Virology

Question 1

How are viruses cultured? 3 ways

Answer 1

Primary cell culture from animal tissue

Diploid cell strains is one type of cell that can divid 100x before dying (used to prepare vaccines)

Continuous cell lines are a single cell type that can be propagated indefinitely

Question 2

What are cytopathic effects?

Answer 2

Changes that virus makes to a cell culture

Includes rounding up, detachment from culture dish, formation of syncytium (group of fused cells)

Question 3

How do you measure the concentration of a virus?

Answer 3

Plaque assay:

(1) prep a plate dilution on susceptible cells
(2) after the cells attach, overaly them with a semi-solid medium to restrict diffusion of virus particles
(3) now the cells are restricted, so they are stuck as a monolayer visible as “plaques”
(4) use a dilution factor to count the plaques

Question 4

What is the lowest energy conformation of a virus particle?

Answer 4

Their minimum free energy conformation is attained when an unfavorable barrier is surmounted, following induction of the irreversible conformational transitions associated with attachment and entry

Question 5

What shapes can the protein coat of the virus be?

Answer 5

Helical

Icosahedral (20 faces of equilateral triangles)

Both are symmetrical shapes

Question 6

What can envelop some viruses?

What can be embedded in this envelope?

Answer 6

A membrane, which comes from the host cell’s membrane

Sometimes there are viral glycoproteins embedded in the lipid bilayer – these have external binding sites, antigenic determinants, sequences that mediate fusion with cell membranes for entry; internal parts are often essential for virus entry i.e. HA protein of influenza A virus

Question 7

What are the 7 categories of viral genomes?

Answer 7

dsDNA

gapped dsDNA

ssDNA

dsRNA

ss+RNA

ss-RNA

ss+RNA with DNA intermediate

Question 8

What is the final piece of genetic info that’s translated to proteins?

Answer 8

+ strand of RNA

Question 9

Where do viruses enter in the body?

Answer 9

Usually the apical surface of epithelial or endothelial cells

Happens especially if tight junctions are loosened

Question 10

Once a virus has entered apical surface of cell, how can it spread?

Answer 10

(1) Laterally, causing localized infection of epithelial/endothelial cells
(2) Tranported to basolateral surface to be released to underlying cells & tissues causing a systemic spread

Question 11

A virion does not infect every cell it incounters.

What determines whether a virion infects a given cell?

Answer 11

A cell must be both susceptible (have the specific receptor which a virion recognizes) and permissive (have the intracellular compartments necessary for replication of a particular virus)

Question 12

How does dsDNA get converted to +RNA?

Answer 12

mRNA is produced by copying the genome by host or viral DNA-dependent RNA polymerase

Question 13

How is gapped DNA converted to mRNA?

Answer 13

(1) gaps must be filled to produce perfect duplexes
(2) mRNA synthesis occurs usually by a virus-encoded reverse transcriptase homologous to that of the retrovirus

Question 14

How is ssDNA converted to +mRNA?

Answer 14

must be converted to a dsDNA template before being made into mRNA (even if the ssDNA is a - strand!!!

ssDNA –> dsDNA –> mRNA

Question 15

How is dsRNA made into +mRNA?

Answer 15

The - strand of this double stranded RNA is converted to +mRNA by a viral RNA-dependent RNA polymerase to produce viral proteins

Question 16

How is +strand RNA that’s part of viral genome prepared to “the” +mRNA?

Answer 16

The genome is replicated into a full length - strand

This - strand is replicated into full length + strand genomes

Question 17

How is a + strand RNA with DNA intermediate prepared to became “the” + strand RNA?

Answer 17

A reverse transcriptase (RNA-dependent DNA polymerase)

The DNA serves as the template for viral mRNA and genome RNA synthesis by cellular enzymes

Question 18

How is - strand RNA prepared to be the + strand RNA?

Answer 18

virus-encoded RNA-dependent RNA polymerases produce mRNAs from the - strand genome

Question 19

Which step do the antiviral medications amantadine/rimantadine target?

Answer 19

the uncoating step: when the virus does endocytosis to get into the cell, it has to uncoat (aka lose the membrane around it)

Question 20

How are viruses assembled?

Answer 20

Make a new genome, new caspid proteins, sometimes even envelope proteins– all occurs in different cellular compartments, so homing signals are required to make sure they get to the right location

Concerted assembly (genome/protein components happen at the same time) or sequential assembly (genome can insert into preformed protein envelope) can occur

Question 21

How are virus particles released?

Answer 21

Lysis (cell bursts) or cell to cell spread (occurs without physical release of the particle)

Question 22

How do NA inhibitors work?

Answer 22

i.e. Tamiflu, Relenza

They are neuroaminidase inhibitors (an enzyme that acts upon sialic acid, which is found on glycoproteins on the surface of normal host cells)

Blocking this enzyme prevents new viral particles from being released by infected cells

Question 23

Where do many viruses enter the body?

Answer 23

Mucosa of respiratory tract, GI tract, UG tract

Outer surface of eye

Skin

Question 24

What features of the respiratory tract protect it from viral entry?

How do viruses get it?

Answer 24

Swallowing, mucociliary blanket, macrophages in alveoli, IgA

Viruses can be found in aerosolized droplets from when somebody sneezes/coughs

These must be swept away by mucus, neutralized by antibody, or destroyed by alveolar macrophages

Question 25

What protects the alimentary tract(GI)?

How do viruses get in here?

What makes it a good point of entry?

Answer 25

Acidic pH of stomach, alkaline pH of intestine, digestive enzymes, bile detergent, mucus lining epithelium, antibodies, phagocytic cells, glycolipids/glycoproteins

Viruses can enter when you eat, drink, kiss

It’s a good point of entry because it can interact with cells of lymphatic, circulatory, and immune system i.e. M cells (membranous epithelial cells) are absorptive cells & point of entry for some viruses

Some viruess use host defense to their advantage i.e. reovirus use proteases to convert into infectious subviral particles

Question 26

What protects the UG tract?

How do viruses enter here?

Answer 26

Musus, low pH of vagina

Normal sexual activity –> microtears which is point of entry for viruses

Some enter epithelial cells & produce local lesions (human pappilomaviruses)

Others gain access to rest of body by infecting cells of immune system (HIV), or neurons (herpes viruses)

Question 27

How do viruses infect eyes?

What protects the eyes?

Answer 27

Enter through sclera and conjunctiva. Blinking protects them

Infected during abrasion: i.e. opthalmic procedures, from environmental contamination. Most are localized infections

Enterovirus 70 can spread to CNS, herpesvirus-1 can infect cornea and cause blindness

Question 28

What protects skin?

How can skin get infected?

Answer 28

Dead outer layer can’t support viral growth

Entry occurs by skin abrasions: insect/animal bites, needle punctures

Some are epidermal and stay localized

If infects deeper at dermis or subdermal tissues, can reach blood vessels, lymphatics, or cells of CNS

Question 29

How do viruses spread?

Answer 29

Basolateral release: Breach basement membrane, reach lymphatic system. phagocytes which help clear it –> can spread & become a systemic infection

Apical release: facilitates virus dispersal; virus doesn’t usually invade underlying tissues

Can spread through blood or through nervous system

Question 30

How does hematogenous spread work?

Answer 30

Entry site –> extracellular fluid –> local lymph (more permeable) –> venous system

Note that once they’re in the lymph they encounter cells of immune sytem & infect lymphoid cells

Provides access to almost every tissue in host

Question 31

What is viremia? (active/passive/primary/secondary)

Answer 31

Like bacteremia: presence of virus particles in the blood; can be free in blood or infected cells such as lymphocytes

Active viremia: produced by virus replication

Passive viremia: when they’re in the blood but not replicating at site of entry

Primary viremia: occurs right after initial replication at site of entry, usually low concentration of virus particles

Secondary viremia: delayed appearance, usually high concentration of virus particles

Question 32

How does neural spread occur?

Answer 32

Viruses enter local nerve endings

Can be definitive characteristic of their pathogenesis i.e. rabies

Can be less frequent/a diversion from their usual site of replication i.e. poliovirus, reovirus

Some can replicate in the brain but spread by hematogenous route i.e. mumps, HIV, measles

Question 33

Neurotropic, neuroinvasive, neurovirulent

Answer 33

Neurotropic: can infect neural cells

Neuroinvasive: can enter CNS after infection at peripheral site

Neurovirulent: can cause disease of nervous tissue

Herpes simplex virus: low neuroinvasiveness, high neurovirulence (always enters peripheral nervous system but rarely enters CNS, but when it does…consequences are very severe)

Mumps: high neuroinvasiveness, low neurovirulence (most invade CNS but neurological disease is mild)

Rabies: high neuroinvasiveness, high neurovirulence (usually infects nervous system & is almost always lethal)

Question 34

How does virus invade organs?

Which organs/how in each organ?

Answer 34

Once it’s in the blood it can spread to almost any organ

Skin: rash when virus leaves blood vessels, macules & papules due to inflammation/cell destruction

Liver/spleen/adrenal glands: sinusoids = open pore capillaires that are lined with macrophages which filter blood but also point of infection i.e. Kupffer cells = liver macrophages where viruses can infect & possibly multiply then ifect underlying hepatocytes –> hepatitis

CNS: some parts has fenestrated epithelium (has windows) and basement membrane is sparse i.e. choroid plexus

Renal glomulerulus, pancreas, ileum, colon: lack sinusoids so virus must adhere to endothelial cells at smallest/thinnest vessels where it’s fenestrated. can also enter via diapedesis of infected monocytes/lymphocytes

Question 35

What is tissue tropism?

Answer 35

The spectrum of itssues infected by a virus; some are limited, others can replicate in many organs

Question 36

What determines tissue tropism?

Answer 36

Cell receptors for viruses

Cellular proteins that regulate viral transcription

Cell proteases: i.e. serine protease called Clara is secreted by non-ciliated Clara cells of the bronchila and bronchiolar epithelia – HA can only infect here bc this protease is required to process it

Question 37

How do you measure viral virulence?

Answer 37

LD50: amount of virus needed to kill 50% of infected animals

Mean time to death

Mean time to appearance of symptoms

etc

Question 38

What determines virulence?

Answer 38

Gene products that alter ability of virus to replicate

Gene products that modify a host’s defense mechanisms i.e. virokenes = mimic cytokines, viroreceptors = homologs of host receptors

Genes that enable virus to spread in the host

Toxic viral proteins

Question 39

How do viral infects injure cells?

Answer 39

Cytopathic effect: alteration of cells, can lead to apoptosis

Inhibition of post protein/RNA synthesis –> loss of membrane integrity –> leakage of enzymes from lysosomes –> cytoplasmic degradation

Question 40

How can immunopathology cause disease?

Give examples of T cell, B cell, and antibody response that makes pathology worse

Answer 40

Too much immune response –> pathology

Example: CD8 T cells cause damage in coxsackie virus B infection which leads to endocarditis

B cells: high concentration/accumulation of virus-antibody complexes at site inaccessible to immune system leads to vasculitis, glomerulonephritis, mental confusion

Antibodies can enhance viral infection: in Degue fever 1st infection is asymptomatic or mild. Antibodies to one doesn’t protect against the other 4 serotypes. Infection by another binds antibodies & enhances infection of Fc receptor monocytes –> cytokine release –> plasma leakage, hemorrhage

NO is produced in virus infected cells as part of innate response but high conc can cause tissue damage by forming free radicals

Question 42

How do chronic viral infections develop?

Answer 42

Reduced cytopathic effects/host defenses –> interfering wtih MHC, escape from antibody/CTL, infect immune cells

Eyes/neurons don’t have immune cells so persistent infection is common (not so bad bc immune response would be detrimental to host)

Some viral infections are here to stay, can be good & confer immunity to other viral infections or can be bad i.e. are associated with disease of inflammation