ANTIGENIC DETERMINANTS OF HUMAN CHORIONIC -GONADOTROPIN (HCG) - IDENTIFICATION OF EPITOPES AND THEIR SIGNIFICANCE FOR BIOLOGICAL FUNCTIONS,FOR THE ESTABLISHMENT OF IMMUNOMETRIC ASSAYS AND THE DEVELOPMENT OF AFERTILITY REGULATING VACCINE

Analysing immunologically relevant structures of human chorionic gonad otropin (hCG), a glycoprotein hormone, has bearing on a variety of cli nical features. These structures are of importance for the establishme nt of immunometric assays for the diagnosis and monitoring of pregnanc y and hCG producing tumours. Moreover, the development of fertility re gulating vaccines by the use of one of the subunits of hCG (hCGbeta) o r a synthetic peptide corresponding to the carboxyl-terminal peptide t hereof (hCGbetaCTP, beta 109-145) requires detailed knowledge of the a ntigenic properties of hCG. It appeared that 16 different antigenic ep itopes are localized on the protein backbone of hCG and that the carbo hydrate moieties do not contribute significantly to the immunological properties of this hormone. Reduced and carboxylmethylated subunits of hCG, i.e., hCGalpha and hCGbeta, express only 1 alpha- and 2 beta-epi topes, the majority of antigenic determinants being destroyed by this chemical procedure. Epitopes of hCG are thus predominantly dependent o n the intact tertiary or quaternary structure of the protein. The urin ary low molecular weight fragment of hCGbeta (hcGbetacf), a predominan t molecular fraction in the urine of pregnant women or patients with v arious tumours expresses 7 out of 9 epitopes of hCGbeta (beta 1-beta 7 ), whereas two antigenic determinants, beta 8 and beta 9, are localize d on the hCGbetaCTP. Certain portions of 5 antigenic determinants were defined by means of synthetic peptides corresponding to hCGalpha- and hCGbeta-sequences: part of the alpha4-epitope is localized within the sequence hCGalpha 17-22; the sequence hCGalpha 33-42 has a share in t he formation of the alpha 6-epitope which is specific for free hCGalph a; the beta 5-epitope is localized within region hCGbeta 45-52 and the two epitopes present on hCGbetaCTP, beta 8 and beta 9, map to aminoac id sequences hCGbeta 135-145 and hCGbeta 108-120, respectively. With r espect to hormone-receptor-interaction it was demonstrated that antibo dies to epitopes on hCGbetaCTP are not able to immunoneutralize hCG. M oreover, it appeared that 3 epitopes are still accessible to antibody binding on receptor-bound hCG, namely beta 3, beta 5 and beta 8. The p ractical significance of the results of this investigation lies in the creation of specific immunometric assays for the measurement of hCG, its free subunits and metabolic products thereof (hCGcf, hCGbetaCTP), as well as in understanding hormone-receptor-interaction. Furthermore, we provide evidence for risks and shortcomings of various approaches by national and international organizations to the development of fert ility regulating vaccines.