On episode #285 of the science show This Week in Virology, Vincent meets up with XJ Meng and Sarah McDonald at Virginia Tech to talk about their work on viruses of swine and rotaviruses.
You can find TWiV #285 at www.twiv.tv.
18 May 2014
Later this month (May 2014) the World Health Assembly will decide whether to destroy the remaining stocks of variola virus – the agent of smallpox – or to allow continued research on the virus at WHO-approved laboratories.
After the eradication of smallpox in 1980, the World Health Organization called for destruction of known remaining stocks of variola virus. The known remaining stocks of the virus are closely guarded in the United States and Russia. These consist not of a single vial of the virus, but of hundreds of different strains, many of which have not been fully characterized, nor has their genome sequence been determined.
It can be argued that there still remains a good deal of work to be done on variola virus, including development of newer diagnostic tests, and identification of additional countermeasures (antivirals and vaccines have been stockpiled in the US). Damon, Damaso, and McFadden have written a summary of the research on variola virus that should be done. We also discussed whether the remaining variola virus stocks should be destroyed on episode #284 of This Week in Virology.
We are interested in what readers of this blog think about this issue – please fill out the poll below.
6 May 2014
This week’s question comes from a graduate student studying virology, who writes:
My professor recently said that really, the MOI doesn’t matter in a culture, it is the concentration of viral particles in the media that matters. Ie: if you have 10 million cells or one cell, but you are infecting the plate with 5mL of 100 million viral particles/mL, then the amount of virus interacting with each cell is not different in either scenario (pretending that it isn’t nearly impossible for that single cell to survive in culture alone). I argued with him, saying that the cytotoxicity to the single cell would certainly be increased. He then said that a student hadn’t argued with him about that in his 15 years of teaching and I promptly decided to get some evidence before I continued the discussion.
I’m not actually sure which side is correct. I know that concentration is certainly a large determinant for infectious events/cell. But, it is hard for me to understand why MOI wouldn’t be more important? The more I think about it the more I think that I may be wrong. But if you have two plates with equal numbers of cells, and you add 5 mL of media to one and 50mL of media to the other – assuming that the media is 100 million infectious particles/mL – would the higher MOI plate not result in more infectious events per cell?
My reply: What first jumps out at me is the fact that the professor is using the no one ever argued with me about that excuse to say that he/she is right. That is the exact role of a student, to ask questions, and it should never be discouraged. Students can ask the best questions because they are frequently unencumbered by the bias of a field.
Please tell your professor that both multiplicity of infection and concentration of viral particles matter, for different reasons. The multiplicity of infection (MOI) is the number of virus particles added per cell. If you add one million virus particles to one million cells in a culture plate, the MOI = 1. If you add ten million virus particles to one million cells, the MOI is 10.
However, if one million virus particles are added to one million cells, each cell will not be infected with one virus particle. How many cells are uninfected, or receive 1, 2, or more virus particles is determined by the Poisson distribution. At an MOI of 1, 37% of the cells are uninfected, 37% receive 1 particle, 18% receive 2 particles, and so on.
In theory, the number of particles that infect each cell is controlled by the MOI, not the virus concentration. However, when the concentration of virus particles is very low, attachment to cells will take a very long time. This is because virus attachment is governed by the concentrations of free virions and host cells. The rate of attachment can be described by the equation
dA/dt = k[V][H]
where [V] and [H] are the concentrations of virions and host cells, respectively, and k is a rate constant.
For a 6 cm culture dish with an area of 113 square cm, we typically infect with virus in a volume no greater than 0.1 – 0.2 ml. In this way virus attachment to cells will be essentially complete within 1 hr at 37 degrees C. If the same amount of virus were added in 10 ml of medium, the attachment would take much longer; however because the MOI is the same in both cultures, at the end of the adsorption period the number of infected and uninfected cells in both cultures would be the same.
To answer the reader’s last question:
But if you have two plates with equal numbers of cells, and you add 5mL of media to one and 50mL of media to the other – assuming that the media is 100 mill infectious particles/mL – would the higher MOI plate not result in more infectious events per cell?
The answer is yes – assuming you wait long enough for the viruses in the more dilute culture to attach to cells.
4 May 2014
The 1918 influenza pandemic was particularly lethal, not only for the very young and the very old (as observed for typical influenza), but unexpectedly also for young adults, 20 to 40 years of age (pictured). It has been suggested that the increased lethality in young adults occurred because they lacked protective immunity that would be conferred by previous infection with a related virus. Reconstruction of the origins of the 1918 influenza virus provides support for this hypothesis.
Analysis of influenza virus genome sequences using a host-specific molecular clock together with seroarchaeology (analysis of stored sera for the presence of antibodies to influenza virus) indicates that the 1918 H1N1 virus arose ~1915 by reassortment of an avian influenza virus with an H1 virus that had previously emerged around 1907. The 1918 virus acquired the HA gene from the 1907 virus, and the NA gene and internal protein genes from an avian virus. This 1918 virus also infected pigs, in which descendants continue to circulate; however the human 1918 virus was displaced in 1922 by a reassortant with a distinct HA gene.
Seroarchaeology and mortality data indicate that an influenza pandemic in 1889-1893 was caused by an influenza H3N8 virus. This virus appears to have circulated until 1900, when it was replaced by a H1N8 virus (the N8 gene originating from the previously circulating H3N8 virus).
How do these events explain the unusual mortality pattern of the 1918 influenza A virus? High mortality among 20-40 year old adults might have been a consequence of their exposure to the H3N8 virus that circulated from 1889-1900. This infection provided no protection against the 1918 H1N1 virus. Protection of other age groups from lethal infection was likely a consequence of childhood exposure to N1 or H1 containing viruses (this may also have resulted in the lower than usual mortality in the elderly population). Influenza is typically highly lethal in very young children due to lack of immunologic memory.
These observations suggest that childhood exposure to influenza virus is a key predictor of virulence of a pandemic strain. Antibodies against the stalk of the HA protein protect against severe disease, but only within groups of HA subtypes (HA groups are determined by phylogenetic analysis). In 1918, antibodies against a group 2 HA subtype virus (H3) did not protect against severe disease caused by a group 1 HA subtype virus (H1). Childhood exposure might also determine mortality of seasonal influenza. For example, the high virulence of currently circulating H3N2 influenza viruses in those older than 65 years might be a consequence of infection with an H1N1 virus at a young age.
This logic can also explain mortality caused by influenza H5N1 and H7N9 viruses. Most fatalities caused by H5N1 viruses (the H5 is a group 1 HA) have been in individuals who were infected as children with an H3 virus (group 2 HA). Most fatalities caused by H7N9 viruses (group 2 HA) have occurred in individuals who were infected as children with H1N1 or H2N2 viruses (group 1 HA).
The practical consequence of this work are clearly stated by the authors:
Immunization strategies that mimic the apparently powerful lifetime protection afforded by initial childhood exposure might dramatically reduce mortality due to both seasonal and novel IAV strains.
27 April 2014
This week’s virology question comes from Eric, who writes:
I’m working on an MPH and in one of my classes we are currently studying the influenza virus. I’d forgotten that the genome is in 8 separate parts. Curious, I’ve been searching but can’t find any information as to why that is?
What evolutionary advantage is conferred by having a segmented genome?
Terrific question! Here is my reply:
It’s always hard to have answers to ‘why’ questions such as yours. We answer these questions from a human-centric view of what viruses ‘need’. We might not be right. But I’d guess there are at least two important advantages of having a segmented RNA genome.
Mutation is an important source of RNA virus diversity that is made possible by the error-prone nature of RNA synthesis. Viruses with segmented genome have another mechanism for generating diversity: reassortment (illustrated).
An example of the evolutionary importance of reassortment is the exchange of RNA segments between mammalian and avian influenza viruses that give rise to pandemic influenza. The 2009 H1N1 pandemic strain is a reassortant of avian, human, and swine influenza viruses.
Having a segmented genome is another way to get around the limitation that eukaryotic mRNAs can only encode one protein. Viruses with segmented RNA genomes can produce at least one protein per segment, sometimes more. There are other ways to overcome this limitation – for example by encoding a polyprotein (picornaviruses), or producing subgenomic RNAs (paramyxoviruses).
Other segmented viral genomes include those of reoviruses, arenaviruses, and bunyaviruses.
There are various ways to achieve genetic variation and gene expression, and viruses explore all aspects of this space.
20 April 2014
13 April 2014
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