Antibody-dependent cellular phagocytosis is responsible for efficacy of anti-CD20 monoclonal antibody therapy in chronic lymphocytic leukemia

Abstract

Abstract Anti-CD20 (αCD20) monoclonal antibodies (mAbs) are important drugs in the management of B cell malignancies, such as chronic lymphocytic leukemia (CLL), and autoimmune diseases, such as systemic lupus erythematosus. However, the mechanisms of αCD20 mAb-induced cytotoxicity of B-cells are not fully understood. The major reported mechanisms are those highly effective at activating innate immune cytotoxic mechanisms, particularly antibody-dependent cellular phagocytosis (ADCP) and antibody-dependent cellular cytotoxicity (ADCC). To determine the role of these mechanisms, we tested αCD20 mAb-mediated cellular cytotoxicity of primary CLL cells by autologous or allogeneic (healthy donor) effector cells (human monocyte-derived macrophages for ADCP, or natural killer cells for ADCC) using flow cytometry or time-lapse video microscopy with fluorescent dye-labeled cells. We found that ADCP is the primary mechanism of cellular cytotoxicity by αCD20 mAb and that ADCC does not reliably predict the ADCP induced by that mAb. Furthermore, we compared ADCP using different αCD20 mAbs and found increased phagocytosis by those mAbs with higher affinity for CD20 or FcγR and by those mAbs with higher activation of complement. Finally, the effect on ADCP of inhibitors of BTK and PI3Kδ, molecules that are targeted by current CLL therapy and also found in the FcγR signaling pathway, were tested. We found that the most specific inhibitors of BTK and PI3Kδ had the least effect on ADCP. These data provide meaningful data to design more effective mAb and inhibitor regimens to best use these targeted agents to improve CLL patient outcome.