Transcript File

Influenza
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Flu Pandemics
• Russian Flu (1889 – 1890)
– Approx. 1 million deaths
– H2N2 subtype
• Spanish Flu (1918 – 1919)
– 40 – 100 million deaths
– H1N1 subtype
• Asian Flu (1957 – 1958)
– 1 – 1.5 million deaths
– H2N2 strain
• Hong Kong Flu (1968- 1969)
– 0.75 to 1 million deaths
– H3N2 subtype
• 2009 H1N1
– Between 151,700 and 575,4000
with 80% of deaths in those 65 and younger
– H1N1 subtype
Flu
• RNA Virus
• 3 Influenza types
– A
• Initial host - wild aquatic birds
but have now infected many different
animals
• Divided into 2 categories
– Low pathogenic and Highly Pathogenic
– B
• Primary host - Humans
– C
• Only hosts - Swine and Humans
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Types A and B cause yearly outbreaks
A mutates the fastest
B mutates at a rate 2-3 times lower than type A
However, since it does mutate, then no lasting immunity
is possible
• C is the slowest to mutate
Influenza A and B
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A is most virulent
A can be subdivided into serotypes
– Hemagglutinin
• 17 subtypes
– Neuraminidase
(Sialidase)
• 10 subtypes
– Genome is generally divided
between 8 separate gene
segments
B can not be broken down
into subtype
http://www.microscopy.fsu.edu/cells/viruses/images/influenzafigure1.jpg
Both have various strains
Current Vaccine help A and B
but not C
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an A/California/7/2009 (H1N1);
an A(H3N2) virus A/Victoria/361/2011;
a B/Massachusetts/2/2012-like.
Quadrivalent vaccine this year
includes a B/Brisbane/60/2008-like
(B/Victoria lineage) virus.
Last year a New Flu – H3N2v which had
Pig to human transmission
Influenza A Nomenclature
http://en.wikipedia.org/wiki/File:Influenza_nomenclature.svg
Human, Swine or Avian Flu?
• Most of the time they all are Influenza Type A
• The subtype can be the same
– for example - H1N1 or H3N2
• But different strains have adapted for the particular host
– Seasonal flu is caused by Influenza A viruses that have changed
to spread in humans (human influenza)
• Currently circulating in humans are H3N2 and H1N1 viruses
– Avian flu is caused by Influenza A viruses that affect birds.
– Swine flu is caused by Influenza A viruses that affect pigs
• Influenza A viruses that typically infect one species can cross over
and cause illness in a different species
– Until 1998 only one H1N1 circulated in pigs then a human H3N2
crossed over
– Recently a H3N8 from horses moved to dogs
• Influenza A viruses “mutate” allowing yearly outbreaks and species
jumps
– Random mutation or reassortments
Antigenic Drift
• Random mutations that
change the antigens in
the virus
• These mutations allow
the virus to evade the
immune response
• Primarily concerned with
mutation in hemagglutinin
which is the cell surface
protein responsible for
entry into cells
Diagram from the National Institute of Allergy
and Infectious Diseases
Antigenic Shift
• Reassortment of two
or more different
strains
• Typically from two
different species
• Often enables a flu
strain to jump from
one animal species to
another
• Could be from direct
or intermediate
transmission with no
genetic changes
Diagram from the National Institute of Allergy
and Infectious Diseases
• Avian Influenza A viruses typically
are transmitted to humans in two
ways
– Through direct contact
– Through an intermediate host
• Avian Flu replicates less efficiently
in humans
• Human less efficiently in birds
• Avian viruses preferentially bind to
N-acetylneuraminic acid- α2,3
galactose
• Human viruses preferentially bind
to N-acetylneuraminic acid- α2,6
galactose
• Swine have BOTH!
• Overlaid images of
α(2,6)- and α(2,3)-linked
sialylpentasaccharides
LSTc and LSTa,
respectively, in the HA
receptor binding site
Hurdles to Cross
• “Species Jump”
– adaptation that allows the strain to
pass to another species
– Infection generally occurs via direct
contact with animals who are ill.
• Human to Human
– development of changed viruses
with the ability to cause infection
and spread in the human population
• Examples
– 1918 flu pandemic which is
believed to have been initially
caused by an antigenic drift from an
avian flu to a human flu
– antigenic shifts from the
reassortment of an avian flu and a
human flu (e.g Asian and Hong
Kong pandemics)
• There may be little or no immunity
in the human population to these
new viruses.
Viral Infection
1. Binding
2. Uncoating
3. Replication/
transcription
4. Translation
5. Protein processing
6. Reassembly
7. Budding
RNA dependent RNA Polymerase
has no proof reading activity, so
Error rate of 105 which results in antigenic drift
Influenza Genes and Proteins
Genes are on 8 separate segments of negative-sense single-stranded RNA that encode 11
known proteins
General category
Surface proteins
Protein(s) encoded
Hemagglutinin (HA)
Neuraminidase (NA)
M2 ion channel
Protein functions
Viral attachment and fusion protein
Virus release
Facilitates viral RNP uncoating
Internal proteins
Matrix (M1) protein
Viral structural protein regulates
RNP nuclear import
Required for viral RNA synthesis
Nucleoprotein (NP)
Polymerase Components
PA
PB1
PB2
Non-structural proteins
NS1
PB1-F2
Nuclear export protein
(NEP or NS2)
Polymerase subunit
Polymerase catalytic subunit
endonuclease activity
Polymerase subunit binds caps
of host cell mRNAs
Evasion of interferon responses
Proapoptotic factor, possible
immune evasion function
Mediates viral RNA nuclear export
Antiviral Research Volume 79, Issue 3, September 2008, Pages 166-178
Hemagglutinin
• Lectin (Glycoprotein) that
mediates binding of virus
to cell
• Viruses bind through hemagglutinin onto sialic acid
sugars on the surface of epithelial cells triggering
uptake of the virus via endocytosis
pH structure
change
Low pH in endosome causes conformational changes that facilitate fusion
of viral and host membranes.
Postulated to be similar to SNARE complexes
Fusion between endosome and virus forms a pore through which the viral genetic material
enter the cytosol
Hemagglutinin
• HA0 must undergo proteolytic cleavage into
HA1 and HA2 to allow the conformational
change required for membrane fusion.
• LPAI have a single basic singe basic
cleavage site
• HPAI have a multibasic cleavage site
• Variations in host protease determines
location and effectiveness of infection
• LPAI is cleaved by a host
trypsin like protease limited to the
respiratory tract
• HPAI is a furin-like protease which
present in virtually all cells thus allowing
HPAI to spread easily throughout the
body often causing fatal systemic infections
The presence of a multibasic cleavage site is not the only determinant
of viral pathogencity . The 1918 Flu had a single site while H5N1 has a
multibasic site
M2 protein
• Proton selective ion channel
• located in the viral envelope
• Transports protons from the endosome into
the virus particle
• Lower pH allows dissociation of the viral
matrix protein M1 from the
ribonucleoprotein RNP
• Crucial step in “uncoating” of the RNP and
exposing the content to the cytoplasm of
the host cell
• The RNP complex is transported to
nucleus where replication and transcription
occurs
• The gene for M2 is susceptible to
mutations, so strains are developing
resistance
M2 Antiviral drugs
• Symmetrel and Flumadine
– function by blocking the channel
– effective against influenza A
– Many strains are resistant too
Neuraminidase
• Glycoprotein
• Glycosidase
• Cleaves the sialic acid
residue on the host cell
allowing the mature virus
to detach
• Flu treatment
– neuraminidase inhibitors
– effective against both
influenza A and B
Neuraminidase Inhibitors
Tamiflu
Relenza
Structure based design
Avian “Bird” Flu
Highly Pathogenic H5N1
which generally results in
severe pneumonia has
– Spanish, Asia and Hong Kong 60% mortality world wide
Most recent are H5N1 and
H7N9 in Asia
Most infections were
contracted after direct
contact with infected bird
Limited evidence of human
to human transmission
Expected to be the next
deadly global pandemic
• Only a few subtypes have
been highly pathogenic in
humans.
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1918 flu pandemic
• H1N1 subtype
• Initially caused by an antigenic drift
• High death rate was due to cytokine storm
– too many immune cells are activated in a
single place
• In lungs fluids and immune cells
accumulate and eventually block off the
airways
• More susceptible were those with healthy
immune systems
Mortality Age Distribution for
1918 epidemic and normal epidemics
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(CDC is author, Taken from "1918 Influenza: the Mother of All Pandemics" Jeffery K. Taubenberger
and David M. Morens, http://www.cdc.gov/ncidod/EID/vol12no01/05-0979.htm )
Asian and Hong Kong Flu
• H2N2
• A mutation in wild ducks combed with a
pre-existing human strain
• H3N2 evolved via antigenic shift and
caused a smaller outbreak
Protein Changes in function that could affect virulence
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Increased virulence of the 1918 H1N1 and H5N1 Bird flu has been linked to:
the HA and NA proteins and the polymerase proteins (PB1,PB2, and PA)
and the Non-structural proteins
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HA
– Altered cleavage site allowing change in tropism;
– altered receptor specificity allowing increased transmissibility and
altered tropism in the airway
NA
– Ability to promote HA cleavage
M2
– Mutations that confer resistance to amantadine and rimantadine
PB1
– Enhanced virus RNA synthesis
PB2
– Enhanced virus RNA synthesis in mammalian cells, enhanced
polymerase function at high temperature
NS1
– Enhanced ability to suppress innate immunity
PB1-F2
– Enhanced pro-apoptotic activity
None suggest so far for
– M1, NP, PA or NS2
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1918 Influenza
• Avian HA consensus sequence
– Six residues are conserved in the
HA1 domains of most avian H1
HAs
– implicating these residues in the
ability to bind to the avian 2,3Gal
receptor
• Comparison of the avian HA
consensus sequence with the 1918
Virus shows that only one or two of
these conserved residues are
different
– 190 and 225
• For H1 1918
– Residue 190 alone determined
shift
• Know already that H5N1 found in
humans have mutations at several
positions which allow better
recognition of the human receptor
H5N1
Might only require a singe BASE change of switch to human receptor
Cell, Volume 153, Issue 7, 1475-1485, 06 June 2013
H7N9
• The hemagglutinin of
H7N9 virus does not
efficiently bind human
receptors
• •A single residue change
in receptor binding site
increases binding to
human receptors
• •Mutations on
hemagglutinin may
reduce the effectiveness
of current H7 vaccines
Cell, Volume 153, Issue 7, 1486-1493, 06 June 2013
Minimum Changes Needed
• 1st better binding to mammalian receptor
• Hemagglutinin
– Only 2-4 amino acid substitutions are required to switch binding
from α2,3 to α2,6
• 2nd Better uptake and release of the virus
• Neuraminidase
– In the 1918 virus, Neuraminidase plays an unusual role in
binding and activating the host protease that cleaves HA
rendering the virus infectious
• Cleavage of the Hemagglutinin precedes the conformation change which
leads to cell fusion
• Cleavage is generally dependent on the type and distribution of the host
trypsin like protease
– H5N1 Neuraminidase is rather slow in cleaving the sialic acid to
release the new virus particle
– A few mutations will take the H5N1 closer to the 1918
neuraminidase
Minimum Changes Needed
• Polymerase proteins (PB1,PB2, and PA)
– The 1918 virus contains only 10 aa changes in the sequences of
the polymerase proteins
– H5N1 already has 7 of the 10
– Important because avian viruses replicate around 41C but
effective replication in mammals is 37C
– Both 1918 and H5N1 have a single mutation, N66S, which has
been linked to increased virulence
2009 H1 N1:Genetics of New strain
• What is special about this
H1N1 outbreak?
• Reassortment of:
– One human strain
– Two pig strains
– One bird strain
• Hemagglutinin gene is
similar to current swine flu
viruses present in U.S. pigs
• Neuraminidase and matrix
protein genes resembled
versions present in European
swine flu isolates
• This is different from the
previous seasonal H1N1 strain
• High rate of Human to Human
transmission
• But mild symptoms so far
N Engl J Med 361:115-119
Infectivity and Virulence
Pathogenic – cause disease
Virulent – extend of pathology
Infectivity – the ability of a organism to invade and replicate
• H5N1 and H7N9 currently has low Human to Human
transmission but are highly virulent
• The 2009 H1N1 quickly became a pandemic (highly
pathogenic) but was not very virulent
• What will be the next pandemic
• Will we see a reassortment (antigenic shift) of 2009
H1N1 and H5N1 or H7N(?
• Will H5N1 have a minor antigenic drift like 1918 H1N1
allowing it to be transmitted human to human?
• Are we prepared?