Structural studies on the membrane attack complex and staphylococcal immune evasion : C8α-MACPF, C5b6 and FLIPr-like-FcγRIIa

The human body is continuously at battle with bacterial pathogens from its surrounding environment. This leads to a strong selective pressure on both the pathogen and the host. Accordingly, humans and bacteria have evolved elaborate measures to fight one another. In humans the complement system forms the first line of defense against invading bacteria. These trigger the activation of the complement system. This leads to various effector functions one of which entails the formation of large lytic pores on the bacterial membrane, thereby directly killing Gram-negative bacteria. These pores are called the membrane attack complex MAC) and their formation is mediated by a group of proteins collectively referred to as the MACPF family which act in concert to drill a hole through the membrane of target cells. We have determined crystal structures of two components of the MAC, the complex between C5b and C6 and the MACPF domain of C8a?. These structures yielded the unexpected result that the MACPF domain resembles cholesterol dependent cytolysins (CDCs), a group of well studied bacterial pore-forming toxins. These structure show for the first time how formation of the MAC is initiated and allow us to construct a model for how the proteins of the MAC go from a water soluble state in the blood to a membrane inserted pore on the surface of bacterial targets. Like CDCs, MACPF domains contain two amphipathic segments folded against the protein. Upon activation these segments can unfold to form extended ?-hairpins that ultimately form the lining of a large ?-barrel pore. The structures have greatly expanded our understanding of how the proteins of the MAC function together to form a bactericidal pore but also form a starting point of biochemical studies that will increase our understanding even further. A second key component humans use to fight of infections is phagocytosis. In this process, white blood cells like neutrophils, recognize antibodies bound to e.g. bacteria through so called Fc? receptors. Staphylococcus aureas, a Gram-positive bacteria that is well known for its numerous immune evasion strategies, has evolved a protein, FLIPr and its homologue, FLIPr-like, that are able to bind to the Fc? receptor most important for phagocytosis by neutrophils, Fc?RIIa This binding inhibits phagocytosis thereby conferring a strong advantage for invading bacteria. We have solved the crystal structure of the complex between FLIPr-like and Fc?RIIa and characterized the binding of the proteins with isothermal calorimetry. The structure give a clear picture of how FLIPr-like inhibits antibody binding to the receptor as their binding sites almost fully overlap. Furthermore, the structure also gives insight into the specificity FLIPr-like has for the different isoforms of Fc? receptors. Collectively the data could aid in the design of novel receptor isoform specific inhibitors to aid in the treatment of auto-immune diseases and inflammatory disorders