Update on the role of innate immune receptors during Brucella abortus infection

The innate immune system constitutes an efficient defense mechanism against invading microbial pathogens. Recent studies have revealed the intracellular signaling cascades involved in the TLR-initiated immune response to Brucella spp. infection. However, there is a piece of the puzzle missing that is the role of non-TLR receptors in innate immunity. The involvement of TLR receptors in brucellosis has been investigated by different research groups. It was demonstrated that TLR2 clearly does not play any role in controlling Brucella abortus infection in vivo, whereas TLR9 has been shown to be required for clearance of this bacterium in infected mice. The participation of adaptor molecules, such as MyD88 and TRIF has also been discussed. Recently, we and others have reported the critical role of MyD88- and not TRIF-mediated signaling in dendritic cell maturation and in vivo resistance during B. abortus infection. However, the relationship between specific Brucella molecules and non-TLR receptors and signal transduction pathways needs to be better understood. It is now clear that the interaction between TLRs and recently identified cytosolic innate immune sensors is crucial for mounting effective immune responses. Finally, this review discusses the mechanisms used by Brucella to escape detection by the host innate immune system.

Proinflammatory caspase-2-mediated macrophage cell death induced by a rough attenuated Brucella suis strain

Brucella spp. are intracellular bacteria that cause an infectious disease called brucellosis in humans and many domestic and wildlife animals. B. suis primarily infects pigs and is pathogenic to humans. The macrophage-Brucella interaction is critical for the establishment of a chronic Brucella infection. Our studies showed that smooth virulent B. suis strain 1330 (S1330) prevented programmed cell death of infected macrophages and rough attenuated B. suis strain VTRS1 (a vaccine candidate) induced strong macrophage cell death. To further investigate the mechanism of VTRS1-induced macrophage cell death, microarrays were used to analyze temporal transcriptional responses of murine macrophage-like J774.A1 cells infected with S1330 or VTRS1. In total 17,685 probe sets were significantly regulated based on the effects of strain, time and their interactions. A miniTUBA dynamic Bayesian network analysis predicted that VTRS1-induced macrophage cell death was mediated by a proinflammatory gene (the tumor necrosis factor alpha [TNF-α] gene), an NF-κB pathway gene (the IκB-α gene), the caspase-2 gene, and several other genes. VTRS1 induced significantly higher levels of transcription of 40 proinflammatory genes than S1330. A Mann-Whitney U test confirmed the proinflammatory response in VTRS1-infected macrophages. Increased production of TNF-α and interleukin 1β (IL-1β) were also detected in the supernatants in VTRS1-infected macrophage cell culture. Hyperphosphorylation of IκB-α was observed in macrophages infected with VTRS1 but not S1330. The important roles of TNF-α and IκB-α in VTRS1-induced macrophage cell death were further confirmed by individual inhibition studies. VTRS1-induced macrophage cell death was significantly inhibited by a caspase-2 inhibitor but not a caspase-1 inhibitor. The role of caspase-2 in regulating the programmed cell death of VTRS1-infected macrophages was confirmed in another study using caspase-2-knockout mice. In summary, VTRS1 induces a proinflammatory, caspase-2- and NF-κB-mediated macrophage cell death. This unique cell death differs from apoptosis, which is not proinflammatory. It is also different from classical pyroptosis, which is caspase-1 mediated.

Toll-like receptor 9 is required for full host resistance to Mycobacterium avium infection but plays no role in induction of Th1 responses

To investigate the role of Toll-like receptor 9 (TLR9) in innate immunity to Mycobacterium avium, TLR9, TLR2, and MyD88 knockout (KO) mice were infected with this bacterium. Bacterial burdens were higher in the spleens, livers, and lungs of infected TLR9 KO mice than in those of C57BL/6 mice, indicating that TLR9 is required for efficient control of M. avium infection. However, TLR9 KO or TLR2 KO spleen cells displayed normal M. avium-induced tumor necrosis factor alpha (TNF-α) and gamma interferon (IFN-γ) responses. This finding was confirmed by determining the number of splenic CD4(+) T cells producing IFN-γ by flow cytometry. Furthermore, TLR2 and MyD88, but not TLR9, played a major role in interleukin-12 and TNF-α production by M. avium-infected macrophages and dendritic cells (DCs). We also found that major histocompatibility complex class II molecule expression on DCs is regulated by TLR2 and MyD88 signaling but not by TLR9. Finally, lack of TLR9, TLR2, or MyD88 reduced the numbers of macrophages, epithelioid cells, and lymphocytes in M. avium-induced granulomas but only MyD88 deficiency affected the number of liver granulomas. In summary, our data demonstrated that the involvement of TLR9 in the control of M. avium infection is not related to the induction of Th1 responses.

RNA interference for improving the outcome of islet transplantation

Islet transplantation has the potential to cure type 1 diabetes. Despite recent therapeutic success, it is still not common because a large number of transplanted islets get damaged by multiple challenges including instant blood mediated inflammatory reaction, hypoxia/reperfusion injury, inflammatory cytokines, and immune rejection. RNA interference (RNAi) is a novel strategy to selectively degrade target mRNA. The use of RNAi technologies to downregulate the expression of harmful genes has the potential to improve the outcome of islet transplantation. The aim of this review is to gain a thorough understanding of biological obstacles to islet transplantation and discuss how to overcome these barriers using different RNAi technologies. This eventually will help improve islet survival and function post transplantation. Chemically synthesized small interferring RNA (siRNA), vector based short hairpin RNA (shRNA), and their critical design elements (such as sequences, promoters, and backbone) are discussed. The application of combinatorial RNAi in islet transplantation is also discussed. Last but not the least, several delivery strategies for enhanced gene silencing are discussed, including chemical modification of siRNA, complex formation, bioconjugation, and viral vectors.

Inhibition of dynamin-dependent endocytosis interferes with type III IFN expression in bacteria-infected human monocyte-derived DCs

Type I IFNs (IFN-α/βs) and type III IFNs (IFN-λ1-3) play an important role in host defense against viral infections. The induction of type I IFNs has recently been found to take place also in bacterial infections, and therefore, this study focuses on analyzing the regulation of type III IFNs in response to bacterial stimulation. We found by quantitative RT-PCR that the expression of IFN-λ1 and IFN-λ2/3 mRNAs, as well as that of IFN-β, was similarly up-regulated in response to stimulation with live Salmonella typhimurium or TLR4 agonist LPS in human moDCs. The induction of IFN-λ mRNAs did not require ongoing protein synthesis, and only IFN-λ1 was detected at the protein level. The induction of IFN-λ mRNAs was sensitive to SB202190, Ly294002, and PDTC, which inhibit p38 MAPK, PI3K, and NF-κB activation, respectively. Furthermore, we observed that blocking dynamin-dependent endocytosis pathways with dynasore led to decreased cell surface expression of CD86 and HLA class II molecules and reduced production of IFN-λ1, CXCL10, and IL-6 when the cells were infected with S. typhimurium. Cytokine production was also impaired in dynasore-treated, Streptococcus thermophilus-stimulated cells. Further, inhibition of dynamin prevented S. typhimurium-induced phosphorylation of IRF3 and the internalization of the bacteria. In summary, induction of type III IFNs in bacteria-infected human moDCs requires multiple signaling pathways and involves bacterial phagocytosis.

Heat-killed and γ-irradiated Brucella strain RB51 stimulates enhanced dendritic cell activation, but not function compared with the virulent smooth strain 2308

Brucella spp. are Gram-negative, facultative intracellular bacterial pathogens that cause abortion in livestock and undulant fever in humans worldwide. Brucella abortus strain 2308 is a pathogenic strain that affects cattle and humans. Currently, there are no efficacious human vaccines available. However, B. abortus strain RB51, which is approved by the USDA, is a live-attenuated rough vaccine against bovine brucellosis. Live strain RB51 induces protection via CD4(+) and CD8(+) T-cell-mediated immunity. To generate an optimal T-cell response, strong innate immune responses by dendritic cells (DCs) are crucial. Because of safety concerns, the use of live vaccine strain RB51 in humans is limited. Therefore, in this study, we analyzed the differential ability of the same doses of live, heat-killed (HK) and γ-irradiated (IR) strain RB51 in inducing DC activation and function. Smooth strain 2308, live strain RB51 and lipopolysaccharide were used as controls. Studies using mouse bone marrow-derived DCs revealed that, irrespective of viability, strain RB51 induced greater DC activation than smooth strain 2308. Live strain RB51 induced significantly (P≤0.05) higher DC maturation than HK and IR strains, and only live strain RB51-infected DCs (at multiplicity of infection 1:100) induced significant (P≤0.05) tumor necrosis factor-α and interleukin-12 secretion.

Well-controlled proinflammatory cytokine responses of Peyer's patch cells to probiotic Lactobacillus casei

SUMMARY

In order to clarify the probiotic features of immunomodulation, cytokine production by murine spleen and Peyer's patch (PP) cells was examined in response to probiotic and pathogenic bacteria. In spleen cells, probiotic Lactobacillus casei induced interleukin (IL)-12 production by CD11b(+) cells more strongly than pathogenic Gram-positive and Gram-negative bacteria and effectively promoted the development of T helper (Th) type 1 cells followed by high levels of secretion of interferon (IFN)-gamma. Although the levels of IL-12 secreted by PP cells in response to L. casei were lower in comparison with spleen cells, Th1 cells developed as a result of this low-level induction of IL-12. However, IFN-gamma secretion by the L. casei-induced Th1 cells stimulated with a specific antigen was down-regulated in PP cells. Development of IL-17-producing Th17 cells was efficiently induced in PP cells by antigen stimulation. Lactobacillus casei slightly, but significantly, inhibited the antigen-induced secretion of IL-17 without a decrease in the proportion of Th17 cells. No bacteria tested induced the development of IL-10-producing, transforming growth factor-beta-producing or Foxp3-expressing regulatory T cells, thus suggesting that certain probiotics might regulate proinflammatory responses through as yet unidentified mechanisms in PP cells. These data show probiotic L. casei to have considerable potential to induce IL-12 production and promote Th1 cell development, but the secretion of proinflammatory cytokines such as IL-12 and IL-17 may be well controlled in PP cells.

Inactivation of formyltransferase (wbkC) gene generates a Brucella abortus rough strain that is attenuated in macrophages and in mice

Rough mutants of Brucella abortus were generated by disruption of wbkC gene which encodes the formyltransferase enzyme involved in LPS biosynthesis. In bone marrow-derived macrophages the B. abortusDeltawbkC mutants were attenuated, could not reach a replicative niche and induced higher levels of IL-12 and TNF-alpha when compared to parental smooth strains. Additionally, mutants exhibited attenuation in vivo in C57BL/6 and interferon regulatory factor-1 knockout mice. DeltawbkC mutant strains induced lower protective immunity in C56BL/6 than smooth vaccine S19 but similar to rough vaccine RB51. Finally, we demonstrated that Brucella wbkC is critical for LPS biosynthesis and full bacterial virulence.

Live Brucella abortus rough vaccine strain RB51 stimulates enhanced innate immune response in vitro compared to rough vaccine strain RB51SOD and virulent smooth strain 2308 in murine bone marrow-derived dendritic cells

Brucella spp. are Gram-negative, coccobacillary, facultative intracellular pathogens. B. abortus strain 2308 is a pathogenic strain affecting cattle and humans. Rough B. abortus strain RB51, which lacks the O-side chain of lipopolysaccharide (LPS), is the live attenuated USDA approved vaccine for cattle in the United States. Strain RB51SOD, which overexpresses Cu-Zn superoxide dismutase (SOD), has been shown to confer better protection than strain RB51 in a murine model. Protection against brucellosis is mediated by a strong CD4+ Th(1) and CD8+ Tc(1) adaptive immune response. In order to stimulate a robust adaptive response, a solid innate immune response, including that mediated by dendritic cells, is essential. As dendritic cells (DCs) are highly susceptible to Brucella infection, it is possible that pathogenic strains could limit the innate and thereby adaptive immune response. By contrast, vaccine strains could limit or bolster the innate and subsequent adaptive immune response. Identifying how Brucella vaccines stimulate innate and adaptive immunity is critical for enhancing vaccine efficacy. The ability of rough vaccine strains RB51 and RB51SOD to stimulate DC function has not been characterized. We report that live rough vaccine strain RB51 induced significantly better (p ≤ 0.05) DC maturation and function compared to either strain RB51SOD or smooth virulent strain 2308, based on costimulatory marker expression and cytokine production.

Toll-like receptor 2 and class B scavenger receptor type I are required for bacterial uptake by trophoblast giant cells

Trophoblast giant (TG) cells, components cells of the mouse placenta, exhibit phagocytic activity, and participate in the placental defense system by extracellular bacterial antigen uptake via phagocytosis. However, the bacterial uptake mechanisms by TG cells remain to be entirely understood. In an attempt to understand these mechanisms, in this study, we investigated the pattern recognition receptors (PRRs) involved in phagocytosis by TG cells. PRRs such as Toll-like receptors (TLRs) and scavenger receptors play a critical role in the immune response to bacterial pathogens. Among these, we selected TLR2 and class B scavenger receptor type I (SR-BI) and then evaluated their properties in TG cells. TLR2 and SR-BI expression is higher in TG cells than in trophoblast stem (TS) cells. Although interferon-gamma treatment activated bacterial uptake in a concentration-dependent manner, it did not induce TLR2 or SR-BI expression. Depletion of TLR2 and SR-BI by siRNA reduced the bacterial uptake ability of TG cells, which was also affected by treatment with the TLR2 agonist triacylated lipopeptide. These results suggested that the phagocytic activity of TG cells is mediated by both TLR2 and SR-BI.

The protein moiety of Brucella abortus outer membrane protein 16 is a new bacterial pathogen-associated molecular pattern that activates dendritic cells in vivo, induces a Th1 immune response, and is a promising self-adjuvanting vaccine against systemic and oral acquired brucellosis

Knowing the inherent stimulatory properties of the lipid moiety of bacterial lipoproteins, we first hypothesized that Brucella abortus outer membrane protein (Omp)16 lipoprotein would be able to elicit a protective immune response without the need of external adjuvants. In this study, we demonstrate that Omp16 administered by the i.p. route confers significant protection against B. abortus infection and that the protective response evoked is independent of the protein lipidation. To date, Omp16 is the first Brucella protein that without the requirement of external adjuvants is able to induce similar protection levels to the control live vaccine S19. Moreover, the protein portion of Omp16 (unlipidated Omp16 [U-Omp16]) elicits a protective response when administered by the oral route. Either systemic or oral immunization with U-Omp16 elicits a Th1-specific response. These abilities of U-Omp16 indicate that it is endowed with self-adjuvanting properties. The adjuvanticity of U-Omp16 could be explained, at least in part, by its capacity to activate dendritic cells in vivo. U-Omp16 is also able to stimulate dendritic cells and macrophages in vitro. The latter property and its ability to induce a protective Th1 immune response against B. abortus infection have been found to be TLR4 dependent. The facts that U-Omp16 is an oral protective Ag and possesses a mucosal self-adjuvanting property led us to develop a plant-made vaccine expressing U-Omp16. Our results indicate that plant-expressed recombinant U-Omp16 is able to confer protective immunity, when given orally, indicating that a plant-based oral vaccine expressing U-Omp16 could be a valuable approach to controlling this disease.

Human brucellosis is characterized by an intense Th1 profile associated with a defective monocyte function

In animal models, a defective Th1 response appears to be critical in the pathogenesis of brucellosis, but the Th1 response in human brucellosis patients remains partially undefined. Peripheral blood from 24 brucellosis patients was studied before and 45 days after antibiotherapy. Twenty-four sex- and age-matched healthy donors were analyzed in parallel. Significantly increased levels of interleukin 1beta (IL-1beta), IL-2, IL-4, IL-6, IL-12p40, gamma interferon (IFN-gamma), and tumor necrosis factor alpha (TNF-alpha), but not of IL-10, in serum and/or significantly increased percentages of samples with detectable levels of these cytokines, measured by enzyme-linked immunosorbent assays (ELISA), were found for untreated brucellosis patients, but these levels were reduced and/or normalized after treatment. Flow cytometry studies showed that the intracytoplasmic expression of IFN-gamma, IL-2, and TNF-alpha, but not that of IL-4, by phorbol myristate-activated CD4(+) CD3(+) and CD8(+) CD3(+) T lymphocytes was significantly increased in untreated brucellosis patients and was also partially normalized after antibiotherapy. The percentage of phagocytic cells, the mean phagocytic activity per cell, and the phagocytic indices for monocytes at baseline were defective and had only partially reverted at follow-up. T lymphocytes from untreated brucellosis patients are activated in vivo and show Th1 cytokine production polarization, with strikingly high serum IFN-gamma levels. In spite of this Th1 environment, we found deficient effector phagocytic activity in peripheral blood monocytes.

The Brucella abortus phosphoglycerate kinase mutant is highly attenuated and induces protection superior to that of vaccine strain 19 in immunocompromised and immunocompetent mice

Brucella abortus is a facultative intracellular bacterial pathogen that causes abortion in domestic animals and undulant fever in humans. The mechanism of virulence of Brucella spp. is not yet fully understood. Therefore, it is crucial to identify new molecules that can function as virulence factors to better understand the host-pathogen interplay. Herein, we identified the gene encoding the phosphoglycerate kinase (PGK) of B. abortus strain 2308. To test the role of PGK in Brucella pathogenesis, a pgk deletion mutant was constructed. Replacement of the wild-type pgk by recombination was demonstrated by Southern and Western blot analyses. The B. abortus Delta pgk mutant strain exhibited extreme attenuation in bone marrow-derived macrophages and in vivo in BALB/c, C57BL/6, 129/Sv, and interferon regulatory factor-1 knockout (IRF-1 KO) mice. Additionally, at 24 h postinfection the Delta pgk mutant was not found within the same endoplasmic reticulum-derived compartment as the wild-type bacteria, but, instead, over 60% of Brucella-containing vacuoles (BCVs) retained the late endosomal/lysosomal marker LAMP1. Furthermore, the B. abortus Delta pgk deletion mutant was used as a live vaccine. Challenge experiments revealed that the Delta pgk mutant strain induced protective immunity in 129/Sv or IRF-1 KO mice that was superior to the protection conferred by commercial strain 19 or RB51. Finally, the results shown here demonstrated that Brucella PGK is critical for full bacterial virulence and that a Delta pgk mutant may serve as a potential vaccine candidate in future studies.

Mice deficient in MyD88 Develop a Th2-dominant response and severe pathology in the upper genital tract following Chlamydia muridarum infection

MyD88, a key adaptor molecule required for many innate immunity receptor-activated signaling pathways, was evaluated in a Chlamydia muridarum urogenital tract infection model. Compared with wild-type mice, MyD88 knockout (KO) mice failed to produce significant levels of inflammatory cytokines in the genital tract during the first week of chlamydial infection. MyD88 KO mice developed a Th2-dominant whereas wild-type mice developed a Th1/Th17-dominant immune response after chlamydial infection. Despite the insufficient production of early inflammatory cytokines and lack of Th1/Th17-dominant adaptive immunity, MyD88 KO mice appeared to be as resistant to chlamydial intravaginal infection as wild-type mice based on the number of live organisms recovered from vaginal samples. However, significantly high numbers of chlamydial organisms were detected in the upper genital tract tissues of MyD88 KO mice. Consequently, MyD88 KO mice developed more severe pathology in the upper genital tract. These results together have demonstrated that MyD88-dependent signaling pathway is not only required for inflammatory cytokine production in the early phase of host response to chlamydial infection but also plays a critical role in the development of Th1/Th17 adaptive immunity, both of which may be essential for limiting ascending infection and reducing pathology of the upper genital tract by chlamydial organisms.

Brucella abortus induces Irgm3 and Irga6 expression via type-I IFN by a MyD88-dependent pathway, without the requirement of TLR2, TLR4, TLR5 and TLR9

The innate immune system senses bacterial pathogens by pattern recognition receptors, such as the well-characterised Toll-like Receptors (TLR). The activation of TLR signalling cascades depends on several adaptor proteins, among which MyD88 plays a key role in triggering innate immune responses. Here, we show in murine macrophages that Brucella abortus triggers expression of the interferon-inducible resistance proteins (IRGs, p47 GTPases) via type-I IFN secretion at late time points, when Brucella has reached its replication niche. This induction requires the adaptor molecule MyD88 but does not involve the TLRs normally implicated in sensing Gram-negative bacteria, namely TLR2, TLR4, TLR5 and TLR9. Brucella mutants lacking the functional VirB type-IV secretion system were not capable of inducing Irgm3 and Irga6 expression, suggesting that the type-IV secretion system is part of the triggering of the activation process. Our data suggest that Brucella is recognized intracellularly by an unknown receptor, different from the conventional ones used for Gram-negative sensing, but one that nevertheless signals through MyD88.

Effects of Brucella abortus biotype 1 infection on the reproductive performance of Sprague-Dawley rats

The aim of this study was to assess the impact of Brucella abortus biotype 1 infection on the reproductive performance using Sprague-Dawley (SD) rat model. Virgin female SD rats (n = 24) were infected intraperitoneally with 0.1 mL of saline containing 1 x 10(11) Colony Forming Unit (CFU) of pathogenic B. abortus biotype 1 Korean bovine isolate. Control rats (n = 24) were inoculated with 0.1 mL of apyrogenic saline. Both inoculated and control rats were divided into six subgroups. Four rats in each subgroup were consistently bred at 3, 7, 14, 21, 28 and 60 days after infection. B. abortus infection induced 41.67% infertility in the infected rats. The mean number of offspring/litter was 8.71 +/- 2.01 for infected rats and 12.87 +/- 1.42 for control rats (p < 0.001). The mean weight of the viable offspring was 6.10 +/- 0.36 g for infected rats and 7.15 +/- 0.40 g for control rats (p < 0.001). The rate of stillbirth was 12.30% in the infected rats. B. abortus biotype 1 was isolated from the uteri of the infected rats. The data of this study indicate that B. abortus biotype 1 infections in SD rat model affect reproduction adversely by causing infertility, stillbirth and loss of number and weight of offspring.

A MyD88-dependent early IL-17 production protects mice against airway infection with the obligate intracellular pathogen Chlamydia muridarum

We found that IL-17, a signature cytokine of Th17, was produced early in the innate immunity phase after an intranasal infection with the obligate intracellular pathogen Chlamydia muridarum. The airway IL-17, which peaked at 48 h after infection, was dependent on live chlamydial organism replication and MyD88-mediated signaling pathways. Treatment with antibiotics or knockout of the MyD88 gene, but not Toll/IL receptor domain-containing adapter-inducing IFN-beta, can block the early IL-17 production. Treatment of mice with an anti-IL-17-neutralizing mAb enhanced growth of chlamydial organisms in the lung, dissemination to other organs, and decreased mouse survival, whereas treatment with an isotype-matched control IgG had no effect. Although IL-17 did not directly affect chlamydial growth in cell culture, it enhanced the production of other inflammatory cytokines and chemokines by Chlamydia-infected cells and promoted neutrophil infiltration in mouse airways during chlamydial infection, which may contribute to the antichlamydial effect of IL-17. These observations suggest that an early IL-17 response as an innate immunity component plays an important role in initiating host defense against infection with intracellular bacterial pathogens in the airway.

Distinct roles for MyD88 and Toll-like receptor 2 during Leishmania braziliensis infection in mice

We have previously reported that Leishmania braziliensis infection can activate murine dendritic cells (DCs) and upregulate signaling pathways that are essential for the initiation of innate immunity. However, it remains unclear whether Toll-like receptors (TLRs) are involved in L. braziliensis-mediated DC activation. To address this issue, we generated bone marrow-derived DCs from MyD88(-/-) and TLR2(-/-) mice and examined their responsiveness to parasite infection. While wild-type DCs were efficiently activated to produce cytokines and prime naïve CD4(+) T cells, L. braziliensis-infected MyD88(-/-) DCs exhibited less activation and decreased production of interleukin-12 (IL-12) p40. Furthermore, MyD88(-/-) mice were more susceptible to infection in that they developed larger and prolonged lesions compared to those in control mice. In sharp contrast, the lack of TLR2 resulted in an enhanced DC activation and increased IL-12 p40 production after infection. As such, L. braziliensis-infected TLR2(-/-) DCs were more competent in priming naïve CD4(+) T cells in vitro than were their controls, findings which correlated with an increased gamma interferon production in vivo and enhanced resistance to infection. Our results suggest that while MyD88 is indispensable for the generation of protective immunity to L. braziliensis, TLR2 seems to have a regulatory role during infection.

Live Borrelia burgdorferi spirochetes elicit inflammatory mediators from human monocytes via the Toll-like receptor signaling pathway

We investigated the mechanisms that lead to the production of proinflammatory mediators by human monocytes when these cells are exposed in vitro to live Borrelia burgdorferi spirochetes. We first focused on myeloid differentiation primary response protein 88 (MyD88), an adapter molecule that is essential in the Toll-like receptor (TLR) pathway. Real-time PCR, flow cytometry, and confocal microscopy experiments revealed that MyD88 was maximally expressed in THP-1 cells after 24-h stimulation of these cells with live B. burgdorferi. Silencing of the MYD88 gene by using small interfering RNA resulted in 24%, 35%, and 84% down-modulation of the production of tumor necrosis factor alpha (TNF-alpha), interleukin-8 (IL-8), and IL-6, respectively, in THP-1 cells stimulated with live B. burgdorferi. Specific silencing of the TLR1, TLR2, or TLR5 gene by RNA interference further revealed that silencing of the TLR1 and TLR2 genes alone or combined, but not the TLR5 gene, caused a downregulation of IL-6, IL-8, and TNF-alpha in live B. burgdorferi-stimulated THP-1 cells. Overall, similar results were obtained for THP-1 cells stimulated with purified lipoproteins. Our results indicate that the TLR pathway mediates, at least in part, the release of inflammatory mediators in human monocytes stimulated with live B. burgdorferi spirochetes and furthermore suggest that the TLR-dependent interaction between these cells and live spirochetes is mediated by spirochetal lipoproteins but not by flagellin.

TLR adaptor MyD88 is essential for pathogen control during oral toxoplasma gondii infection but not adaptive immunity induced by a vaccine strain of the parasite

TLR adaptor MyD88 activation is important in host resistance to Toxoplasma gondii during i.p. infection, but the function of this signaling pathway during oral infection, in which mucosal immunity assumes a predominant role, has not been examined. In this study, we show that MyD88(-/-) mice fail to control the parasite and succumb within 2 wk of oral infection. Early during infection, T cell IFN-gamma production, recruitment of neutrophils and induction of p47 GTPase IGTP (Irgm3) in the intestinal mucosa were dependent upon functional MyD88. Unexpectedly, these responses were MyD88-independent later during acute infection. In particular, CD4(+) T cell IFN-gamma reached normal levels independently of MyD88, despite continued absence of IL-12 in these animals. The i.p. vaccination of MyD88(-/-) mice with an avirulent T. gondii uracil auxotroph elicited robust IFN-gamma responses and protective immunity to challenge with a high virulence T. gondii strain. Our results demonstrate that MyD88 is required to control Toxoplasma infection, but that the parasite can trigger adaptive immunity without the need for this TLR adaptor molecule.

Brucella lipoproteins mimic dendritic cell maturation induced by Brucella abortus

Infection with Brucella abortus induces a pro-inflammatory response that drives T cell responses toward a Th1 profile. The mechanism by which this bacterium triggers this response is unknown. Dendritic cells (DC) are crucial mediators at the host-pathogen interface and are potent Th1-inducing antigen-presenting cells. Thus, we examined the mechanism whereby B. abortus stimulate human DC maturation. B. abortus-infected DC increased the expression of CD86, CD80, CCR7, CD83, MHCII, MHCI and CD40 and induced the production of TNF-alpha, IL-6, IL-10 and IL-12. Both phenomena were not dependent on bacterial viability since they were also induced by heat-killed B. abortus (HKBA). B. abortus LPS was unable to induce markers up-regulation or cytokine production. We next investigated the capacity of the outer membrane protein 19 (Omp19) as a B. abortus lipoprotein model to induce DC maturation. Lipidated Omp19 (L-Omp19), but not its unlipidated form, increased the expression of cell surface markers and the secretion of cytokines. L-Omp19-matured DC also have decreased endocytic activity and displayed enhanced T cell stimulatory activity in a MLR. Pre-incubation of DC with anti-TLR2 mAb blocked L-Omp19-mediated cytokine production. These results demonstrate that B. abortus lipoproteins can stimulate DC maturation providing a mechanism by which these bacteria generate a Th1-type immune response.

The role of innate immune receptors in the control of Brucella abortus infection: toll-like receptors and beyond

Research into intracellular sensing of microbial products is an up and coming field in innate immunity. Toll-like receptors (TLRs) recognize Brucella spp. and bacterial components and initiate mononuclear phagocyte responses that influence both innate and adaptive immunity. Recent studies have revealed the intracellular signaling cascades involved in the TLR-initiated immune response to Brucella infection. TLR2, TLR4 and TLR9 have been implicated in host interactions with Brucella; however, TLR9 has the most prominent role. Further, the relationship between specific Brucella molecules and various signal transduction pathways needs to be better understood. MyD88-dependent and TRIF-independent signaling pathways are involved in Brucella activation of innate immune cells through TLRs. We have recently reported the critical role of MyD88 molecule in dendritic cell maturation and interleukin-12 production during B. abortus infection. This article discusses recent studies on TLR signaling and also highlights the contribution of NOD and type I IFN receptors during Brucella infection. The better understanding of the role by such innate immune receptors in bacterial infection is critical in host-pathogen interactions.