Comparison of the protective efficacy of DNA and baculovirus-derived protein vaccines for EBOLA virus in guinea pigs

The filoviruses Ebola virus (EBOV) and Marburg virus (MARV) cause severe hemorrhagic fever in humans for which no vaccines are available. Previously, a priming dose of a DNA vaccine expressing the glycoprotein (GP) gene of MARV followed by boosting with recombinant baculovirus-derived GP protein was found to confer protective immunity to guinea pigs (Hevey et al., 2001. Vaccine 20, 568–593). To determine whether a similar prime-boost vaccine approach would be effective for EBOV, we generated and characterized recombinant baculoviruses expressing full-length EBOV GP (GP 1,2) or a terminally-deleted GP (GPa-) and examined their immunogenicity in guinea pigs. As expected, cells infected with the GPa- recombinant secreted more GP 1 than those infected with the GP 1,2 recombinant. In lectin binding studies, the insect cell culture-derived GPs were found to differ from mammalian cell derived virion GP, in that they had no complex/hybrid N-linked glycans or glycans containing sialic acid. Despite these differences, the baculovirus-derived GPs were able to bind monoclonal antibodies to five distinct epitopes on EBOV GP, indicating that the antigenic structures of the proteins remain intact. As a measure of the ability of the baculovirus-derived proteins to elicit cell-mediated immune responses, we evaluated the T-cell stimulatory capacity of the GPa- protein in cultured human dendritic cells. Increases in cytotoxicity as compared to controls suggest that the baculovirus proteins have the capacity to evoke cell-mediated immune responses. Guinea pigs vaccinated with the baculovirus-derived GPs alone, or in a DNA prime-baculovirus protein boost regimen developed antibody responses as measured by ELISA and plaque reduction neutralization assays; however, incomplete protection was achieved when the proteins were given alone or in combination with DNA vaccines. These data indicate that a vaccine approach that was effective for MARV is not effective for EBOV in guinea pigs.