Functional characterization of an anti-estradiol antibody by site-directed mutagenesis and molecular modelling: modulation of binding properties and prominent role of the V(L) domain in estradiol recognition.

The high-affinity monoclonal anti-estradiol antibody 9D3 presents a specificity defect towards estradiol-3-sulphate and 3-glucuronide conjugates incompatible with use in direct immunoassays. The corresponding single-chain variable fragment (scFv), cloned and produced in E. coli, exhibited a 10-fold lower affinity for estradiol (K(a)=1.2 x 10(9) M (-1)) and a slightly increased specificity defect for the 3-position. Site-directed mutagenesis revealed critical residues involved in estradiol recognition and produced mutants exhibiting up to a 3-fold increase of the binding affinity for estradiol and up to a 2-fold decrease of the cross-reactivity with estradiol-3-sulphate. A comparative model of the antibody 9D3-estradiol complex was built in which the estradiol D-ring is buried into the binding pocket while the 3-, 6- and 7-positions are solvent exposed, agreeing with the lack of specificity for these three positions. Two potential alternative orientations of the A-ring, one close to CDR H3 and L2 loops, and the other one close to CDR H2 and L3 loops, have been considered for the docking of estradiol, none of which could be unambiguously privileged taking into account data from cross-reactivity measurements, photolabelling and mutagenesis studies. For both orientations, estradiol is stabilized by hydrogen bonding of the 17beta-OH group with TyrL36, His89 and GlnH35 in the first case, or TyrL36, only, in the second case and by van der Waals contacts from TyrL91 with alpha- or beta-face of estradiol, respectively, and from ValH95 and GlyH97 with the opposite face. To elucidate the molecular basis of antibody 9D3 specificity, as compared with that of another anti-estradiol antibody 15H11, single variable domains (V(H) and V(L)) and scFv hybrids have been constructed. The binding activity of V(L)9D3 as well as the specificity of the V(L)9D3/V(H)15H11 hybrid, both similar to antibody 9D3, revealed a prominent role of V(L) in estradiol recognition. These findings establish premises for antibody engineering to reduce cross-reactivity, especially with estradiol-3-conjugates.