Phage-display derived single-chain fragment variable (scFv) antibodies recognizing conformational epitopes of Escherichia coli heat-labile enterotoxin B-subunit

Previously we have described studies on in vitro pentamer assembly of Escherichia coli (E. coli) derived heat-labile enterotoxin B subunit (EtxB) using conventional monoclonal antibodies (Amin et al., JBC 1995: 270, 20143-50 and Chung et al., JBC 2006: 281, 39465-70). To extend these studies further we have used phage-display to select single-chain Fragment variable (scFv) antibodies against different forms of the B-subunit. Two clones exhibiting strong and differential binding were chosen for detailed characterization. A comprehensive sequence analysis was performed to assign the framework and complementary-determining regions and a nonsense mutation present in one of these (scFv-B1.3.9) was corrected. Binding analysis showed that scFv-B1.3.9 bound in ELISA to both heat-denatured monomeric B-subunits (EtxB₁) and also displayed cross-reactivity towards pentameric EtxB (EtxB₅), although there was no reactivity towards monoganglioside (G_M1) captured EtxB₅. Another antibody (scFv-B5.2.14) had a different reactivity profile and, in ELISA, bound only to EtxB₅ but not to EtxB₁ or to EtxB₅ captured via G_M1. Surprisingly, in competition experiments, the assembled pentameric B-subunit inhibited binding of scFv-B5.2.14 to immobilized EtxB₅ only weakly, whereas reduced, but not oxidized, monomeric EtxB₁ was an efficient competitor. These results clearly demonstrate that B1.3.9 and B5.2.14 have different specificities for cryptic epitopes not accessible in the fully assembled G_M1 bound pentameric form of EtxB. Taken together our results show that we were able to successfully isolate and characterize recombinant scFvs that differentially recognize diverse denatured forms or assembly intermediates of the heat-labile enterotoxin B subunit of E. coli.