Development of a Candidate Vaccine for an Enzootic Subtype IIIA Strain of the Venezuelan Equine Encephalitis Virus Complex

Abstract

The Venezuelan equine encephalitis (VEE) virus complex is an antigenically related group of arthropod-borne viruses in the family Togaviridae. Six major subtypes have been isolated with multiple antigenic variants defined in groups I and III. The Mucambo virus is the prototype virus of VEE subtype IIIA. Human infection with Mucambo virus is associated with symptoms including intense muscular pains, severe headache, and motor incoordination, usually lasting 9-10 days with residual symptoms for months. The current VEE vaccine, TC-83, which was derived from VEE IAB does not provide adequate protection against break-through infections with Mucambo virus, and consequently, candidate vaccines for the subtype IIIA viruses are currently being constructed. Comparative sequence analysis using the PE2 glycoprotein precursor gene, the most variable structural gene among VEE viruses, indicated a high degree of sequence conservation among VEE subtype IIIA and IIIB viruses which suggests cross-protection from a IIIA specific vaccine. A full-length chimeric cDNA clone composed of the nonstructural genes of V3000, a clone derived from the VEE IAB Trinidad donkey strain and the structural genes of the Mucambo virus has been constructed for use as a vaccine substrate to derive a suitable vaccine candidate, pV3A-1000. In tissue culture, V3A-1000 replicates as well as the wild-type Mucambo virus, growing to a titer of 1x108 PFU/ml. Attenuating mutations have been introduced into the VEE IAB/IIIA chimeric clones that delete the PE2 furin cleavage site (pV3A-2000) to inhibit reversion to a virulent phenotype. V3A-2000 replicated poorly in tissue culture and as a result could not undergo further studies in animals. However, animal studies in C57/BI6 mice indicated that V3A-1000 is highly attenuated and is capable of stimulating an immune response that provides immunological protection against challenge with high doses of virulent Mucambo virus.