A recombinant vaccinia virus that expresses the
nucleoprotein gene of Lassa virus (Josiah strain) under the control of the P7.5 promoter was constructed using the lacZ coexpression transfer vector pSC11. Southern blot analysis demonstrated that recombination of the sequences inserted within the
thymidine kinase gene of the transfer vector into the HindIII J fragment of vaccina virus genomic
DNA occurred properly. A 63-kDa
protein identical in electrophoretic mobility to authentic Lassa
nucleoprotein was observed in recombinant virus-infected cell lysates. The reactivity of
vaccinia-expressed Lassa
proteins to a panel of
monoclonal antibodies representing multiple
epitopes on each of the N, G1, and G2
proteins was determined by indirect immunofluorescence. Lassa
proteins expressed in recombinant vaccinia virus-infected cells reacted in a manner indistinguishable from that of the
proteins expressed in Lassa virus-infected cells, indicating that there are no significant differences between authentic and recombinant virus-expressed
proteins.
Vaccine efficacy trials in guinea pigs indicated that both the
nucleoprotein and the envelope
glycoproteins are capable of eliciting a protective immune response against a lethal dose of Lassa virus. Ninety-four percent of the animals vaccinated with V-LSN, 79% vaccinated with V-LSGPC, and 58% vaccinated with both recombinant viruses survived a Lassa virus challenge in which only 14% of unvaccinated animals and 39% of animals vaccinated with the New York Board of Health (NYBH) strain of vaccinia virus survived. The protection resulting from vaccination with the recombinant virus
vaccines did not correlate with the levels of prechallenge serum
antibodies, suggesting that a cell-mediated immune response is a critical component of protective immunity to
Lassa fever.