Immune responses to repetitive adenovirus-mediated gene transfer and restoration of gene expression by cyclophosphamide or etoposide

Background. One major concern about adenoviral vectors for repetitive gene delivery is the induction of an immune response to the vector, thus impeding effective gene transduction. Methods. To assess the immune response to the adenoviral vector, repetitive gene dosing was performed into rhesus monkey cervix and C3H mouse skin using the adenoviral vector carrying the lacZ gene. Three repetitive intracervical injections of adenovirus-lacZ were done in the rhesus monkey at the intervals of 4 weeks. Gene expression on the second and third injection was completely suppressed. Results. Anti-adenovirus IgG levels and neutralizing antibody titers in the rhesus monkey significantly increased after the first injection of adenovirus. In the C3H mouse, neutralizing antibody titers significantly increased after the first injection of adenovirus-lacZ at more than 10⁸ plaque-forming unit (PFU). The repetitive expression of lacZ gene in the mouse skin markedly decreased when the second injection is done more than 2 weeks after the first injection. Chronic low-dose treatment with cyclophosphamide or etoposide markedly suppressed neutralizing antibody titers in the mouse serum and restored the gene expression in the mouse skin on the second and third injection. Conclusions. It is suggested that repetitive gene expression by adenovirus-mediated transfer may be reduced by circulating neutralizing antibodies and could be restored by chronic low-dose treatment with cyclophosphamide or etoposide.