Tumour-associated antigenic peptides are present in the HLA class I ligandome of cancer cell line derived extracellular vesicles

The recent success of monoclonal antibody checkpoint inhibitor therapies that enhance the ability of CD8⁺ T cells to detect cancer-related antigenic peptides has refocused the need to fully understand the repertoire of peptides being presented to the immune system. Whilst the peptide ligandome presented by cell surface human leucocyte antigen class I (HLA-I) molecules on cancer cells has been studied extensively, the ligandome of extracellular vesicles (EVs) remains poorly defined. Here, we report the HLA-I ligandome of both the cell surface and EVs from eight breast cancer cell lines (MCF7, MDA-MB-231, MDA-MB-361, MDA-MB-415, MDA-MB-453, HCC 1806, HCC 1395, and HCC 1954), and additionally the melanoma cell line ESTDAB-056 and the multiple myeloma line RPMI 8226. Utilizing HLA-I immunoisolation and mass spectrometry, we detected a total of 6574 peptides from the cell surface and 2461 peptides from the EVs of the cell lines studied. Within the EV HLA-I ligandome, we identified 150 peptides derived from tumour associated antigenic proteins, of which 19 peptides have been shown to elicit T-cell responses in previous studies. Our data thus show the prevalence of clinically relevant tumour-associated antigenic peptides in the HLA-I ligandome presented on EV.

HLA Allele-Specific Quantitative Profiling of Type 1 Diabetic B Lymphocyte Immunopeptidome

Peptide ligands presented by human leukocyte antigen (HLA) molecules on the cell surface represent the immunopeptidome that could be utilized for identification of antigenic peptides for immunotherapy and prevention of autoimmune diseases. Although T-cells are well-known key players in the destruction of pancreatic beta-cells in type 1 diabetes (T1D), increasing evidence points toward a role for B-cells in disease pathogenesis. However, as antigen presenting cells, little is known about the comprehensive immunopeptidome of B cells and their changes in the context of T1D. We performed HLA allele-specific quantitative immunopeptidomics using B lymphocytes derived from T1D patients and healthy controls. Hundreds of HLA-I and HLA-II immunopeptides were identified as differentially regulated in T1D per HLA allele for B cells sharing identical HLA alleles. The results were further validated using additional T1D and healthy B cells with partially overlapped HLA alleles. Differentially expressed immunopeptides were confirmed with targeted proteomics and for reactivity using known T-cell assays in the immune epitope database. Considering samples with identical HLA alleles are difficult to obtain for T1D and other similar HLA-restricted diseases, our work represents a viable approach to better understand HLA allele-specific antigen presentation and may facilitate identification of immunopeptides for therapeutic applications in autoimmune diseases. Data are available via ProteomeXchange with identifier PXD026184.

Low-dose CDK4/6 inhibitors induce presentation of pathway specific MHC ligands as potential targets for cancer immunotherapy

Cyclin dependent kinase 4/6 inhibitors (CDK4/6i) lead to cell-cycle arrest but also trigger T cell-mediated immunity, which might be mediated by changes in human leukocyte antigen (HLA) ligands. We investigated the effects of CDK4/6i, abemaciclib and palbociclib, on the immunopeptidome at nontoxic levels in breast cancer cell lines by biochemical identification of HLA ligands followed by network analyses. This treatment led to upregulation of HLA and revealed hundreds of induced HLA ligands in breast cancer cell lines. These new ligands were significantly enriched for peptides derived from proteins involved in the “G1/S phase transition of cell cycle” including HLA ligands from CDK4/6, Cyclin D1 and the 26S regulatory proteasomal subunit 4 (PSMC1). Interestingly, peptides from proteins targeted by abemaciclib and palbociclib, were predicted to be the most likely to induce a T cell response. In strong contrast, peptides induced by solely one of the drugs had a lower T cell recognition score compared to the DMSO control suggesting that the observed effect is class dependent. This general hypothesis was exemplified by a peptide from PSMC1 which was among the HLA ligands with highest prediction scores and which elicited a T cell response in healthy donors. Overall, these data demonstrate that CDK4/6i treatment gives rise to drug-induced HLA ligands from G1/S phase transition, that have the highest chance for being recognized by T cells, thus providing evidence that inhibition of a distinct cellular process leads to increased presentation of the involved proteins that may be targeted by immunotherapeutic agents.

Characterization of CD8+ T-cell responses to non-anchor-type HLA class I neoantigens with single amino-acid substitutions

CD8⁺ T cells are capable of recognizing mutation-derived neoantigens displayed by HLA class I molecules, thereby exhibiting the ability to distinguish between cancer and normal cells. However, accumulating evidence has shown that only a small fraction of nonsynonymous somatic mutations give rise to clinically relevant neoantigens. The properties of such neoantigens, which must be presented by HLA and immunogenic to induce a T-cell response, remain elusive. In this study, we explored the HLA class I ligandome of a human cancer cell line with microsatellite instability using a proteogenomic approach. The results demonstrated that neoantigens accounted for only 0.34% of the HLA class I ligandome, and most neoantigens were encoded by genes with abundant expression. Thereafter, T-cell responses were prioritized, and immunodominant neoantigens were defined using naive CD8⁺ T cells derived from healthy donors. AKF9, an immunogenic neoantigen with a mutation at a non-anchor position, formed a stable peptide-HLA complex. T-cell responses were analyzed against a panel of AKF9 variants with single amino-acid substitutions, in which mutations did not alter the high HLA-binding affinity and stability. The responses varied across individuals, demonstrating the impact of heterogeneous T-cell repertoires in this human cancer model. Moreover, responses were biased toward a variant group with large structural changes compared to the wild-type peptide. Thus, naive T-cell induction can be attributed to multiple determinants. Combining structural dissimilarity with gene-expression levels, HLA-binding affinity, and stability may further help prioritize the immunogenicity of non-anchor-type neoantigens.

Mild Acid Elution and MHC Immunoaffinity Chromatography Reveal Similar Albeit Not Identical Profiles of the HLA Class I Immunopeptidome

To understand and treat immunology-related diseases, a comprehensive, unbiased characterization of major histocompatibility complex (MHC) peptide ligands is of key importance. Preceding the analysis by mass spectrometry, MHC class I peptide ligands are typically isolated by MHC immunoaffinity chromatography (MHC-IAC) and less often by mild acid elution (MAE). MAE may provide a cheap alternative to MHC-IAC for suspension cells but has been hampered by the high number of contaminating, MHC-unrelated peptides. Here, we optimized MAE, yielding MHC peptide ligand purities of more than 80%. When compared with MHC-IAC, obtained peptides were similar in numbers, identities, and to a large extent intensities, while the percentage of cysteinylated peptides was 5 times higher in MAE. The latter benefitted the discovery of MHC-allotype-specific, distinct cysteinylation frequencies at individual positions of MHC peptide ligands. MAE revealed many MHC ligands with unmodified, N-terminal cysteine residues which get lost in MHC-IAC workflows. The results support the idea that MAE might be particularly valuable for the high-confidence analysis of post-translational modifications by avoiding the exposure of the investigated peptides to enzymes and reactive molecules in the cell lysate. Our improved and carefully documented MAE workflow represents a high-quality, cost-effective alternative to MHC-IAC for suspension cells.

A Non-interventional Clinical Trial Assessing Immune Responses After Radiofrequency Ablation of Liver Metastases From Colorectal Cancer

Background: Radiofrequency ablation (RFA) is an established treatment option for malignancies located in the liver. RFA-induced irreversible coagulation necrosis leads to the release of danger signals and cellular content. Hence, RFA may constitute an endogenous in situ tumor vaccination, stimulating innate and adaptive immune responses, including tumor-antigen specific T cells. This may explain a phenomenon termed abscopal effect, namely tumor regression in untreated lesions evidenced after distant thermal ablation or irradiation. In this study, we therefore assessed systemic and local immune responses in individual patients treated with RFA.

Methods: For this prospective clinical trial, patients with liver metastasis from colorectal carcinoma (mCRC) receiving RFA and undergoing metachronous liver surgery for another lesion were recruited (n = 9) during a 5-year period. Tumor and non-malignant liver tissue samples from six patients were investigated by whole transcriptome sequencing and tandem-mass spectrometry, characterizing naturally presented HLA ligands. Tumor antigen-derived HLA-restricted peptides were selected by different predefined approaches. Further, candidate HLA ligands were manually curated. Peripheral blood mononuclear cells were stimulated in vitro with epitope candidate peptides, and functional T cell responses were assessed by intracellular cytokine staining. Immunohistochemical markers were additionally investigated in surgically resected mCRC from patients treated with (n = 9) or without RFA (n = 7).

Results: In all six investigated patients, either induced immune responses and/or pre-existing T cell immunity against the selected targets were observed. Multi-cytokine responses were inter alia directed against known tumor antigens such as cyclin D1 but also against a (predicted) mutation contained in ERBB3. Immunohistochemistry did not show a relevant influx of immune cells into distant malignant lesions after RFA treatment (n = 9) as compared to the surgery only mCRC group (n = 7).

Conclusions: Using an individualized approach for target selection, RFA induced and/or boosted T cell responses specific for individual tumor antigens were more frequently detectable as compared to previously published observations with well-characterized tumor antigens. However, the witnessed modest RFA-induced immunological effects alone may not be sufficient for the rejection of established tumors. Therefore, these findings warrant further clinical investigation including the assessment of RFA combination therapies e.g., with immune stimulatory agents, cancer vaccination, and/or immune checkpoint inhibitors.

HLA-A24 ligandome analysis of colon and lung cancer cells identifies a novel cancer-testis antigen and a neoantigen that elicits specific and strong CTL responses

This study focused on HLA-A24 and comprehensively analyzed the ligandome of colon and lung cancer cells without the use of MHC-binding in silico prediction algorithms. Affinity purification using the antibody specific to HLA-A24 followed by LC-MS/MS sequencing was used to detect peptides, which harbored the known characteristics of HLA-A24 peptides in terms of length and anchor motifs. Ligandome analysis demonstrated the natural presentation of two different types of novel tumor-associated antigens. The ligandome contained a peptide derived from SUV39H2, a gene found to be expressed in a variety of cancers but not in normal tissues (except for the testis). The SUV39H2 peptide is immunogenic and elicits cytotoxic CD8⁺ T-cell (CTL) responses against cancer cells and is thus a novel cancer-testis antigen. Moreover, we found that microsatellite instability (MSI)-colon cancer cells displayed a neoepitope with an amino-acid substitution, while microsatellite stable (MSS)-colon and lung cancer cells displayed its counterpart peptide without the substitution. Structure modeling of peptide-HLA-A24 complexes predicted that the mutated residue at P8 was accessible to T-cell receptors. The neoepitope readily elicited CTL responses, which discriminated it from its wild-type counterpart, and the CTLs exhibited considerably high cytotoxicity against MSS-colon cancer cells carrying the responsible gene mutation. The specific and strong CTL lysis observed in this study fosters our understanding of immune surveillance against neoantigens.

HLA class I-restricted MYD88 L265P-derived peptides as specific targets for lymphoma immunotherapy

Genome sequencing has uncovered an array of recurring somatic mutations in different non-Hodgkin lymphoma (NHL) subtypes. If affecting protein-coding regions, such mutations may yield mutation-derived peptides that may be presented by HLA class I proteins and recognized by cytotoxic T cells. A recurring somatic and oncogenic driver mutation of the Toll-like receptor adaptor protein MYD88, Leu265Pro (L265P) was identified in up to 90% of different NHL subtype patients. We therefore screened the potential of MYD88L265P-derived peptides to elicit cytotoxic T cell responses as tumor-specific neoantigens. Based on in silico predictions, we identified potential MYD88L265P-containing HLA ligands for several HLA class I restrictions. A set of HLA class I MYD88L265P-derived ligands elicited specific cytotoxic T cell responses for HLA-B*07 and -B*15. These data highlight the potential of MYD88L265P mutation-specific peptide-based immunotherapy as a novel personalized treatment approach for patients with MYD88L265P+ NHLs that may complement pharmacological approaches targeting oncogenic MyD88 L265P signaling.

Developments in cancer vaccines for hepatocellular carcinoma

Hepatocellular carcinoma (HCC) accounts for about 6 % of all new cancers diagnosed worldwide and represents one of the leading causes of cancer-related death globally in men and women, respectively. The overall prognosis for HCC patients is poor, especially in the majority of patients with more advanced stage of disease. Indeed, in such cases immunotherapeutic strategies may represent a novel and effective tool. A few immunotherapy trials conducted for HCC have provided divergent results, urging the scientific community to explore additional paths to improve efficacy of immunotherapeutic approaches. The "Cancer Vaccine development for Hepatocellular Carcinoma"-HEPAVAC Consortium has been funded by the EU within the FP7 with the goal of developing a novel therapeutic peptide-based cancer vaccine strategy for HCC including both "off-the-shelf" and personalized antigens. This will be one of the very few vaccine trials for HCC and the first multi-epitope, multi-target and multi-HLA allele therapeutic cancer vaccine for such a frequent and aggressive disease with a hitherto high unmet medical need. Feasibility, safety and biological efficacy will be evaluated in a randomized, controlled European multicenter phase I/II clinical trial.