Accumulation of secreted antibodies in plant cell cultures varies according to the isotype,host species and culture conditions

Nicotiana tabacum suspension cells have been widely used to produce monoclonal antibodies, but the yield of secreted antibodies is usually low probably because of proteolytic degradation. Most IgGs that have been expressed in suspension cells have been of the human IgG1 isotype. In this study, we examined whether other isotypes displayed the same sensitivity to proteolytic degradation and whether the choice of plant host species mattered. Human serum IgG displayed different degradation profiles when incubated in spent culture medium from N. tabacum, Nicotiana benthamiana or Arabidopsis thaliana suspension cells. Zymography showed that the protease profile was host species dependent. Three human isotypes, IgG1, IgG2 and IgG4, and a mouse IgG2a were provided with the same heavy‐ and light‐chain variable regions from an anti‐human IgM antibody and expressed in N. tabacum cv. BY‐2 and A. thaliana cv. Col‐0 cells. Although all tested isotypes were detected in the extracellular medium using SDS‐PAGE and a functional ELISA, up to 10‐fold differences in the level of intact antibody were found according to the isotype expressed, to the host species and to the culture conditions. In the best combination (BY‐2 cells secreting human IgG1), we reported accumulation of more than 30 mg/L of intact antibody in culture medium. The possibility of using IgG constant regions as a scaffold to allow stable accumulation of antibodies with different variable regions was demonstrated for human IgG2 and mouse IgG2a.     

Cosecretion of protease inhibitor stabilizes antibodies produced by plant roots

A plant-based system for continuous production of monoclonal antibodies based on the secretion of immunoglobulin complexes from plant roots into a hydroponic medium (rhizosecretion) was engineered to produce high levels of single-chain and full-size immunoglobulins. Replacing the original signal peptides of monoclonal antibodies with a plant-derived calreticulin signal increased the levels of antibody yield 2-fold. Cosecretion of Bowman-Birk Ser protease inhibitor reduced degradation of the immunoglobulin complexes in the default secretion pathway and further increased antibody production to 36.4 microg/g root dry weight per day for single-chain IgG1 and 21.8 microg/g root dry weight per day for full-size IgG4 antibodies. These results suggest that constitutive cosecretion of a protease inhibitor combined with the use of the plant signal peptide and the antibiotic marker-free transformation system offers a novel strategy to achieve high yields of complex therapeutic proteins secreted from plant roots.     

Development of single-chain Fv of antibody to DNA as intracellular delivery vehicle

Application of antibodies in most therapeutic area is limited to extracellular or membranous targets because of their impermeability of membrane. For the purpose of biotechnological and therapeutic application, developing intracellular localizing antibody is the invaluable research field. A new recombinant single-chain variable fragment of an anti-dsDNA monoclonal antibody G2-6, IgG of which has been previously shown to have a cell-penetrating activity, was engineered and produced for the use as a delivery vehicle of biomolecule(s). The penetrating capacity of single-chain variable fragment in three mammalian cell lines, L929, NIH/3T3, and COS-7 was analyzed using flow cytometry and confocal microscopy. The results demonstrated that the single-chain variable fragment can effectively internalize all three cell lines, although the internalization level varied. It was also shown that the internalization was time- and concentration-dependent. Moreover, the single-chain variable fragment was located in nuclei as well as cytoplasm of L929 cells. Overall, the G2-6 single-chain variable fragment might be a candidate vehicle which could be used to deliver specific genes or biomolecules for therapy or diagnosis into the cytoplasm or cell nucleus.     

Construction and expression of anti-Tn-antigen-specific single-chain antibody genes from hybridoma producing MLS128 monoclonal antibody

Anti-Tn-antigen monoclonal antibody MLS128 has affinity for three consecutive Tn-antigens (Tn3) more than Tn2. The major aim of this study was to isolate genes encoding MLS128 variable domains to produce a large quantity of recombinant MLS128 antibodies, in turn, allowing the conduct of studies on precise interactions between Tn3- or Tn2-epitopes and MLS128. This study describes cloning of the variable region genes of MLS128, construction of the variable region genes in single-chain variable fragments (scFv) and two scFvs conjugated with human IgG(1) hinge and Fc regions (scFv-Fc) types, and their respective expression in bacterial and mammalian cell. MLS128 scFv protein with the expected specificity and affinity was successfully prepared from inclusion bodies accumulating in Escherichia coli. Construction, expression and purification of two types of MLS128-scFv-Fc proteins with differing linker lengths in Chinese hamster ovary cells demonstrated that the purified scFv-Fc proteins had binding activity specific to the glycoprotein-expressing Tn-antigen clusters. These results revealed that VL and VH genes cloned from the hybridoma represent those of MLS128 and that recombinant antibodies produced from these genes should provide sufficient amounts of binding domains for use in 3D structural studies such as NMR and X-ray analysis.     

Hybridoma cell growth and anti-neuroblastoma monoclonal antibody production in spinner flasks using a protein-free medium with microcarriers

The main disadvantages of foetal calf serum as the world-wide common serum supplement for cell growth are its content of various proteins of variable concentrations between batches as well as its high cost. The use of serum-free and protein-free media is gradually becoming one of the goals of cell culture especially for standardizing culture conditions or for simple purification of cell products like monoclonal antibodies. The mouse hybridoma cells 14/2/1 were cultivated either in protein-free UltraDOMA medium or in serum-containing RPMI medium with and without microcarriers to generate high quantities of monoclonal antibodies against neuroblastoma tumour cells. Cell growth rate, IgG production, viability, glucose and lactate concentrations, attachment rate and doubling time have been used as investigation criteria. Modifications of culture procedures (static or stirred), inoculum density, and microcarrier concentration caused an improvement of monoclonal antibody production. The kinetics of antibody synthesis was best in spinner culture with 2 ml of microcarriers in protein-free medium. These results of short-term microcarrier culture in stirred spinner flasks indicate that IgG yields in protein-free medium 2.5-fold higher to those in serum-supplemented medium can be achieved.     

Generating bispecific human IgG1 and IgG2 antibodies from any antibody pair

Bispecific antibodies and antibody fragments are a new class of therapeutics increasingly utilized in the clinic for T cell recruitment (catumaxomab anti-EpCAM/CD3 and blinatumomab anti-CD19/CD3), increase in the selectivity of targeting, or simultaneous modulation of multiple cellular pathways. While the clinical potential for certain bispecific antibody formats is clear, progress has been hindered because they are often difficult to manufacture, may suffer from suboptimal pharmacokinetic properties, and may be limited due to potential immunogenicity issues. Current state-of-the-art human IgG-like bispecific technologies require co-expression of two heavy chains with a single light chain, use crossover domains to segregate light chains, or utilize scFv (single-chain fragment variable)-Fc fusion. We have engineered both human IgG1 and IgG2 subtypes, with minimal point mutations, to form full-length bispecific human antibodies with high efficiency and in high purity. In our system, the two antibodies of interest can be expressed and purified separately, mixed together under appropriate redox conditions, resulting in a formation of a stable bispecific antibody with high yields. With this approach, it is not necessary to generate new antibodies that share a common light chain, therefore allowing the immediate use of an existing antibody regardless of whether it has been generated via standard hybridoma or display methods. We demonstrate the generality of the approach and show that these bispecific antibodies have properties similar to those of wild-type IgGs, and we further demonstrate the utility of the technology with an example of a CD3/CD20 bispecific antibody that effectively depletes B cells in vitro and in vivo.     

Expression and functional analysis of recombinant scFv and diabody fragments with specificity for human RhD

In an attempt to generate recombinant anti-D reagents for possible diagnostic and therapeutic use we cloned the genes encoding the variable (V) domains of a human anti-D antibody secreted by the lymphoblastoid cell line BTSN4. A single-chain Fv (scFv) fragment was constructed using a 21 amino acid linker to join the genes encoding the variable domains of the BTSN4 heavy (VH) and light chains (VL). A diabody construct was also generated by reducing the length of the scFv linker from 21 to 10 residues. The scFv and diabody constructs were cloned into the pFLAG-CTS vector, expressed in E. coli host cells and the recombinant proteins were affinity-isolated from bacterial culture medium. Analysis of the recombinant proteins indicated that they retained the D antigen binding specificity of the parental BTSN4 IgG. Furthermore, both fragments mediated agglutination of papain-treated D positive erythrocytes in the absence of a cross-linking second antibody. While the agglutinating property of BTSN4 diabody was readily explained by the non-covalent association of this protein as a bivalent dimer, oligomeric forms of BTSN4 scFv were not detected when the protein was analysed by size exclusion chromatography. Thus, the agglutinating property of the scFv is not the result of the formation of non-covalently associated multimeric forms of the antibody fragment.     

Recombinant full-size human antibody to Ebola virus

A full-size human antibody to Ebola virus was constructed by joining genes encoding the constant domains of the heavy and light chains of human immunoglobulin with the corresponding DNA fragments encoding variable domains of the single-chain antibody 4D1 specific to Ebola virus, which was chosen from a combinatorial phage display library of single-strand human antibodies. Two expression plasmids. pCH1 and pCL1, containing the artificial genes encoding the light and heavy chains of human immunoglobulin, respectively, were constructed. Their cotransfection into the human embryonic kidney cell line HEK293T provided the production of a full-size recombinant human antibody. The affinity constant for the antibody was estimated by solid-phase enzyme-linked immunoassay to be 7.7 x 10(7) +/- 1.5 x 10(7) M(-1). Like the parent single-chain antibody 4DI, the resulting antibody bound the nucleoprotein of Ebola virus and did not interact with the proteins of Marburg virus.     

Engineering disulfide bonds within an antibody

Antibodies have evolved to function in oxidative, extracellular environments. A pair of cysteines in close proximity will oxidatively react to form a disulfide bond that fixes and stabilizes the tertiary structure of a protein. Immunoglobulin G (IgG) includes several disulfide bonds, and the patterns of inter-chain disulfide bonds characterize different IgG sub-classes. Moreover, the Ig-fold domains are characterized by a buried intra-domain disulfide bond, which is important for its structural stability. However, the intra-domain disulfide bond can be replaced without crucial effects on the structure and function, if the domain structure is intrinsically stable or has been stabilized by protein engineering. In previous studies, disulfide bonds were removed by amino-acid substitution indicating that Val and/or Ala (i.e. Ala–Ala, Ala–Val, Val–Ala, and Val–Ala) pairs were preferred for cysteine replacement in the Ig-fold domain. As such, these mutations may be useful for the intracellular use of antibodies. Recently, additional intra-domain disulfide bonds have been shown to stabilize Ig-fold domains and whole IgGs. In heavy chain variable or light chain variable domains, the introduction of additional disulfide bonds into the framework region did not reduce antigen-binding affinity, suggesting that generating disulfide bonds may be a method for stabilizing IgG and antibody fragments, such as the antigen-binding fragment, and single-chain and single-domain antibodies. This article is part of a Special Issue entitled: Recent advances in molecular engineering of antibody.     

Development and characterization of a novel fusion protein composed of a human IgG1 heavy chain constant region and a single-chain fragment variable antibody against Venezuelan equine encephalitis virus

Murine monoclonal antibody 1A4A1 has been shown to recognize a conserved neutralizing epitope of envelope glycoprotein E2 of Venezuelan equine encephalitis virus. It is a potential candidate for development of a second generation antibody for both immunodiagnosis and immunotherapy. In order to minimize the immunogenicity of murine antibodies and to confer human immune effector functions on murine antibodies, a recombinant gene fusion was constructed. It encoded a human IgG1 heavy chain constant region and a single-chain fragment variable antibody of 1A4A1. After expression in bacteria as inclusion bodies, the recombinant antibody was purified and refolded in vitro. The recombinant soluble antibody was demonstrated to retain high antigen-binding affinity to Venezuelan equine encephalitis virus and to possess some human IgG crystallizable fragment domain functions, such as recognition by protein G and human complement C1q binding. On non-reducing and reducing gel electrophoresis analysis of proteolytic fragments of the recombinant antibody, disulfide bond formation was found in the hinge region of the antibody. From these data, it was concluded that the recombinant antibody was capable of antigen recognition, and retained several functional activities. This work forms the basis for characterization of the recombinant antibody as to efficacy in vivo.     

Structure-based engineering to restore high affinity binding of an isoform-selective anti-TGFβ1 antibody

Metelimumab (CAT192) is a human IgG4 monoclonal antibody developed as a TGFβ1-specific antagonist. It was tested in clinical trials for the treatment of scleroderma but later terminated due to lack of efficacy. Subsequent characterization of CAT192 indicated that its TGFβ1 binding affinity was reduced by ～50-fold upon conversion from the parental single-chain variable fragment (scFv) to IgG4. We hypothesized this result was due to decreased conformational flexibility of the IgG that could be altered via engineering. Therefore, we designed insertion mutants in the elbow region and screened for binding and potency. Our results indicated that increasing the elbow region linker length in each chain successfully restored the isoform-specific and high affinity binding of CAT192 to TGFβ1. The crystal structure of the high binding affinity mutant displays large conformational rearrangements of the variable domains compared to the wild-type antigen-binding fragment (Fab) and the low binding affinity mutants. Insertion of two glycines in both the heavy and light chain elbow regions provided sufficient flexibility for the variable domains to extend further apart than the wild-type Fab, and allow the CDR3s to make additional interactions not seen in the wild-type Fab structure. These interactions coupled with the dramatic conformational changes provide a possible explanation of how the scFv and elbow-engineered Fabs bind TGFβ1 with high affinity. This study demonstrates the benefits of re-examining both structure and function when converting scFv to IgG molecules, and highlights the potential of structure-based engineering to produce fully functional antibodies.     

Expression of humanized anti-Her2/neu single-chain IgG1-like antibody in mammary glands of transgenic mice

A system for production of single-chain antibody in mammary glands of mice was developed on the basis of a hybrid gene constructed from the coding sequence of anti-Her2/neu single-chain antibody inserted into the first exon of the sheep beta-lactoglobulin gene. Lines of transgenic mice were obtained that expressed humanized single-chain anti-Her2/neu IgG1-like antibody in their milk. These antibodies interact with Her2/neu antigen with high affinity (K_d = 0.4 nM). The expression level of the transgene depended on its integration site in the genome but not on the copy number. The transgene had no toxic effect on the mice and was stably inherited, at least for two generations. The results reveal new opportunities of producing single-chain antibodies in the milk of animals.     

Identification of novel neutralizing single-chain antibodies against vascular endothelial growth factor receptor 2

Human vascular endothelial growth factor (VEGF) and its receptor (VEGFR-2/kinase domain receptor [KDR]) play a crucial role in angiogenesis, which makes the VEGFR-2 signaling pathway a major target for therapeutic applications. In this study, a single-chain antibody phage display library was constructed from spleen cells of mice immunized with recombinant human soluble extracellular VEGFR-2/KDR consisting of all seven extracellular domains (sKDR D1-7) to obtain antibodies that block VEGF binding to VEGFR-2. Two specific single-chain antibodies (KDR1.3 and KDR2.6) that recognized human VEGFR-2 were selected; diversity analysis of the clones was performed by BstNI fingerprinting and nucleotide sequencing. The single-chain variable fragments (scFvs) were expressed in soluble form and specificity of interactions between affinity purified scFvs and VEGFR-2 was confirmed by ELISA. Binding of the recombinant antibodies for VEGFR-2 receptors was investigated by surface plasmon resonance spectroscopy. In vitro cell culture assays showed that KDR1.3 and KDR2.6 scFvs significantly suppressed the mitogenic response of human umbilical vein endothelial cells to recombinant human VEGF(165) in a dose-dependent manner, and reduced VEGF-dependent cell proliferation by 60% and 40%, respectively. In vivo analysis of these recombinant antibodies in a rat cornea angiogenesis model revealed that both antibodies suppressed the development of new corneal vessels (p < 0.05). Overall, in vitro and in vivo results disclose strong interactions of KDR1.3 and KDR2.6 scFvs with VEGFR-2. These findings indicate that KDR1.3 and KDR2.6 scFvs are promising antiangiogenic therapeutic agents.     

Recombinant full-size human antibody to Ebola virus

A full-size human antibody to Ebola virus was constructed by joining genes encoding the constant domains of the heavy and light chains of human immunoglobulin with the corresponding DNA fragments encoding variable domains of the single-chain antibody 4D1 specific to Ebola virus, which was chosen from a combinatorial phage display library of single-strand human antibodies. Two expression plasmids, pCH1 and pCL1, containing the artificial genes encoding the light and heavy chains of human immunoglobulin, respectively, were constructed. Their cotransfection into the human embryonic kidney cell line HEK293T provided the production of a full-size recombinant human antibody. The affinity constant for the antibody was estimated by solid-phase enzyme-linked immunoassay to be 7.7 × 10⁷ ± 1.5 × 10⁷ M^?1. Like the parent single-chain antibody 4D1, the resulting antibody bound the nucleoprotein of Ebola virus and did not interact with the proteins of Marburg virus.     

Functional construction of the anti-mucin core protein (MUC1) antibody MUSE11 variable regions in a bacterial expression system

A bacterial expression system for the variable region fragments (Fvs) of the anti-MUC1 tumor antigen antibody MUSE11 has been constructed. The Fv fragment showed binding specificity toward TFK-1 cells, with slightly reduced affinity compared to its parent IgG. The single-chain Fv fragment was arranged in two orders, VH-linker-VL and VL-linker-VH. However, linking the regions with a flexible peptide linker (GGGGS)(3) or with a shorter linker (GGGGS) led to a dramatic decrease in the biological activity toward the target antigen in both arrangements, suggesting that the MUSE11 antibody loses its activity when the domains are linked with polypeptide linkers. These results indicate that the variable region domains of the anti-MUC1 antibody MUSE11 have specificity only in the Fv form, and that linking the domains strongly reduces the association with its target antigen. Gel filtration analysis indicates that the scFv has a dimeric structure, suggesting that the inactivation of MUSE11 scFv is due to unfavorable intermolecular associations of the scFv chains. To our knowledge, this is the first report of a significant reduction in affinity caused by linking the variable domains in both arrangements, i.e., VH-VL and VL-VH.     

Production of an Anti-Prostate-Specific Antigen Single-Chain Antibody Fragment from Pichia pastoris

Prostate-specific antigen (PSA) is a widely used marker for screening and monitoring prostate cancer. Because PSA levels are normally quite low, an antibody-based assay must be used to detect PSA. However, not all PSA-specific antibodies bind equally well to PSA or to its different isoforms. Therefore, a better understanding of how PSA interacts with PSA-specific antibodies is of considerable clinical interest. B80.3 is a widely used murine monoclonal anti-PSA antibody (IgG), which has very high affinity for both free and α-anti-chymotrypsin complexed PSA. More importantly, its gene sequence is known—making it one of only two anti-PSA antibodies that has been fully cloned and sequenced. To better elucidate the interaction between PSA and B80.3, a single-chain antibody fragment, derived from the variable domain of B80.3 (scFvB80), was cloned into a pPIC9 vector and expressed in Pichia pastoris. The secreted protein was purified using a three-step protocol beginning with a 50% ammonium sulfate precipitation step, followed by a T-gel thio-affinity step and concluding with a simple anion-exchange (DE52) filtration step. NMR studies indicate the protein is correctly folded while competitive enzyme-linked immunosorbant assays show that the purified scFvB80 has approximately 20% of the activity of the full-length B80.3 antibody. The protocol described here provides a quick and convenient route to prepare large quantities of very pure anti-PSA antibody fragments (15–20 mg/L culture medium) for detailed structural and biophysical characterization.     

Production of an anti-prostate-specific antigen single-chain antibody fragment from Pichia pastoris.

Prostate-specific antigen (PSA) is a widely used marker for screening and monitoring prostate cancer. Because PSA levels are normally quite low, an antibody-based assay must be used to detect PSA. However, not all PSA-specific antibodies bind equally well to PSA or to its different isoforms. Therefore, a better understanding of how PSA interacts with PSA-specific antibodies is of considerable clinical interest. B80.3 is a widely used murine monoclonal anti-PSA antibody (IgG), which has very high affinity for both free and alpha-anti-chymotrypsin complexed PSA. More importantly, its gene sequence is known-making it one of only two anti-PSA antibodies that has been fully cloned and sequenced. To better elucidate the interaction between PSA and B80.3, a single-chain antibody fragment, derived from the variable domain of B80.3 (scFvB80), was cloned into a pPIC9 vector and expressed in Pichia pastoris. The secreted protein was purified using a three-step protocol beginning with a 50% ammonium sulfate precipitation step, followed by a T-gel thio-affinity step and concluding with a simple anion-exchange (DE52) filtration step. NMR studies indicate the protein is correctly folded while competitive enzyme-linked immunosorbant assays show that the purified scFvB80 has approximately 20% of the activity of the full-length B80.3 antibody. The protocol described here provides a quick and convenient route to prepare large quantities of very pure anti-PSA antibody fragments (15-20 mg/L culture medium) for detailed structural and biophysical characterization.     

Production of engineered IgM-binding single-chain antibodies inEscherichia coli

Summary Two single-chain antibodies were engineered and tested as novel binding proteins with specificity for immunoglobulin M. Genes for the two single-chain Fv proteins were assembled from the variable light chain cDNA and variable heavy chain cDNA of monoclonal antibodies DA4.4 and Bet 2, which specifically bind human IgM and mouse IgM, respectively. Both single-chain Fv proteins were designed with a 14-amino acid linker which bridged the variable light chain and variable heavy chain domains. The two proteins were expressed inEscherichia coli, purified and assayed for IgM-binding activity. Both proteins demonstrate a binding specificity for their corresponding IgM which is similar to the monoclonal antibodies from which they were derived. These small IgM-binding proteins may have applications in the investigation of the immune response and in the detection and purification of monoclonal antibodies, cell-associated antibodies, and IgM from serum.     

Isothermal titration calorimetry reveals differential binding thermodynamics of variable region-identical antibodies differing in constant region for a univalent ligand

The classical view of immunoglobulin molecules posits two functional domains defined by the variable (V) and constant (C) regions, which are responsible for antigen binding and antibody effector functions, respectively. These two domains are thought to function independently. However, several lines of evidence strongly suggest that C region domains can affect the specificity and affinity of an antibody for its antigen (Ag), independent of avidity-type effects. In this study, we used isothermal titration calorimetry to investigate the thermodynamic properties of the interactions of four V region-identical monoclonal antibodies with a univalent peptide antigen. Comparison of the binding of IgG1, IgG2a, IgG2b, and IgG3 with a 12-mer peptide mimetic of Cryptococcus neoformans polysaccharide revealed a stoichiometry of 1.9-2.0 with significant differences in thermodynamic binding parameters. Binding of this peptide to the antibodies was dominated by favorable entropy. The interaction of these antibodies with biotinylated peptides manifested greater enthalpy than for native peptides indicating that biotin labeling affected the types of Ag-Ab complexes formed. Our results provide unambiguous thermodynamic evidence for the notion that the C region can affect the interaction of the V region with an Ag.     

Expression of anti-tumor recombinant IgG- and IgE-like genes in eukaryotic cells

The tandem of humanized variable VL and VH genes (ScFv fragment 4D5) possessing a high affinity to the HER-2/neu oncogene (the epidermal growth factor receptor expressed in many types of human tumors) was attached through a flexible linker to the second exon of human antibodies of IgG1 or IgE isotypes constant gene. The humanized construct of IgE isotype was generated for the first time. Genes of the recombinant antibodies were cloned into the pCl-neo vector under the control of universal cytomegalovirus (CMV) promoter. Transfected HEK-293 cells efficiently produced antibodies of the corresponding isotypes IgE and IgG1. The results of Western blotting confirmed homogeneity of the expressed antibodies, which had the predicted molecular weight and specifically interacted with the HER-2/neu. The attachment of leader peptide to the 5'-end of the gene resulted in the preferential accumulation of recombinant antibodies in the cultural medium. These results indicate that de novo constructed humanized immunoglobulin genes express functionally active, single-chain recombinant antibodies in eukaryotic cells.