Lecture 13: Animal and human viruses

*T/F: Smallpox has been eradicated

T

Where do we stand now: Polio

Hopefully will soon be the second viral disease wiped out

Where do we stand now: Hepatitis B

An effective worldwide vaccination strategy is being implemented. Liver cancer deaths caused by infection will continue for many decades

Where do we stand now: Measles

Good vaccine since 1963

• Disease could be eliminated with worldwide effort

*Where do we stand now: Influenza

Effective strain-specific vaccines are generated each year, but new variant and potentially dangerous strains emerge each year

*Where do we stand now: HIV

No vaccine; drugs are somewhat effective but are costly and toxic. Drug resistance appears quickly. Worldwide sexual spread continues. More than 50 million people have been infected so far.

Where do we stand now: Ebola

Proved to be the most dangerous virus with mortality rate approaching 70%-80%. Vaccine under development

Which of the following viruses do we currently not have a vaccine for? Polio, Hepatitis B, Measles, Influenza, HIV, Ebola

HIV

Issue with Influenza virus vaccine

An effective strain-specific vaccine must be generated each year. But new variant and potentially dangerous strains emerge each year.

General patterns of infection (4)

• Acute infection (“hit and run” - infection then gone) - most viruses

• Persistent infection

• Latent (inside the cell), reactivating infection - Herpes

• Slow virus infection - measles, HIV

Baltimore and Huang identified [fill in blank] in a negative-stranded RNA virus, vesicular stomatitis virus, that copied viral genome into messenger RNA

RdRp

We eradicated poliovirus in humans

No

*T/F: Memory is only present in the adaptive immune system and NOT in the innate immune system.

True

Course of Viral Infection involves..

• Primary replication

• Systemic spread

• Secondary replication

*Course of viral infection: Primary replication

• The place of primary replication is where the virus replicates after gaining initial entry into the host.

• This frequently determines whether the infection will be localized at the site of entry or spread to become a systemic infection

*Course of viral infection: Systemic spread

Apart from direct cell-to-cell contact, the virus may spread via the blood stream and the CNS

*Course of viral infection: Secondary Replication

Secondary replication takes place at susceptible organs/ tissues following systemic spread

Cell Tropism

Viral affinity for specific body tissues (localized infection)

*What two things determine tropism of a virus?

• Cell receptors for that virus

• Cell transcription factors that recognize viral promoters and enhance sequences → Ability of the cell to support virus replication - Species specificity

Two types of chronic persistent infections

true latency and persistence

True Latency

The virus remains completely latent following primary infection.

• Eg HSV, VZV. Its genome may be integrated into the cellular genome or exist as episomes (never goes away)

Persistent infection

The virus replicates continuously in the body at a very low level. Virus remains associated with the cell without rapidly multiplying or killing the cell

• eg. HIV, HBV, CMV, EBV

*Acute infection

Rapid onset of disease symptoms result in either eradication of the virus or death of the infected animal (cold and flu viruses, ebola)

Three types of persistent infection

1. Virus genome integrates into host genomes. ex. retroviruses

2. Virus is released sporadically but remains in a so-called “latent” state most of the time (Herpes - HSV)

3. Virus is continually released without lysis of host cell. ex. HBV, lambda bacteriophages

Retroviruses display persistent infection by..

Integrating their viral genome into host genomes

Herpes simplex virus (HSV) displays persistent infection by..

Sporadically releasing virus but remain in “latent” state most of the time (inside cell)

Hepatitis B (HBV) displays persistent infection by..

Continually releasing virus without lysis of host cell

Typically, viruses have unique Cell Tropism. The limitation factors are:

(Selected all the apply)

A. Cell transcriptional factors that recognize viral promoters and enhancer sequences

B. Cell receptors for that virus

C. Physical barriers

A and B

How do viruses enter through the skin?

• Not common, but possibly through

  • Small wounds in skin like insect bites (yellow fever virus)

  • Large wounds (rabies from animal bites)

• More generally through epithelial cells

*Basic steps in viral life cycle (6)

1. Absorption: viral surface protein binds to its receptors

2. Penetration: guiding viral particle into cell through membrane

3. Uncoating and eclipse: virus releasing genome to designated replication site

4. Synthesis of viral nucleic acid and proteins

5. Assembly (maturation)

6. Release

Uncoating

• Virus releases genome to designated replication site

• Need to make genome available

• Once uncoating occurs, enter eclipse phage

  • Lasts until first new virus particle formed.

*Where do influenza viruses replicate?

In the nucleus

*Where do smallpox viruses replicate?

In the cytoplasm

*Where do herpes viruses replicate?

In the nucleus

*Attachment proteinfor SARS-CoV virus

ACE2

*Attachment protein for Influenza virus

HA protein

Adsorption of a virus requires.. (3)

• Temperature independent

• Requires viral attachment protein (receptor ligand)

• Cellular receptors

*Penetration of viruses requires..

Fusion with plasma membrane

Is there any advantage between having an envelope versus having no envelope for a virus to penetrate the cell?

No

Uncoating of a viral particles permits [transcription / translation ]

Translation

Uncoating

• Need to make genome available for translation

• Once uncoating occurs, enter eclipse phase

• Eclipse phages lasts until first new virus particle is formed

Synthesis of viral nucleic acid and protein

• Many strategies

• Nucleic acid may be made in nucleus or cytoplasm

• Protein synthesis is always in the cytoplasm

Assembly and maturation occurs at the..

• Nucleus

• Cytoplasm

• At Membrane

*Release

• Lysis

• Budding through plasma membrane

• Not every released virion is infectious

*Transmission of viruses types

• Respiratory transmission (Influenza A virus)

• Faecal-oral transmission (enterovirus)

• Blood-borne transmission (Hepatitis B virus)

• Sexual transmission (HIV ← also blood)

• Animal or insect vectors (Rabies virus)

Transmission of Viruses: For influenza virus, its main transmission is..

Respiratory transmission

*Modes of transmission

• Horizontal transmission: from one organism to another via direct/ indirect contact

• Vertical transmission: Transfer of virus from parent to offspring

Horizontal transmission

Transfer of virus from one organism to another via direct or indirect contact

Vertical transmission

Transfer of virus from parent to offspring

• eg. blood exchange through placenta

• congenitally (HSV)

• direct inheritance of germ line integrated virus

T/F: Most viral infections do not lead to such serious complications

T

Viral Virulence

The ability of a virus to cause disease in an infected host

T/F: A virulent strain causes significant disease while an avirulent or attenuated strain causes no or reduced disease

T

*Virulence depends on.. (4) MS

• Dose

• Virulence strain (genetics)

• Inoculation route - portal of entry

• Host factors - eg. Age SV in adult neurons goes persistent but is lytic in young

Quantitation of virulence to compare strains involves..

• LD50 - lethal dose for 50% kill

• ID50 - infectious dose for 50% of symptom

*Viral genes that affect virulence may..

• Affect the ability of the virus to replicate

• Enable the virus to spread within host or between hosts

• Defeat host defense mechanisms

• Produce products that are directly toxic

Viral genes affect virulence through: (select all that apply)

A. Impact the ability of the virus to replicate

B. Enable the virus to spread within host or between hosts

C. Suppress host defense mechanisms

D. Produce proteins that are directly toxic

All of the above

Cell culture

Most common method used to propagate viruses

• Cells can grow as monolayers or in suspension cultures

Terms that describe viral infection of cells

• Multiplicity of Infection (moi)

• Lytic infection

• PFU: plaque forming unit

Multiplicity of Infection (moi)

• Ratio of input - virus: number of target cells in an infection

• Usually used to describe the infection in a particular cell type grown on a plate or in culture

• Can be used to statistically evaluate the total number of cells that will be infected and the number of viruses that will infect each cell during an infection

In a cell culture, it was found that 5000 viruses infected 1000 cells. What is the m.o.i.?

m.o.i = virus / cell = 5000/ 1000 = 5

Lytic infection

Virus enters cell and usurps cellular machinery to rapidly multiply and in the process kill the cell (many flu and cold viruses)

Plaque forming units (PFU)

• Lysis of cell (lytic infection) creates holes in the monolayer of cells called plaques

• PFU = concentration of viruses that can form plaques in a volume of a virus stock

What does one PFU indicate?

1 PFU = 1 virus particle has infected 1 cell to produce 1 plaque

Who created the Polio vaccine?

Jonas Salk