Engineering IgG1 Fc Domains That Activate the Complement System

Fc-mediated effector functions are important for the clearance of pathologic cells by therapeutic IgG antibodies through two mechanisms: via the activation of the classical complement pathway and through the binding to Fcγ receptors (FcγRs) which mediate clearance of targeted cells by antibody-dependent cellular cytotoxicity (ADCC) and antibody-dependent cellular phagocytosis (ADCP) by effector cells such as macrophages, NK cells, and other leukocytes subsets. Complement activation results in direct cell killing through the formation of the membrane attack complex (MAC, complement-dependent cytotoxicity or CDC) and in the deposition of complement opsonins on pathogen surfaces. The latter are recognized by complement receptors on effector cells in turn triggering complement-dependent cell cytotoxicity and phagocytosis (CDCC and CDCP, respectively). Little is known about the role of CDCC and CDCP on therapeutic antibody function because on the one hand, IgG isotype antibodies bind to both FcγR and C1q to activate the complement pathway, and on the other, immune cells express complement receptor as well as FcγRs. We engineered IgG1 Fc domains that bind with high affinity to C1q but have very little or no binding to FcγR. To this end, we employed display of IgG in E. coli (which lack protein glycosylation machinery) for the screening of very large libraries (>2 × 10⁹) of randomly mutated human Fc domains to isolate Fc variants that bind to C1q. Herein we introduce and describe the method.