Poliovirus (5-6)

Question 1

What are the general features of the Picornavirus family?

Answer 1

Small RNA viruses (30nm caspid)
→ ss +ve RNA (Baltimore Class IV)
→ non-enveloped icosahedral capsid particles, 60 copies of VP1 - 4
→ RNA ~ 7-8kb, single open reading frame with small UTR regions
→ genome has covalently attached protein at 5’ end (VPg)
→ cytoplasmic replication (typically cytopathic)
→ encodes RNA-dependent RNA polymerase

Question 2

What are some human picornaviruses?

Answer 2

Family: Picornaviridae
Genus: Enterovirus - poliovirus, coxsackie virus, echovirus, human rhinovirus
Hepatovirus - hepatitis A
Kobuvirus
Parechovirus

Question 3

What is poliovirus?

Answer 3

Causative agent of polio
→ in the picornaviridae family
→ most people don’t have symptoms
→ some mild flu-like symptoms: high temp, fatigue, headaches, vomiting, step neck ~ 10 days
→ rarely leads to severe symptoms affecting brain and nerves - paralysis

Question 4

During the poliovirus replication cycle how does it attach to a host cell?

Answer 4

Via the poliovirus receptor CD155 (aka PVR)
→ transmembrane anchor part of the Ig superfamily
→ has 3 domains
→ CD155 binds in the poliovirus ‘canyon’

Isolated with cDNA cloning in mice
→ typically polio infection restricted to human and primate cells
→ but addition of CD155 made mouse cells permissive

Question 5

What is the entry-role of receptors during poliovirus infection?

Answer 5

Unzipper → receptor binding triggers an leads to disassembly of virion-caspid (uncoating)
→ virion binding with domain 1 of CD155 causes it to be taken up by endocytosis and causes irreversible conformational change
1. receptor interactions bring VP1 vertex close to the target cell membrane → a part of the VP3 blocks the VP1 and VP4 is held internally
2. conformational change induces by receptor binding moves the VP3 plug from the vertex
→ allows VP1 and VP4 to interact with membrane - forming a pore through which the viral RNA can enter the cell

Question 6

What is the poliovirus canyon?

Answer 6

A depression at each 5-fold vertex which is the binding site for the poliovirus receptor
→ mountain/vertex formed by VP1 petamer
→ at the bottom of the canyon is an opening to a hydrophobic pocket - stabilised by the pocket factor host lipid sphingosine
→ receptor binding displaces the pocket factor

Question 7

How does poliovirus biosynthesis occur?

Answer 7

Virus has + RNA genome
→ can act as mRNA, so genome can be translated straight away by ribosome to make viral proteins
→ it makes RNA replicase (required as host cells don’t have this enzyme) which is used to make copies of the genome - packed with viral proteins to make new viruses

Question 8

How is poliovirus translated?

Answer 8

The genome is one open reading frame so its translated as one long polypeptide which is autocleaved into 10 viral proteins
→ not all cleavage occurs with the same efficiency so different amounts of protein are made

Question 9

What are the functions of the poliovirus viral proteins?

Answer 9

VP4 and VP1 (cleaved from VP0) and VP1, VP3 → proteins of the viral caspid
2A and 3C → proteases which process the poly protein and cleave cellular key proteins
2B and 3A → involved in rearranged cellular membrane vesicles
3D → RNA dependant RNA polymerase - makes multiple copies of the genome (encodes its own polymerase)
3b (VPg) and 2c (helicase) → accessory replication proteins

Question 10

What is the poliovirus IRES element?

Answer 10

Internal ribosome entry site
→ a highly structured piece of RNA at the 5’ end of the poliovirus genome
→ directs poly protein translation - allows RNA to enter site in ribosome and translation in a cap-independent manner

As time passes amount of host proteins synthesised is reduced - host proteins disappear viral proteins appear
→ inhibits translation that is dependant on 5’ cap - 2A cleaves elF4E which is required for ribosome binding of 5’ host translation but not for IRES-dependant translation as IRES can directly bind elF4G - limits host protein synthesis

Question 11

What is RNA-dependent RNA polymerase?

Answer 11

RdRp (aka replicase) → required for replication of RNA viruses
→ not found in cells
→ RNA viruses most encode it
→ produced RNA genome and mRNA from RNA templates

Question 12

How is the poliovirus assembled?

Answer 12

Procaspid formed first → icosahedral shell of 60 copies of each 4 coat proteins (VP1, VP2, VP3, VP4)

Each procaspid aquires viral genome copy with VPg still attached to 5’ end
→ fully assembled virus leaves via host cell lysis

Question 13

What is the pathogenesis of Poliomyelitis (polio virus)?

Answer 13

→ humans are the only known reservoir
→ spread by faecal-oral transmission
→ peaks during warm months in temperate climates
→ poliovirus: 3 serotypes PV-1, PV-2, PV-3 - vaccine must contain all 3

Question 14

How does poliovirus spread throughout the body?

Answer 14

Transmission → primarily faecal-oral
Initial replication → occurs in the oropharyngeal and intestinal mucosa (follicle associated epithelium - M cells, Peyer’s patch)
Virus drains into cervical and mesenteric lymph nodes → drains into the blood
~99% of infection end at this stage - producing non-specific symptoms
~1-2% of virus enters the CNS, replication in motor neurones in spinal chord, brain stem or motor cortex leads to paralysis

Question 15

What are the two types of polio vaccine?

Answer 15

Inactivated poliovirus vaccine (IPV)
→ given by injection
→ doesn’t not cause disease (if properly prepared)
→ used until 1955, 2000-present US

Oral (live attenuated) poliovirus vaccine (OPV)
→ given orally, easy to administer
→ induced intestinal immunity
→ mutant viruses derived from virulent strains
→ unusually reverts during intestinal replication
→ used 2000 US, until 2004 UK
→ in most of the globe, used for eradication

Question 16

What is the timeline of the use of inactivated polio vaccines (IPV)?

Answer 16

IPV is the first available polio vaccine (1955)
→ has been used in Scandinavia (continuously since its invention)
→ ‘cutter incident’ in USA 1955 - Salk vaccines improperly prepared resulting in several children dying or becoming paralysed
→ enhanced-potency IPV was reintroduced as the vaccine of choice in USA, 2000
→ replaced OPV in UK in 2004: component of the 6-part vaccine (diphtheria, pertussis, tetanus, Hib, polio, HBV)

Question 17

What is the polio virus ‘Cutter incidence’?

Answer 17

In April 1995 ~200,000 children were given IPV from Cutter Labs in West and Mid-West states
→ 40,000 cases of disease; 200 paralysed, 10 died
→ due to inadequately inactivated IPV batch (contained virulent virus)

Consequences
→ abandonment of first polio mass vaccination campaign
→ replacement of IPV with OPV for US vaccinations (a ‘dangerous’ vaccine replaced with a more dangerous one)

Question 18

What is the timeline for the OPV?

Answer 18

Oral polio vaccine used in most areas of the world since 1962
→ standard use in USA until 2000, Uk 2004
→ basis for WHO global eradication campaign - live attenuated virus needed for eradication

Question 19

How is the OPV made?

Answer 19

Passage of the virus through nonhuman cells producing spontaneous mutations in the viral genome
→ Albert Sabin used many passes through monkey cells producing 3 serotypes
→ nucleotide substitutions distinguish the attenuated Sabin strains from its virulent parent
→ the primary attenuating factor common to all 3 in a mutation located in the virus’s internal ribosome entry site (IRES) - reduce binding of polyprimidine tract binding protein to the IRES
→ reduces ability of virus to translate its RNA template - small amounts of particle produced but not enough to cause disease
→ replicates in the gut but is unable to replicate in the nervous system
→ multiple doses needed

Question 20

What are the conditions for enabling eradication?

Answer 20

1. Replication must occur only in one host - immunisation of only humans enough
2. Vaccination must induce life-long immunity - no need to vaccinate again

→ only major human disease to be eradicated is smallpox - by smallpox eradication programme 1967, eradicated in 1978

Question 21

Why is polio eradication difficult?

Answer 21

→ 1 in 200 are asymptomatic - source of infection
→ symptoms similar to other diseases
→ vaccine reverts to virulence: disease in 1 in 10^6 recipients
→ poliovirus vaccine differs from smallpox in that it can revert to a wT virulent strain - act as source of infection
→ in countries using OPV, the only source of poliovirus eventually is the vaccine

Question 22

What is vaccine-associated paralytic poliomyelitis?

Answer 22

Sabin (OPV) strains replicate in the gut of immunocompetent individuals for a limited period of time
→ poliovirus excreted for 30-60 days
→ during this time attenuating mutations presenting the vaccines revert rapidly and the virus becomes virulent
→ the reverted strains, vaccine-derivative poliovirus (VDPV), are excreted into environment
→ VDPV can cause vaccine-associated paralytic poliomyelitis in OPV recipients and their close contacts (unvaccinated)

→ vaccination against the vaccine - after eradication immunise global with IPV