Solid phase synthesis of bifunctional antibodies

Enhancing Antibodies' Binding Capacity through Oriented Functionalization of Plasmonic Surfaces

Protein A has long been used in different research fields due to its ability to specifically recognize immunoglobulins (Ig). The protein derived from Staphylococcus aureus binds Ig through the Fc region of the antibody, showing its strongest binding in immunoglobulin G (IgG), making it the most used protein in its purification and detection. The research presented here integrates, for the first time, protein A to a silicon surface patterned with gold nanoparticles for the oriented binding of IgG. The signal detection is conveyed through a metal enhanced fluorescence (MEF) system. Orienting immunoglobulins allows the exposition of the fragment antigen-binding (Fab) region for the binding to its antigen, substantially increasing the binding capacity per antibody immobilized. Antibodies orientation is of crucial importance in many diagnostics devices, particularly when either component is in limited quantities.

GingisKHAN™ protease cleavage allows a high-throughput antibody to Fab conversion enabling direct functional assessment during lead identification of human monoclonal and bispecific IgG1 antibodies

The determination of the binding strength of immunoglobulins (IgGs) to targets can be influenced by avidity when the targets are soluble di- or multimeric proteins, or associated to cell surfaces, including surfaces introduced from heterogeneous assays. However, for the understanding of the contribution of a second drug-to-target binding site in molecular design, or for ranking of monovalent binders during lead identification, affinity-based assessment of the binding strength is required. Typically, monovalent binders like antigen-binding fragments (Fabs) are generated by proteolytic cleavage with papain, which often results in a combination of under- and over-digestion, and requires specific optimization and chromatographic purification of the desired Fabs. Alternatively, the Fabs are produced by recombinant approaches. Here, we report a lean approach for the functional assessment of human IgG1s during lead identification based on an in-solution digestion with the GingisKHAN™ protease, generating a homogenous pool of intact Fabs and Fcs and enabling direct assaying of the Fab in the digestion mixture. The digest with GingisKHAN™ is highly specific and quantitative, does not require much optimization, and the protease does not interfere with methods typically applied for lead identification, such as surface plasmon resonance or cell-based assays. GingisKHAN™ is highly suited to differentiate between affinity and avidity driven binding of human IgG1 monoclonal and bispecific antibodies during lead identification.

Site-directed antibody immobilization techniques for immunosensors

Immunosensor sensitivity, regenerability, and stability directly depend on the type of antibodies used for the immunosensor design, quantity of immobilized molecules, remaining activity upon immobilization, and proper orientation on the sensing interface. Although sensor surfaces prepared with antibodies immobilized in a random manner yield satisfactory results, site-directed immobilization of the sensing molecules significantly improves the immunosensor sensitivity, especially when planar supports are employed. This review focuses on the three most conventional site-directed antibody immobilization techniques used in immunosensor design. One strategy of immobilizing antibodies on the sensor surface is via affinity interactions with a pre-formed layer of the Fc binding proteins, e.g., protein A, protein G, Fc region specific antibodies or various recombinant proteins. Another immobilization strategy is based on the use of chemically or genetically engineered antibody fragments that can be attached to the sensor surface covered in gold or self-assembled monolayer via the sulfhydryl groups present in the hinge region. The third most common strategy is antibody immobilization via an oxidized oligosaccharide moiety present in the Fc region of the antibody. The principles, advantages, applications, and arising problems of these most often applied immobilization techniques are reviewed.

Preparation of Fab' from murine IgG2a for thiol reactive conjugation

Lysyl endopeptidase (LE) from Achromobacter lyticus M497-1 (EC 3.4.21.50) was utilized to prepare F(ab')2 fragments from mouse anti-P-glycoprotein IgG2a obtained from the UIC2 hybridoma. This report describes a novel single step purification procedure for F(ab')2 fragments that eliminates residual LE activity responsible for secondary cleavage of F(ab')2 to Fab fragments. The purification of F(ab')2 and Fc fragments was accomplished utilizing protein G affinity chromatography and either gradient or step changes in the pH/ionic strength for elution of the Fc and F(ab')2 fragments. Residual LE was eluted from the protein G column with buffer containing 200 mM L-lysine prior to elution of F(ab')2 and Fc fragments. The activity of LE was monitored using the fluorogenic substrate Boc-Val-Leu-Lys-7-amido 4-methyl coumarin. A similar purification procedure for F(ab')2 fragments produced following pepsin digestion of IgG2a is also outlined. The ability of Fab' fragments, from reduced F(ab')2 fragments following LE digestion of IgG2a, to conjugate to thiol reactive groups was demonstrated using N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer-meso chlorin e6 mono (N-2-aminoethylamide) (Mce6) conjugates containing reactive maleimide groups. The biological activity of the Fab' targeted HPMA copolymer-Mce6 conjugates was tested against the P-glycoprotein expressing human ovarian carcinoma A2780/AD cell line utilizing a cell survival assay. Fab' targeted HPMA copolymer-Mce6 conjugate demonstrated significantly higher cytotoxicity than either a monoclonal antibody (mAb) targeted HPMA copolymer-Mce6 conjugate or a non-targeted HPMA copolymer-Mce6 conjugate, p < 0.05.

Synthesis of bifunctional antibodies for immunoassays

The synthesis of bifunctional antibodies using the principle of solid-phase synthesis is described. Two Fab' fragments were chemically linked together via a bismaleimide crosslinking reagent. The F(ab')(2) fragments from intact immunoglobulin G (IgG) were prepared using an immobilized pepsin column. Goat, mouse, and human antibodies were digested completely within 4 h. The F(ab')(2) fragments thus produced did not contain any IgG impurities. Fab' fragments were produced by reducing the heavy interchain disulfide bonds using 2-mercaptoethylamine. Use of the solid-phase reactor in the preparation of the bifunctional antibodies eliminated many of the time-consuming separation steps between the fragmentation and conjugation steps. This procedure facilitates the automation of bifunctional antibody preparation and the rapid optimization of reaction conditions.

Bispecific rabbit Fab'-bovine serum albumin conjugate used in hemagglutination immunoassay for beta-microseminoprotein

A polyclonal bispecific (bifunctional) antibody was prepared to develop a hemagglutination immunoassay for beta-microseminoprotein (beta-MSP), a predominant seminal protein. Three types of F(ab')2 fragments of rabbit IgG, affinity-purified anti-human red blood cell (RBC) F(ab')2 nonaffinity-purified anti-beta-MSP F(ab')2 and nonspecific (nonimmunized) F(ab')2, were mixed to obtain a F(ab')2 mixture containing 10% anti-RBC molecules and 10% anti-beta-MSP molecules. Fab' was obtained from the F(ab')2 mixture, and then reacted with maleimide-activated bovine serum albumin (BSA) at a molar ratio of 10:1. As estimated by the decrease in the maleimide content, approximately 7 Fab' molecules were introduced per one BSA molecule. The bispecific (anti-beta-MSP and anti-RBC) Fab'-BSA conjugate thus prepared was incubated successively with a human RBC suspension and with samples. In the presence of beta-MSP, RBCs become agglutinated, providing a test simple for forensic semen identification.