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Background The human leucocyte antigen (HLA) complex controls adaptive immunity by presenting defined fractions of the intracellular and extracellular protein content to immune cells. Understanding the benign HLA ligand repertoire is a prerequisite to define safe T-cell-based immunotherapies against cancer. Due to the poor availability of benign tissues, if available, normal tissue adjacent to the tumor has been used as a benign surrogate when defining tumor-associated antigens. However, this comparison has proven to be insufficient and even resulted in lethal outcomes. In order to match the tumor immunopeptidome with an equivalent counterpart, we created the HLA Ligand Atlas, the first extensive collection of paired HLA-I and HLA-II immunopeptidomes from 227 benign human tissue samples. This dataset facilitates a balanced comparison between tumor and benign tissues on HLA ligand level. Methods Human tissue samples were obtained from 16 subjects at autopsy, five thymus samples and two ovary samples originating from living donors. HLA ligands were isolated via immunoaffinity purification and analyzed in over 1200 liquid chromatography mass spectrometry runs. Experimentally and computationally reproducible protocols were employed for data acquisition and processing. Results The initial release covers 51 HLA-I and 86 HLA-II allotypes presenting 90,428 HLA-I- and 142,625 HLA-II ligands. The HLA allotypes are representative for the world population. We observe that immunopeptidomes differ considerably between tissues and individuals on source protein and HLA-ligand level. Moreover, we discover 1407 HLA-I ligands from non-canonical genomic regions. Such peptides were previously described in tumors, peripheral blood mononuclear cells (PBMCs), healthy lung tissues and cell lines. In a case study in glioblastoma, we show that potential on-target off-tumor adverse events in immunotherapy can be avoided by comparing tumor immunopeptidomes to the provided multi-tissue reference. Conclusion Given that T-cell-based immunotherapies, such as CAR-T cells, affinity-enhanced T cell transfer, cancer vaccines and immune checkpoint inhibition, have significant side effects, the HLA Ligand Atlas is the first step toward defining tumor-associated targets with an improved safety profile. The resource provides insights into basic and applied immune-associated questions in the context of cancer immunotherapy, infection, transplantation, allergy and autoimmunity. It is publicly available and can be browsed in an easy-to-use web interface at https://hla-ligand-atlas.org ., Journal for ImmunoTherapy of Cancer, 9 (4), ISSN:2051-1426

The immunopeptidome, representing the point of contact between somatic and T cells, is key for adaptive immunity. Each presented peptide holds an abundance of information not yet well understood. Up to now, the scientific focus has been the definition of pathogenic or tumor derived epitopes and the deconvolution of HLA peptide motifs of the entire immunopeptidome. Here we go one step further and assess the properties of individual peptides to identify defined HLA allotype-specific and frequently presented peptides. Such allotypic peptides represent a versatile tool to determine HLA allotypes or serve as internal standard for characterization of cancer antigens and differentially processed antigens. Finally, individual tissue- and dignity-specific antigens were defined, and the latter were successfully implemented for molecular tumor testing.Using mass spectrometry based immunopeptidomics a database was generated consisting of ∼900 HLA-typed samples. The identified allotypic peptides enabled a HLA class I allotype determination, which was 95% correct in our in-house dataset and 98% in an external dataset. These abundant peptides were implemented as internal standard for a semi-quantitative investigation of established tumor antigens and antigens processed differentially in malignant and benign tissue. Defined dignity-specific antigens allowed a 87% correct tumor detection across numerous tumor types.In summary, we describe a machine learning approach for mining immunopeptidomic data in order to develop a classification method, allowing to differentiate HLA class I-allotypes of a sample or distinguish between healthy and malignant state of tissues. Furthermore, based on this method, we developed a procedure for the validation of tumor exclusive antigens. Our results support classification of immunopeptidomic data sets using machine learning and highlight their potential utility for biomarker development.

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.

This work demonstrates a novel strategy that enables the identification of HCMV-derived T cell epitopes by mass spectrometry. It provides a panel of novel T cell epitopes and presents evidence for their involvement in physiologic immune control of HCMV infections., In healthy individuals, immune control of persistent human cytomegalovirus (HCMV) infection is effectively mediated by virus-specific CD4+ and CD8+ T cells. However, identifying the repertoire of T cell specificities for HCMV is hampered by the immense protein coding capacity of this betaherpesvirus. Here, we present a novel approach that employs HCMV deletion mutant viruses lacking HLA class I immunoevasins and allows direct identification of naturally presented HCMV-derived HLA ligands by mass spectrometry. We identified 368 unique HCMV-derived HLA class I ligands representing an unexpectedly broad panel of 123 HCMV antigens. Functional characterization revealed memory T cell responses in seropositive individuals for a substantial proportion (28%) of these novel peptides. Multiple HCMV-directed specificities in the memory T cell pool of single individuals indicate that physiologic anti-HCMV T cell responses are directed against a broad range of antigens. Thus, the unbiased identification of naturally presented viral epitopes enabled a comprehensive and systematic assessment of the physiological repertoire of anti-HCMV T cell specificities in seropositive individuals.

The human leukocyte antigen (HLA) complex regulates the adaptive immune response by showcasing the intracellular and extracellular protein content to the immune system, where T cells are able to distinguish between self and foreign. Therefore, a comprehensive map of the entirety of both HLA class I- and class II-presented peptides from different tissues, is a highly sought after resource 1 , as it enables the investigation of basic immunological questions beyond the exome level. In this work, we describe the HLA Ligand Atlas, a comprehensive collection of matched HLA class I and class II ligandomes from 29 non-malignant tissues and 13 human subjects (208 samples in total), covering 38 HLA class I, and 17 HLA*DRB alleles. Nearly 50% of HLA ligands have not been previously described. The generated data is relevant for basic research in diverse fields such as systems biology, general immunology, and molecular biology. Furthermore, the HLA Ligand Atlas provides essential information for translational applications by supporting the development of effective cancer immunotherapies. The characterization of HLA ligands from benign tissues, in particular, is necessary in informing proteogenomic HLA-dependent target discovery approaches. Thus, this data set provides a basis for novel insights into immune-associated processes in the context of tissue and organ transplantation and represents a valuable tool for researchers exploring autoimmunity. The HLA Ligand Atlas is publicly available as a raw data resource but also in the form of a user-friendly web interface that allows users to quickly formulate complex queries against the data set. Both downloadable data and the query interface are available at www.hla-ligand-atlas.org .

3128 Background: The human leukocyte antigen (HLA) complex regulates the adaptive immune response by showcasing the intracellular and extracellular protein content to the immune system, which is the basis for T cell-dependent tumor rejection. Therefore, a comprehensive map of the entirety of both HLA class I- and class II-presented peptides from various benign tissues is a highly sought after resource, as it enables the definition of tumor-association on the immunologically pivotal level of the HLA ligandome. Methods: Human tissue samples were snap frozen post mortem during autopsy. The study was approved by the local IRB. HLA ligands were immunopurified and characterized using an Orbitrap Fusion Lumos mass spectrometer coupled to an Ultimate 3000 RSLC Nano UHPLC System. Data acquisition was performed as technical triplicates in data-dependent mode, and data were analyzed using the containerized, computational pipeline MHCquant. Results: In this work, we describe the HLA Ligand Atlas, a comprehensive collection of matched HLA class I and class II ligandomes from 29 non-malignant tissues and 13 human subjects (208 samples in total), covering 38 HLA class I, and 17 HLA*DRB alleles and comprising 48,381 HLA class I and 16,146 HLA class II peptides. Nearly 50% of HLA ligands have not been previously described. The HLA Ligand Atlas is publicly available as a raw data resource, but also in the form of a user-friendly web interface that allows users to quickly formulate complex queries against the data set. Both downloadable data and the query interface are available at www.hla-ligand-atlas.org. Conclusions: This data set provides a valuable tool for research in diverse fields such as systems biology, general immunology, autoimmune disease and organ transplantation. Most importantly, the HLA Ligand Atlas provides essential information for translational applications in immuno-oncology. The knowledge of HLA ligands from benign tissues strongly supports the informed design of proteogenomic HLA-dependent target discovery approaches.

Background In Germany, approximately 30-50% of Oropharyngeal squamous-cell carcinoma (OPSCC) are related to human papillomaviruses (HPV), of which 90% are caused by HPV-16. The analysis of cancer-associated antigens is needed to develop antigen-specific immunotherapy and to determine the level of personalization required for therapeutic cancer vaccines. Methods Human leucocyte antigen (HLA) ligands from fresh frozen OPSCC biopsies were analyzed. HLA molecules were extracted from tumor tissue using immunoaffinity chromatography. HLA-bound peptides were subjected to high performance liquid chromatography (HPLC) and tandem mass spectrometry (MS/MS). Tumor-exclusive antigens in OPSCC were identified by comparative profiling against an in-house benign and malignant database. Additionally, whole exome sequencing of RNA and DNA isolated from the respective cancer biopsies was performed to complement the database. Results To date, HLA ligandomes from 28 OPSCC (15 HPV+; 13 HPV-) patients were analyzed. Compared to the benign samples database, 148 source proteins were tumor-exclusive for HLA class I and 200 source proteins for HLA class II. The mean number of tumor-exclusive proteins per sample was 6.0 (0-14) for HLA class I and 7.7 for HLA class II (0-41) respectively. Among the tumor exclusive proteins for HLA class I, 23 had not been detected in other malignant tumor ligandomes (n = 703 samples) and 75 for HLA class II (n = 433 samples). A principal component analysis of the HLA-ligandomes from HPV+ and HPV- revealed significant differences between the two groups. For HLA-class I, 85 tumor-exclusive proteins were only found in HPV+ and 55 in HPV-, whereas 8 proteins were found in both groups. Among the 200 HLA class II tumor-exclusive proteins, 96 were only found in HPV+ and 95 in HPV- respectively, whereas 9 were shared by HPV+ and HPV-. Conclusions The analysis of HLA-ligandomes from OPSCC revealed tumor-exclusive and newly discovered antigens. We found clear differences between the HLA ligandomes of HPV+ and HPV- patients. These differences need to be taken into account for therapeutic vaccination strategies. The level of personalization needed for such vaccines remains to be determined. Legal entity responsible for the study The authors. Funding University of Ulm, Clinician Scientist Programme. Disclosure S. Laban: Honoraria (institution), Advisory / Consultancy, Travel / Accommodation / Expenses: Merck Sharp & Dohme (MSD); Honoraria (institution), Advisory / Consultancy, Travel / Accommodation / Expenses: Bristol-Myers Squibb; Honoraria (institution), Advisory / Consultancy, Travel / Accommodation / Expenses: AstraZeneca; Honoraria (institution): Merck Serono. P.J. Schuler: Advisory / Consultancy: Bristol-Myers Squibb. T.K. Hoffmann: Honoraria (institution), Advisory / Consultancy: Merck Sharp & Dohme (MSD); Honoraria (institution): Bristol-Myers Squibb; Honoraria (institution): Merck Serono. H. Rammensee: Advisory / Consultancy, Leadership role, Shareholder / Stockholder / Stock options, Officer / Board of Directors: Immatics; Advisory / Consultancy, Leadership role, Shareholder / Stockholder / Stock options, Officer / Board of Directors: CureVac; Advisory / Consultancy, Leadership role, Shareholder / Stockholder / Stock options, Officer / Board of Directors: Synimmune. All other authors have declared no conflicts of interest.

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.