Adenovirus vectors - new genes, new vaccines

In the 1970s, advances in molecular biology opened the possibility of treating disease using DNA as a drug - gene therapy. This technology applies both to inherited conditions such as haemophilia and cystic fibrosis, where certain genes are defective, and to acquired diseases where DNA could be used for vaccine delivery. In some people’s eyes, gene therapy must be prevented at all costs. Most scientists would agree that modification of the human germ line, even if it is done with the intention of curing genetic diseases, is ethically unacceptable. However, the majority of scientists would also say that it is ethically unacceptable withhold somatic cell therapy which has the potential to save the life of an individual afflicted with a fatal illness.

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A number of different viruses have been investigated as possible vectors which could be used to introduce and express genes into human cells. These include retroviruses, adeno-associated viruses and herpesviruses. Alternative methods of gene therapy have some advantages over the use viruses as vectors to deliver genes. It is simpler to scale up the production of chemicals rather than biologicals, and there is no possibility of accidentally causing infection due to an adverse reaction to the virus vector. Virus infections also result in an immune response to the invader, and this can be problematic as somatic cell therapy of inherited genetic diseases needs to be effective for the lifetime of the patient. In 1999, 17 year old Jesse Gelsinger was the first patient to die in clinical trials of gene therapy. As part of a trial for an inherited genetic liver disease, he was injected with a recombinant adenovirus carrying a corrected gene in the hope that it would manufacture the enzyme he needed. He died four days later, apparently having suffered a massive immune response triggered by the adenovirus vector. This tragic case was a severe setback for virus vectors in gene therapy.

Adenoviruses are non-enveloped viruses with a linear double stranded DNA genome. There are over 40 serotypes of adenovirus, most of which cause relatively harmless respiratory tract infections. The adenovirus replication cycle does not normally involve integration into the host genome, so there is little risk of accidental insertional mutagenesis. The wild type adenovirus genome is approximately 35 kb long, of which more than 30 kb can be replaced with foreign DNA. Conveniently, adenovirus genomes are arranged in a series of “early” genes which encode non-structural proteins with regulatory functions, and a set of “late” genes, which encode structural proteins needed to make up virus particles. Expression of the late transcripts is driven by the a strong promoter which is capable of making large amounts of protein. The first adenovirus vectors were constructed by deletion of parts of the early genes, making the virus defective and unable to replicate unless the missing functions are supplied by a helper cell. The deletion also creates space for insertion of the therapeutic gene(s) under the control of the late promoter. A partial solution to the problem of vector safely is to manipulate the virus so that the main antigenic proteins are absent, culminating in development of “gutless” vectors which contain no adenovirus coding sequences.In general, non-virus mediated methods of somatic cell therapy have gained favour over the last few years. But there is a situation where the immune response to a virus vector may be an advantage - in the development of new vaccines. One of the most successful adenovirus vectors has been AdEasy, a sophisticated vector based on adenovirus 5 (A simplified system for generating recombinant adenoviruses. PNAS USA 1998 95: 2509-14). In this vector, homologous recombination in bacteria is used to generate recombinant clones, and the inserted gene is expressed from a strong cytomegalovirus promoter. Additionally, a green fluorescent protein (GFP) reporter gene allows activity to be monitored easily.

Much of the current interest in adenovirus vectors centres on their development as vaccine delivery vehicles (Adenoviruses as vaccine vectors. 2004 Mol Ther 10: 616–629). They are attractive for vaccine development as they induce both innate and adaptive immune responses in mammalian hosts. Currently, adenovirus vectors are being tested as subunit vaccines for numerous infectious agents ranging from malaria to HIV, and against a multitude of tumours.

The next generation of recombinant adenovirus (rAd) vectors will contain modifications designed to alter their immunogenicity (Mechanism of Ad5 Vaccine Immunity and Toxicity: Fiber Shaft Targeting of Dendritic Cells. 2007 PLoS Pathogens 3: e25). Some components of adenovirus particles are potentially toxic and cause fevers. By manipulating the structure of the virus particle researchers hope to improve the safety and efficacy of recombinant vaccines based on adenovirus vectors. Clearly there is a lot more work to be done before we have reaped the full potential of this useful and potentially lifesaving technology.