A Synergistic Antitumor Effect of Rituximab and Gamma Interferon Combined Therapy on Human CD20＋ B-Cell Lymphoma Cells

Rituximab （RTX） is an anti-CD20 human-mouse chimeric monoclonal antibody that exhibits antibody-dependent, cell-mediated cytotoxicity and complement-dependent cytotoxicity, resulting in an antitumor effect with immune cells or complement. RTX is approved for the treatment of many diseases including B cell lymphoma and rheumatoid arthritis. We examined whether combined RTX and gamma interferon （IFNγ） therapy provides a higher antitumor effect than RTX single therapy using B-cell lymphoma cells. In addition, we investigated the mechanisms underlying the antitumor effect. We treated tumor-derived cell lines with RTX alone, IFNγ alone, or a combination of RTX and IFNγ（RTX-IFNγ）. Untreated cells served as controls. We experimentally examined in vitro cell proliferation, conducted apoptosis and cell cycle assays, performed Western blotting to identify changes in the levels of proteins related to the cell cycle, and investigated tumor growth in a mouse xenograft experiment. Cell proliferation experiments indicated that RTX or IFNγ alone did not significantly suppress cell growth compared with the control, whereas treatment with RTX-IFNγ significantly suppressed cell proliferation. In vivo mouse experiments also showed that the administration of RTX-IFNγ significantly suppressed tumor growth compared to the single therapies. Some tumors in mice treated with RTX-IFNγ were completely resolved. The cell cycle assays revealed a significantly increased rate of cells in the G0/G1 phase following treatment with RTX-IFNγ compared with the other groups, and the levels of p27kip1 increased and the levels of cyclin E and Cdk 2 decreased in cells treated with RTX-IFNγ. Our findings suggested that RTX-IFNγ combined therapy directly affects cells by arresting the cell cycle at the G1/S checkpoint and had a synergistic antitumor effect compared to RTX single treatment of B-cell lymphomas. This combined therapy may change the mortality rate for B-cell lymphomas