Effect of glycosylation on antibody function: implications for genetic engineering

Plant-derived mouse IgG monoclonal antibody fused to KDEL endoplasmic reticulum-retention signal is N-glycosylated homogeneously throughout the plant with mostly high-mannose-type N-glycans

Plants are potential hosts for the expression of recombinant glycoproteins intended for therapeutic purposes. However, N-glycans of mammalian glycoproteins produced in transgenic plants differ from their natural counterparts. The use of the endoplasmic reticulum (ER)-retention signal has been proposed to restrict glycosylation of plantibodies to only high-mannose-type N-glycans. Furthermore, little is known about the influence of plant development and growth conditions on N-linked glycosylation. Here, we report a detailed N-glycosylation profiling study of CB.Hep1, a mouse IgG2b monoclonal antibody (mAb) against hepatitis B surface antigen (HBsAg) currently expressed in tobacco plants (Nicotiana tabacum L.). The KDEL ER-retention signal was fused to the C-terminal of both light and heavy chains. The structures of the N-linked glycans of this mAb produced in transgenic tobacco plants at various growth stages were analysed by high-performance liquid chromatography (HPLC) profiling techniques and matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) and compared with those of murine origin. The high-mannose-type oligosaccharides accounted for more than 80% of the total N-glycans, with Man7GlcNAc2 being the most abundant species. Some complex N-glycans bearing xylose and small amounts of oligosaccharides with both xylose and fucose were identified. No appreciable differences were detected when comparing glycosylation at different leaf ages, e.g. from seedling leaves up to 8 weeks old and top or basal leaves of mature plants, or between leaves, stems and whole plants. A strict retention of glycoproteins to ER by the use of the tetrapeptide KDEL was not sufficient, even though the majority of the resulting N-glycosylation was of the high-mannose type. It is highly likely to be dependent on other factors, which are most probably protein specific.

Plant biopharming of monoclonal antibodies

Recent advances in molecular biology and plant biotechnology have shifted the concept of growing crops as a food source to serving as a bioreactor for the production of therapeutic recombinant proteins. Plants are potential biopharming factories because they are capable of producing unlimited numbers and amounts of recombinant proteins safely and inexpensively. In the last two decades, plant production systems have been developed for monoclonal antibody production, which has been useful in passive immunization of viral or bacterial diseases. Recently, a recombinant monoclonal antibody for rabies prophylaxis was produced in transgenic plants. Rabies virus epidemics remain still problematic throughout the world, and adequate treatment has been hampered by the worldwide shortage and high cost of prophylactic antibodies such as HRIG. Successful mass production of this monoclonal antibody in plants might help to overcome these problems. An effective plant production system for recombinant biologicals requires the appropriate heterologous plant expression system, the optimal combination of gene expression regulatory elements, control of post-translational processing of recombinant products, and efficient purification methods for product recovery. This review discusses recent biotechnology developments for plant-derived monoclonal antibodies and discusses these products as a promising approach to rabies prophylaxis and the consequence for global health benefits.

Heterogeneous conditions in dissolved oxygen affect N-glycosylation but not productivity of a monoclonal antibody in hybridoma cultures

It is known that heterogeneous conditions exist in large-scale animal cell cultures. However, little is known about how heterogeneities affect cells, productivities, and product quality. To study the effect of non-constant dissolved oxygen tension (DOT), hybridomas were subjected to sinusoidal DOT oscillations in a one-compartment scale-down simulator. Oscillations were forced by manipulating the inlet oxygen partial pressure through a feedback control algorithm in a 220-mL bioreactor maintained at a constant agitation. Such temporal DOT oscillations simulate spatial DOT gradients that can occur in large scales. Different oscillation periods, in the range of 800 to 12,800 s (axis of 7% (air saturation) and amplitude of 7%), were tested and compared to constant DOT (10%) control cultures. Oscillating DOT decreased maximum cell concentrations, cell growth rates, and viability indexes. Cultures at oscillating DOT had an increased glycolytic metabolism that was evidenced by a decrease in yield of cells on glucose and an increase in lactate yield. DOT gradients, even several orders of magnitude higher than those expected under practical large-scale conditions, did not significantly affect the maximum concentration of an IgG(1) monoclonal antibody (MAb). The glycosylation profile of the MAb produced at a constant DOT of 10% was similar to that reported in the literature. However, MAb produced under oscillating culture conditions had a higher amount of triantennary and sialylated glycans, which can interfere with effector functions of the antibody. It was shown that transient excursions of hybridomas to limiting DOT, as occurs in deficiently mixed large-scale bioreactors, is important to culture performance as the oscillation period, and thus the time cells spent at low DOT, affected cell growth, metabolism, and the glycosylation pattern of MAb. Such results underline the importance of monitoring protein characteristics for the development of large-scale processes.

O-Linked glycosylation in maize-expressed human IgA1

O-Linked glycans vary between eukaryotic cell types and play an important role in determining a glycoprotein's properties, including stability, target recognition, and potentially immunogenicity. We describe O-linked glycan structures of a recombinant human IgA1 (hIgA1) expressed in transgenic maize. Up to six proline/hydroxyproline conversions and variable amounts of arabinosylation (Pro/Hyp + Ara) were found in the hinge region of maize-expressed hIgA1 heavy chain (HC) by using a combination of matrix-assisted laser-desorption ionization mass spectrometry (MALDI MS), chromatography, and amino acid analysis. Approximately 90% of hIgA1 was modified in this way. An average molar ratio of six Ara units per molecule of hIgA1 was revealed. Substantial sequence similarity was identified between the HC hinge region of hIgA1 and regions of maize extensin-family of hydroxyproline-rich glycoproteins (HRGP). We propose that because of this sequence similarity, the HC hinge region of maize-expressed hIgA1 can become a substrate for posttranslational conversion of Pro to Hyp by maize prolyl-hydroxylase(s) with the subsequent arabinosylation of the Hyp residues by Hyp-glycosyltransferase(s) in the Golgi apparatus in maize endosperm tissue. The observation of up to six Pro/Hyp hydroxylations combined with extensive arabinosylation in the hIgA1 HC hinge region is well in agreement with the Pro/Hyp hydroxylation model and the Hyp contiguity hypothesis suggested earlier in literature for plant HRGP. For the first time, the extensin-like Hyp/Pro conversion and O-linked arabinosylation are described for a recombinant therapeutic protein expressed in transgenic plants. Our findings are of significance to the field of plant biotechnology and biopharmaceutical industry-developing transgenic plants as a platform for the production of recombinant therapeutic proteins.

Engineering Chinese hamster ovary cells to maximize effector function of produced antibodies using FUT8 siRNA

We explored the possibility of converting established antibody-producing cells to cells producing high antibody-dependent cellular cytotoxicity (ADCC) antibodies. The conversion was made by constitutive expression of small interfering RNA (siRNA) against alpha1,6 fucosyltransferase (FUT8). We found two effective siRNAs, which reduce FUT8 mRNA expression to 20% when introduced into Chinese hamster ovary (CHO)/DG44 cells. Selection for Lens culinaris agglutinin (LCA)-resistant clones after introduction of the FUT8 siRNA expression plasmids yields clones producing highly defucosylated (approximately 60%) antibody with over 100-fold higher ADCC compared to antibody produced by the parental cells (approximately 10% defucosylated). Moreover, the selected clones remain stable, producing defucosylated antibody even in serum-free fed-batch culture. Our results demonstrate that constitutive FUT8 siRNA expression can control the oligosaccharide structure of recombinant antibody produced by CHO cells to yield antibodies with dramatically enhanced ADCC.

Inhibition of tumor growth by plant-derived mAb

The tumor-associated antigen EpCAM (GA733-2) is a highly expressed target on adenocarcinoma cells, as defined by murine mAb CO17-1A. We recently developed a transgenic plant system for the safe and inexpensive production of large quantities of mAb CO17-1A as a future source of clinical-grade protein. Although the glycosylation pattern of plant-derived mAb (mAb(P)) CO17-1A differs considerably from that of the mammalian-derived mAb (mAb(M)), we show here that the biological activity of both mAbs is quite similar. mAb(P) heavy and light chains assembled to bind the recombinant antigen GA733-2E and specifically bound to human SW948 colorectal carcinoma cells expressing the antigen GA733-2 to the same extent as mAb(M). mAb(P) was as effective as mAb(M) CO17-1A in inhibiting tumor growth of xenotransplanted SW948 cells in nude mice. These results suggest the promise of transgenic plants as a useful alternative way to produce full-size mAb for cancer immunotherapy.

Comprehensive assessment of N-glycans derived from a murine monoclonal antibody: a case for multimethodological approach

Highly efficient separation techniques, laser-induced fluorescence (LIF) detection, and different mass-spectrometric (MS) measurements were combined in a multimethodological scheme to perform a comprehensive structural characterization of N-linked oligosaccharides in a murine monoclonal antibody (immunoglobulin G (IgG(kappa))). Monosaccharide compositional analysis was carried out through a capillary electrophoresis (CE)-LIF method, in which the chemically and enzymatically released sugars were fluorescently labeled. This analysis provides a preliminary assessment of certain structures, being followed by CE-LIF and matrix-assisted laser desorption/ionization (MALDI)-MS profiling of the intact glycan structures. Linkages and monosaccharide residues were confirmed by MALDI-MS in conjunction with exoglycosidase digestion. MALDI-MS and CE data were effectively combined to reveal the overall structural diversity of both acidic and neutral glycans. Finally, the sites of glycosylation and site occupancies were deduced through the measurements performed with microcolumn liquid chromatography coupled via electrospray to a quadrupole/time-of-flight instrument.

Characterizing biological products and assessing comparability following manufacturing changes

Changes in production methods of a biological product may necessitate an assessment of comparability to ensure that these manufacturing changes have not affected the safety, identity, purity, or efficacy of the product. Depending on the nature of the protein or the change, this assessment consists of a hierarchy of sequential tests in analytical testing, preclinical animal studies and clinical studies. Differences in analytical test results between pre- and post-change products may require functional testing to establish the biological or clinical significance of the observed difference. An underlying principle of comparability is that under certain conditions, protein products may be considered comparable on the basis of analytical testing results alone. However, the ability to compare biological materials is solely dependent on the tests used, since no single analytical method is able to compare every aspect of protein structure or function. The advantages and disadvantages of any given method depends on the protein property being characterized.

Increased levels of galactose-deficient IgG in sera of HIV-1-infected individuals

BACKGROUND

The IgG from sera of patients with chronic inflammatory diseases of autoimmune character or some chronic microbial infections is frequently deficient in galactose on N-linked glycans. However, this phenomenon has not been investigated at length in human viral infections.

OBJECTIVES

To evaluate the glycosylation of serum IgG in HIV-1-positive patients.

METHODS

Psathyrella velutina lectin was used in enzyme-linked immunosorbent and Western blot assays to determine glycosylation. In addition, gas-liquid chromatography and mass spectrometry were utilized to confirm the galactose deficiency observed in the lectin-binding assays.

RESULTS

HIV-1-infected individuals had significantly higher levels of galactose-deficient IgG than healthy controls. In fact, the galactose deficiency of the N-linked glycans observed in other diseases was even more profound in HIV-1 infection. This deficiency was primarily restricted to IgG when total serum glycoproteins were evaluated and IgG1 was the subclass most affected in all patients. Also, a significant increase in lectin binding was observed on IgG2 and IgG4 from HIV-1-positive females compared with HIV-1-negative females.

CONCLUSIONS

Identification of deficient galactosylation of serum IgG from HIV-1-infected patients extended the spectrum of diseases in which this phenomenon has been observed. In addition, the results suggest yet another aspect of immune dysfunction as a result of HIV-1 infection.

The therapeutic potential of plant-derived vaccines and antibodies

The production of recombinant proteins in plants is reviewed with a particular focus on plant-derived vaccines and antibodies for human healthcare. Issues relating to foreign gene expression, such as protein yield, localisation and glycosylation are also considered. Emphasis is placed on reporting progress with preclinical and clinical evaluation of plant-derived vaccines and antibodies. An assessment is made of the likely future direction of research and development in this area.

From the bench to the bedside: ways to improve rituximab efficacy

Rituximab (MabThera, Rituxan) is a chimeric IgG1 monoclonal antibody that specifically targets the CD20 surface antigen expressed on normal and neoplastic B-lymphoid cells. Rituximab is currently used in the treatment of both follicular and aggressive B-cell non-Hodgkin lymphomas. Despite its demonstrated clinical effectiveness, its in vivo mechanisms of action remain unknown and could differ by subtype of lymphoma. Rituximab has been shown to induce apoptosis, complement-mediated lysis, and antibody-dependent cellular cytotoxicity in vitro, and some evidence points toward an involvement of these mechanisms in vivo. Rituximab also has a delayed therapeutic effect as well as a potential “vaccinal” effect. Here, we review the current understanding of the mechanism of action of rituximab and discuss approaches that could increase its clinical activity. A better understanding of how rituximab acts in vivo should make it possible to develop new and more effective therapeutic strategies. (Blood. 2004;104:2635-2642)

Site-specific glycosylation of an aglycosylated human IgG1-Fc antibody protein generates neoglycoproteins with enhanced function

A range of well-defined IgG glycoforms was prepared by employing a combination of synthetic carbohydrate chemistry and genetic engineering. The key aspect of this methodology is the coupling of thioaldoses with cysteine-containing proteins to give disulfide-linked neoglycoproteins. This technology was applied to the synthesis of a series of synthetic N-glycan thioaldoses which were coupled to an aglycosylated IgG1-Fc fragment, engineered to have Cys-297 in place of glycan-linked Asn (Deltah-Fc N297C). Analysis of the resulting Fc neoglycoproteins by mass spectrometry and trypsin digestion showed that the saccharides were site-selectively incorporated at Cys-297 to full occupancy without affecting other Fc protein disulfides. The neoglycoproteins were tested for their ability to interact with human FcgammaRI by inhibiting superoxide production by gamma-interferon-stimulated U937 cells. The neoglycoproteins displayed enhanced superoxide inhibition relative to aglycosylated Deltah-Fc N297C, where increased glycan size correlated positively with increased inhibition.

Defucosylated Chimeric Anti-CC Chemokine Receptor 4 IgG1 with Enhanced Antibody-Dependent Cellular Cytotoxicity Shows Potent Therapeutic Activity to T-Cell Leukemia and Lymphoma

Human IgG1 antibodies with low fucose contents in their asparagine-linked oligosaccharides have been shown recently to exhibit potent antibody-dependent cellular cytotoxicity (ADCC) in vitro. To additionally investigate the efficacy of the human IgG1 with enhanced ADCC, we generated the defucosylated chimeric anti-CC chemokine receptor 4 (CCR4) IgG1 antibody KM2760. KM2760 exhibited much higher ADCC using human peripheral blood mononuclear cells (PBMCs) as effector cells compared with the highly fucosylated, but otherwise identical IgG1, KM3060. In addition, KM2760 also exhibited potent ADCC in the presence of lower concentrations of human PBMCs than KM3060. Because CCR4 is a selective marker of T-cell leukemia/lymphoma, the effectiveness of KM2760 for T-cell malignancy was evaluated in several mouse models. First, to compare the antitumor activity of KM2760 and KM3060, we constructed a human PBMC-engrafted mouse model to determine ADCC efficacy with human effector cells. In this model, KM2760 showed significantly higher antitumor efficacy than KM3060, indicating that KM2760 retains its high potency in vivo. Second, KM2760 suppressed tumor growth in both syngeneic and xenograft mouse models in which human PBMCs were not engrafted. Although murine effector cells exhibited marginal ADCC mediated by KM2760 and KM3060, KM2760 unexpectedly showed higher efficacy than KM3060 in a syngeneic mouse model, suggesting that KM2760 functions in murine effector system in vivo via an unknown mechanism that differs from that in human. These results indicate that defucosylated antibodies with enhanced ADCC as well as potent antitumor activity in vivo are promising candidates for the novel antibody-based therapy.

Anaphylactic and non-anaphylactic murine IgG1 differ in their ability to bind to mast cells: relevance of proper glycosylation of the molecule

We have previously shown that murine IgG1 antibodies comprise two functionally distinct types regarding their ability to induce mast cell degranulation. In this work, we identified two IgG1-producing hybridomas, both with the same antigenic specificity (anti-DNP), but different in vivo anaphylactic activities. Whereas one of them secretes the anaphylactic IgG1 antibody, as assessed by passive cutaneous anaphylaxis, the other produces the non-anaphylactic IgG1 molecule. The evaluation of the ability of both types of IgG1 to bind to and activate a mouse mast cell line revealed that the anaphylactic IgG1 has a higher binding capacity and releases more beta-hexosaminidase from mast cells than the non-anaphylactic IgG1. Aglycosylated IgG1 obtained by treatment of the anaphylactic IgG1-producing hybridoma line with an inhibitor of N-glycosylation failed to elicit anaphylaxis. In addition, a goat anti-mouse IgG1 antibody reacted less with this aglycosylated IgG1 than with the glycosylated form. These results suggest that the anaphylactic activity of IgG1 antibodies is closely related to their structural conformation and the proper N-glycosylation of these molecules. Finally, the difference in the anaphylactic property between the two types of IgG1 seems to be primarily due to binding to the mast cell surface.

Monoclonal C5-1 antibody produced in transgenic alfalfa plants exhibits a N-glycosylation that is homogenous and suitable for glyco-engineering into human-compatible structures

Structural analysis of the N-glycosylation of alfalfa proteins was investigated in order to evaluate the capacity of this plant to perform this biologically important post-translational modification. We show that, in alfalfa, N-linked glycans are processed into a large variety of mature oligosaccharides having core-xylose and core alpha(1,3)-fucose, as well as terminal Lewis(a) epitopes. In contrast, expression of the C5-1 monoclonal antibody in alfalfa plants results in the production of plant-derived IgG1 which is N-glycosylated by a predominant glycan having a alpha(1,3)-fucose and a beta(1,2)-xylose attached to a GlcNAc2Man3GlcNAc2 core. Since this core is common to plant and mammal N-linked glycans, it therefore appears that alfalfa plants have the ability to produce recombinant IgG1 having a N-glycosylation that is suitable for in vitro or in vivo glycan remodelling into a human-compatible plantibody. For instance, as proof of concept, in vitro galactosylation of the alfalfa-derived C5-1 mAb resulted in a homogenous plantibody harbouring terminal beta(1,4)-galactose residues as observed in the mammalian IgG.

Effects of buffering conditions and culture pH on production rates and glycosylation of clinical phase I anti-melanoma mouse IgG3 monoclonal antibody R24

R24, a mouse IgG3 monoclonal antibody (MAb) against ganglioside GD3 (Neu5Acalpha8Neu5Acalpha3Gal beta4Glcbeta1Cer), can block tumor growth as reported in a series of clinical trials in patients with metastatic melanoma. The IgG molecule basically contains an asparagine-linked biantennary complex type oligosaccharide on the C(H)2 domain of each heavy chain, which is necessary for its in vivo effector function. The purpose of this study was to investigate the biotechnological production and particularly the glycosylation of this clinically important MAb in CO(2)/HCO(3) (-) (pH 7.4, 7.2, and 6.9) and HEPES buffered serum-free medium. Growth, metabolism, and IgG production of hybridoma cells (ATCC HB-8445) were analyzed on a 2-L bioreactor scale using fed-batch mode. Specific growth rates (mu) and MAb production rates (q(IgG)) varied significantly with maximum product yields at pH 6.9 (q(IgG) = 42.9 microg 10(-6) cells d(-1), mu = 0.30 d(-1)) and lowest yields in pH 7.4 adjusted batches (q(IgG) = 10.8 microg 10(-6) cells d(-1), mu = 0.40 d(-1)). N-glycans were structurally characterized by high pH anion exchange chromatography with pulsed amperometric detection (HPAEC-PAD), matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF), and electrospray-ionization quadrupole time-of-flight (ESI-QTOF) mass spectrometry (MS). The highest relative amounts of agalacto and monogalacto biantennary complex type oligosaccharides were detected in the pH 7.2 (46% and 38%, respectively) and pH 6.9 (44% and 40%, respectively) cultivations and the uppermost quantities of digalacto (fully galactosylated) structures in the pH 7.4 (32%) and the HEPES (26%) buffered fermentation. In the experiments with HEPES buffering, antibodies with a molar Neu5Ac/Neu5Gc ratio of 3.067 were obtained. The fermentations at pH 7.2 and 6.9 resulted in almost equal molar Neu5Ac/Neu5Gc ratios of 1.008 and 0.985, respectively, while the alkaline shift caused a moderate overexpression of Neu5Ac deduced from the Neu5Ac/Neu5Gc quotient of 1.411. Different culture buffering gave rise to altered glycosylation pattern of the MAb R24. Consequently, a detailed molecular characterization of MAb glycosylation is generally recommended as a part of the development of MAbs for targeted in vivo immunotherapy to assure biochemical consistency of product lots and oligosaccharide-dependent biological activity.

Benefits and risks of antibody and vaccine production in transgenic plants

Phytopharming, the production of protein biologicals in recombinant plant systems, has shown great promise in studies performed over the past 13 years. A secretory antibody purified from transgenic tobacco was tested successfully in humans, and prevented bacterial re-colonization after topical application in the mouth. Rapid production of patient-tailored anti-lymphoma antibodies in recombinant Tobamovirus-infected tobacco may provide effective cancer therapy. Many different candidate vaccines from bacterial and viral sources have been expressed in transgenic plants, and three human clinical trials with oral delivery of transgenic plant tissues have shown exciting results. The use of crop plants with agricultural practice could allow cheap production of valuable proteins, while providing enhanced safety by avoidance of animal viruses or other contaminants. However development of this technology must carefully consider the means to ensure the separation of food and medicinal products when crop plants are used for phytopharming.

Function and glycosylation of plant-derived antiviral monoclonal antibody

Plant genetic engineering led to the production of plant-derived mAb (mAbP), which provides a safe and economically feasible alternative to the current methods of antibody production in animal systems. In this study, the heavy and light chains of human anti-rabies mAb were expressed and assembled in planta under the control of two strong constitutive promoters. An alfalfa mosaic virus untranslated leader sequence and Lys-Asp-Glu-Leu (KDEL) endoplasmic reticulum retention signal were linked at the N and C terminus of the heavy chain, respectively. mAbP was as effective at neutralizing the activity of the rabies virus as the mammalian-derived antibody (mAbM) or human rabies Ig (HRIG). The mAbP contained mainly oligomannose type N-glycans (90%) and had no potentially antigenic alpha(1,3)-linked fucose residues. mAbP had a shorter half-life than mAbM. The mAbP was as efficient as HRIG for post-exposure prophylaxis against rabies virus in hamsters, indicating that differences in N-glycosylation do not affect the efficacy of the antibody in this model.

Secretory IgA N- and O-glycans provide a link between the innate and adaptive immune systems

Secretory IgA (SIgA) is a multi-polypeptide complex consisting of a secretory component (SC) covalently attached to dimeric IgA containing one joining (J) chain. We present the analysis of both the N- and O-glycans on the individual peptides from this complex. Based on these data, we have constructed a molecular model of SIgA1 with all its glycans, in which the Fab arms form a T shape and the SC is wrapped around the heavy chains. The O-glycan regions on the heavy (H) chains and the SC N-glycans have adhesin-binding glycan epitopes including galactose-linked beta1-4 and beta1-3 to GlcNAc, fucose-linked alpha1-3 and alpha1-4 to GlcNAc and alpha1-2 to galactose, and alpha2-3 and alpha2-6-linked sialic acids. These glycan epitopes provide SIgA with further bacteria-binding sites in addition to the four Fab-binding sites, thus enabling SIgA to participate in both innate and adaptive immunity. We also show that the N-glycans on the H chains of both SIgA1 and SIgA2 present terminal GlcNAc and mannose residues that are normally masked by SC, but that can be unmasked and recognized by mannose-binding lectin, by disrupting the SC-H chain noncovalent interactions.

L-SIGN (CD 209L) is a liver-specific capture receptor for hepatitis C virus

Hepatitis C virus (HCV) infects nearly 3% of the population of the world and is a major cause of liver disease. However, the mechanism whereby the virus targets the liver for infection remains unknown, because none of the putative cellular receptors for HCV are both expressed specifically in the liver and capable of binding HCV envelope glycoproteins. Liver/lymph node-specific intercellular adhesion molecule-3-grabbing integrin (L-SIGN) is a calcium-dependent lectin expressed on endothelial cells of liver and lymph nodes. Dendritic cell-specific intercellular adhesion molecule-3-grabbing nonintegrin (DC-SIGN), a homologous molecule expressed on dendritic cells, binds HIV and promotes infection. By using a virus-binding assay, we demonstrate that L-SIGN and DC-SIGN specifically bind naturally occurring HCV present in the sera of infected individuals. Further studies demonstrate that binding is mediated by the HCV envelope glycoprotein E2 and is blocked by specific inhibitors, including mannan, calcium chelators, and Abs to the lectin domain of the SIGN molecules. Thus, L-SIGN represents a liver-specific receptor for HCV, and L-SIGN and DC-SIGN may play important roles in HCV infection and immunity.

Antibody manufacture in transgenic animals and comparisons with other systems

Various forms of recombinant monoclonal antibodies are being used increasingly, mainly for therapeutic purposes. The isolation and engineering of the corresponding genes is becoming less of a bottleneck in the process; however, the production of recombinant antibodies is itself a limiting factor and a shortage is expected in the coming years. Milk from transgenic animals appears to be one of the most attractive sources of recombinant antibodies. None of the production systems presently implemented (CHO cells, insect cells infected by baculovirus, or transgenic animals and plants) has yet been optimized. This review describes the advantages of using milk for antibody production in comparison with the other systems.

Anti-idiotypic antibody as the surrogate antigen for cloning scFv and its fusion proteins

Single-chain variable fragment (ScFv) is a versatile building block for novel targeting constructs. However, a reliable screening and binding assay is often the limiting step for antigens that are difficult to clone or purify. Anti-idiotypic antibodies may be useful as surrogate antigens for cloning scFv and their fusion proteins. 8H9 is a murine IgG(1) monoclonal antibody (MAb) specific for a novel antigen expressed on the cell surface of a wide spectrum of human solid tumors, but not in normal tissues. Rat anti-8H9-idiotypic hybridomas (clones 2E9, 1E12, and 1F11) were produced by somatic cell fusion between rat lymphocytes and mouse SP2/0 myeloma. In direct binding assays enzyme-linked immunosorbant assay--(ELISA)--they were specific for the 8H9 idiotope. Using 2E9 as the surrogate antigen, 8H9-scFv was cloned from hybridoma cDNA by phage display. 8H9scFv was then fused to human-gamma1-CH2-CH3 cDNA for transduction into CHO and NSO cells. High expressors of mouse scFv-human Fc chimeric antibody were selected. The secreted homodimer reacted specifically with antigen-positive tumor cells by ELISA and by flow cytometry, inhibitable by the anti-idiotypic antibody. The reduced size resulted in a shorter half-life in vivo, while achieving comparable tumor to nontumor ratio as the native antibody 8H9. However, its in vitro activity in antibody-dependent cell-mediated cytotoxicity was modest.

Molecular cloning and chimerisation of an inhibitory anti-cathepsin B antibody and its expression in Chinese hamster ovary cells

The lysosomal cysteine protease cathepsin B is one of several proteases that have been linked to tumour progression. Its increased expression and secretion in tumour cells may facilitate the degradation of extracellular matrix proteins, leading to tumour cell invasion and metastasis. Specific inhibitory monoclonal antibodies are a possible alternative to synthetic inhibitors as a therapeutic tool for cancer treatment. An inhibitory monoclonal antibody, which binds to an epitope near the active site of cathepsin B and inhibits its proteolytic activity, was prepared and its effect on invasion of ras-transformed MCF-10A neoT cells was tested in vitro. Here we present the nucleotide sequences of the heavy and light chains of the inhibitory antibody and compare them to the murine immunoglobulin germline sequences for possible somatic hypermutations. Since no harmful mutations were found, a mouse/human chimeric antibody was constructed by fusing murine V(H) and V(L) variable regions of the inhibitory antibody with human gamma 1 and K constant regions, respectively. Chinese hamster ovary K1 cells were co-transfected with expression vectors pcD-NA3L and pcDNA3H and the reactivity of the isolated chimeric antibody was tested by ELISA and Western blotting. We could demonstrate an inhibitory effect of the chimeric antibody.

Lack of fucose on human IgG1 N-linked oligosaccharide improves binding to human Fcgamma RIII and antibody-dependent cellular toxicity

Lec13 cells, a variant Chinese hamster ovary cell line, were used to produce human IgG1 that were deficient in fucose attached to the Asn(297)-linked carbohydrate but were otherwise similar to that found in IgG1 produced in normal Chinese hamster ovary cell lines and from human serum. Lack of fucose on the IgG1 had no effect on binding to human FcgammaRI, C1q, or the neonatal Fc receptor. Although no change in affinity was found for the His(131) polymorphic form of human FcgammaRIIA, a slight improvement in binding was evident for FcgammaRIIB and the Arg(131) FcgammaRIIA polymorphic form. In contrast, binding of the fucose-deficient IgG1 to human FcgammaRIIIA was improved up to 50-fold. Antibody-dependent cellular cytotoxicity assays using purified peripheral blood monocytes or natural killer cells from several donors showed enhanced cytotoxicity, especially evident at lower antibody concentrations. When combined with an IgG1 Fc protein variant that exhibited enhanced antibody-dependent cellular cytotoxicity, the lack of fucose was synergistic.

Secretory component: a new role in secretory IgA-mediated immune exclusion in vivo

Secretory immunoglobulin (Ig) A (SIgA) is essential in protecting mucosal surfaces. It is composed of at least two monomeric IgA molecules, covalently linked through the J chain, and secretory component (SC). We show here that a dimeric/polymeric IgA (IgA(d/p)) is more efficient when bound to SC in protecting mice against bacterial infection of the respiratory tract. We demonstrate that SC ensures, through its carbohydrate residues, the appropriate tissue localization of SIgA by anchoring the antibody to mucus lining the epithelial surface. This in turn impacts the localization and the subsequent clearance of bacteria. Thus, SC is directly involved in the SIgA function in vivo. Therefore, binding of IgA(d/p) to SC during the course of SIgA-mediated mucosal response constitutes a crucial step in achieving efficient protection of the epithelial barrier by immune exclusion.

Biodiversity of mannose-specific lectins within Narcissus species

Mannose-specific lectins (MSLs) were isolated from the bulbs of 27 species of wild Spanish Narcissi and compared to the commercially available MSL from daffodil (Narcissus pseudonarcissus, NPA). Molecular weight analysis showed the monomers of all the MSLs were at, or around, 12.5 kD. Haemagglutination assays showed that the MSLs exhibited activities at up to four times greater than that displayed by NPA and other MSLs derived from other species such as Galanthus nivalus (snowdrop) and Allium ursinum (ramson). Elution profiles from ion exchange chromatography exhibited similarities for species within the same taxonomic section suggesting that this method could aid in species classification. Further analysis by isoelectric focusing showed many isolectins are present in vivo and that even within a single peak from ion exchange chromatography there are numerous isolectins present. The basis of the isolectin heterogeneity is suggested to reside in the tetraploidy (sometime triploidy) nature of Narcissus genes.

The use of high pressure for separation and production of bioactive molecules

Due to its action on the forces governing inter- and intramolecular interactions, the application of high pressure to biopurification or bio-elaboration of a product are of interest. The two closely thermodynamically related parameters, pressure and temperature, render processes based on their action clean, as no chemical reagents have to be added (and thus further removed) when they are applied. The use of high pressure in the development of desorption methods for the purification of bioactive molecules, particularly in the immunoaffinity field, is reviewed and discussed. Also mentioned is the application of the pressure parameter during the synthesis of a bioreagent. Finally, integrated processes relative to the synthesis and purification of these compounds are proposed.

Recombinant immunoglobulin A: powerful tools for fundamental and applied research

The use of monoclonal antibodies has become routine in research and diagnostic laboratories but the potential level of antibodies in use in public health and medical applications is still far from its maximum. From a clinical perspective, topical immunotherapy of mucosal surfaces with monoclonal antibodies can block entry and transmission of bacteria, viruses, fungi and parasites that infect humans, and defeat some key strategies, evolved by many pathogens, to evade the host immune system. The chief antibody at mucosal surfaces is secretory immunoglobulin A (SIgA), a multi-polypeptide complex originating from two cell types. The recent design of heterologous expression systems, coupled with modern biotechnology processes, should form a sound basis for studying the functional properties of SIgAs and evaluate their value as biotherapeutics. Here, we discuss the principles underlying mucosal immunity and review the application of recombinant SIgA to the dissection of mechanisms in passive and active protection at mucosal surfaces.

Improving the efficacy of antibody-based cancer therapies

A quarter of a century after their advent, monoclonal antibodies have become the most rapidly expanding class of pharmaceuticals for treating a wide variety of human diseases, including cancer. Although antibodies have yet to achieve the ultimate goal of curing cancer, many innovative approaches stand poised to improve the efficacy of antibody-based therapies.

Glycosylated and phosphorylated proteins--expression in yeast and oocytes of Xenopus: prospects and challenges--relevance to expression of thermostable proteins

Phosphorylation and glycosylation are important posttranslational events in the biosynthesis of proteins. The different degrees of phosphorylation and glycosylation of proteins have been an intriguing phenomenon. Advances in genetic engineering have made it possible to control the degree of glycosylation and phosphorylation of proteins. Structural biology of phosphorylated and glycosylated proteins has been advancing at a much slower pace due to difficulties in using high-resolution NMR studies in solution phase. Major difficulties have arisen from the inherent mobilities of phosphorylated and glycosylated side chains. This paper reviews molecular and structural biology of phosphorylated and glycosylated proteins expressed in eukaryotic expression systems which are especially suited for large-scale production of these proteins. In our laboratory, we have observed that eukaryotic expression systems are particularly suited for the expression of thermostable light-activated proteins, e.g., bacteriorhodopsins and plastocyanins.

Effect of glycosylation on structure and dynamics of MHC class I glycoprotein: a molecular dynamics study

Complex carbohydrates linked to glycoproteins are recently being implicated to play a variety of biological roles. The lack of well-resolved crystallographic coordinates of the carbohydrates makes it difficult to assess the contributions of the glycan chain on protein structure and dynamics. We have modeled two different oligosaccharides NeuNAc2Gal3Man3GlcNAc5Fuc and Man3GlcNAc4 to generate two glycosylation variants of major histocompatibility complex (MHC) class I glycoprotein. Molecular dynamics simulations of the isolated fourteen- and seven-residue oligosaccharides have been done in vacuo and in solution. The dynamics of the two glycoforms of MHC class I protein have been simulated in solution in the free as well as in the peptide-bound form. Good agreement between the calculated solution conformations of the oligosaccharides in isolated and conjugated forms and the average conformations obtained from x-ray or NMR data was observed for most of the glycosidic linkages. These molecular dynamics simulations of the isolated glycan chains and the glycoconjugates reveal the details of the conformational flexibility of the glycan chains; they also provide atomic level details of protein-carbohydrate interactions and the effect of the ligand binding on the carbohydrate structure and dynamics. It was found that though there is some flexibility in some of the glycosidic linkages in the isolated oligosaccharides, in the protein-conjugated form the linkages adopt more restricted conformations. The glycan chains protrude out into the solvent and might hinder the lateral association of the proteins. The presence of the bulky glycan chains does not affect the average backbone fold of the protein but induces local changes in protein structure and dynamics. It has been noted that the extent of the changes depends upon the nature of the attached glycan chain. The glycan chains do not appear to influence the peptide binding property of the protein directly, but may stabilize the protein residues that are involved in ligand binding.

Influence of growth conditions and developmental stage on N-glycan heterogeneity of transgenic immunoglobulin G and endogenous proteins in tobacco leaves

Plants are regarded as a promising system for the production of heterologous proteins. However, little is known about the influence of plant development and growth conditions on N-linked glycosylation. To investigate this, transgenic tobacco (Nicotiana tabacum cv Samsun NN) plants expressing a mouse immunoglobulin G antibody (MGR48) were grown in climate rooms under four different climate conditions, i.e. at 15 degrees C and 25 degrees C and at either low or high light conditions. N-glycans on plantibodies and soluble endogenous proteins were analyzed with matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry (MS). Antibodies isolated from young leaves have a relatively high amount of high- mannose glycans compared with antibodies from older leaves, which contain more terminal N-acetylglucosamine. Senescence was shown to affect the glycosylation profile of endogenous proteins. The relative amount of N-glycans without terminal N-acetylglucosamine increased with leaf age. Major differences were observed between glycan structures on endogenous proteins versus those on antibodies, probably to be attributed to their subcellular localization. The relatively high percentage of antibody N-glycan lacking both xylose and fucose is interesting.

Expression of single-chain Fv-Fc fusions in Pichia pastoris

Phage display technology makes possible the direct isolation of monovalent single-chain Fv antibody fragments. For many applications, however, it is useful to restore Fc mediated antibody functions such as avidity, effector functions and a prolonged serum half-life. We have constructed vectors for the convenient, rapid expression of a single-chain antibody Fv domain (scFv) fused to the Fc portion of human IgG1 in the methylotrophic yeast Pichia pastoris. The scFv-Fc fusion protein is secreted and recovered from the culture medium as a disulfide-linked, glycosylated homodimer. The increased size of the dimer (approximately 106 kDa vs. approximately 25 kDa for a scFv) results in a prolonged serum half-life in vivo, with t(1/2) of the beta phase of clearance increasing from 3.5 h for a typical scFv to 93 h for a scFv-Fc fusion in mice. The scFv-Fc fusion is capable of mediating antibody-dependent cellular cytotoxicity against tumor target cells using human peripheral blood mononuclear cells as effectors. Finally, the Fc domain is a convenient, robust affinity handle for purification and immunochemical applications, eliminating the need for proteolytically sensitive epitope and/or affinity tags on the scFv.

Immunochemical localisation of cathepsin S, cathepsin L and MHC class II-associated p41 isoform of invariant chain in human lymph node tissue

Antigen presentation by MHC class II molecules requires cysteine proteases (CP) for two convergent proteolytic processes: stepwise degradation of the invariant chain (Ii) and generation of immunogenic peptides. Their activity is controlled by intracellular CP inhibitors, including presumably the p41 isoform of invariant chain (p41 Ii), which is in vitro a potent inhibitor of cathepsin L but not of cathepsin S. In order to evaluate the inhibitory potential of p41 Ii in antigen-presenting cells (APC), these three proteins were stained in lymph node tissue using specific monoclonal and polyclonal antibodies. The most abundant labelling was observed in subcapsular (cortical) and trabecular sinuses of the lymph node. In this area the most frequent APC were macrophages, as confirmed by the CD68 cell marker. Using confocal fluorescence microscopy, co-localisation of p41 Ii with cathepsin S, but not with cathepsin L was found in these cells. Our results are consistent with the hypothesis that cathepsin S participates in degradation of the invariant chain, but they do not support the association between cathepsin L and p41 Ii in APC.

Galactose-extended glycans of antibodies produced by transgenic plants

Plant-specific N-glycosylation can represent an important limitation for the use of recombinant glycoproteins of mammalian origin produced by transgenic plants. Comparison of plant and mammalian N-glycan biosynthesis indicates that beta1,4-galactosyltransferase is the most important enzyme that is missing for conversion of typical plant N-glycans into mammalian-like N-glycans. Here, the stable expression of human beta1,4-galactosyltransferase in tobacco plants is described. Proteins isolated from transgenic tobacco plants expressing the mammalian enzyme bear N-glycans, of which about 15% exhibit terminal beta1,4-galactose residues in addition to the specific plant N-glycan epitopes. The results indicate that the human enzyme is fully functional and localizes correctly in the Golgi apparatus. Despite the fact that through the modified glycosylation machinery numerous proteins have acquired unusual N-glycans with terminal beta1,4-galactose residues, no obvious changes in the physiology of the transgenic plants are observed, and the feature is inheritable. The crossing of a tobacco plant expressing human beta1,4-galactosyltransferase with a plant expressing the heavy and light chains of a mouse antibody results in the expression of a plantibody that exhibits partially galactosylated N-glycans (30%), which is approximately as abundant as when the same antibody is produced by hybridoma cells. These results are a major step in the in planta engineering of the N-glycosylation of recombinant antibodies.

A systematic approach to the validation of process control parameters for monoclonal antibody production in fed-batch culture of a murine myeloma

A systematic approach to the validation of control ranges of control parameters for a cell culture process producing a monoclonal antibody is described. Specifically, the structure and functional activity of a monoclonal IgG1 antibody produced at the outer limits of numerical ranges of fed-batch culture control parameters such as pH and temperature were examined, with the aim of providing assurance that antibody produced under varying culture conditions was of consistent quality based on a carefully defined set of specifications. An experimental design was created using a half-fractional factorial design for fed-batch culture incorporating half of the thirty two possible combinations of five selected control parameters at high and low levels. Statistical analysis of all data gathered from the study allowed an assessment of the effects of the process control parameters at either high or low outer limits on fed-batch culture response variables such as growth rate and specific antibody productivity. Measured values for the responses of growth rate and specific antibody productivity throughout this study ranged from 0.22-0.44 d(-1) and 6.4-32 microg monoclonal antibody/10(6) cells/d respectively. Analytical characterisation of monoclonal antibody purified from each fed-batch culture considered the purity, structure and biological activity of the glycoprotein. All antibody preparations were identical to each other and to the current antibody reference standard or control. Glycosylation analysis of certain samples from the study demonstrated that the distribution of glycoforms of the antibody was not affected by the varying process control conditions of the fed-batch cultures.

Metabolic shifts do not influence the glycosylation patterns of a recombinant fusion protein expressed in BHK cells

BHK-21 cells expressing a human IgG-IL2 fusion protein, with potential application in tumor-targeted therapy, were grown under different nutrient conditions in a continuous system for a time period of 80 days. At very low-glucose (< 0.5 mM) or glutamine (< 0. 2 mM) concentrations, a shift toward an energetically more efficient metabolism was observed. Cell-specific productivity was maintained under metabolically shifted growth conditions and at the same time an almost identical intracellular ATP content, obtained by in vivo (31)P NMR experiments, was observed. No significant differences in the oligosaccharide structures were detected from the IgG-IL2 fusion protein preparations obtained by growing cells under the different metabolic states. By using oligosaccharide mapping and MALDI/TOF-MS, only neutral diantennary oligosaccharides with or without core alpha1-6-linked fucose were detected that carried no, one or two beta1-4-linked galactose. Although the O-linked oligosaccharide structures that are present in the IL2 moiety of the protein were studied with less detail, the data obtained from the hydrazinolysis procedure point to the presence of the classical NeuAcalpha2-3Galbeta1-3GalNAc structure. Here, it is shown that under different defined cellular metabolic states, the quality of a recombinant product in terms of O- and N-linked oligosaccharides is stable, even after a prolonged cultivation period. Moreover, unaffected intracellular ATP levels under the different metabolic states were observed.

Functional human monoclonal antibodies of all isotypes constructed from phage display library-derived single-chain Fv antibody fragments

We have constructed a series of eukaryotic expression vectors that permit the rapid conversion of single chain (sc) Fv antibody fragments, derived from semi-synthetic phage display libraries, into intact fully human monoclonal antibodies (mAb) of each isotype. As a model, a scFv fragment specific for sheep red blood cells (SRBC) was isolated from a semi-synthetic phage antibody (Ab) display library, and used to produce human mAbs of IgM, IgG1-IgG4, IgA1, IgA2m(1) and IgE isotype in vitro in stably transfected cells. N-terminal protein sequence analysis of purified immunoglobulin heavy (H) and light (L) chains revealed precise proteolytic removal of the leader peptide. Biochemical analysis of purified recombinant human mAbs demonstrated that properly glycosylated molecules of the correct molecular size were produced. The IgG and IgA mAbs retained SRBC-binding activity, interacted with different Fc receptor-transfectants, and induced complement-mediated hemolysis and Ab-dependent phagocytosis of SRBC by neutrophils in a pattern consistent with the immunoglobulin (Ig) H chain isotype. We conclude that in vitro produced recombinant human mAbs constructed from phage display library-derived scFv fragments mirror their natural counterparts and may represent a source of mAbs for use in human therapy.

Foreign protein production in plant tissue cultures

Foreign proteins synthesised by plants are now in the marketplace, and clinical trials for plant-derived therapeutic proteins are underway. Economic analysis of plant production systems has helped identify the types of protein that would be most suitable for manufacture using tissue culture methods. The major advantages associated with in vitro plant systems include the ability to manipulate environmental conditions for better control over protein levels and quality, the rapidity of production compared with agriculture, and the use of simpler and cheaper downstream processing schemes for product recovery from the culture medium.

Construction and characterization of a novel recombinant single-chain variable fragment antibody against Western equine encephalitis virus

A novel recombinant single-chain fragment variable (scFv) antibody against Western equine encephalitis virus (WEE) was constructed and characterized. Using antibody phage display technology, a scFv was generated from the WEE specific hybridoma, 10B5 E7E2. The scFv was fused to a human heavy chain IgG1 constant region (CH1-CH3) and contained an intact 6 His tag and enterokinase recognition site (RS10B5huFc). The RS10B5huFc antibody was expressed in E. coli and purified by affinity chromatography as a 70-kDa protein. The RS10B5huFc antibody was functional in binding to WEE antigen in indirect enzyme-linked immunosorbent assays (ELISAs). Furthermore, the RS10B5huFc antibody was purified in proper conformation and formed multimers. The addition of the human heavy chain to the scFv replaced effector functions of the mouse antibody. The Fc domain was capable of binding to protein G and human complement. The above properties of the RS10B5huFc antibody make it an excellent candidate for immunodetection and immunotherapy studies.

Transgenic animal bioreactors

The production of recombinant proteins is one of the major successes of biotechnology. Animal cells are required to synthesize proteins with the appropriate post-translational modifications. Transgenic animals are being used for this purpose. Milk, egg white, blood, urine, seminal plasma and silk worm cocoon from transgenic animals are candidates to be the source of recombinant proteins at an industrial scale. Although the first recombinant protein produced by transgenic animals is expected to be in the market in 2000, a certain number of technical problems remain to be solved before the various systems are optimized. Although the generation of transgenic farm animals has become recently easier mainly with the technique of animal cloning using transfected somatic cells as nuclear donor, this point remains a limitation as far as cost is concerned. Numerous experiments carried out for the last 15 years have shown that the expression of the transgene is predictable only to a limited extent. This is clearly due to the fact that the expression vectors are not constructed in an appropriate manner. This undoubtedly comes from the fact that all the signals contained in genes have not yet been identified. Gene constructions thus result sometime in poorly functional expression vectors. One possibility consists in using long genomic DNA fragments contained in YAC or BAC vectors. The other relies on the identification of the major important elements required to obtain a satisfactory transgene expression. These elements include essentially gene insulators, chromatin openers, matrix attached regions, enhancers and introns. A certain number of proteins having complex structures (formed by several subunits, being glycosylated, cleaved, carboxylated...) have been obtained at levels sufficient for an industrial exploitation. In other cases, the mammary cellular machinery seems insufficient to promote all the post-translational modifications. The addition of genes coding for enzymes involved in protein maturation has been envisaged and successfully performed in one case. Furin gene expressed specifically in the mammary gland proved to able to cleave native human protein C with good efficiency. In a certain number of cases, the recombinant proteins produced in milk have deleterious effects on the mammary gland function or in the animals themselves. This comes independently from ectopic expression of the transgenes and from the transfer of the recombinant proteins from milk to blood. One possibility to eliminate or reduce these side-effects may be to use systems inducible by an exogenous molecule such as tetracycline allowing the transgene to be expressed only during lactation and strictly in the mammary gland. The purification of recombinant proteins from milk is generally not particularly difficult. This may not be the case, however, when the endogenous proteins such as serum albumin or antibodies are abundantly present in milk. This problem may be still more crucial if proteins are produced in blood. Among the biological contaminants potentially present in the recombinant proteins prepared from transgenic animals, prions are certainly those raising the major concern. The selection of animals chosen to generate transgenics on one hand and the elimination of the potentially contaminated animals, thanks to recently defined quite sensitive tests may reduce the risk to an extremely low level. The available techniques to produce pharmaceutical proteins in milk can be used as well to optimize milk composition of farm animals, to add nutriceuticals in milk and potentially to reduce or even eliminate some mammary infectious diseases.

Carbohydrate analysis of a chimeric recombinant monoclonal antibody by capillary electrophoresis with laser-induced fluorescence detection

A general method for the analysis of asparaginyl-linked (N-linked) carbohydrate moieties of an IgG1 monoclonal antibody is described here. The antibody, rituximab, is a mouse/human chimeric antibody to human CD20 antigen. The glycans present on rituximab are neutral complex biantennary oligosaccharides with zero, one, and two terminal galactose residues (G0, G1, and G2, respectively). To monitor the variation of the glycosylation during manufacture, the glycans were first enzymatically released from the antibody via digestion with peptide-N-glycosidase F, then derivatized with a charged fluorophore, 8-aminopyrene-1,3,6-trisulfonic acid and further separated by capillary electrophoresis with laser-induced fluorescence detection. All observed glycans were fully resolved, including the positional isomers of G1. The exact nature of the isomers in terms of the location of the terminal galactose was further characterized via multiple enzymatic digestion steps including mannosidase with activity toward specific Man(alpha 1,3) linkage. The optimization and several key parameters, i.e., enzymatic digestion and derivatization, in the assay development will be discussed. Moreover, to ensure that the assay can be used in routine lot release testing, the assay was validated and found to be accurate and precise. The analytical approach described is suitable for characterization as well as routine testing of the N-linked glycan content in any IgG1 monoclonal antibody and glycoproteins in general.

The effect on immunoglobulin glycosylation of altering in vivo production of immunoglobulin G

The effect on murine immunoglobulin G (IgG) glycosylation of altering IgG production in vivo was assessed in interleukin (IL)-6 transgenic and CD4 knockout mice. C57BL/6 mice carrying the IL-6 transgene showed increased levels of circulating IgG. This was associated with decreased levels of galactose on the IgG oligosaccharides. No decrease in beta4-galactosyltransferase mRNA or in enzyme activity was seen in IL-6 transgenic mice. MRL-lpr/lpr mice normally have elevated levels of circulating IgG, again accompanied by decreased levels of IgG galactose. Disruption of the CD4 gene in MRL-lpr/lpr mice led to a substantial decrease in the concentration of circulating IgG, but IgG galactose levels remained low. Thus, an enforced decrease in IgG levels in the lymphoproliferative MRL-lpr/lpr mice did not alter the percentage of agalactosyl IgG in these mice, suggesting that agalactosyl IgG production is not simply caused by excessive IgG synthesis leading to an insufficient transit time in the trans-Golgi, but rather to a molecular defect in the interaction between galactosyltransferase and the immunoglobulin heavy chain.

Apoptosis of a Human Melanoma Cell Line Specifically Induced by Membrane-Bound Single-Chain Antibodies

CD28 is a key regulatory molecule in T cell responses. Ag-TCR/CD3 interactions without costimulatory signals provided by the binding of B7 ligands to the CD28R appear to be inadequate for an effective T cell activation. Indeed, the absence of B7 on the tumor cell surface is probably one of the factors contributing to the escape of tumors from immunological control and destruction. Therefore, to increase the immunogenicity of tumor cell vaccines, we have expressed anti-CD3 and anti-CD28 single-chain Abs (scFv) separately on the surface of a human melanoma SkMel63 cell line (HLA-A*0201). A mixture of cells expressing anti-CD3 with cells expressing anti-CD28 resulted in a marked activation of allogeneic human PBL in vitro. The apparent induction of a Th1 differentiation pathway was accompanied by the proliferation of MHC-independent NK cells and MHC-dependent CD8+ T cells. PBL that had been cultured together with transfected SkMel63 tumor cells were able to specifically induce apoptosis in untransfected SkMel63 cells. In contrast, three other tumor cell lines expressing HLA-A*0201, including two melanoma cell lines, showed no significant apoptosis. These results provide valuable information for both adoptive immunotherapy and the generation of autologous tumor vaccines.

N-Glycosylation of a mouse IgG expressed in transgenic tobacco plants

Since plants are emerging as an important system for the expression of recombinant glycoproteins, especially those intended for therapeutic purposes, it is important to scrutinize to what extent glycans harbored by mammalian glycoproteins produced in transgenic plants differ from their natural counterpart. We report here the first detailed analysis of the glycosylation of a functional mammalian glycoprotein expressed in a transgenic plant. The structures of the N-linked glycans attached to the heavy chains of the monoclonal antibody Guy's 13 produced in transgenic tobacco plants (plantibody Guy's 13) were identified and compared to those found in the corresponding IgG1 of murine origin. Both N-glycosylation sites located on the heavy chain of the plantibody Guy's 13 are N-glycosylated as in mouse. However, the number of Guy's 13 glycoforms is higher in the plant than in the mammalian expression system. Despite the high structural diversity of the plantibody N-glycans, glycosylation appears to be sufficient for the production of a soluble and biologically active IgG in the plant system. In addition to high-mannose-type N-glycans, 60% of the oligosaccharides N-linked to the plantibody have beta(1, 2)-xylose and alpha(1, 3)-fucose residues linked to the core Man3GlcNAc2. These plant-specific oligosaccharide structures are not a limitation to the use of plantibody Guy's 13 for topical immunotherapy. However, their immunogenicity may raise concerns for systemic applications of plantibodies in human.

Engineered glycoforms of an antineuroblastoma IgG1 with optimized antibody-dependent cellular cytotoxic activity

The glycosylation pattern of chCE7, an antineuroblastoma chimeric IgG1, was engineered in Chinese hamster ovary cells with tetracycline-regulated expression of beta(1,4)-N-acetylglucosaminyltransferase III (GnTIII), a glycosyltransferase catalyzing formation of bisected oligosaccharides that have been implicated in antibody-dependent cellular cytotoxicity (ADCC). Measurement of the ADCC activity of chCE7 produced at different tetracycline levels showed an optimal range of GnTIII expression for maximal chCE7 in vitro ADCC activity, and this activity correlated with the level of constant region-associated, bisected complex oligosaccharides determined by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. The new optimized variants of chCE7 exhibit substantial ADCC activity and, hence, may be useful for treatment of neuroblastoma. The strategy presented here should be applicable to optimize the ADCC activity of other therapeutic IgGs.