How antibodies drive complement activation: Paving the road towards effective therapeutics

Abstract

Infectious diseases are becoming harder to treat, since bacteria become multiresistant against antibiotics. It is therefore important to develop novel therapies against infections. One possibility is antibody therapy in which therapeutic antibodies are administered to activate the human complement system during an infection. The complement system is a group of blood plasma proteins that work together to kill bacteria that have invaded the body. To be effective, these proteins however first need to get activated on the bacterial cell surface. In this thesis it is shown that therapeutic antibodies can be used to stimulate complement activation on bacterial cells. To determine which antibodies are most potent in complement activation, it was studied how different antibody subclasses against artificial and bacterial antigens activate the first complement complex C1qr2s2 and how they affect subsequent steps of complement activation. The artificial model consisted of DNP-labelled beads that are loaded with monoclonal antibodies against DNP, while the bacterial model consisted of Staphylococcus aureus bacteria that were incubated with monoclonal antibodies against an abundant glycopolymer of the bacterial cell wall (namely wall teichoic acid). The studies on the interaction between antibodies and C1qr2s2 revealed a previously unrecognized role for the proteases C1r and C1s in the binding of the C1qr2s2 complex. Association of these proteases with the recognition molecule C1q improved the binding of C1 complex to especially IgG1 and IgG2. C1q binding to IgG could also be improved by introduction of a specific antibody mutation designed to improve the Fc-mediated clustering of antibodies on a surface into hexamers. However, we showed that the mutation does not affect every subclass similarly. Moreover, it was shown that the antibody mutation affected the regulation of C1qr2s2 by C1-inhibitor (C1-INH), the only known direct regulator of C1 activation in the human body. Altogether, this thesis provides new basic knowledge about the complement-activating potency of antibodies and new molecular insights in the (side) effects of antibody engineering that can be very useful for the development of new antibody therapies against infectious disease.