A kinetic model of antigen-dependent IgG oligomerization and complement binding

The classical complement pathway (CCP), an important branch of the mammalian immune system, is initiated through multivalent binding of complement protein C1 to Immunoglobulin G (IgG) antibody oligomers assembled on the surface of pathogens, infected or malignant cells, culminating in the formation of the membrane attack complex (MAC) and subsequent cell lysis. IgG oligomers can further engage immune effector cells through Fcγ receptors or complement receptors, facilitating antibody-dependent cellular cytotoxicity (ADCC) and phagocytosis (ADCP). Detailed knowledge of the factors that drive IgG oligomerization is thus vitally important to establish and improve IgG based therapies. We here focus on the kinetics of antigen-dependent IgG oligomerization and develop a comprehensive model capable of predicting oligomer formation as a function of IgG concentration, antigen density, IgG subclass and Fc point mutants, as well as the presence of Fc-binding and thus oligomerization-inhibiting factors such as staphylococcal protein A (SpA). We characterize the underlying molecular interactions in single molecule force spectroscopy (SMFS) and grating coupled interferometry (GCI) experiments. By fitting experimental data from high-speed atomic force microscopy (HS-AFM) experiments, we further quantify key rate constants and thermodynamic parameters, including free energy changes associated with oligomerization and apply the model to predict complement-mediated lysis in liposomal vesicle-based assays. The presented mechanistic framework may serve as a basis for optimizing antibody engineering and pharmacokinetic/pharmacodynamic modeling in the context of immunotherapies exploiting the CCP.