Chronic lymphocytic leukaemia cells are efficiently killed by an anti-CD20 monoclonal antibody selected for improved engagement of FcgammaRIIIA/CD16

Patients with chronic lymphocytic leukaemia (CLL) treated with a combination of fludarabine, cyclophosphamide and rituximab show a high response rate. However, only a poor response is observed following rituximab monotherapy. The use of chemo-immunotherapy is often associated with haematological and infectious complications. Thus, an antibody with an enhanced ability to kill CLL cells could lead to better clinical responses to antibody monotherapy and the possibility of lowering drug doses during chemo-immunotherapy. We generated a chimeric anti-CD20 monoclonal antibody (mAb), EMAB-6, which has a low fucose content. Apoptosis and complement activities for EMAB-6 were similar to those seen for rituximab. By contrast, EMAB-6 mAb showed improved Fcγ receptor IIIA (FcγRIIIA)/CD16 binding and FcγRIIIA-dependent effector functions. It induced a higher in vitro antibody-dependent cellular cytotoxicity against CLL cells and a higher FcγRIIIA-mediated interleukin-2 production by FcγRIIIA⁺ Jurkat cells in the presence of CLL cells at both low and maximally saturating concentrations. Comparative studies between CLL and lymphoma cells coated with EMAB-6 or rituximab indicated that the difference of efficacy was more pronounced at low doses and when target cells expressed fewer CD20 molecules. Thus, EMAB-6 mAb represents a promising drug candidate for the treatment of CLL by inducing a strong cytotoxicity against tumour cells that express low CD20 levels.