Can isotype switch modulate antigen-binding affinity and influence clonal selection?

Four different monoclonal Ig (MIg) (IgA1 kappa, IgG1 kappa, IgG2 kappa and IgG4 kappa) displaying anti-tubulin activity were detected in the serum fro m a lymphoma patient. The complete sequence of three of these MIg showed id entical V-H and VL domains and the presence of mutations compatible with an antigen-driven process. Surprisingly, despite complete homology in their v ariable domains, IgA1 kappa, IgG1 kappa, or their Fab fragments bound to a common motif recognized in beta tubulin, with significant differences in af finity (IgA1 kappa 1.52x10(-8) M, and IgG1 kappa 2.09x10(-7) M). To substan tiate these results, the V-H and V-L domains from IgA1 kappa :were cloned a nd introduced into expression vectors containing the constant kappa exon an d either the mu or the gamma1 constant exon, and complete recombinant IgM k appa and IgG1 kappa were obtained. Like the lgA1 kappa, the IgM kappa const ruction bound to the tubulin epitope with consistent affinity (7.7x10(-9) M ), whereas the IgG1 kappa. construction displayed a significantly lower aff inity (3.28x10(-7) M). These results provide definitive evidence that isoty pe can influence binding affinity to antigen and suggest that malignant tra nsformation occurred at the germinal center once the mutational process was achieved and the switch process was still active.