Fitgerald-Bocarsly Viruses 2008 Course Slides 2nd...

FLUFLU

Patricia Fitzgerald-BocarslyOctober 23, 2008October 23, 2008

Orthomyxovirusesy

• Orthomyxo virus (ortho = “true” or “correct”)y ( )• Negative-sense RNA virus (complementary to

mRNA)• Five different genera• Five different genera

– Influenza A, B, C– Thogotovirus - Tick-borne

I i (i f ti l i i )– Isavirus (infectious salmon anemia virus)• Segmented RNA allowing for reassortment, but only

within genera• Enveloped

Influenza Virus

• Influenza A, B and C are human pathogens but humans are not the natural host

• Named according to their genus (type), species isolated from (except human), l ti f i l t b f i l tlocation of isolate, number of isolate, year, and (for influenza A, the hemagglutinin(H1-16) and neuraminidase (N1 9) type)16) and neuraminidase (N1-9) type)

Influenza Viruses

Usual host(s)

Transmission Disease Distribution

I fl A H Ai b R i t W ld idInfluenza A Humans, birds, swine

Airborne Respiratory disease

Worldwide

Influenza B

Humans Airborne Respiratory disease

Worldwide

Influenza C

Humans Airborne Respiratory disease

Worldwide

Molecular Properties

• Influenza A, B: 8 gene segments, 14 kb– Encodes 10 proteinsEncodes 10 proteins

• Influenza C: 7 gene segmentsEncodes 9 proteins– Encodes 9 proteins

• Receptor (all): sialic acid (but C uses a different form)different form)

Entryy• Fuses in endocytic compartments

D d l H• Dependent on low pH• Uncoating in endosomes• Fusion requires structural change in the

HA following cleavage of HA0 to HA1 g gand HA2; HA2 then allows fusion

Replication

• Nucleus - unusual for RNA viruses• 8 viral RNA fragments exist as complex with g p

four proteins that all have NLS: viral ribonucleoprotein complexes (vRNP)– RNA– NP - nucleocapsid protein coats RNS

PB1 PB2 PA i l d i iti RNA– PB1, PB2, PA: involved in cap recognition, RNA synthesis

Assembly

• RNA exported• Formation of virionsFormation of virions

– Controversy over how RNA segments segregate:segregate:

• Random packaging of 10 or more segments• Specific packaging of 8 segmentsp p g g g

• Bud from cytoplasmic membrane

Heterogeneity of Flu Virion FormsHeterogeneity of Flu Virion Forms

Neuraminadase

• Cleaves sialic acid residues• Highly variableHighly variable• Function

P t i ti ki b k t ll– Prevents virus sticking back onto cells– Prevents cell clumping

Major Immune Responsesj p• Innate immunity: NK, IFN-alpha, etc.

Ne trali ing antibodies against HA• Neutralizing antibodies against HA– Great deal of variability in HA (also NA)

• CTL

Cytotoxic T Cells in Viral Infectiony

Influenza NS1

• Inhibits IFN induction• Downregulates IRF-3 IRF-7 NF-kBDownregulates IRF 3, IRF 7, NF kB• Inhibits activation of PKR

Fl ith NS1 d l t d iti t• Flu with NS1 deleted very sensitive to IFN

Epidemiology

• Influenza A is the most frequent infection of humans

10 20% ld’ l ti i f t d/– 10-20% world’s population infected/year– 250,000-500,000 deaths– 20,000-30,000 deaths in the US20,000 30,000 deaths in the US

• Major reservoir is birds– In birds, largely asymptomatic– Not much pressure to mutate

• Human farming practices (pigs and fowl) lead to coinfection and reassortment of RNAto coinfection and reassortment of RNA

Flu VaccinesFlu Vaccines

• Whole inactivated - eggs or tissue cultureWhole, inactivated eggs or tissue culture• Live, cold-adapted - FluMist intranasal

– Passaged to be heat sensitiveg– Grows in upper airway– Mimics natural infection - better CTL and

antibody?antibody?• 3 subtypes chosen in Spring: 2 A, 1B

– Last year “missed”Last year missed– This year, three different types in vaccine

• Future vaccine target conserved CTL epitope?

Why do I need a flu vaccineWhy do I need a flu vaccine every year?

• Antigenic shift and antigenic drift: virus escapes immune responsep p

• Short incubation time (2 days)– No time to activate memory cells– No time to activate memory cells– No time to boost antibody levels

Existing antibody might not be protective– Existing antibody might not be protective anyway

1918 Flu

• Pandemic– Pandemic flu arises 3-4 times/century withPandemic flu arises 3 4 times/century with

influenza A (not B)• Unusually high infection (30%) and highUnusually high infection (30%) and high

deaths (20-100 million)• Killed young people at high levels• Killed young people at high levels

Reconstructed 1918 VirusReconstructed 1918 Virus• Tissue samples from Armed Forces Institute

of Pathology London and one frozenof Pathology, London, and one frozen individual buried in permafrost in Alaska

• H1N1, virtually identical in all of the samplesH1N1, virtually identical in all of the samples• Most H1N1 non-pathogenic in mice, but 1918

more pathogenic• In BSL-4 conditions in monkeys, see high

replication rates and extensive spread in the lungslungs– Altered innate imunity– Inflammatory cytokines increased - “cytokine y y y

storm”– Explains high mortality in young adults?

“Bird Flu”Bird Flu

• 18 people in Hong Kong infected and 6 died in 1997

• Avian influenza H5N1• Destruction of 1 6 million domestic birds• Destruction of 1.6 million domestic birds• Reappeared in 2006 and has spread

th h t A i i t Af i d Ethroughout Asia into Africa and Europe– 50% mortality

How is bird flu transmitted totransmitted to people? At the molecular level, what

ld d t hwould need to change to allow the virus to pass directly from ?human-to-human?

??

A “smart” virus does not wipe out its host species. What molecular properties of the H5N1properties of the H5N1 bird flu make it particularly pathogenic in birds? (Or what is knownbirds? (Or, what is known about determinants for pathogenesis in flu?)

Is it possible to develop a protective vaccine against bird flu? What has l d b d d h t d ialready been done and what design

would you propose for a future vaccine?

What are the antiviral drugs that are used in the US for flu infection? Describe their mechanisms of actionDescribe their mechanisms of action and report whether they will be active against bird flu,and why. Do not discuss vaccines in your presentation.

Does Bird Flu constitute a viable terroristic threat? Why? How does the US classify potentialUS classify potential threats?

The range of bird flu has been expanding; with the migration of birds over thousands of miles, what can/should the US becan/should the US be doing to protect Americans and American agriculture from bird flu? [Include thefrom bird flu? [Include the basis of molecular and immunological monitoring in your presentation.]