HIV-1 sub-type C chimaeric VLPs boost cellular immune responses in mice

Abstract

Several approaches have been explored to eradicate HIV; however, a multigene vaccine appears to be the best option, given their proven potential to elicit broad, effective responses in animal models. The Pr55Gag protein is an excellent vaccine candidate in its own right, given that it can assemble into large, enveloped, virus-like particles (VLPs) which are highly immunogenic, and can moreover be used as a scaffold for the presentation of other large non-structural HIV antigens. In this study, we evaluated the potential of two novel chimaeric HIV-1 Pr55Gag-based VLP constructs - C-terminal fusions with reverse transcriptase and a Tat::Nef fusion protein, designated GagRT and GagTN respectively - to enhance a cellular response in mice when used as boost components in two types of heterologous prime-boost vaccine strategies. A vaccine regimen consisting of a DNA prime and chimaeric HIV-1 VLP boosts in mice induced strong, broad cellular immune responses at an optimum dose of 100 ng VLPs. The enhanced cellular responses induced by the DNA prime-VLP boost were two- to three-fold greater than two DNA vaccinations. Moreover, a mixture of GagRT and GagTN VLPs also boosted antigen-specific CD8+ and CD4+ T-cell responses, while VLP vaccinations only induced predominantly robust Gag CD4+ T-cell responses. The results demonstrate the promising potential of these chimaeric VLPs as vaccine candidates against HIV-1.

Figure 1

Purity and quantification of Gag in VLP preparations. Coomassie-stained SDS-PAGE gels of (a) GagTN VLP samples and (b) GagRT VLP samples indicating the purity of the samples. Western blots of (c) GagTN VLP samples and (d) GagRT VLP samples probed with anti-Gag p24 antibody. 0.1 and 0.01 indicate a 1:10 dilution and a 1:100 dilution, respectively, of VLPs loaded per lane. Western blots of Gag p17/p24 (p41) protein (24-240 ng) loaded per lane is indicated and the intensity of these bands was used to determine the Gag content of GagTN and GagRT VLP samples. M = molecular weight marker and arrowheads indicate the migration distance of the respective chimaeric VLPs.

Figure 2

Dose assessment for chimaeric VLP boost. Three groups of five BALB/c mice were primed on day 0 with the DNA vaccine pVRCgrttnC and boosted with (a) GagRT or (b) GagTN on day 28. For all mouse groups spleens were harvested on day 40 and the splenocytes were used in IFN-γ ELISPOT and IL-2 ELISPOT assays with the indicated Gag and RT CD8 and CD4 peptides. Data represent results from one of three representative experiments.

Figure 3

Immunogenicity of GagRT VLPs. BALB/c mice were primed on day 0 with the DNA vaccine pVRCgrttnC then boosted on day 28 with the DNA vaccine (DNA+DNA group) or GagRT VLPs (DNA+GagRT group). A further group was left unprimed then vaccinated on day 28 with GagRT VLPs. For all mouse groups spleens were harvested on day 40 and splenocytes used in IFN-γ ELISPOT and IL-2 ELISPOT assays with the indicated Gag and RT CD8 and CD4 peptides. Cumulative ELISPOT responses to the vaccines are shown and are to the sum of responses to the indicated peptides in the IFN-γ ELISPOT or IL-2 ELISPOT assays with the indicated Gag and RT CD8 and CD4 peptides. Data represent results from one of three representative experiments.

Figure 4

BALB/c mice were primed on day 0 with the DNA vaccine pVRCgrttnC then boosted on day 28 with DNA (DNA+DNA group) or GagTN VLPs (DNA+GagTN group). A further group was left unprimed then vaccinated on day 28 with GagTN VLPs. For all mouse groups spleens were harvested on day 40 and splenocytes used in IFN-γ ELISPOT and IL-2 ELISPOT assays with the indicated Gag CD8 and CD4 peptides. Cumulative ELISPOT responses to the vaccines are shown and are to the sum of responses to the indicated peptides in the IFN-γ ELISPOT or IL-2 ELISPOT assays with the indicated Gag CD8 and CD4 peptides. Data represent results from one of three representative experiments.

Figure 5

Use of a mixture of GagRT and GagTN VLPs as vaccine boosts. BALB/c mice were vaccinated with the DNA vaccine pVRCgrttnC on day 0 and day 28 (DNA+DNA group) or primed on day 0 with the DNA vaccine and boosted on day 28 with a mix of GagRT and GagTN (DNA+VLPmix group). For all mouse groups spleens were harvested on day 40 and splenocytes used in IFN-γ ELISPOT and IL-2 ELISPOT assays with the indicated Gag and RT CD8 and CD4 peptides. Data represent results from one of three representative experiments.