These findings are important, the researchers say, to understanding and preventing the recurrence of the H1N1 influenza virus that caused the 1918 pandemic and to protecting against virulent flu strains in the future, including the H5N1 avian flu virus. Details of the research are available online this week in Proceedings of the National Academy of Sciences.
Gary J. Nabel, M.D., Ph.D., director of the Vaccine Research Center (VRC) at the National Institute of Allergy and Infectious Diseases (NIAID), part of the National Institutes of Health (NIH), led the research team in developing the experimental vaccines and conducting the immunological studies in mice. Terrence Tumpey, Ph.D., of the Centers for Disease Control and Prevention (CDC) conducted vaccine studies in mice involving the live, reconstructed 1918 flu virus in a biosafety level 3-enhanced laboratory at the CDC in Atlanta--one of four types of specialized biosafety labs where scientists study contagious and potentially deadly materials under high-level safety and contamination precautions designed to protect the researchers and prevent microorganisms from entering the environment.
"Understanding why this influenza virus was so deadly is an extremely important question," says NIAID Director Anthony S. Fauci, M.D. "By building upon earlier research where scientists successfully reconstructed the 1918 pandemic flu strain, Dr. Nabel and his colleagues have demonstrated that this virus is vulnerable to intervention. This knowledge will help further our continued efforts to develop treatments and vaccines to protect us against another deadly flu pandemic."
The 1918-1919 influenza pandemic was the most deadly flu outbreak in modern history, killing 50 million or more people worldwide.
"A key to containing pandemic flu viruses is to understand their vulnerabilities and determine whether they can evade immune recognition," says Dr. Nabel. "What we learn about the H1N1 virus that caused the 1918 pandemic is pertinent to other pandemic viruses and to the development of effective and universal vaccines."
Using the genetic sequence information for the 1918 flu virus, Dr. Nabel and his VRC colleagues created plasmids--small strands of DNA designed to express specific characteristics--carrying genes for the virus' hemagglutinin (HA) protein, the surface protein found in all flu viruses that allows the virus to stick to a cell and cause infection. The researchers created two types of plasmids: one to reflect the HA found in the original 1918 flu virus; the other an altered HA protein designed to attenuate (weaken) the virus.
Mice were then injected with a DNA vaccine containing both types of plasmids to determine whether they would generate immune responses to the 1918 virus. The researchers found significant responses both in terms of production of T-cells, the white blood cells critical in the immune system's battle against invading viruses, as well as the production of neutralizing antibodies.
To determine the vaccine's protective effects, the CDC's Dr. Tumpey intranasally exposed a group of mice to live, reconstructed 1918 virus 14 days after they were immunized with the experimental DNA vaccine. All 10 vaccinated mice survived the challenge with the deadly virus. To explore how the vaccine protected the animals, the researchers first depleted other mice of T-cells; however, this had no effect on the immunity of the vaccinated mice to the 1918 virus. In contrast, the researchers discovered that transferring antibody-rich immunoglobulin (IgG) from immunized mice to non-immunized mice resulted in antibody levels in the animals at levels only slightly lower than those that were immunized. Further, when the animals were exposed to the reconstructed 1918 flu virus, 8 of 10 mice that received antibodies from the immunized mice survived; none of the 10 mice that received IgG from the unvaccinated control group survived.
"By using an existing pandemic flu strain, this research will provide the basis for design of alternative vaccines against influenza viruses with enhanced virulence," says Dr. Tumpey.
Although the researchers are not discounting the potential role T-cells may have in combating flu viruses, they concluded that in this study, the experimental DNA vaccine protected the mice by stimulating antibodies capable of neutralizing the 1918 flu virus.
"Who would have imagined five years ago that we'd be able to create a vaccine that protects against one of the deadliest forms of influenza the world has ever seen?" adds Dr. Nabel. "It's because the 1918 flu virus has been reconstructed that we are now able the further understand it. Hopefully, this virus will help us to develop effective vaccine strategies for current pandemic influenza virus threats."
To evaluate the vaccine's antibody-inducing capabilities while minimizing exposure of lab personnel to the 1918 flu virus, Dr. Nabel and his VRC colleagues also created artificial viruses, or pseudoviruses, featuring the HA of the 1918 flu virus but stripped of the ability to cause infection. The pseudoviruses were then incubated with antibody-containing blood samples from the mice immunized with the DNA vaccine and those that were not. The researchers found that the antibodies from the immunized mice neutralized the pseudoviruses while the blood samples from the mice that were not immunized had no effect. This method was also effective in identifying neutralizing antibodies to the H5N1 avian flu virus and could be used to screen for monoclonal antibodies that may be used as an antiviral treatment, according to Dr. Nabel.
"This technique would be very useful in defining evolving serotypes of flu viruses like H5N1 to develop immune sera and neutralizing monoclonal antibodies that may help to contain pandemic flu," says Dr. Nabel.
The study authors indicate that further testing will be needed to determine whether DNA vaccination can confer immune protection in people similar to that seen in the study mice. Additionally, the use of DNA-based vaccines are being explored as a potential strategy for creating vaccines to protect against the H5N1 avian flu virus.
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