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Stalking The Mutating Monster

Stalking The Mutating Monster

Stalking The Mutating Monster (continued)

By Melissa Hendricks

THE CAPRICIOUS NATURE of influenza's protein surface also poses a challenge to vaccine developers. Two classes of proteins figure most significantly. One is called the hemagglutinins (HAs), which influenza uses to latch onto cells. Scientists have identified 16 different HAs, known as H1 through H16. They have also identified nine members of another class of surface proteins known as the neuraminadases (NAs), designated N1 through N9. Flu subtypes are named according to the HAs and NAs that populate their surface. So bird flu is influenza subtype H5N1. Subtypes now circulating in the population include H1N1 and H3N2.

One might suppose that a vaccine targeting the most prominent influenza subtypes would work well and last long. But influenza has not allowed such an easy solution. Thanks to its high rate of mutation, influenza's HA proteins constantly change or "drift" into subtly different forms. This process, known as antigenic drift, results in myriad strains. So, for example, there is not just one strain with the appellation H1N1; there are many.

To craft a seasonal flu vaccine, scientists predict which influenza strains are likely to circulate in the coming season. They then use killed or weakened versions of those strains to construct a vaccine that will stimulate an immune response targeting those particular strains. If all goes well, when a vaccinated person comes into contact with a live version of those influenza strains, their immune system launches an attack that neutralizes the virus.

But this approach has its limitations. First, because flu strains constantly drift, vaccine developers must reformulate their product every year. And even then, the process is not always perfect. Sometimes the experts err in their predictions, and fail to include a strain in the vaccine that ends up causing infections.

Influenza has another trick, however, with even graver consequences, a process called antigenic shift. It occurs when an animal strain of influenza mutates or when a human influenza strain and an animal influenza strain mix genes, giving rise to a novel influenza subtype. Introduced into a population with no baseline immunity against that subtype, the virus would spread extremely quickly and cause severe illness. It would launch a pandemic.

After the 2003 bird flu outbreak and the ensuing fear of a flu pandemic, researchers increased efforts to design a pandemic flu vaccine. That task has proven more difficult than researchers had anticipated, says Ruth Karron, MD, director of the School's Center for Immunization Research. She has overseen clinical trials of several of the candidate live, attenuated vaccines, which included components of viruses thought to have pandemic potential, including H5N1. While the live, attenuated vaccines performed well in animal studies, some were overattenuated (overly weakened) in human volunteers, and did not induce a strong immune response, says Karron, an International Health professor.

Research teams are now using those initial results to improve their vaccines. Several of the candidates look promising, says Karron. Pekosz, however, has taken a different strategy. Instead of focusing on influenza's HAs and NAs, he's turned his attention to a different protein called M2.

It turns out that the M2 protein, unlike the HAs and NAs, is virtually the same in all strains of influenza, including those with pandemic potential. In a biologist's parlance, M2 is "highly conserved." That fact has led Pekosz (and several other researchers in the field) to ponder whether a vaccine built around the M2 protein would stimulate immunity against all strains of influenza.

In other words, it would be a universal influenza vaccine.

With this hope, Pekosz is engineering two types of vaccines based on the M2 principle. One is an attenuated vaccine, a vaccine that contains a live but crippled virus. (The FluMist vaccine available for seasonal flu, for example, is an attenuated vaccine.) For this approach, Pekosz genetically manipulates the M2 gene to result in a virus that contains a longer-than-normal M2 protein. The goal is to craft a virus that can replicate just enough to provoke the immune system, but not enough to cause a full-blown infection.

So far, says Pekosz, animal studies conducted over the past few years have revealed promising results for several of his attenuated vaccines. The experiments involved inoculating laboratory mice with the experimental vaccines, and then exposing the mice to two different strains of influenza that have recently circulated in the human population. The mice remained healthy, while unvaccinated mice serving as a control group succumbed to the infection. In fact, says Pekosz, the experimental vaccines appear to provide even stronger immune protection than conventional flu vaccines.

Pekosz views those results as one of the small victories that sustain scientists. "If you're fortunate, those moments can feed you fuel to go another couple years."

While he's demonstrated a proof of principle, says Pekosz, much work likes ahead. Ultimately, he hopes his strategy will yield a vaccine that protects against three, six, twelve or more strains of the virus. He does not yet know whether his current experimental vaccines will confer such broad immunity. Achieving that goal may take several years. "It's a work in progress," he says.

The tricky part for Andy Pekosz is not knowing if he's pursuing a reasonable but ultimately futile path. "It is certainly possible to go down a dead-end," he says. "The challenge is knowing when to back up."

But other challenges lie ahead. One relates to the fact that mice are not people. While mice can crank out a robust immune response to M2, people do not. No one yet understands why not. So in his second line of vaccine research, Pekosz is seeking a way to boost that weak immune response.

For this approach, the virologist is constructing vaccines that combine the M2 protein with a core protein from the hepatitis virus. Scientists have found that the hepatitis virus is especially skilled at "showing" protein antigen to the immune system. If his plan works as he hopes it will, the hepatitis protein will showcase the M2 protein, and the immune system will "see" the show and will respond by making a generous amount of M2 antibody. Initial tests of the vaccine in mice are encouraging, says Pekosz. Clinical tests of the vaccine may be years away.

The tricky part for Andy Pekosz is not knowing if he's pursuing a reasonable but ultimately futile path. "It is certainly possible to go down a dead-end," he says. "The challenge is knowing when to back up."

Some of Pekosz's colleagues underscore the difficulty of his task. They include his postdoctoral advisor, Northwestern University virologist Robert Lamb, who discovered the M2 gene and protein. If the immune system could target M2, he asks, "wouldn't nature have done it already?"

The ultimate test of his vaccines, however, will be to see if they protect against pandemic strains of influenza. Such studies require extra levels of safety precautions. So for the last leg of his tour, Pekosz takes his visitor to another laboratory that he has been retrofitting as a Biosafety Level 3 lab. The lab must meet the extra levels of safety precautions mandated by the CDC for labs handling potentially lethal respiratory viruses. So workers have been installing special air ventilation systems, door seals, autoclaves and high-level security systems. A box of disposable paper booties sits outside the entrance to the lab's main chamber. Entering the chamber requires full protective gear: foot coverings, coveralls, outer apron, gloves, as well as a hood and face mask connected to an air pressure monitor that sends a constant stream of air flowing away from the face to prevent the inhalation of airborne virus. The protective gear can get hot and uncomfortable. Researchers will have to work for several hours without a break to avoid repeated changes. (Pekosz has calculated each single-use outfit costs about $14.) "It's not easy work," he says.

The goal, however, supersedes the difficulties of the work and uncertainties of the results. "We have a tendency to forget how frightening pandemics are," says Pekosz. Most people have not experienced pandemic flu firsthand. But anyone who has suffered through a bout of seasonal flu has had a milder preview.

Pekosz himself came down with a textbook case of flu about three years ago—high fever, body aches, a dry hacking cough, aches and pains, and listlessness. He was miserable. A pandemic strain would cause a vastly more severe illness, and Pekosz can only imagine such agony.

"I'd been a good boy and had gotten my vaccination," says Pekosz. "I still got flu." Apparently, he'd gotten infected by an antigenic-drift variant. That knowledge compounded his misery at the time, says Pekosz, but also reaffirmed his conviction in his research. A universal flu vaccine might have prevented him from getting sick.

"It's an ongoing battle," he says. "There must be a way to prevent the virus from circulating. It's just a matter of finding the right approaches."

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