ALBERT

Description: The B-cell maturation antigen (BCMA) is selectively expressed by cells of the B-lineage, including multiple myeloma (MM) cells, and constitutes a promising target for immunotherapeutic approaches. At present, BCMA is being evaluated as target for immunotherapeutic approaches, such as CAR T cells and bispecific antibodies, which have demonstrated promising results in phase I clinical trials. The utilization of cytotoxic T cells bearing T-cell receptors against BCMA constitutes an alternative promising approach to target MM cells. Therefore, the identification of BCMA-derived peptides that are naturally presented by human leukocyte antigens (HLA) and thus can serve as target structures for CD8+ T cells, is indispensable. In a previous study, we characterized the immunopeptidomic landscape of MM by mass spectrometry-based analysis of naturally presented HLA ligands from primary MM samples and MM cell lines (Walz et al., Blood, 2015). Comparative HLA peptidome profiling of the MM-derived HLA ligands versus the immunopeptidome of numerous benign samples from different tissues identified several strictly MM-associated antigens. Here, we evaluated this dataset for the presence of BCMA-derived MM-exclusive antigens and identified two HLA class I-restricted, BCMA-derived peptides in the immunopeptidome of our cohort comprising 15 primary MM samples and MM cell lines. Notably, one of these peptides showes strictly MM-associated presentation and was never detected on any benign tissues according to our extensive immunopeptidome database (135,354 HLA ligands originating from 16,626 source proteins detected in 337 samples from various benign tissues including blood, bone marrow, lung, kidney, liver, and spleen). This HLA-B*18-restricted ligand P(BCMA)B*18 is represented in 20% (3/15) of the analyzed MM immunopeptidomes. For immunological characterization of the P(BCMA)B*18 peptide, we performed in vitro artificial antigen-presenting cell-based priming experiments engaging naïve CD8+ T cells obtained from healthy volunteers (HV). Induction of tetramer-positive T-cell populations with frequencies ranging from 0.1-2.9% of viable CD8+ T cells was observed for all analyzed healthy whole blood donors, which demonstrates the immunogenicity of P(BCMA)B*18. Subsequently, we functionally characterized the induced P(BCMA)B*18-specific CD8+ T cells using intracellular cytokine staining. Upon stimulation with P(BCMA)B*18, we observed an increased IFNγ and TNF production specifically in the peptide-specific CD8+ T cells. In addition, the degranulation marker CD107a was found to be upregulated in the analyzed tetramer-positive T cells, confirming the activity of CD8+ T cells upon peptide-stimulation. Priming experiments using naïve CD8+ T cells obtained from MM patients as well as in vitro cytotoxicity assays with polyclonal peptide-specific effector T cells are presently ongoing in order to assess the capacity of P(BCMA)B*18-specific CD8+ T cells to induce antigen-specific cell lysis. Taken together, we identified a naturally presented and MM-associated, BCMA-derived peptide, which constitutes a promising target for tailored T cell-based immunotherapeutic approaches. Disclosures Salih: Several patent applications: Patents & Royalties: e.g. EP3064507A1. Kowalewski:Immatics Biotechnologies GmbH: Employment. Weisel:Amgen, BMS, Celgene, Janssen, and Takeda: Honoraria; Amgen, BMS, Celgene, Janssen, Juno, Sanofi, and Takeda: Consultancy, Membership on an entity's Board of Directors or advisory committees; Amgen, Celgene, Janssen, and Sanofi: Research Funding.

Description: Key Points Direct analysis of the HLA-presented peptidome identifies a distinct antigenic signature in MM. T-cell responses for these antigens are detectable exclusively in MM patients and can be induced in vitro in response-naive patients.

Description: Recent studies underscore that multiple myeloma is an immunogenic disease and suggest that it can be effectively treated by T cell based immunotherapy via immunomodulation. This strategy might be synergistically complemented by therapeutic vaccination, which may help induce and guide specific anti-cancer T cell responses. We have recently conducted a study which directly characterized the antigenic landscape of myeloma by mass spectrometric analysis of naturally presented HLA ligands and identified a panel of T cell epitopes characterized by exquisite myeloma-association (Walz, Stickel et. al., Blood 2015). As standard of care in myeloma includes proteasome inhibitor therapy and the proteasome plays a central role in the generation of MHC-presented peptides, it is of great importance to thoroughly characterize and take into account the effects of this treatment on the antigenic landscape of myeloma cells and implement only robustly presented targets for peptide vaccine design. This is even more important Here we present a mass spectrometry-based study, which longitudinally and semi-quantitatively maps the effects of treatment with the 2nd generation proteasome inhibitor carfilzomib in an in vitro model of multiple myeloma. We observed considerable plasticity of the HLA class I ligandome of MM.1S cells after treatment with carfilzomib with 17.9±1.1.% (mean of 3 biological replicates ± SD) of HLA ligands showing significant modulation (fold change ≥ 4, P ≤ 0.01) at t24h compared to mock-treated controls (down-modulated: 11.5±1.1%, up-modulated: 6.3±0.6%). We were able to longitudinally tracke the abundance of 28 previously defined myeloma antigens, confirming robust (16/28, 57.1%) or even increased presentation (8/28, 28.6%) under treatment for the majority of these peptides. However, - importantly - we observed highly distortive effects of carfilzomib treatment on the HLA allotype distribution of target cells, which manifested as a marked reduction of HLA ligands restricted by HLA-A*23:01 and A*24:02 (-62.5±1.8% and -57.0±0.6%, respectively, at t=24h after treatment). These findings indicate strong allotype-specific effects of carfilzomib on the antigenic landscape of myeloma cells, which we interpret to be a direct reflection of the mechanism of action of this drug. As a significant proportion of the U.S. population are carriers of the affected alleles (A*23:01: 8.2%; A*24:02: 22.6%), these findings could have broad implications for the design or implementation of antigen-specific therapies in patients under proteasome inhibitor treatment. Furthermore, these findings might indicate the possibility of altered cancer immunosurveillance as a consequence of proteasome inhibitor therapy. Disclosures Weisel: Amgen: Consultancy, Honoraria, Other: Travel Support; Onyx: Consultancy, Honoraria; Novartis: Other: Travel Support; Janssen Pharmaceuticals: Consultancy, Honoraria, Other: Travel Support, Research Funding; Noxxon: Consultancy; Celgene: Consultancy, Honoraria, Other: Travel Support, Research Funding; BMS: Consultancy, Honoraria, Other: Travel Support.

Description: Inherited bone marrow failure syndromes comprise a heterogeneous group of genetic disorders characterized by dysfunction of hematopoietic stem or progenitor cells. We present a consanguineous pedigree with two siblings with early-onset, progressive bone marrow failure (neutropenia, anemia, thrombocytopenia) associated with trigonocephaply, hyperplastic gingiva, cataract, accessory mammilla, dental changes and mild neurocognitive developmental delay. Pancytopenia at birth as well as non-compaction cardiomyopathy was observed in one of the patients. Bone marrow histology showed marked hypocellularity with decrease of all hematopoietic cell lines but without cellular atypia, monocytosis, blast excess or fibrosis. Fanconi anemia was excluded by mitomycin C induced chromosomal breakage studies. Immunophenotyping of peripheral blood cells showed a reduction of mature B cells, reduced switched memory B cells, whereas the level of activated B-cells and plasma blasts were increased. T cell proliferation upon PHA and specific antigens was normal. Using whole exome sequencing in patients and both parents we could identify a single potentially disease causing homozygous stop codon mutation (NM_001085487: c.1168G〉T: p.E390*) in MYSM1 (Myb-Like, SWIRM And MPN Domain-Containing Protein 1). A family with two affected patients carrying the same mutation has been reported previously (Alsultan A et al, Blood 122:3844, 2013). MYSM1 is a Histone H2A deubiquitinase that has previously been implicated in controlling hematopoietic stem cells, progenitor B cells, and NK cells in mice (Nijnik A et al, Blood 119:1370, 2012). MYSM1 protein was absent in patients' EBV-transformed B cell lines (EBV-LCLs). MYSM1-deficient EBV-LCLs had elevated γ-H2AX levels, indicative of increased genomic instability. Upon exposure to UV light, we observed a sustained expression of p53 and phospho-p53 as well as p38 MAPK and phospho-p38 MAPK in patients' fibroblasts in comparison to fibroblasts from healthy individuals. In view of the progressive pancytopenia, both siblings underwent allogeneic hematopoietic stem cell transplantation from 10/10 HLA-matched family donors after reduced intensity conditioning with alemtuzumab, fludarabine and treosulfan at 2 and 4 years of age, respectively. The procedure was well tolerated, and both siblings exhibit full donor hematopoietic reconstitution at 21 and 26 months after transplant. In summary, we here identify a novel human bone marrow failure syndrome caused by MYSM1-deficiency that can be cured by allogeneic hematopoietic stem cell transplantation. Disclosures No relevant conflicts of interest to declare.

Description: Chronic myeloid leukemia (CML) is characterized by the translocation t(9;22), which leads to the formation of the BCR-ABL fusion transcript. Several approved tyrosine kinase inhibitors (TKIs) target the resulting fusion protein, leading to an improved prognosis of CML patients. Currently, the main treatment goal is the achievement of a deep molecular response (MR), in which TKI therapy can be terminated. Several studies provide evidence that immunological control plays a major role for the course of CML and contributes to the achievement of deep MR in CML patients under TKI treatment (CMLTKI). This implies that reinforcing these immune responses might sustain long-term TKI-free survival or even cure for CML patients. Besides unspecific immunotherapy, such as interferon or immune checkpoint blocking antibodies, a more specific and minor side effect targeting of CML cells might be achieved by antigen-specific immunotherapy approaches. The prerequisite for such strategies is the identification of T-cell targets represented by tumor-associated human leukocyte antigen (HLA)-presented peptides on malignant cells. In this study, we used a mass spectrometry-based approach to identify naturally presented, CML-associated peptides in primary CML samples (HLA class I, n=21, 11,945 peptides, 5,478 source proteins; class II, n=20, 5,991 peptides, 1,302 proteins). Comparative HLA peptidome profiling using a comprehensive dataset of various benign tissues (e.g. blood, bone marrow, spleen, and lung) revealed frequently presented and strictly CML-associated antigens. In detail, the benign tissue dataset comprises hematological benign samples (class I, n=108, 51,233 peptides, 11,437 proteins; class II, n=88, 42,753 peptides, 4,877 proteins) and non-hematological benign tissues (28 tissues, n=166; class I, 128,590 peptides, 16,405 proteins; class II, 143,652 peptides, 13,410 proteins). We identified 50 CML-associated, HLA class I-restricted peptides with HLA allotype adjusted representation frequencies of ≥38% presented on HLA-A*02, -A*03, -A*11, and -B*07. HLA class II comparative profiling delineated 36 peptides exclusively and frequently presented in the HLA peptidome of ≥20% analyzed CML patients. For immunological characterization, we selected 8 HLA class I- and 6 class II-restricted highly CML-associated antigens. These peptides were analyzed in IFNγ ELISPOT assays using PBMCs from CMLTKI patients and healthy volunteers (HVs). Peptide-specific immune recognition was detected for 1/8 (13%) HLA class I peptides in 2/17 (12%) of CMLTKI patients. We hypothesized that this weak immune response might be due to an impaired CD8+ T cell function that reportedly is caused by TKI treatment. Thus, we compared T-cell responses against viral epitopes in IFNγ ELISPOT assays of CMLTKI patients with that of HVs and chronic lymphocytic leukemia (CLL) patients: in line with our hypothesis, we observed significantly reduced IFNγ release of T cells from CMLTKI patients compared to HVs and CLL patients (p

Description: DN, YZ, AS, & JL contributed equally, as did MF, TW, & BSC The αIIbβ3 antagonist antiplatelet drug abciximab, approved in 1994, is the chimeric antigen-binding fragment (Fab) comprising the variable regions of murine mAb 7E3 and human IgG1 and light chain κ constant domains. In studies involving thousands of patients undergoing percutaneous coronary interventions, abciximab decreased mortality and the risk of recurrent myocardial infarction. Mutagenesis studies conducted by us and others (Puzon-McLaughlin, JBC 2000; Takagi, Biochem 2002; Artoni, PNAS 2004) suggested that abciximab binds to the β3 C177-C184 specificity-determining loop (SDL) and Trp129 on the adjacent β1-α1 helix, and our negative-stain electron microscopy (EM) studies of the complex of mAb 7E3 with αIIbβ3 in nanodiscs (Choi, Blood 2013) supported its binding to the αIIbβ3 head domain. None of these studies, however, had the resolution to assess whether 7E3 or abciximab prevents fibrinogen binding by steric interference, disruption of the αIIbβ3-binding pocket for fibrinogen, or both. To address this knowledge gap, we used cryo-EM to produce a density map at 2.8-Å resolution, which allowed us to build an atomic model of the αIIbβ3-abciximab complex. The interacting surface of abciximab is comprised of residues from all three complementarity determining regions of both the light and heavy chains, with high representation of aromatic residues (Figure). Abciximab buries a total of 1,273 Å2 of solvent-exposed surface on αIIbβ3, of which 1,040 Å2 is with β3 and 218 Å2 is with αIIb. The binding of abciximab does not result in disruption of the ADMIDAS, MIDAS, or SyMBS metal ion regions, but it does produce an ~3.4 Å compression of the SDL. Binding is primarily to the β3 SDL and neighboring residues, the β1-α1 helix, and β3 Ser211-Val212 and Met335. The latter residue interacts with the ADMIDAS metal ion in the unliganded receptor; ligand binding leads to the loss of the interaction and a dramatic swing-out motion of the β3 subunit that produces a high-affinity ligand-binding conformation. Surprisingly, the structure also indicated several abciximab interactions with αIIb. To assess the contribution of individual interactions between αIIbβ3 integrin and abciximab to the stability of the αIIbβ3-abciximab complex, we carried out 4 independent molecular dynamics simulations of the cryo-EM structure as well as the X-ray crystal structure of ligand-free αIIbβ3 integrin in its closed conformation (PDB: 3FCS). These revealed that the dynamic behavior of the RGD peptide-binding pocket was similar between the cryo-EM structure of the αIIbβ3-abciximab complex and the X-ray structure of unbound αIIbβ3 during the total simulation time of 2 microseconds per system. Abciximab-protein interaction analysis of the simulations demonstrated that: 1. The αIIb subunit participated marginally in the interaction with abciximab, with only the Asp159(αIIb)-Arg7(light chain) making contact for 〉0.5 fraction of the simulation time. 2. The β3 residues with 〉0.7 contact fraction values were Lys125, Asp126, Trp129, and Gln132 on the β1-α1 helix; Glu171, Asn175, Tyr178, Lys181, Thr182, and Thr183 on the SDL; and Val212, Met335, and Asp336. We calculated the effect of every possible mutation at each residue involved in significant inter-molecular interaction with abciximab in terms of changes in free energy of binding, and the resulting relative values were compared to experimental mutagenesis data. Thus, we made αIIb Asp159Ala and β3 Met335Asp mutations, the latter producing the analogous murine residue and the mutation predicted to be most disruptive to abciximab binding. We found no effect of either mutation on the binding of either mAb 7E3 or abciximab as judged by flow cytometry. Our data demonstrate unexpected interactions of abciximab with several αIIb residues and β3 Met335. Most importantly, abciximab binding did not alter the atomic structure or dynamics of the RGD-binding pocket in the timescale of the simulation, and so unless it induces allosteric modulation over a longer time scale, it does not appear to disrupt the RGD-binding pocket. Abciximab did, however, compress the SDL, which is not a component of the RGD-binding pocket but contributes to ligand binding by a still undefined mechanism. Thus, our data are most consistent with abciximab preventing ligand binding by steric interference, with a potential contribution via alteration of the SDL. Figure Disclosures Coller: Centocor/Janssen: Patents & Royalties: abxicimab; Accumetrics/Instrumentation Laboratory: Patents & Royalties: VerifyNow assay; Scholar Rock: Consultancy, Equity Ownership; CeleCor: Consultancy, Equity Ownership, Research Funding.

Description: Long-term hematopoietic stem cells (LT-HSC) persist in quiescence to maintain their hematopoietic potential throughout life. In the case of need LT-HSC can be activated to replenish the pool of blood cells. We investigated the impact of acute influenza A virus (IAV) infection on hematopoiesis in C57Bl/6N mice, focusing on the most immature HSC and progenitors. Mice were infected with a lethal dose of IAV PR/8/1934 H1N1 (humane endpoints reached within 6 days post infection (dpi)). In two further groups, mice were treated daily with oseltamivir (antiviral neuraminidase inhibitor, dpi 0-4) or were vaccinated with single-cycle vesicular stomatitis virus replicon particles expressing a miss-matched neuraminidase from influenza A virus Yamaguchi/7/2004 H5N1 four weeks prior to infection. Both treatments rescued mice from infection-induced mortality. Every day 6-9 mice were analyzed for differences in the bone marrow (BM) and blood by flow cytometry and multiplex cytokine assays as well as in the lung to determine viral tissue titers and histopathology. HSC functionality was analyzed in a competitive BM transplantation of infected and non-infected mice. Irrespective of the treatment, high IAV lung tissue titers (≥5x106 tissue culture infectious dose 50) in the first days post infection (dpi 1-5) were associated with activation of HSC into the cell cycle. LT-HSCs (LSK, CD150+, CD34- and CD48-) were 50% less quiescent and shifted into the G1/S-G2-M phase (dpi 2-6) and returned to quiescence state after virus clearance (dpi 10). Furthermore, we detected 1.5-fold increase in proliferation of phenotypic LT-HSC. Differentiation was increased towards lymphoid progenitors (≥3-fold more compared to non-infected mice) during the acute phase of infection in untreated and oseltamivir treated mice and myeloid progenitors were reduced ~50% in all groups (dpi 4-8). We found the inflammatory cytokines IFNγ, IL-1α, IL-6, and TNFα to be significantly upregulated in the BM of untreated and oseltamivir treated mice but less in vaccinated animals (dpi 2-4). IL-1α or IL-6 stimulation of LT-HSCs was sufficient to initiate proliferation in cell culture. In all groups the initial drop of the peripheral platelet count (~30% lower compared to non-infected mice, dpi 2) was replenished with an excessive production of platelets (~45% increased, dpi 8-15). Histopathology and electron microscopy revealed the sequestration and accumulation of platelets in pulmonary capillaries and vessels. Subsequently, we detected twice as many mature megakaryocytes in the BM (dpi 2-4) and elevated CD41 expression on HSCs (LSK, CD150+ and CD34-; 3-fold more compared to non-infected mice dpi 2-6) indicating a myeloid/platelet-biased HSC compartment in response to infection. Competitive whole BM transplantation with activated LT-HSCs from the acute phase of infection vs non-infected mice showed delayed reconstitution of T-cells but a preferential differentiation towards platelets in recipient mice. Taken together, local IAV infection in the lung substantially affected LT-HSC quiescence and differentiation by inflammatory cytokines with systemic consequences and a myeloid/platelet-biased lineage output. Disclosures No relevant conflicts of interest to declare.

Description: Platelet thrombospondin interacts with plasminogen in a specific and saturable manner. Thrombospondin was found to specifically bind to plasminogen and the nonenzyme chain of plasmin. Preincubation of 125I- labeled thrombospondin with 30 mmol/L lysine was without effect in the binding of thrombospondin to immobilized plasminogen; preincubation of 125I-labeled plasminogen with 30 mmol/L lysine, on the other hand, significantly reduced the binding of plasminogen to immobilized thrombospondin, suggesting that the interaction of thrombospondin with plasminogen is not the direct result of the lysine binding sites of plasminogen. Arginine and benzamidine, ligands known to specifically bind to the kringle 5 domain of plasminogen, blocked the binding of thrombospondin to plasminogen. Limited elastase proteolysis of plasminogen and plasmin resulted in the generation of two distinct thrombospondin binding domains, one of which was retained on lysine- agarose. The isolation and amino-terminal analysis of these domains following elastase proteolysis of plasminogen identified them, respectively, as a domain containing kringle structures 4 and 5 and plasmin and the other domain consisting of kringle 5-plasmin. A 16- residue synthetic peptide, which represents the amino acids linking kringle 4 to kringle 5 (residues 435–450 of native plasminogen), was without effect in either binding to thrombospondin or blocking the binding of thrombospondin to plasminogen. Plasminogen, therefore, possesses a single thrombospondin interactive site that is independent of, but influenced by, the lysine binding site containing kringle structures and most likely is located within the kringle 5 domain.

Description: Antileukemia immunity plays an important role in disease control and maintenance of tyrosine kinase inhibitor (TKI)-free remission in chronic myeloid leukemia (CML). Thus, antigen-specific immunotherapy holds promise for strengthening immune control in CML but requires the identification of CML-associated targets. In this study, we used a mass spectrometry–based approach to identify naturally presented HLA class I– and class II–restricted peptides in primary CML samples. Comparative HLA ligandome profiling using a comprehensive dataset of different hematological benign specimens and samples from CML patients in deep molecular remission delineated a panel of novel frequently presented CML-exclusive peptides. These nonmutated target antigens are of particular relevance because our extensive data-mining approach suggests the absence of naturally presented BCR-ABL– and ABL-BCR–derived HLA-restricted peptides and the lack of frequent tumor-exclusive presentation of known cancer/testis and leukemia-associated antigens. Functional characterization revealed spontaneous T-cell responses against the newly identified CML-associated peptides in CML patient samples and their ability to induce multifunctional and cytotoxic antigen-specific T cells de novo in samples from healthy volunteers and CML patients. Thus, these antigens are prime candidates for T-cell–based immunotherapeutic approaches that may prolong TKI-free survival and even mediate cure of CML patients.

Description: In recent years, therapeutic approaches for acute myeloid leukemia (AML) have been improved, however the disease is still characterized by high relapse rates and a poor overall survival mainly in elderly patients aged 60 years and older. The standard therapy for these AML patients involves hypomethylating agents (HMAs) such as decitabine. With this, treatment remission can be achieved in some patients, however effective post-remission therapies are still overdue. Recent data suggests that HMAs induce gene expression of various cancer/testis antigens (CTAs), which could lead to the presentation of cancer/testis antigen-derived peptides on human leukocyte antigen (HLA) molecules. These CTA-derived peptides might serve as prime targets for tailored T cell-based immunotherapy approaches, which could represent an effective post-remission combination therapy. Here we present a mass spectrometry-based study, which longitudinally maps the HLA-presented immunopeptidome and in particular cancer/testis antigens of AML cells under in vitro decitabine treatment. To analyze the impact of decitabine on the presentation of HLA ligands we treated the AML cell lines U937 and MONO-MAC-6 as well as primary AML cells (n = 1) with decitabine for 48 h (t48) and 72 h (t72) in vitro. Upon flow cytometry-based quantification of HLA class I and II surface expression levels, no significant changes of HLA surface molecule levels under decitabine treatment compared to untreated controls were observed. Implementing label-free quantitation mass spectrometry, we then quantitatively assessed HLA class I ligand presentation under decitabine treatment. Only minor effects of decitabine on the whole HLA class I-restricted peptidome were observed: We detected a significant upregulation of 2.6 ± 0.9% of HLA class I ligands (fold change (FC) ≥ 4, p ≤ 0.01) after 48 h of decitabine treatment, whereas 9.6 ± 5.7% of the ligands were altered in their abundance over time without treatment. At t72 similar proportions of decitabine modulation were observed with 4.2 ± 2.7% of up-regulated HLA ligands. A total of 69 HLA class I ligands derived from 31 different CTAs were identified by mass spectrometric analysis, 9 of these ligands were presented exclusively under decitabine treatment. Furthermore, we showed that decitabine exposure caused a significantly increased presentation of 7/69 CTA-derived HLA ligands at least at one time point in the cell lines and the primary AML cells (FC ≥ 4, p ≤ 0.01). From the CTA cyclin A1, two HLA class I-presented peptides were significantly upregulated in U937 cells at t48 (FC 79.0 and 8.2) and t72 (FC 14.1 and 12.4). In primary AML cells, two peptides derived from JARID1B and KIAA0100 were significantly upregulated at either t48 (FC 21.8) or t72 (FC 6.6). In addition, we screened our dataset for HLA ligands derived from previously described decitabine-regulated genes and identified a HLA class I-presented peptide from DAZL, which was significantly upregulated in U937 cells at t72 under decitabine treatment (FC 57.2). Taken together, our results demonstrate a modulatory effect of the hypomethylating agent decitabine on the HLA ligandome of AML cells, enhancing the presentation of CTA-derived peptides on HLA class I molecules. The latter will be further evaluated for their eligibility as targets for tailored peptide-based immunotherapeutic approaches in AML patients undergoing HMA treatment. Disclosures Salih: Several patent applications: Patents & Royalties: e.g. EP3064507A1.