Image credit: Amy Moran, National Library of Medicine
The surface of the influenza (flu) virus is covered with two proteins: hemagglutinin (HA), and neuraminidase (NA). While the immune system can recognize both surface proteins, because HA is the protein that enables viral entry into cells, it is a high priority target for researchers trying to create a long-lasting, universal flu vaccine.
The HA glycoprotein (or spike) has two domains: a globular head domain, which varies significantly across different flu strains, and a stalk domain, which is generally more conserved across strains. When looking for targets that could produce a universal vaccine, the well-conserved stalk domain seems like a likely candidate. However, because it is shielded by the head domain on top, the stalk is more difficult for the immune system to reach.
In a study published in 2013, we used cryo-electron tomography to visualize the HA proteins on virus particles from the H1N1 2009 pandemic flu strain, bound by a stem region-specific antibody, C179. While it was known that C179 could inhibit several flu strains, it wasn’t clear how the antibody could fit in between the tightly-packed HA proteins on the surface of viruses.
By computationally sorting the HA spikes on viruses by whether or not they were bound by C179, we found that about 75% of HA had antibody attached to the stem region. This suggests two things: one, that antibodies can in fact stick to the stem region on real virions; and two, that it is not necessary to inhibit all of the HA molecules in order to stop the virus from entering cells.
More recently, in a collaboration with the laboratories of Peter Palese and Florian Krammer at Mt. Sinai, we used cryo-electron tomography to visualize influenza virus-like particles covered in HA molecules engineered to display the stalk of one subtype with the head domain of a different subtype. While these engineered HA molecules were partially “untwisted”, they could still be bound by neutralizing anti-HA antibodies, suggesting these molecules might help elicit broadly neutralizing antibodies if used in a vaccine.
In a further continuation of this collaboration, we used electron microscopy to visualize the binding sites of two distinct broadly neutralizing antibodies on the surface of influenza B NA particles. While NA is much less abundant on the surface of influenza than HA, it represents another target, which may assist in making influenza vaccines more effective.
The image above is an artistic representation of a flu virus covered in HA proteins (green), some of which are bound by one, two or three stem-specific antibodies (blue).
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Related References: Harris AK, Meyerson JR, et al. Structure and accessibility of HA trimers on intact 2009 H1N1 pandemic influenza virus to stem region-specific neutralizing antibodies. Proc Natl Acad Sci U S A. 2013 Mar;110(12):4592-4597. doi:10.1073/pnas.1214913110.
Tran EEH, Podolsky KA, Bartesaghi A, Kuybeda O, Grandinetti G, Wohlbold TJ, Tan GS, Nachbagauer R, Palese P, Krammer F, and Subramaniam S. Cryo-electron microscopy structures of chimeric hemagglutinin displayed on a universal influenza vaccine candidate. mBio. 2016 Mar;7(2). pii: e00257-16. doi: 10.1128/mBio.00257-16.
Wohlbold TJ, Podolsky KA, Chromikova V, Kirkpatrick E, Falconieri V, Meade P, Amanat F, Tan J, tenOever BR, Tan GS, Subramaniam S, Palese P, and Krammer F. Broadly protective murine monoclonal antibodies against influenza B virus target highly conserved neuraminidase epitopes. Nat Microbiol. 2017 Oct;2(10):1415-1424. doi: 10.1038/s41564-017-0011-8.