In a previous report, we demonstrated that a first-generation (E1- and E3-deleted) recombinant adenovirus can transduce expression of the E. coli lacZ gene into Schwann cells, both in vitro and in vivo, suggesting that this method might be useful for future therapy of peripheral neuropathy, including CMT1. Adenovirus-mediated gene transfer was limited, however, by demyelination and Wallerian degeneration at the site of virus injection, as well as by attenuation of viral transgene expression over time. In our current work we have optimized adenoviral vector-mediated transgene expression after intraneural injection into sciatic nerve. Using an improved injection protocol, peak expression of lacZ occurs between 10 and 14 days after injection of 2-week-old rats, decreases thereafter, and there is minimal associated tissue injury. In contrast, few lacZ-expressing Schwann cells are found in nerve of adult animals 10 days after injection, probably owing to immune clearance of virus-infected cells. Consistent with this notion, high levels of LacZ are found in sciatic nerve 30 days after injection of adult SCID mice, which have a genetic defect in both cellular and humoral immunity, of adult beta2-microglobulin-deficient mice (beta2M4-/-), which have a genetic defect in cellular immunity, or of adult mice treated with the immunosuppressing agent FK506. In addition, adenovirus-infected Schwann cells cocultured with axons in vitro, in the absence of a host immune response, ensheathe axons and express lacZ for at least 8 weeks. These data thus demonstrate that lacZ transgene expression of first-generation recombinant adenovirus in sciatic nerve in adult mice, as in other tissues, is limited mainly by the host cellular immune response to the virus, which can be overcome by attenuation of host cell-mediated immunity. Adenoviral vectors might thus be used to modulate Schwann cell gene expression in patients with peripheral neuropathy after appropriate immunosuppression.