Ratios Of FcγRIIa and FcγRIII Binding Affinities Govern The Efficacy Of PMN-Mediated Cytotoxicity – Implications For Fc-Engineering Approaches Of Therapeutic Antibodies

Antibody-dependent cell-mediated cytotoxicity (ADCC) has been suggested to be an essential effector mechanism for the in vivo activity of tumor-targeting therapeutic monoclonal antibodies (mAbs). Thus, enhancing the affinity of human IgG1 mAbs to NK cell-expressed FcγRIIIa by glyco- or protein-engineering of their Fc moiety has been demonstrated to improve NK cell-mediated ADCC and represents a promising strategy to improve antibody therapy. However, human polymorphonuclear (PMN) cells express the homologous FcγRIIIb isoform, which does not trigger ADCC. The aim of the present study was to analyze a panel of distinct IgG1 mAbs, displaying different affinities for FcγRIIa and FcγRIII, with respect to PMN recruitment for tumor cell destruction.

Affinities of analyzed mAbs to distinct FcγR were determined by surface plasmon resonance technology. Induction of ADCC was assessed by ⁵¹Cr release experiments in the absence or presence of FcγRIII- or FcγRII-blocking agents to unravel the contribution of both receptors in these assays.

Non-fucosylated or protein-engineered IgG1 variants with optimized FcγRIII binding capacities demonstrated the expected benefit in triggering NK cell- mediated ADCC but did not mediate ADCC by PMN, which could be restored by FcγRIIIb blockade. Additionally, eosinophils as well as PMN from paroxysmal nocturnal hemoglobinuria (PNH) patients – expressing no or low levels of FcgRIIIb – mediated effective ADCC with FcgRIII-optimized mAbs. Additional experiments with Fc variants displaying enhanced FcγRIIa binding or with double FcγRIIa/FcγRIII-optimized constructs demonstrated enhanced PMN-mediated ADCC compared to control mAbs. Statistical analyses revealed that FcγRIIa/FcγRIIIb affinity ratios correlated with the extent of human PMN-mediated ADCC.

To conclude, the present study represents novel findings concerning recruitment of PMN for tumor cell destruction by Fc-engineered antibodies. While PMN-mediated ADCC was completely abolished by FcγRIII-optimized antibodies through predominant FcγRIIIb binding, it was potently enhanced by optimization of FcγRIIa binding affinity. Importantly, functional analyses unraveled the ratio between FcγRIIa and FcγRIII binding affinities to control PMN-mediated ADCC activity and therefore opened new alleys in engineering of therapeutic antibodies.

Muchhal:Xencor Inc.: Employment. Desjarlais:Xencor Inc.: Employment.