Purification of essentially RNA free plasmid DNA using a modified Escherichia coli host strain expressing ribonuclease A

Advances in host and vector development for the production of plasmid DNA vaccines

Recent developments in DNA vaccine research provide a new momentum for this rather young and potentially disruptive technology. Gene-based vaccines are capable of eliciting protective immunity in humans to persistent intracellular pathogens, such as HIV, malaria, and tuberculosis, for which the conventional vaccine technologies have failed so far. The recent identification and characterization of genes coding for tumor antigens has stimulated the development of DNA-based antigen-specific cancer vaccines. Although most academic researchers consider the production of reasonable amounts of plasmid DNA (pDNA) for immunological studies relatively easy to solve, problems often arise during this first phase of production. In this chapter we review the current state of the art of pDNA production at small (shake flasks) and mid-scales (lab-scale bioreactor fermentations) and address new trends in vector design and strain engineering. We will guide the reader through the different stages of process design starting from choosing the most appropriate plasmid backbone, choosing the right Escherichia coli (E. coli) strain for production, and cultivation media and scale-up issues. In addition, we will address some points concerning the safety and potency of the produced plasmids, with special focus on producing antibiotic resistance-free plasmids. The main goal of this chapter is to make immunologists aware of the fact that production of the pDNA vaccine has to be performed with as much as attention and care as the rest of their research.

Plasmid DNA production for therapeutic applications

Plasmid DNA (pDNA) is the base for promising DNA vaccines and gene therapies against many infectious, acquired, and genetic diseases, including HIV-AIDS, Ebola, Malaria, and different types of cancer, enteric pathogens, and influenza. Compared to conventional vaccines, DNA vaccines have many advantages such as high stability, not being infectious, focusing the immune response to only those antigens desired for immunization and long-term persistence of the vaccine protection. Especially in developing countries, where conventional effective vaccines are often unavailable or too expensive, there is a need for both new and improved vaccines. Therefore the demand of pDNA is expected to rise significantly in the near future. Since the injection of pDNA usually only leads to a weak immune response, several milligrams of DNA vaccine are necessary for immunization protection. Hence, there is a special interest to raise the product yield in order to reduce manufacturing costs. In this chapter, the different stages of plasmid DNA production are reviewed, from the vector design to downstream operation options. In particular, recent advances on cell engineering for improving plasmid DNA production are discussed.

Automated alkaline lysis for industrial scale cGMP production of pharmaceutical grade plasmid-DNA

Plasmid DNA for biopharmaceutical applications is mainly produced in E. coli cells. The first and most crucial step for recovering the plasmid is the cell lysis. Governed by the physico-chemical properties of the polynucleotide, alkaline lysis has been the lysis-method of choice. This chemical disintegration technique was initially developed for the lab scale and non-pharmaceutical applications. A continuous, fully automated and closed system combining alkaline lysis, neutralization and clarification in one gentle and generic operation was developed. This system consists of a three units. One unit controls mixing and contact time during the alkaline treatment, another one controls the neutralization and the concurrent formation of flocs and a third one the separation of flocs and pDNA containing lysate. Based on optimization experiments the selected process parameters resulted in yields up to 100% and homogeneities comparable to that obtained by gentle manual lysis. The process does not need enzymes and it is scalable and routinely used for cGMP-production of pharmaceutical grade plasmid DNA from 200 L fermentations.

An efficient method of selectable marker gene excision by Xer recombination for gene replacement in bacterial chromosomes

A simple, effective method of unlabeled, stable gene insertion into bacterial chromosomes has been developed. This utilizes an insertion cassette consisting of an antibiotic resistance gene flanked by dif sites and regions homologous to the chromosomal target locus. dif is the recognition sequence for the native Xer site-specific recombinases responsible for chromosome and plasmid dimer resolution: XerC/XerD in Escherichia coli and RipX/CodV in Bacillus subtilis. Following integration of the insertion cassette into the chromosomal target locus by homologous recombination, these recombinases act to resolve the two directly repeated dif sites to a single site, thus excising the antibiotic resistance gene. Previous approaches have required the inclusion of exogenous site-specific recombinases or transposases in trans; our strategy demonstrates that this is unnecessary, since an effective recombination system is already present in bacteria. The high recombination frequency makes the inclusion of a counter-selectable marker gene unnecessary.

Bioprocess engineering issues that would be faced in producing a DNA vaccine at up to 100 m3 fermentation scale for an influenza pandemic

The risk of a pandemic with a virulent form of influenza is acknowledged by the World Health Organization (WHO) and other agencies. Current vaccine production facilities would be unable to meet the global requirement for vaccine. As a possible supplement a DNA vaccine may be appropriate, and bioprocess engineering factors bearing on the use of existing biopharmaceutical and antibiotics plants to produce it are described. This approach addresses the uncertainty of timing of a pandemic that precludes purpose-built facilities. The strengths and weaknesses of alternative downstream processing routes are analyzed, and several gaps in public domain information are addressed. The conclusion is that such processing would be challenging but feasible.

Purification of pharmaceutical-grade plasmid DNA by anion-exchange chromatography in an RNase-free process

Anion-exchange is the most popular chromatography technique in plasmid DNA purification. However, poor resolution of plasmid DNA from RNA often results in the addition of bovine-derived ribonuclease (RNase) A to degrade RNA impurities which raises regulatory concerns for the production of pharmaceutical-grade plasmid DNA. Low capacity for plasmid of most commercial media is another issue affecting the suitability of anion-exchange chromatography for large-scale processing. This study reports the use of anion-exchange chromatography to remove RNA in an RNase-free plasmid purification process. Resolution was achieved through careful selection of adsorbent and operating conditions as well as RNA reduction steps before chromatography. Dynamic capacity for plasmid was significantly increased (to 3.0mg/ml) so that it is now possible to envisage the large-scale manufacturing of therapeutic-grade plasmid DNA in the absence of added RNase using anion-exchange chromatography as a polishing step.

Thiophilic interaction chromatography for supercoiled plasmid DNA purification

Supercoiled plasmid DNA was selectively purified from its open circular form by thiophilic interaction chromatography, performed in the presence of high concentrations of water-structuring salts. To identify optimal conditions for purification, various aromatic thioether ligands were coupled to a chromatographic support and screened for their ability to separate plasmid isoforms from each other and from other host cell contaminants, including RNA, genomic DNA, protein, and endotoxins. Selectivity of the chromatographic medium depended on the structure of the ligands, with characteristics of the substituents on the aromatic ring determining the resolution between the different plasmid DNA isoforms. Optimal resolution was obtained with ligands consisting of an thioaromate, substituted with highly electronegative groups. When 2-mercaptopyridine was used as a ligand, the difference in conductivity for eluting open circular and supercoiled plasmid DNA is only 6 mS/cm. However, with 4-nitrothiophol the resolution for plasmid DNA separation on the media increased, resulting in a 20 mS/cm difference. When used in combination with a prior group separation step, these aromatic thioether ligands facilitated the isolation of highly purified supercoiled plasmid DNA, suitable for use in gene therapy and DNA vaccine applications.

Fractionation of protein, RNA, and plasmid DNA in centrifugal precipitation chromatography using cationic surfactant CTAB containing inorganic salts NaCl and NH4Cl

Centrifugal precipitation chromatography (CPC) is a separation system that mainly employs a moving concentration gradient of precipitating agent along a channel and solutes of interest undergo repetitive precipitation-dissolution, fractionate at different locations, and elute out from the channel according to their solubility in the precipitating agent solution. We report here for the first time the use of a CPC system for fractionation of protein, RNA, and plasmid DNA in clarified lysate produced from bacterial culture. The cationic surfactant cetyltrimethylammonium bromide (CTAB) was initially used as a precipitating agent; however, all biomolecules showed no differential solubility in the moving concentration gradient of this surfactant and, as a result, no separation of protein, RNA, and plasmid DNA occurred. To overcome this problem, inorganic salts such as NaCl and NH(4)Cl were introduced into solution of CTAB. The protein and RNA were found to have higher solubility with the addition of these salts and separated from the plasmid DNA. Decreasing surface charge density of CTAB upon addition of NaCl and NH(4)Cl was believed to lead to lower surfactant complexation, and therefore caused differential solubility and fractionation of these biomolecules. Addition of CaCl(2) did not improve solubility and separation of RNA from plasmid DNA.

Precipitation of RNA impurities with high salt in a plasmid DNA purification process: use of experimental design to determine reaction conditions

The use of high salt solution to precipitate RNA in a pharmaceutical-grade plasmid DNA purification process was investigated. Five antichaotropic salts were tested for their potential to precipitate RNA. Calcium chloride was by far the best precipitant with high RNA removal in a very short incubation time. Calcium chloride precipitation conditions were investigated at two stages of a plasmid purification process using experimental design techniques. The effect of up to five factors on RNA precipitation and plasmid recovery was assessed by statistical modeling. Optimized conditions for calcium chloride precipitation were then introduced to the plasmid purification process resulting in the efficient removal of most impurities (RNA, chromosomal DNA, proteins, and endotoxins).

Synthesis and chromatographic purification of recombinant human pituitary hormones

Recombinant DNA-derived proteins and, in particular, human pituitary hormones, are increasingly used for research, diagnostic and therapeutic purposes. This trend has demanded new synthetic approaches and improved purification techniques. The type and sequence of the purification steps have to be selected in accordance with the cloning and protein expression strategy, the host organism and cellular localization of the protein of interest, with a view to producing the desired product at a required purity, biological activity and acceptable cost. This review article describes and analyzes the main synthetic and purification strategies that have been used for the production of recombinant human growth hormone, prolactin, thyrotropin, luteinizing hormone and follicle-stimulating hormone, giving special consideration to the few published downstream processes utilized by the biotechnology industry. Practically all types of prokaryotic and eukaryotic organisms utilized for this purpose are also reviewed.

Removal of RNA impurities by tangential flow filtration in an RNase-free plasmid DNA purification process

Addition of animal-derived ribonuclease A to degrade RNA impurities is not recommended in the manufacture of pharmaceutical-grade plasmid DNA. Tangential flow filtration (TFF) takes advantage of the significant size difference between RNA and plasmid DNA to remove RNA in the permeate while plasmid remains in the retentate, in an RNase-free plasmid purification process. Operating conditions including transmembrane pressure, membrane pore size, conductivity of the diafiltration buffer, and plasmid load on the membrane were investigated to maximize RNA clearance. Although direct TFF of clarified lysate removed substantial amounts of RNA, the RNA levels left in the retentate were still significant. Calcium chloride is a potent precipitant of high-molecular-weight RNA. The addition of calcium chloride to the clarified lysate combined with the clearance of low-molecular-weight RNA by TFF resulted in complete RNA removal and high plasmid recovery.

A modified Escherichia coli protein production strain expressing staphylococcal nuclease, capable of auto-hydrolysing host nucleic acid

The large-scale production of recombinant biotherapeutics, particularly recombinant proteins, provides significant process and regulatory challenges to the biotechnology industry in order to meet the regulatory agencies stringent requirements in a cost-effective manner. Host cell derived nucleic acid causes problems from both a process and a regulatory perspective, as high molecular weight chromosomal DNA is responsible both for the viscosity of cell lysates, and it is a source of heterologous DNA sequences whose inclusion in the final product must be prevented. We have constructed a modified Escherichia coli JM107 expression host (JMN), containing a staphylococcal nuclease expression cassette, integrated into the host chromosome at the dif locus. The nuclease is expressed as a fusion to the ompA signal peptide, and is translocated to the periplasm of the cell, protecting the cytoplasmic nucleic acid from any toxic activity. The nuclease is released during cell lysis, where it subsequently acts to hydrolyse host nucleic acid present in the lysate. Results with this strain show that sufficient levels of nuclease activity are produced to completely auto-hydrolyse the host's chromosomal DNA to a size non-visible on 1% agarose gel, generating a markedly lower lysate viscosity. This provides a suitable methodology to remove heterologous DNA sequences early in the product stream and decrease lysate viscosity, improving the efficiency of downstream processing and product yield, whilst avoiding the addition of exogenous nuclease and its prohibitive costs at large-scale.