Pathogenic Conversion of Live Attenuated Simian Immunodeficiency Virus Vaccines Is Associated with Expression of Truncated Nef

Rhesus macaques infected with simian immunodeficiency virus (SIV) containing either a large nef deletion (SIVmac239Δ(152)nef) or interleukin-2 in place of nef developed high virus loads and progressed to simian AIDS. Viruses recovered from both juvenile and neonatal macaques with disease produced a novel truncated Nef protein, tNef. Viruses recovered from juvenile macaques infected with serially passaged virus expressing tNef exhibited a pathogenic phenotype. These findings demonstrated strong selective pressure to restore expression of a truncated Nef protein, and this reversion was linked to increased pathogenic potential in live attenuated SIV vaccines.