Two abundant proteasome subtypes that uniquely process some antigens presented by HLA class I molecules

Abstract

Most antigenic peptides presented by MHC class I molecules result from the degradation of intracellular proteins by the proteasome. In lymphoid tissues and cells exposed to IFNγ, the standard proteasome is replaced by the immunoproteasome, in which all of the standard catalytic subunits β1, β2, and β5 are replaced by their inducible counterparts β1i, β2i, and β5i, which have different cleavage specificities. The immunoproteasome thereby shapes the repertoire of antigenic peptides. The existence of additional forms of proteasomes bearing a mixed assortment of standard and inducible catalytic subunits has been suggested. Using a new set of unique subunit-specific antibodies, we have now isolated, quantified, and characterized human proteasomes that are intermediate between the standard proteasome and the immunoproteasome. They contain only one (β5i) or two (β1i and β5i) of the three inducible catalytic subunits of the immunoproteasome. These intermediate proteasomes represent between one-third and one-half of the proteasome content of human liver, colon, small intestine, and kidney. They are also present in human tumor cells and dendritic cells. We identified two tumor antigens of clinical interest that are processed exclusively either by intermediate proteasomes β5i ) or by intermediate proteasomes β1i-β5i (MAGE-A10 [254][255][256][257][258][259][260][261][262] ). The existence of these intermediate proteasomes broadens the repertoire of antigens presented to CD8 T cells and implies that the antigens presented by a given cell depend on their proteasome content.antigen processing | antigenic peptide | immunoproteasome | tumor antigen | MAGE

The proteasome immunosubunits, PA28 and ER‐aminopeptidase 1 protect melanoma cells from efficient MART‐126‐35‐specific T‐cell recognition

“…To determine whether melanoma cells constitutively express IFN-inducible proteasome components and ERAP, seven cell lines were analyzed for the presence of PA28, ERAP1, β1i/LMP2, β2i/MECL-1, and β5i/LMP7. Immunoblot analysis revealed a strong expression of β5i/LMP7 and β1i/LMP2 in almost all cell lines suggesting the presence of mixed proteasome subtypes, as recently described [23]. Significant amounts of β2i/MECL-1 were detected in only one cell line, whereas PA28α/β and ERAP1 were broadly expressed even in the absence of IFN-γ (Fig.…”

“…Since β5i/LMP7 is abundantly expressed in melanoma cell lines, this setup might reflect the in vivo situation (Fig. 1A) [23]. In accordance with published results, we observed that β5i/LMP7-containing proteasomes mainly affected cleavage after V 35 thereby interfering with the C-terminal cut, and that β1i/LMP2-containing proteasomes also impaired the efficient production of the N-terminus.…”

“…These data show that ERAP1/2 negatively contributes to MART-1 [26][27][28][29][30][31][32][33][34][35] epitope generation and that in particular, ERAP1 is responsible for the impairment of epitope presentation by IFN-γ. To study the underlying molecular mechanism, we next performed in vitro trimming experiments using recombinant ERAPs in combination with the N-terminally extended 14-mer epitope precursor peptide MART-1 [22][23][24][25][26][27][28][29][30][31][32][33][34][35] . As shown by reversed-phase HPLC analyses, ERAP1 efficiently trimmed the MART-1 [22][23][24][25][26][27][28][29][30][31][32][33][34][35] precursor peptide.…”

“…In extension of these results, we here demonstrate that the inefficient presentation of the MART-1 [26][27][28][29][30][31][32][33][34][35] epitope is due to its impaired generation and/or destruction by different catalytic activities including proteasome immunosubunits, PA28 and ERAP. Intermediate proteasome types having incorporated either β5i/LMP7 or β1i/LMP2 and β5i/LMP7 represent approximately 17% of the total proteasome content in melanoma cells [23]. Interference with MART-1 epitope generation was reported to be due to the reduced capacity of IPs and intermediate proteasome subtypes to generate the epitope's Cterminus requiring cleavage after V 35 [8].…”

Exogenous Hsp70 delays senescence and improves cognitive function in aging mice

“…Immune subunits and constitutive proteolytic subunits do incorporate into proteasomes together; in healthy organs, from onethird to half of all proteasomes are actually "mixed" proteasomes (for instance, containing catalytic subunits β1, β2, and β5i or β1i, β2, and β5i) (41). The existence of mixed proteasome subtypes is important for the generation of unique peptides for presentation by class I MHC and influences immune recognition (47,48). Moreover, β5i chaperone and proteolytic activity enhanced proteasome maturation and ensured optimal class I MHC expression (25), which is important for synaptic plasticity and refinement in the CNS (49)(50)(51).…”