The administration of naked plasmid DNA into the liver induces antitumor innate immunity in a murine liver metastasis model

BACKGROUND: Gene therapy is a promising strategy against advanced cancer; however, the safety of viral vectors and the effectiveness of non-viral vectors have not yet been established. Recently, a hydrodynamics-based procedure was reported to be an effective and safe method to deliver and transduce DNA into the liver. Herein, we propose a strategy for liver metastasis by a hydrodynamics-based procedure to deliver naked non-coding plasmid DNA (pDNA) into the liver as an immunocompetent organ. METHODS AND RESULTS: Mice received a rapid intravenous (i.v.) injection of naked pDNA in a large volume of saline (0.1 ml/g body weight). The single administration of a naked non-coding pDNA by the hydrodynamics-based procedure before tumor cell inoculation strongly suppressed liver metastasis formation. However, the usual i.v. injection (200 microl/body) of the same dose of naked pDNA could not suppress liver metastasis formation. Following the methylation of CpG sequences within the pDNA using CpG methylase, injection of the methylated pDNA by the hydrodynamics-based procedure could not suppress liver metastasis formation. Gadolinium chloride pretreatment did not interfere with this antitumor effect, but anti-asialo GM1 antiserum treatment did. These findings indicated that natural killer (NK) cells, not Kupffer cells, were involved in this antitumor effect. The NK cytotoxic activities of liver mononuclear cells were strongly enhanced after receiving a naked pDNA by the hydrodynamics-based procedure. CONCLUSIONS: These observations suggest that unmethylated CpG motifs in pDNA stimulated immune cells, resulting in the activation of NK cells in the liver to suppress liver metastases in a murine model.