PA28 and the proteasome immunosubunits play a central and independent role in the production of MHC class I‐binding peptides in vivo

Abstract

Summary Proteasomes play a fundamental role in the processing of intracellular antigens into peptides that bind to MHC class I molecules for presentation to CD8 T cells. Three IFNγ-inducible catalytic proteasome (immuno)subunits as well as the IFNγ-inducible proteasome activator PA28 dramatically accelerate the generation of a subset of MHC class I-presented antigenic peptides. To determine whether these IFNγ-inducible proteasome components play a compounded role in antigen processing, we generated mice lacking both PA28 and the immunosubunits β5i/LMP7 and β2i/MECL-1. Analyses of MHC class I cell surface levels ex vivo demonstrated that PA28-deficiency reduced the production of MHC class I-binding peptides both in cells with and without immunosubunits, in the last cells on top of an already diminished production of MHC ligands in the absence of immunoproteasomes. In contrast, the immunosubunits but not PA28 appeared to be of critical importance for the induction of CD8 T cell responses to multiple dominant Influenza and Listeria-derived epitopes. Taken together, our data demonstrate that PA28 and the proteasome immunosubunits use fundamentally different mechanisms to enhance the supply of MHC class I-binding peptides. However, only the immunosubunit-imposed effects on proteolytic epitope processing appear to have substantial effects on the fine-specificity of pathogen-specific CD8 T cell responses.

Immunoproteasomes

“…The data are compelling, and several reports point to i-proteasome deficiency leading to the lack of suitable peptides for class I occupancy. 47,69 The correlation between MECL-1 and LMP7 deficiency and reduced surface expression of MHC class I is based on two observations: that expression of MHC class I is dependent on occupancy of the cleft in the MHC α chain, 70,71 and that chymotrypsin-like activity of the immune subunit provides a larger pool of precursors. While these hypotheses are sound, direct evidence for a shortage of i-proteasome-generated peptides being the rate-limiting step for maturation of MHC class I molecules and their expression on the cell surface is lacking, as total proteasome content is relatively constant, and standard proteasome would continue to provide peptide precursors, even if the repertoire was altered.…”

Evidence for anti-apoptotic roles of proteasome activator 28γ via inhibiting caspase activity

“…Several studies revealed that proteasomal activators are affecting substrate entry/exit (channeling) [5,6] as well as proteasomal cleavage site preferences. Coincidentally, hetero-heptameric PA28ab complexes and immunoproteasome isoforms differentially affect proteasomal processing and presentation of certain major histocompatibility complex (MHC) class-I epitopes [7].…”

Identification of the Immunoproteasome as a Novel Regulator of Skeletal Muscle Differentiation

“…The binding of PSME1 and PSME2 to the i20S has recently been shown to increase the capacity of i-proteasomes to selectively degrade oxidized proteins (44) and accelerate the generation of a subset of MHC class I-presented antigenic peptides (9,45,46). In addition, in situ PLA showed that both PSME1 associated proteasomes and i-proteasomes were mainly localized in the cytosol of C2C12 cells.…”