Perturbation of antibody bound bifluorescent-ligand probe by polyclonal anti-metatype antibodies interacting with epitopes proximal to the liganded antibody active site

Noncompetitive Immunoassay Detection System for Haptens on the Basis of Antimetatype Antibodies

BACKGROUND

Small molecules classified as haptens are generally measured by competitive immunoassay, which is theoretically inferior to noncompetitive sandwich immunoassay in terms of sensitivity and specificity. We created a method for developing sandwich immunoassays to measure haptens on the basis of antimetatype antibodies.

METHODS

We generated antimetatype monoclonal antibodies against a hapten–antibody immunocomplex using an ex vivo antibody development system, the Autonomously Diversifying Library (ADLib) system. We selected 2 haptens, estradiol (E2) and 25-hydroxyvitamin D [25(OH)D], as analytes. Sandwich immunoassays for these 2 haptens were developed by use of a 96-well microtiter plate and a fully automated chemiluminescence analyzer, and the performances of these immunoassays were investigated.

RESULTS

The developed assays exhibited sensitivity high enough to detect target haptens in serum samples. The limit of detection of the ELISA for E2 was 3.13 pg/mL, and that of the fully automated chemiluminescent enzyme immunoassay (CLEIA) system was 2.1 ng/mL for 25(OH)D. The cross-reactivity with immunoreactive derivatives was effectively improved compared with the competitive assay. The CVs for the sandwich ELISA for E2 were 4.2%–12.6% (intraassay) and 6.2%–21.8% (total imprecision). The CVs for the sandwich CLEIA for 25(OH)D were 1.0%–2.3% (intraassay) and 1.9%–3.5% (total imprecision). In particular, the sandwich CLEIA for 25(OH)D showed correlations of r = 0.99 with both LC-MS/MS and a commercially available 125I RIA.

CONCLUSIONS

Our method represents a potentially simple and practical approach for routine assays of haptens, including vitamins, hormones, drugs, and toxins.

Effects of secondary forces on the ligand binding properties and variable domain conformations of a monoclonal anti-fluorescyl antibody

Biochemical interactions occurring external to the antibody active site or pocket (i.e. secondary forces) that directly effect ligand binding efficiency, and the microenvironment-sensitive spectral properties of bound homologous ligand, residing within the active site of high affinity monoclonal antifluorescyl antibody (mAb) 4-4-20, have been previously reported. This study describes the synthesis and characterization of a series of specially designed and chemically distinct mono-fluoresceinated peptides of equal size (13-mer) as well as the changes in the spectral properties and free energy in the binding of each fluorescein derivatized peptide, upon interaction with mAb 4-4-20. Significant differences in binding efficiency and fluorescence quenching of the ligand, as well as the intrinsic tryptophan fluorescence, were observed for each monofluoresceinated peptide relative to one another and fluorescein ligand. In addition to the effects on the fluorescence quenching of fluorescein and intrinsic tryptophan residues, and the free energy of binding, the conformation of the variable domains of mAb 4-4-20 upon interaction with the fluoresceinated peptides was probed with polyclonal antimetatype (conformational dependent anti-liganded state) antibodies. Studies comparing the results of a solid-phase inhibition assay, with the binding of antimetatype antibodies in solution, suggested that variant metatypic states of mAb 4-4-20 resulted from binding of the various fluorescein derivatized peptides. Depiction of the mAb 4-4-20 active site as a series of thermally averaged substates is proposed as a model and framework to interpret further the results. It was concluded that secondary forces can dictate conformer selection from the various substates. thereby modulating the primary antibody ligand interaction.