Poxviruses

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Introduction: Poxciruses have been known about for centuries - the characteristic "pocks" produced by variola virus (Smallpox) giving their name to all forms of infectious disease "a dose of the pox". Smallpox first appeared in China and the Far East at least 2000 years ago. The Pharaoh Ramses V (left) died of smallpox in 1157 B.C. The disease reached Europe in 710 A.D. and was transferred to America by Hernando Cortez in 1520 - 3.5 million Aztecs died in the next 2 years. In the cities of 18th century Europe, smallpox reached plague proportions and was a feared scourge - highly infectious. Five reigning European monarchs died from smallpox during the 18th century. Smallpox has now been eradicated - the last naturally occurring outbreak of smallpox was in Somalia on 26th October 1977.

Taxonomy:

Morphology:

Thin sections in E.M. reveal that the outer surface is composed of lipid and protein which surrounds the core, which is biconcave (dumbbell-shaped), with two "lateral bodies" (function unknown). The core is composed of a tightly compressed nucleoprotein. To view an electron micrograph of a negatively-stained poxvirus particle click here.
Antigenically, poxviruses are very complex, inducing both specific and cross-reacting antibodies - hence ability to vaccinate against one disease with another virus (below). There are at least 10 enzymes present in the particle, mostly concerned with nucleic acid metabolism/genome replication.

Genome:

Linear, d/s DNA of 130-300kbp. Ends of genome consist of a terminal hairpin loop (no free ends) with several tandem (i.e. direct) repeat sequences (this arrangement is found at the ends of chromosomes from a number of different organisms). The ends of the genome form direct repeats called inverted terminal repeats (ITRs). Several poxvirus genomes have been sequenced. Most of the essential genes are located in the central part of the genome, while non-essential (in tissue culture) genes are located at the ends. There are ~250 genes in the genome.

See the Poxvirus Bioinformatics Resource Center.

Replication:

Receptors are not known, but probably >1 on different cell types. For Vaccinia, one of these is probably the EGF receptor (epidermal growth factor). McFadden G. Poxvirus tropism. Nat Rev Microbiol. 3: 201-213, 2005

Penetration is complex and may also involve >1 mechanism.

Uncoating occurs in two stages, removal of the outer membrane as the particle enters the cell and in the cytoplasm, the particle (minus its outer membrane) is further uncoated and the core passes into the cytoplasm.

Gene expression is carried out (exclusively?) by viral enzymes associated with the core and is divided into 2 phases:

Early genes: ~50% genome, expressed before genome replication

Late genes: expressed after genome replication; late promoters are dependent on DNA replication for activity.

Poxvirus gene expression has been studied in detail because of the interest in the use of Vaccinia virus as a vector for expression of heterologous genes.

Genome replication is believed to involve self-priming, leading to the formation of high m.w. concatemers (isolated from infected cells) which are subsequently cleaved and repaired to make virus genomes. The many virus-encoded enzymes involved in replication (e.g. thymidine kinase - tk) offer potential targets for chemotheraputic agents.

Assembly occurs in the cytoskeleton; the events involved in putting together such a complex particle are not understood, but probably involve interactions with the cytoskeleton (e.g. actin-binding proteins). Inclusions are formed in the cytoplasm which mature into virus particles. Actin 'comet tails' form which shoot IEV through the cytoplasm to the cell surface, and possibly into adjacent cells - these have been timed moving at 3µm/min. This may provide an alternative mechanism for cell to cell spread (c.f. EEV).

Overall, replication of this large, complex virus is rather quick ~12h.

Vaccinia virus shows considerable resistance to the antiviral effects of interferons. One of the early genes of this virus, K3L, encodes a protein which is homologous to eIF-2α which inhibits the action of PKR. In addition, the E3L protein also binds dsRNA and inhibits PKR activation. As well as these, other poxvirus-encoded proteins interfere with the actions of complement, IL-1 and TNFs. Vaccinia infection of cells can confer protection from IFN on other viruses, e.g. Picornaviruses.

Members of the poxvirus family were the first DNA viruses shown to encode secreted versions of cellular cytokine receptors. Virus cytokine binding proteins are generally detected as either secreted glycoproteins or as transmembrane cell surface proteins. Virus cytokine receptor mimics (known as viroceptors) are soluble or membrane-bound proteins that may share significant homology to the ligand binding domains of cellular cytokine receptors and function by sequestering cytokine ligands from interacting to their cognate cellular receptors. Although many virus cytokine binding proteins were previously identified by their sequence relationship to cellular proteins, some virus-encoded soluble proteins have been identified which bind to and inhibit cytokines with high affinity but do not share any sequence identity to known cellular cytokine or immunomodulatory receptors.

Variola and Vaccinia:

At least 9 different poxviruses cause disease in humans, but variola virus (VV) and vaccinia are the best known. VV strains are divided into variola major (25-30% fatalities) and variola minor (same symptoms but less than 1% death rate). "Variolation" = the administration of material from known smallpox cases (hopefully variola minor!!!) to protect recipients - practiced for at least 1000 years (Chinese) but risky - Jenner was nearly killed by variolation in 1756!

The precise origins of vaccinia virus are uncertain:
Edward Jenner, 14th May 1796, used cowpox (Sarah Nemes!) to "vaccinate" 8 year old James Phipps, who he later challenged with VV (ethical?) and showed that he was protected. For more than 100 years, the "vaccine strains" (many origins) were propagated from arm-to-arm, but for at least the last 50 years, Vaccinia has been a distinct virus from cowpox - origin?

Vaccination was almost universally adopted worldwide around 1800, but it took a major commitment from the WHO in 1965 to achieve eradication.
Eradication of smallpox was possible for 3 reasons:

1) There is no other reservoir for VV but man (including primates) (c.f. Arboviruses)

2) VV causes only acute infections, from which the infected person either:

a) dies

b) recovers with life-long immunity (c.f. Herpesviruses)

3) Vaccinia virus is an effective immunogen.

UK Department of Health current vaccination guidelines

Infection with both viruses occurs naturally by the respiratory route and is systemic, producing a variety of symptoms, but most notably with VV characteristic pustules and scarring of the skin. Replication of vaccinia is more localized. Administration of vaccinia to immunocompromised hosts results in systemic infection with neurological damage or death. The vaccine is contraindicated in:

• Eczema, atopic dermatitis or other acute, chronic, or exfoliative skin conditions
• Diseases or treatments which cause immunodeficiency or immunosuppression
• Moderate or severe acute illnesses
• Previous allergic reaction to smallpox vaccine or any of the vaccine components
• Pregnancy
• Breastfeeding mothers
• Infants and children under 12 months of age

Aside for the development of new vaccines, future responses to poxvirus infection are likely to include immune-based therapies such as humanized antibodies or fragments of antibodies produced in animals, and antiviral drugs such as Cidofovir, a DNA polymerase inhibitor developed for Cytomegalovirus retinitis, which has been found to be active in preventing Variola infection in cultured cells and protects mice against lethal virus challenge.
From actually toxic to highly specific - novel drugs against poxviruses. Virol J. 2007 4: 8.

Expression Vectors:

• The proportion of the human population which has already been vaccinated - lifelong protection may result in poor response to recombinant vaccines (?).
• Dangerous in immunocompromised hosts.
The possible solution to this may be to use Avipoxvirus vectors, e.g. fowlpox or canarypox as 'suicide vectors' - undergo abortive replication in mammalian cells:

• can express high levels of foreign proteins
• no danger of pathogenesis
• no natural immunity in humans

Pasteur Merieux Connaught's AIDS vaccine ALVAC-HIV (vCP205) consists of a recombinant canarypox vector expressing various HIV genes (gp120, gag and pro). Phase III clinical trials of this vaccine are currently planned to test it's efficacy.

Bioterrorism or Re-emergence?

Although smallpox has been eradicated, there are concerns about the potential use of variola virus as a weapon of terror. Consequently, destruction of the last (official) remaining smallpox stocks held in Russia and USA has now been postponed indefinitely. The possibility of an orthopoxvirus such as variola, monkeypox, camelpox or taterapox virus emerging or re-emerging as a threat to human health increases as the proportion of the world population that is immunologically naïve for orthopoxviruses increases. The parallel increase in immunosuppression due to AIDS increases the chance of OPVs jumping species and adapting to mankind (Rat-to-Human Transmission of Cowpox Infection). These possibilities justify and the retention of adequate stocks of vaccine (vaccinia) to combat poxvirus infections. Finally, it is unclear whether all, only a few, or just one of the differences between the genomes of viruses such as camelpox and smallpox are responsible for the inability of camelpox to cause human disease. Consequently, genetic modification of camelpox to delete genes that are present in camelpox but absent in smallpox might be highly dangerous. It might also be unwise to insert into camelpox genes encoding Th₂ cytokines, which can cause a dramatic change in ectromelia virus virulence (Gubser C, Smith GL. The sequence of camelpox virus shows it is most closely related to variola virus, the cause of smallpox. J Gen Virol. 83: 855-872, 2002).

• CDC Bioterrorism, Smallpox & Monkeypox sites.
• A Different View of Smallpox and Vaccination
• Search PubMed for all publications on this topic

© MicrobiologyBytes 2007.