Mutations within the US2 and glycoprotein B genes of the equine herpesvirus 1 vaccine strain RacH do not account for its attenuation

The equine herpesvirus 1 (EHV-1) modified live vaccine strain RacH is apath ogenic for both laboratory animals and the natural host. The apathogenicity of RacH was caused by serial passages of the virus in heterologous cells. When compared to the virulent parental strain RacL11 several changes in the RacH genome occured. Previous results have shown that the loss of the IR6 gene correlated with the loss of virulence. Additional important mutations were observed within the US2 gene which is directly adjacent to the IR6 gen e and within the glycoprotein B (gB) gene. To answer the question whether t hese mutations contribute to the attenuation of RacH several recombinant EH V-1 were constructed: The mutated genes in RacH were replaced by the wild-t ype US2 gene or the wild-type gB gene, respectively. In addition, a RacL11 recombinant expressing the mutated (RacH) gB instead of the wild-type gene was generated. All recombinant viruses were tested for virulence using the EHV-1 mouse model. The results were as follows: i) The insertion of the Rac L11 US2 gene into the RacH virus did not restore virulence and none of the infected mice showed typical signs of EHV-l-caused disease (symptoms and bo dy weight loss), ii) Exchanging gB genes between RacL11 and RacH did not al ter their virulence phenotypes remarkably either. Therefore, it is conclude d that attenuation of the EHV-I vaccine strain RacH is caused solely by the absence of the IR6 gene and protein.