Neuronal and glial cell type-specific promoters within adenovirus recombinants restrict the expression of the apoptosis-inducing molecule Fas ligand to predetermined brain cell types, and abolish peripheral liver toxicity.

Gene therapy using Fas ligand (FasL) for treatment of tumours and protection of transplant rejection is hampered because of the systemic toxicity of FasL. In the present study, recombinant replication-defective adenovirus vectors (RAds) encoding FasL under the control of either the neuronal-specific neuronal-specific enolase (NSE) promoter or the astrocyte-specific glial fibrillary acidic protein (GFAP) promoter have been constructed. The cell type-specific expression of FasL in both neurons and glial cells in primary cultures, and in neuronal and glial cell lines is demonstrated. Furthermore, transgene expression driven by the neuronal and glial promoter was not detected in fibroblastic or epithelial cell lines. Expression of FasL driven by a major immediate early human cytomegalovirus promoter (MIEhCMV) was, however, achieved in all cells tested. As a final test of the stringency of transgene-specific expression, the RAds were injected directly into the bloodstream of mice. The RAds encoding FasL under the control of the non-cell type-specific MIEhCMV promoter induced acute generalized liver haemorrhage with hepatocyte apoptosis, while the RAds containing the NSE or GFAP promoter sequences were completely non-toxic. This demonstrates the specificity of transgene expression, enhanced safety during systemic administration, and tightly regulated control of transgene expression of highly cytotoxic gene products, encoded within transcriptionally targeted RAds.