To evaluate the vaccine potential of SHIVs attenuated by deletion of viral accessory genes, seven rhesus macaques were sequentially immunized with ΔvpuΔnef SHIV-4 (vaccine-I) followed by ΔvpuSHIVPPC (vaccine-II). Despite the absence of virological evidence of productive infection with the vaccine strains, based on analysis of infectivity among peripheral blood mononuclear cells (PBMC) of the vaccinated animals, all seven animals developed binding as well as neutralizing antibodies against both vaccine-I and -II. The animals also developed vaccine virus-specific CTLs that recognized homologous as well as heterologous pathogenic SHIVs and SIV, and also soluble inhibitory factors that blocked the in vitro replication of the vaccine strains and different challenge viruses. Virus-specific cellular and humoral responses were sustained throughout a 58-week prechallenge period. To model aspects of natural transmission, the animals received a mucosal (rectal) challenge, with a mixture of three challenge viruses, SHIVKU, SHIV89.6P, and SIVmacR71/17E. Two mock-vaccinated control animals) inoculated with the same mixture of challenge viruses developed large numbers of infectious PBMC, high plasma viremia, and precipitous loss of CD4+ T cells. The control animals did not develop any immune responses and succumbed to AIDS between 6 and 7 weeks postchallenge. All seven vaccinated animals became infected with challenge viruses as indicated by the presence of infectious cells in the PBMC and/or viral RNA in plasma. However, peak plasma viremia in vaccinates was two to nearly five logs lower than in the control animals and later plasma viral RNA became undetectable in all vaccinates. Vaccinated animals maintained normal CD4+ T cell levels throughout the study. Challenge with pathogenic viruses caused massive anamnestic responses as determined by quantitation of virus-specific CD4+ and CD8+ T cells by intracellular IFN-γ staining, and these cells persisted for at least 74 weeks. The study is still in progress and at this time DNA of SIV has become undetectable in lymph nodes of six of the seven vaccinates, SHIV89.6P in five of the seven, and SHIVKU in three of the seven animals.
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