400. CD269 (BCMA)-Specific CAR-Expressing T Cells Dramatically Eradicate Myeloma Cells from Bone Marrow of an Orthotopic Multiple Myeloma Mouse Model

Introduction: Multiple myeloma (MM) is an incurable hematological malignancy of plasma cells. During the past decade, overall survival rate of multiple myeloma have been improved. These improvements are linked to the induction of novel drugs with different mechanism of action such as proteasome inhibitors and immnomodulatory drugs. However, MM remains in most cases an incurable disease, and new therapeutic strategies are urgently required for radical cure or continued disease control. During recent years, some dramatic responses were reported using T cells expressing CD19-targeted chimeric antigen receptors (CAR). Therefore, CAR therapy could also be a promising new therapeutic strategy for MM. CD269 (also known as BCMA: B-cell maturation antigen) is a membrane protein that is selectively expressed on B-cell lineages and on plasma cells including myeloma cells. Our goal of this study is to verify whether treatment with CD269-specific CAR-expressing T cells can eradicate myeloma cells from bone marrow of tumor-bearing NOG mice. Methods: Firstly, we uniquely developed monoclonal antibodies against human CD269. Next, we designed and verified novel CD269-specific CAR. The CD269-CAR recombinant retroviral vector encodes the MoMLV (Moloney murine leukemia virus) retroviral backbone and the 3.3E-28z CAR. The 3.3E-28z CAR consists of an anti-human CD269 scFv that was derived from the 3.3E mouse hybridoma, a portion of the human CD28 molecule and the intracellular domain of the human CD3ζ molecule. Results: CD269-specific CAR-expressing T cells show redirected cytolysis toward CD269-positive U266 human MM-derived cells, but not CD269-negative K562 cells. Luciferase-expressing U266 cells that were injected into the cardiac chamber of NOG mice, selectively infiltrated to bone marrow. Six weeks after tumor inoculation, we injected saline, non-CAR gene-modified T cells, or CAR gene-modified T cells (GMCs) into the cardiac chamber of tumor-bearing mice. In the group of GMC injection, U266 cells were dramatically eradicated from bone marrow. Conclusion: Although the efficacy of BCMA-CAR has been demonstrated by previous study using intradermal models of MM, our challenge is the first report that presents with orthotopic models of MM. Our results are more appropriate for predicting the efficacy of CD269-CAR in the treatment of MM. We conclude that adoptive transfer of CD269-CAR-expressing T cells could be a promising option for patient with MM.