Complement is a blood based immune network that plays an important role in innate and adaptive immune responses. Activation of complement culminates in the formation of a large lytic pore called the Membrane Attack Complex (MAC), which assembles on lipid bilayers and punches holes in target cells. MAC is a versatile and highly effective immune effector as there is no specific receptor directing MAC to pathogens. However, human cells are susceptible to damage by MAC if not properly controlled. Dysregulation of MAC on human cells can have devastating impact for disease pathologies including haemolytic anaemia and immune evasion strategies by cancer cells. Understanding how MAC is controlled during an immune response will enable the design of therapeutics that have the potential to precisely regulate complement activity and improve human health.
The objective of CONTROLLING MAC is to uncover the fundamental molecular principles underpinning MAC regulation on human cells. To achieve this ambitious goal, Controlling MAC uses a range of structural biology and biophysical tools to understand how complement activation impacts cellular pathways that modulate inflammation. The project develops an innovative approach to exploring the interactions between complement proteins and their local lipid environment. The novel insights from these experiments will explain how conformational changes in protein structure are linked with changes in physical properties of cellular membranes. Together, these data will explain how MAC is regulated and define a new role for lipids in immune homeostasis.
The conclusions of the project provide a structural basis for how MAC is regulated on human cells when the immune system switches on. Our cryo electron microscopy (cryoEM) structure of a soluble regulated form of MAC shows how the blood-based chaperone, clusterin, wraps across the polymerizing face of MAC and defines a new conformation of C9. Additionally, our structural work defines a key mechanism of action for CD59, the only membrane-bound regulator of MAC-mediated cell death. By developing a model lipid-based system and using it for structural studies, we explain how CD59 catches and deflects the transmembrane beta-hairpins of MAC proteins to prevent membrane damage. Finally, by developing novel macrocyclic peptides that target CD59 we provide a blueprint for the design of future therapeutics that have the potential to regulate complement activity and improve human health.
