Abstract
Proteasome-catalyzed peptide splicing represents an additional catalytic activity of proteasomes contributing to the pool of MHC-class I-presented epitopes. We here biochemically and functionally characterized a new melanoma gp100 derived spliced epitope. We demonstrate that the gp100mel47–52/40–42 antigenic peptide is generated in vitro and in cellulo by a not yet described proteasomal condensation reaction. gp100mel47–52/40–42 generation is enhanced in the presence of the β5i/LMP7 proteasome-subunit and elicits a peptide-specific CD8+ T cell response. Importantly, we demonstrate that different gp100mel-derived spliced epitopes are generated and presented to CD8+ T cells with efficacies comparable to non-spliced canonical tumor epitopes and that gp100mel-derived spliced epitopes trigger activation of CD8+ T cells found in peripheral blood of half of the melanoma patients tested. Our data suggest that both transpeptidation and condensation reactions contribute to the frequent generation of spliced epitopes also in vivo and that their immune relevance may be comparable to non-spliced epitopes.
Highlights
Proteasome-catalyzed peptide splicing represents an additional catalytic activity of proteasomes contributing to the pool of MHC-class I-presented epitopes
CD8+ T cell responses involve in most cases the proteasome-dependent processing of antigens and cell surface presentation of the resulting peptides by MHC-class I molecules to cytotoxic T lymphocytes (CTLs)
The 20S standard proteasome (s-proteasome) with its active site β -subunits β 1, β 2 and β 5, the immunoproteasome (i-proteasome) with the IFN-γ -induced catalytic β -subunits β 1i/LMP2, β 2i/MECL1 and β 5i/LMP7 or intermediate proteasome types containing both standard and immuno-subunits[1,2] are the catalytic cores of 30S proteasomes, which are formed by the association of two 19S regulator complexes with the 20S core[3]
Summary
Proteasome-catalyzed peptide splicing represents an additional catalytic activity of proteasomes contributing to the pool of MHC-class I-presented epitopes. Five spliced epitopes, derived from the fibroblast growth factor 5 (FGF-5172–176/217–220), melanocyte protein gp100mel (gp100mel40–42/47–52; gp100mel195–202/192), the SP100 nuclear phosphoprotein (SP100296–301/286–289) and tyrosinase (Tyr368–373/336–340) have been identified so far using cancer patient-derived CTLs for epitope identification[11,12,18,19,20] Two of these spliced peptide-specific CD8+ T cells have previously been shown to induce regression of tumors in a clinical setting[20,21] or the engraftment of acute myeloid leukemia cells in non-obese diabetic/SCID mice[11,22], indicating their potential immune relevance
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