Inhibition of Staphylococcal protein A precipitation of heat-aggregated IgG(AIgG) by Clq monomers

Staphylococcal cells containing protein A are used to detect soluble immune complexes (ICX's). This precipitation of ICX's is dependent on the interaction of protein A with the Fc tail of the IgG molecule. Complement has been demonstrated to alter the physicochemical and biological properties of ICX's both in vivo and in vitro. They studied the effect of incorporation of radiolabelled human Clq (Clq131I) into soluble heat aggregates of human IgG (AIgG125I, MW = 107) on the subsequent ability of Staphylococcal cells to precipitate the labelled complexes. Clq was shown to be hemolytically active in preliminary red cell binding and hemolytic studies, and purity demonstrated by SDS-PAGE. Clq was incorporated by reacting AIgG 5 μg with Clq in varying amounts (.2-3 μg) for 25 min at 300C at normal ionic strength. S. aureus cells containing protein A (7 mg) precipitated up to 94% of AIgG125I in the absence of Clq, and up to 10% of Clq131I alone. Clq reduced the amount of AIgG125I precipitated by S. aureus by 10% to 66% depending upon the dose of Clq. In conclusion, Clq, which binds at the C/sub H/2 domain, inhibits the precipitation of model ICX's by protein A, probably because of competitive binding known to be at the C/sub H/2-3 domains of the Fc portion of IgG. This could explain the inability to detect ICX's in the blood of certain patients with autoimmune diseases using this method