Recombinant retrovirus-like particle forming DNA vaccines in prime-boost immunization and their use for hepatitis C virus vaccine development.

Abstract : BACKGROUND: The expression of Moloney murine leukemia virus (Mo-MLV) gag proteins is sufficient to generate retrovirus-like particles (retroVLPs) that can be used as antigen-display platforms by pseudotyping with heterologous envelope proteins or by insertion of epitopes in structural constituents. To circumvent the in vitro production of such retroVLPs, we used DNA plasmids generating recombinant retroVLPs (plasmo-retroVLPs) as immunogens. We previously demonstrated that plasmo-retroVLPs induce significantly better antigen-specific T cell responses and antiviral immune protection than plasmids bearing a single mutation preventing retroVLPs assembly. In the present study, we investigated the possibility of using such plasmo-retroVLPs in prime-boost immunization strategies for hepatitis C virus (HCV) vaccine development. METHODS: To define the best immunization regimen with plasmo-retroVLPs and serotype 5 recombinant adenovirus vectors (rAd5), we used standardized methodologies measuring immune responses to the GP(33-41) 'gold standard' antigen. The protective efficacy of these immunization schedules was also evaluated in mice after tumor challenge. We then applied the optimal prime-boost immunization strategy using vectors expressing HCV-E1/E2 envelope glycoproteins. RESULTS: Using vectors expressing the model antigen, we demonstrated that rAd5(GP33-41)/plasmo-retroVLP(GP33-41) regimen induced significantly higher cellular immune responses than plasmo-retroVLP(GP33-41)/rAd5(GP33-41). Consequently, HCV-specific plasmo-retroVLPs (plasmo-retroVLP(E1E2)) were used as boost in mice primed with rAd5(E1E2) and we observed that plasmo-retroVLP(E1E2) significantly increased E1/E2-specific interferon-gamma cellular responses and E2-specific antibody generation. By contrast, plasmids unable to form E1/E2-pseudotyped retroVLPs had no boosting effect, revealing the importance of presenting E1/E2 in a particulate form. CONCLUSIONS: Altogether, combining plasmo-retroVLPs that represent a new class of genetic vaccines in a heterologous prime-boost vaccination strategy appears to be a promising strategy for HCV vaccine development.