Preferential targeting of conserved Gag regions after vaccination with a heterologous DNA primemodified vaccinia virus Ankara boost HIV-1 vaccine regimen

Prime-boost vaccination strategies against HIV-1 often include multiple variants for a given immunogen for better coverage of the extensive viral diversity. To study the immunologic effects of this approach, we characterized breadth, phenotype, function, and specificity of Gag-specific T cells induced by a DNA-prime modified vaccinia virus Ankara (MVA)-boost vaccination strategy, which uses mismatched Gag immunogens in the TamoVac 01 phase IIa trial. Healthy Tanzanian volunteers received three injections of the DNA-SMI vaccine encoding a subtype B and AB-recombinant Gag_p37 and two vaccinations with MVA-CMDR encoding subtype A Gag_p55. Gag-specific T-cell responses were studied in 42 vaccinees using fresh peripheral blood mononuclear cells. After the first MVA-CMDR boost, vaccine-induced gamma interferon-positive (IFN-γ⁺) Gag-specific T-cell responses were dominated by CD4⁺ T cells (P < 0.001 compared to CD8⁺ T cells) that coexpressed interleukin-2 (IL-2) (66.4%) and/or tumor necrosis factor alpha (TNF-α) (63.7%). A median of 3 antigenic regions were targeted with a higher-magnitude median response to Gag_p24 regions, more conserved between prime and boost, compared to those of regions within Gag_p15 (not primed) and Gag_p17 (less conserved; P < 0.0001 for both). Four regions within Gag_p24 each were targeted by 45% to 74% of vaccinees upon restimulation with DNA-SMI-Gag matched peptides. The response rate to individual antigenic regions correlated with the sequence homology between the MVA- and DNA Gag-encoded immunogens (P = 0.04, r² = 0.47). In summary, after the first MVA-CMDR boost, the sequence-mismatched DNA-prime MVA-boost vaccine strategy induced a Gag-specific T-cell response that was dominated by polyfunctional CD4⁺ T cells and that targeted multiple antigenic regions within the conserved Gag_p24 protein.