Interferon Flashcards
What is interferon?
Transferable factor produced when the cells are exposed to virus
What is the effect of interferon binding to interferon receptors on cells?
It binds to specific receptors and signals the de novo transcription of hundreds of interferon stimulated genes (ISG)
What are the three functions of type I interferons?
- Induce antimicrobial state in infected and neighbouring cells
- Modulate innate immune response to promote antigen presentation and NK cells but inhibit proinflammation
- Activate the adaptive immune response
What are the type I interferons?
IFN-alpha and IFN-beta
What is the first interferon to be produced in a viral infection?
IFN-beta
Which cells produce IFN beta? What corresponding receptor is present?
All cells produce IFN beta and all tissues have IFNAR receptors
What is IFN-beta induction triggered by?
IRF-3
Name a cell type that is specialised for producing IFN alpha.
Plasmacytoid dendritic cells
What do these Plasmacytoid dendritic cells express high levels of constitutively?
IRF-7
How many genes are there for IFN beta? How many different IFN-alpha isotypes are there?
Alpha – 13/14 isotypes
Beta – ONE
What is the type II interferon?
IFN-gamma (specialist immune signalling molecule)
Which cell types produce IFN-gamma?
Produced by activated T cells and NK cells
Which receptor do these IFNs signal through?
IFNGR
What is the type III interferon?
IFN-lambda
Which receptors do type III IFNs signal through?
Where are these receptors mainly present?
L-28 receptors
IL-10 beta receptors
Epithelial surfaces
E.g. respiratory epithelium
Polymorphisms in IFN-lambda is associated with improved outcome from what viral infections?
HCV and HBV
How does the innate immune system recognise non-self?
PRRs (pattern recognition receptors) on innate immune cells recognise PAMPs (pathogen-associated molecular patterns) = e.g. foreign nucleic acid in the cytoplasm
Name two receptors that are involved in detecting the presence of viruses and state where they are found.
RIG-I like receptor (RLRs) – cytoplasmic
Toll-like receptors (TLRs) – plasma membrane + endosomal membrane
Describe how viral RNA is sensed and how this leads to the production of IFN-beta
RIG-I and/or mda-5 will recognise single stranded RNA in the cytoplasm of the cell and it will signal through MAVS = mitochondrial activator of viral signalling (on the mitochondrial membrane).
This will signal further downstream, ultimately leading to the phosphorylation , and hence activation, of IRF-3 which acts as a transcription factor => generation of IFN-beta transcripts.
Describe TLR signalling.
TLR detects nucleic acids in the endosome (this isn’t normal)
- TLR3 signals downstream pathways which join with the RIG-I pathway => IRF-3 phosphorylation
- TLR7 and TLR8 are specific to the Plasmacytoid dendritic cells; they signal through MyD88 => IRF-7 phosphorylation (= transcription factor) => switching on of expression of IFN-alpha (and IFN-beta)
Describe DNA sensing.
Mainly done by cGAS
This is an enzyme that binds to dsDNA in the cytoplasm and synthesises cGAMP (second messenger)
cGAMP binds to STING (found on endoplasmic reticulum)
This triggers phosphorylation of the same sets of transcription factors and signalling molecules the RNA viruses were triggering
Describe the structure of IFN receptors for IFN alpha and IFN beta
They are heterodimers of IFNAR 1 and IFNAR 2
Describe the signalling from IFNAR receptors
IFN binds and the IFN receptor activates Jak and Tyk (through binding at its intracellular domain)
- they go on to phosphorylate STAT molecules
- STAT molecules dimerise and combine with IRF-9
- It then goes to the nucleus, binds to a promoter and regulates transcription of ISGs
State some protein products of Interferon Stimulated Genes (ISGs) and briefly describe what each of them do to interfere with the viral life cycle
PKR Protein Kinase R: inhibits translation of viral gene transcripts and ALL normal cellular gene transcripts.
2’5’OAS: activates RNAse L that destroys any ssRNA (even self ssRNA)
Mx: inhibits incoming viral genomes
ADAR: induces errors during viral replication
Serpine: activates proteases
Viperin: inhibits viral budding
What is IFITM3?
What does it do?
Interferon-induced transmembrane protein 3 (=product of an ISG)
These sit on the membrane of endosomes, in cells that have been previously stimulated by IFN.
- Virus would normally enter cell via an endosome, and it would fuse with the endosomal membrane and deliver its genetic material into the nucleus.
- IFITM3 prevents fusion of the virus membrane with the endosomal membrane so the virus gets trapped in the endosome
NOTE: mice and people lacking IFITM3 get more severe influenza
What are Mx1 and Mx2? How do they generally work? What do each of Mx1 and Mx2 inhibit?
GTPases with a homology to dynamin.
Mx can form multimers that wrap around nucleocapsids of incoming viruses – this nullifies the viral genomes
Mx1 – inhibits influenza
Mx2 – inhibits HIV
The antiviral state in which ISGs are being switched on must self regulate to limit damage. Name a family of genes that suppress the cytokine signalling and turn off the IFN response.
SOCS genes (Suppressor Of Cytokine Signalling)
(note that these are also classed as ISGs
State some mechanisms of viral evasion of the IFN response.
- Avoid detection by hiding the PAMP
- Interfere globally with host cell gene expression and/or protein synthesis
- Block IFN induction cascades
- Inhibit IFN signalling
- Activate SOCS
- Replication strategy that is insensitive to IFN
Explain how hepatitis C controls the interferon response.
Has NS3/4 protease that cleaves MAVS away from the mitochondrial membrane => Hep C is not detected through the RIG-I pathway
Explain how influenza controls the interferon response.
Influenza makes NS1 protein which:
- acts as an antagonist to interferon induction by binding to the RIG-I/TRIM25/RNA complex and preventing activation of the signalling pathway.
- migrates to the nucleus and prevents transcription of newly induced genes
What type of virus are Pox and Herpes viruses?
Large DNA viruses
More than half the pox virus genome is comprised of what?
What do Pox viruses encode specifically that helps control the interferon response?
More than half the pox virus genome is comprised of accessory gees that modify immune response.
They encode soluble cytokine receptors that mop up IFN and prevent it from reaching its receptors.
Describe a potential therapeutic use of this feature
This could be useful in autoimmune or inflammatory conditions where IFN and other cytokines are produced excessively
Ebola evades the IFN response through what three genes? What do they do
VP35, VP24 and VP30
VP35 – inhibits the RIG-I pathway
VP24 – stops IFNAR signalling to the nucleus
The consequences on the body from viral infections are due to…
Damage of cells by the virus and by the immune response itself
Describe how viral infections can cause cytokine storm. What is this typical of?
Differences in clinical outcome may reflect…
Lots of virus propagation –> lots of interferon being produced –> massive release of TNF alpha and other cytokines.
This is typical of Dengue haemorrhagic fever, severe influenza infections and Ebola.
Differences in clinical outcome may reflect vigour of innate immune system, which may vary with age.
What is a serious consequence of cytokine storm?
Pulmonary fibrosis – due to accumulation of immune cells in the lungs
Explain why viruses that cannot control the interferon response can be used as the next generation of live attenuated vaccines.
They will be able to infect the cells and it will replicate sufficiently to be able to mount an immune response but it wont replicate to the extent where it causes disease due to the interferon response.
The downside of this feature of the viruses is that these virus particles can’t be propagated in normal healthy cells. What is the solution to this issue?
Propagate the viruses in cells that are deficient in the IFN response
Explain why interferons are not frequently used as an antiviral therapy.
They stimulate the production of several cytokines and this causes several unpleasant side effects
What disease is IFN used to treat (in combination with)?
Hepatitis C (a combination of pegylated IFN is used with ribavirin)
Explain the reasoning behind using IFN-lambda as a treatment for influenza.
Receptors for IFN lambda are only found on epithelial surfaces (the site of infection of influenza is respiratory epithelium)
IFN lambda cannot signal through immune cells and cause immunopathology
It will only induce an antiviral state in the epithelial cells
Explain how oncolytic viruses would work.
Viruses are engineered that can uniquely replicate in tumour cells and kill them
Generally speaking, cancer cells are deficient in their ability to mount a proper interferon response
So, a virus that is unable to control the IFN response will NOT be able to replicate in normal healthy cells but they will be able to infect and replicate in cancer cells
