Recognition of carbohydrate by major histocompatibility complex class I-restricted, glycopeptide-specific cytotoxic T lymphocytes.

Abstract

SummaryCytotoxic T cells (CTL) recognize short peptide epitopes presented by class I glycoproteins encoded by the major histocompatibility complex (MHC). It is not yet known whether peptides containing posttranslationally modified amino acids can also be recognized by CTL. To address this issue, we have studied the immunogenicity and recognition of a glycopeptide carrying an O-linked N-acetylglucosamine (GlcNAc) monosaccharide-substituted serine residue. This posttranslational modification is catalyzed by a recently described cytosolic glycosyltransferase. We show that glycosylation does not affect peptide binding to MHC class I and that glycopeptides can elicit a strong CTL response that is glycopeptide specific. Furthermore, glycopeptide recognition by cytotoxic T cells is dependent on the chemical structure of the glycan as well as its position within the peptide.T lymphocytes recognize peptide antigens as they are presented on the cell surface by polymorphic proteins encoded in the MHC class I or II. Class I MHC presents peptide fragments ofintracelhlar synthesized protein, whereas class II MHC predominantly presents peptide fragments of extracellular proteins that have been degraded in the endocytic compartment (1). The allelic specificity of peptide binding to MHC is governed by pockets in the MHC binding groove that confer the preferred binding of certain amino acid residues within an allele-specific motif (2).All natural T cell antigens identified to date consist of peptides with unmodified amino acid side chains. However, it is not known whether some of the posttranslational modifications that occur on proteins in vivo contribute to the recognition of peptide antigens by T cells. Thus, it is not known whether sulfated, phosphorylated, carboxylated, or glycosylated peptides can be selected for presentation by MHC with the posttranslational modification intact.Studies on the recognition by CTLs ofpeptides haptenated with trinitrophenyl have demonstrated that T cells will specifically recognize chemically modified peptide antigens (3). It is therefore possible that peptides carrying natural posttranslational modifications, such as glycosylation, might similarly be recognized by T cells.Several different types of protein glycosylation are known including the N-linked glycosylation of asparagine and O-linked glycosylation of serine and threonine occurring in the endoplasmic reticulum (ER) and Golgi apparatus. In addition, a novel O-linked glycosylation, occurring almost exclusively on nuclear and cytosolic proteins, has been described (4). This glycosylation is characterized by substitution of serines or threonines with single O-3-1inked N-acetylglucosamine (GlcNAc) residues catalyzed by a cytosolic N-acetylglucosaminyl transferase (reviewed in reference 5).Since peptide fragments of cytosolic and nuclear proteins are the preferred substrates for antigen presentation by class I MHC, it is possible that glycopeptides derived from O-GlcNAc substituted cytosolic proteins could enter the class I presentation pat...

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