Long-term transgene expression in mice infected with a herpes simplex virus type 1 mutant severely impaired for immediate-early gene expression

The role of viral immediate-early (IE) gene expression in herpes simplex vi rus type I (HSV-1) latency was investigated, The HSV-1 multiple mutant in 1 312, defective for the expression of the virion transactivator VP16 and the IE proteins ICP0 and ICP4, was used as the parent for these studies, The c oding sequences of the Escherichia coli lacZ gene, preceded by the encephal omyocarditis virus internal ribosome entry site, were inserted into the reg ion of in 1312 that encodes the latency-associated transcripts (LATs) such that transcription of the transgene was controlled by the LAT promoter. Thi s insert has previously been shown to direct long-term latent-phase express ion of beta-galactosidase in a wild-type HSV-1 genome (R. H. Lachmann and S . Efstathiou, J. Virol. 71., 3197-3207, 1997). The resulting recombinant, i n 1388, was apathogenic after inoculation into mice via the footpad and did not detectably replicate in dorsal root ganglia (DRG) or footpads. Mutant in 1388 established latency in DRG, and P-galactosidase was expressed in in creasing numbers of neurons over the first 25 days of infection. During lat ency, more than 1% of neurons in ganglia that innervate the footpad express ed P-galactosidase, with the number of positive cells remaining constant fo r at least 5 months, Rescue of the VP16, ICP0, or ICP4 mutations of in 1388 did not affect the number of beta-galactosidase-expressing neurons detecte d during latency. The results demonstrate that HSV-1 mutants severely impai red for IE gene expression are capable of establishing latency and efficien tly expressing a foreign gene product under control of the LAT promoter.