Interaction of human Clq with insoluble immunoglobulin aggregates

Abstract

A system of interaction between 125I-Clq and IgG aggregates insolubilized by glutaraldehyde, which was designed as a model for studying the initial event in activation of the complement sequence, is described. Binding of Clq was shown to be rapid, specific, reversible and saturable, with an average dissociation constant in the order of 1 × 10 −8 M. The binding data produced non-linear Scatchard plots. The binding was favored at an ionic strength below μ = 0.15. a pH range of 5.5–8.5. and was sensitive to low concentrations of organic solvents. Among many compounds which interfered with Clq-IgG aggregate interaction, an anionic compound, suramin, was shown to be a good inhibitor of Clq binding with a sigmoidal inhibition curve and with a K D of Clq-suramin complex in the order of 5 × 10 −5 M. Dissociation of radiolabeled Clq by succinylation caused a loss of binding activity but the succinylated Clq retained the capacity to inhibit 125I-Clq binding to IgG aggregates. The characteristic radioiodination pattern of Clq subunits was shown to be significantly changed when iodination was carried out on the IgG aggregates-bound Clq in such a fashion as to imply an IgG aggregates-induced conformational change in Clq, suggesting the usefulness of this system in exploring the possible mechanism underlying immune-complex mediated Cl activation.