The TIR domain-containing proteins BtpA/Btp1/TcpB and BtpB are translocated into host cells by the facultative intracellular bacterial pathogen Brucella. Here, they interfere with Toll like receptor signalling to temper the host inflammatory response. BtpA has also been found to modulate microtubule dynamics. In both proteins we identified a WxxxE motif, previously shown to be an essential structural component in a family of bacterial type III secretion system effectors that modulate host actin dynamics by functioning as guanine nucleotide exchange factors of host GTPases. We analysed a role for the WxxxE motif in association of BtpA and BtpB with the cytoskeleton.
Unlike BtpA, ectopically expressed BtpB did not show a tubular localisation, but was found ubiquitously in the cytoplasm and the nucleus, and often appeared in discrete punctae in HeLa cells. BtpB was able to protect microtubules from drug-induced destabilisation similar to BtpA. The WxxxE motif was important for the ability of BtpA and BtpB to protect microtubules against destabilising drugs. Surprisingly, ectopic expression of BtpA, although not BtpB, in HeLa cells induced the formation of filopodia. This process was invariably dependent of the WxxxE motif. Our recent resolution of the crystal structure of the BtpA TIR domain reveals that the motif positions a glycine residue that has previously been shown to be essential for interaction of BtpA with microtubules.
Our results suggest a structural role for the WxxxE motif in the association of BtpA and BtpB with microtubules, as with the WxxxE GEF family proteins where the motif positions an adjacent catalytic loop important for interaction with specific Rho GTPases. In addition, the ability of ectopically expressed BtpA to induce filopodia in a WxxxE-dependent manner suggests a novel property for BtpA. A conserved WxxxE motif is found in most bacterial and several eukaryotic TIR domain proteins. Despite the similarity between ectopically expressed BtpA and WxxxE GEFs to modulate host actin dynamics, our results suggest that BtpA is not part of this WxxxE GEF family. The WxxxE motif may therefore be a more common structural motif than thus far described. BtpA may provide clues to cross-talk between the TLR and GTPase signalling pathways.
Electronic supplementary material
The online version of this article (doi:10.1186/s12964-014-0053-y) contains supplementary material, which is available to authorized users.
BtpA; TcpB; BtpB; Brucella; Bacterial TIR domain proteins; TIR domain; WxxxE motif; GTPase signalling; Microtubule dynamics
Several bacterial pathogens have TIR domain-containing proteins that contribute to their pathogenesis. We identified a second TIR-containing protein in Brucella spp. that we have designated BtpB. We show it is a potent inhibitor of TLR signaling, probably via MyD88. BtpB is a novel Brucella effector that is translocated into host cells and interferes with activation of dendritic cells. In vivo mouse studies revealed that BtpB is contributing to virulence and control of local inflammatory responses with relevance in the establishment of chronic brucellosis. Together, our results show that BtpB is a novel Brucella effector that plays a major role in the modulation of host innate immune response during infection.
Brucella; TIR domain; Btp1/BtpA; TLR; DC; NF-κB
Innate immunity recognizes bacterial molecules bearing pathogen-associated molecular patterns to launch inflammatory responses leading to the activation of adaptive immunity. However, the lipopolysaccharide (LPS) of the gram-negative bacterium Brucella lacks a marked pathogen-associated molecular pattern, and it has been postulated that this delays the development of immunity, creating a gap that is critical for the bacterium to reach the intracellular replicative niche. We found that a B. abortus mutant in the wadC gene displayed a disrupted LPS core while keeping both the LPS O-polysaccharide and lipid A. In mice, the wadC mutant induced proinflammatory responses and was attenuated. In addition, it was sensitive to killing by non-immune serum and bactericidal peptides and did not multiply in dendritic cells being targeted to lysosomal compartments. In contrast to wild type B. abortus, the wadC mutant induced dendritic cell maturation and secretion of pro-inflammatory cytokines. All these properties were reproduced by the wadC mutant purified LPS in a TLR4-dependent manner. Moreover, the core-mutated LPS displayed an increased binding to MD-2, the TLR4 co-receptor leading to subsequent increase in intracellular signaling. Here we show that Brucella escapes recognition in early stages of infection by expressing a shield against recognition by innate immunity in its LPS core and identify a novel virulence mechanism in intracellular pathogenic gram-negative bacteria. These results also encourage for an improvement in the generation of novel bacterial vaccines.
Brucellosis is one of the most extended bacterial zoonosis in the world and an important cause of economic losses and human suffering. The causative agents belong to the genus Brucella, a group of highly infectious gram-negative bacteria characterized by their ability to escape early detection by innate immunity. This stealthy behavior effectively delays the development of immunity, creating a gap that is used by the bacterium to penetrate into a variety of cells and to activate complementary virulence mechanisms such as the type IV secretion system. By this manner, the brucellae divert intracellular trafficking to reach a safe multiplication niche and establish chronic infections. Our results show that an inner section of the Brucella LPS (a molecule that in most bacteria is detected by innate immunity), effectively contributes to block recognition by soluble molecules and cellular receptors of the host innate immune system. Accordingly, a mutation disrupting the inner but no other lipopolysaccharide sections generates attenuation by impairing the stealthiness characteristics of this pathogen. This is the first Brucella mutant in which attenuation is specifically linked to the bolstering of immunity against this pathogen. Therefore, this new virulence mechanism opens the way for the development of improved bacterial vaccines.
Brucella melitensis is a facultative intracellular bacterium that causes brucellosis, the most prevalent zoonosis worldwide. The Brucella intracellular replicative niche in macrophages and dendritic cells thwarts immune surveillance and complicates both therapy and vaccine development. Currently, host-pathogen interactions supporting Brucella replication are poorly understood. Brucella fuses with the endoplasmic reticulum (ER) to replicate, resulting in dramatic restructuring of the ER. This ER disruption raises the possibility that Brucella provokes an ER stress response called the Unfolded Protein Response (UPR). In this study, B. melitensis infection up regulated expression of the UPR target genes BiP, CHOP, and ERdj4, and induced XBP1 mRNA splicing in murine macrophages. These data implicate activation of all 3 major signaling pathways of the UPR. Consistent with previous reports, XBP1 mRNA splicing was largely MyD88-dependent. However, up regulation of CHOP, and ERdj4 was completely MyD88 independent. Heat killed Brucella stimulated significantly less BiP, CHOP, and ERdj4 expression, but induced XBP1 splicing. Although a Brucella VirB mutant showed relatively intact UPR induction, a TcpB mutant had significantly compromised BiP, CHOP and ERdj4 expression. Purified TcpB, a protein recently identified to modulate microtubules in a manner similar to paclitaxel, also induced UPR target gene expression and resulted in dramatic restructuring of the ER. In contrast, infection with the TcpB mutant resulted in much less ER structural disruption. Finally, tauroursodeoxycholic acid, a pharmacologic chaperone that ameliorates the UPR, significantly impaired Brucella replication in macrophages. Together, these results suggest Brucella induces a UPR, via TcpB and potentially other factors, that enables its intracellular replication. Thus, the UPR may provide a novel therapeutic target for the treatment of brucellosis. These results also have implications for other intracellular bacteria that rely on host physiologic stress responses for replication.
Brucella melitensis is an intracellular bacterium that invades and replicates within macrophages and dendritic cells. With over 500,000 new infections per year, brucellosis is the most prevalent zoonosis worldwide and incurs significant human morbidity and economic loss. The intracellular location of Brucella renders the organism resistant to antibiotics. A safe and effective human vaccine does not exist. Thus, better understanding of the host-pathogen interactions supporting establishment of the intracellular replicative niche is critical. In this study, we found that infection of macrophages with Brucella induces a host stress response called the Unfolded Protein Response (UPR), a conserved stress response originating in the endoplasmic reticulum (ER). Full induction of the UPR requires live bacteria and expression of a microtubule modulating protein, TcpB. Inhibition of the UPR with the drug tauroursodeoxycholic acid significantly diminished Brucella replication. Together these results suggest Brucella induces the UPR to enable its own replication within host macrophages. Thus the UPR may represent a novel therapeutic target for the treatment of brucellosis.
Brucella species are facultative intracellular bacterial pathogens that cause brucellosis, a global zoonosis of profound importance. Although recent studies have demonstrated that Brucella spp. replicate within an intracellular compartment that contains endoplasmic reticulum (ER) resident proteins, the molecular mechanisms by which the pathogen secures this replicative niche remain obscure. Here, we address this issue by exploiting Drosophila S2 cells and RNA interference (RNAi) technology to develop a genetically tractable system that recapitulates critical aspects of mammalian cell infection. After validating this system by demonstrating a shared requirement for phosphoinositide 3-kinase (PI3K) activities in supporting Brucella infection in both host cell systems, we performed an RNAi screen of 240 genes, including 110 ER-associated genes, for molecules that mediate bacterial interactions with the ER. We uncovered 52 evolutionarily conserved host factors that, when depleted, inhibited or increased Brucella infection. Strikingly, 29 of these factors had not been previously suggested to support bacterial infection of host cells. The most intriguing of these was inositol-requiring enzyme 1 (IRE1), a transmembrane kinase that regulates the eukaryotic unfolded protein response (UPR). We employed IRE1α−/− murine embryonic fibroblasts (MEFs) to demonstrate a role for this protein in supporting Brucella infection of mammalian cells, and thereby, validated the utility of the Drosophila S2 cell system for uncovering novel Brucella host factors. Finally, we propose a model in which IRE1α, and other ER-associated genes uncovered in our screen, mediate Brucella replication by promoting autophagosome biogenesis.
Brucella spp. are facultative intracellular pathogens that cause brucellosis in a broad range of hosts, including humans. Brucella melitensis, B. abortus, and B. suis are highly infectious and can be readily transmitted in aerosolized form, and a human vaccine against brucellosis is unavailable. Therefore, these pathogens are recognized as potential bioterror agents. Because genetic systems for studying host–Brucella interactions have been unavailable, little is known about the host factors that mediate infection. Here, we demonstrate that a Drosophila S2 cell system and RNA interference can be exploited to study the role that evolutionarily conserved Brucella host proteins play in these processes. We also show that this system provides for the identification and characterization of host factors that mediate Brucella interactions with the host cell endoplasmic reticulum. In fact, we identified 52 host factors that, when depleted, inhibited or increased Brucella infection. Among the identified Brucella host factors, 29 have not been previously shown to support bacterial infection. Finally, we demonstrate that the novel host factor inositol-requiring enzyme 1 (IRE1) and its mammalian ortholog (IRE1α) are required for Brucella infection of Drosophila S2 and mammalian cells, respectively. Therefore, this work contributes to our understanding of host factors mediating Brucella infection.
Most bacterial infections induce the activation of polymorphonuclear neutrophils (PMNs), enhance their microbicidal function, and promote the survival of these leukocytes for protracted periods of time. Brucella abortus is a stealthy pathogen that evades innate immunity, barely activates PMNs, and resists the killing mechanisms of these phagocytes. Intriguing clinical signs observed during brucellosis are the low numbers of Brucella infected PMNs in the target organs and neutropenia in a proportion of the patients; features that deserve further attention. Here we demonstrate that B. abortus prematurely kills human PMNs in a dose-dependent and cell-specific manner. Death of PMNs is concomitant with the intracellular Brucella lipopolysaccharide (Br-LPS) release within vacuoles. This molecule and its lipid A reproduce the premature cell death of PMNs, a phenomenon associated to the low production of proinflammatory cytokines. Blocking of CD14 but not TLR4 prevents the Br-LPS-induced cell death. The PMNs cell death departs from necrosis, NETosis and classical apoptosis. The mechanism of PMN cell death is linked to the activation of NADPH-oxidase and a modest but steadily increase of ROS mediators. These effectors generate DNA damage, recruitments of check point kinase 1, caspases 5 and to minor extent of caspase 4, RIP1 and Ca++ release. The production of IL-1β by PMNs was barely stimulated by B. abortus infection or Br-LPS treatment. Likewise, inhibition of caspase 1 did not hamper the Br-LPS induced PMN cell death, suggesting that the inflammasome pathway was not involved. Although activation of caspases 8 and 9 was observed, they did not seem to participate in the initial triggering mechanisms, since inhibition of these caspases scarcely blocked PMN cell death. These findings suggest a mechanism for neutropenia in chronic brucellosis and reveal a novel Brucella-host cross-talk through which B. abortus is able to hinder the innate function of PMN.
The absence of obvious clinical symptoms during the early stages of brucellosis is linked to the Brucella stealthy strategy and its non-canonical PAMPs, which are low PRRs agonists. Still, there are clinical profiles that require explanation. For instance ‒despite the fact that neutrophils readily ingest Brucella during the onset of infection, brucellosis courses without neutrophilia, and just a low number of infected neutrophils are present in target organs. In the chronic phases, a significant proportion of the patients display absolute neutropenia and bone marrow pancytopenia linked to the myeloid cell linage. Examination of the Brucella infected bone marrow reveals granulomas and phagocytosis of myeloid cells. Based on these observations we explored the fate of native neutrophils during their interaction with Brucella. We found that the bacterium induces the premature cell death of neutrophils without inducing proinflammatory phenotypic changes. This event was reproduced by the lipid A of the Brucella LPS and depends on NADPH-oxidase activation and low ROS formation. We believe that this phenomenon explains ‒at least in part‒ the hematological and histological profiles observed during brucellosis. In addition, it may be that dying Brucella-infected neutrophils serve as “Trojan horse” vehicles for infecting phagocytic cells without promoting activation.
The intracellular pathogenic bacterium Brucella generates a replicative vacuole (rBCV) derived from the endoplasmic reticulum via subversion of the host cell secretory pathway. rBCV biogenesis requires the expression of the Type IV secretion system (T4SS) VirB, which is thought to translocate effector proteins that modulate membrane trafficking along the endocytic and secretory pathways. To date, only a few T4SS substrates have been identified, whose molecular functions remain unknown. Here, we used an in silico screen to identify putative T4SS effector candidate proteins using criteria such as limited homology in other bacterial genera, the presence of features similar to known VirB T4SS effectors, GC content and presence of eukaryotic-like motifs. Using β-lactamase and CyaA adenylate cyclase reporter assays, we identified eleven proteins translocated into host cells by Brucella, five in a VirB T4SS-dependent manner, namely BAB1_0678 (BspA), BAB1_0712 (BspB), BAB1_0847 (BspC), BAB1_1671 (BspE) and BAB1_1948 (BspF). A subset of the translocated proteins targeted secretory pathway compartments when ectopically expressed in HeLa cells, and the VirB effectors BspA, BspB and BspF inhibited protein secretion. Brucella infection also impaired host protein secretion in a process requiring BspA, BspB and BspF. Single or combined deletions of bspA, bspB and bspF affected Brucella ability to replicate in macrophages and persist in the liver of infected mice. Taken together, these findings demonstrate that Brucella modulates secretory trafficking via multiple T4SS effector proteins that likely act coordinately to promote Brucella pathogenesis.
Many intracellular parasites ensure their survival and proliferation within host cells by secreting an array of effector molecules that modulate various cellular functions. Among these, Brucella abortus, the causative agent of the worldwide zoonosis brucellosis, controls the intracellular trafficking of its vacuole, the Brucella-containing vacuole (BCV), towards compartments of the secretory pathway via the expression of a Type IV secretion system (T4SS), VirB, which is thought to translocate effector proteins. Here, we have used bioinformatic algorithms and protein translocation reporter assays to identify novel Brucella proteins translocated into host cells, some of which are VirB T4SS substrates and targeted secretory pathway compartments when ectopically expressed in mammalian cells. Three VirB effectors, BspA, BspB and BspF, inhibited protein secretion and contributed to varying degrees to bacterial inhibition of host protein secretion, pathogen intracellular growth and persistence in the liver of infected mice. These findings demonstrate that Brucella modulates secretory trafficking via multiple T4SS effector proteins to promote Brucella pathogenesis.
Brucella is a facultative intracellular pathogen and the etiological agent of brucellosis. In some cases, human brucellosis results in a persistent infection that may reactivate years after the initial exposure. The mechanisms by which the parasite evades clearance by the immune response to chronically infect its host are unknown. We recently demonstrated that dendritic cells (DCs), which are critical components of adaptive immunity, are highly susceptible to Brucella infection and are a preferential niche for the development of the bacteria. Here, we report that in contrast to several intracellular bacteria, Brucella prevented the infected DCs from engaging in their maturation process and impaired their capacities to present antigen to naïve T cells and to secrete interleukin-12. Moreover, Brucella-infected DCs failed to release tumor necrosis factor alpha (TNF-α), a defect involving the bacterial protein Omp25. Exogenous TNF-α addition to Brucella-infected DCs restored cell maturation and allowed them to present antigens. Two avirulent mutants of B. suis, B. suis bvrR and B. suis omp25 mutants, which do not express the Omp25 protein, triggered TNF-α production upon DC invasion. Cells infected with these mutants subsequently matured and acquired the ability to present antigens, two properties which were dramatically impaired by addition of anti-TNF-α antibodies. In light of these data, we propose a model in which virulent Brucella alters the maturation and functions of DCs through Omp25-dependent control of TNF-α production. This model defines a specific evasion strategy of the bacteria by which they can escape the immune response to chronically infect their host.
Brucella are facultative intracellular bacteria that chronically infect humans and animals causing brucellosis. Brucella are able to invade and replicate in a broad range of cell lines in vitro, however the cells supporting bacterial growth in vivo are largely unknown. In order to identify these, we used a Brucella melitensis strain stably expressing mCherry fluorescent protein to determine the phenotype of infected cells in spleen and liver, two major sites of B. melitensis growth in mice. In both tissues, the majority of primary infected cells expressed the F4/80 myeloid marker. The peak of infection correlated with granuloma development. These structures were mainly composed of CD11b+ F4/80+ MHC-II+ cells expressing iNOS/NOS2 enzyme. A fraction of these cells also expressed CD11c marker and appeared similar to inflammatory dendritic cells (DCs). Analysis of genetically deficient mice revealed that differentiation of iNOS+ inflammatory DC, granuloma formation and control of bacterial growth were deeply affected by the absence of MyD88, IL-12p35 and IFN-γ molecules. During chronic phase of infection in susceptible mice, we identified a particular subset of DC expressing both CD11c and CD205, serving as a reservoir for the bacteria. Taken together, our results describe the cellular nature of immune effectors involved during Brucella infection and reveal a previously unappreciated role for DC subsets, both as effectors and reservoir cells, in the pathogenesis of brucellosis.
Brucella are facultative intracellular bacteria chronically infecting humans and animals causing brucellosis, one of the most common zoonotic disease worldwide which can result in infertility and chronic debilitating disease. The cells supporting Brucella growth in vivo remain largely unknown. In order to identify these, we constructed a Brucella melitensis strain expressing a fluorescent protein that allowed us to characterize infected cells by microscopy of the spleen and liver from infected mice. In both tissues, the majority of primary infected cells were cells from the macrophage lineage. The peak of infection correlated with granuloma development. These structures contained the majority of bacteria and were mainly composed of cells expressing CD11b, F4/80, MHC-II, which are specific of activated monocytes/macrophages. A fraction of granuloma cells also expressed CD11c and were similar to inflammatory dendritic cells (DCs). During the chronic phase of infection in susceptible mice, we identified a particular subset of DC expressing CD205 and serving as a reservoir for the bacteria. Overall, our results describe the nature of immune cells infected by Brucella in vivo and reveal an unappreciated role for DC subsets, both as effectors and reservoir cells. These results could help develop new therapeutic strategies to control Brucella infection.
Brucella is a Gram-negative, facultative intracellular bacterium that causes zoonotic brucellosis in humans and various animals. Out of 10 classified Brucella species, B. melitensis, B. abortus, B. suis, and B. canis are pathogenic to humans. In the past decade, the mechanisms of Brucella pathogenesis and host immunity have been extensively investigated using the cutting edge systems biology and bioinformatics approaches. This article provides a comprehensive review of the applications of Omics (including genomics, transcriptomics, and proteomics) and bioinformatics technologies for the analysis of Brucella pathogenesis, host immune responses, and vaccine targets. Based on more than 30 sequenced Brucella genomes, comparative genomics is able to identify gene variations among Brucella strains that help to explain host specificity and virulence differences among Brucella species. Diverse transcriptomics and proteomics gene expression studies have been conducted to analyze gene expression profiles of wild type Brucella strains and mutants under different laboratory conditions. High throughput Omics analyses of host responses to infections with virulent or attenuated Brucella strains have been focused on responses by mouse and cattle macrophages, bovine trophoblastic cells, mouse and boar splenocytes, and ram buffy coat. Differential serum responses in humans and rams to Brucella infections have been analyzed using high throughput serum antibody screening technology. The Vaxign reverse vaccinology has been used to predict many Brucella vaccine targets. More than 180 Brucella virulence factors and their gene interaction networks have been identified using advanced literature mining methods. The recent development of community-based Vaccine Ontology and Brucellosis Ontology provides an efficient way for Brucella data integration, exchange, and computer-assisted automated reasoning.
Brucella; pathogenesis; host immunity; vaccine; systems biology; bioinformatics
Toll/Interleukin-1 like receptors are evolutionarily conserved proteins in eukaryotes that play crucial role in pathogen recognition and innate immune responses. Brucella are facultative intracellular bacterial pathogens causing brucellosis in animal and human hosts. Brucella behaves as a stealthy pathogen by evading the immune recognition or suppressing the TLR signaling cascades. Brucella encode a TIR domain containing protein, TcpB, which suppresses NF-κB activation as well as proinflammatory cytokine secretion mediated by TLR2 and TLR4 receptors. TcpB targets the TIRAP mediated pathway to suppress TLR signaling. With the objective of detailed characterization, we have over expressed and purified TcpB from Brucella melitensis in native condition. The purified protein exhibited lipid binding properties and cell permeability. NF-κB inhibition property of endogenous TcpB has also been demonstrated. The data provide insight into the mechanism of action of TcpB in the intracellular niche of Brucella.
Brucella; TcpB; TIR domain; NF-κB; Phosphoinositide; Inflammation
There is no safe, effective human vaccine against brucellosis. Live attenuated Brucella strains are widely used to vaccinate animals. However these live Brucella vaccines can cause disease and are unsafe for humans. Killed Brucella or subunit vaccines are not effective in eliciting long term protection. In this study, we evaluate an approach using a live, non-pathogenic bacteria (E. coli) genetically engineered to mimic the brucellae pathway of infection and present antigens for an appropriate cytolitic T cell response.
E. coli was modified to express invasin of Yersinia and listerialysin O (LLO) of Listeria to impart the necessary infectivity and antigen releasing traits of the intracellular pathogen, Brucella. This modified E. coli was considered our vaccine delivery system and was engineered to express Green Fluorescent Protein (GFP) or Brucella antigens for in vitro and in vivo immunological studies including cytokine profiling and cytotoxicity assays.
The E. coli vaccine vector was able to infect all cells tested and efficiently deliver therapeutics to the host cell. Using GFP as antigen, we demonstrate that the E. coli vaccine vector elicits a Th1 cytokine profile in both primary and secondary immune responses. Additionally, using this vector to deliver a Brucella antigen, we demonstrate the ability of the E. coli vaccine vector to induce specific Cytotoxic T Lymphocytes (CTLs).
Protection against most intracellular bacterial pathogens can be obtained mostly through cell mediated immunity. Data presented here suggest modified E. coli can be used as a vaccine vector for delivery of antigens and therapeutics mimicking the infection of the pathogen and inducing cell mediated immunity to that pathogen.
Brucella spp. are Gram-negative, facultative intracellular bacteria that cause brucellosis, one of the commonest zoonotic diseases found worldwide in humans and a variety of animal species. While several animal vaccines are available, there is no effective and safe vaccine for prevention of brucellosis in humans. VIOLIN (http://www.violinet.org) is a web-based vaccine database and analysis system that curates, stores, and analyzes published data of commercialized vaccines, and vaccines in clinical trials or in research. VIOLIN contains information for 454 vaccines or vaccine candidates for 73 pathogens. VIOLIN also contains many bioinformatics tools for vaccine data analysis, data integration, and vaccine target prediction. To demonstrate the applicability of VIOLIN for vaccine research, VIOLIN was used for bioinformatics analysis of existing Brucella vaccines and prediction of new Brucella vaccine targets.
VIOLIN contains many literature mining programs (e.g., Vaxmesh) that provide in-depth analysis of Brucella vaccine literature. As a result of manual literature curation, VIOLIN contains information for 38 Brucella vaccines or vaccine candidates, 14 protective Brucella antigens, and 68 host response studies to Brucella vaccines from 97 peer-reviewed articles. These Brucella vaccines are classified in the Vaccine Ontology (VO) system and used for different ontological applications. The web-based VIOLIN vaccine target prediction program Vaxign was used to predict new Brucella vaccine targets. Vaxign identified 14 outer membrane proteins that are conserved in six virulent strains from B. abortus, B. melitensis, and B. suis that are pathogenic in humans. Of the 14 membrane proteins, two proteins (Omp2b and Omp31-1) are not present in B. ovis, a Brucella species that is not pathogenic in humans. Brucella vaccine data stored in VIOLIN were compared and analyzed using the VIOLIN query system.
Bioinformatics curation and ontological representation of Brucella vaccines promotes classification and analysis of existing Brucella vaccines and vaccine candidates. Computational prediction of Brucella vaccine targets provides more candidates for rational vaccine development. The use of VIOLIN provides a general approach that can be applied for analyses of vaccines against other pathogens and infection diseases.
Several intracellular bacterial pathogens have evolved subtle strategies to subvert vesicular trafficking pathways of their host cells to avoid killing and to replicate inside the cells. Brucellae are Gram-negative facultative intracellular bacteria that are responsible for brucellosis, a worldwide extended chronic zoonosis. Following invasion, Brucella abortus is found in a vacuole that interacts first with various endosomal compartments and then with endoplasmic reticulum sub-compartments. Brucella establishes its replication niche in ER-derived vesicles. In the past, it has been proposed that B. abortus passed through the macroautophagy pathway before reaching its niche of replication. However, recent experiments provided evidence that the classical macroautophagy pathway was not involved in the intracellular trafficking and the replication of B. abortus in bone marrow-derived macrophages and in HeLa cells. In contrast, another study showed that macroautophagy favoured the survival and the replication of Brucella melitensis in infected RAW264.7 macrophages. This raises the possibility that B. abortus and B. melitensis followed different intracellular pathways before replicating. In the present work, we have addressed this issue by comparing the replication rate of B. abortus and B. melitensis in embryonic fibroblasts derived from wild-type and Atg5−/− mice, Atg5 being a core component of the canonical macroautophagic pathway.
Our results indicate that both B. abortus S2308 and B. melitensis 16M strains are able to invade and replicate in Atg5-deficient fibroblasts, suggesting that the canonical Atg5-dependent macroautophagic pathway is dispensable for Brucella replication. The number of viable bacteria was even slightly higher in Atg5−/− fibroblasts than in wild-type fibroblasts. This increase could be due to a more efficient uptake or to a better survival rate of bacteria before the beginning of the replication in Atg5-deficient cells as compared to wild-type cells. Moreover, our data show that the infection with B. abortus or with B. melitensis does not stimulate neither the conversion of LC3-I to LC3-II nor the membrane recruitment of LC3 onto the BCV.
Our study suggests that like Brucella abortus, Brucella melitensis does not subvert the canonical macroautophagy to reach its replicative niche or to stimulate its replication.
Electronic supplementary material
The online version of this article (doi:10.1186/s12866-014-0223-5) contains supplementary material, which is available to authorized users.
Brucella abortus; Brucella melitensis; Intracellular trafficking; Replication; Macroautophagy; Atg5
Brucella species are the causative agents of one of the most prevalent zoonotic diseases: brucellosis. Infections by Brucella species cause major economic losses in agriculture, leading to abortions in infected animals and resulting in a severe, although rarely lethal, debilitating disease in humans. Brucella species persist as intracellular pathogens that manage to effectively evade recognition by the host's immune system. Sugar-modified components in the Brucella cell envelope play an important role in their host interaction. Brucella lipopolysaccharide (LPS), unlike Escherichia coli LPS, does not trigger the host's innate immune system. Brucella produces cyclic β-1,2-glucans, which are important for targeting them to their replicative niche in the endoplasmic reticulum within the host cell. This paper will focus on the role of LPS and cyclic β-1,2-glucans in Brucella-mammalian infections and discuss the use of mutants, within the biosynthesis pathway of these cell envelope structures, in vaccine development.
Many pathogenic bacteria use a regulatory process termed quorum sensing (QS) to produce and detect small diffusible molecules to synchronize gene expression within a population. In Gram-negative bacteria, the detection of, and response to, these molecules depends on transcriptional regulators belonging to the LuxR family. Such a system has been discovered in the intracellular pathogen Brucella melitensis, a Gram-negative bacterium responsible for brucellosis, a worldwide zoonosis that remains a serious public health concern in countries were the disease is endemic. Genes encoding two LuxR-type regulators, VjbR and BabR, have been identified in the genome of B. melitensis 16 M. A ΔvjbR mutant is highly attenuated in all experimental models of infection tested, suggesting a crucial role for QS in the virulence of Brucella. At present, no function has been attributed to BabR. The experiments described in this report indicate that 5% of the genes in the B. melitensis 16 M genome are regulated by VjbR and/or BabR, suggesting that QS is a global regulatory system in this bacterium. The overlap between BabR and VjbR targets suggest a cross-talk between these two regulators. Our results also demonstrate that VjbR and BabR regulate many genes and/or proteins involved in stress response, metabolism, and virulence, including those potentially involved in the adaptation of Brucella to the oxidative, pH, and nutritional stresses encountered within the host. These findings highlight the involvement of QS as a major regulatory system in Brucella and lead us to suggest that this regulatory system could participate in the spatial and sequential adaptation of Brucella strains to the host environment.
Some pathogens use the regulatory process termed Quorum Sensing (QS) to synchronize gene expression within bacterial population. We report here the first genome scale study of the Quorum Sensing system of the intracellular pathogen Brucella melitensis. Our combined proteomic and transcriptomic data suggest that Quorum Sensing is involved in the spatial and sequential adaptation of B. melitensis to the host environment.
Brucella; intracellular pathogen; Quorum sensing; LuxR-type regulator; adaptation; proteome; transcriptome; ChIP
Brucella species replicate within host cells in the form of endoplasmic reticulum (ER)-derived vacuoles. The mechanisms by which the bacteria are sequestered into such vacuoles and obtain a continuous membrane supply for their replication remain to be elucidated. In the present study, we provided several lines of evidence that demonstrate the mechanism by which B. abortus acquires the ER-derived membrane. First, during Brucella infection, the IRE1 pathway, but not the PERK and ATF6 pathways, of the unfolded protein response (UPR) was activated in a time-dependent manner, and the COPII vesicle components Sar1, Sec23, and Sec24D were upregulated. Second, a marked accretion of ER-derived vacuoles was observed around replicating bacteria using fluorescent microscopy and electron microscopy. Third, we identified a novel host factor, Yip1A, for the activation of the IRE1 pathway in response to both tunicamycin treatment and infection with B. abortus. We found that Yip1A is responsible for the phosphorylation of IRE1 through high-order assembly of Ire1 molecules at ER exit sites (ERES) under the UPR conditions. In Yip1A-knockdown cells, B. abortus failed to generate the ER-derived vacuoles, and remained in endosomal/lysosomal compartments. These results indicate that the activation of the IRE1 pathway and the subsequent formation of ER-derived vacuoles are critical for B. abortus to establish a safe replication niche, and that Yip1A is indispensable for these processes. Furthermore, we showed that the autophagy-related proteins Atg9 and WIPI1, but not DFCP1, were required for the biogenesis of the ER-derived membrane compartments. On the basis of our findings, we propose a model for intracellular Brucella replication that exploits the host UPR and ER-derived vacuole formation machineries, both of which depend on Yip1A-mediated IRE1 activation.
The genus Brucella is a serious intracellular pathogen that causes brucellosis in a wide range of animals including humans. Infection with Brucella spp. results in a significant economic and health burden due to its high infectivity, chronic nature, and difficulties in vaccine production. Better understanding of the host-pathogen interplay that supports Brucella replication is essential for the development of effective treatments for brucellosis. The unfolded protein response (UPR) has been implicated in the pathogenesis of several viral and bacterial infections. These pathogens modulate individual pathways of the UPR to enable their replication in host cells. Autophagy has also been linked to the survival of several intracellular pathogens. They subvert autophagic machineries of host cells to establish their safe replication niche. In the present study, we show that the activation of the IRE1 pathway of the UPR and the subsequent formation of ER-derived vacuoles are crucial for intracellular survival of B. abortus. In addition, we identified a novel host factor Yip1A that is responsible for these processes. Characterization of the function of Yip1A will provide new insights into the molecular mechanisms by which Brucella spp. replicates in host cells.
Brucella spp., are Gram negative bacteria that cause disease by growing within monocyte/macrophage lineage cells. Clinical manifestations of brucellosis are immune mediated, not due to bacterial virulence factors. Acquired immunity to brucellosis has been studied through observations of naturally infected hosts (cattle, goats), mouse models (mice), and human infection. Even though Brucella spp. are known for producing mechanisms that evade the immune system, cell-mediated immune responses drive the clinical manifestations of human disease after exposure to Brucella species, as high antibody responses are not associated with protective immunity. The precise mechanisms by which cell-mediated immune responses confer protection or lead to disease manifestations remain undefined. Descriptive studies of immune responses in human brucellosis show that TH1 (interferon-γ-producing T cells) are associated with dominant immune responses, findings consistent with animal studies. Whether these T cell responses are protective, or determine the different clinical responses associated with brucellosis is unknown, especially with regard to undulant fever manifestations, relapsing disease, or are associated with responses to distinct sets of Brucella spp. antigens are unknown. Few data regarding T cell responses in terms of specific recognition of Brucella spp. protein antigens and peptidic epitopes, either by CD4+ or CD8+ T cells, have been identified in human brucellosis patients. Additionally because current attenuated Brucella vaccines used in animals cause human disease, there is a true need for a recombinant protein subunit vaccine for human brucellosis, as well as for improved diagnostics in terms of prognosis and identification of unusual forms of brucellosis. This review will focus on current understandings of antigen-specific immune responses induced Brucella peptidic epitopes that has promise for yielding new insights into vaccine and diagnostics development, and for understanding pathogenetic mechanisms of human brucellosis.
T cell epitope; immunology; Brucella; zoonotic diseases; systems biology
Brucella spp. are facultative intracellular Gram-negative coccobacilli responsible for brucellosis, a worldwide zoonosis. We observed that Brucella melitensis is able to persist for several weeks in the blood of intraperitoneally infected mice and that transferred blood at any time point tested is able to induce infection in naive recipient mice. Bacterial persistence in the blood is dramatically impaired by specific antibodies induced following Brucella vaccination. In contrast to Bartonella, the type IV secretion system and flagellar expression are not critically required for the persistence of Brucella in blood. ImageStream analysis of blood cells showed that following a brief extracellular phase, Brucella is associated mainly with the erythrocytes. Examination by confocal microscopy and transmission electron microscopy formally demonstrated that B. melitensis is able to invade erythrocytes in vivo. The bacteria do not seem to multiply in erythrocytes and are found free in the cytoplasm. Our results open up new areas for investigation and should serve in the development of novel strategies for the treatment or prophylaxis of brucellosis. Invasion of erythrocytes could potentially protect the bacterial cells from the host's immune response and hamper antibiotic treatment and suggests possible Brucella transmission by bloodsucking insects in nature.
Brucellosis is a zoonosis caused by Brucella species. Brucellosis research in natural hosts is often precluded by practical, economical and ethical reasons and mice are widely used. However, mice are not natural Brucella hosts and the course of murine brucellosis depends on bacterial strain virulence, dose and inoculation route as well as breed, genetic background, age, sex and physiological statu of mice. Therefore, meaningful experiments require a definition of these variables. Brucella spleen replication profiles are highly reproducible and course in four phases: i), onset or spleen colonization (first 48 h); ii), acute phase, from the third day to the time when bacteria reach maximal numbers; iii), chronic steady phase, where bacterial numbers plateaus; and iv), chronic declining phase, during which brucellae are eliminated. This pattern displays clear physiopathological signs and is sensitive to small virulence variations, making possible to assess attenuation when fully virulent bacteria are used as controls. Similarly, immunity studies using mice with known defects are possible. Mutations affecting INF-γ, TLR9, Myd88, Tγδ and TNF-β favor Brucella replication; whereas IL-1β, IL-18, TLR4, TLR5, TLR2, NOD1, NOD2, GM-CSF, IL/17r, Rip2, TRIF, NK or Nramp1 deficiencies have no noticeable effects. Splenomegaly development is also useful: it correlates with IFN-γ and IL-12 levels and with Brucella strain virulence. The genetic background is also important: Brucella-resistant mice (C57BL) yield lower splenic bacterial replication and less splenomegaly than susceptible breeds. When inoculum is increased, a saturating dose above which bacterial numbers per organ do not augment, is reached. Unlike many gram-negative bacteria, lethal doses are large (≥ 108 bacteria/mouse) and normally higher than the saturating dose. Persistence is a useful virulence/attenuation index and is used in vaccine (Residual Virulence) quality control. Vaccine candidates are also often tested in mice by determining splenic Brucella numbers after challenging with appropriate virulent brucellae doses at precise post-vaccination times. Since most live or killed Brucella vaccines provide some protection in mice, controls immunized with reference vaccines (S19 or Rev1) are critical. Finally, mice have been successfully used to evaluate brucellosis therapies. It is concluded that, when used properly, the mouse is a valuable brucellosis model.
Brucella abortus is a facultative intracellular bacterial pathogen that causes abortion in domestic animals and undulant fever in humans. Recent studies have revealed that Toll-like receptor (TLR)-initiated immune response to Brucella spp. depends on myeloid differentiation factor 88 (MyD88) signaling. Therefore, we decided to study the role of the interleukin-1 receptor-associated kinase 4 (IRAK-4) in host innate immune response against B. abortus. After Brucella infection, it was shown that the number of CFU in IRAK-4−/− mice was high compared to that in IRAK-4+/− animals only at 1 week postinfection. At 3 and 6 weeks postinfection, IRAK-4−/− mice were able to control the infection similarly to heterozygous animals. Furthermore, the type 1 cytokine profile was evaluated. IRAK-4−/− mice showed lower production of systemic interleukin-12 (IL-12) and gamma interferon (IFN-γ). Additionally, a reduced percentage of CD4+ and CD8+ T cells expressing IFN-γ was observed compared to IRAK-4+/−. Further, the production of IL-12 and tumor necrosis factor alpha (TNF-α) by macrophages and dendritic cells from IRAK-4−/− mice was abolished at 24 h after stimulation with B. abortus. To investigate the role of IRAK-4 in mitogen-activated protein kinase (MAPK) and NF-κB signaling pathways, macrophages were stimulated with B. abortus, and the signaling components were analyzed by protein phosphorylation. Extracellular signal-regulated kinase 1 (ERK1) and ERK2 and p38 as well as p65 NF-κB phosphorylation was profoundly impaired in IRAK-4−/− and MyD88−/− macrophages activated by Brucella. In summary, the results shown in this study demonstrated that IRAK-4 is critical to trigger the initial immune response against B. abortus but not at later phases of infection.
Brucella is a facultative intracellular pathogen of various mammals and the etiological agent of brucellosis. We recently demonstrated that dendritic cells (DCs), which are critical components of adaptive immunity, are highly susceptible to Brucella infection. Furthermore, Brucella prevented the infected DCs from engaging in maturation processes and impaired their capacity to present antigen to naive T cells and to secrete interleukin-12 (IL-12). The lipopolysaccharide (LPS) phenotype is largely associated with the virulence of Brucella. Depending on whether they express the O-side chain of LPS or not, the bacteria display a smooth or rough phenotype. Rough Brucella mutants are attenuated and induce a potent protective T-cell-dependent immune response. Due to the essential role of DCs in the initiation of T-cell-dependent adaptive immune responses, it seemed pertinent to study the interaction between rough Brucella strains and human DCs. In the present paper, we report that, in contrast to smooth bacteria, infection of DCs with rough mutants of Brucella suis or Brucella abortus leads to both phenotypic and functional maturation of infected cells. Rough mutant-infected DCs then acquire the capacity to produce IL-12 and to stimulate naive CD4+ T lymphocytes. Experiments with rough and smooth purified LPS of Brucella supported the hypothesis of an indirect involvement of the O-side chain. These results provide new data concerning the role of LPS in Brucella virulence strategy and illuminate phenomena contributing to immune protection conferred by rough vaccine strains.
To unravel the strategy by which Brucella abortus establishes chronic infections, we explored its early interaction with innate immunity.
Brucella did not induce proinflammatory responses as demonstrated by the absence of leukocyte recruitment, humoral or cellular blood changes in mice. Brucella hampered neutrophil (PMN) function and PMN depletion did not influence the course of infection. Brucella barely induced proinflammatory cytokines and consumed complement, and was strongly resistant to bactericidal peptides, PMN extracts and serum. Brucella LPS (BrLPS), NH-polysaccharides, cyclic glucans, outer membrane fragments or disrupted bacterial cells displayed low biological activity in mice and cells. The lack of proinflammatory responses was not due to conspicuous inhibitory mechanisms mediated by the invading Brucella or its products. When activated 24 h post-infection macrophages did not kill Brucella, indicating that the replication niche was not fusiogenic with lysosomes. Brucella intracellular replication did not interrupt the cell cycle or caused cytotoxicity in WT, TLR4 and TLR2 knockout cells. TNF-α-induction was TLR4- and TLR2-dependent for live but not for killed B. abortus. However, intracellular replication in TLR4, TLR2 and TLR4/2 knockout cells was not altered and the infection course and anti-Brucella immunity development upon BrLPS injection was unaffected in TLR4 mutant mice.
We propose that Brucella has developed a stealth strategy through PAMPs reduction, modification and hiding, ensuring by this manner low stimulatory activity and toxicity for cells. This strategy allows Brucella to reach its replication niche before activation of antimicrobial mechanisms by adaptive immunity. This model is consistent with clinical profiles observed in humans and natural hosts at the onset of infection and could be valid for those intracellular pathogens phylogenetically related to Brucella that also cause long lasting infections.
Brucellosis is a chronic infectious disease caused by Brucella spp., a Gram-negative facultative intracellular pathogen that affects humans and animals, leading to significant impact on public health and animal industry. Human brucellosis is considered the most prevalent bacterial zoonosis in the world and is characterized by fever, weight loss, depression, hepato/splenomegaly, osteoarticular, and genital infections. Relevant aspects of Brucella pathogenesis have been intensively investigated in culture cells and animal models. The mouse is the animal model more commonly used to study chronic infection caused by Brucella. This model is most frequently used to investigate specific pathogenic factors of Brucella spp., to characterize the host immune response, and to evaluate therapeutics and vaccines. Other animal species have been used as models for brucellosis including rats, guinea pigs, and monkeys. This paper discusses the murine and other laboratory animal models for human and animal brucellosis.
Brucellosis is an important zoonotic disease of almost worldwide distribution. One significant immune phenomenon of this disease is the ability of the pathogen to hide and survive in the host, establishing long lasting chronic infections. Brucella was found to have the ability to actively modulate the host immune response in order to establish chronic infections, but the mechanism by which the pathogen achieves this remains largely unknown. In our screening for protective antigens of Brucella abortus, 3 proteins (BAB1_0597, BAB1_0917, and BAB2_0431) were found to induce significantly higher levels of gamma interferon (IFNγ) in splenocytes of PBS immunized mice than those immunized with S19. This finding strongly implied that these three proteins inhibit the production of IFNγ. Previous studies have shown that LPS, PrpA, and Btp1/TcpB are three important immunomodulatory molecules with the capacity to interfere with host immune response. They have been shown to have the ability to inhibit the secretion of IFNγ, or to increase the production of IL-10. Due to the role of these proteins in virulence and immunomodulation, they likely offer significant potential as live, attenuated Brucella vaccine candidates. Understanding the mechanisms by which these proteins modulate the host immune responses will deepen our knowledge of Brucella virulence and provide important information on the development of new vaccines against Brucellosis.
Brucella; chronic infection; immune response; live attenuated vaccine; virulence proteins