CS1-Specific Chimeric Antigen Receptor (CAR)-Engineered NK Cells and T Cells Enhance In Vitro and In Vivo Anti-Tumor Activity Against Human Multiple Myeloma

Multiple myeloma (MM) is a B-cell malignancy characterized by the aberrant clonal expansion of plasma cells (PCs) within the bone marrow (BM). Despite the use of proteasome inhibitors and immune-modulating drugs, which have improved overall survival, MM remains an incurable malignancy for which novel therapeutic approaches are urgently needed. Immunotherapy that specifically targets antigens expressed by MM would be a promising approach to treat MM patients refractory to any current treatments. Chimeric antigen receptors (CARs) are engineered fusion proteins containing tumor antigen-recognition moieties and immune cell activation domains. CAR-expressing T cells have been demonstrated successful in the clinic to treat chronic lymphocytic leukemia (CLL) and acute lymphoid leukemia (ALL). However, the potential utility of antigen-specific CAR-engineered natural killer (NK) cells or T cells to target MM-expressed CS1 to treat MM has not been previously explored, and is the focus of our study.

CS1 is a surface glycoprotein and represents an ideal target for the treatment of MM. CS1 is highly, and nearly ubiquitously, expressed on MM cells, while expression remains very low on NK cells, some T-cell subsets, and normal B cells, and also it is almost undetectable on myeloid cells. In addition, monoclonal antibody directed against CS1, elotuzumab, has already been proven safe in phase 1 and 2 clinical trials, and phase 3 trials are ongoing. Therefore, it should be safe to target CS1 for the treatment of MM.

We successfully generated a specific CS1-CAR construct with a lentiviral vector backbone, sequentially containing a signal peptide (SP), a heavy chain variable region (VH), a linker, a light chain variable region (VL), a hinge, CD28 and CD3ζ. Flow cytometry analysis with an antibody against VH and VL regions indicated that CS1-CAR was successfully expressed on the surface of NK cells and T cells transduced with the CAR construct. In vitro, CS1-CAR NK cells and T cells displayed enhanced MM cytolysis (detected by Cr⁵¹ release assay) and augmented production of cytokines (determined by enzyme-linked immunosorbent assay, ELISA), such as IFN-g for NK and T cells and IL-2 for T cells, when co-cultured with CS1-expressing MM cell lines. These effects relied on CS1-dependent recognition of MM cells because CS1-CAR NK or T cells possessed higher activity when they were co-cultured with CS1-expressing cells, but remained much lower activity when they were co-cultured with CS1-negative cells. However, CS1-CAR NK or T cells launched significantly higher killing of MM cells and secrete more abundant cytokines when the CS1-negative MM cells ectopically overexpressed CS1 and were co-cultured with the CAR cells. Ex vivo, compared to NK or T cells transduced with the empty vector, NK or T cells transduced with CS1-CAR also showed significantly enhanced effector functions when responding to purified primary MM tumor cells. More importantly, in an aggressive orthotopic MM xenograft mouse model, when compared to untreated mice or mice treated with empty vector-tranduced NK or T cells, adoptive transfer of 5 × 10⁶ NK or T cells expressing CS1-CAR once every five days efficiently suppressed the growth of human IM9 MM cells and also significantly prolonged survival of mice bearing IM9 MM cells. Our efforts to translate these findings into clinical trials are ongoing.

In summary, CS1 is a promising target for using CAR NK or T cells for MM treatment, and we have generated a CAR that recognizes CS1. We demonstrate that NK cells or T cells armed with this CS1-CAR can recognize and eradicate myeloma cells in vitro and in vivo. Autologous or allogeneic transplantation of these CS1-specific CAR NK cells or CAR T cells may be a promising strategy to treat MM.