Generation and characterization of a high-affinity chimeric anti-OX40 antibody with potent antitumor activity

OX40 is a costimulatory molecule that belongs to the tumor necrosis factor receptor (TNFR) superfamily. OX40 agonist-based combinations are emerging as promising candidates for novel cancer immunotherapy. Clinical trials have shown that OX40 agonist antibodies could lead to better results in cancer patients. Using a hybridoma platform and three different types of immunization strategies, namely recombinant protein, DNA, and overexpressing cells, we identified a chimeric anti-OX40 antibody (mAb035-hIgG1 from DNA immunization) that shows excellent binding specificity, and slightly stronger activation of human memory CD4⁺ T cells and similar potent antitumor activity compared with BMS 986178, an anti-OX40 antibody currently being evaluated for the treatment of solid tumors. This paper further systematically investigates the antigen-specific immune response, the number of binders, epitope bins, and functional activities of antibodies among different immunization strategies. Interestingly, we found that different immunization strategies affect the biological activity of monoclonal antibodies.

Preparation and properties of recombinant Clostridium ramosum IgA proteinase. Isolation of Fc-SC and Fab fragments of human secretory IgA

Immunoglobulin A (IgA) proteinase from Clostridium ramosum is the enzyme which cleaves IgA of both subclasses; in contrast, the other bacterial proteinases cleave only IgA1 proteins. Previous reports characterized the activity of proteinase naturally secreted by C. ramosum specific for the normal human serum IgA of IgA1 and IgA2m(1) subclasses and also for secretory IgA (SIgA). Its amino acid sequence was determined, and the recombinant proteinase which cleaved IgA of both subclasses was prepared. Here we report the optimized expression, purification, storage conditions and activity testing against purified human milk SIgA. The recombinant C. ramosum IgA proteinase isolated in the high degree of purity exhibited almost complete cleavage of SIgA of both subclasses. The proteinase remained active upon storage for more than 10 month at -20 °C without substantial loss of enzymatic activity. Purified SIgA fragments are suitable for studies of all antigen-binding and Fc-dependent functions of SIgA involved in the protection against infections with mucosal pathogens.

An improved protocol for large scale production of high purity 'Fc' fragment of human immunoglobulin G (IgG-Fc)

We describe a simplified approach for purification and characterization of human 'IgG-Fc' fragment used widely as immunochemical tool and for therapeutic purposes. The 'Fc' fragment was purified from human IgG in a 3-stage column chromatography. The purified 'Fc' fragment appeared as a dimer glycoprotein with an apparent molecular mass of 52,981 Dalton (Ultraflex MALDI TOF/TOF). The Size-exclusion HPLC profile of the purified 'Fc' fragment of human IgG matched that of a commercially procured reference 'Fc' fragment material. The purity of the 'Fc' fragments was >99% by SDS-PAGE and size-exclusion HPLC. The results of Western blotting, immunoelectrophoresis, and mass spectrometry analysis indicate a high purity of the 'Fc' fragment. Peptide mass fingerprint analysis of the purified 'Fc' protein yielded peptides that partially match the known database sequences of FCG3B_HUMAN (Uniprot ID: O75015). This method of purification of the 'Fc' fragment is suitable for achieving high purity level of 'Fc' fragment protein. With this purification approach, the cost of the purified 'Fc' fragment of human IgG is significantly reduced as compared with the current market price of IgG-Fc fragment protein in international market. The purified 'IgG-Fc' fragment protein was found to be negative for major viral markers.

Choice of Host Cell Line Is Essential for the Functional Glycosylation of the Fc Region of Human IgG1 Inhibitors of Influenza B Viruses

Abs are glycoproteins that carry a conserved N-linked carbohydrate attached to the Fc whose presence and fine structure profoundly impacts on their in vivo immunogenicity, pharmacokinetics, and functional attributes. The host cell line used to produce IgG plays a major role in this glycosylation, as different systems express different glycosylation enzymes and transporters that contribute to the specificity and heterogeneity of the final IgG-Fc glycosylation profile. In this study, we compare two panels of glycan-adapted IgG1-Fc mutants expressed in either the human endothelial kidney 293-F or Chinese hamster ovary-K1 systems. We show that the types of N-linked glycans between matched pairs of Fc mutants vary greatly and in particular, with respect, to sialylation. These cell line effects on glycosylation profoundly influence the ability of the engineered Fcs to interact with either human or pathogen receptors. For example, we describe Fc mutants that potently disrupted influenza B-mediated agglutination of human erythrocytes when expressed in Chinese hamster ovary-K1, but not in human endothelial kidney 293-F cells.

Mechanisms of precipitate formation during the purification of an Fc-fusion protein

Protein precipitates that arise during bioprocessing can cause manufacturing challenges, but they can also aid in clearance of host-cell protein (HCP) and DNA impurities. Such precipitates differ from many protein precipitates that have been studied previously in their heterogeneous composition, particularly in the presence of high concentrations of the product protein. Here, we characterize the precipitates that form after neutralization of protein A purified and viral-inactivated material of an Fc-fusion protein produced in Chinese hamster ovary cells. The physical growth of precipitate particles was observed by optical microscopy, transmission electron microscopy, dynamic light scattering, and small-angle and ultra-small-angle X-ray scattering to characterize the precipitate microstructure and growth mechanism. The precipitate microstructure is well-described as a mass fractal with fractal dimension approximately 2. The growth is governed by a diffusion-limited aggregation mechanism as indicated by a power-law dependence on time of the size of the principal precipitate particles. Optical microscopy shows that these primary particles can further aggregate into larger particles in a manner that appears to be promoted by mixing. Absorbance experiments at varying pH and salt concentrations reveal that the growth is largely driven by attractive electrostatic interactions, as growth is hindered by an increase in ionic strength. The solution conditions that resulted in the most significant particle growth are also correlated with the greatest removal of soluble impurities (DNA and HCPs). Proteomic analysis of the precipitates allows identification of O ( 100 ) unique HCP impurities, depending on the buffer species (acetate or citrate) used for the viral inactivation. Most of these proteins have pI values near the precipitation pH, supporting the likely importance of electrostatic interactions in driving precipitate formation.

Novel ACE2-Fc chimeric fusion provides long-lasting hypertension control and organ protection in mouse models of systemic renin angiotensin system activation

Angiotensin-converting enzyme 2 (ACE2) is a carboxypeptidase that potently degrades angiotensin II to angiotensin 1-7. Previous studies showed that injection of the enzymatic ectodomain of recombinant ACE2 (rACE2) markedly increases circulatory levels of ACE2 activity, and effectively lowered blood pressure in angiotensin II-induced hypertension. However, due to the short plasma half-life of rACE2, its therapeutic potential for chronic use is limited. To circumvent this, we generated a chimeric fusion of rACE2 and the immunoglobulin fragment Fc segment to increase its plasma stability. This rACE2-Fc fusion protein retained full peptidase activity and exhibited greatly extended plasma half-life in mice, from less than two hours of the original rACE2, to over a week. A single 2.5 mg/kg injection of rACE2-Fc increased the overall angiotensin II-conversion activities in blood by up to 100-fold and enhanced blood pressure recovery from acute angiotensin II induced hypertension seven days after administration. To assess rACE2-Fc given weekly on cardiac protection, we performed studies in mice continuously infused with angiotensin II for 28 days and in a Renin transgenic mouse model of hypertension. The angiotensin II infused mice achieved sustained blood pressure control and reduced cardiac hypertrophy and fibrosis. In chronic hypertensive transgenic mice, weekly injections of rACE2-Fc effectively lowered plasma angiotensin II and blood pressure. Additionally, rACE2-Fc ameliorated albuminuria, and reduced kidney and cardiac fibrosis. Thus, our chimeric fusion strategy for rACE2-Fc is suitable for future development of new renin angiotensin system-based inhibition therapies.

Sialic Acid Derivatization for the Rapid Subclass- and Sialic Acid Linkage-Specific MALDI-TOF-MS Analysis of IgG Fc-Glycopeptides

Matrix-assisted laser desorption/ionization (MALDI)-time-of-flight (TOF)-mass spectrometry (MS) is a highly suitable method for the rapid analysis of IgG glycopeptides, providing a wealth of structural information. A limitation of this approach is that it generates a bias when analyzing sialylated species due to the labile nature of sialic acid glycosidic linkages. One way to overcome this problem is by chemical derivatization of the sialic acids. The method presented here results in both the stabilization of the sialic acids, as well as the differentiation of α2,3- and α2,6-linked sialic acids by mass. Described in this chapter are the isolation of IgG from plasma or serum, tryptic digestion of the samples, derivatization, and finally MALDI-TOF-MS measurement and data analysis.

Analysis and quality control of carbohydrates in therapeutic proteins with fluorescence HPLC

Conbercept is an Fc fusion protein with very complicated carbohydrate profiles which must be carefully monitored through manufacturing process. Here, we introduce an optimized fluorescence derivatization high-performance liquid chromatographic method for glycan mapping in conbercept. Compared with conventional glycan analysis method, this method has much better resolution and higher reproducibility making it excellent for product quality control.

Isolation of a pH-Sensitive IgNAR Variable Domain from a Yeast-Displayed, Histidine-Doped Master Library

In recent years, engineering of pH-sensitivity into antibodies as well as antibody-derived fragments has become more and more attractive for biomedical and biotechnological applications. Herein, we report the isolation of the first pH-sensitive IgNAR variable domain (vNAR), which was isolated from a yeast-displayed, semi-synthetic master library. This strategy enables the direct identification of pH-dependent binders from a histidine-enriched CDR3 library. Displayed vNAR variants contained two histidine substitutions on average at random positions in their 12-residue CDR3 loop. Upon screening of seven rounds against the proof-of-concept target EpCAM (selection for binding at pH 7.4 and decreased binding at pH 6.0), a single clone was obtained that showed specific and pH-dependent binding as characterized by yeast surface display and biolayer interferometry. Potential applications for such pH-dependent vNAR domains include their employment in tailored affinity chromatography, enabling mild elution protocols. Moreover, utilizing a master library for the isolation of pH-sensitive vNAR variants may be a generic strategy to obtain binding entities with prescribed characteristics for applications in biotechnology, diagnostics, and therapy.

Glycosylation of plant produced human antibodies

Human immunoglobulins circulate as highly heterogeneously glycosylated mixture of otherwise homogeneous protein backbones. A series of studies, mainly on IgG, have unequivocally proven that antibodies modulate their effector function through sugars present in the Fc domain. However, our limited technology in producing complex proteins such as antibodies, with defined glycan structures hamper in depths studies. This review introduces a plant based expression platform enabling engineering of antibody glycans. The procedure is based on the simultaneous delivery of appropriate constructs, carrying cDNAs of target proteins (e.g. heavy and light chain of antibodies) in combination with human glycosylation enzymes into plant leaves. Harvesting of recombinant proteins one week post construct delivery allows high speed and flexibility. Major achievements include the production of functional active slialylated pentameric IgMs in tobacco leaves. The system provides a viable approach to the generation of antibodies with defined glycoforms on demand, contributing to studies on antibody glycans and the development of novel antibody based drugs.

NMR-based structural validation of therapeutic antibody produced in Nicotiana benthamiana

KEY MESSAGE

We successfully developed a method for metabolic isotope labeling of recombinant proteins produced in transgenic tobacco. This enabled assessment of structural integrity of plant-derived therapeutic antibodies by NMR analysis. A variety of expression vehicles have been developed for the production of promising biologics, including plants, fungi, bacteria, insects, and mammals. Glycoprotein biologics often experience altered folding and post-translational modifications that are typified by variant glycosylation patterns. These differences can dramatically affect their efficacy, as exemplified by therapeutic antibodies. However, it is generally difficult to validate the structural integrity of biologics produced using different expression vehicles. To address this issue, we have developed and applied a stable-isotope-assisted nuclear magnetic resonance (NMR) spectroscopy method for the conformational characterization of recombinant antibodies produced in plants. Nicotiana benthamiana used as a vehicle for the production of recombinant immunoglobulin G (IgG) was grown in a (15)N-enriched plant growth medium. The Fc fragment derived from the (15)N-labeled antibody thus prepared was subjected to heteronuclear two-dimensional (2D) NMR measurements. This approach enabled assessment of the structural integrity of the plant-derived therapeutic antibodies by comparing their NMR spectral properties with those of an authentic IgG-Fc derived from mammalian cells.

The Biochemical Properties of Antibodies and Their Fragments

Immunoglobulins (Ig) or antibodies are powerful molecular recognition tools that can be used to identify minute quantities of a given target analyte. Their antigen-binding properties define both the sensitivity and selectivity of an immunoassay. Understanding the biochemical properties of this class of protein will provide users with the knowledge necessary to select the appropriate antibody composition to maximize immunoassay results. Here we define the general biochemical properties of antibodies and their similarities and differences, explain how these properties influence their functional relationship to an antigen target, and describe a method for the enzymatic fragmentation of antibodies into smaller functional parts.

Immunological similarity of thyroid stimulating antibody (TSAb) and thyroid blocking antibody (TBAb) with animal IgG

Previously we reported neutralization and partial purification of TSAb and TBAb activity using heterophilic antibody (Ab) to animal IgG from Graves' disease. Thus, we examined immunological similarity of TSAb and TBAb with animal IgG using experimentally generated anti-animal IgG [dog (d), bovine (b), porcine (p) and rabbit (rb)] Abs. TBII activity of TSAb- and TBAb-positive serum was neutralized by these anti-animal IgG Abs. Applied TSAb- or TBAb-IgG protein (purified by Protein A) on these anti-animal IgG Abs-bound column was found mainly in the unbound fraction (UF) (>65%) and partially in the bound fraction(BF) (<35%). The TBII and TSAb activity of TSAb-IgG in the BF showed significantly higher than the UF. Thus, the ratio of TBII activity (U/L)/mg protein in the BF/UF was high. TBII activity of TBAb-IgG was similarly purified by this column. We examined immunological characteristics of TSAb-and TBAb-Fab or F(ab')₂ using rabbit anti-bF(ab')₂ Ab. TBII and TSAb activity of TSAb-Fab or- F(ab')₂ and TBII activity of TBAb-Fab or -F(ab')₂ were neutralized by anti-bF(ab')₂ Ab. Partial purification of TSAb- or TBAb-Fab and -F(ab')₂ by anti-bF(ab')₂ Ab-bound column was also possible. Immunological similarity of TSAb- and TBAb-IgG with animal IgG such as d, b, p, rb by anti-animal IgG Ab, and TSAb- or TBAb-Fab and -F(ab')₂ with bFab by anti-bF(ab')₂ Ab were demonstrated. These fact suggest that both Fab and Fc portion of TSAb- and TBAb-IgG molecule have immunological similarity with animal IgG.

Improved Protein-A separation of V(H)3 Fab from Fc after papain digestion of antibodies

Antibody-binding fragments (Fab) are generated from whole antibodies by treatment with papain and can be separated from the Fc component using Protein-A affinity chromatography. Commercial kits are available, which facilitate the production and purification of Fab fragments; however, the manufacturer fails to report that this method is inefficient for antibodies with V(H)3 domains as a result of the intrinsic variable region affinity for Protein-A. A commercially available, modified Protein-A resin (MabSelect SuRe) has been engineered for greater stability. Here, we report that an additional consequence of the modified resin is the ability to purify V(H)3 family Fab fragments, which cannot be separated effectively from other components of the papain digest by traditional Protein-A resin. This improvement of a commonly used procedure is of significance, as increasingly, therapeutic antibodies are being derived from human origin, where V(H)3 is the most abundantly used variable region family.

T-shaped arrangement of the recombinant agrin G3-IgG Fc protein

Agrin is a large heparin sulphate proteoglycan with multiple domains, which is located in the extracellular matrix. The C-terminal G3 domain of agrin is functionally one of the most important domains. It harbors an α-dystroglycan binding site and carries out acetylcholine receptor clustering activities. In the present study, we have fused the G3 domain of agrin to an IgG Fc domain to produce a G3-Fc fusion protein that we intend to use as a tool to investigate new binding partners of agrin. As a first step of the study, we have characterized the recombinant fusion protein using a multidisciplinary approach using dynamic light scattering, analytical ultracentrifugation and small angle X-ray scattering (SAXS). Interestingly, our SAXS analysis using the high-resolution structures of G3 and Fc domain as models indicates that the G3-Fc protein forms a T-shaped molecule with the G3 domains extruding perpendicularly from the Fc scaffold. To validate our models, we have used the program HYDROPRO to calculate the hydrodynamic properties of the solution models. The calculated values are in excellent agreement with those determined experimentally.

Site-specific chemical modification of a glycoprotein fragment expressed in yeast

Site-specific modification of glycoproteins has wide application in both biochemical and biophysical studies. This method describes the conjugation of synthetic molecules to the N-terminus of a glycoprotein fragment, viz., human immunoglobulin G subclass 1 fragment crystallizable (IgG1 Fc), by native chemical ligation. The glycosylated IgG1 Fc is expressed in a glycosylation-deficient yeast strain. The N-terminal cysteine is generated by the endogenous yeast protease Kex2 in the yeast secretory pathway. The N-terminal cysteine is then conjugated with a biotin thioester to produce a biotinylated, glycosylated IgG1 Fc using native chemical ligation.

Crystallization and preliminary X-ray diffraction studies of an RNA aptamer in complex with the human IgG Fc fragment

Aptamers, which are folded DNA or RNA molecules, bind to target molecules with high affinity and specificity. An RNA aptamer specific for the Fc fragment of human immunoglobulin G (IgG) has recently been identified and it has been demonstrated that an optimized 24-nucleotide RNA aptamer binds to the Fc fragment of human IgG and not to other species. In order to clarify the structural basis of the high specificity of the RNA aptamer, it was crystallized in complex with the Fc fragment of human IgG1. Preliminary X-ray diffraction studies revealed that the crystals belonged to the orthorhombic space group P2(1)2(1)2, with unit-cell parameters a = 83.7, b = 107.2, c = 79.0 A. A data set has been collected to 2.2 A resolution.

Structural and molecular basis for hyperspecificity of RNA aptamer to human immunoglobulin G

Potential applications for functional RNAs are rapidly expanding, not only to address functions based on primary nucleotide sequences, but also by RNA aptamer, which can suppress the activity of any target molecule. Aptamers are short DNA or RNA folded molecules that can be selected in vitro on the basis of their high affinity for a target molecule. Here, we demonstrate the ability of RNA aptamers to recognize--and bind to--human IgG with high specificity and affinity. An optimized 23-nucleotide aptamer, Apt8-2, was prepared, and was shown to bind to the Fc domain of human IgG, but not to other IgG's, with high affinity. Apt8-2 was observed to compete with protein A, but not with the Fcgamma receptor, for IgG binding. NMR chemical-shift analyses localized the aptamer-binding sites on the Fc subdomain, which partially overlaps the protein A binding site but not the Fcgamma receptor binding site. The tertiary structures of the predicted recognition sites on the Fc domain differ significantly between human IgG and other species of IgGs; this, in part, accounts for the high specificity of the selected aptamer. Apt8-2 can therefore be used as a protein A alternative for affinity purification of human IgG and therapeutic antibodies. Using Apt8-2 would have several potential advantages, raising the possibility of developing new applications based on aptamer design.

Isolation of anti-MISIIR scFv molecules from a phage display library by cell sorter biopanning

While cell surface antigens represent the most common targets for antibody-based cancer therapy, isolation of new antibodies specific for these targets from single-chain Fv phage display libraries has been hindered by limitations associated with traditional selection techniques. Solid phase panning is often associated with conformational changes to the target protein due to its immobilization on plastic tubes that can limit the ability of the isolated scFv to bind to conformational epitopes and solution panning methods require the use of secondary tags that often mask desired sequences and create unintended epitopes. Commonly utilized cell-based panning methods typically yield a panel of single-chain Fv (scFv) molecules that are specific for numerous cell surface antigens, often obscuring the desired clones. Here, we describe a novel cell sorter-based system to isolate single-chain Fv molecules specific for defined antigen targets expressed on stably-transformed mammalian cells. We employed these methods to isolate promising scFv clones that bind specifically to the Müllerian inhibiting substance type II receptor, a cell surface ovarian cancer antigen that has proven to be a difficult target for selection strategies.

Purification of an Fc-fusion biologic: Clearance of multiple product related impurities by hydrophobic interaction chromatography

An hydrophobic interaction chromatography step was developed for the large-scale production of an Fc-fusion biologic. Two abundant product-related impurities were separated from the active monomer using a Butyl resin and a simple step-wash and step-elution strategy. Capacity and resolution of the HIC step was optimal when sodium sulfate was employed as the lyotropic salt and pore size of the Butyl resin was 750A. Factorial analysis identified critical parameters for the Butyl chromatography and an operating window capable of delivering high product quality and yield over a broad column loading range.

Binding and enrichment ofEscherichia coli spheroplasts expressing inner membrane tethered scFv antibodies on surface immobilized antigens

Anchored periplasmic expression (APEx) is a new method for the isolation of high affinity ligand-binding proteins from large combinatorial libraries (Harvey et al., 2004, Proc Natl Acad Sci USA 101(25): 9193-9198). In APEx, proteins are expressed as fusions to a membrane anchor that tethers them onto the periplasmic side of the Escherichia coli inner membrane. Conversion of the cells into spheroplasts and incubation with soluble fluorescently conjugated ligands results in the specific labeling of cells expressing ligand-binding proteins and their subsequent isolation by flow cytometry. Here we show that scFv antibody fragments expressed in the APEx format allow the binding of spheroplasts to immobilized ligands. ScFv antibodies specific for the cardiac glycoside digoxin or for the protective antigen (PA) of Bacillus anthracis as a negative control were expressed in E. coli as fusions to either N-terminal or C-terminal membrane anchoring domains. Only the C-terminally anchored fusions resulted in specific recognition and binding of spheroplasts onto TentaGel beads with immobilized antigen. Following three rounds of flow cytometric screening, spheroplasts expressing anti-digoxin scFvs were enriched 950-fold from a large excess (1,000 x) of spheroplasts expressing anti-PA antibodies. These results indicate that the APEx technology may be employed for the screening of libraries based on binding to insoluble antigens possibly including antigens on cell surfaces.

Automated high-throughput purification of antibody fragments to facilitate evaluation in functional and kinetic based assays

Screening antibodies from phage displayed in vitro libraries and from affinity maturation of lead antibodies requires testing of antibody fragments (scFvs and Fabs) for function and binding affinities. Crude scFv or Fab periplasmic preparations from Escherichia coli are often not pure and/or concentrated enough for use in functional and affinity assays. We have developed an automated high-throughput approach for small and large-scale expression and purification of His-tagged scFvs and Fabs using the Qiagen BioRobot 3000 LS with optimized application software. This automated procedure enabled us to rapidly evaluate antibody fragments in functional and surface plasmon resonance (SPR) assays. We have used these procedures to make thousands of purified scFv/Fabs for several antibody maturation campaigns and significantly decreased the time needed to select the best candidates. The assay results from these purified samples were used to prioritize candidates before converting them to IgG. This protocol can process up to 300 small-scale and up to 72 large-scale scFvs or Fabs per week per full-time employee (FTE).

Construction of a novel synergistic system for production and recovery of secreted recombinant proteins by the cell surface engineering

We determined whether the cocultivation of yeast cells displaying a ZZ-domain and secreting an Fc fusion protein can be a novel tool for the recovery of secreted recombinant proteins. The ZZ-domain from Staphylococcus aureus protein A was displayed on the cell surface of Saccharomyces cerevisiae under the control of the GAL1 promoter. Strain S. cerevisiae BY4742 cells displaying the ZZ-domain on their surface were used for cocultivation with cells that produce a target protein fused to the Fc fragment as an affinity tag. The enhanced green fluorescent protein or Rhizopus oryzae lipase was genetically fused to the N and C termini of the Fc fragment of human immunoglobulin G, respectively. Through analysis by fluorescence-activated cell sorting and enzymatic assay, it was demonstrated that these fusion proteins are successfully produced in the medium and recovered by affinity binding with the cell surface displaying the ZZ-domain. These results suggest that the ZZ-domain-displaying cell and Fc fusion protein-secreting cell can be applied to use in synergistic process of production and recovery of secreted recombinant proteins.

Downstream processing of monoclonal antibodies—Application of platform approaches

This paper presents an overview of large-scale downstream processing of monoclonal antibodies and Fc fusion proteins (mAbs). This therapeutic modality has become increasingly important with the recent approval of several drugs from this product class for a range of critical illnesses. Taking advantage of the biochemical similarities in this product class, several templated purification schemes have emerged in the literature. In our experience, significant biochemical differences and the variety of challenges to downstream purification make the use of a completely generic downstream process impractical. Here, we describe the key elements of a flexible, generic downstream process platform for mAbs that we have adopted at Amgen. This platform consists of a well-defined sequence of unit operations with most operating parameters being pre-defined and a small subset of parameters requiring development effort. The platform hinges on the successful use of Protein A chromatography as a highly selective capture step for the process. Key elements of each type of unit operation are discussed along with data from 14 mAbs that have undergone process development. Aspects that can be readily templated as well as those that require focused development effort are identified for each unit operation. A brief description of process characterization and validation activities for these molecules is also provided. Finally, future directions in mAb processing are summarized.

Collaborative study to establish human immunoglobulin BRP batch 3 and human immunoglobulin (molecular size) BRP batch 1

A study was carried out by the European Directorate for the Quality of Medicines (EDQM) as part of the joint Biological Standardisation Programme of the Council of Europe and the European Commission with the aim to establish replacement batches of the European Pharmacopoeia (Ph. Eur.) human immunoglobulin Biological Reference Preparation (BRP) batch 2. Twenty-eight laboratories participated in this study. The suitability of the candidate reference preparations to serve as working references in the tests for distribution of the molecular size, anticomplementary activity and Fc function, in accordance with the specifications of the Ph. Eur. monographs Human normal immunoglobulin for intravenous administration (0918), Human normal immunoglobulin (0338) and Anti-T lymphocyte immunoglobulin for human use, animal (1928) was demonstrated. The candidates were therefore established as human immunoglobulin BRP batch 3 and Human immunoglobulin (molecular size) BRP batch 1. The prescribed use of the latter BRP is limited to the test for distribution of molecular size.

Biochemical characterization of rhEpo-Fc fusion protein expressed in CHO cells

One challenge in biotechnology industry is to produce recombinant proteins with prolonged serum half-life. One strategy for enhancing the serum half-life of proteins includes increasing the molecular weight of the protein of interest by fusion to the Fc part of an antibody. In this context, we have expressed a homodimer fusion protein in CHO cells which consists of two identical polypeptide chains, in which our target protein, recombinant human erythropoietin (rhEpo), is N-terminally linked with the Fc part of a human IgG(1) molecule. In the present study, culture supernatant of a stable clone was collected and purified by affinity chromatography prior characterization. We emphasized product quality aspects regarding the fusion protein itself and in addition, post-translational characterization of the subunits in comparison to human antibodies and rhEpo. However, overproduction of recombinant proteins in mammalian cells is well established, analysis of product quality of complex products for different purposes, such as product specification, purification issues, batch to batch consistency and therapeutical consequences, is required. Besides product quantification by ELISA, N-acetylneuraminic acid quantification in microtiterplates, quantitative isoform pattern and entire glycan profiling was performed. By using these techniques for the characterization of the recombinant human Epo-Fc (rhEpo-Fc) molecule itself and furthermore, for the separate characterization of both subunits, we could clearly show that no significant differences in the core glycan structures compared to rhEpo and human antibody N-glycans were found. The direct comparison with other rhEpo-Fc fusion proteins failed, because no appropriate data were found in the literature.

Isolating recombinant antibodies against specific protein morphologies using atomic force microscopy and phage display technologies

Isolation of antibodies to antigens that are either unstable, exist in multiple morphologies or have very limited availability can be prohibitively difficult. Here we describe a novel technique combining the capabilities of phage display antibody technology and atomic force microscopy (AFM) that is used to isolate antibody fragments that bind to a specific morphology of the target antigen, alpha-synuclein. AFM imaging allows us to both visualize the presence and morphology of the target antigen as well as to monitor the efficiency of each step in the bio-panning process. We demonstrate that phage displayed antibodies specific to the target antigen morphology can be isolated after only two rounds of selection. The target antigen, alpha-synuclein, has been correlated with the Parkinson's disease (PD). Accumulation of alpha-synuclein fibrillar aggregates into Lewy body inclusions is a hallmark feature of PD. While alpha-synuclein can form several different aggregate morphologies including oligomers, protofibrils and fibrils, the role of these morphologies in the progression of PD is not known. The successful selection of the recombinant antibody described here can have potential therapeutic value since the single-chain fragment variable (scFv) can be expressed intracellularly to control folding and toxicity of the specific protein aggregates.

Antibody variable region interactions with Protein A: implications for the development of generic purification processes

In this paper, a wide range of antibodies from various subclasses and subfamilies are employed to evaluate the creation of generic separation processes using Protein A chromatography. The reasons for elution pH differences amongst several IgG1s, IgG2s, antibody fragments, and Fc-fusion proteins during Protein A chromatography are investigated using several complimentary techniques. The results indicate that variable region interactions play a major role in determining elution pH for VH3 subfamily antibodies while using traditional protein A chromatographic materials. On the other hand, experiments with a resin which employs a ligand consisting solely of B domain of Protein A indicate that variable region interactions can be mitigated, enabling the use of a single elution pH for a range of antibodies. Finally, the moderation of elution conditions associated with this engineered ligand are shown to minimize problems associated with low pH induced aggregation. It is expected that the findings reported in this paper will facilitate faster process development cycle times for this important class of human therapeutics.

Application to immunoassays of the fusion protein between protein ZZ and enhanced green fluorescent protein

Enhanced green fluorescent protein (EGFP) from Aequorea victoria was fused to the C terminal region of protein ZZ, an artificial synthetic IgG Fc fragment binding protein derived from tandem repeats of the B domain of protein A. The ZZ-EGFP fusion protein was expressed in Escherichia coli with a His(6) tag and purified in high yield by one-step Ni(2+) chelating affinity chromatography. It was then used in the immunoblot analysis of GST and TNFalpha as well as in immunofluorescent assays of 293T cells transfected with IRF3, an interferon regulatory factor which localized in cytoplasm without virus infection. The fusion protein also performed effectively in FACS analysis of surface integrin beta3 subunit on 293 T cells. The chimeric protein bound various antibodies from different animal sources, directed against a variety of proteins. Thus, ZZ-EGFP showed broad promise in potential immunological applications.

Expression of a functional scFv fragment of an anti-idiotypic antibody with a β-lactam hydrolytic activity

The single chain variable fragment (scFv) of an anti-idiotypic catalytic monoclonal antibody, 9G4H9, displaying a beta-lactamase-like activity was cloned. The recombinant protein was expressed through the periplasm in Escherichia coli in the presence or in the absence of FkpA, a chaperone-like enzyme and tested for its hydrolytic activity. The results show that the catalytic parameters for hydrolysis of ampicillin by scFv9G4H9 are clearly influenced by the presence of FkpA, indicating that the correct folding of the fragment represents a crucial step for catalysis.

Dynamics of the gp130 cytokine complex: a model for assembly on the cellular membrane

Cytokines of the interleukin-6 (IL-6)-type family all bind to the glycoprotein gp130 on the cell surface and require interaction with two gp130 or one gp130 and another related signal transducing receptor subunit. In addition, some cytokines of this family, such as IL-6, interleukin-11, ciliary neurotrophic factor, neuropoietin, cardiotrophin-1, and cardiotrophin-1-like-cytokine, interact with specific ligand binding receptor proteins. High- and low-affinity binding sites have been determined for these cytokines. So far, however, the stoichiometry of the signaling receptor complexes has remained unclear, because the formation of the cytokine/cytokine-receptor complexes has been analyzed with soluble receptor components in solution, which do not necessarily reflect the situation on the cellular membrane. Consequently, the binding affinities measured in solution have been orders of magnitude below the values obtained with whole cells. We have expressed two gp130 extracellular domains in the context of a Fc-fusion protein, which fixes the receptors within one dimension and thereby restricts the flexibility of the proteins in a fashion similar to that within the plasma membrane. We measured binding of IL-6 and interleukin-b receptor (IL-6R) by means of fluorescence-correlation spectroscopy. For the first time we have succeeded in recapitulating in a cell-free condition the binding affinities and dynamics of IL-6 and IL-6R to the gp130 receptor proteins, which have been determined on whole cells. Our results demonstrate that a dimer of gp130 first binds one IL-6/IL-6R complex and only at higher ligand concentrations does it bind a second IL-6/IL-6R complex. This view contrasts with the current perception of IL-6 receptor activation and reveals an alternative receptor activation mechanism.

High level production and one-step purification of biologically active ectodysplasin A1 and A2 immunoadhesins using the baculovirus/insect cell expression system

Ectodysplasin A (EDA) is a ligand of the tumor necrosis factor (TNF) family that has been shown to play a crucial role in ectodermal differentiation. Mutations of the syntenic ectodysplasin A gene (Eda) are responsible for Tabby (Ta) phenotype in mice and human X-linked hypohidrotic ectodermal dysplasia (XLHED). EDA-A1 and EDA-A2 are the two main splice variants of Eda, which differ from each other in only two amino acid residues and engage the tumor necrosis factor (TNF) family receptors EDAR and XEDAR, respectively. We have used the baculovirus/insect cell system to express the recombinant EDA proteins fused to the Fc portion of a truncated human IgG1 immunoglobulin heavy chain. Immunoadhesins (4.5-4.7 mg/L) from crude supernatant could be purified to near homogeneity by using rProtein A affinity chromatography. The purified EDA immunoadhesins were endowed with ligand-binding activity as they could bind EDAR or XEDAR on the surface of 293T cells that had been transiently transfected with the corresponding plasmids. Functional activities of EDA immunoadhesins were demonstrated by their ability to activate the NF-kappaB pathway in cells expressing their cognate receptors. These results open up the possibility of obtaining large amounts of purified EDA proteins to investigate EDAR/XEDAR related signaling pathways and for the treatment of patients with X-linked hypohidrotic ectodermal dysplasia.

Use and Optimization of a Dual-Flowrate Loading Strategy To Maximize Throughput in Protein-A Affinity Chromatography

The effect of an alternate strategy employing two different flowrates during loading was explored as a means of increasing system productivity in Protein-A chromatography. The effect of such a loading strategy was evaluated using a chromatographic model that was able to accurately predict experimental breakthrough curves for this Protein-A system. A gradient-based optimization routine is carried out to establish the optimal loading conditions (initial and final flowrates and switching time). The two-step loading strategy (using a higher flowrate during the initial stages followed by a lower flowrate) was evaluated for an Fc-fusion protein and was found to result in significant improvements in process throughput. In an extension of this optimization routine, dynamic loading capacity and productivity were simultaneously optimized using a weighted objective function, and this result was compared to that obtained with the single flowrate. Again, the dual-flowrate strategy was found to be superior.

Absence of protein G-Fc interaction in ficin-derived mouse IgG1 digests

Typical procedure for IgG fragmentation is based on proteolytic cleavage at the hinge region and usually involves a post-digestion purification step. In mice, IgG1 has been found to bind poorly to protein A. As a result, protein G chromatography could be considered as an alternative for Fc removal. Protein G is generally expected to bind specifically to the Fc region of IgG, but applying protein G for the purification of Fab2 fragment of mouse monoclonal IgG1 under standard physiological conditions, we obtained reproducible clone-independent negligible protein G-Fc reactivity and strong protein G-Fab2 interaction.

Site-specific N-glycosylation of chicken serum IgG

Avian serum immunoglobulin (IgG or IgY) is functionally equivalent to mammalian IgG but has one additional constant region domain (CH2) in its heavy (H) chain. In chicken IgG, each H-chain contains two potential N-glycosylation sites located on CH2 and CH3 domains. To clarify characteristics of N-glycosylation on avian IgG, we analyze N-glycans from chicken serum IgG by derivatization with 2-aminopyridine (PA) and identified by HPLC and MALDI-TOF-MS. There were two types of N-glycans: (1) high-mannose-type oligosaccharides (monoglucosylated 26.8%, others 10.5%) and (2) biantennary complex-type oligosaccharides (neutral, 29.9%; monosialyl, 29.3%; disialyl, 3.7%) on molar basis of total N-glycans. To investigate the site-specific localization of different N-glycans, chicken serum IgG was digested with papain and separated into Fab [containing variable regions (VH + VL) + CH1 + CL] and Fc (containing CH3 + CH4) fragments. Con A stained only Fc (CH3 + CH4) and RCA-I stained only Fab fractions, suggesting that high-mannose-type oligosaccharides were located on Fc (CH3 + CH4) fragments, and variable regions of Fab contains complex-type N-glycans. MS analysis of chicken IgG-glycopeptides revealed that chicken CH3 domain (structurally equivalent to mammalian CH2 domain) contained only high-mannose-type oligosaccharides, whereas chicken CH2 domain contained only complex-type N-glycans. The N-glycosylation pattern on avian IgG is more analogous to that in mammalian IgE than IgG, presumably reflecting the structural similarity to mammalian IgE.

Characterization of anti-cyclin E single-chain Fv antibodies and intrabodies in breast cancer cells: enhanced intracellular stability of novel sFv-F(c) intrabodies

Cyclin E is a critical cell cycle protein in the regulated progression of normal cells to replicate their DNA. Ectopic overexpression of cyclin E results in accelerated G(1) progression, chromosome instability, and a reduced requirement for growth factors. Dysregulated cyclin E expression is found in nearly all breast cancers examined. Toward the goal of developing a system to block cyclin E function in normal and breast cancer cells, we have developed anti-cyclin E single-chain antibodies (sFvs) for use as intrabodies. We have cloned the variable region genes from two hybridoma cell lines that produce anti-human cyclin E antibodies, linked them into sFvs, and showed their ability to bind cyclin E when expressed as sFv-F(c) fusion proteins. Engineering of the sFvs as sFv-F(c) intrabodies resulted in a dramatic increase in the sFv half-life as analyzed by pulse-chase and immunofluorescence, and these fusion intrabodies can be expressed in the cytosol or retargeted to the nucleus of breast cancer cell lines. Stable expression of a nuclear-targeted anti-cyclin E intrabody appears to inhibit the growth of the breast cancer cell line SKBR3. This work sets the stage for the development of intrabody-based inducible or tissue-specific cyclin E knockouts and for identifying cyclin E and its vital cell cycle functions as a potential gene therapy target in breast and other cancers.

[Cloning, expression and bio-activity assay of chimeric fusion protein sTNFRII-IgG Fc]

Tumor necrosis factor (TNF) is a key cytokine in immunology system and is related to many human diseases. In order to inhibit the activity of TNF, cDNA coding for soluble TNF receptor II (sTNFRII) and human IgG Fc were linked using a flexible hinge. This gene was expressed in E. coli as a chimeric protein and purified by metal chelate chromatography. The results show that the fusion protein exists in the physiological form as a dimer, has the ability to bind with TNF and inhibits the cytotoxicity of TNF on L929 cells. Contrasting to monomer sTNFRII, the chimeric protein has an improved bioactivity, and displays potential prospects for research and application.

Process characterization for metal-affinity chromatography of an Fc fusion protein: a design-of-experiments approach

The utility of a design-of-experiments approach was investigated for process characterization of a metal-affinity chromatographic purification process for an Fc fusion protein. This approach gave a better understanding of some of the key process variables as well as robustness for this step in the purification process. Single-variable experiments were employed to screen some of the potentially important variables in this step. Ranges for these variables were set based on prior experience in clinical manufacturing with similar processes. Following these experiments, a fractional factorial study was employed to further investigate the most important variables and their interactions. Key operational variables that had an impact on step yield and eluate purity were identified. In addition, the study helped identify a worst-case scenario for the step purity and helped assure that the rest of the process would successfully purify the product. This paper demonstrates the utility of a design-of-experiments approach for the characterization and validation of process chromatography steps in downstream processing. In addition, this study emphasizes the utility of robustness studies early in process development and establishes a strategy for future robustness studies.

A dual-mode approach to the selective separation of antibodies and their fragments

A novel chromatography method for the separation of antibodies is described. The adsorption of antibodies on the solid phase involves interaction with a ligand that combines mild hydrophobic characteristics and some degree of molecular recognition with a derivative of pyridine. This combined effect results in the adsorption of antibodies in the absence of lyotropic salts. When environmental pH is changed, the ligand becomes ionically charged, allowing the desorption of antibodies. The mechanism of adsorption, involving hydrophobic associations and ionic related interaction, is here qualified as dual-mode. Studies on the determination of the apparent dissociation constant for immunoglobulins G are presented. Adsorption of antibodies from crude feedstocks typically occurs without adjustment of pH or ionic strength. The sorbent is then washed with a buffer to eliminate protein impurities and, when lowering the environmental pH, antibodies are desorbed. The solid-phase material is used for the separation of antibodies from an ascites fluid and from a cell culture supernatant, followed by a polishing step on an hydroxyapatite column. Preliminary studies, related to the ability of the solid phase to separate antibody fragments, are also reported. In these studies, it has been demonstrated that both Fab and Fc fragments from polyclonal IgG are adsorbed to the solid phase under typical binding conditions. Under other defined physico-chemical conditions (ionic strength and pH), separation of both fragments in a single step has been achieved.

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.

The unfolding/denaturation of immunogammaglobulin of isotype 2b and its F(ab) and F(c) fragments

The unfolding and further denaturation of IgG and its F(ab) and F(c) fragments were studied both on a macroscopic and molecular level, using differential scanning calorimetry and circular dichroism spectroscopy, respectively. It was shown that the structural integrity of the F(ab) and F(c) units was retained after fragmentation of the IgG. The F(ab) fragment denatured at approximately 61 degrees C and the F(c) fragment at 71 degrees C. The structural transitions observed in the whole IgG is the sum effect of those determined for the isolated F(ab) and F(c) fragments.

Pairing of oligosaccharides in the Fc region of immunoglobulin G

The Fc portion of immunoglobulin G (IgG) expresses paired oligosaccharides with microheterogeneities, which are associated with efficiencies of effector functions and with pathological states. A comparison of electrospray ionization mass spectrometry data obtained using a variety of Fc fragments derived from human and mouse IgG that do and do not retain the inter-chain disulfide bridge(s) revealed that (1) the Fc portion can be asymmetric as well as symmetric with respect to glycosylation and (2) the ratios of the individual glycoforms are different from what is expected from the random pairing.

Sedimentation equilibrium analysis of recombinant mouse FcRn with murine IgG1

The interaction of mouse IgG1 or IgG1-derived Fc fragment with recombinant, insect cell expressed mouse FcRn has been analyzed using sedimentation equilibrium. This results in a model for the interaction in which the two binding sites for FcRn on Fc or IgG1 have significantly different affinities with macroscopic binding constants of < 130 nM and 6 microM. This data indicates the formation of an asymmetric FcRn:Fc (or IgG1):FcRn complex which is consistent with earlier suggestions that for this form of recombinant FcRn, binding to IgG1 or Fc does not result in a symmetric 2:1 complex in which both binding sites are equivalent.

Immunoadhesins of interleukin-6 and the IL-6/soluble IL-6R fusion protein hyper-IL-6

Signal transduction in response to interleukin-6 (IL-6) results from homodimerization of gp130. This dimerization occurs after binding of IL-6 to its surface receptor (IL-6R) and can also be triggered by the complex of soluble IL-6R and IL-6. We fused IL-6 to the constant region of a human IgG1 heavy chain (Fc). IL-6Fc was expressed in COS-7 cells and purified via Protein A Sepharose. Using three different assays we found that the biological activity of this dimeric IL-6 protein is comparable with monomeric IL-6. Recently, we described the designer cytokine Hyper-IL-6 (H-IL-6) in which soluble IL-6R and IL-6 are connected via a flexible peptide linker. This molecule turned out to be 100-1000 times more effective than unlinked IL-6 and soluble IL-6R. Hyper-IL-6 acts on cells only expressing gp130 and is a potent stimulator of in vitro expansion of early hematopoietic precursors. Here we show that a Fc fusion protein of H-IL-6 (H-IL-6Fc) has the same biological activity on BAF/gp130 cells as H-IL-6. Furthermore, both H-IL-6 forms have a similar ability to induce the synthesis of acute phase proteins in human hepatoma cells HepG2 and in mice in vivo. The introduction of a thrombin cleavage site between H-IL-6 and the Fc portion of H-IL-6Fc made it possible to specifically recover biologically active monomeric H-IL-6 by limited proteolysis of the fusion protein. A more general use of cleavable immunoadhesins expressed in mammalian cells is discussed.

Expression and ligand binding assays of soluble cytokine receptor-immunoglobulin fusion proteins

We have developed a cloning vector for the expression of type I cytokine receptor, NO, extracellular domain (ECD)-mouse IgG1 Fc fusion proteins. The vector has a versatile polylinker that allows in-frame cloning of the receptor ECD with the mouse IgG1 sequence to encode a receptor ECD-IgG1 fusion construct. The receptor-IgG1 fusion proteins are transiently expressed in useful amounts following transfection of the expression vector into COS7 cells and G418 selection. The mouse IgG1 portion of the fusion protein provides a universal handle for purification on an affinity matrix and detection by anti-mouse IgG antibodies in ELISA or Western blot formats. The expressed receptor ECD-IgG1 fusion proteins bind their cognate ligands. In order to demonstrate that the fusion proteins have similar ligand binding affinities as the native receptors, the affinity constants (Kd) for EPOR, TNFR, IL-4R, and IL-6R-IgG1 fusion proteins were measured by surface plasmon resonance and shown to be in good agreement with published values. The TNFR-IgG1 fusion protein was employed in a demonstration of a novel ELISA format for detecting cytokine receptor binding to cytokine.

Limited diversity of human scFv fragments isolated by panning a synthetic phage-display scFv library with cultured human melanoma cells

To broaden the specificity of the Abs recognizing human melanoma-associated Ags (MAAs), we have isolated human single-chain fragment of the V region (scFv) fragments by panning the synthetic phage Ab library (#1) with the human melanoma cell lines S5 and SK-MEL-28. All of the isolated scFv fragments reacted with the mouse mAb defined high molecular weight melanoma-associated Ag (HMW-MAA). scFv #70 immunoprecipitates the two characteristic subunits of HMW-MAA, while scFv #28 only immunoprecipitates its large subunit. These results challenge the current view regarding the structure of HMW-MAA and indicate that it consists of two independent subunits. The human scFv fragments share some similarities with the mouse anti-HMW-MAA mAb. Like mAb 149.53 and 225.28, scFv #28 reacts with rat B49 neural cells that express a homologue of HMW-MAA. scFv #70 reacts with a determinant that is spatially close to the one identified by mAbs 149.53, VT68.2, and VT86. Besides suggesting similarities in the recognition of human melanoma cells by the mouse and human Ab repertoire, these results indicate that the Abs isolated from synthetic Ab libraries resemble those that are found in natural Ab repertoires. The restricted diversity of the scFv fragments that were isolated by panning synthetic Ab libraries with different melanoma cell lines suggests that certain Ags, like HMW-MAA, are immunodominant in vitro. This phenomenon, which parallels the in vivo immunodominance of certain Ags, implies that the antigenic profile of the cells used for panning determines the specificity of the preponderant population of isolated Abs.

Protein A mimetic peptide ligand for affinity purification of antibodies

A peptide mimicking protein A for its ability to recognize the Fc immunoglobulin portion has been identified through screening of a synthetic multimeric peptide library. Screening of the multimeric library, composed of randomized synthetic tripeptide tetramers, has been carried out using a very simple assay, measuring the library ability to interfere with the interaction between protein A and biotinylated immunoglobulins, monitored on solid phase using an enzyme-linked immunosorbent assay format. The tetrameric tripeptide identified after three screening cycles was produced in larger amounts and then immobilized in high yield on preactivated solid support for the preparation of affinity columns, which proved useful for a very convenient one-step purification of antibodies directly from crude sera. Antibody purity after affinity purification was close to 95 per cent, as determined by densitometric scanning of sodium dodecyl sulphate-polyacrylamide gel electrophoresis gels of purified fractions, and up to 2 mg of antibody could be purified from 1 ml of peptide-derivatized affinity support. The ligand was stable to treatment with a vast array of sanitation agents, such as ethanol and 0.1 M sodium hydroxide, and to repeated use, thus making the ligand applicability extremely attractive for the purification of monoclonal antibodies for therapeutic use. Column binding selectivity was similar to that of protein A-affinity columns, since immunoglobulin G from several sources (rabbit, goat, sheep, mouse) was conveniently purified, with no detection of leaked ligand fragments in the purified preparations.

Hydrodynamic studies of a complex between the Fc fragment of human IgE and a soluble fragment of the Fc epsilon RI alpha chain

The interaction between immunoglobulin E (IgE) and its high-affinity receptor Fc epsilon RI is central to allergic disease. The binding site for Fc epsilon RI lies in the third constant region domain of the epsilon heavy chain of IgE (C epsilon 3). Identical epitopes on the two C epsilon 3 domains in the IgE-Fc are predicted to be on opposite sides of the structure, and therefore each could bind independently to a receptor molecule. Titrations, however, reveal that the IgE-Fc forms an equimolar complex with a soluble fragment of the Fc epsilon RI alpha chain (sFc epsilon RI alpha), and the molecular weight of the complex, as determined by sedimentation equilibrium, confirms this stoichiometry. The measured sedimentation coefficients of the two ligands are in good agreement with computed values for a compact IgE-Fc and an elongated sFc epsilon RI alpha structure. The calculated sedimentation coefficients for possible models of a 1:1 complex lead to exclusion of all highly extended geometries for the complex. Possible explanations for the paradoxical stoichiometry of the IgE-Fc/sFc epsilon RI alpha complex, in terms of the curved shape of IgE, a conformational change in IgE when the receptor binds, and steric interference between two molecules of Fc epsilon RI binding to identical sites, are discussed.