Differential recognition of core and terminal portions of oligosaccharide ligands by carbohydrate-recognition domains of two mannose-binding proteins

Back2Basics: animal lectins: an insight into a highly versatile recognition protein

The rapid advancement of molecular research has contributed to the discovery of 'Lectin', a carbohydrate-binding protein which specifically interacts with receptors on surface glycan moieties that regulate various critical cellular activities. The first animal lectin reported was 'the asialoglycoprotein receptor' in mammalian cells which helped analyze how animal lectins differ in glycoconjugate binding. Animal lectins are classified into several families, depending on their diverse cellular localization, and the binding specificities of their Carbohydrate-Recognition Domain (CRD) modules. Earlier characterization of animal lectins classified them into two structural families, the C-type (Ca²⁺-dependent binding) and S-type galectins (sulfhydryl-dependent binding) lectins. The C-type lectin includes the most significant animal lectins, such as endocytic receptors, mannose receptors, selectins, and collectins. The recent developments in research based on the complexity of the carbohydrate ligands, the metabolic processes they perform, their expression levels, and their reliance on divalent cations have identified more than 100 animal lectins and classified them into around 13 different families, such as Calnexin, F-lectin, Intelectin, Chitinase-like lectin, F-box lectin, etc. Understanding their structure and expression patterns have aided in defining their significant functions including cell adhesion, antimicrobial activity, innate immunity, disease diagnostic biomarkers, and drug delivery through specific carbohydrate-protein interactions. Such extensive potential roles of animal lectins made it equally important to plant lectins among researchers. Hence, the review focuses on providing an overview of animal lectins, their taxonomy, structural characteristics, and functions in diverse aspects interconnected to their specific carbohydrate and glycoconjugate binding.

Graphical abstract

β-Glucan-induced cooperative oligomerization of Dectin-1 C-type lectin-like domain

Dectin-1 is a C-type lectin-like pattern recognition receptor that recognizes β(1-3)-glucans present on non-self pathogens. It is of great importance in innate immunity to understand the mechanism whereby Dectin-1 senses β(1-3)-glucans and induces intracellular signaling. In this study, we characterize the ligand binding and ligand-induced oligomerization of murine Dectin-1 using its C-type lectin-like domain (CTLD). Interaction of CTLD with laminarin, a β-glucan ligand, induced a tetramer of CTLD, as evidenced by size exclusion chromatography and multi-angle light scattering. Component analysis suggested a stoichiometry of four CTLD molecules bound to four laminarin molecules. Dimers and trimers of CTLD were not detected suggesting cooperative oligomerization. In order to map the amino acid residues of CTLD involved in β-glucan binding and domain oligomerization, we performed site-directed mutagenesis on surface-exposed and most conserved amino acid residues. Among the mutants examined, W221A, H223A and Y228A abolished oligomer formation. Since these residues are spatially arranged to form a hydrophobic groove, it is likely that W221, H223 and Y228 are directly involved in β-glucan binding. Interestingly, mutation of the residues on the other side of the hydrophobic groove, including Y141, R145 and E243, also exhibited reduced oligomer formation, suggesting involvement in protein-protein interactions guided by laminarin. Ligand titration using intrinsic tryptophan fluorescence revealed that wild-type CTLD binds laminarin cooperatively with a Hill coefficient of ~3, while the oligomer-reducing mutations, inside and outside the putative binding site abolish or decrease cooperativity. We suggest that the ligand-induced cooperative oligomer formation of Dectin-1 is physiologically relevant in sensing exogenous β-glucan and triggering intracellular signaling.

Expression and characterization of recombinant chicken mannose binding lectin

Mannose binding lectin (MBL) is a serum collagenous C-type lectin that plays an important role in the innate immune protection against pathogens. Previously, human and mouse studies have demonstrated that MBL binds a broad range of pathogens that results in their neutralization through agglutination, enhanced phagocytosis, and/or complement activation via the lectin pathway. The role of MBL in chicken is not well understood although the MBL concentration in serum seems to correlate with protection against infections. To investigate the role of MBL in chicken further, recombinant chicken MBL (RcMBL) was produced in HeLa R19 cells and purified using mannan affinity chromatography followed by gel filtration. RcMBL was shown to be structurally and functionally similar to native chicken MBL (NcMBL) isolated from serum. RcMBL is expressed as an oligomeric protein (mixture of trimers and oligomerized trimers) with a monomeric mass of 26kDa as determined by mass spectrometry, corresponding to the predicted mass. Glycan array analysis indicated that RcMBL bound most strongly to high-mannose glycans but also glycans with terminal fucose and GlcNac residues. The biological activity of RcMBL was demonstrated via its capacity to agglutinate Salmonella Typhimurium and to inhibit the hemagglutination activity of influenza A virus. The production of a structurally well-characterized and functionally active RcMBL will facilitate detailed studies into the protective role of MBL in innate defense against pathogens in chicken and other avian species.

Identification, Characterization, and X-ray Crystallographic Analysis of a Novel Type of Mannose-Specific Lectin CGL1 from the Pacific Oyster Crassostrea gigas

A novel mannose-specific lectin, named CGL1 (15.5 kDa), was isolated from the oyster Crassostrea gigas. Characterization of CGL1 involved isothermal titration calorimetry (ITC), glycoconjugate microarray, and frontal affinity chromatography (FAC). This analysis revealed that CGL1 has strict specificity for the mannose monomer and for high mannose-type N-glycans (HMTGs). Primary structure of CGL1 did not show any homology with known lectins but did show homology with proteins of the natterin family. Crystal structure of the CGL1 revealed a unique homodimer in which each protomer was composed of 2 domains related by a pseudo two-fold axis. Complex structures of CGL1 with mannose molecules showed that residues have 8 hydrogen bond interactions with O1, O2, O3, O4, and O5 hydroxyl groups of mannose. The complex interactions that are not observed with other mannose-binding lectins revealed the structural basis for the strict specificity for mannose. These characteristics of CGL1 may be helpful as a research tool and for clinical applications.

A New Surface Plasmon Resonance Assay for In Vitro Screening of Mannose-Binding Lectin Inhibitors

Mannose-binding lectin (MBL) is a circulating protein that acts as a soluble pattern recognition molecule of the innate immunity. It binds to carbohydrate patterns on the surface of pathogens or of altered self-cells, with activation of the lectin pathway of the complement system. Recent evidence indicates that MBL contributes to the pathophysiology of ischemia-reperfusion injury and other conditions. Thus, MBL inhibitors offer promising therapeutic strategies, since they prevent the interaction of MBL with its target sugar arrays. We developed and characterized a novel assay based on surface plasmon resonance for in vitro screening of these compounds, which may be useful before the more expensive and time-consuming in vivo studies. The assay measures the inhibitor's ability to interfere with the binding of murine MBL-A or MBL-C, or of human recombinant MBL, to mannose residues immobilized on the sensor chip surface. We have applied the assay to measure the IC50 of synthetic glycodendrimers, two of them with neuroprotective properties in animal models of MBL-mediated injuries.

Carbohydrate recognition in the immune system: contributions of neoglycolipid-based microarrays to carbohydrate ligand discovery

Oligosaccharide sequences in glycomes of eukaryotes and prokaryotes are enormously diverse. The reasons are not fully understood, but there is an increasing number of examples of the involvement of specific oligosaccharide sequences as ligands in protein-carbohydrate interactions in health and, directly or indirectly, in every major disease, be it infectious or noninfectious. The pinpointing and characterizing of oligosaccharide ligands within glycomes has been one of the most challenging aspects of molecular cell biology, as oligosaccharides cannot be cloned and are generally available in limited amounts. This overview recounts the background to the development of a microarray system that is poised for surveying proteomes for carbohydrate-binding activities and glycomes for assigning the oligosaccharide ligands. Examples are selected by way of illustrating the potential of "designer" microarrays for ligand discovery at the interface of infection, immunity, and glycobiology. Particularly highlighted are sulfo-oligosaccharide and gluco-oligosaccharide recognition systems elucidated using microarrays.

Binding of mouse mannan-binding lectins to different bacterial pathogens of mice

Humans have one mannan-binding lectin (MBL) in circulation but rodents, pigs, rabbits and rhesus monkeys have two, MBL-A and MBL-C. Plasma forms of these proteins have similar mannan-binding activity in vitro, but might differ in their ability to bind other microbial targets. In these studies, we compared carbohydrate-dependent binding of mouse plasma MBL-A and MBL-C to mannan-sepharose beads and to intact bacteria isolated as pathogens from mice. After incubation of mouse plasma with intact bacteria, MBL-A and MBL-C were eluted with N-acetylglucosamine (GlcNAc) and identified in nonreducing SDS-PAGE using Western blot analysis and MBL-A or MBL-C specific monoclonal antibodies. GlcNAc eluates of plasma incubated with mannan-sepharose beads, Klebsiella oxytoca and Staphylococcus aureus contained similar bands (mainly approximately 50kDa) that were immunoreactive with MBL-C antibody. Furthermore, a smaller form of MBL-C (approximately 45kDa) was detected bound to Pseudomonas aeruginosa. By comparison, immunoreactive MBL-A (a ladder of approximately 175kDa and larger bands) was identified in these GlcNAc eluates from mannan-sepharose beads, S. aureus and K. oxytoca but not P. aeruginosa. These studies demonstrate that mouse MBL-A and MBL-C in plasma are not equivalent in their ability to recognize bacteria that are pathogens for mice.

Deficiency of mannose-binding lectin greatly increases susceptibility to postburn infection with Pseudomonas aeruginosa

Burn injury disrupts the mechanical and biological barrier that the skin presents against infection by symbionts like the Pseudomonas aeruginosa, a Gram-negative bacteria. A combination of local factors, antimicrobial peptides, and resident effector cells form the initial response to mechanical injury of the skin. This activity is followed by an inflammatory response that includes influx of phagocytes and serum factors, such as complement and mannose-binding lectin (MBL), which is a broad-spectrum pattern recognition molecule that plays a key role in innate immunity. A growing consensus from studies in humans and mice suggests that lack of MBL together with other comorbid factors predisposes the host to infection. In this study we examined whether MBL deficiency increases the risk of P. aeruginosa infection in a burned host. We found that both wild-type and MBL null mice were resistant to a 5% total body surface area burn alone or s.c. infection with P. aeruginosa alone. However, when mice were burned then inoculated s.c. with P. aeruginosa at the burn site, all MBL null mice died by 42 h from septicemia, whereas only one-third of wild-type mice succumbed (p = 0.0005). This result indicates that MBL plays a key role in containing and preventing a systemic spread of P. aeruginosa infection following burn injury and suggests that MBL deficiency in humans maybe a premorbid variable in the predisposition to infection in burn victims.

Proteins that bind high-mannose sugars of the HIV envelope

A broad range of proteins bind high-mannose carbohydrates found on the surface of the envelope protein gp120 of the human immunodeficiency virus and thus interfere with the viral life cycle, providing a potential new way of controlling HIV infection. These proteins interact with the carbohydrate moieties in different ways. A group of them interacts as typical C-type lectins via a Ca2+ ion. Another group interacts with specific single, terminal sugars, without the help of a metal cation. A third group is involved in more intimate interactions, with multiple carbohydrate rings and no metal ion. Finally, there is a group of lectins for which the interaction mode has not yet been elucidated. This review summarizes, principally from a structural point of view, the current state of knowledge about these high-mannose binding proteins and their mode of sugar binding.

Characterization of the oligomer structure of recombinant human mannan-binding lectin

Mannan-binding lectin (MBL) belongs to a family of proteins called the collectins, which show large differences in their ultrastructures. These differences are believed to be determined by different N-terminal disulfide-bonding patterns. So far only the bonding pattern of two of the simple forms (recombinant rat MBL-C and bovine CL-43) have been determined. Recombinant MBL expressed in human cells was purified, and the structure of the N-terminal region was determined. Preliminary results on human plasma-derived MBL revealed high similarity to the recombinant protein. Here we report the structure of the N-terminal part of recombinant human MBL and present a model to explain the oligomerization pattern. Using a strategy of consecutive enzymatic digestions and matrix-assisted laser desorption ionization mass spectrometry, we succeeded in identifying a number of disulfide-linked peptides from the N-terminal cysteine-rich region. Based on these building blocks, we propose a model that can explain the various oligomeric forms found in purified MBL preparations. Furthermore, the model was challenged by the production of cysteine to serine mutants of the three N-terminally situated cysteines. The oligomerization patterns of these mutants support the proposed model. The model indicates that the polypeptide dimer is the basic unit in the oligomerization.

The potential role of mannan-binding lectin in the clearance of self-components including immune complexes

Mannan-binding lectin (MBL) is a pattern recognition receptor in the innate immune system. It recognizes certain sugar residues arranged in a pattern that enables MBL to bind with sufficient strength. Such sugar patterns are common on the surface of many microorganisms, and MBL has therefore been considered to be an agent that can discriminate between self and nonself. There is, however, increasing evidence supporting that MBL, like many membrane-bound C-type lectin-like receptors, also helps to dispose of various outworn or abnormal body components. Most self-components are protected with sialic acid or galactose that disrupt the pattern of the sugars that MBL can bind, but MBL may be significantly involved in the elimination of self-components that have lost these protective terminal residues. The role of MBL in the clearance of invading pathogens has previously been thoroughly reviewed. Here, we review some findings that support the notion that MBL may contribute to noninflammatory removal of immune complexes and abnormal cells by the reticuloendothelial system. Defects in this clearance mechanism may cause an accumulation of potentially dangerous self-components, thereby increasing the likelihood of chronic inflammation and autoimmunity.

Characterization of beta-glucan recognition site on C-type lectin, dectin 1

Dectin 1 is a mammalian cell surface receptor for (1-->3)-beta-d-glucans. Since (1-->3)-beta-d-glucans are commonly present on fungal cell walls, it has been suggested that dectin 1 is important for recognizing fungal invasion. In this study we tried to deduce the amino acid residues in dectin 1 responsible for beta-glucan recognition. HEK293 cells transfected with mouse dectin 1 cDNA could bind to a gel-forming (1-->3)-beta-d-glucan, schizophyllan (SPG). The binding of SPG to a dectin 1 transfectant was inhibited by pretreatment with other beta-glucans having a (1-->3)-beta-d-glucosyl linkage but not by pretreatment with alpha-glucans. Dectin 1 has a carbohydrate recognition domain (CRD) consisting of six cysteine residues that are highly conserved in C-type lectins. We prepared 32 point mutants with mutations in the CRD and analyzed their binding to SPG. Mutations at Trp(221) and His(223) resulted in decreased binding to beta-glucan. Monoclonal antibody 4B2, a dectin- 1 monoclonal antibody which had a blocking effect on the beta-glucan interaction, completely failed to bind the dectin-1 mutant W221A. A mutant with mutations in Trp(221) and His(223) did not have a collaborative effect on Toll-like receptor 2-mediated cellular activation in response to zymosan. These amino acid residues are distinct from residues in other sugar-recognizing peptide sequences of typical C-type lectins. These results suggest that the amino acid sequence W221-I222-H223 is critical for formation of a beta-glucan binding site in the CRD of dectin 1.

Effect of carbohydrates attached to polystyrene on hepatocyte morphology on sugar‐derivatized polystyrene matrices

Sugar-carrying polymers have been utilized as artificial matrices for cell adhesion in tissue engineering. We have developed sugar-derivatized polystyrenes (PV-sugars) as artificial matrices, which control hepatocyte adhesion and hepatic function. Hepatocytes adhere to PV-sugar matrices in a receptor-mediated manner. In this study, we designed a new galactose-derivatized PV-sugar, poly-(6-O-p-vinylbenzyl-alpha-D-galactose) (PV6Gal) and evaluated the role of carbohydrate attached to polystyrene (PS) backbone in the morphological difference of hepatocyte cultured on PV-sugar matrices. Hepatocytes spread on monosaccharide-derivatized PV-sugars but not on disaccharide-derivatized PV-sugars. The actin filament remained aggregated in the central area of the cell body on disaccharide-derivatized PV-sugars. Hepatocyte cell bodies fully were spread on collagen, and the actin filament was almost completely reorganized. Hepatocyte spreading on monosaccharide-derivatized PV-sugars, however, was caused by protrusive cell-matrix contact like lamellipodia and the actin filament was not completely reorganized. This indicated that hepatocyte spreading on PV-sugar matrices was restricted compared with ECM-mediated cell spreading. In addition, typical spheroid formation of hepatocytes was promoted on disaccharide-derivatized PV-sugars compared with monosaccharide-derivatized PV-sugars. Although hepatocytes adhered with different affinities to PV-sugar matrices, hepatocyte morphology was not affected by the adhesion affinity. We suggest that the type of carbohydrate attached to the PS backbone governs the morphology of hepatocyte cultured on PV-sugar matrices.

Interaction of mannan-binding lectin with Trichinella spiralis glycoproteins, a possible innate immune mechanism

Complex and variable glycoconjugates presented by parasitic nematodes during infection are very important in the host-parasite interplay. Predominantly carbohydrate-rich antigens are involved in the stimulation and modulation of the stage-specific immune response of the host. The non-specific innate immune system, however, acts as the first line of host defence against pathogens, before the appearance of antigen-specific responses. The functional entities of the innate system are lectins that recognize the surface ligands of pathogens: mannan-binding lectin (MBL) is a key recognition element involved in binding oligosaccharide structures exposed on microorganisms. In the present study we investigated whether MBL binds to the parasitic nematode Trichinella spiralis (T. spiralis). Since the parasite is coated with mannose-containing glycans, these structures could represent potential ligands for MBL and contribute to activation of the innate immune response of the host. Histochemical staining revealed MBL on the surface and internal organs of T. spiralis muscle larvae. MBL bound in a mannose-inhibitable manner to both crude extracts of T. spiralis muscle larvae and larvae excretory/secretory products. Western blot analyses showed that MBL recognized glycoproteins from all stages of T. spiralis. In vitro complement activation assays suggested that MBL is capable of fixing complement components on T. spiralis crude extract coated plates and activating the complement cascade through the 'lectin pathway'.

Structural characterization of a mannose-binding protein-trimannoside complex using residual dipolar couplings

The ligand-binding properties of a 53 kDa homomultimeric trimer from mannose-binding protein (MBP) have been investigated using residual dipolar couplings (RDCs) that are easily measured from NMR spectra of the ligand and isotopically labeled protein. Using a limited set of 1H-15N backbone amide NMR assignments for MBP and orientational information derived from the RDC measurements in aligned media, an order tensor for MBP has been determined that is consistent with symmetry-based predictions of an axially symmetric system. 13C-1H couplings for a bound trisaccharide ligand, methyl 3,6-di-O-(alpha-D-mannopyranosyl)-alpha-D-mannopyranoside (trimannoside) have been determined at natural abundance and used as orientational constraints. The bound ligand geometry and orientational constraints allowed docking of the trimannoside ligand in the binding site of MBP to produce a structural model for MBP-oligosaccharide interactions.

Disseminated candidiasis and hepatic malarial infection in mannose-binding-lectin-A-deficient mice

To examine the physiological functions of mannose-binding lectin A (MBL-A), we generated mice that were deficient in MBL-A and examined their susceptibilities to the microbial pathogens Candida albicans and Plasmodium yoelii, an accepted experimental malaria model in mouse. We found no differences in the survival rates and fungal burdens of wild-type and MBL-A(-/-) mice with disseminated C. albicans infection. The two mouse strains were also similar in their abilities to resist hepatic accumulation of P. yoelii parasites. We conclude that MBL-A deficiency does not alter resistance to disseminated candidiasis or initial hepatic invasion by P. yoelii.

High mannose glycans and sialic acid on gp120 regulate binding of mannose-binding lectin (MBL) to HIV type 1

Mannose-binding lectin (MBL) is a C-type lectin of the innate immune system that binds to carbohydrates on the surface of certain microorganisms. Previous studies showed that MBL binds to gp120, the envelope glycoprotein of HIV-1. gp120 is extensively glycosylated, with N-linked complex and high mannose carbohydrates accounting for about half of the molecular weight. The objectives of this study were to determine the types of glycans on gp120 important for MBL binding and to determine if alteration of complex glycans with neuraminidase (NA) could enhance the interaction of MBL with virus. Lectin blot analyses revealed that MBL interacted with recombinant gp120 (rgp120) from both T cell-tropic and M-tropic virus strains. Treatment of rgp120 with endoglycosidase H (eH) or endoglycosidase F1 (eF1) abrogated binding of MBL, but did not decrease binding of wheat germ agglutinin indicating that high mannose and/or hybrid N-linked glycans were required for MBL binding. Removal of sialic acids from rgp120 with NA enhanced MBL binding. Treatment of intact virus from T cell lines or primary isolates with eF1 also significantly decreased HIV binding to MBL, while treatment with NA substantially increased binding. Treatment of virus with both eF1 and NA did not decrease binding compared to NA alone suggesting that NA treatment exposed binding sites on gp120 that are not high mannose glycans. These studies provide evidence that MBL binds to HIV via high mannose carbohydrates on gp120 and shows that the interaction of MBL with virus is regulated by sialylation.

An endogenous Drosophila receptor for glycans bearing alpha 1,3-linked core fucose residues

The genome of Drosophila melanogaster encodes several proteins that are predicted to contain Ca(2+)-dependent, C-type carbohydrate-recognition domains. The CG2958 gene encodes a protein containing 359 amino acid residues. Analysis of the CG2958 sequence suggests that it consists of an N-terminal domain found in other Drosophila proteins, a middle segment that is unique, and a C-terminal C-type carbohydrate-recognition domain. Expression studies show that the full-length protein is a tetramer formed by noncovalent association of disulfide-linked dimers that are linked through cysteine residues in the N-terminal domain. The expressed protein binds to immobilized yeast invertase through the C-terminal carbohydrate-recognition domain. Competition binding studies using monosaccharides demonstrate that CG2958 interacts specifically with fucose and mannose. Fucose binds approximately 5-fold better than mannose. Blotting studies reveal that the best glycoprotein ligands are those that contain N-linked glycans bearing alpha1,3-linked fucose residues. Binding is enhanced by the additional presence of alpha1,6-linked fucose. It has previously been proposed that labeling of the Drosophila neural system by anti-horseradish peroxidase antibodies is a result of the presence of difucosylated N-linked glycans. CG2958 is a potential endogenous receptor for such neural-specific carbohydrate epitopes.

Orientation of bound ligands in mannose-binding proteins. Implications for multivalent ligand recognition

Mannose-binding proteins (MBPs) are C-type animal lectins that recognize high mannose oligosaccharides on pathogenic cell surfaces. MBPs bind to their carbohydrate ligands by forming a series of Ca(2+) coordination and hydrogen bonds with two hydroxyl groups equivalent to the 3- and 4-OH of mannose. In this work, the determinants of the orientation of sugars bound to rat serum and liver MBPs (MBP-A and MBP-C) have been systematically investigated. The crystal structures of MBP-A soaked with monosaccharides and disaccharides and also the structure of the MBP-A trimer cross-linked by a high mannose asparaginyl oligosaccharide reveal that monosaccharides or alpha1-6-linked mannose bind to MBP-A in one orientation, whereas alpha1-2- or alpha1-3-linked mannose binds in an orientation rotated 180 degrees around a local symmetry axis relating the 3- and 4-OH groups. In contrast, a similar set of ligands all bind to MBP-C in a single orientation. The mutation of MBP-A His(189) to its MBP-C equivalent, valine, causes Man alpha 1-3Man to bind in a mixture of orientations. These data combined with modeling indicate that the residue at this position influences the orientation of bound ligands in MBP. We propose that the control of binding orientation can influence the recognition of multivalent ligands. A lateral association of trimers in the cross-linked crystals may reflect interactions within higher oligomers of MBP-A that are stabilized by multivalent ligands.

Conformation of a trimannoside bound to mannose-binding protein by nuclear magnetic resonance and molecular dynamics simulations

A model of the carbohydrate recognition domain of the serum form of mannose-binding protein (MBP) from rat complexed with methyl 3,6-di-O-(alpha-D-mannopyranosyl)-alpha-D-mannopyranoside is presented. Allowed conformations for the bound sugar were derived from simulated annealing protocols incorporating distance restraints computed from transferred NOESY spectra. The resulting sugar conformations were then modeled into the MBP binding site, and these models of the complex were refined using molecular dynamics (MD) simulations in the presence of solvent water. These studies indicate that only one of the two major conformations of the alpha(1-->6) linkage found in solution is significantly populated in the bound state (omega = 60 degrees ), whereas the alpha(1-->3) linkage samples at least two states, similar to its behavior in free solution. The bound conformation allows direct hydrogen bonds to form between the sugar and K182 of MBP, in addition to other water-mediated hydrogen bonds. Estimates of binding constants of candidate complexes based on changes in solvent-accessible surface areas upon binding support the NMR and MD results. These estimates further suggest that the enthalpic gains of the additional sugar-MBP interactions in a trisaccharide as opposed to a monosaccharide are offset by entropic penalties, offering an explanation for previous binding data.

Binding of the Galanthus nivalis agglutinin to thymocytes reveals alterations in surface glycosylation during T-cell development

Surface binding of the Galanthus nivalis agglutinin (GNA) to thymocyte subsets has been studied in pigs and rodents by multicolour flow cytometry. In all the species examined, analogous staining profiles have been recorded. Counter-staining with anti-CD3epsilon, anti-CD4 and anti-CD8 monoclonal antibodies (MoAb) revealed that a significant increase of the GNA targets on the cell surface occurred during early thymocyte differentiation and reached its maximum at the level of the CD3loCD4+CD8+ small cortical thymocyte. This was followed by a decrease in the GNA binding capacity upon terminal maturation to the single positive thymocytes. PAGE analysis has revealed a dominant GNA-binding glycoprotein (molar mass approx. 90 kDa) present on thymocyte plasma membranes and absent on the surface of splenic lymphocytes, although both the whole cell lysates from both organs contained GNA ligands of the same size. Our findings are in agreement with previous data showing that immature thymocytes differ from their mature counterparts and peripheral T lymphocytes in the surface glycosylation pattern, and support the hypothesis that lectin-glycoprotein interaction plays a significant role in the cell-to-cell crosstalk in the thymic cortex.

IgM detection via selective recognition by mannose-binding protein

Ca(2+)-dependent mannose-binding proteins (MBPs) belong to the family of animal lectins isolated from the liver and serum of rabbits, humans and rodents. They perform in vivo as defense molecules that act as opsonins by enhancing the clearance of mannose-rich pathogens and have been used in vitro for the purification of immunoglobulin M (IgM). In this study, we used MBPs as a sensitive and specific reagent for the detection of IgM due to their high specificity for mannose found only in IgM carbohydrate regions. MBP performed as a sensitive alternative to the usually used anti-IgM, where very low concentrations of IgM should be detected. IgM plays a central role in the initial response of the immune system to the invasion of foreign pathogens, as the early detection of the appearance of pathogenic IgM in biological fluids is of great significance in the diagnosis and treatment of many acute pathological cases. The development of a highly sensitive and reliable assay for the detection of low concentrations of IgM based on covalent binding on epoxy film-coated surfaces and selective recognition of IgM by MBP may be of clinical importance.

A novel mechanism of carbohydrate recognition by the C-type lectins DC-SIGN and DC-SIGNR. Subunit organization and binding to multivalent ligands

DC-SIGN and DC-SIGNR are cell-surface receptors that mediate cell-cell interactions within the immune system by binding to intercellular adhesion molecule-3. The receptor polypeptides share 77% amino acid sequence identity and are type II transmembrane proteins. The extracellular domain of each comprises seven 23-residue tandem repeats and a C-terminal C-type carbohydrate-recognition domain (CRD). Cross-linking, equilibrium ultracentrifugation, and circular dichroism studies of soluble recombinant fragments of DC-SIGN and DC-SIGNR have been used to show that the extracellular domain of each receptor is a tetramer stabilized by an alpha-helical stalk. Both DC-SIGN and DC-SIGNR bind ligands bearing mannose and related sugars through the CRDs. The CRDs of DC-SIGN and DC-SIGNR bind Man(9)GlcNAc(2) oligosaccharide 130- and 17-fold more tightly than mannose, and affinity for a glycopeptide bearing two such oligosaccharides is increased by a further factor of 5- to 25-fold. These results indicate that the CRDs contain extended or secondary oligosaccharide binding sites that accommodate mammalian-type glycan structures. When the CRDs are clustered in the tetrameric extracellular domain, their arrangement provides a means of amplifying specificity for multiple glycans on host molecules targeted by DC-SIGN and DC-SIGNR. Binding to clustered oligosaccharides may also explain the interaction of these receptors with the gp120 envelope protein of human immunodeficiency virus-1, which contributes to virus infection.

Biological activities of human mannose-binding lectin bound to two different ligand sugar structures, Lewis A and Lewis B antigens and high-mannose type oligosaccharides

The biological activities of mannose-binding lectin (MBL) which binds to different ligands on mammalian cells were examined using two types of Colo205 cells, a human colon adenocarcinoma cell line: one naturally expressing Lewis A and Lewis B antigens as ligands for MBL (NT-Colo205), and the other modified to express high-mannose type oligosaccharides by treatment with benzyl-2-acetamide-2-deoxy-alpha-galactopyranoside and 1-deoxymannojirimycin (Bz+dMM-Colo205). Although the final lysis was not observed, the deposition of C4 and C3 was observed on both types of Colo205 cells after treatment with MBL and complements as a result of complement activation by MBL. MBL bound to Bz+dMM-Colo205 could also activate human peripheral blood leukocytes and induce superoxide production; however, MBL bound to NT-Colo205 could not. This may be explained by the lower affinity of MBL to Lewis A and Lewis B antigens than to high-mannose type oligosaccharides under physiological conditions, since MBL bound to NT-Colo205 was more easily released from the cell surface than that bound to Bz+dMM-Colo205 at 37 degrees C. These findings suggest that the difference in the affinity of MBL to its ligands could influence the expression of some biological activities of MBL.

Rat mannose-binding protein a binds CD14

Lipopolysaccharide (LPS) has been known to induce inflammation by interacting with CD14, which serves as a receptor for LPS. Mannose-binding protein (MBP) belongs to the collectin subgroup of the C-type lectin superfamily, along with surfactant proteins SP-A and SP-D. We have recently demonstrated that SP-A modulates LPS-induced cellular responses by interaction with CD14 (H. Sano, H. Sohma, T. Muta, S. Nomura, D. R. Voelker, and Y. Kuroki, J. Immunol. 163:387-395, 2000) and that SP-D also interacts with CD14 (H. Sano, H. Chiba, D. Iwaki, H. Sohma, D. R. Voelker, and Y. Kuroki, J. Biol. Chem. 275:22442-22451, 2000). In this study, we examined whether MBP, a collectin highly homologous to SP-A and SP-D, could bind CD14. Recombinant rat MBP-A bound recombinant human soluble CD14 in a concentration-dependent manner. Its binding was not inhibited in the presence of excess mannose or EDTA. MBP-A bound deglycosylated CD14 treated with N-glycosidase F, neuraminidase, and O-glycosidase, indicating that MBP-A interacts with the peptide portion of CD14. Since LPS was also a ligand for the collectins, we compared the characteristics of binding of MBP-A to LPS with those of binding to CD14. MBP-A bound to lipid A from Salmonella enterica serovar Minnesota and rough LPS (S. enterica serovar Minnesota Re595 and Escherichia coli J5, Rc), but not to smooth LPS (E. coli O26:B6 and O111:B4). Unlike CD14 binding, EDTA and excess mannose attenuated the binding of MBP-A to rough LPS. From these results, we conclude that CD14 is a novel ligand for MBP-A and that MBP-A utilizes a different mechanism for CD14 recognition from that for LPS.

Expression in Escherichia coli, folding in vitro, and characterization of the carbohydrate recognition domain of the natural killer cell receptor NKR-P1A

NKR-P1A is a homodimeric type II transmembrane protein of the C-type lectin family found on natural killer (NK) cells and NK-like T cells and is an activator of cytotoxicity. Toward structure determination by NMR, the recombinant carbohydrate-recognition domain (CRD) of NKR-P1A has been expressed in high-yield in Escherichia coli and folded in vitro. The purified protein behaves as a monomer in size-exclusion chromatography and is bound by the conformation-sensitive antibody, 3.2.3, indicating a folded structure. A polypeptide tag at the N-terminus is selectively cleaved from the CRD after limited trypsin digestion in further support of a compact folded structure. The disulfide bonds have been identified by peptide mapping and electrospray mass spectrometry. These are characteristic of a long form CRD. The 1D NMR spectrum of the unlabeled CRD and the 2D HSQC spectrum of the (15)N-labeled CRD are those of a folded protein. Chemical shifts of H(alpha) and NH protons indicate a considerable amount of beta-strand structure. Successful folding in the absence of Ca(2+), coupled with the lack of chemical shift changes upon addition of Ca(2+), suggests that the NKR-P1A-CRD may not be a Ca(2+)-binding protein.

Recombinant antitoxic and antiinflammatory factor from the nonvenomous snake Python reticulatus: phospholipase A2 inhibition and venom neutralizing potential

From the serum of the nonvenomous snake Python reticulatus, a new phospholipase A(2) (PLA(2)) inhibitor termed phospholipase inhibitor from python (PIP) was purified by sequential chromatography and cloned to elucidate its primary structure and fundamental biochemical characteristics. A cDNA clone encoding PIP was isolated from the liver total RNA by reverse transcriptase-polymerase chain reaction (RT-PCR). It contained a 603 bp open reading frame that encoded a 19-residue signal sequence and a 182-residue protein. PIP showed about 60% sequence homology with those PLA(2) inhibitors having a urokinase-type plasminogen activator receptor-like domain structure. PIP was also functionally expressed as a fusion protein in Escherichia coli to explore its potential therapeutic significance. The recombinant PIP was shown to be identical to the native form in chromatographic behavior and biochemical characteristics. Both the native and recombinant PIP appear to exist as a hexamer of 23-kDa subunits having an apparent molecular mass of approximately 140 kDa. PIP showed ability to bind to the major PLA(2) toxin (daboiatoxin, DbTx) of Daboia russelli siamensis at 1-2-fold molar excess of inhibitor to toxin. It exhibited broad spectra in neutralizing the toxicity of various snake venoms and toxins and inhibited the formation of edema in mice. Our data demonstrate the venom neutralizing potential of the recombinant PIP and suggest that the proline-rich hydrophobic core region may play a role in binding to PLA(2).

Molecular determinants of oligomer formation and complement fixation in mannose-binding proteins

Rat serum mannose-binding protein (MBP-A) functions as part of the innate immune system by targetting complement toward potentially pathogenic microorganisms. In order to examine the molecular basis for complement activation, rat MBP-A has been overproduced in Chinese hamster ovary cells. Recombinant protein is post-translationally modified in the same way as the native lectin. Hydrodynamic studies indicate that MBP-A consists predominantly of covalent oligomers containing one to four copies of a subunit that comprises a trimer of polypeptides. These oligomers are non-interconverting and do not assemble into higher order structures at concentrations in excess of those normally found in serum. Disulfide bonds formed between cysteine residues at the N-terminal end of the collagen-like domain link polypeptides to form covalent oligomers. Analysis of wild-type MBP-A and MBP-A containing the substitution Cys6 --> Ser suggests that polypeptides within each trimeric structural unit are mostly linked by disulfide bonds between cysteine residues at positions 13 and 18 arranged in an asymmetrical configuration. Disulfide bonds involving Cys6 connect polypeptides within separate trimers. Analysis of chimeras between MBP-A and rat liver MBP (MBP-C) indicates that residues within the N-terminal region of the collagenous domain and the cysteine-rich domain of MBP-A enable assembly of trimers into higher order oligomers. The activity of MBP-A in a hemolytic complement fixation assay using mannan-coated sheep erythrocytes was approximately 20-fold greater than the activity of MBP-C. Analysis of the MBP chimeras and isolated oligomers of MBP-A reveals that the larger oligomers are more efficient at complement activation. These data indicate that the overall complement fixing activity of MBP-A is a function of the individual molecular activities of oligomers and their relative abundance within the serum.

The C-type lectin superfamily in the immune system

Protein-carbohydrate interactions serve multiple functions in the immune system. Many animal lectins (sugar-binding proteins) mediate both pathogen recognition and cell-cell interactions using structurally related Ca(2+)-dependent carbohydrate-recognition domains (C-type CRDs). Pathogen recognition by soluble collections such as serum mannose-binding protein and pulmonary surfactant proteins, and also the macrophage cell-surface mannose receptor, is effected by binding of terminal monosaccharide residues characteristic of bacterial and fungal cell surfaces. The broad selectivity of the monosaccharide-binding site and the geometrical arrangement of multiple CRDs in the intact lectins explains the ability of the proteins to mediate discrimination between self and non-self. In contrast, the much narrower binding specificity of selectin cell adhesion molecules results from an extended binding site within a single CRD. Other proteins, particularly receptors on the surface of natural killer cells, contain C-type lectin-like domains (CTLDs) that are evolutionarily divergent from the C-type lectins and which would be predicted to function through different mechanisms.

A novel type of binding specificity to phospholipids for rat mannose-binding proteins isolated from serum and liver

Mannose-binding protein (MBP) belongs to the collectin subgroup of C-type lectins with specificity for mannose and N-acetylglucosamine sugars. We investigated whether rat MBPs isolated from serum (S-MBP) and liver (L-MBP) interact with phospholipids using antibody against each MBP. Both S- and L-MBPs bound to phosphatidylinositol coated onto microtiter wells in a concentration- and a Ca2+-dependent manner. L-MBP also bound to phosphatidylglycerol and weakly to phosphatidylserine. MBPs interacted with liposomes composed of these lipids. S- and L-MBPs bound to phosphatidylinositol 4-monophosphate. L-MBP also bound to cardiolipin. These results provide evidence for a novel type of ligand binding specificity for MBPs, and raise the possibility that phospholipids are ligands for collectins.

Difference in the binding mode of two mannose-binding proteins: demonstration of a selective minicluster effect

Serum-type and liver-type mannose-binding proteins (MBP) are both present in higher animals and both are composed of a carbohydrate-recognition domain (CRD) and a collagenous domain. Although known as mannose-binding proteins, these proteins bind N-acetylglucosamine and other related sugars quite well. An earlier specificity study using cloned CRD portions of both types of MBP from rat [Childs, R. A., Feizi, T., Yuen, C.-T., Drickamer, K., & Quesenberry, M. (1990) J. Biol. Chem. 265, 20770-20777] revealed that the liver MBP CRD binds the trimannosyl core structure of N-glycosides, whereas the serum MBP CRD does not. We studied the substrate preferences of these CRDs using both solid and solution phase assays, testing monosaccharides, glycoproteins, and synthetic cluster ligands. While there was no significant difference in the monosaccharide binding specificities of the two CRDs, they displayed very different affinities for natural glycoproteins and mannose-containing cluster glycosides. Most interestingly, synthetic cluster ligands with two terminal GlcNAc moieties have affinity equal to monovalent GlcNAc ligands toward both CRDs, whereas a series of structurally similar Man-terminated divalent ligand displays about 20-fold enhanced affinity toward liver CRD only. A plausible explanation is that the liver MBP CRD has two sugar binding sites per subunit, one of which binds only mannose, and the other, both mannose and N-acetylglucosamine. In contrast, the serum MBP CRD has only one site of the latter type. Results of isothermal titration calorimetry support this hypothesis.

Lectin-carbohydrate interaction in the immune system

The immune system consists of various types of cells and molecules that specifically interact with each other to initiate the host defense mechanism. Recent studies have shown that carbohydrates and lectins (carbohydrate-binding proteins) play an essential role in mediating such interactions. Both lectins and carbohydrates are widely distributed in the mammalian tissues as well as in microorganisms. Carbohydrates, due to their chemical nature, can potentially form structures that are more variable than proteins and nucleic acids. Lectins can exist in either soluble or cell-associated form, and although overall structures vary, invariably possess carbohydrate-recognition domains (CRD) with various specificities. The interaction between lectins and carbohydrates have been shown to be involved in such activities as opsonization of microorganisms, phagocytosis, cell adhesion and migration, cell activation and differentiation, and apoptosis. The number of lectins identified in the immune system is increasing at a rapid pace. The development in this area has opened a new aspect in studying the immune system, and at the same time, provided new therapeutic routes for the treatment and prevention of disease.

A unique CD45 glycoform recognized by the serum mannan-binding protein in immature thymocytes

The serum-mannan binding protein (S-MBP) is a calcium-dependent C-type lectin specific for mannose and N-acetylglucosamine. S-MBP is known as a host defense factor involved in innate immunity, where the target ligands for S-MBP should be on the surface of exogenous microorganisms. In this study, we tried to find endogenous ligands for this endogenous lectin. Among the cells tested, only the lymphocytes from thymus of BALB/c mice expressed ligands for S-MBP on their surface, those from bone marrow, spleen, mesenteric lymph nodes and peripheral blood all being negative. Interestingly, among the thymocytes, only the immature thymocytes with the CD4+CD8+CD3low phenotype expressed ligands for S-MBP, and ligands for S-MBP decreased on their maturation. A major cell surface glycoprotein bearing S-MBP ligands was isolated and identified as CD45RO, which is a transmembrane protein with tyrosine phosphatase activity. Deglycosylation experiments with N-glycanase and endoglycosidase H indicated that the S-MBP ligands on thymic CD45 are high mannose type or hybrid type N-linked oligosaccharides. This unique presentation of S-MBP ligands on this special CD45 isoform suggested the possibility that the oligosaccharide portion of CD45 on immature thymocytes is associated with the maturation, development or selection events of thymocytes.

A novel human serum lectin with collagen- and fibrinogen-like domains that functions as an opsonin

Collectins are C-type animal lectins with both collagenous and carbohydrate recognition domains and are involved in the first line host defense against pathogens. We report here a novel Ca(2+)-dependent and GlcNAc-binding lectin consisting of subunits of 35 kDa (P35) with a collagen-like sequence. When P35 is isolated from human serum, it forms a homopolymer by means of intermolecular disulfide bonding, as is the case with collectins. P35 cDNA was cloned from a human liver cDNA library, and the deduced amino acid sequence of 313 residues revealed that the mature form of P35 consists mainly of collagen- and fibrinogen-like domains. The latter contained two potential Ca(2+)-binding sites that may be involved in carbohydrate binding. The overall sequence of P35 was highly homologous to porcine ficolins alpha and beta. Northern blots of various human tissues showed that the major product of the 1.3-kilobase-long P35 transcript is expressed in liver. P35 enhanced phagocytosis of Salmonella typhimurium by neutrophils, suggesting an opsonic effect via the collagen region. P35 was found to bind to GlcNAc-conjugated bovine serum albumin, a neoglycoprotein, as well as to neoglycolipids containing complex-type oligosaccharides derived from glycoproteins, suggesting that P35 recognizes GlcNAc residues such as those found in microbial glycoconjugates and complex-type oligosaccharides. Therefore, P35 represents a new type of GlcNAc-binding lectin with structural and functional similarities to collectins involved in innate immunity.

Influence of core fucosylation on the flexibility of a biantennary N-linked oligosaccharide

Fluorescence energy transfer was used to study the conformation of each antenna of a complex biantennary oligosaccharide. A core fucosylated biantennary oligosaccharide was converted to a glycosylamine which allowed coupling of a naphthyl donor fluorophore directly to the reducing-end GlcNAc 1. After generating an aldehyde at C-6 of residue 6 or 6' using galactose oxidase, a dansyl ethylenediamine acceptor fluorophore was coupled to either antenna of the oligosaccharide resulting in two donor-acceptor pairs. [Formula: see text] The fluorescence properties of the naphthyl group allowed determination of the end-to-end donor-acceptor distance and antenna flexibility of each isomer by steady-state and time-resolved fluorescence energy transfer at temperatures ranging from 0 to 40 degrees C. Extended (20.6 A) and folded (11.4 A) donor-acceptor distance populations were identified for the isomer containing dansyl attached to Gal 6', whereas only a single extended population (19.7 A) was determined when dansyl was attached to Gal 6. The presence of Fuc 1' had a dramatic effect on the conformation of the 6' antenna. Temperature modulation failed to alter the ratio of extended/folded populations when fucose was present. However, following the removal of fucose, the ratio of the extended/folded populations for 6' exhibited a temperature dependent conformational equilibrium allowing calculation of the enthalpy and entropy of unfolding. These results established a unique conformational property for the 6' antenna of a biantennary oligosaccharide that is influenced by core fucosylation. Comparison of the results obtained for the 6 antenna of biantennary with previous fluorescence energy transfer studies on a triantennary glycopeptide also established conformational differences in this antenna which are dependent on oligosaccharide structure.

Structural analysis of monosaccharide recognition by rat liver mannose-binding protein

The structural basis of carbohydrate recognition by rat liver mannose-binding protein (MBP-C) has been explored by determining the three-dimensional structure of the C-type carbohydrate-recognition domain (CRD) of MBP-C using x-ray crystallography. The structure was solved by molecular replacement using rat serum mannose-binding protein (MBP-A) as a search model and was refined to maximum Bragg spacings of 1.7 A. Despite their almost identical folds, the dimeric structures formed by the two MBP CRDs differ dramatically. Complexes of MBP-C with methyl glycosides of mannose, N-acetylglucosamine, and fucose were prepared by soaking MBP-C crystals in solutions containing these sugars. Surprisingly, the pyranose ring of mannose is rotated 180 degrees relative to the orientation observed previously in MBP-A, but the local interactions between sugar and protein are preserved. For each of the bound sugars, vicinal, equatorial hydroxyl groups equivalent to the 3- and 4-OH groups of mannose directly coordinate Ca2+ and form hydrogen bonds with residues also serving as Ca2+ ligands. Few interactions are observed between other parts of the sugar and the protein. A complex formed between free galactose and MBP-C reveals a similar mode of binding, with the anomeric hydroxyl group serving as one of the Ca2+ ligands. A second binding site for mannose has also been observed in one of two copies in the asymmetric unit at a sugar concentration of 1.3 M. These structures explain how MBPs recognize a wide range of monosaccharides and suggest how fine specificity differences between MBP-A and MBP-C may be achieved.

Cleavage of the third component of complement (C3) by mannose-binding protein-associated serine protease (MASP) with subsequent complement activation

We have previously shown that a novel C1s-like serine protease termed MBP-associated serine protease (MASP) is responsible for activation of the complement cascade initiated by mannose-binding protein (MBP). In this communication, we report that MASP is unique in having the proteolytic capacity to cleave C3 with subsequent activation of the alternative pathway, a capacity which C1s lacks.

alpha 1-Acid glycoprotein binds human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein via N-linked glycans

In the present study, we demonstrate a specific low-affinity interaction between recombinant precursor gp160 (rgp160) or surface unit gp120 (rgp120) of human immunodeficiency virus type 1 (HIV-1) and alpha 1-acid glycoprotein (AGP), a human glycoprotein displaying complex type N-glycans. Binding of rgp160/rgp120 to agarose-coupled AGP was dose-dependent, saturable, calcium-, pH- and temperature-dependent. Binding was inhibited by soluble AGP, asialo-AGP, fetuin, beta-D-GlcNAc47-BSA, alpha-D-Man20-BSA, mannan, complex-type asialo-agalacto-tetraanternary precursor oligosaccharide from human AGP and oligomannose 9 from porcine thyroglobulin; fully deglycosylated AGP was not inhibitory. The three AGP glycoforms separated on immobilized ConA bound rgp160 to the same extent as did unfractionated AGP. These findings extend our previous results on the carbohydrate-binding properties of HIV-1 envelope (Env) glycoprotein in that they demonstrate the involvement of AGP glycan moieties in the binding to rgp160/rgp120. Preincubation of rgp160 with AGP or mannan significantly reduced its binding to monocyte-derived macrophages (MDM), suggesting that AGP may play a role in preventing binding of soluble or virus-bound Env glycoprotein to CD4+ monocytic cells.

Affinity purification of a mannose-binding protein, a sensitive tool in the diagnostics of IgM, via site-directed phosphorylated mannan bound to alumina

Ca2+ -dependent mannose-binding proteins (MBPs) belong to the family of animal lectins. They perform in vivo as defence molecules that act as opsonins by enhancing the clearance of mannose rich pathogens and have been used in vitro for the purification of IgM. MBPs have been previously isolated by methods based on binding the protein moiety of various mannan species to different matrices. However, the mannan-protein complexes did not have a constant protein content and the yield of the isolated MBPs was variable. In the present study we describe a new approach for the affinity purification of MBPs based on the main polysaccharide moiety of the complex. After removal of residual phosphate groups naturally occurring at the C-3 position of the sugar, which interfere with MBP recognition, the mannan was phosphorylated enzymatically at C-6, at which position the OH group is not required for lectin binding. The enzymatically phosphorylated mannan bound to an alumina column was used successfully for MBP separation from rabbit serum. The mannose-binding protein obtained was used in our study for diagnostic purposes in the identification and determination of very low concentrations of IgM.

Recognition of the major cell surface glycoconjugates of Leishmania parasites by the human serum mannan-binding protein

Activation of complement on the surface of parasitic protozoa of the genus Leishmania appears to be important for parasite infectivity in the mammalian host, as it allows these parasites to attach to and invade macrophages via their surface complement receptors. Serum mannan-binding protein (MBP) is a known activator of complement. Therefore, in the present study, we have investigated whether serum MBP binds to live Leishmania parasites, and to mannose-containing saccharides derived from the parasite cell surface. We have observed by fluorescence microscopy that biotinylated MBP binds to the surface of L. major and L. mexicana promastigotes. At this developmental stage the parasites are coated by a mannose-containing lipophosphoglycan (LPG). We have observed that radioiodinated MBP binds in a mannose-inhibitable manner to purified LPG which has been immobilized in plastic microwells, as well as to purified mannose-terminating di-, tri- and tetrasaccharide fragments ('cap' structures) which have been released by mild acid hydrolysis from the outer chains of the LPG, converted into neoglycolipids and resolved by thin-layer chromatography. 125I-MBP also binds in the chromatogram-binding assay to the mannose-containing glycoinositol-phospholipids that are expressed in high copy number on both the promastigote and the intracellular amastigote stages of most Leishmania species. These data suggest that MBP has the potential to opsonize the major developmental stages of Leishmania parasites, and provide a possible mechanism for the antibody-independent activation of complement on their surface.

Collectins: pattern recognition molecules involved in first line host defense

A group of chimeric molecules comprising globular heads, which contain the carbohydrate recognition domain, and collagen tails are defined as collectins. The mannose-binding proteins, pulmonary surfactant apoproteins A and D and conglutinin all qualify as members of this family, whose function appears to be as pattern recognition molecules involved in the first line of defense in the pre-immune host.