Comparison of the virulence of wild-type thymidine kinase (tk)-deficient and tk+ phenotypes of vaccinia virus recombinants after intranasal inoculation of mice.

A recombinant vaccinia virus vector was constructed which expressed the major surface glycoprotein G of human respiratory syncytial virus (RSV) and the thymidine kinase (tk) gene of vaccinia virus. The virulence of this tk+ recombinant virus was compared with that of a tk- recombinant and the wild-type (wt) virus after intranasal inoculation of mice. Respiratory infection with wt virus resulted in a lethal infection with widespread dissemination of virus. In contrast, infection with the tk- recombinant was not lethal and the virus had a reduced ability to disseminate to extrapulmonary tissue compared with wt virus. Insertion of the tk gene restored the virulence of the recombinant virus to the level of that of the wt virus. Despite a dramatic reduction in virulence of the tk- recombinant, virus could occasionally be recovered from the brains of mice. The expression of the attachment glycoprotein of RSV appeared to enhance the ability of the tk- recombinant virus to replicate in the lungs when compared with recombinants expressing fusion or nucleoprotein genes. The results confirm that inactivation of the tk gene results in a dramatic reduction of virulence for mice but suggest that there is still a potential danger of infection of the brain following intranasal administration of virus.