A team of researchers from the University of Wisconsin-Madison and the University of Tokyo has reported in the Proceedings of the National Academies of Science ( PNAS ), a new way to generate genetically altered influenza virus.

The lab-made virus - whose genes are manipulated to disarm its virulent nature - can be seeded into chicken eggs to generate the vaccine used in inoculations, which prepare the human immune system to recognize and defeat the wild viruses that spread among humans in an epidemic or pandemic.

Researchers have described an improved "reverse genetics" technique that makes it easier to make a seed virus in monkey kidney cells, which millions of copies of the disarmed virus to be used to make vaccines.

In nature, viruses commandeer a cell's reproductive machinery to make new virus particles, which go on to infect other cells and make yet more virus particles.

Vaccine makers use a monkey kidney cell line to make non-virulent viruses that serve as the raw material for vaccines.

The technique reported by the Wisconsin team improves upon a previous reverse genetics method ( developed by Kawaoka's group in 1999 ) by significantly reducing the number of plasmid vectors required to ferry viral genes into the monkey kidney cells used to produce the virus particles to make vaccines.

" Compared to other types of cells, which are not approved for vaccine production, it is not always easy to introduce plasmids into the monkey kidney cells, which are approved for such use," says Kawaoka, at UW-Madison's School of Veterinary Medicine.

Monkey kidney cells are used routinely for generation of seed strains for vaccine production because they are not known to carry any unknown infectious agents and do not cause tumors.

According to Kawaoka, "application of the new system may be especially advantageous in situations of outbreaks of highly pathogenic avian influenza viruses."

When a new strain of highly virulent influenza emerges to infect humans, vaccine makers must tailor their vaccines to match it because, genetically, the virus is always different.
The process is a race against time and can take months depending on how quickly new strains are identified, genetically disarmed and subsequently generated in the lab for use to make vaccines in large quantities.

The new technique promises to ensure ready generation of seed strains for the production of vaccines required to blunt the spread of influenza.
In the event of an outbreak of especially virulent strains of influenza, such as the H5N1 or "bird flu" viruses now being monitored by scientists, any efficiency in the manufacture of vaccines will be important.

The method devised by Kawaoka and his colleagues reduces the number of plasmids required to introduce viral genes into the monkey kidney cell lines used to mass produce the deactivated virus for use in vaccine manufacture. " By reducing the number of plasmids, we increase the efficiency of virus production," Kawaoka explains.

Source: University of Wisconsin-Madison, 2005


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