Abstract
Comparative interactions of purified rabbit C1q with 18-2-3, a high affinity (2–3 × 10 10 M −1) anti-fluorescein (anti-Fl) murine monoclonal IgM antibody (pentamer) and constitutive monomeric subunits (IgM s) were studied. Using a solid phase radioimmunoassay (SPRIA), based on immobilized polyvalent antigen, it was shown that the mechanism of C1q binding to IgM was characteristically multiphasic while IgM s yielded monophasic binding curves. The latter compared qualitatively and quantitatively with a monoclonal IgG 2a anti-fluorescein antibody with the same intrinsic affinity of 2–3 × 10 10 M −1. C1q binding efficiency to antibodies was significantly enhanced when the immunoglobulins interacted with immobilized multivalent antigen. Monoclonal IgM antibody bind identically to six Fl-carrier protein conjugates independent of epitope (Fl) density. In contrast, the C1q-antibody interaction binding was dependent upon epitope density. An average distance between Fl epitopes of 80 Å was optimal for C1q binding by IgM. At low concn of IgM, when fluorescein was bound by antigen-binding sites on adjacent subunits of an intact pentamer, C1q appeared to bind IgM intramolecularly.
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