Fc optimization of therapeutic antibodies enhances their ability to kill tumor cells in vitro and controls tumor expansion in vivo via low-affinity activating fcγ receptors

Monoclonal antibodies (mAb) are widely used in the treatment of non-Hodgkin's lymphoma and autoimmune diseases. Although the mechanism of action in vivo is not always known, the therapeutic activity of several approved mAbs depends on the binding of the Fc-gamma regions to low-affinity Fc gamma receptors (Fc gamma R) expressed on effector cells. We did functional genetic screens to identify IgG1 Fe domains with improved binding to the low-affinity activating Fc receptor CD16A (Fc gamma RIIIA) and reduced binding to the low-affinity inhibitory Fc receptor, CD32B (Fc gamma RIIB). Identification of new amino acid residues important for Fc gamma R binding guided the construction of an Fc domain that showed a dramatically enhanced CD16A binding and greater than a 100-fold improvement in antibody-dependent cell-mediated cytotoxicity. In a xenograft murine model of B-cell malignancy, the greatest enhancement of an Fc-optimized anti-human B-cell mAb was accounted for by improved binding to Fc gamma RIV, a unique mouse activating Fc gamma R that is expressed by monocytes and macrophages but not natural killer (NK) cells, consistent with experimental and clinical data suggesting that mononuclear phagocytes, effector cells expressing both activating and inhibitory Fc gamma R, are critical mediators of B-cell depletion in vivo. By using mice transgenic for human CD16A, enhanced survival was observed due to expression of CD16A-158(phe), on monocytes and macrophages as well as on NK cells in these mice. The design of new generations of improved antibodies for immunotherapy should aim at Fc optimization to increase the engagement of activating Fc gamma R present on the surface of tumor-infiltrating effector cell populations.