Enhanced antigen-antibody binding affinity mediated by an anti-idiotypic antibody

We previously described the production of four monoclonal antibodies to the beta-adrenergic receptor antagonist alprenolol. One of these antibodies, 5B7 (IgG2a, kappa), was used to raise anti-idiotypic antisera in rabbits. In contrast to the expected results, one of the anti-idiotypic antisera (R9) promotes [125I]iodocyanopindolol (ICYP) binding to antibody 5B7. In the presence of R9, the dissociation constant decreases 100-fold from 20 to 0.3 nM. This increase in binding affinity of antibody 5B7 for ICYP is not observed in the presence of preimmune, rabbit anti-mouse or anti-idiotypic antisera generated to a monoclonal antibody of a different specificity. Furthermore, R9 in the absence of 5B7 does not bind ICYP. The F(ab) fragments of 5B7 and R9 behaved in a similar manner, and the soluble complex responsible for the high-affinity interaction with ICYP can be identified by gel filtration chromatography. The elution position of the complex is consistent with a 5B7 F(ab)-R9 F(ab) dimer, indicating that polyvalency is not responsible for the enhanced ligand binding. Kinetic analysis of ICYP-5B7 binding revealed that the rate of ICYP dissociation from 5B7 in the presence of R9 is approximately 100 times slower than in the absence of R9 [k-1(+R9) = 0.025 min-1 vs. k-1(-R9) = 2.04 min-1], consistent with the 100-fold change in binding affinity of 5B7 for ICYP. The available data best fit a model in which an anti-idiotypic antibody binds at or near the binding site of the idiotype participating in the formation of a hybrid ligand binding site.(ABSTRACT TRUNCATED AT 250 WORDS)