Novel linear DNA vaccines induce protective immune responses against lethal infection with influenza virus type A/H5N1

DNA synthesis technologies to close the gene writing gap

Synthetic DNA is of increasing demand across many sectors of research and commercial activities. Engineering biology, therapy, data storage and nanotechnology are set for rapid developments if DNA can be provided at scale and low cost. Stimulated by successes in next generation sequencing and gene editing technologies, DNA synthesis is already a burgeoning industry. However, the synthesis of >200 bp sequences remains unaffordable. To overcome these limitations and start writing DNA as effectively as it is read, alternative technologies have been developed including molecular assembly and cloning methods, template-independent enzymatic synthesis, microarray and rolling circle amplification techniques. Here, we review the progress in developing and commercializing these technologies, which are exemplified by innovations from leading companies. We discuss pros and cons of each technology, the need for oversight and regulatory policies for DNA synthesis as a whole and give an overview of DNA synthesis business models.

Linear DNA vaccine prepared by large-scale PCR provides protective immunity against H1N1 influenza virus infection in mice

Linear DNA vaccines provide effective vaccination. However, their application is limited by high cost and small scale of the conventional polymerase chain reaction (PCR) generally used to obtain sufficient amounts of DNA effective against epidemic diseases. In this study, a two-step, large-scale PCR was established using a low-cost DNA polymerase, RKOD, expressed in Pichia pastoris. Two linear DNA vaccines encoding influenza H1N1 hemagglutinin (HA) 1, LEC-HA, and PTO-LEC-HA (with phosphorothioate-modified primers), were produced by the two-step PCR. Protective effects of the vaccines were evaluated in a mouse model. BALB/c mice were immunized three times with the vaccines or a control DNA fragment. All immunized animals were challenged by intranasal administration of a lethal dose of influenza H1N1 virus 2 weeks after the last immunization. Sera of the immunized animals were tested for the presence of HA-specific antibodies, and the total IFN-γ responses induced by linear DNA vaccines were measured. The results showed that the DNA vaccines but not the control DNA induced strong antibody and IFN-γ responses. Additionally, the PTO-LEC-HA vaccine effectively protected the mice against the lethal homologous mouse-adapted virus, with a survival rate of 100% versus 70% in the LEC-HA-vaccinated group, showing that the PTO-LEC-HA vaccine was more effective than LEC-HA. In conclusion, the results indicated that the linear H1N1 HA-coding DNA vaccines induced significant immune responses and protected mice against a lethal virus challenge. Thus, the low-cost, two-step, large-scale PCR can be considered a potential tool for rapid manufacturing of linear DNA vaccines against emerging infectious diseases.

Novel synthetic plasmid and Doggybone DNA vaccines induce neutralizing antibodies and provide protection from lethal influenza challenge in mice

Nucleic acid-based vaccines (NAVs) are a promising alternative to conventional influenza vaccines with the potential to increase influenza vaccine availability due to their simplicity in design and rapid speed of production. NAVs can also target multiple influenza antigens and control flu variants. Traditionally NAVs have been DNA plasmids however, we are continuing to explore new methods that may enhance vaccine efficacy. Recently new focus has been on RNA cassettes as NAVs. RNA vaccines combine conceptual advantages in that they focus on delivery of only the coding cassette. However, RNA vaccines have a short half-life and cause interferon-induced fevers. Here we describe a new NAV approach where we study delivery of a linear DNA cassette [Doggybone™ linear closed DNA [(dbDNA™)] produced by an enzymatic process that yields an antigen expression cassette comprising a promoter, DNA antigen, poly A tail, and telomeric ends. This focused approach has many of the advantages of plasmid DNA as well as a minimal cassette size similar to RNA strategies. For this study, we characterized the specific CD4⁺ and CD8⁺ T cell responses and determined the hemagglutination inhibition (HI) titers induced by dbDNA™ and compared the responses with those of an optimized plasmid DNA (pDNA) vaccine encoding the same H1N1 influenza A/PR/8/34 HA gene. Immunizations with the constructs resulted in similar humoral and cellular immune responses. Both constructs induced high-titer HI antibodies and fully protected animals from lethal viral challenge. The data obtained from this study provides important validation for further development of novel vector approaches.

Influenza A vaccines using linear expression cassettes delivered via electroporation afford full protection against challenge in a mouse model

Alternative DNA vaccine constructs such as fully synthetic linear expressing cassettes (LECs) offer the advantage of accelerated manufacturing techniques as well as the lack of both antibiotic resistance genes and bacterial contaminants. The speed of manufacture makes LEC technology a possible future vaccination strategy for pandemic influenza outbreaks. Previously, we reported on a novel concept of DNA delivery to dermal tissue by a minimally invasive electroporation (EP) surface device powered using low voltage parameters. This device allows electroporation without penetration of electrodes into the skin. In addition to enhancing the delivery of traditional plasmid DNA vaccines, this device may also offer a safe, tolerable and efficient method to administer LECs. To assess immunogenicity and efficacy of EP-enhanced LEC delivery in mice, we designed and tested two influenza antigens in the form of LEC constructs delivered using the newly developed surface dermal EP device. Strong CTL and antibody responses were induced by the LEC versions of the DNA vaccine. When challenged with A/Canada/AB/RV1532/2009 viruses, mice immunized with LEC encoding the M2 and NP antigens recovered faster than naïve or mice immunized ID without EP. Mice immunized with equal-molar doses of LEC encoding the M2 and NP antigens demonstrated 100% survival following a lethal (100× LD50) challenge of the heterologuos and highly pathogenic H5N1 influenza virus (A/Vietnam/1203/04). These results suggest that influenza DNA vaccines based on LEC technology combined with the surface delivery platform are capable of fully protecting mice in a lethal challenge and the LEC based DNA constructs may serve as viable vaccine candidates.

Vaccine protection against lethal homologous and heterologous challenge using recombinant AAV vectors expressing codon-optimized genes from pandemic swine origin influenza virus (SOIV)

The recent H1N1 influenza pandemic and the inevitable delay between identification of the virus and production of the specific vaccine have highlighted the urgent need for new generation influenza vaccines that can preemptively induce broad immunity to different strains of the virus. In this study we have produced AAV-based vectors expressing the A/Mexico/4603/2009 (H1N1) hemagglutinin (HA), nucleocapsid (NP) and the matrix protein M1 and have evaluated their ability to induce specific immune response and protect mice against homologous and heterologous challenge. Each of the vaccine vectors elicited potent cellular and humoral immune responses in mice. Although immunization with AAV-M1 did not improve survival after challenge with the homologous strain, immunization with the AAV-H1 and AAV-NP vectors resulted in survival of all mice, as did inoculation with a combination of all three vectors. Furthermore, trivalent vaccination also conferred partial protection against challenge with the highly heterologous and virulent A/PR/8/34 strain of H1N1 influenza.

Immune response induced by a linear DNA vector: influence of dose, formulation and route of injection

Previously, minimalistic, immunogenetically defined gene expression (MIDGE) vectors were developed as effective and sophisticated carriers for DNA vaccination. Here we evaluate the influence of dose, formulation and delivery route on the immune response after vaccination with MIDGE-Th1 vectors encoding hepatitis B virus surface antigen (HBsAg). An HBsAg-specific IgG1 and IgG2a antibody response was induced in a dose-dependent manner, whereas the IgG2a/IgG1 ratio was independent of the injected DNA dose. Formulation of MIDGE-HBsAg-Th1 with the cationic pyridinium amphiphile SAINT-18 significantly increased antibody levels of IgG1 and IgG2a compared to the unformulated vector. In contrast, SAINT-18 had neither a significant effect on the IgG2a/IgG1 ratio nor on the type and strength of cellular immunity. Overall, the strongest immune response was generated after intradermal injection, followed by intramuscular and subcutaneous (s.c.) injection. The results show that the formulation of MIDGE-Th1 with SAINT-18 increased the efficacy of the MIDGE-Th1 DNA vaccine and is therefore a suitable approach to improve the efficacy of DNA vaccines also in large animals and humans.

Superior efficacy of a recombinant flagellin:H5N1 HA globular head vaccine is determined by the placement of the globular head within flagellin

Transmission of highly pathogenic avian influenza (HPAI) between birds and humans is an ongoing threat that holds potential for the emergence of a pandemic influenza strain. A major barrier to an effective vaccine against avian influenza has been the generally poor immunopotency of many of the HPAI strains coupled with the manufacturing constraints employing conventional methodologies. Fusion of flagellin, a toll-like receptor-5 ligand, to vaccine antigens has been shown to enhance the immune response to the fused antigen in preclinical studies. Here, we have evaluated the immunogenicity and efficacy of a panel of flagellin-based hemagglutinin (HA) globular head fusion vaccines in inbred mice. The HA globular head of these vaccines is derived from the A/Vietnam/1203/04 (VN04; H5N1) HA molecule. We find that replacement of domain D3 of flagellin with the VN04 HA globular head creates a highly effective vaccine that elicits protective HAI titers which protect mice against disease and death in a lethal challenge model.