Scientists Describe Structure of Receptor on Surface of Virus That Caused Deadly 1918 Flu Epidemic

By Jason Socrates Bardi

A team of scientists at The Scripps Research Institute has described for the first time the structure of a protein from the deadly 1918 "Spanish Flu" virus—a virus that took more lives than World War I and became the largest and deadliest influenza outbreak in recorded history. Their work reveals some of the virus's deadly secrets.

"Why was this so devastating an outbreak?" asked lead investigator Ian Wilson, a molecular biology professor and a member of The Skaggs Institute for Chemical Biology at Scripps Research.

Seeking the answer led Wilson, Senior Scripps Research Associate James Stevens, Scripps Research Associate Adam Corper, and several colleagues, including Jeffery K. Taubenberger from the Armed Forces Institute of Pathology in Washington, D.C., and Christopher F. Basler and Peter Palese of Mount Sinai Institute of Medicine in New York, to describe the structure of a protein called hemagglutinin from the 1918 flu virus. This "antigenic" surface protein is the first structure of this extinct virus to be solved.

Their research will be published in an upcoming issue of the journal Science.

A Devastating Infection

Influenza is a common viral infection of the lungs that affects millions of people annually and is a leading cause of death in the United States, contributing to around 50,000 deaths per year. Influenza outbreaks like the one in 1918 occur when a virus adapted to birds jumps directly into humans or reassorts and infects another species, such as the pig, and then jumps into humans. Similar outbreaks occurred in 1957 and 1968.

The 1918 outbreak was remarkable not only because it caused such a large number of deaths—675,000 in the United States and up to 40 million worldwide—but also because it inflicted such high mortality rates, reaching 70 percent in some communities.

Research on the molecular biology of the virus that caused the 1918 outbreak was complicated by the fact that in 1918, the cause of the disease was not known. Viruses were not identified as the cause of influenza until the 1930s, and lung tissue samples taken in 1918 are generally unreliable sources because the virus degrades easily.

However, biopsies from soldiers who died from influenza in 1918 were preserved and maintained in the Armed Forces Institute of Pathology. Another sample was taken from an Inuit woman who had succumbed to the infection and had been buried in the Alaskan permafrost.

Together, these samples yielded a number of pieces of RNA from the virus. A few years ago, Taubenberger and his colleagues at the Armed Forces Institute of Pathology were able to piece together enough fragments to reconstruct the sequence of the gene that coded for the viral protein hemagglutinin. These are the oldest viral sequences that have been reconstructed to date.

Then Basler and Palese at Mount Sinai Institute of Medicine in New York managed to construct an expression system that allowed them to make the hemagglutinin protein. Finally, Wilson and Stevens developed their own systems and made enough of the protein to crystallize and solve the structure using x-ray crystallography.

The structure of the hemagglutinin protein, one of only a handful of proteins made by the virus, reveals details that may be crucial to understanding the 1918 outbreak.

The Hemagglutinin Receptor

A large, glycosylated protein that forms from three identical 550 amino acid chains, hemagglutinin is abundantly displayed on the surface of the influenza virus. It is the receptor responsible for the virus infecting cells of the host organism.

During an infection, the virus enters the airways and travels to the epithelial cells lining the lungs. There, the hemagglutinin on the surface of the virus binds to lung epithelial cell receptors containing sialic acid, which allows the virus to be internalized into the epithelial cell, through something known as the endosomal pathway, and this establishes an infection.

Hemagglutinin is also the main antigenic determinant on the virus—it is what the human immune system primarily recognizes and responds to by making antibodies and mounting an immune defense. How deadly an individual influenza infection is depends on how well one person's immune system recognizes the hemagglutinin.

One of the questions the researchers asked was whether the Spanish Flu entered human populations from pigs or some other mammal that contracted it from an avian source or whether it jumped directly from birds to humans. In order to address this, Stevens, Wilson, and their colleagues solved the structure of the 1918 influenza hemagglutinin, compared it to hemagglutinin proteins from other human, avian, and pig viruses, and described all of this in their Science paper. Another article in this week's Science (by S.J. Gamblin et al) describes the structure of 1918 hemagglutinin and two closely related hemagglutinin proteins with receptor analogues.

The structure that Stevens, Wilson, and their colleagues solved has features primarily found in avian viruses. "It looks more like an avian virus—with some human characteristics," says Wilson. This suggests why the virus may have been so deadly. Avian-to-human transmission is rare and because of this has the potential to be more deadly. Because the surface proteins of the 1918 virus were different from those found on other flu viruses, people's immune systems were unaccustomed to them and unable to fight off the Spanish Flu.

An avian origin of the virus also suggests an explanation for one of the most unusual features of the 1918 outbreak—that mortality was particularly high among young adults, the age group that is usually least impacted by the flu. Influenza is normally more deadly to the elderly and preadolescents within a population, but in 1918, there were a surprising number of deaths among 15 to 34 year olds. The avian nature of the structure suggests the older age group may have been partially protected by exposure to a similar virus in an earlier epidemic.

The research article, "Structure of the uncleaved human H1 hemagglutinin from the extinct 1918 influenza virus," is authored by James Stevens, Adam L. Corper, Christopher F. Basler, Jeffery K Taubenberger, Peter Palese, and Ian A. Wilson and appears in Science Express on February 5, 2004. Science Express provides rapid electronic publication of selected papers in the journal Science. Print versions of these papers will appear in Science after several weeks. See:

The research was funded by the National Institutes of Health, an Ellison Medical Foundation New Scholarship in Global Infectious Diseases, an Ellison Medical Foundation Senior Scholarship, the intramural funds of the Armed Forces Institute of Pathology, and the Skaggs Foundation for Research.



Investigators James Stevens (left), Ian Wilson, and colleagues solved the structure of a surface receptor on the "Spanish Flu" virus. Photo by Kevin Fung.













Ribbon representation of the hemagglutinin HA0 trimer from the 1918 influenza virus. Image courtesy of James Stevens. Click to enlarge.