Live Streptococcus pneumoniae, Haemophilus influenzae, and Neisseria meningitidis activate the inflammatory response through Toll-like receptors 2, 4, and 9 in species-specific patterns

Mucosal clearance of capsule-expressing bacteria requires both TLR and nucleotide-binding oligomerization domain 1 signaling

Expression of capsular polysaccharide by bacterial pathogens is associated with increased resistance to host clearance mechanisms, in particular by evading opsonization and uptake by professional phagocytes. The potential for rapid progression of disease caused by encapsulated bacteria points to the importance of innate immunity at the mucosal surface where infection is initiated. Using a murine model of nasopharyngeal colonization, host immune components that contribute to the mucosal clearance of capsule-expressing bacteria were investigated. Clearance of encapsulated Haemophilus influenzae (Hi) required both TLR and nucleotide-binding oligomerization domain (NOD) signaling pathways, whereas individual deficiencies in each of these signaling cascades did not affect clearance of nonencapsulated strains. Moreover, clearance of Hi-expressing capsular polysaccharide required the recruitment of neutrophils to the site of infection, and ex vivo phagocytic bacterial killing required expression of the NOD1 signaling pathway. Conversely, redundancies within these innate immune pathways of non-neutrophil cells were sufficient to promote mucosal clearance of nonencapsulated Hi. Our findings reveal a role for NOD1 in protection from encapsulated pathogens. In addition, this study provides an example of a microbial virulence determinant that alters the requirements for host signaling to provide effective protection.

Two centuries of meningococcal infection: from Vieusseux to the cellular and molecular basis of disease

Scientific knowledge of meningococcal infection has increased greatly since the epidemic nature of the illness was first described by Vieusseux at the dawn of the nineteenth century. In fact, revolutionary advances have been made in public-health measures, antimicrobial therapy, diagnostic procedures, anti-inflammatory drugs and supportive care facilities. Based on the knowledge accumulated to date, it is generally accepted that the pathogenesis of meningococcal infection involves multiple links that interconnect in a complex web of phenomena from Neisseria meningitidis attachment to meningococcal sepsis or meningitis. In fact, a myriad of strongly interacting inflammatory molecules and cells have been implicated in neisserial infection, illustrating the complexity of meningococcal pathogenesis. In addition, many of these signallers are critically involved in outcomes in the human host. Deciphering the pathogenesis of meningococcal infection could expand our knowledge and provide important clues to the host-pathogen interaction, as well as leading to the development of new therapeutic tools. Herein, we review the history of the discovery and characterization of meningococcal disease, epidemiological features of the disease with an emphasis on recent developments in Brazil, the cellular and molecular basis of disease, and discuss diagnosis and therapy.

Neisseria gonorrhoeae suppresses the oxidative burst of human polymorphonuclear leukocytes

Symptomatic infection with Neisseria gonorrhoeae (Gc) results in a potent polymorphonuclear leukocyte (PMN)-driven inflammatory response, but the mechanisms by which Gc withstands PMN attack are poorly defined. Here we report that Gc can suppress the PMN oxidative burst, a central component of the PMN antimicrobial arsenal. Primary human PMNs remained viable after exposure to liquid-grown, exponential-phase, opacity-associated protein (Opa)-negative Gc of strains FA1090 and MS11 but did not generate reactive oxygen species (ROS), even after bacterial opsonization. Liquid-grown FA1090 Gc expressing OpaB, an Opa protein previously correlated with PMN ROS production, elicited a minor PMN oxidative burst. PMN ROS production in response to Opa(-) and OpaB+ Gc was markedly enhanced if bacteria were agar-grown or if liquid-grown bacteria were heat-killed. Liquid-grown Opa(-) Gc inhibited the PMN oxidative burst elicited by isogenic dead bacteria, formylated peptides or Staphylococcus aureus but did not inhibit PMN ROS production by OpaB+ Gc or phorbol esters. Suppression of the oxidative burst required Gc-PMN contact and bacterial protein synthesis but not phagocytosis. These results suggest that viable Gc directly inhibits PMN signalling pathways required for induction of the oxidative burst, which may contribute to gonococcal pathogenesis during inflammatory stages of gonorrhoeal disease.

Toll-like receptors: their roles in bacterial recognition and respiratory infections

Although respiratory infections cause significant morbidity and mortality throughout the world, the immunologic factors that mediate host susceptibility to these infections remain poorly understood. The lung contains a vast surface at the host-environment interface and acts as a crucial barrier to invading pathogens. The lung is equipped with specialized epithelial and hematopoietic cells, which express pattern recognition receptors that act as both sentinels and mediators of pulmonary innate immunity. Toll-like receptors (TLRs) mediate a particularly critical role in pathogen recognition and subsequent initiation of the host immune response. In this review, we will summarize current knowledge of TLRs and their bacterial ligands and explore their role in respiratory infections. Moreover, we will highlight recent advances in the role of TLRs in pulmonary infections from a human immunogenetics perspective.

Innate immunity to pneumococcal infection of the central nervous system depends on toll-like receptor (TLR) 2 and TLR4

BACKGROUND

Recent studies have suggested that, in addition to Toll-like receptor (TLR) 2, other pattern recognition receptors mediate activation of the immune response after infection of the central nervous system (CNS) with Streptococcus pneumoniae (SP).

METHODS

Using a mouse meningitis model, we investigated the influence of TLR4 single deficiency (TLR4(-/-)), TLR2/TLR4 double deficiency (TLR2/4(-/-)), and TLR2/TLR4/TLR9 triple deficiency (TLR2/4/9(-/-)) on the immune response of the CNS to SP infection. To identify the cell populations that mediate the responses to SP, we generated TLR2/4(-/-)-wild-type (wt) bone marrow (BM) chimeras.

RESULTS

Compared with infected wt mice, infected TLR2/4(-/-) and TLR2/4/9(-/-) mice had similar reductions in brain cytokine levels, pleocytosis, and cerebral pathologic findings, whereas no such effect was noted in infected TLR4(-/-) mice. The attenuated immune response was paralleled by an impaired host defense that resulted in worsening of disease. Analysis of the chimeric mice after infection showed that mere TLR2/4 deficiency, either of radioresistant cells or of transplanted BM-derived cells, was sufficient to mount a substantial cerebral immune response, such as that noted in wt mice.

CONCLUSION

In murine SP meningitis, TLR2 and TLR4 expressed on radioresistant and transplanted BM-derived cells were major cellular sensors of invading SP inducing inflammatory responses.

Streptococcus pneumoniae stabilizes tumor necrosis factor alpha mRNA through a pathway dependent on p38 MAPK but independent of Toll-like receptors

Background

Streptococcus pneumoniae is a human pathogenic bacteria and a major cause of severe invasive diseases, including pneumonia, bacteremia, and meningitis. Infections with S. pneumoniae evoke a strong inflammatory response, which plays a major role in the pathogenesis of pneumococcal disease.

Results

In this study, we have examined how S. pneumoniae affects expression of the inflammatory cytokine tumor necrosis factor (TNF) α, and the molecular mechanisms involved. Secretion of TNF-α was strongly induced by S. pneumoniae, which was able to stabilize TNF-α mRNA through a mechanism dependent on the viability of the bacteria as well as the adenylate uridylate-rich elements in the 3'untranslated region of TNF-α mRNA. The ability of S. pneumoniae to stabilize TNF-α mRNA was dependent on the mitogen-activated protein kinase (MAPK) p38 whereas inhibition of Toll-like receptor signaling via MyD88 did not affect S. pneumoniae-induced mRNA stabilization. P38 was activated through a pathway involving the upstream kinase transforming growth factor-activated kinase 1 and MAPK kinase 3.

Conclusion

Thus, S. pneumoniae stabilizes TNF-α mRNA through a pathway dependent on p38 but independent of Toll-like receptors. Production of TNF-α may contribute significantly to the inflammatory response raised during pneumococcal infection.

Important role for Toll-like receptor 9 in host defense against meningococcal sepsis

Neisseria meningitidis is a leading cause of meningitis and sepsis. The pathogenesis of meningococcal disease is determined by both bacterial virulence factors and the host inflammatory response. Toll-like receptors (TLRs) are prominent activators of the inflammatory response, and TLR2, -4, and -9 have been reported to be involved in the host response to N. meningitidis. While TLR4 has been suggested to play an important role in early containment of infection, the roles of TLR2 and TLR9 in meningococcal disease are not well described. Using a model for meningococcal sepsis, we report that TLR9(-/-) mice displayed reduced survival and elevated levels of bacteremia compared to wild-type mice. In contrast, TLR2(-/-) mice controlled the infection in a manner comparable to that of wild-type mice. TLR9 deficiency was also associated with reduced bactericidal activity in vitro, which was accompanied by reduced production of nitric oxide by TLR9-deficient macrophages. Interestingly, TLR9(-/-) mice recruited more macrophages to the bloodstream than wild-type mice and produced elevated levels of cytokines at late time points during infection. At the cellular level, activation of signal transduction and induction of cytokine gene expression were independent of TLR2 or TLR9 in macrophages and conventional dendritic cells. In contrast, plasmacytoid dendritic cells relied entirely on TLR9 to induce these activities. Thus, our data demonstrate an important role for TLR9 in host defense against N. meningitidis.

Morphine induces defects in early response of alveolar macrophages to Streptococcus pneumoniae by modulating TLR9-NF-kappa B signaling

Resident alveolar macrophages and respiratory epithelium constitutes the first line of defense against invading lung pneumococci. Results from our study showed that increased mortality and bacterial outgrowth and dissemination seen in morphine-treated mice were further exaggerated following depletion of alveolar macrophages with liposomal clodronate. Using an in vitro alveolar macrophages and lung epithelial cells infection model, we show significant release of MIP-2 from alveolar macrophages, but not from lung epithelial cells, following 4 h of exposure of cells to pneumococci infection. Morphine treatment reduced MIP-2 release in pneumococci stimulated alveolar macrophages. Furthermore, morphine treatment inhibited Streptococcus pneumoniae-induced NF-kappaB-dependent gene transcription in alveolar macrophages following 2 h of in vitro infection. S. pneumoniae infection resulted in a significant induction of NF-kappaB activity only in TLR9 stably transfected HEK 293 cells, but not in TLR2 and TLR4 transfected HEK 293 cells, and morphine treatment inhibited S. pneumoniae-induced NF-kappaB activity in these cells. Moreover, morphine treatment also decreased bacterial uptake and killing in alveolar macrophages. Taken together, these results suggest that morphine treatment impairs TLR9-NF-kappaB signaling and diminishes bacterial clearance following S. pneumoniae infection in resident macrophages during the early stages of infection, leading to a compromised innate immune response.

Streptococcus pneumoniae synergizes with nontypeable Haemophilus influenzae to induce inflammation via upregulating TLR2

Background

Toll-like receptor 2 (TLR2) plays a critical role in mediating inflammatory/immune responses against bacterial pathogens in lung. Streptococcus pneumoniae (S. pneumoniae) and nontypeable Haemophilus influenzae (NTHi) were previously reported to synergize with each other to induce inflammatory responses. Despite the relatively known intracellular signaling pathways involved in the synergistic induction of inflammation, it is still unclear if both bacterial pathogens also synergistically induce expression of surface TLR2.

Results

Here we provide direct evidence that S. pneumoniae synergizes with NTHi to upregulate TLR2 expression in lung and middle ear of the mice. Pneumolysin (PLY) appears to be the major virulence factor involved in this synergism. Moreover, S. pneumoniae PLY induces TLR2 expression via a TLR4-MyD88-NF-κB-dependent signaling pathway. Interestingly, tumor suppressor CYLD acts as a negative regulator of S. pneumoniae-induced TLR2 up-regulation via negative-crosstalk with NF-κB signaling.

Conclusion

Our study thus provides novel insights into the regulation of TLR2 expression in mixed bacterial infections.

Live Lactobacillus rhamnosus and Streptococcus pyogenes differentially regulate Toll-like receptor (TLR) gene expression in human primary macrophages

Macrophages are phagocytes that recognize bacteria and subsequently activate appropriate innate and adaptive immune responses. TLRs are essential in identifying conserved bacterial structures and in initiating and mediating innate immune responses. In this work, we have characterized TLR gene expression in human monocyte-derived macrophages in response to stimulation with two live Gram-positive bacteria, a human commensal and probiotic Lactobacillus rhamnosus GG (LGG), and an important human pathogen Streptococcus pyogenes. LGG and S. pyogenes enhanced TLR2 expression in macrophages. LGG and S. pyogenes also required TLR2 for NF-kappaB activation. Only pathogenic S. pyogenes was able to up-regulate TLR3 and TLR7 gene expression. This up-regulation was dependent on IFN-alpha/beta, as neutralizing anti-IFN-alpha/beta antibodies reduced S. pyogenes-induced TLR3 and TLR7 mRNA expression. Our results show that despite similarities, TLR responses of macrophages differ for a Gram-positive probiotic and a pathogen. Our data suggest that macrophages can discriminate between probiotic and pathogenic bacteria by IFN-mediated TLR gene regulation.

Stages of meningococcal sepsis simulated in vitro, with emphasis on complement and Toll-like receptor activation

The clinical presentation of meningococcal disease is closely related to the number of meningococci in the circulation. This study aimed to examine the activation of the innate immune system after being exposed to increasing and clinically relevant concentrations of meningococci. We incubated representative Neisseria meningitidis serogroup B (ST-32) and serogroup C (ST-11) strains and a lipopolysaccharide (LPS)-deficient mutant (the 44/76 lpxA mutant) in human serum and whole blood and measured complement activation and cytokine secretion and the effect of blocking these systems. HEK293 cells transfected with Toll-like receptors (TLRs) were examined for activation of NF-kappaB. The threshold for cytokine secretion and activation of NF-kappaB was 10(3) to 10(4) meningococci/ml. LPS was the sole inflammation-inducing molecule at concentrations up to 10(5) to 10(6) meningococci/ml. The activation was dependent on TLR4-MD2-CD14. Complement contributed to the inflammatory response at >or=10(5) to 10(6) meningococci/ml, and complement activation increased exponentially at >or=10(7) bacteria/ml. Non-LPS components initiated TLR2-mediated activation at >or=10(7) bacteria/ml. As the bacterial concentration exceeded 10(7)/ml, TLR4 and TLR2 were increasingly activated, independent of CD14. In this model mimicking human disease, the inflammatory response to N. meningitidis was closely associated with the bacterial concentration. Therapeutically, CD14 inhibition alone was most efficient at a low bacterial concentration, whereas addition of a complement inhibitor may be beneficial when the bacterial load increases.

The landscape of human proteins interacting with viruses and other pathogens

Infectious diseases result in millions of deaths each year. Mechanisms of infection have been studied in detail for many pathogens. However, many questions are relatively unexplored. What are the properties of human proteins that interact with pathogens? Do pathogens interact with certain functional classes of human proteins? Which infection mechanisms and pathways are commonly triggered by multiple pathogens? In this paper, to our knowledge, we provide the first study of the landscape of human proteins interacting with pathogens. We integrate human-pathogen protein-protein interactions (PPIs) for 190 pathogen strains from seven public databases. Nearly all of the 10,477 human-pathogen PPIs are for viral systems (98.3%), with the majority belonging to the human-HIV system (77.9%). We find that both viral and bacterial pathogens tend to interact with hubs (proteins with many interacting partners) and bottlenecks (proteins that are central to many paths in the network) in the human PPI network. We construct separate sets of human proteins interacting with bacterial pathogens, viral pathogens, and those interacting with multiple bacteria and with multiple viruses. Gene Ontology functions enriched in these sets reveal a number of processes, such as cell cycle regulation, nuclear transport, and immune response that participate in interactions with different pathogens. Our results provide the first global view of strategies used by pathogens to subvert human cellular processes and infect human cells. Supplementary data accompanying this paper is available at http://staff.vbi.vt.edu/dyermd/publications/dyer2008a.html.

MyD88 is necessary for neutrophil recruitment in hypersensitivity pneumonitis

Hypersensitivity pneumonitis is an interstitial lung disease that is characterized by alveolitis, granuloma formation, and in some patients, fibrosis. Using the Saccharopolyspora rectivirgula animal model of Farmer's lung disease, our laboratory has demonstrated that neutrophils play a critical role in IFN-gamma production during the acute phase of the disease. As IFN-gamma is necessary for granuloma formation, it is important to identify the factors that lead to neutrophil recruitment during disease. To begin to identify the pattern recognition receptors (PRRs) that initiate chemokine production, leading to neutrophil recruitment following S. rectivirgula exposure, we examined the role of MyD88 and TLR2. Our results demonstrate that neutrophil recruitment, as measured by flow cytometry and the myeloperoxidase assay, was abolished in the absence of MyD88 following S. rectivirgula exposure. The decrease in neutrophil recruitment was likely a result of a significant decrease in production of neutrophil chemokines MIP-2 and keratinocyte-derived chemokine. These results suggest that S. rectivirgula interacts with PRRs that are upstream of the MyD88 pathway to initiate cytokine and chemokine production. In vitro studies suggest that S. rectivirgula can interact with TLR2, and stimulation of adherent cells from TLR2 knockout (KO) mice with S. rectivirgula resulted in a significant decrease in MIP-2 production. However, TLR2 KO mice did not have a reduction in neutrophil recruitment compared with wild-type mice following S. rectivirgula exposure. The results from our studies suggest that one or more PRR(s) upstream of MyD88 are necessary for neutrophil recruitment following S. rectivirgula exposure.

Lipooligosaccharides containing phosphorylcholine delay pulmonary clearance of nontypeable Haemophilus influenzae

Nontypeable Haemophilus influenzae (NTHi) causes pulmonary infections in patients with chronic obstructive pulmonary disease and other mucociliary clearance defects. Like many bacteria inhabiting mucosal surfaces, NTHi produces lipooligosaccharide (LOS) endotoxins that lack the O side chain. Persistent NTHi populations express a discrete subset of LOS glycoforms, including those containing phosphorylcholine (PCho). In this study, we compared two NTHi strains with isogenic mutants lacking PCho for clearance from mice following pulmonary infection. Consistent with data from other model systems, populations of the strains NTHi 2019 and NTHi 86-028NP recovered from mouse lung contained an increased proportion of PCho+ variants compared to that in the inocula. PCho- mutants were more rapidly cleared. Serial passage of NTHi increased both PCho content and bacterial resistance to clearance, and no such increases were observed for PCho- mutants. Increased PCho content was also observed in NTHi populations within non-endotoxin-responsive C3H/HeJ and Toll-like receptor 4 null (TLR4-/-) mice, albeit at later times postinfection. Changes in bacterial subpopulations and clearance were unaffected in TLR2-/- mice compared to the subpopulations in and clearance from mice of the parental strain. The clearance of PCho- mutants occurred at earlier time points in both strain backgrounds and in all types of mice. Comparison of bacterial populations in lung tissue cryosections by immunofluorescent staining showed sparse bacteria within the air spaces of C57BL/6 mice and large bacterial aggregates within the lungs of MyD88-/- mice. These results indicate that PCho promotes bacterial resistance to pulmonary clearance early in infection in a manner that is at least partially independent of the TLR4 pathway.

Identification of genes particularly sensitive to lipopolysaccharide (LPS) in human monocytes induced by wild-type versus LPS-deficient Neisseria meningitidis strains

Lipopolysaccharide (LPS) in the outer membrane of Neisseria meningitidis plays a dominant role as an inflammation-inducing molecule in meningococcal disease. We have used microarray analysis to study the global gene expression after exposure of human monocytes for 3 h to wild-type N. meningitidis (10(6)), LPS-deficient N. meningitidis (10(6) and 10(8)), and purified N. meningitidis LPS (1 ng [33 endotoxin units]/ml) to identify LPS-inducible genes. Wild-type N. meningitidis (10(6)) induced 4,689 differentially expressed genes, compared with 72 differentially expressed genes induced by 10(6) LPS-deficient N. meningitidis organisms. However, 10(8) LPS-deficient N. meningitidis organisms induced 3,905 genes, indicating a dose-response behavior of non-LPS cell wall molecules. A comparison of the gene expression patterns from 10(6) wild-type N. meningitidis and 10(8) LPS-deficient N. meningitidis organisms showed that 2,401 genes in human monocytes were not strictly LPS dependent. A list of "particularly LPS-sensitive" genes (2,288), differentially induced by 10(6) wild-type N. meningitidis but not by 10(8) LPS-deficient N. meningitidis organisms, showed an early expression of beta interferon (IFN-beta), most likely through the Toll-like receptor-MyD88-independent pathway. Subsequently, IFN-beta may activate the type I IFN signaling pathway, and an unknown number of IFN-beta-inducible genes, such as those for CXCL9, CXCL10, CXCL11, IFIT1, IFIT2, IFIT3, and IFIT5, are transcribed. Supporting this, human monocytes secreted significantly higher levels of CXCL10 and CXCL11 when stimulated by 10(6) wild-type N. meningitidis organisms than when stimulated by 10(8) LPS-deficient N. meningitidis organisms. Plasma CXCL10, but not CXCL11, was positively correlated (r = 0.67; P < 0.01) to LPS in patients (n = 24) with systemic meningococcal disease. Thus, new circulating biomarkers in meningococcal disease may be suggested through LPS-induced gene expression changes in human monocytes.

The presence of capsule in Cryptococcus neoformans influences the gene expression profile in dendritic cells during interaction with the fungus

The aim of this investigation was to study the effect of polysaccharide capsule on the gene expression in dendritic cells (DC) during their interaction with Cryptococcus neoformans. To this end, we used an encapsulated virulent strain of C. neoformans and a cap59 gene-disrupted acapsular avirulent strain derived from the same genetic background. DC were exposed to encapsulated and acapsular C. neoformans strains for 4 h and 18 h, and their transcriptional profiles were analyzed using the Affymetrix mouse gene chip U74Av2. A large number of DC genes were up-regulated after treatment with the acapsular strain. In particular, we observed the up-regulation of the genes involved in DC maturation, such as cell surface receptors, cytokines, and chemokines (interleukin-12 [IL-12], IL-2, IL-1alpha, IL-1beta, IL-6, IL-10, tumor necrosis factor alpha, CCR7, CCL17, CCL22, CCL3, CCL4, CCL7, and CXCL10), membrane proteins, and the genes involved in antigen processing and presentation as well as cell cycle or apoptosis. The chemokine gene expression data were confirmed by real-time reverse transcription-PCR, while the expression of cytokine genes was correlated with their secretion. A completely different pattern of gene expression was observed for DC treated with an encapsulated strain of C. neoformans. In particular, no significant induction was observed in the expression of the genes mentioned above. Moreover, a number of genes, such as those coding for chemokines, were down-regulated. These results suggest that the polysaccharide capsule shrouding the cell wall of C. neoformans plays a fundamental role in inducing DC response, highlighting the molecular basis of the true nature of immune silencing exerted by capsular material.

Toll-like receptors in defense and damage of the central nervous system

Members of the Toll-like receptor (TLR) family play critical roles as regulators of innate and adaptive immune responses. TLRs function by recognizing diverse molecular patterns on the surface of invading pathogens. In the brain, microglial cells generate neuroimmune responses through production of proinflammatory mediators. The upregulation of cytokines and chemokines in response to microbial products and other stimuli has both beneficial and deleterious effects. Emerging evidence demonstrates a central role for TLRs expressed on microglia as a pivotal factor in generating these neuroimmune responses. Therefore, understanding the basis of TLR signaling in producing these responses may provide insights into how activated microglia attempt to strike a balance between defense against invading pathogens and inflicting irreparable brain damage. These insights may lead to innovative therapies for CNS infections and neuroinflammatory diseases based on the modulation of microglial cell activation through TLR signaling.

Immunostimulatory DNA as a vaccine adjuvant

Immunostimulatory DNA containing unmethylated CpG motifs is recognized by Toll-like receptor 9, resulting in the activation of innate immune responses that subsequently amplify the adaptive-immune response. Advances in the characterization of Toll-like receptor 9 signaling have identified immunostimulatory sequences (ISS) with distinct biological activities. Numerous animal models have demonstrated that synthetic ISS are effective adjuvants that enhance both humoral and cellular immune responses in diverse indications, ranging from infectious disease to cancer and allergy. An added benefit supporting the use of ISS as a vaccine adjuvant is that the specific activation of a pathway critical to the regulation of the immune response results in minimal toxicity. To date, clinical testing has largely affirmed the potency and safety of ISS-adjuvanted vaccines.

TLR9 is required for protective innate immunity in Gram-negative bacterial pneumonia: role of dendritic cells

In this study, experiments were performed to determine the contribution of TLR9 to the generation of protective innate immunity against virulent bacterial pathogens of the lung. In initial studies, we found that the intratracheal administration of Klebsiella pneumoniae in wild-type (WT) BALB/c mice resulted in the rapid accumulation of dendritic cells (DC) expressing TLR9. As compared with WT mice, animals deficient in TLR9 (TLR9-/-) displayed significantly increased mortality that was associated with a >50-fold increase in lung CFU and a >400-fold increase in K. pneumoniae CFU in blood and spleen, respectively. Intrapulmonary bacterial challenge in TLR9-/- mice resulted in reduced lung DC accumulation and maturation as well as impaired activation of lung macrophages, NK cells, and alphabeta and gammadelta T cells. Mice deficient in TLR9 failed to generate an effective Th1 cytokine response following bacterial administration. The adoptive transfer of bone marrow-derived DC from syngeneic WT but not TLR9-/- mice administered intratracheally reconstituted antibacterial immunity in TLR9-/- mice. Collectively, our findings indicate that TLR9 is required for effective innate immune responses against Gram-negative bacterial pathogens and that approaches to maximize TLR9-mediated DC responses may serve as a means to augment antibacterial immunity in pneumonia.

Mechanisms of dexamethasone-mediated inhibition of Toll-like receptor signaling induced by Neisseria meningitidis and Streptococcus pneumoniae

Excessive inflammation contributes to the pathogenesis of bacterial meningitis, which remains a serious disease despite treatment with antibiotics. Therefore, anti-inflammatory drugs have important therapeutic potential, and clinical trials have revealed that early treatment with dexamethasone significantly reduces mortality and morbidity from bacterial meningitis. Here we investigate the molecular mechanisms behind the inhibitory effect of dexamethasone upon the inflammatory responses evoked by Neisseria meningitidis and Streptococcus pneumoniae, two of the major causes of bacterial meningitis. The inflammatory cytokine response was dependent on Toll-like receptor signaling and was strongly inhibited by dexamethasone. Activation of the NF-kappaB pathway was targeted at several levels, including inhibition of IkappaB phosphorylation and NF-kappaB DNA-binding activity as well as upregulation of IkappaB alpha synthesis. Our data also revealed that the timing of steroid treatment relative to infection was important for achieving strong inhibition, particularly in response to S. pneumoniae. Altogether, we describe important targets of dexamethasone in the inflammatory responses evoked by N. meningitidis and S. pneumoniae, which may contribute to our understanding of the clinical effect and the importance of timing with respect to corticosteroid treatment during bacterial meningitis.

Interleukin-8 induction via NF-kappaB, p38 mitogen-activated protein kinase and extracellular signal-regulated kinase 1/2 pathways in human peripheral blood mononuclear cells by Alloiococcus otitidis

Alloiococcus otitidis is a newly recognized species of gram-positive bacteria frequently associated with otitis media. Although immunostimulating activity of this organism has been investigated, little is known about the signaling pathways of chemokine/cytokine induction by this organism. We investigated the role of NF-kappaB and mitogen-activated protein (MAP) kinases in chemokine interleukin-8 (IL-8) production by human peripheral blood mononuclear cells (PBMCs) after stimulation with A. otitidis. The organism could induce in vitro IL-8 production in human PBMCs. I kappa B alpha, NF-kappaB, p38 MAP kinase, p44/42 MAP kinase (ERK1/2) became phosphorylated in PBMCs after stimulation with A. otitidis. And, inhibitors of NF-kappaB (caffeic acid phenylethyl ester), p38 (SB 203580), or ERK1/2 (PD 98059) significantly reduced IL-8 induction by the organism. These results were similar to findings in IL-8 induction by Streptococcus pneumoniae, another gram-positive major middle ear pathogen. Our preliminary study suggests that multiple pathways including NF-kappaB, p38, and ERK1/2 were simultaneously activated, and were associated with IL-8 induction by A. otitidis in human PBMCs.

Analysis of the in vitro transcriptional response of human pharyngeal epithelial cells to adherent Streptococcus pneumoniae: evidence for a distinct response to encapsulated strains

Infection of the human host by Streptococcus pneumoniae begins with colonization of the nasopharynx, which is mediated by the adherence of bacteria to the respiratory epithelium. Several studies have indicated an important role for the pneumococcal capsule in this process. Here, we used microarrays to characterize the in vitro transcriptional response of human pharyngeal epithelial Detroit 562 cells to the adherence of serotype 2 encapsulated strain D39, serotype 19F encapsulated strain G54, serotype 4 encapsulated strain TIGR4, and their nonencapsulated derivatives (Deltacps). In total, 322 genes were found to be upregulated in response to adherent pneumococci. Twenty-two genes were commonly induced, including those encoding several cytokines (e.g., interleukin 1beta [IL-1beta] and IL-6), chemokines (e.g., IL-8 and CXCL1/2), and transcriptional regulators (e.g., FOS), consistent with an innate immune response mediated by Toll-like receptor signaling. Interestingly, 85% of genes were induced specifically by one or more encapsulated strains, suggestive of a capsule-dependent response. Importantly, purified capsular polysaccharides alone had no effect. Over a third of these loci encoded products predicted to be involved in transcriptional regulation and signal transduction, in particular mitogen-activated protein kinase signaling pathways. Real-time PCR of a subset of 10 genes confirmed the microarray data and showed a time-dependent upregulation of, especially, innate immunity genes. The downregulation of epithelial genes was most pronounced upon adherence of D39Deltacps, as 68% of the 161 genes identified were repressed only by this nonencapsulated strain. In conclusion, we identified a subset of host genes specifically induced by encapsulated strains during in vitro adherence and have demonstrated the complexity of interactions occurring during the initial stages of pneumococcal infection.

Toll-like receptor 2 contributes to antibacterial defence against pneumolysin-deficient pneumococci

Toll-like receptors (TLRs) are pattern recognition receptors that recognize conserved molecular patterns expressed by pathogens. Pneumolysin, an intracellular toxin found in all Streptococcus pneumoniae clinical isolates, is an important virulence factor of the pneumococcus that is recognized by TLR4. Although TLR2 is considered the most important receptor for Gram-positive bacteria, our laboratory previously could not demonstrate a decisive role for TLR2 in host defence against pneumonia caused by a serotype 3 S. pneumoniae. Here we tested the hypothesis that in the absence of TLR2, S. pneumoniae can still be sensed by the immune system through an interaction between pneumolysin and TLR4. C57BL/6 wild-type (WT) and TLR2 knockout (KO) mice were intranasally infected with either WT S. pneumoniae D39 (serotype 2) or the isogenic pneumolysin-deficient S. pneumoniae strain D39 PLN. TLR2 did not contribute to antibacterial defence against WT S. pneumoniae D39. In contrast, pneumolysin-deficient S. pneumoniae only grew in lungs of TLR2 KO mice. TLR2 KO mice displayed a strongly reduced early inflammatory response in their lungs during pneumonia caused by both pneumolysin-producing and pneumolysin-deficient pneumococci. These data suggest that pneumolysin-induced TLR4 signalling can compensate for TLR2 deficiency during respiratory tract infection with S. pneumoniae.

Epidemic meningitis, meningococcaemia, and Neisseria meningitidis

Meningococcus, an obligate human bacterial pathogen, remains a worldwide and devastating cause of epidemic meningitis and sepsis. However, advances have been made in our understanding of meningococcal biology and pathogenesis, global epidemiology, transmission and carriage, host susceptibility, pathophysiology, and clinical presentations. Approaches to diagnosis, treatment, and chemoprophylaxis are now in use on the basis of these advances. Importantly, the next generation of meningococcal conjugate vaccines for serogroups A, C, Y, W-135, and broadly effective serogroup B vaccines are on the horizon, which could eliminate the organism as a major threat to human health in industrialised countries in the next decade. The crucial challenge will be effective introduction of new meningococcal vaccines into developing countries, especially in sub-Saharan Africa, where they are urgently needed.

Negative regulation of TLR9-mediated IFN-alpha induction by a small-molecule, synthetic TLR7 ligand

Toll-like receptors (TLRs) are a family of molecules that function as sensors for the detection of foreign pathogens through the recognition of nonvariable microbial motifs. Although numerous studies have focused on singular TLRs, less attention has been focused on how simultaneous signaling of multiple TLRs may result in counter-regulation of the effects of each. Here, we examine the counter-regulation that occurs during simultaneous stimulation of TLR7 and TLR9 on human plasmacytoid dendritic cells (PDCs) and B cells. Interestingly, we observed that the capacity for potent IFN-alpha-induction by TLR9 ligands like CpG-C and CpG-A is markedly reduced by concurrent small molecule TLR7 stimulation. However, this inhibition is specific to particular CpG motif-containing immunostimulatory sequence (ISS) functions such as IFN-alpha induction and BDCA-2 down-regulation. Other ISS activities such as PDC expression of CD80/CD86, secretion of IL-6, and B cell proliferation are not altered by the presence of TLR7 ligands (TLR7Ls). In concordance with the ability of TLR7Ls to decrease IFN-alpha secretion induced by ISS, we also find that the expression of interferon regulatory factor-7 (IRF-7), a transcriptional factor critical for IFN-alpha expression, is reduced. Furthermore, down-regulation of TLR9 mRNA expression is accelerated after TLR7 stimulation. These data indicate that TLR7 and TLR9 costimulation do not combine synergistically for IFN-alpha induction and demonstrate that, instead, a negative feedback mechanism has evolved, possibly to prevent levels of IFN-alpha secretion potentially detrimental to the host.

[New aspects of the pathophysiology of pneumonia]

Pneumonia can lead to the critical impairment of gas exchange in the lung. Due to the great variability of pneumonia causing pathogens, a large variety of diverse virulence factors act on the lung. Besides stimulation of unspecific defense mechanisms, activation of receptor-dependent cell-mediated innate immune defense mechanisms are critical for the pulmonary immune defense. Pathogen-associated molecules are detected via transmembraneous and cytosolic receptors of the host. This interaction stimulates the expression of immunomodulatory molecules via signal cascades. Of particular importance, in addition to direct pathogen-caused lung damage, is the overwhelming activation of the inflammatory response which can result in lung barrier failure and impairment of pulmonary gas exchange. In addition to the design of new antibiotics, innovative therapeutic strategies should therefore concentrate on the enhancement of antimicrobial mechanisms by concurrent limitation of inflammation.

NK dendritic cells are innate immune responders to Listeria monocytogenes infection

NK dendritic cells (NKDC) are recently described immunologic cells that possess both lytic and Ag-presenting function and produce prolific quantities of IFN-gamma. The role of NKDC in innate immunity to bacterial infection is unknown. Because IFN-gamma is important in the immune response to Listeria monocytogenes (LM), we hypothesized that NKDC play a critical role during LM infection in mice. We found that LM increased the frequency and activation state of NKDC in vivo. Using in vivo intracellular cytokine analysis, we demonstrated that NKDC are a major source of early IFN-gamma during infection with LM. Adoptive transfer of wild-type NKDC into IFN-gamma-deficient recipients that were subsequently infected with LM decreased bacterial burden in the liver and spleen and prolonged survival. In contrast, NK cells were depleted early during LM infection, produced less IFN-gamma, and conferred less protection upon adoptive transfer into IFN-gamma-deficient mice. In vitro, LM induction of IFN-gamma secretion by NKDC depended on TLR9, in addition to IL-18 and IL-12. Our study establishes NKDC as innate immune responders to bacterial infection by virtue of their ability to secrete IFN-gamma.

Two neisseria meningitidis strains with different ability to stimulate toll-like receptor 4 through the MyD88-independent pathway

Neisseria meningitidis causes acute severe diseases, including sepsis and meningitis, and more benign manifestations such as chronic meningococcemia or colonization of the upper respiratory tract. The inflammatory response, which contributes to the pathogenesis of meningococcal disease, is initiated by pattern recognition receptors, among which Toll-like receptors (TLR)s have been ascribed a particularly important role. We have previously demonstrated that N. meningitidis induce proinflammatory cytokine expression through TLR2 and TLR4. Here we characterize the molecular basis for differential activation of the inflammatory response by two N. meningitidis strains. This difference was due to differential ability to activate signal transduction through TLR4, as HEK293 cells expressing TLR4 produced significantly different levels of interleukin-8 in response to these strains. At the level of signal transduction, the two strains differed substantially in their ability to activate the pathway to nuclear factor kappaB in HEK293-TLR4/MD2 cells at late, but not early, time points. TLR4 activates two signal transduction pathways: one dependent on the adaptor molecule MyD88 and one independent of MyD88, and these pathways induce distinct patterns of gene expression in response to TLR4 ligands. By using macrophages from TLR2-/- mice, we observed that the two strains differed in their ability to activate the TLR4-induced MyD88-independent pathway, but not the MyD88-dependent pathway. This idea was further supported by experiments where either of the two pathways was inhibited and IL-8 secretion was measured. These data therefore provide molecular insight into activation of the inflammatory response by N. meningitidis, which is one of the key events in the pathogenesis of meningococcal disease.

Moraxella catarrhalis is internalized in respiratory epithelial cells by a trigger-like mechanism and initiates a TLR2- and partly NOD1-dependent inflammatory immune response

Moraxella catarrhalis is an important pathogen in patients with chronic obstructive lung disease (COPD). While M. catarrhalis has been categorized as an extracellular bacterium so far, the potential to invade human respiratory epithelium has not yet been explored. Our results obtained by electron and confocal microscopy demonstrated a considerable potential of M. catarrhalis to invade bronchial epithelial (BEAS-2B) cells, type II pneumocytes (A549) and primary small airway epithelial cells (SAEC). Moraxella invasion was dependent on cellular microfilament as well as on bacterial viability, and characterized by macropinocytosis leading to the formation of lamellipodia and engulfment of the invading organism into macropinosomes, thus indicating a trigger-like uptake mechanism. In addition, the cells examined expressed TLR2 as well as NOD1, a recently found cytosolic protein implicated in the intracellular recognition of bacterial cell wall components. Importantly, inhibition of TLR2 or NOD1 expression by RNAi significantly reduced the M. catarrhalis-induced IL-8 secretion. The role of TLR2 and NOD1 was further confirmed by overexpression assays in HEK293 cells. Overall, M. catarrhalis may employ lung epithelial cell invasion to colonize and to infect the respiratory tract, nonetheless, the bacteria are recognized by cell surface TLR2 and the intracellular surveillance molecule NOD1.

Contribution of Toll-like receptors to the innate immune response to Gram-negative and Gram-positive bacteria

Innate recognition of bacteria is a key step in the activation of inflammation and coagulation, and it is dependent on pathogen-associated molecular pattern (PAMP) ligation to Toll-like receptors (TLRs) and CD14. The dominant receptors activated when cells encounter a whole bacterium, which express several PAMPs, are poorly defined. Herein, we have stimulated various human cells with prototypic Gram-negative and Gram-positive bacteria. Receptor-dependent responses to whole bacteria were assessed using both TLR-transfected cells and specific monoclonal antibodies against TLRs, MD-2, and CD14. Enterobacteria-activated leukocytes and endothelial cells in a TLR4/MD-2-dependent manner, most likely via lipopolysaccharide (LPS). TLR2 activation was observed with a high bacterial inoculum, and in epithelial cells expressing TLR2 but not TLR4. Pseudomonas aeruginosa stimulated cells by both TLR2 and TLR4/MD-2. Gram-positive bacteria activated cells only at high concentrations, in a partially TLR2-dependent but TLR4/MD-2-independent manner. Either TLR or CD14 neutralization blocked activation to all bacterial strains tested with the exception of some Gram-positive strains in whole blood in which partial inhibition was noted. This study identifies dominant TLRs involved in responses to whole bacteria. It also validates the concept that host cell activation by bacterial pathogens can be therapeutically reduced by anti-TLR4, -TLR2, and -CD14 mAbs.