Recombinant chimeric western and eastern equine encephalitis viruses as potential vaccine candidates

Chimeric cDNA clones, pMWE1000 and pMWE2000, differing by five nucleotides at their 5' termini, were constructed of the 5' two-thirds of the western equine encephalitis (WEE) virus genome (encoding nonstructural proteins) and the 3' one-third of the eastern equine encephalitis (EEE) virus genome (encoding structural proteins). The WEE virus sequences were derived from full-length cDNA clones, pWE1000 and pWE2000, which were isogenic except for five nucleotide differences at their 5' termini and were responsible for significant differences in mouse virulence. Each cDNA clone was placed downstream from a T7 promoter to allow in vitro transcription of full-length RNA. Transfection of BHK-21 cells with the chimeric RNA by electroporation gave rise to high-titer infectious virus. The in vitro characteristics of each chimera virus were determined by electrophoretic analysis of its structural proteins, plaque morphology, neutralization characteristics, replication kinetics, and rate of viral RNA synthesis. With the exception of plaque morphology, the in vitro characteristics of MWE1000 and MWE2000 were indistinguishable from the parental EEE virus. Subcutaneous inoculation of 5-week-old C57BL/6 mice with varying doses of MWE1000 or MWE2000 virus demonstrated that both chimeric viruses were significantly attenuated compared to the parental WEE virus (Cba 87) and EEE virus (PE-6). Animals infected with 10(5) PFU or more of either MWE1000 or MWE2000 were completely protected from lethal challenge with the virulent EEE virus, FL91-4679, but were not protected from virulent WEE virus Cba 87 challenge. Construction of viable virus chimeras often results in attenuated viruses that may hold promise as genetically engineered alphavirus vaccine candidates.