Immunology I: Pattern recongition receptors and the complementary cascade

Question 1

Principle components of the immune system

Answer 1

a) Recognition – differentiating between pathogen types
b) Response – signalling cascades, cell differentiation, movement, proliferation
c) Effect – lysis of cells, binding molecules (agglutination / opsonisation)
d) Integration of system components

Question 2

Pathogen detection receptors

Answer 2

INNATE receptors respond to AGONISTS
- Pattern recognition receptors
- Lectins, toll-like receptors, NOD-like receptors
- detect pathogen / damage associated molecular patterns PAMP / DAMPs
- 100-200 different receptors
- soluble, transmembrane or cytoplasmic

Example:
Virus- TLR3 for DSDNA
Bacteria- TLR4 foor LPS
Parasite- TLR r for GPI anchor

ADAPTIVE receptors respond to ANTIGENS
- Antibody (soluble), or B / T cell (surface) receptors
- Huge repertoire + clonally expressed
- Made by rearrangement of genomic sequences
- 100 million different receptors
- Soluble or cell surface

Question 3

DAMPS and PAMPS

Answer 3

Inital debate over whether PAMPs or DAMPs lead to immune response

PAMP: pathogen associated melecular protein
-> example: bacteria cell wall
-> evidence: discovery of TOL and NOD like receptors

DAMP: damage associated molecular protein
-> Example: self damage (e.g. DNA)
-> Evidence: sterile activation of the immine response

Question 4

Distribution and Type of PRR

Answer 4

Cell -associated PRR

cell surface:
- Toll like receptors
- Mannose receptors
- mechanism: Cellular activation, Phagocytosis,, Cytokine production, antimicrobial production

Intracellular:
- Nod like receptors
- Mechanisms: Cellular activation, Inflammation, Interferon production, Antiviral cascades

Soluble pattern recognition receptors

Body surface:
- Surfactant protein A and D
- mechanism: opsonisation

Tissue fluid:
- Mannose-binding protein
- mechanism: activate component cascade, opsonisation, lysis, infalamation

Which cells have PRRs
- All nucleated cells express PRRs (detects infection + alerts neighbour cells + cause inflammation)
- Macrophages, polymorphonuclear cells, dendritic cells, B cells have wider array of PRRs
- Same PRR triggered in 2 different cell type get different response eg. TLR7 in B / dendritic cells

Sensory system for same agonist in different compartment so cell can work out where pathogen is.

Question 5

Soluble recognition proteins

Answer 5

They are lectins

The surfactant proteins SpA and SpD
- High level of lung secretion
- Binds to LPS structures and gram + bacteria cell wall
- direct antimicrobial activity and triggers complement cascade by enhancing other PRR (In turn enhances / coordinate cellular response)

Manose binding protein
- Present in tissue fluid
- Binds to mannose at specific concentration
- Triggers complementt cascade (In turn enhances / coordinate cellular response)

Question 6

Complementary cascade

Answer 6

complex network of proteins which cleave and recruit each other in a cascade, resulting in the killing of pathogens (opsonisation/ phagocytosis, terminal compliment cascade and inflamatry response)

All 3 pathways rely on recruitment and cleavage of C molecules to make C3 convertase complex

3 induction pathways -> C3 convertase action -> 3 effector pathways

Induction pathways
1. Classical pathway (ADAPTIVE)
- Antibody binds microbe surface and C1q then recruitment protein until C4b2a made forms (C3 convertase)
- C3 -> C3a and C3b

2. Lectin
   - Mannose binding lectin senses mannose density (mannose only high if pathogen present)
   - recruitment protein until complex C3 convertase forms
   - C3 -> C3a and C3b
3. Alternative
   - C3 spontaneously hydrolyses on pathogen cell surface
   - C3b recruits more molecules to for a convertase accelerating cleavage
   - C3 -> C3a and C3b

Membrane bound C3b recruits more molecules to form a complex acting as a convertase accelerating cleavage -> positive feedback

**Effector pathway **

Opsonisation
- C3b opsonises pathogens

Phagocytosis
- surface bound C3b interacts with other complements forming C5 covertase
- C5 -> C5 a and C5 b
- C5a promotes phagocytosis
- there is also release of inflammatory mediators

Terminal complement cascade
- C5b recruits molecules forming a complex on cell surface
- multiple c9 bind to the complex and form a multimetric poor leading to membrane lesion and entry of lytic enzymes (membrane attack complex)

Question 7

Cell associated recognition proteins: Cell surface

Answer 7

Disitrbution:
- 10-13 TLR in all vertebrates
- Sense extracellular env
- Most -> cell surface
- TLR 7, 8 and 9 -> endosome (compartment made by phago/pinocytosis so contains extracellular material
- TLR 4-> both

Structure:
- Leucine rich repeats in extracellular space (detect pathogen -> vary between TLR)
- transmembrane domain and signalling tail inside PM

Process:
- TLR’s detect surface components OR nucleic acids.
- Different TLR detect and signal different regions of the pathogen so integration indicates the type of pathogen to the immune system.
- > Example: Salmonella (TL4- LPS, TL5- flagella, Tl9- CpG motif)
- Upon ligand binding, adaptor proteins are recruited-> signalling pathway
- Activate cell signalling cascades changing cellular behavior-> promote phagocytosis, change expression of cell surface molecules or antimicrobial production, induce cytokine / interferon / chemokine production

TLR4: detect LPS (DAMPS released by pathogens)
- LPS binding protein (LBP) binds to LPS and interacts with GPI anchored molecule leading to recognision by TLR-4
- TLR4 activity can lead to endotoxic (excessive infam response) but lack of activity leads to death from salmonell (shown in mice study injected with salmonella dn LPS)

Question 8

Cell associated recognition proteins: Intra cellular

Answer 8

Types of receptor:
- NOD-like receptors
- RIG
- AIM

Structure:
- Signalling domain
- Nucleotide binding domain (tend to detect viruses)
- Leucine rich repeats (agonist recognition)

Process:
- NOD like receptor bind to agonist (usually viral DNA)
- Activate cell signalling cascades changing cellular behavior -> promote phagocytosis, change expression of cell surface molecules or antimicrobial production, induce cytokine / interferon / chemokine production

Question 9

Key defintions

Answer 9

Cytokine = soluble mediator produced by cells

Chemokine = type of cytokine that causes cell migration and call cells into particular areas

Interferons = Two classes (1 and 2) with many different functions
- activates non-specific immune cells
- antiviral activity
- cell growth inhibition, immunosuppressive effects
- enhancement of macrophage, natural killer (NK) cell, killer (K) cell and neutrophil functions
- cell differentiation-inducing activity.

Interleukin = soluble mediator produced by white blood cells

Question 10

Overall PRR

Answer 10

Soluble PRR activation -> direct binding to pathogens, enhance / coordinate cellular response, activate extracellular cascades

Cell associated PRR activation -> activate cell signalling cascades, promote phagocytosis, change expression of cell surface molecules or antimicrobial production and nutrient binding molecules, induce cytokine / interferon / chemokine production

Question 11

Overview

Answer 11

Cell associated PRR
-> Intracellular
-> Extra cellular

Soluble PRR
-> Body surface
-> Tissue fluid

These activate the complenetary cascade which leads to inflamatry response, MAC, and phagocytosis