Construction and properties of pseudorabies virus recombinants with altered control of immediate-early gene expression.

To investigate how altered control of expression of the essential immediate-early (IE) gene of pseudorabies virus influences virus replication and virulence, we replaced the IE promoter with the tissue-specific promoters of the bovine cytokeratin IV gene (CKIV), the bovine cytokeratin VIb gene (CKVIb), or the inducible promoter of Drosophila heat shock gene HSP70. We compared expression of the IE gene of the wild-type virus and recombinant viruses in different cell types and at different temperatures and found that IE expression had become cell type or temperature dependent. When a recombinant virus was titrated on nonpermissive cells or was titrated at nonpermissive temperatures in vitro, the plating efficiency was reduced by more than 99%. Mice were inoculated subcutaneously (s.c.), intraperitoneally (i.p.), or intranasally (i.n.) with a dose equal to 100 times the 50% lethal dose of the wild-type virus. After inoculation with temperature-sensitive recombinant N-HSP, two (s.c.), two (i.p.), and four (i.n.) of five mice died. However, at this dose, recombinant N-CKIV, which contains a promoter specific for stratified epithelial tissue of the tongue mucosa, was not lethal when inoculated s.c. or i.p. but killed four mice when inoculated i.n. Recombinant N-CKVIb, which contains a promoter specific for the suprabasal layers of the epidermis, was not lethal after inoculation by any of the three routes. In explant cultures of nasal mucosa of pigs, replication of N-CKIV and N-CKVIb was not markedly reduced in the epithelium. However, in contrast to results obtained with wild-type virus, infection of the stroma was not observed. We conclude that the replicative ability and virulence of pseudorabies virus can be influenced by altering control of expression of the IE gene.