Innate Immune Responses Flashcards
(T/F) Most infections are prevented by innate and adaptive immunity.
False!
Most infections are prevented by physical (anatomical & chemical) barriers.
If viruses bypass these barriers, a series of immune responses (intrinsic, innate, adaptive) are engaged.
Match the following responses to their definitions:
1) Physical barriers
2) Intrinsic
3) Innate
4) Adaptive
A) interferons, apoptosis, etc. cell-autonomous responses (can be achieved by a single cell in isolation). immediate.
B) t/b cells. tailored to pathogen (specific). hours/days.
C) mucus, saliva, tears, acid, etc. continuous. block majority of infections.
D) natural killer cells, complement, antigen-presenting cells, etc. induced by infection (non-specific). minutes/hours.
Physical barriers: mucus, saliva, tears, acid, etc. continuous. block majority of infections.
Intrinsic: interferons, apoptosis, etc. cell-autonomous responses (can be achieved by a single cell in isolation). immediate.
Innate: natural killer cells, complement, antigen-presenting cells, etc. induced by infection (non-specific). minutes/hours.
Adaptive: t/b cells. tailored to pathogen (specific). hours/days.
*intrinsic and innate can be grouped together
How do individual cells detect a virus infection?
Receptor-mediated recognition of MAMPs (microbe-associated molecular patterns).
Then, cell signalling is induced by MAMP-receptor engagement, causing cellular changes to occur (gene expression).
(T/F) Receptors recognize MAMPs only on the cell surface.
False!
They can recognize MAMPs within the cell also. MAMPs can be viral proteins or nucleic acids.
Match the following terms to their definitions:
1) Microbe-associated molecular patterns (MAMPs)
2) Pattern recognition receptors (PRRs)
A) host cell receptors that recognize MAMPs. can be located on host cell surface, endosomal membranes, cytoplasmic, or secreted. examples: Toll-like receptors (TLRs), Rig-1-like receptors (RLRs), and protein kinase R (PKR).
B) MACROMOLECULES shared among groups of microorganisms and are recognized as FOREIGN to host. examples: dsRNA, peptidoglycan, LPS, proteins, flagellin, etc.
Microbe-associated molecular patterns: MACROMOLECULES shared among groups of microorganisms and are recognized as FOREIGN to host. examples: dsRNA, peptidoglycan, LPS, proteins, flagellin, etc.
Pattern recognition receptors (PRRs): host cell receptors that recognize MAMPs. can be located on host cell surface, endosomal membranes, cytoplasmic, or secreted. examples: Toll-like receptors (TLRs), Rig-1-like receptors (RLRs), and protein kinase R (PKR).
(T/F) Toll-like receptors (TLRs) are membrane-bound, while Protein Kinase R receptors are found in the cytoplasm.
True!
MAMP-PRR engagement leads to signalling events that ultimately activate transcription factors such as ______ and the _________.
The major outcome is the expression of ______ genes such as:
NFkB; Interferon Regulatory Factors (IRF3/7)
Cytokine; Inflammatory Cytokines; Type I Interferons (IFNs)
(T/F) PRRs are expressed in equal amounts in all cells.
False!
Expression of PRRs depends on the cell type!
Match the PRRs to their brief description:
1) RIG-1 & MDA5
2) Protein Kinase R (PKR)
3) cGAS
A) Cyclic GMP-AMP synthase; binds to viral dsDNA in the cytoplasm
B) cytoplasmic RNA helicases that function as RNA sensors
C) sensor for viral dsRNA, inhibits cap-dependent translation by eIF2α
RIG-1 & MDA5: cytoplasmic RNA helicases that function as RNA sensors
Protein Kinase R (PKR): sensor for viral dsRNA, inhibits cap-dependent translation by eIF2α
cGAS: Cyclic GMP-AMP synthase; binds to viral dsDNA in the cytoplasm
RIG-1 and MDA5 are part of RIG-1-like receptors (RLRs) that detect slightly different things.
What do each detect?
RIG-1: detects 5’ triphosphate RNA without 5’ cap
MDA5: detects dsRNA
Both RIG-1 and MDA5 contain tandem ___ ________ _______ domains, which are phosphorylated in __________ cells (inactive conformation).
N-terminal CARD; uninfected
What happens when MDA5 and RIG-1 bind to viral RNA?
Binding of viral RNA, CARDs are dephosphorylated by specific phosphatases, and now are in ACTIVE conformations.
In their active conformations, RIG-1 and MDA5 are targeted to the mitochondrial antiviral signalling protein (MAVS).
Then, there is activation of IRF3/7 and NFkB which promotes expression of IFNs and pro-inflammatory cytokines to induce antiviral state.
What is the difference between RIG-1 during inactivation and activation compared to MDA5?
Phosphorylation leads to the inactivation of both. However, RIG-1 is also held in the inactive conformation via INTRAMOLECULAR interactions.
Dephosphorylation leads to the activation of both. However, RIG-1 is also activated by the UBIQUITYLATION of its CARD & C-terminal domains (as signalling molecules not degradation).
How can viruses evade RIG-1 and MDA5 responses (list 4 ways)?
1) Sequestration or modification of viral RNA ligands (prevents recognition by PRR)
2) Manipulation of post-translational modifications on RIG-1, MDA5, MAVS (phosphatase blocked)
3) Cleavage of RIG-1, MDA5, MAVS
4) Sequestration or relocalization of RIG-1, MDA5 (restricting their access)
What is the role of eIF2?
eIF2 promotes the recruitment of tRNA^met to initiate cap-dependent translation when bound to GTP.
Once bound to the ribosome, translation is initiated; GTP –> GDP.
What does PKR do? What is its mechanism of action?
dsRNA-activated protein kinase PKR causes translation arrest.
When PKR binds to viral dsRNA, it undergoes DIMERIZATION and AUTOPHOSPHORYLATION which leads to its activation.
Activated PKR phosphorylates the α subunit of eIF-2 which causes it to remain in an INACTIVE GDP-bound form.
Inactive GDP-bound eIF-2 is not able to recruit Met-tRNA, which leads to an arrest of translation and can cause APOPTOSIS.
How can viruses evade PKR responses?
1) VIral phosphatase can be activated to remove the phosphate added by PKR
2) Viruses can use eIF2 INDEPENDENT translation mechanism
3) Viral protein PKR antagonists can block PKR action
Fill in the blanks regarding cGAS mechanism of action.
1) cGAS binds to viral _______ in the _______.
2) Following binding, cGAS generates ________.
3) This binds and activates _______ which is located on the ____. It dimerizes and is further activated by __________.
4) Activated STING translocated to ________ structures where it promotes expression of Type I IFNs and proinflammatory cytokines.
1) dsDNA; cytoplasm
2) Cyclic GMP-AMP (cGAMP)
3) STING; ER; ubiquitylation (signalling not degradation)
4) Perinuclear
How can viruses evade cGAS responses?
1) Viral manipulation of STING post-translational modifications (no more ubiquitylation)
2) Cleavage of STING
3) Prevent/limit cGAS sensing of nucleic acid ligand (hide dsDNA by viral factories)
1) What are cytokines?
2) What are interferons (IFNs)?
1) Cytokines are small SIGNALLING MOLECULES that are secreted by specific immune cells. They mediate CELL-CELL COMMUNICATION to regulate a range of immune responses.
2) IFNs are a group of cytokines that are generated in response to several pathogens (‘interfere’ with viral infection). There are 3 types: Type I, II, and III.
What are Type I interferons (IFNs)?
Viruses (or viral components) bound by PRRs trigger downstream signalling events that lead to the production of Type I IFNs (α/β).
These cytokines are released from the cell and then bind to IFNAR receptors on the surface of ADJACENT cells to stimulate expression of ‘IFN RESPONSIVE GENES’.
*protects uninfected cells from being infected
*establishes antiviral state
What does ISRE stand for?
IFN-sensitive response element
What is a key molecular/cellular outcome of Type I IFN signalling?
Induces resistance to viral replication in all cells by upregulating antiviral proteins such as Mx proteins, 2’-5’ linked adenosine oligomers, and the kinase PKR.
Match the following antiviral proteins upregulated by Type I IFN signalling to their definitions:
1) PKR
2) 2’,5’-oligo A synthetase
3) Mx GTPases
A) GTPase with diverse roles in the INHIBITION of virion ASSEMBLY.
B) dsRNA sensor that inhibits viral protein synthesis.
C) activated by dsRNA, promotes the production of oligo A which, in turn, activates RNase L. RNase L then promotes DEGRADATION of viral RNA.
PKR: dsRNA sensor that inhibits viral protein synthesis.
2’,5’-oligo A synthetase: activated by dsRNA, promotes the production of oligo A which, in turn, activates RNase L. RNase L then promotes DEGRADATION of viral RNA.
Mx GTPases: GTPase with diverse roles in the INHIBITION of virion ASSEMBLY.
