A murine model based on infection by the respiratory route has been used to study the pathogenesis of recombinant vaccinia viruses. The neurovirulent Western Reserve (WR) strain and the Wyeth smallpox vaccine strain were used as vectors. Recombinant viruses were constructed by insertion of the Epstein—Barr virus membrane glycoprotein 340 gene into the thymidine kinase (TK) gene of each vaccinia virus. Intranasal inoculation of DBA/2 mice with 10 pock-forming units (pk.f.u.) of the WR strain was lethal but mice survived similar infection with the WR recombinant virus. Each virus was recovered from lung, blood and brain but, unlike wild-type virus, the recombinant virus was subsequently cleared. No deaths occurred after similar infection with the Wyeth strain or the Wyeth recombinant virus. There was limited growth of the Wyeth strain in the respiratory tract, low levels of virus in the blood and only sporadic recovery in brain extracts. The Wyeth recombinant virus was cleared rapidly with little viraemia or detectable infection of the central nervous system. No phenotypic character determined could be related consistently to the virulence of wild-type and recombinant viruses. Although the lethal character of the WR strain was affected by its TK phenotype, mice survived infection by intranasal inoculation with 10 pk.f.u. of WR TK recombinant viruses which either expressed the human interleukin 2 gene or had a deficient vaccinia virus growth factor gene.


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