Presentation on theme: "Influenza There are three types of influenza in humans: A,B, C"— Presentation transcript:
1 Influenza There are three types of influenza in humans: A,B, C 1. Flu A: Found in many animal species, in addition to humansClosely related to Type B but not Type CMain type responsible for human epidemicsDemonstrates the greatest antigenic variability (“antigenic drift”)Reservoir in nature is waterfowl2. Flus B and C: Found almost exclusively in humansFlu C can also infect swineFlu C is morphologically and antigenically distinct from A, B3. Flu A strains designated by host from which isolated, where isolated, year of isolation, and type of HA and NA. An isolate (strain) number may also be included if there are multiple isolates.Example: A/goose/Leipzig/137/79 (H7N2)
4 Structure of Influenza Virus Influenza A virus is an enveloped particle that when spherical is about 120 nm in diameterMany particles are not spherical but filamentous in shapeThere are two glycoproteins at the surface in surface “spikes”HA (hemagglutinin) is present as homotrimersNA (neuraminidase) is present as homotetramersProtein M2 forms ion channels in the lipid bilayerThe matrix protein M1 lines the inner side of the lipid bilayerThe genome consists of 8 RNA segments present in helical nucleocapsids
5 Influenza Nucleocapsids NP is the major nucleocapsid protein.It has a major structural roleIt is also required for the switch from mRNA synthesis to genome replication.PA, PB1, and PB2 are minor components of the nucleocapsid and form the RNA synthesis machinery.The function of PA is unknown but may be involved in the switch from mRNA synthesis to genome replicationPB1 is an endonuclease that process the mRNA primer; it also is a polymerase that catalyzes nucleotide additionPB2 recognizes the cap of host cell mRNA required for priming mRNA synthesisM is the matrix protein.It is a peripheral membrane protein that underlies the viral membrane.It interacts with the nucleocapsid and with the tails of HA, NA, and M2
6 Attachment & EntryThe HA spike is a homotrimer with a molecular weight of 110 kDa.HA is synthesized as a 549 aa precursor called HA0 which is anchored in the membrane near the C-terminus.HA0 is cleaved into HA1 (328 aa) and HA2 (221 aa)At the N-terminus of HA is a 16 aa hydrophobic signal peptide for insertion into the ER.A single Arg separates HA1 from HA2 and cleavage is by a cellular trypsin-like proteinaseHA1 and HA2 remain covalently associated after cleavage by a disulfide bridgeThe C terminus of HA2 contains a 26 aa uncharged membrane-spanning domain followed by a 10 aa hydrophilic cytoplasmic domainThe HA polypeptide is glycosylated at specific asparagine residues
7 HA-mediated membrane fusion The HA trimer is stabilized by a hydrophobic core formed between the three stalk regions. Attachment sites for the cellular receptors are located near the top of each large globular region, which also contains neutralization epitopes.The exact glycoprotein(s) that serve as host cell surface receptors has not been identified, but it is known to contain sialic acid.After binding of HA to the cell surface receptor(s) the virus is internalized by endocytosis.The low pH of endosomes ( pH ) results in an irreversible conformational change in HA which results in the extrusion of the highly conserved hydrophobic amino terminus of HA2 from its position in the native protein.This region, termed the ‘fusion peptide’, promotes membrane fusion.The mechanism by which the ‘fusion peptide’ promotes membrane fusion is not completely understood.The subsequent fusion of viral and endosomal membranes allows the release of the viral genome into the cellular cytoplasm
8 Activation of the HA Spike After acid treatmentHA0 precursorCleaved spikeand proteolysis
9 Activation of Fusion Activity of Flu HA0 by Cleavage View of One Monomeric Unit in the Spike
10 Change illustrated for one monomeric unit of the trimeric spike
12 NeuraminidaseNA spike consists of a tetramer. NA is a type 2 glycoprotein with the N terminus inside and the C terminus outside.NA removes sialic acid from oligosaccharides on cell-surface proteins and glycolipids, thus destroying receptors for the virus.Also removes sialic acid from HA so that progeny influenza virions cannot aggregate.Separates virus particles from inhibitory mucopolysaccharides in the respiratory tract allowing efficient infection.
15 Splicing to Produce Influenza A mRNAs Since influenza RNA synthesis occurs in the nucleus, the cellular splicing machinery can be usedIn Flu A two mRNAs are produced from both segments 7 and 8One mRNA is unspliced, the second is spliced
16 Influenza C Has an Esterase Flu C lacks NA and has only 7 segmentsIt has HEF that performs the functions of HA and NA in Flu ABThe receptor for Flu C is 9-O-acetyl-N-acetyl neuraminic acidThe Flu C esterase removes the 9-O-acetyl group to destroy the receptorThe HEF gene is also present in some coronaviruses, which must have obtained it by recombination with Flu C at some time in the past
17 M2 M2 tetramers form ion channels in viral and cellular membranes Exposure to low pH is required to dissociate the nucleocapsid from the matrix protein, allowing the nucleocapsid to be transported to the nucleusM2 also prevents premature activation of the fusion activity of HAAmantadine interferes with the function of M2 and is an effective flu antiviral
18 Virus AssemblyNucleocapsids assemble in the nucleus during genomic RNA synthesisThe encapsidation signal is at the end of the RNA and not present in mRNAsNucleocapsids are exported to the cytoplasm in a process that requires NS2 and M1Glycoproteins are synthesized on the ER and transported to the plasma membraneNucleocapsids bud through the plasma membrane to form virionsMore than 8 segments may be packaged: Ten segments randomly selected would result in ~3% of progeny virions having at least one each of the 8 segmentsRandom selection of segments would mean efficient reassortment during mixed infection, which is known to occur
19 Influenza - Some History Oldest record of an epidemic probably caused by flu: Hippocrates, 412 BC.Epidemics have occurred relatively frequently but at irregular intervalsEpidemics vary in severity but the very young and elderly are most at risk.Epidemics appear to radiate from specific locationsExample: 1781 epidemic that spread across Russia from Asia.Influenza has killed untold millions throughout the centuriesepidemic was particularly severemillion people died, more than died in World War I.3. 80% of US WWI deaths were due to influenza4. A significant factor in the German loss was influenzaFirst human influenza virus was isolated in 1933.Different strains cause different epidemics, but human strains can recirculate
20 Antigenic Shift and Drift in Flu A HA and NA are the major surface antigens of the virusAntigenic drift describes the selection of variants by the immune systemRelatively slowResistance is only partialAntigenic shift describes the results of recombination (reassortment)There are 15 different subtypes of HAThere are 9 different subtypes of HNSubtypes differ by 30% or more in amino acid sequenceA reassortant with a different HA and/or HN may cause a pandemicOnly a few of the subtypes have been isolated from humans
21 Influenza A in Birds The reservoir of influenza A in nature is birds All 15 HA and 9 NA have been found in aquatic birdsIn particular, migratory ducks are important in the maintenance and spread of influenzaInfluenza infection of birds is usually asymtomaticInfluenza replicates in the respiratory tract and the intestinal tract of birdsIt is excreted in the feces and high concentrations have been found in waters in which migratory ducks congregateThe virus appears to be in equilibrium in birds--little or no sequence drift has been found in bird viruses and disease seldom results from infectionIn contrast, the virus drifts rapidly in humans and vaccines must be reformulated yearly, and serious illness is produced
22 Epidemic Influenza Strains YearVirusCommon Name1889H2N21900H3N81918H1N1Spanish1957H2N2Asian1968H3N2Hong Kong1977H1N1RussianWhen a new strain appears the previous strain usually dies outAt present, H3N2 and H1N1 continue to cocirculate in humansAn H1N1 strains has circulated continuously in pigs in the U.S. since 1918
23 Sialic Acid (N-Acetyl Neuraminic Acid) Terminal NANA is attached to galactose by a2,3 or a2,6 linkagesDifferent HAs prefer one or the other linkageAvian intestine contains predominantly a2,3 linkagesHuman trachea contains predominately a2,6 linkagesPig trachea contains both linkages and serves as an efficient intermediate host in which reassortment can take place--pigs are often referred to as mixing chambersOther components also contribute to host specificity, best studied for NP
27 Influenza affects 10-20% of U. S Influenza affects 10-20% of U.S. population each year, causing up to 70,000 deaths. Average death rate in people over 65 is 1/2200 but in it was 1/300.
28 Illness Induced by Influenza Virus Influenza virus infects superficial cells throughout the respiratory tract.There is little or no spread to other organs.High temperatures often accompany the infection, C, that last 3-6 days.Cough and weakness can last 1-2 weeks longer.Extensive destruction of epithelial cells of the LRT can result in primary viral pneumonia.Influenza infection can result in secondary bacterial infection of the LRT resulting in bacterial pneumonia.Death following influenza infection is usually due to pneumonia, whether viral or bacterial or combined.Immunity following influenza infection is incomplete and appears to fade in time.It has been suggested that the high death in young adults in the 1918 pandemic could have resulted from a more active immune response to the virus.
29 Bird InfluenzaAn epidemic of influenza in chickens occurred in Hong Kong in 1997The virus was highly virulent, killing % of infected chickensBird viruses are not normally transmitted to humans but the 1997 Hong Kong virus resulted in 18 humans becoming infectedThis virus was highly virulent in humans--6 of 18 infected people diedThe virus was H5N1 and did not spread in humans--no person to person transmission occurredTo eradicate the virus and to prevent new reassortants from arising that might give rise to epidemic virus by direct person to person transmission, 1.6 million chickens were slaughtered
30 Defenses against Influenza AntiviralsAmantadine and Rimantadine licensed for use and ameliorate symptomsInhibitors of NA being developedVaccinesInactivated vaccines are in widespread useThese vaccines must be reformulated every year because of shift and driftThey are 60-80% effectiveAttempts being made to develop attenuated virus vaccines that could be reformulated yearly by reassortmentAn emergency response to swine flu in 1976 demonstrates the difficulties in preparedness decisions