Contribution of plasmid DNA to hepatotoxicity after systemic administration of lipoplexes

Several studies have demonstrated that intravenous administration of DNA co mplexed with cationic lipid vectors induces the production of large quantit ies of proinflammatory cytokines. In this study we confirm these observatio ns, using cationic lipid DOTAP and cationic phospholipid compounds. Moreove r, we demonstrate that although intravenous administration of lipid-DNA com plexes does not induce toxic effects in the lung, high transgene expression in lung correlates with histopathological lesions in liver, this fact bein g documented by high transaminase levels in serum of treated mice. We exami ne the contribution of various components of the lipoplexes in this observe d liver toxicity, as well as in the increasing level of transaminases, and more particularly the role of nonmethylated CpG sequences of plasmid DNA. W e show that blood samples from animals treated either with cationic lipid a lone, or with cationic lipid complexed with methylated plasmid DNA, contain low levels of transaminases. The significant decrease in transaminase leve ls after injection of cationic lipid-methylated pDNA complexes leads us to believe that nonmethylated CpG sequences could play a major role in this he patoxicity. Similar results were observed when using a vector that did not encode a transgene, demonstrating that the expression of luciferase in lung was not responsible for this liver toxicity. All these observations sugges t that significant work should be devoted to understand more clearly the me chanism of cationic lipid-DNA complex toxicity, and to overcome the problem s subsequent to administration of nonmethylated CpG sequences of plasmid DN A.