Antigen recognition and targeted delivery by the single-chain Fv

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Abstract
The single-chain Fv (sFv) has proven attractive for immunotargeting, both alone and as a targeting element within sFv fusion proteins. This chapter summarizes the features of sFv proteins that have sparked this interest, starting with the conservation of Fv architecture that makes general sFv design practical. The length and composition of linkers used to bridge V domains are discussed based on the sFv literature; special emphasis is given to the (Gly4Ser)3 15-residue linker that has proven of broad utility for constructing Fv regions of antibodies and other members of the immunoglobulin superfamily. The refolding properties of sFv proteins are summarized and examples given from our laboratory. Spontaneous refolding from the fully reduced and denatured state, typified by 26-10 sFv, is contrasted with disulfide-restricted refolding, exemplified by MOPC 315 and R11D10 sFv proteins, which recover antigen binding only if their disulfides have been oxidized prior to removal of denaturant. The medical value of sFv proteins hinges on their reliability in antigen recognition and rapidity in targeted delivery. Detailed analysis of specificity and affinity of antigen binding by the 26-10 antidigoxin sFv has demonstrated very high fidelity to the binding properties of the parent 26-10 sFv. These results gave confidence to the pursuit of more complex biomedical applications of these proteins, which is indicated by our work with the R11D10 sFv for the imaging of myocardial infarctions. Diagnostic imaging and therapeutic immunotargeting by sFv present significant opportunities, particularly as a result of their pharmacokinetic properties. Intravenously administered sFv offers much faster clearance than conventional Fab fragments or intact immunoglobulin with minimal background binding.