Interferon (IFN)-induced immunoproteasomes (i-proteasomes) have been associated with improved processing of major histocompatibility complex (MHC) class I antigens. Here, we show that i-proteasomes function to protect cell viability under conditions of IFN-induced oxidative stress. IFNs trigger the production of reactive oxygen species, which induce protein oxidation and the formation of nascent, oxidant-damaged proteins. We find that the ubiquitylation machinery is concomitantly upregulated in response to IFNs, functioning to target defective ribosomal products (DRiPs) for degradation by i-proteasomes. i-proteasome-deficiency in cells and in murine inflammation models results in the formation of aggresome-like induced structures and increased sensitivity to apoptosis. Efficient clearance of these aggregates by the enhanced proteolytic activity of the i-proteasome is important for the preservation of cell viability upon IFN-induced oxidative stress. Our findings suggest that rather than having a specific role in the production of class I antigens, i-proteasomes increase the peptide supply for antigen presentation as part of a more general role in the maintenance of protein homeostasis.

In TG-2+3, results in boys and girls are superimposable. OPPA-COPP and OEPA-COPDAC seem to be exchangeable regimens in intermediate- and advanced-stage classical HL in pediatric patients.

Immunoproteasomes are considered to be optimised to process Ags and to alter the peptide repertoire by generating a qualitatively different set of MHC class I epitopes. Whether the immunoproteasome at the biochemical level, influence the quality rather than the quantity of the immuno-genic peptide pool is still unclear. Here, we quantified the cleavage-site usage by human standard-and immunoproteasomes, and proteasomes from immuno-subunit-deficient mice, as well as the peptides generated from model polypeptides. We show in this study that the different proteasome isoforms can exert significant quantitative differences in the cleavage-site usage and MHC class I restricted epitope production. However, independent of the proteasome isoform and substrates studied, no evidence was obtained for the abolishment of the specific cleavage-site usage, or for differences in the quality of the peptides generated. Thus, we conclude that the observed differences in MHC class I restricted Ag presentation between standard-and immunoproteasomes are due to quantitative differences in the proteasome-generated antigenic peptides.Keywords: Antigen presentation r Immunoproteasome r MHC class I restricted epitopes r Proteasome r Proteolysis See accompanying Commentary by Zanker and ChenAdditional supporting information may be found in the online version of this article at the publisher's web-site IntroductionThe 20S proteasome is the central proteolytic machinery of the ubiquitin proteasome system, being responsible for the main Correspondence: Dr. Michele Mishto e-mail: michele.mishto@charite.de part of extra-lysosomal protein degradation and generation of MHC class I restricted epitopes [1]. During evolution, the 20S proteasome retained a conserved structure of four stacked seven membered rings (α 7 β 7 β 7 α 7 ). In each β ring, the 20S standard proteasome has three catalytic standard subunits (i.e. β1s, β2s and β5s) that carry an N-terminal threonine residue as a reactive nucleophile. Based on the analysis of yeast 20S proteasome active site mutants with short fluorogenic peptide substrates C 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.eji-journal.eu Eur. J. Immunol. 2014. 44: 3508-3521 Antigen processing 3509 chymotryptic-, tryptic-and caspase-like activities were assigned to the β5, β2 and β1 subunits, respectively [2]. Larger polypeptide substrates bind with their residues surrounding the cleavage site, that is residues in position P4 to P1 (cleavage site) and P1 to P4 , to the non-primed and primed substrate binding sites [3] of the proteasome, respectively. This provides the stability and the orientation of the substrate, thereby determining the cleavage-site usage within a protein substrate [4]. In mammalia, the cytokine IFN-γ induces the expression of three active sites carrying alternative β1i/LMP2, β2i/MECL1 and β5i/LMP7 immuno-subunits, and in consequence the formation of the immunoproteasome isoforms [5].Since the β1i and β5i immuno-subunits are encoded within the MHC class II region in close neighbourhood to ...