Abstract
We characterized prime-boost vaccine regimens using heterologous and homologous vector and gene inserts. Heterologous regimens offer a promising approach that focuses the cell-mediated immune response on the insert and away from vector-dominated responses. Ad35-GRIN/ENV (Ad35-GE) vaccine is comprised of two vectors containing sequences from HIV-1 subtype A gag, rt, int, nef (Ad35-GRIN) and env (Ad35-ENV). MVA-CMDR (MVA-C), MVA-KEA (MVA-K) and MVA-TZC (MVA-T) vaccines contain gag, env and pol genes from HIV-1 subtypes CRF01_AE, A and C, respectively. Balb/c mice were immunized with different heterologous and homologous vector and insert prime-boost combinations. HIV and vector-specific immune responses were quantified post-boost vaccination. Gag-specific IFN-γ ELISPOT, intracellular cytokine staining (ICS) (CD107a, IFN-γ, TNF-α and IL-2), pentamer staining and T-cell phenotyping were used to differentiate responses to inserts and vectors. Ad35-GE prime followed by boost with any of the recombinant MVA constructs (rMVA) induced CD8+ Gag-specific responses superior to Ad35-GE-Ad35-GE or rMVA-rMVA prime-boost combinations. Notably, there was a shift toward insert-focus responses using heterologous vector prime-boost regimens. Gag-specific central and effector memory T cells were generated more rapidly and in greater numbers in the heterologous compared to the homologous prime-boost regimens. These results suggest that heterologous prime-boost vaccination regimens enhance immunity by increasing the magnitude, onset and multifunctionality of the insert-specific cell-mediated immune response compared to homologous vaccination regimens. This study supports the rationale for testing heterologous prime-boost regimens in humans.
Highlights
The development of an efficacious preventive HIV-1 vaccine was given a significant impetus following the promising results of the RV144 Thai trial [1]
Splenocytes from Ad35-GE/MVA-C regimen mounted a statistically significant higher response (p,0.05) to Gag pools and to the immunodominant Gag epitopes, as measured by CD107 and IFN-c expression when compared to splenocytes from homologous MVA-C/MVA-C regimen
In order to understand the respective contributions of the HIV inserts and vectors to the immune responses, we studied the combination of Ad35-GE with other recombinant MVA (rMVA) vectors with inserts that partially match sequences to the Ad35 inserts
Summary
The development of an efficacious preventive HIV-1 vaccine was given a significant impetus following the promising results of the RV144 Thai trial [1]. Modest efficacy against HIV acquisition was conferred by a heterologous prime-boost regimen using a canarypox vector ALVAC-HIV (vCP1521) expressing env, gag and pol genes and AIDSVAX B/E, a bivalent gp120 envelope glycoprotein. This regimen elicited modest interferon-gamma (IFN-c) ELISPOT responses but strong CD4+ lymphoproliferative and humoral immune responses that were better than either product tested alone. Pre-clinical and clinical studies using heterologous prime-boost HIV-1 vaccines have shown the generation of stronger and broader immune responses [9,10,11]. The heterologous prime-boost strategy may apply to either different vectors, different insert sequences, or as tested in this study, a combination of different vectors and different inserts. Combinations of rAd and recombinant MVA (rMVA) have been tested in pre-clinical and some clinical settings with mixed results [16]
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