Abstracts / Molecular Immunology 47 (2010) 2198–2294 2289 86 Impaired ability of complement activation via the alternative pathway C3 convertase caused by a single-point mutation in the beta chain of C3 Georgia Sfyroera a , Daniel Ricklin a , Hui Chen a , Kristina Nilsson- Ekdahl b , Bo Nilsson b , Emilia L. Wu c , Yiannis N. Kaznessis c , John D. Lambris a a Department of Pathology & Laboratory Medicine, University of Penn- sylvania, Philadelphia, PA, USA b Department of Clinical Immunology, The Rudbeck Laboratory, Upp- sala University, Uppsala, Sweden c Department of Chemical Engineering and Materials Science, Univer- sity of Minnesota, Minneapolis, MN, USA A hereditary dysfunction of complement component C3 that induces systemic lupus erythematosus-like syndrome and renders the affected patient more susceptible to meningococcal infections has been described almost two decades ago. Although extensive biochemical research indicated a functional deficit in the activation of C3 by convertases, the genetic cause and molecular consequences of the dysfunction remained elusive. Here we provide these miss- ing links by applying an integrated approach based on genetic, biochemical, biophysical, and computational methods. We could pinpoint the dysfunction to a single point mutation (M351T) in the beta chain of C3, and show that an ominous combination of C3 deficiency and dysfunction in the patient is likely the cause of the observed symptoms. In functional assays, the activity of the alter- native pathway (AP) was completely abolished in patient plasma and could only be reconstituted with external addition of wildtype C3. The M351T mutation was located inside domain MG4, which contains a binding site for complement inhibitor compstatin and has recently been suspected to be involved in the interaction of C3 with the C3 convertase. While M351T is not exposed at the surface of C3, both in silico and in vitro analyses suggested an induction of regional and long-range conformational changes that disturb the integrity of MG4. Indeed, we observed altered binding activ- ities of mutated C3 for both compstatin and surface-immobilized C3b. Most importantly, the presence of M351T in either the C3 substrate or convertase-affiliated C3b critically impaired the initial binding of C3 to the convertase and, consequently, its activation and opsonization. The resulting inability of complement amplification is therefore considered a major cause for the observed symptoms in the patient. Our observations not only reveal the functional mech- anism of a clinically relevant dysfunction of C3 but also shed light into structural details of complement activation by the C3 conver- tase. doi:10.1016/j.molimm.2010.05.268 87 Complement inhibition decreases the procoagulant response and confers organ protection in a baboon model of E. coli sepsis Florea Lupu a,f , Robert Silasi-Mansat a , Hua Zhu a , Narcis I. Popescu a,f , Glenn Peer b , Georgia Sfyroera e , Paola Magotti e , Lacramioara Ivanciu a , Cristina Lupu a , Tom E. Mollnes c,d , Fletcher B. Taylor a,f , Gary Kinasewitz b , John D. Lambris e a Cardiovascular Biology Research Program, Oklahoma Medical Research Foundation, Oklahoma City, OK, USA b Department of Internal Medicine, Pulmonary and Critical Care Divi- sion, Oklahoma University Health Sciences Center, Oklahoma City, OK, USA c Institute of Immunology, Rikshospitalet University Hospital and Uni- versity of Oslo, Norway d Research Laboratory, Nordland Hospital and University of Tromsø, Norway e Department of Pathology and Laboratory Medicine, School of Medicine, University of Pennsylvania, Philadelphia, PA, USA f Department of Pathology, Oklahoma University Health Sciences Cen- ter, Oklahoma City, OK, USA Severe sepsis leads to massive activation of coagulation and complement cascades that could contribute to multiple organ fail- ure (MOF) and death. To investigate the role of complement and its crosstalk with the hemostatic system in the pathophysiol- ogy and therapeutics of sepsis, we have used a potent inhibitor (compstatin) administered early or late post E. coli challenge in a baboon model of sepsis-induced MOF. Compstatin infusion inhib- ited sepsis-induced blood and tissue biomarkers of complement activation, reduced leucopenia and thrombocytopenia, and low- ered the accumulation of macrophages and platelets in organs. Compstatin decreased the coagulopathic response by downregu- lating tissue factor and PAI-1, diminished global blood coagulation markers (fibrinogen, fibrin-degradation products, activated partial thromboplastin time), and preserved the endothelial anticoagulant properties. Compstatin treatment also improved cardiac function and the biochemical markers of kidney and liver damage. Histo- logical analysis of vital organs collected from animals euthanized after 24 h showed decreased microvascular thrombosis, improved vascular barrier function, less leukocyte infiltration and cell death, all consistent with attenuated organ injury. We conclude that complement-coagulation interplay contributes to the progression of severe sepsis and blocking the harmful effects of complement activation products, especially during the organ failure stage of severe sepsis is a potentially important therapeutic strategy. doi:10.1016/j.molimm.2010.05.269 88 Generation of analogues of the clinical complement inhibitor compstatin with improved potency and pharmacokinetics Hongchang Qu a , Paola Magotti a , Daniel Ricklin a , Emilia L. Wu b , Ioannis Kourtzelis a , Maria Lasaosa a , You-Qiang Wu a , Florea Lupu c , Yiannis N. Kaznessis b , John D. Lambris a a Department of Pathology & Laboratory Medicine, University of Penn- sylvania, Philadelphia, PA, USA b Department of Chemical Engineering & Materials Science, University of Minnesota, Minneapolis, MN, USA c Cardiovascular Biology Research Program, Oklahoma Medical Research Foundation, Oklahoma City, OK, USA Recent years have impressively shown that any imbalance between complement activation and regulation may contribute to