CpG methylation of a plasmid vector results in extended transgene product expression by circumventing induction of immune responses.

Gene therapy has the potential to cure inherited diseases if the delivered genes achieve long-term expression at therapeutic levels in the targeted tissues. Expression is commonly short-lived due to induction of cell-mediated immune responses to the gene therapy vehicle and/or the transgene product, which can be perceived as "foreign" by the host's immune system. Plasmid expression vectors have been used to deliver genes. Bacterial DNA carries immunostimulatory sequences in the form of unmethylated CpG motifs, which induce an inflammatory reaction that in turn promotes activation of transgene product-specific B and T cells. Elimination or methylation of immunostimulatory CpG sequences in plasmid expression vectors prevents the stimulation of transgene product-specific immune responses without necessarily reducing transgene expression. In this study, we tested if a CpG-methylated plasmid expression vector expressing the highly immunogenic glycoprotein of rabies virus can achieve prolonged transgene product expression by circumventing immune recognition. Our data show that mice inoculated with a CpG-methylated plasmid expression vector show delayed clearance of transfected cells and fail to mount a strong immune response to the transgene product. Gene transfer with a CpG-methylated plasmid results in a state of immunological low responsiveness to the transgene product, which may facilitate readministration of the transgene. Nevertheless, mice remain responsive to the transgene product delivered by a viral vector.