Vaccination with
DNA is an attractive strategy for induction of pathogen-specific T cells and
antibodies. Studies in humans have shown that
DNA vaccines are safe, but their immunogenicity needs further improvement. As a step towards this goal, we have previously demonstrated that immunogenicity is increased with the use of an alphavirus
DNA-launched replicon (DREP) vector compared to conventional
DNA vaccines. In this study, we investigated the effect of varying the dose and number of administrations of DREP when given as a prime prior to a heterologous boost with poxvirus vector (MVA) and/or HIV
gp140 protein formulated in
glucopyranosyl lipid A (
GLA-AF) adjuvant. The DREP and
MVA vaccine constructs encoded Env and a Gag-Pol-Nef fusion
protein from HIV clade C. One to three administrations of 0.2 μg DREP induced lower HIV-specific T cell and
IgG responses than the equivalent number of immunizations with 10 μg DREP. However, the two doses were equally efficient as a priming component in a heterologous prime-boost regimen. The magnitude of immune responses depended on the number of priming immunizations rather than the dose. A single low dose of DREP prior to a heterologous boost resulted in greatly increased immune responses compared to MVA or
protein antigen alone, demonstrating that a mere 0.2 μg DREP was sufficient for priming immune responses. Following a DREP prime, T cell responses were expanded greatly by an MVA boost, and
IgG responses were also expanded when boosted with
protein antigen. When MVA and
protein were administered simultaneously following multiple DREP primes, responses were slightly compromised compared to administering them sequentially. In conclusion, we have demonstrated efficient priming of HIV-specific T cell and
IgG responses with a low dose of DREP, and shown that the priming effect depends on number of primes administered rather than dose.