Seasonal influenza A is a scourge of the young and old, killing more than
250,000 worldwide each year, while creating an economic burden for millions.
Pandemic influenza, which occurs when a new virus emerges and infects people
globally that have little or no immunity, represents a grave threat to human
health. The recent spread of highly pathogenic avian influenza (HPAI) caused by
the H5N1 strain across Asia, Europe and Africa raises the specter of a new
pandemic, should the virus mutate to become readily transmissible from person
to person. Influenza A is subclassified by its two major surface proteins:
hemagglutinin, which mediates cell entry, first by recognizing host proteins
bearing sialic acid on their surface, and second by triggering the fusion of
viral and host membranes following endocytosis, allowing viral RNA to enter the
cytoplasm; and neuraminidase, which cleaves sialic acid from host and viral
proteins, facilitating cell exit. Hemagglutinin (HA or H), the major antigen on
the flu virus surface, has a large globular head and a thin stem. The head of
HA is constantly mutating, thereby helping the virus evade the immune system.
As a result, the vaccine in the flu shot must be updated every year (Fig 1).
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Fig 1A.
Flu vaccines target the globular head of influenza hemagglutinin (HA) on flu
viruses, preventing them from binding to human cells and injecting viral RNA.
But there are 16 types of HA, and the head of HA mutate constantly. So flu
vaccines are only effective against specific subtype of flu virus and have to
be reformulated each year. (Image: William C. Hwang)
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Fig 1B.
Antibody F10 target the stem of the HA, which rarely mutate and is common among
many different subtypes of flu viruses. F10-bound HA can still bind to a cell,
but it can no longer fuse the viral membrane with human cell membrane and
inject RNA. Mice tested with the antibodies were protected from H5N1 flu in 80
percent of cases. (Image: William C. Hwang) |
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Sui Jianhua and colleagues led by Dr. Wayne Marasco at the Harvard Medical
School and the Dana-Farber Cancer Institute were initially seeking ways to
fight bird flu. She identified 10 antibodies, from a library of 27 billion
human antibodies, against influenza hemagglutinin H5. The antibodies she
isolated can neutralize bird flu in cell cultures when injected into mice
(experiments conducted by Sandra Perez and other scientists led by Dr. Ruben
Donis at the CDC). Remarkably, subsequent studies showed the same antibodies
were also effective against all group 1 influenza viruses tested, including
H5N1 'bird flu' and the 1918 H1N1 'Spanish flu' which killed millions around
the world during World War I. Attempts to create mutant forms of the virus that
could evade the antibody were unsuccessful.
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Fig 2A.
Broad spectrum neutralizing antibody F10 in complex with hemagglutinin H5. F10
heavy chain binds to a highly conserved pocket near the fusion peptide in the
stem region of HA, and prevents fusion between viral and host cell membrane.
(Image: William C. Hwang; Source: Nature Structural & Molecular Biology)
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Fig 2B.
F10 epitope on HA can be classified by structure into two classes,
corresponding the two phylogenetics groups (group1 and group 2) of HA. H1, H5,
and H9 (group 1) are in shades of yellow (PDB 1RU7, 2IBX, and 1JSD); H3 and H7
(group 2) are in shades of green (PDB 1MQL and 1TI8). (Image: William C. Hwang;
Source: Nature Structural & Molecular Biology)
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Using data collected at SSRL, William Hwang and a group of colleagues led by
Dr. Robert Liddington at the Burnham Institute for Medical Research determined
the crystal structure of the hemagglutinin-antibody (H5-F10) complex to
understand why the antibody could neutralize so many different types of
influenza viruses (Fig 2). They found this antibody targets a highly conserved
pocket, which mediates the fusion of viral membrane with host cells, in the
stem region of hemagglutinin. This pocket seems to be critical for influenza's
survival, and so it rarely changes its form, and is similar across many types
of influenza. This explains why this antibody is so effective against so many
different types of flu viruses. In addition, they found the epitope on the HA
can be classified by structure into two classes corresponding to the two
phylogenetics groups (group1 and group 2) of HA. F10 epitope belongs to class 1
(group 1). Experiments have shown that F10 can indeed neutralize all group 1
influenza viruses tested, but not group 2 viruses as expected. Flu viruses may
eventually find a way to dodge this antibody. Nevertheless, this highly
conserved epitope are less prone to mutation, and we can expect fewer escape
mutants should they appear. The problem to defeat the flu virus will be much
more tractable.
Primary Citation:
Sui J, Hwang WC, Perez S, Wei G, Aird D, Chen LM, Santelli E, Stec B, Cadwell
G, Ali M, Wan H, Murakami A, Yammanuru A, Han T, Cox NJ, Bankston LA, Donis RO,
Liddington RC, Marasco WA. Structural and functional bases for broad-spectrum
neutralization of avian and human influenza A viruses. Nat. Struct.
Mol. Biol. 16, 265 - 273 (2009).
Related Videos:
Further Readings:
Single-handed flu combat? by Tia Ghose, The Scientist.
http://www.the-scientist.com/blog/display/55443/
Okuno, Y., Isegawa, Y., Sasao, F. & Ueda, S. A common neutralizing epitope
conserved between the hemagglutinins of influenza A virus H1 and H2 strains. J.
Virol. 67, 2552-2558 (1993).
Smirnov YA, Lipatov AS, Gitelman AK, Okuno Y, Van Beek R, Osterhaus AD, Claas
EC. An epitope shared by the hemagglutinins of H1, H2, H5, and H6 subtypes of
influenza A virus. Acta Virol. 43, 237-244 (1999).
Varecková E, Mucha V, Wharton SA, Kostolansky F. Inhibition of fusion activity
of influenza A haemagglutinin mediated by HA2-specific monoclonal antibodies.
Arch Virol. 2003 Mar;148(3):469-86.
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Highlights Archive
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SSRL is supported
by the Department of Energy, Office of Basic Energy Sciences. The SSRL
Structural Molecular Biology Program is supported by the Department of Energy,
Office of Biological and Environmental Research, and by the National Institutes
of Health, National Center for Research Resources, Biomedical Technology
Program, and the National Institute of General Medical Sciences.
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