Orthomyxoviruses

1. Orthomyxoviruses

2. Characteristics • Name originates from the Greek word ortho (correct), myxo (mucus). Essentially virions infect epithelial cells the right way, as opposed to paramyxoviruses • 3 Types: A, B and C • Hosts: Birds, various mammals and humans • Enveloped virion, 80-120 nm diameter • Linear -ssRNA genome • 6-8 different segments • Total genome length 10-15 Kb • Each segment encodes for 1-2 proteins • Envelope proteins: Hemagglutinin (HA), Neuroaminidase (NA) • Symptoms: High fever, sore throat, cough, headache • Can be fatal, often from secondary bacterial infections

3. Influenza Virions Infecting Epithelial Cells (Upper Resp. Tract)

4. Influenza History • The Name Originates From 18th Century Italy, Disease Thought To Be Due To “Influence” of Stars • In 1918 “Spanish Flu” Caused 20 Million Deaths • Many People Confuse Cold With Flu • Flu is severe! You cannot get out of bed! • Cold is due to rhinoviruses, coronaviruses, adenoviruses • 20,000-30,000 People Die From Flu and Flu Related Complications in USA Every Year Despite Vaccines and Anti-viral Drugs • Pandemics • Spanish Flu, 1918 (20 million deaths) • Asian Flu, 1956 (60,000 deaths) • Hong Kong Flu, 1967

5. Influenza Pathogenicity • Infection and Destruction of Epithelial Cells Leads to Loss of Mucociliary Flow • Loss of Mucociliary Flow Leads to Secondary Bacterial Infections. • Interferon Production Leads to Strong Local and Systemic Inflammatory Responses • These responses cause the severe symptoms • Flu is severe! You cannot get out of bed! • Cold is due to rhinoviruses, coronaviruses, adenoviruses • Spanish Flu Pathogenicity Thought to be Due to “Cytokine Storms” • Affected primarily young people with healthy immune systems, rather unusual • Typically children and older individuals are more vulnerable • Pandemics • Spanish Flu, 1918 (20 million deaths) • Asian Flu, 1956 (60,000 deaths) • Hong Kong Flu, 1967

6. Zanamivir/Oseltamivir Adamantanes Viral Structure

7. Viral Entry and HA • The HA envelop protein is used as an attachment/entry receptor • HA binds to sialic acid residue found on many receptors • Ex. Intestinal cells, lung epithelial cells • Hemaglutinin name derived from the fact that HA can agglutinate RBCs • Other viruses can cause hemaglutination, not just influenza • HA is 549 a/a long, type I transmembrane • C-terminus in cytosol, N-terminus outside virion • HA is initially fusion incompetent • Cleavage by cellular proteases gives rise to 2 subunits • HA1 and HA2

8. Fusion Competent HA

9. HA and Endosomal Virion Release • Once virion enters propagates endosomal pathway, pH decreases progressively • This is the typical endosomal maturation process • Decrease in pH results in conformational change of HA • HA2 extends outwards and inserts into outer endosomal membrane • See next schematic

11. M2 Ion Channel Protein • M2 via formation of a tetramer pore allows H+ to enter virion •  in H+ is needed for nuclecapsids release into cytosol • Release is thought to be due to  interaction between M1 proteins and nucleocapsids • The antiviral drug amantadine is a specific blocker of M2 H+ channel • In presence of amantadine nucleocapsid release is incomplete

12. Nucleocapsids • Influenza nucleocapsids use nucleocapsid protein (NP) as building block • -ve strand RNA rubs around NP proteins • Each nucleocapsid contains a trimeric RNA polymerase complex • PA, PB1 and PB2 • Sequences at 5' and 3' ends are highly conserved in all genome segments • 5' and 3' are complimentary but are not thought to hybridize

13. Nucleocapsids

14. Nucleocapsid Entry Into Nucleus • Orthomyxoviruses replicate inside nucleus • This is contrary to most RNA viruses • Nucleocapsids enter nucleus via nuclear pores • Both NP, PA, PB1 and PB2 contain NLS (nuclear localization sequences) • They all interact with importin- • The complex then interacts with importin- and nuclear translocation occurs

15. Genome Transcription

16. Viral Genome Transcription • Viral polymerase cannot transcribe independently! • Cellular mRNAs used as primers • PB2 binds cellular pre-mRNAs via 5' cap (found on all eukaryotic mRNAs) • PB1 then acts as nuclease cleaving cellular RNAs • Cleaved mRNA is about 10-13 nt • PB1 cleaves at an A or G residue • A residue is hydrogen bonded to U residue on viral genome • PB1 then acts as polymerase moving towards 5' end of genome • Role of PA not understood at this point

17. Viral Genome Transcription

18. Neuraminidase Inhibitors NJEM, 2005

19. Oseltamivir Clinical Trial

22. Vaccines • Most Common Vaccine, TIV (trivalent influenza vaccine) • Comprised of 2 type A and 1 type B influenza viruses • Less Common Vaccine, LAIV (live attenuated influenza virus) • Vaccine May Be Effective For One Year But Not Next Year • 2008-2009 Flu Vaccine • A/Brisbane/59/2007(H1N1)-like virus • A/Brisbane/10/2007 (H3N2)-like virus • B/Florida/4/2006-like virus • Many Companies Manufacture Flu Vaccines Worldwide • CSL Biotherapies (Australia) • Sanofi-Aventis (US, largest worldwide producer) • Novartis Vaccines • Medimmune-Avirion (US) • Chiron Corporation(Global) • Wyeth Lederle (US)

23. Transmission And Humidity • Flu Outbreaks Take Place Predominantly From December to April • Why? • One Recent Study Looked At Relative Humidity As A Key Factor • 2008-2009 Flu Vaccine • A/Brisbane/59/2007(H1N1)-like virus • A/Brisbane/10/2007 (H3N2)-like virus • B/Florida/4/2006-like virus • Many Companies Manufacture Flu Vaccines Worldwide • CSL Biotherapies (Australia) • Sanofi-Aventis (US, largest worldwide producer) • Novartis Vaccines • Medimmune-Avirion (US) • Chiron Corporation(Global) • Wyeth Lederle (US)