Brucella abortus virB12 is expressed during infection but is not an essential component of the type IV secretion system

Copper sensing transcription factor ArsR2 regulates VjbR to sustain virulence in Brucella abortus

Brucellosis, caused by the intracellular pathogen Brucella, is a major zoonotic infection that promotes reproductive disease in domestic animals and chronic debilitating conditions in humans. The ArsR family of transcriptional regulators plays key roles in diverse cellular processes, including metal ion homeostasis, responding to adverse conditions, and virulence. However, little is known about the function of ArsR family members in Brucella. Here, we identified ArsR2 as a nonclassical member of the family that lacks autoregulatory function, but which nevertheless plays a vital role in maintaining copper homeostasis in B. abortus. ArsR2 is a global regulator of 241 genes, including those involved in the VirB type IV secretion system (T4SS). Significantly, ArsR2 regulates T4SS production in B. abortus by targeting VjbR which encodes a LuxR-type family transcriptional regulator. Moreover, copper modulates transcriptional activity of ArsR2, but not of VjbR. Furthermore, deletion of arsR2 attenuated virulence in a mouse model. Collectively, these findings enhance understanding of the mechanism by which ArsR proteins regulate virulence gene expression in pathogenic Brucella species.

Genome-wide analysis of Brucella melitensis growth in spleen of infected mice allows rational selection of new vaccine candidates

Live attenuated vaccines (LAVs) whose virulence would be controlled at the tissue level could be a crucial tool to effectively fight intracellular bacterial pathogens, because they would optimize the induction of protective immune memory while avoiding the long-term persistence of vaccine strains in the host. Rational development of these new LAVs implies developing an exhaustive map of the bacterial virulence genes according to the host organs implicated. We report here the use of transposon sequencing to compare the bacterial genes involved in the multiplication of Brucella melitensis, a major causative agent of brucellosis, in the lungs and spleens of C57BL/6 infected mice. We found 257 and 135 genes predicted to be essential for B. melitensis multiplication in the spleen and lung, respectively, with 87 genes common to both organs. We selected genes whose deletion is predicted to produce moderate or severe attenuation in the spleen, the main known reservoir of Brucella, and compared deletion mutants for these genes for their ability to protect mice against challenge with a virulent strain of B. melitensis. The protective efficacy of a deletion mutant for the plsC gene, implicated in phospholipid biosynthesis, is similar to that of the reference Rev.1 vaccine but with a shorter persistence in the spleen. Our results demonstrate that B. melitensis faces different selective pressures depending on the organ and underscore the effectiveness of functional genome mapping for the design of new safer LAV candidates.

Surface Glycans Regulate Salmonella Infection-Dependent Directional Switch in Macrophage Galvanotaxis Independent of NanH

Salmonella invades and disrupts gut epithelium integrity, creating an infection-generated electric field that can drive directional migration of macrophages, a process called galvanotaxis. Phagocytosis of bacteria reverses the direction of macrophage galvanotaxis, implicating a bioelectrical mechanism to initiate life-threatening disseminations. The force that drives direction reversal of macrophage galvanotaxis is not understood. One hypothesis is that Salmonella can alter the electrical properties of the macrophages by modifying host cell surface glycan composition, which is supported by the fact that cleavage of surface-exposed sialic acids with a bacterial neuraminidase severely impairs macrophage galvanotaxis, as well as phagocytosis. Here, we utilize N-glycan profiling by nanoLC-chip QTOF mass cytometry to characterize the bacterial neuraminidase-associated compositional shift of the macrophage glycocalyx, which revealed a decrease in sialylated and an increase in fucosylated and high mannose structures. The Salmonella nanH gene, encoding a putative neuraminidase, is required for invasion and internalization in a human colonic epithelial cell infection model. To determine whether NanH is required for the Salmonella infection-dependent direction reversal, we constructed and characterized a nanH deletion mutant and found that NanH is partially required for Salmonella infection in primary murine macrophages. However, compared to wild type Salmonella, infection with the nanH mutant only marginally reduced the cathode-oriented macrophage galvonotaxis, without canceling direction reversal. Together, these findings strongly suggest that while neuraminidase-mediated N-glycan modification impaired both macrophage phagocytosis and galvanotaxis, yet to be defined mechanisms other than NanH may play a more important role in bioelectrical control of macrophage trafficking, which potentially triggers dissemination.

The VirB System Plays a Crucial Role in Brucella Intracellular Infection

Brucellosis is a highly prevalent zoonotic disease caused by Brucella. Brucella spp. are gram-negative facultative intracellular parasitic bacteria. Its intracellular survival and replication depend on a functional virB system, an operon encoded by VirB1–VirB12. Type IV secretion system (T4SS) encoded by the virB operon is an important virulence factor of Brucella. It can subvert cellular pathway and induce host immune response by secreting effectors, which promotes Brucella replication in host cells and induce persistent infection. Therefore, this paper summarizes the function and significance of the VirB system, focusing on the structure of the VirB system where VirB T4SS mediates biogenesis of the endoplasmic reticulum (ER)-derived replicative Brucella-containing vacuole (rBCV), the effectors of T4SS and the cellular pathways it subverts, which will help better understand the pathogenic mechanism of Brucella and provide new ideas for clinical vaccine research and development.

Different Impacts of MucR Binding to the babR and virB Promoters on Gene Expression in Brucella abortus 2308

The protein MucR from Brucella abortus has been described as a transcriptional regulator of many virulence genes. It is a member of the Ros/MucR family comprising proteins that control the expression of genes important for the successful interaction of α-proteobacteria with their eukaryotic hosts. Despite clear evidence of the role of MucR in repressing virulence genes, no study has been carried out so far demonstrating the direct interaction of this protein with the promoter of its target gene babR encoding a LuxR-like regulator repressing virB genes. In this study, we show for the first time the ability of MucR to bind the promoter of babR in electrophoretic mobility shift assays demonstrating a direct role of MucR in repressing this gene. Furthermore, we demonstrate that MucR can bind the virB gene promoter. Analyses by RT-qPCR showed no significant differences in the expression level of virB genes in Brucella abortus CC092 lacking MucR compared to the wild-type Brucella abortus strain, indicating that MucR binding to the virB promoter has little impact on virB gene expression in B. abortus 2308. The MucR modality to bind the two promoters analyzed supports our previous hypothesis that this is a histone-like protein never found before in Brucella.

Interaction between Brucella melitensis 16M and small ubiquitin-related modifier 1 and E2 conjugating enzyme 9 in mouse RAW264.7 macrophages

Brucella is an intracellular pathogen that invades a host and settles in its immune cells; however, the mechanism of its intracellular survival is unclear. Modification of small ubiquitin-related modifier (SUMO) occurs in many cellular activities. E2 conjugating enzyme 9 (Ubc9) is the only reported ubiquitin-conjugating enzyme that links the SUMO molecule with a target protein. Brucella's intracellular survival mechanism has not been studied with respect to SUMO-related proteins and Ubc9. Therefore, to investigate the relationship between Brucella melitensis 16M and SUMO, we constructed plasmids and cells lines suitable for overexpression and knockdown of SUMO1 and Ubc9 genes. Brucella 16M activated SUMO1/Ubc9 expression in a time-dependent manner, and Brucella 16M intracellular survival was inhibited by SUMO1/Ubc9 overexpression and promoted by SUMO1/Ubc9 depletion. In macrophages, Brucella 16M-dependent apoptosis and immune factors were induced by SUMO1/Ubc9 overexpression and restricted by SUMO1/Ubc9 depletion. We noted no effect on the expressions of SUMO1 and Ubc9 in B. melitensis 16M lipopolysaccharide-prestimulated mouse RAW264.7 macrophages. Additionally, intracellular survival of the 16M△VirB2 mutant was lower than that of Brucella 16M (p < 0.05). VirB2 can affect expression levels of Ubc9, thereby increasing intracellular survival of Brucella in macrophages at the late stage of infection. Collectively, our results demonstrate that B. melitensis 16M may use the VirB IV secretion system of Brucella to interact with SUMO-related proteins during infection of host cells, which interferes with SUMO function and promotes pathogen survival in host cells.

Type IV Effector Secretion and Subversion of Host Functions by Bartonella and Brucella Species

Bartonella and Brucella species comprise closely related genera of the order Rhizobiales within the class α-proteobacteria. Both groups of bacteria are mammalian pathogens with a facultative intracellular lifestyle and are capable of causing chronic infections, but members of each genus have evolved broadly different infection and transmission strategies. While Brucella spp. transmit in general via the reproductive tract in their natural hosts, the Bartonella spp. have evolved to transmit via arthropod vectors. However, a shared feature of both groups of pathogens is their reliance on type IV secretion systems (T4SSs) to interact with cells in their mammalian hosts. The genomes of Bartonella spp. encode three types of T4SS, Trw, Vbh/TraG, and VirB/VirD4, whereas those of Brucella spp. uniformly contain a single T4SS of the VirB type. The VirB systems of Bartonella and Brucella are associated with distinct groups of effector proteins that collectively mediate interactions with host cells. This chapter discusses recent findings on the role of T4SS in the biology of Bartonella spp. and Brucella spp. with emphasis on effector repertoires, on recent advances in our understanding of their evolution, how individual effectors function at the molecular level, and on the consequences of these interactions for cellular and immune responses in the host.

Structural and Molecular Biology of Type IV Secretion Systems

Type IV secretion systems (T4SSs) are nanomachines that Gram-negative, Gram-positive bacteria, and some archaea use to transport macromolecules across their membranes into bacterial or eukaryotic host targets or into the extracellular milieu. They are the most versatile secretion systems, being able to deliver both proteins and nucleoprotein complexes into targeted cells. By mediating conjugation and/or competence, T4SSs play important roles in determining bacterial genome plasticity and diversity; they also play a pivotal role in the spread of antibiotic resistance within bacterial populations. T4SSs are also used by human pathogens such as Legionella pneumophila, Bordetella pertussis, Brucella sp., or Helicobacter pylori to sustain infection. Since they are essential virulence factors for these important pathogens, T4SSs might represent attractive targets for vaccines and therapeutics. The best-characterized conjugative T4SSs of Gram-negative bacteria are composed of twelve components that are conserved across many T4SSs. In this chapter, we will review our current structural knowledge on the T4SSs by describing the structures of the individual components and how they assemble into large macromolecular assemblies. With the combined efforts of X-ray crystallography, nuclear magnetic resonance (NMR), and more recently electron microscopy, structural biology of the T4SS has made spectacular progress during the past fifteen years and has unraveled the properties of unique proteins and complexes that assemble dynamically in a highly sophisticated manner.

Lable-free based comparative proteomic analysis of secretory proteins of rough Brucella mutants

Brucella rough mutants are reported to induce infected macrophage death, which is type IV secretion system (T4SS) dependent. T4SS and its secretory proteins play a major role in host-bacteria interactions, but the crucial secretory proteins to promote macrophage death during Brucella rough mutant infection have not been characterized. In this study, we found that T4SS components played no role for macrophage death induced by Brucella rough mutant infection, but some T4SS effectors did. Proteomics of secretory proteins from Brucella rough mutants ΔrfbE and ΔrfbEΔvirB123 was analyzed by liquid chromatography/tandem mass spectrometry and 861 unique proteins were identified, among which 37 were differential secretory proteins. Gene ontology and pathway analysis showed that differential secretory proteins involved in cellular process and metabolic process, distributed in the cell and membrane, possessed molecular function of catalytic activity and binding, and were associated with ribosome, NOD-like receptor signaling pathway, two-component system and bacterial secretion system. Cell death analysis showed that T4SS effector VceC, and two differential secretory proteins OmpW family protein (BAB1_1579) and protein BAB1_1185 were associated with Brucella cytotoxicity. This study provides new insights into the molecular mechanisms associated with Brucella cytotoxicity and valuable information for screening vaccine candidates for Brucella. SIGNIFICANCE: Brucella rough mutants induce infected macrophage death, which is T4SS dependent. In the present report, a comparative proteomics analysis revealed 37 differential secretory proteins between Brucella rough mutants ΔrfbE and ΔrfbEΔvirB123. Further study demonstrated OmpW family protein (BAB1_1579) and uncharacterized protein BAB1_1185, two differential secretory proteins, were associated with Brucella cytotoxicity. This study provides novel information of the secretory proteins from the Brucella rough mutants and their effects on the Brucella cytotoxicity.

A novel small RNA Bmsr1 enhances virulence in Brucella melitensis M28

Brucellosis, caused by Brucella spp., is one of the most serious zoonotic bacterial diseases. Small RNAs (sRNAs) are recognized as a key player in bacterial post-transcription regulation, since they participate in many biological processes with high efficiency and may govern the intracellular biochemistry and virulence of some pathogenic bacteria. Here, a novel small regulatory RNA, Bmsr1 (Brucella melitensis M28 small RNA 1), was identified in a virulent Brucella melitensis M28 strain based on bioinformatic analysis, reverse transcription PCR (RT-PCR), and Northern blot. The Bmsr1 expression level was highly induced after infection of macrophage cells RAW264.7 at 48 h, suggesting a role for Bmsr1 during in vitro infection. Indeed, bmsr1 deletion mutant of M28 attenuated its intracellular survival in RAW264.7 at 24 h and 48 h post-infection. In a mouse model of chronic infection, bmsr1 deletion strain displayed decreased colonization in the spleen while Bmsr1-overexpressed strain showed higher colonization levels than wild type pathogen. Isobaric tags for relative and absolute quantification (iTRAQ) revealed that 314 proteins were differentially expressed in M28Δbmsr1 compared with wild type. Functional annotation analysis demonstrated that most of those proteins are involved in biological processes and those proteins in the ribosome and nitrogen metabolism pathways were enriched. iTRAQ results combined with target prediction identified several potential target genes related to virulence, including virB2, virB9, virB10, virB11, and vjbR and many metabolism genes. Taken together, this study revealed the contribution of a novel sRNA Bmsr1 to virulence of B. melitensis M28, probably by influencing genes involved in T4SS, virulence regulator VjbR and other metabolism genes.

Brucella melitensis VirB12 recombinant protein is a potential marker for serodiagnosis of human brucellosis

BACKGROUND

The numerous drawbacks of current serological tests for diagnosis of brucellosis which mainly results from cross reactivity with LPS from other gram-negative bacteria have generated an increasing interest to find more specific non-LPS antigens. Previous studies had indicated that Brucella VirB12 protein, a cell surface protein and component of type IV secretion system, induces antibody response during animal infection. However, this protein has not yet been tested as a serological diagnostic marker in human brucellosis.

METHODS

Recombinant VirB12 protein was prepared and evaluated the efficacy of it in an indirect enzyme-linked immunosorbent assay (ELISA) for brucellosis with sera collected from different region of Iran and the results were compared with a commercial ELISA kit.

RESULTS

Sera from human brucellosis patients strongly reacted to the purified recombinant VirB12. The sensitivity, specificity, accuracy, negative predictive value and positive predictive value of recombinant VirB12-based ELISA related to the commercial-ELISA method were 87.8, 94, 90, 80 and 96.6% respectively.

CONCLUSIONS

We concluded that antigenic VirB12 have a property value that can be considered as a candidate for using in serodiagnostic tests for human brucellosis.

Type IV secretion system of Brucella spp. and its effectors

Brucella spp. are intracellular bacterial pathogens that cause infection in domestic and wild animals. They are often used as model organisms to study intracellular bacterial infections. Brucella VirB T4SS is a key virulence factor that plays important roles in mediating intracellular survival and manipulating host immune response to infection. In this review, we discuss the roles of Brucella VirB T4SS and 15 effectors that are proposed to be crucial for Brucella pathogenesis. VirB T4SS regulates the inflammation response and manipulates vesicle trafficking inside host cells. VirB T4SS also plays crucial roles in the inhibition of the host immune response and intracellular survival during infection. Here, we list the key molecular events in the intracellular life cycle of Brucella that are potentially targeted by the VirB T4SS effectors. Elucidating the functions of these effectors will help clarify the molecular role of T4SS during infection. Furthermore, studying the effectors secreted by Brucella spp. might provide insights into the mechanisms used by the bacteria to hijack the host signaling pathways and aid in the development of better vaccines and therapies against brucellosis.

Indirect ELISA for diagnosis of Brucella ovis infection in rams

Brucella ovis is a major cause of epididymitis in sexually mature rams, resulting in subfertility, infertility, and economic losses for the sheep industry worldwide. The aim of this study was to develop an indirect ELISA (iELISA) using recombinant proteins, namely rBoP59 and rBP26, as antigens for serological diagnosis of B. ovisinfection. The BoP59 and BP26 recombinant proteins were expressed in E. coli and purified by affinity chromatography. Antigenicity was tested by Western blot and iELISA. Standardization of iELISA was performed with 500ng and 1µg BoP59 and rBP26 per well, testing serum from uninfected and experimentally infected rams. rBP26 was effective in distinguishing positive from negative rams. The rBP26 iELISA developed in this study is the first to use a completely purified rBP26 as antigen resulting in high sensitivity (100%) and specificity (90.2%), and an overall accuracy equal to 1.0.

Early transcriptional responses of bovine chorioallantoic membrane explants to wild type, ΔvirB2 or ΔbtpB Brucella abortus infection

The pathogenesis of the Brucella-induced inflammatory response in the bovine placenta is not completely understood. In this study we evaluated the role of the B. abortus Type IV secretion system and the anti-inflammatory factor BtpB in early interactions with bovine placental tissues. Transcription profiles of chorioallantoic membrane (CAM) explants inoculated with wild type (strain 2308), ΔvirB2 or ΔbtpB Brucella abortus were compared by microarray analysis at 4 hours post infection. Transcripts with significant variation (>2 fold change; P<0.05) were functionally classified, and transcripts related to defense and inflammation were assessed by quantitative real time RT-PCR. Infection with wild type B. abortus resulted in slightly more genes with decreased than increased transcription levels. Conversely, infection of trophoblastic cells with the ΔvirB2 or the ΔbtpB mutant strains, that lack a functional T4SS or that has impaired inhibition of TLR signaling, respectively, induced more upregulated than downregulated genes. Wild type Brucella abortus impaired transcription of host genes related to immune response when compared to ΔvirB and ΔbtpB mutants. Our findings suggest that proinflammatory genes are negatively modulated in bovine trophoblastic cells at early stages of infection. The virB operon and btpB are directly or indirectly related to modulation of these host genes. These results shed light on the early interactions between B. abortus and placental tissue that ultimately culminate in inflammatory pathology and abortion.

Molecular cloning of virB12 gene of Brucella melitensis 16M strain in pET28a vector

OBJECTIVE

To clone the virB12 gene in pET28a expression vector for production of recombinant protein to be used as antigenic component for future serological test development.

METHODS

Brucella melitensis (B. melitensis) 16M strain was cultured and bacterial DNA was extracted by Bioneer AccuPrep® Genomic DNA Extraction Kit. Oligonucleotide primer pair was designed based on Brucella virB12 gene sequence with BamHI and HindIII restriction site at 5' end of the forward and reverse primers, respectively. DNA amplification was performed using PrimSTAR® HS DNA polymerase and the PCR product was purified by DNA AccuPrep®Gel Purification Kit. Purified DNA was cloned into pJET1.2 cloning vector. VirB12 gene fragment was excised from pJET1.2 using BamHI/HindIII and subsequently subcloned into pET28a (+).

RESULTS

Brucella virB12 gene was successfully cloned in pJET1.2 and then in pET28a (+) plasmids. PCR and restriction enzyme digestion confirms the procedure.

CONCLUSION

We cloned and expressed the Brucella virB12 gene which could be used as antigenic component for specific serological assay development.

Bacterial Type IV secretion systems: versatile virulence machines

Many bacterial pathogens employ multicomponent protein complexes to deliver macromolecules directly into their eukaryotic host cell to promote infection. Some Gram-negative pathogens use a versatile Type IV secretion system (T4SS) that can translocate DNA or proteins into host cells. T4SSs represent major bacterial virulence determinants and have recently been the focus of intense research efforts designed to better understand and combat infectious diseases. Interestingly, although the two major classes of T4SSs function in a similar manner to secrete proteins, the translocated 'effectors' vary substantially from one organism to another. In fact, differing effector repertoires likely contribute to organism-specific host cell interactions and disease outcomes. In this review, we discuss the current state of T4SS research, with an emphasis on intracellular bacterial pathogens of humans and the diverse array of translocated effectors used to manipulate host cells.

Immunogenic response of Brucella canis virB10 and virB11 mutants in a murine model

The virB locus, which encodes the type IV secretion system, is a major component of virulence in Brucella. A non-polar virB10 mutant and a virB11 deletion mutant were constructed in Brucella canis. In the mouse model, both mutants were cleared at day 21 post-infection, indicating reduced virulence in mice. After challenging with wild-type B. canis, the amounts of CFU recovered at day 15 were significantly lower in the group previously vaccinated with the virB10 mutant. Levels of IgG1, IgG2a, IgG2b, and IgM, the induction of the cytokines IL-2, IL-4, IL-10, and the production of IFN-γ were measured in lymphocyte cultures. All strains elicited similar levels of different antibody isotype profiles, and no significant differences were detected (P < 0.05). The wild-type strain induced a rapid and strong INF-γ response at 24 h, while both mutants induced mild INF-γ responses at 24 h, which remained constant over the course of sampling. Our results suggest that the virB mutants elicit a protective immunity and may be considered as candidates for studies to be conducted in dogs against canine brucellosis.

The small protein CydX is required for function of cytochrome bd oxidase in Brucella abortus

A large number of hypothetical genes potentially encoding small proteins of unknown function are annotated in the Brucella abortus genome. Individual deletion of 30 of these genes identified four mutants, in BAB1_0355, BAB2_0726, BAB2_0470, and BAB2_0450 that were highly attenuated for infection. BAB2_0726, an YbgT-family protein located at the 3′ end of the cydAB genes encoding cytochrome bd ubiquinal oxidase, was designated cydX. A B. abortus cydX mutant lacked cytochrome bd oxidase activity, as shown by increased sensitivity to H₂O₂, decreased acid tolerance and increased resistance to killing by respiratory inhibitors. The C terminus, but not the N terminus, of CydX was located in the periplasm, suggesting that CydX is an integral cytoplasmic membrane protein. Phenotypic analysis of the cydX mutant, therefore, suggested that CydX is required for full function of cytochrome bd oxidase, possibly via regulation of its assembly or activity.

Brucellosis and type IV secretion

Brucellosis is a global disease of domestic and wild mammals that is caused by intracellular bacteria of the genus Brucella. Although humans are not a natural reservoir for Brucella, infection in the human population is common in many countries, and brucellosis is one of the most common zoonotic infections. Brucella species have evolved to avoid the host's immune system and infection is usually characterized by long-term persistence of the bacteria. One important Brucella virulence factor for intracellular survival and persistence in the host is the type IV secretion system. This review will discuss the Brucella type IV secretion system in detail, including current knowledge of architecture and regulation, as well as the newly identified effector substrates that this system transports into host cells.

Comparative proteome analysis of Brucella abortus 2308 and its virB type IV secretion system mutant reveals new T4SS-related candidate proteins

Brucella abortus is an alpha-2 proteobacteria with a type IV secretion system (T4SS) known as virB, which is necessary to gain virulence by building up a replicative vacuole associated with the endoplasmic reticulum of the host cell. A virB T4SS mutant of the B. abortus 2308 strain and its wild-type strain were grown in acid medium in order to obtain and analyze their proteomes, looking for putative proteins that may serve as T4SS substrates and those that may be subjected to T4SS regulation. A total of 47 overexpressed and 22 underexpressed proteins from the virB T4SS mutant strain were selected and sequenced. Some of the 69 analyzed proteins have not been described before either as over or under-expressed in relation to a virB T4SS mutation, whereas some of them have been already described by other groups as potentially important secretory proteins in other Brucella species. An important number of the proteins identified are outer membrane and periplasmic space protein, which makes them become particularly important new T4SS-related candidate proteins.

Quantitative analysis of VirB8-VirB9-VirB10 interactions provides a dynamic model of type IV secretion system core complex assembly

Type IV secretion systems are multiprotein complexes that translocate macromolecules across the bacterial cell envelope. The type IV secretion system in Brucella species encodes 12 VirB proteins that permit this pathogen to translocate effectors into mammalian cells, where they contribute to its survival inside the host. The "core" complex proteins are conserved in all type IV secretion systems, and they are believed to form the channel for substrate translocation. We have investigated the in vitro interactions between the soluble periplasmic domains of three of these VirB components, VirB8, VirB9, and VirB10, using enzyme-linked immunosorbent assays, circular dichroism, and surface plasmon resonance techniques. The in vitro experiments helped in the quantification of the self-association and binary interactions of VirB8, VirB9, and VirB10. Individually, distinct binding properties were revealed that may explain their biological functions, and collectively, we provide direct evidence of the in vitro formation of the VirB8-VirB9-VirB10 ternary complex. To assess the dynamics of these interactions in a simplified in vivo model of complex assembly, we applied the bacterial two-hybrid system in studying interactions between the full-length proteins. This approach demonstrated that VirB9 stimulates the self-association of VirB8 but inhibits VirB10-VirB10 and VirB8-VirB10 interaction. Analysis of a dimerization site variant of VirB8 (VirB8(M102R)) suggested that the interactions with VirB9 and VirB10 are independent of its self-association, which stabilizes VirB8 in this model assay. We propose a dynamic model for secretion system assembly in which VirB8 plays a role as an assembly factor that is not closely associated with the functional core complex comprising VirB9 and VirB10.

VirB3 to VirB6 and VirB8 to VirB11, but not VirB7, are essential for mediating persistence of Brucella in the reticuloendothelial system

The Brucella abortus virB locus contains 12 open reading frames, termed virB1 through virB12, which encode a type IV secretion system. Polar mutations in the virB locus markedly reduce the ability of B. abortus to survive in cultured macrophages or to persist in organs of mice. While a nonpolar deletion of the virB2 gene reduces survival in cultured macrophages and in organs of mice, a nonpolar deletion of virB1 only reduces survival in macrophages, whereas virB12 is dispensable for either virulence trait. Here we investigated the role of the remaining genes in the virB locus during survival in macrophages and virulence in mice. Mutants carrying nonpolar deletions of the virB3, virB4, virB5, virB6, virB7, virB8, virB9, virB10, or virB11 gene were constructed and characterized. All mutations reduced the ability of B. abortus to survive in J774A.1 mouse macrophage-like cells to a degree similar to that caused by a deletion of the entire virB locus. Deletion of virB3, virB4, virB5, virB6, virB8, virB9, virB10, or virB11 markedly reduced the ability of B. abortus to persist in the spleens of mice at 8 weeks after infection. Interestingly, deletion of virB7 did not reduce the ability of B. abortus to persist in spleens of mice. We conclude that virB2, virB3, virB4, virB5, virB6, virB8, virB9, virB10, and virB11 are essential for virulence of B. abortus in mice, while functions encoded by the virB1, virB7, and virB12 genes are not required for persistence in organs with this animal model.

Inactivation of the type IV secretion system reduces the Th1 polarization of the immune response to Brucella abortus infection

The Brucella abortus type IV secretion system (T4SS), encoded by the virB operon, is essential for establishing persistent infection in the murine reticuloendothelial system. To gain insight into the in vivo interactions mediated by the T4SS, we compared host responses elicited by B. abortus with those of an isogenic mutant in the virB operon. Mice infected with the B. abortus virB mutant elicited smaller increases in serum levels of immunoglobulin G2a, gamma interferon (IFN-gamma), and interleukin-12p40 than did mice infected with wild-type B. abortus. Despite equal bacterial loads in the spleen, at 3 to 4 days postinfection, levels of IFN-gamma were higher in mice infected with wild-type B. abortus than in mice infected with the virB mutant, as shown by real-time PCR, intracellular cytokine staining, and cytokine levels. IFN-gamma-producing CD4(+) T cells were more abundant in spleens of mice infected with wild-type B. abortus than in virB mutant-infected mice. Similar numbers of IFN-gamma-secreting CD8(+) T cells were observed in the spleens of mice infected with B. abortus 2308 or a virB mutant. These results suggest that early differences in cytokine responses contribute to a stronger Th1 polarization of the immune response in mice infected with wild-type B. abortus than in mice infected with the virB mutant.

VirB12 is a serological marker of Brucella infection in experimental and natural hosts

The Brucella species type IV secretion system, encoded by the virB1-12 locus, is required for intracellular replication and persistent infection in vivo. The requirement of VirB proteins for infection suggests that they are expressed in vivo and may therefore represent serological markers of infection. To test this idea, we purified recombinant VirB1, VirB5, VirB11, and VirB12 and tested for their recognition by antibodies in sera from experimentally infected mice and goats by using an indirect enzyme-linked immunosorbent assay. Antibody responses to VirB12 but not to VirB1, VirB5, or VirB11 were detected in 20/20 mice experimentally inoculated with Brucella abortus and 12/12 goats experimentally infected with Brucella melitensis. The potential use of VirB12 as a serological tool for the diagnosis of brucellosis was evaluated in the natural bovine host. Serum samples from 145 cattle of known serology (29% negative and 71% positive) were analyzed for the production of antibody responses to VirB12. One hundred two cattle samples (70.3%) were positive for antibodies to VirB12, while 43 samples were negative (29.7%). A positive serological response to VirB12 correlated with positive serology to whole B. abortus antigen in 99% of samples tested. These results show that VirB12 is expressed during infection of both experimental and natural hosts of Brucella species, and they suggest that VirB12 may be a useful serodiagnostic marker for brucellosis.

Cytotoxicity in macrophages infected with rough Brucella mutants is type IV secretion system dependent

Smooth Brucella spp. inhibit macrophage apoptosis, whereas rough Brucella mutants induce macrophage oncotic and necrotic cell death. However, the mechanisms and genes responsible for Brucella cytotoxicity have not been identified. In the current study, a random mutagenesis approach was used to create a mutant bank consisting of 11,354 mutants by mariner transposon mutagenesis using Brucella melitensis rough mutant 16M delta manBA as the parental strain. Subsequent screening identified 56 mutants (0.49% of the mutant bank) that failed to cause macrophage cell death (release of 10% or less of the lactate dehydrogenase). The absence of cytotoxicity during infection with these mutants was independent of demonstrable defects in in vitro bacterial growth or uptake and survival in macrophages. Interrupted genes in 51 mutants were identified by DNA sequence analysis, and the mutations included interruptions in virB encoding the type IV secretion system (T4SS) (n = 36) and in vjbR encoding a LuxR-like regulatory element previously shown to be required for virB expression (n = 3), as well as additional mutations (n = 12), one of which also has predicted roles in virB expression. These results suggest that the T4SS is associated with Brucella cytotoxicity in macrophages. To verify this, deletion mutants were constructed in B. melitensis 16M by removing genes encoding phosphomannomutase/phosphomannoisomerase (delta manBA) and the T4SS (delta virB). As predicted, deletion of virB from 16M delta manBA and 16M resulted in a complete loss of cytotoxicity in rough strains, as well as the low level cytotoxicity observed with smooth strains at extreme multiplicities of infection (>1,000). Taken together, these results demonstrate that Brucella cytotoxicity in macrophages is T4SS dependent.

Mice lacking components of adaptive immunity show increased Brucella abortus virB mutant colonization

The Brucella abortus type IV secretion system (T4SS), encoded by the virB genes, is essential for survival in mononuclear phagocytes in vitro. In the mouse model, a B. abortus virB mutant was initially able to colonize the spleen at the level of the wild type for approximately 3 to 5 days, which coincided with the development of adaptive immunity. To investigate the relationship between survival in macrophages cultivated in vitro and persistence in tissues in vivo, we tested the ability of mutant mice lacking components of adaptive immunity to eliminate the virB mutant from the spleen during a mixed infection with the B. abortus wild type. Ifng(-/-) or beta(2)m(-/-) mice were able to clear the virB mutant to the same degree as control mice. However, spleens of Rag1(-/-) mice and Igh6(-/-) mice were more highly colonized by the virB mutant than control mice after 14 to 21 days, suggesting that, in these mice, there is not an absolute requirement for the T4SS to mediate persistence of B. abortus in the spleen. Macrophages isolated from Igh6(-/-) mice killed the virB mutant to the same extent as macrophages from control mice, showing that the reduced ability of these mice to clear the virB mutant from the spleen does not correlate with diminished macrophage function in vitro. These results show that in the murine model host, the T4SS is required for persistence beyond 3 to 5 days after infection and suggest that the T4SS may contribute to evasion of adaptive immune mechanisms by B. abortus.

Brucella requires a functional Type IV secretion system to elicit innate immune responses in mice

The virB operon, encoding a Type IV secretion system (T4SS), is essential for intracellular survival and persistent infection by Brucella spp. To better understand the role of the T4SS in evading host defence mechanisms and establishing chronic infection, we compared transcriptional profiles of the host response to infection with wild-type and virB mutant Brucella strains. Analysis of gene expression profiles in murine splenocytes 3 days after inoculation with wild-type Brucella strains revealed an inflammatory response, with a prominent upregulation of genes induced by both type I and type II interferons. Real-time RT-PCR showed that a group of genes from these pathways were induced by day 3 post infection and declined to baseline levels by day 7. In contrast, neither of the two virB mutant strains elicited a proinflammatory gene expression profile, demonstrating that the T4SS was required to trigger this response. Infection studies using type I interferon receptor knockout mice showed that a lack of type I interferon signalling did not affect Brucella replication during the first 4 weeks of infection. Thus, induction of type I interferons does not appear to be an essential mechanism by which the T4SS promotes persistent infection by Brucella.

The Brucella suis type IV secretion system assembles in the cell envelope of the heterologous host Agrobacterium tumefaciens and increases IncQ plasmid pLS1 recipient competence

Pathogenic Brucella species replicate within mammalian cells, and their type IV secretion system is essential for intracellular survival and replication. The options for biochemical studies on the Brucella secretion system are limited due to the rigidity of the cells and biosafety concerns, which preclude large-scale cell culture and fractionation. To overcome these problems, we heterologously expressed the Brucella suis virB operon in the closely related alpha(2)-proteobacterium Agrobacterium tumefaciens and showed that the VirB proteins assembled into a complex. Eight of the twelve VirB proteins were detected in the membranes of the heterologous host with specific antisera. Cross-linking indicated protein-protein interactions similar to those in other type IV secretion systems, and the results of immunofluorescence analysis supported the formation of VirB protein complexes in the cell envelope. Production of a subset of the B. suis VirB proteins (VirB3-VirB12) in A. tumefaciens strongly increased its ability to receive IncQ plasmid pLS1 in conjugation experiments, and production of VirB1 further enhanced the conjugation efficiency. Plasmid recipient competence correlated with periplasmic leakage and the detergent sensitivity of A. tumefaciens, suggesting a weakening of the cell envelope. Heterologous expression thus permits biochemical characterization of B. suis type IV secretion system assembly.