1. DNA vs. RNA
2. helical vs. icosahedral
3. naked vs. envelope
4. dimension virion & capsid
Different location to study virus in egg?
How to quantify a virus that does NOT cause Cytopathic effect?
1. Fluorescent focus assay 2. Infectious centers assay
What is cytopathic effect (CPE)?
structural changes in the host cells that are caused by viral invasion
1. cell lysis
2. inability to reproduce
After virus absorption but no detection
time needed from initiation infection to release new infectious particles
Events between eclipse and latent period
1. attachment 2. entry & uncoating 3. viral gene expression 4. viral genome replication 5. assembly and egress
virus - host cell receptor 1. protein - specific 2. carbohydrate - less specific
Requirement for virus entry into host cell?
Energy dependent --> cell MUST metabolically active
Method of entry
1. Endocytosis --> virus endosome
2. Enveloped viruses --> membrane fusion
Virus capsid is removed --> release viral genome
What mechanism do RNA viruses have to express their genome?
RNA-dependent-RNA pol --> production mRNA and replication RNA genome
(+) RNA virus vs. (-) RNA virus
(+) RNA virus: genome can be translated DIRECTLY --> ribosome ready (-) RNA virus: package RdRp --> first transcribe (+) mRNA --> NOT ribosome ready
Helical capsid vs. Icosahedral capsid assembly?
1. Helical capsid: coating genome during synthesis of the genome
2. Icosahedral capsid:
* Coating during genome synthesis, or
* genome is stuffed into preformed capsid
Enveloped vs. naked virus egress?
1. Naked virus: cell lysis
2. Enveloped virus: budding (plasma membrane, Golgi, ER)
Examples indirect cell damage (No CPE)
1. Intergration viral genome
2. Induction mutations host genome
4. Host immune response
Result in destruction of host cell
Slow release of virus w/o cell death
* include enveloped virus
* over long period of time
Delay between infection & showing of symptoms
Oncogenic virus --> induce tumor cell transformation
The importance of APOBEC and Trim5
Cellular factors that block virus infection AFTER entry
1. APOBEC inhibits HIV & HCV
2. Trim5 blocks retroviruses
APOBEC3G and HIV
1. APOEC3g incorporate into HIV particle --> damage viral genetic
2. Vif: HIV protein blocks APOBEC
* binding APOBEC and prevent incorporation
* targeting APOBEC for destruction & elimination
Types of IFN?
1. Type I: IFN alpha, IFN beta
--> secreted by most cells
2. Type II: IFN gamma
--> secreted by T-cells & NK cells
How does body respond to IFN receptor activation?
Through Jak/Stat pathway
1. Type I: Interferon-stimulated response elements
2. Type II: gamma activated site elements
TLRs and their viral molecular pattern
1. TLR3 -- dsRNA
2. TLR7 -- viral RNAs, synthetic ligands
3. TLR9 -- CpG containing DNA
Mediator for IFN?
1. PKR - protein kinase phosphorylates/inhibits translation initation factor --> decrease protein synthesis
2. OAS - activates RNAsase degrade mRN
IFNs bind to specific receptors on cells & induce transcription of genes
How PKR and OAS are made in responding to IFNs signal?
As precursors and activated by dsRNA
--> shut down translation both cellular and viral mRNA
1. Production PKR and OAS
2. Synthesis gene product arresting cell cycle
3. Inducing pro-apoptotic state
4. Inducing syntheis protein presenting viral protein to CTL
Cells of Innate Defenes?
1. Mononuclear phagocyte
2. Dendritic cell
3. NK cells
4. granulocyte - PMN, basophil, eosinophil
Mediator Innate Defense?
1. Cytokines - bind receptors
* integrate innate & adaptive immune response
* Activate phagocyte & lymphocyte
2. Chemokine - chemoattractant cytokine
Virus strategies to evade Host defense?
1. Antigenic variation- point mutation & genome shuffling in antigentic shift
2. Inhibition of the IFN pathway
3. Inhibition of apoptosis and cell cycle control
4. Immune tolerance
5. Infection of immunoprivileged sites such as the brain
6. Direct infection of the immune system
7. Restricted expression of viral genes- invisible to host immune system (latent infection)
8. Production of decoys and inhibitors for immune system
9. Down-regulation of host proteins (MHC I etc..)
Viral evasion through virion?
•Some envelopes can shed spike proteins to serve as decoys to neutralizing Ab.
•Complement-regulatory molecules in the membranes of some enveloped viruses can inhibit virolysis.
Misfolded envelope spikes can be potent immunogens & generate a suboptimal response to native spikes.
Viral evasion of Ab through Infected cells
Infected cells can release misfolded or incomplete envelope complexes that function as decoys.
Viral evasion of Ab through surface glycoprotein (3)
1. A variable loop favors induction of strain-specific rather than strain-cross-neutralizing antibodies.
2. Carbohydrate chains and oligomerization conceal the protein surface, reducing immunogenicity.
3. A recessed receptor site may also reduce immunogenicity.
Method to study virus (5)
1. Electron miscroscopy
2. Cell culture
3. Animal models
Strategies for viral survial (3)
1. House genome in capsid
2. Genome contains all info to initiate and complete infectious cycle
3. Establish relationship with host
Characteristics of virus particles (3)
1. Symmetrical arrangement proteins
2. Non-covalent bond between proteins
3. Highly conserved structural motif capsid
What is the importance of non-convalent interaction between viral particle subunits?
Stable interaction but flexible --> easily taking apart to for new infection
Differ structures of idential vs. non-identical bond between subunits
1. Identical bonds: regular shape
2. Non-identical bonds: clumps or aggregates
Helical capsid (2)
1. Indentical subunits
2. Rotational symmetry
Icosahedral symmetry (3)
1. 20 triangular faces
2. 12 verices
3. 2-, 3-, 5-fold symmetry
Describe the smallest closed icosahedral cell (2)
1. 60 identical subunits
2. 3 subunits per face
How enveloped virus acquires their envelope?
1. By budding through membrane of host cell
* plasma membrane
* Golgi / ER
How non-enveloped virus escapes host cell?
Enveloped virus escapes host cell w/o killing host through help of glycoproteins. List (3) functions of glycoproteins on viral cell cycle
1. Entry & host range determination
2. Assembly & egress
3. Evasion from immune system
T/F: Viruses grow exponentially?
Viruses do NOT grow exponentially but released as a burst because viruses are assembled from preformed compoments
What happen during the eclipse period of viral cell cycle? (6)
1. 0-12 hours of virus infection
2. No virus detection (inside / outside cell)
3. Penetrating cells
4. Releasing genomes
5. No infectious
6. No plaque forming units
What happens during latent period? (6)
1. Start seeing growth
3. Entry & uncoating
4. Gene expression
5. Genome replication
6. Assembly & egress
How do virus attach to host cell?
Specific binding of virus-attachment protein and target receptor
Why carbohydrate receptors less specific than protein receptors?
may occur on many different glycosylated membrane bound molecules
Function of PKR?
Inhibit protein synthesis
Function of OAS?
Cells of innate defense (4)
1. Mononuclear phagocytes
2. Dendritic cells
3. NK cells
Virus of latent infection (4)
4. Virush infecting Multiple sclerosis
Site of dorman for EBV, HSV and HPV
1. EBV in B-lymphocyte
2. HSV in dorsal root ganglia
3. HPV in basal epithelial cell