The Wingless homolog WNT5A and its receptor Frizzled-5 regulate inflammatory responses of human mononuclear cells induced by microbial stimulation

Microarray - assisted gene - expression screens of human macrophages revealed WNT5A, a homolog of Wingless, a key regulator of Drosophila melanogaster embryonic segmentation and patterning, to be consistently up-regulated following stimulation with different mycobacterial species and conserved bacterial structures. The expression of WNT5A required Toll-like receptor signaling and NF-κB activation, which identifies a novel induction pathway for a Wingless homolog. We show that human peripheral-blood mononuclear cells express the WNT5A receptor Frizzled-5 (FZD5). Both WNT5A and FZD5 also were detected in granulomatous lesions in the lungs of Mycobacterium tuberculosis–infected patients. Functional studies showed that WNT5A and FZD5 regulate the microbially induced interleukin-12 response of antigen-presenting cells and interferon-γ production by mycobacterial antigenstimulated T cells. Our findings implicate the evolutionarily conserved WNT/Frizzled signaling system in bridging innate and adaptive immunity to infections.

Endotoxin-like properties of a rhamnolipid exotoxin from Burkholderia (Pseudomonas) plantarii: immune cell stimulation and biophysical characterization

Here we report on the purification, structural characterization, and biological activity of a glycolipid, 2-O-alpha-L-rhamnopyranosyl-alpha-L-rhamnopyranosyl-alpha(R)-3-hydroxytetradecanoyl-(R)-3-hydroxytetradecanoate (RL-2,2(14)) produced by Burkholderia (Pseudomonas) plantarii. RL-2,2(14) is structurally very similar to a rhamnolipid exotoxin from Pseudomonas aeruginosa and identical to the rhamnolipid of Burkholderia pseudomallei, the causative agent of melioidosis. Interestingly, RL-2,2(14) exhibits strong stimulatory activity on human mononuclear cells to produce tumor necrosis factor alpha, the overproduction of which is known to cause sepsis and the septic shock syndrome. Such a property has not been noted so far for rhamnolipid exotoxins, only for bacterial endotoxins (lipopolysaccharide, LPS). Consequently, we analyzed RL-2,2(14) with respect to its pathophysiological activities as a heat-stable extracellular toxin. Like LPS, the cell-stimulating activity of the rhamnolipid could be inhibited by incubation with polymyxin B. However, immune cell activation by RL-2,2(14) does nor occur via receptors that are involved in LPS (TLR4) or lipopeptide signaling (TLR2). Despite its completely different chemical structure, RL-2,2(14) exhibits a variety of endotoxin-related physicochemical characteristics, such as a cubic-inverted supramolecular structure. These data are in good agreement with our conformational concept of endotoxicity: intercalation of naturally originating virulence factors into the immune cell membrane leads to strong mechanical stress on integral proteins, eventually causing cell activation.

Mycobacteria Induce IFN-γ Production in Human Dendritic Cells via Triggering of TLR2

IFN-gamma is of central importance for the induction of robust cell-mediated immunity and for the activation of APC. Recent studies using experimental murine systems have now suggested a fundamental role for APC-derived IFN-gamma during infection with intracellular pathogens. It is currently unknown whether human dendritic cells (DC) can respond to bacterial stimulation with production of IFN-gamma. To test this question, we used human monocyte-derived DC stimulated by Mycobacterium bovis bacillus Calmette-Guérin as a model system. We demonstrate production of IFN-gamma mRNA and protein on the single cell level. IFN-gamma in DC cultures was not simply produced by contaminating lymphocytes because production of DC-IFN-gamma could also be demonstrated in highly purified DC cultures containing virtually no T, B, and NK cells. TLR2 was identified as a key receptor involved in triggering production of DC-IFN-gamma. Interestingly, DC-IFN-gamma seems to participate in an autocrine DC activation loop, and production of DC-IFN-gamma could be enhanced by costimulation of DC with IL-12/IL-15/IL-18. In conclusion, we have demonstrated production of IFN-gamma by human DC on the single cell level, identified TLR2 as a pattern recognition receptor involved in this process, and elucidated some of the functional consequences of autocrine IFN-gamma production by human DC.

In vitro-Generated Viral Double-Stranded RNA in Contrast to Polyinosinic : Polycytidylic Acid Induces Interferon-alpha in Human Plasmacytoid Dendritic Cells

Double-stranded RNA (dsRNA) arises in the cytoplasm during viral replication and was shown to participate in the interferon (IFN)-alpha induction process. Besides the intracellular recognition, released dsRNA from dying, infected cells can function as a pathogen-associated molecular pattern (PAMP) for the innate immune system. In the present study, in vitro-generated dsRNA fragments of genomic sequences of Newcastle disease virus were used to induce IFN-alpha release in human peripheral blood mononuclear cells (PBMC), in immature myeloid dendritic cells (mDC) and in immature plasmacytoid DC (pDC). The extracellular administration of dsRNA fragments but not the application of the corresponding single-stranded RNA (ssRNA) strands led to an IFN-alpha production in PBMC. The synthetic dsRNA analogue polyinosinic acid : polycytidylic acid [Poly(I : C)] could only stimulate IFN-alpha production in enriched mDC but not in pDC. In contrast, dsRNA fragments induced IFN-alpha only in pDC. Complexation of dsRNA fragments with transfection reagents increased the efficiency of IFN-alpha induction and commuted ssRNA molecules into IFN-alpha inducers. However, stimulation of in vitro-generated murine Toll-like receptor 7 (TLR7) knockout DC and human TLR-transfected HEK293 cells with dsRNA fragments gave no evidences for the involvement of pDC-specific TLR7 or TLR9 in the observed IFN-alpha induction.

TLR1- and TLR6-independent recognition of bacterial lipopeptides

Bacterial cell walls contain lipoproteins/peptides, which are strong modulators of the innate immune system. Triacylated lipopeptides are assumed to be recognized by TLR2/TLR1-, whereas diacylated lipopeptides use TLR2/TLR6 heteromers for signaling. Following our initial discovery of TLR6-independent diacylated lipopeptides, we could now characterize di- and triacylated lipopeptides (e.g. Pam(2)C-SK(4), Pam(3)C-GNNDESNISFKEK), which have stimulatory activity in TLR1- and in TLR6-deficient mice. Furthermore, for the first time, we present triacylated lipopeptides with short length ester-bound fatty acids (like PamOct(2)C-SSNASK(4)), which induce no response in TLR1-deficient cells. No differences in the phosphorylation of MAP kinases by lipopeptide analogs having different TLR2-coreceptor usage were observed. Blocking experiments indicated that different TLR2 heteromers recognize their specific lipopeptide ligands independently from each other. In summary, a triacylation pattern is necessary but not sufficient to render a lipopeptide TLR1-dependent, and a diacylation pattern is necessary but not sufficient to render a lipopeptide TLR6-dependent. Contrary to the current model, distinct lipopeptides are recognized by TLR2 in a TLR1- and TLR6-independent manner.

Lipopeptide structure determines TLR2 dependent cell activation level

Bacterial lipoproteins/peptides are composed of di-O-acylated-S-(2,3-dihydroxypropyl)-cysteinyl residues N-terminally coupled to distinct polypeptides, which can be N-acylated with a third fatty acid. Using a synthetic lipopeptide library we characterized the contribution of the lipid portion to the TLR2 dependent pattern recognition. We found that the two ester bound fatty acid length threshold is beyond eight C atoms because almost no response was elicited by cellular challenge with analogues carrying shorter acyl chains in HEK293 cells expressing recombinant human TLR2. In contrast, the amide bound fatty acid is of lesser importance. While two ester-bound palmitic acids mediate a high stimulatory activity of the respective analogue, a lipopeptide carrying one amide-bound and another ester-bound palmitic acid molecule was inactive. In addition, species specific LP recognition through murine and human TLR2 depended on the length of the two ester bound fatty acid chains. In conclusion, our results indicate the responsibility of both ester bound acyl chains but not of the amide bound fatty acid molecule for the TLR dependent cellular recognition of canonical triacylated LP, as well as a requirement for a minimal acyl chain length. Thus they might support the explanation of specific immuno-stimulatory potentials of different microorganisms and provide a basis for rational design of TLR2 specific adjuvants mediating immune activation to distinct levels.

Synthesis of peptidoglycan fragments and evaluation of their biological activity

The peptidoglycan (PG) bacterial cell wall glycoconjugate has been well known as a strong immunopotentiator. Partial structures of PG were chemically synthesized for elucidation of precise biological activities. Effective construction of distinct repeating glycans of PG was accomplished by the coupling of a key disaccharide glucosaminyl-beta(1-4)-muramic acid unit. Stereoselective glycosylation of disaccharide units was achieved by neighboring group participation of the N-Troc (Troc = 2,2,2-trichloroethoxycarbonyl) group and appropriate reactivity of N-Troc-glucosaminyl trichloroacetimidate. By using an efficient synthetic strategy, mono-, di-, tetra- and octasaccharide fragments of PG were synthesized in high yields. The biological activity of synthetic fragments of PG was evaluated by induction of tumor necrosis factor-alpha (TNF-alpha) from human monocytes, and toll-like receptor 2 (TLR2) and Nod2 dependencies by using transfected HEK293 cells, respectively. Here we reveal that TLR2 was not stimulated by the series of synthetic PG partial structures, whereas Nod2 recognizes the partial structures containing the MDP moiety.

Structural and biological characterization of highly purified hepta-acyl lipid A present in the lipopolysaccharide of the Salmonella enterica sv. Minnesota Re deep rough mutant strain R595

One major component of the Salmonella enterica sv. Minnesota Re deep rough mutant (strain R595) lipopolysaccharide is hepta-acyl lipid A (LA(hepta)). In a recent publication [Tanamoto K-I, Azumi S. Salmonella-type heptaacylated lipid A is inactive and acts as an antagonist of lipopolysaccharide action on human line cells. J Immunol 2000; 164: 3149-3156] the corresponding synthetic hepta-acyl lipid A (compound 516) was reported to be agonistically inactive but to rather suppress pro-inflammatory activation by the endotoxic hexa-acyl lipid A (LA(hexa), compound 506) and S-form LPS from Escherichia coli in the human macrophage-like cell lines THP-1 and U937. These results, however, were in contrast to previous findings with human mononuclear cells (hMNC) isolated from peripheral blood, in which compound 516 was found to be an agonist, expressing low, but significant, cytokine-inducing activity as compared to LA(hexa). We have investigated the structure of natural LA(hepta) from the S. enterica sv. Minnesota Re deep rough mutant strain (R595) by TLC immunoblot, MALDI-TOF mass spectrometry and NMR spectroscopy. Using these techniques, the structural identity between LA(hepta) and the synthetic compound 516 was confirmed. In corroboration of previous findings with studies employing compound 516, purified LA(hepta) was found to induce the production of TNF-alpha, IL-1beta and IL-6 in hMNC, thus displaying moderate agonistic activity. Furthermore, we showed that LA(hepta) agonistically activated nuclear translocation of NF-kappaB in THP-1 cells, thus clearly ruling out the possibility that LA(hepta) is an antagonist and that its biological activity is influenced by the type of human myeloid cells used for testing endotoxicity (hMNC versus THP-1 cells).

Role of dendritic cell-derived CXCL13 in the pathogenesis of Bartonella henselae B-rich granuloma

Dendritic cells (DCs) initiate adaptive immunity and regulate the inflammatory response by producing inflammatory chemokines. This study was aimed to elucidate their role in the pathogenesis of the suppurative granuloma induced by Bartonella henselae infection, which characterizes cat scratch disease (CSD). In vitro DC infection by B. henselae results in internalization of bacteria, phenotypic maturation with increased expression of HLA-DR and CD86, and induction of CD83, CD208, and CCR7. In comparison to LPS-activated DCs, B henselae-infected DCs produce higher amounts of IL-10, whereas the production of IL-12p70 is reduced. Infected DCs also produce high levels of CXCL8 and CXCL13, 2 chemokines active respectively on neutrophils and B lymphocytes. These results provide the molecular basis for the morphogenesis of CSD granuloma, which typically contains high numbers of neutrophils and B cells. Remarkably, CSD granulomas in vivo contain CXCL13-producing DCs. We further demonstrate that the B cells in CSD granulomas are represented by monocytoid B cells and, worth noting, they express T-bet, a transcription factor able to induce a T-independent immunoglobulin (Ig) class switch in B lymphocytes. These findings suggest that the humoral immune response to B henselae initiates in the extrafollicular areas of infected lymph nodes and is regulated by DCs.

LPS receptor (CD14): a receptor for phagocytosis of Alzheimer's amyloid peptide

The amyloid beta peptide 42 (Abeta(42)) plays a key role in neurotoxicity in Alzheimer's disease. Mononuclear phagocytes, i.e. microglia, have the potential to clear Abeta by phagocytosis. Recently, the lipopolysaccharide (LPS) receptor CD14 was shown to mediate phagocytosis of bacterial components and furthermore to contribute to neuroinflammation in Alzheimer's disease. Here, we investigated whether this key innate immunity receptor can interact with Abeta(42) and mediate phagocytosis of this peptide. Using flow cytometry, confocal microscopy and two-photon fluorescence lifetime imaging (FLIM) combined with fluorescence resonance energy transfer (FRET), we demonstrated a direct molecular interaction in the range of a few nanometers between Abeta(42) and CD14 in human CD14-transfected Chinese hamster ovary cells. Investigations using cells that were genetically deficient for this receptor showed that in <30 minutes exogenous Abeta(42) added to cultured primary microglial cells was phagocytosed into the cytoplasmic compartment in a CD14-dependent manner. This phagocytosis occurred at Abeta(42) concentration ranges that were considerably lower than the threshold to activate a cellular inflammatory reaction. In contrast, there was no association of CD14 to microglial internalization of microbeads. In complementary clinical experiments, we detected a pronounced CD14 immunoreactivity on parenchymal microglia spatially correlated to characteristic Alzheimer's disease lesion sites in brain sections of Alzheimer's disease patients but not in brain sections of control subjects. By showing a close interaction between CD14 and Abeta(42), demonstrating a direct role of CD14 in Abeta(42) phagocytosis, and detecting CD14-specific staining in brains of Alzheimer's disease patients, our results indicate a role of the LPS receptor in the pathophysiology of Alzheimer's disease, which could be of therapeutic relevance.

Binding of lipopeptide to CD14 induces physical proximity of CD14, TLR2 and TLR1

Lipoproteins or lipopeptides (LP) are bacterial cell wall components detected by the innate immune system. For LP, it has been shown that TLR2 is the essential receptor in cellular activation. However, molecular mechanisms of LP recognition are not yet clear. We used a FLAG-labeled derivative of the synthetic lipopeptide N-palmitoyl-S-[2,3-bis(palmitoyloxy)-(2R,S)-propyl]-(R)-cysteinyl-seryl-(lysyl)(3)-lysine (Pam(3)CSK(4)) to study the roles of CD14, TLR2 and TLR1 in binding and signaling of LP and their molecular interactions in human cells. The activity of Pam(3)CSK(4)-FLAG was TLR2 dependent, whereas the binding was enabled by CD14, as evaluated by flow cytometry and confocal microscopy. Using FRET and FRAP imaging techniques to study molecular associations, we could show that after Pam(3)CSK(4)-FLAG binding, CD14 and Pam(3)CSK(4)-FLAG associate with TLR2 and TLR1, and TLR2 is targeted to a low-mobility complex. Thus, LP binding to CD14 is the first step in the LP recognition, inducing physical proximity of CD14 and LP with TLR2/TLR1 and formation of the TLR2 signaling complex.

Toll-like receptor 6-independent signaling by diacylated lipopeptides

Bacterial lipopeptides are strong immune modulators that activate early host responses after infection as well as initiating adjuvant effects on the adaptive immune system. These lipopeptides induce signaling in cells of the immune system through Toll-like receptor 2 (TLR2)-TLR1 or TLR2-TLR6 heteromers. So far it has been thought that triacylated lipopeptides, such as the synthetic N-palmitoyl-S-[2,3-bis(palmitoyloxy)-(2RS)-propyl]-(R)-cysteine (Pam3)-CSK4, signal through TLR2-TLR1 heteromers, whereas diacylated lipopeptides, like the macrophage-activating lipopeptide from Mycoplasma fermentans (MALP2) or S-[2,3-bis(palmitoyloxy)-(2RS)-propyl]-(R)-cysteine (Pam2)-CGNNDESNISFKEK, induce signaling through TLR2-TLR6 heteromers. Using new synthetic lipopeptide derivatives we addressed the contribution of the lipid and, in particular, the peptide moieties with respect to TLR2 heteromer usage. In contrast to the current model of receptor usage, not only triacylated lipopeptides, but also diacylated lipopeptides like Pam2CSK4 and the elongated MALP2 analog Pam2CGNNDESNISFKEK-SK4 (MALP2-SK4) induced B lymphocyte proliferation and TNF-alpha secretion in macrophages in a TLR6-independent manner as determined with cells from TLR6-deficient mice. Our results indicate that both the lipid and the N-terminal peptides of lipoproteins contribute to the specificity of recognition by TLR2 heteromers and are responsible for the ligand-receptor interaction on host cells.

Recognition of bacterial products by toll-like receptors

Over the past 5 years, our knowledge about how the immune system senses the microbial world has changed fundamentally. It has been known for decades that microbial products such as lipopolysaccharide or bacterial DNA have a profound activity on human cells. Whereas the molecular structure of many different pathogenic microbial compounds has been extensively studied and characterized, the molecular basis of their recognition by the immune system remained elusive for a long time. It was the late Charles Janeway who developed the idea of microbial structures forming pathogen-associated molecular patterns (PAMP) that would be recognized by pattern-recognition receptors [1]. Even if the notion of pattern recognition is challenged today, the discovery of the family of Toll receptors in species as diverse as Drosophila and humans, and the identification of their role in distinguishing molecules and structures that are common to microorganisms has led to a renewed appreciation of the innate immune system. This review focuses on the current knowledge about the different molecules that are recognized by Toll receptors in mammalian cells.

Accumulation of Inhibitory κB-α as a Mechanism Contributing to the Anti-Inflammatory Effects of Surfactant Protein–A

The collectin surfactant protein (SP)-A has been implicated in multiple immunoregulatory functions of innate pulmonary host defense via modulating immune responses both in vitro and in vivo. The aim of the present study was to investigate mechanisms responsible for the anti-inflammatory effects of human (hu) SP-A on the inhibitory kappaB (IkappaB)/nuclear factor (NF)-kappaB signaling pathway in alveolar macrophages (AMs). Initial CD25 expression analysis by flow cytometry of CD14/hu Toll-like receptor 4-transfected Chinese hamster ovary reporter cells demonstrated that SP-A alone does not induce any NF-kappaB-dependent CD25 expression in these cells. In AMs, SP-A pretreatment caused a marked inhibition of lipopolysaccharide (LPS)-induced NF-kappaB activation independent of the LPS chemotype used as determined by electrophoretic mobility shift assay. Western blot analysis revealed that SP-A by itself increased the protein expression of IkappaB-alpha, the predominant regulator for rapidly induced NF-kappaB, in a dose- and time-dependent manner without enhancing IkappaB-alpha messenger RNA as determined by reverse transcription-polymerase chain reaction. SP-A did not interfere with LPS-induced serine(32) phosphorylation of IkappaB-alpha but significantly enhanced IkappaB-alpha abundance under LPS-coupled conditions. The data suggest that anti-inflammatory effects of SP-A on LPS-challenged AMs are associated with a SP-A-mediated direct modulation of the IkappaB-alpha turnover in these cells.

Lipopolysaccharide binding protein binds to triacylated and diacylated lipopeptides and mediates innate immune responses

LPS binding protein (LBP) is an acute-phase protein synthesized predominantly in the liver of the mammalian host. It was first described to bind LPS of Gram-negative bacteria and transfer it via a CD14-enhanced mechanism to a receptor complex including TLR-4 and MD-2, initiating a signal transduction cascade leading to the release of proinflammatory cytokines. In recent studies, we found that LBP also mediates cytokine induction caused by compounds derived from Gram-positive bacteria, including lipoteichoic acid and peptidoglycan fragments. Lipoproteins and lipopeptides have repeatedly been shown to act as potent cytokine inducers, interacting with TLR-2, in synergy with TLR-1 or -6. In this study, we show that these compounds also interact with LBP and CD14. We used triacylated lipopeptides, corresponding to lipoproteins of Borrelia burgdorferi, mycobacteria, and Escherichia coli, as well as diacylated lipopeptides, corresponding to, e.g., 2-kDa macrophage activating lipopeptide of Mycoplasma spp. Activation of Chinese hamster ovary cells transfected with TLR-2 by both lipopeptides was enhanced by cotransfection of CD14. Responsiveness of human mononuclear cells to these compounds was greatly enhanced in the presence of human LBP. Binding of lipopeptides to LBP as well as competitive inhibition of this interaction by LPS was demonstrated in a microplate assay. Furthermore, we were able to show that LBP transfers lipopeptides to CD14 on human monocytes using FACS analysis. These results support that LBP is a pattern recognition receptor transferring a variety of bacterial ligands including the two major types of lipopeptides to CD14 present in different receptor complexes.

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.

Structural polymorphism and endotoxic activity of synthetic phospholipid-like amphiphiles

The physicochemical characteristics and in vitro biological activity of various synthetic hexaacyl phospholipid dimers were compared with the respective behavior of bacterial endotoxins (lipopolysaccharide, LPS). The structural variations of the synthetic amphiphiles include different stereochemical (R,S) configurations about their ester- and amide-linkages for the acyl chains and differences in the length of the serine backbone spacer. The temperature of the gel to liquid crystalline phase transition of the acyl chains (T(c)) lies between 10 and 15 degrees C for the compounds with the shortest backbone and decreases rapidly for the compounds with longer backbones. The phase transition enthalpies (8-16 kJ x mol(-1)) are considerably lower than those of lipid A from hexaacyl endotoxins (28-35 kJ x mol(-1)). In contrast, the dependence of T(c) on Mg(2+) and water content shows a behavior typical for endotoxins: a significant increase with increasing Mg(2+) and decreasing water concentrations. The aggregate structure is sensitively dependent not only on the length of the backbone spacer but also on the different stereochemical variations. It can be directly correlated with the biological activity of the compounds. Thus, as with natural lipid A, the capacity to induce cytokine production in mononuclear cells is directly related to the affinity to form nonlamellar cubic or inverted hexagonal H(II) aggregate structures. Together with the data on the transport and intercalation of the dimers into phospholipid liposomes mediated by the lipopolysaccharide-binding protein (LBP), our conformational concept of endotoxicity and cell activation can be applied to these non-LPS structures: endotoxically active compounds incorporate into membranes of immune cells and cause conformational changes at the site of signaling proteins such as Toll-like receptors or K(+)-channels due to their conical molecular shape.