The spatiotemporal distribution of transgenes determines the therapeutic efficacy of in vivo gene transfer. The important parameters of gene transfer are the level, duration, and cell specificity of expression, and the number of transfected cells. Interaction of vectors with blood cells, antigen-presenting cells,
serum proteins, and other
biological components affects the tissue distribution of vectors and the profile of transgene expression. Although plasmid
DNA is less immunogenic than viral vectors, it can induce inflammatory
cytokine release, due mainly to the presence of unmethylated CpG dinucleotides (CpG motifs). It was clearly demonstrated that
intravenous injection of a plasmid
DNA/cationic
liposome complex resulted not only in the induction of inflammatory
cytokines, but also in the activation of
nuclear factor kappaB (
NF-kappaB) in the lung. Insertion of additional
NF-kappaB-binding sequences into conventional plasmid
DNA resulted in a high transgene expression in the lung, suggesting that the
biological response to vectors can be used to increase transgene expression. In a marked contrast to this strategy, long-term transgene expression was achieved by reducing the number of the CpG motifs in plasmid
DNA. A plasmid encoding murine
interferon (IFN)-beta or IFN-gamma with reduced numbers of CpG motifs was highly effective in inhibiting metastatic
tumor growth in mice. These results clearly demonstrate the importance of the regulation of
biological responses to plasmid vectors to optimize plasmid-based in vivo gene transfer.