Importantly, peptidome-wide analysis delineated clusters of HLA ligands characterized by substantial and sustained down-modulation upon proteasome inhibition. U266 myeloma cells, which exposed significant modulation of a substantial portion of the HLA-presented peptidome. Strikingly, we recognized selective down-modulation of HLA ligands with aromatic C-terminal (R)-3-Hydroxyisobutyric acid anchor amino acids. This particularly manifested like a marked reduction in the demonstration of HLA ligands through the HLA allotypes A*23:01 and A*24:02 on MM.1S cells. These findings implicate that carfilzomib mediates a direct, peptide motif-specific inhibitory effect on HLA ligand processing and demonstration. As a substantial proportion of HLA allotypes present peptides with aromatic C-termini, our results may have broad implications for the implementation of antigen-specific treatment methods in individuals undergoing carfilzomib treatment. Intro Proteasome inhibitors have become a cornerstone in the management of multiple myeloma (MM), efficiently helping to increase disease-free and overall survival of MM individuals over the past decade.1 Carfilzomib, a second-generation proteasome inhibitor, has been approved for individuals with relapsed or refractory disease who have received at least two previous therapies and is currently under investigation like a first-line therapeutic option.2, 3, 4 By specifically and irreversibly binding to the 5-subunit, carfilzomib blocks the chymotrypsin-like specificity of the proteasome resulting in the activation of pro-apoptotic and anti-proliferative pathways5, 6 and the induction of a terminal unfolded protein response.7 As the proteasome has a central part in the generation of MHC-presented peptides,8, (R)-3-Hydroxyisobutyric acid 9, 10 it has long been established that proteasome inhibition can directly effect antigen demonstration by MHC molecules and thereby impair specific T-cell reactions.11, 12, (R)-3-Hydroxyisobutyric acid 13 In MM, the presence of clonally expanded CD8+ T cells has been associated with improved patient survival, pointing to their involvement in tumor monitoring.14, 15 Furthermore, the clinical effectiveness of the immune modulatory drug lenalidomide,16 which has pleiotropic effects including improved cytotoxic T-cell activation,17 indicates the potentially central part of myeloma-specific T cells in disease control. In a recent study, we investigated the underlying specificities of anti-myeloma T-cell reactions by analyzing the antigenic scenery of MM by mass spectrometry and recognized a set of antigens characterized by exquisite myeloma specificity.18 As MM remains a largely incurable disease despite the aforementioned advances,19, 20 the aim of our previous study was to define a panel of broadly presented targets for antigen-specific immunotherapy of MM. Since standard of care in MM comprises proteasome inhibitor therapy, it is of great importance to thoroughly characterize the effects of this treatment within the antigenic scenery of myeloma cells to allow for implementation of robustly offered focuses on for concomitant or subsequent immunotherapy. In the present study, we comprehensively and semi-quantitatively mapped the effect of proteasome inhibition on HLA-restricted antigen demonstration using an model of carfilzomib treatment in myeloma. Quantitation of the demonstration levels of 72 previously defined myeloma antigens under treatment recognized robustly offered focuses on. Importantly, peptidome-wide analysis delineated clusters of HLA ligands characterized by substantial and sustained down-modulation upon proteasome inhibition. Closer investigation of these clusters revealed unique peptide motif-specific inhibitory effects of carfilzomib on HLA-restricted antigen demonstration, which manifested as the designated reduction in the demonstration of antigens with aromatic C-termini. Materials and methods Individuals and bone marrow samples Bone marrow mononuclear cells from MM individuals at the time of analysis or at relapse before therapy were isolated by denseness gradient centrifugation (Biocoll, Biochrom GmbH, Berlin, TSPAN5 Germany) and erythrocyte lysis (EL buffer, Qiagen, Venlo, Netherlands). Informed consent was acquired in accordance with the Declaration of Helsinki (R)-3-Hydroxyisobutyric acid protocol. The study was performed according to the recommendations of the local ethics committee (142/2013BO2). Patient characteristics are provided in Supplementary Table 1. Myeloma cell lines For HLA quantification and HLA ligandome analysis, the myeloma cell lines (MCLs) MM.1S, U266, RPMI8226 and JJN3 were cultured in the recommended cell press (RPMI-1640; Gibco, Carlsbad, CA, USA and IMDM; Lonza, Basel, Switzerland) supplemented with fetal calf serum, 100 IU/l penicillin, 100?mg/l streptomycin and 2?mmol/l glutamine at 37?C and 5% CO2. treatment of MCL and main MM samples Cultured.