Monoclonal antibodies to glycolipids of the isoglobo‐series detect tumor‐associated antigens in rats

Abstract

Monoclonal antibodies (MAbs) directed to 2 neutral glycolipids, isoglobotetraosylceramide and a 6-sugar isogloboneolactoseries hybrid glycolipid, previously shown to be enriched in rat colorectal tumor tissue, were produced by immunization with purified glycolipids. Four MAbs were selected which demonstrate 3 different specificity patterns when tested for binding to purified and crude preparations of neutral glycolipids, cultured tumor cells and fibroblasts and frozen tissue sections. MAbs 14.2 and 14.10, but not 14.3, stained most epithelial colorectal carcinomas, rat testis and a subpopulation of cells in the rat gastric mucosa. However, all 3 MAbs showed strong staining and binding to sections and cultured clones of the cytokeratin-negative tumor of colorectal origin, which was originally used for preparation of the glycolipid immunogens. The observed difference between MAbs 14.2/10 and MAb 14.3 could not be explained by differences in binding to the 2 original glycolipids used for screening. However, MAbs 14.2/10 were demonstrated to bind to high-molecular-weight glycoprotein(s) (HMW-gp's) previously shown to carry determinants for syngeneic antibodies and extracted from epithelial colorectal tumor tissue after extensive lipid extraction. This suggests that a protein-bound carbohydrate determinant, with similarities to the oligosaccharide part of the isoglobo-series glycolipids, is responsible for this cross-reactivity. The staining of rat testis could be explained by the strong expression in this tissue of glycolipids with 8-10 sugar residues bound by the 14.2/10 but not 14.3 MAbs. The cell-surface expression of the 6-sugar hybrid glycolipid was demonstrated by complement-dependent cytotoxicity and immunofluorescent staining of viable cells.