Next generation monoclonal antibodies and their mechanisms of action against B-cell lymphomas

Next generation monoclonal antibodies (mAbs) are unique in that they are specifically designed to enhance their mechanisms of action, primarily complement fixation and antibody-dependent cellular cytotoxicity (ADCC). Recent studies suggest that complement-fixing properties of a mAb can counter its ability to activate NK cells and mediate ADCC. GA101, a third generation (type II anti-CD20) mAb, and rituximab-MAGE (glyco-engineered type I mAb) show enhanced ADCC and direct cell killing; while ofatumumab, a second generation anti-CD20 mAb, shows enhanced complement-mediated cytotoxicity (CMC). These studies set out to determine the primary mechanisms of actions of these various mAbs, and compare the effect of complement on their ability to activate NK cells and mediate ADCC or CMC. We also studied the efficiency of rituximab vs. rituximab-MAGE to deplete B-cells in vivo in mice expressing human transgenic CD20. In vitro, rituximab and ofatumumab fixed more complement and mediated a greater degree of CMC, than GA101 and rituximab-MAGE. Additionally, complement inhibited the ability of both rituximab and ofatumumab to bind to and activate NK cells, whereas, addition of complement to GA101 or rituximab-MAGE did not affect their NK cell activating ability. Complement also blocked rituximab-induced NK-cell mediated ADCC, but not GA101-induced NK-cell mediated ADCC. Finally, GA101 and rituximab-MAGE depleted a higher percentage of B cells in whole blood compared to rituximab and ofatumumab, whereas rituximab-MAGE depleted fewer B cells, in vivo, in a complement-dependent fashion. We conclude from these studies that there are significant differences among these antibodies and that the ability of a given antibody to mediate CMC and complement fixation correlates with the ability of complement to block the interaction between the antibody and NK cells.