Comparative immune responses to native cell envelope antigens and the hot sodium dodecyl sulfate insoluble fraction (PG) of Brucella abortus in cattle and mice

Evaluation of DNA vaccine encoding BCSP31 surface protein of Brucella abortus for protective immunity

Brucellosis is a highly contagious and zoonotic disease and has a considerable impact on animal health and economy of a country, principally in Pakistan, where rural income largely depends upon livestock farming and dairy products. The disease burden is more in underdeveloped/developing countries due to the low economy and limited access to the diagnostic facilities. In Pakistan, the prevalence of Brucella abortus is very high, so it is the need of the hour to control this disease through more advanced methods. This study was designed with the aim to construct the DNA based vaccine of gene encoding antigenic surface protein (BCSP₃₁). For this purpose, the BCSP₃₁ gene was amplified, purified and ligated in pTZ57 R/T (cloning vector). Dubbed BCSP₃₁-pTZ57 R/T vector was transformed into competent cells (DH5α). After plasmid extraction, the plasmid and pET-28a vector was restricted with EcoRI and BamHI. Again, ligation was done and dubbed pET-28a-BCSP₃₁ transformed into E. coli (BL21). After expression, the protein was purified and used for evaluation of immunogenic response. The protective and immunogenic efficacy of the vaccine was evaluated in rabbits (n = 20). The rabbits were divided into four equal groups. Groups A-C were given purified protein diluted in normal saline @ 750, 1500 and 3000 μg/0.2 mL, respectively through intraconjunctival route. Group D was given 0.2 mL normal saline through intraconjunctival route. Specific immunoglobulin G (IgG) responses were measured through indirect ELISA on a weekly basis. The titer of IgG against the antigen was significantly (p < 0.05) higher in vaccinated groups A-C as compared to group D (control group) in a dose dependent manner. Moreover, log units of protection produced by DNA based vaccine in the rabbits (3.02) also indicated the protective efficacy of the DNA vaccine against B. abortus challenge. The response of this vaccine in rabbit suggested its potential effectiveness against Brucella abortus in large animals.

Protective immune-response of aluminium hydroxide gel adjuvanted phage lysate of Brucella abortus S19 in mice against direct virulent challenge with B. abortus 544

The prophylactic efficacies of plain and alum adsorbed lysate were evaluated by direct virulent challenge in mice model. A recently isolated brucellaphage 'ϕLd' was used for generation of lysates. Twenty four h incubated Brucella abortus S19 broth cultures standardized to contain approximately 10(8) CFU/ml were found suitable for generation of lysates. Three lysate batches produced through separate cycles did not show any significant variation with respect to protein and polysaccharide contents, endotoxin level and phage counts, indicating that compositionally stable lysate preparations can be generated through an optimized production process. Three polypeptides of ∼16, 19 and 23 kDa could be identified as immuno-dominant antigens of the lysate which induced both humoral and cell-mediated immune responses in a dose dependent manner. Results of efficacy evaluation trial confirmed dose-dependent protective potencies of lysate preparation. The lysate with an antigenic dose of 0.52 μg protein and 60 μg CHO adsorbed on aluminium gel (0.1 percent aluminium concentration) exhibited the highest protective potency which was greater than that induced by standard S19 vaccine. Phage lysate methodology provides a very viable option through which an improved immunizing preparation with all desirable traits can be developed against brucellosis, and integrated with immunization programmes in a more efficient manner.

A history of the development of Brucella vaccines

Brucellosis is a worldwide zoonosis affecting animal and human health. In the last several decades, much research has been performed to develop safer Brucella vaccines to control the disease mainly in animals. Till now, no effective human vaccine is available. The aim of this paper is to review and discuss the importance of methodologies used to develop Brucella vaccines in pursuing this challenge.

Liposomised recombinant ribosomal L7/L12 protein protects BALB/c mice against Brucella abortus 544 infection

Brucella abortus, a facultative intracellular pathogen, is of tremendous zoonotic importance because of its ability to induce spontaneous abortion in cattle and other livestock. It is also known to cause persistent undulant fever, endocarditis, arthritis, osteomyelitis and meningitis in humans. The available vaccines against this dreadful infection suffer from limitations like short-term immunity, increased risk of hypersensitivity and low prophylactic index in the recipients. In the present study, we have demonstrated that liposomal form of a recombinant ribosomal L7/L12 protein, a B-T cell antigen of B. abortus, activates strong immune response in the host. In contrast, free antigen generates moderate immune response in the immunised animals. The liposomisation of rL7/L12 protein causes tremendous increase in cell-mediated immune response in terms of delayed type hypersensitivity, T-cell proliferation and up-regulation in type I cytokine expression, etc. Moreover, the liposome encapsulated antigen elicited stronger humoral immune response as compared to standard vaccine (S-19) or IFA-L7/L12 combination in the immunised animals. The effectiveness of liposome-based vaccine was also substantiated by better systemic clearance of bacterial load after challenging the animals with B. abortus 544 pathogen. The results of the present study suggest the potential of liposome-based rL7/L12 antigen as prospective and efficient candidate vaccine capable of eliciting both cell mediated as well as humoral immune responses against experimental murine brucellosis.

Bovine T lymphocyte responses to Brucella abortus

The long-held paradigm of T lymphocyte-mediated activation of mononuclear phagocytes (Mø) as the major mechanism of protection against facultative intracellular pathogens such as Brucella has been modified to include killing of infected Mø by various subsets of T lymphocytes. Remnants of killed infected cells are phagocytosed by immunologically-activated Mø, which are much more efficient at killing such pathogens. Most of the detailed information regarding immunity in general and that of brucellosis specifically has been obtained using murine infection models rather than in cattle. However, there has been considerable definition of cellular phenotypes, cytokines and functional characteristics of T lymphocytes in cattle over the last decade. This was mainly due to development of monoclonal antibodies against cell surface markers and application of molecular cloning and polymerase chain reaction (PCR) for isolation, characterization and detection of genes encoding bovine cytokines. This review discusses cellular and molecular immunity in bovine brucellosis as pertains to T lymphocyte interactions with the Mø. Although current knowledge directly obtained from brucellosis immunity studies in the bovine host is limited and incomplete, the many parallels between the bovine and murine immune systems allow for some extrapolation in the description of bovine host defense mechanisms. Direct information from studies with immunized cattle supports the concepts of coordinate activation of uninfected Mø and killing of Brucella-infected Mø by antigen-specific T lymphocytes as major mechanisms of host defense in bovine brucellosis. There also appears to be a bias in the T lymphocyte compartment towards recognition of particular bacterial stress proteins following immunization with live Brucella vaccines.

Brucellosis vaccines: past, present and future

The first effective Brucella vaccine was based on live Brucella abortus strain 19, a laboratory-derived strain attenuated by an unknown process during subculture. This induces reasonable protection against B. abortus, but at the expense of persistent serological responses. A similar problem occurs with the B. melitensis Rev.1 strain that is still the most effective vaccine against caprine and ovine brucellosis. Vaccines based on killed cells of virulent strains administered with adjuvant induced significant protection but also unacceptable levels of antibodies interfering with diagnostic tests. Attempts were made to circumvent this problem by using a live rough strain B. abortus 45/20, but this reverted to virulence in vivo. Use of killed cells of this strain in adjuvant met with moderate success but batch to batch variation in reactogenicity and agglutinogenicity limited application. This problem has been overcome by the development of the rifampicin-resistant mutant B. abortus RB51 strain. This strain has proved safe and effective in the field against bovine brucellosis and exhibits negligible interference with diagnostic serology. Attempts are being made to develop defined rough mutant vaccine strains that would be more effective against B. melitensis and B. suis. Various studies have examined cell-free native and recombinant proteins as candidate protective antigens, with or without adjuvants. Limited success has been obtained with these or with DNA vaccines encoding known protective antigens in experimental models and further work is indicated.

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.

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

The gene coding for the major outer membrane protein (OMP) of 31 to 34 kDa, now designated Omp31, of Brucella melitensis 16M was cloned and sequenced. A B. melitensis 16M genomic library was constructed in lambda GEM-12 XhoI half-site arms, and recombinant phages expressing omp31 were identified by using the anti-Omp31 monoclonal antibody (MAb) A59/10F09/G10. Subcloning of insert DNA from a positive phage into pGEM-7Zf allowed the selection of a plasmid bearing a 4.4-kb EcoRI fragment that seemed to contain the entire omp31 gene under control of its own promoter. omp31 was localized within a region of the EcoRI insert of approximately 1.1 kb. Sequencing of this region revealed an open reading frame of 720 bp encoding a protein of 240 amino acids and a predicted molecular mass of 25,307 Da. Cleavage of the first 19 amino acids, showing typical features of signal peptides for protein export, leaves a mature protein of 221 amino acids with a predicted molecular mass of 23,412 Da. The predicted amino acid sequence of B. melitensis 16M Omp31 showed 35.2% identity with the RopB OMP of Rhizobium leguminosarum bv. viciae 248 and 34.3% identity with Omp25 of B. abortus 544. As in Brucella spp., Omp31 was located in the outer membrane of recombinant Escherichia coli, but its reported peptidoglycan association in Brucella cells was not detected in E. coli. The ability of Omp31 to form oligomers resistant to sodium dodecyl sulfate denaturation at low temperatures, a characteristic described for several bacterial porins, was observed in both B. melitensis and recombinant E. coli. The epitope recognized by the anti-Omp31 MAb A59/10F09/G10, for which a protective activity has been suggested, has been delimited to a region of 36 amino acids of Omp31 covering the most hydrophilic part of the protein. The availability of recombinant Omp31 and the identification of the antigenic determinant recognized by MAb A59/10F09/G10 will allow the evaluation of their potential protective activity and their potential for the development of subcellular vaccines against brucellosis.

Immunization of mice with recombinant L7/L12 ribosomal protein confers protection against Brucella abortus infection

BALB/c mice were immunized with the recombinant Brucella abortus L7/L12 ribosomal protein fused to maltose binding protein (MBP). Vaccinated animals mounted a specific immune response to the recombinant fusion protein as demonstrated by immunoblot analyses. Additionally, B. abortus L7/L12 ribosomal protein conferred a significant degree of protection when compared to mice vaccinated with adjuvant alone, adjuvant plus MBP or B. abortus. These results indicate that a recombinant B. abortus protein, previously identified as T-cell-reactive, engendered protective immunity to mice against brucellosis.

Variation of Brucella abortus 2308 infection in BALB/c mice induced by prior vaccination with salt-extractable periplasmic proteins from Brucella abortus 19

The study compared the immune and protective responses induced in BALB/c mice vaccinated with six salt-extractable periplasmic protein fractions (Brucella cell surface proteins [BCSP]) of Brucella abortus 19 and later challenge exposed with B. abortus 2308. BCSP70 was precipitated with ammonium sulfate at 70% saturation, and BCSP100 was precipitated with ammonium sulfate at 100% saturation by use of supernatant fluid of BCSP70 that had been precipitated with 70% ammonium sulfate. Four subfractions were separated from BCSP100 by anion-exchange high-performance liquid chromatography (HPLC). Monophosphoryl lipid A (MPL) from Salmonella typhimurium Re mutant strain was used as a potential immune response modifier in some vaccines. Reduced or increased numbers of CFU and increased spleen size in the principal groups of mice relative to that of the nonvaccinated control group were considered protectiveness or virulence (survival) criteria. Results indicated that vaccines prepared from BCSP70 and BCSP100 were moderately protective and immunogenic. The subfractions designated BCSP100-A through BCSP100-D purified by anion-exchange HPLC were not protective when MPL was not used as an immune response modifier. However, two subfractions were associated with significant (P < 0.05) increases in CFU per spleen and splenomegaly in vaccinated mice compared with those in nonvaccinated challenge-exposed mice. MPL enhanced protection or was neutral when used with BCSP70, BCSP100, BCSP100-C, and BCSP100-D. Serologic results of an enzyme-linked immunosorbent assay indicated that MPL modulated the immunoglobulin G responses induced by BCSP70, BCSP100, and subfraction BCSP100-B vaccines only. The overall results suggest that certain proteinaceous periplasmic fractions might serve as virulence or survival factors in B. abortus infections.

Alteration of protective and serologic responses in BALB/c mice vaccinated with chemically modified versus nonmodified proteins of Brucella abortus 19

A study was conducted to determine whether the covalent chemical modification of Brucella abortus 19 salt-extractable proteins (BCSP) and BCSP derivatives would modulate the immune responses in BALB/c mice. Salt-extractable proteins BCSP 0-70 and BCSP 70-100 were modified with acetoacetic anhydride, and recombinant proteins rBCSP20 (20 kDa), rBCSP31 (31 kDa), and rBCSP45 (45 kDa) were modified with succinic and dodecanoyl anhydrides. Four weeks after mice were vaccinated with the different preparations, principal and control mice were challenge exposed with a virulent culture of B. abortus 2308, and mice were necropsied 2 weeks later. Serum samples were obtained immediately before mice were challenge exposed and at necropsy. Sera were tested for specific immunoglobulin M (IgM) and G (IgG) antibodies by using an enzyme-linked immunosorbent assay. Acylation decreased the immune responses (increased IgG antibodies and reduced spleen CFU and splenomegaly) induced by both BCSP 0-70 and BCSP 70-100. Modification of the recombinant proteins by dodecanoyl and succinic anhydrides had no effect on the protection induced; however, the IgG serologic responses to the homologous and heterologous proteins were altered. Monophosphoryl lipid A markedly enhanced the immunogenicity of BCSP 0-70.

Antigens of Brucella abortus S19 immunodominant for bovine lymphocytes as identified by one- and two-dimensional cellular immunoblotting

Cellular immune responses are influential for protection against intracellular bacteria such as brucellae. Therefore, identification of Brucella abortus antigens that activate primed bovine lymphocytes is fundamental for discerning the breadth of cellular response in bovine brucellosis. Potentially antigenic components of B. abortus S19 were isolated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by nitrocellulose blotting. Specific one-dimensional blot segments induced proliferation of peripheral blood lymphocytes from all 25 of the vaccinated cattle tested and were defined as immunodominant. Individual proteins that stimulated lymphocyte proliferation were further characterized by two-dimensional cellular immunoblotting by two different approaches. Individual one-dimensional stimulatory blot segments were eluted, concentrated, and then subjected to two-dimensional cellular immunoblotting. Alternatively, entire two-dimensional gels containing all of the B. abortus components were blotted and nitrocellulose sections containing individual proteins were assayed for lymphocyte activation. Thirty-eight Brucella proteins that induced lymphocyte proliferation were resolved by both procedures. Phenotypic analysis of the proliferating cell population demonstrated the presence of CD4+, CD8+, and immunoglobulin M+ lymphocytes. Two immunogenic proteins, 12 and 31 kDa, identified by two-dimensional cellular immunoblotting, were subjected to partial N-terminal amino acid analysis. The 12-kDa protein was within the area of greatest lymphocyte proliferation, while the 31-kDa protein was chosen for comparison with a 31-kDa protein previously reported by others. A search of the National Biomedical Research Foundation protein data bank showed that the sequences were not homologous with other known proteins. Identification of Brucella proteins immunogenic for bovine lymphocytes provides an important step in distinguishing the various proteins involved in pathogenicity and/or disease resistance.

Sodium dodecyl sulfate- and salt-extracted antigens from various Brucella species induce proliferation of bovine lymphocytes

One- and two-dimensional gel electrophoresis and cellular immunoblotting were used to compare the protein profiles and immunogenic capabilities of salt- and sodium dodecyl sulfate-extracted components isolated from different Brucella species. Cellular immunoblotting demonstrated that freshly isolated bovine peripheral blood mononuclear cells proliferated to similar molecular mass components from the various Brucella extractions. One- and two-dimensional gel electrophoresis showed similarities in protein profiles among the different Brucella species that correlated with lymphocyte reactivity. The results presented in this study provide preliminary evidence that common proteins that induce lymphocyte proliferation are present among different Brucella species, suggesting that a genuswide subunit vaccine may be feasible.

Immunopotentiation of cattle vaccinated with a soluble Brucella abortus antigen with low LPS content: an analysis of cellular and humoral immune responses

The adjuvant effects of dimethyl dioctadecyl ammonium bromide (DDA) alone or in combination with trehalose dimycolate (TDM) or muramyl dipeptide (MDP) on bovine humoral and cellular responses to a soluble protein extract of gamma irradiated Brucella abortus strain 19 (SPEBA) were investigated. Thirty-five beef steers were randomly allotted to nine groups. Three of these groups received SPEBA (2 mg protein per dose) subcutaneously in combination with adjuvants, one group received the reduced dose of B. abortus strain 19 (S19), and one group received SPEBA alone. Controls included groups receiving adjuvant preparations only or no vaccine. Immune responses to the various immunizations were assessed sequentially for 56 days using various in vitro and in vivo assays. The humoral response to B. abortus was measured using standard serologic tests, an enzyme-linked immunosorbent assay, and a quantitative fluorometric immunoassay. The cell-mediated immune (CMI) response was measured by antigen-specific lymphoproliferation (LP), interleukin 2 (IL 2) production, and soluble suppressor factor release. Skin testing at day 35 for delayed-type hypersensitivity (DTH) to SPEBA was also performed. Minimal humoral responses were induced with SPEBA alone. The highest and most sustained serum antibody responses to B. abortus antigens were elicited by the S19 vaccine. A combination of SPEBA with DDA + TDM induced higher antibody levels than SPEBA with DDA or SPEBA with DDA + MDP. Responses to DTH among the groups receiving SPEBA were most notable in the SPEBA with DDA + TDM groups. Increased IL 2 production was greatest in the S19 and SPEBA with DDA + TDM vaccinates. The results indicated that a combination of DDA + TDM best potentiated immune responses to a soluble B. abortus antigen preparation and may be useful as adjuvants for future vaccines.

Brucella suis S2, brucella melitensis Rev. 1 and Brucella abortus S19 living vaccines: residual virulence and immunity induced against three Brucella species challenge strains in mice

Live attenuated Brucella suis S2 vaccine was compared to living vaccines B. abortus S19 and B. melitensis Rev. 1 in mice. Residual virulence was estimated by ability to multiply and persist in spleen and lymph nodes. Immunogenicity was estimated by spleen counts of control and vaccinated mice challenged either with the reference B. abortus 544 strain or with virulent B. melitensis H38 and B. suis 1330 strains. S2 vaccine had lower residual virulence; expressed as 50% recovery time, persistence was 4.3 weeks, compared to 7.1 and 9.0 weeks for S19 and Rev. 1 vaccines. Immunity induced by the three vaccines was similar 45 days after vaccination. At 150 days, immunity by S19 and Rev.1 was still similar against the three challenge strains. In contrast, immunity induced by S2 had declined against the B. melitensis strain. Thus, a recall vaccination may be required for vaccination of sheep to confer a long-lasting immunity.

Induction of lymphocyte responsiveness by the outer membrane-peptidoglycan complex of rough strains of Brucella abortus

The outer membrane-peptidoglycan complex (OM-PG) from rough strains of Brucella abortus was tested for its ability to induce lymphocyte responsiveness in cattle. Six groups of heifers were immunized with varying doses and administration schedules of rough OM-PG and assayed for responsiveness of their lymphocytes in proliferation assays in vitro. All OM-PG preparations were emulsified in a commercial adjuvant for administration. Two other groups of heifers were immunized with strain 19 vaccine or adjuvant alone. Three groups of heifers received two inoculations of OM-PG antigens from a naturally-occurring rough strain at a 57-day interval. The doses of OM-PG given in these three groups were 400 micrograms, 1200 micrograms, and 4000 micrograms at each inoculation. The frequency of cows that responded in lymphocyte proliferation assays increased with the dose of OM-PG given. Two groups received single inoculations of OM-PG, either 2400 micrograms or 8000 micrograms. Although there were responsive cows in these immunization groups, their frequency was lower than in the groups receiving the same total dose in two inoculations. A sixth group of cows was inoculated with OM-PG from a rough transposon mutant of B. abortus, and the frequency of responsive cows in this immunization group was comparable to that of responsive cows immunized with the same dose of OM-PG from the spontaneous rough mutant. In comparisons of cows inoculated with strain 19 to those inoculated with OM-PG preparations, differences were observed in the relative responsiveness of their lymphocytes to whole cells and OM-PG in the in vitro lymphocyte proliferation assays. These differences suggested that lymphocytes stimulated by strain 19 vaccination have different specificities than those stimulated by immunization with OM-PG of rough mutant strains of B. abortus.

Immunogenicity of subcellular fractions of Brucella abortus: measurement by in vitro lymphocyte proliferative responses

Five groups of heifers were immunized with various subcellular fractions of Brucella abortus and tested for their responsiveness in lymphocyte proliferative responses in vitro. The five subcellular fractions used as immunogens were: (1) a mixture of recombinant outer membrane proteins fused to Escherichia coli beta-galactosidase, (2) a mixture of outer membrane proteins BaomI, BaomIIB1, and BaomIII1, (3) a mixture of outer membrane proteins 7.5 kDa and 8.8 kDa, (4) a complex of smooth lipopolysaccharide and proteins, and (5) a complex of outer membranes and peptidoglycan (OM-PG complex) from a rough strain. All immunogens were emulsified in adjuvant and administered twice at a 61-day interval. Two other groups of cows were included; one immunized with strain 19 and the other with adjuvant only. Strain 19 and the rough OM-PG complex induced responsiveness in lymphocyte proliferation assays in a high percentage of immunized cows. The smooth lipopolysaccharide-protein complex induced responsiveness in fewer cows. The lowest frequencies of responding cows were found in groups that received either recombinant proteins or purified protein mixtures. Based on these results, we concluded: (1) cellular immunity, as measured by in vitro lymphocyte proliferative responses, can be induced with subcellular fractions of B. abortus and (2) the more complex the immunogen, the greater the frequency of responding cows.

Capacity of passively administered antibody to prevent establishment of Brucella abortus infection in mice

In contrast to immunity against some other facultative intracellular parasites, protective immunity against Brucella abortus is mediated in mice by antibodies as well as by cell-mediated immune responses. It was the purpose of this study to determine whether antibody alone would prevent infection with B. abortus. The majority (82%) of CD-1 outbred mice infected with 100 CFU of virulent B. abortus 2308 preincubated with graded quantities of an O polysaccharide-specific IgG2a monoclonal antibody (MAb) were free of infection 1. 2, 4, and 6 weeks later, based on detection limits of 13 brucellae per spleen and 39 per liver. Infection was present in 95% of control animals. Similar results were obtained with a challenge dose of 500 CFU, but with a challenge dose of 5,000 CFU, infection became established even with the highest concentration of MAb used (50 micrograms of MAb per 5,000 brucellae). Pretreatment with an O polysaccharide-specific IgG1 MAb or with convalescent-phase serum diminished but did not prevent establishment of infection by 100 CFU of B. abortus. A majority of culture-negative mice tested 6 weeks after infection were serologically negative, which could have signified either the absence of previous infection or the early elimination of infection. In an in vitro test system, all of the antibody preparations were efficient in opsonizing B. abortus. Effective killing of the organism by unelicited mouse peritoneal macrophages occurred in conventional but not in endotoxin-free medium, suggesting that activated macrophages were required for killing of opsonized B. abortus. These results emphasize the potential importance of antibodies in the immunoprophylaxis of brucellosis and suggest that the design of a successful vaccine will require the induction of antibodies not only of appropriate specificity but also of the optimal isotype for mediating protective functions.

Effectiveness of natural and synthetic complexes of porin and O polysaccharide as vaccines against Brucella abortus in mice

A single vaccination of mice with a complex of porin and smooth lipopolysaccharide (porin-S-LPS) extracted from virulent Brucella abortus 2308 provided significant protection (P less than 0.01 to P less than 0.001) against challenge with the same strain, equivalent to that achieved by vaccination with living attenuated B. abortus 19. The porin-S-LPS vaccine given without adjuvant or in several adjuvants (trehalose dimycolate and muramyl dipeptide; the pluronic polymer L-121 and muramyl dipeptide; or complexed with Quil A in immunostimulating complexes) provided equivalent protection. In contrast, one vaccination with porin complexed with rough LPS (porin-R-LPS) from a rough mutant of strain 2308 provided no protection with any adjuvant tested. In one experiment, two inoculations with the porin-R-LPS resulted in a low level of protection, probably owing to priming of the animals for production of O-polysaccharide-specific antibodies. However, one vaccination with rough-strain porin covalently bound to purified O polysaccharide conferred protection equal to that obtained with natural complexes of porin-S-LPS or with living strain 19. A synthetic vaccine containing long chains of O polysaccharide was more effective than one prepared with short chains. Protective vaccines caused the formation of increased concentrations of circulating O-polysaccharide-specific antibodies, although there were individual exceptions to the quantitative association between O-polysaccharide-specific antibodies and protection. Antibodies specific for porin or R-LPS were found in negligible quantities in vaccinated mice. These results provide additional evidence that the O polysaccharide will constitute an essential component of an effective subcellular vaccine against B. abortus and that O-polysaccharide-specific antibodies play an important role in protective immunity in brucellosis.