The molecular pathways of IgG oligomerization and classical complement activation on antigenic surfaces

The human immune system consists of powerful defense mechanisms that recognize and eliminate pathogens from our bodies. Immunoglobulin G (IgG) antibodies play a central role in this protection by alerting and activating components of the innate immune system, such as the complement cascade. Here, we employed high-speed atomic force microscopy [1, 2] to conclusively show that IgGs oligomerize on antigenic surfaces via two interconnected Fc-mediated pathways. Bivalently binding IgG molecules were recruited from solution via the formation of Fc-Fc interactions with membrane-bound antibodies, whereas monovalently binding variants [3] also assembled into oligomers through lateral diffusion. All intermediates of the oligomerization process were observed, although only full hexamers bound C1q. Complement dependent cytotoxicity (CDC)-enhancing mutations [3] increased the abundance of higher-order oligomers, while mutations that were shown to decrease CDC also hindered the hexamerization process. Functional assays on cells confirmed that IgG hexamerization was required for maximal complement activation, and that dimers, trimers and tetramers were mostly inactive. We present a dynamic IgG oligomerization model, which provides a framework for immunotherapy optimization and for exploiting the macromolecular assembly of IgGs on antigenic surfaces [4]. 1. Ando, T. et al. A high-speed atomic force microscope for studying biological macromolecules. Proceedings of the National Academy of Sciences of the United States of America 98, 12468–12472 (2001). 2. Preiner, J. et al. IgGs are made for walking on bacterial and viral surfaces. Nature Communications 5, (2014). 3. Diebolder, C. A. et al. Complement Is Activated by IgG Hexamers Assembled at the Cell Surface. Science 343, 1260–1263 (2014). 4. Strasser, J. et al. Unraveling the macromolecular pathways of IgG oligomerization and complement activation on antigenic surfaces. Submitted