In vitro galactosylation of human IgG at 1 kg scale using recombinant galactosyltransferase

Cell-free N-glycosylation of peptides using synthetic lipid-linked hybrid and complex N-glycans

Cell-free, chemoenzymatic platforms are emerging technologies towards generating glycoconjugates with defined and homogeneous glycoforms. Recombinant oligosaccharyltransferases can be applied to glycosylate "empty," i.e., aglycosyalted, peptides and proteins. While bacterial oligosaccharlytransferases have been extensively investigated, only recently a recombinant eukaryotic single-subunit oligosaccharyltransferase has been successfully used to in vitro N-glycosylate peptides. However, its applicability towards synthesizing full-length glycoproteins and utilizing glycans beyond mannose-type glycans for the transfer have not be determined. Here, we show for the first time the synthesis of hybrid- and complex-type glycans using synthetic lipid carriers as substrates for in vitro N-glycosylation reactions. For this purpose, transmembrane-deleted human β-1,2 N-acetylglucosamintransferase I and II (MGAT1ΔTM and MGAT2ΔTM) and β-1,4-galactosyltransferase (GalTΔTM) have been expressed in Escherichia coli and used to extend an existing multi-enzyme cascade. Both hybrid and agalactosylated complex structures were transferred to the N-glycosylation consensus sequence of peptides (10 amino acids: G-S-D-A-N-Y-T-Y-T-Q) by the recombinant oligosaccharyltransferase STT3A from Trypanosoma brucei.

Glycoprotein In Vitro N-Glycan Processing Using Enzymes Expressed in E. coli

Protein N-glycosylation is a common post-translational modification that plays significant roles on the structure, property, and function of glycoproteins. Due to N-glycan heterogeneity of naturally occurring glycoproteins, the functions of specific N-glycans on a particular glycoprotein are not always clear. Glycoprotein in vitro N-glycan engineering using purified recombinant enzymes is an attractive strategy to produce glycoproteins with homogeneous N-glycoforms to elucidate the specific functions of N-glycans and develop better glycoprotein therapeutics. Toward this goal, we have successfully expressed in E. coli glycoside hydrolases and glycosyltransferases from bacterial and human origins and developed a robust enzymatic platform for in vitro processing glycoprotein N-glycans from high-mannose-type to α2-6- or α2-3-disialylated biantennary complex type. The recombinant enzymes are highly efficient in step-wise or one-pot reactions. The platform can find broad applications in N-glycan engineering of therapeutic glycoproteins.

Cell-Free Multi-Enzyme Synthesis and Purification of Uridine Diphosphate Galactose

High costs and low availability of UDP-galactose hampers the enzymatic synthesis of valuable oligosaccharides such as human milk oligosaccharides. Here, we report the development of a platform for the scalable, biocatalytic synthesis and purification of UDP-galactose. UDP-galactose was produced with a titer of 48 mM (27.2 g/L) in a small-scale batch process (200 μL) within 24 h using 0.02 genzyme /gproduct . Through in-situ ATP regeneration, the amount of ATP (0.6 mM) supplemented was around 240-fold lower than the stoichiometric equivalent required to achieve the final product yield. Chromatographic purification using porous graphic carbon adsorbent yielded UDP-galactose with a purity of 92 %. The synthesis was transferred to 1 L preparative scale production in a stirred tank bioreactor. To further reduce the synthesis costs here, the supernatant of cell lysates was used bypassing expensive purification of enzymes. Here, 23.4 g/L UDP-galactose were produced within 23 h with a synthesis yield of 71 % and a biocatalyst load of 0.05 gtotal_protein /gproduct . The costs for substrates per gram of UDP-galactose synthesized were around 0.26 €/g.

Glycan engineering reveals interrelated effects of terminal galactose and core fucose on antibody-dependent cell-mediated cytotoxicity

Antibody-dependent cell-mediated cytotoxicity (ADCC) has been identified as one of the potentially critical effector functions underlying the clinical efficacy of some therapeutic immunoglobin G1 (IgG1) antibodies. It has been well established that higher levels of afucosylated N-linked glycan structures on the Fc region enhance the IgG binding affinity to the FcγIIIa receptor and lead to increased ADCC activity. However, whether terminal galactosylation of an IgG1 impacts its ADCC activity is less understood. Here, we used a new strategy for glycan enrichment and remodeling to study the impact of terminal galactose on ADCC activity for therapeutic IgG1s. Our results indicate that the degree of influence of terminal galactose on in vitro ADCC activity depends on the presence or absence of the core fucose, which is typically linked to the first N-acetyl glucosamine residue of an N-linked glycosylation core structure. Specifically, terminal galactose on afucosylated IgG1 mAbs enhanced ADCC activity with impact coefficients (ADCC%/Gal%) more than 20, but had minimal influence on ADCC activity on fucosylated structures with impact coefficient in the range of 0.1-0.2. Knowledge gained here can be used to guide product and process development activities for biotherapeutic antibodies that require effector function for efficacy, and also highlight the complexity in modulating the immune response through N-linked glycosylation of antibodies.

Comparison of α2,6-sialyltransferases for sialylation of therapeutic proteins

The development of therapeutic proteins for the treatment of numerous diseases is one of the fastest growing areas of biotechnology. Therapeutic efficacy and serum half-life are particularly important, and these properties rely heavily on the glycosylation state of the protein. Expression systems to produce authentically fully glycosylated therapeutic proteins with appropriate terminal sialic acids are not yet perfected. The in vitro modification of therapeutic proteins by recombinant sialyltransferases offers a promising and elegant strategy to overcome this problem. Thus, the detailed expression and characterization of sialyltransferases for completion of the glycan chains is of great interest to the community. We identified a novel α2,6-sialyltransferase from Helicobacter cetorum and compared it to the human ST6Gal1 and a Photobacterium sp. sialyltransferase using glycoprotein substrates in a 96-well microtiter-plate-based assay. We demonstrated that the recombinant α2,6-sialyltransferase from H. cetorum is an excellent catalyst for modification of N-linked glycans of different therapeutic proteins.

Impact of N-glycosylation on Fcγ receptor / IgG interactions: unravelling differences with an enhanced surface plasmon resonance biosensor assay based on coiled-coil interactions

The N-glycosylation profile of immunoglobulin G (IgG) is considered a critical quality attribute due to its impact on IgG-Fc gamma receptor (FcγR) interactions, which subsequently affect antibody-dependent cell-based immune responses. In this study, we investigated the impact of the FcγR capture method, as well as FcγR N-glycosylation, on the kinetics of interaction with various glycoforms of trastuzumab (TZM) in a surface plasmon resonance (SPR) biosensor assay. More specifically, we developed a novel strategy based on coiled-coil interactions for the stable and oriented capture of coil-tagged FcγRs at the biosensor surface. Coil-tagged FcγR capture outperformed all other capture strategies applied to the SPR study of IgG-FcγR interactions, as the robustness and reproducibility of the assay and the shelf life of the biosensor chip were excellent (> 1,000 IgG injections with the same biosensor surface). Coil-tagged FcγRs displaying different N-glycosylation profiles were generated either by different expression systems, in vitro glycoengineering or by size-exclusion chromatography, and roughly characterized by lectin blotting. Of salient interest, the overlay of their kinetics of interaction with several TZM glycoforms revealed key differences on both association and dissociation kinetics, confirming a complex influence of the FcγR N-glycosylation and its inherent heterogeneity upon receptor interaction with mAbs. This work is thus an important step towards better understanding of the impact of glycosylation upon binding of IgGs, either natural or engineered, to their receptors.

Chemoenzymatic Methods for the Synthesis of Glycoproteins

Glycosylation is one of the most prevalent posttranslational modifications that profoundly affects the structure and functions of proteins in a wide variety of biological recognition events. However, the structural complexity and heterogeneity of glycoproteins, usually resulting from the variations of glycan components and/or the sites of glycosylation, often complicates detailed structure-function relationship studies and hampers the therapeutic applications of glycoproteins. To address these challenges, various chemical and biological strategies have been developed for producing glycan-defined homogeneous glycoproteins. This review highlights recent advances in the development of chemoenzymatic methods for synthesizing homogeneous glycoproteins, including the generation of various glycosynthases for synthetic purposes, endoglycosidase-catalyzed glycoprotein synthesis and glycan remodeling, and direct enzymatic glycosylation of polypeptides and proteins. The scope, limitation, and future directions of each method are discussed.

Site-specific immobilization of endoglycosidases for streamlined chemoenzymatic glycan remodeling of antibodies

Chemoenzymatic glycan remodeling of antibodies using an endoglycosidase and its mutant is emerging as an attractive approach for producing homogeneous antibody glycoforms. We report in this paper a site-specific covalent immobilization of the endoglycosidases (Endo-S2 and its glycosynthase mutant D184M) using a recombinant microbial transglutaminase (MTG) and evaluation of the immobilized enzymes in deglycosylation and glycosylation of a therapeutic antibody. The site-specific covalent immobilization was achieved by introduction of a Q-tag at the C-terminus of the recombinant enzymes followed by conjugation of the enzymes to a primary amine-containing solid support through MTG-catalyzed transglutamination. Using rituximab as a model system, we found that the Endo-S2 wild-type and D184M glycosynthase mutant immobilized by this approach were efficient in the two step antibody glycan remodeling to generate homogeneous antibody glycoforms. Notably using the covalently immobilized enzymes can efficiently avoid the need of intermediate purification and eliminate the residual contamination of wild type enzyme for product hydrolysis, thus streamlining the chemoenzymatic Fc glycan remodeling of antibodies.

One-pot N-glycosylation remodeling of IgG with non-natural sialylglycopeptides enables glycosite-specific and dual-payload antibody-drug conjugates

Chemoenzymatic transglycosylation catalyzed by endo-S mutants is a powerful tool for in vitro glycoengineering of therapeutic antibodies. In this paper, we report a one-pot chemoenzymatic synthesis of glycoengineered Herceptin using an egg-yolk sialylglycopeptide (SGP) substrate. Combining this one-pot strategy with novel non-natural SGP derivatives carrying azido or alkyne tags, glycosite-specific conjugation was enabled for the development of new antibody-drug conjugates (ADCs). The site-specific ADCs and semi-site-specific dual-drug ADCs were successfully achieved and characterized with SDS-PAGE, intact antibody or ADC mass spectrometry analysis, and PNGase-F digestion analysis. Cancer cell cytotoxicity assay revealed that small-molecule drug release of these ADCs relied on the cleavable Val-Cit linker fragment embedded in the structure. These results represent a new approach for glycosite-specific and dual-drug ADC design and rapid synthesis, and also provide the structural requirement for their biologic activities.

Solid-Phase Enzymatic Remodeling Produces High Yields of Single Glycoform Antibodies

Antibodies are synthesized in mammalian cell culture as heterogeneous mixtures of glycoforms. Production of single glycoforms remains a challenge despite their value as therapeutics. The authors report a method of sequential enzymatic-based changes to antibodies while immobilized on an affinity column. Various antibodies (monoclonal and polyclonal) are isolated on Protein A or G columns and their glycans modified by sequential addition of enzymes for a desired transformation. Galactosylated antibodies (>90% yield) are produced by a one stage reaction process with sialidase to remove any sialic acid residues and addition of galactose with galactosyltransferase and UDP-Gal. Sialylated antibodies (>90%) are produced by a 2 stage conversion involving α(2,3) sialidase and galactosyltransferase followed by treatment with α(2,6) sialyltransferase in the presence of CMP-NANA. By this method, >90% of a disialylated human-llama antibody (EG2-hFc) and equimolar quantities of monosialylated and disialylated forms of human antibodies (αIL8-hFc and human polyclonal) are produced. Such high levels of sialylation are very difficult to obtain by typical cell culture methods. This method of transformation while the antibody is held on a solid-phase column is superior to previous methods because it allows a series of enzymatic steps without the need for intermediate purification. This is an efficient and rapid method to generate therapeutic antibodies with predefined glycosylation profiles. This should also assist in investigating the structure-function relationship of antibody glycans to find the desired glycosylation profile for high functional activity. With further optimization the method can be used to modify antibodies in large-scale manufacturing.

Chemoenzymatic synthesis of glycoengineered IgG antibodies and glycosite-specific antibody-drug conjugates

Glycoengineered therapeutic antibodies and glycosite-specific antibody-drug conjugates (gsADCs) have generated great interest among researchers because of their therapeutic potential. Endoglycosidase-catalyzed in vitro glycoengineering technology is a powerful tool for IgG Fc (fragment cystallizable) N-glycosylation remodeling. In this protocol, native heterogeneously glycosylated IgG N-glycans are first deglycosylated with a wild-type endoglycosidase. Next, a homogeneous N-glycan substrate, presynthesized as described here, is attached to the remaining N-acetylglucosamine (GlcNAc) of IgG, using a mutant endoglycosidase (also called endoglycosynthase) that lacks hydrolytic activity but possesses transglycosylation activity for glycoengineering. Compared with in vivo glycoengineering technologies and the glycosyltransferase-enabled in vitro engineering method, the current approach is robust and features quantitative yield, homogeneous glycoforms of produced antibodies and ADCs, compatibility with diverse natural and non-natural glycan structures, convenient exploitation of native IgG as the starting material, and a well-defined conjugation site for antibody modifications. Potential applications of this method cover a broad scope of antibody-related research, including the development of novel glycoengineered therapeutic antibodies with enhanced efficacy, site-specific antibody-drug conjugation, and site-specific modification of antibodies for fluorescent labeling, PEGylation, protein cross-linking, immunoliposome formation, and so on, without loss of antigen-binding affinity. It takes 5-8 d to prepare the natural or modified N-glycan substrates, 3-4 d to engineer the IgG N-glycosylation, and 2-5 d to synthesize the small-molecule toxins and prepare the gsADCs.

Integrated process design for biocatalytic synthesis by a Leloir Glycosyltransferase: UDP-glucose production with sucrose synthase

Nucleotide sugar-dependent ("Leloir") glycosyltransferases (GTs), represent a new paradigm for the application of biocatalytic glycosylations to the production of fine chemicals. However, it remains to be shown that GT processes meet the high efficiency targets of industrial biotransformations. We demonstrate in this study of uridine-5'-diphosphate glucose (UDP-glc) production by sucrose synthase (from Acidithiobacillus caldus) that a holistic process design, involving coordinated development of biocatalyst production, biotransformation, and downstream processing (DSP) was vital for target achievement at ∼100 g scale synthesis. Constitutive expression in Escherichia coli shifted the recombinant protein production mainly to the stationary phase and enhanced the specific enzyme activity to a level (∼480 U/g_{cell dry weight} ) suitable for whole-cell biotransformation. The UDP-glc production had excellent performance metrics of ∼100 g_product /L, 86% yield (based on UDP), and a total turnover number of 103 g_UDP-glc /g_{cell dry weight} at a space-time yield of 10 g/L/h. Using efficient chromatography-free DSP, the UDP-glc was isolated in a single batch with ≥90% purity and in 73% isolated yield. Overall, the process would allow production of ∼0.7 kg of isolated product/L E. coli bioreactor culture, thus demonstrating how integrated process design promotes the practical use of a GT conversion. Biotechnol. Bioeng. 2017;114: 924-928. © 2016 Wiley Periodicals, Inc.

Sucrose synthase: A unique glycosyltransferase for biocatalytic glycosylation process development

Sucrose synthase (SuSy, EC 2.4.1.13) is a glycosyltransferase (GT) long known from plants and more recently discovered in bacteria. The enzyme catalyzes the reversible transfer of a glucosyl moiety between fructose and a nucleoside diphosphate (NDP) (sucrose+NDP↔NDP-glucose+fructose). The equilibrium for sucrose conversion is pH dependent, and pH values between 5.5 and 7.5 promote NDP-glucose formation. The conversion of a bulk chemical to high-priced NDP-glucose in a one-step reaction provides the key aspect for industrial interest. NDP-sugars are important as such and as key intermediates for glycosylation reactions by highly selective Leloir GTs. SuSy has gained renewed interest as industrially attractive biocatalyst, due to substantial scientific progresses achieved in the last few years. These include biochemical characterization of bacterial SuSys, overproduction of recombinant SuSys, structural information useful for design of tailor-made catalysts, and development of one-pot SuSy-GT cascade reactions for production of several relevant glycosides. These advances could pave the way for the application of Leloir GTs to be used in cost-effective processes. This review provides a framework for application requirements, focusing on catalytic properties, heterologous enzyme production and reaction engineering. The potential of SuSy biocatalysis will be presented based on various biotechnological applications: NDP-sugar synthesis; sucrose analog synthesis; glycoside synthesis by SuSy-GT cascade reactions.

In vitro glycoengineering of IgG1 and its effect on Fc receptor binding and ADCC activity

The importance and effect of Fc glycosylation of monoclonal antibodies with regard to biological activity is widely discussed and has been investigated in numerous studies. Fc glycosylation of monoclonal antibodies from current production systems is subject to batch-to-batch variability. If there are glycosylation changes between different batches, these changes are observed not only for one but multiple glycan species. Therefore, studying the effect of distinct Fc glycan species such as galactosylated and sialylated structures is challenging due to the lack of well-defined differences in glycan patterns of samples used. In this study, the influence of IgG1 Fc galactosylation and sialylation on its effector functions has been investigated using five different samples which were produced from one single drug substance batch by in vitro glycoengineering. This sample set comprises preparations with minimal and maximal galactosylation and different levels of sialylation of fully galactosylated Fc glycans. Among others, Roche developed the glycosyltransferase enzyme sialyltransferase which was used for the in vitro glycoengineering activities at medium scale. A variety of analytical assays, including Surface Plasmon Resonance and recently developed FcγR affinity chromatography, as well as an optimized cell-based ADCC assay were applied to investigate the effect of Fc galactosylation and sialylation on the in vitro FcγRI, IIa, and IIIa receptor binding and ADCC activity of IgG1. The results of our studies do not show an impact, neither positive nor negative, of sialic acid- containing Fc glycans of IgG1 on ADCC activity, FcγRI, and RIIIa receptors, but a slightly improved binding to FcγRIIa. Furthermore, we demonstrate a galactosylation-induced positive impact on the binding activity of the IgG1 to FcγRIIa and FcγRIIIa receptors and ADCC activity.

Enzyme-mediated methodology for the site-specific radiolabeling of antibodies based on catalyst-free click chemistry

An enzyme- and click chemistry-mediated methodology for the site-selective radiolabeling of antibodies on the heavy chain glycans has been developed and validated. To this end, a model system based on the prostate specific membrane antigen-targeting antibody J591, the positron-emitting radiometal (89)Zr, and the chelator desferrioxamine has been employed. The methodology consists of four steps: (1) the removal of sugars on the heavy chain region of the antibody to expose terminal N-acetylglucosamine residues; (2) the incorporation of azide-modified N-acetylgalactosamine monosaccharides into the glycans of the antibody; (3) the catalyst-free click conjugation of desferrioxamine-modified dibenzocyclooctynes to the azide-bearing sugars; and (4) the radiolabeling of the chelator-modified antibody with (89)Zr. The site-selective labeling methodology has proven facile, reproducible, and robust, producing (89)Zr-labeled radioimmunoconjguates that display high stability and immunoreactivity in vitro (>95%) in addition to highly selective tumor uptake (67.5 ± 5.0%ID/g) and tumor-to-background contrast in athymic nude mice bearing PSMA-expressing subcutaneous LNCaP xenografts. Ultimately, this strategy could play a critical role in the development of novel well-defined and highly immunoreactive radioimmunoconjugates for both the laboratory and clinic.

The impact of glycosylation on monoclonal antibody conformation and stability

Antibody glycosylation is a common post-translational modification and has a critical role in antibody effector function. The use of glycoengineering to produce antibodies with specific glycoforms may be required to achieve the desired therapeutic efficacy. However, the modified molecule could have unusual behavior during development due to the alteration of its intrinsic properties and stability. In this study, we focused on the differences between glycosylated and deglycosylated antibodies, as aglycosyl antibodies are often chosen when effector function is not desired or unimportant. We selected three human IgG1 antibodies and used PNGase F to remove their oligosaccharide chains. Although there were no detected secondary or tertiary structural changes after deglycosylation, other intrinsic properties of the antibody were altered with the removal of oligosaccharide chains in the Fc region. The apparent molecular hydrodynamic radius increased after deglycosylation based on size-exclusion chromatography analysis. Deglycosylated antibodies exhibited less thermal stability for the CH2 domain and less resistance to GdnHCl induced unfolding. Susceptibility to proteolytic cleavage demonstrated that the deglycosylated version was more susceptible to papain. An accelerated stability study revealed that deglycosylated antibodies had higher aggregation rates. These changes may impact the development of aglycosyl antibody biotherapeutics.

Glycosyltransferases as biocatalysts

Glycosyltransferases are useful synthetic tools for the preparation of natural oligosaccharides, glycoconjugates and their analogues. High expression levels of recombinant enzymes have allowed their use in multi-step reactions, on mg to multi-gram scales. Since glycosyltransferases are tolerant with respect to utilizing modified donors and acceptor substrates they can be used to prepare oligosaccharide analogues and for diversification of natural products. New sources of enzymes are continually discovered as genomes are sequenced and they are annotated in the Carbohydrate Active Enzyme (CAZy) glycosyltransferase database. Glycosyltransferase mutagenesis, domain swapping and metabolic pathway engineering to change reaction specificity and product diversification are increasingly successful due to advances in structure-function studies and high throughput screening methods.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: an update for the period 2005-2006

This review is the fourth update of the original review, published in 1999, on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2006. The review covers fundamental studies, fragmentation of carbohydrate ions, method developments, and applications of the technique to the analysis of different types of carbohydrate. Specific compound classes that are covered include carbohydrate polymers from plants, N- and O-linked glycans from glycoproteins, glycated proteins, glycolipids from bacteria, glycosides, and various other natural products. There is a short section on the use of MALDI-TOF mass spectrometry for the study of enzymes involved in glycan processing, a section on industrial processes, particularly the development of biopharmaceuticals and a section on the use of MALDI-MS to monitor products of chemical synthesis of carbohydrates. Large carbohydrate-protein complexes and glycodendrimers are highlighted in this final section.

Post-translational modifications differentially affect IgG1 conformation and receptor binding

Post-translational modifications (PTMs) can have profound effects on protein structure and protein dynamics and thereby can influence protein function. To understand and connect PTM-induced functional differences with any resulting conformational changes, the conformational changes must be detected and localized to specific parts of the protein. We illustrate these principles here with a study of the functional and conformational changes that accompany modifications to a monoclonal immunoglobulin gamma1 (IgG1) antibody. IgG1s are large and heterogeneous proteins capable of incorporating a multiplicity of PTMs both in vivo and in vitro. For many IgG1s, these PTMs can play a critical role in affecting conformation, biological function, and the ability of the antibody to initiate a potential adverse biological response. We investigated the impact of differential galactosylation, methionine oxidation, and fucosylation on solution conformation using hydrogen/deuterium exchange mass spectrometry and probed the effects of IgG1 binding to the FcgammaRIIIa receptor. The results showed that methionine oxidation and galactosylation both impact IgG1 conformation, whereas fucosylation appears to have little or no impact to the conformation. FcgammaRIIIa binding was strongly influenced by both the glycan structure/composition (namely galactose and fucose) and conformational changes that were induced by some of the modifications.

Electrospray ionization quadrupole ion-mobility time-of-flight mass spectrometry as a tool to distinguish the lot-to-lot heterogeneity in N-glycosylation profile of the therapeutic monoclonal antibody trastuzumab

Monoclonal antibodies are typically glycosylated at asparagine residues in the Fc domain, and glycosylation heterogeneity at the Fc sites is well known. This paper presents a method for rapid analysis of glycosylation profile of the therapeutic monoclonal antibody trastuzumab from different production batches using electrospray quadrupole ion-mobility time-of-flight mass spectrometry (ESI-Q-IM-TOF). The global glycosylation profile for each production batch was obtained by a fast LC-MS analysis, and comparisons of the glycoprofiles of trastuzumab from different lots were made based on the deconvoluted intact mass spectra. Furthermore, the heterogeneity at each glycosylation site was characterized at the reduced antibody level and at the isolated glycopeptide level. The glycosylation site and glycan structures were confirmed by performing a time-aligned-parallel fragmentation approach using the unique dual-collision cell design of the instrument and the incorporated ion-mobility separation function. Four different production batches of trastuzumab were analyzed and compared in terms of global glycosylation profiles as well as the heterogeneity at each glycosylation site. The results show that each batch of trastuzumab shares the same types of glycoforms but relative abundance of each glycoforms is varied.

Glycosyltransferase-programmed stereosubstitution (GPS) to create HCELL: engineering a roadmap for cell migration

During evolution of the vertebrate cardiovascular system, the vast endothelial surface area associated with branching vascular networks mandated the development of molecular processes to efficiently and specifically recruit circulating sentinel host defense cells and tissue repair cells at localized sites of inflammation/tissue injury. The forces engendered by high-velocity blood flow commensurately required the evolution of specialized cell surface molecules capable of mediating shear-resistant endothelial adhesive interactions, thus literally capturing relevant cells from the blood stream onto the target endothelial surface and permitting subsequent extravasation. The principal effectors of these shear-resistant binding interactions comprise a family of C-type lectins known as 'selectins' that bind discrete sialofucosylated glycans on their respective ligands. This review explains the 'intelligent design' of requisite reagents to convert native CD44 into the sialofucosylated glycoform known as hematopoietic cell E-/L-selectin ligand (HCELL), the most potent E-selectin counter-receptor expressed on human cells, and will describe how ex vivo glycan engineering of HCELL expression may open the 'avenues' for the efficient vascular delivery of cells for a variety of cell therapies.

The production of biopharmaceuticals in plant systems

Biopharmaceuticals present the fastest growing segment in the pharmaceutical industry, with an ever widening scope of applications. Whole plants as well as contained plant cell culture systems are being explored for their potential as cheap, safe, and scalable production hosts. The first plant-derived biopharmaceuticals have now reached the clinic. Many biopharmaceuticals are glycoproteins; as the Golgi N-glycosylation machinery of plants differs from the mammalian machinery, the N-glycoforms introduced on plant-produced proteins need to be taken into consideration. Potent systems have been developed to change the plant N-glycoforms to a desired or even superior form compared to the native mammalian N-glycoforms. This review describes the current status of biopharmaceutical production in plants for industrial applications. The recent advances and tools which have been utilized to generate glycoengineered plants are also summarized and compared with the relevant mammalian systems whenever applicable.

N-linked glycosylation is an important parameter for optimal selection of cell lines producing biopharmaceutical human IgG

We studied the variations in N-linked glycosylation of human IgG molecules derived from 105 different stable cell lines each expressing one of the six different antibodies. Antibody expression was based on glutamine synthetase selection technology in suspension growing CHO-K1SV cells. The glycans detected on the Fc fragment were mainly of the core-fucosylated complex type containing zero or one galactose and little to no sialic acid. The glycosylation was highly consistent for the same cell line when grown multiple times, indicating the robustness of the production and glycan analysis procedure. However, a twofold to threefold difference was observed in the level of galactosylation and/or non-core-fucosylation between the 105 different cell lines, suggesting clone-to-clone variation. These differences may change the Fc-mediated effector functions by such antibodies. Large variation was also observed in the oligomannose-5 glycan content, which, when present, may lead to undesired rapid clearance of the antibody in vivo. Statistically significant differences were noticed between the various glycan parameters for the six different antibodies, indicating that the variable domains and/or light chain isotype influence Fc glycosylation. The glycosylation altered when batch production in shaker was changed to fed-batch production in bioreactor, but was consistent again when the process was scaled from 400 to 5,000 L. Taken together, the observed clone-to-clone glycosylation variation but batch-to-batch consistency provides a rationale for selection of optimal production cell lines for large-scale manufacturing of biopharmaceutical human IgG.

Emerging methods for the production of homogeneous human glycoproteins

Most circulating human proteins exist as heterogeneously glycosylated variants (glycoforms) of an otherwise homogeneous polypeptide. Though glycan heterogeneity is most likely important to glycoprotein function, the preparation of homogeneous glycoforms is important both for the study of the consequences of glycosylation and for therapeutic purposes. This review details selected approaches to the production of homogeneous human N- and O-linked glycoproteins with human-type glycans. Particular emphasis is placed on recent developments in the engineering of glycosylation pathways within yeast and bacteria for in vivo production, and on the in vitro remodeling of glycoproteins by enzymatic means. The future of this field is very exciting.

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.

Site-specific linking of biomolecules via glycan residues using glycosyltransferases

The structural information on glycosyltransferases has revealed that the sugar-donor specificity of these enzymes can be broadened to include modified sugars with a chemical handle that can be utilized for conjugation chemistry. Substitution of Tyr289 to Leu in the catalytic pocket of bovine beta-1,4-galactosyltransferase generates a novel glycosyltransferase that can transfer not only Gal but also GalNAc or a C2-modified galactose that has a chemical handle, from the corresponding UDP-derivatives, to the non-reducing end GlcNAc residue of a glycoconjugate. Similarly, the wild-type polypeptide-N-acetyl-galactosaminyltransferase, which naturally transfers GalNAc from UDP-GalNAc, can also transfer C2-modified galactose with a chemical handle from its UDP-derivative to the Ser/Thr residue of a polypeptide acceptor substrate that is tagged as a fusion peptide to a non-glycoprotein. The potential of wild-type and mutant glycosyltransferases to produce glycoconjugates carrying sugar moieties with chemical handle makes it possible to conjugate biomolecules with orthogonal reacting groups at specific sites. This methodology assists in the assembly of bio-nanoparticles that are useful for developing targeted drug-delivery systems and contrast agents for magnetic resonance imaging.

Refolding of human beta-1-2 GlcNAc transferase (GnT1) and the role of its unpaired Cys 121

Human beta1-2N-acetylglucosaminyltransferase (hGnT1) lacking the first 103 amino acids was expressed as a maltose binding protein (MBP) fusion protein in inclusion bodies (IBs) in Escherichia coli and refolded using an oxido-shuffling method. GnT1 mutants were prepared by replacing a predicted unpaired cysteine (C121) with alanine (C121A), serine (C121S), threonine (C121T) or aspartic acid (C121D). A double mutant R120A/C121H, was generated to mimic Gly14, the Caenorhabditis elegans GnT1 counterpart to hGNT1. Each mutant hGnT1 was constructed as an MBP fusion protein and resultant IBs were isolated and refolded. Wild type hGnT1 and mutants C121A, C121S and R120A/C121H transferred UDP-GlcNAc to the glycoprotein acceptor Man(5)-RNAse B, whereas mutants C121T and C121D were inactive. These findings indicated that cysteine 121 has a structural role in maintaining active site geometry of hGnT1, rather than a catalytic role, and illustrates for the first time the potential utility of E. coli as an expression system for hGnT1.

Modifications of therapeutic proteins: challenges and prospects

The production of therapeutic proteins is one of the fastest growing sectors of the pharmaceutical industry. However, most proteins used in drug therapy require complex post-translational modifications for efficient secretion, drug efficacy and stability. Common protein modifications include variable glycosylation, misfolding and aggregation, oxidation of methionine, deamidation of asparagine and glutamine, and proteolysis. These modifications not only pose challenges for accurate and consistent bioprocessing, but also may have consequences for the patient in that incorrect modifications or aggregation may lead to an immune response to the protein therapeutic. This review provides examples of analytical and preventative advances that have been devised to meet these challenges, and insights into how further advances can improve the efficiency and safety in manufacturing recombinant proteins.

GlycoPEGylation of recombinant therapeutic proteins produced in Escherichia coli

Covalent attachment of polyethylene glycol, PEGylation, has been shown to prolong the half-life and enhance the pharmacodynamics of therapeutic proteins. Current methods for PEGylation, which rely on chemical conjugation through reactive groups on amino acids, often generate isoforms in which PEG is attached at sites that interfere with bioactivity. Here, we present a novel strategy for site-directed PEGylation using glycosyltransferases to attach PEG to O-glycans. The process involves enzymatic GalNAc glycosylation at specific serine and threonine residues in proteins expressed without glycosylation in Escherichia coli, followed by enzymatic transfer of sialic acid conjugated with PEG to the introduced GalNAc residues. The strategy was applied to three therapeutic polypeptides, granulocyte colony stimulating factor (G-CSF), interferon-alpha2b (IFN-alpha2b), and granulocyte/macrophage colony stimulating factor (GM-CSF), which are currently in clinical use.