Viruses and host defensesViruses and host defenses

How the successful virus beats the body

Learning objectivesLearning objectives

• Describe the role of interferons, natural killer cells, innate and adaptive immune system in recovery from virus infection

• Explain how various viruses evade host immune strategies

• Interpret data from experiments designed to determine virus evasion mechanisms

Host outcomeHost outcome

• Recovery

• Persistence

• Rapid death

What prevents most viruses from causing What prevents most viruses from causing disease?disease?

• Skin• Nonspecific innate defenses

– Natural killer cells– Interferon– Complement– Apoptosis

• Acquired immunity– Antibodies– Cytotoxic T cells– Lymphokines

How do we know the importance of each host How do we know the importance of each host defense mechanism in recovery?defense mechanism in recovery?

• Infect and measure responses over time

• Use mutant animals/genetic defects in humans

• Infect and block specific defenses

• Do in vitro studies

Innate defenses: we survive most virus infectionsInnate defenses: we survive most virus infections

skin: impenetrable barrier unless insect vector or wound

• mucous membranes: live target cells

• inflammatory response: neutrophils

• complement

• macrophages

– HIV integration/destruction;

– Dengue virus immune enhanced uptake

• cytokines

NK cellsNK cells

Overcoming NK cellsOvercoming NK cells

• CMV makes MHC homolog

• How does it prevent cytotoxic T cells from killing virus infected cell?

Interferon: host proteins induced by virus in vertebrates that Interferon: host proteins induced by virus in vertebrates that

interfere with viral replicationinterfere with viral replication (type 1)(type 1)

• bind to receptor on host cells (determines species specificity)

• through signal transduction induces synthesis of antiviral proteins (AVPs) controlled by interferon-stimulated response elements (ISRE) which protect cell from viral replication

• What would happen if you microinject IF into a cell?

• What would happen in presence of Actinomycin D?

• Evidence for IF role in recovery from infection

– Time

– Place

– Exogenous transfer experiments

– Loss

Antiviral effects of IF-induced AVPsAntiviral effects of IF-induced AVPs

• Oligoadenylate synthetase activated in presence of dsRNA

– ATP-----> oligoA up to 15 residues

– oligoA activates RNAase that cleaves host and viral mRNA

• Protein kinase (PKR) activated in presence of dsRNA phosphorylates eIF2 needed for translation

Host cell responses are not always protectiveHost cell responses are not always protective

• Results of microarrays using high and low virulence strains

– Red are upregulated

– Green are downregulated

– Black - no change

• Increased inflammatory, apoptotic, oxidative damage responses

antibodyantibody

• neutralization: antigenic shifts and drifts(variation)

• complement mediated lysis: vaccinia makes homolog of inhibitor

• HSV makes receptor for a complement component

cell mediated immunity: requires viral protein displayed cell mediated immunity: requires viral protein displayed in MHC1in MHC1

• downregulates transport of MHC to surface (Adenovirus; herpesvirus; measles)

• Increases endocytosis of MHC

• Neurons express little MHC

• Mutation to new epitopes

• latency

cell apoptosiscell apoptosis

• Blocked by viral proteins which may bind to host proteins promoting apoptosis

• Produce mimic proteins that inhibit apoptosis

Multiple stolen genes of KSHV - HHV8Multiple stolen genes of KSHV - HHV8

• V-FLIP - FLICE inhibitory protein

• Decoy receptor• Bcl-2 - anti-apoptotic factor• complement binding protein• IL6-like cytokine• three chemokines• interferon regulatory factor• D-type cyclin• G-protein coupled receptor

How to make a killer virusHow to make a killer virus

• What characteristics should a biological weapon have?

• How can it be constructed?

• Ectromelia virus causes mousepox

• Recovery due to CTL death of infected cells via perforin pathway mousepox virus produces inhibitors of caspases

• Vaccinia virus does not inhibit caspases so they are killed by two mechanisms

• Il4 skews immune response to ab production and shuts down perforin pathway

Ebola strategiesEbola strategies

• Releases soluble glycoprotein (portion of transmembrane protein)

• Hypothesis: It may bind to a host cell and prevent its activity

• How do you show binding to specific white blood cell?

• Gel depletion

sGP vs GP binding to neutrophils vs endothelial sGP vs GP binding to neutrophils vs endothelial cellscells

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neutrophils

endothelials

GP pseudotyped virus

Infection by GP pseudotyped luciferase virus

Ebola and IF induced gene expressionEbola and IF induced gene expression

• Northern blots

• Which genes are affected by EZ?

• How would you determine which EZ gene is responsible?

The Many Roles of NefThe Many Roles of Nef

• Hypothesis: Downregulation of MHC1

• How do you show this?

• What can cause MHC downregulation?

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CD4 vs MHC downregulationCD4 vs MHC downregulation

• Nef genes from WT, nonprogressors, slow progressors and rapid progressors

• What is advantage to virus of CD4 downregulation?

• How can you determine whether the same or different parts of the Nef protein are responsible for each activity?

Increases viral replication/budding by interacting with signal transduction and TCR

Nef binds to p53 - what is Nef binds to p53 - what is affect on apoptosis?affect on apoptosis?

• How would you determine which proteins Nef binds to?

• Nef also binds to several cellular signal transduction elements -What is role?

• How do you show affect on apoptosis?

Nef binds to p53. (a) A GST-Nef fusion protein specifically coprecipitates p53. Purified recombinant GST-Nef (lane1) and GST (lane 6) alone were incubated with purified recombinant p53, affinity purified with glutathione-Sepharose beads, electrophoresed, and transferred to nitrocellulose. Purified p53 was electrophoresed and transferred to nitrocellulose as a control (lane 7). Nitrocellulose membranes were then reacted with anti-p53 in Western blotting. For competition of the GST-Nef-p53 interaction by purified Nef protein, p53 was incubated with purified Nef protein at 0.3- (lane 2), 3- (lane 3), 10- (lane 4), and 30-fold (lane 5) molar excess before reaction with GST-Nef and processing as described above

MOLT-4 cells which had been electroporated with purified recombinant full-length Nef or GST or which had been mock electroporated were either exposed to a lethal dose of UV light (+) for approximately 30 s or were not exposed (-) and then returned to culture for 12 h. Hirt DNA was extracted from each of the samples and analyzed by gel electrophoresis for the presence of fragmented DNA.

The APOBEC enzyme family The APOBEC enzyme family deaminates specific cytidine (C) deaminates specific cytidine (C) residues in either DNA or mRNA, residues in either DNA or mRNA, converting them to uridine (U) converting them to uridine (U) residuesresidues

• Viruses replicating in cells producing APOBEC3G need Vif gene for cells to be permissive

• Vif- can grow in cells without APOBEC

Anti IF strategy of HCVAnti IF strategy of HCV

• NS5a binds to PKR and inactivates

• E2 gene has 12 aa homology to autophosphorylation site of PKR and eIF2a

• How do IFres and Ifsens differ?

• How might that help the virus?

Do PKR and E2 bind?Do PKR and E2 bind?

• His tag binds to beads

• Isolate and run on gel

• Wt PKR

• K296 = mutant in ATP binding domain

• E2-C - no Phos site

• Hn - cell protein control

Does E2 interfere with PKR activity?Does E2 interfere with PKR activity?

• ATP- P32

• PKR +/- E2 and in presence of dsRNA activator and substrate H2a

HSV blocks IF in several waysHSV blocks IF in several waysvhs degrades mRNA, ICP27 prevents splicingvhs degrades mRNA, ICP27 prevents splicing

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Virus dephosphorylates eIF2a

Virus destroys Jak1, disperses ND10 and disrupts PML