New vaccine platforms are needed to address the time gap between pathogen emergence and vaccine licensure. RNA-based vaccines are an attractive candidate for this role: they are safe, are produced cell free, and can be rapidly generated in response to pathogen emergence. Two RNA vaccine platforms are available: synthetic mRNA molecules encoding only the antigen of interest and self-amplifying RNA (sa-RNA). sa-RNA is virally derived and encodes both the antigen of interest and proteins enabling RNA vaccine replication. Both platforms have been shown to induce an immune response, but it is not clear which approach is optimal. In the current studies, we compared synthetic mRNA and sa-RNA expressing influenza virus hemagglutinin. Both platforms were protective, but equivalent levels of protection were achieved using 1.25 μg sa-RNA compared to 80 μg mRNA (64-fold less material). Having determined that sa-RNA was more effective than mRNA, we tested hemagglutinin from three strains of influenza H1N1, H3N2 (X31), and B (Massachusetts) as sa-RNA vaccines, and all protected against challenge infection. When sa-RNA was combined in a trivalent formulation, it protected against sequential H1N1 and H3N2 challenges. From this we conclude that sa-RNA is a promising platform for vaccines against viral diseases.

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Tissue resident memory T cells (Trm) act as sentinels and early responders to infection. RSV specific Trm have been detected in the lungs after human RSV infection, but whether they play a protective role is unknown. To dissect the protective function of Trm, BALB/c mice were infected with RSV; infected mice developed antigen specific CD8 + Trm (CD103 + /CD69 + ) in the lungs and airways.Intranasally transferring cells from the airways of previously infected animals to naïve animals reduced weight loss on infection in the recipient mice. Transfer of airway CD8 cells led to reduced disease and viral load and increased IFNγ in the airways of recipient mice, whilst CD4 transfer reduced TNFα in the airways. Because DNA vaccines induce a systemic T cell response, we compared vaccination with infection for the effect of memory CD8 cells generated in different compartments. Intramuscular DNA immunization induced RSV specific CD8 T cells, but they were immunopathogenic not protective. Notably, there was a marked difference in the induction of Trm; infection but not immunization induced antigen specific Trm in a range of tissues. These findings demonstrate a protective role for airway CD8 against RSV and support the need for vaccines to induce antigen specific airway cells.3

The use of DNA to deliver vaccine antigens offers many advantages, including ease of manufacture and cost. However, most DNA vaccines are plasmids and must be grown in bacterial culture, necessitating elements which are either unnecessary for effective gene delivery (e.g. bacterial origins of replication) or undesirable (e.g. antibiotic resistance genes). Removing these elements may improve the safety profile of DNA for the delivery of vaccines. Here we describe a novel, double-stranded, linear DNA construct produced by an enzymatic process that solely encodes an antigen expression cassette, comprising antigen, promoter, polyA tail and telomeric ends. We compared these constructs (called ‘Doggybones’ because of their shape) with conventional plasmid DNA. Using luciferase-expressing constructs, we demonstrated that expression levels were equivalent between Doggybones and plasmids both in vitro and in vivo. When mice were immunized with DNA constructs expressing the HIV envelope protein gp140, equivalent humoral and cellular responses were induced. Immunizations with either construct type expressing haemagluttinin were protective against H1N1 influenza challenge. This is the first example of an effective DNA vaccine which can be produced on a large scale by enzymatic processes.

DNA plasmids can be used to induce a protective (or therapeutic) immune response by delivering genes encoding vaccine antigens. That naked DNA (without the refinement of coat proteins or host evasion systems) can cross from outside the cell into the nucleus and be expressed is particularly remarkable given the sophistication of the immune system in preventing infection by pathogens. As a result of the ease, low cost, and speed of custom gene synthesis, DNA vaccines dangle a tantalizing prospect of the next wave of vaccine technology, promising individual designer vaccines for cancer or mass vaccines with a rapid response time to emerging pandemics. There is considerable enthusiasm for the use of DNA vaccination as an approach, but this enthusiasm should be tempered by the successive failures in clinical trials to induce a potent immune response. The technology is evolving with the development of improved delivery systems that increase expression levels, particularly electroporation and the incorporation of genetically encoded adjuvants. This review will introduce some key concepts in the use of DNA plasmids as vaccines, including how the DNA enters the cell and is expressed, how it induces an immune response, and a summary of clinical trials with DNA vaccines. The review also explores the advances being made in vector design, delivery, formulation, and adjuvants to try to realize the promise of this technology for new vaccines. If the immunogenicity and expression barriers can be cracked, then DNA vaccines may offer a step change in mass vaccination.