Bacterial lipoprotein induces resistance to Gram-negative sepsis in TLR4-deficient mice via enhanced bacterial clearance

TLRs are highly conserved pathogen recognition receptors. As a result, TLR4-deficient C3H/HeJ mice are highly susceptible to Gram-negative sepsis. We have previously demonstrated that tolerance induced by bacterial lipoprotein (BLP) protects wild-type mice against polymicrobial sepsis-induced lethality. In this study, we assessed whether pretreatment of C3H/HeJ mice with BLP could induce resistance to a subsequent Gram-negative Salmonella typhimurium infection. Pretreatment with BLP resulted in a significant survival benefit in TLR4-deficient C3H/HeJ mice (p < 0.0002 vs control C3H/HeJ) after challenge with live S. typhimurium (0.25 x 10(6) CFU/mouse). This survival benefit was associated with enhanced bacterial clearance from the circulation and in the visceral organs (p < 0.05 vs control C3H/HeJ). Furthermore, pretreatment with BLP resulted in significant increases in complement receptor type 3 (CR3) and FcgammaIII/IIR expression on polymorphonuclear neutrophils (PMNs) and macrophages (p < 0.05 vs control C3H/HeJ). There was impaired bacterial recognition and phagocytosis in TLR4-deficient mice compared with wild-type mice. However, a significant augmented uptake, ingestion, and intracellular killing of S. typhimurium by PMNs and peritoneal macrophages was evident in BLP-pretreated C3H/HeJ mice (p < 0.05 vs control C3H/HeJ). An up-regulation of inducible NO synthase and increased production of intracellular NO were observed in peritoneal macrophages from BLP-pretreated C3H/HeJ mice (p < 0.05 vs control C3H/HeJ). Depletion of PMNs did not diminish the beneficial effects of BLP with regard to both animal survival and bacterial clearance. These results indicate that BLP, a TLR2 ligand, protects highly susceptible TLR4-deficient mice from Gram-negative sepsis via enhanced bacterial clearance.     

MyD88 is pivotal for the early inflammatory response and subsequent bacterial clearance and survival in a mouse model of Chlamydia pneumoniae pneumonia

Chlamydia pneumoniae is the causative agent of respiratory tract infections and a number of chronic diseases. Here we investigated the involvement of the common TLR adaptor molecule MyD88 in host responses to C. pneumoniae-induced pneumonia in mice. MyD88-deficient mice were severely impaired in their ability to mount an acute early inflammatory response toward C. pneumoniae. Although the bacterial burden in the lungs was comparable 5 days after infection, MyD88-deficient mice exhibited only minor signs of pneumonia and reduced expression of inflammatory mediators. MyD88-deficient mice were unable to up-regulate proinflammatory cytokines and chemokines, demonstrated delayed recruitment of CD8+ and CD4+ T cells to the lungs, and were unable to clear the pathogen from their lungs at day 14. At day 14 the MyD88-deficent mice developed a severe, chronic lung inflammation with elevated IL-1beta and IFN-gamma leading to increased mortality, whereas wild-type mice as well as TLR2- or TLR4-deficient mice recovered from acute pneumonia and did not show delayed bacterial clearance. Thus, MyD88 is essential to recognize C. pneumoniae infection and initiate a prompt and effective immune host response against this organism leading to clearance of bacteria from infected lungs.     

Integrin beta 1 regulates phagosome maturation in macrophages through Rac expression

Phagocytosis and subsequent phagosome maturation by professional phagocytes are essential in the clearance of infectious microbial pathogens. The molecular regulation of phagosome maturation is largely unknown. We show that integrin beta(1) plays critical roles in the phagocytosis of microbial pathogens and phagosome maturation. Macrophages lacking integrin beta(1) expression exhibit reduced phagocytosis of bacteria, including group B streptococcus and Staphylococcus aureus. Furthermore, phagosomes from macrophages lacking integrin beta(1) show lowered maturation rate, defective acquisition of lysosome membrane markers, and reduced F-actin accumulation in the periphagosomal region. Integrin beta(1)-deficient macrophages exhibit impaired bactericidal activity. We found that the expression of the Rho family GTPases Rac1, Rac2, and Cdc42 was reduced in integrin beta(1)-deficient macrophages. Ectopic expression of Rac1, but not Cdc42, in integrin beta(1)-deficient macrophages restored defective phagosome maturation and F-actin accumulation in the periphagosomal region. Importantly, macrophages lacking Rac1/2 also exhibit defective maturation of phagosomes derived from opsonized Escherichia coli or IgG beads. Taken together, these results suggest that integrin beta(1) regulates phagosome maturation in macrophages through Rac expression.     

MyD88/IL-18-dependent pathways rather than TLRs control early parasitaemia in non-lethal Plasmodium yoelii infection

Plasmodium falciparum GPI contributes to malaria pathology by inducing cytokine release. It has been shown to be recognized through TLR2 and to a lesser extent TLR4 in vitro. However, previous findings on the role of TLRs in parasite clearance or pathology in vivo are conflicting. Thus, we analyzed the impact of TLR-signalling on protection using the P. yoelii infection model. Deficiency of single TLRs as well as triple TLR2/4/9-deficiency had no impact on parasitaemia. In contrast, mice deficient for the adaptor protein MyD88 were more susceptible to P. yoelii infection in that they exhibited an increased parasitaemia in the early phase of the infection and a higher lethality. This phenotype was caused mainly by impaired IL-18 signalling since parasitaemia in IL-18-deficient mice was also increased at early time points during P. yoelii infection compared to wild-type control mice. However, no lethality was observed in IL-18-deficient mice. Since parasitaemia in IL-1R-deficient mice was also slightly increased during P. yoelii infection, impaired IL-1R signalling contributed to the increased susceptibility of MyD88-deficient mice to a lesser extent. These findings correlated with a reduced IFN-gamma production in MyD88- and IL-18-deficient mice, but not in TLR2/4/9-deficient mice. We conclude that mainly IL-18/MyD88-dependent signalling but not TLR2/4/9-signalling is important for early parasite control in our model.     

Involvement of toll-like receptor 4 in innate immunity to respiratory syncytial virus

The mammalian Toll-like receptor 4, TLR4, is an important component in the innate immune response to gram-negative bacterial infection. The role of TLR4 in antiviral immunity has been largely unexplored. In this study, the in vivo immune responses to respiratory syncytial virus (RSV) and influenza virus infection were examined in TLR4-deficient (C57BL/10ScNCr) and TLR4-expressing (C57BL/10Sn) mice. TLR4-deficient mice challenged with RSV, but not influenza virus, exhibited impaired natural killer (NK) cell and CD14(+) cell pulmonary trafficking, deficient NK cell function, impaired interleukin-12 expression, and impaired virus clearance compared to mice expressing TLR4. These findings suggest that Toll signaling pathways have an important role in innate immunity to RSV.     

Cutting edge: myeloid differentiation factor 88 deficiency improves resistance against sepsis caused by polymicrobial infection

Toll-like receptors (TLRs) are important for the activation of innate immune cells upon encounter of microbial pathogens. The present study investigated the potential roles of TLR2, TLR4, and the signaling protein myeloid differentiation factor 88 (MyD88) in polymicrobial septic peritonitis. Whereas both TLR2 and TLR4 were dispensable for host defense against septic peritonitis, MyD88-deficient mice were protected in this infection model. Recruitment of neutrophils to the septic focus and bacterial clearance were normal in MyD88-deficient mice. In contrast, the systemic inflammatory response was strongly attenuated in the absence of MyD88. Surprisingly, MyD88 deficiency did not alter cytokine and chemokine production in spleen, but markedly reduced the inflammatory response in liver and lung. Production of monocyte chemoattractant protein-1 and macrophage-inflammatory protein-1alpha was entirely independent of MyD88. These results imply a central role of MyD88 for the systemic immune pathology of polymicrobial sepsis and show that cytokine production in spleen and induction of certain chemokines are MyD88 independent.     

Enhanced virulence of Chlamydia muridarum respiratory infections in the absence of TLR2 activation

Chlamydia trachomatis is a common sexually transmitted pathogen and is associated with infant pneumonia. Data from the female mouse model of genital tract chlamydia infection suggests a requirement for TLR2-dependent signaling in the induction of inflammation and oviduct pathology. We hypothesized that the role of TLR2 in moderating mucosal inflammation is site specific. In order to investigate this, we infected mice via the intranasal route with C. muridarum and observed that in the absence of TLR2 activation, mice had more severe disease, higher lung cytokine levels, and an exaggerated influx of neutrophils and T-cells into the lungs. This could not be explained by impaired bacterial clearance as TLR2-deficient mice cleared the infection similar to controls. These data suggest that TLR2 has an anti-inflammatory function in the lung during Chlamydia infection, and that the role of TLR2 in mucosal inflammation varies at different mucosal surfaces.     

KIM-1-/TIM-1-mediated phagocytosis links ATG5-/ULK1-dependent clearance of apoptotic cells to antigen presentation

Phagocytosis of apoptotic cells by both professional and semi-professional phagocytes is required for resolution of organ damage and maintenance of immune tolerance. KIM-1/TIM-1 is a phosphatidylserine receptor that is expressed on epithelial cells and can transform the cells into phagocytes. Here, we demonstrate that KIM-1 phosphorylation and association with p85 results in encapsulation of phagosomes by lipidated LC3 in multi-membrane organelles. KIM-1-mediated phagocytosis is not associated with increased ROS production, and NOX inhibition does not block LC3 lipidation. Autophagy gene expression is required for efficient clearance of apoptotic cells and phagosome maturation. KIM-1-mediated phagocytosis leads to pro-tolerogenic antigen presentation, which suppresses CD4 T-cell proliferation and increases the percentage of regulatory T cells in an autophagy gene-dependent manner. Taken together, these data reveal a novel mechanism of epithelial biology linking phagocytosis, autophagy and antigen presentation to regulation of the inflammatory response.     

Redundant Toll-like receptor signaling in the pulmonary host response to Pseudomonas aeruginosa

Activation of pulmonary defenses against Pseudomonas aeruginosa requires myeloid differentiation factor 88 (MyD88), an adaptor for Toll-like receptor (TLR) signaling. To determine which TLRs mediate recognition of P. aeruginosa, we measured cytokine responses of bone marrow cells from wild-type mice and mice lacking TLR2 (TLR2(-/-)), TLR4 (TLR4(-/-)), TLR2 and TLR4 (TLR2/4(-/-)), or MyD88 (MyD88(-/-)) to wild-type P. aeruginosa and to fliC P. aeruginosa, which lacks the TLR5 ligand flagellin. Mice also were challenged with aerosolized bacteria to determine cytokine responses, lung inflammation, and bacterial clearance. TNF induction required MyD88 and was absent in TLR2/4(-/-) cells in response to fliC but not wild-type P. aeruginosa, whereas TLR2(-/-) cells exhibited augmented responses. In vivo, TLR4(-/-) mice responded to wild-type P. aeruginosa with reduced cytokine production and inflammation, but intact bacterial clearance, while TLR2(-/-) mice had partially impaired cytokine responses and delayed bacterial killing despite normal inflammation. When challenged with fliC, MyD88(-/-) mice failed to mount early cytokine and inflammatory responses or control bacterial replication, resulting in necrotizing lung injury and lethal disseminated infection. TLR4(-/-) and TLR2/4(-/-) mice responded to fliC infection with severely limited inflammatory and cytokine responses but intact bacterial clearance. TLR2(-/-) mice had partially reduced cytokine responses but augmented inflammation and preserved bacterial killing. These data indicate that TLR4- and flagellin-induced signals mediate most of the acute inflammatory response to Pseudomonas and that TLR2 has a counterregulatory role. However, MyD88-dependent pathways, in addition to those downstream of TLR2, TLR4, and TLR5, are required for pulmonary defense against P. aeruginosa.     

Toll-like receptor 9 acts at an early stage in host defence against pneumococcal infection

Toll-like receptor 9 (TLR9) induces an inflammatory response by recognition of unmethylated CpG dinucleotides, mainly present in prokaryotic DNA. So far, TLR9-deficient mice have been shown to be more sensitive than wild-type mice to viral, but not to bacterial infections. Here, we show that mice deficient in TLR9 but not in TLR1, TLR2, TLR4 and TLR6 or IL-1R/IL-18R are more susceptible to a respiratory tract bacterial infection caused by Streptococcus pneumoniae. Intranasal challenge studies revealed that TLR9 plays a protective role in the lungs at an early stage of infection prior to the entry of circulating inflammatory cells. Alveolar as well as bone marrow-derived macrophages deficient in either TLR9 or the myeloid adaptor differentiation protein MyD88 were impaired in pneumococcal uptake and in pneumococcal killing. Our data suggest that in the airways, pneumococcal infection triggers a TLR9 and MyD88-dependent activation of phagocytic activity from resident macrophages leading to an early clearance of bacteria from the lower respiratory tract.     

Lipopolysaccharide from Coxiella burnetii is involved in bacterial phagocytosis, filamentous actin reorganization, and inflammatory responses through Toll-like receptor 4

The role of Toll-like receptors (TLRs) in the recognition of extracellular and facultative intracellular bacteria by the innate immune system has been extensively studied, but their role in the recognition of obligate intracellular organisms remains unknown. Coxiella burnetii, the agent of Q fever, is an obligate intracellular bacterium that specifically inhabits monocytes/macrophages. We showed in this study that C. burnetii LPS is involved in the uptake of virulent organisms by macrophages but not in that of avirulent variants. The uptake of virulent organisms was dependent on TLR4 because it was reduced in macrophages from TLR4(-/-) mice. In addition, LPS was responsible for filamentous actin reorganization induced by virulent C. burnetii, which was prevented in TLR4(-/-) macrophages. In contrast, the intracellular fate of C. burnetii was not affected in TLR4(-/-) macrophages, suggesting that TLR4 does not control the maturation of C. burnetii phagosome and the microbicidal activity of macrophages. These results are consistent with in vivo experiments because the pattern of tissue infection and the clearance of C. burnetii were similar in wild-type and TLR4(-/-) mice. We also showed that the number of granulomas was decreased in the liver of infected TLR4(-/-) mice, and the formation of splenic granulomas was only transient. The impaired formation of granulomas was associated with decreased production of IFN-gamma and TNF. Taken together, these results demonstrate that TLR4 controls early events of C. burnetii infection such as macrophage phagocytosis, granuloma formation, and cytokine production.     

In vivo function of airway epithelial TLR2 in host defense against bacterial infection

Decreased Toll-like receptor 2 (TLR2) expression has been reported in patients with chronic obstructive pulmonary disease and in a murine asthma model, which may predispose the hosts to bacterial infections, leading to disease exacerbations. Since airway epithelial cells serve as the first line of respiratory mucosal defense, the present study aimed to reveal the role of airway epithelial TLR2 signaling to lung bacterial [i.e., Mycoplasma pneumoniae (Mp)] clearance. In vivo TLR2 gene transfer via intranasal inoculation of adenoviral vector was performed to reconstitute TLR2 expression in airway epithelium of TLR2(-/-) BALB/c mice, with or without ensuing Mp infection. TLR2 and lactotransferrin (LTF) expression in airway epithelial cells and lung Mp load were assessed. Adenovirus-mediated TLR2 gene transfer to airway epithelial cells of TLR2(-/-) mice reconstituted 30-40% TLR2 expression compared with TLR2(+/+) cells. Such airway epithelial TLR2 reconstitution in TLR2(-/-) mice significantly reduced lung Mp load (an appropriate 45% reduction), coupled with elevated LTF expression. LTF expression in mice was shown to be mainly dependent on TLR2 signaling in response to Mp infection. Exogenous human LTF protein dose-dependently decreased lung bacterial load in Mp-infected TLR2(-/-) mice. In addition, human LTF protein directly dose-dependently decreased Mp levels in vitro. These data indicate that reconstitution of airway epithelial TLR2 signaling in TLR2(-/-) mice significantly restores lung defense against bacteria (e.g., Mp) via increased lung antimicrobial protein LTF production. Our findings may offer a deliverable approach to attenuate bacterial infections in airways of asthma or chronic obstructive pulmonary disease patients with impaired TLR2 function.     

Chlamydia trachomatis induces expression of IFN-gamma-inducible protein 10 and IFN-beta independent of TLR2 and TLR4, but largely dependent on MyD88

IFN-gamma-inducible protein 10 (IP-10) is a chemokine important in the attraction of T cells, which are essential for resolution of chlamydial genital tract infection. During infections with Gram-negative bacteria, the IP-10 response mediated through type I IFNs usually occurs as a result of TLR4 stimulation by bacterial LPS. However, we found that levels of IP-10 in genital tract secretions of Chlamydia trachomatis-infected female wild-type mice were similar to those of infected TLR2- and TLR4-deficient mice but significantly greater than those of infected MyD88-deficient mice. We investigated the mechanism of IP-10 and IFN-beta induction during chlamydial infection using mouse macrophages and fibroblasts infected ex vivo. The induction of IP-10 and IFN-beta was unchanged in Chlamydia-infected TLR2- and TLR4-deficient cells compared with wild-type cells. However, infection of MyD88-deficient cells resulted in significantly decreased responses. These results suggest a role for MyD88-dependent pathways in induction of IP-10 and IFN-beta during chlamydial infection. Furthermore, treatment of infected macrophages with an endosomal maturation inhibitor significantly reduced chlamydial-induced IFN-beta. Because endosomal maturation is required for MyD88-dependent intracellular pathogen recognition receptors to function, our data suggest a role for the intracellular pathogen recognition receptor(s) in induction of IFN-beta and IP-10 during chlamydial infection. Furthermore, the intracellular pathways that lead to chlamydial-induced IFN-beta function through TANK-binding kinase mediated phosphorylation and nuclear translocation of IFN regulatory factor-3.     

Multiple MyD88-dependent responses contribute to pulmonary clearance of Legionella pneumophila

MyD88-dependent signalling is important for secretion of early inflammatory cytokines and host protection in response to Legionella pneumophila infection. Although toll-like receptor (TLR)2 contributes to MyD88-dependent clearance of L. pneumophila , TLR-independent functions of MyD88 could also be important. To determine why MyD88 is critical for host protection to L. pneumophila , the contribution of multiple TLRs and IL-18 receptor (IL-18R)-dependent interferon-gamma (IFN-γ) production in a mouse was examined. Mice deficient for TLR5 or TLR9, or deficient for TLR2 along with either TLR5 or TLR9, were competent for controlling bacterial replication and had no apparent defects in cytokine production compared with control mice. MyD88-dependent production of IFN-γ in the lung was mediated primarily by natural killer cells and required IL-18R signalling. Reducing IFN-γ levels did not greatly affect the kinetics of L. pneumophila replication or clearance in infected mice. Additionally, IFN-γ-deficient mice did not have a susceptibility phenotype as severe as the MyD88-deficient mice and were able to control a pulmonary infection by L. pneumophila . Thus, MyD88-dependent innate immune responses induced by L. pneumophila involve both TLR-dependent responses and IL-18R-dependent production of IFN-γ by natural killer cells, and these MyD88-dependent pathways can function independently to provide host protection against an intracellular pathogen.     

Toll-like receptor 7 is required for effective adaptive immune responses that prevent persistent virus infection

TLR7 is an innate signaling receptor that recognizes single-stranded viral RNA and is activated by viruses that cause persistent infections. We show that TLR7 signaling dictates either clearance or establishment of life-long chronic infection by lymphocytic choriomeningitis virus (LCMV) Cl 13 but does not affect clearance of the acute LCMV Armstrong 53b strain. TLR7(-/-) mice infected with LCMV Cl 13 remained viremic throughout life from defects in the adaptive antiviral immune response-notably, diminished T?cell function, exacerbated T?cell exhaustion, decreased plasma cell maturation, and negligible antiviral antibody production. Adoptive transfer of TLR7(+/+) LCMV immune memory cells that enhanced clearance of persistent LCMV Cl 13 infection in TLR7(+/+) mice failed to purge LCMV Cl 13 infection in TLR7(-/-) mice, demonstrating that a TLR7-deficient environment renders antiviral responses ineffective. Therefore, methods that promote TLR7 signaling are promising treatment strategies for chronic viral infections.     

Innate immune responses to Rhodococcus equi

We examined innate immune responses to the intracellular bacterium Rhodococcus equi and show that infection of macrophages with intact bacteria induced the rapid translocation of NF-kappa B and the production of a variety of proinflammatory mediators, including TNF, IL-12, and NO. Macrophages from mice deficient in MyD88 failed to translocate NF-kappa B and produced virtually no cytokines in response to R. equi infection, implicating a TLR pathway. TLR4 was not involved in this response, because C3H/HeJ macrophages were fully capable of responding to R. equi infection, and because RAW-264 cells transfected with a dominant negative form of TLR4 responded normally to infection by R. equi. A central role for TLR2 was identified. A TLR2 reporter cell was activated by R. equi, and RAW-264 cells transfected with a dominant negative TLR2 exhibited markedly reduced cytokine responses to R. equi. Moreover, macrophages from TLR2(-/-) mice exhibited diminished cytokine responses to R. equi. The role of the surface-localized R. equi lipoprotein VapA (virulence-associated protein A), in TLR2 activation was examined. Purified rVapA activated a TLR2-specific reporter cell, and it induced the maturation of dendritic cells and the production of cytokines from macrophages. Importantly, TLR2(-/-)-deficient but not TLR4(-/-)-deficient mice were found to be compromised in their ability to clear a challenge with virulent R. equi. We conclude that the efficient activation of innate immunity by R. equi may account for the relative lack of virulence of this organism in immunocompetent adults.     

IL-1 receptor-mediated signal is an essential component of MyD88-dependent innate response to Mycobacterium tuberculosis infection

MyD88, the common adapter involved in TLR, IL-1, and IL-18 receptor signaling, is essential for the control of acute Mycobacterium tuberculosis (MTB) infection. Although TLR2, TLR4, and TLR9 have been implicated in the response to mycobacteria, gene disruption for these TLRs impairs only the long-term control of MTB infection. Here, we addressed the respective role of IL-1 and IL-18 receptor pathways in the MyD88-dependent control of acute MTB infection. Mice deficient for IL-1R1, IL-18R, or Toll-IL-1R domain-containing adaptor protein (TIRAP) were compared with MyD88-deficient mice in an acute model of aerogenic MTB infection. Although primary MyD88-deficient macrophages and dendritic cells were defective in cytokine production in response to mycobacterial stimulation, IL-1R1-deficient macrophages exhibited only a reduced IL-12p40 secretion with unaffected TNF, IL-6, and NO production and up-regulation of costimulatory molecules CD40 and CD86. Aerogenic MTB infection of IL-1R1-deficient mice was lethal within 4 wk with 2-log higher bacterial load in the lung and necrotic pneumonia but efficient pulmonary CD4 and CD8 T cell responses, as seen in MyD88-deficient mice. Mice deficient for IL-18R or TIRAP controlled acute MTB infection. These data demonstrate that absence of IL-1R signal leads to a dramatic defect of early control of MTB infection similar to that seen in the absence of MyD88, whereas IL-18R and TIRAP are dispensable, and that IL-1, together with IL-1-induced innate response, might account for most of MyD88-dependent host response to control acute MTB infection.     

Intracellular invasion of Orientia tsutsugamushi activates inflammasome in asc-dependent manner

Orientia tsutsugamushi, a causative agent of scrub typhus, is an obligate intracellular bacterium, which escapes from the endo/phagosome and replicates in the host cytoplasm. O. tsutsugamushi infection induces production of pro-inflammatory mediators including interleukin-1β (IL-1β), which is secreted mainly from macrophages upon cytosolic stimuli by activating cysteine protease caspase-1 within a complex called the inflammasome, and is a key player in initiating and maintaining the inflammatory response. However, the mechanism for IL-1β maturation upon O. tsutsugamushi infection has not been identified. In this study, we show that IL-1 receptor signaling is required for efficient host protection from O. tsutsugamushi infection. Live Orientia, but not heat- or UV-inactivated Orientia, activates the inflammasome through active bacterial uptake and endo/phagosomal maturation. Furthermore, Orientia-stimulated secretion of IL-1β and activation of caspase-1 are ASC- and caspase-1- dependent since IL-1β production was impaired in Asc- and caspase-1-deficient macrophages but not in Nlrp3-, Nlrc4- and Aim2-deficient macrophages. Therefore, live O. tsutsugamushi triggers ASC inflammasome activation leading to IL-1β production, which is a critical innate immune response for effective host defense.     

Plasmacytoid dendritic cells promote host defense against acute pneumovirus infection via the TLR7-MyD88-dependent signaling pathway

Human respiratory syncytial virus (RSV) is the leading cause of lower respiratory tract infection in infants. In human infants, plasmacytoid dendritic cells (pDC) are recruited to the nasal compartment during infection and initiate host defense through the secretion of type I IFN, IL-12, and IL-6. However, RSV-infected pDC are refractory to TLR7-mediated activation. In this study, we used the rodent-specific pathogen, pneumonia virus of mice (PVM), to determine the contribution of pDC and TLR7 signaling to the development of the innate inflammatory and early adaptive immune response. In wild-type, but not TLR7- or MyD88-deficient mice, PVM inoculation led to a marked infiltration of pDC and increased expression of type I, II, and III IFNs. The delayed induction of IFNs in the absence of TLR7 or MyD88 was associated with a diminished innate inflammatory response and augmented virus recovery from lung tissue. In the absence of TLR7, PVM-specific CD8(+) T cell cytokine production was abrogated. The adoptive transfer of TLR7-sufficient, but not TLR7-deficient pDC to TLR7 gene-deleted mice recapitulated the antiviral responses observed in wild-type mice and promoted virus clearance. In summary, TLR7-mediated signaling by pDC is required for appropriate innate responses to acute pneumovirus infection. It is conceivable that as-yet-unidentified defects in the TLR7 signaling pathway may be associated with elevated levels of RSV-associated morbidity and mortality among otherwise healthy human infants.     

Cutting edge: TLR2-deficient and MyD88-deficient mice are highly susceptible to Staphylococcus aureus infection

Toll-like receptor (TLR) family acts as pattern recognition receptors for pathogen-specific molecular patterns. We previously showed that TLR2 recognizes Gram-positive bacterial components whereas TLR4 recognizes LPS, a component of Gram-negative bacteria. MyD88 is shown to be an adaptor molecule essential for TLR family signaling. To investigate the role of TLR family in host defense against Gram-positive bacteria, we infected TLR2- and MyD88-deficient mice with Staphylococcus aureus. Both TLR2- and MyD88-deficient mice were highly susceptible to S. aureus infection, with more enhanced susceptibility in MyD88-deficient mice. Peritoneal macrophages from MyD88-deficient mice did not produce any detectable levels of cytokines in response to S. aureus. In contrast, TLR2-deficient macrophages produced reduced, but significant, levels of the cytokines, and TLR4-deficient macrophages produced the same amounts as wild-type cells, indicating that S. aureus is recognized not only by TLR2, but also by other TLR family members except for TLR4.