Characterization of a macaque recombinant monoclonal antibody that binds to a CD4-induced epitope and neutralizes simian immunodeficiency virus

A potent neutralizing Fab fragment from a long-term survivor of simian immunodeficiency virus (SIVsm) infection was used to construct a recombinant macaque immunoglobulin G1kappa (IgG1kappa) molecule, designated IgG1-201. A Chinese hamster ovary cell line expressing IgG1-201 was derived by stable transfection and optimized for antibody secretion by methotrexate selection and dihydrofolate reductase gene amplification. IgG1-201 effectively neutralized the homologous, molecularly cloned SIVsmH4 virus but had no activity against the heterologous SIVmac251/BK28 virus. The previously characterized, neutralization-resistant SIVsmE543-3 virus was also not neutralized by IgG1-201. Binding to SIVsmH4 gp120 was enhanced in the presence of recombinant soluble CD4, suggesting that IgG1-201 bound a CD4-induced epitope. IgG1-201 immunoprecipitated the SIVsmH4 but not the SIVsmE543-3 envelope despite a close relationship between these two clones. Immunoprecipitation of a panel of SIVsmH4/SIVsmE543-3 chimeric viruses tentatively assigned the neutralization epitope to the third constant domain, immediately C terminal to the V3 loop. These findings suggest the presence of at least one CD4-induced neutralization epitope on SIV, as is the case with human immunodeficiency virus type 1.