Abstract: Vaccinia virus (VACV or VV) is a large, complex, enveloped virus belonging to the poxvirus family. It has a linear, double-stranded DNA genome approximately 190 kbp in length, and which encodes for approximately 250 genes. The dimensions of the virionare roughly 360 × 270 × 250 nm, with a mass of approximately 5-10 fg. Vaccinia virus is well known for its role as a vaccine (its namesake) that eradicated the smallpox disease, making it the first human disease to be successfully eradicated by science. This endeavour was carried out by the World Health Organization under the Smallpox Eradication Program. Post eradication of smallpox, scientists study Vaccinia virus to use as a tool for delivering genes into biological tissues (gene therapy and genetic engineering).
In the early 21st century, due to concerns about smallpox being used as an agent for bioterrorism, there was renewed interest in studying the Vaccinia virus.
Vaccinia infections may be divided into the following types:
1. Generalized vaccinia
2. Eczema vaccinatum
3. Progressive vaccinia (Vaccinia gangrenosum, Vaccinia necrosum)
4. Roseola vaccinia
Vaccinia virus is closely related to the virus that causes cowpox; historically the two were often considered to be one and the same. The precise origin of vaccinia virus is unknown, however, due to the lack of record-keeping as the virus was repeatedly cultivated and passaged in research laboratories for many decades. The most common notion is that vaccinia virus, cowpox virus, and variola virus (the causative agent of smallpox) were all derived from a common ancestral virus. There is also speculation that vaccinia virus was originally isolated from horses.
Poxviruses are unique among DNA viruses because they replicate only in the cytoplasm of the host cell, outside of the nucleus. Therefore, the large genome is required for encoding various enzymes and proteins involved in viral DNA replication and gene transcription. During its replication cycle, VV produces four infectious forms which differ in their outer membranes: intracellular mature virion (IMV), the intracellular enveloped virion (IEV), the cell-associated enveloped virion (CEV) and the extracellular enveloped virion (EEV). Although the issue remains contentious, the prevailing view is that the IMV consists of a single lipoprotein membrane, while the CEV and EEV are both surrounded by two membrane layers and the IEV has three envelopes. The IMV is the most abundant infectious form and is thought to be responsible for spread between hosts. On the other hand, the CEV is believed to play a role in cell-to-cell spread and the EEV is thought to be important for long range dissemination within the host organism.
Vaccinia contains within its genome several proteins that give the virus resistance to interferons. K3L is a protein with homology to the protein eukaryotic initiation factor 2 (eIF-2alpha). K3L protein inhibits the action of PKR, an activator of interferons. E3L is another protein encoded by Vaccinia. E3L also inhibits PKR activation; and is also able to bind to double stranded RNA.
Use as a Vaccine
A Vaccinia virus infection is very mild and is typically asymptomatic in healthy individuals, but it may cause a mild rash and fever. Immune responses generated from a Vaccinia virus infection protects the person against a lethal smallpox infection. For this reason, Vaccinia virus was, and is still being used as a live-virus vaccine against smallpox. Unlike vaccines that use weakened forms of the virus being vaccinated against, the Vaccinia virus vaccine cannot cause a smallpox infection because it does not contain the smallpox virus. However, certain complications and/or vaccine adverse effects occasionally arise. The chance of this happening is significantly increased in people who are immunocompromised. Approximately one in one million individuals will develop a fatal response to the vaccination. Currently, the vaccine is only administered to health care workers or research personnel who have a high risk of contracting the varioloa virus, and to the military personnel of the United States of America. Due to the present threat of smallpox-related bioterrorism, there is a possibility the vaccine may have to be widely administered again in the future. Therefore, scientists are currently developing novel vaccine strategies against smallpox which are safer and much faster to deploy during a bioterrorism event.
On September 1, 2007, the U.S. Food and Drug Administration (FDA) licensed a new vaccine ACAM2000 against smallpox which can be produced quickly upon need. Manufactured by Acambis ofCambridge, England, and Cambridge, Massachusetts, the U.S. Centers for Disease Control and Prevention stockpiled 192.5 million doses of the new vaccine (see list of common strains below).
The original vaccine for smallpox, and the origin of the idea of vaccination, was Cowpox, reported on by Edward Jenner in 1796. The Latinterm used for Cowpox was variola vaccina, essentially a direct translation of "cow-related pox". That term lent its name to the whole idea of vaccination. When it was realized that the virus used in smallpox vaccination was not, or was no longer, the same as the Cowpox virus, the name 'vaccinia' stayed with the vaccine-related virus. (See OED.) Vaccine potency and efficacy prior to the invention of refrigerated methods of transportation was unreliable. The vaccine would be rendered impotent by heat and sunlight, and the method of drying samples on quills and shipping them to countries in need often resulted in an inactive vaccine. Another method employed was the "arm to arm" method. This involved vaccinating an individual then transferring it to another as soon as the infectious pustule forms, then to another, etc. This method was used as a form of living transportation of the vaccine, and usually employed orphans as carriers. However, this method was problematic due to the possibility of spreading other blood diseases, such as hepatitis and syphilis. 41 Italian children contracted syphilis after being vaccinated by the arm to arm method in 1861.
In 1913, E. Steinhardt, C. Israeli, and R. A. Lambert grew vaccinia virus in fragments of guinea pig corneal tissue culture.
In 1939 Alan Downie showed that the smallpox vaccines being used in the 20th century and cowpox virus were not the same, but some sorts of cousins.
In March 2007, a 2-year-old Indiana boy and his mother contracted a life-threatening vaccinia infection from the boy's father. The boy developed the telltale rash over 80 percent of his body after coming into close contact with his father, who was vaccinated for smallpox before being deployed overseas by the United States Army. The United States military resumed smallpox vaccinations in 2002. The child acquired the infection due to eczema, which is a known risk factor for vaccinia infection. The boy was treated with intravenous immunoglobulin, cidofovir, and an experimental drug being developed by SIGA Technologies. On April 19, 2007, he was sent home with no after effects except for possible scarring of the skin.
In 2010, the Centers for Disease Control and Prevention (CDC) reported that a woman in Washington had contracted vaccinia virus infection after digital vaginal contact with her boyfriend, a military member who had recently been vaccinated for smallpox. The woman had a history of childhood eczema, but she had not been symptomatic as an adult. The CDC indicated that it was aware of four similar cases in the preceding 12 months of vaccinia infection after sexual contact with a recent military vaccinee.
This is a list of some of the well-characterized vaccinia strains used in research and immunizations.
1. Western Reserve
3. Dryvax (also known as "Wyeth"): the vaccine strain previously used in the United States, produced by Wyeth. It was replaced in 2008  by ACAM2000 (see below), produced by Acambis. It was produced as preparations of calf lymphwhich was freeze-driedand treated with antibiotics.
4. ACAM2000: The current strain in use in the USA, produced by Acambis. ACAM2000 was derived from a clone of a Dryvax virus byplaque purification. It is produced in cultures of Vero cells.
5. Modified vaccinia Ankara: a highly attenuated (not virulent) strain created by passaging vaccinia virus several hundred times inchicken embryo fibroblasts. Unlike some other vaccinia strains it does not make immunodeficient mice sick and therefore may be safer to use in humans who have weaker immune systems due to being very young, very old, having HIV/AIDS, etc (Wikipedia, 2012).
Title: Vaccinia Virus Tricks Its Way Into Hosts By Mimicking Dead Cells
Date: April 25, 2008
Abstract: If it looks like a dead cell and it feels like a
dead cell, be careful – it could be a virus. Viruses are experts at
infiltrating and exploiting cells but some are so big that they need to use
special tricks. The Vaccinia virus is one of these. It belongs to the same family as the more
infamous variola virus that causes smallpox. This group are among the largest
of viruses, dwarfing many other types by a factor of ten. But despite its size,
Vaccinia relies on stealth rather than brute force.
It’s a mimic and it disguises itself as cellular flotsam. Vaccinia carries a molecular tag on its surface called phosphatidylserine, which is usually found on the remains of cells that have died naturally. Its presence dupes other host cells into absorbing the virus as part of their general clean-up duties.
Jason Mercer and Ari Helenius from the EHT in Zurich discovered this piece of molecular trickery by tagging individual virus particles with a protein that glowed yellow, to track their movements as they stormed into a cell. Each invasion began as a virus latched onto thin protrusions called filopodia and used these as anchors to inch its way towards the main cell body.
When the virus reached the cell proper, it triggered a process called macropinocytosis, that cells normally use to import large volumes of fluid or molecules that are too big to pull in through other means. The cell’s membrane developed spherical blisters called “blebs”, which swelled outward only to collapse again within half a minute. The touch of a single virus was enough to produce about a hundred blebs all over the cell’s surface and as they retracted, they smuggled the virus in with them.
When Mercer and Helenius treated the cells with a drug that prevents bleb formation (amusingly called blebbistatin), they reduced rates of infection by a factor of three. So the blebs are the virus’s gateway into the cell, but they only form because the virus has the right key – phosphatidylserine, a molecule that plays a key role in the death of cells.
It’s in our bodies’ interests to get rid of malfunctioning, damaged or unnecessary cells and they do this through a programmed process of cell suicide called apoptosis. In the cell’s final moments, it pushes out the phosphatidylserine molecule to its surface. There, it acts as an identity tag and it reads, “I’m a piece of junk. Clean me up.” Other cells react to it by absorbing the tagged fragment.
But Vaccinia is rife with phosphatidylserines. A third of its surface is peppered with the molecule, which it uses to blag its way into cells. To show how important this process is, Mercer and Helenius incubated Vaccinia with a protein that binds to PS and caps the protruding tag. That simple move blocked infection rates by 90%. Likewise, the virus was unable to enter cells if its outer layer (where PS resides) was removed, and regained the ability when the layer was reconstituted.
The virus’s trick relies on help from the inside, specifically from the proteins of the deceived cell. After analysing over 7,000 of these, Mercer and Helenius found that one in particular, PAK1, was especially important in the initial stages of infection. It was always found within the blebs and when the researchers lowered the levels of PAK1 by silencing its gene, infection rates fell by about 70%.
Looking like a piece of dead cell allows the virus to enter actual cells, but it has other advantages too. A side effect of macropinocytosis is a stifling of a host’s immune response so not only can Vaccinia gain easy access into host cells, it can also move between them undetected.
The strategy of mimicking dead cells is a first for viruses, but it may be shared by other poxviruses. Vaccinia itself causes mild infection, usually little more than rashes, fever and aches. Smallpox, on the other hand, managed to kill up to half a billion people in the 20th century before it was finally eradicated. Ironically, the closely related Vaccinia virus was used as part of the vaccine.
Mercer and Helenius cite the threat of bioterrorist
attacks using poxviruses as a reason for studying the ways of Vaccinia. An
equally valid reasnonis the possibility of emerging poxviruses that cause new
diseases. Just this week, researchers who were searching for new poxviruses in
Uganda found one in pretty much the first place they
looked – the bodies of red
colobus monkeys (Discovery, 2008).