Virus neutralization by germ-line vs. hypermutated antibodies

Mice infected with vesicular stomatitis virus (VSV), a cytopathic virus clo sely related to rabies virus, mount a virus-neutralizing antibody response protecting against lethal disease, VSV-neutralizing monoclonal IgGs isolate d from primary immune responses were devoid of somatic mutations, whereas m ost secondary and all hyperimmune response IgGs tested were hypermutated. A comparative analysis of recombinant single-chain antibody fragments (scFv- C kappa) revealed that even the germ-line precursor of one hypermutated ant ibody bound and neutralized VSV. Four somatic amino acid substitutions in V H increased by 300-fold the binding strength of monovalent scFv-C kappa, Th e multivalent binding avidity of germ-line scFv-C kappa was increased by mo re than 10-fold compared with the monovalent binding strength. In contrast, hypermutated scFv-C kappa did not show such avidity effects, Thus the over all binding difference between the germ-line and the hypermutated VSV-neutr alizing antibody was only 10- to 15-fold. This may explain why primary germ -line antibodies and secondary hypermutated antibodies directed against pat hogens such as viruses and bacteria expressing repetitive antibody determin ants show rather similar binding qualities, whereas monovalently binding ha pten-specific antibodies can show "affinity maturation" effects of up to 10 00-fold.