The HLA-A*0201-Restricted H-Y Antigen Contains a Posttranslationally Modified Cysteine That Significantly Affects T Cell Recognition

Abstract

A peptide recognized by two cytotoxic T cell clones specific for the human minor histocompatibility antigen H-Y and restricted by HLA-A*0201 was identified. This peptide originates from SMCY, as do two other H-Y epitopes, supporting the importance of this protein as a major source of H-Y determinants in mice and humans. In naturally processed peptides, T cells only recognize posttranslationally altered forms of this peptide that have undergone modification of a cysteine residue in the seventh position. One of these modifications involves attachment of a second cysteine residue via a disulfide bond. This modification has profound effects on T cell recognition and also occurs in other class I MHC-associated peptides, supporting its general importance as an immunological determinant.

Cysteinylation of MHC Class II Ligands: Peptide Endocytosis and Reduction Within APC Influences T Cell Recognition

“…Here through studies of a cysteine-containing peptide from the Ag human Ig , we have demonstrated that cysteine modification can regulate T cell responses to class II-restricted epitopes. Although cysteinylation of ligands for MHC class I has been reported previously (35)(36)(37), this marks the first demonstration that such a modification can alter class II-restricted T cell responses. Furthermore, these studies demonstrate that endocytosis and processing of the cysteinylated peptide by viable APC was necessary to restore antigenicity.…”

“…Although dimerization and oxidation of these cysteine-containing peptides was detected, by far the most common modification was peptide cysteinylation. In vivo cysteinylation of peptides is highly likely due to the high circulating levels of cystine in serum (0.1 mM) and the reactivity of free sulfhydryls (36,62). The importance of cysteinylation in regulating class II-restricted T cell activation, has not been investigated despite elegant studies pointing to the role of disulfide reduction and cysteine residues in Ag unfolding, processing, and class II presentation (29,30,39,42,63,64).…”

“…Thus we would predict that cysteine residue within the I peptide serves as a contact for MHC as well as TCR. Similarly, studies of class I ligands indicate that epitope cysteinylation can alter either or both TCR contact and binding to MHC proteins (35,36,71). Depending upon the position and number of cysteine residues, cysteinylation may influence epitope association with class II molecules potentially explaining the lack of MHC binding reported for cysteine-rich insulin (30) and hen egg lysozyme (29) epitopes.…”

“…Experiments using a broad panel of protease and peptidase inhibitors failed to block the ability of viable APC to convert the synthetic I peptide to a functional epitope (data not shown), prompting a search for alternate modifications that might influence T cell recognition of this peptide. Studies of class I-restricted viral and tumor epitopes have revealed modifications of cysteine residues, which alter CD8 ϩ T cell responses (35,36). The I peptide (KHKVYACEVTHQGLSS) contains a central cysteine residue, potentially susceptible to disulfide linkage or oxidation.…”

Cutting Edge: MHC Class II-Restricted Peptides Containing the Inflammation-Associated Marker 3-Nitrotyrosine Evade Central Tolerance and Elicit a Robust Cell-Mediated Immune Response

“…If conversion of tyrosine to nitrotyrosine in autologous proteins can render these proteins recognizable as immunogenic autoantigens, this could have important implications for autoimmune diseases. Recent studies have indicated that a host of post-translational modifications, including glycosylation (19,20), phosphorylation (21,22), and cysteinylation (23,24), can all affect T cell immunoreactivity. Indeed, it has been proposed that these modifications may play a role in the initiation and/or maintenance of autoimmune disease by contributing to the breakdown of immunological tolerance (25,26).…”