3 - Interferon (05.03.2020) Flashcards
(47 cards)
1
Q
What are the barriers to viral infection?
A
- Intrinsic immunity (barriers)
- innate immunity (not specific, this is what makes you feel sick)
- acquired immunity (needed for clearance)
2
Q
Intrinsic immunity: CpG and ZAP
A
- if a virus has high levels of CpG this is detected by ZAP which activates RNA exosome to degrade viral RNA and prevent replication
- could be used to make attenuated live vaccines! (engineer viruses to have too much CpG and then the immune system would catch them more)
- not specific, just recognises something very generic
3
Q
ZAP
A
zinc-finger antiviral protein
4
Q
CpG
A
C followed by G in the genome
5
Q
What is interferon?
A
- a substance that is produced by cells infected with a virus and has the ability to inhibit viral growth
- Binds to specific receptors and signals activation of de novo transcription of hundreds of Interferon Stimulated Genes, ISGs (warns the cells that there is a problem in the surrounding and that they should prepare for a virus)
- active against many different viruses
6
Q
ISG
A
interferon stimulated genes
7
Q
Type 1 interferon
A
- switch on earliest/first
- T1 interferon receptors are found on all cells int he body
- Polypeptides secreted from infected cells
- Three major functions:
- Induce antimicrobial state in infected and neighbouring cells
- modulate innate response to promote Ag presentation and NK
- Activate the adaptive immune response
= antiviral state + recruit troops
- IFN beta is secreted by all cells and IFNAR receptor is present on all tissues. IFNb induction is triggered by IRF-3.
- Plasmacytoid dendritic cells pdcs are specialist IFN a secreting cells. They express high levels of IRF-7 constitutuvely.
- One gene for IFN b, 13/14 isotypes of IFNa.
- primary T1 interferon is interferon beta
8
Q
Which interferon do dendritic cells and macrophages mainly secrete?
A
interferon alpha (macrophages also secrete interferon beta)
9
Q
What are the types of interferon?
A
Type I IFNs are IFN alpha and IFN beta
Type II IFN is IFN gamma
Type III IFN is IFN lambda
10
Q
What receptor does type 1 IFN bind to?
A
IFNAR (present on all tissues)
11
Q
Type 2 IFN
A
Type II IFN is IFN gamma
- Produced by activated T cells and NK cells
- Signals through a different receptor IFNGR.
12
Q
Type 3 IFN
A
Type III IFN is IFN lambda
- Signals through receptors IL28R and IL10b that are mainly present on epithelial surfaces -> doesn’t affect immune cells
- important in the early events of viral infection going through respiratory surfaces and the liver
- e.g. outcome of HepB/C is influenced bu IFN-lambda polymorphism
13
Q
How many IFN alpha / betas are there?
A
1 beta
around 13/14 isotypes for IF alpha
14
Q
How do cells differentiate self from non-self?
A
- Pathogen Associated Molecular Patterns, PAMPs
- Pattern Recognition Receptors, PRRs
- Often sense foreign nucleic acid
- cytoplasmic RIG-I like receptors RLRs, endosomal Toll like receptors TLRs
- Cytoplasmic nucleotide oligomerization domain receptors NLRs
15
Q
Pathogen sensors detect viral nucleic acid
A
=> PRRs
Examples:
- TLR - endosomal
- RLR (Rig-1 like Receptor) - RIG1, MDA5, LGP2 - cytoplasmic
- NLR - cytoplasmic nucleotide oligomerization domain receptors
- DNA sensors (cGAS)
16
Q
Interferon induction via the RIG pathway
A
- RIG1 and/or MDA5 are in the cytoplasm
- if they see RNA that is wrong (e.g. no cap or poly A tail) they bind to it and change conformation
- then binds to MAVS (mitochondrial activator of viral signal), this is stuck on the mitochondrial membrane
- downstream signals and cascades follow
- phosphorylation of IRF-3 -> dimerises and moves to the nucleus where it is the TF for IFN-beta
17
Q
What pathways can induce interferon production?
A
- RLR (RNA ins cytoplasm, all cells have this)
- TLR (RNA in endoscope, mainly in plasmacytoid dendritic cells)
- cGAS (DNA)
18
Q
Interferon induction via TLR
A
- TLRs sit in endosomes
- if there is viral RNA in an endosome this will be picked up by TLRs (TLR3,7,8)
TLR3:
- joins the sam pathway as RIG-1, causes phosphorylation of IRF-3 and the production of IFN-beta
TLR7 and TLR8
- signal through Myd88
- cause phosphorylation of IRF-7 (constitutively expressed in plasmacytoid dendritic cells)
- TF for production of IFN-alpha and IFN-beta
=> strong response burst with those 2 IFNs
This occurs mainly in more specialised cells (plasmacytoid dendritic cells
19
Q
cGAS - STING pathway induction of interferon
A
- DNA is sensed by cGAS that signals through STING
- cGAS is am enzyme and it is activated by binding to dsDNA in the cytoplasm
- cGAMP acts as a second messenger binding to STING which sits on the membrane of the ER
- via TBK-1 there is increased phosphorylation of IRF-3 and increased production of Type-1 IFN
20
Q
STING
A
stimulator of interferon genes
21
Q
What kind of signalling does interferon beta do?
A
autocrine
paracrine
-> secreted by an infected cell and then binds to INFAR on that cell or on neighbouring cells
22
Q
What triggers INF-beta induction?
A
phosphorylation of IRF-3 which is a TF for the IFN-beta gene
23
Q
Summarise the effects of interferon beta production
A
- IFN-beta is produced and secreted by infected cells
- binds to INFAR (present on all cells)
- > autocrine and paracrine signalling
- de novo synthesis of around 300 or 400 INF stimulated genes
- this costs a lot of energy and can be quite toxic in terms of normal cell function so they have to be turned on and off quite rapidly.
24
Q
Proof for the importance of IFN
A
- in mutations in genes in the pathway (e.g. IRF-7) you may not be able to fight off a viral infection
- might have a reaction to an attenuated vaccine
25
Herpes simplex encephalitis
- HSE is the most common cause of sporadic encephalitis in the Western world.
- Prevalence of 1/10,000.
- Most common in childhood, affecting previously healthy individuals on primary infection with HSV-1
Inborn errors in at least 6 genes implicated in HSE
- TLR3, UNC93B1, TRIF, TRAF3, TBK1, and IRF3.
- Impair CNS intrinsic interferon a/b response to HSV infection.
- uncontrolled viral replication in the brain -> encephalitis
26
Structure of INF R
heterodimer (e.g. IFNAR1 and IFNAR2)
27
What happens when INF binds to a receptor?
- IFN binds to IFN-R (e.g. INFAR)
- Jak and STAT are on the inside of the cell
- STAT phosphorylation and dimerisation
- STAT dimer enters the nucleus and binds to the promoter region of 100s genes that respond to the stat dimer
- transcription and translation occurs
- 300-400 proteins made
28
Which proteins are made in response to IFN binding?
- PKR Protein Kinase R: inhibits translation (viruses are dependant on the host machinery)
- 2’5’OAS: activates RNAse L that destroys ss RNA
- Mx: inhibits incoming viral genomes
- ADAR : induces errors during viral replication
- Serpine: activates proteases
- Viperin: inhibits viral budding
- IFITM3 (IFN induced transmembrane protein 3) restricts virus entry through endosomes, sits on endosomes and restricts them from fusing e.g. in flu.
=> very drastic response (better to kill the cell and block the virus)
29
IFITM3
=IFN induced transmembrane protein 3
- sits on endosomes and restricts them from fusing e.g. in flu
- restricts virus entry through endosomes
- mutation makes you more susceptible to influenza
- 25% Chinese and 50% Japanese carry it making them more susceptible to influenza (compared to <1 not so severe affected and around 6% hospitalised europeans)
- remember: this is an interferon stimulated gene
30
Antiviral mediators: Mx1 and Mx2
- GTPase with homology to dynamin
- Mx can form multimers which wrap around the nucleocapsids of incoming viruses.
- Mx1 inhibits influenza
- Mx2 inhibits HIV
- stops viruses from entering the nucleus
31
Self-regulation of antiviral response
- to limit damage
- IFN response may only be maintained for several hours
- Subsequently the ability to response to IFN is lost due to negative regulation
- SOCS suppressor of cytokine signalling genes turn off the response.
32
SOCS
- interferon regulated genes
| - suppressors of cytokine signalling
33
How do viruses evade IFN response?
- Avoid detection by hiding the PAMP (e.g. replicate within membrane bound compartments)
- Interfere globally with host cell gene expression and/or protein synthesis (e.g. blocks transcribing)
- Block IFN induction cascades by destroying or binding
- Inhibit IFN signalling (e.g. STAT proteins)
- Block the action of individual IFN induced antiviral enzymes
- Activate SOCS prematurely
- Replication strategy that is insensitive to IFN (e.g. so fast before the cell has mounted the response)
34
Examples of interferon control by viruses: stopping activation
Hepatitis C virus:
- NS3/4 protease acts as antagonist to IFN induction by cleaving MAVS.
Influenza virus:
- NS1 protein acts as antagonist to IFN induction by binding to RIG-I /TRIM25/RNA complex and preventing activation of signalling pathway
- also prevents nuclear processing of newly induced genes.
35
accessory genes
- genes that the virus does not necessarily need to replicate inside us
- if it has them it is good at shutting the cell down and blocking the immune repsonse
- makes it more successful
- half of the pox virus genome is genes that modify the host immune repsonse
36
Pox virus - why is it successfu?
- Pox viruses and herpes viruses are large DNA viruses
- More than half the pox virus genome is comprised of accessory gees that modify immune response.
- Pox viruses encode soluble cytokine receptors (vaccinia virus B18), that are being developed as possible future immune therapies e.g. for AI disease
- this soluble receptor mops up signalling molecules so that they don't bind to cells.
37
Ebola virus cycle and immune evasion mechanisms
- has ways of stopping being sensed.
- encodes VP35 and VP30
- > cell makes less INF
- VP24 stops INF from having its action in the nucleus and INF induced genes are not transcribed
- 3 of 7 or 8 genes in the ebola genome are dedicated to blocking the INF system
38
How many genes does the Ebola virus have?
- 3 of them are dedicated to blocking the INF system
| - 7 or 8 (I think 7 according to a website with the. genome)
39
Consequences of innate immunity: Viral pathology
- A combination of damage of infected cells by virus and
| - damage of infected and bystander cells by the immune response (saving whole body by local inflammatory response)
40
How do viruses skew the immune response by interfering with interferons?
- Many viruses modulate the immune response, presumably to increase their own replication and transmission.
- This can result in inadvertant pathology.
- The effects of interferon can vary from protective to immunopathologic. This may depend on how much IFN is made- 100 times more IFN is required for IL-6 induction than for Mx.
- IL-6 might make you feel sick and have a fever and may cause cytokine storm
41
Cytokine storm
- innate immunopathology of virus infections
- Virus replicates, induces high IFN accompanied by massive TNFa and other cytokines.
- Differences in clinical outcome may reflect vigour of innate immune system, which may vary with age.
- > more INF, more IL-6 does more damage to the cell in many organs than the virus would have done in the patient.
- This is typical of Dengue haemorrhagic fever, severe influenza infections and Ebola.
42
What do mediators of cytokine storm cause?
- leaky endothelium / endothelial dysfunction
- inappropriate inflammatory responses
- can cause pulmonary fibrosis after respiratory infections
43
Consequences of the balance between viruses and interferons
Host range barriers
Therapeutics
Vaccines
oncolytic viruses
44
IFN as an antiviral treatment
- IFN as a treatment (HCV, pegylated IFN often used with ribavirin -> now not used anymore because we have quite good directly acting antivirals)
- Associated with unpleasant side effects (you feel like you have flu)
45
IFN lambda as an influenza therapeutic?
- IFN-lambda only signals to receptors on epithelial cells
- by giving local IFN-lambda you protect the cells from viral infection
- smaller amount of cytokines etc. because not immune cells
- may be a better therapeutic in the longer run
46
Viruses that lack ability to control interferon as a new generation of live attenuated vaccines
- Viruses deficient in control of IFN are attenuated in IFN competent cells .
- The high IFN levels they induce can also recruit useful immune cells, IFN acting as an ‘adjuvant’.
- you would have to grow the flu cells lacking NS1 in an environment without INF (otherwise they don't survive) and use them as a live attenuated vaccine)
- Cells naturally or engineered to be deficient in IFN response can be used to grow these attenuated virus strains.
47
IFN and cancer
- cancer cells don't make a good IFN response
- genetically engineer viruses that kill cancer cells but don't infect healthy cells
'healthy cells make IFN and can control viruses. Cancer cells can't make interferon and the viruses kill them'
