virB-Mediated survival of Brucella abortus in mice and macrophages is independent of a functional inducible nitric oxide synthase or NADPH oxidase in macrophages

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

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

Diagnostic utility of in-house loop-mediated isothermal amplification and real-time PCR targeting virB gene for direct detection of Brucella melitensis from clinical specimens

AIMS

In this present study, the utility of a newly developed loop-mediated isothermal amplification (LAMP) and real-time PCR assays designed to amplify the virB gene region of Brucella melitensis was evaluated from human clinical specimens.

METHODS AND RESULTS

Fifty-four culture-confirmed cases of brucellosis and 54 culture negative but clinically suspected cases of brucellosis were included in the study. Whole blood, serum and other nonblood specimens were collected and subjected to blood culture using automatic blood culture system, serological tests, LAMP assay and real-time PCR. Overall sensitivities of LAMP and real-time PCR assays were 67·5 and 68·3% respectively. For nonblood clinical specimens, we noticed a marked increase in the sensitivities of LAMP (88·9%) and real-time PCR (100%) assays.

CONCLUSIONS

Performance of LAMP and real-time PCR was not satisfactory for whole-blood specimens because of the low abundance of bacteria or DNA. On the other hand, using nonblood specimens, both the assays showed higher sensitivity and specificity which makes them a good alternative for the rapid diagnosis of human brucellosis.

SIGNIFICANCE AND IMPACT OF THE STUDY

The developed LAMP and real-time PCR assays are a specific and rapid diagnostic tool for direct and early detection of Brucella in clinical specimens.

Brucella abortus nitric oxide metabolite regulates inflammasome activation and IL-1β secretion in murine macrophages

NLRP3 inflammasome is a protein complex crucial to caspase-1 activation and IL-1β and IL-18 maturation. This receptor participates in innate immune responses to different pathogens, including the bacteria of genus Brucella. Our group recently demonstrated that Brucella abortus-induced IL-1β secretion involves NLRP3 inflammasome and it is partially dependent on mitochondrial ROS production. However, other factors could be involved, such as P2X7-dependent potassium efflux, membrane destabilization, and cathepsin release. Moreover, there is increasing evidence that nitric oxide acts as a modulator of NLRP3 inflammasome. The aim of this study was to unravel the mechanism of NLRP3 inflammasome activation induced by B. abortus, as well as the involvement of bacterial nitric oxide (NO) as a modulator of this inflammasome pathway. We demonstrated that NO produced by B. abortus can be used by the bacteria to modulate IL-1β secretion in infected murine macrophages. Additionally, our results suggest that B. abortus-induced IL-1β secretion depends on a P2X7-independent potassium efflux, lysosomal acidification, cathepsin release, mechanisms clearly associated to NLRP3 inflammasome. In summary, our results help to elucidate the molecular mechanisms of NLRP3 activation and regulation during an intracellular bacterial infection.

Infection-generated electric field in gut epithelium drives bidirectional migration of macrophages

Many bacterial pathogens hijack macrophages to egress from the port of entry to the lymphatic drainage and/or bloodstream, causing dissemination of life-threatening infections. However, the underlying mechanisms are not well understood. Here, we report that Salmonella infection generates directional electric fields (EFs) in the follicle-associated epithelium of mouse cecum. In vitro application of an EF, mimicking the infection-generated electric field (IGEF), induces directional migration of primary mouse macrophages to the anode, which is reversed to the cathode upon Salmonella infection. This infection-dependent directional switch is independent of the Salmonella pathogenicity island 1 (SPI-1) type III secretion system. The switch is accompanied by a reduction of sialic acids on glycosylated surface components during phagocytosis of bacteria, which is absent in macrophages challenged by microspheres. Moreover, enzymatic cleavage of terminally exposed sialic acids reduces macrophage surface negativity and severely impairs directional migration of macrophages in response to an EF. Based on these findings, we propose that macrophages are attracted to the site of infection by a combination of chemotaxis and galvanotaxis; after phagocytosis of bacteria, surface electrical properties of the macrophage change, and galvanotaxis directs the cells away from the site of infection.

Brucella neotomae Recapitulates Attributes of Zoonotic Human Disease in a Murine Infection Model

Members of the genus Brucella are Gram-negative pathogens that cause chronic systemic infection in farm animals and zoonotic infection in humans. Study of the genus Brucella has been hindered by the need for biosafety level 3 select agent containment. Brucella neotomae, originally isolated from the desert pack rat, presented an opportunity to develop an alternative, non-select agent experimental model. Our prior in vitro work indicated that the cell biology and type IV secretion system (T4SS) dependence of B. neotomae intracellular replication were similar to observations for human-pathogenic select agent Brucella species. Therefore, here, we investigated the pathobiology of B. neotomae infection in the BALB/c mouse. During a sustained infectious course, B. neotomae replicated and persisted in reticuloendothelial organs. Bioluminescent imaging and histopathological and PCR-based analysis demonstrated that the T4SS contributed to efficient early infection of the liver, spleen, and lymph nodes; granuloma formation and hepatosplenomegaly; and early induction of Th1-associated cytokine gene expression. The infectious course and pathologies in the murine model showed similarity to prior observations of primate and native host infection with zoonotic Brucella species. Therefore, the B. neotomae BALB/c infection model offers a promising system to accelerate and complement experimental work in the genus Brucella.

Quantification and comparison of TLR2 activity in monocyte-derived macrophages of zebu and crossbred cattle

The present study was conducted to quantify and compare TLR2 (toll-like receptor 2) activity in monocyte-derived macrophages of zebu (Tharparkar) and crossbred (Holstein-Friesian × Jersey × Brown Swiss × Hariana) cattle. The cells were either induced with Pam3CSK4 or kept as control. The TLR2 activity was quantified in terms of IκB-α inhibitory subunit (NFKBIA) messenger RNA (mRNA) copies using real-time, one-step reverse transcription-polymerase chain reaction (RT-PCR). Toll-like receptor 2 activity of induced cells was in the range of 1060421 ± 477937 (n=3) to 3514715 ± 290222 (n=3) copies for Tharparkar cattle (n=7) and in the range of 1365532 ± 47243 (n=3) to 3016510 ± 172340 (n=3) copies for the crossbred cattle (n=7). For uninduced cells, this activity was within the range of 117 ± 51 (n=3) to 293 ± 103 (n=3) copies for the Thraparkar cattle (n=7), and in the range of 182 ± 122 (n=3) to 296 ± 88 (n=3) copies for the crossbred cattle (n=7). The TLR2 activity of induced cells in both groups was found to be significantly higher than that of the respective uninduced cells (P<0.0001). Furthermore, upon comparison, TLR2 activities of induced and uninduced cells of the Tharparkar were not found to be significantly different from those of the crossbred cattle (P=0.8154 and P=0.6670). In the present study, we have quantified and compared, for the first time, TLR2 activity in terms of NFKBIA mRNA copies in monocyte-derived macrophages of Tharparkar and crossbred cattle and found that both have equivalent TLR2 activity.

Inositol-Requiring Enzyme 1-Dependent Activation of AMPK Promotes Brucella abortus Intracellular Growth

AMP-activated protein kinase (AMPK) is a serine/threonine kinase that is well conserved during evolution. AMPK activation inhibits production of reactive oxygen species (ROS) in cells via suppression of NADPH oxidase. However, the role of AMPK during the process of Brucella infection remains unknown. Our data demonstrate that B. abortus infection induces AMPK activation in HeLa cells in a time-dependent manner. The known AMPK kinases LKB1, CAMKKβ, and TAK1 are not required for the activation of AMPK by B. abortus infection. Instead, this activation is dependent on the RNase activity of inositol-requiring enzyme 1 (IRE1). Moreover, we also found that B. abortus infection-induced IRE1-dependent activation of AMPK promotes B. abortus intracellular growth with peritoneal macrophages via suppression of NADPH-derived ROS production.

IMPORTANCE

Previous studies showed that B. abortus infection does not promote any oxidative burst regulated by NADPH oxidase. However, the underlying mechanism remains elusive. We report for the first time that AMPK activation caused by B. abortus infection plays important role in NADPH oxidase-derived ROS production.

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

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

A new method for simultaneous gene deletion and down-regulation in Brucella melitensis Rev.1

In this study, our aim was to integrate an antisense expression cassette in bacterial chromosome for providing a long-term expression down-regulation in a bid to develop a new approach for simultaneous deletion and down-regulation of target genes in bacterial system. Therefore, we were used this approach for simultaneous deletion of the perosamine synthetase (per) gene and down-regulation of the virB1 expression in Brucella melitensis Rev.1. The per gene, which is one of the LPS O-chain coding genes, was replaced by homologous recombination with an antisense virB1 expression cassette together with kanamycin resistance cassette (kan(R)). Deletion of the per gene was characterized by PCR analysis and DNA sequencing. The expression of antisense virB1 cassette was confirmed by RT-PCR. Down-regulation of the virB1 mRNA expression was quantified by real-time RT-PCR using virB1 specific primers relative to the groEL reference gene. The survival rate of mutant strain was evaluated by CFU count in the BALB/c mice. The virB1 mRNA expression was down-regulated on average 10-fold in mutant strain as compared to parental strain. The loss of per gene function and decrease of the virB1 mRNA expression resulted in reduced entry and survival of the mutant Rev.1 strain in BALB/c mice splenocytes. We propose that this method can be used for simultaneous regulation of multiple genes expression.

Inhibition of mCD14 inhibits TNFα secretion and NO production in RAW264.7 cells stimulated by Brucella melitensis infection

Brucellosis, caused by Brucella spp., is an important disease affecting not only human health, but also a number of animal species around the world. A receptor for LPS of Brucella and important innate immune molecule, CD14, has been implicated in the initiation of the inflammatory response to sepsis. Evidence indicates that upstream inhibition of the LPS initiated inflammatory pathway is an effective therapeutic approach for attenuating damaging immune activation. The aim of this study was to explore the possibility of using RNA interference (RNAi) targeting mCD14 as a strategy for inhibiting the secretion of tumor necrosis factor alpha (TNFα) and the production of nitric oxide (NO) from Brucella-stimulated RAW264.7 cells and attenuating damaging immune activation. The current study stably incorporated mCD14-shRNA-224 into the RAW264.7 cell line by lentiviral gene transfer to successfully knockdown mCD14, and was then challenged with Brucella melitensis M5-90. The secretion of TNFα, interleukin (IL)-12, CXCL1/KC, and inducible nitric oxide synthase (iNOS) protein expression, and NO production were evaluated. The mCD14-shRNA-224 knockdown was shown to effectively inhibit B. melitensis M5-90-stimulated TNFα release, iNOS protein expression, and NO production, but no significant differences were observed for IL-12 and CXCL1/KC. These findings provide the basis for the development of RNAi-based prophylaxis and therapy for B. melitensis induced inflammatory disease.

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

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

Extended safety and efficacy studies of the attenuated Brucella vaccine candidates 16 M(Delta)vjbR and S19(Delta)vjbR in the immunocompromised IRF-1-/- mouse model

The global distribution of brucellosis and high incidence in certain areas of the world warrant the development of a safer and efficacious vaccine. For the past 10 years, we have focused our attention on the development of a safer, but still highly protective, live attenuated vaccine for human and animal use. We have demonstrated the safety and protective efficacy of the vaccine candidates 16 MΔvjbR and S19ΔvjbR against homologous and heterologous challenge in multiple immunocompetent animal models, including mice and deer. In the present study, we conducted a series of experiments to determine the safety of the vaccine candidates in interferon regulatory factor-1-knockout (IRF-1(-/-)) mice. IRF-1(-/-) mice infected with either wild-type Brucella melitensis 16 M or the vaccine strain Brucella abortus S19 succumb to the disease within the first 3 weeks of infection, which is characterized by a marked granulomatous and neutrophilic inflammatory response that principally targets the spleen and liver. In contrast, IRF-1(-/-) mice inoculated with either the 16 MΔvjbR or S19ΔvjbR vaccine do not show any clinical or major pathological changes associated with vaccination. Additionally, when 16 MΔvjbR- or S19ΔvjbR-vaccinated mice are challenged with wild-type Brucella melitensis 16M, the degree of colonization in multiple organs, along with associated pathological changes, is significantly reduced. These findings not only demonstrate the safety and protective efficacy of the vjbR mutant in an immunocompromised mouse model but also suggest the participation of lesser-known mechanisms in protective immunity against brucellosis.

Innate immune encounters of the (Type) 4th kind: Brucella

In humans, pathogenic Brucella species cause a febrile illness known as brucellosis. A key pathogenic trait of this group of organisms is their ability to survive in immune cells and persist in tissues of the reticuloendothelial system, a process that requires the function of a Type IV secretion system. In contrast to other well-studied Gram-negative bacteria, Brucella spp. do not cause inflammation at the site of invasion, but have a latency period of 2-4 weeks before the onset of symptoms. This review discusses several mechanisms that allow Brucella spp. both to evade detection by pattern recognition receptors of the innate immune system and suppress their signalling. In contrast to these stealth features, the VirB Type IV secretion system, which mediates survival within phagocytic cells, stimulates innate immune responses in vivo. The responses stimulated by this virulence factor are sufficient to check bacterial growth, but not to elicit sterilizing immunity. The result is a stand-off between host and pathogen that results in persistent infection.

Engagement of Toll-like receptor 2 in mouse macrophages infected with Mycobacterium avium induces non-oxidative and TNF-independent anti-mycobacterial activity

Toll-like receptor (TLR) 2 plays an important role in the immune response to mycobacterial infections, being required for optimal immunity against certain virulent Mycobacterium avium strains. Here we analyzed the role of TLR2 in the intra-macrophagic growth of M. avium, using macrophages from TLR2-deficient mice. We found that the engagement of TLR2/TLR6 and/or TLR2/TLR1 receptors induced bacteriostasis of M. avium inside bone marrow-derived macrophages in a MyD88-dependent way. Additionally, lipoproteins from the cell envelope of M. avium with a molecular mass of 20-25 kDa triggered this TLR2 pathway, leading to a decrease in the growth of the mycobacteria. Although TLR2 engagement induced the production of TNF, this cytokine as well as nitric oxide and superoxide molecules were not necessary for TLR2-mediated bacteriostasis. Finally, TLR ligation did not induce the expression of the 47-kDa guanosine triphosphatase (LRG-47) but it promoted an increased maturation of the phagosome with regards to acquisition of LAMP1. Our data show that triggering TLR2 inhibited M. avium growth by an as-yet-unknown mechanism that may involve increased phagosome maturation.

The LysR-type transcriptional regulator Hrg counteracts phagocyte oxidative burst and imparts survival advantage to Salmonella enterica serovar Typhimurium

LysR-type transcriptional regulators (LTTRs) are one of the key players that help bacteria adapt to different environments. We have designated STM0952, a putative LTTR in Salmonella enterica serovar Typhimurium (S. Typhimurium), as hydrogen peroxide resistance gene (hrg). A hrg knockout mutant of S. Typhimurium was sensitive to oxidative products of the respiratory burst, specifically to H(2)O(2). The hrg mutant is profoundly attenuated in a murine model of infection and showed decreased intracellular proliferation in macrophages. It also induced increased amounts of reactive oxygen species and co-localization with gp91phox in the macrophage cell line, when compared to the wild-type. A strain overexpressing the hrg gene showed a survival advantage over the wild-type Salmonella under H(2)O(2)-induced stress. Microarray analysis suggested the presence of an Hrg regulon, which is required for resistance to the toxic oxidative products of the reticuloendothelial system.

RNAi screen of endoplasmic reticulum-associated host factors reveals a role for IRE1alpha in supporting Brucella replication

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.

VirB type IV secretory system does not contribute to Brucella suis' avoidance of human dendritic cell maturation

Dendritic cells (DCs), which are critical components of adaptive immunity, are highly susceptible to infection with the intracellular bacteria Brucella. Infection with living Brucella prevents infected human DCs from engaging in maturation processes, thus impairing their capacity to present antigens to naïve T cells and to secrete IL-12. Recently, we have established that several attenuated mutants of Brucella (rough, omp25, bvrR) are unable to control DCs maturation and thus effectively stimulate naïve T cells, which could be the origin of the protective immunity elicited by these mutants in vivo. In this study, we investigate the interactions of a VirB-defective Brucella mutant with human DCs to determine whether its attenuation could be attributed to the induction of an adaptive immune response. We show here that in contrast to previously studied strains and similar to wild-type strains, this virB mutant was unable to trigger significant DC maturation. Together with recently published data describing infection with virB mutants in vivo, these results suggest that Brucella T4SS VirB is not involved in the control of DC maturation and does not interfere with the establishment of a T-helper type 1 adaptive immune response.

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

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

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

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

Brucella abortus uses a stealthy strategy to avoid activation of the innate immune system during the onset of infection

BACKGROUND

To unravel the strategy by which Brucella abortus establishes chronic infections, we explored its early interaction with innate immunity.

METHODOLOGY/PRINCIPAL FINDINGS

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-alpha-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.

CONCLUSION/SIGNIFICANCE

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.

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

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

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

The Brucella abortus virB operon, consisting of 11 genes, virB1 to virB11, and two putative genes, orf12 (virB12) and orf13, encodes a type IV secretion system (T4SS) that is required for intracellular replication and persistent infection in the mouse model. This study was undertaken to determine whether orf12 (virB12) encodes an essential part of the T4SS apparatus. The virB12 gene was found to encode a 17-kDa protein, which was detected in vitro in B. abortus grown to stationary phase. Mice infected with B. abortus 2308 produced an antibody response to the protein encoded by virB12, showing that this gene is expressed during infection. Expression of virB12 was not required for survival in J774 macrophages. VirB12 was also dispensable for the persistence of B. abortus, B. melitensis, and B. suis in mice up to 4 weeks after infection, since deletion mutants lacking virB12 were recovered from splenic tissue at wild-type levels. These results show that VirB12 is not essential for the persistence of the human-pathogenic Brucella spp. in the mouse and macrophage models of infection.

Interaction between Brucella abortus and cellular prion protein in lipid raft microdomains

A wide variety of pathogens employ lipid raft microdomains to infect host cells. Here, we review selected aspects of interaction between Brucella abortus and cellular prion protein, one of the lipid raft-associated molecules on the plasma membrane, when bacteria infect macrophages, and discuss the correlates of proliferation in mice.

Differential requirements for VirB1 and VirB2 during Brucella abortus infection

The Brucella abortus virB operon, encoding a type IV secretion system (T4SS), is required for intracellular replication and persistent infection in the mouse model. The products of the first two genes of the virB operon, virB1 and virB2, are predicted to be localized at the bacterial surface, where they could potentially interact with host cells. Studies to date have focused on characterization of transposon mutations in these genes, which are expected to exert polar effects on downstream genes in the operon. In order to determine whether VirB1 and VirB2 are required for the function of the T4SS apparatus, we constructed and characterized nonpolar deletion mutations of virB1 and virB2. Both mutants were shown to be nonpolar, as demonstrated by their ability to express the downstream gene virB5 during stationary phase of growth in vitro. Both VirB1 and VirB2 were essential for intracellular replication in J774 macrophages. The nonpolar virB2 mutant was unable to cause persistent infection in the mouse model, demonstrating the essential role of VirB2 in the function of the T4SS apparatus during infection. In contrast, the nonpolar virB1 mutant persisted at wild-type levels, showing that the function of VirB1 is dispensable in the mouse model of persistent infection.

Inducible nitric oxide synthase promoter polymorphism in human brucellosis

Nitric oxide (NO), produced by the inducible nitric oxide synthase (NOS2) protein, is a defence mechanism against various pathogens, including bacteria of the genus Brucella. The aim of this study was to investigate the possible association between the NOS2 gene promoter polymorphism, TAAA(n) and CCTTT(n) microsatellites, and the predisposition to human brucellosis. We performed a case-control study in 85 patients with brucellosis and 100 healthy individuals, matched for age and sex, living in the same geographic area, in whom the NOS2 promoter was genotyped by using a polymerase chain reaction (PCR)-based method combined with fluorescent technology. No statistically significant differences were observed in the distribution of TAAA(n) alleles between the groups under study. When the overall NOS2 CCTTT(n) allele distribution of the brucellosis patients was compared with that of the control subjects, a significant skewing was observed (P = 0.04, by chi(2) test from 2 x 9 contingency table). Interestingly, we observed a trend towards Brucella infection protection with the 9 repeat (181 bp) allele (1.8% patients vs. 7.5% controls; P = 0.01, odds ratios = 0.22, 95% confidence interval = 0.05-0.83), which turned out to be non-significant after applying multiple testing. We concluded that the NOS2 microsatellite polymorphism might not have a major effect on brucellosis; nevertheless, the fact that a non-significant trend towards protection was detected in the CCTTT(n) alleles may be an indication for a follow-up study.

Brucella evades macrophage killing via VirB-dependent sustained interactions with the endoplasmic reticulum

The intracellular pathogen Brucella is the causative agent of brucellosis, a worldwide zoonosis that affects mammals, including humans. Essential to Brucella virulence is its ability to survive and replicate inside host macrophages, yet the underlying mechanisms and the nature of the replicative compartment remain unclear. Here we show in a model of Brucella abortus infection of murine bone marrow-derived macrophages that a fraction of the bacteria that survive an initial macrophage killing proceed to replicate in a compartment segregated from the endocytic pathway. The maturation of the Brucella-containing vacuole involves sustained interactions and fusion with the endoplasmic reticulum (ER), which creates a replicative compartment with ER-like properties. The acquisition of ER membranes by replicating Brucella is independent of ER-Golgi COPI-dependent vesicular transport. A mutant of the VirB type IV secretion system, which is necessary for intracellular survival, was unable to sustain interactions and fuse with the ER, and was killed via eventual fusion with lysosomes. Thus, we demonstrate that live intracellular Brucella evade macrophage killing through VirB-dependent sustained interactions with the ER. Moreover, we assign an intracellular function to the VirB system, as being required for late maturation events necessary for the biogenesis of an ER-derived replicative organelle.

Cellular prion protein promotes Brucella infection into macrophages

The products of the Brucella abortus virB gene locus, which are highly similar to conjugative DNA transfer system, enable the bacterium to replicate within macrophage vacuoles. The replicative phagosome is thought to be established by the interaction of a substrate of the VirB complex with macrophages, although the substrate and its host cellular target have not yet been identified. We report here that Hsp60, a member of the GroEL family of chaperonins, of B. abortus is capable of interacting directly or indirectly with cellular prion protein (PrPC) on host cells. Aggregation of PrPC tail-like formation was observed during bacterial swimming internalization into macrophages and PrPC was selectively incorporated into macropinosomes containing B. abortus. Hsp60 reacted strongly with serum from human brucellosis patients and was exposed on the bacterial surface via a VirB complex-associated process. Under in vitro and in vivo conditions, Hsp60 of B. abortus bound to PrPC. Hsp60 of B. abortus, expressed on the surface of Lactococcus lactis, promoted the aggregation of PrPC but not PrPC tail formation on macrophages. The PrPC deficiency prevented swimming internalization and intracellular replication of B. abortus, with the result that phagosomes bearing the bacteria were targeted into the endocytic network. These results indicate that signal transduction induced by the interaction between bacterial Hsp60 and PrPC on macrophages contributes to the establishment of B. abortus infection.