Immunologic mapping of glycomes: implications for cancer diagnosis and therapy

Abstract

Cancer associated glycoconjugates are important biomarkers, as exemplified by globo-H, CA125, CA15.3 and CA27.29. However, the exact chemical structures of many such biomarkers remain unknown because of technological limitations. In this article, we propose the "immunologic mapping" of cancer glycomes based on specific immune recognition of glycan structures, which can be hypothesized theoretically, produced chemically, and examined biologically by immuno-assays. Immunologic mapping of glycans not only provides a unique perspective on cancer glycomes, but also may lead to the invention of powerful reagents for diagnosis and therapy.

Figure 1

Molecular identification of an immune epitope. mAb, monoclonal antibody; TCR, T cell receptor; BCR, B cell receptor.

Figure 2

Structural basis of diverse glycosidic linkages. A. Numbering of representative hexose sugars (galactose and N-acetylneuraminic acid). Hydroxyl groups at different positions (OH-2, OH-3, OH-4, and OH-6) of a typical hexose acceptor may be involved in glycosidic linkages. B. Two possible anomeric configurations (α versus β) and branching are the basis for further structural diversity. In this example, the T antigen (Galβ1,3GalNAc) is branched with an α-N-acetylneuraminic acid moiety at position 6 of N-acetylgalactosamine.

Figure 3

GSLs are heterogeneous because of variations in their ceramide parts. A The ceramide part of GSLs includes a sphingosine and an N-fatty acyl chain. Both the sphingosine and N-fatty acyl chains may be modified by hydroxyl groups and unsaturation. B. Immunostaining of GSLs separated by thin layer chromatography (TLC). Lane 1, leukemia cell line RBL, which expresses iGb3 and other α1,3Gal-terminated GSLs, as stained by a monoclonal antibody specific for Galα1,3Gal; Lane 2, a chemically synthesized iGb3 from Alexis Biochemicals, CA; Lane 3, a chemically synthesized Gb3 from Alexis Biochemicals, CA. The retention factor of GSLs on thin layer chromatography is dependent on its sugar moiety (hydrophilic) and ceramide moiety (hydrophobic). The hydroxylation and unsaturation of a GSL also influences its retention factor. a, a chemically synthesized iGb3 (h18:1/C26:0, which means it contains one hydroxylation and one unsaturation in the sphingosine chain, and no unsaturation in N-fatty acyl chain); b, iGb3 expressed in a leukemia cell line, which has a different ceramide part (mixed d18:1/C24:0 and d18:1/C24:1); c, multiple bands representing other α1,3Gal-terminated GSLs expressed by RBL cells.

Figure 4

The assembly line of glyco-enzymes. Glycans are assembled in a stepwise manner catalyzed by glycosyltransferases, instead of from DNA or RNA templates. In different cells (here represented as X and Y), different glycan structures are formed by different glycosyltransferases (T1-T4 in cell X, and T5-T8 in cell Y). The presence of specific glycan structures cannot be predicted by gene expressions, because each glycan can be converted to another structure by downstream glyco-enzyme modification.

Figure 5

Generation of hypotheses in immunologic mapping of the glycome. Glycan structures are collected from immune cells and compared to a database of known glycans. Candidate novel glycans are isolated and their structures hypothesized. Novel structures are then chemically synthesized and examined for immunological activity.