The adoptive transfer of highly cytotoxic natural killer (NK) cells is an emerging tool for cancer immunotherapy. Antibody-dependent cellular cytotoxicity (ADCC) has recently been identified as one of the critical factors for the clinical efficacy of anticancer antibodies, in which NK cells are the major effectors of ADCC. NK cells were expanded from PBMC by a feeder-cell-free expansion method. NK cell expansion efficiency was evaluated within a period of 21 days. The kinetics of NK cell expansion and the expression of activating and inhibitory receptors on NK cells were monitored. NK cells producing IFN-γ and TNF-α were detected by intracellular cytokine staining. The cytotoxicity of expanded NK cells against various cancer cells was compared with that of freshly isolated NK cells. The ADCC functions of expanded NK cells in combination with rituximab against CD20+ lymphoma cell lines were evaluated. Our method efficiently expanded NK cells ex vivo, which showed a much higher activity to induce the expression of activating receptors and to produce IFN-γ and TNF-α as well as cytotoxicity against various cancer cell lines including CD133+ primary cancer cells than freshly isolated NK cells. We observed a synergistic cytotoxicity of our expanded NK cells against CD20+ B lymphoma cell lines as well as higher IFN-γ and TNF-α production when combined with rituximab. Our results suggest that the adoptive transfer of a large number of ex vivo expanded NK cells, particularly in combination with monoclonal antibody drugs, is a useful tool for cancer immunotherapy.
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