Latency associated promoter transgene expression in the central nervous system after stereotaxic delivery of replication-defective HSV-1-based vectors

The herpes simplex virus type I (HSV-1) latency associated promoter (LAP) h as been shown to sustain long-term reporter gene expression within sensory neurones. Its activity within the CNS is, however, less well understood. In this study we characterise the activity of the LAP after stereotaxic deliv ery of recombinant HSV-1-based vectors to the brain. Two classes of vectors were utilised in these studies: (1) a replication-defective vector lacking the glycoprotein H and thymidine kinase genes, designated CS1, and (2) a v irus mutant severely impaired for immediate-early (IE) gene expression whic h lacks functional VP16, ICP4 and ICP0 genes, designated in 1388. Both vect ors contain the LacZ gene under the control of the LAP. Following delivery of either vector to the striatum, beta -gal expression was detected within anatomically related CNS regions distal to the site of injection. At these sites the number of beta -gal-positive cells increased with time and remain ed stable up to 4 weeks p.i. beta -Gal expression could not be detected at the site of injection after delivery of CS1 but beta -gal expression within neurones located at this site was observed after delivery of in 1388, indi cating reduced toxicity of this severely disabled virus. Transgene expressi on decreased dramatically with both vectors at later time-points (>4 weeks after delivery), but PCR analysis demonstrated that viral genomes were stab ly maintained for up to 180 days following delivery, indicating that the lo ss of beta -gal-positive neurones was not likely to be due to a loss of vec tor-transduced cells. Moreover, after delivery of an equivalent virus to th e rat striatum in situ hybridisation analysis showed a similar decrease in the number of neurones expressing the endogenous LA Ts with time. These dat a indicate that although the HSV-I LAP can drive the expression of foreign genes in a variety of CNS neurones, in these cells there is a slow down-reg ulation of the viral promoter which eventually results in the loss of detec table transgene expression.