Errors in which genes can lead to increased fever and/or death?
Increased fever and/or death due to errors in:
- - IRF-1 genes – results in increased interferon production meaning they feel very ill.
- - IFNAR2 gene – this is an interferon synthesis receptor gene that is faulty so they don’t make interferon.
- - IRF-3 gene – important in the stimulatory pathway to create interferon.
What is the most common cause of sporadic encephalitis in the Western World?
In who is this most common?
Herpes simplex encephalitis
Most common in childhood – affecting previously healthy individuals on primary infection with HSV-1
- Inborn gene errors implicated in HSE – TLR3, UNC93B1, TRIF, TRAF3, TBK1, IRF3.*
- People that get infected with HSE normally have an effective INF response but sick HSE people may have inborn errors in the activation of this pathway.*
What is interferon?
What is the effect of interferon binding to interferon receptors on cells?
“A transferable factor produced by exposure of eggs to a heat-inactivated influenza virus that protected new cells from infection”
“A soluble cytokine secreted from infected cells, which binds to specific receptors to stimulate transcription of interferon-stimulated gene”
- Binds to specific receptors and signals activation of de novo transcription of hundreds of Interferon Stimulated Genes, ISGs
What are type 1 interferons and what are their functions?
Polypeptides secreted from infected cells
Three major functions:
- Induce antimicrobial state in infected and neighboring cells
- modulate innate response to promote Ag presentation and NK
- Activate the adaptive immune response
What are the types of Type 1 interferons?
Type 1 - IFNa (IRF-7) and IFNb (IRF-3)
Which interferons is produced first in a viral infection and what is this induced by?
IFNβ is secreted first and by ALL cells (IFNAR is the receptor) and production is induced by IRF-3
- IFNβ is secreted by ALL CELLS and the IFNAR receptor is present on ALL tissues
- IFNβ induction is triggered by IRF-3
Name a cell type that is specialised for producing IFN alpha.
What do these cells express high levels of?
Plasmacytoid dendritic cells (pDC) are specialist IFNa-secreting cells and express high levels of IRF-7 constitutively
(pDC and IRF-7)
How many genes are there for IFN alpha and IFN beta?
- *Alpha** – 13/14 isotypes of genes
- *Beta** – one
What are type 2 interferons?
What are they produced by and what receptor do they use?
Type II Interferon = IFN γ
- This is a much more specialist immune signalling molecule
- It is produced by immune cells - activated T cells and NK cells
- It signals through a different receptor - IFNGR (interferon-gamma receptor)
What are type 3 interferons?
What receptors do they use and where are these present?
Type III Interferon = IFN λ
- Signals through receptors: IL28R receptor and IL10β
- These are mainly present on epithelial surfaces (e.g. respiratory epithelium and gut)
- normally expressed during respiratory tract infections and liver infections.*
What is the role of IFNlamba in the liver and what is the result of a polymorphism in the gene?
Important in the liver/at epithelial surfaces
- Polymorphisms in IFNlamba associated with the improved outcome from HCV and HBC both spontaneous clearance and response to antiviral therapy
- Polymorphisms in the IFN lambda gene are associated with quite different outcomes from liver viruses (e.g. Hep B and Hep C) in terms of some people being able to spontaneously clear the virus if they have good IFN lambda responses
- It is also important in terms of response to antiviral therapy
How does the innate immune system differentiate self from non-self?
PRRs (pattern recognition receptors) on innate immune cells/ sit inside cells and detect PAMPs
PAMPs (pathogen-associated molecular patterns) – often sense foreign nucleic acids (e.g. dsRNA)
Name receptors that are involved in detecting the presence of viruses and state where they are found.
- RLRs – RIG-I-like Receptors (cytoplasmic) - bind to Mavs (found on mitochondria) and stimulate signalling and IFN-b production - recognise viruses that uncoat within the cell
- TLRs – Toll-Like Receptors (endosomal) - found in endosomes and make IFNa - membrane proteins so recognise things coming into the cell
- NLRs – NOD (Nucleotide Oligomerisation Domain) Like Receptors (cytoplasmic)
Describe RIG-I signalling.
- RIG-I like receptors will recognise single-stranded RNA in the cytoplasm of the cell and it will signal through MAVS (mitochondrial)
- This will signal further downstream, leading to generation of IFN-beta transcripts
Describe TLR signaling.
- TLR detects nucleic acids in the endosome (this isn’t normal)
- It will signal to molecules outside the endosome (MyD88) and send various transcription factors to the nucleus
- It will result in the switching on of expression of IFN alpha
Describe DNA sensing.
Mainly done by cGAS
- This is an enzyme that binds to dsDNA in the cytoplasm and synthesises cGAMP (second messenger)
- cGAMP diffuses 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
IFN receptors when activated are heterodimers of IFNAR1 and IFNAR2
and the IFNs are SOLUBLE cytokines and so can signal in a paracrine manner.
Describe the signalling from IFNAR receptors
This could be an example of an adjacent cell responding to IFN.
- 1. IFNAR1 and AR2 dimerise and JAK1 and TYK2 cross-phosphorylate.
- 2. STAT proteins are activated and this activates:
- a. Antiviral response (ISRE).
- b. Inflammatory response (GAS).
- c. Repressors of the inflammatory pathways (GAS).
Give examples of interferon-stimulated genes
Interferons induce transcription of HUNDREDS of antiviral mediators – hence why you get a fever and feel sick.
Interferon stimulated genes include –
- - PKR - protein kinase R: inhibits translation
- - 2’5’OAS: activates RNAse L that destroys ssRNA
- - Mx: inhibits incoming viral genomes
- - ADAR: induces errors during viral replication
- - Serpine: activates proteases
- - Viperin: inhibits viral budding
What is IFITM3?
Interferon-induced transmembrane protein 3
- restricts virus entry through endosomes by stopping them escaping so the virus is broken down by the acidic pH
- 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
- NOTE: mice and people lacking IFITM3 get more severe influenza*
What are Mx1 and Mx2?
- 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
Describe the actions of Protein Kinase R.
When is PKR activated by cells?
- 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
When is PKR activated by cells? - It is an extreme measure and a last resort – only activated when the cell has no other option - if they don’t switch on these genes, the cells will be infected by the virus and the virus could kill the cell
Name a family of genes that suppress the cytokine signalling and turn off the response.
SOCS
- The IFN response cannot last (maintained for a few hours) and the ability to respond to IFN is lost gradually due to negative regulation – SOCS (Suppressor of Cytokine Signalling) genes turn OFF the IFN response.
- SOCS genes suppress the cytokine signalling and turn off the response - if SOCS genes are switched on, even if the IFN is bound to the receptor the signals won’t get through and no new PKR will be made
What is viral evasion mediated by?
State some mechanisms of viral evasion of the IFN response.
Viral evasion is mediated by:
- Virus avoid detection by hiding the PAMPS – e.g. inside vesicles.
- Interfere globally with host cell gene expression (or protein synthesis).
- Block IFN induction cascades (by destroying or binding) → NS3/4 protease (HCV)
- Inhibit IFN signalling directly → NS1 protein (influenza)
- Block action of individual IFN-induced antiviral enzymes.
- Activate SOCS.
- Replication strategy that is insensitive to IFN.
