Role of Calcium Signaling in GA101-Induced Cell Death in Malignant Human B Cells

GA101/obinutuzumab is a novel type II anti-CD20 monoclonal antibody (mAb), which is more effective than rituximab (RTX) in preclinical and clinical studies when used in combination with chemotherapy. Ca(2+) signaling was shown to play a role in RTX-induced cell death. This report concerns the effect of GA101 on Ca(2+) signaling and its involvement in the direct cell death induced by GA101. We reveal that GA101 triggered an intracellular Ca(2+) increase by mobilizing intracellular Ca(2+) stores and activating Orai1-dependent Ca(2+) influx in non-Hodgkin lymphoma cell lines and primary B-Cell Chronic Lymphocytic Leukemia (B-CLL) cells. According to the cell type, Ca(2+) was mobilized from two distinct intracellular compartments. In Raji, BL2, and B-CLL cells, GA101 induced a Ca(2+) release from lysosomes, leading to the subsequent lysosomal membrane permeabilization and cell death. Inhibition of this calcium signaling reduced GA101-induced cell death in these cells. In SU-DHL-4 cells, GA101 mobilized Ca(2+) from the endoplasmic reticulum (ER). Inhibition of ER replenishment, by blocking Orai1-dependent Ca(2+) influx, led to an ER stress and unfolded protein response (UPR) which sensitized these cells to GA101-induced cell death. These results revealed the central role of Ca(2+) signaling in GA101’s action mechanism, which may contribute to designing new rational drug combinations improving its clinical efficacy.