Prime-boost vaccinations using recombinant flavivirus replicon and vaccinia virus vaccines: an ELISPOT analysis

Recombinant Kunjin replicon virus-like particle (VLP), vaccinia virus (rVV) and DNA vaccines were tested in a large series of prime-boost vaccinations using interferon (IFN)γ ELISPOT assays that reflected effector (E), effector memory (EM) and central memory (CM) responses. All vaccine constructs encoded the murine polytope immunogen and responses to four CD8 T-cell epitopes (TYQRTRALV, SYIPSAEKI, YPHFMPTNL and RPQASGVYM) were measured. VLP/rVV out performed (by 14- to 20-fold) DNA/rVV for induction of CM responses, whereas EM responses were only marginally increased. DNA/VLP induced more EM, but not CM responses, than VLP alone, illustrating that DNA priming is not universally beneficial. rVV/VLP gave comparable results to VLP/rVV combinations, although the former induced approximately threefold more E responses, illustrating the utility of poxvirus priming in this setting. Although higher doses of VLP and rVV increased responses after single immunizations, such dose increases provided only marginal benefit in heterologous prime-boost settings. Triple combinations also provided no benefit over two vaccinations. DNA vaccination was associated with broad CM, but not EM responses, and the breadth of EM and E responses was significantly improved by increasing viral vector dose. VLP/rVV, rather than DNA priming, induced T cells with consistently high IFNγ secretion profiles across all ELISPOT measures. Vector-specific CD8 T-cell responses generally correlated well with immunogen-specific responses, although, as expected, single use of each vector reduced the relative levels of vector-specific responses. These experiments illustrate the utility of replicons in heterologous prime-boost vaccinations, and illustrate the diversity of data that can be obtained from ELISPOT analyses.