Specific phenotypic restoration of an attenuated virus by knockout of a host resistance gene

To produce disease, viruses must enter the host, multiply locally in host t issues, spread from the site of entry, and overcome or evade host immune re sponses. At each stage in this infectious process, specific microbial and h ost genes determine the ultimate virulence of the virus. Genetic approaches have identified many viral genes that play critical roles in virulence and are presumed to target specific components of the host innate and acquired immune response. However, formal proof that a virulence gene targets a spe cific protein in a host pathway in vivo has not been obtained. Based on cel l culture studies, it has been proposed that the herpes simplex virus type 1 gene ICP34.5 (ICP, infected cell protein) enhances neurovirulence by nega ting antiviral functions of the IFN-inducible double-stranded RNA-dependent protein kinase R or PKR [Chou, J,, Chen, J,J,, Gross, M. & Roizman, B. (19 95) Proc. Natl, Acad. Sci, USA 92, 10516-10520], Herein, we show that a vir us that has been attenuated by deletion of ICP34.5 exhibits wild-type repli cation and virulence in a host from which the PKR gene has been deleted. We show that restoration of virulence is specific to ICP34.5 and PKR by using additional host and viral mutants. The use of recombinant viruses to infec t animals with null mutations in host defense genes provides a formal genet ic test for identifying in vivo mechanisms and targets of microbial virulen ce genes.