title: Autoantibodies against C1q in systemic lupus erythematosus are antigen-driven.
abstract: Autoantibodies against complement C1q (anti-C1q Abs) were shown to strongly correlate with the occurrence of severe nephritis in patients with systemic lupus erythematosus (SLE), suggesting a potential pathogenic role by interfering with the complement cascade. To analyze the humoral immune response against C1q at the molecular level, we screened a bone marrow-derived IgGkappa/IgGlambda Fab phage display library from a SLE patient with high anti-C1q Ab titer against purified human C1q. Six Fabs that exhibited strong binding to C1q in ELISA were isolated. The anti-C1q Fabs recognized neoepitopes that were only exposed on bound C1q and not present on soluble C1q mapping to different regions of the collagen-like region of C1q. Analysis of the genes encoding the variable H and L chains of the IgG-derived anti-C1q Fab revealed that all the variable H and L chain regions were highly mutated, with nucleotide and amino acid homologies to the closest germline in the range of 71-97% (average 85 +/- 4) and 72-92% (average 88 +/- 6), respectively. In addition, the variable region of the Fabs exhibited high replacement to silent ratios. The six anti-C1q Fabs were shown to be of high affinity, with a K(d) ranging from of 8.4 x 10(-8) M to 1.4 x 10(-7) M, comparable to an antiviral immune response. Our data underlines the notion that the development of anti-C1q Abs in SLE is the consequence of an Ag-driven, affinity-matured immune response. Those anti-C1q Fabs are unique tools to address how complement C1q is implicated in the pathogenesis of SLE.
authors.affiliation: Department Biomedicine, Laboratory of Clinical Immunology, University Hospital Basel, Basel, Switzerland. monica.schaller@unibas.ch
authors.forename: Monica
authors.initials: M
authors.lastname: Schaller
authors.forename: Cornelia
authors.initials: C
authors.lastname: Bigler
authors.forename: Doris
authors.initials: D
authors.lastname: Danner
authors.forename: Henrik J
authors.initials: HJ
authors.lastname: Ditzel
authors.forename: Marten
authors.initials: M
authors.lastname: Trendelenburg
mesh_terms: D000595:Amino Acid Sequence; D000911:Antibodies, Monoclonal / Q000096:biosynthesis / Q000302:isolation & purification / Q000378:metabolism; D000915:Antibody Affinity; D000918:Antibody Specificity; D051017:Apoptosis Regulatory Proteins / Q000096:biosynthesis / Q000302:isolation & purification / Q000378:metabolism; D001323:Autoantibodies / Q000096:biosynthesis* / Q000302:isolation & purification / Q000378:metabolism; D001324:Autoantigens / Q000276:immunology / Q000378:metabolism / Q000502:physiology*; D015922:Complement C1q / Q000276:immunology / Q000378:metabolism / Q000502:physiology*; D006801:Humans; D007140:Immunoglobulin Fab Fragments / Q000096:biosynthesis / Q000302:isolation & purification / Q000378:metabolism; D008180:Lupus Erythematosus, Systemic / Q000276:immunology*; D008969:Molecular Sequence Data
chemical_list: D000911:Antibodies, Monoclonal; D051017:Apoptosis Regulatory Proteins; D001323:Autoantibodies; D001324:Autoantigens; D007140:Immunoglobulin Fab Fragments; D015922:Complement C1q