Antibody Fab display system that can perform open-sandwich ELISA

Previously, immunological detection of small haptens or peptides was only possible in a competitive format, which needed competitor antigen either labeled by a reporter or attached to a carrier protein. Beside this, open-sandwich immunoassay (OS-IA) is a simple but powerful immunoassay that can noncompetitively determine monovalent antigen concentration by measuring the antigen-dependent increase in V_H/V_L interaction of an antibody. However, the procedure to obtain suitable assay reagents for OS-IA for a target antigen has not been straightforward because of the lack of easy-to-use antibody selection/manipulation methods. Here we devise a new Fab antibody phage display system that is useful for rapidly evaluating and selecting suitable antibody Fv fragments to OS-IA. The system is based on a phagemid vector in which two identical restriction sites were incorporated into both ends of a human constant region domain. After selection of the M13 phage displaying a Fab fragment, the vector can be easily converted to the vector that can simultaneously produce the V_H-displaying phage and the light chain in the culture supernatant, which can be directly used for OS-ELISA. The successful results of model selection as well as conversion to OS format show the potential in developing various OS-IA for clinically and environmentally important targets.