Most currently used antiviral vaccines are based on homologous viruses,i.e.inactivated or live attenuated viruses that are inoculated to prevent infection by their live wild type cognate.Their successful prevention of a large number of viral diseases establishes that packaging antigens into/onto particles is a highly efficient way to trigger immune responses.Recent vaccine development aiming to improve vaccine safety and efficacy has seen promising results using recombinant heterologous viruses or DNA plasmids to act as safe carriers to deliver genes encoding antigens of interest.In this line,we engineered recombinant retrovirus-based virus-like particles (rVLP) to serve as vaccine platforms.We found that such VLPs,engineered to display a heterologous virus antigen,were extremely efficient immunogens.They generated strong cytotoxic T-cell responses at concentrations 106 lower than those required for peptides formulated with incomplete Freunds adjuvant.