Construction of an extended three-dimensional idiotope map by electron microscopic analysis of idiotope-anti-idiotope complexes.

A three-dimensional map of the positions of four idiotypic determinants (idiotopes or Ids) and an isotypic determinant was derived by transmission electron microscopy of negatively stained immune complexes. Each complex was composed of a monoclonal Id-expressing IgG and one or two varieties of monoclonal anti-Id (or anti-isotype) Fab fragment or IgG. Data from the various combinations of Id and anti-Id (and anti-isotype) were used to construct a low-resolution three-dimensional model that revealed not only the approximate locations of Ids on the surface of the antibody variable domains but also details of the geometry of Id-anti-Id interactions not otherwise available. The Ids were shown to be dispersed over the variable domains, extending from the complementarity-determining region to near the variable-constant switch region. Thus, immunoelectron microscopy is a useful complement to serologic, biochemical, and genetic strategies for the topographical analysis of immunoglobulin Ids or other epitopes. This same approach should be of broader applicability in the study of epitopes and receptor sites on other macromolecules.