Fc-specific and covalent conjugation of a fluorescent protein to a native antibody through a photoconjugation strategy for fabrication of a novel photostable fluorescent antibody

Fabrication of cysteine-modified antibodies with Fc-specific conjugation for covalent and oriented immobilization of native antibodies

Covalent and oriented immobilization of antibodies (Abs) can substantially improve the sensitivity and stability of solid-phase immunoassays. By modifying the natural Abs with functional groups that provide unique handles for further conjugation, Abs could be immobilized onto the solid matrices with uniform orientation. Herein, an effective approach for Fc-specific modification of Abs was developed for the oriented and covalent immobilization of Abs. Twelve photoreactive Z-domain variants, incorporated with a photoactivable probe (p-benzoyl-L-phenylalanine, Bpa) at different positions and carrying a C-terminal Cys-tag (i.e. ZBpa-Cys variants), were individually constructed and produced in Escherichia coli and tested for photo-cross-linking to various IgGs. The different ZBpa-Cys variants demonstrated large differences in photo-conjugation efficiency for the tested IgGs. The conjugation efficiencies of 17thZBpa-Cys ranged from 90 % to nearly 100 % for rabbit IgG and mouse IgG2a, IgG2b and IgG3. Other variants, including 5thZBpa-Cys, 18thZBpa-Cys, 32thZBpa-Cys, and 35thZBpa-Cys, also displayed conjugation efficiencies of 61 %-83 % for mouse IgG1, IgG2a and IgG3. Subsequently, the photo-modified Abs, namely IgG-Cys conjugates, were covalently immobilized onto a maleimide group-functionalized solid-phase carrier on the basis of the reaction of sulfhydryl and maleimide. Thus, a generic platform for the controlled and oriented immobilization of Abs was developed, and the efficacy and potential of the proposed approach for sensitive immunoassays was demonstrated by detecting human α-fetoprotein.

Chemoselective Methionine Labelling of Recombinant Trastuzumab Shows High In Vitro and In Vivo Tumour Targeting

A highly effective 2-step system for site-specific antibody modification and conjugation of the monoclonal antibody Herceptin (commercially available under Trastuzumab) in a cysteine-independent manner was used to generate labelled antibodies for in vivo imaging. The first step contains redox-activated chemical tagging (ReACT) of thioethers via engineered methionine residues to introduce specific alkyne moieties, thereby offering a novel easy way to fundamentally change the process of antibody bioconjugation. The second step involves modification of the introduced alkyne via azide-alkyne cycloaddition 'click' conjugation. The versatility of this 2-step approach is demonstrated here by the selective incorporation of a fluorescent dye but can also be applied to a wide variety of different conjugation partners depending on the desired application in a facile manner. Methionine-modified antibodies were characterised in vitro, and the diagnostic potential of the most promising variant was further analysed in an in vivo xenograft animal model using a fluorescence imaging modality. This study demonstrates how methionine-mediated antibody conjugation offers an orthogonal and versatile route to the generation of tailored antibody conjugates with in vivo applicability.

Development of site-specific antibody-conjugated immunoliposomes for sensitive detection of disease biomarkers

Liposome-based immunoassay (LIA) is an attractive protocol for amplifying the detection signals because of the excellent ability of liposomes to encapsulate signal marker compounds. The antigen-binding activity of the conjugated antibodies on the liposomal surface is crucial for the specificity and sensitivity of LIA. We present here a general platform to ensure that antibodies can conjugate onto the surface of liposomes in a site-specific and oriented manner. A His-handle-modified antibody with Fc region-specific and covalent conjugation was first fabricated using a photoactivatable Z_Bpa-His tag that was engineered using the aminoacyl-tRNA synthetase/suppressor tRNA technique. Based on the high affinity between the His tag and divalent metal ions, the novel His-modified antibody was oriented onto the surface of nickel ion-modified liposomes encapsulating horseradish peroxidase. With the prostate-specific antigen as a model, the detection efficiency of the new immunoliposomes was evaluated by chemiluminescence immunoassay. The immunoliposomes exhibited a limit of detection of 0.2 pg mL^-1, which was a six time improvement compared with that of the chemical-coupled antibody-liposome conjugates. Thus, the proposed immunoliposomes are expected to hold potential applications for the sensitive detection of various biomarkers in complicated serum samples.