Optimization of bovine coronavirus hemagglutinin-estrase glycoprotein expression in E3 deleted bovine adenovirus-3

Adenoviral vectors expressing foreign genes have many desirable properties in applications such as vaccination. Recently, we have generated replication-competent (E3 deleted) bovine adenovirus-3 (BAV-3) recombinants expressing significant amounts of glycoprotein D (gD) of bovine herpesvirus-1 (a DNA virus). However, attempts to express the RNA virus genes using the same strategy were not successful. In an effort to optimize the expression, we have constructed several BAV-3 recombinants carrying the hemagglutinin esterase (HE) gene of bovine coronavirus (BCV) in the E3 region with or without exogenous transcription control elements. The expression studies suggest that the introduction of a 137 bp chimeric intron upstream of the HE cDNA is able to increase the level of HE gene expression. The introduction of a SV40 early promoter or human cytomegalovirus (HCMV) immediate early (IE) promoter into the expression cassette changed the kinetics of the HE expression. However, the recombinant BAV-3 containing HE under the HCMV IE promoter replicated less efficiently than the wild-type BAV-3. These studies should prove useful in expression of other RNA viral genes in the E3 region of BAV-3 expression system.