Induction of immune responses by DNA vaccines in large animals
Introduction
It has been just over 10 years since the first reports regarding genetic (polynucleotide) immunization were published [1], [2], [3]. The original excitement of developing vaccines using this technology even heralded the technology as the “third generation of vaccinology” [4]. As a result, over 1000 different reports of this technique have been published with antigens from different bacteria, viruses, and parasites (reviewed in [5], [6]). Unfortunately, the majority of the most successful demonstrations of the efficacy of DNA immunization have been performed in the mouse. When similar approaches were used in humans or large animals the results were not as encouraging. Although polynucleotide vaccines have not been very efficient in large animals or in humans there continues to be an interest in developing this technology for these species since DNA vaccination has many advantages over conventional vaccines. These advantages include the endogenous production of antigen. This is a tremendous advantage, especially for viral antigens where all the post translation modifications are similar following DNA immunization as those present during infection. As a result, the antigens are authentic with all the conformational epitopes required for protection being expressed. A second advantage is that since the animal acts as a bioreactor, there is no need for downstream processing of the vaccine after the plasmid is purified. Antigen purification is often a laborious and expensive process. Thus, DNA-based vaccination should be economical to produce. Since the antigen is produced endogenously there is also no need for adjuvants or problems associated with injection site reactions produced by adjuvants [7]. Because of the endogenous expression of antigens, plasmid-based vaccines induce a more balanced Th1/Th2 like immune response which is important for clearance of many viral infections [8]. Possibly one of the major advantages of DNA vaccines is that they can induce immune responses in neonates even in the presence of passive antibody [9], [10]. Recently, we have demonstrated that it is even possible to induce immunity in utero [11]. This has the real advantage of insuring that individuals are born immune to pathogens which they may experience early in their life. For example, transmission of herpes simplex, hepatitis B, human immunodeficiency virus, Group B Streptococcus, Haemophilus influenzae type B, and Chlamydia sp. occurs shortly before, during, or after birth. To reduce the risk of vertical disease transmission of these diseases, Cesarean sections, antibiotic treatments, or maternal antiviral therapy during the last trimester are used where available, together with improved neonatal care. Unfortunately, none of these approaches completely eliminates the risk of neonatal infection. Therefore, there is a need for developing vaccines that could be employed in these instances. Finally, since DNA vaccines are simple to purify, and technologies are available for purification the risk of extraneous contaminating agents, which are a major problem in conventional live vaccines is eliminated [12], [13].
The reasons for the lower efficacy of DNA vaccines in humans or large animals are not currently known, but it could be related to the transfection efficiency. In mouse cells, transfection appears to occur with much more efficiency than in larger animals species. Thus, improving delivery of the plasmid to enhance cellular transfection and subsequent expression of proteins will be a critical factor in developing vaccines for large animals and humans. The present report describes the progress made in the last few years in addressing some of the factors influencing the efficacy of DNA vaccines, specifically focusing on improving transfection efficiency and gene expression by improved vectors and delivery systems.
Section snippets
Enhancing antigen expression
The simplest concept of a DNA based vaccine incorporates a promoter, the gene of interest for use in the vaccine, and a backbone for delivery of the cassette into cells. Manipulation of each of these different components of the plasmid has been shown to alter the efficacy of DNA vaccination. Since it is generally believed that the level of gene expression should correlate with the level of immunity, the plasmid construct should be one that maximizes gene expression in vitro. If the transfection
Delivery
Although it is difficult to quantitate the number of plasmids that enter cells or are degraded before they enter the nucleus and initiate gene expression it is believed that in excess of 90% of the DNA never gets into the cytoplasm and of this 10% less than 1% enter the nucleus where gene expression occurs [39], [69]. Thus, there have been numerous approaches used to enhance plasmid uptake not only into the cell but into the nucleus. The earlier approaches focused on lipid based delivery
Mucosal delivery
Since the majority of pathogens enter through mucosal surfaces of the gastrointestinal tract, respiratory and reproductive tracts, delivery of vaccines to induce mucosal immunity would be a major achievement [89], [90]. Thus, if it was possible to prevent infection, by neutralizing the pathogen at the site of entry, this would be more effective than preventing disease after infection, as is the case with many vaccines given parenterally. Secondly, by inducing mucosal immunity, the amount of
References (92)
- et al.
Nucleic acids: vaccines of the future
Parasitol. Today
(1995) - et al.
Nucleic acid vaccines: research tools or commercial reality
Vet. Immunol. Immunopathol.
(2000) - et al.
DNA vaccines: a review
Adv. Res.
(1999) A survey of mycoplasma detection in veterinary vaccines
Vaccine
(1986)- et al.
Effect of different promoters on immune responses elicited by HIV-1 gag/env multigenic DNA vaccine in Macaca mulatta and Macaca nemestrina
Vaccine
(2000) - et al.
Comparison of various expression plasmids for the induction of immune response by DNA immunization
Mol. Cells
(1997) - et al.
Immune responses and protection induced by DNA vaccines encoding bovine parainfluenza virus type 3 glycoproteins
Virology
(1999) - et al.
Evaluation of eukaryotic promoters for the construction of DNA vaccines for aquaculture
Genet. Anal.
(1999) - et al.
A single immunization with a plasmid encoding hepatitis C virus (HCV) structural proteins under the elongation factor 1-alpha promoter elicits HCV-specific cytotoxic T-lymphocytes (CTL)
Vaccine
(1999) - et al.
Muscle-specific expression of hepatitis B surface antigen: no effect on DNA-raised immune responses
Virology
(1999)