Emergence of antigenic variants of simian immunodeficiency virus (SIVmac) in a seronegative macaque SIVmac239 infection

Infection with the macaque strain of the simian immunodeficiency virus (SIVmac) induces simian immunodeficiency syndrome in rhesus macaques. This report describes the isolation and identification of antigenic variants of SIVmac in one of the infected monkeys (macaque No. 22803). Eight naive rhesus monkeys were inoculated with a titered viral stock of the molecularly cloned SIVmac239. Standard serological analysis revealed that all but two were seroconverted. Western blot analysis confirmed the seronegativity of macaque No. 22803. In addition, sera recovered from this monkey were not able to neutralize the parent STVmac239. However, virus could be isolated from all of the infected animals, including macaque No. 22803. Sera recovered were reactive to the autologous virus. The results suggest that the virus from macaque No. 22803 may have undergone extensive antigenic shift in vivo. To test this hypothesis, a portion of the gag gene was amplified by the polymerase chain reaction (PCR), cloned, and sequenced. Sequence analysis revealed amino acid changes that were clustered between amino acids 200-245. Evaluation of the possible selective pressures contributing to the observed viral mutation revealed that in comparison with the other SIVmac239-infected monkeys, macaque No. 22803 produced an unusually high T cell proliferative response toward mitogen stimulation before infection, and continued to display a persistently high plasma viremia titer after infection.