The effect of the interferon‐γ‐inducible processing machinery on the generation of a naturally tumor‐associated human cytotoxic T lymphocyte epitope within a wild‐type and mutant p53 sequence context

The human wild‐type (wt) p53.264–272 peptide is a universal tumor antigen and recognized by HLA‐A*0201 (A2.1)‐restricted CTL. Generation of this epitope by constitutive 20S proteasomes is prevented by a p53 R to H hotspot mutation at the C‐terminal flanking residue 273. We report on the impact of the interferon‐γ (IFN‐γ)‐inducible proteasomal activator PA28 (11S regulator) and the immunoproteasome on the in vitro and cellular processing of wt and mutant (mut) p53 substrates. We found that production of the antigenic 264–272 peptide from wt p53 by constitutive as well as immunoproteasomes is accelerated and amplified by the PA28 activator. PA28 and (immuno)proteasomes were not capable to reconvert the resistance of epitope release from mut p53. Maximum and accelerated antigen production in vitro and on the cellular level required the IFN‐γ‐inducible interaction of immunoproteasomes and PA28. We conclude that efficient processing of p53.264272 from wt p53 is governed by the proteasome/PA28 complex. These studies have important implications for p53‐specific cancer immunotherapy and demonstrate that the effects of the immunoproteasome and PA28 are influenced by the individual epitope and its flanking sequence context.