Virology Flashcards
How are viruses cultured? 3 ways
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
What are cytopathic effects?
Changes that virus makes to a cell culture
Includes rounding up, detachment from culture dish, formation of syncytium (group of fused cells)
How do you measure the concentration of a virus?
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
What is the lowest energy conformation of a virus particle?
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
What shapes can the protein coat of the virus be?
Helical
Icosahedral (20 faces of equilateral triangles)
Both are symmetrical shapes
What can envelop some viruses?
What can be embedded in this envelope?
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
What are the 7 categories of viral genomes?
dsDNA
gapped dsDNA
ssDNA
dsRNA
ss+RNA
ss-RNA
ss+RNA with DNA intermediate
What is the final piece of genetic info that’s translated to proteins?
+ strand of RNA
Where do viruses enter in the body?
Usually the apical surface of epithelial or endothelial cells
Happens especially if tight junctions are loosened
Once a virus has entered apical surface of cell, how can it spread?
(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
A virion does not infect every cell it incounters.
What determines whether a virion infects a given cell?
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)
How does dsDNA get converted to +RNA?
mRNA is produced by copying the genome by host or viral DNA-dependent RNA polymerase
How is gapped DNA converted to mRNA?
(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
How is ssDNA converted to +mRNA?
must be converted to a dsDNA template before being made into mRNA (even if the ssDNA is a - strand!!!
ssDNA –> dsDNA –> mRNA
How is dsRNA made into +mRNA?
The - strand of this double stranded RNA is converted to +mRNA by a viral RNA-dependent RNA polymerase to produce viral proteins
How is +strand RNA that’s part of viral genome prepared to “the” +mRNA?
The genome is replicated into a full length - strand
This - strand is replicated into full length + strand genomes
How is a + strand RNA with DNA intermediate prepared to became “the” + strand RNA?
A reverse transcriptase (RNA-dependent DNA polymerase)
The DNA serves as the template for viral mRNA and genome RNA synthesis by cellular enzymes
How is - strand RNA prepared to be the + strand RNA?
virus-encoded RNA-dependent RNA polymerases produce mRNAs from the - strand genome
Which step do the antiviral medications amantadine/rimantadine target?
the uncoating step: when the virus does endocytosis to get into the cell, it has to uncoat (aka lose the membrane around it)
How are viruses assembled?
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
How are virus particles released?
Lysis (cell bursts) or cell to cell spread (occurs without physical release of the particle)
How do NA inhibitors work?
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
Where do many viruses enter the body?
Mucosa of respiratory tract, GI tract, UG tract
Outer surface of eye
Skin
What features of the respiratory tract protect it from viral entry?
How do viruses get it?
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
What protects the alimentary tract(GI)?
How do viruses get in here?
What makes it a good point of entry?
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
What protects the UG tract?
How do viruses enter here?
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)
How do viruses infect eyes?
What protects the eyes?
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
What protects skin?
How can skin get infected?
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
How do viruses spread?
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
How does hematogenous spread work?
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
What is viremia? (active/passive/primary/secondary)
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
How does neural spread occur?
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
Neurotropic, neuroinvasive, neurovirulent
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)
How does virus invade organs?
Which organs/how in each organ?
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
What is tissue tropism?
The spectrum of itssues infected by a virus; some are limited, others can replicate in many organs
What determines tissue tropism?
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
How do you measure viral virulence?
LD50: amount of virus needed to kill 50% of infected animals
Mean time to death
Mean time to appearance of symptoms
etc
What determines virulence?
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
How do viral infects injure cells?
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
How can immunopathology cause disease?
Give examples of T cell, B cell, and antibody response that makes pathology worse
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
How do chronic viral infections develop?
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
