[Monoclonal antibodies to human spleen ferritin. II. Localization of epitopes and quantitative parameters of antigen binding].

The interaction of three monoclonal antibodies with human spleen ferritin has been studied. Using titration of various monoclonal antibody-containing media with the immobilized antigen, the specific content of active antibodies capable of binding to ferritin was determined, which was 22-27% for ascitic fluids, 35-50% for total mouse IgG and 88% for affinity-purified HSF102 antibody. Using [125I]ferritin and a novel computer-aided technique for determining the antigen-antibody binding, the affinity constants were obtained which ranged from 6.10(8) to 3.10(9) M-1. The monoclonal antibodies inhibited by 77-95% the binding of rabbit polyclonal antibodies to ferritin. This result is suggestive of a compact distribution on the ferritin surface of immunodominant epitopes forming clusters of closely related antigenic sites recognized by monoclonal and rabbit polyclonal antibodies. Competitive binding and additivity assays revealed that the epitope for the HSF102 antibody was sterically remote from the epitopes recognized by HSF101 and HSF103 antibodies to allow for noncompetitive binding of two different monoclonal antibodies. The competition between HSF101 and HSF103 antibodies pointed to the overlapping of their epitopes. It was found that no more than four [125I]IgG molecules could simultaneously be bound to one ferritin molecule which reflected the maximal valency of this antigen during its interaction with IgG antibodies.