Resistance to obinutuzumab-induced antibody-dependent cellular cytotoxicity caused by abnormal Fas signaling is overcome by combination therapies

Obinutuzumab, a Type II anti-CD20 antibody, is used to treat follicular lymphoma. A major mode of action of obinutuzumab is antibody-dependent cellular cytotoxicity (ADCC). Knowledge of the mechanisms of resistance to obinutuzumab is important for the development of next-line strategies to follow obinutuzumab-containing therapy, including obinutuzumab retreatment. Unfortunately, the mechanisms by which tumor cells acquire resistance to ADCC are still poorly understood. To address this, we examined the mechanisms of resistance to obinutuzumab-induced ADCC and the combination efficacy of obinutuzumab and clinically available agents in the established resistant cells.We established cells resistant to obinutuzumab-induced ADCC using the non-Hodgkin lymphoma cell line RL and examined their mechanisms of resistance and the combination efficacy of obinutuzumab and clinically available agents. Comprehensive analysis by RNA sequencing of resistance mechanisms revealed that abnormal Fas signaling decreased sensitivity to ADCC in resistant clones. Combination treatment with prednisolone, a component of CHOP and CVP, was found to enhance ADCC sensitivity of RL cells and resistant clones and to significantly suppress tumor growth in xenograft models. Treatment with prednisolone upregulated expression of CD20 and an apoptosis-inducing protein BIM, which might augment perforin/granzyme B-mediated cell death. Furthermore, pretreatment of the effector cells with bendamustine enhanced ADCC activity, and treatment with obinutuzumab plus bendamustine showed significant antitumor efficacy in xenograft models. It was speculated that bendamustine upregulates ADCC activity by potentiating granules-mediated cell killing.Our study revealed a novel mechanism underlying obinutuzumab-induced ADCC resistance and indicated that ADCC resistance could be overcome by combining obinutuzumab with prednisolone or bendamustine. This study provides a scientific rationale for obinutuzumab-retreatment in combination with clinically available chemotherapeutic agents for obinutuzumab resistant follicular lymphoma.