Affimer proteins inhibit immune complex binding to Fc gamma RIIIa with high specificity through competitive and allosteric modes of action

Protein-protein interactions are essential for the control of cellular functions and are critical for regulation of the immune system. One example is the binding of Fc regions of IgG to the Fc gamma receptors (Fc gamma Rs). High sequence identity (98%) between the genes encoding Fc gamma RIIIa (expressed on macrophages and natural killer cells) and Fc gamma RIIIb (expressed on neutrophils) has prevented the development of monospecific agents against these therapeutic targets. We now report the identification of Fc gamma RIIIa-specific artificial binding proteins called Affimer that block IgG binding and abrogate Fc gamma RIIIa-mediated downstream effector functions in macrophages, namely TNF release and phagocytosis. Cocrystal structures and molecular dynamics simulations have revealed the structural basis of this specificity for two Affimer proteins: One binds directly to the Fc binding site, whereas the other acts allosterically.