Development and characterization of an infectious cDNA clone of the modified live virus vaccine strain of equine arteritis virus.

A stable full-length cDNA clone of the modified live virus (MLV) vaccine strain of equine arteritis virus (EAV) was developed. RNA transcripts generated from this plasmid (pEAVrMLV) were infectious upon transfection into mammalian cells, and the resultant recombinant virus (rMLV) had 100% nucleotide identity to the parental MLV vaccine strain of EAV. A single silent nucleotide substitution was introduced into the nucleocapsid gene (pEAVrMLVB), enabling the cloned vaccine virus (rMLVB) to be distinguished from parental MLV vaccine as well as other field and laboratory strains of EAV by using an allelic discrimination real-time reverse transcription (RT)-PCR assay. In vitro studies revealed that the cloned vaccine virus rMLVB and the parental MLV vaccine virus had identical growth kinetics and plaque morphologies in equine endothelial cells. In vivo studies confirmed that the cloned vaccine virus was very safe and induced high titers of neutralizing antibodies against EAV in experimentally immunized horses. When challenged with the heterologous EAV KY84 strain, the rMLVB vaccine virus protected immunized horses in regard to reducing the magnitude and duration of viremia and virus shedding but did not suppress the development of signs of EVA, although these were reduced in clinical severity. The vaccine clone pEAVrMLVB could be further manipulated to improve the vaccine efficacy as well as to develop a marker vaccine for serological differentiation of EAV naturally infected from vaccinated animals.