Ability of the v3 loop of simian immunodeficiency virus to serve as a target for antibody-mediated neutralization: Correlation of neutralization sensitivity, growth in macrophages, and decreased dependence on CD4

To better define the effects of sequence variation and tropism on the abili ty of the simian immunodeficiency virus SIVmac V3 loop to act as a target o f antibody-mediated neutralization, a series of experiments were performed. Three SIV strains, SIVmac239, SIVmac316, and SIVmac155/T3, each with defin ed differences in env sequence and tropism, were used to construct a panel of viruses chimeric for a portion of envelope that includes the V2 and V3 r egions, Peptides with sequences corresponding to the V3 loops of the parent al viruses were used to immunize rabbits. The polyclonal rabbit antibodies and plasma from SIVmac239-infected animals were then used to assess the neu tralization sensitivity of the parental and chimeric viruses. One of the pa rental viruses, SIVmac316, which is able to replicate to high titer in alve olar macrophages and can infect cells in a CD4-independent fashion, was hig hly sensitive to neutralization by plasma from SIVmac-infected rhesus macaq ues, with average 50% neutralization titers of 1:20,480; this same strain w as also sensitive to neutralization by the anti-V3 loop peptide sera. Other parental and chimeric viruses were less sensitive to neutralization with t his same panel of antibodies, but as seen with SIVmac316, those viruses tha t were able to productively replicate in alveolar macrophages were more sen sitive to antibody-mediated neutralization. To further define the amino aci ds involved in increased sensitivity to neutralization, a panel of viruses was constructed by changing envelope residues in SIVmac316 to the correspon ding SIVmac239 amino acids. The increased neutralization sensitivity observ ed for SIVmac316 was mapped principally to three amino acid changes spread throughout gp120, In addition, the increased sensitivity to neutralization by V3-directed antibodies correlated with the ability of the various viruse s to replicate to high levels in alveolar macrophage cultures and a CD4-neg ative cell line, BC7/CCR5, These results demonstrate that the V3 loop of SI Vmac Env can act as an efficient target of neutralizing antibodies in a fas hion that is highly dependent on sequence context. In addition, these studi es suggest a correlation between decreased dependence on CD4 and increased sensitivity to antibody-mediated neutralization.