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41 p.-11 fig. ; C3 is the central component of the complement system. Upon activation, C3 sequentially generates various proteolytic fragments, C3a, C3b, iC3b, C3dg, each of them exposing novel surfaces, which are sites of interaction with other proteins. C3 and its fragments are therapeutic targets and markers of complement activation. We report the structural and functional characterization of four monoclonal antibodies (mAbs) generated by immunizing C3-deficient mice with a mixture of human C3b, iC3b and C3dg fragments, and discuss their potential applications. This collection includes three mAbs interacting with native C3 and inhibiting AP complement activation; two of them by blocking the cleavage of C3 by the AP C3-converase and one by impeding formation of the AP C3-convertase. The interaction sites of these mAbs in the target molecules were determined by resolving the structures of Fab fragments bound to C3b and/or iC3b using electron microscopy. A fourth mAb specifically recognizes the iC3b, C3dg, and C3d fragments. It binds to an evolutionary-conserved neoepitope generated after C3b cleavage by FI, detecting iC3b/C3dg deposition over opsonized surfaces by flow cytometry and immunohistochemistry in human and other species. Because well-characterized anti-complement mAbs are uncommon, the mAbs reported here may offer interesting therapeutic and diagnostic opportunities. ; Work in this report has been funded by the Spanish "Ministerio de Economía y Competitividad" (SAF2011-26583 and SAF2015-66287-R to SRdC and SAF2014- 52301-R to OL) and the Seventh Framework Programme European Union Project EURenOmics (305608) to SRdC. In addition, this work has been supported by a grant from the Autonomous Region of Madrid (S2010/BMD-2316) to SRdeC and OL. ; Peer reviewed

21 p.-6 fig. ; The proteolytic cleavage of C3 to generate C3b is the central and most important step in the activation of Complement, a major component of innate immunity. The comparison of the crystal structures of C3 and C3b illustrates large conformational changes during the transition from C3 to C3b. Exposure of a reactive thio-ester group allows C3b to bind covalently to surfaces such as pathogens or apoptotic cellular debris. The displacement of the thio-ester containing domain (TED) exposes hidden surfaces that mediate the interaction with complement factor B to assemble the C3- convertase of the alternative pathway (AP). In addition, the displacement of the TED domain and its interaction with the macroglobulin 1 (MG1) domain generates a extended surface in C3b where the complement regulators factor H (FH), decay accelerating factor (DAF), membrane cofactor protein (MCP) and complement receptor 1 (CR1) can bind, mediating accelerated decay of the AP C3-convertase and proteolytic inactivation of C3b. In the last few years, evidence has accumulated revealing that the structure of C3b in solution is significantly more flexible than anticipated. We review our current knowledge on C3b structural flexibility to propose a general model where the TED domain can display a collection of conformations around the MG ring, as well as a few specialised positions where the TED is held in one of several fixed locations. Importantly, this conformational heterogeneity in C3b impacts complement regulation, by affecting the interaction with regulators. ; This work was funded by the Spanish Government (SAF2011- 22988 and SAF2014-52301-R to OL;SAF2011-26583 to SRdeC), the EU (Eurenomics to SRdC) and by the Government of the Autonomous Region of Madrid (S2010/BMD-2316 to OL and SRdC). ; Peer reviewed

C3 is the central component of the complement system. Upon activation, C3 sequentially generates various proteolytic fragments, C3a, C3b, iC3b, C3dg, each of them exposing novel surfaces, which are sites of interaction with other proteins. C3 and its fragments are therapeutic targets and markers of complement activation. We report the structural and functional characterization of four monoclonal antibodies (mAbs) generated by immunizing C3-deficient mice with a mixture of human C3b, iC3b and C3dg fragments, and discuss their potential applications. This collection includes three mAbs interacting with native C3 and inhibiting AP complement activation; two of them by blocking the cleavage of C3 by the AP C3-converase and one by impeding formation of the AP C3-convertase. The interaction sites of these mAbs in the target molecules were determined by resolving the structures of Fab fragments bound to C3b and/or iC3b using electron microscopy. A fourth mAb specifically recognizes the iC3b, C3dg, and C3d fragments. It binds to an evolutionary-conserved neoepitope generated after C3b cleavage by FI, detecting iC3b/C3dg deposition over opsonized surfaces by flow cytometry and immunohistochemistry in human and other species. Because well-characterized anti-complement mAbs are uncommon, the mAbs reported here may offer interesting therapeutic and diagnostic opportunities. ; Work in this report has been funded by the Spanish "Ministerio de Economía y Competitividad" (SAF2011‐26583 and SAF2015‐66287‐R to SRdC and SAF2014‐52301‐R to OL) and the Seventh Framework Programme European Union Project EURenOmics (305608) to SRdC. In addition, this work has been supported by a grant from the Autonomous Region of Madrid (S2010/BMD‐2316) to SRdeC and OL. ; Sí