The ISOBM TD-7 Workshop on hCG and Related Molecules

Abstract

The ISOBM TD-7 hCG Workshop was established to characterize the molecular epitope structure and specificities of a panel of diagnostically relevant monoclonal antibodies (MAbs) directed against human chorionic gonadotropin (hCG) and its derivatives, and to consider how this information could be used to improve comparability of immunoassay results for these analytes. In this multicenter study, 27 MAbs have been characterized in detail as to their main and fine specificities by direct binding-, competitive- and sandwich-RIA, -ELISA, BIAcore^® and Western blotting. Antigens used in the study included the upcoming first WHO reference reagents for immunoassay, i.e. nick-free hCG (hCG), nicked hCG (hCGn), hCG α-subunit (hCGα), hCG β-subunit (hCGβ), nicked hCG β-subunit (hCGβn), hCG β-core fragment (hCGβcf), synthetic peptides of hCGβ C-terminal peptide (hCGβCTP), and homologous hormones, luteinizing hormone (LH) and subunits (LHβ) from various species. Correct classification of blinded internal controls demonstrated the reliability of the MAb referencing approach. Three-dimensional molecular epitope assignment was possible in many instances by comparing immunoreactivity of the ISOBM MAbs (n = 27) to a large panel of MAbs (n = 18) previously well characterized in the Innsbruck (P.B.) and Paris (J.M.B.) laboratories. All three major antibody specificities (α, n = 1; β, n = 21; αβ, n = 5) were represented in the TD-7 MAb panel. HCGβ MAbs could further be subdivided into (i) those recognizing hCGβ only (epitopes: β₆, n = 1; β₇, n = 2; β₁₄, n = 1) and (ii) those recognizing hCGβ + hCG (β₁, β₂, β₄, β₅, n = 10; β₈ and β₉, n = 9). Members of the latter group were specific either for hCG + hCGβ + hCGβcf (β₁, n = 3) or hCG + hCGβ + hCGβCTP (β₈, n = 6; β₉, n = 1) or in addition to hCG + hCGβ + hCGβcf recognized hLH/hLHβ to a minor (β₂, n = 3; β₄, n = 3) or similar degree (β₅, n = 1). Epitopes were (i) located on the first and third loops protruding from the cystine knot of hCGβ (β₂–β₆, aa hCGβ20–25 and 68–77), (ii) presumably centered around the knot itself (β₁), or (iii) on hCGβCTP (epitope β₈ = hCGβ141–144, β₉ = hCGβ113–116). The ISOBM panel of MAbs represents all major epitope specificities suitable for the design of specific sandwich immunoassays. High analyte variability in serum and urine during the course of pregnancy and tumor development favors certain epitope combinations. For routine diagnostic purposes, assays recognizing a broad spectrum of hCG/hCGβ variants such as hCG + hCGn + hCGβ + hCGβn + hCGβcf + –CTPhCG + –CTPhCGβ may be useful. Low cross-reactivity against related glycoprotein hormones (e.g. hLH) and their derivatives is mandatory. These criteria are best met by combinations of MAbs directed against epitopes located around the cystine knot (β₁) and against those encompassing the top of loops 1 and 3 on hCGβ (β₂, β₄). The first WHO reference reagents for immunoassay of hCG and hCG-related molecules being prepared by the IFCC should facilitate characterization of what assays for ‘hCG’ are measuring. The next step towards improving between-laboratory comparability of measurements of hCG/hCG derivatives in pregnancy and oncology is provided by results of this TD-7 Workshop.