Structural and functional consequences of peptide-carbohydrate mimicry. Crystal structure of a carbohydrate-mimicking peptide bound to concanavalin A

The functional consequences of peptide-carbohydrate mimicry were analyzed on the basis of the crystal structure of concanavalin A (ConA) in complex with a carbohydrate-mimicking peptide, DVFYPYPYASGS. The peptide binds to the non-crystallographically related monomers of two independent dimers of ConA in two different modes, in slightly different conformations, demonstrating structural adaptability in ConA-peptide recognition. In one mode, the peptide has maximum interactions with ConA, and in the other, it shows relatively fewer contacts within this site but significant contacts with the symmetry-related subunit. Neither of the peptide binding sites overlaps with the structurally characterized mannose and trimannose binding sites on ConA. Despite this, the functional mimicry between the peptide and carbohydrate ligands was evident. The peptide-inhibited ConA induced T cell proliferation in a dose-dependent manner. The effect of the designed analogs of the peptide on ConA-induced T cell proliferation and their recognition by the antibody response against alpha-d-mannopyranoside indicate a role for aromatic residues in functional mimicry. Although the functional mimicry was observed between the peptide and carbohydrate moieties, the crystal structure of the ConA-peptide complex revealed that the two peptide binding sites are independent of the methyl alpha-d-mannopyranoside binding site.

Phosphatidylinositol mannosides: Synthesis and adjuvant properties of phosphatidylinositol di- and tetramannosides

Phosphatidylinositol mannosides (PIMs) isolated from mycobacteria have been identified as an important class of glycolipids with significant immune modulating properties. We present here the syntheses of phosphatidylinositol dimannoside (PIM2, 1) and phosphatidylinositol tetramannoside (PIM4, 2) and evaluate their adjuvant properties in a transgenic mouse model. The key step in the synthetic methodology for the synthesis of 2 relies on the selective glycosylation of diol 3 with mannosyl donor 11. Both synthetic PIMs were effective at enhancing IFN-gamma when given as adjuvants with a model antigen, with PIM2 being the more active. These data suggest that in this assay the PIM core structure is responsible for the observed biological activity.

Cryptosporidium parvum sporozoite pellicle antigen recognized by a neutralizing monoclonal antibody is a beta-mannosylated glycolipid

The protozoan parasite Cryptosporidium parvum is an important cause of diarrhea in humans, calves, and other mammals worldwide. No approved vaccines or parasite-specific drugs are currently available for the control of cryptosporidiosis. To effectively immunize against C. parvum, identification and characterization of protective antigens are required. We previously identified CPS-500, a conserved, neutralization-sensitive antigen of C. parvum sporozoites and merozoites defined by monoclonal antibody 18.44. In the present study, the biochemical characteristics and subcellular location of CPS-500 were determined. CPS-500 was chloroform extractable and eluted with acetone and methanol in silicic acid chromatography, consistent with being a polar glycolipid. Following chloroform extraction and silicic acid chromatography, CPS-500 was isolated by high-pressure liquid chromatography for glycosyl analysis, which indicated the presence of mannose and inositol. To identify which component of CPS-500 comprised the neutralization-sensitive epitope recognized by 18.44, the ability of the monoclonal antibody to bind CPS-500 treated with proteases, or with alpha- or beta-glycosidases, was determined. Monoclonal antibody 18.44 did not bind antigen treated with beta-D-mannosidase but did bind antigen treated with alpha-D-mannosidase, other alpha- or beta-glycosidases, or a panel of proteases. These data indicated that the target epitope was dependent on terminal beta-D-mannopyranosyl residues. By immunoelectron microscopy, 18.44 binding was localized to the pellicle and an intracytoplasmic tubulovesicular network in sporozoites. Monoclonal antibody 18.44 also bound to antigen deposited and released onto substrate over the course travelled by gliding sporozoites and merozoites. Surface localization, adhesion and release during locomotion, and neutralization sensitivity suggest that CPS-500 may be involved in motility and invasion processes of the infective zoite stages.

Probable presence of beta(1-2)-linked oligomannosides that act as human immunoglobulin G3 epitopes and are distributed over a Candida albicans 14- to 18-kilodalton antigen

Kinetic analysis of candidosis patients' immunoglobulin G3 response has shown that reactivity towards beta(1-2)-linked mannan-derived oligomannosides was associated with the recognition through metaperiodate-sensitive epitopes of a 14- to 18-kDa Candida albicans antigen unreactive with concanavalin A.

Novel adjuvant action of lipopolysaccharides that possess mannose homopolysaccharides as O-specific polysaccharides on immune responses to nonimmunogenic autoantigens in mice

The adjuvant action of various lipopolysaccharides on immune responses to syngeneic tissue extract in mice was examined. Only lipopolysaccharides possessing the linear mannose homopolysaccharides as O-specific polysaccharides exhibited definite adjuvant action on immune responses to the autoantigens. The intensity of this adjuvant activity of lipopolysaccharide from Klebsiella O3 seemed to be the strongest.

Mapping of beta-1,2-linked oligomannosidic epitopes among glycoconjugates of Candida species

The distribution of beta-1,2-linked oligomannosides among glycoconjugates of various Candida species was investigated by Western blotting, using monoclonal and polyclonal antibodies which react with these epitopes. Expression of beta-1,2-linked oligomannosidic epitopes on a 14-18 kDa polydisperse antigen nonreactive with concanavalin A (ConA), previously identified as a C. albicans serotype A phospholipomannan (PLM), appeared to be restricted to C. albicans serotypes A and B (including var. C. stellatoidea types I and II) and C. tropicalis. In C. albicans, beta-1,2-linked oligomannosidic epitopes also appeared to be slightly associated with high molecular mass (> 100 kDa) polydisperse ConA-reactive mannoproteins. For all the other Candida strains investigated, belonging to the species C. parapsilosis, C. krusei, C. glabrata and C. robusta (S. cerevisiae), beta-1,2-linked oligomannosidic epitopes were found to be present in association with medium molecular mass (18-100 kDa) and high molecular mass ConA-reactive mannoproteins, giving reproducible labelling profiles that varied between species.

Analysis of binding of mannosides in relation to Langerin (CD207) in Langerhans cells of normal and transformed epithelia

Tandem-repeat C-type lectins (pattern-recognition receptors) with specificity for mannosides are intimately involved in antigen recognition, uptake, routing and presentation in macrophages and dendritic cells. In Langerhans cells, Langerin (CD207), a type-II transmembrane protein with a single C-type carbohydrate recognition domain attached to a heptad repeat in the neck region, which is likely to establish oligomers with an alpha-coiled-coil stalk, has been implicated in endocytosis and the formation of Birbeck granules. The structure of Langerin harbours essential motifs for Ca2+-binding and sugar accommodation. Lectin activity has previously been inferred by diminished antibody binding to cells in the presence of the glycan ligand mannan. In view of the complexity of the C-type lectin/lectin-like network, it is unclear what role Langerin plays for Langerhans cells in binding mannosides. In order to reveal in frozen tissue sections to what extent mannose-binding activity co-localizes with Langerin, we have used a synthetic marker, i.e. a neoglycoprotein carrying mannose maxiclusters, as a histochemical ligand, and computer-assisted fluorescence monitoring in a double-labelling procedure. Mannoside-binding capacity was detected in normal epithelial cells. Double labelling ensured the unambiguous assessment of the binding of the neoglycoprotein in Langerhans cells. Light-microscopically, its localization profile resembled the pattern of immunohistochemical detection of Langerin. This result has implications for suggesting rigorous controls in histochemical analysis of this cell type, because binding of kit reagents, i.e. mannose-rich glycoproteins horseradish peroxidase or avidin, to Langerin (or a spatially closely associated lectin) could yield false-positive signals. To show that recognition of carbohydrate ligands in dendritic cells is not restricted to mannose clusters, we have also documented binding of carrier-immobilized histo-blood group A trisaccharide, a ligand of galectin-3, which was not affected by the presence of a blocking antibody to Langerin. Remarkably, access to the carbohydrate recognition domain of Langerin appeared to be impaired in proliferatively active environments (malignancies, hair follicles), indicating presence of an endogenous ligand with high affinity to saturate the C-type lectin under these conditions.

Association of concentration of asbestos and asbestos-like fibers with the patient's survival and the binding capacity of lung parenchyma to galectin-1 and natural alpha-galactoside- and alpha-mannoside-binding immunoglobulin G subfractions from human serum

Our aim in this study was to search for lung parenchyma alterations associated with asbestos and/or asbestos-like fiber concentration. This was done by means of immuno- or glycohistochemistry. The hot-ashing technique determined the asbestos and asbestos-like fiber concentrations in the lung tissues of 100 patients of whom 52 were treated for primary lung and 25 for secondary lung tumors; fiber concentration was also measured for 23 patients whose disease was benign. The results were correlated to smoking habits, survival of the patients, and expression of binding capacities for endogenous lectins, natural carbohydrate-binding and lectin-specific antibodies. The cohort with proven asbestos exposure revealed a mean fiber concentration 114 f/g compared to 95 f/g in the non-exposed group. An increased asbestos fiber concentration was correlated to galectin-1-binding and the presence of epitopes for natural immunoglobulin G subfractions with selectivity to alpha-galactosides and alpha-mannosides. The survival of patients with primary and secondary lung tumors was negatively associated with the fiber concentration. The data indicate that increased presence of asbestos is correlated with an alteration of defined glycohistochemical features of alveolar lining cells.

Isolation of glycopeptides with skin test activity from dermatophytes

By using ethylene glycol extraction of whole submerged cultures followed by Sephadex G-200 and diethylaminoethyl-Sephadex chromatography, we isolated four distinct glycopeptides from Trichophyton mentagrophytes, T. rubrum, and Microsporum canis. Chemical analyses revealed that these glycopeptides contained mostly carbohydrate (42.5 to 81.6%) and protein (4.3 to 11.3%), with lesser amounts of phosphorus (0.4 to 6.0%) and hexosamines (0.3 to 0.6%). Based upon total carbohydrate and monosaccharide content, these dermatophyte glycopeptides could be divided into two chemical groups: glucopeptides (DSI1) and mannopeptides (DSI2, DSII1, and DSII2). The mannopeptides and glucopeptides of each species of dermatophyte were not significantly different chemically from those derived from the other two dermatophyte species studied. Skin testing of DSI1-glycopeptides or DSI2-mannopeptides in immunized guinea pigs indicated that only the DSI2-mannopeptides elicited a delayed hypersensitivity reaction. Skin testing T. mentagrophytes 62-infected guinea pigs with the four purified DS-glycopeptides, as well as earlier fractions from the purification scheme, derived from T. mentagrophytes, T. rubrum, and M. canis, again indicated that only the DSI2-mannopeptides of the two Trichophyton species elicited a delayed hypersensitivity reaction. The number of infections or duration of infection had no effect on the size of the skin test response. DSI2-mannopeptides were non-cross-reactive between genera when tested in Trichophyton-immunized or -infected guinea pigs and Microsporum-immunized guinea pigs.

Liposomes as immunological carriers for the preparation of antimannosyl antibodies

Antiserum was raised against an aminophenyl derivative of D-mannose grafted on to a liposomal surface. As characterized by immunodiffusion, quantitative precipitation and hapten inhibition, the antiserum was found to contain mannose specific antibodies in addition to antibodies against the aromatic phenyl group.

Carbohydrate-specific antibodies in normal human sera. II. Structural definition of antigenic determinants

IgG and IgM antibodies against several sugars have been characterized in sera of normal donors by passive hemagglutination and a quantitative hemagglutination inhibition test. These antibodies distinguish between the equatorial and axial OH groups at C2, C3, or C4 positions of the glycopyranose configuration, differences between the anomers, linkage types, changes in the primary alcohol group at C6, and OH substitution. In the examples of antibodies to mannose, galactose, and glucose investigated, specificities were usually directed against the beta-anomers. In disaccharides, the antibodies appeared to react only with 1 of the 2 sugar subunits, but unlike monosaccharides, the glycosidic linkages also seemed to be a part of the reaction site. Thus, the reacting moiety in gentiobiose was beta-D-glucopyranosyl with 1----6 linkage, in cellobiose with beta-D-glucopyranosyl with 1----4 linkage, in meliboise was alpha-D-galactopyranosyl with 1----6 linkage, and in lactose the reaction was directed against beta-D-galactopyranosyl with 1----4 linkage. In the maltose-dependent hemagglutination, alpha-D-glucose appeared to be the main reaction site. ManNAc exemplified the specificity determined by OH group substitution. Antibody to D-Fucose represented example of specificity evolving from substitution of the primary alcohol.

Influence of antibodies to mannophosphoinositides on phospholipid synthesis in Mycobacterium smegmatis ATCC 607

Antiserum to mannophosphoinositides had a significant inhibitory effect on phospholipid synthesis in Mycobacterium smegmatis ATCC 607 at different phases of growth; however, the inhibition was more pronounced in the exponential phase. The rate of synthesis of total phospholipids and individual phospholipids, viz., cardiolipin, phosphatidylethanolamine, and mannophosphoinositides, decreased in the presence of antiserum as compared with normal serum. The maximum inhibitory effect of antiserum was exerted on the synthesis of mannophosphoinositides, particularly on tetraacylated dimannophosphoinositide. Antiserum also had a growth inhibitory effect as compared with normal serum.

Antibodies directed against the thiomannose moiety of a glycoconjugate of 2-imino-2-methoxyethyl 1-thio-alpha-D-mannopyranoside and bovine serum albumin

Anti-thiomannose antibodies were induced in rabbits immunized with a glycoconjugate of 2-imino-2-methoxyethyl 1-thio-alpha-D-mannopyranoside (Man-S) and bovine serum albumin (BSA). Also anti-BSA antibodies directed against the BSA moiety of the glycoconjugate were detected in low concentrations in the immune serum. However, antibodies against the combinatorial epitope of the hapten group and the carrier protein were not detected. The anti-thiomannose and the anti-BSA antibodies were isolated in pure forms by affinity chromatography on Sepharose 4B-bearing thiomannosyl-BSA ligands or BSA ligands. The anti-thiomannose antibodies constituted the major fraction of the antibodies, and these antibodies were isolated in pure form for the first time. The specificity of the thiomannose antibodies was established from data of experiments of periodate oxidation, perpropionic acid oxidation, hapten inhibition, and agar diffusion, Isoelectrofocusing showed that the anti-thiomannose antibody preparation consisted of at least six isomeric proteins, all of which exhibited antibody activity against the glycoconjugate of thiomannose and BSA.

Specificity analysis of antibodies formed in rabbits to a mannosyl trisaccharide: similarity with lectin binding activity. Para-aminophenyl O-alpha-D-mannopyranosyl-(1-->2)-alpha-D-mannopyranosyl- (1-->6)-alpha-D-mannopyranoside linked to bovine serum albumin as an antigen

The para-aminophenyl derivative of Man alpha 1-->2Man alpha 1-->6Man alpha 1-->was coupled via a diazotization reaction to bovine serum albumin, and the resulting glycoconjugate was used to immunize two rabbits. The resultant antisera were tested for reactivity with a number of related mono, di- and trisaccharides to determine the immunodominant portion of this trisaccharide. Two populations of antibody resulted, one of which required the reducing end mannose, and could react with either an N-acetylglucosamine or a mannose as the penultimate sugar. The other population reacted with the Man alpha 1-->2Man alpha 1-->6Man alpha 1-->. The aglycone moiety and its configurations play an important role in determining the specificity of antibodies to this synthetic antigen. The similarity of this reactivity to the reactivity of mannose binding lectins is discussed.

Modification of the biological activities of concanavalin A by anti-concanavalin A

Concanavalin A (Con A) bound to cell membrane glycoproteins, may be dissociated from the membrane receptors by competitive ligands such as alpha-methyl-D-mannoside. Addition of antibody to Con A to the system forms complexes of antibody and Con A which are still bound to the membrane receptors. Such complexes are not dissociable from the membrane by alpha-methyl-D-mannoside. Presence of the complexes on the membrane is monitored by radioactive label or by passive lysis of the cells with guinea pig complement. Antibody to Con A can completely suppress Con A mediated stimulation of lymphocytes as measured by incorporation of tritiated thymidine. However production of lymphokines involved in migration inhibition, enhancement of plaque forming cells or blastogenesis are differentially suppressed by antibody to Con A.

CD1c restricts responses of mycobacteria-specific T cells. Evidence for antigen presentation by a second member of the human CD1 family

Previous studies suggest that CD1 is a family of Ag-presenting molecules distantly related to those encoded by the MHC. However, of the four known human CD1 proteins, only CD1b has been shown to restrict Ag-specific T cell responses. In this study, we have shown that a second member of the human CD1 family, CD1c, could also mediate Ag presentation to T cells. Three T cell lines recognizing mycobacterial Ags in a CD1c-restricted manner were isolated from normal donor blood. These T cells were MHC unrestricted, and their recognition of Ag was independent of the products of the transporter associated with Ag presentation-1/2 and DMA/B genes that are generally required for Ag presentation by MHC-encoded Ag-presenting molecules. Furthermore, unlike MHC-restricted responses to peptides, the CD1c-restricted T cell lines recognized protease-resistant mycobacterial lipid Ags. These T cell lines also showed significant cytotoxicity toward CD1c-expressing target cells even in the absence of mycobacterial Ags, which was shown by clonal analysis to be mediated by a subpopulation of T cells directly reactive to CD1c molecules. Our findings establish the ability of a second member of the CD1 family to restrict responses of Ag-specific T cells, and thus support the general hypothesis that the CD1 family comprises a third lineage of Ag-presenting molecules that presents a novel class of foreign and self Ags to MHC-unrestricted T cells.

Cell type-specific glycoforms of Fc gamma RIIIa (CD16): differential ligand binding

Fc gamma RIIIa, considered an intermediate affinity receptor, can variably bind monomeric IgG and appears to have a higher affinity for IgG than the lower affinity Fc gamma Rs, Fc gamma RII and Fc gamma RIIIb. We explored this property for both NK cell and monocyte Fc gamma RIIIa and found higher affinity ligand binding by Fc gamma RIIIa expressed on NK cells compared with Fc gamma RIIIa on monocytes. In normal whole blood or plasma (containing 8-11 mg/ml IgG), NK cell Fc gamma RIIIa was fully blocked, but in monocytes Fc gamma RIIIa showed approximately 60% blockade of the binding of mAb 3G8, which binds in or near the ligand binding site. The ligand binding site of NK cell Fc gamma RIIIa was blocked with as little as 2 mg/ml of human IgG, while monocyte Fc gamma RIIIa was only partially (30%) blocked by 2 mg/ml of human IgG. In contrast, plasma containing approximately 26 mg/ml of IgG (obtained from patients receiving therapeutic gamma-globulin) showed complete saturation of monocyte Fc gamma RIIIa with blockade of mAb 3G8 binding. These binding differences are not due to allelic polymorphisms or primary sequence differences between donors. Although NK cell and monocyte Fc gamma RIIIa have identical protein cores, they each undergo differential cell type-specific glycosylation. NK cell Fc gamma RIIIa is glycosylated with high mannose- and complex-type oligosaccharides, while monocyte Fc gamma RIIIa has no high mannose-type oligosaccharides. These results indicate that natural glycoforms of Fc gamma RIIIa (cell type-specific glycosylation variants) bind ligand differently, conferring a lower affinity on monocyte/macrophage Fc gamma RIIIa, which makes the receptor ideal for initial immune complex capture and sensitive to moderate changes in serum IgG levels.

Activation of mouse peritoneal macrophages by mannophosphoinositides of mycobacteria

Peritoneal macrophages isolated from mannoside-methylated bovine serum albumin (MBSA)-immunized mice showed significantly enhanced phagocytosis of Mycobacterium smegmatis compared to control or MBSA-immunized groups. Immune macrophages also exhibited bacteriostatic activity against M. smegmatis. Pretreatment of mycobacteria with mannoside antibodies did not further alter the phagocytosis and bacteriostatic effect of immune macrophages.