The role of isotype and epitope specificity of monoclonal antibody mixtures in immunodiffusion reactions.

Twenty monoclonal antibodies to human alpha-fetoprotein have been characterized in terms of IgG subclass, epitope specificity and immunodiffusion properties. The Ig molecules consisted of six G1, ten G2a and four G2b isotypes. The epitope analysis was conducted by a solid-phase RIA employing 125I-labelled AFP. The RIA analysis resulted in the identification of seven determinants, two of which were specific for one antibody each, while others were specific for two or more antibodies. In Ouchterlony tests with the antibodies, none formed bands of immunoprecipitate when diffused individually against the antigen. Tests with all possible mixtures of pairs of the antibodies (190) resulted in positive immunodiffusion responses with only two mixtures. These each contained antibodies of the 2b isotype and demonstrated distinct epitope specificities. The immunodiffusion of mixtures of three antibodies from a group of 12 that were selected to represent equal numbers of isotypes resulted in 84 positive responses and 136 negative responses, i.e. from a total of 220 mixtures. A striking correlation was noted between positive immunodiffusion tests and composition of antibody isotypes and of epitope specificities of the mixtures. Each mixture was found to contain antibodies of at least one 2b isotype and of different epitope specificities. None of the mixtures that lacked a 2b isotype (56) responded in immunodiffusion tests. Similarly, in instances (46) in which the epitope specificities of the antibodies in the mixture were the same, i.e. duplicated or triplicated, the results were again negative despite the presence of a 2b isotype. The comparison of these studies with similar studies of a smaller group of antibodies to pregnancy-specific beta 1-glycoprotein strongly suggests that the G3 antibody isotype may have an immune precipitation enhancing effect similar to the G2b isotype.