Single-chain Fv fragments derived from an anti-11-deoxycortisol antibody. Affinity, specificity, and idiotype analysis.

Single-chain Fv fragments (scFvs) against a corticosteroid, 11-deoxycortisol (11-DC), have been generated as a template antibody fragment from which a comprehensive mutated antibody library containing various anti-steroid antibodies could be constructed. The cDNAs encoding variable heavy (V(H)) and light (V(L)) domains of a mouse anti-11-DC antibody (CET-M8), were amplified by RT-PCR, combined via a common linker to construct the sequence of 5'-V(H)-(Gly(4)Ser)(3)-V(L)-3', and cloned into a phagemid vector, pEXmide 5. The phage clones exhibiting binding activity to 11-DC were isolated after single panning against a hapten-immobilizing immunotube. The scFv gene in one of these clones was reamplified to introduce the ochre codons, and then expressed in the bacterial periplasm as the soluble antibody fragment. Two different scFvs (#6 and #12) were cloned, whose binding characteristics were examined by a radioimmunoassay using a tritium-labeled 11-DC. Both of them showed high affinity (K(a)=1.3x10(10)M(-1)) and practical specificity (cross-reactivity: cortisol, <0.2%; cortisone, <0.3%) to 11-DC, and furthermore, strong reactivity with an anti-idiotype antibody which recognizes the paratope of CET-M8. These results suggest that the present scFvs retain the three-dimensional structure of the paratope of the original monoclonal antibody.