THE MAKING OF THE DOMINANT MHC CLASS-I LIGAND SYFPEITHI

The proteasome contributes to the generation of most of the peptide li gands of MHC class I molecules. To compare the identity of the peptide s generated by the proteasome with those finally presented by MHC clas s I molecules, we generated a monoclonal antibody recognizing the C-te rminal part of the dominant H2-K-d ligand SYFPEITHI derived from the J AK1 tyrosine kinase. Immunoprecipitations of lysates from H2-K-d-expre ssing or non-expressing cells revealed that only in the presence of H2 -K-d SYFPEITHI could be isolated. No longer potential precursor peptid e containing SYFPEITHI could be detected. Surprisingly, a peptide lack ing the first two amino acids, FPEITHI, was isolated independently of the presence of H2-K-d molecules. The detection of only SYFPEITHI and FPEITHI in cell lysates corresponded with the strong generation of the se two peptides in in vitro digests of elongated SYFPEITHI-containing peptides with purified 20S proteasomes. Our results indicate that MHC ligands can be generated directly by the proteasome in vivo and that a t least for SYFPEITHI the expression of the corresponding MHC molecule is critical for protection of the ligand in vivo.