The Scientific Clues to Reducing Flu Epidemics

Our understanding of vaccines has come a long way since the 1918 flu epidemic, and scientists continue to advance the research in this field.

Published March 1, 2002

By Lorrence H. Green, PhD
Academy Contributor

In 1918 a global influenza pandemic is estimated to have killed between 20 and 40 million people. Today, influenza –– a negative stranded RNA virus that causes respiratory disease–– is responsible for about 20,000 deaths a year in the United States. In severe epidemics, the death toll can be much higher.

Because the genetic structure of the influenza virus changes each year, due to genetic drift, new vaccines must be in constant development. Significant genetic shifts occur about every 20-40 years, resulting in major genome changes and influenza pandemics. At a recent Microbiology Forum held at The New York Academy of Sciences (the Academy), Dr. Adolfo Garcia-Sastre, of New York’s Mount Sinai School of Medicine, described molecular research being undertaken to design improved influenza virus vaccines.

Garcia-Sastre explained that the influenza genome contains eight genetic segments coated by nucleoproteins and that it is surrounded first by a matrix, and then by a lipid bilayer envelope. In its replication cycle, the virus first binds to receptors on the cell surface; then it is incorporated. Following this, the negative strand RNA is copied, forming a double stranded structure.

An Unusual RNA Virus

Influenza is unusual in that it is an RNA virus that is replicated in the infected cell nucleus, as opposed to the cytoplasm. Important influenza proteins include: hemagglutinin (HA), which is responsible for binding to the cell receptor; neuraminidase (NA), which is responsible for budding off new influenza viral particles; the matrix (M1) and membrane proteins (M2); the nucleoprotein (NP), which is associated with the RNA genome; the transcriptase components (PB1, PB2 and PA), which are responsible for copying the negative RNA strand; and the NS proteins (NS1, NS2), whose functions were discussed.

Garcia-Sastre noted that negative stranded influenza RNA is not infective without its associated proteins. He described research in which genetic influenza material was inserted into a plasmid system. Then a second plasmid was developed that contained the genetic information the influenza proteins required for replication. By using both plasmids to transfect a cell, one could get influenza replication. This system could be used, he said, to specifically genetically alter the influenza genes and develop viral strains that would be useful as vaccines.

A Critical Protein

Using this system, Garcia-Sastre said NS1 was found to be a critical protein that could be exploited for vaccine development. Deletion experiments found that the function of NS1 was to allow the influenza virus to avoid the effects of the anti-viral protein, interferon. The NS1 protein was purified and studied. It was found to have three domains: an RNA binding domain, an eIF-4GI binding domain, and an effector domain. Further deletion experiments were conducted and showed that the anti-interferon activity was confined to a portion of the RNA binding domain. As one deleted more of the RNA binding domain, the influenza virus lost its pathogenicity.

Exploiting this finding to develop vaccine strains presented a problem, however. As larger portions of this domain were deleted, the virus that was generated became less immunogenic. The experiments suggested the possibility of removing just enough of the genetic coding material for the RNA binding domain of NS1 to create a vaccine virus that was not pathogenic, but that could still be used to induce long-term protection. These experiments are currently underway

Also read:  Antibodies, Vaccines and Public Health