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About viruses and viral disease
The TWiV team covers outbreaks of eastern equine encephalitis virus in the US and poliovirus in the Philippines, and explain how a chemokine induced by HIV-1 infection helps release more virus particles from cells.
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A new breed of vaccines is on the horizon: they replicate in one type of cell, allowing for their production, but will not replicate in humans. Two different examples have recently been described for influenza and chikungunya viruses.
The influenza virus vaccine is produced by introducing multiple amber (UAG) translation stop codons in multiple viral genes. Cloned DNA copies of the mutated viral RNAs are not infectious in normal cells. However, when introduced into specially engineered ‘suppressor’ cells that can insert an amino acid at each amber stop codon, infectious viruses can be produced. These viruses will only replicate in the suppressor cells, not in normal cells, because the stop codons lead to the production of short proteins which do not function properly.
When inoculated into mice, the stop-codon containing influenza viruses infect cells, and although they do not replicate, a strong and protective immune response is induced. Because the viral genomes contain multiple mutations, the viruses are far less likely than traditional infectious, attenuated vaccines to sustain mutations that allow them to replicate in normal cells. It’s a clever approach to designing an infectious, but replication-incompetent vaccine (for more discussion, listen to TWiV #420).
Another approach is exemplified by an experimental vaccine against chikungunya virus. The authors utilize Eilat virus, a virus that only replicates in insects. The genes encoding the structural proteins of Eilat virus were replaced with those of chikungunya virus. The recombinant virus replicates in insect cells, but not in mammalian cells. The virus enters the latter cells, and some viral proteins are produced, but genome replication does not take place.
When the Eilat-Chikungunya recombinant virus in inoculated into mice, there is no genome replication, but a strong and protective immune response is induced. The block to replication – viral RNA synthesis does not occur – is not overcome by multiple passages in mice. Like the stop-codon containing influenza viruses, the Eilat recombinant virus is a replication-incompetent vaccine.
These are two different approaches to making viruses that replicate in specific cells in culture – the suppressor cells for influenza virus, and insect cells for Eilat virus. When inoculated into non-suppressor cells (influenza virus) or non-insect cells (Eilat virus), a strong immune response is initiated. Neither virus should replicate in humans, but clinical trials have to be done to determine if they are immunogenic and protective.
The advantage of these vaccine candidates compared with inactivated vaccines is that they enter cells and produce some viral proteins, likely resulting in a stronger immune response. Compared with infectious, attenuated vaccines, they are far less likely to revert to virulence, and are easier to isolate.
These two potential vaccine technologies have been demonstrated with influenza and chikungunya viruses, but they can be used for other virus. The stop-codon approach is more universally applicable, because the mutations can be introduced into the genome of any virus. The Eilat virus approach can only be used with viruses whose structural proteins are compatible with the vector – probably only togaviruses and flaviviruses. A similar approach might be used with insect-specific viruses in other virus families.
Why do I call these vaccines ‘paradoxical’? Because they are infectious and non-infectious, depending on the host cell that is used.
Note: The illustration is from a t-shirt, and the single letter code of the protein spells out a message. However the title, ‘the gene stops here’, is wrong. It should be ‘the protein stops here. The 3’-untranslated region, which continues beyond the stop codon, is considered part of the gene.
On episode #316 of the science show This Week in Virology, Vincent, Alan, Rich and Kathy discuss how interleukin 10 modulation of Th17 helper cells contributes to alphavirus pathogenesis.
You can find TWiV #316 at www.microbe.tv/twiv.
On episode #223 of the science show This Week in Virology, Vincent, Alan, and Kathy discuss new influenza virus NA inhibitors, detection of EEEV antibody and RNA in snakes, and replication of the coronavirus EMC in human airway epithelial cells.
You can find TWiV #223 at www.microbe.tv/twiv.
I recall learning about chikungunya1 virus when I was a Ph.D. student in the late 1970s – only because its exotic name made an impression on me. The virus, first identified in Tanzania in 1953, causes severe rashes and joint pains, but is rarely fatal, and the infection was considered benign. The outbreaks were massive, but largely confined to developing countries in Africa and Asia. I was surprised to discover several years ago that the virus had moved into Europe and was threatening the United States. What changed to bring this third-world viral disease into the forefront of public concern?
Since its discovery over 50 years ago, infection by chikungunya virus was known to be spread by mosquitoes, primarily Aedes aegypti, which feeds almost exclusively on humans. In 2004, an outbreak of chikungunya disease started in Kenya and spread to islands in the Indian Ocean and finally India, where it had not been reported for 32 years. On Réunion, one of the Indian Ocean Islands, nearly 40% of the population of 785,000 fell ill. An outbreak in Italy three years later, the first ever in Europe, was started by an infected traveler from India. Cases of chikungunya have been reported in travelers returning to the US, although transmission within the country has not been reported.
The rapid global movement of chikungunya virus appears to be a consequence of a change in its mosquito vector. Some time during 2005 a virus was selected with a single amino acid change in the envelope glycoprotein which allows efficient replication in Aedes albopictus, the predominant mosquito in Réunion. This mosquito was never a good host for chikungunya virus, partly because it bites so many different animal species. The amino acid change enhances viral replication in the mosquito, leading to much higher levels of virus in the salivary gland. Consequently the virus is more likely to be transmitted upon biting a new host.
To further complicate matters, not only is Aedes albopictus now a good host for chikungunya virus, but the mosquito is spreading across the globe from eastern Asia to Europe and the United States. The mosquito was first found in the New World in 1985 when it was isolated in Houston, Texas. It probably traveled there from northern Asia in ships carrying scrap tires. But there are at least five other reasons to worry about Aedes albopictus: the mosquito has also been found to carry eastern equine encephalitis, Keystone, Tensaw, Cache Valley, and Potosi viruses.
1Chikungunya is believed to be derived from an East African dialect describing the contorted posture of patients with the severe joint pain characteristic of this disease.
Enserink, M. (2007). INFECTIOUS DISEASES: Chikungunya: No Longer a Third World Disease Science, 318 (5858), 1860-1861 DOI: 10.1126/science.318.5858.1860
Vazeille, M., Moutailler, S., Coudrier, D., Rousseaux, C., Khun, H., Huerre, M., Thiria, J., Dehecq, J., Fontenille, D., Schuffenecker, I., Despres, P., & Failloux, A. (2007). Two Chikungunya Isolates from the Outbreak of La Reunion (Indian Ocean) Exhibit Different Patterns of Infection in the Mosquito, Aedes albopictus PLoS ONE, 2 (11) DOI: 10.1371/journal.pone.0001168
Tsetsarkin, K., Vanlandingham, D., McGee, C., & Higgs, S. (2007). A Single Mutation in Chikungunya Virus Affects Vector Specificity and Epidemic Potential PLoS Pathogens, 3 (12) DOI: 10.1371/journal.ppat.0030201