Secondary rearrangements and hypermutation generate sufficient B cell diversity to mount protective antiviral immunoglobulin responses

Variable (V) region gene replacement was recently implicated in B cell repe rtoire diversification, but the contribution of this mechanism to antibody responses is still unknown. To investigate the role of V gene replacements in the generation of antigen-specific antibodies, we analyzed antiviral imm unoglobulin responses of "quasimonoclonal" (QM) mice. The B cells of QM mic e are genetically committed to exclusively express the anti-(4-hydroxy-3-ni trophenyl) acetyl specificity. However, similar to 20% of the peripheral B cells of QM mice undergo secondary rearrangements and thereby potentially a cquire new specificities. QM mice infected with vesicular stomatitis virus (VSV), lymphocytic choriomeningitis virus, or poliovirus mounted virus-spec ific neutralizing antibody responses. In general, kinetics of the antiviral immunoglobulin responses were delayed in QM mice; however, titers similar to control animals were eventually produced that were sufficient to protect against VSV-induced lethal disease. VSV neutralizing single-chain Fv fragm ents isolated from phage display libraries constructed from QM mice showed VH gene replacements and extensive hypermutation. Thus, our data demonstrat e that secondary rearrangements and hypermutation can generate sufficient B cell diversity in QM mice to mount protective antiviral antibody responses , suggesting that these mechanisms might also contribute to the diversifica tion of the B cell repertoire of normal mice.