Interferon Flashcards
Summarise the immune responses to viruses
Intrinsic defences (mucosal and physical barriers)- lesion in these or just enough viruses- will cross these to infect the vulnerable cells and tissues Here, the innate immune system will act to destroy the virus- via phagocytosis and production of cytokines- these interfere with normal cell function- which can make us fee ill an lethargic- in healthy patients- this stage will be asymptomatic- and the virus will be cleared here However, the virus may become systemic- activating the adaptive immune response (B cells and T cells)
Describe the role of CpG and ZAP in intrinsic immunity
Non-Specific host defence strategy
ZAP recognises viruses (particularly RNA viruses) which have a different nucleotide ratio to that of human genetic material- more specifically they have an abnormally high ratio of CpG.
The genetic material of the virus binds to the zinc-finger domains of ZAP, resulting in its degradation in an RNA exosome
Summarise the discovery of interferon
Discovered more than 50 years ago by Isaacs and Lindenmann
A transferable factor produced by exposure of eggs to heat inactivated influenza virus that protected new cells from infection (both to active influenza and measles)- non-specific defence
Binds to specific receptors and signals activation of de novo transcription of hundreds of Interferon Stimulated Genes, ISGs
Describe the functions of type 1 interferons
Polypeptides secreted from infected cells- earliest and fastest-acting interferons
Three major functions:
1. Induce antimicrobial (and antiviral) state in infected and neighbouring cells
2. modulate innate response to promote Ag presentation and NK - act as adjuvants- modulate antigen presentation via MHC
3. Activate the adaptive immune response
Essentially, what are interferons
Transferrable factor produced when the cells are exposed to virus
Polypeptides
Interferons: soluble polypeptide transferable factors produced by infected cells to protect nearby cells, binding to specific receptors to signal activation of transcription of interferon stimulated genes - danger signal that triggers anti-viral state in nearby cells
Describe how type 1 interferons act in response to a respiratory virus
Respiratory virus infects epithelial cells.
Activation of PRRs will lead to production of interferon-stimulated genes- which will induce an antiviral state in that infected cell and also neighbouring cells- IFNbeta
Will also induce ISGs in fibroblasts- which will produce IGFbeta to promote an antiviral state in the epithelial barrier
PAMP can also bind to PRRs in sentinel cells (pDC, DC and macrophages)
pDC- IFNa
DC and macrophages - IFNa and IFNb
These induce ISGs in infected cells and neighbouring cells, increase antibody production and chemokine production in innate cells- increase signal that there is an infection
increased effector T cell immune responses- adaptive immunity
Summarise the type 1 interferons
Type I IFNs are IFN α and IFN β.
IFN β is secreted by all cells and IFNAR receptor is present on all tissues. IFNβ induction is triggered by IRF-3.
Plasmacytoid dendritic cells pdcs are specialist IFN α secreting cells. They express high levels of IRF-7 constitutuvely.
One gene for IFN β, 13/14 isotypes of IFNα
IFNAR present in all tissues
IFNb- produced by all cells in the body
Summarise the type 2 interferons
Type II IFN is IFNγ.
Produced by activated T cells and NK cells
Signals through a different receptor IFNGR.
Important for the adaptive immune system
Summarise the type 3 interferons
Type III IFN is IFNλ.
Signals through receptors IL28R and IL10β that are mainly present on epithelial surfaces in the liver and lungs
§ So normally expressed during respiratory tract infections and liver infections.
§ Polymorphisms in IFNl are associated with IMPROVED outcomes from HCV and HBV with both spontaneous clearance and response to antiviral therapy.
Ultimately, how do we distinguish between self and non-self
Pathogen Associated Molecular Patterns, PAMPs
Pattern Recognition Receptors, PRRs
NOTE: they often sense nucleic acids
Genetic material of viruses will be different to that of humans:
i.e no DNA in cytoplasm normally- this will be detected
RNA may not have 5’ cap or 3’ polyadenylated tail- PRRs can pick up these differences.
Describe some PRRs
RIG-I like receptor (RLRs) – cytoplasmic
Toll-like receptors (TLRs) – plasma membrane + endosomal membrane
Cytoplasmic nucleotide oligomerization domain receptors NLRs
What do TLRs, RLRs and DNA sensors sense
RNA -TLRs and RLRs
DNA- DNA sensors
Describe interferon induction via RLRs
RNA is sensed by RIG-I and/or mda-5- if it differs from mRNA
it will signal through MAVS (mitochondrial)
This will then switch on kinases (Tbk1) - which will phosphorylate Irf3 and Irf7
these will dimerise and act as a transcrition factor in the nucleus- bind to PRDIII domain (promoter)- stimulates the production of IFN-B
Describe interferon induction via TLRs
TLRs also sense viral RNA, especially in dendritic cells and signal through Myd88
Tlr3- will sense dsRNA and signal through Trif to phosphorylate iRF3
Trl7,8 and 9 wil sense ssRNA siRNA and CpG motifs and signal through Myd88 to stimulate NFkB and Irf7
IRF7 translates to the nucleus to stimulate gene expression of IFNa and IFNb
Also up-regulate the gene expression of inflammatory cytokines (TNF-a, IL-6 and IL-12).
Describe interferon induction via cGAS
cGAS is an enzyme- binds to viral dsDNA
Promotes production of cGAMP
cGAMP then binds to sting- found on the surface of ER
STING then binds to TBK1 receptor on perinuclear puncta- which phosphorylates IRF-3, which dimerises and transolcates to the nucleus to up-regulate the production of type 1 interferons.
Describe the actions of the released IFN
IFN is a soluble cytokine
Binds to IFNAR on its own cell, and neighbouring cells- allowing autocrine and paracrine functions respectively,
Up-regulates ISGs- which promote inflammation and disturb normal functioning of the cell- too much and we can get ill
Describe a monogenic inborn error of the IFR7 gene
Noncosanginous parents of French descent each with heterozygous mutation in IRF-7 revealed by exome sequencing
Greater risk of viral infections
Describe defects in the IFNAR2 gene
Proband and newborn sibling have autosomal recessive IFNAR2 gene
Defect in the receptor- thus defect in type 1 interferon signalling
Could have Fata response to MMR vaccine
Describe defects in IRF3
Patient P1 has heterozygous mutation in IRF-3
Describe 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.
Describe the inborn errors associated with Herpes Simplex Encephalitis
TLR3,UNC93B1,TRIF, TRAF3,TBK1, and IRF3.
Impair CNS intrinsic interferon a/b response to HSV infection.
Describe the structure of the IFNAR for type 1 interferon
They are heterodimers of IFNAR 1 and IFNAR 2
Describe the type 1 interferon signalling cascade
They are heterodimers of IFNAR 1 and IFNAR 2- these two monomers dimerise upon IFNb/IFNa binding.
Auto-phosphorylation of JAK1.
Phosphorylation of STAT
Phosphorylated STAT dimerises, translocates to the nucleus and leads to the transcription of ISGs:
PKR
Mx, IFITM3, miRNAs, ADAR, apoptosis, cell cycle arrest, etc.
mac
2’5’OAS- RNAase L
MHC
Describe the action of some ISGs
PKR Protein Kinase R: inhibits translation- will also prevent the host from making proteins
2’5’OAS: activates RNAse L that destroys ss RNA- so will also destroy host mRNA
Mx: inhibits incoming viral genomes
ADAR : induces errors during viral replication
Serpine: activates proteases
Viperin: inhibits viral budding
Describe how IFITM3 restricts viral entry through endosomes
Interferon-induced transmembrane protein 3
These sit on the membrane of endosomes, in cells that have been previously stimulated by IFN
It prevents fusion of the virus membrane with the endosomal membrane so the virus gets trapped in the endosome and thus destroyed by the acidic pH- virus can no longer enter the nucleus to replicate
NOTE: mice and people lacking IFITM3 get more severe influenza
Describe 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
Prevents the virus from getting into the nucleus once it has been released from the endosome
Describe the self-regulation of the anti-viral state
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.
Describe the different strategies by which viruses can evade IFN responses
Avoid detection by hiding the PAMP- may replicate inside membrane bound organelles or vacuoles - PRRs can’t detect PAMPs
Interfere globally with host cell gene expression and/or protein synthesis
Block IFN induction cascades by destroying or binding
Inhibit IFN signalling
Block the action of individual IFN induced antiviral enzymes
Activate SOCS
Replication strategy that is insensitive to IFN
Describe how Hepatitis C evades the immune response
Hepatitis C virus: NS3/4 protease acts as antagonist to interferon induction by cleaving MAVS.
HCV protease NS3/4A cleaves MAVS.
Thus destroying the sensor system of viral RNA
Describe how the influenza virus evades IFN signalling
Influenza virus: NS1 protein acts as antagonist to interferon induction by binding to RIG-I /TRIM25/RNA complex and preventing activation of signalling pathway, and also prevents nuclear processing of newly induced genes- inhibits the transcription factor CPSF30
Describe the action of PKR
It phosphorylates the alpha subunit of eIF2 (initiation factor) that is important in translation
This prevents ribosomes from binding to mRNA so NO NEW GENES WILL BE TRANSLATED
It also phosphorylates NFkB, which is an important transcription factor that is part of the interferon and inflammatory response
It is an extreme measure and a last resort – only activated when the cell has no other option
Describe Pox viruses
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
Describe the immune evasion mechanisms of ebola
o Ebola – evades the IFN response by VP35, VP24 and VP30:
§ VP35 – blocks RIG-I Like complexes and RNAi expression. - stops IFN production
§ VP30 – blocks RNAi expression. - stops IFN production
§ VP24 – directly blocks IFN signalling (prevents STAT from translocating to the nucleus)- no antigen presentation, no IFITM, Tetherin or ISGs
Describe the consequences of innate immunity
A combination of damage of infected cells by virus and
damage of infected and bystander cells by the immune response
Innate immunity is what makes us feel ill
Describe the consequences of viruses interfering with IFNS
Skew the immune response
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 interferon is made- 100 times more IFN is required for IL-6 induction than for Mx.- this will make us feel really ill
Describe cytokine storm
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.
This is typical of Dengue haemorrhagic fever, severe influenza infections and Ebola.
o Cytokines involved – TNF, IFN(a/b), IFN(gamma), IL-1, IL-6, CCL2.
What is the paradox of the cytokine storm
o Differences in the clinical outcome may vary with age and health – so the less ill you get (healthier), paradoxically, the more severe the clinical outcome as you’re better at producing IFN.
Describe the consequences of the cytokine storm
Endothelial dysfunction:
Altered endothelial physiology and barrier function
Increased permeability syndrome
Endothelial cytokine storm infiltration and amplification loop
Inflammatory responses:
Systemic Cytokine Circulation
Sepsis syndrome (hypotension and leukocytosis)
Pulmonary Fibrosis:
Recruitment of fibrocytes
Progressive deposition of ECM
T-cell mediated immunopatholgy
Describe IFNs as an antiviral treatment
IFN as a treatment (HCV, pegylated IFN often used with ribavirin)
Associated with unpleasant side effects
They stimulate the production of several cytokines and this causes several unpleasant side effects
Explain the reasoning IFNlambda as an anti-viral 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
Describe how viruses that cannot control the innate immune response can not be used as LAV
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’.
Cells naturally or engineered to be deficient in IFN response can be used to grow these attenuated virus strains.
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
Describe 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
What does innate immunity comprise
Barriers: epithelial surfaces are major points of entry for virus into host. They are covered with protective secretions that contain mucus and collectins which can act as decoy receptors.Influenza virus neuraminidase can enzymatically remove the decoy receptors from mucus allowing virus to gain access to the ‘real’ receptor at the cell surface.
List the different type of IFNs
Interferon response: All cells can make and respond to type I interferons, IFN alpha and IFN beta. These are soluble cytokines that are secreted when the cell detects a foreign pattern. Type II interferon is IFN γ. This is produced by cells of the immune system. IFN lambda (type III) can be produced by all cells but only acts at epithelial cells.
Explain how type 1 IFN synthesis is regulated
Detection of the virus or other invading pathogens is by pattern recognition receptors, PRRs. These can be at the cell surface, for example Toll Like receptors TLRs.
Or they can be intracellular, for example RIG-I.
The PRRs detect unusual nucleic acids that are nonself because they are different to those made by the host or they are in the wrong place. These are the pathogen associated molecular patterns or PAMPs.
When the PAMP is detected, a signaling cascade is set in motion that results in the transcription of the IFN gene
Outline the canonical type 1 IFN pathway
Newly transcribed and translated IFN is secreted from the infected cells and acts on specific receptors on the surface of the infected cell or neighbouring cells and signals the synthesis of hundreds of new genes that have antiviral effects.
Explain the mechanisms of action of some important type- 1 ISGs
PKR will shut down protein translation, a host function on which viruses are absolutely dependent.
Mx will bind and nullify incoming virus genomes.
IFITM3 will prevent the entry of enveloped viruses.
Tetherin will prevent the budding of many enveloped viruses.
In addition IFN will recruit other arms of the immune response such as dendritic cells and NK cells.
Give examples of how viruses control or evade the type I interferon system and how this knowledge can be harnessed to make new therapies.
Because this IFN response is so powerful, many viruses have had to evolve strategies to antagonize it.
Describe the potential therapeutic use of soluble cytokine receptors
This could be useful in autoimmune or inflammatory conditions where IFN and other cytokines are produced in abundanc
i.e rheumatoid arthrirts
Which two techniques could be used to show the skewing of cytokine production by viruses
Transcriptomimics – shows changes in mRNA production
Proteomimics – shows changes in protein expression
Name two proteins produced by HIV that helps them deal with restriction factors
Vif – APOBEC
Vpu – Tetherin
What is the normal action of APOBEC
APOBEC is involved in the innate immune resistance to retroviruses and hepadnaviruses
APOBEC modifies some of the nucleotides in reverse transcription and makes them into the wrong version
APOBEC deaminates dC to dU in the minus strand of viral cDNA during reverse transcription
This leads to G to A hypermutation resulting in ERROR CATASTROPHE
This results in so many mutations that the viral genome becomes nonsense and the virus can’t replicate
Vif counteracts the activity of APOBEC and targets it for degradation
This removes the interference of APOBEC with reverse transcription
Describe a Tetherin
Tetherin sits on the cell surface of infected cells and binds to viruses that try to escape the cell to go and infect other cells
This limits the spread of viral infection
Vpu pulls tetherin back from the cell surface and targets it for degradation
