T-CELL RECOGNITION OF TUMOR-ASSOCIATED CARBOHYDRATES - THE NATURE OF THE GLYCAN MOIETY PLAYS A DECISIVE ROLE IN DETERMINING GLYCOPEPTIDE IMMUNOGENICITY

Aberrant glycosylation is one of the most constant traits of the malig nant cell phenotype, To study T-cell responses to tumor-associated gly cans, the mouse hemoglobin-derived decapeptide Hb(67-76), which binds well to the MHC class II molecule E-k and is nonimmunogenic in CBA/J m ice, was either O- or N-glycosylated at its primary T-cell receptor co ntact residue, position 72, with different glycans attached to either threonine, serine, or asparagine. The carbohydrate moieties included t umor-associated mucins, i.e., the Tn and T antigens, mucin-related gly cans, and mucin-unrelated glycans, The side chain of the amino acid in position 72 points away from the MHC binding site when the Hb(67-76) peptide is bound to E-k, so the assumption was that this was also the case for glycans attached to this position, The glycosylated Hb(67-76) peptide analogues were then studied for binding to E-k and for immuno genicity in CBA/J mice, All 16 glycopeptides bound well to E-k, althou gh those with the more complex carbohydrates bound more weakly than th ose with monosaccharides. Six of 12 O-glycosylated and 0 of 4 N-glycos ylated glycopeptides were able to induce a T-cell proliferative respon se with a stimulation index above 3.0, Some glycopeptides were not imm unogenic, suggesting that there may be holes in the T-cell repertoire due to a lack of T-cell receptor regions accommodating certain glycan structures, The four strongest immunogenic glycopeptides were all O-gl ycosylated, and interestingly, three of them carried the tumor-associa ted Tn or T antigen, On the other hand, the Hb(67-76) peptide analogue with the natural mucin Core2 structure attached did not elicit any T- cell response, T cells primed to a glycopeptide with a simple glycan s tructure such as Tn did not cross-respond significantly to other glyco peptides, indicating a high degree of carbohydrate specificity in T-ce ll recognition, T cells primed to a glycopeptide carrying the more com plex T antigen showed a complicated pattern of cross-responses to glyc opeptides with simpler glycan moieties, The fact that it is possible t o raise MHC class II-restricted T-cell responses against tumor-associa ted carbohydrate structures opens new perspectives for the designing o f cancer vaccines.