title: Structural and functional analysis of a C3b-specific antibody that selectively inhibits the alternative pathway of complement.
abstract: Amplification of the complement cascade through the alternative pathway can lead to excessive inflammation. Targeting C3b, a component central to the alternative pathway of complement, provides a powerful approach to inhibit complement-mediated immune responses and tissue injury. In the present study, phage display technology was employed to generate an antibody that selectively recognizes C3b but not the non-activated molecule C3. The crystal structure of C3b in complex with a Fab fragment of this antibody (S77) illustrates the structural basis for this selectivity. Cleavage of C3 to C3b results in a plethora of structural changes within C3, including the rearrangement of macroglobulin domain 6 enabling binding of S77 to the adjacent macroglobulin domain 7 domain. S77 blocks binding of factor B to C3b inhibiting the first step in the formation of the alternative pathway C3 convertase. In addition, S77 inhibits C5 binding to C3b. This results in significantly reduced formations of anaphylatoxins and membrane-attack complexes. This study for the first time demonstrates the structural basis for complement inhibition by a C3b-selective antibody and provides insights into the molecular mechanisms of alternative pathway complement activation.
authors.affiliation: Departments of Immunology, Antibody Engineering, Protein Engineering, Assay & Automation Technology, and Protein Chemistry, Genentech Inc., 1 DNA Way, South San Francisco, California 94080, USA.
authors.forename: Kenneth J
authors.initials: KJ
authors.lastname: Katschke
authors.forename: Scott
authors.initials: S
authors.lastname: Stawicki
authors.forename: Jianping
authors.initials: J
authors.lastname: Yin
authors.forename: Micah
authors.initials: M
authors.lastname: Steffek
authors.forename: Hongkang
authors.initials: H
authors.lastname: Xi
authors.forename: Lizette
authors.initials: L
authors.lastname: Sturgeon
authors.forename: Philip E
authors.initials: PE
authors.lastname: Hass
authors.forename: Kelly M
authors.initials: KM
authors.lastname: Loyet
authors.forename: Laura
authors.initials: L
authors.lastname: Deforge
authors.forename: Yan
authors.initials: Y
authors.lastname: Wu
authors.forename: Menno
authors.initials: M
authors.lastname: van Lookeren Campagne
authors.forename: Christian
authors.initials: C
authors.lastname: Wiesmann
mesh_terms: D000818:Animals; D000906:Antibodies / Q000737:chemistry / Q000378:metabolism; D051561:Complement C3 Convertase, Alternative Pathway / Q000378:metabolism; D003179:Complement C3b / Q000737:chemistry / Q000235:genetics / Q000276:immunology* / Q000378:metabolism; D051566:Complement C5 Convertase, Alternative Pathway / Q000378:metabolism; D003170:Complement Pathway, Alternative / Q000502:physiology*; D018360:Crystallography, X-Ray; D004791:Enzyme Inhibitors / Q000737:chemistry / Q000378:metabolism; D004795:Enzyme Stability; D006801:Humans; D007140:Immunoglobulin Fab Fragments / Q000737:chemistry / Q000378:metabolism; D008253:Macaca mulatta; D008958:Models, Molecular; D008969:Molecular Sequence Data; D019151:Peptide Library; D010456:Peptides, Cyclic / Q000737:chemistry / Q000378:metabolism; D011487:Protein Conformation; D017463:Receptors, Complement 3b / Q000737:chemistry / Q000378:metabolism
chemical_list: D000906:Antibodies; C509991:CR1 protein, human; D004791:Enzyme Inhibitors; D007140:Immunoglobulin Fab Fragments; D019151:Peptide Library; D010456:Peptides, Cyclic; D017463:Receptors, Complement 3b; C111828:compstatin; D003179:Complement C3b; D051561:Complement C3 Convertase, Alternative Pathway; D051566:Complement C5 Convertase, Alternative Pathway