T-Cell Immunotherapy with a Chimeric Receptor Against CD38 Is Effective in Eliminating Myeloma Cells

Abstract 885

Myeloma cells themselves are not merely less sensitive to chemotherapy due to their dormancy but prone to becoming drug-resistant. Thus, new therapeutic options are needed for patients with myeloma, especially those over 60 years old. Myeloma cells show the strong expression of CD38. Thus, we investigated whether the retroviral vector-mediated transduction of an anti-CD38-chimeric antigen receptor (CAR) gene contributed to the abrogation of myeloma cells. We had previously designed the retroviral vector carrying anti-CD38-CAR and GFP. T-cells obtained from healthy donors were transduced with retroviral supernatant containing the anti-CD38-CAR vector. The anti-CD38-CAR was expressed with high efficiency. The median percentage of transduced cells was 65.89% (range, 48.98%–89.98%). We confirmed the expression of the anti-CD38-CAR by flow cytometry using a goat-anti-mouse polyclonal antibody conjugated to biotin followed by streptavidin-PerCP, which reacts with the extracellular antibody-derived single-chain variable domain (scFv) of the anti-CD38-CAR. To assess their cytotoxicity, the T-cells transduced with the anti-CD38-CAR were incubated with the KMM1 and RPMI8226 myeloma cell lines for 3 days at a variety of effector (E): target (T) ratios. The transduced cells exerted a highly cytotoxic effect on RPMI8226 cells in a time-dependent manner. The chimeric receptor-mediated cytotoxicity was dose-dependent. The specific cytotoxicity of T-cells with the anti-CD38-CAR to myeloma cell lines at an E: T ratio of 0.5:1. By contrast, there was no detectable cytotoxicity in experiments with the U266-A4 cells, lacking CD38 expression (0.05%). These results demonstrated that CD38-specific T-cells from donors effectively exerted a cytotoxic effect on myeloma cell lines in a dose- and time-dependent fashion. We next evaluated the effect of the anti-CD38-specific T-cells on myeloma cells from the patients. Myeloma cells freshly isolated from the five patients accounted for over 80% of the cells in bone marrow. After myeloma cells were co-cultured with T-cells bearing the anti-CD38-CAR for 3 days, cytotoxic activity was assessed as described above. Intriguingly, mean specific cytotoxicity observed at an E: T ratio of 0.5: 1 for 3 days was >90% in individual patients. Moreover, we obtained the similar results in the autologous settings as well. These results showed that T-cells harboring the anti-CD38-CAR definitively eliminated myeloma cells from the patients as well as myeloma cell lines. Anti-CD38 antibodies or antibody variants have been shown to provide therapeutics for the treatment of CD38-positive hematological malignancies. Introduction of the antibody against CD38 may, however, raise concerns of side effects such as an infusion reaction and pharmacological phenomena associated with the long half-life of the antibody. Although antibodies against CD38 have been administered to patients with myeloma or B-cell lymphoma, there has been no report of adverse effects. Indeed, daratumumab, a novel therapeutic human CD38 antibody and humanized monoclonal CD38 antibody, is currently in clinical trials. In this study, we demonstrated that human T-cells with the anti-CD38-CAR were highly cytotoxic against myeloma cells strongly expressing CD38. Improvements to immunotherapy using autologous T-cells transduced with the anti-CD38-CAR might shed further light on the treatment of myeloma patients with a poor prognosis.