The molecular basis for epitopes on the free beta-subunit of human chorionic gonadotrophin (hCG), its carboxyl-terminal peptide and the hCG beta-core fragment

The molecular basis for antigenic determinants on the free beta-subunit of human chorionic gonadotrophin (hCG beta), its carboxyl-terminal peptide (hCG beta CTP) and the hCG beta-core fragment (hCG beta cf) was elucidated by means of monoclonal antibodies (MCAs). The objective of the present study was to resolve the antigenic topography of these three molecules in terms of epitope identification at different levels of structural organization as well as analysis of their spatial arrangement. An hCG beta cf preparation, a synthetic peptide corresponding to the hCG beta CTP (beta 109-145), overlapping synthetic peptides spanning the entire amino acid sequence of hCG beta, and a reduced and alkylated hCG beta preparation were assayed in a solid-phase one-site enzyme-linked immunoassay and in a soluble-phase direct-binding radioimmunoassay (RIA) or competitive RIA. The antigenic topography was mapped by incorporating the MCAs into two-site binding assays. On the surface of free hCG beta, nine different epitopes (beta 1-beta 9), arranged in three spatially distinct domains, could be distinguished. Epitopes beta 1-beta 7 were located in a single large domain on both hCG beta and the hCG beta cf whereas hCG beta CTP contained two topographically distant determinants, designated beta 8 and beta 9 respectively. All but the two epitopes located on hCG beta CTP (beta 8 and beta 9) were destroyed by reducing and alkylating hCG beta, suggesting that most antigenic determinants are predominantly non-contiguous and require an intact tertiary structure whereas the molecular structure of hCG beta CTP is linear. At a molecular level, amino acid residues spanning hCG beta 45-52, hCG beta 137-144 and hCG beta 113-116 contributed to the formation of epitopes beta 5, beta 8 and beta 9 respectively. We have also shown that the hCG beta cf represents the immunodominant part of the free beta-subunit of hCG, containing seven mainly conformationally determined epitopes, one of which has a share of the sequence beta 45-52. The hCG beta CTP does not play a critical role in the immunologically important tertiary structure of hCG beta and was itself found to be a predominantly continuous sequence also within the native hormone, expressing two spatially distant antigenic determinants located within residues beta 113-116 and beta 137-144 respectively.

Free alpha subunit of human chorionic gonadotrophin: molecular basis of immunologically and biologically active domains

Immunochemical studies were undertaken to identify surface-orientated epitopes of the free alpha subunit of human chorionic gonadotrophin (hCG-alpha) at the amino acid sequence level. We investigated the molecular organization of these epitopes, resolved the immunological topography in terms of spatial arrangement of antigenic domains and related structures to functions such as subunit association or receptor binding. Overlapping synthetic peptides covering the entire amino acid sequence of hCG-alpha, an enzymatically digested hCG-alpha subunit, and a reduced and alkylated hCG-alpha preparation were assayed in a solid-phase one-site enzyme-linked immunoassay, and in a solution-phase competitive radioimmunoassay (RIA). The antigenic topography was mapped by monoclonal antibodies (MCAs) in two-site binding assays (sandwich RIA). On the surface of hCG-alpha, seven different epitopes (alpha 1-alpha 7), arranged in four spatially distinct domains, could be distinguished: A, alpha 1,2,4; B, alpha 3,5; C, alpha 6; D, alpha 7. The peptides spanning hCG-alpha(13-18), hCG-alpha(17-22) and hCG-alpha(33-42) appeared to contribute to the formation of epitopes alpha 2, alpha 4 and alpha 6 respectively. Since epitope alpha 6 is present only on the free non-assembled subunit of different species, we concluded that the region hCG-alpha(33-42), which is evolutionarily highly conserved, represents a subunit assembly site. All but one epitope (alpha 7) are destroyed by reducing and alkylating hCG-alpha. In contrast, chymotryptic digestion of hCG-alpha, leading to release of the heptapeptide hCG-alpha(41-47), did not affect epitope expression, indicating that this sequence is not involved in the formation of antigenic determinants. Addressing the biological properties of hCG-alpha epitopes by radioreceptor assay revealed that the three hCG-alpha peptides corresponding to epitopes alpha 2, alpha 4 and alpha 6 did not displace radiolabelled hCG from its receptor, whereas any of the MCAs directed against determinants (alpha 1-alpha 5), shared by hCG and hCG-alpha, totally inhibited binding. Consistent with this, the antibodies neutralized the biological activity of hCG in terms of testosterone production in a mouse Leydig cell in vitro bioassay. We therefore concluded that hormone antibody-binding sites differ from those of hormone receptor binding, revealing no essential congruence of immunologically and biologically active domains.

Functional and immunological relevance of the COOH-terminal extension of human chorionic gonadotropin beta: implications for the WHO birth control vaccine

The World Health Organisation (WHO) Task Force on Birth Control Vaccines has selected the pregnancy hormone human chorionic gonadotropin (hCG) as a target molecule for a contraceptive vaccine. A synthetic peptide antigen corresponding to the amino acid sequence 109-145 of the carboxyl-terminal portion of the hCG beta-subunit (hCG beta CTP), which is supposed to elicit hCG-immunoneutralizing antibodies, has been submitted to clinical trials. Recent findings suggest that hCG beta CTP does not play a role in the biological activity of hCG. This raises the question concerning the assumed mechanism of action of the hCG beta CTP-based birth control vaccine. We therefore investigated the immunoneutralizing capacity of antibodies directed against hCG beta CTP. Although it is possible to generate specific monoclonal and polyclonal antibodies for hCG by using hCG beta CTP as an immunogen, it appeared that the biological response to hCG was not affected by such antibodies. The reason for this is that the hCG-antibody-complex is still able to bind to target cell receptors and therefore the intended contraceptive effect should not occur. In addition there is a risk of hazardous possible side effects such as an autoimmune reaction against the ovary because we found that at least one epitope is still accessible for antibody binding on receptor-bound hCG. We conclude from our results that both the efficacy and safety of the WHO vaccine are not yet ensured.

Serum glycoprotein hormones and their free alpha-subunit in a healthy elderly population selected according to the SENIEUR protocol. Analyses with ultrasensitive time resolved fluoroimmunoassays

The SENIEUR protocol was elaborated by a working party of European Community's Concerted Action Programme on Aging (EURAGE) to define strict admission criteria for 'healthy' elderly subjects and young controls for immunogerontological studies. This protocol, which is based on case history, laboratory values and drug consumption, intends to limit the influence of underlying disease and/or medication in order to allow analyses of the aging process per se. In a group of 38 male and 37 female individuals we determined the impact of age and classification according to the SENIEUR protocol on luteinizing hormone (LH), follicle stimulating hormone (FSH), human chorionic gonadotropin (hCG) and free glycoprotein hormone alpha-subunit serum values. Analyses were performed by a set of ultrasensitive time-resolved immunofluorometric assays (IFMA) using our own panel of monoclonal antibodies (MCA). HLH and hFSH, but also hCG and free alpha serum levels increased highly significantly with age in the female population (P < 0.001). In males hFSH, hLH hCG and the free alpha-subunit increased with age. However, only the rise of hFSH and of Free alpha was statistically significant (P < 0.01). The influence of the SENIEUR status on the respective hormone serum levels was determined using two factor analysis of variance, which revealed no statistically significant difference (P > 0.01) between SENIEUR and NON-SENIEUR individuals for all four analytes in both sexes. We conclude that the age related increase of hLH, hFSH, hCG and free alpha is an intrinsic age-dependent phenomen and is not modified by or due to underlying disease or medication as demonstrated by analyses of SENIEUR individuals. Since SENIEUR and NON-SENIEUR individuals had comparable hormone values, a randomly chosen, 'apparently healthy' population seems to be sufficient for physiological studies on serum GPH levels. Lastly, these age related hormonal changes in an extremely well defined healthy population underline the need for age adjusted 'normal' hormone values as elaborated in this communication.

Time-resolved immunofluorometry and other frequently used immunoassay types for follicle-stimulating hormone compared by using identical monoclonal antibodies

The influence of assay design and quantification system on assay performance was investigated by developing, optimizing, and comparing a time-resolved immunofluorometric assay (IFMA), an immunoenzymometric assay (IEMA), an immunoradiometric assay (IRMA), and a competitive radioimmunoassay (RIA), all performed with the same monoclonal antibodies (MCA) directed against human follicle-stimulating hormone (hFSH). The lowest detection limit (2 ng/L for hFSH-I-3, corresponding to 2.5 mIU of 1st International Reference Preparation of hFSH 78/549 per liter), the widest measuring range (2-160,000 ng/L), and the greatest signal-to-noise ratio (13,000:1 at 160,000 ng/L) were obtained in the IFMA. For analysis of serum samples from 101 male (ages 2-91 years) and 99 female (ages 2-90 years) individuals at a single dilution, 100% of samples were within the measuring range of the IFMA, whereas only 87%, 55%, 32%, and 8% of the sera were for the IRMA, the IEMA evaluated with double-wavelength measurement, the conventional IEMA, and the competitive MCA-based RIA, respectively. These studies demonstrate clear advantages of the IFMA in sensitivity and assay range, which allows reliable and cost- and time-effective determination of hFSH in individuals from infancy to senescence.

Time-resolved immunofluorometry and other frequently used immunoassay types for follicle-stimulating hormone compared by using identical monoclonal antibodies

The influence of assay design and quantification system on assay performance was investigated by developing, optimizing, and comparing a time-resolved immunofluorometric assay (IFMA), an immunoenzymometric assay (IEMA), an immunoradiometric assay (IRMA), and a competitive radioimmunoassay (RIA), all performed with the same monoclonal antibodies (MCA) directed against human follicle-stimulating hormone (hFSH). The lowest detection limit (2 ng/L for hFSH-I-3, corresponding to 2.5 mIU of 1st International Reference Preparation of hFSH 78/549 per liter), the widest measuring range (2-160,000 ng/L), and the greatest signal-to-noise ratio (13,000:1 at 160,000 ng/L) were obtained in the IFMA. For analysis of serum samples from 101 male (ages 2-91 years) and 99 female (ages 2-90 years) individuals at a single dilution, 100% of samples were within the measuring range of the IFMA, whereas only 87%, 55%, 32%, and 8% of the sera were for the IRMA, the IEMA evaluated with double-wavelength measurement, the conventional IEMA, and the competitive MCA-based RIA, respectively. These studies demonstrate clear advantages of the IFMA in sensitivity and assay range, which allows reliable and cost- and time-effective determination of hFSH in individuals from infancy to senescence.

Number and topography of epitopes of human chorionic gonadotropin (hCG) are shared by desialylated and deglycosylated hCG

A previously established map of the surface epitopes of human chorionic gonadotropin (hCG) served as template for the present study in which we investigated the antigenic surfaces of two glycosylation variants of hCG, i.e. desialylated hCG (asialo-hCG) and deglycosylated hCG (degly-hCG). This map allocates five epitopes to the alpha subunit, five to the beta subunit and four alpha beta epitopes to structures formed only by the alpha/beta heterodimer holo-hCG (Schwarz et al. (1986) Endocrinology 118, 189-197; Berger et al. (1990) J. Endocrinol. 125, 301-309). Here it is described that both variants complied with this template: each of the 14 distinct monoclonal antibodies with which the epitopes of hCG were defined reacted with radiolabeled asialo-hCG and degly-hCG as well and generally bound degly-hCG with greater affinity than hCG. Moreover, every combination of capture and radiolabeled detection antibody that was either compatible or incompatible on unlabeled hCG was so also on unlabeled asialo-hCG and degly-hCG. It thus appears that alterations of the carbohydrate structure of hCG can be associated with a change in affinity between some antibodies and their respective epitopes but not with a loss of an epitope or with a change in the topographical relationships of the 14 epitopes.