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 imme diate-early (IE) gene of pseudorabies virus influences virus replicati on and virulence, we replaced the LE promoter with the tissue-specific promoters of the bovine cytokeratin TV gene (CKIV), the bovine cytoke ratin VIb gene (CKVIb), or the inducible promoter of Drosophila heat s hock gene HSP70. We compared expression of the LE gene of the wild-typ e virus and recombinant viruses indifferent cell types and at differen t temperatures and found that IE expression had become cell type or te mperature dependent. When a recombinant virus was titrated on nonpermi ssive cells or was titrated at nonpermissive temperatures in vitro, th e plating efficiency was reduced by more than 99%. Mice were inoculate d subcutaneously (s.c.), intraperitoneally (i.p.), or intranasally (i. n.) with a dose equal to 100 times the 50% lethal dose of the wild-typ e virus. After inoculation with temperature-sensitive recombinant N-HS P, 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 supra basal layers of the epidermis, was not lethal after inoculation by any of the three routes. In explant cultures of nasal mucosa of pigs, rep lication of N-CKIV and N-CKVIb was not markedly reduced in the epithel ium. However, in contrast to results obtained with wild-type virus, in fection of the stroma was not observed. We conclude that the replicati ve ability and virulence of pseudorabies virus can be influenced by al tering control of expression of the IE gene.