Long-Lasting Protection by Live Attenuated Simian Immunodeficiency Virus in Cynomolgus Monkeys: No Detection of Reactivation after Stimulation with a Recall Antigen

The infection of cynomolgus monkeys with an attenuated simian immunodeficiency virus (SIV) (C8) carrying a deletion in the nefgene results in a persistent infection associated with an extremely low viral burden in peripheral blood mononuclear cells. The aim of this study was to determine (1) the breadth of the protection after repeated challenges of monkeys with SIV homologous strains of different pathogenicity, (2) the genotypic stability of the live virus vaccine, (3) whether the protection might depend on cellular resistance to superinfection, and (4) whether immunogenic stimuli such as recall antigens could reactivate the replication of the C8 virus. To address these goals, the monkeys were challenged at 40 weeks after C8 infection with 50 MID50of cloned SIVmac251, BK28grown on macaque cells. They were protected as indicated by several criteria, including virus isolation, anamnestic serological responses, and viral diagnostic PCR. At 92 weeks after the first challenge, unfractionated peripheral blood mononuclear cells from protected monkeys were susceptible to the in vitroinfection with SIVmac32H, spl. At 143 weeks after C8 infection, the four protected monkeys were rechallenged with 50 MID50of the pathogenic SIVmac32H, splgrown on macaque cells. Once again, they were protected. The C8 virus remained genotypically stable, and depletion of CD4+cells was not observed during ∼3 years of follow-up. In contrast, it was found that the infection with SIVmac32H, splinduced CD4+cell depletion in three of three control monkeys. Of importance, stimulation with tetanus toxoid, although capable of inducing specific humoral and T cell proliferative responses, failed to induce a detectable reactivation of C8 virus.