Glycophenotyping of osteoarthritic cartilage and chondrocytes by RT-qPCR, mass spectrometry, histochemistry with plant/human lectins and lectin localization with a glycoprotein

INTRODUCTION

This study aimed to characterize the glycophenotype of osteoarthritic cartilage and human chondrocytes.

METHODS

Articular knee cartilage was obtained from nine osteoarthritis (OA) patients. mRNA levels for 27 glycosyltransferases were analyzed in OA chondrocytes using RT-qPCR. Additionally, N- and O-glycans were quantified using mass-spectrometry. Histologically, two cartilage areas with Mankin scores (MS) either ≤ 4 or ≥ 9 were selected from each patient representing areas of mild and severe OA, respectively. Tissue sections were stained with (1) a selected panel of plant lectins for probing into the OA glycophenotype, (2) the human lectins galectins-1 and -3, and (3) the glycoprotein asialofetuin (ASF) for visualizing β-galactoside-specific endogenous lectins.

RESULTS

We found that OA chondrocytes expressed oligomannosidic structures as well as non-, mono- and disialylated complex-type N-glycans, and core 2 O-glycans. Reflecting B4GALNT3 mRNA presence in OA chondrocytes, LacdiNAc-terminated structures were detected. Staining profiles for plant and human lectins were dependent on the grade of cartilage degeneration, and ASF-positive cells were observed in significantly higher rates in areas of severe degeneration.

CONCLUSIONS

In summary, distinct aspects of the glycome in OA cartilage are altered with progressing degeneration. In particular, the alterations measured by galectin-3 and the pan-galectin sensor ASF encourage detailed studies of galectin functionality in OA.

Inhibition of IgE binding to cross-reactive carbohydrate determinants enhances diagnostic selectivity

Background

Allergy diagnosis by determination of allergen-specific IgE is complicated by clinically irrelevant IgE, of which the most prominent example is IgE against cross-reactive carbohydrate determinants (CCDs) that occur on allergens from plants and insects. Therefore, CCDs cause numerous false-positive results. Inhibition of CCDs has been proposed as a remedy, but has not yet found its way into the routine diagnostic laboratory. We sought to provide a simple and affordable procedure to overcome the CCD problem.

Methods

Serum samples from allergic patients were analysed for allergen-specific IgEs by different commercial tests (from Mediwiss, Phadia and Siemens) with and without a semisynthetic CCD blocker with minimized potential for nonspecific interactions that was prepared from purified bromelain glycopeptides and human serum albumin.

Results

Twenty two per cent of about 6000 serum samples reacted with CCD reporter proteins. The incidence of anti-CCD IgE reached 35% in the teenage group. In patients with anti-CCD IgE, application of the CCD blocker led to a clear reduction in read-out values, often below the threshold level. A much better correlation between laboratory results and anamnesis and skin tests was achieved in many cases. The CCD blocker did not affect test results where CCDs were not involved.

Conclusion

Eliminating the effect of IgEs directed against CCDs by inhibition leads to a significant reduction in false-positive in vitro test results without lowering sensitivity towards relevant sensitizations. Application of the CCD blocker may be worthwhile wherever natural allergen extracts or components are used.

Rhamnogalacturonan II structure shows variation in the side chains monosaccharide composition and methylation status within and across different plant species

A paradigm regarding rhamnogalacturonans II (RGII) is their strictly conserved structure within a given plant. We developed and employed a fast structural characterization method based on chromatography and mass spectrometry, allowing analysis of RGII side chains from microgram amounts of cell wall. We found that RGII structures are much more diverse than so far described. In chain A of wild-type plants, up to 45% of the l-fucose is substituted by l-galactose, a state that is seemingly uncorrelated with RGII dimerization capacity. This led us to completely reinvestigate RGII structures of the Arabidopsis thaliana fucose-deficient mutant mur1, which provided insights into RGII chain A biosynthesis, and suggested that chain A truncation, rather than l-fucose to l-galactose substitution, is responsible for the mur1 dwarf phenotype. Mass spectrometry data for chain A coupled with NMR analysis revealed a high degree of methyl esterification of its glucuronic acid, providing a plausible explanation for the puzzling RGII antibody recognition. The β-galacturonic acid of chain A exhibits up to two methyl etherifications in an organ-specific manner. Combined with variation in the length of side chain B, this gives rise to a family of RGII structures instead of the unique structure described up to now. These findings pave the way for studies on the physiological roles of modulation of RGII composition.

Expression of functionally active sialylated human erythropoietin in plants

Recombinant human erythropoietin (rhEPO), a glycohormone, is one of the leading biopharmaceutical products. The production of rhEPO is currently restricted to mammalian cell expression systems because of rhEPO's highly complex glycosylation pattern, which is a major determinant for drug-efficacy. Here we evaluate the ability of plants to produce different glycoforms of rhEPO. cDNA constructs were delivered to Nicotiana benthamiana (N. benthamiana) and transiently expressed by a viral based expression system. Expression levels up to 85 mg rhEPO/kg fresh leaf material were achieved. Moreover, co-expression of rhEPO with six mammalian genes required for in planta protein sialylation resulted in the synthesis of rhEPO decorated mainly with bisialylated N-glycans (NaNa), the most abundant glycoform of circulating hEPO in patients with anemia. A newly established peptide tag (ELDKWA) fused to hEPO was particularly well-suited for purification of the recombinant hormone based on immunoaffinity. Subsequent lectin chromatography allowed enrichment of exclusively sialylated rhEPO. All plant-derived glycoforms exhibited high biological activity as determined by a cell-based receptor-binding assay. The generation of rhEPO carrying largely homogeneous glycosylation profiles (GnGnXF, GnGn, and NaNa) will facilitate further investigation of functionalities with potential implications for medical applications.

Plant species and organ influence the structure and subcellular localization of recombinant glycoproteins

Many plant-based systems have been developed as bioreactors to produce recombinant proteins. The choice of system for large-scale production depends on its intrinsic expression efficiency and its propensity for scale-up, post-harvest storage and downstream processing. Factors that must be considered include the anticipated production scale, the value and intended use of the product, the geographical production area, the proximity of processing facilities, intellectual property, safety and economics. It is also necessary to consider whether different species and organs affect the subcellular trafficking, structure and qualitative properties of recombinant proteins. In this article we discuss the subcellular localization and N-glycosylation of two commercially-relevant recombinant glycoproteins (Aspergillus niger phytase and anti-HIV antibody 2G12) produced in different plant species and organs. We augment existing data with novel results based on the expression of the same recombinant proteins in Arabidopsis and tobacco seeds, focusing on similarities and subtle differences in N-glycosylation that often reflect the subcellular trafficking route and final destination, as well as differences generated by unique enzyme activities in different species and tissues. We discuss the potential consequences of such modifications on the stability and activity of the recombinant glycoproteins.

Characterizing the link between glycosylation state and enzymatic activity of the endo-β1,4-glucanase KORRIGAN1 from Arabidopsis thaliana

Defects in N-glycosylation and N-glycan processing frequently cause alterations in plant cell wall architecture, including changes in the structure of cellulose, which is the most abundant plant polysaccharide. KORRIGAN1 (KOR1) is a glycoprotein enzyme with an essential function during cellulose biosynthesis in Arabidopsis thaliana. KOR1 is a membrane-anchored endo-β1,4-glucanase and contains eight potential N-glycosylation sites in its extracellular domain. Here, we expressed A. thaliana KOR1 as a soluble, enzymatically active protein in insect cells and analyzed its N-glycosylation state. Structural analysis revealed that all eight potential N-glycosylation sites are utilized. Individual elimination of evolutionarily conserved N-glycosylation sites did not abolish proper KOR1 folding, but mutations of Asn-216, Asn-324, Asn-345, and Asn-567 resulted in considerably lower enzymatic activity. In contrast, production of wild-type KOR1 in the presence of the class I α-mannosidase inhibitor kifunensine, which abolished the conversion of KOR1 N-glycans into complex structures, did not affect the activity of the enzyme. To address N-glycosylation site occupancy and N-glycan composition of KOR1 under more natural conditions, we expressed a chimeric KOR1-Fc-GFP fusion protein in leaves of Nicotiana benthamiana. Although Asn-108 and Asn-133 carried oligomannosidic N-linked oligosaccharides, the six other glycosylation sites were modified with complex N-glycans. Interestingly, the partially functional KOR1 G429R mutant encoded by the A. thaliana rsw2-1 allele displayed only oligomannosidic structures when expressed in N. benthamiana, indicating its retention in the endoplasmic reticulum. In summary, our data indicate that utilization of several N-glycosylation sites is important for KOR1 activity, whereas the structure of the attached N-glycans is not critical.

Production, characterization, and antigen specificity of recombinant 62-71-3, a candidate monoclonal antibody for rabies prophylaxis in humans

Rabies kills many people throughout the developing world every year. The murine monoclonal antibody (mAb) 62-71-3 was recently identified for its potential application in rabies postexposure prophylaxis (PEP). The purpose here was to establish a plant-based production system for a chimeric mouse-human version of mAb 62-71-3, to characterize the recombinant antibody and investigate at a molecular level its interaction with rabies virus glycoprotein. Chimeric 62-71-3 was successfully expressed in Nicotiana benthamiana. Glycosylation was analyzed by mass spectroscopy; functionality was confirmed by antigen ELISA, as well as rabies and pseudotype virus neutralization. Epitope characterization was performed using pseudotype virus expressing mutagenized rabies glycoproteins. Purified mAb demonstrated potent viral neutralization at 500 IU/mg. A critical role for antigenic site I of the glycoprotein, as well as for two specific amino acid residues (K226 and G229) within site I, was identified with regard to mAb 62-71-3 neutralization. Pseudotype viruses expressing glycoprotein from lyssaviruses known not to be neutralized by this antibody were the controls. The results provide the molecular rationale for developing 62-71-3 mAb for rabies PEP; they also establish the basis for developing an inexpensive plant-based antibody product to benefit low-income families in developing countries.

Generation of biologically active multi-sialylated recombinant human EPOFc in plants

Hyperglycosylated proteins are more stable, show increased serum half-life and less sensitivity to proteolysis compared to non-sialylated forms. This applies particularly to recombinant human erythropoietin (rhEPO). Recent progress in N-glycoengineering of non-mammalian expression hosts resulted in in vivo protein sialylation at great homogeneity. However the synthesis of multi-sialylated N-glycans is so far restricted to mammalian cells. Here we used a plant based expression system to accomplish multi-antennary protein sialylation. A human erythropoietin fusion protein (EPOFc) was transiently expressed in Nicotiana benthamiana ΔXTFT, a glycosylation mutant that lacks plant specific N-glycan residues. cDNA of the hormone was co-delivered into plants with the necessary genes for (i) branching (ii) β1,4-galactosylation as well as for the (iii) synthesis, transport and transfer of sialic acid. This resulted in the production of recombinant EPOFc carrying bi- tri- and tetra-sialylated complex N-glycans. The formation of this highly complex oligosaccharide structure required the coordinated expression of 11 human proteins acting in different subcellular compartments at different stages of the glycosylation pathway. In vitro receptor binding assays demonstrate the generation of biologically active molecules. We demonstrate the in planta synthesis of one of the most complex mammalian glycoforms pointing to an outstanding high degree of tolerance to changes in the glycosylation pathway in plants.

Glycan profiles of the 27 N-glycosylation sites of the HIV envelope protein CN54gp140

Hope rests on the envelope proteins of human immunodeficiency virus (HIV) as protective vaccines and thus their antibody binding sites are of prime interest. 2G12 and other human antibodies bind to a cluster of oligomannose N-glycans. Owing to the extreme number and density of N-glycosylation sites gp160 and its recombinant form gp140 represent challenging tasks for site-specific glycosylation analysis. We have conducted a glycosylation analysis of CN54gp140 by liquid chromatography-electrospray ionization-mass spectrometry (LC-ESI-MS) using an ion trap as well as a Q-TOF instrument and standard software for glycopeptide identification. First, a deglycosylated sample of the protease digest served to locate the elution positions of peptides covering all of the 27 potential N-glycosylation sites. Then, the assignments of the similarly eluting glycopeptides were verified by collision-induced decay MS/MS experiments with elevated fragmentation energy. The acquisition of site-specific glycan profiles was facilitated by the use of buffered eluent, which rounds up all glycoforms of a peptide into one peak. Calculation of the molecular mass drawn on the weighted averages of the glycans at each site led to the actual mass of gp140 of approximately 120 kDa.

"Cross-glycosylation" of proteins in Bacteroidales species

While it is now evident that the two Bacteroidales species Bacteroides fragilis and Tannerella forsythia both have general O-glycosylation systems and share a common glycosylation sequon, the ability of these organisms to glycosylate a protein native to the other organism has not yet been demonstrated. Here, we report on the glycosylation of heterologous proteins between these two organisms. Using genetic tools previously developed for Bacteroides species, two B. fragilis model glycoproteins were expressed in the fastidious anaerobe T. forsythia and the attachment of the known T. forsythia O-glycan to these proteins was demonstrated by liquid chromatography electrospray ionization tandem mass spectrometry. Likewise, two predominant T. forsythia glycoproteins were expressed in B. fragilis and glycosylation with the B. fragilis O-glycan was confirmed. Purification of these proteins from B. fragilis allowed the preliminary characterization of the previously uncharacterized B. fragilis protein O-glycan. Based on mass spectrometric data, we show that the B. fragilis protein O-glycan is an oligosaccharide composed of nine sugar units. Compositional and structural similarities with the T. forsythia O-glycan suggest commonalities in their biosynthesis. These data demonstrate the feasibility of exploiting these organisms for the design of novel glycoproteins.

Myrosinases TGG1 and TGG2 from Arabidopsis thaliana contain exclusively oligomannosidic N-glycans

In all eukaryotes N-glycosylation is the most prevalent protein modification of secretory and membrane proteins. Although the N-glycosylation capacity and the individual steps of the N-glycan processing pathway have been well studied in the model plant Arabidopsis thaliana, little attention has been paid to the characterization of the glycosylation status of individual proteins. We report here the structural analysis of all N-glycans present on the endogenous thioglucoside glucohydrolases (myrosinases) TGG1 and TGG2 from A. thaliana. All nine glycosylation sites of TGG1 and all four glycosylation sites of TGG2 are occupied by oligomannosidic structures with Man₅GlcNAc₂ as the major glycoform. Analysis of the oligomannosidic isomers from wild-type plants and mannose trimming deficient mutants by liquid chromatography with porous graphitic carbon and mass spectrometry revealed that the N-glycans from both myrosinases are processed by Golgi-located α-mannosidases.

Generation of hypoallergenic neoglycoconjugates for dendritic cell targeted vaccination: a novel tool for specific immunotherapy

The incidence of allergic disorders and asthma continuously increased over the past decades, consuming a considerable proportion of the health care budget. Allergen-specific subcutaneous immunotherapy represents the only intervention treating the underlying causes of type I allergies, but still suffers from unwanted side effects and low compliance. There is an urgent need for novel approaches improving safety and efficacy of this therapy. In the present study we investigated carbohydrate-mediated targeting of allergens to dermal antigen-presenting cells and its influence on immunogenicity and allergenicity. Mannan, high (40 kDa) and low (6 kDa) molecular weight dextran, and maltodextrin were covalently attached to ovalbumin and papain via mild carbohydrate oxidation resulting in neoglycocomplexes of various sizes. In particular, mannan-conjugates were efficiently taken up by dendritic cells in vivo leading to elevated humoral immune responses against the protein moiety and a shift from IgE to IgG. Beyond providing an adjuvant effect, papain glycocomplexes also proved to mask B-cell epitopes, thus rendering the allergen derivative hypoallergenic.

The present data demonstrate that carbohydrate-modified allergens combine targeting of antigen presenting cells with hypoallergenicity, offering the potential for low dose allergen-specific immunotherapy while concomitantly reducing the risk of side effects.

Engineering of sialylated mucin-type O-glycosylation in plants

Proper N- and O-glycosylation of recombinant proteins is important for their biological function. Although the N-glycan processing pathway of different expression hosts has been successfully modified in the past, comparatively little attention has been paid to the generation of customized O-linked glycans. Plants are attractive hosts for engineering of O-glycosylation steps, as they contain no endogenous glycosyltransferases that perform mammalian-type Ser/Thr glycosylation and could interfere with the production of defined O-glycans. Here, we produced mucin-type O-GalNAc and core 1 O-linked glycan structures on recombinant human erythropoietin fused to an IgG heavy chain fragment (EPO-Fc) by transient expression in Nicotiana benthamiana plants. Furthermore, for the generation of sialylated core 1 structures constructs encoding human polypeptide:N-acetylgalactosaminyltransferase 2, Drosophila melanogaster core 1 β1,3-galactosyltransferase, human α2,3-sialyltransferase, and Mus musculus α2,6-sialyltransferase were transiently co-expressed in N. benthamiana together with EPO-Fc and the machinery for sialylation of N-glycans. The formation of significant amounts of mono- and disialylated O-linked glycans was confirmed by liquid chromatography-electrospray ionization-mass spectrometry. Analysis of the three EPO glycopeptides carrying N-glycans revealed the presence of biantennary structures with terminal sialic acid residues. Our data demonstrate that N. benthamiana plants are amenable to engineering of the O-glycosylation pathway and can produce well defined human-type O- and N-linked glycans on recombinant therapeutics.

Moss-based production of asialo-erythropoietin devoid of Lewis A and other plant-typical carbohydrate determinants

Protein therapeutics represent one of the most increasing areas in the pharmaceutical industry. Plants gain acceptance as attractive alternatives for high-quality and economical protein production. However, as the majority of biopharmaceuticals are glycoproteins, plant-specific N-glycosylation has to be taken into consideration. In Physcomitrella patens (moss), glyco-engineering is an applicable tool, and the removal of immunogenic core xylose and fucose residues was realized before. Here, we present the identification of the enzymes that are responsible for terminal glycosylation (α1,4 fucosylation and β1,3 galactosylation) on complex-type N-glycans in moss. The terminal trisaccharide consisting of α1,4 fucose and β1,3 galactose linked to N-acetylglucosamine forms the so-called Lewis A epitope. This epitope is rare on moss wild-type proteins, but was shown to be enriched on complex-type N-glycans of moss-produced recombinant human erythropoietin, while unknown from the native human protein. Via gene targeting of moss galactosyltransferase and fucosyltransferase genes, we identified the gene responsible for terminal glycosylation and were able to completely abolish the formation of Lewis A residues on the recombinant biopharmaceutical.

O-Glycosylation of snails

The glycosylation abilities of snails deserve attention, because snail species serve as intermediate hosts in the developmental cycles of some human and cattle parasites. In analogy to many other host-pathogen relations, the glycosylation of snail proteins may likewise contribute to these host-parasite interactions. Here we present an overview on the O-glycan structures of 8 different snails (land and water snails, with or without shell): Arion lusitanicus, Achatina fulica, Biomphalaria glabrata, Cepaea hortensis, Clea helena, Helix pomatia, Limax maximus and Planorbarius corneus. The O-glycans were released from the purified snail proteins by β-elimination. Further analysis was carried out by liquid chromatography coupled to electrospray ionization mass spectrometry and – for the main structures – by gas chromatography/mass spectrometry. Snail O-glycans are built from the four monosaccharide constituents: N-acetylgalactosamine, galactose, mannose and fucose. An additional modification is a methylation of the hexoses. The common trisaccharide core structure was determined in Arion lusitanicus to be N-acetylgalactosamine linked to the protein elongated by two 4-O-methylated galactose residues. Further elongations by methylated and unmethylated galactose and mannose residues and/or fucose are present. The typical snail O-glycan structures are different to those so far described. Similar to snail N-glycan structures they display methylated hexose residues.

Rapid high yield production of different glycoforms of Ebola virus monoclonal antibody

BACKGROUND

Fc-glycosylation of monoclonal antibodies (mAbs) has profound implications on the Fc-mediated effector functions. Alteration of this glycosylation may affect the efficiency of an antibody. However, difficulties in the production of mAbs with homogeneous N-glycosylation profiles in sufficient amounts hamper investigations of the potential biological impact of different glycan residues.

METHODOLOGY/PRINCIPAL FINDINGS

Here we set out to evaluate a transient plant viral based production system for the rapid generation of different glycoforms of a monoclonal antibody. Ebola virus mAb h-13F6 was generated using magnICON expression system in Nicotiana benthamiana, a plant species developed for commercial scale production of therapeutic proteins. h-13F6 was co-expressed with a series of modified mammalian enzymes involved in the processing of complex N-glycans. Using wild type (WT) plants and the glycosylation mutant ΔXTFT that synthesizes human like biantennary N-glycans with terminal N-acetylglucosamine on each branch (GnGn structures) as expression hosts we demonstrate the generation of h-13F6 complex N-glycans with (i) bisected structures, (ii) core α1,6 fucosylation and (iii) β1,4 galactosylated oligosaccharides. In addition we emphasize the significance of precise sub Golgi localization of enzymes for engineering of IgG Fc-glycosylation.

CONCLUSION

The method described here allows the efficient generation of a series of different human-like glycoforms at large homogeneity of virtually any antibody within one week after cDNA delivery to plants. This accelerates follow up functional studies and thus may contribute to study the biological role of N-glycan residues on Fcs and maximizing the clinical efficacy of therapeutic antibodies.

Arabidopsis thaliana alpha1,2-glucosyltransferase (ALG10) is required for efficient N-glycosylation and leaf growth

Assembly of the dolichol-linked oligosaccharide precursor (Glc(3) Man(9) GlcNAc(2) ) is highly conserved among eukaryotes. In contrast to yeast and mammals, little is known about the biosynthesis of dolichol-linked oligosaccharides and the transfer to asparagine residues of nascent polypeptides in plants. To understand the biological function of these processes in plants we characterized the Arabidopsis thaliana homolog of yeast ALG10, the α1,2-glucosyltransferase that transfers the terminal glucose residue to the lipid-linked precursor. Expression of an Arabidopsis ALG10-GFP fusion protein in Nicotiana benthamiana leaf epidermal cells revealed a reticular distribution pattern resembling endoplasmic reticulum (ER) localization. Analysis of lipid-linked oligosaccharides showed that Arabidopsis ALG10 can complement the yeast Δalg10 mutant strain. A homozygous Arabidopsis T-DNA insertion mutant (alg10-1) accumulated mainly lipid-linked Glc(2) Man(9) GlcNAc(2) and displayed a severe protein underglycosylation defect. Phenotypic analysis of alg10-1 showed that mutant plants have altered leaf size when grown in soil. Moreover, the inactivation of ALG10 in Arabidopsis resulted in the activation of the unfolded protein response, increased salt sensitivity and suppression of the phenotype of α-glucosidase I-deficient plants. In summary, these data show that Arabidopsis ALG10 is an ER-resident α1,2-glucosyltransferase that is required for lipid-linked oligosaccharide biosynthesis and subsequently for normal leaf development and abiotic stress response.

The two endo-β-N-acetylglucosaminidase genes from Arabidopsis thaliana encode cytoplasmic enzymes controlling free N-glycan levels

Endo-β-N-acetylglucosaminidases (ENGases) cleave N-glycans from proteins and/or peptides by hydrolyzing the O-glycosidic linkage between the two core-N-acetylglucosamine (GlcNAc) residues. Although, two homologous genes potentially encoding ENGases have been identified in Arabidopsis thaliana, their respective substrate specificity, their subcellular and their organ specific localization was hitherto unknown. In order to investigate the role of ENGases in this model plant species, we transiently expressed the two A. thaliana genes in Nicotiana benthamiana and determined the substrate specificities, as well as the Km values, of the purified recombinant enzymes. The assumed predominantly cytosolic localisation of both enzymes, here referred to as AtENGase85A and AtENGase85B, was determined by confocal microscopy of plant leaves expressing the respective GFP-fusion constructs. For the individual characterization of the two enzymes expression patterns in planta, single knock-out plants were selected for both genes. Although both enzymes are present in most organs, only AtENGase85A (At5g05460) was expressed in stems and no ENGase activity was detected in siliques. A double knock-out was generated by crossing but-like single knock-out plants-no apparent phenotype was observed. In contrast, in this double knock-out, free N-glycans carrying a single GlcNAc at the reducing end are completely absent and their counterparts with two GlcNAc-visible only at a trace level in wild type-accumulated dramatically.

Characterization and scope of S-layer protein O-glycosylation in Tannerella forsythia

Cell surface glycosylation is an important element in defining the life of pathogenic bacteria. Tannerella forsythia is a Gram-negative, anaerobic periodontal pathogen inhabiting the subgingival plaque biofilms. It is completely covered by a two-dimensional crystalline surface layer (S-layer) composed of two glycoproteins. Although the S-layer has previously been shown to delay the bacterium's recognition by the innate immune system, we characterize here the S-layer protein O-glycosylation as a potential virulence factor. The T. forsythia S-layer glycan was elucidated by a combination of electrospray ionization-tandem mass spectrometry and nuclear magnetic resonance spectroscopy as an oligosaccharide with the structure 4-Me-β-ManpNAcCONH(2)-(1→3)-[Pse5Am7Gc-(2→4)-]-β-ManpNAcA-(1→4)-[4-Me-α-Galp-(1→2)-]-α-Fucp-(1→4)-[-α-Xylp-(1→3)-]-β-GlcpA-(1→3)-[-β-Digp-(1→2)-]-α-Galp, which is O-glycosidically linked to distinct serine and threonine residues within the three-amino acid motif (D)(S/T)(A/I/L/M/T/V) on either S-layer protein. This S-layer glycan obviously impacts the life style of T. forsythia because increased biofilm formation of an UDP-N-acetylmannosaminuronic acid dehydrogenase mutant can be correlated with the presence of truncated S-layer glycans. We found that several other proteins of T. forsythia are modified with that specific oligosaccharide. Proteomics identified two of them as being among previously classified antigenic outer membrane proteins that are up-regulated under biofilm conditions, in addition to two predicted antigenic lipoproteins. Theoretical analysis of the S-layer O-glycosylation of T. forsythia indicates the involvement of a 6.8-kb gene locus that is conserved among different bacteria from the Bacteroidetes phylum. Together, these findings reveal the presence of a protein O-glycosylation system in T. forsythia that is essential for creating a rich glycoproteome pinpointing a possible relevance for the virulence of this bacterium.

Silencing β1,2-xylosyltransferase in Transgenic Tomato Fruits Reveals xylose as Constitutive Component of Ige-Binding Epitopes

Complex plant N-glycans containing β1,2-xylose and core α1,3-fucose are regarded as the major class of the so-called "carbohydrate cross-reactive determinants" reactive with IgE antibodies in sera of many allergic patients, but their clinical relevance is still under debate. Plant glycosyltransferases, β1,2-xylosyltransferase (XylT), and core α1,3-fucosyltransferase (FucT) are responsible for the transfer of β1,2-linked xylose and core α1,3-linked fucose residues to N-glycans of glycoproteins, respectively. To test the clinical relevance of β1,2-xylose-containing epitopes, expression of the tomato β1,2-xylosyltransferase was down-regulated by RNA interference (RNAi) in transgenic plants. Fruits harvested from these transgenic plants were analyzed for accumulation of XylT mRNA, abundance of β1,2-xylose epitopes and their allergenic potential. Based on quantitative real-time PCR analysis XylT mRNA levels were reduced up to 10-fold in independent transgenic lines as compared to untransformed control, whereas no xylosylated N-glycans could be revealed by MS analysis. Immunoblotting using anti-xylose-specific IgG antibodies revealed a strong reduction of β1,2-xylose-containing epitopes. Incubating protein extracts from untransformed controls and XylT_RNAi plants with sera from tomato allergic patients showed a patient-specific reduction in IgE-binding, indicating a reduced allergenic potential of XylT_RNAi tomato fruits, in vitro. To elucidate the clinical relevance of β1,2-xylose-containing complex N-glycans skin prick tests were performed demonstrating a reduced responsiveness of tomato allergic patients, in vivo. This study provides strong evidence for the clinical relevance of β1,2-xylose-containing epitopes in vivo.

Inconsistent results of diagnostic tools hamper the differentiation between bee and vespid venom allergy

Background

Double sensitization (DS) to bee and vespid venom is frequently observed in the diagnosis of hymenoptera venom allergy, but clinically relevant DS is rare. Therefore it is sophisticated to choose the relevant venom for specific immunotherapy and overtreatment with both venoms may occur. We aimed to compare currently available routine diagnostic tests as well as experimental tests to identify the most accurate diagnostic tool.

Methods

117 patients with a history of a bee or vespid allergy were included in the study. Initially, IgE determination by the ImmunoCAP, by the Immulite, and by the ADVIA Centaur, as well as the intradermal test (IDT) and the basophil activation test (BAT) were performed. In 72 CAP double positive patients, individual IgE patterns were determined by western blot inhibition and component resolved diagnosis (CRD) with rApi m 1, nVes v 1, and nVes v 5.

Results

Among 117 patients, DS was observed in 63.7% by the Immulite, in 61.5% by the CAP, in 47.9% by the IDT, in 20.5% by the ADVIA, and in 17.1% by the BAT. In CAP double positive patients, western blot inhibition revealed CCD-based DS in 50.8%, and the CRD showed 41.7% of patients with true DS. Generally, agreement between the tests was only fair and inconsistent results were common.

Conclusion

BAT, CRD, and ADVIA showed a low rate of DS. However, the rate of DS is higher than expected by personal history, indicating that the matter of clinical relevance is still not solved even by novel tests. Furthermore, the lack of agreement between these tests makes it difficult to distinguish between bee and vespid venom allergy. At present, no routinely employed test can be regarded as gold standard to find the clinically relevant sensitization.

N-glycosylation engineering of plants for the biosynthesis of glycoproteins with bisected and branched complex N-glycans

Glycoengineering is increasingly being recognized as a powerful tool to generate recombinant glycoproteins with a customized N-glycosylation pattern. Here, we demonstrate the modulation of the plant glycosylation pathway toward the formation of human-type bisected and branched complex N-glycans. Glycoengineered Nicotiana benthamiana lacking plant-specific N-glycosylation (i.e. β1,2-xylose and core α1,3-fucose) was used to transiently express human erythropoietin (hEPO) and human transferrin (hTF) together with modified versions of human β1,4-mannosyl-β1,4-N-acetylglucosaminyltransferase (GnTIII), α1,3-mannosyl-β1,4-N-acetylglucosaminyltransferase (GnTIV) and α1,6-mannosyl-β1,6-N-acetylglucosaminyltransferase (GnTV). hEPO was expressed as a fusion to the IgG-Fc domain (EPO-Fc) and purified via protein A affinity chromatography. Recombinant hTF was isolated from the intracellular fluid of infiltrated plant leaves. Mass spectrometry-based N-glycan analysis of hEPO and hTF revealed the quantitative formation of bisected (GnGnbi) and tri- as well as tetraantennary complex N-glycans (Gn[GnGn], [GnGn]Gn and [GnGn][GnGn]). Co-expression of GnTIII together with GnTIV and GnTV resulted in the efficient generation of bisected tetraantennary complex N-glycans. Our results show the generation of recombinant proteins with human-type N-glycosylation at great uniformity. The strategy described here provides a robust and straightforward method for producing mammalian-type N-linked glycans of defined structures on recombinant glycoproteins, which can advance glycoprotein research and accelerate the development of protein-based therapeutics.