New players in TLR-mediated innate immunity: PI3K and small Rho GTPases

Inhibition of Rac activity alleviates lipopolysaccharide-induced acute pulmonary injury in mice

BACKGROUND

Rac small GTPases play important roles in cytoskeleton and many cell functions including cell cycle, cell growth, cell adhesion and gene transcription. Here, we investigated the roles of Rac including Rac1 and Rac2 in lipopolysaccharide (LPS)-induced pulmonary injury.

METHODS

After LPS was intratracheally instilled to lungs in mice, Rac, CDC42 and RhoA activation assay by pull-down and West blot, inflammatory cell infiltration assay by counting cell numbers and lung histological examination, pro-inflammatory mediator mRNA expression assay by quantitative RT-PCR, measurement of myeloperoxidase (MPO) activity, Evans Blue and albumin accumulation by spectrophotometry were performed to evaluate the roles of Rac in pulmonary injury by using its specific inhibitor, NSC23766.

RESULTS

LPS challenge led to increases of both Rac1 and Rac2, but not CDC42 or RhoA activities in lungs, and intraperitoneal administration with NSC23766 inhibited both Rac1 and Rac2, but not CDC42 or RhoA activities. Treatment with NSC23766 at 1 or 3mg/kg not only reduced the inflammatory cells infiltration and MPO activities, but also inhibited pro-inflammatory mediators, tumor necrosis factor-α and interleukin-1β, mRNA expression. Moreover, in vitro neutrophil migration assay and in vivo microvascular permeability assay indicated that NSC23766 not only inhibited neutrophil transwell migration toward a chemoattractant, fMLP, but also reduced Evans Blue and albumin accumulation in LPS-challenged lungs. LPS activated both Rac1 and Rac2, but not CDC42 or RhoA activities in lungs, and specific inhibition of Rac activities by NSC23766 effectively alleviated LPS-induced injury.

GENERAL SIGNIFICANCE

Rac could be a potential target for therapeutic intervention of pulmonary inflammation.

Bone marrow mononuclear cells up-regulate toll-like receptor expression and produce inflammatory mediators in response to cigarette smoke extract

Several reports link cigarette smoking with leukemia. However, the effects of cigarette smoke extract (CSE) on bone marrow hematopoiesis remain unknown. The objective of this study was to elucidate the direct effects of cigarette smoke on human bone marrow hematopoiesis and characterize the inflammatory process known to result from cigarette smoking. Bone marrow mononuclear cells (BMCs) from healthy individuals when exposed to CSE had significantly diminished CFU-E, BFU-E and CFU-GM. We found increased nuclear translocation of the NF-κB p65 subunit and, independently, enhanced activation of AKT and ERK1/2. Exposure of BMCs to CSE induced IL-8 and TGF-β1 production, which was dependent on NF-κB and ERK1/2, but not on AKT. CSE treatment had no effect on the release of TNF-α, IL-10, or VEGF. Finally, CSE also had a significant induction of TLR2, TLR3 and TLR4, out of which, the up-regulation of TLR2 and TLR3 was found to be dependent on ERK1/2 and NF-κB activation, but not AKT. These results indicate that CSE profoundly inhibits the growth of erythroid and granulocyte-macrophage progenitors in the bone marrow. Further, CSE modulates NF-κB- and ERK1/2-dependent responses, suggesting that cigarette smoking may impair bone marrow hematopoiesis in vivo as well as induce inflammation, two processes that proceed malignant transformation.

Chronic stress promotes lymphocyte reduction through TLR2 mediated PI3K signaling in a β-arrestin 2 dependent manner

Physical and psychological stress can alter the immune system in both humans and animals. Stress is a known risk factor for numerous human diseases, such as infectious and autoimmune diseases, and cancer. Toll-like receptors (TLRs) play a pivotal role in the induction of innate and adaptive immune response. Our previous studies have shown that TLR4 deficiency prevents stress-induced splenocyte reduction. However, the role of TLR2 in stress-mediated lymphocyte reduction is unknown. In this study, we investigated the effects of TLR2 ligands on stress-induced lymphocyte reduction. We also defined whether the phosphoinositide 3-kinases (PI3Ks)/Akt pathway contributes to TLR2-mediated lymphocyte numbers altered by stress. Our data have shown that stimulation of TLR2 by TLR2 ligands peptidoglycan (PGN) or Pam3CSK4 (Pam3) attenuates stress-induced reduction in lymphocyte numbers. However, TLR2 ligand-induced protection from stress-induced lymphocyte reduction is lost in TLR2 deficiency in mice. Furthermore, stimulation of TLR2 by PGN induces protection from stress-induced reduction in the number of splenocytes through PI3K. Moreover, PGN dramatically increases the level of phosphorylation of Akt through a PI3K-dependent manner. Moreover, we found that stimulation of TLR2 by PGN induced protection from stress-induced reduction in splenocyte numbers is abolished in β-arrestin 2 deficient mice. In addition, PGN-induced immune protection in stress-induced changes of cytokine levels appears to require β-arrestin 2, a multifunctional adaptor and signal transducer. Collectively, our study thus demonstrates that stimulation of TLR2-mediated PI3K signaling attenuates splenocyte reduction induced by stress, and that β-arrestin 2 modulates TLR2-mediated immune response following stress.

TLR2-dependent pathway of heterologous down-modulation for the CC chemokine receptors 1, 2, and 5 in human blood monocytes

During innate immune responses, the inflammatory CC chemokine receptors CCR1, CCR2, and CCR5 mediate the recruitment of blood monocytes to infected tissues by promoting cell migration in response to chemokines CCL2-5. Toll-like receptors also play an essential role, allowing pathogen recognition by the recruited monocytes. Here, we demonstrate that Toll-like receptor 2 (TLR2) stimulation by lipoteichoic acid (LTA) from Staphylococcus aureus leads to gradual down-modulation of CCR1, CCR2, and CCR5 from the plasma membrane of human blood-isolated monocytes and inhibits chemotaxis. Interestingly, LTA does not promote rapid desensitization of chemokine-mediated calcium responses. We found that the TLR2 crosstalk with chemokine receptors is not dependent on the Toll/interleukin-1 receptor domain-containing adaptor protein, but instead involves phospholipase C, the small G protein Rac1, and is phorbol ester sensitive. Activation of this pathway by LTA lead to β-arrestin-mediated endocytosis of Ser349-phosphorylated CCR5 into recycling endosomes, as does CCL5 treatment. However, LTA-induced internalization of CCR5 is a slower process associated with phospholipase C-mediated and phorbol ester-sensitive phosphorylation. Overall, our data indicate that TLR2 negatively regulates CCR1, CCR2, and CCR5 on human blood monocytes by activating the machinery used to support chemokine-dependent down-modulation and provide a molecular mechanism for inhibiting monocyte migration after pathogen recognition.

An integrative approach to the identification of Arabidopsis and rice genes involved in xylan and secondary wall development

Xylans constitute the major non-cellulosic component of plant biomass. Xylan biosynthesis is particularly pronounced in cells with secondary walls, implying that the synthesis network consists of a set of highly expressed genes in such cells. To improve the understanding of xylan biosynthesis, we performed a comparative analysis of co-expression networks between Arabidopsis and rice as reference species with different wall types. Many co-expressed genes were represented by orthologs in both species, which implies common biological features, while some gene families were only found in one of the species, and therefore likely to be related to differences in their cell walls. To predict the subcellular location of the identified proteins, we developed a new method, PFANTOM (plant protein family information-based predictor for endomembrane), which was shown to perform better for proteins in the endomembrane system than other available prediction methods. Based on the combined approach of co-expression and predicted cellular localization, we propose a model for Arabidopsis and rice xylan synthesis in the Golgi apparatus and signaling from plasma membrane to nucleus for secondary cell wall differentiation. As an experimental validation of the model, we show that an Arabidopsis mutant in the PGSIP1 gene encoding one of the Golgi localized candidate proteins has a highly decreased content of glucuronic acid in secondary cell walls and substantially reduced xylan glucuronosyltransferase activity.

Atrial natriuretic peptide protects against Staphylococcus aureus-induced lung injury and endothelial barrier dysfunction

Lung inflammation and alterations in endothelial cell (EC) permeability are key events to development of acute lung injury (ALI). Protective effects of atrial natriuretic peptide (ANP) have been shown against inflammatory signaling and endothelial barrier dysfunction induced by gram-negative bacterial wall liposaccharide. We hypothesized that ANP may possess more general protective effects and attenuate lung inflammation and EC barrier dysfunction by suppressing inflammatory cascades and barrier-disruptive mechanisms shared by gram-negative and gram-positive pathogens. C57BL/6J wild-type or ANP knockout mice (Nppa-/-) were treated with gram-positive bacterial cell wall compounds, Staphylococcus aureus-derived peptidoglycan (PepG) and/or lipoteichoic acid (LTA) (intratracheal, 2.5 mg/kg each), with or without ANP (intravenous, 2 μg/kg). In vitro, human pulmonary EC barrier properties were assessed by morphological analysis of gap formation and measurements of transendothelial electrical resistance. LTA and PepG markedly increased pulmonary EC permeability and activated p38 and ERK1/2 MAP kinases, NF-κB, and Rho/Rho kinase signaling. EC barrier dysfunction was further elevated upon combined LTA and PepG treatment, but abolished by ANP pretreatment. In vivo, LTA and PepG-induced accumulation of protein and cells in the bronchoalveolar lavage fluid, tissue neutrophil infiltration, and increased Evans blue extravasation in the lungs was significantly attenuated by intravenous injection of ANP. Accumulation of bronchoalveolar lavage markers of LTA/PepG-induced lung inflammation and barrier dysfunction was further augmented in ANP-/- mice and attenuated by exogenous ANP injection. These results strongly suggest a protective role of ANP in the in vitro and in vivo models of ALI associated with gram-positive infection. Thus ANP may have important implications in therapeutic strategies aimed at the treatment of sepsis and ALI-induced gram-positive bacterial pathogens.

The phosphoproteome of toll-like receptor-activated macrophages

Recognition of microbial danger signals by toll-like receptors (TLR) causes re-programming of macrophages. To investigate kinase cascades triggered by the TLR4 ligand lipopolysaccharide (LPS) on systems level, we performed a global, quantitative and kinetic analysis of the phosphoproteome of primary macrophages using stable isotope labelling with amino acids in cell culture, phosphopeptide enrichment and high-resolution mass spectrometry. In parallel, nascent RNA was profiled to link transcription factor (TF) phosphorylation to TLR4-induced transcriptional activation. We reproducibly identified 1850 phosphoproteins with 6956 phosphorylation sites, two thirds of which were not reported earlier. LPS caused major dynamic changes in the phosphoproteome (24% up-regulation and 9% down-regulation). Functional bioinformatic analyses confirmed canonical players of the TLR pathway and highlighted other signalling modules (e.g. mTOR, ATM/ATR kinases) and the cytoskeleton as hotspots of LPS-regulated phosphorylation. Finally, weaving together phosphoproteome and nascent transcriptome data by in silico promoter analysis, we implicated several phosphorylated TFs in primary LPS-controlled gene expression.

Sphingosine kinase-1 is required for toll mediated beta-defensin 2 induction in human oral keratinocytes

Background

Host defense against invading pathogens is triggered by various receptors including toll-like receptors (TLRs). Activation of TLRs is a pivotal step in the initiation of innate, inflammatory, and antimicrobial defense mechanisms. Human β-defensin 2 (HBD-2) is a cationic antimicrobial peptide secreted upon Gram-negative bacterial perturbation in many cells. Stimulation of various TLRs has been shown to induce HBD-2 in oral keratinocytes, yet the underlying cellular mechanisms of this induction are poorly understood.

Principal Findings

Here we demonstrate that HBD-2 induction is mediated by the Sphingosine kinase-1 (Sphk-1) and augmented by the inhibition of Glycogen Synthase Kinase-3β (GSK-3β) via the Phosphoinositide 3-kinase (PI3K) dependent pathway. HBD-2 secretion was dose dependently inhibited by a pharmacological inhibitor of Sphk-1. Interestingly, inhibition of GSK-3β by SB 216763 or by RNA interference, augmented HBD-2 induction. Overexpression of Sphk-1 with concomitant inhibition of GSK-3β enhanced the induction of β-defensin-2 in oral keratinocytes. Ectopic expression of constitutively active GSK-3β (S9A) abrogated HBD-2 whereas kinase inactive GSK-3β (R85A) induced higher amounts of HBD-2.

Conclusions/Significance

These data implicate Sphk-1 in HBD-2 regulation in oral keratinocytes which also involves the activation of PI3K, AKT, GSK-3β and ERK 1/2. Thus we reveal the intricate relationship and pathways of toll-signaling molecules regulating HBD-2 which may have therapeutic potential.

FoxO1 - the key for the pathogenesis and therapy of acne?

Five main factors play a pivotal role in the pathogenesis of acne: androgen dependence, follicular retention hyperkeratosis, increased sebaceous lipogenesis, increased colonization with P. acnes, and inflammatory events. This paper offers a solution for the pathogenesis of acne and explains all major pathogenic factors at the genomic level by a relative deficiency of the nuclear transcription factor FoxO1. Nuclear FoxO1 suppresses androgen receptor, other important nuclear receptors and key genes of cell proliferation, lipid biosynthesis and inflammatory cytokines. Elevated growth factors during puberty and persistent growth factor signals due to Western life style stimulate the export of FoxO1 out of the nucleus into the cytoplasm via activation of the phos-phoinositide-3-kinase (PI3K)/Akt pathway. By this mechanism, genes and nuclear receptors involved in acne are derepressed leading to increased androgen receptor-mediated signal transduction, increased cell proliferation of androgen-dependent cells, induction of sebaceous lipogenesis and upregulation of Toll-like-receptor-2-dependent inflammatory cytokines. All known acne-inducing factors exert their action by reduction of nuclear FoxO1 levels. In contrast, retinoids, antibiotics and dietary intervention will increase the nuclear content of FoxO1, thereby normalizing increased transcription of genes involved in acne. Various receptor-mediated growth factor signals are integrated at the level of PI3K/Akt activation which finally results in nuclear FoxO1 deficiency.

Renal CD14 expression correlates with the progression of cystic kidney disease

Monocyte and macrophage markers are among the most highly overexpressed genes in cpk mouse kidneys with severely progressive renal cystic disease. We show here that one of these markers, CD14, is abnormally transcribed, activated and shed in cystic kidneys. However, these abnormalities were not associated with an increased number of interstitial CD14-positive mononuclear cells. Instead, we found that most non-cystic and cystic renal tubular epithelia were CD14-positive; even distal nephron-derived principal cells. Cd14 was significantly overexpressed in the kidneys of 5-day-old cpk mice and further increased as the disease progressed. In the cpk model with variable rates of cystic kidney enlargement (due to an intercross of two distinct genetic backgrounds), Cd14 expression positively correlated with kidney volume, exceeding the correlation with MCP-1, an established marker of autosomal-dominant polycystic kidney disease (ADPKD). In 16 patients with ADPKD, the baseline urinary CD14 level showed some tendency to correlate with the 2-year change in total kidney volume; however, the tendency was not statistically significant. But the association was significant when the analysis was confined to males. Clearly more studies need to be done to evaluate the utility of CD14 as a marker for outcomes in ADPKD.

Myeloid PTEN promotes inflammation but impairs bactericidal activities during murine pneumococcal pneumonia

Phosphatidylinositol 3-kinase has been described as an essential signaling component involved in the chemotactic cell influx that is required to eliminate pathogens. At the same time, PI3K was reported to modulate the immune response, thus limiting the magnitude of acute inflammation. The precise role of the PI3K pathway and its endogenous antagonist phosphatase and tensin homolog deleted on chromosome 10 (PTEN) during clinically relevant bacterial infections is still poorly understood. Utilizing mice lacking myeloid cell-specific PTEN, we studied the impact of PTEN on the immune response to Streptococcus pneumoniae. Survival analysis disclosed that PTEN-deficient mice displayed less severe signs of disease and prolonged survival. The inflammatory response to S. pneumoniae was greatly reduced in macrophages in vitro and in vivo. Unexpectedly, neutrophil influx to the lungs was significantly impaired in animals lacking myeloid-cell PTEN, whereas the additional observation of improved phagocytosis by alveolar macrophages lacking PTEN ultimately resulted in unaltered lung CFUs following bacterial infection. Together, the absence of myeloid cell-associated PTEN and consecutively enhanced PI3K activity dampened pulmonary inflammation, reduced neutrophil influx, and augmented phagocytic properties of macrophages, which ultimately resulted in decreased tissue injury and improved survival during murine pneumococcal pneumonia.

Scaffolding adaptor protein Gab1 is required for TLR3/4- and RIG-I-mediated production of proinflammatory cytokines and type I IFN in macrophages

RIG-I-like helicases and TLRs are critical sensors in the induction of type I IFN and proinflammatory cytokines to initiate innate immunity against invading pathogens. However, the mechanisms for the full activation of TLR and RIG-I-triggered innate response remain to be fully investigated. Grb2-associated binder 1 (Gab1), a member of scaffolding/adaptor proteins, can mediate signal transduction from many receptors, however, whether and how Gab1 is required for TLR and RIG-I-triggered innate responses remain unknown. In this study, we demonstrated that Gab1 significantly enhances TLR4-, TLR3-, and RIG-I-triggered IL-6, IL-1beta, and IFN-alpha/beta production in macrophages. Gab1 knockdown in primary macrophages or Gab1 deficiency in mouse embryonic fibroblasts significantly suppresses TLR3/4- and RIG-I-triggered production of IL-6, IL-1beta, and IFN-alpha/beta. Consistently, Gab1 deficiency impairs vesicular stomatitis virus (VSV) infection-induced IFN-alpha/beta production. In addition to promoting both MyD88- and TLR/IL-1 receptor domain-containing adaptor protein inducing IFN-beta-dependent MAPKs and NF-kappaB activation, Gab1 enhances PI3K/Akt activation by directly binding p85 in TLR signaling and VSV infection. Accordingly, Gab1 inhibits VSV replication and VSV infection-induced cell damage by inducing type I IFNs and IFN-inducible gene expression via PI3K/Akt pathway. Therefore, Gab1 is needed for full activation of TLR3/4- and RIG-I-triggered innate responses by promoting activation of PI3K/Akt, MAPKs, and NF-kappaB pathways.

High-throughput screening for TLR3-IFN regulatory factor 3 signaling pathway modulators identifies several antipsychotic drugs as TLR inhibitors

TLR3 is one of the major innate immune sensors of dsRNA. The signal transduction pathway activated by TLR3, upon binding to dsRNA, leads to the activation of two major transcription factors: NF-kappaB and IFN regulatory factor (IRF) 3. In an effort to identify specific chemical modulators of TLR3-IRF3 signal transduction pathway, we developed a cell-based readout system. Using the IFN-stimulated gene 56 promoter-driven firefly luciferase gene stably integrated in a TLR3-expressing HEK293 cell line, we were able to generate a cell line where treatment with dsRNA resulted in a dose-dependent induction of luciferase activity. A screen of two pharmacologically active compound libraries using this system identified a number of TLR3-IRF3 signaling pathway modulators. Among them we focused on a subset of inhibitors and characterized their mode of action. Several antipsychotic drugs, such as sertraline, trifluoperazine, and fluphenazine, were found to be direct inhibitors of the innate immune signaling pathway. These inhibitors also showed the ability to inhibit IFN-stimulated gene 56 induction mediated by TLR4 and TLR7/8 pathways. Interestingly, they did not show significant effects on TLR3-, TLR7-, and TLR8-mediated NF-kappaB activation. Detailed analysis of the signaling pathway indicated that these drugs might be exerting their inhibitory effects on IRF3 via PI3K signaling pathway. The data presented in this study provide mechanistic explanation of possible anti-inflammatory roles of some antipsychotic drugs.

IL-6 increases B-cell IgG production in a feed-forward proinflammatory mechanism to skew hematopoiesis and elevate myeloid production

Src homology 2 domain-containing inositol 5-phosphatase (SHIP(-/-)) animals display an age-related increase in interleukin-6 (IL-6), a decrease in B lymphopoiesis, and an elevation in myelopoiesis. We investigated the origin of the IL-6 production and show that it is largely produced by peritoneal and splenic macrophages. IL-6 production by these macrophages is not a direct result of the loss of SHIP: IL-6 production is not spontaneous, is absent from bone marrow-derived macrophages, declines with prolonged culture of macrophages, and requires a stimulus present in vivo. The IL-6-rich peritoneal cavity of SHIP(-/-) mice shows more than 700-fold more immunoglobulin G (IgG) than wild-type, approximately 20% of which is aggregated or in an immune complex and contains B220(+) cells that secrete IgG. The SHIP-deficient peritoneal macrophages show evidence of IgG receptor stimulation. Animals lacking both the signal-transducing gamma-chain of IgG receptors and SHIP or Ig and SHIP produce less IL-6. The data indicate a feed-forward process in which peripheral macrophages, responding through IgG receptors to secreted IgG, produce IL-6, to support further B-cell production of IgG. Because of the proinflammatory phenotype of SHIP(-/-) animals, these findings emphasize the importance of IL-6-neutralizing strategies in autoimmune and proinflammatory diseases.

The pathophysiology of septic shock

There is a profound cellular dysfunction in sepsis, that clinically manifests as a continuum from simple, uncomplicated sepsis to severe sepsis, and finally to septic shock. Septic shock remains a significant challenge for clinicians. Recent advances in cellular and molecular biology have significantly improved our understanding of its pathogenetic mechanisms. These improvements will translate to better care and improved outcomes for these patients.

TLR2 ligands attenuate cardiac dysfunction in polymicrobial sepsis via a phosphoinositide 3-kinase-dependent mechanism

Myocardial dysfunction is a major consequence of septic shock and contributes to the high mortality of sepsis. In the present study, we examined the effect of Toll-like receptor 2 (TLR2) ligands, peptidoglycan (PGN), and Pam3CSK4 (Pam3) on cardiac function in cecal ligation and puncture (CLP)-induced sepsis in mice. We also investigated whether the phosphoinositide 3-kinase (PI3K)/Akt signaling pathway is involved in the effect of TLR2 ligands on cardiac function in CLP mice. PGN was administered to C57B6/L mice 1 h before the induction of CLP. Sham surgically operated mice served as a control. Cardiac function indexes (rate of change in left ventricular pressure, stroke work, cardiac output, and ejection fraction) were examined by a microconductance pressure catheter. Cardiac function was significantly decreased 6 h after CLP-induced sepsis compared with sham-operated control. In contrast, PGN administration attenuated CLP-induced cardiac dysfunction. Importantly, the therapeutic treatment with Pam3 1 h after CLP also significantly attenuated cardiac dysfunction in CLP mice. However, the beneficial effect of TLR2 ligands on cardiac dysfunction in CLP-mice was abolished in TLR2-deficient mice. PGN administration significantly increased the levels of phospho-Akt and phospho-GSK-3beta in the myocardium compared with the levels in untreated CLP mice. PI3K inhibition abolished the PGN-induced attenuation of cardiac dysfunction in CLP mice. In conclusion, these data demonstrate that the administration of TLR2 ligands, PGN, or Pam3 attenuates cardiac dysfunction in septic mice via a TLR2/PI3K-dependent mechanism. More significantly, Pam3 therapeutic treatment will have a potential clinical relevance.

Toll-like receptor signaling in cell proliferation and survival

Toll-like receptors (TLRs) are important sensors of foreign microbial components as well as products of damaged or inflamed self tissues. Upon sensing these molecules, TLRs initiate a series of downstream signaling events that drive cellular responses including the production of cytokines, chemokines, and other inflammatory mediators. This outcome results from the intracellular assembly of protein complexes that drive phosphorylation and other signaling cascades ultimately leading to chromatin remodeling and transcription factor activation. In addition to driving inflammatory responses, TLRs also regulate cell proliferation and survival which serves to expand useful immune cells and integrate inflammatory responses and tissue repair processes. In this context, central TLR signaling molecules, such as the mitogen-activated protein kinases (MAPK) and phosphoinositide 3-kinase (PI3K), play key roles. In addition, four major groups of transcription factors which are targets of TLR activation also control cell fate. This review focuses on the role of TLR signaling as it relates to cell proliferation and survival. This topic not only has important implications for understanding host defense and tissue repair, but also cancer which is often associated with conditions of chronic inflammation.

Impaired osteoclastogenesis by staphylococcal lipoteichoic acid through Toll-like receptor 2 with partial involvement of MyD88

Degenerative bone disease, marked by excessive loss of calcified matrix, is often associated with bacterial infections. Osteoclasts, which mediate the bone-resorptive process, are derived mainly from myeloid precursor cells of the monocyte/macrophage lineage, from which cells with phagocytic and inflammatory capacities may alternatively arise. Here, we investigated the effect of LTA, a major cell-wall virulence factor of Gram-positive bacteria, on osteoclast differentiation. Osteoclast precursors were prepared from C57BL/6 mouse BM using M-CSF and RANKL. When osteoclastogenesis was induced in the presence of staphylococcal LTA, LTA dose-dependently inhibited the differentiation of osteoclast precursors to mature osteoclasts. A corresponding inhibition of bone-resorptive function was observed in the reduced resorption area on calcium phosphate-coated culture plates. In contrast, the phagocytic and inflammatory potential of the osteoclast precursors increased in the presence of LTA. TLR2, known to recognize LTA, might be essential for the LTA inhibition of osteoclastogenesis, as the inhibition did not occur in the precursors from TLR2-deficient mice. Importantly, MyD88-dependent and MyD88-independent pathways would participate in the inhibition, as determined using MyD88-deficient cells. Moreover, LTA inhibited phosphorylation of ERK and JNK in osteoclast precursors stimulated with M-CSF and RANKL, concomitantly with a decreased DNA-binding activity of AP-1. These results suggest that staphylococcal LTA inhibits osteoclast differentiation primarily through TLR2 but also in part through MyD88 signaling, which in turn, inhibits activation of ERK, JNK, and AP-1.

Rho proteins are negative regulators of TLR2, TLR3, and TLR4 signaling in astrocytes

The family of Toll-like receptors (TLRs) expressed by innate immune cells recognizes a spectrum of microbial components as well as molecules released from injured tissues. TLR ligation activates intracellular signaling cascades that culminate in the up-regulation of proinflammatory genes. We have recently demonstrated that the up-regulation of inflammatory cytokines mediated by TLR4 in astrocytes is negatively controlled by the monomeric GTPases of Rho subfamily. The present study was undertaken to examine further the involvement of Rho proteins in the inflammatory response of astrocytes elicited by the ligation of three TLRs that use divergent signaling pathways. Astrocyte cultures established from newborn rat brains were challenged with ligands of TLR2, TLR3, and TLR4. The expression of genes encoding interleukin (IL)-1beta, IL-6, tumor necrosis factor-alpha (TNFalpha), interferon-beta (IFNbeta), and inducible nitric oxide synthase (NOS2) was up-regulated 24 hr after the challenge as determined by real-time RT-PCR. Pretreatment of the cells with toxin B, which specifically inactivates Rho proteins, enhanced the up-regulation of gene expression. The extent of this enhancement was both receptor and gene dependent. The enhancing effect of Rho protein inactivation was also evident at the protein level of IL-6 and NOS2 as revealed by ELISA and immunoblot analyses, respectively. These results suggest that Rho proteins control TLR-mediated up-regulation of inflammatory genes in astrocytes by interfering with multiple events along the signaling pathways.

Rapamycin enhances LPS induction of tissue factor and tumor necrosis factor-alpha expression in macrophages by reducing IL-10 expression

Bacterial lipopolysaccharide (LPS) induces monocytes/macrophages to express proinflammatory cytokines and tissue factor (TF), the primary activator of the coagulation cascade. Anti-inflammatory signaling pathways including the phosphatidylinositol-3-kinase (PI3K)-Akt pathway inhibit proinflammatory and TF gene expression in macrophages. We determined the role of Akt, the mammalian target of rapamycin (mTOR) and interleukin-10 in the inhibition of LPS-induced proinflammatory cytokine and TF gene expression in peritoneal macrophages (PMs). We used wild type (WT) peritoneal macrophages (PMs), and PMs from PTEN(flox/flox)/LysMCre mice (PTEN(-/-) PMs), which have increased Akt activity. Pharmacologic inhibition of mTOR with rapamycin inhibited LPS induction of IL-10 mRNA and protein, and enhanced the expression of TF and the proinflammatory cytokine TNFalpha in WT PMs. Furthermore, neutralizing IL-10 with anti-IL-10 antibody enhanced LPS induction of TNFalpha and TF expression in WT PMs. The addition of recombinant IL-10 abolished rapamycin enhancement of LPS-induced TNFalpha and TF expression in WT PMs. Consistent with enhanced Akt activation, LPS-induced IL-10 expression was increased in PTEN(-/-) PMs compared to WT PMs. In contrast, LPS-induced TNFalpha and TF expression was significantly reduced in PTEN(-/-) PMs compared to WT PMs. However, the neutralizing IL-10 antibody did not completely prevent inhibition of LPS-induced TNFalpha and TF expression in PTEN(-/-) PMs. The results indicate that mTOR dependent IL-10 expression leads to inhibition of LPS induction of TF and the proinflammatory cytokine TNFalpha in WT macrophages. In contrast, the decrease in LPS-induced TNFalpha and TF expression in PTEN(-/-) PMs also requires an IL-10-independent pathway.

Chronic restraint stress promotes immune suppression through toll-like receptor 4-mediated phosphoinositide 3-kinase signaling

Stress, either psychological or physical, can have a dramatic impact on the immune system. Toll-like receptors (TLRs) play a pivotal role in the induction of innate and adaptive immune response. We have reported that stress modulates the immune response in a TLR4-dependent manner. However, the mechanisms underlying TLR4-mediated signaling in stress modulation of immune system have not been identified. Here, we demonstrate an essential role for the TLR4-mediated phosphoinositide 3-kinase (PI3K)/Akt signaling. PI3K inhibition by inhibitors wortmannin or LY294002 abrogated protection of stress-induced immune suppression in TLR4-deficient mice compared with TLR4-deficient mice that did not receive the inhibitors. The mechanisms by which PI3K are increased in the TLR4-deficient lymphocytes may involve increased phosphorylation of Akt as well as increased phosphorylation of glycogen synthase kinase-3 beta (GSK-3 beta). The stress-mediated suppression of T help 1 (Th1) cytokine and increased production of Th2 cytokine was greatly reduced in TLR4 deficient mice compared with the wild type mice. Moreover, inhibition of PI3K diminished protection of the above Th1 and Th2 changes caused by stress in TLR4-deficient mice compared with non-stressed mice and the wild type mice. Our data demonstrated that TLR4 negatively regulates PI3K activity in wild type mice, leading to the observed the stress-induced immune response. The higher levels of PI3K prevent TLR4 deficient mice from the stress-induced immune response. Therefore, stress modulates the immune system through TLR4-mediated PI3K/Akt signaling.

Molecular biology of inflammation and sepsis: a primer

BACKGROUND

Remarkable progress has been made during the last decade in defining the molecular mechanisms that underlie septic shock. This rapidly expanding field is leading to new therapeutic opportunities in the management of severe sepsis.

AIM

To provide the clinician with a timely summary of the molecular biology of sepsis and to better understand recent advances in sepsis research.

DATA SELECTION

Medline search of relevant publications in basic mechanisms of sepsis/severe sepsis/septic shock, and selected literature review of other manuscripts about the signalosome, inflammasome, apoptosis, or mechanisms of shock. DATA SYNTHESIS AND FINDINGS: The identification of the toll-like receptors and the associated concept of innate immunity based upon pathogen- or damage-associated molecular pattern molecules allowed significant advances in our understanding of the pathophysiology of sepsis. The essential elements of the inflammasome and signal transduction networks responsible for activation of the host response have now been characterized. Apoptosis, mitochondrial dysfunction, sepsis-related immunosuppression, late mediators of systemic inflammation, control mechanisms for coagulation, and reprogramming of immune response genes all have critical roles in the development of sepsis.

CONCLUSIONS

Many of these basic discoveries have direct implications for the clinical management of sepsis. The translation of these "bench-to-bedside" findings into new therapeutic strategies is already underway. This brief review provides the clinician with a primer into the basic mechanisms responsible for the molecular biology of sepsis, severe sepsis, and septic shock.

Regulation of TLR4-mediated signaling by IBP/Def6, a novel activator of Rho GTPases

TLRs play a fundamental role in innate immune responses. Although Rho GTPases have been implicated in TLR-mediated signaling pathways, the molecules that control their activation in response to TLR engagement are largely unknown. IFN regulatory factor-4-binding protein (IBP; which is encoded by the gene Def6) is a unique type of activator for Rac that plays a crucial role in TCR-mediated signaling and adaptive immune responses. Here, we demonstrate that IBP/Def6 also controls innate immune responses by modulating TLR-induced signaling events. Mice deficient in IBP/Def6 are protected from LPS-induced septic shock. This protection is associated with a decrease in the production of proinflammatory cytokines and is accompanied by diminished activation of MAPKs and NF-kappaB. Our results thus identify IBP/Def6 as a novel component of the TLR4-induced signaling cascade that controls the production of proinflammatory cytokines.

Toll-like receptor-4 coordinates the innate immune response of the kidney to renal ischemia/reperfusion injury

Toll-like receptors (TLRs) can detect endogenous danger molecules released upon tissue injury resulting in the induction of a proinflammatory response. One of the TLR family members, TLR4, is constitutively expressed at RNA level on renal epithelium and this expression is enhanced upon renal ischemia/reperfusion (I/R) injury. The functional relevance of this organ-specific upregulation remains however unknown. We therefore investigated the specific role of TLR4 and the relative contribution of its two downstream signaling cascades, the MyD88-dependent and TRIF-dependent cascades in renal damage by using TLR4−/−, MyD88−/− and TRIF-mutant mice that were subjected to renal ischemia/reperfusion injury. Our results show that TLR4 initiates an exaggerated proinflammatory response upon I/R injury, as reflected by lower levels of chemokines and infiltrating granulocytes, less renal damage and a more preserved renal function in TLR4−/− mice as compared to wild type mice. In vitro studies demonstrate that renal tubular epithelial cells can coordinate an immune response to ischemic injury in a TLR4-dependent manner. In vivo we found that epithelial- and leukocyte-associated functional TLR4 contribute in a similar proportion to renal dysfunction and injury as assessed by bone marrow chimeric mice. Surprisingly, no significant differences were found in renal function and inflammation in MyD88−/− and TRIF-mutant mice compared with their wild types, suggesting that selective targeting of TLR4 directly may be more effective for the development of therapeutic tools to prevent I/R injury than targeting the intracellular pathways used by TLR4. In conclusion, we identified TLR4 as a cellular sentinel for acute renal damage that subsequently controls the induction of an innate immune response.

CC chemokine receptor 4 modulates Toll-like receptor 9-mediated innate immunity and signaling

The present study addressed the modulatory role of CC chemokine receptor 4 (CCR4) in Toll-like receptor (TLR) 9-mediated innate immunity and explored the underlying molecular mechanisms. Our results demonstrated that CCR4-deficient mice were resistant to both septic peritonitis induced by cecal ligation and puncture (CLP) and CpG DNA/D-galactosamine-induced shock. In bone marrow-derived macrophages (BMMPhi) from CLP-treated CCR4-deficient mice, TLR9-mediated pathways of MAPK/AP-1, PI3K/Akt, and IkappaB kinase (IKK)/NF-kappaB were impaired compared to wild-type (WT) cells. While TLR9 expression was not altered, the intensity of internalized CpG DNA was increased in CCR4-deficient macrophages when compared to WT macrophages. Pharmacological inhibitor studies revealed that impaired activation of JNK, PI3K/Akt, and/or IKK/NF-kappaB could be responsible for decreased proinflammatory cytokine expression in CCR4-deficient macrophages. Interestingly, the CCR4-deficient BMMPhi exhibited an alternatively activated (M2) phenotype and the impaired TLR9-mediated signal transduction responses in CCR4-deficient cells were similar to the signaling responses observed in WT BMMPhi skewed to an alternatively activated phenotype. These results indicate that macrophages deficient in CCR4 impart a regulatory influence on TLR9-mediated innate immunity.

Leishmania donovani infection down-regulates TLR2-stimulated IL-12p40 and activates IL-10 in cells of macrophage/monocytic lineage by modulating MAPK pathways through a contact-dependent mechanism

The failure of Leishmania, an intracellular pathogen, to stimulate a pro-inflammatory response following entry into macrophages has been well reported. This occurs in spite of the fact that ligands for the toll-like receptors (TLR) have been recently shown on the parasite surface and their role in disease protection well documented. The outcome of infection in leishmaniasis is determined by the Th1 versus Th2 nature of the effector response and the generation of IL-12 and IL-10 by the infected macrophages is important for this decision. We evaluated the effect of L. donovani infection of monocytes (cell line THP-1, and monocytes derived from human peripheral blood) on Pam3cys (TLR2 ligand) and lipopolysaccharide (TLR4 ligand) stimulated production of IL-12p40 and IL-10. L. donovani infection caused suppression of TLR2 and TLR4-stimulated IL-12p40, with an increase in IL-10 production. Parasites also modulated the TLR2-stimulated mitogen-activated protein kinase (MAPK) pathway by suppressing MAPK P(38) phosphorylation and activating extracellular regulated kinase (ERK)1/2 phosphorylation. These effects could be reversed either by using a MAPK P(38) activator, anisomycin, or ERK1/2 inhibitor, U0126. L. donovani caused modulation of TLR2-stimulated MAPK pathways in a contact-dependent mechanism. In addition parasite structural integrity but not viability was required for suppression of TLR2-stimulated IL-12p40 and activation of IL-10. These observations suggest that L. donovani has evolved survival strategies that subvert the pro-inflammatory response generated through TLRs.

Extracellular high-mobility group box 1 acts as an innate immune mediator to enhance autoimmune progression and diabetes onset in NOD mice

OBJECTIVE

The implication of innate immunity in type 1 diabetes development has long been proposed. High-mobility group box 1 (HMGB1), an evolutionarily conserved chromosomal protein, was recently recognized to be a potent innate inflammatory mediator when released extracellularly. We sought to test the hypothesis that HMGB1 acts as an innate immune mediator implicated in type 1 diabetes pathogenesis.

RESEARCH DESIGN AND METHODS

Eight- and 12-week-old NOD mice were treated with an HMGB1 neutralizing antibody once a week until 25 weeks of age and monitored for insulitis progression and diabetes onset. The underlying mechanisms of HMGB1 regulation of autoimmune response were further explored.

RESULTS

During autoimmunity, HMGB1 can be passively released from damaged pancreatic beta-cells and actively secreted by islet infiltrated immune cells. Extracellular HMGB1 is potent in inducing NOD dendritic cell maturation and stimulating macrophage activation. Blockade of HMGB1 significantly inhibited insulitis progression and diabetes development in both 8- and 12-week-old NOD mice. HMGB1 antibody treatment decreased the number and maturation of pancreatic lymph node (PLN) CD11c(++)CD11b(+) dendritic cells, a subset of dendritic cells probably associated with autoantigen presentation to naïve T-cells, but increased the number for PLN CD4(+)Foxp3(+) regulatory T-cells. Blockade of HMGB1 also decreased splenic dendritic cell allo-stimulatory capability associated with increased tolergenic CD11c(+)CD8a(+) dendritic cells. Interestingly, the number of CD8(+)interferon-gamma(+) (Tc1) T-cells was increased in the PLNs and spleen after blockade of HMGB1, which could be associated with retarded migration of activated autoreactive T-cells into the pancreatic islets.

CONCLUSIONS

Extracellular HMGB1 functions as a potent innate immune mediator contributing to insulitis progression and diabetes onset.

Genetic analysis of the role of the PI3K-Akt pathway in lipopolysaccharide-induced cytokine and tissue factor gene expression in monocytes/macrophages

LPS stimulation of monocytes/macrophages induces the expression of genes encoding proinflammatory cytokines and the procoagulant protein, tissue factor. Induction of these genes is mediated by various signaling pathways, including mitogen-activated protein kinases, and several transcription factors, including Egr-1, AP-1, ATF-2, and NF-kappaB. We used a genetic approach to determine the role of the phosphatidylinositol-3-kinase (PI3K)-protein kinase B (Akt) pathway in the regulation of LPS signaling and gene expression in isolated macrophages and in mice. The PI3K-Akt pathway is negatively regulated by the phosphatase and tensin homologue (PTEN). We used peritoneal exudate cells from Pik3r1-deficient mice, which lack the p85alpha regulatory subunit of PI3K and have reduced PI3K activity, and peritoneal macrophages from PTEN(flox/flox)/LysMCre mice (PTEN(-/-)), which have increased Akt activity. Analysis of LPS signaling in Pik3r1(-/-) and PTEN(-/-) cells indicated that the PI3K-Akt pathway inhibited activation of the ERK1/2, JNK1/2, and p38 mitogen-activated protein kinases and reduced the levels of nuclear Egr-1 protein and phosphorylated ATF-2. Modulating the PI3K-Akt pathway did not affect LPS-induced degradation of IkappaBalpha or NF-kappaB nuclear translocation. LPS induction of TNF-alpha, IL-6, and tissue factor gene expression was increased in Pik3r1(-/-) peritoneal exudate cells and decreased in PTEN(-/-) peritoneal macrophages compared with wild-type (WT) cells. Furthermore, LPS-induced inflammation and coagulation were enhanced in WT mice containing Pik3r1(-/-) bone marrow compared with WT mice containing WT bone marrow and in mice lacking the p85alpha subunit in all cells. Taken together, our results indicate that the PI3K-Akt pathway negatively regulates LPS signaling and gene expression in monocytes/macrophages.

ROCK, PAK, and Toll of synapses in Alzheimer's disease

Alzheimer's disease is a neurodegenerative disorder where the cognitive deficit is the hallmark symptom reflecting the progression of the disease. Synaptic dysfunction is a sensitive parameter of the AD pathology. Rho GTPases and the Rho kinases, ROCK1/2, and PAK1-3, are important regulators of synaptic plasticity, especially in maintaining the actin cytoskeleton of dendritic spines. Recent studies have revealed that beta-amyloid oligomers can inhibit PAK and stimulate ROCK-mediated signaling. Both of these effects enhance the disassembly of synaptic actin filaments and ultimately evoke synaptic loss. Brain tissue in AD recognizes the beta-amyloid peptide oligomers as foreign protein particles and mounts an inflammatory defense via Toll-like receptor (TLR) signaling which causes synaptic impairment. We will review here the dysfunction of ROCK, PAK, and Toll signaling associated with AD pathology. The protection of synapses in AD may provide new therapeutic approaches to combatting the cognitive impairment in AD.

Sphingosine 1-phosphate 1 and TLR4 mediate IFN-beta expression in human gingival epithelial cells

IFN-beta production is a critical step in human innate immune responses and is primarily controlled at the transcription level by highly ordered mechanisms. IFN-beta can be induced by pattern-recognition receptors such as the TLR4. S1P1 is a G protein-coupled receptor, which has a high affinity for sphingosine 1-phosphate (S1P). Although many of the receptors and signaling pathways leading to the expression of IFN-beta have been identified and characterized, it is still unclear how IFN-beta is regulated in primary human gingival epithelial cells (HGECs). In this study, we demonstrate that S1P1 and TLR4, acting in unison, play an important role in IFN-beta expression at the protein and mRNA level in HGECs. We demonstrate that the expression of both IFN-beta and IFN-inducible protein-10 (CXCL-10) is significantly up-regulated by LPS and S1P or LPS and a specific S1P1 agonist. This enhanced innate immune response is attenuated in HGECs by small interfering RNA knockdown of either TLR4 or S1P1. Moreover, we show that triggering of TLR4 results in the increased expression of S1P1 receptors. Furthermore, we found that IFN-regulatory factor 3 activation was maximized by LPS and S1P through PI3K. Our data show that triggering TLR4 increases S1P1, such that both TLR4 and S1P1 acting through PI3K enhancement of IFN-regulatory factor 3 activation increase IFN-beta expression in epithelial cells. The functional association between TLR4 and the S1P1 receptor demonstrates a novel mechanism in the regulation of IFN-beta and CXCL-10 in human primary gingival epithelial cells.

The covalent modification and regulation of TLR8 in HEK-293 cells stimulated with imidazoquinoline agonists

The mammalian TLRs (Toll-like receptors) mediate the rapid initial immune response to pathogens through recognition of pathogen-associated molecular patterns. The pathogen pattern to which TLR8 responds is ssRNA (single-stranded RNA) commonly associated with ssRNA viruses. TLR8 also responds to small, purine-like molecules including the imidazoquinoline IRMs (immune-response modifiers). The IRMs include molecules that selectively activate TLR7, selectively activate TLR8 or non-selectively activate both TLR7 and TLR8. Using HEK-293 cells (human embryonic kidney cells) stably expressing an NF-kappaB (nuclear factor kappaB)/luciferase promoter-reporter system as a model system, we have examined the regulation of TLR8 using the non-selective TLR7/8 agonist, 3M-003. Using conservative tyrosine to phenylalanine site-directed mutation, we show that of the 13 tyrosine residues resident in the cytosolic domain of TLR8, only three appear to be critical to TLR8 signalling. Two of these, Tyr898 and Tyr904, reside in the Box 1 motif and the third, Tyr1048, lies in a YXXM putative p85-binding motif. TLR8 is tyrosine-phosphorylated following 3M-003 treatment and TLR8 signalling is inhibited by tyrosine kinase inhibitors. Treatment with 3M-003 results in the association of the p85 regulatory subunit of PI3K (phosphoinositide 3-kinase) with TLR8 and this association is inhibited by tyrosine to phenylalanine mutation of either the YXXM or Box 1 motifs. As a further consequence of activation by 3M-003, TLR8 is modified to yield both higher and lower molecular mass species. These species include a monoubiquitinated form as deduced from ubiquitin peptide sequencing by HPLC/MS/MS (tandem MS).

AKAP13, a RhoA GTPase-specific guanine exchange factor, is a novel regulator of TLR2 signaling

Members of the guanine exchange factor (GEF) family of scaffold proteins are involved in the integration of signal flow downstream of many receptors in adaptive immunity. However, the full complement of GEFs that function downstream of Toll-like receptors (TLRs) requires further identification and functional understanding. By systematically integrating expression profiles from immune and epithelial cells with functional studies, we demonstrate that protein kinase A anchoring protein 13 (AKAP13), a scaffold protein with GEF activity, is an activator of NF-kappaB downstream of TLR2 signaling. Stimulation of the human macrophage cell line THP-1 and epithelial cells with a TLR2 ligand caused a significant up-regulation in AKAP13 mRNA, corresponding to an increase in protein expression. Analysis of TLR2 reporter cell lines deficient in AKAP13 expression revealed significantly reduced NF-kappaB activation and reduced secretion of interleukin-8 and MCP-1 in response to specific ligand stimulation. Furthermore, NF-kappaB activation was partially inhibited by a GEF-deficient AKAP13 mutant. AKAP13 was also involved in phosphorylation of JNK but not of extracellular signal-regulated kinase ERK1 and -2 following ligand stimulation. Together, our results suggest that AKAP13 plays a role in TLR2-mediated NF-kappaB activation and suggest that GEF-containing scaffold proteins may confer specificity to innate immune responses downstream of TLRs.

RhoE participates in the stimulation of the inflammatory response induced by ethanol in astrocytes

Astroglial cells are involved in the neuropathogenesis of several inflammatory diseases of the brain, where the activation of inflammatory mediators and cytokines plays an important role. We have previously demonstrated that ethanol up-regulates inflammatory mediators in both brain and astroglial cells. Since Rho GTPases are involved in inflammatory responses of astrocytes where loss of stress fibers takes place and RhoE/Rnd3 disorganizes the actin cytoskeleton, the aim of the present study was to investigate the implication of this protein in the stimulation of inflammatory signaling induced by ethanol. Our findings show that RhoE expression induces a decrease in both RhoA and Rac. In addition, RhoE not only induces actin cytoskeleton disorganization but it also stimulates both the IRAK/ERK/NF-kappaB pathway and the COX-2 expression associated with the inflammatory response in these cells. Our results also show that ethanol exposure induces RhoE signaling in astrocytes. Preincubation of astrocytes with GF109203X, an inhibitor of PKCs, reduces the RhoE levels and abolishes the ethanol-induced activation of IRAK, NF-kappaB and the COX-2 expression. Furthermore, RhoE overexpression restores ethanol responses in astrocytes treated with the PKCs inhibitor. Altogether, our findings suggest that this small GTPase is involved in the stimulation of the inflammatory signaling induced by ethanol in astrocytes. These findings provide new insights into the molecular mechanism involved in the inflammatory responses in astrocytes.

Altered innate immune functioning of dendritic cells in elderly humans: a role of phosphoinositide 3-kinase-signaling pathway

Aging represents a state of paradox where chronic inflammation is associated with declining immune responses. Dendritic cells (DCs) are the major APCs responsible for initiating an immune response. However, DC functions in aging have not been studied in detail. In this study, we have compared the innate immune functions of monocyte-derived myeloid DCs from elderly subjects with DCs from young individuals. We show that although phenotypically comparable, DCs from the aging are functionally different from DCs from the young. In contrast to DCs from the young, DCs from elderly individuals display 1) significantly reduced capacity to phagocytose Ags via macropinocytosis and endocytosis as determined by flow cytometry; 2) impaired capacity to migrate in vitro in response to the chemokines MIP-3beta and stromal cell-derived factor-1; and 3) significantly increased LPS and ssRNA-induced secretion of TNF-alpha and IL-6, as determined by ELISA. Investigations of intracellular signaling revealed reduced phosphorylation of AKT in DCs from the aging, indirectly suggesting decreased activation of the PI3K pathway. Because the PI3K-signaling pathway plays a positive regulatory role in phagocytosis and migration, and also functions as a negative regulator of TLR signaling by inducing activation of p38 MAPK, this may explain the aberrant innate immune functioning of DCs from elderly subjects. Results from real-time PCR and protein expression by flow cytometry demonstrated an increased expression of phosphatase and tensin homolog, a negative regulator of the PI3K-signaling pathway, in DCs from the aging. Increased phosphatase and tensin homolog may thus be responsible for the defect in AKT phosphorylation and, therefore, the altered innate immune response of DCs from elderly humans.

TLR2 Is Constitutively Expressed within the Kidney and Participates in Ischemic Renal Injury through Both MyD88-Dependent and -Independent Pathways

TLRs are an evolutionarily conserved family of cell membrane proteins believed to play a significant role in innate immunity and the response to tissue injury, including that induced by ischemia. TLR signaling pathways activate transcription factors that regulate expression of prosurvival proteins, as well as proinflammatory cytokines and chemokines through one of two proximal adapter proteins, MyD88 or Toll/IL-1R domain-containing adaptor-inducing IFN-beta (Trif). Our study defines the constitutive protein expression of TLR2 in kidneys of humans and mice, and provides insight into the signaling mechanisms by which a deficiency of TLR2 protects from ischemic organ injury. Our study compared and contrasted the effects of renal ischemia in wild-type mice and mice deficient in TLR2, MyD88, Trif, and MyD88xTrif. TLR2 protein was evident in many cell types in the kidney, including renal tubules of the outer stripe of the medulla, glomeruli, and in the renal vasculature. The pattern of protein expression was similar in humans and mice. The absence of TLR2, MyD88, and MyD88xTrif conferred both physiologic and histologic protection against sublethal ischemia at 24 h. Interestingly, TLR2-deficient mice were better protected from ischemic renal injury than those deficient for the adapter protein MyD88, raising the intriguing possibility that TLR-2-dependent/MyD88-independent pathways also contribute to kidney injury. We conclude that TLR2 protein is constitutively expressed in the kidney and plays an important role in the pathogenesis of acute ischemic injury by signaling both MyD88-dependent and MyD88-independent pathways.

Toll-like receptor 2 controls mucosal inflammation by regulating epithelial barrier function

BACKGROUND & AIMS

Toll-like receptors (TLRs) represent a class of transmembrane pattern recognition receptors essential for microbial recognition and control of innate immune responses. Commensal bacteria play an important role in maintaining tolerance and active stability of the intestinal epithelial barrier by suppressing intestinal inflammation, yet the mechanisms of action are unknown. The aim of this study was to determine the functional relevance of TLR2 to control tight junction (TJ)-associated intestinal epithelial barrier integrity to balance mucosal homeostasis against inflammatory stress-induced damage.

METHODS

TLR2 ligand (synthetic Pam(3)Cys-SK4 [PCSK])-induced activation of signaling cascades and TJ-associated distribution was assessed by using Western blotting and confocal microscopy combined with functional transfection and inhibitor studies in model intestinal epithelial cell (IEC) lines (IEC-6, Caco-2) or primary IEC cultured short-term ex vivo. DSS colitis was induced by standard protocol in wild-type, TLR2-/-, and MyD88-/- mice. Spontaneous apoptosis was assessed by terminal deoxinucleotidyl-transferase-mediated dUTP-biotin nick end-labeling.

RESULTS

Data from in vitro and ex vivo models of intestinal epithelial cells revealed that TLR2 stimulation effectively preserves TJ-associated barrier assembly against stress-induced damage through promotion of PI3K/Akt-mediated cell survival via MyD88. Furthermore, in vivo studies underscored that TLR2-mediated TJ regulation critically determines susceptibility to intestinal injury and inflammation. Inflammatory stress in mice deficient of TLR2 or MyD88 induced early TJ-associated disruption interrelated with anti-apoptotic failure of the intestinal epithelial barrier. Oral treatment of colitis with the TLR2 ligand PCSK significantly suppressed mucosal inflammation and apoptosis by efficiently restoring TJ-associated integrity of the intestinal epithelium in vivo.

CONCLUSION

TLR2 may provide a target to pharmacologically modulate mucosal injury and intestinal inflammation.

Extracellular hmgb1 functions as an innate immune-mediator implicated in murine cardiac allograft acute rejection

Hmgb1, an evolutionarily conserved chromosomal protein, was recently re-discovered to be an innate immune-mediator contributing to both innate and adaptive immune responses. Here, we show a pivotal role for Hmgb1 in acute allograft rejection in a murine cardiac transplantation model. Extracellular Hmgb1 was found to be a potent stimulator for adaptive immune responses. Hmgb1 can be either passively released from damaged cells after organ harvest and ischemia/reperfusion insults, or actively secreted by allograft infiltrated immune cells. After transplantation, allografts show a significant temporal up-regulation of Hmgb1 expression accompanied by inflammatory infiltration, a consequence of graft destruction. These data suggest the involvement of Hmgb1 in acute allograft rejection. In line with these observations, treatment of recipients with rA-box, a specific blockade for endogenous Hmgb1, significantly prolonged cardiac allograft survival as compared to those recipients treated with either rGST or control vehicle. The enhanced graft survival is associated with reduced allograft expression of TNFalpha, IFNgamma and Hmgb1 and impaired Th1 immune response.