Protecting Against Infectious Diseases

The mutations in the influenza virus that complicate the development of seasonal vaccines could in fact hold the key to improving the effectiveness of flu vaccination. It’s an interesting twist that Nicholas Wu, PhD, a research associate at The Scripps Research Institute in La Jolla, Calif., and colleagues recently discovered by introducing random mutations to the receptor binding site of hemagglutinin — the protein found on the surface of the influenza virus.

They discovered that various individual mutations and combinations of mutations prevented some viruses from infecting cells, while some mutations allowed the viruses to infect the cells and continued to replicate. Figuring out which mutation to target with antibodies requires staying one step ahead of the ever-changing flu virus, according to Dr. Wu, who took a few minutes to discuss how his promising new findings could be used to build better defenses against the flu and other infectious diseases.

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Why did you focus on the receptor binding site of hemagglutinin?

We zeroed in on that area because it’s the first to engage the virus for entry into host cells. Hemagglutinin allows the virus to stick to and fuse with cell membranes, and its receptor binding site mediates the first step of that binding process. The site is flu’s first point of contact and also where a great deal of antibody response takes place — antibodies generated in the human body target the site and neighboring regions. On the one hand, the site is important for function of the flu virus, but, on the other hand, it’s always under immune pressure.

Why did you concentrate on assessing influenza mutations?

One critical issue in influenza research is evolution of the virus, which necessitates yearly updates to the seasonal flu vaccine. We wanted to identify possible mutations that could be generated by the virus to stay one step ahead of that evolution. We genetically engineered approximately 20,000 mutations onto the receptor binding site of influenza H1N1 and H3N2 strains to determine which mutations impacted replication of the virus. We found that nearly all single mutations proved lethal to the virus, but approximately 20% of the mutations allowed the virus to thrive when they worked in combination with other mutations. That was interesting.

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Your study mentioned “epistatic effect.” What is that?

It means the combined effect of two individual mutations can’t be predicted ahead of time. It also means the effect of a single mutation depends on the presence of other mutations. In nature, mutations occur by chance. A lot of single mutations would kill the flu virus, but when combined mutations occur, the virus can survive, and the possible mutation diversity at the receptor binding site is unexpectedly high, because of the epistatic effect. That’s something pharmaceutical manufactures must consider when developing new drugs and vaccines.

What’s the next step in your research?

We examined just one example of how mutations might impact virus replication. There are many, many more sites to assess. Moving forward, we want to determine how the epistatic effect impacts functional sites that are being considered as vaccine targets and to look at flu diversity, not only in single mutations, but also in combinations of mutations. I’d imagine no site is 100% intolerant, but the question regarding vaccine development is figuring out how to manage the mutation diversity. The site that turns out to be the most intolerant to mutations will likely be the best site to target with future forms of vaccines.

How will your findings impact the development of more effective vaccines?

A better understanding of mutation combinations that are permissible — and which are not — could help researchers narrow down the spectrum of mutations that should be targeted with antibodies and antiviral molecules. This study showed that the evolution of the influenza virus can surprise us, but if we know ahead of time what kind of mutations — single or in combinations — can be accommodated at a particular receptor site on the virus, we'll have a better idea of how to develop drugs or antibodies to target that site. We also need to consider the epistasis effect in other viruses such as HIV.

—Dan Cook