Interactions between complement and cellular mediated mechanisms of monoclonal antibody therapy

Monoclonal antibodies (mAbs) have become an important part of therapy for a number of cancers. The first mAb to be approved for clinical use is rituximab, which is currently used for the treatment of various B cell malignancies. Despite its clinical value, the mechanisms in which rituximab induces tumor regression are unclear. Growing evidence suggests that multiple mechanisms involving complement-dependent cytotoxicity (CDC) and antibody-dependent cellular cytotoxicity (ADCC) are involved. However, the direct interactions between CDC and ADCC have yet to be investigated.

My studies examine the relationship between complement fixation and the activation of NK cells by utilizing in vitro assays, a syngeneic murine lymphoma model, and clinical samples from patients. Using these systems, I demonstrate that the initiation of the complement cascade inhibits NK cell activation and ADCC induced by rituximab in vitro. I also show that depletion of complement enhances the activation of NK cells and improves the efficacy of mAb therapy in a murine model. Lastly, I demonstrate that NK cell activation correlates with decreased complement activity in patients after rituximab treatment.

The studies described in this dissertation have furthered the understanding of the mechanisms involved in antibody therapy. These results have described a novel inhibitory role for complement activity in the anti-tumor responses of mAbs. Furthermore, these findings suggest that strategies to circumvent the inhibitory effect of complement may improve how current mAbs are used and the how mAbs are designed in the future.