antibodies (MAbs) has shown that the major OMPs appear to be of little relevance as antigens in smooth (S) B. abortus or B. melitensis infections i.e. low or no protective activity in the mouse model of infection and low or no immunogenicity during host infection. However, group 3 proteins, in particular Omp31, appear as immunodominant antigen in the course of rough (R) B. ovis infection in rams and as important protective antigen in the B. ovis mouse model of infection. The major OMP genes display diversity and specific markers have been identified for Brucella species, biovars, and strains, including the recent marine mammal Brucella isolates for which new species names have been proposed. Recently, Omp25 has been shown to be involved in virulence of B. melitensis, B. abortus and B. ovis. Mutants lacking Omp25 are indeed attenuated in animal models of infection, and moreover provide levels of protection similar or better than currently used attenuated vaccine strain B. melitensis Rev.1. Therefore, these mutant strains appear interesting vaccine candidates for the future. The other group 3 proteins identified in the genome merit also further investigation related to the development of new vaccines.

Paradigm shifts in vaccine development: lessons learned about antigenicity, pathogenicity and virulence of Brucellae

As part of a program to support the USDA Animal Plant Health Inspection Service Bovine Brucellosis Eradication Program, the Brucellosis Research Unit of the National Animal Disease Center (NADC) sought to develop a bovine brucellosis vaccine that would allow vaccinated animals to be distinguished from virulent field infected animals. In order to meet that goal, several avenues of research were undertaken to construct and test candidate vaccines, including Brucella abortus RB51. In early vaccine development studies, a subunit preparation obtained by extracting B. abortus with salts was studied as a candidate subunit vaccine. Later, molecular biological techniques were used both to clone genes encoding products found in the salt extract (BCSP31 and Cu-Zn SOD) and genes encoding proteins of B. abortus that were antigenic (HtrA) or possibly essential (two-component systems) for full virulence of B. abortus. In vitro systems using mammalian cells lines such as HeLa and macrophage-related were used along with the mouse model and host animal models. Results obtained at NADC and in other Brucellosis research laboratories, using survival in mammalian cell lines and the mouse model to access pathogenicity and virulence of genetically engineered strains, do not necessarily identify loci that are essential for full virulence or pathogenicity in the natural host, the bovine. Studies at NADC and other brucellosis laboratories showed that antigenicity was not a predictor of the effectiveness of a protein as a subunit vaccine.

Effect of omp10 or omp19 deletion on Brucella abortus outer membrane properties and virulence in mice

The distinctive properties of Brucella outer membrane have been considered to be critical for Brucella sp. virulence. Among the outer membrane molecules possibly related to these properties, Omp10 and Omp19 are immunoreactive outer membrane lipoproteins. Moreover, these proteins of Brucella could constitute a new family of outer membrane proteins specifically encountered in the family RHIZOBIACEAE: We evaluated the impact of omp10 or omp19 deletion on Brucella abortus outer membrane properties and virulence in mice. The omp10 mutant was dramatically attenuated for survival in mice and was defective for growth in minimal medium but was not impaired in intracellular growth in vitro, nor does it display clear modification of the outer membrane properties. Significantly fewer brucellae were recovered from the spleens of mice infected with the omp19 mutant than from those of mice infected with the parent strain at 4 and 8 weeks postinfection. The omp19 mutant exhibited an increase in sensitivity to the polycation polymyxin B and to sodium deoxycholate. These results indicate that inactivation of the omp19 gene alters the outer membrane properties of B. abortus.

Brucella species lacking the major outer membrane protein Omp25 are attenuated in mice and protect against Brucella melitensis and Brucella ovis

To aid in the development of novel efficacious vaccines against brucellosis, Omp25 was examined as a potential candidate. To determine the role of Omp25 in virulence, mutants were created with Brucella abortus (BA25), Brucella melitensis (BM25), and Brucella ovis (BO25) which contain disruptions in the omp25 gene (Deltaomp25 mutants). Western immunoblot analysis and PCR verified that the Omp25 protein was not expressed and that the omp25 gene was disrupted in each strain. BALB/c mice infected with B. abortus BA25 or B. melitensis BM25 showed a significant decrease in mean CFU/spleen at 18 and 4 weeks post-infection, respectively, when compared to the virulent parental strain (P<0.05, n=5). Mice infected with B. ovis BO25 had significantly lower mean CFU/spleen counts from 1 to 8 weeks post-infection, at which point the mutant was cleared from the spleens (P<0.01, n=5). Murine vaccination with either BM25 or the current caprine vaccine B. melitensis strain Rev. 1 resulted in more than a 2log(10) reduction in bacterial load following challenge with virulent B. melitensis (P<0.01, n=5). Vaccination of mice with the B. ovis mutant resulted in clearance of the challenge strain and provided 2.5log(10) greater protection against virulent B. ovis than vaccine strain Rev. 1. Based on these data, the B. melitensis and B. ovis Deltaomp25 mutants are interesting vaccine candidates that are currently under study in our laboratory for their safety and efficacy in small ruminants.

Single-step purification and evaluation of recombinant BP26 protein for serological diagnosis of Brucella ovis infection in rams

To investigate the value of the BP26 protein in the serological diagnosis of ovine brucellosis caused by Brucella ovis, recombinant BP26 protein was produced in Echerichia coli and purified for use in an indirect enzyme-linked immunosorbent assay (I-ELISA). The majority of the recombinant protein was recovered from the supernatant of sonicated recombinant E. coli cells in a soluble form. This facilitated the purification of the recombinant BP26 protein which was achieved by using ion-exchange chromatography. After one step of purification, the purity of the recombinant BP26 protein was analyzed by using SDS-PAGE, Coomassie blue staining, and Western blot with a monoclonal antibody (MAb) directed against the BP26 protein. The degree of purity appeared satisfactory so that it could be directly used in I-ELISA. Although the recombinant BP26-ELISA appeared less useful than I-ELISA using the B. ovis hot saline (HS) extract as antigen, the high number of sera from B. ovis infected rams found positive (90%) in the recombinant BP26-I-ELISA indicated that the BP26 protein may be an additional suitable antigen for serological diagnosis of B. ovis infection in rams.

Pathogenicity and protective activity in pregnant goats of a Brucella melitensis Deltaomp25 deletion mutant

The Brucella melitensis mutant BM 25, which lacks the major 25 kDa outer membrane protein Omp25, has previously been found to be attenuated in the murine brucellosis model. In the present study, the capacity of the Deltaomp25 mutant to colonise and cause abortions in the caprine host was evaluated. The vaccine potential of BM 25 was also investigated in goats. Inoculation of nine pregnant goats in late gestation with the B. melitensis mutant resulted in 0/9 abortions, while the virulent parental strain, B. melitensis 16M, induced 6/6 dams to abort (P<0.001, n=6). BM 25 also colonised fewer adults (P<0.05, n=6) and kids (P<0.01, n=6) than strain 16M. The Deltaomp25 mutant was found capable of transient in vivo colonisation of non-pregnant goats for two weeks post-infection. Owing to the ability of BM 25 to colonise both non-pregnant and pregnant adults without inducing abortions, a vaccine efficacy study was performed. Vaccination of goats prior to breeding with either BM 25 or the current caprine vaccine B. melitensis strain Rev. 1 resulted in 100 per cent protection against abortion following challenge in late gestation with virulent strain 16M (P<0.05, n=7). However, unlike strain Rev. 1, BM 25 does not appear to cause abortions in late gestation based on this study with a small number of animals. The B. melitensis Deltaomp25 mutant, BM 25, may be a safe and efficacious alternative to strain Rev. 1 when dealing with goat herds of mixed age and pregnancy status.

Characterization of a Brucella species 25-kilobase DNA fragment deleted from Brucella abortus reveals a large gene cluster related to the synthesis of a polysaccharide

In the present study we completed the nucleotide sequence of a Brucella melitensis 16M DNA fragment deleted from B. abortus that accounts for 25,064 bp and show that the other Brucella spp. contain the entire 25-kb DNA fragment. Two short direct repeats of four nucleotides, detected in the B. melitensis 16M DNA flanking both sides of the fragment deleted from B. abortus, might have been involved in the deletion formation by a strand slippage mechanism during replication. In addition to omp31, coding for an immunogenic protein located in the Brucella outer membrane, 22 hypothetical genes were identified. Most of the proteins that would be encoded by these genes show significant homology with proteins involved in the biosynthesis of polysaccharides from other bacteria, suggesting that they might be involved in the synthesis of a Brucella polysaccharide that would be a heteropolymer synthesized by a Wzy-dependent pathway. This polysaccharide would not be synthesized in B. abortus and would be a polysaccharide not identified until present in the genus Brucella, since all of the known polysaccharides are synthesized in all smooth Brucella species. Discovery of a novel polysaccharide not synthesized in B. abortus might be interesting for a better understanding of the pathogenicity and host preference differences observed between the Brucella species. However, the possibility that the genes detected in the DNA fragment deleted in B. abortus no longer lead to the synthesis of a polysaccharide must not be excluded. They might be a remnant of the common ancestor of the alpha-2 subdivision of the class Proteobacteria, with some of its members synthesizing extracellular polysaccharides and, as Brucella spp., living in association with eukaryotic cells.

Minor nucleotide substitutions in the omp31 gene of Brucella ovis result in antigenic differences in the major outer membrane protein that it encodes compared to those of the other Brucella species

The gene coding for the major outer membrane protein Omp31 was sequenced in five Brucella species and their biovars. Although the omp31 genes appeared to be highly conserved in the genus Brucella, nine nucleotide substitutions were detected in the gene of Brucella ovis compared to that of Brucella melitensis. As shown by differential binding properties of monoclonal antibodies (MAbs) to the two Brucella species, these nucleotide substitutions result in different antigenic properties of Omp31. The antigenic differences were also evidenced when sera from B. ovis-infected rams were tested by Western blotting with the recombinant B. melitensis or B. ovis Omp31 proteins. Twelve available sera reacted with recombinant B. ovis Omp31, but only four of them reacted with recombinant B. melitensis Omp31. These results validate previous evidence for the potential of Omp31 as a diagnostic antigen for B. ovis infection in rams and demonstrate that B. ovis Omp31, instead of B. melitensis Omp31, should be used to evaluate this point. The antigenic differences between the B. melitensis and B. ovis Omp31 proteins should also be taken into account when Omp31 is evaluated as a candidate for the development of subcellular vaccines against B. ovis infection. No reactivity against recombinant B. melitensis Omp31 was detected, by Western blotting, with sera from B. melitensis-infected sheep. Accordingly, Omp31 does not seem to be a good diagnostic antigen for B. melitensis infections in sheep. Two immunodominant regions were identified on the B. ovis Omp31 protein by using recombinant DNA techniques and specific MAbs. Sera from B. ovis-infected rams that reacted with the recombinant protein were tested by Western blotting against one of these immunodominant regions shown to be exposed at the bacterial surface. Only 4 of the 12 sera reacted, but with strong intensity.

Attenuation of a Brucella abortus mutant lacking a major 25 kDa outer membrane protein in cattle

OBJECTIVE

To determine the virulence of a Brucella abortus mutant, BA25, lacking a major 25 kd outer membrane protein (Omp25) in cattle.

ANIMALS

20 mixed-breed heifers in late gestation.

PROCEDURE

10 heifers were inoculated with 1 x 10(7) colony-forming units of the Omp25 mutant via the conjunctival sac, and an equal number were infected with the virulent parental strain B. abortus 2308. The delivery status of the dams was recorded, and colonization was assessed following necropsy. The ability of BA25 to replicate inside bovine phagocytes and chorionic trophoblasts was also evaluated in vitro because of the propensity of virulent brucellae to replicate inside these cells in vivo.

RESULTS

The parental strain induced abortions in 5 of 10 inoculated cattle, whereas only 1 of 10 dams exposed to BA25 aborted. Brucella abortus strain 2308 colonized all of the cow-calf pairs and induced Brucella-specific antibodies in 100% of the dams. In contrast, BA25 was isolated by bacteriologic cultural technique from 30% of the calves and 50% of the inoculated dams (n = 10). Of the 10 heifers inoculated with BA25, 4 did not develop Brucella-specific antibodies nor were they colonized by the mutant strain. In bovine macrophages and chorionic trophoblasts, BA25 replicated in significantly lower numbers than the virulent parental strain (n = 3).

CONCLUSIONS AND CLINICAL RELEVANCE

The 25 kd outer membrane protein may be an important virulence factor for B. abortus in cattle. The attenuation of the Omp25 mutant in cattle may involve the inability of BA25 to replicate efficiently in bovine phagocytes and chorionic trophoblasts.

Major outer membrane protein Omp25 of Brucella suis is involved in inhibition of tumor necrosis factor alpha production during infection of human macrophages

Brucella spp. can establish themselves and cause disease in humans and animals. The mechanisms by which Brucella spp. evade the antibacterial defenses of their host, however, remain largely unknown. We have previously reported that live brucellae failed to induce tumor necrosis factor alpha (TNF-alpha) production upon human macrophage infection. This inhibition is associated with a nonidentified protein that is released into culture medium. Outer membrane proteins (OMPs) of gram-negative bacteria have been shown to modulate macrophage functions, including cytokine production. Thus, we have analyzed the effects of two major OMPs (Omp25 and Omp31) of Brucella suis 1330 (wild-type [WT] B. suis) on TNF-alpha production. For this purpose, omp25 and omp31 null mutants of B. suis (Deltaomp25 B. suis and Deltaomp31 B. suis, respectively) were constructed and analyzed for the ability to activate human macrophages to secrete TNF-alpha. We showed that, in contrast to WT B. suis or Deltaomp31 B. suis, Deltaomp25 B. suis induced TNF-alpha production when phagocytosed by human macrophages. The complementation of Deltaomp25 B. suis with WT omp25 (Deltaomp25-omp25 B. suis mutant) significantly reversed this effect: Deltaomp25-omp25 B. suis-infected macrophages secreted significantly less TNF-alpha than did macrophages infected with the Deltaomp25 B. suis mutant. Furthermore, pretreatment of WT B. suis with an anti-Omp25 monoclonal antibody directed against an epitope exposed at the surface of the bacteria resulted in substancial TNF-alpha production during macrophage infection. These observations demonstrated that Omp25 of B. suis is involved in the negative regulation of TNF-alpha production upon infection of human macrophages.

Molecular, antigenic, and functional analyses of Omp2b porin size variants of Brucella spp

Omp2a and Omp2b are highly homologous porins present in the outer membrane of the bacteria from the genus Brucella, a facultative intracellular pathogen. The genes coding for these proteins are closely linked in the Brucella genome and oriented in opposite directions. In this work, we present the cloning, purification, and characterization of four Omp2b size variants found in various Brucella species, and we compare their antigenic and functional properties to the Omp2a and Omp2b porins of Brucella melitensis reference strain 16M. The variation of the Omp2a and Omp2b porin sequences among the various strains of the genus Brucella seems to result mostly from multiple gene conversions between the two highly homologous genes. As shown in this study, this phenomenon has led to the creation of natural Omp2a and Omp2b chimeric proteins in Omp2b porin size variants. The comparison by liposome swelling assay of the porins sugar permeability suggested a possible functional differences between Omp2a and Omp2b, with Omp2a showing a more efficient pore in sugar diffusion. The sequence variability in the Omp2b size variants was located in the predicted external loops of the porin. Several epitopes recognized by anti-Omp2b monoclonal antibodies were mapped by comparison of the Omp2b size variants antigenicity, and two of them were located in the most exposed surface loops. However, since variations are mostly driven by simple exchanges of conserved motifs between the two genes (except for an Omp2b version from an atypical strain of Brucella suis biovar 3), the porin variability does not result in major antigenic variability of the Brucella surface that could help the bacteria during the reinfection of a host. Porin variation in Brucella seems to result mainly in porin conductivity modifications.

Potential symbiosis-specific genes uncovered by sequencing a 410-kilobase DNA region of the Bradyrhizobium japonicum chromosome

The physical and genetic map of the Bradyrhizobium japonicum chromosome revealed that nitrogen fixation and nodulation genes are clustered. Because of the complex interactions between the bacterium and the plant, we expected this chromosomal sector to contain additional genes that are involved in the maintenance of an efficient symbiosis. Therefore, we determined the nucleotide sequence of a 410-kb region. The overall G+C nucleotide content was 59.1%. Using a minimum gene length of 150 nucleotides, 388 open reading frames (ORFs) were selected as coding regions. Thirty-five percent of the predicted proteins showed similarity to proteins of rhizobia. Sixteen percent were similar only to proteins of other bacteria. No database match was found for 29%. Repetitive DNA sequence-derived ORFs accounted for the rest. The sequenced region contained all nitrogen fixation genes and, apart from nodM, all nodulation genes that were known to exist in B. japonicum. We found several genes that seem to encode transport systems for ferric citrate, molybdate, or carbon sources. Some of them are preceded by -24/-12 promoter elements. A number of putative outer membrane proteins and cell wall-modifying enzymes as well as a type III secretion system might be involved in the interaction with the host.

Hemin-binding surface protein from Bartonella quintana

Bartonella quintana, the agent of trench fever and a cause of endocarditis and bacillary angiomatosis in humans, has the highest reported in vitro hemin requirement for any bacterium. We determined that eight membrane-associated proteins from B. quintana bind hemin and that a approximately 25-kDa protein (HbpA) was the dominant hemin-binding protein. Like many outer membrane proteins, HbpA partitions to the detergent phase of a Triton X-114 extract of the cell and is heat modifiable, displaying an apparent molecular mass shift from approximately 25 to 30 kDa when solubilized at 100 degrees C. Immunoblots of purified outer and inner membranes and immunoelectron microscopy with whole cells show that HbpA is strictly located in the outer membrane and surface exposed, respectively. The N-terminal sequence of mature HbpA was determined and used to clone the HbpA-encoding gene (hbpA) from a lambda genomic library. The hbpA gene is 816 bp in length, encoding a predicted immature protein of approximately 29.3 kDa and a mature protein of 27.1 kDa. A Fur box homolog with 53% identity to the Escherichia coli Fur consensus is located upstream of hbpA and may be involved in regulating expression. BLAST searches indicate that the closest homologs to HbpA include the Bartonella henselae phage-associated membrane protein, Pap31 (58.4% identity), and the OMP31 porin from Brucella melitensis (31.7% identity). High-stringency Southern blots indicate that all five pathogenic Bartonella spp. possess hbpA homologs. Recombinant HbpA can bind hemin in vitro; however, it does not confer a hemin-binding phenotype upon E. coli. Intact B. quintana treated with purified anti-HbpA Fab fragments show a significant (P < 0.004) dose-dependent decrease in hemin binding relative to controls, suggesting that HbpA plays an active role in hemin acquisition and therefore pathogenesis. HbpA is the first potential virulence determinant characterized from B. quintana.

DNA polymorphism in the genus Brucella

The genus Brucella has been described as consisting of six species, three of them including several biovars, which display a high degree of DNA homology by DNA-DNA hybridization. However, DNA polymorphism able to differentiate the six Brucella species and some of their biovars has been shown to exist. This work reviews the DNA variability in the genus Brucella and discusses the relationships between its members according to this genetic variability and a proposal for their evolution based on genetic diversity of the omp2 locus.

Identification of protective outer membrane antigens of Brucella ovis by passive immunization of mice with monoclonal antibodies

Outer membrane proteins (OMPs) and rough lipopolysaccharide (R-LPS), the main surface antigens of Brucella ovis, display surface-exposed epitopes. Mixtures of monoclonal antibodies (mAbs) to both antigens were previously shown to protect mice against a B. ovis challenge. To further identify the antigens involved, seven mAbs against Brucella OMPs (Omp10, Omp16, Omp19, Omp25, Omp31, Omp2b and Omp1) and three to R-LPS were tested for protection either individually or in combinations. Significant reduction in spleen infection in challenged mice, relative to controls, was used as the protection criteri. Controls included nonimmunized mice and mice given an irrelevant, anti-O-polysaccharide (OPS), mAb. For comparison, a group received a mouse serum containing antibodies to both OMPs and R-LPS; this serum was prepared by immunization with a B. ovis hot-saline extract which, as described previously, induces protective immunity in mice and rams. Significant protection was observed with both mAbs to OMPs and R-LPS. mAbs to Omp16, Omp19 and Omp31 afforded the highest protection and prevented the development of splenomegaly. The protective effect of mAb to Omp31 was not interfered with by nonprotective mAbs in different mixtures. The data presented confirm the protective role of antibodies to OMPs and R-LPS against B. ovis, and identify several OMPs, especially Omp31, which are promising candidates for a subunit vaccine against ram epididymitis.

Outer membrane proteins Omp10, Omp16, and Omp19 of Brucella spp. are lipoproteins

The deduced sequences of the Omp10, Omp16, and Omp19 outer membrane proteins of Brucella spp. contain a potential bacterial lipoprotein processing sequence. After extraction with Triton X-114, these three proteins partitioned into the detergent phase. Processing of the three proteins is inhibited by globomycin, a specific inhibitor of lipoprotein signal peptidase. The three proteins were radioimmunoprecipitated from [(3)H]palmitic acid-labeled Brucella abortus lysates with monoclonal antibodies. These results demonstrate that Omp10, Omp16, and Omp19 are lipoproteins.

Molecular characterization of a Brucella species large DNA fragment deleted in Brucella abortus strains: evidence for a locus involved in the synthesis of a polysaccharide

A Brucella melitensis 16M DNA fragment of 17,119 bp, which contains a large region deleted in B. abortus strains and DNA flanking one side of the deletion, has been characterized. In addition to the previously identified omp31 gene, 14 hypothetical genes have been identified in the B. melitensis fragment, most of them showing homology to genes involved in the synthesis of a polysaccharide. Considering that 10 of the 15 genes are missing in B. abortus and that all the polysaccharides described in the Brucella genus (lipopolysaccharide, native hapten, and polysaccharide B) have been detected in all the species, it seems likely that the genes described here might be part of a cluster for the synthesis of a novel Brucella polysaccharide. Several polysaccharides have been identified as important virulence factors, and the discovery of a novel polysaccharide in the brucellae which is probably not synthesized in B. abortus might be interesting for a better understanding of the pathogenicity and host preference differences observed between the Brucella species. However, the possibility that the genes described in this paper no longer encode the synthesis of a polysaccharide cannot be excluded. Brucellae belong to the alpha-2 subdivision of the class Proteobacteria, which includes other microorganisms living in association with eucaryotic cells, some of them synthesizing extracellular polysaccharides involved in the interaction with the host cell. The genes described in this paper might be a remnant of the common ancestor of the alpha-2 subdivision of the class Proteobacteria, and the brucellae might have lost such extracellular polysaccharide during evolution if it was not necessary for survival or for establishment of the infectious process. Nevertheless, further studies are necessary to identify the entire DNA fragment missing in B. abortus strains and to elucidate the mechanism responsible for such deletion, since only 9,948 bp of the deletion was present in the sequenced B. melitensis DNA fragment.

Immunogenicity of recombinant Escherichia coli expressing the omp31 gene of Brucella melitensis in BALB/c mice

BALB/c mice were immunized with recombinant Escherichia coli expressing the omp31 gene of Brucella melitensis, a gene coding for a major outer membrane protein. Immunization resulted in the production of specific antibodies to B. melitensis in the serum, the production of which was considerably increased after boosting with a dose ten times lower than the first. A significant specific proliferative response of immune spleen cells to B. melitensis was observed 5 weeks after the first immunization but this response did not persist. Despite the induction of systemic humoral and cellular immune responses by recombinant E. coli expressing the B. melitensis omp31 gene, no significant protection against a challenge with smooth B. melitensis H38S was observed in immunized mice. These results demonstrate that despite the strong antibody response induced in mice, immunization with the recombinant Omp31 of B. melitensis does not confer any protective effect against a virulent smooth B. melitensis. However, its potential protective effect for protection against rough Brucella would be worth testing.

Identification of the perosamine synthetase gene of Brucella melitensis 16M and involvement of lipopolysaccharide O side chain in Brucella survival in mice and in macrophages

Brucella organisms are facultative intracellular bacteria that may infect many species of animals as well as humans. The smooth lipopolysaccharide (S-LPS) has been reported to be an important virulence factor of these organisms, but the genetic basis of expression of the S-LPS O antigen has not yet been described. Likewise, the role of the O side chain of S-LPS in the survival of Brucella has not been clearly defined. A mini-Tn5 transposon mutant library of Brucella melitensis 16M was screened by enzyme-linked immunosorbent assay (ELISA) with monoclonal antibodies (MAbs) directed against the O side chain of Brucella. One mutant, designated B3B2, failed to express any O side chain as confirmed by ELISA, Western blot analysis, and colony coloration with crystal violet. Nucleotide sequence analysis demonstrated that the transposon disrupted an open reading frame with significant homology to the putative perosamine synthetase genes of Vibrio cholerae O1 and Escherichia coli O157:H7. The low G+C content of this DNA region suggests that this gene may have originated from a species other than a Brucella sp. The survival of B. melitensis mutant strain B3B2 in the mouse model and in bovine macrophages was examined. The results suggested that S-LPS or, more precisely, its O side chain is essential for survival in mice but not in macrophages.

The regulated outer membrane protein Omp21 from Comamonas acidovorans is identified as a member of a new family of eight-stranded beta-sheet proteins by its sequence and properties

Omp21, a minor outer membrane protein of the soil bacterium Comamonas acidovorans, was purified from a spontaneous mutant lacking a surface layer and long-chain lipopolysaccharide. Omp21 synthesis is enhanced by oxygen depletion, and the protein has a variable electrophoretic mobility in sodium dodecyl sulfate-polyacrylamide gel electrophoresis due to its heat-modifiable behavior. The structural gene omp21 encodes a precursor of 204 amino acids with a putative signal peptide of 21 amino acids. Mature Omp21 is a typical outer membrane protein with a high content of beta structure as determined by infrared spectroscopy. Sequence comparisons show that it belongs to a new outer membrane protein family, characterized by eight amphipathic beta strands, which includes virulence proteins, such as the neisserial opacity proteins, Salmonella typhimurium Rck, and Yersinia enterocolitica Ail, as well as the major outer membrane proteins OmpA from Escherichia coli and OprF from Pseudomonas aeruginosa.

Characterization of an immuno-dominant antigen in Brucella ovis and evaluation of its use in an enzyme-linked immunosorbent assay

A panel of 45 Brucella ovis serologically positive sera were tested in immunoblots against B. ovis outer membrane proteins Omp31 and Omp25, purified by preparative SDS-gel electrophoresis. Forty-three sera reacted with Omp31, while only 11 reacted with Omp25, suggesting that Omp31 is identical to the previously reported immuno-dominant 29-kDa protein. Attempts to purify Omp31 on a larger scale by using procedures such as ion exchange-, reversed phase-, affinity- and gel filtration chromatography suggested that the outer membrane proteins were aggregated with rough lipopolysaccharide. Only denaturing SDS-gel filtration chromatography was able to separate proteins of about 29 kDa from rough lipopolysaccharide but did not separate Omp31 from Omp25 in B. ovis preparations. When used in an enzyme-linked immunosorbent assay, this 29-kDa protein preparation was less sensitive and less specific than the routinely used heat-extracted B. ovis antigen. A readily available recombinant E. coli, expressing the gene for Omp31 from Brucella melitensis 16 M, was used to extract and enrich recombinant Omp31 by a temperature-dependent Triton X-114-based technique. When this material was used in immunoblots with the 45 sera from B. ovis-infected sheep and with 10 monoclonal antibodies, raised against B. ovis Omp31, major differences in the antibody reactivity between the recombinant B. melitensis Omp31 and the B. ovis Omp31 were found. Such differences were unexpected because of the known structural and immunological relatedness of outer membrane proteins from various Brucella species. These results indicated that the antibody-response in B. ovis naturally-infected sheep against the immuno-dominant Omp31 was directed against epitopes which were only accessible when the protein was aggregated with rough lipopolysaccharides, or which were formed after aggregation but were not present in the recombinant protein.

DNA polymorphism at the omp-31 locus of Brucella spp.: evidence for a large deletion in Brucella abortus, and other species-specific markers

The omp-31 gene, encoding a major outer-membrane protein in Brucella melitensis, was PCR-amplified from Brucella strains representing all species and known biovars by using primers selected according to the B. melitensis 16M omp-31 published sequence. Amplification of omp-31 was achieved from DNA of all Brucella species with the exception of Brucella abortus, the only Brucella species where expression of omp-31 was not detected by reactivity with an mAb specific for an epitope located in Omp-31. Southern blot hybridization of plasmid probes, bearing inserts (4.4-17 kb) containing B. melitensis 16M omp-31 and adjacent DNA of different sizes, with HindIII-digested total DNA showed that a large fragment, comprising the entire omp-31 gene and flanking DNA, was actually absent in B. abortus strains. The size of this DNA fragment has been determined to be about 10 kb. Southern blot hybridization with the different plasmid probes identified species-specific markers for B. abortus and B. melitensis. At the biovar level, a specific marker for B. melitensis bv. 1 was also identified. Additionally, PCR-RFLP studies of omp-31 revealed specific markers for Brucella ovis, Brucella canis and Brucella suis bv. 2. Using a combination of omp-31 PCR-RFLP patterns and Southern blot hybridization profiles Brucella species were differentiated with the sole exception of Brucella neotomae which was not differentiated from B. suis bv. 1, 3, 4 and 5. Results presented in this paper demonstrate the potential of omp-31 for differentiating the brucellae and show that B. abortus lacks a large DNA fragment of about 10 kb containing omp-31 and flanking DNA. In such a large deletion, other genes in addition to omp-31 are probably involved. Sequencing of this DNA fragment will help to identify the missing genes in B. abortus which could possibly be involved in the differences of pathogenicity and host preference seen in Brucella species.

Identification and characterization of Brucella ovis immunogenic proteins using two-dimensional electrophoresis and immunoblotting

In a previous report, proteins from Brucella melitensis were characterized by two-dimensional polyacrylamide gel electrophoresis (2-D PAGE) and N-terminal microsequencing. In the present report, we have extended this study to the second etiologic agent in ovine brucellosis, B. ovis, responsible for ram epididymitis and infertility. The combination of 2-D gel electrophoresis and protein microsequencing facilitated the location and identification of the major proteins of B. ovis on the 2-D pattern. These proteins comprised cytoplasmic, periplasmic, and some membrane proteins except the major outer membrane proteins. By comparing 2-D gel profiles of B. ovis with that of B. melitensis described previously, a few proteins with different expression levels were readily identified. Serum from a ram naturally infected with B. ovis was used in immunoblotting studies to identify immunogenic proteins recognized during the course of infection. This serum showed antibody reactivity against approximately 82 protein spots. Twenty-one of these proteins were identified either by use of monoclonal antibodies or by N-terminal microsequencing. Several proteins previously described in earlier Brucella works were identified: the 89 kDa outer membrane protein, DnaK, GroEL, BP26, and Cu-Zn superoxide dismutase. Eight proteins had amino acid sequences homologous to those of various proteins from other bacteria found in protein databases; NikA, dihydrolipoamide succinyltransferase, a hypothetical 31 kDa protein, malate dehydrogenase, succinyl-CoA synthetase alpha subunit, an amino acid ABC type transporter, Leu/Ile/Val-binding protein precursor, and ClpP. The remaining eight proteins had N-terminal sequences lacking similarity to existing databases entries. Thus, the 2-D PAGE analysis provided a convenient first approach in the characterization of immunogenic proteins.

Molecular and immunological characterization of the major outer membrane proteins of Brucella

The major outer membrane proteins (OMPs) of Brucella spp. were initially identified in the early 1980s by selective extraction techniques and classified according to their apparent molecular mass as 36-38 kDa OMPs or group 2 porin proteins and 31-34 kDa and 25-27 kDa OMPs which belong to the group 3 proteins. Variation in apparent molecular mass is essentially due to association with peptidoglycan subunits of different sizes. Two genes, omp2a and omp2b, which are closely linked in the Brucella genome, and which share a great degree of homology (> 85%), encode the 36 kDa porin proteins, now named Omp2a and Omp2b proteins respectively. Two genes code for the group 3 OMPs and are named omp25 and omp31. The predicted amino acid sequences of omp25 and omp31 share 34% identity. Furthermore, all Brucella major OMPs share amino acid sequence homology with the major OMPs RopA or RopB of Rhizobium leguminosarum, which supports the close genetic relationship of brucellae with members of the alpha-2 subdivision of the class Proteobacteria. Another characteristic common to the major OMPs of R. leguminosarum and Brucella is that they are tightly, probably covalently, associated with the peptidoglycan. The major OMP genes display diversity among Brucella species, biovars and strains allowing their differentiation, and the polymorphic markers identified have brought new insights into the evolutionary development of the genus Brucella, antigenic variability of brucellae, and future prospects in the field of vaccine development.