Background: To determine the frequency of T cell responses of Tanzanian HIV-1 seronegative participants in a recently concluded multi-clade HIV-1 DNA plasmid/Adenovirus-5 vector (rAd5) vaccine trial.

Methods: Sixty participants were enrolled in a phase I/II randomized, double-blind, placebo-controlled trial to assess the safety and immunogenicity of a multi-clade (A, B and C) HIV-1 DNA plasmid vaccine, boosted with HIV-1 clade-matched rAd5 vectors (Vaccine Research Center, NIH, USA). The vaccine:placebo ratio was 1:1. DNA was administered at weeks 0, 4 and 8, and the rAd5 boost at 24 weeks. Interferon-gamma (IFN-g) ELISpot assays were performed blinded to immunisation status on a sub-set of participants (N=40) with freshly isolated peripheral blood mononuclear cells (PBMC). Assays were conducted at 0, 6, 10, 26, 30, 39 and 48 weeks to HIV-1 Env (clades A and B), Pol and Nef (clade B) peptide pools. A positive IFN-g ELISpot response was defined as at least 55 SFC/10⁶ PBMC and 4 times the media treated wells. Pre-immunisation Ad5 titers were measured using a validated assay.

Results: Twenty participants received the placebo and 20 the vaccine. There were no positive ELISpot responses to HIV peptides prior to immunization and in the placebos at any time point tested. Positive responses were observed after the second DNA immunization in 6/20 (30%) vaccinees, and increased to 10/20 vaccinees following the third DNA immunization, prior to rAd5 boosting. Two weeks after the rAd5 boost, the cumulative frequency of positive responders increased to 16/20 (80%), accompanied by an increased magnitude of the immune response to Env (Mean: 128 SFC/10⁶PBMC pre-rAD5 versus 344 SFC/10⁶PBMC post-rAd5). Six months following the adenovirus boost, the cumulative frequency of positive responders increased to 17/20 (85%). Positive responses were predominantly to Env (16/20; 80%), with 5/20 (25%) and 3/20 (15%) vaccinees responding to Nef and Pol, respectively. Thirteen vaccinees (65%) were positive at multiple time points. The titer of Ad5 neutralising antibody did not affect the frequency of HIV-specific immune responses as 9/9 volunteers with titers >5000 demonstrated positive HIV-1 specific IFN-g responses, compared to 8/11 with titers <5000.

Conclusion: This vaccine regimen induced robust cellular immune responses at multiple time points in the majority of vaccinees. However, recent concerns using rAd5 vectors will require further research to supports its safety in settings with high baseline titers of Ad5 neutralizing antibodies.