Oct. 19, 2005

Unlocking the puzzle of how avian flu virus works may be key to preventing widespread infection

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In trying to prevent a worldwide epidemic of human influenza derived from avian flu, scientists have taken on a formidable opponent. The influenza virus is one of the most extensively studied and best-understood viruses around. It is also one of the most adaptable and elusive.

If the avian flu jumps from chickens and migratory birds to humans, early diagnosis will be critical, according to Gary Whittaker, associate professor of microbiology and immunology in Cornell University's College of Veterinary Medicine. But containment of the virus, he emphasizes, will rely both on early diagnosis and getting a start on creating a vaccine. That means gaining a greater understanding of how the flu virus enters human cells.

The mechanism by which the influenza virus works has been studied in great detail, and although parts of the puzzle are still missing, it has been possible to develop some stopgap measures. Early this year, Whittaker and graduate student Victor C. Chu added a new insight about how the virus infects cells that may eventually lead to new weapons.

Gary Whittaker
Whittaker

A virus consists of a package of genetic material surrounded by a protein and lipid shell. The type A influenza virus -- the family to which the new, closely watched strain of bird flu belongs -- consists of 10 proteins and eight strands of ribonucleic acid (RNA), which carry the code for making the proteins.

To invade a host, the virus shell includes certain proteins that bind to receptors on the outside of cells in the airways and lungs of victims. The act of binding draws the virus into the cell membrane. The virus shell fuses with the cell membrane and moves through it, emerging into the cytoplasm of the cell, where the shell opens, releasing the RNA inside.

The viral RNA is "negative sense" RNA -- a mirror image of the messenger RNA the cell uses to make its own proteins. The RNA moves into the nucleus of the cell, where the cell's machinery makes positive copies that travel back out into the cytoplasm. The cell treats them like any other messenger RNA and uses them to make copies of the viral proteins. At the same time, other positive copies of the viral RNA inside the nucleus act as templates to make more negative viral RNA.

The new viral RNA then moves back into the cytoplasm where it joins with the newly made proteins to form new copies of the complete virus. The assembly occurs on the inside of the cell membrane, and as the process is completed, the virus moves through the cell wall and "buds" on the outside. The new virus is either released into the airway to find another cell to infect or is ejected in a cough or sneeze and launched to find a new host.

Eventually the virus replication takes over so much of the cell's machinery that the cell dies. Dead cells in the airways result in a runny nose and scratchy throat. Too many dead cells in the lungs result in death.

receptor on the cell membrane
Kathryn Seely/Cornell University
After binding to a receptor on the cell membrane, the influenza virus moves into the cytoplasm where the virus shell opens, releasing RNA. In the nucleus, viral RNA is copied to messenger RNA which moves back to the cytoplasm as a template to make more viral proteins. Copies of the viral RNA join with the viral proteins to make more viruses, which bud on the outer surface of the cell and seek new cells to infect.

The "shape" of receptors in the cell wall is a little different from one species to another, so a virus that can latch onto a chicken cell usually can't infect a human. But the process isn't exact, and there are minor variations from one organism to another, even within a species. The 100 or so people in Southeast Asia who have died of avian flu infections over the past year may have had just enough variation in their cell structure to allow the avian virus to attach. Or, a few copies of the avian virus may have mutated enough to infect a human. The fact that only a few cases have been reported argues for the former explanation.

Some species have the misfortune to be about halfway between birds and humans. Pigs, for example, are susceptible to both avian and human strains of flu. What scientists fear is that a pig somewhere may be infected with both viruses at the same time. With proteins and RNA strands from both viruses floating around inside the cell, new viruses might assemble containing a reassortment of components, perhaps with the proteins that attach to a human cell but with other features that give it the virulence of the avian virus, including the ability to infect cells outside the respiratory tract.

Antiviral drugs can interrupt the process by which a virus reproduces at several stages. Amantadine, for example, prevents the virus particle from opening after it enters the cell and can inhibit the manufacture of virus proteins. Relenza and Tamiflu stop the virus from budding out of the cell. But antiviral drugs are helpful only if they are given within a day or two after the onset of the disease. Their most important application is to prevent the spread of disease within a family or community by giving them to people who have been in contact with an infected person. Again, notes Whittaker, this approach can only be used if the disease is diagnosed promptly.

Whittaker and Chu have found that attaching to a single receptor is not sufficient for the flu virus to enter a cell. Another receptor or some other process must be involved. The primary receptor, already extensively studied, varies from one virus to another, but whatever the additional step is, it seems to be the same for many different flu viruses, and perhaps for all. "It's too soon to be sure, but we've tested it with a wide range of viruses, and they all behave the same," Whittaker says. Understanding the process could lead to the development of new antiviral drugs or even a vaccine effective against all influenza viruses, he says, adding that such a result is still a long way off.

Even with the limited vaccines now being deployed, Whittaker points out, our vaccination strategy needs to change. Presently, the strategy is to give a vaccine to the elderly and others most at risk. But many experts are now advocating that we first vaccinate children, because schools and day-care centers are the prime transmission points for disease. A new vaccine that is inhaled as an aerosol spray may make this easier -- children don't like needles -- but so far it has been approved only for adults.

This is the third of five articles in the Chronicle Online series detailing Cornell researchers' roles in the global battle against avian flu.