What is the most common cause of sporadic encephalitis worldwide?
Herpes simplex encephalitis
Which subset of the population is herpes encephalitis most common in?
Most common in childhood – affecting previously healthy individuals on primary infection with HSV-1
What is interferon?
Transferrable polypeptide factor produced and secreted from cells 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?
Which cells produce IFN beta?
All cells produce IFN beta and all tissues have IFNAR receptors
What is IFN beta induction triggered by?
Name a cell type that is specialised for producing IFN alpha.
Plasmacytoid dendritic cells
What do Plasmacytoid dendritic cells express high levels of?
How many genes are there for IFN alpha and IFN beta?
Alpha – 13/14 isotypes Beta – ONE
Which IFN comes under type II interferon?
IFN-gamma - specialist immune signalling molecule
Which cell types produce type II IFN?
Produced by activated T cells and NK cells
Which receptor do these type II IFN signal through?
Which IFN falls under type III IFN?
Which receptors do type III IFNs signal through?
IL-28 receptors IL-10 beta receptors
Where are the receptors for type III inteferons mainly present?
Epithelial surfaces E.g. respiratory epithelium and gut
Which organ is IFN lambda very important in?
How does the innate immune system recognise non-self?
PRRs (pattern recognition receptors) on innate immune cells recognise PAMPs (pathogen-associated molecular patterns) NOTE: they often sense nucleic acids
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 Both examples of PRRs
Describe RIG-I signalling.
RIG-I like receptors will recognise single stranded RNA in the cytoplasm of the cell It will signal through MAVS (mitochondrial) This will signal further downstream, leading to phosphorylation of IRF3 and IRF7 Theses will dimerise and enter the nucleus to act as a transcription factor Leads to the generation of IFN-beta transcripts Note: Mavs may also activate pathways leading to NFkB activation or Jun kinase activation
Describe TLR signalling.
- TLR detects nucleic acids in the endosome (this isn’t normal) or outside the cell
- It will signal to one of 2 molecules outside the endosome
- MyD88 or Trif
- Trif activates IRF3
- Myd88 activates NFkB and IRF7
- It will result in the switching on of expression of INF alpha and beta
- MyD88 or Trif
Describe DNA sensing.
- Mainly done by cGAS
- This is an enzyme that binds to dsDNA in the cytoplasm
- Then synthesises cGAMP (second messenger)
- cGAMP diffuses to STING (found on endoplasmic reticulum)
- This triggers phosphorylation of IRF-3
- which then dimerises and acts as a transcription factor
- which then dimerises and acts as a transcription factor
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
- This phosphorylates the STAT molecules
- STAT molecules dimerise and combine with IRF-9
- It then goes to the nucleus, binds to a promoter and regulates transcription
What is IFITM3?
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 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 is an extreme measure and a last resort – only activated when the cell has no other option
Name a family of genes that suppress the cytokine signalling and turn off the response.
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.
NS3/4 This is a protease that cleaves MAVS MAVS is important in detecting Hep C through the RIG-I pathway So Hep C is not detected
Explain how influenza controls the interferon response.
NS1 protein Acts an antagonist to interferon induction by binding to the RIG-I/TRIM25/RNA complex and preventing activation of the Mavs signalling pathway NS1 also migrates to the nucleus where it prevents the export of newly synthesised genes e.g. IFN beta genes and ISGs in neighboring cells
What type of virus are Pox and Herpes viruses?
Large DNA viruses
What do Pox viruses encode that helps deal with the interferonresponse?
They encode soluble cytokine receptors that mop up IFN and prevent it from reaching its receptors
Describe a potential therapeutic use of this feature (pox way of dealing with interferon)
This could be useful in autoimmune or inflammatory conditions where IFN and other cytokines are produced in abundance
What are two proteins produced by Ebola virus that are particularly important in dealing with the immune response?
What do these ebola proteins do?
- VP35 – inhibits the RIG-I pathway
- VP24 – inhibits interferon signalling to the nucleus
Describe how viral infections can cause cytokine storm.
Lots of virus propagation --> lots of interferon being produced --> massive release of TNF alpha and other cytokines
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 can beused 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
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?
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
What is the role of ZAP?
Protein that determins how likley a nucleic acid sequence is likley to belong to the host Detects how many CpG sites there are If there is lots then ZAP binds to the nucleic acid and targets it for degredation
Does ZAP play much of a role in defence from human viruses?
No as human viruses have evolved to down regulate their CpG sites However, non-human viruses haven't so ZAP plays a key role in defence against these viruses
What kind of cells secrete interferon beta?
Immune cells: Macrophages Dendritic cells Plasmacytoid dendritic cells (pdcs)
Breifly describe 6 genes stimulated by inteferons
PKR Protein Kinase R: inhibits translation 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