The Glycosylation of Human Serum IgD and IgE and the Accessibility of Identified Oligomannose Structures for Interaction with Mannan-Binding Lectin

Examination of glycan profiles from IgG-depleted human immunoglobulins facilitated by microscale affinity chromatography

Among the most important proteins involved in disease and healing processes are the immunoglobulins (Igs). Although many of the Igs have been studied through proteomics, aside from IgG, immunoglobulin carbohydrates have not been extensively characterized in different states of health. It seems valuable to develop techniques that permit an understanding of changes in the structures and abundances of Ig glycans in the context of disease onset and progression. We have devised a strategy for characterization of the glycans for the Ig classes other than IgG (i.e., A, D, E, and M) that contain kappa light chains that requires only a few microliters of biological material. First, we designed a microcolumn containing recombinant Protein L that was immobilized on macroporous silica particles. A similarly designed Protein G microcolumn was utilized to first perform an online depletion of the IgG from the sample, human blood serum, and thereby facilitate enrichment of the other Igs. Even though only 3 μL of serum was used in these analyses, we were able to recover a significantly enriched fraction of non-IgG immunoglobulins. The enrichment properties of the Protein L column were characterized using a highly sensitive label-free quantitative proteomics LC-MS/MS approach, and the glycomic profiles of enriched immunoglobulins were measured by MALDI-TOF MS. As a proof of principle, a comparative study was conducted using blood serum from a small group of lung cancer patients and a group of age-matched cancer-free individuals to demonstrate that the method is suitable for investigation of glycosylation changes in disease. The results were in agreement with a glycomic investigation of whole blood serum from a much larger lung cancer cohort.

Affinity chromatography as a tool for antibody purification

The global antibody market has grown exponentially due to increasing applications in research, diagnostics and therapy. Antibodies are present in complex matrices (e.g. serum, milk, egg yolk, fermentation broth or plant-derived extracts). This has led to the need for development of novel platforms for purification of large quantities of antibody with defined clinical and performance requirements. However, the choice of method is strictly limited by the manufacturing cost and the quality of the end product required. Affinity chromatography is one of the most extensively used methods for antibody purification, due to its high selectivity and rapidity. Its effectiveness is largely based on the binding characteristics of the required antibody and the ligand used for antibody capture. The approaches used for antibody purification are critically examined with the aim of providing the reader with the principles and practical insights required to understand the intricacies of the procedures. Affinity support matrices and ligands for affinity chromatography are discussed, including their relevant underlying principles of use, their potential value and their performance in purifying different types of antibodies, along with a list of commercially available alternatives. Furthermore, the principal factors influencing purification procedures at various stages are highlighted. Practical considerations for development and/or optimizations of efficient antibody-purification protocols are suggested.

Biochemical and biophysical characterization of humanized IgG1 produced in Pichia pastoris

The first full length IgG produced in Pichia pastoris was reported in late 1980. However, use of a wild-type Pichia expression system to produce IgGs with human-like N-linked glycans was not possible until recently. Advances in glycoengineering have enabled organisms such as Pichia to mimic human N-glycan biosynthesis and produce IgGs with human glycans on an industrial scale. Since there are only a few reports of the analytical characterization of Pichia-produced IgG, we summarize the results known in this field, and provide additional characterization data generated in our laboratories. The data suggest that Pichia-produced IgG has the same stability as that produced in Chinese hamster ovary (CHO) cells. It has similar aggregation profiles, charge variant distribution and oxidation levels as those for a CHO IgG. It contains human N-linked glycans and O-linked single mannose. Because of the comparable biophysical and biochemical characteristics, glycoengineered Pichia pastoris is an attractive expression system for therapeutic IgG productions.

Fibrosis severity and mannan-binding lectin (MBL)/MBL-associated serine protease 1 (MASP-1) complex in HCV-infected patients

BACKGROUND AND STUDY AIMS

Mannan-binding lectin (MBL) is a collectin synthesised in the liver and secreted into the bloodstream. It binds micro-organisms via interactions with glycans on the target surface. Bound MBL subsequently activates MBL-associated serine protease proenzymes (MASPs). Several studies have investigated the possible role for MBL in hepatitis C virus (HCV) infection by examining MBL levels and polymorphisms in relation to disease progression and in response to treatment. The aim of this study was to investigate the relation of the activity of MBL and MBL/MASP-1 complex in sera of patients with mild and severe chronic HCV infection and outcome of HCV infection.

PATIENTS AND METHODS

Serum level of MBL and functional assays for MBL/MASP-1 complex activity were assayed in sera of 80 patients with chronic HCV infection. Patients were divided into two groups according to the results of the liver biopsy, group I (40 HCV patients had mild hepatic fibrosis, Ishak fibrosis stages 0-1) and group II (40 HCV patients had severe hepatic fibrosis, Ishak fibrosis stages 5-6), in addition to 20 control subjects as group III. The analysis of the MBL/MASP-1 complex activity at 0, 3 and 6 months was performed in all patients.

RESULTS

Serum levels of MBL and MBL/MASP-1 complex activity were higher in sera of patients with chronic HCV liver disease compared to those in control subjects. There was a correlation between the activity of the MBL/MASP-1 complex and the severity of fibrosis (P=0.003). MBL/MASP-1 complex activity was associated more significantly with severe fibrosis in comparison to MBL concentration.

CONCLUSION

MBL and MBL/MASP-1 complex activities play a key role in first-line host defence mechanism against certain infectious agents including HCV infection. However, it is also likely that the role of MBL and MBL/MASP-1 complex activity extends beyond this restricted infection-related view in that it appears to be a key regulator of inflammation.

Conformational changes in IgE contribute to its uniquely slow dissociation rate from receptor FcɛRI

Among antibody classes, IgE has a uniquely slow dissociation rate from, and high affinity for, its cell surface receptor FcɛRI. We show the structural basis for these key determinants of the ability of IgE to mediate allergic hypersensitivity through the 3.4-Å-resolution crystal structure of human IgE-Fc (consisting of the Cɛ2, Cɛ3 and Cɛ4 domains) bound to the extracellular domains of the FcɛRI α chain. Comparison with the structure of free IgE-Fc (reported here at a resolution of 1.9 Å) shows that the antibody, which has a compact, bent structure before receptor engagement, becomes even more acutely bent in the complex. Thermodynamic analysis indicates that the interaction is entropically driven, which explains how the noncontacting Cɛ2 domains, in place of the flexible hinge region of IgG antibodies, contribute together with the conformational changes to the unique binding properties of IgE.

Specific interaction of hepatitis C virus glycoproteins with mannan binding lectin inhibits virus entry

Mannan-binding lectin (MBL) is a soluble innate immune protein that binds to glycosylated targets. MBL acts as an opsonin and activates complement, contributing to the destruction and clearance of infecting microorganisms. Hepatitis C virus (HCV) encodes two envelope glycoproteins E1 and E2, expressed as non-covalent E1/E2 heterodimers in the viral envelope. E1 and E2 are potential ligands for MBL. Here we describe an analysis of the interaction between HCV and MBL using recombinant soluble E2 ectodomain fragment, the full-length E1/E2 heterodimer, expressed in vitro, and assess the effect of this interaction on virus entry. A binding assay using antibody capture of full length E1/E2 heterodimers was used to demonstrate calcium dependent, saturating binding of MBL to HCV glycoproteins. Competition with various saccharides further confirmed that the interaction was via the lectin domain of MBL. MBL binds to E1/E2 representing a broad range of virus genotypes. MBL was shown to neutralize the entry into Huh-7 cells of HCV pseudoparticles (HCVpp) bearing E1/E2 from a wide range of genotypes. HCVpp were neutralized to varying degrees. MBL was also shown to neutralize an authentic cell culture infectious virus, strain JFH-1 (HCVcc). Furthermore, binding of MBL to E1/E2 was able to activate the complement system via MBL-associated serine protease 2. In conclusion, MBL interacts directly with HCV glycoproteins, which are present on the surface of the virion, resulting in neutralization of HCV particles.

N-glycan moieties of the crustacean egg yolk protein and their glycosylation sites

Vitellogenin (Vg) is the precursor of the egg yolk glycoprotein of crustaceans. In the prawn Macrobrachium rosenbergii, Vg is synthesized in the hepatopancreas, secreted to the hemolymph, and taken up by means of receptor-mediated endocytosis into the oocytes. The importance of glycosylation of Vg lies in its putative role in the folding, processing and transport of this protein to the egg yolk and in the fact that the N-glycan moieties could provide a source of carbohydrate during embryogenesis. The present study describes, for the first time, the structure of the glycan moieties and their sites of attachment to the Vg of M. rosenbergii. Bioinformatics analysis revealed seven putative N-glycosylation sites in M. rosenbergii Vg; two of these glycosylation sites are conserved throughout the Vgs of decapod crustaceans from the Pleocyemata suborder (N 159 and N 660). The glycosylation of six putative sites of M. rosenbergii Vg (N 151, N 159, N ,168 N ,614 N 660 and N 2300) was confirmed; three of the confirmed glycosylation sites are localized around the N-terminally conserved N-glycosylation site N 159. From a theoretical three-dimensional structure, these three N-glycosylated sites N 151, N 159, and N 168 were localized on the surface of the Vg consensus sequence. In addition, an uncommon high mannose N-linked oligosaccharide structure with a glucose cap (Glc1Man9GlcNAc2) was characterized in the secreted Vg. These findings thus make a significant contribution to the structural elucidating of the crustacean Vg glycan moieties, which may shed light on their role in protein folding and transport and in recognition between Vg and its target organ, the oocyte.

Activation of mannan-binding lectin-associated serine proteases leads to generation of a fibrin clot

The lectin pathway of complement is activated upon binding of mannan-binding lectin (MBL) or ficolins (FCNs) to their targets. Upon recognition of targets, the MBL-and FCN-associated serine proteases (MASPs) are activated, allowing them to generate the C3 convertase C4b2a. Recent findings indicate that the MASPs also activate components of the coagulation system. We have previously shown that MASP-1 has thrombin-like activity whereby it cleaves and activates fibrinogen and factor XIII. MASP-2 has factor Xa-like activity and activates prothrombin through cleavage to form thrombin. We now report that purified L-FCN-MASPs complexes, bound from serum to N-acetylcysteine-Sepharose, or MBL-MASPs complexes, bound to mannan-agarose, generate clots when incubated with calcified plasma or purified fibrinogen and factor XIII. Plasmin digestion of the clot and analysis using anti-D-dimer antibodies revealed that the clot was made up of fibrin and was similar to that generated by thrombin in normal human plasma. Fibrinopeptides A and B (FPA and FPB, respectively) were released after fibrinogen cleavage by L-FCN-MASPs complexes captured on N-acetylcysteine-Sepharose. Studies of inhibition of fibrinopeptide release indicated that the dominant pathway for clotting catalysed by the MASPs is via MASP-2 and prothrombin activation, as hirudin, a thrombin inhibitor that does not inhibit MASP-1 and MASP-2, substantially inhibits fibrinopeptide release. In the light of their potent chemoattractant effects on neutrophil and fibroblast recruitment, the MASP-mediated release of FPA and FPB may play a role in early immune activation. Additionally, MASP-catalysed deposition and polymerization of fibrin on the surface of micro-organisms may be protective by limiting the dissemination of infection.

Abnormal IgD and IgA1 O-glycosylation in hyperimmunoglobulinaemia D and periodic fever syndrome

In order to determine the glycosylation pattern for IgD, and to examine whether there are changes in the pattern of IgD and IgA1 O-glycosylation in patients with hyperimmunoglobulinaemia D and periodic fever syndrome (HIDS) during acute febrile attacks and during periods of quiescence, serum was obtained from 20 patients with HIDS and 20 control subjects. In the HIDS group, serum was obtained either during an acute febrile episode (n = 9) or during a period of quiescence (n = 11). The O-glycosylation profiles of native and desialylated IgA1 and IgD were measured in an ELISA-type system using the lectins Helix aspersa and peanut agglutinin, which bind to alternative forms of O-glycan moieties. IgD is more heavily O-galactosylated and less O-sialylated than IgA1 in healthy subjects. HIDS is associated with more extensive O-galactosylation of IgD and a reduction in O-sialylation of both IgD and IgA1. These changes are present both during acute febrile attacks and periods of quiescence. The T cell IgD receptor is a lectin with binding affinity for the O-glycans of both IgD and IgA1. The observed changes in IgD and IgA1 O-glycosylation are likely to have a significant effect on IgD/IgA1-T cell IgD receptor interactions including basal immunoglobulin synthesis, and possibly myeloid IgD receptor-mediated cytokine release.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2003-2004

This review is the third update of the original review, published in 1999, on the application of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings the topic to the end of 2004. Both fundamental studies and applications are covered. The main topics include methodological developments, matrices, fragmentation of carbohydrates and applications to large polymeric carbohydrates from plants, glycans from glycoproteins and those from various glycolipids. Other topics include the use of MALDI MS to study enzymes related to carbohydrate biosynthesis and degradation, its use in industrial processes, particularly biopharmaceuticals and its use to monitor products of chemical synthesis where glycodendrimers and carbohydrate-protein complexes are highlighted.

Evaluation of the serum N-linked glycome for the diagnosis of cancer and chronic inflammation

The identification of serum biomarkers has lead to improvements in the detection and diagnosis of cancer, and combinations of these biomarkers have increased further their sensitivity and specificity. Glycosylation is the most common PTM of secreted proteins and the identification of novel serum glyco-biomarkers has become a topic of increasing interest because the glycan processing pathways are frequently disturbed in cancer cells. A future goal is to combine current biomarkers with glyco-biomarkers to yield further improvements. Well characterised N-glycosylation changes in the serum glycome of cancer patients include changes in the levels of tri- and tetra-antennary glycan structures, sialyl Lewis X epitopes and agalactosylated bi-antennary glycans. Several of these glycosylated markers have been linked to chronic inflammatory diseases, promoting questions about the links between inflammation and cancer. In this review, the glycoproteins which display these glycan epitopes, the glycosyl transferases which can generate them, their potential functions and their use as biomarkers are evaluated.

Crystal structure and carbohydrate analysis of Nipah virus attachment glycoprotein: a template for antiviral and vaccine design

Two members of the paramyxovirus family, Nipah virus (NiV) and Hendra virus (HeV), are recent additions to a growing number of agents of emergent diseases which use bats as a natural host. Identification of ephrin-B2 and ephrin-B3 as cellular receptors for these viruses has enabled the development of immunotherapeutic reagents which prevent virus attachment and subsequent fusion. Here we present the structural analysis of the protein and carbohydrate components of the unbound viral attachment glycoprotein of NiV glycoprotein (NiV-G) at a 2.2-A resolution. Comparison with its ephrin-B2-bound form reveals that conformational changes within the envelope glycoprotein are required to achieve viral attachment. Structural differences are particularly pronounced in the 579-590 loop, a major component of the ephrin binding surface. In addition, the 236-245 loop is rather disordered in the unbound structure. We extend our structural characterization of NiV-G with mass spectrometric analysis of the carbohydrate moieties. We demonstrate that NiV-G is largely devoid of the oligomannose-type glycans that in viruses such as human immunodeficiency virus type 1 and Ebola virus influence viral tropism and the host immune response. Nevertheless, we find putative ligands for the endothelial cell lectin, LSECtin. Finally, by mapping structural conservation and glycosylation site positions from other members of the paramyxovirus family, we suggest the molecular surface involved in oligomerization. These results suggest possible pathways of virus-host interaction and strategies for the optimization of recombinant vaccines.

Recapitulation of IVIG anti-inflammatory activity with a recombinant IgG Fc

It is well established that high doses of monomeric immunoglobulin G (IgG) purified from pooled human plasma [intravenous immunoglobulin (IVIG)] confer anti-inflammatory activity in a variety of autoimmune settings. However, exactly how those effects are mediated is not clear because of the heterogeneity of IVIG. Recent studies have demonstrated that the anti-inflammatory activity of IgG is completely dependent on sialylation of the N-linked glycan of the IgG Fc fragment. Here we determine the precise glycan requirements for this anti-inflammatory activity, allowing us to engineer an appropriate IgG1 Fc fragment, and thus generate a fully recombinant, sialylated IgG1 Fc with greatly enhanced potency. This therapeutic molecule precisely defines the biologically active component of IVIG and helps guide development of an IVIG replacement with improved activity and availability.

The impact of glycosylation on the biological function and structure of human immunoglobulins

Immunoglobulins are the major secretory products of the adaptive immune system. Each is characterized by a distinctive set of glycoforms that reflects the wide variation in the number, type, and location of their oligosaccharides. In a given physiological state, glycoform populations are reproducible; therefore, disease-associated alterations provide diagnostic biomarkers (e.g., for rheumatoid arthritis) and contribute to disease pathogenesis. The oligosaccharides provide important recognition epitopes that engage with lectins, endowing the immunoglobulins with an expanded functional repertoire. The sugars play specific structural roles, maintaining and modulating effector functions that are physiologically relevant and can be manipulated to optimize the properties of therapeutic antibodies. New molecular models of all the immunoglobulins are included to provide a basis for informed and critical discussion. The models were constructed by combining glycan sequencing data with oligosaccharide linkage and dynamics information from the Glycobiology Institute experimental database and protein structural data from "The Protein Data Bank."

Severe fibrosis in hepatitis C virus-infected patients is associated with increased activity of the mannan-binding lectin (MBL)/MBL-associated serine protease 1 (MASP-1) complex

Mannan-binding lectin (MBL) binds microorganisms via interactions with glycans on the target surface. Bound MBL subsequently activates MBL-associated serine protease proenzymes (MASPs). A role for MBL in hepatitis C virus (HCV) infection had been indicated by previous studies examining MBL levels and polymorphisms in relation to disease progression and response to treatment. We undertook this study to investigate a possible relationship between disease progression and functional MBL/MASP-1 complex activity. A functional assay for MBL/MASP-1 complex activity was employed to examine serum samples from patients with chronic HCV infection, non-HCV liver disease and healthy controls. Intrapatient consistency of MBL/MASP-1 complex activity levels was assessed in sequential samples from a subgroup of patients. Median values of MBL/MASP-1 complex activity were higher in sera from patients with liver disease compared with healthy controls. MBL/MASP-1 complex activity levels correlate with severity of fibrosis after adjusting for confounding factors (P = 0.003). MBL/MASP-1 complex activity was associated more significantly with fibrosis than was MBL concentration. The potential role of MBL/MASP-1 complex activity in disease progression is worthy of further study to investigate possible mechanistic links.

Potato lectin activates basophils and mast cells of atopic subjects by its interaction with core chitobiose of cell-bound non-specific immunoglobulin E

A major factor in non-allergic food hypersensitivity could be the interaction of dietary lectins with mast cells and basophils. Because immunoglobulin E (IgE) contains 10-12% carbohydrates, lectins can activate and degranulate these cells by cross-linking the glycans of cell-bound IgE. The present objective focuses on the effect of potato lectin (Solanum tuberosum agglutinin; STA) for its ability to release histamine from basophils in vitro and mast cells in vivo from non-atopic and atopic subjects. In this study, subjects were selected randomly based on case history and skin prick test responses with food, pollen and house dust mite extracts. Skin prick test (SPT) was performed with STA at 100 microg/ml concentration. Histamine release was performed using leucocytes from non-atopic and atopic subjects and rat peritoneal exudate cells. SPT on 110 atopic subjects using STA showed 39 subjects positive (35%); however, none showed STA-specific IgE; among 20 non-atopic subjects, none were positive by SPT. Maximal histamine release was found to be 65% in atopic subjects (n = 7) compared to 28% in non-atopic subjects (n = 5); the release was inhibited specifically by oligomers of N-acetylglucosamine and correlates well with serum total IgE levels (R(2) = 0.923). Binding of STA to N-linked glycoproteins (horseradish peroxidase, avidin and IgG) was positive by dot blot and binding assay. As potato lectin activates and degranulates both mast cells and basophils by interacting with the chitobiose core of IgE glycans, higher intake of potato may increase the clinical symptoms as a result of non-allergic food hypersensitivity in atopic subjects.

Human Follicular Lymphoma Cells Contain Oligomannose Glycans in the Antigen-binding Site of the B-cell Receptor

Expression of surface immunoglobulin appears critical for the growth and survival of B-cell lymphomas. In follicular lymphoma, we found previously that the Ig variable (V) regions in the B-cell receptor express a strikingly high incidence of N-glycosylation sequons, NX(S/T). These potential glycosylation sites are introduced by somatic mutation and are lymphoma-specific, pointing to their involvement in tumor pathogenesis. Analysis of the V region sugars from lymphoma-derived IgG/IgM reveals that they are mostly oligomannose and, remarkably, are located in the antigen-binding site, possibly precluding conventional antigen binding. The Fc region contains complex glycans, confirming that the normal glycan processing pathway is intact. Binding studies indicate that the oligomannose glycans occupying the V regions are accessible to mannose-binding lectin. These findings suggest a potential contribution to lymphoma pathogenesis involving antigen-independent interaction of surface immunoglobulin of the B-cell receptor with mannose-binding molecules of innate immunity in the germinal center.

The DMalpha and DMbeta chain cooperate in the oxidation and folding of HLA-DM

HLA-DM (DM) is a heterodimeric MHC molecule that catalyzes the peptide loading of classical MHC class II molecules in the endosomal/lysosomal compartments of APCs. Although the function of DM is well-established, little is known about how DMalpha and beta-chains fold, oxidize, and form a complex in the endoplasmic reticulum (ER). In this study, we show that glycosylation promotes, but is not essential for, DMalphabeta ER exit. However, glycosylation of DMalpha N15 is required for oxidation of the alpha-chain. The DMalpha and beta-chains direct each others fate: single DMalpha chains cannot fully oxidize without DMbeta, while DMbeta forms disulfide-linked homodimers without DMalpha. Correct oxidation and subsequent ER egress depend on the unique DMbeta C25 and C35 residues. This suggests that the C25-C35 disulfide bond in the peptide-binding domain overcomes the need for stabilizing peptides required by other MHC molecules.

Discrete MBL-MASP complexes show wide inter-individual variability in concentration: data from UK vs Armenian populations

Mannan-binding lectin (MBL) circulates in plasma in complex with MBL-associated serine proteases (MASP) -1, -2 and -3 and a smaller component, MAp19. When MBL binds to the surface of foreign material (microorganisms), MASP-1, -2, -3 are activated. MASP-2 then activates the complement system. MASP-1 and -3 may activate other (unidentified) systems. MBL levels, MBL-bound MASP-1 and MBL-bound MASP-2 activities have been evaluated in healthy individuals from UK and Armenian populations. MBL-bound MASP-2 activity declines in aging (P<0.04). MBL correlates with smoking (P<0.02). There were significant differences between the two populations in MBL-bound MASP-1 activity and in MBL, but no difference in MBL-bound MASP-2 activity. When MASP activities were normalised to MBL (i.e. MASP-1 activity/MBL, MASP-2 activity/MBL), normalised MASP-2 activity in UK individuals was more than 2 fold higher than in Armenians. The difference in normalised MASP-2 activity level between these two Caucasoid populations, suggests that concentration of the MBL-(MASP-2) complex, and therefore the function of activating complement, depends not only on the quantity of MBL in serum and its oligomeric state, but also on the quantity of MASP-2 in serum. It is likely that in individuals with high MBL concentration there is excess free MBL not occupied by MASPs, particularly not by MASP-2.

Investigations on the Involvement of the Lectin Pathway of Complement Activation in Anaphylaxis

BACKGROUND

Systemic anaphylaxis is the most severe form of immediate hypersensitivity reaction. The activation of the complement system occurs during anaphylactic shock. The purpose of this study was to determine in a mouse model whether the lectin pathway of complement activation is involved in anaphylaxis.

METHODS

To see whether the lectin pathway is involved in anaphylactic shock, serum mannan-binding lectin (MBL) levels were measured after passive anaphylaxis. Also MBL expression and binding to potential ligands were investigated. To determine whether complement or mast cell activation is essential for hypothermia in anaphylactic shock, mouse strains deficient in MBL-A and MBL-C, C1q, factors B and C2, C5, C5aR, or mast cells were tested.

RESULTS

After antigenic challenge a marked drop in body temperature as well as a rapid decrease in serum MBL levels were observed. The decrease of serum MBL levels in shock could not be attributed to MBL binding to immune complexes or tissues, but an interaction of MBL with mast cell-derived proteoglycans was seen. In contrast to mast cell-deficient mice, none of the complement-deficient mouse strains were protected from shock-associated hypothermia.

CONCLUSIONS

These results indicate that neither MBL nor activation of the complement cascade is crucial for the induction of anaphylaxis. In contrast mast cell activation is associated with the development of hypothermia and possibly the observed decrease in serum MBL levels.

Effect of horse gram lectin (Dolichos biflorus agglutinin) on degranulation of mast cells and basophils of atopic subjects: identification as an allergen

Horse gram (Dolichos biflorus) is widely consumed in the tropical south Asian countries including rural areas of India. Since D. biflorus agglutinin (DBA) is an important dietary lectin in horse gram, we have studied its effect on the degranulation of mast cells and basophils of atopic subjects. Allergy to horse gram lectin has not been reported so far. Skin prick test (SPT) was performed with 100 microg/mL of DBA. DBA-specific IgE was detected by dot-blot, and ELISA. Histamine release (HR) assay was carried out using leukocytes from non-atopic and atopic subjects, and rat peritoneal exudate cells. Among the atopic group, 10 of 48 subjects (21%) were found to be positive for DBA by SPT, and none were positive in the non-atopic group (n=20). Two subjects out of the ten who tested positive for DBA by SPT were found to be sensitized to DBA as revealed by the presence of specific IgE by ELISA and dot-blot. The HR was found to be 2- to 3-fold higher in DBA-allergic subjects than in non-atopic and atopic subjects. Basophil HR by DBA was found to be similar in both non-atopic and atopic subjects. However, DBA induces activation of mast cells in vivo in a sub-population (21%) of atopic subjects. Two subjects have been identified as having food allergy to horse gram based on the presence of DBA-specific IgE with a positive correlation to basophil HR. This is the first report of food allergy to horse gram, and DBA has been identified as an allergen.

Increased complement classical and mannan-binding lectin pathway activities in schizophrenia

Schizophrenia is a severe mental disorder, with worldwide prevalence of 1-1.5%. Immunological research in schizophrenia indicates that infectious or autoimmune processes might play a role in the etiopathogenesis. The complement system is a major mediator of innate immune defence against infection and contributes to many functions of the immune system including inflammation, opsonization and cell lysis. Mannan-binding lectin (MBL) activates the complement system via the lectin pathway. Inherited MBL deficiency, common in most human populations, predisposes to infectious and autoimmune diseases. We measured total complement activity (CH50), C4 activity (C4 CH50), MBL level and the activities of MBL-associated serine proteases, MASP-1 and MASP-2 in sera of 45 schizophrenic patients and in 62 healthy volunteers. We found that schizophrenic patients and healthy volunteers have statistically similar MBL levels and MASP-1 activity. However, MBL-bound MASP-2 activity and therefore MBL and MASP-2-mediated complement activation capacity is increased in schizophrenic patients compared with healthy volunteers (P<0.01). The increase was accompanied by increased CH50 (P<0.02) and C4 CH50 (P<0.02). Our results support the idea that complement system alterations may be involved in schizophrenia.

Degalactosylated and/or Denatured IgA, but Not Native IgA in Any Form, Bind to Mannose-Binding Lectin

Mannose-binding lectin (MBL) is reported to bind to agalactosyl IgG, but not to normally galactosylated (native) IgG. It was recently reported that serum polymeric IgA in its native form reacts with MBL, whereas a more recent report has claimed that native IgD and IgE, and possibly IgM, do not. This led us to investigate whether IgA is truly reactive with MBL. To accomplish this, we collected purified human Igs, of various classes, subclasses, and allotypes, and tested their ability to bind to MBL using an ELISA method. Among these preparations, only one (monoclonal IgA2m(2):Kur) exhibited significant MBL binding. In particular, polymeric or monomeric forms of our normal serum IgA preparation lacked any ability to bind to MBL whatsoever. However, all the Ig preparations which had not bound to MBL became able to do so when they were degalactosylated with a galactosidase treatment, and the binding was further enhanced by acidic denaturation of the Igs. Among the degalactosylated and/or acid-denatured IgA, the IgA2 subclass exhibited a higher level of MBL binding than did IgA1. Our results suggest that MBL does not bind to native Igs (viewed in principle as "self" components), and that only Igs with abnormal glycosylation (degalactosylated forms) and/or denaturation would be MBL reactive.

Variable region heavy chain glycosylation determines the anticoagulant activity of a factor VIII antibody

BACKGROUND

N-glycosylation occurs in the variable region of about 10% of antibodies but the role of carbohydrate at this location is still poorly understood.

OBJECTIVES

We investigated the function of N-glycosylation in the variable region of the heavy chain of a human monoclonal antibody, mAb-LE2E9, that partially inhibits factor VIII (FVIII) activity during coagulation.

METHODS AND RESULTS

Enzymatic deglycosylation indicated that the oligosaccharides do not determine the affinity of the antibody but enhance its FVIII neutralizing activity. A mutant antibody lacking the N-glycosylation site in the variable region of the heavy chain inhibited FVIII activity by up to 40%, while inhibition by the native antibody was 80%. To evaluate the physiological effect of such a FVIII inhibition, we investigated the ability of the mutant antibody devoid of N-glycosylation in the variable region to prevent thrombosis in mice with a strong prothombotic phenotype resulting from a type II deficiency mutation in the heparin binding site of antithrombin. Despite its moderate inhibition of FVIII activity, the mutant antibody significantly prevented thrombosis in treated animals. We also carried out glycan analysis of native and mutant antibodies.

CONCLUSIONS

Modification of glycosylation in the variable region of antibodies contributes to the diversity of FVIII type II inhibition possibly by steric hindrance of the active site of FVIII by glycans, and may provide a novel strategy to modulate the functional activity of therapeutic antibodies.

Interaction of Mannan Binding Lectin with α2 Macroglobulin via Exposed Oligomannose Glycans

The serum collectin mannan-binding lectin (MBL) binds to oligomannose and GlcNAc-terminating glycans present on microorganisms. Using a commercial affinity chromatography resin containing immobilized MBL we screened human and mouse serum for endogenous MBL-binding targets. We isolated the serum protease inhibitor alpha(2) macroglobulin (alpha2M), a heavily glycosylated thiol ester protein (TEP) composed of four identical 180-kDa subunits, each of which has eight N-linked glycosylation sites. alpha2M has previously been reported to interact with MBL; however, the interaction was not characterized. We investigated the mechanism of formation of complexes between alpha2M and MBL and concluded that they form by the direct binding of oligomannose glycans Man(5-7) occupying Asn-846 on alpha2M to the lectin domains (carbohydrate recognition domains) of MBL. The oligomannose glycans are accessible for lectin binding on both active alpha2M (thiol ester intact) and protease-cleaved alpha2M (thiol ester cleaved). We demonstrate that MBL is able to interact with alpha2M in the fluid phase, but the interaction does not inhibit the binding of MBL to mannan-coated surfaces. In addition to alpha2M, two other members of the TEP family, C3 and C4, which also contain oligomannose glycans, were captured from human serum using the MBL resin. MBL binding may be a conserved feature of the TEPs, dating from their ancestral origins. We suggest that the inhibition of proteases on the surface of microorganisms by an ancestral alpha2M-like TEP may generate "arrays" of oligomannose glycans to which MBL or other lectins can bind. Binding would lead to opsonization or activation of enzyme systems such as complement.

Semi-extended solution structure of human myeloma immunoglobulin D determined by constrained X-ray scattering

Human immunoglobulin D (IgD) occurs most abundantly as a membrane-bound antibody on the surface of mature B cells (mIgD). IgD possesses the longest hinge sequence of all the human antibody isotypes, with 64 residues connecting the Fab and Fc fragments. A novel rapid purification scheme of secreted IgD from the serum of an IgD myeloma patient using thiophilic (T-gel) and lectin affinity chromatography gave a stable, homogeneous IgD preparation. Synchrotron X-ray scattering and analytical ultracentrifugation of IgD identified the solution arrangement of its Fab and Fc fragments, and thereby its hinge structure. The Guinier X-ray radius of gyration R(G) of 6.9(+/-0.1)nm showed that IgD is more extended in solution than the immunoglobulin subclass IgA1 (R(G) of 6.1-6.2nm). Its distance distribution function P(r) showed a single peak at 4.7nm and a maximum dimension of 23nm. Velocity experiments gave a sedimentation coefficient of 6.3S, which is similar to that for IgA1 at 6.2S. The complete IgD structure was modelled using molecular dynamics to generate IgD hinge structures, to which homology models for the Fab and Fc fragments were connected. Good scattering curve fits were obtained with 18 semi-extended best fit IgD models that were filtered from 8500 trial models. These best-fit models showed that the IgD hinge does not correspond to an extended polypeptide structure. The averaged solution structure arrangement of the Fab and Fc fragments in IgD is principally T-shaped and flexible, with contribution from Y-shaped and inverted Y-shaped structures. Although the linear sequence of the IgD hinge is much longer, comparison with previous scattering modelling of IgA1 and IgA2(m)1 suggests that the hinge of IgA1 and IgD are more similar than might have been expected, Both possess flexible T-shaped solution structures, probably reflecting the presence of restraining O-linked sugars.

Heterogeneity of MBL-MASP complexes

In order to study aspects of the stoichiometry and composition of human MBL-MASP complexes in the population, MBL-MASP complexes were bound from sera of 152 healthy individuals onto mannan-coated microtitre plates. Bound mannan-binding lectin (MBL) was measured by ELISA, and the enzyme activities of MBL-bound MASP-1 and MASP-2 were measured by an amidolytic assay and a C4 fixation assay, respectively. MASP-1 activity correlated with MBL concentration, as did MASP-2 activity (in both cases: p<0.0001). This is expected since MASP-1 and MASP-2 are bound to the mannan via MBL. However, when MASP activities were normalised to MBL concentration (i.e. MASP-1 activity/[MBL], MASP-2 activity/[MBL]) MASP-1 activity was inversely correlated with MASP-2. This means on average that high MASP-1 correlates with low MASP-2 and vice-versa, and confirms the hypothesis that native MBL-MASP complexes on average do not have fixed MBL-(MASP-1)-(MASP-2) stoichiometry. The findings are consistent with separate populations of MBL-MASP-1 complexes and MBL-MASP-2 complexes, the concentrations of which show wide inter-individual variation.

Human serum IgM glycosylation: identification of glycoforms that can bind to mannan-binding lectin

The glycoprotein IgM is the major antibody produced in the primary immune response to antigens, circulating in the serum both as a pentamer and a hexamer. Pentameric IgM has a single J chain, which is absent in the hexamer. The mu (heavy) chain of IgM has five N-linked glycosylation sites. Asn-171, Asn-332, and Asn-395 are occupied by complex glycans, whereas Asn-402 and Asn-563 are occupied by oligomannose glycans. The glycosylation of human polyclonal IgM from serum has been analyzed. IgM was found to contain 23.4% oligomannose glycans GlcNAc2Man5-9, consistent with 100% occupancy of Asn-402 and 17% occupancy of the variably occupied site at Asn-563. Mannan-binding lectin (MBL) is a member of the collectin family of proteins, which bind to oligomannose and GlcNAc-terminating structures. A commercial affinity chromatography resin containing immobilized MBL has been reported to be useful for partial purification of mouse and also human IgM. Human IgM glycoforms that bind to immobilized MBL were isolated; these accounted for only 20% of total serum IgM. Compared with total serum IgM, the MBL-binding glycoforms contained 97% more GlcNAc-terminating structures and 8% more oligomannose structures. A glycosylated model of pentameric IgM was constructed, and from this model, it became evident that IgM has two distinct faces, only one of which can bind to antigen, as the J chain projects from the non-antigen-binding face. Antigen-bound IgM does not bind to MBL, as the target glycans appear to become inaccessible once IgM has bound antigen. Antigen-bound IgM pentamers therefore do not activate complement via the lectin pathway, but MBL might have a role in the clearance of aggregated IgM.