Interactions between complement and cellular mediated mechanisms of monoclonal antibody therapy

Antibody dependent cellular cytotoxicity (ADCC) and complement fixation both appear to play a role in mediating anti-tumor effects of monoclonal antibodies (mAbs), including rituximab. I evaluated the relationship between rituximab-induced complement fixation, NK cell activation, and NK cell mediated ADCC. Down-modulation of NK cell CD16 and NK cell activation induced by rituximab-coated target cells was blocked by human serum but not heat-inactivated serum. This inhibition was also observed in the absence of viable target cells. C1q and C3 in the serum were required for these inhibitory effects, while C5 was not. An antibody that stabilizes C3b on the target cell surface enhanced the inhibition of NK cell activation induced by rituximab-coated target cells. Binding of NK cells to rituximab-coated plates through CD16 was inhibited by the fixation of complement. C5-depleted serum failed to induce complement-mediated lysis but blocked NK cell-mediated ADCC. These data suggest that C3b deposition induced by rituximab-coated target cells inhibits the interaction between the rituximab Fc and NK cell CD16, thereby limiting the ability of rituximab-coated target cells to induce NK activation and ADCC. Introduction Monoclonal antibodies (mAbs) are now a mainstay of therapy for a number of cancers. Rituximab was the first chimeric mAb to be approved for clinical use and remains the most extensively utilized mAb in cancer therapy. Rituximab binding of CD20 has been shown to signal apoptosis in a subset of lymphoma cell lines (121). However, there is little evidence that signaling plays an important role in clinical