The Acquisition of Complement-Dependent Cytotoxicity by the Type II Anti-CD20 Therapeutic Antibody Obinutuzumab

Simple Summary Therapeutic anti-CD20 monoclonal antibodies (mAbs) are divided into two types based on their dominant effector mechanism. In contrast to type I specimens, obinutuzumab, a representative of type II mAbs, poorly activates the complement system. Recent studies explained that the structure of the antigen-antibody complex characteristic for type II antibodies precludes oligomer formation, which otherwise supports an efficient complement activation by human mAbs. Herein, we provide evidence that obinutuzumab's ability to activate complement can be rescued at later stages of the cascade, as observed in the presence of hyperactive complement convertase components. Such modulation, which enforces additional effector mechanisms, may be an alternative way of improving a killing repertoire of already existing drugs rather than designing their novel versions.Abstract Rituximab, a prototypic anti-CD20 mAb, and the third-generation anti-CD20 mAb obinutuzumab differ in their ability to activate the complement system. According to recent studies, this contrast stems from the architecture of the antigen-antibody complex formed by these two mAbs that facilitates (rituximab) or disables (obinutuzumab) further oligomerization, leading to engagement of the initial classical complement pathway component C1q. We examined whether a gain-of-function C2 variant that acts downstream of C1q and enforces the formation of complement convertase resistant to physiological decay can impact complement activation by obinutuzumab. Co-application of the C2 variant with obinutuzumab and human serum resulted in complement-dependent cytotoxicity equal to or higher than attainable for rituximab. This effect was observed either in serum or hirudin-anticoagulated whole blood. Long-term (24 h) overall cytotoxicity of obinutuzumab was improved in target cells of moderate sensitivity to complement but diminished in cells of low sensitivity. Our results demonstrate that the ability of complement activation of a given antibody is not ultimately determined at the stage of initial interactions with its target antigen but is modulable at later stages of the cascade and that the benefit of the acquisition of this new effector mechanism by obinutuzumab depends on the target cell characteristics.