title: Low-molecular weight C1q-binding immunoglobulin G in patients with systemic lupus erythematosus consists of autoantibodies to the collagen-like region of C1q.
abstract: The majority of C1q-binding IgG in sera of some patients with systemic lupus erythematosus (SLE) cosediments with monomeric IgG. This study was undertaken to provide definitive proof that the low-molecular weight C1q-binding IgG consists of autoantibodies to C1q. Monomeric C1q-binding IgG was isolated from five SLE plasmas by C1q affinity chromatography and gel filtration. All C1q-binding IgG preparations and their F(ab')2 fragments bound to both C1q and the collagen-like region of C1q by an ELISA. To rule out the possibility that small DNA-antiDNA immune complexes caused this binding activity, Fab' fragments of the C1q-binding IgG preparations were digested with DNase I to degrade any DNA. The Fab' fragments continued to bind to C1q and its collagen-like region after this treatment. C1q-binding IgG was heterogenous on isoelectric focusing. Interaction of C1q-binding IgG with solid-phase C1q was retained in 1 M NaCl, whereas the binding of DNA or heat-aggregated IgG to solid-phase C1q was abrogated or markedly diminished. The association constant of C1q-binding IgG with solid-phase C1q was 2.7 X 10(7) M-1. We conclude that low-molecular weight C1q-binding IgG in the studied patients with SLE consists of autoantibodies to the collagen-like region of C1q.
authors.affiliation: Department of Medicine, University of Washington, Seattle 98195.
authors.forename: S
authors.initials: S
authors.lastname: Uwatoko
authors.forename: M
authors.initials: M
authors.lastname: Mannik
mesh_terms: D001323:Autoantibodies / Q000302:isolation & purification*; D002352:Carrier Proteins; D003094:Collagen / Q000276:immunology*; D003166:Complement Activating Enzymes / Q000378:metabolism*; D003172:Complement C1 / Q000378:metabolism*; D015922:Complement C1q; D004347:Drug Interactions; D004591:Electrophoresis, Polyacrylamide Gel; D006801:Humans; D018960:Hyaluronan Receptors; D007118:Immunoassay; D007074:Immunoglobulin G / Q000276:immunology / Q000302:isolation & purification* / Q000378:metabolism; D007525:Isoelectric Focusing; D008180:Lupus Erythematosus, Systemic / Q000276:immunology* / Q000378:metabolism; D008562:Membrane Glycoproteins; D024101:Mitochondrial Proteins; D008970:Molecular Weight; D011951:Receptors, Complement / Q000032:analysis*; D012967:Sodium Dodecyl Sulfate
chemical_list: D001323:Autoantibodies; C087299:C1QBP protein, human; D002352:Carrier Proteins; D003172:Complement C1; D018960:Hyaluronan Receptors; D007074:Immunoglobulin G; D008562:Membrane Glycoproteins; D024101:Mitochondrial Proteins; D011951:Receptors, Complement; C032402:complement 1q receptor; D012967:Sodium Dodecyl Sulfate; D015922:Complement C1q; D003094:Collagen; D003166:Complement Activating Enzymes