Construction, expression, and activity of a bivalent bispecific single-chain antibody

This report describes the design, construction, and expression of a bivalent bispecific single-chain antibody (SCA) protein in Escherichia coli. The bispecificity of the bivalent protein was based on two previously constructed monovalent single-chain antibody molecules possessing distinct specificities, SCA 4-4-20 (anti-fluorescein) and SCA 04-01 (anti-single-stranded DNA). A flexible linker, modeled after a secreted fungal cellulase protein, was incorporated as the interdomain linker covalently joining the two active sites. Bivalent bispecific SCA protein that accumulated in bacteria as insoluble inclusion bodies was harvested, denatured, refolded, and affinity-purified in vitro. Affinity-purified bivalent bispecific SCA showed nearly identical ligand binding properties at each site relative to the individual monovalent single-chain antibody prototype molecules. In both solid and solution phase binding assays, the bivalent bispecific single-chain antibody simultaneously bound both ligands (fluorescein and (dT)6). Construction of a model bivalent bispecific molecule provides a foundation for future assembly of similar molecules designed to identify parameters involved in enhanced binding of antibodies due to avidity and dual specificity.