Background: Live, attenuated simian immunodeficiency virus (SIV) elicited protective immunity in rhesus monkeys. This animal model provides evidence that protective immunity can be elicited by vaccination. However, this vaccine approach was hindered by safety concerns that the live attenuated HIV-1 may cause disease in humans. To ensure the safety and maintain the immunogenicity of a live, attenuated vaccine, we have developed a replication defective HIV-1pseudotyped with Vesicular Stomatitis Virus G protein (VSV-G) as vaccine candidate.

Methods: In the first vaccine construct, the polymerase gene of HIV-1 was truncated in the replication-defective HIV-1. In the second vaccine construct, an interferon gamma gene was co-expressed with HIV-1 proteins. Replication defective viruses were produced by co-transfecting vaccine constructs with constructs encoding functional pol and VSV-G genes. The pseudotyped HIV-1 can infect many cell types including human and simian cells, and only undergoes single cycle of replication. Four rhesus monkeys were immunized with replication-defective HIV-1 through multi-routes (oral, subcutaneous, and intramuscular). Immunized animals and naïve control were challenged with SHIVku-2. To further test the therapeutic effect of vaccine, the naïve control animal was immunized with replication defective vaccines one year after challenge.

Results: Strong immune responses (humoral and cell-mediated) can be detected by ELISA, immunoblotting, neutralizing antibody, and chromium release assay in all immunized animals. After challenging with SHIVku-2, all immunized animals and naïve control have transient viremia by RT-PCR and co-culture assay. However, virus load becomes undetectable in the vaccinated animals 2 months after challenge by sensitive co-culture and PCR as compared to persistent infection in control animals for over one year. The level of CD4 cells also maintains at normal level in the vaccinated animals as compared to 40% drop in control animals after 44 weeks postchallenge. Furthermore, viremia was not detected in the immunized animals after re-challenge one year after initial challenge. To measure the therapeutic effects, virus load becomes undetectable in the challenged naïve control animal two months after receiving replication defective vaccines.

Conclusions: These studies indicate that replication-defective HIV-1 is highly immunogenic and elicits protective immunity in rhesus monkeys. Furthermore, this vaccine also provides therapeutic effect in persistent infected animals. Therefore, replication defective HIV-1 may provide a safe vaccine candidate for human clinical trials.