Influencia del proteasoma y de la tripeptidil peptidasa II en la generación del reperterio peptídico presentado por HLA-B27

Abstract

HLA-B*2705 is one of the MHC class I molecules whose surface expression is less dependent on proteasome activity. A combination of stable isotope tagging and mass spectrometry was used to determine the percentage, structural features and parental proteins of proteasome independent B27 ligands. About 30% of the 104 molecular species examined was generated in the presence of the proteasome inhibitor epoxomicin. Proteasome-dependent and -independent ligands showed few differences in their overall chemical character or residue usage. Moreover, no significant differences in their flanking sequences or in the subcellular location of the parental proteins were detected. Strikingly, while the former set of peptides arose from proteins whose size and isoelectric point roughly reflected those in the human proteome, proteasomeindependent ligands, other than a few coming from endoplasmic reticulum signal sequences, almost exclusively derived from basic proteins of low molecular weight (<16,5 kDa) which account only for the 6.6% of the human proteome. The possible implication of tripeptidyl peptidase II (TPPII) in the generation of proteasome-independent ligands was analyzed by monitoring the re-expression of HLAB27 after acid stripping in the presence of two distinct TPPII inhibitors, butabindide and Ala-Ala-Phe-chloromethylketone (AAF-cmk). None of these inhibitors decreased the level of B27 re-expression under conditions in which TPPII was largely inhibited. This result contrasted with a significant reduction of HLA-B27 expression in the presence of epoxomicin. The failure of TPPII inhibition to block the surface expression of B27 was replicated in the HLA-B27 negative cell line Mel JuSo. Actually, HLA class I reexpression in these cells increased progressively as a function of butabindide concentration, which is consistent with a role of TPPII in destroying HLA class I ligands. Thus, a novel proteolytic pathway, unrelated to the proteasome and TPPII, and specialized in the degradation of small proteins accounts for a significant fraction of the HLA-B27-bound peptide pool.