Immune Responses to Adenoviral Vectors During Gene Transfer in the Brain

Abstract

We have investigated the immune response to E1-deleted adenovirus vectors encoding the lacZ gene introduced into the brains of adult mice. Injection of these nonreplicating vectors caused a marked inflammatory response in the brain as assessed by immunocytochemistry and flow cytometry of leukocytes. Infiltrating leukocytes were detectable within 2 days of injection and reached a maximum by 9 days. Thereafter, the number of infiltrating cells decreased, but a small number persisted in the brain until day 60. Between 2 and 4 days after injection, the percentage of CD8⁺ cells detectable increased whereas the percentage of CD4⁺ cells present in the infiltrating population did not significantly increase until day 6, peaking on day 15. Activated CD25⁺ T cells were detectable between days 6 and 15. β-Galactosidase (β-Gal), the product of the lacZ gene encoded by the vector, was also detected, both at the injection site in the striatum and also in the substantia nigra. Expression peaked between 4 and 6 days but a small number of β-Gal⁺ cells was still seen at 60 days after injection. This study demonstrates that a quantitative analysis of the immune responses caused by a nonreplicating adenovirus vector is possible in the brain. E1-deleted adenoviral vectors trigger a strong inflammatory response in the brain, but this immune response is not sufficient to eliminate completely expression of genes encoded by the adenoviral construct. Quantitative and qualitative analyses of inflammatory cells using flow cytometry and immunocytochemistry revealed a marked inflammatory response in the brain after injection of E1-deleted adenoviral vectors, and characterized the timing and cellular characteristics of these immune responses in detail. β-Galactosidase (β-Gal), the product of the lacZ gene encoded by the vector, was detected both at the injection site in the striatum and also in the substantia nigra. β-Gal expression persisted for at least 60 days in spite of the strong immune response to adenoviral vectors. These data demonstrate that the immune response triggered by injection of E1-deleted adenoviral vectors into the brain is insufficient to eliminate completely expression of genes encoded by the adenoviral vectors.