Type IV secretion system of Brucella spp. and its effectors

Bacteria-Killing Type IV Secretion Systems

Bacteria have been constantly competing for nutrients and space for billions of years. During this time, they have evolved many different molecular mechanisms by which to secrete proteinaceous effectors in order to manipulate and often kill rival bacterial and eukaryotic cells. These processes often employ large multimeric transmembrane nanomachines that have been classified as types I-IX secretion systems. One of the most evolutionarily versatile are the Type IV secretion systems (T4SSs), which have been shown to be able to secrete macromolecules directly into both eukaryotic and prokaryotic cells. Until recently, examples of T4SS-mediated macromolecule transfer from one bacterium to another was restricted to protein-DNA complexes during bacterial conjugation. This view changed when it was shown by our group that many Xanthomonas species carry a T4SS that is specialized to transfer toxic bacterial effectors into rival bacterial cells, resulting in cell death. This review will focus on this special subtype of T4SS by describing its distinguishing features, similar systems in other proteobacterial genomes, and the nature of the effectors secreted by these systems and their cognate inhibitors.

A small non-coding RNA facilitates Brucella melitensis intracellular survival by regulating the expression of virulence factor

Brucella species are the causative agents of brucellosis, a worldwide zoonotic disease that affects a broad range of mammals and causes great economic losses. Small regulatory RNAs (sRNAs) are post-transcriptional regulatory molecules that participate in the stress adaptation and pathogenesis of Brucella. In this study, we characterized the role of a novel sRNA, BSR1141, in the intracellular survival and virulence of Brucella melitensis. The results show that BSR1141 was highly induced during host infections and under in vitro stress situations that simulated the conditions encountered within host phagocytes. In addition, a BSR1141 mutant showed reduced survival both under in vitro stress conditions and in mice, confirming the role of BSR1141 in Brucella intracellular survival. Bioinformatic and experimental approaches revealed that BSR1141 affects the expression of many target genes, including the Brucella virulence component virB2. These data indicate that BSR1141 could influence the expression of virB2, which is important for B. melitensis pathogenesis and intracellular survival. This work provides new insight into the mechanism of adaptation to environmental stress and into the pathogenesis of intracellular pathogens.

Intracellular Pathogens: Host Immunity and Microbial Persistence Strategies

Infectious diseases caused by pathogens including viruses, bacteria, fungi, and parasites are ranked as the second leading cause of death worldwide by the World Health Organization. Despite tremendous improvements in global public health since 1950, a number of challenges remain to either prevent or eradicate infectious diseases. Many pathogens can cause acute infections that are effectively cleared by the host immunity, but a subcategory of these pathogens called "intracellular pathogens" can establish persistent and sometimes lifelong infections. Several of these intracellular pathogens manage to evade the host immune monitoring and cause disease by replicating inside the host cells. These pathogens have evolved diverse immune escape strategies and overcome immune responses by residing and multiplying inside host immune cells, primarily macrophages. While these intracellular pathogens that cause persistent infections are phylogenetically diverse and engage in diverse immune evasion and persistence strategies, they share common pathogen type-specific mechanisms during host-pathogen interaction inside host cells. Likewise, the host immune system is also equipped with a diverse range of effector functions to fight against the establishment of pathogen persistence and subsequent host damage. This article provides an overview of the immune effector functions used by the host to counter pathogens and various persistence strategies used by intracellular pathogens to counter host immunity, which enables their extended period of colonization in the host. The improved understanding of persistent intracellular pathogen-derived infections will contribute to develop improved disease diagnostics, therapeutics, and prophylactics.

Canine Brucellosis: Old Foe and Reemerging Scourge

The genus Brucella is a primary cause of reproductive diseases. Widely known as a problem in livestock, Brucella is gaining notoriety as a cause of canine reproductive disease and as a scourge to dog breeders. Only within the last few decades has the risk of severe brucellosis in dogs, and the people who own and work with them, been more fully appreciated. This review summarizes the epidemiology, clinical signs, and advances in diagnosis and management of Brucella canis. Canine brucellosis prevention, owner education, and possible therapies for the future are also discussed.

Deletion of the Type IV Secretion System Effector VceA Promotes Autophagy and Inhibits Apoptosis in Brucella-Infected Human Trophoblast Cells

Brucellosis is the most common zoonotic disease that caused by intracellular parasitic bacteria Brucella. The survival and replication of Brucella in the host depend on the type IV secretion system (T4SS). The T4SS system of Brucella has many components and secreted proteins. But the mechanism helped Brucella to evade the host defense is still not clear. The objective of the present study was to investigate the effects of VceA on autophagy and apoptosis in Brucella-infected embryonic trophoblast cells. We constructed the VceA mutant strain (2308ΔVceA) and complementary strain (2308ΔVceA-C) of Brucella abortus 2308 (S2308). The human trophoblast cells (HPT-8 cells) and mice were infected by S2308, 2308ΔVceA and 2308ΔVceA-C. The cell autophagy and apoptosis were detected. The Atg5, LC3-II and Bcl-2 mRNA expression were significantly increased in 2308ΔVceA group than the S2308 group, and mRNA expression of P62 and Caspase-3 were significantly decreased than the S2308 group. Western blotting, qPCR and flow cytometry analysis showed that 2308ΔVceA promoted autophagy and inhibited apoptosis. Mouse immunohistochemistry experiments showed that P62 protein was scattered coloring and Cytochrome C protein was scarcely in 2308ΔVceA group at the myometrium. These results indicated that 2308ΔVceA promoted autophagy and inhibited apoptosis in HPT-8 cells during Brucella infection.

Transcriptomic Analysis of the Brucella melitensis Rev.1 Vaccine Strain in an Acidic Environment: Insights Into Virulence Attenuation

The live attenuated Brucella melitensis Rev.1 (Elberg-originated) vaccine strain is widely used to control the zoonotic infection brucellosis in small ruminants, but the molecular mechanisms underlying the attenuation of this strain have not been fully characterized. Following their uptake by the host cell, Brucella replicate inside a membrane-bound compartment—the Brucella-containing vacuole—whose acidification is essential for the survival of the pathogen. Therefore, identifying the genes that contribute to the survival of Brucella in acidic environments will greatly assist our understanding of its molecular pathogenic mechanisms and of the attenuated virulence of the Rev.1 strain. Here, we conducted a comprehensive comparative transcriptome analysis of the Rev.1 vaccine strain against the virulent reference strain 16M in cultures grown under either normal or acidic conditions. We found 403 genes that respond differently to acidic conditions in the two strains (FDR < 0.05, fold change ≥ 2). These genes are involved in crucial cellular processes, including metabolic, biosynthetic, and transport processes. Among the highly enriched genes that were downregulated in Rev.1 under acidic conditions were acetyl-CoA synthetase, aldehyde dehydrogenase, cell division proteins, a cold-shock protein, GroEL, and VirB3. The downregulation of these genes may explain the attenuated virulence of Rev.1 and provide new insights into the virulence mechanisms of Brucella.

Brucella Hijacks Host-Mediated Palmitoylation To Stabilize and Localize PrpA to the Plasma Membrane

Brucellaceae are a group of pathogenic intracellular bacteria with the ability to modulate the host response, both at the individual cell level and systemically. One of the hallmarks of the virulence process is the capacity of the bacteria to downregulate the adaptive and acquired host immune response through a plethora of virulence factors that directly impact several key signaling cascades. PrpA is one of those virulence factors that alters, via its polyclonal B-cell activity, the humoral and cellular immune responses of the host, ultimately favoring the establishment of a chronic infection. Even though PrpA affects B cells, it directly targets macrophages, triggering a response that ultimately affects B lymphocytes. In the present article we report that PrpA is S-palmitoylated in two N-terminal cysteine residues by the host cell and that this modification is necessary for its biological activity. Our results demonstrate that S-palmitoylation promotes PrpA migration to the host cell plasma membrane and stabilizes the protein during infection. These findings add a new mechanism exploited by this highly evolved pathogen to modulate the host immune response.

Comprehensive assessment and performance improvement of effector protein predictors for bacterial secretion systems III, IV and VI

Bacterial effector proteins secreted by various protein secretion systems play crucial roles in host-pathogen interactions. In this context, computational tools capable of accurately predicting effector proteins of the various types of bacterial secretion systems are highly desirable. Existing computational approaches use different machine learning (ML) techniques and heterogeneous features derived from protein sequences and/or structural information. These predictors differ not only in terms of the used ML methods but also with respect to the used curated data sets, the features selection and their prediction performance. Here, we provide a comprehensive survey and benchmarking of currently available tools for the prediction of effector proteins of bacterial types III, IV and VI secretion systems (T3SS, T4SS and T6SS, respectively). We review core algorithms, feature selection techniques, tool availability and applicability and evaluate the prediction performance based on carefully curated independent test data sets. In an effort to improve predictive performance, we constructed three ensemble models based on ML algorithms by integrating the output of all individual predictors reviewed. Our benchmarks demonstrate that these ensemble models outperform all the reviewed tools for the prediction of effector proteins of T3SS and T4SS. The webserver of the proposed ensemble methods for T3SS and T4SS effector protein prediction is freely available at http://tbooster.erc.monash.edu/index.jsp. We anticipate that this survey will serve as a useful guide for interested users and that the new ensemble predictors will stimulate research into host-pathogen relationships and inspiration for the development of new bioinformatics tools for predicting effector proteins of T3SS, T4SS and T6SS.

Transposon Sequencing of Brucella abortus Uncovers Essential Genes for Growth In Vitro and Inside Macrophages

Brucella abortus is a class III zoonotic bacterial pathogen able to survive and replicate inside host cells, including macrophages. Here we report a multidimensional transposon sequencing analysis to identify genes essential for Brucella abortus growth in rich medium and replication in RAW 264.7 macrophages. The construction of a dense transposon mutant library and mapping of 929,769 unique mini-Tn5 insertion sites in the genome allowed identification of 491 essential coding sequences and essential segments in the B. abortus genome. Chromosome II carries a lower proportion (5%) of essential genes than chromosome I (19%), supporting the hypothesis of a recent acquisition of a megaplasmid as the origin of chromosome II. Temporally resolved transposon sequencing analysis as a function of macrophage infection stages identified 79 genes with a specific attenuation phenotype in macrophages, at either 2, 5, or 24 h postinfection, and 86 genes for which the attenuated mutant phenotype correlated with a growth defect on plates. We identified 48 genes required for intracellular growth, including the virB operon, encoding the type IV secretion system, which supports the validity of the screen. The remaining genes encode amino acid and pyrimidine biosynthesis, electron transfer systems, transcriptional regulators, and transporters. In particular, we report the need of an intact pyrimidine nucleotide biosynthesis pathway in order for B. abortus to proliferate inside RAW 264.7 macrophages.

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.

An optimal set of features for predicting type IV secretion system effector proteins for a subset of species based on a multi-level feature selection approach

Type IV secretion systems (T4SS) are multi-protein complexes in a number of bacterial pathogens that can translocate proteins and DNA to the host. Most T4SSs function in conjugation and translocate DNA; however, approximately 13% function to secrete proteins, delivering effector proteins into the cytosol of eukaryotic host cells. Upon entry, these effectors manipulate the host cell’s machinery for their own benefit, which can result in serious illness or death of the host. For this reason recognition of T4SS effectors has become an important subject. Much previous work has focused on verifying effectors experimentally, a costly endeavor in terms of money, time, and effort. Having good predictions for effectors will help to focus experimental validations and decrease testing costs. In recent years, several scoring and machine learning-based methods have been suggested for the purpose of predicting T4SS effector proteins. These methods have used different sets of features for prediction, and their predictions have been inconsistent. In this paper, an optimal set of features is presented for predicting T4SS effector proteins using a statistical approach. A thorough literature search was performed to find features that have been proposed. Feature values were calculated for datasets of known effectors and non-effectors for T4SS-containing pathogens for four genera with a sufficient number of known effectors, Legionella pneumophila, Coxiella burnetii, Brucella spp, and Bartonella spp. The features were ranked, and less important features were filtered out. Correlations between remaining features were removed, and dimensional reduction was accomplished using principal component analysis and factor analysis. Finally, the optimal features for each pathogen were chosen by building logistic regression models and evaluating each model. The results based on evaluation of our logistic regression models confirm the effectiveness of our four optimal sets of features, and based on these an optimal set of features is proposed for all T4SS effector proteins.

The Highest Cited Papers in Brucellosis: Identification Using Two Databases and Review of the Papers' Major Findings

Citation classics represent the highest impact work in a given field. We aim to identify and analyze the most frequently cited papers on brucellosis. We used the databases Scopus and Web of Science to determine the most frequently cited papers. The most cited fifty papers in each database were identified. We then ranked the papers according to the highest citation count recorded from any of the two databases. The most frequently cited paper received 964 citations and was by DelVecchio VG et al. reporting the complete genomic sequencing of Brucella melitensis. The papers were published in 30 journals led by the “Infection and Immunity” journal and the “Veterinary Microbiology” journal (each had 7 papers). Citation classics in brucellosis were all in English except one in French and were mostly of basic science type. In addition, we noticed that 12 articles that were identified among the highest fifty articles in one database were missed by the other database and vice versa. Therefore, we suggest that searching in more than one database would detect additional citation classics.

Mitochondrial fragmentation affects neither the sensitivity to TNFα-induced apoptosis of Brucella-infected cells nor the intracellular replication of the bacteria

Mitochondria are complex organelles that participate in many cellular functions, ranging from ATP production to immune responses against viruses and bacteria. This integration of a plethora of functions within a single organelle makes mitochondria a very attractive target to manipulate for intracellular pathogens. We characterised the crosstalk that exists between Brucella abortus, the causative agent of brucellosis, and the mitochondria of infected cells. Brucella replicates in a compartment derived from the endoplasmic reticulum (ER) and modulates ER functionality by activating the unfolded protein response. However, the impact of Brucella on the mitochondrial population of infected cells still requires a systematic study. We observed physical contacts between Brucella containing vacuoles and mitochondria. We also found that B. abortus replication is independent of mitochondrial oxidative phosphorylation and that mitochondrial reactive oxygen species do not participate to the control of B. abortus infection in vitro. We demonstrated that B. abortus and B. melitensis induce a drastic mitochondrial fragmentation at 48 hours post-infection in different cell types, including myeloid and non-myeloid cells. This fragmentation is DRP1-independent and might be caused by a deficit of mitochondrial fusion. However, mitochondrial fragmentation does not change neither Brucella replication efficiency, nor the susceptibility of infected cells to TNFα-induced apoptosis.

Brucella abortus Senses the Intracellular Environment through the BvrR/BvrS Two-Component System, Which Allows B. abortus To Adapt to Its Replicative Niche

Brucella abortus is a facultative extracellular-intracellular pathogen belonging to a group of Alphaproteobacteria that establishes close interactions with animal cells. This bacterium enters host cells in a membrane-bound compartment, avoiding the lysosomal route and reaching the endoplasmic reticulum through the action of the type IV secretion system, VirB. In this work, we demonstrate that the BvrR/BvrS two-component system senses the intracellular environment to mount the transcriptional response required for intracellular life adaptation. By combining a method to purify intracellularly extracted bacteria with a strategy that allows direct determination of BvrR phosphorylation, we showed that upon entrance to host cells, the regulatory protein BvrR was activated (BvrR-P) by phosphorylation at aspartate 58. This activation takes place in response to intracellular cues found in early compartments, such as low pH and nutrient deprivation. Furthermore, BvrR activation was followed by an increase in the expression of VjbR and VirB. The in vitro activation of this BvrR-P/VjbR/VirB virulence circuit rescued B. abortus from the inhibition of intracellular replication induced by bafilomycin treatment of cells, demonstrating the relevance of this mechanism for intracellular bacterial survival and replication. All together, our results indicate that B. abortus senses the transition from the extracellular to the intracellular milieu through BvrR/BvrS, allowing the bacterium to transit safely to its replicative niche. These results serve as a working model for understanding the role of this family of two-component systems in the adaptation to intracellular life of Alphaproteobacteria.

Type IV secretion in Gram-negative and Gram-positive bacteria

Type IV secretion systems (T4SSs) are versatile multiprotein nanomachines spanning the entire cell envelope in Gram-negative and Gram-positive bacteria. They play important roles through the contact-dependent secretion of effector molecules into eukaryotic hosts and conjugative transfer of mobile DNA elements as well as contact-independent exchange of DNA with the extracellular milieu. In the last few years, many details on the molecular mechanisms of T4SSs have been elucidated. Exciting structures of T4SS complexes from Escherichia coli plasmids R388 and pKM101, Helicobacter pylori and Legionella pneumophila have been solved. The structure of the F-pilus was also reported and surprisingly revealed a filament composed of pilin subunits in 1:1 stoichiometry with phospholipid molecules. Many new T4SSs have been identified and characterized, underscoring the structural and functional diversity of this secretion superfamily. Complex regulatory circuits also have been shown to control T4SS machine production in response to host cell physiological status or a quorum of bacterial recipient cells in the vicinity. Here, we summarize recent advances in our knowledge of 'paradigmatic' and emerging systems, and further explore how new basic insights are aiding in the design of strategies aimed at suppressing T4SS functions in bacterial infections and spread of antimicrobial resistances.

Overexpression of Cu-Zn SOD in Brucella abortus suppresses bacterial intracellular replication via down-regulation of Sar1 activity

Brucella Cu-Zn superoxide dismutase (Cu-Zn SOD) is a periplasmic protein, and immunization of mice with recombinant Cu-Zn SOD protein confers protection against Brucella abortus infection. However, the role of Cu-Zn SOD during the process of Brucella infection remains unknown. Here, we report that Cu-Zn SOD is secreted into culture medium and is translocated into host cells independent of type IV secretion systems (T4SS). Furthermore, co-immunoprecipitation and immunofluorescence studies reveal that Brucella abortus Cu-Zn SOD interacts with the small GTPase Sar1. Overexpression of Cu-Zn SOD in Brucella abortus inhibits bacterial intracellular growth by abolishing Sar1 activity in a manner independent of reactive oxygen species (ROS) production.

Brucella induces unfolded protein response and inflammatory response via GntR in alveolar macrophages

Brucella is an intracellular bacterium that causes the zoonosis brucellosis worldwide. Alveolar macrophages (AM) constitute the main cell target of inhaled Brucella. Brucella thwarts immune surveillance and evokes endoplasmic reticulum (ER) stress to replicate in macrophages via virulence factors. The GntR regulators family was concentrated as an important virulence factor in controlling virulence and intracellular survival of Brucella. However, the detailed underlying mechanism for the host-pathogen interaction is poorly understood. In this study the BSS2_II0438 mutant (ΔGntR) was constructed. The type IV secretion system (T4SS) virulence factor genes (VirB2, VirB6, and VirB8) were down-expression in ΔGntR. ΔGntR could infect and proliferate to high titers in GAMs without a significant difference compared with the parental strain. ΔGntR infection increased the expression of ER stress marker genes GRP78, ATF6, and PERK in the early stages of its intracellular cycle but decreased the expression of these genes in the late stages. ΔGntR increased greatly the number of Brucella CFUs in the inactive ER stress state in GAMs. Meanwhile, ΔGntR infection increased the levels of IFN-γ, IL-1β, and TNF-α, indicating ΔGntR could induce the secretion of inflammatory but not anti-inflammatory cytokines IL-10. Taken together, our results clarified the role of the GntR in B. suis. S2 virulence expression and elucidated that GntR is potentially involved in the signaling pathway of the Brucella-induced UPR and inflammatory response in GAMs.

Effective prediction of bacterial type IV secreted effectors by combined features of both C-termini and N-termini

Various bacterial pathogens can deliver their secreted substrates also called as effectors through type IV secretion systems (T4SSs) into host cells and cause diseases. Since T4SS secreted effectors (T4SEs) play important roles in pathogen-host interactions, identifying them is crucial to our understanding of the pathogenic mechanisms of T4SSs. A few computational methods using machine learning algorithms for T4SEs prediction have been developed by using features of C-terminal residues. However, recent studies have shown that targeting information can also be encoded in the N-terminal region of at least some T4SEs. In this study, we present an effective method for T4SEs prediction by novelly integrating both N-terminal and C-terminal sequence information. First, we collected a comprehensive dataset across multiple bacterial species of known T4SEs and non-T4SEs from literatures. Then, three types of distinctive features, namely amino acid composition, composition, transition and distribution and position-specific scoring matrices were calculated for 50 N-terminal and 100 C-terminal residues. After that, we employed information gain represent to rank the importance score of the 150 different position residues for T4SE secretion signaling. At last, 125 distinctive position residues were singled out for the prediction model to classify T4SEs and non-T4SEs. The support vector machine model yields a high receiver operating curve of 0.916 in the fivefold cross-validation and an accuracy of 85.29% for the independent test set.

The Mosaic Type IV Secretion Systems

Escherichia coli and other Gram-negative and -positive bacteria employ type IV secretion systems (T4SSs) to translocate DNA and protein substrates, generally by contact-dependent mechanisms, to other cells. The T4SSs functionally encompass two major subfamilies, the conjugation systems and the effector translocators. The conjugation systems are responsible for interbacterial transfer of antibiotic resistance genes, virulence determinants, and genes encoding other traits of potential benefit to the bacterial host. The effector translocators are used by many Gram-negative pathogens for delivery of potentially hundreds of virulence proteins termed effectors to eukaryotic cells during infection. In E. coli and other species of Enterobacteriaceae, T4SSs identified to date function exclusively in conjugative DNA transfer. In these species, the plasmid-encoded systems can be classified as the P, F, and I types. The P-type systems are the simplest in terms of subunit composition and architecture, and members of this subfamily share features in common with the paradigmatic Agrobacterium tumefaciens VirB/VirD4 T4SS. This review will summarize our current knowledge of the E. coli systems and the A. tumefaciens P-type system, with emphasis on the structural diversity of the T4SSs. Ancestral P-, F-, and I-type systems were adapted throughout evolution to yield the extant effector translocators, and information about well-characterized effector translocators also is included to further illustrate the adaptive and mosaic nature of these highly versatile machines.

Brucella Rough Mutant Induce Macrophage Death via Activating IRE1α Pathway of Endoplasmic Reticulum Stress by Enhanced T4SS Secretion

Brucella is a Gram-negative facultative intracellular pathogen that causes the worldwide zoonosis, known as brucellosis. Brucella virulence relies mostly on its ability to invade and replicate within phagocytic cells. The type IV secretion system (T4SS) and lipopolysaccharide are two major Brucella virulence factors. Brucella rough mutants reportedly induce the death of infected macrophages, which is T4SS dependent. However, the underlying molecular mechanism remains unclear. In this study, the T4SS secretion capacities of Brucella rough mutant and its smooth wild-type strain were comparatively investigated, by constructing the firefly luciferase fused T4SS effector, BPE123 and VceC. In addition, quantitative real-time PCR and western blotting were used to analyze the T4SS expression. The results showed that T4SS expression and secretion were enhanced significantly in the Brucella rough mutant. We also found that the activity of the T4SS virB operon promoter was notably increased in the Brucella rough mutant, which depends on quorum sensing-related regulators of VjbR upregulation. Cell infection and cell death assays revealed that deletion of vjbR in the Brucella rough mutant absolutely abolished cytotoxicity within macrophages by downregulating T4SS expression. This suggests that up-regulation of T4SS promoted by VjbR in rough mutant ΔrfbE contribute to macrophage death. In addition, we found that the Brucella rough mutant induce macrophage death via activating IRE1α pathway of endoplasmic reticulum stress. Taken together, our study provide evidence that in comparison to the Brucella smooth wild-type strain, VjbR upregulation in the Brucella rough mutant increases transcription of the virB operon, resulting in overexpression of the T4SS gene, accompanied by the over-secretion of effecter proteins, thereby causing the death of infected macrophages via activating IRE1α pathway of endoplasmic reticulum stress, suggesting novel insights into the molecular mechanisms associated with Brucella rough mutant-induced macrophage cytotoxicity.

Multivalent Fusion DNA Vaccine against Brucella abortus

As an alternative brucellosis prevention method, we evaluated the immunogenicity induced by new multivalent DNA vaccines in BALB/c mice. We constructed the vaccines by fusion of BAB1_0273 and/or BAB1_0278 open reading frames (ORFs) from genomic island 3 (GI-3) and the Brucella abortus 2308 sodC gene with a link based on prolines and alanines (pV273-sod, pV278-sod, and pV273-278-sod, resp.). Results show that immunization with all tested multivalent DNA vaccines induced a specific humoral and cellular immune response. These novel multivalent vaccines significantly increased the production of IgM, IgG, and IgG2a antibodies as well as IFN-γ levels and the lymphoproliferative response of splenocytes. Although immunization with these multivalent vaccines induced a typical T-helper 1- (Th1-) dominated immune response, such immunogenicity conferred low protection levels in mice challenged with the B. abortus 2308 pathogenic strain. Our results demonstrated that the expression of BAB1_0273 and/or BABl_0278 antigens conjugated to SOD protein can polarize mice immunity to a Th1-type phenotype, conferring low levels of protection.

Systems Biology Analysis of Temporal In vivo Brucella melitensis and Bovine Transcriptomes Predicts host:Pathogen Protein-Protein Interactions

To date, fewer than 200 gene-products have been identified as Brucella virulence factors, and most were characterized individually without considering how they are temporally and coordinately expressed or secreted during the infection process. Here, we describe and analyze the in vivo temporal transcriptional profile of Brucella melitensis during the initial 4 h interaction with cattle. Pathway analysis revealed an activation of the "Two component system" providing evidence that the in vivo Brucella sense and actively regulate their metabolism through the transition to an intracellular lifestyle. Contrarily, other Brucella pathways involved in virulence such as "ABC transporters" and "T4SS system" were repressed suggesting a silencing strategy to avoid stimulation of the host innate immune response very early in the infection process. Also, three flagellum-encoded loci (BMEII0150-0168, BMEII1080-1089, and BMEII1105-1114), the "flagellar assembly" pathway and the cell components "bacterial-type flagellum hook" and "bacterial-type flagellum" were repressed in the tissue-associated B. melitensis, while RopE1 sigma factor, a flagellar repressor, was activated throughout the experiment. These results support the idea that Brucella employ a stealthy strategy at the onset of the infection of susceptible hosts. Further, through systems-level in silico host:pathogen protein-protein interactions simulation and correlation of pathogen gene expression with the host gene perturbations, we identified unanticipated interactions such as VirB11::MAPK8IP1; BtaE::NFKBIA, and 22 kDa OMP precursor::BAD and MAP2K3. These findings are suggestive of new virulence factors and mechanisms responsible for Brucella evasion of the host's protective immune response and the capability to maintain a dormant state. The predicted protein-protein interactions and the points of disruption provide novel insights that will stimulate advanced hypothesis-driven approaches toward revealing a clearer understanding of new virulence factors and mechanisms influencing the pathogenesis of brucellosis.

Molecular prevalence of putative virulence-associated genes in Brucella melitensis and Brucella abortus isolates from human and livestock specimens in Iran

Molecular prevalence of nine putative virulence factors in two more prevalent Brucella species in Iranian patients and livestock was investigated. During five years (2010-2015), 120 human and animal specimens were collected from three geographical areas of Iran. All samples were cultured in blood culture media and subcultured into Brucella agar medium. Nine primer pairs were designed for detection of VirB2, VirB5, VceC, BtpA, BtpB, PrpA, BetB, BPE275 and BSPB virulence factors using PCR and sequence analysis. Totally, 68 Brucella isolates including 60 B. melitensis and 8 B. abortus were isolated from the human and animal specimens examined. Approximately, all B. melitensis and B. abortus strains were positive (100%) regarding btpA, btpB, virB5, vceC, bpe275, bspB, and virB2 genes except for prpA and betB that were detected in 86% and 97% of the strains, respectively. Significant relationships were found between the presence of prpA and human B. melitensis isolates (P = 0.04), and also between the presence of betB and human isolates of B. abortus (P = 0.03). In conclusion, our results revealed that Iranian Brucella strains, regardless of human or animal sources, are extremely virulent due to high prevalence of virulence attributes in almost all strains studied.

Brucella Melitensis 16M Regulates the Effect of AIR Domain on Inflammatory Factors, Autophagy, and Apoptosis in Mouse Macrophage through the ROS Signaling Pathway

Brucellosis is a highly contagious zoonosis caused by Brucella. Brucella can invade and persist inside host cells, which results in chronic infection. We constructed AIR interference and overexpression lentiviruses to acquire AIR interference, overexpression, and rescue stable expression cell lines. We also established a Brucella melitensis 16M-infected macrophage model, which was treated with either the vehicle control or NAC (ROS scavenger N-acetylcysteine (NAC) for 0, 3, 6, 12, and 24 h. Confocal laser microscopy, transmission electron microscopy, fluorescence quantitative PCR, flow cytometry, ELISA, and Western blot were used to detect inflammation, cell autophagy and apoptosis-related protein expression levels, ROS levels, and the distribution of mitochondria. It was found that after interference and overexpression of AIR, ROS release was significantly changed, and mitochondria became abnormally aggregated. B. melitensis 16M activated the NLRP3/AIM2 inflammatory complex, and induced RAW264.7 cells to secrete IL-1β and IL-18 through the ROS pathway. B. melitensis 16M also altered autophagy-related gene expression, increased autophagy activity, and induced cell apoptosis through the ROS pathway. The results showed that after B. melitensis 16M infection, ROS induced apoptosis, inflammation, and autophagy while AIR inhibited autophagosome maturation and autophagy initiation. Autophagy negatively regulated the activation of inflammasomes and prevented inflammation from occurring. In addition, mitophagy could promote cell apoptosis.

Inhibition of MHC-I by Brucella abortus is an early event during infection and involves EGFR pathway

Brucella abortus is able to persist inside the host despite the development of potent CD8⁺ T-cell responses. We have recently reported the ability of B. abortus to inhibit the interferon-γ-induced major histocompatibility complex (MHC)-I cell surface expression on human monocytes. This phenomenon was due to the B. abortus-mediated retention of MHC-I molecules within the Golgi apparatus and was dependent on bacterial viability. However, the implications of bacterial virulence or replicative capacity and the signaling pathways remained unknown. Here we demonstrated that the B. abortus mutant strains RB51 and virB10^- are able to inhibit MHC-I expression in the same manner as wild-type B. abortus, even though they are unable to persist inside human monocytes for a long period of time. Consistent with this, the phenomenon was triggered early in time and could be observed at 8 h postinfection. At 24 and 48 h, it was even stronger. Regarding the signaling pathway, targeting epidermal growth factor (EGF) receptor (EGFR), ErbB2 (HER2) or inhibition of tumor necrosis factor-α-converting enzyme, one of the enzymes which generates soluble EGF-like ligands, resulted in partial recovery of MHC-I surface expression. Moreover, recombinant EGF and transforming growth factor-α as well as the combination of both were also able to reproduce the B. abortus-induced MHC-I downmodulation. Finally, when infection was performed in the presence of an extracellular signal-regulated kinase 1/2 (Erk1/2) inhibitor, MHC-I surface expression was significantly recovered. Overall, these results describe how B. abortus evades CD8⁺ T-cell responses early during infection and exploits the EGFR-ERK signaling pathway to escape from the immune system and favor chronicity.

The Rickettsia type IV secretion system: unrealized complexity mired by gene family expansion

Many prokaryotes utilize type IV secretion systems (T4SSs) to translocate substrates (e.g. nucleoprotein, DNA, protein) across the cell envelope, and/or to elaborate surface structures (i.e. pili or adhesins). Among eight distinct T4SS classes, P-T4SSs are typified by the Agrobacterium tumefaciens vir T4SS, which is comprised of 12 scaffold components (VirB1-VirB11, VirD4). While most P-T4SSs include all 12 Vir proteins, some differ from the vir archetype by either containing additional scaffold components not analogous to Vir proteins or lacking one or more of the Vir proteins. In a special case, the Rickettsiales vir homolog (rvh) P-T4SS comprises unprecedented gene family expansion. rvh contains three families of gene duplications (rvhB9, rvhB8, rvhB4): RvhB9,8,4-I are conserved relative to equivalents in other P-T4SSs, while RvhB9,8,4-II have evolved atypical features that deviate substantially from other homologs. Furthermore, rvh contains five VirB6-like genes (rvhB6a-e), which are tandemly arrayed and contain large N- and C-terminal extensions. Our work herein focuses on the complexity underpinned by rvh gene family expansion. Furthermore, we describe an RvhB10 insertion, which occurs in a region that forms the T4SS pore. The significance of these curious properties to rvh structure and function is evaluated, shedding light on a highly complex T4SS.