Selective Targeting of Multiple Myeloma By Bcma-Specific Central Memory CD8⁺ cytotoxic T Lymphocytes: A Potential Immunotherapeutic Application in Multiple Myeloma and Other Plasma Cell Disorders

Background:

Multiple myeloma (MM) is a B-cell malignancy characterized by the clonal proliferation and accumulation of malignant plasma cells in the bone marrow and development of osteolytic bone lesions. Despite recent advances in treatment using novel therapeutics, MM remains incurable with high mortality rates. We have demonstrated in preclinical studies that CD8⁺ cytotoxic T lymphocytes (CTL) generated with immunogenic HLA-A2 or HLA-A24 peptides targeting XBP1(X-box binding protein 1), CD138 (Syndecan-1) and CS1 (SLAMF7) antigens induces robust cytotoxic activities against MM. The Phase 1/2a trials were completed or are in progress in the patients with smoldering multiple myeloma or triple negative breast cancer, respectively, using the HLA-A2 XBP1/CD138/CS1 multipeptide vaccine. The clinical data demonstrated that the multipeptide vaccine is safe and induces the XBP1/CD138/CS1-specific immune responses, evidenced by expansion of peptides-specific Tetramer⁺/CD45RO⁺memory CTL and Th-1 specific immune responses. Moreover, clinical trials combining with Lenalidomide or checkpoint inhibitor enhanced the CD8⁺ CTL activities induced by multipeptide vaccine, indicating the benefit of combination therapy. To expand the breadth and extent of the antigens-specific immunotherapy beyond XBP1/CD138/CS1, we have recently identified additional tumor-associated antigens (TAA) on tumor cells obtained from newly diagnosed MM patients (N=616). Here, we introduce a novel heteroclitic peptides specific to BCMA, the receptor forbinding of B cell activating factor (BAFF) and a proliferation-inducing ligand (APRIL). Due to its restricted expression pattern on MM cells and plasma cells along with its critical role in promoting MM cell growth, survival and drug resistance, we are currently in development of novel immunotherapeutic to target BCMA on MM cells with different therapeutic approaches.

Objective:

The aims of current study were to target BCMA as a TAA by generating the antigen-specific memory CD8⁺ CTL to induce effective and long-lasting immune response against MM.

Findings:

We report on immunogenic HLA-A2-restricted peptides derived from BCMA, which are capable of evoking antigen-specific immune responses against MM. The heteroclitic BCMA peptides displayed improved binding affinity/stability to HLA-A2 molecules from their native BCMA peptides. To define immunogenicity of the selected peptides, we generated BCMA-specific CTL (BCMA-CTL) by repeated stimulationof CD3⁺ T cells with respective heteroclitic peptide. The BCMA-specific engineered peptides evoked the expansion of antigen-specific CD8⁺ CTL and generated BCMA-CTL displaying high T cell activation (CD38, CD69) and co-stimulatory (CD40L, OX40, GITR) molecules expression. Additionally, a gradual expansion was observed in BCMA-specific memory CD8⁺ T cells, with a corresponding decrease in naïve CD8⁺ T cells. The BCMA-CTL demonstrated robust poly-functional immune responses with Th1-specific anti-MM activities [high IFN-g/IL-2/TNF-aproduction, CD8⁺ T cells proliferation, cytotoxicity] in antigen-specific and HLA-A2-restricted manner. The functional activities were directly correlated with the expansion of central memory CD8⁺ CTL in the BCMA-CTL generated from different HLA-A2⁺ individuals (Donor 1 BCMA-CTL: 81.0%, Donor 2 BCMA-CTL: 82.6%, Donor 3 BCMA-CTL: 67.0%). Finally, the combination with checkpoint inhibitor (anti-LAG3) or immune agonist (anti-OX40) enhanced the anti-tumor activities of BCMA-CTL, along with the induction of cytotoxic activities by central memory CD8⁺ T cell subset. Therefore, these studies suggest that heteroclitic BCMA peptides offer a therapeutic potential to effectively generate BCMA-specific CD8⁺ CTL targeting MM.

Significance:

Here, we introduce novel immunogenic engineered heteroclitic BCMA peptides capable of inducing antigen-specific memory CD8⁺ CTL with robust poly-functional immune responses against MM. These results provide the framework for therapeutic application of heteroclitic BCMA peptides in MM patients. They further support combination treatment options incorporating BCMA peptides-specific vaccine or BCMA peptides-specific adoptive T cells immunotherapy with anti-LAG3 and/or anti-OX40for patients with myeloma or other diseases expressing BCMA.