title: Characterization of C1q-binding IgG complexes in systemic lupus erythematosus.
abstract: The molecular size of C1q-binding immunoglobulin (Ig) G complexes in systemic lupus erythematosus (SLE) sera was studied by gel filtration using C1q solid-phase radioimmunoassay (C1q SPRIA). All 15 SLE sera tested contained predominantly small-sized IgG complexes, cofractionating with monomeric IgG. In contrast to heat-aggregated IgG, these small-sized IgG complexes retained C1q-binding activity even after pepsin digestion, exposure to low pH, or reduction and alkylation, suggesting that the F(ab')2 region is involved in C1q-binding activity of these complexes. To see whether anti-C1q antibodies or small antigen-IgG complexes, which bind to C1q via their antigens, are responsible for C1q-binding activity via the F(ab')2 region, the pepsin-digested Ig fractions of SLE sera were fractionated at high salt. C1q-binding activity in the fractions corresponding to the F(ab')2 region increased 2.5- to 3.9-fold at high salt. These results suggest that the C1q-binding, small-sized IgG complexes may be comprised mostly of anti-C1q antibodies and that some of the antibodies, which are dissociated with their antigens at high salt, might be cross-reactive with C1q.
mesh_terms: D000328:Adult; D000478:Alkylation; D000936:Antigen-Antibody Complex / Q000032:analysis*; D001666:Binding Sites, Antibody; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D003166:Complement Activating Enzymes / Q000378:metabolism*; D015922:Complement C1q; D004247:DNA / Q000276:immunology; D005260:Female; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007074:Immunoglobulin G / Q000032:analysis / Q000378:metabolism*; D008180:Lupus Erythematosus, Systemic / Q000150:complications / Q000276:immunology*; D008875:Middle Aged; D010434:Pepsin A / Q000494:pharmacology; D011507:Proteinuria / Q000209:etiology / Q000276:immunology
chemical_list: D000936:Antigen-Antibody Complex; D007074:Immunoglobulin G; D015922:Complement C1q; D004247:DNA; D003166:Complement Activating Enzymes; D010434:Pepsin A