Effect of protein A and its fragment B on the catabolic and Fc receptor sites of IgG

Radiolabeled protein A from Staphylococcus aureus (SpA) injected i.v. into mice and rabbits forms a soluble [(IgG)2-(SpA)1]2 complex (Mr = 684 000) which is identical in composition to that formed by SpA in vitro with an equivalent amount or an excess of IgG. A soluble rabbit IgG-SpA complex injected into a mice or rabbits dissociates completely in vivo and a new complex is formed with the IgG of the recipient animal. The half-life of SpA administered to a mouse or a rabbit is therefore the half-life of the IgG-SpA complex formed in vivo. In mice and rabbits the half-life of the complexes formed is 9 and 30 h, respectively, whereas the half-life of rabbit IgG in these animals is 106 and 153 h, respectively. Fragment B of SpA (fSpA) reacts with IgG of mouse and rabbit and forms an (IgG)1-(fSpA)1 complex. Complexes of identical composition are formed if fSpA is injected i.v. into mice and rabbits. The half-life of the complexes in mice and rabbits are much shorter than those of the corresponding free IgG in these animals (up to 15 times). This result suggests that the binding of fSpA to the CH2 and the CH3 domains of IgG alters the function of the site, which controls the catabolism of IgG and is located in the CH2 domain. By contrast, fSpA does not change the Fc receptor-binding site of IgG, indicating that the Fc receptor site and the catabolic site are unrelated to each other.

Immunogenicity and effector functions of glutaraldehyde-treated rabbit and mouse immunoglobulin G

Rabbit and mouse IgG treated with glutaraldehyde (GA) were immunogenic in homologous species. Glutaraldehyde treatment induced in the IgG molecule two types of antigenic determinants. One of them was found on the monomeric fraction of GA-treated rabbit IgG (haptenic determinant) and the other on the polymeric fraction (structural determinant). The haptenic determinants were found also on monoaldehyde-treated rabbit IgG and GA-treated Fab and Fc fragments. It was demonstrated that rabbit and mouse antibodies are specific for GA-treated IgG and have species specificity. While GA treatment did not alter the antigen binding capacity of rabbit IgG antibody, its effector functions (except protein A binding) were much affected. Thus it was found that GA treatment enhances IgG ability to react with rheumatoid factor, reduces drastically its capacity to activate the complement system, abolishes the cytophilic properties of IgG and accelerates its catabolic rate. The possible blocking effect of GA on the amino acid residues (mainly Lys) situated in or very close to the effector sites of the IgG molecule is suggested.