Protection against Mucosal SHIV Challenge by Peptide and Helper-Dependent Adenovirus Vaccines

Abstract

Groups of rhesus macaques that had previously been immunized with HIV-1 envelope (env) peptides and first generation adenovirus serotype 5 (FG-Ad5) vaccines expressing the same peptides were immunized intramuscularly three times with helper-dependent adenovirus (HD-Ad) vaccines expressing only the HIV-1 envelope from JRFL. No gag, pol, or other SHIV genes were used for vaccination. One group of the FG-Ad5-immune animals was immunized three times with HD-Ad5 expressing env. One group was immunized by serotype-switching with HD-Ad6, HD-Ad1, and HD-Ad2 expressing env. Previous work demonstrated that serum antibody levels against env were significantly higher in the serotype-switched group than in the HD-Ad5 group. In this study, neutralizing antibody and T cell responses were compared between the groups before and after rectal challenge with CCR5-tropic SHIV-SF162P3. When serum samples were assayed for neutralizing antibodies, only weak activity was observed. T cell responses against env epitopes were higher in the serotype-switched group. When these animals were challenged rectally with SHIV-SF162P3, both the Ad5 and serotype-switch groups significantly reduced peak viral loads 2 to 10-fold 2 weeks after infection. Peak viral loads were significantly lower for the serotype-switched group as compared to the HD-Ad5-immunized group. Viral loads declined over 18 weeks after infection with some animals viremia reducing nearly 4 logs from the peak. These data demonstrate significant mucosal vaccine effects after immunization with only env antigens. These data also demonstrate HD-Ad vectors are a robust platform for vaccination.

Figure 1.

Protein sequence alignment of envelope antigens used in this study. The JRFL gp140 immunogen expressed by the HD-Ad vectors was aligned to the SF162P3 env protein of the challenge virus. Boxes indicate the locations of the six env peptides that were used to vaccinate the macaques prior to HD-Ad vaccination.

Figure 2.

Neutralizing Antibody Responses Against Ad. Plasma samples taken at the indicated times were incubated with Ad5 expressing luciferase for 1 hour at 37°C prior to addition to A549 cells. 24 hours later, luciferase activity was measured and gene delivery was compared to untreated Ad5 vector. Data is expressed as geometric mean titers that reduced Ad luciferase activity 50%.

Figure 3.

IFN-γ ELISPOT of PBMCs from macaques during HD-Ad vaccination and after SHIV challenge. PBMCs were stimulated with SF162P3 env peptide pools, the six conserved env peptides, or Ad5 or Ad6 viruses. Spot forming cells (SFC) as measured by ELISPOT are shown relative to the y axis, with the time point of assay before and after vaccination and challenge shown below each graph. The HD-Ad5 group is shown in lower panels and the serotype-switched (HD-Ad6, 1, 2) group is shown in the top panels. On the x-axis, HD-Ad designates time points 2 weeks after each vaccination. Arrows indicate the time of SHIV challenge. SHIV+2, SHIV+4, and SHIV+18 designate weeks 2, 4, and 18 after SHIV challenge.

Figure 4.

Plasma viral loads after rectal SHIV-SF162P3 challenge. Three control macaques and the eight HD-Ad vaccinated macaques were challenged rectally by atraumatic administration of 1,000 TCID₅₀ of SHIV-SF162P3. Viral loads were assessed by quantitative realtime PCR of viral genomes from the blood at the indicated times after challenge. A) Viral loads over 18 weeks after challenge. B) Comparison of peak viral titers at 2 weeks after challenge. Rh51 was excluded from the analysis due to high probability that the animal was not infected with SHIV rather than sterilizing immunity was generated.

Figure 4.

Plasma viral loads after rectal SHIV-SF162P3 challenge. Three control macaques and the eight HD-Ad vaccinated macaques were challenged rectally by atraumatic administration of 1,000 TCID₅₀ of SHIV-SF162P3. Viral loads were assessed by quantitative realtime PCR of viral genomes from the blood at the indicated times after challenge. A) Viral loads over 18 weeks after challenge. B) Comparison of peak viral titers at 2 weeks after challenge. Rh51 was excluded from the analysis due to high probability that the animal was not infected with SHIV rather than sterilizing immunity was generated.