Enhanced T-cell immunogenicity and protective efficacy of a human immunodeficiency virus type 1 vaccine regimen consisting of consecutive priming with DNA and boosting with recombinant fowlpox virus

Abstract

The induction of human immunodeficiency virus (HIV)-specific T-cell responses is widely seen as critical to the development of effective immunity to HIV type 1 (HIV-1). Plasmid DNA and recombinant fowlpox virus (rFPV) vaccines are among the most promising safe HIV-1 vaccine candidates. However, the immunity induced by either vaccine alone may be insufficient to provide durable protection against HIV-1 infection. We evaluated a consecutive immunization strategy involving priming with DNA and boosting with rFPV vaccines encoding common HIV-1 antigens. In mice, this approach induced greater HIV-1-specific immunity than either vector alone and protected mice from challenge with a recombinant vaccinia virus expressing HIV-1 antigens. In macaques, a dramatic boosting effect on DNA vaccine-primed HIV-1-specific helper and cytotoxic T-lymphocyte responses, but a decline in HIV-1 antibody titers, was observed following rFPV immunization. The vaccine regimen protected macaques from an intravenous HIV-1 challenge, with the resistance most likely mediated by T-cell responses. These studies suggest a safe strategy for the enhanced generation of T-cell-mediated protective immunity to HIV-1.

Figures

FIG. 1

HIV-1 antibodies in DNA-rFPV-vaccinated macaques. Four HIV-1-vaccinated macaques (M2, M3, M4, and M5) (closed symbols) seroconverted to HIV-1 after the second epidermal administration of plasmid DNA pNL4.3dpol as assessed by a particle agglutination assay. HIV-1 antibodies titers declined (>4-fold) thereafter despite three vaccinations with rFPV expressing HIV-1 Env and Gag-Pol antigens. Vaccination times are noted on the x axis. Four control macaques (M7, M8, M9, and M10) (open symbol) received DNA and FPV vaccines not expressing HIV-1 antigens and all had no detectable HIV-1-specific antibodies.

FIG. 2

HIV-1-specific Th responses following DNA-rFPV vaccination. (A) rFPV boosting of DNA-primed macaques resulted in a rise in lymphoproliferative responses to both Gag (p24) and homologous (LAI) and heterologous (SF2) subtype B HIV-1 Env antigens. The SIs (means ± standard errors of the means) of PBMC obtained from four macaques receiving HIV-1-expressing vaccines at week 10 following two DNA vaccinations (open bars) and at week 34 following three rFPV boosts (solid bars) are shown in comparison to those for four unvaccinated macaques (hatched bars). (B) PBMC from macaques vaccinated consecutively with HIV-1-expressing DNA and rFPV secrete significant IFN-γ but minimal IL-4 in response to recombinant HIV-1LAI gp160 protein stimulation. The means ± standard errors of the means of triplicate measurements on the four vaccinated macaques (M2 to -5) after two DNA vaccinations alone (week 14) and after three rFPV boosts (week 38) are shown. PBMC from the same animals had an opposite pattern of cytokine secretion in response to stimulation with tetanus toxoid.

FIG. 3

HIV-1-specific CTL responses following DNA-rFPV vaccinations. (A) Net specific lysis of Env-, Gag-, or Pol-expressing autologous BLCL by antigen-stimulated bulk PBMC from three HIV-1-vaccinated macaques and three control macaques was assessed. CTL activities were determined after two DNA vaccinations alone (expressing Env and Gag antigens; week 10 following initial vaccination) or three subsequent rFPV boosting vaccinations (expressing Env, Gag, and Pol antigens; week 34 following initial vaccination). Greater than 5% net specific lysis (dotted line) was considered significant. E:T, effector/target. (B) Quantification of CTL precursors to Env, Gag, and Pol antigens was done by a limiting-dilution assay following DNA and rFPV vaccinations. CTL frequencies were assessed at week 14 (after DNA vaccinations alone) and week 34 (after rFPV boosting). Recognition of control targets expressing VV antigens alone was 6 PBMC and has been subtracted. CTL precursor frequencies and 95% confidence intervals were calculated by a maximum-likelihood analysis method.

FIG. 4

HIV-1-specific CTL responses following HIV-1 challenge. (A) HIV-1 Env, Gag, and Pol CTL precursor frequencies in two HIV-1-vaccinated macaques (closed symbols) and three control macaques (open symbols) were assessed serially over time following HIV-1 challenge by limiting-dilution analysis. Error bars show 95% confidence intervals for CTL precursor estimation. (B) HIV-1 Nef-specific CTL were determined 2 weeks following HIV-1 challenge in bulk PBMC cultures from two vaccinated macaques (M2 and M3) and two control macaques (M8 and M9). E:T, effector/target.