Characterization of C1q-binding IgG complexes in systemic lupus erythematosus

Clin Immunol Immunopathol. 1984 Jan;30(1):104-16. doi: 10.1016/0090-1229(84)90011-4.


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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adult
  • Alkylation
  • Antigen-Antibody Complex / analysis*
  • Binding Sites, Antibody
  • Chemical Phenomena
  • Chemistry, Physical
  • Complement Activating Enzymes / metabolism*
  • Complement C1q
  • DNA / immunology
  • Female
  • Humans
  • Hydrogen-Ion Concentration
  • Immunoglobulin G / analysis
  • Immunoglobulin G / metabolism*
  • Lupus Erythematosus, Systemic / complications
  • Lupus Erythematosus, Systemic / immunology*
  • Middle Aged
  • Pepsin A / pharmacology
  • Proteinuria / etiology
  • Proteinuria / immunology


  • Antigen-Antibody Complex
  • Immunoglobulin G
  • Complement C1q
  • DNA
  • Complement Activating Enzymes
  • Pepsin A