Transcript Folie 1

Harro Frauendorf
Introduction
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Infection: microbes vs. host
Viruses try to secure a niche for replication
Host must limit pathogen's advance
Type 1 interferons are key players
– many members: IFN-α/β
• Virus evolved tricks to avoid antiviral
effects of IFNs (co evolution)
Induction of type 1 IFNs by viruses
• Two functional classes:
– Extracytoplasmic pathway
– Cytoplasmic pathway
– Localization: either cell membrane or
cytoplasm consequence: IFN production
either in infected cells and/or before viral
contact
Extracytoplasmic pathway
• TLR-dependent
PRR
viral PAMP
general sensor
TLR3
Endosomal dsRNA
TLR7
ssRNA
TLR8
ssRNA
TLR9
Unmethylated DNA
constitutively expressed in a subset
of cells, the plasmacytoid dendritic
cells (PDCs),
(TLR2 and TLR4 can also detect viral products such Core, NS3
or F-protein, Env-protein.)
Extracytoplasmic pathway by TLR3
• TLR3 become activated and transmit signals through their
cytoplasmic Toll/ interleukin-1 receptor (TIR) domains
• TIR domain–containing adaptor inducing IFN-β (TRIF)
• TRIF mediates the activation of IκB kinase ε (IKK ε) and tankbinding
kinase 1 (TBK1)
• which phosphorylate IFN regulatory factor 3 (IRF3), resulting in its
dimerization and nuclear translocation´where it promotes gene
transcription
• activation of neural factor κB (NF-κB) and activating protein 1 (AP1)
through the kinase complex IKKα/β/γ and the mitogen-activated
protein kinase (MAPK) cascade
 Interferon
Extracytoplasmic pathway by
TLR7/8/9
• myeloid differentiation primary response protein 88 (MyD88)
• interleukin-1 receptor–associated kinases IRAK1 and IRAK4, and
the tumor necrosis factor receptor–associated factor 6 (TRAF6)
• IRF7 initiates the general induction of the IFN-a genes
• TLR7/8/9 and IRF7 appear to be constitutively expressed in only a
subset of cells, the plasmacytoid dendritic cells (PDCs), which are
characterized by high IFN production
 Interferon
Cytoplasmic pathway
• TLR-independent
• DExD/H box RNA helicase that contain
caspase-recruiting domains (CARDs)
• Intracellular recognition of viruses by:
– Retinoic acid-inducible gene I (RIG-I)
– melanoma differentiation associated gene 5
(mda5).
Cytoplasmic pathway
• RNA helicases, upon binding to dsRNA, interact with a downstream
molecule
(named independently by four different groups as mitochondrial
antiviral signaling protein (MAVS), IFN-b promoter stimulator 1 (IPS1), virus-induced signaling adaptor (VISA), and CARD adaptor–
inducing IFN-β (CARDIF))
• next steps are not well define
 Interferon
PRR
Viral PAMP
RIG-I
Cytoplasmic 5'-triphosphate dsRNA
MDA5
Cytoplasmic dsRNA
IFN-mediated effects on defense
IFN-mediated effects on defense
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type 1 IFNs regulate a range of immune
responses through the type 1 IFN receptor
IFN-a receptor 1 (IFNAR1) and IFNAR2
subunit dimerization and activation of
kinases that associate with their cytoplasmic
tails: the Janus-activated kinase 1 (JAK1)
and tyrosine kinase 2 (TYK2)
tyrosine phosphorylation activates the signal
transducers and activators of transcription 1
and 2 (STAT1 and STAT2), to form a trimeric
STAT1-STAT2-IRF9 complex, also known as
IFN-stimulated gene factor 3 (ISGF3)
STAT1 homodimer complex, known as the
IFN-γ–activated factor (GAF)
Both complexes translocate to the nucleus
and bind to DNA regulatory sequences
containing IFN-stimulated response
elements (ISREs) and IFN-g–activated sites
(GAS)
IFN-mediated effects on defense
Antiviral activities of ISGs
Protein kinase R
(myxovirus-resistance A)
2′,5′-oligoadenylate synthetase 1
myxovirus-resistance A (MxA)
• The MxA protein accumulates
in the cytoplasm on
intracellular membranes (such
as the endoplasmic reticulum,
ER) as oligomers formed by
association between the
leucine zipper (LZ) domain and
central interactive domain of
the protein.
• Following viral infection, MxA
monomers are released and
bind viral nucleocapsids or
other viral components, to trap
and then degrade them.
Protein kinase R (PKR)
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Protein kinase R is constitutively
expressed, and is also induced by
type I interferons (IFNs).
The kinase accumulates in the
nucleus and cytoplasm as an
inactive monomer, which is
activated directly by viral RNAs
(and by several other ligands
Following activation, PKR
monomers are phosphorylated
and dimerize to form the active
enzyme.
a crucial function of PKR in viral
defence is the inhibition of
translation by phosphorylation of
eukaryotic translation initiation
factor 2α (EIF2α).
2′,5′-oligoadenylate synthetase 1
(OAS1)
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OAS1 is expressed at low constitutive
levels and is upregulated by type I
interferons (IFNs).
OAS1 protein accumulates in the cell
cytoplasm as an inactive monomer.
activation by viral double-stranded
RNA (dsRNA), the enzyme
oligomerizes to form a tetramer
Synthesize of 2′,5′-oligoadenylates
activate constitutivelyexpressed
inactive ribonuclease L (RNaseL).
The binding of 2′,5′-oligoadenylates to
RNaseL triggers the dimerization of
enzyme monomers, through their
kinase-like domains, and this then
enables RNAseL to cleave cellular
(and viral) RNAs.
Regulation of immune responses
• They amplify their own expression through two independent
mechanisms:
– the induction of IRF7 to extend IFN gene expression to a broader range
of IFN-αs
– and the accumulationof PDCs, major contributors to IFN-a/b responses
• Activate natural killer (NK) cells and induce IL-15 to promote NK cell
proliferation
• At high concentrations, type 1 IFNs inhibit IL-12 and NK cell
responsiveness for IFN-γ expression
• IL-15 contributes short-term proliferation of memory CD8 T cells
• STAT1 acts to limit nonspecific CD8 T cell expansion
• antigen-specific CD8 T cells with lower relative levels of STAT1 are
induced and preferentially undergo proliferation
• the type 1 IFN receptor helps with long-term maintenance of the
CD8 T cell pool
Viral evasion of IFN responses
viral IFN antagonists focus inhibition on at least one of three key
pathways: the IRF3, the JAKSTAT, and the PKR pathways
Antagonism of type 1 IFN induction
• Viral inhibition of IRF3
• influenza and poxviruses encode dsRNA binding proteins NS1 and
E3L that prevent IRF3 activation
 RIG-I and mda5 can't detect viral ds RNA.
• direct binding to mda5 of a viral IFN antagonist resulting in mda5
inhibition (V protein of several paramyxoviruses)
• MAVS/IPS-1/VISA/ CARDIF is the target for cleavage by the
NS3/4A protease of hepatitis C virus. This protease also cleaves
TRIF.
blocks both TLR3- and RIG-I–mediated activation of IFN.
• The human herpesvirus 8 encodes several analogs of IRF, known
as viral IRFs, some of which act as dominant negative mutants of
IRF3 action.
Antagonism of type I IFN signaling
• The JAK/STAT pathway is also targeted at multiple levels by viral
IFN antagonists.
• Poxviruses secrete a soluble form of the IFNAR that sequesters type
1 IFN before it can bind to the natural IFNAR
• Inhibition of the JAK kinases has been documented for several
viruses.
• STATs appear to be a preferred target for many paramyxoviruses
 accessory V and W proteins bind to these factors and prevent
their activation
– different specificities for STATs, with some of them inhibiting STAT1,
STAT2, STAT3, or a combination of these factors.
– degradation of the STATs is seen with a subset of paramyxoviruses.
• V proteins of several paramyxoviruses, and the NS1 and NS2
proteins of respiratory syncytial virusof inhibit both IRF3 and STAT
activation
Antagonism of type 1 IFN-inducible genes
and their products
• PKR product appears to be a common target for many viral IFN
antagonists
• PKR inhibition pathway are very diverse
– sequestration of the PKR-activating dsRNA
– expression of dsRNA mimics
– binding to PKR preventing its dimerization and activation
• Influenza virus infection activates a cellular inhibitor of PKR(p58IPK)
• herpesviruses encode a protein, g34.5, that recruits a cellular
phosphatase for the dephosphorylation of eIF2a, reverting the PKRmediated translational block
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