A view of the human idiotypic repertoire. Electron microscopic and immunologic analyses of spontaneous idiotype-anti-idiotype dimers in pooled human IgG.

It has previously been reported that up to 40% of the molecules of human IgG from pooled plasma (100,000 donations) spontaneously dimerize, whereas IgG prepared from a single donor contains only trace quantities of dimer. We have conducted immunoelectron microscopic analyses on such samples and have verified that the majority of dimers are composed of complexes in which two arms of each molecule are bound in a reciprocal fashion at or near the distal tips of their respective arms as previously seen in bona fide Id-anti-Id complexes. A significant role for Fc was ruled out by showing the formation of dimeric ring structures in purified F(ab')2 samples. Spontaneous dimerization was also observed in pooled bovine or mouse IgG, but not in that from single animals. Radiolabeled monomeric, single donor human IgG was used as a probe to investigate dimer formation; this material readily codimerized with IgG from other donors or from pooled plasma (either human or bovine), but did not dimerize with IgG from the same donor. Subclass analysis of multiple donor human IgG revealed that the most flexible subclass (IgG3) was overrepresented in the dimer fraction, whereas one of the less flexible subclasses (IgG2) was underrepresented. Although IgG2 could dimerize to some extent with IgG of other subclasses, it could not self-dimerize. These data suggest that structural constraints (hinge flexibility) may play a role in limiting dimerization. This suspicion was confirmed by showing that an additional 15.5% of the monomeric IgG fraction from a multidonor sample could dimerize after a chemically induced increase in hinge flexibility. Our results are interpreted to show that, although the number of functionally distinct Id produced by a species is immense, it is nontheless finite. Moreover, the Id repertoire of an individual is much smaller than that of the species. Pooling the IgG from a number of individuals increases the Id diversity, which increases the chance that any given Id-bearing molecular will encounter a complementary partner.