Traditional biodefense vaccines were designed by using either live attenuated or inactivated vaccine approaches. As discussed in the above sections, live attenuated vaccines have the benefit of normally inducing stronger protective immunity than inactivated vaccines but are not ideal candidates when facing the challenges related to manufacturing processes or more critically, to improve the overall safety profile of biodefense vaccines. Furthermore, regulatory authorities expect to see the mechanism of attenuation to be well-characterized. On the other hand, inactivated vaccines, although safe in general, are usually not very immunogenic, require multiple immunizations to reach the protective levels of immunity, and have not been shown to be good inducers of T cell immunity when used alone.
Given that it takes 18-20 years to develop a vaccine in many cases, there are major initiatives to develop new platform technologies where selected protective antigens, or genes for such antigens, but not the whole pathogen as in the cases of live attenuated and inactivated vaccines, can be incorporated to generate a vaccine against a biodefense agent at short notice. At the present time, there is still a long way to achieve this goal but there are a number of promising platform technologies.
In the near term, recombinant protein-based vaccines are attractive alternatives because they can be produced using a highly standardized manufacturing process, are safer than using the entire pathogen (either live or inactivated), and should be no less immunogenic than inactivated vaccines if a proper adjuvant is included in the final formulation. Examples of the successes of such platform technology in the public health arena are hepatitis B virus (HBV) and human papillomavirus (HPV) vaccines. Recombinant protein-based vaccines represent several forerunners of the newer generation of biodefense vaccines, especially for bacterial pathogens with well-characterized protective antigens. For plague vaccines, recombinant proteins F1 and V have been well-established as key protective antigens and vaccines based on these proteins have entered clinical studies [16
]. Vaccines based on the recombinant protein PA antigen have been the leading candidate for a newer generation of anthrax vaccines [8
]. For certain toxin-producing bacterial pathogens, recombinant protein-based subunit vaccines based on modified toxins are also replacing the traditional toxoid as the protective antigen, such as in the case of Botulinum
]. Despite the advantages for recombinant protein vaccines, one key weakness for subunit-based vaccines is their poor immunogenicity for T cell responses.
Gene-based vaccines, on the other hand, have emerged in the last decade as a completely novel strategy for vaccination [18
]. At first, their ability to induce antigen-specific T cell responses was considered as the main strength. Over time, however, it became clear that gene-based vaccines are also effective in eliciting antibody responses. Gene-based vaccines include DNA vaccines and vector-based vaccines. Vectors can be either viral or bacterial, but more biodefense vaccines use viral vectors over bacterial vectors. Although both DNA vaccines and vector-based vaccines incorporate a natural or modified gene from a pathogen, which encodes the protective antigen, they differ in many ways. First, DNA vaccines can be delivered directly in the form of plasmids whereas the vector approach usually required the production of a large stock of highly concentrated, packaged vector vaccines. Second, most DNA vaccines generally do not contain unrelated proteins in the construct. The vaccinated hosts will only generate immune responses against the biodefense antigen expressed by the DNA vaccines. In contrast, vector-based vaccines express other antigens as part of the original vector virus or bacteria. Immune responses against vector components can generate several negative effects. For hosts who have been exposed to the same vector in the past, such as in the case serotype 5 of adenovirus (Ad5), pre-existing immune responses against the Ad5 can reduce the ability of a vaccine using Ad5 vector as the delivery system [20
]. Anti-vector immunity may also interfere with protection of vector-based vaccines, as was observed during the STEP trial, a large international HIV vaccine clinical trial co-sponsored by the National Institute of Allergy and Infectious Diseases and the pharmaceutical company Merck & Co. Inc., in which people with high pre-existing anti-Ad5 antibody responses had a higher chance of being infected by HIV-1 when these people were immunized with an Ad5-based HIV-1 vaccine [21
As shown in , the DNA vaccine approach has been tested for almost every biodefense pathogen due to the relatively simple nature of this approach. Vector-based vaccines have also been tested for many biodefense pathogens. We have included in key references of published work on these gene-based vaccination approaches for each of the key pathogens included in the current Supplement. Most of these studies were conducted in small animal models and were successful in eliciting positive immunogenicity results against these biodefense pathogens, and some obtained data on protection in the cases where validated animal models exist.
The key information missing for many of these studies is the comparison between gene-based vaccines and other vaccines using traditional approaches. Such comparison would provide valuable information on the actual improvement of immunogenicity with these gene-based vaccination approaches over the existing and imperfect vaccines. However, the main challenge for gene-based vaccines is the overall low immunogenicity demonstrated in human studies for these vaccines [22
]. But significant progress has been made in at least three key areas in recent years to address this issue. Molecular adjuvants, in the form of genes coding for immune-stimulating cytokines, have been shown to significantly improve the overall immunogenicity of DNA vaccines in non-human primate models [23
]. Physical delivery approaches, such as gene gun and electroporation, were shown more effective in eliciting higher levels of immune responses than the traditional needle injection method [25
]. Finally, the prime-boost strategies in which DNA or vector based vaccines were used in combination with another form of vaccines, such as in the case of DNA prime-protein boost [27
], or DNA prime-Ad5 vector boost [28
], have shown real promise in eliciting high and balanced antibody and T cell immune responses in both animal and human studies.