Part II - why vaccine viruses are weaker that their pathogenic cousins

I've talked about this before (here with mumps virus) but just what makes vaccines so good at being vaccines? - that is, what makes them safe yet immunogenic? The great thing about this is that maybe if we understand these processes a bit better we may even be able to develop safer, more effective and cheaper vaccines that will ultimately save more and more peoples lives. A lot of research at the minute is cuurently attempting to tease apart the roles of hundreds of virus genes in infection and disease with many groups focusing on specifically how these genes affect attenuation, that is how they make the virus weaker and less dangerous so it can be used as a vaccine.

So it starts off like this: we know vaccines are safe and effective when after we give them to people/other animals very little get sick and a lot of them are immunized against infection with that particular virus yet the 'wild-type', normal virus is able to infect and cause disease - so just what makes these different? Well to find out we have to compare the two viruses - the weak one and the virulent one and somewhere in their genomes lie differences that will possibly shed light on the important molecular mechanisms behind virus attenuation and pathogenesis.

Canine distemper in dogs. http://www.dogs-info.net

In a recent paper, Dietzel et al investigated these processes using a great model system, specifically canine distemper virus - or CDV, a virus able to infect and cause life-threatening disease in a whole range of mammalian species in a domestic or wild-life situation and hence is obligatorily given as part of a multivalent vaccine to all domestic dogs. The group began by comparing the matrix - or M- gene between a vaccine strain of a virus known as canine distemper virus or CDV and a wild-type one. This M gene encodes a protein which plays a major role in the assembly of new virus particles, cell-cell spread and particle stability within CDV-infected cells through co-ordinating genome interactions with virion-surface proteins. Its role in CDV pathogenesis - and other viruses - has barely been looked at. The two M proteins differed at only six different amino acid positions (3%) so to look into the role of them both during CDV infection the group genetically engineered a CDV virus based on the wild-type but carrying the vaccine-derived M gene in place of its own. The growth and physical characteristics of the three viruses :wild-type, wild-type + vaccine M and vaccine strain viruses were looked at as well as physically characteristics of the produced particles, how they are assembled and what happens during animal infection.


The 3 viruses grow equally well in cell culture conditions.


Despite the three viruses growing equally well under cell culture conditions (see growth above and how the infection looks in tissue culture - all are pretty much similiar), the two differed markedly in their particle-infectivity ratio, that is - the number of virus particles capable to carry out a successful infection compared to the total number of particles in a given volume, i.e some particles may be non-functional for some reason, maybe because they are less physically stable or damaged.

virus particle release from the tops or bottoms of the cells

The vaccine virus also differed in the way new CDV particles were released from cells: the wild-type virus is released from the tops of polarised epithelial cells while the vaccine strain is released from both the top and the bottom. However, transfer of the vaccine M gene wasn't able to confer bipolar release onto the wild-type virus indicating the function of other CDV genes in this phenotype. To determine a mechanism for this bipolar release the group analysed the intracellular distribution of the key CDV proteins, H and F involved in particle production and cell-cell spread. They found that this proteins differed in their location between the three viruses: the wild-type only had them on the tops of cells while the vaccine strain had them on both sides, the chimeric virus had an intermediate distribution. These results highlight the primary ability of the CDV matrix protein to coordinate protein distribution within the cell.

CDV protein intracellular distibution

All these results are good and all but what effect do they actually have in vivo - in a situation where the virus is infecting multiple cell types and is constantly battling the immune system? To determine the effect of vaccine M gene, groups of ferrets (an excellent model of CDV infection that are naturally infected by the virus) were inoculated intranasally with the three viruses and disease severity, body temperature, disease symptoms and fatality was recorded. Interestingly, based on these measures, the wild-type with the vaccine M was more attenuated that the vaccine virus as shown below with the measure of ferret blood leukocytes - the more blood cells the less pathogenic the virus.

A measure of CDV disease - the percent of ferret blood cells - a CDV target cell


The process of pathogen attenuation for the generation of protective vaccines has worked well in the past for many diseases yet many viruses and bacteria do not prove to be responsive to this method. Other more targeted processes may have to be designed in order to create vaccines for against these diseases. Work investigating the molecular basis of virus pathogenesis and attenuation may allow us to rapidly and successfully attenuate these hard-to-vaccinate viruses in a more targeted fashion - and one that may work better than the previously tried methods.

Dietzel, E., Anderson, D., Castan, A., von Messling, V., & Maisner, A. (2011). Canine Distemper Virus Matrix Protein Influences Particle Infectivity, Particle C
omposition, and Envelope Distribution in Polarized Epithelial Cells and Modulates Virulence Journal of Virology, 85 (14), 7162-7168 DOI: 10.1128/JVI.00051-11


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