Dissection of C1q capability of interacting with IgG. Time-dependent formation of a tight and only partly reversible association.
C1q-bearing immune complexes have been observed in diseases such as rheumatoid arthritis and human immunodeficiency virus infection-associated neuropathy. For the purpose of understanding better the phenomenon of C1q-bearing immune complexes, we investigated the constancy of the C1q-IgG interaction. An enzyme-linked immunosorbent assay was developed in which wells were coated with IgG to mimic antigen-complexed IgG. Serial dilutions of C1q were applied for distinct time intervals, and bound C1q was detected either directly or after exposure to one of several elution buffers. Our results show that a part of C1q attached to IgG forms a tight association that is not reversible under treatment with buffers containing usually protein-protein interaction-dissociating reagents such as 3 M NaCl, 5 M urea, sodium dodecyl sulfate, or beta-mercaptoethanol. The formation of the highly stable C1q-IgG complex was found to be time-, temperature-, and pH-dependent and to proceed with bound C1q even in the absence of free C1q in the supernatant. In ligand blotting experiments we demonstrate for the first time directly that all three chains of C1q can individually bind IgG. Altogether, our results provide a suitable explanation for the formation and persistence of C1q-bearing immune complexes.
Kaul
M
M
Institute of Medical Microbiology and Hygiene, Johannes Gutenberg University, Hochhaus am Augustusplatz, 55101 Mainz, Germany.
Loos
M
M
D000936:Antigen-Antibody Complex / Q000737:chemistry / Q000378:metabolism; D015922:Complement C1q / Q000378:metabolism*; D003181:Complement C4 / Q000378:metabolism; D004797:Enzyme-Linked Immunosorbent Assay; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007074:Immunoglobulin G / Q000378:metabolism*; D066298:In Vitro Techniques; D011485:Protein Binding; D012965:Sodium Chloride / Q000378:metabolism; D013997:Time Factors
D000936:Antigen-Antibody Complex; D003181:Complement C4; D007074:Immunoglobulin G; D012965:Sodium Chloride; D015922:Complement C1q