Toll-Like Receptors

Question 1

Define PAMPs

Answer 1

Pathogen-Associated Molecular Patterns:
- recognized by receptors of innate immune system

Question 2

PAMPs on viruses

Answer 2

• glycoproteins
• ssRNA, dsRNA, genomic DNA (HIV, covid)

Question 3

PAMPs on bacteria

Answer 3

• lipopeptides, lipoteichoic acid (mycobacterium)
• LPS
• flagellin
• CpG DNA (non-methylated)

Question 4

PAMPs on protozoa

Answer 4

• GPI anchors
• dsRNA
• haemozoin (waste product of malaria)

Question 5

Define PRR. Provide an example.

Answer 5

Pattern Recognition Receptors:
- receptors that recognize PAMPs
- signals cellular/ inflammatory response
Ie. Toll-like receptors

Question 6

Define TLRs. Where are they found ?

Answer 6

Toll-like Receptors:
- type of PRR
- on cell surfaces (dendritic cells, macrophages)
- inside cells (endosomes, lysosomes)

Question 7

Describe TLR structure

Answer 7

Exterior:
- leucine-rich repeats
- recognize and binds PAMP

Interior:
- Toll/ IL-1 receptor
- activates signalling in cell to generate response

Question 8

TLR2: ligand and organisms

Answer 8

Ligand: peptidoglycan
Organism: gram pos bacteria, parasites, fungus

Question 9

TLR3: ligand and organisms

Answer 9

Ligand: dsRNA
Organism: virus

Question 10

TLR4: ligand and organisms

Answer 10

Ligand: LPS
Organism: gram neg bacteria

Question 11

TLR5: ligand and organisms

Answer 11

Ligand: flagellin
Organism: bacteria

Question 12

TLR9: ligand and organisms

Answer 12

Ligand: unmethylated CpG DNA
Organism: bacteria, viruses

Question 13

Describe TLR dimerization

Answer 13

• TWO+ receptors need to bind their ligand and form dimers
• both exterior (LRR) and interior (TIR) are needed for signalling

LRR = leucine-rich receptors
TIR = Toll/ IL-1 Receptor

Question 14

Outline TLR signalling (4):

Answer 14

• LRR of TLR recognizes PAMP
• dimerization of TLRs
• conformational change in TIR
• recruitment of adaptor molecules (MyD88 or TRIF)

LRR = Leucine-rich receptor
TIR = Toll/ IL-1 receptor

Question 15

TLR homodimers vs heterodimers

Answer 15

Homodimers = dimerization of the same TLR type ie. TLR4/ TLR4

Heterodimers = dimerization of different TLR types ie. TLR1/ TLR 2

Question 16

Outline MyD88-dependent TLR signalling

Answer 16

• facilitates IL-1 receptor
  1. MYD88 binds TIR = recruits IRAK
  2. MYD88 phosphorylates TRAF6
  3. IRAK and TRAF6 dissociate and interact with TAK1
  4. TAK1 induces NF-κB and AP-1 transcription factors

Question 17

Outline TRIF-dependent TLR signalling

Answer 17

• endosomal pathway for TLR3 and TLR4:
  1. TRIF activates TRAF3 and TBK-1-IKKe
  2. TRAF3 complex phosphorylates IRF7 and IRF3
  3. IRF7 and IRF3 induce transcription of IFN-α and IFN-β

Question 18

TLR signalling results in: (4)

Answer 18

• antimicrobial peptides
• cytokines
• chemokines
• enzymes

Question 19

Cytokines produced through MyD88-dependent TLR signalling

Answer 19

MyD88 > NF-κB and AP-1:
- TNF = activates macrophages, vascular permeability, induces acute-phase reactants, fever
- IL-1 and IL-6 = neutrophil release from BM, vascular permeability, induces acute-phase reactants, fever

NOTE: pro-inflammatory

Question 20

Cytokines produced through TRIF-dependent TLR signalling

Answer 20

TRIF > IRF7 and IRF3:
- Type 1 interferons (INF-α, INF-β): inhibits viral replication within cells & infection to neighbouring cells, promotes MHC I expressions, activates NK cells
- IL-10: regulates macrophages and suppresses Th1 cells

NOTE: anti-inflammatory

Question 21

Other proteins produced due to TLR

Answer 21

• chemokines ie. IL-8 attract neutrophils to infection site
• defensins: cause permeability to prokaryotic membranes
• enzymes ie. Nitric Oxide Synthase (NOS) for respiratory burst against intracellular bacteria

Question 22

How do TLR assist the T cell response ?

Answer 22

A). TLR4 = recognize LPS = produce IL-12 = Th1 response

B). TLR2 = recognize peptidoglycan = Th2 response
= IL-10 = stimulates B cells, mast cells, BUT inhibit Th1 cells