PEPTIDE ANCHOR RESIDUE GLYCOSYLATION - EFFECT ON CLASS-I MAJOR HISTOCOMPATIBILITY COMPLEX BINDING AND CYTOTOXIC T-LYMPHOCYTE RECOGNITION

This study extends our previous observation that glycopeptides bind to class I major histocompatibility complex (MHC) molecules and elicit c arbohydrate-specific CTL responses. The Sendai virus nucleoprotein wil d-type (WT) peptide (FAPGNYPAL) binds H-2D(b) using the PS-Asn as an a nchor. The peptide K2 carrying a P5 serine substitution did not bind D -b. Surprisingly, glycosylation of the serine (K2-O-GlcNAc) with N-ace tylglucosamine (GlcNAc), a novel cytosolic O-linked glycosylation, par tially restored peptide binding to D-b. We argue that the N-acetyl gro up of GlcNAc may fulfil the hydrogen bonding requirements of the D-b p ocket which normally accomodates PS-Bsn. Glycosylation of the PS-Asn r esidue itself abrogated binding similar to K2, probably for steric rea sons. The peptide K2-O-GlcNAc readily elicited D-b-restricted cytotoxi c T lymphocytes (CTL), which did not cross-react with K2 or WT. Howeve r, all D-b-restricted CTL raised against K2-O-GlcNAc cross-reacted str ongly with another glycopeptide, K3-O-GlcNAc, where the GlcNAc substit ution is on a neighboring P4-Ser. Furthermore, D-b-restricted CTL clon es raised against K3-O-GlcNAc or K3-O-GlcNAc displayed a striking TCR conservation. Our interpretation is that the carbohydrate of K2-O-GlcN Ac not only mediates binding to Dh, but also interacts with the TCR in such a way as to mimic K3-O-GlcNAc. This unusual example of molecular mimicry extends the known effects of peptide glycosylation from what we and others have previously reported: glycosylation may create a T c ell neo-epitope, or, conversely abrogate recognition. Alternatively, g lycosylation may block peptide binding to MHC class I and finally, as reported here, restore binding, presumably through direct interaction of the carbohydrate with the MHC molecule.