A mouse monoclonal antibody that reacts specifically with immune-complexed human IgG

After human IgG binds to antigen, it attains biological functions that are not properties of monomeric, uncomplexed IgG, including the ability to activate complement and to bind to cellular receptors. Associated with antigen binding, we have recently demonstrated that IgG itself has neoantigenic epitopes. Antibodies to these neoantigens on immune-complexed IgG may represent a significant proportion of circulating anti-human IgG in rabbits immunized with immune complexes. In contrast, mice immunized in an identical fashion have very little circulating anti-neoantigen antibody. This is true whether the mice are genetically easy to tolerize to monomeric human IgG (DBA/2 and C57BL/6) or difficult to tolerize (BALB/c). Fusions were made between the NS-1 myeloma cell line and spleen cells from mice of each strain, which had been made tolerant to monomeric human IgG and then immunized with immune complexes containing IgG. Like the serum antibody, antibodies made by these fusions showed little specificity for immune complexes since 99% of the hybridoma antibodies that recognized IgG in immune complexes also bound to uncomplexed IgG. Only 1 hybridoma produced antibody that preferentially recognized human IgG in immune complexes. This antibody, called CE9, is an IgM that binds to IgG in plate-bound immune complexes with 100–1000-fold greater avidity than it does to plate-bound uncomplexed IgG. Because CE9 will not bind to immune complexes made with F(ab') 2 antibody, the epitope it recognizes requires the Fc fragment of IgG. The minimal binding of CE9 to uncomplexed IgG is easily inhibited by soluble aggregates of IgG, but binding to immune complexes is not inhibited by aggregated IgG. CE9 does recognize fluid-phase immune complexes as well as solid-phase immune complexes. We conclude that, while mice produce much less anti-immune complex antibody than rabbits, anti-neoantigen is still a component of their response to immunization with immune complexes. Using hybridoma techniques to amplify these anti-neoantigen antibodies, we have shown that they resemble rheumatoid factors in their isotype and binding properties.