Binding of monoclonal antibodies with wild-type or optimized Fc domains to FcγRs. Representative surface plasmon resonance traces for binding of fixed concentrations of soluble human FcγRs or C1q (a), cynomolgus monkey FcγRs (b), or murine FcγRs (c) to ch4D5 (contains wild-type-Fc domain) or ch4D5-0264 (contains MGFc0264) captured on immobilized recombinant human HER2 protein are shown. hCD16A-158V, hCD16A-158F, and hCD64 were analyzed as soluble monomeric extracellular domains (ECDs), whereas hCD32A-131R, hCD32A-131H, and hCD32B were analyzed as soluble dimeric extracellular domain Fc-fusions (FcN297Q or FcD265A). Values of the equilibrium dissociation constant (KD) from full-range titration studies were determined by the fitting of equilibrium responses to a steady-state affinity model for hCD16 and hCD32 receptors or by a global fit to 1:1 binding model for hCD64 interactions that did not reach a steady state. FcγR, Fc-gamma receptor; RU, relative units.