Abstract
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It was 1993 when the first cluster of papers started to appear in major biomedical journals reporting the success of DNA vaccines in various animal models against a wide range of pathogens. DNA vaccine was quickly considered the third generation of vaccines after the traditional live-attenuated and inactivated vaccines (the first generation) and the recombinant protein or subunit vaccines (the second generation).
However, a quarter century later, we still do not have a licensed DNA vaccine for human applications, although solid progress has been made, including one major DNA vaccine that was just licensed a few months ago by China's Ministry of Agriculture to prevent H5 subtype avian influenza infection in chickens.
The HGT Special Issue on DNA vaccines does not intend to provide a comprehensive update in this dynamic field. Rather, a collection of work included here reflects the key questions being asked at the current moment. Several authors are pioneers of DNA vaccines, and since the early 1990s, their work has made major contributions to the advancement of this field. It is our hope that papers in the HGT Special Issue reflect the most updated thinking on the future of DNA vaccines.
To help readers who did not follow the original DNA vaccine work, Harriet Robinson et al. provide a historical reflection on the DNA vaccine development in those early days. 1 Same as other true technology breakthroughs, DNA immunization initially was met with great skepticism. At one point, the whole lab was surviving on a bridging departmental funding. “We knew we were onto something, and kept going,” write the authors. The persistent work from Robinson et al. not only finally convinced the world that DNA vaccination is real science, but also brought several HIV vaccine formulations into human studies to confirm the ultimate clinical utility.
Similar to other gene-based therapies or vaccines, the delivery technology plays a key role in the final efficacy of DNA vaccines. Studies in both animals and humans in the last two decades have demonstrated that the physical delivery approaches such as gene gun and electroporation (EP) are at least 10 times more potent than using the traditional needle injections, even with the optimized lipid or nanoparticle formulations. In the current issue, two papers investigated the other optimal delivery of DNA vaccines.
Zhang et al. explored the use of microneedles to improve the immunogenicity of DNA vaccines. 2 They mainly monitored the antigen expression of a DNA vaccine at various time points after immunization (6–144 h) when it was delivered at the epidermis by a 6 × 6 array of microneedles (220 μm high), as depicted in the cover art of this issue. This delivery approach led to substantial greater expression of antigen than the traditional needle injection, and elevated antigen expression was followed by 6–10 times higher levels of antibody and T-cell responses.
Robin Shattock's group conducted detailed analysis on the influence of intradermal (i.d.) and intramuscular (i.m.) injections, with or without EP, to the quality of induced cellular and humoral immune responses following the immunization of a DNA vaccine expressing HIV-1 clade C Env antigen. 3 Their data indicate that the concurrent i.d./i.m. vaccination with EP is a highly activator of both cellular and humoral response. This approach generated both T- and B-cell responses that were dramatically amplified by a homologous protein boost, even in the absence of any adjuvant.
Besides the optimization of delivery approaches, the prime-boost approach has emerged as another major strategy to enhance the immune responses elicited by a DNA vaccine. In such designs, the DNA vaccines were used as the priming immunization, followed with the boost immunization using a matched or similar recombinant protein vaccine. DNA prime can also be combined with viral vector vaccines. Felber et al. compared the relative efficacy between DNA and recombinant MVA vector as the boost in nonhuman primates that were immunized with a DNA vaccine expressing conserved elements of HIV-1 Gag. 4 Their results show that both DNA and rMVA can boost such immune responses. Vaccine regimens that employ DNA vaccine expressing conserved Gag antigen as the prime hold promise for the application in HIV prevention and therapy.
The discovery of DNA vaccine also provides the opportunity to improve certain currently available vaccines. A good example is the influenza vaccines. While influenza vaccines have been used in humans for a long time, the existing influenza vaccines are far from ideal. In particular, new influenza vaccines have to be developed to replace the existing ones due to antigen drift within a given subtype or a major type of influenza viruses. The immunogenicity of existing vaccines is also weak, especially for inactivated influenza vaccines. There is significant interest in developing novel influenza vaccines. In the current HGT issue, two papers tried to use DNA vaccination approach for influenza applications.
David Weiner, another DNA vaccine pioneer, led his group to develop four micro-consensus antigens designed to mimic the sequence and antigenic diversity of H3 HA antigen. 5 Synthetic plasmid DNA constructs were formed to express each micro-consensus immunogen, and the combination of these four DNA vaccines formed a DNA vaccine cocktail. Immunization in mice with this cocktail formulation induced comprehensive, potent humoral responses against diverse seasonal H3 subtype viruses ranging from 1968 to the present. Antigen-specific T-cell responses were also induced. These immunized mice were protected against lethal challenge with two distinct H3 viruses, supporting further studies to validate the use of micro-consensus based influenza DNA vaccines.
Ivanova et al. developed a novel DNA vaccine that expresses a fusion protein antigen, including a scFV fragment from mouse anti-human FcγRI μoνoχλoναλ antibody and a sequence encoding a T- and B-cell epitope-containing influenza A virus HA antigen. 6 The role of anti-FcγRImonoclonal antibodies (mAbs) is to serve as a strong adjuvant. The authors immunized the immunodeficient NOD-SCID gamma (NSG) mice that received normal human lymphoid cells as a humanized mouse model. Immunization of this DNA vaccine, and the prime boost with a matching recombinant protein antigen, induced significant serum anti-influenza antibodies and strong CTL activity against influenza virus-infected cells in humanized mice.
The utility of DNA immunization is not limited to preventive vaccines against infectious diseases. Wu's group reported the use of DNA as therapeutic vaccines against human papillomavirus and associated diseases. 7 This is a quite comprehensive review covering almost every key question in this application. It focuses on the current state of therapeutic HPV DNA vaccines, including results from recent and ongoing clinical trials. It also outlines different strategies that have been employed to improve their potencies. The use of DNA vaccine to treat cancer holds great potential as an innovative treatment strategy.
Finally, DNA immunization is a powerful tool to elicit challenging mAbs. Liu et al. summarize their progress in using DNA immunization to produce mAbs in mice, rabbits, and humans. 8 DNA immunization provided two unique benefits over the traditional protein- or peptide-based immunizations for the induction of mAbs. First, DNA immunization is highly efficient in eliciting antigen-specific B-cell development due to the ability of DNA vaccines to enhance innate immunities, in addition to activation of germinal center B cells through the function of Tfh cells. Second, antigens delivered by DNA immunization and expressed in vivo would have a much better chance of preserving the sensitive conformation of antigens, especially to those antigens difficult to produce in vitro. With the exciting progress in mAb-based immune therapies, there are great demands for more potent and functional mAbs as therapeutic agents. DNA immunization will be able to contribute to this new application.
Eight papers included in the current HGT issues provide a nice snapshot of the current DNA vaccine field. It is no longer an issue whether DNA vaccine is immunogenic in humans. As a gene-based interventional approach, the optimization of delivery will remain important. More importantly, DNA vaccine needs to find its unique place in the overall biomedical field, whether being the solution for a dreamed HIV vaccine, an improved influenza vaccine, a novel immunotherapy to cancers, or a tool to develop special mAbs for broad indications. The discovery made 25 years ago will have a much broader and brighter future than what the pioneers dreamed.