Allele-selective effect of PA28 in MHC class I antigen processing

PA28 is an IFN-gamma-inducible proteasome activator and its genetic ablation causes complete loss of processing of certain Ags, but not all of them. The reason why this occurs and how PA28 influences the formation of peptide repertoires for MHC class I molecules remains unknown. In this study, we show the allele-specific role of PA28 in Ag processing. Retrovirus-transduced overexpression of PA28 alpha decreased expression of K-d (D-d) while it increased K-b and L-d on the cell surface. By contrast, overexpression of PA28 alpha Delta C5, a mutant carrying a deletion of its five C-terminal residues and capable of attenuating the activity of endogenous PA28, produced the opposite effect on expression of those MHC class I molecules. Moreover, knockdown of both PA28a and 1; by small-interfering RNA profoundly augmented expression of K-d and D-d, but not of L-d, on the cell surface. Finally, we found that PA28-associated proteasome preferentially digested within epitopic sequences of Kd, although correct C-terminal flankings were removed, which in turn hampered production of K-d ligands. Our results indicate that whereas PA28 negatively influences processing of K-d (D-d) ligands, thereby, down-regulating Ag presentation by those MHC class I molecules, it also efficiently produces K-b (L-d) epitopes, leading to up-regulation of the MHC molecules.