Induction of immune response in BALB/c mice with a DNA vaccine encoding bacterioferritin or P39 of Brucella spp

LDH as an adjuvant makes Brucella outer-membrane vesicles and outer-membrane vesicle-associated proteins highly protective in mice

Objectives

Existing Brucella vaccines are attenuated and can cause vaccine-associated brucellosis; and these safety concerns have affected their application. Although subunit vaccines have the advantages of safety, efficacy, low cost, and rapid production, they are usually poorly immunogenic and insufficient to trigger persistent immunity. Therefore, we added layered double hydroxide (LDH) as an adjuvant to Brucella subunit vaccine formulations to enhance the immune response to the antigen.

Materials and Methods

LDH and Freund's adjuvant were combined with Brucella outer-membrane vesicles (OMVs) and OMV-associated proteins to form a subunit vaccine, respectively. The immunogenicity of LDH as an adjuvant was assessed in BALB/c mice. We examined levels of immunoglobulin G, G1, and G2a (IgG, IgG1, and IgG2a) antibodies (aBs); percentages of Cluster of Differentiation 4-positive (CD4⁺) and CD8⁺ T cells in peripheral-blood lymphocytes; and secretion of cytokines in mouse spleen lymphocytes. Finally, splenic index and splenic bacterial load were assessed via Brucella challenge experiments on mice.

Results

The LDH subunit vaccine also produced high levels of specific aBs in mice compared with Freund's adjuvant subunit vaccine and induced mainly T-helper 1 cell (T_h1)-type immune responses. In addition, mice in the LDH subunit vaccine group had significantly lower bacterial loads in their spleens than those in the Freund's adjuvant subunit vaccine group, and the LDH-OMV vaccine offered a higher level of protection against Brucella attack.

Conclusion

LDH as an adjuvant-paired vaccine provided a high level of protection against Brucella infection.

Proteomics of Brucella

Brucella spp. are Gram negative intracellular bacteria responsible for brucellosis, a worldwide distributed zoonosis. A prominent aspect of the Brucella life cycle is its ability to invade, survive and multiply within host cells. Comprehensive approaches, such as proteomics, have aided in unravelling the molecular mechanisms underlying Brucella pathogenesis. Technological and methodological advancements such as increased instrument performance and multiplexed quantification have broadened the range of proteome studies, enabling new and improved analyses, providing deeper and more accurate proteome coverage. Indeed, proteomics has demonstrated its contribution to key research questions in Brucella biology, i.e., immunodominant proteins, host-cell interaction, stress response, antibiotic targets and resistance, protein secretion. Here, we review the proteomics of Brucella with a focus on more recent works and novel findings, ranging from reconfiguration of the intracellular bacterial proteome and studies on proteomic profiles of Brucella infected tissues, to the identification of Brucella extracellular proteins with putative roles in cell signaling and pathogenesis. In conclusion, proteomics has yielded copious new candidates and hypotheses that require future verification. It is expected that proteomics will continue to be an invaluable tool for Brucella and applications will further extend to the currently ill-explored aspects including, among others, protein processing and post-translational modification.

The advances in brucellosis vaccines

Brucellosis is a worldwide zoonosis affecting animal and human health. Till now, there is no effective vaccine licensed for brucellosis in humans. Although M5, H38 and 45/20 vaccines were used to prevent animal brucellosis in the early stages, the currently used animal vaccines are S19, Rev.1, S2, RB51 and SR82. However, these vaccines still have several drawbacks such as residual virulence and interfering conventional serological tests. With the development of DNA recombination technologies and the completion of the sequence of Brucella genome, much research focuses on the search for potential safer and more effective vaccines. Preliminary studies have demonstrated that new vaccines, including genetically engineered attenuated vaccines, subunit vaccines and other potential vaccines, have higher levels of protection, but there are still some problems. In this paper, we briefly review the main vaccines that have been used in controlling the brucellosis for decades and the progress in the development of new brucellosis vaccines.

Identification of Cross-Protective Potential Antigens against Pathogenic Brucella spp. through Combining Pan-Genome Analysis with Reverse Vaccinology

Brucellosis is a zoonotic infectious disease caused by bacteria of the genus Brucella. Brucella melitensis, Brucella abortus, and Brucella suis are the most pathogenic species of this genus causing the majority of human and domestic animal brucellosis. There is a need to develop a safe and potent subunit vaccine to overcome the serious drawbacks of the live attenuated Brucella vaccines. The aim of this work was to discover antigen candidates conserved among the three pathogenic species. In this study, we employed a reverse vaccinology strategy to compute the core proteome of 90 completed genomes: 55 B. melitensis, 17 B. abortus, and 18 B. suis. The core proteome was analyzed by a metasubcellular localization prediction pipeline to identify surface-associated proteins. The identified proteins were thoroughly analyzed using various in silico tools to obtain the most potential protective antigens. The number of core proteins obtained from analyzing the 90 proteomes was 1939 proteins. The surface-associated proteins were 177. The number of potential antigens was 87; those with adhesion score ≥ 0.5 were considered antigen with "high potential," while those with a score of 0.4-0.5 were considered antigens with "intermediate potential." According to a cumulative score derived from protein antigenicity, density of MHC-I and MHC-II epitopes, MHC allele coverage, and B-cell epitope density scores, a final list of 34 potential antigens was obtained. Remarkably, most of the 34 proteins are associated with bacterial adhesion, invasion, evasion, and adaptation to the hostile intracellular environment of macrophages which is adjusted to deprive Brucella of required nutrients. Our results provide a manageable list of potential protective antigens for developing a potent vaccine against brucellosis. Moreover, our elaborated analysis can provide further insights into novel Brucella virulence factors. Our next step is to test some of these antigens using an appropriate antigen delivery system.

Development of new generation of vaccines for Brucella abortus

Brucella abortus is a Gram-negative facultative and intracellular bacteria, it causes bovine brucellosis, a zoonotic disease that is responsible for considerable economic loss to owners of domesticated animals and can cause problems in otherwise healthy humans. There are a few available live attenuated vaccines for animal immunization against brucellosis; however, these have significant side effects and offer insufficient protective efficacy. Thus, the need for more research into the Molecular pathobiology and immunological properties of B. abortus that would lead to the development of better and safer vaccines. In this paper we have reviewed the main aspects of the pathology and the responsive immunological mechanisms, we have also covered current and new prospective vaccines against B. abortus.

Brucellosis vaccines based on the open reading frames from genomic island 3 of Brucella abortus

Brucella abortus is the etiological agent of brucellosis, a zoonotic disease affecting cattle and humans. This disease has been partially controlled in cattle by immunization with live attenuated B. abortus S19 and RB51 strains. However, use of these vaccine strains has been associated with safety issues in animals and humans. New vaccines have since emerged in the prevention of brucellosis, particularly DNA vaccines, which have shown effectiveness and a good safety profile. Their protection efficacy in mice is associated with the induction of Th1 type and cytotoxic T cell mediated immune response against structural antigens and virulence factors expressed during B. abortus infection. Some antigenic candidate for vaccine design against brucellosis (mainly DNA vaccines) have been obtained from genomic island 3 (GI-3) of B. abortus, which encodes several open reading frames (ORFs) involved in the intracellular survival and virulence of this pathogen. The immunogenicity and protection conferred by these DNA vaccines in a murine model is reviewed in this article, suggesting that some of them could be safe and effective vaccine candidates against to prevent B. abortus infection.

Intradermal active full-length DNA Aβ42 immunization via electroporation leads to high anti-Aβ antibody levels in wild-type mice

Aβ immunotherapies with anti-Aβ antibody responses have high potential as possible prevention treatment for Alzheimer's disease. We have previously shown that active DNA Aβ1-42 immunization via gene gun delivery led to a non-inflammatory immune response resulting in decreased Aβ levels in brains of an immunized AD mouse model. To make DNA vaccination more applicable for clinical use, we used here intradermal electroporation. With fine tuning of the electropulse parameters, high antibody levels and low levels of inflammatory cytokines in the cellular immunoassays were observed. Full-length DNA Aβ1-42 immunization delivered via electroporation has potential to be used in the clinical setting.

Meta-Analysis and Advancement of Brucellosis Vaccinology

Background/Objectives

In spite of all the research effort for developing new vaccines against brucellosis, it remains unclear whether these new vaccine technologies will in fact become widely used. The goal of this study was to perform a meta-analysis to identify parameters that influence vaccine efficacy as well as a descriptive analysis on how the field of Brucella vaccinology is advancing concerning type of vaccine, improvement of protection on animal models over time, and factors that may affect protection in the mouse model.

Methods

A total of 117 publications that met the criteria were selected for inclusion in this study, with a total of 782 individual experiments analyzed.

Results

Attenuated (n = 221), inactivated (n = 66) and mutant (n = 102) vaccines provided median protection index above 2, whereas subunit (n = 287), DNA (n = 68), and vectored (n = 38) vaccines provided protection indexes lower than 2. When all categories of experimental vaccines are analyzed together, the trend line clearly demonstrates that there was no improvement of the protection indexes over the past 30 years, with a low negative and non significant linear coefficient. A meta-regression model was developed including all vaccine categories (attenuated, DNA, inactivated, mutant, subunit, and vectored) considering the protection index as a dependent variable and the other parameters (mouse strain, route of vaccination, number of vaccinations, use of adjuvant, challenge Brucella species) as independent variables. Some of these variables influenced the expected protection index of experimental vaccines against Brucella spp. in the mouse model.

Conclusion

In spite of the large number of publication over the past 30 years, our results indicate that there is not clear trend to improve the protective potential of these experimental vaccines.

A Critical Role of Bacterioferritin in Salmonella pullorum-Induced IFN-β Expression in DF-1 Cells

Salmonella enterica serovar Pullorum (S. pullorum) causes pullorum disease in poultry and results in great economic losses to the poultry industry. Although an eradication program has been successfully performed in some countries, it remains a major threat to countries with poor poultry disease surveillance. Currently there are no effective control measures for pullorum disease except eradication. In particular, the pathogenesis of S. pullorum infection is still largely unknown. Here we identified bacterioferritin (Bfr) as a major antigen of S. pullorum to elicit a humoral immune response. Furthermore, we demonstrate that Bfr induces activation of IFN-β promoter and mRNA expression in DF-1 cells, and that the amino acids 1-50 form a critical domain involved in IFN-β expression. Moreover, we found that the p38 MAPK signaling pathway was essential for Bfr-induced IFN-β expression. Importantly, S. pullorum-induced IFN-β expression was totally abolished by deficiency of Bfr in the bacteria, indicating that Bfr plays a critical role in S. pullorum induced IFN-β expression in DF-1 cells. Our findings provide new insights into the molecular mechanisms of the host response to S. pullorum infection.

Recent advances in Brucella abortus vaccines

Brucella abortus vaccines play a central role in bovine brucellosis control/eradication programs and have been successfully used worldwide for decades. Strain 19 and RB51 are the approved B. abortus vaccines strains most commonly used to protect cattle against infection and abortion. However, due to some drawbacks shown by these vaccines much effort has been undertaken for the development of new vaccines, safer and more effective, that could also be used in other susceptible species of animals. In this paper, we present a review of the main aspects of the vaccines that have been used in the brucellosis control over the years and the current research advances in the development of new B. abortus vaccines.

Immunization with Brucella VirB proteins reduces organ colonization in mice through a Th1-type immune response and elicits a similar immune response in dogs

VirB proteins from Brucella spp. constitute the type IV secretion system, a key virulence factor mediating the intracellular survival of these bacteria. Here, we assessed whether a Th1-type immune response against VirB proteins may protect mice from Brucella infection and whether this response can be induced in the dog, a natural host for Brucella. Splenocytes from mice immunized with VirB7 or VirB9 responded to their respective antigens with significant and specific production of gamma interferon (IFN-γ), whereas interleukin-4 (IL-4) was not detected. Thirty days after an intraperitoneal challenge with live Brucella abortus, the spleen load of bacteria was almost 1 log lower in mice immunized with VirB proteins than in unvaccinated animals. As colonization reduction seemed to correlate with a Th1-type immune response against VirB proteins, we decided to assess whether such a response could be elicited in the dog. Peripheral blood mononuclear cells (PBMCs) from dogs immunized with VirB proteins (three subcutaneous doses in QuilA adjuvant) produced significantly higher levels of IFN-γ than cells from control animals upon in vitro stimulation with VirB proteins. A skin test to assess specific delayed-type hypersensitivity was positive in 4 out of 5 dogs immunized with either VirB7 or VirB9. As both proteins are predicted to locate in the outer membrane of Brucella organisms, the ability of anti-VirB antibodies to mediate complement-dependent bacteriolysis of B. canis was assessed in vitro. Sera from dogs immunized with either VirB7 or VirB9, but not from those receiving phosphate-buffered saline (PBS), produced significant bacteriolysis. These results suggest that VirB-specific responses that reduce organ colonization by Brucella in mice can be also elicited in dogs.

Protective effect of a DNA vaccine containing an open reading frame with homology to an ABC-type transporter present in the genomic island 3 of Brucella abortus in BALB/c mice

The immunogenicity of a DNA vaccine containing an open reading frame (ORF) of genomic island 3 (GI-3), specific for Brucella abortus and Brucella melitensis, has been examined. Intramuscular injection of plasmid DNA carrying the open reading frame with homology to an ABC-type transporter (pV278a) into BALB/c mice elicited both humoral and cellular immune responses. Mice injected with pV278a had a dominant immunoglobulin G2a (IgG2a) response. This DNA vaccine elicited a T-cell-proliferative response and induced significant levels of interferon gamma (INF-γ) upon restimulation with recombinant 278a protein. Upon stimulation with an appropriate recombinant protein or crude Brucella protein, the vaccine did not induce IL-4, suggesting a typical T-helper (TH1) response. Furthermore, the vaccine induced protection in BALB/c mice when challenged with the virulent strain Brucella abortus 2308. Taken together, these data suggest that DNA vaccination offers an improved delivery of the homologous of an ABC-type transporter antigen, and provides the first evidence of a protective effect of this antigen in the construction of vaccines against B. abortus.

Construction of an expression plasmid (vector) encoding Brucella melitensis outer membrane protein, a candidate for DNA vaccine

BACKGROUND

DNA vaccination with plasmid encoding bacterial, viral, and parasitic immunogens has been shown to be an attractive method to induce efficient immune responses. Bacteria of the genus Brucella are facultative intracellular pathogens for which new and efficient vaccines are needed.

METHODS

To evaluate the use of a DNA immunization strategy for protection against brucellosis, a plasmid containing the DNA encoding the Brucella melitensis (B. melitensis) 31 kDa outer membrane protein, as a potent immunogenic target, was constructed.

RESULTS

The constructed plasmid, pcDNA3.1+omp31, was injected intramuscularly into mice and the expression of omp31 RNA was assessed by RT-PCR. The integrity of the pcDNA3.1+omp31 construct was confirmed with restriction analysis and sequencing. Omp31 mRNA expression was verified by RT-PCR.

CONCLUSION

Our results indicate that the pcDNA3.1+omp31 eukaryotic expression vector expresses omp31 mRNA and could be useful as a vaccine candidate.

Evaluation of protective effect of DNA vaccines encoding the BAB1_0263 and BAB1_0278 open reading frames of Brucella abortus in BALB/c mice

The immunogenicity of two DNA vaccines encoding open reading frames (ORFs) of genomic island 3 (GI-3), specific for Brucella abortus and Brucella melitensis, has been examined. Intramuscular injection of plasmid DNA carrying the BAB1_0263 and BAB1_0278 genes (pVF263 and pVF278, respectively) into BALB/c mice elicited both humoral and cellular immune responses. Mice injected with pVF263 or pVF278 had a dominant immunoglobulin G2a (IgG2a) response. In addition, both DNA vaccines elicited a T-cell-proliferative response, but only pVF263 induced significant levels of interferon gamma (INF-γ) upon restimulation with recombinant 263 protein. Neither DNA vaccine induced interleukin (IL)-10, nor IL-4, upon stimulation with an appropriate recombinant protein or crude Brucella protein, suggesting the induction of a typical T-helper 1 (Th1)-dominated immune response. Furthermore, the pVF278 DNA vaccines induced protection in BALB/c mice against challenge with the virulent strain B. abortus 2308. Taken together, these data suggest that DNA vaccination offers an improved delivery strategy for the BAB1_0278 antigen, and provide the first evidence of a protective effect of this antigen.

DNA vaccination of bison to brucellar antigens elicits elevated antibody and IFN-γ responses

Brucella abortus remains a threat to the health and well-being of livestock in states bordering the Greater Yellowstone Area. During the past several years, cohabitation of infected wildlife with cattle has jeopardized the brucellosis-free status of Idaho, USA; Wyoming, USA; and Montana, USA. Current livestock B. abortus vaccines have not proven to be efficacious in bison (Bison bison) or elk (Cervus elaphus nelsoni). One problem with the lack of vaccine efficacy may stem from the failure to understand wildlife immune responses to vaccines. In an attempt to understand their immune responses, bison were vaccinated with eukaryotic DNA expression vectors encoding the Brucella periplasmic protein, bp26, and the chaperone protein, trigger factor (TF). These DNA vaccines have previously been shown to be protective against Brucella infection in mice. Bison were immunized intramuscularly at weeks 0, 2, and 4 with bp26 and TF DNA vaccines plus CpG adjuvant or empty vector (control) plus CpG. Blood samples were collected before vaccination and at 8, 10, and 12 wk after primary vaccination. The results showed that bison immunized with bp26 and TF DNA vaccines developed enhanced antibody, proliferative T cell, and interferon-gamma (IFN-γ) responses upon in vitro restimulation with purified recombinant bp26 or TF antigens, unlike bison immunized with empty vector. Flow cytometric analysis revealed that the percentages of CD4(+) and CD8(+) T lymphocytes from the DNA-vaccinated groups were significantly greater than they were for those bison given empty vector. These data suggest that DNA vaccination of bison may elicit strong cellular immune responses and serve as an alternative for vaccination of bison for brucellosis.

Co-immunization with interlukin-18 enhances the protective efficacy of liposomes encapsulated recombinant Cu-Zn superoxide dismutase protein against Brucella abortus

Brucellosis is a worldwide zoonotic disease caused by Brucella abortus and a number of closely related species. Brucellosis has severe impact on the health and economic prosperity of the developing countries due to the persistent nature of infection and unavailability of effective control measures. The Cu-Zn superoxide dismuatse (SOD) protein of Brucella have been extensively studied as a major antigen involved in bacterial evading mechanism of host defence. Being a critical pro-inflammatory cytokine interleukin-18 (IL-18) plays key role in induction of immune mediated protection against intracellular pathogens. In the present study, we aimed to investigate the immunogenic potential of fusogenic liposomes (escheriosomes) encapsulated recombinant Cu-Zn SOD (rSOD) protein alone or in combination with recombinant IL-18 (rIL-18). Escheriosomes encapsulated rSOD mediated immune responses were further increased upon co-immunization with rIL-18. Furthermore, immunization with escheriosomes encapsulated rSOD alone or in combination with rIL-18, increased resistance in mice against challenge with B. abortus 544.

Liposomal delivery of p-ialB and p-omp25 DNA vaccines improves immunogenicity but fails to provide full protection against B. melitensis challenge

Background

We have previously demonstrated protective efficacy against B. melitensis using formulations of naked DNA vaccines encoding genes ialB and omp25. The present study was undertaken to further understand the immune response generated by the protective vaccination regimens and to evaluate cationic liposome adsorption as a delivery method to improve vaccine utility.

Methods

The protective efficacy and immunogenicity of vaccines delivered as four doses of naked DNA, a single dose of naked DNA or a single dose of DNA surface adsorbed to cationic liposomes were compared using the BALB/c murine infection model of B. melitensis. Antigen-specific T cells and antibody responses were compared between the various formulations.

Results

The four dose vaccination strategy was confirmed to be protective against B. melitensis challenge. The immune response elicited by the various vaccines was found to be dependent upon both the antigen and the delivery strategy, with the IalB antigen favouring CD4+ T cell priming and Omp25 antigen favouring CD8+. Delivery of the p-ialB construct as a lipoplex improved antibody generation in comparison to the equivalent quantity of naked DNA. Delivery of p-omp25 as a lipoplex altered the profile of responsive T cells from CD8+ to CD4+ dominated. Under these conditions neither candidate delivered by single dose naked DNA or lipoplex vaccination methods was able to produce a robust protective effect.

Conclusions

Delivery of the p-omp25 and p-ialB DNA vaccine candidates as a lipoplex was able to enhance antibody production and effect CD4+ T cell priming, but was insufficient to promote protection from a single dose of either vaccine. The enhancement of immunogenicity by lipoplex delivery is a promising step toward improving the practicality of these two candidate vaccines, and suggests that this lipoplex formulation may be of value in situations where improvements to CD4+ responses are required. However, in the case of Brucella vaccine development it is suggested that further modifications to the candidate vaccines and delivery strategies will be required in order to deliver sustained protection.

Francisella tularensis infection-derived monoclonal antibodies provide detection, protection, and therapy

Francisella tularensis is the causative agent of tularemia and a potential agent of biowarfare. As an easily transmissible infectious agent, rapid detection and treatment are necessary to provide a positive clinical outcome. As an agent of biowarfare, there is an additional need to prevent infection. We made monoclonal antibodies to the F. tularensis subsp. holarctica live vaccine strain (F. tularensis LVS) by infecting mice with a sublethal dose of bacteria and, following recovery, by boosting the mice with sonicated organisms. The response to the initial and primary infection was restricted to immunoglobulin M antibody directed solely against lipopolysaccharide (LPS). After boosting with sonicated organisms, the specificity repertoire broadened against protein antigens, including DnaK, LpnA, FopA, bacterioferritin, the 50S ribosomal protein L7/L12, and metabolic enzymes. These monoclonal antibodies detect F. tularensis LVS by routine immunoassays, including enzyme-linked immunosorbent assay, Western blot analysis, and immunofluorescence. The ability of the antibodies to protect mice from intradermal infection, both prophylactically and therapeutically, was examined. An antibody to LPS which provides complete protection from infection with F. tularensis LVS and partial protection from infection with F. tularensis subsp. tularensis strain SchuS4 was identified. There was no bacteremia and reduced organ burden within the first 24 h when mice were protected from F. tularensis LVS infection with the anti-LPS antibody. No antibody that provided complete protection when administered therapeutically was identified; however, passive transfer of antibodies against LPS, FopA, and LpnA resulted in 40 to 50% survival of mice infected with F. tularensis LVS.

Vaccination with recombinant Semliki Forest virus particles expressing translation initiation factor 3 of Brucella abortus induces protective immunity in BALB/c mice

Recombinant replicons of Semliki Forest virus (SFV) can be used to induce high-level, transient expression of heterologous proteins in vivo. We constructed infectious but replication-deficient SFV particles carrying recombinant RNA encoding the Brucella abortus translation initiation factor 3 (IF3). The recombinant SFV particles (SFV-IF3 particles) were then evaluated for their ability to induce immune responses and to protect BALB/c mice against a challenge with B. abortus 2308 following vaccination. Animals inoculated with SFV-IF3 developed IF3-specific IgM antibodies at day 14 post-immunization. In vitro stimulation of splenocytes from vaccinated mice with either recombinant IF3 (rIF3) or crude Brucella protein extracts resulted in a T-cell proliferative response and induction of interferon gamma secretion, but not interleukin-4. In addition, mice immunized with SFV-IF3 exhibited a significant level of resistance against challenge with the virulent B. abortus strain 2308 (P<0.01). These findings indicate that an SFV-based vector carrying RNA encoding Brucella IF3 has potential for use as a vaccine to induce protection against B. abortus infections.

Enzymatic, immunological and phylogenetic characterization of Brucella suis urease

BACKGROUND

The sequenced genomes of the Brucella spp. have two urease operons, ure-1 and ure-2, but there is evidence that only one is responsible for encoding an active urease. The present work describes the purification and the enzymatic and phylogenomic characterization of urease from Brucella suis strain 1330. Additionally, the urease reactivity of sera from patients diagnosed with brucellosis was examined.

RESULTS

Urease encoded by the ure-1 operon of Brucella suis strain 1330 was purified to homogeneity using ion exchange and hydrophobic interaction chromatographies. The urease was purified 51-fold with a recovery of 12% of the enzyme activity and 0.24% of the total protein. The enzyme had an isoelectric point of 5, and showed optimal activity at pH 7.0 and 28-35 degrees C. The purified enzyme exhibited a Michaelis-Menten saturation kinetics with a Km of 5.60 +/- 0.69 mM. Hydroxyurea and thiourea are competitive inhibitors of the enzyme with Ki of 1.04 +/- 0.31 mM and 26.12 +/- 2.30 mM, respectively. Acetohydroxamic acid also inhibits the enzyme in a competitive way. The molecular weight estimated for the native enzyme was between 130-135 kDa by gel filtration chromatography and 157 +/- 7 kDa using 5-10% polyacrylamide gradient non-denaturing gel. Only three subunits in SDS-PAGE were identified: two small subunits of 14,000 Da and 15,500 Da, and a major subunit of 66,000 Da. The amino terminal sequence of the purified large subunit corresponded to the predicted amino acid sequence encoded by ureC1. The UreC1 subunit was recognized by sera from patients with acute and chronic brucellosis. By phylogenetic and cluster structure analyses, ureC1 was related to the ureC typically present in the Rhizobiales; in contrast, the ureC2 encoded in the ure-2 operon is more related to distant species.

CONCLUSION

We have for the first time purified and characterized an active urease from B. suis. The enzyme was characterized at the kinetic, immunological and phylogenetic levels. Our results confirm that the active urease of B. suis is a product of ure-1 operon.

Virulence factors in brucellosis: implications for aetiopathogenesis and treatment

Brucella species are responsible for the global zoonotic disease brucellosis. These intracellular pathogens express a set of factors - including lipopolysaccharides, virulence regulator proteins and phosphatidylcholine - to ensure their full virulence. Some virulence factors are essential for invasion of the host cell, whereas others are crucial to avoid elimination by the host. They allow Brucella spp. to survive and proliferate within its replicative vacuole and enable the bacteria to escape detection by the host immune system. Several strategies have been used to develop animal vaccines against brucellosis, but no adequate vaccine yet exists to cure the disease in humans. This is probably due to the complicated pathophysiology of human Brucella spp. infection, which is different than in animal models. Here we review Brucella spp. virulence factors and how they control bacterial trafficking within the host cell.

A Combined DNA Vaccine Encoding BCSP31, SOD, and L7/L12 Confers High Protection Against Brucella abortus 2308 by Inducing Specific CTL Responses

We constructed a combined DNA vaccine comprising genes encoding the antigens BCSP31, superoxide dismutase (SOD), and L7/L12 and evaluated its immunogenicity and protective efficacy. Immunization of mice with the combined DNA vaccine offered high protection against Brucella abortus (B. abortus) infection. The vaccine induced a vigorous specific immunoglobulin G (IgG) response, with higher IgG2a than IgG1 titers. Cytokine profiling performed at the same time showed a biased Th1-type immune response with significantly increased interferon-gamma and tumor necrosis factor-alpha stimulation. CD8(+), but not CD4(+), T cells accumulated at significantly higher levels after administration of the vaccine. Granzyme B-producing CD8(+) T cells were significantly higher in number in samples prepared from combined DNA-vaccinated mice compared with S19-vaccinated mice, demonstrating that the cytotoxicity lysis pathway is involved in the response to Brucella infection. The success of our combined DNA vaccine in a mouse model suggests its potential efficacy against brucellosis infection in large animals.

Nasal immunization with recombinant Brucella melitensis bp26 and trigger factor with cholera toxin reduces B. melitensis colonization

bp26 and trigger factor (Tf) DNA vaccines have previously been shown to protect against Brucella infection. In this study, purified bp26 and Tf proteins were tested in BALB/c mice for immunity and protection. The results showed that intranasal (i.n.) immunization with bp26 and Tf in conjunction with cholera toxin (CT) adjuvant elicit both elevated mucosal and systemic immune responses. While nasal immunization with either bp26 or Tf elicited elevated antibody responses, co-immunization with both enhanced anti-Tf immunity, suggesting bp26 adjuvant activity. Evaluation of serum IgG subclass responses showed elevated IgG1 titers. Further analysis to discern the source of immune B cells revealed effective immunization of respiratory tissues. However, Tf stimulated a significantly higher level of cytokine-forming cells (CFC) than bp26. These results imply that co-immunization of bp26 and Tf proteins elicits synergistic cooperation to stimulate the immune system. When immunized mice were challenged with B. melitensis 16M, bp26-plus Tf-immunized mice showed no difference in splenic weights but harbored three-fold less bacterial CFU when compared to sPBS-immunized control mice.

The identification of two protective DNA vaccines from a panel of five plasmid constructs encoding Brucella melitensis 16M genes

Five candidate genes from the Brucella melitensis 16M genome were selected. Eukaryotic expression plasmids encoding these antigens were constructed and expression was verified in vitro from transfected Cos7 cells. Each vaccine was assessed for protective efficacy in a BALB/c mouse brucellosis infection model. From these experiments two protective DNA vaccines were identified: p-omp25 and p-ialB. The Omp25 antigen (BMEI1249) has previously been studied in terms of Brucella virulence, serodiagnosis and as a protective antigen. However, this study represents the first report of a significant protective effect achieved against B. melitensis 16M challenge using the Omp25 antigen in a DNA vaccine approach. The other protective vaccine identified in this study was p-ialB. The ialB candidate (BMEI1584) was selected based upon its' putative function as an invasion protein which was assigned due to shared identity with the invasion protein B (ialB) of Bartonella bacilliformis. This candidate has not previously been investigated with regard to Brucella virulence or pathogenesis. This study is the first report to identify the Brucella invasion protein B (BMEI1584) as a novel protective antigen for brucellosis.

Protective immunity elicited by a divalent DNA vaccine encoding both the L7/L12 and Omp16 genes of Brucella abortus in BALB/c mice

This study was designed to evaluate the immunogenicity and the protective efficacy of a divalent fusion DNA vaccine encoding both the Brucella abortus L7/L12 protein (ribosomal protein) and Omp16 protein (outer membrane lipoprotein), designated pcDNA3.1-L7/L12-Omp16. Intramuscular injection of this divalent DNA vaccine into BALB/c mice elicited markedly both humoral and cellular immune responses. The specific antibodies exhibited a dominance of immunoglobulin G2a (IgG2a) over IgG1. In addition, the dual-gene DNA vaccine elicited a strong T-cell proliferative response and induced a large amount of gamma interferon-producing T cells upon restimulation in vitro with recombinant fusion protein L7/L12-Omp16, suggesting the induction of a typical T-helper-1-dominated immune response in vivo. This divalent DNA vaccine could also induce a significant level of protection against challenge with the virulent strain B. abortus 544 in BALB/c mice. Furthermore, the protection level induced by the divalent DNA vaccine was significantly higher than that induced by the univalent DNA vaccines pcDNA3.1-L7/L12 or pcDNA3.1-Omp16. Taken together, the results of this study verify for the first time that the Omp16 gene can be a candidate target for a DNA vaccine against brucellosis. Additionally, a divalent genetic vaccine based on the L7/L12 and Omp16 genes can elicit a stronger cellular immune response and better immunoprotection than the relevant univalent vaccines can.

Evaluation of Brucella abortus DNA vaccine by expression of Cu–Zn superoxide dismutase antigen fused to IL-2

The Cu-Zn superoxide dismutase (SOD) antigen of Brucella abortus was previously identified to be a T cell antigen which induces both proliferation of and gamma interferon (IFN-gamma) secretion by T cells from infected mice. In an earlier study, we demonstrated that intramuscular injection of mice with a plasmid DNA carrying the gene for SOD leads to the development of significant protection against B. abortus challenge. It has been reported that the antigen-specific immune responses generated by a DNA vaccine can be enhanced by co-delivery of certain cytokine genes. In this study, we evaluated the effect of delivering IL-2 on the efficacy of SOD DNA vaccine by generating a plasmid (pSecTag-SOD-IL2) that codes for a secretory fusion protein of SOD and IL-2. Another plasmid (pSecTag-SOD) that codes for only SOD as a secretory protein was used for comparison. BALB/c mice injected intramuscularly with pSecTag-SOD or pSecTag-SOD-IL2, but not the control plasmid pSecTag, developed SOD-specific antibody and T cell immune responses. Upon in vitro stimulation with recombinant SOD (rSOD) antigen, T cells from mice immunized with pSecTag-SOD-IL2, in comparison with those from mice immunized with pSecTag-SOD, exhibited a lower proliferation response but produced significantly higher concentrations of IFN-gamma. Both DNA vaccines, however, induced similar levels of SOD-specific antibodies and cytotoxic T cell response. Although mice immunized with pSecTag-SOD-IL2 showed increased resistance to challenge with B. abortus virulent strain 2308, this increase was not statistically significant from that of pSecTag-SOD vaccinated mice. These results suggest that a SOD DNA vaccine fused to IL2 did not improve protection efficacy.

A DNA vaccine coding for the Brucella outer membrane protein 31 confers protection against B. melitensis and B. ovis infection by eliciting a specific cytotoxic response

The development of an effective subunit vaccine against brucellosis is a research area of intense interest. The outer membrane proteins (Omps) of Brucella spp. have been extensively characterized as potential immunogenic and protective antigens. This study was conducted to evaluate the immunogenicity and protective efficacy of the B. melitensis Omp31 gene cloned in the pCI plasmid (pCIOmp31). Immunization of BALB/c mice with pCIOmp31 conferred protection against B. ovis and B. melitensis infection. Mice vaccinated with pCIOmp31 developed a very weak humoral response, and in vitro stimulation of their splenocytes with recombinant Omp31 did not induced the secretion of gamma interferon. Splenocytes from Omp31-vaccinated animals induced a specific cytotoxic-T-lymphocyte activity, which leads to the in vitro lysis of Brucella-infected macrophages. pCIOmp31 immunization elicited mainly CD8(+) T cells, which mediate cytotoxicity via perforins, but also CD4(+) T cells, which mediate lysis via the Fas-FasL pathway. In vivo depletion of T-cell subsets showed that the pCIOmp31-induced protection against Brucella infection is mediated predominantly by CD8(+) T cells, although CD4(+)T cells also contribute. Our results demonstrate that the Omp31 DNA vaccine induces cytotoxic responses that have the potential to contribute to protection against Brucella infection. The protective response could be related to the induction of CD8(+) T cells that eliminate Brucella-infected cells via the perforin pathway.

Selection of protective epitopes for Brucella melitensis by DNA vaccination

The Brucella melitensis 16M genome was examined for proteins in excess of 100 amino acids and for immunogenicity-associated genes. One subset of 32 annotated genes or open reading frames was identified, and each of these were cloned into the eukaryotic vector pcDNA3.1. Purified recombinant plasmids were used to intramuscularly (i.m.) immunize BALB/c mice. After challenge with B. melitensis 16M strain, two protective antigens were found: the periplasmic protein, bp26, and the chaperone protein, trigger factor (TF). Protective efficacy was confirmed with DNA vaccines for these two B. melitensis proteins and, when combined, protection against wild-type challenge was significantly enhanced. Both proteins were found to be immunogenic since elevated serum immunoglobulin G (IgG) antibodies without a specific IgG subclass bias were induced subsequent to i.m. DNA immunization. Antigen-restimulation assays revealed that bp26 and TF stimulated gamma interferon and only bp26 induced interleukin-4 (IL-4), IL-5, and IL-6 cytokines as measured by cytokine enzyme-linked immunospot assay. These collective results suggest that both bp26 and TF are excellent candidates for use in future vaccination studies against brucellosis.

An RNA vaccine based on recombinant Semliki Forest virus particles expressing the Cu,Zn superoxide dismutase protein of Brucella abortus induces protective immunity in BALB/c mice

We constructed infectious but replication-deficient Semliki Forest virus (SFV) particles carrying recombinant RNA encoding Brucella abortus Cu,Zn superoxide dismutase (SOD). The recombinant SFV particles (SFV-SOD particles) were then evaluated for their ability to induce a T-cell immune response and to protect BALB/c mice against a challenge with B. abortus 2308. Intraperitoneal injection of mice with recombinant SFV-SOD particles did not lead to the induction of SOD-specific antibodies, at least until week 6 after immunization (the end of the experiment). In vitro stimulation of splenocytes from the vaccinated mice with either recombinant Cu,Zn SOD (rSOD) or crude Brucella protein resulted in a T-cell proliferative response and the induction of gamma interferon secretion but not interleukin-4. In addition, the splenocytes exhibited significant levels of cytotoxic T-lymphocyte activity against Brucella-infected cells. The SFV-SOD particles, but not the control virus particles, induced a significant level of protection in BALB/c mice against challenge with B. abortus virulent strain 2308. These findings indicated that an SFV-based vector carrying the SOD gene has potential for use as a vaccine to induce resistance against B. abortus infections.

The physiological role of ferritin-like compounds in bacteria

Iron, as the ferrous or ferric ion, is essential for the life processes of all eukaryotes and most prokaryotes; however, the element is toxic when in excess of that needed for cellular homeostasis. Ferrous ions can react with metabolically generated hydrogen peroxide to yield toxic hydroxyl radicals that in turn degrade lipids, DNA, and other cellular biomolecules. Mechanisms have evolved in living systems for iron detoxification and for the removal of excess ferrous ions from the cytosol. These detoxification mechanisms involve the oxidation of excess ferrous ions to the ferric state and storage of the ferric ions in ferritin-like proteins. There are at least three types of ferritin-like proteins in bacteria: bacterial ferritin, bacterioferritin, and dodecameric ferritin. These bacterial proteins are related to the ferritins found in eukaryotes. The structure and physical characteristics of the ferritin-like compounds have been elucidated in several bacteria. Unfortunately, the physiological roles of the bacterial ferritin-like compounds have been less thoroughly studied. A few studies conducted with mutants indicated that ferritin-like compounds can protect bacterial cells from iron overload, serve as an iron source when iron is limited, protect the bacterial cells against oxidative stress and/or protect DNA against enzymatic or oxidative attack. There is very little information available concerning the roles that ferritin-like compounds might play in the survival of bacteria in food, water, soil, or eukaryotic host environments.

Intraspleen delivery of a DNA vaccine coding for superoxide dismutase (SOD) of Brucella abortus induces SOD-specific CD4+ and CD8+ T cells

In the development of vaccines capable of providing immunity against brucellosis, Cu-Zn superoxide dismutase (SOD) has been demonstrated to be one of the protective immunogens of Brucella abortus. In an earlier study, we provided strong evidence that intramuscular injection with a plasmid DNA carrying the SOD gene (pcDNA-SOD) was able to induce a protective immune response. The present study was designed to characterize T-cell immune responses after an intraspleen (i.s.) vaccination of BALB/c mice with pcDNA-SOD. Animals vaccinated with pcDNA-SOD did not develop SOD-specific antibodies, at least until week 4 after immunization (the end of the experiment), and in vitro stimulation of their splenocytes with either recombinant Cu-Zn SOD or crude Brucella protein induced the secretion of gamma interferon (IFN-gamma), but not interleukin-4, and elicited the induction of cytotoxic-T-lymphocyte activity. Upon analyzing the SOD-specific T-cell responses, the pcDNA-SOD vaccination was found to be stimulating both CD4(+)- and CD8(+)-T-cell populations. However, only the CD4(+) population was able to produce IFN-gamma and only the CD8(+) population was able to induce cytotoxic activity. Nevertheless, although i.s. route vaccination induces a significant level of protection in BALB/c mice against challenge with the virulent B. abortus strain 2308, vaccination by the intramuscular route with a similar amount of plasmid DNA does not protect. Based on these results, we conclude that i.s. immunization with pcDNA-SOD vaccine efficiently induced a Th1 type of immune response and a protective response that could be related to IFN-gamma production and cytotoxic activity against infected cells by SOD-specific CD4(+) and CD8(+) T cells, respectively.

Brucella lumazine synthase elicits a mixed Th1-Th2 immune response and reduces infection in mice challenged with Brucella abortus 544 independently of the adjuvant formulation used

The immunogenicity and protective efficacy of recombinant lumazine synthase from Brucella spp. (rBLS) administered with different adjuvants was evaluated in mice. Mice were immunized with rBLS in the absence or the presence of aluminum hydroxide gel (BLS-Al), monophosphoryl lipid A (BLS-MPA), or incomplete Freund's adjuvant (BLS-IFA). rBLS per se induced a vigorous immunoglobulin G (IgG) response, with high titers of IgG1 as well as IgG2. All the adjuvants increased this response; the BLS-IFA formulation was the most effective at inducing BLS-specific IgG antibodies. In addition, after in vitro stimulation with rBLS, spleen cells from BLS-IFA-, BLS-Al-, or BLS-MPA-immunized mice proliferated and produced interleukin-2 (IL-2), gamma interferon (IFN-gamma), IL-10, and IL-4, suggesting the induction of a mixed Th1-Th2 response. Immunization with rBLS protected mice against challenge with B. abortus 544. The levels of protection in the spleen were similar for all adjuvants, but only BLS-Al and BLS-IFA were effective in the liver. Our results indicate that BLS might be a useful candidate for the development of subunit vaccines against brucellosis, since it elicits antigen-specific cellular responses, with production of IFN-gamma and protection, independently of the adjuvant formulation used.

A DNA vaccine encoding Cu,Zn superoxide dismutase of Brucella abortus induces protective immunity in BALB/c mice

This study was conducted to evaluate the immunogenicity and protective efficacy of a DNA vaccine encoding Brucella abortus Cu,Zn superoxide dismutase (SOD). Intramuscular injection of plasmid DNA carrying the SOD gene (pcDNA-SOD) into BALB/c mice elicited both humoral and cellular immune responses. Animals injected with pcDNA-SOD developed SOD-specific antibodies which exhibited a dominance of immunoglobulin G2a (IgG2a) over IgG1. In addition, the DNA vaccine elicited a T-cell-proliferative response and also induced the production of gamma interferon, but not interleukin-10 (IL-10) or IL-4, upon restimulation with either recombinant SOD or crude Brucella protein, suggesting the induction of a typical T-helper-1-dominated immune response in mice. The pcDNA-SOD (but not the control vector) induced a strong, significant level of protection in BALB/c mice against challenge with B. abortus virulent strain 2308; the level of protection was similar to the one induced by B. abortus vaccine strain RB51. Altogether, these data suggest that pcDNA-SOD is a good candidate for use in future studies of vaccination against brucellosis.

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.

Molecular and cellular interactions between Brucella abortus antigens and host immune responses

Host protection against Brucella abortus, is thought to be mediated primarily by a Th1 type immune response. Unfortunately, only few specific bacterial antigens involved in stimulating protective cellular immunity against Brucella are known. Therefore, identifying bacterial proteins that induce a T-lymphocyte mediated response is critical to determine Brucella immunity. Several library screening methods are discussed that have been used to identify Brucella proteins that stimulate T lymphocytes including cellular immunoblotting, Escherichia coli expressed Brucella proteins, green fluorescence reporter systems, and signature tagged mutagenesis. Future studies would likely examine how bacterial proteins expressed within host cells aid pathogen survival and/or induce host responses. Some of these newly identified bacterial gene products may serve as antigens to activate a protective host immune response. Also, identifying Brucella proteins expressed at particular times during infection will also yield insights into Brucella pathogenesis.

Comparative proteome analysis of Brucella melitensis vaccine strain Rev 1 and a virulent strain, 16M

The genus Brucella consists of bacterial pathogens that cause brucellosis, a major zoonotic disease characterized by undulant fever and neurological disorders in humans. Among the different Brucella species, Brucella melitensis is considered the most virulent. Despite successful use in animals, the vaccine strains remain infectious for humans. To understand the mechanism of virulence in B. melitensis, the proteome of vaccine strain Rev 1 was analyzed by two-dimensional gel electrophoresis and compared to that of virulent strain 16M. The two strains were grown under identical laboratory conditions. Computer-assisted analysis of the two B. melitensis proteomes revealed proteins expressed in either 16M or Rev 1, as well as up- or down-regulation of proteins specific for each of these strains. These proteins were identified by peptide mass fingerprinting. It was found that certain metabolic pathways may be deregulated in Rev 1. Expression of an immunogenic 31-kDa outer membrane protein, proteins utilized for iron acquisition, and those that play a role in sugar binding, lipid degradation, and amino acid binding was altered in Rev 1.

Yersinia enterocolitica as a vehicle for a naked DNA vaccine encoding Brucella abortus bacterioferritin or P39 antigen

Brucella is a facultative intracellular parasite that causes brucellosis in animals and humans. The protective immune response against Brucella involves both humoral and cell-mediated immunity. In previous studies, we demonstrated that the T-dominant Brucella antigens bacterioferritin (BFR) and P39 administered either as CpG adjuvant recombinant proteins or as naked-DNA plasmids induced a specific Th1-biased immune response in mice. In order to improve the protection conferred by the BFR and P39 vaccines and to evaluate the additive role of antilipopolysaccharide (anti-LPS) antibodies, we used live attenuated Yersinia enterocolitica serotypes O:3 and O:9 as delivery vectors for naked-DNA plasmids encoding these BFR and P39 antigens. Following two intragastric immunizations in BALB/c mice, the Yersinia vectors harboring a DNA vaccine encoding BFR or P39 induced antigen-specific serum immunoglobulin and Th1-type responses (both lymphocyte proliferation and gamma interferon production) among splenocytes. Moreover, as expected, antibodies recognizing Brucella abortus 544 lipopolysaccharide were detected in O:9-immunized mice but not in O:3-treated animals. Animals immunized with O:9 organisms carrying pCI or with O:9 organisms alone were found to be significantly resistant to infection by B. abortus 544. Our data demonstrated that pCI plasmids encoding BFR or P39 and delivered with live attenuated strains of Yersinia O:3 or O:9 can trigger Th1-type responses. The fact than only O:9 vectors induced a highly significant protective immunity against B. abortus 544 infection pointed out the crucial role of anti-LPS antibodies in protection. The best protection was conferred by a serotype O:9 strain carrying pCIP39, confirming the importance of the P39 T-cell antigen in this mechanism.