Orthomyxoviruses

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

Influenza Virions Infecting Epithelial Cells (Upper Resp. Tract)

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

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

Viral Structure

Adamantanes

Zanamivir/Oseltamivir

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

Fusion Competent HA

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

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

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

Nucleocapsids

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

Genome Transcription

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

Viral Genome Transcription

Neuraminidase Inhibitors

NJEM, 2005

Oseltamivir Clinical Trial

• 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)

Vaccines

• 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)

Transmission And Humidity