Comparative Analysis of Antigen-Targeting Sequences Used in DNA Vaccines

DNA Vaccines Against Maedi-Visna Virus

Maedi-visna virus (MVV) is an ovine retrovirus of the Lentivirus genus, responsible for a chronic and progressive disease of sheep with a high prevalence all over the world. Therefore, measures aiming at the control of MVV infection are necessary, and the development of DNA vaccines may be the ideal approach. A DNA vaccine is an antigen-encoding bacterial plasmid designed to mimic infections safely, with ability to generate both humoral and cellular long-lasting immune responses once it is delivered to the host.Here, we describe the development and evaluation of DNA vaccines against ovine maedi-visna virus. The first step is the design of the vaccines, including the choice of the backbone vector and the nucleotide sequences to use as antigen-encoding sequences. Once constructed, the vaccines may be produced with high quality for use in in vitro and in vivo tests. In vitro assays are performed through transfection of animal cells to confirm the expression of the protein, while in vivo tests are carried out by mouse and/or sheep immunization in order to check humoral and cellular responses to the vaccines and conclude about their efficiency. Several approaches may be later performed in order to enhance the effectiveness of the vaccines, such as the introduction of targeting sequences, the use of a prime-boost strategy, the administration of a combined vaccine, and the use of liposomes as delivery vehicle.

Using Plasmids as DNA Vaccines for Infectious Diseases

DNA plasmids can be used to induce a protective (or therapeutic) immune response by delivering genes encoding vaccine antigens. That naked DNA (without the refinement of coat proteins or host evasion systems) can cross from outside the cell into the nucleus and be expressed is particularly remarkable given the sophistication of the immune system in preventing infection by pathogens. As a result of the ease, low cost, and speed of custom gene synthesis, DNA vaccines dangle a tantalizing prospect of the next wave of vaccine technology, promising individual designer vaccines for cancer or mass vaccines with a rapid response time to emerging pandemics. There is considerable enthusiasm for the use of DNA vaccination as an approach, but this enthusiasm should be tempered by the successive failures in clinical trials to induce a potent immune response. The technology is evolving with the development of improved delivery systems that increase expression levels, particularly electroporation and the incorporation of genetically encoded adjuvants. This review will introduce some key concepts in the use of DNA plasmids as vaccines, including how the DNA enters the cell and is expressed, how it induces an immune response, and a summary of clinical trials with DNA vaccines. The review also explores the advances being made in vector design, delivery, formulation, and adjuvants to try to realize the promise of this technology for new vaccines. If the immunogenicity and expression barriers can be cracked, then DNA vaccines may offer a step change in mass vaccination.

Antigen Delivery System II

This chapter reviews papers mostly written since 2005 that report results using live attenuated bacterial vectors to deliver after administration through mucosal surfaces, protective antigens, and DNA vaccines, encoding protective antigens to induce immune responses and/or protective immunity to pathogens that colonize on or invade through mucosal surfaces. Papers that report use of such vaccine vector systems for parenteral vaccination or to deal with nonmucosal pathogens or do not address induction of mucosal antibody and/or cellular immune responses are not reviewed.

Strategies for improving DNA vaccine performance

The goal of active vaccination is to induce all the immune effector pathways and to establish immunological memory allowing prolonged surveillance against pathogens or cancer cells. DNA vaccination platform is an intriguing strategy owing to its ability to mobilize both branches of the immune system (i.e., innate immunity as well as adaptive immunity). Since plasmids offer several advantages for biotechnological applications due to their modular structure and easy manipulation, a wide range of strategies can be applied to improve DNA vaccine performance. This chapter discusses this topic in detail taking into account antigen/epitope selection and optimization, inclusion of intracellular targeting sequences and genetic adjuvants, and provision of T cell help.

Enhancement of DNA vaccine efficacy by intracellular targeting strategies

Immune response against an encoded antigenic protein can be elicited by including targeting sequences to DNA vaccines that promote protein sorting to processing pathways, related with antigen presentation by major histocompatibility complexes (MHC). Candidate DNA vaccines coding for neuraminidase 3 of the avian influenza virus were designed to encode different sequences that direct the protein to specific cellular compartments such as endoplasmic reticulum (i.e., adenovirus E1A), lysosomes (i.e., LAMP), and the combination of protein targeting to the endoplasmic reticulum and lysosome (i.e., E1A-LAMP). The DNA vaccine prototypes were engineered by biomolecular techniques and subsequently produced in E. coli cells. The biological activity of the vaccines was tested firstly in vitro, in Chinese hamster ovary cells, through flow cytometry and real-time polymerase chain reaction analysis. Then, an essential in vivo study was performed in chickens, in order to evaluate the efficacy of DNA prototype vaccines, by measuring the antibody production by enzyme-linked immunosorbent assay.

The future of human DNA vaccines

DNA vaccines have evolved greatly over the last 20 years since their invention, but have yet to become a competitive alternative to conventional protein or carbohydrate based human vaccines. Whilst safety concerns were an initial barrier, the Achilles heel of DNA vaccines remains their poor immunogenicity when compared to protein vaccines. A wide variety of strategies have been developed to optimize DNA vaccine immunogenicity, including codon optimization, genetic adjuvants, electroporation and sophisticated prime-boost regimens, with each of these methods having its advantages and limitations. Whilst each of these methods has contributed to incremental improvements in DNA vaccine efficacy, more is still needed if human DNA vaccines are to succeed commercially. This review foresees a final breakthrough in human DNA vaccines will come from application of the latest cutting-edge technologies, including "epigenetics" and "omics" approaches, alongside traditional techniques to improve immunogenicity such as adjuvants and electroporation, thereby overcoming the current limitations of DNA vaccines in humans.

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.