Immunology III: Innate Immunity & Inflammation 2 (Part 2: PRRs)

Question 1

Toll-like receptors: What do they detect?

Answer 1

TLR1 – can detect mycobacteria (like TB) and gram-negative bacteria
TLR2 – can detect peptidoglycans – major component of cell wall of gram-positive bacteria
TLR3 – double-stranded RNA  only found in viruses
TLR4 – lipopolysaccharide (LPS), major component of gram negative bacteria
Toll-like receptors tend to recognize PAMPs in the ECF or in endosomes (not in the cytosol)

Question 2

C-type Lectin receptors: what do they detect? Where are they found? (PAMP)

Answer 2

C-type Lectin receptors – detect carbohydrate components of many microbes (viruses, fungi, mycobacteria, parasites, some bacteria)
Found on the cell membrane of a wide variety of immune cells (macrophages, dendritic cells, neutrophils, lymphocytes)
These receptors are also found on the cell membrane, so they don’t detect pathogens in the cytosol

Question 3

NOD-like receptors (NLRs): where are they present? What do they detect? What does their activation lead to? (PAMP)

Answer 3

NOD-like receptors (NLRs) are present in the cytosol of a wide range of immune cells (dendritic cells, neutrophils, macrophages, lymphocytes) and non-immune cells such as epithelial cells

Most detect bacteria or parasite cell wall components that are present in the cytosol
Some bacteria and parasites reproduce and spend part of their life cycle inside cells
Some NLRs seem to be able to detect viruses as well

Like TLRs, activation of NLRs leads to activation of NF-KB and AP-1

Question 4

RIG-like receptors (RLRs): where are they present? What do they detect? (PAMP)

Answer 4

RIG-like receptors (RLRs) are present in the cytosol of many (if not all) immune cells as well as non-immune cells (i.e. endothelial, epithelial cells)

RLRs detect viral RNA (particularly double-stranded RNA) and activate:
NF-KB

transcription factors that lead to the production of particular “antiviral” cytokines known as interferons

Question 5

All of these PRRs activate NF-KB, what additional function do Lectin receptors have and RIG-like receptors? (PAMP)

Answer 5

phagocytosis=lectin

interferon production=RIG-like

Question 6

QUESTIONS

Answer 6

What are the main ligands for each type of major PAMP-receptor?
What cellular compartment/structure is the PAMP-receptor found in?
How does this relate to the ligand that it binds?

Question 7

What are the 3 main PAMPS?

Answer 7

Toll-like
C-type Lectin
RIG-like
NOD-like

Question 8

What is a PAMP?

Answer 8

Pathogen-Associated Molecular Pattern: molecular structures that are commonly found on the surface of pathogens such as bacteria, viruses, fungi, and other microorganisms. These patterns are recognized by the innate immune system as signals of the presence of a potential threat, helping the immune system to identify and respond to invading pathogens.

NOT present in host

Question 9

What is a PRR?

Answer 9

pattern recognition receptors (PRRs) present on various immune cells. These receptors are capable of recognizing specific PAMPs, initiating a cascade of events that lead to the activation of the immune response. The recognition of PAMPs and the subsequent immune response are crucial for the body’s defense against infections.

Question 10

What is a DAMP? Examples?

Answer 10

Danger-Associated Molecular Patterns

DAMPs are molecular “signals” that are present when a cell is damaged

EXAMPLES: Loss of intracellular K+ due to loss of cell membrane integrity
High concentrations of free radicals
Extracellular ATP (leakage from cells due to loss of membrane integrity)
Unfolded proteins (suggested by some studies)

DAMPs can also be crystals that “shouldn’t be there”
Cholesterol crystals (cellular damage or oxidized LDL)
Uric acid crystals (gout)

Question 11

Which NOD-like receptor is capable of detecting a wide range of DAMPS as well as PAMPs?

Answer 11

NLRP3

Question 12

When a DAMP activates NLRP3, what does it associate with/activate? What is formed? What is it called?

Answer 12

When a DAMP activates NLRP3, it associates with caspase 1 and activates it by forming a large molecular complex known as an inflammasome

Question 13

Caspase 1 activation has two major effects:

Answer 13

Activation of pro-IL-1-beta to IL-1-beta

Insertion of a cell membrane pore – gasdermin - into the cell membrane

Question 14

What are inflammasomes? What do they do?

Answer 14

Inflammasomes are a molecular assembly that activates caspase 1 
1. Conversion of pro-IL-1 to IL-1
2. Activation of a cellular pore known as gasdermin
IL-1 tends to leave the cell via gasdermin pores

If enough gasdermin is produced, then the cell (usually a macrophage) will lyse and die  release of more DAMPs
This can lead to activation of neighbouring cells
This type of cell death is known as pyroptosis

Question 15

If enough gasdermin is produced what happens? What can this lead to? What is this called?

Answer 15

If enough gasdermin is produced, then the cell (usually a macrophage) will lyse and die  release of more DAMPs
This can lead to activation of neighbouring cells
This type of cell death is known as pyroptosis

Question 16

What is one of the most important pro-inflammatory cytokines and has a wide range of effects? It is released in very large quantities (for a cytokine) by macrophages in response to significant infection or tissue damage.

Answer 16

IL-1-beta

Question 17

IL-1 is generated and released in a two-step process:

Answer 17

Step 1 (priming) – detection of PAMPs or DAMPs  synthesis and storage of large quantities of pro-IL-1

Step 2 (release) – continued or larger DAMP or PAMP stimuli  NLRP3 activation  caspase 1 activation  conversion of pro-IL-1 to IL-1  IL-1 release

Question 18

Resident macrophages

Answer 18

In someone without disease or infection, circulating monocytes often enter a wide range of tissues and differentiate to macrophages to serve as a “cellular sentinel”
Langerhans cells – dermis and epidermis
Kupffer cell – liver
Alveolar macrophages - lung
Microglia – brain
Resident macrophages also found in all connective tissue
Spleen (and other secondary lymphoid organs)
These cells express most PAMP and DAMP receptors and can secrete large quantities of pro-inflammatory cytokines (often due to NF-K activation)

Question 19

Where is the resident macrophage? Dermis and epidermis?

Answer 19

Langerhans cells

Question 20

Where is the resident macrophage? Liver?

Answer 20

Kupffer

Question 21

Where is the resident macrophage? Lung?

Answer 21

Alveolar macophages

Question 22

Where is the resident macrophage? Brain?

Answer 22

Microglia

Question 23

Where is the resident macrophage? Connective tissue?

Answer 23

In the connective tissue

Question 24

Where is the resident macrophage? Spleen?

Answer 24

In the spleen and other secondary lymphoid organs

Question 25

What cells are important DAMP and PAMP detectors in blood and lymph vessels? What do they do?

Answer 25

Endothelial cells

These lining cells express TLRs and RLRs – when they detect a PAMP they:
Increase expression of ICAMs and selectins Increase the production of pro-inflammatory cytokines and chemokines

Question 26

What cells are important DAMP and PAMP detectors in the skin and mucosal surfaces? What do they do?

Answer 26

PRRs tend to result in increased production of local anti-microbial peptides
Chemokines and cytokines can also be produced if more leukocytes need to be recruited from the circulation