Lipid-restricted recognition of mycobacterial lipoglycans by human pulmonary surfactant protein A: a surface-plasmon-resonance study

Selected factors of the innate immunity in Polish patients suffering from pulmonary tuberculosis

We conducted a prospective study of 453 Polish patients suffering from pulmonary tuberculosis and 267 healthy controls. Selected polymorphisms of the genes encoding for collectins, ficolins and MBL-associated serine protease 2 were investigated as were serum concentrations of mannose-binding lectin, surfactant protein D, ficolin-1 and ficolin-3. The number of MBL2 gene exon 1 variant allele carriers was significantly higher in patients, compared with controls. The homozygosity for SFTPA2 +26 C > A SNP variant allele occurred less commonly within TB, while homozygosity for the FCN1 -542 G > A major allele was less frequent within the control group. Two patients were found MASP-2-deficient. Serum concentrations of MBL, SP-D and ficolin-1 were higher amongst patients while the converse was found for ficolin-3. Ficolin-1 had high specificity to differentiate between individuals with tuberculosis and healthy persons and therefore may be considered potential disease marker.

Biomimetic Dextran-Based Hydrogel Layers for Cell Micropatterning over Large Areas Using the FluidFM BOT Technology

Micropatterning of living single cells and cell clusters over millimeter-centimeter scale areas is of high demand in the development of cell-based biosensors. Micropatterning methodologies require both a suitable biomimetic support and a printing technology. In this work, we present the micropatterning of living mammalian cells on carboxymethyl dextran (CMD) hydrogel layers using the FluidFM BOT technology. In contrast to the ultrathin (few nanometers thick in the dry state) CMD films generally used in label-free biosensor applications, we developed CMD layers with thicknesses of several tens of nanometers in order to provide support for the controlled adhesion of living cells. The fabrication method and detailed characterization of the CMD layers are also described. The antifouling ability of the CMD surfaces is demonstrated by in situ optical waveguide lightmode spectroscopy measurements using serum modeling proteins with different electrostatic properties and molecular weights. Cell micropatterning on the CMD surface was obtained by printing cell adhesion mediating cRGDfK peptide molecules (cyclo(Arg-Gly-Asp-d-Phe-Lys)) directly from aqueous solution using microchanneled cantilevers with subsequent incubation of the printed surfaces in the living cell culture. Uniquely, we present cell patterns with different geometries (spot, line, and grid arrays) covering both micrometer and millimeter-centimeter scale areas. The adhered patterns were analyzed by phase contrast microscopy and the adhesion process on the patterns was real-time monitored by digital holographic microscopy, enabling to quantify the survival and migration of cells on the printed cRGDfK arrays.

Carbohydrate-dependent binding of langerin to SodC, a cell wall glycoprotein of Mycobacterium leprae

Langerhans cells participate in the immune response in leprosy by their ability to activate T cells that recognize the pathogen, Mycobacterium leprae, in a langerin-dependent manner. We hypothesized that langerin, the distinguishing C-type lectin of Langerhans cells, would recognize the highly mannosylated structures in pathogenic Mycobacterium spp. The coding region for the extracellular and neck domain of human langerin was cloned and expressed to produce a recombinant active trimeric form of human langerin (r-langerin). Binding assays performed in microtiter plates, by two-dimensional (2D) Western blotting, and by surface plasmon resonance demonstrated that r-langerin possessed carbohydrate-dependent affinity to glycoproteins in the cell wall of M. leprae. This lectin, however, yielded less binding to mannose-capped lipoarabinomannan (ManLAM) and even lower levels of binding to phosphatidylinositol mannosides. However, the superoxide dismutase C (SodC) protein of the M. leprae cell wall was identified as a langerin-reactive ligand. Tandem mass spectrometry verified the glycosylation of a recombinant form of M. leprae SodC (rSodC) produced in Mycobacterium smegmatis. Analysis of r-langerin affinity by surface plasmon resonance revealed a carbohydrate-dependent affinity of rSodC (equilibrium dissociation constant [KD] = 0.862 μM) that was 20-fold greater than for M. leprae ManLAM (KD = 18.69 μM). These data strongly suggest that a subset of the presumptively mannosylated M. leprae glycoproteins act as ligands for langerin and may facilitate the interaction of M. leprae with Langerhans cells.

Pulmonary Collectins in Diagnosis and Prevention of Lung Diseases

Pulmonary surfactant is a complex mixture of lipids and proteins, and is synthesized and secreted by alveolar type II epithelial cells and bronchiolar Clara cells. It acts to keep alveoli from collapsing during the expiratory phase of the respiratory cycle. After its secretion, lung surfactant forms a lattice structure on the alveolar surface, known as tubular myelin. Surfactant proteins (SP)-A, B, C and D make up to 10% of the total surfactant. SP-B and SPC are relatively small hydrophobic proteins, and are involved in the reduction of surface-tension at the air-liquid interface. SP-A and SP-D, on the other hand, are large oligomeric, hydrophilic proteins that belong to the collagenous Ca²⁺-dependent C-type lectin family (known as “Collectins”), and play an important role in host defense and in the recycling and transport of lung surfactant (Awasthi 2010) (Fig. 43.1). In particular, there is increasing evidence that surfactant-associated proteins A and -D (SP-A and SP-D, respectively) contribute to the host defense against inhaled microorganisms (see 10.1007/978-3-7091-1065_24 and 10.1007/978-3-7091-1065_25). Based on their ability to recognize pathogens and to regulate the host defense, SP-A and SP-D have been recently categorized as “Secretory Pathogen Recognition Receptors”. While SP-A and SP-D were first identified in the lung; the expression of these proteins has also been observed at other mucosal surfaces, such as lacrimal glands, gastrointestinal mucosa, genitourinary epithelium and periodontal surfaces. SP-A is the most prominent among four proteins in the pulmonary surfactant-system. The expression of SP-A is complexly regulated on the transcriptional and the chromosomal level. SP-A is a major player in the pulmonary cytokine-network and moreover has been described to act in the pulmonary host defense. This chapter gives an overview on the understanding of role of SP-A and SP-D in for human pulmonary disorders and points out the importance for pathology-orientated research to further elucidate the role of these molecules in adult lung diseases. As an outlook, it will become an issue of pulmonary pathology which might provide promising perspectives for applications in research, diagnosis and therapy (Awasthi 2010).

Synthesis, biological studies of linear and branched arabinofuranoside-containing glycolipids and their interaction with surfactant protein A

Oligoarabinofuranoside-containing glycolipids relevant to mycobacterial cell wall components were synthesized in order to understand the functional roles of such glycolipids. A series of linear tetra-, hexa-, octa- and a branched heptasaccharide oligoarabinofuranosides, with 1 → 2 and 1 → 5 α-linkages between the furanoside residues, were synthesized by chemical methods from readily available monomer building blocks. Upon the synthesis of glycolipids, constituted with a double alkyl chain-substituted sn-glycerol core and oligosaccharide fragments, biological studies were performed to identify the effect of synthetic glycolipids on the biofilm formation and sliding motilities of Mycobacterium smegmatis. Synthetic glycolipids and arabinofuranosides displayed an inhibitory effect on the growth profile, but mostly on the biofilm formation and maturation. Similarly, synthetic compounds also influenced the sliding motility of the bacteria. Further, biophysical studies were undertaken, so as to identify the interactions of the glycolipids with a pulmonary surfactant protein, namely surfactant protein A (SP-A), with the aid of the surface plasmon resonance technique. Specificities of each glycolipid interacting with SP-A were thus evaluated. From this study, glycolipids were found to exhibit higher apparent association constants than the corresponding oligosaccharide portion alone, without the double alkyl group-substituted glycerol core.

Structural analysis of an unusual bioactive N-acylated lipo-oligosaccharide LOS-IV in Mycobacterium marinum

Although lipo-oligosaccharides (LOSs) are recognized as major parietal components in many mycobacterial species, their involvement in the host-pathogen interactions have been scarcely documented. In particular, the biological implications arising from the high degree of structural species-specificity of these glycolipids remain largely unknown. Growing recognition of the Mycobacterium marinum-Danio rerio as a specific host-pathogen model devoted to the study of the physiopathology of mycobacterial infections prompted us to elucidate the structure-to-function relationships of the elusive end-product, LOS-IV, of the LOS biosynthetic pathway in M. marinum. Combination of physicochemical and molecular modeling methods established that LOS-IV resulted from the differential transfer on the caryophyllose-containing LOS-III of a family of very unusual N-acylated monosaccharides, naturally present as different diastereoisomers. In agreement with the partial loss of pathogenecity previously reported in a LOS-IV-deficient M. marinum mutant, we demonstrated that this terminal monosaccharide conferred to LOS-IV important biological functions, including macrophage activating properties.

The effect of a cholesterol liquid crystalline structure on osteoblast cell behavior

To investigate the effect of a liquid crystalline structure on cell behavior, polymethylsiloxane-graft-(10-cholesteryloxydecanol) was specially designed to get a thermotropic liquid crystalline polymer. Results of Fourier transform infrared (FT-IR) spectroscopy, proton nuclear magnetic resonance (1H-NMR) spectroscopy and gel permeation chromatography (GPC) indicated that cholesterol was successfully covalently grafted onto polymethylhydrosiloxane via decamethylene 'flexible spacer'. Differential scanning calorimetry (DSC) and polarized optical microscopy (POM) investigations revealed that the copolymer with 44.9% mesogenic unit showed obvious thermotropic liquid crystalline transition at about 124.9 degrees C. Polymer films were prepared by spin coating on clean glass plates from 5 mg ml(-1) toluene solutions of the copolymers. The POM investigation indicated that while the unannealed films (SC15, SC45) showed no liquid crystalline structure, the films which were annealed in vacuo at 140 degrees C for 9 h and then quenched to room temperature (SC15C, SC45C) formed discrete island-like liquid crystalline and continuous liquid crystalline structures, respectively. Osteoblast cells (MC3T3) were chosen to test the cell behavior of annealed and unannealed films. In comparison to unannealed films, the annealed films with a cholesterol liquid crystalline structure could promote osteoblast cell attachment and growth significantly.

Lung surfactant proteins A and D as pattern recognition proteins

Lung surfactant proteins A and D belong to a group of soluble humoral pattern recognition receptors, called collectins, which modulate the immune response to microorganisms. They bind essential carbohydrate and lipid antigens found on the surface of microorganisms via low affinity C-type lectin domains and regulate the host's response by binding to immune cell surface receptors. They form multimeric structures that bind, agglutinate, opsonise and neutralize many different pathogenic microorganisms including bacteria, yeast, fungi and viruses. They modulate the uptake of these microorganisms by phagocytic cells as well as both the inflammatory and the adaptive immune responses. Recent data have also highlighted their involvement in clearance of apoptotic cells, hypersensitivity and a number of lung diseases.

Synthesis and mycobacterial growth inhibition activities of bivalent and monovalent arabinofuranoside containing alkyl glycosides

Arabinofuranosides constitute one of the important components of cell wall structures of mycobacteria. With this importance of arabinofuranosides in mind, alkyl glycosides bearing arabinofuranoside trisaccharides were prepared, wherein the sugars were presented either in the monovalent or bivalent forms. Following the synthesis, the monovalent and bivalent alkyl glycosides were tested for their activities in a mycobacterial growth assay. The growth of the mycobacterial strain M. smegmatis was assessed in the presence of the alkyl glycosides and it was realized that the alkyl glycosides acted as inhibitors of the mycobacterial growth. The inhibition of the growth, caused by the above alkyl glycosides, was not observed for the arabinofuranose trisaccharide alone, without the alkyl groups, and for an alkyl glycoside bearing maltose as the sugar component.

Acylation determines the toll-like receptor (TLR)-dependent positive versus TLR2-, mannose receptor-, and SIGNR1-independent negative regulation of pro-inflammatory cytokines by mycobacterial lipomannan

Mycobacterium tuberculosis lipomannans (LMs) modulate the host innate immune response. The total fraction of Mycobacterium bovis BCG LM was shown both to induce macrophage activation and pro-inflammatory cytokines through Toll-like receptor 2 (TLR2) and to inhibit pro-inflammatory cytokine production by lipopolysaccharide (LPS)-activated macrophages through a TLR2-independent pathway. The pro-inflammatory activity was attributed to tri- and tetra-acylated forms of BCG LM but not the mono- and di-acylated ones. Here, we further characterize the negative activities of M. bovis BCG LM on primary murine macrophage activation. We show that di-acylated LMs exhibit a potent inhibitory effect on cytokine and NO secretion by LPS-activated macrophages. The inhibitory activity of mycobacterial mannose-capped lipoarabino-mannans on human phagocytes was previously attributed to their binding to the C-type lectins mannose receptor or specific intracellular adhesion molecule-3 grabbing nonintegrin (DC-SIGN). However, we found that di-acylated LM inhibition of LPS-induced tumor necrosis factor secretion by murine macrophages was independent of TLR2, mannose receptor, or the murine ortholog SIGNR1. We further determined that tri-acyl-LM, an agonist of TLR2/TLR1, promoted interleukin-12 p40 and NO secretion through the adaptor proteins MyD88 and TIRAP, whereas the fraction containing tetra-acylated LM activated macrophages in a MyD88-dependent fashion, mostly through TLR4. TLR4-dependent pro-inflammatory activity was also seen with M. tuberculosis LM, composed mostly of tri-acylated LM, suggesting that acylation degree per se might not be sufficient to determine TLR2 versus TLR4 usage. Therefore, LM acylation pattern determines the anti-inflammatory versus pro-inflammatory effects of LM through different pattern recognition receptors or signaling pathways and may represent an additional mean of regulating the host innate immunity by mycobacteria.

Identification by surface plasmon resonance of the mycobacterial lipomannan and lipoarabinomannan domains involved in binding to CD14 and LPS-binding protein

The mycobacterial lipoglycans, lipomannan (LM) and lipoarabinomannan (LAM), regulate host defence mechanisms through their interaction with pattern recognition receptors such as Toll-like receptors (TLRs). We have developed a surface plasmon resonance assay to analyse the molecular basis for the recognition of Mycobacterium kansasii LM or LAM, by immobilized CD14 and LPS-binding protein (LBP) both being capable to promote presentation of bacterial glycolipids to TLRs. The affinity of either LM/LAM was higher to CD14 than to LBP. Kinetic and Scatchard analyses were consistent with a model involving a single class of binding sites. These interactions required the lipidic anchor, but not the carbohydrate domains, of LM or LAM. We also provide evidence that addition of recombinant LBP enhanced the stimulatory effect of LM or LAM on matrix metalloproteinase-9 expression and secretion in macrophages, through a TLR1/TLR2-dependent mechanism.

Collectins: sentinels of innate immunity

Collectins, present in plasma and on mucosal surfaces, are humoral molecules of the innate immune system. They were discovered a hundred years ago in 1906 as the first association of an animal lectin with the immune system. They are a family of calcium-dependent lectins that recognize pathogen-associated molecular patterns. They share a similar modular domain architecture consisting of four regions; a cysteine-rich N-terminal domain, a collagen-like region, an alpha-helical neck domain and a C-terminal carbohydrate recognition domain. There have been eight collectins members defined so far, of which, MBL, SP-A and SP-D are the most characterized. Collectins represent the first line of host defense. Upon recognition of the infectious agents, collectins put into action effector mechanisms like direct opsonization, neutralization, agglutination, complement activation and phagocytosis to curb the microbial growth. In addition, they also modulate inflammatory and allergic responses and apoptotic cell clearance. These functions limit infection and subsequently modulate the adaptive immune responses. The role of collectins, their structure, function, characteristics and clinical significance are reviewed in this article.

The Mycobacterium tuberculosis cell-surface glycoprotein apa as a potential adhesin to colonize target cells via the innate immune system pulmonary C-type lectin surfactant protein A

Tuberculosis is still a major health problem, and understanding the mechanism by which Mycobacterium tuberculosis (Mtb) invades and colonizes its host target cells remains an important issue for the control of infection. The innate immune system C-type lectins (C-TLs), including the human pulmonary surfactant protein A (PSP-A), have been recently identified as determinant players in the early recognition of the invading pathogen and in mounting the host defense response. Although the antigenic lipoglycan mannosylated lipoarabinomannan is currently considered to be the major C-TL target on the mycobacterial surface, the recognition by some C-TLs of the only mycobacterial species composing the "Mtb complex" indicates that mannosylated lipoarabinomannan cannot account alone for this specificity. Thus, we searched for the mycobacterial molecules targeted by human PSP-A, focusing our attention on the Mtb surface glycoproteins. We developed an original functional proteomic approach based on a lectin blot assay using crude human bronchoalveolar lavage fluid as a source of physiological PSP-A. Combined with selective cell-surface protein extraction and mass spectrometry peptide mapping, this strategy allowed us to identify the Apa (alanine- and proline-rich antigenic) glycoprotein as new potential target for PSP-A. This result was supported by direct binding of PSP-A to purified Apa. Moreover, EDTA addition or deglycosylation of purified Apa samples completely abolished the interaction, demonstrating that the interaction is calcium- and mannose-dependent, as expected. Finally, we provide convincing evidence that Apa, formerly considered as mainly secreted, is associated with the cell wall for a sufficiently long time to aid in the attachment of PSP-A. Because, to date, Apa seems to be restricted to the Mtb complex strains, we propose that it may account for the selective recognition of those strains by PSP-A and other immune system C-TLs containing homologous functional domains.

Fine discrimination in the recognition of individual species of phosphatidyl-myo-inositol mannosides from Mycobacterium tuberculosis by C-type lectin pattern recognition receptors

The Mycobacterium tuberculosis (M.tb) envelope is highly mannosylated with phosphatidyl-myo-inositol mannosides (PIMs), lipomannan, and mannose-capped lipoarabinomannan (ManLAM). Little is known regarding the interaction between specific PIM types and host cell C-type lectin pattern recognition receptors. The macrophage mannose receptor (MR) and dendritic cell-specific ICAM-3-grabbing nonintegrin on dendritic cells engage ManLAM mannose caps and regulate several host responses. In this study, we analyzed the association of purified PIM families (f, separated by carbohydrate number) and individual PIM species (further separated by fatty acid number) from M.tb H(37)R(v) with human monocyte-derived macrophages (MDMs) and lectin-expressing cell lines using an established bead model. Higher-order PIMs preferentially associated with the MR as demonstrated by their reduced association with MDMs upon MR blockade and increased binding to COS-1-MR. In contrast, the lower-order PIM(2)f associated poorly with MDMs and did not bind to COS-1-MR. Triacylated PIM species were recognized by MDM lectins better than tetra-acylated species and the degree of acylation influenced higher-order PIM association with the MR. Moreover, only higher-order PIMs that bind the MR showed a significant increase in phagosome-lysosome fusion upon MR blockade. In contrast with the MR, the PIM(2)f and lipomannan were recognized by DC-SIGN comparable to higher-order PIMs and ManLAM, and the association was independent of their degree of acylation. Thus, recognition of M.tb PIMs by host cell C-type lectins is dependent on both the nature of the terminal carbohydrates and degree of acylation. Subtle structural differences among the PIMs impact host cell recognition and response and are predicted to influence the intracellular fate of M.tb.

Surface plasmon optical studies of carboxymethyl dextran brushes versus networks

The conformational changes of carboxymethyl dextran (CMD) substrates induced by variations in pH and ionic strength were studied using surface plasmon field-enhanced fluorescence spectroscopy (SPFS). A typical response was an increase in swelling upon increasing the pH and decreasing the ionic strength. Furthermore, the effects of the surface charge and cross-link density of the CMD on the degree of stimulus responses were investigated. The swelling/collapse ratio decreased with decreasing carboxyl group surface concentration and increasing cross-link density.

A lipomannan variant with strong TLR-2-dependent pro-inflammatory activity in Saccharothrix aerocolonigenes

Lipomannans (LMs) are powerful pro-inflammatory lipoglycans found in mycobacteria and related genera, however the molecular bases of their activity are not fully understood. We report here the isolation and the structural and functional characterization of a new lipomannan variant present in the Pseudonocardineae, Saccharothrix aerocolonigenes, designated SaeLM. Using a range of chemical degradations, NMR experiments, and mass spectrometry analyses, SaeLM revealed a mannosylphosphatidyl-myo-inositol (MPI) anchor glycosylated by an original carbohydrate structure whereby an (alpha1-->6)-Manp backbone is substituted at >80% of the O-2 position by side chains composed of Manp-(alpha1-->2)-Manp-(alpha1-->. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry analysis indicated a distribution of SaeLM glyco-forms ranging from 19 to 61 Manp units, which centered on species containing 37 or 40 Manp units. SaeLM induced a Toll-like receptor 2 (TLR-2)-dependent production of tumor necrosis factor-alpha (TNF-alpha) by human THP-1 monocyte/macrophage cell lines and interestingly was found to be the strongest inducer of this pro-inflammatory cytokine when compared with other LAM/LM-like molecules. We previously established that a linear (alpha1-->6)-Manp chain, linked to the MPI anchor, is sufficient in providing pro-inflammatory activity. We demonstrate here that by adding side chains and increasing their size, one may potentiate this activity. These findings should enable a better understanding of the structure/function relationships of TLR-2-dependent lipoglycan signaling.

Highly Ordered Supra-Molecular Organization of the Mycobacterial Lipoarabinomannans in Solution. Evidence of a Relationship Between Supra-Molecular Organization and Biological Activity

The complex mycobacterial mannosylated lipoarabinomannans (ManLAMs) are currently considered to be the major virulence factors of the pathogenic Mycobacterium tuberculosis. The recognition and the interaction of ManLAMs with immune system receptors have been shown to promote M.tuberculosis phagocytosis but also to down-regulate the bactericidal immune response of the host in favor of the survival of the pathogenic bacilli. To date these original biological activities were mainly associated to the presence of mannose residues capping the non-reducing ends of the ramified polysaccharide moiety of these complex lipoglycans. However, we demonstrated recently that the molecular recognition of ManLAM terminal mannose units by human pulmonary surfactant protein A (hSP-A) carbohydrate recognition domains depends on the presence of the lipid moiety of the ManLAMs as proposed by Sidobre et al. in 2002. Thus, we investigated the putative role of the ManLAM aglycon moiety. The data presented here, indicate that the hydrophobic aglycon part of ManLAM is associated to a characteristic concentration-dependent supra-molecular organization of these complex molecules. Furthermore, we observed that the deacylated ManLAMs or the lipid-free mannosylated arabinomannans, which do not exhibit characteristic ManLAM activities, do not display this supra-molecular organization. These observations strongly suggest that the ManLAMs immunomodulatory activities might be associated to their particular organization. Finally, the determination of the critical micellar concentration of ManLAMs obviously supports the notion that this supra-molecular organization may be responsible for the specific biological activities of these complex molecules.

Novel prenyl-linked benzophenone substrate analogues of mycobacterial mannosyltransferases

PPM (polyprenol monophosphomannose) has been shown to act as a glycosyl donor in the biosynthesis of the Man (mannose)-rich mycobacterial lipoglycans LM (lipomannan) and LAM (lipoarabinomannan). The Mycobacterium tuberculosis PPM synthase (Mt-Ppm1) catalyses the transfer of Man from GDP-Man to polyprenyl phosphates. The resulting PPM then serves as a donor of Man residues leading to the formation of an alpha(1-->6)LM intermediate through a PPM-dependent alpha(1-->6)mannosyltransferase. In the present study, we prepared a series of ten novel prenyl-related photoactivatable probes based on benzophenone with lipophilic spacers replacing several internal isoprene units. These probes were excellent substrates for the recombinant PPM synthase Mt-Ppm1/D2 and, on photoactivation, several inhibited its activity in vitro. The protection of the PPM synthase activity by a 'natural' C(75) polyprenyl acceptor during phototreatment is consistent with probe-mediated photoinhibition occurring via specific covalent modification of the enzyme active site. In addition, the unique mannosylated derivatives of the photoreactive probes were all donors of Man residues, through a PPM-dependent mycobacterial alpha(1-->6)mannosyltransferase, to a synthetic Manp(1-->6)-Manp-O-C(10:1) disaccharide acceptor (where Manp stands for mannopyranose). Photoactivation of probe 7 led to striking-specific inhibition of the M. smegmatis alpha(1-->6)mannosyltransferase. The present study represents the first application of photoreactive probes to the study of mycobacterial glycosyltransferases involved in LM and LAM biosynthesis. These preliminary findings suggest that the probes will prove useful in investigating the polyprenyl-dependent steps of the complex biosynthetic pathways to the mycobacterial lipoglycans, aiding in the identification of novel glycosyltransferases.

A survey of the year 2002 commercial optical biosensor literature

We have compiled 819 articles published in the year 2002 that involved commercial optical biosensor technology. The literature demonstrates that the technology's application continues to increase as biosensors are contributing to diverse scientific fields and are used to examine interactions ranging in size from small molecules to whole cells. Also, the variety of available commercial biosensor platforms is increasing and the expertise of users is improving. In this review, we use the literature to focus on the basic types of biosensor experiments, including kinetics, equilibrium analysis, solution competition, active concentration determination and screening. In addition, using examples of particularly well-performed analyses, we illustrate the high information content available in the primary response data and emphasize the impact of including figures in publications to support the results of biosensor analyses.

Tsukamurella paurometabola Lipoglycan, a New Lipoarabinomannan Variant with Pro-inflammatory Activity

The genus Tsukamurella is a member of the phylogenetic group nocardioform actinomycetes and is closely related to the genus Mycobacterium. The mycobacterial cell envelope contains lipoglycans, and of particular interest is lipoarabinomannan, one of the most potent mycobacterial immunomodulatory molecules. We have investigated the presence of lipoglycans in Tsukamurella paurometabola and report here the isolation and structural characterization of a new lipoarabinomannan variant, designated TpaLAM. Matrix-assisted laser desorption ionization-mass spectrometric analysis revealed that TpaLAM had an average molecular mass of 12.5 kDa and consequently was slightly smaller than Mycobacterium tuberculosis lipoarabinomannan. Using a range of chemical degradations, NMR experiments, capillary electrophoresis, and mass spectrometry analyses, TpaLAM revealed an original carbohydrate structure. Indeed, TpaLAM contained a mannosylphosphatidyl-myo-inositol (MPI) anchor glycosylated by a linear (alpha1-->6)-Manp mannan domain, which is further substituted by an (alpha1-->5)-Araf chain. Half of the Araf units are further substituted at the O-2 position by a Manp-(alpha1-->2)-Manp-(alpha1--> dimannoside motif. Altogether, TpaLAM appears to be the most elaborated non-mycobacterial LAM molecule identified to date. TpaLAM was found to induce the pro-inflammatory cytokine tumor necrosis factor (TNF)-alpha when tested with either human or murine monocyte/macrophage cell lines. This induction was completely abrogated in the presence of an anti-toll-like receptor-2 (TLR-2) antibody, suggesting that TLR-2 participates in the mediation of TNF-alpha production in response to TpaLAM. Moreover, we established that the lipomannan core of TpaLAM is the primary moiety responsible for the observed TNF-alpha-inducing activity. This conclusively demonstrates that a linear (alpha1-->6)-Manp chain, linked to the MPI anchor, is sufficient in providing pro-inflammatory activity.

Collectins

Collectins are a family of collagenous calcium-dependent defense lectins in animals. Their polypeptide chains consist of four regions: a cysteine-rich N-terminal domain, a collagen-like region, an alpha-helical coiled-coil neck domain and a C-terminal lectin or carbohydrate-recognition domain. These polypeptide chains form trimers that may assemble into larger oligomers. The best studied family members are the mannan-binding lectin, which is secreted into the blood by the liver, and the surfactant proteins A and D, which are secreted into the pulmonary alveolar and airway lining fluid. The collectins represent an important group of pattern recognition molecules, which bind to oligosaccharide structures and/or lipid moities on the surface of microorganisms. They bind preferentially to monosaccharide units of the mannose type, which present two vicinal hydroxyl groups in an equatorial position. High-affinity interactions between collectins and microorganisms depend, on the one hand, on the high density of the carbohydrate ligands on the microbial surface, and on the other, on the degree of oligomerization of the collectin. Apart from binding to microorganisms, the collectins can interact with receptors on host cells. Binding of collectins to microorganisms may facilitate microbial clearance through aggregation, complement activation, opsonization and activation of phagocytosis, and inhibition of microbial growth. In addition, the collectins can modulate inflammatory and allergic responses, affect apoptotic cell clearance and modulate the adaptive immune system.

Toll-Like Receptor 2 (TLR2)-Dependent-Positive and TLR2-Independent-Negative Regulation of Proinflammatory Cytokines by Mycobacterial Lipomannans

Lipoarabinomannans (LAM) and lipomannans (LM) are integral parts of the mycobacterial cell wall recognized by cells involved in the innate immune response and have been found to modulate the cytokine response. Typically, mannosylated LAM from pathogenic mycobacteria have been reported to be anti-inflammatory, whereas phosphoinositol-substituted LAM from nonpathogenic species are proinflammatory molecules. In this study, we show that LM from several mycobacterial species, including Mycobacterium chelonae, Mycobacterium kansasii, and Mycobacterium bovis bacillus Calmette-Guérin, display a dual function by stimulating or inhibiting proinflammatory cytokine synthesis through different pathways in murine primary macrophages. LM, but none of the corresponding LAM, induce macrophage activation characterized by cell surface expression of CD40 and CD86 and by TNF and NO secretion. This activation is dependent on the presence of Toll-like receptor (TLR) 2 and mediated through the adaptor protein myeloid differentiation factor 88 (MyD88), but independent of either TLR4 or TLR6 recognition. Surprisingly, LM exerted also a potent inhibitory effect on TNF, IL-12p40, and NO production by LPS-activated macrophages. This TLR2-, TLR6-, and MyD88-independent inhibitory effect is also mediated by LAM from M. bovis bacillus Calmette-Guérin but not by LAM derived from M. chelonae and M. kansasii. This study provides evidence that mycobacterial LM bear structural motifs susceptible to interact with different pattern recognition receptors with pro- or anti-inflammatory effects. Thus, the ultimate response of the host may therefore depend on the prevailing LM or LAM in the mycobacterial envelope and the local host cell receptor availability.

Disruption of Cg-Ppm1, a polyprenyl monophosphomannose synthase, and the generation of lipoglycan-less mutants in Corynebacterium glutamicum

The glycosyl donor, polyprenyl monophosphomannose (PPM), has been shown to be involved in the biosynthesis of the mycobacterial lipoglycans: lipomannan and lipoarabinomannan. The mycobacterial PPM synthase (Mt-ppm1) catalyzes the transfer of mannose from GDP-mannose to polyprenyl phosphates. Based on sequence homology to Mt-ppm1, we have identified the PPM synthase from Corynebacterium glutamicum. In the present study, we demonstrate that the corynebacterial synthase is composed of two distinct domains; a catalytic domain (Cg-ppm1) and a membrane domain (Cg-ppm2). Through the inactivation of Cg-ppm1, we observed a complex phenotype that included altered cell growth rate and inability to synthesize PPM molecules and lipoglycans. When Cg-ppm2 was deleted, no observable phenotype was noted, indicating the clear organization of the two domains. The complementation of the inactivated Cg-ppm1 strain with the corresponding mycobacterial enzyme (Mt-Ppm1/D2) led to the restoration of a wild type phenotype. The present study illustrates, for the first time, the generation of a lipoglycan-less mutant based on a molecular strategy in a member of the Corynebacterianeae family. Lipoglycans are important immunomodulatory molecules involved in determining the outcome of infection, and so the generation of defined mutants and their subsequent immunological characterization is timely.

Identification of the required acyltransferase step in the biosynthesis of the phosphatidylinositol mannosides of mycobacterium species

Fatty acyl functions of the glycosylated phosphatidylinositol (GPI) anchors of the phosphatidylinositol mannosides (PIM), lipomannan (LM), and lipoarabinomannan (LAM) of mycobacteria play a critical role in both the physical properties and biological activities of these molecules. In a search for the acyltransferases that acylate the GPI anchors of PIM, LM, and LAM, we examined the function of the mycobacterial Rv2611c gene that encodes a putative acyltransferase involved in the early steps of phosphatidylinositol mannoside synthesis. A Rv2611c mutant of Mycobacterium smegmatis was constructed which exhibited severe growth defects and contained an increased amount of phosphatidylinositol mono- and di-mannosides and a decreased amount of acylated phosphatidylinositol di-mannosides compared with the wild-type parental strain. In cell-free assays, extracts from M. smegmatis overexpressing the M. tuberculosis Rv2611c gene incorporated [14C]palmitate into acylated phosphatidylinositol mono- and di-mannosides, and transferred cold endogenous fatty acids onto 14C-labeled phosphatidylinositol mono- and di-mannosides more efficiently than extracts from the wild-type strain. Cell-free extracts from the Rv2611c mutant of M. smegmatis were greatly impaired in these respects. This work provides evidence that Rv2611c is the acyltransferase that catalyzes the acylation of the 6-position of the mannose residue linked to position 2 of myo-inositol in phosphatidylinositol mono- and di-mannosides, with the mono-mannosylated lipid acceptor being the primary substrate of the enzyme. We also provide the first evidence that two distinct pathways lead to the formation of acylated PIM2 from PIM1 in mycobacteria.

Lipomannans, but not lipoarabinomannans, purified from Mycobacterium chelonae and Mycobacterium kansasii induce TNF-alpha and IL-8 secretion by a CD14-toll-like receptor 2-dependent mechanism

Lipoarabinomannans (LAMs) are glycolipids from the mycobacterial cell wall that exhibit various biological activities, including proinflammatory and anti-inflammatory responses. However, little is known about the properties of lipomannans (LMs), considered to be precursors of LAMs. In this study, we provide evidence that LMs purified from Mycobacterium chelonae and a clinical strain of Mycobacterium kansasii stimulated mRNA expression and secretion of TNF-alpha and IL-8 from human macrophage-like differentiated THP-1 cells. In contrast to LMs, LAMs were not able to induce a significant cytokine-inducing effect. The mechanism of activation by LMs was investigated using various Abs raised against surface receptors for multiple bacterial products. The presence of anti-CD14 or anti-Toll-like receptor 2 (TLR2) Abs profoundly affected production of TNF-alpha and IL-8, suggesting that both CD14 and TLR2 participate in the LM-mediated activation process. Furthermore, stimulation of cells was dependent on the presence of the LPS-binding protein, a plasma protein that transfers glycolipids to CD14. Chemical degradation of the arabinan domain of mannose-capped LAM from M. kansasii, which presented no cytokine-eliciting effect, restored the cytokine-inducing activity at a level similar to those of LMs. These results support the hypothesis that the presence of an arabinan in LAMs prevents the interaction of these glycolipids with TLR2/CD14 receptors. In addition, we found that phosphatidylinositol dimannosides isolated from M. kansasii did not induce cytokine secretion. This study suggests that LMs isolated from different mycobacterial species participate in the immunomodulation of the infected host and that the D-mannan core of this glycolipid is essential for this function.

The cell surface receptor DC-SIGN discriminates between Mycobacterium species through selective recognition of the mannose caps on lipoarabinomannan

Interactions between dendritic cells (DCs) and Mycobacterium tuberculosis, the etiological agent of tuberculosis, most likely play a key role in anti-mycobacterial immunity. We have recently shown that M. tuberculosis binds to and infects DCs through ligation of the DC-specific intercellular adhesion molecule-3-grabbing nonintegrin (DC-SIGN) and that M. tuberculosis mannose-capped lipoarabinomannan (ManLAM) inhibits binding of the bacilli to the lectin, suggesting that ManLAM might be a key DC-SIGN ligand. In the present study, we investigated the molecular basis of DC-SIGN ligation by LAM. Contrary to what was found for slow growing mycobacteria, such as M. tuberculosis and the vaccine strain Mycobacterium bovis bacillus Calmette-Guérin, our data demonstrate that the fast growing saprophytic species Mycobacterium smegmatis hardly binds to DC-SIGN. Consistent with the former finding, we show that M. smegmatis-derived lipoarabinomannan, which is capped by phosphoinositide residues (PILAM), exhibits a limited ability to inhibit M. tuberculosis binding to DC-SIGN. Moreover, using enzymatically demannosylated and chemically deacylated ManLAM molecules, we demonstrate that both the acyl chains on the ManLAM mannosylphosphatidylinositol anchor and the mannooligosaccharide caps play a critical role in DC-SIGN-ManLAM interaction. Finally, we report that DC-SIGN binds poorly to the PILAM and uncapped AraLAM-containing species Mycobacterium fortuitum and Mycobacterium chelonae, respectively. Interestingly, smooth colony-forming Mycobacterium avium, in which ManLAM is capped with single mannose residues, was also poorly recognized by the lectin. Altogether, our results provide molecular insight into the mechanisms of mycobacteria-DC-SIGN interaction, and suggest that DC-SIGN may act as a pattern recognition receptor and discriminate between Mycobacterium species through selective recognition of the mannose caps on LAM molecules.