H5N1 threat puts human flu back in...( t the same animal and exchange genes. I...),Health

t the same animal and exchange genes. In 1957, a human H1N1 and avian H2N2 infected the same animal and swapped some genes--a process called reassortment. The resulting viral offspring caused that year's severe epidemic. According to the paper's authors, even in the absence of a quick reassortment, the right antigenic drift could give influenza A viruses the ability to spread to new species, including humans.

The prospect of a human pandemic of H5N1 is alarming considering what can happen to people infected by this virus, according to the St. Jude investigators. The immune system response to H5N1 can run amok, with immune cells spewing out inflammatory chemicals called cytokines in a "cytokine storm" that causes airways to become inflamed and the alveoli to fill with fluid. The result can be rapid death. According to the investigators, this finding at least partially explains why so many young, otherwise healthy people succumbed to the 1918-1919 pandemic, as do many victims of H5N1 today; their young, healthy immune systems generate a strong cytokine storm.

However, breakthroughs in understanding the details of the battle between the immune system and influenza A viruses hold the promise of better therapies and vaccines.

"A key challenge to immunologists is learning how to exploit the exquisite sensitivity of CD8+ cells to different targets on H5N1 and other flu viruses," said Richard Webby, Ph.D., an assistant member of the St. Jude Department of Infectious Diseases and co-author of the paper. "We could use that knowledge to design better vaccines."

Vaccines made of live viruses like Flumist have the advantage of stimulating the memories of previously alerted CD8+ T cells, according to the authors. And, because these weakened viruses can still replicate themselves in the body, lower doses are effective. However, regardless of whether virus used as a vaccine is alive or dead, the virus targeted by the vaccine can mutate an immune
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Contact: Kelly Perry
media@stjude.org
901-495-3306
St. Jude Children's Research Hospital
4-May-2006

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