Effect of Exogenous Nerve Growth Factor on Neurotoxicity of and Neuronal Gene Delivery by a Herpes Simplex Amplicon Vector in the Rat Brain

We have previously shown that local destruction of neural tissue by wild-type herpes simplex virus type 1 (HSV-1) is attenuated by intracerebral infusion of nerve growth factor (NGF). To investigate the effect of NGF on the extent of neurolysis and efficacy of neuronal gene transfer mediated by an HSV-1 amplicon vector system in vivo, rats were stereotaxically injected in the striatum with an amplicon preparation, pHSVlac. This amplicon contains the Escherichia coli lacZ gene under the transcriptional control of the HSV-1 immediate early 4/5 promoter and is packaged by an HSV-1 helper virus carrying a deletion in the immediate early 3 gene. Vector injection was followed by continuous intracerebral infusion of NGF-β (total dose 5 μg) or vehicle solution over 7 days. Animals were sacrificed at the end of the 7-day infusion period for histological analysis of the brains.

A distinct zone of inflammation and necrosis surrounded the injection site in all vector-inoculated animals. The volume of striatal tissue destruction was significantly smaller in NGF-treated animals (1.27 ± 0.19 mm³; mean ± SEM) than in the vehicle-treated controls (2.16 ± 0.37 mm³; P < 0.05 by t-test). Immunohistochemical staining for HSV and β-galactosidase (β-Gal) in vehicle-treated animals revealed that many striatal cells harbored HSV antigens (3,678 ± 636), but only a small number expressed the reporter gene at 7 days post-injection (294 ± 60). NGF infusion did not significantly affect the number of HSV-immunoreactive cells (4,224 ± 618), or the number of cells expressing β-Gal (330 ± 72) at this time. In some animals in both groups, a disseminated pattern of HSV immunoreactivity and reporter gene expression was seen throughout the brain. We conclude that exogenous NGF reduces the local cytopathologic effects of this HSV-1-derived vector in the rat striatum, but does not affect the number of HSV-immunoreactive cells or the short-term expression of the transgene.

Overview summary

The neurotoxicity of many HSV-1-derived vectors limits their usefulness for gene delivery to the central nervous system. The present study represents an attempt to quantify systematically the local neurotoxicity of an HSV amplicon vector, and to diminish this toxicity by manipulating the local host microenvironment. We show that intracerebral infusion of NGF significantly reduces the extent of local brain tissue destruction by this vector, but does not affect the number of cells exhibiting HSV immunoreactivity or expressing the reporter gene at 1 week. This novel use of a trophic factor in combination with a viral vector may prove to be a promising approach for gene delivery to the brain.