A carbohydrate epitope associated with human squamous lung cancer

A monoclonal antibody, 43-9F, specifically recognizes a tumor-associated antigen expressed both on surface membrane glycoproteins and on secreted soluble mucins of human squamous lung carcinoma (SLC) cells, and the corresponding antigen can be detected as a circulating tumor marker in plasma of SLC patients. Thin-layer chromatography immunostaining of neutral glycolipids extracted from SLC cells reveals a 43-9F-reactive glycolipid whose carbohydrate structure, as determined by fast atom bombardment-mass spectrometry, is identical with that of an Lea-active pentaglycosylceramide described previously: Gal beta 1-3[Fuc alpha 1-4]-GlcNAc beta 1-3Gal beta 1-4Glc-Cer. However, the Lea-active oligosaccharide hapten, lacto-N-fucopentaose II, with the same carbohydrate structure, fails to inhibit binding of 43-9F, and a well-characterized anti-Lea monoclonal antibody blocks only 40% of 43-9F binding sites on SLC cells, suggesting that the major epitope recognized by 43-9F is more complex than the Lea epitope. To search for a higher affinity 43-9F epitope among more complex oligosaccharides, a mixture of tritiated neutral oligosaccharide alditols from pooled human milk was passed through a 43-9F affinity column. A major retarded oligosaccharide was purified by high-performance liquid chromatography and shown by fast atom bombardment-mass spectrometry to have the following structure: Gal beta 1-3[Fuc alpha 1-4]GlcNAc beta 1-3Gal beta 1-4[Fuc alpha 1-3] GlcNAc beta 1-3Gal beta 1-4Glc. Oligosaccharides containing this sugar sequence are at least 100-fold more active than lacto-N-fucopentaose II as competitive inhibitors of 43-9F. Thus, antibody 43-9F binds to the above difucosyl Lea-X determinant with high affinity and weakly cross-reacts with the Lea antigen under some conditions such as occurs in thin-layer chromatography and enzyme-linked immunosorbent assay where multiple weak interactions of the decavalent IgM antibody may occur.