Effects of inflammatory response on in vivo transgene expression by plasmid DNA in mice

To examine the effects of inflammatory response to plasmid DNA (pDNA) on transgene expression, serum tumor necrosis factor-alpha (TNF-alpha) was measured after intravenous injection of pDNA or calf thymus DNA (CT DNA) in the naked or complexed form with cationic liposomes (lipoplex). pDNA with many CpG motifs induced TNF-alpha production regardless of the forms. No significant TNF-alpha production was detected when CT DNA or methylated pDNA was injected. Clodronate liposomes and dexamethasone were used to deplete phagocytes or to inhibit inflammatory responses, respectively. Transient depletion of phagocytes, such as liver Kupffer cells and splenic macrophages, by clodronate liposomes slightly altered the tissue distribution of (32)P-pDNA lipoplex, but significantly reduced the TNF-alpha production and transgene expression. Dexamethasone significantly inhibited the initial transgene expression, but increased the duration of the expression slightly. Use of NF-kappaB activity-dependent plasmid vector suggested that the inhibition of NF-kappaB activation is involved in the reduced expression by these treatments. These findings indicate that tissue macrophages are closely involved in the CpG motif-dependent TNF-alpha production. It is also suggested that TNF-alpha activates NF-kappaB and increases transgene expression by pDNA having many NF-kappaB binding sites, but TNF-alpha also reduces transgene expression at later time periods, leading to short-term transgene expression.