Brucella ovis lacking a species-specific putative ATP-binding cassette transporter is attenuated but immunogenic in rams

Rev1Δwzm vaccine candidate is safe in young and adult sheep and protects against Brucella ovis infection in rams

Small ruminants affected by brucellosis, caused mainly by Brucella melitensis and B. ovis, suffer reproductive disorders, leading to significant economic losses worldwide. Vaccination is an essential tool to prevent the disease in ovine and caprine livestock, but the only vaccine recommended to date is B. melitensis Rev1, which in sheep is only safe for use in lambs aged 3-4 months. This restriction poses considerable practical challenges for the implementation of Rev1 in countries with endemic brucellosis and/or limited resources, where there is a need for mass vaccination with a safe vaccine to control the disease in both animals and humans. We recently developed a B. melitensis strain Rev1Δwzm showing superior vaccine properties in mice and safety in pregnant ewes. Here, we report that Rev1Δwzm (i) is safe in young and adult sheep, both male and female; (ii) induces a transient serological response in the Rose Bengal test in ≤50 % of sheep, confirmed to some extent by the complement fixation test, and a stronger, more persistent anti- rough-LPS response; and (iii) protects rams against a B. ovis challenge 25 weeks after vaccination. To resolve the problem of serological interference, the use of green fluorescent protein tagging strategy allowed us to identify vaccinated sheep with only a single inoculation. These results, together with the previously reported safety in pregnant ewes, position Rev1Δwzm as a firm vaccine candidate and a promising alternative to Rev1. Further experiments are warranted to assess its efficacy against B. melitensis in pregnant ewes.

Pathogenesis of Brucella ovis in pregnant mice and protection induced by the candidate vaccine strain B. Ovis ΔabcBA

Ovine brucellosis caused by Brucella ovis is a major cause of reproductive failure in sheep. This study aimed to evaluate transplacental infection and pathogenicity of B.ovis wild type strain ATCC 25,840 (WT B.ovis) and the candidate vaccine strain B.ovis ΔabcBA in pregnant mice. A total of 40 BALB/c mice were equally divided into 4 groups: (i) non immunized and uninfected control mice (3/10 mice became pregnant); (ii) non immunized and challenged with WT B.ovis (5/10 pregnant); (iii) inoculated only with B.ovis ΔabcBA (6/10 pregnant); (iv) immunized with B.ovis ΔabcBA and challenged with WT B.ovis (5/10 pregnant). Female mice bred, and five days after visualization of the vaginal plug, they were inoculated intraperitoneally (ip) with 100 µL of sterile PBS, 100 µL of 1 × 10⁶ CFU of B.ovis ΔabcBA, or 100 µL of 1 × 10⁶ CFU of B.ovis WT, according to each group. At the 17th day of gestation, samples of spleen, liver, uterus, placenta, fetus and mammary gland were obtained for bacteriology, histopathology and immunohistochemistry. Non immunized mice challenged with B.ovis WT developed necrotizing placentitis as well as microgranulomas in the liver and spleen. These findings support the notion that B.ovis infection in pregnant mice induces lesions that are similar to those caused by B.abortus in the same animal model. B.ovis ΔabcBA was not recovered from any of the sampled organs, and it did not cause any gross or microscopic lesions, indicating that it is a safe and attenuated strain in this experimental model. In addition, B.ovis ΔabcBA was induced protective immunity as demonstrated by decreased numbers of B.ovis WT in the liver, uterus and fetuses of immunized mice after the challenge with B.ovis WT.

Canine Brucellosis: An Update

Canine brucellosis is an infectious and zoonotic disease caused by Brucella canis, which has been reported worldwide, and is a major public health concern due to close contact between dogs and humans. In dogs, canine brucellosis manifests with abortion outbreaks, reproductive failure, enlargement of lymph nodes, and occasionally affects the osteoarticular system, although the occurrence of asymptomatic infections in dogs are not uncommon. In humans, the disease is associated with a febrile syndrome, commonly with non-specific symptoms including splenomegaly, fatigue, and weakness. Infection of dogs occurs mostly by the oronasal route when in contact with contaminated tissues such as aborted fetuses, semen, urine, and vaginal secretions. In humans, contact with contaminated fluids from infected dogs is an important source of infection, and it is an occupational risk for veterinarians, breeders, laboratory workers, among other professionals who deal with infected animals or biological samples. The diagnosis in dogs is largely based on serologic methods. However, serologic diagnosis of canine brucellosis remains very challenging due to the low accuracy of available tests. Molecular diagnostic methods have been increasingly used in the past few years. Treatment of infected dogs is associated with a high frequency of relapse, and should be employed only in selected cases. Currently there are no commercially available vaccines for prevention of canine brucellosis. Therefore, development of novel and improved diagnostic methods as well as the development of efficacious and safe vaccination protocols are needed for an effective control of canine brucellosis and its associated zoonotic risk.

The candidate vaccine strain Brucella ovis ∆abcBA is protective against Brucella melitensis infection in mice

Brucellosis is a major zoonotic disease, and Brucella melitensis is the species most often associated with human infection. Vaccination is the most efficient tool for controlling animal brucellosis, with a consequent decrease of incidence of human infections. Commercially available live attenuated vaccines provide some degree of protection, but retain residual pathogenicity to human and animals. In this study, Brucella ovis ∆abcBA (Bo∆abcBA), a live attenuated candidate vaccine strain, was tested in two formulations (encapsulated with alginate and alginate plus vitelline protein B [VpB]) to immunize mice against experimental challenge with B. melitensis strain 16M. One week after infection, livers and spleens of immunized mice had reduced numbers of the challenge strain B. melitensis 16M when compared with those of nonimmunized mice, with a reduction of approximately 1-log₁₀ of B. melitensis 16M count in the spleens from immunized mice. Moreover, splenocytes stimulated with B. melitensis antigens in vitro secreted IFN-γ when mice had been immunized with Bo∆abcBA encapsulated with alginate plus VpB, but not with alginate alone. Body and liver weights were similar among groups, although spleens from mice immunized with Bo∆abcBA encapsulated with alginate were larger than those immunized with Bo∆abcBA encapsulated with alginate plus VpB or nonimmunized mice. This study demonstrated that two vaccine formulations containing Bo∆abcBA protected mice against experimental challenge with B. melitensis.

Disruption of pyruvate phosphate dikinase in Brucella ovis PA CO2-dependent and independent strains generates attenuation in the mouse model

Brucella ovis is a non-zoonotic rough Brucella that causes genital lesions, abortions and increased perinatal mortality in sheep and is responsible for important economic losses worldwide. Research on virulence factors of B. ovis is necessary for deciphering the mechanisms that enable this facultative intracellular pathogen to establish persistent infections and for developing a species-specific vaccine, a need in areas where the cross-protecting ovine smooth B. melitensis Rev1 vaccine is banned. Although several B. ovis virulence factors have been identified, there is little information on its metabolic abilities and their role in virulence. Here, we report that deletion of pyruvate phosphate dikinase (PpdK, catalyzing the bidirectional conversion pyruvate ⇌ phosphoenolpyruvate) in B. ovis PA (virulent and CO₂-dependent) impaired growth in vitro. In cell infection experiments, although showing an initial survival higher than that of the parental strain, this ppdK mutant was unable to multiply. Moreover, when inoculated at high doses in mice, it displayed an initial spleen colonization higher than that of the parental strain followed by a marked comparative decrease, an unusual pattern of attenuation in mice. A homologous mutant was also obtained in a B. ovis PA CO₂-independent construct previously proposed for developing B. ovis vaccines to solve the problem that CO₂-dependence represents for large scale production. This CO₂-independent ppdK mutant reproduced the growth defect in vitro and the multiplication/clearance pattern in mouse spleens, and is thus an interesting vaccine candidate for the immunoprophylaxis of B. ovis ovine brucellosis.

Brucella ovis mutant in ABC transporter protects against Brucella canis infection in mice and it is safe for dogs

BACKGROUND/OBJECTIVES

Vaccination is the most important tool for controlling brucellosis, but currently there is no vaccine available for canine brucellosis, which is a zoonotic disease of worldwide distribution caused by Brucella canis. This study aimed to evaluate protection and immune response induced by Brucella ovis ΔabcBA (BoΔabcBA) encapsulated with alginate against the challenge with Brucella canis in mice and to assess the safety of this strain for dogs.

METHODS

Intracellular growth of the vaccine strain BoΔabcBA was assessed in canine and ovine macrophages. Protection induced by BoΔabcBA against virulent Brucella canis was evaluated in the mouse model. Safety of the vaccine strain BoΔabcBA was assessed in experimentally inoculated dogs.

RESULTS

Wild type B. ovis and B. canis had similar internalization and intracellular multiplication profiles in both canine and ovine macrophages. The BoΔabcBA strain had an attenuated phenotype in both canine and ovine macrophages. Immunization of BALB/c mice with alginate-encapsulated BoΔabcBA (108 CFU) induced lymphocyte proliferation, production of IL-10 and IFN-γ, and protected against experimental challenge with B. canis. Dogs immunized with alginate-encapsulated BoΔabcBA (109 CFU) seroconverted, and had no hematologic, biochemical or clinical changes. Furthermore, BoΔabcBA was not detected by isolation or PCR performed using blood, semen, urine samples or vaginal swabs at any time point over the course of this study. BoΔabcBA was isolated from lymph nodes near to the site of inoculation in two dogs at 22 weeks post immunization.

CONCLUSION

Encapsulated BoΔabcBA protected mice against experimental B. canis infection, and it is safe for dogs. Therefore, B. ovis ΔabcBA has potential as a vaccine candidate for canine brucellosis prevention.

Pathogenic potential of Brucella ovis field isolates with different genotypic profile and protection provided by the vaccine strain B. ovis ΔabcBA against B. ovis field isolates in mice

ABSTRACT: Brucella ovis causes economic and reproductive losses in sheep herds. The goal of this study was to characterize infection with B. ovis field isolates in a murine model, and to evaluate protection induced by the candidate vaccine strain B. ovis ΔabcBA in mice challenged with these field isolates. B. ovis field strains were able to colonize and cause lesions in the liver and spleen of infected mice. After an initial screening, two strains were selected for further characterization (B. ovis 94 AV and B. ovis 266 L). Both strains had in vitro growth kinetics that was similar to that of the reference strain B. ovis ATCC 25840. Vaccination with B. ovis ΔabcBA encapsulated with 1% alginate was protective against the challenge with field strains, with the following protection indexes: 0.751, 1.736, and 2.746, for mice challenged with B. ovis ATCC25840, B. ovis 94 AV, and B. ovis 266 L, respectively. In conclusion, these results demonstrated that B. ovis field strains were capable of infecting and inducing lesions in experimentally infected mice. The attenuated vaccine strain B. ovis ΔabcBA induced protection in mice challenged with different B. ovis field isolates, resulting in higher protection indexes against more pathogenic strains.

Characterization of Cell Envelope Multiple Mutants of Brucella ovis and Assessment in Mice of Their Vaccine Potential

Brucella ovis is a non-zoonotic Brucella species lacking specific vaccine. It presents a narrow host range, a unique biology relative to other Brucella species, and important distinct surface properties. To increase our knowledge on its peculiar surface and virulence features, and seeking to develop a specific vaccine, multiple mutants for nine relevant cell-envelope-related genes were investigated. Mutants lacking Omp10 plus Omp19 could not be obtained, suggesting that at least one of these lipoproteins is required for viability. A similar result was obtained for the double deletion of omp31 and omp25 that encode two major surface proteins. Conversely, the absence of major Omp25c (proved essential for internalization in HeLa cells) together with Omp25 or Omp31 was tolerated by the bacterium. Although showing important in vitro and in vivo defects, the Δomp10Δomp31Δomp25c mutant was obtained, demonstrating that B. ovis PA survives to the simultaneous absence of Omp10 and four out seven proteins of the Omp25/Omp31 family (i.e., Omp31, Omp25c, Omp25b, and Omp31b, the two latter naturally absent in B. ovis). Three multiple mutants were selected for a detailed analysis of virulence in the mouse model. The Δomp31Δcgs and Δomp10Δomp31Δomp25c mutants were highly attenuated when inoculated at 10⁶ colony forming units/mouse but they established a persistent infection when the infection dose was increased 100-fold. The Δomp10ΔugpBΔomp31 mutant showed a similar behavior until week 3 post-infection but was then totally cleared from spleen. Accordingly, it was retained as vaccine candidate for mice protection assays. When compared to classical B. melitensis Rev1 heterologous vaccine, the triple mutant induced limited splenomegaly, a significantly higher antibody response against whole B. ovis PA cells, an equivalent memory cellular response and, according to spleen colonization measurements, better protection against a challenge with virulent B. ovis PA. Therefore, it would be a good candidate to be evaluated in the natural host as a specific vaccine against B. ovis that would avoid the drawbacks of B. melitensis Rev1. In addition, the lack in this attenuated strain of Omp31, recognized as a highly immunogenic protein during B. ovis infection, would favor the differentiation between infected and vaccinated animals using Omp31 as diagnostic target.

The CO2-dependence of Brucella ovis and Brucella abortus biovars is caused by defective carbonic anhydrases

Brucella bacteria cause brucellosis, a major zoonosis whose control requires efficient diagnosis and vaccines. Identification of classical Brucella spp. has traditionally relied on phenotypic characterization, including surface antigens and 5–10% CO₂ necessity for growth (CO₂-dependence), a trait of Brucella ovis and most Brucella abortus biovars 1–4 strains. Although molecular tests are replacing phenotypic methods, CO₂-dependence remains of interest as it conditions isolation and propagation and reflects Brucella metabolism, an area of active research. Here, we investigated the connection of CO₂-dependence and carbonic anhydrases (CA), the enzymes catalyzing the hydration of CO₂ to the bicarbonate used by anaplerotic and biosynthetic carboxylases. Based on the previous demonstration that B. suis carries two functional CAs (CAI and CAII), we analyzed the CA sequences of CO₂-dependent and -independent brucellae and spontaneous mutants. The comparisons strongly suggested that CAII is not functional in CO₂-dependent B. abortus and B. ovis, and that a modified CAII sequence explains the CO₂-independent phenotype of spontaneous mutants. Then, by mutagenesis and heterologous plasmid complementation and chromosomal insertion we proved that CAI alone is enough to support CO₂-independent growth of B. suis in rich media but not of B. abortus in rich media or B. suis in minimal media. Finally, we also found that insertion of a heterologous active CAII into B. ovis reverted the CO₂-dependence but did not alter its virulence in the mouse model. These results allow a better understanding of central aspects of Brucella metabolism and, in the case of B. ovis, provide tools for large-scale production of diagnostic antigens and vaccines.

Comparative genomic analysis between newly sequenced Brucella suis Vaccine Strain S2 and the Virulent Brucella suis Strain 1330

Background

Brucellosis is a bacterial disease caused by Brucella infection. In the late fifties, Brucella suis vaccine strain S2 with reduced virulence was obtained by serial transfer of a virulent B. suis biovar 1 strain in China. It has been widely used for vaccination in China since 1971. Until now, the mechanisms underlie virulence attenuation of S2 are still unknown.

Results

In this paper, the whole genome sequencing of S2 was carried out by Illumina Hiseq2000 sequencing method. We further performed the comparative genomic analysis to find out the differences between S2 and the virulent Brucella suis strain 1330. We found premature stops in outer membrane autotransporter omaA and eryD genes. Single mutations were found in phosphatidylcholine synthase, phosphorglucosamine mutase, pyruvate kinase and FliF, which have been reported to be related to the virulence of Brucella or other bacteria. Of the other different proteins between S2 and 1330, such as Omp2b, periplasmic sugar-binding protein, and oligopeptide ABC transporter, no definitive implications related to bacterial virulence were found, which await further investigation.

Conclusions

The data presented here provided the rational basis for designing Brucella vaccines that could be used in other strains.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-3076-5) contains supplementary material, which is available to authorized users.

The abcEDCBA-Encoded ABC Transporter and the virB Operon-Encoded Type IV Secretion System of Brucella ovis Are Critical for Intracellular Trafficking and Survival in Ovine Monocyte-Derived Macrophages

Brucella ovis infection is associated with epididymitis, orchitis and infertility in rams. Most of the information available on B. ovis and host cell interaction has been generated using murine macrophages or epithelial cell lines, but the interaction between B. ovis and primary ovine macrophages has not been studied. The aim of this study was to evaluate the role of the B. ovis abcEDCBA-encoded ABC transporter and the virB operon-encoded Type IV Secretion System (T4SS) during intracellular survival of B. ovis in ovine peripheral blood monocyte-derived macrophages. ΔabcBA and ΔvirB2 mutant strains were unable to survive in the intracellular environment when compared to the WT B. ovis at 48 hours post infection (hpi). In addition, these mutant strains cannot exclude the lysosomal marker LAMP1 from its vacuolar membrane, and their vacuoles do not acquire the endoplasmic reticulum marker calreticulin, which takes place in the WT B. ovis containing vacuole. Higher levels of nitric oxide production were observed in macrophages infected with WT B. ovis at 48 hpi when compared to macrophages infected with the ΔabcBA or ΔvirB2 mutant strains. Conversely, higher levels of reactive oxygen species were detected in macrophages infected with the ΔabcBA or ΔvirB2 mutant strains at 48 hpi when compared to macrophages infected with the WT strain. Our results demonstrate that B. ovis is able to persist and multiply in ovine macrophages, while ΔabcBA and ΔvirB2 mutations prevent intracellular multiplication, favor phagolysosome fusion, and impair maturation of the B. ovis vacuole towards an endoplasmic reticulum-derived compartment.

Encapsulated Brucella ovis Lacking a Putative ATP-Binding Cassette Transporter (ΔabcBA) Protects against Wild Type Brucella ovis in Rams

This study aimed to evaluate protection induced by the vaccine candidate B. ovis ΔabcBA against experimental challenge with wild type B. ovis in rams. Rams were subcutaneously immunized with B. ovis ΔabcBA encapsulated with sterile alginate or with the non encapsulated vaccine strain. Serum, urine, and semen samples were collected during two months after immunization. The rams were then challenged with wild type B. ovis (ATCC25840), and the results were compared to non immunized and experimentally challenged rams. Immunization, particularly with encapsulated B. ovis ΔabcBA, prevented infection, secretion of wild type B. ovis in the semen and urine, shedding of neutrophils in the semen, and the development of clinical changes, gross and microscopic lesions induced by the wild type B. ovis reference strain. Collectively, our data indicates that the B. ovis ΔabcBA strain is an exceptionally good vaccine strain for preventing brucellosis caused by B. ovis infection in rams.

Protection Provided by an Encapsulated Live Attenuated ΔabcBA Strain of Brucella ovis against Experimental Challenge in a Murine Model

This study aimed to evaluate the Brucella ovis ΔabcBA strain as a vaccine candidate in the murine model. BALB/c mice were subcutaneously or intraperitoneally immunized with a single dose or three doses of the B. ovis ΔabcBA strain and then were challenged with wild-type B. ovis. Single or multiple immunizations provided only mild protection, with significantly smaller numbers of wild-type B. ovis CFU in the livers of immunized mice but not in the spleens. Encapsulation of B. ovis ΔabcBA significantly improved protection against experimental challenges in both BALB/c and C57BL/6 mice. Furthermore, immunization with encapsulated B. ovis ΔabcBA markedly prevented lesions in the spleens and livers of experimentally challenged mice. These results demonstrated that the encapsulated B. ovis ΔabcBA strain confers protection to mice; therefore, this strain has potential as a vaccine candidate for rams.

The predicted ABC transporter AbcEDCBA is required for type IV secretion system expression and lysosomal evasion by Brucella ovis

Brucella ovis is a major cause of reproductive failure in rams and it is one of the few well-described Brucella species that is not zoonotic. Previous work showed that a B. ovis mutant lacking a species-specific ABC transporter (ΔabcBA) was attenuated in mice and was unable to survive in macrophages. The aim of this study was to evaluate the role of this ABC transporter during intracellular survival of B. ovis. In HeLa cells, B. ovis WT was able to survive and replicate at later time point (48 hpi), whereas an ΔabcBA mutant was attenuated at 24 hpi. The reduced survival of the ΔabcBA mutant was associated with a decreased ability to exclude the lysosomal marker LAMP1 from its vacuolar membrane, suggesting a failure to establish a replicative niche. The ΔabcBA mutant showed a reduced abundance of the Type IV secretion system (T4SS) proteins VirB8 and VirB11 in both rich and acid media, when compared to WT B. ovis. However, mRNA levels of virB1, virB8, hutC, and vjbR were similar in both strains. These results support the notion that the ABC transporter encoded by abcEDCBA or its transported substrate acts at a post-transcriptional level to promote the optimal expression of the B. ovis T4SS within infected host cells.

Mutants in the lipopolysaccharide of Brucella ovis are attenuated and protect against B. ovis infection in mice

Brucella spp. are Gram-negative bacteria that behave as facultative intracellular parasites of a variety of mammals. This genus includes smooth (S) and rough (R) species that carry S and R lipopolysaccharides (LPS), respectively. S-LPS is a virulence factor, and mutants affected in the S-LPS O-polysaccharide (R mutants), core oligosaccharide or both show attenuation. However, B. ovis is naturally R and is virulent in sheep. We studied the role of B. ovis LPS in virulence by mutating the orthologues of wadA, wadB and wadC, three genes known to encode LPS core glycosyltransferases in S brucellae. When mapped with antibodies to outer membrane proteins (Omps) and R-LPS, wadB and wadC mutants displayed defects in LPS structure and outer membrane topology but inactivation of wadA had little or no effect. Consistent with these observations, the wadB and wadC but not the wadA mutants were attenuated in mice. When tested as vaccines, the wadB and wadC mutants protected mice against B. ovis challenge. The results demonstrate that the LPS core is a structure essential for survival in vivo not only of S brucellae but also of a naturally R Brucella pathogenic species, and they confirm our previous hypothesis that the Brucella LPS core is a target for vaccine development. Since vaccine B. melitensis Rev 1 is S and thus interferes in serological testing for S brucellae, wadB mutant represents a candidate vaccine to be evaluated against B. ovis infection of sheep suitable for areas free of B. melitensis.

Evaluation in mice of Brucella ovis attenuated mutants for use as live vaccines against B. ovis infection

Brucella ovis causes ram contagious epididymitis, a disease for which a specific vaccine is lacking. Attenuated Brucella melitensis Rev 1, used as vaccine against ovine and caprine brucellosis caused by B. melitensis, is also considered the best vaccine available for the prophylaxis of B. ovis infection, but its use for this purpose has serious drawbacks. In this work, two previously characterized B. ovis attenuated mutants (Δomp25d and Δomp22) were evaluated in mice, in comparison with B. melitensis Rev 1, as vaccines against B. ovis. Similarities, but also significant differences, were found regarding the immune response induced by the three vaccines. Mice vaccinated with the B. ovis mutants developed anti-B. ovis antibodies in serum of the IgG₁, IgG_2a and IgG_2b subclasses and their levels were higher than those observed in Rev 1-vaccinated mice. After an antigen stimulus with B. ovis cells, splenocytes obtained from all vaccinated mice secreted similar levels of TNF-α and IL12(p40) and remarkably high amounts of IFN-γ, a crucial cytokine in protective immunity against other Brucella species. By contrast, IL-1α -an enhancer of T cell responses to antigen- was present at higher levels in mice vaccinated with the B. ovis mutants, while IL-10, an anti-inflammatory cytokine, was significantly more abundant in Rev 1-vaccinated mice. Additionally, the B. ovis mutants showed appropriate persistence, limited splenomegaly and protective efficacy against B. ovis similar to that observed with B. melitensis Rev 1. These characteristics encourage their evaluation in the natural host as homologous vaccines for the specific prophylaxis of B. ovis infection.