We have previously developed an antigen delivery system based on recombinant parvovirus-like particles (PPV-VLPs) formed by self-assemblage of a capsid protein of porcine parvovirus (PPV), carrying a foreign sequence at its N terminus. These pseudo-particles are produced by insertion of exogenous peptides into VP2 protein (67 KDa), which is one of three structural viral proteins of porcine parvovirus. VP2 self-assembles into 25 nm pseudo viral particles.
After immunization of adult mice with PPV-VLPs carrying a CD8+ T cell epitope of the lymphocytic choriomeningitis virus nucleoprotein (PPV-VLPs-LCMV) or ovalbumin (PPV-VLPs-OVA) without adjuvant, mice developed a strong cytotoxic T lymphocyte (CTL) response against peptide-coated or virus-infected target cells. Recombinant PPV-VLPs induce a long-lasting memory CTL response which can be evidenced even 9 months after the last immunization. Besides the i.p. and i.v. routes, mucosal immunization with PPV-VLPs was also demonstrated to be efficient. Indeed, intranasal immunization of mice with PPV-VLPs induces seric as well as mucosal neutralizing IgG and IgA antibodies specific for PPV-VLPs. Intranasal immunization also induces a CTL response against LCMV peptide-coated-target cells. The CTL response induced by PPV-VLPs-OVA was shown to be B and CD4+ T cell independent, as demonstrated using knock out mice.
The long-term CTL response obtained by immunization with PPV-VLPs-LCMV was shown to be protective. Indeed, mice immunized with PPV-VLPs-LCMV survived to an intra-cerebral injection of LCMV that normally killed non-immunized mice in 7 days. This protection was mediated by CD8+ T cells, as demonstrated by in vivo depletion studies and is due to the induction of high-avidity and high frequency CTL responses.
One main result of Neovac concerns the demonstration of the ability of this non-replicative delivery system based on parvovirus-like particles to induce CTL responses in the neonatal period. A single immunization of 1-week-old BALB/c mice with recombinant VLP carrying a CD8+ T cell determinant from lymphocytic choriomeningitis virus (VLP-LCMV) induced antigen-specific CD8+ cytotoxic T cells that were similar to those elicited by adult immunization as assessed by cytotoxic activity, IFN-g secretion, cytotoxic precursor cell frequencies, avidity for antigen and protective activity against viral challenge. These CTL responses are elicited within 2 weeks of a single immunization, in the absence of any adjuvant and independently of the presence and help of CD4+ T cells, highlighting the potential of VLP as vaccine candidates in early life.