The C3b Component of Complement Inhibits NK Cell Activation Induced by Rituximab-Coated Target Cells.

Rituximab (RTX) is an accepted therapy for B cell malignancies, but there is still much to learn about the mechanisms responsible for the observed responses and the potential interactions between various mechanisms of action. Some studies suggest that complement fixation followed by lysis through the membrane attack complex contributes to the anti-tumor effects of RTX. Other investigations indicate that antibody dependent cellular cytotoxicity (ADCC) mediated by NK cells is central to the response of therapy. In prior studies, we found that RTX-coated target cells activate NK cells as indicated by NK cell modulation of CD16, upregulation of CD54 and CD69, and production of IFNγ. NK activation induced by RTX-coated target cells was dependent on the affinity of multivalent interactions between Fc γ receptors III (CD16) of the NK cell and Fc regions of cell-bound RTX molecules. We used these in vitro assays to assess the relationship between complement fixation, and the ability of RTX-coated target cells to activate NK cells. Normal human serum inhibited the modulation of NK cell CD16, and also blocked upregulation of CD54, induced by RTX-coated target cells. The ability of serum to inhibit NK activation was dose dependent and was abrogated upon heat inactivation. Serum depleted of C1q or C3 also failed to inhibit NK cell activation. The inhibitory activity of serum depleted of these complement components was restored when purified C1q or C3 were added back respectively. In addition, the level of NK cell inhibition was dependent on the amount of C3b deposited on the target cells. An antibody that stabilizes C3b on the target cell surface (3E7, DiLillo et al., Molec Immunol 2006) further enhanced the inhibition of NK cell activation induced by RTX-coated target cells. One possible explanation for these findings is that complement-mediated lysis destroyed the RTX-coated target cells before they had the opportunity to induce activation of the NK cells. To assess this possibility, lymphoma cells were killed, fixed with formaldehyde, and washed prior to their use as target cells. These RTX-coated and fixed target cells were able to induce modulation of CD16 on the NK cells, which was again inhibited by normal human serum. These findings indicate that the observed inhibition of NK activation by complement is unlikely to be a consequence of complement mediated lysis of the target cells. Instead, these data suggest that C3b deposition on RTX-coated target cells inhibits the interaction between the Fc portions of RTX and CD16 on the NK cells, and so limits the ability of RTX-coated target cells to induce NK activation. These results could have implications in our understanding of the relationship between complement fixation and ADCC, and their relative roles in potentiating destruction of malignant cells in the blood and tissues.