Mediators of innate immune recognition of bacteria concentrate in lipid rafts and facilitate lipopolysaccharide-induced cell activation

Polycationic lipids inhibit the pro-inflammatory response to LPS

Lipopolysaccharide (LPS) is a major component of the outer membrane of Gram-negative bacteria. As such, it signals monocytes, macrophages and neutrophils to up-regulate phagocytic functions and to release pro-inflammatory cytokines. Despite the established role of CD14 as the main LPS receptor, the precise nature of the LPS signalling complex and its compartmentalization remain unknown. Interactions of LPS with other cell surface molecules such as TLR-4 and MD-2, and its subsequent internalization are required for LPS signalling. Here, we show that the polycationic lipid LipoFectamine causes inhibition of the LPS-induced MAPK activation and lack of pro-inflammatory cytokine production, despite proper localization of CD14 within lipid rafts and massive LPS internalization. The ability of LipoFectamine to inhibit LPS induced pro-inflammatory responses may be due to uncoupling of CD14 from TLR-4/MD-2 in the LPS signalling complex of mouse macrophages/microglial cells, as suggested by inhibition of LPS-induced concomitant internalization of these surface molecules. Thus, LipoFectamine may be a useful tool to dissect the molecular interactions leading to LPS signalling, and identifies a potential therapeutic strategy for LPS clearance.

TLR signaling: an emerging bridge from innate immunity to atherogenesis

Chronic inflammation and disordered lipid metabolism represent hallmarks of atherosclerosis. Considerable evidence suggests that innate immune defense mechanisms might interact with proinflammatory pathways and contribute to development of arterial plaques. The preponderance of such evidence has been indirect clinical and epidemiologic studies, with some support from experimental animal models of atherosclerosis. However, recent data now directly implicate signaling by TLR4 in the pathogenesis of atherosclerosis, establishing a key link between atherosclerosis and defense against both foreign pathogens and endogenously generated inflammatory ligands. In this study, we briefly review these and closely related studies, highlighting areas that should provide fertile ground for future studies aimed at a more comprehensive understanding of the interplay between innate immune defense mechanisms, atherosclerosis, and related vascular disorders.

Lipid rafts in cytokine signaling

Lipid rafts are established as critical structures for a variety of cellular processes, including immune cell activation. Beyond their importance for initial immune cell activation at the immunological synapse, lipid rafts are now also being recognized as important sites for cytokine and growth factor signal transduction, both in immune cells as part of secondary regulatory processes, and in non-immune cells. This review summarizes current knowledge regarding the roles of rafts in cytokine signaling and emphasizes the need for measures to better standardize the study of rafts.

Coxsackievirus B4-induced cytokine production in pancreatic cells is mediated through toll-like receptor 4

Coxsackievirus B4 (CBV4), a member of the Picornavirus genus, has long been implicated in the development of insulin-dependent diabetes mellitus (IDDM) caused by virus-induced pancreatic cell damage. The progressive destruction of pancreatic beta cells is responsible for the development of IDDM. It has recently been suggested that CBV4 infection can induce the production of proinflammatory cytokines, and these cytokines seem to be involved in the damage to the insulin-producing cells. In this study we investigated whether toll-like receptors (TLRs) are responsible for triggering the proinflammatory cytokine production in human pancreatic cells in response to CBV4. Here we demonstrate that CBV4 triggers cytokine production through a TLR4-dependent pathway. This interaction seems to be independent of virus attachment and cell entry.

Disruption of membrane cholesterol stimulates MyD88-dependent NF-kappaB activation in immature B cells

Agents that extract or sequester membrane cholesterol stimulate IkappaB degradation and lead to NF-kappaB activation in a subset of B cells. Although the extraction of cholesterol by methyl-beta-cyclodextrin is the most potent stimulus of NF-kappaB, other agents that sequester cholesterol have similar effects. B cells and B cell lines with an immature phenotype are significantly more sensitive to the effects of cholesterol perturbation than their mature B cell counterparts. NF-kappaB activation does not involve signaling from the B cell receptor complex. Instead, the disruption of membrane cholesterol activates NF-kappaB through a MyD88-dependent pathway involving the pattern recognition receptor, Toll-like receptor 4. We suggest that lipid raft microdomains may serve not only to orchestrate receptor signaling, but to sequester signaling components one from one another, which serves to prevent receptor-mediated signaling from occurring. A role for this process during B cell development is suggested.

Lipid-raft-dependent Coxsackievirus B4 internalization and rapid targeting to the Golgi

Coxsackievirus B4 (CBV4), a member of the Picornavirus genus, has long been implicated in the development of insulin-dependent diabetes mellitus (IDDM), by viral-induced pancreatic cell damage. Although the pancreotropic nature of this virus is well documented, the early stages of CBV4 viral infection that involve the attachment of virions to the cell surface by binding to their cellular receptors followed by entry into the cell, are poorly understood. In this study, we show that the entry of CBV4 requires functional lipid rafts as the site of virus attack. In addition, we show that this virus is endocytosed independently of clathrin-associated machinery and is delivered to the Golgi via a lipid-raft-dependent mechanism.

Suppression of innate immunity by acute ethanol administration: a global perspective and a new mechanism beginning with inhibition of signaling through TLR3

Excessive consumption of ethanol (EtOH) suppresses innate immunity, but the mechanisms have not been fully delineated. The present study was conducted to determine whether EtOH suppresses TLR signaling in vivo in mice and to characterize the downstream effects of such suppression. Degradation of IL-1R-associated kinase 1 induced by a TLR3 ligand in peritoneal cells ( approximately 90% macrophages) was suppressed by EtOH. Phosphorylation of p38 kinase in peritoneal macrophages (F4/80(+)) was suppressed, as was nuclear translocation of p-c-Jun and p65 in peritoneal cells. EtOH decreased IL-6 and IL-12 (p40), but did not significantly affect IL-10 in peritoneal lavage fluid or in lysates of peritoneal cells. Changes in cytokine mRNAs (by RNase protection assay) in macrophages isolated by cell sorting or using Ficoll were generally consistent with changes in protein levels in cell lysates and peritoneal lavage fluid. Thus, suppression of TLR signaling and cytokine mRNA occurred in the same cells, and this suppression generally corresponded to changes in i.p. and intracellular cytokine concentrations. DNA microarray analysis revealed the suppression of an IFN-related amplification loop in peritoneal macrophages, associated with decreased expression of numerous innate immune effector genes (including cytokines and a chemokine also suppressed at the protein level). These results indicate that EtOH suppresses innate immunity at least in part by suppressing TLR3 signaling, suppressing an IFN-related amplification loop, and suppressing the induction of a wide range of innate effector molecules in addition to proinflammatory cytokines and chemokines.

Transplantation studies in C3-deficient animals reveal a novel role of the third complement component (C3) in engraftment of bone marrow cells

Mice deficient in complement C3 (C3(-/-)) are hematologically normal under steady-state conditions, and yet displayed a significant delay in hematopoietic recovery from either irradiation or transplantation of wild-type (WT) hematopoietic stem/progenitor cells (HSPC). Transplantation of histocompatible WT Sca-1(+) cells into C3(-/-) mice resulted in a (i) decrease in day 12 CFU-S, (ii) 5-7-day delay in platelet and leukocyte recovery, and (iii) reduced number of BM CFU-GM progenitors at day 16 after transplantation. Nevertheless, HSPC from C3(-/-) mice engrafted normally into irradiated WT mice, suggesting that there was a defect in the hematopoietic environment of C3(-/-) mice. Since C3(-/-) mice cannot activate/cleave C3, the C3 fragments C3a, C3a(des-Arg), and iC3b were examined for a role in HSPC engraftment. Liquid-phase C3a and C3a(des-Arg) increased CXCR4 incorporation into membrane lipid rafts (thus potentiating HSPC responses to SDF-1 gradients), whereas iC3b was deposited onto irradiated BM cells and functioned to tether CR3(CD11b/CD18)(+)HSPC to damaged stroma. The activity of C3a(des-Arg) suggested that C3aR(+)HSPC also expressed the C5L2 (receptor for C3a and C3a(des-Arg)) and this was confirmed. In conclusion, a novel mechanism for HSC engraftment was identified, which involves complement activation and specific C3 fragments that promote conditioning for transplantation and enhance HSPC engraftment.

Lipoteichoic Acid and Toll-like Receptor 2 Internalization and Targeting to the Golgi Are Lipid Raft-dependent

Lipoteichoic acid (LTA), a key cell wall component of Gram-positive bacteria, seems to function as an immune activator with characteristics very similar to lipopolysaccharide from Gram-negative bacteria. It has been shown that LTA binds CD14 and triggers activation via Toll-like receptor 2, but whether the activation occurs at the cell surface or internalization is required to trigger signaling has yet to be demonstrated. In this work we have investigated LTA binding and internalization and found that LTA and its receptor molecules accumulate in lipid rafts and are subsequently targeted rapidly to the Golgi apparatus. This internalization seems to be lipid raft-dependent because raft-disrupting drugs inhibited LTA/Toll-like receptor 2 colocalization in the Golgi. Similarly to lipopolysaccharide, LTA activation occurs at the cell surface, and the observed trafficking is independent of signaling.

Selective priming to Toll-like receptor 4 (TLR4), not TLR2, ligands by P. acnes involves up-regulation of MD-2 in mice

Lipopolysaccharide (LPS) triggers cytokine production through Toll-like receptor 4 (TLR4), which shares downstream signaling pathways with TLR2. We investigated the roles of TLR2 and TLR4 in Propionibacterium acnes (P. acnes)-primed, LPS-induced liver damage using selective TLR ligands. Stock LPS induced interleukin 8 in both TLR4- and TLR2-expressing human embryonic kidney (HEK) 293 cells. Purified LPS (TLR4 ligand) activated HEK/TLR4 cells, while peptidoglycan and lipoteichoic acid (TLR2 ligands) activated HEK/TLR2 cells, respectively. In mice, P. acnes priming resulted in increased liver messenger RNA (mRNA) and serum levels of tumor necrosis factor alpha, interleukin 12, and interferon gamma (IFN-gamma) by both stock LPS and purified LPS challenges compared with nonprimed controls. In contrast, P. acnes failed to sensitize to TLR2 ligands (peptidoglycan + lipoteichoic acid). In the liver, P. acnes-priming was associated with up-regulation of TLR4 and MD-2 proteins, and subsequent LPS challenge further increased MD-2 and CD14 mRNA levels. The lack of sensitization to TLR2 ligands by P. acnes correlated with no increase in hepatic TLR1 or TLR6 mRNA. In vitro, P. acnes pretreatment desensitized RAW macrophages to a secondary stimulation via both TLR2 and TLR4. However, IFN-gamma could selectively prevent desensitization to TLR4 but not to TLR2 ligands. Furthermore, P. acnes induced production of IFN-gamma in vivo as well as in isolated splenocytes. In vitro, P. acnes-primed Hepa 1-6 hepatocytes but not RAW macrophages produced increased MD-2 and CD14 mRNA levels after an LPS challenge. In conclusion, P. acnes priming to selective TLR4-mediated liver injury is associated with up-regulation of TLR4 and MD-2 and is likely to involve IFN-gamma and prevent TLR4 desensitization by P. acnes.

Lipid Rafts Regulate Lipopolysaccharide-induced Activation of Cdc42 and Inflammatory Functions of the Human Neutrophil

Lipid rafts are cholesterol-rich membrane microdomains that are thought to act as coordinated signaling platforms by regulating dynamic, agonist-induced translocation of signaling proteins. They have been described to play a role in multiple prototypical cascades, among them the lipopolysaccharide pathway, and to host multiple signaling proteins, including kinases and low molecular weight G-proteins. Here we report lipopolysaccharide-induced activation of the Rho family GTPase Cdc42, and we show its activation in the human neutrophil to be mediated by a p38 mitogen-activated protein kinase-dependent mechanism. Subcellular fractionation reveals that lipopolysaccharide induces translocation of Cdc42 to lipid rafts, where it and p38 are both found to be activated. By contrast, lipopolysaccharide causes translocation of Rac from the polymorphonuclear leukocyte (PMN) rafts and does not induce its activation. With the use of methyl-beta-cyclodextrin, a cholesterol-depleting agent that reversibly disrupts rafts, we confirm an important regulatory role for rafts in the activation state of p38 and Cdc42 and in the Rho GTPase-dependent functions superoxide anion production and actin polymerization. Methyl-beta-cyclodextrin induces activation of p38 and Cdc42, but not Rac, in the nonstimulated PMN, yet inhibits subsequent lipopolysaccharide-induced activation of p38 and Cdc42. In parallel, methyl-beta-cyclodextrin primes the human PMN for subsequent superoxide release triggered by the formylated bacterial tripeptide formyl-Met-Leu-Phe, and induces actin polymerization in a subcellular distribution distinct from that induced by lipopolysaccharide. In sum, these findings provide evidence for an important regulatory role of cholesterol in both transmission of the lipopolysaccharide signal and the inflammatory phenotype of the human neutrophil.

Implications of lipid raft disintegration: enhanced anti-inflammatory macrophage phenotype

BACKGROUND

Lipid rafts are membrane microdomains characterized by an enriched cholesterol environment and appear to serve as a platform for signaling. Their role within the macrophage during endotoxin exposure is unknown.

METHODS

THP-1 cells were subjected to lipopolysaccharide stimulation with or without methyl-beta-cyclodextrin (MbetaCD) pretreatment, a cholesterol depleting agent. Cell surface expression of toll-like receptor-4 (TLR4) and platelet-activating factor receptor (PAFr) was determined by flow cytometry. Membrane receptor components and activation of the mitogen-activated protein kinases (MAPK) was determined from lipid raft and cellular protein by immunoblot. Inflammatory mediator production was determined from harvested supernatants by enzyme-linked immunosorbent assay.

RESULTS

Surface expression of TLR4 and PAFr was not affected by MbetaCD. Lipopolysaccharide stimulation led to TLR4 mobilization to lipid rafts, MAPK activation, and inflammatory mediator production. Pretreatment with MbetaCD did not affect TLR4 mobilization to lipid rafts, but did result in lost lipid raft expression of the PAFr coupled G-protein, Galpha1. MbetaCD treatment led to selective attenuation of MAPK activation through ERK 1/2. This dysregulated signaling was associated with attenuated production of tumor necrosis factor-alpha, but increased production of interleukin-10.

CONCLUSION

Lipid raft disintegration results in lost expression of Galpha1, dysregulated MAPK signaling, and selective anti-inflammatory mediator production. Therefore, modulation of lipid raft cholesterol content may represent a potential mechanism for regulation of macrophage phenotypic differentiation.

Lipid Raft-Associated GTPase Signaling Controls Morphology and CD8+T Cell Stimulatory Capacity of Human Dendritic Cells

Their eponymous morphology and unique ability to activate naive T cells are hallmark features of dendritic cells (DCs). Specific properties of the actin cytoskeleton may define both characteristics. In search for regulators that coordinate DC phenotype and function, we observed strongly increased expression of the actin-remodeling GTPases Cdc42 and Rac1 during DC development from human stem cells. Cdc42 and Rac1 are constitutively active in immature DCs, and their activity is further up-regulated by maturational stimuli such as LPS or CD40L. Activation of Rac1 is associated with its rapid recruitment into lipid rafts. Cdc42 is not recruited into rafts, but readily activated by raft-associated moieties. The functional interplay of rafts, GTPases, and cortical actin is further shown by GTPase activation and actin remodeling after pharmacological disruption of lipid rafts and by the loss of the actin-based DC morphology by transfection of dominant-negative Cdc42 and Rac1. Both Cdc42 and Rac1 also control the transport of essential immunostimulatory molecules to the DC surface. Transfection with dominant-negative GTPases led to reduced surface expression of MHC class I and CD86. Consecutively, DCs display a reduced stimulatory capacity for CD8(+) T cells, whereas MHC class II-dependent stimulation of CD4(+) T cells remains unperturbed. We conclude that Cdc42 and Rac1 signaling controls DC morphology and conditions DCs for efficient CD8(+) T cell stimulation.

Combinational clustering of receptors following stimulation by bacterial products determines lipopolysaccharide responses

The innate immune system has the capacity to recognize a wide range of pathogens based on conserved PAMPs (pathogen-associated molecular patterns). In the case of bacterial LPS (lipopolysaccharide) recognition, the best studied PAMP, it has been shown that the innate immune system employs at least three cell-surface receptors: CD14, TLR4 (Toll-like receptor 4) and MD-2 protein. CD14 binds LPS from Enterobacteriaceae and then transfers it to MD-2, leading to TLR4 aggregation and signal transduction. LPS analogues such as lipid IVa seem to act as LPS antagonists in human cells, but exhibit LPS mimetic activity in mouse cells. Although TLR4 has been shown to be involved in this species-specific discrimination, the mechanism by which this is achieved has not been elucidated. The questions that remain are how the innate immune system can discriminate between LPS from different bacteria as well as different LPS analogues, and whether or not the structure of LPS affects its interaction with the CD14-TLR4-MD-2 cluster. Is it possible that the 'shape' of LPS induces the formation of different receptor clusters, and thus a different immune response? In the present study, we demonstrate using biochemical as well as fluorescence-imaging techniques that different LPS analogues trigger the recruitment of different receptors within microdomains. The composition of each receptor cluster as well as the number of TLR4 molecules that are recruited within the cluster seem to determine whether an immune response will be induced or inhibited.

TLR4 Is the Signaling but Not the Lipopolysaccharide Uptake Receptor

TLR4 is the primary recognition molecule for inflammatory responses initiated by bacterial LPS (endotoxin). Internalization of endotoxin by various cell types is an important step for its removal and detoxification. Because of its role as an LPS-signaling receptor, TLR4 has been suggested to be involved in cellular LPS uptake as well. LPS uptake was investigated in primary monocytes and endothelial cells derived from TLR4 and CD14 knockout C57BL/6 mice using tritiated and fluorescein-labeled LPS. Intracellular LPS distribution was investigated by deconvolution confocal microscopy. We could not observe any difference in LPS uptake and intracellular LPS distribution in either monocytes or endothelial cells between TLR4(-/-) and wild-type cells. As expected, CD14(-/-) monocytes showed a highly impaired LPS uptake, confirming CD14-dependent uptake in monocytes. Upon longer incubation periods, the CD14-deficient monocytes mimicked the LPS uptake pattern of endothelial cells. Endothelial cell LPS uptake is slower than monocyte uptake, LBP rather than CD14 dependent, and sensitive to polyanionic polymers, which have been shown to block scavenger receptor-dependent uptake mechanisms. We conclude that TLR4 is not involved in cellular LPS uptake mechanisms. In membrane CD14-positive cells, LPS is predominantly taken up via CD14-mediated pathways, whereas in the CD14-negative endothelial cells, there is a role for scavenger receptor-dependent pathways.

Lipopolysaccharide-Induced Macrophage Inflammatory Response Is Regulated by SHIP

LPS stimulates monocytes/macrophages through TLR4, resulting in the activation of a series of signaling events that potentiate the production of inflammatory mediators. Recent reports indicated that the inflammatory response to LPS is diminished by PI3K, through the activation of the serine/threonine kinase Akt. SHIP is an inositol phosphatase that can reverse the activation events initiated by PI3K, including the activation of Akt. However, it is not known whether SHIP is involved in TLR4 signaling. In this study, we demonstrate that LPS stimulation of Raw 264.7 mouse macrophage cells induces the association of SHIP with lipid rafts, along with IL-1R-associated kinase. In addition, SHIP is tyrosine phosphorylated upon LPS stimulation. Transient transfection experiments analyzing the function of SHIP indicated that overexpression of a wild-type SHIP, but not the SHIP Src homology 2 domain-lacking catalytic activity, up-regulates NF-kappaB-dependent gene transcription in response to LPS stimulation. These results suggest that SHIP positively regulates LPS-induced activation of Raw 264.7 cells. To test the validity of these observations in primary macrophages, LPS-induced events were compared in bone marrow macrophages derived from SHIP(+/+) and SHIP(-/-) mice. Results indicated that LPS-induced MAPK phosphorylation is enhanced in SHIP(+/+) cells, whereas Akt phosphorylation is enhanced in SHIP(-/-) cells compared with SHIP(+/+) cells. Finally, LPS-induced TNF-alpha and IL-6 production was significantly lower in SHIP(-/-) bone marrow-derived macrophages. These results are the first to demonstrate a role for SHIP in TLR4 signaling, and propose that SHIP is a positive regulator of LPS-induced inflammation.

Peptidoglycan and lipoteichoic acid in gram-positive bacterial sepsis: receptors, signal transduction, biological effects, and synergism

In sepsis and multiple organ dysfunction syndrome (MODS) caused by gram-negative bacteria, lipopolysaccharide (LPS) initiates the early signaling events leading to the deleterious inflammatory response. However, it has become clear that LPS can not reproduce all of the clinical features of sepsis, which emphasize the roles of other contributing factors. Gram-positive bacteria, which lack LPS, are today responsible for a substantial part of the incidents of sepsis with MODS. The major wall components of gram-positive bacteria, peptidoglycan and lipoteichoic acid, are thought to contribute to the development of sepsis and MODS. In this review, the literature underlying our current understanding of how peptidoglycan and lipoteichoic acid activate inflammatory responses will be presented, with a focus on recent advances in this field.

Targeting of scavenger receptor class B type I by synthetic amphipathic alpha-helical-containing peptides blocks lipopolysaccharide (LPS) uptake and LPS-induced pro-inflammatory cytokine responses in THP-1 monocyte cells

Human scavenger receptor class B type I, CLA-1, mediates lipopolysaccharide (LPS) binding and internalization (Vishnyakova, T. G., Bocharov, A. V., Baranova, I. N., Chen, Z., Remaley, A. T., Csako, G., Eggerman, T. L., and Patterson, A. P. (2003) J. Biol. Chem. 278, 22771-22780). Because one of the recognition motifs in SR-B1 ligands is the anionic amphipathic alpha-helix, we analyzed the effects of model amphipathic alpha-helical-containing peptides on LPS uptake and LPS-stimulated cytokine production. The L-37pA model peptide, containing two class A amphipathic helices, bound with high affinity (K(d) = 0.94 microg/ml) to CLA-1-expressing HeLa cells with a 10-fold increased capacity when compared with mock transfected HeLa cells. Both LPS and L-37pA colocalized with anti-CLA-1 antibody and directly bound CLA-1 as determined by cross-linking. SR-BI/CLA-1 ligands such as HDL, apoA-I, and L-37pA efficiently competed against iodinated L-37pA. Bacterial LPS, lipoteichoic acid, and hsp60 also competed against iodinated L-37pA. Model peptides blocked uptake of iodinated LPS in both mock transfected and CLA-1-overexpressing HeLa cells. Bound and internalized Alexa-L-37pA and BODIPY-LPS colocalized at the cell surface and perinuclear compartment. Both ligands were predominantly transported to the Golgi complex, colocalizing with the Golgi markers bovine serum albumin-ceramide, anti-Golgin97 antibody, and cholera toxin subunit B. A 100-fold excess of L-37pA nearly eliminated BODIPY-LPS binding and internalization. L-37pA and its d-amino acid analogue, D-37pA peptide were similarly effective in blocking LPS, Gram-positive bacterial wall component lipoteichoic acid and bacterial heat shock protein Gro-EL-stimulated cytokine secretion in THP-1 cells. In the same culture media used for the cytokine stimulation study, neither L-37pA nor D-37pA affected the Limulus amebocyte lysate activity of LPS, indicating that LPS uptake and cytokine stimulation were blocked independently of LPS neutralization. These results demonstrate that amphipathic helices of exchangeable apolipoproteins may represent a general host defense mechanism against inflammation.

Rotavirus RRV associates with lipid membrane microdomains during cell entry

Rotavirus cell entry is a multistep process, not completely understood, which requires at least four interactions between the virus and cell surface molecules. In this work, we investigated the role of the sphingolipid- and cholesterol-enriched lipid microdomains (rafts) in the entry of rotavirus strain RRV to MA104 cells. We found that ganglioside GM1, integrin subunits alpha2 and beta3, and the heat shock cognate protein 70 (hsc70), all of which have been implicated as rotavirus receptors, are associated with TX-100 and Lubrol WX detergent-resistant membranes (DRMs). Integrin subunits alpha2 and beta3 were found to be particularly enriched in DRMs resistant to lysis by Lubrol WX. When purified RRV particles were incubated with cells at 4 degrees C, about 10% of the total infectious virus was found associated with DRMs, and the DRM-associated virus increased to 37% in Lubrol-resistant membrane domains after 60-min incubation at 37 degrees C. The virus was excluded from DRMs if the cells were treated with methyl-beta-cyclodextrin (MbetaCD). Immunoblot analysis of the viral proteins showed that the virus surface proteins became enriched in DRMs upon incubation at 37 degrees C, being almost exclusively localized in Lubrol-resistant DRMs after 60 min. These data suggest that detergent-resistant membrane domains play an important role in the cell entry of rotaviruses, which could provide a platform to facilitate the efficient interaction of the rotavirus receptors with the virus particle.

Molecular mechanisms of macrophage activation and deactivation by lipopolysaccharide: roles of the receptor complex

Bacterial lipopolysaccharide (LPS), the major structural component of the outer wall of Gram-negative bacteria, is a potent activator of macrophages. Activated macrophages produce a variety of inflammatory cytokines. Excessive production of cytokines in response to LPS is regarded as the cause of septic shock. On the other hand, macrophages exposed to suboptimal doses of LPS are rendered tolerant to subsequent exposure to LPS and manifest a profoundly altered response to LPS. Increasing evidence suggests that monocytic cells from patients with sepsis and septic shock survivors have characteristics of LPS tolerance. Thus, an understanding of the molecular mechanisms underlying activation and deactivation of macrophages in response to LPS is important for the development of therapeutics for septic shock and the treatment of septic shock survivors. Over the past several years, significant progress has been made in identifying and characterizing several key molecules and signal pathways involved in the regulation of macrophage functions by LPS. In this paper, we summarize the current findings of the functions of the LPS receptor complex, which is composed of CD14, Toll-like receptor 4 (TLR4), and myeloid differentiation protein-2 (MD-2), and the signal pathways of this LPS receptor complex with regard to both activation and deactivation of macrophages by LPS. In addition, recent therapeutic approaches for septic shock targeting the LPS receptor complex are described.

On the mechanism and significance of ligand-induced internalization of human neutrophil chemokine receptors CXCR1 and CXCR2

It is well established that leukocyte chemotactic receptors, a subset of G protein-coupled receptors, undergo endocytosis after stimulation by ligand. However, the significance of this phenomenon to cell motility and other important leukocyte functions induced by chemoattractants has not been clearly defined. Here we show that in primary human neutrophils, the threshold levels of agonist required for endocytosis of the chemotactic receptors CXCR1 and CXCR2 were approximately 10-fold or higher than those needed for maximal chemotactic and calcium flux responses. Moreover, when stimulated by agonists at concentrations that are high enough for chemotaxis but too low for receptor endocytosis, neutrophil CXCR1 and CXCR2 could be reactivated in response to repeated application of the same agonist. Both receptors were excluded from Triton X-100-insoluble lipid rafts, and at high agonist concentrations were rapidly endocytosed by a clathrin/rab5/dynamin-dependent pathway. These data support the conclusion that neutrophil migration in response to CXCR1 or CXCR2 agonists is not dependent on endocytosis of CXCR1 or CXCR2. Rather than being integral to the process of cell migration, receptor endocytosis may be a terminal stop signal when cells reach the focus of inflammation where the chemoattractant concentrations are the highest.

Dynamic redistribution of raft domains as an organizing platform for signaling during cell chemotaxis

Spatially restricted activation of signaling molecules governs critical aspects of cell migration; the mechanism by which this is achieved nonetheless remains unknown. Using time-lapse confocal microscopy, we analyzed dynamic redistribution of lipid rafts in chemoattractant-stimulated leukocytes expressing glycosyl phosphatidylinositol-anchored green fluorescent protein (GFP-GPI). Chemoattractants induced persistent GFP-GPI redistribution to the leading edge raft (L raft) and uropod rafts of Jurkat, HL60, and dimethyl sulfoxide-differentiated HL60 cells in a pertussis toxin-sensitive, actin-dependent manner. A transmembrane, nonraft GFP protein was distributed homogeneously in moving cells. A GFP-CCR5 chimera, which partitions in L rafts, accumulated at the leading edge, and CCR5 redistribution coincided with recruitment and activation of phosphatidylinositol-3 kinase gamma in L rafts in polarized, moving cells. Membrane cholesterol depletion impeded raft redistribution and asymmetric recruitment of PI3K to the cell side facing the chemoattractant source. This is the first direct evidence that lipid rafts order spatial signaling in moving mammalian cells, by concentrating the gradient sensing machinery at the leading edge.

Innate immune responses to mycobacteria and the downregulation of atopic responses

PURPOSE OF REVIEW

Exposure to certain environmental microorganisms can promote the induction of T regulatory cells via the innate immune system. This review explores the possibility that reduced exposure to such organisms is leading to increased immunoregulatory disorders in a subset of individuals in whom this regulatory T-cell-inducing pathway is less efficient. We concentrate on mycobacteria and on asthma, because these are well documented.

RECENT FINDINGS

The blood cells of the children of farmers, who are partly protected from allergies, express increased levels of messenger RNA encoding CD14 and TLR2, and polymorphisms of CD14 are linked to allergic manifestations in some studies. Polymorphisms of TLR2 (which recognizes mycobacterial components in concert with CD14) are involved in the pattern of response to mycobacteria, and in the type of leprosy that develops. Similarly, polymorphisms of Nramp1, which affect the response to mycobacteria, are linked with the diseases of immunodysregulation that are increasing in parallel with allergic disorders. Moreover, congenic mice bearing different variants of Nramp1 differ in their allergic responses. These parallels are suggestive, in view of the observation that a saprophytic environmental mycobacterium is a potent inducer of regulatory T cells, and has shown significant effects in several phase I/II studies in man.

SUMMARY

The components of the innate immune system that are involved in responses to mycobacteria overlap with those implicated in allergic disorders. Polymorphisms might define the subset of individuals who develop immunoregulatory disorders. Understanding the role of the innate immune system will facilitate the design of clinical trials using microbial products.

Saturated fatty acid activates but polyunsaturated fatty acid inhibits Toll-like receptor 2 dimerized with Toll-like receptor 6 or 1

Toll-like receptor 4 (TLR4) and TLR2 agonists from bacterial origin require acylated saturated fatty acids in their molecules. Previously, we reported that TLR4 activation is reciprocally modulated by saturated and polyunsaturated fatty acids in macrophages. However, it is not known whether fatty acids can modulate the activation of TLR2 or other TLRs for which respective ligands do not require acylated fatty acids. A saturated fatty acid, lauric acid, induced NFkappaB activation when TLR2 was co-transfected with TLR1 or TLR6 in 293T cells, but not when TLR1, 2, 3, 5, 6, or 9 was transfected individually. An n-3 polyunsaturated fatty acid (docosahexaenoic acid (DHA)) suppressed NFkappaB activation and cyclooxygenase-2 expression induced by the agonist for TLR2, 3, 4, 5, or 9 in a macrophage cell line (RAW264.7). Because dimerization is considered one of the potential mechanisms for the activation of TLR2 and TLR4, we determined whether the fatty acids modulate the dimerization. However, neither lauric acid nor DHA affected the heterodimerization of TLR2 with TLR6 as well as the homodimerization of TLR4 as determined by co-immunoprecipitation assays in 293T cells in which these TLRs were transiently overexpressed. Together, these results demonstrate that lauric acid activates TLR2 dimers as well as TLR4 for which respective bacterial agonists require acylated fatty acids, whereas DHA inhibits the activation of all TLRs tested. Thus, responsiveness of different cell types and tissues to saturated fatty acids would depend on the expression of TLR4 or TLR2 with either TLR1 or TLR6. These results also suggest that inflammatory responses induced by the activation of TLRs can be differentially modulated by types of dietary fatty acids.

Endotoxin recognition molecules, Toll-like receptor 4-MD-2

Toll-like receptors (TLRs) are innate pathogen recognition molecules for microbial products. Lipopolysaccharide (LPS), a membrane constituent of Gram-negative bacteria, is one of the most potent microbial products. LPS is recognized by TLR4 and MD-2. TLR4 is a transmembrane protein, the extracellular domain of which is composed of a protein motif called leucine-rich repeats (LRR). MD-2 is an extracellular molecule that is associated with the extracellular LRR of TLR4. MD-2 has a role in cell surface expression of TLR4 and interaction with LPS. TLR4-MD-2 contributes to containment of infections by Gram-negative bacteria by activating immune responses.

Study on the relationship between heat shock protein 70 and toll-like receptor-4 of monocytes

To explore the relation between human heat shock protein 70 (hsp70) and TLR4 in human monocytes in vitro, human monocytes were stimulated with various concentrations of HSP70, and TNF-alpha production in supernatants was measured by ELISA. Pre-incubated with or without anti-TLR4 mAb, and stimulated with hsp70 (5.0 microg/ml), NF-kappaB p65 of human monocytes in different time points were detected by immunohistochemistry and monocyte surface expression of TLR4 was measured by flow cytometry. After the human monocytes were pre-incubated with various concentrations of anti-TLR4 and stimulated with hsp70 (5.0 microg/ml), TNF-alpha production in supernatants was measured. The results showed that hsp70 enhanced NF-kappaB activation, which was clearly inhibited by anti-TLR4, with the positive cell ratios being 67.44%, 39.17%, 31.56% and 28.05 %, respectively. TLR4 was rapidly down-regulated in the presence of hsp70. MFI of TLR4 on monocytes in different time points were 87.77 +/- 5.38, 78.16 +/- 6.01 and 45.17 +/- 4.97 (P<0.05), 26.98 +/- 5.83 (P<0.01), respectively. Moreover, hsp70-induced TNF-alpha production by human monocytes was inhibited by anti-TLR4. It is suggested that TLR4 is involved in the hsp70-mediated activation of innate immunity.

Lipopolysaccharide-induced c-Jun NH2-terminal kinase activation in human neutrophils: role of phosphatidylinositol 3-Kinase and Syk-mediated pathways

Polymorphonuclear leukocytes (neutrophils) respond to lipopolysaccharide (LPS) through the up-regulation of several pro-inflammatory mediators. We have recently shown that LPS-stimulated neutrophils express monocyte chemoattractant protein 1 (MCP-1), an AP-1-dependent gene, suggesting that LPS activates the c-Jun N-terminal kinase (JNK) pathway in neutrophils. Previously, we have shown the activation of p38 MAPK, but not JNK, in suspended neutrophils stimulated with LPS but have recently shown activation of JNK by TNF-alpha in an adherent neutrophil system. We show here that exposure to LPS activates JNK in non-suspended neutrophils and that LPS-induced MCP-1 expression, but not tumor necrosis factor-alpha (TNF-alpha) or interleukin-8 (IL-8), is dependent on JNK activation. In addition, LPS stimulation of non-suspended neutrophils activates Syk and phosphatidylinositol 3-kinase (PI3K). Inhibition of Syk with piceatannol or PI3K with wortmannin inhibited LPS-induced JNK activation and decreased MCP-1 expression after exposure to LPS, suggesting that both Syk and PI3K reside in a signaling pathway leading to LPS-induced JNK activation in neutrophils. This Syk- and PI3K-dependent pathway leading to JNK activation after LPS exposure in non-suspended neutrophils is specific for JNK, because inhibition of neither Syk nor PI3K decreased p38 activation after LPS stimulation. Furthermore we show that PI3K inhibition decreased LPS-induced Syk activation suggesting that PI3K resides upstream of Syk in this pathway. Finally, we show that Syk associates with Toll-like receptor 4 (TLR4) upon LPS stimulation further implicating Syk in the LPS-induced signaling pathway in neutrophils. Overall our data suggests that LPS induces JNK activation only in non-suspended neutrophils, which proceeds through Syk- and PI3K-dependent pathways, and that JNK activation is important for LPS-induced MCP-1 expression but not for TNF-alpha or IL-8 expression.

p38 activation through Toll-like receptors modulates IFN-gamma-induced expression of the Tap-1 gene only in macrophages

Although interferon-gamma (IFN-gamma) induces the transporter associated with antigen processing (Tap)-1 expression in macrophages, cooperation with lipopolysaccharide signaling through Toll-like receptor 4 (TLR4) accelerates the kinetics and increases the overall levels of this gene. In this report, we show that peptidoglycan signaling through TLR2 and bacterial CpG DNA signaling through TLR9 are functionally equivalent at synergizing with IFN-gamma in regulating Tap-1 expression in macrophages. Activation of the p38 mitogen-activated protein kinase is necessary for this response, which correlates with increased phosphorylation of signal transducer and activator of transcription-1 on serine 727. Activation of p38, however, is not sufficient, as this signaling event does not affect the response to IFN-gamma in HeLa cells. The cooperation between these different signaling pathways also requires membrane fluidity. These data suggest that macrophages possess an ability to coordinate the signaling between the IFN-gamma and TLR receptors.

Lipid raft microdomains: key sites for Coxsackievirus A9 infectious cycle

Lipid rafts have an important property to preferentially concentrate some proteins, while excluding others. Lipid rafts can also act as functional platforms for multiple signalling and trafficking processes. Several reports have shown that lipid rafts play a crucial role in the assembly of several enveloped viruses and possibly their cell entry. In this study we investigated the importance of lipid raft formation in Coxsackievirus A9 (CAV-9) entry and cell infection. Here by using a variety of biochemical and biophysical methods, we report that receptor molecules integrin alphavbeta3 and GRP78, which are implicated in CAV-9 infection as well as accessory molecules such as MHC class I, are accumulated in increased concentrations in lipid rafts following CAV-9 infection. In addition our studies revealed that raft integrity is essential for this virus since CAV-9 activates the Raf/MAPK signalling pathway within the raft and raft-disrupting drugs such as nystatin and MCD can successfully inhibit CAV-9 infection.

Oxidative stress reprograms lipopolysaccharide signaling via Src kinase-dependent pathway in RAW 264.7 macrophage cell line

Oxidative stress generated during ischemia/reperfusion injury has been shown to augment cellular responsiveness. Whereas oxidants are themselves known to induce several intracellular signaling cascades, their effect on signaling pathways initiated by other inflammatory stimuli remains poorly elucidated. Previous work has suggested that oxidants are able to prime alveolar macrophages for increased NF-kappa B translocation in response to treatment with lipopolysaccharide (LPS). Because oxidants are known to stimulate the Src family of tyrosine kinases, we hypothesized that the oxidants might contribute to augmented NF-kappa B translocation by LPS via the involvement of Src family kinases. To model macrophage priming in vitro, the murine macrophage cell line, RAW 264.7, was first incubated with various oxidants and then exposed to low dose LPS. These studies show that oxidant stress is able to augment macrophage responsiveness to LPS as evidenced by earlier and increased NF-kappa B translocation. Inhibition of the Src family kinases by either pharmacological inhibition using PP2 or through a molecular approach by cell transfection with Csk was found to prevent the augmented LPS-induced NF-kappa B translocation caused by oxidants. Interestingly, while Src kinase inhibition was able to prevent the LPS-induced NF-kappa B translocation in oxidant-treated macrophages, this strategy had no effect on NF-kappa B translocation caused by LPS in the absence of oxidants. These findings suggested that oxidative stress might divert LPS signaling along an alternative signaling pathway. Further studies demonstrated that the Src-dependent pathway induced by oxidant pretreatment involved the activation of phosphatidylinositol 3-kinase. Involvement of this pathway appeared to be independent of traditional LPS signaling. Together, these studies provide a novel potential mechanism whereby oxidants might prime alveolar macrophages for altered responsiveness to subsequent inflammatory stimuli and suggest different cellular targets for immunomodulation following ischemia/reperfusion.

Intracellular recognition of lipopolysaccharide by toll-like receptor 4 in intestinal epithelial cells

Toll-like receptor (TLR)4 has recently been shown to reside in the Golgi apparatus of intestinal crypt epithelial m-IC_cl2 cells, colocalizing with internalized lipopolysaccharide (LPS). Here we demonstrate that disruption of the integrity of the Golgi apparatus significantly reduced LPS-mediated nuclear factor κB activation. Also, the TLR4 adaptor protein MyD88 and the serine/threonine kinase IRAK-1 were rapidly recruited to the Golgi apparatus upon stimulation. LPS-mediated activation required lipid raft formation and intact clathrin-dependent internalization. In contrast to macrophages, prevention of ligand internalization by use of LPS-coated beads significantly impaired recognition by epithelial cells. The localization of TLR4 to the Golgi apparatus was abrogated by expression of a genetically modified form of the TLR4 binding chaperone gp96. Thus, our data provide evidence that in contrast to the situation in macrophages, LPS recognition in intestinal epithelial cells may occur in the Golgi apparatus and require LPS internalization.

Binding and entry of respiratory syncytial virus into host cells and initiation of the innate immune response

Respiratory syncytial virus (RSV) is the most common cause of severe lower respiratory tract infection in infants and the elderly. There is currently no effective antiviral treatment for the infection, but advances in our understanding of RSV uptake, especially the role of surfactant proteins, the attachment protein G and the fusion protein F, as well as the post-binding events, have revealed potential targets for new therapies and vaccine development. RSV infection triggers an intense inflammatory response, mediated initially by the infected airway epithelial cells and antigen-presenting cells. Humoral and cell-mediated immune responses are important in controlling the extent of infection and promoting viral clearance. The initial innate immune response may play a critical role by influencing the subsequent adaptive response generated. This review summarizes our current understanding of RSV binding and uptake in mammalian cells and how these initial interactions influence the subsequent innate immune response generated.

The antifungal drug amphotericin B promotes inflammatory cytokine release by a Toll-like receptor- and CD14-dependent mechanism

Amphotericin B is the most effective drug for treating many life-threatening fungal infections. Amphotericin B administration is limited by infusion-related toxicity, including fever and chills, an effect postulated to result from proinflammatory cytokine production by innate immune cells. Because amphotericin B is a microbial product, we hypothesized that it stimulates immune cells via Toll-like receptors (TLRs) and CD14. We show here that amphotericin B induces signal transduction and inflammatory cytokine release from cells expressing TLR2 and CD14. Primary murine macrophages and human cell lines expressing TLR2, CD14, and the adapter protein MyD88 responded to amphotericin B with NF-kappaB-dependent reporter activity and cytokine release, whereas cells deficient in any of these failed to respond. Cells mutated in TLR4 were less responsive to amphotericin B stimulation than cells expressing normal TLR4. These data demonstrate that TLR2 and CD14 are required for amphotericin B-dependent inflammatory stimulation of innate immune cells and that TLR4 may also provide stimulation of these cells. Our results provide a putative molecular basis for inflammatory responses elicited by amphotericin B and suggest strategies to eliminate the acute toxicity of this drug.

α-Melanocyte-stimulating hormone inhibits lipopolysaccharide-induced biological responses by downregulating CD14 from macrophages

Monocytes/macrophages are the first cells involved in inflammation. alpha-Melanocyte-stimulating hormone (alpha-MSH) is known to possess an anti-inflammatory role induced by a variety of stimuli; however, the molecular mechanisms underlying these effects are not clearly defined. In this report we provide evidence that alpha-MSH inhibited serum-activated lipopolysaccharide (SA-LPS)-induced proteolytic enzyme release, oxidative burst response, reactive oxygen intermediate generation, nitric oxide production, and adhesion molecule expression in monocyte-derived macrophages. alpha-MSH also inhibited SA-LPS-induced nuclear transcription factor kappaB activation not only in macrophages, but also in a T-cell line and human neutrophils isolated from fresh blood. alpha-MSH downregulated CD14, but not interleukin-1 receptor, tumor necrosis factor receptor 1 or 2 from the surface of macrophages. Anti-CD14 antibody was unable to protect alpha-MSH-mediated downregulation of CD14. Overall, our results suggest that alpha-MSH exerts its anti-inflammatory effect by a novel mechanism in macrophages through downregulating of the endotoxin receptor CD14.

Disease-associated dendritic cells respond to disease-specific antigens through the common heat shock protein receptor

The most abundant intracellular proteins, heat shock proteins (HSPs), serve as molecular chaperones for regulatory and maturation pathways. Diverse families of HSPs have been shown to bind antigenic peptides and to play major roles in innate and adaptive immune responses through the common HSP receptor, CD91. HIV-1+ patients with Kaposi sarcoma (KS) were matched for CD4 count and HIV-1 RNA viral load to HIV-1+ patients without Kaposi sarcoma (and negative for Kaposisarcoma-associated herpesvirus). We then investigated the pathways used by tumor lysates, viral lysates, and viral particles in their activation. In particular, we observed immune responses after HSP depletion using antitumor antibiotics and blockade of the common HSP receptor, CD91. Despite the impaired functional capacity of dendritic cells (DCs) derived from patients with KS, DCs retain the ability to prime the adaptive arm of the immune system through the common HSP receptor, leading to phenotypic activation and stimulation of tetramer-positive CD8+ cytotoxic T cells. We also show that interferon-producing plasmacytoid DCs are selectively depleted in KS-positive compared with matched KS-negative HIV-1-infected patients. Functionally impaired DCs can effectively cross-present immune responses through the common HSP receptor. These results have important implications for the etiopathogenesis of KS and for the development and design of any compounds, including vaccines, derived from cellular lysates.

Clustering of the B Cell Receptor Is Not Required for the Apoptotic Response

We examined the role of BCR cell membrane redistribution in anti-IgM-induced apoptosis in three human B cell lines, RA#1, 2G6, and MC116, that differ in their relative levels of sIgM expression. The apoptotic response was found to be dependent on the nature of the anti-IgM and the cell line. In the cell lines, RA#1 and MC116, sIgM aggregated into patches that were insensitive to the disruption of cholesterol-rich membrane microdomains by nystatin or beta-MCD. The B cell line 2G6 was able to reorganize sIgM into a tight coalescent cap upon anti-IgM treatment. However, in this case, the lipid raft inhibitors nystatin and beta-MCD disrupted the patching. In 2G6 cells, BCR-mediated apoptosis was not affected by nystatin treatment, whereas it increased in beta-MCD pretreated cells. Thus, no evident correlation was found between apoptosis and BCR cell membrane redistribution or lipid raft formation in either of the three cell lines. The data indicate that the apoptotic signal transduction pathway is independent of BCR translocation into lipid rafts and/or aggregation.

Pathogens: raft hijackers

Throughout evolution, organisms have developed immune-surveillance networks to protect themselves from potential pathogens. At the cellular level, the signalling events that regulate these defensive responses take place in membrane rafts--dynamic microdomains that are enriched in cholesterol and glycosphingolipids--that facilitate many protein-protein and lipid-protein interactions at the cell surface. Pathogens have evolved many strategies to ensure their own survival and to evade the host immune system, in some cases by hijacking rafts. However, understanding the means by which pathogens exploit rafts might lead to new therapeutic strategies to prevent or alleviate certain infectious diseases, such as those caused by HIV-1 or Ebola virus.

Specific phospholipid oxidation products inhibit ligand activation of toll-like receptors 4 and 2

OBJECTIVE

We have previously shown that phospholipid oxidation products of 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphorylcholine (ox-PAPC) inhibit lipopolysaccharide (LPS)-induced E-selectin expression and neutrophil binding in human aortic endothelial cells (HAECs). The current studies identify specific phospholipids that inhibit chemokine induction by Toll-like receptor-4 (TLR4) and -2 (TLR2) ligands inECs and macrophages.

METHODS AND RESULTS

Measurements of interleukin (IL)-8 and monocyte chemotactic protein-1 levels secreted from ox-PAPC- and LPS-cotreated ECs indicate that ox-PAPC inhibits activation of TLR4 by LPS. The effects of IL-1beta and tumor necrosis factor-alpha, which utilize the same intracellular signaling molecules, were not inhibited. Cell fractionation and immunofluorescence analyses demonstrate that LPS induces membrane translocation of the LPS receptor complex to a lipid raft/caveolar fraction in ECs. Ox-PAPC inhibits this translocation and alters caveolin-1 distribution. Supporting an important role for caveolae in LPS action, overexpression of caveolin-1 enhanced LPS-induced IL-8 synthesis. Ox-PAPC also inhibits the effect of TLR2 and TLR4 ligands in human macrophages.

CONCLUSIONS

These studies report a novel mechanism that involves alterations to lipid raft/caveolar processing, by which specific phospholipid oxidation products inhibit activation by TLR4 and TLR2 ligands. These studies have broader implications for the role of ox-PAPC as a regulator of specific lipid raft/caveolar function.

Expression of the molecular chaperone Hsp70 in detergent-resistant microdomains correlates with its membrane delivery and release

Accumulating evidence suggests that some heat shock proteins (Hsps), in particular the 72-kDa inducible Hsp70, associate to the cell membrane and might be secreted through an unknown mechanism to exert important functions in the immune response and signal transduction. We speculated that specialized structures named lipid rafts, known as important platforms for the delivery of proteins to the cell membrane, might be involved in the unknown mechanism ensuring membrane association and secretion of Hsp70. Lipid rafts are sphingolipid-cholesterol-rich structures that have been mainly characterized in polarized epithelial cells and can be isolated as detergent-resistant microdomains (DRMs). Analysis of soluble and DRM fractions prepared from unstressed Caco-2 epithelial cells revealed that Hsp70, and to a lesser extent calnexin, were present in DRM fractions. Increased expression of Hsps, through heat shock or by using drugs acting on protein trafficking or intracellular calcium level, induced an efficient translocation to DRM. We also found that Hsp70 was released by epithelial Caco-2 cells, and this release dramatically increased after heat shock. Drugs known to block the classical secretory pathway were unable to reduce Hsp70 release. By contrast, release of the protein was affected by the raft-disrupting drug methyl-beta-cyclodextrin. Our data suggest that lipid rafts are part of a mechanism ensuring the correct functions of Hsps and provide a rational explanation for the observed membrane association and release of Hsp70.

Subversion and utilization of the host cell cyclic adenosine 5'-monophosphate/protein kinase A pathway by Brucella during macrophage infection

Brucella spp. are intramacrophage pathogens that induce chronic infections in a wide range of mammals, including domestic animals and humans. Therefore, the macrophage response to infection has important consequences for both the survival of phagocytosed bacteria and the further development of host immunity. However, very little is known about the macrophage cell signaling pathways initiated upon infection and the virulence strategy that Brucella use to counteract these responses and secure their survival. In a previous study, we have shown that macrophages activated by SR141716A, a ligand of the cannabinoid receptor CB1, acquired the capacity to control Brucella and observed that the CB1 receptor-triggering engages the microbicidal activity of phagocytes. To analyze the perturbation of cell signaling pathway during macrophage infection by Brucella, we hypothesized that SR141716A provides cell signaling that interferes with the bacterial message leading to inhibition of macrophage functions. As CB1 receptor belongs to the family of G protein-linked receptors, we explored the cAMP signaling pathway. In this study, we show that the CB1 ligand inhibited the bacteria-induced cell signaling. Taking advantage of this result, we then demonstrated that Brucella infection elicited a rapid activation of the cAMP/protein kinase A pathway. This activation resulted in a prolonged phosphorylation of the transcription factor CREB. We finally demonstrate that the activation of the cAMP/protein kinase A pathway is crucial for the survival and establishment of Brucella within macrophages. For the first time in phagocytes, we thus characterized a primordial virulence strategy of Brucella involving the host signaling pathway, a novel point of immune intervention of this virulent pathogen.

The roles of membrane microdomains (rafts) in T cell activation

Detergent-resistant membrane microdomains enriched in sphingolipids, cholesterol and glycosylphosphatidylinositol-anchored proteins play essential roles in T cell receptor (TCR) signaling. These 'membrane rafts' accumulate several cytoplasmic lipid-modified molecules, including Src-family kinases, coreceptors CD4 and CD8 and transmembrane adapters LAT and PAG/Cbp, essential for either initiation or amplification of the signaling process, while most other abundant transmembrane proteins are excluded from these structures. TCRs in various T cell subpopulations may differ in their use of membrane rafts. Membrane rafts also seem to be involved in many other aspects of T cell biology, such as functioning of cytokine and chemokine receptors, adhesion molecules, antigen presentation, establishing cell polarity or interaction with important pathogens. Although the concept of membrane rafts explains several diverse biological phenomena, many basic issues, such as composition, size and heterogeneity, under native conditions, as well as the dynamics of their interactions with TCRs and other immunoreceptors, remain unclear, partially because of technical problems.

Lipopolysaccharide rapidly traffics to and from the Golgi apparatus with the toll-like receptor 4-MD-2-CD14 complex in a process that is distinct from the initiation of signal transduction

Mammalian responses to LPS require the expression of Toll-like receptor 4 (TLR4), CD14, and MD-2. We expressed fluorescent TLR4 in cell lines and found that TLR4 densely localized to the surface and the Golgi. Similar distributions were observed in human monocytes. Confocal imaging revealed rapid recycling of TLR4-CD14-MD-2 complexes between the Golgi and the plasma membrane. Fluorescent LPS followed these trafficking pathways in CD14-positive cells. The TLR4- adapter protein, MyD88, translocated to the cell surface upon LPS exposure, and cross-linking of surface TLR4 with antibody induced signaling. Golgi-associated TLR4 expression was disrupted by brefeldin A, yet LPS signaling was preserved. We conclude that LPS signaling may be initiated by surface aggregation of TLR4 and is not dependent upon LPS trafficking to the Golgi.

Cag pathogenicity island-specific responses of gastric epithelial cells to Helicobacter pylori infection

Helicobacter pylori infects over half the world's population and causes a wide range of diseases, including gastritis, peptic ulcer, and two forms of gastric cancer. H. pylori infection elicits a variety of phenotypic responses in cultured gastric epithelial cells, including the expression of proinflammatory genes and changes in the actin cytoskeleton. Both of these responses are mediated by the type IV secretion system (TFSS) encoded by the cag pathogenicity island (cag PAI). We used human cDNA microarrays to examine the temporal transcriptional profiles of gastric AGS cells infected with H. pylori strain G27 and a panel of isogenic mutants to dissect the contributions of various genes in the cag PAI. Infection with G27 induced expression of genes involved in the innate immune response, cell shape regulation, and signal transduction. A mutant lacking the cagA gene, which encodes an effector molecule secreted by the TFSS and required for the host cell cytoskeletal response, induced the expression of fewer cytoskeletal genes. A mutant lacking cagE, which encodes a structural component of the TFSS, failed to up-regulate a superset of host genes, including the cagA-dependent genes, and many of the immune response genes. A mutant lacking the entire cag PAI failed to induce both the cagE-dependent genes and several transiently expressed cagE independent genes. Host cell transcriptional profiling of infection with isogenic strains offered a detailed molecular picture of H. pylori infection and provided insight into potential targets of individual virulence determinants such as tyrosine kinase and Rho GTPase signaling molecules.

Hsp90 interactions and acylation target the G protein Galpha 12 but not Galpha 13 to lipid rafts

The heterotrimeric G proteins, G(12) and G(13), are closely related in their sequences, signaling partners, and cellular effects such as oncogenic transformation and cytoskeletal reorganization. Yet G(12) and G(13) can act through different pathways, bind different proteins, and show opposing actions on some effectors. We investigated the compartmentalization of G(12) and G(13) at the membrane because other G proteins reside in lipid rafts, membrane microdomains enriched in cholesterol and sphingolipids. Lipid rafts were isolated after cold, nonionic detergent extraction of cells and gradient centrifugation. Galpha(12) was in the lipid raft fractions, whereas Galpha(13) was not associated with lipid rafts. Mutation of Cys-11 on Galpha(12), which prevents its palmitoylation, partially shifted Galpha(12) from the lipid rafts. Geldanamycin treatment, which specifically inhibits Hsp90, caused a partial loss of wild-type Galpha(12) and a complete loss of the Cys-11 mutant from the lipid rafts and the appearance of a higher molecular weight form of Galpha(12) in the soluble fractions. These results indicate that acylation and Hsp90 interactions localized Galpha(12) to lipid rafts. Hsp90 may act as both a scaffold and chaperone to maintain a functional Galpha(12) only in discrete membrane domains and thereby explain some of the nonoverlapping functions of G(12) and G(13) and control of these potent cell regulators.