Analysis of the Multiple Myeloma HLA Peptidome Identifies a Naturally Presented Bcma-Derived Peptide As an Immunogenic T-Cell Epitope for Immunotherapeutic Approaches

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 specificall