IDENTIFICATION OF THE GASTROINTESTINAL AND PANCREATIC CANCER-ASSOCIATED ANTIGEN DETECTED BY MONOCLONAL-ANTIBODY 19-9 IN THE SERA OF PATIENTS AS A MUCIN

Abstract

Monoclonal antibody 19-9 detects an antigen in the serum from most patients with gastrointestinal and pancreatic cancer. The epitope of this antibody is a carbohydrate with the sugar sequence .**GRAPHIC**. in which NeuNAc is N-acetylneuraminic acid, Gal is galactose, GlcNAc is N-acetylglucosamine, and Fuc is fucose. In colon carcinoma cells and many gastrointestinal and pancreatic cancers, this sequence occurs in monosialoganglioside containing a sialylated Lea-active pentasaccharide (sialylated lacto-N-fucopentaose II, IV3-.alpha.-NeuNAc-III4-.alpha.-Fuc-LcOse4, in which LcOse4 is Gal.beta.1-3GlcNAc.beta.1-3Gal.beta.1-4Glc). However, the antigen in the sera of patients occurs mainly as a mucin, not a ganglioside, based on the following evidence. Little antigen is extracted by organic solvents from sera, and that which is extracted remains at the origin under conditions of thin-layer chromatography where the ganglioside antigen migrates up the plate. Upon gel filtration of serum on Sephacryl S-400, the antigen is eluted in the void volume, indicating a MW of .gtoreq. 5 .times. 106. Incubation for 5 h at 37.degree. C in 0.1 N NaOH destroys the serum antigen but does not affect the ganglioside antigen. The density of the serum antigen is determined in a CsCl gradient is 1.50 g/ml, while in 4 M guanidine-HCl its density is 1.43 g/ml. Antigen affinity purified by antibody 19-9 from the serum of a cancer patient belonging to the Le(a-b+) blood group contains Leb antigen, consistent with the multiple antigenic specificities exhibited by mucins.