The future of human DNA vaccines

Abstract

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.

Figure 1. Mechanisms of DNA vaccines

Recent studies found that TLR-9 is dispensable for DNA vaccines, while TBK1 and AIM2 pathways were shown to be critical for plasmid DNA induced innate and adaptive immune responses. However, the upstream bona fide B DNA sensor still remains unknown.

Figure 2. Potential design strategies and technologies for future DNA vaccine development

(a) With the ease of DNA synthesis and manipulation, a lot of high throughput technologies, e.g. gene expression arrays, proteomics, genomics, transcriptomics, reverse vaccinology and RNAi screening platforms, which were accompanied by comprehensive bioinformatics tools, can efficiently map and find the DNA sequences encoding optimal antigens for vaccine development. (b) After selecting the candidate pathogen DNA sequences, codon optimization and promoter design are the main two steps before cloning into expression vectors. Simple codon conversion and using the strong viral promoter will most likely result in higher gene expression, but controversial effects were also reported. The more precise algorithm is anticipated for this purpose in the field of plasmid DNA based therapy or vaccination. (c) The optimized DNA fragments are then cloned into expression vector to test expression. To further optimize the DNA vaccine, minicircle DNA technology (d) will be used to completely remove the bacterial elements and incorporate S/MAR sequence (e) to enhance expression and safety. (f) and (g) Epigenetics mechanisms are closely related with most of the above steps and will be applied in DNA vaccine design. (h) The immunogenicity of the DNA vaccine construction could be further increased by injection of the optimized construction using new delivery devices (e.g. EP) and/or by using prime/boost regimen. See the article for details.