Phosphorylated peptides are naturally processed and presented by major histocompatibility complex class I molecules in vivo

Posttranslational modification of peptide antigens has been shown to alter the ability of T cells to recognize major histocompatibility complex (MHC) class I-restricted peptides. However, the existence and origin of naturally processed phosphorylated peptides presented by MHC class I molecules have not been explored. By using mass spectrometry, significant numbers of natur ally processed phosphorylated peptides were detected in association with se veral human MHC class I molecules. In addition, CD8(+) T cells could be gen erated that specifically recognized a phosphorylated epitope. Thus, phospho rylated peptides are part of the repertoire of antigens available for recog nition by T cells in vivo.