Structural insights into complement regulation

As an important tool of the innate immune system, the complement system rapidly recognizes and clears invading microbes and host debris. Furthermore, it stimulates B- and T-cell development through cross talks with the adaptive immune system. Complement is activated through specific danger pattern recognition by the classical and lectin pathways, unspecific initiation and amplification by the alternative pathway, resulting in cell lysis by terminal pathway. The classical pathway, lectin and alternative pathways induce opsonization of C3b on targeted cells, which ultimately results in the clearance of targets. The complement system is tightly controlled to avoid unwanted damage of healthy host cells. Over-activation of complement due to mutations in complement proteins or regulators, or the presence of autoantibodies against the regulators, has been linked to diseases like atypical hemolytic uremic syndrome (aHUS), age-related macular degeneration (AMD) and paroxysmal nocturnal hemoglobinuria (PNH). ‘Regulators of complement activity’ (RCA) proteins form a group of complement regulators. They down-regulate the activity of the central enzyme complexes of the complement system, i.e. C3 convertases, through two mechanisms: (1) they assist serine protease Factor I to induce degradation of C3b and C4b (cofactor activity) and (2) they accelerate the irreversible dissociation of C3 convertases, C3bBb and C4b2a (decay acceleration activity). Many viruses functionally mimic the host complement-regulator proteins to avoid complement attacks. Investigations on complement regulation elucidated the mechanisms of how regulators bind to C3b. The recent published structures of C3b in complex with factor H (FH), complement receptor 1 (CR1), membrane cofactor protein (MCP), smallpox inhibitor of complement enzymes (SPICE) and decay accelerating factor (DAF) imply that all the regulators utilize a common platform in C3b for binding. However, the molecular mechanisms of complement regulation, especially the activity control of C3 convertases, are still largely unexplored. In this thesis, we investigated the molecular basis of cofactor activity and decay accelerating activity in the alternative pathway, as well as the regulatory mechanisms in the classical pathway, through determining a series crystal structures and performing systematic biochemical and biophysical experiments.