Lecture 4: Innate Sensors Flashcards
Why is innate immunity important?
Plays crucial role in early recognition of pathogens and triggers proinflammatory response to invading pathogens
What does the innate immune system do?
Signals to adaptive arms, resolves infection, and promotes angiogenesis
What are examples of sensor cells?
Phagocytes and APCs: macrophages, neutrophils, monocytes, dendritic cells, and mast cells
What are sensor cells and what do they do?
Cell types that detect inflammatory mediators through expression of many innate recognition receptors, which remain constant over individuals lifetime
What is the result of PAMPs detecting damage via PRRs?
Activation, movement, effector function and cytokine production
What is phagocytosis?
Process of ingestion of a microorganism by a phagocyte via its plasma membrane
What causes phagocytosis?
Occurs after ligation of cell surface receptors
What are the 4 steps of phagocytosis?
Tasting: receptor mediated binding
Feeling: actin dependent internalisation
Swallowing: phagolysosome degradation
Digesting: antigen processing and presentation
What are toll-like receptors?
Receptors which are evolutionarily conserved
What are the 3 domains of TLRs?
- N-terminal domain (outside)
- Middle helix transmembrane domain
- C-terminal domain (inside)
How does dimerisation affect TLRs?
- are monomers or weak dimers during homeostasis
- activation forms homodimers (TLR4) or heterodimers (TLR1/2)
Dimerisation = stability = signalling
How many TLRs exist and which ones are species specific?
13 types
TLR10: humans, TLR11/12/13: mice
What are LRR?
Leucine rich repeats found in the TLRs, consisting of 20-25 amino acids
What is the TIR domain and what does it do?
Toll-IL-1 receptor, which interacts with signalling molecules
Why do mice have additional TLRs?
- evolutionary adaptation, diverse ecological niches, and specialised immune response
Where are TIR domains located?
Cytosolic side of the plasma membrane
How do TIR domains lead to cell death?
TIR domains have weak transient interactions until they self-associate, then create a scaffold that facilitates signal transduction, which leads to immune response
What pathways is TLR signalling divided into?
MyD88-dependent and TRIF-dependent
What are the 4 main TIR-domain containing adaptor proteins?
MyD88, TRIF, TIRAP/MAL, and TRAM
What is the MyD88-dependent pathway?
Activates NF-kB and MAPKs for the induction of inflammatory cytokine genes, used by all TLRs
What is the TRIF-dependent pathway?
TRIF interrupted to TLR3/4 and promotes an alternative pathway that leads to the activation of IRF3, NK-kB, and MAPKs for induction of type I IFN and inflammatory cytokine genes
What is NF-kB?
Nuclear Factor Kappa B is a protein which controls transcription of DNA
What does NF-kB do?
Is an indictable transcription factor that targets genes involved in inflammation development and progression
How does NF-kB target inflammation?
- by directly increasing the production of inflammatory cytokines, chemokines and adhesion molecules
- by regulating the cell proliferation, apoptosis, morphogenesis, and differentiation
What does TLR1/2 & TLR2/6 target?
Lipoproteins
What does TLR1/2 detect?
Triacyl lipopeptides
What does TLR2/6 detect?
Diacyl lipopetides
Where are TLR1/2 and 2/6 found?
In the cell wall of gram positive bacteria
What are alarmins?
Endogenous ligands that can be detected by TLR1/2 and TLR2/6, such as heat shock proteins and B-defensin-3
How do TLRs 1&2 form a dimer?
The convex surface of TLRs 1&2 have binding sites for lipid side chains of triacyl lipopeptides, binding of each TLR indices dimerisation which brings their cytoplasmic TIR domains into close proximity
What does TLR3 target?
dsRNA in viruses and poly I:C
How does homodimerisation activate IRF3?
Homidimerisation allows for signalling via TRIF/TRAF3 to activate IRF3
How does TLR3 induce gene expression?
TLR3 in the endosome binds dsRNA and signals via TRIF to induce IFN gene expression
What does TLR4 target?
Lipopolysaccharides
How does TLR4 use MD-2 accessory protein?
- LPS has multiple fatty acetyl chains linked to a Lucan head = 5 bind to MD-2 pocket, 1 remains free
- free chain of LPS binds to outer surface of another TLR4 molecule to form dimer
- dimer is stabilised when another LPS molecule binds to other TLR4
How does TLR4 use accessory proteins LBP and CD14?
- LBP binds a monomer of LPS from gram -ve bacteria
- LBP delivers to soluble/membrane bound CD14 molecule
- CD14 transfers LPS to the ectodomain of TLR4/MD-2 complex which leads to homodimerisation
- change in confirmation leads to dimerisation of TIR-domain that provides binding site for MyD88
In what 2 ways can TLR4 initiate transcription of proinflammatory cytokines?
- MyD88 activates NF-kB and MAPK signalling, resulting in transcription of proinflammatory cytokines
- Endocytosis of the LPS.TLR4/MD-2 complex leads to TRIF dependent signalling pathway that mediates induction of IRF3 and IFN-I
What does TLR5 target?
Flagellin
What types of cell is TLR5 expressed in?
Monocytes, immature DCs, and epithelial cells
How does TLR5 activate IKK complex and NFkB?
- binds flagellin
- conformational change in TIR allows recruitment of MyD88
- signals via IRAK, TRAF6, and TAKI
- leads to activation of IKK complex and NF-kB
TLR5 is expressed on basal side of cells - why is this useful?
Forms a physical isolation from luminal contents, which helps prevent uncontrolled inflammation by symbiotic microbes
What effect does TLR5 activation have?
- Decreases epithelial barrier resistance and reduces expression of tight junction proteins
- Produces chemokine
- Attracts immune cells and makes space for the immune cells to access the luminal space
What do TLR7 and TLR8 target?
GU rich ssRNA
What are TLR& and TLR8?
Endosomal receptors important for response to viral infection
Ho do TLR7 and TLR8 activate cytokine production?
- Signalling is MyD88 dependent, which is mediated by nuclear translocation of AP-1, NF-kB, and IRFs
- Phosphorylation of IRFs promote induction of IFN-stimulated response elements (ISREs) with expression of type I IFN
- NF-kB activation results in production of pro-inflammatory cytokines
What are IRFs?
Interferon regulatory factors
What does TLR9 target?
Nucleic acid sensors in DNA (unmethylated CpG)
What type of ssDNA does TLR9 recognise?
Viral and bacterial
What does CGAS-STING target?
nucleic acid sensors in dsDNA (detects DNA in the cytosol)
How does cGAS induce expression of IFNs?
- dsDNA from viruses activates cGAS to produce cGAMP from ATP and GTP
- cGAMP binds to STING dimer present on the ER membrane and activates its signalling
- STING activates the kinase TBKI to phosphorylate IRF3
- IRF3 then enters the nucleus then induces expression of IFN genes
What does AIM2 target?
Nucleic acid sensors in dsDNA
How does AIM2 activation result in pro-ILI or cell death?
- AIM2 inflammasome activates caspase-1
- Caspase-1 cleaves pro-IL-113 and gasdermins to mediate inflammation and pyroptosis
- Triggers inflammatory responses through cytokine maturation and pyroptotic cell death
What are RIG-I-Like Receptors?
Retanoic acid-inducible gene-I lie receptors
What are NOD-like receptors?
Nucleotide binding oligomerisation domain
What are NOD-like receptors?
Cytoplasmic innate sensors which initiate NF-kB, and have a CARD domain for Caspase-1 recruitment
What do NOD-like receptors target?
Cell wall peptidoglycans
How do NOD-like receptors initiate NF-kB activation?
- NOD proteins reside in the cytoplasm in inactive form
- binds to bacterial ligands and induces recruitment of RIP2
- RIP2 activates TAKI, leading to NF-kB activation
How does the NLP inflammasome release inflammatory cytokines?
- potassium efflux induces dissociates of chaperones which keep NLRP3 in inactive form
- NLRP3 forms oligomers with ASC causing proteolytic cleavage of caspase-I
- caspase-I releases mature cytokines such as IL-I and IL-18
