Submitted February 16, 2007
Accepted November 8, 2007
NK cell activation and antibody dependent cellular cytotoxicity induced by rituximab-coated target cells is inhibited by the C3b component of complement
Siao-Yi Wang, Emilian Racila, Ronald P Taylor, and George J. Weiner*
Graduate Program in Immunology, University of Iowa, Iowa City, IA, United States
Holden Comprehensive Cancer Center and Department of Internal Medicine, University of Iowa, Iowa City, IA, United States
The Department of Biochemistry and Molecular Genetics, Unviersity of Virginia, Charlottesville, VA, United States
* Corresponding author; email: george-weiner{at}uiowa.edu.
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. We 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 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. Further studies are needed to define in more detail the impact of complement fixation on ADCC, and whether mAb that fail to fix complement will be more effective at mediating ADCC.
