Genetic immunization: what's in a name?

DNA electroporation of multi-agent vaccines conferring protection against select agent challenge: TriGrid delivery system

Effective multi-agent/multivalent vaccines that confer protection against more than one disease are highly desirable to the patient and to health-care professionals. Electroporation of DNA vaccines, whereby tissues injected with DNA are subjected to localized electrical currents, is an ideal platform technology that achieves protective immune responses to multivalent vaccination. Here, we describe an electroporation-based immunization technique capable of administering a cocktail of DNA vaccinations in vivo. Immune response measurements, including protection from pathogen challenge and induction of antigen-specific antibody responses and cell-mediated immune responses, are also discussed.

Evolution of electroporated DNA vaccines

Vaccines have evolved for hundreds of years, but all utilize the premise that safely pre-exposing the host to some component of a pathogen allows for enhanced immune recognition, and potential protection from disease, upon encountering the pathogen at a later date. Early vaccination strategies used inactivated or attenuated vaccines, many of which contained toxins and other components that resulted in reactogenicity or risk of reversion to virulence. DNA vaccines supplant many of the issues associated with inactivated or attenuated vaccines, but these vaccines tend to provide weak immunological responses, particularly in primates. DNA Electroporation may prove to be the "missing link" in the evolution of DNA vaccines allowing for enhanced immune responses from DNA vaccination in humans thereby resulting in protection from disease post-pathogen exposure.

Rational design of DNA vaccines for the induction of human papillomavirus type 16 E6- and E7-specific cytotoxic T-cell responses

Many DNA vaccine candidates have been developed for the treatment of human papillomavirus type 16 (HPV16)-induced malignancies. Most of these vaccines consist of a fusion of E7 with a "carrier-protein" that functions to increase the potency of the vaccine. The nature of these carrier-proteins varies widely, and the mechanisms proposed to explain the enhanced immunogenicity of such fusions are often linked to the biological function of the carrier-protein. However, the potentiating effect of these carrier-proteins might also be explained by more general mechanisms, such as the provision of CD4+ T-cell help, increased antigen stability, or altered subcellular localization of the antigen. To assess whether these more generic mechanisms could suffice to generate highly immunogenic DNA vaccines, we evaluated a series of modular HPV16 E7 DNA vaccines in which the presence of CD4+ T-cell help, the presence of an endogenous carrier-protein, and the subcellular localization of the antigen could be systematically altered. Using this approach, we demonstrate that the addition of an element that provides CD4+ T-cell help, elements that enforce endoplasmic reticulum (ER) localization/retention are both necessary and sufficient to create markedly effective HPV16 E7-directed DNA vaccines. Importantly, the resulting design rules also apply to an HPV16 E6-directed DNA vaccine. The developed "HELP(ER)" HPV DNA vaccines encode only very limited additional sequences besides the antigen, thereby reducing the risk of antigenic competition and/or autoimmunity.

DNA vaccines and intradermal vaccination by DNA tattooing

Over the past two decades, DNA vaccination has been developed as a method for the induction of immune responses. However, in spite of high expectations based on their efficacy in preclinical models, immunogenicity of first generation DNA vaccines in clinical trials was shown to be poor, and no DNA vaccines have yet been licensed for human use. In recent years significant progress has been made in the development of second generation DNA vaccines and DNA vaccine delivery methods. Here we review the key characteristics of DNA vaccines as compared to other vaccine platforms, and recent insights into the prerequisites for induction of immune responses by DNA vaccines will be discussed. We illustrate the development of second generation DNA vaccines with the description of DNA tattooing as a novel DNA delivery method. This technique has shown great promise both in a small animal model and in non-human primates and is currently under clinical evaluation.

DNA as therapeutics; an update

Human gene therapy is the introduction of new genetic material into the cells of an individual with the intention of producing a therapeutic benefit for the patient. Deoxyribonucleic acid and ribonucleic acid are used in gene therapy. Over time and with proper oversight, human gene therapy might become an effective weapon in modern medicine's arsenal to help fight diseases such as cancer, acquired immunodeficiency syndrome, diabetes, high blood pressure, coronary heart disease, peripheral vascular disease, neurodegenerative diseases, cystic fibrosis, hemophilia and other genetic disorders. Gene therapy trials in humans are of two types, somatic and germ line gene therapy. There are many ethical, social, and commercial issues raised by the prospects of treating patients whose consent is impossible to obtain. This review summarizes deoxyribonucleic acid-based therapeutics and gene transfer technologies for the diseases that are known to be genetic in origin. Deoxyribonucleic acid-based therapeutics includes plasmids, oligonucleotides for antisense and antigene applications, deoxyribonucleic acid aptamers and deoxyribonucleic acidzymes. This review also includes current status of gene therapy and recent developments in gene therapy research.

Improved HIV-1 specific T-cell responses by short-interval DNA tattooing as compared to intramuscular immunization in non-human primates

The new intradermal DNA delivery technique, termed DNA tattooing might overcome the discrepancy between the encouraging immunogenicity results obtained with DNA vaccines in murine studies and the poor results obtained in non-human primates and humans, the so called "simian barrier". Here, we demonstrate a 10- to 100-fold increase in the magnitude of vaccine specific T-cell responses in peripheral blood from DNA tattooed rhesus macaques, as compared to T-cell responses in animals immunized via intramuscular (IM) route. A marked increase in the magnitude of the antigen specific T-cell responses as well as an increase in the number of animals responding to the immunogens was observed. These findings in non-human primates suggest that similar results may be observed in humans. Clinical trials are planned to validate tattooing as an optimal method of DNA vaccine delivery in humans.

Antibody-based Proteomics: From bench to bedside

Over the past 75 years, antibodies have gone from being recognized as disease biomarkers to being used as very powerful therapeutic tools. This evolution has been accelerated by the identification of mAb and the extensive use of immunological tools both at fundamental and clinical levels. In this review, we evaluate how antibodies can be used to assess the proteome of cells or tissues and their relevance for clinical applications. These antibody-based proteomics approaches also require analytical and technological pipelines as well as specific enabling tools which are described. Our first objective was to establish how large-scale datasets (provided by high-throughput studies such as proteomics and transcriptomics) can be integrated with literature searches and clinical data to identify potentially relevant markers against which antibodies should be raised. Then based on an extensive literature review and our experience, we compare the methodologies developed to produce specific antibodies either in vivo or in vitro. This is followed by the description of the validation tools currently available and it also includes the use of antibody-based approaches in the establishment of molecular signatures utilized at the bench and soon available for bedside use.

Protection against heartwater by DNA immunisation with four Ehrlichia ruminantium open reading frames

We have reported previously that a recombinant DNA vaccine consisting of four Ehrlichia ruminantium (Welgevonden) open reading frames (ORFs) known as the 1H12 cocktail provided protection against a virulent E. ruminantium (Welgevonden) needle challenge in sheep. In this study, we have investigated the vaccine effectiveness of two other cocktails of E. ruminantium (Welgevonden) ORFs, as well as single ORFs from the 1H12 cocktail, to protect sheep against a virulent needle challenge with the homologous strain. Each individual 1H12 ORF provided protection, but all the animals vaccinated with the other cocktails succumbed to the challenge.

Optimization and delivery of plasmid DNA for vaccination

Vaccination with DNA is one of the most promising novel immunization techniques against a variety of pathogens and tumors, for which conventional vaccination regimens have failed. DNA vaccines are able to stimulate both arms of the immune system simultaneously, without carrying the safety risks associated with live vaccines, therefore representing not only an alternative to conventional vaccines but also significant progress in the prevention and treatment of fatal diseases and infections. However, translation of the excellent results achieved in small animals to similar success in primates or large animals has so far proved to be a major hurdle. Moreover, biosafety issues, such as the removal of antibiotic resistance genes present in plasmid DNA used for vaccination, remain to be addressed adequately. This review describes strategies to improve the design and production of conventional plasmid DNA, including an overview of safety and regulatory issues. It further focuses on novel systems for the optimization of plasmid DNA and the development of diverse plasmid DNA delivery systems for vaccination purposes.

Characterization of monoclonal antibodies against prostate specific antigen produced by genetic immunization

Prostate specific antigen (PSA) is the most important marker for prostate cancer. Antibodies against minor variants of PSA may be useful in the development of novel diagnostic tests for prostate cancer, but it has been difficult to produce such antibodies by protein immunization. In this study, we have compared the characteristics of monoclonal antibodies (MAbs) obtained by genetic immunization with those obtained by protein immunization. The whole coding region of PSA-cDNA was cloned in a mammalian expression vector pCDNA-3. Six mice were immunized four times by intra-muscular (i.m.) injection of the PSA-pCDNA3 plasmid. The MAbs produced were characterized with respect to subclass, epitope specificity, binding to various molecular forms of PSA and affinity. After intra-muscular injection of DNA, anti-PSA antibodies were detected in the serum of all mice, but the antibody titers were markedly lower than after protein immunization. After fusion of the spleen cells from the mice, five hybridomas producing MAbs to PSA were obtained. The MAbs were of IgG1 and IgG2a isotype and they all recognized equally different forms of free PSA, namely enzymatically active, nicked and proPSA. Epitope mapping showed that these MAbs reacted with the same antigenic regions as those obtained by protein immunization. Thus, genetic immunization leads to production of anti PSA MAbs with similar characteristics to those obtained by immunizing with PSA protein. As applied in the present study, it is less efficient than protein immunization, but it is a useful technique when the antigen is not available in the quantities needed for immunization.

DNA-based therapeutics and DNA delivery systems: a comprehensive review

The past several years have witnessed the evolution of gene medicine from an experimental technology into a viable strategy for developing therapeutics for a wide range of human disorders. Numerous prototype DNA-based biopharmaceuticals can now control disease progression by induction and/or inhibition of genes. These potent therapeutics include plasmids containing transgenes, oligonucleotides, aptamers, ribozymes, DNAzymes, and small interfering RNAs. Although only 2 DNA-based pharmaceuticals (an antisense oligonucleotide formulation, Vitravene, (USA, 1998), and an adenoviral gene therapy treatment, Gendicine (China, 2003), have received approval from regulatory agencies; numerous candidates are in advanced stages of human clinical trials. Selection of drugs on the basis of DNA sequence and structure has a reduced potential for toxicity, should result in fewer side effects, and therefore should eventually yield safer drugs than those currently available. These predictions are based on the high selectivity and specificity of such molecules for recognition of their molecular targets. However, poor cellular uptake and rapid in vivo degradation of DNA-based therapeutics necessitate the use of delivery systems to facilitate cellular internalization and preserve their activity. This review discusses the basis of structural design, mode of action, and applications of DNA-based therapeutics. The mechanisms of cellular uptake and intracellular trafficking of DNA-based therapeutics are examined, and the constraints these transport processes impose on the choice of delivery systems are summarized. Finally, the development of some of the most promising currently available DNA delivery platforms is discussed, and the merits and drawbacks of each approach are evaluated.

Genetic vaccination against murine cysticercosis by using a plasmid vector carrying Taenia solium paramyosin

A plasmid vector carrying the immunoprotective amino-terminal fragment of Taenia solium paramyosin (VW2-1) was designed for genetic vaccination studies. Mice that were genetically immunized with VW2-1 and challenged by intraperitoneal inoculation of Taenia crassiceps cysticerci showed 43 to 48% reductions in the parasite burden, values which were similar to values obtained previously when the recombinant protein was used.

Immobilization of plasmid DNA in bacterial ghosts

The development of novel delivery vehicles is crucial for the improvement of DNA vaccine efficiency. In this report, we describe a new platform technology, which is based on the immobilization of plasmid DNA in the cytoplasmic membrane of a bacterial carrier. This technology retains plasmid DNA (Self-Immobilizing Plasmid, pSIP) in the host envelope complex due to a specific protein/DNA interaction during and after protein E-mediated lysis. The resulting bacterial ghosts (empty bacterial envelopes) loaded with pDNA were analyzed in detail by real time PCR assays. We could verify that pSIP plasmids were retained in the pellets of lysed Escherichia coli cultures indicating that they are efficiently anchored in the inner membrane of bacterial ghosts. In contrast, a high percentage of control plasmids that lack essential features of the self-immobilization system were expelled in the culture broth during the lysis process. We believe that the combination of this plasmid immobilization procedure and the protein E-mediated lysis technology represents an efficient in vivo technique for the production of non-living DNA carrier vehicles. In conclusion, we present a "self-loading", non-living bacterial DNA delivery vector for vaccination endowed with intrinsic adjuvant properties of the Gram-negative bacterial cell envelope.

Type 1 interferon augments DNA-based vaccination against hepatitis C virus core protein

Eradication of chronic hepatitis C virus (HCV) infection depends upon a broad-based cellular immune response. Genetic immunization stimulates such a response, but the resultant activity is generally weak. Type 1 interferons (IFNs), which are known for their direct anti-viral and anti-proliferative properties, possess vigorous immunomodulatory properties. The aim of this study was to assess the capacity of IFN-alpha to augment the cellular immune response to DNA vaccination against HCV core protein. Three types of IFN-alpha were investigated: the non-species-specific hybrid IFN A/D, human pegylated IFN-alpha, and a plasmid that expressed murine IFN-alpha. Low doses of hIFN-A/D and hPegIFN-alpha augmented three to fourfold the cellular immune response to DNA-based vaccination, determined in conventional CTL assays, as well as in an in vivo tumor challenge model. Importantly, augmentation occurred within a narrow concentration range; a further increase in IFN dosage suppressed the CTL response significantly. Humoral immunity showed a very similar pattern of augmentation. These findings demonstrate that the immunomodulatory properties of IFN-alpha can be exploited to augment DNA based immunization, but it is important to consider the effects of dose on both cellular and humoral immune response for optimal augmentation.

Molecular adjuvants for mucosal immunity

Mucosal surfaces represent the entry route of a multitude of viral pathogens. For many of these viruses, such as the herpes simplex viruses and human immunodeficiency virus, no effective vaccine exists. Hence, it is important that prospective vaccines engender maximal immunity at these susceptible sites. Genetic vaccines encoding adjuvant molecules represent one approach to optimize mucosal as well as systemic immunity. Promising candidates include various inflammatory cytokines and chemokines that might be used to enhance the primary response to a level sufficient for protection. Encouraging studies involving cytokines such as granulocyte/macrophage colony-stimulating factor, interleukin-2 (IL-2), IL-12, IL-18, and many others are examined. Notable chemokines that may offer hope in such efforts include IL-8, RANTES, CCL19, CCL21, and a few others. Combinatorial approaches utilizing several cytokines and chemokines will most likely yield the greatest success. In addition, as more is discovered regarding the requirements for memory development of T cells, boosters involving key cytokines such as IL-15 and IL-23 may prove beneficial to long-term maintenance of the memory pool. This review summarizes the progress in the use of genetic vaccines to achieve mucosal immunity and discusses the needed strategies to maximize long-term prospective immunity at this vulnerable entry site.

Approaches to enhance the efficacy of DNA vaccines

DNA vaccines consist of antigen-encoding bacterial plasmids that are capable of inducing antigen-specific immune responses upon inoculation into a host. This method of immunization is advantageous in terms of simplicity, adaptability, and cost of vaccine production. However, the entry of DNA vaccines and expression of antigen are subjected to physical and biochemical barriers imposed by the host. In small animals such as mice, the host-imposed impediments have not prevented DNA vaccines from inducing long-lasting, protective humoral, and cellular immune responses. In contrast, these barriers appear to be more difficult to overcome in large animals and humans. The focus of this article is to summarize the limitations of DNA vaccines and to provide a comprehensive review on the different strategies developed to enhance the efficacy of DNA vaccines. Several of these strategies, such as altering codon bias of the encoded gene, changing the cellular localization of the expressed antigen, and optimizing delivery and formulation of the plasmid, have led to improvements in DNA vaccine efficacy in large animals. However, solutions for increasing the amount of plasmid that eventually enters the nucleus and is available for transcription of the transgene still need to be found. The overall conclusions from these studies suggest that, provided these critical improvements are made, DNA vaccines may find important clinical and practical applications in the field of vaccination.