Currently available flu vaccines are based on the rapidly changing viral hemagglutinin (HA) antigen, and are therefore quickly outdated. Thus new vaccines need to be developed each year.

Now, a new study published in the journal Science on October 25, 2019, reports on a set of three novel antibodies that bind to another type of viral cell surface antigen called neuraminidase (NA) that is necessary for viral replication. These antibodies bind to NA from multiple strains and subtypes of the virus, indicating a broad range of protective action. This could propel the development of both preventive and therapeutic modes against influenza.

The conventional path – anti-HA antibodies

Many scientists have tried to find a broadly protective vaccine that will be effective against more than just one or a very few strains of the virus. The issue with HA-based vaccines is the frequent changes that occur in the protein with mutations in the viral DNA. This is why a new vaccine must be devised with each flu season, updating the HA antigen to match that of the viral strains in current circulation. Researcher Ali Ellebedy, who first discovered the antibodies in a blood sample, says, “There are many strains of influenza virus that circulate so every year we have to design and produce a new vaccine to match the most common strains of that year.”

The innovative route – anti-NA antibodies

On the other hand, NA antigens change at a much slower rate, and this inspired the researchers in the current study to base their vaccine development on these proteins. The use of these antigens could well improve the efficacy of preventive vaccines and therapies against influenza. Says Ellebedy, “One vaccine that protected against all influenza strains, including human, swine and other highly lethal avian influenza viruses… this antibody could be the key to design of a truly universal vaccine.”

The clinical efficacy of anti-NA antibodies was shown in previous years when the history of previous infection with earlier strains of the virus protected individuals against the current strains as well.

The study

The researchers studied antibody-producing lymphocytes from the serum of a flu-infected volunteer patient who had become sick with the H3N2 strain in 2017. The samples were taken on the fifth day of infection, as part of a broader study of the immune response to flu.

They looked for monoclonal antibodies (mAbs), derived from a single cell line. These are specifically designed to bind to a single protein only. The mAbs were then screened to test their ability to bind to various influenza antigens. In this case, the mAbs bound not only to the major HA antigen but also to many other antigens. They found that among 45 mAbs, there were three which were anti-NA antibodies, able to bind to the H3N2 strain of the virus. On further testing, they found that these antibodies were binding to NA antigens of all known types – from flu viruses isolated from humans as well as animals.

“At first, we didn’t believe it.”

This widespread activity was a great surprise to the researchers, despite knowing that NA antibodies are typically directed against multiple viruses within a subtype. Why the astonishment? It was because these mAbs acted against different subtypes of the virus – binding NA from both influenza A and influenza B. Indeed, neuraminidase specialist and researcher Florian Krammer says, “At first, we did not believe our results. It is amazing what the human immune system is capable of if presented with the right antigens.”

Tamiflu, which is the antiviral drug most extensively used to fight severe influenza, works by suppressing NA activity. However, there are several NA variants and drug resistance is also developing, which means that the drug doesn’t always work as well as expected.

Successful testing in mice
In light of this phenomenon, the researchers wanted to test if these three mAbs could prevent the development of influenza infection in mammalian cells. They selected the mouse as an experimental mammalian model and infected them with different strains of influenza.

The results showed that multiple NA proteins derived from multiple strains of the virus in humans, birds and pigs were blocked successfully by these antibodies. In other words, they were protective against influenza virus A groups 1 and 2, as well as some influenza B viruses.

Moreover, in mice, severe influenza was prevented in most cases. Even the injection of lethal doses of the H3N2 virus failed to kill mice who were treated with low doses of the three mAbs at up to 72 hours after injection.

Ellebedy says joyfully, “They definitely got sick and lost weight, but we still saved them. It was remarkable. It made us think that you might be able to use this antibody in an intensive care scenario when you have someone sick with flu and it’s too late to use Tamiflu.” This situation is often seen, because Tamiflu is only effective within 24 hours of the onset of symptoms.

How anti-NA antibodies work
However, there is a lot of work to be done before such an approach can be designed based on these results. The first step was to see if they could find out how the antibody bound to the NA antigen. Structural biologist Ian Wilson stepped in here to help map, with Xueyong Zhu, the antibody structure within the antigen-antibody complex – that is, in bound form. They found that each of the three mAbs had a long loop that inserted right into the active site of the NA antigen, disabling it. This prevented the NA enzyme from releasing newly synthesized viral particles from the cell surface, and this in turn prevented further viral replication, bringing the infection to a halt over time.

This unique mechanism of action allows a single antibody to disable multiple variants of NA because these loops insert into the largely unchanging active site of the protein, but keep away from the surrounding highly variable regions that cause viral diversity and antigenic shift between pandemics. This helps them achieve “much greater breadth against the neuraminidase of different influenza viruses than we have seen before.”

Implications
These promising results indicate the potential for a new influenza therapy for flu patients, based on these powerful antibodies. Moreover, they could point the way to developing new types of vaccines that will give rise to similar immune reactions, thus protecting the vaccinee against influenza caused by multiple strains and for a longer time than the current breed of vaccines do. Ellebedy sums up: “We have an alternative approach to start designing novel vaccines that induce antibodies like this. And that could be really important if we are going to figure out how to design a truly universal vaccine.”

Sources:

• Nih.gov. (2019). Broadly protective antibodies could lead to better flu treatments and vaccines. https://www.nih.gov/news-events/news-releases/broadly-protective-antibodies-could-lead-better-flu-treatments-vaccines
• By targeting flu-enabling protein, newly discovered antibody may protect against wide-ranging strains, https://www.scripps.edu/news-and-events/press-room/2019/20191024-wilson-neuraminidase.html

Journal reference:

Daniel Stadlbauer, Xueyong Zhu, Meagan McMahon, Jackson S. Turner, Teddy J. Wohlbold, Aaron J. Schmitz, Shirin Strohmeier, Wenli Yu, Raffael Nachbagauer, Philip A. Mudd, Ian A. Wilson, Ali H. Ellebedy, and Florian Krammer. Broadly protective human antibodies that target the active site of influenza virus neuraminidase. Science. DOI: 10.1126/science.aay0678 (2019). https://science.sciencemag.org/content/366/6464/499