Government researchers reported this week that they have developed a vaccine that protects mice from the deadly 1918 "Spanish flu" virus, demonstrating that immunization against it is possible.

The scientists, led by Gary J. Nabel, MD, PhD, of the National Institute of Allergy and Infectious Diseases (NIAID), created a vaccine using a piece of DNA from the 1918 virus, called H1N1. Mice that were injected with the vaccine and later exposed to the 1918 virus all survived, according to the report in the Proceedings of the National Academy of Sciences (PNAS).

The study also showed that the vaccine induced the production of protective antibodies and that these antibodies could be used to protect unvaccinated mice from the virus.

In addition, the researchers report that they devised a new technique for identifying antibodies that can neutralize dangerous influenza viruses, using an approach that reduces lab workers´ exposure to those viruses.

"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," NIAID Director Dr. Anthony S. Fauci commented in a news release.

The Spanish flu pandemic of 1918-19 killed as many as 100 million people worldwide. In recent years scientists have reconstructed the 1918 virus by analyzing samples from frozen bodies of flu victims in Alaska and from an Army collection of autopsy specimens. They have studied its effect on mice in an effort to learn why it was so lethal.

Nabel, director of the NIAID´s Vaccine Research Center, told CIDRAP News, "There has always been this question, Was there something special about this virus in terms of its ability to evade immunity and resist what have become traditional approaches to vaccination? Do we have to do something differently to get a good vaccine?"

The study shows, he said, "It´s in fact reasonably straightforward to develop the vaccine strategy that can handle the virus pretty readily, and by doing that it gives us an opportunity to ask questions about how that occurs from the mechanistic point of view."

To create a vaccine, the team created plasmids, or short strands of DNA, carrying genes for the 1918 virus´s hemagglutinin (HA) protein, the surface protein that enables the virus to attach to and enter host cells, according to an NIAID news release. (In contrast, conventional flu vaccines typically use killed whole flu viruses or live but weakened viruses rather than small pieces of viral DNA.) The researchers created one plasmid reflecting the normal (wild) form of the virus´s HA and one that was modified to represent a weakened form.

Mice that were injected with vaccines containing the plasmids showed both cellular and humoral immune responses, in the form of T-cells and neutralizing antibodies, according to the report.

Two weeks after vaccination, in a Biosafety Level 3 lab at the Centers for Disease Control and Prevention, the vaccinated mice and unvaccinated controls were given large intranasal doses of the 1918 virus. All the vaccinated mice survived, while the control mice all died.

To assess how the vaccine worked to protect the mice, the researchers depleted one group of vaccinated mice of T-cells and then dosed them with the virus; they all survived. Further, the team transferred antibodies, in the form of purified immunoglobulin-G (Ig-G), from vaccinated mice into unvaccinated mice and then exposed the latter to the virus. Eight of 10 of these mice survived. In contrast, mice that received Ig-G from an unvaccinated control group all died.

"Antibodies confer all of the protection against the virus, we learned from the study," said Nabel. As in mice, immunity in humans probably would depend on antibodies, but the possibility of a role for cellular immunity in humans can´t be excluded, the PNAS article states.

The study provides "proof of concept" that immunization against the 1918 virus is possible, but it doesn´t mean that a human vaccine for the virus would have to be a DNA vaccine, according to Nabel.

"Once we know the mechanism, that it´s antibody-mediated, it tells us that there are many different ways to get this type of antibody vaccine," he said. "I think there´s no need for us to rely on a DNA vaccine in humans. It should be applicable to a variety of platforms." The article says that DNA vaccines generally have been much less effective in humans than in rodents.

To test the 1918 vaccine´s antibody-inducing power while limiting the risk to lab workers, the scientists created artificial viruses, or "pseudoviruses," by grafting the 1918 virus HA onto a weakened form of lentivirus (the virus family that includes HIV).

The pseudoviruses were incubated with blood samples from immunized and nonimmunized mice. The researchers found that the antibodies from the immunized mice neutralized the pseudoviruses, while the samples from the nonimmunized mice had no effect, the NIAID said.

"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," Nabel said in the NIAID release.

"The other advantage of this new assay is that it´s much more sensitive than the previous method of detecting antibodies: hemagglutination inhibition or microneutralization," Nabel told CIDRAP News. "And there´s a very good correlation so far between what we see in this assay and the traditional ones." He also said the technique is straightforward enough to be widely used.

Nabel said he thinks there´s a "remote chance" that the 1918 virus could once again infect humans, perhaps through a lab accident. Some experts think that people might have some natural protection from it because of past exposure to H1 viruses, he added.

"I think the one scientific piece of information is that we certainly can generate a vaccine that would be protective, and we know how it works," he said.

Kong W, Hood C, Yang A, et al. Protective immunity to lethal challenge of the 1918 pandemic influenza virus by vaccination. Proc Nat Acad Sci 2006;103(43):15987-91 [Full text]