Affinity and Specificity Maturation by CDR Walking

Random mutagenesis and phage display selection provide excellent tools for rapid engineering of the binding properties of antibodies for different applications. One strategy to optimise the antibody combining site is to mutate the CDRs in a stepwise manner. The CDRs are mutated and selected sequentially, after every round the best mutant is used as the template for the subsequent round of mutagenesis and selection. Yang et al. (1995) developed a high affinity anti-HIV gp 120 Fab by the CDR walking strategy in four steps. They were able to further increase the affinity (altogether 420-fold, Kd=1.5xlO_11M) by combining independently selected mutations, however, in this case only one of the six combinations constructed resulted in higher affinity demonstrating the unpredictable behaviour of parallel CDR optimisation. By stepwise optimisations of the light and heavy CDR3 regions and by using low, decreasing amount of biotinylated antigen in the selection steps Schier et al (1996) isolated an anti-c-erbB-2 scFv with picomolar affinity (Kd=1.3×lO_11M). The CDR walking and parallel optimisation strategies have also been used to improve the affinity and specificity of a monoclonal anti-testosterone antibody in the development of a clinically useful recombinant antibody (Hemminki et al, 1998a, b).