A DNA vaccine is a plasmid produced in Escherichia coli that contains an antigen gene controlled by a promoter that functions only in animal cells, usually the cytomegalovirus promoter. The DNA vaccine plasmid must be delivered to antigen-presenting cells (APCs) of the host, where the antigenic protein is expressed, processed, and presented to the immune system. This approach works efficiently in mice and other animal models when DNA is delivered by intramuscular injection, and DNA vaccines have been licensed for veterinary applications, yet no DNA vaccine has been approved for use in humans. A key problem is achieving the delivery of sufficient plasmid to the APCs. This article discusses the approaches designed to achieve delivery, including a focus on the use of live bacterial vectors.
Since DNA vaccination was developed in the early 1990s, the most common method for immunization has been intramuscular injection of DNA. The DNA is usually dissolved in water or an isotonic saline solution, with the inclusion of an adjuvant if necessary. The aim is to get the plasmid into APCs, which in muscle tissue are primarily dendritic cells. There, the DNA is transported to the nucleus for expression, and the resulting polypeptide is processed and presented on the cell surface in a major histocompatibility complex (MHC). DNA vaccination can therefore produce a protective CD8+ cytotoxic T lymphocyte (CTL) response via MHC Class I (1). Also important are bystander effects, whereby antigens expressed and released from adjacent myocytes (i.e., muscle cells) are taken up by APCs, thus triggering antibody production through MHC Class II. This effect enables DNA vaccines to be developed to target a wide range of bacterial, viral, and parasitic diseases, in addition to the rapidly expanding field of cancer immunotherapy.The first licensed DNA vaccine was West Nile-Innovator DNA, approved in 2005 for the immunization of horses against West Nile Virus. It was given in two intramuscular doses 2–4 weeks apart, then as a single dose annually (2). However, this vaccine has now been discontinued by Pfizer following their acquisition of the developers, Wyeth's Fort Dodge Animal Health. Also in 2005, Novartis Animal Health (Basel, Switzerland) gained approval for Apex-IHN, a vaccine against infectious hematopoietic necrosis virus in salmon, which encodes a viral glycoprotein and is administered by a single intramuscular dose of 10 µg DNA (3).
Despite promising results in animals, DNA vaccine efficacy has been disappointing in human clinical trials. This is generally because of a much lower specific immune response generated by injected plasmid DNA alone, and has led to the adoption of a prime-boost vaccination strategy whereby the plasmid DNA injection is followed by a boost with the same antigen gene in a viral vector. This heterologous boosting strategy has significantly increased the specific immune response. Commonly-used attenuated viral vectors include those based on modified vaccinia ankara (derived from the smallpox vaccine), lentivirus (mainly derived from HIV), and adenoviruses of primate origin. Viral vectors are expensive and complex to manufacture compared with DNA, thus eliminating the advantages of the DNA vaccine approach such as lower cost and simple, generic production processes. An additional disadvantage is the potential of a host immune response directed against the vector. Development of the ideal DNA vaccine therefore requires the optimization of delivery strategies and plasmid vectors.