Transgene sequences free of CG dinucleotides lead to high level, long-term expression in the lung independent of plasmid backbone design.

Non-viral aerosol gene therapy offers great potential for treating chronic lung diseases of the airways such as cystic fibrosis (CF). Early clinical trials showed that transgene expression in the airways was transient whereas maximal duration of transgene expression is essential in order to minimise the frequency of aerosol treatments. Improved vector design, such as careful selection of the promoter/enhancer, can lead to more persistent levels of transgene expression, but multiple factors affect expression in vivo. Following aerosol delivery to the lungs of mice, we measured reporter gene expression from a CpG-free luciferase transgene cassette in the context of both a plasmid and minicircle vector configuration and showed that the vector backbone had no effect on expression. Transgene activity was affected by the vector backbone however, when a similar, but sub-optimal CpG-containing transgene was used, suggesting that aspects of the plasmid backbone had a negative impact on transgene expression. Similar studies were performed in Toll-like receptor-9 (TLR9) knockout mice to investigate a potential role for the TLR9 signalling pathway in detecting CpGs in the vector sequence. Even in the absence of TLR9, persistent expression could only be achieved with a CpG-free transgene. Together, these data indicate that in order to achieve high levels of persistent expression in vivo, a CpG-free transgene cassette is required.

The use of CpG-free plasmids to mediate persistent gene expression following repeated aerosol delivery of pDNA/PEI complexes.

Aerosol gene therapy offers great potential for treating acquired and inherited lung diseases. For treatment of chronic lung diseases such as cystic fibrosis, asthma and emphysema, non-viral gene therapy will likely require repeated administration to maintain transgene expression in slowly dividing, or terminally differentiated, lung epithelial cells. When complexed with plasmid DNA (pDNA), the synthetic polymer, 25 kDa branched Polyethylenimine (PEI), can be formulated for aerosol delivery to the lungs. We show that pDNA/PEI aerosol formulations can be repeatedly administered to airways of mice on at least 10 occasions with no detectable toxicity. Interestingly, peak reporter gene activity upon repeated delivery was significantly reduced by up to 75% compared with a single administration, despite similar pDNA lung deposition at each subsequent aerosol exposure. Although the precise mechanism of inhibition is unknown, it is independent of mouse strain, does not involve an immune response, and is mediated by PEI. Importantly, using a dosing interval of 56 days, delivery of a fourth-generation, CpG-free plasmid generated high-level, sustained transgene expression, which was further boosted at subsequent administrations. Together these data indicate that pDNA/PEI aerosol formulations offer a versatile platform for gene delivery to the lung resulting in sustained transgene expression suitable for treatment of chronic lung diseases.

The significance of plasmid DNA preparations contaminated with bacterial genomic DNA on inflammatory responses following delivery of lipoplexes to the murine lung.

Non-viral gene transfer using plasmid DNA (pDNA) is generally acknowledged as safe and non-immunogenic compared with the use of viral vectors. However, pre-clinical and clinical studies have shown that non-viral (lipoplex) gene transfer to the lung can provoke a mild, acute inflammatory response, which is thought to be, partly, due to unmethylated CG dinucleotides (CpGs) present in the pDNA sequence. Using a murine model of lung gene transfer, bronchoalveolar lavage fluid was collected following plasmid delivery and a range of inflammatory markers was analysed. The results showed that a Th1-related inflammatory cytokine response was present that was substantially reduced, though not abolished, by using CpG-free pDNA. The remaining minor level of inflammation was dependent on the quality of the pDNA preparation, specifically the quantity of contaminating bacterial genomic DNA, also a source of CpGs. Successful modification of a scalable plasmid manufacturing process, suitable for the production of clinical grade pDNA, produced highly pure plasmid preparations with reduced genomic DNA contamination. These studies help define the acceptable limit of genomic DNA contamination that will impact FDA/EMEA regulatory guidelines defining clinical grade purity of plasmid DNA for human use in gene therapy and vaccination studies.

Strategies for long-term expression of transgenes in the respiratory epithelium.

Lung gene therapy is being developed to treat acute and chronic airway diseases, and many viral and non-viral gene transfer vectors have been evaluated in the airway epithelium lining the nose and lung. Stem cells have not been clearly defined in the airways and, currently, it is only possible to target progenitor cells to proliferate and repair the epithelium after inducing epithelial damage. However, the majority of airway epithelial cells are slowly dividing or terminally differentiated, thus necessitating repeated administration of gene transfer vectors for life-long transgene expression. Many improvements to adeno-associated virus and lentivirus vectors have led to increased airway transduction efficiencies, although achieving consistent repeated administration remains problematic because of immune system activation. Non-viral vectors appear to be less efficient, but can be successfully re-administered. The modification of plasmid sequences also offers maximum flexibility in increasing and extending the duration of transgene expression in the airways. This review describes recent developments in achieving persistent transgene expression in the airways by specifically targeting the cells of the respiratory epithelium lining the nose and lung.

CpG-free plasmids confer reduced inflammation and sustained pulmonary gene expression.

Pulmonary delivery of plasmid DNA (pDNA)/cationic liposome complexes is associated with an acute unmethylated CG dinucleotide (CpG)-mediated inflammatory response and brief duration of transgene expression. We demonstrate that retention of even a single CpG in pDNA is sufficient to elicit an inflammatory response, whereas CpG-free pDNA vectors do not. Using a CpG-free pDNA expression vector, we achieved sustained (>or=56 d) in vivo transgene expression in the absence of lung inflammation.