Glycopeptides Bind MHC Molecules and Elicit Specific T Cell Responses

Carbohydrates are T cell independent antigens because they do not bind to MHC molecules. However, glycopeptides might potentially bind to MHC molecules via their peptide component for presentation to T cells. We have conjugated the disaccharide galabiose [Galα(1-4)Galβ] to the amino terminus of a T cell peptide determinant from hen egg-white lysozyme [HEL(52-61)]. The resulting glycopeptide (Gal₂-52-61) and a nonglycosylated analogue containing tyrosine and glutamic acid at the amino-terminus (YE-52-61) bound equally well to purified l-A^k. T cell hybridomas were produced after immunization with Gal₂-52-61. Many of the T cell hybridomas were glycopeptide-specific and responded to Gal₂-52-61 but not to nonglycosylated synthetic peptidesorto HEL presented by APC, indicating that the carbohydrate moiety influenced T cell recognition. Recognition was lost with the amino terminal attachment of the disaccharide to a peptide six amino acids longer at the amino terminus than HEL(52-61). Recognition also was lost with peptides containing only a single galactosyl residue or with galabiose bound to a different l-A^k binding peptide. T cells directed to Gal₂-52-61 recognized glycopeptides having significant variation in the disaccharide structure, such as HEL(52-61) glycopeptides carrying lactose, cellobiose, or hepta-o-acetylated galabiose. Peptide residues were important features of the T cell epitope; Ala substitutions of two critical T cell contact residues of HEL(52-61) (Tyr⁵³ and Leu⁵⁶) abrogated T cell reactivity to the glycopeptides without affecting binding to l-A^k. In conclusion, we propose that these T cells recognize a peptide conformation specific to glycopeptide-l-A^k complexes and that this recognition does not involve specific interaction between the carbohydrate moiety and the T cell receptor.