The role of somatic mutation in the generation of the protective humoral immune response against vesicular stomatitis virus

During most clinically relevant infections with cytopathic viruses, neutralizing antibodies are generated early, i.e., within the first week of infection, As early as 4 days after immunization of mice with vesicular stomatitis virus (VSV), a cytopathic virus closely related to rabies virus, hybridomas could be isolated that secreted virus-neutralizing IgGs. Such antibodies were devoid of somatic mutations, showed high binding avidities (similar to 10(9) M(-1)), and used V gene fragments predominantly belonging to the V(H)Q52 and V(K)19-28 families. In contrast, most secondary and hyperimmune response IgGs isolated 12 and 150 days after infection used several additional V gene combinations. These, which used the V(H)Q52/V(K)19-28 combination of early IgGs, were point mutated but showed only marginally enhanced binding avidities. Since all V(H)Q52/V(K)19-28-positive IgGs bound to one subsite within the major antigenic site of VSV-G irrespective of the presence or absence of somatic point mutations, fine specificity diversification of secondary and hyperimmune responses was achieved by newly appearing V gene combinations.