Critical Gene a Suspect in Lethal Epidemic

October 7, 2004


By recreating the influenza virus that killed up to 50 million people in 1918-19, researchers may have identified the gene that turned it into one of the most lethal in human history.

The gene, one of eight in the virus, seems to have an unexpected capacity for sending the body's immune system into overdrive, causing inflammation, hemorrhage and death, the scientists report today in the journal Nature.

The research team, led by Dr. Yoshihiro Kawaoka of the University of Wisconsin, has been trying to determine just why the 1918 virus was so lethal and how defenses could be devised if a similar virus appeared in the future. Although the virus has long since perished, Dr. Kawaoka and his colleagues were able to recreate it because the composition of its genes had been reconstructed from the preserved tissue of victims. The genes have been reconstituted over the last few years by Dr. Jeffery K. Taubenberger and colleagues at the Armed Forces Institute of Pathology in Washington.

Dr. Kawaoka, Dr. Taubenberger and others have been reinserting the 1918-type genes into ordinary flu viruses to see if they can pinpoint which of the genes made the virus so lethal and how it did so. In the latest of these experiments, which Dr. Kawaoka reports today, a gene called the hemagglutinin or HA gene seems to be largely responsible for the dire effects of Spanish flu, as the 1918 epidemic is also known.

Recreating such a dangerous organism is not an experiment to be undertaken lightly. Dr. Kawaoka's approach required replacing the HA and another gene in a mild flu virus with the Spanish flu versions and infecting mice with the novel agent. Because of the obvious hazards, he at first conducted the work in the most secure type of biological laboratory, designated Biosafety Level 4, one of which was available at the National Microbiological Laboratory in Winnipeg, Canada.

He said that after satisfying himself that the souped-up virus was susceptible to an antiviral agent known as Tamiflu, he transferred the research to a Biosafety Level 3 laboratory at the University of Wisconsin.

Dr. R. Timothy Mulcahy, chairman of the university's biosecurity task force, said that the chances of escape from the Biosafety Level 3 facility were minimal and that Dr. Kawaoka had been "extremely prudent" in starting out at the higher level. "If there were an escape there would be treatments," Dr. Mulcahy said. He noted that another group of researchers had already worked with similar engineered flu viruses in a Level 3 facility owned by the Department of Agriculture in Athens, Ga.

The HA gene studied by Dr. Kawaoka's team is well known to flu experts because it changes from year to year. Since the protein made by the gene is the one singled out for attack by the immune system, the body's defenses are caught off guard each year as flu virus arrives with a novel version of the protein to which the body has no prior immunity.

The HA protein's role is to latch onto the surface of human cells and then help the virus merge into the cell's outer membrane. Researchers recently worked out the exact three-dimensional structure of the Spanish flu version of the HA protein, but could see no other function that it was designed to serve. The same is true of the other Spanish flu genes recovered by Dr. Taubenberger. In the current state of knowledge, the genes betray no clear hint of what makes them so lethal.

That makes necessary experiments like Dr. Kawaoka's, in which researchers physically reconstruct the virus and try to understand how it works. What he has now found is that the Spanish flu version of the HA gene, in addition to its break-in and enter role, seems able to trigger the release of cytokines, the signaling agents with which the immune system gears itself up for massive attack against an infectious agent.

Uncontrolled overdrive can make the immune system kill the body in order to save it, through excessive inflammation. The virus carrying the Spanish flu version of the HA gene produced high levels of cytokines in mice, Dr. Kawaoka says, and this is probably what led to the inflammation and lung damage that killed them.

Dr. Adolfo Garcia-Sastre, a flu expert at the Mount Sinai School of Medicine who has constructed a similar virus, said the HA gene might be causing extra virulence simply by helping the virus replicate better, not because of any special effect on cytokine production. But either way, the finding helped focus attention on the gene's role, he said.

Survivors of the 1918 epidemic have high levels of antibody to the engineered virus, Dr. Kawaoka reports, but people infected recently with a similar class of flu virus do not. "Thus, a large section of the population would be susceptible to an outbreak of a 1918-like influenza virus," he and his colleagues conclude.