In atherosclerosis and Alzheimer's disease, deposition of the altered self components oxidized low-density lipoprotein (LDL) and amyloid-beta triggers a protracted sterile inflammatory response. Although chronic stimulation of the innate immune system is believed to underlie the pathology of these diseases, the molecular mechanisms of activation remain unclear. Here we show that oxidized LDL and amyloid-beta trigger inflammatory signaling through a heterodimer of Toll-like receptors 4 and 6. Assembly of this newly identified heterodimer is regulated by signals from the scavenger receptor CD36, a common receptor for these disparate ligands. Our results identify CD36-TLR4-TLR6 activation as a common molecular mechanism by which atherogenic lipids and amyloid-beta stimulate sterile inflammation and suggest a new model of TLR heterodimerization triggered by coreceptor signaling events.

Lipopolysaccharide (LPS) activates innate immune responses through TLR4/MD-2. LPS binds to the MD-2 hydrophobic pocket and bridges the dimerization of two TLRs/MD-2 complexes to activate intracellular signaling. However, exactly how lipid A, the endotoxic moiety of LPS, activates myeloid-lineage cells remain unknown. Lipid IVa, a tetra-acylated lipid A precursor, has been used widely as a model for lipid A activation. For unknown reasons, lipid IVa activates proinflammatory responses in rodent cells but inhibits the activity of LPS in human cells. Using stable TLR4-expressing cell lines and purified monomeric MD-2, as well as MD-2 deficient bone marrow derived macrophages, we found that both mouse TLR4 and mouse MD-2 are required for lipid IVa activation. Computational studies suggested that unique ionic interactions exist between lipid IVa and TLR4 at the dimerization interface in the mouse complex only. The negatively charged 4-phosphate on lipid IVa interacts with two positively charged residues on the opposing mouse, but not human, TLR4 (K367 and R434) at the dimerization interface. When replaced with their negatively charged human counterparts E369 and Q436, mouse TLR4 was no longer responsive to lipid IVa. In contrast, human TLR4 gained lipid IVa responsiveness when ionic interactions were enabled by charge reversal at the dimerization interface, defining the basis of lipid IVa species-specificity. Thus, using lipid IVa as a selective lipid A agonist, we successfully decoupled and coupled two sequential events required for intracellular signaling: receptor engagement and dimerization, underscoring the functional role of ionic interactions in receptor activation.

Specialized protein translocation systems are used by many bacterial pathogens to deliver effector proteins into host cells that interfere with normal cellular functions. How the host immune system recognizes and responds to this intrusive event is not understood. To address these questions, we determined the mammalian cellular response to the virulence-associated type III secretion system (T3SS) of the human pathogen Yersinia pseudotuberculosis. We found that macrophages devoid of Toll-like receptor (TLR) signaling regulate expression of 266 genes following recognition of the Y. pseudotuberculosis T3SS. This analysis revealed two temporally distinct responses that could be separated into activation of NFkappaB- and type I IFN-regulated genes. Extracellular bacteria were capable of triggering these signaling events, as inhibition of bacterial uptake had no effect on the ensuing innate immune response. The cytosolic peptidoglycan sensors Nod1 and Nod2 and the inflammasome component caspase-1 were not involved in NFkappaB activation following recognition of the Y. pseudotuberculosis T3SS. However, caspase-1 was required for secretion of the inflammatory cytokine IL-1beta in response to T3SS-positive Y. pseudotuberculosis. In order to characterize the bacterial requirements for induction of this novel TLR-, Nod1/2-, and caspase-1-independent response, we used Y. pseudotuberculosis strains lacking specific components of the T3SS. Formation of a functional T3SS pore was required, as bacteria expressing a secretion needle, but lacking the pore-forming proteins YopB or YopD, did not trigger these signaling events. However, nonspecific membrane disruption could not recapitulate the NFkappaB signaling triggered by Y. pseudotuberculosis expressing a functional T3SS pore. Although host cell recognition of the T3SS did not require known translocated substrates, the ensuing response could be modulated by effectors such as YopJ and YopT, as YopT amplified the response, while YopJ dampened it. Collectively, these data suggest that combined recognition of the T3SS pore and YopBD-mediated delivery of immune activating ligands into the host cytosol informs the host cell of pathogenic challenge. This leads to a unique, multifactorial response distinct from the canonical immune response to a bacterium lacking a T3SS.

Inflammation is the coordinated immune response to harmful stimuli that appear during infections or after tissue damage. Cells of the innate immune system are the central players in mediating inflammatory tissue responses. These cells are equipped with an array of signaling receptors that detect foreign molecular substances or altered endogenous molecules that appear under situations of stress. This review provides an overview of recent progress in elucidating the molecular mechanisms that lead to inflammatory reactions. We discuss the current knowledge of the mechanisms leading to the activation of cytoplasmic, multimolecular protein complexes, termed "inflammasomes," which regulate the activity of caspase-1 and the maturation and release of IL-1beta.

Although a clear role for the adaptor protein myeloid differentiation factor-88 (MyD88) adaptor-like (Mal, or TIRAP) in TLR4 signaling has been demonstrated, there is limited information on its role in TLR2 signaling. Here we have systematically analyzed the role of Mal in signaling by TLR2, TLR4, and as a control TLR3 in murine macrophages and dendritic cells. Mal was not required for the induction of IL-6 or NFkappaB activation at high concentrations of the TLR1/2 ligand Pam(3)Cys-Ser-(Lys)(4) or the TLR2/6 ligand macrophage-activating lipopeptide-2 and was required for these responses only at low ligand concentrations. Similarly, induction of IL-6 by Salmonella typhimurium, which is sensed by TLR2, required Mal only at low levels of bacteria. Mal was required for IL-6 induction at all concentrations of the TLR4 ligand LPS. Mal deficiency boosted IL-6 induction by the TLR3 ligand polyinosinic-polycytidylic acid. Activation of JNK, but not p38 or IkappaB degradation, was similarly potentiated in response to polyinosinic-polycytidylic acid in Mal-deficient macrophages. MyD88 was vital for all TLR2 and TLR4 responses and, similar to Mal, was also inhibitory for TLR3-dependent IL-6 and JNK induction. MyD88 interacted with the Toll/IL-1R domains of TLR1, TLR2, TLR4, and TLR6. Mal interacted with the Toll/Il-1R domains of TLR1, TLR2, and TLR4 but not with TLR6. Our study, therefore, reveals that Mal is dispensable in TLR2 signaling at high ligand concentrations in macrophages and dendritic cells, with MyD88 probably coupling to the TLR2 receptor complex at sufficient levels to allow activation. An inhibitory role for Mal in TLR3 signaling to JNK was also demonstrated.

Toll/interleukin-1 (TIR)receptor-containing adapters are critical in orchestrating the different signal transduction pathways following Toll-like receptor (TLR) activation. MyD88 adapter-like (Mal), also termed TIRAP, is involved in bridging MyD88 to the receptor complex for TLR-2 and TLR4 signaling in response to bacterial infection. We have previously reported an interaction between Mal and tumor necrosis factor receptor-associated factor 6 (TRAF6) via a TRAF6-binding motif, the disruption of which inhibited TLR-mediated NF-kappaB-luciferase reporter activity. Given the recent report of intracellular TRAM localization promoting sequential signaling in TLR4 responses, we further characterized Mal interaction with TRAF6, the cellular localization, and the outcomes of disrupting this association on TLR inflammatory responses. We found that Mal and TRAF6 directly interact in response to TLR2 and TLR4 stimulation, although membrane localization is not necessary to facilitate interaction. Critically, reconstitution of murine Mal-deficient macrophages with MalE190A, containing a mutation within the TRAF6-binding motif, fails to reconstitute the proinflammatory response to TLR2 and TLR4 ligands compared with wild type Mal. Furthermore, Mal interaction with TRAF6 mediates Ser phosphorylation of the p65 subunit of NF-kappaB and thus controls transcriptional activation but not nuclear translocation of NF-kappaB. This study characterizes the novel role for Mal in facilitating the direct recruitment of TRAF6 to the plasma membrane, which is necessary for TLR2- and TLR4-induced transactivation of NF-kappaB and regulation of the subsequent pro-inflammatory response.

The recognition of bacterial lipoproteins by toll-like receptor (TLR) 2 is pivotal for inflammation initiation and control in many bacterial infections. TLR2-dependent signalling is currently believed to essentially require both adaptor proteins MyD88 (myeloid differentiation primary response gene 88) and Mal/TIRAP (MyD88-adapter-like/TIR-domain-containing adaptor protein). TLR2-dependent, but MyD88-independent responses have not been described yet. We report here on a novel-signalling pathway downstream of TLR2, which does not adhere to the established model. On stimulation of the TLR2/6 heterodimer with diacylated bacterial lipoproteins, Mal directly interacts with the regulatory subunit of phosphoinositide 3-kinase (PI3K), p85alpha, in an inducible fashion. The Mal-p85alpha interaction drives PI3K-dependent phosphorylation of Akt, phosphatidylinositol(3,4,5)P3 (PIP(3)) generation and macrophage polarization. MyD88 is not essential for PI3K activation and Akt phosphorylation; however, cooperates with Mal for PIP(3) formation and accumulation at the leading edge. In contrast to TLR2/6, TLR2/1 does not require Mal or MyD88 for Akt phosphorylation. Hence, Mal specifically connects TLR2/6 to PI3K activation, PIP(3) generation and macrophage polarization.

The adapter protein MyD88 adapter-like (Mal), encoded by TIR-domain containing adapter protein (Tirap) (MIM 606252), is the most polymorphic of the five adapter proteins involved in Toll-like receptor signaling, harboring eight non-synonymous single nucleotide polymorphisms in its coding region. We screened reported mutations of Mal for activity in reporter assays to test the hypothesis that variants of Mal existed with altered signaling potential. A TIR domain variant, Mal D96N (rs8177400), was found to be inactive. In reconstituted cell lines, Mal D96N acted as a hypomorphic mutation, with impaired cytokine production and NF-kappaB activation upon lipopolysaccharide or PAM2CSK4 stimulation. Moreover, co-immunoprecipitation studies revealed that Mal D96N is unable to interact with MyD88, a prerequisite for downstream signaling to occur. Computer modeling data suggested that residue 96 resides in the MyD88 binding site, further supporting these findings. Genotyping of Mal D96N in three different cohorts suggested that it is a rare mutation. We, thus, describe a rare variant in Mal that exerts its effect via its inability to bind MyD88.

During screening of genes up-regulated by lipopolysaccharide (LPS; endotoxin) treatment of bone marrow derived mouse macrophages it was unexpectedly found that cholesterol 25-hydroxylase (Ch25h) was strongly up-regulated. Treatment of macrophages with 10 ng/ml of LPS for two hours resulted in a 35-fold increase in the expression of Ch25h. In contrast, LPS treatment did not increase the expression of Cyp27a1 or Cyp7b1. The increased Ch25h expression was found to be independent of Myeloid differentiation protein 88 (MyD88) signalling, but dependent on Toll-like receptor (TLR) 4 signalling. LPS treatment of macrophages caused a 6-7 fold increase in cellular 25-hydroxycholesterol concentration. When macrophages were treated with increasing concentrations of 25-hydroxycholesterol, a dose-dependent release of CCL5 into the culture medium was observed. Intravenous injection of LPS in eight healthy volunteers resulted in an increase in plasma 25-hydroxycholesterol concentration. The possibility is discussed that 25-hydroxycholesterol may have a role in the inflammatory response, in addition to its more established role in the regulation of cholesterol homeostasis.

Mycobacterium tuberculosis (Mtb) signals through Toll-like receptor 2 (TLR2) to regulate antigen presenting cells (APCs). Mtb lipoproteins, including LpqH, LprA, LprG and PhoS1, are TLR2 agonists, but their co-receptor requirements are unknown. We studied Mtb lipoprotein-induced responses in TLR2(-/-), TLR1(-/-), TLR6(-/-), CD14(-/-) and CD36(-/-) macrophages. Responses to LprA, LprG, LpqH and PhoS1 were completely dependent on TLR2. LprG, LpqH, and PhoS1 were dependent on TLR1, but LprA did not require TLR1. None of the lipoproteins required TLR6, although a redundant contribution by TLR6 cannot be excluded. CD14 contributed to detection of LprA, LprG and LpqH, whereas CD36 contributed only to detection of LprA. Studies of lung APC subsets revealed lower TLR2 expression by CD11b(high)/CD11c(low) lung macrophages than CD11b(low)/CD11c(high) alveolar macrophages, which correlated with hyporesponsiveness of lung macrophages to LpqH. Thus, lung APC subsets differ in TLR expression, which may determine differences in responses to Mtb.

Group B streptococcus (GBS), the most frequent single isolate in neonatal sepsis and meningitis, potently activates inflammatory macrophage genes via myeloid differentiation antigen 88 (MyD88). However, events parallel to and downstream of MyD88 that instruct the macrophage response are incompletely understood. In this study, we found that only MyD88, not the Toll-like receptor (TLR) adapter proteins MAL/TIRAP, TRIF, and TRAM, essentially mediates the cytokine (tumor necrosis factor [TNF] and interleukin-6) and chemokine (RANTES) responses to whole GBS organisms, although MAL, TRIF, and TRAM have been shown to mediate the responses to substructures in other gram-positive and gram-negative bacteria. GBS-induced, MyD88-dependent phosphorylation of the mitogen-activated protein kinase p38 activated the transcription factor AP-1 and early growth response factor 1 (Egr-1) but not NF-kappaB. Furthermore, phosphorylation of Ets-like molecule 1 (Elk-1) was mediated by p38. However, in contrast to Egr-1 and AP-1, Elk-1 was dispensable for transcriptional activation of TNF by GBS organisms. Studies of macrophages from Elk-1-deficient mice revealed that Elk-1 was furthermore nonessential for the TNF responses to purified TLR2 and TLR4 agonists, which was in notable contrast to what was revealed in studies employing in vitro expression systems. In conclusion, MyD88, p38, and Egr-1, but not Elk-1, essentially mediate the inflammatory cytokine response to GBS organisms.

We previously reported that rOv-ASP-1, a recombinant Onchocerca volvulus activation associated protein-1, was a potent adjuvant for recombinant protein or synthetic peptide-based Ags. In this study, we further evaluated the adjuvanticity of rOv-ASP-1 and explored its mechanism of action. Consistently, recombinant full-length spike protein of SARS-CoV or its receptor-binding domain in the presence of rOv-ASP-1 could effectively induce a mixed but Th1-skewed immune response in immunized mice. It appears that rOv-ASP-1 primarily bound to the APCs among human PBMCs and triggered Th1-biased proinflammatory cytokine production probably via the activation of monocyte-derived dendritic cells and the TLR, TLR2, and TLR4, thus suggesting that rOv-ASP-1 is a novel potent innate adjuvant.

Malaria-induced sepsis is associated with an intense proinflammatory cytokinemia for which the underlying mechanisms are poorly understood. It has been demonstrated that experimental infection of humans with Plasmodium falciparum primes Toll-like receptor (TLR)-mediated proinflammatory responses. Nevertheless, the relevance of this phenomenon during natural infection and, more importantly, the mechanisms by which malaria mediates TLR hyperresponsiveness are unclear. Here we show that TLR responses are boosted in febrile patients during natural infection with P. falciparum. Microarray analyses demonstrated that an extraordinary percentage of the up-regulated genes, including genes involving TLR signaling, had sites for IFN-inducible transcription factors. To further define the mechanism involved in malaria-mediated "priming," we infected mice with Plasmodium chabaudi. The human data were remarkably predictive of what we observed in the rodent malaria model. Malaria-induced priming of TLR responses correlated with increased expression of TLR mRNA in a TLR9-, MyD88-, and IFNgamma-dependent manner. Acutely infected WT mice were highly susceptible to LPS-induced lethality while TLR9(-/-), IL12(-/-) and to a greater extent, IFNgamma(-/-) mice were protected. Our data provide unprecedented evidence that TLR9 and MyD88 are essential to initiate IL12 and IFNgamma responses and favor host hyperresponsiveness to TLR agonists resulting in overproduction of proinflammatory cytokines and the sepsis-like symptoms of acute malaria.

Type I interferon (IFN) is an important host defense cytokine against intracellular pathogens, mainly viruses. In assessing IFN production in response to group B streptococcus (GBS), we find that IFN-beta was produced by macrophages upon stimulation with both heat-killed and live GBS. Exposure of macrophages to heat-killed GBS activated a Toll-like receptor (TLR)-dependent pathway, whereas live GBS activated a TLR/NOD/RIG-like receptor (RLR)-independent pathway. This latter pathway required bacterial phagocytosis, proteolytic bacterial degradation, and phagolysosomal membrane destruction by GBS pore-forming toxins, leading to the release of bacterial DNA into the cytosol. GBS DNA in the cytosol induced IFN-beta production via a pathway dependent on the activation of the serine-threonine kinase TBK1 and phosphorylation of the transcription factor IRF3. Thus, activation of IFN-alpha/-beta production during infection with GBS, commonly considered an extracellular pathogen, appears to result from the interaction of GBS DNA with a putative intracellular DNA sensor or receptor.

TLR9 is critical in parasite recognition and host resistance to experimental infection with Trypanosoma cruzi. However, no information is available regarding nucleotide sequences and cellular events involved on T. cruzi recognition by TLR9. In silico wide analysis associated with in vitro screening of synthetic oligonucleotides demonstrates that the retrotransposon VIPER elements and mucin-like glycoprotein (TcMUC) genes in the T. cruzi genome are highly enriched for CpG motifs that are immunostimulatory for mouse and human TLR9, respectively. Importantly, infection with T. cruzi triggers high levels of luciferase activity under NF-kappaB-dependent transcription in HEK cells cotransfected with human TLR9, but not in control (cotransfected with human MD2/TLR4) HEK cells. Further, we observed translocation of TLR9 to the lysosomes during invasion/uptake of T. cruzi parasites by dendritic cells. Consistently, potent proinflammatory activity was observed when highly unmethylated T. cruzi genomic DNA was delivered to the endo-lysosomal compartment of host cells expressing TLR9. Thus, together our results indicate that the unmethylated CpG motifs found in the T. cruzi genome are likely to be main parasite targets and probably become available to TLR9 when parasites are destroyed in the lysosome-fused vacuoles during parasite invasion/uptake by phagocytes.

The fibrillar peptide amyloid-beta (A beta) has a chief function in the pathogenesis of Alzheimer's disease. Interleukin 1 beta (IL-1 beta) is a key cytokine in the inflammatory response to A beta. Insoluble materials such as crystals activate the inflammasome formed by the cytoplasmic receptor NALP3, which results in the release of IL-1 beta. Here we identify the NALP3 inflammasome as a sensor of A beta in a process involving the phagocytosis of A beta and subsequent lysosomal damage and release of cathepsin B. Furthermore, the IL-1 beta pathway was essential for the microglial synthesis of proinflammatory and neurotoxic factors, and the inflammasome, caspase-1 and IL-1 beta were critical for the recruitment of microglia to exogenous A beta in the brain. Our findings suggest that activation of the NALP3 inflammasome is important for inflammation and tissue damage in Alzheimer's disease.

BACKGROUND: The deleterious effects of the accumulation of supragingival plaque are well known, but the role of the proinflammatory property of supragingival plaque in periodontal diseases has not been completely elucidated. The aim of this study was to determine the relevance of Toll-like receptor (TLR)2- and TLR4-stimulating activity of supragingival plaque to periodontal parameters. METHODS: We isolated 144 supragingival plaque samples and analyzed TLR2- and TLR4-stimulating activity using genetically engineered Chinese hamster ovary reporter cells that express a reporter molecule upon activation of nuclear factor-kappa B through TLR2 or TLR4. The numbers of Porphyromonas gingivalis, Aggregatibacter actinomycetemcomitans (previously Actinobacillus actinomycetemcomitans), and Streptococcus mutans cells in each plaque sample were determined by real-time polymerase chain reaction. RESULTS: The activity to induce TLR4-mediated stimulation, but not TLR2-mediated stimulation, was positively associated with the plaque score and bleeding on probing score of the teeth from which the plaque samples were taken. The activity to induce TLR2-mediated stimulation, but not TLR4-mediated stimulation, was negatively associated with probing depth and clinical attachment level. The ratio of TLR4-/TLR2-mediated stimulation was positively associated with all of those parameters. The number of P. gingivalis cells in each plaque sample was associated with the plaque score and clinical attachment level, but no strong association was observed between the ratio of examined bacteria in each plaque sample and the activity to induce TLR2- or TLR4-mediated stimulation, except for a weak correlation between the ratio of A. actinomycetemcomitans cells and the activity to induce TLR4-mediated stimulation. CONCLUSION: The TLR2- and TLR4-stimulating activity of supragingival plaque is associated with clinical parameters for gingivitis and periodontitis.

Group B streptococcus (GBS) is the most important cause of neonatal sepsis, which is mediated in part by TLR2. However, GBS components that potently induce cytokines via TLR2 are largely unknown. We found that GBS strains of the same serotype differ in released factors that activate TLR2. Several lines of genetic and biochemical evidence indicated that lipoteichoic acid (LTA), the most widely studied TLR2 agonist in Gram-positive bacteria, was not essential for TLR2 activation. We thus examined the role of GBS lipoproteins in this process by inactivating two genes essential for bacterial lipoprotein (BLP) maturation: the prolipoprotein diacylglyceryl transferase gene (lgt) and the lipoprotein signal peptidase gene (lsp). We found that Lgt modification of the N-terminal sequence called lipobox was not critical for Lsp cleavage of BLPs. In the absence of lgt and lsp, lipoprotein signal peptides were processed by the type I signal peptidase. Importantly, both the Deltalgt and the Deltalsp mutant were impaired in TLR2 activation. In contrast to released factors, fixed Deltalgt and Deltalsp GBS cells exhibited normal inflammatory activity indicating that extracellular toxins and cell wall components activate phagocytes through independent pathways. In addition, the Deltalgt mutant exhibited increased lethality in a model of neonatal GBS sepsis. Notably, LTA comprised little, if any, inflammatory potency when extracted from Deltalgt GBS. In conclusion, mature BLPs, and not LTA, are the major TLR2 activating factors from GBS and significantly contribute to GBS sepsis.

In response to invading pathogens, Toll-like receptors (TLR) play a critical role in the initiation of the innate immune response, which can be either beneficial or detrimental to the host. In the present study, we demonstrated that central nervous system (CNS) glial cells are activated by Lymphocytic Choriomeningitis Virus (LCMV) in a TLR2-MyD88/Mal-dependent manner. Specifically, in response to LCMV, both astrocytes and microglial cells isolated from wild-type (WT) mice produced chemokines, such as MCP-1, RANTES and TNF-alpha. Similar responses occurred in TLR3 KO and TLR4 KO glial cells. In striking contrast, both astrocytes and microglial cells isolated from mice deficient in TLR2, MyD88, and Mal did not produce any of these chemokines. In addition, LCMV infection of glial cells induced up-regulation of TLR2, MHC class-I and II, CD40, CD86 in a MyD88-dependent manner. These results define a functional role for TLR signaling in viral infection-induced activation of CNS glial cells as well as for the immunopathology in the CNS.

Lipoteichoic acid (LTA) is a major outer cell wall component of Gram-positive bacteria that has been implicated as an important factor in the inflammatory response following bacterial infection. In vitro data indicate roles for TLR2, platelet-activating factor receptor (PAFR), CD14, and LPS-binding protein (LBP) in cellular responsiveness to LTA, whereas the mechanisms contributing to LTA effects in vivo have never been investigated. Using mice deficient for LBP, CD14, TLR2, TLR4, or PAFR, we now examined the role of these molecules in pulmonary inflammation induced by highly purified LTA in vivo. Although pulmonary LBP increased dose-dependently following administration of LTA, the inflammatory response was unaltered in LBP-/- mice. TLR2 proved to be indispensable for the initiation of an inflammatory response, as polymorphonuclear cell influx, TNF-alpha, keratinocyte-derived chemokine, and MIP-2 release were abolished in TLR2-/- mice. Minor effects such as moderately decreased TNF-alpha and MIP-2 levels were observed in the absence of CD14, indicating a role for CD14 as a coreceptor. Quite surprisingly, the absence of TLR4 greatly diminished pulmonary inflammation and the same phenotype was observed in PAFR-/- animals. In contrast to all other mice studied, only TLR4-/- and PAFR-/- mice displayed significantly elevated IL-10 pulmonary concentrations. These data suggest that TLR2 is the single most important receptor signaling the presence of LTA within the lungs in vivo, whereas TLR4 and PAFR may influence lung inflammation induced by LTA either by sensing LTA directly or through recognition and signaling of endogenous mediators induced by the interaction between LTA and TLR2.