Comparison of plasmid DNA preparation methods for direct gene transfer and genetic immunization

Localization of CD8+ cells specific for hepatitis B virus surface protein in the liver of immunized mice

DNA plasmids are potent inducers of long-lasting antigen-specific CTL responses. Little is known about the distribution of antigen-specific CD8+ T cells in the lymphoid tissue and the non-lymphoid tissue after DNA immunization. HBsAg-specific CD8+ T cells in peripheral blood mononuclear cells, spleen, lymph nodes, and the liver of Balb/c mice have been quantified after injection with a DNA plasmid expressing the major S protein of hepatitis B virus (HBV). The kinetics of CD8+ T-cell responses in the circulation were measured after priming and boosting, showing that antigen-specific CD8+ T cells undergo first expansion and then decline to a sustainable level in the circulation, although the frequencies of HBsAg-specific CD8+ T cells in the circulation were lower than for the spleen. The greater frequencies of HBsAg-specific CD8+ T cells were found in the liver, whereas the largest numbers of antigen-specific CD8+ T cells were found in the spleen. By day 100 after priming, HBsAg-specific CD8+ T cells were still detected in the circulation, the spleen and the liver. After boosting with the same plasmid DNA immunogen, HBsAg-specific CD8+ T cells proliferated quickly and vigorously. By 150 days after boosting, HBsAg-specific memory CD8+ T cells were sustained at higher levels than those recorded after the first, primary injection, both in the spleen and the liver: anti-HBs antibody-secreting plasma cells persisted in the bone marrow and in the spleen, consistent with the detection of anti-HBs antibodies detected in the blood. These findings indicate that DNA immunization has considerable potential for inducing specific T cell responses in the liver and offers a strategy for the development of post-exposure immunotherapy against persistent hepatitis B infections.

Nuclear accumulation of plasmid DNA can be enhanced by non-selective gating of the nuclear pore

One of the major obstacles in non-viral gene transfer is the nuclear membrane. Attempts to improve the transport of DNA to the nucleus through the use of nuclear localization signals or importin-β have achieved limited success. It has been proposed that the nuclear pore complexes (NPCs) through which nucleocytoplasmic transport occurs are filled with a hydrophobic phase through which hydrophobic importins can dissolve. Therefore, considering the hydrophobic nature of the NPC channel, we evaluated whether a non-selective gating of nuclear pores by trans-cyclohexane-1,2-diol (TCHD), an amphipathic alcohol that reversibly collapses the permeability barrier of the NPCs, could be obtained and used as an alternative method to facilitate nuclear entry of plasmid DNA. Our data demonstrate for the first time that TCHD makes the nucleus permeable for both high molecular weight dextrans and plasmid DNA (pDNA) at non-toxic concentrations. Furthermore, in line with these observations, TCHD enhanced the transfection efficacy of both naked DNA and lipoplexes. In conclusion, based on the proposed structure of NPCs we succeeded to temporarily open the NPCs for macromolecules as large as pDNAs and demonstrated that this can significantly enhance non-viral gene delivery.

Production of clinical-grade plasmid DNA for human Phase I clinical trials and large animal clinical studies

The use of plasmid DNA as vaccines for the treatment of cancer and infectious diseases is on the rise. In order to facilitate the manufacture of clinical-grade plasmid DNA for Phase I clinical trials, we developed a process whereby >200 mg plasmid could be produced in a single production run under Good Manufacturing Practices. A dedicated cleanroom (Class 10,000 with Class 100 biosafety cabinet) is utilized for production of the bacterial cell bank, fermentation, harvest/lysis of the biomass, and downstream purification. Fermentation requires three 16-18 h runs (approximately 12 L each) in shaker-flasks, yielding approximately 60 g bacterial paste following batch centrifugation. The biomass is alkaline-lysed, pooled, and the resulting flocculent precipitate is separated by a novel vacuum step, followed by depth-filtration. Downstream processing includes anion-exchange chromatography, utilizing Qiagen silica-based resin, and precipitation with isopropanol. Following precipitation, the DNA is harvested by centrifugation, dried, formulated, and sterile-filtered using a Sartorius Sartobran 150 filter prior to Final-Filling. All processing steps utilize sterilized, single-use components. This process results in a product manufactured according to regulatory guidelines. The plasmid DNA is sterile with >or=95% supercoiled DNA, an A260/A280 ratio>or=1.9, undetectable or extremely low residual endotoxin, RNA, genomic DNA, protein, and antibiotic. Residual solvent levels are negligible. The product yields the predicted profile upon restriction-enzyme digestion, is biologically active upon transfection and remains stable for several years at -20 degrees C. We have therefore developed a reproducible and cost effective process to manufacture clinical-grade plasmid DNA. This process can be adapted by other academic centers for human or large animal clinical trials.

Chromatography of plasmid DNA

Liquid chromatography plays a central role in process-scale manufacturing of therapeutic plasmid DNA (pDNA) for gene therapy and DNA vaccination. Apart from its use as a preparative purification step, it is also very useful as an analytical tool to monitor and control pDNA quality during processing and in final formulations. This paper gives an overview of the use of pDNA chromatography. The specificity of pDNA purification and the consequent limitations to the performance of chromatography are described. Strategies currently used to overcome those limitations, as well as other possible solutions are presented. Applications of the different types of chromatography to the purification of therapeutic pDNA are reviewed, and the main advantages and disadvantages behind each technique highlighted.

Spermine compaction is an efficient and economical method of producing vaccination-grade DNA

Plasmid DNA inoculations can induce both humoral and cellular immunity, and this technique is now being employed in developing vaccination regimens for a large number of applications. DNA vaccination studies require the preparation of large amounts of purified plasmid DNA with low endotoxin contamination, and the cost burden for multiple injections, multiple animal or large animal studies is significant. We recently reported that selective compaction with spermine can be used to purify large quantities of DNA. We wanted to determine whether this method would produce DNA suitable for vaccination. Endotoxin levels for spermine-compacted DNA were 0.3+/-0.01 endotoxin units (EU)/microg, well within the accepted range (less than 3 EU/microg) for in vivo use. When injected intramuscularly into mice, column-purified and spermine-compacted DNA induced an equivalent antigen-specific CD8+ T-cell response. The labor and time involved in purifying 5 mg of DNA by each method were similar, but the cost of spermine-compacted DNA was only 20% of the cost of column-purified DNA. We conclude that spermine compaction is an efficient and economical method for preparing vaccination-grade DNA.

DNA preparation method can influence outcome of transgenic animal experiments

In our continuing quest to improve the efficiency of producing transgenic animals, we have compared the influence of two transgene purification techniques on the efficiency of creating transgenic sheep and mice. Three hundred eighty-seven sheep zygotes and 2,737 mouse zygotes were microinjected with one of four transgenes. Transgenes were isolated from plasmid sequences either by agarose gel electrophoresis followed by gel extraction or by a single step sodium chloride gradient fractionation technique. Four transgenic sheep and 61 transgenic mice were produced. Both sheep and mice embryos responded similarly to transgene preparation methods. Overall, pregnancy rate was higher for recipients that received embryos injected with NaCl purified DNA (mean +/- SEM: 64 +/- 7% vs. 38 +/- 7%). Furthermore, offspring per zygote transferred (NaCl, 22 +/- 3% vs. Gel, 12 +/- 3%) and transgenics born per zygote transferred (NaCl, 3.9 +/- 0.6% vs. Gel, 1.5 +/- 0.6%) were higher when the NaCl purified DNA was used. However, the proportion of offspring born that were identified as transgenic did not differ between transgene purification methods. Transgenes responded differently to methods of preparation. One of the four genes yielded a significantly higher proportion of transgenics when the transgene was prepared by NaCl purification. These data suggest that on average the NaCl gradient purification technique results in a higher embryo survival rate to term for both sheep and mice, but the technique has no influence on rate of transgene integration.

Immunization of animals: from DNA to the dinner plate

Recently, there has been a great deal of interest in polynucleotide vaccination also referred to as DNA vaccines or genetic immunization for inducing long-term immunity in various animals and humans. The main attraction of this technology is the possibility to induce a broad range of immune responses without the use of conventional adjuvants. To date, most of the studies (>500 reports) have focused on DNA vaccination in mice. The present report summarizes the limited number of trials that have used target animal species to not only test the immune responses but also correlate them to protection.

DNA vaccines: a review

Therapeutic and prophylactic DNA vaccine clinical trials for a variety of pathogens and cancers are underway (Chattergoon et al., 1997; Taubes, 1997). The speed with which initiation of these trials occurred is no less than astounding; clinical trials for a human immunodeficiency virus (HIV) gp160 DNA-based vaccine were underway within 36 months of the first description of "genetic immunization" (Tang et al., 1992) and within 24 months of publication of the first article describing intramuscular delivery of a DNA vaccine (Ulmer et al., 1993). Despite the relative fervor with which clinical trials have progressed, it can be safely stated that DNA-based vaccines will not be an immunological "silver bullet." In this regard, it was satisfying to see a publication entitled "DNA Vaccines--A Modern Gimmick or a Boon to Vaccinology?" (Manickan et al., 1997b). There is no doubt that this technology is well beyond the phenomenology phase of study. Research niches and models have been established and will allow the truly difficult questions of mechanism and application to target species to be studied. These two aspects of future studies are intricately interwoven and will ultimately determine the necessity for mechanistic understanding and the evolution of target species studies. The basic science of DNA vaccines has yet to be clearly defined and will ultimately determine the success or failure of this technology to find a place in the immunological arsenal against disease. In a commentary on a published study describing DNA vaccine-mediated protection against heterologous challenge with HIV-1 in chimpanzees, Ronald Kennedy (1997) states, "As someone who has been in the trenches of AIDS vaccine research for over a decade and who, together with collaborators, has attempted a number of different vaccine approaches that have not panned out, I have a relatively pessimistic view of new AIDS vaccine approaches." Kennedy then goes on to summarize a DNA-based multigene vaccine approach and the subsequent development of neutralizing titers and potent CTL activity in immunized chimpanzees (Boyer et al., 1997). Dr. Kennedy closes his commentary by stating. "The most exciting aspect of this report is the experimental challenge studies.... Viraemia was extremely transient and present at low levels during a single time point. These animals remained seronegative ... for one year after challenge" and "Overall, these observations engender some excitement". (Kennedy, 1997). Although this may seem a less than rousing cheer for DNA vaccine technology, it is a refreshingly hopeful outlook for a pathogen to which experience has taught humility. It has also been suggested that DNA vaccine technology may find its true worth as a novel alternative option for the development of vaccines against diseases that conventional vaccines have been unsuccessful in controlling (Manickan et al., 1997b). This is a difficult task for any vaccine, let alone a novel technology. DNA-based vaccine technology represents a powerful and novel entry into the field of immunological control of disease. The spinoff research has also been dramatic, and includes the rediscovery of potent bacterially derived immunomodulatory DNA sequences (Gilkeson et al., 1989), as well as availability of a methodology that allows extremely rapid assessment and dissection of both antigens and immunity. The benefits of potent Th1-type immune responses to DNA vaccines must not be overlooked, particularly in the light of suggestions that Western culture immunization practices may be responsible for the rapid increases in adult allergic and possibly autoimmune disorders (Rook and Stanford, 1998). The full utility of this technology has not yet been realized, and yet its broad potential is clearly evident. Future investigations of this technology must not be hindered by impatience, misunderstanding, and lack of funding or failure of an informed collective and collaborative effort.

Effect of CpG methylation on isotype and magnitude of antibody responses to influenza hemagglutinin-expressing plasmid

We previously showed that intramuscular saline DNA immunizations favor the development of an IgG2a-dominant Th1 immune response, whereas gene gun DNA immunizations stimulate the production of an IgG1-dominant Th2 immune response. Several studies have implicated immunostimulatory CpG sequences as the causative factor in the development of Th1 immune responses to saline DNA immunization. To determine whether the Th1 cytokine-inducing properties of CpG sequences in plasmid DNA (pDNA) were responsible for the induction of a Th1 immune response, in vitro methylated and untreated (nonmethylated) hemagglutinin-expressing pDNA were compared for immunogenicity. Methylation abrogated the immunostimulatory activity of pDNA for cultured splenocytes and significantly reduced antigen expression. However, methylation of pDNA was not associated with a change from the induction of IgG2a to IgG1. After immunization with the methylated plasmid, the magnitude of the immune response was reduced. However, the decline in the total antibody response matched the decline in antigen expression. The dose of DNA or the presence of lipopolysaccharide in pDNA likewise did not affect the preferential development of an IgG2a antibody response. Our findings reveal that high levels of CpG sequences are not required for raising IgG2a-predominant, Thl-biased immune responses to intramuscular injections of hemagglutinin-expressing DNA.

Main features of DNA-based immunization vectors

DNA-based immunization has initiated a new era of vaccine research. One of the main goals of gene vaccine development is the control of the levels of expression in vivo for efficient immunization. Modifying the vector to modulate expression or immunogenicity is of critical importance for the improvement of DNA vaccines. The most frequently used vectors for genetic immunization are plasmids. In this article, we review some of the main elements relevant to their design such as strong promoter/enhancer region, introns, genes encoding antigens of interest from the pathogen (how to choose and modify them), polyadenylation termination sequence, origin of replication for plasmid production in Escherichia coli, antibiotic resistance gene as selectable marker, convenient cloning sites, and the presence of immunostimulatory sequences (ISS) that can be added to the plasmid to enhance adjuvanticity and to activate the immune system. In this review, the specific modifications that can increase overall expression as well as the potential of DNA-based vaccination are also discussed.

DNA vaccines for viral diseases

DNA vaccines, with which the antigen is synthesized in vivo after direct introduction of its encoding sequences, offer a unique method of immunization that may overcome many of the deficits of traditional antigen-based vaccines. By virtue of the sustained in vivo antigen synthesis and the comprised stimulatory CpG motifs, plasmid DNA vaccines appear to induce strong and long-lasting humoral (antibodies) and cell-mediated (T-help, other cytokine functions and cytotoxic T cells) immune responses without the risk of infection and without boost. Other advantages over traditional antigen-containing vaccines are their low cost, the relative ease with which they are manufactured, their heat stability, the possibility of obtaining multivalent vaccines and the rapid development of new vaccines in response to new strains of pathogens. The antigen-encoding DNA may be in different forms and formulations, and may be introduced into cells of the body by numerous methods. To date, animal models have shown the possibility of producing effective prophylactic DNA vaccines against numerous viruses as well as other infectious pathogens. The strong cellular responses also open up the possibility of effective therapeutic DNA vaccines to treat chronic viral infections.

Direct gene transfer to the respiratory tract of mice with pure plasmid and lipid-formulated DNA

Direct gene transfer into the respiratory system could be carried out for either therapeutic or immunization purposes. Here we demonstrate that cells in the lung can take up and express plasmid DNA encoding a luciferase reporter gene whether it is administered in naked form or formulated with cationic liposomes. Depending on the lipid used, the transfection efficiency with liposome-formulated DNA may be higher, the same as, or less than that with pure plasmid DNA. Tetramethyltetraalkylspermine analogs with alkyl groups of 16 or 18 carbons and DMRIE/cholesterol formulations proved particularly effective. Similar results for reporter gene expression in the lung were obtained whether the DNA (naked or lipid formulated) was administered by indirect, noninvasive intranasal delivery (inhaled or instilled) or by invasive, direct intratracheal delivery (injected or via a cannula). Reporter gene expression peaks around 4 days, then falls off dramatically by 9 days. The dose-response is linear, at least up to 100 microg plasmid DNA, suggesting better transfection efficiencies might be realized if there was not a volume limitation. For a given dose of DNA, the best results are obtained when the DNA is mixed with the minimum amount of lipid that can complex it completely. These results are discussed in the context of direct gene transfer for either gene therapy or delivery of a mucosal DNA vaccine.

Manufacturing and quality control of plasmid-based gene expression systems

DNA plasmid-based gene expression systems are being widely investigated for the potential treatment of genetic and acquired disease and for DNA-based vaccination. A number of human clinical trials are in progress using plasmid-based drugs. The regulatory framework that has been applied to biologicals such as recombinant DNA-derived proteins has proven to be generally applicable for regulating plasmid-based drugs as well. This was recently emphasized by the inclusion of therapeutic DNA plasmid products in the U.S. Food and Drug Administration's list of well-characterized biotechnology products. Present techniques for manufacturing and characterizing plasmids have been adapted from large-scale protein purification and from traditional molecular biology. Production of multi-gram quantities of plasmid, at purities of 95% or more, is currently possible, but further development of both manufacturing and analytical techniques is required. This review describes the approaches and methods currently used to manufacture and characterize DNA plasmids for pharmaceutical use, as well as recent changes in the regulatory environment that will impact future development and marketing of plasmids as human drugs.