Adenoviral Vector-Mediated Gene Expression in the Nervous System of Immunocompetent Wistar and T Cell-Deficient nude Rats: Preferential Survival of Transduced Astroglial Cells in nude Rats

In the present paper, we examined the effect of the adenoviral vector dosage, the role of T cells, and the influence of the presence of replication-competent adenovirus (RCA) in adenoviral vector stocks, on the efficacy of adenoviral vector-directed transgene expression in the facial nucleus of immunocompetent Wistar and athymic nude rats. A small number of motor neurons and glial cells was transduced at low dosages of viral vector (1 × 106 pfu) and in the absence of RCA, and transgene-expressing cells persisted throughout the 3-week period of observation. Intraparenchymal infusion of 2 × 107 pfu of a recombinant adenoviral vector free of RCA was required for optimal transduction of facial motor neurons. In Wistar rats, a biphasic immune response occurred at higher dosages of the vector (5 × 106 and 2 × 107 pfu) that was characterized by early infiltration of macrophages and the occurrence of T cells during the second week after injection of the vector. The immune response was associated with the loss of transduced neural cells. In nude rats, administration of an adenoviral vector free of RCA resulted in a macrophage response comparable to that in the Wistar rat and long-term survival of transduced astroglial cells. However, transduced motor neurons degenerated according to a similar time course as observed in Wistar rats. Small amounts of RCA (2 × 105 pfu) injected with 2 × 107 pfu recombinant viral vector particles resulted in an accelerated T cell response and a rapid elimination of transduced cells within 1 week in Wistar rats, whereas in nude rats transgene expression continued during this period. Taken together, these observations suggest that at the high viral vector loads necessary to achieve optimal transduction of the facial nucleus, T cells play a role in the degeneration of adenoviral vector-transduced astroglial cells. The adverse effects on neurons appear to be due to the observed inflammatory response or to direct adenoviral vector toxicity. Adenoviral vectors have been shown to transduce neural cells in vitro and in vivo. The prevailing view is that adenoviral vector-directed gene transfer to the brain results in long-lasting transgene expression. We show that following administration of low dosages of viral vector (1 × 106 pfu) relatively small numbers of neurons expressed the transgene for at least 3 weeks. At moderate (5 × 106 and 2 × 107 pfu) and high dosages (1 × 108 pfu), transduction efficiency of neural cells improved significantly; however, this was accompanied by a biphasic immune response (comparable to that observed in lung and liver). In T cell-deficient nude rats, transduced astroglial cells persisted for at least 30 days but motor neurons degenerated as in Wistar rats, suggesting that T cells are involved in the elimination of transduced glial cells. Neuronal degeneration is apparently due to direct adverse effects of the viral vector or to the parenchymal inflammatory response.