Development of monoclonal antibodies directed against different epitopes of human chorionic gonadotropin.

Human chorionic gonadotropin (HCG) is a 40,000 dalton glycoprotein composed of two non-identical alpha- and beta-subunits. HCG and other related pituitary hormones, such as human luteinizing hormones (HLH), human follicle stimulating hormone (HFSH), and human thyroid stimulating hormone (HTSH), consist of nearly identical alpha-chains. However, their beta-chains show a variable degree of amino acid sequence homology. Detection and subsequent quantitative determination of HCG in human biological fluids is useful in early diagnosis of pregnancy and in monitoring of tumor patients. For these applications monoclonal antibodies (McAbs) with defined specificity are required. Several hybridomas secreting McAbs to HCG have been isolated. The hybridoma cells have been developed by fusion of NS1 myeloma cells with spleen cells of Balb/C mice immunized with HCG. With the aid of an enzyme linked immunosorbent assay, six McAbs were characterized. McAbs A-73, A-76 and A-112 recognize an epitope present on the alpha-HCG subunit. McAbs B-68, B-69 and B-106 recognize an antigenic determinant associated with the beta-HCG subunit. Two of these McAbs: B-68 and B-69 are directed against an epitope on the B subunit specific to the HCG molecule and B-106 McAb towards an epitope common to HCG, TSH, LH and FSH molecule. In a hemagglutination test, only the A-73 McAb is capable of inducing agglutination of sheep red blood cells coated with HCG, thus suggesting that this McAb recognizes a repeating epitope on the alpha-HCG molecule.