Antigenic studies on an enzymatically sialylated carbohydrate: NeuAc(alpha 2-3)Gal(beta 1-3)[GlcNAc(beta 1-6)]GalNAc

The CA 125 antigenic determinant has been shown to be elevated in the serum of ovarian cancer patients and is a useful prognostic marker. The chemical epitope which characterizes the CA 125 antigen is found on a high molecular weight glycoprotein and has been suggested to be carbohydrate in nature. This study was undertaken to establish the relationship of a carbohydrate to the epitope recognized by the monoclonal antibody, OC 125. Along this line a carbohydrate structure conjugated to bovine serum albumin (BSA), was enzymatically sialylated using a purified sialyltransferase to yield NeuAcGal[GlcNAc]GalNAc-OC6H4N = N-BSA. This sialylated product was used to immunize a goat and two rabbits for the development of polyclonal antisera. The resultant antisera were tested against related carbohydrates in EIA biased competitive inhibition assays. It was determined that the GlcNAc(beta 1-6)GalNAc residue was immunologically dominant for all antisera tested. As the immune response matured (greater than 40 days), there was an increase in the proportion of the antibodies that were directed to the NeuAc(alpha 2-3)Gal(beta 1-3)GalNAc residue compared to the nonsialylated form. Known CA 125 molecules did not inhibit the binding of raised antisera to the sialylated product, nor did the sialylated product react with OC 125 monoclonal antibodies. It was therefore concluded that the carbohydrate structure in question is not the epitope, or is not a large enough part of the epitope to be recognized in these assays by the OC 125 monoclonal antibodies.